Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W1: Supersolids
Sponsoring Units: DCMPChair: Moses Chan, Pennsylvania State University
Room: Spirit of Pittsburgh Ballroom A
Thursday, March 19, 2009 11:15AM - 11:51AM |
W1.00001: The role of dislocations in supersolid Helium-4 Invited Speaker: Disorder and crystallographic defects play a major role in the supersolid phase of ${}^4$He. Monte Carlo simulations addressed the physics of vacancies [1], grain boundaries [2] and dislocations [3], which are the focus of this talk. We find that certain types of edge and screw dislocations are superfluid while other remain insulating, depending on their orientation, Burgers vector and possible splitting of the core. The mechanism for superfluidity is provided by the strain near the core of the defect exceeding a threshold value [4]. Superfluid dislocations can build a network of phase coherent tubes (the so-called Shevchenko state) [3], which might lead to an observable mass decoupling in experiment. I will also look at the interactions between a Helium-3 impurity atom and a screw dislocation [5] and make contact with recent experiments. \\[4pt] [1] M. Boninsegni, A. Kuklov, L. Pollet, N. Prokof'ev, B. Svistunov and M. Troyer, {\it The fate of vacancy-induced supersolidity in ${}^4{\rm He}$}, Phys. Rev. Lett. {\bf 97}, 080401 (2006).\\[0pt] [2] L. Pollet, M. Boninsegni, A. B. Kuklov, N. V. Prokof'ev, B. V. Svistunov, and M. Troyer, {\it Superfluidity of Grain Boundaries in solid $^4$He}, Phys. Rev. Lett. {\bf 98}, 135301 (2007).\\[0pt] [3] M. Boninsegni, A. B. Kuklov, L. Pollet, N. V. Prokof'ev, B. V. Svistunov, and M. Troyer, {\it Luttinger Liquid in the Core of Screw Dislocation in Helium-4}, Phys. Rev. Lett. {\bf 99}, 035301 (2007).\\[0pt] [4] L. Pollet, M. Boninsegni, A. B. Kuklov, N. V. Prokofev, B. V. Svistunov, and M. Troyer, {\it Local stress and superfluid properties of solid Helium-4}, Phys. Rev. Lett. {\bf 101}, 097202 (2008).\\[0pt] [5] P. Corboz, L. Pollet, N. V. Prokof'ev, and M. Troyer, {\it Binding of a $^3$He impurity to a screw dislocation in solid $^4$He}, Phys. Rev. Lett. {\bf 101}, 155302 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W1.00002: Elastic Behavior and Dislocations in Solid Helium Invited Speaker: Torsional oscillator experiments show decoupling of solid $^4$He below 200 mK, the signature of the ``non-classical rotational inertia'' which would characterize a supersolid phase of matter. Solids are distinguished from liquids by their non-zero elastic shear modulus and we have developed a new technique to measure this modulus at very low frequencies and amplitudes. Our measurements show a large and unexpected stiffening in the shear modulus of $^4$He below 200 mK. It has the same dependence on temperature, frequency, amplitude and $^3$He impurity concentration as the decoupling seen in torsional oscillator measurements and the two phenomena are clearly related. The elastic behavior is explained in terms of the motion of dislocations and their pinning by impurities, suggesting that these defects play an important role in supersolidity. We have now extended our elastic measurements to both bcc and hcp $^3$He. These measurements, and comparisons to new torsional oscillator results, clarify the roles of quantum statistics and crystal structure in the behavior of solid helium. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W1.00003: The effect of order, disorder, and confinement on the NCRI response in solid $^{4}$He Invited Speaker: After a multitude of experimental and theoretical efforts over the past few years attempting to explain the microscopic origin of non-classical rotational inertia (NCRI) signals seen in torsional oscillator (TO) experiments, disorder has emerged as a crucial factor for determining the supersolid behavior. In an attempt to discover the type of disorder relevant to the NCRI effect we have performed TO experiments on solid $^{4}$He samples grown at constant pressure or temperature from the superfluid [1], a method known to reliably produce large helium crystals. We show that similar NCRI signals are seen for samples with grain boundary areas that differ by orders of magnitude, indicating that grain boundaries are largely irrelevant to the supersolid phenomenon. In addition, we probe the effect of confining the helium crystals within several restricted geometries. [1] A.C. Clark, J.T. West, and M.H.W. Chan, Phys. Rev. Lett. \textbf{99}, 135302 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W1.00004: X-ray scattering experiments on solid helium Invited Speaker: Using x-ray synchrotron radiation, we have studied the nature of crystals of solid $^{4}$He at temperatures down to 50 mK. Measurements of peak intensities and lattice parameters do not show indications of the supersolid transition. Between 50 mK and 0.6K the relative change in the lattice parameters is less than 2$\times $10$^{-5}$ and that in $\left\langle {u^2} \right\rangle $ less than 4$\times $10$^{-3}$. Scanning with a small (down to 10 x 10 $\mu $m$^{2})$ beam, we resolve a mosaic structure within these crystals consistent with numerous small angle grain boundaries. The mosaic shows significant motion even at temperatures far from melting. When grown in aerogel, solid $^{4}$He polycrystalline, with an hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high quality crystals grown from the superfluid phase. The low temperature supersolid fraction is only $\sim $3x10$^{-4}$ and the onset temperature is $\sim $~100~mK. Work done in collaboration with C.A. Burns, M.H.W. Chan, C.N. Kodituwakku, L.B. Lurio, A. Said and J.T. West. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W1.00005: Torsional oscillator experiments on helium films on graphite; the search for a two dimensional supersolid Invited Speaker: $^{4}$He films adsorbed on graphite have been investigated by torsional oscillator methods in the temperature range 2.5mK to 3.5K, focussing primarily on the behaviour of the second layer. The second layer atoms move in a $^{4}$He lattice potential created by the compressed first solid layer. In our experiments, we observe an anomalous response consistent with that previously discussed by Crowell and Reppy [1]. We have made precise measurements of the film decoupling and its contribution to the dissipation over this wide temperature range. These allow us to infer $\rho _{s}(T$ = 0) as a function of second layer density. The results indicate a supersolid response of the second layer in the vicinity of the putative $\surd $7 $\times \surd $7 triangular superlattice phase. The triangular lattice, unlike the square lattice, is predicted to support a supersolid phase. Cold atoms in a triangular lattice provide a candidate system to stabilise this new phase of matter, but this has yet to be realized experimentally. The ``super''-response of the second $^{4}$He layer as a function of filling of the underlying lattice potential will be discussed. \\[4pt] [1] P A Crowell and J D Reppy, Phys. Rev. B53, 2701 (1996) [Preview Abstract] |
Session W2: Progress in Understanding the Nature of the 5/2 Fractional Quantum Hall State
Sponsoring Units: DCMP GQIChair: Steven Simon, Alcatel-Lucent
Room: Spirit of Pittsburgh Ballroom BC
Thursday, March 19, 2009 11:15AM - 11:51AM |
W2.00001: Finite-Layer Thickness Stabilizes the Pfaffian State for the 5/2 Fractional Quantum Hall Effect: Wave Function Overlap and Topological Degeneracy Invited Speaker: The fractional quantum Hall effect (FQHE) in the second orbital Landau level at even-denominator filling factor 5/2 remains mysterious and is currently motivating many scientists not only because of its connection to a possible implementation of a fault tolerant topological quantum computer (Das Sarma et al., PRL 94, 166802(2005)). In this work, we theoretically consider the effect of the quasi-two-dimensional nature of the experimental fractional quantum Hall systems on a number of FQHE states in the lowest three orbital Landau levels. Our primary result is that the finite width of the quasi-two-dimensional systems produce a physical environment sufficient to stabilize the Moore-Read Pfaffian state thought to describe the FQHE at filling factor 5/2. This conclusion is based on exact calculations performed in the spherical and torus geometries, studying wave function overlap and ground state degeneracy. Furthermore, our results open the possibility of creating optimal experimental systems where the 5/2 FQHE state would more likely be described by the Moore-Read Pfaffian. We also discuss the role of the three-body interaction Hamiltonian that produces the Moore-Read Pfaffian as an exact ground state and particle-hole symmetry in the FQHE at 5/2. We acknowledge support from Microsoft Project Q. Work done in collaboration with Sankar Das Sarma, Thierry Jolicoeur, and Kwon Park. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W2.00002: Understanding the 5/2 fractional quantum Hall effect without the Pfaffian wave function Invited Speaker: The fractional quantum Hall effect (FQHE) in the second Landau level has attracted attention, because the lowest Landau level theories do not straightforwardly generalize to these states, and several of the porposed models feature excitations with non-Abelian braiding statistics, with possible applications in topological quantum computing. In particular, the FQHE states at $\nu=5/2$ and 7/2, which have no lowest Landau level analogons, are usually understood in terms of the paired composite fermion model proposed by Moore and Read. I present an alternative understanding of the 5/2 FQHE within the composite fermion theory. I argue that the residual interaction between composite fermions plays a crucial role in establishing incompressibility at 5/2. The low-energy spectrum and the activation gap are estimated with the help of a perturbative procedure that incorporates inter-composite- fermion interactions. This approach is amenable to systematic improvement, and produces ground as well as excited states. It, however, does not relate to non-Abelian statistics in any obvious manner. The emergence of incoompressiblity due to inter-composite-fermion interactions is also observed other fractions in the second Landau level, notably at $\nu=2+2/5$ and 2+3/8. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W2.00003: Density Matrix Renormalization Group Studies of Incompressible Fractional Quantum Hall States Invited Speaker: In this talk I introduce a powerful technique, the density-matrix renormalization group (DMRG), for studying ground and excited state properties of incompressible FQH states on the sphere. This method not only reproduces the numerical results obtained earlier in the exact diagonalization studies, but we are able to extend our understanding of the ground-state and low-lying excited state properties of these FQH states to substantially larger system sizes. We address a very important open problem in fractional quantum Hall physics, namely, if the half-filled second Landau level state can exhibit non-Abelian statistics. By studying large systems on the sphere, and extrapolating to the thermodynamic limit, we determine that the ground state for this filling fraction is fully polarized for shifts corresponding to both the Moore-Read Pfaffian state and its particle-hole conjugate (anti-Pfaffian). This result is found to be robust against small variations of the interaction, strongly supporting the argument favoring a non-Abelian state. We further extend the application of this technique to identify other incompressible Hall states in the second Landau level. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W2.00004: Intrinsic Gap of the 5/2 Fractional Quantum Hall State and Tilted Field Experiments Invited Speaker: Nearly twenty years since the first discovery of a an even denominator fractional quantum Hall state (FQHE), a complete understanding of the the 5/2-state continues to be among the most important questions in semiconductor physics. It is widely believed that this unique state of matter is theoretically best described by the Moore-Read Pfaffian wavefunction,resulting from a BCS-like pairing of composite fermions [1]. In recent years this wavefunction has received special interest owing to its non-abelian quantum statistics which underlies a new paradigm for topological (fault tolerant) quantum computation. However, in spite of several important theoretical advancements, an unequivocal experimental verification of the Moore-Read description is still missing. We present results from a study of the 5/2 state in a sample with the lowest electron density reported to date (by nearly a factor of two) [2]. This allows for the observation of the 5/2 in a regime where the cyclotron energy is smaller than the Coulomb interaction energy. We discuss our results in the context of previous work, and we examine the role of disorder on the activation energy gap. Measurements of the energy gap for the 5/2 and the 7/3 FQH states in a tilted field geometry also reveals an unexpected and contrasting dichotomy between the two. Whereas the 7/3 FQH gap is observed to be enhanced by an applied parallel magnetic field, the 5/2 gap is strongly suppressed, in spite of the two gaps being energetically comparable at zero parallel fields in our sample. This calls into question the prevailing theoretical belief that they should behave similarly if both are spin-polarized, and raises doubt as to whether or not the 5/2 state is indeed described by a spin-polarized Pfaffian Moore-Read wavefunction. \\[4pt] [1] G. Moore and N. Read, Nucl. Phys. B 360, 362 (1991).\\[0pt] [2] C.R. Dean, B.A. Piot, P. Hayden, S. Das Sarma, G. Gervais, L.N. Pfeiffer, K.W. West, Phys. Rev. Lett. 100, 146803 (2008).\\[0pt] [3] C.R. Dean, B.A. Piot, P. Hayden, S. Das Sarma, G. Gervais, L.N. Pfeiffer, K.W. West, Phys. Rev. Lett. 101, 186806 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W2.00005: Observations of Low-lying Collective Excitations in Quantum Phases of the Second Landau Level Invited Speaker: Astonishing quantum phases emerge in partially populated higher Landau levels of 2D electron systems. There are even-denominator fractional quantum Hall fluids in the second (N=1) Landau level, and quantum Hall phases that mingle and compete with alternate ground states. Such findings are striking manifestations of novel collective states that emerge from fundamental interactions in 2D. This talk considers results of inelastic light scattering studies of low-lying excitation modes of quasiparticles in these quantum Hall regimes. Experiments in the states of the lowest (N=0) Landau level uncover interplay from interactions in the spin and charge degrees of freedom and the impact of spin textures is clearly seen in spectra of low-lying spin excitations [1,2]. More recent work, explores excitations of the quantum Hall fluids that reside in the N=1 Landau level [3]. These experiments discovered the intriguing collapse of the long wavelength ferromagnetic spin wave for filling factors that are slightly below filling factor nu=3. While the collapse of the spin wave at fillings near nu=1had been linked to loss of spin polarization due to formation of spin textures, the low-lying excitation modes seen in the N=1 level below nu=3 seem much more complex because a sharp spin wave does not recover for fractional quantum Hall states such as nu=8/3 and 5/2. These measurements suggest significant roles for quasiparticle spin in the competing quantum phases that emerge in the second Landau level. This work is a collaboration with T. D. Rhone, J. Yan, Y. Gallais, I. Dujovne, C. Hirjibehedin, J. Groshaus, B.S. Dennis, L.N. Pfeiffer and K.W. West. \\[4pt] [1] J. Groshaus et al., Physical Review Letters 100, 086806 (2008). \\[0pt] [2] Y. Gallais et al., Physical Review Letters 100, 046804 (2008). \\[0pt] [3] T.D. Rhone et al., contributed talk at this meeting. [Preview Abstract] |
Session W3: Physics of Circulating Tumor Cells and Metastasis
Sponsoring Units: DBPChair: Jerry Lee, National Cancer Institute
Room: 301/302
Thursday, March 19, 2009 11:15AM - 11:51AM |
W3.00001: Single-molecule Force Spectroscopy of Intercellular Adhesion in Cancer Invited Speaker: The progression of several human cancers correlates with the loss of a-catenin from E-cadherin-rich intercellular junctions and loss of adhesion. However, the potential role of a-catenin in directly modulating the adhesive function of individual E-cadherin molecules in human cancer is unknown. Here we use single-molecule force spectroscopy to probe the tensile strength, lifetime, and interaction energy between live human parental breast cancer cells lacking a-catenin and these cells where a-catenin is re-expressed. We find that the tensile strength and lifetime of single E-cadherin bonds between parental cells are significantly lower over a wide range of loading rates. Statistical analysis of the force-displacement spectra reveals that single cadherin bonds between cancer cells feature an exceedingly low energy barrier against tensile forces and low molecular rigidity. These results suggest that the loss of a-catenin drastically reduces the adhesive force between individual cadherin pairs on adjoining cells, explain the global loss of cell adhesion in human breast cancer cells and show that the forced expression of a-catenin in cancer cells can restore both higher intercellular avidity and intermolecular E-cadherin affinity. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W3.00002: Studying Cell Motility and Cell Mechanics with ``Designer Cells" Invited Speaker: Micro/nanopatterning allows for the creation of cells of identical morphologies and with ``designed'' organization of the cytoskeleton. Analysis of such ``Designer Cells'' via high-resolution microscopy allows for studying the intracellular processes related to cytoskeletal dynamics and cancer invasiveness in quantitative detail. In addition, three-dimensional imaging can be used to reconstruct cell shapes and describe these shapes by mathematical functions - it is found that cells are constant-curvature surfaces corresponding to the minima of relatively simple energy functionals describing cell micromechanics. These and other results have implications for physical assays with which to diagnose the metastatic form of cancer. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W3.00003: Nanostructured Substrates for Capturing Circulating Tumor Cells in Whole Blood Invited Speaker: Over the past decade, circulating tumor cells (CTCs) has become an emerging ``biomarker'' for detecting early-stage cancer metastasis, predicting patient prognosis, as well as monitoring disease progression and therapeutic outcomes. However, isolation of CTCs has been technically challenging due to the extremely low abundance (a few to hundreds per ml) of CTCs among a high number of hematologic cells (109 per mL) in the blood. Our joint research team at UCLA has developed a new cell capture technology for quantification of CTCs in whole blood samples. Similar to most of the existing approaches, epithelial cell adhesion molecule antibody (anti-EpCAM) was grafted onto the surfaces to distinguish CTCs from the surrounding hematologic cells. The uniqueness of our technology is the use of nanostructured surfaces, which facilitates local topographical interactions between CTCs and substrates at the very first cell/substrate contacting time point. We demonstrated the ability of these nanostructured substrates to capture CTCs in whole blood samples with significantly improved efficiency and selectivity. The successful demonstration of this cell capture technology using brain, breast and prostate cancer cell lines encouraged us to test this approach in clinical setting. We have been able to bond our first validation study with a commercialized technology based on the use of immunomagnetic nanoparticles. A group of clinically well-characterized prostate cancer patients at UCLA hospital have been recruited and tested in parallel by these two technologies. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W3.00004: A portable circulating tumor cell capture microdevice Invited Speaker: Sensitive detection of earliest metastatic spread of tumors in a minimally invasive and user-friendly manner will revolutionize the clinical management of cancer patients. The current methodologies for circulating tumor cell (CTC) capture and identification have significant limitations including time, cost, limited capture efficiency and lack of standardization. We have developed and optimized a novel parylene membrane filter-based portable microdevice for size-based isolation of CTC from human peripheral blood. Following characterization with a model system to study the recovery rate and enrichment factor, a comparison of the microdevice with the commercially available system using blood from cancer patients demonstrated superior recovery rate and the promise of clinical utility of the microdevice. The development of the microdevice and its potential clinical applicability will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W3.00005: Detection and Characterization of Circulating Tumor Cells Invited Speaker: Circulating tumor cells (CTCs) occur in blood below the concentration of 1 cell in a hundred thousand white blood cells and can provide prognostic and diagnostic information about the underlying disease. While numeration of CTCs has provided useful information on progression-free and overall survival, it does not provide guidance of treatment choice. Since CTCs are presumed contain features of the metastatic tissue, characterization of cancer markers on these cells could help selection of treatment. At such low concentrations, reliable location and identification of these cells represents a significant technical challenge. Automated digital microscopy (ADM) provides high levels of sensitivity, but the analysis time is prohibitively long for a clinical assay. Enrichment methods have been developed to reduce sample size but can result in cell loss. A major barrier in reliable enrichment stems from the biological heterogeneity of CTCs, exhibited in a wide range of genetic, biochemical, immunological and biological characteristics. We have developed an approach that uses fiber-optic array scanning technology (FAST) to detect CTCs. Here, laser-printing optics are used to excite 300,000 cells/sec, and fluorescence from immuno-labels is collected in an array of optical fibers that forms a wide collection aperture. The FAST cytometer can locate CTCs at a rate that is 500 times faster than an ADM with comparable sensitivity and improved specificity. With this high scan rate, no enrichment of CTCs is required. The target can be a cytoplasm protein with a very high expression level, which reduces sensitivity to CTC heterogeneity. We use this method to measure expression levels of multiple markers on CTCs to help predict effective cancer treatment. [Preview Abstract] |
Session W4: Recent Progress in Spin-Spiral Ferroelectricity
Sponsoring Units: DCMP GMAGChair: Sang-Wook Cheong, Rutgers University
Room: 306/307
Thursday, March 19, 2009 11:15AM - 11:51AM |
W4.00001: Magnetically driven spiral ferroelectrics with high transition temperature Invited Speaker: In the past few years, a new class of multiferroics have been discovered, wherein non-collinear spiral magnetic order induces ferroelectricity. In these multiferroics, it is not too much to say that the origin of the ferroelectricity is driven by magnetism and is completely different from that in conventional ferroelectrics. However, most of known magnetically driven spiral ferroelectrics operate only at low temperature [ferroelectric Curie temperature ($T_{C}) \quad <$ 40K]. To develop magnetically driven ferroelectrics with higher $T_{C}$, we combined studies of `high $T_{c}$ superconductivity in cuprates' and `multiferroism'. We propose that cuprates having large magnetic superexchange interactions can be good candidates for magnetically driven ferroelectrics with high $T_{C}$. In fact, we demonstrate ferroelectricity accompanied by a spiral magnetic ordering in an simple copper oxide, CuO, which is known as a starting material for the synthesis of high-$T_{c}$ cuprates. CuO shows a spiral magnetic ordering and multiferroics nature below 230K [1]. This result provides a new route to develop magnetically driven ferroelectrics with high $T_{C}$. This work is in collaboration with Y. Sekio, H. Nakamura, T. Siegrist, A. P. Ramirez, W. B Wu, and D. J. Huang. \\[4pt] [1] T. Kimura et al., Nature Mater. 7, 291 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W4.00002: Static and dynamic magnetoelectric coupling in frustrated magnets Invited Speaker: The recently discovered multiferroic materials, where ferroelectricity is induced by spin orders breaking inversion symmetry, show strong sensitivity of electric polarization and dielectric constant to applied magnetic fields. Most of these multiferroics are frustrated magnets with incommensurate spiral spin structures, in which case a polar lattice distortion is driven by the Dzyaloshinskii-Moriya interaction between non-collinear spins. Since this interaction originates from the relatively weak spin-orbit coupling, the induced electric polarization in spiral multiferroics is small compared with that of proper ferroelectrics. Much larger polarizations were predicted for multiferroics where electric dipoles are induced by superexchange interactions between spins. This mechanism of magnetoelectric coupling works for spin structures commensurate with the crystal lattice and does not require non-collinear spins. In many frustrated magnets incommensurate spiral and commensurate collinear spin states compete. Furthermore, in materials such as orthorhombic rare earth manganites RMnO$_{3}$ and RMn$_{2}$O$_{5}$, both types of magnetic states are ferroelectric. This competition has important implications for the dynamic magnetoelectric coupling between spin waves and polar phonons resulting in mixed electromagnon excitations. I will discuss microscopic mechanisms of the single-magnon and bi-magnon excitation by an electric field of light in multiferroic and magnetoelectric materials, focusing in particular on the recently observed electromagnon peaks in orthorhombic manganites and Kagome magnets carrying monopole and toroidal magnetic moments. I will show that optical studies can provide useful information about competing multiferroic states in frustrated magnets. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W4.00003: Multiferroic domain wall and its relevance to magnetoelectric phenomena in ferroelectric helimagnets Invited Speaker: Recently, magnetically induced ferroelectricity and the giant magnetoelectric (ME) effect in helimagnets (HMs) have attracted much attention. In the ferroelectric HMs, the ferroelectric domain walls (DWs) may be clamped with the DWs of the magnetization, the helical plane direction, and/or the wave vector $k$ of HM. In this talk, we show the role of the multiferroic domain wall motion in the giant magnetoelectric effect. We have observed the $P$ under $H$ unparallel to $k$ in a proper screw HM ZnCr$_2$Se$_4$. The origin of the $P$ can be ascribed to the rotation of the conical spin structure. In the high $H$ region, we observe the discontinuous change of the $P$ due to the $k$-flop in this material. The $k$-flop is driven by the DW of $k$. There are two types of the $k$-DW. The stability of the DWs determines the sign of the spin helicity after the $k$-flop. Another example of the ME phenomena related to DW is $P$-flop in DyMnO$_3$. In DyMnO$_3$, the magnetic field along $b$-axis induces the $P$-flop from $P||c$ to $P||a$. The dielectric constant shows a large enhancement in the course of the $P$-flop. We have investigated the dielectric dispersion of the giant magnetocapacitance (GMC) effect and found that the GMC is attributable to the motion of the DW between $bc$ plane spin cycloid ($P||c$) and $ab$ plane spin cycloid ($P||a$) domains. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W4.00004: Electric field control of magnetism and ferroelectricity in single crystals of multiferroic BiFeO$_3$ Invited Speaker: BiFeO$_3$ is a room-temperature multiferroic combining large electric polarization (P) with long-wavelength spiral magnetic order. Significant efforts have been devoted to studies of thin- film BiFeO$_3$ model multiferroic devices, and local control of magnetization by an electric field has been demonstrated recently. However, the extant thin films consist of a poorly controlled patchwork of ferroelastic domains severely impeding experimental work. We report growth of mm-sized single crystals consisting of a single ferroelectric (FE) domain. Switching between two (out of 8) unique directions of P by an electric field is demonstrated. Magnetic moments are strongly coupled to the lattice, and rotate together with P when the field is applied. Electric field can be used to control the populations of the 3 equivalent magnetic domains with different directions of the spiral wave vector. In particular, a FE monodomain with a single-wave-vector magnetic spiral can be prepared. The spiral has the same helicity in the entire sample. All these effects are reversible. Thus, electric field can be used to control the ferroelectric and magnetic states, and even the magnetic helicity of the sample. This level of control, so far unachievable in thin films, makes single- crystal BiFeO$_3$ a promising object for investigation of physics of magnetoelectric coupling in multiferroics, as well as for model multiferroic device research. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W4.00005: Study of Multiferroic Manganites using Double-Exchange Models Invited Speaker: The double exchange (DE) model, supplemented by lattice distortions and superexchange between the $t_{\rm 2g}$ spins, has been very successful in describing the physics of manganites, such as La$_{1-x}$Ca$_{x}$MnO$_3$, including the presence of colossal magnetoresistance in Monte Carlo simulations [1]. In this presentation, we describe the first steps toward the application of the theoretical framework previously used for CMR manganites now to the study of multiferroic manganites. An encouraging result was recently obtained when S. Dong {\it et al.} [2] showed that the addition to the DE model of a next-nearest-neighbor antiferromagnetic $t_{\rm 2g}$ coupling $J_2$ was found to produce a phase diagram that correctly predicts a transition from an A-type AF to a spiral phase and finally to an E-type AF state with increasing $J_2$, as in experiments. This result paves the way for a variety of investigations and theoretical predictions now varying both the hole doping $x$ and $J_2$. Other issues in the area of multiferroics will also be addressed in this presentation, including the prediction of ferroelectricity in the spin zigzag E-type AF state [3]. \vskip 0.2cm [1] C. \c{S}en {\it et al.}, Phys. Rev. Lett. {\bf 98}, 127202 (2007); R. Yu {\it et al.}, Phys. Rev. B {\bf 77}, 214434 (2008); and references therein. \vskip 0.1cm [2] S. Dong, R. Yu, S. Yunoki, J.-M. Liu, and E. Dagotto, Phys. Rev. B {\bf 78}, 155121 (2008). \vskip 0.1cm [3] I. A. Sergienko {\it et al.}, Phys. Rev. Lett. {\bf 97}, 227204 (2006); S. Picozzi {\it et al.}, Phys. Rev. Lett. {\bf 99}, 227201 (2007). [Preview Abstract] |
Session W5: Physics Meets Art
Sponsoring Units: FPSChair: Philip Taylor, Case Western Reserve University
Room: 401/402
Thursday, March 19, 2009 11:15AM - 11:51AM |
W5.00001: Quasicrystals in Medieval Islamic Architecture Invited Speaker: The conventional view holds that girih (geometric star-and-polygon) patterns in medieval Islamic architecture were conceived by their designers as a network of zigzagging lines, and drafted directly with a straightedge and a compass. I discuss our recent findings that, by 1200 A. D., a conceptual breakthrough occurred in which girih patterns were reconceived as tessellations of a special set of equilateral polygons (girih tiles) decorated with lines. These girih tiles enabled the creation of increasingly complex periodic girih patterns, and by the 15th century, the tessellation approach was combined with self-similar transformations to construct nearly-perfect quasicrystalline patterns. These patterns have remarkable properties; they do not repeat periodically, and have special symmetry---and were not understood in the West until the 1970s. I will discuss some of the properties of Islamic quasicrystalline tilings, and their relation to the Penrose tiling, perhaps the best known quasicrystal pattern. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W5.00002: The Story of the Water Cube Invited Speaker: The National Aquatics Center or ``Water Cube,'' constructed for the Beijing Olympics, is unusual in that its very structure has a physical significance. It consists of a massive framework of steel beams that are arranged as in the Weaire-Phelan structure of an ideal foam, with an outer facing of transparent ``cushions.'' Brilliantly conceived by Tristram Carfrae of the Arup Corporation, it makes a spectacular impression on those who enter. It provokes thoughts on aesthetics, order/disorder, optimisation, and the frequent recurrence of bubbles/foams in our literary and artistic culture. The story of the Water Cube will start in the nineteenth century, when William Thomson (Lord Kelvin) first posed the problem: what kind of foam of equal-sized bubbles minimises area (or energy)? [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W5.00003: The Drip Paintings of Jackson Pollock: Are They Really fractal? Invited Speaker: It has been claimed the drip paintings of late Abstract Expressionist painter Jackson Pollock can be usefully characterized as fractal, and that fractal analysis can be used to authenticate works of unknown origin. This academic issue has become of more general interest following the recent discovery of a cache of disputed Pollock paintings. I will demonstrate that this hypothesis of ``Fractal Expressionsim'' is fundamentally flawed, and that fractal analysis as an authentication tool yields inconsistent and unreliable results. This work has also led to two new results in fractal analysis of more general scientific significance. First, the composite of two fractals is not generally scale invariant and exhibits complex multifractal scaling in the small distance asymptotic limit. Second the statistics of box-counting and related staircases provide a new way to characterize geometry and distinguish fractals from Euclidean objects. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W5.00004: Learning from Monet: A Fundamentally New Approach to Image Analysis Invited Speaker: The hands and minds of artists are intimately involved in the creative process, intrinsically making paintings complex images to analyze. In spite of this difficulty, several years ago the painter David Hockney and I identified optical evidence within a number of paintings that demonstrated artists as early as Jan van Eyck (c1425) used optical projections as aids for producing portions of their images. In the course of making those discoveries, Hockney and I developed new insights that are now being applied in a fundamentally new approach to image analysis. Very recent results from this new approach include identifying from Impressionist paintings by Monet, Pissarro, Renoir and others the precise locations the artists stood when making a number of their paintings. The specific deviations we find when accurately comparing these examples with photographs taken from the same locations provide us with key insights into what the artists' visual skills informed them were the ways to represent these two-dimensional images of three-dimensional scenes to viewers. As will be discussed, these results also have implications for improving the representation of certain scientific data. Acknowledgment: I am grateful to David Hockney for the many invaluable insights into imaging gained from him in our collaboration. [Preview Abstract] |
Session W6: Progress on Quantum Optics with Circuit Quantum Electrodynamics
Sponsoring Units: GQIChair: Steve Girwin, Yale University
Room: 406
Thursday, March 19, 2009 11:15AM - 11:51AM |
W6.00001: Controlling Photons, Qubits and their Interactions in Superconducting Electronic Circuits Invited Speaker: A combination of ideas from atomic physics, quantum optics and solid state physics allows us to investigate the fundamental interaction of matter and light on the level of single quanta in electronic circuits. In an approach known as circuit quantum electrodynamics, we coherently couple individual photons stored in a high quality microwave frequency resonator to a fully controllable superconducting two-level system (qubit) realized in a macroscopic electronic circuit [1]. In particular, we have recently observed the simultaneous interaction of one, two and three photons with a single qubit. In these experiments, we have probed the quantum nonlinearity of the qubit/light interaction governed by the Jaynes-Cummings hamiltonian, clearly demonstrating the quantization of the radiation field in the on-chip cavity. We have also performed quantum optics experiments with no photons at all. In this situation, i.e. in pure vacuum, we have resolved the renormalization of the qubit transition frequency - known as the Lamb shift - due to its non-resonant interaction with the cavity vacuum fluctuations [3].\\[4pt] [1] A. Wallraff et al., Nature (London) 431, 162 (2004)\\[0pt] [2] J. Fink et al., Nature (London) 454, 315 (2008)\\[0pt] [3] A. Fragner et al., Science 322, 1357 (2008) [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W6.00002: Nonlinear response of the vacuum Rabi resonance Invited Speaker: On the level of single atoms and photons, the coupling between atoms and the electromagnetic field is typically very weak. By employing a cavity to confine the field, the strength of this interaction can be increased many orders of magnitude to a point where it dominates over any dissipative process. This strong-coupling regime of cavity quantum electrodynamics has been reached for real atoms in optical cavities, and for artificial atoms in circuit QED and quantum-dot systems. A signature of strong coupling is the splitting of the cavity transmission peak into a pair of resolvable peaks when a single resonant atom is placed inside the cavity -- an effect known as vacuum Rabi splitting. The circuit QED architecture is ideally suited for going beyond this linear response effect. Here, we show that increasing the drive power results in two unique nonlinear features in the transmitted heterodyne signal: the supersplitting of each vacuum Rabi peak into a doublet, and the appearance of additional peaks with the characteristic $\sqrt{n}$ spacing of the Jaynes-Cummings ladder. These constitute direct evidence for the coupling between the quantized microwave field and the anharmonic spectrum of a superconducting qubit acting as an artificial atom. Work done in collaboration with L.S. Bishop, J.M. Chow, A.A. Houck, M.H. Devoret, E. Thuneberg, S.M. Girvin, and R.J. Schoelkopf. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W6.00003: Preparation of arbitrary quantum states in a microwave resonator Invited Speaker: Two-level systems, or qubits, can be prepared in arbitrary quantum states with exquisite control, just using classical electrical signals. Achieving the same degree of control over harmonic resonators has remained elusive, due to their infinite number of equally spaced energy levels. Here we exploit the good control over a superconducting phase qubit by using it to pump photons into a high-$Q$ coplanar wave guide resonator and, subsequently, to read out the resonator state. This scheme has previously allowed us to prepare and detect photon number states (Fock states) in the resonator [1]. Using a generalization of this scheme [2] we can now create arbitrary quantum states of the photon field with up to approximately 10 photons. We analyze the prepared states by mapping out the corresponding Wigner function, which is the phase-space equivalent to the density matrix and provides a complete description of the quantum state.\\[2ex] [1] MH {\it et al.}, Nature {\bf 454}, 310 (2008)\\[1ex] [2] Law and Eberly, Phys.\ Rev.\ Lett.\ {\bf 76} 1055 (1996) [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W6.00004: Resonant Two-Qubit Gates and Mesoscopic Shelving Qubit Readout in Circuit QED Invited Speaker: We consider the implementation of universal sets of resonant one-qubit and two-qubit gates for superconducting qubits inside microwave resonators in Circuit QED, aiming at the speed-up of gate operations [1]. We study also the implementation of mesoscopic shelving readout of a superconducting qubit inside a microwave resonator, where a high-fidelity measurement may be achieved [2]. In both proposals we borrow from inspiring quantum-optical tools and concepts, exploiting the advantages of three-level physics and adapting electron-shelving readout in a novel manner in the context of multi-cavity physics [3], and in particular two-cavity Circuit QED [4]. \\[4pt] REFERENCES: \\[0pt] [1] G. Haack, F. Helmer, M. Mariantoni, J. von Delft, F. Marquardt, and E. Solano, ``Resonant toolbox of universal quantum gates in Circuit QED'', in preparation. \\[0pt] [2] B. Englert, G. Mangano, M. Mariantoni, R. Gross, J. Siewert, and E. Solano, ``Mesoscopic Shelving Qubit Readout in Circuit QED'', in preparation. \\[0pt] [3] F. Helmer, M. Mariantoni, A. G. Fowler, J. von Delft, E. Solano, and F. Marquardt, ``Two-dimensional cavity grid for scalable quantum computation with superconducting circuits'', arXiv:0706.3625. \\[0pt] [4] M. Mariantoni, F. Deppe, A. Marx, R. Gross, F. K. Wilhelm, and E. Solano, ``Two-resonator circuit quantum electrodynamics: A superconducting quantum switch'', Phys. Rev. B 78, 104508 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W6.00005: Lasing, Cooling, and Nonequilibrium Photon States in Circuit QED Invited Speaker: Several of the concepts originally introduced in quantum electrodynamics (QED) have been reproduced and extended in recent experiments with superconducting quantum circuits. In these systems a single qubit (or few) is coupled to a microwave resonator. Lasing and cooling of the resonator as well as nonequilibrium photon states, incl. Fock states, have been observed. Apart from the similarities to quantum optics there exist important differences, some of which will be addressed in this talk: (i) Circuit QED provides realizations of ``single-atom lasers,'' with the atom being replaced by a superconducting qubit. The low number of degrees of freedom makes the quantum nature of the field visible. As a result in single atom lasers the lasing transition is smeared. On the other hand, the permanent qubit - resonator coupling allows exploring the specific properties of single-atom lasing. (ii) The coupling between qubit and resonator may be very strong, giving rise to qualitatively new effects. For instance, strong coupling can lead to multiple optimal regimes for lasing and a double peak structure in the resonator output spectrum. In addition higher order correlations gain quantitative importance. (iii) Decoherence and relaxation effects need to be accounted for. The large line-width observed in single-qubit lasers is mainly due to low-frequency noise, which renders the line-shape Gaussian rather than Lorentzian. (iv) Circuit QED offers new ways to drive the qubit, e.g., the qubit may consist of a driven superconducting single-electron transistor, or to engineer resonators with specific anharmonicities. (v) A variety of resonant conditions can be exploited, including the situation where the low Rabi-frequency is in resonance with a slow oscillator. [Preview Abstract] |
Session W7: Information Theory in Biology
Sponsoring Units: DBPChair: Ned Wingreen, Princeton University
Room: 407
Thursday, March 19, 2009 11:15AM - 11:51AM |
W7.00001: Optimizing information flow in biological networks Invited Speaker: The generation of physicists who turned to the phenomena of life in the 1930s realized that to understand these phenomena one would need to track not just the flow of energy (as in inanimate systems) but also the flow of information. It would take more than a decade before Shannon provided the tools to formalize this intuition, making precise the connection between entropy and information. Since Shannon, many investigators have explored the possibility that biological mechanisms are selected to maximize the efficiency with which information is transmitted or represented, subject to fundamental physical constraints. I will survey these efforts, emphasizing that the same principles are being used in thinking about biological systems at very different levels of organization, from bacteria to brains. Although sometimes submerged under concerns about particular systems, the idea that information flow is optimized provides us with a candidate for a real theory of biological networks, rather than just a collection of parameterized models. I will try to explain why I think the time is right to focus on this grand theoretical goal, pointing to some key open problems and opportunities for connection to emerging experiments. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W7.00002: Form, Function, and Information Processing in Stochastic Regulatory Networks Invited Speaker: The ability of a biological network to transduce signals, e.g., from chemical information about the abundance of small molecules into regulatory information about the rate of mRNA expression, is thwarted by numerous sources of noise. A great amount has been learned and conjectured in the last decade about the extent to which the form of a network --- specified by the connectivity and sign of regulation --- constrains or guides the networks function --- the particular noisy input-output relation(s) the network is capable of executing. In parallel, a great amount of research has sought to elucidate the role of inescapable or 'intrinsic' noise arising from the finite copy number of the participating molecules, which sets physical limits on information processing in small cells. I'll discuss how information theory may help illuminate these topics by providing a framework for quantifying function which does not rely on specifying the particular task to be performed a priori, as well as by providing a measure for the extent to which form follows function. En route I hope to show how stochastic chemical kinetics, modeled by the (linear) master equation describing the probability of copy counts for all reactants, benefits from the same spectral approaches fundamental to solving the (linear) diffusion equation. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W7.00003: Decision boundaries for maximizing information transmission in neural circuits Invited Speaker: Everything we know about the world around us is represented in the nervous system in sequences of discrete electrical pulses termed spikes. One attractive theoretical idea, going back to 1950s, is that these representations are efficient in the sense of information theory. I will describe an approach for finding the optimal coupling strengths between different neurons that is based on a concept of a decision boundary [1]. In this framework, neural circuit responses are described by specifiying for each neuron the decision boundary that separates multi-dimensional signals that elicit a spike in that neuron from those signals that do not elicit a spike. The shape and position of individual neurons' boundaries determine the amount of mutual information that the neural circuit can transmit about the incoming signals. Correspondingly, the optimal configuration of the decision boundaries depends on the probability distribution of incoming signals. Signals typical of our natural sensory environment are known to be strongly correlated and to possess large-amplitude deviations that are often better described by an exponential rather than a Gaussian distribution. Considering exponentially distributed signals, we find that optimal decision boundaries of neurons are curved, and that they exhibit sharp discontinuities when decision boundaries of different neurons intersect. This, in turn, corresponds to non-zero coupling constants when these neural circuits are described using the pairwise maximum entropy models. \\[4pt] [1] T. Sharpee and W. Bialek, PLoS One, e646 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W7.00004: Information processing and signal integration in bacterial quorum sensing Invited Speaker: Bacteria communicate with each other using secreted chemical signaling molecules called autoinducers (AIs) in a process known as quorum sensing. Quorum sensing enables bacteria to collectively regulate their behavior depending on the number and/or species of bacteria present. The quorum-sensing network of the marine-bacteria {\it Vibrio harveyi} consists of three AIs encoding distinct ecological information, each detected by its own histidine-kinase sensor protein. The sensor proteins all phosphorylate a common response regulator and transmit sensory information through a shared phosphorelay that regulates expression of downstream quorum-sensing genes. Despite detailed knowledge of the {\it Vibrio} quorum-sensing circuit, it is still unclear how and why bacteria integrate information from multiple input signals to coordinate collective behaviors. Here we develop a mathematical framework for analyzing signal integration based on Information Theory and use it to show that bacteria must tune the kinase activities of sensor proteins in order to transmit information from multiple inputs. This is demonstrated within a quantitative model that allows us to quantify how much {\it Vibrio}'s learn about individual inputs and explains experimentally measured input-output relations. Furthermore, we predicted and experimentally verified that bacteria manipulate the production rates of AIs in order to increase information transmission and argue that the quorum-sensing circuit is designed to coordinate a multi-cellular developmental program. Our results show that bacteria can successfully learn about multiple signals even when they are transmitted through a shared pathway and suggest that Information Theory may be a powerful tool for analyzing biological signaling networks. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W7.00005: to be determined by you Invited Speaker: |
Session W8: Forefront Detectors for Synchrotron Radiation
Sponsoring Units: GIMSChair: Timothy Graber, University of Chicago
Room: 414/415
Thursday, March 19, 2009 11:15AM - 11:51AM |
W8.00001: Advanced X-ray Detector Development at NSLS Invited Speaker: Several detector development projects are underway at Brookhaven National Laboratory's NSLS. These projects are close collaborations between NSLS and BNL's Instrumentation Division, and address various synchrotron radiation techniques such as diffraction, spectroscopy and imaging. They are all based on custom microelectronics for high-density readout and custom sensors fabricated in-house at BNL. The talk will describe selected examples of these developments. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W8.00002: Novel Detector developments for the European XFEL Invited Speaker: The source properties of the European XFEL to be built in Hamburg impose extremely demanding requirements for the X-ray detectors that will be used in the experiments. The high luminosity of European XFEL, with many more pulses per second as compared to the American and Japanese projects, is one of the strong points that for sure will be used to the advantage in the experiments. The time structure is however such that the pulses are not distributed uniformly in time but are delivered in bunch trains (with up to 3000 bunches in a train) of 0.6 msec followed by 99.4 msec with no beam. This means that up to 3000 images will have to be recorded during the bunch train of 0.6 msec. This can only be achieved by temporarily storing the images in the detector, and reading them out during the 99.4 msec intervals. Furthermore, for every pulse of less than a 100 fsec a complete image has to be recorded, one can not use photon counting (``all photons arrive at the same time''), and one has to use integrating detectors, that record the total deposited X-ray energy, but with sufficiently low noise, so that one is able to distinguish between 0, 1, 2, 3, ... photons. On top of this one also wants to be able to record up to 10$^{4}$ photons, meaning a true dynamic range of more than 10$^{4}$, which is far from trivial. I will show various experimental examples, illustrating the specific detector challenges that follow from the above requirements. I will also discuss one solution, currently under development, which is the Adaptive Gain Integration Pixel Detector (AGIPD) project (DESY, PSI, Uni-Bonn, Uni-Hamburg). This detector is based on a classical Hybrid pixel array detector with a dynamically switcheable gain stage to cope with the dynamic range, and an analogue pipeline to store the recorded images during the 0.6 msec bunch train. Two other projects, LPD, and DEPFET will also be mentioned briefly. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W8.00003: Integrating Pixel Array Detector Development Invited Speaker: X-ray experiments are very frequently detector limited at storage ring synchrotron radiation sources, and will be even more so at future x-ray free electron laser and energy recovery linac sources. Limitations most frequently arise from the inability of detectors to efficiently collect and process data at the rates at which the data can be generated. Two bump-bonded silicon pixel array detectors (PADs) are being developed at Cornell University that will greatly enhance data collection capabilities. In these PADs x-rays are converted to electrical signals in a pixelated layer of high resistivity silicon, each pixel of which is connected by a metal solder ``bump'' to a corresponding pixel in a CMOS silicon integrated circuit. Each CMOS pixel contains its own data handling and processing electronics. Since all pixels operate in parallel, the PAD is capable of handling extremely high data throughput. The PAD pixels feature integrating analog front-end electronics which allow extremely high instantaneous count-rates, yet sufficiently high signal-to-noise to be able to detect single x-ray photons. The first PAD is designed for coherent x-ray imaging experiments at the Linac Coherent Light Source (LCLS) at SLAC. This detector frames continuously at the LCLS rate of 120 Hz, where the data for each frame can arrive in femtoseconds. The second detector, a result of a collaboration with the Area Detector Systems Corporation, is designed for high throughput protein crystallography experiments. Both detectors are described, and test data is provided. The capabilities of the detectors suggest a variety of new applications, some of which will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W8.00004: CMOS Hybrid Pixel Detectors for Scientific, Industrial and Medical Applications Invited Speaker: Crystallography is the principal technique for determining macromolecular structures at atomic resolution and uses advantageously the high intensity of 3rd generation synchrotron X-ray sources . Macromolecular crystallography experiments benefit from excellent beamline equipment, recent software advances and modern X-ray detectors. However, the latter do not take full advantage of the brightness of modern synchrotron sources. CMOS Hybrid pixel array detectors, originally developed for high energy physics experiments, meet these requirements. X-rays are recorded in single photon counting mode and data thus are stored digitally at the earliest possible stage. This architecture leads to several advantages over current detectors: No detector noise is added to the signal. Readout time is reduced to a few milliseconds. The counting rates are matched to beam intensities at protein crystallography beamlines at 3rd generation synchrotrons. The detector is not sensitive to X-rays during readout; therefore no mechanical shutter is required. The detector has a very sharp point spread function (PSF) of one pixel, which allows better resolution of adjacent reflections. Low energy X-rays can be suppressed by the comparator At the Paul Scherrer Institute (PSI) in Switzerland the first and largest array based on this technology was constructed: The Pilatus 6M detector. The detector covers an area of 43.1 x 44.8 cm2 , has 6 million pixels and is read out noise free in 3.7 ms. Since June 2007 the detector is in routine operation at the beamline 6S of the Swiss Light Source (SLS). The company DETCRIS Ltd, has licensed the technology from PSI and is commercially offering the PILATUS detectors. Examples of the wide application range of the detectors will be shown. [Preview Abstract] |
Session W9: Systems Far from Equilibrium III
Sponsoring Units: GSNPChair: Narayanan Menon, University of Massachusetts
Room: 303
Thursday, March 19, 2009 11:15AM - 11:27AM |
W9.00001: Depinning transition in failure of disordered materials Laurent Ponson Crack propagation is the fundamental process leading to
material failure. However, its dynamics is far from being fully
understood. In this work, we investigate both experimentally
and theoretically the far-from-equilibrium propagation of a
crack within a disordered brittle material. The variations of
its growth velocity $v$ with respect to the external driving
force $G$ are carefully measured on a brittle rock of average
fracture energy $\langle \Gamma \rangle$. The crack dynamics is
shown to display two regimes, well described by a sub-critical
creep law $v \sim e^{-\frac{c}{(G- \langle \Gamma \rangle)^
{\mu}}}$ with $\mu \simeq 1$ for $G |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W9.00002: Random Organization and Irreversibility at Plastic Depinning Charles Reichhardt, Cynthia Reichhardt We provide evidence that plastic depinning falls into the same class of phenomena as the random organization which was recently studied for periodically driven particle systems by L.~Corte {\it et al.} [Nature Phys. {\bf 4}, 420 (2008)]. In the plastic flow system that we consider, the pinned regime corresponds to a quiescent state while the moving regime corresponds to a fluctuating state. Upon the sudden application of an external force, the system organizes into one of these two states and the time scale required to reach the final state diverges as a power law when approaching a nonequilibrium transition. We propose a simple experiment to test for this transition in colloidal systems with random disorder and in superconducting vortex systems. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W9.00003: Static Avalanches in a Random Landscape A. Alan Middleton, Pierre Le Doussal, Kay J. Wiese We study jumps or avalanches in a model of a $d$-dimensional elastic interface that is pinned by disorder and tied to a harmonic spring. The interface configuration is the most stable one, given the disorder and spring position: as the spring is moved, this most stable configuration undergoes discrete jumps or shocks. We carry out numerical simulations to study these shocks and find: (1) detailed qualitative and quantitative verification of the validity of the functional renormalization group analysis of such interfaces and (2) that the distribution of avalanche sizes is numerically consistent with our new calculation of the exact shape of the avalanche distribution, computed in an $\epsilon=4-d$ expansion. The results are quite similar to those seen for dynamic avalanches, where the drive pushes interface configurations between metastable (not globally stable) states. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W9.00004: A simple model for deformation in solids with universal predictions for stress-strain curves and slip avalanches Karin Dahmen, Yehuda Ben-Zion, Jonathan Uhl A basic model for deformation of solids with only one tuning parameter (weakening epsilon) is introduced. The model can reproduce observed stress-strain curves, acoustic emissions and related power spectra, event statistics, and geometrical properties of slip, with a continuous phase transition from brittle to ductile behavior. Exact universal predictions are extracted using mean field theory and renormalization group tools. The results agree with recent experimental observations and simulations of related models for dislocation dynamics, material damage, and earthquake statistics. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W9.00005: Hysteresis loop area of the kinetic Ising model with next-nearest neighbor interaction William Baez, Trinanjan Datta, Christian Poppeliers We investigate the effects of the next-nearest neighbor interaction on the hysteresis loop area of the two-dimensional kinetic Ising model subject to a time dependent magnetic field. For the nearest neighbor model it is known that the loop area, $A(H_{o},f)$, has a dispersion relationship given by $A(H_ {o},f) \propto H^{2/3}_{o}f^{1/3}$ in the low frequency limit, $f \rightarrow 0$, where $H_{o}$ is the external magnetic field amplitude. Using the Metropolis algorithm we explore the hysteresis dispersion in the low frequency limit for various external magnetic fields. We find that the hysteresis relationship changes, as compared to the nearest neighbor model, in the presence of next-nearest neighbor interaction. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W9.00006: Decay of metastable states in the N-neighbor Ising model Ranjit Chacko, Harvey Gould, W. Klein We study the decay of metastable states in the N-neighbor Ising model in which each spin equally interacts with all other spins. Previous work has shown that near the pseudospinodal in an Ising model with long-range interactions nucleation occurs when many clusters which span a correlation volume coalesce to form the nucleating droplet. By observing the decay of a metastable state in the N-neighbor Ising model we can study the effect of the pseudospinodal on nucleation in a model which does not possess a length scale. This study has implications for spin-crossover materials. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W9.00007: Physical criteria for comparing length and time scales of coarsening models Benjamin Vollmayr-Lee A variety of models have been introduced to study the dynamics of phase separation, ranging from sub-critical kinetic Ising models to phase-field models to Oono and Puri's cell dynamical systems (CDS). These models have in common that at asymptotic late times the dynamics reduces to that of sharp interfaces driven by a surface tension. In practical terms, one is typically interested in simulating these models into the asymptotic late-time regime, but it is not clear how to compare the rates of approach to asymptotia. Additionally, while the sharp interface dynamics have a high degree of universality, it is not clear to what degree this applies to the sub-asymptotic dynamics. A scheme is presented to address these questions. Essentially, one first identifies the relevant parameters that determine the asymptotic dynamics and leading sub-asymptotic dynamics. From these, the appropriate dimensionless measures of effective convergence can be obtained. The technique will be illustrated by a comparison of CDH to the Cahn-Hilliard phase field model. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W9.00008: Dynamics of Nucleation in the Ising model Seunghwa Ryu, Wei Cai While several theories have been developed to describe the kinetics of first order phase transitions, the range of applicability of each theory is not fully understood due to uncertainties in experiments and numerical difficulties in rare event simulations. In this study, we compute the decay rate of meta-stable states of the Ising model to test the validity of several existing nucleation theories. We employ advanced sampling methods to compute the nucleation rate, which spans a range over ten orders of magnitude, as a function of temperature and external field. Investigation of the critical nuclei and the pre-exponential factor reveals that nucleation in the 2d Ising model is well described by the field-theoretic model of Langer (1969). However, discrepancies between theory and numerical results are observed in the 3d Ising model. This discrepancy points to the importance of the shape of the critical nuclei to the nucleation kinetics. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W9.00009: Scaling of the Island Density, Size Distribution and Capture Numbers in 3D Nucleation and Growth John Royston, Jacques Amar The results of kinetic Monte Carlo (KMC) simulations of a model of the irreversible nucleation and growth of fractal islands in 3D are presented along with a comparison with rate-equation (RE) results and mean-field (MF) theory. In previous work for point-islands in 3D it was found that both the scaled island-size distribution (ISD) and capture-number distribution (CND) approach the MF prediction of a diverging ISD and size-independent CND in the limit of large $D/F$ (where $D$ is the monomer diffusion rate and $F$ is the deposition rate). In contrast, here we find that the divergence of the ISD with increasing $D/F$ is much weaker for the case of fractal islands while the scaled CND $C(s/S)$ (where $S$ is the average island size) is not constant but increases linearly with island size $s$. We also find that the exponent $\chi$ describing the dependence of the peak island-density on $D/F$ (e.g. $N_{pk} \sim (D/F)^{-\chi}$) deviates significantly from the standard prediction $\chi = 1/3$. Self-consistent RE results for the average island and monomer densities which give good agreement with simulations are also presented, along with an analytical expression for the exponent $\chi$. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W9.00010: The effects of spatial symmetry breaking on unstable state evolution Rachele Dominguez, Kipton Barros, W. Klein We develop a theory that predicts two distinct stages for the early unstable kinetics of systems with spatial symmetry breaking transitions. In the first stage the kinetics is dominated by symmetry preserving dynamics which acts on a short time scale. In the second stage, which shares some characteristics with the Cahn-Hilliard-Cook theory, noise driven fluctuations break the symmetry of the initial phase on a time scale that is large compared to the first stage for systems with an effective long-range interaction. Our simulations of the initial evolution of a long-range antiferromagnetic Ising model quenched into an unstable region are consistent with our predictions. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W9.00011: Liquid to solid nucleation through onion-structure droplets Kipton Barros, William Klein We start from a Landau-Ginzburg free energy and develop a theory of crystal nucleation for metastable liquids. Saddle points of the free energy represent nucleating droplets and are obtained analytically and numerically. We find nucleating droplets with hexagonal symmetry in two dimensions and bcc and icosahedral symmetries in three dimensions. Surprisingly, we also find nucleating droplets in three dimensions with a spherically symmetric structure resembling the layers of an onion. These onion-structure objects are the preferred nucleating droplets near the spinodal. We discuss recent experiments and simulations which are consistent with our predictions. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W9.00012: Dynamical non-ergodic scaling in continuous finite-order quantum phase transitions Shusa Deng, Gerardo Ortiz, Lorenza Viola We investigate the emergence of universal dynamical scaling in quantum critical spin systems adiabatically driven out of equilibrium, with emphasis on quench dynamics which involves non-isolated critical points ( i.e., critical regions) and cannot be a priori described through standard scaling arguments nor time-dependent perturbative approaches. Comparing to the case of an isolated quantum critical point, we find that non-equilibrium scaling behavior of a large class of physical observables may still be explained in terms of equilibrium critical exponents. However, the latter are in general non-trivially path-dependent, and detailed knowledge about the time-dependent excitation process becomes essential. In particular, we show how multiple level crossings within a gapless phase may completely suppress excitation depending on the control path. Our results typify non-ergodic scaling in continuous finite-order quantum phase transitions. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W9.00013: A deposition model with temperature dependent diffusion Yen-Liang Chou, Michel Pleimling We study a deposition process where the deposed particles are allowed to hope to their neighboring sites with a probability that depends both on the temperature and on the height difference. Changing the temperature, the model evolves from the random deposition model with surface relaxation at zero temperature to the random deposition model at infinite temperature. A generalized dynamic scaling of the surface width as a function of the lattice size, the deposition time, and the temperature is given. The response to a sudden change in temperature is studied. Two types of quenching behavior are observed: a power law decay within the Edwards-Wilkinson regime and an exponential decay in the saturation regime. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W9.00014: Hydrodynamic limit of a model of unstable diffusive interface growth Matteo Nicoli, Mario Castro, Rodolfo Cuerno Recently we have proposed a stochastic moving boundary model to describe the morphological evolution of a large class of diffusive growth systems, with thin film production by Chemical Vapor Deposition and Electrochemical Deposition (ECD) as relevant physical examples. The model has a direct connection with measurable experimental parameters. In order to study the hydrodynamic limit of this model we have performed a small slopes expansion (SSE) that leads to an effective interfacial stochastic equation (ISE). In case of attachment kinetics much larger than the mean growth velocity the kinetic roughening exponents of this ISE are completely different from those of standard universality classes. This equation is a particular instance of a new class of nonlocal interface equations whose novel properties we have studied by numerical and RG techniques. In order to study the model beyond the SSE we have mapped it into an equivalent phase-field model. Numerical simulations of the latter show a remarkable quantitative agreement with ECD experiments. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W9.00015: The Isothermal Dendritic Growth Experiment Archive Matthew Koss The growth of dendrites is governed by the interplay between two simple and familiar processes---the irreversible diffusion of energy, and the reversible work done in the formation of new surface area. To advance our understanding of these processes, NASA sponsored a project that flew on the Space Shuttle Columbia is 1994, 1996, and 1997 to record and analyze benchmark data in an apparent-microgravity ``laboratory.'' In this laboratory, energy transfer by gravity driven convection was essentially eliminated and one could test independently, for the first time, both components of dendritic growth theory. The analysis of this data shows that although the diffusion of energy can be properly accounted for, the results from interfacial physics appear to be in disagreement and alternate models should receive increased attention. Unfortunately, currently and for the foreseeable future, there is no access or financial support to develop and conduct additional experiments of this type. However, the benchmark data of 35mm photonegatives, video, and all supporting instrument data are now available at the IDGE Archive at the College of the Holy Cross. This data may still have considerable relevance to researchers working specifically with dendritic growth, and more generally those working in the synthesis, growth {\&} processing of materials, multiscale computational modeling, pattern formation, and systems far from equilibrium. [Preview Abstract] |
Session W10: Insulators and Dielectrics: Electronic Structure and Optical Properties
Sponsoring Units: DCMPChair: Dillon Fong, Argonne National Laboratory
Room: 304
Thursday, March 19, 2009 11:15AM - 11:27AM |
W10.00001: First-principles theory of coloration of WO$_3$ upon charge insertion Yu Xue, Peihong Zhang Tungsten trioxide is one of the most extensively studied electrochromic materials. Here we report density functional theory (DFT) investigations of the coloration mechanism of WO$_3$ upon charge insertion. Our results explains very well the systematic change in color of Na$_x$WO$_3$ from blue to golden-yellow with increasing sodium concentration. We find that proper accounts for the free-carriers contribution to the optical response are critical for a quantitative understanding of the coloration mechanism in this system. We thank Dr. Yong Zhang for his helpful discussion. We thank Dr. M. D. Jones for his assistance in coding. We acknowledge the computational support provided by the Center for Computational Research at the University at Buffalo, SUNY. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W10.00002: First principles studies of Ce and Eu-doped inorganic Andrew Canning, Anurag Chaudhry, Rostyslav Boutchko, Stephen Derenzo This work presents the results of first principles electronic structure calculations for europium and Ce doped inorganic compounds performed using the pseudopotential method based on the local spin density approximation (LSDA) and generalized gradient approximation+U (GGA+U) in density functional theory. The positions of the europium and cerium 4f and 5d states relative to the valence band maximum and conduction band minimum of the host material are determined. Qualitative predictions of the brightness of scintillation in the doped material is made based on the following criteria: (1) The size of the host material bandgap (2) The energy difference between the VBM (Valence Band Maximum) of the host material and the dopant 4f level (3) The energy difference between the occupied Eu or Ce 5d excited state and the host material CBM (Conduction Band Minimum) (4) The level of localization of the 5d excited state on the dopant atom. We have validated this theoretical approach on examples of known bright scintialltors and non-activated scintillators. We have performed calculations on new Eu doped compounds to determine if they are candidates for Eu2+ activated scintillators [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W10.00003: Electronic structure of the quasi-two-dimensional spin-gap system SrCu$_2$(BO$_3$)$_2$ Andres Saul, Guillaume Radtke, H. Dabkowska, B. Gaulin, G. Botton During the last decade, a lot of theoretical and experimental work has been devoted to the study of the magnetic properties of SrCu$_2$(BO$_3$)$_2$. This compound crystallizes in a tetragonal structure where layers of CuBO$_3$ alternate with planes of Sr atoms along the(001) direction. Due to this unusual structure where Cu$\sp{2+}$ atoms are arranged in the layers to form a network of orthogonal dimers, SrCu$_2$(BO$_3$)$_2$ appears as the first realization of a 2D Heisenberg model known as the Shastry-Sutherland model and exhibits a number of unique features such as a spin gap behavior, unusual magnetic excitations or a magnetization {\it plateaux}. In this work, the electronic structure of this system has been investigated using first-principles band structure calculations within the local-density approximation (LDA)+U method as implemented in the {\tt wien2k} code. The comparison of our calculations with available experimental data (exchange integrals estimated from magnetic susceptibility measurements, optical gap from reflectance measurements, and O-K edge recorded in EELS) shows that the (LDA)+U method with a single value of the parameter $U$ can provide an accurate description of both low-energy-scale (magnetic) and high-energy-scale (electronic) properties of this magnetic insulator. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W10.00004: PDMS-BaTiO$_{3}$ Composites with Mechanically Tunable Optical Properties. Nasser Mohamed, Moises Hinojosa, Virgilio Gonzalez Novel composites that show visible light transmittance, mechanically tunable refractive index and good mechanical properties based on PDMS and BaTiO$_{3}$ (BT) nanoparticles (NP), were prepared in 2 steps. First, NP were obtained via mechanical milling; the BT was used as-purchased. Average particle sizes of $\sim $100nm were selected. Second, the NP were embedded into PDMS by in-situ polymerization. PDMS from Dow Corning (Sylgard 184) was supplied as a kit containing 2 components: the Base and the Curing Agent. The BT content was varied up to 1.0wt{\%}. Finally, thick films were prepared by solvent casting and cured in a vacuum furnace, where the trapped air and solvent were extracted. Weight content of the NP was examined. XRD and Raman confirmed the desired tetragonal phase of BT NP. Average particle size was determined by SEM. EDS maps revealed a homogeneous dispersion of the NP. UV-Vis analysis showed transmittances of $\sim $70{\%}. The ellipsometry results revealed that the wt{\%} of BT significantly influences the optical response of the composite when it is stressed; however the response is not linear. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W10.00005: Measuring Quantum Efficiency of Organic Dyes Encapsulated in Dielectric NanoSpheres Timothy Russin, Erhan Altinoglu, James Adair, Peter Eklund We present results of a fluorescent quantum efficiency ($\Phi )$ study on the encapsulation of the near infrared dye indocyanine green (ICG) in calcium phosphate (CP) nanoparticles (dia$\sim $50 nm). The quantum efficiency ($\Phi $, described as the ratio of photons emitted to photons absorbed) provides a quantitative means of describing the fluorescence of an arbitrary molecule. However, standard quantum efficiency measurement techniques provide only $\Phi $ of the smallest fluorescing unit -- in the case of a nanoparticle suspension, the nanoparticle itself. This presents a problem in accurately describing the quantum efficiency of fluorophores embedded in a nanoparticle. We have developed a method to determine the quantum efficiency of the constituent fluorescent molecules embedded in such a nanoparticle, which provides a more meaningful comparison with the unencapsulated fluorophore. While applicable to generic systems, we present results obtained by our method for the ICG/CP nanoparticles in phosphate buffer solution, revealing a dramatic improvement in per-molecule $\Phi $ driven by encapsulation. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W10.00006: Dynamical mean-field analysis of the photo-induced insulator-metal transition in correlated electron systems --- pump-probe spectroscopy Naoto Tsuji, Takashi Oka, Hideo Aoki Recent pump-probe spectroscopy experiments have revealed that photo-excitation can trigger a `phase transition' from an insulator to a metal in various strongly correlated materials. The transition, occurring inherently out of equilibrium, is distinct from conventional phase transitions. In order to identify the nature of the states emerging during the irradiation of an intense laser, we employ the dynamical mean-field theory combined with the Floquet technique for ac fields, which enables us to take account of both the electron correlation and the nonlinear electric-field effect, two essential ingredients in the photo-induced phenomenon. We apply the method to the Falicov-Kimball model, one of the simplest models of correlated electrons, coupled to an ac pump light. The derived optical conductivity spectrum exhibits a Drude-like peak in the low-energy region indicative of metallization. We have also obtained the nonequilibrium distribution of electrons, which turns out to very much deviate from the Fermi distribution, so that the phenomenon is distinct from the heating effect picture. Interestingly, a dip structure is found to emerge in the charge transfer peak, which is shown to come from the vertex correction. We also discuss the dependence of the optical conductivity on the photon energy of the pump light. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W10.00007: Understanding of Nuclear Quadruple Interaction of $^{ 19}$F* and Binding Energies of Solid Fluorine at the First-Principles Level D.R. Mishra, M.M. Aryal, N.P. Adhikari, S.R. Badu, R.H. Pink, R.H. Scheicher, Lee Chow, T.P. Das We have studied the binding energy (BE) and nuclear quadrupole interaction (NQI) parameters for the $^{19}$F* excited nuclear state in solid fluorine as part of our investigation [1] of the properties of solid halogens using the first principles Hartree-Fock Cluster procedure combined with many-body perturbation theory (MBPT), implemented by the Gaussian 03 set of programs. Our results show that Van der Waals interaction obtained from intermolecular electron correlation has dominant effect on the BE but negligible effect on the NQI parameters. For the latter, our e$^{2}$qQ is 117.7MHz forQ($^{19}$F*), 0.072 *10$^{-28 }$m$^{2}$ [2] and $\eta $ is essentially zero.. The influence of vibrational effects on e$^{2}$qQ is being investigated using a first-principles procedure [3] to bridge the small remaining difference with experiment. [1] M.M. Aryal et al., Hyperfine Interact, 176, 51 (2007). [2] K.C.Mishra et al.,Phys. Rev.B25, 3389(1982). [3] N. Sahoo et al. Phys. Rev. Lett. 50, 913(1983) [4] H. Barfuss et al., Phys. Lett. 90A, 33(1982). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W10.00008: Group IIIA doping in \textit{$\alpha $}-Fe$_{2}$O$_{3}$ for PEC hydrogen production Muhammad N. Huda, Aron Walsh, Yanfa Yan, Su-Huai Wei, Yong-Sheng Hu, Alan Kleiman-Shwarsctein, Eric McFarland, Mowafak Al-Jassim Among iron oxides, \textit{$\alpha $}-Fe$_{2}$O$_{3}$ is the most abundant on earth. Because it has a band gap of approximately 2 eV, it is stable and inexpensive to process, \textit{$\alpha $}-Fe$_{2}$O$_{3}$ has been considered as a potential photoelectrocatalyst for solar driven photoelectrochemical (PEC) water splitting to make hydrogen. However, as \textit{$\alpha $}-Fe$_{2}$O$_{3}$ is a charge-transfer type insulator, the poor conduction properties have limited its efficiency as a PEC material. We will present our study on the doping of group IIIA elements in \textit{$\alpha $}-Fe$_{2}$O$_{3}$ to improve its performance. All the calculations were done with DFT+$U$. The main electronic features of \textit{$\alpha $}-Fe$_{2}$O$_{3}$ remained almost unchanged for group IIIA doping. While for Al-doping, the band gap remained almost the same, for Ga and In substitution the band gap marginally increased. However, increased conduction and PEC efficiency has been experimentally reported for Al-doped \textit{$\alpha $}-Fe$_{2}$O$_{3}$. It will be shown that the change in volume plays an important role in this behavior. A dramatic increase in photo-response cannot be expected for this type of doping in \textit{$\alpha $}-Fe$_{2}$O$_{3}$. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W10.00009: The band gap of ultra-thin amorphous and well-ordered Al$_{2}$O$_{3}$ films on CoAl(100) Volker Rose, Rene Franchy Understanding the insulating properties of thin oxide films is key to developing novel devices. In this work, the band gaps of ultra-thin amorphous and well-ordered alumina films on CoAl(100) were investigated by means of scanning tunneling spectroscopy (STS). The ordered intermetallic alloy CoAl(100) exhibits a magnetic surface, although the bulk is nonmagnetic. Such a material is extremely attractive for innovative technical applications. Utilizing selective oxidation, by which the oxidation of CoAl leads to surface segregation of the element with higher oxygen affinity, thin high-quality Al$_{2}$O$_{3}$ films are formed. Oxidation at 300 K leads to the growth of amorphous oxide, while well-ordered films result at elevated temperatures. In both cases, the self-limiting thickness of the oxide film amounts to around 1 nm. The analysis yields band gaps of 2.8 and 3.6 eV for amorphous and well-ordered Al$_{2}$O$_{3}$, respectively. The with respect to the bulk oxide reduced band gap can be explained by the appearance of defect induced states localized in the band gap. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W10.00010: Spectroscopic analysis of ALD-coated 3D structures and origin of the Berreman effect Giovanna Scarel, Jeong-Seok Na, Kevin Hyde, Gregory Parsons The Berreman effect shed light on various phenomena in 2D systems. However, coatings of 3D systems in soft-lithography and photonic devices, or 3D fibers suggest that the Berreman effect in 3D structures could be different. Experimental and computational infrared spectroscopy studies of 3D structures conformally coated with Al$_{2}$O$_{3}$ and ZnO layers using atomic layer deposition support this conclusion. In 2D systems, defining $\theta _{0}$ the macroscopic incidence angle of the IR beam on a sample, the LO mode absorbance increases as [Sin($\theta _{0})$]$^{4}$ when $\theta _{0}$ becomes grazing. On the other hand, in 3D systems a linear combination of [Sin($\theta _{0})$]$^{4}$ with appropriate coefficients must be considered. Accounting for Snell's law in the simulation model is essential to explain these results and the origin of the Berreman effect. We conclude that sample geometry determines infrared absorbance of LO modes versus $\theta $ and vice-versa Our results promise a new tool to investigate topography of insulating ionic oxide layers. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W10.00011: Dielectric properties of solids in the regular and split charge equilibration formalisms Razvan Nistor, Martin M{\" u}ser We investigate the generic dielectric properties of solids in which atomic charges are assigned within the split-charge equilibration (SQE) method, which contains the regular charge equilibration method as a limiting case. It is shown that the latter always mimics ideal conductors, while any positive bond hardness, which is introduced in the SQE method, turns the solid into a dielectric. Crystals with simple cubic and rocksalt structure are considered explicitly. For these symmetries we map the split-charge formalism onto a continuum model, which can be solved analytically, e.g., we provide simple analytical expressions for how the dielectric constant and penetration depth depend on atomic hardness, bond hardness, and lattice constant. This mapping may prove useful when having to solve the dielectric response of a heterogeneous system to external electrical fields not only on the atomic but also on a coarse-grained scale. Successful comparison of numerical data to analytical solutions is made, including those that contain discretization corrections to the continuum solution. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W10.00012: Charge and Bonding States of Ag Atoms in Superionic Conductor ${\alpha}$-AgI Masato Ito, Kazuo Tsumuraya The fast migration mechanism of the cations in the superionic conductors has been little known up to now. In the case of ${\alpha}$-AgI, the charge states of the Ag atoms and the bonding states between Ag and I atoms during the migration remain to be explained. No explanation has also been given for the origin of both the positions and the asymmetric first peaks of the Ag-I and Ag-Ag pair distribution functions. We investigate the electronic states of AgI using the first principles electronic structure calculations. We use the Bader analysis to evaluate the charges that belong to each atom and obtain the ionicity of the atoms. The stability of the cation pairs in the conductor will be discussed using their binding energies. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W10.00013: Multiplets and Crystal Fields: Systematics for X-Ray Spectroscopies Francois Vernay, Bernard Delley Recently Soft X-ray spectroscopies such as XAS and RIXS, became tools of choice to investigate transition metal oxides. The current resolving power is such that it is nowadays possible to investigate multiparticle excitations like, for instance, bi-magnon dispersion throughout the entire Brillouin zone. Yet, these spectroscopies are strongly linked to local physics: the absorption of a photon and creation of a localized core-hole opens up a shell and therefore a multiplet structure becomes apparent in the spectra. From here we see that it becomes crucial, while interpreting the experimental data, to have a systematic, user-friendly and transparent way of computing the multiplet spectra in order to disentangle in the experiment the information arising from single-particle excitations from the information relevant to collective excitations. We present our approach for arbitrary core-valence multiplets arising from a single configuration. The method covers the full range LS-intermediate-jj and allows to introduce splitting by an arbitrary crystal field easily. [Preview Abstract] |
Session W11: Focus Session: Transport Properties of Nanostructures VI: Kondo Phenomena
Sponsoring Units: DMPChair: Aditi Mitra, New York University
Room: 305
Thursday, March 19, 2009 11:15AM - 11:27AM |
W11.00001: Very high Kondo temperature ($T_{K}\sim $ 80 K) in single self-assembled InAs quantum dots coupled to metallic nanogap electrodes Kenji Shibata, Kazuhiko Hirakawa We have studied electron tunneling through single self-assembled InAs quantum dots (QDs) laterally coupled to metallic nanogap electrodes. Lateral electron tunneling structures were fabricated by forming nanogap metallic electrodes directly upon single self-assembled InAs QDs grown on GaAs surfaces. The n-type substrate was used as a backgate electrode. Although no intentional tunneling barriers were introduced, the fabricated samples worked as single electron transistors and exhibited Coulomb blockade effect. Furthermore, a clear spin-half Kondo effect was observed when strong coupling between the electrodes and the QDs was realized using a large QD with a diameter of $\sim $100 nm. From the temperature dependence of the linear conductance at the Kondo valley, the Kondo temperature, $T_{K}$, was determined to be $\sim $ 81 K. This is the highest $T_{K}$ ever reported for artificial semiconductor nanostructures. This high Kondo temperature is due to strong QD-electrode coupling and large charging/orbital-quantization energies in our self-assembled InAs QD structures. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W11.00002: Reduced Kondo conductance in a quantum dot by a high-biased quantum point contact nearby Kenichi Hitachi, Akira Oiwa, Seigo Tarucha A quantum point contact (QPC) near a quantum dot (QD) can be used for detecting the charge state in a QD. Also a single spin in a QD can be monitored by pulsed gate operation. However it has been shown that applied QPC source-drain bias voltage induces undesirable charge and spin fluctuations in a QD, such as photon-assisted like tunneling in a Coulomb blockade regime or suppressing conductance at spin-half Kondo valley. In this experiment, we examined the influence of Kondo valley and inelastic cotunneling at Coulomb valley in detail. We found that decreasing conductance at Kondo valley can be explained by the increase of local temperature, which is estimated by the conductance at inelastic cotunneling. We predict that this local increase of temperature is caused by the back-action between a QD and a QPC. This gives an alternative explanation of suppressing conductance at Kondo valley, which was thought to be the effect of dephasing a spin singlet between the dot and the lead. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W11.00003: A magnetic field-induced crossover to a non-universal regime in a Kondo dot Andrei Kogan, Tai-Min Liu, Bryan Hemingway, Steven Herbert, Michael Melloch We have measured the magnetic splitting, $\Delta_K$, of a Kondo peak in the differential conductance of a Single-Electron Transistor while tuning the Kondo temperature, $T_K$, along two different paths in the parameter space: varying the dot-lead coupling at a constant dot energy, and vice versa. At a high magnetic field, $B$, the changes of $\Delta_K$ with $T_K$ along the two paths have opposite signs, suggesting that $\Delta_K$ is not a universal function of $T_K$. At low $B$, we observe a decrease in $\Delta_K$ with $T_K$ along both paths. Detailed $\Delta_K(B)$ data for two different $T_K$ show consistency for the splitting onset. Furthermore, we find $\Delta_K/\Delta<1$ at low $B$ and $\Delta_K/\Delta>1$ at high $B$, where $\Delta$ is the Zeeman energy of the bare spin. We discuss an approximate scaling of $\Delta_K$ with $B/T_K$ at low $B$ and compare the findings to previous measurements and theory. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W11.00004: Correlated Wavefunction Description of Kondo States on Metal Surfaces Sahar Sharifzadeh, Patrick Huang, Emily A. Carter At low temperatures, a variety of magnetic impurities adsorbed on metal surfaces form a Kondo state, where the conduction electrons are thought to screen out the spin on the impurity to yield a many-body singlet, based on analogy with bulk Kondo physics in which magnetic quenching is observed at low temperatures.~ In scanning tunneling spectroscopy (STS), this state manifests as a narrow resonance in the density of states at the Fermi level.~ However, qualitative differences in the Kondo resonance lineshape are seen between specific adatom-substrate systems, for reasons that are not understood.~ We present a many-body correlated wavefunction study of Co on transition metal surfaces.~ We apply an embedded configuration interaction (CI) approach, where a finite cluster containing the impurity is described by a many-body CI wavefunction, while the effects of the extended background are included via a periodic density functional theory-based embedding potential.~ We discuss the nature of the correlated wavefunction and impurity orbital structure on different surfaces, and discuss implications for the observed STS data. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W11.00005: Kondo effect in single-molecule magnet transistors Gabriel Gonzalez, Michael Leuenberger, Eduardo Mucciolo We present a careful and thorough microscopic derivation of the anisotropic Kondo Hamiltonian for single-molecule magnet (SMM) transistors. When the molecule is strongly coupled to metallic leads, we show that by applying a transverse magnetic field it is possible to topologically induce or quench the Kondo effect in the conductance of a SMM with either an integer or a half-integer spin S$>$1/2. This topological Kondo effect is due to the Berry-phase interference between multiple quantum tunneling paths of the spin. We calculate the renormalized Berry-phase oscillations of the two Kondo peaks as a function of a transverse magnetic field by means of the poor man's scaling approach. We illustrate our findings with the SMM Ni4, which we propose as a possible candidate for the experimental observation of the conductance oscillations. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W11.00006: Frequency-dependent Full Counting Statistics of Electron Transport in Double Quantum Dots Ramon Aguado, David Marcos, Clive Emary, Tobias Brandes Full Counting Statistics is a powerful tool to study correlations in stochastic processes. It has been applied in the last years to characterize nanoscale transport. We present a technique that allows to calculate finite frequency high-order correlators of the electronic current through an interacting nanostructure. We illustrate our technique by calculating the frequency-dependent shot noise (second order) and skewness (third order) of a double quantum dot. Our results demonstrate that the frequency- dependent skewness contains useful information about the internal quantum dynamics of the nanostructure in bias voltage regimes where the second-order correlations are dominated by thermal fluctuations. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W11.00007: Transport properties of a superconducting single-electron transistor coupled to a nanomechanical oscillator Verena Koerting, T.L. Schmidt, C.B. Doiron, C. Bruder, B. Trauzettel Superconducting single-electron transistors (SSETs) are known to constitute a very sensitive probe for the position measurement of a nanomechanical resonator (NR) which can provide near quantum-limited accuracy. The laws of quantum mechanics, however, also require a backaction of the SSET on the resonator, which limits the sensitivity. Recent experiments have confirmed that the backaction gives rise to an effective thermal bath which has the potential to cool or drive the resonator. Our research attempts to gain a better understanding of this system by examining the action of the NR on the SSET. In particular, we investigate the effect on transport properties of the SSET. We focus on the double Josephson quasiparticle (DJQP) resonance where an especially strong back-action can be observed due to the appearance of two coherent Cooper pair tunneling events. We argue that a measurement of for example the current, the charge noise and the shot noise (Fano factor) provides a direct way of gaining information on the state of the NR. In addition to an analytical discussion of the linear response regime we discuss results of higher order approximation schemes and a full numerical solution. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W11.00008: Spin-dependent tunneling and the Kondo effect in quantum dots.* S. E. Ulloa, A. Ngo, E. Vernek Many-body effects have a significant role in electronic transport of nanoscale systems. Particular interesting systems are quantum dots coupled to electronic reservoirs via quantum point contacts. Due to strong spin-orbit interactions [1,2], quantum point contacts can exhibit spin dependent hybridization of the QD states, opening the possibility for generating spin-polarized transport. In this work we study electronic transport of a single level quantum dot connected to polarizing quantum point contacts (QPCs) in both the Coulomb blockade and Kondo regimes. We study how QPCs generate spin-polarized currents by using scattering matrix methods and the equations-of-motion technique. We calculate the electronic Green's function, conductance and spin polarization in different parameter regimes. Our results show that both Hubbard and Kondo regimes exhibit high spin-polarized conductance. We analyze how the spin-dependent hybridization of the QPC modifies the Kondo resonance, as well as the density of states of the system. These effects are controllable by lateral gate voltages applied on QPCs, as in recent experiments [2]. [1] A. Reynoso \textit{et al.,} Phys. Rev, B \textbf{75}, 085321 (2007). [2] P. Debray \textit{et al.}, unpublished (2008). * Supported by NSF-DMR WMN. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W11.00009: Andreev transport through side-coupled double quantum dots Yoichi Tanaka, Norio Kawakami, Akira Oguri We study the transport through side-coupled double quantum dots, connected to normal and superconducting (SC) leads with a T-shape configuration, using the numerical renormalization group. We find that the Coulomb interaction in the side dot suppresses the destructive interference effect typical of the T-shape geometry, and enhances the conductance substantially in the Kondo regime. This behavior stands in stark contrast to a wide Kondo valley seen in the normal transport. Moreover, the SC proximity penetrating into the interfacial dot pushes the Kondo clouds, which screens the local moment in the side dot, towards the normal lead to make the singlet bond long. The conductance shows a peak of the unitary limit as the cloud expands. It is further elucidated that two separate Fano structures appear in the gate-voltage dependence of the Andreev transport, and the corresponding line shape is quite different from the Fano-Kondo plateau observed in the normal transport. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W11.00010: Zeeman vs resonance splitting effects in a double quantum dot system N. Sandler, E. Vernek, L.G.G.V. Dias da Silva, K. Ingersent, S.E. Ulloa Electron correlations in quantum dot (QD) systems have many intriguing consequences. At low temperatures, the coupling between confined and conduction electrons is known to realize the Kondo effect. This phenomenon exhibits new and interesting features when electrons in an interacting QD hybridize with a non-flat conduction band. For example, when the QD is side-connected to external leads via a second large (noninteracting) QD, the effective density of states coupling to the interacting QD can have a peak at or near the Fermi level. In this regime, interference between the many-body Kondo state in the interacting dot and the single-particle resonance on the other dot causes splitting of the Kondo resonance [1] Here, we use the numerical renormalization group method to study this double-QD system in the presence of an external \textit{in-plane} magnetic field. We explore the interplay between different energy scales and discuss the behavior of the Kondo resonance in the presence of competing interactions. The in-plane field suppresses the Kondo effect, although this requires a stronger field than for a single QD, and the conductance decreases with field in a non-universal fashion. [1] L. G. G. V. Dias da Silva \textit{et al.}, Phys. Rev. Lett.\textbf{ 9}7, 096603 (2006). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W11.00011: Shot noise in a Mn-doped quantum dot nanomagnet L.D. Contreras-Pulido, J. Fernandez-Rossier, R. Aguado A single-electron transistor (SET) based upon a II-IV semiconductor quantum dot doped with a single Mn ion behaves as a nanomagnet whose magnetic properties can be controlled electrically, and the effective exchange between the Mn and the carriers depends whether the SET is operated in the electron or the hole region. For holes, the Ising coupling for symmetric dots in absence of spin-flip Mn-hole exchange, results in Coulomb Blockade oscillations which depend on the spin state of the Mn atom [1]. We extended such analysis and studied finite-frequency shot noise through the SET [2]. Shot noise shows various regimes which, as a function of gate and bias voltages, reflect different magnetic configurations of the nanomagnet. We find super-Poissonian noise in a region of bias and gate voltages where the competing dynamics between slow and fast channels (corresponding to different orientations between the hole and the Mn ion) results in bunching. This behavior appears as a resonance around zero frequency, reflecting charge relaxation dynamics. We also discuss the role of transverse spin-flip terms. [1] J. Fernandez-Rossier and R. Aguado, Phys. Rev. Lett. 98, 106805 (2007) [2] D. Contreras-Pulido, J. Fernandez-Rossier and R. Aguado, in preparation [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W11.00012: Kondo effects in triangular triple quantum dots Akira Oguri, Takahide Numata, Yunori Nisikawa, A.C. Hewson We study the conductance through a triangular triple quantum dot, which is connected to two noninteracting leads, using the numerical renormalization group (NRG). It is found that the system shows a variety of Kondo effects depending on the filling of the triangle. The SU(4) Kondo effect occurs at half-filling, and a sharp conductance dip due to a phase lapse appears in the gate-voltage dependence. Furthermore, when four electrons occupy the three sites on average, a local $S=1$ moment, which is caused by the Nagaoka mechanism, is induced along the triangle. The temperature dependence of the entropy and spin susceptibility of the triangle shows that this moment is screened by the conduction electrons via two separate stages at different temperatures. The two-terminal and four-terminal conductances show a clear difference at the gate voltages, where the SU(4) or the $S=1$ Kondo effects occur[1]. We will also discuss effects of deformations of the triangular configuration, caused by the inhomogeneity in the inter-dot couplings and in the gate voltages. \\[4pt] [1] T.Numata, Y.Nisikawa, A.Oguri, and A.C.Hewson: arXiv:0808.3496. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W11.00013: Interference in triple quantum dot systems George Martins, Edson Vernek, Carlos Busser, Enrique Anda, Sergio Ulloa, Nancy Sandler Transport properties of an interacting triple quantum dot system coupled to three leads in a triangular geometry has been studied in the Kondo regime. Applying mean-field finite-U slave boson and embedded cluster approximations to the calculation of transport properties of this system unveils a set of very rich features associated to its particular symmetry. In the case where just two leads are present, interference effects between degenerate molecular levels are studied, as well as an $S=1$ Kondo effect. The introduction of a third lead does not affect the coherence of propagating electrons, but rather results in an `amplitude leakage' phenomenon, which alters the interference effects. There is a good overall agreement between the two techniques employed. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W11.00014: Bloch oscillations in lateral periodic nanostructure arrays W. Pan, S.K. Lyo, J.L. Reno, J.A. Simmons, D. Li, S.R.J. Brueck In a periodic structure of electron potential, under an external electric field $E$, if an electron can reach the boundary of the Brillouin zone (BZ) without being scattered, it undergoes Bragg reflection, passing back into the BZ on the opposite side. This results in a high frequency oscillation of electrons, i.e., Bloch oscillation (BO). Recently, BO has gained a renewed interest, as a Bloch$^{ }$oscillator can be utilized as a frequency-tunable THz source. Work on BO has mainly been carried out in quantum well superlattices. On the other hand, a surface superlattices patterned into a two dimensional electron system has long been proposed as an alternative structure to generate BO. Here, we report our experimental results on the negative differential conductance and Bloch oscillation induced edge magnetoplasma resonance in a series of lateral superlattices. Results from the so-called reversed Bloch oscillations measurements and bolometric measurements will also be presented and discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. The facilities of the NSF-sponsored NNIN node at UNM were used for the fabrication. [Preview Abstract] |
Session W12: Steps, Islands and Nanostructures
Sponsoring Units: DMP DCMPChair: Ray Phaneuf, University of Maryland
Room: 308
Thursday, March 19, 2009 11:15AM - 11:27AM |
W12.00001: Relaxation of Terrace-width Distributions: Novel Analysis and Features T.L. Einstein, Ajmi BH. Hamouda, A. Pimpinelli We describe a Fokker-Planck scheme to describe the relaxation of the terrace-width distribution (TWD) on a vicinal surface toward the generalized Wigner form describing equilibrium.\footnote{A.\ BH.\ Hamouda, A.\ Pimpinelli, \& TLE, Surface Sci. 602 (2008) 3569} We performed KMC calculations on the standard minimal SOS model to show that the time constant gives physical information, in particular the energy barrier of the rate-determining process. For close-packed steps, this involves kink-antikink generation, breaking 3 rather than the expected 2 lateral bonds (the latter associated with equilibrium fluctuations). We discuss strengths and limitations of this FP procedure, higher moments of the distribution beyond the variance, and generalizations to other step orientations. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W12.00002: Narrowing of Terrace-width Distributions During Growth on Vicinals Ajmi BH. Hamouda, A. Pimpinelli, T.L. Einstein Using kinetic Monte Carlo simulations for a generic minimal SOS model of vicinal surfaces, we compute the terrace-width distributions (TWDs) as a function of incident flux during homoepitaxial growth. We show that the distribution narrows markedly,\footnote{ Ajmi BH. Hamouda, Ph.D. thesis, U.B.P.–Clermont 2} as though there were a flux-dependent repulsion between steps, until the step picture fails at high flux. Using a Fokker-Planck approach\footnote{A.\ BH.\ Hamouda, A. Pimpinelli, \& TLE, Surface Sci. 602 (2008) 3569 }, we analyze the evolution and saturation of this narrowing. We compare with a 1D model\footnote{H.-J. Gossman et al., J. Appl. Phys. 67 (1990) 745} and with our simulations for narrowing due to an Ehrlich-Schwoebel barrier. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W12.00003: Stability and mobility of vacancy nanoclusters on Cu(111) surface: An {\it ab initio} study Alireza Akbarzadeh, Zhengzheng Chen, Nicholas Kioussis We used {\it ab initio} calculations to study stability and mobility of vacancy nanoclusters on Cu(111) surface. We found that the formation energies for single vacancies in the vicinity of surface are $\approx$0.3\,eV lower than that in bulk. Interestingly, calculations yield strongly bonded 1$^{st}$NN divacancy on the surface than in bulk. In addition a trivacancy binds very strong on the surface, indicating that formation of loop-like vacancy nanoclusters are most energetically favored on the surface. These findings imply the ease of nucleation of vacancy nanoclusters on the surface. We also examined migration of mono-, di- and trivacancy on the surface. A zigzag motion for divacancy diffusion on the surface is predicted with the migration barrier higher on the surface than in the bulk due to larger binding energy and elastic contribution. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W12.00004: Two-dimensional island ripening on the basal plane of ice Shu Nie, Norm Bartelt, Konrad Thurmer Despite the importance of ice surfaces to many natural phenomena there have been no accurate measurements of \textit{surface} self-diffusion coefficients of ice. To provide this needed basic information, we applied the newly discovered capability of STM to image thick ice films on Pt [1], and tracked the evolution of 2-dimensional ice islands grown on the basal plane of ice. Uniform 5 nm thick ice films grown at 145 K were used as a template to study surface self-diffusion. By depositing a fraction of a monolayer of water onto these films at 115 K, we created arrays of two-dimensional islands with diameters of 5-10 nm. Remarkably, when annealed to temperatures between 115 and 135 K, these island arrays coarsened. By fitting the average island area to the t$^{2/3}$ growth law expected for diffusion-controlled ripening we extract an activation energy for surface self-diffusion of 0.4 $\pm $0.1 eV, which is on the order of the energy of a hydrogen bond and much less than the value measured for bulk diffusion (0.7 eV) [2]. This work is supported by U. S. DOE, OBES, Division of Materials Sciences under contract DE-AC04-94AL8500. [1] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. B \textbf{77}, 195425 (2008). [2] D. E. Brown and S. M. George, J. Phys. Chem. \textbf{100}, 15460 (1996). [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W12.00005: Ferromagnetic-Semiconductor Interfacial Order Suppression: Self Assembled Fe$_{3}$Ga Island Structures on GaAs(001) Philip Ryan, Jong Woo Kim, Justin Shaw, Charles Falco, Lahsen Assoufid, Richard Rosenberg, David Keavney The practical development of spintronics requires a new class of multifunctional microelectronic components, involving electronic device mechanisms dependent upon ferromagnetic materials. The Fe-GaAs(001) system has been extensively studied as the prototypical spin injection junction for spintronic device mechanisms. Increasing spin injection efficiency has been calculated to be dependent upon the structural order of an abrupt interfacial junction between a ferromagnet and semiconductor. Room temperature low coverage Fe deposition on GaAs(001) reveals the formation of fully strained, epitaxial Fe$_{3}$Ga domains. An iron interfacial layer adheres fully coherent to the buried substrate surface. The adlayer is mediated through the back-bonding of the Fe to substrate terminating As. This structural environment is tied to the suppression of interfacial order. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W12.00006: Snow flake shaped gold nanostructures templated on graphene: an avenue to fabricate novel nano electronic devices Kabeer Jasuja, Vikas Berry Non spherical gold nanoparticles such as rods, multipods, polygons, cubes, stars and branched nanostructures have generated significant research attention in the past few years. Such anisotropic nano structures have been shown to exhibit size and shape dependent properties which are either significantly different or highly pronounced from their spherical counterparts. The unique properties of anisotropic nanostructures (such as localized surface plasmon resonance and surface enhanced fluorescence) make these ideal candidates for a broad range of emerging applications in photonics, opto-electronics, biomedical labeling, sensing and imaging. One of the foremost challenges in utilizing such properties is integrating the anisotropic gold nanostructures into devices which can justifiably tap these properties. Here we demonstrate a simple colloidal synthetic route that results in the formation of snow-flake shaped nanostructures of gold (Au SFs) templated on the nano-sheets of Graphene-oxide(GO). Graphene nanosheets have generated renewed interest in recent years due to their unique 2-dimensional nature and associated electronic, physical and chemical properties. An assembly of Au SFs supported on GO sheets will not only give way to the next generation electronic and optoelectronic nanodevices but will also find wide ranging applications in a number of industrially relevant reactions such as catalysis, fuel cell technology and pollution control. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W12.00007: Large-area nanocrystal superlattice films by surface-tension mediated self-assembly. Angang Dong, Christopher Murray We report a facile and general approach based on the dynamic self-assembly of nanocrystals on the liquid/liquid interface to fabricate hierarchically ordered nanocrystal superlattice films with areas up to several square centimeters. In addition to the close-packed ordering of nanocrystals at nanometer scale, the film exhibits exceptionally ordered stripe patterns at micrometer scale. The stripes are formed by the controlled, repetitive stick-slip motion of the liquid-liquid contact line. Both the film thickness and the stripe periodicity are tunable by changing the nanocrystal concentration. The final nanocrystal film, supported on the liquid surface, can be readily transferred to arbitrary substrates for device fabrication. The methodology reported here not only provides a simple and highly reproducible approach for production of large-area nanocrystal superlattice films, but also opens up a new avenue for lithography-free patterning of nanocrystal arrays for applications in optical, electronic, and magnetic devices. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W12.00008: Mechanism of Asymmetric Growth of Wurtzite Nanostructures: A Case Study of CdSe Through Ab Initio Computations Ghanshyam Pilania, Ramamurthy Ramprasad An interesting and potentially useful phenomenon observed in wurtzite semiconductor nanocrystals is asymmetric anisotropic growth. This property has been exploited in the preferential creation of nanorods, nanoribbons and nanosaws over spherical nanocrystals. However, the details of the mechanism underlying this phenomenon of asymmetric anisotropic growth remain poorly understood. Here, we use CdSe as a prototypical wurtzite system, and oxygen as an agent that encourages asymmetric anisotropic growth. This study focus on the impact of the ordering of the surface energies of several polar and nonpolar surface facets as a function of (i) the chemical potential of Cd (i.e., precursor concentration), (ii) the presence of oxygen adsorbates, (iii) the binding modes of oxygen at the surface, and (iv) the density of oxygen adsorbates on the surfaces, using density functional theory (DFT). Our results show that by controlling the ordering of the surface energies (e.g., though proper choices of precursor concentration, temperature, and surfactants), novel growth modes such as asymmetric growth can be made possible. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W12.00009: \textit{Ab Initio} Study of the Effects of Surface Chemistry and Size on Xray Absorption Spectra of CdSe Nanoparticles Heather Whitley, David Prendergast, Tadashi Ogitsu, Eric Schwegler The specificity of their opto-electronic properties with respect to size, shape, and surface chemistry, as well as cost-effective solution based methods of synthesis, make CdSe nanoparticles a material of choice for use in novel opto-electronic devices, such as photovoltaics and field effect transistors. Developing methods by which these nanomaterials can be systematically engineered to meet specific device goals is largely dependent on understanding how surface passivation and reconstruction affect the properties of a given nanomaterial. Xray absorption spectroscopy (XAS) is an ideal method for structural analysis, but its application to studying nanomaterial surfaces is nontrivial due to the convolution of the absorption of surface atoms with those within the nanomaterial. We utilize \textit{ab initio} methods to investigate the dependence of the Cd L-edge xray absorption cross-section on the size and passivation for Cd atoms both at the surface and within the core of CdSe nanomaterials. We aim to enable routine surface characterization of CdSe nanomaterials via XAS. Prepared by LLNL under Contract DE-AC52 07NA27344. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W12.00010: Microwave induced in-situ deposition of Gold and Silver nanoparticles on chemically modified sheets of graphene: Avenue to build Graphene-metal interface Kabeer Jasuja, Vikas Berry In recent years there has been a great interest in the architecture of 2-D sheets of graphene which have been shown to display remarkable electronic, physical and chemical properties. An extremely high conductivity of graphene sheets along with the ease these can be prepared, has already made graphene as the material of choice for applications in several electronic, optoelectronic and biodevices. There is a great deal of interest in interfacing graphene sheets with other low dimensional nano structures for building novel hybrids. Coupling such low dimensional materials at nano scale yield novel composites with interesting properties. In this study we synthesized nanoparticles of gold and silver on the sheets of graphene-oxide using a one step microwave heating method. Our results indicate that the sheets of chemically modified graphene act as excellent templates for in-situ formation of gold and silver nanoparticles. The advantage of this present synthetic route lies in not using the conventional low molecular weight stabilizing agents which can otherwise react with the graphene sheets leading to impurities. This simple processing approach opens up a new way to synthesize hybrid sheets of graphene decorated with gold and silver nanoparticles which can be used in developing novel catalysts and composites. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W12.00011: Efficient sticking of surface-passivated Si nanospheres via phase-transition plasticity Traian Dumitrica, Mayur Suri Large-scale atomistic simulations considering a $5$~nm in radius H-passivated Si nanosphere that impacts with relatively low energies onto a H-passivated Si substrate reveal a transition between two fundamental collision modes. At impacting speeds of less than $\sim1000$~m/s {\it particle-reflection} dominates. At increased speeds the partial onset in the nanosphere of a $\beta$-tin phase on the approach followed by $a$-Si phase on the recoil is an efficient dissipative route that promotes {\it particle-capture}. In spite of significant deformation, the integrity of the deposited nanosphere is retained. Our result explains the efficient fabrication of nanoparticulate films by hypersonic impaction, where the nanoparticle impact velocities equal $1000$--$2000$~m/s. \\[3pt] [1] M. Suri and T. Dumitric\u{a}, {\it Efficient Sticking of Surface-Passivated Si Nanospheres via Phase-Transition Plasticity}, Physical~Review~B [Rapid~Communication] {\bf 78}, 081405 (2008). \\[0pt] [2] P. Valentini and T. Dumitric\u{a}, {\it Microscopic Theory for Nanoparticle-Surface Collisions in Crystalline Silicon}, Physical Review B {\bf 75}, 224106 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W12.00012: Electronic Nano-Structures as Ionic Barriers: A New Corrosion Prevention Concept Sreeya Sreevatsa, Haim Grebel Corrosion is a longstanding problem which costs the economy billions of dollars annually. The simplest way to prevent corrosion is to use paint thereby blocking diffusion of corrosive component towards the metallic surface. Here we consider a new concept - the electronic barrier -- for corrosion prevention. The barrier is an electronic p-n junction made by topping one film of functionalized carbon nanotubes on another. The barrier is constructed such that the positive ions in the electrolyte are prohibited from reaching the metallic surface through electronic screening. Potentiodynamic tests, Raman spectroscopy and inspection by scanning electron microscope revealed that the order of the layers (namely, p-n or n-p with respect to the metal surface does determine whether the metal corrodes or not. Numerical analysis of the structure will be provided as well. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W12.00013: Ion-beam-assisted nano-texturing of halite-structure thin films Vladimir Matias We study biaxial crystalline texturing at early film growth in a variety of compounds during ion-beam-assisted deposition (IBAD). We have found that many different halite-structure compounds share the ion-beam texturing ability at nucleation and early film growth. This includes numerous oxides and nitrides. Fluorite-structure compounds also exhibit the possibility of fast IBAD texturing. For these materials IBAD texturing can be achieved within the first few nanometers of deposited material. We examine the detailed texture evolution for MgO. To perform these experiments we developed a unique experimental methodology based on linear combinatorial research. Three different texture development regions can be identified in MgO texture evolution. The first stage where biaxial texture first appears is during grain nucleation. There is evidence of a phase transition in this region. With additional IBAD texture continues to improve by grain alignment up to a certain point. Further improvement in crystalline alignment can be achieved by a third stage of epitaxial overgrowth. We find that the IBAD texture development is very sensitive to the nucleation surface conditions, both chemical species and surface morphology. An in-plane texture of less than 2\r{ } and an out-of-plane texture of less than 1\r{ } are attainable in an artificially textured MgO layer on an amorphous substrate This work is supported by the DOE Office of Electricity Delivery {\&} Energy Reliability. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W12.00014: Intercalant Based E-Beam Lithography on a Layered Dichalcogenide Surface Timothy Kidd, Tyler Rash, Laura Strauss We have developed a novel method for creating surface structures on the surface of dichalchogenides using a scanning electron microscope. Using a single shot chemical vapor transport method, large (several mm diameter) single crystals of TiSe2 were synthesized with thin films of CuI upon their surfaces. The films were locally ordered, with distinct pyramidal nanostructures detected using atomic force microscopy (AFM) and scanning electron microscopy (SEM). After removing the surface layers via cleaving, the CuI films at the surface could be renewed by heating the sample. However, it was also found that the formation of these secondary CuI films could be impeded locally by controlled scanning of the SEM. In this way, one could create artificial micro- or nano- structured films upon the dichalcogenide surface in a manner similar to that of standard E-beam lithography. This technique could be used in the development of unique electro-optical devices on dichalchogenide crystal or thin film substrates. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W12.00015: Mesoscopic Modeling of Nanostructured Strained Films: Single-Component vs. Alloy Systems Zhi-Feng Huang, Ken Elder The instability and nanostructure formation in strained solid films are examined through a mesoscopic approach that we developed recently to incorporate both the film crystalline structure and standard continuum theory. It is based on the phase field crystal (PFC) model and particularly the corresponding amplitude equation analysis of the slowly varying film surface profile, for both single-component and binary alloy films. A universal scaling relation for strained island size is identified, showing a crossover from the continuum elasticity result at the weak strain limit to a behavior governed by the ``perfect'' lattice relaxation condition [1]. We also analyze the coupling between the film composition distribution and the evolution of film morphology and nanostructures in alloy systems. Our results indicate the breakdown of conventional continuum approaches even at relatively large scales due to the discrete nature of the film crystalline structure. \\[3pt] [1] Z.-F. Huang and K. R. Elder, Phys. Rev. Lett. 101, 158701 (2008). [Preview Abstract] |
Session W13: Focus Session: Iron Pnictides and Other Novel Superconductors XIV: ARPES
Sponsoring Units: DCMPChair: Norman Mannella, University of Tennessee, Knoxville
Room: 309
Thursday, March 19, 2009 11:15AM - 11:27AM |
W13.00001: Electronic structure of electron doped BaAs$_{2}$Fe$_{2}$ superconductors revealed by Angle Resolved Photoemission P. Vilmercati, I. Vobornik, M. Unnikrishnan, A. Fedorov, A. Goldoni, G. Panaccione, A. Safa-Sefat, R. Jin, M.A. McGuire, B.C. Sales, D.J. Singh, D. Mandrus, N. Mannella The electronic structure in the normal state of Co-doped BaAs$_{2}$Fe$_{2}$ superconductors has been measured by Angle Resolved photoemission (ARPES). Co doping on the Fe site results in electron doping [A. S. Sefat et al., Phys. Rev. Lett. 101, 117004 (2008)]. The data qualitatively reveal that Co-doping results in raising the chemical potential, as expected with electron doping. The Fermi surface topology and the possible relevance to the mechanism of spin fluctuation will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W13.00002: ARPES Study of the Electronic Structure of the Fe Pnictides Ming Yi, Donghui Lu, Ruihua He, Sung-Kwan Mo, James Analytis, Jiun-Haw Chu, Ann Erickson, David Singh, Zahid Hussain, Ted Geballe, Ian Fisher, Xingjiang Zhou, G.F. Chen, Jianlin Luo, Nanlin Wang, Zhi-Xun Shen The iron-based layered superconductors have galvanized explosive interest in the field of high temperature superconductivity since its discovery early this year. With transition temperatures as high as 55K, this new family of compounds not only ended the monopoly of copper oxides in the high T$_{c}$ field, but also provides us a new direction to better understand the phenomenon of high temperature superconductivity. Here we present recent angle-resolved photoemission results on these iron-based layered superconductors, including direct measurements of the electronic band structures and Fermi surface topology. This new class of superconductors is different from the cuprates in that they have a high density of states near the Fermi level and have multiple bands that cross the Fermi level, which make ARPES an ideal technique to study them because of its unique capability to resolve and capture the rich information on the electronic structure in momentum space. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W13.00003: Study of band structure and Fermi Surface of SrFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2 }$by angle-resolved photoemission spectroscopy Madhab Neupane, Y.-M. Xu, Z. Wang, P. Richard, S. Souma, K. Nakayama, T. Sugawara, T. Arakane, Y. Sekiba, A. Takayama, T. Sato, T. Takahashi, X. Dai, Z. Fang, G.F. Chen, J.L. Luo, J. Bowen, N.L. Wang, H. Ding Recently superconductivity has been discovered in many iron pnictides when they are properly doped with charge carriers. Thus it is important to understand the undoped parent compounds that also have a puzzling collinear antiferromagnetic ground state. We have performed a systematic angle-resolved photoemission study on some of the parent compounds, mostly on SrFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$, to investigate their electronic structure and Fermi surface. We will report our experimental results and the comparisons to first-principle band calculations. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W13.00004: Fermi surface and superconducting gap of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ studied by high-resolution ARPES K. Nakayama, T. Sato, Y. Sekiba, P. Richard, S. Souma, M. Neupane, Y.-M. Xu, Z. Wang, X. Dai, Z. Fang, G. F. Chen, J. L. Luo, N. L. Wang, H. Ding, T. Takahashi The electronic states near the Fermi level are the key ingredient to understand the superconducting mechanism of iron-based superconductor. Although electrons in the iron orbitals have been found to play a key role to the occurrence of the superconductivity, the microscopic origin of high-$T_{c}$ superconductivity is still unclear. To address this important issue, we report our recent high-resolution ARPES results on hole-doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, and demonstrate the Fermi-surface-sheet and momentum dependence of the superconducting gap as well as the doping evolution of the Fermi surface and band structure. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W13.00005: ARPES study of doping dependence of the superconducting gap in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ Y.-M. Xu, M. Neupane, P. Richard, K. Nakayama, Y. Sekiba, T. Qian, S. Souma, T. Sato, T. Takahashi, H.-H. Wen, Z. Wang, H. Ding High transition temperature superconductivity has been discovered recently in many doped iron pnictides which join the cuprates in the family of high-Tc superconductors. It is very important to understand the nature of the superconducting gap and its doping dependence, as in the case of the cuprates, in order to understand this new class of superconductors. A systematic angle-resolved photoemission spectroscopy (ARPES) study has been performed on the iron pnictide Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ at different K concentrations, to determine its doping dependence of the superconducting gap. We will report our ARPES results and their implications. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W13.00006: Momentum dependence of superconducting gap, strong-coupling dispersion kink, and tightly bound Cooper pairs in the high-Tc (Sr,Ba)1-x(K,Na)xFe2As2 superconductors Lewis Wray, Dong Qian, David Hsieh, Yuqi Xia, Ali Yazdani, N. Phuan Ong, Nanlin Wang, M. Zahid Hasan We present a systematic angle-resolved photoemission spectroscopic study of the high-T$c$ superconductor class (Sr/Ba)$_{1-x}$(K/Na)$_x$Fe$_2$As$_2$. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, $\Delta(\overrightarrow{k})$. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25-55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 $\AA$, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40$\pm$15 meV kink likely reflects contributions from the frustrated spin excitations in a J$_1$-J$_2$ magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W13.00007: Momentum dependence of the superconducting gap in NdFeAsO$_{1-x}$F$_x$ single crystals measured by angle resolved photoemission spectroscopy Takeshi Kondo, A.F. Santander-Syro, O. Copie, Chang Liu, M.E. Tillman, J. Schmalian, S.L. Bud'ko, P.C. Canfield, A.D. Kaminski We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO$_{1-x}$F$_x$ single crystals. We find that the $\Gamma$ hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 $\pm$ 3 meV and its anisotropy around the hole pocket is smaller than 20$\%$ of this value. This is consistent with an isotropic or anisotropic s-wave symmetry of the order parameter or exotic d-wave symmetry with nodes located off the Fermi surface sheets. This is a significant departure from the situation in the cuprates, pointing to possibility that the superconductivity in the iron arsenic based system arises from a different mechanism. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W13.00008: 3D band structure determination of BaFe2As2, CaFe2As2 and SrFe2As2 Qiang Wang, Zhe Sun, Filip Ronning, Eric Bauer, Suchitra Sebastian, Daniel Dessau The band structure of the parent compounds of iron-arsenic superconductors BaFe2As2, CaFe2As2 and SrFe2As2 are investigated by angle-resolved photoemission spectroscopy. The dispersion of predominant Fe 3d bands has been successfully resolved and compared with theoretical calculations. Although the overall band structure is in line with nonmagnetic DFT computations, the Fe 3d band dispersions strongly deviate from calculations, and the Fermi surface topology differs from theoretical results. These results suggest that some significant correlations have not been correctly involved in the current understanding of these new materials. The kz dependence of the band structure has also been studied for these quasi-2D materials. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W13.00009: Band Structure and Fermi Surface of Extremely Overdoped Iron-Based Superconductors Takafumi Sato, K. Nakayama, Y. Sekiba, P. Richard, S. Souma, Y.-M. Xu, G. F. Chen, J. L. Luo, N. L. Wang, H. Ding, T. Takahashi The discovery of superconductivity at 26 K in LaFeAsO$_{1-x}$F$_{x}$ has triggered intensive researches on the high-temperature ($T_{c})$ superconductivity of iron pnictides and opened a new avenue for high-$T_{c}$ material research beside cuprates. To elucidate the mechanism of high-$T_{c}$ superconductivity in terms of the electronic structure, previous angle resolved photoemission spectroscopy (ARPES) studies have been performed on both hole and electron-doped compounds in the optimally- an non(under)-doped region. On the other hand, little is known about the electronic states in the overdoped region. We report ARPES measurements on heavily overdoped pnictides. Our results indicate that the electronic states around the M point play an important role in the high-$T_{c}$ superconductivity of these materials and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the $T_{c}$ value in the overdoped region. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W13.00010: Electronic structure of electron-doped BaFe$_{2-x}$Co$_x$As Superconductor class studied by ARPES Dong Qian, N.L. Wang, M.Z. Hasan State-of-art high resolution angle-resolved photoemission spectroscopic studies have been carried out on the electron doped BaFe$_{2-x}$Co$_x$As Superconductor (Tc=26K). Electronic band structure, Fermi surface topology and superconducting gap evolution would be reported in this presentation. Nature of the spin sensity wave (SDW) state would be discussed from a band nesting point of view. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W13.00011: Observation of an orbital selective electron-mode coupling in Fe-based high-Tc superconductors Pierre Richard, T. Sato, K. Nakayama, S. Souma, Y.-M. Xu, G.F. Chen, J.L. Luo, N.L. Wang, H. Ding, T. Takahashi The recent discovery of Fe-based superconductors with critical temperatures up to 56 K raises the prospect of unconventional superconducting pairing mechanism. While the electronic pairing in conventional superconductors is mediated by phonons, its nature in the Fe-based high-$T_{c}$ superconductors is unknown. A direct signature of an electron-mode coupling is an anomaly in the electronic energy dispersion (kink). For example, previous angle-resolved photoelectron spectroscopy (ARPES) studies revealed a kink in cuprates, which is believed to be linked to the pairing. We report an ARPES observation of a kink around 25 meV in the dispersion of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ that nearly vanishes above $T_{c}$. The energy scale of the related mode (13$\pm $2 meV) and its strong dependence on orbital and temperature indicates that it is unlikely related to phonons. Moreover, the momentum locations of the kink can be connected by the antiferromagnetic wavevector. Our results point towards an electronic origin of the mode and the superconducting pairing in the Fe-based superconductors, and strongly support the anti-phase s-wave pairing symmetry. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W13.00012: Electronic properties of CaFe$_{2}$As$_{2}$ Chang Liu, Takeshi Kondo, Ari Palczewski, German Samolyuk, Yongbin Lee, Ni Ni, Sergey Bud'ko, Paul Canfield, Adam Kaminski, Aaron Bostwick, Eli Rotenberg CaFe$_{2}$As$_{2}$ is a parent compound of a new family of FeAs based high-$T_{c}$ superconductors. It undergoes a first-order structural transition from low-$T$ orthorhombic to high-$T$ tetragonal phase [Ni \textit{et al.}, Phys. Rev. B \textbf{78}, 014523]. Moderate pressure lowers the transition temperature, and turns on the superconductivity [Torikachvili \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 057006]. Study on its electronic properties is of crucial importance for understanding the pairing mechanism of the FeAs based superconductors. Here we present angle-resolved photoemission spectroscopy (ARPES) results on both the orthorhombic and the tetragonal phase of CaFe$_{2}$As$_{2}$. In the orthorhombic phase, we find strong $k_{z}$ dispersion on the Fermi surfaces, showing a three dimensional electronic structure. We also find dramatic difference of the Fermi surface structure between the orthorhombic and the tetragonal phase. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W13.00013: Fermi surface of the parent compound of iron-based superconductor T. Shimojima, Y. Ishida, N. Katayama, K. Ohgushi, K. Ishizaka, T. Kiss, M. Okawa, T. Togashi, X.-Y. Wang, C.-T. Chen, S. Watanabe, T. Oguchi, S. Shin Fermi surface of the parent compound of iron-based superconductor BaFe2As2 is studied by angle-resolved photoemission spectroscopy using VUV-laser. This compound shows structural and magnetic phase transition around TN = 140 K [1]. We found the transformation of Fermi surface across TN. We will discuss its origin comparing with the first principle band calculation. \\[4pt] [1] M. Rotter et al., Phys. Rev B 78, 020503 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W13.00014: APRES study of in-plane element substituted iron-based superconductors Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Ni$_{x})_{2}$As$_{2}$ Jonathan Bowen, Kensei Terashima, Pierre Richard, Takafumi Sato, Takashi Takahashi, Zhuan Xu, Hong Ding Much excitement has surrounded the recent discovery of the doped iron pnictides which exhibit high temperature superconductivity. These new materials are generally grouped into either 1111 or 122 compounds by the stoichiometric formula of the parent compounds. Understanding how doping with in-plane elements with different valence affects their superconducting properties is a vital element of working toward a complete picture of these interesting new compounds. To that end, we have conducted a high-resolution ARPES study of two 122 compounds: Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Ni$_{x})_{2}$As$_{2 }$and will report our results. [Preview Abstract] |
Session W14: Granular Flow
Sponsoring Units: DFDChair: Arshad Kudrolli, Clark University
Room: 315
Thursday, March 19, 2009 11:15AM - 11:27AM |
W14.00001: Clustering in a Dense, Freely-Falling Granular Stream John R. Royer, Scott R. Waitukaitis, Daniel J. Evans, Heinrich M. Jaeger We investigate the breakup of a freely-falling granular stream into discrete, compact clusters of grains. This breakup, occurring for grain diameters less than about 200 microns falling out of a hopper opening, is reminiscent of the breakup of a liquid stream, though granular materials are generally thought of as lacking a surface tension. Our experiments employ high-speed video imaging in the co-moving frame, which allows us to track the onset of clustering and the subsequent cluster evolution in detail. Varying the material, size, roughness, and wetting properties of the grains as well as the surrounding gas pressure and the hopper opening diameter, we investigate the role of capillary, electrostatic and van der Waals forces in the clustering process. We find that the clustering provides a window to observe very weak cohesive forces between the grains which are masked in other experiments. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W14.00002: Vector Force Measurements of a Dense Granular Flow Kevin Facto, Tom Schicker, Narayanan Menon We have made force measurements at the wall of a dense granular flow. The data was acquired at rate of 800 Hz in all three spatial directions. The fluctuations in the forces were examined for a wide range of flow speeds. Correlations in the forces decay by the time the flow moves one ball diameter. The force along the flow direction is highly correlated with the force normal to the wall. For a given value of normal force, the force along the flow has a gaussian distribution about the tangential force that would be predicted from a constant friction angle. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W14.00003: Space-Time Structure of Granular Flows in a Rough Vertical Channel Donald Candela, Kevin Facto We report measurements using PFG-NMR of the space and time structure of steady granular flows through a long vertical channel of circular cross section with roughened walls. The granular sample consisted of seeds approximately 400$~\mu$m in diameter, flowing through a 9.8~mm ID tube to which was adhered a monolayer of glass beads similar in diameter to the grains. Data was acquired from a region approximately 50 channel diameters higher than the aperture at the channel bottom used to control the flow rate. The mean velocity of the grains as well as the RMS fluctuations in the grain motion were measured as functions of the radial coordinate and for time intervals in the range 5-200~ms, for several different granular flow speeds. For some flow regimes the displacement distributions are distinctly non-Gaussian, at odds with a ``molecular fluid'' model of the granular medium. The time dependence of the fluctuation distribution provides clues to the mechanism by which the gravitational body force is transmitted to the channel walls. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W14.00004: Vibrheology of Granular Matter Joshua Dijksman, Geert Wortel, Martin van Hecke We show how weak agitations substantially modify the rheology of granular materials. We experimentally probe dry granular flows in a weakly vibrated split bottom shear cell -- the weak vibrations act as the agitation source. By tuning the applied stress and vibration strength, and monitoring the resulting strain, we uncover a rich phase diagram in which non-trivial transitions separate a jammed phase, a creep flow case, and a steady flow case. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W14.00005: Shear zones at the walls of a 2D gravity-driven flow of grains Kelsey Hattam, Nalini Easwar, Narayanan Menon We study the flow of spherical grains under gravity in a vertical, straight-walled 2-dimensional hopper, where the flow velocity is controlled by a taper at the outlet. We perform these studies both for monodisperse steel spheres as well as for a bidisperse system of equal numbers of spheres with a ratio of diameters of 1.25. The monodisperse system shows crystalline order even in flow, whereas there is no obvious structural order in the bidisperse system. The velocity profile across the flow is profoundly different in the two systems: the wall shear zone in the monodisperse system extends only a few particle diameters, and there are only small velocity gradients in the bulk of the flow. In contrast to this nearly-plug-like flow, there are significantly broader shear zones in the disordered flow. We report these profiles as a function of the width of the hopper in order to study the scaling of the shear zone with the system size, and with the flow rate. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W14.00006: A Void Diffusion Model of Granular Flow Jayanta Rudra, Paul Vieth In an earlier paper$^{1}$ we derived a nonlinear diffusion equation to describe the dynamics in granular flow based on a Diffusion Void Model (DVM). The equation was successfully used to describe the flow of a homogeneous granular material through the hole of a container under gravity. It also properly described similar flow in the presence of a flat horizontal barrier placed above the hole. Recently, however, we have found out that the above nonlinear equation does not lead to correct static equilibrium. For example, the stability of the free surface of a granular aggregate cannot be described by the equation. The equation also fails to describe, say, how an unstable vertical column of a granular material will change to a stable $\Lambda $-shaped pile at the angle of repose. In this paper work we derive an equation using an appropriate current density of voids that can explain all the observed dynamical characteristics of a simple granular state. $^{1}$Jayanta K. Rudra and D. C. Hong, Phys. Rev. E47, R1459(1993). [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W14.00007: The Role of Extensional Viscosity in Sedimentation of Dense Suspensions Theodore Brzinski, Douglas Durian When two particles in a viscous fluid approach contact the motion of the interstitial fluid is dominated by extensional flows. We are interested in how the details of these flows influence the sedimentation of sense suspensions. To highlight the effects of extensional flows on particle motion we compare systems in which the fluids have the same shear viscosities, but drastically different extensional viscosities. We enhance the extensional viscosity by adding a flexible, high molecular weight polymer. In the case of a system without polymer there is a dense, static packing which grows from the bottom of the container, a region which remains at the initial grain density and settles at a constant velocity, and a clear supernatant at the top. In the polymeric fluid particles settle more slowly, and rather than sedimenting directly from the initial density to a static packing there is a long consolidation process during which the particle density increases at a constant rate. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W14.00008: Local Rearrangements in a Dense Granular Medium During Steady and Oscillatory Shear Steven Slotterback, Krisztian Ronaszegi, Wim Van Saarloos, Wolfgang Losert Cooperative motion is a hallmark of dense granular media. Using the laser sheet scanning method described in [1], we are able to track the motions of all particles in a dense packing of spheres in three dimensions. We analyze the motions of all particles within a split bottom shear cell. We study both steady and oscillatory shearing processes. We compare relative motions of neighboring particles using a measure, P(cos($\alpha ))$, based on a measure originally used by Ellenbroek et al [1]. The angle, $\alpha $, is the angle between the relative displacements of neighboring particles and their bond vectors. A pair of neighboring particles where cos($\alpha )$=0 is called a rolling contact. We find that particles in contact tend to roll past one another, which is consistent with the findings made by Ellenbroek et al for systems close to jamming. We also find that the number of rolling contacts drops at the onset of a shear reversal. [1] Slotterback et al, to appear in Phys Rev Lett [2] Ellenboek et al., Phys Rev Lett, 97 258001 (2006) [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W14.00009: Three-dimensional Order and Self-Diffusion in a Cyclically Sheared Granular System Andreea Panaitescu, Arshad Kudrolli We investigate the structure and dynamics of a dense granular packing (consisting of one millimeter diameter spherical glass beads) undergoing cyclic shear obtained by smoothly deforming a parallelepiped shaped cell. Using a fluorescent refractive index matched particle tracking technique, we obtain the three dimensional position of particles in the central region of the shear cell as a function of shear cycle. The granular packing is observed to evolve towards crystallization over thousands of shear cycles and the packing fraction is correspondingly observed to increase smoothly from loose packing fraction. We obtain the Voronoi cell volume distributions from the measured positions, and compare them with various models which predict a Gamma-distribution and help us define a regularity factor. Further, we discuss the measured radial distribution and the bond-order parameter Q6 which are widely used to quantify local order in spherical particle systems. We find that the initial self-diffusion of the particles is anisotropic with diffusion greater in the flow direction compared with the velocity gradient direction which in turn is greater than the vorticity direction. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W14.00010: Dynamic effective mass of granular media David Johnson, Rohit Ingale, John Valenza, Chaur-Jian Hsu, Nicolas Gland, Hernan Makse We report an experimental and theoretical investigation of the frequency-dependent effective mass, $\tilde{M}(\omega)$, of loose granular particles which occupy a rigid cavity to a filling fraction of 48\%, the remaining volume being air of differing humidities. We demonstrate that this is a sensitive and direct way to measure those properties of the granular medium that are the cause of the changes in acoustic properties of structures containing grain-filled cavities. Specifically, we apply this understanding to the case of the flexural resonances of a rectangular bar with a grain-filled cavity within it. The dominant features of $\tilde{M}(\omega)$ are a sharp resonance and a broad background, which we analyze within the context of simple models. We find that: a) These systems may be understood in terms of a height-dependent and diameter-dependent effective sound speed ($\sim 130$ m/s) and an effective viscosity ($\sim 2\times 10^4$ Poise). b) There is a dynamic Janssen effect in the sense that, at any frequency, and depending on the method of sample preparation, approximately one-half of the effective mass is borne by the side walls of the cavity and one-half by the bottom. c) On a fundamental level, dissipation is dominated by adsorbed films of water at grain-grain contacts in our experiments. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W14.00011: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W14.00012: Visualization of displacement fields in sheared granular systems Kinga Lorincz, Peter Schall The jamming transition, i.e. the transition in a granular system from rest to flow is a fundamental problem of great importance to the understanding of a wide class of disordered materials: grains, clay and glassy materials such as molecular glasses and gels. We visualize the particles in a sheared three-dimensional granular packing immersed in an index matching liquid using confocal microscopy and laser sheet imaging. These experimental methods allow for an accurate determination of the displacement field of the particles at the onset of flow. [Preview Abstract] |
Session W15: Biologically Inspired Physics: Self-Assembly, Filaments, Membranes
Sponsoring Units: DFDChair: Norm Wagner, University of Delaware
Room: 316
Thursday, March 19, 2009 11:15AM - 11:27AM |
W15.00001: Assembly and melting of DNA nanotubes and tile lattices Thomas Sobey, Stephan Renner, Friedrich Simmel Programmable molecular self-assembly using DNA is allowing the demonstration of increasingly novel nanoscale structures such as lattices and tubes. Understanding the assembly and melting pathways of these will allow us to develop more complex and/or stable structures, and potentially useful nanomaterials. We experimentally show differences in these pathways by correlating temperature-controlled UV absorption measurements with atomic force microscopy, fluorescence microscopy, and transmission electron microscopy measurements. The three-dimensional nanotubes assemble in several hierarchical steps but melt in a single step, and this contrast is proposed to arise from the fundamental distinction between three-dimensional closed tubes and two-dimensional open lattices. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W15.00002: Response of a self-assembling to mechanical stress Yves Dubief, Ross Packard, Sreedhar Manchu, Leonie Cowley Coarse-grained molecular dynamics is used to characterize the mechanical properties of a solution of phospholipids and polyelectrolytes under shear and compression. DPPC (1,2-Dipalmitoylphosphatidylcholine), polyelectrolyes and water are coarse-grained using the MARTINI force field. Simulations are performed using both GROMACS and LAMMPS. In our simulation, the solution is confined by two rigid walls. The objective of this work is (i) to study influence of the electrostatic nature of the wall on the self-assembling structure of the solution and (ii) to define the rheological and structural response of the solution under shear and compression by moving one wall. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W15.00003: Self-assembling structures resulting from the presence of polyelectrolytes in a solution of phospholipids Ross Packard, Yves Dubief The objective of this study is the characterization of self-assembled structures formed by the combination of phospholipids and polyelectrolytes. Coarse-grained molecular dynamics is used to simulate solutions of DPPC (1,2-Dipalmitoylphosphatidylcholine) and polyelectrolyes in three dimensional periodic domain. The MARTINI database defines the topology of coarse-grained macromolecules and water and simulations are performed using GROMACS. The interaction between negatively charged polyelectrolytes and positively charged hydrophilic heads of DPPC causes the disruption of lipid bilayer membranes and vesicles. The study attempts to define the conditions necessary for the formation of vesicles or organized networks of lipid bilayers that encapsulate the polyelectrolytes. Such structures are suspected to play an important role in biological fluids subject to large mechanical stress. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W15.00004: Self-organized Gels in DNA/F-Actin mixtures without Crosslinkers John Butler, Ghee Hwee Lai, Olena Zribi, Ivan Smalyukh, Thomas Angelini, Kirstin Purdy, Ramin Golestanian, Gerard C. L. Wong Interactions between flexible chains and rigid rods govern a broad range of soft matter systems. As a model system of like-charged rigid rods and flexible chains, we examine mixtures of DNA and filamentous actin (F-actin). Confocal microscopy reveals the formation of elongated nematic F-actin domains reticulated via defect-free vertices into a network embedded in a mesh of random DNA. Synchrotron small-angle x-ray scattering (SAXS) indicates that the DNA mesh squeezes the F-actin domains into a nematic state with an inter-actin spacing that decreases with increasing DNA concentration. Salt strongly influences the domain sizes and transitions the system from a counterion-controlled regime to a depletion-controlled regime, both mechanisms of which are entropic in origin. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W15.00005: Modeling the hydrophobic effect by coupling solutes to a lattice gas Amish Patel, David Chandler In problems of biological assembly, manifestation of the hydrophobic effect is complex depending on the size as well as the conformation of the solute. The solute disrupts the inherent structure of the solvent by causing an unbalancing of attractive interactions experienced by the solvent molecules The extent of this disruption determines the relative ease with which the solute is solvated. The theory of Lum, Chandler and Weeks (LCW) successfully describes this rich interplay between the solute and solvent structures by coarse-graining the solvent density and analytically integrating out solvent fluctuations on length-scales smaller than the coarse-graining length ($L_{c})$. Since the implementation of LCW theory can be computationally very demanding the coarse-grained solvent density was mapped onto a lattice gas by ten Wolde, Sun and Chandler. In this work, we further improve upon the theory by relaxing certain assumptions about the unbalancing of attractive interactions on length scales smaller than $L_{c}$. In addition to a brief overview of the theory, results obtained by application of the theory to several pertinent problems of hydrophobic assembly will be presented. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W15.00006: Direct Measurement of Inter-filament Forces in Neurofilament Networks: Synchrotron X-ray Diffraction Study under Osmotic Pressure R. Beck, J. Deek, C.R. Safinya Neurofilaments (NFs) are the major protein constituents in neuronal processes (axons and dendrites) that impart mechanical stability and act as structural scaffolds. The filaments assemble from 3 different subunit proteins (NF-L, NF-M, NF-H) to form a 10 nm diameter flexible polymer with radiating unstructured sidearms. Recent work, showed that at high protein concentration, the NFs form a nematic hydrogel network with a well-defined interfilament spacing as can be measured by synchrotron small angle x-ray scattering (SAXS) [1]. In order to directly elucidate the interfilament forces responsible for the mechanical properties of NFs hydrogel, we conducted a SAXS-osmotic pressure study, which yielded pressure-distance curves at different subunit compositions and monovalent salts. We show that filaments composed with NF-L and NF-M strongly attract each other through their polyampholyte sidearms, in particularly at high monovalent salt. However, filaments comprised of NF-L and NF-H, show a distinctly different pressure-distance dependency, with much larger interfilament spacing and weaker salt dependence. Supported by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0803103, and the Human Frontier Science Program organization. [1] J.B. Jones, C.R. Safinya, Biophys. J. \textbf{95, }823 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W15.00007: Flexible ferromagnetic filaments and the interface with biology Andrejs Cebers, Mihails Belovs, Kaspars Erglis Flexible ferromagnetic filaments exist in Nature (magnetotactic bacteria use them for the navigation purposes in the magnetic field of the Earth) and may be synthesized artificially by linking the functionalized ferromagnetic particles by DNA fragments of definite length. Ferromagnetic filaments allow to mimic self-propulsion of microorganisms by using AC magnetic fields. It is investigated both theoretically and experimentally. The elastic properties of the filaments are studied by kinetics of their orientation in an AC magnetic field of enough high frequency and allow to describe the observed deformation of the filaments at reversal of the magnetic field. By numerical analysis the Floquet coefficients for the dynamics of ferromagnetic filaments are calculated and the existence of stationary oscillations of U-like shapes is confirmed. These shapes self-propel perpendicularly to the AC magnetic field. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W15.00008: Electrokinetic effects near a membrane David Lacoste We discuss the electrostatic and electrokinetic contribution to the elastic moduli of a cell or artificial membrane placed in an electrolyte and driven by a DC electric field. The field drives ion currents across the membrane, through specific channels, pumps or natural pores. In steady state, charges accumulate in the Debye layers close to the membrane, modifying the membrane elastic moduli. We first study a model of a membrane of zero thickness, later generalizing this treatment to allow for a finite thickness and finite dielectric constant. Our results clarify and extend the results presented in [D.\ Lacoste, M.\ Cosentino Lagomarsino, and J.\ F.\ Joanny, Europhys. Lett., {\bf 77}, 18006 (2007)], by providing a physical explanation for a destabilizing term proportional to $kps^3$ in the fluctuation spectrum, which we relate to a nonlinear ($E^2$) electro-kinetic effect called induced-charge electro-osmosis (ICEO). Recent studies of ICEO have focused on electrodes and polarizable particles, where an applied bulk field is perturbed by capacitive charging of the double layer and drives flow along the field axis toward surface protrusions; we predict similar ICEO flows around driven membranes, due to curvature-induced tangential fields within a non-equilibrium double layer, which hydrodynamically enhance protrusions. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W15.00009: Piezoelectricity of Fluid Lipid Lamellar Phases and Their Chirality Dependence John Harden, Nicholas Diorio, Alexander Petrov, Antal Jakli The effects of chirality of membrane-forming lipids, has been largely ignored at present. Here we demonstrate that the chirality of phospholipids makes fluid lipid bilayers piezoelectric. This implies that chiral lipids would play a central role in the functioning of cell membranes as active mechano-transducers. By periodically shearing and compressing nonaqueous lamellar phases of left (L-alpha-Phosphatidylcholine), right (D-alpha-Phosphatidylcholine) and racemic (DL-alpha-Phosphatidylcholine) lipids, we induced a tilt of the molecules with respect to the bilayer's normal and produced an electric current perpendicular to the tilt plane with the chiral lipids but not with a racemic mixture. This effect occurs because the lipids from a SmA* phase liquid crystal structure of the bilayers. Under molecular tilt, a ferroelectric SmC* phase is formed, creating a polarization which is normal to the tilt plane. This coupling allows for a wide variety of sensory possibilities of cell membranes such as mechano-reception, magneto-sensitivity, as well as in-plane proton membrane transport and related phenomena like ATP-synthesis, soft molecular machine performance, etc. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W15.00010: Biomotor-functionalized Nanowires for Nanobio-mechanical Applications Dong Shin Choi, Kyung-Eun Byun, Eunhee Cho, Moon-Sook Lee, Seunghun Hong Protein motors such as actomyosin have shown the possibility as a building block for bio-inspired nanomechanical applications such as protein motor-based nanoscale engines. For such applications, it is crucial to combine protein motors with inorganic nanostructure such as nanowires. However, it has been difficult to functionalize nanowires/nanotubes with biological motors due to the incompatibility of such nanostructures with biomotors. Herein, we present a method to functionalize nanowires with biomotors while maintaining their functionalities. Significantly, we successfully demonstrated various motility assays using biomotor-functionalized nanowires, such as the delivery of nanowires functionalized with actin filaments on solid substrates. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W15.00011: Non-monotonic mobility vs. length dependence observed in electrophoretic separation of 25 bp DNA ladder in Pluronic gels. Seungyong You, David Van Winkle We electrophoresed a double-stranded DNA ladder first in an agarose gel, then in gels of Pluronic F-127 at room temperature. The DNA ladder consisted of 19 discrete fragments ranging in length from 25 to 450 bp at 25 bp increments plus 500 bp. The DNA fragments were first separated in agarose gel and stacked normally with 25 bp having the highest mobility. A single lane of the separated DNA ladder in the agarose gel was inserted at the edge of a Pluronic gel slab. The DNA was electrophoresed from the agarose into the Pluronic gels perpendicular to the original separation axis. Mobilities of DNA fragments increased from 25 bp to 175 bp and then decreased from 175 bp to 500 bp. The 25 bp and 500 bp bands of the ladder had approximately the same mobility in several different Pluronic gel concentrations. Both were slower than most bands in between. The highest mobility fragments with length of 175 bp have 59.5 nm contour length which is about 3.5 times the diameter of a micelle (17 nm). This result suggests a crossover from chromatographic separation to electrophoretic separation for these short DNAs. This research is supported by the state of Florida (Martech) and Research Corporation. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W15.00012: Partition function and space-filling fractal-like networks of branching tubes Samir Lipovaca We may think of the probability in quantum mechanics as a sort of fluid that flows from one point to another continuously and without loss or gain. We will utilize this fluid idea and imagine that probability flows through a space-filling fractal-like networks of branching tubes similar to the networks of a general model for the origin of allometric scaling laws in biology. In the general model, scaling laws arise from the interplay between physical and geometric constraints. This model provides a complete analysis of scaling relations for mammalian circulatory systems that are in agreement with data. We will show that there is a connection between a quantum system in thermal equilibrium and space-filling fractal-like networks. The relationship will be revealed through the calculation of the total fluid (probability) network volume. We will show that this total volume is proportional to the partition function of the related quantum system. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W15.00013: The impact of conformational fluctuations on self-assembly: Cooperative aggregation of archaeal chaperonin proteins Stephen Whitelam, Carl Rogers, Andrea Pasqua, Chad Paavola, Jonathan Trent, Phillip Geissler Protein complexes called rosettasomes self-assemble in solution to form large-scale filamentous and planar structures. The relative abundance of these aggregates varies abruptly with environmental conditions and sample composition. Our simulations of a model of patchy nanoparticles can reproduce this sharp crossover, but only if particles are allowed to switch between two internal states favoring different geometries of local binding. These results demonstrate how local conformational adaptivity can fundamentally influence the cooperativity of pattern-forming dynamics. [Preview Abstract] |
Session W16: Bosons in Optical Lattices I
Sponsoring Units: DAMOPChair: Selim Jochim, Max Planck Institute-Heidelberg
Room: 317
Thursday, March 19, 2009 11:15AM - 11:27AM |
W16.00001: Generic Phase Diagram for Bose-Einstein Condensation of Weakly Interacting Symmetric Bosonic Mixtures A.B. Kuklov, T. Blanchard, B.V. Svistunov Weakly interacting Bose gas represents {\it strongly} correlated classical field within a domain (determined by the gas parameter ) of its Bose-Einstein condensation (BEC) temperature $T=T_c$. Thus, $N$-component {\it weakly} interacting mixtures representing some symmetry can potentially exhibit rich phase diagram (PD). In particular, it can feature {\it quasi-molecular} phases preceding actual formation of the ODLRO in the vicinity of $T_c$. However, realization of a specific part of the PD depends on details of interactions. As examples, we consider mixtures characterized by O(2)$\times$O(2) symmetry ($N=2$) and spin $S=1$ with the symmetry reduced to U(1)$\times$U(1) ($N=3$). Monte Carlo simulations of these systems find a {\it single} line of the respective two- and three-component BEC transitions which has tricritical point separating II and I order transitions. No {\it quasi-molecular} phases have been found despite that na\"ive mean field (with one loop correction) predicts it. We discuss how such phases can emerge above the actual $N$-component BEC transition. One suggestion relies on Feschbach resonance detuned into negative inter-specie scattering length even when the gas parameter remains small. We acknowledge support from NSF grants PHY 0653135, 0653183 and CUNY grant 80209-0914. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W16.00002: Universal state diagrams for harmonically trapped bosons in optical lattices Marcos Rigol, George G. Batrouni, Valery G. Rousseau, Richard T. Scalettar We use quantum Monte-Carlo simulations to obtain universal zero temperature state diagrams for strongly correlated lattice bosons in one and two dimensions under the influence of a harmonic confining potential. Since harmonic traps generate a coexistence of superfluid and Mott insulating domains, we use local quantities like the quantum fluctuations of the density and a local compressibility to identify the phases present in the inhomogeneous density profiles. We emphasize the use of the 'characteristic density' to produce a universal state diagram which is relevant to experimental optical lattice systems, regardless of the number of bosons or trap curvature. We show that the critical value of U/t at which Mott insulating domains appear in the trap depends on the filling in the system, and it is in general greater than the value in the homogeneous system. Recent experimental results by Spielman et al. [Phys. Rev. Lett. 100, 120402 (2008)] are analyzed in the context of our two-dimensional state diagram, and shown to exhibit a value for the critical point in good agreement with simulations. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W16.00003: Occupation Statistics of a Bose-Einstein Condensate in a Driven Double Well Potential K. Smith-Mannschott, M. Chuchem, M. Hiller, T. Kottos, D. Cohen We consider the occupation statistics $P_t(n)$ of a Bose-Einstein condensate consisting of $N$ particles loaded in a double-well trap with intersite coupling $K$. Two dynamical scenaria are investigated: a) wavepacket dynamics and b) linear variation of the bias between the onsite energies of the two wells. In the latter case, we resolve three different behaviors as we increase the driving rate for intermediate values of the interatomic interaction $K/N< U < NK$: quantum adiabatic, diabatic, and sudden regime. We find that during the adiabatic to diabatic crossover, many-body Landau-Zener transitions play a dominant role, resulting in oscillations of the second moment of the occupation statistics. In contrast, the crossover to the sudden regime is characterized by a broad distribution $P_n(t\rightarrow\infty)$ which is reflected in a global maximum of the second moment. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W16.00004: Supersolidity of Cold Atomic Bose-Fermi mixtures in optical lattices Peter P. Orth, Doron L. Bergman, Karyn Le Hur An important possible mechanism for boson supersolidity in a Bose-Fermi mixture is the existence of a nested Fermi surface. Fermions then tend to exhibit a density wave at the nesting wavevector and imprint this order via boson-fermion interactions onto the bosons, which already support superfluidity. This coexistence of bosonic superfluidity and density wave order is a signature of the supersolid phase. We present new results concerning a cold mixture confined to a triangular optical lattice. For a fermionic density of $n_f=3/4$ per lattice site, the Fermi surface exhibits both a van-Hove singularity and nesting. With a Landau-Ginzburg and a microscopic mean-field analysis, we predict the supersolid parameter regime in current experimental realizations of Bose-Fermi mixtures, and make comparisons with the square lattice geometry. We also discuss competing low-temperature phases such as a phase separated and a Mott insulating regime. Finally, we consider the case of spatially anisotropic hopping, which allows us to explore a quasi 1d regime of supersolidity. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W16.00005: Magnetic phases of two-component lattice bosons at nonzero temperature Stephen Powell The realization of magnetically-ordered phases in optical lattices is set to be one of the next major experimental advances in the field of ultracold atoms. In the limit of strong repulsion and weak tunneling between lattice sites, perturbation theory predicts that two-component fermions form a N\'eel state with a two-sublattice structure, while bosons will tend to form a ferromagnetic insulator. This perturbative approach is, however, ill-suited for describing the physics above zero temperature and away from the strong-coupling limit. Here we address the phase diagram of two-component bosons at nonzero temperature using an approach that takes as its basis the standard mean-field theory for spinless bosons. This allows spin and charge excitations to be treated on an equal footing, and elucidates the competition between the possible magnetic and superfluid orders in the lattice. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W16.00006: Ground State Phase Diagram of the Two-Component Bose-Hubbard Model Sebnem Gunes Soyler, Barbara Capogrosso-Sansone, Nikolay Prokof'ev, Boris Svistunov We have performed path integral Monte Carlo simulations of the two-component hard-core Bose-Hubbard model on a square lattice at half-integer filling factor for each component. This system can be realized experimentally with heteronuclear bosonic mixtures in optical lattices with tunable interspecies interactions. Our results disagree with preexisting analytical treatments both quantitatively and qualitatively. We reveal the existence of an additional solid+superfluid phase for strong anisotropy between the hopping amplitudes which radically changes the topology of the ground-state phase diagram. The new phase is a direct consequence of effective interactions between ``heavy'' atoms mediated by the ``light'' superfluid component. Remarkably, mediated interactions are sign-alternating and thus lead to a rich variety of yet to be discovered quantum phases. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W16.00007: Counterflow and paired superfluidity in one-dimensional Bose mixtures Anzi Hu, Ludwig Mathey, Ippei Danshita, Carl Williams, Charles Clark Experimental progress in recent years has made it possible to realize mixtures of cold atoms in optical lattices. In this talk,we present our work on two types of superfluidity in 1D Bose mixtures: the counterflow superfluid and the paired superfluid phase, each of which can coexist with charge-density wave order. We predict and identify these phases both with Luttinger liquid theory and with numerical simulations. Specifically, we show the phase diagram as a function of the filling fraction and the inter-species interaction. We address the question of realizability and detectability of these phases by adding a trap potential, and by calculating various quantities that can be measured in experiment. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W16.00008: Determination of mixing or demixing state of a two-component BEC system Chao-Chun Huang, W. C. Wu In a stable ultracold trapped two-component BEC system, it is shown that the condition $U_{11}U_{22}-U_{12}^2>0$ holds as long as the intra-species $s$-wave interactions $g_{11}$ and $g_{22}$ are both repulsive. Here $U_{ij} = g_{ij} \int d{\bf r} |\Psi_i({\bf r})|^2 |\Psi_ j({\bf r})|^2$ with $\Psi_i({\bf r})$ the wave function of species $i$. The condition is valid no matter the system is in a single-trap or in an optical lattice. Based on the variational approach, the condition has been applied to determine whether the system is in mixing or demixing state, both for the single-trap and optical-lattice cases. Phonon modes of the optical-lattice system are also shown to be intimately related to the above condition. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W16.00009: Quantum Monte Carlo simulation of disordered Bose-Hubbard model in a 3D optical lattice Fei Lin, ShengQuan Zhou, Matthew Pasienski, Brian DeMarco, David Ceperley We perform large scale quantum Monte Carlo (QMC) simulations of disordered Bose-Hubbard model defined in a 3D optical lattice and confined in a spherical harmonic trap. Our system size is comparable to the experiment ($60^3$) [1], and our simulation temperature is as low as around 3 nK. We shall show QMC results on particle density distribution inside the trap, superfluid density as a function of disorder strength, and compare our condensate fraction to the experimentally measured values [1]. [1] M. White, M. Pasienski, D. McKay, S. Zhou, D. Ceperley, and B. DeMarco, arxiv.org/abs/0807.0446 [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W16.00010: Supersolidity in a commensurate mixture of one-dimensional hardcore bosons with mass imbalance Tassilo Keilmann, Juan Ignacio Cirac, Tommaso Roscilde We report on numerical simulations of an attractive mixture of mass-imbalanced hardcore bosons in a one-dimensional optical lattice. At a commensurate filling with 2-to-1 filling ratio we observe the formation of a crystal of \emph{trimers} (made of two heavy and one light particle) which shows quasi-condensation and superfluidity for \emph{both} particle species - hence a two-species supersolid. Supersolidity is observed both in the ground state of the system, as well as out of equilibrium in the stationary state that the system attains in the Hamiltonian evolution, after having being prepared into trimers by a superlattice. These two situations correspond to two different preparation protocols (simple adiabatic loading into an optical lattice, and release from a superlattice) which can both lead to the observation of supersolidity in optical lattice experiments. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W16.00011: A numerical study of the phase diagram and dynamics of spin 1 bosons in a one dimensional optical lattice Subroto Mukerjee, Frank Pollmann, Joel Moore We study the phase diagram and dynamics of spin 1 bosons in a one dimensional optical lattice using iTEBD (inifinite Time Evolved Block Decimation), a numerical technique that allows us to perform calculations in the thermodynamic limit. We compare our results with previous numerical work on these systems using the conventional DMRG (Density Matrix Renormalization Group) technique and analytic calculations based on the non-linear sigma model. We also investigate the possibility of the existence of a condensate of paired singlets, a Mott insulator with dimer order and a supersolid phase between the two. Our numerical technique allows us to efficiently calculate various dynamical properties of these phases to shed light on the nature of excitations. In particular, we do this for the case of one boson per site and strong interactions when the model can be mapped on to the dimerized phase of the spin-1 Heisenberg chain. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W16.00012: Localization in a Quasi-Periodic One Dimensional System John Biddle, Donald Priour, Sankar Das Sarma We study mobility edges and Anderson-like localization in a disorder-free, one-dimensional quasi-periodic system. In particular, we study a bichromatic sinusoidal lattice potential where a primary periodic lattice is perturbed by a secondary lattice with a period that is incommensurate with that of the primary lattice. This interesting potential admits both extended and localized states without the presence of disorder. We examine the transitions between extended and localized states by numerically solving the Schrodinger equation over a broad spatial domain, and the Lyapunov exponents are obtained from the localized eigenstates. From calculations based on the single-particle eigenstates, we report how mobility edges will be manifest in cold atom experiments in bichromatic incommensurate optical lattices. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W16.00013: Quantum gas microscope Jonathon Gillen, Waseem Bakr, Amy Peng, Simon Foelling, Markus Greiner Ultracold quantum gases in optical lattices have opened the door to studying fundamental questions of modern condensed matter physics with atomic physics experiments. The idea is to build an enlarged model of a quantum material, with ultracold atoms in the lattice playing the role of electrons or cooper pairs in real materials. In this way it becomes possible to experimentally realize condensed matter Hamiltonians and simulate for example bosonic and fermionic Hubbard models. In my talk I will focus on our new experiment in which we build a quantum gas microscope that we plan to use as a quantum simulator. This experiment will allow us to control the quantum gas on a single lattice site level, paving the way to simulating a wide variety of Hamiltonians. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W16.00014: Observation of Coherence between Superfluid Spheres with Quantum Monte Carlo Simulation Yasuyuki Kato, Naoki Kawashima We execute quantum Monte Carlo simulation based on the directed- loop algorithm (DLA) for the Bose-Hubbard model with an external harmonic field, which is effective for modeling a cold atomic Bose gas trapped in an optical lattice. While the general idea of DLA has very broad applicability, its straight forward application to boson systems is not efficient. To simulate large systems efficiently, we modified the DLA. [1,2] We treat a system consisting of $1.8\times10^5$ particles in a $64^3$ lattice and observe a four-tiered large ``wedding-cake'' structure in Ref. [2]. These numbers are comparable to those in the pioneering experimental work by Greiner {\it et al}. We focus on cases of the two-tiered wedding-cake systems, which have two superfluid spherical regions separated by Mott insulator region at low temperature. We show an evidence of the coherence between the two superfluid spheres. [1]. Y. Kato, T. Suzuki, and N. Kawashima: Phys. Rev. E 75, 066703 (2007) [2]. Y. Kato and N. Kawashima unpublished [Preview Abstract] |
Session W17: Quantum Algorithms, Simulation, and Error Correction
Sponsoring Units: GQIChair: Yaakov Weinstein, Mitre Corporation
Room: 318
Thursday, March 19, 2009 11:15AM - 11:27AM |
W17.00001: Time dependent DMRG for spectral functions of Heisenberg chains Steven White, Ian Affleck, Rodrigo Pereira Recently developed real-time DMRG techniques allow the calculation of space and time dependent spin-spin correlation functions for spin chain systems. These correlation functions can be Fourier transformed to obtain momentum and frequency dependent spectral functions. The growth of entanglement in the simulation as a function of time prevents extremely long simulation times, limiting the frequency resolution. We have found that the long time behavior can be extrapolated using either of two different techniques, allowing us to obtain very high resolution spectra with very high accuracy. We demonstrate these techniques for the S=1 Heisenberg chain and the XXZ S=1/2 chain. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W17.00002: High-threshold surface code quantum computing: threshold calculation Peter Groszkowski, Austin Fowler, Ashley M. Stephens Surface codes are topological quantum error correcting codes. In such codes, information is encoded in a collection of physical qubits arranged on a lattice, with only nearest-neighbor interaction required for processing and readout. In this talk we present a detailed account of a numerical threshold calculation for a planar surface code with boundaries (arXiv:0803.0272). In the end we find a threshold value that's approaching 1{\%}. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W17.00003: Error threshold in topological quantum-computing models with color codes Helmut Katzgraber, Hector Bombin, Miguel A. Martin-Delgado Dealing with errors in quantum computing systems is possibly one of the hardest tasks when attempting to realize physical devices. By encoding the qubits in topological properties of a system, an inherent protection of the quantum states can be achieved. Traditional topologically-protected approaches are based on the braiding of quasiparticles. Recently, a braid-less implementation using brane-net condensates in 3-colexes has been proposed. In 2D it allows the transversal implementation of the whole Clifford group of quantum gates. In this work, we compute the error threshold for this topologically-protected quantum computing system in 2D, by means of mapping its error correction process onto a random 3-body Ising model on a triangular lattice. Errors manifest themselves as random perturbation of the plaquette interaction terms thus introducing frustration. Our results from Monte Carlo simulations suggest that these topological color codes are similarly robust to perturbations as the toric codes. Furthermore, they provide more computational capabilities and the possibility of having more qubits encoded in the quantum memory. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W17.00004: Performing measurement based quantum computation on ground states Andrew Doherty, Stephen Bartlett One of the most exciting developments in quantum computing in recent years has been the realisation that there exist states of quantum many-body systems that can serve as a universal resource for quantum computing, where computation proceeds solely through single-qubit measurements. Although currently only a few isolated examples of such universal resource states are known, we discuss the possibility that there exist models of interacting spin systems in which an ordered phase is characterized by the ability to perform measurement-based quantum computation (MBQC). To identify such phases, we propose to use nonlocal correlation functions that quantify the fidelity of quantum gates performed between well separated qubits. The quantum computing phase is then characterized by set of order parameters corresponding to a universal set of quantum gates. We investigate a simple spin-lattice system based on the cluster-state model for MBQC by using a series of dualities with better studied models. We demonstrate that the model possesses a zero temperature phase transition between a disordered phase and an ordered ``cluster phase'' in which it is possible to perform a large class of one and two qubit quantum gates. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W17.00005: Operator Theoretic Quantum Fault Tolerance Gerald Gilbert, Yaakov S. Weinstein, Vaneet Aggarwal, A. Robert Calderbank We outline the advantages of an operator approach to quantum fault tolerance. Operator quantum fault tolerance is based on an explicitly stated halting condition, exact solutions of quantum error correction code dynamics, and as accurate and realistic descriptions as possible of the error models. This allows the proper integration of error correction and concatenation strategies with the system dynamics so as to better allocate quantum computational resources such as qubits, quantum gates, and computation time for quantum circuit design. We demonstrate these characteristics of the operator approach with an example of an asymmetric error model. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W17.00006: Parallel Environment for Quantum Computing Frank Tabakin, Bruno Julia Diaz To facilitate numerical study of noise and decoherence in QC algorithms,and of the efficacy of error correction schemes, we have developed a Fortran 90 quantum computer simulator with parallel processing capabilities. It permits rapid evaluation of quantum algorithms for a large number of qubits and for various ``noise'' scenarios. State vectors are distributed over many processors, to employ a large number of qubits. Parallel processing is implemented by the Message-Passing Interface protocol. A description of how to spread the wave function components over many processors, along with how to efficiently describe the action of general one- and two-qubit operators on these state vectors will be delineated.Grover's search and Shor's factoring algorithms with noise will be discussed as examples. A major feature of this work is that concurrent versions of the algorithms can be evaluated with each version subject to diverse noise effects, corresponding to solving a stochastic Schrodinger equation. The density matrix for the ensemble of such noise cases is constructed using parallel distribution methods to evaluate its associated entropy. Applications of this powerful tool is made to delineate the stability and correction of QC processes using Hamiltonian based dynamics. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W17.00007: Relation of operator Schmidt decomposition and CNOT complexity Mark Coffey, Ron Deiotte We consider two-qubit operators and provide a correspondence between their Schmidt number and controlled-NOT (CNOT) complexity, where the CNOT complexity is up to local unitary operations [1]. The results are obtained by complementary means, and a number of examples are given. Additionally, we present results for exact decompositions of two-qubit operators in terms of CNOT [2]. Instances of these results are applicable to superconducting-flux qubit and other systems. [1]M. W. Coffey and R. Deiotte, Quant. Info. Proc. 7, 117 (2008). [2]M. W. Coffey and R. Deiotte, preprint (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W17.00008: New Method for the Calculation of Qubit Decoherence in the Presence of 1/f Noise Dong Zhou, Robert Joynt We present a new mathematical method for the calculation of qubit decoherence subject to classical noise coming from an ensemble of two-level fluctuators. The time evolution of the qubit density matrix is governed by a non-Hermitian quasi-Hamiltonian, mapping the problem onto a system consisting of a spin-1 particle (the qubit) coupled to spin-1/2 particles (the fluctuators). The method gives non-perturbative results for the energy relaxation, free induction decay (FID) and spin echo pulse measurements. This extends the range of known results to strong coupling, beyond the range of validity of Redfield theory and the commonly-used Gaussian approximation. New functional forms are suggested to explain the recent experiments by Kakuyanagi [PRL 98, 047004 (2007)] and Yoshihara [PRL 97, 167001 (2006)] on qubit decoherence with 1/f noise. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W17.00009: Quantum Darwinism for mixed-state environment Haitao Quan, Michael Zwolak, Wojciech Zurek We exam quantum darwinism when a system is in the presence of a mixed environment, and we find a general relation between the mutual information for the mixed-state environment and the change of the entropy of the fraction of the environment. We then look at a particular solvable model, and we numerically exam the time evolution of the ``mutual information" for large environment. Finally we discuss about the exact expressions for all entropies and the mutual information at special time. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W17.00010: Static environments in open quantum systems Ian Durham An open quantum system is one that interacts in some way with another quantum system external to itself, e.g. an environment. In some cases this environment is constrained to be static under unitary transformations. It turns out that there are severe limitations on the types of systems and environments that can interact in such cases. For instance, we find that the system may only be in a pure state under most such transformations. In addition we find that a static environment cannot serve as a sub-system of a Bell state. It \emph{may} serve as a sub-system of a GHZ state in most cases, but its feasibility is dependent upon the unitary transformation that is applied to any part of the system. We note an implication that these results have for recent studies of quantum computation in the presence of closed time-like curves. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W17.00011: Decoherence in the hypercube quantum walk Frederick Strauch A new model of decoherence in the hypercube quantum walk will be presented, in which dephasing occurs between every vertex of the hypercube. Surprisingly, in this model the hitting probability remains bounded for arbitrarily large hypercubes. This result can be obtained by a simple analytical argument, and has implications for perfect quantum state transfer in qubit networks. This argument, and related numerical and perturbative results, will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W17.00012: Phase transitions in random quantum satisfiability Chris Laumann, Roderich Moessner, Antonello Scardicchio, Shivaji Sondhi The potential power of quantum computers is a subject of great current interest and the raison d'etre for the intense effort and progress to build them. Naturally much theoretical interest has focused on algorithms that outperform their classical counterpart but recent developments in quantum complexity theory suggest that we already know problems, those shown to be QMA-complete, whose worst case instances would take a quantum computer exponentially long to solve. As in classical complexity theory the supposed difficulty of QMA complete problems follows from the existence of polynomial transformations relating any of the large class of QMA problems to instances of QMA-complete questions. This does not directly address the question of why this problem has hard instances and what features they posses. In this work we attempt to investigate the features of hard instances of a QMA complete problem introduced by S. Bravyi: quantum k-SAT. We use techniques of statistical physics of disordered systems in order to study a random ensemble of quantum k-SAT instances parametrized by clause density $\alpha$ in a program that is analogous to recent studies of classical random k-SAT. We establish a phase transition in satisfiability as a function of clause density and show that the problem almost always reduces to identifying a classical graph property. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W17.00013: Asymptotic convergence rates for statistical moments of pseudorandom quantum circuits Winton Brown, Lorenza Viola We investigate the statistical moments of pseudorandom quantum circuits acting on an n-qubit system. We show that for pseudorandom quantum circuits that are invariant under arbitrary permutations of the qubit labels, there exists a representation of the linear map which describes the evolution of moments of fixed order, t, such that the dimension of the map scales polynomially in the number of qubits. The long time asymptotic convergence rate for low-order moments may be obtained by means of a perturbation expansion, shedding light on the question of how well pseudorandom quantum circuits approximate unitary t-designs. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W17.00014: Quantum annealing for the ground state problem via exactly solvable models Yohei Saika, Jun-ichi Inoue In this study, in order to clarify the efficiency of quantum annealing for optimization, we study the ground state problem using solvable spin systems, such as the spin 1/2 quantum Ising-XY chain under the Lorentzian field. First, we exactly estimate static properties, such as the ground state energy and the energy gap. We find that the ground state energy depends on the selection of the control field, although the ground state energy is same if the control field vanishes respectively. We also find that the energy gap between the ground state and the first excited state is inversely proportional to the system size and becomes zero in the thermodynamic limit. Also, via the numerical simulation on the Schrodinger equation, we clarify that the quantum annealing using various control fields, such as the transverse field, the ferromagnetic Ising interaction, is available of the ground state problem not also for the spin 1/2 quantum Ising-XY chain and also for the random field Ising model. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W17.00015: Quantum adiabatic algorithm for the Ising model on a Bethe lattice Erin Handberg, Sergey Knysh, Vadim Smelyanskiy, Andre Petukhov We investigate the average-case complexity of the quantum adiabatic algorithm (QAA) for the Ising model on the Bethe lattice by studying the asymptotic scaling of the minimum gap. For $N < 30$, this is done using direct diagonalization of the quantum Ising Hamiltonian with randomly chosen Gaussian exchange couplings. We use the Mezard-Parisi definition of the Bethe lattice for finite $N$ with the connectivity graph chosen uniformly at random among all $k$-regular graphs. The results of the direct diagonalization are compared to those from the parallel tempering version of quantum Monte Carlo (QMC) algorithm, and we use the QMC to study larger values of $N$. [Preview Abstract] |
Session W18: Focus Session: Mechanical Force Spectroscopy and Device Applications of Polymeric and Biological Materials
Sponsoring Units: DPOLYChair: Denis Pristinski, National Institute of Standards and Technology
Room: 319
Thursday, March 19, 2009 11:15AM - 11:51AM |
W18.00001: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W18.00002: Transitions of a tethered chain under tension Jutta Luettmer-Strathmann, Wolfgang Paul, Kurt Binder When the end of a polymer chain tethered to an attractive surface is pulled away from the surface, the force required to extend the chain depends on interactions with the surface as well as intrachain interactions. Similarly, when the chain is held fixed and the temperature is reduced, both adsorption and collapse transitions have a signature in the force-extension curve. In this work, we performed Monte Carlo simulations of the bond-fluctuation model with a Wang-Landau algorithm to determine the density of states in the state space of monomer-monomer contacts, monomer-surface contacts, and chain extension. We study the effect of tension on the collapse and adsorption transitions and calculate force-extension curves that may be compared with experimental data. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W18.00003: Velocity Dependency of Dragging Force and Wetting Properties of High Viscous Liquids Using Constant Diameter Nanoneedle-Tipped AFM Probes. Mehdi Yazdanpanah, Mahdi Hosseini, Santosh Pabba, Charles Walter, Jayan Hewaparakrama, Robert Cohn A high aspect ratio and constant diameter Ag$_{2}$Ga nanoneedle grown on an AFM cantilever was used to perform F-D experiments on four different molecular weights of PDMS surfaces. The needle is partially inserted into and retracted from the liquid surface in various scan speeds. The viscous drag force causes the cantilever to deflect and recorded as a function of vertical displacement of the needle for each scan speed. The viscosity of the liquid is calculated by fitting a model into experimental data. The results show that the viscosity has strong correlation with the scan speed. Due to simple geometry of the needles, F-D curves are also interpreted to study the wetting properties (i.e. dynamic contact angle, meniscus height) of the PDMS at different scan speeds. Also, F-D curves are interpreted for polymer fiber formation during the capillary thinning and meniscus stretching that shown a strong correlation between the fiber length and the stretching velocity. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W18.00004: Mechanical Properties of Individual Microgel Particles Sara Hashmi, Eric Dufresne Microgels are important materials for both basic science and engineering and have wide applications from the study of phase transitions to the delivery of drugs. These micron and sub-micron particles, made of hydrogel materials, respond to solvent conditions. The most common microgels are environmentally sensitive, responding to temperature and pH. Our material of interest, poly(N-isopropylacrylamide) or NIPAM, undergoes a deswelling transition above a critical temperature. The deswelling behavior of this polymeric material has been thoroughly studied in ensemble microgel systems as well as in bulk hydrogel samples. We present measurements of the elastic properties of single microgel particles using atomic force microscopy. We observe a stiffening of the Young's modulus by an order of magnitude at temperatures well above the transition, where the cross-linked polymer network has fully collapsed. Interestingly, near the transition we observe a comparable softening of the material. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W18.00005: Intrinsically Disordered Titin PEVK as a Molecular Velcro: Salt-Bridge Dynamics and Elasticity Jeffrey Forbes, Wanxia Tsai, Richard Wittebort, Kuan Wang Titin is a giant modular protein (3-4 MDa) found in the muscle sarcomere, where the intrinsically disordered and elastic PEVK segment plays a major role in the passive tension of skeletal and heart tissues. We have proposed that salt-bridges play a central role in the elasticity of PEVK. The 50 kDa engineered PEVK polyprotein shows well-resolved NMR spectra at all concentrations. From long-range NOE's, we observed stable K to E salt-bridges. Simulated annealing with NMR restraints yielded a manifold of structures for an exon 172 trimer. Steered molecular dynamics simulations were done to study how the manifold of salt-bridges evolves during the stretching experiment. Repeated SMD simulations at slow velocity (0.0005 nm/ps) showed force spectra consistent with experimental AFM force spectra of the polyprotein. SMD shows that salt-bridges occur even at high degrees of stretch and that these short range interactions are in integral part of the mechanical properties of PEVK. We propose that the long-range, non-stereospecific nature of electrostatic interactions provide a facile mechanism to tether and untether the flexible chains, which in turn affect elasticity as well as control the accessibility of protein-protein interaction to these nanogel-like proteins. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W18.00006: Mechanical Signal Filtering by Viscoelastic Properties of Cuticle in a Wandering Spider Michael E. McConney, Clemens Schaber, Michael Julian, Joseph A.C. Humphrey, Friedrich Barth, Vladimir V. Tsukruk As recently found, in mechano-sensors of wandering spiders (Cupiennius salei) viscoelastic materials are important in signal filtering. We used atomic force microscopy to probe the time dependent mechanical behavior of these materials in live animals. We measured Young's modulus of a rubbery material located between a vibration receptor and the stimulus source. Earlier electrophysiological studies had demonstrated that the strain needed to elicit a sensory response (action potential) increased drastically as stimulus frequencies went below 10 Hz. Our surface force spectroscopy data similarly indicated a significant decrease in stiffness of the cuticular material and therefore less efficient energy transmission due to viscoelastic effects, as the frequency dropped to around 10 Hz. The stimulus transmitting cuticular material is acting as a high-pass filter for the mechanical stimulus on its way to the strain receptors. Again our results indicate that viscoelastic mechanical signal filtering is an important tool for arthropods to specifically respond to biologically relevant stimulus patterns. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W18.00007: Bacterial Cell Wall Peptidoglycan at Single Molecule Resolution Ahmed Touhami, Manfred Jericho, Valerio Matias, Anthony Clarke, Terry Beveridge, John Dutcher The major structural component of bacterial cell walls is the peptidoglycan sacculus, which is one of nature's strongest and largest macromolecules that maintains the large internal pressure within the cell while allowing the transport of molecules into and out of the cell and cell growth. The three-dimensional structure of this unique biopolymer is controversial, and two models have been proposed: the planar model, in which the glycan strands lie in the plane of the cell surface, and the scaffold model, in which the glycan strands lie perpendicular to the cell surface. We have used atomic force microscopy to investigate the high resolution structure of isolated, intact sacculi of Escherichia coli K12 bacteria. Atomic force microscopy-single molecule force spectroscopy was performed on single sacculi exposed to the tAmiB enzyme which cleaves the peptide-glycan bonds. Surprisingly, the measurements revealed individual strands of up to 250 nm in length. This finding combined with high resolution AFM images recorded on hydrated sacculi provide evidence for the validity of the planar model for the peptidoglycan structure in Gram-negative bacteria. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W18.00008: Unfolding polyelectrolytes in a strong DC electric field Pai-Yi Hsiao, Kun-Mao Wu The behavior of single polyelectrolytes in multivalent salt solutions under the action of an electric field is investigated by computer simulations. The variation of chain size against the strength of electric field displays a sigmoidal transition, which defines a critical field $E^\ast $ to unfold a chain. Above $E^\ast $, the chain is unfolded into a rodlike structure, aligned parallel to the field direction. We show that $E^\ast $ is a function of salt concentration and depends on the chain length via the scaling law $V^{-1/2}$ where $V$is the ellipsoidal volume occupied by the chain. Moreover, the magnitude of the electrophoretic mobility of chain drastically increases during the unfolding. These findings provide a solid foundation to a newly proposed mechanism to separate long charged homopolymers by their chain length in free-solution electrophoresis via the unfolding transition of globule polyelectrolytes condensed by multivalent salt. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W18.00009: Liquid Drop Pinning on Micro-patterned Surfaces Ahmed Soliman, Yevgeniy Kalinin, Robin Baur, Robert Thorne Pinning of liquid drops on surfaces is important in many areas of biotechnology. Micro-patterned surfaces provide a way to control drop pinning, and to investigate the mechanisms of pinning on real (rough) surfaces. Continuous circular rings on silicon wafers produced by etching the interior and surrounding silicon are shown to dramatically increase contact line pinning. The critical apparent contact angles and liquid drop volumes are measured and correlated with parameters that describe the ring geometry, such as ring-wall height and width, as well as with ring surface energy (hydrophilicity / hydrophobicity). Micro-patterning of surfaces in this way can be used to improve drop pinning, shape reproducibility and imaging in high-throughput protein crystallization. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W18.00010: An Optical Biosensing Platform using Reprecipitated Polyaniline Microparticles Louis Nemzer, Arthur Epstein A great deal of effort remains focused on the goal of developing a continuous \textit{in vivo} glucose monitoring system for patients with \textit{diabetes mellitus.} We report a proof-of-concept study on a reagentless optical biosensing platform that circumvents the problems usually associated with direct glucose detection by utilizing the UV-VIS absorption properties of polyaniline, a biocompatible polymer. When the enzyme glucose oxidase is entrapped within reprecipitated polyaniline microparticles, a glucose molecule readily donates two protons and two electrons to the polyaniline, reversibly altering the polymer's oxidation state. The resultant change can be monitored by measuring the absorption at wavelengths that fall within the ``optical window'' for skin. The micro-structured morphology also insures a high surface-area to volume ratio. Data from \textit{in vitro} prototype devices indicate that in the low enzyme-loading regime, the response can be fit to the Michaelis-Menten model for enzyme kinetics, but at higher enzyme loading, diffusion effects dominate. As a biosensing platform, the system also has the potential to be adapted to detect other biologically relevant analytes, including cholesterol and ethanol. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W18.00011: Patterning of Ferritin Nanoparticles on Gold Posts of Silicon Substrate Yunxia Hu, Dian Chen, Soojin Park, Todd Emrick, Thomas Russell Patterning and immobilizing protein nanoparticles with nanometer-scale control has been proven integral to a range of applications in the development of biochip arrays, biosensor and electronic devices. Protein nanoparticles, such as ferritin nanoparticles, have a uniform size distribution and shape that can be used to construct well-defined patterns with nanoscale features. Here, the gold posts on silicon were produced using block copolymer PS (47.6 kg/mol)-b-P4VP (20.9 kg/mol) (PDI: 1.14) as a template and then gold chloride solution~was loaded into P4VP domain. After reducing gold salt into gold and removing the block copolymer using anoxygen plasma, producing a pattern of gold posts. Thiol modified horse spleen ferritin are anchored to gold posts of silicon substrate by the binding of thiol and gold. Scanning electron microscopy (SEM) shows that the feature size of gold posts decreased from 30 nm to 13 nm after attached with modified ferritin nanoparticles, which is consistent with size of modified ferritin. Also XPS result shows nitrogen and ion elements on ferritin-attached gold posts, and the signal of gold was attenuated after ferritin attached. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W18.00012: Rapid Hydrogel Microactuator Using Elastic Instability Howon Lee, Chunguang Xia, Nicholas Fang Rapid Hydrogel Microactuator Using Elastic Instability Inspired by rapid movement of sensitive plants such as Venus flytrap [1], we present an innovative way to enhance actuation speed of hydrogel micro devices by exploiting elastic instability. In this work, hydrogel micro devices in doubly curved shape are designed and fabricated using projection micro-stereolithography[2], with embedded microfluidic channels on the surface. Local swelling of hydrogel around channels causes bending which subsequently induces stretching of the soft structure. Such coupling gives rise to elastic instability, the onset of which triggers rapid conversion of stored elastic energy into kinetic energy in fast motion. We further designed a set of devices with different dimensions, which leads to different coupling of elastic energy in bending and stretching [1]. Our experimental results verified the critical coupling parameter that triggers snap-buckling motion. Ongoing experiments are investigating the actuation speed as a function of coupling parameter. This novel approach promises new potential applications for hydrogel based devices in various fields of study including microfluidics, soft robotics, artificial muscle, and drug delivery. Reference [1] Forterre, Y., et al, Nature, 433, 421-425 (2005) [2] Sun, C., et al, Sensors and Actuators A, 121:1, 113-120 (2005) [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W18.00013: Effect of Single Bacterium Cell and DNA Attachment on the Electrical Properties of Chemically Modified Graphene Sheets Nihar Mohanty, Vikas Berry Chemically modified graphene (CMG) sheets are expected to have a considerably different electrical sensitivity to molecular attachment than the pristine graphene sheets. Here we present the electrical-interfacing properties of (a) CMG's hybrids with single bacterial cells, (b) CMG with DNA (single and double stranded) tethered on graphene-surface and (c) CMG with polyelectrolyte-layer assembled on surface. These hybrids function as: (a) single bacterium devices, (b) DNA hybridization sensor and (c) charge polarity sensitive chemical-detector, respectively. A single bacterium attachment leads to generation of $\sim $1400 holes on a CMG while hybridization of $\sim $4 DNA molecules on graphene-DNA-carpets lead to generation of one hole. Further explanation of the attachment-potential, system-reversibility and sensitivity will also be presented. [Preview Abstract] |
Session W19: The Physics of Polymer Nanocomposites: Grafting and Dispersion
Sponsoring Units: DPOLYChair: Arthi Jayaraman, University of Colorado, Boulder
Room: 320
Thursday, March 19, 2009 11:15AM - 11:27AM |
W19.00001: A theoretical study of polymer grafted nanoparticles as fillers in polymer nanocomposites Arthi Jayaraman, Kenneth Schweizer We have generalized the microscopic Polymer Reference Interaction Site Model (PRISM) theory to study the structure and phase behavior of polymer-tethered spherical nanoparticles (fillers) in a homopolymer matrix. In the absence of a polymer matrix, melts of polymer-tethered nanoparticles show strong concentration fluctuations indicative of aggregate formation and/or a tendency for microphase separation as the total packing fraction and/or nanoparticle attraction strength increase. In the presence of a polymer matrix there is competition between nanoparticle attractions, steric repulsion between grafted polymers, and polymer matrix induced depletion-like attraction. For single tethered particles, volume of the tether being equal to the volume of the nanoparticle, the apparent microphase spinodal curve exhibits both dilution-like and depletion-like features, and a non-monotonic dependence on matrix chain length. As the particle size and tether length are increased, such that the total space filling volume of the tether continues to equal the nanoparticle volume, the shape of the microphase spinodal curve remains unchanged, but the effect of matrix polymer chain length on the spinodal temperature diminishes. The effect of various parameters on the spinodal temperature will be presented. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W19.00002: Highly-branched anisotropic hybrid nanoparticles at surfaces. Vladimir Tsukruk We present a brief overview of our recent studies on combined hybrid anisotropic structures composed of inorganic nanoparticles and highly branched molecules such as modified silsesquioxanes polyhedra cores (POSS) with mixed hydrophobic-hydrophilic tails and silver nanowires with functionalized star block copolymer with embedded gold nanoparticles (nanocobs). We demonstrate two-stage melting of that branched POSS and their ability to form monolayer and multilayered LB structures. On the other hand, we observed that silver-BCP-gold nanocobs display extremely bright Raman scattering caused by surface enhanced Raman effect with very different longitudinal and transversal optical properties as revealed by high-resolution confocal Raman microscopy. To study these hybrid nanostructures we applied combined AFM, SEM, TEM, XPS, SERS, UV-vis, and XR techniques. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W19.00003: Particle Dynamics within Self-Assembling Polymer-Grafted Spherical Nanoparticles Pinar Akcora, Sanat K. Kumar, Yu Li, Brian Benicewicz, Suresh Narayanan We have recently shown that the self-assembly of polymer grafted spherical nanoparticles can be achieved by varying the brush grafting density and chain length. The mechanical behavior of these nanocomposites with various states of particle dispersion has been explored using x-ray photon correlation spectroscopy. Nanoscale and macroscopic dynamic measurements show that mechanical reinforcement results from the percolated and also strongly entangled brushes forming strong networks. Particle dynamics within various polymeric nanostructures will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W19.00004: Thermally Stable Au Nanoparticles via Photo-crosslinkable Polymeric Stabilizers Joona Bang, Misang Yoo, Bumjoon J. Kim Polymer nanocomposites consisting of polymers and inorganic nanoparticles (NPs) have attracted many interest due to their applications such as solar cell, sensors, catalysts and ferroelectric devices. To integrate NPs into polymer matrix in the controlled manner, thiol-terminated stabilizers have been used to tune the surface property of NPs such as Au, Pt, CdSe, etc. However, a practical use of such particles in the nanocomposites is very limited by thermal instability even at $\sim $90 $^{\circ}$C, leading to the agglomeration of NPs. To impart the thermal stability of NPs, we modified Au NPs surface using UV-crosslinkable polymeric stabilizers. After UV-crosslinking, it was found that the Au NPs exhibit the excellent stability at high temperature ($\sim $180 $^{\circ}$C) in both solution and thin-film states. Furthermore, we demonstrate that thermally stable Au NPs can be used as compatibilizers in PS/PMMA blends. The NPs at the PS/PMMA interface produced the dramatic reduction in the droplet size after 1 day of thermal annealing at 180 $^{\circ}$C, in which the particle size is unchanged. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W19.00005: Design of Polymer-Grafted Particles for Biocompatability David Trombly, Venkat Ganesan Drug designers often coat drug particles with grafted polymers in order to introduce a net repulsion between the particles and blood proteins. This net repulsion results from the energy cost of compressing grafted chains on approach of proteins. It thus overcomes the Van Der Waals attraction between drug and protein which would otherwise cause particle-protein agglomeration and ultimately thrombosis. This study proposes to develop a fundamental understanding of the role of different features in controlling the efficacy of the grafted layers. We address this issue by developing a framework to predict the interactions between a polymer-coated spherical particle and a bare spherical particle. In order to fully capture the two-sphere system, a numerical solution of polymer mean field theory is used in a bispherical coordinate system. Results for protein-particle interaction energies for different design parameters will be presented. For biological applications, polyethylene glycol is often used as the grafted polymer. The unique properties of this molecule will be accounted for using the n-cluster model. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W19.00006: Synthesis of Polystyrene-Silica Composite Particles via One-Step Nanoparticle-Stabilized Emulsion Polymerization Lenore Dai, Huan Ma Polystyrene-silica core-shell composite particles are prepared by one-step emulsion polymerization with a nonionic initiator VA-086, solely stabilized by silica nanoparticles. The silica nanoparticles are successfully incorporated into as the shell, likely due to the fact that the nanoparticles are thermodynamically favorable to self-assemble and remain at the liquid-liquid interfaces during the emulsion polymerization. The silica content, determined by thermogravimetric analysis, is approximately 20 wt% in the composite particles. In addition, we further explore the polymerization mechanism by studying the particle growth as a function of initiator concentration and reaction time: when the silica/monomer ratio is increased from 0.83 wt% to 2.5 wt%, the particle size at 24 hour reaction time decreases for a fixed monomer amount, probably due to a larger number of nuclei at the initial stage of polymerization. Further increasing the initiator/monomer ratio to 4.2 wt% does not continually decrease the particle size, which may be limited by the stabilization provided by a fixed concentration of silica nanoparticles. The surface coverage also changes with initiator concentration and reaction time although the underlying mechanism is not fully understood. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W19.00007: Effects of Grafted Chain Density on Nanoparticle and Melt Structure Joshua Kalb, Sanat Kumar, Robert S. Hoy, Gary S. Grest Applications of nanoparticles have increased dramatically over the last few years with uses ranging from scratch proof glass to lubricants to fighting cancer. Grafting polymer chains to these systems further increases the range of their properties, but still much remains to understand about the behavior of 'brush grafted nanoparticle' systems, particularly in their interaction and entanglement with a polymer melt. Previous works where polymer brushes were attached to a flat surface have demonstrated that entanglements between the attached chains and the polymer melt depend strongly on coverage and length of the attached chains. Allowing for a curved grafted nanoparticle surface allows for a wider range of interactions with the melt. Here we present molecular dynamics simulations of the structure of grafted nanoparticles and their entanglements~with a highly entangled melt. Individual entanglements are identified using a modified version of primitive path analysis. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W19.00008: Polyethylene/organically-modified layered-silicate nanocomposites with antimicrobial activity P. Songtipya, M.M. Jimenez-Gasco, E. Manias Despite the very intensive research on polymer nanocomposites, the opportunities for new functionalities possible by nanofillers still remain largely untapped. Here, we present polyethylene/inorganic nanocomposites that exhibit strongly enhanced mechanical performance and, at the same time, also an antimicrobial activity originating from the organo-filler nature. Specifically, PE/organically-modified layered-silicate nanocomposites were prepared via melt-processing, and antimicrobial activity was designed by proper choice of their organic modification. Their antimicrobial activity was measured against three micotoxinogen fungal strains (\textit{Penicillium roqueforti} and \textit{claviforme}, and \textit{Fusarium graminearum}) as model soil-borne plant and food contaminants. Montmorillonite-based organofillers, which only differ in their organic modification, were used to exemplify how these surfactants can be designed to render antifungal activity to the nanocomposites. The comparative discussion of the growth of fungi on unfilled PE and nanocomposite PE films is used to demonstrate how the antimicrobial efficacy is dictated by the surfactant chemistry and, further, how the nanocomposites' inhibitory activity compares to that of the organo-fillers and the surfactants. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W19.00009: Modeling of block copolymer/nanoparticle nano-composites Marco Pinna, Ignacio Pagonabarraga, Andrei Zvelindovsky We develop a coarse grained simulation technique to study dynamics in soft nano-composites. The system consists of block copolymer melt with embedded nano-size particles. The time evolution of the system is described by a hybrid method combining a field based simulation for block copolymer component and a particle based method for nano-colloids. The block copolymer is modelled by cell dynamics simulation technique, and nano-particles are modelled as soft particles with prescribed density profile. A cross interaction term is controlling the interplay of dynamics of both components. The influence of nano-particles on block copolymer morphology is investigated. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W19.00010: Interfacial Slip in Polymer Blends with Nanoparticles Joseph Ortiz, Eihab Jaber, Dilip Gersappe The interfacial region in polymer blends has been identified as a low viscosity region in which considerable slip can occur when the blend is subjected to shear forces. Here we use Molecular Dynamics simulations to establish the role that added nanoparticle fillers play in modifying the interfacial rheology. By choosing conditions under which the fillers are localized, either in the two phases or at the interface, we can look at the interplay between the strengthening capability of nanoparticles and the change in the interfacial slip behavior. We examine particle size, attraction between the particle and the polymer component, and the amount of filler in the material. Our studies are performed both above and below the point at which the filler particles form a transient network in the blend. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W19.00011: Synthesis of composite polymer nanoparticles . Edward Van Keuren, Maki Nishida We have been developing composite nanoparticles using the reprecipitation method or miniemulsion polymerization. These methods enable the combination of multiple functional components, such as large metal or metal oxide clusters and molecular species such as fluorophores, into polymer nanoparticles. The incorporation of these into the polymer or monomer precursors requires a detailed understanding of the mutual solubility of the components. We present fluorescence correlation spectroscopy measurements of molecular solubility and results from dynamic light scattering, electron microscopy and Raman spectroscopy that reveal the morphology and composition of these particles. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W19.00012: Relaxation behaviors of nanoparticles in polymer composites: influence of local frictions by polymer chains Byeongdu Lee, PAPPANNAN Thiyagarajan, Suresh Narayanan, Alec Sandy, Vilas Pol, Chieh-Tsung Lo, David Bohnsack The dynamics of Au nanoparticles (AuNP) tethered with thiol-terminated polystyrene (PS) in the composites with poly(styrene-b-2-vinylpyridine) diblock copolymers (PS-PVP) have been studied by x-ray photon correlation spectroscopy and small-angle x-ray scattering. Relaxation behaviors of nanoparticles located selectively in PS domain due to enthalpic interaction, interestingly, are not correlated with those of matrix polymer chains, i.e., their relaxation times are not dependent on the molecular weights of PS-PVP. They relax faster in PS-PVP than in PS homopolymer having the same molecular weight as the PS brush of PS-PVP. On the other hand, the influence of morphological structures of PS-PVP, however, is significant: AuNP moves faster in the lamellae phase than those in the cylinder phase. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W19.00013: Nano-particle distribution in a polymer nano-composite Panagiotis Maniadis, Ioannis N. Tsimpanogiannis, Edward M. Kober, Turab Lookman We use the hybrid particle-Self Consistent Field calculation (hybrid particle-SCF) to study the distribution of particles in a multi-block copolymer nano-composite. Using the static approach, we first find the effective interaction potential between the nano-particles and the polymer. The interaction has an entropic and an enthalpic component. The dynamical simulation confirms that the distribution of particles has a maximum at the minima of the interaction potential. We also study the situation where the nano-particles are distributed in a blend of AB diblock and A homopolymer. In this case, for large homopolymer concentration (larger than 20\%), an interface is created between components that are identical, but they come from different types of polymer chains (i.e. the AB diblock or the A homopolymer). We find that the interaction potential has a minimum in this A/A interface which is of pure entropic origin. Furthermore the dynamical simulation reveals that the distribution of nano-particles has a maximum in the area around this interface. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W19.00014: Thermoresponsive Self-Assembling Nanocomposites Kari Thorkelsson, Yue Zhao, Thomas Schilling, Alexander Mastroianni, Joseph M. Luther, Yue Wu, A. Paul Alivisatos, Ting Xu Nanoparticles have significant potential for use in fields including photovoltaics and memory storage, but to realize this potential, their distribution must be finely controlled. We present here a verstile method to achieve such control, using a diblock copolymer supramolecule composed of polystyrene-block-poly(4-vinylpyridene) (PS-b-P4VP) and 3-pentadecylphenol (PDP). The PDP hydrogen bonds to the P4VP block, forming a comb block. This change in morphology causes the PS-b-P4VP(PDP) supramolecule to force the nanoparticles into well-organized rows one nanoparticles thick at the center of the P4VP(PDP) domains. Furthermore, the morphology of the supramolecule-nanoparticle composite changes with temperature as hydrogen bonding is broken and the PDP becomes soluble in the PS block. This provides a useful path for the production of polymer-based thermoresponsive nanocomposites. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W19.00015: Self-Assembly of Polymer-Decorated Nanoparticles in the Bulk and in a Nanometric Confinement Damien Maillard, Sanat Kumar, Pinar Akcora As shown previously by simulation and TEM studies in the bulk, PS grafted nanoparticles when mixed a PS matrix self-assemble into a range of superstructures. These self-assembled structures can be regrouped into a phase diagram in which the leading parameters are the particles grafting density and the molecular weight ratio of the grafted and free matrix chains. Depending on those parameters the particles can be well dispersed or aggregated in one (strings), two (interconnected sheets) or three (spherical aggregates) dimensions. Here we consider the corresponding behavior in thin films (100 nm thick) using in-situ phase contrast AFM. In addition to yielding the morphologies, this protocol allows us directly visualize the aggregation process of the particles. [Preview Abstract] |
Session W20: Theory and Simulation III
Sponsoring Units: DPOLYChair: Jack Douglas, National Institute of Standards and Technology
Room: 321
Thursday, March 19, 2009 11:15AM - 11:27AM |
W20.00001: Conformational Study of Di-Substituted \textit{para-polyphenyleneethylene} (PPE) in Dilute Solutions Sabina Maskey, Flint Pierce, Dvora Perahia, Gary Grest Molecular dynamics (MD) simulations have been used to study the conformation of highly rigid di-substituted \textit{para-polyphenyleneethylenes} (PPEs) polymers, electro-active polymers, in dilute toluene solutions. The conformation of PPEs determines the conjugation of the polymer and their assembly mode which in-turn affect the electro-optical properties. In solution, the conformation is determined by molecular parameters including the length of the polymer and the nature of the side chain, coupled with the interaction of the molecules. The present study investigates the effects of molecular weight and the nature of the side chain in toluene solutions. Toluene is a good solvent for the backbone and a poor solvent for the substituting side chains. Small angle neutron studies have shown that short alkyl PPEs are fully stretched out. With increasing molecular weights they assume a worm like configuration. The current study provides further insight into the factors that determines the conformation of the PPEs. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W20.00002: Crystal and Rotator Phases of n-alkanes: a Molecular Dynamics Study Nathaniel Wentzel, Scott T. Milner The odd $n$-alkanes exhibit a wide variety of solid phase behavior; experimentally observed phases include an orthorhombic crystal phase, in which the molecules show long range herringbone order, and rotator phases in which the molecules do not diffuse but display various degrees of disorder. The rotator phases are of interest because they are implicated in the nucleation of $n$-alkane and polyethylene crystals. C$_{23}$ has been found experimentally to have two stable rotator phases, orthorhombic $R_{I}$ and hexagonal $R_{II}$, at temperatures between the crystal and melt. The crystal--$R_{I}$ and $R_{I}$--$R_{II}$ phase transitions are observed to be weakly first order. Simulations of C$_{23}$ to date have found the $R_{I}$ phase but not the $R_{II}$ phase, and have not much characterized the phases or the transitions between them. We report our results for local order and pretransitional fluctuations of rotator phases, from our all-atom molecular dynamics simulations of thin layers of C$_{23}$. We also comment on how these properties relate to the experimentally observed phase transitions. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W20.00003: Computational Modeling of Polymers and the Influence of Molecular Level Structural Features on Mechanical Properties Thomas Clancy, Sarah-Jane Frankland The role of molecular structure on the mechanical properties of polymer based materials is investigated through atomistic based molecular dynamics simulations. Models of crosslinked polymers were built with a range of moisture content in order to study the effect of environmental exposure on mechanical properties. Another key structural parameter, the degree of crosslinking, was also varied. The molecular structural features associated with these parameters are studied for their influence on the mechanical properties. The relative motion of crosslink points and the influence of penetrants such as water are investigated under deformation conditions. The mobility of penetrants within the polymer matrix is studied under equilibrium and deformation conditions in order to assess the role of these structural features on the mechanical properties as well as to assess the influence of deformation on diffusion rates. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W20.00004: Hydrogen Bonding Structure in Hyperbranched Aliphatic Polyesters Studied by MD Simulations Brian Olson, Mukul Kaushik, Sergei Nazarenko Hyperbranched aliphatic polyesters based on dimethoxy propionic acid (bis-MPA) as the repeating unit and ethoxylated pentaerythritol as the tetrafunctional core gained widespread attention due to their unusual structure and properties. These globular macromolecules possess a large number of hydroxyl functional groups in particular on their periphery. These hydroxyl groups interact readily through hydrogen bonding (HB) and form clusters responsible for many physical properties of this system. The structure of these clusters however remains unclear. Therefore MD simulations have been used to elucidate the structure of these clusters. MD simulations revealed that peripheral hydroxyl groups form linear hydrogen bond clusters (strings) similar to those proposed in hydrofluoric acid (HF) but considerably shorter consisting of 4-10 hydroxyl groups per cluster. Simulations also led to prediction of WAXS pattern for these hyperbranched polyesters in the bulk with the characteristic peak at 2$\theta $ =30\r{ } due to O-O correlations similar to those in water. The predictions were in excellent agreement with the experimental WAXS data. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W20.00005: Phases of functionalized polymer-inorganic composites in solution studied via molecular dynamics Joshua Anderson, Rastko Sknepnek, Alex Travesset Using self-assembling polymer systems to direct the formation of inorganic crystals, polymer-inorganic composite materials offer new opportunities in materials design. Molecular dynamics simulations allow for an exploration of the wide range of phases in these systems. Amphiphilic ABA triblocks with A hydrophilic, B hydrophobic, and functional ends with an affinity to inorganic particles are modeled to capture the minimum physics needed to describe polymer-inorganc systems currently being investigated by experiment. A number of phases are formed in solution as the attraction strength between the inorganic particles and the affinity of those particles to the functional end beads of the polymer are varied. Some of the phases found include hexagonal, square columnar, lamellar, perforated lamellar, and the gyroid. Polymer stretching plays an important role in each of the phases found, with a characteristic multi-modal behavior in the polymer end to end distance distribution. In the gyroid phase, for instance, the peaks correspond to the polymers being in two preferred conformations: v-shaped with a small end to end distance and fully extended in a line with the largest possible end to end distance. At high interaction strengths, inorganic particles are found to crystallize and form plate-like structures. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W20.00006: The effect of chain stiffness on the structure and phase behavior of diblock copolymer melts G. Leuty, Mesfin Tsige In block copolymers the covalent bond joining the different immiscible block segments prevents the occurrence of macroscopic phase separation of the different components of a copolymer chain. Instead, the block segments give rise to well-organized periodic domain nanostructures whose size and shape mainly depend on the dimensions of the blocks and the segment-segment interaction parameters. Variations in the stiffness of the different block segments can directly affect the morphology of the system and may result in a very rich phase behavior. To the best of our knowledge, there is no theory at the atomic or molecular level that explains how variations in the stiffness of the different block segments can affect the dynamics and morphology of these systems. We have studied the microphase separation of symmetric diblock copolymers with variable block stiffness and different block chain length using coarse-grained molecular dynamics simulations. The morphology of the diblock systems we studied is found to be strongly dependent on the relative stiffness of the two block segments. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W20.00007: Rigidity transition with increasing crosslinking of a single macromolecule Jiwu Liu, Phillip Duxbury A nano-particle can be formed by the intramolecular crosslinking of a polymer chain. In this process the rigidity of the system increases with the crosslinking density. We carried out extensive molecular dynamics simulations of the intramolecular crosslinking on six different models to study their rigidity transitions. It was found the crosslinking satisfiablity of the system is greatly affected by its rigidity. A facile analysis of floppy modes of the system was employed to determine the rigidity transition threshold and a good agreement with simulation data was obtained. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W20.00008: Spreading of Droplets on Viscous Polymer Liquids Flint Pierce, Dvora Perahia, Gary Grest Significant strides have been made in understanding the spreading of liquid droplets on solid surfaces. However from biological complexes to polymeric interfaces, the surfaces are not ideal; explicitly, the surfaces may dynamically respond as spreading takes place and the droplets may partially penetrate. Molecular dynamic simulations were carried out to investigate the spreading of liquid droplets of short chains on films of viscous polymer melts. Unlike the spreading on solid surfaces, the droplets simultaneously spread and penetrate. The degree of penetration and the magnitude of damping from the film depend on the viscosity of the underlying liquid and the relative interaction of the two constituents. At the interface with viscous films a precursor foot spreads ahead of the droplet whereas on top of less viscous interfaces, the droplets penetrate while spreading with no precursor foot. A kinetic model described the spreading of shorter chain length droplets, while a hydrodynamic model better expresses the spreading of longer chain length liquid. Supported in part under DOE contract No. ER46456. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W20.00009: Shear rate threshold for the onset of boundary slip in dense polymer films Nikolai Priezjev Molecular dynamics simulations are carried out to investigate the dynamic behavior of the slip length in thin polymer films confined between atomically smooth surfaces. The polymer melt is modeled as a collection of bead-spring chains of 20 Lennard- Jones monomers interacting through the FENE potential. We found that at high melt densities and low shear rates the fluid velocity profiles acquire a pronounced curvature near the walls and the slip length is approximately equal to the thickness of the immobile boundary layer. The linearity of the fluid velocity profiles is restored at higher shear rates where the slip length increases rapidly as a function of shear rate. We will show that the friction coefficient at the interface between a polymer melt and a solid wall follows power law decay as a function of the slip velocity. At large slip velocities the friction coefficient is determined by the product of the value of surface induced peak in the structure factor and the contact density of the first fluid layer near the solid wall. A relation to recent slip flow experiments is discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W20.00010: Direct Numerical Evaluation of Plateau Modulus of Entangled Polymer Melts via Multi-Scale Molecular Dynamics(MD) Won Bo Lee, Kurt Kremer Plateau modulus and viscosity of entangled polymer melts can be calculated by off-diagonal elements of stress tensor, which are connected by the Green-Kubo relation and tube theory. However, direct numerical evaluation of plateau modulus via stress autocorrelation function (SAF) from MD simulation is a big challenge in a computational point of view due to the following reasons: strong fluctuations, long relaxational times and large spatial scales. In the present work, SAFs of entangled polymer melts are calculated by coarse-grained MD. We find that the use of time-averaged stress helps to reduce strong noise in SAF while capturing most local chain relaxations. Plateau values by SAF are compared with plateau values predicted from the entanglement length evaluated via primitive path analysis (PPA). The importance of well equilibrated melts for such an analysis is shortly discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W20.00011: Cole-Davidson Glassy Dynamics in Simple Chain Models John McCoy, Joanne Budzien, Taylor Dotson, Douglas Adolf, Jonathan Brown Rotational relaxation functions of the end-to-end vector of short, freely jointed and freely rotating chains were determined from molecular dynamics simulations. The associated response functions were obtained from the one-sided Fourier transform of the relaxation functions. The Cole-Davidson function was used to fit the response functions. For the systems studied, the Cole-Davidson function provided remarkably accurate fits (as compared to the transform of the Kohlrausch-Williams-Watts (KWW) function). The only appreciable deviations from the simulation results were in the high frequency limit and were due to ballistic, or free rotation, effects. The accuracy of the Cole-Davidson function appears to be the result of the transition in the time-domain from stretched exponential behavior at intermediate time to single exponential behavior at long time. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W20.00012: Glass transition temperature of PIB, PDMS and PMMA from small-time simulations Solomon Duki, Mesfin Tsige, Philip Taylor We have applied some new techniques to obtain predictions of the glass transition temperatures $T_g$ of poly(isobutylene), poly(dimethyl-siloxane), and poly(methyl methacrylate) from small-time atomistic molecular dynamics simulations. The different fragilities of these materials are reflected in the results of the simulations. One approach involved measurement of the apparent softening of the ``cage'' in which a monomer is bound, while another involved studying autocorrelation of a convolution of the velocity with a smoothing function in order to detect the frequency of escapes from the ``cage.'' To check the accuracy of the short-time methods, the $T_g$ of the polymers was also found using conventional diffusion simulations in which the rate of increase of the root mean squared displacement of an atom, monomer, or molecule is measured at very long times. The economical short-time simulations yielded results for $T_g$ that were identical to those of the computer-intensive long-time simulations. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W20.00013: Translocation of a Polymer Through a Nanopore in the Presence of Obstacles Hendrick W. de Haan, Gary W. Slater The translocation of a polymer through a nanopore is interesting both as a process of fundamental biological importance and as relevant to the development of next-generation DNA sequencing technology. Due to the time and length scale of typical systems and events, computer simulations are well suited to study this problem and have been used extensively to study different aspects of the translocation process. In this work, we present results from a system in which a polymer and a membrane containing a nanopore are placed in a medium containing obstacles. Using the Espresso Molecular Dynamics simulation package, simulations are performed in which the translocation events are driven by: i) an obstacle concentration gradient and ii) a varying amount of disorder. Results indicating the establishment of a preferential direction and assessing the impact of the system configuration on details such as the translocation time will be given. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W20.00014: Role of RNA in the self-assembly of virus:A coarse-grained Brownian Dynamics study J.P. Mahalik, Murugappan Muthukumar Assembly of a single viral capsid (Icosahedral T1 type) was studied in the absence and presence of RNA. A coarse-grained rigid body model was used to represent the capsomer units and a flexible polyelectrolyte model was used to represent RNA. Brownian Dynamics simulation was used to study the assembly process. The rate of assembly was found to be enhanced in the presence of RNA. The free energy contribution of the RNA in the self-assembly process was computed using weighted histogram analysis method. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W20.00015: Probability of adsorption of peptide (CR3-1, S2) chains on clay minerals by coarse-grained Monte Carlo simulation Ras B. Pandey, Hendrik Heinz, Barry L. Farmer, Sharon Jones, Lawrence F. Drummy, Rajesh R. Naik A coarse-grained description is used to study the structure and dynamics of peptide chains (CR3-1, S2) in presence of a clay surface on a cubic lattice. A peptide chain is represented by the specific sequence of amino acids. Specificity of residues is captured via an interaction matrix based on the insight gained from the atomistic simulation, i.e., each residue interacts with surrounding residues, solvent, and the clay surface with a unique interaction potential. We use a standard LJ potential with its coefficient controlled by the interaction matrix. Simulations are performed with a number of peptide chains. Along with the global energy and dynamics of peptides, we keep track of mobility, energy (total and adsorption), and correlation with the local structure from the density profiles of each residue. Based on the analysis of local and global quantities, we are able to assess the probability of adsorption of peptides to clay surface in agreement with experiment. The probability of adsorption of S2 is found to be much higher than that of CR3-1 in which S2 is anchored by Lysine. The procedure is complementary to biopanning experiments since it allows screening a large number of peptides (more than 10E+5) on the surface to estimate their binding potential. [Preview Abstract] |
Session W21: Dopants and Defects in Semiconductors IV
Sponsoring Units: DMPChair: Anderson Janotti, University of California Santa Barbara
Room: 323
Thursday, March 19, 2009 11:15AM - 11:27AM |
W21.00001: Temperature Dependence of the Minority Carrier Lifetime in n$^{-}$ Epilayers of 4H-SiC Paul Klein, Amitesh Shrivastava, Tangali Sudarshan Controlling the lifetime of minority carriers in n$^{-}$ epilayers of 4H-SiC is of great current interest, as short lifetimes lead to a high forward voltage drop in high-voltage, bipolar switching devices. As such devices operate at elevated temperatures, the temperature dependence of the carrier lifetime is of particular interest. For materials where the lifetime is controlled by trapping at deep defects (e.g. Z1/Z2), increasing the temperature results in the thermal emission of trapped carriers, leading to an increase in the carrier lifetime. In this work the temperature dependence of the carrier lifetime, measured by time-resolved photoluminescence decay at low injection, has been studied in a range of epitaxial layers. In addition to the classic temperature dependence, some samples exhibit a lifetime that decreases rapidly with temperature in a thermally activated manner. This behavior is believed to result from the fact that the deep defect concentration is too low in these samples to limit the lifetime, and that other processes have become dominant. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W21.00002: Issues with Deep Defect Spectra in Electron Irradiated 4H SiC F. Yan, R.P. Devaty, W.J. Choyke, T. Kimoto, T. Ohshima, G. Pensl Recently, Steeds \textit{et al.} [1] discussed deep levels induced by high fluence electron irradiation in 4H SiC. We have also observed the particular triplet assigned to di-carbon antisites using both ion implantation and electron irradiation. Here we specifically address data obtained by 170 keV electron irradiation at a fluence of 5x10$^{16 }$cm$^{-2}$. We shall discuss details of the no phonon lines of the triplet as well as two sets of vibrational modes well beyond the highest energy of the SiC lattice spectrum. Theory suggests that one should observe four no phonon lines and groups of four lines for each observed localized mode. Our high resolution spectra reveal differences in the LVM spectra with respect to those reported by Steeds \textit{et al.} We obtain strong spectra at a fluence of 5x10$^{16 }$cm$^{-2}$ whereas Steeds \textit{et al.} report that they do not see the triplet in the irradiated area using greater than 10$^{19}$ cm$^{-2}$, but do see it beyond the periphery of the TEM beam. We can explain this in terms of the transverse straggling of the electrons in his TEM beam. Finally, we report the reappearance of this triplet due to an anneal at 1100\r{ }C after it had already been annealed out at 1400\r{ }C. [1] J. W. Steeds\textit{ et al.}, Phys. Rev. B \textbf{77}, 195203 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W21.00003: Charge transfer kinetics of carbon vacancy defect in 4H-SiC J. Dashdorj, M.E. Zvanut, J.G. Harrison There has been much detailed work aimed at understanding carbon vacancy related defects and their complexes in SiC, but there are no reports of charge transfer kinetics between the carbon vacancy and other defect centers. In this study, optical cross sections of the positively charged carbon vacancy, V$_{c}^{+}$, in high purity semi-insulating 4H-SiC were measured by time-dependent photo-electron paramagnetic resonance, EPR. The measurements were performed by a X-band EPR spectroscopy at 80 K. Selected photon energies used in this study were between 0.8 and 3.13 eV. A single defect model considering only capture and emission of electrons from V$_{c}^{+}$ was shown to fit well the measured data. The photon energy-dependence of the cross sections exhibit threshold value of 1.6 eV and peak value of 2.15 eV for the capture, and threshold value of 1.9 eV and peak value of 2.45 eV for the emission processes, respectively. In this talk, we will discuss the above results in terms of charge transfer mechanisms including the effects of the electronic density of states and participation of phonons. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W21.00004: EPR Study of SiC Defects Related to N$_{2}$ and O$_{2}$ Annealing. Sarah Thomas, Mary Ellen Zvanut SiC is a promising replacement for Si in future high power, high temperature electronic devices. It is well known that the Si/SiO$_{2}$ interface in MOSFETs has electronically active defects, and recent work has shown the same is true for SiC. Our research focuses on identifying the cause and location of defects in thermally treated SiC substrates using EPR at 9.8 GHz. Samples underwent isochronal anneals from 400 to 1000 $^{o}$C in high purity dry ($<$0.9 ppm H$_{2}$O) N$_{2}$ or O$_{2}$. Room temperature EPR spectra showed two defects, defect A and defect B, with line-widths of 4G and 10G, respectively. The temperature dependence was similar for the N$_{2}$ and O$_{2}$ anneals until 800 $^{o}$C, when the concentration of defect A, which stayed constant in N$_{2}$, decreased in O$_{2}$. In both ambients defect B was eliminated, and it was determined that this defect was due to cutting. That the amount of defect A decreased during the O$_{2}$ anneals, but not during the N$_{2}$, suggests that oxidation, perhaps through etching, removes the signal. During the talk we will compare the results of oxidation and reactive ion etching studies, as these will give a better understanding of the location of defect A. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W21.00005: Pulsed ENDOR at 240 GHz of nitrogen centers in 4H-SiC: towards a detailed description of the wavefunction. Johan van Tol, Mary-Ellen Zvanut SiC is a very suitable semiconductor material for high power and high temperature applications. The electronic properties of many different defects and dopants in various polytypes have been studied by electron paramagnetic resonance (EPR), including various common nitrogen substitutional defects. In particular, \textit{high frequency} EPR has proven very powerful in separating the EPR signals of different sites, while the nuclear transitions of hyperfine coupled $^{29}$Si and $^{13}$C that are observed by electron nuclear double resonance (ENDOR) can be well separated. Here we report on new data on the hexagonal nitrogen center in 4H-SiC, for which the hyperfine interaction with the surrounding silicon and carbon shells was measured by pulsed ENDOR at 240 GHz. While these measurements give precise values for the electron spin density on the surrounding atoms, the assignment of these densities to particular atomic sites has proven challenging. The multivalley structure of the conduction band in this indirect semiconductor complicates the analysis. We will discuss the observed results and propose a tentative assignment on the basis of the Kohn-Luttinger theory. Supported by the NSF though grants DMR-0654118 and NSF DMR-0520481. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W21.00006: X-ray photoelectron spectroscopy of Ni doped boron carbides Nina Hong, M.A. Langell, S. Adenwalla Ni acts as an n-type dopant for semiconducting boron carbide (BC). A series of samples with increased Ni doping were grown on Si substrates using plasma enhanced chemical vapor deposition (PECVD) and characterized using IV measurements and X-ray photoelectron spectroscopy (XPS). Increased Ni doping leads to a linear increase in Ni concentration as evidenced by the intensity of the Ni 2p photoemission peak relative to that of the B 1s peak; concomitantly, the IV curves indicate that the BC becomes increasingly n doped. B1s peak shapes shows B-C and B-B bonding structure, and the C1s peak shows B$_{11}$C icosahedra bonds and C-B-C chain bonds in all samples. The overall binding energies for B and C agree with the results from sputter deposited stoichiometric B$_{4}$C [1]; in these PECVD grown samples, however, the graphite peak commonly seen in the sputter deposited B$_{4}$C is absent. [1] I. Jimenez, L. J. Terminello, \textit{et al.} J. Elect. Spec. Relat. Phenom., 101-103, 611-615 (1999). [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W21.00007: First-principles study of Oxygen vacancies in Mg$_{x}$Zn$_{1-x}$O alloys Adisak Boonchun, Walter Lambrecht A first principles study of oxygen vacancies in Mg$_{x}$Zn$_{1-x}$O alloys has been carried out within the LDA$+U_s+U_d$ FP-LMTO approach. Different types of oxygen vacancies are distinguished by their number of Mg and Zn nearest neighbors. We find that the energy of formation is lowest for oxygen vacancies surrounded by four Zn nearest neighbors. Because of the Boltzmann factor this implies that the probability of finding oxygen vacancies with one or more Mg as nearest neighbors is strongly suppressed. Unlike in pure ZnO and MgO, we do not find negative U behavior but this may in part be because of the small size of the supercell. The 2+/+ and +/0 transitions level gradually move to higher energy as the number of nearest neighbor Mg atoms increases. Defect levels in rocksalt MgO are also presented. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W21.00008: Cation dopant distributions in Mn-doped ZnO nanostructures and thin films: experiment and Monte Carlo simulations T.C. Droubay, D.J. Keavney, S.M. Heald, T.C. Kaspar, B.P. Kaspar, C.M. Wang, C.A. Johnson, K.M. Whitaker, D.R. Gamelin, S.A. Chambers Anion or cation doping at relatively high concentrations of several atomic percent is frequently suggested to realize synthetic materials with qualitatively new functionality. While the statistical probability of obtaining singles, dimers, and trimers has been determined for bulk lattices, these distributions are significantly altered in nanostructures and thin films due to the presence of under-coordinated surface sites. The dopant distributions in nanostructures and thin films of doped wurtzite ZnO have been determined from Monte Carlo simulations. Using empirical expressions derived from the MC simulations that accurately predict dopant bonding configurations as a function of surface-to-volume ratio and concentration, experimental results for epitaxial films of Mn-doped ZnO will be discussed. X-ray absorption and x-ray magnetic circular dichroism revealed that Mn(II) substituted for Zn in the Mn:ZnO films, which were deposited by PLD using targets created from Mn:ZnO nanoparticles. However, while substitutional, the Mn distribution is not stochastic but rather tends to segregate, yielding higher local concentrations than anticipated. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W21.00009: Weak localization effects in Al-doped ZnO films Priya.V Chinta*, Q.Y. Chen*, O. Lozano*, P.V. Wadekar*, W.K. Chu, S.W. Yeh, N.J. Ho, L.W. Tu, Y.S. Chang, W.Y. Pang, I. Lo, H.W. Seo Metal-semiconductor transitions (MST) at low temperatures were studied for (0001)-oriented Zn$_{1-x}$Al$_{x}$O thin films deposited by simultaneous RF magnetron sputtering of ZnO and Al onto (11-20)-oriented Al$_{2}$O$_{3}$ substrates. The MST occurs at 190K, 102K and 260K for x=2{\%}, 3{\%} and 10{\%} of Al-doping, respectively. The samples display negative magnetoresistance at low temperatures with zero-field electrical resistivity being as low as 3.3 $\times$ 10$^{-4} \quad \Omega $-cm for x=3{\%}. The charge scattering mechanisms below the MST will be discussed in light of weak localization and coulomb interactions due to disorder in the system. *Also with Dept of Physics, NSYSU, Taiwan. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W21.00010: First-Principles Theoretical Analysis of Dopant Adsorption and Diffusion on Surfaces of II-VI Compound Semiconductor Nanocrystals Tejinder Singh, T. J. Mountziaris, Dimitrios Maroudas We present a first-principles theoretical analysis of dopant adsorption and diffusion on facets of II-VI semiconductor nanocrystal surfaces and discuss its implications for dopant incorporation into growing nanocrystals. We focus on ZnSe nanocrystals with diameters d$\sim $5 nm that have polyhedral shapes with well-defined facets. Using density functional theory calculations, we find that ZnSe(001)-(2$\times $1) is the energetically favorable surface facet for dopant binding, with multiple adsorption sites. We find that the binding energy for Mn adsorption onto various sites of the ZnSe(001)-(2$\times $1) surface increases with increasing dopant surface concentration. This low binding energy at low dopant surface concentration provides an explanation for doping difficulties during nanocrystal growth. In addition, we have analyzed several dopant migration pathways for Mn diffusion on the ZnSe(001)-(2$\times $1) surface and calculated the corresponding activation barriers as a function of dopant surface concentration. We find that Mn atoms can migrate fast along the Se dimer rows. However, Mn migration to a trough site is governed by a high-barrier process that may lead to dopant incorporation into the ZnSe nanocrystal. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W21.00011: Effect of Oxygen Vacancies in In$_2$O$_3$ Kalum Palandage, Adil-Gerai Kussow, Alkim Akyurtlu, Gayanath Fernando In order to assess the effect of oxygen vacancies on its electronic structure, we have calculated the band structure of In$_2$O$_3$ (in the Ia$_3$ structure) with and without oxygen vacancies using density functional theory within the local density approximation. A $4\times4\times4$ Monkhorst-Pack grid of k- points were used to sample the Brillouin zone while permitting full structural relaxation and self-consistency. A noticeable change that is observed is in the nature of the direct band gap of In$_2$O$_3$ at the zone center, which becomes indirect with the addition of a single oxygen vacancy to an ideal 40-atom unit cell. There is also a clear tendency toward metallic behavior with the inclusion of a single oxygen vacancy, which appears to be independent of the location of the vacancy. In addition, spin-polarized calculations reveal negligible magnetization due to the introduction of these vacancies. The threshold vacancy concentration necessary for metallic behavior, our results from a symmetry analysis of the relevant valence and conduction band states, which are crucial for optical transitions, as well as effects due to Cr-doping will be presented. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W21.00012: Transport properties of transparent conducting oxide thin film, Nb:In2O3 O. Lozano*, Q.Y. Chen*, P.V. Chinta*, P.V. Wadekar*, L.H. Chu, D. Wijesundera, Wei-Kan Chu, H.W. Seo, L.W. Tu, Y.S. Chang, W.Y. Pang, I.K. Lo, S.W. Yeh, N.J. Ho Thin films of Nb-doped In$_{2}$O$_{3}$ were deposited on YSZ(001) by magnetron co-sputtering. The well-oriented thin films were studied as a function of Nb doping by x-ray diffraction, optical absorption spectroscopy and magneto-transport measurement. The optical transparency in the visible and infrared spectral ranges is 97-99{\%} while the electrical resistivity is about 0.4 m$\Omega $-cm. The variation of these properties with respect to doping will be discussed in the context of scattering and optical transition mechanisms. *Also with Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W21.00013: A First-Principle Density-Functional Theory of BxGa1-xBiyAs1-y Quaternary Alloys Zuozi Chen, Lei Liu, Peter Y. Yu, Zhi Xun Ma, S.S. Mao Both B and Bi are isovalent impurities in GaAs when they substitute for Ga and As, respectively, at low concentrations. At higher concentrations they can form alloys with GaAs. They have opposite effects on the host GaAs crystal in terms of the lattice constant and band gap. B is smaller than Ga and will increase the band gap of GsAs in addition to converting it from a direct band gap semiconductor into an indirect one. On the other hand, Bi is larger than As and will decrease the band gap of GaAs, turning it into a semi-metal at high concentration. In principle, by incorporating B and Bi into GaAs in appropriate concentration one can tune the band gap of the alloy over a large range of values from the far infra-red to the near uv. We have performed a first-principle density-functional calculation of the total energy, lattice parameters and the band gap of the cubic BxGa1-xBiyAs1-y alloy system. A generalized quasi-chemical approach is adopted to handle the disorder effects induced by alloying. The constant band gap energy surface Eg(x,y) of the quaternary alloy was found to display a two-dimensional bowing in the x-y plane. The range of compositions for which the alloy is lattice-matched to GaAs is also obtained. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W21.00014: Diffusion of point defects in CdTe John Jaffe, Charles Henager We have investigated the mobility of isolated native point defects in CdTe by first-principles calculations. Cd vacancies and interstitials, Te interstitials and Te-on-Cd antisites were considered. Diffusion barriers were found by the NEB (nudged- elastic-band) technique within the PAW-LDA method as implemented in the VASP code. Diffusion constants are estimated, and some implications for the growth of radiation detector material are suggested, especially in regard to the formation of Te precipitates. Comparisons to experimental and earlier theoretical studies are also provided. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W21.00015: Engineering Oxygen Vacancy Distribution by Exteranl Strain Da-Jun Shu, Shu-Ting Ge, Mu Wang, Nai-Ben Ming The most common defects on surfaces of transition metal oxides are oxygen vacancies, which play critical roles in applications such as heterogeneous catalysis, photoelectrolysis, biocompatibility, etc.. If the nature and distribution of the oxygen vacancies can be controlled, the surface properties will then be modified for different applications. For this purpose, one needs to understand both the influence of oxygen vacancies on the surface properties and the responses of oxygen vacancies to different external fields. We have conducted comprehensive first principles calculations on the surface energy of strained rutile TiO2(110) with oxygen vacancies. The formation energy of each type of oxygen vacancy is calculated as a function of external strain. We find that the type of the most easily formed oxygen vacancy can be tuned by the strain and therefore suggest that the distribution of oxygen vacancies can be engineered by external strain, which helps to improve the applications of TiO2 surface where oxygen vacancies play important roles. The dependence of surface elastic properties on the type of oxygen vacancy is found to be responsible for the interplay between external strain and oxygen vacancies. [Preview Abstract] |
Session W22: Focus Session: Dilute Magnetic Nitride Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Roland Kawakami, University of California Riverside
Room: 324
Thursday, March 19, 2009 11:15AM - 11:27AM |
W22.00001: Nitrogen defects and ferromagnetism in Cr-doped AlN Bang-Gui Liu It is believed that N defects play important roles in achieving high-temperature ferromagnetism in Cr-doped AlN. We use state-of-the-arts DFT method to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies $V_{N}$. Our total-energy calculations show that the nearest Cr-Cr pair with the two spins in parallel is the most favorable and the nearest Cr-$V_{N}$ pair makes a stable complex. Our formation energies indicate that $V_{N}$ regions can be formed spontaneously under N-poor condition, or Cr-doped regions can be formed under N-rich condition. Hence real samples should be inhomogeneous. Both of the single Cr and $V_{N}$ create filled electronic states in the semiconductor gap of AlN. N vacancies enhance the ferromagnetism by enlarging Cr moment, but reduce the ferromagnetic exchange constants between the spins in the nearest Cr-Cr pairs. These calculated results are in agreement with experimental observations and facts. Phys. Rev. B 78, 195206 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W22.00002: Optical and magnetic properties of Tm-doped doped AlGaN alloys N. Nepal, S.M. Bedair, J.M. Zavada, N.A. El-Masry, D.S. Lee, A.J. Steckl, A. Sedhain, J.Y. Lin, H.X. Jiang Trivalent RE-ions in AlGaN alloys have been shown to emit narrow intra-4$f$ transitions over the entire infra red to ultraviolet (UV) spectral range. Also, unpaired 4$f$-electrons of RE ions can align along an easy axis giving magnetic properties to these RE-doped semiconductors. Thulium is one of the RE element which has special optical and magnetic characteristics. We present optical and magnetic properties of Tm doped Al$_{x}$Ga$_{1-x}$N (0 $\le $ x $\le $ 1) alloys grown by solid-source molecular beam epitaxy. Hysteresis measurements on these alloys show ferromagnetic behavior at room temperature. The measured magnetization was strongly dependent upon the Al content and reached a maximum for x = 0.62. Previous photoluminescence measurements on these films yielded a blue emission at 465 nm with peak intensity at the same Al content. Our experimental results indicate that both optical and magnetic properties are directly correlated with the alloy compositional fluctuation found in undoped Al$_{x}$Ga$_{1-x}$N alloys. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W22.00003: The Failure of LDA and GGA to describe Relative Stability, Electronic Structure and Magnetism of MnN and (Ga,Mn)N Alloys Jennifer Chan, Zhe Liu, Hannes Raebiger, Stephan Lany, Alex Zunger Pure MnN and (Ga,Mn)N alloys are studied using {\it ab initio} generalised gradient approximation +$U$ (GGA+$U$) or hybrid-exchange density functional (B3LYP) methods which predict dramatically different electronic structure, magnetic behavior and relative stabilities compared to local-density calculations. A unique structural anomaly of MnN, in which local-density calculations fail to predict the experimentally observed rocksalt as the ground state, is resolved with GGA+U and B3LYP. The phase-separation of zinc-blende (Ga,Mn)N alloys is examined using a mixed-basis cluster expansion based on the corrected GGA total energies. The predicted asymmetric spinodal phase diagram indicates that (Ga,Mn)N precipitates contain $\sim$5\% or $\sim$50\% Mn at typical growth temperatures. Thus, 100\% pure MnN, that suppresses the Curie temperature, will not be formed. The Curie temperature for the x$_{Mn}$=50\% phase is estimated to be T$_C$=$\sim$300~K indicating that high T$_C$ ferromagnetism in zinc-blende (Ga,Mn)N alloys is due to precipitates. \\ Chan {\it et al.} Phys. Rev. B {\bf 78}, 184109 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W22.00004: Origin and control of ferromagnetism in magnetically doped semiconductors. The case of (Ga,Fe)N Invited Speaker: The comprehensive search for materials exhibiting spintronic functionalities has resulted in the discovery of a number of magnetically doped or nominally undoped wide-band gap semiconductors and oxides showing ferromagnetic features persisting up to high temperatures. In order to shed light on the origin of the high-TC ferromagnetism in these materials systems, we have undertaken studies of MOVPE-grown (Ga,Fe)N, either undoped or co-doped with Si or Mg, combining the magnetic (SQUID and EPR), magnetooptical, and XANES investigation with a thorough structural and chemical characterization (SIMS, TEM, EDS, synchrotron XRD), that provides information on the Fe distribution at the nanoscale. In this talk, we first discuss our quantitative study of the exchange coupling between the spins S = 5/2 localized on the Fe ions and of the effective mass electrons. Our results point to an anomalous p-d exchange splitting of the valence band [1], that we explain in terms of a renormalization of extended states occurring if the impurities perturb strongly the crystal potential. We then show that the Fe ions are incorporated in the nitride matrix in a way giving rise either to a diluted random alloy or to ferromagnetic nanocrystals that aggregate by precipitation or by spinodal decomposition into regions more or less rich in the magnetic component, and that can be controlled by the growth parameters and co-doping with shallow donors and acceptors [2].\\[4pt] [1] W. Pacuski, P. Kossacki, D. Ferrand, A. Golnik, J. Cibert, M. Wegscheider, A. Navarro-Quezada, A. Bonanni, M. Kiecana, M. Sawicki, T. Dietl, Phys. Rev. Lett. 100, 037204 (2008).\\[0pt] [2] A. Bonanni, A. Navarro-Quezada, Tian Li, M. Wegscheider, R.T. Lechner, G. Bauer, Z. Matej, V. Holy, M. Rovezzi, F. D'Acapito, M. Kiecana, M. Sawicki, and T. Dietl, Phys. Rev. Lett. 101, 135502 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W22.00005: Magnetic, structural and optical properties of Mn-based and Cr-based diluted magnetic semiconductors and alloys A. Alsaad We have implemented supercell approach by using local spin density functional theory for Mn-doped GaN, Mn-doped ScN and the linear muffin-tin orbital method to predict the structural and magnetic properties of these novel diluted magnetic semiconductors and their Ga$_{x}$Mn$_{1-x}$N and Sc$_{x}$Mn$_{1-x}$N alloys. The global energy minimum of MnN is obtained for zinc-blende structure. If the compound is compressed by 6 {\%} the energy minimum corresponds to the NaCl structure in disagreement with the experimentally observed a slightly tetragonally distorted rocksalt structure, known as ? phase. The rocksalt structure of CrN at about 8 {\%} lattice expansion becomes stable in the ferromagnetic (FM) state and has a global minimum energy at a lattice constant of 3.9 {\AA}. We have observed an isostructural phase transition for Sc$_{x}$Mn$_{1-x}$N alloys from zince-blende phase to hexagonal phase that occurs at a hydrostatic pressure of 17.5 GPa. Moreover, the structural and optical properties of single crystal CrN/ScN superlattices and Cr$_{1-x}$Sc$_{x}$N alloys are studied in details. We report an isostructural phase transition from wurtzite ($w-$CrN) to hexagonal ($h$-ScN) at a hydrostatic pressure of 21 GPa. We have also used first-principles methods to study the electronic, optical and magnetic properties of MnN and MnAs compounds in the hypothetical cubic zinc-blende phase, a phase in which the two MnN and MnAs binaries have the same local environment as that they have in GaMnN and GaMnAs alloys. We show that MnN exhibits antiferromagnetic (AFM) ground state and MnAs adopts ferromagnetic (FM) ground state. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W22.00006: Role of the Localized Defect States in the Unconventional Magnetism of GaN and ZnO Pratibha Dev, Peihong Zhang The cation defects -vacancies and the appropriate substitutionals - introduce localized defect states chiefly centered around the four surrounding anions in GaN and ZnO. This defect-induced magnetism in these otherwise nonmagnetic semiconductors is studied using ab-initio methods. The defects investigated include the cation vacancy, substitutional acceptors, and acceptor-like defect complexes. The defect states show two opposing attributes -one one hand, they are strongly localized on the anions surrounding the defect site, leading to local magnetic moment formation, while on the other hand, the extended tails of their wavefunctions lead to the long-ranged exchange interaction between the local moments. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W22.00007: Defect induced ferromagnetism in Gd doped GaN Chandrima Mitra, Walter Lambrecht We review various suggested mechanisms for the ferromagnetism in Gd-doped GaN using local spin density approximation supercell calculations. The spin splitting of the conduction band induced by the Gd $s-f$ coupling is found to decrease linearly with Gd concentration and hence colossal magnetic moments cannot be explained by filling this spin-split band with ionized donor electrons. Furthermore we find the Gd-Gd interactions to be antiferromagnetic except in p-type material. Although, Ga vacancies can induce long range interactions and up to three Bohr magneton moments per vacancy in the neutral charge state, we note that these defects only are favorable to form in n-type materials and then should predominantly occur in a $3-$ charge state which has no magnetic moment. N-interstitials are likely to form in conjunction with N vacancies for mid gap Fermi levels consistent with the semi-insulating nature of the samples and have a magnetic moment for the corresponding charge state. We find that Gd in the presence of N interstitials alone prefer antiferromagnetic coupling but in the presend of both N-interstitials and N vacancies prefer ferromagnetic coupling. We find that oxygen tends to seggregate toward an interstitial site near the Gd and in that case can induce a strong ferromagnetic coupling between Gd. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W22.00008: Ferromagnetism in GdN: an antiferromagnet in disguise. Walter R. L. Lambrecht, Chandrima Mitra We analyze the exchange interactions in GdN and Gd pnictides GdX with X=P,As,Sb,Bi using full-potential linearized muffin-tin orbital calculations as well as the linear response approach in the atomic sphere approximation. We show that in GdN, the ferromagnetism arises from the small induced opposite magnetic moments on Gd-d and N-p orbitals. When these form a perfect antiferromagnetic arrangement on the rocksalt lattice, it pins the large magnetic 4f moments in a ferromagnetic arrangement through the on-site f-d coupling. In contrast, in the other pnictides, the AFM-II (111)-ordered state is preferred, in which case there is no moment induced on N. The N\'eel temperatures as well as the Curie-Weiss temperatures extracted from this model are in good agreement with experiment for the pnictides, but the Curie temperature of GdN in this model at only about 10 K strongly underestimates the experimentally observed Curie temperature of about 70 K. Linear response calculations give an alternative view including exchange interactions with empty spheres but give consistent estimates of $T_c$ and also give a reasonable Curie temperature for metallic Gd. We find that adding n-type doping by shifting the Fermi level does not increase $T_c$ in GdN substantially but adding N-vacancies explicitly does. Thus, it seems that defects play a significant role in establishing the Curie temperature of GdN. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W22.00009: Ferromagnetism and Photoluminescence in Rare-Earth doped GaN via Diffusion M. Oliver Luen, N. Nepal, S.M. Bedair, J.M. Zavada, Ei Ei Brown, U. Hommerich, P. Frajtag, N.A. El-Masry Rare-earth doped GaN is attracting attention both as a diluted magnetic semiconductor (DMS) material and for optical devices useful in communications and multi-color semiconductor display technology. GaN's large band gap (3.4 eV) gives rise to optical transparency over a wide spectral range, from the infrared (IR) to the ultraviolet. These properties make it an optimum host for the various emissions that are possible from rare-earth (RE) ions. Recently, rare-earth doped GaN also has demonstrated above room temperature ferromagnetism. In this study, we report the diffusion of RE (Nd, Sm, Gd and Er) into undoped, Mg-doped and Si-doped GaN templates. Room temperature optical and ferromagnetic properties were studied using photoluminescence (PL) and alternating gradient magnetometer, respectively. Ferromagnetic properties show a preference for undoped and n-type GaN. PL spectra exhibit RE ion inner shell transitions in the visible and infrared regions. The mechanisms for above room temperature ferromagnetism and emission intensity related to the RE concentration, is discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W22.00010: Dilute Magnetic and Electronic Properties of Mn$_{x}$Sc$_{(1-x)}$N/ScN(001)/MgO(001) Films Grown by Molecular Beam Epitaxy Costel Constantin, Kangkang Wang, Abhijit Chinchore, Arthur Smith, Han-Jong Chia, John Markert In this study, we report the magnetic and electronic properties of Mn$_{x}$Sc$_{(1-x)}$N films grown by molecular beam epitaxy. Recently, theoretical calculations predicted a Curie temperature above 350 K for ScN films with up to 20{\%} Mn impurity concentrations[1]. The magnetic hysteresis data suggests ferromagnetic behavior for Mn$_{0.03}$Sc$_{0.97}$N and Mn$_{0.15}$Sc$_{0.85}$N films with Curie temperatures of 383 K and 361 K, respectively. Furthermore, the measured electron concentrations for the Mn$_{0.03}$Sc$_{0.97}$N and Mn$_{0.15}$Sc$_{0.85}$N films are 6.51$\times $10$^{19}$ cm$^{-3}$ and 6.17$\times $10$^{19}$ cm$^{-3}$, respectively. These measured carrier concentration agree well with the prediction of Herwadkar \textit{et al.} that ferromagnetism above room temperature in Mn$_{x}$Sc$_{(1-x)}$N should be possible by keeping the electron concentration below 10$^{20}$ cm$^{-3}$. This work is supported by: Seton Hall: University Research Council; Ohio University: DOE-BES Grant No. DE-FG02-06ER46317 and NSF Grant No. 0730257; and UT Austin: NSF Grant Nos. DMR-0605828 and DGE-0549417, Welch Foundation Grant No. F-1191. [1] A. Herwadkar (\textit{et al}.), \textit{Phys. Rev. B }\textbf{\textit{77}}\textit{, 134433 (2008).} [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W22.00011: Vibrational properties of ScN and rare-earth nitrides: theory and and Raman spectra T.R. Paudel, W.R.L. Lambrecht, C. Meyer, H.J. Trodahl, J. Zhang, A.R.H. Preston, S.E. Granville, B.J. Ruck, G.V.M. Willams Frozen phonon calculations are presented for the phonons at $\Gamma$, $L$ and $X$ points in the rare-earth nitrides using the FP-LMTO and LSDA+U. The method is found to be in good agreement with linear response pseudopotential calculations for the closely related ScN. Comparison of the calculated phonon DOS in ScN with the Raman spectra (RS) reported in literature, show that the spectum corresponds to disordered induced first order Raman scattering and emphasizes the zone-boundary modes, in particular the LO(L) mode, because the latter correspond to a breathing mode and has the strongest electron-phonon coupling for above band gap Raman excitation. We present the measured RS for SmN, GdN, DyN, ErN, LuN thin films measured with 633 and 514 nm excitation grown by evaporation of the metals in ultrapure N$_2$ gas. No significant changes were found for GdN below the T$_{\rm c}$, indicating that the mechanism is not spin-disorder related but rather to disorder originated in the presence of N vacancies. The main Raman line in the RE-N is found to correspond to the pure N-like LO(L) mode. [Preview Abstract] |
Session W23: Focus Session: Extreme Conditions and High Pressure IV: Equations of State and Dynamics
Sponsoring Units: DCOMP GSCCMChair: Kyle Caspersen, Lawrence Livermore National Laboratory
Room: 325
Thursday, March 19, 2009 11:15AM - 11:27AM |
W23.00001: Pressure induced structural transitions in the potential hydrogen storage compound NH$_{3}$BH$_{3}$ Ravhi Kumar, Jianzhong Zhang, Monika Hartl, Zhijun Lin, Sven Vogel, Luke Daemen, Andrew Cornelius, Malcolm Nicol, Yushen Zhao Ammonia borane has received much attention in recent years as it is reported to have up to 19.6 wt {\%} of hydrogen [1-2]. Hydrogen is released in a three step process when heated above 100$^{\circ}$C. To understand the structural stability of this compound under compression, we have performed high pressure angle dispersive x-ray diffraction experiments up to 27 GPa using synchrotron x-rays at HPCAT, Advanced Photon Source. Two successive pressure induced structural phase transitions were observed. The ambient tetragonal structure transforms to an orthorhombic structure around 1.2 GPa and then to another high pressure phase above 8 GPa. Complementary neutron diffraction experiments performed up to 5 GPa at LANSCE are in good agreement with the x-ray results. The structural details of the high pressure phases will be presented.\\[4pt] [1] Z. Xiong et al., Nature, 7 (2008) pp 034508\\[0pt] [2] J.B.Yang etal., Appl.Phys.Lett, 92 (2008) pp 091916 [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W23.00002: Pressure-dependent structures of amorphous red phosphorus and the origin of first sharp diffraction peaks. Joseph Zaug, Alan Soper, Simon Clark Characterizing the nature of medium range order (MRO) in liquids and disordered solids is important for understanding their structure and transport properties. However, accurately portraying MRO, as manifested by the first sharp diffraction peak (FSDP) in neutron and X-ray scattering measurements, has remained elusive for more than 80 years. Here, using X-ray diffraction of amorphous red phosphorus (a-rP) compressed to 6.30 GPa, supplemented with micro-Raman scattering studies, we build three-dimensional structural models consistent with the diffraction data. We discover that the pressure dependence of the FSDP intensity and line position can be quantitatively accounted for by a characteristic void distribution function, defined in terms of average void-size, void-spacing, and void-density. This work provides a template to unambiguously interpret atomic and void-space MRO across a broad range of technologically promising network-forming materials. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W23.00003: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W23.00004: First-principles calculation of lattice anharmonicity and lattice thermal conductivity of MgSiO3 perovskite Xiaoli Tang, Abby Kavner, Jianjun Dong MgSiO3 perovskite (Mg-pv) is likely the most abundant material in the Earth's lower mantle, and its thermal conductivity at the high pressure and high temperature conditions of the Earth's interior plays an important role in governing heat transport and thus the whole Earth evolution. Measurement of the lattice thermal conductivity ($\kappa )$ of this important material is still not available at lower mantle conditions. We will present a theoretical study of $\kappa $ for Mg-pv calculated with a parameter-free method which combines first-principle techniques, quantum scattering theory, and kinetic transport equation. We have explicitly calculated the pressure dependence of both harmonic phonon spectra and the third order lattice anharmonicity tensors. A preliminary analysis based on the single relaxation time approximation suggests that the increase of phonon frequencies at high pressures contributes to the increase of $\kappa $ at the rate of $1\% GPa^{-1}$ at 1000K and $1.3\% GPa^{-1}$ at 3000K. The pressure dependence of phonon relaxation time and its implication for the pressure dependence of $\kappa $ will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W23.00005: Theoretical study of pressure dependence of lattice thermal conductivity in MgO Jianjun Dong, Xiaoli Tang We have recently developed a computation method that combines first-principles methods and transport theories to directly calculate lattice thermal conductivities for MgO at high pressure and high temperature conditions. Within the simple single-phonon-lifetime approximation, we estimate that the blueshifts in phonon frequencies lead to an increase of conductivity at high pressure at a rate of 1.3{\%}GPa$^{-1}$ (300K). A further calculation using quantum scattering theory shows that the anharmonicity-induced phonon scattering rate decreases at high pressure. The estimated pressure effect related to the phonon lifetime increase is comparable to that due to the frequency blueshifts. Our final calculation results will be discussed with comparison to available experiment data and the damped-harmonic-oscillator-phonon-gas model. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W23.00006: Effects of Ionizing Radiation on Capacitors Harold P. Hjalmarson, Rudolph J. Magyar, Paul S. Crozier, E. Frederick Hartman Irradiation of an insulator by energetic ionizing radiation creates hot electrons and holes. These species cool by creating additional hot electrons and holes. In this presentation, the consequences of these hot carriers will be computed using a continuum transport method in which the carriers are assumed to be described by a quasi-equilibrium temperature much larger than the lattice temperature. The electrical effects of carrier recombination at defects for simple capacitors under irradiation will be described. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the United States Department of Energy under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W23.00007: Quasi-isentropic Compression Waves Generated by Shock Waves into Sapphire W. J. Nellis, G. I. Kanel, S. V. Razorenov, A. S. Savinykh, A. M. Rajendran For sixty years it has tacitly been assumed that a shock wave incident on a material will propagate as a shock wave in that material. Between 15 and 80 GPa a shock wave cannot propagate in sapphire, the first material demonstrated not to have a Hugoniot. Wave profiles of sapphire crystals with three orientations and two thicknesses were measured at incident shock stresses of 14, 24 and 87 GPa. 14 GPa generates elastic shocks that are overdriven at $\sim $90 GPa. Elastic-precursor decay occurs at 24 and 87 GPa. At 24 GPa all three orientations have plastic-compression waves with rise times of 200-300 ns. Long rise times are probably caused by strong bonds that break heterogeneously and statistically over a relatively long time interval. This slow damage-induced increase in pressure causes quasi-isentropic compression. Since the Hugoniot and isotherm of sapphire are essentially coincident up to 340 GPa, dissipative energy probably goes primarily into entropy of disordering the crystal rather than temperature. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W23.00008: Micron scale simulations of a Kelvin-Helmholtz instability: a direct comparison between molecular dynamics and Navier-Stokes hydrodynamics. Kyle Caspersen, Robert Rudd, David Richards, Jim Glosli, William Cabot, Paul Miller, Fred Streitz The modeling of hydrodynamic phenomena has largely been the purview of continuum mechanics, such as through the solution of the Navier-Stokes equations. Nevertheless, at small length scales, where atomistic effects become important, it is not clear that this continuum approach provides a complete description of fluid behavior. To understand the effects of atomistics, we have performed a 9 billion atom quasi-2D molecular dynamics simulation, and the corresponding Navier-Stokes hydrodynamic simulation, of an interface of copper and aluminum in a strong shear layer. The applied shear flow of 2 km/s produces complex phenomena associated with a Kelvin-Helmholtz (KH) instability. In this presentation we compare and contrast the initiation and early evolution of the KH instability modeled both by molecular dynamics and continuum hydrodynamics. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W23.00009: Transition of deformation modes in Shocked Tantalum Luke Hsiung Shock-induced twinning and $\alpha $ (bcc) $\to \quad \omega $ (hexagonal) phase transition in tantalum, which exhibits no clear solid-state phase transformation under hydrostatic pressure conditions, have been investigated. Since the domains of deformation twin and $\omega $ phase were frequently observed in regions containing high-density screw dislocations without dislocation cell structures, it is suggested that the shock-induced shear transformations (twinning and phase transformation) occur as alternative deformation modes to accommodate insufficient dislocation flow resulting from the exhaustion of dislocation multiplication when dynamic recovery processes for dislocation annihilation and cell formation become largely suppressed under dynamic pressure conditions. A physical mechanism based upon the overlapping of closely spaced dislocation loops nucleated from a jogged screw dislocation is proposed to rationalize the shock-induced shear transformations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W23.00010: Hydrogen at Extreme Conditions Subramanian Natarajan, Alexander Goncharov, Somayazulu Maddury, Russell Hemley Vast regions of the P-T phase diagram of H$_{2}$, especially in the high P-T region, remain to be explored for melting behavior and exotic phenomena related to disassociation of the H$_{2}$ molecule, metallicity and superconductivity. In recent years, few experiments employing either laser-heating or resistive heating techniques in conjunction with in situ spectroscopic experiments using Diamond Anvil Cells (DAC) have been reported attempting to address some of these. A key problem that faces experimenters is to confine the hot and reactive H$_{2}$ in the small DAC sample chamber at high pressures long enough to make meaningful measurements of physical properties. Recently, we have made considerable progress in confining hot and dense hydrogen while not compromising on the ability to make spectroscopic measurements using a complex sample assembly. With this, it has been possible to perform in situ Raman spectroscopy on H$_{2}$ and D$_{2}$ while simultaneously doing double-sided laser heating at P-T conditions of more than 1Mbar and 1500K. Typically, we are now able to perform laser heating and in situ Raman spectroscopy over several heating/cooling cycles without loss of H$_{2}$ in the Mbar range. Results of these experiments will be presented; along with details of the methodology we adopted to successfully confine hot and dense hydrogen. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W23.00011: An accurate high pressure scale from quantum Monte Carlo Kenneth Esler, R.E. Cohen, Burkhard Militzer, Jeongnim Kim We have developed a fundamental high-temperature and high-pressure scale based on cubic boron nitride (cBN) using a combination of Quantum Monte Carlo (QMC) for the static contribution along with density functional perturbation theory (DFPT) for the thermal pressure. The anharmonic Raman frequency was determined as a function of pressure by solving the Schrodinger equation for the vibrational well determined using QMC with the frozen phonon method. The use of cBN as a pressure scale has a number of advantages. Unlike ruby, its structure is highly constrained by symmetry and stable well beyond 100 GPa, and it has a well-separated Raman spectrum with sufficient pressure dependence to allow accurate pressure calibration. While the cBN EOS from density functional theory (DFT) gives reasonable agreement with experiment, the results from different approximate functionals disagree. Quantum Monte Carlo is a first principles simulation method which circumvents approximate functionals and is widely regarded to provide the most accurate predictions of the properties of solids available. We utilize state-of-the-art QMC methods to obtain the static EOS with the QMCPACK code. We include a novel correction based on all-electron wave functions to eliminate pseudopotential error. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W23.00012: Softening of ultra-incompressible CrN at high pressure Francisco Rivadulla, Manuel Banobre, Alberto Pineiro, Victor Pardo, Daniel Baldomir, Arturo Lopez-Quintela, Jose Rivas We report a dramatic softening of CrN above 1.5 GPa, (the bulk modulus decreasing from K$_{0}$=413(20) GPa to 243(10) GPa) associated to a structural transition. From the structural and magnetic data under pressure, and ab-initio calculations we suggest that this effect is purely electronic, driven by the proximity of the ionic Cr-Cr bond to itinerant electron limit. Our results help to understand fundamental aspects of the chemical bond that gives superhard materials their superior al properties, and could be useful to preserve the mechanical properties of CrN. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W23.00013: Correlating cation coordination, stiffness, phase transition pressures, and smart materials behavior in metal phosphates Dmitry Shakhvorostov, Nicholas Mosey, Yang Song, Peter Norton, Martin Mueser In this study, we present X-ray diffraction data on zinc- and calcium phosphates. The experiments reveal that low-coordinated zinc phosphates are relatively soft at ambient conditions but stiffen dramatically with pressure, p, exhibiting smart materials behavior, while high-coordinated zinc and calcium phosphates have higher initial bulk moduli and stiffen considerably less rapidly with increasing p. All systems amorphize when their bulk modulus reaches a value near 210~$\pm$~40 GPa, where the precise value depends on chemical details, indicating that phosphate networks become unstable when their bulk modulus reaches that value. Our ab initio simulations of zinc $\alpha$-phosphate support the idea that the elastic properties are controlled by the motion of rigid phosphate units, which becomes more hindered under densification, with or without increasing cation coordination. It is discussed how these results may explain why low-coordinated zinc phosphates are good anti-wear agents. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W23.00014: Novel Numerical Computations for the Equation of State of Hard Particle Systems from Gaseous to Extreme High Densities Uduzei Edgal A special form of the ``Reduced Monte Carlo Scheme'' (RMCS) used for numerical computation of the EOS of the hard particle system (2D and 3D cases) will be discussed. A major advantage of the numerical scheme is that it does not lead to difficulties with meta-stable states as do traditional MC methods. In particular, RMCS calculations (in the special form) provide results from the lowest (fluid phase) to the highest (solid phase) densities which show a first order phase transition in the hard particle system [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W23.00015: X-ray diffraction of electrodeposited nanocrystalline Ni under high-pressure Christian Grant, Jonathan Crowhurst, Tom Arsenlis, Eduardo Bringa, Morris Wang, James Hawreliak, Peter Pauzauskie, Simon Clark We studied the compressibility of monolithic fully-dense electrodeposited nanocrystalline Ni (29 nm grain size) under both quasi-hydrostatic and non-hydrostatic conditions up to a nominal pressure of 50 GPa using angle-dispersive x-ray diffraction. We obtained an equation of state consistently and unambiguously from each measured reflection. The apparent bulk modulus measured under non-hydrostatic conditions is larger than that of the corresponding coarse grained-material under either type of compression, but is nearly the same when measured under quasi-hydrostatic conditions. Our results suggest that the strength, but not the bulk modulus, of 29 nm nanocrystalline Ni is enhanced relative to its coarse-grained counterparts. [Preview Abstract] |
Session W24: Focus Session: Computational Nanoscience V: Transport
Sponsoring Units: DMP DCOMPChair: Hyoung Joon Choi, Yonsei University, Korea
Room: 326
Thursday, March 19, 2009 11:15AM - 11:51AM |
W24.00001: Quantum Monte Carlo Studies of Interaction-Induced Localization in Quantum Dots and Wires Invited Speaker: We investigate interaction-induced localization of electrons in both quantum dots and inhomogeneous quantum wires using variational and diffusion quantum Monte Carlo methods. Quantum dots and wires are highly tunable systems that enable the study of the physics of strongly correlated electrons. With decreasing electronic density, interactions become stronger and electrons are expected to localize at their classical positions, as in Wigner crystallization in an infinite 2D system. (1) Dots: We show that the addition energy shows a clear progression from features associated with shell structure to those caused by commensurability of a Wigner crystal. This cross-over is, then, a signature of localization; it occurs near $r_s \sim 20$. For higher values of $r_s$, the configuration symmetry of the quantum dot becomes fully consistent with the classical ground state. (2) Wires: We study an inhomogeneous quasi-one-dimensional system -- a wire with two regions, one at low density and the other high. We find that strong localization occurs in the low density quantum point contact region as the gate potential is increased. The nature of the transition from high to low density depends on the density gradient -- if it is steep, a barrier develops between the two regions, causing Coulomb blockade effects. We find no evidence for ferromagnetic spin polarization for the range of parameters studied. The picture emerging here is in good agreement with the experimental measurements of tunneling between two wires. Collaborators: C. J. Umrigar (Cornell), Hong Jiang (Fritz Haber Institut), Amit Ghosal (IISER Calcutta), and H. U. Baranger (Duke). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W24.00002: Magnetic impurities in Cu nanocontacts: Kondo effect and conductance from first principles David Jacob, Kristjan Haule, Gabriel Kotliar We present {\it ab initio} calculations of the electronic structure and coherent transport properties of Cu nanocontacts hosting a single magnetic impurity (Fe,Co or Ni) in the contact region. The strong electron correlations of the impurity $3d$-electrons are fully taken into account by combining density functional calculations with a dynamical treatment of the impurity $3d$-shell in the so called one-crossing approximation. We find that for all three impurities the strong electron correlations give rise to Kondo resonances at the Fermi level which in turn lead to Fano lineshapes in the coherent transport characteristics of the nanocontact. The exact shape of the Kondo and Fano lineshapes, however, depends strongly on the impurity type and the geometry of the contact. This is in agreement with recent experiments measuring the conductance of magnetic impurities on noble metal surfaces [1-4]. [1] P. Wahl {\it et al.}, Phys. Rev. Lett. 93, 176603 (2004). [2] N. N\'eel {\it et al.}, Phys. Rev. Lett. 98, 016801 (2007). [3] L. Vitali {\it et al.}, Phys. Rev. Lett. 101, 216802 (2008). [4] N. N\'eel {\it et al.}, arXiv:0810.0236 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W24.00003: Iterative real-time path integral approach to nonequilibrium quantum transport Michael Thorwart, Stephan Weiss, Jens Eckel, Reinhold Egger We have developed a numerical approach to compute real-time path integral expressions for quantum transport problems out of equilibrium. The scheme is based on a deterministic iterative summation of the path integral (ISPI) for the generating function of the nonequilibrium current. Self-energies due to the leads, being non-local in time, are fully taken into account within a finite memory time, thereby including non-Markovian effects, and numerical results are extrapolated both to vanishing (Trotter) time discretization and to infinite memory time. This extrapolation scheme converges except at very low temperatures, and the results are then numerically exact. The method is applied to nonequilibrium transport through an Anderson dot. [1] S. Weiss, J. Eckel, M. Thorwart, and R. Egger, Phys. Rev. B {\bf 77}, 195316 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W24.00004: Time dependent transport in nanostructures Kalman Varga Using the Lagrange-function representation [1] we present time-dependent density functional calculations of the transport properties of nanostructures. To avoid the complications related to the semiinfinite leads a complex absorbing potential (CAP) is added to the Hamiltonian [2,3]. This transformation leads to an effectively closed system which is computationally manageable. We will compare the results of the time dependent approach to those of time independent approaches for prototypical molecular devices such as benzene ring between gold electrodes and nanotubes.\\[4pt] [1] K. Varga, Z. Zhang, S.T. Pantelides, Phys. Rev. Lett. \textbf{93}, 176403 (2004).\\[0pt] [2] K. Varga, S.T. Pantelides, Phys. Rev. Lett. \textbf{98}, 076804 (2007).\\[0pt] [3] J. A. Driscoll, K. Varga, Phys. Rev. B. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W24.00005: Effective capacitance of small molecules and nanoscale devices in an electric circuit Xiaoguang Zhang, Jun-Qiang Lu, Sokrates Pantelides A quantum-mechanical definition of the capacitance of a molecule or nanodevice between two electodes is complicated by the fact that one cannot unambiguously partition the electron density between the metal electrodes and the molecule or device. We introduce a procedure that leads to an unambiguous partitioning and to practical calculations using a linear response formalism for alternating current (AC) transport. The linear response theory is derived for a closed quantum system including the molecule and two electrodes with a finite length. The mutual capacitance between the two electrodes in the absence of a molecule or device is subtracted to obtain an effective capacitance for the molecule in the presence of the electrodes. Numerical calculations show that the effective capacitance converges with the increasing length of the electrodes. The converged results for single molecules of CO$_2$, CO, CH$_4$, NH$_3$, H$_2$, H$_2$O, and benzene range from 0.18 to 2.832 ($10^{-22}$ F). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W24.00006: Theoretical Study of Electron Transport across Carbon Nanotube Junctions Decorated with Au Nanoparticles Khoonghong Khoo, James Chelikowsky In recent years, there has been extensive research on carbon nanotube networks owing to their potential for applications in transparent electronics, and several experimental studies have found that electrical conductivity across these networks can be increased by metal nanoparticle doping. To aid in understanding the mechanism of this conductance increase, we have performed first-principles calculations on nanotube junctions decorated with small Au nanoparticles. Our calculations show that the conductance of nanotube junctions is significantly increased by the introduction of odd-numbered Au nanoparticles, and electron transport is mediated by resonant tunneling through Au nanoparticle states. In addition, we find that interesting interference effects modulate conduction across doped nanotube junctions that connect near nanotube tips. This work was supported in part by NSF under DMR-0551195 and the U.S. Department of Energy under DE-FG02-06ER46286 and DE-FG02-06ER15760. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W24.00007: Mechanism of Current-Induced Switching in Naphthalocyanine Molecular Device Tesfaye Abtew, Jerry Bernholc, Wenchang Lu Current-induced switching of inner cavity hydrogen atoms in a naphthalocyanine molecule has been reported experimentally [1]. The experiment shows a rotation of the lowest unoccupied molecular orbital (LUMO) image by 90$^{o }$when the switching occurs. We study transport properties and energetics of a naphthalocyanine molecule sandwiched between gold leads using a parallel real space multigrid method. A non-equilibrium Green's function formalism in a basis of optimized localized orbitals is employed to evaluate the current-voltage characteristics. Current-induced forces are evaluated and used to obtain bias-induced relaxations. The current-voltage characteristics indeed reveal contrasting high and low conductances depending on the orientation of the hydrogen atoms. However, a high energy barrier restrains the hydrogens from switching. We propose an alternative atomic configuration, which yields a much lower energy barrier for switching but still results in LUMO images that agree with the experimental results. [1]. P. Liljeroth, J. Repp, and G. Meyer, Science 317, 1203 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W24.00008: Negative differential resistance in molecular junctions: The effect of the electrodes electronic structure Natalya Zimbovskaya, Mark Pederson We have carried out calculations of electron transport through a metal-molecule-metal junction with metal nanoclusters taking the part of electrodes. We show that negative differential resistance peaks could appear in the current-voltage curves. The peaks arise due to narrow features in the electron density of states of the metal clusters. The proposed analysis is based on the ab initio computations of the relevant wave functions and energies within the framework of the density functional theory using NRLMOL software package. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W24.00009: Transport Properties of DNA Bases Placed in Graphene Nano-gap Christian Wolowiec, Nick Kioussis, Dmitri Novikov There has been significant demand and research activity for the development of new DNA sequencing technologies employing transverse transport techniques. We present systematic first principles studies based on Density Functional Theory of the transport properties and current-voltage characteristics of nucleotide molecules of the DNA bases, placed in 1.2 nm gap formed between the zigzag edges of graphene nano-electrodes. The linear dispersion of the graphene electrons and the local spin-polarization associated with the zigzag edges allow the exploration of both the charge- and spin-current signatures of the DNA bases to sequence DNA. We will present results in the tunneling regime of the charge- and spin-transport properties as the geometrical conformation of the bases is varied. Such signatures may be used experimentally for developing an efficient means of sequencing larger strands of DNA. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W24.00010: Quantum mechanical pseudopotential atomistic simulations of nanosized CMOS devices Lin-Wang Wang, Xiang-Wei Jiang, Hui-Xiong Deng We have used empirical pseudopotential to calculate the electronic structures of million atom CMOS systems. This is done by using the linear combination of bulk band (LCBB) method. For a nonequilibirum CMOS system with an applied source-drain bias, we have devised three different ways to calculate the inverse carrier charge densities and the corresponding currents. The first is to use partition functions extended from source and drain using their respective Fermi energies. The second is to use a spatially dependent local quasi-Fermi energy, and the third is to calculate the current using Bardeen's tunneling current formula. In this talk, we will compare the results of these three different methods. We will also compare the quantum mechanical results with classical simulation results. This work was supported by U.S. Department of Energy under Contract No. DE-AC02-05CH11231. It has also been supported by Chinese National Natural Science Foundation. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W24.00011: Nagaoka instabilities and coherent pairing in various cluster topologies Armen Kocharian, Gayanath Fernando, Kalum Palandage, James Davenport Electron pairing and formation of various types of magnetic correlations in the ensemble of small clusters of different geometries are studied with emphasis on tetrahedron, square pyramid, etc under variation of interaction strength, electron doping and temperature. These exact calculations of charge and spin collective excitations and pseudogaps yield intriguing insights into level crossing degeneracies, phase separation, condensation and spatial inhomogeneities. Separate condensation of electron charge and spin degrees offers a new route to superconductivity in inhomogeneous HTSC systems, different from the BCS scenario. Phase diagrams resemble a number of inhomogeneous, coherent and incoherent nanoscale phases seen recently in high Tc cuprates, manganites and CMR nanomaterials. \\[4pt] [1] A.~N.~Kocharian, G.~W.~Fernando, K.~Palandage, and J.~W.~Davenport, Phys.~Rev. B{\bf 78} 075431 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W24.00012: Calculation of complex band structure for low symmetry lattices Manoj Srivastava, Xiaoguang Zhang, Hai-Ping Cheng Complex band structure calculation is an integral part of a first-principles plane-wave based quantum transport method. [1] The direction of decay for the complex wave vectors is also the transport direction. The existing algorithm [1] has the limitation that it only allows the transport direction along a lattice vector perpendicular to the basal plane formed by two other lattice vectors, e.g., the c-axis of a tetragonal lattice. We generalize this algorithm to nonorthogonal lattices with transport direction not aligned with any lattice vector. We show that this generalization leads to changes in the boundary conditions and the Schrodinger's equation projected to the transport direction. We present, as an example, the calculation of the complex band structure of fcc Cu along a direction perpendicular to the (111) basal plane. [1] Hyoung Joon Choi and Jisoon Ihm, Phys. Rev. B 59, 2267 (1999). [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W24.00013: Reduced Bloch mode expansion for fast band structure calculations Mahmoud Hussein In this paper, we present reduced Bloch mode expansion for fast band structure calculations in lattice dynamics. The expansion employs a natural basis composed of a selected reduced set of Bloch eigenfunctions. The reduced basis is selected within the irreducible Brillouin zone at high symmetry points determined by the medium's crystal structure and group theory. At each of the reciprocal lattice selection points, a number of Bloch eigenfunctions are selected up to the frequency/energy range of interest for the band structure calculations. Being in line with the well known concept of modal analysis, the proposed approach maintains accuracy while reducing the computation time by up to two orders of magnitudes or more depending on the size and extent of the calculations. Results are presented for Si-Ge quantum dot superlattice band structures. [Preview Abstract] |
Session W25: Focus Session: Graphene XV: Scanning Probes II and Hall Effect
Sponsoring Units: DMPChair: Pablo Jarillo-Herrero, Massachusetts Institute of Technology
Room: 327
Thursday, March 19, 2009 11:15AM - 11:27AM |
W25.00001: Atomically Flat Graphene on Mica Substrates Li Liu, Chun Hung Lui, Kin Fai Mak, George Flynn, Tony Heinz Much recent interest has focused on the question of the intrinsic flatness of monolayers of exfoliated graphene. In studies of both suspended graphene [\textit{Meyer et al, Nature 446 (2007)}] and graphene deposited on SiO$_{2}$ substrates [\textit{Stolyarova et al, PNAS 104 (2007)}], graphene monolayers exhibited clear variations in height. For suspended films, this variation was attributed to an intrinsic rippling instability [\textit{Meyer et al, Nature 446 (2007)}]. In the case of graphene on SiO$_{2}$ substrates, the role of intrinsic and substrate- induced effects remained unclear because of the corrugation of the substrate. In this paper we present results of a detailed study of the morphology of exfoliated graphene monolayers deposited on the atomically flat terraces of cleaved mica surfaces. Using high-resolution atomic force microscopy (AFM), we demonstrate that graphene monolayers on mica, when measured with lateral spatial resolution of $\sim $ 6nm, are flat to over micron lateral length scales to within the instrumental sensitivity of 50 pm. These results stand in sharp contrast to the behavior reported for both suspended graphene and graphene on SiO$_{2}$ substrates. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W25.00002: Scattering in monolayer graphene on SiO$_{2 }$observed by STS Aparna Deshpande, Brian LeRoy, Wenzhong Bao, Feng Miao, Chun Ning Lau The intrinsic ripples in graphene and its distinctive band structure make graphene a novel two dimensional system with intriguing structural and electronic properties. To probe the influence of graphene structure on its electronic properties we have carried out scanning tunneling spectroscopy (STS) measurements on exfoliated graphene on SiO$_{2}$ with an ultra high vacuum scanning tunneling microscope (UHV STM) at 4.2 K. Atomically resolved local density of states (LDOS) images show an interference pattern due to scattering. 2D Fourier transforms of the LDOS maps reveal two types of scattering wave vectors corresponding to long range intravalley scattering and short range intervalley scattering. Intervalley scattering due to short range potential variations leads to a $\sqrt 3 \times \sqrt 3 $ R30$^{o}$ interference pattern in the LDOS while intravalley scattering causes long range disorder in the LDOS images. Our measurements present a comprehensive picture of scattering mechanisms in exfoliated graphene and underline the contribution of random impurities, defects and SiO$_{2}$ morphology to the electronic properties of graphene. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 12:15PM |
W25.00003: STM on Gate-Tunable Graphene Invited Speaker: We have successfully performed atomically-resolved scanning tunneling microscopy and spectroscopy (STS) on mechanically exfoliated graphene samples having tunable back-gates. We have discovered that the tunneling spectra of graphene flakes display an unexpected gap-like feature that is pinned to the Fermi level for different gate voltages, and which coexists with another depression in density-of-states that moves with gate voltage. Extensive tests and careful analysis show that the gap-feature is due to phonon-assisted inelastic tunneling, and the depression directly marks the location of the graphene Dirac point. Using tunneling spectroscopy as a new tool, we further probe the local energetic variations of the graphene charge neutral point (Dirac point) to map out spatial electron density inhomogeneities in graphene. Such measurements are two orders of magnitude higher in resolution than previous experiments, and they can be directly correlated with nanometer scale topographic features. Based on our observation of energy-dependent periodic electronic interference patterns, our measurements also reveal the nature of impurity scattering of Dirac fermions in graphene. These results are significant for understanding the sources of electron density inhomogeneity and electron scattering in graphene, and the microscopic causes of graphene electron mobility. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W25.00004: Effect of disorder on electron tunneling in graphene layers through potential barriers Vrinda Thareja, Manish Sharma, Sankalpa Ghosh Electrons at the fermi level in Graphene monolayer behave like massless Dirac fermions. Using a coherent potential approximation, we study the tunneling of such electrons through a double barrier potential in presence of disorder. We subsequently extend this study in the case of periodic lattice potentials. Our approach involves using the Green's function calculation and is particularly amenable to studying the effect of disorder, impurities and defects on electron propagation through Graphene. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W25.00005: Scanning Gate Microscopy on Patterned Graphene Nanoribbons Andrei Garcia, Markus Koenig, Kathryn Todd, David Goldhaber-Gordon Graphene-based electronic devices are of interest due to the unique nature of the graphene band structure. Bulk graphene exhibits a gapless linear dispersion near the Fermi level. When graphene is etched to form a narrow ribbon a transport gap opens at the Dirac point. The origin of this transport gap in patterned graphene nanoribbons remains an unresolved problem. Two possible explanations for the origins of the gap are confinement in the direction perpendicular to the length of the ribbon and localization due to disorder along imperfectly formed ribbon edges. We explore the local properties of this gap in nanoribbons using a scanning gate microscope. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W25.00006: Scanning tunneling microscopic (STM) studies of strain-induced local density of states modulations in single-layer graphene on SiO$_{2}$ A.P. Lai, M.L. Teague, C.R. Hughes, A.D. Beyer, N.-C. Yeh, M.W. Bockrath, J. Velasco, C.N. Lau We report strain-induced spatial modulations in the electronic density of states (DOS) of single-layer graphene on SiO$_{2}$. Spatially resolved topographic and spectroscopic measurements were performed simultaneously at 77 K and at pressures $<$ 10$^{-7}$ torr. Fourier transformation of local topography shows a distorted hexagon with lattice vectors ranging from a$_{0}$=3.0 $\pm $0.2{\AA} to 2.1$\pm $0.2 {\AA} as the result of surface corrugation from the roughness of the underlying substrate. A spatially varying strain map derived from local distortions of the lattice constants correlates well with the surface topography. Strained graphene, due to three dimensional surface corrugations of $\pm $ 5 {\AA} over 10 nm lateral distance, show parabolic ``U-shaped'' conductance vs. biased voltage spectra rather than the Dirac-like ``V-shaped'' spectra. In contrast, for regions of relaxed graphene, Dirac-like spectra are recovered. The Dirac voltage, V$_{D}$, determined from the biased voltage of conductance minimum, appears to be position independent at V$_{D}$=36$\pm $5 meV, while the minimum conductance and the degree of derivation from the Dirac-like spectra at low energies appear to correlate directly with the topography. This work was supported by NSF/NRI under Caltech/CSEM. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W25.00007: SPM measurements of graphene corrugation and spatial correlation William Cullen, Jianhao Chen, Masa Ishigami, Ellen Williams, Michael Fuhrer In order to determine the effect of graphene corrugation on electronic transport, it is most important to know the spatial correlation properties of the corrugated graphene structure. In spite of much experimental effort, there is still contentious debate about the structure of graphene, both in supported and suspended geometries. It has frequently been asserted that a graphene monolayer exfoliated onto a SiO2 substrate may display ``intrinsic'' corrugation -- rippled structure which is not derived from the topography of the underlying substrate. Here, we report recent UHV NC-AFM and STM results which show that anomalous corrugation may be observed due to local interaction between the tip and the graphene monolayer. Our results show that non-perturbative NC-AFM measurement reveals a graphene topography which is as smooth as the underlying SiO2, with height-height correlation exponent 2H = 1. STM measurement of graphene, due to uncontrolled tip-sample forces, may exhibit anomalous corrugation depending on tip condition. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W25.00008: Probing localized states in graphene by Scanning Gate Microscopy Markus Koenig, Andrei Garcia, Kathryn Todd, David Goldhaber-Gordon In various experiments, patterned graphene devices like nano-ribbons exhibit a complete suppression of transport for the Fermi level in the vicinity of the Dirac point. However, the experimental results cannot be explained purely by confinement based on the geometry of the devices. Usually, the transport gap is wider than would be expected for an energy gap based only on the confinement. Among other mechanisms, localization of charge carriers at imperfect sample edges has been proposed to contribute to the increased transport gap. We use Scanning Gate Microscopy to explore localized states at etched edges and within constrictions, and discuss their impact on the transport in patterned graphene devices. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W25.00009: Phonon mediated tunneling into graphene Tim Wehling, Ilya Grigorenko, Alexander Lichtenstein, Alexander Balatsky Recent scanning tunneling spectroscopy experiments [V. W. Brar et al., Appl. Phys. Lett. 91, 122102 (2007); Y. Zhang et al., Nature Phys. 4, 627 (2008)] on graphene reported an unexpected gap of about $\pm 60$\,meV around the Fermi level. Here, we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab-initio theory and present a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasi free electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated and its effects on tunneling into graphene are discussed. In particular, good agreement of the tunneling density of states within our model and the experimental d$I$/d$U$ spectra is found. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W25.00010: Infrared Hall Conductivity in Graphene C.T. Ellis, M.-H. Kim, T. Wu, G. Sambandamurthy, J. Cerne, V. Lee, S. Banerjee Among the many different techniques which have revealed graphene's remarkable properties, infrared conductivity ($\sigma_{\mathrm{xx}}$) (Jiang, PRL 2007) and the DC Hall effect (Novoselov, Nature 2005; Zhang, Nature 2005; Zhang, PRL 2006) have provided new insights into this material. In our study we determine the infrared Hall conductivity ($\sigma_{\mathrm{xy}}$) for graphene in the 120-1000 meV range at temperatures down to 7K and magnetic fields up to 7T using Faraday measurements. Unlike $\sigma_{\mathrm{xx}}$, which measures the sum of the optical responses for left and right circularly polarized light, $\sigma_{\mathrm{xy}}$ measures the difference and therefore is sensitive to small changes in symmetry. We compare graphene samples that are prepared using several methods, including cleaving from parent materials such as highly ordered pyrolytic graphite, as well as sonication-assisted solution-phase exfoliation of natural flake graphite powder. The films are then deposited onto $Si/SiO_2$ substrates for infrared measurements. This work is supported by the NSF-CAREER-DMR0449899, also GS and SB thank the UB-IRDF for financial support. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W25.00011: Optical Hall conductivity in QHE systems Takahiro Morimoto, Yasuhiro Hatsugai, Hideo Aoki While the quantum Hall effect is among the most remarkable static properties of two-dimensional electron systems at low temperatures in magnetic fields, recent advances in optics in the THz region make spectroscopic measurements of the Hall angle possible in magnetic field of a few tesla. So a natural question we pose here is: can the quantum Hall effect, a topological phenomenon, evolve into an ``optical Hall conductivity'' in the ac regime, especially in the THz region which is the cyclotron energy scale. Motivated by this, we have theoretically calculated the optical Hall conductivity $\sigma_{xy}(\omega)$ for the ordinary quantum Hall system with Kubo formula, where the effect of disorder is taken into account with the self- consistent Born approximation. The result shows that the Hall plateaus do remain in the optical (THz) region when the disorder is not too large. Next we have extended the calculation to the graphene QHE system, and found that the optical Hall conductivity $\sigma_{xy}(\omega)$ reflects the massless Dirac dispersion and the associated Landau level structure. While the Hall palteaus are again retained in the ac region against disorder, the structure at the central ($N=0$) Landau level is particularly robust. We predict such phenomena should be measurable through an accurate detection of the Hall angle in the THz regime. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W25.00012: Magnetically induced low density phases near the Dirac point Xu Du, Ivan Skachko , Eva Y. Andrei We demonstrated techniques for suspending graphene that isolate the charge carriers from substrate-induced potential fluctuations. The suspended samples provide access to the intrinsic properties of pristine graphene close to the Dirac point. Magneto-transport measurements on these samples show low temperature mobilities exceeding 200,000 cm$^{2}$/Vs for carrier densities below 5x10$^{9}$ cm$^{-2}$, values not attainable in semiconductors or non-suspended graphene$^{\ast }$. At sub-Kelvin temperatures and in magnetic fields up to 7T, we observed unconventional quantum Hall plateaus indicative of interaction effects. Near the Dirac point, coulomb blockade-like transmission was observed. $^{\ast }$\underline { }Xu Du, Anthony Barker, Ivan Skachko, and Eva Y. Andrei, \textit{Nature Nanotechnology}, Vol.3, 491, 2008 [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W25.00013: Enhancement of nearest neighbor spin-singlet correlations in d-wave SNS graphene Josephson junctions Annica Black-Schaffer, Sebastian Doniach Using the self-consistent tight-binding Bogoliubov-de Gennes (BdG) formalism we investigate the effect of nearest neighbor spin-singlet bond (SB) correlations in a graphene SNS Josephson junction with d-wave superconducting contacts. All p$\pi$-bonded planar organic molecules, of which graphene is the infinite extension, show a preference for SB over polar configurations, as originally captured by Pauling's idea of resonating valence bonds. At strong enough coupling and/or high doping levels, these correlations will give rise to a d-wave superconducting state. However, the estimated coupling strength in graphene would require a doping level not currently experimentally achievable by a gating bias. We demonstrate that by creating a graphene SNS Josephson junction with d-wave contacts, for example by depositing a high-Tc cuprate on top of the graphene, it should be possible to enhance the effect of the SB correlations and see clear signatures of d-wave pairing in proximity effect, superconducting decay length, and supercurrent. [Preview Abstract] |
Session W26: Focus Session: Graphene XVI: Functionalization and Growth I
Sponsoring Units: DMPChair: Xiaosong Wu, Georgia Institute of Technology
Room: 328
Thursday, March 19, 2009 11:15AM - 11:27AM |
W26.00001: Impact of Atomic Hydrogen Adsorption on Transport Properties of Graphene Jyoti Katoch, Jianhao Chen, Masa Ishigami We have measured transport properties of graphene as a function of surface coverage by atomic hydrogen in ultra high vacuum. Hydrogen adsorption is reversible at moderate temperatures and alters electronic properties of graphene at atomic scale. We will discuss dependence of minimum conductivity and field-effect carrier mobility on the density of adsorbed hydrogen. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W26.00002: Determination of the Crystallographic Orientation of Graphene by Raman Spectroscopy Mingyuan Huang, Hugen Yan, Changyao Chen, Daohua Song, Tony Heinz, James Hone We present a systematic study of the Raman spectra of the G band in graphene monolayers under tunable uniaxial tensile stress. The G band splits into two distinct sub-bands (G$^{+}$, G$^{-})$ because of the strain-induced symmetry breaking. Raman scattering from the G$^{+}$ and G$^{-}$ bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W26.00003: Growth and Characterization of CVD Graphene Alfonso Reina, Xiaoting Jia, John Ho, Daniel Nezich, Hyungbin Son, Vladimir Bulovic, Mildred S. Dresselhaus, Jing Kong Large-area ( $\sim $cm$^{2})$ graphene films are grown by ambient pressure chemical vapor deposition (CVD) on evaporated Ni films. We show that proper engineering of the Ni film properties, such as grain structure and surface roughness, and the use of ultra-diluted hydrocarbon flow yield films consisting of 1 to $\sim $10 graphene layers in thickness. Furthermore, the produced graphene can be transferred, by wet-etching the underlying Ni, to a variety of substrates allowing graphene coverage over large areas on different materials such as glass, polymers or other semiconductors. Raman Spectroscopy, electron diffraction and transmission electron microscopy suggest disordered stacking of regions with multilayer graphene. Growth mechanisms will also be discussed. Opto-electronic properties and ambipolar transfer characteristics of the produced material is also demonstrated. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W26.00004: Chemical exfoliation procedure for graphene deposition Michelle Zimmermann, Mahito Yamamoto, Brad Conrad, Jianhao Chen, Ellen Williams Mechanical exfoliation techniques for graphene production yield flakes which are too small and too rare for feasible large-scale experiments or commercial device fabrication. We present a systematic evaluation of the steps involved in chemical exfoliation of graphite to generate suspended graphene sheets. The approach is based on the solubilizing of a graphite intercalation compound in a polar solvent, analogous to solubilization of CNT salts [1] and recently reported for graphene [2]. A shift in the Raman G peak of graphite provides a metric of the degree of intercalation of lithium and naphthalene into graphite flakes. To optimize deposition onto SiO$_{2}$ substrates, we compare drop casting, spin-coating and dip-coating, as well as the effects of different surface treatments (UV ozone, oxygen plasma, functionalization). [1] A. P\'{e}nicaud, et al., \textit{J. Am. Chem. Soc. }\textbf{127}, 8 (2005). [2] C. Vall\'{e}s, et al., \textit{J. Am. Chem. Soc.} \textbf{130}, 15802 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W26.00005: Anomalously high conductivity in bromine-intercalated graphite A.F. Hebard, S. Tongay, J. Hwang, D.B. Tanner, D. Maslov We have found that when graphite is intercalated with bromine, the \textit{ab}-plane ($c$-axis) conductivity sharply increases (decreases). Characterization of the Br intercalated samples by exposure time, weight uptake, sputter Auger spectroscopy and X-ray diffraction show a Br concentration that is uniformly distributed within a graphite host having an expanded interplanar spacing $d_{c}$. The \textit{ab}-plane conductivity is enhanced by several orders of magnitude in the temperature range from 300~K down to 1.7~K and shows no sign of saturation with increasing Br concentration. Hall measurements confirm a pronounced increase in the density of negative carriers consistent with an increased optical reflectivity (below 3000~cm$^{-1})$. The inferred plasma frequencies and extrapolated dc conductivities are consistent with the transport measurements. The diamagnetic susceptibility decreases with increasing Br concentration and follows a temperature dependence from which a Fermi energy that increases with increasing Br concentration is extracted. By increasing $d_{c}$, the \textit{ab}-plane conductivity of Br intercalated graphite begins to resemble the additive contributions of parallel connected doped graphene sheets and thus has implications for carbon based electronics. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W26.00006: Influence of substrates on graphene layers: Raman study Jorge Camacho, Liyuan Zhang, Tony Valla, Igor Zaliznyak Electrical contacts and a substrate can significantly influence electronic and physical properties of graphene. Charge transfer, strain, introduction of various impurities and defects are some of the factors that can alter graphene properties. Therefore, the interaction with substrate and contacts has to be considered in any real graphene-based device. Here we use Raman spectroscopy to study effects of different substrates and adsorbates on graphene Raman-active modes. We find that the intensity, frequency and line-width of some modes are very sensitive to the chemical environment of graphene sheets, reflecting the changes in interactions of these modes with charge carriers and degree of disorder introduced in the system. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W26.00007: Doping effect of electrode on Graphene Keyu Pi, Kathy McCreary, Wei Han , Yan Li, WenZhong Bao, Chun Ning Lau, Roland Kawakami Graphene as a carrier tunable transport media has drawn a lot of interest since its discovery. It has recently been reported that invasive electrode contacts cause electron-hole asymmetry [1] which will affect the transport properties. To study this effect, we developed an in-situ measurement system that combines Molecular Beam Epitaxy (MBE) with transport measurement. Fine control of the material deposition rate allows us to study the doping effect on graphene at the early stages of electrode formation. [1] B. Huard et al., PRB. \textbf{78}. 121402(R), 2008 [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W26.00008: Multi-layer graphene derived from graphite fluoride S.H. Cheng, K. Zou, A. Gupta, H.R. Gutierrez, P. Eklund, J.O. Sofo, J. Zhu, F. Okino We produce multi-layer graphene through the reduction of graphene fluoride. Graphite fluoride (CF) is synthesized by reacting F$_2$ with graphite at 500 - 600$^{o}$C. We obtain few- layer CF sheets through mechanical exfoliation and characterize their properties with electron diffraction, TEM, AFM, Raman and transport measurements. Electron diffraction spectra of fluorinated few-layer CF show the persistence of six-fold hexagonal symmetry and long-range in-plane crystalline order. Domains of varying thickness in both AFM and TEM measurements suggest an incomplete fluorination. Raman spectra of few-layer CF show the appearance of a D band ($\sim$1350 cm$^{-1}$) as expected from sp$^{3}$ bonding. Few-layer CF sheets are defluorinated in flowing H$_2$/Ar (10$\%$/90$\%$) at 500 - 600$^ {o} $C. AFM studies of defluorinated CF show a pronounced decrease in roughness and thickness, suggesting the removal of fluorine. Raman spectra of defluorinated CF show a reduced background with an enhanced 2D peak ($\sim$2700 cm$^{-1}$). We present transport measurements in field effect transistors fabricated from CF and defluorinated sheets and compare with that of pristine graphene and graphene-oxide reduced graphene. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W26.00009: Size Selection of Metal Nanoparticles on Few Layer Graphene Luke A. Somers, Zhengtang Luo, E.J. Mele, A.T. Charlie Johnson We find layer number dependence in the size of metal nanoparticles grown on 1 to 10 layer graphene. Graphene is an attractive substrate for investigating and using nanoparticles due to its loose interaction with them. To preserve this condition it is ideal to grow particles in place rather than deposit them from solution. We find that annealing of evaporated metal nanoparticles on graphene and few layer graphene surfaces tightens their size distribution. The number of graphene layers changes the selected size. These results are in quantitative agreement with a model incorporating surface, bulk, and coulomb free energies. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W26.00010: Growth of atomically smooth MgO films on graphene by molecular beam epitaxy. Kathleen McCreary, Wei Han, Wei-Hua Wang, Keyu Pi, Wenzhong Bao, Feng Miao, Roland Kawakami, Chun-Ning Lau Graphene has been the focus of many recent studies involving both electronic and spintronic devices due to its tunable charge carriers, high mobility, and possibility of long spin coherence lifetimes. To improve the spin injection into graphene spintonic devices, dielectric layers, such as MgO, are often used to minimize the conductivity mismatch between graphene and electronic contacts. We investigate the growth of MgO films on graphene by molecular beam epitaxy and find that surface diffusion promotes a rough morphology. To reduce the mobility of surface atoms, the graphene surface is dressed by Ti atoms prior to MgO deposition. With as little as 0.5 ML monolayer of Ti, the MgO overlayer becomes atomically smooth. Single layer graphene has been patterned into nanoscale devices to study the effect of the Ti dressing layer and MgO overlayer on the electronic and spintronic properties. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W26.00011: Optimization of high quality epitaxial graphene growth on SiC Ming Ruan, Michael Sprinkle, Yike Hu, Claire Berger, Walter de Heer We have developed an RF inductance furnace to grow epitaxial graphene(EG) of very high quality on SiC. EG has attracted much attention during the past years due to its potential as next generation of material for electronic devices. The formation of graphene on silicon carbide by sublimation of Si is a complicated process that is not well understood yet. We present here our latest result on the growth of epitaxial graphene in low vacuum. The research is carried out for multilayer graphene growth on 4H SiC (000$\overline{1}$) and (0001) face. Atomic force microscopy(AFM), ellipsometry, Raman spectroscopy and resistivity measurement are adopted as characterization tools. The stepped terrace structure on the SiC surface changes with the graphene layer growth conditions. Terraces larger than 20$\mu$m with flat graphene layers extended over the steps are achieved. We demonstrated a path towards mass production of high quality EG chips. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W26.00012: Growing Epitaxial Graphene on an Insulator by MBE Chandra Mohapatra, James Eckstein We have used electron beam evaporation of solid carbon (C) to deposit graphene on MgO $<$111$>$ at 850C. The growth appears epitaxial as observed by in-situ RHEED which also reveals that the hot scattering surface transitions from an insulator to a conductor after deposition of 1 monolayer of C. Growth at higher temperatures gives better crystallinity. We further characterize the film by ex-situ Raman spectroscopy, AFM and transport. Raman reveals all the characteristic G, D and 2D peaks of graphene and the 2D peak can be fit to a single lorentzian typical for graphene. AFM pictures show that the surface consists of flat connected domains, which are uniform across the substrate. Electrical transport shows insulating behavior with resistance (R) varying as $1/T^2$. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W26.00013: The effect of self-assembled monolayers on graphene conductivity and morphology T. L. Moore, J. H. Chen, B. Riddick, E. D. Williams Graphene transport properties are limited by charge defects in SiO$_{2}$, and by large charge density due to strong interaction with SiC. To modify these effects we have treated 300 nm SiO$_{2}$ with tricholosilanes with different termination groups including pure and fluoro and amino-terminated hydrocarbons for use as substrates for mechanical exfoliation of graphene. XPS measurements verify the presence of the expected termination groups. AFM measurements reveal modified monolayer roughness and correlation lengths; for a fluorinated carbon chain the RMS roughness is 0.266 $\pm $ 0.017 nm and the correlation length is 10.2 $\pm $ 0.7 nm compared to 0.187 $\pm $ 0.011 nm and 19.8 $\pm $ 2.5 nm for SiO$_{2}$. Surface free energies of the monolayers and the SiO$_{2}$ blank have been computed from static contact angle measurements and all decrease the SiO$_{2 }$surface free energy; for the fluorinated carbon chain monolayer a decrease of 20 mJ/m$^{2}$ from SiO$_{2}$. We will discuss the ease of exfoliation, and the morphology and conductivity of graphene on these monolayers. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W26.00014: The effect of SiO$_{2}$ surface states on the electronic characteristics of graphene FET devices Jorge Sofo, Ning Shen, Hugo Romero, Peter Eklund Electronic states localized at the surface of oxide semiconductors are a common cause of their low ionization potential. We study the properties of the SiO$_{2}$ surface states using density functional theory (DFT) and show that they strongly affect the intrinsic doping of graphene on oxidized silicon substrates. We present simple empirical model that it is parameterized from the DFT calculations. The model demonstrates that Dirac voltages as large as 50~V and intrinsic n-doping are produced by the presence of these surface states. We extend it to include the effect of other adsorbates, such as water, that modify the dielectric properties of the device. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W26.00015: Efficient manipulation of zigzag and armchair edges in graphene nanoribbons by joule heating Xiaoting Jia, Mario Hofmann, Vincent Meunier, Bobby Sumpter, Jessica Campos-Delgado, Jose Romo-Herrera, Hyungbin Son, Ya-Ping Hsieh, Alfonso Reina, Jing Kong, Mauricio Terrones, Mildred Dresselhaus Edge study in graphene nanoribbons has attracted lots of interest in recent years, due to the different electronic properties of the ribbons arising from zigzag and armchair edges. Here we demonstrate and monitor an efficient crystallization process for graphite nanoribbon edges by Joule heating inside an integrated transmission electron microscope (TEM) equipped with a scanning tunneling stage STM (TEM-STM system). With this system we were able to produce for the first time atomically smooth zigzag or armchair edges from defective rough edges present in graphite nanoribbons, by applying a controlled voltage, while observing the structural behavior in-situ. Edge motion along certain preferred crystallographic orientations is observed, and the transformational effects of Joule heating and applied electric field are described. This work demonstrates both the possibility of self-eliminating lattice defects by applying a bias voltage, and an effective way to produce clean zigzag and armchair edges, which could be useful for both fundamental studies of edge reactivity, magnetism and the development of future electronics applications. [Preview Abstract] |
Session W27: Hydrogen Storage: Sorbents
Sponsoring Units: FIAPChair: Channing Ahn, California Institute of Technology
Room: 329
Thursday, March 19, 2009 11:15AM - 11:27AM |
W27.00001: Functionalized Heterofullerenes for Hydrogen Storage Puru Jena, Qian Wang, Qiang Sun Using density functional theory we show that Li decorated B doped heterofullerene (Li12C48B12) has the desired properties of a hydrogen storage material: (1) The Li atoms remain isolated. (2) Through charge transfer to electron deficient C48B12 heterofullerene, the Li atoms become positively charged. (3) Each Li atom is able to bind up to three H2 molecules which remain in molecular form, and the binding energies of successive H2 molecules are in the range of 0.135 to 0.172 eV/H2, suitable for ambient temperature storage; (4) The gravimetric density reaches the 9 wt {\%} limit necessary for applications in the mobile industry. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W27.00002: Can Silicon Carbide Nanotubes Be Effective Storage Medium for Hydrogen Storage Souptik Mukherjee, Asok Ray A systematic study of molecular hydrogen adsorption on three different atomic configurations of armchair SICNTs has been performed. In the first stage of our study, first principles calculations using both density functional theory (DFT) and hybrid density functional theory (HDFT) as well as the finite cluster approximation have been performed to study the adsorption of molecular hydrogen on three types of armchair (9, 9) silicon carbide nanotubes. The distances of molecular hydrogen from the outer wall of the nanotubes have been optimized manually using the B3LYP and PW91 functionals and results have been compared in detail with published literature results. In the second part of our study, hydrogen molecule has been adsorbed from both inside as well as from the outer wall of nanotubes ranging from (3, 3) to (6, 6) for all three types. A detailed comparison of the binding energies, equilibrium positions and Mulliken charges has been performed for all three types of nanotubes and for all possible sites in those nanotubes. In the third phase, co-adsorption of two hydrogen molecules has been carried out. Possibilities of hydrogen storage have been explored in detail. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W27.00003: Nanostructured Adsorbents for Hydrogen Storage Lin Simpson To meet the DOE goals for hydrogen storage, NREL and our partners have focused development efforts on the use of nanomaterials with hydrogen binding energies between $\sim $4 and 40 kJ/mol. The use of these types of materials enables hydrogen to be reversibly adsorption/desorption with moderate to low temperatures and pressures, and greatly simplifies the refueling/ regeneration process. NREL is investigating multiple approaches to obtain high hydrogen sorption materials with the common goal of determining the underlying mechanisms and applying a fundamental basis to intelligently design advanced materials. NREL will provide detailed hydrogen capacity/performance and reproducible processing information for promising nanostructured materials. This will include detailing the potential for hydrogen storage by nanostructures, the effects of dopants, demonstrate materials with greater than 4 wt{\%} hydrogen uptake, and discuss the potential to develop materials with 9 wt{\%} or more hydrogen storage. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W27.00004: A comparative investigation of H$_2$ adsorption energy in Cd- and Zn-based metal organic framework-5 Pornjuk Srepusharawoot, Carlos Moys\'{e}s Ara\'{u}jo, Andeas Blomqvist, Ralph Scheicher, Rajeev Ahuja Density functional theory has been used to study the physisorption energies of hydrogen at all possible adsorption sites near the metal oxide cluster in both Cd- and Zn-based Metal Organic Framework-5 (MOF-5). Three types of exchange- correlation functionals (LDA, GGA-PW91, and GGA-PBE) were compared. The binding for all adsorption sites in Cd-based MOF- 5 was found to be generally stronger than in Zn-based MOF-5. In particular, the hydrogen adsorption energy at the secondary adsorption sites of Cd-based MOF-5 is increased by about 25\% compared to Zn-based MOF-5. This result suggests that Cd-based MOF-5 might be better suited to store hydrogen at a given temperature than Zn-based MOF-5. See also: {\it J. Chem. Phys.} {\bf 129}, 164104 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W27.00005: Design of Multi-Decker Incorporated Metal Organic Frameworks for Hydrogen Storage Kiran Boggavarapu, Anil Kandalam Metal Organic Frameworks (MOFs) are a new class of rationally designed microporous hybrid (organic-inorganic) materials. They have recently gained attention as potential hydrogen storage systems with gravimetric density meeting the DOE 2015 targets of 9 wt{\%}. However, due to weak interaction between the molecular hydrogen and the host MOF (see figure), high pressures are required to reach the target storage levels. Recently, multi-decker organometallic complexes are shown to exhibit the ideal thermodynamics and kinetics for hydrogen storage. However, it is not clear if these multi-decker complexes can retain their hydrogen storage capability when assembled into a bulk-material. In this presentation, we investigate the hydrogen storage capability of a new class of materials by combining the strengths of MOFs and decker complexes. An ideal way to integrate these two systems is to incorporate the multi-deckers into the structural framework of MOFs. In these hybrid materials, the multi-decker units are expected to maintain their structural integrity and there by retaining the hydrogen storage capacity with an added advantage of being a part of stable porous MOF back-bone. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W27.00006: Hydrogen Adsorption by High Surface Area Micro- porous Carbon Synthesized from Phenol-Formaldehyde Qingyuan Hu, Gregory P. Meisner A high surface area microporous carbon material can be synthesized by mixing the activation reagent potassium hydroxide into a carbon precursor solution of phenol-formaldehyde oligomers. Some polymerization of the carbon precursor occurs during the initial mixing, and further polymerization is completed by heating to 160$^{o}$C. Carbonization and activation is accomplished by heating to 500$^{o}$C - 900$^{o}$C in an inert atmosphere. The porosity and surface area of the resulting carbon material depends predominantly on the amount of activation reagent added to the carbon precursor solution and on the carbonization/activation temperature and time. Optimized synthesis conditions yield a microporous carbon with a very high BET specific surface area of nearly 3000 m$^{2}$/g and a narrow pore size distribution. This new synthesis approach yields surface areas dramatically larger than those typically obtained by traditional chemical activation methods for porous carbon where solid carbon precursors are soaked in activation reagent solutions. Hydrogen absorption up to 5.75 wt{\%} at 77 K and above 20 bars hydrogen pressure is observed for this new microporous carbon material. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W27.00007: Ab-initio study of hydrogen storage of Titanium-decorated organic systems with hydroxyl groups Manh Cuong Nguyen, Jisoon Ihm Using first-principles calculation, we study hydrogen storage of Titanium-decorated organic systems with hydroxyl groups, such as propane-1,3-diol. The results show that Ti atom is bound selectively to hydroxyl groups with the binding energy of 3.0 eV. The first hydrogen molecule adsorbed on Ti is dissociated and then Ti can bind three hydrogen molecules in molecular form more with the binding energy suitable for reversible processes (adsorption and desorption) in hydrogen storage at ambient temperature and pressure. Using thermodynamics, the usable number of hydrogen molecules per Ti atom is almost three due to the proper binding energy of the last three hydrogen molecules on Ti. Based on this result, we can design organic systems with hydroxyl groups to store hydrogen with the reduction of the tendency of transition metal clustering. We also explain the mechanism of multi-hydrogen molecules adsorption on Ti by generalizing the Kubas model. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W27.00008: First-principles study of dihydrogen interaction of porphyrin-like nitrogen-doped graphenes Woon Ih Choi, Seung-Hoon Jhi, Yong-Hyun Kim We have studied electronic structure and dihydrogen (H$_{2})$ binding characteristics of porphyrin-like nitrogen-doped graphenes (PNGs) by performing first-principles total energy calculations based on the density functional theory. The stability of PNGs and the H$_{2}$ binding ability of the PNG metal centers (Ca and 3$d$ transition metals from Sc to Zn) have been analyzed within the generalized gradient approximation. We have found that Ca, Sc, Ti, Co, and Ni can be incorporated relatively easily into the nitrogen-doped graphenes, while V, Fe, Cu, and Zn are less likely to be. The PNGs can be used for active building blocks of hydrogen storing metal-organic frameworks. Due to the unique crystal field splitting of the planar PNGs, approaching dihydrogen exclusively interacts with the $d$z$^{2}$ orbitals of the core metals. We also found that intra-$d$-orbital charge transfer plays a key role in the dihydrogen binding. Finally, we will discuss how such dihydrogen binding can be modified by external strain. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W27.00009: Magnetic Properties of High-Surface-Area Carbons and Their Effect on Adsorbed Hydrogen Jimmy Romanos, Matthew Beckner, Michael Kraus, Jacob Burress, Peter Pfeifer We report the discovery that a large number of nanoporous carbon samples, made from corn cob and exhibiting high hydrogen storage capacities (Pfeifer et al, Mater. Res. Soc. Symp. Proc. 1041 R02-02 (2008)), show unexpected magnetic properties, due to iron impurities in the samples. Magnetization curves are consistent with ferromagnetic and/or super-paramagnetic behavior. Magnetic susceptibilities, saturation magnetizations, coercivities and remanence magnetizations, from measurements on a SQUID magnetometer, will be presented, and their temperature dependence will be discussed. Results will be presented regarding the presence of small iron clusters, magnetic properties of iron-leached samples, and hydrogen binding energies as a function of iron leaching. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG-08GO18142. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W27.00010: Boron-Doped Carbon Nanospaces for High-Capacity Hydrogen Storage Matthew Beckner, Jacob Burress, Carlos Wexler, Zhi Yang, Fred Hawthorne, Peter Pfeifer The Alliance for Collaborative Research in Alternative Fuel Technology (ALL-CRAFT, http://all-craft.missouri.edu) has been optimizing high surface area [$>$3,000 m2/g] activated carbon nanospaces for high capacity hydrogen storage. Boron-doped samples have been produced using solid, liquid, and vapor phase boron doping. The boron-doped samples were analyzed using sub-critical nitrogen adsorption to determine surface areas and the effect that boron-doping and annealing, as a function of temperature, has on the microporous structure of the samples. Results will be presented for hydrogen storage capacity (excess adsorption) per unit area of boron-doped surface, and for hydrogen binding energies at 77 K and 293 K, as a function of boron concentration and annealing temperature. This material is based on work supported by the U.S. Department of Defense under Awards No. N00164-07-P-1306 and N00164-08-C-GS37. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W27.00011: Hierarchical Pore Structure of Engineered Carbon Nanospaces for Use in Hydrogen Storage. Michael Kraus, Jacob Burress, Matthew Beckner, Carlos Wexler, Peter Pfeifer High-surface-area activated carbons are promising material for hydrogen storage. Mapping the pore structure at the nanometer scale is fundamental for the understanding of adsorptive properties. Structural analyses of pore spaces in nanoporous carbons, using subcritical nitrogen adsorption, supercritical methane adsorption, and small-angle x-ray scattering (SAXS), are presented. Adsorption isotherms provide pore-size distributions, while SAXS provides information about the spatial arrangement of pores. At large length scales, $\sim $20-2000 nm, our samples exhibit an extended regime of surface fractal behavior with a fractal dimension of $\sim $2.3, corresponding to a mild external roughness of the samples. At small length scales, the samples exhibit an abundance of pores 0.5-1.5 nm wide. An illustrative case exhibits cylindrical pores with average width of 0.9 nm and average length 1.6 nm, in excellent agreement with structural data inferred from adsorption isotherms. Comparison of pore data from SAXS and nitrogen isotherms will be compared with hydrogen uptake isotherms. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG-08GO18142. Use of the Advanced Photon Source was supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W27.00012: Analysis of Hydrogen Adsorption in Engineered Carbon Nanospaces Jacob Burress, Matthew Beckner, Nick Kullman, Raina Cepel, Carlos Wexler, Peter Pfeifer We present a survey of how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities ($\sim $80 g H2/kg carbon, $\sim $50 g H2/liter carbon, at 50 bar and 77 K). The H2 gas-carbon surface interface physics was investigated using supercritical hydrogen isotherms. Experimental case studies, with surface areas as high as 3500 m2/g, in which 40{\%} of all surface sites reside in pores of width $\sim $0.7 nm and binding energy $\sim $9 kJ/mol, and 60{\%} of sites in pores of width $>$1.0 nm and binding energy $\sim $5 kJ/mol, are also presented. We experimentally distinguish between molecules exhibit mobile or local adsorption, how lateral dynamics affect the hydrogen storage capacity, and how the two situations are controlled by the vibrational frequencies of adsorbed hydrogen molecules parallel and perpendicular to the surface. In our samples, adsorption is mobile at 293 K, and localized at 77 K. These findings present evidence hydrogen storage capacities in nanoporous carbons can be increased, without any chemical surface functionalization, by more than a factor of two by suitable engineering of the nanopore space. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W27.00013: Quantum energy levels of hydrogen adsorbed on nanoporous carbons: an intrinsic probe for pore structure, and improving Monte Carlo simulations of adsorption R. Cepel, B. Kuchta, L. Firlej, P. Pfeifer, C. Wexler Hydrogen is the lightest molecule in nature, making both rotational and translational degrees of freedom eminently quantum mechanical (especially at low temperatures). For isolated molecules the first excited (degenerate) rotational states are at about 175 K above the (non-degenerate) ground state. When the hydrogen molecule is adsorbed, however, interaction with the substrate partially eliminates this degeneracy due to the different adsorption strengths of the different rotational states of the molecule. In this talk, we consider the adsorption of hydrogen in nanometer-size pores in carbon. We show that the rotation-vibration energy levels are strongly dependent on the pore structure (geometry and size). This dependence may be probed by inelastic neutron scattering as a local, non-destructive, probe intrinsic to the system, to characterize nanopores (in fact, using H$_2$ as the probe makes sure that the pore structure probed is relevant for H$_2$ adsorption). The rotation-vibration energy levels were also used as input for grand canonical Monte Carlo simulations of H$_2$ adsorption, improving the accuracy of the simulations. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W27.00014: Structural and energetic factors in designing a perfect nano-porous sorbent for hydrogen storage Bogdan Kuchta, Lucyna Firlej, Raina Cepel, Peter Pfeifer, Carlos Wexler Carbons are one of potentially promising groups of materials for hydrogen storage by adsorption. However, the heat of hydrogen physisorption in such materials is low, in the range of about 4-8 kJ/mol which limits the total amount of hydrogen adsorbed at P = 100 bar to $\sim$2 wt\% at room temperature and about $\sim$10 wt\% at 77 K. To get better storage capacity, the adsorbing surfaces must be modified, either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. Here we analyze the variation of interaction energy between a molecule of hydrogen and graphene-based sorbents prepared as hypothetical modifications of the graphene layer. In particular, we show that partial substitution of carbons (for example, by boron) modifies both the symmetry of the energy landscape and strength of hydrogen physisorption. The effect of substituent extends over several sites of graphene lattice making the surface more heterogeneous. [Preview Abstract] |
Session W28: Focus Session: Device Applications of Multiferroic Structures
Sponsoring Units: FIAP DMPChair: Ichiro Takeuchi, University of Maryland
Room: 330
Thursday, March 19, 2009 11:15AM - 11:51AM |
W28.00001: Multiferroic Microwave and Millimeter Wave Devices Invited Speaker: Layered composites of ferrites and ferroelectrics are magneto-electric (ME) multiferroics and are of interest for studies on the physics of ME interactions and for novel signal processing devices. There are two types of interactions. (i) \textit{ME coupling in bound ferrite-piezoelectrics:} An electric field E applied to the composite produces a mechanical deformation in the piezoelectric phase that in turn is coupled to the ferrite, resulting in a shift in the ferromagnetic resonance field. The strength of the interactions is measured from the FMR shifts. (ii) \textit{ME interactions in unbound ferrite-ferroelectrics}: This is a proximity effect in which hybrid spin-electromagnetic waves are formed. An electric field applied to the ferroelectric will result in a change in the permittivity and a shift in the hybrid modes. We performed studies on the nature of ME interactions at 1-110 GHz in bilayers of epitaxial yttrium iron garnet (YIG) films, single crystal spinel ferrites or hexagonal ferrites and single crystal lead magnesium niobate-lead titanate (PMN-PT) or polycrystalline lead zirconium titanate (PZT). A stripline structure or a cavity resonator was used. Electric fields effects were investigated on magnetostatic waves, uniform precession modes or hybrid modes in the ferrite. We found evidence for strong microwave ME coupling. The coupling strength has been found to be dependent on magnetic field orientation, the nature of piezoelectric coupling and volume for both phases [1]. The high frequency ME effect is of importance for dual electric and magnetic field tunable ferrite-ferroelectric devices. We will discuss the design and characterization of ME resonators, phase shifters, delay lines and filters [2]. The work is supported by grants from the Army Research Office and the office of Naval Research. \\[4pt] [1] ``Multiferroic magnetoelectric composites: Historical perspective, status, and future direction,'' Ce-Wen Nan, M. I. Bichurin, S. Dong, D. Viehland, and G. Srinivasan, J. Appl. Phys.\textbf{ 103}, 031101 (2008). \\[0pt] [2] ``Magnetoelectric interactions in a ferromagnetic-piezoelectric layered structures: Phenomena and devices,'' M. I. Bichurin, D. Viehland and G. Srinivasan, J. Elec. Ceramics \textbf{19}, 243 (2007). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W28.00002: Electric-field-induced magnetic domain wall motion in bilayer FeGa/BaTiO3 thin film structures John Cumings, T. Brintlinger, S.-H. Lim, Y. Qi, L. Salamanca-Riba, I. Takeuchi We have studied electromechanical coupling induced magnetic domain motion in unclamped FeGa/BaTiO3 thin film bilayer structures. Magnetostrictive FeGa layers were sputter-deposited on epitaxially grown BaTiO3 films on SrTiO3 substrates. Focused ion-beam milling was used to remove the substrate from underneath the BaTiO3 film, and electrodes were patterned in the metallic FeGa film to apply electric field across a patterned gap (1 micron). Lorentz microscopy was used to monitor the magnetic domains in FeGa, while electric field is applied to the piezoelectric BaTiO3. Lorentz microscopy allows direct and dynamic observation of magnetic domain motions. Reversible electric field induced magnetic domain motion was observed, and the results will be compared to micromagnetic simulations of the domain wall structure. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W28.00003: Ferroelectric control of magnetization in BiFeO3/CoFe heterostructures. Martin Gajek, Lane Martin, John Heron, Jan Seidel, Ramamoorthy Ramesh The cross coupling between ferroic order parameters in multiferroics opens an alternative for the control of magnetism in magnetoelectric devices by purely electrical means. We first report on the exchange coupling between BiFeO3, an antiferromagnetic ferroelectric , and CoFe. We then show that the domain structure of the ferromagnet can be changed by poling the ferroelectric layer. Finally, we will discuss the implementation of our findings into possible device schemes. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W28.00004: Induced magnetization in ferroelectric-antiperovskite heterostructure Pavel Lukashev, Renat Sabirianov We theoretically predict the linear magnetoelectric effect (ME) in ferroelectric-antiperovskite PbTiO$_{3}$/Mn$_{3}$GaN heterostructure. The effect is caused by the recently reported piezomagnetic nature of the Mn$_{3}$GaN. Elastic deformations in the Mn$_{3}$GaN are due to the surface strain and the soft mode atomic displacements from~ferroelectric to the antiperovskite (AP) phase. Both mechanisms lower the symmetry of the AP component, which results in the induced magnetization. Reversal of the polarization direction in the ferroelectric phase results in the magnetization reversal in Mn$_{3}$GaN, thus the observed effect is linear. We study few interface geometries to account for the electrostatic complementarity at the surface. Those interfaces, which are electrostatically incompatible exhibit strong tetragonal distortion of the cell. The induced magnetization depends on the termination of the components of the heterostructure, and ranges from 0.25 $\mu _{B}$ to 0.6 $\mu _{B}$ per unit cell of Mn$_{3}$GaN. All calculations were performed by projector augmented wave method. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W28.00005: Magnetoelectric Coupling in Complex Oxide Heterostructures Jason Hoffman, Carlos Vaz, Hajo Molegraaf, Jean-Marc Triscone, Charles Ahn Current efforts to use materials with multifunctional capabilities have renewed interest in multiferroics, which display a coupling between ferroic order parameters. Engineered structures that combine dissimilar magnetic and ferroelectric systems epitaxially have been shown to exhibit enhanced magnetoelectric coupling. In this work, off-axis RF magnetron sputtering was used to deposit epitaxial ferroelectric Pb(Zr,Ti)O$_3$ (PZT) / La$_{0.8}$Sr$_{0.2}$MnO$_3$ (LSMO) heterostructures with high crystalline quality and atomically smooth surfaces. X-ray diffraction shows c-axis oriented growth of PZT, with a typical root-mean-square (RMS) surface roughness of ~5\AA. We employ magneto-optic Kerr effect (MOKE) magnetometry to study directly the local magnetic state of the LSMO as a function of the PZT polarization state. We demonstrate direct control of magnetism via applied electric fields, including on/off switching of magnetism. The coupling between magnetic and electric order parameters in ferroelectric / Sr-doped lanthanum manganite heterostructures is illustrated by hysteretic M-E (magnetization vs. electric field) loops, with a measured magnetoelectric susceptibility of $\alpha \sim 1 $Oe cm / kV$^{-1}$. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 1:15PM |
W28.00006: GaMnAs-based hybrid multiferroic memory device Invited Speaker: A rapidly developing field of spintronics is based on the premise that substituting charge with spin as a carrier of information can lead to new devices with lower power consumption, non-volatility and high operational speed. Despite efficient magnetization detection, magnetization manipulation is primarily performed by current-generated local magnetic fields and is very inefficient. Here we report a novel non-volatile hybrid multiferroic memory cell with electrostatic control of magnetization based on strain-coupled GaMnAs ferromagnetic semiconductor and a piezoelectric material. We use the crystalline anisotropy of GaMnAs to store information in the orientation of the magnetization along one of the two easy axes, which is monitored via transverse anisotropic magnetoresistance. The magnetization orientation is switched by applying voltage to the piezoelectric material and tuning magnetic anisotropy of GaMnAs via the resulting stress field. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W28.00007: Exchange Coupling across BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Interface Pu Yu, M. Huijben, M.B. Holcomb, C.H. Yang, Q. He, Y. H. Chu, J. X. Zhang, L. W. Martin, R. Ramesh, Per-Anders Glans, J. H. Guo Controlling the magnetic state by using electric field is a central topic for spintronics and has piqued intense interest. The coupling of antiferromagnetic and ferroelectric order parameters of multiferroic BiFeO3 (BFO) provides a new approach to achieve this goal by using exchange coupling effect. Our previous studies have shown negative exchange bias between epitaxial ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and BFO heterostructures and magnetoelectric coupling at these interfaces. The coupling mechanism between these two materials is still a mystery due to the complexity of the G-type spin structure. In this work, by controlling the domain structures of BFO and studying the corresponding exchange coupling effect, the possible coupling mechanism of antiferromagnetic and ferromagnetic spin structures will be proposed. Additionally, the coupling of orbitals across the interface will also be discussed, which will give us another possible clue to understand the spin coupling mechanism, since usually orbital and spin orders are coupled together. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W28.00008: Electric field tuning of magnetic properties in FeGa films on ferroelastic Pb(Zr,Ti)O$_{3}$ thin films probed by ferromagnetic resonance Arun Luykx, Samuel Lofland, Varatharajan Anbusathaiah, Valanoor Nagarajan, Fransiska Kartawidjaja, John Wang, Ichiro Takeuchi In order to investigate the possibility of fabricating electric field tunable thin film magnetic devices using a multiferroic transduction effect, we have patterned Fe$_{0.7}$Ga$_{0.3}$ (FeGa) films sputter-deposited on PbZr$_{0.3}$Ti$_{0.7}$O$_{3}$ (PZT(30/70))/PbZr$_{0.7}$Ti$_{0.3}$O$_{3}$ (PZT(70/30)) tetragonal/rhombohedral bilayers on Pt/Ti/SiO$_{2}$/Si wafers. Previous piezoforce microscopy studies have shown that the PZT bilayers exhibit presence of ferroelastic domains where the fraction of the local $c$/$a$ domain ratio can be tuned by an applied electric field. The FeGa top layer was patterned into 20 $\mu $m x 20 $\mu $m capacitor devices in order to apply electric field to the multilayers, and ferromagnetic resonance (FMR) measurements at 9.2 GHz were performed. Typically, a relatively sharp FMR signal observed before application of the electric field would get substantially broadened after initial application of +4 kV/cm. Angular dependent FMR indicates that magnetic anisotropy in the FeGa is indeed affected by application of electric field. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W28.00009: Magnetoelastic/piezoelectric laminated structures for tunable remote contact-less magnetic sensing and energy harvesting Peter Finkel We report a method for tunable, contact-less, magnetic field sensing using magnetoelastic coupling properties of the magnetoelastic/piezoelectric laminated composite structure. The magnetically tunable, flexural resonant mode in the bimorph FeNi36{\%} (invar) /PVDF clamped cantilever has been investigated as a function of stress and external magnetic field using Doppler laser spectroscopy. Here we demonstrated that this bimorph structure can be used for low frequency contact-less detection of magnetic field fluctuation and magnetic field monitoring. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W28.00010: Electrical and magnetic properties of BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanotube composite Chandran Sudakar, Ambesh Dixit, Moodakare Bheema Sahana, Gavin Lawes, Ratna Naik, Vaman M. Naik We report the electrical and magnetic properties of BiFeO$_{3}$ and CoFe$_{2}$O$_{4}$ nanotube composite multiferroics. CoFe$_{2}$O$_{4}$ nanotubes were prepared on Pt coated Si substrates using a template assisted method, yielding nanotubes with 20-50 nm thick walls and an outer diameter of 200 to 400 nm. These nanotubes were then uniformly coated by a BiFeO$_{3}$ layer by a metal organic decomposition method to yield the composite multiferroics. We observed ferroelectric switching behavior with saturated hysteresis loops with $P_{r}$ and $E_{c}$ values of approximately 0.08 \textit{$\mu $}C/cm$^{2}$ and 15 kV/cm, respectively, for a maximum applied electric field of 50 kV/cm. For pure BiFeO$_{3}$ thin films the hysteresis curves do not show any saturating trend and the E$_{c}$ is three times smaller than that of the composite. The magnetic measurements show that the pure BiFeO$_{3}$ is non-ferrimagnetic, while the composite shows a clear hysteresis with saturation magnetization of $\sim $12 emu/cm$^{3}$. These composite BiFeO$_{3}$ -- CoFe$_{2}$O$_{4}$ structures provide an approach for studying magnetoelectric coupling at the interfaces between different ferroic materials. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W28.00011: Multiferroic tunnel junctions: Prediction of four resistance states from first-principles J.P. Velev, C.-G. Duan, J.D. Burton, A. Smogunov, M.K. Niranjan, E. Tosatti, S.S. Jaswal, E.Y. Tsymbal Electron tunneling and ferroelectricity have had long but separate histories. In the past decade both attracted significant interest due to application in electronic devices such as magnetic tunnel junctions (tunneling) and ferroelectric capacitors (ferroelectricity) relevant to non-volatile random-access memories. Recently, driven by demonstrations of ferroelectricity in ultrathin films, it was proposed to combine these two phenomena in a multiferroic tunnel junction (MFTJ) utilizing a ferroelectric barrier between two magnetic electrodes. Due to sensitivity of the conductance to both the magnetization alignment of the electrodes (magnetoresistance) and orientation of the polarization in the ferroelectric barrier (electroresistance), this junction can serve as a four-state resistance device. Here based on first-principles calculations we demonstrate the existence of the four resistance states in SrRuO$_{3}$/BaTiO$_{3}$/SrRuO$_{3}$ MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal exciting prospects of MFTJs for application in multifunctional electronic devices. [Preview Abstract] |
Session W29: Focus Session: Current-Induced Oscillations
Sponsoring Units: GMAG DMP FIAPChair: Chun-Yeol You, Inha University
Room: 333
Thursday, March 19, 2009 11:15AM - 11:27AM |
W29.00001: Competition between orbital torques and spin polarization in controlling FMR linewidths Sezen Demirtas, Ali R. Koymen, Myron B. Salamon We have investigated temperature dependent dynamic magnetic properties of rare earth (Gd, Tb, Sm)/Ag/transition metal (Fe, Co, Ni and Py) trilayers by ferromagnetic resonance technique. We found that Fe and Co among TM (transition metals) show narrower magnetic resonance linewidths in rare earth (RE)/Ag/TM/Ag thin film trilayers compared to the values for Ag/TM/Ag, while Ni and Py in the trilayer films show equal or larger linewidths. We attribute this behavior to the relative contributions of intraband and interband scattering to the Gilbert damping parameter. The Y/Ag/(Fe, Co) trilayers seems not to change the resonance linewidth from the bulk value, suggesting that the magnetic moments for the f-electrons play a significant role. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W29.00002: Current-induced dynamics in almost symmetric magnetic nanopillars Sergei Urazhdin, Weng Lim, Andrew Higgins Magnetic nanodevices usually include a free layer whose configuration can be changed by spin-polarized current via the spin transfer (ST), and a fixed reference (polarizing) layer. The polarizer is usually made much larger than the free layer to minimize the effects of ST. However, it is presently not known what makes a specific magnetic layer behave as a fixed polarizer or a free layer driven by ST. Little is also known about the dynamics in bilayers with thin polarizers, where the effects of ST on both layers are significant. We will discuss our spectroscopic measurements of current-induced dynamics in nanopillars with similar thicknesses of the extended polarizer and the nanopatterned free layer. We demonstrate coherent precession for both polarities of current in symmetric devices. However, even slightly asymmetric devices exhibit a rapid suppression of precession for one of the current polarities. We interpret our results in terms of the dynamical coupling between magnetic layers due to spin transfer, completely suppressing precession of the thicker layer. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W29.00003: Measurements of out-of-plane dynamics induced by spin transfer in magnetic nanopillars Weng Lim, Sergei Urazhdin Current-induced spin transfer (ST) can induce dynamical states in magnetic multilayer nanopillars not accessible by any other techniques. For-in plane magnetic field, the predicted dynamical regimes include elliptical, clamshell, and out-of-plane precession. The first two regimes have been demonstrated and extensively analyzed. However, the out-of-plane precession has so far been elusive. Calculations [1] show that dynamical coupling between ferromagnets due to ST can result in suppression of coherent out-of-plane precession in nanopillars with a patterned polarizing layer, which is the geometry studied so far. We will discuss our measurements of current-induced dynamics in nanopillars with extended polarizer, in which the decoherence caused by the coupling between magnetic layers is minimized. We demonstrate coherent out-of-plane precession, whose dependence on current and the direction of the magnetic field is consistent with micromagnetic simulations. Most surprisingly, our data are asymmetric with respect to reversal of the magnetic field, which is explained by a combination of the Oersted field and sample shape imperfections. [1] S. Urazhdin, Phys. Rev. B 78, 060405(R) (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W29.00004: Spin Torque Dynamics of Nanomagnets with Weak Magnetic Anisotropy Hoang Yen T. Nguyen, Xiao Cheng, Carl Boone, Jian Zhu, Ilya Krivorotov We study switching and persistent precession of magnetization induced by spin transfer torque in Co(4 nm)/ Cu(6 nm)/ Co(0.7 nm)/Pt nanopillar spin valves where perpendicular magnetic anisotropy at the Co/Pt interface nearly cancels the easy-plane shape anisotropy of the free Co layer. We find that in this system with weak total magnetic anisotropy, spin torque can switch magnetization of the free layer between the in-plane and the out-of-plane static magnetic states. In the regime of current-driven persistent magnetization precession, we observe unusual non-monotonic dependence of the precession frequency on current. Simulations show that these unusual features of spin torque dynamics are due to the second-order perpendicular magnetic anisotropy term at the Co/Pt interface. Our work demonstrates a method for controllable switching of magnetization of a nanomagnet between stable in-plane and out-of-plane magnetic configurations by spin-polarized current. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W29.00005: Slonczewski windmill with dissipation and asymmetry Yaroslaw Bazaliy J. Slonczewski invented spin-transfer effect in layered systems in 1996. Among his first predictions was the regime of the ``windmill motion'' of a perfectly symmetric spin valve. In this regime magnetizations of the layers rotate in a fixed plane keeping the angle between them constant. Since ``windmill'' was predicted to happen in the case of zero magnetic anisotropy, while in most experimental setups the anisotropy is significant, the phenomenon was not a subject of much research. However, the behavior of the magnetically isotropic device is related to the interesting question of current induced ferromagnetism and is worth more attention. Here we study the windmill regime in the presence of dissipation, exchange interaction, and layer asymmetry. It is shown that the windmill rotation is almost always destroyed by those effects, except for a narrow interval of electric current, determined by the parameters of the device. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:51PM |
W29.00006: Spin-dependent tunneling effects in magnetic tunnel junctions Invited Speaker: It has long been known that current extracted from magnetic electrodes through ultra thin oxide tunnel barriers is spin polarized. This current gives rise to two important properties: tunneling magnetoresistance (TMR) when the tunnel barrier is sandwiched between two thin magnetic electrodes and, spin momentum transfer, which can be used to manipulate the magnetic state of the magnetic electrodes. In the first part of my talk I show how the structure of thin CoFe layers can be made amorphous by simply sandwiching them between two amorphous layers, one of them the tunnel barrier. No glass forming elements are needed. By slightly changing the thickness of these layers or by heating them above their glass transition temperature they become crystalline. Surprisingly, the TMR of the amorphous structure is significantly higher than of its crystalline counterpart. The tunneling anisotropic magnetoresistance, which has complex voltage dependence, is also discussed. In the second part of my talk I discuss the microwave emission spectrum from magnetic tunnel junctions induced by spin torque from spin polarized dc current passed through the device. We show that the spectrum is very sensitive to small variations in device structures, even in those devices which exhibit similarly high TMR ($\sim $120{\%}) and which have similar resistance-area products ($\sim $4-10 $\Omega \mu $m$^{2})$. We speculate that these variations are due to non-uniform spatial magnetic excitation arising from inhomogeneous current flow through the tunnel barrier. [In collaboration with Xin Jiang, M. Hayashi, Rai Moriya, Brian Hughes, Teya Topuria, Phil Rice, and Stuart S.P. Parkin] [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W29.00007: Frequency-doubling spin-torque microwave oscillator Graham Rowlands, Ilya Krivorotov We describe a new type of spin torque oscillator with two free layers that is capable of emitting high microwave power ($>$ 1 $\mu $W) at high frequency ($>$ 50 GHz) in zero external field. This device has two perpendicular-anisotropy fixed ferromagnetic layers and two easy-plane free layers sandwiched between the fixed layers, with all of the magnetic layers separated from each other by non-magnetic spacers. We simulate current-driven magnetization dynamics in this structure in the macrospin approximation, taking into account spin-torque interactions between adjacent ferromagnetic layers. Our simulations show that for both fixed layers magnetized in the same direction perpendicular to the plane of the sample, spin-torque induces clockwise rotation of one of the layers and counterclockwise rotation of the other. This type of current-driven dynamics gives rise to large-amplitude microwave signal with the frequency that is the sum of the precession frequencies of the free layers. We study the effect of dipolar coupling, shape anisotropy and external field on the dynamics of this spin torque oscillator and determine the optimal device parameters for high-amplitude high-frequency microwave signal generation. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W29.00008: Spectral Line Shape and Line Width of a Single-Mode Spin Torque Oscillator Ilya Krivorotov, Carl Boone, Jian Zhu, Xiao Cheng, Jordan Katine, Jeff Childress Spin torque auto-oscillators are strongly nonlinear dynamical systems that are highly susceptible to external perturbations such as spin-polarized current and temperature. To understand the effect of thermal fluctuations on the oscillator dynamics, we measure power spectrum of single-mode spin torque oscillators based on a GMR nanocontact to a permalloy nanowire. Our measurements reveal deviations of the power spectral line shape from a simple Lorentzian. These deviations can be understood in terms of dephasing induced by the oscillator amplitude fluctuations. The measured spectral line shape is in a good agreement with a recent analytic theory of spin torque oscillator dynamics at a non-zero temperature [1]. We show that precise measurements of the line shape give information on important oscillator parameters such as Gilbert damping in the large-amplitude regime of current-driven magnetization dynamics. [1] V. S. Tiberkevich, A. N. Slavin, J.-V. Kim, Phys. Rev. B 78, 092401 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W29.00009: Time domain studies of aperiodicity in spin-torque driven vortex oscillations Vlad Pribiag, B. Williams, A. Stehura, D.C. Ralph, R.A. Buhrman Previous studies of current-driven magnetic vortex oscillations in nanopillars [1] and point contact geometries [2] have been restricted to detection of the \textit{average} envelope of the oscillations. In this talk we discuss aperiodic features of the vortex oscillations that were studied based on single-shot time domain measurements of the oscillating GMR signal. These measurements reveal stochastic mode jumping at 10's of $\mu $s mean duty cycles between several closely spaced frequencies. The power spectrum of the time traces indicates that the shape and amplitude of the oscillation's spectral peaks change abruptly as the function of time, corresponding to aperiodic modulation of these oscillations on the $\mu $s time scale. Due to the very narrow \textit{long-time }linewidths of the oscillations it is possible to detect clearly these fine modulations of the peak shape, frequency and amplitude. From these studies of the spin-torque-driven vortex oscillator stability we seek to obtain insights for the design and fabrication of spin-torque vortex oscillators with even narrower linewidths. [1] V.S. Pribiag \textit{et al.,} \textit{Nature Phys.} \textbf{3}, 498 (2007). [2] Q. Mistral \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{100}, 257201 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W29.00010: The Role of Spin-Motive Forces in Spin-Valve Dynamics Jun'ichi Ieda, Sadamichi Maekawa, Stewart E. Barnes A spin-motive force (smf) is the counterpart of an electro-motive force, which couples to the spin rather than charge degrees of freedom of electrons. Here we discuss how smfs work in spin-valves. When the magnetization makes a sudden change, there often appears a large peak in dV/dI, i.e., a voltage jump that is better interpreted in terms of smfs. To see this, we model spin-valves using an equivalent circuit that involves magnetic dissipation represented by smfs as well as electric dissipation through ordinary resisters for both majority and minority currents. There are four possible conduction paths, e.g., the majority electrons hop to the majority band, or to the minority band and vice versa. The first path adds an up electron to the free layer and causes a rotation in a certain sense, while the second path adds a down electron and a rotation in the opposite sense. Since the rotations are in opposite senses so is the work done on the free layer and hence the smf. By solving the circuit problem and the Landau-Lifshitz equations supplemented with the Slonczewski torque-transfer term simultaneously we find the spin-transfer effect is dramatically modified by smfs. With the relevant parameters a stable large angle precession and a voltage signal are predicted. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W29.00011: Mutual phase-locking and frustration in arrays of interacting spin-torque nano-oscillators Andrei Slavin, Vasil Tiberkevich We developed a perturbation theory describing collective dynamics of spin-torque nano-oscillator (STNO) arrays in a weak-coupling limit. In this limit each STNO is described by a single dynamical variable -- effective phase $\phi _j $, which satisfies the equation ${d\phi _j } \mathord{\left/ {\vphantom {{d\phi _j } {dt}}} \right. \kern-\nulldelimiterspace} {dt}=\omega _j +\sum\nolimits_k {\lambda _{j,k} \sin (\phi _k -\phi _j +\beta _{j,k} )} $. Here $\omega _j $ is the free-running (unperturbed) frequency of the $j$-th oscillator, $\lambda _{j,k} $ is the effective coupling amplitude of $j$-th and $k$-th oscillators, and $\beta _{j,k} $ is the \textit{frustration angle} of the oscillators' interaction. The frustration angles $\beta _{j,k} $ are determined by the intrinsic nonlinearity of STNO and by the delay of coupling signals. The frustration angles can be controlled by changing the distance between STNOs and/or by adding reactive elements to the STNO circuit. We have analyzed collective dynamics of STNO arrays in the case of global coupling, i.e. when coupling amplitudes and frustration angles for all STNOs are equal, $\lambda _{j,k} =\lambda $, $\beta _{j,k} =\beta $. We have shown that STNO array mutually phase-locks only when $\cos (\beta )>0$. The critical coupling amplitude $\lambda _{cr} $, at which phase-locking starts, has a minimum for $\cos (\beta )=1$ (i.e., for $\beta =2n\pi )$ and increases with the decrease of $\cos (\beta )$. For $\cos (\beta )<0$ the mutual phase-locking of more than two STNOs is impossible, and the STNO array enters a \textit{frustrated state}, in which the output power becomes vanishingly small due to the destructive interference between individual STNOs. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W29.00012: Spin-torque-driven ferromagnetic resonance in a nonlinear regime Wenyu Chen, G. de Loubens, J-M. L. Beaujour, J. Z. Sun, A. D. Kent Spin-torque-driven ferromagnetic resonance (ST-FMR) is a quantitative tool for studying spin-transfer interactions in nanojunctions. Using this method we have studied Co/Cu/CoNi spin valves, in which the CoNi synthetic free layer has perpendicular magnetic anisotropy. Perpendicular field swept resonance lines were measured under a large amplitude GHz current excitation, which drove ST-FMR into a nonlinear regime and produced a large angle precession of the free layer magnetization. With increasing rf power, the resonance lines deviate from a Lorentzian shape and became asymmetric, with a lower resonance field and a larger linewidth. A non-hysteretic step jump in ST-FMR voltage signal was also observed at high powers. The comparison of the experimental results to the foldover and the nonlinear damping theories will be presented. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W29.00013: Microwave oscillation generation in a Co/Cu/Co nano-contact without external magnetic field Kwun Hung Cheung, Xin Xiao, Hong Wen Jiang Using spin-transfer-torque effect to generate microwave oscillation at zero magnetic- field is of recent interest. Here, we report the observation a resistive oscillation at microwave-frequencies ranging from 1.5 to 3 GHz in a nano-contact formed on a Co/Cu/Co tri-layer structure without any external field. The observed oscillation modes have frequencies that are much higher than that reported in other similar experimental systems [1,2]. We have studied the evolution of the oscillation as a function of the DC excitation current and the effect of a small in-plane field. Micromagnetic simulations support the notion that the oscillation is as a result of the translational motion of a vortex-core underneath the nano-contact, due the competition of the circular Orstead field and the spin-transfer torque, both induced by the DC current passing through the nano-contact. The work was supported by the Western Institute of Nanoelectronics (WIN). [1] M. R. Pufall et al., Phys. Rev. B 75, 140404(R) (2007). [2] Q. Mistral et al, Phys. Rev Lett. 100, 257201 (2008). [Preview Abstract] |
Session W30: Focus Session: Ruthenates
Sponsoring Units: DMP GMAGChair: Zhiqiang Mao, Tulane University
Room: 334
Thursday, March 19, 2009 11:15AM - 11:51AM |
W30.00001: Ca$_{3}$(Ru$_{1-x}$Cr$_{x}$)$_{2}$O$_{7}$: A new paradigm for spin valves Invited Speaker: A spin valve is a device structure whose electrical resistance can be manipulated by controlling the relative spin alignment of adjacent metallic, magnetic layers separated by nonmagnetic insulating layers. The spin valve effect is a quantum phenomenon so far only realized in multilayer thin films or heterostructures. Here we report a \textit{novel}, \textit{strong} spin valve effect existing in \textit{bulk} single crystals of Ca$_{3}$(Ru$_{1-x}$Cr$_{x})_{2}$O$_{7}$ having an anisotropic, bilayered crystal structure [1]. This discovery opens new avenues to understand the underlying physics of spin valves, and fully realize its potential in practical devices. \\[4pt] [1] G. Cao, V. Durairaj, S. Chikara, and L.E. DeLong and P. Schlottmann, \textit{Phys. Rev. Lett.} \textbf{100}, 016604 (2008) [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W30.00002: Antiferromagnetism and bulk spin valve effect in Ca$_{3}$(Ru$_{1-x}$Ti$_{x}$)$_{2}$O$_{7}$ J. Peng, T.J. Liu, Z. Qu, E. Vehstedt, B. Qian, D. Fobes, L. Spinu, W. Bao, Z.Q. Mao Ca$_{3}$Ru$_{2}$O$_{7}$ has generated growing interest. It shows an antiferromagnetic (AFM) transition at $T_{N}=56$ K, followed by a metal-insulator (MI) transition at $T_{MI}=48$ K $\left[1\right]$. Giant magnetoresistance (GMR) across its metamagnetic transition is also observed. We have determined the magnetic structures of Ca$_{3}$Ru$_{2}$O$_{7}$ under magnetic fields using neutron scattering $\left[2\right]$. Our results demonstrate that the GMR in this material originates from a bulk spin-valve effect, and clarify the origin for the puzzling observation that the GMR occurs under easy axis field alignment, while a colossal magnetoresistance appears with hard axis field alignment $\left[1\right]$. In addition, we have studied the effect of Ti doping on Ca$_{3}$Ru$_{2}$O$_{7}$. We find that Ti doping dramatically affects both the AFM and the MI transition by shifting them to much higher temperatures, e.g. $T_{N}=114$ K and $T_{MI}=107$ K for 10\% Ti doping. We will discuss possible origins for this remarkable doping effect. This work is supported by the NSF under grant DMR-0645305, the DOE under DE-FG02-07ER46358.\\ $\left[1\right]$ G.Cao \textit{et al}., Phys. Rev. Lett. \textbf{78}, 1751 (1997)\\ $\left[2\right]$ Wei Bao \textit{et al}., Phys. Rev. Lett. \textbf{100}, 247203 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W30.00003: Antiferromagnetic Metallic State And Spin Valve Effect in Doped (Ca$_{1-x}$ A$_{x})_{3}$Ru$_{2}$O$_{7}$ (A = Sr, Ba) Single Crystals S. Chikara, O.B. Korneta, T.F. Qi, S. Parkin, G. Cao, W.P. Song, W.P. Crummett Bilayered Ca$_{3}$Ru$_{2}$O$_{7}$ is a highly anisotropic system [1] characterized by orbitally-driven colossal magnetoresistance$^{2}$ and an unusual antiferromagnetic metallic (AFM-M) state [2]. We report transport and thermodynamic properties of (Ca$_{1-x}$ A$_{x})_{3}$Ru$_{2}$O$_{7}$ (A = Sr, Ba) single crystals as a function of temperature and applied magnetic field. While Ba doping shows a far stronger impact, both Sr and Ba substitution for Ca induce a large array of interesting phenomena. Among them, a bulk spin-valve effect occurs in the AFM-M range, which is largely broadened due to the doping. This effect in bulk crystals is a novel phenomenon first observed in Ca$_{3}$(Ru$_{1-x}$Cr$_{x})_{2}$O$_{7}$ single crystals [3]. The spin-valve effect in (Ca$_{1-x}$A$_{x})_{3}$Ru$_{2}$O$_{7}$ single crystals opens new avenues to understand the underlying physics and realize the potential of spin valves in practical devices.\\[0pt] [1] G. Cao et al., \textit{PRL} \textbf{78}, 1751 (1997)\\[0pt] [2] X. N. Lin et al., \textit{PRL} \textbf{95}, 017203 (2005)\\[0pt] [3] G. Cao et al., \textit{PRL} \textbf{100}, 016604 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W30.00004: Microscopic theory of metamagnetism and nematic order in Sr$_3$Ru$_2$O$_7$ Srinivas Raghu, Arun Paramekanti, Eun-Ah Kim, Steven Kivelson The bilayer ruthenate compound Sr$_3$Ru$_2$O$_7$ exhibits a remarkable set of low temperature electronic properties. In an externally applied magnetic field, ultra-pure crystals of the compound undergo a metamagnetic transition at a temperature which can be tuned towards zero as B $\parallel$ c approaches a critical value of $\sim$8T. This putative metamagnetic quantum critical point, however, is preempted by a nematic fluid phase with order one resistive anisotropy in the ab plane. In this talk, we consider the microscopic origins of metamagnetism and the accompanying nematic order, focusing primarily on the quasi-one-dimensional bands in a bilayer model. Making use of local Coulomb interactions in conjunction with the sharp divergence of the density of states near a van-Hove singularity, we construct a phase diagram which enables our system to traverse a metamagnetic transition into a nematic phase followed by a second metamagnetic transition into a phase which preserves C$_4$ rotational symmetry, with increasing magnetic field. We treat quantum nematic fluctuations in the vicinity of the metamagnetic transitions to 1-loop order and consider the extent to which nematic fluctuations can give rise to the observed ``pseudogap'' in the local density of states of this material. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W30.00005: Probing orbital-dependent magnetism in layered perovskite ruthenates through angle-dependent magnetoresistivity D. Fobes, T.J. Liu, Z. Qu, H.Q. Yuan, M. Salamon, M. Zhou, J. Hooper, Z.Q. Mao The orbital degree of freedom plays a very important role in layered perovskite ruthenates, leading to unusual magnetic states. The triple layered Sr$_{4}$Ru$_{3}$O$_{10}$ contains a ferromagnetic ground state, and undergoes a metamagnetic transition under moderate in-plane magnetic fields $\left[1\right]$. By analyzing the anisotropy in angle-dependent directional magnetoresistance measurements we can extract orbital information of magnetism. In Sr$_{4}$Ru$_{3}$O$_{10}$ we have found that below the metamagnetic transition the 4$d_{xy}$ orbitals are already polarized whereas the 4$d_{xz,yz}$ orbitals are not; \textit{$\rho $}$_{ab}$(H) exhibits 2-fold anisotropic symmetry indicating ferromagnetism and \textit{$\rho $}$_{c}$(H) exhibits anisotropy consistent with Fermi surface warping. Above the transition field, under polar rotation we observe a first order phase transition in \textit{$\rho $}$_{c}$(H) for angles close to the \textit{ab}-plane clearly indicating a polarization of the 4$d_{xz,yz}$ orbitals. Additionally, we have performed similar studies on Sr$_{3}$Ru$_{2}$O$_{7}$, and have observed preliminary evidence of orbital-dependent magnetic correlations. This work is supported by the NSF under grant DMR-0645305, the DOE under DE-FG02-07ER46358.\\ $\left[1\right]$ Cao \textit{et al}., Phys. Rev. B \textbf{68}, 174409 (2003) [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W30.00006: Electronic phase diagram in double layered ruthenates (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ Z.Q. Mao, Z. Qu, J. Peng, T.J. Liu, D. Fobes, B. Qian, L. Spinu We previously established a magnetic phase diagram for (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ ($0 \leq x \leq 1$) using high quality single crystals grown by a floating-zone method $\left[1\right]$. This phase diagram exhibits rich magnetic properties. The magnetic ground state ranges from an itinerant metamagnetic state $(0 \leq x < 0.08)$, to an unusual heavy-mass, nearly ferromagnetic (FM) state $(0.08 < x < 0.4)$, and finally to an antiferromagnetic (AFM) state $(0.4 < x \leq 1)$. In this talk we report the electronic properties of these magnetic states. We will show that the electronic and magnetic properties are strongly coupled in this system. The electronic ground state evolves from an AFM quasi-two-dimensional metal for $x = 1.0$, to an Anderson localized state for the AFM region $0.4 < x < 1.0$, and then to a weakly localized state, induced by magnetic scattering, for the nearly FM region $0.08 < x < 0.4$. When \textit{x} approaches the critical composition 0.08, the localization weakens and non-Fermi liquid (FL) behavior occurs. The system eventually transforms into a FL ground state when the magnetic ground state switches to the itinerant metamagnetic state for $x < 0.08$. These results demonstrate the delicate balance among the charge, spin, lattice and orbital degrees of freedom in ruthenates.\\ $\left[1\right]$ Z. Qu \textit{et al}., Phys. Rev B \textbf{78}, 180407 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W30.00007: Inhomogeneous magnetic phases: a LOFF-like phase in Sr$_3$Ru$_2$O$_7$ Andrew Berridge, Andrew Green, Santiago Grigera, Ben Simons The phase diagram of Sr$_3$Ru$_2$O$_7$ contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We show - through a Ginzburg- Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous - that a Stoner model with electronic band dispersion can reproduce this phase diagram and transport behaviour. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W30.00008: Crystal-Field Level Inversion in Lightly Mn-Doped Sr$_3$Ru$_2$O$_7$ Muhammed Hossain, Z. Hu, M.W. Haverkort, T. Burnus, C.F. Chang, S. Klein, J.D. Denlinger, H.-J. Lin, C.T. Chen, R. Mathieu, Y. Kaneko, Y. Tokura, S. Satow, H. Takagi, Y. Yoshida, A. Tanaka, I.S. Elfimov, G.A. Sawatzky, L.H. Tjeng, A. Damascelli Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$, in which 4$d$-Ru is substituted by the more localized 3$d$-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra $e_g$ electron occupies the in-plane $3d_{x^2-y^2}$ orbital instead of the expected out-of-plane $3d_{3z^2-r^2}$. We propose that the 3$d$-4$d$ interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state. Published: Phys. Rev. Lett. 101, 016404 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W30.00009: Strong spin-orbit coupling effects on the Fermi surface of Sr$_{2}$RuO$_{4}$ and Sr$_{2}$RhO$_{4}$ Andrea Damascelli, Maurits Haverkort, Ilya Elfimov, Hao Tjeng, George Sawatzky We present a first-principle study of spin-orbit coupling effects on the Fermi surface of Sr$_{2}$RuO$_{4}$ and Sr$_{2}$RhO$_{4}$ [1]. For nearly degenerate bands, spin- orbit coupling leads to a dramatic change of the Fermi surface with respect to non-relativistic calculations; as evidenced by the comparison with experiments on Sr$_{2}$RhO$_{4}$, it cannot be disregarded. For Sr$_{2} $RuO$_{4}$, the Fermi surface modifications are more subtle but equally dramatic in the detail: spin-orbit coupling induces a strong momentum dependence, normal to the RuO$_2$ planes, for both orbital and spin character of the low-energy electronic states. These findings have profound implications for the understanding of unconventional superconductivity in Sr$_{2}$RuO$_{4}$. [1] M.W. Haverkort {\it et al.}, Phys. Rev. Lett. \textbf{101}, 026406 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W30.00010: Dimensionality and doping effect on the Core-level X-ray photoemission satellites in layered ruthenates. Haizhong Guo, Yi Li, Biao Hu, Rongying Jin, E.W. Plummer, Jiandi Zhang, D. Urbina, Tijiang Liu, David Fobes, Zhiqiang Mao Core-level photoelectron spectra of the layered perovskite crystal Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$ (n = 1, 2, and 3) and Mn-doped Sr$_{3}$Ru$_{2}$O$_{7}$ are investigated by x-ray photoemission spectroscopy (XPS) techniques. The Sr 3$d$ and Ru 3$d$ core-level spectra exhibit a two-peak structure, screened and unscreened peaks, indicating strong correlation effects among Ru 4$d$ electrons. However, there are little changes of the core-level satellite features with n, suggesting the electron-electron correlation is mainly confined in the RuO$_{2}$ plane. On the other hand, doping of Mn will drastically affect the core-level spectral weight, reflecting the doping-induced metal-to-insulator transition in the doped system. The position of Ru-core levels remain the same, thus, indicating no doping-induced change of Ru valence. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W30.00011: Magnetic Ordering in Ba$_{2}$DyRuO$_{6}$ J. Lamsal, W. Yelon, H. Blackstead, M. Smylie, Q. Cai, W. James, J. Yang Magnetization measurements and neutron diffraction (ND) studies have been carried out on the double perovskite ruthenate, Ba$_{2}$DyRuO$_{6}.$ The low field magnetization data indicate ordering around 50K, a sharp rise below 8K and a possible transition around 25K. Rietveld analysis of ND data confirms antiferromagnetic ordering at 48K. The temperature dependence of the Ru moment appears to follow a Brillouin type curve down to the lowest temperature accessible, (12K), and there is no evidence for a transition around 25K. In contrast, the Dy moment is found to be proportional to the square of the Ru moment, implying that the Ru moment is the primary order parameter and that Dy ordering is driven by the Ru-Dy coupling. The ND experiment could not reach the temperature at which the magnetization rises sharply (8K), but following similar arguments, we suggest that this point represents the temperature at which the Dy-Dy interactions become larger than the Ru-Dy interactions, and may, thus, lead to a new structure. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W30.00012: Resistance noise in the bad metal SrRuO$_{3}$ Francoise Kidwingira, Michael Rozler, Gertjan Koster, Wolter Siemons, Rik Groenen, Malcolm Beasley SrRuO$_{3}$ (SRO) is a strongly correlated electrons system with some interesting properties. It is an itinerant ferromagnet below 150K and it transitions from a bad metal at high temperature to a Fermi Liquid at low temperature. In SRO thin films, there is evidence that even a slight presence of Ru deficiencies increases the degree of electron correlations [1]. Using Scanning Tunneling Potentiometry [2], we have studied the local transport properties of this material. The measured resistance has a noise level well above Johnson noise that depends both on the method of synthesis and on the voltage across the sample. We will attempt to characterize these resistance fluctuations with respect to the various unusual properties of the material. [1] W. Siemons \textit{et. al.}, Phys. Rev. B \textbf{76}, 075126 (2007) [2] M. Rozler and M. R. Beasley, Rev. Sci. Inst \textbf{79}, 073904 (2008) [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W30.00013: Nyquist noise as probe of hot-electron effects in the ferromagnetic insulating state of manganites Sudeshna Samanta, Arup K. Raychaudhuri Hole-doped rare-earth manganites (like La$_{1-x}$Ca$_{x}$MnO$_{3})$ in the ferromagnetic insulating (FMI) state show large non-linear conductance. Such non-linear conductance can arise due to hot-electron effect which originates from decoupling of the electron and lattice temperatures at high power level. The non-linear conductance manifests as electro-resistance or current induced resistance change. We report here low frequency temperature dependent noise measurement which allows us to estimate the electronic temperature by measuring Nyquist noise (``white noise'' in contrast to 1/f noise) in La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ single crystals which has a distinct FMI state below 100K. The measurement was performed with low ac biasing current which was mixed with a high current density d.c that leads to electron heating. We observed that in the insulating state, above a certain input d.c power, the Nyquist noise increases by a large extent and this is coupled to the onset of non-linear conduction as signalled by the power dependence of the differential conductance. The experiment establishes a direct link between hot-electron effect and non-linear conductance. [Preview Abstract] |
Session W31: Focus Session: Magnetic Nanoparticles and Nanowires
Sponsoring Units: DMP GMAGChair: Amanda Petford-Long, Argonne National Laboratory
Room: 335
Thursday, March 19, 2009 11:15AM - 11:27AM |
W31.00001: Comparative studies of Co nanowires of different diameters electroplated into porous aluminum oxide membranes Zuxin Ye, Haidong Liu, Zhiping Luo, Han-Gil Lee, Wenhao Wu, D. G. Naugle, I. Lyuksyutov The correlation between the structural and magnetic properties of template-electroplated Co nanowires has been investigated. Co nanowires of diameters either 65 or 200 nm were fabricated by electroplating Co into the pores of anodic aluminum oxide membranes. Strikingly different structures were observed in these two types of Co nanowires. The 65 nm-thick Co nanowires are composed of long Co single crystal segments with a hexagonal close-packed major phase, while the 200 nm-thick Co nanowires are composed of hexagonal close-packed and face center cubic Co single crystal segments. Correspondingly, different magnetic properties were revealed in these two types of Co nanowires. The 65 nm-thick Co nanowires have a magnetic hysteresis that is significantly larger than that of the 200 nm-thick Co nanowires. Spontaneous nanowire magnetic moments are parallel to the nanowires in the 65 nm-thick Co nanowires but are transverse to the nanowires in the 200 nm-thick Co nanowires, as observed by the magnetic force microscopy. The correlation between their different magnetic properties and microstructures is discussed. This work was supported by DOE No. DE-FG02-07ER46450, NSF No. DMR-0606529, and the Robert A. Welch Foundation A-0514. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W31.00002: Self-Assembled Superparamagnetic Binary Nanoparticle Superlattices J. Chen, X. Ye, Y. Zhang, J.M. Kikkawa, C.B. Murray We report binary nanoparticle superlattices (BNSLs) composed of two different types of superparamagnetic nanoparticles (NPs). Since the magnetic properties of these NPs depend both on size and composition, two strategies are used to form BNSLs. First, we use different sizes of the same material (e.g.-10.5 nm and 5.6 nm diameter Fe$_{3}$O$_{4}$ NPs). Second, we use different materials, such as 14.2 nm Fe$_{3}$O$_{4}$ NPs and 6 nm FePt NPs, or 14.2 nm Fe$_{3}$O$_{4}$ NPs and 7.1 nm CoPt$_{3}$ NPs. We observe the formation of large scale BNSLs (up to several $\mu $m) due to the high uniformity of these nanoparticles. Using a serial tilting capability of our TEM tomography holder we confirm that the BNSLs are icosohedral NaZn$_{13}$ and AlB$_{2}$ type structures, which are thermodynamically stable due to their high packing density. We further measured the magnetic properties of these BNSLs samples, and single component samples, by SQUID magnetometry. Dipolar and/or exchange coupling between two components is studied. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W31.00003: Magnetic domain formation in monolayer nanoparticle films Brian Maranville, Kathryn Krycka, Julie Borchers, Charles Hogg, Sara Majetich, Yumi Ijiri Self-assembled magnetic nanoparticle films offer promise as data storage media, but an understanding of the interactions is missing. Modified Langmuir-Blodgett methods were used to prepare monolayer films of 7 and 11 nm diameter Fe$_3$O$_4$ nanoparticles with large structural domains. Small-angle neutron scattering (SANS) shows a peak at a wavevector $Q$ corresponding to the particle size and spacing, and scattering at intermediate $Q$ indicating possible long-range correlations. We extend to lower $Q$ with off-specular neutron reflectivity, achieving high intensity by sacrificing resolution along one in-plane direction $y$ while retaining high resolution in the other in-plane direction $x$ and the normal direction $z$. We measure in saturation and zero field to extract magnetic scattering. In high fields, the specular scattering ($Q_x=0$) is increased, consistent with aligned moments. Preliminary results show weak magnetic scattering for nonzero $Q_x$ . Since the maximal $Q_x$ roughly corresponds to the lowest $Q$ in SANS, the combination of these techniques allows us to quantify field-dependent magnetic domain size. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W31.00004: Finite size effects and long wavelength magnetic structures in Mn$_{3}$O$_{4}$ nanoparticles R. Regmi, R. Tackett, G. Lawes Mn$_{3}$O$_{4}$ (Hausmannite) having normal spinel structure with Mn$^{2+}$ ion at tetrahedral A site and Mn$^{3+}$ ion at octahedral B site orders ferrimagnetically to Yafet-Kittel phase at 42K. The interplay between the different magnetic ions leads to additional magnetic transitions in bulk, including incommensurate and commensurate phases developing at 40K and 34K respectively. We have investigated the magnetic properties of Mn$_{3}$O$_{4}$ nanoparticles through both thermodynamic and magnetic studies. Both of these measurements observe only a single magnetic transition at 42K; the transitions at 40K and 34K appear to be completely suppressed. We motivate this suppression by comparing the long wavelength of the magnetic structure in the lower temperature phases with the particle size. These nanoparticles also exhibited superparamagnetic blocking near 40K and frequency dependent magnetic loss at 30K, which we attribute to surface spin effects. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W31.00005: Fresnel Lorentz Microscopy Imaging of Domains in Fe3O4 Nanoparticle Arrays S. A. Majetich, E. R. Evarts, C. Hogg, K. Yamamoto, T. Hirayama Fresnel Lorentz microscopy was used to study the magnetic domain structures of self-assembled nanoparticle arrays as a function of temperature, from 24 to 605 \r{ }C. 11 nm diameter Fe3O4 nanoparticles with an edge-to-edge spacing of 2.5 nm form magnetic domains through magnetostatic interactions alone. At room temperature stripe domains were evident in monolayer arrays. The average domain size in monolayer regions is larger than that in bilayers. Mean field theories predict a reduced stabilization energy for bilayers, relative to that for monolayers. The domain wall positions were fairly stable up to 500 \r{ }C, though the contrast in the walls diminished, indicating reduced magnetic order. Above 500 \r{ }C there were large temperature-dependent changes. The walls surrounding the smaller domains disappeared at lower temperatures than those of the larger domains. Some magnetic contrast was visible up to 575 \r{ }C, close to the Curie temperature of Fe3O4 (585 \r{ }C). Transmission electron microscopy after cooling showed that the particle shape and position in the ordered arrays had been preserved during the high temperature imaging experiments. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W31.00006: Magnetic imaging of individual nanomagnets B. Kalisky, J. R. Kirtley, L. Qian, N. Koshnick, M. E. Huber, K. A. Moler \newline Characterization of nanomagnets is usually done in ensembles, which is problematic because their magnetic properties are inherently sensitive to small variations in volume, shape and structure. Our aim is to detect and characterize \textit{individual} nanomagnets using scanning microscopy, which allows gathering statistics about the behavior of many individual particles under the same conditions. Scanning SQUID is a suitable tool for this challenge because it has sensitivity of $\sim $800 spins. We built a scanning microscope for this purpose, which is intended to measure the nanomagnets up to their superparamagetic state while keeping the SQUID superconducting. We will present on our preliminary efforts to measure FePt particles. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W31.00007: FePt nanoparticles as high resolution magnetic force microscope (MFM) probes Lisa Qian, Jaemin Kim, John Kirtley, Beena Kalisky, Shouheng Sun, Kathryn Moler Current MFM probes are often fabricated by sputtering a magnetic thin film across the entirety of an atomic force microscope (AFM) cantilever, limiting their spatial imaging resolution to about 30nm. We report our progress on improving this resolution by using single crystal, high-coercivity ferromagnetic FePt nanoparticles as magnetic sensors for MFM. By attaching nanomagnets 5-10 nm in diameter to the end of a functionalized AFM tip, this technique has potential for an image resolution of under 10nm. We are attempting to characterize the magnetic properties of a single nanomagnet using a novel scanning SQUID susceptometer capable of raising the sample temperature well above the SQUID temperature, with a SQUID pickup loop diameter and sensor-sample spacing well below a micron. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W31.00008: Mechanical Measurement of Magnetization Reversal in a Single Iron Filled Carbon Nanotube Palash Banerjee, M. Herman, K.C. Fong, D.V. Pelekhov, Yu. Obukhov, P. Chris Hammel, F. Wolny, U. Weissker, T. M\"{u}hl, A. Leonhardt, Bernd B\"{u}chner The hysteresis loop and switching behavior of an {\em individual} Fe-filled carbon nanotube (FeCNT) has been measured at low temperatures using cantilever magnetometry. From the magnetometry data, we are able to extract the total moment of the nanotube and the effective anisotropy field arising from the extreme aspect ratio of the nanotube (length $\sim 13~\mu$m, diameter $\sim$ 25 nm). We find the magnetization reversal in the FeCNT occurs at a well defined switching field $H_{sw}$ and in a single step. These switching fields ($H_{sw}$ = 2245 G at 4.2 K) are characterized by a narrow distribution ($\sigma_{sw} \leq$ 1 G) and their measured temperature dependence is consistent with a thermally activated process of magnetization reversal. This work was supported by the NSF Materials World Network grant DMR-0807093 and a NSF I2CAM Grant DMR-0645461. P.B. acknowledges support of the ICAM Branches Cost Sharing Fund for a postdoctoral fellowship. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W31.00009: Nanowire-Based Magnetorheological Elastomers Richard C. Bell, Jacob L. Planinsek, Joseph A. Filer II, Hyun J. Song, Norman M. Wereley Magnetorheological elastomers (MREs) are composite materials consisting of ferromagnetic particles aligned within an elastomer matrix. The stiffness of the elastomer can be controlled by varying the magnitude of an applied magnetic field. In this study, we present the static and dynamic characteristics of nanowire-based MREs and compare their response to those containing conventional particles. The MRE samples were fabricated using various ferromagnetic materials (iron, cobalt, and nickel) and particle loadings in a silicone rubber matrix and their characteristics evaluated using a material test machine. The static and dynamic properties of the MREs were evaluated under a compressive load for the various compositions. The equivalent damping coefficient of the MRE samples was measured and compared under various magnetic field intensities. The dynamic characteristics, including the dynamic stiffness and loss factor, were measured under sinusoidal excitation in the frequency domain. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W31.00010: Functional nanocomposite polymer films with uniform magnetic nanoparticle dispersions K. Stojak, S. Pal, M.J. Miner, H. Srikanth, S. Skidmore, J. Wang, T. Weller Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential in applications such as tunable microwave devices, EMI shielding, and flexible electronics. The challenge comes with evenly dispersing the nanoparticles once they are embedded in the polymer matrix. To avoid clustering of particles in the polymer nanocomposites and achieve excellent dispersion, competition between polymer-polymer and polymer-particle interactions must be balanced. In earlier work, we demonstrated the synthesis of 2$\mu $m thick, spin-coated nanocomposite PMMA films with Fe$_{3}$O$_{4}$ (mean size 15nm) nanoparticles embedded that displayed superparamagnetic behavior. In this work we will report on the successful extension of this strategy to 20 $\mu $m thick films that are needed for microwave applications. In addition to Fe$_{3}$O$_{4}$, we have also functionalized the films with other ferrite nanoparticles. Magnetic characterization and microstructural studies of the polymer nanocomposites will be presented and discussed. Microwave response of these films using a coplanar waveguide fixture will also be reported. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W31.00011: Effects of varying surfactant chain lengths on the magnetic, optical and hyperthermia properties of ferrofluids Corneliu Rablau, Prem Vaishnava, Rajesh Regmi, Chandran Sudakar, Correy Black, Gavin Lawes, Ratna Naik, Melissa Lavoie, David Kahn We report studies of the structural, magnetic, magneto-thermal and magneto-optic properties of dextran, oleic acid, lauric acid and myristic acid surfacted Fe$_{3}$O$_{4}$ nanoparticles of hydrodynamic sizes ranging from 32 nm to 92 nm. All the samples showed saturation magnetization of $\sim $50 emu/g, significantly smaller than the bulk value for Fe$_{3}$O$_{4}$, together with superparamagnetic behavior. The ac magnetization measurements on the dextran coated nanoparticles showed frequency dependent blocking temperature, consistent with superparamgnetic blocking. The ferrofluid heating rates in a 250 Gauss, 100 kHz ac magnetic field varied with the chain lengths of the surfactants, with higher heating rates for longer chains. DC-magnetic-field-induced light scattering patterns produced by two orthogonal He-Ne laser beams passing through the ferrofluid sample revealed different optical signatures for different surfactants. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W31.00012: Metallic Iron Nanoparticles for MRI Contrast Enhancement Hafsa Khurshid, Michael Bonder, Srinivasan Balakrishnan, Costas Hadjipanayis, George Hadjipanayis This study is focused on our chemically synthesized iron nanoparticles, coated with carboxyl-methyl terminated polyethylene glycol to make them biocompatible and water dispersible. The particles have an average size of 14 nm and a magnetization of 110 emu/g. TEM studies revealed their core shell structure with iron in the core and iron oxide in the shell. The effects of these nanoparticles on MRI contrast enhancement were studied in vitro using a clinical MRI scanner at a magnetic field of 1.5 T. Both the r$_{2}$ (1/T$_{2})$ and r$_{2}^{\ast }$(1/T$_{2}^{\ast })$ were found to be significantly higher than those of iron oxide nanoparticles with a similar size. This behavior is attributed to their stronger magnetic susceptibility, leading to spin dephasing and shortening of T2 effects and thus darkening of the MRI contrast. These results suggested that the iron nanoparticles are expected to be more useful for MRI contrast enhancement and other biomedical applications than the currently used iron oxide nanoparticles. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W31.00013: RIE-based Pattern Transfer Using Nanoparticle Arrays as Etch Masks Chip Hogg, Sara A. Majetich, James A. Bain Nanomasking is used to transfer the pattern of a self-assembled array of nanoparticles into an underlying thin film, for potential use as bit-patterned media. We have used this process to investigate the limits of pattern transfer, as a function of gap size in the pattern. Reactive Ion Etching (RIE) is our chosen process, since the gaseous reaction products and high chemical selectivity are ideal features for etching very small gaps. Interstitial surfactant is removed with an O$_2$ plasma, allowing the etchants to penetrate between the particles. Their pattern is transferred into an intermediate SiO$_2$ mask using a CH$_4$-based RIE. This patterned SiO$_2$ layer is finally used as a mask for the MeOH-based RIE which patterns the magnetic film. We present cross-sectional TEM characterization of the etch profiles, as well as magnetic characterization of the film before and after patterning. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W31.00014: Magneto-Transport in quantum dot films Alexandre Pourret, Philippe Guyot-Sionnest Colloidal semiconductor quantum dots are being studied intensely because of their tunable optical properties. Quantum dot solids (e.g. films) present further interesting possibilities for designing novel materials with control of the electronic properties at the nanometer scale. While neutral CdSe or CdSe/CdS nanocrystal films are quite insulating, photo excitation or doping the film electrochemically leads to higher conductivity. In this talk I will present photoconductivity and conductivity measurements of charged CdSe and CdSe/CdS nanocrystal films at low temperature under a magnetic field. The combination of electrochemistry and spectroscopy enables the precise control and detection of electrons injected into the quantum dot films. The temperature and electric field dependent conductivity is analyzed with the variable range hopping model of Efros and Shklovskii and the magneto-conductivity is discussed in terms of a spin-blockade. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W31.00015: Surface and Size Manipulation of the Magnetic Properties of CdSe Quantum Dots. Robert Meulenberg, Jonathan Lee, Scott McCall, Louis Terminello, Tony van Buuren The appearance of magnetism in otherwise non-magnetic materials has recently been reported for a number of nanoscale materials. Coupled with the size-dependent optical and electronic properties of the nanocrystalline materials, this magnetic behavior opens the possibility for an extended range of technological applications. As such, identifying the origin of the magnetism is an extremely important goal, yet this remains the subject of some controversy in the literature. We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the particle size and surface ligands. Using SQUID magnetometry and x-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand endgroup functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed. [Preview Abstract] |
Session W32: Focus Session: Theory and Simulation of Spin-Dependent Effects and Properties II
Sponsoring Units: GMAG DCOMP DMPChair: Aldo Romero, Cinvestav Querétaro
Room: 336
Thursday, March 19, 2009 11:15AM - 11:27AM |
W32.00001: The Ising model for the bcc, fcc and diamond lattices; a comparison Per H\aa kan Lundow, Klas Markstr\"om, Anders Rosengren A large scale Monte Carlo simulation study of the Ising model for the simple cubic lattice was recently performed (Adv. Phys. \textbf{56}, 653--755 (2007)). We have complemented that with a study of the bcc, fcc and diamond lattices. Both the canonical and microcanonical ensembles were employed. We present estimates of the critical temperature and other quantities in the critical region. An analysis of the critical behaviour suggests distinct high- and low-temperature exponents, especially for the specific heat, as was obtained also for the simple cubic lattice. This discrepancy is briefly discussed. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W32.00002: Thermodynamics of magnetic systems from first principles: gWL-LSMS Markus Eisenbach, G. Malcolm Stocks, Don M. Nicholson, Thomas Schulthess Density Functional Calculations have proven to be a useful tool to study the ground state of many materials. For finite temperatures the situation is less ideal an one is often forced to rely on models with parameters either fitted to first principles or experimental results. This approach is especially unsatisfactory in inhomogeneous systems, nano particles or other systems where the model parameters should vary significantly from one site to another. Here we describe a possible solution to this problem by combining classical thermodynamic Monte Carlo calculations - The Wang-Landau method in this case [F Wang and DP Landau, PRL 86, 2050 (2001)] - with a first principles electronic structure calculation, specifically our locally selfconsistent multiple scattering code. The combined code shows superb scaling behavior on massively parallel computers and first tests on Fe systems provide a proof of principle. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W32.00003: Quantum Monte Carlo calculates bulk properties and magnetic ordering in iron William D. Parker, John W. Wilkins Quantum Monte Carlo (QMC) models electronic systems with high accuracy but its computational demands limit wider use. Few QMC calculations exist for solid-state systems and none comparing the energetic ordering of different spin configurations (magnetic states). Density-functional calculations with generalized-gradient-approximation exchange-correlation give correct magnetic ordering and accurate lattice constants and bulk moduli for bcc iron. However, the predicted cohesive energy differs from experiment by 0.5-1.0 eV. QMC-calculated bulk elastic properties for pure iron in the bcc, hcp and fcc phases compare with properties in the ferromagnetic, antiferromagnetic and nonmagnetic spin configurations. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W32.00004: Current-driven vortex oscillations in metallic nanocontacts Invited Speaker: In this paper, we performed full micromagnetics simulations of metallic nano-contacts from the TUNAMOS consortium, by solving the Landau Lifshitz Gilbert Slonctewski equation simultaneously with quasi-static Maxwell equations. We take into account the spatially inhomogeneous current distribution flowing through the magnetic free layer and consequently use the Oersted field generated by this current for the magnetization dynamics. The system we simulated was a trilayer CoFe 3$.$5 nm/Cu 3nm/NiFe 4nm stack. The saturation magnetization of the free layer is taken to be the same as the experimental value \textit{Ms }=1$.$1 T, and a GMR ratio of 1{\%} is used. We account for the inhomogeneous current distribution flowing through the free layer by computing the local current density from the local angle between the free and fixed layer magnetizations. The Oersted field is computed with the Biot-Savart law from this current distribution [2], and an asymmetric Slonczewski term for the spin transfer is used [3]. We observe that the additional spin torque drives the vortex out of the contact area and towards a stable orbit around the contact. These simulations reveal that the oscillations observed are related to the large-amplitude translational motion of a magnetic vortex. In contrast to the nanopillar geometry in which the vortex core precesses within the confining part of the Oersted field [1], the dynamics here correspond to an orbital motion \textit{outside }the contact region. This behavior can be likened to planetary orbital motion under the influence of a gravitational field; the spin-transfer torque leads to a centripetal motion of the vortex core, which is counterbalanced by the attractive potential provided by the Oersted field. Good quantitative agreement between the simulation and experimental frequencies is achieved [4]. \\[4pt] [1] V. S. Pribiag et al., Nat. Phys. \textbf{3}, 498 (2007) \\[0pt] [2] O. Ertl \textit{et al.}, J. Appl. Phys. \textbf{99}, 08S303 (2006). \\[0pt] [3] J. Xiao, A. Zangwill, and M. D. Stiles, Phys. Rev. B \textbf{70}, 172405 (2004). \\[0pt] [4] Q. Mistral, M. van Kampen, G. Hrkac, et al. PRL \textbf{100, }257201 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W32.00005: Monte Carlo simulation of incommensurate helical ordering in a frustrated FCC lattice of Heisenberg spins Seongweon Park, Ch. M. Sullivan, G. Schneider, T.M. Giebultowicz The zincblende structure of MnSe can be stabilized in thin films and is expected to exhibit Type III FCC antiferromagnetic ordering. The expected magnetic order is indeed observed in MnSe/ZnSe superlattices where the MnSe layers experience compressive strain. However, in the MnSe/ZnTe system, in which MnSe layers experience tensile strain, the Mn spins form incommensurate helical structures \footnote{T.M.Giebultowicz et al, Phys Rev B, 46, 12076-12079 (1992)}. Mean field theory can explain the basic mechanism leading to helical ordering but cannot explain details such as the temperature dependence of the pitch of the helical ordering. We report results of Monte Carlo simulations using classical 3D Heisenberg spins and ``free'' boundary conditions. The simulations were performed for a range of systems with different sizes (including ``bulk'' thickness) and exchange constants. The change of helical pitch with temperature is correctly reproduced in our results and our data indicate that it is at least partially a finite-thickness effect. We compare our results with earlier simulations using XY spins and ``self-determined'' boundary conditions\footnote{M. Collins and W.M. Saslow, Phys Rev B 53, 8533-8538 (1996)}. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W32.00006: Dynamical magnetizations of nanomagnets with strong magnetic anisotropy Bang-Gui Liu We developed a non-equilibrium Monte Carlo method to investigate dynamical spins and magnetizations of nanomagnets with strong magnetic anisotropy and applied it to Co spin chains on Pt surface and a composite spin system (Phys. Rev. B 73, 174418; Phys. Rev. Lett. 96, 217201; Front. Phys. China 2, 424). Here we report on our exploration for universal dynamical magnetic properties of spin chains and single-layered nanomagnets with strong magnetic anisotropy. Furthermore, we investigate representative systems composed of sub-10nm nanomagnets with large uniaxial anisotropy for magnetic data storage, finding various magnetization memory effects and aging effect in such single systems of the interacting nanomagnets with the same easy axis, and study the exchange bias and training effect observed in composite films and heterostructures. This method is proved to be effective and reliable in simulating dynamical magnetism in nanomagnets with strong magnetic anisotropy. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W32.00007: Tailoring Effective Exchange Interactions via Domain Walls in Coupled Heisenberg Rings Vanita Srinivasa, Jeremy Levy The nature of the exchange coupling variation in an antiferromagnetic spin-1/2 system can be used to tailor its ground-state properties. In particular, dimerized Heisenberg rings containing domain walls have localized states which can serve as ``flying spin qubits'' when the domain walls are moved (PRB \textbf{76}, 094411 (2007)). We show theoretically that, when two of these rings are coupled, the movement of the domain walls leads to modulation of the effective exchange interaction between the qubits. Appropriately chosen configurations of domain walls can give rise to ferromagnetic effective exchange. We describe how these spin rings may be used as basic building blocks to construct quantum spin systems whose properties are tunable by virtue of the exchange variation within the rings. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W32.00008: Electron magnetism of antiferromagnetic conductors. Revaz Ramazashvili Essential momentum dependence of the electron g-tensor in an antiferromagnet turns the common Zeeman term into a spin-orbit coupling. I will discuss some of the remarkable experimental consequences of this phenomenon. The predictions may be relevant to antiferromagnetic conductors from chromium to electron- and hole-doped cuprates, borocarbides, pnictides, organic and heavy fermion materials. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W32.00009: On the Electric-Field-Controlled Surface Ferromagnetic Transition in Metals Igor V. Ovchinnikov, Kang L. Wang It is widely believed that in metals, unlike in the dilute magnetic semiconductors, the control of the ferromagnetic ordering by an external voltage is hardly achievable. We compare the two materials and show that there is no obvious reason why metals are less preferable for this phenomenon. Similar effect in metals, however, has a different physical picture and should be identified as a voltage-induced surface ferromagnetic transition. We study its properties within the theory of the surface critical phenomena and discuss possible difficulties on the way to its experimental realization. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W32.00010: Effect of fluctuations on effective Hamiltonians of anisotropic frustrated pyrochlore antiferromagnets. Paul McClarty, Michel Gingras The rare earth pyrochlore magnets R$_{2}$Ti$_{2}$O$_{7}$ exhibit a wide range of puzzling features. Tb$_{2}$Ti$_{2}$O$_{7}$, a weakly Ising-like antiferromagnet, is a cooperative paramagnet down to, at least, 50 mK despite having a -20 K Curie-Weiss temperature. Er$_{2}$Ti$_{2}$O$_{7}$, which has magnetic ions with a strong easy plane anisotropy, has a transition to an ordered phase but the origin of a long-range ordered state with discrete broken symmetry is not understood. Recent experimental work has also uncovered interesting field-induced phases in both of these materials. We construct effective Hamiltonians, derived from microscopic models, for these two frustrated antiferromagnets by considering the effects of quantum fluctuations out of the classical ground states of these models to assess the stability of these states, the nature of the excitations and possible mechanisms of degeneracy breaking. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W32.00011: Ferromagnetic spin coupling as the origin of $0.7$ anomaly in quantum point contacts Karan Aryanpour, Jong E. Han We study one-dimensional itinerant electron models with ferromagnetic coupling to investigate the origin of $0.7$ anomaly in quantum point contacts (QPC). Linear conductance calculations using the Kubo formula from the quantum Monte Carlo (QMC) technique for spin interactions of different spatial range suggest that $0.7\times(2e^{2}/h)$ anomaly results from a strong interaction of low-density conduction electrons to ferromagnetic fluctuations formed across the potential barrier. The conductance plateau results due to the strong incoherent scattering at high enough temperatures when the electron traversal time through the gate voltage barrier matches the time-scale of dynamic ferromagnetic excitations (magnons). In addition, our model also captures the correct evolution of the anomalous plateau as a function of temperature and Zeeman magnetic field. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W32.00012: Intrinsic Perturbation of the Landau Levels in Metals and Semiconductors at Low Temperatures Ayodeji Awobode The de Haas--van Alphen effect in non-superconducting metals and semiconductors at very low temperatures is proposed as a test of an intrinsic perturbative term which appears in the Landau equation sequel to the modification of the Pauli equation. Corrections to the frequency (or period) of the de Haas--van Alphen oscillation in metals is calculated and shown to depend on the Fermi energy and the measured anomalous part of the electron magnetic moment. Precision measurement of the magneto-optical properties which arise from the motion of electrons in binary semiconductors placed in a weak magnetic field is also proposed as a means of observing very small changes in the. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W32.00013: Variation of magnetic H field in closed loop magnetic circuits: problems with the standard equation Esaindang Umenei, Eugene Melikhov, David Jiles We have developed a reliable method for calculating the variation of magnetic field H in closed circuits. This offers advantages over standard numerical Finite Element Modeling which requires meshing of the spatial domain. Such calculations can consume enormous computational resources and time. Analytical models work much faster but are only applicable in restricted cases. The well known ``standard model'' for the relationship between current I and magnetic field H derived from Ampere's Law is$H=\frac{N\,I}{L}$, where $N$ I is magnetomotive force and $L$ is the length of the magnetic path. However, this formula fails to describe the variation in magnetic field with position. In fact H is usually inhomogeneous around a closed path unless special precautions have been taken to ensure uniformity. In order to describe the magnetic field around a closed circuit we have introduced extensions to the standard formula for a finite coil in a closed circuit. This includes parameters for location and shape of core to enhance the accuracy. This analytic model produces fast and accurate predictions for the variation of H with position. Results are comparable with FEM calculations that take much longer to generate. [Preview Abstract] |
Session W33: Superconductor-Insulator Transitions
Sponsoring Units: DCMPChair: Michael Osofsky, Naval Research Laboratory
Room: 403
Thursday, March 19, 2009 11:15AM - 11:27AM |
W33.00001: Mott transition, magnetism and d-wave superconductivity on lattices with frustration A.-M.S. Tremblay, Bumsoo Kyung By tuning band parameters and choosing appropriate lattices, it is possible to frustrate antiferromagnetism to reveal the competing d-wave superconducting (d-SC) phase, the normal phase and the Mott insulating phase. We study the nature of the competition between these phases by using Cellular Dynamical Mean-Field Theory for the Hubbard model on the anisotropic triangular lattice and on the square lattice with second-neighbor hopping. The phase diagram in the $T=0$ plane is drawn as a function of interaction strength $U/t$ and frustration $t'/t$. The critical interaction strength for the Mott transition is found as a function of frustration. At half-filling and for intermediate coupling we find that outside the Mott insulating phases, d-SC appears at an optimal level of frustration. We also identify spin fluctuations as the source of pair formation. We find that components of the spin susceptibility involved in binding are mostly centered in the quarter of the Brillouin zone closest to $(\pi,\pi)$. We conclude that retarded short-range spin fluctuations are crucial for d-SC even in the presence of frustration and that there are optimal values of frustration that favor d-SC. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W33.00002: Superconductor-Metal-Insulator Phase Transition in Ta Films Yize Li, Jongsoo Yoon We have reported the magnetically induced metallic phase in superconducting Ta films which intervenes the superconducting and insulating phases [1]. Recently, we studied the electronic transport properties of Ta film with various degrees of disorders which were controlled by film thickness. We found that as sample thickness decreases, the films undergo a superconductor-metal-insulator phase transition. Each phase exhibits distinct nonlinear current-voltage (I-V) characteristics, similar to those of magnetically induced superconductor-metal-insulator transition. We have measured the evolution of nonlinear I-V with changing magnetic field (B) and temperature (T), for representative samples with different degrees of disorders, which leads to the phase diagram in B-T-Disorder space. [1] Y.Qin et al., Phys. Rev. B 73, 100505(R) (2006). [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W33.00003: Dissipation-driven quantum phase transition in superconductor-graphene systems Roman Lutchyn, Victor Galitski, Gil Refael, Sankar Das Sarma We show that a system of Josephson junctions coupled via low-resistance tunneling contacts to graphene substrate(s) may effectively operate as a current switching device. The effect is based on the dissipation-driven superconductor-to-insulator quantum phase transition, which happens due to the interplay of the Josephson effect and Coulomb blockade. Coupling to a graphene substrate with gapless excitations further enhances charge fluctuations favoring superconductivity. The effect is shown to scale exponentially with the Fermi energy in graphene, which can be controlled by the gate voltage. We develop a theory, which quantitatively describes the quantum phase transition in a two-dimensional Josephson junction array, but it is expected to provide a reliable qualitative description for one-dimensional systems as well. We argue that the local effect of dissipation-induced enhancement of superconductivity is very robust and a similar sharp crossover should be present in finite Josephson junction chains. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W33.00004: Superfluid response of a gated LaAlO$_3$ / SrTiO$_3$ heterostructure Shashank Misra, Lukas Urban, Stefan Thiel, Christoph Richter, German Hammerl, Jochen Mannhart, Ali Yazdani Disordered two-dimensional superconductors undergo a quantum phase transition into an insulating phase, with an unusual intervening metallic phase, upon the introduction of sufficiently large amounts of disorder or the application of a sufficiently strong magnetic field. The LaAlO$_3$ / SrTiO$_3$ heterostructure, because it can be gated, provides a new opportunity: to see how two-dimensional superconductivity is destroyed continuously as a function of carrier concentration. We build on the electrical transport measurements in other works, which demonstrated the existence of a superconductor-insulator transition upon decreasing the carrier concentration, by using a two coil mutual inductance technique to measure the complex ac conductivity. We will track the superfluid density, which can be derived from the complex conductivity, throughout parts of the carrier concentration- temperature- magnetic field phase diagram, and make comparisons with data from the field-tuned superconductor-insulator transitions in MoGe and InOx. This work is supported by the DOE and the DFG via SFB484. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W33.00005: Giant enhancement of superconductivity in ultrathin $a$-Pb films by a parallel magnetic field: effect of magnetic impurity Ashwani Kumar, H. Jeffrey Gardner, Liuqi Yu, Peng Xiong An ultrathin superconductor containing paramagnetic impurities is predicted$^{1,2}$ to exhibit \textit{enhancement} of superconductivity ($T_{c}$ and $I_{C})$ when subject to a parallel magnetic field. We have recently observed a pronounced enhancement of superconductivity in ultrathin homogeneous amorphous Pb films without any (intentionally added) magnetic impurities in the presence of a parallel magnetic field; the $T_{c}$ enhancement is as large as 13{\%} and persists in field as high as 8 T. Our experiments are carried out in a modified dilution refrigerator capable of \textit{in situ} film growth, sample rotation, and incremental deposition of magnetic (Cr) impurities, which allows for a systematic, unambiguous elucidation of the effect of paramagnetic impurities on the field-enhancement of superconductivity. With increasing Cr density on a Pb film, the magnitude of the $T_{c}$ enhancement is progressively suppressed, contrary to the theoretical predictions. $^{1}$ Kharitonov \textit{et al}., JETP Lett. 82, 473 (2005). $^{2}$ Wei \textit{et al}., Europhys. Lett. 75, 943 (2006). [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W33.00006: A direct transition between a Neel ordered Mott insulator and a $d_{x2-y2}$ superconductor on the square lattice Ying Ran, Ashvin Vishwanath, Dung-Hai Lee In this paper we study a bandwidth-controlled direct, continuous, phase transition from a Mott insulator, with easy plane Neel order, to a fully gapped $d_{x2-y2}$ superconductor with a doubled unit cell on the square lattice, a transition that is forbidden according to the Landau paradigm. This transition is made possible because the vortices of the antiferromagnet are charged and the vortices of the superconductor carry spins. These nontrivial vortex quantum numbers arise because the ordered phases are intimately related to a topological band insulator. We describe the lattice model as well as the effective field theory. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W33.00007: Superconductor to Quantum Metal Transitions in Ultra Thin Films Yen-Hsiang Lin, Allen M. Goldman Homogeneous films of amorphous bismuth have been continuously tuned from the superconducting state by increasing a perpendicular magnetic field. Electrical transport and Hall measurements show that the non-superconducting states of the films are quantum-corrected metals. In the vicinity of transition field, the resistance can be fit by an Arrhenius type of conduction at high temperatures but this form fails at lower temperatures where the resistance is a non-monotonic function of temperature. This suggests that a two-phase regime develops near criticality. Theories suggest that this is in the form of superconducting puddles embedded in a normal matrix$^{1,2}$. $^{1}$B. Spivak, P. Oreto, and S. A. Kivelson, Phys. Rev. B \textbf{77}, 214523 (2008) $^{2}$Y. Dubi, Y. Meir, and Y. Avishai, Nature \textbf{449}, 876-880 (2007) [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W33.00008: The superconductor-insulator transition: is there a new insulating state? Maoz Ovadia, Benjamin Sacepe, Dan Shahar We present nonlinear conductivity measurements on the insulating side of the superconductor-insulator transition in amorphous indium oxide. The results agree with previous data$^{1,2}$, and show conductance jumps at well-defined voltage bias thresholds. The current in the sample changes by as much as a factor of 10$^ {6}$ at the threshold, from our noise floor of 3x10$^{-14}$A to over 10$^{-8}$A. The jumps disappear above a magnetic-field- dependent temperature T$^{*}$, which is 0.11K or lower. The threshold voltage changes from 20$\mu$V to over 0.2V (4 orders of magnitude) by application of a magnetic field. We ask whether a true zero conductance state exists in our samples. DC measurements reveal pseudo-exponential I-V characteristics, which can be extrapolated to find the high Ohmic resistance of these samples at low temperatures. The extrapolated R(T) curves typically show a sub-activated trend at low T. Our results suggest that our samples have zero conductance only at the absolute zero of temperature. \\[4pt] (1) Sambandamurthy et al. PRL 92, 107005\\[0pt] (2) Baturina et al. Nature Letters 452, p613 [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W33.00009: Metal-insulator transition and superconductivity in the Mott insulator GaTa$_{4}$Se$_{8}$ : towards a tuning of the Mott transition by electric pulses E. Janod, C. Vaju, V. Dubost, B. Corraze, T. Cren, P. Moreau, F. Debontridder, D. Braithwaite, D. Roditchev, L. Cario We have recently discovered the existence of a non-volatile electric-pulse-induced resistive switching (EPI-RS) in the spinel Mott insulator GaTa$_{4}$Se$_{8}$ [1]. The origin of this effect is different from other EPI-RS mechanisms identified to date [2]. A granular superconducting state below $T_{C}$ = 5-7 K, ascertained by critical current and critical field data obtained on single crystals, appears in the EPI ``metallic'' state. This transition is reminiscent of the bulk superconductivity at 5-8 K obtained under pressure [3]. Interestingly, STM experiments have revealed a puzzling electromechanical coupling between the tip voltage and the GaTa$_{4}$Se$_{8}$ sample surface. All these results may therefore indicate that, beside electronic doping and pressure, electric pulses, through an electrostrictive effect, could be a relevant parameter to tune the Mott metal-insulator transition. [1] C. Vaju \textit{et al.}, Adv. Mater. 20, 2760 (2008) [2] R. Waser, M. Aono, Nature Mat. 6, 833 (2007) [3] M.M. Abd-Elmeguid \textit{et al.}, PRL 93, 126403 (2004) [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W33.00010: Enhanced Suppression of Superconductivity in Amorphous Films with Nanoscale Patterning M.D. Stewart, Jr., H.Q. Nguyen, S.M. Hollen, Aijun Yin, J.M. Xu, J.M. Valles, Jr. We have measured the thickness dependence of the superconducting critical temperature, $T_c(d_{\rm Bi})$, in amorphous Bi/Sb films patterned with a regular array of holes as well as nanoscale thickness variations. We find that the mean field $T_c$ is suppressed relative to simultaneously produced unstructured films of the same thickness. Surprisingly, however, the functional form for $T_c(d_{\rm Bi})$, remains unaffected. The role of the thickness variations in suppressing $T_c$ is compared to the role of the holes, through parameterization of the surface, as measured through AFM/SEM and a proximity effect calculation. These results suggest that these two nanoscale modifications suppress $T_c$ about equally and are consistent with $T_c$ being determined on a microscopic length scale. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W33.00011: Giant enhancement of superconductivity in ultrathin $a$-Pb films by a parallel magnetic field: effect of film thickness H. J. Gardner, L. Yu, A. Kumar, P. Xiong We have observed a pronounced enhancement of superconductivity in ultrathin homogeneously disordered amorphous Pb films in the presence of a parallel magnetic field. Our experiments are carried out in a modified dilution refrigerator capable of \textit{in situ} film growth and sample rotation, thus allowing for a detailed examination of the effects of perpendicular and parallel magnetic field on the same film at incrementally larger thicknesses. With increasing thickness, the film is tuned from an insulator to a superconductor where its zero-field T$_{c}$ increases with the thickness. For these films we observe enhancement of T$_{c}$ by a parallel magnetic field, while a perpendicular magnetic field of any strength causes a degradation of T$_{c}$. The magnitude of the T$_{c}$ enhancement appears to exhibit a non-monotonic dependence on the film thickness (zero-field T$_{c})$. The enhancement is as large as 13{\%} and persists in fields as high as 8 T for certain thicknesses. We will discuss the possible origin of the field-enhancement of superconductivity. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W33.00012: Superconductor-insulator transitions in films patterned with a disordered nanohoneycomb hole array H.Q. Nguyen, S.M. Hollen, M.D. Stewart, Jr., Aijun Yin, J.M. Shainlin, J.M. Xu, J.M. Valles, Jr. On both sides of the Superconductor-Insulator Transition (SIT), ultrathin Bi films patterned with an ordered array of holes exhibit magnetoresistance (MR) oscillations with a period set by the superconducting flux quantum$[1]$. This observation implies that the insulating phase consists of localized Cooper pairs. To probe further this localized Cooper pair phase we have investigated samples patterned with disordered hole arrays. We have found that disorder reduces the number of MR oscillations and weakens the magnetic field tuned SIT. We will present these results and discuss their implications for the Cooper pair insulating phase. \\ $[1]$ M.D Stewart, Jr., A. Yin, J.M. Xu, and J.M. Valles, Jr., Science \textbf{318}, 1273 (2007) [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W33.00013: High field magnetoresistance peak near the superconductor insulator transition in amorphous Bi films patterned with a nanohoneycomb array of holes S.M. Hollen, H.Q. Nguyen, M.D. Stewart, Jr., J.M. Shainline, Aijun Yin, J.M. Xu, J.M. Valles, Jr. The spectacular magnetoresistance (MR) peak that appears on the insulating side of the Superconductor-Insulator Transition (SIT) in In Oxide films [1] has received much attention. It has been taken as a sign that Cooper pairs persist into their insulating phase. We have observed a similar MR peak in ultrathin amorphous Bi films patterned with a disordered nanohoneycomb array of holes. This peak increases in magnitude with decreasing thickness and moves to lower field with decreasing temperature. Most importantly, it coexists with MR oscillations at lower fields that reveal the presence of Cooper pairs [2]. We will present our latest investigations of this peak and contrast our results with the behavior of unpatterned amorphous film systems.\\[3pt] [1] G. Sambandamurthy, \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{92}, 107005 (2004). \\[0pt] [2] M. D. Stewart, Jr., \textit{et al.}, \textit{Science} \textbf{318}, 1273 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W33.00014: Superconductor-Insulator Phase Transitions in Current-Biased Arrays of Small Josephson Junctions Christopher Porter, David Stroud We present a variational approach to treat the metastable superconducting state in an array of small Josephson junctions driven by an applied current. The approach is a generalization of one previously used to treat such an array at zero applied current. The array Hamiltonian is treated variationally using the Gibbs-Bogoliubov inequality, using a set of harmonic ``phase phonons'' as the variational state. We find that, for a given J/U, where J and U are the Josephson and charging energies, a superconducting (S) to insulating (I) transition occurs as a function of applied current, or by varying the direction of the applied current at fixed magnitude. The critical values of J/U are calculated for a square, triangular, and simple cubic lattices of Josephson junctions as a function of bias current. The resulting critical J/U is found to be highly sensitive to changes in bias current magnitude, and somewhat less sensitive to current direction, for all geometries studied. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W33.00015: Biot-Savart correlations in layered superconductors Kumar Raman, Vadim Oganesyan, Shivaji Sondhi We discuss the superconductor to normal phase transition in an infinite layered type-II superconductor in the limit where Josephson coupling between layers is negligible. We model each plane as a neutral gas of thermally excited pancake vortices and assume the Biot-Savart interaction between vortices is the dominant mechanism for coupling the layers. Using the real-space renormalization group, we demonstrate that the transition in this model is a Kosterlitz-Thouless transition driven by the unbinding of pancake vortices. We study the high temperature phase using a Debye-Huckel type mean field theory. We find that while the long range interaction leads to correlations between the planes, the screening within the individual layers is not significantly different from an isolated two-dimensional system. This overall picture places some claims expressed in the literature on a more secure analytical footing and also resolves some conflicting views. Experimental implications will be discussed. [Preview Abstract] |
Session W34: Superconducting Nanowires
Sponsoring Units: DMPChair: Zhili Xiao, Argonne National Laboratory
Room: 404
Thursday, March 19, 2009 11:15AM - 11:27AM |
W34.00001: Current-induced anomalous negative magnetoresistance in Zn nanowires. Yu Chen, Stephen Snyder, Allen Goldman Quasi-one dimensional superconducting Zn nanowires, connected with wide Zn electrodes, were fabricated using electron-beam lithography. The Zn was deposited with the substrates held at liquid nitrogen temperature. At temperatures well below the nanowires' transition temperature, four-terminal measurements of resistance showed an anomalous negative magnetoresistance when applying a current slightly higher than its critical current in zero magnetic field. The magnitude of this negative magnetoresistance can be as large as 50 percent even with applied field as low as few Oersted. This effect was found over a narrow range of temperatures, which depended upon the applied current. At this writing, the origin of the effect is unclear. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W34.00002: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W34.00003: Electronic structure of quasi-one-dimensional edge-sharing cuprate LiCu$_{2}$O$_{2}$ single crystals measured by angle-resolved photoemission spectroscopy K.-D. Tsuei, C.-M. Cheng, J.-Y. Yuh, K.W. Yeh, M.K. Wu We have carried out a high resolution angle-resolved photoemission study on edge-sharing quasi-one-dimensional (1D) chain cuprate LiCu2O2 single crystals at room temperature. The low energy electron diffraction of cleaved (001) surfaces show a well ordered (2x1) pattern with single domain. Absence of photon energy dependence of high lying peaks just below the Fermi energy in the normal emission spectra suggests localization within the ab-pannar layers. One can identify three dispersive bands between 0.5 eV and 2 eV binding energies along the high symmetry directions along with the off-normal spectra. The highest energy peak is observed at the Y-point with a binding energy 0.55 eV bearing a $d_{xy}$ symmetry based on a polarization dependent selection rule, and can be associated with a hybridized state of primarily Cu 3$d_{xy}$ and O 2$p $orbitals, in agreement with a LDA band calculation. We observed no indication of a band maximum at half integral position along GY, predicted by a 1D $t-J$ model. Another peak at 1.3 eV shows a strong dispersion along GX, normal to the chain direction. Its identity will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W34.00004: Synthesis and characterization of superconducting, single-crystal Al nanowires using template based electrodeposition Meenakshi Singh, Jian Wang, Mingliang Tian, Alexis Pereira, Nitesh Kumar, Thomas Mallouk, Moses Chan Al nanowires (ANW) have been fabricated using template based electrodeposition at room temperature for the first time. An anhydrous electrolyte comprised of AlCl$_{3}$ and LiAlH$_{4}$ in tetrahydrofuran with a Pt anode and Ag cathode was used to electrodeposit Al in an anodic aluminum oxide template. X-ray diffraction and electron diffraction show that the ANW are single crystal with (110) as the growth direction. Transmission electron microscopy shows that the wires have uniform diameters with an oxidation layer a few nanometers thick around them. Four electrode transport measurements on a single $70$ nm thick ANW have shown significant enhancement in the critical temperature and the critical field and decrease in the critical current density from the bulk value. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W34.00005: Resistance in Superconducting Epitaxial Niobium Nanowires and Films Timothy McArdle, Allison Dove, Kevin Inderhees, Mitrabhanu Sahu, Alexey Bezryadin, Paul Goldbart, James Eckstein The thermally activated phase slip (TAPS) description of resistance in one-dimensional superconducting wires is unable to explain additional resistance observed in extremely narrow nanowires well below the critical temperature. We fabricate nanowires using electron beam lithography from single-crystal niobium films grown by ultra-high vacuum molecular beam epitaxy. Since the resulting wires are single crystal and homogenous, the role of disorder is reduced and neither weak links nor grains are present. The epitaxial films are 10 to 30 nm thick, while the finished wires range in length from 1 to 10 $\mu $m, and in width from 35 to 200 nm. Transport measurements on the nanowires of varying widths show a range of distinct temperature dependencies below the critical temperature that cannot be accounted for by the single exponential form of the TAPS model. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W34.00006: Two superconducting transitions and periodic magnetoresistance oscillations in the crystalline Au nanowire with superconducting electrodes Jian Wang, Chuntai Shi, Mingliang Tian, Jainendra Jain, Qi Zhang, Nitesh Kumar, Meenakshi Singh, Thomas Mallouk, Moses Chan Transport measurements were made on individual crystalline Au nanowire with four focused ion beam (FIB) deposited tungsten (W) electrodes, which are superconducting below 5 K. It was found that the 1.2 microns long (between the inner edges of the two voltage electrodes) Au nanowire is superconducting. Instead of a single sharp drop to zero resistance as seen in usual superconductors, here the resistance drops in two steps. Below TC, with a magnetic field applied perpendicular to the axis of the nanowire, we found ``magnetoresistance mini-gaps'' in low field regime. In addition, clear periodic magnetoresistance oscillations in the superconducting to normal transition region were observed. The 1 micron and 1.9 microns Au nanowires were also investigated to further reveal the anomalous superconductivity we found. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W34.00007: Vortex blockade and conductance fluctuations of superconducting strips in magnetic fields Paul Goldbart, Gil Refael, David Pekker Recent experiments on the conductance of a thin, narrow superconducting strip found periodic fluctuations as a function of the perpendicular magnetic field, with the period corresponding to approximately two flux quanta per strip area [1]. Using vortex-charge duality, we explore the possibility that the superconducting strip is the dual of a quantum dot, with vortices playing the role of the electrons, the magnetic field appearing as the gate voltage, and the applied current replacing the source-drain voltage. As with a quantum dot, extrema of the conductance are obtained when configurations with $n$ and $n+1$ vortices have equal energy; in the bias-current versus magnetic-field plane, the conductance displays blockade diamonds. Furthermore, we find that there is a simple relation between the linear-response conductance and the critical current, as they are both set by the barrier to vortex tunneling on to and off of the strip. [1] A. Johansson et al. Phys. Rev. Lett. 95, 116805 (2005). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W34.00008: Tuning of the critical current of superconducting nanowires by application of voltage pulses Thomas Aref, Alexey Bezryadin The critical current of superconducting nanowires may be dependent on a number of factors such as dimensions of the nanowire, the wire's normal resistance or the presence of magnetic impurities. Determining which factors are most important is key to developing a detailed understanding of the underlying physics of 1D superconductivity. It is experimentally difficult to alter and probe such properties in situ at cryogenic temperatures. We have developed a method for tuning the critical current of a nanowire in situ inside a helium cryostat by the application of short, high bias voltage pulses. These pulses alter the critical current of the nanowire in a controlled manner by annealing or electromigration. Thus, for a single nanowire, we can vary critical current and normal resistance in situ. This process may have applications in developing future superconducting nanowire circuitry such as qubits where precise control of the nanowire's critical current is required. We also investigate the dependence of the critical current on the normal resistance of the wire and compare to existing theories. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W34.00009: Flux-quantization effect in superconducting niobium loops Suhong Yu*, Zhili Xiao*, Alexandra Imre, Jiong Hua*, Ulrich Welp, Wai-Kwong Kwok Superconducting loops have periodical oscillation of critical temperature $T_{c}$ as a function of applied perpendicular magnetic field $H$ The corresponding periodicity is related to superconducting flux quantization, $h/2e$, due to size constraint. When the loop size shrinks, however, new phenomena can appear. For example, the oscillation can show a $h$/e rather than $h/2e$ periodicity if the hole diameter is comparable to the superconducting coherence length. We present experimental investigation of flux-quantization effect in mesoscopic superconducting niobium loops. We developed a new approach to fabricate high quality loops by combining electron-beam lithography with focused-ion-beam (FIB) milling techniques. Periodic oscillations were observed in both the $H-T$ phase diagram and the magnetoresistance. Analysis of the data with various theories will be presented [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W34.00010: Anisotropic magnetoresistance of a one-dimensional superconducting niobium strip Jiong Hua*, Zhili Xiao*, Alexandra Imre, Suhong Yu*, Umesh Patel*, Leo Ocola, Ralu Divan, Alexei Koshelev, John Pearson, Ulrich Welp, Wai-Kwong Kwok We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross-section. When the strip's transverse dimensions are comparable to the superconducting coherence length, we find the angle dependent magentoresistances at a fixed temperature can be scaled as R($\theta $, H) = R(H /H$_{c\theta })$ where H$_{c\theta }$ = H$_{c0}$ (cos$^{2}\theta +\gamma ^{-2}$sin$^{2}\theta )^{-1/2}$ is the angular dependent critical field, $\gamma $ = w/d is the width to thickness ratio of the strip, and H$_{c0}$ is the out-plane critical field at $\theta $ = 0\r{ }. Our results can be understood in terms of the anisotropic diamagnetic energy of a one-dimensional superconductor in a magnetic field. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W34.00011: Quasi-reentrant resistive behavior in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{x}$ whiskers Sevda Avci, Umesh Patel, Suhong Yu, Zhili Xiao, Ralu Divan, Ulrich Welp, Wai-Kwong Kwok, Milind Kunchur BSCCO (2212) whiskers were fabricated via a melt-quench-growth method and their morphology was characterized with scanning/transmission electron microscopy and atomic force microscopy. Four-probe magneto-transport measurements were conducted as a function of temperature and current. In low magnetic fields and currents, the resistance decreases monotonically and vanishes at a temperature of $\sim $ 80 K. However, at large currents and magnetic fields the resistance shows a non-monotonic dependence on temperature, even showing values that are higher than the normal state resistance for certain ranges of the parameters We attribute the observed behavior to the brick-wall morphology of the whiskers leading to a competition between normal and superconductive tunnelings that is known to take place in granular superconductive systems. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W34.00012: Quenching of Meissner Diamagnetism in Superconducting Nanocrystals Helena Moreira, Irene Resa, Benoit Mahler, Benoit Dubertret, Herve Aubin We developed a new chemical synthesis for the preparation of high quality monodisperse superconducting Lead (Pb) nanocrystals. They are obtained from the alcohol reduction of Lead carboxylates in a hot organic solution and lead to colloids stabilized and protected from oxidation by organic ligands. Large quantities of nanocrystals with tunable diameter (8 to 30 nm) can be obtained. This new material allows the study of the effects of quantum confinement on superfluid response with unprecedented size resolution. Magnetic susceptibility measurements show that the large critical field of the particles increases from 2 to 5 T as the diameter is reduced down to 16 nm. This critical field results from the competition between the kinetic energy for Cooper pairs and superfluid condensation energy. Below the diameter of 16 nm, no Meissner effect remains in the particles, but only the signature of residual superconducting fluctuations. Remarkably the size scale below which the superfluid response disappears (16 nm) is significantly larger than the value expected from Anderson criterion. This implies that, in the regime of quantum confinement, there are distinct size-scales for the formation of Cooper pairs and the establishment of the superfluid response. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W34.00013: Thin Superconducting Rings and Cylinders in a Magnetic Field Robert Beaird, Daniel E. Sheehy, Ilya Vekhter We examine the magnetic field dependence of the critical temperature ($T_c$) for a thin superconducting ring and thin- walled superconducting cylinder (of radius $R$), in the presence of a magnetic field ($B$). We include both the orbital effect and the Zeeman splitting of the quasiparticle bands. We derive a Ginzburg-Landau free energy functional and allow for the appearance of the spatially-modulated (Fulde-Ferrell-Larkin- Ovchinnikov) state. We explore the competition between the orbital effect and Zeeman splitting as a function of the ratio of $R$ to the superconducting coherence length, the orientation of $B$ with respect to the plane of the ring, and the Maki parameter (the ratio of the orbital and paramagnetic critical fields). We focus on the interplay of the periodicity and the overall suppression of $T_c$ with applied $B$. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W34.00014: Order and Creep in Flux Lattices and CDWs Pinned by Planar Defects Aleksandra Petkovic, Thomas Nattermann The influence of randomly distributed point impurities \emph{and} planar defects on the order and transport in type-II superconductors and related systems is considered theoretically. For random planar defects of identical orientation the flux line lattice exhibits a new glassy phase with diverging shear and tilt modulus, a transverse Meissner effect, large sample to sample fluctuations of longitudinal magnetic susceptibility and an exponential decay of translational long range order. The flux creep resistivity for currents $J$ parallel to the defects is $\rho(J)\sim \exp-(J_0/J)^{\mu}$ with $\mu=3/2$. Strong disorder enforces an array of dislocations to relax shear strain. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W34.00015: Crossed Andreev reflection dominated subgap transport in normal metal/superconducting hybrid structures Andreas Kleine, Andreas Baumgartner, Jelena Trbovic, Christian Schonenberger We report on a systematic study of the non-local Andreev process (also called cross Andreev reflection = CAR) in planar mesoscopic N-S devices that consist of a superconducting (S) Al wire with several normal metal (N) fingers contacting the superconductor via tunneling barriers. We measure the non-local voltage appearing at a N (detector) contact located outside the current path, while a bias current is driven from another N (injector) contact to the S wire. This non-local differential signal has been studied as a function of bias and temperature for several samples with different N-S contact transparencies. In addition to CAR, elastic co-tunneling (EC) and charge imbalance (CI) appears in the measurements. We ob-serve a systematic dependence of the relative magnitude of CAR, EC and CI. Most strikingly, CAR can dominate the subgap transport for all energies below the superconducting gap for a certain kind of sam-ple. If the tunneling resistance R$_t$ is increased, EC starts to dominate over CAR. This contribution is limited to small subgap biases, whereas CAR remains dominating at higher subgap biases. This de-pendence is explained by Coulomb blockade that becomes more prominent for increasing R$_t$. [Preview Abstract] |
Session W35: Superconducting Theory II
Sponsoring Units: DCMPChair: Michael Norman, Argonne National Laboratory
Room: 405
Thursday, March 19, 2009 11:15AM - 11:27AM |
W35.00001: Gap anisotropy and universal pairing scale in a spin fluctuation model for cuprates Artem Abanov, Andrey Chubukov, Michael Norman We consider the evolution of $d_{x^2-y^2}$ pairing, mediated by nearly critical spin fluctuations, with the coupling strength. We show that the onset temperature for pairing, $T^*$, smoothly evolves between weak and strong coupling, passing through a broad maximum at intermediate coupling. At strong coupling, $T^*$ is of order the magnetic exchange energy $J$. We argue that for all couplings, pairing is confined to the vicinity of the Fermi surface. We also find that thermal spin fluctuations only modestly reduce $T^*$, even at criticality, but they substantially smooth the gap anisotropy. The latter evolves with coupling, being the largest at weak coupling. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W35.00002: Magneto-oscillations in Underdoped Cuprates Chandra Varma The conventional interpretation of the recent magneto-oscillation experiments in underdoped Cuprates, requires that there be a state of altered translational symmetry in the pseudogap state which is not supported by structural and Angle Resolved Photoemission Spectroscopy (ARPES) experiments. I show that the observed oscillations may be reconciled with the conclusion arrived in ARPES experiments that the fermi-surface, suitably defined, has the shape of four arcs which shrink to four points as the temperature T approaches 0. Experiments, including infrared absorption in a magnetic field, are suggested to distinguish between such a state from that obtained by the conventional interpretation of the magneto-oscillations. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W35.00003: Phase Diagrams for Stripe Phases with a Spin gap Akbar Jaefari, Siddhartha Lal, Eduardo Fradkin We consider the problem of competing orders in a stripe phase with a large spin gap. In developing the phase diagram, we discuss the phases arising from the stabilization of the Superconducting (SC) and Charge Density Wave (CDW) orders by inter-stripe couplings. This is particularly relevant for stripe phases in High Temperature Superconductivity arising from the Josephson tunneling between neighboring stripes, and is essentially a problem of dimensional crossover. Using inter-chain Mean Field Theory, we present results for the gaps, critical temperatures, and critical exponents in terms of the inter-chain couplings and interaction parameters of the model. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W35.00004: Phase separation of electrons strongly coupled with phonons in cuprates and manganites Sasha Alexandrov Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron-electron correlations. I argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron-phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott-Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons. They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Froehlich EPI as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR and high and low-resistance phase coexistence near the ferromagnetic transition of doped manganites. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W35.00005: Coherent Lattice Vibrations, Kohn Anomalies, and Pseudogaps in Superconductors Alan M. Kadin A recent analysis has proposed [1] that the superconducting state is associated with charge density standing waves at k=2k$_{F}$, coupled to coherent lattice vibrations at 2k$_{F}$-G, where G is a reciprocal lattice vector. Independently, Aynajian et al. [2] have recently observed phonon spectral anomalies in Nb and Pb that correspond to Kohn anomalies in the Fermi surface, at energies matching the low T energy gap 2$\Delta $(0). Since Kohn anomalies are also defined by k=2k$_{F}$-G, these observations appear consistent with [1]. This also suggests that Kohn anomalies and an associated pseudogap provide a universal precursor of the superconducting state. Further experiments are proposed that should provide direct evidence of the coherent lattice vibrations in the superconducting state of conventional electron-phonon superconductors, and of alternative coherent oscillations (spin waves, etc.) in unconventional materials. \\[0pt] [1] A.M. Kadin, ``Coherent Lattice Vibrations in Superconductors'', Physica C 468, 255 (2008); http://arxiv.org/abs/0706.0338. \\[0pt] [2] P. Aynajian, et al., ``Energy gaps and Kohn anomalies in elemental superconductors'', Science 319, 1509 (2008); http://arxiv.org/abs/0808.1028. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W35.00006: Phase-fluctuations model for the pseudogap of high temperature superconductors Wonkee Kim, Yan Chen, C. S. Ting Within the phase fluctuation picture for the pseudogap state of a high-$T_{c}$ superconductor, we incorporate the phase fluctuations generated by the classical XY model with the Bogoliubov-de Gennes formalism utilizing a field-theoretical method. This picture delineates the essential characteristics of spatially varying local order parameters observed in high-$T_{c}$ superconductors above $T_{c}$. We also compute the local density of states near a non-magnetic impurity with a strong scattering potential. The resonance peak smoothly evolves as temperature increases through $T_{c}$ without showing any sudden broadening, which is consistent with recent experimental findings. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W35.00007: Antiferromagnetic to singlet transition in the quarter-filled band R.T. Clay, S. Mazumdar One of the greatest challenges in constructing a theory of superconductivity in the presence of strong electron-electron (e-e) interactions is to describe how a transition can occur from antiferromagnetic to singlet order. Transitions between antiferromagnetism (AFM) and singlet order are well known in several specific cases such as the spin-Peierls (SP) transition, dimerization in the presence of antiferromagnetic nearest neighbor and second neighbor couplings, and the rung-based singlet in the rectangular spin ladder. In all three examples, the transition is a consequence of confinement within a quasi-one-dimensional lattice. Similar AFM/singlet transitions have not been found in the two dimensional (2D) 1/2-filled band. We present evidence that an AFM/singlet transition can occur in a 2D 1/4-filled anisotropic triangular lattice. A key difference is that at 1/4 filling, inhomogeneity in the form of coexisting charge, bond, and spin orders occur due to e-e and electron-phonon interactions. We show that with increasing lattice frustration the ground state of the 1/4-filled band anisotropic triangular lattice changes from AFM to a charge-ordered state with local singlets. Our results have direct implications for the 1/4-filled organic superconductors as well as related inorganic materials such as Na$_x$CoO$_2$, LiTi$_2$O$_4$, CuRh$_2$S$_4$. Supported by DOE grant DE-FG02-06ER46315. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W35.00008: Collective Fluctuations of the Loop-Current Phase in Cuprates Arkady Shekhter, Vivek Aji, Chandra Varma We have calculated the collective modes of the loop-current ordered phase observed in underdoped Cuprates. Besides the modes given by the fluctuations of the Ashkin- Teller model, we find that the current fluctuations introduce a mode whose properties are similar to that of electro-magnetic vector potentials. We calculate the coupling of such a mode to the Ashkin-Teller modes and to the Fermions. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W35.00009: Superconductivity in Spin-Chain Ladder Cuprate Shigeru Koikegami, Takashi Yanagisawa We study the superconductivity in the three-dimensional d-p model with the quasi-one-dimensional structure in which CuO$_2$- chain and Cu$_2$O$_3$-ladder are alternately stacked with each other. When we control the hole density on each Cu site in our model, we have two or three Fermi surfaces, on which the fully- gapped superconductivity develops. Both the inter-band nesting and the large density of states around Van Hove singulality points play essential roles to achieve the superconductivity, and these two factors can coexist easily owing to the electron transfer between chain and ladder. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W35.00010: Superconducting Fluctuations in Strongly Correlated Electronic Systems William Putikka Superfluid behavior is relatively common in strongly correlated fermion systems. This suggests there is a common reason for this behavior rooted in the strong correlations. I propose such a mechanism, developed in the context of the 2D $t$-$J$ model, where $d_{x^2-y^2}$ superconducting fluctuations have recently been observed$^1$. The $d_{x^2-y^2}$ fluctuations are {\it not} due to antiferromagnetic fluctuations; the AF fluctuations compete with superconducting fluctuations. Pair fluctuations have their own, separate origin based in the strong correlations. If the on site repulsion is strong enough it can affect the electronic degrees of freedom while the entropy still dominates the free energy. This requires the entropy to be maximized under the constraint of no double occupancy, thereby rearranging the electronic degrees of freedom into separate spin and charge excitations. These excitations have different statistics and very different energy scales, allowing the charges to develop pair correlations before the spin degrees of freedom become coherent. Below the spin coherence temperature, the spins determine the symmetry of the pair wave function for the {\it electronic} pair fluctuations. The symmetry which best {\it avoids} the AF fluctuations on a square lattice is $d_{x^2-y^2}$.\\ 1. WO Putikka and MU Luchini, PRL{\bf 96}, 247001 (2006). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W35.00011: The quantum effective potential and the condensation of topological excitations in Josephson junction arrays. Said Sakhi I analyze the radiative corrections to the effective potential for an Abelian gauge theory relevant to Josephson junction arrays (JJA). This model consists of two disorder fields related to electric and magnetic charges coupled to topologically gauge fields described by Maxwell terms and a mixed Chern-Simons term. The symmetry of the ground state is studied through the effective potential which takes into account radiative corrections in the theory. Here zero condensates for the topological charge excitations describe [1] insulating phases of JJA, and nonzero condensates describe superconducting phases. The gauge fields contribution to the one-loop effective potential is evaluated and its effect on the spontaneous symmetry breaking is examined. Effects of dissipation driven coupling in JJA systems connected to a reservoir of gapless single-particle excitations are also studied. Coupling to gapless fermions is shown to induce radiative corrections in the effective potential which favor transitions between an insulating state and a superconducting state. [1] S. Sakhi, Europhys. Lett. 73 (2), 267 (2006). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W35.00012: Topological Confinement and Superconductivity Khaled Al-Hassanieh, Cristian Batista, Pinaki Sengupta, Adrian Feiguin We derive a Kondo Lattice model with a correlated conduction band from a two-band Hubbard Hamiltonian. This mapping allows us to describe the emergence of a robust pairing mechanism in a model that only contains repulsive interactions. The mechanism is due to topological con?nement and results from the interplay between antiferromagnetism and delocalization. By using Density-Matrix-Renormalization Group (DMRG) we demonstrate that this mechanism leads to dominant superconducting correlations in a 1D-system. [1] K. A. Al-Hassanieh, C. D. Batista, P. Sengupta, and A. E. Feiguin, preprint arXiv:0808.3735. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W35.00013: Vortex lattice structures of spin triplet superconductors Daniel Agterberg, Suk Bum Chung, Eun-Ah Kim Motivated by recent interest in spin triplet superconductors, we investigate the vortex lattice structures for this class of unconventional superconductors. We discuss how the order parameter symmetry can give rise to U(1)$\times$U(1) symmetry in same sense as in spinor condensates, making the half- quantum vortex (1/2-qv) topologically stable. We then calculate the vortex lattice structure of 1/2-qv's, with particular attention on the roles of the crystalline lattice, the Zeeman coupling, and Meissner screening, all absent in spinor condensates. Finally, we consider how spin-orbit coupling leads to a breakdown of the U(1) $\times$ U(1) symmetry and the fate of the 1/2-qv lattice. As examples, we consider models for spin-triplet superconductivity in Sr$_2 $RuO$_4$ and more speculative spin-triplet models for Na$_x$CoO$_2 \cdot y$H$_2$O and Bechgaard salts. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W35.00014: Strong coupling limit of superconductivity in anti-ferromagnetic phase: Extended hardcore boson picture of d-wave order and phase fluctuation Yucel Yildirim, Wei Ku Strong coupling limit (local-pairing $>>$ kinetic energy) of the superconductivity in High-$T_c$ superconductors is investigated within the anti-ferromagnetic phase. An extended hardcore boson picture consisting of paired holes results from general considerations of paired fermions, in which directional near-neighbor occupations of bosons are forbidden. By use of Wannier function of the low-energy sector of the bosonic Hilbert space, our simple picture provides a natural separation of the phase of the superconducting order parameter into local and external ones. Within a realistic parameter range, the local structure is found to be of d-wave symmetry, driven by the kinetic energy. On the other hand, the genuine behavior of the superconductivity is controlled by the phase coherence of the external phase, which leads to experimentally observed linear reduction of super fluid density. Interestingly, due to the competition with p-wave symmetry, the effective mass of the boson is enormously enhanced from that of the Fermion, explaining the very small stiffness. Connections to recent observation of 4-period d-wave CDW in the ``stripe'' phase, and C2-symmetry bond-centered charge profile will also be addressed. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W35.00015: A new method for solving the inhomogeneous Bogoliubov - de Gennes equations Lucian Covaci, Mona Berciu Inhomogeneities (surface, interfaces, impurities, etc.) in superconductors give rise to interesting phenomena, like broken time-reversal states, bound states near surfaces, etc. Numerical solutions of the self-consistent Bogoliubov-de Gennes mean field equations become computationally intensive for systems whose translational symmetry is broken. We propose a new method of solving the mean-field equations based on the Kernel Polynomial Method. We expand the Green's functions in terms of Chebyshev polynomials and calculate the order parameters self-consistently. Because the most expensive operation is only the sparse matrix-vector multiplication, the benefits of this method are multiple: usage of large systems, easy implementation of symmetries, multiple bands. Although we apply this method to a specific example (formation of Andreev states in 2D superconductors), it is applicable to any mean-field calculation. [Preview Abstract] |
Session W36: Nanoscale Complex Structures
Sponsoring Units: DCMPChair: Wei Pan, Sandia National Laboratories
Room: 408
Thursday, March 19, 2009 11:15AM - 11:27AM |
W36.00001: Conductance of a fully equilibrated quantum wire Tobias Micklitz, Jerome Rech, K. A. Matveev We study electronic transport properties of a long weakly interacting homogeneous quantum wire, connected to non-interacting leads. From Galilean invariance of the system we infer that in a state with a finite electric current, the electrons reach thermal equilibrium in a frame moving with their drift velocity. At non-zero temperature the resulting distribution function inside the wire is slightly different from the distribution supplied by the leads. This gives rise to a small correction to the quantized value of conductance $2e^2/h$, which can be found by performing a careful analysis of the conservation laws. The correction is of the order of $\left(T/E_F\right)^2$ and does not depend on the details of the electron-electron interaction. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W36.00002: Simulation of electron conduction in a prototypical three-terminal molecular transistor Haiying He, Ravindra Pandey, Shashi Karna In a single molecule, electronic charge can be modulated either by electrical field or by chemical effects, thereby opening up the possibility of their use as active elements in electronic devices. In this talk, we present the results of a theoretical study on the electronic conduction of a novel, three-terminal molecular architecture, analogous to a heterojunction bipolar transistor. In this architecture, two diode arms consisting of donor-acceptor molecular wires fuse through a ring, while a gate modulating wire is a $\pi $-conjugated wire. The calculated results show the enhancement or depletion mode of a transistor by applying a gate field along the positive or negative direction. A small gate field is required to switch on the current in the proposed architecture. The changes in the electronic conduction can be attributed to the intrinsic dipolar molecular architecture in terms of the evolution of molecular wavefunctions, specifically the one associated with the terphenyl group of the modulating wire in the presence of the gate field. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W36.00003: Synthesis and metal-to-semiconductor conversion of carbon nanotubes by light irradiation Lewis Gomez De Arco, Akshay Kumar, Yi Zhang, Koungmin Ryu, Alexander Badmaev, Chongwu Zhou We report on the synthesis of aligned nanotubes on Sapphire and quartz substrates, transfer, device fabrication and scalable metal-to-semiconductor conversion of carbon nanotubes on field-effect transistor (CNTFETs) channels by broadband light irradiation at environmental conditions. Inactivation of metallic nanotubes in the channels was achieved as a consequence of a light-assisted photochemical process that led to a controlled sp$^{2}$ to sp$^{3}$ transition in the nanotubes structure, and hence localization of $\pi $-electrons. Stronger gate bias dependence with improvements in the drain current On/Off ratio up to 10$^{5}$ was found in around 90 percent of the CNTFETs. The possibility of fabricating mostly semiconducting carbon nanotube transistors by simple light irradiation in air over entire wafers constitutes an important achievement in terms of assembly, integration and large scale fabrication of nanotube-based circuits. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W36.00004: Spectroscopic and structural studies of L-arginine doped Potassium Dihydrogen Phosphate crystals Jayesh Govani, Cristian Botez, William Durrer, Felicia Manciu We report in this study the spectroscopic and structural characterization of standard and L-arginine doped potassium dihydrogen phosphate crystals synthesized by a solution growth technique. The infrared absorption and Raman results demonstrate chemical functionalization between the amino (NH$_{3}^{+})$ groups of the organic material and the phosphate units of the inorganic crystals. This affirmation, which also implies the achievement of successful doping, is supported by the existence of extra vibrational lines in the IR and Raman spectra of L-arginine doped potassium dihydrogen phosphate crystals; these vibrational lines exhibit shifting towards lower frequencies as compared with the characteristic bands of L-arginine. Incorporation of the amino acid into the structure of the inorganic material is revealed by X-ray diffraction results also, where the shifting of diffraction lines and the appearance of a new one are observed. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W36.00005: Heterborane Analogs of Silicon clusters: Experimental and Theoretical Studies on Bi$_{2}$Si$_{n}$ Kiran Boggavarapu, Miley Jackson, Xiang Li, Andrej Grubisic, Di Wang, Kit Bowen, Anil Kandalam, Haopeng Wang Despite numerous studies, silicon clusters continue to fascinate. Part of the intrigue comes from the fact that, unlike metallic clusters which have strongly delocalized electrons and prefer to follow simple electron counting rules such as those originating from Jellium models, there are no simple rules of thumb that can be used to understand the diverse structures of silicon clusters. However, over the last couple of decades, there have been attempts to connect the structure and bonding of silicon clusters to a large class of well-studied three dimensional boron hydride compounds namely, \textit{closo-}boranes, B$_{n}$H$_{n}^{2-}$. By equating the $\sigma $-lone pair of divalent silicon to a B-H bond, it was shown that the frontier orbitals of both units are similar. Theoretical studies have concluded that the silicon clusters (Sin$^{2-})$ adopt similar structural patterns to those of boranes, when n = 5, 6, 7, 8, 10 and 13. The question then arises, whether neutral analogs of Si$_{n}^{2-}$ and neutral heteroboranes, X$_{2}$B$_{n}$H$_{n}$ (X = N, P, Sb, Bi), can be envisioned. Here, we present the scope and limitation such analogy based on our recent theoretical (DFT) and experimental (anion-photoelectron spectroscopy) studies on Bi$_{2}$Si$_{n}$ (n = 4-8). In particular, we show that that both Bi$_{2}$Si$_{5}$ and Bi$_{2}$B$_{5}$H$_{5}$ adopt similar pentagonal bipyrmidal (PBP) geometries and have analogous orbital energy patterns. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W36.00006: In situ characterization of crystal structure and physical properties of individual nanostructures in as-fabricated devices Marcel Lucas, Zhong Lin Wang, Elisa Riedo Nanostructures have potential applications as electronic components, catalysts, sensors, biomarkers, and energy harvesters. Control over their morphology and structure is essential, since their physical properties depend on their dimensions and crystallographic structure. Although in situ transmission electron microscopy can correlate the structure and physical properties of individual one-dimensional nanostructures, it usually damages the sample and is unable to recover the characterized nanostructure for next-step device fabrication and application. Here, we demonstrate a method combining atomic force microscopy and polarized Raman spectroscopy to characterize in situ the morphology, crystal structure and physical properties of individual nanostructures that can be as-fabricated devices without sample damage. Based on scanning probe microscopy, our method can be extended to study the electronic, mechanical, and tribological properties of inorganic/biological nanostructures. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W36.00007: Simulating self-assembly of porphyrin nanorods Gregory K. Guthe, Adam V. Subhas, Walter F. Smith, Joshua Schrier Diacid meso-tetra(4-sulfonatophenyl)porphine (TPPS$_{4}^{2-}$ ) monomers have been shown to self assemble into nanorods with well-defined cross-section$^{1}$ and intriguing photoelectronic properties$^{2}$. However, the structure and conduction mechanism of these nanorods is poorly understood, and questions remain about the aggregation process. Using density functional theory (DFT), we first obtain optimized geometries and atomic-charges for the monomers, which we then use for subsequent molecular dynamics (MD) simulations to observe the initial stages of the self-assembly process. This work uses the resources of the National Energy Research Scientific Computing Center. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W36.00008: Electric field directed growth of cuprous oxide nanostructures for photon sensing Sangeeta Sahoo, Saroj Nayak, Pulickel Ajayan We demonstrate an electro-deposition technique to synthesize cuprous oxide nanomaterials in various types of nanostructural form at room temperature. We apply an electric field under de-ionized water between two electrodes one of which is made of Cu. Using this method, direct growth of nanostructures has been achieved on different types of substrates. We show that the structural evolution depends strongly on the electric potential between the electrodes and also on the type of substrates. We have studied the growth mechanism on flat Si substrate and on Transmission electron Microscopy grid. A variety of structures from simple one dimensional nanowires to different complex two and three dimensional structures are successfully grown directly on substrates with this method. Direct integration of these nanostructures on Si substrate brings us one step ahead towards the fabrication of electronic devices. Taken together, this novel technique of Cu$_{2}$O nanostructure production is highly reproducible, catalysts free, fast and a low cost simple process. In addition, the electrical characteristics indicate the usefulness of these structures for photo-sensing and optoelectronic applications. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W36.00009: Ca$_{1+\varepsilon }$Co$_{4}$B$_{4}$ and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$: New Borides with One-Dimensional Channel Structures Yukari Katsura, Hiraku Ogino, Yutaka Matsumura, Kazumasa Sugiyama, Toetsu Shishido, Shigeru Horii, Jun-ichi Shimoyama, Kohji Kishio We discovered two novel borides Ca$_{1+\varepsilon }$Co$_{4}$B$_{4}$ (\textit{$\varepsilon $} $\sim $0.10) and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$ (\textit{$\varepsilon $} $\sim $0.18) as the first members of Gd$_{1+\varepsilon }$Fe$_{4}$B$_{4}$- and Pr$_{1+\varepsilon }$Re$_{4}$B$_{4}$-type ternary borides with a divalent metal at the rare earth sites. In these compounds, tetrahedral chains of transition metals and boron form tetragonal channel structures, which contain single atomic chains of Ca. These are composite structures of Ca sublattice and CoB/RuB sublattice, with common $a$-axis lengths and independent $c$-axis lengths. The two structural types are distinguished by configurations of the tetrahedral chains. Resistivity and magnetization measurements showed that these compounds are paramagnetic metals down to 2 K. Preliminary first-principle calculations indicated the presence of covalent bonds between transition metals and boron, and electrical conductivity originating from the $d$-bands of the transition metals. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W36.00010: Photoluminescence Studies of Hydrogenated a-Silicon Carbide Somilkumar Rathi, Feng Zhu, Joshua Gallon, Brian Simonds, George Radziszewski, P. Craig Taylor A series of PECVD grown silicon carbide samples differing in their carbon content was investigated by a near-IR FT-Photoluminescence (PL) technique. The goal of this study was to establish a correlation between carbon concentration in a-SiC and the observed luminescence signal. Variations in the observed temperature dependences of the PL signals in a-SiC are attributed to differences in the carbon content. The samples, initially kept at 18 K on a closed-cycle helium cryostat, were excited with Ar-ion laser light at 514.5 nm (with power ranging 5-30 mW; and a fluence of 0.5 to 3 W/cm$^{2})$, and the luminescence was recorded with an InGaAs detector over the range of 1.5 eV to 0.67 eV. There was no change in PL intensity below 50 K; above 50 K the intensity decayed logarithmically up to room temperature. The increase of carbon content caused an increase in the PL on the higher energy side of the emission peak (approximately centered at 1.29 eV; FWHH = 0.3 eV) with concurrent intensity decrease on the lower energy side. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W36.00011: Electrical transport in YSi$_{2}$ nanowires V. Iancu, P.R.C. Kent, T.-H. Kim, A.-P. Li, L.D. Menard, J.M. Ramsey, H.H. Weitering When a small amount of yttrium is deposited onto a gently heated Si(100)2x1 surface in ultrahigh vacuum, the yttrium atoms self-assemble into highly uniform silicide nanowires with dimensions of the order of 0.4x1.1x1000 nm$^{3}$. These YSi$_{2}$ nanowires are among the thinnest silicide structures fabricated to date. Their electrical properties have been explored using a four-probe scanning tunneling microscope (STM). The wires exhibit ohmic conductance at room temperature but the conductance decreases at lower temperature. STS measurements [1] indicated a small gap opening at low temperature in the thinnest YSi$_{2}$ wires, which appears to be associated with the charge-order fluctuations seen in STM. The YSi$_{2}$ nanowires not only represent an interesting model system for exploring 1D quantum transport, but they can also be used as electrodes or interconnects in nanoscale electronic devices on a silicon platform. The research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, U.S. DOE. [1] C. Zeng \textit{et al. }Nat. Mat. 7, 539 (2008) [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W36.00012: Gate-tunable magnetic exchange and giant g-factor fluctuations in InAs nanowire quantum dots Szabolcs Csonka, Lukas Hofstetter, Frank Freitag, Stefan Oberholzer, Christian Schonenberger, Thomas Sand Jespersen, Martin Aagesen, Jesper Nygard We use the spin-1/2 Kondo effect, which is observed in every other charge ground state with odd elec-trons, to measure the field-induced splitting of the spin-doublet, and hence the $g$- factor. We do this in hybrid quantum dots using both normal (N), ferromagnetic (F) and superconducting (S) contacts. Unlike to previous studies, the $g$-factors of neighboring states can vary a lot: $g$ can scatter between $2$ and $18$ and can therefore be even larger than in the bulk ($g \sim 15$). We demonstrate further the electric gate tunability of the $g$- factor in a single charge state. When using F contacts, a zero- field split-ting is induced. This proximity induced exchange field has recently been measured for the first time by Hauptmann et al. (Nature Physics \textbf{4}, (2008)) in carbon nanotubes. Here, we show the same ef-fect in a semiconducting nanowire, demonstrating that this effect is universal. Employing a pair of S and F contacts, the proximity-induced exchange shows up as a minigap in superconducting spectroscopy. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W36.00013: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W36.00014: First principles study of very thin TiO nanowires Oguz Gulseren, Deniz Cakir We have systematically investigated structural, electronic and magnetic properties of very thin TiO$_{x}$ ($x$=1,2) nanowires as well as bulk-like (110) rutile nanowires by using the first principles plane wave pseudopotential calculations based on density functional theory. A large number of different possible structures have been searched via total energy calculations in order to find the ground state structures of these nanowires. Three dimensional structures are more energetic than planar ones for both of the stoichiometries (i.e. $x$=1,2). The stability of TiO$_x$ nanowires enhances with its increasing radius, thus reaching sufficient coordination number of Ti and O atoms. All stoichiometric TiO$_{2}$ nanowires studied exhibit semiconducting behavior and have nonmagnetic ground state. There is a correlation between binding energy ($E_b$) and energy band gap ($E_g$) of TiO$_{2}$ nanowires. In general, $E_b$ increases with increasing $E_g$. In TiO nanowires, both metallic and semiconductor nanowires are resulted. In this case, in addition to paramagnetic TiO nanowires, there are also ferromagnetic ones. We have also studied the structural and electronic properties of bulk-like rutile (110) nanowires. There is a crossover in terms of energetics and bulk-like nanowires are more stable than the thin nanowires for larger radius wires after a critical diameter. These (110) rutile nanowires are all semiconductors. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W36.00015: Giant Flexoelectric Effect In Two-Dimensional Boron-Nitride Layers Ivan Naumov, Alexander Bratkovsky, Vivek Ranjan The direct conversion of ambient motion into electrical energy is a challenging fundamental and technological problem that is currently a focus of research. Boron-Nitride non-centrosymmetric monolayers are piezoelectrics that can sustain much larger structural and produce very large (a few Volts) voltage drop across flexed nanostrips. We show, by way of first-principles calculations, the existence of giant nonlinear flexoelectric effect in BN 2D strips. The induced polarization is quadratic in amplitude of atomic displacements $A$, yet the dipole moment per unit cell is about four times larger compared to PbZrTiO3 [1]. The new effect may find a variety of practical applications and, in particular, as nanogenerators and tactile sensors powered by an ambient motion or agitation. BN material is inert and can be used in biological environment.\\[3pt] [1] I.Naumov, A.Bratkovsky, V.Ranjan, arXiv:0810.1775 (2008). [Preview Abstract] |
Session W37: Theoretical Methods and Algorithms
Sponsoring Units: DCPChair: Mohammad Sahrapour, University of Illinois at Urbana-Champaign
Room: 409
Thursday, March 19, 2009 11:15AM - 11:27AM |
W37.00001: X-ray absorption spectra of ice and water: a first principles study with the GW method Xifan Wu, Wei Chen, Roberto Car We calculated the X-ray absorption spectra of ice and liquid water by adopting an approach based on the GW method to describe the excited electron in presence of a frozen core hole. We used the static Coulomb-hole and screened exchange approximation for the self-energy and used Maximally Localized Wannier functions to make GW calculations feasible in the large supercell needed to model a disordered system like water. The calculated spectra considerably improve the agreement with experiment, compared with previous DFT calculations. In particular, the three main features observed in experiments are well reproduced in terms of position and intensity for both ice and water. We also find that the difference between the ice and water spectra can be understood in terms of the electronic structures of these systems, manifested by a distorted, tetrahedral hydrogen bond network in the liquid. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W37.00002: Iterative Monte Carlo for Quantum Dynamics Vikram Jadhao, Nancy Makri We present a fully quantum mechanical methodology for calculating complex-time correlation functions by evaluating the discretized path integral expression iteratively on a grid selected by a Monte Carlo procedure [1]. Both the grid points and the summations performed in each iteration utilize importance sampling, leading to favorable scaling with the number of particles, while the stepwise evaluation of the integrals circumvents the exponential growth of statistical error with time.\newline [1] V. Jadhao and N. Makri J.Chem.Phys. 129, 161102 (2008) [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W37.00003: Resonating Valence Bond wave function with molecular orbitals: first application to dimers Sandro Sorella, Sam Azadi, Mariapia Marchi, Michele Casula We introduce a method for accurate quantum chemical calculations based on a single determinant wave function, the Antisymmetrized Geminal Power (AGP), and a real space correlation factor (the so called Jastrow factor), that can be efficiently sampled by means of standard quantum Monte Carlo techniques. This allows to obtain a very accurate description of the chemical bond even in extremely difficult cases (such as $Be_2$, $N_2$ and $C_2$) where strong dynamical correlations and/or weak vdW interactions are present. The method is based on a constrained variational optimization, obtained with an appropriate number $n$ of molecular orbitals in the AGP wavefunction. It is shown that the most relevant dynamical correlations are correctly reproduced, once $n$ is univocally determined by the requirement to have size consistent results upon atomization to correlated Hartree-Fock Slater determinants in presence of the Jastrow factor. We apply this method to the Iron dimer molecule and obtain an accurate description of the ground state energy and excitations of this molecule, which is compatible with the experimental findings. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W37.00004: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W37.00005: Representing quantum environments Michael Zwolak Understanding dissipative and decohering processes is fundamental to the study of non-equilibrium systems and quantum computing, and such processes can even induce quantum phase transitions. A typical construction is to have a system connected to a continuum environment, which acts as the source of dissipation or decoherence, or as a reservoir of particles. If the connection is strong or the environment has long- range correlations in time, the system dynamics are not easily separated from the dynamics of the environment. To study this situation numerically, one option is to simulate both the system and environment. This is a viable option so long as an efficient finite representation of the environment can be constructed. We will discuss recent results on constructing finite representations of environments for use in computational simulations. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W37.00006: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W37.00007: Application of the Finite-Element Space-Time Algorithm to Bound States Charles Weatherford, Albert Wynn, Daniel Gebremedhin, Xingjun Zhang The implementation of the Dirac representation is facilitated by the finite element space-time algorithm.[1] Multicenter integral computations are also facilitated by this same algorithm. The present work is the first application of this original algorithm to the computation of bound states of atoms and molecules. The Dirac representation is employed such that H$_0$ is the sum of the one-electron operators while the residual H$_1$ is the sum of the two-electron operators. Soft-Coulomb geminals are then used as the basis for the time-dependent calculation of a superposition of the bound-states. The eigenstates and eigenvalues are then extracted by filter-diagonalization. An addition theorem is given for the soft-coulomb geminals and the geminals are translated again using the space-time algorithm, so that multicenter integrals may be computed. Several small atoms and molecules are considered as an illustration of the method. [1]D.H. Gebremedhin, C.A. Weatherford, X. Zhang, A. Wynn III, and G. Tanaka, ``Evaluation of the matrix exponential function using finite elements in time,'' arXiv:0811.2612v1 [math-ph] 17 Nov 2008. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W37.00008: Test of Current Variational Procedures for Electronic Structures and Properties of Molecular and Solid State Systems by application to Atomic Systems-H$^{- }$Ion Hari Paudel, Archana Dubey, R.H. Scheicher, S.R. Badu, R.H. Pink, T.P. Das Electronic properties of atomic systems are obtainable using Linked Cluster Many-Body Perturbation Theory(LCMBPT) with high accuracy and excellent agreement with experiment, using complete sets of states obtained by differential equation procedures [1,2]. Unfortunately such procedures are not practicable for multi-center molecular and solid state problems and variational procedures have to be used for obtaining the occupied and excited one electron states to work on electronic properties of the latter systems. With the aim to assess the accuracies of the latter procedures with Gaussian basis states, like the first principles Hartree-Fock procedure together with many body perturbation theory, and density functional based procedures, we are testing them for both energy and wave function dependent properties of atoms. Results will be presented for H$^{-}$ ion, where Hartree-Fock theory predicts instability with respect to auto ionization to H atom and electron correlation effects obtained by the LCMBPT procedure [3] restore stability, providing nearly exact experimental affinity for H$^{-}$.[1] Alfred Owusu et al., Phys. Rev. A\underline {56}, 305(1997) [2] T.Lee et al., Phys. Rev. A4 1410(1971) [3]C.M. Dutta et al., Phys. Rev. A2, 2289(1970) [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W37.00009: Computation of Nonlinear Impedance Spectra in Samaria Doped Ceria Francesco Ciucci Samarium Doped Ceria (SDC) electrodes are currently of great interest for solid oxide fuel cells (SOFC) applications. For example, ceria-containing anodes can be operated directly on hydrocarbons without coking, and in addition can be used at lower temperatures than Ni/YSZ. In order to design, optimize, and characterize electrodes, it is very useful to have models to aid in interpreting experimental results. In this work, we present a non-linear, time-dependent model for the study of SDC. This model allows us to compute species concentrations, electric potential and currents under medium bias conditions. A regular perturbation of the drift diffusion equations and Poisson's equation is used to derive the model for the behavior of bulk of the material. We also include the kinetics of reactions occurring at the SDC-gas surface where the SDC is exposed to a spatially uniform hydrogen-water-argon mixture at fixed total pressure. The numerical procedure allows for fast computations and for the direct determination of fast and rate limiting steps. Impedance spectra are computed in the 2D case and a quantitative comparison between experimental (symmetric cell) and numerical results is presented. Our model can be naturally extended to the non-symmetric case, i.e. the case under which the two sides of the SDC assembly are exposed to different atmospheres. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W37.00010: An Analytical Approach to Computing Biomolecular Electrostatic Potential Andrew Fenley, John Gordon, Alexey Onufriev Analytical approximations to fundamental equations of continuum electrostatics on simple shapes can lead to computationally inexpensive prescriptions for calculating electrostatic properties of realistic molecules. Here, we derive a closed form, analytical approximation to the Poisson equation for an arbitrary distribution of point charges and a spherical dielectric boundary. The simple, parameter-free formula defines continuous electrostatic potential everywhere in space and is obtained from the exact infinite series (Kirkwood) solution by an approximate summation method that avoids truncating the infinite series. We show that keeping all the terms proves critical for the accuracy of this approximation, which is fully controllable for the sphere. We apply the approximation to 580 biomolecules under realistic solvation conditions, where the effects of mobile ions are included at the Debye-H\"{u}ckel level. The accuracy of the approximation as applied to the biomolecules is assessed through comparisons with numerical Poisson-Boltzmann (NPB) reference solutions. For each structure, the deviation from the reference is computed for a large number of test points placed near the dielectric boundary (molecular surface). The accuracy of the approximation is within 1 $kT$ per unit charge for 91.5\% of the individual test points. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W37.00011: {\em Ab Initio} Study of Atomic and Molecular Polarizabi-lities Igor Vasiliev, James R. Chelikowsky We calculate the static electric dipole polarizabilities for a variety of atoms and molecules using a finite field method implemented in the framework of an {\it ab initio} density functional formalism. Our calculations employ several different representations of the exchange-correlation potential, including the local density approximation, generalized gradient approximation, and asymptotically correct functionals introduced by Leeuwen-Baerends [1] and Casida-Salahub [2]. We observe that the computed values of polarizabilities are strongly influenced by the asymptotic behavior of the density functional exchange-correlation potential. The accuracy of theoretical atomic and molecular polarizabilities is substantially improved by the use of asymptotically correct exchange-correlation functionals. This result can be explained in terms of electronic excitation energies and the polarizability sum rule.\\[0pt] [1] R. van Leeuwen and E. J. Baerends, Phys. Rev. A 49, 2421 (1994).\\[0pt] [2] M. E. Casida and D. R. Salahub, J. Chem. Phys. 113, 8918 (2000). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W37.00012: Theoretical predictions of the impact of nuclear dynamics and environment on core-level spectra of organic molecules David Prendergast, Craig Schwartz, Janel Uejio, Richard Saykally Core-level spectroscopy provides an element-specific probe of local electronic structure and bonding, but linking details of atomic structure to measured spectra relies heavily on accurate theoretical interpretation. We present first principles simulations of the x-ray absorption of a range of organic molecules both in isolation and aqueous solvation, highlighting the spectral impact of internal nuclear motion as well as solvent interactions. Our approach uses density functional theory with explicit inclusion of the core-level excited state within a plane-wave supercell framework. Nuclear degrees of freedom are sampled using various molecular dynamics techniques. We indicate specific cases for molecules in their vibrational ground state at experimental conditions, where nuclear quantum effects must be included. Prepared by LBNL under Contract DE-AC02-05CH11231. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W37.00013: Efficient free energy calculations of quantum systems through computer simulations Alex Antonelli, Rafael Ramirez, Carlos Herrero, Eduardo Hernandez In general, the classical limit is assumed in computer simulation calculations of free energy. This approximation, however, is not justifiable for a class of systems in which quantum contributions for the free energy cannot be neglected. The inclusion of quantum effects is important for the determination of reliable phase diagrams of these systems. In this work, we present a new methodology to compute the free energy of many-body quantum systems [1]. This methodology results from the combination of the path integral formulation of statistical mechanics and efficient non-equilibrium methods to estimate free energy, namely, the adiabatic switching and reversible scaling methods. A quantum Einstein crystal is used as a model to show the accuracy and reliability the methodology. This new method is applied to the calculation of solid-liquid coexistence properties of neon. Our findings indicate that quantum contributions to properties such as, melting point, latent heat of fusion, entropy of fusion, and slope of melting line can be up to 10\% of the calculated values using the classical approximation. \noindent [1] R. M. Ramirez, C. P. Herrero, A. Antonelli, and E. R. Hern$\rm \acute{a}$ndez, Journal of Chemical Physics \textbf{129}, 064110 (2008) [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W37.00014: Applications of a novel QM/MM method incorporating a polarizable force field. Christopher Williams, John Herbert In conventional QM/MM methods the MM region is modeled by a force field that uses a set of point charges to represent the electrostatics. However, recently developed force fields use multipole expansions combined with polarizable sites to to represent electrostatic interactions. A novel algorithm is presented to interface this class of force fields with a QM region by allowing the QM region and the MM region to polarize each other self-consistently. It is implemented using the QChem electronic structure code and the AMOEBA force field as implemented in the software package TINKER. The algorithm is general and can be used with a variety of QM methods including MP2 and DFT. Examples of both ground state and excited state calculations are presented, including the investigation of the effectiveness of many-body expansions in modeling the solvation of charged species and the effect of charged environments on biomolecules. [Preview Abstract] |
Session W38: Focus Session: Ion Channel Physics and Chemical Physics I
Sponsoring Units: DCPChair: Rob Coalson, University of Pittsburgh
Room: 410
Thursday, March 19, 2009 11:15AM - 11:51AM |
W38.00001: Numerical modeling of fast gate-coupled ion permeation in ClC channels Invited Speaker: We have developed a three dimensional Brownian dynamics (BD) and discrete-state model to couple ion permeation to the motion of a putative fast gate in the ClC channels. The model channel is designed so as to represent certain essential features of ClC chloride channel, in which a glutamate side chain moves from an open state to a closed state (blocking the channel pore at a position which also acts as a binding site for Cl$^{-}$ ions moving through the channel). Both BD and the discrete-state model generate results in qualitative agreement with experimental observations and consistent with the foot-in-the-door mechanism. Furthermore, we have formulated a numerical approach to calculate the discrete rate constants in our model channel using BD. The discrete-state model with the rate constants solved via BD produces results consistent with the (continuous space) multi-ion BD simulations. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W38.00002: Modeling Ion Solvation and Transport through Chloride Transport Proteins Invited Speaker: Ion channels and transporters are membrane proteins that selectively conduct ions either passively (channels) or actively using a chemical gradient of one ion (transporters). The H+/Cl- transporter, first discovered in bacteria, has now been shown to also occur in animals and plants. This talk will discuss computational approaches aimed at understanding the ion transit pathways through the bacterial chloride transporter. A Monte Carlo method (TransPath) that uses the crystal structure as input and exhaustively searches the protein for open pore spaces and favorable electrostatic domains has been developed. The algorithm successfully predicted pathways for the motion of chloride ions and protons. In order to better understanding the free energies along the predicted transport pathways, we have developed a new statistical mechanical approach for computing absolute solvation free energies in restricted environments based on a quasi-chemical approach. Anion free energy results employing the new method will be presented. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W38.00003: Snug-fit, fluctuations, and metal-ion hydration in the selectivity of potassium ion channels Invited Speaker: On the basis of molecular simulation, an identification of a single dominating physical factor responsible for Na+/K+ selectivity of the KcsA channel has been contentious. The potential distribution theorem and quasi-chemical theory cast new light on the factors responsible for Na+/K+ selectivity. In that context, we argue that an alternative definite formulation of the molecular statistical thermodynamic problem can help in achieving a concensus view of selectivity. We summarize the necessary new theoretical ingredients and published numerical results in working toward that concensus view. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W38.00004: Conformation changes in the Glutamate receptor as studied by LRET Invited Speaker: Glutamate receptors are the primary mediators of excitatory neurotransmission in the mammalian central nervous system. Glutamate binding to an extracellular ligand binding domain initiates a series of conformational changes that results in the formation of cation selective transmembrane ion channels that ultimately desensitize. We have used luminescence resonance energy transfer to determine the conformational changes that underlie the allosteric process of glutamate mediated gating in the receptor. These investigations showed that agonist binding induced cleft closure in the ligand binding domain confirming that this change observed in the isolated ligand binding domain of the receptor is one of the mechanisms by which agonist mediates activation. The LRET investigations also allowed a study of the conformational changes between the subunits. The apo state of the protein showed a dimer interface that was open. The dimer interface was brought together only in the activated state, suggesting that cleft closure drives the formation of the contacts at dimer interface, which in turn transiently stabilizes the open channel. At longer times, the stress induced by the transmembrane segments, ultimately drives the breakdown of the interface, leading to channel closure and receptor desensitization. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W38.00005: Electric and Molecular Characteristics of Ion Channels Richard DeSantis A galvanic cell's electrolyte is an insulator. A conductive electrolyte would quickly discharge a dry cell's voltage. Voltage-producing paths within an electrolyte can develop spontaneously. A voltage-producing path must bridge from the anode to the cathode, to export voltage out of the insulating electrolyte. Doubling cathode to anode distance does not decrease the cell's fixed output voltage. The fixed voltage indicates superconductor-like behavior. Gaps between voltage-producing molecules would isolate the anode from the cathode, preventing superconductor-like behavior. Gating activity within membrane protein complexes can prevent or allow voltage-producing paths. A voltage-producing path is a single molecule containing both anode and cathode reactants. Only combined anode-cathode reactions within a single molecule can produce the cell's fixed exterior voltage. While within the single molecule, atoms can relocate and react. In a lead acid cell, charging voltage raises the molecule's energy state. The extra energy allows Pb and PbO2 to form during the molecule's collapse. For discharging, an external circuit provides an outlet for the molecule's voltage, which lowers the molecule's energy state to a level that permits PbSO4 production. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W38.00006: Coulombic dragging of molecular assemblies on nanotubes Petr Kral, Kyaw Sint, Boyang Wang We show by molecular dynamics simulations that polar molecules, ions and their assemblies could be Coulombically dragged on the surfaces of single-wall carbon and boron-nitride nanotubes by ionic solutions or individual ions moving inside the nanotubes [1,2]. We also briefly discuss highly selective ionic sieves based on graphene monolayers with nanopores [3]. These phenomena could be applied in molecular delivery, separation and desalination.\\[3pt] [1] Boyang Wang and Petr Kral, JACS 128, 15984 (2006). \\[0pt] [2] Boyang Wang and Petr Kral, Phys. Rev. Lett. 101, 046103 (2008). \\[0pt] [3] Kyaw Sint, Boyang Wang and Petr Kral, JACS, ASAP (2008). [Preview Abstract] |
Session W39: Quantitative Biology
Sponsoring Units: DBPChair: Michael Deem, Rice University
Room: 411
Thursday, March 19, 2009 11:15AM - 11:27AM |
W39.00001: Synchrony in embryogenesis via an active medium R. Scott McIsaac, Anirvan Sengupta, Ned Wingreen In developing embryos of the frog \emph{Xenopus}, mitotic divisions occur at 8-minute intervals. After the initial rounds of division, nuclei divide in near-perfect synchrony throughout the embryo. Given a typical protein diffusion constant of $10 \frac{\mu m^2}{sec}$, and an embryo length of $\approx 1mm$, it would take diffusion many hours to propagate a signal across the embryo. Therefore, synchrony cannot be attained by diffusion alone. We hypothesize that known autocatalytic reactions of cell-cycle components make the embryo an ``active medium'' in which waves propagate much faster than diffusion, enforcing synchrony. Furthermore, developing embryos are found to be very robust, meaning that their spatial and temporal patterns are highly repeatable over a broad range of environmental conditions and despite biochemical noise. We report on robust synchronization of oscillations for a coupled two-species system consisting of diffusing activator and repressor molecules. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W39.00002: Cell migration on ridges and cliffs Meghan Driscoll, Colin McCann, Rael Kopace, John Watts, Tess Homan, Wolfgang Losert The amoeba \textit{Dictyostelium discoideum} is a model system for the study of cellular migration, an important physiological process that occurs in embryonic development, wound healing, and cancer metastasis. We study the motion of \textit{D. discoideum} on surfaces with various topographies, particularly those that affect the direction of cellular migration. Topographical features, such as ridges and cliffs, were fabricated using multiphoton absorption polymerization. As the cells encountered these topographical features, we tracked their overall motions and shapes, as well as the locations and intensities of certain intracellular signals. We found that when cells undergoing chemokinesis, random migration in response to a chemical signal, encounter a ridge, they tend to move along that ridge, even if the ridge is shorter than the cell. When cells undergoing chemotaxis, directed migration in response to a chemical signal, are directed off of a cliff, they do not fall off the cliff. Instead, they search for new attachment points, eventually change direction, and continue moving along the edge of the cliff. Both ridges and cliffs affect more than just the motion of a cell; they also affect its shape. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W39.00003: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W39.00004: Mutational robustness emerges in a microscopic model of protein evolution Konstantin Zeldovich, Eugene Shakhnovich The ability to absorb mutations while retaining structure and function, or mutational robustness, is a remarkable property of natural proteins. We use a computational model of organismic evolution [Zeldovich et al, PLOS Comp Biol 3(7):e139 (2007)], which explicitly couples protein physics and population dynamics, to study mutational robustness of evolved model proteins. We compare evolved sequences with the ones designed to fold into the same native structures and having the same thermodynamic stability, and find that evolved sequences are more robust against point mutations, being less likely to be destabilized, and more likely to increase stability upon a point mutation. These results point to sequence evolution as an important method of protein engineering if mutational robustness of the artificially developed proteins is desired. On the biological side, mutational robustness of proteins appears to be a natural consequence of the divergence-mutation- selection evolutionary process. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W39.00005: Kinetics of Slow Axonal Transport and Shape of Axon Peter Jung, Chen Ying, Yinyun Li, Anthony Brown The mechanical integrity of the axon in mature axons is provided by neurofilaments(NF). NFs move through the axon at the average slow rate of $0.5$mm/day, characterized by bursts of movement and extended pauses in between. The local number of NFs determines the local axonal caliber and as a result, the kinetics of NF movement determines the overall shape of the axon. We developed a kinetic model for movement of NFs based on live cell-imaging (J Neurosci. 2007,27:507, Mol Biol Cell. 2005, 16:4243). We use this model to predict changes in axonal morphology upon local modifications of the kinetics by e.g. factors released by myelin. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W39.00006: Group behavior in cell migration Wolfgang Losert, Carole Parent, Colin McCann Cell migration up an external chemical gradient is a crucial element in many biological processes, such as embryogenesis and cancer metastasis. The aim of our study is to quantify chemotaxis of groups of cells. We find that at high cell densities (i.e. low cell-cell distances) cells migrate together in streams either spontaneously or in response to an externally applied chemical gradient. Analysis of cell tracks outside and within streams shows that cells do not speed up or slow down when moving as a group. In addition the persistence of motion appears unaffected by the formation of streams. At large cell-cell distances cells do not form streams in response to externally applied chemical gradients, and fewer cells move. At very low cell plating density cells are unable to respond to a chemical signal, even close to the signal source. We confirm that this lack of motion is not due to signal relay. Our results indicate that a quorum sensing mechanism exists which is closely coupled to chemotaxis. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W39.00007: Effect of Recombination in the Evolutionary Dynamics of HIV under the Surveillance of Immune System Weiqun Peng, Wenjing Yang, Guanyu Wang Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS), which has become one of the most destructive pandemics in history. The fact that HIV virus evolves very fast plays a central role in AIDS immunopathogenesis and the difficulty we face in finding a cure or a vaccine for AIDS. A distinguishing feature of HIV is its high frequency of recombination. The effect of recombination in the HIV evolution is not clear. We establish a mathematical model of the evolutionary dynamics. This model incorporates both point mutation and recombination for genetic diversity, and employs a fitness function developed by Wang and Deem (PRL 97, 188106, 2006) that accounts for the effect of immune system. Using this model, we explore the role of recombination in the battle between the virus population and the immune system, with a special focus on the condition under which recombination helps the virus population to escape from the immune system. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W39.00008: A collective mechanism for phase variation in biofilms Nicholas Chia, Carl Woese, Nigel Goldenfeld Understanding how microbes gather into biofilm communities and maintain diversity remains one of the central questions of microbiology, requiring an understanding of microbes as communal rather then individual organisms. Phase variation plays an integral role in the formation of diverse phenotypes within biofilms. We propose a collective mechanism for phase variation based on gene transfer agents, and apply the theory to predict the population structure and growth dynamics of a biofilm. Our results describe quantitatively recent experiments, with the only adjustable parameter being the rate of intercellular horizontal gene transfer. Our approach derives from a more general picture for the emergence of cooperation between microbes. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W39.00009: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W39.00010: $\lambda$-prophage induction modeled as a cooperative failure mode of lytic repression Nicholas Chia, Ido Golding, Nigel Goldenfeld We analyze a system-level model for lytic repression of $\lambda$-phage in {\it E. coli\/} using reliability theory, showing that the repressor circuit comprises 4 redundant components whose failure mode is prophage induction. Our model reflects the specific biochemical mechanisms involved in regulation, including long-range cooperative binding, and its detailed predictions for prophage induction in {\it E. coli\/} under ultra-violet radiation are in good agreement with experimental data. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W39.00011: Statistical Physics of Vaccine Design Michael Deem I will define a new parameter to quantify the antigenic distance between two H3N2 influenza strains. I will use this parameter to measure antigenic distance between circulating H3N2 strains and the closest vaccine component of the influenza vaccine. For the data between 1971 and 2004, the measure of antigenic distance correlates better with efficacy in humans of the H3N2 influenza A annual vaccine than do current state of the art measures of antigenic distance such as phylogenetic sequence analysis or ferret antisera inhibition assays. I suggest that this measure of antigenic distance can be used to guide the design of the annual flu vaccine. I will describe combining this measure of antigenic distance with a multiple-strain avian influenza transmission model to study the threat of simultaneous introduction of multiple avian influenza strains. For H3N2 influenza, the model is validated against observed viral fixation rates and epidemic progression rates from the World Health Organization FluNet - Global Influenza Surveillance Network. I find that a multiple-component avian influenza vaccine is helpful to control a simultaneous multiple introduction of bird-flu strains. I introduce Population at Risk (PaR) to quantify the risk of a flu pandemic, and calculate by this metric the improvement that a multiple vaccine offers. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W39.00012: Spatial coordination in memrane proximal signaling in T-cells Maxim N. Artyomov, Mieszko Lis, Arup Chakraborty Membrane-proximal signaling initiates signaling networks of the T-cell which ultimately lead to the T-cell activation. Signal formation requires assembly of the several membrane proteins and successful cooperative interactions inside the complex. Diffusion and chemical reactions involved in the process are characterized by substantially different timescales. In this work we consider how the reaction-diffusion system described by the wide spectrum of timescales can be selective for the minute amounts of the signal (cognate peptide-MHC complex) over the large amounts of irrelevant targets (non-cognate peptide-MHC complex). Note that single distinction between relevant and irrelevant targets - the affinity to the T-cell receptor, is nonetheless sufficient to discriminate between two groups of targets. Moreover, proposed mechanism allows for signal cooperativity with non-cognate peptides amplifying the signal from cognate ones even though they can not signal by themselves. This kind of cooperativity has been observed in recent experiments. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W39.00013: Digital signaling, signal filters and central tolerance in thymocytes Ashok Prasad, Julie Zikherman, Jayajit Das, Jeroen Roose, Arthur Weiss, Arup Chakraborty T cells are characterized by the immense diversity of the antigen binding receptors (TCR's) they bear. TCR's carried by immature T cells (thymocytes) are made in the thymus by a stochastic process, followed by testing against self-peptides. Thymocytes that do not respond to self-peptides die through neglect (positive selection); those that respond too strongly die through apoptosis (negative selection). We present a new molecular explanation of this phenomenon via a computational model, which we also test by experiments. We show that Ras activation in thymocytes is characterized by the presence of a digital molecular switch due to a positive feedback loop in a Ras-activating enzyme. We also show how an important adaptor protein, LAT, acts as a filter, sending weak TCR signals along a pathway that leads to Ras activation via a graded mechanism, and sending stronger signals along another path that activates Ras via the molecular switch. Our model yields a new mechanism for digital signaling of the Erk protein in mammalian cells, and has important implications for autoimmunity. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W39.00014: A biophysical model of prokaryotic diversity in geothermal hot springs Suzanne Amador Kane, Anna Klales, James Duncan, Elizabeth Janus Nett Photosynthetic bacteria living in geothermal hot spring environments have surprisingly complex ecosystems with an unexpected level of genetic diversity. In particular, their thermal gradients support genetically distinct bacterial strains that differ in their preferred temperatures for reproduction and photosynthesis. Each region along the thermal gradient exhibits multiple strains of photosynthetic bacteria adapted to several distinct thermal optima, rather than the expected single thermal strain adapted to the local environmental temperature. Here we analyze microbiology data from several ecological studies to show that the thermal distribution field data exhibit several universal features independent of location and specific bacterial strain. These include the distribution of optimal temperatures of different thermal strains and the functional dependence of the net population density on temperature. We present a simple population dynamics model of these systems that explains the observed diversity of different strains of the photosynthetic bacteria, the observed thermal population distributions and certain features of population dynamics observed in laboratory studies of the same organisms. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W39.00015: Understanding Original Antigenic Sin with a Dynamical System Keyao Pan, Michael Deem Original antigenic sin is the phenomenon in which prior exposure to an antigen leads to a subsequent suboptimal immune response to a related antigen. Immune memory normally allows for an improved and rapid response to antigens and is the mechanism by which vaccination works. We here develop a dynamical system model of the mechanism of original antigenic sin, clarifying and explaining the detailed spin-glass treatment of original antigenic sin [1]. The dynamical system describes the virus load as it propagates through healthy and infected cells, the naive and memory B cell concentrations, and the affinity of the immune response. Explicit correspondences between the microscopic variables of the spin-glass model and the dynamical system model will be given. The dynamical system model reproduces the phenomenon of original antigenic sin, and describes how competition between different B-cells compromises the overall effect of the immune system. The trade off between the naive and memory immune responses as a function of antigenic distance between the initial and subsequent antigens is displayed. A suboptimal immune response, the original antigenic sin, is observed for intermediate antigenic distances. [1] Deem MW, Lee H-Y. Sequence space localization in the immune system response to vaccination and disease. Phys Rev Lett 2003;91:068101. [Preview Abstract] |
Session W40: Single Molecule Biophysics
Sponsoring Units: DBPChair: David Lederman, University of West Virginia
Room: 412
Thursday, March 19, 2009 11:15AM - 11:27AM |
W40.00001: Crowding Effects on the Unfolding of Ubiquitin David Pincus, Devarajan Thirumalai Using a coarse-grained representation of polypeptide chains, we probed the mechanical stability of Ubiquitin (Ub) monomers and trimers ((Ub)$_3$) in the presence of monodisperse spherical crowding agents. Our findings indicate that crowding increases the volume fraction ($\Phi_c$)-dependent average force ($\langle f_u(\Phi_c) \rangle$), relative to the value at $\Phi_c = 0$, needed to unfold Ub and the polyprotein. Furthermore, we found that average unfolding forces increase with decreasing crowder diameter ($\sigma_c$). The average unfolding force $\langle f_u(\Phi_c) \rangle$ depends on the ratio $\frac{D}{R_g}$, where $D \approx \sigma_c (\frac{\pi}{6 \Phi_c})^{\frac{1}{3}}$ with $R_g$ being the radius of gyration of Ub (or (Ub)$_3$) in the unfolded state. Examination of the unfolding pathways shows that, relative to $\Phi_c = 0$, crowding promotes reassociation of ruptured secondary structural elements. Both the nature of the unfolding pathways and $\langle f_u(\Phi_c) \rangle$ for (Ub)$_3$ are altered in the presence of crowding particles with the effect being most dramatic for the subunit that unfolds last. We predict that $\langle f_u(\Phi_c) \rangle$ scales in a simple manner with $\Phi_c$. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W40.00002: Electron Transfer in Myoglobin-based Single-Electron Transistors Debin Li, Peter Gannett, David Lederman The mechanism of electron transfer by myoglobin was investigated using nanometer-gap platinum electrodes fabricated by breaking a small junction by electromigration at cryogenic temperatures. The experimental results suggest single electron transport behavior is mediated by resonance of the electronic levels of the heme group in a single myoglobin protein. Evidence for a two-step electron tunneling process, resulting from the structural relaxation of the protein with the addition of a single electron, was observed. Our experimental results show that the slow protein relaxation may result in resonant tunneling and the fast protein relaxation is the condition of two-step resonant tunneling behavior. The conformation and orientation of myoglobin in the gap of electrodes may significantly affect the conductance of these devices. The calculation for the conductance graph as a function of gate voltage and bias voltage was performed with the rate equations for electron tunneling via discrete quantum states and considering the two-step process. The results of calculation match those of our experiment. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W40.00003: Surface-enhanced photocycle studied in a single photoreceptor protein molecule Kaan Kalkan, Aihua Xie Photoactive yellow protein (PYP) functions as a blue light sensor for bacterial vision (phototaxis). The photocycle of PYP is initiated by the absorption of a blue photon (absorption peak at 446 nm) by its para-coumaric acid (pCA) chromophore. The photon energy is stored in the pCA through photoisomerization which is subsequently transferred to the rest of the protein through a series of conformational states, finally leading to its partial unfolding (signaling). The present work captures the distinct conformational changes of PYP at the single molecule level, during the execution of its photocycle. In particular, the present work employs surface-enhanced Raman scattering (SERS) active substrates and non-resonant excitation at 514 nm. As we confirm with regular Raman spectroscopy, the photocycle of PYP cannot be excited under 514 nm irradiation. On the contrary, 514 nm photons can excite the photocycle when PYP is adsorbed on silver, as we evidence from single molecule as well as ensemble-averaged SERS. In this case, the optical absorption of PYP shows a dramatic broadening (full width at half maximum shifting from 0.4 to 0.9 eV) such that electronic excitation can occur significantly at 514 nm. Therefore, the origin of the observed ``surface-enhanced photocycle'' is understood to be of the same as ``chemical enhancement'' in SERS in view of the ``adsorbate-induced resonance states'' model (Persson, 1993). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W40.00004: Rigidity effects and mechanical unfolding of proteins Oleg Vorov, Dennis Livesay, Donald Jacobs We describe a new method that shows promise for evaluating the partition function for a protein under an applied external force within a Distance Constraint Model (DCM). This approach is based on an approximate account for the rigidity effects due to hydrogen bond crosslinking using Maxwell constraint counting. Within a mean-field treatment, the free energy is estimated accurately over an ensemble of accessible conformations conditional upon the breaking of various weakest-link distance constraints, as they successively break due to a series of mini structural transitions. These calculations are performed using an exact transfer matrix approach combined with a combinatorial partitioning of the structure into different parts based on separating lines of unfolding pathways. The various shortest paths over an ensemble of structures that ``crack'' open in different ways are used to obtain the appropriate Boltzmann weight, related to the work done by the external pulling force. For structures with beta-hairpin geometry, all permutations of unfolding pathways are enumerated exactly. For a simple minimal DCM, results for extension-force curves agree markedly well with experiment. Using computational methods, this approach can be used to describe single-molecule experiments on mechanical protein unfolding under different settings, such as fixed extension, or constant force conditions. This work is supported by NIH R01 GM073082. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W40.00005: Physical Principles of Virus Templating through Single Molecule Dynamic Force Spectroscopy Raymond Friddle, Selim Elhadj, George Gilmer, Aleksandr Noy, James De Yoreo The use of macromolecular scaffolds for hierarchical organization of molecules and materials is a common strategy in living systems that leads to emergent behavior. One characteristic of this strategy is that it generates micron-scale structures from nm-scale building blocks, possessing high-density functionality, defined at angstrom-scales by active sites; a typical example being viral capsids. We are systematically determining the physical variables necessary to consistently pattern virus particles on to nanoscale templates. This presentation will focus on our theoretical and experimental findings regarding our Dynamic force spectroscopy (DFS) measurements; a technique in which fundamental parameters related to interaction potentials can be determined. Here we present a novel theory for determining kinetic desorption rates and equilibrium free energies using DFS in which two well-defined states exist. We compare the results with force spectra measured between individual MS2 virions and chemically modified AFM tips. We also investigate the effects of solution additives, such as PEG, on microscopic kinetics and free energies. Finally, we discuss the relation of single-molecule measurements with the ensemble, and show a connection between the two in the case of bimolecular dissociation. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W40.00006: Probing Protein Folding Kinetics with High-resolution, Stabilized Optical Tweezers Wesley Wong, Ken Halvorsen Single-molecule techniques provide a powerful means of exploring molecular transitions such as the unfolding and refolding of a protein. However, the quantification of bi-directional transitions and near-equilibrium phenomena poses unique challenges, and is often limited by the detection resolution and long-term stability of the instrument. We have developed unique optical tweezers methods that address these problems, including an interference-based method for high-resolution 3D bead tracking ($\sim$1 nm laterally, $\sim$0.3 nm vertically, at $>$ 100 Hz), and a continuous autofocus system that stabilizes the trap height to within 1-2 nm longterm [1,2]. We have used our instruments to quantify the force-dependent unfolding and refolding kinetics of single protein domains (e.g. spectrin in collaboration with E. Evans). These single-molecule studies are presented, together with the accompanying probabilistic analysis that we have developed. References: 1. W.P. Wong, V. Heinrich, E. Evans, Mat. Res. Soc. Symp. Proc., 790, P5.1-P5.10 (2004). 2. V. Heinrich, W.P. Wong, K. Halvorsen, E. Evans, Langmuir, 24, 1194-1203 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W40.00007: Electrostatic signatures of single protein dynamics for detection with carbon nanotube sensors G. Schneider, L. Prisbrey, E. Minot Observing single molecule dynamics in real time at atomic resolution is crucial to study enzyme function, which is closely linked to the intrinsic dynamics of the enzyme and molecular interactions between enzyme and substrate. At present, techniques such as nuclear magnetic resonance (NMR) and single molecule fluorescence energy transfer (FRET) are used together with computer simulations to study single molecule dynamics. Recent progress in point-functionalization of single wall carbon nanotubes\footnote{B. R. Goldsmith et al, Science 315, 77-81 (2007).} (CNT) opens up the possibility of electronic detection of single molecule dynamics.\footnote{B. R. Goldsmith et al, Nano Lett 8, 189-194 (2008).} CNTs are ideal candidates for electronic sensing of single protein dynamics. Typical CNT diameters are 1-2 nm, comparable to both the size of proteins in solution and the electrostatic screening length in physiological solutions. CNT sensors based on point defects have potential advantages over FRET including better time resolution. We report results for the electrostatic signature of several proteins in solution, both in substrate free and bound forms, and discuss the potential for electronic detection of biologically relevant single protein dynamics using functionalized carbon nanotubes. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W40.00008: Experimental Investigation of the Velocity Convergence of Jarzynski's Equality Using Single-Molecule AFM Pulling of Titin I27 Nolan Harris, Ching-Hwa Kiang Single molecule atomic force microscopy (AFM) of individual biomolecules allows one to observe high energy conformations and transitions between equilibrium states that are not otherwise observable. Jarzynski's equality has been used to extract important equilibrium information, such as free energy surfaces, from these nonequilibrium AFM measurements. However, the convergence behavior of Jarzynski's equality, i.e. the number of AFM trajectories required to adequately sample the nonequilibrium work distribution of a process, depends nontrivially on the AFM pulling schedule. Here we study the velocity dependent nature of Jarzynski's equality in AFM experiments. We reconstructed the free energy surfaces for the forced unfolding of the I27 domain of human cardiac titin via AFM using different pulling velocities. We found that the number of experimental trajectories required for convergence of Jarzynski's equality increases roughly exponentially as experimental pulling velocity is increased. We suggest optimal pulling velocities for pulling titin I27 and discuss the obstacles involved with using extreme pulling velocities. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W40.00009: Real Time Single Molecule Imaging of Protein-Surface Interactions Shannon Kian G. Zareh, Shawn H. DeCenzo, Y.M. Wang We study the dynamics of the adsorption of protein to surfaces using real time Total Internal Reflection Fluorescence microscopy (TIRF). We have observed two mechanisms responsible for protein adsorption on surfaces: Reversible and Irreversible binding. The irreversible binding occurs on the deposition step induced by the initial deposition flow, and the reversible binding is the equilibrium binding between the proteins and the surfaces. Our study has shown that the irreversible binding is the main contribution to the surface adsorption of proteins. We will discuss the energy for GFP and fused-silica surface interaction, and also a method to prevent protein adsorption onto surfaces. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W40.00010: Single molecule image deconvolution. I. Standard deviation analysis of immobile fluorescent molecules Michael C. DeSantis, Shawn H. DeCenzo, Y.M. Wang Single molecule fluorescence imaging has been a powerful technique in studying individual processes not accessible by bulk, ensemble-averaged measurements [1]. Improvements in image analysis are required for high temporal and spatial precision in the localization of single fluorescent molecules. We present the first thorough standard deviation analysis for point spread functions (PSFs) of single immobile fluorescent molecules. Using this new single molecule image deconvolution (SMID) method, we show that 3D localization of individual molecules with sub-nanometer precision can be achieved. We have derived an expression estimating the standard error of the PSF's standard deviation, incorporating experimental effects of the number of collected photons, finite pixel size, and background noise. The localization precision obtained via this expression is approximately 1.5 times better than the current available methods. The use of SMID to extract subexposure dynamics of mobile molecules will also be discussed.\\ $[1]$. Wang, Y. M, R. H, Austin, \& Cox, E. C. 2006 \textit{Physical Review Letters} \textbf{97}, 048302(1-4). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W40.00011: Monte Carlo simulation of single-molecule trapping via electrophoresis William Robinson, Lloyd Davis For many biophysical studies, there is a need to observe a molecule for an extended duration without immobilizing it on a surface. The problem of trapping a single fluorescent molecule in solution is examined here via Monte Carlo numerical simulation.~ Optical forces are insufficient for trapping small molecules. Instead, trapping is executed by sensing the position and applying real-time feedback of flow to compensate diffusional displacement. Using a nanochannel as the volume of interest reduces the problem to one dimension, and with such a configuration the position of the molecule can be measured from its fluorescence in the presence of a two-focus irradiance pattern.~ The collected photons are analyzed by an algorithm developed for a field-programmable gate array controller for experimental implementation, and an electrophoretic flow provides the trapping mechanism.~ Trapping is also possible in three dimensions with two-photon excitation of the molecule from a four-focus irradiance pattern arranged as a tetrahedron or with a single focus scanning over a three-dimensional volume. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W40.00012: Sub-diffraction limit differentiation of single fluorophores using Single Molecule Image Deconvolution (SMID) Shawn H. DeCenzo, Michael C. DeSantis, Y. M. Wang In order to better understand biological systems, researchers demand new techniques and improvements in single molecule differentiation. We present a unique approach utilizing an analysis of the standard deviation of the Gaussian point spread function of single immobile fluorescent molecules. This technique, Single Molecule Image Deconvolution (SMID), is applicable to standard TIRF instrumentation and standard fluorophores. We demonstrate the method by measuring the separation of two Cy3 molecules attached to the ends of short double-stranded DNA immobilized on a surface without photobleaching. Preliminary results and further applications will be presented. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W40.00013: Selection of optimal variants of Go-like models of proteins through studies of stretching Joanna Sulkowska, Marek Cieplak The Go-like models of proteins are constructed based on the knowledge of the native conformation. However, there are many possible choices of a Hamiltonian for which the ground state coincides with the native state. Here, we propose to use experimental data on protein stretching to determine what choices are most adequate physically. This criterion is motivated by the fact that stretching processes usually start with the native structure, in the vicinity of which the Go-like models should work the best. Our selection procedure is applied to 62 different versions of the Go model and is based on 28 proteins. We consider different potentials, contact maps, local stiffness energies, and energy scales -- uniform and non-uniform. In the latter case, the strength of the nonuniformity was governed either by specificity or by properties related to positioning of the side groups. Among them there is the simplest variant: uniform couplings and no$ i,i+2 $contacts. This choice also leads to good folding properties in most cases. We elucidate relationship between the local stiffness described by a potential which involves local chirality and the one which involves dihedral and bond angles. The latter stiffness improves folding but there is little difference between them when it comes to stretching. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W40.00014: Discontinuities at the DNA supercoiling transition Bryan Daniels, Scott Forth, Maxim Sheinin, Michelle Wang, James Sethna While slowly turning the ends of a single molecule of DNA at constant applied force, a discontinuity was recently observed at the supercoiling transition, when a small plectoneme is suddenly formed. This can be understood as an abrupt transition into a phase in which stretched and plectonemic DNA coexist. We argue that there should be discontinuities in both the extension and the torque at the transition, and provide experimental evidence for both. To predict the sizes of these discontinuities and how they change with the overall length of DNA, we organize a theory for the coexisting plectonemic state in terms of four length-independent parameters. We also test plectoneme theories, including our own elastic rod simulation, finding discrepancies with experiment that can be understood in terms of the four coexisting state parameters. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W40.00015: Pressure-driven single-file transport of DNA molecules along linear arrays of nanopits embedded in a slit-like nanochannel. Jackson Del Bonis-O'Donnell, Walter Reisner, Anders Kristensen, Derek Stein Due to the growth in nanobiofluidic technology for DNA manipulation and analysis there is growing interest in understanding the physics of DNA in nanoconfined environments. Using fluorescence video microscopy we study the transport of DNA in slit-like nanochannels with an embedded nanotopology consisting of linear arrays of nanopit structures. The nanopit structures are made via a two level fabrication process: (1) An ebeam lithography and etching step to make the nanopits followed by (2) a photolithography step to fabricate the slit. Under an applied pressure drop the DNA molecules are observed to move single-file down the nanopit array undergoing sequential pit-to-pit hops. We make systematic measurements of pressure dependent nanopit velocity. We observe two distinct transport regimes depending on whether the molecule configuration can occupy a single pit or must subtend multiple pits. We interpret our results in terms of a simple scaling picture of the free energy of chains in the linear array. [Preview Abstract] |
Session W41: Kondo Physics; Dynamical Mean-Field Theory
Sponsoring Units: DCMPChair: Chris Marianetti, Columbia University
Room: 413
Thursday, March 19, 2009 11:15AM - 11:27AM |
W41.00001: The 2d Kondo effect in p-type Quantum Wells in GaAs Theodore Castner Two groups [Huang et al. [1] and Hamilton et al. [2]] have observed minima in the resistivity $\rho$(T) at very low T well below $\rho$$_{max}$ at T$_{max}$ in metallic samples (p$>$p$_{c}$). Minima in [1] were found at 32 mK for p$\sim$ 2.1 p$_{c}$, 25 mK for p$\sim$ 1.76 p$_{c}$, while no minimum was observed down to 0.5 mK for p$\sim$ 1.24 p$_{c}$. Both groups have interpreted their results in terms of a crossover to insulating (nonmetallic) behavior for T $<$ T$_{m}$ where d$\rho$/dT $<$ 0. An alternative explanation arises from a 2d Kondo effect. Using the Hamann function [3] for the magnetic scattering contribution from localized magnetic moments and a term $\rho$(0)CT/T$_{F}$ from screening (interactions) one obtains an expression for T$_{m}$(p) which is very close to the Kondo temperature T$_{K}$(p) given by (E$_{F}$/k)x exp(-1/N(E$_{F}$absJ). The very strong p-dependence of T$_{m}$(p) and T$_{K}$(p) is dominated by N(E$_{F}$) which is shown to approach zero as p approaches p$_{c}$ because of the pseudogap in the DOS. Good agreement is obtained T$_{m}$(p) for both [1] and [2]. The data in [1] supports metallic behavior down to 0.5 mK. The implications for a true 2d MIT as a QPT are discussed. 1) J. Huang et al. Phys.Rev.Lett.98, 226801 (2007); 2) A. R. Hamilton et al. Phys.Rev.Lett.82, 1542 (1999); D. R. Hamann, Phys.Rev.158, 570 (1967). [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W41.00002: Quantum phase transition in the XY-anisotropic Bose-Fermi Kondo model Mengxing Cheng, Kevin Ingersent Using the numerical renormalization group, we study the quantum phase transition induced by dissipation in the XY-anisotropic Bose-Fermi Kondo model for a spin-one-half magnetic impurity coupled both to the on-site spin of a conduction electron band and, via its x and y spin components, to a bath of vector bosons. We focus on the case of a sub-Ohmic bath characterized by a power-law spectral exponent $s < 1$. Upon increasing the coupling of the impurity to the bosonic bath (at fixed fermionic coupling), the system exhibits a continuous quantum phase transition from a Kondo-screened ground state to a state in which the impurity moment is localized by the dissipation. We probe the quantum-critical behavior in the vicinity of this transition through the calculation of critical exponents describing the static and dynamical response to a local magnetic field both at absolute zero and at finite temperatures. Critical comparisons are made with analytical renormalization-group results obtained previously through expansion around the Ohmic case $s = 1$. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W41.00003: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W41.00004: Validity of Equation-of-Motion Approach to Kondo Problem in the Large N limit Yunong Qi, Jian-Xin Zhu, Chin-Sen Ting The Anderson impurity model for Kondo problem is investigated for arbitrary orbit-spin degeneracy $N$ of the magnetic impurity by the equation of motion method (EOM). By employing a new decoupling scheme, a set self-consistent equations for the one-particle Green function are derived and numerically solved in the large-$N$ approximation. For the particle-hole symmetric Anderson model with finite Coulomb interaction $U$, we show that the Kondo resonance at the impurity site exists for all $N \geq 2$. The approach removes the pathology in the standard EOM for $N=2$, and has the same level of applicability as non-crossing approximation. For $N=2$, an exchange field splits the Kondo resonance into only two peaks, consist with the result from more rigorous numerical renormalization group (NRG) method. The temperature dependence of the Kondo resonance peak is also discussed. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W41.00005: Evolution of a Large Fermi Surface in the Kondo Lattice Junya Otsuki, Hiroaki Kusunose, Yoshio Kuramoto We investigate the Kondo lattice model with use of the continuous-time quantum Monte Carlo method, combined with the dynamical mean-field theory. The antiferromagnetic phase diagram is determined from a divergence of the corresponding susceptibility [1]. In the paramagnetic phase, crossover behavior is traced quantitatively to a heavy Fermi-liquid state from the local-moment state at high temperatures [2]. The momentum distribution in the low-temperature limit acquires a discontinuity at the location that involves the local-spin degrees of freedom. Even without the charge degrees of freedom for local electrons, the excitation spectra exhibit hybridized bands similar to those in the Anderson lattice. Temperature dependence in the zero-energy component of the self-energy is crucial in forming the Fermi-liquid state with the large Fermi surface. [1] J. Otsuki, H. Kusunose and Y. Kuramoto, arXiv:0808.3829; arXiv:0811.1102. [2] J. Otsuki, H. Kusunose and Y. Kuramoto, arXiv:0811.2613 to appear in Phys. Rev. Lett. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W41.00006: A linear in $\beta$ solver for Cluster Dynamical Mean Field Theory Ehsan Khatami, Mark Jarrell, Che-Rung Lee, Richard Scalettar We develop a Quantum Monte Carlo (QMC) cluster solver for the Dynamical Cluster Approximation (DCA) which scales linearly in the inverse temperature, $\beta$, and has the same minus sign problem as conventional methods. Determinantal QMC (DQMC) used in this method is modified by adding non-interacting bands to mimic the coupling to the host. The DCA hybridization function is fitted to the non-interacting band parameters. We prove that the sign problem has the same statistics as in the Hirsch-Fye (HF) algorithm in the limit of a large number of bath bands ($N_{\alpha}$). Whereas the HFQMC scales as $\beta^3$, this DQMC-based method scales linearly in $\beta$. We demonstrate rapid convergence of the sign to the HF result for different cluster sizes and model parameters as $N_{\alpha}$ increases. We also present results for the convergence of other physical quantities to their HFQMC counterparts. This method can be used to solve other embedded cluster problems including those in Dynamical Mean Field Theory (DMFT), and cellular DMFT. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W41.00007: The relationship between Hirsch-Fye and weak coupling diagrammatic Quantum Monte Carlo methods Karlis Mikelsons, Alexandru Macridin, Mark Jarrell Two weak coupling Continuous Time Quantum Monte Carlo (CTQMC) methods are shown to be equivalent for Hubbard-like interactions. A relation between these CTQMC methods and the Hirsch-Fye Quantum Monte Carlo (HFQMC) method is established, identifying the latter as an approximation within CTQMC and providing a diagrammatic interpretation of HFQMC. Both HFQMC and CTQMC are shown to be equivalent when the number of time slices in HFQMC becomes infinite, implying the same degree of fermion sign problem in this limit. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W41.00008: Cluster Dynamical Mean Field Theory of the Mott Transition Hyowon Park, Kristjan Haule, Gabriel Kotliar We address the nature of the Mott transition in the Hubbard model at half-filling using cluster dynamical mean field theory (DMFT). We compare cluster DMFT results with those of single site DMFT. We show that inclusion of the short range correlations on top of the on-site correlations does not change the order of the transition between the paramagnetic metal and the paramagnetic Mott insulator, which remains first order. However, the short range correlations reduce substantially the critical $U$ and modify the shape of the transition lines. Moreover, they lead to very different physical properties of the metallic and insulating phases near the transition point. Approaching the transition from the metallic side, we find an anomalous metallic state with very low coherence scale. The insulating state is characterized by the narrow Mott gap with pronounced peaks at the gap edge. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W41.00009: Nearly frozen Coulomb liquids Yohanes Pramudya, Sergey Pankov, Efstratios Manousakis, Vladimir Dobrosavljevic We show that very long range repulsive interactions of a generalized Coulomb-like form \textbf{\textit{V(R) $\sim $ R}}$^{-a}$, with \textbf{\textit{a $<$ d}} (\textbf{\textit{d}}-spatial dimensionality), typically introduce very strong frustration, resulting in extreme fragility of any charge-ordered state. An ``almost frozen'' liquid then survives in a broad dynamical range above the (very low) melting temperature \textbf{\textit{T}}$_{c}$. Using a combination of extended dynamical mean-field theory [1] and Monte Carlo simulations we study classical lattice gas models with such long range interaction, focusing on the behavior at \textbf{\textit{T $>$ T}}$_{c}$. We find that a soft, temperature-dependent pseudo-gap emerges in this regime, reflecting strong short-range correlations that persist above the melting temperature. This ``pseudo-gap'' phase is characterized by unusual insulating-like, but very weakly temperature dependent transport, similar to experimental findings [2] in many low carrier density systems. [1] S.Pankov and V. Dobrosavljevic, Phys. Rev. Lett.\textbf{ 94}, 046402 (2005). [2] K. Lai, W. Pan, D. C. Tsui, S. Lyon, M. Muhlberger, and F. Schaffler, Phys. Rev. B \textbf{75}, 033314 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W41.00010: Quantum criticality out of equilibrium: Thermopower and shot noise in a ferromagnetic single electron transistor Jedediah Pixley, Stefan Kirchner, Qimiao Si The low-energy properties of a single electron transistor attached to ferromagnetic leads are described by the Bose-Fermi Anderson model. This model can undergo a continuous T=0 dissipative phase transition and it was shown in [1] that the transistor can be tuned through this quantum phase transition. The out-of-equilibrium scaling properties near the quantum critical point and in the adjacent phases of the spin and charge response and their fluctuation-dissipation ratios were recently studied in [2]. In this contribution, we study the thermopower and shot noise, which probe the quantum criticality in a way that goes beyond the current-voltage characteristics. The thermoelectric properties of a quantum dot in the Kondo regime can be directly measured [3]. Bulk thermopower measurements of heavy fermion compounds near their quantum critical point provide valuable information about the Fermi surface [4] and are related to our work through the Extended Dynamical Mean Field Theory that maps the Kondo lattice onto a Bose-Fermi Kondo model augmented by a self-consistency condition. [1] S. Kirchner et al., PNAS 102, 18824 (2005). [2] S. Kirchner and Q. Si, arXiv:0805.3717 (2008). [3] R. Scheibner et al, PRL 95, 176602 (2005). [4] S. Hartmann et al, to be published (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W41.00011: Mottness scenario for non-Fermi liquid behavior in the periodic Anderson model within dynamical mean-field theory Giovanni Sordi, Adriano Amaricci, Marcelo Rozenberg We study the Mott metal-insulator transition in the periodic Anderson model within dynamical mean-field Theory (DMFT). Near the quantum transition, we find a non-Fermi liquid metallic state down to a vanishing temperature scale. We identify the origin of the non-Fermi liquid behavior as due to magnetic scattering of the doped carriers by the localized moments. The non-Fermi liquid state can be tuned by either doping or external magnetic field. Our results show that the coupling to spatial magnetic fluctuations (absent in DMFT) is not a prerequisite to realize a non-Fermi liquid scenario for heavy fermion systems. Refs: {\sl Phys. Rev. Lett.} {\bf 99}, 196403 (2007); {\sl Phys. Rev. Lett.} {\bf 101}, 146403 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W41.00012: Percolation transition in nanowire magnetorheological fluids Josh Karli, Darin Zimmerman, Joseph Filer, Richard Bell, Norman Wereley We measure the yield stress of magnetorheological (MR) fluids that employ cobalt nanowires as the ferromagnetic component and observe a percolation transition in the yield stress at a critical value of the cobalt-nanowire volume fraction, $p_c $. The critical volume fraction depends not only on the particle size and aspect ratio (as expected) but also on the external magnetic field applied to the MR-fluid sample. We fit the yield-stress data using McLachlan's generalized effective medium (GEM) model to determine $p_c $ and the percolation exponents $s$ and $t$ that describe the transition behavior below and above $p_c $, respectively. The phase transition from low- to high-yield stress at low magnetic-particle volume fraction ($<1\% )$ has potential application to the development of precision magnetic sensors and actuators. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W41.00013: Anderson localization transition in thin films of gadolinium R. Misra, A.F. Hebard, K.A. Muttalib, P. Woelfle \textit{In situ} temperature-dependent transport studies have been performed on a series of gadolinium (Gd) films deposited onto sapphire substrates having sheet resistance $R_{0}$~$\equiv $~$R_{xx}$(5K) varying over the range 4011~$\Omega $ ($\sim $35{\AA}) to 132 K$\Omega $ ($<$ 20 {\AA}). The disorder strength, as measured by $R_{0}$, is sufficiently high so that quantum corrections to the classical Boltzmann conductivity are no longer observed. In this region of moderately strong disorder, we find a temperature-dependent conductivity of the form \textit{$\sigma $}($T)$~ = \textit{A~+BT}$^{p}$ where $A$ and $B$ are disorder-dependent constants and $p$ is a power with value 0.4. We find that $A$ is proportional to (1-$R_{0}/R_{c})^{s}$ where the conductivity exponent $s~$=1 and the critical resistance $R_{c }$= 22.7 k$\Omega $. This change in sign of $A$ with unity exponent at critical disorder describes the critical regime of an Anderson localization transition[1] with the temperature-dependent localization length sufficiently small so that the Gd films can be considered to be in the 3D regime, rather than the 2D regime where metallic behavior does not occur [2]. [1] Lee {\&} Ramakrishnan, RMP 57, 287 (1985); Belitz {\&} Kirkpatrick, RMP 66, 261 (1994) [2] Abrahams, Anderson, Licciardello {\&} Ramakrishnan, PRL 42, 673 (1979) [Preview Abstract] |
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