Session H1: Spin-Triplet Supercurrents in Superconductor/Ferromagnet/Superconductor Josephson Junctions

Odd Triplet superconductivity in Superconductor-Ferromagnet hybrid structures

Since the prediction of a long-range triplet component of the superconducting condensate in ferromagnetic-superconductor (S/F) proximity structures in 2001 [1], the activity in the field has increased considerably, both theoretically and experimentally The coexistence of conventional singlet superconductivity and ferromagnetism in S/F structures is closely related with the appearence of a triplet component of the condensate, which is odd in frequency and even in momentum and therefore insensitive to nonmagnetic impurities. The presence of the triplet component leads to new effects as for example, long-range Josephson coupling in SFS junctions [1], flow of a supercurrent through a half-metallic link [2] and screening of the magnetic moment of ferromagnetic particles embbeded in a superconductor [3]. In this talk I will review the main issues of the odd-triplet superconductivity, its manifestation in physical properties, and briefly discuss the relevant experiments in the field [4]. \\[4pt] [1] F.S. Bergeret, A. F. Volkov and K. B. Efetov, Phys. Rev. Lett. 86, 4096 (2001); see also by the same authors Rev. Mod. Phys. 77, 1321 (2005).\\[0pt] [2] M. Eschrig and T. L\"ofwander, Nature Phys. 4, 138 (2008).\\[0pt] [3] F. S. Bergeret, A. F. Volkov and K. B. Efetov, Phys. Rev. B 69, 174504 (2004).\\[0pt] [4] R. S. Keizer et al, Nature 439, 825 (2006); T. S. Khaire et al., Phys. Rev. Lett. 104, 137002 (2010); J. W. A. Robinson, J. D. S. Witt and M. G. Blamire, Science 329, 59 (2010), Jian Wang et al, Nature Phys. 6, 389 (2010).   

Proximity effect-induced superconductivity in crystalline metallic and ferromagnetic nanowires

In a single crystal gold nanowire of 1.2 microns contacted by superconducting contacts, the proximity effect induced superconductivity was found to appear in two distinct steps. The superconducting and normal regions are separated by a mini-gap state of low critical field. We suggest that a superposition of two distinct magnetic-flux states, which correspond to quantum flux 0 and 1 trapped in the nanowire, can explain the mini-gap state. Furthermore, we observed clear periodic differential magnetoresistance oscillations in the superconducting to normal transition region, which corresponds to the generation or annihilation of one vortex. In crystalline ferromagnetic Co and Ni nanowires, unexpected long-range proximity effect was observed. Josephson current associated with weakly damped singlet superconducting correlations or triplet correlations produced by the contact regions may lead to the observed long ranged proximity effect. In addition, a large and sharp resistance peak around the transition temperature was observed in the wires exhibiting incomplete superconductivity. Further theoretical model needs to be developed to reveal the physics behind the ``peak effect.''   

Observation of spin-triplet supercurrent in Co-based Josephson junctions

When a superconductor (S) and a ferromagnet (F) are put into contact with each other, the combined S/F system may exhibit altogether new properties. There is a proximity effect where pair correlations from S penetrate into F, but these correlations decay over a very short distance due to the large exchange splitting between the spin-up and spin-down electron bands in F. Theory predicts that, under certain conditions, electron pair correlations can be generated with spin-triplet rather than spin-singlet symmetry [1]. The two electrons in such a spin-triplet pair have parallel spins and are not subject to the exchange splitting in F; hence they propagate long distances. We have measured a long-range supercurrent in Josephson junctions of the form S/X/N/SAF/N/X/S, where S is a superconductor (Nb), N is a normal metal (Cu), SAF is a synthetic antiferromagnet of the form Co/Ru/Co, and X is a thin ferromagnetic layer necessary to induce spin-triplet correlations in the structure [2]. Spin-triplet correlations are generated due to non-collinearity of the magnetizations in each X layer and the nearest Co layer. Using X = PdNi, CuNi, and Ni, we observe enhancements of the critical current of up to 300 times relative to similar samples lacking the X layers. We also observe a large additional enhancement of the spin-triplet supercurrent after the samples are magnetized in a large field. This result is counter-intuitive, since one would expect magnetizing the samples to suppress the occurrence of non-collinear magnetization. We will present a model of the SAF magnetization structure that explains these intriguing results. \\[4pt] [1] F.S. Bergeret, A.F. Volkov, and K.B. Efetov, Phys. Rev. Lett., 86, 4096 (2001).\\[0pt] [2] T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., N.O. Birge, Phys. Rev. Lett. 104, 137002 (2010).   

Triplet supercurrents in ferromagnets

In almost all superconductors the pairs of electrons which carry the charge are in the so-called singlet state in which the quantum spin of the two electrons is antiparallel. During the past five years there has been increasing evidence that proximity coupling between singlet superconductors and ferromagnets can sometimes generate triplet pairs within the ferromagnet in which the spins of the electrons are parallel rather than antiparallel -- the evidence being that supercurrents can be passed through thicknesses of ferromagnetic material which are simply too large for singlet pairs to survive. The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. For strong ferromagnets such as Co, a thickness of only a few nanometres is sufficient to almost completely suppress the critical current of a Nb/Co/Nb Josephson junction. Here we report experiments in which a conical magnet (holmium) is placed at the interface between the superconductor and ferromagnet. The results showed that a long-ranged supercurrent can occur through the ferromagnetic Co layer but only for certain critical thicknesses of the Ho [1]. These thicknesses correspond to maximum magnetic inhomogeneity on the Ho and are therefore consistent with models which predict that a spin-mixing interface between the superconductor and ferromagnet can generate triplet pairs which are long-ranged in the ferromagnet. This paper reports recent experiments which aim to understand further the behaviour of triplet pairs in superconductor / ferromagnet heterostructures. \\[4pt] [1] J. W. A. Robinson, J. D. S. Witt, and M. G. Blamire, ``Controlled Injection of Spin-Triplet Supercurrents into a Strong Ferromagnet'' Science \textbf{329}, 59-61 (2010).   

Long-ranged supercurrents through half-metallic ferromagnetic CrO$_2$

In the last few years, the scenery in the physics of superconductor/ ferromagnet hybrids has changed considerably with the realization that spin triplets may be induced in the ferromagnet through the mechanism of odd-frequency pairing. Since the equal-spin component of the triplet is not susceptible to pair breaking by the exchange field, such correlations can sustain supercurrents over long lengths, in particular in fully spin polarized materials where only one spin band is available. In halfmetallic ferromagnetic CrO$_2$ for instance, where superconducting contacts were deposited on top of the ferromagnetic films, we observed the current to flow over 700~nm at 4.2~K [1]. Still, we also have fabricated devices where the supercurrent is absent, which indicates that the mechanism of triplet generation is not yet well in hand. The presence of non-homogeneous magnetization is important, and here the grain structure of the film appears to play a key role, as can be illustrated with data for films grown on different substrates (TiO$_2$ and Al$_2$O$_3$). Moreover, recent data will be presented which suggest that triplet generation can be improved by using an additional ferromagnetic layer in the contact area. \\ \newline [1] M. S. Anwar, F. Czeschka, M. Hesselberth, M. Porcu, and J. Aarts, Phys. Rev. B {\bf 82}, 100501(R) (2010).   

Session H2: New Materials for Spin Quantum Hall Effect and Topological Insulators

Bulk Topological Insulators and Superconductors: Discovery and the new Frontiers

While most known phases of matter are characterized by broken symmetries, the discovery of quantum Hall effects (1980s) revealed that there exists an organizational principle based on topology rather than broken symmetry. In the past few years, theory and experiments have suggested that new types of topological states of matter exist in certain bulk insulators without any applied magnetic field. These topological insulators are characterized by a full band gap in their bulk and gap-less conducting edge or surface states protected by time-reversal symmetry. Unlike the quantum Hall systems, the bulk 3D topological insulators can be doped into superconductors and magnets revealing the interplay between topological-order and broken-symmetry-order [Rev. Mod. Phys 82, 3045 (2010)]. In this talk, I will highlight the experimental observations and focus on recent experimental developments on bulk topological insulators. I will then draw connections between the topological physics and their potential applications in electronics and the emergent new frontiers in fundamental physics. Work in collaboration with D. Hsieh, Y. Xia, L. Wray, D. Qian, C.L. Kane, H. Lin, A. Bansil, D. Grauer, R.J. Cava, Y.S. Hor, J. H. Dil, F. Meier, L. Patthey, J. Osterwalder, A.V. Fedorov.   

Tunable multifunctional topological insulators in ternary Heusler and related compounds

Recently the quantum spin Hall effect was theoretically predicted and experimentally realized in quantum wells based on the binary semiconductor HgTe. The quantum spin Hall state and topological insulators are new states of quantum matter interesting for both fundamental condensed-matter physics and material science. Many Heusler compounds with C1b structure are ternary semiconductors that are structurally and electronically related to the binary semiconductors. The diversity of Heusler materials opens wide possibilities for tuning the bandgap and setting the desired band inversion by choosing compounds with appropriate hybridization strength (by the lattice parameter) and magnitude of spin--orbit coupling (by the atomic charge). Based on first-principle calculations we demonstrate that around 50 Heusler compounds show band inversion similar to that of HgTe. The topological state in these zero-gap semiconductors can be created by applying strain or by designing an appropriate quantumwell structure, similar to the case of HgTe. Many of these ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the rare-earth element Ln, which can realize additional properties ranging from superconductivity (for example LaPtBi) to magnetism (for example GdPtBi) and heavy fermion behaviour (for example YbPtBi). These properties can open new research directions in realizing the quantized anomalous Hall effect and topological superconductors. Heusler compounds are similar to a stuffed diamond, correspondingly, it should be possible to find the ``high Z'' equivalent of graphene in a graphite-like structure with 18 valence electrons and with inverted bands. Indeed the ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. These materials have a gap at the Fermi energy and are therefore candidates for 3D-topological insulators. Additionally they are centro-symmetric, therefore, it is possible to determine the parity of their wave functions, and hence, their topological character. Surprisingly, the compound KHgSb with the strong SOC is topologically trivial, whereas LiAuSe is found to be a topological non-trivial insulator.   

Quantized Anomalous Hall Effect in Magnetic Topological Insulators

The anomalous Hall effect is a fundamental transport process in solids arising from the spin-orbit coupling. In a quantum anomalous Hall insulator, spontaneous magnetic moments and spin-orbit coupling combine to give rise to a topologically nontrivial electronic structure, leading to the quantized Hall effect without an external magnetic field. Based on first-principles calculations, we predict that the tetradymite semiconductors Bi$_2$Te$_3$, Bi$_2$Se$_3$, and Sb$_2$Te$_3$ form magnetically ordered insulators when doped with transition metal elements (Cr or Fe), in contrast to conventional dilute magnetic semiconductors where free carriers are necessary to mediate the magnetic coupling. In two-dimensional thin films, this magnetic order gives rise to a topological electronic structure characterized by a finite Chern number, with the Hall conductance quantized in units of e$^2$/h. References:\\[4pt] [1] R. Yu, W. Zhang, H.J. Zhang, S. C. Zhang, X. Dai, Z. Fang, ``Anomalous Hall Effect in Magnetic Topological Insulators,'' Science 329, 61 (2010).\\[0pt] [2] Y. Zhang, K. He, C. Z. Chang, et.al., ``Crossover of the 3D topological insulator Bi$_2$Se$_3$ to the 2D limit,'' Nature Physics 6, 584 (2010)   

Search for Topological Insulators in Ternary Chalcogenides

A topological insulator (TI) is a novel quantum state, which is a bulk insulator but has gapless surface states. Recently, binary chalcogenides, Bi2Te3, Bi2Se3 and Sb2Te3 have been theoretically predicted and experimentally observed to be a family of TIs [1]. In this talk, we extend our search of TIs to ternary chalcogenides by replacing some of Bi or Sb atoms by other atoms, such as thallium and rare earth atoms. It is found that for thallium-based materials [2], only TlSbS2 is trivial and all the others are TIs, while for rare earth-based materials[3], LaBiTe3 is a TI and the others are trivial. The search in ternary chalcogenides not only bring new members of TIs in the family of chalcogenides but also may provide candidates for other new topological states such as topological superconductor, quantum anomalous Hall insulator, axionic insulator and topological Kondo insulator. \\[4pt] Reference:\\[0pt] [1] Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface, Haijun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Xi Dai, Zhong Fang and Shou-ChengZhang, Nature Physics 5, 438 - 442 (2009).\\[0pt] [2] Theoretical prediction of topological insulators in thallium-based III-V-VI2 ternary chalcogenides, Binghai Yan, Chao-Xing Liu, Hai-Jun Zhang, ChiYung Yam, Xiao-Liang Qi, Thomas Frauenheim and Shou-Cheng Zhang, Europhysics Letters, 90, 37002 (2010).\\[0pt] [3] Theoretical prediction of topological insulator in ternary rare earth chalcogenides, Binghai Yan, Hai-Jun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Thomas Frauenheim, Shou-Cheng Zhang, Phys. Rev. B 82, 161108(R) (2010).   

Magnetotransport studies of new topological insulators: Bi$_2$Te$_2$Se and others

A topological insulator (TI) is a material that has a gapped insulating bulk and a gapless metallic surface. However, presently available TI materials are not truly insulating, making surface transport measurements to be a challenge. The second generation of TIs, Bi$_2$Se$_3$ and related compounds, turned out to be more suitable for the experimental studies of the topological 2D states than the first discovered Bi-Sb alloys, due to a much larger bulk gap and a simpler surface state consisting of a single Dirac cone. Unfortunately, near-stoichiometric Bi$_2$Se$_3$ is always a metallic n-type material owing to a finite amount of Se vacancies. We searched for new TI materials that are better suited for achieving a bulk insulating state and found that Bi$_2$Te$_2$Se, which has an ordered tetradymite structure with the Te-Bi-Se-Bi-Te layer sequence, is a very promising material. It was found that high-quality single crystals of Bi$_2$Te$_2$Se show a high resistivity exceeding 1 $\Omega$cm, together with a variable-range hopping behavior which is a hallmark of an insulator; yet, they present Shubnikov-de Haas oscillations which signify the 2D surface state consistent with the topological one observed by photoemission spectroscopy. Moreover, we are able to clarify both the bulk and surface transport channels, establishing a comprehensive understanding of the transport in this material. Our results demonstrate that Bi$_2$Te$_2$Se is the best material to date for studying the surface quantum transport in a topological insulator. Transport properties of other new TI materials are also presented.   

Session H3: Collective Effects in Molecular Magnets

Experimental realization of random-field Ising ferromagnetism in a molecular magnet

The longitudinal magnetic susceptibility of single crystals of the molecular magnet Mn$_{12}$-acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase at $\sim $ 0.9 K [1,2]. With increasing magnetic field applied transverse to the easy axis, a marked change is observed in the temperature dependence of the susceptibility, with a considerably more rapid suppression of the Curie-Weiss temperature than predicted by mean-field theory for an ordered single crystal. Our results can instead be fit by a Hamiltonian for a random-field Ising ferromagnet in a transverse magnetic field, where the randomness derives from the intrinsic distribution of locally tilted magnetic easy axes known to exist in Mn$_{12}$-acetate crystals. Mn$_{12}$-ac and other single molecule magnets may thus serve as clean model systems for the study of random field ferromagnetism where the random fields are controllable and considerably larger than typical hyperfine fields. This discovery promises to enable widespread and convenient experimental study of magnetism in a random field in a broad class of new materials.\\[4pt] Work performed by and in collaboration with: Bo Wen, and Lin Bo, Physics Dept. City College of New York, CUNY (funded by NSF-DMR-0451605), P. Subedi and A. D. Kent, Physics Dept., NYU, (funded by NSF-DMR-0506946 and ARO-W911NF-08-1-0364) Y. Yeshurun, Physics Dept., Bar Ilan U, (funded by Deutsche Forschungsgemeinschaft), A. J. Millis, Physics Dept. Columbia U. (funded by DMR DMR-0705847), C. Lampropoulos and G. Christou, Chemistry Dept., U. of Florida (funded by NSF -CHE-0910472). \\[4pt] [1] B. Wen et al., Phys. Rev. B 82, 014406 (2010). \\[0pt] [2] Luis, et al., Phys. Rev. Lett. 95, 227202 (2005).   

Pure and Random--field quantum criticality in dipolar Ising magnets

A theoretical model for $Mn_{12}$ acetates and related materials is derived. Isomer effects present in some families of host acetate materials are argued to lead to a random field of a strength which may be tuned by a magnetic field applied in a direction perpendicular to the easy axis of the $Mn_{12}$ unit. A mean field phase diagram is presented and consequences of beyond-mean-field physics are outlined. Measureable consequences in the experimentally accessible high temperature regime are presented and in this regime the importance of a complete treatment of the molecular level structure is emphasized. Open theoretical problems are described. Work reported in Phys. Rev. B 82, 014406 (2010) and Phys. Rev. B 82, 174405 (2010)). and performed in collaboration with: M. Sarachik, Bo Wen, and Lin Bo, Physics Dept. City College of New York, CUNY (funded by NSF-DMR-0451605), P. Subedi and A. D. Kent, Physics Dept., NYU, (funded by NSF-DMR-0506946 and ARO-W911NF-08-1-0364) Y. Yeshurun, Physics Dept., Bar Ilan U, (funded by Deutsche Forschungsgemeinschaft), C. Lampropoulos and G. Christou, Chemistry Dept., U. of Florida (funded by NSF -CHE-0910472). \\[4pt] [1] Phys. Rev. B 82, 014406 (2010).\\[0pt] [2] Phys. Rev. B 82, 174405 (2010)).   

Deflagration, fronts of tunneling, and dipolar ordering in molecular magnets

Although there is no exchange interaction in crystals of molecular magnets characterized by a giant effective spin $S$ ($S$ =10 for Mn$_{12}$, and Fe$_{8})$, magnetic field $B^{(D)}$ generated by magnetic moments \textit{g$\mu $}$_{B}S$ of magnetic molecules creates energy bias $W^{(D) }$=2\textit{Sg$\mu $}$_{B} B^{(D)}$ on a molecule that largely exceeds the tunnelling splitting $\Delta $ of matching quantum states on different sides of the anisotropy barrier. Thus the dipolar field has a profound influence on the processes of tunnelling and relaxation in molecular magnets. Both theoretical and experimental works showed a slow non-exponential relaxation of the magnetization in both initially ordered and completely disordered states since most of the spins are off tunneling resonance at any time. Recently a new mode of relaxation via tunneling has been found, the so-called fronts of tunneling, in which (within a 1$d$ theoretical model) dipolar field adjusts so that spins are on resonance within the broad front core. In this ``laminar'' regime fronts of tunnelling are moving fast at speeds that can exceed that of the temperature-driven magnetic deflagration, if a sufficiently strong transverse field is applied. However, a ``non-laminar'' regime has also been found in which instability causes spins to go off resonance and the front speed drops. In a combination with magnetic deflagration, the laminar regime becomes more stable and exists in the whole dipolar window 0$\le W \quad \le W^{(D)}$ on the external bias $W$, where the deflagration speed strongly increases. Another dipolar effect in molecular magnets is dipolar ordering below 1 K that has recently been shown to be non-uniform because of formation of magnetic domains. An object of current research is possible non-uniformity of magnetic deflagration and tunneling fronts via domain instability that could influence their speed.   

Experiments on Magnetic Deflagration

Magnetic deflagration was first observed in molecular magnets [1,2] and then in glassy magnetic materials like manganites [3,4] and intermetallic systems like Gd$_{5}$Ge$_{4}$ [5]. The role of the chemical energy is played by the magnetic energy of the material. In the case of a molecular magnet, this is Zeeman energy, while in manganites and Gd$_{5}$Ge$_{4}$ the free energy is a combination of the Zeeman energy and the energy of the metastable magnetic phase. In molecular magnets both the ignition process and the speed of the flame are assisted by quantum spin reversal [2]. There also exists some evidence of the transition from deflagration to detonation [6]. Various experimental techniques have been used to detect the speed of the magnetic flame. They include SQUID magnetometry, Hall bars and coils. Magnetic deflagration has been ignited by local heating, application of external fields, by surface acoustic waves and microwaves. High frequency EPR measurements of the population of spin levels permitted observation of magnetic deflagration in real time. The talk will review these experiments and their interpretation. \\[4pt] [1] Y. Suzuki et al. Phys. Rev. Lett. 95, 147201 (2005). \\[0pt] [2] A. Hernandez-Minguez et al. Phys. Rev. Lett. 95, 217205 (2005). \\[0pt] [3] F. Macia et al. Phys. Rev. B79, 092403 (2009). \\[0pt] [4] F. Macia et al. Phys. Rev. B76, 174424 (2007). \\[0pt] [5] S. Velez et al. Phys. Revc. B81, o64437 (2010). \\[0pt] [6] W. Decelle et al. Phys. Rev. let. 102, 027203 (2009)   

Electronic Structure and Transport Through Single Molecule Magnets

Over the past decade, single-molecule magnets have drawn considerable attention due to observed magnetic quantum tunneling and interference and a possibility of using them for information storage or devices. There have been so far significant experimental efforts to build and characterize monolayers of single-molecule magnets on various surfaces or single-molecule magnets connected to electrodes. There is need to understand changes of electronic and magnetic properties of single-molecule magnets in those environments using quantum mechanical simulations. We simulate, within density-functional theory, a nanostructure in which prototype single-molecule magnets Mn12 are adsorbed onto a gold surface. We investigate coupling between the Mn12 and the surface and discuss electronic structure and magnetic anisotropy of the Mn12 on a gold surface in comparison to an isolated Mn12. In addition, we present electron transport properties through a Mn12 bridged between gold electrodes, using the nonequilibrium Green's function method in conjunction with density-functional theory. We discuss a possibility of using a Mn12 molecule as a spin filter and an effect of interface geometry and bonding type on transport across a Mn12.   

Session H4: Polymer Physics Prize

Polymer Physics Prize Talk: Polymer Brushes: Why do we still care?

Polymer molecules have been widely used to modify the properties of surfaces including its adhesion. Among the most studied have been polymer brushes, in which polymer chains are grafted at one end to a surface and immersed in a small molecule solvents. Experiments and simulations have shown that the conformation of the chains grafted onto a flat surface depends on the grafting density and the interaction of the polymer with the solvent. As the molecular weight of the solvent increases, the structure of the brush changes. Consequently the brush chains are expelled from the solvent due to entropic loss that originate from the fact that the melt chains penetrating the brush cannot overcome the translational, or mixing, entropy. This crossover from wetted to non-wetted brushes, has important implications for polymer adhesion, where the phase separation of melt and brush chains reduces entanglements at the interface. As polymers are grafted to nanoparticles, the curvature of the surface offers the polymer brush chains a significantly larger space to explore compared to a flat surface, reducing the tendency for autophobic dewetting. Using large scale molecular dynamics simulations we have studied the interface between brush coated nanoparticles and a polymer melt. Effects of chain length of the brush, and that of the polymer melt, the coverage of the nanoparticle and its curvature on the brush/melt interface will be discussed. The role of individual entanglements, between the brush chains and the melt, as identified by primitive path analysis will be introduced. These simulations provide insight into the structure of the brush/polymer interface which is not accessible through other theoretical or experimental means.   

Polymer Physics Prize Talk: Topological Constraints Matter -- or Back to the Origin

Topological constraints, being permanent or temporal, influence many properties of soft matter, especially polymers. While at a first glance the simple Rouse models describes the motion of short chains surprisingly well, the fact that chains cannot cut through each other dominates the dynamics of long chain melts, ring polymers and the relaxation in networks and gels. Furthermore new phenomena in special melts and mixtures even make this more obvious. The talk will review some developments and will also address new problems linked to material science as well as biology. To illustrate the importance of topological constraints, numerical simulations for a melt on non concatenated ring polymers with and without linear contaminants will be presented. While the static properties of long rings can be rationalized by the concept of a crumpled globule, dynamic properties are much less understood. Our simulations clearly show that diffusion and stress relaxation in such a system of globules decouple. In addition the first results for non concatenated rings added to a melt of linear polymers and for a few linear polymers added to a melt of rings will be discussed.   

Bottle-brush polymers versus worm-like chains: Do we understand the stiffness of macromolecules?

Bottle-brush polymers contain a long flexible macromolecule as a backbone to which flexible side-chains are grafted. Through the choice of the grafting density of the length of the side chains to the local stiffness of this cylindrical molecular brush can be controlled. However, understanding mesoscopic length scales (cross-sectional radius, persistence length, contour length) of these semiflexible cylindrical brushes poses a challenging problem. While self-avoiding walks of variable stiffness show a crossover to the Kratky-Porod worm-like chain model, and hence a (pre-asymptotic) regime of Gaussian behavior, bottle-brushes under good solvent conditions are not compatible with this model. Consequences for the description of chain stiffness in terms of the concept of the persistence length are discussed, as welll as pertinent experiments.   

Entanglements and the Mechanical Properties of Glassy Polymers

The response of glassy polymers to shear or tensile strain is strongly influenced by the entanglement network that is inherited from the melt. Molecular dynamics simulations are used to probe the microscopic origins of stress-strain curves and their connection to entanglements. The latter are identified in real space by examining topological constraints along the primitive path. The first part of the talk will consider the process of craze formation, where the entanglement density is correlated to the volume increase during crazing. Simulations show that entanglements are preserved during crazing, but the craze density does not correspond to pulling chains taut between entanglements. The second part of the talk will examine the effect of entanglements on strain hardening under uniaxial strain. The stress is directly associated with the degree of orientational order along the strain axis, and nearly independent of order along perpendicular directions [1]. Studies with mixtures of short and long chains show that the degree of order is independent of the surrounding chains [2]. The final part of the talk will examine the strength of welds formed by diffusion across polymer interfaces. The shear stress follows the bulk response until chains are pulled taut on the scale of the length of segments that have diffused across the interface. When this length is several times the entanglement length, the maximum shear stress saturates at the bulk value and chains fail through scission. Similar trends are found for the fracture energy in tensile loading.\\[4pt] [1] T. Ge and M. O. Robbins, J. Polymer Sci. B: Polymer Physics {\bf 48}, 1473-1482 (2010).\\[0pt] [2] R. S. Hoy and M. O. Robbins, J. Chem. Phys. {\bf 131}, 244901 (2009)   

Conjugated Polymer Nanoparticle Hybrids: Structure, Dynamics and Forces

While nanoparticles (NPs) have unique tunable elctro-optical properties and exceptional mechanical strength, it remains a challenge to integrate them into devices while retaining the advantages of the nanoscale. Tethering polymeric materials to the NPs surfaces has the potential to stabilize single NPs and direct their assembly. The polymers may serve in several capacities from a simple tether to a matrix to directed assembly tool taking advantage of the inherent structure of the polymers and as an active component in a complex material. However confining a large molecule to a highly curved surface affects the inherent configuration of the polymer. These effects are of particular interests in conjugated polymer-nanoparticle hybrids, where the conformation of the polymers affects not only the assembly of the nanoparticles but also the optical and electronic communication between the NPs. Using molecular dynamic simulations we have studied the structure of a single hybrid of \textit{para} dialkyl phenylene ethynelyne (PPE) grafted nanoparticles. PPEs are polymers whose conformation determines their degree of conjugation and therefore their electro-optical response. Using simulations coupled with neutron scattering studies we have shown that PPE is a rigid polymer that is fully extended in dilute solutions in good and theta solvents but can be forced into a collapsed configuration in a poor a solvent. When confined to a nanoparticle surface, the PPE chains are fully extended but cluster as the solvent quality is reduced. Results for the conformation of grafted PPE molecules on a single nanoparticle and the forces between two nanoparticles as a function of chain length and solvent quality will be presented. These simulations provide insight to the interactions that result in formation of tunable hybrids.   

Session H5: Drowning in Carbon: The Imperative of Nuclear Power

A Strategy for Expanded Nuclear Power: The Role of the U.S. Department of Energy

This abstract not available.   

What happened to the US nuclear renaissance?

While nuclear power generation is seeing a distinct revival internationally, especially in Asia, a corresponding revival within the United States has not yet occurred. I will discuss the various reasons for this difference, as well as the consequences - some distinctly unintended - for the future U.S. role in the spread of nuclear power generation as well as in non-proliferation internationally.   

Used Nuclear Fuel: From Liability to Benefit

Nuclear power has proven safe and reliable, with operating efficiencies in the U.S. exceeding 90{\%}. It provides a carbon-free source of electricity (with about a 10{\%} penalty arising from CO$_{2}$ released from construction and the fuel cycle). However, used fuel from nuclear reactors is highly toxic and presents a challenge for permanent disposal -- both from technical and policy perspectives. The half-life of the ``bad actors'' is relatively short (of the order of decades) while the very long lived isotopes are relatively benign. At present, spent fuel is stored on-site in cooling ponds. Once the used fuel pools are full, the fuel is moved to dry cask storage on-site. Though the local storage is capable of handling used fuel safely and securely for many decades, the law requires DOE to assume responsibility for the used fuel and remove it from reactor sites. The nuclear industry pays a tithe to support sequestration of used fuel (but not research). However, there is currently no national policy in place to deal with the permanent disposal of nuclear fuel. This administration is opposed to underground storage at Yucca Mountain. There is no national policy for interim storage---removal of spent fuel from reactor sites and storage at a central location. And there is no national policy for liberating the energy contained in used fuel through recycling (separating out the fissionable components for subsequent use as nuclear fuel). A ``Blue Ribbon Commission'' has been formed to consider alternatives, but will not report until 2012. This paper will examine alternatives for used fuel disposition, their drawbacks (e.g. proliferation issues arising from recycling), and their benefits. For recycle options to emerge as a viable technology, research is required to develop cost effective methods for treating used nuclear fuel, with attention to policy as well as technical issues.   

Fuel Cycle R\&D Requirements for Future Nuclear Power

Recently, DOE Nuclear Energy completed its Road Map for a science-based approach to future nuclear energy development. Fuel cycle R\&D is a central element of the Road Map, which covers nuclear energy through the period 2040-2050 and perhaps beyond. Examples of fuel cycle R\&D activities will be presented, along with an outline of the types of research facilities needed to support this effort. Experimental facilities within several areas of the DOE, including DOE's Office of Nuclear Energy and its Office of Science, will be required for this task. In addition, advanced modeling and simulation will play a growing major role in these activities.   

Session H6: Ultracold Molecules and Quantum Many Body Physics

Control of dipolar collisions of polar molecules in the quantum regime

Ultracold polar molecular quantum gases promise to open new research directions ranging from the study of ultra-cold chemistry, precision measurements to novel quantum phase transitions. Based on the preparation of high-phase space density gases of polar KRb molecules, I will discuss the control of dipolar collisions and chemical reactions of polar molecules in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play a dominant role. In particular, I will discuss the crucial role of electric dipole-dipole interactions and external confinement in determining the chemical reaction rate. Finally, I will discuss prospects of reaching quantum degeneracy in bi-alkali samples of polar molecules and prospects for these systems as novel dipolar quantum many-body systems.   

Ultracold high-density samples of rovibronic ground-state molecules in an optical lattice

Ultracold molecules controlled at the level of single quantum states with respect to all internal and external degrees of freedom will enable a series of fundamental studies in physics and chemistry, ranging from novel quantum gas experiments and cold controlled chemistry to quantum information and quantum simulation. Ultracold molecules trapped in an optical lattice at high density and prepared in their lowest internal quantum state are an ideal starting point for these studies. We create ultracold and dense samples of molecules in a single hyperfine sublevel of the rovibronic ground state while each molecule is individually trapped in the motional ground state of an optical lattice well [1,2]. Starting from an atomic Mott-insulator state with optimized double-site occupancy, weakly bound Cs dimer molecules are efficiently formed on a Feshbach resonance and subsequently transferred to the rovibronic ground state by a stimulated 4-photon process with the Stimulated Raman Adiabatic Passage (STIRAP) technique. The molecules are trapped in the lattice with a lifetime of 8 s. We aim at producing Bose-Einstein condensates of ground-state molecules by adiabatically removing the lattice. Our results, when suitably generalized to heteronuclear molecules, present an important step towards the realization of dipolar quantum-gas phases in optical lattices. I will report on recent progress in Innsbruck on the formation of RbCs ground state molecules. \\[4pt] [1] Science \textbf{321}, 1062 (2008) \\[0pt] [2] Nature Physics \textbf{6}, 265 (2010)   

Theory of ultracold heteronuclear polar molecules

This abstract not available.   

Laser Cooling of a Diatomic Molecule

Laser cooling techniques to produce ultracold (T$<$1$\mu$K) atoms have lead to rapid advances in a wide array of fields. However, extending laser cooling to molecules has remained elusive. The primary problem is that laser cooling requires a large number ($>$104) of photon absorption/emission cycles. Molecules, however, have vibrational and rotational degrees of freedom, which typically lead to high branching probabilities into a large number of unwanted sublevels. Here we report on experiments demonstrating the laser cooling of a diatomic molecule which have overcome this problem. We use the molecule strontium monofluoride (SrF) where only three lasers and a magnetic field are necessary to scatter $>$105 photons. We have demonstrated 1-D transverse cooling of a beam of SrF, dominated by Doppler or Sisyphus-type cooling forces depending on experimental parameters. We observe a reduction in the velocity distribution by a factor of 3 or more, corresponding to final 1-D temperature T $<$ 1 mK. This transverse cooling may be useful for a variety of experiments; in addition, our results open a path to trapping and 3D cooling of SrF to the ultracold regime.   

Meta-stable 1-D gases of polar molecules with attractive dipole forces

The recent achievements in the formation and manipulation of ultracold polar molecules have opened the gate to exciting new studies in several fields of physical sciences. Polar molecules could find uses in quantum information science and in precision measurements, while dense samples could provide a fertile ground for novel quantum gases because of their long-range and anisotropic interactions. Until now, stable dipolar gases were thought to require a repulsive dipole-dipole interaction, such as provided by parallel dipoles aligned perpendicularly to a two-dimensional (2-D) trap. However, to observe interesting new correlations and condensed matter phases, attractive interactions are needed. Here, we explore how meta-stable one-dimensional (1-D) samples of ultracold polar molecules could be created with attractive long-range dipole-dipole interaction. We show that a repulsive barrier due to a strong quadrupole interaction can stabilize a gas of ultracold KRb molecules and even lead to long-range wells supporting bound states. The properties of these wells can be controlled by external electric fields, allowing the formation of long chains of KRb polymers, and the further study of Luttinger liquid transition. We also discuss the general molecular properties necessary for the existence of a repulsive barrier.   

Session H7: Physics of Proteins II: Dynamics and Functions

Energy Landscapes Encoding Function in Enzymes Investigated Over Broad Time Scales

The operating hypothesis of much of our current work is that atomic motion, over broad time scales (femtoseconds to milliseconds, the latter being the time scale of most enzyme catalyzed reactions), contributes to enzymic catalysis in proteins. It is clear from our work that specific types of motions are important in binding of ligands to proteins and transition state formation in enzymatic catalysis. Since new experimental and theoretical approaches are needed to understand the dynamical nature of proteins broadly and enzymatic catalysis specifically, we have employed time-resolved ``pump-probe'' spectroscopic techniques because of the sensitivity of these type of approaches to all relevant time scales. And we have also developed and applied new theoretical methods. The talk will focus on how lactate dehydrogenase brings about catalysis based on current experimental and theoretical studies.   

NMR investigations of molecular dynamics

NMR spectroscopy is a powerful experimental approach for characterizing protein conformational dynamics on multiple time scales. The insights obtained from NMR studies are complemented and by molecular dynamics (MD) simulations, which provide full atomistic details of protein dynamics. Homologous mesophilic (\textit{E. coli}) and thermophilic (\textit{T. thermophilus}) ribonuclease H (RNase H) enzymes serve to illustrate how changes in protein sequence and structure that affect conformational dynamic processes can be monitored and characterized by joint analysis of NMR spectroscopy and MD simulations. A Gly residue inserted within a putative hinge between helices B and C is conserved among thermophilic RNases H, but absent in mesophilic RNases H. Experimental spin relaxation measurements show that the dynamic properties of \textit{T. thermophilus} RNase H are recapitulated in \textit{E. coli} RNase H by insertion of a Gly residue between helices B and C. Additional specific intramolecular interactions that modulate backbone and sidechain dynamical properties of the Gly-rich loop and of the conserved Trp residue flanking the Gly insertion site have been identified using MD simulations and subsequently confirmed by NMR spin relaxation measurements. These results emphasize the importance of hydrogen bonds and local steric interactions in restricting conformational fluctuations, and the absence of such interactions in allowing conformational adaptation to substrate binding.   

Kinetic vs. Thermodynamic Control of Bacteriorhodopsin Pumping

Bacteriorhodopsin is a transmembrane proton pump that converts light energy to a transmembrane electrochemical gradient. Retinal, bound in the center of the protein, absorbs light and isomerizes from the all-trans to 13-cis configuration. A series of conformational changes and proton transfers then restores the structure to the all-trans ground state while pumping one proton from the high pH cell interior to the low pH exterior, saving energy in an electrochemical gradient. Poorly understood gating elements control key steps where incorrect proton transfer would return the protein to the ground state without pumping. The gate's barrier height determines how much the pump leaks. Analysis of high-resolution structures trapped in different intermediates has produced ideas for how bacteriorhodopsin ensures pumping. There are two contrasting strategies, one primarily thermodynamic and the other relying on kinetic control to ensure that protons are moved uphill. With thermodynamic control, residue protonation states always remain in quasi-equilibrium. Relatively slow conformational changes shift the energy landscape modifying site pKas. Residues then change ionization remaining in equilibrium in each metastable intermediate. The sequence of intermediates imparts the directionality to the transfers. Alternatively, the direction of transfer is determined by the accessibility of low energy pathways so is thus is under kinetic control. We will discuss which steps in the bacteriorhodopsin photocycle are under thermodynamic or under kinetic control. The role of three specific conformational changes (retinal isomerization, Arg82 reorientation and Glu194 and 204 separations) on the degree of proton transfer will be described. Supported by NFS MCB 1022208.   

Beller Lectureship Talk: Ultrafast Excitation Energy Transfer and the Mechanism of Non-Photochemical Quenching in Plant Photosynthesis

The success of photosynthesis relies on two ultrafast processes: excitation energy transfer in the light-harvesting antenna followed by charge separation in the reaction center. LHCII, the peripheral light-harvesting complex of Photosystem II, plays a major role. At the same time, the same light-harvesting system can be `switched' into a quenching state, which effectively protects the reaction center of Photosystem II from over-excitation and photodamage. In this talk I will demonstrate how LHCII collects and transfers excitation energy. Using single molecule spectroscopy we have discovered how LHCII can switch between this light-harvesting state, a quenched state and a red-shifted state. We show that the switching properties between the light-harvesting state and the quenched state depend strongly on the environmental conditions, where the quenched state is favoured under `NPQ-like' conditions. It is argued that this is the mechanism of non-photochemical quenching in plants.   

Protein Dynamics, Ligand Binding, and Biological Function

Dynamics is essential for protein function. To demonstrate this point, this talk presents three studies. (1) For a ligand-gated ion channel, ligand binding leads to channel activation by modulating the dynamics of the channel protein. A common theme that emerges from different families of ligand-gated ion channels is that agonist binding closes the ligand-bidning domain (LBD), leading to pore opening in the transmembrane domain (TMD); in contrast, antagonist binding opens the LBD, leading to pore closing in the TMD [1]. (2) When the structure [2] and gating dynamics [3] of the influenza M2 proton channel are accounted for, the calculated rate of ion transport is in quantitative agreement with experimental data [4]. (3) In enzymes, gating dynamics afford substrate selectivity [5]. \\[4pt] [1] M. Yi, H. Tjong, and H.-X. Zhou (2008). Spontaneous conformational change and toxin binding in $\alpha $7 nicotinic acetylcholine receptor: insight into channel activation and inhibition. Proc. Natl. Acad. Sci. 105, 8280-8285. \\[0pt] [2] M. Sharma, M. Yi, H. Dong, H. Qin, E. Peterson, D. D. Busath, H.-X. Zhou, and T. A. Cross (2010). Insight into the mechanism of the influenza A proton channel from a structure in a lipid bilayer. Science 330, 509-512. \\[0pt] [3] M. Yi, T. A. Cross, and H.-X. Zhou (2009). Conformational heterogeneity of the M2 proton channel and a structural model for channel activation. Proc. Natl. Acad. Sci. USA 106, 13311-13316. \\[0pt] [4] H.-X. Zhou (2010). Diffusion-influenced transport of ions across a transmembrane channel with an internal binding site. J. Phys. Chem. Lett. 1, 1973-1976. \\[0pt] [5] H.-X. Zhou, S. T. Wlodek, and J. A. McCammon (1998). Conformation gating as a mechanism for enzyme specificity. Proc. Natl. Acad. Sci. USA 95, 9280-9283.   

Session H8: Science, Art and Culture

Robotics in the World of Entertainment

This abstract not available.   

XPower plus the Physics of Rodeo

This abstract not available.   

Singing Tesla Coils

This abstract not available.   

The Science of Barbecue (Texas Style)

This abstract not available.   

Session H9: Colloids: Experimental

Universality in the delayed failure of colloidal gels

The mechanical failure of heterogeneous solids is not always instantaneous with the application of a load, but can be significantly delayed. We use colloidal gels, a prototypic heterogeneous material, to unravel the microscopic mechanisms behind this delayed failure. A universal behavior is revealed; the delay time depends only on the magnitude of the applied stress not on its origin. Whether the gel succumbs to internal tension, gravitational compression or shear stresses, the behavior can be quantitatively explained using a generalized bond-rupture model that describes the microscopic events triggering macroscopic failure.   

Glassy dynamics of 2D colloid crystals in a random pinning potential

Recently, we have demonstrated that a monolayer charged colloidal crystal confined to a rough charged surface provides a realization of the Larkin-Ovchinnikov random-pinning model in two dimensions [1]. The statics of the system is found to agree with Larkin's prediction of balkanization into small ordered domains. However, the dynamics are in disagreement with the collective creep model. Detailed analysis of the particle trajectories suggest that collective creep is preempted by channel flow. We also find that the velocity response to a step-like driving force shows a stretched exponential behavior similar to that found in structural glasses. Here, we provide a detailed analysis of this process.\\[4pt] [1] A. Pertsinidis and X.S. Ling PRL {\bf 100} \normalfont 028303 (2008)   

Field-driven pattern formation of charged particles in nonpolar solvent

We combine microfluidics and high-speed imaging to investigate transport dynamics of charged colloidal particles in a nonpolar solvent as the polarity of an external electric field is switched periodically. Immediately following a switch, particles which were initially all packed against one electrode move towards the opposite electrode in an unstable manner; instead of remaining uniform, the particle front develops undulations. This results in a heterogeneous deposition of particles on the electrode wall. For a range of wait times between switches, we find that the particles localize at exceptionally well-defined periodic modes and we offer a simple physical model to account for this pattern formation.   

Colloids with magnetic patches: synthesis and self-assembly

We developed a new class of colloidal particles that programmably and reversibly self-assemble into well-defined clusters by virtue of ``magnetic patches'' carrying a permanent magnetic dipole moment. The resulting clusters form spontaneously in a zero external field, and their geometry is entirely determined by the interplay between magnetic, steric, and electrostatic interactions. Imposing an external magnetic field enables the clusters to unbind or change their geometry allowing, in principle, to create materials with tunable structural arrangements.   

Melting Dynamics of Colloidal Thin Films on Patterned Substrates

We present results of experiments on the melting dynamics of colloidal crystals formed on patterned substrates. Our system consists of micron-sized colloidal particles and a tunable short- range attractive depletion interaction that can be controlled by small temperature changes. We investigate the melting rates of crystalline islands that form on substrates with square and hexagonal symmetry. We find that crystals with square symmetry melt significantly slower than those with hexagonal symmetry despite the fact that particles at the edge of the hexagonal crystal are on average bound more strongly than those at the edge of a square crystal. We find that the symmetry of the substrate affects the ability of particles to diffuse away from a melting crystal, and these differences in single-particle diffusion rates account for the difference in melting rates.   

Dynamics of Transient Vorticity Aligned Structures in Attractive Colloidal Suspensions

Shear rate jumps from high to low flow rates in an attractive colloidal suspension of carbon black particles in a non-polar solvent result in the formation of transient log-like structures aligned in the vorticity direction. Optical microscopy in situ with bulk rheology shows that the appearance of these aggregates is attended by an increase in the suspension viscosity. The viscosity shows a peak and then gradually recedes with passage of time under flow in concordance with the disappearance of the log-like structures. The time at which the viscosity reaches its maximum scales inversely with the shear rate applied to the system. This emergence of the peak in viscosity appears to be controlled by a critical strain and rescaling in these terms produces a common response across several different shear rates. Alteration of the attraction strength between particles by the addition of surfactant severely inhibits the structure formation. We present a simple model to account for these observations.   

Two-dimensional Fibonacci spiral optical thermal ratchets

A novel two-dimensional optical thermal ratchet has been implemented with holographic optical trapping arrays structured as the ``Fibonacci spiral'' for diffusing colloidal particles. Periodically rotating the optical trapping array by an angle in a three-step cycle yields a two-dimensional time-varying optical landscape that acts either as (1) a deterministic pump when traps are closely dispersed in space, whose induced radial and azimuthal fluxes can be quantitatively mapped out according to the geometry of Fibonacci spiral, or else as (2) an optical thermal ratchet when traps are widely dispersed, whose transport property depends on the competition between the temporal evolution in optical landscapes and Brownian particles' diffusivity. The Fibonacci ratchet displays independent flux reversals in both the radial and azimuthal directions as a function of the cycle frequency and the inter-trap separation.   

Using shear to assemble colloidal strings

Sheared colloidal suspensions exhibit various fascinating phases under the influence of hydrodynamic, interparticle and thermal interactions. These shear-induced phases have been intensively studied for suspensions well above the crystalline threshold, but remain relatively unexplored for amorphous suspensions. Here, we report a novel string phase in less concentrated colloidal suspensions under shear, where particles assemble into long strings normal to the plane of shear. This finding contradicts previous numerical results that predict the formation of particle strings along the shear velocity direction. We systematically investigate how the phase depends on the shear rates, the confinement of shear plates, and the volume fractions of samples. We demonstrate the relation between the string phase of low volume fraction samples and the shear-induced crystallization of high volume fraction samples. A simple mechanism for the formation of this novel phase is suggested.   

Biomembrane-mediated control of like-charge colloidal attraction

The nature of attractions observed between like-charged colloidal particles near a confining wall is still mysterious, due in part to the lack of experimental systems with tunable inter-particle interactions. Biomembranes are appealing candidates for colloidal functionalization, enabling access to electrostatic and chemical properties that influence inter-particle relations. Previous optical-trap based examinations of lipid membrane functionalized particles revealed a surprising linear relationship between the magnitude of the attractive pair potential and the particle charge in presence of a wall of constant charge density. Here, using lipid membranes to also functionalize the confining wall, thereby controlling its charge density, we find a non-linear relationship between inter-particle attraction and charge. Our results highlight the role of substrate-induced fields in controlling pair interactions between colloidal microparticles.   

Colloidal Gas-Liquid Condensation induced by the Critical Casimir Effect

We explore a new temperature control over colloidal phase formation by using the Critical Casimir effect. This effect allows direct control over particle interactions via temperature-dependent solvent fluctuations: In analogy to the confinement of fluctuations of the electromagnetic field between two dielectrics (quantum mechanical Casimir effect), the confinement of fluctuations of a critical solvent leads to an attraction between surfaces that are immersed in this solvent. This allows exquisite temperature control over the interactions of colloidal particles that are suspended in this critical solvent. We show that this temperature control allows us to ``freeze'' a dilute colloidal gas into a dense colloidal liquid, and a crystalline solid. By using confocal microscopy, we follow these phase transitions directly in real space, and we measure the particle pair potential. We show that we can quantitatively account for the gas-liquid condensation by using Van der Waals theory. We study the growth of colloidal liquid droplets by following the mean droplet radius $<$R$>$ with dynamic light scattering. We find $<$R$>$ $\sim$ t1/2 and $<$R$>$ $\sim$ t1/3 indicating that the droplets form by nucleation, followed by diffusion limited growth.   

Comparison of different analysis techniques in inline holographic video microscopy

Holographic video microscope can be analyzed on a frame-by-frame basis to track individual colloidal particles' three-dimensional motions with nanometer resolution. In this work, we compare the performance of two complementary analysis techniques, one based on fitting to the exact Lorenz-Mie theory and the other based on phenomenological interpretation of the scattered light field reconstructed with Rayleigh-Sommerfeld back-propagation. Although Lorenz-Mie tracking provides more information and is inherently more precise, Rayleigh-Sommerfeld reconstruction is faster and more general.   

Dynamics of colloidal particles in ice

Solidification of the solvent phase of a colloidal suspension occurs in many natural and technological settings and is becoming a popular technique for creating microporous structures and composite materials. During freezing, regions of high particle density can form as particles are rejected from the growing solid and guided into a variety of macroscopic morphologies. The particles in the high density regions form an amorphous colloidal solid that deforms in response to internal and external stresses. Using X-ray Photon Correlation Spectroscopy, we studied this deformation for silica particles in polycrystalline ice. We found that the particles in the high density regions underwent ballistic motion coupled with a non-exponential decay of the intensity autocorrelation function (ACF) that transitions from a stretched to a compressed exponential with increasing scattering vector q. While ballistic motion and a compressed exponential decay of the ACF is common, the coupling with a stretched exponential decay is very rare and a transition with increasing q has not previously been reported. We explain this behavior in terms of ice grain boundary migration.   

Imaging the microscopic structure of shear thinning and thickening colloidal suspensions

The viscosity of colloidal suspensions varies by orders of magnitude depending on how quickly they are sheared. Such non- Newtonian behavior arises from the arrangement of suspended particles and their mutual interactions. Although numerical simulations and various scattering experiments have revealed much about the local and average suspension structures, particle dynamics at mesoscopic length scales, where non- Newtonian behaviors are believed to originate, are still poorly understood. Here, by combining fast confocal microscopy with simultaneous rheological measurements, we systematically investigate changes in suspension structure over a range of length scales, as the suspension transitions through regimes with different rheological signatures. Our measurements bridge previous simulation and scattering results, and unambiguously show that shear thinning is coupled to particle layering, that shear thickening is decoupled from suspension order-to-disorder transitions, and that there exists a novel phase where particles self-assemble into strings oriented normal to the plane of shear.   

Cubic crystals from cubic colloids

We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing water-soluble polymers as depletion agents. We show that under certain conditions the cubes can self-organize into crystals with a simple cubic symmetry, which is set by the size of the depletant. The dynamic of crystal nucleation and growth is investigated monitoring the samples in time by optical microscopy. Furthermore, by using temperature sensitive microgel particles as depletant it is possible to fine tune depletion interactions as to induce crystal melting. Assisting crystallization with an alternating electric field improves the uniformity of the cubic pattern allowing the preparation of macroscopic (almost defect-free) crystals that show visible Bragg colors.   

Spin-coating of rapidly dried colloidal suspensions: model and experiments

The study of the formation of colloidal crystals has been a very active field in recent years. The spin-coating technique has proven to be a highly reproducible process to form large area colloidal crystals. Here, we present recent results on spin- coating of rapidly dried colloidal suspension. We show that the dynamics observed can be represented by an extension of classical Emslie model to highly volatile fluids. We obtained this extension while maintaining the explicit solution for the temporal evolution of the fluid thickness. We observed that the dynamics can be separated in two regimes: one dominated by non-evaporative effects and a second dominated by evaporative effects. The transition between these two dynamical regimes corresponds well with the transition between two symmetries observed during the fluid phase (six and four-fold). Similarly, the quality of the deposited structure is also well correlated to the relative strength of the capillary forces with respect to the viscous ones.   

Session H10: Semiconductor Surfaces and Interfaces

Reversible vertical manipulation of Ag atoms on Si(111)-(74$\times$7) at room temperature

We have demonstrated a technique to conduct reproducible and reversible vertical manipulation of Ag atoms on the Si(111)-(7$\times$7) surface at room temperature using a scanning tunneling microscope tip. The direction of the transfer of Ag atoms between the sample surface and the tip is simply controlled by the polarity of the bias voltage. Using the 7$\times$7 unit cell as a nanometer size template, complex Ag nano-clusters could be assembled or disassembled by adding or removing Ag atoms in an atom-by-atom manner. With controlled number of Ag atoms filled in a half unit cell, we can construct Ag clusters with up to 25 Ag atoms. The precise control of the number of Ag atoms in the Ag clusters can provide critical information for understanding their physical and chemical properties, and form a fundamental base for the relevant studies of the Ag/Si(111)-(7$\times$7) system and for fabricating nano-devices.   

Probing surface electronic structure with conductance measurements on Si nanomembranes

The surface electronic structure of nanostructures has a strong, sometimes dominant, influence on their transport properties, because of their large surface to volume ratios. Different surface terminations result in different transport behavior, and therefore conductance measurements on nanostructures can be used to study surface and interface electronic spectra. In our experiments, the conductance of the thin (200nm or less in thickness) top Si layer in silicon-on-insulator is measured as the back gate voltage is varied, for both hydrogen terminations and clean reconstructed surfaces in UHV. Experimental results on samples of different thicknesses are compared systematically with simulations to understand the role of the Si/SiO$_{2}$ interface and the electronic structure of the front surface. We explain why the transport behavior of NMs with a clean Si(001) surface is distinct from that with hydrogen termination. Donor type surface states are present in the majority on the hydrogenated surface, and their concentration is on the order of 10$^{12}$ cm$^{-2}$. On the reconstructed Si surface, instead, pseudo-pinning of the Fermi level occurs because of the high density of states of the clean-surface band (2x1 reconstruction) and the presence of surface defect states.   

Experimental evidence for trapped volumes in thin Ag films on Si(111)-7x7

Thin films of Ag on Si(111)-7x7 were prepared in UHV at room temperature by vapor depositing Ag at a glancing angle with respect to the surface normal. Comparing x-ray reflectivity and atomic force microscopy (AFM) measurements, it is found that both techniques give the same height distribution at the surface. However, for a given deposition angle, x-ray reflectivity measurements reveal that there is a significant portion of trapped volume that goes undetected by the AFM. Also, by increasing the deposition angle from normal to glancing incidence angles, both the roughness and the maximum height distribution profile increase. Experimental evidence for trapped volumes in Ag/Si(111)-7x7 will be discussed. Research funding is supported by NSF DMR-0706278. The Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82.   

Growth of Iridium and Silver on Ge(111) Studied by STM

We have used scanning tunneling microscopy (STM) to characterize the growth of iridium and silver onto Ge(111) as a function of coverage and annealing temperature. Ir was deposited onto the Ge(111) c(2x8) surface at different coverages less than 1ML. The Ir forms islands with a ($\surd $3x$\surd $3)R30\r{ } phase and island size increasing with increasing annealing temperature. Stranski-Krastanov growth was observed at most coverages. Ag deposited onto the Ge(111) c(2x8) surface and annealed at 450K forms both a (4x4) phase and a (3x1) phase. The Ge(111) surface reorganizes to a (2x2) phase after deposition of both Ir and Ag. High resolution images have been obtained allowing direct observation of the different phases.   

One-dimensional Mn atom chains templated on a Si(001) surface

Single-atom chains on a wide gap substrate are a very attractive embodiment of a truly one-dimensional system to explore the remarkable physical properties emerging in such low dimensions. We present self-assembled single-atom Mn chains on a Si(001) surface with Bi nanolines, which serve to increase greatly the average length of the Mn chains. They grow perpendicular to the Si(001) dimer rows, at densities which can be adjusted by means of the growth parameter. High resolution scanning tunneling microscopy (STM) micrographs are in perfect agreement with density functional theory (DFT), providing detailed insight into the chain structure. We further discuss low temperature STM spectroscopy and spin dependent DFT modeling suggesting Mn-chains are indeed a suitable candidate to observe electronic and magnetic properties in one-dimension experimentally.   

Low energy alkali ion-surface charge exchange for Si(111) as a function of doping

Alkali ion-surface charge exchange, which can be used to probe surface electronic states, is well understood within the context of the resonant charge transfer (RCT) model. Recent studies have extended the use of alkali ion scattering and the RCT model from metal surfaces to semiconductors and insulators. In the present work, we measure the effect of doping type and concentration on the neutralization probability of alkali ions scattered from semiconductors. Si(111) surfaces were prepared in UHV, and the neutralization probability of scattered Li$^+$ ions was measured for projectiles that were singly scattered from Si atomic sites. For the clean Si(111)-$7 \times 7$ surface, the neutralization is determined by the surface electronic states [1] and is independent of doping. Samples were then dosed with atomic hydrogen in order to passivate the surface states and unpin the Fermi level. This affects the neutralization probabilities and reveals differences between n and p-type materials. \\[4pt] [1] Y. Yang and J.A. Yarmoff, Phys. Rev. Lett. 89, 196102 (2002).   

Binding sites and diffusion barriers of a Ga adatom on the $\textrm{GaAs(001)-\emph{c}(}\textbf{4}\times\textbf{4}\textrm{)}$ surface from first-principles computations

The Ga adatom adsorption and diffusion processes on the $\textrm{GaAs(001)-\emph{c}(}4\times4\textrm{)}$ surface were studied using \emph{ab initio} density-functional-theory computations in the local density approximation. Two distinct sets of minima and transition sites were discovered for a Ga adatom relaxing from heights of 3 and 0.5 {\AA} from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers. An electronic signature of the differences in this interaction was identified. We computed the energetic barriers to diffusion for various adsorption sites. From these, we propose three pathways for diffusion of a Ga adatom on this surface which indicate anisotropic diffusion along different directions.\footnote{Supported by NSF DMR 0705464.}   

Surface Structure of Bi-terminated GaAs Grown with Molecular Beam Epitaxy

Control of III-V semiconductor surfaces is crucial for high-quality device production. A means of interface control involves the use of Bi as a surfactant, which both smooths the surface and alters the surface reconstruction. We examined the effects of Bi deposition via molecular beam epitaxy on the GaAs(001) surface structure using reflective high energy electron diffraction and scanning tunneling microscopy. After $\sim $0.2 ML of Bi deposition, scanning tunneling microscopy revealed a disruption in the initial c(4$\times $4) reconstruction. Coverages of $\sim $0.4 ML and $\sim $0.6 ML produced a (1$\times $3) and a (2x3) diffraction pattern, respectively, and an atomic surface structure consisting of a disordered row reconstruction, $\beta $2(2$\times $4) reconstruction rows, and surface clusters, with 1 ML deep pits at a coverage of $\sim $0.6 ML. Calculations show these changes in surface structure and morphology are likely not the result of As desorption, but due to the presence of Bi on the surface. These observations may help explain the origin of Bi clusters that give GaAsBi most of its unique properties.   

Design of carbide thin film coatings from first principles

Transition metal carbides have many interesting physical properties and have therefore been used in many technological applications, e.g. as thin film metal coatings. A commonly studied thin film coating material is nc-TiC/a-C, where nanocomposites (nc-) of TiC are dispersed in an amorphous (a-) C matrix. An interesting feature of these types of materials is the possibility to design the material to obtain new functionality, e.g. by tuning the C to Ti content. In this talk we will present results obtained by first principles density functional theory calculations of a different approach, where various metals have been alloyed into TiC. Depending on the alloying metals ability to form carbides this will yield different effects. One of these effects is the creation of a driving force for the release of C from the carbide. This C release has been shown to yield favorable lubricating properties of nc-(Ti,Al)C/a-C thin films. We will show that the C release can be tuned by a careful selection of the alloying metal in order to optimize the properties of these types of thin film carbide coatings.   

THz optical Hall-effect and MIR-VUV ellipsometry characterization of 2DEG properties in HfO$_{2}$ passivated AlGaN/GaN HEMT structures

We present non-contact, optical measurements of free-charge carrier mobility, sheet density, and effective mass parameters of the 2DEG for different HfO$_{2}$ passivated AlGaN/GaN high electron mobility transistor structures at room temperature. Spectroscopic ellipsometry (SE) in the spectral range from THz and Mid-IR to the VUV and THz optical Hall-effect (generalized ellipsometry in magnetic fields) (OHE) are employed. Changes in the HfO$_{2}$ layer growth conditions are found to drastically influence the electron density of the channel. The sheet density and the carrier mobility obtained by the optical investigations are in excellent agreement with results from electrical Hall-effect measurements. The electron effective mass parameters determined here using the OHE corroborate previous SdH and cyclotron resonance studies. The surface sensitivity of VUV-SE in combination with OHE allows for correlation of surface passivation and changes in the 2DEG properties.   

Low Temperature Epitaxial Growth of Ge Quantum Dots on Si(100)

The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted in ultrahigh vacuum. A chirped pulse amplified Ti:sapphire laser with $\sim $60 femtosecond pulse width, center wavelength $\sim $800 nm, and operating at 1 kHz repletion rate was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. \textit{In situ} reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and \textit{ex situ} atomic force microscopy (AFM) were used to study the morphology of the grown QDs. For Ge coverage of 12 monolayer, it was observed that the excitation laser reduces the epitaxial growth temperature to 70 \r{ }C, at which no epitaxy is possible without excitation. By using nanosecond Nd:YAG laser for ablation and excitation, it was shown that applying the excitation laser to the substrate during the growth changes the QD morphology and island density and improves the size uniformity of the QDs at 390 \r{ }C. RHEED recovery curves show that the excitation laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved.   

Large scale atomic engineering of silicon (100) surfaces

Control of atomic morphology at the micrometer scale has been a long term challenge to enable atomically precise manufacturing. In this presentation we describe our method to pattern micrometer scale, ordered features on a Si surface with subsequent etch and high temperature processing. Following high temperature UHV processing, high quality atomically-ordered surfaces are imaged using atomic-resolution STM. A significant attribute of these surfaces is that the micrometer scale features evolve, but persist, allowing external location of nanometer scale features as well as comprehensive control of atomic terrace sizes and step bunching. A multi step thermal process is used, resulting in surface with symmetric, reproducible step-terrace patterns and very wide atomically flat regions. A kinetic Monte Carlo (KMC) model is used to simulate the current induced electromigration process which is primarily responsible for the long range evolution of surfaces.   

Theory of Scanning Tunneling Microscopy of Dangling Bonds on Silicon Surfaces

Silicon surface dangling bonds (DBs) are electronic gap states with eigenenergy close to the middle of the bandgap of bulk silicon and can be explored as quantum dots. During exploratory fabrication and characterization of DB-structures on H-Si(100) surfaces, scanning tunneling microscopy (STM) imaging showed sharp halo-like features around single DBs that cannot be explained by the standard STM theory. Halo appearance varies with sample doping level and imaging conditions (sample bias and current). Here we investigate the nature of such features in the STM imaging of DBs. We propose a theory of image formation based on non-equilibrium charge transfer balance (via elastic and inelastic channels), from the STM tip to DB on one hand, and from DB to bulk Si on the other. For empty-state imaging mode, in the immediate proximity of a DB ($<$1nm) tip-induced band bending shifts the DB-level to a value between the Fermi levels of the STM tip and of the sample. Consequently, a steady-state of charge flow is established through the DB state, which dictates the time-average amount of charge on the DB. This in turn affects the total STM current in that proximity leading to the appearance of a halo.   

The Electronic and Transport Properties of Si(111)-7$\times $7 and Related Reconstructions

The 7$\times $7 reconstruction of Si(111) has the interesting property of being metallic despite bulk Si being a semiconductor. This surface has a complex reconstruction that takes on a dimer-adatom stacking fault (DAS) structure composed of adatoms, rest atoms, and several other key features. It is believed that the conductivity occurs through the dangling bonds of the adatoms, and that it is entirely a surface effect. To elucidate the details of this mechanism, we have investigated a set of related Si(111) reconstructions of increasing complexity in order to resolve the effect of the different DAS features on the electronic and transport properties of the Si(111)-7$\times $7 surface. Density functional theory (DFT) calculations have been carried out on the $\surd $3$\times \surd $3-$R$30\r{ }, 2$\times $2, 5$\times $5, and 7$\times $7 reconstructions of Si(111). Additionally, our work is extended to electron transport simulations employing the non-equilibrium Green's function technique coupled with DFT (NEGF-DFT) to calculate the conductance for these systems. Finally, the effect of atomic steps and adsorbates on the conductive properties will also be discussed.   

Size dependent superconductivity of Pb islands grown on Si (111)

The superconductivity of nano-sized Pb islands grown on Si (111) with different size at 9 monolayer thickness was studied by low temperature scanning tunneling spectroscopy. By measuring the zero bias conductance as a function of temperature, for larger islands we observed a transition from pseudogap state at high temperature to superconductivity state at low temperature through two distinct slopes, where the superconductivity transition temperature ($T$c) of the island can be determined. For island size of $\sim $58 nm$^{2}$, a large drop in $T$c is found; when the size is further reduced to about 30 nm$^{2}$, no superconducting state was observed down to the measured temperature of 3.2 K. By properly subtracting the background and pseudogap effect, information on the temperature dependent superconductivity gap can be obtained. The ratio of $\frac{2\Delta _0 }{k_B T_c }$ decreased from 4.5 to 3.3 with the reduction of island size, showing that the electron-phonon coupling becomes weaker as the size decreases.   

Session H11: SPS Undergraduate Research III

Nanodynamics of Ferroelectric Ultrathin Films

An active area of research in nanoscale science is the study of ferroelectric ultrathin films. We will report a first-principles-based study of the nanodynamics in ferroelectric Pb(Zr$_{0.4}$Ti$_{0.6}$)O$_3$ films with thickness 20-192 nm. In our computational experiment we first anneal such films under realistic conditions of partial screening of the surface charge to obtain the ground state nanodomain pattern. After that the films are subjected to $ac$ electric fields with frequencies varying from 0.1 THz to 4.0 THz and close to nanodomain resonance frequency. The domain evolution is then studied as a function of time, electric field frequency, and film thickness in order to quantitatively characterize the laws and parameters associated with it. This allows us to reveal for the first time ever intrinsic high-frequency dynamics of ferroelectric nanostripe domains.   

Thermal Conductivity of Random Multilayer Thin Films

Thermoelectric based energy scavenging has tremendous potential for the recovery of waste heat and temperature regulation. Manufactured thermoelectric devices today are limited in efficiency, and therefore widespread use, by high lattice thermal conductivity. In an effort to minimize lattice conductivity with respect to electrical conductivity, opportunities for utilizing the Anderson localization of phonons have been explored. In particular, the thermal conductivity of a model random multilayer thin film with Lennard-Jones bonding has been determined using classical reverse non-equilibrium molecular dynamics as a function of mass induced disorder. Results indicate that the inclusion of random planes in which the atomic mass has been increased by a factor of ten can produce reductions in lattice thermal conductivity by over a factor of one hundred. The dependence of thermal conductivity on the magnitude and nature of this disorder has been measured. Finite size effects have been quantified and a length scale has been determined on which they can be neglected. These results indicate that the pursuit of nanostructured thermoelectric materials in the form of random multilayers may provide a path to efficient and sustainable thermoelectric materials.   

Controlling the microstructure of binary carbide films with elemental substitutions

We report on experiments to control the microstructure of textured binary carbide thin films deposited by reactive magnetron sputter deposition. Controlling the microstructure in these materials is important as the microstructure of these films provides a template for the resulting carbide-derived carbon (CDC) film and impacts their performance. Specifically, a combinatorial approach is used to add chromium to TiC films creating a compositional gradient as a function of position. We present a measurement of surface roughness as a function of material composition. The resulting materials, (Ti$_{1-x}$Cr$_{x})$C films, are significantly smoother than their pure TiC counterparts and the resulting CDC's have correlated defects which will improve the performance of the CDC in supercapacitor applications.   

Polypeptide Chirality Influences Multilayer Thin Film Growth and Structure

Polypeptide multilayer thin films are being developed for a variety of applications.These include coatings for implant devices and systems for drug delivery in thebiomedical sciences, and optical coatings. Subsequent polymer adsorption steps involve polymers of opposite polarity. Here, the polymers were polypeptides. This project compared the consequences of changing polypeptide chirality on film growth and structure. The peptides were poly(L-glutamic acid), its right-handed counterpart, poly(D-glutamic acid), and poly(lysine-tyrosine). The first two are negatively charged at neutral pH, the third one is positively charged. Poly(lysine-tyrosine)/poly(L-glutamic acid) films and poly(lysine-tyrosine)/poly(D-glutamic acid) films werefabricated on 1 mm-thick quartz plates. In one experiment, films were grown to 34layers. The UV absorption spectrum was taken after each layer deposited to determinethe rate of polymer self-assembly. Separately, UV or visible wavelength spectra wereobtained for films stained with a dye cooled/heated in the range 4-65 \r{ }C. In anotherexperiment, a mixture of poly-L-glutamic acid and poly-D-glutamic acid was used as thepolyanion for film buildup. The data show that poly(lysine-tyrosine)/poly(L-glutamicacid) films built up at a higher rate than the corresponding right-handed films.   

Thermoelectric Properties and Microstructure of Ca$_{3}$Co$_{4}$O$_{9}$ thin films on SrTiO$_{3}$ and Al$_{2}$O$_{3}$ Substrates

Ca$_{3}$Co$_{4}$O$_{9}$ (CCO), a misfit layered structure exhibiting large Seebeck coefficient at temperatures up to 1000K has attracted increasing attention as a novel high-temperature thermoelectric material. In this work, we investigate CCO thin films grown on SrTiO$_{3}$ (001) and Al$_{2}$O$_{3}$ (0001) using pulsed laser deposition. Quality of the thin films was examined using high-resolution transmission electron microscopy and thermoelectric transport measurements. HRTEM images show incommensurate stacks of CdI$_{2}$-type CoO$_{2}$ layer alternating with rock-salt-type Ca$_{2}$CoO$_{3}$ layer along the c-axis. Perovskite buffer layer about 10nm thick was found present between CCO and SrTiO$_{3}$ accompanied by higher density of stacking faults. The CCO grown on Al$_{2}$O$_{3}$ exhibited numerous misoriented grains and presence of Ca$_{x}$CoO$_{2}$ phase. Seebeck coefficient measurements yield an improvement for both samples compared to the bulk value. We suggest that thermoelectric properties of CCO increase due to additional phonon scattering at the stacking faults as well as at the film surfaces/interfaces.   

Antireflective Coatings using Layer-by-Layer Self Assembly of Silica and Titania Nanoparticles

It is known that glass substrates (borosilicate glass) reflect about 4{\%} of light at each air/glass interface and thus, they transmit only 92{\%} of light. For some devices like camera lenses, it is important to maximize the amount of transmitted light. Previous research has demonstrated that it is possible to do so by adding antireflective coatings to the substrates. Our research aimed to deposit thin films on glass substrates that would minimize the reflectance of light and thus, maximize its transmittance. The thin films consisted of multiple alternating layers of silica and titania nanoparticles following the theory behind double-quarter periodic systems and were deposited on the substrates via the ISAM (ionically self-assembled monolayers) technique. Several experiments were conducted in order to investigate the factors that affected the quality of the coatings and some of the significant factors observed were the pH and the molarity of the silica, titania and PDDA solutions. A number of factor-level combinations yielded transmittances in excess of 96{\%}, well above the value for uncoated substrates.   

Compositional dependence of the narrow band emission from zinc oxide nanowires

Zinc oxide is a versatile platform thanks to the unique combination of optical, semiconducting, and piezoelectric properties of ZnO. The properties can be further diversified by creating microstructures and by varying the Zn/O ratio in the crystallites. We are illustrating this concept for the case of narrow band emission for ZnO nanostrutures grown through chemical vapor transport. The samples were characterized by photoluminescence spectroscopy (pulsed and continuous wave), scanning electron microscopy, and energy dispersive x-ray spectroscopy. Narrow band emission has been observed in the pulsed excitation mode. The narrowing is intensity dependent suggesting a mechanism of stimulated emission. The emission properties were correlated with the degree of oxidation of the ZnO nanocrystallites and with the presence of optically active defects. The influence of different oxidizing agents on the emission properties of the ZnO nanocrystals will be shown.   

Dependence of the band gap of highly confined CdSe and PbSe nanocrystals on temperature

We have recorded fluorescence spectra from PbSe and CdSe quantum dots in hexane/toluene respectively between 5K and 300K in order to investigate the temperature dependence of the electronic band gap of these highly confined nanostructures. The band gap for CdSe follows the known blue shift with decreasing temperature. We have measured the temperature dependence of the band gap of PbSe quantum dots for two different diameters below 4 nm and indeed observe a red shift of the band gap with decreasing temperature, which is stronger for the smaller size quantum dots. Such behavior would contradict the expected blue shift of the band gap with decreasing temperature. The origin of this peculiar behavior is not well understood and we are pursuing further theoretical and experimental studies in order to elucidate the mechanism behind it. Using the method of single-nanostructure laser spectroscopy will allow us to observe individual nanostructures while simultaneously removing ensemble averaging effects due to quantum interactions between multiple structures.   

Synthesis and characterization of ZnO nanocrystals co-doped with Ce$^{3+}$ and Tb$^{3+}$

Rare earth doped zinc oxide nanocrystals produce visible emissions under ultraviolet excitation. Using a sol-gel process, we synthesized a series of ZnO nanocrystals doped with Tb$^{3+}$ and Ce$^{3+}$ in silica glass, keeping the ZnO/SiO$_2$ ratio constant at 10/90 and doping with 1\% rare earth by weight, with varying relative concentrations of Tb$^{3+}$ and Ce$^{3+}$. The nanocrystals were characterized using photoexitation and emission spectroscopy, time-resolved photoluminescence, UV/VIS spectroscopy, and transmission electron microscopy. We determined that co-doping with cerium enhanced the visible terbium emissions to a point, with the most effective enhancement occurring at mid-range Ce$^{3+}$ concentrations.   

Growth and Morphology of High Mobility Organic Semiconductors

We utilize atomic force microscopy (AFM) to image the growth and morphology of chemically modified, solution-deposited anthradithiophene transistors. We discuss the effects of backbone modifications on crystal structure, film properties, and electrical device performance. These devices display a mobility of 0.001 cm$^2$/Vs to 1 cm$^2$/Vs. Crystal orientation and film structures, such as film thickness, grain size, and growth modes will be discussed. In addition, AFM images are related to diffraction data and conduction channel crystallographic information is extracted.   

Effect of the polymer concentration on the ON/OFF states of a TN-LCD: polyvinyl alcohol vs. soy lecithin

In this work we study the response of a Twisted Nematic Liquid Crystal Display (TN-LCD) by varying both the concentration and the polymer used for the microgroove. We compare the performance of two polymers: polyvinyl alcohol and soy lecithin. In particular, the light transmission for the ON/OFF states is evaluated. The polyvinyl alcohol is a polymer widely used in LCDs while lecithin soy is a natural polymer.   

Tunable Schottky diodes fabricated from electrospun crossed SnO$_{2}$/PEDOT-PSSA nanoribbons

Hardware in most solid state devices contains at least one interface between a $p$-type and an $n$-type semiconductor. Such hetero-junctions are typically fabricated from all inorganic Si based materials. In the past two decades however, devices fabricated from organic-inorganic semiconductors that are not Si based, or from all organic semiconductors have been the focus of much research. Semiconducting $n$-doped metal oxides are also attractive for use in devices and of particular interest is tin oxide (SnO$_{2})$ as it is stable in air and is optically transparent with a band gap of $\sim $3.4 eV. The $p$-doped conducting polymer PEDOT-PSSA is also stable in air and is widely used in flexible devices. We shall report on the electrospinning technique to fabricate in air Schottky diodes, by simply crossing $n$-doped SnO$_{2}$ and $p$-doped PEDOT-PSSA nanoribbons. The device parameters could be tuned by a back gate bias and by shining UV light. The diode parameters were calculated using the standard thermionic emission model of a Schottky and was tested as a half wave rectifier.   

UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics

Organic photovoltaic cells are prepared using an active layer containing a functionalized C60 molecule, [6-6]-phenyl C61 butyric acid octadecyl ester (PCBOD); and a conjugated polymer, poly(3-hexylthiophene) (P3HT). PCBOD functions as an electron acceptor in conjunction with P3HT, the electron donor. Both current-voltage (IV) transport data of solar cells and spectroscopic absorption data of the corresponding active layer are collected at regular time intervals for periods up to several days. IV data show changes in power conversion efficiency which are strongly dependent on device preparation (stoichiometry, annealing, etc.). Ultraviolet and visible light absorption exhibits similar time dependence. Recent results show that annealing the active layer up to 200\r{ }C substantially improves device performance. Further spectroscopic studies, such as Carbon-13 NMR spectroscopy, are ongoing.   

Modeling of Quantum Cascade lasers with different waveguide profiles

Quantum Cascade (QC) laser-based sensor systems help us monitor the environment through the detection of trace chemicals that have optical spectra in the mid-infrared. For the laser to become more efficient and usable, the thermal management and the optical and electrical properties of the laser waveguides need to be more closely examined. The performances of QC lasers with different waveguide profiles have so far not been systematically compared and the device optimization for the three design components has not yet been coupled together. Here, we use a finite element solver to calculate the active region peak core temperature, the optical confinement factor and waveguide loss, and the local current density, and compare these for QC lasers with dry- and wet-chemical etch profiles, i.e. with vertical or sloped sidewalls, respectively. Initial results show a preference for wet-etched profiles under thermal conductivity considerations.   

Photoluminescence studies of WO$_{3}$ and WO$_{3-x }$ single crystals

WO$_{3}$ is an important material to study, not only due to its interesting electronic properties, but also because it has other applications in both electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades [1]. We have studied two WO$_{3}$ single crystals, a transparent and doped WO$_{3-x}$, in an attempt to understand this effect. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not appear to be correlated with temperature. However, polarization studies reveal at least two polarization dependent components of the center. \\[4pt] [1] Satyen K. Deb, Solar energy materials and solar cells \textbf{92}, 245 (2008), and the references therein   

Session H12: Focus Session: Dopants and Defects in Semiconductors: Silicon

Isotopic Fingerprints in the Luminescence of Deep Defects in Silicon

In a series of recent papers [1, 2] we have shown that the dramatic improvements in spectral resolution made possible in highly enriched $^{28}$Si can provide surprising new information on the detailed constituents of deep luminescence centers. The `isotopic fingerprints' reveal the presence, and number, of different chemical species involved in the deep centers. While many of these luminescence centers have been studied for decades, this new technique revealed that \textit{none} of these was what it was thought to be. Armed with this new information, many new centers have been discovered, containing either four or five atoms chosen from among: Cu, Ag, Au, Pt and Li. There is at present no theoretical explanation for the stability and ubiquity of these centers in rapidly thermally quenched silicon. \\[4pt] [1] M. Steger et al., Phys. Rev. B 81, 235217-1-6 (2010). \\[0pt] [2] M. Steger et al., Phys. Rev. Lett. 100, 177402-1-4 (2008).   

Theoretical study of the Cu$_{\rm PL}$ defect in Si

Copper is a common contaminant in Si processing. When in supersaturation, a fraction of 1\%\ of the Cu in the sample forms an electrically-active defect easily seen by photoluminescence. This Cu$_{\rm PL}$ defect in Si has a no-phonon line at 1014 meV. It has long been believed to consist of an interstitial copper (Cu$_i$) weakly bound to a substitutional copper (Cu$_s$) : The \{Cu$_s$Cu$_i$\} pair. However, PL studies in isotopically pure $^{28}$Si crystals have shown that the defect contains not two but four copper atoms [1]. We examine the possibility that the core of the defect consists of not one but two adjacent substitutional Cu atoms. This core traps two Cu$_i$ atoms, resulting in defect with $D_{3d}$ symmetry. We will discuss its formation mechanism and stability, and show that they are consistent with the conditions at which Cu$_{\rm PL}$ is observed. If this model is correct, then then DLTS lines associated with Cu$_s$ should be re-assigned to \{Cu$_s$Cu$_s$\}. \\[4pt] [1] M. Steger {\it et al}., Phys. Rev. Lett. 100, 177402 (2008)   

EPR parameters of the dangling bond defect in crystalline and amorphous silicon: A DFT-study

Thin-film a-Si:H solar cells are considered as low-cost alternatives to bulk crystalline silicon (c-Si) solar cells. A disadvantage of these devices is that their efficiency is severely limited by light-induced defects (Staebler-Wronski effect). In this context, electron-paramagnetic resonance (EPR) is a key technique to probe for the local atomic structure of defects with unpaired spins such as the silicon dangling bond. However, the assignment of the EPR signal to a specific defect structure requires comparison to theoretical models. Using density-functional theory, we address structure-property relationships by combining systematic studies for idealized dangling-bond models in c-Si with a statistical analysis of a variety of dangling bonds in a-Si:H supercells. Our studies reveal the influence of the local geometry on sp-hybridization and delocalization. Yet, the structural variability of a-Si:H cannot be captured by these idealized defect models alone. Rather, our calculations indicate that a relatively broad distribution of dangling-bond like structures gives rise to the experimental signal supporting a recent re-evaluation of EPR parameters from multifrequency EPR.   

Electrically Detected Pulsed ENDOR in Phosphorus-Doped Silicon

We demonstrate the electrical detection of X-band electron nuclear double resonance (ENDOR) in phosphorus-doped silicon at 4\,K. A pulse sequence analogous to Davies ENDOR in conventional electron spin resonance is used to measure the nuclear spin transition frequencies of the $^{31}$P nuclear spins, where the $^{31}$P electron spins are detected electrically via spin-dependent transitions through Si/SiO$_2$ interface states. In addition, electrical detection of coherent nuclear spin oscillations is shown, demonstrating the feasibility to electrically read out the spin states of possible nuclear spin qubits. Combining the enhanced sensitivity of electrically-detected magnetic resonance and the wide range of applications of pulsed ENDOR, this techniques could be a versatile tool to study paramagnetic defects in semiconductor nanostructures.   

Electroelastic Hyperfine Tuning of Phosphorus Donors in Silicon

We demonstrate an electroelastic control of the hyperfine interaction between nuclear and electronic spins opening an alternative way to address and couple spin-based qubits. The hyperfine interaction is measured by electrically detected magnetic resonance in phosphorus-doped silicon epitaxial layers employing a hybrid structure consisting of a silicon-germanium virtual substrate, a piezoelectric actuator, and a loop-terminated coplanar strip line for on-chip microwave magnetic-field generation. By applying a voltage to the actuator, the hyperfine interaction is changed by up to 0.9~MHz, which would be enough to address spin-qubits in isotopically purified $^{28}$Si with a sufficient fidelity under optimized conditions.   

Characterizing Individual Group V donors in Silicon

The study of dopants in silicon has been rapidly growing in importance because the dimensions of semiconductor devices have now decreased to the point where their functionality relies upon only a few atoms. Group V donors are especially interesting due to their potential application in spintronics and quantum computing. Whereas P dopants have been extensively studied, comparatively little is known about the characteristics of other group V donors. Using a combination of ion implantation and cross-sectional scanning tunneling microscopy (XSTM) and, we study individual Bi and Sb atoms in the cleaved Si(111)2x1 surface. High-resolution STM topography images and scanning tunneling spectroscopy (STS) data allow us to probe the structural and electronic properties of these individual dopants in silicon. Density functional theory (DFT) calculations further support our structural assignments.   

Ab initio shallow impurity level calculations in semiconductors

Binding energies of B, Al, Ga, In and Tl shallow acceptors in bulk Si were calculated using a GW + Semi-empirical procedure. Within the procedure, both density functional theory calculation within local density approximation (LDA) and GW calculation were performed. In the LDA calculation, a large supercell containing tens of thousands of Si atoms and the center impurity atom was constructed from a potential patching procedure. The central potential of this system was further corrected by 64 atom GW calculations. The folded spectrum method was used to calculate the eigen energies of this large supercell containing the center impurity. The calculated binding energies show good agreement with experimental impurity binding energies. This procedure represents an efficient approach to study shallow impurity levels which are important for semiconductor devices.   

First principles study of phosphorus and boron defects in Si-XII

We present a first-principles study of phosphorus and boron substitutional defects in Si-XII, a polytype of silicon in the R8 structure. Recent results from nanoindentation experiments reveal that this phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. We examine the formation energies of the B and P defects at the two distinct atomic sites in the R8 structure. We also calculate the thermodynamic transition levels of each defect in its relevant charge states.   

Stability of donor-pair defects in $Si_{1-x}Ge_x$ alloy nanowires

Semiconductor nanowires (NWs) have attracted much attention because of the quantum confinement effect, large surface-to- volume ratio, and compatibility with the existing Si technology. Although impurity doping is important for applications to optoelectronic devices, it is generally difficult to dope nanostructures due to segregation of dopants to the surface, high activation energies induced by the surrounding low dielectric medium, and compensation by defects such as surface dangling bonds. Furthermore, compared with bulk Si, electrically deactivating donor-pair defects are energetically more favorable than isolated shallow donors in NWs. In this work, we perform first-principles density functional calculations to study the stability of donor-pair defects and the doping efficiency in $Si_{1-x}Ge_x$ alloy NWs doped with P impurities. The stability of donor-pair defects is enhanced as the Ge concentration increases. Consequently, the doping efficiency in $Si_{1-x}Ge_x$ alloy NWs is expected to be suppressed by the formation of donor-pair defects, similar to previous calculations for Si NWs with small diameters. The effects of reduced dimensionality, Ge chemical bonding, and strain on the stability of donor-pair defects in alloy NWs are discussed.   

New electronic effects observed on n-type Si(111)2x1 using cross-sectional STM

Cross-sectional scanning tunneling microscopy (XSTM) of in-situ cleaved semiconductor surfaces has two distinct advantages over STM experiments where studies are performed on the surface of the annealed and/or sputtered semiconductor wafers. Firstly, the cleaving process exposes a clean surface without the usual need for high temperature annealing, thus revealing a surface that has not been driven to its thermodynamic minimum energy state. Secondly, the surface being imaged is perpendicular to the surface of the original wafer, which is of particular value for the study of implanted or epitaxially overgrown wafers. We use XSTM measurements, spatially resolved scanning tunneling spectroscopy (STS) and density functional theory (DFT) to study the electronic properties of the cleaved (111)2x1 surface of silicon. We examine bulk-doped, and ion-implanted samples. Our studies reveal new, long range, electronic effects that have implications for future nanoscale devices in silicon.   

Charged Defects in the Si(001) Surface

The Si(001) surface has been the subject of intense research for decades due to its ubiquitous use in the semiconductor industry, its applicability as a model semiconductor surface, and proposals for its use in novel quantum devices. Surprisingly, atomic-scale investigations using scanning tunneling microscopy and spectroscopy (STM/STS) continue to produce new insights into the structural and electronic properties of this deceptively simple semiconductor surface. Tip- and charge-induced band bending are generally considered to play only minor roles in measurements of silicon surfaces due to Fermi level pinning by surface states and defects. However, such effects become important when investigating charged defects and/or surfaces that have had their surface states removed through chemical passivation. We present high resolution STM images and spectroscopy data of defects in the Si(001) surface. We include band bending and charge state in the discussion of the results.   

Near surface dopant depletion in UHV prepared H-Si(100): spectroscopic and imaging effects

Dangling bonds (DBs) have been shown to be useful in directing chemical reactions on silicon and for atom scale electronics such as quantum cellular automata. One enabling aspect of DBs is that they can assume various charge states depending on the type and level of crystal doping. We have found that for degenerately doped n-type silicon, the scanning tunneling spectroscopy (STS) and imaging characteristics H-Si(100) surfaces and DBs varies depending on the preparation method. Samples heated to 1050\r{ }C were found to have a consistent level of doping throughout the bulk and near surface regions. Samples heated to 1250\r{ }C showed a reduced dopant concentration in the near surface region. STS showed shifted I/V spectra. The loss of degeneracy was indicated by the loss of tunneling through dopant states in the band gap. These results show that UHV prepared silicon does not have a consistent dopant profile and that the bulk dopant density should not be assumed in the near surface region. This has important ramifications for DB imaging and modeling.   

Ab initio study of water molecule dissociation on the hydrogenated Si(100) surface

The reaction mechanisms for the H$_2$O molecule dissociation at the Si surface, and the resulting oxidation sites, are still object of debate. Here, we present a detailed theoretical investigation of the reaction pathways for the dissociation of water on the Si(100)(2x1):H surface, starting from different initial ``attack'' sites and leading to different final, oxidized configurations. We use extended-surface (slab) models, working within DFT, with pseudo-potentials and plane wave basis set in the quantum-espresso code. The pathways were mapped using the CI-NEB method with both local and gradient corrected exchange-correlation functionals in order to obtain a fair estimate of energy barriers. Our results indicate that the oxidation routes suggested by earlier experimental works are not favored. We propose two new oxidation routes, with simultaneous release of one H2 molecule: one related to chemisorption of the oxygen atom on the Si-Si dimer bond, and another related to the absorption on the back-bond. Analysis of energy barriers showed that these two new possibilities are both kinetically and energetically viable. We also present analyses of the profiles obtainable through STM for the investigated structures, which should help experimental identification.   

Session H13: Focus Session: Jamming Theory and Experiment I

Jamming in Disordered and Ordered States: From RLP to FCC

The concept of jamming was originally introduced in the context of zero-temperature, frictionless sphere packings through which the jamming transition was identified with the more familiar idea of random close packing. More recently, the jamming behaviour for particles with friction has led to a practical definition of the less well-defined random loose packed limit. However, there are a number of subtleties associated with jamming that extend these concepts further. Here we implement a range of protocols to generate jammed packings both with and without friction, and find that the jamming transition actually consists of a finite region in packing fraction depending on the protocol used to create the jammed state. Furthermore, we examine how it is possible to tune the structural properties of jammed packings from the disordered regime through to the ordered face centred cubic lattice, and the subsequent changes in the jamming properties as the structure is manipulated.   

Long wavelength behavior of the static structure factor in jammed packings

There are several features associated with the jamming transition in monodisperse sphere packings. One recently reported property is the anomalous long wavelength behavior of the static structure factor, $S(k)$. An unusual linear dependence with the wavenumber $k$, becomes increasingly pronounced on approach to the jamming transition. However, it remains unclear how polydispersity and force model affect this behavior. Here, we study the structure factor of jammed disordered bidisperse sphere packings using computer simulations, especially its behavior in the long wavelength regime ($k \to 0$). We evaluated the structure factor using an appropiate formalism for polydisperse systems and extract information on the susceptibility in the low-$k$ limit.   

Topology of the force field in a jammed granular systems exposed to an intruder

It is well known that the structure of forces and stresses in granular systems goes through significant changes close to jamming. In this talk, we will present precise and objective measures of these changes based on topological properties of the force field in granular systems exposed to compression and shear. Then, we will discuss how these measures evolve in granular systems during an impact of a large intruder. We will particularly concentrate on the role of packing, polydispersity, and friction on structure of the force field.   

Critical Scaling of Shearing Rheology at the Jamming Transition of Soft Core Frictionless Disks

We perform numerical simulations to determine the shear stress and pressure of steady-state shear flow in a soft-disk model in two dimensions at zero temperature in the vicinity of the jamming transition $\phi_J$. We use critical point scaling analyses to determine the critical behavior at jamming, and we find that it is crucial to include {\it corrections to scaling} for a reliable analysis. We find that the relative size of these corrections are much smaller for pressure than for shear stress. We furthermore find a superlinear behavior for pressure and shear stress above $\phi_J$, both from the scaling analysis and from a direct analysis of pressure data extrapolated to the limit of vanishing shear rate.   

Glassiness, Rigidity and Jamming of Frictionless Soft Core Disks

The jamming of frictionless bi-disperse soft core disks is considered, using a variety of different protocols to produce the jammed state. We find, consistent with earlier works, that cooling and compression can lead to a broad range of jamming packing fractions $\phi_J$, depending on cooling or compression rate and on initial configuration. Such $\phi_J$ show no clear upper bound as the cooling or compression rate decreases. In contrast, we show that shearing leads to a jamming transition to a disordered solid, with a well-defined, non-trivial, value of $\phi_J$ as the shearing rate vanishes. We show that shearing breaks up the particle clustering (the precursor to phase separation) that can lead to increasing values of $\phi_J$ under slow cooling or compression, and argue that the process of shearing creates a well-defined ensemble that is independent of the starting configuration.   

Exact tools for 2D granular packings

Building on the loop force formulation of Ball and Blumenfeld\footnote{PRL 88 115505, 2002}, a new, exact potential formulation is given for two dimensional, static packings of frictional, monodisperse disks. Using degree-of-freedom counting and explicit constructions, it is shown that the natural graph for analysis of stress distribution in such packings is the Delaunay triangulation. Edges of this graph which do not correspond to contacts yield ``virtual contact'' vectors, which are shown to be of great physical importance. In particular, the new potential satisfies force and torque balance identically and is subject only to the Coulomb constraint and a new set of physically transparent constraints on the ``virtual contacts.'' Using the new coordinates, previous results on the contact force distribution are rationalized, and a unified framework is presented for understanding the sources of correlation between contact forces. A new maximum-entropy argument is presented to derive the contact force distribution, and the dependence on shear, friction, and coordination number is discussed.   

Particle-to-Particle Dynamics in a Granular Pile Subject to Cyclic Shear

We report a study of the particulate motions of a granular pile under cyclic shear and how they relate to the bulk rheological properties of the pile. Using a laser sheet scanning technique, we track the trajectories of all of the particles within a section of a split-bottom shear cell. We shear the pile quasistatically to ensure rate independence of shear stress. Immediately after reversal of the shear direction, we observe a transient drop in shear stress of the pile over a characteristic strain. We construct a network of nearest neighbors that roll or slide past one another between frames. We find that, for strain amplitudes less than the aforementioned characteristic strain, rolling/sliding links are extinguished with higher frequency than for larger amplitudes. We also report other particle level measures, such as mean squared displacements, for various amplitudes of oscillatory shear.   

Ratio of effective temperature to pressure controls the dynamics of sheared hard spheres

Using molecular dynamics simulations, we calculate the effective temperature, $T_{\rm eff}$, and the pressure, $p$, of steadily sheared mixtures of hard spheres of mass $m$ and diameters $\sigma$ and $1.4 \sigma$ in contact with a thermal reservoir at temperature $T$. We vary the packing fraction, $\phi$, and the shear stress, $\Sigma$. We define $T_{\rm eff}$ from the ratio of correlations to response and show that different correlation-response relations yield a consistent numerical value $T_{\rm eff}\ge T$ that reduces to $T_{\rm eff}=T$ when $\Sigma=0$. We show that the effective temperature represents the limiting value of the effective temperature for soft spheres in the limit $p\sigma^3/\epsilon\rightarrow 0$, where $\epsilon$ is the repulsive energy scale. We find that the dimensionless ratio $T_{\rm eff}/p \sigma^3$ controls the dynamic jamming transition that occurs with decreasing shear stress and increasing packing fraction. In particular, we find that the dependence of the dimensionless relaxation time, $\tau \sqrt{p \sigma/m}$, on $T_{\rm eff}/p \sigma^3$ as shear stress is varied is quantitatively similar to the dependence of $\tau \sqrt{p \sigma/m}$ on $T/p \sigma^3$ in equilibrium.   

Shear-jammed states in granular materials

For frictionless particles with purely repulsive interactions, there is a critical packing fraction $\phi_J$ below which no jammed states exist. Experiments by Zhang \& Behringer on physical granular systems show jammed states in the regime of $\phi < \phi_J$ can be created by the application of shear stress. Compared to the states above $\phi_J$, the shear-jammed states are mechanically more fragile, but they resist shear. These shear-jammed states cannot exist under isotropic stress. Rather, their formation require the anisotropic contact network as a backbone which is created by an applied shear stress. The anisotropic components of the stress tensor and contact network fabric tensor form a classic hysteresis loop suggesting an analogy to ferromagnetic behavior and critical phenomena. These new states must be incorporated into a more general jamming picture. We also carry out extensive analysis on shear-jammed states and find local stress fluctuations are controlled by their respective global pressures. To explain the scaling of local stress fluctuations, we construct a mean-field model based on the entropy of stress configurations.   

Quasistatic flows near jamming: The role of inertia and dissipation

We perform massively parallel computer simulations of granular particles at fixed shearing rate and density near the onset of jamming. The microscopic dynamical model contains two types of damping; one which damps the \emph{absolute} motion of a particle with respect to a homogeneously shearing background (as in SLLOD type approaches) and another which damps the \emph{relative} motion of a particle with respect to its near-neighbors (as in discrete element approaches). We study how the damping mechanism and its strength affects the collective particle dynamics through the statistics of local particle displacements and local strains. In particular, we show that for strong, \emph{absolute} damping, the single particle displacement statistics can be similar for systems at different distances from jamming while the short-time plastic activity can vary dramatically.   

Viscoelastic response near the jamming transition

We use numerical and theoretical methods to investigate oscillatory rheology in soft sphere packings, which serve as a minimal model for foams, emulsions, and other complex fluids that undergo a jamming transition. Although the zero frequency (elastic) properties of jammed media are well documented, far less is known about their viscoelastic response. We demonstrate that the frequency-dependent storage and loss moduli display critical scaling with distance to the jamming point. This behavior is governed by a diverging time scale that separates quasistatic response from a critical regime in which viscous and elastic forces contribute equally to the stress. We provide scaling arguments for all of the relevant critical exponents.   

Phase transition kinetics in the site dilute Ising model

We consider the phase transition kinetics of a quenched site dilute Ising model. To date, most studies of this model have focused on dilution-averaged quantities, such as the critical temperature and the associated critical exponents. In this talk we study how the spatial distribution of the dilution affects the local growth of the stable phase after an instantaneous quench. For an off critical quench, we find growth occurs most rapidly in areas of increased dilution for both unstable state decay and nucleation. Conversely, growth after a critical quench is accelerated in environments with relatively few vacant sites. Additionally, we consider the role of the range of interaction in these processes.   

A first-order phase transition defines the random close packing of hard spheres

Randomly packing spheres of equal size into a container consistently results in a static configuration with a density of $\sim$64\%. The ubiquity of random close packing (RCP) rather than the optimal crystalline array at 74\% begs the question of the physical law behind this empirically deduced state. Indeed, there is no signature of any macroscopic quantity with a discontinuity associated with the observed packing limit. Here we show that RCP can be interpreted as a manifestation of a thermodynamic singularity, which defines it as the ``freezing point'' in a first-order phase transition between ordered and disordered packing phases. Despite the athermal nature of granular matter, we show the thermodynamic character of the transition in that it is accompanied by sharp discontinuities in volume and entropy. This occurs at a critical compactivity, which is the intensive variable that plays the role of temperature in granular matter. This approach is useful since it maps out-of-equilibrium problems in complex systems onto simpler established frameworks in statistical mechanics.   

Session H14: Focus Session: Friction, Fracture and Deformation Across Length Scales I: Sliding Friction and Asperities

Nanoscale friction anisotropy controlled by interface inhomogeneous slip and lattice defects

Stick-slip behavior observed from nanoscale asperity friction experiments is often simulated by the one-degree-of-freedom Tomlinson model, which is unable to explain well the effects of lattice structure and interface defects, particularly the friction anisotropy. Using our recently developed Rice-Peierls framework, we study the relative sliding of two elastic half-spaces with a circular contact for two types of interplanar potential: i) triangular lattice potential (3-fold); ii) rectangle potential (2-fold). Our major findings are as follows: first, one can construct friction anisotropy from the interface interaction potential; second, one can modulate the friction anisotropy by controlling the sliding direction and the ratio of contact radius to lattice spacing. We identify that for both cases, when a/b is small, the frictional behavior approaches the Tomlinson limit, while, when a/b is large, the frictional behavior is governed by interface defects. The latter case and its resulting friction anisotropy are very sensitive to the degree of interface incommensurability.   

Frictional Sliding of Amorphous Contacts over Six Decades of Velocity

Our understanding of the nanoscale origins of sliding friction primarily arises from theories of idealized crystalline surfaces in contact. However, many if not most tribological interactions involve one or more surfaces that are disordered in structure. The role that the amorphous nature of these surfaces plays in mediating friction is poorly understood. We apply an emerging simulation methodology, hyperdynamics, for the first time to friction, examining sliding between an oxidized silicon tip and surface over a previously inaccessibly wide range of sliding velocities. The simulations replicate interesting temperature dependent change in the velocity dependence of the friction force observed in this system experimentally, and reveal the nature of the intermediate state-switching transitions responsible for this behavior. A theory based on these transitions is developed and used to describe the experimental and simulated data. We conclude that this type of transition must be quite common in frictional sliding when one or more of the involved surfaces are not perfectly crystalline.   

Sliding Over a Phase Transition

The frictional response experienced by a stick-slip slider when a phase transition occurs in the underlying solid substrate is a potentially exciting, poorly explored problem. We show, based on 2-dimensional simulations modeling the sliding of a nanotip, that indeed friction may be heavily affected by a continuous structural transition. First, friction turns nonmonotonic as temperature crosses the transition, peaking at the critical temperature $T_c$ where fluctuations are strongest. Second, below Tc friction depends upon order parameter directions, and is much larger for those where the frictional slip can cause a local flip. This may open a route towards control of atomic scale friction by switching the order parameter direction by an external field or strain, with possible application to e.g., displacive ferroelectrics such as BaTiO$_3$, as well as ferro- and antiferro-distortive materials.   

Onset of Sliding in Single Asperity Contacts

Continuum models of friction often assume that sliding initiates at the edge of a contact, and gradually spreads across the contact. However these partial slip models make simple assumptions about friction laws and must break down at atomic scales. Molecular dynamics simulations are used to analyze the nature of atomistic effects and the variation of partial slip with length scale. In continuum theory there are singularities in tangential force at the edge of the contact that initiate slip. The discrete spacing between atoms and interfacial elasticity reduce these singularities in small contacts. Elastic coupling within the contact also limits partial slip and favors coherent slip across the interface. The variation of these effects with length scale, atomic geometry and the presence of adsorbed monolayers is described.   

Finite size effects at high speed frictional interfaces

Non-Equilibrium Molecular Dynamics simulations have exhibited characteristic velocity weakening for the tangential frictional force at smooth single crystal interfaces for velocities greater than a critical velocity, v$_{c}$. This behavior has been seen in a number of material pairs including Cu-Ag, Ta-Al and Al-Al. Expressions for v$_{c}$ that characterize this behavior depend on system size. We discuss the size dependence for Al-Al single crystal interfaces for two cases: an Al(111)/Al(001) interface sliding along [1-10],N=1.5M, and an Al(110)[001]/Al(110)[1-10] interface sliding along [001], N=7.5x10$^{6}$ corresponding to a three-fold increase in system size normal to the sliding direction. We find agreement with an inverse size scaling for v$_{c}$. We discuss the similarities in behavior for a highly defective plastically deformed sample with Al(110)[001]/Al(110)[1-10] orientation having the same normal dimension and N= 16.0x10$^{6}$.   

The Tribological Properties of Nanocrystalline Metals

Materials that perform well in electrical contacts usually exhibit high adhesion during frictional contacts. An excellent example of this phenomenon is pure gold, which has extremely low electrical contact resistance, but generally has a high friction coefficient. The exception to this, however, is nanocrystalline gold alloyed with minute amounts of Ni or Co, which in addition to its low contact resistance can also show low friction. The mechanism for this remains poorly understood. We carried out large scale molecular dynamics (MD) simulations to study the tribological response of both single crystal and nanocrystalline gold or silver films in contact with curved probe tips or flat slabs under a variety of sliding conditions. Results show that grain reorientation and coalescence across the contact interface under compressive load or during shearing are responsible for the observed high friction in these contacts. In metallic alloys of silver, the addition of other elements such as copper introduces lattice mismatch and hinders the grain coalescence, which reduces friction during sliding.   

Tribo-induced melting transitions at sliding tungsten/gold-nickel asperity contacts

Tribo-induced nanoscale surface melting mechanisms have been investigated by means of a combined QCM-STM technique [1] for a range of Au and Au-Ni alloys with varying compositional percentages and phases. A transition from solid- solid to solid-``liquid like'' contact[1] was observed for each sample at sufficiently high asperity sliding speeds. Pure gold, solid-solution and two-phase Au-Ni (20 at.\% Ni) alloys were compared, which are materials of great relevance to MEMS RF switch technology [2]. The transition points agree favorably with theoretical predictions for their surface melting characteristics. We acknowledge NSF and AFOSR support for this research. \\[4pt] [1] B. D. Dawson, S. M. Lee, and J. Krim, Phys. Rev. Lett. 103, 205502 (2009)\\[0pt] [2] Zhenyin Yang; Lichtenwalner, D.J.; Morris, A.S.; Krim, J.; Kingon, A.I, Journal of Microelectromechanical Systems, April 2009, Volume: 18 Issue:2, 287-295   

\textit{In-situ }study of AFM tip wear by contact resonance force microscopy

The size and shape of an atomic force microscope (AFM) tip strongly influence the resolution and accuracy of the instrument. Here we present a new means to directly measure tip wear \textit{in situ} during contact-mode AFM scanning. Tip wear is observed from changes in contact radius determined by contact resonance force microscopy (CR-FM). In CR-FM, a flexural eigenmode of the cantilever is excited and tracked while the tip is in contact with a sample. As the tip wears, the resonant frequency increases, corresponding to increased contact radius. We demonstrate excellent agreement between quantitative tip wear results from CR-FM and from established \textit{ex-situ} techniques such as scanning electron microscope imaging. Even for compliant cantilevers scanned at very low forces, we are able to resolve subnanometer changes in contact radius. Overall, benefits of our wear-monitoring approach are that CR-FM provides quantitative values of contact radius, allows continuous measurements, affords high spatial resolution, and does not adversely influence the wear rate.   

A physical basis for frictional ageing using single-asperity measurements

Rate and state friction laws are widely used to model laboratory data and reproduce a variety of phenomena in earthquake modeling, and in other multi-asperity contacts. However, these laws lack a physical basis. To identify mechanisms underlying the time dependence of friction, especially the ageing effect, atomic force microscopy (AFM) was employed to probe friction for nanometer-scale single asperity contacts between oxidized silicon AFM tips and a set of substrates. Similar to macroscopic rock friction experiments, `slide-hold-slide' (SHS) experiments on silica revealed a linear increase in friction with the log of the hold time. However, SHS experiments on chemically inert substrates showed little to no ageing. This indicates that the ageing mechanism is related to interfacial chemical reactions, and not plastic deformation of asperities. Ageing in silica-silica contacts is more than an order of magnitude higher than for macroscopic interfaces. However, modeling of slip in multi-asperity contacts suggests that the single- and multi-asperity results agree, since the magnitude of the ageing effect in multi-asperity contacts is reduced by asperity interactions. These results provide the first asperity-level insights into possible mechanisms behind rate and state friction laws.   

Friction at the nanoscale:theory and experiment

Bowden and Tabor established more than 50 years ago that friction is due to populations of asperities. In recent years, increasingly detailed experiments have begun to document the dynamics of these asperities during sliding, and to show that several different modes of motion are possible. I will discuss experiments that probe slipping motion of macroscopic samples down to the nanoscale, and show that the small slow slipping motions are described by the rate and state theory of friction that was developed for very different length and time scales.   

Stiffness of Contacts of Self-Affine Surfaces

The presence of roughness on a wide range of scales has a profound effect on the contact area and interfacial stiffness between contacting surfaces. In turn, the interfacial stiffness dominates the response of many macroscopic systems. Molecular dynamics simulations are used to characterize contacts between self-affine fractal surfaces with different roughness exponents. A unified framework describes the relation between roughness, system size, surface separation, stiffness, and contact area for a wide variety of systems. The contact area and normal stiffness rapidly approach Persson's continuum theory with increasing system size [1]. The lateral stiffness and friction are much more sensitive to atomic-scale effects. Atomic scale displacements at the interface can greatly reduce lateral stiffness and may explain the low lateral stiffness observed in some experiments.\\[4pt] [1] B. N. J. Persson Phys. Rev. Lett. 99, 125502 (2007).   

Stick-Slip and the Transition to Sliding in a 2D Granular Medium and a Fixed Particle Lattice

We report an experimental study of stick-slip to steady sliding for a solid object pulled via a spring across 2D granular substrates of photoelastic disks that are either fixed in a solid lattice or unconstrained, i.e. a disordered granular bed. We observe a progression of friction regimes with increasing sliding speed: single, mixed, and double slip-stick regimes, steady sliding, and inertial oscillations. For the granular bed, we characterize frictional behavior for the low speed stick-slip regime, including spring and elastic energy dependencies. For the granular solid, we explore friction with/without externally applied vibrations, and compare to sliding on a granular bed. We find that external vibration reduces transition values for both the single to double slip transition and the stick-slip to steady sliding transition. Moreover, we observe that the effect of packing disorder on granular friction appears similar to the effect of vibration induced disorder, a result that to our knowledge has not been reported previously in the experimental literature.   

A simple model fault system

The Gutenberg-Richter distribution, which characterizes the frequency-magnitude statistics collected over earthquake fault systems, has lead seismologists as well as physicists and geophysicists to propose various simple models to explain this empirical scaling relation. To date, these models have been limited to the description of a single fault. We discuss a model of an earthquake fault system made up of non-interacting faults that are represented as damaged, Olami-Feder-Christensen models. The frequency-magnitude statistics do not, in general, scale on a single fault with some realization of damage; however, these statistics follow a simple distribution that can also be used to describe the data collected from actual earthquake faults. What is more, by varying the amount of damage on each fault in the system, as well as the relative frequency with which a fault with a given amount of damage occurs within the system, we obtain a one-parameter family of models, all of which produce Gutenberg-Richter-like statistics. This parameter is a measure of the stress dissipation within the fault system, a quantity known to vary with various geological properties, and offers an explanation for the range of $b$-values observed by seismologists.   

Session H15: Focus Session: Spins in Semiconductors - Ferromagnetic Semiconductors

Interplay of confinement and spin-orbit interaction in ferromagnetic semiconductors

We review experimental and theoretical works which show a surprising influence of confinement on properties of III-V ferromagnetic semiconductors related to the spin-orbit interaction. In particular, according to SQUID studies, magnetization of (Ga,Mn)As thin films show two distinct components of orthogonal in-plane easy axes, whose relative strength can be controlled by the gate voltage [M. Sawicki et al. Nature~Phys.~6\textbf{, }22 (2010)]. Furthermore, in high $T_{C}$ structures a confinement leads to an unanticipated collapse of the anomalous Hall effect [D. Chiba et al. Phys.~Rev.~Lett.~104\textbf{, }106601 (2010)]. A possibility of a non-trivial interplay of confinement and spin orbit interaction is further highlighted by the theoretical prediction of a non-collinear spin arrangement in thin films of (Ga,Mn)As [A. Werpachowska and T. Dietl, Phys.~Rev.~B 82\textbf{, }085204 (2010)]. Finally, we show how a large energy separation between the heavy and light hole subbands in compressively strained gated InAs:Mn quantum wells leads to hysteretic behavior even in the single Mn impurity limit [U. Wurstbauer et al. Nature Phys. (2010) doi:10.1038/nphys1782].   

Room Temperature Ferromagnetism in GaN-AlN Quantum Confined Heterostructures

GaN and AlN two dimensional electron gas heterostructures were grown by plasma assisted molecular beam epitaxy. During growth, a $\delta$-doped layer of Gd was introduced at a controlled distance from the interface of the GaN and AlN layers. Resulting magnetic and structural properties were characterized by a variety of complementary methods including X-ray diffractometry, atomic force microscopy, transmission electron microscopy, superconducting quantum interference device magnetometry, photoluminescence spectroscopy and measurement of the anomalous Hall effect. Doping with Gd was observed to give rise to a colossal magnetic moment of 200$\mu_{\rm{B}}$/Gd with a Curie point in excess of 300K. To elucidate the mechanism by which introduction of Gd results in such a large moment, a wide variety of heterostructure geometries and growth conditions have been explored.   

Digital magnetic heterostructures based on GaN

Delta doped heterostructures based on GaN are possible wide band gap spintronic material with high temperature ferromagnetic transition [Hory et al, Physica B 324, 142 (2002); Reed et al, Appl. Phys. Lett. 79, 3473 (2001)]. They are formed by a thin magnetic layer or submonolayer, embedded into thick semiconductor layers [Marques et al, Phys. Rev. B 73, 224409 (2006)]. In this work we studied digital magnetic heterostructures with isolated magnetic monolayers of V, Cr, Mn, Fe, Co, Ni, or Cu embedded in GaN. We performed first-principles calculations within the density functional theory with self-energy, to account for excitation energies [Ferreira et al, Phys. Rev. B 78, 125116 (2008)]. The technique allowed us to calculate the GaN band gap as $E^{calculated}_{gap}=3.53eV$ with no adjustable parameters. GaN:V and GaN:Cr do have ferromagnetic ground states with high Curie temperatures, as predicted by a mean-field approximation. GaN:Cr presents a 2D half-metallic ground state with 100\% spin polarization. GaN:V has an unusual ground state, with a 2.0$\mu_B$ magnetic moment, no states at the Fermi energy, a minority spin gap of 3.0eV, and a majority spin gap of only 0.3eV.   

Intrinsic ferromagnetism at AlN-MgB$_2$ interfaces

Spin polarization without magnetic elements is possible utilizing dangling bonds. In particular, nitrogen-dangling bonds of cation vacancies are responsible for spin polarization in nitride semiconductors. Enhancement of ferromagnetism due to cation vacancies by Gd dopants has also been found [1], which is consistent with colossal magnetic moments per Gd observed in experiments. However, randomness of point-defect distribution is not feasible to control magnetisation. In this situation, ordered structures of spin sites are highly desirable for the control of magnetization. In this study, we demonstrate by means of first-principles calculations that nitride-boride interfaces could be a candidate for such ordered spin sites. Partially occupied N $p$ states at AlN-MgB$_2$(0001) interfaces exhibits two-dimensional spin polarization, which cannot be anticipated from the atomic structure because the N dangling bonds are apparently saturated by Mg. Hund's coupling of the two N $p_{||}$ orbitals as well as high density of spin-unpolarized states at the Fermi energy contribute to strong itinerant ferromagnetism. Roles of metal-induced gap states [2] will also be discussed. \\[4pt] [1] Y. Gohda and A. Oshiyama, Phys. Rev. B 78, 161201(R) (2008). \\[0pt] [2] Y. Gohda, S. Watanabe, and A. Gro\ss, Phys. Rev. Lett. 101, 166801 (2008).   

Room temperature ferromagnetism in cluster free, Co doped Y$_{2}$O$_{3}$ dilute magnetic oxide

Diluted magnetic oxides (DMO) displaying the ferromagnetic behavior far above room temperature has attracted much attention for potential spintronic applications. Our study using low temperature co-deposition has produced uniformly doping of transitional metal (TM) Co (2-10 at.{\%}) in Y$_{2}$O$_{3}$ films without formation of clusters or second phases, stable up to 450\r{ }C anneals under most ambient. This was confirmed by EXAFS local structural analysis, XANES, and XMCD measurement. Ferromagnetic behavior of magnetic moment was observed at 300K, and the Co saturation magnetization was modulated by the concentration of oxygen vacancies under various post treatments. The observation is consistent with the impurity band exchange model to account for apparent ferromagnetism in these nearly insulating DMO films. One surprising implication from this model is the occurrence of ferromagnetic insulator behavior in TM doped HfO$_{2}$ is more likely than the widely studied TM doped ZnO, TiO$_{2}$, and SnO$_{2}$ systems of smaller band gaps for doping concentrations kept under cation percolation threshold.   

No role of magnetic impurities in observed ferromagnetism in Ti$_{1-x}$Ta$_{x}$O$_{2}$ thin film

Recently, the idea of cationic-vacancy-induced FM in wide band-gap semiconducting oxides was proposed on theoretical grounds. Experimentally, we observed ferromagnetism in thin films of anatase Ti$_{1-x}$Ta$_{x}$O$_{2}$ grown by PLD. Ta incorporation gives rise to cationic defects, acting as magnetic centers and free charge carriers as detected by various spectroscopic and transport measurements. To confirm that the ferromagnetism is an intrinsic property of Ti$_{1-x}$Ta$_{x}$O$_{2}$ and to rule out any impurity issues such as presence of magnetic elements and clustering, we carried out in-depth analysis based on Rutherford backscattering spectroscopy (RBS), Proton induced X-Ray Emission spectroscopy (PIXE) and Secondary Ion Mass Spectroscopy (SIMS). Form these results we concluded that the observed FM was not due to magnetic impurities. Rather it is an intrinsic property of Ti$_{1-x}$Ta$_{x}$O$_{2}$ thin film.   

Magnetism of DMS SnO$_{2}$:Co Thin Films Grown by RF Sputtering

SnO$_{2}$:Co thin films with dopant concentrations ranging from 2-15at{\%} were deposited on r-cut sapphire substrates via RF sputtering, to examine the origin of the room temperature ferromagnetism (RTFM) observed in such materials. Films deposited with 9:1 Ar/O$_{2}$ partial pressure ratio have a saturation moment of $\sim $0.34$\mu _{B}$/Co. Utilizing Coey's generalized grain boundary model [1], this implies that only $\sim $(2 $\pm $ 0.5){\%} of the sample is FM. Furthermore, XPS studies reveal that the cobalt valance is 2+, suggesting it exists in the form of an oxide, either Co substitutional or as CoO clusters. Furthermore, angle dependent measurements indicate no sign of phase segregation. We speculate that the FM is due to the spontaneous magnetization of uncompensated spins at the surface of the CoO antiferromagnetic nanocrystals [2] in the host SnO$_{2}$. This model may also explain the large anisotropy observed in some of our films.\\[4pt] [1] Coey et. al IEEE Trans. Magn. (2010)\\[0pt] [2] Dietl et. al PRB \textbf{76}, 155312 (2007)   

Observation of room-temperature ferromagnetism in Cu:ZnO films part I; soft X-ray Magnetic Circular Dichroism

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient Cu:ZnO films by using soft x-ray magnetic circular dichroism and x-ray absorption [1]. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu$^{2+}$ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order.\\[4pt] [1] T.S. Herng et al, Phys. Rev. Lett. {\bf 105}, 207201 (2010)   

Observation of room-temperature ferromagnetism in Cu:ZnO films part II; a theoretical study

To better understand the observation of room-temperature ferromagnetic ordering in O-deficient Cu:ZnO films [1], we calculated the configuration-averaged spectral function $\langle A(k,\omega)\rangle$ of ZnO with 2\% Cu impurities and 1\% O vacancies within the ``LDA+U'' approximation, solving the Hamiltonian only within the low energy Hilbert space, defined via the first-principles Wannier functions [2]. Based on these first principles results we proposed a microscopic ``indirect double-exchange'' model for the FM in Cu:ZnO that explains our main experimental findings.\\[4pt] [1] T.S. Herng et al, Phys. Rev. Lett. {\bf 105}, 207201 (2010)\\[0pt] [2] T. Berlijn et al, arXiv:1004.1156 (2010)   

Electrodeposition of Co-doped Cu$_{2}$O layers with high Curie temperature

In this work, we have studied the magnetic properties of room temperature electrodeposited Cu$_{2}$O layers doped with Co. These layers were growth from electrolytes containing lactic acid and copper sulfate, with the addition of cobalt sulfate for the doping process. The layers are considered as a diluted magnetic semiconductor, showing ferromagnetic behavior above room temperature and saturation magnetization proportional to the concentration of cobalt sulfate. The decrease of lattice parameter and resistivity with the increase of the band gap for doped samples were results that pointed out to the Co incorporation to the growing layers as individual atoms. In addition, no evidences for the existence of superparamagnetic particles were observed from ZFC and FC curves, hysteresis loops and HRTEM images. The magnetic behavior is associated to Co atoms diluted in the Cu$_{2}$O lattice and a promising Curie temperature for spintronic application of 550 K was determined.   

The spin polarized electronic structures and magnetic properties of Co doped and Ga codoped ZnO

The understanding of the magnetic property of Co doped ZnO (ZnO:Co) has been inconclusive with confusing experimental observations. Here, spin-polarized first-principles calculations have been performed for ZnO:Co to better understand its magnetic property. Without O and Zn vacancies, the total energy per Co ion in ZnO:Co in the ferromagnetic (FM) state was found to be only 6meV lower than that in the antiferromagnetic (AFM) state, which suggests that at room or higher temperature ZnO:Co is in the spin glass state. O vacancies and co-doping with Ga ions were found to enhance Co-Co FM coupling by induced delocalized states in the vicinity of Co 3d bands. The O vacancy was found to have a greater effect of FM enhancement than the co-doped Ga ion. While Zn vacancies were found to lower the Fermi level and enhance hybridization between Co 3d and valence-band O 2p states, which enhances super-exchange coupling and renders ZnO:Co to be in the AFM state. The opposite effects of O and Zn vacancies imply that the magnetic property of ZnO:Co depends strongly on the relative concentrations of O and Zn vacancies.   

Magnetotransport in the amorphous Ge(1-x)Mn(x) with self-assembled nanostructures

Mn ions have been reported to segregate into intermetallic precipitates or form coherent clusters in crystalline Ge(1-x)Mn(x) thin films. In this study, we investigated the microstructure of amorphous Ge(1-x)Mn(x) synthesized using low temperature molecular beam epitaxy, and observed the self-assembly of Mn rich nanostructures in Ge matrix with both cluster and column shapes by varying the Mn concentration. The magnetotransport properties were found to closely correlate with the magnetism. Negative magnetoresistance (MR) showed a dominant effect in as-grown materials, and required a very high magnetic field to saturate, whereas only positive MR was observed in post-annealed Ge(1-x)Mn(x). The anisotropic behavior in magnetism and magnetoresistance will also be discussed.   

Session H16: Focus Session: Spins in Carbon-Based Materials-- Spin Valves and Interfaces

Spin filtering effect of ferromagnetic metal-organic interfaces

The study of the spin properties of organic semiconductors (OSC) is recently receiving great attention. Being characterized by moderate spin-relaxation lengths, one of the most promising routes to employ OSC for spintronics applications is probably to exploit the high spin injection achievable across ferromagnetic metal-organic interfaces [1,2]. Combined with the extreme flexibility and tunability of OSC, it is expected that such hybrid interfaces will constitute a fundamental building block for advanced spintronics devices, where spin-injection is controlled by fine-tuning of the interface physical ad chemical properties. An example has been recently presented in [3], where doping of the OSC copper phthalocyanine (CuPc) has been successfully used to tune the spin functionality of a cobalt-CuPc interface. In particular, the presence of a spin-polarized hybrid interface state, acting as a spin-filter at the interface, has been used to enhance the efficiency of spin injection to values above 100{\%}. In order to exploit such great potential of hybrid organic-inorganic interfaces, fundamental knowledge about their spin-dependent properties is essential. Besides the cobalt-CuPc interface, we have studied the iron-CuPc, cobalt- tris[8-hydroxyquinoline]aluminium (Alq3) and iron-Alq3 interfaces. We applied several complementary experimental techniques, namely spin polarized scanning tunnelling microscopy and spectroscopy together with spin polarized ultraviolet photoemission spectroscopy and spin- and time-resolved two-photon photoemission. We found evidence for spin-polarized interface states and show that they act as a spin-filter for electrons crossing the interface between the ferromagnetic metal and the OSC. Correspondingly, we observed a pronounced spin-dependency of the lifetime of electrons injected in the above mentioned hybrid spin-polarized interface states. \\[4pt] [1] M. Cinchetti et al., Nature Materials 8, 115-119 (2009); \\[0pt] [2] H. Ding et al., Phys. Rev. B 78, 075311 (2008); \\[0pt] [3] M. Cinchetti et al., Phys. Rev. Lett. 104, 217602 (2010).   

Spin valve effect and high field magnetoresistance in hybrid magnetic tunnel junction of V(TCNE)$_{x~2}$/rubrene/ La$_{2/3}$Sr$_{1/3}$MnO$_{3}$

Molecule/organic-based magnets, that allow chemical tuning of electronic and magnetic properties, are a promising new class of magnetic materials for future spintronics [1]. V (TCNE:tetracyanoethylene)$_{x}$ ($x \sim$ 2) is the room temperature organic-based magnetic semiconductor ($T_c \sim$ 400 K). It has ferrimagnetic coupling between the spins in the TCNE$^{-}$ anions and spins in V$^{\rm II}$ leading highly spin- polarized valence and conduction bands. In this talk, we present realization of an organic-based magnetic as an electron spin polarizer in the standard spin valve device geometry [2]. The room temperature organic-based magnet, V(TCNE)$_x$ was successfully incorporated into the standard magnetic tunnel junction devices in tandem with LSMO (La$_{2/3}$Sr$_{1/3}$MnO$_3$) film. Beside spin valve effect, the device exhibits large negative high-field magnetoresistance, which may be associated with anomalous field dependent Fermi level shift in LSMO.\\[4pt] [1] A.J. Epstein, MRS Bull. ${\b 28}$, 492 (2003)\\[0pt] [2] Yoo et al., Nature Materials ${\b 9}$, 638 (2010)   

Organic Spin Valves with Characteristics of Inelastic Tunneling and Hopping Transport

We report on the inelastic scattering characteristics of an organic-based spin valve with a thin organic barrier of 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) dusted with alumina at organic/ferromagnetic interfaces. Spin injection with magnetoresistance up to 12\% at room temperature was achieved. In the inelastic tunneling spectrum, the observation of characteristic vibrational loss peak of organic spacer provides direct evidence of the interplay between the spin-polarized electrons and the organic molecules. The spin-dependent transport mechanism can be further described with a model of combined tunneling and hopping processes as verified by experiments as a function of organic layer thickness.   

Coulomb blockade magnetoresistance in organic spin transport device

Using buffer-layer-assisted growth, we successfully fabricated organic spin transport devices with a discontinuous granular magnetic layer centered in an organic spacer film. The Coulomb blockade magnetoresistance (MR) effects were observed, as predicted by X.-G. Zhang \textit{et al} (Phys. Rev. B. 81, 155122, 2010). The spin-dependent Coulomb blockade voltage arises from the coupled magnetic dots inside the organic material and correlate with the observed MR effect.   

Spin-polarized tunnel injection and extraction effects on magneto-resistance in organic semiconductor spin valves

Experimental evidence of large magneto-resistance has been reported for organic spin valves. An organic spin valve consists of a conjugated hydrocarbon semiconductor sandwiched between two ferromagnetic contacts. Tunnel injection of charge carriers from a ferromagnetic contact can be strongly spin-polarized. The process is modeled as tunneling through a thin interfacial layer into localized molecular states of the organic semiconductor near the equilibrium Fermi level, and subsequent thermally activated hopping of the charge carriers out of these localized states into the bulk of the semiconductor, where the transport can be described by macroscopic device equations. The extraction of charge carriers follows an analogues process at the collecting contact. We explore the consequences of parallel or anti-parallel alignment of contact magnetizations on the spin-polarization and the magneto-resistance associated with the spin-polarized current in the device.   

Spin-orbit coupling, spin relaxation, and spin diffusion in organic solids: applications to Alq$_3$ and CuPc

We develop a systematic approach of quantifying spin-orbit coupling (SOC) and a rigorous theory of carrier spin relaxation caused by the SOC in disordered organic solids. The SOC mixes up- and down-spin in the polaron states and can be characterized by an admixture parameter $\gamma^2$. The spin mixing effects spin flips as polaron hops from one molecule to another even through the interaction that facilitate hopping is spin-indepdent. The spin relaxation time is $\tau_{sf}= \bar{R}^2/(16 \gamma^2 D)$ and the spin diffusion length is $L_s=\bar{R}/4|\gamma|$, where $\bar{R}$ is the mean polaron hopping distance and $D$ the carrier diffusion constant. We show that the SOCs in tris-(8-hydroxyquinoline) aluminum (Alq$_3$) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in the latter. The theory quantitatively explains the recent observed spin diffusion lengths in Alq$_3$ from muon measurements and in CuPc from two-photon photoemission.   

Triplet exciton controlled current in an organic semiconductor

Organic materials like MEH-PPV have relatively low spin-orbital coupling leading to long spin lifetimes and good spin-selection rules. As a result, the rates of recombination of an ensemble of polaron pairs can be changed by resonant manipulation of either the P$^{+}$ or the P$^{-}$ spins, leading to a flopping between the singlet to triplet manifolds - this can be observed as a small change in the device current [1]. In this study we have used pulsed electrically detected magnetic resonance with electron-rich OLED devices to investigate the possibility of other spin-dependent processes like an exciton-polaron interaction. We expect that devices with excess electrons will show signals from free electrons interacting with long lived triplet excitons. Coherent Rabi nutation experiments were carried out from room temperature down to 5K. At room temperature only a single polaron-pair resonance is observed at g$\sim $2.003. However, as the temperature is decreased a signal at g$\sim $4(triplet exciton resonance) is observed along with a second signal at g$\sim $2.003 corresponding to the rotation of a single polaron.\\[4pt] [1] D. R. McCamey, \textit{et. al.} Nature Mater. \textbf{7}, 723, (2008)   

STM studies of Lanthanide Phthalocyanine molecules on metallic and thin-insulating surfaces

Molecules deposited onto surfaces are of interest because of their potential use as nano-scale electronic components. More recently, the magnetic properties of these systems have also become accessible. Using scanning tunneling microscopy (STM), it is possible to study both sets of properties, and to examine the local environment of the molecules. For example, large magnetic anisotropies have been observed for transition metal Phthalocyanine (Pc) molecules on thin insulators, which decouple the spin from the underlying metal. We present STM imaging and spectroscopy data on lanthanide double-decker Pcs. We explore the different binding configurations and study how these can influence the properties of these molecules on surfaces.   

STM Studies of Iron Phthalocyanine on Fe(110) Films

We have observed molecular-scale-resolution arrays of Iron Phthalocyanine (FePc) molecules which we adsorbed at room temperature on thin ($\sim $5-10 ML) films of Fe(110). These molecular layers were grown in a UHV Omicron/AFM/STM/ multi-probe system at NC State in the Physics Department at pressures of $\sim $10$^{-10}$ torr. Our results indicate a strong inter-molecular interaction that produces well-ordered films at monolayer coverage. For lower coverage ($\sim $0.2 -- 0.6 ML) the FePc-Fe substrate interaction strongly dominates and the STM image morphology has only small clusters of 2-6 molecules. Our data clearly shown that the FePc molecules are lying flat on the surface in the ordered $\sim $1 ML samples since we see evidence of the carbon-ring ligands in some images. We discuss the possibility of spin-dependent effects between the molecular Fe and the substrate Fe as an example of potential molecular-modified spin-based devices. Initial STM-spectroscopy including both I vs. V and Z vs. V results are consistent with our structural observations.   

Scanning Tunneling Microscopy and Spectroscopy Studies of a Model Organic Spintronic Interface: Alq3 on Cr(001)

Scanning tunneling microscopy was used to observe coverage-dependent structure during growth of the first monolayer of Alq3 on a Cr(001) surface. No long range molecular ordering is observed, though molecules tend to form randomly oriented chain-like aggregates even at the lowest coverages. This illustrates that the well-known amorphous nature of Alq3 films begins even in the first layer, but that the disorder in films have subtle local correlations relevant to electronic and spintronic device modeling. Scanning tunneling spectroscopy was used to locate the LUMO-derived transport state above the Fermi level and correlate its position with local film structure.   

STM studies of a novel organic/inorganic interface: TCNE/GaAs(110)

Recent efforts in the field of spintronics have focused on the integration of organic molecular magnets with inorganic semiconductors. Little is known, however, about the interfacial chemistry and physics that occurs between the organic spin injector and the inorganic device structure. We are therefore studying tetracyanoethylene/GaAs(110) as a model system to gain a basic understanding of the properties that emerge upon integration of these materials. Utilizing low temperature (7 K) ultrahigh vacuum scanning tunneling microscopy we are able to identify both bonding geometries and bonding sites for isolated TCNE molecules on the unreconstructed GaAs(110) surface. Scanning tunneling spectroscopy can provide a detailed look at the interfacial electronic structure, including alignment of individual molecular orbitals with respect to the band structure of the underlying substrate. Single transition metal--TCNE complexes can be realized and investigated via atomic/molecular manipulation.   

Magnetic Field Effects Generated by Inter-molecular Excited States in Organic Semiconductors

It has experimentally found that an external magnetic field can change electroluminescence, electric current, and photocurrent, generating magnetic field effects (MFEs) in non-magnetic organic semiconductors. Our photoluminescence studies have found that the intermolecular excited states are accountable for the MFEs while the intra-molecular excited states exhibit negligible MFEs. Our experimental studies further indicated that inter-molecular excited states can exhibit tunable spin-orbital coupling and exchange interaction based on materials mixing. We observed that tuning inter-molecular spin-orbital coupling and exchange interaction can largely modify the MFEs through spin-dependent formation and intersystem crossing in inter-molecular excited states. Therefore, the use of inter-molecular excited states presents a new mechanism to generate magnetic responses in non-magnetic organic semiconductors.   

Charge transport through single alkanedithiol molecules on an ultrathin insulating film: Influence on an atomic Kondo resonance

Studies of charge/spin transport through single molecules are important for understanding organic-based electronic and memory devices. We have realized a single molecule wire comprising an alkanedithiol molecule and a single Co atom contact using a low temperature scanning tunneling microscope. This wire is formed on an ultrathin insulating layer (Cu2N on Cu(100)). A Kondo resonance observed on isolated Co atoms on Cu2N indicates minimal contact to the Cu substrate. However, increased contact to Cu is achieved by connecting the Co atom via the alkanedithiol molecule. A change in the Kondo lineshape on the Co atom indicates an open conduction channel through the molecule. ~This result provides an opportunity to study charge/spin transport through single molecules with atomically precise contacts. We acknowledge financial support from NSF CAREER Award No. DMR-0645451 and NSF MRSEC-0820414. http://www.physics.ohio-state.edu/$\sim $jgupta.   

Session H17: Focus Session: Bulk Properties of Complex Oxides - Ferrites

Inelastic neutron scattering measurements on the triangular lattice antiferromagnet CuFe1-xGaxO2 in the paraelectric and multiferroic phases

The intensive efforts to study multiferroic materials in recent years have led to a better understanding of the fundamental physical processes and interactions leading to the complex behavior in those compounds. In this talk, I will focus on the recent neutron scattering measurements and calculations of the spin dynamics of the triangular lattice antiferromagnet CuFe$_{1-x}$Ga$_{x}$O$_{2}$. In pure CuFeO$_{2}$ a low-temperature~collinear spin structure is stabilized by long range magnetic interactions. The spin wave spectra show dynamics precursory to the multiferroic phase. When this system is doped with a few percent of nonmagnetic gallium, its low-temperature phase has a complex noncollinear (CNC) spin order and becomes multiferroic. The CNC phase appears to have distorted screw-type magnetic configuration that is stabilized by the displacement of the oxygen atoms. The spin dynamics and the spin order in this material, as well as their implications to the origin of the ferroelectricity will be discussed [1,2]. \\[4pt] [1] J. T. Haraldsen, et al., \textit{Phys. Rev. B}, \textbf{82}, 020404 (2010). \\[0pt] [2]. Research was sponsored by the U.S. DOE/BES, Division of Materials Sciences and Engineering and the Division of Scientific User Facilities.   

Investigating the spin dynamics of the non-collinear magnetic phase of 3.5\% Ga-doped CuFeO$_2$

We examine the evolution of the non-collinear phase of a hexagonal lattice antiferromagnet to help understand the inelastic neutron scattering measurements for the multiferroic ground state of 3.5\% Ga-doped CuFeO$_2$. With the complex ground state stabilized by the displacement of the oxygen atoms, the multiferroic coupling is explained by the predicted ``spin-driven'' model. By comparing the observed and calculated spectrum of spin excitations for multiple spin configurations, we conclude that the magnetic ground state is a distorted screw-type spin configuration with a distribution of turn angles produced by lattice distortions.   

Long-period solitonic lattice in the rare earth orthoferrite TbFeO3

Rare earth orthoferrites show a variety of magnetic transitions and spectacular magneto-electric effects originating from the coupling between the iron and rare earth magnetic sublattices. Our recent single-crystal neutron diffraction measurements revealed the presence of an unusual incommensurate phase in TbFeO3, which is induced by the magnetic field along the c axis and has a period of about 100 unit cells. We also present the results of model calculations, which explain the origin of this novel phase and reproduce the magnetic phase diagram of TbFeO3.   

Magnetic and Raman spectroscopic studies of yttrium substituted BiFeO$_{3}$

As a room temperature multiferroic BiFeO$_{3 }$is attractive for a number of applications. However the relatively weak magnetism limits its suitability for applications. Bi$_{1-x}$Y$_{x}$FeO$_{3}$ (x=0, 0.05, 0.10, 0.15 and 0.20) ceramic samples have been prepared by sol-gel technique and thin films by metalorganic decomposition. X-ray diffraction, X-ray photoelectron and Raman spectroscopy measurements confirm that these ceramic and thin film samples are of single phase. More remarkably these Y modified BiFeO$_{3}$ samples exhibit enhanced room temperature magnetic and magnetodielectric properties, with the saturation magnetization approaching 25 emu cm$^{-3}$ for x=0.20. We discuss the correlation between the enhanced room temperature magnetization and two-phonon Raman modes in the context of investigating the origin of the magnetic properties in Bi$_{1-x}$Y$_{x}$FeO$_{3}$.   

Relation between electric phase and magnetic ordering of Y-type Hexaferrite

Y-type hexaferrite Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$Fe$_{12}$O$_{22}$(BSZFO), one of multiferroic materials, we could acquire magnetic field-induced commensurate phase, changing of magnetic phase of BSZFO and Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$(Fe$_{1-x}$Al$_{x}$)$_{12}$O$_{22}$ (x = 0.08)(BSZFAO) using resonant soft X-ray scattering(RSXS) experiment. Also we could confirm that magnetic ordering changing has some relation with electric phase transition, q=1.5 at ferroelectric phase not only BSZFO but also BSZFAO. This research results were acquired by using 2A EPU beamline at PAL.   

Magnetic Spectra on Charge Ordering in $R_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ ($R$ = La, Pr, and Nd)

$R_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ ($R$ = La, Pr, and Nd) compounds are reported to have the same charge ordering (CO) and Neel temperatures. Inelastic neutron scattering is applied to study the magnetic energy effect on the CO state in this system. Based on the ratio of the ferromagnetic exchange energy ($J_{F})$ and antiferromagnetic exchange energy ($\vert J_{AF}\vert )$, the magnetic exchange energy can stabilize the CO state in La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ and Pr$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ alone; with the smaller $R^{3+}$ substitution, $\vert J_{AF}\vert $ increases a lot from La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ to Nd$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$, but the CO of Fe ions could still be driven by magnetic energy itself with the correction on magnetic energy ratio.   

First-principles calculations for XAS of infinite-layer iron oxides

The oxygen defect perovskite SrFeO$_{3-x}$ shows various properties such as the giant magnetoresistance effect and the thermoelectric effect. It had been believed that the oxygen content in SrFeO$_{3-x}$ changes up to $x=0.5$. Recently, Tsujimoto $et$ $al.$ have succeeded in synthesizing the infinite-layer iron oxide SrFeO$_2$. SrFeO$_2$ has a square-planar oxygen coordination, while the iron oxides usually have the tetrahedral and octahedral coordination. CaFeO$_2$ has also infinite layer structure and the same magnetic ordering as SrFeO$_2$. However, it is suggested that the oxygen coordination of CaFeO$_2$ is different from that of SrFeO$_2$. In order to investigate the electronic structure of iron in (Ca, Sr)FeO$_2$, the x-ray absorption spectroscopy (XAS) spectrum has been measured. In this work, we perform the calculation for XAS spectrum near the Fe-K edge of (Ca, Sr)FeO$_2$ using the first-principles calculations. We compare the results with the experiment and discuss the electronic structure of iron in (Ca, Sr)FeO$_2$.   

Canted-spin-caused electric dipoles: a local symmetry theory

A pair of magnetic atoms with canted spins ${\rm {\bf S}}_a \,\mbox{and}\,{\rm {\bf S}}_b $can give rise to an electric dipole moment \textbf{P. } Several forms for the dependence of \textbf{P} on the spins have been derived from various microscopic models, some of which have been invoked to explain experimental results found in some multiferroic materials. The forms are ${\rm {\bf P}}_1 \propto {\rm {\bf R}}\times ({\rm {\bf S}}_{\rm {\bf a}} \times {\rm {\bf S}}_b ),\,{\rm {\bf P}}_2 \propto {\rm {\bf S}}_{\rm {\bf a}} \times {\rm {\bf S}}_b ,\,{\rm {\bf P}}_3 \propto {\rm {\bf S}}_{\rm {\bf a}} ({\rm {\bf S}}_{\rm {\bf a}} \cdot {\rm {\bf R}})-{\rm {\bf S}}_b ({\rm {\bf S}}_b \cdot {\rm {\bf R}})$, where \textbf{R} is the relative position of the atoms. To unify and generalize these various forms we consider \textbf{P }as the most general quadratic function of the spin components that vanishes whenever ${\rm {\bf S}}_{\rm {\bf a}} \,\mbox{and }{\rm {\bf S}}_b $are collinear. The study reveals new forms. We generalize to the vector \textbf{P}, Moriya's symmetry considerations on the (scalar) DM energy ${\rm {\bf D}}\cdot {\rm {\bf S}}_{\rm {\bf a}} \times {\rm {\bf S}}_b $(which led to restrictions on \textbf{D}). This provides a rigorous proof that \textbf{P}$_{1}$ is allowed no matter how high the symmetry of the atoms plus environment, and gives restrictions for all other contributions. The analysis leads to the suggestion of new terms omitted in the existing microscopic models, and predicts an unusual antiferroelectric ordering in the antiferromagnetically and ferroelectrically ordered phase of RbFe(MoO$_{4})_{2}$.   

Structural and magnetic ordering in bulk Sr2FeMoO6 synthesized by planetary ball mill: The effects of grinding.

The standard solid-state synthesis procedure has been widely used to make bulk complex oxides, including half-metallic double perovskite Sr2FeMoO6. However, although it is generally recognized that multi-step grinding and heating are crucial for synthesis of high quality materials, little has been done to quantitatively characterize the effect of grinding on the quality of the final products. We systematically varied the level of grinding, ranging from poor grinding by hand for a short period of time ($\sim $10 min) to very fine grinding and mixing by a planetary ball mill for many hours which produces uniform sub-micron particles. XRD, SEM and VSM were used to characterize the structural and magnetic properties. The Sr2FeMoO6 samples made by different grinding methods exhibit drastically different structural and magnetic ordering. The highest quality Sr2FeMoO6 is from the most thorough grinding and gives a close to ideal Fe/Mo ordering and magnetic moment close to 4 Bohr magneton per formula unit.   

Theory of ferromagnetic double perovskites

We derive and validate an effective classical spin model which describes the magnetic properties of double perovskites (DP) like Sr$_{2}$FeMoO$_{6}$, including the effects of disorder and carrier concentration. This model generalizes the Anderson-Hasegawa model for manganites to DP's. We validate our effective spin model by making detailed comparisons with the results obtained from a quantum Hamiltonian of itinerant electrons interacting with spins on the Fe-sites. We show that the conduction electron polarization at the chemical potential $P(T)$ tracks the temperature-dependence of the total magnetization $M(T)$. We point out the importance of Coulomb correlation $U$ on Mo-sites and of direct Mo-Mo hopping $t^\prime$ on stabilizing the ferromagnetic phase as a function of electron doping (by La substitution of Sr). We show how the small parameters $U$ and $t^\prime$ are crucial in understanding the experimental results for T$_c$ as a function of carrier concentration. We predict how the ferromagnetic $T_c$ can be raised substantially (up to 40\%), without sacrificing the polarization $P$, by a combination of excess Fe and La-doping.   

Role of disorder and doping in ferromagnetic double perovskites

We use an effective classical spin model, which we have recently derived and validated, to examine the effect of disorder on the magnetic properties of double perovskites like Sr$_{2}$FeMoO$_{6}$ (SFMO). We compute the effects of excess Fe, excess Mo, and anti-site disorder on the magnetization $M(T)$ and the ferromagnetic transition temperature $T_c$, and show that our results are quantitatively consistent with available experiments. We then use these results to propose routes to increase the ferromagnetic $T_c$ above that of pure SFMO (420K). We predict that, by adding excess Fe on Mo sites and compensating for the change in conduction electron density by La substitution on the Sr sites, we can raise $T_c$ by as much as 40\%, without substantially sacrificing the conduction electron polarization $P$ at the chemical potential.   

Session H18: Electronic Structure II

Spectral element solution of the Kohn-Sham atom

Electronic structure calculations of atoms are important in nuclear physics, and are necessary input for most methods to construct first-principles effective potentials (i.e., pseudopotentials and projector augmented wave potentials). The standard method to solve the atomic problem within Kohn-Sham density functional theory is the shooting method. In this work, the more robust spectral element method is applied to the 1D atomic radial equation. The spectral element method provides a strict, upper-bound on the absolute error in the Kohn-Sham eigenvalues and wavefunctions enabling the solution to be converged to a well controlled accuracy. The results of this method are compared to the extensive ``NIST Atomic Reference Data for Electronic Structure Calculations'' database for elements H to U, providing a more rigourous assessment of this dataset than previously available.   

Ab initio calculations of optical absorption spectra: Solution of the Bethe-Salpeter equation within density matrix perturbation theory

We present an approach to compute optical absorption spectra from first principles, which is suitable for the study of large systems and gives access to spectra within a wide energy range. In this approach, the quantum Liouville equation is solved iteratively within first order perturbation theory, with a Hamiltonian containing a static self-energy operator [1]. This is equivalent to solving the Bethe-Salpeter equation. Explicit calculations of single particle excited states and inversion of dielectric matrices are avoided using techniques based on Density Functional Perturbation Theory [1,2]. The calculation and inclusion of GW quasi-particle corrections within this framework are discussed. The efficiency and accuracy of our approach are demonstrated by computing optical spectra of solids, nanostructures and dipeptide molecules exhibiting charge transfer excitations. \\[4pt] [1] D.Rocca, D.Lu and G.Galli, J. Chem. Phys. 133, 164109 (2010). \\[0pt] [2] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B , 78, 113303, (2008).   

Ab-initio calculations of absorption spectra of nanowires by solving the Bethe-Salpeter Equation

A first principle approach to the solution of the Bethe Salpeter equation without empty electronic states has been recently developed [1], which makes possible the calculations of absorption spectra of relatively large systems (with several hundreds of electrons). We present applications of this approach to quasi-one dimensional systems, including chains of hydrogen molecules and Si nanowires. We discuss techniques to further improve the performance of absorption spectra calculations, and present a general scheme to accurately integrate the divergence in the screened exchange integrals. Finally, in the case of Si nanowires, we discuss the effect of surface reconstruction in shaping optical absorption spectra.\\[4pt] [1] D. Rocca, D. Lu and G. Galli, J. Chem. Phys. 133, 164109 (2010)   

Efficient GW calculations using eigenvalue-eigenvector decomposition of the dielectric matrix

During the past 25 years, the GW method [1] has been successfully used to compute electronic quasi-particle excitation spectra of a variety of materials. It is however a computationally intensive technique, as it involves summations over occupied and empty electronic states, to evaluate both the Green function (G) and the dielectric matrix (DM) entering the expression of the screened Coulomb interaction (W). Recent developments have shown that eigenpotentials of DMs can be efficiently calculated without any explicit evaluation of empty states [2]. In this work, we will present a computationally efficient approach to the calculations of GW spectra by combining a representation of DMs in terms of its eigenpotentials [3] and a recently developed iterative algorithm [4]. As a demonstration of the efficiency of the method, we will present calculations of the vertical ionization potentials of several systems. [1] L. Hedin, Phys. Rev. 139, A796 (1965). [2] H.-V. Nguyen and S. de Gironcoli, Phys. Rev. B 79, 205114 (2009); H. F. Wilson, D. Lu, F. Gygi, and G. Galli, Phys. Rev. B 79, 245106 (2009). [3] D. Lu, F. Gygi, and G. Galli, Phys. Rev. Lett. 100, 147601 (2008). [4] P. Umari, G. Stenuit, and S. Baroni, Phys. Rev. B 81, 115104 (2010)   

A TDLDA+U approach on strongly hybridized Frenkel excitons in Mott insulators and implications to TDDFT and GW+BSE

The applicability of nowadays first-principles approach on local excitations of strongly correlated systems is unknown. We therefore derived the dynamical linear response of LDA+U functional within the framework of TDDFT.\footnote{Chi-Cheng Lee et al., Phys. Rev. B 82, 081106(R) (2010).} The strength and weakness of LDA+U functional in describing charge excitations of strongly interacting Mott insulators is examined via this TDLDA+U method. Formulated using real-space Wannier functions, a computationally inexpensive framework gives detailed insights into the formation of tightly bound Frenkel excitons with reasonable accuracy. Specifically, a strong hybridization of multiple excitons is found to significantly modify the exciton properties. Furthermore, our study exposes a significant generic limitation of adiabatic approximation in TDDFT with hybrid functionals and in existing Bethe-Salpeter-equation approaches, advocating the necessity of strongly energy-dependent kernels in future development. Finally, a superatom approach beyond TDLDA+U will also be discussed.   

Level alignment at covalently bonded metal-organic interfaces within the GW approximation

Accurate calculations of orbital energies for molecules chemisorbed on metal surfaces are important for understanding energy conversion, molecular scale transport, and charge transfer events at metal electrodes. Here, using density functional theory (DFT) and many-body perturbation theory within the GW approximation (GWA), we report the orbital energies of a well-studied molecule, benzene diamine (and derivatives), covalently bonded to aluminum and gold surfaces. For chemisorbed derivatives on Al surfaces, we predict a shift in the highest occupied molecular orbital resonance energy greater than 1 eV relative to the DFT result. We discuss our GWA results in the context of a model self-energy approach based on prior work [1], which can be applied to larger systems at greatly reduced computational cost. \\[4pt] [1] J. B. Neaton, M.S. Hybertsen, and S.G. Louie, PRL, 97, 216405 (2006)   

GW approach to degenerate systems

Many-body perturbation theory based on the GW approximation to the electron self energy describes accurately in first-principles calculations the electronic (quasiparticle) excited states of solids, clusters and molecules. However, despite the multitude of important systems with degenerate ground states, ranging from open-shell atoms and molecules to magnetic defects in solids, the GW approach has been applied almost exclusively to closed-shell systems. In this talk, we discuss some of the problems with existing GW calculations for degenerate systems, such as spin contamination, the multiplet problem, and the proper definition of the Green function in open-shell systems. Different formulations to overcome these problems are explored.   

Ab-initio theory of spin fluctuations in magnets

We propose a framework for a true ab initio theory of magnetism, based on many-body perturbation theory (MPBT). It fits in naturally with methods based MPBT such as the GW approximation; but the approach can be implemented as an extension to any existing static method for electronic structure such as the local spin density approximation to density functional theory, to include spin fluctuations. Initially we calculated the spin fluctuation contributions using random phase approximation. The self consistency procedure similar to the one used in Moryia-Kawabata theory can be naturally implemented. The fluctuation dissipation theorem is used to calculate the reduction of the mean field magnetic moment in itinerant magnets. The applications of the technique includes traditional 3d ferromagnetic metals, their alloys and compounds and 5f systems.   

A GW-based many-body perturbation theory approach for investigating materials with strong spin-orbit coupling

Spin-orbit coupling is an essential ingredient in understanding the electronic properties of materials of recent interest. We have developed a means of incorporating spin-orbit coupling to the quasiparticle excitations in solids within the GW approach. We apply our method to the properties of materials with heavy ion cores. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC.   

Self-consistent band-structure calculations at GW quality and DFT expense

GW provides rather accurate quasi particle energies where approximate DFT methods tend to fail, yet are too computer intensive for complex~inorganic materials (large systems or dense Brillouin zone sampling) that are currently of interest, e.g. for energy conversion. We explore the possibility that the trends of the GW quasi-particle energy corrections due to the non-local and energy dependent self-energy $\Sigma $(r,r',$E)$ can be captured by atomic potentials that do not significantly increase the computational effort of a standard DFT calculation. We proceed in 4 steps: (i) Perform GW reference calculations for II-VI and III-V semiconductors (ii) Define atomic potentials that are added to the DFT Hamiltonian. Here we extend the concept of the non-local external potentials (NLEP) of Ref. [1], now allowing for two parameters per atom type and angular momentum. (iii) Fit the NLEP parameters to the GW test set. (iv) Finally, we test transferability by applying the potentials to the III$_{2}$-VI$_{3}$ and II$_{3}$-V$_{2}$ compounds that were not included in the fitting set. \\[4pt] [1] S. Lany, H. Raebiger, A. Zunger, Phys. Rev. B 77, 241201(R) (2008).   

The sc-COHSEX+GW and the static off-diagonal GW approaches to quasiparticle wavefunctions and energies

Within the conventional GW approach, density functional theory (DFT) is typically used as a mean-field starting point; the self-energy operator is evaluated to 1st order in the DFT Green's function G$_{0}$ and screened Coulomb interaction W$_{0}$. The quasiparticle energies are calculated from diagonal elements of $\Sigma$ in the DFT orbital basis. This approach works extraordinarily well for many materials but has limitations when the DFT states are far from the quasiparticle wavefunctions. In such cases, off-diagonal elements of $\Sigma$ in the mean-field basis are large and the full $\Sigma$ matrix is needed. The slow convergence of the off-diagonal elements make approaches requiring the explicit construction of this matrix prohibitively expensive. We present two alternative approaches based on the static (COHSEX) approximation that efficiently include the mean-field off-diagonal matrix element effects: a sc-COHSEX+GW approach where a renormalized basis is obtained from a self-consistent evaluation of quasiparticle wavefunctions in the static approximation and a less intensive treatment of just the off-diagonal elements within the COHSEX approximation. We show examples of the approaches for molecules and crystalline systems. Support by NSF DMR10-1006184, DOE DE-AC02-05CH11231.   

Constructing unoccupied states for G$_0$W$_0$ quasiparticle calculations from plane-waves

Standard methods of first-principles calculations of the quasiparticle energies within the G$_0$W$_0$ scheme require summing over large numbers of unoccupied states. The generation of these states within the ab initio pseudopotential plane-wave density functional theory (DFT) quickly becomes a bottleneck of the calculation with increasing system size, especially in low-dimensional systems. In this work, we propose a method for approximating the high-energy continuum and resonant states in low-dimensional systems. The continuum and resonant states above a chosen energy are replaced with symmetrized plane-waves and localized DFT states computed with short-range localized basis functions (such as in the SIESTA code), respectively. The Gram-Schmidt process is used to orthogonalize these constructed high-energy unoccupied states. The method opens a route towards precise G$_0$W$_0$ quasiparticle calculations in large low-dimensional systems using a small number of unoccupied DFT states. This work was supported by NSF Grant No. DMR10-1006184, the U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NSF through TeraGrid at NICS and DOE at LBNL's NERSC.   

On-site screened Coulomb interactions for localized electrons in transition metal oxides and defect systems

Electronic and structural properties of strongly correlated material systems are largely determined by the strength of the on-site Coulomb interaction. Theoretical models devised to capture the physics of strongly correlated materials usually involve screened Coulomb interactions as adjustable parameters. We present first-principles results for the screened on-site Coulomb and exchange energy for transition metal oxides. The dielectric screening is calculated within the random phase approximation and the localized electrons are represented by maximally localized Wannier functions. We further extend our study to calculate on-site Coulomb interactions for localized defect states in semiconductors. We acknowledge the computational support provided by the Center for Computational Research at the University at Buffalo, SUNY. This work is supported by the National Science Foundation under Grant No. DMR-0946404 and by the Department of Energy under Grant No. DE-SC0002623.   

Optimizing Generalized Norm-Conserving Pseudopotentials

The ``generalized'' method permits the construction of norm-conserving pseudopotentials at energies that do not correspond to bound atomic states, giving added flexibility in the treatment of angular-momentum channels for which no bound states exist.\footnote{D. R. Hamann, Phys. Rev. B \textbf{40}, 2980 (1989).} An effective method for optimizing the convergence of pseudopotential calculations with plane-wave-basis cutoff energy requires atomic wave functions with decaying tails, and has not been applicable to such ``generalized'' states.\footnote{A. M. Rappe,\textit{ et al.}, Phys. Rev. B \textbf{41}, 1227 (1990).} By introducing a potential well outside the core radius for selected angular-momenta, an artificial decaying tail can be produced for positive-energy states. This permits the application of the optimization method, and we find convergence behavior comparable to that for ordinary bound states. In practice, we terminate the positive-energy all-electron wave function smoothly with an exponential or Gaussian tail, and never need to treat the implied well potential explicitly. The projectors to form fully-nonlocal operators\footnote{L. Kleinman and D. M. Bylander, Phys. Rev. Lett. \textbf{48}, 1425 (1982).} can be terminated at the core radii as usual, despite differences of the semi-local potentials outside the well radii.   

Development of a Semi-empirical Hamiltonian for Phosphorus for Quantum Mechanics Based Simulations of Phosphorous-based Nanostructures

We have developed a parameterized semi-empirical Hamiltonian for phosphorous for simulation studies of phosphorous-based nanostructures including phosphorous-doped silicon nanowires.This Hamiltonian models the environment-dependent electron-ion and ion-ion interactions and electron-electron correlations, by capturing the salient features of \textit{ab initio} Hamiltonians/\textit{ab initio} methods, ($e.g$., electron screening and charge self-consistency).Such a semi-empirical Hamiltonian has been shown to be successful in predicting the properties of intermediate-sized silicon, boron, and carbon clusters and extended structures of boron and silicon [1-4]. We optimized the parameters of our Hamiltonian for phosphorous by fitting the properties of bulk (black phosphorous) and small clusters (P$_{2}$ to P$_{10})$ as obtained by our method to \textit{ab initio} calculations. It is expected that such a Hamiltonian will have the predictive power to enable the study of larger phosphorous based nanostructures that are not possible via \textit{ab initio} studies. \\[4pt] [1] C. Leahy, et al, Phys. Rev. B 74,155408 (2006). [2 ]P. Tandy, et al, Bulletin of the APS,2009 APS March Meeting Vol. 54, Num.1, Sess. D26, [3] Ming Yu, et al, J. Chem. Phys. 130,184708 (2009). [4] Ming Yu, S.Y. Wu, and C.S. Jayanthi, Physica E 42, 1 (2009).   

Session H19: Focus Session: Spin Transport & Magnetization Dynamics in Metals III

Large and inverted spin signals in nonlocal spin valves

For a metallic nonlocal spin valve (NLSV), usually the nonlocal resistance value is high for the parallel (P) state of spin injector and detector and the value is low for the antiparallel (AP) state. The difference between two states is known as the spin signal. We show that a 6 miliohms spin signal was observed in a typical NLSV device. However, in another NLSV device with apparently similar structure and dimensions as the previous one, we found a very large spin signal of 90 milliohms with an inverted sign, meaning that the P state yields a low value and the AP state yields a high value. The resistance between the spin detector and the Cu channel is extremely large, exceeding mega-ohms. We conclude that a break-junction is formed at the detector/Cu interface due to static discharge. The large magnitude of the spin signal is due to the spin-charge coupling at the low-conductance break-junction interface. The inverted sign is due to a very different spin-dependent density of states near the break-junction. Work supported by DOE grant No. DE-FG02-07ER46374.   

Spin transfer effects in non-local spin valves with sustained d.c. currents

We utilized pure spin current in a nonlocal spin valve (NLSV) for spin-transfer. The submicron lateral device consists of a Py spin injector (80 nm wide), a Py spin detector (60 nm wide), and a Cu nonmagnetic channel (100 nm wide). The thickness of the spin detector is 3.5 nm, and a nanoscale magnetic domain (60 nm by 100 nm) in the detector underneath the Cu channel can be switched by spin-transfer. We explore reversible spin-transfer switching over a wide temperature range and using both d.c. current pulses and sustained dc currents. Since a d.c. current changes the baseline of the nonlocal resistance, spin-transfer in NLSV has only been explored by d.c. current pulses. In this work, we achieved NLSV spin-transfer with sustained d.c. currents. The hysteresis of nonlocal resistance as a function of the sustained current is clearly observed, despite the baseline variations. High field and polarity-dependent features in the nonlocal MR curves indicate evidence of spin-transfer induced magnetization dynamics. Work supported by US DOE grant No. DE-FG02-07ER46374.   

Spin injection into ferromagnetic insulators

Spin current in a junction of a normal metal and a ferromagnetic insulator is theoretically studied. At the interface, spins of conduction electrons in a normal metal interact with localized spins in ferromagnetic insulator through the exchange interaction. When a spin accumulation is present in the normal metal, accumulated spins decay into magnons via spin-flip scattering of conduction electrons at the interface, thereby creating a magnon spin current in the ferromagnet. Using the linear response theory, we calculate the spin current across the interface and find that spin accumulation plays a role of spin voltage for generating the spin current thorough the junction. Using the spin Hall effect, the spin current injection from a normal metal into a ferromagnetic insulator is demonstrated. We also discuss the spin current in the presence of temperature difference between the normal metal and the ferromagnetic insulator.   

Quantifying Spin Hall Effects in Nonmagnetic Metals

Spin Hall effects intermix spin and charge currents even in nonmagnetic materials and, therefore, offer the possibility to generate and detect spin currents without the need for using ferromagnetic materials. In order to gain insight into the underlying physical mechanism and to identify technologically relevant materials, it is important to quantify the spin Hall angle $\gamma$, which is a direct measure of the charge-to-spin (and vice versa) conversion efficiency. Towards this end we utilized non-local transport measurements with double Hall bars fabricated from gold and copper.\footnote{G.~Mihajlovi\'{c}, J.~E.~Pearson, M.~A.~Garcia, S.~D.~Bader, and A.~Hoffmann, Phys.Rev.\ Lett.\ {\bf 103}, 166601 (2009).} In principle, this geometry permits the study of spin currents both generated and detected via spin Hall effects. We observe an unusual non-local resistivity that changes sign as a function of temperature. However, this results is quantitatively similar in gold and cooper, indicating that the non-local signals are not due to spin transport. An analysis of the data based on a combination of diffusive and quasi-ballistic transport leads to an upper limit of $\gamma< 0.027$ for gold at room temperature. Therefore we developed an approach based on spin pumping, which enables us to quantify even small spin Hall angles with high accuracy. Spin pumping utilizes microwave excitation of a ferromagnetic layer adjacent to a normal metal to generate over a macroscopic area a homogeneous {\em dc} spin current, which can be quantified from the line-width of the ferromagnetic resonance. In this geometry voltages from spin Hall effects scale with the device dimension and therefore good signal-to-noise can be obtained even for materials with small spin Hall angles. We integrated ferromagnet/normal metal bilayers into a co-planar waveguide and determined the spin Hall angle for a variety of non-magnetic materials (Pt, Pd, Au, and Mo) at room temperature. Of these materials Pt shows the largest spin Hall angle with $\gamma = 0.013\pm0.002$.\footnote{O.~Mosendz, V.~Vlaminck, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, arXiv:1009.5089; O.~Mosendz, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.Lett.{\bf 104}, 046601 (2010).}   

Giant spin Hall effect of Au films with Pt impurities: Surface-assisted skew scattering

We show theoretically a novel route to obtain giant room temperature spin Hall effect (SHE) using surface-assisted skew scattering. By a combined approach of density functional theory and the quantum Monte Carlo (QMC) method, we have studied the SHE due to a Pt impurity in different Au hosts. We show that the spin Hall angle (SHA) could become larger than 0.1 on the Au (111) surface, and decreases by about a half on the Au (001) surface, while it is small in bulk Au. The QMC results show that the spin-orbit interaction (SOI) of the Pt impurity on the Au (001) and Au (111) surfaces is enhanced, because the Pt 5 levels are lifted to the Fermi level due to the valence fluctuations. In addition, there are two SOI channels on the Au (111) surface, while only one for Pt either on the Au (001) surface or in bulk Au.   

Spin Hall angle in Pd below the spin diffusion length

The spin-orbit coupling gives rise to an inter-conversion of spin and charge currents. A pure spin current is accompanied by a charge accumulation perpendicular to both the spin polarization and spin current, so-called inverse spin Hall effect (ISHE). We report measurements of the ISHE in a permalloy/palladium (Py/Pd) bilayer integrated with a coplanar wave-guide by pumping a pure spin current via ferromagnetic resonance (FMR) [1]. The magnetization precession creates a spin accumulation at the Py/Pd interface that diffuses into the normal metal and partially scatters back into the permalloy when the Pd thickness is smaller than the spin diffusion length. We observe an increasing broadening of the FMR linewidth with increasing thickness of Pd from which we extract the spin diffusion length in Pd and an average spin mixing conductance. The resultant pure spin current induces, in turn, a spin Hall voltage that is measured across the metallic layer. The spin Hall angle obtained from fitting the dc voltage [1] remains fairly constant even for thickness smaller than the spin diffusion length.\\[4pt] [1] O. Mosendz et al., Phys. Rev. B (in press). arXiv: 1009.5089   

Spin Torque Ferromagnetic Resonance Induced by the Spin Hall Effect

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current. As an independent check, we also apply a DC charge current to the Pt/NiFe bilayer while measuring the FMR signal. The effective damping of the NiFe layer can be increased or decreased depending on the relative angle between the magnetic moment and the injected spin. The amplitude of spin current extracted from this measurement agrees quite well with that obtained from the FMR lineshape. The self-calibration nature of this new technique makes it an excellent solution for a quantitative measurement of the SHE in a ferromagnetic/non-magnetic metal bilayer.   

Spin-orbit dichroism in SX-ARPES of Pt(111)

We study the bulk electronic structure of Pt(111) using polarization dependent soft x-ray (SX)-ARPES ($h\nu$=450--610 eV). Pt is known to exhibit the largest spin Hall conductivity of all metals, which is derived from its large spin orbit coupling [1,2]. We have measured band dispersions along $\Gamma$-K-X ($h\nu$=466 eV) with clockwise and counterclockwise circularly polarized x-rays and obtained circular dichroism (CD) in the valence band of Pt. A comparison with calculated band dispersions including spin-orbit coupling gives a very good match with the experimental results [3,4], thus establishing the role of spin-orbit coupling in the electronic structure of Pt. Our results also identify (i) a hybridization gap with symmetry switching dichroism and (ii) strong CD of bands at the Fermi level, which provide the carriers responsible for SHE.\\[0pt] [1] T.~Kimura, {\it et al.}, Phys.~Rev.~Lett.~{\bf 98}, 156601 (2007). [2] M.~Morota, {\it et al.}, arXiv:1008.0158v1. [3] G.~Y.~Guo, {\it et al.}, Phys.~Rev.~Lett.~{\bf 100}, 096401 (2008). [4] A.~D.~Corso and A.~M.~Conte, Phys.~Rev.~B {\bf 71}, 115106 (2005).   

Anisotropic Spin Hall Effect from First Principles

We present first principles calculations [1] of the intrinsic non-dissipative spin Hall conductivity (SHC) for 3$d$, 4$d$ and 5$d$ transition metals focusing in particular on the anisotropy of the SHC in nonmagnetic hcp metals and in antiferromagnetic Cr. For the metals of this study we generally find large anisotropies. We derive the general relation between the SHC vector and the direction of spin-polarization and discuss its consequences for hcp metals. Especially, it is predicted that for systems where the SHC changes sign due to the anisotropy the spin Hall effect may be tuned such that the spin polarization is parallel either to the electric field or to the spin current. Additionally, we describe our computational method [2,3] emphasizing the Wannier interpolation technique and the definition of the conserved spin current.\\[4pt] [1] e-print: http://arxiv.org/abs/1011.2714\\[0pt] [2] F. Freimuth et al. Phys. Rev. B \textbf{78}, 035120 (2008)\\[0pt] [3] www.flapw.de   

Detection of the transverse voltage associated with the spin Seebeck effect in ferromagnetic thin films

The spin Seebeck effect, the generation of spin current in response to an applied thermal bias across a sample, is a novel effect involving spin current that is being researched in nanostructures for advances in spin caloritronics. Understanding the fundamental physics governing heat transport at the nanoscale is challenging because thermal properties of nanostructures are often difficult measurements to make. We present a novel technique for detecting the presence of a thermally generated spin current based on a micromachined thermal isolation platform. Our technique offers advantages including the ability to measure this effect in a reduced dimension sample, to reverse the thermal gradient, and to generate a large $\triangle T$ across the sample. We present results for a range of thin films and compare to previously reported similar larger scale structures. We discuss future experiments to probe the local nature of the spin Seebeck effect, additional thermal properties including the traditional Seebeck effect and thermal conductivity, and the application of our technique to an array of nanowires.   

Gigantic enhancement of spin Seebeck effect by phonon drag

We investigate both theoretically and experimentally a gigantic enhancement of the spin Seebeck effect [K. Uchida et al., Nature 455, 778 (2008); C. M. Jaworski et al., Nature Mater. 9, 898 (2010); K. Uchida et al., Nature Mater. 9, 894 (2010)] in a prototypical magnet LaY$_2$Fe$_5$O$_{12}$ at low temperatures. Our theoretical analysis sheds light on the important role of phonons; the spin Seebeck effect is enormously enhanced by nonequilibrium phonons that drag the low-lying spin excitations. We further argue that this scenario gives a clue to understand the observation of the spin Seebeck effect that is unaccompanied by a global spin current, and predict that the substrate condition affects the observed signal.   

Two Exponentials Associated with Temperature in Spin-Seebeck Effect Geometry

Recent experiments report the observation of a Spin-Seebeck effect, where an applied thermal gradient along (x) a very thin (z), narrow (y) ferromagnetic sample F is associated with a spin current.\footnote{K. Uchida et al, Nature 455, 778 (2008).} In present geometries this spin current is measured indirectly via a Pt bar above (z) the sample; a voltage difference $V$ along y is measured and interpreted as being due to a spin current $j_s$ into (z) the Pt, which then causes an inverse Spin Hall effect ($j_s$ causes transverse $V$). Measured voltages often show a $\sinh(x/s)$ dependence, where $s$ is long compared to any relevant spin-diffusion length.\footnote{C. M. Jaworski et al, Nature Materials 9, 898 (2010).} The spin current has been interpreted as accompanying a temperature disequilibrium between the phonons and magnons in F.\footnote{J. Xiao et al, Phys. Rev. B 81, 214418 (2010).} The present work uses irreversible thermodynamics to include magnon-phonon equilibration in F and the thermal properties of the (non-magnetic) substrate S. We find two exponentials describing the overall thermal response along x, the second one associated with equilibration between F and S. If the thermal coupling between F and S is poor, then the second length can be rather long.   

Dissipationless mechanism of skyrmion Hall effect in two-dimensional double-exchange ferromagnets

We revisit a theory of nonequilibrium single-skyrmion transport in two-dimensional double-exchange ferromagnets with the Rashba spin-orbit interaction. Combining the collective-coordinate approach with the Keldysh formalism and an effective U(1) gauge theory, the velocity of a skyrmion core is calculated under the electric field. Then, it is found that the emergent Chern-Simons term and the associated intrinsic anomalous Hall can produce a dissipationless skyrmion Hall current through the coupling between electrons and localized spins, which takes the form of the spin transfer torque. In metals, this is additive to the conventional dissipative motion of magnetic vortices, which relies on phenomenological damping terms in the Landau-Lifshitz-Gilbert equation.   

Session H20: Focus Session: Physics of Energy Storage Materials III -- Hydrogen Storage Adsorbents

Strategies for Hydrogen Storage in Nanoporous Metal-Organic Framework Materials

Storing hydrogen by physisorption in porous materials is a challenging problem of great interest for future vehicle technology. Metal-organic frameworks (MOFs) are a new class of nanoporous materials that have demonstrated exciting potential for solving this problem. MOFs are synthesized by the self-assembly of metal nodes and connecting organic linker molecules to create stable, porous frameworks. The synthetic chemistry opens the possibility to create an almost unlimited number of MOFs and to tailor them for particular applications, such as hydrogen storage. The diversity of MOFs also creates an opportunity to learn more about the fundamentals of hydrogen adsorption in porous materials. We have used a combination of classical Monte Carlo simulations and quantum mechanical approaches to investigate fundamental questions about hydrogen storage in MOFs and to design new materials with improved storage capabilities. Relationships have been elucidated between hydrogen uptake and properties such as the MOF surface area, void volume, degree of catenation, enthalpy of adsorption, and cation content. Introduction of cations is a promising strategy to improve hydrogen uptake at room temperature, and different metal cations and different strategies for introducing them into MOFs have been screened computationally.   

Increased hydrogen uptake of MOF-5 by powder densification

The metal-organic framework MOF-5 has attracted significant attention due to its ability to store large quantities of H$_{2}$ by mass, up to 10 wt.{\%} absolute at 70 bar at 77K. On the other hand, since MOF-5 is typically obtained as a bulk powder, it exhibits a low volumetric density and poor thermal conductivity---both of which are undesirable characteristics for a hydrogen storage material. Here we explore the extent to which powder densification can overcome these deficiencies, as well as to characterize the impact of densification on crystallinity, pore volume, surface area, and crush strength. MOF-5 powder was processed into cylindrical tablets with densities up to 1.6 g/cm$^{3}$ by mechanical compaction. We find that optimal hydrogen storage properties are achieved for $\rho \quad \sim $ 0.5 g/cm$^{3}$, yielding a 350{\%} increase in volumetric H$_{2}$ density with only a modest 15{\%} reduction in gravimetric H$^{2}$ excess in comparison to the powder. Higher densities result in larger reductions in gravimetric excess. Total pore volume and surface area decrease commensurately with the gravimetric capacity, and are linked to an incipient amorphization transformation. Nevertheless, a large fraction of MOF-5 crystallinity remains intact in densities up to 0.75 g/cm$^{3}$, as confirmed from powder XRD.   

First-principles study of hydrogen adsorption in metal-doped COF-10

Covalent organic frameworks (COFs), due to their low-density, high-porosity, and high-stability, have promising applications in gas storage. In this study we have explored the potential of COFs doped with Li and Ca metal atoms for storing hydrogen under ambient thermodynamic conditions. Using density functional theory we have performed detailed calculations of the sites Li and Ca atoms occupy in COF-10 and their interaction with hydrogen molecules. The binding energy of Li atom on COF-10 substrate is found to be about 1.0 eV and each Li atom can adsorb up to three H2 molecules. However, at high concentration, Li atoms cluster and, consequently, their hydrogen storage capacity is reduced due to steric hindrance between H2 molecules. On the other hand, due to charge transfer from Li to the substrate, O sites provide additional enhancement for hydrogen adsorption. With increasing concentration of doped metal atoms, the COF-10 substrate provides an additional platform for storing hydrogen. Similar conclusions are reached for Ca doped COF-10.   

Enhanced Dihydrogen-Metal Interaction in Transition Metal Exposed Paddle-Wheel Frameworks

The experimentally observed enhancement of hydrogen adsorption in Cu$_{2}$-tetracarboxylate paddle-wheel frameworks is investigated by first-principles density-functional theory calculations [1]. We reveal that the puzzling enhancement is due to the effective orbital coupling between the occupied H$_{2}$ \textit{$\sigma $} and the unoccupied Cu 4$s$-derived states. The nontrivial dihydrogen-metal \textit{$\sigma $s} interaction is enabled by a strong localization of the Cu 4$s$ orbital after hybridizing with the neighboring oxygen 2$p$ orbitals. Based on this understanding, we predict that the dihydrogen-metal interaction can be further increased by alloying Cu with $s$-orbital element Zn or Mg. We will also discuss on the enhanced dihydrogen adsorption on other 3$d$-transition-metal paddle wheel frameworks.\\[4pt] [1] Y.-H. Kim, J. Kang, and S.-H. Wei, Phys. Rev. Lett., in press (2010).   

Iron decorated - functionalized MOF for high-capacity hydrogen storage: First-principles calculations

We perform electronic structure calculations for the Fe-decorated, OH-functionalized isoreticular metal organic framework 16 (IRMOF16) to investigate the hydrogen storage capacity. Because of the relatively strong Kubas interaction between Fe and H$_{2}$, hydrogen molecule can be adsorbed on the proposed MOF even at room temperature. The reversibly usable storage capacity under ambient conditions reaches 6.0 wt{\%}. Fe has a much lower oxidation tendency than other metals (e.g., Ti, Ca, or Li) used for decorating backbone structures and therefore far more convenient in practical implementation. We also find that the spin flip, which comes from the competition between exchange field splitting and ligand field splitting, plays a significant role in the interaction between Fe and H$_{2}$.   

Study on Ca32C60 Cluster for Hydrogen Storage

Using first-principles calculations within density functional theory (DFT), we study the assembly of Ca32C60, the most desirable metal-coating fullerene as hydrogen storage medium. We first explore possible structures of Ca32C60 dimer with different initial configurations, and find a surprisingly large binding energy up to 2.8 eV. Our further analysis on electronic structures indicates that such a large binding strength stems from the enhanced chemical reactivity of Ca due to the Ca-3s valence electrons partially transferred to the fullerene. We then systematically investigate the alkali and alkali earth elements coated on fullerene, and find that the chemical reactivity of these metal elements can be tuned due to the large electron affinity of C60. Based on this finding, we then extend our studies to the bulk form and two-dimensional structures of Ca32C60, and propose an optimum assemble structure for hydrogen storage. These results shall facilitate designing and optimizing carbon-based materials for hydrogen storage.   

Interaction potential and IR absorption of endohedral H$_2$ in C$_{60}$

We measured the IR spectra of a H$_2$ molecule trapped inside a C$_{60}$ cage at temperatures from 6 to 300\,K and analyzed the spectra by using a model of a vibrating rotor in a spherical potential. The electric dipole moment of IR transitions is induced by the translational motion of H$_2$. The rotation of H$_2$ is unhindered but coupled to the translational motion. The isotropic and translation-rotation coupling part of the potential are anharmonic and different in the ground and excited vibrational states of H$_2$. The vibrational frequency and the rotational constant of endohedral H$_2$ are smaller than in the gas phase. The assignment of IR lines to ortho- and para-H$_2$ is confirmed by measuring spectra of a para enriched H$_2$@C$_{60}$ and is consistent with the earlier interpretation of the low temperature IR spectra [ S. Mamone \textit{et al.}, J. Chem. Phys.\textbf{ 130}, 081103 (2009) ].   

Metallacarboranes: Towards promising hydrogen storage metal organic framework

Using first principles calculations we show the high hydrogen storage capacity of metallacarboranes,\footnote{A. K. Singh, A. Sadrzadeh, and B. I. Yakobson, Metallacarboranes: Toward Promising Hydrogen Storage Metal Organic Frameworks, JACS 132,14126 (2010).} where the transition metal (TM) atoms bind hydrogen via Kubas interaction. The average binding energy of $\sim $0.3 eV/H favorably lies within the reversible adsorption range The Sc and Ti are found to be the optimum metal atoms maximizing the number of stored H$_{2}$ molecules. Depending upon the structure, metallacarboranes can adsorb up to 8 wt{\%} of hydrogen, which exceeds DOE goal for 2015. Being integral part of the cage, TMs do not suffer from the aggregation problem. Furthermore, the presence of carbon atom in the cages permits linking the metallacarboranes to form metal organic frameworks (MOF), thus able to adsorb hydrogen via Kubas interaction, in addition to van der Waals physisorption.   

Anomalous Characteristics of a PVDC Carbon Adsorbant

Nanoporous carbon produced by the pyrolisis of poly(vinylidene chloride-co-vinyl chloride) shows remarkably high adsorption of molecular hydrogen despite its relatively low surface area. In particular, its room temperature volumetric storage is significantly higher than other carbons with surface areas four times higher. In this talk we will present experimental hydrogen adsorption isotherms (and low-temperature isosteric heats of adsorption), subcritical nitrogen adsorption, real space images (TEM), and inelastic neutron scattering. In all cases, the sample characteristics are quite unusual. Whereas the sample under consideration is quite unusual in its high hydrogen sorption capacity, other samples in the literature also show similar unusual characteristics, suggesting the presence of phenomena not fully understood by standard adsorption theory.   

Inelastic Neutron Scattering from Hydrogen Adsorbed in Carbon

Inelastic neutron scattering (INS) from adsorbed hydrogen offers a powerful tool to probe the local adsorption environment of storage material. We will show recently measured INS spectra of hydrogen adsorbed on four different carbon samples and discuss the interpretation of their spectral features, using previous theoretical calculations [1]. Both rotational and vibrational transitions can be observed, along with free recoil scattering parallel to the adsorption plane. The spectra from carbon nanotubes and activated carbon are well explained by theory. However, the spectra from PVDC carbon is quite unusual. \\[4pt] [1] R. Olsen, L. Firlej, B. Kuchta, H. Taub, P. Pfeifer, and C. Wexler; Sub-Nanometer Characterization of Activated Carbon by Inelastic Neutron Scattering; Carbon (under review).   

Analysis of hydrogen sorption characteristics of boron-doped activated carbons

There is significant interest in the properties of boron-doped activated carbons for their potential to improve hydrogen storage.\footnote{See http://all-craft.missouri.edu} Boron-doped activated carbons have been produced using a novel process involving the pyrolysis of a boron containing compound and subsequent high-temperature annealing. In this talk we will present a systematic study of the effect of different boron doping processes on the samples' surface area, micropore structure, and hydrogen sorption. Experimental results include boron content from prompt gamma neutron activation analysis, boron-carbon chemistry from Fourier transform infrared spectroscopy (FTIR), nitrogen subcritical adsorption, and 80K and 90K hydrogen adsorption isotherms which allow us to evaluate the hydrogen binding energy for each sorptive material.   

The effect of KOH:C and activation temperature on hydrogen storage capacities of activated carbons

The Alliance for Collaborative Research in Alternative Fuel Technologies (ALL-CRAFT\footnote{See http://all-craft.missouri.edu}) has been producing high surface area activated carbons. Here we will investigate the effect of the ratio of activating agent to carbon and activation temperature on hydrogen sorption characteristics and sample structure. Results show that a ratio of 3:1 KOH:C and an activation temperature of 790 C are the ideal activation conditions for hydrogen storage applications. Hydrogen sorption measurements are completed using a volumetric instrument that operates at pressures up to 100 bar and at temperatures of 80 K, the sublimation temperature of dry ice (-78.5 C), and room temperature. Specific surface area and pore size distributions are measured using subcritical nitrogen isotherms.   

Session H21: Focus Session: Advances in Scanned Probe Microscopy II -- High Frequencies and Optical Techniques

Rapid Serial Prototyping and Analysis of Nanomagnet-Tipped Attonewton-Sensitivity Cantilevers for Magnetic Resonance Force Microscopy

Magnetic resonance force microscopy offers exciting possibilities for imaging protons and electrons in native and spin-labeled biomolecules. The central component of a magnetic resonance force microscope experiment is a custom-fabricated attonewton-sensitivity cantilever with an overhanging magnetic-nanorod tip. We have recently developed a method for making precision tips which involves 1) fabricating overhanging magnetic tips on shortened mock cantilevers, 2) using focused ion beam milling and deposition (FIB/FID) to cut the mock cantilever (and attached tip) free from the substrate, and then 3) attaching the released structure to a full-length high-sensitivity cantilever. The resulting magnets have been characterized by cantilever magnetometry, high-resolution transmission electron microscopy (HR-TEM), and nanometer-resolution electron energy loss spectroscopy (EELS). This approach to fabrication and analysis is allowing us to optimize tips for proposed single-electron-spin imaging experiments in a very short time. Rapid access to such high-quality tips will significantly advance our ability to image individual biomolecules and macromolecular complexes.   

Development of Magnet-on-Oscillator Low Temperature NMR Force Microscopy

We report recent advances for our nuclear magnetic resonance force microscopy (NMRFM) experiment. Force detection of nuclear spins is made possible by coupling NMR spin flip sequences to a mechanical oscillator. Periodic inversion of the spins in a magnetic field gradient provides the ac coupling force. The force sensitivity for NMRFM improves with decreasing distance between a small gradient magnet and the spins in a sample. Adapting a perpendicular oscillator orientation allows us to decrease the magnet-to-sample distance, providing increased sensitivity. We've also adapted a magnet-on-oscillator design. With this approach, we can perform experiments using oscillating cantilever-driven adiabatic reversal, a technique which has been used to detect single electron spins below the surface of a solid [1]. We've integrated an optical fiber interferometer to measure an oscillator's motion with sub nanometer precision. We can routinely measure the resonance frequencies, quality factors, and spring constants of various oscillators.\\[4pt] [1] Rugar D et al. Nature. 2004;430:329--332   

Correlation of Noise in Multiple Cantilever Modes

The ultrasensitive cantilevers utilized in frequency detection MRFM schemes are susceptible to noncontact interactions between the cantilever tip and the sample. Large efforts have been undertaken to comprehensibly understand the surface related dissipation mechanisms. We propose that sample dielectric fluctuations near DC frequencies should effect all the driven cantilever's oscillatory modes similarly. Utilizing the fundamental and second harmonic mode of the cantilever we have demonstrated this. The correlation between the frequency deviations of the two modes of a driven cantilever increases as the tip-to-sample distance decreases and the measured 1/f noise due to dielectric fluctuations becomes manifest. This result provides additional support for the theory developed by Yazdanian, Marohn and Loring, J. Chem. Phy. 128, 224706 (2008) explaining the origins of noncontact friction and frequency jitter.   

Nanoscale scanning probe ferromagnetic resonance imaging using localized modes

We report the demonstration of scanned probe ferromagnetic resonance imaging (FMRI), a new technique based on Magnetic Resonance Force Microscopy that offers a window into nanoscale properties of buried ferromagnets. Images have been obtained with a current resolution of 200 nm, and significant improvements are straightforwardly possible. Ferromagnetic Resonance (FMR) is a powerful spectroscopic tool for studying internal magnetic fields, interactions and dynamic magnetic properties of ferromagnetic systems, but conventional FMR measures global properties of an entire sample. In FMRI the ``magnetic field well'' created by the probe tip field confines the spin wave modes; these can then be scanned to obtain FMR images. This new microscope is unique in its ability to map internal magnetic fields in buried ferromagnets with spectroscopic precision and nanoscale resolution. First images in permalloy films reveal the ability to image inhomogeneities in magnetic properties with field resolution of approximately 1 Gauss/$\sqrt{\rm {\mathbf Hz}}$. We report a first application to imaging the internal exchange bias field in exchange-biased films.   

MRFM based spectroscopy of GaAs

The apparent contradiction of how to perform NMR spectroscopy given the large magnetic field gradients present in MRFM is resolved by removing the magnetic field gradient while RF based NMR spectroscopic pulses are applied to the sample. This is accomplished by 1) shuttling (move) the sample away from the magnetic particle mounted on the cantilever, 2) apply RF spectroscopic pulse sequences to the sample, 3) store a component of the free induction decay along the z-axis, 4) shuttle the sample back to the cantilever, and 5) read out the magnetization stored on the z-axis with MRFM using an adiabatic rapid passage protocol (ARP). We will describe our progress on performing shuttle based spectroscopy of GaAs using MRFM. We will describe our measurements of T1 of Ga69 in GaAs with an inversion-recovery experiment. Using a single ARP sweep, the polarization is inverted and its recovery is monitored with a driven cantilever using the CERMIT protocol.   

Development of Variable Temperature NMR Force Microscopy

We report our progress on the construction of a variable temperature NMR force microscopy probe and the development of its control system for three dimensional nanoscale scanning. The probe contains two 3-axis piezo-driven slip-stick motion stages for fiber interferometer and for gradient magnet positioning. The control station is a LabView software based control system capable to perform signal generation and data acquisition. Preliminary scan position dependent NMR Force measurements on ammonium sulfate (NH$_{4})_{2}$SO$_{4}$ were performed at room temperature in a sample-on-oscillator configuration. Both piezo-driven and thermal noise cantilever motion have been analyzed to determine resonant frequencies $\omega _{c}$, quality factor Q, and spring constants k; a typical cantilever yielded $\omega _c =1494.40\pm 0.10$Hz, $k=0.039\pm 0.004N/m$, $Q=93.$ RF frequency-modulation-driven artifact effects have been observed and measured during analysis.   

Nitrogen Impurities in Diamond Studied using Magnetic Resonance Force Microscopy

Spin-bearing defects and impurities in diamond have attracted much attention in recent years, with the N-V center defect being a good example. A related defect in the diamond lattice is comprised of a substitutional nitrogen alone and is known as the P1 center with an electron spin S = 1/2 localized on a N-C bond with a strong hyperfine coupling to the $^{14}$N nuclear spin I = 1. We have used Magnetic Resonance Force Microscopy (MRFM) to study the properties of a small collection of P1 centers in diamond. By operating with large field gradients approaching a few Gauss per nanometer, we are able to couple fewer than 100 spins and probe their relaxation properties with a sensitivity approaching a few spins. We have seen that spin lifetimes in the rotating frame are dependent on impurity concentration. We'll show long spin lifetimes ($>$2 s) while undergoing tens of thousands adiabatic spin flips. We also show that spin lifetimes are shorter in diamond implanted with nitrogen ions to create P1 centers. This work was supported by The Army Research Office under W911NF-07-1-0305 and the National Science Foundation under DMR-0807093.   

Parametric Amplification Protocol for Frequency-Modulated Magnetic Resonance Force Microscopy Signals

We present data and theoretical signal and noise calculations for a protocol using parametric amplification to evade the inherent tradeoff between signal and detector frequency noise in force-gradient magnetic resonance force microscopy signals, which are manifested as a modulated frequency shift of a high- $Q$ microcantilever. Substrate-induced frequency noise has a $1/f$ frequency dependence, while detector noise exhibits an $f^2 $ dependence on modulation frequency $f$. Modulation of sample spins at a frequency that minimizes these two contributions typically results in a surface frequency noise power an order of magnitude or more above the thermal limit and may prove incompatible with sample spin relaxation times as well. We show that the frequency modulated force-gradient signal can be used to excite the fundamental resonant mode of the cantilever, resulting in an audio frequency amplitude signal that is readily detected with a low-noise fiber optic interferometer. This technique allows us to modulate the force-gradient signal at a sufficiently high frequency so that substrate-induced frequency noise is evaded without subjecting the signal to the normal $f^2 $ detector noise of conventional demodulation.   

Nonlinear Near-Field Microwave Microscopy for RF Defect Localization in Nb-Based Superconducting Radio Frequency Cavities

Niobium Superconducting Radio Frequency (SRF) cavities are very sensitive to localized defects that give rise to quenches at high accelerating gradients. In order to identify these defects via scanning microscopy, and to further understand the origins of the quench under high radio frequency excitation (1-3 GHz), a scanning probe with localized and up to $\sim $200 mT RF magnetic field is required for low temperature microscopy to achieve sub-micron resolution. For this purpose, we developed a micro loop probe on silicon substrate with outer diameter 20 $\mu $m and inner diameter 17 $\mu $m and successfully fabricated it by lithography. The probe has been used to identify a signal arising from the nonlinear Meissner effect in a Nb thin film. In addition, a magnetic write head is another promising candidate to achieve this goal of understanding localized defect behavior under high RF magnetic field at low temperatures [1]. We will discuss and compare both types of probe for nonlinear scanning microscopy and RF defect localization in superconductors. \\[4pt] [1] Tamin Tai, X. X. Xi, C. G. Zhuang, Dragos I. Mircea and Steven M. Anlage, ``Nonlinear Near-Field Microwave Microscope For RF Defect Localization in Superconductors'' (http://arxiv.org/abs/1008.2948)   

Super-rolloff electron tunneling transduction of nanomechanical motion using frequency downmixing

Electron tunneling transduction has high sensitivity for detecting the motion of nanomechanical devices, but the relatively low detection bandwidth of a few 10's of kHz has limited its development. Here we demonstrate a novel downmixing transduction scheme which eliminates the detection bandwidth problem of electron tunneling transduction. With this technique, the high frequency vibration modes ($\sim $ 1 MHz) of a MEMS doubly clamped beam are measured. This measurement is 2 orders of magnitude above the electronic bandwidth of our readout circuitry with no fundamental limitation anticipated up to microwave frequencies. The displacement sensitivity is 40 fm/Hz$^{1/2}$ comparable to state-of-the-art low finesse free-space optical interferometry. The back-action force induced by the STM tip on the MEMS device is also explored and is shown to have a small effect on the measurement resonance frequency, causing slight resonance frequency shifts of order 1{\%}.   

Toward Atomic-Scale Optical Probes with UHV STM

We present the details of a variable temperature ultra-high vacuum (UHV) scanning tunneling microscopy (STM) apparatus with optical access for the investigation of optically active materials at the atomic scale. Local field enhancement in close proximity to the ultra-sharp STM tip enables the observation of optical signals from a very small number of surface adsorbates and even single molecules, which, combined with the electronic sensitivity and high spatial resolution of STM, allows the simultaneous optical, electronic, and topographic analysis of nanoscale systems. A high-numerical-aperture (NA) optic is integrated into the STM to achieve sharp and stable focusing of the laser excitation while maintaining polarization integrity and high collection efficiency. The initial findings of investigations carried out on epitaxial graphene grown on SiC and operational characteristics of the apparatus are discussed. A next-generation optically accessible 4K UHV STM apparatus under development is also introduced.   

Protecting TERS Probes While Keeping Extreme Enhancement

Protecting the probes used for tip enhanced Raman spectroscopy (TERS) with alumina coatings reduces chemical, mechanical and thermal degradation that otherwise limit the applicability of this emerging technique. Protected plasmonic structures are of interest for surface enhanced Raman spectroscopy (SERS) generally, especially for SERS-based sensors, but we focus particularly on the special case of TERS. Most recently we have focused on the detailed effect of the protective coating for cases in which the enhancement is particularly strong. ``Blinking,'' which is characteristic of extreme enhancement, has been observed with TERS on polymer films for the first time. An Al$_{2}$O$_{3}$ coating prolongs the duration of blinking from 20 minutes to 2 days without significant detriment to the extreme enhancement. The fact that blinking has been observed in the presence of the alumina coating allows us to eliminate chemical enhancement as a major mechanism of the extreme enhancement evidenced by blinking.   

Session H22: Heavy Fermions

Field-induced Spin Fluctuations in Intermetallic Ce$X_{2}$Ge$_{2 }(X$= Cu, Ag, Au)

Intermetallic rare-earth compounds containing a lattice of 4 $f$ or 5 $f$- electrons are prototypical systems to study the magnetic quantum phase transition which mainly results from the fluctuation of the antiferromagnetic moment at T= 0 K. Therefore, understanding the mechanism behind spin fluctuations is important towards a meaningful universal formulation of the QPT phenomena. We have performed magnetic, thermodynamic and neutron scattering measurements on Ce$X_{2}$Ge2 (X = Cu, Ag, Au) compounds in single crystal form to further understand the mechanism behind spin fluctuations. Ce$X_{2}$Ge$_{2}$ crystallize in a ThCr$_{2}$Si$_{2}$-type tetragonal crystal structure and undergo antiferromagnetic transitions at T$_{N}$ = 4.2 K (Cu), 4.6 K (Ag) and 13.5 K (Au). Detail measurements of Q-vectors associated with the long-range order and the numerical modeling of the data revealed the propagation of amplitude modulated spin density wave in CeCu$_{2}$Ge$_{2}$ and CeAg$_{2}$Ge$_{2}$ with the propagation vectors of (0.29,0.29,0.52) and (0,0.705,0.11) respectively. Dynamic measurements of Ce$X_{2}$Ge$_{2}$ compounds in applied magnetic field, exhibiting the varying nature of spin fluctuations as $X$ changes, will be discussed and compared with other Ce-based intermetallic compounds.   

Growth and properties of heavy fermion thin films and superlattices

We have grown thin films of the heavy fermion phases CeCu$_{2} $Ge$_{2}$ (CCG) and CeFe$_{2}$Ge$_{2}$ (CFG) on MgO and DySrO3 substrates using molecular beam epitaxy. We find that the growth begins via island nucleation leading to a granular morphology, since there are two equivalent registrations of the film with the substrate. After nucleating, the grains grow flat with c-axis orientation. These single phase films show similar temperature(T) dependent transport behavior as seen in single crystals of the materials, including for CCG Kondo scattering and the emergence of coherent coupling of the heavy fermion transport channel at low T and for CFG a monotonic decrease in resistivity as the temperature is lowered. Superlattices combining CCG and CFG in different supercell architectures were also grown. In transport, they show a systematic evolution with composition between the distinct R(T) behavior of the two parent phases. A correlation between spectroscopic measurements and resistivity was found and details will be presented.   

Neutron Scattering Study of the Field Induced Non-Fermi-Liquid Behavior in CeAuSb$_2$

The modestly heavy Fermion compound CeAuSb$_2$ ($\gamma = 90 $~mJ/K$^2$~mol) was reported to exhibit highly anisotropic magnetic properties with an antiferromagnetic transition temperature $T_N \approx 5$~K~[1]. In addition, the unconventional temperature dependence of the resistivity and specific heat, observed when an external magnetic field suppresses $T_N$ to 0~K, has lead to the identification of CeAuSb$_2$ as a system showing possible magnetic field-induced quantum critical behavior [2]. Here we report on neutron scattering measurements of CeAuSb$_2$ in magnetic fields up to 9~T applied along the $\left$ direction. Both elastic and inelastic measurements were carried out to track the evolution of the magnetic structure and spin fluctuations as a function of applied field. \\[4pt] [1] A. Thamizhavel et al., Phys. Rev. B \textbf{68}, 054427 (2003).\\[0pt] [2] L. Balicas et al., Phys. Rev. B \textbf{72}, 064422 (2005).   

Shubnikov-de Haas Effect measured on single crystals of CeOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$ along the high symmetry directions

The filled skutterudite compounds CeOs$_4$Sb$_{12}$, PrOs$_4$Sb$_{12}$, and NdOs$_4$Sb$_{12}$ are respectively a 1~K antiferromagnetic (AFM) Kondo insulator, a 1.85~K unconventional superconductor, and a 1~K mean-field type ferromagnet (FM), suggesting that superconductivity in PrOs$_4$Sb$_{12}$ may result from proximity to AFM and FM quantum-critical points. Fermi-surface measurements of NdOs$_4$Sb$_{12}$ and CeOs$_4$Sb$_{12}$ could therefore give insights into the pairing mechanism. We have used a MHz skin-depth technique to observe Shubnikov-de Haas oscillations (SdHos) in single crystals of these materials at fields of up to 60~T. In CeOs$_4$Sb$_{12}$ for {\bf H} // [001], a previously-unobserved semimetal-to-metal transition was detected at $\approx 25$~T; above this, a series of SdHos with a frequency of 1700~T and $ m_{\rm CR} \approx 3.6 m_{\rm e}$ emerge. For {\bf H} // [011] in NdOs$_4$Sb$_{12}$, a single series of SdHos, frequency $\approx 874$~T, was found. These may correspond to the $\beta$ band in PrOs$_4$Sb$_{12}$, but with a much smaller $m_{\rm CR} \approx 1.5 m_{\rm e}$. Research at CSU-Fresno is supported by RC CCSA $\#$7669 and the start-up fund; at NHMFL by DOE, NSF, and FL.; at UCSD by NSF$\#$0802478 and US DOE DE FG02-04ER46105; at Hokkaido U by MEXT, Japan.   

The non-centrosymmetric heavy fermion ferromagnet Sm$_2$Fe$_{12}$P$_7$

The investigation of quantum critical points (QCPs) in heavy fermion compounds (HF) has proven to be a useful tool in gaining insight into strongly correlated electron physics. However, the body of work on HF systems mainly focuses on antiferromagnetic QCPs. We report measurements of the electrical resistivity, magnetization and specific heat on single crystals of the non- centrosymmetric compound Sm$_2$Fe$_{12}$P$_7$, that exhibits ferromagnetic (FM) order below T$_{M,1}$ = 6.3 K. The ratio of the effective magnetic moment in the paramagnetic state, to the saturation magnetic moment in the ordered state indicates that the ordered state is associated with itinerant electrons. An enhanced value for the coefficient of the electronic speci?c heat $\gamma$ $\sim$ 450 mJ mol$^{-1}$K$^{-1}$ is observed, that is accompanied by a large coefficient $A$ of the $T^2$ term in the electrical resistivity, suggesting a HF ground state. Three consecutive magnetic phase transitions, indicative of competing magnetic energy scales, and the observation of a metamagnetic transition additionally suggest proximity to a QCP. Thus, we propose that Sm$_2$Fe$_{12}$P$_7$ is a possible candidate to study a FM QCP in a HF compound.   

Multiple regions of quantum criticality in YbAgGe

YbAgGe is a stoichiometric heavy fermion antiferromagnet that exhibits field-induced quantum criticality.~ We present and discuss thermal expansion and magnetostriction measurements that reveal a new field-induced state. On the low-field side of this state we find evidence for a first-order phase transition and suggest that YbAgGe may be close to a quantum critical end point at 4.5 T. On the high-field side we find evidence for a second-order phase transition suppressed to a quantum critical point near 7.2 T. We will discuss these results in light of global phase diagrams proposed for Kondo lattice systems. Work at Occidental College was supported by the National Science Foundation under DMR-1006118. Work at Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358.   

Quasiparticle duality in the Kondo-screened state of YbInCu4

We present a study of the excitation spectra of YbInCu4. This system exhibits a first- order isoelectronic phase transition which separates regimes with very different $T/T_K$. Using infrared optics, we were first able to demonstrate the existence of a hybridization gap feature which is ubiquitous in heavy fermion systems. More recently, using the burgeoning technique of resonant inelastic X-ray scattering (RIXS) at the Yb M5 edge, we identify a feature at the same energy, strongly suggesting that components of this excitation have mixed itinerant and localized character. Prospects for the future studies using the RIXS technique in the context of heavy fermion materials will be discussed.   

High-resolution angle-resolved photoemission studies of YbRh$_2$Si$_2$ using 7 eV laser

We present angle-resolved photoemission spectra of prototypical heavy fermion compound YbRh$_2$Si$_2$ measured with 7 eV ultraviolet laser. Much improved energy and momentum resolutions enable us to resolve the sharp weakly dispersing peaks at the lowest energy of single-electron spectra. This coherent state grows in intensity and weight as temperature decreases below a characteristic temperature. The characteristic temperature is not only different from the single-ion Kondo temperature of YbRh$_2$Si$_2$ derived from thermodynamic measurements, it is of the same scale as the energy and the lifetime of the coherent state.   

Zeeman-driven Lifshitz transition: A scenario for the Fermi-surface reconstruction in YbRh2Si2

The heavy-fermion metal YbRh$_2$Si$_2$ displays a field-driven quantum phase transition where signatures of a Fermi-surface reconstruction have been identified, often interpreted as breakdown of the Kondo effect. We argue that instead many properties of the material can be consistently described assuming a Zeeman-driven Lifshitz transition of heavy-fermion bands. Using a suitable quasiparticle model, we find a smeared jump in the Hall constant and maxima in susceptibility and specific heat, very similar to experimental data. An intermediate non-Fermi liquid regime emerges due to the small effective Fermi energy near the transition. Further experiments to discriminate the different scenarios are proposed.   

Neutron magnetic form factor in strongly correlated materials

We introduce a formalism to compute the neutron magnetic form factor F(q) within a first-principles Density Functional Theory (DFT) + Dynamical Mean Field Theory (DMFT). We use our method to compute the form factor of PuCoGa5. We find that the local physics of this material is described by a mixed valence mechanism of the type observed in elemental Plutonium. This picture explains nicely the experimental neutron form factor of PuCoGa5 and it is consistent with the photo-emission spectra shape and the value of the specific heat linear coefficient.   

Formation of heavy electron bands by ordering in two-channel Kondo lattice

Itinerant and localized characters of electrons are one of the most fundamental problems in condensed matter physics. In typical Kondo lattice systems, the f electrons are localized in the high temperature region, and acquire the itinerancy by the interaction between f and conduction electrons with decreasing temperature. In the present work, however, we show that the localized character of f electrons changes into itinerant one at the transition point in two-channel Kondo lattice systems. We have analyzed the system using the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. With one conduction electron per site, which corresponds to the quarter filling of each band, a channel order emerges in wide parameter region with metal-insulator transition. At the same time, the heavy electron bands are formed, which indicates the itinerant f-electron states. Since f electrons acquire the itinerancy only below the transition temperature, this behavior can be regarded as itinerant-localized transition of electronic states. We will discuss these behaviors through temperature dependence of the single-particle spectrum.   

Hard X-Ray Photoelectron Spectroscopic Analysis of single crystal UPd$_{3}$, UGe$_{2}$, and USb$_{2}$

Hard X-ray Photoelectron Spectroscopy (HAXPES) with 7.6 keV photons has been performed on single crystals of UPd3, UGe2, and USb2 at the European Synchrotron Radiation Facility (ESRF). A potential correlation between the localization/itinerancy of the 5f electrons and the core levels of these materials is investigated. The greatly reduced surface sensitivity of HAXPES enabled observation of the bulk core levels in spite of some surface oxidation. An 800 meV splitting of the Sb 3d and 4d core levels was observed. The splitting of the Sb core levels is attributed to manifestations of two distinct binding modes within the USb2 single crystal as supported by consideration of interatomic distances and charge transfer calculations.   

Small Angle Neutron Scattering and the Vortex Lattice of UPt$_3$

UPt$_3$ is among the most well studied of the unconventional superconductors. However, there are still many unanswered questions, two of which are: understanding chirality in the superconducting B-phase and understanding the nature of the B-C transition. Central to theories describing both of these areas are predictions for unusual vortex structures. Small angle neutron scattering (SANS) provides a unique way to explore the bulk vortex lattice (VL) and thus can be used to investigate the bulk superconducting state without electronic surface scattering which complicates results from other probes. Ongoing SANS experiments on high quality single crystals in a novel geometry seek to explore the the relationship between field history and the VL in UPt$_3$. Preliminary results show well defined diffraction patterns and narrow rocking curves at fields well into the C-phase and interesting behavior for a variety of field histories. These results will shed new light on chirality, the B-C transition, and VL (meta)stability.   

Unconventional Anomalous Hall Effect in UCu$_{5}$

Field-dependent resistivity, magnetization, and specific heat measurements have been carried out on the heavy fermion compound UCu$_{5}$. We find an unconventional anomalous Hall resistance below the lower temperature magnetic transition at T$_{2} \quad \sim $ 1 K that is proportional to neither the magnetization nor the longitudinal resistivity. We discuss the origin of this resistance in terms of the itinerant carriers' interaction with the magnetically ordered U cations. Complementary measurements on Lu$_{1-x}$U$_{x}$Cu$_{5}$, x = 0 to 0.15 show how non-magnetic dilution affects the Hall resistance and magnetic phase diagram. Interestingly, light Lu substitution for U appears to stabilize the low temperature magnetic phase.   

Temperature Dependent Hybridization Gaps

A number of heavy-fermion/mixed-valent materials show hybridization gaps either at the Fermi-energy or close to the Fermi-energy. In the former case, a heavy-fermion semiconducting state ensues and in the later case, the system remains metallic at low temperatures. In either case, the electronic structure is extremely temperature-dependent. It has been observed that the gap closes and the heavy quasiparticle bands disappear at high temperatures. The magnitude of the gaps scale with effective quasiparticle masses. A phenomenological model is presented that exhibits a temperature-dependence which is consistent with the above behavior. The model is based on a periodic array of Anderson impurities in which the electron correlations are represented by the coupling to bosons with an Einstein spectra. The model can be solved via systematic approximation. The solution describes the temperature- dependence of coherent and incoherent structures in the electronic excitation spectra. The predicted hybridization gaps for the metallic case are compared with data from photoemission experiments on UPd$_2$Al$_3$.   

Session H23: Superconductivity: Mainly ARPES

Doping dependence of the electron-phonon and electron-spin fluctuation interactions in Bi-2212

In the BCS theory of conventional superconductors, electrons form (Cooper) pairs via interactions with the underlying crystal lattice. In the cuprate superconductors, it is not clear whether Cooper pairing takes place via electrons interacting with phonons, spin fluctuations, or whether a bosonic mechanism is necessary at all. Though time-integrated optical measurements on the cuprates can give information on the coupling strength between electrons and an effective boson, it is difficult to tell whether one or more bosons are involved, or the nature of these bosons. We report measurements of time-resolved quasiparticle relaxation of Bi$_2 $Sr$_2 $CaCu$_2 $O$_{8+\delta } $ single crystals (hole concentration $p$ = 0.10-0.22). Our data indicate that \textit{two} bosonic modes are associated with superconductivity: the electron-phonon coupling constant ($\lambda _{e-ph} )$ peaks at optimal doping, while the electron-spin fluctuation coupling constant ($\lambda _{e-sf} )$ decreases monotonically with doping.   

Time-Resolved ARPES Study of Non-Equilibrium Quasiparticle Dynamics in Cuprate Superconductors

We use Time- and Angle-Resolved Photoemission (TR-ARPES) to measure the relaxation dynamics of low energy excitations in the cuprate superconductor Bi-2212. We find an as-yet unreported temperature dependence in nodal quasiparticle spectral weight which is sensitive to the critical temperature. We also find possible evidence for non-thermal transient electronic behavior.   

Ultra High Resolution Pump Probe Angle Resolved Photoemission Experiments on High Temperature Superconductor Bi2Sr2CaCu2O8

Ultra high resolution laser-based pump probe angle-resolved photoemission measurements have been carried out on underdoped Bi2Sr2CaCu2O8 high temperature superconductor. In this talk, we will report on the observation and analysis of quasiparticle relaxation in Bi2Sr2CaCu2O8.   

Bands, spin fluctuations and traces of Fermi surfaces in ARPES intensities on high-$T_C$ cuprates

The band structures of pure and hole doped La$_2$CuO$_4$ with anti-ferro magnetic (AFM) spin-fluctuations are calculated and compared to spectral weights of ARPES. It is shown that the observation of coexisting Fermi surface (FS) arcs and closed FS pockets are consistent with antiferromagnetic spin fluctuations of varying wave lengths. The FS signal of the underlying non-magnetic material is mixed with echos of FS-breaks from domains with AFM spin waves. Large variations of strong spin fluctuations make the outer part of the FS break diffuse at low doping. This part of the FS is suppressed at high doping when spin fluctuations becomes weak. The resulting superimposed spectral weight has features both from FS arcs and closed pockets. This makes a connection between results of ARPES and neutron scattering, and it implies that spin-phonon coupling is an important mechanism for cuprate properties.   

Origin of magnetic resonance spectrum in cuprate high-temperature superconductors and related issues

A distinct low energy magnetic mode has been observed in almost all the cuprate materials in a broad range of experiments including ARPES, Raman, optical, STM, RIXS, as well as neutron scattering. This mode is enhanced in the superconducting (SC) state and its energy scales universally as $\omega_{res} \propto2\Delta$, suggesting that these modes play an important role in the mechanism of superconducting pairing. Here we study this resonance via first-principle susceptibility calculations in a Hubbard model with $d-$wave superconductivity [1]. The resulting excitation mode produces the universal $\omega_{res}\propto 2\Delta$ relation as well as the puzzling `hour-glass' dispersion and the 45$^{\circ}$ rotation of the spin excitations with energy in a series of cuprates in accord with experiments [2]. Work supported by US DOE.\\[4pt] [1] Tanmoy Das, R.S. Markiewicz, and A. Bansil, Phys. Rev. B {\bf 81}, 174504 (2010).\\[0pt] [2] A. Bansil, {\it et al.} Journal of Physics and Chemistry of Solids (2010).   

Particle-hole asymmetric components of QPI in the pseudogap phase of underdoped Bi-2212

QPI visualized by SI-STM became an extremely useful tool in the study of complex electronic matter. Particle-hole(p-h) symmetric QPI observed in the superconducting cuprates revealed many interesting phenomena including the disappearance of the QPI signal around the reduced zone boundary[1], and the persisting QPI signal above the T$_{c}$[2]. Recently, the most dominating band of Sr$_{3} $Ru$_{2}$O$_{7}$ was identified above the metamagnetic nematic phase transition temperature by analyzing p-h asymmetric QPI [3]. Also p-h asymmetric QPI in the parent compound of the ferropnictide superconductor revealed a nematic like electronic structure[4]. Within the same rationale, it is of great interest to find the QPI signature of the band before the superconducting gap opens in the cuprates. Here we explore QPI with particle-hole asymmetric dispersion in the pseudogap phase of underdoped Bi$_ {2}$Sr$_{2}$CaCu$_{2}$O$_{8}$; it appears to disperse continuously through E$_{F}$. Our measured value of the v$_{F}$ of this dispersion is 0.2$\times$10$^{6} $m/s which compares well with the reported value 1.7eV{\AA} from ARPES. We will discuss the possible origin of this QPI by examining its symmetry and dispersion near the zone boundary using theoretical models and currently available experimental data from other probes. [1] Y. Kohsaka et al., Nature(2008) [2] Jhinhwan Lee et al., Science(2009) [3] Jinho Lee et al., Nature Physics(2009) [4] T.-M. Chuang et al., Science(2010)   

Oxygen reduction effects on the electronic structures of electron doped cuprates: investigating the mechanism of the metal insulator transition

In electron doped cuprates, oxygen reduction process not only induces superconductivity but also causes changes in many physical properties. In order to understand these oxygen reduction effects, we performed ARPES studies on as-grown and de-oxygenated superconducting electron doped cuprates, PLCCO, NCCO and SCCO. We observe Fermi surface topology change and pseudo gap filling due to weakening of AFM as reported in other studies. In addition, sharp quasi-particles (QP) appear out of broad incoherent features as the as-grown samples are de-oxygenated through the oxygen reduction process. We believe that this behavior of the QP peak closely related to the insulator to metal transition in the reduction process. We attribute the suppression of the QP states in as-grown sample to the Anderson localized electron states due to strong disorder and impurity scattering.   

Renormalization of f-levels away from the Fermi energy in electron excitation spectroscopies: Density functional results of Nd$_{2-x}$Ce$_x$CuO$_4$

Relaxation energies for photemission, when an occupied electronic state is excited, and for inverse photoemission, when an empty state is filled, are calculated within the density functional theory with application to Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO). The associated relaxation energies are obtained by computing differences in total energies between the ground state and an excited state in which one hole or electron is added into the system. The relaxation energies of f-electrons are found to be of the order of several eV's, indicating that f-bands will appear substantially away from the Fermi energy ($E_F$) in their spectroscopic images, even if these bands lie close to the $E_F$ in the ground state of NCCO. Our analysis explains why it would be difficult to observe f electrons at the $E_F$ even in the absence of strong electronic correlations. Work supported by the US DOE.   

Differential heat capacity studies of Nd$_{2-x}$Ce$_{x}$CuO$_{4}$

The electronic heat capacity of several \textit{hole}-doped cuprate systems has been determined accurately over a wide temperature range using a unique differential calorimeter. It gives important thermodynamic information about the electronic excitations and the pseudogap [1] that is difficult to obtain in other ways, so it is clearly of interest to extend these studies to some \textit{electron}-doped materials. Here we report progress in measuring the specific heat capacity of a series of polycrystalline Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ samples with x varying from 0.14 to 0.18 in steps of 0.01, between 2K and 100K, in magnetic fields from 0 -- 13T and complementary magnetic and transport data. The aims of this work are to look for possible signatures of the pseudogap and to compare our results with recent quantum oscillation studies [2]. \\[4pt] [1] For example, J. W. Loram \textit{et al}., J. Phys. Chem. Solids \textbf{62}, 59 (2001). \\[0pt] [2] T. Helm \textit{et al}., Phys. Rev. Lett. \textbf{103}, 157002 (2009).   

Consequences of two-dimensionality for the quantum oscillations in underdoped YBCO

We report new high resolution measurements on underdoped YBCO over an unprecedented magnetic field range with a high signal- to-noise ratio. The reduced-dimensionality of the Fermi surface is found to strongly influence the quantum oscillations and result in unusual properties. Careful analysis of these unconventional properties is found to severely constrain the Fermi surface topology.   

Lifshitz transitions in the underdoped cuprates with spin-density wave order

It has recently been proposed that a neck-disrupting Lifshitz transition can explain the disappearance of quantum oscillations and diverging cyclotron mass observed in underdoped YBCO. We found that both pocket-disappearing and neck-disrupting types of Lifshitz transitions can be realized in two-dimensional spin-density wave models for underdoped cuprates. Close to Lifshitz transitions, the impurity relaxation rate acquires strong energy-dependence. The thermoelectric power is strongly enhanced, and behaves differently for the two types of transitions.   

Observation of Electronic Structures on Alkali-earth Metal Intercalated Superconducting GICs

We synthesized alkali-metal intercalated GICs(Ca, Ba, Sr). To compare electronic structure of these GICs, we performed high resolution angle-resolved photoemission spectroscopy (ARPES) on it. Through quantitative analysis of band structure, we can see alkali metal dependent band structure.   

Evidence of nodes in the non-centrosymmetric superconductor Y$_{2}$C$_{3}$

In a non-centrosymmetric superconductor, antisymmetric spin-orbit coupling (ASOC) can admix spin-singlet and spin-triplet pairing states, leading to accidental nodes in the energy gap [1]. Y$_{2}$C$_{3}$ is such a superconductor with a T$_{c}$ of 18K. Early NMR and $\mu $SR results [2] indicated a gap structure incompatible with either BCS s-wave or typical d-wave behavior. To further elucidate its superconducting properties, we have measured the temperature dependence of the magnetic penetration depth using a tunneling-diode oscillator technique. While the high temperature penetration depth, and therefore its corresponding superfluid density, can be well described by a two-gap BCS model, as discussed in Ref. [2], the low temperature penetration depth follows a linear temperature dependence, indicating possible existence of nodes in the energy gap. Together with the large upper critical field observed in Y$_{2}$C$_{3}$ [3], the existence of nodes, we argue, might be attributed to the ASOC as a result of absent inversion symmetry even though other possibilities cannot be excluded. [1] H. Q. Yuan et al, Phys. Rev. Lett. \textbf{97}, 017006 (2006). [2] A. Harada et al, J. Phys. Soc. Jpn. \textbf{76}, 023704(2007); S. Kuroiwa et al, Phys. Rev. Lett. \textbf{100}, 097002 (2008). [3] H. Q. Yuan et al, J. Phys. Chem. Solids (in press).   

Session H24: Focus Session: What is Computational Physics? I

Computational Physics' Greatest Hits

The digital computer, has worked its way so effectively into our profession that now, roughly 65 years after its invention, it is virtually impossible to find a field of experimental or theoretical physics unaided by computational innovation. It is tough to think of another device about which one can make that claim. In the session ``What is computational physics?'' speakers will distinguish computation within the field of computational physics from this ubiquitous importance across all subfields of physics. This talk will recap the invited session ``Great Advances...Past, Present and Future'' in which five dramatic areas of discovery (five of our ``greatest hits'') are chronicled: The physics of many-boson systems via Path Integral Monte Carlo, the thermodynamic behavior of a huge number of diverse systems via Monte Carlo Methods, the discovery of new pharmaceutical agents via molecular dynamics, predictive simulations of global climate change via detailed, cross-disciplinary earth system models, and an understanding of the formation of the first structures in our universe via galaxy formation simulations. The talk will also identify ``greatest hits'' in our field from the teaching and research perspectives of other members of DCOMP, including its Executive Committee.   

Nicholas Metropolis Award Talk for Outstanding Doctoral Thesis Work in Computational Physics: Computational biophysics and multiscale modeling of blood cells and blood flow in health and disease

Computational biophysics is a large and rapidly growing area of computational physics. In this talk, we will focus on a number of biophysical problems related to blood cells and blood flow in health and disease. Blood flow plays a fundamental role in a wide range of physiological processes and pathologies in the organism. To understand and, if necessary, manipulate the course of these processes it is essential to investigate blood flow under realistic conditions including deformability of blood cells, their interactions, and behavior in the complex microvascular network. Using a multiscale cell model we are able to accurately capture red blood cell mechanics, rheology, and dynamics in agreement with a number of single cell experiments. Further, this validated model yields accurate predictions of the blood rheological properties, cell migration, cell-free layer, and hemodynamic resistance in microvessels. In addition, we investigate blood related changes in malaria, which include a considerable stiffening of red blood cells and their cytoadherence to endothelium. For these biophysical problems computational modeling is able to provide new physical insights and capabilities for quantitative predictions of blood flow in health and disease.   

Computational Physics Across the Disciplines

In this informal talk, I will present two case studies of the unexpected convergence of computational techniques across disciplines. First, the marriage of neutron star astrophysics and the materials theory of the mechanical and thermal response of crystalline solids. Although the lower reaches of a neutron star host exotic nuclear physics, the upper few meters of the crust exist in a regime that is surprisingly amenable to standard molecular dynamics simulation, albeit in a physical regime of density order of magnitude of orders of magnitude different from those familiar to most condensed matter folk. Computational results on shear strength, thermal conductivity, and other properties here are very relevant to possible gravitational wave signals from these sources. The second example connects not two disciplines of computational physics, but experimental and computational physics, and {\it not} from the traditional direction of computational progressively approaching experiment. Instead, experiment is approaching computation: regular lattices of single-domain magnetic islands whose magnetic microstates can be exhaustively enumerated by magnetic force microscopy. There resulting images of island magnetization patterns look essentially like the results of Monte Carlo simulations of Ising systems... statistical physics with the microstate revealed.   

Role of Electronic Structure Calculations in Understanding Superconductors

Superconductivity remains one of the most challenging and exciting areas in condensed matter physics. It is a field that often sees surprises. These come in the form of new superconducting materials with unprecedented properties that need explanation. Here we briefly discuss the role that computational electronic structure studies have played in understanding some of these new systems over the years. The materials discussed are high temperature cuprates, borocarbides, Sr$_{2}$RuO$_{4}$, MgB$_{2}$, and the iron-based superconductors. Computation has played a key role in understanding properties of these materials and in some but not all cases pointing directly to the mechanism of superconductivity.   

What is computational physics? An embarrassment of riches for teaching computational physics

The first decade of the 21$^{st}$ century has provided a wealth of exceptional resources for teaching computational physics, including numerous textbooks, libraries of computer codes (visual as well as numerical), and high-level interfaces for accessing these libraries. We are now faced with the very real challenge of choosing which of these resources to incorporate into the finite number of courses available in a given curriculum. This choice depends on several factors: How much time can be allocated to teaching computational methods and at what stage in the curriculum? What are the goals? (Learning physics better? Learning to individually implement numerical solutions for small-scale problems? Being prepared to work in research labs studying large-scale problems?) Are commercial packages an appropriate option for your student population? There are no right and wrong answers to these questions, and I will present more questions than answers! However, in recent years I have taught three undergraduate computational physics courses per year, and I will discuss some of the decisions that have been made regarding those courses.   

Computational Physics? Some perspectives and responses of the undergraduate physics community

Any of the many answers possible to the evocative question ``What is ...'' will likely be heavily shaded by the experience of the respondent. This is partly due to absence of a canon of practice in this still immature, hence dynamic and exciting, method of physics. The diversity of responses is even more apparent in the area of physics education, and more disruptive because an undergraduate educational canon uniformly accepted across institutions for decades already exists. I will present evidence of this educational community's lagging response to the challenge of the current dynamic and diverse practice of computational physics in research. I will also summarize current measures that attempt respond to this lag, discuss a researched-based approach for moving beyond these early measures, and suggest how DCOMP might help. I hope this will generate criticisms and concurrences from the floor.   

High-performance scientific computing in the cloud

Cloud computing has the potential to open up high-performance computational science to a much broader class of researchers, owing to its ability to provide on-demand, virtualized computational resources. However, before such approaches can become commonplace, user-friendly tools must be developed that hide the unfamiliar cloud environment and streamline the management of cloud resources for many scientific applications. We have recently shown that high-performance cloud computing is feasible for parallelized x-ray spectroscopy calculations.\footnote{J.J. Rehr et al., CiSE, {\bf 12}, 34 (2010)} We now present benchmark results for a wider selection of scientific applications focusing on electronic structure and spectroscopic simulation software in condensed matter physics. These applications are driven by an improved portable interface that can manage virtual clusters and run various applications in the cloud. We also describe a next generation of cluster tools, aimed at improved performance and a more robust cluster deployment.   

Characterizing Large-Scale Computational Physics

Large-scale computational physics calculations typically share some of a number of basic characteristics: \begin{itemize} \item Brute-force approaches: Atomistic molecular dynamics, particle-in-cell plasma physics, particle-mesh cosmological simulations, DNS of turbulence, lattice QCD, Monte Carlo, {\ldots}. \item Wide range of relevant scales: Angstroms to millimeters in molecular dynamics, ion/electron cyclotron period to seconds or minutes in plasmas, galaxy to observable universe in cosmology, high Reynolds number turbulence, {\ldots}. \item Obvious need for yet larger scale: higher resolution, larger simulation domain, more particles, {\ldots}. \item Code is named, parallel, community, long-lived (but evolving). \end{itemize} This talk views the computational physics landscape from the perspective a physicist who has worked at three DOE large-scale computing centers: the Argonne Leadership Computing Facility, the (former) Advanced Computing Laboratory, and NERSC. The ``usual suspects'' at the large-scale end of computational physics are remarkably persistent, even in the face of an ever-increasing definition of large-scale.   

Discussion of DCOMP Activities

The APS Division of Computational Physics, founded in 1986, explores the use of computers in physics research and education as well as the role of physics in the development of computer technology. Its goals are to promote research and development in computational physics, enhance the prestige and professional standing of its members, encourage scholarly publication, and promote international cooperation in these activities. This talk will mainly invite an open discussion of these objectives and how best to achieve them.   

Session H25: Superconductivity: Tunneling Spectroscopy

Pairing Glue in High Tc Cuprates from Tunneling Spectroscopy

Break junction tunneling spectroscopy data in Bi2212 over a wide range of doping are fit using a d-wave Eliashberg model. Self consistency is achieved as the electron-boson spectral function, $\alpha ^{2}$F($\omega )$, that fits the tunneling conductance dip feature also leads to the correct superconducting gap. The anomalous negative dI/dV observed in break junctions on optimal doped Bi2212 is also reproduced in the analysis. The diagonal and off-diagonal self energies, $\Sigma (\omega )$ and $\phi (\omega )$, respectively are generated in the analysis and they show trends with doping which are in agreement with numerical simulations of the Hubbard model. The peak in $\alpha ^{2}$F($\omega )$ is consistent with the resonance mode in the spin fluctuation spectrum. Tunneling data of other cuprates are also discussed.   

How Topological Defects Couple the Smectic and Nematic Electronic Structure of the Cuprate Pseudogap States

We study the recently discovered coexisting smectic and nematic broken symmetries in the pseudogap-energy electronic structure of underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. By visualizing their spatial components separately, we discover 2$\pi $ topological defects throughout the phase-fluctuating smectic states. Imaging the locations of large numbers of these topological defects simultaneously with the fluctuations of the electronic nematicity about its average, reveals strong empirical evidence for a coupling between them. We also found the same phenomenology in a single layer compound of Bi$_{2}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta }$. From these observations, we propose a Ginzburg-Landau free energy describing the quantum nematic/smectic coupling and demonstrate how it can explain the coexistence of these states and correctly predict their interplay at the atomic scale. This theoretical understanding of the coupling between the quantum nematic and smectic broken symmetries can lead to unraveling the complexities of the phase diagram of cuprate high-$T_{c}$ superconductors[1]. [1]A. Mesaros, K. Fujita, H. Eisaki, S. Uchida, J. C. Davis, S. Sachdev, J. Zaanen, M. J. Lawler, and Eun-Ah Kim, \textit{Submitted} (2010).   

Universal properties of disordered electron nematics from surface probes

When the electronic degrees of freedom break the rotational symmetry of the host crystal, i.e. from C$_4$ to C$_2$, the resulting state is an electronic Ising nematic. However the combination of reduced dimensionality and material disorder can forbid the formation of a long- range-ordered electron nematic, especially in strongly layered materials. Nevertheless, large domains are still possible. In this talk we will present results from a new kind of analysis for Scanning Tunneling Microscopy (STM) experiments as well as other surface probes. We map the locally broken C$_4$ to C$_2$ rotational symmetry of the electronic degrees of freedom to an Ising-type order parameter,use the local order parameter configuration to shed light on the universality class controlling the local pattern formation in cuprate superconductors.   

Vacuum-aging effect on electronic structure of YBa$_{2}$Cu$_{3}$O$_{6+x}$ thin film: a STM/STS study

It is well known that oxygen plays a key role in the occurrence of superconductivity in high-temperature cuprate superconductors. Variation of oxygen content changes carrier concentration and directly affects electronic structure and superconducting properties of cuprate superconductors. Majority of previous studies relied on the intake process of oxygen to change the oxygen content in samples, while the reverse process, oxygen depletion, was rarely investigated. Nevertheless, the escape of oxygen from the surface of cuprate sample that was kept at room temperature under ultrahigh vacuum for extended period of time might lead to significant degradation of its superconducting properties due to the decrease of the carrier concentration. Here, we report this so-called vacuum-aging effect in YBa$_{2}$Cu$_{3}$O$_{6+x}$ thin films grown by laser-MBE technique. In particular, we use variable-temperature scanning tunneling microscopy/spectroscopy to follow the evolution of superconductivity and pseudogap states in this material as a function of aging time and tip position on the surface.   

Influence of the tunneling property on the noise thermometry using a metal-insulator-metal tunnel junction

We are developing a noise thermometry setup based on precision RF measurement, where temperature can be inferred from the noise of a tunnel junction as a function of the bias voltage. We measure the electrical noise of an Al-AlOx-Al tunnel junction around 1 GHz with a bandwidth of a few hundred MHz. In this presentation, as an analysis on the source of error in thermometry, we studied the influence of the junction quality and the inelastic process on the temperature measurement. We compared the noise of an as-fabricated tunnel junction with that of a degraded tunnel junction after thermal cycling. Except for the junction degradation, all measurement environments were kept exactly same. We observed an apparently higher noise value near the zero-bias, which leads to an overestimation of temperature. We present a simple model to describe how the inelastic process in a tunnel junction affects the temperature measurement.   

Mapping the Pseudogap by Fourier Transform Scanning Tunneling Spectroscopy in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$

The relationship between pseudogap and superconductivity in cuprate superconductors remains an important open question. To shed light on this issue, we have used Fourier-transform scanning tunneling microscopy to map quasiparticle interference (QPI) patterns as well as the ``checkerboard,'' a weak charge modulation associated with the pseudogap, as a function of doping and temperature in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ (Bi-2212). We can extract the doping dependence of the pseudogap transition temperature T* within the superconducting dome. Our results strongly suggest that the pseudogap is a competing order.   

Phenomenological model for extracting local energy scales across dopings and temperatures in cuprates

One of the key questions that remains unanswered in the cuprate high temperature superconductors is the nature of the pesudogap phase that exists above the superconducting transition temperature in underdoped materials. In order to differentiate the different proposed origins of this phase, a detailed phenomenology of the different energy scales that characterize it and the superconducting phase is required. In addition, many of these materials (BSCCO-2212 in particular) have shown striking inhomogeneity that further complicates observations of these scales. Spectroscopic imaging scanning tunneling microscopy has the unique ability to resolve density of states features with the needed spatial resolution for seeing through this inhomogeneity. We will present a new phenomenological model for extracting three different energy scales that are present with atomic resolution across several dopings. In addition, preliminary data on the temperature dependence of these scales will be shown. Lastly, how these different scales relate to the different phases present in the underdoped cuprates will be discussed.   

Understanding the Measurement of the K-Space Gap Using Spectroscopic Imaging Scanning Tunneling Microscopy

Two of the many tools used to probe the High Tc cuprates are Angle Resolved Photo Emission (ARPES) and Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM). While the two probes have had many qualitative agreements recently there has been a movement in the field to strive for quantitative agreement in order to better understand the phase diagram of the cuprates. When looking for quantitative agreement we are met with striking disagreements such as, the measurement of the super conducting gap by both probes and the observation of Fermi arcs. We have generated a preliminary simulation based on a superconducting tight binding model where we can tune different parameters in order to begin exploring some of these issues. With our STM just beginning to take data our simulation is allowing us to understand the type of data we need to shed some light on these issues.   

a-axis NIS tunnel junctions using LaSrCuO4

Planar tunneling in NIS structures with crystalline order reveals the density of states largely focused in the tunneling direction. Earlier experiments with a-axis YBCO showed an unexpected strongly broken particle hole symmetry in the CuO bond direction. Here we report on work to make similar structures with a-axis LaSrCuO4.   

Determining the pseudogap Dirac point in the underdoped cuprate superconductors using FT-STS and AC-ARPES

Prominent in the underdoped cuprate superconductors is the existence of a pseudogap in the excitation spectrum which opens above $T_c$ but below a temperature $T^*$. Whether this gap is the same as the superconducting energy gap or is a manifestation of a competing order independent of the superconductivity remains an open and central question. If there are two distinct gaps of $d$-wave symmetry, they each will exhibit a Dirac point at a different energy and momentum in the band structure. We demonstrate how to find the pseudogap Dirac point by using quasiparticle interference (QPI) measurements from Fourier transform scanning tunneling spectroscopy (FT-STS) or by extrapolating information from the autocorrelation function of angle-resolved photoemission spectroscopy (AC-ARPES) to positive energies. Current examination of photoemission data supports our proposal and suggests that a Dirac point exists at positive energy relative to the Fermi level.   

Spatial Variations in the Fermi Surface of Bi-2212

In cuprate superconductors, scanning tunneling microscopy can be used to see variations in the Fermi surface on a nanometer length scale caused by doping inhomogeneity. Prior STM studies show that the local wavelength of the checkerboard, a weak charge modulation ascribed to antinodal Fermi surface nesting, varies with the size of the pseudogap in Bi$_2$Sr$_2$CuO$_{6+\delta}$ (Bi-2201) [1]. Here we report similar STM measurements in Bi-2212. We therefore confirm the local relationship between pseudogap energy and charge ordering wavevector in a second high-Tc superconductor.\\[4pt] [1] W. D. Wise, et al. Nature Physics 5, 213 (2009).   

Vortex-core structure in d-wave superconductors with weak triplet pairing attraction

The quasiparticle states found in the vortex core of an d-wave high-$T_c$ cuprate superconductor may be probed by STM experiments. Results of such experiments have revealed typical spectra that are quite different from what is seen in conventional low-$T_c$ superconductors. In particular the Caroli-deGennes-Matricon state at $E\sim 0$ in the core center is not seen. Instead, in a high-$T_c$ vortex core, quasiparticle states are found at energies that are at a sizable fraction of the gap energy. One explanation for this could be that a finite amplitude of a competing orderparameter stabilizes in the vortex-core center. We explore the possibility of nucleating a vortex-core state that locally breaks inversion symmetry. The vortex-core orderparameter is of mixed parity, in our case a (d + ip)-wave, and the quasiparticle spectra in the core center lacks the $E = 0$ states. We compare our results with available experimental data.   

Possible mechanism of enhanced pairing correlation near dopant oxygen in cuprate

Recent experiments on Bi-based cuprate superconductors have revealed an unexpected enhancement of the pairing correlations near the interstitial dopant oxygens. We propose a mechanism by which the dopant oxygens strongly enhance the interaction $J$ locally [1]. We notice that there is a strong covalency between the dopant oxygen and closely located apical oxygens, forming a molecular orbital complex. By considering virtual $p$-$d$ and $d$-$d$ charge transitions within the Cu-O-Cu bond that lead to the spin exchange $J$, we will show that the corresponding excitation energies are screened by the polarization of molecular orbitals hence enhancing $J$. The effect is greatly amplified due to cooperative response of the spatially extended oxygens complex. We will also show, by an exact diagonalization of the $t$-$J$ model, that local enhancement of $J$ leads to the spatial variations in density of electronic states observed in STM experiments. Our findings suggest an interesting possibility of quantum-chemistry control of the key interaction $J$ in cuprates. \\[4pt] [1] G. Khaliullin, M. Mori, T. Tohyama, and S. Maekawa, arXiv:1008.0435   

Session H26: Focus Session: Iron Based Superconductors -- Anisotropic Spin Dynamics

Impact of the Spin Density Wave Order on the Superconducting Gap of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We report a doping dependent electronic Raman scattering measurements on iron-pnictide superconductor Ba(Fe$_{1-x} $Co$_x$)$_2$As$_2$ single crystals. The B$_{2g}$ Raman spectrum at optimal doping is consistent with a strongly anisotropic gap on the electron pocket. Upon entering the coexistence region between superconducting (SC) and spin-density-wave (SDW) orders, the effective pairing energy scale is strongly reduced. Our results are interpreted in terms of a competition between SC and SDW orders for electronic states at the Fermi level. Our findings advocate for a strong connection between the SC and SDW gaps anisotropies which are both linked to interband interactions.   

Frustrated square lattice Heisenberg model and magnetism in Iron Telluride

We have measured spin excitations in iron telluride Fe1.1Te, the parent material of (1,1) family of iron-based superconductors. It has been recognized that J1-J2-J3 frustrated Heisenberg model on a square lattice might be relevant for the unusual magnetism and, perhaps, the superconductivity in cuprates [1,2]. Recent neutron scattering measurements show that similar frustrated model might also provide reasonable account for magnetic excitations in iron pnictide materials. We find that it also describes general features of spin excitations in FeTe parent compound observed in our recent neutron measurements, as well as in those by other groups. Results imply proximity of magnetic system to the limit of extreme frustration. Selection of spin ground state under such conditions could be driven by weak extrinsic interactions, such as lattice distortion, or strain [3]. Consequently, different nonuniversal types of magnetic order could arise, both commensurate and incommensurate. These are not necessarily intrinsic to an ideal J1-J2-J3 model, but might result from lifting of its near degeneracy by weak extrinsic perturbations.\\[4pt][1] A. V. Chubukov, Phys. Rev. B 48, 5588 (1992). [2] P. A. Lindgard, Phys. Rev. Lett. 95, 217001 (2005). [3] I. A. Zaliznyak, Phys. Rev. B 68, 134451 (2003); ibid. 69, 092404 (2004).   

Superconductivity and Magnetism in the Checkerboard Models for Iron-based Superconductors

We study three different checkerboard models for iron-based superconductors and obtain their phase diagrams in the solvable limit of weakly coupled checkerboards. We demonstrate that the strongest superconducting pairing is in the $A_{1g}$-S wave channel and the development of the superconductivity (SC) is correlated with the emergence of the next nearest neighbor antiferromagnetism (AFM). Moreover, this study suggests that the superconductivity and magnetism are orbital-selective. In the three-band model, the AFM is more robust in the $d_{xy}$ orbital and the superconductivity is easier to be generated in the $d_{xz}$ and $d_{yz}$ orbitals. Comparisons between our theoretical results and current experimental measurements are discussed.   

Anisotropy of the spin dynamics in hole and electron-doped 122

This abstract not available.   

Magnetic properties in the Mott-insulating iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$OSe$_{2}$

The role of electron correlation and magnetism in high-temperature superconductivity of the iron pnictides has been a topic of discussion. It has also motivated interest to compare related compounds with the iron pnictides and chalcogenides. Recently both electronic structure calculations and experimental measurements have indicated that the iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$OSe$_{2}$, which contains an Fe square lattice with an enlarged unit cell, has a larger U/t and is a Mott insulator [1]. We focus here on the understanding of the magnetism of this system. Within the density functional theory, we consider the magnetic phase diagram. Using an effective frustrating spin-exchange model in a doubled checker-board lattice, we study the magnetic excitation spectrum. Our theoretical results are compared with the emerging elastic and inelastic neutron scattering data in this compound. \\[4pt] [1] J.-X. Zhu, R. Yu \textit{et. al}, Phys. Rev. Lett. \textbf{104}, 216405 (2010).   

Consistent model of magnetism in ferropnictides

The character of magnetic interactions and spin fluctuations in ferropnictides has until now resisted understanding within any conventional model of magnetism. We show that the most puzzling features can be naturally reconciled within a rather simple effective spin model with biquadratic interaction, which is consistent with electronic structure calculations. While preserving the symmetry of the lattice, this model spin Hamiltonian stabilizes the collinear stripe ground state and generates an anisotropic spin wave spectrum. A natural reinterpretation of the measured spin wave spectra in ferropnictides is presented based on this model. Classical Monte Carlo simulations with experimentally motivated parameters produce reasonable Neel temperatures for 122 compounds. The model predicts that the phase transition to the paramagnetic phase changes from second to first order as the magnitude of the biquadratic term is increased. This property agrees with the observed behavior of the 122 compounds under doping. A clear signature of the separation of the nematic and antiferromagnetic phase transitions is also found. Preprint: arXiv:1011.1715.   

Spin-resolved electron-phonon coupling in FeSe

FeSe is one of the simplest iron-based superconductors. There are previous studies indicating that including the iron magnetic moment ordering has a significant effect on electron-phonon interactions and thus might be important for superconductivity. To explore the role of spin-dependent phonon induced pairing of the electrons, we apply first principle techniques based on the pseudopotential density functional approach and the local spin density approximation to calculate the electron-phonon coupling properties of FeSe. Our results indicate that introducing magnetic moments leads to a significant increase in coupling at least for certain phonon modes. At the gamma point in the Brillouin zone the increase is two-fold. Both phonon renormalization and electron-phonon matrix elements increases are present.   

Spin-phonon coupling and superconductivity in iron pnictides

Early electron-phonon (el-ph) coupling calculations for iron pnictide system based on standard non-spin-polarized perturbation theory indicate that conventional el-ph coupling cannot explain the observed high Tc in these systems. However, the experimental phonon spectrum indicates features which are not produced in the standard linear response non-magnetic phonon calculations. The magnetic phonon calculations clearly indicate that the observed phonon-DOS at room temperature is much closer to the magnetic phonon-DOS rather than non-magnetic DOS and Fe-magnetism must present in the iron-pnictide systems all the time [1-2]. Thus we need to calculate the magnetic el-phonon coupling with the Fe-spins included before we can rule out any type of phonon-mediated mechanism. In order to carry out such complex self-consistent magnetic el-ph coupling calculations we are developing a finite-displacement method in which both the phonon energies and the corresponding el-ph coupling constant are easily calculated. Implications of our results on the mechanism of superconductivity in iron pnictides will be discussed. Finally, we will compare our calculations with the available phonon energy and line-width measurements. \\[4pt] [1] T. Yildirim, Phys. Rev. Lett. 102, 037003 (2009). \\[0pt] [2] T. Yildirim, Physica C 469, 425-441 (2009).   

Gap structure of the iron-pnictide superconductor LiFeAs via low-temperature thermal conductivity

The thermal conductivity of the stoichiometric iron-pnictide superconductor LiFeAs was measured at temperatures down to $T \sim 50$~mK in magnetic fields up to $H=17$~T on high-quality single crystals with $T_c \simeq 18$~K. The absence of any residual linear term at $T \to 0$ shows that there are no nodal quasiparticles. The slow increase of thermal conductivity with magnetic field shows that the gap is large everywhere on the Fermi surface. The same behaviour is observed for both in- plane and out-of-plane directions. We conclude that the superconducting gap in LiFeAs is basically isotropic. This is similar to what has been found in the iron-pnictide superconductors Ba$_{1- x}$K$_x$Fe$_2$As$_2$ [1] and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ [2] at optimal doping (maximal $T_c$). \\[4pt] [1] X.-G. Luo {\it et al.}, Phys. Rev. B {\bf 80}, 140503 (2009). \\[0pt] [2] J.-Ph. Reid {\it et al.}, Phys. Rev. B {\bf 82}, 064501 (2010).   

NMR Study of the SDW ordering and the Spin Fluctuations on NaFeAs single crytals

In iron pnictides, the nature of the spin density wave (SDW) ordering is still not clear. Recently, increasing attention has been drawn to the correlation between the SDW transition and the high-temperature tetragonal to the low-temperature orthorhombic structure transition. In NaFeAs, the magnetic moment is small and both transitions are well separated, and therefore NaFeAs could be a good candidate to study the interplay of different degrees of freedom microscopically. In this talk, we report our $^{23}$Na and $^{75}$As NMR observations on NaFeAs single crystals. We found that 1) the spin fluctuations are largely enhanced below the structure transition; 2) the SDW transition temperature and the magnetic moment increase significantly with pressure; and 3) the NMR linewidth and the temperature/field dependence of the spin- lattice relaxation rate show signatures of an incommensurate SDW ordering in a limited temperature range just below the SDW transition. Based on these results, we discuss the coupling between the magnetism and the lattice/band structure in NaFeAs.   

Spin excitation in LiFeAs

We used inelastic neutron scattering to study the spin excitations in LiFeAs. Clear spin excitations were found but there was no spin resonance. Surprisingly, very big spin gap exists in this material.   

Neutron Diffraction Studies of PrFe(1-x)Ru(x)AsO

We report neutron powder diffraction (NPD) studies of Ru doped PrFe(1- x)Ru(x)AsO. The parent compound PrFeAsO undergoes a structural transition as well as magnetic transitions involving Fe and Pr moments upon cooling. Previous measurements (M. A. McGuire et al, Jrnl of Solid State Chem, 182- 8, 2326-2331) showed that Ru doping suppresses the above transitions. However, unlike most 1111's, this does not lead to superconductivity. To investigate the origin of this odd behavior we performed NPD measurements as a function of temperature for values of x up to 0.75. The results showed that although the structural and magnetic transitions are suppressed, the c axis displayed apparent negative thermal expansion (NTE) for all values of x. Such NTE has been seen in the parent compound (S. A. J. Kimber et al, PRB 78-140503), but to our knowledge there are no reports of NTE in superconducting samples. This suggests that the mechanism producing the NTE could also be responsible for the absence of superconductivity. We also report data on the magnetic transitions for lightly doped samples with x up to 0.1.   

Interactions between rare earth and iron magnetism in \textit{RE}FeAsO single crystals

In iron-based pnictides high-temperature superconductors, magnetic fluctuations and magneto-elastic effects are believed to be important for the superconducting electron pairing mechanism. To gain insight into the interplay between the different ordering phenomena and the underlying couplings we studied the tetragonal-to-orthorhombic distortion and the magnetic order by x-ray and neutron diffraction on \textit{RE}FeAsO single crystals. The onset of rare earth (\textit{RE} = Nd, Pr) magnetic order is coupled to changes in the iron magnetic structure without affecting the lattice distortion. High-resolution neutron and x-ray resonant magnetic scattering measurements down to 0.4 K revealed complex magnetic structures with multiple propagation vectors at low temperatures.   

Session H27: Focus Session: Semiconductor Qubits- Silicon Spin Qubits

Considerations for spin-based quantum computing in the solid-state

We give an update on recent work towards practical quantum computing using gated spins in semiconductors, especially in silicon.   

Fast initialization of a silicon spin qubit via an excited orbital state

We present data showing the initialization and measurement of individual electron spins in a silicon quantum dot. Spectroscopy of the electronic excited states of the dot reveals a relatively low-lying excited orbital state that is much more strongly coupled to the reservoir than the ground orbital state. As a function of an applied magnetic field, Zeeman splitting is observed for both the ground and the excited orbital states. By tuning a gate voltage, electron spins can be preferentially loaded into the quantum dot via any of these spin-split orbital states. Loading at either of the excited orbital states is measured to be over an order of magnitude faster than loading at directly into the orbital ground state. We use single-shot readout to measure the spin state of the loaded electrons. We observe two clear peaks in the fraction of spin-up electrons that are loaded, and these peaks correlate with loading through the spin-up ground or excited orbitals.   

Measurement of the electron spin relaxation time in a silicon quantum dot using single-shot readout

Electron spins in Si/SiGe quantum dots are promising candidates as qubits for quantum information processing, because spins in silicon couple weakly to the host material. We present a measurement of the spin lifetime for electrons in a silicon quantum dot. The spin state of individual electrons is measured using single-shot charge readout and spin-to-charge conversion: only spin-up electrons will tunnel off the quantum dot. Charge sensing is performed with an integrated quantum point contact that detects single electron tunnel events as steps in current. We determine the relaxation time by measuring the fraction of measurements that contain spin-up tunneling events as a function of the time that the electron spins are held on the quantum dot. We observe a clear decay in this spin-up fraction versus time, and an exponential fit yields $\mathrm{T_1}\sim 2.8~\mathrm{seconds}$ at a magnetic field of $1.85~\mathrm{T}$.   

Measurement of the Spin Relaxation Lifetime (T$_{1}$) in a One-Electron Strained-Si Accumulation-Mode Quantum Dot

We report measurements of the spin-relaxation lifetime (T$_{1}$) as a function of magnetic field in a strained-Si, accumulation-mode quantum dot. An integrated quantum-point contact (QPC) charge sensor was used to detect changes in dot occupancy as a function of bias applied to a single gate electrode. The addition spectra we obtained are consistent with theoretical predictions starting at N=0. The conductance of the charge sensor was measured by applying an AC voltage across the QPC and a 3 k$\Omega$ resistor. Lifetime measurements were conducted using a three-pulse technique consisting of a load, read, and flush sequence. T$_{1}$ was measured by observing the decay of the spin bump amplitude as a function of the load pulse length. We measured decay times ranging from approximately 75 msec at 2T to 12 msec at 3T, consistent with previous reports and theoretical predictions. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.   

Undoped Si/SiGe Depletion-Mode Few-Electron Double Quantum Dots

We have successfully formed a double quantum dot in the sSi/SiGe material system without need for intentional dopants. In our design, a two-dimensional electron gas is formed in a strained silicon well by forward biasing a global gate. Lateral definition of quantum dots is established with reverse-biased gates with $\sim $40 nm critical dimensions. Low-temperature capacitance and Hall measurements confirm electrons are confined in the Si-well with mobilities $>$10$^{4}$ cm$^{2}$/V-s. Further characterization identifies practical gate bias limits for this design and will be compared to simulation. Several double dot devices have been brought into the few-electron Coulomb blockade regime as measured by through-dot transport. Honeycomb diagrams and nonlinear through-dot transport measurements are used to quantify dot capacitances and addition energies of several meV. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.   

Transport, Charge Sensing, and Quantum Control in Si/SiGe Double Quantum Dots

Si/SiGe quantum dots hold great promise as ultra-coherent qubits [1]. In comparison with the GaAs system, Si has a weaker hyperfine interaction due to the zero nuclear spin of $^{28}$Si and smaller spin-orbit coupling due to its lighter atomic weight [2]. However, the fabrication of highly controllable Si/SiGe quantum dots is complicated by valley degeneracy, the larger effective electron mass, and the difficulty of obtaining high quality samples [3]. Here we develop a robust fabrication process for depletion mode Si/SiGe quantum dots, demonstrating high quality ohmic contacts and low-leakage Pd top gates. We report DC transport measurements as well as charge sensing in single and double quantum dots. The quantum dot gate electrode pattern allows a relatively high level of control over the confinement potential, tunneling rates, and electron occupation. \\[4pt] [1] C. B. Simmons \textit{et al.}, arXiv:1010.5828v1 (2010). \\[0pt] [2] R. Hanson \textit{et al.}, Rev. Mod. Phys. \textbf{79}, 1217 (2007). \\[0pt] [3] F. Sch\"affler, Semicond. Sci. Tech. \textbf{12}, 1515 (1997).   

Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry

We present transport measurements of a tunable silicon metal- oxide-semiconductor double quantum dot device with lateral geometry. Experimentally extracted gate-to-dot capacitances show that the device is largely symmetric under the gate voltages applied. Intriguingly, these gate voltages themselves are not symmetric. Comparison with numerical simulations indicates that the applied gate voltages serve to offset an intrinsic asymmetry in the physical device. We also show a transition from a large single dot to two well isolated coupled dots, where the central gate of the device is used to controllably tune the interdot coupling. \\ This work was supported by the LDRD program at Sandia National Laboratories, a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary Lockheed-Martin Company, for the U. S. DOE NNSA under Contract No. DE-AC04-94AL85000   

The effect of donors on lateral gated quantum-devices in Si/SiGe heterostructures

Much activity has focused on the development of quantum dots in Si/SiGe because of its potentially very long decoherence times (T2). However, to fabricate well-controlled quantum dots in Si/SiGe heterostructures, one must overcome complications that do not arise in GaAs/AlGaAs heterostructures. We demonstrate that switching charge noise and donor-layer conduction can lead to instability and cross-coupling among the tunnel barriers, thus making it difficult to achieve highly stable and tunable quantum devices in a Si/SiGe heterostructure. In particular, we have used an integrated charge-sensing quantum point contact to investigate the charge motion that originates from the excess donors, and present a systematic capacitance measurement to show how the donor layer affects device function in devices with large ($\sim $100 $\mu$m$^2$) gates as well as nanometer-size ones.   

Pauli Spin Blockade and Lifetime-Enhanced Transport in a Si/SiGe double quantum dot

We analyze electron transport data through a Si/SiGe double quantum dot in terms of spin blockade and lifetime-enhanced transport (LET), which is transport through excited states that is enabled by long spin relaxation times. We present a series of low-bias voltage measurements showing the sudden appearance of a strong tail of current that we argue is an unambiguous signature of LET appearing when the bias voltage becomes greater than the singlet-triplet splitting for the (2,0) electron state. We present eight independent data sets, in both forward and reverse bias regimes, and show that excellent fits to all data sets were obtained using one consistent set of parameters. We also obtain quantitative estimates for the tunneling rates and currents in the reverse bias regime using the Lindblad formalism. [Ref: arXiv:1008.5398v1]   

Fabrication of Few-Electron Carbon Nanotube Single and Double Quantum Dots

We discuss fabrication methods for carbon nanotube quantum dot devices designed to satisfy the requirements of spin qubit applications. These requirements include low disorder for reliable access to the few-electron regime, detection of charge states, and rapid manipulation with multiple gates. Nanotube growth occurs at or near the end of the fabrication process, a scheme that has been shown previously to produce clean devices for transport studies. In our devices the nanotubes are grown over pre-patterned gates or the nanotubes are located and gates are placed on top. A new atomic layer deposition process was developed to coat the nanotubes in a high-k dielectric for effective gating and suppression of electron interactions. We find in these devices that disorder on the length scale of the quantum dot is made small enough for routine occupancy with few charges, but disorder with sufficiently short range to couple valleys remains an uncontrolled parameter that is important for qubit applications of nanotubes. We acknowledge support from NSF-MWN, IBM, and Harvard University.   

Undoped Heterostructure Materials for SiGe Quantum Devices

Quantum well heterostructures, widely used for the fabrication of quantum dots and related devices, typically make use of modulation doping. Removal of the dopants, by use of globally ``field-gated'' and/or back-gated heterostructure designs, eliminates the dominant sources of scattering, charge noise and instability in devices intended for low-temperature operation. In this talk we present recent progress in designing and fabricating undoped quantum well heterostructures in sSi/SiGe. A combination of simulation based modeling and experimental work has enabled us to successfully engineer materials for stable and quiet quantum dot operation. Specific topics to be presented include the important role of substrate and buffer layer background doping, concurrent MOS accumulation, leakage to front and back gates via barrier tunneling, and the expected range of electric fields that determine valley mixing in quantum dots. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.   

Heterostructure surface effects on Si/SiGe 2DEGs

We present the results of Hall and Shubnikov-de Haas measurements of the two-dimensional electron gas (2DEG) in Si/SiGe heterostructures at 2 K. We demonstrate that the condition of the surface has significant effects on the carrier density and mobility of electrons in the quantum well. Results from multiple samples show that the carrier density and mobility decrease with the amount of time that the samples are exposed to air. Surface treatment via a forming gas anneal or by dipping the samples in HF restores the carrier density and mobility of the degraded samples, and storing the samples in vacuum slows the rate of degradation. We believe that the reduction in carrier density of the 2DEG is a result of interface traps that form in the surface native oxide. Forming gas anneal passivates the interface traps, and HF strips the oxide. Illuminating the degraded samples at 2 K also improves the carrier density and mobility, possibly by activating electrons out of trap states. Deposition of AL2O3 on the surface using ALD caused a severe reduction in carrier density, which we believe is the result of a high trap density.   

Density and Depth of Natural Quantum Dots in Silicon MOS Structures

Electron spins in MOS structures have shown promise as qubits for quantum information processing. Typically, characteristics such as mobility, mid-gap interface states and oxide fixed charge are considered figures of merit for the Si/SiO$_{2}$ interface, however, other properties may be important. Recently, we have shown that, by biasing the gate above threshold and then reducing V$_{G}$ to 0V, we freeze electrons into natural quantum dots, where 2D electrons are confined by interface disorder. The depth of these dots is determined by the temperature and can be extracted using a Schottky-Hall-Read model. Additionally, we measure the density of confined electron states from the magnitude of the ESR signal. These measurements offer us a means to characterize the interface disorder in these MOS structures. Experiments have been performed on devices from different labs. Preliminary results from industrial quality devices fabricated at Sandia National Laboratories indicate a shallower dot depth, though a similar mobility. The shallower confinement suggests a higher quality for single-electron quantum devices.   

Real time electron counting through wavelet edge detection

We have recently demonstrated single-shot measurements of individual electron spins in a Si/SiGe quantum dot. These experiments were analyzed using a wavelet-based technique that allows detection of charging events in real time. An alternative method, based on level thresholding, is not well suited for real time detection, due to drifting background currents in the charge sensor. In contrast, the wavelet technique relies on edge detection and is hence robust against drifting currents levels. In this talk, we describe our wavelet algorithm and its applications for charge sensing. We benchmark the performance of the algorithm under realistic signal noise conditions.   

Triangulating the source of tunneling resonances in a point contact with nanometer scale sensitivity

We observe resonant tunneling in split gate point contacts defined in a double gate enhancement mode Si-MOS device structure. We determine the capacitances from the resonant feature to each of the conducting gates and the source/drain two dimensional electron gas regions. In our device, these capacitances provide information about the resonance location in three dimensions. Semi-classical electrostatic simulations of capacitance, already used to map quantum dot size and position [Stalford et al., IEEE Nanotechnology], identify a combination of location and confinement potential size that satisfy our experimental observations. The sensitivity of simulation to position and size allow us to triangulate possible locations of the resonant level with nanometer resolution. We discuss our results and how they may apply to resonant tunneling through a single donor. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

Session H28: Focus Session: Carbon Nanotubes and Related Materials: Devices I

Individual SWCNT based ionic field effect transistor

Here we report that the ionic current through a single-walled carbon nanotube (SWCNT) can be effectively gated by a perpendicular electrical field from a top gate electrode, working as ionic field effect transistor. Both our experiment and simulation confirms that the electroosmotic current (EOF) is the main component in the ionic current through the SWCNT and is responsible for the gating effect. We also studied the gating efficiency as a function of solution concentration and pH and demonstrated that the device can work effectively in the physiological relevant condition. This work opens the door to use CNT based nanofluidics for ion and molecule manipulation.   

Chemically-Driven Two Level Fluctuations in Single-Walled Carbon Nanotubes (SWCNTs) with Defects

When a SWCNT conductor contains a defect, its electronic fluctuations are sensitive indicators of the surrounding chemical environment and of the chemical state of the defect itself. We demonstrate this effect by fabricating single SWCNT devices and then engineering their defect condition through the method of electrochemical point-functionalization. By characterizing the same SWCNT before and after the introduction of a point defect, we clearly establish the defect's contribution to the overall device noise. Carboxylate defects are particularly interesting because they have a deprotonated state that is sensitive to pH, electrolyte, and electrochemical potential. Large amplitude, two level fluctuations are observed from carboxylate sites when probed under conditions near the dissociation constant pKa, and the occupation statistics can be reversibly tuned by either pH or potential. We interpret the fluctuation in terms of the controlled protonation and deprotonation of the defect site, and describe a simple electrostatic gating model that supports this conclusion.   

Gas detection mechanism for single-walled carbon nanotube networks

We study field-effect transistors fabricated with carbon nanotube (CNT) networks to determine whether the gas sensing mechanism is due to molecules adsorbed on the nanotubes, or changes at the interface between the nanotubes and the contacts. Our previous work showed that in devices made with isolated CNT, the response to nitrogen dioxide was mainly due to the contact interfaces [1]. Here, we focus on CNT networks and use SU-8 layers patterned with e-beam lithography to passivate the contact interfaces, while leaving the network exposed. We look to investigate possible differences in sensing mechanism for devices made with isolated tubes versus networks. \\[4pt] [1] J. Zhang, A. Boyd, A. Tselev, M. Paranjape, and P. Barbara, \textit{Mechanism of NO}$_{2}$\textit{ detection in carbon nanotube field effect transistor chemical sensors}, Applied Physics Letters \textbf{88,} 123112-123115 (2006)   

Effects of adsorbed gases on the conductance of individual carbon nanotubes

We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer density\footnote{Z. Wang et al, Science 327, 552 (2010)} while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.   

Anomalous Current-Voltage Characteristics in Suspended Carbon Nanotubes in Various Gas Environments

Electrically-heated suspended, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or ``kink'') in various gaseous environments. We study the effect of different gas molecules on these $I-V $characteristics while simultaneously monitoring the changes in the nanotube vibrational structure under high bias voltages using Raman spectroscopy. When the nanotube is electrically biased at the kink, the $G$ band Raman mode is observed to downshift, as is typical of electrically heated devices. However, the $G$ band frequency at the kink ($\omega _G^{kink})$ lies in the narrow range between 1575 and 1579cm$^{-1}$ for all samples measured, regardless of gas environment. The voltage at which the kink occurs depends on the type of the gas environment with the following dependence: $V_{kink}^{Ar} [Preview Abstract]

Heat Dissipation from Suspended Carbon Nanotubes to their Surrounding Gas Environment

The assistance of gas molecules to dissipate heat in 5-$\mu $m-long, electrical heated suspended carbon nanotubes (CNTs) is observed by comparing the $G $band Raman phonon temperature profiles measured in different gas environments and in vacuum. The measurement results show that 50-60{\%} of the heat generated in the CNT is carried away by its surrounding gas molecules. By analyzing the temperature profiles investigated in different gases, the thermal boundary conductance (TBC) between the gas molecules and the CNT can also be extracted. We find the TBC to be higher in carbon dioxide than in nitrogen, argon and helium.\footnote{I Kai Hsu \textit{et al.} \textit{Journal of Applied Physics }\textbf{2010,} 108, (084307).} Moreover, we report another optical method to explore the heat spreading behavior on a longer suspended CNTs in air, in which one laser is used as a heat source while another laser is used as a local temperature probe. A fin-shape thermal transport model is applied to fit the exponentially decaying temperature profiles measured away from the heat source. These results yield a heat decay length and TBC for air to be around 6.5 $\mu $m and 3$\times $10$^{5}$ W/m$^{2 }$ \textbullet K, respectively.   

Electroluminescence from a single nanotube-molecule-nanotube junction

The reliable fabrication of metallic singlewall carbon nanotube (mSWNT) electrode pairs with sub-10 nm spacing allows us to contact organic molecules (M) via dielectrophoresis and to form mSWNT-M-mSWNT junctions. For this purpose we used specific designed molecules which have an appropriate length to bridge the SWNT electrode gap, and a sufficient polarizability to allow the molecule deposition between the SWNT electrodes via DC-dielectrophoresis. The molecules comprise a fluorescent chromophore subunit. During transport measurements several mSWNT-M-mSWNT junctions showed light emission at voltages $>$ 4 V. The electroluminescence spectrum from the junction is very similar to the photoluminescence signal of the molecules on HOPG-surfaces. This result together with control experiments indicates that light is emitted from the chromophore core of the mSWNT contacted molecule [1]. If time allows I will also report on a related work about phonon-assisted electroluminescence from biased metallic single wall carbon nanotubes (SWNT), multi wall carbon nanotube (MWNT) and few layer graphene (FLG) devices [2]. \\[4pt] [1] C.W. Marquardt, S. Grunder, A. Blaszczyk, S. Dehm, F. Hennrich, H. v. L\"{o}hneysen, M. Mayor, R. Krupke, Nature Nanotechnology 2010; DOI: 10.1038/NNANO.2010.230 \\[0pt] [2] S. Essig et al., Nano Letters 10, 1589 (2010)   

Thermal Emission of Suspended Carbon Nanotube

We study the thermal emission spectra of individual suspended carbon nanotube induced by electrical heating. Semiconducting and metallic devices exhibit different spectra, based on their distinctive bandstructures. These spectra are compared with the ideal blackbody emission spectrum. In the response region of our detector, i.e. visible to near infrared, the thermal emission spectra of semiconducting devices agree well with Planck's law, while the spectra of metallic devices show an additional peak around 1.65 eV. For semiconducting devices, the temperature of the nanotube was fitted to Planck's law, and was compared with the temperature fitted from the G band downshift as well as the Stokes:anti-Stokes intensity ratio. For devices showing thermal non-equilibrium, the electron temperature agrees well with G+ downshift, but deviates from G- downshift. Finally, for metallic devices, partially polarized IR emission was observed, and possible mechanisms are discussed.   

Electric field dependence of photoluminescence from individual single-walled carbon nanotubes

Using suspended single-walled carbon nanotubes, we investigate electric field effects on photoluminescence. Trenches are fabricated on SiO$_{2}$/Si substrates, and Pt is deposited for electrical contacts. Carbon nanotubes are grown by patterned chemical vapor deposition. These devices operate as back-gate field effect transistors, allowing application of electric fields on as-grown ultraclean nanotubes. Individual suspended carbon nanotubes are identified by taking photoluminescence images using a home-built laser-scanning confocal microscope. After determining the chirality by photoluminescence excitation spectra, we measure gate voltage dependence of photoluminescence. We observe quenching of photoluminescence intensity and shifts of emission wavelength as gate voltages are applied.   

Simultaneous Rayleigh and Raman spectroscopy on suspended single-walled carbon nanotubes under electrostatic gating

The optical properties of single-walled carbon nanotubes (SWNTs) under electrostatic gating are of great interest for fundamental understanding of one-dimensional physics and for their application as optoelectronics devices. Here, we report how the electronic transitions are modified by gating conditions through direct measurements of Rayleigh (elastic) light scattering from individual suspended SWNTs [1]. With increasing gate voltage, we observed both a broadening and shift of the excitonic resonances in the Rayleigh scattering spectra. The influence of carrier doping on the optical resonances and, as gauged through simultaneous Raman measurements, on vibrational transitions will be discussed.\\[4pt] [1] M. Y. Sfeir et al., Science 306, 1540 (2004).   

Photoconductivity measurements of single-walled carbon nanotube field effect transistors

Photoconductivity measurements are performed on carbon nanotube field effect transistors. Carbon nanotubes are grown on SiO$_{2}$/Si substrate by patterned chemical vapor deposition using ethanol as carbon source. Next, electron beam lithography, metal deposition, and liftoff processes are performed to form source and drain electrodes. The Si substrate is used as a back-gate in these devices. Wavelength tunable Ti:sapphire laser is focused onto the sample with an objective lens, and the laser spot is scanned with a steering mirror. A lock-in amplifier is used to detect the photoconductivity signal of carbon nanotube field effect transistors.   

Role of defects in optical phonon decay, softening and 1/f noise resonance in carbon nanotubes

Scattering and relaxation of optical phonons are especially important processes in carbon nanotubes. Strong phonon softening near the Dirac point in metallic nanotubes occurs by coupling of carrier excitation to optical phonon transitions. Current saturation and negative differential conductance in the high bias regime in nanotube devices are attributed optical phonon absorption and emission. Cooling of hot carriers occurs mostly via optical phonons which eventually decay anharmonically into acoustic phonons. Whether intentional or unavoidable, defects will strongly influence these fundamentally important processes. In this talk, how defects affect optical phonon scattering will be discussed. In particular, defect-dependent optical phonon lifetime and resonant 1/f noise associated with phonon softening via the Kohn anomaly will be discussed.   

Micro-scale ``air-gap'' circuitry with conducting carbon nanotube-copper composite

The ability of water-assisted CVD to produce aligned close-packed single wall carbon nanotubes(CNT) with superior thermal and mechanical properties make them ideal materials for use in microelectronics. However, their poor electrical conductivity has been a major obstacle in realizing this. To overcome this, we report the synthesis of conducting CNT-copper composite (conductivity $\sigma $=10$^{5}$ Scm$^{-1})$ through a novel organic phase electrodeposition. The conductivity enhancement (10$^{3}$ times over CNT) is due to the high, uniform filling of Cu in the aligned CNT matrix. Micro-scale, three-dimensional lithographic engineering of CNT-Cu, involving fully suspended CNT-Cu beams, is achieved for microelectronic applications. Multi-tier CNT-Cu circuits are also fabricated, with the constituent lines separated by air (replaceable with vacuum). This ``vacuum-separation'' exists in the horizontal and vertical directions providing unique multi-tier ``air-gap'' circuits. This realization of dielectric-less, air-gap circuits with CNT-Cu is thought to be a breakthrough for developing faster and efficient microelectronic devices.   

Session H29: Focus Session: Quantum Information for Quantum Foundations - Axiomatics and Toy Models

Toward a conceptual foundation of Quantum Information Processing

Quantum Information Science has brought to light an enormous amount of new protocols showing that the structure of quantum theory dramatically impacts the way in which information can be processed. It also made clear that the rules of information processing are dictated by physics and that different physical theories entail different models of information processing. Quantum Information poses an exciting challenge to foundational research: the challenge is to reduce the multiplicity of quantum protocols to a small number of basic physical principles and to answer questions like ``What are the physical roots of the power of quantum information?'' A satisfactory answer to these questions calls for the solution of a long-standing problem: deriving quantum theory from physical principles, as opposed to the abstract mathematical principles of the Hilbert space formulation. In this talk I will show that quantum theory can be derived from few principles about information processing. The central principle of the derivation will be the purification principle, stating that ignorance about a part (subsystem) is always compatible with maximal knowledge of the whole (compound system). A large number of quantum information features, including e.g. teleportation and no-cloning, are direct consequences of the purification principle, which appears a strong candidate for the conceptual foundation of Quantum Information Processing. Moreover, the derivation of quantum theory from purely informational principles provides a rigorous justification of the diffuse claim that quantum theory is ultimately a theory of information.   

Physics as Information

The experience from Quantum Information has lead theorists to look at Quantum Theory and the whole Physics from a different angle. A new information-theoretic paradigm is emerging, long time ago prophesied by John Archivald Wheeler with his popular coinage ``It from bit.'' Theoretical groups are now addressing the problem of deriving Quantum Theory from informational principles, and similar lines are investigated e.g. in new approaches to Quantum Gravity. In my talk I will review some recent advances on these lines. The general idea synthesizing the new paradigm is that there is only Quantum Theory (without quantization rules), and the whole Physics---including space-time and relativity--is emergent from the quantum-information processing. And, since Quantum Theory itself is made with purely informational principles, the whole Physics must be reformulated in information-theoretical terms. The review is divided into four parts: a) Very short review of the informational axiomatization of Quantum Theory; b) How space-time and relativistic covariance emerge from the quantum computation; c) What is the information-theoretical meaning of inertial mass and Planck constant, and how the Dirac field emerges; d) Observable consequences of the new theory. I will then conclude with some possible future research lines.   

A derivation of quantum theory from physical requirements

Quantum theory is usually formulated by postulating the mathematical structure and representation of states, transformations, and measurements. The general physical consequences that follow (like violation of Bell-type inequalities, the possibility of performing state tomography with local measurements, or factorization of integers in polynomial time) come as theorems which use the postulates as premises. In this work, this procedure is reversed: we impose five simple physical requirements, and this suffices to single out quantum theory and derive its mathematical formalism uniquely. This is more similar to the usual formulation of special relativity, where two simple physical requirements ---the principles of relativity and light speed invariance--- are used to derive the mathematical structure of Minkowski space-time and its transformations.   

Homogeneous Self-Dual Cones and the Structure of Quantum Theory

This talk reviews recent and on-going work with Howard Barnum on the origins of the Jordan-algebraic structure of finite-dimensional quantum theory. I begin by surveying various principles --- e.g., that every state of a bipartite system arise as the marginal of a ``steering" bipartite state -- - that force the cone of (un-normalized) states of a finite-dimensional probabilistic system to be homogenous and {\em weakly} self-dual, that is, isomorphic to its dual cone. Where this weak self-duality can be implemented by an inner product, the cone is {\em strongly} self dual. In this case, classical results of Koecher and Vinberg show that it is isomorphic to the cone of squares in a formally real Jordan algebra. If this is the case, then (using a theorem of H. Hanche-Olsen) one can show that the only locally-tomographic theory containing at least one qubit is finite-dimensional Complex QM. I conclude with a brief discussion of how one might motivate strong self-duality.   

Quaternions and the Quantum

Birkhoff and von Neumann pointed out that quantum probability calculi could be formulated over rings admitting involutory anti-automorphisms [1]. We discuss a model for generalized quantum measurements and quantum states based on quaternionic matrix algebras. We show that the usual Born rule for calculating probabilities for outcomes of quantum measurements can be carried over into quaternionic quantum theories within a Jordan-algebraic framework. We exploit a group isomorphism between Sp(1) and SU(2) to show that single-system unitary dynamics and generalized measurements in a quaternionic quantum theory can be simulated by corresponding processes in usual quantum mechanics. We resurvey the divide between quaternionic and complex quantum theories given this quadit-qudit correspondence. Reference: [1] G. Birkhoff and J. Von Neumann, ``The logic of quantum mechanics'', Ann. Math., 37, 823-843 (1936).   

Quantum theory cannot be extended

Predictions made by quantum theory are generally not deterministic: the theory tells us only how to calculate the probabilities with which measurement outcomes occur. This indeterminism is one of the key differences from classical mechanics and one can ask whether this is the best any theory can offer, or whether observable quantities could be better predicted by some higher theory. In a famous work, Bell considered extensions of quantum theory in the form of local hidden variables and showed that these cannot determine the outcomes of measurements on maximally entangled particles. Here, we go beyond the case of such classical extensions and ask whether any improved predictions can be achieved by any extension of quantum theory. We answer this question in the negative. More precisely, under the assumption that measurement settings can be chosen freely, there cannot exist any extension of quantum theory that provides us with any additional information about the outcomes of future measurements.   

Eliminating remnants of classical mechanics and the birth of the Schr\"odinger equation

We show that the Schr\"odinger equation emerges from the Hamilton-Jacobi equation for a specific choice of the amplitude $R$ of a wave $\psi\equiv R\exp[I S/\hbar]$ where $S$ is the classical action. This choice eliminates in the wave equation for $\psi$ all remnants of classical mechanics associated with $S$ but at the same time builds via the wave equation for $R$ a bridge to classical mechanics and to the de Broglie pilot wave theory.   

Modal Quantum Theory

We present a class of toy model theories similar in structure to ordinary quantum mechanics. Some of these models are based on finite fields instead of complex amplitudes. The interpretation of such theories involves only the ``modal'' concepts of possibility and necessity rather than quantitative probability measures. Despite its very simple structure, our toy model nevertheless includes many of the key phenomena of actual quantum systems: interference, complementarity, entanglement, nonlocality, and the impossibility of cloning. These results are detailed in arXiv:1010.2929 and arXiv:1010.5452.   

Time-asymmetry and causal structure

We consider devices with two inputs and two outputs, Alice and Bob each having access to one input and one output. To such a device we associate time-reverses by exchanging the roles of the inputs and the outputs. We find that there are devices which admit a local hidden variable representation, but for which time- reverses enable perfect signaling between Alice and Bob. That is, a ``perfect channel in one time direction'' becomes a ``non-channel in the other direction.'' Also, for PR boxes time-reverses enable signaling between Alice and Bob, but never as a perfect channel. This result has several consequences. Firstly, it establishes that the arrow of time can be read from signaling structure: signaling means backward in time. It undermines the representation of causal structures as partial orders or similar `time-symmetric structures', as is often assumed in search of a theory of quantum gravity. They also provide new insights into the structure of the polytope of generalized probabilistic correlations, hence on theories more general than quantum theory. Finally, it contributes to the growing area of research into quantum information processing in relativistic spacetimes. Ref: arXiv:1010.4572   

Topos formulation of History Quantum Theory

In this talk I will describe a topos formulation of consistent histories obtained using the topos reformulation of standard quantum mechanics put forward by Doering and Isham. Such a reformulation leads to a novel type of logic with which to represent propositions. In the first part of the talk I will introduce the topos reformulation of quantum mechanics. I will then explain how such a reformulation can be extended so as to include temporally-ordered collection of propositions as opposed to single time propositions. Finally I will show how such an extension will lead to the possibility of assigning truth values to temporal propositions.   

Causal Tapestries

Causal sets provide one of many approaches to the problem of quantum gravity. Causal tapestries generalize the concept of a causal set, extending the range of putative dynamics from sequential growth to include iterative and non deterministic methods, and the range of embedding manifolds to include those with curvature. Like causal sets, causal tapestries are manifestly Lorentz invariant in spite of possessing a form of ``transient now''. It is shown that the order relations of the local causal structures must possess an order theoretic (Dushnik {\&} Miller) dimension not exceeding the topological dimension of the embedding manifold and the finite free dimension is bounded by the number of elementary processes generating the causal relations.   

The quantal algebra and abstract equations of motion

Classical and quantum mechanics common characteristics reveal core physics features that are hidden by the details related to the realizations of those theories in phase and Hilbert space respectively. The quantal algebra combines classical and quantum mechanics into an abstract structurally unified structure. It is based on two observations which can be made about classical and quantum mechanics. The first observation is that classical and quantum mechanics use two products: one symmetric and one anti-symmetric. The second observation is that classical and quantum mechanics obey the so-called composability principle: any two physical systems can interact with each other. The local structure of spacetime is contained in the quantal algebra without having been postulated. We will generalize classical and quantum mechanics equations of motion to abstract equations of motion in which the anti-symmetric product of the quantal algebra plays a central role. We will express the defining identities of the quantal algebra in terms of the abstract derivation. In this form it is easy to see that the first defining identity (the Jacobi identity) captures the essence of the Bianchi identity in general relativity which is one set of gravitational field equations for the curvature tensor.   

Session H30: Graphene: Synthesis and Characterization

Electrical breakdown of graphene and few-layer graphene structures

The electrical breakdown of graphene and few-layer graphene (FLG) structures are investigated. To better understand the dynamics of these nano-scale thermal effects, we investigate graphene and FLG nanowires of various dimensions and find that significant joule heating occurs inducing the structures to evolve. A distinct change in the behavior during electrical stressing indicates that different mechanisms occur at the various stages of evolution. The results are compared to detailed thermal modeling of our structures and could have implications on the development of high current carrying nanoscale graphene devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.   

Designing all-graphene nano-junctions by covalent functionalization

We study the effect of covalent edge functionalization, with organic functional groups, on the opto-electronic properties of graphene nano-flakes and nano-junctions. We work within the frame of Hartree-Fock-based semi-empirical methods [1,2]. Our study shows that functionalization can be designed to tune electron affinities and ionization potentials of graphene flakes. The stability of the proposed mechanism is analyzed with respect to the functional groups, the functionalization rate and the width of graphene nanostructures. We show that this effect can be exploited to realize type-II all-graphene nano-junctions. Different frontier orbital alignments can be engineered varying the functionalization, leading to specific optical properties: The conditions to obtain charge transfer excitations are investigated.\\[4pt] [1] Dewar, et al., J. Am. Chem. Soc. 107, 3902 (1985)\\[0pt] [2] Ridley and Zerner, Theoret. Chim. Acta 32, 111 (1973)   

Schottky diode via dielectrophoretic assembly of reduced graphene oxide sheets between dissimilar metal contacts

Reduced graphene oxide (RGO) has attracted significant attention due to its ability to produced graphene nanostructures in large quantities. It has been also considered as a promising building block for future generation of electronic and optoelectronic devices. Here we demonstrate fabrication of RGO Schottky diodes with high yield via dielectrophoretic (DEP) assembly between two dissimilar metal contacts. Titanium (Ti) was used to make a Schottky contact, while palladium (Pd) was used to make an Ohmic contact. From the current - voltage characteristics, we obtain rectifying behavior with a rectification ratio of up to 600. The ideality factor was high (4.9) due to the presence of a large number of defects. The forward biased threshold voltage was 1 V while the reverse bias breakdown voltage was - 3.1 V which improved further upon mild annealing at 200\r{ } C and can be attributed to an increase of work function of RGO due to annealing.   

Graphitic carbon molecular beam epitaxy on dielectric substrates

We report on growth of thin large area graphitic layers on dielectric substrate materials by means of molecular beam epitaxy (MBE) under UHV conditions. This solid source MBE technique offers highly controllable conditions without the need of gas precursors or metal surfaces. Our initial experiments on dielectric substrates such as mica, SiO$_2$ and BN clearly demonstrates the potential of this new growth technique. NEXAFS studies show that the binding mechanism in our sheets is dominated by sp$^2$ bonds and the Raman spectra confirm their graphitic nature. We will also describe STM measurements of the topography and local electronic structure of these films.   

Stability of epitaxial graphene on pristine Si(111)

Incorporation of carbon nanostructures with silicon-based nanoelectronics will involve the direct integration of graphene with silicon chips, but so far graphene has not been grown on pristine silicon surfaces. Because usual synthesis routes would likely lead to the formation of silicon carbide, we calculate the binding energy of graphene transferred onto the Si(111) surface and also analyze its stability at various temperatures. Our calculations based on (commensurate) moir\'e superstructures with periodic boundary conditions show a strong graphene--substrate binding, about 1.5 eV/carbon atom, over a wide range of in-plane orientations of the graphene layer. Molecular dynamics simulations based on bond-order and reactive force field interatomic potentials suggest that the graphene binds to the substrate where carbon is rehybridized sp$^{3}$, and that this rehybridized graphene structure does not lead to the decomposition of graphene into silicon carbide even at temperatures as high as 80\% of the substrate melting temperature.   

Synthesis and characterization of graphene patterned with Fe$_{3}$O$_{4}$ nanoparticles

Graphene has emerged as a very exciting material with its outstanding physical, chemical, and mechanical properties. Due to the presence of excess free electrons on a graphene surface, the possibility of graphene-mediated long-range interactions between magnetic nanoparticles would open up new avenues of research and device development. Our studies aimed to deposit $\sim $9 nm Fe$_{3}$O$_{4}$ NPs on graphene layers to understand the role of the metallic interface in mediating the magnetic interactions between the particles. We successfully grew the high-quality graphene on Ni films using CVD and used the Langmuir-Blodgett technique to pattern different layers of the Fe$_{3}$O$_{4}$ nanoparticles on the graphene sheets. The samples were well structurally characterized by XRD, TEM, AFM, and Raman spectroscopy. Interestingly we have observed strong variation in the magnetic properties such as magnetic anisotropy of the NPs pattered graphene samples when compared to just the NPs. These results point to the important role of the metallic interface in mediating the magnetic interactions between the Fe$_{3}$O$_{4}$ nanoparticles.   

Scaled-Up Synthesis and Characterization of High-Purity Graphene

Graphene, a two-dimensional, single-atom sheet of carbon atoms, discovered in 2004, has emerged as a new class of novel nano-scale material due to its unique chemical and physical properties, and potential applications in a wide range of civilian and military technologies. However, a major challenge in its technological application is a lack of chemical/physical method(s) to produce/synthesize high-purity graphene in viable quantity. Another challenge in the technological application of graphene is a lack of detailed understanding of its structure-property relationship. In order to address these issues, we have developed a chemical exfoliation method that yields high-purity graphene in bulk quantity. The method is scalable to produce large quantities of high purity graphene. In this paper, we present the results of our synthetic approach and structure-property characterization of graphene.   

Two-Dimensional Molecular Crystals of Phosphonic Acids on Graphene

The synthesis and characterization of two-dimensional (2D) molecular crystals comprised of long and linear phosphonic acids atop graphene is reported. Using scanning probe microscopy in combination with first-principles calculations, we show that these true 2D crystals are oriented along graphene armchair direction only, thereby enabling an easy determination of graphene flake orientation. We have also compared the doping level of graphene flakes via Raman spectroscopy. The presence of the molecular crystal atop graphene induces a well-defined shift in the Fermi level, corresponding to hole doping, which is in agreement with our ab initio calculations.   

Carbon chains grown perpendicularly on graphene: Nanobrush

We predict a peculiar growth process, where carbon adatoms adsorbed to graphene readily diffuse above room temperature and form linear chains. These chains grow longer on graphene through insertion of carbon atoms one at a time from the bottom end. Through this growth process two allotropic forms of carbon, namely graphene and polyyne are combined to make several novel nanostructures. Brush like graphene sheets with protruding polyynes can achieve chemical activity and attain new functionalities.   

Size Control of Nanographene Directly Grown On Glass

We directly deposited very thin carbon film was on glass by thermal chemical deposition at 750 $^{\circ}$C without using any catalyst. The directly deposited carbon film consists of nanographene with an in-plane crystal size of $\sim $ 15 nm. The in-plane crystal size of nanographene increased up to 23 nm by annealing of the post-deposited nickel film on the nanographene film at 300 $^{\circ}$C to 500 $^{\circ}$C. This study suggests that nanographene can be directly deposited on glass at low temperature and that the crystalline size can be controlled.   

Session H31: Focus Session: Materials at High Pressure I: Molecular and Simple Materials

Structural investigation and shock Hugoniot calculations of methane under high temperatures and pressures

The behavior of methane under pressures and temperatures spanning 0.02-7.75 Mbar and 300-30,000 K was studied using density functional molecular dynamics. The structural properties of fluid and crystalline methane were analyzed with simulations at various (P,T) conditions. These simulations were also used to calculate the shock Hugoniot curves of methane for a range of initial densities between 0.4-0.57 g/cc. These curves allow us to make predictions of state and phase that correspond to future methane shock experiments.   

Proton Exchange Reactions in Deuterium Water Mixtures

Binary mixtures of water and hydrogen under pressure are of interest both as fundamental systems in physics and chemistry and due to their applicability to fuel cells. Their behaviors at extreme pressures and temperatures are also of significance to understanding the interaction of chemical species in the interiors of giant gas planets and other planetary objects. In this talk, we will present high-pressure Raman data of deuterium water mixtures, which provides both kinetic information regarding the proton exchange reactions and the structure of deuterium in the mixtures.   

Zero-Temperature Structures of Atomic Metallic Hydrogen

Since the first prediction of an atomic metallic phase of hydrogen by Wigner and Huntington over 75 years ago, there have been many theoretical efforts aimed at determining the crystal structures of the zero-temperature phases. We present results from ab initio random structure searching with density functional theory performed to determine the ground state structures from $500$ GPa to $5$ TPa. We estimate that molecular hydrogen dissociates into a monatomic body-centered tetragonal structure near $500$ GPa ($r_s = 1.225$), which then remains stable to $2.5$ TPa ($r_s = 0.969$). At higher pressures, hydrogen stabilizes in an $...ABCABC...$ planar structure that is remarkably similar to the ground state of lithium, which compresses to the face-centered cubic lattice beyond $5$ TPa ($r_s < 0.86$). Our results provide a complete ab initio description of the atomic metallic crystal structures of hydrogen, resolving one of the most fundamental and long outstanding issues concerning the structures of the elements.   

Phase Diagram of Carbon Dioxide at High Pressure and Temperatures: Implications to the Deep Carbon Cycle

Carbon dioxide is an important terrestrial volatile often considered to exist in the deep interior of the Earth. The phase diagram of carbon dioxide is critical to validate such hypothesis. In this study, we will present the phase diagram of carbon dioxide including the most recent finding of coesite-like carbon dioxide, a missing analog to SiO$_{2}$, address several controversies in terms of phase metastabilities and thermal path dependent transitions, and discuss about the implication to the deep carbon cycle.   

Structural and optical properties of liquid CO$_2$ up to 1 terapascal

The properties of liquid CO$_2$ have been studied through first-principles molecular dynamics simulations in the pressure-temperature range of 0-1 TPa and 200-100,000 K. The resulting equation of state data is used to predict shock Hugoniots for several initial conditions. Comparison with available experimental data up to 70 GPa is excellent. We find a gradual phase transition characterized by the destabilization of CO$_2$ molecules and the formation of other molecular compounds. The liquid phase diagram is divided into several regimes based on a thorough analysis on changes in bonding, structural properties, and chemical composition. Calculations of optical properties such as conductivity and reflectivity will also be discussed.   

Density Functional Theory (DFT) simulations of CO2 under shock compression and design of liquid CO2 experiments on Z

Quantitative knowledge of the thermo-physical properties of CO2 at high pressure is required to confidently model the structure of gas-giants like Neptune and Uranus and the deep carbon cycle of the earth. DFT based molecular dynamics has been established as a method capable of yielding high fidelity results for many materials, including shocked gases, at high pressure and temperature. We predict the principal Hugoniot for liquid CO2 up to 500GPa. Our simulations also show that the plateau in shock pressure identified by Nellis and co-workers [1] is the result of dissociation. At low temperatures we validate the DFT results by comparing with diffusion Monte Carlo calculations. This allows for a more accurate determination of the initial conditions for the shock experiments. We also describe the design of upcoming flyer-plate experiments on the Z-machine aimed at providing high-precision shock compression data for CO2 between 150 and 600 GPa. [1] W. Nellis, et. al. , J. Chem. Phys. {\bf 95}, 5268 (1991).   

Novel phases of simple substances at megabar pressures

Under megabar pressures solids can be strongly compressed: volume of solid hydrogen decreases in $\sim $20 times, even diamond is 1.5 fold compressed at achievable pressures of $>$300 GPa. This dramatically changes interatomic distances in materials eventually leading to metallization in a number of presenting substances. Metals under compression supposedly remain in metallic state. But at high densities the core electrons come in to play and the electronic structure significantly departs from the simple metal as it was demonstrated for lithium. We present an ultimate case: sodium - simple metal - becomes transparent at pressures of $\approx $200 GPa transforming into ionic- electride-like state. We will present also our recent studies on nitrogen and nitrogen-rich materials: ammonia, azides and others, and progress on studies at pressures $\sim $400 GPa.   

Diamond as a high pressure gauge up to 2.7 MBar

Diamond anvil cell (DAC) technique has become a very important method of probing materials behaviour under pressure in various fields of research ranging from physics to biology and geoscienses. Optical methods of pressure determining in DAC experiments are based on fluorescent markers or calibrated pressure dependence of the Raman shift of suitable materials. Diamond has been since long recognised as a good pressure marker in experiments conducted in a diamond anvil cell. It is stable at ultra-high pressures that allows one to use the pressure dependence of the Raman frequency of the LTO optical phonon of diamond as a pressure gauge. A pressure gauge is a key issue of any high pressure experiment in a diamond anvil cell. Here we present a method of \textit{in situ} synthesis of microcrystals of diamond that can be further used as a pressure standard in course of the same DAC experiment. Calibration curve of the Raman shift \textit{vs} pressure is extended up to 270 GPa and experimental results are compared with those of \textit{ab initio} calculations.   

High-pressure and high temperature deformation studies of polycrystalline diamond

With Vicker's hardness 120 GPa, shear modulus 535 GPa, diamond is the hardest material known to mankind. However, because diamond is difficult to deform, little is known with regard to its constitutive properties such as yield strength. In this work, we present results obtained at NSLS using deformation-DIA on polycrystalline diamond at different P-T conditions. As expected, even at total strains up to 20{\%}, we did not observe the yield point of diamond at room temperature and a confining pressure of 4 GPa. However, for deformation at 1000 and 1200 $^{\circ}$C, we observed an plastic flow of diamond at total strains of 10{\%} and 5{\%}, respectively, indicating that diamond weakens rapidly when temperature is over 1000 $^{\circ}$C. We further derived the micro stress of diamond from peak width analysis, and found that the micro and macro stresses show similar variations with total strain at both room temperature and 1000 $^{\circ}$C. However, at 1200 $^{\circ}$C, the micro stress remains constant in entire deformation, indicating that stress is uniformly distributed within diamond particles at 1200 $^{\circ}$C. We also carried out SEM studies on the recovered samples to investigate the miscrostructures, and find that the grain size of diamond decreases substantially during the deformation, from the initial microns to sub-microns for the room temperature deformation, however, almost doesn't change for the 1200 $^{\circ}$C.   

New primary pressure calibrants for high pressure and temperature scale: SiC-3C and cBN are possible candidates

Since the invention of a diamond-anvil cell, various high-pressure scales for in situ pressure measurements have been realized. Ruby-based pressure scale (Mao et al., 1986) is the best known and high-pressure scientific community has been using it for over two decades. However, it has limited use at elevated temperatures, due to the weakening and broadening of the ruby fluorescence line. The recent developments in the field of high temperature, high pressure physics and geophysics require some alternative pressure scale, capable of measuring pressures at temperatures up to 3000 K. Cubic boron nitride (cBN) was recently proposed as the possible pressure calibrant. It has been suggested that the simultaneous use of x-ray diffraction to measure density and Brillouin spectroscopy to obtain elastic properties of the crystal can be used to construct the pressure scale independent of any other pressure standards. However, the acoustic velocities of cBN are very close to those of diamond and, therefore, are hard to resolve in experiment in diamond-anvil cell. Another possible primary pressure calibrant is cubic silicon carbide (SiC-3C). We performed single crystal x-ray diffraction and Brillouin spectroscopy up to 1 Mbar in pressure at room temperature in the diamond-anvil cell and show that cBN and SiC-3C, indeed, can be used in constructing reliable and accurate high-pressure, high-temperature scale.   

Formation and superconductivity of hydrides under pressure

Hydrogen is the lightest and smallest element in the periodic table. Despite its simplest electronic structure, enormous complexity can arise when hydrogen participates in the formation of solids. Pressure as a controllable parameter can provide an excellent platform to investigate novel physics of hydrides because it can induce structural transformation and even changes in stoichiometry accompanied with phenomena such as metallization and superconductivity. In this presentation, we will briefly overview contemporary high-pressure research on hydrides and show our most recent results on predicting crystal structures of metal hydrides under pressure using ab initio random structure searching. Our findings allow for a better understanding of pressure-induced metallization/superconductivity in hydrides which can help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials.   

Mechanical properties of icosahedral boron carbide explained from first principles

An exhaustive DFT study of the structural defects of icosahedral B$_{4}$C and of their behavior under high pressure has been performed. Among the possible atomic structures, the lowest value of the formation energy has been found for the \textit{polar} model B$_{4}$C$^{P}$, which consists of one distorted icosahedron and of one CBC chain. This result, together with the inspection of the vibrational and NMR spectra, has proved that B$_{4}$C$^{P}$ is the proper structural model for B$_{4}$C.[1,2] Consequently, B$_{4}$C$^{P}$ has been used as a matrix to isolate the defects. The native defects have been identified and shown to be energetically stable at high pressure. Most vacancy locations in B$_{4}$C$^{P}$ are found to be energetically unstable and only a boron vacancy in the CBC chain is stable. A cluster of this vacancy is shown to induce a dynamical instability of the icosahedra when the pressure is increased. The dynamical failure of shocked B$_{4}$C [3] is attributed to the increase in the concentration of these unstable vacancies under plastic deformation. 1. R. Lazzari, N.Vast, J.M. Besson, S. Baroni and A. Dal Corso, Phys. Rev. Lett. 83 (1999) 3230. 2. F. Mauri, N. Vast and C.J. Pickard, Phys. Rev. Lett. 87 (2001) 085506. 3. T. Vogler, W. Reinhart and L. Chhabildas, J. Appl. Phys. 95 (2004) 4173.   

Session H32: Focus Session: Photonic Crystals, Metamaterials and Other Optical Systems

Disordered Hyperuniform Photonic Band Gap Materials

Until recently, the only materials known to have complete photonic band gaps were photonic crystals, periodic structures, and it was generally assumed that long-range periodic order was instrumental in the band gap formation. We have shown that there exists a more general class of systems, called hyperuniform photonic structures, which exhibit large and complete photonic band gaps. This classification includes not only crystalline structures, but also non-crystalline materials, ranging from quasicrystals with crystallographically-forbidden rotational symmetries to isotropic, translationally-disordered structures. Remarkably, we find that the photonic band gaps in hyperuniform disordered structures are not only comparable to those found in photonic crystals, but also display a high degree of isotropy. These new materials possess unique photonic and physical properties that provide important advantages for applications. Our results show that hyperuniform disordered structures enable the realization of optical cavities with ultimate isotropic confinement of the electromagnetic radiation, lossless waveguides with arbitrary bending angles and flexible optical insulator platforms.   

Experimental observation of photonic bandgaps in two dimensional hyperuniform disordered materials

We report the first experimental demonstration of photonic bandgaps (PBGs) in 2D hyperuniform disordered materials and show that is possible to obtain isotropic, disordered, photonic materials of arbitrary size with complete PBGs. There are only limited numbers of allowed rotational symmetries in periodic or quasiperiodic structures. Periodicity and Bragg scattering lead to different stop gap center frequencies in different directions, since periodicities change in different directions. Hyperuniformity together with short range geometric order and uniform local topology are enough to give raise to an isotropic PBG. Hyperuniform systems have a variance in mass or particle number which varies with distance, r, from an arbitrary point less rapidly than the d dimensional volume. We present a new class of photonic materials posessing PBGs that have a number of advantages, including: isotropy, robustness against disorder (they are already disordered), flexibility (can fit arbitrary regions of space in which one may have trouble putting a periodic system), and possibly lower minimum dielectric contrast.   

Photonic density of states of 2D quasicrystals from level set equations and decorated quasiperiodic tiling patterns

The TE and TM photonic band gaps (PBG) of 8mm, 10mm, and 12mm rotationally symmetric 2D quasicrystals (QCs) were numerically investigated for families of morphologies generated from level set equations and from quasiperiodic tiling patterns decorated with cylindrical rods, respectively. We discovered a 12mm QC with 56.5{\%} TE PBG, which is the largest reported TE PBG for aperiodic crystals to date. Further, we find that the TM PBG of 2D QC is highly related to the shape of the structural features comprised by the QC morphology. Two physical models are presented to explain the decrease of the center frequency of PBG as dielectric filling ratio increases.   

Measurement of photonic band diagram in non-crystalline photonic band gap (PBG) materials

Non-crystalline PBG materials have received increasing attention recently and sizeable PBGs have been reported in quasi-crystalline structures or even in disordered structures. Band calculations for periodic structures produce accurate dispersion relations in them and refraction properties at their surfaces. However, band calculations for non-periodic structures employ large super-cells of N $>$100 building blocks, and provide little useful information other than the PBG frequency and width. Since band is folded into N bands, within the first Brillouin zone of the supper-cell. Using stereolithography, we construct various quasi-crystalline or disordered PBG materials and perform transmission measurements. The dispersion relations of EM wave (band diagrams) are reconstructed from the measured phase data. Our experiments not only verify the existence of sizeable PBGs in these structures, but also provide detailed information of the effective band diagrams, dispersion relation, group velocity vector, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study presents a powerful tool to investigate photonic properties of non-crystalline structures and provides important dispersion information, which is otherwise impossible to obtain.   

MWIR tunable polarmetric scatterometery applied to a fishnet structure

Understanding how light is scattered by a material, such as a metamaterial, which is engineered to have specific optical properties is necessary for a better understanding of the design parameters and for refining designs. Because of their high irradiance and small spot size, lasers are an ideal light source for these scatter measurements. However, lasers are highly monochromatic and it can be very difficult to manufacture metamaterials to resonate at such specific wavelengths. By modifying a Schmitt Measurement System's Complete Angle Scatter Instrument (CASI) with the addition of 6 external cavity Quantum Cascade Lasers by Daylight Solutions we were able to have a tunable laser light source from 4.35 to 9.71 $\mu $m with a small exclusion from 6.54 to 7.40 $\mu $m. The CASI system was further modified with the addition of a dual rotating retarder which allows the full Mueller matrix to be calculated for both specular scatter and off specular scatter. This makes the system unique to commercially available systems like Woollam's IR-VASE which can only measure the Mueller matrix elements for the specular reflection. This unique system was used to measure a fishnet structure at both resonate and off resonate frequencies. The fishnet sample was also measured using an IR-VASE system to compare specular results.   

Electromagnetism in multicoaxial negative-index metamaterial cables

By using an elegant Green [or response] function theory, which does not require matching of the messy boundary conditions, we investigate the surface plasmon excitations in the multicoaxial cylindrical cables made up of negative-index metamaterials. The multicoaxial cables with {\em dispersive} metamaterial components exhibit rather richer (and complex) plasmon spectrum with each interface supporting two modes: one TM and the other TE for (the integer order of the Bessel function) $m \ne 0$. The cables with {\em nondispersive} metamaterial components bear a different tale: they do not support simultaneously both TM and TE modes over the whole range of propagation vector. The computed local and total density of states enable us to substantiate spatial positions of the modes in the spectrum. Such quasi-one dimensional systems as studied here should prove to be the milestones of the emerging optoelectronics and telecommunications systems.   

Universal shift of the Brewster angle in stratified random media

We study theoretically the propagation and the Anderson localization of p-polarized electromagnetic waves incident obliquely on randomly stratified dielectric media with weak uncorrelated Gaussian disorder. Using the invariant imbedding method, we calculate the localization length and the disorder-averaged transmittance in a numerically precise manner. We find that the localization length takes an extremely large maximum value at some critical incident angle, which we call the generalized Brewster angle. The disorder-averaged transmittance also takes a maximum very close to one at the same incident angle. Even in the presence of an arbitrarily weak disorder, the generalized Brewster angle is found to be substantially larger than the ordinary Brewster angle in uniform media. It is a rapidly increasing function of the average dielectric permittivity and approaches 90 degrees when the average relative dielectric permittivity is slightly larger than two. We find that the dependence of the generalized Brewster angle on the average dielectric permittivity is universal in the sense that it is independent of the strength of disorder and the wave frequency. We also make a surprising observation that when the p wave is incident from an optically denser region, the localization length and the average transmittance become larger for stronger disorder in a wide range of incident angle.   

Unidirectional suppresion of Bragg reflection in grated PT-symmetric media

We study the scattering properties of light through optical fibers with grating that involves gain/loss modulation that respect Parity-Time (PT) symmetry. We derive analytical expressions for transmission and reflection coefficients both in the presence and absence of Kerr non-linearity. At the spontaneous PT-symmetric point we have found that Bragg reflection is suppressed once the light is injected from the left, while it is amplified (with respect to the passive medium) if the fiber is illuminated from the right. Our results are robust for a large interval of the detuning parameter away from the Bragg wavelength.   

Application of Iterative Time-Reversal for Electromagnetic Wave Focusing in a Wave Chaotic System

Time-reversal mirrors exploit the time-reversal invariance of the wave equation to achieve spatial and temporal focusing, and they have been shown to be very effective sensors of perturbations to wave chaotic systems. The sensing technique is based on a classical analogue of the Loschmidt echo [1]. However, dissipation results in an imperfect focusing, hence we created a sensing technique employing exponential amplification to overcome this limitation [1,2]. We now apply the technique of iterative time-reversal, which had been demonstrated in a dissipative acoustic system, to an electromagnetic time-reversal mirror, and experimentally demonstrate improved temporal focusing. We also use a numerical model of a network of transmission lines to demonstrate improved focusing by the iterative technique for various degrees and statistical distributions of loss in the system. The application of the iterative technique to improve the performance and practicality of our sensor is explored.\\[4pt] [1] B. T. Taddese, et al., Appl. Phys. Lett. 95, 114103 (2009).\\[0pt] [2] B. T. Taddese, et al., J. Appl. Phys. 108, (2010) in press; arXiv:1008.2409.   

Optimizing energy transfer efficiency in highly branched nanoplasmonic waveguides

Energy transfer in highly branched nanoplasmonic particle waveguides is simulated and optimized by varying the waveguide branching geometry and composition. The periodically branched nanostructures provide a new route towards efficient nanoscale light concentration and local field enhancement. On the one hand, they mimick the analogous randomly branched plasmonic nanostructures which have been previously used for surface-enhanced optical spectroscopy such as SERS. On the other hand, the design is inspired by branched molecular aggregates used for energy funneling. The proposed nanostructures may find applications in sensing, light harvesting and nanophotonics.   

A Novel Nanoscale Coaxial Optical Microscope by Converging Array of Subwavelength Waveguides

A novel nanoscale coaxial optical microscope (NCOM) is proposed by constructing a converging array of coaxial subwavelength optical waveguides (nanocoax). This new design has potential for deep subdiffraction limit resolution, essentially independent of wavelength of the light source. The coaxial structure also has the capability of modal confinement, which can be utilized to extract phase information in the imaging plane. The transmittance and energy dissipation properties of a single nanocoax are obtained, in the visible light range, by numerical simulation. Optical properties of a converging nanocoax array are also investigated numerically. Finally, progress toward an experimental realization of this novel NCOM is discussed.   

Trap rainbow in a self-similar coaxial optical waveguide

We report in this work that the light waves with different frequencies can be selectively guided and spatially separated in a self-similar dielectric waveguide, where a hollow core is surrounded by a coaxial Thue-Morse multilayer. Due to the self-similar furcation feature in the photonic band structure, the transmission multibands are achieved. More interestingly, this dielectric waveguide supports cladding modes, which are spatially separated and confined along the waveguide. Consequently, a rainbow can be trapped (spatial confined but not stopped) in the Thue-Morse waveguide. The finding can be applied to designing miniaturized compact photonic devices, such as spectroscopy on a chip. Reference: Qing Hu, Jin-Zhu Zhao, Ru-Wen Peng, Feng Gao, Rui-Li Zhang, and Mu Wang, Appl. Phys. Lett. (2010) 96, 161101.   

Optical Transmission through Archimedeam Spiral Nanotrenches in Ti Film

We study the optical transmission of circularly polarized light through nanoscale Archimedean spiral trenches in Ti film through experiments and numerical simulations; the focus of these studies is on the effect of radial repetition of the spiral nanotrenches. Experimental measurements show that the left and right circularly polarized light exhibit different transmission through the spiral nanotrenches, and the transmission difference decays when the number of the radial periods of the spiral trenches is increased. Numerical simulations reproduce this interesting phenomenon. The underlying physical mechanism of the radial period dependence is attributed to the absorption difference at the center of the spirals.   

Bio inspired replication and mimicry of optical structure from nature

The optical response from some insects and animals is not from dye or pigment but from their complex structure. The so-called structural color involves interference, diffraction, scattering and photonic crystal effect in various combinations. Structures associated with the structural color have been invasively attended because they have been considered as essentials of optical and photonic devices. The diffraction grating was replicated from beetles by the atomic layer deposition (ALD), and the optical response of resulting structures was characterized. We also present our result on mimicry of the structure of Papilio butterfly. To mimic the structure in the butterfly, we created the basic cup-like structure from polymer films having ordered array of holes, and coated it with an alternating multilayer of the materials. The optical properties of the mimicked structures are also investigated.   

Carrier recombination lifetime in InGaN/GaN multiquantum well LED

Carrier dynamics in InGaN/GaN multiquantum well LED with 5{\%} In content in InGaN wells is studied by time-resolved photoluminescence (TRPL) using time-correlated single photon counting detection system. The excitation energy is 3.06 eV, frequency-doubled output of a Ti: sapphire laser operating at 808 nm (1.53 eV) with 100 fs pulse width and a repetition rate of 80 MHz. TRPL spectra are fitted biexponentially to obtain decay times. The fast decay process is carrier relaxation and slow decay is the carrier recombination process. The fast relaxation decay time shows insignificant variation with the photon energies and pump fluences. On the other hand carrier recombination time increases with increase of photon energies attaining maximum near photoluminescence peak energy and then decreases again on further increase of photon energies. The carrier recombination life time shows increasing behavior with increase of pump fluences and is obtained as long as $\sim $ 7 ns at pump fluence of 0.21 $\mu $J/cm$^{2}$ at room temperature. As the temperature decreases, the carrier recombination life time increases indicating the dominating nature of radiative decay process at low temperatures.   

Session H33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Multiferroics & Magnetoelectrics I

Changing Dielectrics into Multiferroics---Alchemy Enabled by Strain

Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials. The properties of what few compounds simultaneously exhibit these phenomena pale in comparison to useful ferroelectrics or ferromagnets: their spontaneous polarizations ($P_{s})$ or magnetizations ($M_{s})$ are smaller by a factor of 1000 or more. The same holds for (magnetic or electric) field-induced multiferroics. Recently, however, Fennie and Rabe proposed a new route to ferroelectric ferromagnets\footnote{C.J. Fennie and K.M. Rabe, \textit{Phys. Rev. Lett.} \textbf{97} (2006) 267602.}---transforming magnetically ordered insulators that are neither ferroelectric nor ferromagnetic, of which there are many, into ferroelectric ferromagnets using a single control parameter: strain. The system targeted, EuTiO$_{3}$, was predicted to simultaneously exhibit strong ferromagnetism ($M_{s}$~$\sim $~7~$\mu _{B}$/Eu) and strong ferroelectricity ($P_{s}$~$\sim $~10~$\mu $C/cm$^{2})$ under large biaxial compressive strain. These values are orders of magnitude higher than any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression, we show$^{3}$ both experimentally and theoretically the emergence of a multiferroic state under biaxial \textit{tension} with the unexpected benefit that even lower misfits are required, thereby enabling higher quality crystalline films. The resulting genesis of a strong ferromagnetic ferroelectric points the way to high temperature manifestations of this spin-phonon coupling mechanism.\footnote{J.H. Lee and K.M. Rabe, \textit{Phys. Rev. Lett.} \textbf{104} (2010) 207204.} Our work\footnote{J.H. Lee, L. Fang, E. Vlahos, X. Ke, Y.W. Jung, L. Fitting Kourkoutis, J-W. Kim, P.J. Ryan, T. Heeg, M. Roeckerath, V. Goian, M. Bernhagen, R. Uecker, P.C. Hammel, K.M. Rabe, S. Kamba, J. Schubert, J.W. Freeland, D.A. Muller, C.J. Fennie, P. Schiffer, V. Gopalan, E. Johnston-Halperin, and D.G. Schlom, \textit{Nature} \textbf{466} (2010) 954-958.} demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics.   

Magnetoelectric coupling in the strain-induced multiferroic BiMnO$_{3}$

BiMnO$_{3}$ is a rare single phase, multiferroic compound which displays both ferromagnetic and ferroelectric properties. We have grown thin films of BiMnO$_{3 }$on SrTiO$_{3}$ (100) substrates using pulsed laser deposition that display the presence of both order parameters. The ferroelectricity is found to be highly tunable, modulated by both magnetic fields (decreasing by more than 10{\%}) and external strain (increasing by more than 50{\%}). Time dependent ferroelectric measurements in addition to dielectric characterizations reveal BiMnO$_{3}$ is a relaxor ferroelectric. The polar-nano-regions (PNRs) responsible for relaxor ferroelectricity are shown to reside at the island edges where strain is inherently high. Understanding the PNR properties is shown to be essential for understanding the magnetoelectric and strain couplings.   

Magnetoelectric coupling in layered perovskites from first principles

The rational design of a multiferroic with a large polarization and a strong coupling between the polarization and the magnetization remains a challenge. Recognizing the limitations of bulk materials, we attempt to design a strongly coupled multiferroic by focusing on artificial layered materials. In particular, strained Sr-Ti-O layered perovskites have recently been shown to have ferroelectric lattice instabilities that can be controlled by altering the effective dimensionality of the layered system. We use a combination of density-functional theory and group theoretical methods to investigate the interplay of magnetization with ferroelectricity when a layer of magnetic transition metal ions are introduced into this highly tunable dielectric superlattice.   

Electrical field control of interface magnetic anisotropy

The interface magnetic anisotropy of ferromagnetic metals comes from the spin-orbit interaction. By explicitly taking into account the interaction between the symmetry-broken interface potential and the spin-dependent electric dipoles of the Bloch states, we find that the interface spin-orbit coupling can be modeled by the Rashba spin-orbit Hamiltonian (RSOH). Due to the presence of the RSOH, the spin up and down states of the ferromagnet are spin mixed at the interface. Among other consequences, the RSOH induces a perpendicular surface magnetic anisotropy whose magnitude is comparable to the observed values in transition metals. When an external electric field is applied across the interface, the induced screening potential modifies the RSOH and thus the perpendicular anisotropy can be manipulated. Our calculated results are in agreement with the experiments [1]. \\[4pt] [1] Endo et al., Appl. Phys. Lett. 96, 212503 (2010); T. Nozaki et al, Appl. Phys. Lett. 96, 022506 (2010).   

First Principles Studies of Electronic structure and Lattice Dynamics of Multiferroic GaFeO$_{3}$

GaFeO$_{3}$ (GFO) is a room temperature piezoelectric material with antiferromagnetic ordering in the ground state. However, experimental observation reports ferrimagetic behavior below the magnetic transition temperature, attributed to the site disorder of Fe and Ga sites. This transition occurs at temperatures close to room temperature, depending upon the Fe content of the material. Previous structural characterization studies indicate that the room temperature crystal structure (\textit{Pc2}$_{1}n)$ is retained at least until 4 K. While there are a few experimental studies on this compound, there is no well established understanding of its electronic structure and lattice dynamics which can give insight into the piezoelectric and magnetic properties of the material. From this perspective, we started our calculations with the experimental lattice parameters of stoichiometric GFO assuming no partial occupancies of the constituent ions. The calculations are carried out using local spin density approximation (LSDA+U). Electronic structure and Born effective charges were calculated based on the ground state structure. First principles density functional theory based calculations using small displacement method was adopted to calculate the phonon dispersion relations for the material. On the basis of the dispersion relations modes were assigned.   

Ferroelectricity-ferromagnetism coexistence and electromagnons in multi-band electron systems

While it has been established that noncoliner spin textures can realize multiferroics through magnetoelectric effect, here we look into another senario. Namely, in multi-band systems that comprise odd- and even-parity orbits, homogeneous ferromagnetism and ferroelectricity can coexist as proposed by Batista. In multi-orbital systems Hund's exchange coupling is obviously expected to play an important role, but this has yet to be studied in the above scenario. We have here determined the finite-temperature phase diagram for the quarter-filled two-band Hubbard model in the strong coupling limit. A ferroelectric-ferromagnetic phase appears in multi-band insulator phases, where Hund's coupling, neglected in previous researches, is indeed found to be important for the multiferroic phase. We have further explored low-lying excitation spectrum in the multiferroic phase, since collective excitations should be an interesting experimental probe in the multiferroics. Similar to previous studies for the SU(4) Kugel-Khomskii model, magnon-pseudomagnon bound states appear as electromagnetic excitations due to a cross correlation in the homogeneous multiferroic phase.   

Electromagnon in TbMnO$_3$ under magnetic field by Raman scattering

Magnetoelectric excitations in the multiferroic TbMnO$_3$ have been investigated by Raman spectroscopy. Our observations reveal electromagnons excitations at 30 cm$^{-1}$ and at 60 cm$^{-1}$ with electric polarization of light parallel to the a axis [1]. When a magnetic field is applied along the c axis, no flop of the spiral plane or polarization is observed but TbMnO$_3$ becomes paraelectric and a simple antiferromagnetic phase is developed. We show that the dipole character of the electromagnons disappears whereas their magnon compound appears immediately when the spins spiral is destabilized with a magnetic field along the c axis. The magnon dispersion curve associated with the electromagnons is preserved before the construction of the magnon dispersion of the simple antiferromagnetic phase at higher magnetic field. The effect of the phase transition on the phonon modes shows that the Mn-O distance is the key that controls the polar character of the electromagnons.\\[4pt] [1] P. Rovillain et al., PRB 81, 054428 (2010)   

Dielectric and Resistive Response in Multiferroic Superlattices

Building superlattices (SLs) with alternate layers of ultra thin films of ferroelectric and ferromagnetic materials is one of the ways to engineer magnetoelectric multiferroic materials. Alternate thin layers of ferroelectric PbZr$_{0.52}$Ti$_{0.48}$O$_{3}$ (PZT) and ferromagnetic La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) materials were grown on LaAlO$_{3}$ (001) substrates using Pulsed laser deposition technique. X-ray diffraction patterns displayed the typical satellite peaks confirming SLs formation. The surface topography indicates homogeneous films with average surface roughness of $\sim $ 1.5 nm. Well saturated polarization and high dielectric tunability were observed at room temperature. Piezo-force microscopy (PFM) measurements revealed switching of polarization under the external DC bias field. The zero field cooled (ZFC) and field cooled (FC) magnetic measurements revealed the ferromagnetic behavior of SLs, and it undergoes phase transition at lower temperature compared to the bulk and thin films of pure LSMO. To gain further understanding of the electrical properties of the SLs, impedance spectroscopy, dielectric permittivity and ac conductivity were investigated. We observed dynamic magneto-resistive and magneto-dielectric effects around the LSMO metal-insulator and ferromagnetic phase transition temperature   

Enhanced resonant magnetoelectric coupling in frequency-tunable composite multiferroic bimorph structures

We report on a giant tunable enhanced resonant magnetoelectric (ME) coupling in multiferroic magnetostrictive/piezoelectric composite based on Fe-Ni/PVDF and Metglas/PZT-fiber bimorph structures. The approach was shown to provide more than a tenfold gain in the ME coefficient, and a magnetic/electric field assisted stress-reconfigurable resonance frequency tuning, up to 100{\%}. The studies were performed by laser Doppler spectroscopy. We also show that this principle of a continuously tuned resonance that might be used to enhance sensitivity and to reject noise for ME magnetic sensors.   

Interface magnetoelectric effect in ferroelectric/antiperovskite heterostructures

We present results of the first principles calculations of the magnetoelectric effect in thin film layered heterostructures of typical ferroelectric (FE), such as PbTiO3, with Mn-based antiperovskite (AP), such as Mn3GaN. Mn-based antiperovskite materials are interesting due to a non-trivial magnetic order and a linear magnetic response to applied strain that makes them piezomagnetic. The symmetry breaking produces a net magnetization at the FE/AP heterostructure interfaces. This magnetization can be controlled by reversing the polarization of the FE layer. Our calculations show that for the positive FE polarization the induced net magnetization is 3.8 $\mu_{B}$ at the PbO/GaMn and 0.6 $\mu_{B}$ at the TiO2/Mn2N interface, while the corresponding values are 1.6 $\mu_{B}$ and 1.2 $\mu_ {B}$ for the negative FE polarization and 2.2 $\mu_{B}$ and 0.4 $\mu_{B}$ for the zero FE polarization. Thus, the FE/AP interface magnetization exhibits a strong dependence on the direction of the FE polarization, with difference as large as by a factor of 2. The presented novel approach to electrically control the magnetic properties of thin-film layered ferroelectric/piezomagnetic heterostructures may be interesting for practical applications. Therefore, we hope that our results will stimulate experimental work on the FM/AP thin-film layered heterostructures.   

Electric field tuning of magnetic domains in permalloy thin films using elastic coupling with ferroelectric PZT bilayers

We are investigating electric field controlled magnetic domain motion in permalloy films deposited on Pb(Zr$_{x}$Ti$_{(1-x)}$) O$_{3}$ (PZT) bilayers. Previously, we have shown that bilayered heterostructures consisting of a tetragonal PbZr$_ {0.3}$Ti$_{0.7}$O$_{3}$ film (70 nm) deposited on a rhombohedral PbZr$_{0.7}$Ti$_{0.3}$O$_{3}$ (70 nm) display large ferroelastic domains in the top tetragonal PZT layer (Adv. Mat. 21, 3497, 2009). The reversible non-volatile twin boundary motion in this layer can serve as a basis for inducing controlled strain on magnetic thin films deposited on top. We find that permalloy films (50 nm) sputtered on top of the ferroelastic layer exhibit out-of-plane magnetization whose domains can be imaged by magnetic force microscopy (MFM). Voltage pulses are applied between patterned pads of the permalloy film and the bottom electrode underneath the PZT bilayer. This results in different twin configurations in the tetragonal PZT layer, which in turn leads to changes in magnetic domains in the permalloy film as monitored by MFM.   

Magnetism of Fe nanostructure on ultrathin BaTiO$_{3}$ film

A study of Fe nanostructure on BaTiO$_{3}$ (BTO) thin films with variable temperature scanning tunneling microscopy (STM) and X-Ray Magnetic Circular Dichroism (XMCD) under ultrahigh vacuum is presented. Fe/BTO is a prototype system for the study of magneto-electric effects but it is experimentally challenging to achieve high quality metal-oxide interfaces. Our approach is to use atomically flat, unreconstructed and stoichiometric BTO films of 13 unit cell thickness on SrTO$_{3}$, and to deposit Fe impurity atoms and small clusters with molecular beam epitaxy at T = 8 K and compact nanometer clusters by buffer layer assisted growth for comparison. The magnetism of both systems was studied by XMCD at the Fe L3/2 absorption edges. The key observation is that even isolated Fe atoms on the BTO have a sizeable magnetic moment, which quickly increases with increasing coverage. This, together with a detailed analysis of the L 3/2 line shape, is evidence that intermixing and oxidation at the Fe/BTO interface is strongly suppressed. The interface quality achieved can thus potentially be exploited to experimentally observe a magneto-electric interface effect predicted by Tsymbal et al. [Phys. Rev. Lett. 97, 047201 (2006)].   

Voltage Manipulation of Magnetic Anisotropy in MgO/Ferromagnet/Ag system

Recently, the development of new types of memory storage and processing devices has led to great interest in voltage-induced manipulation of magnetic properties in ferromagnetic metals (FM). We investigate the voltage-induced changes in the magnetic properties of a FM in an Indium Tin Oxide (ITO)/Poly(methyl methacrylate) (PMMA)/MgO/FM/Ag system. Samples are fabricated through molecular beam epitaxy (MBE) synthesis and PMMA resist is used as a dielectric layer. ITO is used for the top transparent conductive electrode and magnetic properties are examined through magneto-optic Kerr effect (MOKE) measurements. We report our results and observations of voltage-induced manipulation of the magnetic anisotropy in ITO/PMMA/MgO/FM/Ag system.   

Session H34: Focus Session: Atomic, Molecular, and Memristive Junctions

Memory effects in nanoscale systems

Memory emerges quite naturally in systems of nanoscale dimensions: the change of state of electrons and ions is not instantaneous if probed at specific time scales, and it generally depends on the past dynamics. This means that the resistive, capacitive and/or inductive properties of these systems generally show interesting time-dependent (memory) features when subject to time-dependent perturbations. In other words, nanoscale systems behave as a combination of (or simply as) memristors, memcapacitors or meminductors, namely circuit elements whose resistance, capacitance and inductance, respectively, depend on the past states through which the system has evolved. After an introduction to the theory and properties of memristors, memcapacitors and meminductors, I will discuss several memory phenomena in nanostructures associated to charge, ion and spin dynamics and their far-reaching applications ranging from information storage to computation to biologically-inspired systems. Work supported in part by NSF, NIH, and DOE.   

Multiple switching modes and multiple level states in memristive devices

As one of the most promising technologies for next generation non-volatile memory, metal oxide based memristive devices have demonstrated great advantages on scalability, operating speed and power consumption. Here we report the observation of multiple switching modes and multiple level states in different memristive systems. The multiple switching modes can be obtained by limiting the current during electroforming, and related transport behaviors, including ionic and electronic motions, are characterized. Such observation can be rationalized by a model of two effective switching layers adjacent to the bottom and top electrodes. Multiple level states, corresponding to different composition of the conducting channel, will also be discussed in the context of multiple-level storage for high density, non-volatile memory applications.   

Stochastic memory: getting memory out of noise

Memory circuit elements, namely memristors, memcapacitors and meminductors [1], can store information without the need of a power source. These systems are generally defined in terms of deterministic equations of motion for the state variables that are responsible for memory. However, in real systems noise sources can never be eliminated completely. One would then expect noise to be detrimental for memory. Here, we show that under specific conditions on the noise intensity memory can actually be enhanced. We illustrate this phenomenon using a physical model of a memristor in which the addition of white noise into the state variable equation improves the memory and helps the operation of the system. We discuss under which conditions this effect can be realized experimentally, discuss its implications on existing memory systems discussed in the literature, and also analyze the effects of colored noise. \\[4pt] [1] M. Di Ventra, Y.V. Pershin, L.O. Chua, Circuit elements with memory: memristors, memcapacitors and meminductors, Proc. IEEE 97, 1717 (2009).   

Nano-Ionic in Molecular Nanojunctions

Metal filament growth can be induced on the surfaces of electrochemically active metals such as silver and copper by an external electric potential. We study the voltage-driven formation and dissolution of nanoscale silver filaments in silver/molecular-layer/gold junctions. In this system, an applied electric voltage causes oxidation at the silver surface and the resulting silver cations from the reaction are transported away from the silver surface toward the gold surface. The silver cations are reduced at the gold surface and from the accumulation of the transported silver particles, filaments are formed from the gold surface. We found that the energy required for silver nanofilament formation depends critically on the thickness and electrostatic property of the molecular layer, while the energy required for the dissolution of silver nanofilaments is virtually molecule-independent. I will discuss what our results tell us about chemical reactions in the nanoscale, and the practical application of our results in the area of electronic memory and chemical sensors.   

Bias dependent shot noise measurement in STM style Au junction at room temperature

Shot noise in nanoscale junctions is suppressed strongly near certain conductance values because of nearly fully transmitted modes. Using a gold tip in a STM style motion as the source, combining with high-frequency techniques, we simultaneously measured the conductance and the mean square current noise in nanoscale junctions at a series of voltage biases at room temperature. We have observed peaks in the conductance histogram and related shot noise suppression at different biases near integer multiples of the conductance quantum G0, especially the first three. It demonstrates that quantized electronic transport through quantum channels takes place. We will discuss the relevant noise processes and their evolution with bias across the junctions.   

Large bandwidth measurements of break junctions for molecular electronics at microwave frequencies

The controlled breaking of a thin gold wire, by mechanical stress or by electromigration, has not only been successfully employed to produce the nm-spaced electrodes necessary for creating single molecule devices, but has also attracted attention as the means to produce optical field enhancement for surface enhanced Raman scattering on the few to single molecule level. Although frequently employed, such break junctions usually have a low bandwidth when performing electrical transport measurements of single molecule devices. We investigate such junctions and single molecule devices by using microwave reflectrometry, where a break junction is integrated into a superconducting impedance matching circuit. This allows the impedance and thus the state of the junction to be deduced from the measured reflection coefficient with a bandwidth of 10-100 MHz. We electrically characterize such impedance matching circuits at microwave frequencies as well as gold break junctions at DC. We also show measurement results of the combined system, where the break junction is formed either by electromigration or by mechanical means. This setup will be used to study the transport properties of single molecule devices with a bandwidth larger than that of standard low frequency techniques.   

Shot Noise Measurements in Individual Electromigrated Nanoscale Jsunctions

Shot noise provides insight into the correlated motion of electrons in nanostructures. Previous measurements have examined shot noise in mechanically controlled break junctions (MCBJs), looking at a large ensemble of junction configurations. Electromigrated, lithographically created Au structures at liquid nitrogen and helium temperatures allow for the shot noise measurement of individual junction configurations. High frequency excess noise is amplified by a rf amplifier chain and measured in combination with lock-in techniques simultaneously with the dc conductance. Preliminary noise measurements across bias and temperature are compared to previous experiments preformed with MCBJs at room temperature, with an emphasis on the dependence of the noise on bias conditions.   

Entanglement dynamics within the micro-canonical approach to transport

When a central barrier is located between two biased electrodes, the tunneling of electrons may build a quasi steady-state current and the entanglement entropy between the two sides increases. We study these quantities using the micro-canonical picture of transport [1]. The quasi steady-state current from our simulations agree with that obtained from single-particle scattering theory. In addition, we find that the entanglement entropy increases linearly in time and with bias, so long as the barrier is only partially transmitting, which agrees qualitatively with previous results derived under restrictive assumptions. The micro-canonical approach also allows us to investigate this system highly out-of-equilibrium and under a range of conditions. We present further results on barriers with different tunneling probabilities, biases, and time-dependent fields. \\[4pt] [1] M. Di Ventra and T. N. Todorov, J. Phys. Cond. Matt. 16, 8025 (2004).   

The number of transmission channels through a single-molecule junction

We calculate transmission eigenvalue distributions for Pt-benzene-Pt and Pt-butadiene-Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting pi-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level. Thus for Pt-benzene-Pt junctions we predict two dominant transmission channels, and for Pt-butadiene-Pt junctions only one. Pt-buckyball-Pt junctions are predicted to exhibit up to five dominant transmission channels.   

Vibrational heating in molecular junctions

Energy injection, distribution and dissipation are of great important in understanding molecular electronics. One method of characterizing the distribution of energy in a system is to measure the effective temperature. Using surface-enhanced Raman spectroscopy (SERS) of molecular nanojunctions, we measure the effective vibrational temperatures of a molecular nanojunction as a function of bias. We observe significant mode-specific vibrational pumping by both optical excitation and DC current, with effective temperature changes exceeding several hundred Kelvin. These measurements provide direct information about heat generation and dissipation in molecular-scale junctions and allow direct comparisons with theories of nanoscale heating.   

Effective field theory of interacting pi electrons in molecular junctions

We present an effective field theory that allows the two-body Hamiltonian for a $\pi$ electron system to be expressed in terms of three effective parameters: the $\pi$ orbital quadrupole moment, the on-site repulsion, and a dielectric constant. As an application of this theory, we present a model of screening in single-molecule junctions based on the image charge method, and use this technique to calculate the van der Waals interaction between a neutral molecule and a metallic conductor.   

Temperature dependence of charge transport in conjugated single molecule junctions

Over the last decade, the break junction technique using a scanning tunneling microscope geometry has proven to be an important tool to understand electron transport through single molecule junctions. Here, we use this technique to probe transport through junctions at temperatures ranging from 5K to 300K. We study three amine-terminated (-NH$_{2})$ conjugated molecules: a benzene, a biphenyl and a terphenyl derivative. We find that amine groups bind selectively to undercoordinate gold atoms gold all the way down to 5K, yielding single molecule junctions with well-defined conductances. Furthermore, we find that the conductance of a single molecule junction increases with temperature and we present a mechanism for this temperature dependent transport result.   

Charge transport in mechanically controlled single-molecule break-junctions

We present inelastic electron tunneling spectroscopy (IETS) measurements carried out on single molecules incorporated into a mechanically controllable break-junction (MCBJ) at low temperature. The single molecules contacted with a MCBJ or with a STM show various conductance values under stretching depending on the contact geometry and the molecular conformation ($e.g.$, \textit{trans} or \textit{gauche}). In such single-molecule devices, the metal of electrodes ($e.g.$, gold or platinum) and anchoring groups ($e.g.$, thiol (-SH) or amine (-NH$_{2}))$ can also significantly influence the charge transport through the single molecules. Here we demonstrate how these individual aspects influence the conductance properties of single-molecule junctions.   

Session H35: Topological Insulators: General

Topological Insulators as Substrates for CO Oxidation Catalysis by Ultrathin Au Films

We propose a novel application of three dimensional topological insulators (3DTIs) in heterogeneous catalysis based on first- principles calculations within density functional theory. We use a Bi$_2$Se$_3$ substrate as the support of an ultrathin Au film, and show that the Au adatoms are strongly bound to and able to wet the surface of Bi$_2$Se$_3$. More importantly, we find the topological surface states of Bi$_2$Se$_3$ are robust against Au deposition, and it can enhance the interaction between Au and CO, O$_2$ molecules by acting as an $``$electron bath$"$. The present study may broaden the potential technological applications of 3DTIs, and shine some new light on the understanding of the role of surface states in heterogeneous catalysis.   

Topological insulators on the ruby lattice

We study a tight-binding model on the two-dimensional ruby lattice. This lattice supports two types of second neighbor spin-dependent hopping parameters in an s-band model that preserves time-reversal symmetry. We discuss the phase diagram of this model for various values of the hopping parameters, and note an interesting interplay between the two spin-orbit terms that individually would drive the system to a Z2 topological insulating phase. The fidelity of each phase is also calculated.   

Extension of the Kitaev model on the square lattice

We study an extension of the Kitaev model [1] on the square lattice, where two types of Gamma matrices on neighboring sites have interaction that respects time reversal symmetry. A family of Kitaev models can be classified as the topological insulator/superconductor when described by Majorana fermions [2]. Our model is in class DIII in Altland-Zirnbauer classification, and thus a Z$_{2}$ invariant characterizes two distinct phases. There appear helical Majorana edge modes in the topological phase. The same model on the one-dimensional ladder is also studied. \\[4pt] [1] A. Kitaev, Annals of Physics 321, 2 (2006).\\[0pt] [2] S. Ryu, Phys. Rev. B 79, 075124 (2009).   

Electrical manipulation and measurement of spin properties of quantum spin Hall edge states

The existence of the quantum spin Hall state has been confirmed in a series of experiments performed in HgTe quantum wells but a quantitative observation of the helical edge structure is still lacking. We study an electrical manipulation and measurement of helicity properties of the edge states by employing the Rashba spin-orbit interaction (SOI). Specifically, we show that a spatially uniform Rashba SOI, controllable by the gate voltage, can be utilized in tuning the spin orientation of the edge modes (J. I. V\"ayrynen and T. Ojanen, arXiv:1010.1353). We introduce a point-contact geometry where helicity of the edge modes can be accessed by two- terminal conductance measurements.   

Electronic Transport in Exfoliated Bismuth Selenide

Recent theoretical and experimental work has identified bismuth selenide as a promising candidate for studies of three-dimensional topological insulators due to its large bulk semiconducting gap crossed by topological Dirac surface states. We report on the fabrication and measurement of mesoscale exfoliated bismuth selenide devices, including the effects of electric-field-effect gating and magnetic field on transport and possible signatures of topological states. We will also discuss fabrication strategies to mitigate surface disorder and doping   

Approaching the insulating state in Ca-doped Bi$_{2}$Se$_{3}$ nanodevices

We report a systematic tuning of the carrier density in Ca-doped Bi$_{2}$Se$_{3}$ nanodevices. A clear transition from the metallic to insulating state is observed as both the Ca-concentration and the gate voltage are tuned. At high temperatures, the devices behave metallic as indicated by the linear temperature dependence of the electrical resistivity as the devices are initially cooled. As the temperature is lowered, the resistivity shows a minimum then increases. This insulating behavior can be modeled by a thermal activated conductivity, which is taken over by saturation at the lowest temperatures. At 1.5 K, the resistivity undergoes a $\sim $5-fold increase as a gate voltage is swept from -60 V to 60 V. The field-effect mobility is found to be about $\sim $ 5000 cm2/Vs. We have also observed a systematic evolution of the magnetoresistance as the chemical potential is tuned via the gate voltage. The combination of the chemical and electronic dopings provides an effective way to access the low carrier density gap states in Bi2Se3 topological insulator nanodevices. This work was supported in part by DOE and NSF.   

Topological Response Theory of Doped Topological Insulators

We generalize the topological response theory of three-dimensional topological insulators (TI) to metallic systems -- specifically, doped TI with finite bulk carrier density and a time-reversal symmetry breaking field near the surface. We show that there is an inhomogeneity-induced Berry phase contribution to the surface Hall conductivity that is completely determined by the occupied states and is independent of other details such as band dispersion and impurities. In the limit of zero bulk carrier density, this intrinsic surface Hall conductivity reduces to the half-integer quantized surface Hall conductivity of TI. Our theory is directly related to the TI materials currently under experimental investigation, which have finite residual bulk carrier densities.   

Topological states in one dimensional solids and photonic crystals

We show that the band structure of a one-dimensional solid with particle-hole symmetry may be characterized by a topological index that owes its existence to the non-trivial homotopy of the space of non-degenerate real symmetric matrices. Moreover we explicitly demonstrate a theorem linking the topological index to the existence of bound states on the surface of a semi-infinite one dimensional solid. Our analysis is a one-dimensional analogue of the analysis of topological insulators in two and three dimensions by Balents and Moore; our results may be relevant to long molecules that are the one dimensional analogue of topological insulators. We propose the realization of this physics in a one-dimensional photonic crystal. In this case the topology of the bandstructure reveals itself not as a bound surface state but as a Lorentzian feature in the time delay of light that is otherwise perfectly reflected by the photonic crystal.   

Robustness of topologically protected surface states in layering of Bi$_2$Te$_3$ thin films

Recently, topological insulators with time-reversal symmetry have drawn great attention due to their topologically protected states. Topological insulators differ from band insulators in that a bulk energy gap opens up due to strong spin-orbit coupling and that metallic surface states reside in the energy gap. The surface states of topological insulators are topologically protected in that impurities preserving time-reversal symmetry neither destroy the surface states nor impact the topological nature of the surface states. Recently, bulk bismuth-based alloys were shown to be topological insulators. Thin films offer valuable probes of topological insulators as well as device applications. Additionally, bismuth-based thin films of a thickness of a few nanometers were fabricated on substrates or suspended across trenches. We investigate surface states of Bi$_2$Te$_3$(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi$_2$Te$_3$ thin films, we examine the topological nature of the surface states as a function of the film thickness and compare our results with experimental data and other theoretical reports.   

Spin and Charge Transport on the Surface of a Topological Insulator

We derive diffusion equations, which describe spin-charge coupled transport on the helical metal surface of a three-dimensional topological insulator. The main feature of these equations is a large magnitude of the spin-charge coupling, which leads to interesting and observable effects. In particular, we predict a new magnetoresistance effect, which manifests in a nonohmic correction to a voltage drop between a ferromagnetic spin-polarized electrode and a nonmagnetic electrode, placed on top of the helical metal. This correction is proportional to the cross-product of the spin polarization of the ferromagnetic electrode and the charge current between the two electrodes. We also demonstrate tunability of this effect by applying a gate voltage, which makes it possible to operate the proposed device as a transistor.   

Spin and Charge Transport in Thin Films of Topological Insulators

We develop a theory of spin-charge coupled transport in thin films of topological insulator materials, when the top and bottom surfaces of the sample hybridize. We find significant differences from the case of transport on unhybridized surfaces. In particular, we find significant reduction of the spin relaxation rates, which enhances all the spin-related transport effects, compared to the case of a single surface. We also find that the out-of-plane component of the spin, which is absent from the hydrodynamic transport equations in the single surface case, reappears when the surfaces are hybridized.   

Conductance of a helical edge liquid coupled to a magnetic impurity

In a quantum spin Hall system, which can be realized in HeTe/(Hg,Cd)Te semiconductor quantum wells [1], helical edge states carry a current and the conductance takes the universal value of $2e^2/h$. This is because an impurity without internal degrees of freedom cannot backscatter an electron at the edge in the presence of time-reversal symmetry [2]. On the other hand, backscattering by a magnetic impurity is allowed. We study the effect of backscattering from a magnetic impurity on the conductance of a quantum spin Hall system [3], and obtain the correction $\delta G(\omega)$ to the electrical conductance as a function of frequency $\omega$. We find that the correction $\delta G(\omega)$ vanishes in the dc limit ($\omega \to 0$), when our model conserves the total spin $S_z$. Another interesting transport property is the thermal conductance, which is affected by the coupling to the magnetic impurity even at $\omega\to 0$. We find that the temperature dependence of the thermal conductance shows a non-monotonic behavior with a minimum occurring at the Kondo temperature. \\[4pt] [1] M. Konig {\it et al.}, Science 318, 766 (2007). \\[0pt] [2] C. Wu, B. A. Bernevig and S. C. Zhang, Phys. Rev. Lett. 96, 106401 (2006); C. Xu and J. E. Moore, Phys. Rev. B 73, 045322 (2006). \\[0pt] [3] J. Maciejko {\it et al.}, Phys. Rev. Lett. 102, 256803 (2009).   

Design Principles and Coupling Mechanisms in the 2D Quantum-Well Topological Insulator HgTe/CdTe

We present atomistic band structure calculations revealing a different mechanism than recently surmised via {$\bf k\cdot p$} calculations about the evolution of the topological state (TS) in HgTe/CdTe. We show that {\it 2D interface} (not {\it 1D edge}) TS are possible. We find that the transitions from a topological insulator at critical HgTe thickness of $n= 23$ ML (62.5 {\AA}) to a normal insulator at smaller $n$ is due to the crossing between two interface localized states: one derived from the S-like $\Gamma_{6c}$ and one derived from the P-like $\Gamma_{8v}$ light-hole, not because of the crossing of an interface state and an extended QW state. These atomistic calculations suggest that a 2D TS can exist in a 2D system, even without truncating its symmetry to 1D, thus explaining the otherwise surprising similarity between the 2D dispersion curves of the TS in HgTe/CdTe with those of the TS in 3D bulk materials such as Bi$_2$Se$_3$. Ref: J.W. Luo and A. Zunger, Phys. Rev. Lett. {\bf 105}, 176805 (2010).   

Towards Quantum Spin Hall Effect in InAs/GaSb Quantum Wells

Recently, it has been proposed that inverted InAs/GaSb composite quantum wells (CQWs) should exhibit the Quantum Spin Hall Effect (QSHE), characterized by the energy gap in the bulk and gapless edge modes which are protected from backscattering by time reversal symmetry. We have successfully fabricated a double-gated device on high-quality MBE-grown InAs/GaAs CQWs in the inverted regime, in which we were able to vary the band structure via an electrical field, and tune the Fermi level into mini-gap regime. We observed clear evidence for an energy gap in the inverted regime, with values of the gap consistent with those theoretically predicted; however, the mini-gap exhibits residual conductivity of non-trivial origin, which complicates transport investigation of proposed edge channels. We note that the InAs surface states around the sample edges may play a role in the observed resistivity features. In ongoing work, we pursue Cooper pair injection experiments by proximity to an s-wave superconductor, which should provide a novel probe of the proposed helical edge modes. We will discuss our progress towards observing QSHE in this unique material system. The work at Rice was supported by grants from NSF, Keck Foundation, and Hackerman Advanced Research Program.   

Session H36: Photovoltaics: Compound Semiconductors and Organics

Effects of doping on the band gap of iron pyrite

Iron pyrite (FeS$_{2})$, a highly abundant materials in Earth's upper continental crust, is of great interests in photovoltaic and photo-electrochemical applications, due to its wide band gap of 0.9 - 0.95 eV and large absorption coefficient of $\alpha >$10$^{5}$ cm-1 for $\lambda <$10$^{3}$ nm. However, its electron/hole mobility is typically very low, caused by the presence of sulfur deficiency in crystalline FeS$_{2}$ bulks or nanostructures. In principle, doping electrons or holes by exotic elements may not only compensate sulfur deficiency but also create mobile carriers. In this work, we systematically studied the dopabilities of N, P, F and Cl in bulk FeS$_{2}$, by using the first-principles calculations. First of all, we found that these elements substitute S under sulfur poor condition in the FeS$_{2}$ bulk and nanostructures. While N, P and F dopants merely induce deep localized defect levels, doping 1.6{\%} Cl not only generates a 0.996 $\mu _{B}$ local magnetic moment per Cl and but also increases the carrier density by 3x10$^{18}$/cm$^{-3}$. The defect states are delocalized and hence doping of Cl also improve the carrier mobility in FeS$_{2}$. We found that incorporation of Cl also leads to significant structural distortions around Cl atom.   

The reason FeS$_2$ is not a good PV absorber

FeS$_2$ is representative of an ideal earth-abundant candidate absorber for thin film PV, because of its appropriate band gap, high absorption coefficient and good electron/hole mobility. Yet, despite $\sim$15 years of research, the promise of FeS$_2$ as an absorber layer has been unrealized, manifesting as a low open circuit voltage which has been attributed to E$_f$ pinning arising from bulk sulfur vacancies. Our first-principles calculations and experimental thermogravimetirc analyses, however, show that S vacancies and other point defects have rather high formation energies. Hence, they are unlikely to form and pin E$_f$. We find that the widely observed S deficiency in FeS$_2$ is accommodated by phase-coexistence of a few Fe$_{1-x}$S compounds, rather than S vacancies. These minority phases are metallic and detrimental for PV. We find select ternary Fe sulfides do not have thermodynamically-mandated phase-coexistence like FeS$_2$, yet they retain optimal band gaps and high absorption strengths comparable to FeS$_2$. These properties and associated surface-defect calculations will be discussed.   

Direct measurement of the built-in potential in a nanoscale heterostructure

Recently synthesized heterostructured nanorods are a promising material for applications such as photovoltaics. Understanding the electronic structure of these materials is both an interesting scientific question and vitally important for applications. We present the measurement of the built-in potential across individual Cu2S-CdS heterostructured nanorods by combining transmission electron microscopy with electrostatic force microscopy. This represents the first experimental determination of the electrostatic potential across an isolated nanostructure. We observe a variation of built-in potentials, ranging from 100 to 920 mV, with an average of 250 mV. Nanorods of a uniform composition with no heterojunction do not show built-in potential, as expected.   

First Principles Calculation of Optical Properties of Ternary Semiconductors Cu$_3$PSe$_4$ and Cu$_3$PS$_4$

The ternary semiconducting compounds Cu$_3$PSe$_4$ and Cu$_3$PS$_4$ are of interest as potential optoelectronic materials. Of particular interest for solar photovoltaic devices is Cu$_3$PSe$_4$, as its band gap lies in the desired 1.0 to 1.6 eV range for an absorber. We have theoretically calculated the optical properties of these materials using density functional theory with GGA and hybrid exchange-correlation functionals, as well as with the GW$_0$ approximation from many-particle theory. We find that Cu$_3$PSe$_4$ has a direct band gap with relatively strong optical absorption above 2 eV, indicating that this compound is a candidate photovoltaic absorber. Cu$_3$PS$_4$ has a larger band gap of approximately 2.4 eV, placing it outside consideration as a solar absorber.   

Crystalline ordered states and local surface potential variations of photovoltaic Cu(In,Ga)Se$_{2}$ thin-films

Structural and electrical properties of CuInSe$_{2 }$(CIS), Cu(In,Ga)Se$_{2}$ (CIGS) and CuGaSe$_{2}$ (CGS) grown by co-evaporation were studied. Intriguing morphology and grain growth behaviors were found in the surface of the films. X-ray diffraction of the films exhibited phase formation of the stoichiometric chalcopyrite while Cu$_{2}$Se and CuSe$_{2}$ were observed. Using Raman scattering spectroscopy, shift of A$_{1}$ mode was observed from 177 cm$^{-1}$ for CIS to 189 cm$^{-1}$ for CGS as Ga content increased. It is very interesting that two different crystalline ordered states with chalcopyrite (CH) and CuAu structure (CA) were found. Effects of the grain boundaries on local electrical properties of the films with different chemical contents were examined. Local current mapping and surface potential distribution were obtained in the film by conductive atomic force microscopy and Kelvin probe microscopy. Minority carrier transport behaviors and local variations of potential values on and near the grain boundaries were characterized. These results suggested that a local built-in potential is possibly formed on positively charged grain boundaries.   

Minority-Carrier Lifetimes in GaInP

Minority-carrier lifetimes are very important to the performance of photovoltaic materials and are quite sensitive to the structure of the material. The impact of lifetimes can be readily illustrated using computer modeling of cell performance, and a brief discussion of the results of our modeling will be given. AlInP/GaInP double heterojunctions of varying thickness and doping concentration were grown on GaAs substrates by metallorganic chemical vapor deposition (MOCVD). Lifetimes were measured using time-resolved photo-luminescence. Carrier concentrations were determined using capacitance-voltage measurements. Here we report on the minority carrier lifetime as a function of active layer thickness and doping concentration for n-type and p-type GaInP that is lattice-matched to the substrate.   

Photvoltaic effects in ferroelectrics due to nonlinear optical processes

The physical mechanism for the bulk photovoltaic effect that appears in noncentrosymmetric materials, especially in ferroelectric devices, is not well understood. A promising candidate for a truly bulk photovoltaic effect is non-linear optical processes -- most notably ``shift current,'' which describes the net motion of coherently excited electrons in the absence of inversion symmetry, and has been described analytically several times in the literature. Shift current is also of interest due to the appearance of a gauge invariant phase describing the carrier mobility. We have developed an expression for shift current suitable for efficient computation and analysis utilizing wavefunctions of arbitrary origin, and calculated the response for several prominent ferroelectrics -- including LiNbO$_3$, BaTiO$_3$, and PbTiO$_3$ -- using KS eigenstates. The calculated short-circuit currents appear to be in rough agreement with available experimental results where they exist. Furthermore, they indicate a more subtle relationship between polarization and band gap than has heretofore been presumed, with strong implications for the materials design process, as well as shift current's overall viability as a mechanism for efficiently harvesting solar energy.   

Nanostructured multiferroic materials for optoelectronics and energy-related nanodevices

Combining properties into multifunctional materials is one of innovative ways explored by the modern technology to achieve high miniaturization of integrated devices. In this context, besides their exciting physics, multiferroic materials hich combine two or more ferroic order offer opportunities for potential applications in emerging fields of spintronics, optoelectronics and data storage. For such applications, successful integration of these multifunctional materials needs to develop adequate fabrication processes and to the coexistence in singe phase of robust properties at room temperature (RT). Furthermore, a synergistic interaction between magnetic and electric orders leads to additional freedom for designing related devices. Here we review recent progress of our group in growth and nanopatterning of multiferroic thin films developed to overcome those drawbacks. We will present the fabrication of RT-multiferroic Bi$_{2}$FeCrO$_{6 }$thin films. Successful nanopatterning of these complex oxides by a versatile and generic approach and their photovoltaic properties will be also discussed.   

Energy Transfer in Organic Photovoltaic Cells and its Impact on Measurements of the Exciton Diffusion Length

In order to generate photocurrent from an organic photovoltaic cell (OPV), the optically generated exciton must be dissociated into its constituent charge carriers. This process is carried out at the interface between electron donating and accepting materials. Consequently, photocurrent is generated only at the donor-acceptor (D-A) interface, and exciton diffusion to the interface is a critical step in the photoconversion process. The focus of this work is on the development of methods that permit the accurate measurement of the exciton diffusion length, and realizing architectures that demonstrate enhanced exciton harvesting. In measuring the exciton diffusion length, emphasis is placed on quantifying the role of excitonic energy transfer in the dissociation process by explicitly measuring the F\"{o}rster radius between donor and acceptor materials. Many of the techniques currently used to estimate the exciton diffusion length incorrectly ignore these effects, potentially leading to overestimates. Efforts to overcome the short diffusion length are focused on small molecule OPVs that contain a continuously graded D-A film composition as a means to simultaneously optimize both exciton diffusion and charge collection. In a properly optimized graded heterojunction OPV, power conversion efficiencies $>$4{\%} can be realized, exceeding the performance of conventional planar and uniformly mixed structures.   

Effect of Thin Polymer Layers on the Performance of ZnO/Cu$_2$O Solar Cells

Transition metal oxides are a class of stable, non-toxic, and inexpensive semiconductors with great potential in low-cost photovoltaics (PV) applications. Cu$_2$O is a versatile p-type oxide that absorbs visible light and can be solution-processed at low temperatures. ZnO is a wide-E$_g$ n-type material with good electronic properties and has already been widely incorporated into other low-cost PV technologies such as organic and dye-sensitized solar cells. While ZnO/Cu$_2$O devices have large theoretical efficiencies (as high as 20\%) [1], practical devices do not reach their full potential due to poor charge collection and recombination. ZnO/Cu$_2$O PV's can be improved by optimizing deposition conditions, such as solution pH and temperature, and device geometry, such as layer thickness [2]. This talk, however, will discuss how semiconducting polymer layers can further enhance performance for scalable device fabrication. In particular, polymer type and the Cu$_2$O/polymer interface will be discussed as routes to better performance. \\[4pt] [1] J. Nelson. \emph{The Physics of Solar Cells}. Imperial College Press, 2003 \newline [2] Musselman et al., unpublished   

Nanoscale Morphology and Charge Transport in Hybrid Solar Cells by Conducting Probe Atomic Force Microscopy

Measurements of the dependence of photoactive response on nanoscale morphology provide essential insights to further improve processing and achieve morphologies with enhanced device performance. To study the correlation between local morphology and photoactive response, we have fabricated hybrid polymer/zinc oxide thin films and have characterized their electrical properties at nanoscale resolution with conducting probe atomic force microscopy (c-AFM). The charge carrier mobilities were extracted based on local IV characteristics. The surface morphology and current mapping were recorded simultaneously under various illumination and biasing conditions, enabling direct study of morphology dependent transport processes in these photoactive devices.   

Development of Phase Stable Organic Photon Upconverters

Recently, high efficiency photon upconversions (UCs) utilizing triplet-triplet annihilation (TTA) in aromatic hydrocarbon molecules, applicable to sunlight intensity, have been actively studied for the purpose of improving external quantum efficiencies of photovoltaics. These studies have been using volatile organic solvents as media in order for TTA to occur, which are currently hindering its applications. I have discovered that those aromatic molecules can be stably dispersed by a simple method within certain class of ionic liquids (ILs), which are non-volatile and thermally stable up to several hundred degree C, to form unprecedented organic photon upconverters with improved phase stability [1,2]. The proposed mechanism for the molecular stabilization in ILs as well as the UC quantum efficiencies is presented.\\[4pt] [1] Y. Murakami and I. Sato, Patent 2010-230938JP (pending)\\[0pt] [2] Y. Murakami et al., submitted.   

Charge transport and absorption study of metal nanoparticle plasmonics for organic photovoltaics

A hybrid plasmonic nanostructure of an optically sensitive heterojunction organic film incorporating metal nanoparticles is fabricated. From the Charge Extraction in Linearly Increasing Voltage (CELIV) measurements, the mobility of this hybrid plasmonic nanostructure has been experimentally extracted to be at least one order of the magnitude higher than that of the organic film without metal nanoparticles. The measured absorption spectrum also shows the increasing of the intensity by around 28{\%} as well as the broadening of the spectrum. The theoretical calculation confirms this broadband optical absorption enhancement results from localized surface plasmon resonance. The optimization of the density of the metal nanoparticles has been done to achieve the best performance for the photovoltaic devices.   

The Effect of a Self Assembled Monolayer in Small Molecule Organic Solar Cells

We have previously found that a Self Assembled Monolayer (SAM) of Flouroalkyl TrichloroSilane (FTS) molecules on Single-Walled and Multi-Walled Carbon Nanotubes (SWCNT {\&} MWCNT) can greatly improve the conductivity [1]. In present work we have studied the effect of SAM modified carbon nanotubes in Small molecule organic photovoltaic cells. (OPV) We have fabricated and characterized OPV of the general structure: CNT(FTS)/CuPC/C60/BCP/Al. We observed improvement of the performance of the OPV with CNT anodes with FTS SAM both for SW and MW CNT. The major effect is an improvement of the open circuit voltage and also small improvements in both short circuit current and filling factor. The increase in open circuit voltage is likely due to modifications of the carbon nanotube work function by the strong dipole moments of the FTS molecules. The improvements in short circuit current and filling factor is probably due to improved active layer morphology and removal of absorbed water from the substrate. \\[4pt] [1] Cook, Alexander; Lee, Bumsu; Kuznetsov, Alexander; Podzorov, Vitaly; Zakhidov, Anvar. Self Assembled Dipole Monolayers on CNTs: Effect on Transport and Charge Collectio. Oral Presentation APS March Meeting 2010   

Inverted Polymeric Photovoltaic Cells and Parallel Tandems with Transparent Single Wall Carbon Nanotubes Interlayer

We demonstrate an organic photovoltaic (OPV) monolithic multi junction cell in a parallel electrical configuration utilizing polymers with complementary absorption spectra and transparent single wall CNT (SWCNT) as an interlayer electrode (IE). Parallel tandem cells are of importance because they can append to the limited spectral coverage of available polymers and because there is no need balance current as is the case with in-series configurations. Devices comprise of polymeric sub cells where one is inverted using ZnO nanoparticles and a MoO$_{3}$ buffer layers, this inverted structure allows for the SWCNT IE to function as a cathode. Each sub cell is characterized independently and the short circuit current of the tandem device is shown to increase. Overall increase in efficiency is observed and attributed to enhanced spectral coverage by spectrally complimentary polymers and the effective use of parallel tandem architecture. We also demonstrate a semi transparent inverted OPV structure with a SWCNT electrode and a efficiency of over 3{\%}.   

Session H37: Focus Session: Graphene Structure, Dopants, and Defects: Transport I

Probing the nature of impurity scattering in graphene

Since the very first investigations of the electronic properties of graphene the nature of defects has been shown to play an essential role in determining the carrier density dependence of the conductance. Impurity scattering is characterized by two different times the transport and elastic scattering times which are sensitive to the mass less energy dispersion of graphene. The analysis of the ratio between these two times gives insight on the nature (neutral or charged) and range of the scatterers. We will discuss how to extract these two times from magneto-transport measurements in macroscopic samples and analyze their differences in monolayer and bilayer Graphene in relation with the different symmetry properties of their band structure and wave functions. \\[4pt] \textit{``Transport and Elastic Scattering Times as Probes of the Nature of Impurity Scattering in Single-Layer and Bilayer Graphene}'' M.~Monteverde, C. Ojeda-Aristizabal, R. Weil, K. Bennaceur, M.~Ferrier, S. Gu\'{e}ron, C. Glattli, H. Bouchiat, J. N. Fuchs, and D. L. Maslov Phys. Rev. Lett. \textbf{104}, 126801 (2010).   

Transport and optical measurements on Graphene - hBN heterostructures

Placing graphene on hexagonal BN (hBN) substrates has recently been shown to lead to improved device quality.~ In addition, the planar nature of the h-BN allows for the realization of novel device architectures with high mobility graphene, including dual-gated devices.~ I will discuss recent transport and optical measurements on such graphene-hBN heterostructures, focusing on high mobility dual gated bilayer graphene, in which the carrier density and the band-gap can be tuned independently.   

Fabrication and transport characterization of Graphene/Hexagonal Boron Nitride sandwich structures

High quality, large size hexagonal Boron Nitride (h-BN) thin films and single layer graphene were grown on metal substrates via the chemical vapor deposition method (CVD) and transferred to form Graphene/h-BN sandwich structures. High resolution transmission microscopy (TEM) was performed on Graphene and h-BN, confirming highly-ordered crystalline structures of both. The electronic transport properties of various sandwich configurations were investigated at low temperature and high magnetic field.   

Tunable band gaps in bilayer graphene-BN heterostructures

We investigate band-gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV, and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. Our calculations suggest that graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.   

Dielectric thickness dependence of carrier mobility in graphene with Al$_{2}$O$_{3}$ and HfO$_{2}$ top dielectrics

We study the carrier mobility in graphene with high-$k$ top dielectrics, as a function of the dielectric thickness and temperature. Metal-oxide high-$k$ films, Al$_{2}$O$_{3}$ ($k\sim $8.4) and HfO$_{2}$ ($k\sim $16), are deposited on graphene by atomic layer deposition (ALD), at deposition temperatures of 200-250 \r{ }C. A considerable ($\sim $50{\%}) mobility drop is observed after the deposition of the first 2-4 nm of dielectric. For thicker dielectrics the mobility is relatively insensitive to thickness. The carrier mobility has a weak temperature dependence, which indicates that phonon scattering is not the primary mobility limiting factor in these devices. Our results suggest that Coulomb scattering caused by fixed charged impurities located in the high-$k$ dielectric, and in proximity to the graphene layer plays a significant role in mobility degredation. The ALD grown high-$k$ films are inherently oxygen deficient, and oxygen vacancies form donor levels which become positively charged in the proximity of the graphene layer. We speculate that Coulomb scattering due to charged point defects is the mobility limiting mechanism in graphene devices with Al$_{2}$O$_{3}$ or HfO$_{2}$ high-$k$ dielectrics.   

Coulomb Drag in Independently Contacted Graphene Bilayers

Two graphene layers placed in close proximity, and electrically isolated, offer a unique system to investigate interacting electron physics. In such graphene bilayer, the interlayer spacing can be reduced to values much smaller than otherwise achievable in semiconductor heterostructures. Moreover, the zero energy band-gap allows the realization of coupled hole-hole, electron-hole, and electron-electron two-dimensional systems in the same sample. Here we demonstrate the realization of independently contacted graphene bilayers. We probe the resistance and density of each layer, and quantitatively explain their dependence on the back-gate bias. We experimentally measure the Coulomb drag between the two graphene layers, by flowing current in one layer and measuring the voltage drop in the opposite layer. The drag resistivity gauges the momentum transfer between the two layers, which in turn probes the interlayer coupling. The temperature dependence of the Coulomb drag above temperatures of 77K reveals that the ground state in each layer is a Fermi liquid. Below 77K we observe mesoscopic fluctuations of the drag resistivity, as a result of the interplay between coherent transport in the graphene layer and interlayer interaction.   

Probing charge scattering mechanisms in suspended and supported graphene by varying dielectric environment

The electronic properties of graphene are drastically affected by its environment, such as the substrate underneath it and the impurities near it. To elucidate the effect of scattering due to the environment, we used the Hall probe technique to study the electronic transport properties and the quantum capacitance of single layer graphene devices in environments with different dielectric susceptibility $\kappa$. We have varied the susceptibility by i) using solvents of different dielectric constants, ii) mixing two miscible solvents of different dielectric strengths and iii) varying the temperature of the solvent. To eliminate the effects due to a substrate, we have also studied suspended graphene devices. We have observed enhancement in the Hall mobility and reduction in the minimal conductivity in both supported and suspended devices as the static dielectric constant is increased from $\kappa\sim2$ to $\kappa\sim30$. This suggests stronger screening of charge scattering in higher $\kappa$ dielectric environment. Our results support the conjecture that the dominant scattering mechanism in graphene is the long range Coulomb interaction due to the charge impurities.   

Charge transport studies in graphene devices: a focus on mobility behavior

Graphene has been the subject of extensive electrical characterization since 2004. As in semiconductor based FETs, mobility ($\mu )$ is used as the parameter to gauge and compare the device performance. Typically reported is the effective mobility, $\mu _{eff}$, extracted from I$_{d}$ -- V$_{g}$ characteristics or the channel mobility ($\mu _{H})$ extracted from Hall measurements, which can be especially illuminating when more than one carrier type is participating in the charge transport process. The dependence of the mobility on parameters such as applied field, dielectric type, underlying oxide thickness, channel dimensions and temperature is not well understood. A study of $\mu _{H}$ and the accompanying magnetoresistance as a function of the above mentioned parameters in low to moderate magnetic fields was performed, as well as $\mu _{eff}$ on the same devices, the results of which will be compared and presented. The dependence on graphene type (grown vs. exfoliated) will also be discussed.   

Graphene nanoelectronics from ab initio theory

We employ atomic first principles theory to study the properties of realistic graphene nanoelectronic systems. We focus on the role of different contact materials and their effect on the transport properties at the graphene/metal junction. The current-voltage characteristics were calculated using density functional theory (DFT) combined with non-equilibrium Green's functions (NEGF).   

Charge transport in two-terminal graphene junctions with bonding metal contacts

One has to attach graphene to metal leads to measure charge transport characteristics. In a number of experiments a thin Ti film is grown on top of graphene and additional metal leads (e.g., Au or Al) are created on top of this film. Ti forms covalent bonds with graphene, destroying the linear dispersion at the Ti/graphene contact. For practical reasons (i.e., the use of Hamiltonians with a linear dispersion) most theoretical approaches either consider only the effects of the inhomogeneities caused by the insulating substrate, or, when two-terminal calculations exist, use assumptions that preclude quantitative modeling. These assumptions include: (i) extremely large bias steps separating the leads and the central region of the device, or (ii) unusually large imaginary contributions to the self-energies representing metal leads. We depart from these models and follow an atomistic approach to compute transmission characteristics of Ti/graphene/Ti junctions. From these calculations we identify the key physical ingredients determining the transport features, and extract parameters to be used in a quantitative effective model that describes accurately the electronic structure and the transmission probabilities of charge carriers. This work complements our previous results in metal/graphene/metal junctions where the metal does not bond covalently to graphene (PRL {\bf 104}, 076807 (2010)).   

Transport in Metal/Graphene Tunnel Junctions

We present a technique to fabricate thin oxide barriers between graphene and Al and Cu to create tunnel junctions and directly probe graphene in close proximity to a metal. We map the differential conductance of our junctions versus probe and back gate voltage, and observe fluctuations in the conductance that are directly related to the graphene density of states. We develop a simple theory of tunneling into graphene to extract experimental numbers, and take into account the electrostatic gating of graphene by the tunneling probe. Results of measurements in magnetic fields will also be discussed, including evidence for incompressible states in the Quantum Hall regime.   

Quantum transport in high-quality Bilayer Graphene pnp Junctions

Using high-quality bilayer grapheme pnp junctions with suspended top gates, we perform transport measurements. At a magnetic field B=0, by an applied perpendicular electric field, band gap opens at 260mK. Within the band gap, we demonstrate the conductance decreases exponentially by 3 orders of magnitude with increasing electric field and this can be explained by variable range hopping with a gate-tunable density of states, effective mass, and localization length.   

Session H38: Focus Session: Quantum Coherence in Biology I

Indicators of quantum coherence in light-harvesting dynamics

Characterizing quantum dynamics of electronic excitations in a variety of light-harvesting systems is currently of much interest [1]. In particular, it is important to identify measures that appropriately quantify the strength of coherent dynamics and its impact on different time scales of the light-harvesting process. In this talk I will discuss quantum transport performance measures that are defined based on the probability for the dynamics to successfully distinguish different initial photo-excitation conditions. I will also discuss how initial state distinguisability can provide information on spatially correlated phonon fluctuations as well as on the non-markovian character of the quantum dynamics. The prototype systems considered here are cryptophyte light-harvesting antennae isolated from marine algae [2, 3]. Experimental quantification of state distinguishability can be realized by monitoring the evolution of selected off-diagonal density matrix elements and therefore it could be achieved with current two-dimensional spectroscopy techniques. \\[4pt] [1] A. Olaya-Castro and G. D. Scholes, ``Energy transfer from F\"{o}rster-Dexter theory to quantum coherent light-harvesting'', to appear in Int. Rev. Phys. Chem. (2010) \\[0pt] [2] E. Collini, C.Y. Wong, K.E. Wilk, P.M.G. Curmi, P. Brumer and G.D. Scholes, ``Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature'' Nature, 463, 644-647 (2010) \\[0pt] [3] A. Kolli, A Nazir, F. Fassioli, R. Dinshaw, G D Scholes, and A Olaya-Castro, ``Energy transfer dynamics in cryptophyte antennae proteins'', submitted for publication (2010)   

Phonon-mediated path-interference in electronic energy transfer

Motivated by the recent observations of quantum coherence in light-harvesting antennae, we present a formalism to quantify the contribution of path-interference in phonon-mediated electronic energy transfer. The transfer rate between two molecules is computed by considering the quantum mechanical amplitudes associated with pathways connecting the initial and final sites. This includes contributions from classical pathways, but also terms arising from their interference. By treating the vibrational modes of the molecules as a non-Markovian harmonic oscillator bath, we compute the first-order path-interference correction to the classical transfer rate. We show that the correction due to path-interference may have either a vibrational or an electronic character, and may exceed the contribution of the indirect classical pathways.   

Efficiency of the energy transfer in the FMO complex using hierarchical equations on Graphics Processing Units

We study the efficiency of the energy transfer in the Fenna-Matthews-Olson complex solving the non-Markovian hierarchical equations (HE) proposed by Ishizaki and Fleming in 2009, which include properly the reorganization process. We compare it to the Markovian approach and find that the Markovian dynamics overestimates the thermalization rate, yielding higher efficiencies than the HE. Using the high-performance of graphics processing units (GPU) we cover a large range of reorganization energies and temperatures and find that initial quantum beatings are important for the energy distribution, but of limited influence to the efficiency. Our efficient GPU implementation of the HE allows us to calculate nonlinear spectra of the FMO complex. References see www.quantumdynamics.de   

Efficient GPU calculation of 2D-echo spectra of excitonic energy-transfer in systems with large reorganization energy

Using the Fenna-Matthews-Olson light harvesting complex as example, we calculate the two dimensional echo spectra (2D echo) of a multi-site system coupled to phonon baths using the propagation scheme suggested by Ishizaki and Fleming in 2009 which works for large system-bath couplings. We study the anti-correlations in the shapes of the 2D spectrum peaks which are seen as evidence for exciton energy transfer. This computationally demanding calculation uses 2.6 h GPU (graphics processing unit) time compared to 2.8 weeks time on a high performance conventional CPU cluster. The efficient implementation of the exact hierarchical equations obliterates the need for approximative methods and facilitates the interpretation and comparison of theory and experiment for systems with large reorganization energies. References see www.quantumdynamics.de   

Simulation of dissipative quantum dynamics in the presence of strongly-interacting and structured environments: a many-body approach to memory effects

Quantum systems which interact strongly with complex and structured environments are receiving increasing attention due to their importance in contexts such as solid-state quantum information processing and bio-molecular quantum dynamics. Unfortunately, these systems are difficult to simulate as the system-bath interactions cannot be treated perturbatively, and standard approaches are invalid or inefficient. Here we combine time-dependent density matrix renormalization group methods with techniques from the theory of orthogonal polynomials to provide an efficient method for simulating open quantum systems at zero and finite temperatures. Using this technique we demonstrate a number of novel dynamical effects which result from long bath memories induced by either sharp spectral structures or strong coupling, and comment on how these can be exploited to drive efficient transport in small networks. We also show how our technique can be used to find the equilibrium properties of excitations in strongly renormalizing environments, and present some results on the quantum phase transition in the sub-Ohmic spin-boson model.   

Optimal Excitation energy transfer dynamics in light-harvesting systems

With the facilitation of surrounding protein environments, excitation energy transfer (EET) in photosynthetic systems can be highly efficient and robust. This talk compares different descriptions of dissipative exciton dynamics, discusses the generic mechanism of optimal energy transfer, and explores its implications for light-harvesting systems. (i) The generalized Bloch-Redfield equation provides a reliable description of exciton dynamics over a broad range of parameter space. (ii) The generic mechanism of optimal efficiency allows us to examine the interplay of quantum coherence, dynamics noise, and static disorder in a unified conceptual framework. (iii) The topological symmetry and network structures in photosynthetic systems reveal useful insights for the optimal design of artificial energy transfer systems.   

A quantum landscape study of energy transfer efficiency in light-harvesting complexes

Over billion years of evolution some photosynthetic complexes have turned into highly efficient light energy harvesting systems. In this work, we demonstrate optimality and robustness of energy transfer in the Fenna-Matthews-Olson (FMO) protein complex with respect to all the relevant parameters of system and environmental interactions. To this end we developed an efficient technique for studying the dynamics of energy transfer in a non-Markovian and non-perturbative regime. For the FMO protein of green sulfur bacteria we find that all the relevant natural parameters to lay within the optimal and robust regimes of energy transfer process. This suggests a peculiar interplay of internal and external forces in order to have a system that functions optimally while being robust under physiological conditions.   

Fast and efficient excitation transfer across disordered molecular networks

In this talk, we will present our statistical investigations on coherent excitation transfer through finite-size disordered molecular networks. As we have found, there exist certain molecular conformations that exhibit fast and highly efficient transport -- mediated by constructive quantum interference. We will discuss the properties of these optimal conformations which go along with the enhancement of efficiency. These insights may be relevant for explaining efficient energy transfer in the photosynthetic FMO complex.   

Regenerative quantum coherence in photosynthesis under natural conditions

Recent experiments provide compelling evidence for the feasibility of quantum coherent beating in photosynthetic light harvesting complexes, even at room temperature. However, whether this coherence arises \emph{in vivo} and its biological function (if any) have remained unclear. Here we present theoretical evidence for the creation and regeneration of electronic coherence under natural conditions. We show how such regenerated coherence may contribute to energy transfer efficiency in the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria.   

Enhanced exciton diffusion length via cooperative quantum transport

The energy transfer rate in biomolecular systems is typically calculated from the transition probability of an excitation hopping from one molecule to another using F\"orster energy transfer based on dipole-dipole interaction of individual molecules in the perturbative regime. However, due to strong interactions of among a group of molecules the excitation can become highly delocalized leading to an effective large dipole moment with an enhanced oscillator strength. Under certain symmetries, this could lead to an enhancement in exicton transfer rate via cooperative donation or acceptance of an excitation. Here, we explore this phenomenon in various multichromophoric geometries, under different symmetries, initial conditions, and dynamics. We study the behavior of the exciton diffusion length under the effects of disorders and environmental fluctuations and quantify the crossover from ballistic to diffusive regimes. Specifically, for a quasi-1 D array of rings containing N chromophores interacting with a bosonic bath, an interplay of time scales dictates the exciton dynamics. In the ``far-field'' regime, environmental interactions are dominating and the system properties are approaching those of the incoherent equilibrium Gibbs state. However, in the ``near-field'' the coherent interactions among dipole aggregates dominate other time scales and exciton diffusion length is enhanced by a factor of $\sqrt{N}$.   

Concatenated quantum codes in biological systems

This talk investigates how biological systems such as photosynthetic bacteria use quantum coding techniques such as decoherent subspaces, noiseless subsystems, and concatenated quantum codes to engineer long exitonic lifetimes and rapid energy transport. The existence of hierarchical structures in photosynthetic complexes is associated with concatenated quantum codes. A concatenated code is one that combines two or more codes to construct a hierarchical code that possesses features of all its constituent codes. In photosynthetic complexes, structures at the smallest level use quantum coding techniques to enhance exciton lifetimes, and structures at higher scales possess symmetries that enhance exciton hopping rates. The result is a concatenated quantum code that simultaneously protects excitons and enhances their transport rate. All known quantum codes can be described within the framework of group representation theory. This talk reviews the relationship between symmetry and quantum codes, and shows how photosynthetic bacteria and plants put quantum coding techniques to use to improve the efficiency of photosynthetic transport.   

A possible mechanisms for quantum coherence assisted ion transport in ion channels

Recently it was demonstrated that long-lived quantum coherence exists during excitation energy transport in photosynthesis. It is a valid question up to which length, time and mass scales quantum coherence may extend, how to one may detect this coherence and what if any role it plays for the dynamics of the system. Ion-channels are involved in many physiological processes. In the nervous system their coordinated opening and closing generates action potentials that form the basis for intra-neural communication which are essential for information representation and processing. We have recently suggested that the selectivity filter of ion channels may exhibit quantum coherence which might be relevant for the process of ion selectivity and conduction. I will discuss some of our current experimental efforts in this direction and show that quantum resonances could provide a viable approach to probe these quantum coherences. The emergence of resonances in the conduction of ion channels that are modulated periodically by time varying external fields can serve as signatures of quantum coherence in such a system.   

Session H39: Focus Session: Physics of Cancer

Nanotechnology Approaches to Studying Epigenetic Changes in Cancer

Placing polyelectrolytes into confined geometries has a profound effect on their molecular configuration. For instance, placing long DNA molecules into channels with a cross-section of about 100 nm$^2$ stretches them out to about 70\% of their contour length. We are using this effect to map epigenetic changes on single DNA and chromatin strands. This mapping on single molecules becomes central in the study of the heterogeneity of cell population in cancer, since rapid change of epigenetic makeup, propagated through rare cancer stem cells, is a hallmark of its progression. We demonstrate the basic building blocks for the single-molecule epigenetic analysis of genomic sized DNA. In particular, we have achieved the mapping of methylated regions in DNA with heterogeneous 5-methyl cytosine modification using a specific fluorescent marker. We further show that chromatin with an intact histone structure can be stretched similar to DNA, and that the epigenetic state of histone tails can be detected using fluorescent antibodies.   

Cancer Evolution under Drug-Induced Stress-Gradients

The lack of long term success in eliminating cancer cells while avoiding the evolution of drug resistance indicates that our understanding of how cells evolve in response to stress is still incomplete. We interpret this not as a failure of the current approaches, but rather as an indication that new research venues should be undertaken, where conventional wisdom is challenged in order to drive forward our understanding of cancer. Of particular importance, we believe that the powerful role of evolution in the origin of drug resistance is ill-understood. We do not ask whether evolution occurs, but rather how. We do not describe molecular mechanisms underlying drug resistance at the single cell level, but rather ask how does resistance spread in cancerous tissues and metastatic lesions. We attempt to answer these questions by studying the population-wide dynamics of drug evolution and the collective stress response of cancer cells in a microfluidics device. We use microfluidics technologies to impose high levels of stress on cancer cell metapopulation by create smoothly varying gradients of either oxygen, chemotherapeutic drug, nutrient or pH. We present long-term studies of the adaptation of tumorigenic cancer cells to drug- induced stress gradients.   

Are biomechanical changes necessary for tumor progression?

With an increasing knowledge in tumor biology an overwhelming complexity becomes obvious which roots in the diversity of tumors and their heterogeneous molecular composition. Nevertheless in all solid tumors malignant neoplasia, i.e. uncontrolled growth, invasion of adjacent tissues, and metastasis, occurs. Physics sheds some new light on cancer by approaching this problem from a functional, materials perspective. Recent results indicate that all three pathomechanisms require changes in the active and passive cellular biomechanics. Malignant transformation causes cell softening for small deformations which correlates with an increased rate of proliferation and faster cell migration. The tumor cell's ability to strain harden permits tumor growth against a rigid tissue environment. A highly mechanosensitive, enhanced cell contractility is a prerequisite that tumor cells can cross its tumor boundaries and that this cells can migrate through the extracellular matrix. Insights into the biomechanical changes during tumor progression may lead to selective treatments by altering cell mechanics. Such drugs would not cure by killing cancer cells, but slow down tumor progression with only mild side effects and thus may be an option for older and frail patients.   

A cellular automaton model for tumor growth in heterogeneous environment

Cancer is not a single disease: it exhibits heterogeneity on different spatial and temporal scales and strongly interacts with its host environment. Most mathematical modeling of malignant tumor growth has assumed a homogeneous host environment. We have developed a cellular automaton model for tumor growth that explicitly incorporates the structural heterogeneity of the host environment such as tumor stroma. We show that these structural heterogeneities have non-trivial effects on the tumor growth dynamics and prognosis.   

Microfabricated Tepui: probing into cancer invasion, metastasis and evolution in a 3D environment

Cancer metastasis and chemotherapeutic resistance are the major reasons why cancer remains recalcitrant to long-term therapy. We are interested to know: 1. How cancer cells invade tissues and metastasize in a 3D spatial environment? 2. How cancer cells evolve resistance to chemotherapeutic therapy? Answering these fundamental questions will require spatially propagating cancer cells in a 3D \textit{in vitro }micro environment with dynamically controlled chemical stress. Here we attempt to realize this micro environment with a three-dimentional topology on a micro-chip which consist of isolated highlands (Tepui) and deep lower lands. Cancer cells are patterned in the lower lands and their spatial invasion to the mesas of Tepui is observed continuously with a microscope. Experiments have demonstrated that the cell invasion potential is time dependent, which is not only determined by cell motility, but also cell number and spatial stress. Quantitative analysis shows that the invasion rate fits logistic equation. Further more, we have also imbedded collagen based Extracellular Matrix (ECM) inside these structures and established a robust chemical gradient in a vertical space. With merit of real-time confocal imaging, cell propagation, metastasis and evolution in the 3D environment are studied with time as a model for cell behavior inside tissues.   

Study of breast cancer cell behavior under chemical stress using microfluidic gradient generator

Understanding the behavior of cancer cells in gradients of chemotherapeutic agents is important in studying the evolution of cancer drug resistance. Compared to traditional in-vitro methods, microfluidic gradient generators better control temporal and spatial profile of gradients. However, maintaining chemical gradients requires high flow rate of liquid (10ul/hr) in microfluidic chip while culturing mammalian cells demands slow flow rate of liquid (1ul/hr). In this paper, we modify a microfluidic gradient generator (Jeon et al, Langmuir, 2001) to overcome the challenge of maintaining slow flow rate and stable gradients simultaneously based on numerical simulations, and culture metastatic breast cancer cell line (MDA-MB-231) in the chip. To characterize the stability of gradients, we visualize the gradient profile by infusing fluorescein. Finally, we will report the response of the on-chip culture under the stress of chemical gradients, observing for cellular phenotypic changes including death, proliferation, morphology, and migration.   

Superficial dose distribution in breast for tangential radiation treatment of breast cancer

The superficial (0-2 cm) dose distribution in a cylindrical phantom is examined theoretically and experimentally when irradiated by tangential photon beams. The lateral superficial part of the phantom is shown to receive full dose beyond 2 mm whereas the build-up region is up to 7 mm where the beams enter. Eclipse AAA calculations agree well with the experimental and Monte Carlo data while Eclipse PBC underestimates the entrance dose the first 3-4 mm and fails to give a correct lateral dose close to the surface up to 10 mm depth. The performance of the Eclipse algorithms is evaluated in a number of clinical cases with Monte Carlo results. Examples are given to illustrate how differences in geometrical presentation of the body structure in the treatment planning system and the Monte Carlo simulation as well as the patient voxelization may affect the evaluation results.   

Origin of using cisplatin over transplatin for cancer treatment: An ab initio study

Eventhough cisplatin has been used as a chemotherapy anti-cancer drug for over 40 years the thermodynamics and kinetics of the reactions are still largely unknown. Cisplatin molecules are known to be attacked by water molecules before they react with DNA. As a result, two Cl atoms are eliminated. The active piece in the cell, therefore, is not cisplatin but (NH$_{3})_{2}$Pt$^{2+}$. To explain why only cisplatin but not transplatin functions as anticancer drug, we used first principles method to study the dechlorination process in cis- and transplatin. Although transplatin molecule is more stable than cisplatin by 0.52 eV, we found cisplatin to be more favorable for reaction due to the following reasons: 1) the energy cost to remove a Cl atom is less from cisplatin than transplatin. 2) cis-form (NH$_{3})_{2}$Pt$^{2+}$ derived from cisplatin with N-Pt-N angle of 97\r{ } is lower in energy than trans-form derived from transplatin with N-Pt-N angle of 180\r{ }. The rotation barrier for N-Pt-N changing from 180\r{ } to 97\r{ } is about 1.0 eV. 3) When cis-form of (NH$_{3})_{2}$Pt$^{2+}$ reacts with two Guanines in DNA, the two N atoms in Guanines can readily bind to the Pt atom in cisplatin. The transplatin due to steric reasons does not provide that opportunity.   

Actinomycin D binding mode reveals the basis for its potent HIV-1 and cancer activity

Actinomycin D (ActD) is one of the most studied antibiotics, which has been used as an anti-cancer agent and also shown to inhibit HIV reverse transcription. Initial studies with ActD established that it intercalates double stranded DNA (dsDNA). However, recent studies have shown that ActD binds with even higher affinity to single stranded DNA (ssDNA). In our studies we use optical tweezers to stretch and hold single dsDNA molecule at constant force in the presence of varying ActD concentrations until the binding reaches equilibrium. The change in dsDNA length upon ActD binding measured as a function of time yields the rate of binding in addition to the equilibrium lengthening of DNA. The results suggest extremely slow kinetics, on the order of several minutes and 0.52 $\pm $ 0.06 $\mu $M binding affinity. Holding DNA at constant force while stretching and relaxing suggests that ActD binds to two single strands that are close to each other rather than to pure dsDNA or ssDNA. This suggests that biological activity of ActD that contributes towards the inhibition of cellular replication is due to its ability to bind at DNA bubbles during RNA transcription, thereby stalling the transcription process.   

Session H40: Multi-cellular Processes and Development

Dynamics of asexual reproduction in planarians

Planaria research is experiencing a resurgence due to the development of molecular tools, the Planarian genome project and database resources. Despite the resulting progress in planarian biology research, an extensive study of their physical properties remains to be undertaken. We developed a method to collect a large amount of data on the dynamics of clonal reproduction in the freshwater planarian S.mediterranea. The capability of planarians to regenerate an entire organism from a minuscule body part is based on a homogeneously distributed stem cell population that comprises 25-30{\%} of all cells. Due to this stem cell contingent, planarians can reproduce spontaneously by dividing into a larger head and a smaller tail piece, which then will rebuild the missing body parts, including a central nervous system, within about a week. Time-lapse imaging allows us to characterize the fission process in detail, revealing the stages of the process as well as capturing the nature of the rupture itself. A traction force measurement setup is being developed to allow us to quantify the forces planarians exert on the substrate during reproduction, a macroscopic analog to the Traction Force Microscopy setups used to determine local cellular forces. We are particularly interested in the molecular processes during division and the interplay between tissue mechanics and cell signaling.   

Quantifying cell behaviors during embryonic wound healing

During embryogenesis, internal forces induce motions in cells leading to widespread motion in tissues. We previously developed laser hole-drilling as a consistent, repeatable way to probe such epithelial mechanics. The initial recoil (less than 30s) gives information about physical properties (elasticity, force) of cells surrounding the wound, but the long-term healing process (tens of minutes) shows how cells adjust their behavior in response to stimuli. To study this biofeedback in many cells through time, we developed tools to quantify statistics of individual cells. By combining watershed segmentation with a powerful and efficient user interaction system, we overcome problems that arise in any automatic segmentation from poor image quality. We analyzed cell area, perimeter, aspect ratio, and orientation relative to wound for a wide variety of laser cuts in dorsal closure. We quantified statistics for different regions as well, i.e. cells near to and distant from the wound. Regional differences give a distribution of wound-induced changes, whose spatial localization provides clues into the physical/chemical signals that modulate the wound healing response.   

Keeping track of embryo development: new insights in the coupling between local and global changes

Modern imaging techniques allow us to study biological systems such as Drosophila in vivo during early development. Between the ninth and fourteenth cell cycles of the Drosophila embryo, nuclei are positioned at the embryo's surface and are observed to divide at the end of each cycle in a highly synchronized fashion. We have implemented a new tracking technique that allows us to determine the shapes of the nuclei as they elongate and divide, and to follow their motion on the surface. We find that during each cycle, the nuclei shapes evolve with time in a consistent way from nucleus to nucleus. These shape changes spread as waves with a well-defined wave velocity through the embryo, coupling local (nucleus) and collective (entire embryo) development. The waves in turn induce collective motions of the nuclei, not just after division but also before it.   

A cellular Potts model of germband retraction and dorsal closure

Germband retraction and dorsal closure are critical morphogenetic events in fruit fly embryogenesis. Both involve the coordinated reshaping of two epitheloid tissues -- germband (GB) and amnioserosa (AS). The GB is initially curled into a U-shape with the AS between the arms of the U. Retraction leaves the embryo's dorsal surface covered by AS cells which then contract to pull lateral parts of the GB up to cover the dorsal surface. We have simulated these events using a cellular Potts model. The model is 3D with several generalized cell types: a central yolk; a surrounding monolayer of AS and GB cells with epithelial polarization; and an outer vitelline membrane enclosing the cells and a perivitelline fluid. The model also incorporates several critical cell behaviors: polarized apical constriction of AS cells; controlled relaxation of stretched GB cells; and differentiation of GB cells at the GB-AS interface so that these cells then contract a supracellular purse-string and extend filopodia that reach across the AS and zip together the GB's approaching lateral flanks. We will discuss how all of these components are necessary to reproduce normal tissue motions and those observed during laser microsurgery experiments.   

Developmental and Metabolite Transport Strategies to Optimize the Growth of Filamentous Cyanobacteria

Individual cells of filamentous cyanobacteria share nutrients through cytoplasmic and/or periplasmic connections. Under conditions of low fixed-nitrogen some cells terminally differentiate into heterocysts, which fix nitrogen for the remaining photosynthetic vegetative cells. Heterocysts are observed to occur in a regular pattern separated by clusters of vegetative cells. Using a quantitative model of nitrogen uptake, consumption and transport together with vegetative cell growth and division, we explore how the overall growth rate of the filament depends on different heterocyst positioning patterns and on particular strategies of nitrogen transport.   

Elasticity of developing cardiac tissue

Proper development and function of the heart from the tissue to cellular scale depends on a compliant ECM. Here we study the maturation of embryonic cardiac tissue mechanics in parallel with the effects of extracellular mechanics on individual cardiomyocyte function throughout early development. We used micropipette aspiration to measure local and bulk elastic moduli (E) of embryonic avian heart tissue from days 2-12. We observe stiffening of the early heart tube from E = 1 kPa at day 1 to E = 2 kPa at day 4, reaching neonatal values by day 12. Treating heart tubes with blebbistatin led to 30{\%} decrease in E, indicating a significant but partial actomyosin contribution to mechanics at these stages. We performed a proteomic analysis of intact and decellularized 2-4 day heart tubes by mass spectrometry to quantify the ECM present at these stages. Isolated cardiomyocytes from 2-4 day chick embryos were cultured on collagen-coated PA gels of various stiffnesses. Beating magnitude was modulated by substrates with E = 1-2 kPa, similar to physiological E at those stages.   

ECM ordering effects as a marker for early tissue formation on artificial substrates - a sum-frequency-generation spectroscopy study

The in situ monitoring of the interphase between a substrate and a cellular layer is of great interest as it allows determination of changes in surface properties and extracellular matrix (ECM) organization. The latter is an early indicator of major cellular processes like migration, adhesion, proliferation, metastasis, tissue formation, and gain or loss of differentiation. We demonstrated recently that vibrational sum-frequency-generation (SFG) spectroscopy can be used to probe the layer between living cells and a solid substrate. In this contribution we will report on the investigation of ordering phenomena within the ECM of fibroblasts allowing to track early stages of tissue formation. SFG spectroscopy offers a unique way to observe these correlated changes of order in real-time without the need of labeling or disruption.   

Distinguishing Pattern Formation Phenotypes: Applying Minkowski Functionals to Cell Biology Systems

Spatial Clustering of proteins within cells or cells themselves frequently occur in cell biology systems. However quantifying the underlying order and determining the regulators of these cluster patterns have proved difficult due to the inherent high noise levels in the systems. For instance the patterns formed by wild type and cyclic-AMP regulatory mutant \textit{Dictyostelium} cells are visually distinctive, yet the large error bars in measurements of the fractal number, area, Euler number, eccentricity, and wavelength making it difficult to quantitatively distinguish between the patterns. We apply a spatial analysis technique based on Minkowski functionals and develop metrics which clearly separate wild type and mutant cell lines into distinct categories. Having such a metric facilitated the development of a computational model for cellular aggregation and its regulators.   

Measuring the viscosity of embryonic epithelia \textit{in vivo} by magnetic tweezers

During early development, sheets of epithelial cells are reshaped by cellular forces. Several recent investigations in fruit fly (\textit{Drosophila}) embryos have used laser microsurgery and video force microscopy to measure these forces; however, these measurements are actually limited to force/viscosity ratios because the effective viscosity of epithelial cells in a living embryo is largely unknown. This effective viscosity may vary spatially within the embryo and temporally as development progresses. To address this issue, we use microinjection, magnetic tweezers and confocal microscopy to measure the effective viscosity of epithelial cells in fruit fly embryos \textit{in vivo}. We inject fluorescent magnetic beads (2-$\mu $m diameter) into GFP-labeled embryos at the multi-nuclear syncytial blastoderm stage. The beads are pulled to embryo's surface by a permanent magnet and become engulfed by individual epithelial cells during cellularization. During later stages of development, we supply current pulses to an electromagnet to apply force pulses to the beads with a magnitude of $\sim $100 pN. The effective viscosity is inferred from the movement of these beads as tracked by confocal microscopy. We will report initial results on amnioserosa cells during dorsal closure.   

Modeling of Endothelial Glyccalyx via Dissipative Particle Dynamics

We employ Dissipative Particle Dynamics (DPD) to simulate flow in small vessels with the endothelial glycocalyx attached to the wall. Of particular interest is the quantification of the slip velocity at the edge of glycocalyx and of the increased pressure drop at different crafting densities, stiffness and height of the glycocalyx. Results will be presented for capillaries and small arterioles, and interactions with discrete red blood cells will be included in the modeling. In addition to the physical insight gain for this important but relatively unexplored bioflow, simple models for the slip velocity will be proposed that can be used in continuum simulations of blood flow in micro-vessels.   

An approach to collective behavior in cell cultures: modeling and analysis of ECIS data

We review recent results in which statistical measures of noise in ECIS data distinguished healthy cell cultures from cancerous or poisoned ones: after subtracting the ``signal,'' the $1/f^\alpha$ noise in the healthy cultures shows longer short-time and long-time correlations. We discuss application of an artificial neural network to detect the cancer signal, and we demonstrate a computational model of cell-cell communication that produces signals similar to those of the experimental data. The simulation is based on the $q$-state Potts model with inspiration from the Bak-Tang-Wiesenfeld sand-pile model. We view the level of organization larger than cells but smaller than organs or tissues as a kind of ``mesoscopic'' biological physics, in which few-body interactions dominate, and the experiments and computational model as ways of exploring this regime.   

Session H41: Irving Langmuir Prize Session: Ultrafast Dynamics

Irving Langmuir Prize in Chemical Physics Talk: Attosecond Electron Dynamics

Isolated attosecond pulses are produced by the process of high order harmonics, and these pulses are used as a soft X-ray probe in wavelength-dispersed transient absorption. Inner shell core-level spectroscopic transitions are thus used to analyze the chemical and electronic environment of specific atomic states as a function of time following ionization and dissociation. High field ionization processes, using 800 nm pulses, result in spin-orbit electronic state populations, alignment, and electronic wave packet superpositions, all of which are investigated by the spectrally-resolved X-ray probe. By using isolated attosecond pulses as the probe, high field ionization events on a subfemtosecond timescale are investigated. The generality of the transient absorption method for attosecond dyamics is described, as well as the challenges during the pump-probe pulse overlap time period. The results are compared to theoretical calculations by collaborators.   

Time- and Angle-Resolved Photoemission Spectroscopy: Ultrafast Dynamics of Electronic Structure

Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental tool for condensed matter systems as it measures the single-particle spectral function. Using femtosecond laser pulses in a pump-probe scheme, ARPES can be extended into the time domain. Here we report the construction of a time-resolved ARPES (trARPES) system. We utilize a Ti:Sapphire oscillator to produce infrared pump pulses, while ultraviolet probe pulses are generated by frequency quadrupling. A hemispherical electron analyzer measures the photoemission spectrum as a function of pump-probe delay. We present results on Gallium Arsenide, which displays hot electron dynamics on two distinct timescales in the unoccupied states. Interestingly, the signal in the occupied states has high temporal contrast, resembling a step-function with dynamics at negative delays. These properties make Gallium Arsenide a versatile tool for trARPES system characterization, allowing for calibration of pump-probe temporal and spatial overlap, as well as determination of time resolution.   

Beyond the Frontiers of Time-Resolved Spectroscopy

Time-resolved spectroscopy experiments typically require the measurement of at least two (``pump'' and ``probe'') interactions of a field (e.g. a laser pulse) with the spectroscopic target system (e.g. atom, molecule) at variable but known temporal delays. It is often assumed that the shortest dynamics measurable with such techniques is on the order of the pulse duration of the pump and probe events. In this talk, it will be shown that attosecond electron wavefunction beating can, in principle, be resolved by employing a nonlinear interferometry concept with phase-stabilized femtosecond pulses that does not require attosecond pulses for pumping nor probing. The perfectly coherent and reproducible electric fields of the pump and probe pulses thus seem the ultimate technical goal to achieve the highest temporal resolution in science. By contrast, however, it will be shown that statistically varying (colored-noise) partially coherent pulses typically produced at free-electron lasers (FELs) can be beneficial in resolving dynamics beyond their average pulse duration. These findings may carry general implications for the future development of time-resolved spectroscopy.   

Quantitative imaging of ultrashort photoelectron pulse dynamics

Understanding and mitigating the space charge effects is a pressing issue in the development of ultrafast electron diffraction and imaging. Using a novel ultrafast projection imaging technique, quantitative imaging of transient space charge effects in the generation of high density ultrashort electron pulses is performed, which offers a means to directly compare with multi-electron calculations. We establish that the pulse width exhibits a fractional power-law scaling with the sheet density of the emitted electron pulses. By comparing to multi-electron simulations, the initial longitudinal phase space of the photoelectrons is extracted, demonstrating a strong dependence of the initial momentum spread on the sheet density. Multielectron effects are treated using a simple extension of single electron photoemission theory yielding qualitatively correct estimates of the quantum efficiency.   

Electron and ion dynamics in the melting of two-dimensional charge density waves

The cause of local lattice distortion in the formation of charge density waves (CDW) in 1D materials is often attributed to the Peierls mechanism, while for 2D system, such as CeTe3, it is not precisely known, due to imperfect nesting of the Fermi surface and a rather large CDW gap observed. Using ultrafast electron crystallography, the femtosecond electronic melting and recrystallization of CDW is investigated by following the superlattice peaks (order parameter) originated from the long-range charge ordering and the accompanying lattice distortion. We find that the reconstitution of CDW is subject to a bottleneck effect that can be attributed to the distinctively separated dynamical properties of electrons and ions in the short time scale, revealing the complexity of 2D CDW formation.   

Nonperturbative Rydberg excitations triggered by electrons or photons

Recently investigated processes with autoionizing Rydberg atoms or molecules will be discussed. In one class of processes, two or more Rydberg state are dressed by a laser field that couples them nonperturbatively, after which the coupled states are subsequently probed by XUV photons in a transient absorption experiment. This class will be discussed in the context of two recent experiments involving doubly-excited autoionizing states of atomic helium. In the second class of processes, the Rydberg states are initially created when electrons collide with molecular ions in a plasma environment, then get trapped temporarily in a high Rydberg state after giving up part of their energy to vibrational or rotational degrees of freedom. The Rydberg molecules then have competitive decay pathways, via photon emission, autoionization, or dissociation. The theory will be discussed in the context of experiments that bear on this second class of dynamical processes, which have been performed in Berkeley and also in Prague.   

Molecular structures studied using laser induced electron diffraction

In a strong laser field, the field ionized electrons from molecules can be returned to the parent molecules by the laser field. These electrons are then scattered off their parent ions. Such phenomena can be used to study molecular structures like the conventional electron diffraction technique, with much better temporal resolution (a few femtoseconds). In this study, we demonstrated its capability to retrieve static structures of molecules using a simple experimental setup. We obtained electron diffraction patterns from spatially aligned oxygen, nitrogen and carbon dioxide in a strong laser field with intensity around 7*10$^{13}$ W cm$^{-2}$. Excellent energy and angular resolutions were achieved by using velocity map imaging detection of electron momentum. The analysis shows that in order to extract the structure information, two kinds of interferences have to be considered: in the first kind, the electrons are ionized and scattered from the same atom; in the second kind, they are ionized from one atom but scattered off another atom in the molecule. We were able to account for the main features in the diffraction patterns of all three molecules and thus obtained the internuclear distances. In the future, we will apply this technique to retrieve structures of polyatomics and also plan to study molecular dynamics exploiting its superb temporal resolution.   

Session H42: Focus Session: Polymers for Energy Storage and Conversion -- Physics of Ion Conductivity in Polymers

Broadband Dielectric Spectroscopy and Quasi-Elastic Neutron Scattering on Single-Ion Polymer Conductors

The application of solid polymer electrolytes in rechargeable batteries has not been fully realized after decades of research due to its low conductivity. Dramatic increases of the ion conductivity are needed and this progress requires the understanding of conduction mechanism. We address this topic in two fronts, namely, the effect of plasticizer additives and geometric confinement on the charge transfer mechanism. To this end, we combine broadband dielectric spectroscopy (BDS) to characterize the ion mobility and quasi-elastic neutron scattering (QENS) to quantify segmental motion on a single-ion model polymer electrolyte. Deuterated small molecules were used as plasticizers so that the segmental motion of the polymer electrolyte could be monitored by QENS to understand the mechanism behind the increased conductivity. Anodic aluminum oxide (AAO) membranes with well defined channel sizes are used as the matrix to study the transport of ions solvated in a 1D polymer electrolyte.   

A Quasi Elastic Neutron Scattering study of polymer dynamics in PEO based sulfonate ionomers: Effect of ion content and ion identity

We present Quasi Elastic Neutron Scattering (QENS) data for characterizing dynamics in ion containing polymers (ionomers) with varying ion content (cation to ether oxygen ratio) and ion identity. To remove electrode reverse polarization, the anion is immobilized by covalently bonding it to the PEO backbone through an `ionizable' isophthalate co-monomer unit and only the cation contributes to the conductivity. We vary the ion content in two ways: changing the ratio of neutral to ionized co-monomer units, and changing the length of the PEO spacer separating the co-monomer units. In neutral ionomers, we observe two segmental processes; PEO segments in the spacer midpoint are one order of magnitude faster than those near the isophthalate groups. In ionized samples, cross-linking between ionic groups considerably slows the dynamics of PEO segments near the isophthalate group. The extent of cross linking depends on the ion content and spacer length. This effect is also ion dependent, which indicates that cations have different binding capacities and formation of this complex controls the availability of free cations for conduction.   

Structure and Dynamics of Proton-Conducting Azoles Confined within Metal-Organic Frameworks

Efficient polymer electrolyte membrane (PEM) fuel cells are one of the most promising candidates to power our vehicles of the future. Hydrated sulfonated polymers are currently the preferred membrane material because of their excellent conductivity and gas diffusion characteristics. The intrinsic water dependence in these systems limits the operating temperature to 100 C, leading to reduced electrode kinetics and increased CO poisoning. If water can be replaced by a small molecule with a higher boiling point, the overall efficiency of the system can be improved. To this end, we have investigated a set of new host/guest materials based on metal-organic frameworks (MOFs) loaded with a variety of azoles. The thermally and chemically stable frameworks provide a well-defined porous structure that accommodates the proton conduction pathways formed by the azole networks. We will present the structure of the azole networks as well as insight into the proton motion dynamics as a result of a variety of neutron scattering experiments.   

Ion solvation thermodynamics in polymer blends and block copolymers

There is much current interest in ion-containing polymers as materials for energy applications. For example, a promising system for rechargeable battery applications consists of diblock copolymers of an ion-dissolving block, typically polyethylene oxide (PEO) and a nonconducting block such as polystyrene. The addition of lithium salts has been shown to significantly alter the order-order and order-disorder transition temperatures, which reflects a change in the miscibility between the two polymer blocks. In this talk, I discuss some simple theoretical ideas for explaining and predicting the change in polymer miscibility due to the addition of salt ions for both polymer blends and block copolymers. A key effect is the solvation energy of the ions by the polymers, which we approximate using the Born solvation model. The difference in the Born energy of the ions between different polymers provides a driving force towards phase separation, whereas the translational entropy of the ions favors keeping the polymers mixed. In the case of lithium salts added to systems containing PEO, we develop a complexation model in which the lithium ions are tightly bound to the oxygen groups in the EO monomers, while the anions can either be free or form ion pairs with the lithium. For PEO-PS blends or block copolymers, we show that adding lithium salts leads to significant increase in the effective $\chi$ parameter between the two polymers. Our theory predicts that the effect should weaken with increasing radius of the anion, in agreement with available experimental data. Furthermore, we show that the domain spacing in microphase separated block copolymers should increase, also in agreement with experiments. We also examine the issue of ion distribution using self-consistent field theory.   

Understanding Ion Transport in Plasticized Polymer Electrolytes using Dielectric Spectroscopy

A challenge facing the development of new renewable energy storage materials is the low ionic conductivity within polymer matrices. Most materials development must overcome two main hurdles: Increase the ionic mobility and maximize the conducting ion concentration. The main role of small molecule plasticizers is not only to improve flexibility and segmental motion, which consequently lowers the T$_{g}$ and increase ion mobility, but also to solvate the counterion through some specific interaction, which increase the conducting ion content. In this study, we add plasticizers to polysiloxane-based ionomers that have anions covalently attached to the polymer chain, with Li$^{+}$ counterions. Using the 1953 Macdonald model it is possible to separate the conductivity of plasticized ionomers into the number density of conducting ions and their mobility, allowing us to quantify these vital quantities as functions of plasticizer content and temperature.   

Ionic conductivity of mesoporous block copolymer membranes in liquid electrolyte as a function of copolymer and homopolymer molecular weight

Mesoporous block copolymer membranes have been synthesized using poly(styrene-block-ethylene-block-polystyrene) (SES). A series of symmetric SES copolymers and PS homopolymers have been studied at different blending fractions. Ionic conductivities of the porous films in a liquid electrolyte, 1.0 M LiPF$_{6}$ in ethylene carbonate/diethyl carbonate, compare favorably to conventional battery separators and generally increase with internal surface area, as measured by nitrogen adsorption. Characterization of the effects of pore structure and SES morphology on conductivity will be presented.   

Cation/Anion Associations and Transport in Ionic Polymer Membranes

Ionic polymer membranes and ionic liquids (ILs) find fruitful applications in a range of ion conduction applications, from electromechanical ``artificial muscles'' to organic batteries. Various intermolecular interactions determine local structure and dynamics in these ion-dense media. In particular, ion aggregation can drastically affect ion transport, especially since neutral species (dipoles, quadrupoles...) will not be driven by electric fields. We are investigating mixtures of different ILs, ILs with water, and ILs swollen into ionomer membranes, using pulsed-gradient NMR to probe diffusion and electrophoretic mobility. We observe strong dependencies of the cation/anion diffusion coefficient ratio (ranging from 3X to 0.25X) on mixture and membrane properties, which we relate to ion association phenomena. We will further discuss NMR for transport and dynamics studies, especially regarding chemically resolved transport of various mobile species, probing a range of length and time scales, and quantifying ion aggregation.   

Correlation between cation conduction and ionic morphology in a PEO-based single ion conductor

We use molecular dynamics simulation to study ion transport and backbone mobility of a PEO-based single ion conductor. Ion mobility depends on the chemical structure and the local environment of the ions, which consequently impact ionic conductivity. We characterize the aggregation state of the ions, and assess the role of ion complexes in ionomer dynamics. In addition to solvated cations and pairs, higher order ion clusters are found. Most of the ion clusters are in string-like structure and cross-link two or more different ionomer chains through ionic binding. Ionic crosslinks decrease mobility at the ionic co-monomer; hence the mobility of the adjacent PEO segment is influenced. Na ions show slow mobility when they are inside large clusters. The hopping timescale for Na varies from 20 ns to 200. A correlation is found between Na mobility and the number of hops from one coordination site to another. Besides ether oxygens, Na ions in the ionomer also use the anion and the edge of the cluster as hopping sites. The string-like structure of clusters provide less stable sites at the two ends thus ions are more mobile in those regions. We observed Grotthus like mechanism in our ionomer, in which the positive charge migrates within the string-like cluster without the cations actually moving.   

Decoupling Between Ionic Conductivity and Segmental Dynamics in Polymers

The idea of solid polymer-based electrolytes (SPE) with high ionic conductivity at room temperature is known in scientific community for more than three decades. The interest is caused by unique advantages these materials may offer: mechanical flexibility, high power density, enhanced environmental and operational safety, etc. However, even after several decades of studies, the main challenge remains -- there is no ``dry'' SPE with conductivity of $\approx $ 10$^{-2}$ -- 10$^{-3}$ S/cm at room temperature. Ionic conductivity is controlled by two parameters, number of ions and their diffusion. Traditional views relate the diffusion of ions in a polymer to the segmental relaxation. Thus, when segmental dynamics freeze the ion motion halts, leading to low conductivity in solid state. A very good example of a material with such behavior is poly(ethylene oxide). In this work we demonstrate that the temperature dependence of ionic conductivity and segmental relaxation can be decoupled in a material specific way. Degree of the observed decoupling exhibits strong correlation with the steepness of the temperature dependence of structural relaxation (fragility). We predict that more fragile materials can have higher ionic conductivity in the solid state than the strong polymers (e.g. PEO).   

Molecular dynamics simulations of ionic aggregates in a coarse-grained ionomer melt

Ionomers (polymers with a small fraction of covalently bound ionic groups) have potential application as solid battery electrolytes. Understanding ion transport is essential for such applications. A key question is how molecular properties affect ionic aggregation and counterion dynamics. Recent experimental advances allowed synthesis and extensive characterization of ionomers with a precise spacing of charged groups, which is ideal for comparison with simulations. We use coarse-grained molecular dynamics to simulate ionomers with charged beads placed periodically either in the polymer backbone or pendant to the backbone. The polymers, along with counterions, are simulated at melt densities. Pendant ions at low dielectric form roughly spherical aggregates with liquidlike interaggregate order, qualitatively different from the aggregate morphology of analogous linear ionomers. The effects of dielectric constant and backbone spacing of charged beads on ionic structure and diffusion will be discussed.   

Session H43: Focus Session: Thin Film Block Copolymers II

Directed self-assembly of Si-containing block copolymer thin films in topographical templates

Block copolymer films in which one block contains Si are attractive for nanolithographic applications due to the high etch contrast and etch resistance of the Si-rich block. We describe the microphase separation of thin films of such polymers on topographic templates made either by electron-beam writing or by etching of another block copolymer film. The self-assembled morphology is governed by the commensurability between the block copolymer and the template, and both periodic and aperiodic patterns such as meanders and junctions can be directed by appropriate template designs. Different morphologies can be formed in one block copolymer film by sequential solvent anneal steps. Results for directed assembly of diblock copolymers and triblock terpolymers are understood through 3D self consistent field theory modeling.   

Thermal Manipulation of Block Copolymer Morphology by Focused Laser Spike (FLaSk) Annealing

Block copolymer (BCP) thin films have a high potential as a pattern transfer medium for ultra-fine ($<$10nm) features. We introduce a novel technique for performing rapid local annealing of BCP films by focused laser spike (FLaSk) heating using visible wavelengths. This process may be viewed as imposing a local instantaneous landscape in both block mobilities and interaction parameters corresponding to the temperature profile. By controlling the duration and intensity of the dose, either the rapid local perfection of the equilibrium microdomain morphology or the controlled incorporation of metastable architectures is possible. Moreover, the ultra-short FLaSk process can limit polymer degradation, allowing faster microdomain manipulation by enabling higher temperature anneals. Utilization of a direct write stage allows for deliberate control of arbitrary thermal patterns and subsequent BCP ordering, with line width near the diffraction limit. FLaSk can be applied to nearly any BCP system and performed with other ordering techniques. Direct write experiments were combined with thermal finite elements simulations to probe the various material and process parameters necessary to enhance control of spherical and cylindrical BCPs and address challenges such as the use of thicker films.   

Tunable Cosolvent Annealing Affects on Block Copolymer Morphology

Being able to precisely and reproducibly control block copolymer (BCP) morphology is of interest for lithographic applications due to the techniques ability to result in feature sizes ranging from 10-100nm. We explore the morphological phase behavior that thin films (30-40nm) of poly(styrene-b-dimethylsiloxane) (PS-PDMS, 45kg/mol, $\sim $0.26 segmental Flory-Huggins interaction parameter) exhibit under different cosolvent vapors of toluene and heptane. Variation in the solvent conditions results in selective swelling of the different blocks of the copolymer depending on relative Hildebrand solubility parameters (e.g. PS- 18.5, toluene-18.3 (MPa)$^{1/2}$) resulting in cylinders, spheres, lamella, and perforatted lamella self-assembled features which can be revealed by selectively etching the PS with an oxygen plasma (50W CF4). Here we describe precision solvent vapor control while doing in situ spectral reflectometry (230-1500nm) to track swelling of the BCP films as a function of time to gain insight into this BCP system.   

Annealing Techniques for Obtaining Ordered Morphologies in Poly(methacrylic acid)-Poly(methyl methacrylate) Diblock Copolymer Thin Films

The microphase separation of block copolymers in thin films continues to be of great value for the fabrication of nanostructured materials. While highly ordered arrays of microdomains can be easily achieved in some block copolymers, proper processing of others are more challenging. Obtaining ordered morphologies in poly(methacrylic acid)-poly(methyl methacrylate) (PMAA-PMMA), a diblock possessing polyelectrolyte functionality, offers unique associative properties and aqueous reaction chemistries otherwise inaccessible by most other block copolymer films. Due to the limited choices of suitable solvents with sufficiently high vapor pressure and the thermal degradation temperature of PMAA being lower than its glass transition temperature, direct solvent and thermal annealing of PMAA-PMMA are not ideal for generating ordered nanostructures. Here, we begin by solvent annealing poly(tert-butyl methacrylate)-poly(methyl methacrylate) (PtBMA-PMMA) films at room temperature. We then thermally anneal the films to convert the PtBMA block to PMAA. We present results from atomic force microscopy (AFM) and grazing-incidence small-angle x-ray scattering (GISAXS) studies.   

High-Speed Block Copolymer Self-Assembly under Ambient Conditions

Self-assembled block copolymer (BC) nanopatterns have critical application impacts on nanotemplates and scaffolds for the fabrication of nanometer scale periodic arrays, nanostructured networks and membranes for fuel cells, and high-density information storage media in computers and related devices. To achieve such application potentials, well-aligned BC nanopatterns should be reliably engineered in a thin film on a variety of functional substrates within a practical time-scale for industrial production. Here, we illustrate an exceedingly high-speed BC self-assembly under ambient conditions, which is not readily achievable in a vacuum. Only in a few seconds, BC nano-cylinders perpendicular to an energetically preferential surface have been spontaneously developed in a thin BC film under air. The time-scale for the BC self-assembly under air is at least 1000 times faster than that under vacuum. However, a micro-scale film instability that seriously impairs BC nanostructures has also rapidly evolved under air prior to the lateral self-organization of BC nano-cylinders. To suppress the evolution of micro-scale film instability and also to enhance the lateral order of BC nano-cylinders, we have imposed geometric confinements during the thermal annealing process of a thin BC film. Consequently, only in a few minutes, we have prepared hexagonally well-aligned BC nano-cylinders perpendicular to the bottom surface of geometric confinements under ambient conditions.   

Directed self-assembly with density multiplication of block copolymer via controlled solvent annealing

We report density multiplication of chemically patterned template employing highly segregating polyhedral oligomeric silsesquioxane (POSS) containing block copolymer (PMMA-bPMAPOSS) for extending the technique to smaller dimensions than that attained by PS-b-PMMA. PMMA-b-PMAPOSS which self-assembles into hexagonally closed packed (hcp) array of dots with lattice spacing d=13nm was spin coated on the chemical template with doubled hcp lattice spacing d=26nm, and annealed under controlled CS$_{2}$ atmosphere. By tuning the swell ratio of PMMA-b-PMAPOSS, ordered array of dots with d=13nm, which correspond to 3.5Tbit/in$^{2}$ was obtained by multiplying pattern density of the chemical template in a factor of 4. This work was supported by New Energy and Industrial Technology Development Organization, Japan.   

Temperature Gradient effects in Directed Assembly of Block Copolymer Films via Cold Zone Annealing

Vertically oriented micro-phases of block copolymers (BCPs) are generally preferable for applications like organic photovoltaic devices and nanoscale lithography. Here we demonstrate a Cold Zone Annealing (CZA) technique that produces a very sharp thermal gradient in contrast to our previous studies that produced well-ordered parallel BCP microphases [1]. Under these conditions, initial experiments on cold zone annealed PS-b-PMMA BCP films, yielded long range vertical orientation order in PMMA cylinders. GISAXS analysis indicates that the vertical morphology is maintained throughout the film thickness. Comparison of the CZA with conventional oven annealed samples show a magnitude of improvement in the ordering of BCP phases. \\[4pt] [1] Berry et al., \textit{Nano Lett}., \textbf{7}, pg 2789 (2007)   

The dynamics of lamellar re-orientation in free-standing diblock copolymer films: flipping the morphology from edge-on to flat-on

Many exquisite structures formed by diblock copolymers have been studied rigorously over the past two decades. Using a symmetric polystyrene-poly(methyl methacrylate) diblock copolymer, we prepare thin films on a substrate which form lamellae oriented perpendicular to the film interfaces. These ``edge-on'' samples are subsequently transferred, by floating onto water, to produce free-standing films with a symmetric boundary condition. Upon annealing these free-standing films, the lamellae switch from edge-on, to ``flat-on'' such that the domains are oriented parallel to the interface. Using atomic force microscopy, we study the dynamics of pattern formation as lamellae flip from the edge-on to flat-on morphology.   

New approaches to directing self-assembly and alignment of block copolymer

Directed self-assembly of block copolymers (BCPs) has been explored extensively using a variety of methods to simultaneously develop long-range order and exert orientational control over microphase separated structures. Here we propose two new routes for directing self-assembly in BCPs. First we discuss solvent vapor permeation which is based on pressure driven transport of a solvent vapor through a free-standing film. We demonstrate that alignment of BCP interfaces parallel to the vapor flux may be achieved rapidly in mm-scale thick films of high molecular weight BCP. Secondly, we present the use of electrospray for controlled deposition of block copolymer thin films. We speculate that morphology can be dictated by thermal equilibration in the presence of a pre-existing pattern or substrate template and that the ultra-slow growth afforded by electrospray permits persistence of this pattern beyond the 1 micron scale where conventional surface directed morphologies degenerate.   

A novel approach to achieve perpendicular long range order alignment in lamella PS-b-PEO system

Here, we introduce a novel approach for obtaining perpendicular alignment in lamella forming PS-b-PEO system. The vertical alignment of layers in diblock copolymer thin films has great potential for producing nanowires used in nanofabrication of electronic devices. However, due to selective surface interaction of the polymers with the substrate, perpendicular alignment usually requires neutralisation of the surface by means of brushes or making pre-pattern substrates which could be complicated and time consuming. Applying our novel approach named ``combinatorial annealing'' which consists of two stages of thermal and solvent annealing process, we have successfully created parallel lines (without a brush). After selective etching of one block, the remaining template is pattern transferred to a silicon substrate leading to manufacturing of sub 20 nm silicon nanowires.   

Session H44: Surfaces, Interfaces, and Polymer Thin Films I

Diffusion and Filtration Properties of Self-assembled Close-packed Nanocrystal Membranes

Small dyes are known to be able to penetrate through randomly packed nanoparticle monolayers, but a detailed understanding of the mechanisms for transport through the interstices between nanoparticles is still lacking. We report on systematic measurements of molecular transport across monolayers of close- packed, 5 nm diameter gold nanocrystals ligated with dodecanethiol. For water we find a filtration coefficient two orders of magnitude larger than for polymer-based nanofiltration membranes, while the self-diffusion coefficient is more than 100x smaller than in films of pure hydrocarbons. As we confirm by molecular dynamics simulations, larger molecules (tested molecular weight range: 200 - 43000) are unable to diffuse through the ligands. Instead, they most likely move through nm-sized regions of reduced ligand density, which are formed by slight variations in the local packing configuration and orientation of neighboring nanocrystals. In this intermediate size range we also find a pronounced dependence of the rejection rate on the molecules' charge. Molecules with cross-section above 2 nm are totally rejected.   

Diffusion of Small Penetrants in Polybutadienes

The diffusion coefficient $D$ in the dilute limit for three different penetrants--- oxygen, water, and methanol---in three different conformations of polybutadiene (all cis-1,4, all trans-1,4, and a random copolymer containing 50\% trans-1,4, 40\% cis- 1,4, and 10\% vinyl-1,2 repeat units) has been computed using molecular dynamics simulations for temperatures in the range $T=300$--$400$\,K. Simulations runs of 25 and 50 ns made for each of the 45 combinations of penetrant, conformation, and temperature studied. Over this temperature range the density of the all-cis-1,4 conformation is higher than that of the all-trans-1,4 and random copolymer conformations, which are approximately equal. For all three conformations, $D$ for oxygen and water are comparable and larger than that of methanol. However, for a given penetrant, strong differences were observed in the rate of increase of $D$ for the three conformations. We find that the activation barriers for the three penetrants are generally between 20 and 30 kJ/mol, in agreement with experimental results. The magnitude of the activation energy is directly proportional to the size, rather than the mass, of the penetrant molecule. (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.)   

Control dispersion of water in thin films of semi-fluorinated polymer/POSS nanocomposites

The permeation and distribution of solvents in polymer nanocomposites is governed by the way the nanoparticles (NP) associate within the matrix polymer. We have previously shown that in thin films interfacial effects affect the distribution of the NP. The current work focuses on the response of a semi-fluorinated random copolymer, Biphenyl Perfluorocyclobutane, and Polyhedral Oligomeric Silsesquioxane (POSS) NP modified with fluorinated or protonated side chains, to presence of D$_{2}$O. POSS was introduced either as a free NP or tethered to a polymer chain. We found that the presence of POSS reduces the overall uptake of D$_{2}$O. It also changes the distribution of water in the film as well. In the pristine polymer film the water mainly accumulated at the substrate/polymer interface. In the nanocomposite, the water distribution is correlated with the NP distribution, where NP at the air interface minimize water penetration.   

Water Desorption from Ferroelectric and Dipole-Oriented Polymers

Herein we compare the water absorption/ adsorption on three different polymer films: the ferroelectric co-polymer poly(vinylidene fluoride with trifluoroethylene) P(VDF-TrFE), the strongly dipole oriented polymer poly(methyl vinylidene cyanide) (PMVC) [1] and the dipole oriented poly(methyl methacrylate) PMMA. We investigate the dipole-dipole interaction of the water molecule and the ferroelectric/ dipole oriented polymer films and we propose that the dipole interactions may affect the surface chemistry at these polymer surfaces. Surface dipoles can affect the binding site of water species adsorbed at the surface and sterically hinder or enhance desorption of adsorbed and absorbed water.\\[4pt] [1] Dowben, P.A., Rosa, Luis G., Ilie, C.C., \textit{Zeitschrift f\"{u}r Physikalische Chemie} 222 (2008) 755-778.   

Role of Diffusion in Scaling of Polymer Chain Aggregates Found in Vapor Deposition Polymerization

Linear polymer chain aggregates grown by 1+1D Monte Carlo simulations of vapor deposition polymerization (VDP) were studied. The behavior of chain length distribution $n_{s}(t)$ as a function of chain length (s) and deposition time (t) was examined for relevant model parameters. The scaling of $n_{s}(t)$ was found to be sensitive to the ratio $G = D/F$ of deposition rate (F) and free monomer diffusion (D). A systematic approach is presented to isolate the dependence of $n_{s}(t)$ on $t$, $s$, and $G$. We found power law dependence of $n_{s}(t)$ on $t$ with exponent $\omega=1.01 \pm 0.02$ that was invariant with changes in $G$. For small $s$ and deposition time of $t$ = $1 \times10^3$, $5 \times 10^3$, and $10 \times10^3$, $n_{s}(t)$ showed a power-law decrease with $s$ and exponent $\tau=-0.58 \pm 0.02$. We observed a strong influence of $G$ on the rescaled $n_{s}(t)$ data that prevented the manifestation of unique scaling function for varying $G$. The dependence of scaling function of $n_{s}(t)$ on $G$ was found to be a characteristic of VDP and elucidates the sensitivity of polymer chain aggregates to $G$.   

Structure and dynamics of dense polymer chains in 2D

Self-avoiding polymers in two-dimensional melts are known to adopt compact and segregated configurations. Compactness does obviously not imply Gaussian chain statistics nor does segregation of chains impose disk-like shapes minimizing the average perimeter length of the chains. Using scaling arguments and molecular dynamics simulations with chain length up to 2048 we show that the chain perimeters are highly irregular and characterized by a fractal line dimension 5/4. This result may be verified experimentally from the power-law scaling of the intrachain form factor in the intermediate wavevector regime in agreement with a generalized Porod law for a compact object of fractal border [1]. The dynamics of dense polymer chains exhibits two interesting features: the incompressibility induces long range correlations in the displacement auto-correlations and a relaxation channel due to friction at the fractal contours of compact sub-segments leads to relaxation faster than a Rouse model would predict [2].\\[4pt] [1] H. Meyer et al Phys. Rev. E 79 050802(R) (2009); J. Chem. Phys. (2010)\\[0pt] [2] J. Wittmer et al. Phys. Rev. Lett 105 (2010) 037802.   

Dynamic surface tension effects from molecular dynamics simulations

Effects of dynamic surface tension have been studied in a model system using molecular dynamics simulations. The model system has been made of an artificially expanding liquid droplet, with the rate of change of the external surface area being comparable with the gas-liquid interface formation characteristic time, obtained from the estimates of macroscopic theories. The size of the liquid droplet has been chosen to have about 5.000-7,000 identical molecules, each having between 10-20 beads, to obtain well developed and separated the bulk and surface phases. The methodology of surface tension evaluation has been verified against the Laplace Law in a stationary state of the liquid drop. The results of the MD simulations will be discussed in comparison with the estimations obtained from macroscopic experiments on dynamic wetting using a sharp interface formation theory for different chain length of molecules and strength of intermolecular interactions.   

Hierarchical roughness of sticky and non-sticky superhydrophobic surfaces

The importance of superhydrophobic substrates (contact angle $>$150\r{ } with sliding angle $<$10\r{ }) in modern technology is undeniable. We present a simple colloidal route to manufacture superstructured arrays with single- and multi-length-scaled roughness to obtain sticky and non-sticky superhydrophobic surfaces. The largest length scale is provided by (multi-)layers of silica spheres (1$\mu $m, 500nm and 150nm diameter). Decoration with gold nanoparticles (14nm, 26nm and 47nm) gives rise to a second length scale. To lower the surface energy, gold nanoparticles are functionalized with dodecanethiol and the silica spheres by perfluorooctyltriethoxysilane. The morphology was examined by helium ion microscopy (HIM), while wettability measurements were performed by using the sessile drop method. We conclude that wettability can be controlled by changing the surface chemistry and/or length scales of the structures. To achieve truly non-sticky superhydrophobic surfaces, hierarchical roughness plays a vital role.   

Alkane Self Assembling

Self-assembling of organic molecules has awaken scientific and technological interest. In this work we study the self-assembling process of long chain hydrocarbons, mainly $n$-dotriacontane ($n$-C32H66). We dip-coated C32 monolayers onto silicon wafers covered by their native silicon oxide layer (Si(100)/SiO2). Our results show that withdrawing speed affects the coverage and morphology of the C32 films. For slow withdrawing speeds, alkanes formed islands with a dragon-fly shape, while for fast withdrawing alkanes assembled in stripes with widths in the order of microns. When we quantified coverage and morphology versus withdrawing speed, we found an inflection, which we associate with a transition between two film deposition kinetics. These transitions have been previously described by de Gennes [1]. For slow withdrawing, film deposition follows the Langmuir-Blodget process and above a threshold speed, solution on the solid enters a Landau-Levich regime. This work opens the possibility for growing microstructures with nanometric thickness using a very simple method. These organic microstructures could be used as templates or as grids for optical diffraction. \\[0pt] [1] P.G. de Gennes, Colloid {\&} Polymer Sci. 264, 463-465 (1986).   

A Thermodynamic Treatment of Polymer Thin Film Glasses

We have recently developed a mean field equation of state (EOS) approach to model the thermodynamic properties of polymer thin films. The model is analytic and transparent yielding characteristic film properties as a ``whole sample'' average. We focus on the properties of freestanding thin films and, parameterizing only with bulk data, demonstrate how the EOS leads to predictions of film properties as a function of film thickness under varied thermodynamic conditions. We share some thoughts on how to use this model for the prediction of the thickness-dependent depression of the thin film glass transition temperature.   

Non-equilibrium behavior of spin-cast films

The behavior of polystyrene films cast from various solvents using an electric field to weakly perturb the free surface of the polymer melt was examined. The effective viscosity and residual stresses of the as-spun films strongly depend on the casting solvent. As-cast films had a substantially reduced viscosity compared to annealed films, with the greatest reduction in films cast from solutions near $\theta$-temperature. The reduced viscosity is explained in terms of non-equilibrium effects from the film formation process; rapid quenching during spin-coating results in a lower entanglement density of chains compared to an equilibrium melt. The difference in films spun from the various solvents is explained by changes in chain conformations in the initial solutions and the vitrification point. The wavelength of the instabilities in as-cast films was higher than expected, indicating a weak stabilizing pressure. This is attributed to frozen-in normal stresses resulting from an asymmetric deformation of the chains due to evaporation of residual solvent after vitrification. The results show the non-equilibrium nature of as-cast polymer films and that processing conditions strongly influence their behavior.   

Session H45: Exotic Quantum Phases in Optical Lattices: FFLO, P-band Physics, and Beyond

Stable Fulde-Ferrell-Larkin-Ovchinnikov pairing states in 2D and 3D optical lattices

We present the study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states in the $p$-orbital bands in both two and three-dimensional optical lattices. Due to the quasi one-dimensional band structure which arises from the unidirectional hopping of the orthogonal $p$-orbitals, the pairing phase space is not affected by spin imbalance. Furthermore, interactions build up high dimensional phase coherence which stabilizes the FFLO states in 2D and 3D optical lattices in a large parameter regime in phase diagram. These FFLO phases are stable with imposing the inhomogeneous trapping potential. Their entropies are comparable to those of the normal states at finite temperatures.   

Robust Larkin-Ovchinnikov-Fulde-Ferrell phases in a wide class of lattice models

We consider BCS pairing of fermions on lattice whose normal state breaks both time-reversal and spatial inversion symmetries. Due to the asymmetric band structure, unusual pairing states exist: Cooper pairs condense at finite momentum, which is known as the Fulde- Ferrel-Larkin-Ovchinnikov (FFLO) state. A one-dimensional lattice model of spinless fermions is studied in detail and two types of FFLO states are found: (1) a FF state with spontaneous supercurrent and (2) a nodeless LO state where the amplitude of order parameter oscillates. This conclusion is obtained via mean-field theory, bosonization, and exact diagonalization. The transition between the two phases can be tuned by the filling. We also find that the FF state is a topological superconductor. We further consider a generalization to two dimensions, where similar physics is realized.   

Spectral Functions of FFLO states in coupled chains

Polarized Fermi gases hold the possibility of an exotic and fragile modulated superfluid known as a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Quasi-one-dimensional systems of ultracold fermions are the ideal place to look for FFLO physics. Using various methods [1] (including determinant quantum Monte Carlo, stochastic Green function, and Bogoliubov-de Gennes methods), we study the correlation functions and quantum dynamics of polarized Fermi gases in single chains and coupled chains. Our results indicate that fluctuating domain walls lead to spectral weight near the Fermi energy in the spin-resolved density of states, that are a signature of Andreev reflections and fluctuating bound states. We derive bounds for the optimal interchain coupling to maximize the critical temperature of the FFLO state, in order to aid detection of these FFLO states in cold atom experiments [2].\\[4pt] [1] Y.-L. Loh and N. Trivedi, Phys. Rev. Lett. {\bf 104}, 165302 (2010).\\[0pt] [2] Y-an. Liao et. al Nature {\bf 467}, 567-569 (2010).   

Understanding the Mass-Imbalanced Highly-Polarized Fermi Gases

The phase diagram of spin-polarized single atomic species Fermi gases has been well-studied theoretically and experimentally. However, cold gases containing multiple atomic species open up the possibility of seeing more exotic states. Recent variational calculations (arXiv:1002.0101v2 [cond-mat.quant-gas]) suggest a complicated phase diagram for a light impurity interacting via a short-range potential with a sea of heavier fermions. In particular, at large mass ratio the polaron is expected to give way to more complicated many-body bound states, such as the trimer or the FFLO molecule. We extend these results beyond this variational ansatz, sampling over many-body states with an arbitrary number of particle-hole pairs. We will discuss the phase diagram resulting from these simulations, including implications for the stability of the trimer and FFLO phases.   

Spectroscopy of the soliton lattice formation in quasi-one-dimensional fermionic superfluids with population imbalance

Motivated by recent experiments in low-dimensional trapped fermionic superfluids we study quasi-1D superfluid with imbalanced populations between two hyperfine states and analyze its properties using the exact mean field solution for the order parameter. When population imbalance exceeds some critical value the superfluid order parameter develops spatial inhomogeneities and can be described by a soliton lattice formation. Emergence of the soliton lattice is accompanied by the formation of the spin density wave with the majority fermions residing at the points in space where Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) order parameter vanishes. We show that the presence of the spin density wave leads to the formation of the band of the ``subgap states,'' which serves as a hallmark of the quasi-1D FFLO state. We employ the soliton lattice description to discuss the possibilities for the experimental detection of the quasi-1D FFLO phase: elastic and inelastic optical Bragg scattering experiments and radio-frequency spectroscopy. We demonstrate that these measurements allow one to extract necessary information about the inhomogeneous superfluid phase to unambiguously identify quasi-1D FFLO state.   

$(2k_F, 2k_F)$ density-wave orders of interacting p-orbital fermions in square optical lattice

We study instabilities of spinless fermionic atoms in the p- orbital bands in two dimensional optical lattices at non- integer filling against interactions. Stripe charge-density- wave or orbital-density-wave orders are found for attractive and repulsive interactions, respectively. A surprising result is that the superfluid phase, usually expected of attractively interacting fermions, is less energetically favored. Nesting quasi-one-dimensional Fermi surfaces in such systems are independent of filling, which ensures that the stripe density- wave orders occur in a large parameter regime.   

$U(1) \times Z_2$ transition from the Mott insulator to $p_x+ip_y$ Bose-Einstein superfluid phase

Motivated by the recent experiment on p-band bosons in optical lattices [arXiv:1006.0509 (2010)], we study theoretically the quantum phases and phase transition of a two-dimensional extended Bose-Hubbard model with p-orbital degrees of freedom. The system features a novel superfluid phase with transversely staggered orbital current at weak interaction, and a Mott insulator phase with antiferro-orbital order at strong coupling and commensurate filling. We derive an effective theory from a microscopic model to describe the quantum phase transition from Mott to superfluid phase. We also calculate the excitation spectra near quantum critical point and find two gapless modes away from the Mott tip but four gapless modes at the tip point. We describe how the phase coherence builds up in the Mott regime when approaching the critical point.   

Induced $p$-wave superfluidity at unitarity in strongly imbalanced Fermi gases

We compute the induced interaction among the majority spin-up fermions, due to the presence of the minority spin-down fermions, in a population imbalanced Fermi gas. This interaction leads to an instability of the spin-polaron Fermi liquid, favoring a $p$-wave superfluid. For the majority component, near unitarity, the transition temperature is found to be within experimental reach, of order a few percent of the Fermi energy. As a probe of this phase, the radio-frequency spectroscopic line-shape is calculated for the $p_{x}+ip_{y}$ ground state.   

Polaron Metastability

We investigate the metastability associated with the first order transition from normal to superfluid phases along the BEC-BCS crossover in partially polarised Fermi gases. The momentum thresholds and rates of key decay processes involved are presented in the context of the system's phase diagram, together with metastability regions. In the limit of a single polaron, this region extends from the interaction strength at which a polarised phase of molecules becomes the groundstate ($\frac{1}{k_{F\uparrow}a} 0.73$), to the value of the crossing point from a single polaron to molecule groundstate ($\frac{1}{k_{F\uparrow}a} 0.9$). Finally, we propose experiments to explore the metastability of this Fermi liquid and the various decay processes, and to observe the $\frac{1}{k_{F\uparrow}a} 0.9$ value.   

Possibility of $\pi$-Josephson junction and spontaneous current in a spin-polarized Fermi gas

We theoretically propose an idea to realize a $\pi$-phase in a superfluid Fermi gas, where the phase of the superfluid order parameter differs by $\pi$ across a Josephson junction. When a weak nonmagnetic potential barrier is embedded in a superfluid Fermi gas with population imbalance ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of atoms with pseudospin $\sigma=\uparrow, \downarrow$), this barrier may be {\it magnetized} in the sense that some of excess atoms $N_\uparrow-N_\downarrow>0$ are localized around it. This magnetic barrier behaves like a {\it ferromagnetic junction} discussed in superconductivity literature, which twists the phase of superfluid order parameter by $\pi$. We confirm this idea by solving an attractive Hubbard model within the mean-field theory at $T=0$. We also show that, when this ferromagnetic barrier is realized in a ring-shaped (or torus) trap, the system becomes the so-called $\pi$-Josephson junction, where spontaneous circulating current flows due to the phase twist at the juntion.   

Many-body spectral moment sum rules for the Bose Hubbard model

Exact results for many-body interacting systems are rare. Here we derive a series of exact results for the single-band Bose-Hubbard model. In particular, we derive spectral moment sum rules for the Green's functions of the Bose-Hubbard model. Unlike the fermionic sum rules, the bosonic ones depend on complicated expectation values of the bosons that go beyond just needing to know the local particle density. Nevertheless, they can be used to benchmark the quality of different numerical calculations of spectral functions. These sum rules hold with arbitrary values of the interaction strength and even into nonequilibrium situations, similar to what is seen for the fermionic case. We present some case studies comparing the exact moments to those found with other numerical techniques like the VCA approximation.   

Bosonic Hubbard-Holstein model and its realization in optical lattices

The Hubbard-Holstein (HH) model describes the interplay between the Coulomb interaction and the electron-phonon coupling for fermionic systems. Motivated by the recent experimental progresses in optical lattices, we investigate a bosonic version of the HH model, where the two competing many-body interactions of the HH model become a bosonic two-body interaction and a boson-phonon coupling. In the regime of weak boson-phonon coupling, the mean-field phase diagram shows that overall effects of the phonons is to expand the domain of superfluidity. This bosonic Hubbard-Holstein (BHH) model can be realized in a pair of overlapping optical lattices, where bosonic particles trapped in one optical lattice are perturbed by more massive particles trapped in the other lattice.   

Number Density Distributions of Ultracold Bosons in 3D Optical Lattices

We calculate the probability, $P(n)$, of finding $n$ bosons at a site and the probability of hopping in a uniform optical lattice as a function of the temperature, $T$, and the repulsive interaction between bosons, $U/t$, as a function of hopping energy. We examine the characteristic $P(n)$ distribution for the Mott Insulator, quantum critical region and superfluid and determine its behavior across thermal and quantum phase transitions using quantum Monte Carlo. The behavior of the local kinetic energy is estimated using the probability of hopping. These results illuminate number squeezing in the Mott Insulator and the quantum critical region described in [1].\\[4pt] [1] Y. Kato, et al., {\em Nature Physics} {\bf 4}, 617 (2008).   

Surface Majorana modes in ultra-cold fermion systems with unconventional Cooper pairings

The rapid progress of dipolar fermions provides a new opportunity to investigate unconventional Cooper pairings and exotic topological properties. We study the zero energy modes for the single and multiple-component dipolar gases along the surface perpendicular to the z-direction, which are a flat band of Majorana fermions. Under time-reversal symmetry breaking perturbations, such as vortices, the degeneracy of the surface Majorana modes is lifted. We also investigated the spontaneous time-reversal symmetry breaking effect in such systems.   

Attractive Bose-Hubbard model with three-body constraint

We numerically study the quantum and thermal phase transitions of the Bose-Hubbard model with particle numbers per site restricted to less than three. The bosons experience on-site attractions while the nearest-neighbor interactions are repulsive. Using particular two-loops algorithm in the QMC simulations, we study the exotic dimer superfluid at small hopping and low density regime. The nature of the phase transitions between the dimer superfluid and the atomic superfluid will be discussed.