Session Y1: New Insights Into the Mott Transition

Renewed Understanding on Doped Mott Insulators

Mott transitions and nearby underdoped metals in two dimensions remain a major challenge in condensed matter physics, because of large spatial and quantum fluctuations, with its relevance to cuprate superconductors. Recent theoretical and computational developments have renewed its understanding with a unified picture for the unconventional metals. We overview historical backgrounds followed by a recent coherent picture obtained by path-integral renormalization group, many-variable variational Monte Carlo methods, and cluster-type dynamical mean-field theory [1]. Coexisting zeros and poles of the single-particle Green's function hold a key for Mott physics. Non-Fermi-liquid caused by topological transitions of Fermi surface including Lifshitz transitions naturally emerges. The energy-momentum dependent spectra reproduce the arc/pocket and pseudogap formation. We propose that the pseudogap in the cuprates is d-wave-like only below the Fermi level while it retains s-wave-like full gap above the Fermi energy even in the nodal point. In addition, the spectral asymmetry, back-bending and waterfall dispersions as well as the low-energy kink emerge within the same framework in agreement with the underdoped cuprates, excluding the scenarios by preformed pairs and d-density-waves, but supporting the proximity to the Mott insulator. We also propose that an extension of the exciton concept to doped Mott insulators by using cofermions accounts for the above unconventionality and superconductivity [2].\\[4pt] [1] S. Sakai et al., Phys. Rev. Lett. 102, 056404 (2009); Phys. Rev. B 82, 134505 (2010)\\[0pt] [2] Y. Yamaji and M. Imada, arXiv:1009.1197.   

Quantum criticality in the Hubbard model

In large scale dynamical cluster quantum Monte Carlo simulations of the two-dimensional (2D) Hubbard model with only nearest neighbor hopping, we find a quantum critical point (QCP) at finite doping separating a Fermi liquid region at low filling from a non-Fermi liquid pseudogap region near half-filling. Marginal Fermi liquid behavior is seen in the thermodynamics and single-particle properties for a wide range of doping and temperatures above the QCP. The QCP is due to the second-order terminus of a line of first order phase separation transitions that is driven to zero temperature as the next near-neighbor hopping t' vanishes. The superconducting dome surrounds the QCP. The proximity the QCP and the dome is due to an algebraic divergence, replacing the BCS log divergence, of the bare pairing polarization. This behavior is captured with a simple variation of the quantum critical BCS formalism.   

Cluster Dynamical Mean Field Methods and the Momentum-selective Mott transition

Innovations in methodology and computational power have enabled cluster dynamical mean field calculations of the Hubbard model with interaction strengths and band structures representative of high temperature copper oxide superconductors, for clusters large enough that the thermodyamic limit behavior may be determined. We present the methods and show how extrapolations to the thermodynamic limit work in practice. We show that the Hubbard model with next-nearest neighbor hopping at intermediate interaction strength captures much of the exotic behavior characteristic of the high temperature superconductors. An important feature of the results is a pseudogap for hole doping but not for electron doping. The pseudogap regime is characterized by a gap for momenta near Brillouin zone face and gapless behavior near the zone diagonal. for dopings outside of the pseudogap regime we find scattering rates which vary around the fermi surface in a way consistent with recent transport measurements. Using the maximum entropy method we calculate spectra, self-energies, and response functions for Raman spectroscopy and optical conductivities, finding results also in good agreement with experiment.   

Finite doping signatures of the Mott transition in the two-dimensional Hubbard model

The evolution from the conventional metal at high doping to the Mott insulator at zero doping remains a central problem in physics of copper-oxide superconductors. Here we solve the cellular dynamical mean-field equations [1,2] for the two-dimensional Hubbard model on a plaquette with continuous-time quantum Monte Carlo [3,4]. The normal-state phase diagram as a function of temperature T, interaction strength U, and filling n reveals that, upon increasing n towards the Mott insulator, there is a surface of first-order transition between two metals at nonzero doping. That surface ends at a finite temperature critical line originating at the half-filled Mott critical point [5,6]. There is a maximum in scattering rate associated with this transition. These findings suggest a new scenario for the normal-state phase diagram of the high temperature superconductors. The criticality surmised in these systems can originate not from a T=0 quantum critical point, nor from the proximity of a long-range ordered phase, but from a very low temperature transition between two types of normal state metals at finite doping. The influence of Mott physics extends well beyond half-filling. \\[4pt] [1] G. Kotliar et al., Rev. Mod. Phys. 78, 865 (2006).\\[0pt] [2] T. Maier et al., Rev. Mod. Phys. 77, 1027 (2005).\\[0pt] [3] P. Werner and A.J. Millis, Phys. Rev. B 74, 155107 (2006).\\[0pt] [4] K. Haule, Phys. Rev. B 75, 155113 (2007).\\[0pt] [5] G. Sordi, K. Haule, and A.-M.S. Tremblay, Phys. Rev. Lett. 104, 226402 (2010).\\[0pt] [6] G. Sordi, K. Haule, and A.-M.S. Tremblay, unpublished (2010).   

Computational Studies of Realistic Multiband Models of the Copper Oxides

High temperature superconductivity was achieved by introducing holes in a parent compound consisting of copper oxide layers separated by spacer layers. It is possible to dope some of the parent compounds with electrons, and their physical properties are bearing some similarities but also significant differences from the hole doped counterparts. Here, we use a modern first principles method, to study the electron doped cuprates and elucidate the deep physical reasons why their behavior is so different than the hole doped materials. We find that electron doped compounds are Slater insulators, e.g. a material where the insulating behavior is the result of the presence of magnetic long range order. This is in sharp contrast with the hole doped materials, where the parent compound is a Mott charge transfer insulator, namely a material which is insulating due to the strong electronic correlations but not due to the magnetic order. In particular, we point out that both hole and electron doped compounds are located close to the charge-transfer insulator to metal transition, and we discuss the consequences for optical and specific heat measurements done for the normal state, and additional consequences for the magnetic and superconducting orders of electron and hole doped copper oxides.\\[4pt] Work done in collaboration with Kristjan Haule and Gabriel Kotliar, Rutgers University.   

Session Y2: Topological Insulators: Transport and Interactions

Quantum oscillations and Hall anomaly of surface electrons on Topological Insulators

The investigation of Topological Insulators (TI) by transport experiments is a challenge, because the surface currents cannot be well-resolved when the bulk conductance is dominant, as in most crystals. I will review the progress starting from Ca-doped Bi$_2$Se$_3$, and proceeding to Bi$_2$Te$_3$ and to Bi$_2$SeTe$_2$. Using Ca dopants in Bi$_2$Se$_3$, we succeeded in lowering the Fermi energy $E_F$ into the bulk gap. However, in non-metallic crystals, the substantial dopant-induced disorder precluded observation of Shubnikov-de Haas (SdH) oscillations. Fortunately, $E_F$ in undoped Bi$_2$Te$_3$ can be tuned into the gap by heat treatment. The non-metallic samples display a bulk resistivity $\rho$ = 4-12 m$\Omega$cm at 4 K. In these crystals, weak SdH oscillations are observed below 10 K. We confirmed that these oscillations arise from a 2D Fermi Surface by tilting the magnetic field $\bf H$. From the behavior of the SdH amplitude versus temperature $T$ and $H$, we infer a surface Fermi velocity $v_F$ = 3.7-4.2 $\times 10^5$ m/s, and a high surface mobility $\mu$ = 10,000 cm$^2$/Vs. The high mobility of the surface electrons is confirmed by the appearance of an unusual weak-field anomaly in the Hall conductance $G_{xy}$. I will discuss recent progress in further lowering the bulk conductance in the new TI Bi$_2$Se$_3$, in which a Se layer is sandwiched between two Te layers in each quintuplet unit cell. In these crystals, $\rho$ at 4 K is a factor of 1000 larger (6 $\Omega$cm). The interesting pattern of SdH oscillations in this new system will be reported.\\[4pt] Collaborators: D.X. Qu, J. M. Checkelsky, Y. S. Hor, J. Xiong, R. J. Cava   

Dirac Fermions in HgTe Quantum Wells

Replace this text with your abstract. Narrow gap HgTe quantum wells exhibit a band structure with linear dispersion at low energies and thus are very suitable to study the physics of the Dirac Hamiltonian in a solid state system. In comparison with graphene, they boast higher mobilities and, moreover, by changing the well width one can tune the effective Dirac massfrom positive, through zero, to negative. Negative Dirac mass HgTe quantum wells are 2-dimensional topological insulators and, as a result, exhibit the quantum spin Hall effect. In this novel quantum state of matter, a pair of spin polarized helical edge channels develops when the bulk of the material is insulating, leading to a quantized conductance. I will present transport data provide very direct evidence for the existence of this third quantum Hall effect: when the bulk of the material is insulating, we observe a quantized electrical conductance. Apart from the conductance quantization, there are some further aspects of the quantum spin Hall state that warrant experimental investigation. Using non-local transport measurements, we can show that the charge transport occurs through edge channels - similar to the situation in the quantum Hall effect. However, due to the helical character of the quantum spin Hall edge channels, inhomogeneities in the potential profile of the experimental devices have a much stronger effect on the transport properties. Moreover, the quantum spin Hall edge channels are spin polarized. We can prove this fact in split gate devices that are partially in the insulting and partly in the metallic regime, making use of the occurrence of the metallic spin Hall effect to convert the magnetic spin signal into an electrical one. Finally, I will address another aspect of Dirac Fermion physics: HgTe quantum wells at a critical thickness of 6.3 nm are zero gap systems and exhibit transport physics that is very similar to that observed over the past few years in graphene. However, zero gap HgTe wells have a higher mobility than graphene, and also have only a single Dirac valley. This makes them especially suitable to study quantum interference effects under a Dirac Hamiltonian.   

Band Topology, Electron Correlations and 3D Dirac Metal in Pyrochlore Iridates

We study consequences of strong spin orbit interaction in a class of correlated systems. We discuss the possibility of novel phases such as a $\pi $ axion insulator, protected by inversion, rather than time reversal symmetry and a gapless topological phase, the three dimensional Dirac semimetal. The latter phase has unusual surface states that take the form of `Fermi Arcs', that cannot be realized in any two dimensional band structure. The pyrochlore iridates, (such as Y$_{2}$Ir$_{2}$O$_{7})$ according to LDA+U calculations and existing experimental data, are argued to be promising materials for realizing these states. This work was done in collaboration with Xiangang Wan (Nanjing U.), Sergey Savrasov (UC Davis) and Ari Turner (UC Berkeley).   

General Theory of interacting Topological insulators

In this talk, I shall first briefly review the theory of topological insulators and the experimental status. I will then discuss the general theory of an interacting topological insulator, whose topological order parameter is expressed in terms of the full interacting Green function. This topological order parameter is also experimentally measurable in terms of the quantized magneto-electric effect. I shall discuss various applications of this theory to realistic materials which could realize the topological Mott insulator state. \\[4pt] ``Topological Field Theory of Time-Reversal Invariant Insulators,'' Phys. Rev. B. {\bf 78}, 195424, (2008). \\[0pt] ``General theory of interacting topological insulators,'' arXiv:1004.4229. \\[0pt] ``Dynamical Axion Field in Topological Magnetic Insulators,'' Nature Physics {\bf 6}, 284 (2010). \\[0pt] ``Quantum Spin Hall Effect in a Transition Metal Oxide Na2IrO3,'' Phys. Rev. Lett. {\bf 102}, 256403 (2009). \\[0pt] ``Topological Mott Insulators,'' Phys. Rev. Lett. {\bf 100}, 156401, (2008).   

Classification of Topological Insulators and Superconductors: the ``Ten-Fold Way''

We review the exhaustive ten-fold classification scheme of topological insulators and superconductors. It is found that the conventional (i.e.: ``$Z_2$'', or `spin-orbit') topological insulator, experimentally observed in 2D (`Quantum Spin Hall') and in 3D materials, is one of a total of five possible classes of topological insulators or superconductors which exist in every dimension of space. Different topological sectors within a given class can be labeled, depending on the case, by an integer winding number, or by a ``binary'' $Z_2$ quantity. The topological nature of the bulk manifests itself through the appearance of ``topologically protected'' surface states. These surface states completely evade the phenomenon of Anderson localization due to disorder. Examples of the additional topological phases in 3D include topological superconductors (i) with spin-singlet pairing, and (ii) with spin-orbit interactions, as well as ${}^{3}{\rm He \ B}$. -- The classification of topological insulators (superconductors) in d dimensions is reduced to the problem of classifying Anderson localization at the (d-1)-dimensional sample boundary which, in turn, is solved. The resulting five symmetry classes of topological insulators (superconductors) found to exist in every dimension of space correspond to a certain subset of five of the ten generic symmetry classes of Hamiltonians introduced 16 years ago by Altland and Zirnbauer in the context of disordered systems (generalizing the three well-known ``Wigner-Dyson'' symmetry classes). For a significant part of the phases of topological insulators (superconductors) of the classification a characterization can be given in terms of the responses of the system. For these, the responses are described by a field theory possessing a [gauge, gravitational (thermal), or mixed] anomaly. This implies that these phases are well defined also in the presence of inter-fermion interactions.   

Session Y3: Recent Developments in Solid 4He

Dynamics, Defects and Deformation in Solid Helium

The shear modulus of solid $^4$He shows remarkable softening above 100 mK, the same temperature range in which the apparent supersolid disappears in torsional oscillator experiments. We have measured helium's shear modulus and dissipation at frequencies from 0.5 to 8500 Hz. The onset temperature for softening/stiffening is broad, frequency dependent, and is accompanied by a dissipation peak - features typical of a dynamical crossover in a disordered system rather than a true phase transition. This behavior can be qualitatively explained if dislocations are mobile at high temperatures but are pinned by $^3$He impurities below 100 mK. To better understand the role of dislocations, we have plastically deformed crystals by rapid thermal quenching and used pressure gradient measurements to study subsequent annealing. In our most recent experiments we have sheared solid helium mechanically and looked at the effect of large deformations on the helium's elastic properties.   

New evidence of supersolidity in rotating solid helium

The irrotationality of superfluid causes it to decouple form the container, leading to a reduction in the rotational inertia. This is more technically known as non-classical rotational inertia (NCRI). Although it is intuitively most natural to associate superflow only with the liquid phase, a decrease in the resonant period of a torsional oscillator (TO) was detected in solid helium below about 200 mK and interpreted as the appearance of NCRI. However, the resonant period may be also reduced for reasons other than supersolidity, such as the temperature dependence of the elastic modulus of solid helium. Unusual increase in the shear modulus with striking resemblance to those of NCRI supports the non-superfluid explanations. We superimposed dc rotation onto oscillatory measurements to distinguish between the supersolidity and classical elastic modulus change effects. We performed such simultaneous measurements of the TO and the shear modulus, and observed substantial change in the resonant period with rotational speed where the modulus remained unchanged. This contrasting behavior suggests that the decrease in the TO period is a result of supersolidity. This work is performed by collaboration with H. Choi, D. Takahashi, and K. Kono.   

Quantum plasticity and supersolidity

We have discovered that, in the total absence of impurities, helium 4 crystals are anomalously soft [1]. In our opinion, this is a consequence of the quantum properties of their dislocation lines which are able to move macroscopic distances (typically a fraction of a millimeter) at high speed (several meters per second) as a response to very small applied stresses (one microbar). Moreover, this quantum plasticity appears to be closely related to another astonishing property of quantum crystals, namely their ``supersolidity,'' that is the possible superflow of a fraction of the crystal mass through the rest which remains elastic, actually more rigid than in the normal state [2]. Very tiny traces of helium 3 impurities are sufficient to pin the dislocations below about 100 mK and destroy the quantum plasticity. By studying rotational and elastic properties of crystals with various qualities and variable helium 3 content, we are now checking that supersolidity is a consequence of matter flowing along dislocation lines but only if these dislocations are pinned by impurities. \\[4pt] [1] X. Rojas, A. Haziot, V. Bapst, H.J. Maris, and S. Balibar, Anomalous softening of helium 4 crystals, Phys. Rev. Lett. 105, 145302 (2010). \\[0pt] [2] S. Balibar, The enigma of supersolidity, Nature 464, 176 (2010).   

Mass Flow in Solid $^4$He as Observed by Fountain Effect Measurements

We have created an experimental cell in which solid helium is sandwiched between two Vycor rods which are each in turn in contact with reservoirs of superfluid $^4$He [1]. Application of a temperature difference between the two reservoirs creates a thermo-mechanical effect, which causes a flux of atoms from one reservoir to the other through the solid helium, which is off the melting curve. The flux is measured to increase with falling temperature below about 650 mK, fall precipitously near 80 mK and then rise again at lower temperatures [2]. Results of these experiments as well as the behavior of solid growth will be presented and discussed in the context of recent theoretical work. \\[4pt] [1] M. Ray and R.B. Hallock, Phys. Rev. Letters 100, 235301 (2008); Phys. Rev. B 79, 224302 (2009).\\[0pt] [2] M. Ray and R.B. Hallock, Phys. Rev. Letters 105, 145301 (2010).   

Superclimb of Dislocations in Solid $^4$He

Edge dislocation with superfluid core can perform {\it superclimb} -- non-conservative motion (climb) assisted by superflow along its core. Such dislocation, with Burgers vector along the C-axis, has been found in {\it ab initio} simulations of {\it hcp} solid $^4$He [1]. Uniform network of superclimbing dislocations can induce {\it isochoric compressibility} $\chi = dN/d\mu $ which is finite (in contrast to ideal solid where it vanishes) and, practically, independent of the network density. Here $N$ is total number of atoms and $\mu$ is chemical potential [1]. Such giant response has been observed by Ray and Hallock during superfluid flow events through solid He4 [2]. Study [3] of superclimbing dislocation within the model of Granato-L\"ucke string, subjected to Peierls potential and to vanishing bias by $\mu$, has found that $\chi$ exhibits wide peak in the intermediate range of temperatures (T) - above some $T_p$ determined by Peierls energy and below $T_s \sim 0.5$K above which superfluidity of the core essentially vanishes. Non-Luttinger type behavior characterized by $\chi \sim L^b$ scaling as some power $1< b \leq 2$ of dislocation length $L$ is observed in the wide peak region. Biasing superclimbing dislocation by finite $\mu$ (due to a contact with liquid $^4$He through vycor electrodes [2],[4]) can induce core roughening caused by thermally assisted tunneling of jog-antijog pairs through the barrier produced by combination of Peierls potential and the bias [5]. The threshold for this effect scales as $\mu_c\sim 1/L^a$ with some power $a\approx 1.7$. The roughening is found to be hysteretic below some temperature $T_{\rm hyst}$. At $T_{\rm hyst}< T < T_R$, with $T_R$ determining temperature of thermal roughening, $\chi$ exhibits strong and narrow resonant peak leading to a dip in the core superfluid sound velocity. This mechanism is proposed as an explanation for a strong and narrow dip observed in critical superflow rate [4]. It is found that the dip characteristics are sensitive to the bias by $\mu$ and, therefore, this can be used as a test for the proposed mechanism. It is also predicted that the dip depth at given $T$ should be periodic in $\mu$ with the period $\sim \mu_c$. \\[4pt] [1] S. G. S\"oyler, et. al., PRL {bf 103}, 175301 (2009).\\[0pt] [2] M. W. Ray and R. B. Hallock, PRL {\bf 100}, 235301 (2008) ; PRB {\bf 79}, 224302 (2009); PRB {\bf 81}, 214523 (2010); Phys. Rev. {\bf B82}, 012502 (2010);\\[0pt] [3] D. Aleinikava, et al., JLTP, to be published;\\[0pt] [4] M. W. Ray and R. B. Hallock , Phys. Rev. Lett. {\bf 105}, 145301 (2010); \\[0pt] [5] D. Aleinikava and A.B. Kuklov, unpublished.   

Session Y4: Polymer Colloids: Structure, Function and Dynamics

Colloidal photonic crystals: Beyond optics, beyond spheres

Monodisperse and symmetrically shaped colloidal particles tend to form ordered aggregates. When the particle size is in the hundreds of nanometres, such highly ordered structures exhibit fascinating optical properties, hence their name and fame as colloidal ``photonic crystals'' or as ``photonic bandgap material'', because they exhibit a forbidden energy band for photons, very much like semiconductor crystals are characterized by a bandgap for electrons. Photonic bandgap engineering is possible by a proper choice of the size and nature of the ``photonic atom'', and by a proper combination of different kinds of particles. The fame of monodisperse colloidal spheres as photonic atoms is largely based on the self-assembling capabilities into inherently three-dimensional photonic crystals. Colloidal photonic crystals can hence be used as an easy photonic crystal platform to demonstrate proof-of-principle for effects such as reduced local density of states for photons on their emission probability. We have induced spectral narrowing for emission from dye molecules and enhanced energy transfer between light-absorbing molecules in colloidal photonic crystals. By inserting superparamagnetic particles in the tens of nanometres range, it is possible to additionally impart magnetic properties to the photonic crystal. Tuning and enhancing Faraday rotation was possible by careful nanoscale bandgap engineering at two different nanoscales. One disadvantage of colloidal spheres for photonic crystals is the incomplete bandgap that is typical for the highly symmetrical crystal structures that are commensurable with dense packing of spheres. A number of approaches allow deviating from this paradigm towards a complete bandgap in the visible. Etching of material allows a less dense crystal, while non-spherical colloidal particles provide alternate crystal structures. Orientational ordering of such anisotropic particles in an anisotropic photonic crystal requires an additional handle on the particles, the colloidal assembly providing the positional order. Magnetism again provides this handle. Post-formation processing of crystals of positionally ordered spheres into orientationally anisotropic crystals represents another approach.   

Near Wall Dynamics in Colloidal Suspensions Studied by Evansescent Wave Dynamic Light Scattering

The dynamics of dispersed colloidal particles is slowed down, and becomes anisotropic in the ultimate vicinity of a flat wall due to the wall drag effect. Although theoretically predicted in the early 20th century, experimental verification of this effect for Brownian particles became possible only in the late 80s. Since then a variety of experimental investigations on near wall Brownian dynamics by evanescent wave dynamic light scattering (EWDLS) has been published. In this contribution the method of EWDLS will be briefly introduced, experiments at low and high colloid concentration for hard-sphere suspensions, and the theoretical prediction for measured initial slopes of correlation functions will be discussed. On increasing the particle concentration the influence of the wall drag effect is found to diminishes gradually, until it becomes negligible at volume fractions above $\phi> 0.35$. The effect that a wall exerts on the orientational dynamics was investigated for different kinds of colloids. Experiments, simulations and a virial expansion theory show that rotational dynamics is slowed down as well. However, the effect is prominent in EWDLS only if the particles' short axis is of the order of the evanescent wave penetration depth.   

Assembly of Dimer-Based Photonic Crystals

Recent advances in colloid synthesis to prepare monodisperse shape anisotropic particles provide the opportunity to address challenges related to structural diversity in ordered colloidal solids. In particular, computational simulations and mechanical models suggest that upon system densification nonspherical dimer colloids undergo disorder-order and order-order phase transitions to unconventional solid structures including, base-centered monoclinic crystals, degenerate aperiodic crystals, plastic crystal or rotator, etc. based on free energy minimization. The particle systems have notable analogy to molecular systems, where the shape of molecules and their packing density has been shown to critically influence structural phase behavior and lead to a rich variety of structures, both natural and synthetic. The materials engineering challenges have been in attaining sufficiently monodisperse (size uniformity) colloidal building blocks, as well as the lack of understanding and control of self-assembly processes for non-spherical colloids. This talk highlights our investigations of how particle shape programs the self-organization of colloidal structures. Methods including evaporation mediated assembly and confinement provide a platform to understand the formation of complex colloidal structures from non-spherical building blocks (silica-coated iron oxide, polystyrene, hollow silica shell). Optical property simulations for unconventional 2D and 3D structures with nonspherical particle bases will also be discussed.   

Directed self-assembly of small colloidal clusters

We study the formation and structure of equilibrium colloidal clusters at small particle number ($N \sim 10$) using optical microscopy. Our experimental system consists of isolated groups of colloidal microspheres with short-ranged attractions. With non-specific depletion interactions, we observe that the number of configurations increases sharply with $N$. The most favorable states are those with the lowest symmetry. With specific DNA-mediated attractions, the number of states is reduced. Experiments and theoretical calculations suggest that it is possible to direct the assembly of specific structures through multiple competing DNA-mediated interactions.   

Convective microsphere monolayer deposition

There is perhaps no simpler way of modifying surface chemistry and morphology than surface deposition of particles. Micron-sized microspheres were deposited into thin films via rapid convective deposition, similar to the `coffee ring effect' using a similar method to that studied by Prevo and Velev, Langmuir, 2003. By varying deposition rate and blade angle, the optimal operating ranges in which 2D close-packed arrays of microspheres existed were obtained. Self-assembly of colloidal particles through a balance of electrostatic and capillary forces during solvent evaporation was revealed. These interactions were explored through a model comparing the residence time of a particle in the thin film and the characteristic time of capillary-driven crystallization to describe the morphology and microstructure of deposited particles. Co-deposition of binary suspensions of micron and nanoscale particles was tailored to generate higher-quality surface coatings and a simple theory describes the immergence of instabilities that result in formation of stripes. Optical and biomedical applications that utilize the described nanoscale control over surface morphology will also be discussed.   

Session Y5: Opening the Gap: Chemical Functionalization and Substitution in Graphene

Magnetic Moment and Electronic Correlations in Chemically Functionalized Graphene

Magnetic moments in extended systems are the result of local electronic correlations. In the case of graphene functionalized with chemisorbed atoms such as hydrogen, fluorine, or oxygen, the Anderson Model picture, where correlations in a localized state are responsible for the formation of a magnetic moment, has to be modified to properly describe the magnetic moment formation and their interactions. We use a tight-binding model with local correlations to analyze the results obtained with Density Functional Theory calculations for these systems. The model allows the treatment of local correlations beyond the mean field level and the investigations of a possible Kondo effect. We find that the Coulomb repulsion at the carbon atoms near the impurity play a crucial role in the magnetic moment formation. External doping with a gate voltage can control the nature of the binding and the formation of the magnetic moment. This effect could be observed in transport experiments as the scattering of the graphene electrons at the Fermi energy strongly depends on the structure of the defect.   

A theoretical study of chemical functionalisation of graphene: graphane and graphXene

Chemical functionalisation of graphene is reported from a first principles, theoretical study [1]. The electronic structure, including band gap, of H adsorbed on graphene (i.e. graphane) is discussed in this presentation [2]. In addition, adsorption of Group VII elements on graphane (named graphXene) is also reported [3]. Similarities and differences in the chemical binding and electronic structure of graphane and graphXene are analyzed. The adsorption on graphene is found to, depending on adatoms, result in sp2 or sp3 binding, where in general the sp3 bonded systems show a bandgap. The theoretical calculations make use of both GGA functionals as well as the GW approximation. In addition to large graphene layers, theoretical analysis of functionalised graphene nano-ribbons will also be presented [4]. References: \\[4pt] [1] V. A. Coleman, et al.,J. Phys. D: Appl. Phys. 41,062001 (2008); S. H. M. Jafri, et al, J. Phys. D 43, 45404 (2010). \\[0pt] [2] S.Lebegue, et al., Phys. Rev. B 79, 245117 (2009). \\[0pt] [3] M.Klintenberg, et al., Phys. Rev. B 81, 85433 (2010). \\[0pt] [4] S. Bhandary, et al., Phys. Rev. B 82, 165405 (2010).   

Gap control via graphene solid-state reactions

While a gapless dispersion law of Dirac fermions in graphene does warrant admiration, to serve as useful semiconductor graphene needs a gap. Relatively inert, it can nevertheless be induced to react. A generic outcome of a reaction, C + A -$>$ C$_{1-x}$A$_{x}$ is a transition of some C-atoms from their sp$^ {2}$- into sp$^{3}$-state, corresponding to a situation of the insulating, ultimate (mono- or few-layer) diamond slab [1]. Computations support a concept that the product of such reactions (A = H, F, O, Cl, etc.) forms a well-defined phase [2], permitting a patterning of 2D-geometries with useful properties: interconnects-nanoroads [3], quantum isles-dots [4], etc. Comparison of hydrogenation (A = H) into graphAne with fluorination (A = F) into 2D-teflon, shows the former as hindered by nucleation barrier and reversible (H-storage), in contrast to barrier-less reaction into a stable CF in the latter. *** In collaboration with F. Ding, E. Penev, M.A. Ribas, and A.K. Singh. ***\\[4pt] [1] E. Munoz, et al., Diamond \& Related Mater., 19, 368, 2010.\\[0pt] [2] Y. Lin, et al., Phys. Rev. B, 78, 041402(R), 2008.\\[0pt] [3] A.K. Singh and BIY, Nano Lett., 9, 1540, 2009.\\[0pt] [4] A.K. Singh, et al., ACS Nano, 4, 3510, 2010.\\[0pt] [5] ``Patterning on fluorinated graphene,'' M. Ribas, et al., Nano Res. (2010).   

Spin-polarized semiconductor induced by magnetic impurities in graphene

Magnetic impurities adsorbed on graphene sheets are coupled antiferromangetically via the itinerant electrons in the graphene. We study this interaction and its impact on the electrons' spectral density by use of unbiased Monte-Carlo simulations. The antiferromagnetic order breaks the symmetry between the sublattices, and a gap for the itinerant electrons opens. Our simulations show that the itinerant states below and above the gap are not dispersionless states trapped by the impurities, but are instead mobile states with a large dispersion. We compare various scenarios for the impurity distribution and find that random doping produces a standard semiconductor. If, on the other hand, all or most of the impurities are localized in the same sublattice, the spin degeneracy is lifted and the conduction band becomes spin-polarized. We also discuss the properties of edge states at edges or magnetic domain boundaries.\\[4pt] M.~Daghofer, N.~Zheng, A.~Moreo; Phys.~Rev.~B {\bf 82}, 121405(R) (2010)   

Graphene monofluoride: a wide bandgap material derived from graphene

Fluorination provides an effective way of controlling the properties of carbon materials. In this talk, I will describe our experimental and theoretical work on the synthesis, structural, electrical and optical properties of fully fluorinated graphene and graphite, i. e., graphene monofluoride CF and graphite monofluoride (CF)$_n$. (CF)$_n$ is synthesized by reacting HOPG graphite with F$_2$ gas at high temperature. Transmission electron microscopy and electron diffraction measurements show crystalline few-layer CF with a lattice constant 4\% larger than that of graphene, in good agreement with first principle calculations. We observe the E$_g$ and A$_{1g}$ Raman modes of graphene monofluoride using UV Raman spectroscopy. Photoluminescence measurements of (CF)$_n$ using variable excitation wavelength (244-514 nm) and temperature (5-295 K) show several emission modes in the visible spectrum, which likely originate from mid-gap defect states. The absence of the band edge emission suggests a large band gap of greater than 5 eV. Partially fluorinated graphene fluoride exhibits non-linear, strongly insulating transport with variable-range hopping temperature dependence, consistent with the presence of localized states due to missing fluorine atoms. Highly conductive graphene can be recovered by annealing CF in Ar/H$_2$ at high temperature, resulting in a conductance improvement of five orders of magnitude. As a transparent and atomically thin insulator, graphene monofluoride may find its use in graphene electronics and photonics. In collaboration with: Bei Wang, Shih-Ho Cheng, Justin Sparks, Humberto Gutierrez, Ke Zou, Ning Shen, Youjian Tang, Qingzhen Hao, Awnish Gupta, Peter Eklund, Vincent Crespi, Jorge Sofo and Fujio Okino (Shinshu University, Japan). References: Cheng et al, ``Reversible fluorination of graphene: towards a two-dimensional wide band gap semiconductor,'' Phys. Rev. B 81, 205435 (2010) Wang et al, ``Photoluminescence from nanocrystalline graphite monofluoride,'' Appl. Phys. Lett. 97, 141915 (2010)   

Session Y6: Ultrafast Magnetization Dynamics: Where Are We Today?

The x-ray few of femtosecond spin-orbit excitations in ferromagnets

Polarized soft x-rays have been used over the past 20 years to obtain fascinating new insights into nanoscale magnetism. The separation of spin and orbital magnetic moments, for instance, enabled detailed insights into the interplay of exchange and spin-orbit interactions at the atomic level. The now available polarized soft x-ray pulses with only 100 fs duration allow us to observe the magnetic interactions at work in real time. The ultimate goal of such studies is to understand how spins may be manipulated by ultrashort magnetic field, spin polarized current or light pulses. In this talk I will focus on fs laser induced spin-orbit dynamics in 3d transition metals. Using fs x-ray pulses from the BESSY II femtoslicing facility I will show how fs excitation of the electronic system modifies the spin-orbit interaction enabling ultrafast angular momentum transfer between spin, orbital and lattice degrees of freedom.   

Possibility of Nanoscale Imaging of Ultrafast Magnetization Dynamics

Understanding the microscopic mechanisms driving the magnetization dynamics on the fs time scale is of essential importance for manipulating and controlling the macroscopic state in magnetic storage devices. The demagnetization in ferromagnetic films by an ultrashort laser excitation on a time scale of a few hundred fs raised controversies about the effective path to dissipate angular momentum to the lattice, see e.g. [1]. Even more intriguing is the demonstration of all-optical magnetization reversal in ferrimagnetic compounds using circularly polarized, fs laser pulses [2]. Until only recently, the field of ``Femto-magnetism'' has naturally been driven by all-optical pump-probe techniques. Femtosecond time-resolved X-ray magnetic circular dichroism spectroscopy has been utilized to unambiguously determine the ultrafast quenching of spin and orbital moments after ultrashort laser excitation [3]. While all-optical pump-probe techniques allow ultrafast excitations (pump) and the study of their evolution (probe) on the macroscopic scale by use of the magneto-optical Kerr or Faraday effect, little is known about the microscopic processes on nano- and sub-nanometer length scales because of the lack of real or momentum space resolution of optical techniques. By combining resonant coherent resonant magnetic scattering with the unique high peak-brightness, short pulse structure, and fully transverse coherence of the new x-ray free-electron lasers, the dynamics of magnetic fluctuations and magnetization relaxation processes can be studied on the nanometer scale with sub-picosecond time resolution. We demonstrate the possibility of nondestructive single shot imaging of the magnetization in Co/Pd multilayers at LCLS. \\[4pt] [1] Koopmans, B, et al., Nature materials 9, 259 (2010). \\[0pt] [2] Stanciu, C.D., et al., Phys. Rev. Lett. 99, 047601 (2007). \\[0pt] [3] Stamm, C., et al., Nature materials 6, 740 (2007).   

Imaging Magnetization Dynamics on the Nanoscale Using X-ray Microscopy

We aim at time- and spatially resolved imaging of excitations in ferromagnetic materials such as spin waves, the motion of domain walls and the gyration of magnetic vortices and antivortices. Special emphasize is given to the interaction of electrical currents with magnetic inhomogeneities like domains walls and vortices. The spin-polarized current can give rise to a spin torque on spatially inhomogeneous magnetization configurations. With magnetic transmission X-ray microscopy we observe a current-driven oscillation of an individual domain wall on its genuine time scale. In the framework of an analytical model insight into the domain-wall motion and its characteristic damping time is gained by examination of different phase spaces [1]. Current-induced depinning of a domain wall from a pinning site depends on the temporal shape of the current pulse. Apart from resonant excitation of the wall this effect arises from an additional force on the wall due to a fast changing current. Efficient depinning is achieved for rise times smaller than the damping time of the domain wall [2]. Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of vortices and antivortices. They behave as two-dimensional oscillators with a gyrotropic eigenmode which can be resonantly excited by spin currents and magnetic fields [3]. It is shown that the two excitation types couple in an opposing sense of rotation in case of resonant antivortex excitation with circular-rotational currents [4]. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. Financial support by the DFG via SFB 668 and via GK 1286 as well as by the City of Hamburg via the Landesexzellenzcluster Nano-Spintronics is gratefully acknowledged. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy. \\[4pt] [1] L. Bocklage et al., Phys. Rev. B \textbf{81}, 054404 (2010) \\[0pt] [2] L. Bocklage et al. Phys. Rev. Lett. \textbf{103}, 197204 (2009) \\[0pt] [3] A. Drews et al., Phys. Rev. B \textbf{77}, 094413 (2008); M. Bolte et al., Phys. Rev. Lett. \textbf{100}, 176601 (2008). \\[0pt] [4] T. Kamionka et al., Phys. Rev. Lett. \textbf{105}, 137204 (2010).   

Ultrafast magnetization dynamics in lanthanide ferromagnets: From bulk to surfaces

The intense research on femtosecond laser-induced magnetization dynamics resulted in rich ultrafast phenomena [1]. A microscopic description of the underlying elementary processes, however, remains a challenge. Most efforts focus on the $3d$ transition metal ferromagnets and related compounds. This talk discusses recent work on the lanthanide ferromagnets Gd and Tb. Their magnetic moment is dominated by $4f$ electrons which are localized at the ion core. Their spin-lattice coupling is determined by the angular momentum of the $4f$ electrons. Using femtosecond x-ray magnetic circular dichroism at the femtosecond slicing facility at the BESSY II storage ring in Berlin, Germany, we measure the ultrafast change in the magnetic moment, which occurs on two specific timescales [2]. The faster one is 0.75~ps. It is driven by hot electrons and is identical for both lanthanides. The slower one is different for Gd (40~ps) and Tb (8~ps) due to the stronger spin-lattice coupling in Tb. The talk also discusses time-resolved non-linear optical studies on Gd(0001) and Tb(0001) surfaces [3]. We find a coherent surface phonon which is strongly coupled with the ultrafast magnetic response and pronounced differences compared to the bulk dynamics which are attributed to spin-polarized transport effects. \\[4pt] [1] A. Kirilyuk, A. V. Kimel, Th. Rasing, Rev. Mod. Phys. \textbf{82}, 2731(2010).\\[0pt] [2] M. Wietstruk {\it et~al.}, http://arxiv.org/abs/1010.1374.\\[0pt] [3] A. Melnikov and U. Bovensiepen, in {\it Dynamics at solid state surfaces and interfaces} Vol. 1, edited by U. Bovensiepen, H. Petek, M. Wolf (Wiley-VCH, Weinheim, 2010); A. Melnikov {\it et~al.}, J. Phys. D: Appl. Phys. \textbf{41}, 164004 (2008).   

Ultrafast magnetization dynamics in a system with tunable angular momentum

Many peculiarities of the magnetization dynamics are related to the fact that a certain amount of angular momentum is associated with magnetic moment. Here the dynamics of angular momentum is studied in ferrimagnetic rare-earth -- transition metal alloys, e.g. GdFeCo, where both magnetization and angular momenta are temperature dependent. Depending on their composition, such ferrimagnets can exhibit a magnetization compensation temperature TM where the magnetizations of the sublattices cancel each other and similarly, an angular momentum compensation temperature TA where the net angular momentum vanishes. At the latter point, the frequency of the homogeneous spin precession diverges. As a consequence, ultrafast heating of a ferrimagnet across its compensation points may result in a subpicosecond magnetization reversal [1]. Moreover, we have experimentally demonstrated that the magnetization can be manipulated and even reversed by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field [2,3]. This optically induced ultrafast magnetization reversal is the combined result of laser heating of the magnetic system and circularly polarized light acting as a magnetic field with amplitudes of up to several Teslas. The direction of this opto-magnetic switching is determined only by the helicity, i.e. angular momentum, of light. This novel reversal pathway (see [4]) is shown to crucially depend on the net angular momentum reflecting the balance of the two opposite sublattices. \\[4pt] [1] C.D. Stanciu et al., Phys. Rev. B 73, 220402 (2006); Phys. Rev. Lett. 99, 217204 (2007). \\[0pt] [2] C.D. Stanciu et al, Phys. Rev. Lett. 99, 047601 (2007). \\[0pt] [3] A. Kirilyuk, A.V. Kimel, and Th. Rasing, Rev. Mod. Phys. 82, 2731 (2010). \\[0pt] [4] K. Vahaplar et al., Phys. Rev. Lett. 103, 117201 (2009).   

Session Y8: Experiences and Issues for Young Physicists in the International Arena: Impact on the Future of Physics followed by Panel Discussion

Impact of Visa Issues on an International Physics Graduate Student in the U.S.

More than 35 percent of the physics graduate students in the US are temporary visa holders. Many of these students work in large international collaborations and must travel abroad for research and international conferences, sometimes more than once a year. In many cases, students have to reapply for their visas in order to return to the U.S., a process that can be time-consuming and costly. Furthermore, many international students cannot leave the U.S. even in the case of an emergency because a slow visa process may mean deferring for a semester or losing financial support. Thus visa issues affect not only the scholastic life of students but also their personal lives. Finding ways to resolve these issues could positively affect the quality of graduate research by eliminating these extra hurdles to the progress of international physics graduate students.   

Overcoming the Cultural Barrier: An International Physicist's Experience

Doing experimental physics in the midst of an international community, a necessity in certain fields due to the breadth and the complexity of the projects involved, is a task that presents many advantages but also challenges. I will be reviewing some of these from the point of view of an international physicist working in China. I will also be sharing my personal experiences in overcoming the cultural barriers and in transitioning from a country that is traditionally underrepresented in science.   

Life In a large scientific collaboration

I will be talking about life in a large scientific collaboration. The dynamics of dealing with many groups, collaborating with people from various linguistic and cultural origins can be a daunting experience. However, it is exactly this diversity of culture and learning that can make it an invigorating journey. You need to find your place in terms of professional contribution as well as personal liaisons to be productive and innovative in a large work culture. Scientific problems today are not solved by one person hunched over an old notebook. It is solved by sharing computer codes, experimental infrastructure and your questions over coffee with your colleagues. An affinity to take in and impart healthy criticism is a must for productive throughput of work. I will discuss all these aspects as well as issues that may arise from adjusting to a new country, customs, food, transportation or health-care system. The purpose of the talk is to familiarize you with what I have learned through my past five years of stay at CERN and working in the ATLAS collaboration.   

Perspectives from an International Female Physicist in Academia

I will bring my perspective as an international physicist in academia to the discussion of issues facing international physicists. I will also talk about issues facing women physicists worldwide.   

Panel Discussion: Issues Facing International Physicists and the Future of Physics

The panel will discuss the challenges and key issues faced by today's young physicists, especially when participating in international collaborations.   

Session Y9: Motility, Locomotion and Cellular Fluid Mechanics

Coordinated Swimming: Hydrodynamic interactions between multi-flagellated bacteria

Multi-flagellated bacteria, such as Escherichia coli, often have flagella attached at random locations to the cell body, which drive swimming behavior. To study the effect of hydrodynamic interactions on the swimming behavior, we develop a bead-spring model which represents both the body and the flagella using up to 240 Stokeslets, or hydrodynamic drag centers. These beads are bonded by 1) a spring potential, 2) a bending potential, and 3) a torsional potential to adjacent beads. This modeled bacterium swims by rotating the flagella with constant torques. We find that the number and arrangement of the flagella along the bodies of the swimmers affects how two such swimmers approach each other, when swimming either in a line, or side by side, and affects whether or not flagellar rotations are synchronized or not. We show how the flow field generated by each swimmer can be represented using a low order multipole expansion, which can capture the qualitative features of their interactions. With this simple low order expansion, simulations of hundreds or thousands of such swimmers can be carried out, allowing the effects of numbers and locations of flagella on swimming pattern formation to be captured.   

Remote Powering and Steering of Self-Propelling Microdevices by Modulated Electric Field

We have demonstrated a new class of self-propelling particles based on semiconductor diodes powered by an external uniform alternating electric field [1]. The millimeter-sized diodes floating in water rectify the applied voltage. The resulting particle-localized electroosmotic flux propels them in the direction of the cathode or the anode depending on their surface charge. These particles suggest solutions to problems facing self-propelling microdevices, and have potential for a range of additional functions. The next step in this direction is the steering of these devices. We will present a novel technique that allows on-demand steering of these self-propelling diodes. We control remotely their direction of motion by modifying the duty cycle of the applied AC field. The diodes change their direction of motion when a DC component (wave asymmetry) is introduced into the AC signal. The DC component leads to redistribution of the counterions near the diode surface. The electric field resulting from this counterion redistribution exerts a torque on the dipole across the diode, causing its rotation. Thus, the reversal of the direction of the electroosmotic flux caused by field asymmetry leads to reversal of the direction of diode motion. This new principle of steering of self-propelling diodes can find applications in MEMs and micro-robotics. [1] S. T. Chang, V. N. Paunov, D. N. Petsev, O. D. Velev, Nat Mater, 6, 235-240 (2007).   

Motility of rotating flagella in viscoelastic fluids

Bacteria achieve motility by eluding the constraints of kinematic reversibility, for instance, by rotating a helical flagellum. We study experimentally the motility of the flagellum with a scaled-up model system, a motorized helical coil that rotates along its axial direction. The rotating helix is tethered on a linear stage that advances at a predetermined speed along the axial direction. A free-swimming speed is obtained when the net force on the helix is zero. In the Newtonian case, the free-swimming speed of the helix is always proportional to its rotation rate. We show how such motility is affected by the presence of the viscoelasticity of the fluid, a ubiquitous environment for living bacteria.   

Electrical Control of Microtubule Translocation on Graphene

Motor protein systems such as a kinesin-microtubule complex play an important role in intracellular cargo transport by directly converting a chemical energy into a mechanical work. For exploiting their high energy efficiency, there have been considerable efforts to integrate them with various nanostructures to build nanoscale biodevices such as an advanced nano-transportation system. Herein, we demonstrated a successful motility assay of microtubules on a kinesin-functionalized graphene electrode which has a good transparency and conductivity. By applying a voltage bias onto the graphene electrode, we could spatially control the translocation of the microtubules. Our result clearly shows that graphene can be used not only as a good substrate for a motor-protein motility assay but also as a key component for a nano-mechanical system based on biomotors.   

Swimming speed of an oscillating sheet in Newtonian and viscoelastic fluids

We discuss a mechanical experimental model of a flexible sheet swimming with a prescribed wave pattern - a Taylor swimmer - through a fluid. Our study is motivated by a need for a fundamental understanding of microorganism locomotion through non-Newtonian fluids. In order to simplify the problem, we suspend a tall flexible cylindrical sheet concentric within a cylindrical tank filled with the fluid. Torque free boundary conditions are imposed by supporting the flexible sheet and the tank with friction-free ball-bearings. A traveling wave is imposed on the sheet with a pair of rollers in the azimuthal direction. We first demonstrate a linear response in the swimming velocity of the sheet with respect to its phase velocity in a viscous Newtonian fluid. Further, we show that the analytical system is essentially two dimensional by varying the height of fluid in the tank. We then discuss measurements of swimming speed in Polyox-water mixtures as a function of wave speed. We demonstrate that the swimming speed in this viscoelastic fluid decrease relative to the Newtonian case as wave speed is increased. We will further discuss the dependence of swimming speed on Deborah number and other characteristics of the fluid.   

Highly-Controllable Near-Surface Swimming of Magnetic Nanorods

Directed manipulation of nanomaterials has significant implications in the field of nanorobotics, nanobiotechnology, microfluidics, and directed micro- and nano-object assembly. With this in mind, we present a simple, efficient method for the fabrication and controlled manipulation of rod-shaped micro-scaled swimmers in a low-Reynolds environment. We demonstrate fine spatial control of the swimmers' motion and we approach, capture, and manipulate a polystyrene microbead as proof of principle. The swimmers consist of 200-nm-diameter gold nanowires which are grown by electrodeposition in an AAO template. The template is removed via dissolution in NaOH, and a layer of nickel (50 nm) is subsequently evaporated onto the surface of the wires. These wires settle near the floor of an enclosed water-filled cell and are observed via optical microscopy. Rotation is induced via an external magnetic field provided by a permanent magnet. The field is rotated in a plane nearly parallel to the floor; a small tilt out-of-plane results in symmetry-breaking, with the end of the rod nearest the floor experiencing an enhanced drag coefficient due to the presence of the boundary. The imbalance in drag forces between the two ends of the rotating rod results in a net translation. We use resistive force theory to develop an analytical model which describes the motion of these swimmers and correlate this model with experimental results.   

Textured boundaries and their effects on ciliary locomotion

Many microorganisms in nature propel themselves by creating coordinated motion of the cilia and often interact with each other through hydrodynamic interactions. We study the behavior of these organisms near boundaries of different topography and rationalize the hydrodynamic effects involved. Various geometries like wavy, rough or solid walls are simulated using micro fabrication and their effects on the locomotory traits are observed. Finally a comprehensive discussion on the effect of different boundaries on the swimming characteristics of the organism is presented.   

Motion of Elastic Microcapsules on Compliant Surfaces with Adhesive Ligands

By integrating mesoscale models for hydrodynamics, micromechanics and adhesion, we examine the fluid driven motion of elastic microcapsules on compliant surfaces. The capsules, modeled as three-dimensional fluid-filled elastic shells, represent polymeric microcapsules or biological cells. Our combined integrated Lattice Boltzmann model/Lattice spring model (LBM/LSM) approach allows for a dynamic interaction between the elastic capsule's wall and surrounding fluid. To capture the interaction between the shell and the surface, we adopt the Bell model, used previously to describe the interaction of biological cell like leukocytes rolling on surfaces under the influence of an imposed shear. The surface of the microcapsule contains receptors with an affinity to adhesive ligands of the substrate. We examine how the parameters of adhesion and rigidity of the capsules and the substrate affect movement of the capsules. The findings provide guidelines for creating smart surfaces that could regulate the microcapsules' motion.   

Modelling the dynamics of colloidal nanorods in a spatially varying electric field

The behavior of anisotropic nanoparticles is of great current interest in the design of optical metamaterials. We have carried out numerical simulations to model the dynamical behavior of metallic nanorods, dispersed in an isotropic solvent, under the influence of a radially varying electric field. Diffusive and convective transport is considered both in orientation and position space. The Smoluchowski equation governing the spatial and orientational probability density function (PDF) was derived. Discretization was carried out using a finite-volume method on a mesh generated via Voronoi tesselation and regularization on the unit sphere. The time evolution of the PDF was obtained using a combination of operator splitting and a stable biconjugate gradient method. We present the results of our numerical experiments. We report interesting and anomalous behavior, where, due to the coupling of orientation and translational mobility, the applied field depopulates certain orientational states, similar to 'orientational hole burning' in nonlinear optics.   

Designing self-propelling micro-swimmer that navigates in microfluidic channels

Using a fully-coupled computational approach that integrates the lattice Boltzmann model for the hydrodynamics and the lattice spring model for the micromechanics of deformable solids, we design a synthetic micro-swimmer that not only self-propels but also successfully navigates in a low Reynolds number environment of a microfluidic channel. The swimmer body consists of a responsive polymer gel that undergoes periodical swelling and shrinking. Two thin elastic flappers are attached to the opposite sides of the swimmer body. The flappers wiggle driven by swimmer body oscillations and, in this fashion, propel the micro-swimmer through its highly viscous fluid environment. Third, light sensitive flapper is attached in the front of the swimmer and serves to steer its trajectory in microchannel. When exposed to light, the steering flap bends towards the light source. We show that this swimmer can either move straight or turn in the required direction following light signals. Thus, guided by light, the micro-swimmer can successfully navigate towards the target in a microfluidic channel.   

Azobenzene Crystal Shooting and Shape Behavior in the Context of Time Dependent Ginzburg-Landau Equations

Blends of azobenzene chromophore and diacrylate monomer show novel nucleation instability. Once a crystal nucleates near a larger growing crystal, it shoots away from the growing front. This shooting phenomenon is explained in the context of ``Marangoni propulsion,'' an imbalance of surface energies at the leading and trailing crystal edges. A concentration gradient is established during the course of diffusion-controlled crystal growth; as the crystal front pulls azobenzene molecules in and rejects acrylate solvent molecules. Thus, crystal growth dynamics influence the concentration gradient build up at the advancing front, as well as the crystal's shape. The time dependent Ginzburg-Landau model C equation was used to simulate crystal growth using a free energy expression which combines Flory-Huggins theory of liquid-liquid demixing and the phase field free energy of crystallization. We have also established a theoretical phase diagram by self-consistently solving the free energy expression. Crystal shape and shooting character will be explained in the context of the phase diagram.   

Flagellar generated flow mediates attachment of {\it Giardia lamblia}

{\it Giardia lamblia} is a protozoan parasite responsible for widespread diarrheal disease in humans and animals worldwide. Attachment to the host intestinal mucosa and resistance to peristalsis is necessary for establishing infection, but the physical basis for this attachment is poorly understood. We report results from TIRF and confocal fluorescence microscopy that demonstrate that the regular beating of the posterior flagella generate a flow through the ventral disk, a suction-cup shaped structure that is against the substrate during attachment. Finite element simulations are used to compare the negative pressure generated by the flow to the measured attachment force and the expected performance of the flagellar pump.   

Probing the directional structure and intracellular microrheology of vascular endothelial cells

The magnitude of the rheological properties of cytoplasm is important because it sets the level of intracellular deformation in response to stress. The directionality is equally important because it allows the cell to modulate the stress-strain relation differently along different directions. We aim to elucidate the relation between the structural organization of the cytoplasm and the directionality of its rheological properties by 1) measuring the local orientation of fluorescently labeled intracellular filaments and 2) determining the local directions of the maximum and minimum intracellular viscosity. For this purpose, we improved current microrheology measurements by studying the drag force experienced by a microsphere in an anisotropic viscoelastic network permeated by a liquid. In the limit of strong frictional coupling between network and liquid, the flow around the sphere is modeled with a generalized Stokes equation using several viscosity parameters. We solve this equation analytically to provide new closed-form microrheology formulae relating the resistance measured experimentally to the anisotropic properties of the network. Tracking the random motion of endogenous particles in 2D and using these novel microrheology formulae we measured directional intracellular viscosities.   

Non-equilibrium fluctuations of cell membranes: The effect of cytoskeletal motor activity on membrane dynamics

The mechanics and non-equilibrium (i.e. molecular motor-driven) fluctuation spectrum of living cells remains an open problem. In this talk, we explore the question: What can one infer about the action of endogenous motors in the cytoskeleton by observing the height fluctuations of cell membrane? To address this, we treat the cytoskeleton as a uniform elastic half-space bounded by a membrane with a finite bending modulus and driven out of equilibrium by molecular motors (i.e. myosin). These motors produce transient and stochastic contractile stresses in the elastic bulk. We first calculate the induced undulations of the membrane-bound surface due to the action of a single molecular motor. Then, making assumptions about the spectrum of motor force fluctuations, we calculate the expected non-thermal contribution to the cellular membrane fluctuations due to the action of an ensemble of such motors. We discuss extensions of this simple model to include, e.g. the effect spatially inhomogeneous coupling between the cytoskeleton and the membrane. We also mention ongoing experimental tests of these ideas.   

Session Y10: Electronic Structure of Surfaces and Interfaces

Interface structure and magnetic anisotropy of Fe/Pd(001) and Pd/Fe/Pd(001) monatomic films

Fe and Pd are known to form L1$_{0}$-ordered alloy, which exhibits easy magnetization axis perpendicular to the atomic stacking plane. In order to reveal the origin of the uniaxial magnetic anisotropy in the point of view of atomic structure, we performed the experiments on bare and Pd-covered Fe monatomic films on Pd(001) surface. Interface structure analysis was done by means of intensity-voltage analysis of low-energy electron diffraction (LEED I-V), and the magnetic anisotropy was investigated by X-ray magnetic circular dichroism (XMCD). Sample fabrication and XMCD experiments were performed at HiSOR-BL14 of Hiroshima Synchrotron Radiation Center, Hiroshima University. It is revealed that the intermixing between Fe films and Pd substrate occurs at room temperature growth, and Pd-overlayer compresses the interlayer distance around Fe layer. Fe thickness dependent XMCD revealed that the spin reorientation transition from perpendicular to in-plane direction occurs in bare Fe/Pd(001) with Fe thickness increase. On the contrary, in-plane magnetic anisotropy is stable in Pd/Fe/Pd(001). We attributed the perpendicular magnetic anisotropy in Fe/Pd(001) to the L1$_{0}$-like interface structure which realized in this system.   

Enhancement of Kondo effect through Rashba spin-orbit interactions

The role of Rashba spin-orbit (RSO) interactions on the Kondo regime has been a topic of debate since resistivity measurements on Pt doped Cu:Mn compounds were interpreted as evidence for suppression of the Kondo effect by SO scattering. Subsequent theoretical and experimental activity has yielded conflicting results. Thus, the question: what is the role of SO interactions in the Kondo regime? remains open. To provide a definite answer we obtain an exact solution of an Anderson magnetic impurity model in a two-dimensional metallic host with RSO interactions. We show that the Hamiltonian reduces to an effective two-band Anderson model coupled to a S=1/2 impurity. An appropriate Schrieffer-Wolff transformation produces an effective 2-channel Kondo model plus a Dzyaloshiinski-Moriya (DM) interaction term. The exact solution reveals that the impurity couples to the bath with ferro- and antiferromagnetic couplings. DM interactions, that vanish at half-filling and at the Hubbard U-infinity limits, introduce an exponential increase in the value of the Kondo temperature.   

Correlation induced charge ordering metal-insulator transition in a two-dimensional triangular lattice

Mott insulators are one of the clearest examples on how electronic correlations limit the band theory. Semiconducting surfaces offer an ideal playground to study correlation effects in two dimensions. We report here a combined experimental and theoretical analysis on correlation effects in an atomically ordered reconstruction of 1/3 ML of Sn on Ge(111). This interface exhibits a Mott metal insulator transition below 30 K [1,2]. We find a novel phase between the known metallic and insulating phases, settled by electronic correlations and characterized as a charge ordering insulator (COI) that competes with the lower temperature Mott phase. We describe here the electronic mechanism behind the stabilization of the COI-phase, the role of atomic vibrations in the process, and interpret these findings on the basis of DMFT theoretical calculations. These results explain recent controversies [3] on the interpretation of the nature of the low temperature phase. [1] A. Tejeda et al. Phys. Rev. Lett. 100 (026103) 2008 [2] R. Cortes et al. Phys. Rev. Lett. 96 (126103) 2006 [3] H. Morikawa et al. Phys. Rev. B 78 (245307), 2008; S. Colonna et al., Phys. Rev. Lett. 101 (186102) 2008   

Passage from Spin-Polarized Surface States to Unpolarized Quantum Well States in Topologically Nontrivial Sb Films

Topological insulators, which possess robust gapless surface states as a result of strong spin-orbit coupling, have attracted much interest because of their unusual surface spin structures. When such materials are reduced to ultrathin films, the spin-split surface states must connect, by analytic continuation, to quantum well states, which are spin-unpolarized in centrosymmetric systems. We report herein a combined experimental and theoretical study of this passage from polarized to unpolarized states in Sb films. Bulk Sb is semimetallic with a negative band gap; nevertheless, it shares the same topological order as Bi$_{1-x}$Sb$_{x}$ (0.07$<$ x$<$ 0.2), the first material identified as a three-dimensional topological insulator. Angle-resolved photoemission (ARPES) from Sb films, aided by first-principles calculations, shows smooth dispersion relations associated with this passage; the spin polarizations of the two states fade away, while the energy splitting is maintained through the emergence of different charge density patterns of the resulting quantum well states.   

Electron-grain boundary scattering and the resistivity of nanometric metallic structures

The resistivity of metallic structures depends on electron-grain boundary and electron-surface scattering. By tuning the grain size, we have been able to separate the contribution to the resistivity originating in electron-grain boundary scattering, from that arising in electron-surface scattering. The resistivity of gold films approximately 54 nm thick deposited onto mica substrates under high vacuum, was measured between 4 and 300 K. It exhibits a cross over, in samples where the average grain diameter d $>$ 38 nm and the resistivity is determined by electron-surface plus electron-phonon scattering, to a regime where it is determined by electron-grain boundary plus electron phonon scattering , in samples where d $<$ 38nm. l(300)=38 nm is the electron mean free path in the bulk at 300 K. The resistivity can described by Drude's model. It can be described as well by Mayadas's theory using the grain boundary reflectivity R \textit{as the only adjustable parameter.} Funded by FONDECYT 1085026. \textbf{References.} R. Henriquez et al., Phys. Rev. \textbf{B82} (2010) 113409.   

Atomic and Electronic Structures of the Cu2O/TiO2 Heterostructure Interface

Earth-abundant metal oxides have great potentials in replacing Si in semiconductor solar cells and meeting the terawatt scale global energy demand. The structural and electronic properties of the heterojunction interface in oxide-based thin film solar cells, which is of great importance to the energy conversion efficiency, however, is not well understood yet. In this talk, we will present our experimental and theoretical work on the atomic and electronic structures of the interface of Cu$_2$O and anatase TiO$_2$. Despite the large lattice mismatch of 13\%, Cu$_2$O can be grown epitaxially on TiO$_2$(001) in the cube-on-cube orientation by pulsed laser deposition. The interface is found to form a regular coincidence lattice of 8 Cu$_2$O and 9 TiO$_2$ unit cells in each in-plane direction. The relaxed structure of this coincidence lattice is simulated using density functional theory calculations. The local density of states along the interface is found to shift as much as 0.4 eV, depending on the local alignment of the two lattices. As a result, the valence band and conduction band edge wave functions are well separated.   

Response of the Shockley surface state on Cu(111) to an external electrical field: A density-functional theory study

We study the response of the Cu(111) Shockley surface state to an external electrical field E by combining a density-functional theory calculation for a finite slab geometry with an analysis of the Kohn-Sham wavefunctions to obtain a well-converged characterization. We find that the surface state displays isotropic dispersion, quadratic until the Fermi wave vector but with a significant quartic contribution beyond. We find that the shift in band minimum and effective mass depend linearly on E. Most change in electrostatic potential profile, and charge transfer occurs outside the outermost copper atoms, and most of the screening is due to bulk electrons. Our analysis is facilitated by a method used to decouple the Kohn-Sham states due to the finite slab geometry, using a rotation in Hilbert space. We discuss applications to tuning the Fermi wavelength and so the many patterns attributed to metallic surface states.   

Measuring Transport Properties of Thin Films Under Isotropic and Anisotropic Strain Using Piezoelectric Substrates

Thin film systems have been of great technological interest in the last few decades due to their unique properties. It is crucially important to understand transport properties of such films under strain for some applications such as in strain gauges. Piezoelectric materials have been used in the past to study the isotropic strain-dependent properties of magnetotransport devices. We have extended this technique using one of the shearing modes of a Lead Magnesium Niobate-Lead Titanate (PMN-PT) crystal poled in the $<$011$>$ direction to study anisotropic strain in thin films. A double Hall Bar pattern oriented along the eigenaxes of the piezoelectric shearing mode permits the characterization of the film in both directions simultaneously. A uniform field in the piezoelectric substrate may be achieved for patterned devices by growing a metal surface surrounding the entire pattern. We will discuss how the changes in the carrier density and electron mobility associated with strain can be characterized in thin metal films deposited directly on the PMN-PT substrate.   

Symmetry-induced confinement in reconstructed Au(100)

The clean reconstructed Au(100) surface was investigated by angle-resolved photoemission spectroscopy, Low Energy Electron Diffraction and Scanning Tunneling Microscopy. The reconstruction can be described as a floating, corrugated hexagonal layer on top of the bulk-terminated substrate, as in the case of Ir(100) and Pt(100) surfaces. We determine a superperiodicity of (5x26). The substrate Shockley surface state survives the reconstruction and becomes an interfacial surface state. Compelling evidence supports that the overlayer behaves as a quasi-1D system. The presence of quasi-1D states and Shockley surface states are both a consequence of a certain degree of vertical electronic confinement induced by the different symmetry of the hexagonal overlayer and the square bulk-terminated Au(100). The existence of quasi-1D states reveals a significant lateral confinement perpendicular to the atomic chains of the reconstructed Au layer.   

Surface states localization induced by single adatoms at metal surfaces

The perturbation introduced at metal surfaces by the adsorption of a single adatom affects the surface states. Due to their two dimensional character, localized bound states could result at energy lower then the pristine surface states. The extent of such a localization depends on a variety of aspects such as the attractive strength of the adatom induced potential, the adsorption distance, the nature of the surface state. We investigate this effect through a density functional theory approach that accounts for the semi-infinite character of the substrate and which reproduces the experimental surface states and the surface projected energy gap. The results obtained for different kind of adatoms (both magnetic and paramagnetic) on metal surfaces are discussed, focusing on the localization of the Shockley state and of the image states for different adsorption configurations. The spin splitting of the localized bound state will be also analyzed for magnetic adatoms.   

Structure, Morphology and SRF Characteristics of Superconducting Niobium Thin Films on Ceramic Substrates

The need to improve superconducting thin film coatings for radio frequency (SRF) cavities used in linear accelerators has inspired recent niobium thin film research. To better understand the SRF properties in thin film niobium, correlated studies of structure, surface morphology and SRF performance are examined. Recent work on epitaxial growth of niobium on insulating ceramic substrates --- a-plane sapphire and MgO (001) --- anticipates Superconducting / Insulating / Superconducting (SIS) multilayer structures, which have been proposed as a means to achieve higher field gradients in SRF cavities, overcoming the intrinsic SRF limitations of bulk niobium. A fundamental study correlating structure, morphology and SRF superconducting properties of niobium thin films is an imperative first step towards realizing next generation SRF materials.   

The Electronic Structure and Properties of Different Surface Terminations of Li2B4O6 Single Crystal

The electronic structure of the(100) and (110) surfaces of Li2B4O6 single crystal was investigated by combined angle- resolved photoemission and inverse photoemission spectroscopies. The obtained results are in a qualitative agreement with the available model bulk band structure calculations.Together with some common features, they reveal clear differences between the two surfaces. For both of them the observed dispersion of the conduction band is much greater than that of valence band and both surfaces are of n-type, though the feature is more pronounced for (100) surface, which, on the whole, is more polar. However, the (110) surface demonstrates much more sofisticated properties exhibiting, in particular, the true surface states and complicated temperature and time dependent photovoltaic charging behaviour. For this surface, in the temperature range of(80-280)K, the off-axis pyroelectric effect was observed with strongly temperature dependent currents in the $<$110$>$ direction and much smaller pyroelectric coefficient than that measured in the $<$001$>$ direction.   

Correlated electron effects in low energy alkaline earth ion scattering

The spin correlations of many electrons can lead to emergent phenomena that cannot be extrapolated from the behavior of independent electrons. The role of such multi-electron processes in charge exchange during atom-surface collisions remains a challenging and unsolved problem. Two prior independent theoretical investigations predicted that when a projectile has a single unpaired electron or hole, this localized spin impurity would induce a Kondo resonance at the Fermi energy leading to a mixed-valent state in the metal conduction band. The occupancy of this sharp state would be a strong function of the surface temperature, which would cause an anomalous temperature dependence of the neutralization probability in a scattering experiment. We demonstrate such dependence for low energy Sr$^{+}$ scattered from clean polycrystalline gold. This unusual temperature dependence is amplified when the metal work function is reduced by embedding Sr atoms into the material.   

Session Y13: Granular Materials I

Homogeneous linear shear of a two dimensional granular system

Using a novel shear device, we experimentally study the response of dry granular materials to quasi-static shear. Our apparatus is capable of creating linear strain profiles over the entire width of the two dimensional shear cell. By eliminating the usual tendency of granular shear to localize in non-uniform shear bands, we can study the poorly understood nature of granular flows in great detail. We employ photo elastic particles, fluorescent labelling and high resolution imaging to obtain information about particle positions, rotation and inter particle forces. We discuss our results in the context of the jamming scenario and also look at various measures capable of elucidating the physics of dense granular flows.   

Constitutive relations for granular fluid of smooth inelastic hard spheres, to Burnett order

In the framework of kinetic theory for dilute granular gases, we have generalized the work of Sela \& Goldhirsch (1998) by including body force (gravity) term in the Boltzmann equation. In order to derive the constitutive relations for flows of smooth inelastic hard spheres in three dimensions, the Boltzmann equation is perturbatively solved by performing generalized Chapman-Enskog (double expansion) in two small parameters, the Knudsen number and the degree of inelasticity. We have derived the constitutive relations till Burnett order (up to second order in small parameters). In this talk I would like to present the methodology for obtaining the constitutive relations.\\[4pt] Ref: Sela, N. \& Goldhirsch, I. 1998 Hydrodynamic equations for rapid flows of smooth inelastic spheres, to Burnett order. \textit{J. Fluid Mech.} \textbf{361}, 41--74.   

Random to ordered granular sphere packings through cyclic shear

We investigate the structure of a dense granular packing submitted to quasi-static cyclic shear deformations using a fluorescent liquid refractive index matching method. This technique allows us to obtain the three dimensional position of 1mm glass spheres in the bulk during each cycle. The granular packing is observed to evolve towards crystallization over hundreds of thousands of shear cycles and the packing fraction is correspondingly observed to increase from loose packing fraction, 0.59, to above random close packing, 0.634. The appearance and the propagation of the crystalline order was studied using the orientational order metric, Q$_{6}$. In the early stages of nucleation the particles belonging to the nucleating crystallites are predominantly in hexagonal close packed configuration. When the packing volume fraction approaches a value close to random close packing, a rapid increase of the global Q$_{6}$ and the number of particles with local face centered cubic order is observed. Following the evolution of the crystallites, we find the critical nuclei size to be between 10-50 particles, surprisingly consistent with transitions observed with thermal elastic frictionless spheres. A detailed description of the crystalline clusters and their development will be presented.   

Shearbanding Instability and Patterns in Granular Shear Flows

When a (dense) granular material is sheared in shear-cell experiments, shearing remains confined to a narrow localized zone (``shearband'') near the moving boundary. Such shear-banding has also been realized in the molecular dynamics simulations of granular plane Couette flow for a range of densities (even without gravity) in the rapid flow regime. In this talk I will present the shear-banding instability of granular shear flow via an order parameter equation. \\[4pt] [1] Weakly nonlinear theory of shear-banding instability in granular plane Couette flow: analytical solution, comparison with numerics and bifurcation, Priyanka Shukla and Meheboob Alam, Journal of Fluid Mechanics 2010, {\bf 665}, p. 1-50. \\[0pt] [2] Landau-type order parameter equation for shear banding in granular Couette flow, Priyanka Shukla and Meheboob Alam, Physical Review Letters, {\bf 103}, 068001, 2009. \\[0pt] [3] Universality of shear-banding instability and crystallization in sheared granular fluid, Meheboob Alam, Priyanka Shukla and Stefan Luding, Journal of Fluid Mechanics, {\bf 615}, p. 293-321, 2008.   

Shearing granular media: from elasticity to compaction

A granular system is able to behave like a solid (a sand pile for example) or like a liquid depending on the deformation imposed on the material. Using rheometry measurements we investigate the response of a granular bed to an imposed deformation or an imposed stress as a function of its packing fraction. We observed different regimes: elastic and plastic behaviors, flow regime and finally compaction. The dependence of these regimes on the packing fraction and on the pressure allows us to delineate the phase diagram of granular media.   

Microscopic rearrangements and macroscopic stress fluctuations in dense emulsion flow

One characteristic of dense granular materials is they can resist small stresses but start to flow under large stresses. During granular flow, the stress exerted on the boundaries of the flow can have large fluctuations. These fluctuations are thought to originate from internal rearrangements and from changes of force chains; however, the connection between these internal microscopic changes and the macroscopic influences seen at the boundaries is not yet clear. We experimentally study the shear flow of oil-in-water emulsion droplets in a Hele-Shaw cell with a hopper shape. Due to the thinness of the Hele-Shaw cell, the droplets are deformed into quasi-2D pancakes, somewhat analogous to soft photoelastic disks. As droplets approach the hopper exit, they shear past one another and droplets are forced to rearrange. We focus on a typical plastic rearrangement called T1 event, where local four particles have neighbor exchanges. Simultaneously, we use the deformation of the droplets to determine the interdroplet forces, which also change as the sample is sheared. These forces fluctuate over large regions as expected. Our analysis of this emulsion system shows a direct and local relationship between microscopic T1 rearrangements and macroscopic stress ?uctuations.   

Flow and Sedimentation of particulate suspensions in Fractures

Suspended particles are commonly found in reservoir fluids. They alter the rheology of the flowing liquids and may obstruct transport by narrowing flow channels due to gravitational sedimentation. An understanding of the dynamics of particle transport and deposition is, therefore, important to many geological, enviromental and industrial processes. Realistic geological fractures usually have irregular surfaces with self-affine structures, and the surface roughness plays a crucial role in the flow and sedimentation processes. Recently, we have used the lattice Boltzmann method to study the combined effects of sedimentation and transport of particles suspended in a Newtonian fluid in a pressure-driven flow in self-affine channels, which is especially relevant to clogging phenomena where sediments may block fluid flows in narrow constrictions of the channels. The lattice Boltzmann method is flexible and particularly suitable for handling irregular geometry. Our work covers a broad range in Reynolds and buoyancy numbers, and in particle concentrations. In this talk, we focus on the transitions between the ``jammed'' and the ``flow'' states in fractures, and on the effects of nonuniform particle size distributions.   

Dilation of Granular Packings of Spheres and Non-Spherical Particles under Shear

A parallelepiped shear cell is used to experimentally measure the dilation of particles prepared at different initial volume fractions from relatively loose assemblies to densely packed ones. The samples consist of spherical marbles, plastic ellipsoids and tetrahedral dice at the centimeter scale and specially prepared particles at the millimeter scale. Under quasi-static shear, loosely packed samples compact while densely packed particles dilate, as in previous studies. For small shear amplitudes, both the dilation and compaction of the tetrahedral packings is significantly larger than that of spheres.   

Dynamic crystallization in granular flow

We explore dynamic crystallization in simulations of two dimensional (2D) inelastic frictional hard disks as a model for granular materials. Previous simulations and experiments show formation of hexagonal structures in quasi-2D systems under vibration, rotation, and shearing. In experiments of a uniform but non-equilibrium steady-state (UNESS) under constant pressure the gas-crystal transition shows all of the classic signs of a first-order sublimation phase transition including discontinuous change in density, rate dependent hysteresis, and an equation of state consistent with sublimation. We use molecular dynamics to simulate steady shear under a variety of boundary conditions to determine a dynamic equation of state in the in the density range of the crystallization transition. We compare the dynamic equation of state with that found in non-flowing UNESS experiments, simulations, and theory.   

The path to fracture: dynamics of broken-link networks in granular flows

Capturing the dynamics of granular flows on intermediate length scale can often be difficult. We propose the broken-links network as a new tool to study fracture at the intermediate scale. Using experimental data on the 3D tracks of all particles in a region, we calculate the dynamically evolving broken-links network and observe a second-order, percolation-like phase transition in the formation of the giant component as links are broken. We implement a velocity gradient model of link breakages and find that the model demonstrates a faster growth of the giant component than the data. Surprisingly, the broken-links network formed in the model is also more highly clustered than our empirical observations.   

Hockey night in phase space

In order to explore the possibility of developing a statistical mechanics for dissipative ensembles, we have performed an experiment in which we track the translational and rotational velocities of pucks on an air hockey table. The pucks are driven by bumpers at the boundaries and are bidisperse to prevent crystallization. At packing fractions of 60\% we find that the system distributes rotational and translation energy according to the equipartition theorem. However, as the packing fraction increases, the ratio of rotational energy to translational energy also increases to a value larger than predicted by equipartition. The translational and angular velocity distributions are approximately exponential and the distributions of the translational velocity are the same for both large and small particles. In contrast, the distribution of the angular velocities is broader for the small particles than for the large.   

Rotational statistics in dense granular flows of smooth cylindrical particles

We report the results of an experiment to investigate the dissipation in the rotational degree of freedom for smooth cylindrical particles in a dense, driven granular flow. The flow is studied in a rotating drum of radius R = 30 cm for particles of radius r = 0.635 cm while the cell is rotated at speeds between 0.25 and 0.75 Hz. The 2D geometry of the experimental design allows for the measurement of two translational degrees of freedom as well as the rotation of the disks within the driven flow. The rotational velocity statistics demonstrate non-Gaussian behavior as well as a significant amount of energy being dissipated within the flow via the tangential friction between the particles. The results of this experiment are significant in that many driven granular experiments use smooth cylindrical or spherical particles to investigate granular dynamics, but the contribution from the rotational degrees of freedom are often unmeasured. A novel imaging technique is used to extract both the translational and rotational velocity statistics to a high degree of precision in the entire cell during the experiment.   

Size Segregation of Granular Materials

Segregation of granular materials due to size difference while flowing/energized is a very well known but poorly understood phenomena. Despite of some good understanding of the mechanism of size segregation, predictive models for size segregation are not available. Size segregation of binary granular mixtures flowing over inclined plane is studied by means of DEM simulations. Buoyant force acting on trace particles of a bigger size is obtained by varying the density of the trace particles rising/sinking in the granular flow. We show that moderately big trace particles of same density as that of the light particles tend to rise because of higher buoyancy forces than the weight of the trace particles. For very big trace particles of same density, however, the buoyant force becomes smaller than the weight of the particles causing the particles to settle down. Drag force on the trace particle is found to be given by Stokes' law. Friction drag is found to almost $10-12\%$ of the weight of the trace particles. Incorporating the Stokes' law and balancing the segregation and diffusion flux of big particles, we are able to predict the number fraction of the big particles in terms of viscosity and diffusivity for moderately dilute binary mixture of different size particles. The proposed theory is tested against DEM simulation results and very good agreement has been found with the simulation results.   

Session Y14: Focus Session: Statistical Mechanics of Complex Networks II

Universal features of dynamic small-world networks

In a dynamic small-world contact network, an individual has fixed short range links within its local neighborhood and time-varying stochastic long range links outside of that neighborhood. The probability of a long range link occurring ($p$, in analogy with the standard small-world rewiring parameter) is a measure of the mobility of the population. In this study we investigate the epidemic to non-epidemic phase transition that occurs in a susceptible-infected-recovered (SIR) disease spreading model located on this type of dynamic network. We first derive the finite-valued critical mobility p$_{c}$ and find excellent agreement with numerical simulations. Close to p$_{c}$ the outbreak size scales as (p-p$_{c})^{\beta }$ since it is a continuous transition; we find that $\beta $ is near 2, but varies as a function of the infectivity and recovery rates. At the critical point our study shows that the distribution of outbreak sizes scales as $\sim $ N$^{-\alpha }$ with $\alpha $ = 1.5$\pm $0.03. We compare these critical exponents to those found in related small-world and dynamic small-world networks and comment on potential universality.   

Robustness and dynamics of networks of coupled modules

Many systems, from power grids and the internet, to the brain and society, can be modeled using networks of coupled overlapping modules. The elements of these networks perform individual and collective tasks such as generating and consuming electrical load or transmitting data. We study the robustness of these systems using percolation theory: a random fraction of the elements fail which may cause the network to lose global connectivity. We show that the modules themselves can become isolated or uncoupled (non-overlapping) well before the network falls apart. This has important structural and dynamical consequences for these networks and may explain how missing information hides pervasive overlap between communities in real networks.   

Explosive Percolation in the Human Protein Homology Network

We study the explosive character of the percolation transition in a real-world network. We show that the emergence of a spanning cluster in the Human Protein Homology Network (H-PHN) exhibits similar features to an Achlioptas-type process and is markedly different from regular random percolation. The underlying mechanism of this transition can be described by slow-growing clusters that remain isolated until the later stages of the process, when the addition of a small number of links leads to the rapid interconnection of these modules into a giant cluster. Our results indicate that the evolutionary-based process that shapes the topology of the H-PHN through duplication-divergence events may occur in sudden steps.   

Power Spectrum of the Finite Kuramoto Model

We study the synchronization of oscillators in the finite Kuramoto model, a simple model for coupled phase oscillators that exhibits a phase transition. The usual self-consistent approach used in studying the Kuramoto model gives a prediction for the distribution of modified frequencies that includes a Dirac delta at the synchronized frequency and a depletion of nearby frequencies. For finite systems, the prediction adequately describes the distribution of frequencies averaged over very long durations, but the accompanying power spectrum of the order parameter looks very different. The sharp peak at the synchronization frequency has a finite width and oscillators that are otherwise entrained manage to occasionally escape. The resulting harmonics of these escaped oscillators leads to a power spectrum with an exponential drop-off from the peak, rather than the originally predicted depletion.   

Recruitment dynamics in adaptive social networks

We model recruitment in social networks in the presence of birth and death processes. The recruitment is characterized by nodes changing their status to that of the recruiting class as a result of contact with recruiting nodes. The recruiting nodes may adapt their connections in order to improve recruitment capabilities, thus changing the network structure. We develop a mean-field theory describing the system dynamics. Using mean-field theory we characterize the dependence of the growth threshold of the recruiting class on the adaptation parameter. Furthermore, we investigate the effect of adaptation on the recruitment dynamics, as well as on network topology. The theoretical predictions are confirmed by the direct simulations of the full system.   

The spread of opinion on co-evolving networks

We discuss a model of opinion formation in co-evolving networks. In realistic scenarios, the network constantly changes structure favoring connections between similar individuals (homophily). Here we allow the opinions to co-evolve with the reorganization of links in the network. This dynamical nature of the network impedes the spreading of opinions. We study how this resistance to the spread can be overcome and consensus can be achieved by randomly distributing a few committed agents (-nodes that are not influenceable in their opinions). In this model adjacent nodes influence each other if they are similar on at least Q attributes, where Q is the influence threshold. Nodes will rewire their existing links if they are not similar enough. We demonstrate through simulations that in the absence of committed agents, time to reach consensus in opinions diverges exponentially with system size N. However, as committed agents are added, beyond a small value of committed fraction, the consensus time becomes a slowly varying function of N. (Ref- F. Vazquez et al. - Phys. Rev. E76, 046120 -2007)   

Network resilience to real-world disasters: Eyjafjallaj\"okull and 9/11

We investigate the resilience of the the world-wide air transportation network (WAN) to the 9/11 terrorist attacks and the recent eruption of the volcano Eyjafjallaj\"okull. Although both disasters caused wide-spread disruption, the number of airports that were closed and the volume of interrupted traffic were well below the percolation threshold predicted by the classical theory. In order to quantify and visualize network deformation before breakdown, we introduce a framework based on the increase in shortest-path distance and homogenization of shortest-path structure. These real-world disasters are a new type of disruption because the removal of all vertices (airports) is geographically compact. Our framework incorporates the dual perspective of individual airports and geopolitical regions to capture how the impact interacts with the sub-network structure.We find that real-world events have an impact signature which is qualitatively different from that of random or high-centrality attacks. Furthermore, we find that the network is more resilient to the 9/11 disaster, although it removed more airports and traffic than the volcanic ash-cloud. This is due to the network roles of Europe and North America. We discuss how regional roles influence resilience to a region's removal.   

Stochastic Moment Equations - Case Closed

Reaction networks frequently appear in many natural systems, such as chemistry, biology and ecology. The modelling of these networks is commonly based on rate equations models, incorporating the law of mass action kinetics. However, when the system is microscopic, it becomes governed by fluctuations, the law of mass action kinetics no longer applies, and the rate equations fail. To obtain an accurate description of microscopic reaction networks, one must refer to stochastic methods based on the master equation. The problem is that the number of equations rises exponentially with the number of species, rendering the treatment of the master equation infeasible. Moment equations are known to be more efficient, however the equations are not closed, and become prohibitively complicated when moments of high order are included. In this talk we present the binomial moment equations. The binomial moments are linear combinations of the ordinary moments related to the population size of the reactive species. They capture the essential combinatorics of the reaction processes reflecting their stoichiometric structure. This leads to a simple and transparent form of the equations, allows a highly efficient and surprisingly simple truncation scheme and enables the inclusion of moments up to any desired order. The result is a set of equations that enables an equation-based stochastic analysis of reaction networks under a very broad range of conditions.   

Using a Projector to Control BZ Drops: Attractor Selection by Pattern Entrainment

An emulsion consisting of drops in the 100$\mu$m diameter range containing the Belousov-Zhabotinsky (BZ) oscillatory chemicals can interact via diffusive inhibition and can be thought of as coupled phase oscillators. For weak coupling, a 2-D hexagonal lattice of these drops naturally develop regions of attractor states of sequential oscillations with phase differences of plus/minus $2\pi/3$ much like the 2D anti-ferromagnetic Heisenberg spin model. An untrained system of these oscillators will develop unstable regions of both attractors that grow and compete. We use photo-initiated inhibition to optically entrain the system with a projected $+2\pi/3$ pattern in an attempt to force the system into the $+2\pi/3$ attractor state. However, both the left and right handed variants of the $2\pi/3$ attractor are present in the entrained system. Defining an order parameter $e^{i 3 \phi}$ allows for a quantitation of the purity of the $2\pi/3$ attractor state in the final system.   

Complexity facilitates perturbation of a coherent dynamical process

We discuss the influence of perturbation on networks of globally coupled three state stochastic oscillators. When coupled, the system shows intermittent behavior characterized by a waiting time distribution which reveals both inverse power-law and coherent dynamical properties. Specifically, we compare the results of perturbation realized with a periodic signal to those obtained using perturbation provided by a matching system. We find that the SNR (signal-to-noise ratio) does not depend on the frequency of the perturbing signal. We also observe that the second approach results in higher values of SNR. We discuss how those findings cannot be explained by either classical or statistical resonance theory. With the help of the fluctuation-dissipation theorem [1] we determine the role of the scaling dynamics in the system under investigation. \\[4pt] [1] Aquino G., Bologna M., Grigolioni P., West B.J., PRL 105, 040601 (2010)   

Coupled Oscillations in a 1D Emulsion of Belousov-Zhabotinsky Droplets

We experimentally and computationally study the dynamics of interacting oscillating Belousov-Zhabotinsky (BZ) droplets of $\sim $120 $\mu $m diameter separated by perfluorinated oil and arranged in a one-dimensional array (1D). The coupling between BZ droplets is dominated by inhibition and is strongest at low concentrations of malonic acid (MA) and small droplet separations. A microfluidic chip is used for mixing the BZ reactants, forming monodisperse droplets by flow-focusing and directing them into a hydrophobized 100 $\mu $m diameter capillary. For samples composed of many drops and in the absence of well defined initial conditions, the anti-phase attractor, in which adjacent droplets oscillate 180\r{ } out of phase, is observed for strong coupling. When the coupling strength is reduced the initial transients in the phase difference between neighboring droplets persist until the BZ reactants are exhausted. In order to make quantitative comparison with theory, we use photosensitive Ru(bipy)$_{3}^{2+}$-catalyzed BZ droplets and set both boundary and initial conditions of arrays of small numbers of oscillating BZ droplets with a programmable illumination source. In these small collections of droplets, transient patterns decay rapidly and we observe several more complex attractors, including ones in which some adjacent droplets are in-phase.   

Quarantine generated phase transition in epidemic spreading

We study the critical effect of quarantine on the propagation of epidemics on an adaptive network of social contacts. For this purpose, we analyze the susceptible-infected-recovered (SIR) model in the presence of quarantine, where susceptible individuals protect themselves by disconnecting their links to infected neighbors with probability w, and reconnecting them to other susceptible individuals chosen at random. Starting from a single infected individual, we show by an analytical approach and simulations that there is a phase transition at a critical rewiring (quarantine) threshold $w_c$ separating a phase ($w= w_c$) where the disease does not spread out. We find that in our model the topology of the network strongly affects the size of the propagation, and that $w_c$ increases with the mean degree and heterogeneity of the network. We also find that $w_c$ is reduced if we perform a preferential rewiring, in which the rewiring probability is proportional to the degree of infected nodes.   

Session Y15: Focus Session: Spins in Semiconductors - Spin Currents IV

Electrically-generated electron spin polarization for non-reciprocal integrated photonic devices

Electron spin polarization based photonic devices offer promising advantages over current technologies. Spin orbit coupling allows for an all-electrical means of control over light in semiconductor waveguides. Electrically generated spin polarization results in a circular dichroism near the absorption edge which results in non-reciprocal Faraday rotation. We investigate the requirements for manipulating light in semiconductor waveguides using electrically-generated spin polarization. Ultimately, device performance will be limited by the magnitude of achievable Faraday rotation, birefringence, and absorption. We show that one can limit birefringence by appropriate waveguide design and that substantial Faraday rotation is accessible sufficiently far below the band edge for material absorption to be minimal.   

Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet-Semiconductor Heterostructures

We demonstrate a spatially-confined magnetic field gate to modulate the Larmor frequency of an optically-injected spin ensemble drifting down a GaAs channel [1]. The gate is activated either optically or electrically and polarizes GaAs nuclear spins at the interface between a lithographically-defined MnAs island and the channel via the ferromagnetic proximity polarization effect. We measure the rotation angle of the spin ensemble as it emerges from the polarized region using time-resolved Kerr rotation. The ensemble's spin rotation angle can be tuned by up to 5$\pi$ radians as the spins travel over 30 $\mu $m by controlling the nuclear field strength and adjusting the drift velocity. \\[4pt] [1] M.E. Nowakowski, et. al., Phys. Rev. Lett. 105, 137206 (2010)   

Large and Small Signal Analyses of Spin Modulation in Lasers

We have developed a set of rate equations for semiconductor spin-lasers that contain spin-polarized carriers (electrons and holes) in the active region due to spin-polarized (electrical or optical) injection. Previous studies consider the steady-state regime [1-5], showing advantages of the spin-lasers over its conventional counterparts such as threshold reduction and enhanced emission intensity [6]. We suggest a further improvement of spin lasers under dynamical operation. We use both large and small signal analyses to show that the spin-polarized injection can lead to an enhanced bandwidth and desirable switching properties of spin-lasers. Supported by ONR, AFOSR, NSF-ECCS CAREER. \\[4pt] [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). \\[0pt] [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \\[0pt] [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). \\[0pt] [4] C. Gothgen, R. Oszwaldowski, A. Petrou, I. Zutic, Appl. Phys. Lett. 93, 042513 (2008). \\[0pt] [5] I. Vurgaftman et al., Appl. Phys. Lett. 93, 031102 (2008). \\[0pt] [6] J. Lee, W. Falls, R. Oszwaldowski, and I. Zutic, Appl. Phys. Lett. 97, 041116 (2010).   

Mapping between Quantum-Dot and Quantum-Well Spin-Lasers

It has been demonstrated that performance of semiconductor lasers with a quantum-well (QW) active region can be improved by injecting spin-polarized carriers [1-3]. Their rate-equation models have been developed [4-5], however, description of a quantum-dot (QD) spin-laser, demonstrated recently [6], is more complicated [7]. Here, we present a method which allows to employ the simple QW rate equations to study the QD spin-lasers. With this method, one can easily extract QW-like parameters such as differential gain, gain compression factor and time constants. This effort is worthwhile, because the QW spin-laser rate equations have exact analytical solutions, unlike their QD counterparts [7]. Supported by US ONR, AFOSR, DOE-BES, and NSF-ECCS CARRER. [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). [4] C. Gothgen, R. Oszwaldowski, A. Petrou, I. Zutic, Appl. Phys. Lett. 93, 042513 (2008). [5] I. Vurgaftman et al., Appl. Phys. Lett. 93, 031102 (2008). [6] D. Basu et al., Appl. Phys. Lett. 92, 09119 (2008). [7] R. Oszwaldowski, C. Gothgen, and I. Zutic, Phys. Rev. B 82, 085316 (2010).   

High frequency dynamics and output polarization of a spin laser

The dynamic characteristics of a spin laser have been studied theoretically and experimentally. Calculations with the coupled carrier and photon rate equations show that the small signal modulation bandwidth of the preferred polarization mode is enhanced due to spin injection. The large signal modulation characteristics show temporally separated relaxation oscillations corresponding to the two polarization modes. More importantly, it is shown that an output polarization of 100{\%} can be obtained, with appropriate biasing conditions, irrespective of the degree of spin injection. This is experimentally verified in a quantum dot spin-vertical cavity surface emitting laser (spin VCSEL), where an output polarization of $\sim $ 60{\%} is measured with a 5-6{\%} carrier spin polarization in the active region.   

Consequences of spin transport in heterogeneous environments

Understanding behavior of spins in spatially varying environments such as magnetic fields, spin lifetime and gyromagnetic ratio is very important for real spintronic devices [1]. We present here numerical solutions of the spin diffusion equation in such situations. We show that local magnetic fields can be useful as an imaging tool for spin properties such as spin lifetime. It can also complicate the interpretation of experimental results in the case of spin injection from a ferromagnet into a semiconducting channel through a rough interface [1,2]. \\[4pt] [1] S.P.Dash et.al, Electrical creation of spin polarization in silicon at room temperature, Nature 462, 491-494\\[0pt] [2] V.P. Bhallamudi et.al, Spin transport and imaging opportunities in inhomogeneous environments, arXiv:1010.3747v1 [cond-mat.mes-hall]   

Robust Level Coincidences in the Subband Structure of Quasi 2D Systems

Recently, level crossings in the energy bands of crystals have been identified as a key signature for topological phase transitions. In general, three independent parameters must be tuned appropriately to bring two quantum levels into degeneracy. Using realistic models we show that for Bloch electrons in a crystal the parameter space controlling the occurrence of level coincidences has a much richer structure than anticipated previously. In particular, we identify cases where level coincidences depend on only two independent parameters thus making the level coincidences robust, i.e., they cannot be removed by a small perturbation of the Hamiltonian compatible with the crystal symmetry. We consider HgTe/CdTe quantum wells as a specific example. (See arXiv:1011.xxxx)   

Anomalous spin-resolved point-contact transmission of holes due to cubic Rashba spin-orbit coupling

We present experimental and theoretical evidence for the crossing at finite wave vector of the two lowest one-dimensional spin-split subbands in quantum point contacts fabricated from two-dimensional hole gases with strong spin-orbit interaction. We derive the existence of such crossing point from a two-dimensional spin-orbit interaction with a cubic momentum dependence, appropriate for asymmetric quantum wells. This phenomenon provides an explanation for the anomalous sign of the spin polarization filtered by the point contact, as observed in magnetic focusing experiments. Anticrossing in the one-dimensional spin subbands is introduced by a magnetic field parallel to the channel or an asymmetric potential transverse to it. Controlling the magnitude of the spin-splitting affords a novel mechanism for inverting the sign of the spin polarization.   

Energy Spectra and Spin Properties of Electrons in Spin-Orbit Superlattice Quantum Wires

We calculate the energy spectra of electrons in quantum wires with spatially uniform and modulated spin-orbit coupling. The effects of Rashba spin-orbit coupling arising from asymmetric confinement in perpendicular and lateral directions with respect to the plane containing the wire are considered. We investigate the resulting interplay of strong lateral confinement, a periodic one-dimensional superlattice potential, and spin-orbit coupling in two orthogonal directions. The implications for the spin-dependent properties of electrons confined within these quantum wires are discussed. A potential realization of such systems within narrow nanowires at the interface of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures is also described.   

Unexpected Anisotropy of Electron g-factor in GaAs/AlGaAs(110) Quantum Well

Semiconductor spin qubit is a promising candidate for solid state quantum computation. A lot of effort has been devoted to study spin dynamics in semiconductors ever since a revival of research interest in this field in the late 1990s. Spin lifetime longer than 1ns at room temperature has been discovered in GaAs/AlGaAs(110) quantum wells (QW) as a result of the absence of a predominant spin scattering mechansim (DP mechanism), which also leads to a strong anisotropy of electron spin decoherence in such QWs, with the spin lifetime of spins along the growth direction 10 times bigger than that of spins perpendicular to the growth direction. However, not much is known about the (an)isotropy of spin-related processes in the (110) QW plane, despite that it may offer useful information about spin relaxation. Utilizing a time-resolved Kerr rotation (TRKR) system with a rotatable in-plane magnetic field, we studied the spin processes in GaAs/AlGaAs (110) QWs and found an unexpected anisotropy of electron g-factor in such QWs. The g-factor as measured with the magnetic field along the [1-10] axis is some 10\% larger than that along the [001] axis. Such a strong anisotropy is not only unexpected for QWs, but also much bigger than that found in InGaAs/GaAs quantum dots. An explanation for these results is still in demand but it may give some hints to improve our understanding of spin dephasing mechanisms in semiconductors.   

High Resolution Magneto-Optic Measurements in GaAs using a Sagnac Interferometer

The Sagnac Interferometer is a tool which measures the Polar Kerr effect--a direct indicator of magnetism. Using 820 nm light from a superluminescent diode, we probe GaAs structures and measure the Kerr angle with sub-microradian resolution. ~By utilizing diffraction limited optics and a piezoelectric scanner, we also achieve high spatial resolution. Our measurements are performed at cryogenic temperatures and offer a way to measure the Spin Hall Effect in the DC regime along with other forms of magnetic order.   

Temperature-dependent spin- and phase coherence measured via h/e and h/2e quantum oscillations in resistance of mesoscopic ring arrays in an InAs 2DES

We investigate electron spin- and phase coherence in an array of quasi-ballistic InAs quantum well mesoscopic rings through observation of Aharonov-Bohm h/e oscillations (AB) and Altshuler-Aronov-Spivak h/2e oscillations (AAS). The temperature dependence of the AAS oscillations is characterized through a single effective coherence length, $L_{\rm{eff}}$, following the formalism of Dou\c{c}ot and Rammal, from which the phase coherence length, $L_\phi$ and the spin coherence length as limited by spin-orbit interaction, $L_{\rm{SO}}$, are extracted. AB oscillations are also present, and can be separated from AAS by Fourier transformation. We contrast the AAS method of extracting the coherence lengths with analysis of the AB oscillation amplitudes. Previous studies have examined $L_\phi$ from AB signals in single ballistic rings, or by using AAS amplitudes in large networks, or have observed AB and AAS in single rings with spin-orbit interaction. Here the presence of both AB and AAS in an array with spin-orbit interaction allows for study of both $L_\phi$ and $L_{\rm{SO}}$, and enables direct juxtaposition of different quantum coherence phenomena as means for measuring coherence lengths (DOE DE-FG02-08ER46532).   

Weak Antilocalization and Spin-Orbit Coupling in InAlN/AlN/GaN Heterostructures

Spin-orbit coupling is investigated by magnetotransport and weak antilocalization (WAL) measurements in In$_x$Al$_{1-x}$N/AlN/GaN heterostructures in the carrier density ranges extending from 1.22$\times$10$^{13}$ cm$^{-2}$ to 1.41$\times$10$^{13}$ cm$^{-2}$ and from 1.99$\times$10$^{13}$ cm$^{-2}$ to 2.15$\times$10$^{13}$ cm$^{-2}$. By combining the data from AlGaN/AlN/GaN samples, we find that the spin-orbit field is not a constant at high carrier densities and the electron spin-splitting energies show a deviation from linear behavior with Fermi wavefactor. However, the spin-splitting energies extracted from WAL oscillations, even in this high carrier density regime, were found to be much smaller than the previous reports based on Shubnikov-de Haas (SdH) measurements. We will discuss how the nonuniformities in the carrier density can lead to beating features in SdH oscillations, which can then be misinterpreted as large spin-splitting energies. This finding may resolve the long-standing discrepancy between the WAL and SdH results.   

Spin Coulomb Drag in the Hubbard Chain

The spin Coulomb drag is the decay of the spin current in a metal as a consequence of the Coulomb interaction between up- and down-spin carriers and is a distinctive feature of spin- polarized transport. The current of majority spins can induce a current of minority spin carriers via the transresistivity. This friction reduces the current but does not change the spin- polarization.\footnote{I. D'Amico and G. Vignale, Phys. Rev. B {\bf 62}, 4853 (2000).} We calculate the critical exponents of the resistivity for up- and down-spin electrons and the transresistivity for the spin-polarized Hubbard chain with nonmagnetic impurities within the Kubo formalism using (1) bosonization techniques\footnote{P. Schlottmann, Phys. Rev. B {\bf 80}, 205110 (2009).} and (2) the Bethe ansatz solution and conformal invariance.\footnote{P. Schlottmann, Phys. Rev. B {\bf 82}, 075103 (2010).} The charge-spin separation in 1D is strictly valid only in the absence of spin-polarization. Due to the Luttinger liquid properties the temperature dependence of the transport correlation functions follow power laws of $T$ with non-universal exponents. A large spin polarization is more favorable for a sustained spin current than a small magnetization.   

Spin Texture in a Cold Exciton Gas

We report on the observation of a spin texture in a cold exciton gas in a GaAs/AlGaAs coupled quantum well structure. The spin texture is observed around the rings in the exciton emission pattern. The observed phenomena include: a ring of linear polarization, a vortex of linear polarization with polarization perpendicular to the radial direction, an anisotropy in the exciton flux, a skew of the exciton fluxes in orthogonal circular polarizations and a corresponding four-leaf pattern of circular polarization, and a periodic spin texture. These phenomena emerge when the exciton gas is cooled below a few Kelvin.   

Session Y16: Focus Session: Spins in Carbon-Based Materials -- Magnetoresistance, Magneto-Electric Effect

Quantum Linear Magnetoresistance and Extraordinary Magnetoresistance in Graphene

Graphene, a single atomic layer of hexagonally arranged carbon atoms, presents the optimal platform to study rarely-observed magnetoresistance (MR) effects because of its temperature-independent mobility and linear band structure with zero band gap. Linear magnetoresistance (LMR), which is characterized as a large, non-saturating linear MR, is one such unusual effect. Normally, the resistance of a conductor in an applied magnetic field increases quadratically with field and then saturates at a relatively low value. Models that explain LMR behavior have been proposed that include both quantum and classical origins, but most systems studied thus far can be explained by a purely classical model. However, we find that quantum LMR is observed in multilayer epitaxial graphene grown on SiC at temperatures as high as 300 K and with a magnitude greater than 200{\%} at 12 Tesla (T). In addition, a phenomenon closely related to classical LMR called extraordinary magnetoresistance (EMR) and characterized by even larger MR, can be realized in metal-shunted graphene devices. Here, due to the different magnetic-field-dependent resistances of the metallic shunt, graphene, and shunt-graphene interface, current flows easily through the shunt in zero and low magnetic field, while in high magnetic field, more current flows around the shunt and is redistributed in the graphene. Devices made from chemical vapor deposition (CVD) graphene grown on copper and transferred to a SiO$_{2}$/Si substrate with Ti/Au shunts display gate-tunable longitudinal MR of $\sim $600{\%} at 12 T and also show promise for use as Hall sensors. Graphene magnetoresistance devices have many possible applications including magnetic field sensors and magnetic read-heads. In contrast with the many proposed electronic uses for graphene, which necessitate the creation of a band-gap, graphene magnetoresistance devices that exploit LMR or EMR provide a use for as-grown or deposited graphene.   

Frequency dependence of organic magnetoresistance

Organic magnetoresistive (OMAR) devices show a large enough magnetoresistive response (typically 10{\%}) for potential applications as magnetic field sensors. However, applications often require sensing high frequency magnetic fields, and the examination of the frequency-dependent magnetoresistive response is therefore required. Analysis of time constants that limit the frequency response may also shed light on the mechanism behind the OMAR effect, because different OMAR mechanisms occur at different time scales In our experiments, the AC magnetic field is supplied by a coil with a ferrite core which is driven by a function generator The AC magnet shows a frequency response that is almost flat up to 1MHz. We found that the OMAR frequency limit is about 10 kHz for a typical organic semiconductor device and at least 100 kHz for devices made from a doped polymer film. We also performed capacitance and conductance vs. frequency measurements to understand the origin of the observed limit frequencies.   

Room Temperature Ferromagnetic Polymer and the Correlated Anomalous Magnetoresistance Phenomenon

Organic magnetoresistance (OMAR) has been observed in organic semiconductor devices where resistance can change in a relatively small external magnetic field at room temperature. Since a weak magnetic field is involved, the hyperfine interaction (HFI) is employed to explain OMAR in the reported literatures. None of these issues consider the magnetic properties of the organic semiconductors themselves. However, the we recently discovered that polymer semiconductors, such as poly(3-hexylthiophene) P3HT, can have room temperature (RT) ferromagnetic properties in their crystalline phase and when mixed with phenyl-C61-butyric acid methyl ester (PCBM). Here, we will report the possible correlation between the ferromagnetic property of the P3HT:PCBM and anomalous OMAR phenomenon including the anisotropic and hysteretic OMAR behavior. The magnetic property of the polymer including the anisotropic and photo induced change of magnetism will be also discussed to explore the possible mechanism of the room temperature ferromagnetism.~   

Magnetic fringe field control of electronic transport in an organic film

Random nuclear hyperfine fields in organic materials dramatically affect electronic transport properties such as the electrical (photo)conductivity and electroluminescence. The influence of these nuclear hyperfine fields can be overwhelmed by a uniform external applied magnetic field. As a result, in applied magnetic fields of about 10mT the kinetics of exciton formation, bipolaron formation, and carrier hopping are all modified, leading to changes in room-temperature electrical transport properties in excess of 10 {\%} in many materials. Here we demonstrate a new method of controlling the electronic transport in an organic film, using the spatially-varying magnetic fringe fields of an unsaturated ferromagnetic electrode. The effect of these magnetic fringe fields is hysteretic, anisotropic, and depends sensitively on the distance of the organic material from the ferromagnetic electrode; all these effects appear in the magnetic-field dependences of electronic transport in these films. Such structures, which do not rely on spin injection or spin-valve behavior, may provide a simple approach to integrating magnetic metals and organics for hybrid spintronic devices.   

Spin-Boson Theory of Organic Magnetoresistance

The discovery of room-temperature, low-field magnetoresistance (MR) in organic light-emitting devices was one of major achievements of spintronics in the last decade. Compared to its inorganic counterpart, a sizable organic MR (OMR) is relatively easy to be obtained, showing extensive potential in magnetically controlled applications. Yet, after years of intense research, a comprehensive understanding of this magnetic field effect out of these nonmagnetic materials is still lacking. In this work, we present a spin-boson theory for magnetotransport in organic semiconducting materials, on the basis of a coupling between charge carriers' spin and a local bosonic environment, which is shown to be an irreducible ingredient in understanding of the anomalous OMR. Among those compose this environment triplet excitons play a basic role. The incoherent hopping rate between molecules is calculated to give out the basic behavior of OMR. The underlying mechanism is revealed from the calculation of entanglement, represented by the von Neumann entropy, between the carrier's spin and bosons. We also obtain the dependence of OMR on both of the bias voltage and the spin-boson coupling. The results obtained from the theory are in good agreement with experiments.   

Electronic and transport properties of Cobalt-based valence tautomeric molecules and polymers

The advancement of molecular spintronics requires further understandings of the fundamental electronic structures and transport properties of prototypical spintronics molecules and polymers. Here we present a density functional based theoretical study of the electronic structures of Cobalt-based valence tautomeric molecules Co$^{III}$(SQ)(Cat)L Co$^{II}$(SQ)$_{2}$L and their polymers, where SQ refers to the semiquinone ligand, and Cat the catecholate ligand, while L is a redox innocent backbone ligand. The conversion from low-spin Co$^{III}$ ground state to high-spin Co$^{II}$ excited state is realized by imposing an on-site potential U on the Co atom and elongating the Co-N bond. Transport properties are subsequently calculated by extracting electronic Wannier functions from these systems and computing the charge transport in the ballistic regime using a Non-Equilibrium Green's Function (NEGF) approach. Our transport results show distinct charge transport properties between low-spin ground state and high-spin excited state, hence suggesting potential spintronics devices from these molecules and polymers such as spin valves.   

Spin polarized transport properties of impurity induced Carbon nanostructures

We study spin polarized transport on structures consisting of Carbon wires including impurities. We perform first principle calculations on these structures using the nonequilibrium Green Function formalism combined with the density functional theory. The different impurity induced Carbon nanostructures are found to depend strongly on the geometrical disorder. Through the conductance, transmission spectra, density of states and current-voltage characteristics the numerical results of spin polarized calculations are discussed.   

Spin transport in organic semiconductor single crystals

Organic semiconductors have been attracting much attention as potential spin transport media due to their weak spin-orbit and hyperfine interactions that promise long spin lifetimes. However, to date most studies have focused on amorphous, or polycrystalline thin-film based organic semiconductors. In addition, short transport distances equal to or less than a couple of hundred nanometers have been measured despite the prediction of long spin transport distance. We have investigated spin injection and transport in high purity single-crystal organic semiconductors, especially rubrene(5,6,11,12-tetraphenylnaphthacene). We will present and discuss our experimental results obtained in both vertical and lateral transport geometries. Great care is needed to understand and avoid possible spurious effects in these studies.   

Correlation of electric polarization and magnetic ordering in cobalt chloride thiourea

The coupling between electricity and magnetism in magneto-electric multiferroics has been intensively investigated in a wide range of transition metal oxides. Recently the material classes have been extended to organo-metallic insulators (sometimes known as metal-organic frameworks or molecular magnets) such as NiCl$_{2}$-4[SC(NH$_{2})_{2}$], which provides a new arena for designing magneto-electric multiferroics. We have grown single crystals of cobalt chloride thiourea, CoCl$_{2}$-n[SC(NH$_{2})_{2}$], which forms two different crystal structures with n = 2 and 4. The compound CoCl$_{2}$-2[SC(NH$_{2})_{2}$] has a triclinic crystal structure with strong magnetic anisotropy and $\sim $ 3 $\mu _{B}$/Co ion, indicating \textbf{S} = 3/2 Co spins, and the compound CoCl$_{2}$-4[SC(NH$_{2})_{2}$] has a tetragonal structure with almost no magnetic anisotropy and 1 $\mu _{B}$/Co ion, indicating \textbf{S} = 1/2 Co spins. We will present details of the magnetic field-induced electric polarizations and magnetic properties of these compounds.   

Magneto-electric multiferroic behavior in a metal-organic framework

We will discuss strong magneto-electric coupling in the metal-organic compound NiCl$_{2}$-4SC(NH$_{2})_{2 }$(also known as a metal-organic framework or molecular magnet). Magneto-electric multiferroic behavior is traditionally investigated in transition-metal oxides, however we are expanding the field to metal-organics, which are designable materials with soft lattices and electrically polar organic elements. In this material we observe a magnetic field-induced change in the electric polarization of 50 $\mu $C/m$^{2}$ driven by ordering of the Ni S = 1 spins. We can model it in terms of a combination of exchange striction and crystal electric fields, and Quantum Monte Carlo simulations of these effects provide an excellent fit to the data. We find that the induced electric polarization is a sum of $<$S$_{z}^{2}>$ and the nearest neighbor correlation function $<$S$_{i}$ dot S$_{j}>$ The presence of electrically polar thiourea molecules [SC(NH$_{2})_{2}$] amplifies the effect of small magnetically induced crystal distortions on the electric polarization.   

Manipulating singlet-triplet equilibrium in organic biradical materials

We investigated the tunability of the singlet-triplet equilibrium population in 1,4-phenylenedinitrene via magneto-optical spectroscopy. Both temperature- and magnetic field-induced spectral changes in this organic biradical are sensitive to magnetic energy scales, specifically the spin gap, demonstrating the important interplay between charge, and magnetism in this system. These measurements also establish the value of local-probe photophysical techniques for extraction of magnetic properties data in systems where a traditional Curie law analysis has intrinsic limitations. *This work is supported by the National Science Foundation.   

Reversible mechanism for spin crossover in transition-metal cyanides

Spin transitions generally occur in compounds of octahedrally coordinated 3$d$ transition metal ions. These transitions can be induced by external perturbations such as light, heat, pressure, magnetic field, and chemical substitution. Transition metal cyanides are one such material, which exhibit {\em reversible} spin transition while perturbed with light at $T < $10 K. Here we report the first-principles (DFT+U) study of anhydrated KCoFe(CN)$_6$. We find that the complete spin transition from the low spin ground sate ($S=0$) to a high spin ($S=2$) state takes place due to intra-atomic and inter-atomic charge transfers in two steps. In the first step a $d$-electron is transferred from Fe to Co through cyanide ligand, which is followed by the $d$-electron rearrangement in the Co. This spin transition is strongly correlated with the internal lattice, and we find as large as 10\% extension of the Co$-$N bond via a Jahn-Teller active (tetragonally distorted) lattice in the intermediate spin ($S=1$) state. The calculated energy required for this transition is in agreement with experiments. We further predict that this spin transition in such materials can be induced, and further tuned, by external pressure to enable realization of such reversible transitions at ambient temperatures.   

Psuedo-entanglement between nuclear spins in photoexcited functionalized fullerenes

Optically excited triplet electron spins can be used to polarise, manipulate, couple and measure nuclear spins. Here we present photoexcited pulsed magnetic resonance experiments for the characterization of functionalized fullerene structures with homo and hetero nuclear spins. We use density functional theory in order to predict the hyperfine interaction between the photoexcited triplet and various nuclear spins in the structure, and then use magnetic resonance (ENDOR) to investigate these values experimentally. In addition to the hyperfine coupling strength, we measure the relevant relaxation rates and initial hyperpolarisation of the triplet in order to understand the possible degree of entanglement of nuclear spins through the optically excited mediator spin. We measure an increased nuclear-nuclear coupling in the presence of the triplet which permits fast nuclear controlled-NOT gates.These operations, in conjunction with the transfer of electron polarisation to the nucleus, allow the demonstration of nuclear-nuclear pseudo-entanglement, measured using quantum state tomography.   

Session Y18: Focus Session: Low D/Frustrated Magnetism - More Frustrated Magnets

Field-induced thermal transport in BEC antiferromagnets

Recent experiments in BEC quantum magnets exhibit a dramatic evolution of the thermal conductivity of these materials in magnetic field. By considering various relaxation mechanisms of bosonic excitations in the vicinity of the BEC quantum-critical point at finite temperature we provide a detailed explanation of several unusual features of the data. We identify the leading impurity-scattering interaction and demonstrate that its renormalization due to quantum fluctuations of the paramagnetic state compensates the related mass renormalization effect. This explains the enigmatic absence of the asymmetry between the two critical points in the low-$T$ thermal conductivity data, while such an asymmetry is prominent in many other physical quantities. The observed characteristic ``migration'' of the peak in thermal conductivity away from the transition points as a function of temperature is explained as due to a competition between an increase in the number of heat carriers and an enhancement of their mutual scattering. An important role of the three-boson scattering processes within the ordered phase of these systems is also discussed. Other qualitative and quantitative features of the experiment are clarified and the future directions are sketched.   

Investigation of the magnetic susceptibility of the disordered BEC system NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ at ultralow-temperatures

We report measurements of the magnetic susceptibility of a disordered BEC system of magnons for single crystals of NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ (with 15{\%} Cl atoms replaced by Br). NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ is a potential candidate for a Bose glass (BG) phase of the spins adjacent to a region of Bose-Einstein condensation (BEC). The BG to BEC phase is the bosonic analog of a metal-insulator transition for fermions. The measurements were carried out for temperatures down to 1mK and for applied magnetic fields up to 14.5T. The results show that the critical fields $H_{c}$ do not obey the conventional 3D universality class for a BEC, $H_{c}(T)$ -- $H_{c}(0) \quad \sim $ $T^{\alpha }$, where $\alpha $ = 1.5 [1]. The values of $\alpha $ changes from $\alpha $ = 0.52 for T $>$ 300 mK to $\alpha $ = 0.91 for T $<$ 250 mK and then again at 70$\sim $90mK to $\alpha $ = 0.48 for T$<$ 70 mK, indicating a crossover to possible BG behavior.   

Bose-Einstein Condensation in Han Purple - a NMR Study

NMR study of the two quasi-2D coupled spin-1/2 dimer compound, BaCuSi$_2$O$_6$ (Han Purple) [1], is presented. $T_{\rm{BEC}}$ varies as $(H-H_{c1})^{2/d}$, where $d$ is the dimensionality of the system, and $H_{c1}$ the critical field which closes the gap. BaCuSi$_2$O$_6$ was claimed to exhibit an reduction of d from 3D to 2D at low T [2]. However, due to a structural transformation at 90 K, different intradimer exchange couplings and different gaps ($\Delta_{\rm{B}}$/$\Delta_{\rm{A}}$=1.16) exist in every second plane along the c axis [3]. In our first NMR experiments [3], we have shown that the population of bosons in the B planes $n_{\rm{B}}$ was much smaller than $n_{\rm{A}}$, but finite in the field range $\Delta_{\rm{A}}/g\mu_{\rm{B}} < H < \Delta_{\rm{B}}/g\mu_{\rm{B}}$ where $n_{\rm{B}}$ = 0 is expected in a naive model of uncoupled planes. Recently, a new model has been presented [4] which takes into account both frustration and quantum fluctuations. This leads to a non-zero population $n_{\rm{B}}$ of \emph{uncondensed} bosons in the $B$ plane, increasing quadratically with $(H-H_{c1})$, as compared to the linear dependence of $n_{\rm{A}}$. We compare our new NMR results to these predictions. $[$1$]$ M. Jaime \textit{et al.}, PRL \textbf{93},087203 (2004). $[$2$]$ S. E. Sebastian \textit{et al.}, Nature \textbf{441}, 617 (2006). $[$3$]$ S. Kr\"{a}mer \textit{et al.}, PRB \textbf{76}, 100406(R) (2007). $[$4$]$ N. Laflorencie and F. Mila, PRL \textbf{102}, 060602 (2009).   

Magnetic neutron scattering of a Prussian blue analogue photomagnet

Since the discovery of photoinduced magnetism in cobalt hexacyanoferrate (CoFe) Prussian blue analogues (PBAs) in 1996,$^1$ there have been many, multifarious studies that elucidated the nature of the photoeffect. However, the magnetization in CoFe has proven difficult to model quantitatively using macroscopic data due to the presence of multiple magnetic species, magnetic bistability, superexchange, and unquenched orbital angular momentum. To investigate the ordered magnetization directly, we have studied dueterated powders of CoFe using unpolarized and polarized neutron diffraction, and observed magnetic neutron scattering for the first time in this compound. A model for the magnetic structure based upon neutron diffraction, elemental analysis, infrared spectroscopy, and macroscopic magnetization will be presented. \newline [1] O. Sato, et al., Science 272, 704 (1996).   

Photoinduced Magnetism in Nanoscale Heterostructures of Prussian Blue Analogues*

Nanometer-scale cubic heterostructures of two Prussian blue analogues, ferromagnetic K$_j$Ni$_k$[Cr(CN)$_6$]$_l \cdot$\emph{n}H$_2$O (\textbf{A}) with $T_c\sim70$~K and photo-active ferrimagnetic Rb$_a$Co$_b$[Fe(CN)$_6$]$_c \cdot$\emph{m}H$_2$O (\textbf{B}) with $T_c\sim20$~K, have been studied.\footnote{M. F. Dumont \emph{et al.}, Inorg. Chem., submitted.} These samples exhibit a persistent photoinduced decrease in magnetization at temperatures up to $T_c\sim70$~K of the \textbf{A} constituent, resembling results from analogous \textbf{ABA} heterostructured films.\footnote{D. M. Pajerowski \emph{et al.}, J. Am. Chem. Soc. \textbf{132}, 4058 (2010).} This net decrease suggests that the photoinduced structural transition in the \textbf{B} layer generates a strain-induced decrease in the magnetization of the \textbf{A} layer, similar to a pressure-induced decrease previously observed in the pure \textbf{A} material.\footnote{M. Zentkov\'{a} \emph{et al.}, J. Phys.: Condens. Matter \textbf{19}, 266217 (2007).} Core-shell and core-shell-shell configurations \textbf{AB}, \textbf{BA}, \textbf{ABA}, and \textbf{BAB} have been characterized by TEM, FTIR, XRD, and SQUID magnetometry.\\[4pt]*Supported, in part, by NSF DMR-0701400 (MWM) and DMR-1005581 (DRT), NSERC, CFI, the NHMFL, and the State of Florida.   

Structural and Magnetic Interplay in Molecule-based Magnets with Photocontrollable Properties

Understanding the cooperative effects, such as electron-lattice interactions, in molecule-based magnetic coordination complexes possessing photoinduced phase transitions is an important step to being able to rationally tune the variables governing the process.\footnote{H.~Watanabe \emph{et al.}, Phys.~Rev.~B {\bf 79} (2009) 180405.} Specifically, variable temperature FTIR spectroscopy and magnetometry have been used to explore the temperature and photocontrollable spin transitions in Co-Fe Prussian blue analogues, $A_j$Co$_k$[Fe(CN)$_6$]$_{\ell}\cdot n$H$_2$O, where $A$ is an alkali ion, and in new Fe spin-crossover complexes. By studying nanoparticles\footnote{M.F.~Dumont \emph{et al.}, Inorg.~Chem., submitted.} and heterostructures,\footnote{D.M.~Pajerowski \emph{et al.}, J.~Am.~Chem.~Soc.~{\bf 132} (2010) 4058.} the data provide insight into the roles played by restricted lattice geometries and strain-pressure effects.   

Interplay between charge fluctuations and magnetic order in a stacked triangular-Kagome lattice : Applications to FeCrAs

The recently studied antiferromagnet FeCrAs [Wu et al, EPL, 85 17009 (2009)] exhibits a surprising combination of experimental signatures, with Fermi liquid like specific heat but resistivity showing strong non-Fermi liquid character. From the material properties we motivate a minimal model for the low energy degrees of freedom, and study its properties using slave-rotor mean field theory. Using this approach we find a variety of exotic phases and propose that the features of FeCrAs can be qualitatively explained by a spin liquid proximate to a metal-insulator transition.   

Giant Anomalous Hall Effect in (Ba,Sr)T$_{2+x}$Ru$_{4-x}$O$_{11}$ (T=Fe,Co,Mn) Ferrites

Hexagonal R-type ferrites (Ba,Sr)T$_{2+x}$Ru$_{4-x}$O$_{11}$ are promising spintronic materials that exhibit collinear ferrimagnetic order at unusually high critical temperatures T$_{C} \quad \le $ 490 K for Fe-bearing compositions, and an in-plane, ``all-in/all-out'' order at T$_{C}$'s $<<$ 300 K due to frustrated antiferromagnetic interactions within the Kagome basal plane in metallic Co or Mn compositions. A strong, nonmonotonic field dependence of the anomalous Hall effect is observed in metallic ferrites, which is generated by non-zero scalar spin chirality and the Berry phase acquired by carriers moving in the ``topologically nontrivial'' spin background of the Kagome plane. The FM semiconductor BaFe$_{3.4}$Ru$_{2.6}$O$_{11}$ (T$_{C}$ = 440 K) exhibits a giant Hall resistivity = 77 $\mu \Omega $-cm at 300 K, with a low-temperature sign change and monotonic field dependence that are consistent with a strong Berry phase curvature (gauge field) acquired by carriers in momentum space.   

Coexistence of ferromagnetic and antiferromagnetic orders in Ba-doped cobalt perovskites studied by neutron scattering

Cobalt-containing oxide compounds have attracted a great deal of interest in recent years due to the variety of magnetic and electrical properties. We performed single crystal neutron diffraction on 6T2 at~the~LLB in France and~the~HB3A four-circle diffractometor at~the~HFIR of ORNL. The Ba-doped cobalt perovskite (La$_{0.8}$Ba$_{0.2}$CoO$_{3})$ crystal was measured in the temperature range of 2-250 K. At temperature $T<$ 200 K, a set of ferromagnetic peaks ($k_{1}$ = 0) onsets and then antiferromagnetic peaks with $k_{2}$* = (1/2 0 1/2) and (0 0 3/2) join in at $T<$ 100 K. Both ferromagnetic and antiferromagnetic peaks saturate at $T\approx $ 40 K. By refining the peaks collected for $k_{1}$ and $k_{2}$ sets, magnetic structures were determined.   

Density matrix renormalization group study of optical conductivity in the one-dimensional Mott insulator Sr$_2$CuO$_3$

We examine the optical conductivity of Sr$_2$CuO$_3$ by using zero and finite temperature dynamical density matrix renormalization group (DMRG) methods. Employing a Hubbard- Holstein model containing Holstein-type coupling of electron to the Einstein phonons, we reproduce both the Mott-gap excitation and phonon-assisted spin excitation observed experimentally [1,2] by using the dynamical DMRG method combined with a regulated polynomial expansion [3]. We find a parameter set describing Sr$_2$CuO$_3$. Furthermore, by using a low- temperature dynamical DMRG method which is recently developed by present authors [4], we examine the temperature effect of the Mott-gap excitation to clarify the effect of optical phonons on spectral shape at finite temperatures. We find that the presence of phonons induces the enhancement of the width of an excitonic peak in the optical conductivity. [1] M. Ono, K. Miura, A. Maeda, H. Matsuzaki, H. Kishida, Y. Taguchi, Y. Tokura, M. Yamashita, and H. Okamoto, Phys. Rev. B {\bf 70}, 085101 (2004). [2] H. Suzuura, H. Yasuhara, A. Furusaki, N. Nagaosa, Y. Tokura: Phys. Rev. Lett. {\bf 76}, 2579 (1996). [3] S. Sota and M. Itoh, J. Phys. Soc. Jpn. {\bf 76}, 054004 (2007). [4] S. Sota and T. Tohyama, Phys. Rev. B {\bf 78}, 113101 (2008).   

A Compton scattering study of the magnetic structure of NbFe$_2$

NbFe$_2$ displays a diverse phase diagram over a narrow compositional range, possibly due to close proximity to a Quantum Critical Point. The ground state of NbFe$_2$ has been the subject of a number of recent theoretical investigations [1,2], but for near-stoichiometric compositions its exact nature remains ambiguous. We have probed the low temperature magnetic structure by performing Magnetic Compton Scattering (MCS) measurements on polycrystalline Nb$_{1+y}$Fe$_{2-y}$ samples with $y=-0.02,0.00$ and $0.03$. MCS is able to measure how the magnetic electrons within a sample are distributed in momentum space, and can be a useful tool for resolving site-specific contributions to the spin moment. The interpretation of these measurements was aided by electronic structure calculations, which favour a ferrimagnetic ground state. The results are presented with reference to the possible presence of spin fluctuations. \\[4pt] [1] Subedi and Singh, PRB {\bf 81}, 024422 (2010) \\[0pt] [2] Tompsett et al, PRB {\bf 82}, 155137 (2010)   

Impurity-entanglement in dimerized spin chains

To quantify the entanglement caused by an impurity in an $S=\frac{1}{2}$ dimerized $J_1-J_2$ quantum spin chain, several different entanglement-measures have been utilized. We present the results of variational calculations of the impurity entanglement entropy as well as the negativity for a chain with an impurity attached at one end. We compare the results for both of these measures.   

On the low-temperature behavior of a geometrically frustrated Heisenberg antiferromagnet

The thermodynamic behavior of the Heisenberg antiferromagnet on the kagome lattice and the effects of its geometrical frustration are widely understood [1]. At low temperatures planar spin configurations due to multiple zero-modes (so-called Weathervane loops) are favored entropically. These modes occur when spin clusters are bounded by spins pointing in a similar direction, so that the cluster spins revolve freely around this direction. However, it remains unclear if with decreasing temperature the number of these modes continues to increase and if this eventually leads to a highly ordered $\sqrt{3}\!\times\!\sqrt{3}$ state. In order to investigate this system we applied the simulated tempering method, combined with the Heatbath algorithm for single spins and a Metropolis loop-flip Monte Carlo move. We examined the thermodynamic properties for temperatures $\frac{k_BT}{J}\!\ge\!10^{-6}$; and found that once the planar state is attained, the out-of-plane excitations are reduced with decreasing temperature but no further order is established. Hence, the prevailing spin structure represents a temperature independent entropy maximum where any entropy gain produced by additional zero modes is neutralized by an entropy loss in the Weathervane loop structure.\\[4pt] [1] J.\ N.\ Reimers and A.\ J.\ Berlinsky, Phys. Rev. B {\bf 48}, 9539 (1993).   

Topological phases and quenches in spin-ladder systems

We show that a ladder version of Kitaev's honeycomb model can be directly mapped to a one-dimensional $p$-wave superconducting system. The ladder system is characterized by $Z_2$ vortices at every unit cell; the presence of vortices is encoded in the sign of the local chemical potential in the $p$-wave system. Compared to recently studied phases in topological superconductors, we show that certain vortex patterns in this ladder system can result in new topological phases and can alter the universality classes for associated phase transitions. We discuss the effect of performing time-dependent quenches in these new phases.   

Multi-spin exchange model for a quantum spin liquid on the honeycomb lattice

Recently, a possible quantum spin liquid (QSL) state has been found through quantum Monte Carlo studies of Hubbard model on the honeycomb lattice. The obtained QSL does not show long range correlation of any known type, which has a finite spin gap and a short range dimer-dimer correlation pattern resembling the short range resonant-valence-bond (RVB) state. Given the intensive current interest in such an exotic QSL, it is natural and timely to ask a question: what is the effective spin model to capture the essential low-energy physics near this QSL region? We report here a comparative numerical study based on finite-size exact diagonalizations (ED) of the Hubbard model, and a multi-spin exchange model with two-, four- and six-spin exchange terms. The latter model is derived from the strong coupling expansion of the former one. Through extensive ED calculations of low-energy spectra and ground-state correlation functions of both models, we try to establish connections between them, especially near the QSL region. Furthermore, the phase diagram of the multi-spin exchange model is explored in details.   

Session Y20: Focus Session: Thermoelectric Materials: Chalcogenides and 1D/2D Systems

Resonant Energy Levels and the Thermoelectric Figure of Merit

Distortions of the electronic density of states are a potent mechanism to increase the thermopower and ZT of thermoelectric semiconductors. Band-structure engineering approaches will be reviewed that can be used to do this, namely quantum size effects, hybridization effects in strongly correlated electron systems, and resonant impurity levels. The properties of known resonant impurities for PbTe, SnTe, Bi$_{2}$Te$_{3}$ and GaSb will also be reviewed. They can increase the thermoelectric power through 2 mechanisms, (1) the increase in density of states, and (2) resonant scattering. The first increases the thermopower in a nearly temperature-independent way; the second results in an electron energy filtering effect that increases the thermopower, but only at cryogenic temperatures where the electron-phonon interactions are weaker. An analysis of the thermomagnetic tensor components makes it possible to dissociate the two contributions experimentally.   

The Effect of Sintering on the Thermoelectric Properties of Chemically Synthesized Nano-Bulk Bi$_{2-x}$Sb$_{x}$Te$_{3}$

Considerable research effort has gone into improving the performance of traditional thermoelectric (TE) materials such as Bi$_{2-x}$Sb$_{x}$Te$_{3}$ through a variety of nanostructuring approaches. Bottom-up, chemical approaches have the potential of producing very small nanoparticles ($<$ 50 nm) with narrow size distributions and controlled shape. For this study, nanocrystalline powder of Bi$_{2-x}$Sb$_{x}$Te$_{3}$ with x = 0 -- 1.5 has been synthesized using a ligand- assisted chemical method, and consolidated into bulk pellets with cold pressing followed by sintering. These materials have the interesting property that a wide range of carrier concentrations are accessible through different Bi/Sb ratios, with low values of x being n-type and higher values becoming p-type. In this work, we present the thermoelectric transport measurements from 6 -- 300 K as a function of sintering temperature, and a beneficial effect is found. The samples are also characterized by Hall effect, XRD, and compositional analysis. We will present results on the structure-property relations, and discuss strategies for optimization of this class of TE materials for high performance.   

Coherent Phase Stability of IV-VI Rocksalt Semiconductor Alloys

The creation of nanoscale precipitates via phase separation provides a mechanism for decreasing the lattice thermal conductivity of some bulk thermoelectric materials. The IV-VI semiconductor alloy systems may phase separate by either a spinodal decomposition or nucleation and growth mechanism. To better understand these phase transformations, we use first-principles density functional theory (DFT) calculations to investigate the coherent and incoherent phase stability of a series of IV-VI rocksalt semiconductor alloys (IV=Pb, Sn, Ge and VI = S, Se, Te). We use mixing enthalpies derived from calculations of special quasirandom structures (SQS), along with coherency strain energies to model the thermodynamic driving forces for incoherent and coherent phase separation. By fitting these inputs to a sub-regular mixing enthalpy model and including an ideal mixing entropy term, we calculate incoherent and coherent phase diagrams. We show the incorporation of coherency strain energies cause large depressions of the coherent spinodals for each system. The depressions are large enough that at realistic processing temperatures, the dominant precipitation mechanism of phase separation is nucleation and growth.   

Electronic structure of PbTe doped with K and Na

PbTe is a well-known thermoelectric which shows excellent thermoelectric performance (for both {\it p}- and {\it n}-type) in the temperature range between ambient and 600$^{\circ}$C. Thermopower (S) of PbTe can be enhanced with proper doping. Hermann {\it et al}. have found the figure of merit ZT=1.5 at 773~K with 2~\% Tl doping in PbTe. They ascribe this to the enhancement of the density of states (DOS) caused by Tl-induced resonance level in the valence band. This is in agreement with the {\it ab initio} studies of Ahmad {\it et al}., who also found an enhanced DOS associated with K defects in PbTe. Recently Androulakis {\it et al}. have looked for resonant states in the valence band associated with Na/K impurities in PbTe. Although they observe an increase in power factor at high temperature, they do not find any evidence of resonant states. We have reexamined this issue by carrying out detailed band structure calculations in the presence of K and Na defects in PbTe using 64-atom supercells. The question of the existence of resonant states and the origin of the enhanced DOS near the valence band maximum will be discussed.   

Ho Doped $\mathbf{Bi_xSb_y}$ Nanopolycrystalline Alloys

Department of Physics, Boston College, Chestnut Hill, Massachusetts, 02467. Bismuth-Antimony alloys have been shown to have high ZT values below room temperature, especially for single crystals. For polycrystalline samples, impurity doping and magnetic field have proven to be powerful tools in the search for understanding and improving thermoelectric performance. Nanopolycrystalline $\mathrm{Bi_xSb_y}$ doped with 1 and 3$\%$ Ho were prepared by ball milling and dc hot pressing technique. Electrical resistivity, Seebeck coefficient, thermal conductivity, carrier concentration, mobility, and magnetization are measured in a temperature range of 5-350 K and in magnetic fields up to 9 Tesla. The effects of Ho doping on the thermoelectric properties of $\mathrm{Bi_xSb_y}$ in magnetic field will be discussed.   

Thermoelectric properties of Quintuple Layer Bi$_2$Te$_3$

Motivated by recent experimental results,\footnote{D. Teweldebrhan, V. Goyal and A. A. Balandin, Nano Lett. 10, 1209 (2010); D. Teweldebrhan, V. Goyal, M. Rahman, and A. A. Balandin, Appl. Phys. Lett. 96, 053107 (2010); Y. Zhang et al., Nat. Phys. 6, 584 (2010). } we derive the thermoelectric parameters of a Bi$_{2}$Te$_{3}$ film of one quintuple layer thickness. Our results show approximately ten times increase in the figure of merit (ZT) for the thin film (ZT = 7.2) compared to that for the bulk (ZT = 0.68). The large enhancement in ZT results from the change in the distribution of the valence band density of modes brought about by the quantum confinement in the thin film. Our theoretical model uses ab initio electronic structure calculations as implemented in the VASP software package combined with a Landauer approach for calculating the linear-response transport coefficients. We employ two fitting parameters: a rigid shift of the conduction and valence bands to match the known bulk bandgap (i.e. a `scissors operator'), and an energy independent electron mean free path for the phonon scattering inside the device. With these two fitting parameters, our results show excellent agreement with the known experimental values for bulk Bi$_{2} $Te$_{3}$.   

Phase diagram of thermoelectric Bi2S3-Bi2Se3-Bi2Te3 system

It is well known that the highest ZT value, at an optimized carrier concentration, is mainly determined by a material parameter $\beta =\mu $(m*/m0)3/2/$\kappa $lat, where $\mu $(m*/m0)3/2 and $\kappa $lat are the weighted carrier mobility and lattice thermal conductivity, respectively. In order to explore some new compositions in Bi2S3-Bi2Se3-Bi2Te3 system, we propose a compositional thermoelectric phase diagram (TPD), including weighted carrier mobility, lattice thermal conductivity, and material parameter, for the 1{\%} copper doped Bi2S3-Bi2Se3-Bi2Te3 solid solution fabricated by MA-HP method. Here, the $\mu $(m*/m0)3/2 and $\kappa $lat values could be deduced from the measured electrical resistivity, Seebeck coefficient, and thermal conductivity. The alloying effect on the thermoelectric phase diagram will be discussed from varying atomic size, chemical bond, lattice structure, etc.   

Exploration of effects of various impurities in Bismuth and its extension to Bismuth-Antimony alloys

While Te and Se are known donors and Pb and Sn known acceptors in elemental bismuth, little is known about other possible dopants. The effect of various impurities on thermoelectric properties of elemental bismuth is investigated here. Impurities investigated encompass the transition metals, group III and IV elements, and the chalcogens. The thermoelectric power, electrical resistivity and Hall coefficients of Bi samples doped with these impurities are measured from room temperature to 2K. Indium is found to be an acceptor, which is surprising because it is mostly trivalent. A calculation of the band structure subsequently performed at the AGH University of Science and Technology in Cracow reveals that In gives an excess density of states in the valence band. This finding in elemental Bi is extended to the case of bismuth-antimony alloys which have superior thermoelectric efficiency at cryogenic temperatures.   

Thermoelectric properties of Sn1-xEuxTe

SnTe has potential in thermoelectric application for intermediate temperature [1]. However, the figure of merit ZT of SnTe is limited because that it always has a high hole concentration owing to Sn vacancies. As a result, the Seebeck coefficient of SnTe is low and it is very difficult to get SnTe to the optimized doping level required to get a good figure of merit. SnTe also has heavy valence band close to the light valence band edge. We know from theoretical calculations that degenerate bands are preferable than bands separated by an energy difference for thermoelectric application. EuTe has a much higher band gap than SnTe. Recent results [2] show that in Sn1-xEuxTe films prepared by hot-wall epitaxy, the direct L-point bandgap first closes with x, and then opens. In this presentation, we report on the synthesis of bulk Sn1-xEuxTe samples, and report on their Seebeck coefficient, Hall coefficient, resistivity and thermal conductivity. A simplified model is proposed to explain the experimental data. The results confirm the results of the previous study, and point towards the possibility of finding a high-ZT formulation in these compounds. The work is supported by ZTPlus. \\[4pt] [1] V. P. Vedeneev et al., Semiconductors, 32, 241 (1998) \\[0pt] [2] Akihiro Ishida et al., J. Appl. Phys. 107, 123708 (2010)   

Incipient Ferroelectricity in Thermoelectric Lead Telluride

PbTe, is the parent compound of currently the most important thermoelectric (TE) materials in applications just above room temperature [1]. It has an anomalously low thermal conductivity resulting in a rather high TE figure of merit. Our neutron total scattering and atomic pair distribution function analysis shows the existence of a novel paraelectric state at and above room temperature. However, on cooling the structural dipoles do not order, but disappear resulting in an undistorted rock-salt ground-state. We suggest that new thermoelectrics should be sought among materials that, like PbTe [2], are close to a ferroelectric instability.\\[4pt] [1] Z.H. Dughaish, Physica B v.322, pp205 (2002).\\[0pt] [2] E.S. Bozin et al, Science (to be published).   

Enhancement of thermoelectric figure-of-merit in a wide temperature range in In$_4$Se$_{3-x}$Cl$_y$ bulk crystals

Recently, we proposed that the charge density wave is a new pathway for high thermoelectric performance in bulk crystalline materials [1,2]. Through the quasi one-dimensional lattice distortion (Peierls distortion) in In$_4$Se$_{3-x}$ bulk single crystals, we have achieved a high thermoelectric figure-of-merit {\it ZT} of 1.48 at 705 K. From the Boltzman transport calculation, it was confirmed that the reported {\it ZT} could be further increased if we could increase the chemical potential of the In$_4$Se$_{3-x}$ crystals. Here we report the significant increase of {\it ZT} over a wide temperature range from 50 $^{\circ}$C to 425 $^{\circ}$C by chlorine doping in the In$_4$Se$_{3-x}$, which comes from the improvement of crystal quality and increase of chemical potential, resulting in the power factor enhancement and the thermal conductivity reduction. \\[4pt] [1] J.-S. Rhyee et al., J. Appl. Phys. {\bf 105}, 053712 (2009). \\[0pt] [2] Jong-Soo Rhyee et al., Nature (London) {\bf 459}, 965 (2009).   

Thermoelectricity in the ultra-thin limit

Hicks and Dresselhaus [1] predicted an enhanced thermoelectric power factor due to quantum confinement. In the past, superlattices have been employed to demonstrate this effect but the results have remained controversial. Sustained efforts on surface termination and treatment of single crystalline oxide substrates has enabled growth of high quality thin films using techniques like pulsed laser deposition and molecular beam epitaxy. In this work, we explore the nature of thermoelectric response for ultra thin layers ($\sim $ 1 -- 100 nm) of model thermoelectric oxides such as doped SrTiO$_{3}$ and Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y }$grown by pulsed laser deposition. Thermopower, resistivity and Hall measurements were carried out as a function of thickness to understand the role of quantum confinement and other extraneous effects like surface depletion etc. on the thermoelectric response. References: [1] L.D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993).   

Zero-dimensional nanostructured material with metallic bismuth nanoparticles: a new route for thermoelectrics

The thermoelectric figure of merit ZT has so far not exceeded the value ZT=3 need to compete with mechanical energy conversion systems. However, theoretical work has shown that it is possible to reach values of ZT higher than this. One of the most promising routes is nanostructured materials, which offer the opportunity to tailor physical properties such as electrical and heat transport, due to the effects of electron filtering and phonon confinement. Dresselhaus \textit{et al. (ref.?)} were among the first to show that 2D and 1D structures are capable of reaching ZT values higher than 2. The thermoelectric materials of current interest are in the form of nanotubes, nanodots and, more generally, superlattices composed of a matrix and nanoparticles. In our work we synthesize a periodic network of bismuth nanoparticles in a matrix of mesoporous SiO$_{2}$. We find that in this form bismuth transforms from a rhombohedral to a cubic structure, with improved filtering of electrons and phonons.   

Session Y21: High Magnetic Field Measurements, Novel sensors, and Neutron Diffraction

Stand alone experimental setup for measurements of magnetoresistance tensor by dc reversal technique

Several years ago Keithley Instruments, Inc. created a combination of a Current Source and a Nanovoltmeter (Model 6221 and Model 2182A, respectively) for low level transport measurements. That nanovoltmeter/ current source combination was designed for measurements of \textit{one} voltage only. Proposed are the setups assembled from several nanovoltmeter/current source pairs which allow to measure simultaneous \textit{several} voltages associated with the same current. The setups utilize specific wiring and a unique triggering sequence. Several milliseconds delays incorporated into triggering sequence secure stable triggering and proper data flow. The delays might be assured by selection of specific time parameters in the current sources and nanovoltmeters. The setups allow utilizing the built-in functions of the devices. Tested setups consisted of up to four pairs, allowing measurements of up to four voltages.\footnote{A. V. Suslov, Rev. Sci. Instrum. 81, 075111 (2010).} Application of the setups to simultaneous measurements of magnetoresistance tensor components will significantly simplify the experiment, increase precision, and decrease consumption of resources.   

High Magnetic Field Characterization of Cu-Sn Alloys for Distortion-free MRI Probes

For a wide-range of reasons, magnetic resonance imaging (MRI) of brain activity is now exploiting minituraized electrodes and cannulas. However, common construction materials such as stainless steel cause significant distortion of the MRI signals.\footnote{F.M.~Martinez-Santiesteban \emph{et al.}, Phys.~Med.~Biol.~{\bf 52} (2007) 2073.} With the goal of developing brain-susceptibility-matched electrodes and cannula for distortion-free MRI in fields up to 11~T, we have investigated the magnetic properties of a spectrum of Cu-Sn alloys. The results of various characterization studies, including SQUID magnetometry up to 7~T and MRI studies up to 11 T, will be reported and related to the stoichiometric composition of the Cu-Sn solutions. Extensions to device development and other metal alloy combinations will be discussed.   

Addressing signal recovery challenges in pulsed field environment

We review approaches to recovering weak electric signals in the challenging environment of pulsed magnetic fields implemented at the National High Magnetic Field Laboratory Pulsed Field Facility (NHMFL-PFF). Recent technique developments including AC-specific heat measurements at sub-kelvin temperatures, nanosecond-scale resistivity measurements, as well as customized instrumentation and computer-assisted signal detection using Field Programmable Gate Arrays will be discussed.   

The National High magnetic Field Laboratory Pulsed Field Facility. An overview of high field magnet operations and scientific techniques

The National High magnetic Field Laboratory -- Pulsed Field Facility (NHMFL-PFF) is the home to the pulsed field user facility which routinely delivers 85T pulses for user science using a 1.4GW motor generator. The facility also houses a 60T shaped waveform magnet, 65T short pulse and 50T mid pulse capacitor driven magnets. Many techniques are available to users including, Transport, magnetization, calorimeter and cantilever techniques. I will describe the facilities and the measurement techniques available to users.   

Extreme Material Physical Properties and Measurements above 100 tesla

The National High Magnetic Field Laboratory (NHMFL) Pulsed Field Facility (PFF) at Los Alamos National Laboratory (LANL) offers extreme environments of ultra high magnetic fields above 100 tesla by use of the Single Turn method as well as fields approaching 100 tesla with more complex methods. The challenge of metrology in the extreme magnetic field generating devices is complicated by the millions of amperes of current and tens of thousands of volts that are required to deliver the pulsed power needed for field generation. Methods of detecting physical properties of materials are essential parts of the science that seeks to understand and eventually control the fundamental functionality of materials in extreme environments. De-coupling the signal of the sample from the electro-magnetic interference associated with the magnet system is required to make these state-of-the-art magnetic fields useful to scientists studying materials in high magnetic fields. The cutting edge methods that are being used as well as methods in development will be presented with recent results in Graphene and High-Tc superconductors along with the methods and challenges.   

Sub 100 nm ballistic sensors for ultra high spatial resolution magnetic field detection

There is an ongoing drive to develop non-invasive magnetic field sensors with ultra high spatial resolution (UHSR) of 100 nm or less for numerous applications.$^{1,2}$ Conventional field sensors e.g. based on the Hall effect, rely on diffusive transport, where high mobility III-V semiconductors offer the best field sensitivity (T/Hz$^{0.5}$).$^{2}$ For UHSR, the critical dimensions of the device must be reduced below the mean free path where transport is ballistic, and the detection properties are not preserved, e.g. the Hall response can be suppressed and/or nonlinear. We report sub 100 nm sensors utilizing the negative bend resistance of InSb/InAlSb ballistic structures at elevated temperatures.$^{3}$ These devices exhibit an enhanced responsivity that is tunable by geometric design and extremely attractive for the detection of ultra small magnetic fields. Our smallest device studied to date has an active sensor area of 35 x 35 nm$^{2}$, and a sensitivity of 0.87 $\mu$T/Hz$^{0.5}$ at 100 K. The performance and detection properties are reviewed with respect to state-of-the-art technologies.\\ $^{1}$P. Manandhar, Nanotechnol. 20, 355501 (2009). $^{2}$A. Sandhu, Microelectron. Eng. 73, 524 (2004). $^{3}$A. M. Gilbertson, et al., Submitted to Appl. Phys. Letts. (2010).   

dc SQUIDs as displacement detectors for embedded micromechanical resonators

Superconducting quantum interference devices (SQUIDs) can detect tiny amounts of magnetic flux and are also used to study macroscopic quantum effects. We employ a dc SQUID as a linear detector of the displacement of an embedded micromechanical resonator with femtometer sensitivity. We have also measured the backaction of the dc SQUID on the resonator, where the resonance frequency and damping of the resonator can be tuned with bias current and applied magnetic flux. The backaction can tune the resonator from strongly damped to self-sustained oscillation and may be used to cool the resonator.   

A Novel Ambient Operating Force and Acceleration Detector

An investigation to develop a novel accelerometer capable of operating under ambient conditions without any cryogenics is in progress in our laboratory. In this device the proof mass comprises of magnetic or diamagnetic materials. This mass is freely suspended in stable equilibrium under gravity by the combined actions of magnetic attraction and repulsion forces. Stability is achieved along all three Cartesian axes even at zero frequency. For highly dynamical onboard platforms, realtime nulling by active control at high-frequency is desirable. A description of prototypes and measurements will be discussed. Sensitivity in the $\sim $ 0.1ngal regime to both kinematic and gravitational accelerations and $\sim $ pN force resolution is observed. Our initial results including (i) detection of tidal changes in the gravitational background, (ii) seismic tremors, (iii) Fourier analysis of time displacement data and (iv) design considerations for enhanced sensitivity and improved performance will be presented. Several scientific and technological implications will be suggested.   

Driving electronics for a z-positioner in a new SPM design.

We use a modified Pan-type walker as the z coarse approach mechanism in our new SPM design. We developed new electronics for driving and exercising the walker with the main circuit consisting of six 12V relays. Connecting the relays in series produces a timing cascade due to the mechanical delay in each relay. The traditional slow linear ramp has been replaced with the charge and discharge behavior of the RC circuits, where C is the capacitance of the piezolelectric plates. Initial tests with a 6Hz frequency input showed 10 nm step size and a 3 millimeter range. A single 555 timer serves as our frequency generating source. A highly stabilized square wave can be generated in its monostable mode, with the output frequency determined by two external resistors and a capacitor. We also replace the high voltage supply with a voltage quadrupler circuit that is compact and inexpensive, with 64V and 128V DC output in the final configuration.   

A unique 30 Tesla single-solenoid pulsed magnet instrument for x-ray studies

We present a dual-cryostat pulsed-magnet instrument at the Advanced Photon Source (APS) with unique capabilities. The dual-cryostat independently cools the solenoid (Tohoku design) using liquid nitrogen and the sample using a closed-cycle refrigerator, respectively. Liquid nitrogen (LN) cooling allows a repetition rate of seven minutes for peak fields of 30 Tesla. The system is unique in that the LN cryostat incorporates a double-funnel vacuum tube passing through the solenoid's bore preserving the entire angular range allowed by the magnet. This scheme is advantageous in that it allows the applied magnetic field to be parallel to the scattering plane complementing typical split-pair magnets with fields normal to the scattering plane. Performance of the coils along with preliminary x-ray diffraction and spectroscopic studies will be presented.   

Applications of superconducting trapped field magnets for x-ray scattering experiments

Two long standing problems in x-ray and neutron scattering studies in applied magnetic fields are, 1) limited optical access and 2) practical impossibility to apply magnetic field parallel to x-ray (neutron) momentum transfer. In order to overcome these obstacles we have developed an application of Type-II superconducting magnets. In this approach, a small and thin plate-like single crystal sample is mounted on the surface of a melt-textured superconductor (SC). The SC is magnetized by cooling it from temperature above its superconducting critical temperature ($T_{c})$ in an applied magnetic field. Below $T_{c}$, magnetic flux gets trapped inside the SC disk after the removal of the external magnetic field. The SC disk acts as a permanent magnet with applied field normal to the flat surface of the disk providing unrestricted optical access to the entire hemisphere allowing a magnetic field parallel to the x-ray momentum transfer.   

Efficient conversion of $^{3}$He($n$,\textit{tp}) and $^{10}$B($n$,\textit{$\alpha $}$^{7}$Li) reaction energies into far-ultraviolet radiation by noble gas excimers

Previous work$^{1,2 }$showed that the $^{3}$He($n$,\textit{tp}) reaction in a cell of $^{3}$He at atmospheric pressure generated tens of far-ultraviolet (FUV) photons per reacted neutron. Here we report amplification of that signal by factors of 1000 when noble gases are added to the cell. Calibrated filter-detector measurements show that this large signal is due to noble-gas excimer emissions, and that the nuclear reaction energy is converted to FUV radiation with efficiencies of up to 30{\%}. Our results have been placed on an absolute scale through calibrations at the NIST SURF III Synchrotron and Center for Neutron Research.$^{3}$ We have also seen large neutron-induced FUV signals when the $^{3}$He gas in our system is replaced with a $^{10}$B film target; an experiment on substituting $^{3}$He with BF$_{3}$ is underway. Our results suggest possibilities for high-efficiency, non-$^{3}$He neutron detectors as an alternative to existing proportional counters. $^{1}$A. K. Thompson, \textit{et al., J. Res. Natl. Inst. Stand. Technol.} \underline {\textbf{113}}\underline {, 69} (2008) $^{2}$M. A. Coplan, A. K. Thompson and C. W. Clark, \underline {U.S. Patent No. 7,791,045} (2010) $^{3}$P.P. Hughes, \textit{et al.,} \underline {arXiv:1009.4707} (\textit{Appl. Phys. Lett. }in press, 2010)   

First Results from the Triple-axis Spectrometer at OPAL

The thermal triple-axis spectrometer TAIPAN is the first instrument for inelastic scattering at Australian research reactor OPAL. TAIPAN started operation in February 2009 and is in full user service since November 2010. The instrument can operate with variable incident or final energies and has a secondary spectrometer with a single detector. Presently the PG (002) double-focusing monochromator and analyzer are in use. The incident energy range on the TAIPAN is from $\sim $ 5 meV up to $\sim $ 100 meV with neutron flux at sample position of $\sim $ 10$^{8 }$n/cm$^{2}$/s [1]. First experiments were performed with superionic conductor Cu$_{2-x}$Se [2]. The measurements reveal a presence of soft mode in addition to the flat optic-like phonon branch. The DFT calculations show that unstable soft mode is related to ordering of Cu atoms followed by $\alpha -\beta $ phase transition at a lower temperature. The evolution of the magnetic structure with temperature in magnetically modulated FePt$_{3}$ thin film was investigated in the diffraction mode of TAIPAN. The results show that the film fabricated by modulation of the chemical order parameter consists of a magnetic FM/AFM superlattice in single-crystalline FePt$_{3}$ [3]. [1] S.A. Danilkin et al., Neutron News, 20 (2009) 37; [2] S.A. Danilkin et al., J. Phys. Soc. Jpn. 79 (2010) Suppl. A, 25; [3] T. Saerbeck et al., Phys. Rev. B 82 (2010) 134409.   

Session Y22: Ferroelectric and Structural Phase Transitions

Magnetic and magnetoelectric excitations in multiferroic BiFeO$_{3}$

Ferroelectric antiferromagnet BiFeO$_{3}$ combines ferroelectricity with an antiferromagnetic order at room temperature. A control of its magnetic state by voltage has been demostrated both in bulk and in thin film BiFeO$_{3}$. The distortion of the cubic perovskite lattice leads to two effects through the Dzyaloshinski-Moriya magnetic interaction: the ferroelectric distortion results in the observed incommensurate spiral spin structure, and the rotation of oxygen octahedra with alternating sense on neighboring Fe ions results in a local canting of spins. We present a terahertz spectroscopic study of magnetic excitations in BiFeO$_{3}$. We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's spiral antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction and the local spin canting lead to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the spiral magnetic structure of BiFeO$_{3}$.   

Size-dependent infrared phonon modes and ferroelectric phase transition in BiFeO$_3$ nanoparticles

One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO$_3$, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is \emph{Pm}\=3\emph{m} and the lowest frequency A$_1$ feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red shifted band gap in nanoscale BiFeO$_3$ compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide- based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides.   

Remarkably robust ferroelectric state in multiferroic Mn$_{1-x}$Zn$_x$WO$_4$

Zinc doping in Mn$_{1-x}$Zn$_x$WO$_4$ is equivalent to the removal of Mn spins and a dilution of the magnetic system. The multiferroic (ferroelectric) phase of MnWO$_4$ is stabilized through Zn substitution and the low-temperature commensurate phase (up-up-down-down phase) is completely suppressed at a Zn concentration of more than 5\%. The magnetic and ferroelectric phases as well as the multiferroic properties are studied through magnetic, heat capacity, polarization, and neutron scattering experiments. The multiferroic phase is remarkably stable and it still exists for Zn substitution levels up to and above 50\%. At low doping (2\%) the incommensurate helical and the commensurate low-T phases coexist. External magnetic fields do lift the phase degeneracy and stabilize either one of the two ground states, depending on the direction of the field.   

Ferroelectricity in CaTiO$_{3}$ Single Crystal Surfaces and Thin Films and Probed by Nonlinear Optics and Raman Spectroscopy

Bulk CaTiO$_{3}$ has a centrosymmetric point group and is \textit{not }polar or ferroelectric. However, we present surprising results that show highly regular polar domains in single crystals of CaTiO$_{3}$. Confocal Second Harmonic Generation (SHG) and Raman imaging studies were carried out on perovskite CaTiO$_{3}$ crystal surfaces. They reveal large, crystallographic polar domains at room temperature, with in-plane polarization components delineated by twin walls. SHG analysis indicates that the highest symmetry of the polar surface is $m $(space group P$c)$ with polarization in the $m$ plane. In addition, we present results of the polar domain structure imaged before and after the application of an external electric field. Finally, we present the SHG studies of CaTiO$_{3}$ thin films grown using reactive Molecular Beam Epitaxy (MBE); these films are predicted by theory to be ferroelectric and are shown experimentally, both with SHG and in-plane dielectric measurements, to be ferroelectric for temperatures less than $\sim $150 K with group symmetry \textit{mm}2.   

Phase transitions in relaxors as seen by neutron scattering

Relaxors have a broad temperature and frequency-dependent peak in the dielectric permittivity that is not necessarly linked to a structural phase transition. A model relaxor is PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ (PMN) doped with PbTiO$_{3}$ (PT). We report neutron studies of the low-energy spectra of (1-x)PMN-xPT crystals. Apart from phonons which do not show a soft mode, there are two components of the diffuse scattering: one is quasi-elastic (QE) and the other static. The energy width of the QE scattering decreases as the peak of the susceptibility is approached. The static component behaves like an order parameter. In the crystals that become ferroelectric it is maximal at the ferroelectric phase transition, but in PMN it steadily increases on cooling. We discuss previously reported and new results in terms of a random-field model of the cubic crystal.   

Elastic collapse and avalanche criticality near a Mott transition

We study some dynamic aspects of a Mott transition in a rare-earth alloy Ce$_{0.90}$Th$_{0.10}$ by resonant-ultrasound spectroscopy (RUS), electrical-transport, and thermal-expansion measurements. In the temperature range spanning the first-order transition, we observe a stiffening of the elastic response that is associated with a continuous front propagation ($e.g.$ solitons). A defining characteristic of a mixed phase regime, slow scanning rates (0.01 K/min) show these solitons to be superimposed with jerks and avalanches in all three data sets: RUS, resistivity, and thermal expansion data. Analysis of the avalanche data give power law distributions with critical exponents $P(E)=E^{n}$ for energy, in the case of thermal expansion data and length, in the case of electrical transport data.   

Calculated temperature dependence of elastic constants and phonon dispersion of hcp and bcc beryllium

Conventional first principle methods for calculating lattice dynamics are unable to calculate high temperature thermophysical properties of materials containing modes that are entropically stabilized. In this presentation we use a relatively new approach called self-consistent \textit{ab initio} lattice dynamics (SCAILD) to study the hcp to bcc transition (1530 K) in beryllium. The SCAILD method goes beyond the harmonic approximation to include phonon-phonon interactions and produces a temperature-dependent phonon dispersion. In the high temperature bcc structure, phonon-phonon interactions dynamically stabilize the N-point phonon. Fits to the calculated phonon dispersion were used to determine the temperature dependence of the elastic constants in the hcp and bcc phases.   

Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy

In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. In this study [1], we consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100~K below their Curie temperature, to demonstrate from \textit{ab-initio} calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte-Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data. [1] Ekholm et al., Phys. Rev. Lett. 105:167208 (2010)   

Structural and magnetic properties of Ni$_2$MnGa from first-principles

Ni$_2$MnGa is the prototype magnetic shape-memory alloy. In this work we use ab-initio calculations to characterize structural and magnetic transitions and to identify possible strategies to tune them towards the same critical point. To this aim both the austenite and the martensite phases of the Ni$_2$MnGa alloy are studied with particular attention to the electronic factors controlling their stability and the onset of the structural transition. Our results indicate that, in spite of its metallic character, electronic correlations play an important role in determining the behavior of this compound and, in particular, the entity (and sign) of the deformation accompanying the transition from the austenite phase to the martensite one. The vibrational properties of the austenite phase are also studied and structural instabilities (soft modes) are investigated as possible signatures of intermediate ``modulated'' structures.   

Origin of ``aging'' in shape-memory alloys

For more than half a century it has been widely observed that a majority of shape-memory alloys exhibit a gradual change in physical properties with time in the martensitic phase, and this is referred to as ``aging.'' However, its microscopic mechanism has remained controversial due to lack of experiments that can probe atomic level processes. We clarify the atomic mechanism for how shape memory alloys ``age'' in time using a combination of molecular dynamics and Monte-Carlo simulations. Through analysis of the atomic configurations during aging, we find that the observed phenomenon is associated with a gradual change in the short range order of point defects so that the defect short range order tends to adopt the same ``symmetry'' as the crystal symmetry of the host martensite lattice. The results provide atomic-level evidence for the symmetry-conforming short-range order model, and may provide new insight into how to control aging to design aging-free shape memory alloys. Reference: 1). J. Deng, X. Ding, T. Lookman, et al, Physical Review B , \textbf{81}, 220101(R), 2010 2). J. Deng, X. Ding, T. Lookman, et al, Physical Review B, \textbf{82},184101, 2010 3). J. Deng, X. Ding, T. Lookman, et al, Applied Physics Letters, \textbf{97},171902, 2010   

Microstructure from ferroelastic transitions using strain pseudospin clock models in two and three dimensions

We show how microstructure can arise in first-order ferroelastic structural transitions, in two and three spatial dimensions, through a local mean-field approximation of their pseudospin Hamiltonians, that include anisotropic elastic interactions. Such transitions have symmetry-selected physical strains as their order parameters, with Landau free energies that have a single zero-strain ``austenite'' minimum at high temperatures, and spontaneous-strain ``martensite'' minima of structural variants at low temperatures. The total free energy also has gradient terms, and power-law anisotropic effective interactions, induced by ``no-dislocation'' St Venant compatibility constraints. In a reduced description, the strains at Landau minima induce temperature dependent, clocklike Hamiltonians, with strain- pseudospin vectors S pointing to discrete values including zero. We study elastic texturing in five such first-order structural transitions through a local mean-field approximation of their pseudospin Hamiltonians, that include the power-law interactions. The local mean-field solutions in 2D and 3D yield or oriented domain- wall patterns as from continuous-variable strain dynamics, showing the discrete- variable models capture the essential ferroelastic texturings.   

Symmetry breaking in amorphous solids undergoing martensitic phase transformation - a relation to Landau's theory

Martensitic phase transformation can be classified as displacive solid -solid phase transformations, where the symmetry of the high temperature phase (austenite) breaks when phase transformation occurs. The martensitic phase (low temperature phase ) and its variants are products of symmetry breaking in solids. Based on a quasiparticle statistical mechanics approach the canonical free energy of a representative solid volume element consisting of several quasiparticles (representative mole number) can be derived. The symmetry breaking order parameter of the system is the total strain which is an ensemble mean value in the statistical mechanics concept. In the current theory the order parameter is a macroscopic strain in a sense that the representative volume element stands for the macroscopic level, whereas the lattice parameter changes are considered in the hamiltonian definition of each quasiparticle. Computational results of the developed theory correspond to experimentally observed phenomena in materials undergoing martensitic phase transformation. The present study is focusing the region nearby the phase transformation and shows how the developed theory for describing symmetry breaking and order parameter changes correspond to Landau's phenomenological theory of phase transitions.   

Probing driven first order structural transitions with resistivity noise

We study the avalanche-mediated driven first order structural transition in nickel titanium shape memory alloys with time-dependent fluctuations in electrical resistivity. Higher order statistics of the fluctuations, or noise, has been used as a kinetic detector of the underlying two stage athermal phase transition. We have found that the non-gaussian component of the higher order statistics carries significant information about the transition parameters and is coupled to the microscopic origin of the phase transition. The results can be explained with a model based on three competing time scales dependent on avalanche relaxation, thermal fluctuations and drive rate. The transition temperature was found to decrease with increasing drive rate indicative of the increased possibility of the system being driven towards the athermal limit. Moreover, the magnitude of the non-gaussian component is found to have signatures of the extent of correlations in the system and hence a viable tool to detect any overlap of avalanches in space or time. The study establishes noise as a sensitive tool to probe the kinetics of driven structural transitions which can be exploited in a variety of other systems. References: U. Chandni et. al, Phys. Rev. Lett. 102, 025701 (2009) U. Chandni and A. Ghosh, Phys. Rev B. \textbf{81}, 134105 (2010)   

Session Y23: Superconductivity: Proximity Effects

Supercurrent-Induced Magnetization Dynamics

We investigate supercurrent-induced magnetization dynamics in a Josephson junction with two misaligned ferromagnetic layers, and demonstrate a variety of effects by solving numerically the Landau-Lifshitz-Gilbert equation. In particular, we demonstrate the possibility to obtain supercurrent-induced magnetization switching for an experimentally feasible set of parameters, and clarify the favorable condition for the realization of magnetization reversal. These results constitute a superconducting analogue to conventional current-induced magnetization dynamics and indicate how spin-triplet supercurrents may be utilized for practical purposes in spintronics.   

Superfluid Densities in Superconducting/Ferromagnetic (Nb/NiV/Nb) Heterostructures

Superfluid density measurements allow us to probe the superconducting structure of thin films below T$_{c}$ with remarkable detail. They yield information not only of the inherent robustness of the superconducting state, but also about the homogeneity of the sample and possible ``hidden'' transitions at temperatures lower than the initial T$_{c}$. For this reason multiple transitions in superconducting heterostructures are revealed to us. We use superfluid density measurements on Nb/Ni$_{0.95}$V$_{0.05}$/Nb trilayers to study the interplay between two superconducting films separated by the destructive proximity effects of a ferromagnet. We show there are trilayers with strong coupling, which produces a single transition, that become decoupled to the point of separation into two transitions as the ferromagnetic layer thickness increases. We discuss the difficulties in observing the second transition in $\sigma _{1}$, while obvious in $\lambda ^{-2}$.   

Magnetic-state-controlled proximity effect across high-T$_{C}$ superconductor/ferromagnetic interfaces.

We have investigated the electronic density of states of a ferromagnet (F: a Co/Pt superlattice) in contact with a c-axis YBCO film. This was done by measuring the current-perpendicular-to-plane differential conductance across vertical junctions of area down to 6 $\mu $m$^{2}$, which were fabricated using optical lithography and ion etching. We have found salient features of the leakage of the superconducting order parameter into the F layer, such as a zero-bias conductance peak which can be modulated by the magnetic state of the ferromagnet. We discuss the possibility of triplet superconducting correlations induced in the F layer as the origin of this behavior.   

Long-Range Superconducting Proximity Effect in Template-Fabricated Single-Crystal Nanowires

We study a superconducting proximity effect observed in single-crystal nanowires of Zn, Sn, and Pb of length up to 60 $\mu $m. These nanowires are electrochemically deposited into the pores of anodic aluminum oxide membranes and polycarbonate membranes. Using an \textit{in situ} self-contacting method, single nanowires are electrically contacted on both ends to a pair of macroscopic film electrodes of Au, Sn, or Pb pre-fabricated on both surfaces of the membranes. Superconductivity in the nanowires is strongly suppressed when Au electrodes are used. When electrodes having higher superconducting transition temperatures are used, the nanowires become superconducting at the transition temperatures of the electrodes. Microscopy analyses of the structure and the chemical composition of the nanowires will be presented. Measurements of sample resistance and $I-V$ characteristics at various temperatures and magnetic fields will also be presented. Furthermore, the effects of the length, the diameter, and the residual resistance ratio of the nanowires on the proximity induced superconductivity will be analyzed and discussed.   

Radiative Interband Transition of Cooper Pairs in a Semiconductor

Interactions of photons and superconductors have been a hot topic for superconducting (SC) qubit operations. The relevant photon energies were limited below the superconducting gap of superconductors, that is, microwave frequencies. The possibility of electron Cooper-pair interactions with photons with much higher energies was discussed theoretically [1]. In this talk we will demonstrate that Cooper pairs penetrated into a semiconductor from an adjacent superconductor by the proximity effect play a major role in interband radiative recombinations in the semiconductor experimentally. SC Nb electrodes were formed on an InGaAs/InP light emitting diode (LED) and electroluminescence (EL) around 1.55um was observed from a slit formed on the surface Nb electrode. EL was drastically enhanced below the Nb SC critical temperature (T$_{c})$ of $\sim $8K [2]. The reduction of radiative recombination lifetime consistent with the observed EL enhancement was observed below T$_{c}$[3]. These results are well explained with the theory [1]. We will discuss the possibility of generating entangled photon pairs based on this new scheme. [1] Y. Asano et al., PRL 103 (2009) 187001. [2] Y. Hayashi et al., Appl. Phys. Express 1 (2008) 011701. [3] I. Suemune et al., APEX 3 (2010) 054001.   

Robustness of Majorana modes and minigaps in a spin-orbit-coupled semiconductor-superconductor heterostructure

We study the robustness of Majorana zero energy modes and minigaps of quasiparticle excitations in a vortex by numerically solving Bogoliubov-deGennes equations in a heterostructure composed of an \textit{s} -wave superconductor, a spin-orbit-coupled semiconductor thin film, and a magnetic insulator. This heterostructure was proposed recently as a platform for observing non-Abelian statistics and performing topological quantum computation. The dependence of the Majorana zero energy states and the minigaps on various physics parameters (Zeeman field, chemical potential, spin-orbit coupling strength) is characterized. We find the minigaps depend strongly on the spin-orbit coupling strength. In certain parameter region, the minigaps are linearly proportional to the \textit{s}-wave superconducting pairing gap $\Delta_{s}$, which is very different from the $\Delta_{s}^{2}$ dependence in a regular \textit{s-} or \textit{p}-wave superconductor. We characterize the zero energy chiral edge state at the boundary and calculate the STM signal in the vortex core that shows a pronounced zero energy peak. We show that the Majorana zero energy states are robust in the presence of various types of impurities. We find the existence of impurity potential may increase the minigaps and thus benefit topological quantum computation.   

Topological superconductivity and Majorana fermions in half-metal / superconductor heterostructure

A half-metal is by definition spin-polarized at its Fermi level and therefore was conventionally thought to have little proximity effect to an $s$-wave superconductor. Here we show that if there is spin-orbit coupling at the interface between a single-band half-metal and an $s$-wave superconductor, $p_x + ip_y$ superconductivity would be induced on the half-metal. This can give us topological superconductor with a single chiral Majorana edge state. We show that two atomic layers of CrO$_2$ or CrTe gives us the single-band half-metal and is thus a candidate material for realizing this physics.   

Superconducting proximity effects of Pb nano-islands

Superconductivity in systems with spatial dimensions smaller than the coherence length has been the subject of intense interest for decades. We systematically address how superconducting nano-islands interact each other via a detailed scanning tunneling microscopy/spectroscopy (STM/STS). By measuring the spatial mapping of the local superconducting gap, an intriguing lateral proximity effect is observed in an island containing regions of different thicknesses and different superconducting strength which shows a gradual but evident change of local superconductivity at the thickness boundary. This must be due to a lateral proximity effect caused by the tunneling of Cooper pairs with different binding energies across the boundary. We were also able to experimentally determine a proximity length. When an island is smaller than the proximity length, it is found that superconductivity within the island is rather uniform, indicating the rigidity of the order parameter on the scale of proximity length.   

S -N-S junction formed by graphene with lead (Pb) contacts

We fabricate lead (Pb) contacts to graphene that allow us to observe supercurrent in the Pb-graphene-Pb structure up to temperatures of $\sim $3K. The measured critical current is much smaller than a naive expectation based on calculations for a superconductor-insulator-superconductor (S-I-S) junction. Hysteresis is seen in the switching current despite the fact that the junction is made to be overdamped. The behavior of the Pb-graphene-Pb structure is qualitatively explained by considering it as an S-N-S junction.   

Switching and retrapping behavior in graphene proximity-effect superconducting junctions

Since the pioneering work by R. Holm and W.Meissner [Z. Physik.~86, 787 (1933)], who observed zero resistance in SNS pressed contacts, many manifestations of the superconducting proximity effect have been reported. Recently it was shown that when closely spaced superconducting leads are placed on graphene, the proximity effect is induced and a supercurrent can flow between the electrodes. Here we fabricate graphene proximity-effect junctions (GPJ) and compare them to Josephson junctions (JJ). As the bias current is increased to near the critical current, a thermal escape from the washboard potential can occur driving the junction into the runaway voltage state. ~The standard deviation of the switching current is measured as a function of temperature and compared to the thermal and quantum escape models for JJs. ~We find that the temperature dependence of the standard deviation of switching currents of graphene proximity junctions is qualitatively different from the well-studied behavior of the insulator-based JJs. Possible reasons will be discussed.   

Interface Engineering of Thin Film Superconductor Heterostructures

Thin film superconductivity is a subject with great scientific and technological importance. The previous works demonstrated that the superconductivity exits in extreme two-dimensional lead film with a thickness of only two atomic layers. Most strikingly, the Tc is only slightly suppressed from the bulk value. However, when the film is pseudomorphically strained, the Tc is suppressed further, implying the importance of the interface. In this work we explore thin film superconductivity in a new direction by engineering superconductor/normal metal heterostructures with atomically flat interface. Using in-situ scanning tunneling microscopy/spectroscopy, we explore the superconductivity of the Pb/Ag heterostructure by independently tuning the thicknesses of the atomically flat Ag films and superconducting Pb films respectively. The intriguing role of the Ag thin films on the superconductivity of Pb thin films will be reported.   

Tunneling Measurements of the Exchange Field in Superconducting Al-EuS Bilayers

We present tunneling density of states measurements of the proximity-induced exchange field in superconducting Al-EuS bilayers. By depositing thin Al films onto an insulating EuS layer we demonstrate that one can induce an exchange field of several tesla in the superconducting Al. Contrary to expectations, this exchange field is a strong function of the applied field below 2 T. This applied-field dependence is not associated with the alignment of domains in the EuS, but instead appears to be an intrinsic effect. In addition, we show that the exchange field decreases significantly with increasing temperature below 1 K.   

Exploring the Parameter Space of the Anti-Proximity Effect

The anti-proximity effect, in which the superconductivity of a nanowire is weakened by the superconductivity of measuring bulk electrodes, has been reported in Zn nanowires (Tian et. al., PRL 95, 076802 (2005), Chen et. al., PRL 103, 127002 (2009)). The mechanism of this effect is not completely understood. We have studied the anti-proximity effect in Al nanowires in a variety of configurations. The effect is found to manifest only when the critical temperature (T$_{c})$ of the nanowire is greater than the T$_{c}$ of its bulk form. The range of the effect is found to be $\sim $ 1$\mu $m. The effect is seen in single nanowires in the absence of a magnetic field establishing that the effect depends on the nature of the measuring electrodes and is not caused by an external magnetic field. The anti-proximity effect has also been seen with the bulk superconductor not connected to the measurement circuitry.   

Session Y24: Density Functional Theory II

Exact Exchange calculations for periodic systems: a real space approach

We present a real-space method for exact-exchange Kohn-Sham calculations of periodic systems. The method is based on self-consistent solutions of the optimized effective potential (OEP) equation on a three-dimensional non-orthogonal grid, using norm conserving pseudopotentials. These solutions can be either exact, using the S-iteration approach, or approximate, using the Krieger, Li, and Iafrate (KLI) approach. We demonstrate, using a variety of systems, the importance of singularity corrections and use of appropriate pseudopotentials.   

Dissociation of diatomic molecules and the exact-exchange Kohn-Sham potential: the case of LiF

The incorrect fractional-charge dissociation of stretched diatomic molecules, predicted by semi-local exchange-correlation functionals, is revisited. This difficulty can be overcome with asymptotically correct non-local potential operators, but should also be absent in exact Kohn-Sham theory, where the potential is local. Here, we show, for the illustrative case of the LiF dimer, that the exact-exchange local Kohn-Sham potential, constructed within the Krieger, Li, and Iafrate (KLI) approximation, can lead to binding energy and charge dissociation curves that are qualitatively correct. This correct behavior is traced back to a characteristic ``step'' structure in the local exchange potential and its relation to the Kohn-Sham eigenvalues is analyzed.   

A Projector Augmented Wave Treatment of Fock Exchange in Hartree-Fock and Optimized Effective Potential Calculations

The use of orbital-dependent exchange-correlation functionals within electronic structure calculations has recently received renewed attention as a means of improving accuracy. In particular, the Fock exchange functional exactly cancels the electron self-interaction error which can be particularly significant in transition metals and other materials with localized orbitals. Since the Projector Augmented Wave (PAW) formalism\footnote{P. E. Bl\"{o}chl, {\em{Phys. Rev. B}} {\bf{50}}, 17953 (1994).} accurately evaluates the interaction integrals including all multiple moments, it is a natural choice for representing the Fock exchange functional within an efficient pseudopotential-like scheme. We have adapted the PAW formalism for use both in Hartree-Fock (HF) theory and in the KLI approximation to Optimized Effective Potential theory.\footnote{J. B. Krieger, Y. Li and G. J. Iafrate {\em{Phys. Rev. A}} {\bf{45}} 101 (1992).} We show that the effects of core electrons are significant and can be accurately treated within a frozen core orbital approximation.\footnote{Xiao Xu and N. A. W. Holzwarth, {\em{Phys. Rev. B}} {\bf{81}} 245105 (2010).} PAW-HF and PAW-KLI basis, projector, and pseudopotential functions are presented for several elements throughout the periodic table.   

Localized resolution of identity for efficient Hartree-Fock exchange, based on numeric atom-centered orbitals

Methods based on an exact exchange operator (EX) are increasingly popular, but are still restricted to analytical basis functions (e.\,g. Gaussians) for medium system sizes. We here introduce a localized resolution-of-identity approach for the two-electron Coulomb operator, based on expanding single-particle basis function products separately into auxiliary atom-centered basis sets that are restricted to two centers. Our approach produces accurate results for all-electron EX, can be applied both to analytical and numeric basis functions, requires only ${\mathcal{O}}(N^2)$ intermediate storage and retains a path towards ${\mathcal{O}}(N)$ EX for large systems. We demonstrate a total-energy accuracy of $<1$\,meV/atom for systems including Alanine chains and the S22 benchmark molecule set [1], using the numeric atom-centered orbital based all-electron electronic structure code FHI-aims [2].\\[4pt] [1] P. Jure\v{c}ka \emph{et al.}, Phys. Chem. Chem. Phys.~\textbf{8}, 1985 (2006).\\[0pt] [2] V. Blum \emph{et al.}, Comput. Phys. Comm.~\textbf{180}, 2175 (2009).   

Analytic Treatment of the Pair Density in Kohn-Sham Density Functional Theory

We have developed a new treatment of the LDA functional in Kohn-Sham density functional theory which is expressed in terms of the pair density of a non-interacting system of particles, thus avoiding from the outset self-interaction effects. The pair density is expressed explicitly in terms of the density using a orthonormal and complete basis expressed as a functional of the density. This allows its functional differentiation with respect to the density by analytic means. The method is illustrated with numerical results for the potential in the case of one and three dimensional systems and is compared to the potentials obtained from the Hartree term.   

Wave Function Functionals for the Density

In recent work we have developed\footnote{Pan, Slamet, and Sahni, Phys. Rev. A \textbf{81}, 042524 (2010).} a constrained-search variational method for the construction of wave functions that are functionals of a function $\chi: \Psi = \Psi [\chi]$. These wave function functionals are \emph{simultaneously} normalized, reproduce the \emph{exact} expectation of either single- or two-particle operators, and lead to rigorous upper bounds to the energy. In this paper we extend this method to the construction of wave function functionals $\Psi[\chi]$ that are simultaneously normalized, reproduce the density \emph{exactly}, and lead to rigorous upper bounds to the energy. We apply the method to the ground state of the He atom to obtain wave function functionals that reproduce the density of an accurate correlated wave function. The wave function functionals as expected give rise to the exact expectation of non-differential single particle operators, and lead to accurate two-particle expectations and highly accurate energies.   

Koopmans' condition for density-functional theory

In approximate Kohn-Sham density-functional theory, self-interaction manifests itself as the dependence of the energy of an orbital on its fractional occupation. Here, we first examine self-interaction in terms of the discrepancy between total and partial electron removal energies, and then highlight the importance of imposing the generalized Koopmans' condition to resolve this discrepancy. In the process, we derive a correction to approximate functionals that, in the frozen-orbital approximation, eliminates the unphysical occupation dependence of orbital energies up to the third order in the single-particle densities. This non-Koopmans correction brings physical meaning to single-particle energies; when applied to common local or semilocal density functionals it provides results that are in excellent agreement with experimental data while providing an explicit total energy functional that preserves or improves on the description of established structural properties.   

Density Functional Theory for Open Systems

We provide a formal proof of the convexity relation, $2E[N] \leq E[N - 1] + E[N + 1]$, characterizing the total energy of interacting $N$-electron systems under the action of a given external potential, hitherto assumed to be true only on experimental grounds. This is used to prove the inequality, $I(N) - A(N) \geq 0$, where $I(N)$ and $A(N)$ are, respectively, the ionization potential and electron affinity of a $N$-electron system.   

Volume effects in band gap predictions for solids

The \textit{ab initio} prediction of band gaps for solids is important for fundamental and practical reasons. Many approaches exist to remedy the ``band gap problem'' in Density Functional Theory (DFT) in which band gaps are severely underestimated. We recently proposed the $\Delta$-sol method [1], an adaptation of the $\Delta$SCF method towards solids, in which the fundamental gap is evaluated using total energies from DFT. Using $\Delta$-sol, we obtained band gaps for over 100 crystalline semiconductors at accuracies similar to those of hybrid functionals such as HSE, but at significantly smaller computational costs. However, the accuracy of band gap predictions from first principles remains dependent on accurate determination of lattice parameters and cell volumes. In this talk, we discuss the effects of the accuracy in lattice parameters on predicted band gaps. We present results on the accuracy of cell volumes determined using several exchange-correlation functionals: LDA, PBE, HSE and AM05, and compare the dependence of Kohn-Sham gaps and band gaps predicted using $\Delta$-sol on cell volumes. Finally we discuss optimal approaches for predicting band gaps for compounds with unknown lattice parameters. \\[4pt] [1] M. K. Y. Chan and G. Ceder, Phys. Rev. Lett. 105, 196403 (2010)   

Fundamental gaps in finite systems from the eigenvalues of generalized kohn-sham method

We present a broadly-applicable, physically-motivated first-principles approach to determining the fundamental gap of finite systems. The approach is based on using a range-separated hybrid functional within the generalized Kohn-Sham approach to density functional theory. Its key element is the choice of a range-separation parameter such that Koopmans' theorem for both neutral and anion is obeyed as closely as possible. We demonstrated the validity, accuracy, and advantages of this approach on first, second and third row atoms, the oligoacene family of molecules, and a set of hydrogen-passivated silicon nanocrystals. This extends the quantitative usage of density functional theory to an area long believed to be outside its reach.   

Self-optimizing Kohn-Sham hybrid functional

Recent work using range-separated hybrid functionals has confirmed the importance of including long-range exchange in treatments of phenomena such as charge transfer reactions. Using a self-optimizing [1,2] form of the BNL [3] functional, we present results for structural, electronic, and thermochemical properties of a large set of molecules (including the G2 and G3 test sets). The success of this approach, as well as its ability to describe reaction barriers, will be discussed. \\[4pt] [1] T. Stein, L. Kronik, and R. Baer, JACS, 131 (8), 2818, 2009 \newline [2] T. Stein, H. Eisenberg, L. Kronik, and R. Baer, ``Fundamental gaps of finite systems from the eigenvalues of a generalized Kohn-Sham method'', Phys. Rev. Lett., in press. \newline [3] E. Livshits and R. Baer, PCCP, 9, 2932 , 2007   

TDDFT and qualitative properties of excited states: three illustrative applications using DMol$^3$

Three applications of DMol$^3$ TDDFT [1] are presented to show possible new frontiers in each case. First, excitations involving multiplet structure for the example of the Ti$^{4+}$ ion are discussed, showing that atomic multiplet splitting is fully exhibited within TDDFT. This approach to multiplets exhibits notable similarities and also notable differences with a first principles based Condon-Shortley-Cowan multiplet theory. Second, UV-VIS spectra of benzene and derivative molecules are discussed by comparing experimental log plots of molar extinction with a TDDFT results completed by the Gaussian envelope model for the vibrational progression. The envelope model provides a natural scale for comparing TDDFT excitations with measured absorption spectra. In the third example, excited states of (Fe(CN)$_5$NO)$^{-2}$ are studied along the reaction coordinate connecting the long lived metastable states that can be produced by optical excitation.\\[4pt] [1] B. Delley, J. Phys. Cond. Mat. 22, 384208, 2010.   

Exact Time-Dependent Kohn-Sham Potential for an Interacting Few-Body System

Time-dependent density functional theory enables practical simulations of the dynamic many-electron systems, but one of the biggest obstacles to reliable application is the quality of the approximate potential. It is often difficult to determine whether ever-more sophisticated approximations properly include new physics, as there exist few benchmark exact potentials. Towards this ends, we have developed and tested a scheme to extract the exact (non-adiabatic) time-dependent Kohn-Sham potential for few body systems. We will present some examples on 1D model systems. The approach is general and can be used to back engineer high-level quantum mechanical simulations to gain insight into TDDFT on a broad scale. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of the Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

TDDFT approach to study nonlinear excitonic effects in Four-Wave Mixing processes

We develop a time-dependent density-functional theory (TDDFT) formalism to study nonlinear effects in the processes in Four-Wave Mixing experiments. Namely, we generalize our recently proposed approach to calculate excitonic and biexcitonic eigen-energies within TDDFT on the dynamical case, which includes nonlinear effects, the like exciton-exciton interaction. In particular, we derive the TDDFT version of the nonlinear time-dependent equation for excitonic polarization with terms which correspond to exciton-exciton correlations through the memory functions. We have obtained the formula which relates the memory functions with the TDDFT exchange- correlation (XC) kernel. To test the formalism, we calculate the 2D Fourier spectra of a GaAs multi-quantum well system and compare them with experimenatal results in the case of several XC kernels. In addition, we compare the results with the ones obtained within a many-body method for non-linear effects in semiconductors. It is shown that the results obtained within the TDDFT approach may reproduce semi-quantively the 2D Fourier spectra, including the nonlinear effects, in the case of several phenomenological potentials.   

Long-range corrected time-dependent density functional theory with spin-orbit couplings

Relativistic time-dependent density functional theory (TDDFT) is a powerful tool to include both of relativistic and correlation effects with low computational cost. However, TDDFT with conventional exchange functionals have severe problems in e.g. the reproducibility of charge transfer (CT) and Rydberg excitation energies and oscillator strengths. These problems are due to the lack of long-range exchange interactions in conventional exchange functionals. We have proposed long-range corrected (LC) DFT and have overcome these problems. Especially, LC-TDDFT succeeds in describing CT excitations with remarkable accuracy. CT excitations often play a major role in spin-forbidden transitions, because the spin-orbit couplings are significant for excitations inducing the changes in electron distributions. In this study, LC-DFT has been applied to a spin-orbit TDDFT to describe spin-forbidden transitions appropriately by TDDFT. Our results have demonstrated that LC-DFT accurately reproduces the splitting of ionization energies of heavy atoms and spin-forbidden excitation energies for which electrons are moved to widely-distributed orbitals.   

Session Y25: Superconductivity: Mainly HTSC Theory

The underdoped cuprates as fractionalized Fermi liquids

We model the underdoped cuprates using fermions moving in a background with local antiferromagnetic order. The antiferromagnetic order fluctuates in orientation, but not in magnitude, so that there is no long-range antiferromagnetism, but a `topological' order survives. The normal state is described as a fractionalized Fermi liquid (FL*), with electron-like quasiparticles coupled to the fractionalized excitations of the fluctuating antiferromagnet. The FL* and its mother state, algebraic charge liquid, reveal interesting features in the underdoped cuprates such as shift of the Fermi pocket center from the magnetic Brillouin zone boundary . Also, with transition to superconductivity, the normal states can explain puzzling experiment data such as a nodal-anti-nodal `dichotomy' identifying characteristics of the two gaps. Implication of our model and extensions are discussed.   

Collective Modes in the Loop Ordered Phase of Cuprates

We show that the two branches of collective modes discovered recently in under-doped Cuprates with huge spectral weight are a necessary consequence of the loop-current state. Such a state has been shown in earlier experiments to be consistent with the symmetry of the order parameter competing with superconductivity in four families of Cuprates. We also predict a third branch of excitations and suggest techniques to discover it. Using parameters to fit the observed modes, we show that the direction of the effective moments in the ground state lies in a cone at an angle to the c-axis as observed in experiments.   

Unconventional superconductivity in honeycomb lattice

Motivated by results of DMRG and tensor network simulations on doped $t$-$J$ model on honeycomb lattice, we study superconductivity of singlet and triplet pairing in this model. We show that a coexistence of singlet and triplet pairing superconductivity is induced by antiferromagnetic order near half-filling. The superconducting state we obtain is a topological superconductor.   

Superconductivity as a Condensate of Collective Cooper Pairs

Along the last century, the fascinating phenomenon of superconductivity has recurrently been considered as a Bose-Einstein condensation (BEC) of Cooper pairs. However, creation and annihilation operators of the Cooper pairs do not satisfy the bosonic commutation relations [1] and then, the superconductivity theories based on the BEC have a weakness in their foundation. In this work, for the dilute limit we prove the bosonic nature of collective Cooper pairs (CCP), defined as linear combinations of Cooper pairs [2]. This bosonic nature is given rise from their diffuse character on the Cooper pairs, which allows the accumulation of many collective pairs at a single quantum state. Moreover, the superconducting ground state proposed by Bardeen, Cooper and Schrieffer (BCS) can be written in terms of these CCP, leading to a possible BEC theory of superconductivity. Finally, the energy spectra of CCP are calculated for a mixture of bosons and fermions, which permit to determine the condensation critical temperature as well as other thermodynamic properties of the CCP condensate. \\[4pt] [1] J. Bardeen, L.N. Cooper and J.R. Schrieffer, Phys. Rev. 108, 1175 (1957). \\[0pt] [2] C. Ramirez and C. Wang, Phys. Lett. A 373, 269 (2009).   

Self-consistent Eliashberg theory, $T_c$, and the gap function in electron-doped cuprates

We consider normal state properties, the pairing instability temperature, and the structure of the pairing gap in electron-doped cuprates. We assume that the pairing is mediated by collective spin excitations, with antiferromagnetism emerging with the appearance of hot spots. We use a low-energy spin-fermion model and Eliashberg theory up to two-loop order. We justify ignoring vertex corrections by extending the model to $N >>1$ fermionic flavors, with $1/N$ playing the role of a small Eliashberg parameter. We argue, however, that it is still necessary to solve coupled integral equations for the frequency dependent fermionic and bosonic self-energies, both in the normal and superconducting state. Using the solution of the coupled equations, we find an onset of $d-$wave pairing at $T_c \sim 30$ K. To obtain the momentum and frequency dependent $d$-wave superconducting gap, $\Delta ({\vec k}_F, \omega_n)$, we derive and solve the non-linear gap equation. We find that $\Delta ({\vec k}_F, \omega_n)$ is a non-monotonic function of momentum along the Fermi surface, with its node along the zone diagonal and its maximum some distance away from it. We obtain $2\Delta_{\mathrm{max}} (T\rightarrow0) /T_c \sim 4$. We argue that the value of $T_c$, the non-monotonicity of the gap, and $2\Delta_{\textrm{max}}/T_c$ ratio are all in good agreement with the experimental data on electron-doped cuprates.   

Spin-space entangled orbitals in a Hartree-Fock scheme predicting the AF and insulator properties of La2CuO4

Its is argued that spin-orbit entangled single particle states in a Hartree-Fock scheme can describe the insulator and antiferromagnetic nature of La2CuO4, as independent particle properties. Therefore, a currently considered as a Mott insulator material, is represented as a Slater one. This curious outcome is not strange if we consider that, strictly speaking, correlation quantities should be defined by the differences between the exact result and the ``best'' Hartree-Fock one. The discussion opens a road for understanding the connections between the successful phenomenological Mott picture and the First Principle (Slater) schemes of calculations. The results also furnish a simple framework for further studying the normal state properties of HTc superconductors. In particular, the microscopic structure of the antiferromagnetic order and the isolator size of the gap in La2CuO4 are both explained as coherent effects coming from the entangled ``spin-orbit'' structure of the single particle Hartree-Fock states. The possibility of the stability of the isolator gap when temperature rises up to the experimental Neel value is argued to be allowed by the same entanglement effect.   

Anomalous Isotope Effect in Low and High Tc Superconductors: the contribution of the electronic structure

Some of the low and high Tc superconductors exhibit an anomalous isotope effect, where the exponent ($\alpha )$ for the isotope effect is much smaller than $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. We present first-principles calculations of the electronic structures of the selected superconductors, including Zirconium (Zr) and YBa$_{2}$Cu$_{3}$O$_{7 }$(YBCO). The characteristically narrow electron bands around the Fermi levels (E$_{f})$ in these materials suggest that the rapid variations of the densities of states around E$_{f}$, within the range of phonon energy, can have a noticeable effect on the total coupling matrix elements. Such effect may explain the anomalous isotope effect on Tc in these superconductors. The work is funded in part by NSF and the Air Force Office of Scientific Research.   

Electronic Specific Heat and Dissipative Viscosity of Hole-Doped Cuprates

We investigate a d-density wave (DDW) mean field model Hamiltonian in the momentum space suitable for the hole-doped cuprates, such as YBCO, in the pseudo-gap phase to obtain the Fermi surface(FS)topologies, including the elastic scattering by disorder potential ($\vert $v$_{0}\vert )$. For the chemical potential $\mu =-$ 0.27 eV (at 10{\%} doping level), and $\vert $v$_{0}\vert \quad \ge \quad \vert $t$\vert $ (where $\vert $t$\vert $ = 0.25 eV is the first neighbor hopping), at zero/non-zero magnetic field (B) the FS on the first Brillouin zone is found to correspond to electron pockets around anti-nodal regions and barely visible patches around nodal regions. We next relate our findings regarding FS to the entropy per particle(S), the electronic specific heat C$_{el}$ and the dissipative viscosity ($\eta )$. The magneto-quantum oscillations in C$_{el}$ are shown to take place in the moderate disorder regime ($\vert $v$_{0}\vert \quad \sim $0.2 eV) only for B $\sim $ 40 T. For the density of viscosity $\eta $(\textbf{k}) on the first Brillouin zone, we find that whereas the negative contribution arises from the electron pockets in the anti-nodal region, the positive contributions are from the hole-pockets in the nodal region. The KSS bound ($\eta $/S $\ge $ h/4$\pi $k$_{B })$is easily satisfied for the moderately strong disorder potential. The viscosity is found to be proportional to the magnetic field up to B $\sim $ 50 T.   

The stripe-like collective excitation in cuprates: A variational Monte-Carlo study

In this study we report variational Monte-Carlo calculations of collective excitations for the extended $t-J$ model. We found a particular collective excitation involving modulation of charge, spin and pair field have a fairly small stiffness constant. These very easily excitable excitations are the same as the stripe-like states observed in our previous results for the $t- J$ model. This anomalous low rigidity from these low-lying collective excitations may provide a better understanding of the ubiquitous nature of the stripe states in cuprates.   

Pairing theory of striped superconductivity

Starting from a pairing Hamiltonian with an attractive interaction for electrons on nearest-neighbor sites on a square lattice we present a Hartree-Fock scheme which allows for spin and charge density order simultaneously with d-wave superconductivity. Specifically for filling 7/8 the stable groundstate solution is a striped superconductor with a stripe wavelength of eight lattice constants and $\pi$-shifted order parameters for d-wave pairing and antiferromagnetism. The superconducting state contains Cooper pairs with finite center of mass momenta ${\bf q}$ and $-{\bf q}$ corresponding to half the wavelength for the stripe pattern of the charge density. Despite the d-wave symmetry of the local pairing amplitude the striped superconductor is fully gapped. We characterize the striped superconducting state in real-space and in momentum space and discuss its possible relevance to La$_{1.875}$Ba$_{0.125}$CuO$_4$.   

Pair Density Wave correlations in the Kondo-Heisenberg Model

We show, using density matrix renormalization group calculations complemented by field theoretic arguments, that the spin gapped phase of the one dimensional Kondo-Heisenberg model exhibits quasi-long range superconducting correlations {\it only} at a non-zero momentum. The local correlations in this phase resemble those of the pair density wave state which was recently proposed to describe the phenomenology of the striped ordered high temperature superconductor {La$_{2-x}$Ba$_x$CuO$_4$}, in which the spin, charge, and superconducting orders are strongly intertwined.   

Uniform and pair-Density-Wave SC states in asymmetric ladders

We consider the problem of the superconducting state in ladder fermionic systems and focus on two possible types of condensates: a uniform (``d-wave'') state and a pair-density wave state (PDW). The uniform SC state is known to occur generically in symmetric hole-doped ladders. Recently it was shown that the PDW state occurs in the Kondo-Heisenberg chain on a broad range of parameters. The Kondo-Heisenberg chain is an extreme version of an asymmetric two-leg ladder. These facts suggest that there must be a quantum phase transition between these two states as a function of the relative doping of the two legs of a ladder. We investigate the nature of this quantum phase transition in the weak coupling limit, by taking advantage of bosonization methods available for 1D systems, and investigate the mechanism of this phase transition. We speculate on the relevance of these results to 2D systems. References: S. R. White, R. M. Noack and D. J. Scalapino: Phys. Rev. Lett 73 (1994) A.E. Sikkema, I. Affleck, and S.R. White, Phys. Rev. Lett. 79, 929 (1997) E. Berg, E. Fradkin, S. A. Kivelson, Phys. Rev. Lett. 105, 146403 (2010) O. Zachar and A. M. Tsvelik, Phys. Rev. B 64, 033103 (2001)   

Unconventional superconductivity nearby antiferromagnetism in quasi-1D conductors:the role of electron-phonon interaction

The stabilization of unconventional superconductivity (SCd) close to a spin-density-wave state (SDW) under pressure in organic conductors like the Bechgaard salts points out the primary importance of the repulsive Coulomb term in the origin of these phases. However, the electron-(acoustic) phonon interaction is known to be finite in practice, as borne out for example by diffuse X-ray scattering experiments. The question then arises about the role of this coupling, if any, in the mechanism of interaction between SDW and SCd orders in such materials. In this work, we address this issue using the renormalization group method. This is done in the framework of the quasi-1D electron gas model with repulsive direct Coulomb terms and weak retarded electron-phonon interaction, which are treated on equal footing. The impact of electron-phonon interaction on the SDW and SCd instability lines of the phase diagram and on the strength of spin correlations in the normal phase are analyzed at arbitrary phonon frequency, and discussed in connection with experiments in organic superconductors like the Bechgaard salts.   

Resonant alteration of supercurrent in guiding structures with complex de Gennes distance and its magnetic-field-induced restoration

Properties of the superconducting 2D disk and 3D wire are calculated within the framework of linearized Ginzburg-Landau theory with the complex de Gennes distance $\Lambda$ in the boundary condition. As a result, the self-adjointness of the Hamiltonian is lost, its eigenvalues $E$ become complex too and the discrete bound states of the disk turn into the quasibound states with their lifetime defined by the eigeneneries imaginary parts $E_i$. Accordingly, the longitudinal supercurrent undergoes alteration with its attenuation/amplification being $E_i$-dependent too. It is shown that $E_i$ as a function of the de Gennes imaginary part $\Lambda_i$ exhibits a pronounced sharp extremum with its magnitude being the largest for the zero real part $\Lambda_r$ of the de Gennes distance. Increasing magnitude of $\Lambda_r$ quenches the $E_i-\Lambda_i$ resonance and at large $\Lambda_r$ the eigenenergies $E$ approach the asymptotic real values independent of the de Gennes length imaginary component. The extremum is also wiped out by the applied longitudinal uniform magnetic field. The finite lifetime of the disk quasibound states stems from the $\Lambda_i$-induced currents flowing through the superconductor boundary. The effect can be observed in the superconductors by applying to them the external electric field.   

A real space study of the effect of disorder on superconductivity

Our method of studying the effect of disorder on superconductivity is based on the augmented space formalism that goes beyond mean-field approximations for configuration averaging and effectively deals with the influence of configuration fluctuations of the neighbourood of an atom. In the regime of validity of Anderson's theorem our results for $s$- and $d$-wave dirty superconductors has excellent agreement with existing results. The formalism is extended and tested for random negative {\bf U} Hubbard model. Having verified the reliability of our method we use it to study environment dependent, inhomogeneous randomness in disordered superconducting systems. Our model can be easily extended to study multi-band systems which takes us a step closer to studying real materials.   

Session Y26: Focus Session: Iron Based Superconductors -- Orbital Order

Effect of doping on the crystalline structure and superconductivity properties in Ca$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2 }$ single crystals

We have studied the crystalline structure and the superconducting properties of Ca$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2}$ single crystals for various levels of the chemical doping (x). We performed a comparative analysis of the angular dependent $H_{c2}(\Theta )$ (where $\Theta $ is the angle between the magnetic field and the c axis) for x$\sim $0.5 and x $\sim $ 0.75, corresponding to T$_{c} \quad \sim $ 19 K and 33 K, respectively. We found that in both cases $H_{c2}(\Theta )$ near T$_{c}$ exhibits a single band character with the same anisotropy $\gamma \quad \sim $ 1.8. In the crystal with T$_{c} \quad \sim $ 33 K we detected the presence of a narrow vortex-liquid phase, in agreement with the expectation from estimates based on the Lindemann criterion and the Ginzburg number. We also found large and anisotropic flux creep rates, with temperature dependences that indicate glassy relaxation. We analyzed those results in terms of single vortex and collective pinning regimes associated with random and correlated disorder.   

First-principles study of orbital-selective magnetism in FeAs-based superconductors

LaFeAsO$_{1-x}$F$_{x}$ and related compounds show unconventional superconductivity (SC) in the vicinity of the antiferromagnetism (AFM). These compounds are featured with multiple Fermi surfaces with strong orbital characters. We perform first-principles calculations of the electronic and magnetic properties in LnFeAsO (Ln=La, Ce, Pr, Nd, Sm, and Gd) as a function of Fe magnetic moment to study material-dependent interplay between orbitals and magnetic moments. With this approach, we show orbital-selective magnetic phases in small-Fe-moment regime: $d_{xy}$ magnetic phase, which is itinerantly driven by orbital selection of Fermi-surface nesting, and $d_{yz}$ magnetic phase, which is driven by local interactions. The Fe magnetic moments in the two phases show different coupling strengths to Fermi-surface electrons orbital-selectively, suggesting different roles in SC and in AFM, and making orbital characters of the Fe magnetic moment resolvable by measuring the electronic structures. This work was supported by the NRF of Korea (Grant No. 2009-0081204). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2008-S02-0004)   

Orbital characters of the iron-pnictides

The orbital degree of freedom plays an important role in the physics of iron-based high-temperature superconductors and their parent compounds. For example, possible orbital ordering has been associated with the spin density wave, and we recently found that the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries. We studied the orbital characters of the electronic structure in optimally electron-doped BaFe$_{1.85}$Co$_{0.15}$As$_{2}$ by exploiting the polarization-sensitivity of the orbitals in angle resolved photoemission spectroscopy. The orbital characters of the low energy electronic structure and Fermi surface in three dimensional momentum space are determined. Our results indicate that the previous orbital assignments of band structure calculations are just partially correct. Particularly, the contributions of the $d_{xy}$ and $d_{x2-y2}$ orbitals were not right. Our results lay the foundation for constructing realistic microscopic models of iron-based superconductors. Furthermore, we studied the transport properties and electronic structure of magnetically detwinned NaFeAs, and AEFe$_{2}$As$_{2}$. We identify the roles of various orbitals in the spin density wave formation.   

Dichotomy between Large Local and Small Ordered Magnetic Moments in Iron-Based Superconductors

We study a four-band model for iron-based superconductors within the local density approximation combined with dynamical mean-field theory (LDA+DMFT). This successfully reproduces the results of models which take As p degrees of freedom explicitly into account and has several physical advantages over the standard five d-band model. Our findings reveal that the new superconductors are more strongly correlated than their single-particle properties suggest. Two-particle correlation functions unveil the dichotomy between local and ordered magnetic moments in these systems, calling for further experiments to better resolve the short time scale spin dynamics.   

High-energy x-ray diffraction studies of \textit{AE}Fe$_{2}$As$_{2}$ compounds

The relationship between structure, magnetism and superconductivity has become a major theme in studies of the iron arsenide family of superconductors. We have used high-energy x-ray diffraction, together with two-dimensional area detectors, to image large regions of reciprocal space in order to gain further insight into the structural transitions in the \textit{AE}Fe$_{2}$As$_{2}$ (\textit{AE} = Ca, Sr, Ba) compounds. Here we present results of our study of the impact of annealing and temperature on the structure of these materials.   

High Resolution X-ray Scattering Studies of Structural Phase Transitions in BaFe$_{2-x}$Cr$_x$As$_2$

While the effects of electron-doping on the parent compounds of the 122 family of Fe-based superconductors have been extremely well-studied in recent years, far less is known about the influence of hole-doping in compounds such as BaFe$_{2-x}$Cr$_x$As$_2$. In contrast to the electron-doped 122 systems, the hole-doped compounds do not become superconducting. Furthermore, while the hole-doped compounds exhibit similar structural and magnetic phase transitions, they appear to be much less sensitive to dopant concentration. We have performed high resolution x-ray scattering and magnetic susceptibility measurements on single crystal samples of BaFe$_{2-x}$Cr$_x$As$_2$ for Cr concentrations ranging from 0 $\le$ x $\le$ 0.67. These measurements allow us to determine the magnetic and structural phase transitions for this series and map out the low temperature phase diagram as a function of doping. In particular, we have carried out detailed measurements of the tetragonal (I4/mmm) to orthorhombic (Fmmm) structural phase transition which reveal how the orthorhombicity of the system evolves with increasing Cr concentration and how this correlates with the values of T$_s$ and T$_m$.   

Orbital Order and Spontaneous Orthorombicity in Iron Pnictides

Phase diagram of the Iron Pnictide families of superconductors show a tetragonal to orthorhombic transition, sometimes coincident with the antiferromagnetic phase transition and sometimes at temperatures clearly above the antiferromagnetic phase transition. Inelastic neutron scattering spectra show exchange constants with strong spatial anisotropy. Recent photoemission measurements in mechanically detwinned samples show clear evidence of unequal orbital occupation and strikingly different spectra for dxz and dyz iron orbitals. Scanning Tunneling Microscopy and transport measurements have also shown substantial orthorhombicity in these materials. We discuss a simple microscopic picture for coupled spin and orbital degrees of freedom as the root cause for such an anisotropy and discuss the extent to which this picture can be distinguished from spin-frustration induced Ising nematic fluctuations as being the dominant driver for this phenomena.   

Commensurate antiferromagnetic ordering in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ determined by x-ray resonant magnetic scattering at the Fe $K$ edge

We describe x-ray resonant magnetic diffraction measurements at the Fe $K$ edge of both the parent BaFe$_{2}$As$_{2}$ and superconducting Ba(Fe$_{0.953}$Co$_{0.047})_{2}$As$_{2}$ compounds. From these high-resolution measurements we conclude that the magnetic structure is commensurate for both compositions. The energy spectrum of the resonant scattering is in reasonable agreement with theoretical calculation using the full-potential linear augmented plane-wave method with a local density functional. The calculation suggests that the resonant scattering at the Fe $K$ edge in the $\sigma $-to-$\pi $ scattering channel arises from dipole allowed transitions from the core 1$s$ states to the unoccupied 4$p$ states that are spin polarized due to hybridization with the 3$d$ states close to the Fermi energy.   

Antiferromagnetic ordering in the absence of a structural distortion in Ba(Fe$_{1-x}$Mn$_{x})_{2}$As$_{2}$

Neutron and x-ray diffraction studies of Ba(Fe$_{1-x}$Mn$_{x})_{2}$As$_{2}$ for low doping concentrations ($x \le $ 0.176) reveal that at a critical concentration, 0.102$ [Preview Abstract]

An empirical method to account for spin-fluctuation suppression of magnetism in Fe pnictides

Parent materials of Fe-based superconductors, such as BaFe$_2$As$_2$, are itinerant antiferromagnets, and as such should be better described by LDA calculations than are strongly-correlated cuprates. To an extent, this is true, but LDA, being a mean-field approximation, underestimates the suppression of the long-range magnetism due to spin fluctuations. These can be accounted for within Moria's self-consistent renormalization theory, which, however, includes unknown parameters such as the mean amplitude of the spin-fluctuations. We propose to include Moria's renormalization empirically, through a scaling of the LDA exchange-correlation magnetic field by a uniform constant factor, tuned so as to reproduce the observed phase diagram. This is a much more physical method to produce electronic bands with a proper exchange splitting, than adding an artificial ``negative-U'' term within an LDA+U formalism, a technique used now. We will report the results of such renormalized calculations for BaFe$_2$As$_2$ and, for comparison, for a prototypical itinerant magnet, ZrZn$_2$.   

In-plane resistivity anisotropy in underdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$

Underdoped Fe arsenide superconductors suffer a structural transition that is either coincident with, or precedes the onset of long range antiferromagnetic order. Crystals tend to form a dense array of twins upon cooling through the structural transition, but uniaxial pressure can be used to almost completely detwin samples, enabling measurement of the associated in-plane electronic anisotropy. Initial experiments on detwinned samples of Ba(Fe$_{1- x}$Co$_x$)$_2$As$_2$ revealed a large in-plane resistivity anisotropy which varied non-monotonically with cobalt concentration. Here we present data extending the initial study to include detwinned samples of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$. The composition-dependence of the resistivity anisotropy $\rho_b/\rho_a$ reveals a striking correlation with that of the Hall coefficient for all three substitution series.   

Quasiparticle interference of C$_{2}$-symmetric surface states in LaOFeAs parent compound

We present scanning tunneling microscopy studies on the LaOFeAs parent compound of iron pnictide superconductors [1]. High resolution spectroscopic imaging reveals strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. Fourier analysis shows that the distribution of scattering wavevectors exhibits pronounced two-fold (C$_{2})$ symmetry, strongly reminiscent of the nematic electronic state found in CaFe$_{1.94}$Co$_{0.06}$As$_{2}$ [2]. The implications of these results to the electronic structure of the pnictide parent states will be discussed. \\[4pt] [1] X.Zhou \textit{et al.} arXiv:1008.2642 \\[0pt] [2] T.M.Chuang \textit{et al.} Science \textbf{327}, 181 (2010)   

Session Y27: Focus Session: Semiconductor Qubits- In Search of Majorana

Non-Abelian statistics and topological quantum information processing in 1D wire networks

Topological quantum computation provides an elegant way around decoherence, as one encodes quantum information in a non-local fashion that the environment finds difficult to corrupt. Here we establish that one of the key operations---braiding of non-Abelian anyons---can be implemented in one-dimensional semiconductor wire networks. Previous work [Lutchyn et al., arXiv:1002.4033 and Oreg et al., arXiv:1003.1145] provided a recipe for driving semiconducting wires into a topological phase supporting long-sought particles known as Majorana fermions that can store topologically protected quantum information. Majorana fermions in this setting can be transported, created, and fused by applying locally tunable gates to the wire. More importantly, we show that networks of such wires allow braiding of Majorana fermions and that they exhibit non-Abelian statistics like vortices in a p+ip superconductor. We propose experimental setups that enable the Majorana fusion rules to be probed, along with networks that allow for efficient exchange of arbitrary numbers of Majorana fermions. This work paves a new path forward in topological quantum computation that benefits from physical transparency and experimental realism.   

Majorana fermions in nanowires without gating superconductors

Majorana fermions have been proposed to be realizable at the end of the semiconductor nanowire on top of an s-wave superconductor [1,2]. These proposals require gating the nanowire directly in contact with a superconductor which may be difficult in experiments. We analyze [1,2] in configurations where the wire is only gated away from the superconductor. We show that some signatures of the Majorana mode remain but the Majorana mode is not localized and hence not suitable for quantum computation. Therefore we propose an 1D periodic heterostructure which can support localized Majorana modes at the end of the wire without gating on the superconductor. \\[4pt] [1] Jay D. Sau et al., arXiv:1006.2829, Phys Rev B (in press)\\[0pt] [2] Roman M. Lutchyn et al., Phys. Rev. Lett. 105, 077001 (2010)   

Effects of Interactions on a Topological Phase Exhibiting Majorana Fermions in Quantum Wires

The ability to create and manipulate Majorana fermions in condensed matter systems is not only of fundamental interest for understanding topological phases but also provides a realistic route toward quantum computation. Recently, a series of devices have been proposed that could realize exotic Majorana physics in relatively conventional settings; among the most promising is a superconducting wire system with strong spin-orbit coupling. Because superconducitivity is induced in this system by proximity effect, the system remains superconducting even with net repulsive interactions. The effects of such interactions on this system have until now remained unexplored. Using the Density Matrix Renormalization Group method, we explore the fate of the topological phase in the presence of interactions. Obtaining a matrix product state representation of the degenerate ground states is especially helpful as it allows us to determine detailed properties of the Majorana edge states. Furthermore, we find that interactions significantly expand the topological region of the phase diagram, a result which strengthens proposals to realize Majorana fermions in such wire systems experimentally.   

The exchange statistics of Majorana fermions in quasi-one-dimensional networks

Under appropriate external conditions a semiconductor with strong spin-orbit coupling in proximity to an $s$-wave superconductor can be in a topological superconducting (TS) phase. In the topological phase, various defects of the order parameter trap zero energy excitations called Majorana bound states. In a wire geometry the relevant defects are the two ends of the topological region, and each traps a localized zero energy excitation. A network of such wires allows the pairwise exchange of the Majorana bound states. Alicea et al. have shown that these bound states obey non- Abelian exchange statistics, and have proposed [1] such a system as a platform for topological quantum computation (TQC). Here we show that the particular realization of non- Abelian statistics produced in a Majorana wire network is highly dependent on the local properties of individual wire junctions. For a simply connected network, the possible realizations can be characterized by the chirality of individual junctions. We demonstrate how this chirality may be calculated for a particular junction. There is in general no requirement for junction chiralities to remain consistent across a wire network. Careful control of the junction chirality is required for TQC applications of Majorana wire networks. [1] J. Alicea et al., arXiv:1006.4395.   

Interferometry and topological quantum computation using Majorana Fermions at semiconductor/superconductor interfaces

Majorana Fermions are hitherto unobserved exotic Fermionic excitations, which are their own anti-particles. Recently, a lot of excitement has been generated by proposals to realize Majorana fermions in topological superconductors in a rather general class of topological superconductors, some of which may be as simple as the interface 1D or 2D InAs and Al in the appropriate parameter regime might have exotic topological properties and Majorana Fermions [1]. In my talk, I will discuss recent proposals for performing interferometry in 2D and 1D versions of such systems [2] together with ideas for performing Quantum Computation [3] using such robust Majorana fermion based qubits. \\[4pt] [1] J. Sau, S. Tewari, R. Lutchyn, T. Stanescu, S. Das Sarma, arxiv:1006.2829 PRB (in press). [2] J. Sau, S. Tewari, S. Das Sarma, arxiv:arXiv:1004.4702. [3] J. Sau, S. Tewari, S. Das Sarma, arxiv:arXiv:1007.4204 PRA(in press)   

Topological Phases in Dissipative Quantum Transport

Recently, a new type of topological quantization was discovered in dissipative quantum transport on a one dimensional bipartite lattice with decay [1]. The transition between distinct topological phases is accompanied by a discontinuous change in the expected displacement covered by a particle before it decays. Here we show that this behavior extends to a much wider family of models, and provide a prescription for computing the topological invariant which distinguishes all of the phases which arise in the general case. When the underlying hopping problem without decay possesses time reversal symmetry, we show that the expected displacement, averaged with respect to all initial states, is quantized. The topological nature of this phenomenon, which is unique to systems with decay, places it on a similar footing as other robust topological phenomena such as the quantization of the Hall conductance [2], or of the adiabatically-pumped charge in periodically-driven 1D systems [3]. Correspondingly, here we find that quantization is robust against a range of perturbations and certain types of decoherence. Similarities and differences with the phases of one-dimensional topological insulators will be discussed. [1] M. S. Rudner and L. S. Levitov, Phys. Rev. Lett. 102, 065703 (2009). [2] D. J. Thouless, M. Kohmoto, M. P. Nightingale, and M. den Nijs, Phys. Rev. Lett. 49, 405 (1982). [3] D. J. Thouless, Phys. Rev. B 27, 6083 (1983).   

Counting Majorana zero modes in superconductors

We present a counting formula for computing the number of (Majorana) zero modes bound to topological point defects. The counting formula is evaluated in a gradient expansion for systems with charge-conjugation symmetry. We will consider examples that include Dirac fermions and the chiral p-wave superconductor in two-dimensional space. In all cases, we explicitly relate the counting of zero modes to Chern numbers.   

Non-Abelian order in s-wave superconductors: Phases and quantum transitions

Non-Abelian topological superconductivity has been predicted to occur in s-wave superconductors with a sizable spin-orbit (SO) coupling. As is now well known, such a system can be used for topological quantum computation. When an external Zeeman splitting crosses a critical value, the system passes from a regular, non-topological, superconducting phase to a topological one. On the other hand, in the absence of SO coupling this critical value corresponds to the Zeeman splitting above which the system loses its s-wave superconductivity. We are thus led to the paradoxical conclusion that the topological superconducting phase appears in a parameter regime at which the system actually is non- superconducting in the absence of SO coupling. In this work we resolve this paradox.   

Induced Chiral f-wave Superconducting Pairing and Majorana Fermions in a Hole-doped Semiconductor

We show that a chiral f + if-wave superconducting pairing may be induced in the lowest heavy hole band of a hole-doped semiconductor thin film through proximity contact with an s-wave superconductor. The chirality of the pairing originates from the 3$\pi$ Berry phase accumulated for a heavy hole moving along a close path on the Fermi surface. There exist three chiral gapless Majorana edge states, in consistence with the chiral f + if-wave pairing. We show the existence of zero energy Majorana fermions in vortices in the semiconductor-superconductor heterostructure by solving the Bogoliubov-de-Gennes equations numerically as well as analytically in the strong confinement limit. The proposed semiconductor/superconductor heterostructure can be used as a platform for observing non-Abelian statistics and performing TQC.   

Anyonic entanglement renormalization

We introduce a family of variational ansatz states for chains of anyons which optimally exploits the structure of the anyonic Hilbert space. This ansatz is the natural analog of the multi-scale entanglement renormalization ansatz for spin chains. In particular, it has the same interpretation as a coarse-graining procedure and is expected to accurately describe critical systems with algebraically decaying correlations. We numerically investigate the validity of this ansatz using the anyonic golden chain and its relatives as a testbed. This demonstrates the power of entanglement renormalization in a setting with non-abelian exchange statistics, extending previous work on qudits, bosons and fermions in two dimensions.   

Protected phase gates for superconducting qubits

Quantum systems with inherent error-correcting properties offer a powerful tool for building quantum computers to be insensitive to the effects of errors. Kitaev [arXiv:cond-mat/0609441] has proposed an intrinsically fault-tolerant qubit design based on superconducting systems. The phase gate $\Lambda(i)$ in this system is performed by coupling the qubit to a quantum $LC$ oscillator for a period of time. The evolution of the oscillator can be understood as being protected by a family of continuous variable quantum codes at every point in its evolution, providing natural robustness against random variations in the duration and strength of the coupling. We present the results of numerical simulations of this system which investigate the fidelity of the phase gate operation as a function of the duration mistiming. We discuss the robustness of the gate under the effect of anharmonic perturbations to the oscillator and oscillator coupling, and adiabaticity requirements for this scheme to properly function.   

Resilience of Topological Codes to Depolarization

Standard error correction is based on redundant storage of quantum information. However, in topological quantum error correction decoherence effects are prevented by encoding logical qubits in nonlocal degrees of freedom, while actively correcting for errors that occur locally in the system. Previous studies have shown that the two hallmark topological codes---the toric code and color codes---are stable against bit-flip/phase-flip and measurement errors. In this work we study the effects of the depolarizing channel to both the toric code and topological color codes. By mapping the quantum problem onto a disordered statistical-mechanical 8-vertex model we compute the error tolerance of these systems using large-scale Monte Carlo simulations. Our results show that the error threshold increases significantly for both the toric code and color codes.   

Local equivalence of topological order: Kitaev's code and color codes

We demonstrate that distinct topological codes can be mapped onto each other by local transformations. The existence of such a local mapping can be interpreted as saying that these codes belong to the same topological phase. When used as quantum error correcting codes, the local mapping also enables us to use any decoding algorithm suitable for one of these codes to decode other codes in the same topological phase. We illustrate this idea with the topological color code and the topological subsystem color code that are found to be locally equivalent to two copies of Kitaev's toric code. We are therefore able to decode these two codes that had no previously known efficient decoding algorithm, and find error thresholds comparable to previously estimated optimal values. These local mappings could have additional use for fault-tolerant quantum computation. In particular, one could in principle take advantage of the features (transversal gates, topological gates, etc.) of all the codes that are locally equivalent by switching between them during the computation in a fault-tolerant fashion.   

Exactly solvable 3D quantum model with finite temperature topological order

We present a family of exactly solvable spin-$\frac{1}{2}$ quantum hamiltonians on a 3D lattice. The degenerate ground state of the system is characterized by a quantum error correcting code whose number of encoded qubits are equal to the second Betti number of the manifold. These models 1) have solely local interactions 2) admit a strong-weak duality relation with an Ising model on a dual lattice 3) have topological order in the ground state, some of which survive at finite temperature. The associated quantum error correcting codes are all non-CSS stabilizer codes.   

Universal Behavior of Entanglement in 2D Quantum Critical Dimer Models

We examine the scaling behavior of the entanglement entropy for the 2D quantum dimer model (QDM) at criticality and derive the universal finite sub-leading correction $\gamma_{QCP}$. We compute the value of $\gamma_{QCP}$ without approximation working directly with the wave function of a generalized 2D QDM at the Rokhsar-Kivelson QCP in the continuum limit. Using the replica approach, we construct the conformal boundary state corresponding to the cyclic identification of $n$-copies along the boundary of the observed region. We find that the universal finite term is $\gamma_{QCP}=\ln R-1/2$ where $R$ is the compactification radius of the bose field theory quantum Lifshitz model, the effective field theory of the 2D QDM at quantum criticality. We also demonstrated that the entanglement spectrum of the critical wave function on a large but finite region is described by the characters of the underlying conformal field theory. It is shown that this is formally related to the problems of quantum Brownian motion on $n$-dimensional lattices or equivalently a system of strings interacting with a brane containing a background electromagnetic field and can be written as an expectation value of a vertex operator.   

Session Y29: Focus Session: Superconducting Qubits - Coherence and Materials II

Investigating decoherence in the transmon qubit using a 3D resonator

We studied the coherence times of transmon qubits using three-dimensional resonators. The three-dimensional (3D) superconducting resonant cavity is machined with aluminum alloy, whose quality factor is higher than 5 million at 10 mK inside a magnetic shield. The transmons are fabricated on sapphire substrates whose internal Q was not lower than 2 million when evaluated in the 3D resonator. We measured the relaxation and dephasing times of the qubits and were able to draw a lower bound on these numbers.   

Decoherence in Improved Transmon Qubits

The transmon is a simple superconducting qubit which has less dependence on the usual sources of 1/f noise, and has coherence which is mostly limited by a source of anomalous dissipation. The quality factors of transmon qubits on sapphire are observed to be $\sim$ 50,000, similar to that of transmission line resonators made with the same geometry. It is likely that both these devices may be limited by surface dielectric losses. We will report on the design and characterization of transmon qubits which are fabricated with reduced dielectric losses to possibly increase coherence times.   

Measurements of quasiparticle tunneling rate in a superconducting transmon qubit

A practical quantum computer requires qubits with long coherence times in order to perform many quantum gates. For a superconducting qubit, non-equilibrium quasiparticle tunneling is one possible source of decoherence. Spectroscopy measurements of a superconducting transmon qubit can be used to set a bound on the quasiparticle tunneling rate. When operated in the low $E_J$/$E_C$ regime, the transmon qubit transition frequency switches between two well-resolved branches due to quasiparticle tunneling. A selective $\pi$ pulse applied to one of these two branches can excite the qubit only if the qubit is at that frequency. Thus by repeatedly applying $\pi$ pulses to interrogate the qubit state, the quasiparticle dynamics can be studied. We will present our results on the quasiparticle tunneling rate in a transmon qubit.   

Dynamical decoupling and noise spectroscopy with a superconducting flux qubit

We demonstrate dynamical decoupling in a superconducting flux qubit with a long energy-relaxation time, $T_1 = 12\,\mu$s. Low-frequency noise acts to dephase the qubit, reducing its transverse coherence time $T_2$. At the noise-optimal bias point we observe a free-induction decay time $T_2^* = 2.5\,\mu$s and $T_1$-limited spin-echo decay, $T_{2E} = 2\,T_1$. Biased away from this point, the increased sensitivity to flux noise leads to increased echo and free-induction decay rates. We moderate the dephasing effects of this noise by applying dynamical-decoupling sequences with up to 200 $\pi$-pulses. Using the CPMG sequence, we achieve a more than 50-fold enhanced decay time over $T_2^*$, and Gaussian pure-dephasing times $T_\varphi > 100\,\mu$s. We use the filtering property of this pulse sequence to facilitate spectroscopy of the environmental noise and reconstruct its $1/f$ power spectral density, which we independently confirm by a Rabi-spectroscopy approach. We characterize the noise sources coupling to the energy-bias and tunnel-coupling terms of the Hamiltonian.   

Multi-mode circuit quantum electrodynamics

In circuit QED experiments with low anharmonicity superconducting qubits, like the transmon, it has been shown how the many-level structure of the qubits can give rise to non-trivial effects. Examples are the straddling regime [1] and high-power qubit readout induced by qubit nonlinearities [2]. In the same spirit, there are also clear experimental evidences to the effect that higher resonator modes play an important role in setting the size of the qubit-qubit flip-flop interaction mediated by virtual resonator photons [3] and the qubit decay rate due to the Purcell effect [4]. In this talk we explore how these higher modes can be taken into account in a theoretical description of the system, and how they affect the flip-flop and Purcell decay rates. \\[4pt] [1] Houck et al., Phys Rev. A 76, 042319 (2007); Srinivasan et al, V26.00006, 2010 March Meeting. \\[0pt] [2] Reed et al, Phys. Rev. Lett. 105, 173601 (2010), Bishop et al, Phys. Rev. Lett. 105, 100505 (2010), Boissonneault et al, Phys. Rev. Lett. 105, 100504 (2010). \\[0pt] [3] Filipp et al, arXiv:1011.3732v1 \\[0pt] [4] Houck et al, PRL 101, 080502 (2008)   

Dissipation in the ultra-strong coupling regime

It has recently been shown that the ultra-strong coupling regime, in which the rotating-wave approximation breaks down, can be obtained using a flux qubit coupled to a transmission line [1]. This regime has been observed experimentally in [2, 3]. We will show the usual quantum optics master equation fails in this context and give a more accurate one. We will also explain how non-trivial properties of the ground state could be experimentally studied.\\[3pt] [1] J. Bourassa et al, Phys. Rev. A 80, 32109 (2009)\\[0pt] [2] T. Niemczyk et al, Nature Physics 6, 772-776 (2010)\\[0pt] [3] P. Forn-D\'iaz et al., arXiv:1005.1559v1 (2010)\\[0pt]   

Strong frequency dependence of coupling of a Cooper- pair box qubit to Quantum Noise

Our system consists of an Al/AlO$_{\mbox{x}}$/Al Cooper-pair box (CPB) charge qubit coupled to a lumped element resonator, which in turn is coupled to a transmission line. From the measured Rabi frequency, for a given microwave frequency $f$ and amplitude in the transmission line, we can extract the coupling of qubit to the transmission line. We observe an order of magnitude variation in this coupling over the range of $f$ = 4 to 8GHz which is in agreement with the variation of our measured lifetimes. Assuming that our qubit is coupled directly to a $50 \Omega $ impedance with the measured coupling, we find that for $f = $ 6 to 7 GHz the lifetime of $30 \mu$s measured at the charge sweet spot can be well explained by quantum noise. At $f = 4$GHz, we observe an order of magnitude weaker coupling and a $T_1$ of $ 200\mu$s .   

Dephasing Measurements of a Cooper-pair box

We present data on the dephasing properties of our Al/AlO$_{\mbox{x}}$/Al Cooper-pair box (CPB) qubit. The CPB had a charging energy $E_{C}/h$ = 6.25 GHz and a maximum $E_{J}/h$ = 19 GHz which was decreased by an external magnetic field to an effective $E_{J}/h$ of 6.1 GHz. The qubit was capacitively coupled to a lumped element microwave resonator ($f_{0} = 5.446$ GHz, $Q_{L} = 1.8\times10^{4}$) which was in turn coupled to a transmission line. To manipulate the qubit, a microwave pulse at 6.1 GHz was sent to the transmission line. The state of the qubit was then measured by sending a second microwave pulse at $f_{0}$ and measuring the amplitude and phase of the transmitted power. We observed Rabi oscillations with Rabi frequencies from 1.94 to 5.32 MHz decay with time constants in the range T' = 0.6 to 1.6 $\mu$s. We measured an inhomogeneous dephasing time ($T_{2}${*}) of 322 ns by performing a Ramsey fringe experiment. Assuming $1/f$ charge noise is the dominant dephasing mechanism we extracted a $1/f$ charge noise amplitude of 1.6$\times$10$^{-3} e/\sqrt{Hz}$ at 1 Hz.   

Improved T2 in Josephson Phase Qubits

Phase qubit gate fidelities are limited by individual device dephasing times (T2). Reduction of dephasing is therefore an important immediate goal for phase qubit experiments. A simple way to reduce dephasing is to increase the device loop inductance in order to lower the noise currents driven by magnetic flux noise; T2 should scale linearly with loop inductance. Surface spin models for flux noise also predict that wider loop traces should reduce the noise. We present data on T2 for phase qubits with varied loop inductance and trace width. We present data from experiments in which we find that doubling the loop inductance increases T2 by 25\%.   

Evidence for coherent quantum phase slips from dephasing of fluxonium qubit

Phase slips are events in which the phase across a superconducting wire changes by 2$\pi$. The thermally activated phase slips at high temperatures are well understood but the coherent phase slips caused by quantum fluctuations well below the critical temperature have, so far, eluded observation. We report new decoherence data for the fluxonium qubit [1] that provide evidence for coherent quantum phase slips across the qubit inductance, implemented with a long array of Josephson tunnel junctions. Coherent quantum phase slips result in broadening of the qubit transition frequency due to Aharonov-Casher interference of multiple phase slip paths (or flux tunneling through different junctions) encircling random offset charges on array islands [2]. \\[4pt] [1] V.E. Manucharyan et al., Science 326, 113 (2009).\\[0pt] [2] D. Ivanov et al., Phys. Rev. B 65, 024509 (2002).   

Relaxation mechanisms of the fluxonium qubit

Fluxonium is a highly anharmonic artificial atom, which utilizes an inductance formed by an array of large Josephson junctions to shunt the junction of a Cooper-pair box. The first excited state transition frequency is widely tunable with flux, yet can be read out over the entire five octave range due to interactions of the 2nd excited state with the readout cavity, enabling a dispersive readout. We present T1 times of several fluxonium samples over the full range of flux dependent transition energies. By mapping out the qubit lifetimes we are able to distinguish between the contributions due to the Purcell effect and quantify dissipation internal to the qubit. With this understanding, we can design a qubit with minimized contribution from internal losses, which should push lifetimes further into the tens of microseconds. [1] V. E. Manucharyan et al., Science 326, 113 (2009).   

1/f noise and susceptibility-magnetization correlation in disordered ferromagnets

We consider a strongly disordered ferromagnet modeled by Ising spins placed at random in 2D with ferromagnetic interactions decaying exponentially with inter-site distance. Ferromagnetic phase in this model arises due to formation of infinite percolation cluster of strongly interacting spins. Fractal nature of the percolation cluster manifests itself in the dynamics of the system in the vicinity of the percolation transition. Simulating the dynamics with single spin flip Monte Carlo algorithm we observe 1/f power spectra of magnetization noise in a wide temperature range near the transition. Subjected to external AC magnetic field the system shows significant cross-correlation between susceptibility and magnetization in the ferromagnetic phase. This results suggest a possible explanation of the inductance-flux cross-correlation recently observed in SQUIDs [1]. \\[4pt] [1] S. Sendelbach, D. Hover, M. Muck, and R. McDermott, Phys. Rev. Lett. 103, 117001 (2009)   

Are ``pinholes'' the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconductor---insulator---superconductor (SIS) and superconductor---insulator---normal-metal (SIN) junctions, ``excess'' subgap current is usually observed. We have studied subgap conductance in Al/AlO$_x$/Al and Al/AlO$_x$/Cu tunnel junctions. In the former, we observed a huge (two orders of magnitude) decrease in subgap conductance upon the transition from the SIS to the SIN regime. In the latter, we observed several signatures of coherent diffusive two-particle transport. We use the quasiclassical Keldysh-Green function theory to quantify the contributions of the single- and two-particle processes on subgap conductance. Our observations indicate insignificance of highly transparent microscopic defects (``pinholes'') in the tunneling barrier, and we therefore argue that the common ``pinhole'' scenario is not the explanation for the observed excess subgap current in SIS tunnel junctions.   

Critical current noise and junction resonators in Josephson junction from interacting trap states

We analyze the impact of trap states in the oxide layer of superconducting tunnel junctions on the fluctuation of the Josephson current. These are known to inhibit the coherent operation of superconducting qubits. These have a twofold effect: Occupying trap states blocks out parts of the critical current of the Josephson junction. Electrons can also cross the junction via hopping across a trap. We are extending previous studies of noninteracting traps to the case where the traps have on-site electron repulsion. We use second order perturbation theory which allows to obtain analytical results but limited to small and intermediate repulsion. Remarkably, it still reproduces the main features of the model as identified from the Numerical Renormalization group. We present analytical formulations for the subgap bound state energies, the singlet-doublet phase boundary, and the spectral weights, which are in agreement with recent Numerical Renormalization Group analysis. We show that interactions can reverse the supercurrent across the trap. We finally work out the resonance noise spectrum in the presence of on-site repulsive electrons and suggest a criteria for the fabrication of parameters that may help to suppress low frequency noise from superconducting quantum computation devices.   

Energy relaxation mechanisms in capacitively shunted flux qubits

Energy losses in superconducting qubits remain a major object of study in the road towards scalable, highly coherent qubit devices. The current understanding of the loss mechanisms in these devices is far from being complete and it is sometimes difficult to experimentally separate the different contributions to decoherence. Here we compare a traditional three Josephson-junction flux qubit to the recently implemented capacitively shunted flux qubit [1], whose energy decay is thought to be limited by dielectric losses arising from native oxides in the shunting capacitor. Keeping all parameters identical except for the shunting capacitance, we obtain energy relaxation times that are comparable for both types of qubit. This suggests that the energy relaxation time is not limited by junction losses in capacitively shunted flux qubits. We discuss some other possible loss mechanisms present in these devices. \\[4pt] [1] M. Steffen \textit{et al}. Phys. Rev. Lett. \textbf{105}, 100502 (2010)   

Session Y30: Nanowires and Nanotubes: Fundamentals and Applications

Apparent Power-Law Behavior of Conductance in Disordered Quasi-One-Dimensional Systems

Observation of power-law dependence of conductance on temperature and voltage has been reported for a wide variety of low-dimensional systems(nano-wires, nano-tubes, and conducting polymers). This behavior has been attributed to the Luttinger liquid effects expected in a pure one-dimensional wire. However, the systems studied were neither one-dimensional nor defect-free. Using numerical simulations we show that the power-law behavior can arise from variable-range hopping in an ensemble of non-interacting disordered wires connected in parallel. This power-law behavior holds in restricted ranges of voltage and temperature, typical of experimental situations. Physically, it comes from rare, but highly conducting hopping paths that appear by chance in some members of the ensemble. The power-law exponents and their dependence on system parameters are consistent with the great majority of available empirical data.   

Equilibration of a One-Dimensional Wigner Crystal

Equilibration of a one-dimensional system of interacting electrons requires processes that change the numbers of left- and right-moving particles. At low temperatures such processes are strongly suppressed, resulting in slow relaxation towards equilibrium. We study this phenomenon in the case of spinless electrons with strong long-range repulsion, when the electrons form a one-dimensional Wigner crystal. We find the relaxation rate by accounting for the umklapp scattering of phonons in the crystal. For the integrable model of particles with inverse-square repulsion, the relaxation rate vanishes. We apply our results to calculation of the correction to the conductance of a long quantum wire due to the equilibration processes.   

STM images of carbon-nanotube quantum dots: Seeing a Wigner molecule of correlated electrons

The paradigm of few-electron complexes in quantum dots (QDs) relies on the idea that the lowest quantized levels are filled according to Pauli's exclusion principle. If Coulomb repulsion is sufficiently strong to overcome the kinetic energy cost of localization, a different scenario is predicted: a ``Wigner'' molecule (WM) forms, made of electrons frozen in space according to a geometrical pattern. Despite considerable experimental effort, evidence of the WM in semiconductor QDs has been elusive so far. Here we demonstrate theoretically that WMs occur in gate-defined QDs embedded in typical semiconducting carbon nanotubes (CNTs). The unambiguous signatures of the WM state must be searched in the scanning tunneling microscopy (STM) images of the electrons. Through exact diagonalisation (ED) calculations, we unveil the inherent features of the electron molecular states. We show that, like nuclei in a usual molecule, electrons have localized wave functions and hence negligible exchange interactions. ED results for single and double QDs provide a simple interpretation for transport experiments in ultraclean CNTs.   

All-Semiconducting nanotube networks Thin Film Transistors: An insight towards High Performance Printed Nanoelectronics

In this work, we present our progress towards devices fabrication using all semiconducting nanotubes as the starting material. A critical issue is the ink formulation and dependence of electronic properties on the nanotube density after deposition. These are some of the first spin-on, all semiconducting nanotube devices ever made and initial results are quite promising for printed RF electronics. Semiconducting single-walled nanotube (99{\%}) ink was used to deposit nanotube network on APTES modified Si/SiO$_{2}$ substrate. Following the nanotube deposition, source and drain electrodes (Pd/Au) were deposited using standard photolithography and E-beam evaporation. The Si wafer was used for back gating and SiO$_{2}$ as the gate dielectric. The impact of density of nanotube was studied for 3 random densities. We also studied the effect of gate length on mobility, and on/off ratio, for devices with different gate lengths (10$\sim $100 $\mu $m). DC characterization of devices shows a high mobility, up to 40 cm$^{2}$/V-s, and good on/off ratio up to the order of 10$^{4}$ in some cases. Since we are using 99{\%} semiconducting ink, a high on/off ratio is expected, which is true in our devices. The on/off ratio of more than 1000 and mobilities up to $\sim $40 cm$^{2}$/V-s were observed in almost all devices.   

Spin-valley blockade and electron valley resonance in carbon-based quantum dots

The spin blockade effect in the electric conduction through two semiconductor quantum dots connected in series has allowed the monitoring of spin-breaking effects, notably single-spin rotations induced via external fields in electron spin resonance (ESR) and spin decoherence due to the hyperfine coupling to the nuclear spin environment. Electrons in double quantum dots in carbon nanotubes and graphene comprise a valley isospin in addition to their spin. We show that this can lead to a spin-valley blockade which is sensitive to both spin and valley breaking effects. The hyperfine interaction due to residual $^{13}C$ nuclear spins turns out to be both spin- and valley-breaking, while non-magnetic atomic impurities can lead to pure valley-breaking. We study the magnetic-field dependent leakage current in the spin-valley blockade, also taking into account the spin-orbit interaction in carbon nanotubes. In analogy to ESR, we propose a resonance effect for the valley isospin (electron valley resonance) driven by an oscillatory electric field in a graphene or carbon nanotube quantum dot. References: A. Palyi and G. Burkard, Phys. Rev. B 80, 201404 (2009); Phys. Rev. B 82, 155424 (2010); arXiv: 1010.4338 (2010).   

Single tube electric transport properties of synthesized Titania nanotubes

Titania nanotube arrays fabricated by means of electrochemical anodization is currently the main interest of several research groups due to its promising applications. The high aspect ratio, durability, cheap and scalable fabrication technique make it highly attractive material for efficient solar cell. In this regard extensive research work is being carried out to investigate its properties. In our previous work we were able to find a mechanism for separating a single titania nanotube from the titania nanotube arrays and to measure its electric transport properties using e-beam lithography technique, In this work we investigated the effect of thermal annealing on the transport properties, we studied the effect of different annealing temperatures, heating and cooling rates, and in different gases. As a result, we were able to find the optimal annealing conditions to enhance the transport properties in blank titania nanotube. Under these optimal conditions, we were able to study the effect of coating TNTs with N719 dye and gold nanoparticles on the transport properties. As a result of our work we were able to optimize the treatments for more efficient solar cell fabrication.   

Computer Simulated Cold Welding of Gold Nanowires

Recently cold welding was achieved in gold and silver nanowires (NWs) with diameters in the range of 4 to 10 nm [1]. Since metallic contacts are of great importance in electronic devices, the ability of welding them without temperature change is quite remarkable and of interest. In the present work we use computer simulations to produce cold welding in gold NWs at room temperature. We used molecular dynamics with many body effective potentials based on the embedded-atom method EAM using the LAMMPS code to simulate first the braking of gold NWs, the two produced NWs are then cold welded and similarly as the experiments, the newly welded NWs showed fcc structures as the pristine samples. The structural analysis is done with two independent methods [2] and strain stress curves of the breaking and welding are present. Our computer simulation compare very well with the experiments. \\[4pt] [1] Y. Lu, \textit{et al}. Nature Nanotechnology 5, 218 - 224 (2010) \\[0pt] [2] E. Z. da Silva and Z.S. Pereira, Phys. Rev. B \textbf{81}, 195417 (2010).   

Nanowire FET as a measurement tool: A method for distiguishing molecular configurations using Debye Screening effect

Silicon nanowires/nanoribbons configured as field effect transistors (FETs) with receptor modified surface can be utilized for sensing of charged biomolecular species due to surface potential modulation upon receptor-ligand binding. However, charged ionic species of the sensing buffers interfere with a sensing process by lowering the effective charge of the bound molecules sensed by an FET. In this work, we exploit the Debye screening effect on the device signal by modulating the ionic strength of the sensing buffer i.e. the Debye length, to distinguish between the different configurations of the receptor-ligand complex. We compare our experimental data with a theoretical model and are able to extract characteristic length parameters of the receptor-ligand system. We will discuss the use of the suggested method for the sensing of conformational changes of biomolecules. References Sorensen M. H., Mortensen N. A., Brandbyge M., Appl. Phys. Lett. 91, 102105 (2007) Stern E., Vacic A., Rajan N. K., et al. Nature Nanotechnology 5, 138 (2010)   

Silicene Nano-Ribbons: Strong Resistance Towards Oxidation due to sp$^{2}$ Hybridization of the Si Valence Orbitals

We have synthesized for the first time silicene, that is, a new silicon allotrope analogous to graphene recently theoretically predicted [1], in the form of a massively parallel array of quantized zigzag nano-ribbons with a common ``magic'' width of 1.6 nm. They display characteristic linear band dispersions similar to the Dirac cones of graphene, in correspondence with their hexagonal arrangement seen in STM imaging [2]. Here we show, through the angle-dependence of REEL spectra taken at the Si L$_{2,3}$ edge, the typical signatures of 2p $\to \quad \pi $* and 2p $\to \quad \sigma $* transitions associated with sp$^{2}$ hybridization of the Si valence orbitals. We further show through high-resolution synchrotron radiation Si 2p core-level spectroscopy measurements that the afore mentioned silicene grating is very resistant toward oxidation. Typically, the oxygen uptake starts at about 10$^{4}$ higher doses than on the clean Si(111)7x7 surface. Indeed, this striking behavior is directly related to the sp$^{2}$ bonding, an additional confirmation of the silicene (i.e., graphene-like) nature of the nano-ribbons. \\[4pt] [1] S. Cahangorov et al., Phys. Rev. Lett., \textbf{102}, 236804 (2009). \\[0pt] [2] P. De Padova et al., Appl. Phys. Lett. \textbf{96}, 261905 (2010).   

Low- Temperature Magneto-conductance In Carbon Nano-tubes

Angular dependent magneto-resistance (MR) has been studied in Multi- walled carbon nano-tubes (MWNTs). In case of thin MWNTs, the flux cancelation has been observed in the low temperature MR. Based on the theoretical studies on the scattering behaviors in the carbon nano-tube (CNT) and flux cancelation in one dimensional transport in CNT, we can analyze an intrinsic carrier scattering based on our MR result of the angular dependence between the angle of thin MWNT axis in parallel and perpendicular directions to the magnetic fields. Also, we can discuss on the positive MR appeared in the perpendicular field direction into the thin MWNT axis.   

Session Y31: Amorphous Solids, Glasses & Liquids II

Synthesis, Homogenization and Molecular Structure of Chalcogenide glasses

Over the years, bulk glasses have been synthesized by reacting starting materials in evacuated (10$^{-5}$ Torr to 10$^{-7}$Torr) quartz tubings for various periods at suitable elevated temperatures. The lack of a non-invasive structural probe to track spatial heterogeneity of samples during synthesis has been an impediment to tune synthesis conditions, and obtain a homogeneous product. We have developed a Raman profiling technique to understand the homogenization kinetics of Ge$_x$Se$_{100-x}$ melts, and find dry samples (2 gram size) take 7 days of reaction at 950$^{\circ}$C to homogenize on a scale of 50 microns, while wet ones homogenize quicker ($\sim$3 days), but possess physical properties measurably different from their dry counterparts. Rotating sample tubes during synthesis assists in homogenization of samples incrementally but not dramatically. A score of compositions were homogenized across the 10\% $<$ x $<$ 33.3\% range, and calorimetric, Raman scattering, and molar volume data accumulated. These data provide clear evidence for three distinct regimes of behavior as a function of Ge content, which are identified with the three elastic phases discussed earlier.\footnote{P.Boolchand et al. J.Non-Cryst. Solids 293, 348 (2001).}   

Elastic phases in Ge$_x$Sb$_x$Se$_{100-2x}$ ternary glasses

The rigidity and stress phase transitions in titled ternary glasses are examined in Raman scattering, modulated DSC and molar volume measurements, and found to occur at x$_c$(1) = 14.9\% (rigidity) and x$_c$(2) = 17.5\% (stress). Raman scattering provides evidence of the structural motifs populated in these networks. Using Size Increasing Cluster Agglomeration, Rigidity theory and the decoded structural motifs, we have calculated the rigidity and stress transitions in the first step of agglomeration to occur at x$_c$(1)$^t$ = 15.2\% and x$_c$(2)$^t$ = 17.5\% respectively, in reasonable accord with experiments. Theory predicts and experiments confirm that these transitions will coalesce if edge-sharing Ge- tetrahedral motifs were absent in the structure, a circumstance that prevails in the Ge-deficient Ge$_7$Sb$_x$Se$_{93-x}$ ternary, where a narrow IP is reported.\footnote{B.J. Madhu et al. Eur. Phys. J. B 71,21 (2009).} These results underscore the central role played by topology in determining the elastic phases of network glasses.   

Short Range Order Signature in Crystalline and Amorphous GeSbTe Xanes Spectra

A new implementation of XANES spectra calculations within DFT and PAW potentials is used to compute the XANES spectra of various amorphous and crystalline GeSbTe structures. A clear correlation between the local order, either tetrahedral or distorted octahedral, and the shape of the XANES signal is observed. These calculations provide a new interpretation of past XANES measurements, relating essentially the phase change mechanism to a moderate modification of the local environment of the Ge atoms.   

Photocrystallization in a-Se films with and without As-Se buffer layers

Photo-induced crystallization is studied for temperatures of 250 - 435 K in amorphous Se (a-Se) HARP* imaging targets using Raman spectroscopy to detect the appearance and growth-rate of trigonal Se in these thin-film structures. We observe striking differences between HARP films in which a thin buffer layer of As-Se alloy is, or is not, deposited prior to growth of the principal a-Se photoconductive layer. Films containing an As-Se buffer appear to be much more stable; no photocrystallization is found within the temperature range studied, even after 3.5 hours of laser exposure (0.17W/mm$^{2}$ at 647nm). Whereas for films with no As-Se buffer, photocrystallization readily occurs in two temperature regimes below and above the a-Se glass transition (Tg $\sim $310K), and there is a regime in the neighborhood of Tg where photocrystallization is absent. We discuss these results in terms of a polymerization model under the competing effects of shear-strain at the substrate and As-cross-linking in the buffer layer, which, respectively, tend to promote and inhibit crystallization in the thick Se over-layer. *\textit{High-gain Avalanche Rushing Photodetector}.   

Diffraction studies of short- and intermediate-range order of phosphorus-selenium glasses

We present state-of-the-art neutron and X-ray diffraction data that provide a definitive picture of the short- and intermediate-range structure of P-Se glasses spanning both glass regions. Specific goals were (1) to obtain detailed information about the development with increasing of intermediate-range order on the length scale around 10 {\AA}, based on the behavior of the first sharp diffraction peak; and (2) to obtain a reliable statistical picture of the short-range order, using the information about types and concentrations of local structural units provided by recent NMR measurements to interpret the trends observed as the P concentration is varied. Particular attention is given to the fine structure of the first peak in the pair distribution function and to a feature in the structure factor at 7.5 {\AA}$^{-1}$, highlighted by Sergi et al. as a signature of molecular units.   

Structure investigation of ultra-small CdSe nanoparticles using the atomic PDF

The size-dependent structure of CdSe nanoparticles, with diameter ranging from 1.5 to 3.6 nm, has been studied using the atomic pair distribution function (PDF) method. The samples are prepared by the methods of Peng \textit{et al} [1], with modifications. The structure of the smallest stable size, ($\sim $1.5 nm), have been found to posses locally distorted wurtzite structure, with no clear evidence of a heavily disordered surface region [2]. The PDF data of the smallest particle show an extra structural peak appears around r = 3.5 A indicates there is structure modification happened in this sample. This peak start appearing the nanoparticles PDF data gradually as nanoparticle size decreases. The structural parameters are reported quantitatively. We measure a size-dependent strain on the Cd-Se bond which reaches 1.0{\%} at the smallest particle size [3]. The size of the well-ordered core extracted directly from the data agrees with the size determined from other methods. \\[0pt] [1] Peng, et al, \textit{JACS}., 120, 5343-5344 (1998). [2] Gilbert et al, \textit{Science}, 305, 651-654 (2004). [3] Masadeh et al. \textit{ PRB} \textbf{76}, 115413 (2007).   

Evidence of an Intermediate Phase in bulk alloy oxide glass sysem

Reversibility windows have been observed in modified oxides (alkali-silicates and -germanates) and identified with Intermediate Phases(IPs).\footnote{V. Rompicharla J. Phys.: Condens. Matter 20, 202101 (2008).} Here we find preliminary evidence of an IP in a ternary oxide glass, (B$_2$O3)$_5$(TeO$_2$)$_{95-x}$(V2O5)$_x$, which is composed of network formers. Bulk glasses are synthesized across the 18\% $<$ x $<$ 35\% composition range, and examined in Raman scattering, modulated DSC and molar volume experiments. Glass transition temperatures T$_g$(x) steadily decrease with V$_2$O$_5$ content x, and reveal the enthalpy of relaxation at T$_g$ to show a global minimum in the 24\% $<$ x $<$ 27\% range, the reversibility window (IP). Molar volumes reveal a minimum in this window. Raman scattering reveals a Boson mode, and at least six other vibrational bands in the 100 cm$^{-1}$ $<$ $\nu$ $<$ 1700 cm$^{-1}$ range. Compositional trends in vibrational mode strengths and frequency are established. These results will be presented in relation to glass structure evolution with vanadia content and the underlying elastic phases.   

Glasses, Stress, Attenuation and Thermal Conductivity

A wide variety of amorphous materials exhibit similar behavior in their thermal properties. Examples include universal features in the specific heat,thermal conductivity, and ultrasonic attenuation. Recent experiments from the Parpia group at Cornell find that high stress silicon nitride thin film resonators exhibit a remarkably high Q factor, exceeding that of amorphous SiO$_2$ by 2 to 3 orders of magnitude over a broad range of temperatures, and even exceeding that of single crystal silicon at room temperature. We present a model of why the stress reduces the attenuation. The basic assumption is that high stress increases the potential barriers of the excitations of defects that produce the loss, thus reducing the effective density of lossy fluctuators. We predict that high stress could lead to high thermal conductivity and low dielectric loss, making high stress SiN an excellent candidate as a substrate for integrated circuits.   

Highly stable glasses as a general phenomenon: Physical vapor depositions of four different trisnaphthylbenzene isomers

Glasses of each of the trisnaphthylbenzene (TNB) isomers, a low molecular weight glass forming family of four isomers, were created by physical vapor deposition. These glasses were analyzed using differential scanning calorimetry and wide angle x-ray scattering, and then compared to glasses prepared by quenching each melt. All four isomers produced stable glasses (increased onset temperature, large enthalpy overshoot, and excess x-ray scattering) when vapor-deposited at 0.85 T$_{g}$ and at low deposition rates. This result is surprising as one of the TNB isomers readily crystallizes when cooled as a liquid. When coupled with previous experiments, these results show that stable glasses are not just produced by a small set of good glass forming molecules but seem to be a general phenomenon. Thus physical vapor deposition can be used as a general route to create unusual glasses for future scientific exploration and technological uses.   

In-situ characterization of vapor-deposited glasses of toluene by differential AC chip nanocalorimetry

We use ac nanocalorimetry to investigate extraordinarily stable glasses of toluene prepared by vapor deposition. For that purpose we've built a vapor deposition chamber that allows in-situ characterization of vapor-deposited organic glasses down to liquid nitrogen temperature. With highly sensitive nanocalorimeters in a differential setup, we are able to measure ng-samples over a frequency range from 0.1 Hz up to 8 kHz. The device was used to investigate the transformation of as-deposited stable toluene glasses into ordinary glasses. For films about 100 nm thick, the transformation was studied as a function of time at constant temperature above the common glass transition and as function of temperature at constant heating rate. The stability of the thin films was investigated as a function of substrate temperature and deposition rate.   

Thermal Evolution of Defects and Hydrogenated Surfaces in nc-Si:H

Photovoltaics research has created a push for new materials and nanotechnology is a primary focus. The most familiar of the nanomaterials is hydrogenated nanocrystalline silicon (nc-Si:H). nc-Si:H has less light-induced degradation than a-Si:H and is cheaper to make than crystalline silicon. X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and electron spin resonance (ESR) experiments explored the crystallite size, orientation and defect density on nc-Si:H samples with varying crystalline volume fraction (CVF). Samples with CVF $\sim $ 50{\%} show preferential [220] crystallite orientation, whose microstructure changes with thermal annealing. Modeling of SAXS data for as-grown material shows that the crystallite surfaces are 20{\%} to 40{\%} hydrogenated. After high temperature annealing, hydrogen leaves these surfaces and the ESR signal increases by about 10 times. We discuss these results and then speculate on the relationship between hydrogen, defects, and microstructure.   

Session Y33: Cold Fusion

Justifying Condensed Matter Nuclear Phenomena Using Hot Fusion Data

The selective resonant tunneling model [1] has been successful in describing 6 major fusion cross-section data (d+T, d+D, d+He3, t+T, t+He3, p+D). The new formula needs only 3 parameters; however, it gives much better results than what were given by the 5-parameter formula in NRL Plasma Formulary. It provides an opportunity to find the resonance energy level which is necessary to explain the Condensed Matter Nuclear Phenomena in metal-hydrides. The proton-lithium fusion data, the astrophysical S-factor data, the K-electron capture data of beryllium, and the anomalous ratio of the isotope abundance of lithium in palladium-hydride (7Li/6Li) will be presented as an example for this justification. Thus, selective resonant tunneling model explains not only the 3 puzzles in Condensed Matter Nuclear Science (i.e. tunneling the Coulomb barrier, excess heat without commensurate neutron radiation, and the missing gamma radiation), but also 7 sets of hot fusion data. It predicts that there must be neutrino radiation accompanied with Condensed Matter Nuclear Phenomena in metal-hydrides. \\[4pt] [1] Xing Z. Li, et al., Nucl. Fusion 48 125003 (2008).   

-dimensional Symmetry Catalysts for A-Z Gas Loading Fusion

An epitaxial mating of a metal layer to a chemically stable ionic crystal minimizes system energy for cold fusion based on Bloch function symmetry and using gas loading and nm-Pd at a favored interface.[1] To achieve epitaxy second and third metal layers need to have imperfections. One thinks of the stable ionic crystal as a template and the nano-Pd solid as a malleable lattice. The interior volume of the nano-Pd solid has a face-centered cubic structure. ZrO2 was the template ionic crystal used in A-Z gas loading studies at elevated T in (2005). A template crystal using the sapphire crystal equivalent of a double-layer graphene crystal is suggested. Impurity Rh and Ru are suggested as impurity atoms in the nano-metal (as in gem-quality Zircon) and a amall amount of interstitial H in addition to dominant D as involved in diffusion. Ref. [1] ``Interface Modeling of Cold Fusion,'' Talbot A. Chubb, Proc. ICCF14, Book 2, pp 534-539 (2008).   

Comparison of Calorimetry: MIT and Fleischmann-Pons Systems

The history of cold fusion shows that the MIT heat conduction calorimetry in 1990 reported a sensitivity of 40 mW while the Fleischmann-Pons Dewar calorimetry achieved a sensitivity of 0.1 mW. Additional information about the MIT calorimetry allows a more detailed analysis. The major finding is that the MIT calorimetric cell was so well insulated with glass wool (2.5 cm in thickness) that the major heat transport pathway was out of the cell top rather than from the cell into the constant temperature water bath. It can be shown for the MIT calorimeter that 58\% of the heat transport was through the cell top and 42\% was from the cell into the water bath. Analysis of the Fleischmann-Pons Dewar cell shows that under conditions similar to the MIT experiments, almost all of the heat flow would be from the Dewar calorimetric cell to the constant temperature water bath. Furthermore, the sensitivity of the Fleischmann- Pons temperature measurements was 0.001 K versus 0.1 K for the MIT calorimetric cell. Evaluations of the calorimetric equations and data analysis methods leads to the conclusion that the Fleischmann-Pons calorimetry was far superior to that of MIT.   

Can LENR Energy Gains Exceed 1000?

Energy gain is defined as the energy realized from reactions divided by the energy required to produce those reactions. Low Energy Nuclear Reactions (LENR) have already been measured to significantly exceed the energy gain of 10 projected from ITER,possibly 15 years from now. Electrochemical experiments using the Pd-D system have shown energy gains exceeding 10. Gas phase experiments with the Ni-H system were reported to yield energy gains of over 100. Neither of these reports has been adequately verified or reproduced. However, the question in the title still deserves consideration. If, as thought by many, it is possible to trigger nuclear reactions that yield MeV energies with chemical energies of the order of eV, then the most optimistic expectation is that LENR gains could approach one million. Hence, the very tentative answer to the question above is yes. However, if LENR could be initiated with some energy cost, and then continue to ``burn,'' very high energy gains might be realized. Consider a match and a pile of dry logs. The phenomenon termed ``heat after death'' will be examined to see if it might be the initial evidence for nuclear ``burning.''   

Lattice Assisted Nuclear Reactions From Nanostructured Metamaterials Electrically Driven at Their Optimal Operating Point

In lattice assisted nuclear reactions, hydrogen-loaded alloys enable near room temperature deuterium fusion and other nuclear reactions (1). The structural metamaterial shape of some D-loaded Pd nanostructures and deuterium flux (2) through them, driven by an applied electric field, appear to play decisive roles. The spiral Phusor$^{\textregistered}$-type cathode with open helical cylindrical geometry in a high electrical resistance solution is a LANR metamaterial design creating intrapalladial deuteron flow. Optimal operating point technology allows improved and more reproducible operation (3). LANR power gain can be considerable. In situ imaging has revealed that the excess power gain is linked to non-thermal near-IR emission when the LANR devices are operated at their OOP. LANR devices have shown power gains more than 200\%, and short term power gains to $\sim$8000\%. 1. Swartz, M, J. Sci. Exploration, 23, 4, 419-436 (2009). 2. Swartz, M, Fusion Technology, 22, 2, 296-300 (1992); 26, 4T, 74-77 (1994); 32, 126-130 (1997). 3. Swartz. M, Fusion Technology, 31, 63-74 (1997).   

The Use of SSNTD's in the Pd-D Co-deposition Experiment

An early derivative experiment of the original Fleischman-Pons electrochemical experiment [1-3] was that of Szpak et al [4-5]. Szpak et al. chose to electro- deposit bulk metal palladium on a conductive metal substrate from a deuterium oxide (D2O) solution of a Pd salt, as opposed to electrolytically loading a bulk Pd cathode in a D2O solution. Recent work, by Boss et al [6] has concentrated on using solid state nuclear track detectors (SSNTD, specifically CR-39) to search for evidence of nuclear particles. In most of these experiments the CR-39 was immersed in the electrolyte, which makes the interpretation of the tracks potentially ambiguous because of the possibility of chemical damage. However, different interpretations of results presented have concluded that the data argue for the generation of alpha particles, protons, and/or neutrons. We have chosen to reproduce one version of these recent experiments using CR-39 immersed and separated from the electrolyte with a 6 $\mu$m thick piece of Mylar$^{\textregistered}$ film. A 60 $\mu$m thick piece of polyethylene, used as a protective cover during handling, was occasionally allowed to remain on the film to facilitate thermalization of possible product neutrons. 1. Fleischmann, M., S. Pons, and M. Hawkins, ``Electrochemically induced nuclear fusion of deuterium''. J. Electroanal. Chem., 1989. 261, 301   

LENR BEC Clusters on and below Wires through Cavitation and Related Techniques

During the last two years I have been working on BEC cluster densities deposited just under the surface of wires, using cavitation, and other techniques. If I get the concentration high enough before the clusters dissipate, in addition to cold fusion related excess heat (and other effects, including helium-4 formation) I anticipate that it may be possible to initiate transient forms of superconductivity at room temperature.   

Use of Helium Production to Screen Glow Discharges for Low Energy Nuclear Reactions (LENR)

My working hypothesis of the conditions required to observe low energy nuclear reactions ( LENR ) follows: 1) High fluxes of deuterium atoms through interfaces of grains of metals that readily accommodate movement of hydrogen atoms interstitially is the driving variable that produces the widely observed episodes of excess heat above the total of all input energy. 2) This deuterium atom flux has been most often achieved at high electrochemical current densities on highly deuterium-loaded palladium cathodes but is clearly possible in other experimental arrangements in which the metal is interfacing gaseous deuterium, as in an electrical glow discharge. 3) Since the excess heat episodes must be producing the product(s) of some nuclear fusion reaction(s) screening of options may be easier with measurement of those ``ashes'' than the observance of the excess heat. 4) All but a few of the exothermic fusion reactions known among the first 5 elements produce He-4. Hence helium-4 appearance in an experiment may be the most efficient indicator of some fusion reaction without commitment on which reaction is occurring. This set of hypotheses led me to produce a series of sealed tubes of wire electrodes of metals known to absorb hydrogen and operate them for $>$100 days at the $<$1 watt power level using deuterium gas pressures of $\sim$100 torr powered by 40 Khz AC power supplies. Observation of helium will be by measurement of helium optical emission lines through the glass envelope surrounding the discharge. The results of the first 18 months of this effort will be described.   

Conventional Physics can Explain Excess Heat in the Fleischmann-Pons Cold Fusion Effect

In 1989, when Fleischmann and Pons (FP) claimed they had created room temperature, nuclear fusion in a solid, a firestorm of controversy erupted. Beginning in 1991, the Office of Naval Research began a decade-long study of the FP excess heat effect. This effort documented the fact that the excess heat that FP observed is the result of a form of nuclear fusion that can occur in solids at reduced temperature, dynamically, through a deuteron (d)+d?helium-4 reaction, without high-energy particles or ? rays. This fact has been confirmed at SRI and at a number of other laboratories (most notably in the laboratory of Y. Arata, located at Osaka University, Japan). A key reason this fact has not been accepted is the lack of a cogent argument, based on fundamental physical ideas, justifying it. In the paper, this question is re-examined, based on a generalization of conventional energy band theory that applies to finite, periodic solids, in which d's are allowed to occupy wave-like, ion band states, similar to the kinds of states that electrons occupy in ordinary metals. Prior to being experimentally observed, the Ion Band State Theory of cold fusion predicted a potential d+d?helium-4 reaction, without high energy particles, would explain the excess heat, the helium-4 would be found in an unexpected place (outside heat- producing electrodes), and high-loading, x?1, in PdDx, would be required.   

Electrochemical and Electron Probe Microanalysis Measurements on Nanostructured Palladium

The hydrogen region of nanostructured Pd in the cyclic voltammetry in 1 M H2SO4 was more resolved than that of plain Pd because of the thin walls of the nanostructure and the high surface area. We could distinguish the hydrogen adsorption and absorption processes. The permeation of hydrogen into the Pd metal lattice occurs with fast kinetics when the Pd surface is blocked by either crystal violet or Pt. We believe that the hydrogen absorption process takes place without passing through the adsorbed state so that hydrogen diffuses directly into the Pd bulk. This process speeds up when the formation of adsorbed hydrogen is suppressed by the coverage of poisons. These results were compared to those obtained in a heavy water solution to which the Pd electrode was exposed. Adsorption characteristics of deuterium on the Pd metal surface are slightly different to those obtained for hydrogen in previous studies. Diffusion of deuterium into the Pd metal lattice works with fast kinetics under appropriate surface modification. We are also interested in studying the Pd structure before and after long term electrolysis in light and heavy water using electron probe micronanalysis (EPMA) with a energy dispersive spectrometer (EDS)   

Session Y35: Topological Insulators: Applications

Physical Vapor Deposition Growth of Topological Insulator Nanostructures

Nanostructures consisting of strong topological insulators are of interest for the fabrication of devices in which surface state transport is dominant. We report Bi$_2$Se$_3$ nanoribbon and nanoplatelet growth using a multi-zone furnace.\footnote{D. Kong \textit {et al.}, Nano Lett. \textbf{10}, 329 (2010).} Nanoribbons are grown by the vapor-liquid-solid method, using Au nanoparticles or Au thin films ($\sim$5 nm) as catalysts, while nanoplatelets are grown on bare silicon. We systematically vary the growth parameters, including the temperatures of the powdered Bi$_2$Se$_3$ precursor and growth substrate, the growth pressure and duration, the rate of the Argon carrier gas flow, size of the gold catalyst, and the quantity of Bi$_2$Se$_3$ source material. Typical nanoribbon growth occurs at 450$^\circ$C and 350 Torr, with the precursor held at 530$^\circ$C in an Argon carrier gas flow rate rate of 140 sccm. Typical platelet growth occurs at lower pressures and temperatures. High resolution transmission electron microscopy, diffraction, and energy dispersive x-ray analysis are used to characterize the synthesized structures.   

Topological insulators for high performance terahertz to infrared applications

Topological insulators in the Bi2Se3 family have an energy gap in the bulk and a gapless surface state consisting of a single Dirac cone. Low frequency optical absorption due to the surface state is universally determined by the fine structure constant. When the thickness of these three dimensional topological insulators is reduced, they become quasi-two dimensional insulators with enhanced absorbance. The two dimensional insulators can be topologically trivial or non-trivial depending on the thickness, and we predict that the optical absorption is larger for topological non-trivial case compared with the trivial case. Since the three dimensional topological insulator surface state is intrinsically gapless, we propose its potential application in wide bandwidth, high performance photo-detection covering a broad spectrum ranging from terahertz to infrared. The performance of photodetection can be dramatically enhanced when the thickness is reduced to several quintuple layers, with a widely tunable band gap depending on the thickness.   

Two Dimensional Transport Induced Linear Magneto-resistance in Topological Insulator Bi$_{2}$Se$_{3}$ Nanoribbons

Bulk Bi$_{2}$Se$_{3}$ has been proposed and confirmed as a type of three-dimensional (3D) topological insulators (TI's) with a single Dirac cone for the surface state. Although the existence of topological surface state in Bi$_{2}$Se$_{3}$ has been established by surface sensitive techniques (ARPES, STM), the transport properties of two dimensional (2D) surface state in 3D TI's has been plagued by the dominating conductivity from bulk carriers. Here, we report the study of a novel linear magneto-resistance (MR) under perpendicular magnetic fields in Bi$_{2}$Se$_{3}$ nanoribbons, and show that this linear MR is purely due to 2D transport by angular dependence experiments. The 2D magneto-transport induced linear MR in Bi$_{2}$Se$_{3}$ nanoribbons is in agreement with the recently discovered linear MR from topological surface state in bulk Bi$_{2}$Te$_{3}$, and the MR of other gapless semiconductors and graphene. We further show that the linear MR of Bi$_{2}$Se$_{3}$ nanoribbons persists up to room temperature, underscoring the potential of exploiting TI's for room temperature magneto-electronic applications.   

Quantum impurity spin in Majorana edge modes

We show that Majorana edge modes of two-dimensional spin-triplet topological superconductors (superfluids) have Ising-like spin density whose direction is determined by the d-vector characterizing the spin-triplet pairing symmetry in the bulk. Thus, when a quantum impurity spin is introduced at the edge of the spin-triplet topological superconductors (superfluids), the exchange coupling between this impurity spin and the Majorana modes becomes Ising-type. Observing this, we argue that, under the external magnetic fields, this quantum impurity spin exhibits anisotropic dissipative quantum dynamics due to the `background' massless Majorana edge modes. We also discuss how the magnetic response of this impurity spin can serve as a local probe for spin-triplet superconducting order parameter in the bulk.   

Surface state transport in Bi2Se3 nanodevices

We report on electronic transport measurements on thin ($<$100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel surface and bulk contributions.   

Thermoelectric transport of edge/surface states of topological insulators

In my talk we theoretically study thermoelectric properties of topological insulators (TI) [1], where novel properties of edge/surface states are expected to appear. As compared to the number of bulk states, the edge/surface states are very few; we therefore consider a narrow ribbon for 2D and a thin slab for 3D TI to make the edge/surface-state transport larger. By considering edge/surface and bulk transport together, we calculate the charge and heat conductivity, and Seebeck coefficient. We find that in 2D TI the bulk and edge transport compete each other in the thermoelectric transport. By lowering temperature, the thermoelectric figure of merit ZT has a minimum, corresponding to the bulk-to-edge crossover, and then increases again at low temperature where the edge state dominates. The crossover is estimated to be at around 5K-10K for 10nm-width ribbon. We also discuss surface state transport for 3D TI as well.\\[4pt] [1] R. Takahashi and S. Murakami, Phys. Rev. B81, 161302 (R) (2010).\\[0pt] [2] S. Murakami, R. Takahashi, O. A. Tretiakov, Ar. Abanov, J. Sinova, arXiv:1010.2304.   

Mobility and thermopower of surface and bulklike charges in Bi and Sb nanowires

Topological insulators (TI) surface charges are predicted to have high mobilities and other properties. Bi and Sb, that are classified as TI trivial and true, respectively, are interesting candidates but are not very good bulk insulators. However, in very thin nanowires, quantum confinement opens a gap for the bulk states that is not expected to change the material's TI character. We studied the electronic transport of 18-nm to and 200-nm diameter nanowires in arrays, fabricated by Bi injection in porous alumina, via coupled measurements of resistance and thermopower (4-300 K). Surface carriers and holes Landau level spectra were analyzed to extract densities. The nanowires low temperature thermopower (T$<$100 K) is -1 T microvolt/(K$^2$) consistent in sign and magnitude with surface electrons. Coexistence of bulklike holes with surface electrons, consistent with the carrier's hybridization that is expected in Bi, is observed. Results for Sb will be presented also.   

Raman spectroscopy of exfoliated Bi2Se3

The study of topological insulators is often frustrated by the presence of a residual bulk conductivity arising from defects which makes isolating the surface contribution to a given measurement difficult. Nanoscale topological insulators are therefore an appealing alternative to bulk crystals, as a small volume should emphasize surface contributions and allow the suppression of the residual bulk carriers by gating. To this end we have produced, via mechanical exfoliation, nanocrystals as thin as a 2 nm of the topological insulator Bi2Se3 on Mica substrates. Exfoliated crystals of a variety of thicknesses have been characterized by optical, Raman, and atomic force microscopies. We observe an emergent mode at 158 cm-1 which is attributed to the breaking of inversion symmetry at the Bi2Se3 surfaces. The utility of this emergent mode for determining nanocrystal thickness is discussed.   

Mechanical Exfoliation and Electron Transport of Topological Insulator Nanoribbons

Bismuth selenide (Bi$_{2}$Se$_{3}$), a stoichiometric material of a single Dirac-cone band structure, is one of the most promising candidates to realize the topologically non-trivial surface state protected by time reversal symmetry. Especially, many exotic physical phenomena are predicted to emerge in low dimensional nanostructures of Bi$_{2}$Se$_{3}$, such as the crossover between 3D to 2D topological insulator. Due to the weak Van der Waals interaction between adjacent quintuple layers (QLs), Bi$_{2}$Se$_{3}$ can be exfoliated down to a few QLs. We will present the mechanical exfoliation of topological insulator nanoribbons by an atomic force microscope (AFM) tip, which enables ultra-thin topological insulator down to a single QL. Electron transport measurement on low dimensional topological insulator will be also discussed, as well as the conductivity mapping experiment using a microwave scanning probe technique.   

Topological surface states along antiwires

Surface states along amorphous columnar defects in three-dimensional topological insulators are considered. The response of these states to a magnetic field and their contribution to thermoelectric transport will be discussed.   

Bias dependence of h/e and h/2e Aharonov-Bohm oscillations in topological insulators

Recently Aharonov-Bohm (AB) oscillations were observed in Bi$_{2}$Se$_{3}$ nanoribbons by Peng \textit{et al}. [1] as a direct evidence for the existence of surface states in topological insulator. However, the resistance showed only h/e oscillations with a minimum in resistance at zero flux while the ballistic and diffusive theory predicts either h/e oscillations with a maximum in resistance at zero flux or h/2e oscillations with a minimum in resistance at zero flux respectively [2]. A possible explanation of the results of Peng \textit{et al.} was given in the theory of disordered topological insulators proposed by Bardarson \textit{et al }.[2] and Zhang \textit{et al.} [3] where they attributed the results of Peng \textit{et al.} to presence of weak disorder. Furthermore authors of [2] and [3] studied dependence of h/e and h/2e oscillations on disorder strength and doping using their proposed theory. In this work we look at the effect of doping by studying bias dependence of AB oscillations using a gated device and observe both h/e and h/2e oscillations whose relative strength depends on the applied bias and compare the proposed theory of ref. [2] and [3] with the experimental results. [1] H. Peng, \textit{et al}. Nature Mater. 9, 225 (2010).[2] J. Bardarson, \textit{et al}, Phys. Rev. Lett. 105, 156803 (2010).[3] Y. Zhang and A. Vishwanathan, Phys. Rev. Lett. 105, 206601 (2010).   

Spin polarization and transport of surface states in the topological insulators Bi$_2$Se$_3$ and Bi$_2$Te$_3$ from first principles

We investigate the band dispersion and the spin texture of topologically protected surface states in the reference bulk topological insulators Bi$_2$Se$_3$ and Bi$_2$Te$_3$ by using a first-principles approach. Exceptionally strong spin-orbit interaction in these materials entangles the electronic states across broad energy ranges thus reducing the spin-polarization of the topologically protected surface states to $\sim$50\% in both cases. This reduction is absent in simple phenomenological models but of important implications to essentially any application of bulk topological insulators in spintronics and likely to some other phenomena. We further propose a way of controlling the magnitude of spin polarization associated with a charge current in thin films of topological insulators by means of an external electric field. The proposed dual-gate device configuration provides new possibilities for electrical control of spin.   

Manipulating surface states in topological insulator nanostructures

Topological insulators show unique properties resulting from massless, Dirac-like surface states that are protected by time-reversal symmetry. Theory predicts that the surface states exhibit quantum spin Hall effect that allows for spins to transport without scattering. However, to date, the direct manipulation of these states with external means remains a significant challenge owing to the predominance of bulk carriers. Here we show the first experimental evidence of surface-state modulation through the observation of voltage-controlled quantum oscillations in Bi2Te3 nanostructures. The surface conduction can be dramatically enhanced with external gate bias. Up to 51 percent of the total conductance is attributed to the surface states. The ability to manipulate the surface states mark an important milestone in the development of TI materials and may further open up exciting and novel applications in nanoelectronics and spintronics.   

Aharonov-Bohm oscillations in disordered topological insulator nanowires

A direct signature of electron transport at the metallic surface of a topological insulator is the Aharonov-Bohm oscillation observed in a recent study of Bi$_2$Se$_3$ nanowires [Peng {\it et~al.}, Nature Mater.\ 2010] where conductance was found to oscillate as a function of magnetic flux $\phi$ through the wire, with a period of $h/e$ and {\it maximum} conductance at zero flux. This seemingly agrees neither with diffusive theory (period of $h/2e$) nor with ballistic theory, which in the simplest form predicts a period of $h/e$ but a {\it minimum} at zero flux due to a nontrivial Berry phase. We show how the magneto-conductance depends on doping and disorder strength, provide a possible explanation for the experiments, and discuss further experiments that could verify the theory.   

Terahertz conductivity of Bi$_2$Se$_3$ topological insulator thin films

We report a study of high quality MBE grown Bi$_2$Se$_3$ topological insulator thin films. We have measured the ac conductivity in the terahertz region using time domain terahertz spectroscopy. By measuring films with different thickness we can set limits on the value of the bulk and surface conductivites. We will also report on measurements of the Faraday rotation using polarized light at these frequencies.   

Session Y36: Graphene: Optical Properties II

Raman spectroscopy of graphite in high magnetic fields

Recently, much attention has been paid to electron-phonon coupling in graphene. In particular, significant re-normalization and broadening of long-wavelength optical phonons are predicted to occur through resonant interaction with Landau-quantized Dirac fermions. We report here on a high-field magneto-Raman spectroscopy study of highly-oriented pyrolytic graphite (HOPG) and natural graphite at temperatures down to 5 K and in magnetic fields up to 45 T. The E$_{2g}$ graphite phonon line exhibits anticrossing-like behavior at approximately 30 T, which we attribute to the magneto-phonon resonance (MPR) of graphite's massless holes at the H-point. Additionally, we observed features related to inter-Landau-level transitions at the K-point of graphite. We also observed weak graphene-like signatures of MPR, indicating the existence of graphene flakes on the graphite surface.   

Raman measurements of graphene in magnetic fields

Electron phonon interactions in graphene are effectively measured using Raman spectroscopy. For example the G-Band of graphene grown on SiC shows characteristic anticrossings when tuning an external magnetic field exactly at the resonances between the G-Band phonon and the electronic Landau Levels. We measure the micro Raman spectra of mechanically exfoliated graphene lithographically prepared as field effect devices. Unlike prior high magnetic field studies, this provides charge tunability and allows simultaneous Raman and transport measurements under variable B-field. Our initial results show a Landau Level dependent splitting of the G-band for magnetic fields B $>$ 10T. We present our latest results of studies of the Raman G and 2D Band and single and bilayer graphene at T=4.2K and fields to 12T.   

Raman Maps of Carbon Nanoscrolls and their Bundles

Recent theoretical simulations and experimental data have shown that carbon nanoscrolls, formed by rolling a single layer of graphene, exhibit different electronic and optical properties from carbon nanotubes or the planar graphene sheet. They hold promise in a variety of applications areas such as energy storage and nanomechanics. Here, we present our investigation on the formation of carbon nanoscrolls and their bundles from large-area graphene originally grown on copper foil by chemical vapor deposition. When graphene is transferred from a copper foil onto a silicon wafer using wet chemistry, both filled and unfilled carbon nanoscrolls are produced upon the rupture of graphene. These nanoscrolls can further form bundles. The results from Raman mapping, optical microscopy, scanning electron microscopy, and atomic force microscopy measurements will be presented and the formation mechanism will be discussed.   

Controlling Inelastic Light Scattering Quantum Pathways in Graphene

Graphene exhibits unique tunable optical properties. Researchers have observed infrared absorptions in graphene interband transitions as well as intraband transitions can be modified substantially through electrostatic gating. At the same time, inelastic Raman scattering from few layer graphene is readily observable and widely used to characterize graphene quality, and to probe graphene electron-phonon interactions. In strongly gated graphene, Raman scattering from graphene can also be varied from electrical doping through direct change of electronic transitions. In this talk, I will describe how the Raman intensity of G-mode and 2D-mode Raman varies with the Fermi energy in doped graphene.   

Ab initio calculation of double-resonant Raman spectra for bilayer graphene

The discovery that the application of an external electric field induces a band gap opening in bilayer graphene attracted a lot of interest on this system, due to important applications in nanoelectronics [1]. Raman spectroscopy is one of the most important experimental techniques for the characterisation of carbon based materials, providing informations on carriers concentration [2], disorder [3], number of layers on multi-layers graphene systems [4], and phonon properties. Most of the theoretical studies on multi-layers graphene are performed using a Tight Binding (TB) model, and full calculation of Raman matrix elements to obtain frequencies, intensities and linewidths of Raman bands has not been performed up to now. The developpement of a fully ab initio theoretical tool to compute Raman spectra is therefore higly desirable and particularly relevant for systems where a simple TB parametrization of the electronic structure and of the electron-phonon interaction is not available. In this talk I will discuss a recently developped methodology to compute fully ab initio double-resonant Raman spectra and I will present results for bilayer graphene.\\[4pt] [1] Ohta et al, Science {\bf 313}, 951 (2006), [2] Malard et al, PRL {\bf 101}, 257410 (2008), [3] Lucchese et al, Carbon {\bf 48}, 1592 (2010), [4] Ferrari et al, PRL {\bf 97}, 187401 (2006)   

Observation of out-of-plane vibrations in few-layer graphene using combination and overtone Raman modes

We have studied three distinct higher-order Raman features, appearing at $\sim $ 1660, 1730 and 1760 cm$^{-1}$, in graphene samples of 1-6 layers thickness and both Bernal and rhombohedral stacking. By detailed analysis of the measured dispersions of these lines using double-resonance theory, we have identified the features, respectively, as the LO+ZA, LO+ZO' combination modes and the 2ZO overtone mode. Here LO, ZA, and ZO, and ZO' denote, respectively, the in-plane longitudinal optical mode, the out-of-plane acoustic, optical and layer-breathing modes. All three of these Raman features are absent in single-layer graphene, which lacks the layer-breathing vibration and exhibits particularly high symmetry. The line shape of LOZO' mode shows a dramatic dependence on the stacking order of the layers and can serve as a means of identifying stacking order in few-layer graphene. In addition, the LOZO' mode allows us to access the properties of the low-energy layer-breathing (ZO') mode in few-layer graphene samples.   

Infrared Kerr and Faraday Measurements in Gated, Multi-Layer SiC Graphene

Magneto-optical Kerr and Faraday measurements are used to probe the Landau level structure of SiC graphene in the mid- and far-infrared regimes (100-1000 meV and 3-10 meV, respectively). Transmittance/reflectance spectroscopy probes the longitudinal conductivity ($\sigma _{xx})$, which is related to the sum of chiral response. In contrast, polarization sensitive techniques provide new insights into the electronic structure by probing the Hall conductivity ($\sigma _{xy})$, which reflects the difference in the chiral response. Samples, which are studied in applied fields (B) up to 7T and temperatures ranging from 10-300K, show robust features arising from two distinct sets of Landau level transitions. One set displays transition energies that are $\sqrt B $ dependent as expected of monolayer graphene. Interestingly, below a critical photon energy ($\sim $100 meV) these features become symmetric with B. The other set is consistent with expectations of bilayer graphene and graphite, showing a linear B dependence and the expected odd symmetry in B. Further investigation of Landau level behavior is accomplished by tuning the Fermi energy in samples with a gate. Work supported by NSF-DMR1006078.   

Landau level-phonon resonances in graphene and their spectroscopic signatures in magneto-optical measurements

The excited states and the optical spectra of a two-dimensional electronic system under a magnetic field are strongly influenced by the electron-phonon interaction when the energy spacing of the Landau levels is resonant with the frequency of an optical phonon. We have performed a theoretical study of these excited states in graphene, and have calculated the optical absorption spectra for a range of magnetic fields. Electron-electron interactions are found to redistribute the spectral weight of the coupled modes and have important consequences for the absorption spectra. Our results are in good agreement with recent magneto-optical transmission experiments on epitaxial graphene on SiC. This work was supported by NSF Grant No. DMR10-1006184 and U.S. DOE Contract No. DE- AC02-05CH11231.   

Probing plasmons in graphene

Plasmon behaviour in graphene is important for the understanding of many body interaction of 2D Dirac fermions. It also provides physical background for potential graphene applications in optoelectronics and ultrahigh speed THz electronics. In this talk, we will describe our study of plasmon behaviour in graphene using far-infrared spectroscopy and compare our experimental results to theoretical predictions.   

Plasmon-Enhanced Photocurrent in a Graphene Nanoconstriction

A plasmonic nanostructure can act like an optical antenna, concentrating light into a deep sub-wavelength volume and enabling manipulation of light-electron interactions at the nanometer scale. Achieving efficient coupling from such antennas to functional electrical devices has been challenging, because the region of field enhancement is so small. We report the use of a use a self- aligned fabrication process to couple a gold break junction acting as a plasmonic antenna with a sub-10-nm graphene constriction. The nonlinear electrical characteristics of the graphene device allow it to serve as a photodetector. We observe a photocurrent that is peaked at the plasmon frequency and strongly modulated by the polarization direction of the incident light. The enhancement of the local optical-frequency electric field induced by the plasmon is a factor of 1.5-10.   

Thermal broadening effects on unstable plasmons in extrinsic graphene with injected carriers

We study theoretically the charge density collective oscillations (plasmons) of an extrinsic ({\em i.e.}, doped) graphene system into which charge carriers (either electrons or holes) are injected. When the injected carriers are sharply peaked so that the distribution function of the injected carriers can be well approximated by $f_{\mbox{\tiny inject}}(\mathbf p) = n\delta(\mathbf p - \mathbf p_0)$, some of the plasmons in the system become unstable, in the sense that the amplitudes of these plasmons grow exponentially in time (at least initially, in the linear-response regime). This effect is analogous to the two-stream instability that is seen in classical plasma systems. As with the classical plasma system, thermal broadening of the injected carriers tends to suppress the instability. We report a theoretical study of the effect of the thermal broadening of the injected carriers on the plasmon instability in graphene, and we delineate the parameters where the thermal effects completely suppress the instability.   

Electromagnetic wave propagation through a graphene-based photonic crystal

A novel type of photonic crystal formed by embedding a periodic array of constituent stacks of alternating graphene and dielectric discs into a background dielectric medium is proposed [1]. The frequency band structure of a 2D photonic crystal with the square lattice of the metamaterial stacks of the alternating graphene and dielectric discs is obtained. The electromagnetic wave transmittance of such photonic crystal is calculated. The graphene-based photonic crystals have the following advantages that distinguish them from the other types of photonic crystals. They can be used as the frequency filters and waveguides for the far infrared region of spectrum at the wide range of the temperatures including the room temperatures. The photonic band structure of the graphene-based photonic crystals can be controlled by changing the thickness of the dielectric layers between the graphene discs and by the doping. The sizes of the graphene-based photonic crystals can be much larger than the sizes of metallic photonic crystals due to the small dissipation of the electromagnetic wave. The advantages of the graphene-based photonic crystal are discussed.\\[4pt] [1] O. L. Berman, V. S. Boyko, R. Ya. Kezerashvili, A. A. Kolesnikov, and Yu. E. Lozovik, Phys. Letts. A 374, 4784 (2010).   

Effects of Layer Stacking on the Combination Raman Modes in Graphene

We have observed new combination modes in the range from 1650 -- 2300 cm$^{-1}$ in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. The M band at $\sim $1750 cm$^{-1}$ is suppressed for both SLG and IBLG. A peak at $\sim $1860 cm$^{-1}$ (iTALO$^{-})$ is observed due to a combination of the iTA and LO phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent in bulk graphite. Two previously unidentified modes at $\sim $1880 cm$^{-1}$ (iTALO$^{+})$ and $\sim $2220 cm$^{-1}$ (iTOTA) in SLG are tentatively assigned as combination modes around the K point of the graphene Brillouin zone. The peak frequencies of the iTALO$^{+}$ (iTOTA) modes are observed to increase (decrease) linearly with increasing graphene layers.   

Session Y38: Focus Session: Non-Equilibrium Insights into Single Molecules and Cell Function II

Can Simple Biophysical Principles Yield Complicated Biological Functions?

About once a year, a new regulatory paradigm is discovered in cell biology. As of last count, eukaryotic cells have more than 40 distinct ways of regulating protein concentration and function. Regulatory possibilities include site-specific phosphorylation, epigenetics, alternative splicing, mRNA (re)localization, and modulation of nucleo-cytoplasmic transport. This raises a simple question. Do all the remarkable things cells do, require an intricately choreographed supporting cast of hundreds of molecular machines and associated signaling networks? Alternatively, are there a few simple biophysical principles that can generate apparently very complicated cellular behaviors and functions? I'll discuss two problems, spatial organization of the bacterial chemotaxis system and nucleo-cytoplasmic transport, where the latter might be true. In both cases, the ability to precisely quantify biological organization and function, at the single-molecule level, helped to find signatures of basic biological organizing principles.   

Single-Molecule Analysis of Protein Large-Amplitude Conformational Transitions

Proteins have evolved to harness thermal fluctuations, rather than frustrated by them, to carry out chemical transformations and mechanical work. What are, then, the operation and design principles of protein machines? To frame the problem in a tractable way, several basic questions have been formulated to guide the experimental design: (a) How many conformational states can a protein sample on the functionally important timescale? (b) What are the inter-conversion rates between states? (c) How do ligand binding or interactions with other proteins modulate the motions? (d) What are the structural basis of flexibility and its underlying molecular mechanics? Guided by this framework, we have studied protein tyrosine phosphatase B, PtpB, from M. tuberculosis (a virulence factor of tuberculosis and a potential drug target) and adenylate kinase, AK, from E. coli (a ubiquitous energy-balancing enzyme in cells). These domain movements have been followed in real time on their respective catalytic timescales using high-resolution single-molecule F\"{o}rster resonance energy transfer (FRET) spectroscopy. It is shown quantitatively that both PtpB and AK are capable of dynamically sampling two distinct states that correlate well with those observed by x-ray crystallography. Integrating these microscopic dynamics into macroscopic kinetics allows us to place the experimentally measured free-energy landscape in the context of enzymatic turnovers.   

Unfolding proteins with mechanical forces: From toy models to atomistic simulations

The remarkable combination of strength and toughness, displayed by certain biological materials (e.g. spider silk) and often unmatched by artificial materials, is believed to originate from the mechanical response of individual load-bearing protein domains. Single-molecule pulling experiments carried out during the last decade showed that those proteins, when loaded, respond in a non-equilibrium fashion and can dissipate large amounts of energy though the breaking of sacrificial bonds. In my talk, I will discuss what structural properties correlate with mechanical strength and toughness at the single-molecule level, how thermodynamic stability is related to the mechanical stability, and why both atomistic simulations and simple models seem to fail to reconcile the mechanical responses of the same proteins measured under varied loading regimes. I will further discuss whether it is easier to unfold a protein mechanically by pulling at its ends or by threading it through a narrow pore. The latter process is believed to commonly occur in living organisms as an intermediate step in protein degradation.   

Non-equilibrium microrheology of living cells

Intracellular stresses generated by molecular motors can actively modify cytoskeletal network and change intracellular mechanical properties. We study the out-of-equilibrium microrheology in living cells using endogenous organelle particles as probes. This paper reports measurements of the intracellular mechanical properties using passive, particle-tracking and active, optical tweezers-based microrheology approaches. Using arguments based on the fluctuation-dissipation theorem, we compared the results from both approaches to distinguish thermal and non-thermal mechanical fluctuations in living cells.   

Dissecting the heterogeneity of gene expressions in mouse embryonic stem cells

A population of genetically identical cells, of the same nominal cell type, and cultured in the same petri dish, will nevertheless often exhibit varying patterns of gene expression. Taking mouse embryonic stem (ES) cells as a model system, we use immunofluorescence and flow cytometry to examine in detail the distribution of expression levels for various transcription factors key to the maintenance of the ES cell identity. We find the population-level distribution of many proteins, once rescaled by the average expression level, have very similar shapes. This suggest the largest component of observed heterogeneity comes from a single source. More subtly, we find the expression many of genes appears to modulate with the cell cycle. This may suggest that the program for maintaining ES cell identity is tightly coupled to the cell cycle machinery.   

Synchronization of Cell Cycle Oscillator by Multi-pulse Chemical Perturbations

Oscillators underlie biological rhythms in various organisms and provide a timekeeping mechanism. Cell cycle oscillator, for example, controls the progression of cell cycle stage and drives cyclic reproduction in both prokaryotes and eukaryotes. The understanding of the underlying nonlinear regulatory network allows experimental design of external perturbations to interact and control cell cycle oscillation. We have previously demonstrated in experiment and in simulation that the cell cycle coherence of a model bacterium can be progressively tuned by the level of a histidine kinase. Here, we present our recent effort to synchronize the division of a population of bacterium cells by external pulsatile chemical perturbations. We were able to synchronize the cell population by phase-locking approach: the external oscillator (i.e. periodic perturbation) entrains the internal cell cycle oscillator which is in analogous to the phase-locking of circadian clock to external light/dark oscillator. We explored the ranges of frequencies for two external oscillators of different amplitudes where phase-locking occurred. To our surprise, non-periodic chemical perturbations could also cause synchronization of a cell population, suggesting a Markovian cell cycle oscillation dynamics.   

Analysis of Cell Cycle Phase Response Captures the Synchronization Phenomena and Reveals a Novel Cell Cycle Network Topology

Cell cycle progression requires a succession of temporally-regulated sub-processes, including chromosome replication and cell division, which are each controlled by their own regulatory modules. The modular design of cell cycle regulatory network allows robust environmental responses and evolutionary adaptations. It is emerging that some of the cell cycle modules involve their own autonomous periodic dynamics. As a consequence, the realization of robust coordination among these modules becomes challenging since each module could potentially run out of sync. We believe that an insight into this puzzle resides in the coupling between the contributing regulatory modules. Here, we measured the phase response curve (PRC) of the cell cycle oscillator by driving the expression of a master regulator of the cell cycle in a pulsatile manner and measuring the single cell phase response. We constructed a return map that quantitatively explains the synchronization phenomena that were caused by periodic chemical perturbation. To capture the measured phase response, we derived a minimalist coupled oscillator model that generalizes the basic topology of the cell cycle network. This diode-like coupling suggests that the cell is engineered to ensure complete coordination of constituent events with the cell cycle.   

Swimming Response of Individual Paramecia to Variable Forces

Experiments demonstrate that swimming paramecia exhibit a negative force-kinetic response. In particular, upward swimming paramecia exert a stronger propulsive force as they fight their tendency to sediment. This response is remarkable because it suggests that paramecia can sense forces as small as their apparent weight, which is less than 100 pN. We are investigating the origins of this response by applying variable magnetic forces to individual swimming paramecia and measuring how their swimming trajectories change. We conduct the experiments at the National High Magnetic Field Laboratory where it is possible to achieve forces sufficient to stall the swimmers. We will present our latest data on how paramecia adjust the geometry of their helical trajectories under varying forces.   

Session Y39: Focus Session: Imaging and Interfaces in Energy Science

Imaging Interfacial Structure and Reactivity with X-ray Reflectivity and Microscopy

A fundamental understanding of interfacial reactions is best achieved with ability to observe the systems of interest directly, ideally with molecular-scale resolution and/or sensitivities. X-ray-based approaches offer broad opportunities for probing complex interfaces in environments (e.g., liquids) that are normally inaccessible. I will describe two complimentary approaches for imaging interfaces. The first, X-ray reflection interface microscopy (XRIM), uses the weak interface-reflected X-ray beam to image laterally heterogeneous interfacial structures and processes using a full-field imaging approach. This approach incorporates all of the sensitivities of X-ray reflectivity (XR, including sensitivity to interfacial topography, structure and composition) as potential contrast mechanisms. Recent applications of XRIM will be described, including the abilities to observe: elementary surface topography (i.e., 6.5 {\AA}-high steps) with $\sim $100 nm spatial resolution; interfacial reactivity; and liquid-solid interfaces, in-situ. A second, complementary, approach images the vertical distributions of element-specific sub-structures at an interface through the use of resonant dispersion at X-ray energies close to element's absorption edge (resonant anomalous X-ray reflectivity, RAXR). Recent applications of RAXR will be described including the ability to image element-specific distributions (i.e., ions near a charged liquid-solid interface) and its sensitivity for probing oxidation state specific structures at interfaces. The use of these techniques to observe charge transport at interfaces with respect to energy-related processes will be discussed.   

Small-Pore Molecular Sieves SAPO-34 with Chabazite Structure: Theoretical Study of Silicon Incorporation and Interrelated Catalytic Activity

The catalytic conversion of methonal to olefin (MTO) has attracted attention both in industrial and academic fields. Strong evidence shows that small-pore molecular sieves with certain amount silicon incorporated (SAPO) present promising high catalytic activity in MTO conversion. Using DFT, we study the structural and electronic properties of chabazite SAPO-34. Although there are extensively experimental results show that silicon incorporation does not change the overall structure as the original AlPO structure, local structural changes are still created by silicon substitution, which probably accounted for the high catalytic activity. It is noted that the catalytic activity of SAPO-34 presents increasing trend along with the silicon incorporation amount increasing and maintain a flat peak even with more silicon incorporated. Hence, there is an optimal silicon incorporation amount which possibly yields the highest catalytic MTO conversion.   

Nanosecond Scanning Tunneling Microscopy: resolving spin dynamics at the atomic scale

With the advent of nanoelectronics, functional electronic elements advance towards atomic dimensions and analysis techniques need to keep pace. Scanning tunneling microscopes (STM) have evolved into standard tools to measure the static electronic properties of nanostructures, molecules and atoms. Here we show how the STM can be used to access the equally important dynamical properties on time scales ranging from pico- to nanoseconds. We combine inelastic electron tunneling spectroscopy (IETS) with an all-electronic pump-probe measurement scheme and record the dynamical evolution of magnetic atoms on surfaces in the time domain [1]. We focus on the dynamics of electron spin relaxation in transition metal atoms placed onto a copper nitride decoupling layer on Cu(100). On this surface Fe atoms experience large magneto-crystalline anisotropy [2] that enables long spin lifetimes. At the same time the quantum mechanical nature of the discrete spin states allows for an additional path of spin relaxation: quantum tunneling of magnetization. We probe the dynamic behavior associated with this process and find that placing a Cu atom close to a Fe atom boosts the uniaxial anisotropy energy and creates a long-lived spin state with relaxation times in excess of 200 ns. The ability to probe individual nanostructures with atomic spatial and nanosecond temporal resolution opens a new avenue to explore spin dynamics and other dynamical phenomena on the intrinsic length scale of the underlying interactions. \\[4pt] [1] S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 329 1628 (2010). \\[0pt] [2] C.F. Hirjibehedin, et al., Science 317, 1199 (2007).   

Tracking Oxygen Vacancies in Thin Film SOFC Cathodes

Oxygen vacancies have been proposed to control the rate of the oxygen reduction reaction and ionic transport in complex oxides used as solid oxide fuel cell (SOFC) cathodes [1,2]. In this study oxygen vacancies were tracked, both dynamically and statically, with the combined use of scanned probe microscopy (SPM) and scanning transmission electron microscopy (STEM). Epitaxial films of La$_{0.8}$Sr$_{0.2}$CoO$_{3}$ (LSC$_{113})_{ }$and LSC$_{113}$/LaSrCoO$_{4 }$(LSC$_{214})$ on a GDC/YSZ substrate were studied, where the latter showed increased electrocatalytic activity at moderate temperature. At atomic resolution, high angle annular dark field STEM micrographs revealed vacancy ordering in LSC$_{113}$ as evidenced by lattice parameter modulation and EELS studies. The evolution of oxygen vacancy concentration and ordering with applied bias and the effects of bias cycling on the SOFC cathode performance will be discussed.   

First principles study of GaN(10\underline{1}0)/Water interface

GaN/ZnO alloy semiconductors have been shown to be promising materials to serve as photo-anode in photocatalytical fuel cells. In recent study by Shen et al\footnote{X. Shen, Y.A. Small, J. Wang, P.B. Allen, M.V. Fernandez-Serra, M.S. Hybertsen and J.T. Muckerman \textit{J. Phys. Chem. C }\textbf{114(32)}, 13695 (2010)}, the non polar GaN(10\underline{1}0) surface has been studied with atomistic modeling and a sequence of intermediate steps for the water oxidation process at the interface are proposed. Here we present a first principles molecular dynamics study of the GaN(10\underline{1}0)/Water interface. We found dissociation events happen within 1ps and we show a detailed analysis of the changes in structure and dynamics of water molecules interacting with a dissociating wet surface. The complex hydrogen bond network near the surface is also analyzed in detail, including a throughout study of the proton diffusion processes. We perform a detailed analysis of the dynamics of the hole localization. The link between water surface dissociation and quantum efficiency will be discussed.   

Ions at interfaces and their spectroscopic consequences

The affinity of relatively small ions for air-water interfaces challenges our basic understanding of the basic driving forces for solvation. Here I will show that this adsorption is a general phenomenon for ions in polar solvents. Its physical origin lies in a precarious and unexpected balance of strong nonlinear contributions. The statistics of solvent electric fields suggests a key role for interfacial fluctuations. I will also present an intuitive perspective on surface-specific vibrational spectroscopy, and discuss observable signatures for ion adsorption at aqueous interfaces.   

First-Principles Studies of Functionalized Si(111) in Air and in Water

We have investigated structural, electronic and vibrational properties of hydrogen and methyl-terminated Si(111) surfaces both in air and in contact with water, by combining density functional theory and many-body perturbation theory within the GW approximations. The computed surface dipole moments for both H-Si(111) and CH3-Si(111) surfaces were found to be consistent with measured electron affinities (EAs), and can be explain by simple electronegative trends. While GW self-energy corrections greatly improve the absolute values of EAs, the EA difference of the two surfaces remains overestimated by about 0.3 eV. The variations in CH3 frequencies, e.g. the umbrella mode and CH stretching mode, for the surface in air and water are also well reproduced by our calculations. The influence exerted by the adsorption of water molecules on the hydrophobic H-Si(111) and CH3-(111) surfaces, in particular, on the EAs and the surface vibrational frequencies will be discussed and compared with recent experiments.\\[4pt] [1] A. Aliano, Y. Li, G. Cicero and G. Galli, J. Phys. Chem. C 114, 11898 (2010).\\[0pt] [2] Y. Li and G. Galli, Phys. Rev. B 82, 045321 (2010).   

Interaction between surfaces with ionizable sites

A key factor controlling the interaction between surfaces in aqueous solutions is the surface charge density. Surfaces typically become charged though a titration process where surface groups can become ionized based on their dissociation constant and the pH of the solution. In this work we use a Monte Carlo method to treat this process explicity in a system with two planar surfaces in a salt solution. We find that the surface charge density changes as the surfaces come close to contact due to interactions between the ionizable groups on each surface. In addition, we observe an attraction between the surfaces above a threshold surface charge, in good agreement with previous theoretical predictions based on uniformly charged surfaces. However, close to contact we find the force is significantly different than the uniformly charged case. We also explore the role of salt concentration and the density of the ionizable sites.   

Session Y40: Focus Session: Nanocomposite Physics II-Polymer Dynamics

Particles Bridge the Gap -- Relevance of Polymer Graft Architecture on the Properties of Particle Brush Assemblies

Current interest in the assembly of ligand-coated nanoparticles into 2D and 3D array structures is driven by the opportunities for novel material technologies that derive from the interactions within nanoparticle superlattice structures. A common challenge in the solution-based assembly of particle superlattice structures is the propensity of hard-sphere type particle assemblies to crack formation and brittle fracture during solvent evaporation. Recent progress in controlled radical polymerization offers novel opportunities for polymer-stabilized particle systems (particle brushes) as building blocks of particle superlattice structures. This contribution will discuss synthetic strategies to realize particle brush systems with well defined polymer graft-architecture in the dense or semi-dilute brush regime and discuss the effect of polymer grafting on the structure formation and cohesive interactions in particle brush assemblies. In particular, it will be demonstrated chain entanglements between surface-grafted chains give rise to fracture through polymer-like crazing thus dramatically increasing the toughness and flexibility of the particle assembly. The modulus and toughness of polymer nanocomposites synthesized by self-assembly of particle brush systems will be shown to exceed those of ``conventional'' particle-filled polymer composites -- a result that will be interpreted as a consequence of the particular conformational constraints of surface grafted chains.   

How do Macromolecules Diffuse Through Pathways imposed by Nanoparticles?

Macromolecular motion slows down in crowded biological and engineered systems. Polymer nanocomposites (PNC) containing nanotubes and nanospheres are ideal systems for probing the underlying mechanisms of diffusion in a crowded system. Here, we review the current experimental studies of tracer diffusion in PNC. For silica nanospheres (12nm and 28nm), the normalized diffusion coefficients fall on a master curve when plotted against the interparticle separation divided by the probe size. The entropic barrier model accounts for the reduced diffusion by the loss of chain entropy due to the constrictive bottlenecks between nanoparticles. A new flux-based model depicts confined chains as diffusing along narrow pathways arranged by the NPs. This model captures experimental results while accounting for the distribution of particle separations inherent to real PNC.   

Macromolecular Diffusion in Polymer Nanocomposites

Macromolecular diffusion in crowded systems is important in biological and engineered systems. We have studied macromolecular diffusion through a model polymer nanocomposite (PNC) containing phenyl grafted silica nanoparticles (NPs), randomly distributed in a polystyrene matrix. Over a wide range of NP loading and tracer molecular weight (M), the scaling of the diffusion coefficient with M is in excellent agreement with the entropic barrier model (EBM) previously used to describe diffusion of DNA through confined media (e.g., gels and nanopores). To investigate the effect of NP size, diffusion was measured in PNC's with silica NPs having diameters of 28 and 12 nm. The normalized diffusion coefficients ($D$/$D_{0})$ plotted against the interparticle separation relative to probe size (i.e., \textit{ID}/2$R_{g})$ collapse on a master curve. Diffusion in a poly(methyl methacrylate):silica NP system was also investigated to understand how attractive interactions (i.e., enthalpy) perturb motion relative to the polystyrene and phenyl-silica NP system which is athermal. Finally, a flux-based model is proposed and compared with experimental results.   

Role of Particle -- Polymer Interactions on the Dynamics of Polymer Nanocomposites

Understanding the physics of polymers in the presence of nanoparticle fillers is of crucial importance, since it can lead to the formulation of truly engineered, functional nanocomposites with unique features and broad commercial utilization. The thermomechanical behavior of polymer nanocomposites qualitatively resembles those of polymer films confined to the nanoscale. It has been recently hypothesized that the suppression of physical aging in PMMA/silica nanocomposites is primarily due to hindered mobility of polymer molecules resulting from hydrogen bonding with hydroxyl units on the silica. Further, when solid nanoparticles are dispersed in polymer melts adsorption of polymer chains on the surface of nanoparticles alters the mobility of the chains far into the bulk, and several non-continuum effects are observed. We therefore investigate the effects of polymer-particle interactions on the relaxation dynamics and viscoelastic properties of a model nanocomposite based on a mixture of silica nanoparticles and poly(methyl acrylate). Narrow molecular weight distribution PMA was synthesized using ATRP and mixed with nanoparticles at different concentrations. Alterations in the viscoelastic behavior are attributed to filler structuring and interactions with the host polymer.   

The relationship between the $T_{g}$ depression and the speeding up of physical aging in polystyrene/gold nanocomposites

The effect of gold nanoparticles on the segmental dynamics, glass transition ($T_{g})$ and physical aging of polystyrene (PS) was studied in PS/Gold nanocomposites samples containing 5 and 15 wt.{\%} of 60 nm spherical gold nanoparticles, surface-treated with thiolated-PS. While the segmental dynamics of PS, as assessed by broadband dielectric spectroscopy (BDS), was found to be unchanged in presence of gold nanoparticles, the calorimetric $T_{g}$ of PS was shown to decrease with increasing the amount of nanoparticles in the samples. Furthermore, the physical aging of PS, monitored by measuring the enthalpy relaxation below $T_{g }$ by means of DSC, was shown to speed up with increasing the nanoparticles weight fraction, i.e. the amount of PS/Gold interface in the hybrid material. Thus, the main conclusion of our work is that PS molecular mobility and out-of-equilibrium dynamics are decoupled in these nanocomposites. The significant effect of the amount of PS/Gold interface on both the physical aging rate of PS and the calorimetric $T_{g}$ depression are quantitatively accounted for by a model based on the diffusion of free volume holes towards polymer interfaces, with a diffusion coefficient depending only on the molecular mobility.   

Nanoparticle-directed self-assembly of amphiphilic block-copolymers

The self-assembly of nanoparticles and amphiphilic polymers provides a powerful tool for the fabrication of functional composite materials for a range of applications spanning from nanofabrication to medicine. Here, we present how the incorporation of nanoparticles affects the self-assembly behavior of amphiphilic block-copolymers and how to control the morphology of nanoparticle-encapsulating polymer assemblies. Based on the approach, we have prepared various types of well-defined nanoparticle-encapsulating polymeric nanostructures, including polymersomes packed with magnetic nanoparticles and unique cavity-like quantum dot assembles. We found that the incorporation of nanoparticles drastically affects the self-assembly structure of block-copolymers by modifying the relative volume ratio between the hydrophobic block and the hydrophilic block. In addition, the nanoparticle-polymer and nanoparticle-solvent interactions impact the arrangement and the hybridization of nanoparticles in polymer matrix. These findings should form the basis for the design rules of the self-assembly of nanoparticles and polymer amphiphiles, which will allow one to create new hybrid structures with predesigned morphology and properties. Furthermore, we demonstrated that the morphology of nanoparticle-encapsulating polymer assemblies significantly affects their properties such as magnetic relaxation properties, underscoring the importance of the overall self-assembly structure and the nanoparticle arrangement in polymer matrixes.   

Isothermal Crystallization of Poly(ethylene oxide) / Single Walled Carbon Nanotube Nanocomposites

The isothermal crystallization behavior of poly(ethylene oxide)/single walled carbon nanotubes (PEO/SWNT) nanocomposites were studied with a focus on the \textit{overall crystallization kinetics} and the \textit{morphological} evolution of PEO using differential scanning calorimetry and in-situ small angle x-ray scattering measurements, respectively. The overall crystallization process of the PEO was strongly affected by lithium dodecyl sulfate (LDS) stabilized carbon nanotubes. Further, analysis of the overall crystallization kinetics showed that the PEO chains were topologically constrained by the presence of LDS with an increased energy barrier associated with nucleation and crystal growth, and the nanotubes further act as a barrier to chain transport or enhance the LDS action on the PEO chains. The energy penalty and diffusional barrier to chain transport in the nanocomposites disrupt the PEO crystal helical conformation. This destabilization leads to formation of thinner crystal lamellae and suggests that the crystallization kinetics is primarily controlled by the growth process. This study is particularly interesting considering the suppression of the PEO crystallinity in presence of small amounts of Lithium ion based surfactant and carbon nanotubes.   

MD Simulations of DNA-Programmable Nanoparticle Self-Assembly

Self-assembly through linker mediated hybridization is a powerful technique to control self-assembly at the nanoscale. Recent experiments with complementary ssDNA attached to Au nanoparticles have shown crystallization into BCC and FCC crystals. We give a brief overview of a coarse grained model and present molecular dynamics simulations of the model. We discuss its static and dynamical properties.   

Hierarchical Superstructures from the Self-assembly of Giant Surfactants in Condensed State

Giant surfactants are a class of tadpole-shaped hybrid nanomaterials with a functional nanoparticle as the head group and a polymer chain as the tail, such as perfluorochain-functionalized polyhedral oligomeric silsesquioxane end-capped poly($\varepsilon $-caprolactone) (FPOSS-PCL). The self-assembly of FPOSS-PCL with different composition in bulk were studied using DSC, SAXS, WAXD, and TEM. The compact arranagement of the perfluorochains on the POSS nanoparticles clearly distinguishes them from the polymer chain, leading to the formation of nanophase-separated supramolecular structures such as spheres, cylinders, and bilayered lamellaes. This physical picture is rather unusual and quite reminescent to that observed in the aggregates of small-molecule surfactants. The striking similarity indicates the importance in tuning the interactions to control the hierarchical structure formation in hybrid nanomaterials.   

Session Y41: Elastomers and Gels

Swelling Kinetics of a Microgel Shell

Tanaka's approach to swelling kinetics of a solid gel sphere is extended to a spherical microgel shell. The boundary condition at the inner surface is obtained from the minimization of shear elastic energy. Temporal evolution of a shell is represented in a form of expansion over eigenfunctions of the corresponding diffusion equation. The swelling of Tanaka's solid spherical gel is recovered as a special case of our general solution if the inner radius approaches zero. To test our theoretical model, we prepared monodisperse poly-N-isopropylacrylamide (PNIPAM) hydrogel shells using a microfluidic device. The temporal dependence of the inner and outer radii of the shell was measured and the data was fitted to our theoretical model. As a result, we obtained the collective diffusion constants for shrinking and for swelling processes. The obtained values for microgel shells are in excellent agreement with the previous results obtained for sub-millimeter PNIPAM solid spheres in the same temperature interval. Our model shows that the characteristic swelling time of a gel shell should be proportional to the square of its outer radius---just as with Tanaka's model.   

Indentation of polydimethylsiloxane submerged in organic solvents

This study uses a method based on indentation to characterize a polydimethylsiloxane (PDMS) elastomer submerged in an organic solvent (decane, heptane, pentane, or cyclohexane). An indenter is pressed into a disk of a swollen elastomer to a fixed depth, and the force on the indenter is recorded as a function of time. By examining how the relaxation time scales with the radius of contact, one can differentiate the poroelastic behavior from the viscoelastic behavior. By matching the relaxation curve measured experimentally to that derived from the theory of poroelasticity, one can identify elastic constants and permeability. The measured elastic constants are interpreted within the Flory-Huggins theory. The measured permeabilities indicate that the solvents migrate in PDMS by diffusion, rather than by convection. This work confirms that indentation is a reliable and convenient method to characterize swollen elastomers.   

Synthesis and mechanical properties of resilin-like hydrogels

Resilience measures a material's efficiency for mechanical energy storage. Many materials exhibit high resilience at low strains, but relatively few can maintain this performance at high strain levels. One of the most notable examples of a resilient material is resilin, a protein used strategically when Nature requires elasticity with minimal loss over large deformations. Similar to resilin in many aspects, we present a novel hydrogel network with well-defined architecture by introducing hydrophobic polydimethylsiloxane (PDMS) into hydrophilic polyethylene glycol (PEG)-based network. As a function of the PDMS to PEG ratio, we demonstrate that maximum water content can range from 97{\%} to 80{\%} and Young's modulus from 5kPa to 75kPa. Across this full range of network compositions and water content, the resiliency is nearly 100{\%} for uniaxial strains exceeding 80{\%}. This unique balance of properties is associated with two network attributes: uniformity in network connectivity and negligible secondary structures.   

Pre-Stressed Double Network Elastomers And Hydrogels

A new approach to prepare and characterize pre-stressed double network elastomers and hydrogel systems is investigated. In one example, a styrene-butadiene-styrene (SBS) tri-block copolymer system containing physical cross-links is used to achieve a pre-stressed double network by additional chemical crosslinking in a strained state using ultra-violet (UV) light. Unusual physical and mechanical properties that result from the interactions between each network are presented. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical properties, as well as lower permanent set in both low and high strain regimes along with lower hysteresis. These networks exhibit lower modulus, along with lower coefficient of thermal expansion, still showing lower swelling ratios, which results from a competition of the networks. In another example, a new two-step curing schedule is utilized for Polyacrylamide based hydrogels, where a strain is induced in the middle of curing reaction. The final mechanical properties of these double network hydrogels are studied and compared to both first network and the single network formed without any step strain.   

Topological effects on viscoelasticity of polyacrylamide hydrogels

Viscoelastic behavior of long linear chains in a concentrated solution is governed by the topology of the molecules and interchain excluded volume interaction. As a consequence, chain diffusive motion is significantly retarded and such an assembly of chains exhibits highly pronounced entropy elastic behavior. In this contribution, two types of additional chain confinements imposed on a concentrated solution of linear polyacrylamide (L-PAA) will be discussed. The confinement was realized either by adding silica nano-filler into the concentrated solution of L-PAA or by cross-linking of acrylamide in the concentrated solution of L-PAA. While in the first case the trapped entanglement interaction is caused by interaction of chains with large nano-filler surface, in the second case the L-PAA chains are trapped among the cross-links of the PAA network. Viscoelastic response of both types of composite systems exhibited generic characteristics. In both cases, the trapped entanglement interaction significantly changed the relaxation spectrum of the matrix polymer solution and considerably enhanced the linear elastic modulus.   

Memory Effects in Strained Polymer Networks Caused By Multiple Stages of Crosslinking

Polymer networks crosslinked in multiple strain states usually are analyzed with the independent network model. For networks that undergo scission in addition to crosslinking, however, the networks have been shown not to be completely independent. Even with complete removal of all crosslinks from a given network reacted in a particular strain state, the system still responds as though a portion of the original network remains. This talk will present simulation results of a coarse-grained model that has multiple networks with crosslinking and scission occurring in stages.   

Viscoelastic Properties and Ionic Conductivity of Block Copolymer-Based Ion Gel Electrolytes

The viscoelastic properties and ionic conductivity of block copolymer-based ion gels were investigated with polymer concentrations of 10 -- 50 wt{\%} over a temperature range of 25 -- 200 $^{\circ}$C. Ion gels were prepared through the self-assembly of poly(styrene-$b$-ethylene oxide-$b$-styrene) (SOS) and poly(styrene-$b$-methyl methacrylate-$b$-styrene) (SMS) triblock copolymers in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMI][TFSI]). The S end-blocks associate into micelles, whereas the O and M midblocks are well-solvated by this ionic liquid. Under oscillatory mechanical shear, two relaxation modes have been observed in the SMS ion gels. The faster mode corresponds to the relaxation of the M midblocks in the ionic liquid, while the slow mode reflects motion of the S end blocks within their micellar cores. Comparison of the solid gels and the liquid homopolymer solutions showed that the reduction of ionic conductivity of the gels with respect to that of the solutions is relatively small, and depends primarily on the volume fraction of S micelles.   

Fracture Behavior of High-Toughness, Ionically Cross-linked Triblock Copolymer Hydrogels

Mechanisms for enhancing energy dissipation and hence toughness are important for the generation of robust synthetic soft materials for biomedical applications. Ionic cross-linking in particular has been explored in triblock copolymer hydrogels and affords a remarkable change in mechanical performance comparable to non-cross-linked analogs. Here we employ a physically associated base triblock copolymer network composed of hydrophobic poly(methyl methacrylate) endblocks and a hydrophilic poly(methacrylic acid) midblock capable of complexing with divalent cations. Increases in stiffness and strength have previously been reported, with the extent dependent upon the identity, concentration, and pH of a cross-linking cation solution. We delineate the measured toughness in such systems using tensile tear tests and relate the mechanical performance to a damage zone model reminiscent of loading behavior observed in double network hydrogels.   

Origin of the Toughness and the Elastomeric Properties of Gels from Block Copolymers with Semicrystalline Syndiotactic Polypropylene Blocks

The exceptional toughness and elastomeric properties of gels from block copolymers with semicrystalline syndiotactic polypropylene blocks have been reported. From results obtained from small angle and wide angle X-ray scattering experiments simultaneously performed with step cycle tensile stretching, the toughness can be attributed to the formation, orientation and elongation of crystalline fibrils along the tensile direction. The evolution of the crystalline structure during deformation is confirmed by FTIR measurements and the crystalline morphology is characterized by polarized microscopy imaging. Both polypropylene crystals and the rubbery phase play a role in the elasticity of the gels. Due to the viscoelasticity of the rubbery phase, an increase in the elastic recovery is observed when the gels are allowed to relax at zero load before starting the next cycle.   

Chemomechanical Characterization of Autonomic Polyacrylamide Gels

Autonomic behavior is a distinctive attribute of complex biological systems. Like biological tissue, self-oscillating hydrogels driven by the Belousov-Zhabotinsky (BZ) reaction can convert chemical signals into a mechanical response. Under appropriate conditions BZ gels exhibit sustained mechanical swell-deswell oscillations; and arrays of these gels have the potential to form networks of coupled oscillators. One of the key challenges to developing criteria for device design and assessing practical performance limits of these materials is the need for detailed knowledge of the chemomechanical characteristics of the BZ gels at various states of autonomic behavior. Recently we developed an easily synthesized BZ gel system based on polyacrylamide. Here in, the swell-deswell amplitude, mechanical forces produced during uniform oscillations, and the chemical response to external loads are discussed in context with current poly(N-isopropylacrylamide)-based systems. These studies establish the parameter space leading to robust chemomechanical oscillations and provide an experimental foundation to refine currently available theoretical models to guide the design of autonomic materials and devices.   

Block copolymer photonic crystal gels for mechanochromic sensing

Block copolymer based photonic crystal gels (BCPG) have been previously demonstrated for chemical sensing by taking advantage of dynamic changes in structural color upon interactions with their environment. With their high degree of tunability in structural color and mechanical properties, these materials can function as mechanochromic sensors with the potential application for measuring local mechanical deformation such as cell adhesion and mechanics. In this work, we demonstrate the application of a BCPG for local mechanical sensing by investigating the changes in structural color in response to mechanical deformation. The BCPG consists of a hydrophobic block (polystyrene) -- hydrophilic polyelectrolyte (poly(2-vinyl pyridine)) block copolymer that self-assembles into a one- dimensional periodic lamellar structure and functions as a one dimensional Bragg reflector. Contact adhesion testing is used to measure and relate the changes in structural color of the BCPG films as a function of mechanical deformation. We explore the effects of solvent conditions and applied mechanical deformation in determining the relationships between structural color changes and mechanical strain.   

Theoretically Informed Coarse-Grained Simulations of Polymer Nanogels

Nanoscale finite-sized polymer networks (nanogels) are smart responsive materials that undergo large reversible volume changes with moderate changes in environmental conditions such as temperature, pH, light, and electric field. We develop a coarse-grained model of nanogels in terms of experimentally measurable physical quantities, and perform a theoretically informed Monte Carlo simulation that combines ideas from both the particle and continuum approaches of polymer physics. The elastic interactions are treated through beads connected by harmonic springs (``particles''), and the van der Waals and electrostatic interactions are treated by weighted densities (``fields''). Our simulations predict high degrees of swelling and a discontinuous volume phase transition in ionic nanogels, in contrast to moderate swelling and a continuous volume phase transition for the non-ionic case. We analyze the effects of mesh-size, polymer charge fraction, ionic strength, and solvent quality, on the swelling behavior of nanogels. A comparison is made with the results of a simplified continuum model, where the electrostatic interactions are treated using the Poisson-Boltzmann approximation.   

Session Y42: Focus Session: Dynamics of Polymers--Phenomena due to Confinement--Theory, Wrinkling, and Glass Transitions

Twinkling Fractal Analysis of Confinement Effects on the Glass Transition of Thin Films

The Twinkling Fractal Theory (TFT) of the Glass Transition has recently been verified experimentally [J.F. Stanzione, et al., ``Observing the twinkling fractal nature of the glass transition'', J. Non-Cryst. Solids (2010), doi:10.1016/j.jnoncrysol.2010.06.041] Here we apply the TFT to understand nanoconfinement effects on T$_{g}$ for amorphous thin films of thickness h with free and adhered surfaces. The TFT states that T$_{g}$ occurs when the dynamic clusters percolate rigidity at the rate of testing $\gamma $. The lifetime $\tau $ of these fractal clusters of size R behaves as $\tau \sim $R$^{\delta }$ exp $\Delta $E/kT, where $\delta $=D$_{f}$/d$_{f}$ in which D$_{f}$ is the fractal dimension and d$_{f}$=4/3 is the fracton dimension for the vibrational density of states g($\omega )\sim \omega ^{df}$. The activation energy $\Delta $E = $\beta $[T$^{\ast 2}$-T$_{g}^{2}$] in which $\beta \quad \approx $ 0.3 cal/mol $^{o}$K$^{2}$ and T*$\approx $1.2T$_{g}$. In confined spaces, only clusters of size R$<$h can exist and these have a very fast relaxation time compared to the bulk. Thus, for free surfaces, T$_{g}$ must be dropped at that test rate to percolate rigidity and we obtain the familiar expression T$_{g}$(h)/T$_{g\infty } \quad \approx $ [1-(B/h)$^{\delta }$] where $\delta \approx $1.8 when D$_{f} \quad \approx $2.5 and B is a known constant. For thin films adhered to solid substrates, T$_{g}$ increases in accord with the adhesion energy$\Delta $A as $\Delta $E$\to \Delta $E+$\Delta $A and the adhered cluster lifetime increases. As the rate of testing $\gamma $ increases, the confinement effects diminish as T$_{g}$ increases in accord with T$_{g}(\gamma )$ = T$_{go}$ + (k/2$\beta )$ ln $\gamma $/$\gamma _{o}$.   

Mechanical properties of thin polymer films close to the glass transition: a mesoscale model

Polymer dynamics slows down in the vicinity of a solid substrate (when interactions are sufficiently strong), as can be evidenced experimentally by measuring the glass transition temperature Tg in thin films. We extend here the Long and Lequeux model which quantitatively accounts for this effect. We describe the mechanical properties of the polymers on the scale of dynamical heterogeneities (of a few nanometers). We propose a constitutive relation regarding the local relaxation time, the local stress, and the deformation history. The mechanical equations coupled to these constitutive relations are solved, allowing to reach a scale of a few tens of nanometers and macroscopic time scales. In particular, we measure the elastic modulus G' as a function of temperature, for various films thicknesses. This measurement allows for measuring the glass transition temperature of the film as a function of thickness. The results show that the glass transition temperature is shifted as compared to the bulk (corresponding to large film thickness), depending on the strength of the polymer/substrates interaction, with values which are consistent with experimental results.   

A Simple Approach to Free Volume Transport in Molten/Glassy Material

A key component of microscopic models for the glass transition, in polymer thin films and more generally, is the local dynamics of free volume, which governs what portions of a near-glassy liquid are mobile at a given instant in time. For example, our recent Delayed Glassification (DG) model implements a proposal of de Gennes that segment-sized kinks of free volume may travel from a free surface into a film along polymeric loops or bridges, helping to plasticize material within some accessible distance from the surface. Recently, we have constructed a simple model for `the mobility of mobility', i.e., how local mobility is itself transported through a dense liquid slightly above Tg. Our simple model results in growing cooperativity lengths and intermittency timescales as Tg is approached from above. If time permits, we shall also describe how the model may be adapted to describe the approach to glassy behavior in supported and freestanding films.   

Deviations in mechanical properties of ultrathin polymer films via surface wrinkling

In ultrathin polymer films (h $<$ 100 nm), the measurement of stress relaxation and Young's modulus is a difficult problem due to the delicate nature of such thin films and the lack of appropriate measurement tools for this length scale. Recent work has shown that the Young's modulus of ultrathin glassy polymer films can be measured by a wrinkling-based metrology. Interestingly, the modulus of such thin films was observed to deviate considerably from their bulk counterparts. Building off these observations, we have shown that the rate of structural relaxation and the temperature dependence of the film modulus can also be obtained by following the `relaxation' of strain-induced wrinkling patterns back to their flat equilibrium state. By measuring the decay or relaxation of surface undulations in compressed thin films, we demonstrate that the structural relaxation of the polymer film is highly thickness-dependent and obeys Arrhenius temperature dependence with an activation energy that decreases progressively with decreasing film thickness. This gives rise to an overall broadening of glass transition and to a relatively weak temperature dependence of structural relaxation.   

Calorimetry of Thin Films -- From Single Layer Glass Transitions to Inter-layer Diffusion in Double Layers

Nanocalorimetry allows studying the glass transition in nanometer thin films. One of the striking results of fast scanning (FSC) as well as alternating current (AC) calorimetry is the commonly observed constant Tg in thin films down to a few nm. Blends of polystyrene and poly(phenylene oxide) (PS/PPO) confined in thin films (down to 6 nm) were investigated by AC nanocalorimetry. For this blend, we see even for the thinnest films (6 nm, corresponding to about half of PPO's radius of gyration Rg) only one unchanged glass transition. The good miscibility between PS and PPO remains even in ultrathin films. Finally, we show that our chip calorimeter is sensitive enough to study the inter-layer diffusion in ultrathin films. The PS chains in a 150 nm PS/PPO double layer that is prepared by spin coating PPO and PS thin films in tandem gradually diffuse into the PPO layer when heated above the Tg of PS, forming a PSxPPO100-x blend. However, on top of the PSxPPO100-x blend, there exists a stable pure PS like layer (ca. 30nm in our case) that does not diffuse into the blend beneath even staying at its liquid state over 10 hours.   

$T_{g}$ depression and segmental dynamics of polystyrene thin films

The glass transition temperature ($T_{g})$ of polymer thin films has been a subject of intense debate in the last two decades. (Pseudo)thermodynamic determinations, such as calorimetry and ellipsometry, generally suggest a significant depression of $T_{g}$, whereas the dynamic $T_{g}$, measured by techniques such broadband dielectric spectroscopy and AC-calorimetry directly probing the molecular mobility, is found to be unchanged. The present study provides a resolution to this controversy on polystyrene by showing that the experimental relaxation time obtained from (pseudo)thermodynamic techniques, and the intrinsic molecular relaxation time can be rescaled on a master curve, only accounting for the thickness of the film. Furthermore the thickness and cooling rate dependence of the (pseudo)thermodynamic $T_{g}$ is quantitatively captured by the free volume holes diffusion model. In this framework, the $T_{g}$ depression emerges from the ability of thinner films to maintain equilibrium, due to the shortest distance free volume holes have to diffuse to the polymer interface.   

Evidence for Two Simultaneous Mechanisms Causing Tg Reductions in High Molecular-Weight Free-Standing Films Observed as Dual Glass Transitions More Than 30 K Apart in a Single Film

Glass transition temperature (Tg) changes seen in nanoconfined polymer films have been well documented over the past 15+ years. Supported films exhibit a molecular-weight (MW) independent Tg reduction that manifests itself as a gradient in dynamics emanating from the free surface. Low MW free-standing films show qualitatively the same Tg reduction as supported films, but with the presence of two free surfaces resulting in a Tg reduction that is twice as large for a given film thickness. In contrast, high MW free-standing films exhibit a qualitatively different behavior with a linear reduction in Tg that is MW dependent, potentially described by de Gennes' sliding mode theory. These observations suggest that there may exist two separate mechanisms which can propagate enhanced mobility from the free surface into the film. With ellipsometry measurements over an extended temperature range, we have observed two reduced Tgs more than 30 K apart in individual high MW free-standing polystyrene films suggesting that both mechanisms act simultaneously within a film. These results may explain recent studies on high MW free-standing films using different experimental techniques that contradict the original literature.   

Demonstration of Glass Transition Temperature Depression in Thin Supported Polystyrene Films Using Internal Standard

Clear evidence for glass transition temperature (Tg) depression in $\sim$ 5 nm thick atactic polystyrene (Mw = 212 kg/mol) films supported on silicon substrates is demonstrated by ellipsometry in vacuum [1]. Transition in polystyrene droplets formed by dewetting is used as an internal reference. Both temperature-modulated [2] and linear temperature scanning techniques are utilized; measurements are performed at $10^{-6} - 10^{-8}$ torr residual gas pressure. The method is sensitive enough to observe glass transition in 1 $-$ 2 nm thick supported polystyrene films. Our recent study shows appreciable reduction of Tg in less than 10 $-$ 20 nm thick samples; Tg versus thickness function is found to follow a step-like curve originally reported by [3]. The curve is characterized by moderate (about 17 K) constant Tg depression for thickness less than 7 $-$ 8 nm. References: [1]. M. Y. Efremov, S. S. Soofi, A. V. Kiyanova, C. J. Munoz, P. Burgardt, F. Cerrina, and P. F. Nealey, Rev. Sci. Instrum., 79, 043903 (2008). [2]. M. Y. Efremov, A. V. Kiyanova, and P. F. Nealey, Macromolecules, 41, 5978 (2008). [3] T. Miyazaki, K. Nishida, and T. Kanaya, Phys. Rev. E, 69, 061803 (2004).   

Effect of Packing Density on the Measurement of Glass Transition Temperatures in Thin Film

In the measurement of \textit{Tg} of polymers, a break or jump in some properties is seen at the transition temperature. For bulk polymer, the measurement of \textit{Tg} by different methods has similar result. However, the results reported for thin films have shown quite disagreement among different experimental methods. We used NMR and fluorescence spectroscopy to detect interchain distance and found that the thin film and the freeze-dried polymers show reduced packing densities. And we also found no thickness dependence of \textit{Tg} in thin film and no changes of \textit{Tg} in the freeze-dried polymer measured by calorimetric method or by dynamic mechanical thermal analysis. However the \textit{Tg} in the same samples measured by thermo-mechanical analysis or by positron annihilation lifetime spectroscopy is significantly lower than that in bulk polymers. We argue that the reduction in packing density is a major factor which causes the disagreement among \textit{Tg} measured by different methods for thin films. During the processes of some measurements, an unjamming transition is proposed to take place, which reduces \textit{Tg}.   

The Effect of Molecular Weight on the Glass Transition Temperature of Polymer Thin Films

The thickness dependence of glass transition temperature (Tg) of polymer thin films has attracted considerable attention in both technological and scientific fields. With decreasing polymer film thickness d, the Tg(d) can decrease, increase or remain constant lying on the details of measurement techniques and sample preparation, etc. Using the recently developed differential alternating current chip calorimeter, we directly measured the calorimetric Tg of polystyrene thin films with various molecular weights. We found that when the molecular weight of polystyrene is below the critical chain entanglement, its Tg in a thin film with a thickness of 15 nm can reduce by 20 degrees (compared to bulk sample). However, the Tg of polystyrene film above the entanglement molecular weight remains constant as the film thickness changes. We argue that the molecular weight plays an important role in the thickness dependence of glass transition temperature of polymer thin films.   

Glassy dynamics within surface-bound molecular monolayers

Dynamics within a monolayer collection of surface-bound substituted-alkyl chains are studied with narrow-band dielectric spectroscopy. A transition from independent (intra-molecular) motion in low density systems to complex, glassy (inter-molecular) motion as the density is increased is observed. At high density, both the glassy mode [1,2] and the sub-Tg relaxation [3] have direct analogy to equivalent relaxations in polyethylene. Thus this experimental approach enables observation of the formation of a fragile glass as an explicit function of density; in addition by altering the molecular characteristics and surface arrangement, resultant changes in the nature of the glass transition (its glass transition temperature Tg and fragility m) can be determined. The effects of packing efficiency, chain length, and molecule-molecule interactions, as tuned by altering dipoles within the chain, will be discussed.\\[4pt] [1] M. C. Scott\textit{ etal.}, \textit{ACS Nano} \textbf{2}, 2392 (2008);\\[0pt] [2] M. Beiner, and H. Huth, \textit{Nature Materials} \textbf{2}, 595 (2003);\\[0pt] [3] Q. Zhang\textit{ et al.}, \textit{J. Phys. Chem. B} \textbf{110}, 4924 (2006).   

Interchain Coupling in Thin Polymer Film Studied by Fluorescence Nonradiative Energy Transfer

According to many views, the glass transition temperature (Tg) changes with decreasing polymer film thickness. There is an ongoing debate on the origin of the changes. As an important parameter, however, the interchain distance of thin film was still a challenge. We used Non-radiative energy transfer (NET) method to characterize polymer interchain proximity and association in polymer thin film, by attaching carbazolyl probe (donor) or anthryl probe (accepter) to the side groups of polymethyl methacrylate (PMMA) chain, respectively. We measured the NET results of PMMA films on silicon and found that the NET results decreased with decreasing film thickness. The NET results represented the interchain distance or density. With decreasing film thickness h, the density of the films decreased, which caused an increment of the polymer chain mobility. That might help us to understand the physical nature Tg changes.   

Impact of annealing and adsorption on the distribution of segmental mobility and tracer diffusivity of ultrathin films of polystyrene

We show experimental evidence that the changes ultrathin films undergo during annealing are strongly correlated to the amount of chains irreversibly adsorbed at the interface. A careful analysis of the time evolution of the dielectric function during annealing steps above Tg revealed three different regimes: at times much shorter than the adsorption time, the thickness of the adsorbed layer is constant and the interface mimics the effect of a free surface (packing frustration); upon increase of surface coverage, the films undergo a series of metastable states characterized by the largest changes in the deviations from bulk behavior; finally, when the thicknesses of the irreversibly adsorbed layer doubles its starting value, the system approach a new equilibrium whose properties are fixed by the new interfacial configurations. Our picture is further confirmed by the effect of annealing on the distribution of glass transition temperatures [1], dielectric relaxation strength and tracer diffusivity at different distances from the adsorbing interface. \\[4pt] [1] Rotella, Napolitano et al. Macromolecules, 2010, 43, 8686-8691   

Session Y43: Molecules, Solutions, Networks, & Gels

Distinct Tensile Response of Model Semi-flexible Elastomer Networks

Through coarse-grained molecular modeling, we study how the elastic response strongly depends upon nanostructural heterogeneities in model networks made of semi-flexible chains exhibiting both regular and realistic connectivity. Idealized regular polymer networks have been shown to display a peculiar elastic response similar to that of super-tough natural materials (e.g., organic adhesives inside abalone shells). We investigate the impact of chain stiffness, and the effect of including tri-block copolymer chains, on the network's topology and elastic response. We find in some systems a dual tensile response: a liquid-like behavior at small deformations, and a distinct saw-tooth shaped stress-strain curve at moderate to large deformations. Additionally, stiffer regular networks exhibit a marked hysteresis over loading-unloading cycles that can be deleted by heating-cooling cycles or by performing deformations along different axes. Furthermore, small variations of chain stiffness may entirely change the nature of the network's tensile response from an entropic to an enthalpic elastic regime, and micro-phase separation of different blocks within elastomer networks may significantly enhance their mechanical strength.   

Elastically tuned defect mode in cholesteric elastomers

We consider an axially elongated cholesteric elastomer having a twist defect. We show that its localized mode can be mechanically tuned, and the scaling of the inverse relative line width can be largely enhanced when the values of the deformation and shape anisotropy are near the pseudo isotropic curve. This choice causes a tremendous variation in the behavior of the photon dwell time in the defect mode, which then grows linearly versus the sample thickness. The shift of the defect wavelength, the reflection band width, and the angle between the electric and magnetic fields are also calculated.   

Glassy structure and thermal fluctuations of amorphous nematogenic solids

Amorphous nematogenic solids (ANS) are media comprising rod-like nematogens that have been randomly linked to form macroscopic, elastically deformable networks. Classes of ANS include chemical nematogen gels (i.e., networks of small molecules) and liquid crystalline elastomers (built from crosslinked nematogen-containing macromolecules), as well as biophysical networks, such as those composed of actin filaments. One common feature of these systems is that the linking process introduces into them a new type of random field, consisting of a conventional static part along with a new, thermal-fluctuation-induced, dynamic part. We develop a phenomenological model of ANS which shows how this composite random field, together with the coupling between the orientational and positional fluctuation that nematogens exhibit, leads to the occurrence of decaying but also oscillatory correlations of the thermal fluctuation, and also shows how these correlations influence the glassy structure of ANS.   

Thermal and Mechanical Properties of Sequential and Simultaneous Thiol-Ene-Isocyanate Networks

Ternary networks containing having stoichiometrically balanced thiol /(ene+isocyanate) ranging from 0 to 20 mol% isocyanate were synthesized via sequential or simultaneous thiol/ene and thiol/isocyanate click reactions. The effects of cross-link density were studied using three thiols, GDMP (difunctional), 3T (trifunctional) and 4T (tetrafunctional) respectively. TEA catalyzes the isocyanate-thiol coupling and chain extension, while the photoinitiator DMPA initiates a radical thiol-ene crosslinking process. Real-time FTIR was used to study kinetics of both light and dark reactions utilizing thiol, ene and isocyanate peaks which appear independently. It was found that difunctional thiols and isocyanates reacted initially, forming chain extended prepolymers end-capped with thiol functionalities. Upon UV irradiation, thiol functionalized prepolymers reacted with TTT, a trifunctional ene, forming networks containing incorporated thiourethane linkages. Initial DSC results indicated higher Tgs for higher cross-linked networks; however, isocyanate content has significant effects on each system. Films were also be thermally characterized via DMA and mechanical properties measured using MTS.   

Computational and Experimental Investigation of Morphology of Polymer Gels

Thermo-reversible polymer gels based on block copolymers represent a remarkable class of materials for a wide range of applications. An efficient approach to control and modify the properties of these gels is to use multicomponent mixtures of self-assembling block copolymers differing in architecture, length and chemical nature. As a result of microphase separation, ``mixed'' or ``pure'' micelles, containing block copolymers of the same or different types, are developed. Here, we present a dissipative particle dynamics (DPD) study of the morphology of a binary mixture of AB/ABA block copolymers differing in length of the insoluble blocks in B-selective solvent. We have observed numerous morphologies of AB/ABA blends, which are characterized by formation of pure and mixed micelles of various compositions, structures and sizes. We have discovered that changing the copolymer ratio and processing conditions impacts morphology of these blends. Finally, we have established factors that affect an intermicellar distance and a bridge fraction which ultimately determines the mechanical properties of the gels. Results of our computations were compared with our experimental findings based on atomic force microscopy and the other experimental and theoretical studies and demonstrated a good agreement.   

Theory of volume phase transitions of polyelectrolyte gels

We will present theoretical results for the effect of charge regularization accompanying volume phase transitions of polyelectrolyte gels. Our theoretical formulation of the cascade effect that couples the effective degree of ionization and the polymer density leads to significant deviations from the classical Flory-type theories.   

Plasticity and slow relaxation phenomena in reinforced rubbers

Elastomers reinforced with nanometric solid particles or aggregates exhibit remarkable properties: temperature dependent reinforcement of the modulus in the linear regime, non linear effects, irreversibility and hysteretic effects. Important progress has been achieved recently in modeling these properties, based on glassy layers around filler particles. In some cases, reinforcement as a function of temperature and filler volume fraction was explained quantitatively. We shall focus here on the plasticity and related slow relaxation phenomena which occur in these systems. We show that the amplitude of plasticity is correlated to the reinforcement amplitude, and that plasticity relaxes with a very broad distribution of relaxation times (similar to an ageing phenomenon), in the same way as the stress relaxes at high strain amplitude. These experimental observations of long time evolution are well described by the mesoscale model for reinforced rubbers that we have proposed.   

Migration of chemical additives in a rubber under UV irradiation

The evolution of the chemical composition of a rubber, in particular that of its surface, is governed by several factors including temperature, oxidation and migration of additives. Oxidation mechanisms alone do not account for all the phenomena observed, for example the appearance of deposits on the surface. We have studied the effects of temperature and photo-oxidation on the migration of chemical additives on a rubber surface. The morphological and chemical evolution were followed by AFM and XPS, respectively. 3D reconstruction with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) combined with the AFM and XPS results enabled us to establish a relation between the oxidizing degradation of the rubber and the surface migration of the additives. This finding is supported by a kinetic study of the surface evolution.   

Probing the sliding interactions between bundled actin filaments

Assemblies of filamentous biopolymers are hierarchical materials in which the properties of the overall assemblage are determined by structure and interactions between constituent particles at all hierarchical levels. For example, the overall bending rigidity of a two bundled filaments greatly depends on the bending rigidity of, and the adhesion strength between individual filaments. However, another property of importance is the ability for the filaments to slide freely against one another. Everyday experience indicates that it is much easier to bend a stack of papers in which individual sheets freely slide past each other than the same stack of papers in which all the sheets are irreversibly glued together. Similarly, in filamentous structures the ability for local re-arrangement is of the utmost importance in determining the properties of the structures observed. In order to study this phenomenon we create bundles of biopolymers by inducing attractive interactions between actin filaments via the depletion mechanism. We find that bundles of actin filaments to do not slide freely across one another. In order to characterize these sliding interactions, we perform active experiments using laser tweezers to pull one filament across the other at constant velocity.   

Apparent Yield Stress and Interfacial Viscoelasticity of Globular Protein Solutions

Globular proteins influence the dynamics, phase behavior and transport of biomolecules and drugs in the mammalian body. In conventional rheological studies conducted on torsional rheometers, protein solutions are commonly reported to have a solid-like response at concentrations as low as 0.03{\%} by weight. In this study, we probe the bulk and interfacial viscoelasticity of bovine serum albumin (BSA) solutions as a canonical example of a globular protein system. Using a stress-controlled rotational rheometer, augmented by microfluidic rheometry and interfacial rheometry, we demonstrate that the origin of the yield-like response reported in bulk viscometric flows lies in the formation of a film of adsorbed protein, formed spontaneously at the solution/gas interface. We directly measure the concentration-dependent interfacial viscoelasticity of the adsorbed protein and we describe a coherent means of extracting the interfacial contribution from bulk viscosity measurements. Finally, we demonstrate how the presence of surfactants changes both the interfacial and bulk rheology of pharmaceutical formulations based on protein solutions.   

The enhanced bubble formation in short dsDNA loops

Recent experiments have shown the dsDNAs readily bend and loop over the nanometer scale much shorter than its persistence length (50 nm). Motivated by this, we study possibility of enhanced bubble formation in short dsDNA loops by evaluating free energy of bubble formation analytically, and also by simulating the breathing DNA model. We analyze the bubble size distribution and the average bubble size as a function of the loop length, which are compared with those of the linear DNA of the same length. 1) T. E. Cloutier and J Widom, Mol. Cell \textbf{14}, 355 (2004). 2) P. A. Wiggins \textit{et al}., Nat. Nanotechnol. 1, 137 (2006). 3) O. Lee, J. H. Jeon and W. Sung, Phys. Rev. E 81, 021906 (2010)   

Universality in the timescales of internal loop formation in unfolded proteins and single-stranded oligonucleotides

Understanding the rate at which various parts of a molecular chain come together to facilitate the folding of a biopolymer (e.g., a protein) into its functional form remains an elusive goal. Here we use experiments, simulations, and theory to study the kinetics of internal loop closure in disordered biopolymers such as single-stranded DNA and unfolded proteins. We present theoretical arguments and computer simulation data to show that the relationship between the timescale of internal loop formation and the positions of the monomers enclosing the loop can be recast in a form of a universal master dependence. Our measurements of the loop closure times in single-stranded oligonucleotides, as well the internal loop closure kinetics in unfolded proteins reported by others, are all well described by this theoretically predicted dependence. Experimental deviations from the master dependence can then be used as a sensitive probe of dynamical and structural order in unfolded proteins and other biopolymers.   

Topological interactions between ring polymers

The detailed topological and entropic forces between loops still remain elusive. We have quantitatively determined the potential of mean force between the centers of mass of two ring polymers, i.e. loops. We find that the transition from a linear to a ring polymer induces a strong increase in the entropic repulsion between these two polymers. On top, topological interactions such as the non-catenation constraint further reduce the number of accessible conformations of close-by ring polymers by about 50 percent, resulting in an additional effective repulsion.   

Modulation of DNA condensation by cation valence

Aggrecan is a negatively charged bottlebrush-shaped proteoglycan in the extracellular matrix, with unique polyelectrolyte properties. Aggrecan-hyaluronic acid aggregates are responsible for the compressive resilience of articular cartilage. Unlike linear polyelectrolytes such as DNA, aggrecan is insensitive to the presence of multivalent counterions (e.g., calcium ions) and self-assembles into micro-gels in near-physiological salt solutions. These features are preserved by aggrecan adsorbed on mica surfaces. To probe both the nature of aggrecan assemblies in solution and their surface interactions, we image the aggrecan assemblies adsorbed on mica surface using Atomic Force Microscopy The effect of counterion valence on the hydration-dehydration properties of the aggrecan assemblies will be discussed.   

Combining DNA Nanotechnology and Fluorescence Polarization Microscopy to Determine the Orientation of DNA-bound Fluorophores

We describe a technique to measure the axis of the transition dipole moment of a fluorophore bound to dsDNA and compare results with existing techniques. We use DNA nanotubes to present the dsDNA in a known orientation and query a variety of intercalating (e.g., YO-YO, TO-TO), groove-binding (e.g. DAPI) or covalently linked (e.g., Fluorescein, Cy3, Cy5) dyes. A de S\'{e}narmont prism in front of the camera generates simultaneous images of fluorescence polarized perpendicular and parallel to the DNA nanotube axis, allowing for ratio measurements that are insensitive to bleaching. We suggest the use of technique to detect helical supertwist, and possibly other nanoscale structural features, of DNA nanostructures.   

Session Y44: Focus Session: Organic Electronics and Photonics -- Exciton and charge separation physics

Charge transport studies in organic semiconductors using carrier extraction by linearly increasing voltage (CELIV) technique

Organic optoelectronic devices, such as solar cells, light emitting diodes and transistors, share a common feature: their performance critically depends on the efficiency with which charge carriers (electrons and/or holes) move in the material. Understanding and improving the charge transport is the main goal when improving the device performance or designing novel organic compounds through chemical engineering. Due to low carrier mobility in disordered films, as well as due to its time, electric field and carrier density dependence, standard measurement technique like Hall effect and Time-of-Flight are either inapplicable or limited in applicability. Charge Carrier Extraction by Linearly Increasing Voltage (CELIV) technique has become a world standard used by many scientific groups to measure charge transport and recombination in inorganic and organic semiconductors. The method can be used to study the charge carrier mobility dependence on time, carrier concentration, electric field, temperature, film thickness and morphology directly in the operational devices. However, the latest research have shown that CELIV current transients and extraction maximum used for mobility evaluation is strongly dependent on experimental conditions such as carrier density, light absorption profile and electric field. Procedure, allowing estimating the correction factor in mobility relation will be presented. In contrast to inorganic crystalline semiconductors, the long-range disorder in the films of organic devices makes the charge transport properties strongly dependent on the degree of disorder and nanomorphology of the films. Carrier density, electric field and temperature dependent mobility in disordered organic semiconductors is shown to obey Arrhenius-type, Poole-Frenkel-type, Meyer-Neldel rule, and Gill's law. Stochastic transport theories are used to describe charge carrier hopping within localized Density-Of-States as opposed to delocalized band-transport in the crystals.   

Direct determination of energy level alignment of organic-organic bulk heterojunction: cases of the P3HT:PCBM and P3HT:FLN-i blend

Using photoemission spectroscopy (UPS{\&}IPES) combined near edge X-ray absorption fine structure method, we have determined the surface compositions and electronic alignments of the blend films comprising poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester. Given the fact that the surface of the blend film is a nearly pure P3HT wetting layer, we use a lift-off method to access the originally buried surface, which is rich in both P3HT and PCBM and thus representative of the BHJ. We show that the donor/acceptor LUMO-HOMO gap is 1.46 eV, implying a 0.5-0.6 eV interface dipole barrier between the two materials. As far as we know, this is the first report of the direct determination of electronic structure of the blend. The combined measurement and lift-off method are standard and can be applied to other organic blend films, like P3HT and FLN-i.   

Direct Determination of Energy Level Alignment and Charge Transport at Metal/Alq$_3$ Interfaces via Ballistic-Electron-Emission Spectroscopy (BEES)

In organic electronic devices, the difference between the electrode work function and the organic lowest unoccupied molecular orbital (LUMO) or highest occupied molecular orbital (HOMO) is a crucial parameter in determining the nature of charge transport. However, experimental determination of LUMO is challenging.\footnote{ J. C. Scott, J. Vac. Sci. Tech, A {\bf21}, 521 (2003).} For the archetypal electroluminescent organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq$_3$), various techniques gave significantly different HOMO-LUMO gap values.\footnote{I. H. Campbell, D. L. Smith, Appl. Phys. Lett. {\bf74}, 561 (1999); I. G. Hill \emph{et al.} Chem. Phys. Lett. {\bf327}, 181 (2000); S. F. Alvarado \emph{et al.} IBM J. Res. Dev. {\bf45}, 89 (2001).} Using BEES, we directly determined the energy barrier for electron injection at clean interfaces of Alq$_3$ with Al and Fe to be 2.1 eV and 2.2 eV, respectively. We quantitatively modeled the sub-barrier BEES spectra with an accumulated space charge layer, and found that the transport of non-ballistic electrons is consistent with random hopping over the injection barrier. Supported by U.S. DOE Office of Science Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Band alignment optimization of bulk rrP3HT/C$_{60}$ heterojunction

Organic solar cells could overcome the cost limitation of traditional solar cells by using large-scale fabrication techniques associated with polymers. However, a better understanding of the bulk heterojunctions (BHJs) electronic properties used in these devices is required to reach an efficiency of 10\%. DFT and GW computations were used to study BHJs formed by the inclusion of C$_{60}$ in a regioregular poly(3- hexylthiophene) polymer (rrP3HT) crystal. An increasing packing density in the BHJ extent the energy separation between the C$_{60}$-LUMO and the rrP3HT-HOMO, which is proportional to the open circuit voltage of the device (Voc). This trend is consistent with the induced dipole moment variation observed at a pentacene- C$_{60}$ junction upon reduction of the intermolecular distance [1]. In contrast, an increasing size of rrP3HT crystal domain leads to decrease both Voc and rrP3HT bandgap, in a similar fashion than upon the formation of rrP3HT crystallite along the annealing of BHJs [2]. \\[4pt] [1] M. Linares, et al., J. Phys. Chem. C, 114 (2010), 3215.\\[0pt] [2] G. Dennler, et al., Adv. Mater., 21 (2009), 1323.   

Strong interface p-doping and band bending in C$_{60}$

C$_{60}$ is a strongly n-type material with its lowest unoccupied molecular orbital very close to the Fermi level, and p-doping C$_{60}$ has been a challenging issue. We measured the electronic energy level evolution of C$_{60}$ on molybdenum oxide (MoO$_{x})$/ conducting indium tin oxide (ITO) interfaces with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). We found that MoO$_{x}$ strongly p-dopedC$_{60}$at the interface, resulting in an inversion layer in C$_{60}$. The energy levels of C$_{60}$relax gradually as the thickness of C$_{60}$ increases, and the band bending region is observed to be greater than 400 {\AA} in C$_{60}$. The root mean square (RMS) roughness measured with AFM of 581 {\AA} thick C$_{60}$ film was 68 {\AA}, slightly increased from that of the ITO substrate of 55 {\AA}. We have also investigated the effect of exposing the MoO$_{x}$ air, and found that it eliminated the doping effect.   

Understanding the role of interfaces in small molecule organic photovoltaics

This abstract not available.   

Photophysics of Poly(3-dodecylthinylenevinylene) with Controlled Regioregularity

Poly(thienylene vinylene) (PTV) is a conductive polymer with potential applications for use in photovoltaics owing to its low-band gap, good hole-mobility and low oxidation potential. It is generally considered a non-luminescent material and reports suggest its emissive properties are highly dependent upon the excitation, conjugation length, alkyl side group and regio-regularity, complicating the interpretation of the non-radiative decay routes for photo-generated excitations. Better understanding of this behavior could explain the low efficiencies so far observed in PTV based solar cells and lead to improved performance. We have studied photoluminescence of Poly(3-dodedyl -2,5-thienylene vinylene) as a function of regio-regularity in thin films and solutions. By varying the excitation, temperature, and also by controlling the morphology through the use of different solvents, concentrations, and film preparation techniques, we hope to deduce the physical mechanisms competing with radiative recombination. Complimentary characterization of films through XRD and electro-absorption yield detailed information about the semi-crystalline structure and electronic levels, respectively.   

Magneto-Optical Studies of Internal Photovoltaic Processes in Organic Solar Cells

It has been found that exciton dissociation inevitably forms electron-hole pairs, namely charge-transfer (CT) complexes, at donor-acceptor interfaces due to Coulomb attraction in organic solar cells. In particular, the dissociation of CT complexes is a critical process that is accountable for the generation of photocurrent. However, it is a challenging issue to study the CT complexes formed at donor-acceptor interfaces. Here, we use magneto-optical measurements: magnetic field effects of photocurrent (MFE$_{PC})$ and light-assisted dielectric response (LADR) as effective experimental tools to experimentally examine the formation of CT complexes and the related photovoltaic processes. Our studies reveal that internal electrical drifting and local Coulomb interaction can largely change the binding energy and dissociation probability of CT complexes through intrinsic electrical polarization in donor-acceptor interpenetrating network. This experimental finding indicates that intrinsic electrical polarization plays an important role in controlling charge dissociation, transport, and collection in organic solar cells.   

Photoluminescence Influenced by Chain Conformation in Thin Conjugated Polymer Films by Spin Coating and Dewetting

Motivated by recent observations of photoluminescence (PL) enhancement by molecular constraints, the chain conformation effect was explored. It was found that PL efficiency decreased with film thickness under a constant spin speed but increased under a constant solution concentration indicating that prolonged solvent evaporation, and hence more open entangled coils, improved PL efficiencies. Strong substrate dependence was observed in the ultrathin regime, revealing the role of substrate-polymer interactions during the condensation process. Upon annealing, the thin film dewetted and resulted multi-fold PL enhancement. As revealed by micro-PL spectroscopy, the PL efficiency was about 10 times greater in the residual layer ($\sim $3 nm) than that in the droplets and demonstrated independence of substrate quenching effect, unveiling important optoelectronic features of the molecular constrained states.   

Session Y45: Atom Interactions with Molecules, Surfaces, and X-Rays

Dissipative Effects on Quantum Sticking

Using variational mean-field theory, many-body dissipative effects on the threshold law for quantum sticking and reflection of neutral particles are examined. For the case of an ohmic bosonic bath, we study the effects of the infrared divergence on the probability of sticking and obtain an analytic expression for the rate of sticking as an asymptotic expansion in the incident energy $E$. The low-energy threshold law for quantum sticking is found to be robust with respect to many-body effects and remains a universal scaling law to leading order in $E$. Non-universal many-body effects alter the coefficient of the rate law and the exponent of a subdominant term.   

Breaking Quantum Mirrors with Thermal Fluctuations

We study ultracold atoms interacting with a surface at finite temperature. For the case where the surface is out of thermal equilibrium with the environment, the asymptotic form of the Casimir-Polder potential decays as an inverse square law and can be either attractive or repulsive, depending on the temperature difference. We analyze the effect of this interaction on the threshold law for quantum sticking, the probability that an atom will stick to the surface $s(E)$ as the incident energy tends to zero. We predict a new threshold law for neutral atoms interacting with a surface out of thermal equilibrium with its environment: $s(E)\sim E^\gamma$ as $E\to 0$ where $\gamma$ ($0\le\gamma\le 1/2$) depends on the strength of the non-equilibrium Casimir-Polder interaction which can be tuned with temperature.   

Temperature dependence of the depolarization rates of Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms induced by He atom collisions

Our theoretical depolarization rates for the disalignment, disorientation, and alignment relaxation of Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms at temperatures between 10 K and 3000 K are compared with various experiments. We perform quantum close-coupling many-channel calculations using a new model potential for the interaction between Ne$^{\ast }$(2p$_{i}$ [J=1]) and He atoms [1]. We analyze isotropic collisions in a gaseous mixture at thermal equilibrium, and find excellent agreement between our calculations and the experimental data above 77 K [1, 2]. We explain the temperature dependence of the depolarization rates using the anisotropy of the collisional channels [2]. For T $<$ 77 K, our disalignment rates for the Ne$^{\ast }$(2p$_{2}$ [J=1]) and Ne$^{\ast }$(2p$_{10}$ [J=1]) atoms are larger than the experimental data. The experiment predicts a linear variation of the intra-multiplet cross sections to zero-energy. Our calculations indicate that for the 2p$_{2}$ and 2p$_{10}$ states, at low collision energies, the nuclear rotation at large atomic separation has a stronger influence in the molecular Hamiltonian than the electrostatic interaction. This situation does not occur for the 2p$_{5}$ and 2p$_{7}$ states, where the agreement between theory and experiment is found even at 20K [1]. [1] Bahrim C and Khadilkar V 2009 \textit{Phys Rev A} \textbf{79} 042715. [2] Khadilkar V and Bahrim C 2010 \textit{J Phys B }\textbf{43 }(in press).   

Dipole Transitions for the hydrogen molecule using Fully Nonadiabatic Wavefunctions

Using variational Monte Carlo and simple, explicitly-correlated fully-nonadiabatic wavefunctions we have computed highly accurate trial wavefunctions for the lowest rovibrational state of several states of the hydrogen molecule. With these wavefunctions we have calculated the transition moments for all possible dipole transitions and we compare our results with those from more traditional calculations.   

Formation of the negative molecular ion MH- by radiative association of a neutral molecule M with H-

We consider the formation of negative molecular ions MH$^{-}$ through the reaction of radiative association: M+H$^{-} \quad \to $ MH$^{-}$ + ?$\omega $, where M is a diatomic or triatomic neutral molecule. We present a theoretical approach to calculate the cross-section and the rate constant for the reaction and apply the theory to study formation of molecular ions from H$^{-}$ and neutral molecules abundant in the interstellar medium (ISM): We consider H$_{2}$, CO, and H$_{2}$O as possible candidates to form negative ions. Such ions have never been observed in the ISM. Their eventual observation would serve as a proof of presence of H$^{-}$ in the ISM too. The H$^{-}$ ion cannot be detected directly by the photoabsorption spectroscopy. Supported by Triangle de la Physique contract QCCM and the National Science Foundation grant PHY-0855622   

Terahertz Time-Domain Spectroscopy of Ices of N$_{2}$, CO$_{2}$, and Ar

We used Terahertz Time-Domain Spectroscopy (THz- TDS) to study thin ice films of N$_{2}$, CO$_{2}$, and Ar from 0.1 -1.6THz. We observed an absorption line for N$_{2}$ ice films at 1.46THz in the temperature range of 10-28K. Ar ice films have absorption lines at 0.47THz and 0.97THz in the temperature range of 10-30K. We observed no absorption line for CO$_{2}$ ice films in the temperature range of 10-40K from 0.1-1.6THz. These results will be helpful in analyzing the data terms from observations of THz radiation from astronomical sources impinging upon interstellar materials.   

Collision-Induced Infrared Absorption by Collisional Complexes in dense Hydrogen-Helium gas mixtures at Thousands of Kelvin

The interaction-induced absorption by collisional pairs of H$_{2}$ molecules is an important opacity source in the atmospheres of the outer planets and cool stars. The emission spectra of cool white dwarf stars differ significantly in the infrared from the expected blackbody spectra of their cores, which is largely due to absorption by collisional H$_{2}$--H$_{2}$, H$_{2}$--He, and H$_{2}$--H complexes in the stellar atmospheres. Using quantum-chemical methods we compute the atmospheric absorption from hundreds to thousands of kelvin [1]. Laboratory measurements of interaction-induced absorption spectra by H$_{2}$ pairs exist only at room temperature and below. We show that our results reproduce these measurements closely [1], so that our computational data permit reliable modeling of stellar atmosphere opacities even for the higher temperatures [1]. \\[4pt] [1] Xiaoping Li, Katharine L. C. Hunt, Fei Wang, Martin Abel, and Lothar Frommhold, ``Collision-Induced Infrared Absorption by Molecular Hydrogen Pairs at Thousands of Kelvin'', International Journal of Spectroscopy, vol. 2010, Article ID 371201, 11 pages, 2010. doi: 10.1155/2010/371201   

Size dependent ionization dynamics of argon clusters in intense x-ray pulses

Free Electron Lasers open the door for novel experiments in many science areas ranging from ultrafast chemical dynamics to single shot imaging of molecules. For the success of virtually all experiments with free electron lasers a detailed understanding of the light - matter interaction in the x-ray regime is pivotal. The Linac Coherent Light Source (LCLS) free electron laser in Stanford allows for the first time to study innershell ionization dynamics of intense x-ray pulses on a femtosecond time scale. We performed experiments on the ionization dynamics of Argon clusters at different pulse length using the slotted spoiler foil in the second LCLS bunch compressor [1]. The Auger rate of argon clusters is predicted to be size dependent and lower than in atoms due to delocalization of the valence electrons [2]. We observe a dependence of the ionization dynamics on pulse length and cluster size. The results are discussed and also compared to recent atomic and molecular data from LCLS.\\[4pt] [1] P. Emma et al. PRL 92, 074801 (2004)\\[0pt] [2] U. Saalmann, JM Rost PRL 89, 14 (2002)   

{\it Ab initio} calculations of atomic coherence excited by optical pulses: CEP effects and generation of X-ray radiation

Recent progress in ultrashort, e.g. attosecond, laser technology allows to obtain ultra-strong fields which can be of the same order of magnitude as the electric field created by an atomic nucleus. Interaction of such strong and broadband field with atomic systems even under the action of a far-off resonance strong pulse of laser radiation should be revisited. As we have shown, such pulses can excite remarkable coherence on high frequency transitions. We have found and analyzed analitical solutions for various pulse shapes. We have developed new mechanisms of efficient atomic coherent excitation by using two-frequency laser pulses and via tunneling through electric fields. We have done {\it ab initio} calculations using TDDFT for several atoms and simple molecules interacting with strong optical fields. We compare efficiency generation with the efficiency of high harmonic generation approach, and discuss the CEP effects and possible applications of the results obtained to cooperative generation of XUV radiation. The efficiency of XUV generation is calculated for particular candidates for XUV radiation such as H (100 nm) and He (50 nm) atoms and H-like ions (Li$^{2+}$ (30 nm), as well as Ar$^{8+}$ and Xe$^{8+}$ (30-50 nm).   

XAS measurements at LCLS: Investigating Electronic Damage at an X-Ray FEL

As X-ray FEL sources such as the LCLS ramp up scientific studies, the damage caused by the intense x-ray pulses has become a central question. X-ray FEL investigations of solid-state materials must consider the change in the electronic system during the x-ray pulse, in contrast to proposed biomolecular imaging experiments which must suppress atomic motion.\footnote{Neutze, R. et. al. Nature 406, 752 (2000).} The potential electronic damage to the system is also amplified in many materials investigations which probe absorption edges. Therefore, a key need of all studies involving materials research with X-ray FELs is to mitigate or overcome the electronic damage when probing the system. We report the first x-ray absorption spectroscopy (XAS) results from LCLS, which show significant line shape changes dependent on the fluence and x-ray pulse length. We employ a technique previously developed at FLASH which also allows us to visualize the beam dispersion.\footnote{Bernstein, D.P. et al. Appl. Phys. Lett. 95, 134102 (2009).} Our spectroscopy results from LCLS demonstrate a safe fluence and pulse length regime at which material investigations can be conducted without perturbing the ground state of the system during the probing x-ray pulse.   

Berry phase-like effect near DOS singularity in continuum models coupled with discrete states

Threshold effects in a continuum model (cut-off frequency in a waveguide or the band edge in tight-binding chains) may significantly modify the single-particle discrete eigenvalue spectrum resulting from coupled discrete states. Focusing on tight-binding chains as an example we reveal a Berry phase-like effect as the system parameters are adiabatically varied about certain exceptional points (non-analytic points in the eigenvalue spectrum) that are related to the threshold (van Hove) singularity in the density of states. We show that this effect is related to the form of the eigenvalue expansion in the vicinity of the band edge. In particular, for a semi-infinite model with a side-coupled impurity the eigenvalues in this vicinity may be expanded in powers of the coupling $g$, rather than the more usual $g^{2}$. In another example, in the case of an infinite tight-binding chain with a side-coupled impurity (or a two-level atom traveling in an infinite waveguide) the DOS singularity results in a $g^{4/3}$ amplification of the decay width of the resonant state [1, 2]. \\[4pt] [1] Phys. Rev. B \textbf{73}, 115340 (2006). \\[0pt] [2] Phys. Rev. Lett \textbf{94}, 043601 (2005).   

A Novel Geometric Effect of the Sunbeam (NGES) and Geometric Spin Hall Effect of Light (GSHEL) due to the Earth Rotation

Recently there are reports of NGES\footnote{S. B. Nam, arXiv \textbf{0910.5767} (2009).} based on geometrical optics, and GSHEL\footnote{A. Aiello, et al., Phys. Rev. Lett. \textbf{103}, 100401 (2009).} based on electrodynamics in vacuum. Here, I discuss the nature of NGES, using the Berry notion of the classical parallel transport,\footnote{M. V. Berry, ``Geometric Phases in Physics'' (Ed. by A. Shapere and F. Wilczek, Singapore, 1989) \textbf{pp7-28}.} and present GSHEL due to the earth rotation. For both NGES and GSHEL, the observing frame (detecting plane) should be tilted with respect to the light beam propagation direction. Setups to detect simultaneously both GNES and GSHEL are discussed. Descriptions given here are applicable to any beams such as electronic and atomic beams.   

Observation of a Passive PT Phase Transition

In 1998 Bender and Boettcher discovered that the spectrum of a system with PT-symmetric Hamiltonian can still be entirely real. This subject attracts more and more attention during the last few years. One of the intriguing characteristics of PT-symmetric systems is the possibility of a\textit{ phase transition} beyond which the spectrum ceases to be entirely real. This symmetry breaking occurs suddenly once the imaginary component of the potential exceeds a certain critical level. Here we report the observation of a phase transition in a passive PT-symmetric optical structure once the loss exceeds a certain critical value. This counterintuitive loss-enhanced transmission is purely an outcome of a spontaneous PT symmetry breaking.