Session M1: Invited Session: Tunable, Intense, Coherent THz Emission From a High Temperature Superconductor

Hot Spot and THz Wave Generation in Bi$_2$Sr$_2$CaCu$_2$O$_8$ Intrinsic Josephson Junction Stacks

Stacks of intrinsic Josephson junctions made of the high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ have been shown to emit coherent radiation at THz frequencies [1]. Emission is observed both in a low bias regime and a high bias regime. While at low bias the temperature of the stack is close to the bath temperature, at high bias a hot spot and a standing wave, formed in the ``cold'' part of the stack, coexist [2-5]. THz radiation is very stable in this regime, exhibiting a linewidth which is much smaller than expected from a purely cavity-induced synchronization mechanism [6]. We investigate the interaction of hot spots and THz waves using a combination of transport measurement, direct electromagnetic wave detection and low temperature scanning laser microscopy (LTSLM). In this talk recent developments will be presented, with a focus on the mechanism of hot spot formation.\\[4pt] In collaboration with B. Gross, S. Gu\'{e}non, M. Y. Li, J. Yuan, N. Kinev, J. Li, A. Ishii, K. Hirata, T. Hatano, R. G. Mints, D. Koelle, V. P. Koshelets, H. B. Wang and P. H. Wu.\\[4pt] [1] L. Ozyuzer, et al., Science \textbf{318}, 1291 (2007).\\[0pt] [2] H.~B. Wang, et al., Phys. Rev. Lett. \textbf{102}, 017006 (2009).\\[0pt] [3] H. B. Wang, et al., Phys. Rev. Lett. \textbf{105}, 057002 (2010).\\[0pt] [4] S. Guenon, et al, Phys. Rev B \textbf{82}, 214506 (2010).\\[0pt] [5] B. Gross, et al., Phys. Rev. B \textbf{86}, 094524 (2012).\\[0pt] [6] M. Y. Li, et al., Phys. Rev. B \textbf{86}, 060505 (R) (2012).   

Towards practical applications of powerful and widely-tunable THz sources made of layered superconductors

Terahertz (THz) emission from intrinsic Josephson junction stacks made of high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ have been obtained both in a low bias and a high bias regime [1, 2]. While at low bias the temperature distribution in the stack is almost homogeneous, at high bias an over-heated part (hot spot area) and a cold part of the sample coexist [2, 3]. Previous resolution-limited measurements indicated that the linewidth $\Delta f$ of THz emission may be below 1 GHz, showing no difference between two regimes. In this talk, we report on measurements of the linewidth of THz radiation using a Nb/AlN/NbN integrated receiver for detection [4]. While at low bias we found $\Delta f$ to be not smaller than $\sim$500 MHz, at high bias $\Delta f$ turned out to be as narrow as a few MHz. We attribute this to the hot spot acting as a synchronizing element. Also thanks to the variable size of the hot spot and the temperature rise due to the self-heating, the emission frequency can be tuned over a wide range of up to 500 GHz. Last but not least, the emission power was measured to be above 25 $\mu$W. All these properties imply that THz sources made of layered cuprate superconductors can be employed for practical applications.\\[4pt] [1] L. Ozyuzer, et al., Science 318, 1291 (2007).\\[0pt] [2] H. B. Wang, et al., Phys. Rev. Lett. 105, 057002 (2010).\\[0pt] [3] S. Gu\'enon, et al., Phys. Rev. B 82, 214506 (2010).\\[0pt] [4] M. Y. Li, et al., Phys. Rev. B.86, 060505(R) (2012).   

THz Radiation from Mesas of Intrinsic Josephson Junction of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ under Extreme Thermal Inhomogeneity

After the discovery of intense, coherent and continuous electromagnetic waves at terahertz frequencies (THz waves) in 2007,\footnote{L. Ozyuzer et al.,Science 318 (2007) 1291, Kadowaki et al., Physica C468 (2008) 634.} a number of experimental and theoretical works have been carried out to understand the THz radiation phenomena from mesa structure of layered high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (hereafter abbreviated as Bi2212). At present after five year intensive studies, the basic mechanism of the THz wave emission can be understood by two principles: one is the ac-Josephson effect working in-between individual intrinsic Josephson junctions in the mesa of Bi2212 and the other is the cavity resonance effect associated with both the geometrical shape and the electromagnetic properties of the mesa structures of Bi2212. However, the precise conditions to obtain strong THz radiation are not yet established well at the stage of mesa fabrication.\footnote{M. Tsujimoto et al., PRL 108 (2012) 107006.} Moreover, it appears that our recent results of measurement of the inhomogeneous temperature distribution due to the hot-spot formation producing gigantic Joule heat in the mesa may give us much more complicated situations to understand physics of the THz radiation.\footnote{H. minami et al., submitted to PRL.} In this talk based on the experimental results we will provide a unified picture of the THz radiation phenomena in spite of highly nonequilibrium thermal condition, which hopefully will give us a hint to improve the performance and the efficiency of the emission power exceeding 1 mW from a single mesa structure. This will be also useful for various applications.   

Modelling the coherent THz radiation from Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesas of various geometries

Mesa structures of the high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ have been prepared in a variety of ways. Groove mesas have so far been made in rectangular, square, circular, triangular, and pentagonal shapes. There are distinct differences in the properties of the radiation that depend strongly on the type and shape of the mesas. Two types of experiments have provided information regarding the mechanism of the coherent radiation: Angular distribution studies and frequency spectrum measurements. In analyzing the angular distribution measurements, we used the Love equivalence principles to model the radiation as arising from two effective sources: the uniform $ac$ Josephson current source, and the radiation from the excitation of an EM cavity mode, and modelled the substrate by a simple image model. We generally found the fractions of the output from these two sources to be comparable in magnitude, implying that the quality factor $Q$ of the EM cavity is very low, allowing for a high degree of output frequency tunability. The largest tunability observed to date from the outer current-voltage characteristic branch was found for an acute isosceles triangular mesa shape. In several geometries, radiation was observed at frequencies far from EM cavity mode frequencies.   

Intrinsic line shape of electromagnetic radiation from a stack of intrinsic Josephson junctions synchronized by an internal cavity resonance

Stacks of intrinsic Josephson-junctions are realized in mesas fabricated out of layered superconducting single crystals, such as Bi$_2$Sr$_2$CaCu$_2$O$_8$ (BSCCO). Synchronization of phase oscillations in different junctions can be facilitated by the coupling to the internal cavity mode leading to powerful and coherent electromagnetic radiation in the terahertz frequency range. An important characteristic of this radiation is the shape of the emission line. A finite line width appears due to different noise sources leading to phase diffusion. We investigated the intrinsic line shape caused by the thermal noise for a mesa fabricated on the top of a BSCCO single crystal. In the ideal case of fully synchronized stack the finite line width is coming from two main contributions, the quasiparticle-current noise inside the mesa and the fluctuating radiation in the base crystal. We compute both contributions and conclude that for realistic mesa's parameters the second mechanism typically dominates. The role of the cavity quality factor in the emission line spectrum is clarified. Analytical results were verified by numerical simulations. In real mesa structures part of the stack may not be synchronized and chaotic dynamics of unsynchronized junctions may determine the real line width.   

Session M2: Invited Session: Interaction-Driven Quantum Hall States in Graphene

Unconventional Sequence of Fractional Quantum Hall States in Graphene

Electronic compressibility is a powerful tool for the study of correlated electron phases in two-dimensional electron systems. Using a scanning single-electron transistor, we have measured the local electronic compressibility of suspended graphene in the quantum Hall regime. The local nature of the measurement technique allows us to probe exceptionally clean regions of graphene, revealing delicate many-body effects that are obscured by disorder in global transport studies. In this talk, I will review recent measurements of the fractional quantum Hall effect (FQHE) in graphene. We observe a multitude of FQH states that follow the standard composite fermion sequence between $\nu =$ 0 and 1, but only occur at even-numerator fractions between $\nu =$ 1 and 2, suggesting that an underlying symmetry remains. Moreover, we observe a series of phase transitions in the FQH states between $\nu =$ 0 and 1 that are marked by a decreased energy gap and a narrow region of negative compressibility that cuts across the FQH state. We use a simple model based on crossing composite fermion Landau levels with different internal degrees of freedom to reproduce much of the experimental behavior. Our results provide insight into the interplay between electron-electron interactions and the spin and valley symmetries of graphene.   

Phase diagram and edge excitations of the $\nu=0$ quantum Hall state in graphene

The interaction-induced broken-symmetry incompressible quantum Hall states in graphene at integer and fractional filling factors have by now been firmly established in transport and compressibility measurements. However, identifying their precise nature (e.g., how the symmetry is broken) still remains a tough challenge: on the experimental side, transport and compressibility probes do not provide direct information about the physical order; on the theoretical side, the presence of additional to spin discrete degrees of freedom, valleys, results in a variety of competing phases in this multicomponent system. As the prime example of this rich behavior, I will present a generic phase diagram for the intriguing $\nu=0$ state, obtained within the framework of quantum Hall ``ferromagnetism.'' The diagram consists of the canted antiferromagnetic, ferromagnetic, charge-density-wave (charge-layer-polarized), and Kekul\'{e} (interlayer-coherent) phases in monolayer (bilayer). I will then discuss the edge excitations of the $\nu=0$ state. Remarkably, the edge excitations are nonuniversal (e.g., can be gapped or gapless) and crucially depend on which phase is realized in the bulk of the system. Besides being of considerable theoretical interest, these unprecedented properties simultaneously allow one to infer about the nature of the phases from the transport experiments. I will present arguments based on this analysis and existing data why the insulating $\nu=0$ state realized in real bilayer (and possibly, monolayer) graphene is likely to be canted antiferromagnetic. Finally, I will mention how this theoretical framework can be generalized to fractional quantum Hall states in graphene, which could shed light on some of the puzzling features of the recent experiments.   

Tunable electron interactions and robust non-Abelian quantum Hall states in graphene and other Dirac materials

Discovery of the fractional quantum Hall effect inspired a concept of quasiparticles with non-Abelian exchange statistics. However, a major limitation for experimental studies of non-Abelian quasiparticles in traditional GaAs-based 2d systems is their lack of tunability: the effective electron interactions in such systems are fixed at values which make non-Abelian states either absent of very fragile. Therefore it is desirable to find alternative, tunable 2d systems that host robust non-Abelian quantum Hall states. In this talk, we will discuss the phase diagram of fractional quantum Hall states in recently discovered 2d Dirac materials (graphene, bilayer graphene, topological insulators). We will show that the effective interactions in these materials can be naturally tuned in a broad range, in contrast to GaAs. This tunability is achieved by external fields that control the mass gap of Dirac fermions. Alternatively, the effective interactions can be controlled by engineering the dielectric environment of the 2d Dirac electron gas. We will demonstrate that the tunability of interactions in Dirac materials allows one to stabilize non-Abelian states, as well as to drive phase transitions between various correlated phases (quantum Hall states, Fermi-liquid-like states, and states with broken translational symmetry) in a controlled manner. Connecting to experiments, we will argue that a very promising candidate material for tuning interactions and stabilizing non-Abelian states is bilayer graphene, where the gap can be naturally controlled by perpendicular electric field. Our study provides a realistic route towards engineering robust fractional and non-Abelian quantum Hall states in graphene and other Dirac materials. \\[4pt] [1] Z. Papic, R. Thomale, D. A. Abanin,Phys. Rev. Lett. 107, 176602 (2011).\\[0pt] [2] Z. Papic, D. A. Abanin, Y. Barlas, R. N. Bhatt, Phys. Rev. B 84, 241306(R) (2011).\\[0pt] [3] D. Abanin, Z. Papic, Y. Barlas, R. N. Bhatt, New J. Phys. 14, 025009 (2012).   

Superconducting states in graphene

In spite of the remarkable electronic properties of graphene, which include the existence of massless Dirac quasiparticles, the low density of states near the Dirac points seems to conspire against the formation of new many body ground states. In this context, the search for intriscic superconductivity in graphene has involved either combining graphene with other materials [1], or else exploring ways to modify the electronic density of states at the Fermi level. In this talk, after discussing the classification of symmetry states in the honeycomb lattice and analysing the general thermodynamic properties for Dirac fermion superconductors [2], I will describe a few promissing mechanisms to induce superconductivity in graphene. In particular, I will show that in the situation where strain effects lead to a reconstruction of the vacuum into a discrete spectrum of Landau levels due to pseudo magnetic fields, which preserve overall time reversal symmetry, superconductivity is quantum critical at integer filling of the Landau levels, when the system is incompressible. At partial filling, the quenching of the kinetic energy due to the Landau levels leads to a crossover to a non-Fermi liquid regime, where the critical temperature scales linearly with the coupling in the weak coupling limit. I will show that the critical temperature can be orders of magnitude larger than in conventional weak coupling superconductors, and may be triggered by phonons.\\[4pt] [1] B. Uchoa, A. H. Castro Neto, Physical Review Letters 98, 146801 (2007);\\[0pt] [2] V. N. Kotov, B. Uchoa et al., Reviews of Modern Physics 84, 1067 (2012).   

Session M3: Invited Session: Novel Quantum Phases in Artificial Lattices and Networks

Mott-Hubbard Physics in a Patterned GaAs Heterostructure with Honeycomb Topology

This talk considers efforts directed towards the design and exploration of novel collective electron states in artificial lattice structures that are realized in semiconductor heterostructures by nanofabrication methods. These studies reveal striking interplays between electron interactions and geometrical constraints (topology). We focus on the honeycomb topology, or ``artificial graphene'' (AG) [1,2], that supports Dirac fermions. Dirac fermions and the emergence of quantum phases, such as spin liquids and topologically protected states, can be studied by highly demanding inelastic light scattering methods and by electrical transport at low temperatures [3,4]. In particular, we probed the excitation spectrum of electrons in the honeycomb lattice in a magnetic field identifying collective modes that emerged from the Coulomb interaction [4], as predicted by the Mott-Hubbard model [5]. These observations allow us to determine the Hubbard gap and suggest the existence of a Coulomb-driven ground state [4]. Studies of electrons confined to artificial lattices should provide key perspectives on strong electron correlation in condensed matter science. \\[4pt] [1] M. Gibertini et al. Phys. Rev. B RC 79, 241406 (2009)\\[0pt] [2] C.H. Park and S.G. Louie, Nano Lett. 9, 1793 (2009).\\[0pt] [3] G. De Simoni et al. Appl. Phys. Lett. 97, 132113 (2010)\\[0pt] [4] A. Singha et al. Science 332, 1176 (2011)\\[0pt] [5] J. Hubbard. Proc. R. Soc. Lond. A 281, 401 (1964)   

Dirac Fermions in a Nanopatterned Two-Dimensional Electron Gas

If a lateral periodic potential with triangular (or honeycomb) lattice symmetry is applied to a conventional two-dimensional electron gas (2DEG), the charge carriers behave like massless Dirac ferions [1,2]. A very interesting and useful point of these newly-generated massless Dirac fermions is that, unlike the case of graphene, their properties can be tuned through the external periodic potential. In this presentation, I will review the electronic properties of those newly-generated massless Dirac fermions in an artificial 2DEG superlattice system and will discuss how the elecctronic structure of those massless Dirac fermions changes depending on the external periodic potential [3]. \\[4pt] [1] C.-H. Park and S. G. Louie, Nano Lett. 9, 1793 (2009).\\[0pt] [2] M. Gibertini et al., Phys. Rev. B 79, 241406 (2009).\\[0pt] [3] C.-H. Park et al., in preparation.   

Designer Dirac Fermions, Topological Phases, and Gauge Fields in Molecular Graphene

The observation of massless Dirac fermions in monolayer graphene has propelled a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Using low-temperature scanning tunneling microscopy and spectroscopy, we show the emergence of Dirac fermions in a fully tunable condensed-matter system---molecular graphene---assembled via atomic manipulation of a conventional two-dimensional electron system in a surface state. We embed, image, and tune the symmetries underlying the two-dimensional Dirac equation into these electrons by sculpting the surface potential with manipulated molecules. By distorting the effective electron hopping parameters into a Kekul\'e pattern, we find that these natively massless Dirac particles can be endowed with a tunable mass engendered by the associated scalar gauge field, in analogy to the Higgs field. With altered symmetry and texturing of the assembled lattices, the Dirac fermions can be dressed with gauge electric or magnetic fields such that the carriers believe they are in real fields and condense into the corresponding ground state, as confirmed by tunneling spectroscopy. Using these techniques we ultimately fabricate a quantum Hall state without breaking time-reversal symmetry, in which electrons quantize in a gauge magnetic field ramped to 60 Tesla with zero applied laboratory field. We show that these and other chiral states now possible to realize have direct analogues in topological insulators, and can be used to guide or confine charge in nontrivial ways [1]. \break\break [1] Kenjiro K. Gomes, Warren Mar, Wonhee Ko, Francisco Guinea, and Hari C. Manoharan, ``Designer Dirac Fermions and Topological Phases in Molecular Graphene,'' Nature \textbf{483}, 306--310 (2012).   

Electron-electron interactions in artificial graphene

Recent advances in the creation and modulation of graphenelike systems are introducing a science of ``designer Dirac materials.'' In its original definition, artificial graphene is a man-made nanostructure that consists of identical potential wells (quantum dots) arranged in an adjustable honeycomb lattice in the two-dimensional electron gas. As our ability to control the quality of artificial graphene samples improves, so grows the need for an accurate theory of its electronic properties, including the effects of electron-electron interactions. Here we determine those effects on the band structure and on the emergence of Dirac points, and discuss future investigations and challenges in this field.   

Quantum Simulation with Circuit QED

Superconducting circuits and circuit quantum electrodynamics provide an excellent toolbox for non-equilibrium quantum simulation. In circuit QED, the strong interaction of light with a single qubit can lead to strong qubit-mediated photon-photon interactions. Recent theoretical proposals have predicted phase transitions in arrays of these cavities, demonstrating that complex matter-like phenomena can emerge with such interacting photons. Due to inevitable photon dissipation and the ease of adding photons through driving, these systems are fundamentally open and a useful tool for studying non-equilibrium physics. I will discuss recent experimental and theoretical progress towards realization of these non-equilibrium quantum simulators. I will focus on a localization-delocalization crossover in a pair of coupled cavities, and discuss preliminary measurements of large cavity arrays. I will discuss a variety of available measurements in these systems, including transport, photon number statistics, and a scanned local quantum probe.   

Session M4: Invited Session: Quantum Simulation with Photons

High Orbital Exciton-Polariton Condensates in Two-Dimensional Lattices

Microcavity exciton-polaritons are hybrid quantum quasi-particles as admixtures of cavity photons and quantum-well excitons. The inherent light-matter duality provides experimental advantages to undergo a phase change to condensation at high temperatures (e.g. 4-10 K in GaAs and room temperatures in GaN materials) due to the extremely light effective mass and stimulated scattering processes, and the dynamical nature in the open-dissipative condition allows us to control orbital symmetries of condensates. We have engineered two-dimensional polariton-lattice systems for the investigation of exotic quantum phase order arising from high orbital bands. Via photoluminescence signals in both real and momentum coordinates, we have observed $d$-orbital meta-stable condensation, vortex-antivortex phase order, linear Dirac dispersion, and flattened band structures in square, honeycomb, triangular and kagome lattices respectively. We envision that the polariton-lattice systems will be promising solid-state quantum emulators in the quest for understanding strongly correlated materials and in the development of novel optoelectronic devices.   

Quantum Hall physics with light

Quantum Hall physics provides a variety of novel phenomena in both the integer and fractional domain, with applications in metrology, technology, and quantum computation. I will discuss implementing quantum Hall physics with optical systems by means of synthetic gauge fields and photon-photon interactions. First, in the integer quantum Hall regime, I consider our theoretical and experimental efforts using established photonics technology to see expected phenomena, such as edge states of light. I will then consider the nonlinear regime, where photon-photon interactions via optical or microwave nonlinearities enable the potential realization of fractional quantum Hall states, and indicate challenges and solutions for examining pumped, non-equilibrium systems that do not admit a mean-field description. Finally, potential applications of these ideas in passive and active photonics will be examined.   

Many body physics with light

Systems of strongly interacting atoms and photons, which can be realized wiring up individual Cavity QED (CQED) systems into lattices, are perceived as a new platform for quantum simulation [1-3]. While sharing important properties with other systems of interacting quantum particles, the nature of light-matter interaction gives rise to unique features with no analogs in condensed matter or atomic physics setups. Such Lattice CQED systems operate on polaritonic quasi-particles that are hybrids of light and matter in a controllable proportion, combining long-range coherence of photons and strong interactions typically displayed by massive particles. In this talk, I will discuss our recent efforts [4-6] on the possibility of observing quantum many body physics and quantum phase transitions in Lattice CQED systems. Unavoidable photon loss coupled with the ease of feeding in additional photons through continuous external driving renders such lattices open quantum systems [5]. Another key aspect of many body physics with light that I will focus on is the particle number non-conserving nature of the fundamental light-matter interaction [6] and the question of what quantity, if not the chemical potential, can stabilize finite density quantum phases of correlated photons.\\[4pt] [1] M. J. Hartmann, F. G. Brandao, and M. B. Plenio, Laser and Photonics Reviews {\bf 2}, 527 (2008).\\[0pt] [2] A. Tomadin and R. Fazio, JOSA B {\bf 27}, A130 (2010).\\[0pt] [3] A. Houck, H. E. Tureci, and J. Koch, Nature Phys. {\bf 8}, 292 (2012).\\[0pt] [4] S. Schmidt, D. Gerace, A. A. Houck, G. Blatter, and H. E. Tureci, Physical Review B {\bf 82}, 100507 (2010).\\[0pt] [5] F. Nissen, S. Schmidt, M. Biondi, G. Blatter, H. E. Tureci, J. Keeling, Phys. Rev. Lett. {\bf 108}, 233603 (2012).\\[0pt] [6] M. Schiro, M. Bordyuh, B. Oztop, H. E. Tureci, Phys. Rev. Lett. {\bf 109}, 053601 (2012).   

Bose-Einstein condensation of photons

In recent work, we have observed Bose-Einstein condensation (BEC) of a two-dimensional photon gas in an optical microcavity [1]. Here, the transversal motional degrees of freedom of the photons are thermally coupled to the cavity environment by multiple absorption-fluorescence cycles in a dye medium, with the latter serving both as a heat bath and a particle reservoir. The photon energies in this system are found to follow a Bose-Einstein distribution at room temperature. Upon reaching a critical total photon number, a condensation into the transversal ground state of the resonator sets in, while the population of the transversally excited modes roughly saturates. The critical photon number is experimentally verified to agree well with theoretical predictions. Owing to particle exchange between the photon gas and the dye molecules, grandcanonical experimental conditions can approximately be realized in this system. Under these conditions, two markedly different condensate regimes are theoretically expected [2]. On the one hand, this includes a condensate with Poissonian photon number statistics, being the analog to present atomic Bose condensates. Additionally, we predict a second regime with anomalously large condensate fluctuations accompanied by a Bose-Einstein-like photon number distribution that is not observed in present atomic BEC experiments. The crossover between these two regimes, corresponding to the emergence of second-order coherence, depends on the size of the molecular reservoir (e.g. the dye concentration) and is expected to occur at a temperature below the BEC phase transition. In my talk, I will give an update on our experimental work.\\[4pt] [1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, \textit{Nature} \textbf{468}, 545 (2010)\\[0pt] [2] J. Klaers, J. Schmitt, T. Damm, F. Vewinger, and M. Weitz, \textit{Phys. Rev. Lett.} \textbf{108}, 160403 (2012)   

From Mott transitions to interacting relativistic theories with light: A brief history of photonic quantum simulators

I will start by reviewing our early works for observing photon-blockade induced Mott transitions in coupled cavity QED systems [1]. After briefly touching on the idea of simulating spin-models and the Fractional Hall effect [2], I will analyze more recent developments in realizing continuous 1D models in nonlinear optical fibers exhibiting electromagnetically induced transparency nonlinearities. Here the concept of the ``photonic Luttinger liquid'' will be introduced, along with a proposal to observe spin-charge separation with polarized photons in a nonlinear slow light set up [3]. I will continue by presenting our recent efforts in simulating 1D lattice models in the non-relativistic regime, such as the sine-Gordon and Bose-Hubbard [4], and the efforts for simulations of out of equilibrium phenomena using driven systems [5,6]. I will conclude by presenting ongoing work on interacting relativistic models (Thirring)[7]. Possible experimental implementations in quantum optical systems such as photonic crystals, optical fibers coupled to cold atoms, and Circuit QED will be discussed.\\[4pt] [1] D.G. Angelakis, M.F. Santos and S. Bose, Phys. Rev. A \textbf{76}, 031805(R) (2007); D.G. Angelakis, Reports in Progress in Phys., IOP (2012) to appear.\\[0pt] [2] J. Cho, D.G. Angelakis, Phys. Rev. Lett \textbf{101}, 246809 (2008).\\[0pt] [3] D.G. Angelakis, M.-X. Huo, E. Kyoseva and L.C.Kwek, Phys. Rev. Lett. \textbf{106}, 153601 (2011).\\[0pt] [4] M.-X. Huo, D.G. Angelakis, Phys. Rev. A \textbf{85} 023821 (2012)\\[0pt] [5] T. Gruzic, S. R. Clark, D. G. Angelakis. Dieter Jacksh, New Jour,. of Phys. \textbf{14}, 103025 (2012). [6] P. Das, C. Noh, D.G. Angelakis, arXiv:1208.0313.\\[0pt] [7] D.G. Angelakis, M.-X. Huo, D. Chang, L.C. Kwek, V. Korepin arXiv:1207.7272.   

Session M5: Focus Session: Computational Discovery and Design of New Materials: Semiconductors, Molecular Systems and Interfaces

ReaxFF-based molecular dynamics studies on reactions at complex material surfaces

The ReaxFF method provides a highly transferable simulation method for atomistic scale simulations on chemical reactions at the nanosecond and nanometer scale. It combines concepts of bond-order based potentials with a polarizable charge distribution. Since it initial development for hydrocarbons in 2001, we have found this concept to be highly transferable, leading to applications to elements all across the periodic table, including all first row elements, metals, ceramics and ionic materials. In this presentation we will provide an overview of recent developments of the ReaxFF method for reactions at the complex material interfaces, in particular TiO$_2$/water, silica/water and graphite/oxygen interfaces. We will describe the ReaxFF parameter development process and show how, by employing parallel molecular dynamics methods, ReaxFF can assist in bridging the gap between atomistic-scale simulations and experiment. We will also discuss new developments in metadynamics and Monte Carlo based implementations of ReaxFF, which enable us to extend molecular dynamics simulation times to beyond hundreds of nanoseconds.   

Reliable Modeling of Complex Organic/Metal Interfaces

The understanding of electronic properties of complex organic/metal interfaces requires a reliable method for the prediction of their structure and stability. The bonding at complex interfaces arises from delicate balance between covalent bonds, van der Waals (vdW) forces, charge transfer, and Pauli repulsion. We developed a method based on density-functional theory with vdW interactions (PBE+vdW$^{\rm surf}$ [1]) to accurately model adsorbates on surfaces, by a synergetic linkage of the PBE+vdW [2] for intermolecular interactions with the Lifshitz-Zaremba-Kohn theory [3] for the dielectric screening within the substrate surface. This method is demonstrated to reliably model a multitude of molecules on metal surfaces [1,4], leading to an accuracy of 0.1 {\AA} in adsorption heights and 0.1 eV in binding energies wrt experiments. To demonstrate the predictive power of the PBE+vdW$^{\rm surf}$, we design a novel type of single-molecule push button switch, by carefully controlling the stability and activation barrier between a chemically bound state and a physically bound state for benzene derivatives adsorbed on metal surfaces.\\[4pt] [1] Ruiz, \textit{et al.}, PRL (2012).\\[0pt] [2] Tkatchenko and Scheffler, PRL (2009).\\[0pt] [3] Zaremba and Kohn, PRB (1976).\\[0pt] [4] Wagner, \textit{et al.}, PRL (2012).   

A High-Throughput Computational Search for New Transparent Conducting Oxides

Transparent conducting oxides (TCOs) are critical to many technologies from solar cells to electronics. However, finding materials that combine the two antagonistic properties of large conductivity and transparency to the visible light can be extremely challenging. In this talk, we will present a high-throughput screening approach aimed at discovering new high-performance TCOs. Combining different \emph{ab initio} techniques from density functional theory to GW, we evaluated thousands of oxides in terms of essential TCO properties (e.g., band gap and carrier transport). From these results, we will present new interesting compounds as well as discuss the chemistries likely to form high performance TCOs.   

Generation and analysis of the largest ab initio database for metal borides

Boron-based materials have been observed in a remarkable variety of crystal structures with outstanding superconducting, mechanical, and refractory properties. Aiming to provide a systematic description of known compounds and to identify new synthesizable candidate materials, we have generated an extensive ab initio database spanning over 40 binary and ternary metal boride systems at ambient and gigapascal pressures. The considered crystal structures include known prototypes listed in the ICSD as well as brand-new prototypes found with an evolutionary search implemented in MAISE [1]. Having examined over 15,000 entries of calculated formation enthalpies, we find a number of surprising disagreements between theory and experiment regarding the ground state crystal structures and identify over a dozen systems in which novel compounds are expected to form under high pressures. Data mining of the ab initio information has revealed trends in the electronic, magnetic, vibrational, and elastic properties which can help fine-tune the metal boride materials for specific applications. [1] Module for Ab Initio Structure Evolution, \underline {http://maise-guide.org}   

Mechanistic Design of New Materials and Processes through Multifunctional Atomic-Scale Simulations

Multifunctional systems that contain heterogeneous interfaces are ubiquitous in numerous applications, including catalysis, electronic devices, friction, and coatings. Traditionally, computational studies of these complex interfacial systems have relied on methods such as first-principles density functional theory (DFT), because of the difficulty in describing the changes in bonding environment with empirical approaches. Here, empirical, charge optimized many-body (COMB) potentials are used in classical, atomic-scale simulations to examine several model systems that involve heterogeneous material interfaces or surface reactions at size scales that are much larger than are currently tractable with traditional DFT methods. . The COMB potentials allow for dynamic charge transfer between atoms and across interfaces, and are demonstrated to describe metallic, covalent, and ionic bonding across interfaces and at surfaces. The simulations yield mechanistic insights that allow for the design of materials and optimization of process conditions for several applications, including catalysis, thin-film growth, and supported two-dimensional materials with well-defined interfacial interactions.   

Towards Reliable Predictions of Molecular Materials

While dispersion interactions are known to be essential to the stability and accurate prediction of molecular-crystal structures, the vast majority of computational methods use simple pairwise approximations to model these interactions, ignoring the non-additive, many-body nature of long-range electron correlation. Here we use the recently developed many-body dispersion (MBD) method (PRL 108, 236402; PNAS 109, 14791) together with a representative database of molecular crystals, to illustrate how important electrodynamic screening and many-body contributions are to crystal stability. Crucially, these MBD contributions allow DFT calculations to reach the highly coveted ``chemical accuracy'' with respect to high-level calculations and experiments in both the crystalline and gaseous phases. This ability to treat molecular solids and their components on such an accurate and equal footing is essential for controled and informed design of complex materials.   

Property optimization in isovalent and aliovalent semiconductor alloys based on MnO

Materials for solar energy conversion need to fulfill specific targets in regard of the band-structure, optical properties, carrier transport, and doping. In order to design or discover novel materials that satisfy multiple requirements, we employ design principles to select a range of material compositions where those properties are likely to occur, and then evaluate them computationally. Here we are addressing the design of semiconductor alloys based on the d$^{\mathrm{5}}$ oxide MnO, which was recently identified as an interesting base material for semiconducting transition metal oxides [PRB 85, 201202(R)]. To calculate the properties for different alloy compositions with the many-body GW method, we modeled the alloy systems by searching for special quasi-random structures (SQS). In isovalent alloys, the SQS was chosen such that the correlation functions were as close as possible to the ideal random alloy. For aliovalent alloys where strong short-range-ordering is expected, the target correlation functions for the SQS search were determined by a Monte-Carlo simulation based on a cluster expansion of the total energy for the alloy. The optical properties determined from GW calculations for such SQS alloy structures are compared with available experimental data.   

Abundant defects and defect clusters in kesterite Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$

Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$ are drawing intensive attention as the light-absorber materials in thin-film solar cells. A large variety of intrinsic defects can be formed in these quaternary semiconductors, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. We will present our first-principles calculation study on a series of intrinsic defects and defect clusters in Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$, and discuss: (i) strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the dominant Cu$_{\mathrm{Zn}}$ antisites and Cu vacancies which determine the intrinsic p-type conductivity, and their dependence on the elemental ratios; (iii) the high population of charge-compensated defect clusters (like V$_{\mathrm{Cu}}+$Zn$_{\mathrm{Cu}}$ and 2Cu$_{\mathrm{Zn}}+$Sn$_{\mathrm{Zn}})$ and their contribution to non-stoichiometry ; (iv) the deep-level defects which act as recombination centers. Based on the calculation, we will explain the experimental observation that Cu poor and Zn rich conditions give the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu$_2$ZnSn(S,Se)$_4$ cells with high S composition.   

Strain Induced Photoabsorption of CuGa$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$O$_2$

Delafossite oxides are a family of materials that hold promise for photocatalytic, thermoelectric, and other cutting edge applications. These materials are of interest because they exhibit a disparity between their optical and electronic band gaps due to inversion symmetry according to the Laporte selection rule. Though they appear transparent, their electronic structure suggests that they should absorb visible light, aside from conduction and valence band parity. We use B-site substitution to break inversion symmetry and allow the absorption of visible light. Here we present computational and experimental electronic and optical results of B-site substitution of the delafossite CuGaO$_{2}$ with Fe which supports the inversion symmetry theory of the band gap disparity. Included are experimental and computational absorption spectra for CuGa$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$O$_{2}$. We find and explain an interesting increase optical absorption in the visible range at the 5{\%} Fe substitution level. To the best of our knowledge computational results to this degree of percentage accurate substitution or alloying have not been performed on this or similarly complicated systems.   

Massive computational search for n-type organic semiconductors

In a search for n-type organic materials, which are rare compared to p-type, we calculate the optimized geometries and electronic structures for millions of conjugated oligomers. A good n-type material (electron conductor) must have a low-lying LUMO level (high electron affinity) in order to avoid chemical reactions that create electron traps. For high conductivity, it must have a low barrier to electron hopping, indicated by an internal reorganization energy in the meV range. The calculations use the adapted Su-Schrieffer-Heeger tight-binding Hamiltonian and include both neutral and singly charged structures. The group of structures with a combination of low-lying LUMO levels and small internal reorganization energies is presented as candidates for n-type organic semiconductor materials. The data are also used to directly compute the optical band gaps and exciton binding energies, while the HOMO and LUMO levels are used to estimate cyclic voltammetry oxidation and reduction potentials. Application of the methodology to other organic materials searches is discussed.   

Theoretical study of LaOXS \{X=Cu, Ag\} layered oxide sulphides

The ternary oxides, owing to the mismatch between the energy levels of the transition metal {\em d}-orbitals and the deep oxygen {\em p}-orbitals, typically show a limited dispersivity of the valence band maxima (VBM) and relatively heavy masses that make them not favorable in applications as p-type transparent conducting oxides (TCOs). In a hope to increase the {\em p}-{\em d} hybridization and preserve large band gaps in oxides with the addition of sulphur atoms, we studied the reported layered quarternary oxysulphides (LaCuOS, LaAgOS) using density functional theory with G0W0 self energy corrections. We confirmed that the VBM is mainly contributed by the antibonding state of Cu/Ag-{\em d} and S-{\em p} and the hole effective mass increases upon Cu substitution by Ag, which has a deeper {\em d} level than the Cu {\em d} one.   

Session M6: Graphene: Multilayer and Tunneling

Quantumn Hall Effect in single-, bi- and tri-layer graphene

Quantum Hall Effect has been extensively studied in single layer, bilayer and trilayer graphene. Our recent studies showed intrinsic gapped state at the charge neutrality point in bilayer and trilayer graphene. Here we describe the fabrication of high-quality single-bilayer and bi-trilayer hybrid graphene devices, and present results from magneto-transport measurements.   

Velocity renormalization in multilayer graphene

Multilayer graphene has recently attracted considerable attention because of its chiral electronic structure which is sensitive to stacking sequences, and its possible use as the basis of new electronic devices. Furthermore, as sample quality improves, it is expected that electron-electron interactions play a significant role which was hidden by disorder. In this talk, we study velocity renormalization in multilayer graphene due to electron-electron interactions. After analyzing velocity renormalization in the chiral two-dimensional electron gas which is a low-energy effective model of graphene systems, we discuss its implication for multilayer graphene.   

Emergent Electromagnetism in Bilayer Graphene

Recently atomically flat layers of carbon known as graphene have become the rising star in spintronics as their electrons carry not only the ordinary spin degree of freedom, but they also have a pseudospin degree of freedom tied to the electrons' orbital motion which could enable new routes for spintronics. Here we focus on bilayer graphene (BLG). Using group theory we have established a complete description of how electrons in BLG interact with electric and magnetic fields. We show that electrons in BLG experience an unusual type of matter-field interactions where magnetic and electric fields are virtually equivalent: every coupling of an electron's degrees of freedom to a magnetic field is matched by an analogous coupling of the same degrees of freedom to an electric field. This counter-intuitive duality of matter-field interactions allows novel ways to create and manipulate spin and pseudo-spin polarizations via external fields that are not available in other materials. See arXiv:1206.4761.   

Coexisting massive and massless Dirac fermions in quasi-freestanding bilayer graphene

The most widely accepted theoretical model to describe charge carriers in bilayer graphene is ``massive Dirac fermions'', characterized by a nearly parabolic band pair touching each other at the Dirac energy. This electronic structure of bilayer graphene is widely believed to be unstable towards symmetry breaking either by structural distortions, such as twist and strain, or electronic interactions. In this work, we investigate quasi-freestanding bilayer graphene by angle-resolved photoemission spectroscopy, which shows an unexpected electronic spectrum, consisting of both massive and massless Dirac fermions. The latter has a unique band topology with a chiral pseudospin texture, and its origin will be discussed in terms of symmetry breaking induced by a native imperfection of bilayer graphene.   

Quasiparticle Energy and Excitonic Effects of Gated Bilayer Graphene

By employing the first-principles GW-Bethe-Salpeter Equation simulation, we obtain the accurate quasiparticle (QP) band gap and optical absorption spectra of gated bilayer graphene (GBLG). Many-electron effects are shown to be extremely important for understanding these excited-state properties; enhanced electron-electron interactions dramatically enlarge the QP band gap; infrared optical absorption spectra are dictated by bright bound excitons. In particular, these QP band gaps, exciton binding energies, and even the exciton spectra can be tuned in a wide range by the gate field. Our results satisfactorily explain recent experiments. Moreover, our calculation predicts exotic excitonic effects that have not been observed yet, which can be of interest for optoelectronics applications based on GBLG.   

A theoretical study of symmetry-breaking organic overlayers on single- and bi-layer graphene

An ``overlayer'' of molecules that breaks the AB symmetry of graphene can produce (modify) a band gap in single- (bi-) layer graphene.\footnote{M. Li et al., Phys. Rev. B 76, 155438 (2007)} Since the triangular shaped trimesic acid (TMA) molecule forms two familiar symmetry breaking configurations, we are motivated to model TMA physisorption on graphene surfaces in conjunction with experiments by Groce et al. at UMD. Using VASP, with ab initio van der Waals density functionals (vdW-DF), we simulate adsorption of TMA onto a graphene surface in several symmetry-breaking arrangements in order to predict/understand the effect of TMA adsorption on experimental observables.   

Vortex zero mode and charge of mass skyrmion in graphene

We investigate the skyrmion formed by the mass order parameters in graphene and bilayer graphene. The skyrmion out of the three quantum anomalous spin Hall order parameters carries charge of 2e and 4e, respectively, in graphene and BA-stacking bilayer graphene. The origin of the above is related to the counting of vortex zero-mode and the representation of Clifford algebra imposed on the mass order parameters. The doubling of charge in bilayer case is due to the Kramers's degeneracy implied by the pseudo time-reversal symmetry, which is a result of the quadratic band touching at low-energy.\\[4pt] [1] Chi-Ken Lu and Igor F. Herbut, Phys. Rev. Lett. {\bf 108}, 266402 (2012)\\[0pt] [2] Igor F. Herbut, Chi-Ken Lu, and Bitan Roy, Phys. Rev. B {\bf 86} 075101 (2012).   

Broken Symmetry Phases in ABC Trilayer Graphene

We study the effects of electron-electron interaction in ABC-stacked trilayer graphene (TLG) within the framework of weak coupling renormalization group (RG). We find that, when the interaction is mainly in the forward scattering channel, the system orders into a gapless phase characterized by breaking of the TLG lattice mirror symmetries. A presence of small but finite back scattering changes the nature of the leading instability and results in gapped phases. The repulsive back scattering favors layered anti-ferromagnetic order, while the attractive back scattering yields the quantum spin Hall phase (gapped in bulk only). By classifying order parameters in TLG according to irreducible representations of the TLG space group, we conclude that any orders that break the rotational symmetry (e.g., the nematic state) in TLG are disfavored compared to the orders that do not break the lattice trifold rotational symmetry. The results are discussed in the context of present experiments on TLG.   

Unravelling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene

Extensive scanning microscopy and spectroscopy experiments completed by first principles and parameterized tight binding calculations provide a clear answer to the existence, origin and robustness of van Hove singularities in twisted grapheme layers. Our results are conclusive: vHs due to interlayer coupling are present in abroad range of rotation angles. From the variation of the energy separation of the vHs with rotation angle we recover the Fermi velocity of the grapheme monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments and calculations which test the role of the periodic modulation and absolute value of the interlayer distance. We clarify the origin of the moir\'{e} corrugation observed in the STM images.   

Study on Metal/Metal oxide/Graphene Tunnel Junctions

Metal/metal-oxide/graphene (Metal $=$ Al, Ti, Hf, Zr) tunnel junctions were fabricated by transferring single-layer graphene grown by chemical vapor deposition on Cu onto metal strips by either a wet or dry approach. The metal strips were prepared by dc magnetron sputtering through a shadow mask and were exposed to air for about 10 minutes for native oxides to grow prior to the transfer. Good tunneling properties were observed for all the junctions fabricated by either means of graphene transfer. The zero-bias resistance of these junctions all increases with time to a final value, indicating continuing oxidation of the metals with a self-limited oxidation rate. Some junctions show the final area-normalized zero-bias resistances and self-limited oxidation time scales for Al, Ti, Hf, Zr are about 0.15, 0.2, 6000, 1000 k$\Omega $cm$^{2}$ and 25, 90, 6, 9 hour, respectively. The tunneling spectra were studied at various temperature down to 4.2 K and analyzed by the Brinkman-Dynes-Rowell model to get the height and width of the tunnel barriers, taking into account the electron structure of graphene. The junctions are good candidates for chemical sensing applications.   

Tunneling Spectroscopy of Graphene using Planar Pb Probes

We show that evaporating lead directly on graphene can create high-quality tunnel probes. By monitoring and comparing the resistances of probes made from Pb, Al and Ti/Au, we have found unique and robust behavior of the Pb probes: the contact resistance between the Pb and graphene first increases and then saturates over a time period of approximately one week. Characterization via transport measurements at low temperature shows that after oxidation a well-formed tunnel barrier is created between the Pb and the graphene. Tunneling spectroscopy using the Pb probes manifests energy-dependent features such as scattering resonances and localization behavior, and can thus be used to probe the microscopic electronics of graphene.   

Mg/MgO/Graphene Tunnel Junctions Made by Dry Transfer of Graphene in Vacuum

Mg/MgO/Graphene junctions were fabricated by dry transfer of single layer graphene film grown by chemical vapor deposition on Cu Mg strips were deposited onto Si/SiO$_{2}$ or glass substrates by thermal evaporation through a shadow mask. The tunnel barrier MgO was formed by exposing deposited Mg for about 10 minutes in air prior to the graphene transfer. To prevent degradation of MgO by liquids, a dry transfer technique is used. First a graphene film was transfer onto a free-standing 4$\mu $m-thick Cu film using the traditional wet method, then pressed onto a transparent and flexible PDMS stamp followed by etching away the Cu film in FeCl$_{3}$ solution, and finally stamped onto the Mg strips in vacuum to prevent any gas bubbles that may form between graphene and Mg strips. The dry-transferrd graphene has similar properties to traditional wet-transferred graphene, characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and transport measurements. It has a sheet resistance of 1.6 $\sim$ 3.4 k$\Omega$/$\Box$, charge carrier density of 4.1 $\sim$ 5.3 $\times$ 10$^{12}$ /cm$^{2}$ and mobility of 460 $\sim$ 760 cm$^{2}$/Vs without doping at room temperature. Mg/MgO/graphene junctions show good tunneling characteristics at temperatures down to 4.2 K. The barrier height and width were obtained by fitting with the Brinkman-Dynes-Rowell trapezoid-shaped barrier model with consideration of graphene electron structure.   

Intrinsic Dirac Point Energy Level and Band Offset of Graphene/SiO$_2$ interface

Advancing toward the rational design, fabrication, and implementation of graphene(GR)-based electronic and optical devices, the intrinsic barrier height of undoped GR (the Dirac point of GR to the conduction band(CB) edge of an insulator), as well as the intrinsic work function(WF) of GR must be accurately determined. We present an internal photoemission (IPE) investigation of a unique semi-transparent metal/high-k/GR/SiO$_{2}$/Si structure, and focus our study on the photoemission phenomena at the GR/SiO$_{2}$ interface. By taking advantage of the optical interference of the SiO$_2$ cavity, the enhanced photoemission from GR was observed. As a result, a complete electronic band alignment at the GR/SiO$_{2}$/Si interfaces is established. The intrinsic positions of the undoped GR Dirac point with respect to the CB of SiO$_2$, 3.58 eV (Al$_2$O$_3$ TG) and 3.60 eV (HfO$_2$ TG), are obtained. The intrinsic WF of graphene is found to be 4.50 eV. The determination of the WF of GR is of significant importance to the engineering of GR-base devices and the IPE spectroscopy, combined with specific interference cavity structures, would be a valuable measurement technique for other GR-like2-D material systems.   

Point Contacts to Graphene for Corbino Disk Geometry Devices

A Corbino disk geometry raises new possibilities for observing novel phenomena for Dirac electrons in graphene [1]. For example, Recent theoretical work has suggested the possibility of observing a quantum relativistic Corbino effect in which the conductance of a graphene layer measured in a Corbino disk geometry shows magneto-oscillations related to to the number of flux quanta threading the area of the disk [2]. We will discuss a technique we have developed for making air-bridge contacts to graphene layers based on a multi-layer resist technique [3]. The air bridge enables a Corbino disk geometry in the absence of topside dielectric layers, potentially facilitating annealing techniques in conjunction with placement on, for example, BN substrates to enable high mobility devices. The latest transport results will be discussed.\\[4pt] [1] Zhao et al., Phys. Rev. Lett. 108, 106804 (2012). \newline [2] Rycerz, Phys. Rev. B 81, 121404?[U+0351]R (2010). \newline [3] Liu et al., Appl. Phys. Lett. 92, 203103 (2008).   

Session M7: Focus Session: Graphene Devices VII

Negative refractive index electron `optics', pseudospintronics and chiral tunneling in graphene pn junction -- beating the Landauer switching limit?

We use atomistic quantum kinetic calculations to demonstrate how graphene PN junctions can switch with high ON currents, low OFF currents, steep gate transfer characteristics and unipolar rectification. The physics of such unconventional switching relies on (a) field-engineering with patterned gates to create a \textit{transmission gap,} by sequential filtering of all propagating modes, and (b) using tilted junctions to suppress Klein tunneling under appropriate gate biasing, making that transmission gap \textit{gate tunable}. The doping ratio of the junction dictates the energy range over which the tilt angle exceeds the critical angle for transmission, generating thereby a gate tunable transmission gap that enables switching at voltages less than the Landauer-Shannon thermal limit. The underlying physics involves a combination of `electron optics' driven by Snell's law, negative index metamaterial with a PN junction, and pseudospin driven chiral tunneling, for which we also present experimental verification. [Sajjad et al, APL 99, 123101 (2011); Sajjad et al, PRB 86, 155412 (2012)].   

All-carbon optical diode

Optical diodes that allow unidirectional transport of light, similar to an electronic p-n junction diode, are vital to manipulate and control light for information processing. These ``optical diodes'' have already been realized using photonic crystals (PC) with engineered periodicity. However, an important criterion for the functioning of a PC-based optical diode is that the periodicity of the PC should be on the same length scale as half the wavelength of the electromagnetic waves used. For the visible region of the electromagnetic spectrum, this periodicity must be $\sim$ 200-350 nm making the fabrication of PCs expensive, cumbersome and complicated. An optical diode based on the transmission of optical pulses through structures with an abrupt variation in the longitudinal nonlinear absorption coefficient, as opposed to periodic variation of refractive index or dielectric constant is demonstrated. In particular, we present the studies performed on an all carbon optical diode with C60 and graphene coated on quartz cover slips. We find that the reverse saturable absorption of C60 and the saturable absorption of graphene can be combined to obtain modest reciprocity factors for a solid-state all-carbon optical diode.   

Photoconductivity of biased graphene

The origin of photosensitivity of graphene devices has been attributed to either thermoelectric, photovoltaic, or bolometric effects. Here we report on the intrinsic photoresponse of electrically biased, but otherwise homogeneous single-layer graphene. In this simple, yet unstudied experimental condition, the photocurrent shows polarity reversal, as it alternates between two of these effects while sweeping the electronic potential. Near the Dirac point, the photovoltaic effect dominates, and the photocurrent adds to the transport current. Away from the Dirac point, the bolometric effect dominates, and reduces the transport current. Magnitude and polarity of the photocurrent allow us to infer the hot carrier and phonon temperatures under light illumination. The electron temperature is found to be an order of magnitude higher than the phonon temperature, shedding light on energy loss pathways other than via intrinsic graphene phonons. (M. Freitag et al., Nature Photonics, accepted for publication (2012).)   

Far-IR Spectroscopy and FDTD Simulations of Graphene Plasmonic Structures

Plasmonics, the field of manipulating charge density waves, is uniquely suited to graphene due to graphene's high mobility and tunable plasma frequency in the THz range. Graphene microstructures, such as strips, discs, and rings confine plasmon modes, leading to plasma resonances with THz frequencies. These micro- and nanostructures form the building blocks of graphene plasmonic devices for tunable terahertz generation, detection, filtering, and switching. We present experimental results on the spectroscopy of plasmon resonances in the far-IR wavelength range in various graphene microstructures. Analytical methods of modeling even the simplest graphene plasmonic structures are not quantitatively accurate, and as such, we developed a 3D finite-difference time-domain (FDTD) tool for simulating the plasmon modes. By fitting simulations to the measured data, we have quantitatively extracted the parameters characterizing graphene's intraband conductivity and carrier scattering time with good accuracy. We have also investigated the interaction between plasmon modes of nearby structures and found them to be strong when the distance between structures is less than the dimension of the structures. FDTD simulations enable a quantitative characterization of such interactions.   

Ultra-Amplification of Surface Plasmon Coupled Emission in Graphene-Silver Hybrid Films

Surface Plasmon Coupled Emission (SPCE) stems from an interaction between fluorophores and thin metallic films and leads to strongly directional $p$-polarized emission with signal intensities that are 10-1000 times greater than isotropic fluorescence emission. Conventional SPCE methods use silver thin films with a SiO$_{2}$ spacer layer to prevent oxidation of silver, and the latter has no role in the signal generation. Here we employ single- and bi- layer graphene (SLG-BLG) as the spacer layer and demonstrate a 10 fold enhancement in comparison to the isotropic fluorescence intensity for rhodamine B fluorophore doped in PVA matrix. A fiber optic spectrometer was used to record the emission which was strongly directional (at 50$^{\circ}$ relative to the incident excitation) and 97{\%} $p$-polarized. Base on our preliminary simulations, we attribute the synergistic interaction between the $\pi $-plasmons of graphene and the surface plasmons of silver as the most important factor in the amplification of the SPCE.   

Single layer graphene plasmonic detector for broadband THz spectroscopy

Among many possible applications of graphene, THz detection is one of the most promising. The Drude-type absorption of THz radiation by free carriers is much stronger than the frequency-independent ~2.3\% absorption for interband transitions. By patterning the graphene sheet strips the Drude-type response is transformed into a Lorentzian peak corresponding to a THz plasmon resonance on the width $w$ of each strip. The plasmon resonance frequency $\omega_0 \propto n^{1/4} w^{1/2}$, where $n$ is carrier concentration which is tunable by gate(s) as was reported in Ref. 1 for graphene grown by chemical vapor deposition. We have reproduced results of Ref. 1 on our single layer graphene on Si-face SiC with electrolyte top gate. The next step to a detector is extraction of DC photocurrent without destroying plasmons. We will present our solution to this problem and compare the performance of our room-temperature detector to existing THz detector technologies. Other aspects of our graphene photodetectors such as device fabrication, response time, and response mechanism will be presented in other talks at this meeting. [1] L. Ju et al, Nature Nanotechnology, 6 (2011) 630-634.   

Graphene electrically reconfigurable patterns for THz imaging applications

THz waves are attractive for several imaging applications, since they can propagate through non metallic media such as paper, cloth, plastics, and ceramics, and do not scatter over nano-scale defects or ionize the material under imaging -as might shorter wavelengths do- while offering an image resolution similar to that of the human eye. In this work we propose and experimentally demonstrate electrically reconfigurable patterns for single-pixel terahertz imaging based on arrays of graphene THz electro-absorption modulators. In an optical setup, in conjunction with mirrors, the modulator array can transform the output radiation from a CW THz source into a pixelated and collimated beam of illumination. Single-atom-thick graphene is employed as the active element of these modulators, achieving a modulation of the THz wave reflectance \textgreater 50{\%} with a potential modulation depth approaching 100{\%} (i.e. each region of the pixelated collimated beam can be potentially completely turned-off). Although the proof-of-concept device here discussed only consists of 4x4 elements, we foresee that this technology can enable low-cost video rate THz imaging systems.   

Graphene Based Tunable SPR Sensors

Today's highly mobile world requires widely deployable disease detection and monitoring systems. We need compact, sensitive, and cost-effective biosensors, which can also tolerate a wide range of operating conditions to be field-deployable. Especially for point-of-care diagnostics, where the testing environment can be highly variable, it would be advantageous to have sensors with tunable operating ranges. To address this need, we propose tunable, localized surface plasmon resonance (SPR) based biosensors using graphene layers and metal nanoparticle arrays. Tuning capability is achieved by bias voltage applied to the thin layers of the substrate, where on metal nanoparticle arrays are fabricated. The key component of the design is graphene. The applied voltage changes not only optical properties of graphene but also the induced dipole moment of each nanoparticle and hence the resonance wavelength of the sensor. For the modeling of proposed tunable biosensors, we use both a frequency domain approximate solver (layer medium coupled dipole approximation) and a full wave time-domain electromagnetic solver (Wavenology). Numerical results obtained with these two independent solvers reveal the tuning capability of the proposed structures.   

Ferroelectric-Gated Terahertz Plasmonics on Graphene

Inspired by recent advancement of low-power ferroelectic-gated memories and transistors, we propose a design of ferroelectic-gated nanoplasmonic devices based on graphene sheets clamped in ferroelectric crystals. We show that the two-dimensional plasmons in graphene strongly couple with the phonon-polaritons in ferroelectrics at terahertz frequencies, leading to characteristic modal wavelength of the order of 100--200 nm at only 3--4 THz. By patterning the ferroelectrics into different domains, one can produce compact on-chip plasmonic waveguides, which exhibit negligible crosstalk even at 50 nm separation distance. Harnessing the memory effect of ferroelectrics, low-power electro-optical switching can be achieved on these plasmonic waveguides.   

Graphene nano-photonics and carrier dynamics

Graphene, a two-dimensional sheet of carbon atoms, has recently emerged as a novel material with unique electrical and optical properties, with great potential for novel opto-electronic applications, such as ultrafast photo-detection, optical switches, strong light-matter interactons etc. In the first part of this talk, I will review recent experimental work on exploiting graphene as a host for guiding, switching and manipulating light and electrons at the nanoscale [1]. This is achieved by exploiting surface plasmons: surface waves coupled to the charge carrier excitations of the conducting sheet. Due to the unique characteristics of graphene, light can be squeezed into extremely small volumes and thus facilitate strongly enhanced light-matter interactions. Additionally, I will discuss novel types of hybrid graphene photodetectors [2] and recent findings on carrier dynamics and hot carrier multiplication in graphene. By studying the ultrafast energy relaxation of photo-excited carriers after excitation with light of varying photon energy, we find that electron-electron scattering dominates the energy relaxation cascade rather than electron-phonon interaction [3]. This solves a long- standing debate on the relative contribution of electron-electron scattering versus optical phonon emission.\\[4pt] [1] J. Chen, M. Badioli, P. Alonso-Gonz\'{a}lez, S Thongrattanasiri, F Huth, J Osmond, M. Spasenovic, A. Centeno, A. Pesquera, P. Godignon, A. Zurutuza, N. Camara, J. Garcia de Abajo, R. Hillenbrand, F. Koppens, ``Optical nano- imaging of gate-tuneable graphene plasmons'', Nature (2012).\\[0pt] [2] G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, P. Garcia de Arquer, F. Gatti, F. Koppens, ``Hybrid graphene-quantum dot phototransistors with ultrahigh gain'', Nature Nanotechnology (2012).\\[0pt] [3] Photo-excitation Cascade and Multiple Carrier Generation in Graphene. K.J. Tielrooij, J.C.W. Song, S.A. Jensen,~A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L.S. Levitov, and F.H.L. Koppens. ArXiv 1210.1205 (2012)   

High-performance Photoconductive devices based on Graphene-Nanowire Hybrid Structures

The photoconductivity effect in various semiconducting materials has been extensively utilized for optoelectronic applications. However, conventional photoconductive channels exhibited rather slow responses to external light pulses because the photogenerated electrons and holes survive for a rather long time even after the lights are turned off. On the other hand, single-layer graphene (SLG) was reported to exhibit quite a fast photoconductivity, while its rather small photocurrent levels may limit the practical applications. Herein, we developed graphene-CdS nanowire (NW) hybrid structures for high-speed photoconductivity and large photoresponse. The hybrid structure consists of CdS NWs which were selectively grown in specific regions on a SLG sheet. The photosensor based on graphene-CdS NW hybrid structures exhibited rather large photocurrents as well as much faster operation speed than those based only on CdS NW networks. This simple but efficient strategy takes advantages of both graphene and NWs, and it should enable the fabrication of high performance optoelectronic devices for practical applications.   

Liquid-Gated Epitaxial Graphene: How Leakage Currents Affect Ionic Strength Sensing

Graphene is a promising material for the fabrication of miniaturized biological and chemical sensors. Epitaxial graphene is an exciting candidate due to its compatibility with standard processing techniques and its intrinsic robustness. We have fabricated liquid-gated FET-like devices based upon sub-millimeter wide epitaxial graphene strips defined using optical lithography methods. The devices exhibit a bipolar conductance versus gate voltage behavior with the minimum conductance point being dependent upon the ionic strength of a KCl solution. Measurements of the graphene conductance and gate-leakage currents during the stepping of the gate voltage demonstrate the presence of time dependent nA-scale leakage currents which limit signal stability at short times. Notably, these currents depend upon the gate voltage and the composition of the gate electrode. These and other electrode dependent effects have ramifications for graphene sensor design and implementation such as the need to limit gate voltage operating windows as and carefully design electrodes. With high transconductance and controlled doping, such devices should be able to function at low gate voltages if a full understanding of charge and charge transport at the graphene interface is obtained.   

Graphene as a Platform for Hybrid Optomechanical Devices

Graphene is known for providing a flat 2D material with outstanding optical, electrical and mechanical properties. We propose to take advantage of all three features by developing an optomechanical platform based on cantilevers made of freestanding multilayer graphene connected to an electrode. In this talk I will present several examples of a simple optomechanical systems involving a multilayer graphene suspended cantilevers that can act as a mirror closing an optical cavity. By varying the gate voltage applied on the mirror, its angle can be adjusted on a wide range (exceeding the wavelength of the incoming light) and its motion can be actuated and followed in real time from DC up to the tens of MHz range. Detection of elastic and inelastic scattered light can be performed. It allows simultaneous detection of motion, local stress and temperature of the membrane. A fully spectral detection of NEMS resonance is presented (1) and allows a novel optomechanical scheme based on coupling between motion and light through the dynamic mechanical stress. Further applications are presented as well such as a gate tunable enhancement of the Raman signal of molecular species adsorbed on the graphene platform. (1) Reserbat-Plantey, A., et al, Nature Nanotechnology, vol. 7, 151-155. (2012).   

Session M8: Focus Session: Graphene - Twisted Layers, Stacking

Single-layer behavior and its breakdown in twisted graphene layers

Stacking order plays a major role in the electronic properties of graphene layers because hopping between carbon atoms in neighboring layers is a key ingredient in their band structure. Twisting the layers away from the equilibrium Bernal stacking, which produces the superstructures known as Moir\'{e} patterns in scanning tunneling microscopy, decreases the overlap between atoms in adjacent layers and therefore significantly alters their electronic properties. Using scanning tunneling microscopy and spectroscopy, we obtained direct evidence for the electronic structure of twisted graphene layers.\footnote{G. Li, A. Luican, J.M. B. Lopes dos Santos, A. H. Castro Neto, A. Reina, J. Kong and E.Y. Andrei, Nature Physics 6, 109 ( 2010).} The samples were membranes of CVD grown graphene and graphite crystals which contain areas with various twist angles. In topographic images the regions where layers are twisted away from Bernal stacking exhibit Moir\'{e} patterns with periods which depend on the twist angle. We find that the density of states on the twisted layers develops two Van Hove singularities that symmetrically flank the Dirac point at an energy that depends on the twist angle. High magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers reveal that for twist angles exceeding $\sim $3 degrees the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20 degrees the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent.\footnote{Luican, G. Li, A. Reina, J. Kong, R. R. Nair, K. S. Novoselov, A. K. Geim, E.Y. Andrei, Phys. Rev. Lett. 106, 126802 (2011).} These results are compared with theoretical predictions.   

Interacting Dirac Fermions and Neutrino-Like Oscillation in Twisted Bilayer Graphene

The low-energy quasiparticles in graphene can be described by a Dirac Hamiltonian for massless fermions, hence graphene has been proposed to be an effective medium to study exotic phenomena originally predicted for particle physics, such as Klein tunneling and Zitterbewegung. In this work, we show that another important particle-physics phenomenon -- the neutrino oscillation can be studied and observed in a particular graphene system, namely, twisted bilayer graphene. It has been found that graphene layers grown epitaxially on SiC or by the chemical vapor deposition (CVD) method on metal substrates display a stacking pattern with adjacent layers rotated by an angle with respect to each other. The quasiparticle states in two distinct graphene layers act as neutrinos with two flavors, and the interlayer interaction between them induces an appreciable coupling between these two ``flavors'' of massless fermions, leading to neutrino-like oscillations. In addition, anisotropic transport properties manifest in this specific energy window, which is accessible in experiment for twisted bilayer graphene. We demonstrate that combining two graphene layers enables us to probe the rich physics involving multiple interacting Dirac fermions.   

Landau level splitting in rotationally faulted multilayer graphene

In this work we explore theoretically whether the interlayer motion of electrons in rotationally faulted multilayer graphene can break the valley degeneracy. We show that in~the presence of a magnetic field and interlayer commensurations~this is indeed possible. It leads to the splitting of Landau levels linear in the field. Our theoretical work is motivated by a recent experiment [1] on epitaxially grown multilayer graphene where a splitting of Landau levels was observed. This Landau level splitting was found to be linear in the field at moderate fields. We consider both bilayer and trilayer configurations and find that in both cases a linear splitting can occur. The predicted lack of valley degeneracy is due to a simultaneous breaking of time-reversal symmetry and inversion symmetry by~the magnetic field and interlayer commensurations, respectively. [1] Y. J. Song, et al., Nature 467, 185 (2010).~   

Simultaneous investigation of magnetoresistance (MR) and twisted angle of twisted bilayer graphene

We have measured magnetoresistance (MR) and twisted angle of twisted bilayer graphene, simultaneously. Twisted angle was measured by transmission electron microscopy (TEM) diffraction experiment on SiN$_{x}$ substrate. We performed Raman spectroscopy experiment and observed enhanced G mode which results from double resonance scattering process near van Hove singularity (vHs). MR shows superposition of two Shubnikov de Haas (SdH) oscillations and is analyzed by Landau fan diagram.   

Probing electronic and vibrational interactions in few-layer graphene by optical spectroscopy

Graphene possesses remarkable physical properties and great potential for novel applications.~ As more than one graphene layers are stacked on one another, the properties of the few-layer system can be strongly modified by the interactions between electrons and lattice vibrations in different graphene layers. We have investigated, by means of infrared and Raman spectroscopy, the electronic and vibrational properties of few-layer graphene with different layer thickness and stacking sequence. Our results reveal the critical roles of these degrees of freedom in defining the properties of few-layer graphene. We show how optical spectroscopy offers important routes to characterizing the thickness and stacking order of the graphene samples as well as probing the material's response to external perturbations. In particular, we will describe the use of Raman spectroscopy to identify the interlayer breathing modes in few-layer graphene of up to 20 layers in thickness, and the use of Infrared spectroscopy to probe the modulation of electronic structure and electron-phonon interactions in few-layer graphene with varying thickness, stacking order and doping level. This work was performed at Columbia University in collaboration with L. Brus, E. Cappelluti, G. L. Carr, Z.Y. Chen, Z.Q. Li, K.F. Mak, L.M. Malard and R. Saito.   

Resonance profile of Moire-pattern Raman peaks in twisted graphene layers

In this work, we study the Raman spectra of graphene samples grown by CVD on a Cu foil, with different laser excitation lines. The spectra exhibit a number of extra sharp Raman peaks, classified in different families, each one associated with Moire patterns of graphene layers twisted with different angles. The presence of these extra peaks is theoretically analyzed considering the interlayer potential perturbation, that gives rise to a set of wavevectors within the interior of the Brillouin zone of graphene, activating special selective double-resonance (DR) Raman modes, in a so-called umklapp DR (u-DR) process. The resonance Raman profile of the Moire peaks obtained experimentally by changing the laser energy is compared with the calculations of the u-DR process, showing that Raman spectroscopy is useful to characterize Moire patterns in graphene systems.   

Interaction Induced Symmetry Breaking in ABA Trilayer Graphene

We present a mean-field phase diagram of dual-gated ABA trilayer graphene which is obtained by numerically solving the self-consistent Hartree-Fock equations. A metal-insulator phase transition occurs in neutral ABA trilayers at interaction strength $\alpha=0.18$ which is not associated with broken lattice symmetries. ABA trilayers do not possess the inversion symmetry present in bilayers, but do possess a mirror-plane symmetry which remains unbroken for realistic values of alpha for the case of spinless, valley-less fermions. The manner in which SU(4) spin-valley symmetry breaks depends on doping, interlayer bias, and the surrounding dielectric medium. We compare interaction effects in ABA graphene with those in the more familiar chirally-stacked multilayers.   

Quantized Strain Channels in Bilayer Graphene

For bilayer graphene, Bernal stacking presents the lowest energy configuration. However, when the two layers are free to translate, there are two mirrored Bernal stacking orders with degenerate energies [1]. In large-area bilayer systems grown by chemical vapor deposition domains of both stacking configurations have been observed [2], although the precise structure of their boundaries was not understood. Here, we image such structures with atomic resolution using scanning transmission electron microscopy (STEM). We find that domain boundaries are formed by continuous strain of one layer with respect to the other, while the direction and magnitude of their displacements are quantized by the energy landscape. Finally, we extend their characterization over many microns with standard dark-field TEM imaging and discover that the strain regions form long channels that can perhaps be exploited for their electronic properties in the future. 1. Lebedeva et al., J. Chem. Phys. 134, 104505 (2011) 2. Brown et al., Nano Lett. 12, 1609 (2012)   

Electronic structure of multilayer graphene with a mixture of Bernal and rhombohedral stacking

We propose a general scheme to describe the electronic band structure of multilayer graphene with an arbitrary mixture of Bernal and rhombohedral stacking. The system can be viewed as a series of finite Bernal graphite sections connected by rhombohedral-type stacking faults. We find that the low-energy eigenstates are mostly localized in each Bernal section, and the whole spectrum is well approximated by a collection of the spectra of independent sections. In the ensemble-averaged electronic structure, there are frequently-appearing linear bands and quadratic bands with particular band velocities or curvatures, corresponding to finite Bernal sections and their combinations.   

Electronic dispersion from long-range atomic ordering and periodic potentials in two overlapping graphene sheets

A worldwide effort is underway to learn how to build devices that take advantage of the remarkable electronic properties of graphene and other two-dimensional crystals. An outstanding question is how stacking two or a few such crystals affects their joint electronic behavior. Our talk concerns ``twisted bilayer graphene (TBG),'' that is, two graphene layers azimuthally misoriented. Applying angle-resolved photoemission spectroscopy and density functional theory, we have found van Hove singularities (vHs) and associated mini-gaps in the TBG electronic spectrum, which represent unambiguous proof that the layers interact. Of particular interest is that the measured and calculated electronic dispersion manifests the periodicity of the moir\'e superlattice formed by the twist. Thus, there are vHs not just where the Dirac cones of the two layers overlap, but also at the boundaries of the moir\'e superlattice Brillouin zone. Moir\'es, ubiquitous in hybrid solids based on two-dimensional crystals, accordingly present themselves as tools for manipulating the electronic behavior.   

Session M9: Invited Session: A History of Physics in Industry followed by Panel Discussion

Commercial Scholarship: Spinning Physics Research into a Business Enterprise

The American Institute of Physics' Center for History of Physics has conducted a three year NSF funded study of physicist entrepreneurs during which we interviewed 140 physicists who have founded ninety-one startups. Forty of those companies have spun research out of twenty-some universities. Startups spun out of university research tend to be technology push companies, creating new potentially disruptive technologies for which markets do not yet clearly exist, in contrast to market pull companies founded to address innovations responding to market demands. This paper addresses the unique issues found in university spinout companies and their responses to them. While technology push companies are generally considered to be higher risk compared to market pull companies, the university spinouts in our study had a higher rate of both SBIR and venture capital funding than did the market pull companies in our study.   

A Place for Materials Science: University of Pennsylvania's Laboratory for Research on the Structure of Matter

The University of Pennsylvania's Laboratory for Research on the Structure of Matter (LRSM) opened its doors in 1965. Constructed to house cutting-edge research on Materials Science, the LRSM building was designed to foster interdisciplinary research among physicists, chemists and metallurgical engineers. Each of the five floors of the new building included a central facility, including a high magnetic field center, an analytical chemistry research center and an electron microscopy center. While primarily funded by the Department of Defense's Advanced Research Projects Agency, the LRSM also was also partly sponsored by industry. The LRSM received funding from Philadelphia Electric Company, General Electric Company, and IBM, among others. In this paper, I will study how the building was designed to encourage interdisciplinary collaboration, while also becoming a place of intersection among academic, private, and governmental interests. This project is a collaboration with Hyungsub Choi.   

Dad's in the Garage: Santa Barbara Physicists in the Long 1970s

American physicists faced many challenges in the 1970s: declining research budgets; public skepticism of scientific authority; declining student enrollments; and pressure to shift to topics such as biomedicine, environmental remediation, alternative energy, public housing and transport, and disability technologies. This paper examines the responses to these challenges of a small group of Santa Barbara physicists. While this group is not representative of the American physics profession, the success and failure of their responses to changed conditions tells us something about how American physicists got through the 1970s, and about the origins of some features of American physics today. The three physicists examined here are Philip Wyatt, David Phillips, and Virgil Elings. In the late `60s, Wyatt left a defense think tank to found an instrumentation firm. The Santa Barbara oil spill and other factors pushed that firm toward civilian markets in biomedicine and pollution measurement. Phillips joined Wyatt's firm from UCSB, while also founding his own company, largely to sell electronic devices for parapsychology. Phillips was also the junior partner in a master's of scientific instrumentation degree curriculum founded by Elings in order to save UCSB Physics' graduate program. Through the MSI program, Elings moved into biomedical research and became a serial entrepreneur. By the 1990s, Wyatt, Phillips, and Elings' turn toward academic entrepreneurship, dual military-civilian markets for physics start-ups, and interdisciplinary collaborations between physicists and life scientists were no longer unusual. Together, their journey through the `70s shows how varied the physics' profession's response to crisis was, and how much it pivoted on new interactions between university and industry.   

Industrial Physics---Southern California Style

Only in Southern California did space-age style really come into its own as a unique expression of Cold War scientific culture. The corporate campuses of General Atomic in San Diego and North American Aviation in Los Angeles perfectly expressed the exhilarating spirit of Southern California's aerospace era, scaling up the residential version of California modernism to industrial proportion. Architects William Pereira and A.C. Martin Jr., in collaboration with their scientific counterparts, fashioned military-industrial `dream factories' for industrial physics that embodied the secret side of the space-age zeitgeist, one the public could only glimpse of in photographs, advertisements, and carefully staged open houses. These laboratories served up archetypes of the California dream for a select audience of scientists, engineers, and military officers, live-action commercials for a lifestyle intended to lure the best and brightest to Southern California. Paradoxically, they hid in plain sight, in the midst of aerospace suburbs, an open secret, at once visible and opaque, the public face of an otherwise invisible empire. Now, at the end of the aerospace era, these places have become an endangered species, difficult to repurpose, on valuable if sometimes highly polluted land. Yet they offer an important reminder of a more confident time when many physicists set their sights on the stars.   

Panel Discussion - Perspectives on the History of Industrial Physics

This panel discussion provides the speakers and the audience an opportunity to explore the common themes these papers exhibit in greater detail.   

Session M10: Invited Session: Physics Jobs in Government and Science Policy followed by Panel Discussion

Off the Beaten Path: A Journey to a Career Beyond the Laboratory

This presentation will provide insights on how a scientific graduate degree can lead to opportunities that combine scientific expertise with diverse interests such as business, international affairs, and science policy. The speaker will talk about potential challenges for PhD scientists working outside of a traditional research environment and the professional skills that help ensure success in careers beyond the laboratory.   

``Political'' Science

Politics and policy affect all of us, both as scientists and as citizens, and issues ranging from laboratory budgets to arms control treaties clearly require research problem-solving skills and technical expertise. There is a critical role for scientists in each aspect of the political system, and in fact, we as a society need more scientists to take part in politics. Furthermore, the research we pursue has important societal applications and is fascinating! We have a right and a responsibility to share our scientific knowledge not only with each other, but with the general public as well. So, why are we as a community of scientists reticent in the public arena, hesitant to enter politics, and even at times unsupportive of our peers who transition into governmental roles? In this time of fiscal constraint, when difficult research funding (and de-funding) choices are regularly being made, we as scientists must step up to the plate, reach across the aisle, and explain why what we do is fascinating, inspiring, and important, not just to us, but to society as a whole. A range of policy-relevant roles exists inside and outside the laboratory, such as Congressional Fellowships. Each year the Congressional Fellowships program brings together approximately thirty scientists at all stages of their careers to serve as scientific advisors in a variety of offices in the U.S. Senate and House of Representatives. Although the jump from lab to lobbying meetings can be frustrating, the transition can also be intriguing. Firsthand experience with the ``how'' and ``why'' (or lack thereof) of politics and policy is invaluable and provides a unique opportunity to expand and broaden one's background. The opportunity to work on Capitol Hill is unparalleled, particularly because our nation has a definite need for scientists with the inclination and interest to inform and develop policy. But, whatever role you decide to take, from contributing scientific news to local publications to running for Congress, it's high time to show that we as scientists have important contributions to make both inside and outside the laboratory. We as scientists can and should contribute to ongoing political discussions, and there is no better time than now to speak up and apply our expertise to the policy issues at hand.   

PANEL DISCUSSION

Panelists: Tyler Glembo and Hugh Van Horn   

Session M11: Invited Session: Polymer Electrolytes for Energy Storage

Effect of Ion Clusters on Transport in Hydrated Block Copolymers

Transport through hydrated membranes is important for a wide variety of applications including desalination, artificial photosynthesis, and hydrogen fuel cells. Model membranes for these applications can be created by self-assembly of block copolymers containing an ion-containing hydrophilic block and a nonionic hydrophobic block that provides the membrane with structural integrity in the hydrated state. The formation of ordered microdomains such as lamellae and cylinders in block copolymers is well-established. The ion-containing microdomains also contain nanoscale ionic aggregates. The talk will focus on the effect of morphology on transport of protons and hydroxide ions. We pay particular attention to the effect of clusters on ion transport.   

New Approaches to Conjugated Polymer Electrodes for Organic Energy Storage

Conjugated polymers have been explored as electrodes in batteries and pseudocapacitors for over 30 years. Yet, their widespread implementation has been hindered for several reasons such as oxidative stability, low capacity, and rate limitations associated with ionic mobility relative to current state-of-the-art. On the other hand, conjugated polymers have much to offer because of their good electronic conductivity, high Coulombic efficiency, and theoretical capacities comparable to those of metal oxides. Our lab's current goal is to overcome the aforementioned challenges, so that conjugated polymeric electrodes can be suitable used in energy storage for applications such as mechanically flexible energy storage and structural power system. This talk will present several approaches towards synthesis and processing of polyaniline that achieve oxidatively stable, high capacity, ionically mobile electrodes. These approaches include template polymerization, synthesis of nanofibers, and layer-by-layer assembly.   

Ionomer Design, Synthesis and Characterization for Ion-Conducting Energy Materials

For ionic actuators and battery separators, it is vital to utilize single-ion conductors that avoid the detrimental polarization of other ions; the commonly studied dual-ion conductors simply will not be used in the next generation of materials for these applications. \textit{Ab initio} quantum chemistry calculations at 0 K in vacuum characterize ion interactions and ion solvation by various functional groups, allowing identification of constituents with weak interactions to be incorporated in ionomers for facile ion transport. Simple ideas for estimating the ion interactions and solvation at practical temperatures and dielectric constants are presented that indicate the rank ordering observed at 0 K in vacuum should be preserved. Hence, such \textit{ab initio} calculations are useful for screening the plethora of combinations of polymer-ion, counterion and polar functional groups, to decide which are worthy of synthesis for new ionomers. Single-ion conducting ionomers are synthesized based on these calculations, with low glass transition temperatures (facile dynamics) to prepare ion-conducting membranes for ionic actuators and battery separators. Characterization by X-ray scattering, dielectric spectroscopy, NMR and linear viscoelasticity collectively develop a coherent picture of ionic aggregation and both counterion and polymer dynamics. Examples are shown of how \textit{ab initio} calculations can be used to understand experimental observations of dielectric constant, glass transition temperature and conductivity of polymerized ionic liquids with counterions being either lithium, sodium, fluoride, hydroxide (for batteries) or bulky ionic liquids (for ionic actuators).   

Thermodynamics of salt-doped polymers

There is much current interest in salt-doped polymers as materials for energy applications. For example, a promising system for rechargeable battery applications consists of diblock copolymers of an ion-dissolving block, such as polyethylene oxide (PEO) and a nonconducting block such as polystyrene. Experimentally, it has been shown that the addition of lithium salts significantly alters the order-order and order-disorder transition (ODT) temperatures. In particular, the ODT temperature can increase substantially upon adding even a small amount of lithium salt, and the domain spacing in the ordered phases also increases significantly. Both changes are found to depend on the anion type. In this talk, I describe a simple theory for explaining these phenomena. A key effect is the solvation energy of the anions by the polymers, which we approximate using the Born solvation model. The difference in the Born energy between different polymers provides a driving force towards phase separation. By studying the shift in the mean-field spinodal of the disordered phase, we can identify an effective $\chi$ parameter, with a systematic dependence on the anion radius, in agreement with available experimental data. Furthermore, by studying the behavior of the domain spacing with salt concentration, we clarify the relationship between different definitions of the effective $\chi$ parameter. We propose that the effective $\chi$ parameter determined from the structure factor of the disordered phase is a more robust measure of the change in miscibility between the two blocks. Finally, we demonstrate that salt doping induces a strongly first-order transition from the disordered phase to the lamellar phase, with different salt concentrations in the two phases.   

Session M12: Topological Insulators: Topological States in Superconductors

Engineering Majorana modes in MBE grown III-V semiconductor heterostructures

Several theoretical proposals for realizing Majorana fermions in condensed matter systems have created much excitement and are being intensely followed by experimental groups. A common feature of all these proposals is the large size of the parameter space. We are pursuing a proposal based on coupling a semiconductor nanowire with strong spin-orbit coupling to an s-wave superconductor. Considering only the energy landscape, the size of the induced quasiparticle gap depends on the spin-orbit coupling, Zeeman energy, mobility, coupling between the two materials, and the s-wave superconducting gap. We find that Majorana modes can only be realized through carefully engineered materials. We explore this parameter space and discuss the feasibility of realizing Majorana modes based on measured parameters in our MBE grown semiconductor heterostructures.   

Tunneling spectroscopy of topological superconducting states -- toward detection of Majorana fermions

Topological insulators and superconductors have attracted much research interest recently. These materials are known to possess exotic electronic structures that cannot be adiabatically transformed to topologically trivial ones. The spin-momentum locked (helical) Dirac fermions form surface conduction bands while the bulk is insulating. When they become superconducting, charge-neutral zero-energy modes, the so-called Majorana fermion modes, are predicted to emerge due to the unique quasiparticle properties in such a superconducting state. Aiming at detect them, we investigate two novel superconducting systems using tunneling spectroscopy: i) thin film Nb which is proximity-coupled to the helical Dirac fermions in (Bi,Sb)$_{2}$Se$_{3}$; ii) (Sn,In)Te, a potential topological superconductor. Our measurements reveal unusual conductance features in the background and near zero bias. We will report results on their temperature and magnetic field dependences and discuss their implications.   

Transport properties of topological superconductor-Luttinger liquid junctions

Devices involving topological superconductor-Luttinger liquid junctions have been fabricated recently [1,2] to detect Majorana zero-energy modes. One of the signatures of Majoranas in such systems is the so-called ``zero-bias anomaly'' - a quantization of the tunneling conductance at zero temperature. We have developed a framework based on Keldysh formalism to study the corrections to the tunneling conductance due to finite temperature and voltage. Our results are important for understanding the experimental data. \\[4pt] [1] V. Mourik et al., Science 25 May 2012: 336 (6084);\\[0pt] [2] Das et al., arXiv:1205.7073 (2012)   

Interface currents in topological superconductor-ferromagnet junctions

Both fully gapped and nodal pairing states of noncentrosymmetric superconductors (NCS) display non-trivial topological properties, manifested by topologically protected dispersing and flat-band surface states [1,2]. Using a 2D model of an NCS, we show that the surface states typically have strong spin-polarization $s_{\mu=x,z}(k_y)$, which is odd in the surface-Brillouin-zone momentum $k_y$. Upon placing the NCS in proximity contact with a ferromagnet, the coupling to the exchange field gives a perturbative correction to the energy of these states $\propto s_\mu(k_y)$, thus generating an interface charge current $\propto \partial_{k_y}s_\mu(k_y)$ in the NCS. This is most clearly realized in a nodal NCS, where the weak dispersion acquired by the singly degenerate zero-energy flat bands leads to a strong enhancement of the interface current at low temperatures. We argue that this effect is a ``smoking-gun'' signature of the singly degenerate flat bands.\\[4pt] [1] A. P. Schnyder and S. Ryu, Phys. Rev. B {\bf 84}, 060504(R) (2011).\\[0pt] [2] P. M. R. Brydon, A. P. Schnyder, and C. Timm, Phys. Rev. B {\bf 84}, 020501(R) (2011); A. P. Schnyder, P. M. R. Brydon, and C. Timm, Phys. Rev. B {\bf 85}, 024522 (2012).   

Majorana fermions in spin-singlet nodal superconductors with coexisting non-collinear magnetic order

Realizations of Majorana fermions in solid state materials have attracted great interests recently in connection to topological order and quantum information processing. We propose a novel way to create Majorana fermions in superconductors. We show that an incipient non-collinear magnetic order turns a spin-singlet superconductor with nodes into a topological superconductor with a stable Majorana bound state (MBS) in the vortex core or on the edge. Moreover the topologically-stable point defect of non-collinear magnetic order also hosts a zero-energy MBS. We argue that such an exotic non-Abelian phase can be realized in extended $t$-$J$ models on the triangular and square lattices. Our proposal suggests a new avenue for the search of Majorana fermions in correlated electron materials where nodal superconductivity and magnetism are two common caricatures.   

Tuning between s-wave and p-wave superconductors as well as emerging Majorana fermions in extended Hubbard lattices

We study spin-half fermions in one dimensional extended Hubbard lattices in which the superconducting pairing orders are induced by the tuning of nearest-neighbor charge and spin interactions. We derive gap equations for three p-wave (triplet) as well as one s-wave (singlet) pairing orders and obtain a phase diagram characterizing these orders as a function of interaction couplings. We find that the system can evolve between s-wave and p-wave pairing states, accompanied with the emergence of Majorana fermions in the p-wave regime, identified as a time-reversal invariant Kitaev Majorana chain. Finally we discuss the effects on the topological non-trivial states when time-reversal or SU(2) symmetry breaks.   

Robustness of Majorana modes in multiband topological superconductors

We investigate the robustness of Majorana modes in a multiband topological superconductor model belonging to symmetry class DIII, against various perturbations. In the three dimensional case, we find that in topological phases where an even number of Kramer pairs of Majorana modes exist on each boundary, these modes may become gapped under a boundary perturbation, despite time-reversal invariance being preserved. Conversely, in two dimensions, the gapless Majorana modes may remain gapless in the presence of certain time-reversal breaking fields or impurities. However, upon changing the strength of an applied longitudinal Zeeman field, a transformation from helical Majorana modes to chiral Majorana modes may be induced, accompanied by a quantum phase transition in the bulk.   

Majorana end modes in STM Fabricated Atomic Chains on the Surface of a Superconductor: Theory \& Experiment

The search for Majorana fermions (MF) in solid state devices has been hampered by the possible affects of disorder which may induce signatures similar to those expected by novel MF boundary states. Therefore it is important to identify clean solid state systems in which MF modes can be easily distinguished from disorder related effects. In this talk, we will present theoretical calculations and preliminary experimental results on chains of magnetic atoms on the surface of an s-wave superconductors. The theoretical efforts show that surprisingly short magnetic chains (20 atoms long or more) support MF under specific conditions depending on spins of the magnetic atoms and their coupling. We will describe these theoretical results along with experiments in which a scanning tunneling microscopy (STM) has been used to assemble chains of magnetic atoms (3d transition metals) on Nb and Pb single crystals. Presence of Majorana boundary modes in these structures can be probed using spatially-resolved STM spectroscopy.   

Topological defects and subgap excitations in two-band superconductors

Phase solitons are topological defects peculiar to two-band superconductors, which are associated with a $2\pi$ winding of the relative phase of the two superconducting condensates. The order parameter phase variation in each of the bands leads to the quasiparticle bound states whose energies are below the bulk gap. We calculate the single soliton energy as well as the interaction energy of two solitons, at arbitrary temperature. Applications to a similar system -- one or more domain walls in a chiral $p$-wave superconductor -- are discussed.   

Symmetry Protected Majorana fermions in topological superconductors

Recently, there are considerable interests in Majorana fermions in topological superconductors. It has been found that promising schemes to realize Majorana fermions is to break some of symmetries of the system. Indeed, by inducing the spin-orbit interaction and the Zeeman coupling which break inversion and time-reversal symmetries, conventional s-wave superconductors may support Majorana fermions on the boundaries. Moreover, by breaking the spin-rotation symmetry, spin-triplet superconductors may support Majorana fermions. Therefore, one might expect that symmetry is an obstruction to detect Majroana fermions. In this talk, however, we will show that this is not always the case. We show that symmetry may protect Majorana fermions in topological superconductors. As an example, we will show that Majorana Ising charater , which gives a detectable signal of Majorana fermion , is stabilized by symmetry of the system. We will also discuss some other roles of symmetry for Majorana fermions in topological crystalline superconductors.   

Majorana fermions in 3DTI with superconductivity

We study the problem of a strong 3D topological insulator (TI) with intrinsic superconductivity (SC). Particularly we present microscopic calculations using a low energy model of bulk massive Dirac fermions with mean field s-wave SC pairing. Introducing a kink in the mass in one spatial direction we can verify the appearance of localized (around the kink) states which correspond to the TI surface states and, with the further introduction of a vortex in the SC pairing, we are able to bind Majorana zero-modes (MZM's). The MZM's are known to be elusive particles in the sense that they are hard to detect. We then introduce a Majorana representation to the system Hamiltonian described above and propose an artificial doubling of this system which gives rise to a O(2) symmetry and allows us to define a conserved charge that can be used to probe for the presence of the MZM's. This doubled Majorana system then becomes an interesting playground, allowing us to search for masses which mix the different Hilbert spaces and study the behavior of this charge. We finish with a path-integral formulation of the problem through which we can integrate out the fermions and find an effective action for both, the electromagnetic as well as the corresponding to the O(2) conserved charge, gauge fields.   

Josephson-Majorana cycle in topological single-electron hybrid transistors

Charge transport through a small topological superconducting island in contact with a normal and a superconducting electrode occurs through a cycle which involves coherent oscillations of Cooper pairs and tunneling in/out the normal electrode through a Majorana bound state, the Josephson-Majorana cycle. We illustrate this mechanism by studying the current-voltage characteristics of a superconductor - topological superconductor - normal metal single-electron transistor. At low bias and temperature the Josephson-Majorana cycle is the dominant mechanism for transport. We discuss a three-terminal configuration that constitutes a direct probe of the non-local character of the Majorana bound states. Non-local cotunneling dominates over the local contributions and the current noise is maximally correlated independently of the length of the wire. Preprint: arXiv:1202.6357   

Superconducting Klein tunneling and AC Josephson effect in superconductor/topological insulator/superconductor junctions

We consider superconductor(S)/surface state of topological insulator(TI)/superconductor junctions (S) where the S regime describes the surface state of the TI with the proximity with the s-wave superconductor. The novelty of such S/TI/S junctions originates from the electron spin helicity (locking of the mometum and the spin for a surface of TIs) which leads to both the s-wave singlet and the p-wave triplet pairing on the surface underneath the superconductor. Existence of these two superconducting channels lead to interesting features in transport through these junctions. In particular we show that superconducting Klein tunneling and topological Andreev bound state (ABS) (state of hybridized two Majorana fermions)) occur for the normal incidence where ABS is protected against backscattering. For transport channels different than for the normal incidence, the scattering from the junction barrier generates an energy gap in the spectrum supporting non-topological ABSs. Due to mixed order parameter, the AC Josephson effect is fractional showing higher odd harmonics. We conclude that favorable conditions for the observation of the topological ABS exist in narrow TI links with a small number of open channels close to one.   

Josephson currents through topological insulator surfaces

Motivated by recent experiments carried out on superconductor -- 3D topological insulator -- superconductor junctions, we study the transport properties of these junctions. Transport is believed to be dominated by the surface states of the topological insulator, and we discuss the effects of the junctions geometry on the Josephson supercurrent in the presence of a magnetic field.   

X-ray absorption spectroscopy of doped Bi2Se3 and Bi2Te3

Topological insulators are a prototypical system to investigate correlated electron physics. Analogous to quantum hall states, these remarkable materials have conducting surface/edge states surrounding an insulating in the bulk state. Unlike quantum hall systems the conducting states of topological insulators do no arise from an applied magnetic field but instead emerge as a result of spin-orbit interactions. Furthermore, doping with different 3d-metals can significantly alter the electronic structure, inducing superconductivity in the case of CuxBi2-xSe3, and ferromagnetism in Bi2-xMnxTe3. In an effort to elucidate the role of the local bonding environment on the electronic structure in the chalchogenide topological insulators, Bi2Te3 and Bi2Se3 with various transition metal as dopants, we have preformed a series of soft x-ray absorption spectroscopy measurements.   

Session M13: Focus Session: Topological Materials - Surface Effects

Photoelectron spin-flipping and texture manipulation in a topological insulator

A hallmark characteristic of the recently discovered topological insulators is their protected metallic surface states. Electrons in these surface states are spin polarized with their spins governed by their momentum, resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser, we found that the spin polarization of the resulting photoelectrons exhibits rich phenomena previously unobserved. These surprising results provide insight into the physics of these fascinating materials and the use of spin-resolved photoemission in general.   

Interaction between Dirac fermions and phonons on the (001) surface of the strong 3D topological insulator Bi$_2$Te$_3$

We report on studies of the interaction of Dirac fermion quasiparticles with phonons on the (001) surface of the strong 3D topological insulator Bi$_2$Te$_3$. Studying this coupling is essential for determining the technological viability of this new class of materials. We employed inelastic helium atom scattering to determine surface phonon dispersions along the $\Gamma$M and $\Gamma$KM directions. In contrast to our previous studies on Bi$_2$Se$_3$,\footnote{Zhu, et al. Phys. Rev. Lett. 107, 186102, 2011.} which exhibited a strong Kohn anomaly at $2k_F \approx 0.2$\AA$^{-1}$ in a low-lying optical phonon branch, the current results show a weaker Kohn anomaly at $2k_F \approx 0.1$\AA$^{-1}$ in a similarly low-lying branch. The lower value of $k_F$ is consistent with the smaller carrier concentration in Bi$_2$Te$_3$ as evidenced by Hall conductivity measurements. Our results are further substantiated by lattice dynamical calculations performed within the pseudo-charge model. We also report on a detailed analysis of the electron-phonon coupling as a function of phonon branch index and wave vector utilizing the methods we recently developed.\footnote{Zhu, et al. Phys. Rev. Lett. 108, 185501, 2012. }   

Polarization-Dependent Scanning Photocurrent Microscopy of Bi2Se3

We measured the spatially-resolved response of Bi2Se3 topological insulator to polarized light by means of scanning photocurrent microscopy. A polarized laser spot of 1 um diameter is raster scanned over a gate-controlled Bi2Se3 two-contact device oriented at 45 degrees to the plane of incidence, and the photo-generated current is measured at each point for varying light polarization states from linearly polarized to right-handed circularly polarized to left-handed circularly polarized. The data is, in turn, used to differentiate the contributions from helicity-dependent spin-orbit coupling effects and helicity-independent photovoltaic and photothermoelectric effects, as well as map their spatial distributions over the device. The experiment is repeated for different carrier densities, through varying the voltage of the back gate, to investigate the dependence of the photoresponse on the carrier density.   

Investigation of Positron Sticking to the Surfaces of Topological Insulators

We describe experiments aimed at probing the sticking of positrons to the surfaces of topological insulators. In these experiments, a magnetically beam will be used to deposit positrons at the surface of Bi$_{2}$Te$_{2}$Se. The energy spectra and intensities of electrons emitted as a result of Positron Annihilation induced Auger electron Spectroscopy (PAES) provides a distinct element specific signal which can be used to determine if positrons can be trapped efficiently into a surface localized bound state. The experiments are aimed at determining the practicality of using positron annihilation to selectively probe the critically important top most layer of topological insulator system.   

Emergent quantum size effects at topological insulator surfaces

Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial Z$_2$ topology. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure\footnote{P.D.C. King et al., Phys. Rev. Lett., 107 (2011) 096802 } whose origin and properties have proved controversial. Here we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states.\footnote{M.S. Bahramy, P.D.C. King et al., Nature Commun. 3 (2012) 1159} In combination with circular dichroism in angle-resolved photoemission experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study sheds new light on the surface-bulk connectivity in topological insulators, and reveals how this is modified by quantum confinement.   

Surface metal doping of topological insulator Bi$_{2}$Se$_{3}$ thin films

Three-dimensional topological insulators have attracted much attention due to their spin-momentum locked surface Dirac states, which have been proposed as the basis for spintronics and quantum computing. In the case of Bi$_{2}$Se$_{3}$, thin films grown by molecular beam epitaxy are typically heavily doped n-type, which places the Fermi level outside its band gap, making it challenging to develop devices that rely on the behavior of surface Dirac fermions. In this work, we grow high quality Bi$_{2}$Se$_{3}$ films and tune the topological surface state by metal doping on the surface. The atomic structure and morphology of the metal/Bi$_{2}$Se$_{3}$ are investigated by \textit{in situ} scanning tunneling microcopy. Furthermore, scanning tunneling spectroscopy reveals that the position of Dirac energy can be shifted by as much as 150 meV. These results and comparison with first-principles calculations will be discussed at the meeting.   

Carrier control via charge transfer at the topological-insulator/organic-molecule interface

A topological insulator is a material that behaves as an insulator as a bulk state, while permitting metallicity on its Dirac cone surface state. One of the most serious issues of recent researches in this field, however, has been the fact that the Fermi levels in many TIs actually fall in either the conduction or valence band due to the naturally occurring defects and must be controlled by further doping. We report here that the major electron carriers on the SS of a Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$(BSTS) single crystal can be converted to the hole carriers via interface control using 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F4-TCNQ), with strong electron affinity. The evidence can be elucidated using a detailed three-carrier model. The results apparently demonstrate that the charge transfer at the TI/organic-molecule interface is very efficient in order to control the carrier density of TIs, particularly on the SS. Our present results will be very important for studying the fundamental aspects of TIs as well as their future device applications.   

The Effects of Different Ambient Environments on the Electrical Properties of Bi$_{2}$Se$_{3}$ Thin Films over Time

It has been recognized recently that the Bi$_{2}$Se$_{3}$ surface is highly susceptible to environmental doping at room temperature when exposed to ambient air. The change in conductivity is correlated to oxidation of the surface; however, the roles of O$_{2}$ and residual H$_{2}$O in the process are not fully understood. In this study, we investigated the effects of different ambient environments (air, O$_{2}$, N$_{2}$, H$_{2}$O) on the electrical properties of Bi$_{2}$Se$_{3}$ thin films grown by hybrid physical-chemical vapor deposition. Hall measurements were performed on samples exposed to each of the gases over a period of several hours to days. The electron concentration of the Bi$_{2}$Se$_{3}$ films initially decreased upon exposure to air but began to rapidly increase and continued to do so over the next several hours. The use of an O$_{2}$ purge resulted in a large initial decrease in electron concentration suggesting that O$_{2}$ rapidly diffuses into Bi$_{2}$Se$_{3}$ and partially compensates the native donors. Over time, however, the electron concentration began to rise rapidly in a similar manner to that observed in air. Exposure of the surface to water vapor resulted in nearly identical behavior to that obtained in air. In contrast, measurements carried out under a N$_{2}$ purge demonstrate a small initial decrease in electron concentration but do not exhibit an appreciable increase in electron concentration even after 24 hours. The mechanism of surface oxidation and conductivity change will be discussed.   

Modeling electron dynamics at the topological insulator-metal interface

The surface environment of the topological insulators possesses ideal properties such as spin-polarized conductivity and suppressed scattering for advanced electronics applications. A major key missing ingredient in this connection is lack of understanding of how topologically ordered electrons respond to the presence of interfaces and various surface terminations that constitute device components at the nanometer scale. To explore these issues we have developed a Green's function implementation of the $k \cdot p$ model to numerically simulate junctions and surfaces of topological insulator $\rm Bi_2Se_3$ based on experimentally measured bulk electron kinetics. Our model explains a number of interesting features observed in ARPES experiments for surface deposition in $\rm Bi_2Se_3$.   

Interaction of Dirac fermions with surface lattice excitations and electron-phonon coupling on topological insulator surfaces

Surface Dirac fermions are robust against backscattering, but other scattering events can affect their anticipated ballistic behavior. Technical improvements may minimize or eventually eliminate surface defects, but phonons are always present. Consequently, coupling to phonons should be the dominant scattering mechanism for Dirac fermions on these surfaces at finite temperatures. Recent measurements of phonon dispersion curves on the (001) surfaces of several binary and ternary topological insulators were carried out using coherent inelastic helium beam surface scattering techniques. The dispersion curves reveal similar features among these materials: first, the absence of long-wavelength Rayleigh waves. Second, the appearance of a low-lying optical phonon branch with isotropic convex dispersive character in the vicinity of the $\Gamma$-point. Lattice dynamics calculations based on the pseudo-charge model show that the optical phonon branch appears with a concave shape when Dirac fermions are absent, but its dispersion changes to a convex shape when Dirac fermions are present. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model. Moreover, this optical branch displays a V-shaped minimum at approximately $2\mathbf{k}_F$ that defines a Kohn anomaly. Using a Hilbert transform, we are able to obtain the imaginary part of the phonon self-energy from the real part fitted to the dispersion curve of the surface optical phonon branch. From this imaginary part of the self-energy we obtain a branch-specific electron-phonon coupling constant as a function of wave-vector. The average electron phonon coupling associated with this branch is found to be strong, especially for Bi$_2$Se$_3$, reflecting the pronounced renormalization described above.   

Direct Real Space Imaging of Quantum Spin Hall Edge States in HgTe Quantum Well

Microscopic real space imaging of the helical edge states is an important milestone to fully elucidate quantum spin Hall effect as a new state of quantum matter. By employing a unique cryogenic microwave impedance microscope, we directly imaged quantum spin Hall edges in a gapped HgTe quantum well. The edge state size increases monotonically as the Fermi level is tuned from p-type across the Dirac point into n-type. Whereas this result is counter-intuitive within any particle-hole symmetric model, it actually agrees well with the 8-band model of real material. Real space evolution of the edge states shows surprising dependence on the magnetic field which could not be explained by Landau level physics assuming a clean system. Alternative scenarios will be discussed.   

Layer-by-layer entangled spin-orbital texture of the topological surface state in Bi$_2$Se$_3$

With their spin-helical metallic surface state, topological insulators (TI) define a new class of materials with a strong application potential in quantum electronic devices. Technological exploitation depends on the degree of spin polarization of the topological surface state (TSS) - assumed to be 100$\%$ in phenomenological models. Yet in real materials, spin- and angle-resolved photoemission spectroscopy (ARPES) showed that the TSS spin polarization varies over a wide range: 20-85$\%$. This striking variation in TSS spin polarization has remained unexplored, leaving an undefined application prospect of TIs. Here we present a light-polarization study of ARPES momentum maps to unveil the entangled spin-orbital texture of the TSS in Bi$_2$Se$_3$. By determining the layer-by-layer evolution of this spin-orbital entanglement, we solve the puzzle of the observed TSS spin polarization and also provide means to manipulate the spin polarization of photoelectrons and photocurrents in TI devices.   

Session M14: Focus Session: Patterned Magnetic Nanostructures

Magnetic ToF GISANS on self-assembled nanoparticles

Nanoparticle superlattices can be considered as novel type of materials with controllable electronic, optical and magnetic properties. Their building blocks are nanoparticles (or ``nanocrystals'') from a metallic, metal-oxide, or semiconducting material or hybrid between different materials. Using self-assembling techniques it is possible to create a large amount of highly ordered 3D structures, which we have investigated for their structural and magnetic properties. The lateral ordering is quantified using electron microscopy and grazing incidence small angle X-ray scattering (GISAXS) [1,2,4]. The macroscopic magnetic behavior and correlations are investigated by superconducting quantum interference device (SQUID) magnetometry [1,3]. Utilizing the time of flight (ToF) magnetism reflectometer at SNS the magnetic correlations have been studied with polarized GISANS and PNR. \\[4pt] [1] M. J. Benitez et al., J. Phys.: Condens. Matter 23, 126003 (2011).\\[0pt] [2] G. A. Badini Confalonieri et al., Nanotechnology 22, 285608 (2011).\\[0pt] [3] A. Ebbing et al., Phys. Rev. B. 84, 012405 (2011).\\[0pt] [4] D. Mishra et al., Nanotechnology 23, 055707 (2012).\\[0pt] [5] E. Josten et al. (unpublished).   

Anomalous Magnetoresistance Effect in Topographical Nanoengineered Material

Recent developments in nanofabrication allow for the engineering of a broad range of topographical materials with strong implications in spin caloritronics of condensed matter physics. We have applied the top down approach to create a series of nanoengineered materials, which consists of locally hexagonal periodic array of Co dots (12 nm in diameter and 3 nm in thickness, with a periodicity of 28 nm) in direct multidirectional contact with encapsulating thin layer of polycrystalline Cu film (15-30 nm). The electrical transport measurements on the nanoengineered materials unveiled a host of interesting properties that includes the giant thermal hysteresis, which is onset above the room temperature, and anomalous magnetoresistance (MR) behavior. The thermal hysteresis exhibits strong magnetic field dependence, applied perpendicular to the substrate. The most unusual behavior, perhaps, is manifested by MR oscillations, which occur only in the initial field scan in a very unusual temperature range of 100 K \textless T \textless 200 K. The qualitative interpretation of the experimental results suggests that the spin-orbit-type coupling between giant localized moments in periodic sites and the surrounding conduction electrons play important role in the anomalous MR oscillation.   

Magnetic Dipole Interaction on a Square Lattice

We have studied interactions and phase transitions of circular magnetic islands with dipole character on a square lattice. By lithographic means we have prepared square patterns of periodicity 300 nm decorated with circular islands of 150 nm diameter using Pd0.87Fe0.13 as magnetic alloy. Below the Curie temperature of 260 K each island is in a ferromagnetic, single domain state with dipolar character and zero in-plane anisotropy. Below a second transition temperature the dipoles start to interact. MOKE measurements show a characteristic change in the magnetic hysteresis for temperatures below 160 K with increasing coercivity for decreasing temperatures. Furthermore, below the second transition the in-plane hysteresis becomes anisotropic, having an easy axis along [10] direction and a hard axis along [11] direction. SPEEM experiments at BESSY II of the HZB with circularly polarized incident photons tuned to the Fe L3 - edge show clearly the development of dipolar chains below the second phase transition that increase in length with decreasing temperature. Neighbouring chains are found to be oriented parallel as well as antiparallel.   

Real-time imaging of magnetic-field gradient directed self-assembly of magnetic nanoparticles into patterns using magnetic recording media

We employ enormous magnetic field gradients at the surface of disk drive media to self-assemble ferrite nanoparticles from a colloidal fluid onto the medium surface. Thus we ``nanomanufacture'' a user-programmed and magnetically-recorded pattern with demonstrated 25 nm precision. Using a low-noise CCD camera for bright-field microscopy with a 40x water dipping lens, we demonstrate real-time optical imaging of the pattern formation. By introducing concentrated ferrofluid to a water solution covering the recording medium, we observe both diffusion of the ferrofluid as well as self-assembly of nanoaprticles onto the magnetic field pattern recorded on the disk. The average intensity of the nanoparticle pattern increases exponentially and then saturates, while the overall brightness of the image decreases exponentially, over both patterned and unpatterned regions. These results hint at interesting nanoparticle dynamics during the initial ferrofluid diffusion and after the nanoparticle assembly process occurs on the disk medium surface. We suggest real-time optical microscopy can help explain the dynamics of colloidal magnetic nanoparticles in the presence of extreme magnetic field gradients which are not employed in typical magnetophoretic assembly.   

Self-assembly of magnetic nanoparticles in a liquid-crystalline media

We investigate the self-assembly of magnetic Fe$_{3}$O$_{4}$ nanoparticles (NPs) dispersed in a liquid crystal (LC) matrix. The NP assembly is driven by the temperature-induced transition of the LC from the isotropic to the nematic phase. Using magneto-optical Kerr effect (MOKE) and polarized optical miscroscopy, we observe that the NPs are mostly expelled into the isotropic regions, finally ending up clustered around LC defect points when the transition is complete. We use NPs with diameters between 10-30 nm and the concentration of NPs in the LC media range from 0.02{\%} to 0.2{\%} by weight. We find that the resulting NP assemblies exhibit superparamagnetic and ferrimagnetic behavior, depending on their sizes.   

Toward Dynamic Control over Ordered Nanoparticle Monolayer Fabrication by Electrophoretic Deposition

A primary challenges to the implementation of nanoparticles into device applications is the rapid production of densely packed, ordered films of these materials. The ordered arrangement of the nanomaterials is required for applications that rely on the collective interactions of the constituents or on the high density of the materials for information storage or surface protection. Rapid fabrication is a manufacturing demand to reduce operation costs and to streamline production. We have achieved a substantial milestone toward the mass production of macroscopic monolayers and thin films of colloidal nanocrystals on various substrates, including conducting metals and doped-semiconducting substrates. Our approach combines the advantages of liquid-phase, colloidal suspension approaches with the superior deposition rate, size scalability, and cost effective features of electrophoretic deposition (EPD) to achieve monolayer-by-monolayer deposition control over nanocrystal films with various degrees of internal order. Such work has the potential for the fabrication of industrial scale quantities and surface areas of these colloidal solids. Our recent research activities have demonstrated film formation with titanium dioxide nanoparticles and core/shell iron oxide nanoparticles.   

Magnetic Nanostructures by Templated Self Assembly

Self assembly techniques provide a route to the rapid synthesis of nanostructures whose long range order and registration can be controlled by pre-patterning the substrate lithographically. This presentation will focus on two processes. First, masks made from templated block copolymer films are used for patterning of metallic magnetic films and multilayers into arrays of lines or dots with feature sizes of 10 nm and above. Second, codeposition of spinel and perovskite oxide phases leads to epitaxial thin film nanocomposites in which ferrimagnetic cobalt ferrite pillars are embedded in a ferroelectric bismuth orthoferrite matrix. The pillars form a regular array when templated by pits of pitch 60 nm and above, and have a strong magnetoelastic anisotropy. Magnetic properties of the resulting nanostructured materials are described.   

Assembly and manipulation of planar ordered magnetic micro-bead clusters

The driving forces for many complex systems in nature often rely on the competition and cooperation between interacting simple components. These natural systems yield a framework to develop artificial phenomena and devices. In this vein we have investigated interacting micrometer sized beads containing superparamagnetic particles where competing deterministic and stochastic forces are tuned to create ordered clusters that are then maneuvered in a cooperative manner. Ferromagnetic microwires patterned on a silicon surface are utilized to regulate the magnetic interactions by confining the fluid-borne beads to a planar surface. Oriented weak external magnetic fields yield repulsive inter-particle forces that compete with local forces directed toward trap sites whose locations are determined by the underlying magnetic microwire pattern. The self-assembled ordered ``clusters'' of interacting dipolar beads are also subject to observable Brownian fluctuations. The geometrical order and inter-bead spacing within individual clusters are magnetically tuned, while entire clusters can be transported to nearby traps and reform into predictable shapes upon arrival. These features offer the potential for interesting engineering and biophysics studies.   

Characterization of barium hexaferrite thick films deposited by aerosol deposition method

We present results on the first deposition of nano-crystalline barium hexaferrite (BaFe$_{\mathrm{12}}$O$_{\mathrm{19}})$ (BaM) powder onto copper, silicon, and sapphire substrates using the aerosol deposition method (ADM). BaM is an important magnetic compound with many applications, including, permanent magnets, magnetic recording, and components in electronic circuits. Advantages of the ADM include the ability to form up to hundreds of microns thick, dense ceramic films at room temperature at high deposition rate on a variety of substrates. Deposition is achieved by creating a pressure gradient that accelerates particles in the aerosol to high velocity. Upon impact with the target the particles fracture and embed. Continual deposition forms the thick compacted film. Scanning electron microscopy and profilometry suggest that the film is compact and well adhered to the substrate surface. We compare magnetization curves of the raw nano-crystalline powder, pressed sintered powder, and deposited film. Our typical values of magnetic saturation are about 60 emu/g, coercive field 2 kOe, remnant magnetization 30 emu/g, and squareness 0.5. The similarity between the deposited films suggests comparable deposition quality across this range of substrate hardness. The reduction in remnance and saturation compared with the powder may suggest a more random orientation of moments and an increase in fracturing of the particles. We conclude with preliminary attempts to magnetically align particles during deposition.   

FMR Study of Quasicrystalline Arrays of Antidots in Permalloy Films

We have used electron beam lithography to pattern permalloy films of thickness 25 nm with \textbf{\textit{quasiperiodic, }}five-fold rotationally symmetric Penrose tilings of antidots (AD). Two samples were fabricated with AD kites and darts having long (d$_{\mathrm{1}})$ and short edges (d$_{\mathrm{2}})$ equal to 1620 nm or 810 nm, and 1000 nm or 500 nm, respectively, with fixed Py line width of 100 nm. We have studied broad-band (RF frequencies 10 MHz \textless f \textless 15 GHz, DC applied fields -3.5 kOe \textless H \textless 3.5 kOe) and narrow-band FMR (f $=$ 9.7 GHz, 0 \textless H \textless 8 kOe) for various angles between the in-plane DC field and the array edge. BBFMR spectra for f \textless 4 GHz exhibit rich, highly reproducible structure, in spite of low-field (\textbar H\textbar \textless 500 Oe) hysteresis, including a \textbf{\textit{frequency-independent}} (implying localized) mode near H $=$ 0 Oe. Both low-field FMR data and dynamic simulations exhibit two-fold rotational symmetry instead of the expected five-fold symmetry, which we attribute to an unsaturated state. Higher-field (\textbar H\textbar \textless 12 kOe) simulations exhibit ten-fold rotational symmetry, which we attribute to the symmetry of the demagnetization fields.   

Magnetization Reversal Study of Geometrically Frustrated, Quasiperiodic Antidot Arrays

We have used electron beam lithography to pattern quasiperiodic AD arrays in permalloy films of thickness 25 nm. Two five-fold rotationally symmetric Penrose tilings were fabricated with AD kites and darts having long (d1) and short edges (d2) equal to 1620 nm or 810 nm, and 1000 nm or 500 nm, respectively, with fixed Py line width of 100 nm. Two eight-fold Ammann tilings were patterned with square and rhomboid AD of edge lengths of 1000 nm or 2000 nm, resp. Magnetization reversal was studied at various angles between the in-plane, applied DC magnetic field H and the quasiperiodic array. We observed very reproducible hysteresis curves with low-field anomalies not present in our previous studies of periodic, square arrays of square-, circular- and diamond-shaped AD; e.g., for the Penrose tilings, we observed four reproducible knee anomalies (both for 81 \textless H \textless 331 Oe, and for -19 \textgreater H \textgreater -71 Oe). Micromagnetic simulations exhibit systematic evolution of domain walls (DW) in the hysteretic regime due to DW pinning by edges of the quasicrystalline pattern, which correlates DW evolution with observed features in magnetic hysteresis.   

Observation of Novel Low-Field FMR modes in Permalloy Antidot Arrays

Permalloy films of thickness 23 nm were patterned with square arrays of square antidots (AD) with feature size D $=$ 120 nm, and lattice constants d $=$ 200, 300, 500 and 700 nm (total sample area $=$ 2 mm x 2mm), using electron beam lithography. Our broad-band (frequencies f $=$ 10 MHz-15 GHz) and narrow-band (9.7 GHz) FMR measurements of even dilute (D/d \textless \textless 1) AD lattices (ADL) reveal remarkably reproducible absorption spectra in the low-frequency, hysteretic regime in which disordered domain wall (DW) patterns and unsaturated magnetization textures are expected for unpatterned films, but in the present case are strongly affected by the periodic ADL. Other modes in the saturated regime exhibit strong dependence on the angle between the applied DC field H and the ADL axes, as confirmed by our micromagnetic simulations. Novel modes are observed at DC fields above that of the uniform mode, which simulations indicate are localized at AD edges. Other novel modes are observed for DC fields below that of the uniform mode, which simulated power and phase maps indicate are confined to ADL interstices oriented parallel to H. These results show even dilute AD concentrations can effect strong control of DW evolution.   

Micromagnetic Simulations of Quasiperiodic (Penrose Tiling) Antidot Arrays

We have performed static and dynamic micromagnetic simulations of permalloy antidots (AD) patterned on quasiperiodic arrays of 25 nm film thickness. Two Penrose tilings (five-fold rotationally symmetric) were simulated with AD kites and darts with long (d$_{\mathrm{1}})$ and short edges (d$_{\mathrm{2}})$ equal to 1620 nm or 810 nm, and 1000 nm or 500 nm, respectively, and fixed Py line width of 100 nm. Two Ammann tilings were patterned with square and rhomboid AD of edge lengths 1000 nm or 2000 nm, and line width of 100 nm. Our simulations exhibit FMR modes not previously predicted; for example, power and phase maps for Penrose tilings exhibit three bulk modes (at angles $\varphi \quad =$ 0$^{\mathrm{\thinspace o}}$, 72$^{\mathrm{\thinspace o}}$ and 144$^{\mathrm{o}}$ with respect to in-plane applied DC field H) and two edge modes ($\varphi \quad =$ 72$^{\mathrm{\thinspace o}}$ and 144$^{\mathrm{o}})$ for H $=$ 1.2 kOe. Static micromagnetic simulations exhibit highly repeatable evolution of domain walls (DW) with apparent long-range order in the \textbf{\textit{hysteretic regime}}. We attribute this remarkable reproducibility in a \textbf{\textit{geometrically frustrated, aperiodic system}} to magnetic reversal controlled by DW pinning by AD edges.   

Session M15: Focus Session: Spin/orbital Frustration and Short-range Order

Spin-orbital quantum liquid on the honeycomb lattice

The symmetric Kugel-Khomskii can be seen as a minimal model describing the interactions between spin and orbital degrees of freedom in transition-metal oxides with orbital degeneracy, and it is equivalent to the SU(4) Heisenberg model of four-color fermionic atoms. We present simulation results for this model on various two-dimensional lattices obtained with infinite projected-entangled pair states (iPEPS), an efficient variational tensor-network ansatz for two dimensional wave functions in the thermodynamic limit. This approach can be seen as a two-dimensional generalization of matrix product states - the underlying ansatz of the density matrix renormalization group method. We find a rich variety of exotic phases: while on the square and checkerboard lattices the ground state exhibits dimer-N\'eel order and plaquette order, respectively, quantum fluctuations on the honeycomb lattice destroy any order, giving rise to a spin-orbital liquid. Our results are supported from flavor-wave theory and exact diagonalization. Furthermore, the properties of the spin-orbital liquid state on the honeycomb lattice are accurately accounted for by a projected variational wave-function based on the pi-flux state of fermions on the honeycomb lattice at 1/4-filling. In that state, correlations are algebraic because of the presence of a Dirac point at the Fermi level, suggesting that the ground state is an algebraic spin-orbital liquid. This model provides a good starting point to understand the recently discovered spin-orbital liquid behavior of Ba$_3$CuSb$_2$O$_9$. The present results also suggest to choose optical lattices with honeycomb geometry in the search for quantum liquids in ultra-cold four-color fermionic atoms.   

Spin-orbital short-range order in the honeycomb-based quantum magnet Ba$_{3}$CuSb$_{2}$O$_{9}$

The realization of quantum correlated matter beyond one dimension has been vigorously pursued in geometrically frustrated spin systems for decades. In frustrated magnetic materials, however, symmetry breaking of orbital and chemical origin is usually found to induce semi-classical spin freezing. In this talk, I present a contrast case where spins and possibly orbitals remain in a liquid state down to low temperature even in a highly disordered structure of 6H-perovskite Ba$_{3}$CuSb$_{2}$O$_{9}$. Our comprehensive experimental analysis indicates that the geometrical frustration of Wannier's Ising antiferromagnet on a triangular lattice can be exploited to build a nano-structured bipartite honeycomb lattice from electric dipolar spin-1/2 molecules. Despite a strong local Jahn-Teller distortion about the Cu$^{2+}$ ion, the resulting spin-orbital random bond lattice not only retains hexagonal symmetry averaged over time and space, but it supports a gapless excitation spectrum without spin freezing down to ultralow temperatures. This is the work based on the collaboration with K. Kuga, K. Kimura, R. Satake, N. Katayama, E. Nishibori, H. Sawa, R. Ishii, M. Hagiwara, F. Bridges, T. U. Ito, W. Higemoto, Y. Karaki, M. Halim, A. A. Nugroho, J. A. Rodriguez-Rivera, M. A. Green, C. Broholm.   

Spin-orbital entanglement due to dynamical Jahn-Teller effect

Quantum spin liquid (QSL) state is one of the fascinating themes in correlated electron systems. Recently, a new candidate of the QSL state is experimentally reported in a layered copper oxide Ba$_{3}$CuSb$_{2}$O$_{9}$. In this material, a Cu$^{2+}$ has the $e_{g}$ orbital degree of freedom and the dynamical Jahn-Teller effect (DJTE) is suggested to play a key role for the emergence of the QSL state. Motivated from the recent experiments in Ba$_{3}$CuSb$_{2}$O$_{9}$, we study the DJTE in the spin-orbital coupled system and examine a possibility of the QSL state in a spin-orbital system with lattice vibrations. In particular, we focus on the competitive or cooperative phenomena between the superexchange (SE) interaction and the DJTE. A SE interaction Hamiltonian is derived from the $d$-$p$ type Hamiltonian and the DJTE Hamiltonian for the low-lying vibronic states is represented by the orbital pseudo-spin and the lattice vibration. We analyze the model, where these two interactions are taken into account on a honeycomb lattice, by using the cluster mean-field approximation with the exact diagonalization (ED) method and the combined method of the quantum Monte-Carlo method and ED method. We find that magnetic orders are unstable in a wide parameter region and a spin-singlet dimer state associated with an orbital order is realized. With increasing the DJTE, the orbital order is strongly suppressed and a resonance state of the spin-orbital dimers appears. We confirm that the spin and orbital degrees of freedom are strongly entangled with each other in this resonance state.   

Raman phonon study of Jahn-Teller distortion in Ba$_3$CuSb$_2$O$_9$

The frustrated magnet Ba$_3$CuSb$_2$O$_9$ does not exhibit either structural or magnetic ordering down to the lowest measured temperatures and is of great current interest as a spin-liquid candidate. It has been proposed recently that the lack of ordering is due to a static or dynamic Jahn-Teller distortion that leads to orbital disorder [1]. We use phonon Raman scattering at temperatures between 20 and 380 K to investigate Jahn-Teller distortion in crystals with different Sb:Cu stoichiometry. We focus on phonons in the range of 500-800 cm$^{-1}$ attributable to oxygen vibrations. In addition to signatures of the strong disorder due to Cu-Sb site mixing present in these materials, we observe mode-splitting due to a static Jahn-Teller distortion below 200 K in samples that undergo a transition to an orthorhombic phase. In contrast, samples that remain hexagonal to the lowest temperatures do not show such mode splitting. References: [1] S. Nakatsuji et al. Science 336, 559 (2012)   

Orbital short range correlation in Ba$_3$CuSb$_2$O$_9$

Ba$_3$CuSb$_2$O$_9$ is consist of short range honeycomb lattice of $S=1/2$ Cu$^{2+}$ with the Weiss temperature $-55$~K[1]. Because of the similar energy scale of the spin and orbital degrees of freedom, the interaction between them is important in this system. We have studied the behavior of the orbital degree of freedom, which can fluctuate under an effect of frustrated spin system, by means of x-ray diffuse scattering method. Measurements were performed with a four-circle diffractometer at BL-3A of the Photon Factory, KEK, Japan. Clear Huang scattering that reflects lattice strain induced by the Jahn-Teller distortion was observed. The orbital correlation provides additional scattering intensity around the $\Gamma$ point in low temperatures. The lifetime of the strain field was examined by inelastic x-ray experiments performed at BL-35XU of the SPring-8, Japan. Quasielastic intensity corresponding to the Huang scattering had slightly broader energy width than the instrumental resolution, and the lifetime was estimated as 3 picoseconds.\\[4pt] [1] S. Nakatsuji et al., Science, {\bf 336}, 559 (2012).   

Local Probe Studies of the Quantum Honeycomb Antiferromagnet Ba$_3$CuSb$_2$O$_9$

The 6H-perovskites, Ba$_3M$Sb$_2$O$_9$, have generated an enormous amount of interest in the last two years following the possible discovery of quantum spin liquid physics in two such materials. We present local probe studies (muon spin rotation and nuclear magnetic resonance) on the spin-1/2 honeycomb antiferromagnet Ba$_3$CuSb$_2$O$_9$. We show that the system presents no spin freezing down to temperatures as low as 20 mK. NMR measurements show evidence of a spin gap and suggest that the material has a random singlet ground state rather than the alternative spin-orbital liquid state.   

Electronic structure and the suppression of the Jahn-Teller distortion in the quantum antiferromagnet Ba$_3$CuSb$_2$O$_9$

In recent years, the field of geometrically frustrated materials have regained interest by the dicsovery of several candidates for quantum spin liquids. The antiferromagnet Ba$_3$CuSb$_2$O$_9$ is one such material where the $S=\frac{1}{2}$ on a triangular (more recently hexagonal) lattice leads to frustration. Using density functional methods, we study the electronic structure of the material, both in the triangular lattice as well as the honeycomb structure. For both structures, a simple tight-binding description involving the Cu ($e_g$) orbitals describes the band structure rather well, confirming the central role of these orbitals in the physics of the problem. It has been suggested that the Jahn-Teller effect could play an important role in the properties of the system. We find that in spite of the presence of the Cu ($d^9$) ion, the Jahn-Teller coupling is surprisingly weak in the material, which suppresses any Jahn-Teller distortion of the CuO$_9$ octahedra in the compound.   

Ba NMR studies of the triangular lattice antiferromagnets Ba$_3$MSb$_2$O$_9$ (M=Co, Ni)

Ba$_3$MSb$_2$O$_9$, with M=Co, Ni are triangular lattice magnetic systems with near-neighbor antiferromagnetic exchange. For M=Co (S=1/2), the ground state is ordered and there are field-induced changes to the symmetry, whereas for the Ni (S=1) system there is no evidence for a phase transition to a lower-symmetry phase. Here we report on Ba nuclear magnetic resonance (NMR) spectroscopy and spin-lattice relaxation measurements for both systems. For example, the temperature dependence of the relaxation rate is independent of temperature for the Ni-based compound, and is similar to what is observed for the high-symmetry phase of the Co compound. The spin structures for the ordered phases of the Co material are also explored.   

Physical Properties of new A$_{2}T$O$_{3}$ ($A = $ Na, Li, $T = $ Ru, Rh, Ir) materials

The layered iridates $A_{2}$IrO$_{3}$ ($A \quad =$ Na, Li) have recently been suggested to be spin-orbit driven Mott insulators with their magnetism being consistent with an extended Kitaev-Heisenberg model [1-6]. While Na$_{2}$IrO$_{3}$ was found to lie deep in a magnetically ordered region, Li$_{2}$IrO$_{3}$ was suggested to lie close to the spin-liquid state expected in the strong Kitaev limit [6]. To explore the effect of chemical pressure and the effect of varying the spin-orbit coupling we have synthesized the new materials Li$_{2}$RhO$_{3}$, Na$_{2}$RuO$_{3}$, and Na$_{2}$Ir$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$O$_{3}$. We will present magnetic, electrical transport, and heat capacity measurements on these materials.\\[4pt] [1] Y. Singh and P. Gegenwart, Phys. Rev. B \textbf{82}, 064412 (2010).\\[0pt] [2] Y. Singh et al., Phys. Rev. Lett. \textbf{108}, 127203 (2012)\\[0pt] [3] S. K. Choi et al., Phys. Rev. Lett. \textbf{108}, 127204 (2012).\\[0pt] [4] F. Ye et al., Phys. Rev. B \textbf{85}, 180403 (2012)\\[0pt] [5] R. Commin, et al., Phys. Rev. Lett. (in press) 2012.\\[0pt] [6] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev.Lett. \textbf{105}, 027204 (2010).   

Neutron Scattering Study on the Spin-Orbital Coupling in Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$(x$=$0.2, 0.4, and 0.6)

Two consecutive magnetic transitions have been reported in MnV$_{2}$O$_{4}$ compounds: the first transition is collinear and is from paramagnetic to ferrimagnetic state; The second transition, which is noncollinear, is accompanied by a tetragonal distortion, which produces an excitation gap in the magnetic spectrum [1]. However, the V-V distance is interfered with Co doping, and there is no structural phase transition observed in CoV$_{2}$O$_{4}$ down to 10 K [2]. In order to study the Co-doping effects on the structural and magnetic properties of Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$, elastic and inelastic neutron scattering is applied in our experiments and the interaction between magnetism and orbital will be discussed.\\[4pt] [1] V. O. Garlea, et al.,Phys. Rev. Lett. \textbf{100}, 066404 (2008);\\[0pt] [2] A. Kismarahardja, et al., Phys. Rev. Lett. \textbf{106}, 056602 (2011)   

Vibronic excitations in the orbitally active A-site spinels FeSc$_2$S$_4$, FeCr$_2$O$_4$, and FeCr$_2$S$_4$

We investigated the low-lying excitations of the spinels FeSc$_2$S$_4$, FeCr$_2$O$_4$, and FeCr$_2$S$_4$ by THz spectroscopy. FeSc$_2$S$_4$ reportedly is in a spin-orbital singlet ground state [1,2], while the other two compounds exhibit complex magnetically ordered ground states and orbital ordering transitions [3]. In all compounds we observed excitations which we assign to transitions between vibronic levels of the Fe2+ ions in tetrahedral environment. We will discuss the evolution of these excitations in the case of orbital ordering transition and the competition of spin-orbit coupling and electron-phonon interaction as a source for (spin-)orbital frustration in these systems. \\[4pt] [1] A. Krimmel et al. Phys Rev Lett. 94, 237402 (2005).\\[0pt] [2] G. Chen et al. Phys Rev Lett. 102, 096406 (2009)\\[0pt] [3] V. Tsurkan, et al., Phys. Rev. B 81, 184426 (2010).   

Session M16: Focus Session: Ferromagnetic Chains/Nanostructures

Terahertz excitations near the quantum critical point in the 1D Ising chain quantum magnet CoNb$_2$O$_6$

The one-dimensional magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of a one-dimensional quantum Ising spin chain. It has been shown to undergo a quantum phase transition in a magnetic field oriented transverse to its ferromagnetically aligned spin chains. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb$_2$O$_6$ in an external magnetic field. For static magnetic fields oriented transverse to the spin chains, the terahertz excitations show evidence of the phase transitions that occur near the quantum critical magnetic field. Additional spin flip excitations are also observed for longitudinally oriented magnetic fields.   

Low Temperature 1D-Ising-like Behaviour of Cobalt Niobate

Cobalt niobate, CoNb$_{2}$O$_{6}$, is a material that exhibits 1D-Ising-like behaviour at low temperatures, based primarily on chains of spins of the Co$^{2+}$ atoms. Specific heat and magnetic susceptibility measurements on cobalt niobate have found magnetic transitions at 1.9 K and 2.9 K, in agreement with previous work. Specifically, we have performed specific heat measurements in zero field down to 330 mK and have mapped some of the field dependence of the specific heat above 2 K. The low temperature specific heat measurements show an increasingly long relaxation time, implying that the spins become increasingly decoupled from the lattice with decreasing temperature. We have also been the first group to examine the magnetic properties of this material with muon spin rotation ($\mu$SR). This work found that the cobalt moments remain largely dynamic on the microsecond timescale for temperatures well below 1.9 K, indicating that the ground state of CoNb$_{2}$O$_{6}$ is more complex than previously thought.   

Dispersion relations near quantum criticality in the quasi one-dimensional Ising chain CoNb$_2$O$_6$ in transverse magnetic field

The Ising chain in a transverse magnetic field is one of the canonical examples of a quantum phase transition. We have recently realized this model experimentally in the quasi-one-dimensional (1D) Ising-like ferromagnet CoNb$_2$O$_6$ [1]. Here, we present single-crystal inelastic neutron scattering measurements of the magnetic dispersion relations in the full three-dimensional (3D) Brillouin zone for magnetic fields near the critical point and in the high field paramagnetic phase. We explore the gap dependence as a function of field and quantify the cross-over to 3D physics at the lowest energies due to the finite interchain couplings. We parametrize the dispersion relations in the high-field paramagnetic phase to a spin wave model to quantify the sub-leading terms in the spin Hamiltonian beyond the dominant 1D Ising exchange. [1] R. Coldea, D.A. Tennant, E.M. Wheeler et al, Science 327 177-180 (2010).   

Electron-phonon and magnetoelastic interactions in ferromagnetic Co[N(CN)$_2$]$_2$

Many of the most attractive properties of multifunctional materials can be traced to the competition between charge, structure, and magnetism. The discovery that these interactions can be tuned with various physical stimuli has accelerated investigation of their behavior under extreme conditions. In this work, we combined Raman and infrared vibrational spectroscopies with complementary lattice dynamics calculations and magnetization measurements to highlight the signatures of two different coupling processes in the molecule-based magnet Co[N(CN)$_2$]$_2$. In addition to a large anisotropy, our work reveals electron-phonon coupling as a field-driven avoided crossings of the low-lying Co$^{2+}$ electronic excitation with the ligand phonons and a magnetoelastic effect that signals a flexible local CoN$_6$ environment. These findings broaden our understanding of charge-lattice-spin interactions under extreme conditions and demonstrate rich new aspects of multifunctionality in tunable molecular materials.   

Colossal reduction in Curie temperature due to finite-size effects in CoFe2O4 nanoparticles

In this talk I will show the tremendous size effect on the ordering transition temperature, $T_{O}$, in samples of CoFe$_{2}$O$_{4}$ nanoparticles with diameters ranging from 1 to 9 nm. Samples were characterized by HRTEM and XRD analyses and show a bimodal particle size distribution centered at 3 nm and around 6 nm for ``small'' and ``large'' particles, respectively. The results and their interpretation are derived from studies of the magnetization dependence of the samples on temperature at low and high magnetic fields and relaxation times using both DC and AC fields. The large particles show a typical superparamagnetic behavior with blocking temperatures, $T_{B}$, arround 100K and a Curie temperature, $T_{C}$, above room temperature. The small particles, however, show a colossal reduction of their magnetic ordering temperature and display paramagnetic behavior down to about 10K. At lower temperatures these small particles are blocked and show both exchange and anisotropy field values above 5T. The order of magnitude reduction in $T_{O}$ demonstrates a heretofore unreported magnetic behavior for ultrasmall nanoparticles of CoFe$_{2}$O$_{4}$, suggesting its further study as an advanced material.   

The effect of shape, spin, and grain boundary on the vibrational properties of iron nanoparticles

We have performed both spin-polarized and nonspin-polarized density functional theory (DFT) calculations of vibrational modes for Fe113 of either rectangular or spherical shape. We also have calculated them for a spherical nanoparticle with a single grain boundary ($\sum $3(111)) to understand the effect of grain boundary. We used both classical molecular dynamics and DFT to optimize the geometry of the Fe113 nanoparticles. Regarding the vibrational density of states (VDOS) of the nonspin-polarized Fe nanoparticles, the spherical shape exhibits a slightly enhanced VDOS in high frequency modes as compared to rectangular shape. The grain boundary brings about remarkable changes in the VDOS in all frequency ranges (as compared to the VDOS of Fe nanoparticles without a grain boundary: (1) enhanced VDOS in low frequency range (10-15 meV) (2) peak shift to higher frequency in middle range (20 -- 35 meV) (3) new peaks in high frequency range (40 - 55 meV). Most remarkable changes occur when spin is taken into account for Fe113 nanoparticle. The average magnetic moment (per atom) of the spherical Fe113 nanoparticle calculated by DFT is 2.7 Bohr magneton, which is already close to that of iron bulk (2.2 Bohr magneton). The spin-induced features in VDOS (as compared to non-spin cases) are: remarkable (1) increase in the low and middle frequency regions (7-30 meV) and (2) decrease in the high frequency regions. These spin effects are possibly correlated to spin-induced Fe-Fe bond softening (Fe-Fe bond length expansion). Work supported by DOE Grant No. DE-FG02-07ER46354.   

Controlling Magnetism by Light in Nanoscaled Heterostructures of Cyanometallate Coordination Networks: the role of increased complexity

Nanometer-sized heterostructures of the Prussian blue analogues A$_j$Co$_k$[Fe(CN)$_6$]$_{\ell} \cdot n$H$_2$O (Co-Fe PBA, with A = K, Rb) and Rb$_a$Ni$_b$[Cr(CN)$_6$]$_c \cdot m$H$_2$O (Ni-Cr PBA) have been investigated, and new phenomena, not observed for the constituent bulk phases, have been observed.\footnote{ D.M.~Pajerowski, M.J.~Andrus, J.E.~Gardner, E.S.~Knowles, M.W.~Meisel, D.R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 132} (2010) 4058.}$^,$\footnote{ M.F.~Dumont, E.S.~Knowles, A.~Guiet, D.M.~Pajerowski, A.~Gomez, S.W.~Kycia, M.W.~Meisel, D.R.~Talham, \emph{\mbox{Inorg.}~Chem.~}{\bf 50} (2011) 4295.}$^,$\footnote{ D.M.~Pajerowski, J.E.~Gardner, M.J.~Andrus, S.~Datta, A.~Gomez, S.W.~Kycia, S.~Hill, D.R.~Talham, M.W.~Meisel, \emph{Phys.~Rev.~B} {\bf 82} (2010) 214405.}$^,$\footnote{ E.S.~Knowles, M.F.~Dumont, M.K.~Peprah, M.W.~Meisel, C.H.~Li, M.J.~Andrus, D.R.~Talham, arxiv:1207.2623 (2012).} A crucial aspect of the ability to photocontrol the persistent magnetism up to 70~K is the role of the strain coupling present at the interfaces between the nanoscaled regions of the constituents. Increasing the morphological complexity of the samples has the potential to provide materials possessing novel combinations of properties. In parallel, the interplay between long-range magnetic order and structural coherence is an important consideration in our attempts to design new systems. Open, unresolved issues will be discussed, and potential future paths will be sketched.   

Effects of Pressure on the Magnetic Properties of Prussian Blue Analogue Heterostuctures

Magnetic studies on the Prussian blue analogues (PBAs), Li$_x$Cu[Fe(CN)$_6$]$_y\cdot$$m$H$_2$O (CuFe-PBA) and Li$_k$Ni[Cr(CN)$_6$]$_l \cdot$$n$H$_2$O (NiCr-PBA), as well as CuFe@NiCr-PBA core-shell heterostructures, have been conducted under pressures ranging from ambient to $\approx$ 1.4~GPa and at temperatures of 2 - 90~K. Our results for the single phase CuFe-PBA indicate robust magnetic properties under the range of pressures studied: a \textit{T$_c$} of 20 K was observed at all pressures.\footnote{M. Verdaguer, G. S. Girolami, Magnetism: Molecules to Materials V, (Wiley 2005) p 303; M. Okubo \textit{et al.}, Angew. Chem. Int. Ed. \textbf{50} (2011) 6269.} However, our pressure studies of single phase NiCr-PBA are consistent with the results of Zentkov\'a \textit{et al.} up to 1~GPa.\footnote{ M. Zentkov\'a \textit{et al.}, J. Phys.: Condens. Matter \textbf{19} (2007) 266217.} At pressures above 1.0 GPa, the decrease in magnetization is accompanied by a decrease in the \textit{T$_c$}, an indication of changes in the superexchange value, an effect not reported by Zentkov\'a \textit{et al.} Lastly, our results on the effects of pressure on the magnetic properties of heterostructed PBAs, specifically CuFe@NiCr-PBA, will be presented.   

Strain-Mediated Photocontrol in Core-Shell Prussian Blue Analogue Particles

The Prussian blue analogue (PBA), A$_i$Ni[Cr(CN)$_6$]$_j\cdot n$H$_2$O (\textbf{A}), has been shown to exhibit a pressure-induced decrease in magnetization under both external isotropic pressure\,\footnote{M. Zentkov\'a \emph{et al.}, J. Phys.: Condens. Matter \textbf{19} (2007) 266217; \\ M.~K. Peprah \emph{et al.}, in preparation.} and internal photoinduced structural strain when layered with Rb$_i$Co[Fe(CN)$_6$]$_j\cdot n$H$_2$O (\textbf{B}).\footnote{M. F. Dumont \emph{et al.}, Inorg. Chem. \textbf{50} (2011) 4295; D. M. Pajerowski \emph{et al.}, J. Am. Chem. Soc. \textbf{132} (2010) 4058.} Current investigations of a series of core-shell PBAs, consisting of the photoactive ferrimagnetic \textbf{B} surrounded by ferromagnetic \textbf{A}, quantitatively model this photoinduced phenomenon, which is shown to affect both the magnetic moment and superexchange of roughly half the volume of the \textbf{A} shells. An accurate understanding of the mechanism of strain-mediated photocontrol in these heterostructures will allow the pursuit of rationally designed room temperature photocontrol systems by incorporating pressure-sensitive materials with higher magnetic ordering temperatures.   

First-Principles Modeling of Bonding and Magnetic Exchange in the Metal-TCNE Magnet Family

The chemical bond and its role as a mediator of magnetic exchange interaction remains a crucial aspect in the study of molecular magnetism. Within the M-TCNE (M$=$3$d$ metal; TCNE$=$tetracyanoethylene) class of organic-based magnets, only V[TCNE]$_{\mathrm{x}}$ (x$\sim $2) orders magnetically above room-temperature ($T_{\mathrm{c}}\sim $400 K), while structural factors underlying this exceptional behavior remain elusive. Conversely, Mn-TCNE complexes of diverse crystal structure, e.g., 1D-chain MnTPP[TCNE] ($T_{\mathrm{c}}\sim $10 K), 2D-layer [Mn(TCNE)(NCMe)$_{2}$]SbF$_{6}$ ($T_{\mathrm{c}}\sim $75 K), and 3D-network [Mn(TCNE)$_{1.5}$](I$_{3}$)$_{0.5}(T_{\mathrm{c}}\sim $170 K) have recently become available. Using this structural data, hybrid DFT simulations has been performed and the spin-polarized electronic structures resolved. The nature of bonding and non-bonding orbital interactions crucial for understanding magnetic behavior was revealed. Orbital ordering, hybridization, and trends in spin-density transfer (bonding/backbonding) as well as the formation of exchange/superexchange pathways have been identified and interpreted in terms of the dimensionality of magnetic interaction. The role of these and additional factors in establishing high-$T_{\mathrm{c}}$ magnetism in the broader M-TCNE class will be discussed.   

Neutron-Scattering Evidence for the Spin State of a Molecule-Based Magnet with Interpenetrating Sublattices

The molecule-based magnet [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] contains two interpenetrating cubic sublattices. Each sublattice is magnetically frustrated by the easy-plane anisotropy of the spin-3/2 diruthenium (II/III) paddlewheel complexes, which lie at the middle of each cube edge and are antiferromagnetically coupled by the exchange interaction J$_c \sim $ 1.7 meV to two spin-3/2 Cr(III) ions at the cube corners. Symmetry considerations suggest that each cubic sublattice has a non-collinear spin state with net moment along one of the cubic diagonals. The moments of the two interpenetrating sublattices are antiferromagnetically coupled at small magnetic fields and become aligned above a critical field of about 1000 Oe $\sim $ K$_c$/$\mu_B$, where K$_c \sim $ 2 x 10$^{-3}$ meV is the weak dipolar coupling between sublattices. Powder neutron-diffraction measurements on a deuterated sample confirm that the sublattice moments lie along the cubic diagonals and provide indications for substantial quantum corrections to the spin state of each sublattice.   

Approach to criticality in disorder-tuned antiferromagnetic manganese thin films

Using a specialized high vacuum deposition/characterization chamber, we study the \textit{in situ} temperature-dependent conductivity $\sigma (T$,$R_{0})$~of thin magnetic films ( Gd, Cr {\&} Mn) prepared at different stages of disorder where disorder is characterized by the sheet resistance $R_{0}$ measured at $T$~=~5~K. The temperature dependence of normalized conductivity in these thin-films follows power-law dependence of the form, $\sigma(T$,$R_{0})$~=~$A$~+~$\textit{BT}^{P}$. The fitting parameters $A$, $B$ and $P$ vary systematically with increasing disorder. For Mn the parameter $A$ asymptotically approaches zero but always remains positive on the metallic side of a possible metal-insulator transition (MIT) for this material. In contrast, for Gd the parameter $A$ crosses from positive (metal) to negative (insulator) values at critical disorder ($A$ = 0) with a critical disorder strength $R_{0}$ = $R_{C}$ = 22.67~k$\Omega$ at the MIT. The behavior of Mn is strikingly different when compared with Gd, where the MIT occurs before granularity emerges. Most likely this difference of behavior occurs because the inelastic phase breaking length $L_{\phi }$ is not sufficiently high in antiferromagnet Mn to reach the 3D limit where $L_{\phi}$ is less than the film thickness $b$.   

Determination of ground state in potassium intercalated polyacenes

Intercalated compounds of polycyclic aromatic hydrocarbons have been drawing much attention from the view point of new type of organic superconductors. The mechanism of superconductivity in these materials is still unclear, and therefore the true ground states with various carrier concentrations must be understood. The antiferromagnetic ground states were reported particularly on K-doped pentacene, a typical polyacene. In the present study, we focus on the synthesis and the magnetic properties of K-intercalated polyacenes, such as anthracene, tetracene, and pentacene. The improved synthetic method based on the conventional solid state reaction was employed to obtain high quality bulk samples. The X-ray powder diffraction profiles of doped samples showed new stable phases. Interestingly, a pronounced hump at 150 K was observed in the temperature dependence of magnetic susceptibility of K$_1$anthracene. In ESR measurements the linewidth of the signals decreased significantly with a decrease in temperature below 150 K and no Pauli magnetic contribution was detected. These results clearly indicate that charge transfer occurs but the most stable ground state is still insulating via antiferromagnetic interactions. Further discussion will be made among these K-intercalated polyacenes.   

Session M17: Focus Session: Frustrated Multiferroics

Magnetic Order and Spin Correlations in the Multiferroic Sr$_{0.56}$Ba$_{0.44}$MnO$_{3}$

Neutron diffraction and inelastic scattering measurements have been carried out on a polycrystalline sample of Ferroelectric Sr$_{0.56}$Ba$_{0.44}$MnO$_{3}$ (T$_{\mathrm{F}}=$400 K) using the BT-7 and SPINS triple-axis spectrometers. The system orders antiferromagnetically at 190 K with an order parameter that varies smoothly with temperature. Inelastic measurements at base temperature reveal an energy gap of 1.7 meV, with a continuous distribution of magnetic scattering above the gap that exhibits a weak peak at 7.5 meV. The inelastic scattering is strongly peaked at the magnetic reciprocal lattice position up to the highest energy of 15 meV measured so far, indicating strong spin correlations. With increasing temperature the magnetic scattering increases in intensity as expected according to the Bose-Einstein thermal population factor for spin waves. Above Tn strong correlations persist, but the scattering does noticeably broaden.   

Magnetic field effects on the multiferroic phases and the ferroelectric polarization of Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_4$

MnWO$_4$ is a classical multiferroic where ferroelectricity is induced by an inversion symmetry breaking helical spin order. The origin of the helical order is found in competing magnetic exchange interactions with strong uniaxial anisotropy, resulting in magnetic frustration. The extreme sensitivity of the multiferroic state with respect to chemical substitution of Fe, Zn, or Co for Mn was recently shown and Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_4$ (0 \textless\ x \textless\ 0.3) has the most complex phase diagram with multiple polarization flops upon increasing Co content. We report the effects of external magnetic fields on the multiferroic phases in Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_4$ and show that, depending on the Co content, magnitude and orientation of the ferroelectric polarization can be continuously controlled and even complete reversals of the polarization as function of temperature or field are observed. The experimental results are discussed in terms of the external field tuning of the helical or conical spin structures giving rise to the multiferroic state.   

Magnetic properties of multiferroic hexagonal LuFeO$_3$ thin film

We present magnetic properties of multiferroic hexagonal LuFeO$_{3}$ single crystalline thin films grown on Al$_{2}$O$_{3}$(0001) substrates using~ pulsed laser deposition(PLD) technique. Neutron diffraction and superconducting quantum interference device (SQUID) measurements suggest that the hexagonal LuFeO$_{3}$ thin film displays an antiferromagnetic order above room temperature and a second magnetic phase transition at lower temperature. The possible magnetic structures of this system are discussed.   

Origin of ferroelectricity and exotic magnetism in frustrated LiCuVO$_4$

The spin-1/2 Heisenberg chain with competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest neighbor ($J_2$) interactions is probably one the simplest, yet richest model in frustrated magnetism. It is experimentally realized in a diversity of Mott insulators, in particular in copper-oxide materials built-up from edge-sharing CuO$_6$ octahedra. The quasi-1D compound LiCuVO$_4$ stands out for the diverse emergent magnetic and multiferroic phenomena it displays, its simple crystal structure and its availability as high-quality single crystals. I will review recent elastic neutron scattering works [1,2] on LiCuVO$_4$ which elucidate the nature of its ground-state as a function of applied electric field and magnetic field up to 14~T. Below 3.5~T [1], a model long-range ordered ferroelectric spin-cycloid is unveiled, its chirality fully controlled by an applied electric field, and the corresponding magnetoelectric coupling in excellent agreement with the predictions of a purely electronic mechanism based on spin currents. Above 8~T [2], a transition to a new quantum state is observed. This new phase resembles the longitudinal density-wave of magnon-pairs ($p$=2 SDW) predicted in the purely 1D case but is characterized by the intriguing absence of long-ranged dipolar correlations.\\[4pt] [1] M. Mourigal {\it et al.}, PRB {\bf 83}, 100409R (2011).\\[0pt] [2] M. Mourigal {\it et al.}, PRL {\bf 109}, 027203 (2012)   

Magnetism and Ferroelectricity in the frustrated spin chain compound Ca$_{3}$CoMnO$_{6}$

In many multiferroics, there is little or no net magnetism coupled to electric polarization. Ca$_{3}$CoMnO$_{6}$ is unusual among multiferroics since it has a net, hysteretic magnetization coupled to electric polarization, which is important for many applications. Thus, understanding the origin of the magnetic behavior and its coupling to the electric polarization is important. Up to now the arrangement of magnetic exchange interactions, the size of the Co spin, and the origin of magnetic hysteresis were not completely understood. We show magnetization, magnetostriction, electric polarization, and magnetocaloric effect data up to 100 T, including notably a 1/2 and a 2/3 plateau in the magnetization and non-monotonic magnetostriction behavior. We determine that the spin state of Co is definitely \textbf{S} $=$ 3/2 at both high fields and low fields. We show that this behavior is consistent with an ANNNI-like model with antiferromagnetic interactions in the hexagonal \textbf{ab-}plane, and ferromagnetic interactions along \textbf{c-}axis. The model takes into account Ising-like Co spins and Heisenberg-like anisotropic Mn spins. The evolution of the Ising-like Co spins accounts for the hysteresis and steps in the physical properties up to 20 T, and also produces a positive magnetostriction, whereas alignment of the Heisenberg-like Mn spins produce non-hysteretic behavior up to saturation at $\sim$85 T, as well as negative magnetostriction.   

Magnetic field switching of ferroelectricity in spiral magnet CuCrO$_{2}$

The triangular lattice antiferromagnet CuCrO$_{2}$ show ferroelectricity induced by a proper-screw spiral magnetic structure, where spins in form 120$^{\circ}$ angles with neighboring spins due to frustration. CuCrO$_{2}$ is thought to be a rare example of the Arima mechanism for multiferroic behavior. In addition, it has been shown that the magnetoelectric coupling can be tuned by both an electric and a magnetic field along \textbf{ab-}plane. We test a prediction for the magnetic field-evolution of the physical properties of CuCrO$_{2}$ via magnetization and electric polarization measurements up to 65 T. We explore the complicated $H-T$ phase diagram along different crystalline directions. In zero field, a spontaneous electric polarization in CuCrO$_{2}$ is coupled to antiferromagnetic ordering below 24 K without an accompanying structural phase transition. In high fields, we observe electric polarization flops for magnetic fields applied along both the \textbf{ab-}plane and the \textbf{c}-axis, although at different magnetic fields than predicted. By contrast no noticeable anomaly is detected in magnetization isotherms, which are linear in fields up to 65 T. The electric polarization reversal is highly sensitive to the external magnetic field for both the \textbf{ab}-plane and \textbf{c}-axis due to a 3-dimensional proper-screw structure. We find that additional interactions may be necessary to explain our observed results.   

The Effect of Electric Field on Multiferroic Ba$_{0.5}$Sr$_{1.5}$Zn$_2$(Fe$_{0.92}$Al$_{0.08}$)$_{12}$O$_{22}$ Investigated by NMR

Multiferroic helimagnet Ba$_{0.5}$Sr$_{1.5}$Zn$_2$(Fe$_{0.92}$Al$_{0.08}$)$_{12}$O$_{22}$ (Al-BSZFO) shows extremely high magnetoelectric susceptibility so that the critical field for switching electric polarization is less than 1 mT below 90 K [1]. Recently, a large macroscopic magnetization was successfully induced by the electric field ($\pm$2 $\mu_B$/f.u. by $\pm$2 MV/m) in properly annealed Al-BSZFO [2]. To reveal the microscopic origin, a study on the magnetic domain structure is needed. In the magnetic material, NMR intensity is enhanced by the coupling between the electron magnetic susceptibility and the nuclear magnetic susceptibility. Hence if we trace out the amount of NMR intensity enhancement, we would get the information of the magnetic domain configuration. By measuring both the magnetic field and the electric field dependence of NMR intensity enhancement, we found the area of the magnetic domains is actually tuned by the electric field. [1] S. H. Chun et al., Phys. Rev. Lett. 104, 037204 (2010). [2] K. H. Kim, The 19th International Conference on Magnetism (2012); Y. S. Chai et al., unpublished.   

Gigantic ferroelectric polarization and magnetoelectric coupling in a ferrimagnetic oxide CaBaCo$_{4}$O$_{7}$

From both fundamental and applications points of view, improper ferroelectrics that exhibiting a strong coupling between polarization and magnetic structure are challenging the scientific community. Several multiferroics belonging to that category have been reported; however, they exhibit rather small values of electric polarization combined with low magnetic ordering temperatures. Only the CuO (tenorite), the ordered perovskites LBaCuFeO$_{5}$ and the Z-type hexaferrites display magnetic ordering temperatures near room temperature, but they all suffer from polarization much smaller than that of proper ferroelectrics. Here, we report a ferrimagnetic cobaltite, CaBaCo$_{4}$O$_{7}$, crystallizing in a polar space group, which enters an improper ferroelectric phase below T$_{\mathrm{C}}=$ 64 K. Single crystals of CaBaCo$_{4}$O$_{7}$ demonstrate the highest polarization value reported among improper ferroelectrics to date, reaching 10 mC/m$^{2}$ at T$_{\mathrm{C}}$ and approaching 16 mC/m$^{2}$ at 8 K. Moreover a large magnetoelectric coupling coefficient is also evidenced near T$_{\mathrm{C}}$. This result points to routes for exploring new multiferroics among ferrimagnetic phases.   

Spin Wave Excitations in the Multiferroic Ba$_2$CoGe$_2$O$_7$

Ba$_2$CoGe$_2$O$_7$ is a multiferroic material where spin waves exhibit giant directional dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric effect [S. Bordacs et al., Nature Physics {\bf 8}, 734 (2012)]. We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba$_2$CoGe$_2$O$_7$ in high magnetic fields up to 33 T [Penc et al., Phys. Rev. Lett. {\bf 108}, 257203 (2012)]. In the ESR and THz absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. A multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active.   

Muller matrix ellipsometry of dynamic magnetoelectric effects in multiferroics

Far-IR spectra of magneto-electric (ME) and multiferroic materials are in the focus of modern experimental and theoretical studies. Bi-anisotropic optical properties of these materials require consideration of not only dielectric susceptibility tensor $\hat{{\varepsilon }}(\omega )$ but also magnetic permeability $\hat{{\mu }}(\omega )$ and ME $\hat{{\alpha }}(\omega)$ tensors that cannot be distinguished from a single transmission or reflection spectrum. We report on the application of Mueller matrix spectroscopic ellipsometry (MM-SE) for studies of elementary excitations in multiferroic materials such as TbMnO$_{3}$, TbMn$_{2}$O$_{5}$, and TbFe$_{3}$(BO$_{3})_{4}$ single crystals. We show that magnetic, electric, and ME dipole excitations, such as magnons, phonons, and electromagnons can be distinguished from each other using a single MM measurement without introducing any modeling arguments. The fit of MM spectra based on the Berreman's $4\times 4$ propagation matrix formalism allowed us to determine parameters of electromagnon excitations separating the electric $\hat{{\varepsilon }}(\omega )$ and ME $\hat{{\alpha }}(\omega )$ tensors components.   

Reinvestigation of the linear magnetoelectric effect in Cr$_{2}$O$_{3}$ single crystals

Cr$_{2}$O$_{3}$ is not only the first experimentally confirmed magnetoelectric compound but also a rare example compound in which the magnetoelectric effect occurs at room temperature. It is worthwhile to revisit this compound from the standpoint of recently developed ``multiferroic'' where electric and magnetic orders coexist. Thus, we grew single crystals of Cr$_{2}$O$_{3}$ and measured their magnetodielectric and magnetoelectric effects. We found that the temperature dependence of the dielectric constant measured in a magnetic field shows a sharp peak around Neel temperature 307 K. Furthermore, we observed that the electric polarization induced by a magnetic field is reversed by sweeping an electric field at room temperature. In this talk, we present our experimental results on electric and magnetic properties in Cr$_{2}$O$_{3}$, and discuss the origins from current point of view.   

Temperature-dependent electrical and electro-optical properties of LuFe$_{2}$O$_{4}$ thin films

We present temperature-dependent electrical properties of LuFe$_{2}$O$_{4}$ (LFO) thin films deposited on (001) sapphire substrates. The Hall-effect measurements of LFO thin films showed the p-type conductivity at temperatures above 440 K, which is the 2D charge-ordered (CO) state of LFO. In the 3D CO stated below 340 K, we observed complex electrical properties of LFO thin films: dc voltage-current measurements displayed a hysteresis behavior and transient response of voltage-under-current pulses showed a nonlinear voltage-current relationship. We also present the electro-optical effects of LFO in the photon energy range of 0.5 - 6 eV. At 170 K, LFO thin films show the electro-optical effects of size up to 8{\%} near Fe$^{2+}$ d to d on-site electronic transitions. The electrical and electro-optical properties of LFO thin films could be associated with the changes of the ferroelectric polarization in applied electric fields through the interplay of the spin, charge, and lattice degrees of freedom in the multiferroic state of LFO. We will discuss the measured data in the view of the Maxwell-Wagner effects at the contacts, and demonstrate that LFO does show the ferroelectric state below 330 K.   

Crystal field splitting and optical band gap of hexagonal LuFeO$_3$ films

In order to study the electronic structures, we have characterized the hexagonal LuFeO$_3$ films (grown by pulsed laser deposition) using x-ray absorption and optical spectroscopy. The crystal splitting of Fe$^{3+}$ is extracted as $E_{e'}-E_{e''}$=0.7 eV and $E_{a_1'}-E_{e'}$=0.9 eV and a 2.0 eV optical band gap is determined assuming a direct gap. First-principles calculations confirm the experiments that the relative energies of crystal field splitting states do follow $E_{a_1'}>E_{e'}>E_{e''}$ with slightly underestimated values and a band gap of 1.35 eV.   

Session M18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Diamond

Nitrogen-vacancy centers in diamond: a local probe to study magnetic oxides

We report on the development of a diamond-based scanning probe magnetometer (SPM) that operates over a wide range of temperature from 300 K to 4 K. The magnetic sensor is a nitrogen-vacancy (NV) center in diamond, which is read out via optically detected magnetic resonance. This sensor promises non-invasive imaging with single spin sensitivity and spatial resolution down to $\sim$ 10 nm. We have fabricated single-crystal diamond scanning probes with an embedded RF antenna for coherent manipulation of the NV electronic spin. The SPM is integrated into a variable temperature transport set-up in order to study interface magnetism in complex oxide heterostructures.   

Probing dynamics of a spin ensemble of P1 centers in diamond using a superconducting resonator

Solid-state spin ensembles are promising candidates for realizing a quantum memory for superconducting circuits. Understanding the dynamics of such ensembles is a necessary step towards achieving this goal. Here, we investigate the dynamics of an ensemble of nitrogen impurities (P1 centers) in diamond using magnetic-field controlled coupling to the first two modes of a superconducting (NbTiN) coplanar waveguide resonator. Three hyperfine-split spin sub-ensembles are clearly resolved in the 0.25-1.2 K temperature range, with a collective coupling strength extrapolating to 23 MHz at full polarization. The coupling to multiple modes allows us to distinguish the contributions of dipolar broadening and magnetic field inhomogeneity to the spin linewidth. We find the spin polarization recovery rate to be temperature independent below 1 K and conclude that spin out-diffusion across the resonator mode volume provides the mechanism for spin relaxation of the ensemble. Furthermore, by pumping spins in one sub-ensemble and probing the spins in the other sub-ensembles, we observe fast steady-state cross-relaxation (compared to spin repolarization) across the hyperfine transitions. These observations have important implications for using the three sub-ensembles as independent quantum memories.   

High-Resolution Correlation Spectroscopy of $^{13}$C Spins Near a Nitrogen-Vacancy Center in Diamond

We use a pulse protocol to monitor the time evolution of the 13C ensemble in the vicinity of a NV center. We observe time correlations in the nuclear spin dynamics that extend over several milliseconds exceeding the color center coherence lifetime by more than an order of magnitude. Upon Fourier transform, we separate 13C spins exhibiting differing coupling constants with a frequency resolution inversely proportional to the NV spin-lattice relaxation time. Further, we use the nuclear spin of the host nitrogen as a quantum register during the correlation interval and demonstrate that hyperfine-shifted resonances in this spectrum can be separated from the bare carbon peak upon proper initialization of the NV. Intriguingly, we find that the amplitude of the correlation signal exhibits a sharp dependence on the applied magnetic field, virtually disappearing below a critical field common to all centers. The value of this transition field can be `tuned' by properly adjusting the timing within our correlation scheme. We discuss these observations in the context of the `quantum-to-classical' transition proposed recently to explain the combined dynamics of the NV spin and the 13C bath at variable magnetic field.   

Measurement and control of single spins in diamond above 600 K

The nitrogen vacancy (NV) center in diamond stands out among spin qubit systems in large part because its spin can be controlled under ambient conditions whereas most other solid state qubits operate only at cryogenic temperatures. However, despite the intense interest in the NV center's room temperature properties for nanoscale sensing and quantum information applications, the ultimate thermal limits to its measurement and control have been largely unknown. We demonstrate that the NV center's spin can be optically addressed and coherently controlled at temperatures exceeding 600 K and show that its addressability is eventually limited by thermal quenching of the optical spin readout [1]. These measurements, in combination with computational studies, provide important information about the electronic states that facilitate the optical spin measurement and, moreover, suggest that the coherence of the NV center's spin states could be utilized for thermometry. We infer that single spins in diamond offer temperature sensitivities better than 100 mK/$\mathrm{\sqrt{Hz}}$ up to 600 K using conventional sensing techniques and show that advanced measurement schemes provide a pathway to reach 10 mK/$\mathrm{\sqrt{Hz}}$ sensitivities. Together with diamond's ideal thermal and mechanical properties, these results suggest that NV center thermometers could be applied in cellular thermometry and scanning thermal microscopy. \\[4pt] [1] D. M. Toyli*, D. J. Christle*, A. Alkauskas, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, \emph{Phys. Rev. X} \textbf{2}, 031001 (2012).   

Approach to Dark Spins Initialization in Nanodiamond

Diamond nanoparticles host a number of paramagnetic point defects and impurities--many of them adjacent to the surface--whose response to external stimuli could help probe the complex dynamics of the particle and its local, nanoscale environment. Here we use a Hartman-Hahn protocol to demonstrate spin polarization transfer from a single, optically-polarized nitrogen-vacancy (NV) center to the ensemble of paramagnetic defects hosted by an individual diamond nanocrystal (30 nm in diameter). Owing to the strong NV-bath coupling, the transfer takes place on a short, microsecond time scale. Upon fast repetition of the pulse sequence we observe strong polarization transfer blockade, which we interpret as an indication of spin bath cooling. Numerical simulations indicate that the spin bath polarization is non-uniform throughout the nanoparticle averaging approximately 2\% over the crystal volume, but reaching up to 20\% in the immediate vicinity of the NV. These observations may prove relevant to the planning of future bath-assisted magnetometry tests.   

Engineering shallow spins in diamond with nitrogen delta-doping

The excellent spin properties of diamond nitrogen-vacancy (NV) centers motivate applications from sensing to quantum information processing. Still, external electron and nuclear spin sensing are limited by weak magnetic dipole interactions, requiring NVs be within a few nm of the surface and retain long spin coherence times ($T_2$). We report a nitrogen delta-doping technique to create artificial NVs meeting these requirements. Isotopically pure $^{15}$N$_2$ gas is introduced to form a thin N-doped layer (1--2 nm thick) during chemical vapor deposition of a diamond film. Post growth electron irradiation creates vacancies and subsequent annealing forms NVs while mitigating crystal damage. We identified doped NVs through the hyperfine signature of the rare $^{15}$N isotope in electron spin resonance measurements. We confirm the doped NV depth dispersion is less than 4 nm by doping NVs in the $^{12}$C layer of an isotopically engineered $^{13}$C/$^{12}$C/$^{13}$C structure and probing the coupling between the doped NVs and the $^{13}$C nuclear spins. Furthermore, despite their surface proximity, doped NVs embedded in $^{12}$C films 5 (52) nm below the surface show $T_2$ greater than 100 (600) $\mu$s [1].\\[4pt] [1] K. Ohno \emph{et al.}, Appl. Phys. Lett. \textbf{101}, 082413 (2012).   

Optically trapped nanodiamonds with nitrogen-vacancy center spins for scanning magnetometry and thermometry

Nanodiamonds with nitrogen-vacancy (NV) centers are a versatile sensing platform that combines the optically addressable atom-like properties of embedded NV centers, which are sensitive to electromagnetic fields and temperature, with the physical size and mobility necessary for nanometer-scale spatial resolution. We constructed an optical tweezers apparatus that accomplishes position control of nanodiamonds in solution within a microfluidic circuit and enables simultaneous optical measurement and microwave manipulation of the NV centers' ground-state spins [1]. We observe nanodiamond fluorescence and trapping stability over many hours, and infer high d.c. magnetic field and temperature sensitivities from measured spin resonance spectra. Scanning the position of the trapped nanodiamonds enables us to map the magnetic field of current-carrying wires and magnetic nanostructures, and perform thermometry in liquid. This work provides an approach to three-dimensional spin-based scanning probe magnetometry and thermometry in fluids for applications in the biological and physical sciences. \\[4pt] [1] V.R. Horowitz, B.J. Alem\'{a}n, D.J. Christle, A.N. Cleland, and D.D. Awschalom, \emph{Proc. Natl. Acad. Sci. USA}, \textbf{109}, 13493 (2012).   

Exchange Interaction of Transition Metal Dopants in Diamond

Advances in single-ion implantation and spectroscopy have permitted direct observation of the exchange interaction between two dopant spins in a semiconductor[1], which is accurately described by tight-binding models of the semiconducting host[1,2]. These advances suggest controllable fabrication and utilization of few-dopant structures to explore fundamental properties and for applications[3]. Transition metal substitutional dopants in tetrahedrally-bonded semiconductors are good candidates for controllable spin manipulation and spin-spin interaction because they offer both highly-localized and much more extended spin-polarized states. For example, both the Ni and Cr dopant have spin-1 ground states in diamond, but with differing spatial extent[4]. We calculate the exchange interaction between pairs of Ni and Cr dopants in diamond using the technique of Ref. 2, but with an spds* tight-binding model. We find strong exchange interactions between pairs of Ni, and pairs of Cr, which are influenced by the differing symmetry of the dopants' ground state. [1] D. Kitchen et al., Nature 442, 436 (2006). [2] J.-M. Tang \& M.E. Flatt\'e, Phys. Rev. Lett. 92, 047201 (2004). [3] P. Koenraad \& M.E. Flatt\'e, Nat. Mat. 10, 91 (2011). [4] T. Chanier, et. al., Phys. Rev. B 86, 085203 (2012).   

Detection and Manipulation of Single NV Centers in Diamond

We use a scanning confocal microscope to investigate the fluorescence emission from nitrogen vacancy (NV) centers in diamond, a promising building block for quantum computing due to its long coherence time at room temperature. We demonstrate detection and coherent manipulation of a single NV center spin in synthetic diamond. Rabi oscillation data shows a modulation in the amplitude that is accounted for by simulating NV center spin dynamics in the presence of a proximal $^{14}$N nuclear spin. The hyperfine interaction opens up the possibility of coupling the electronic spin of an NV center to nearby nuclear spins, forming multi-qubit systems for quantum computation. For applications where a long coherence time is necessary, decoherence caused by the hyperfine interaction can be suppressed using a spin-echo pulse sequence, resulting in electron spin coherence times of over 1 $\mu$s at room temperature in type Ib diamond of high impurity content.   

Improving the Collection Efficiency of Bulk Diamond NV Center Fluorescence with Solid Immersion Lenses

The spin-dependent fluorescence of nitrogen vacancy (NV) centers in diamond makes them promising systems for a variety of applications ranging from magnetic field sensing to quantum information processing. The fidelity of optical detection of NV center spin states is therefore dependent on the collection efficiency of the NV fluorescence. While the crystal structure of diamond is useful in allowing for stable, room temperature measurements, its high index of refraction leads to a shallow critical angle of total internal reflection ($\sim24^\circ$) significantly limiting the optical collection efficiency. Here we develop a method for fabricating a solid immersion lens (SIL) on the surface of bulk diamonds. The hemispherical SILs, milled with high-energy gallium ions, are positioned such that the NV center of interest is at the origin of the sphere, thereby utilizing the full numerical aperture of the objective lens in the confocal microscope. Our lenses have already improved the collection efficiency by a factor of 2-3. With simple first order corrections to the milling process, higher collection efficiencies should be attainable. Further improvements in the lenses will allow single-shot readout of the spin states of NV centers.   

Towards Nuclear Polarization of Nanodiamond

Nanoparticles with long nuclear spin relaxation times [1] are candidates for use in targeted therapeutic delivery [2] and magnetic resonance imaging [3]. We report progress towards the development of contrast agents based on 13C in nanodiamond. Nuclear relaxation and electron spin resonance data is presented. We describe the development of a DNP setup at X band frequencies based on an ENDOR cavity, together with a novel brute force setup that combines milli-Kelvin temperatures of a dilution refrigerator, high magnetic fields and fast sample exchange. [1] J. Aptekar, {\it et al.}, ACS Nano, 3, 4003-4008 (2009). [2] H. Huang , E. Pierstorff, E. Osawa, and D. Ho, Nano Lett, 7, 3305-3314 (2007). [3] L Manus T. J. Meade, Nano Lett, 10, 484-489 (2010).   

Effective spin-orbit Hamiltonians and spin lifetimes for diamond and strontium titanate

The long spin coherence times of spin centers in diamond and the large Rashba coefficients and spin injection efficiencies in strontium titanate-based two-dimensional systems makes these wide band-gap semiconductors strong candidates for spintronics applications. To calculate the spin properties of these inversion-symmetric materials we have constructed a low-energy Hamiltonian, making use of a tight-binding model with atomic spin-orbit interactions. Furthermore we have derived and calculated the tensor that controls the form of the effective spin-orbit interaction in the non-spherical conduction bands of these materials. Finally we have computed the spin relaxation rates via the Elliott-Yafet mechanism through impurity scattering for diamond and uniaxially strained strontium titanate as a function of temperature and carrier density. Long spin lifetimes suggest the potential for novel spintronic applications of these wide bandgap semiconductors. This work was supported by an ARO MURI and an AFOSR MURI.   

Dynamic nuclear polarization of single nitrogen isoelectronic centers in GaAs

Due to their very long coherence time, nuclear spins of atomic systems represent good candidates for spin-based qubits in semiconductors. In this work, the dynamic nuclear polarization of isoelectronic centers formed from two nitrogen impurities in GaAs is investigated as a function of the external magnetic field and the polarization ellipticity of the exciting light. The nuclear spins of a single center are probed by the Overhauser shift of the neutral exciton and negatively charged exciton bound states. A nuclear magnetic field of 25 mT is measured at low external magnetic field and it decreases with this external field, indicating an efficiency loss in the exciton-nucleus spin-flip process. A peculiar Overhauser shift, scaling as the square of the ellipticity, is found for the exciton. A strong hysteretic behavior is also observed for both the neutral and charged excitons. These effects are believed to originate from the complex dynamic of the hyperfine interaction between the different excitonic spin states and nuclei. Our results show that dynamic nuclear polarization, much studied in quantum dots, is scalable to a single atomic-sized system. These results represent a first step towards the optical control of single nuclear spins in semiconductors.   

Session M19: Strongly Correlated Electron Systems and Phase Transitions

Quantum criticality in the pseudogap two-channel Anderson and Kondo models

The two-channel Anderson and Kondo impurity models with a density of states $\rho(E) \propto |E|^r$ that vanishes at the Fermi energy ($E=0$) is of current interest in connection with impurities in graphene and in unconventional superconductors. The phase diagram of these models has been established previously [1,2]. We study the low-temperature static and dynamical properties of the models using the numerical renormalization-group method, and compare our results against exact and perturbative analytical theories [2], and against calculations performed within the non-crossing approximation. In the vicinity of the quantum critical points separating local-moment and non-Fermi liquid phases, the static local spin susceptibility is characterized by a set of critical exponents that satisfy the hyperscaling relations expected of an interacting system below its upper critical dimension. The dynamical local susceptibility and the impurity spectral function exhibit forms consistent with frequency-over-temperature scaling, another feature associated with interacting quantum critical points. [1] C. Gonzalez-Buxton and K. Ingersent, Phys. Rev. B 57, 14254 (1998). [2] I. Schneider et al., Phys. Rev. B, 84, 125139 (2011).   

Phase Diagram of a Correlated Band Insulator

The effect of on-site electron-electron repulsion U in a band insulator is explored for a bilayer Hubbard Hamiltonian with opposite sign hopping on the two sheets. Unlike the case of the ionic Hubbard model, which has a closely related noninteracting dispersion relation, no evidence is found for a metallic phase intervening between the Mott and band insulators: The gap in the spectral function monotonically increases with U from its initial band insulating value. The origin of such difference can be traced to the fact that the local interaction in a bilayer favors the formation of independent singlets whereas in the ionic model is responsible for a homogenization of the density and a consequent reduction of band structure effects. We found that the formation of singlets between the planes, and the resulting destruction of antiferromagnetic order occurs much more rapidly than in the case of a symmetric Hubbard bilayer, which has the same sign of hopping in the two sheets.   

Quantum Griffiths singularities in ferromagnetic metals

We present a theory of the quantum Griffiths phases associated with the ferromagnetic quantum phase transition in disordered metals. For Ising spin symmetry, we study the dynamics of a single rare region within the variational instanton approach. For Heisenberg symmetry, the dynamics of the rare region is studied using a renormalization group approach. In both cases, the rare region dynamics is even slower than in the usual quantum Griffiths case because the order parameter conservation of an itinerant ferromagnet hampers the relaxation of large magnetic clusters. The resulting quantum Griffiths singularities in ferromagnetic metals are stronger than power laws. For example, the low-energy density of states $\rho(\epsilon)$ takes the asymptotic form $\exp[\{-\tilde{\lambda}\log (\epsilon_0/\epsilon)\}^{3/5}]/\epsilon$ with $\tilde{\lambda}$ being non-universal. We contrast these results with the antiferromagnetic case in which the systems show power-law quantum Griffiths singularities in the vicinity of the quantum critical point. We also compare our result with existing experimental data of ferromagnetic alloy ${\rm{Ni}}_{x}{\rm{V}}_{1-x}$.   

Quantum Criticality of Charged Particles in Polar Liquids

We propose a general theory for the interaction of electrons with polarizable media for which the dynamical structure factor for charge fluctuations is known. The theory is based on a generalization of Leggett's method for the construction of path integral functionals for electrons in dissipative media. We apply the method to the case of electrons in polar liquids using a dynamical structure factor obtained by numerical simulations. The functional integrals are approximated using Feynman's variational method. At low temperatures, a dynamical structure factor with local spatial structure along with a Debye-like decaying frequency dependence, as suggested by the simulations, produces a first-order transition at a critical coupling constant. This is in contrast with the Feynman polaron theory, which does not have local structure formation, where no transition takes place. We also find a line of continuous quantum criticality.   

Unbinding of giant vortices in states of competing order

We consider a two-dimensional system with two order parameters, one with O(2) symmetry and one with O($M$), near a point in parameter space where they couple to become a single O($2+M$) order. While the O(2) sector supports vortex excitations, these vortices must somehow disappear as the high symmetry point is approached. We develop a variational argument which shows that the size of the vortex cores diverges as $1/\sqrt{\Delta}$ and the Berezinskii-Kosterlitz-Thouless transition temperature of the O(2) order vanishes as $1/\ln(1/\Delta)$, where $\Delta$ denotes the distance from the high-symmetry point. Our physical picture is confirmed by a renormalization group analysis which gives further logarithmic corrections, and demonstrates full symmetry restoration within the cores.   

Numerical study of a mobile magnetic impurity in a one-dimensional quantum liquid

We study a mobile spin-1/2 impurity, coupled antiferromagnetically to a one-dimensional gas of fermions. Combining perturbative ideas and extensive density matrix renormalization group calculations, we study the interplay between the screening of the impurity by the electrons and the kinetic and magnetic properties of the impurity. We show that this problem displays a quantum phase transition between one- and two-channel Kondo physics. Using finite-size scaling, we construct a ground-state phase diagram and discuss various non-trivial regimes.   

Critical fluctuations in $N$-component superconductor models

Inspired by recent conflicting views on the order of the phase transition from an antiferromagnetic N\'eel state to a spin liquid or valence bond solid, we use the functional renormalization group to reconsider the $N$-component superconductor models, in which a dynamic gauge field is minimally coupled to $N$ bosonic complex fields. In contrast to previous work, we only expand in covariant derivatives and use a truncation in which the full field dependence of all wave-function renormalization functions is kept. As a consequence, we find non-trivial RG fixed points for all positive integer $N$.   

Zigzag Quantum Phase Transition in Quantum Wires

We use Quantum Monte Carlo (QMC) techniques to study the quantum phase transition of interacting electrons in a quantum wire to a quasi-one-dimensional zigzag phase. Interacting electrons confined to a wire by a transverse harmonic potential form a linear Wigner crystal at low densities; as density increases, symmetry about the axis of the wire is broken and the electrons undergo a transition to a quasi-one-dimensional zigzag phase. The phase diagram of particles with Coulomb interaction that undergo a linear to zigzag transition is relevant to electrons in quantum wires and ions in linear traps. We characterize this phase transition by using QMC to study the order parameter, correlation functions, pair density, power spectrum, and addition energies.   

Construction of local order parameters from non-vanishing mutual information

In the recent decades, raising attention has been paid in the study of quantum phase transitions (QPTs) from quantum information perspectives. In this talk, we will present a scheme in constructing the local order parameters by investigating the spectra of the reduced density matrices that are used to calculate the mutual information. We will briefly review the relation between non-vanishing mutual information and the presence of long-range correlation in a system. In particular, we will illustrate our scheme using the numerical exact diagonalization result of the one-dimensional Hubbard model.   

Columnar and superfluid order in an extended Shastry-Sutherland model

The low temperature magnetic properties of several rare-earth tetraborides have been shown to be well-characterized by an extension of the Shastry-Sutherland model (SSM). This extension includes additional next-nearest-neighbor bonds, and the exchange interaction along all bonds is anisotropic with strictly ferromagnetic transverse exchange. The extended SSM is thus equivalent to a system of hard-core bosons and is free of the quantum Monte Carlo (QMC) sign problem. Using large scale QMC simulations, we study the phase diagram of the extended SSM in a new parameter regime that stabilizes a zero-field columnar antiferromagnetic state. We show how application of an external magnetic field can induce a phase transition to a spin supersolid phase. We compare the overall magnetization process to experimental observations of ErB$_4$, a rare-earth tetraboride with ground state columnar antiferromagnetic ordering. Finally, we speculate that if the zero-field columnar order present in ErB$_4$ is driven by similar interactions it may also possess a field-induced supersolid phase.   

Resummation of divergent fluctuations near to metallic ferromagnetic quantum criticality

Fluctuations near to the metallic ferromagnetic quantum critical point can have profound effects. They lead to new quantum critical scaling at high temperatures, which gives way to reconstruction of the phase diagram at lower temperatures. In the vicinity of the quantum critical point, new spatially modulated magnetic or spin nematic phases appear. These new phases may be revealed by means of non-analytic corrections to Hertz-Millis theory [1], or in the recently-developed quantum order-by-disorder approach [2]. Here we demonstrate a re-summation of all the leading divergences in the latter approach to extend the analysis from the finite-temperature tricritical point down to zero temperature.\\[4pt] [1] D. Belitz, T.R. Kirkpatrick and T. Vojta, Rev. Mod. Phys. 77, 579 (2005); D. V. Efremov, J.J. Betouras, A.V. Chubukov Phys. Rev. B 77, 220401(R), (2008)\\[0pt] [2] G.J. Conduit, A.G. Green \& B.D. Simons Phys. Rev. Lett. 103, 207201 (2009)   

Tractable Crossing-symmteric Equations Formalism and Applications in Two Dimensions

The tractable crossing symmetric formalism is developed for the 2D case. We first consider circular Fermi surfaces and then extend this to 2D square lattice systems. Limiting cases, such as small $(q,\omega)$, vanishing momentum-energy transfer $(q\rightarrow0, \omega\rightarrow0)$, vanishing q but non-zero $\omega$ are considered. This is applied to the study of various properties of 2D Fermi systems. Of particular interest is the physics near Pomeranchuk instabilities: in Fermi systems, interactions can cause symmetry-breaking deformations of the Fermi surface, called Pomeranchuk instabilities. In Fermi liquid theory language, this occurs when one of the Landau harmonics $F_{\ell}^{s,a}\rightarrow -(2\ell+1)$; e.g. $F_{0}^{s,a}\rightarrow-1$ are related to ferromagnetic transition (a), and density instabilities(s). The corresponding points in parameter space may be viewed as quantum critical points. Using graphical and numerical methods to solve coupled non-linear integral equations that arise in the crossing symmetric equation scheme, we obtain results in the 2D case close to Pomeranchuk instabilities. We compare our 2D results for various response functions and instabilities with the results of recent calculations in the 3D case, which will also be discussed.   

Strongly-correlated phases in a flatband with incommensurate filling

We explore strongly-correlated electronic phases in flatband systems (such as on the kagome lattice) with incommensurate filling, in the presence of spin-orbit interactions and ferromagnetism. The competition between Fermi-liquid, charge-density wave and superconducting phases in this system is examined.   

Session M20: Focus Session: Metamaterials - Plasmonics

Quantum Plasmonics: Electron transfer processes

Plasmon energies can be tuned across the spectrum by simply changing the geometrical shape of a nanostructure. Plasmons can efficiently capture incident light and focus it to nanometer sized hotspots which can enhance electronic and vibrational excitations in nearby structures.[1] Another important but still relatively unexplored property of plasmons, is that they can be efficient sources of hot energetic electrons which can transfer into nearby structures and induce a variety of processes. This process is a quantum mechanical effect: the decay of plasmon quanta into electron-hole pairs. I will discuss how plasmon induced hot electrons can be used in various applications: such as to induce chemical reactions in molecules physisorbed on a nanoparticle surface;[2] to inject electrons directly into the conduction band of a nearby substrate;[3] and to induce local doping of a nearby graphene sheet.[4] References [1] N.J. Halas \textit{et al.}, Adv. Mat. 24(2012)4842 [2] R. Huschka \textit{et al.}, JACS 133(2011)12247; S. Mukherjee \textit{et al.} TBP 2012 [3] M. W. Knight \textit{et al.}, Science 332(2011)702, Z.Y. Fang \textit{et al.}, NL 12(2012)3808 [4] Z.Y. Fang \textit{et al.}, ACS Nano 6(2012)10.1021/nn304028b   

Plasmonic electron injection drives ultrafast phase transition by catastrophic phonon collapse I: experiment

Phase transitions in quantum materials such as vanadium dioxide (VO$_{\mathrm{2}})$ can provide functionality in nanophotonic devices. Here we report on a novel all-optical mechanism to trigger phase transformation (PT) of VO$_{\mathrm{2}}$ faster than its intrinsic single phonon period. By optically exciting a spectrally resonant sparse mesh of plasmonic gold nanoparticles, hot electrons created are ballistically injected across the Au/VO$_{\mathrm{2}}$ interface to assist the sub-picosecond PT, lowering the switching threshold by a factor of five. As confirmed by density functional calculations, the injected electrons cause a catastrophic collapse of the 6 THz phonon mode in VO$_{\mathrm{2}}$, essential for triggering its PT (next abstract). This demonstration of plasmon-induced hot-electron-driven PT controlled by this ultrafast technique represents a critical step towards developing hybrid nanomaterials with optimal switching thresholds.   

Plasmonic electron injection drives ultrafast phase transition by catastrophic phonon collapse II: theory

The ultrafast photo-induced phase transition in VO$_{2}$ is promising for data storage and sensing applications. Our experimental work (the previous talk) shows that in a Au/VO$_{2}$ hybrid nanostructure, electrons excited in the Au photocathode by an ultrafast laser trigger the insulator-to-metal transition in VO$_{2}$. Here we report first-principles density-functional calculations showing that the collapse of a 6 THz optical phonon, corresponding to a twisting motion of V atoms, is responsible for the ultrafast phase transition. Above a concentration threshold, we find that injected electrons from Au induce collapse of the VO$_{2\, }$phonon, which stimulates the monoclinic-to-rutile structural phase transition. We also show that hole-doping can induce the same effect. The abrupt change of the critical phonon results from the weakening of the V-V bonds induced by the combined flux of injected electrons and holes. Thus, our results explain the experimental finding of plasmonic-electron-driven ultrafast phase transition and represent a step towards manipulating the photo-induced phase transition by surface modification.   

Optical circulation and power flow rotation with nonreciprocal plasmonic structure

In this work we propose a concept for tailoring the near-zone optical field with the plasmonic nanostructures mixed with MO materials, and demonstrate a novel effect of a subwavelength power flow circulation. We study both analytically and numerically plasmonic nanostructures embedded into magneto-active media, and analyze their resonances and corresponding eigenmode spectra. We show that when the structure is degenerate the magneto-optical activity, when introduced, causes strong interaction between these modes. Such intermodal interaction leads to a formation of a novel set of rotating states and to a frequency splitting between them. We study the plane wave excitation of such nanostructures and reveal a strong power flux circulation around such structures in the presence of magneto-optical activity. We will discuss a possible application of the observed effect and propose a subwavelength optical circulator. In particular, we study numerically a plasmonic nanostructure embedded into the core of the Y-junction formed by single mode optical waveguides. We show that mixing the plamonic nanostructures with magneto-optical materials it is possible to break significantly the symmetry between the output arms of the junction and almost completely isolate one of them.   

Mode matching for optimal plasmonic nonlinear generation

Nanostructures and metamaterials have attracted interest in the nonlinear optics community due to the possibility of engineering their nonlinear responses; however, the underlying physics to describe nonlinear light generation in nanostructures and the design rules to maximize the emission are still under debate. We study the geometry dependence of the second harmonic and third harmonic emission from gold nanostructures, by designing arrays of nanostructures whose geometry varies from bars to split ring resonators. We fix the length (and volume) of the nanostructure on one axis, and change the morphology from a split ring resonator on the other axis. We observed that the optimal second harmonic generation does not occur at the morphology indicated by a nonlinear oscillator model with parameters derived from the far field transmission and is not maximized by a spectral overlap of the plasmonic modes; however, we find a near field overlap integral and mode matching considerations accurately predict the optimal geometry.   

Theory of plasmon-enhanced metal photoluminescence

Metal photoluminescence (MPL) originates from radiative recombination of photoexcited core holes and conduction band electrons. In metal nanostructures, MPL is enhanced due to surface plasmon local field effect. We identify another essential process in plasmon-assisted MPL - excitation of Auger plasmons by core holes - that hinders MPL from small nanostructures. We develop a microscopic theory of plasmon-enhanced MPL that incorporates both plasmonic enhancement and suppression mechanisms and derive enhancement factor for MPL quantum efficiency. Our numerical calculations of MPL from Au nanoparticles are in excellent agreement with experiment.   

Probing light-matter interactions in plasmonic nanostructures with a single quantum dot

Understanding and controlling the interactions between single quantum emitters and plasmonic nanostructures is important for a wide variety of applications in quantum optics and nanophotonics. Metal nanostructures provide subwavelength confinement of electromagnetic fields in the form of surface plasmon polaritons, which can enhance optical nonlinearities for improved light-matter interactions. In this talk we will present recent results on nano-manipulation of single colloidal quantum dots (QDs) for deterministic probing of light-matter interactions in plasmonic nanostructures. Single QDs are manipulated using a combination of microfluidics and engineered fluid chemistry. We achieve deterministic positioning with 50 nm accuracy and demonstrate probing of the surface plasmon mode of a silver nanowire. Spatially variant interactions are quantified by measuring the coupling rate of the QD into the wire mode as well as changes to the QD emission lifetime. The resulting interactions are resolved with nanoscale resolution and reveal features such as the evanescent field decay away from the wire surface and interference along the wire length.   

Giant circular dichroism of a molecule in a plasmonic nanoparticle dimer

We report on giant circular dichroism (CD) of a molecule inserted into a plasmonic hot spot. Naturally occurring molecules and biomolecules have typically CD signals in the UV range, whereas plasmonic nanocrystals exhibit strong plasmon resonances in the visible spectral interval. Therefore, excitations of chiral molecules and plasmon resonances are typically off-resonant. Nevertheless, we demonstrate theoretically that it is possible to create strongly-enhanced molecular CD utilizing the plasmons. Specifically, by employing a nanoparticle dimer, we gain simultaneously a strong plasmonic enhancement and a shift of optical CD from the UV range to the visible. The associated mechanism of giant CD comes from the Coulomb interaction which is greatly amplified in a plasmonic hot spot. Two key factors play a role in the described effect: One is the Coulomb interaction within the molecule-dimer complex giving rise to the plasmon peak in the CD spectrum, whereas the other one is the plasmonic enhancement of the absorption process in a chiral molecule. We propose that, by using the hot spot effect and plasmon-induced CD signals, one can design optical sensors to study chirality of biomolecules.   

Exciton-plasmon coupling in monolayer molybdenum disulfide

Two-dimensional materials such as monolayer molybdenum disulfide (MoS$_{2})$ represent a unique platform for investigating the dynamics of exciton-plasmon coupling. We report on the generation and modulation of coherent and incoherent coupled states between excitons in monolayer MoS$_{2}$ and plasmons in an array of gold nanoparticle deposited onto the surface of MoS$_{2}$. We study the behavior of these coherent states, termed plexcitons using a combination of photoluminescence, extinction and ultrafast spectroscopies. The close proximity of the two characteristic exciton bands of MoS$_{2}$ presents multiple coherent coupling configurations, including A-or-B exciton-plasmon, and A-and-B exciton-plasmon interactions. These configurations of plexciton formation that are shown to modulate both the extinction and photoluminescence spectra of the hybrid system. This includes broadband photoluminescence and Fano-type resonances. This behavior is distinct from the spectral response of the MoS$_{2}$ and plasmonic components of the system. Incoherent exciton-plasmon coupling, achieved by detuning from the plasmon extinction peaks, enhances the interaction of MoS$_{2}$ with light by focusing the plasmon energy. Depending on which coupling configuration is chosen, our results show that the MoS$_{2}$/plasmon hybrid systems can act as high efficiency light harvesters, broadband emitters and as tunable visible and NIR photodetectors.   

Optical Properties of Graphene Plasmons in Periodic Gate Structures

Plasmons in graphene have been shown to be tunable in a wide frequency range including the THz regime. Room temperature, narrow plasmon modes have been demonstrated in graphene ribbons arranged periodically on surfaces. Here we present computational results of localized modes in continuous graphene layers with periodic arrangements of gates that modulate spatially the charge density. These induce boundary conditions different from those in graphene ribbons and open the possibility of electrical injection. We discuss the optical absorption and reflection spectra for different gate voltages and for a range of gate widths and spacing. We also discuss different regimes of electrical injection and the role of substrates in coupling to plasmons and in heat dissipation.   

Optical Properties of Epitaxially Grown Silver Films

One major obstacle in the advancing field of plasmonics is the loss in metals. A sizable contribution of this loss comes from grain boundaries and surface roughness introduced during thin film growth using conventional deposition methods. A novel epitaxial growth technique is used to produce silver (Ag) thin films free of such flaws. We investigate the optical properties--namely the dielectric optical constants--of these new epitaxial films in the bulk region and in the ultrathin film limit where quantum mechanical behaviors emerge due to energy quantization in the growth direction. The values for the dielectric optical constants are extracted from the spectral ellipsometry (SE) measurements over a wide range of optical frequencies. By using an adequate model of the sample structure and initial values of the fitting parameters (i.e. the real and imaginary parts of the optical constants), we can extract these measured values for the new Ag films. We have confirmed that in the bulk region, the optical constants converge with the well-known Johnson and Christy measurements [1]. In the ultrathin film limit, however, we observed significant changes near the D-band transition likely due to a quantum well-like density of states.\\[4pt] [1] P.B. Johnson and R.W. Christy, PRB 6 4370 (1972)   

Hamiltonian Optics Approach for Hybridized Surface Plasmon Polariton in Graded Metal-Dielectric-Metal Waveguide with Periodically Varying Index

In a complex plasmonic nanostucture, it is possible to support several elementary modes of surface plasmon polariton due to the multi-surface configuration. Hybridized surface plasmon polariton (HSPP) is formed when those modes interact with each others. The dispersion curves of these complex plasmon modes will be shifted from the original ones. As the shifting depends strongly on the geometry of the structure, it allows one to manage the properties of light inside the structure with much higher flexibility and complexity. We have studied the properties of HSPP in a graded metal-dielectric-metal (MDM) waveguide with the refractive index of the dielectric varying periodically, using the Hamiltonian optics approach, to investigate the feasibility of light manipulation inside this structure. We have extracted the allowed phase orbits using the quantization condition. The time series of position and wavevector of HSPP were also simulated by solving the Hamiltonian equations of motion. The results revealed two possible orbits of the HSPP inside the waveguide: confinement and propagation. The range of angular frequency such that the phase orbits become singular is also determined. In this regime, the photon energy is efficiently converted into surface plasmon energy.   

Self-Complementary Plasmonic Structures for High Efficiency Broadband Absorber in the Visible Range

We demonstrate, by simulation, that a planar 3-layer structure on a metal substrate can highly absorb electromagnetic radiation in the entire visible range, which can become a potential platform for high-efficiency broadband absorber. Such a structure consists of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference layer. It is shown that the perforated metallic film and the ultrathin absorber form an effective metamaterial film, which negatively refracts light in this broad frequency range. Our quantitative simulation confirms that the absorption bandwidth is maximized at the self-complementary pattern of the percolation threshold. If amorphous silicon (a-Si) is selected as the ultrathin semiconducting material, the absorbance of the structure with a checkerboard-patterned perforated metallic film is about 90{\%} in the visible range (from 400 nm to 700 nm), where 80{\%} goes into the a-Si layer and the other 10{\%} being absorbed by other layers. Further simulation shows that for a single p-i-n a-Si junction, the energy conversion efficiency of an optimized structure can exceed 12{\%}.   

Plasmonic halos: optical surface plasmon drumhead modes

We present the discovery and systematic study of a novel optical phenomenon, wherein optically-pumped surface plasmons on circular silver microcavities form confined drumhead modes that, under off-resonant conditions, transform to colorful far field radiation at their circumferential boundaries. We call this phenomenon the ``plasmonic halo.'' We demonstrate both experimentally and theoretically that such circular microcavities integrated with perimeter step gaps can generate surface plasmon cavity modes, and modulate optical transmission/emission through/from the device, yielding the plasmonic halo effect. Via the tuning of geometric and/or material parameters, optical properties of this device can be manipulated in the visible range, leading to promising applications in biomedical plasmonics, dielectric constant sensing and discrete optical filtering, among others.   

Tunable Fano resonance due to interaction between molecular vibrational modes and a double-continuum of a plasmonic metamolecule

We have fabricated and characterized a plasmonic system comprised of an array of asymmetric cross-shaped apertures in a metallic film coated with poly(methyl methacrylate) (PMMA). The apertures (called plasmonic metamolecules) produce localized surface plasmon (LSP) resonances that can be tuned by varying the polarization of incident light. Arrays of these nano-scale apertures, designed to have resonances at infrared wavelengths, were fabricated using electron beam lithography and argon ion milling of a gold film. Filling the apertures with PMMA allowed its C=O bond resonance to interact with tunable LSP modes. The transmission, reflection and absorption spectra of the system were measured using FTIR. Coupling between the LSPs and the C=O bond is shown to produce a Fano resonance that can be tuned in situ. The system was investigated theoretically using (a) rigorous electromagnetic calculations and (b) a quantum mechanical model that describes the interaction between a discrete state (the C=O bond) and multiple continua (the LSPs of the plasmonic metamolecule). We demonstrate that the predictions of the quantum model are in good agreement with the experimental data and show that the model allows an intuitive interpretation, at the quantum level, of the plasmon-molecule coupling.   

Session M21: Focus Session: Relaxors, Nanostructures and Morphotropic Phase Boundaries

Spontaneous ferroelectric-ferroelectric phase transitions and giant electro-mechanical energy conversion in [011] cut relaxor ferroelectric crystals

We report on giant electro-mechanical energy conversion is demonstrated under a ferroelectric/ferroelectric phase transformation in [011] cut and poled lead titanate-based relaxor perovskite morphotropic Pb(In$_{1/2}$Nb$_{1/2})$O$_{3}$-Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-PbTiO$_{3}$ (PIN-PMN-PT). single crystals. It is found that under mechanical pre-stress, a relatively small oscillatory stress drives the material reversibly between rhombohedral and orthorhombic phases with a remarkably high polarization and strain jumps induced at zero bias electric field and room temperature. The measured electrical output per cycle is more than an order of magnitude larger than that reported for linear piezoelectric materials. Ideal thermodynamic cycles are presented for this electro-mechanical energy conversion followed by a presentation and discussion of the experimental data. The stress dependence of thermally driven polarization change is reported for a ferroelectric rhombohedral to ferroelectric orthorhombic phase transformation in [011] cut and poled. A giant jump in polarization and strain is associated with the phase transformation of the ferroelectric material. The phase transition temperature can be tuned, over a broad temperature range, through the application of bias stress. This phenomenon results in a new approach to applications in the field of energy harvesting   

Successive pressure-induced structural transitions in relaxor Pb(In$_{1/2}$Nb$_{1/2})$O$_{3}$

We employed Raman scattering and x-ray diffraction to investigate the behavior of disordered Pb(In$_{1/2}$Nb$_{1/2})$O$_{3}$ (PIN) under pressure up to 50 GPa at 300 K. The sharp peak centered at 370 cm$^{-1}$ increases its intensity with pressure. Two Raman peaks around 550 cm$^{-1}$ merge at 16 GPa and their linewidths increase with pressure. The structural phase transition is associated with a splitting of the 50 cm$^{-1}$ peak above 16 GPa. In most Pb-based relaxors, in contrast to PIN, the 50 cm$^{-1}$ peak shows a slight hardening with pressure and no splitting is observed. The pressure evolution of the diffraction patterns for PIN shows obvious splittings above 16 GPa, particularly for the pseudo-cubic [110], [111] and [220] diffraction peaks, indicative of a symmetry-lowering transition. Our results demonstrate that PIN undergoes successive structural phase transitions. The transition at 6 GPa is similar to that observed in other Pb-based relaxors and related to the octahedra tilting; the transition at 16 GPa could be a rhombohedral to orthorhombic transition, and the transition at 38 GPa is assigned to an orthorhombic to a monoclinic transition.   

Strain, composition tuning and size effect in Pb$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_3$ piezoelectric thin films and nanostructures

Optimizing the piezoelectric performance at the nanoscale is one of the main challenges for future piezoelectric applications, especially in the field of vibrational energy harvesting. In this work, we have investigated the combined influence of epitaxial strain, compositional variation and size reduction on the crystallographic structure, ferroelectric domain configuration and piezoelectric properties of Pb$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_{3}$ thin films and nanostructures epitaxially grown by Pulsed Laser Deposition on SrRuO$_{3}$-buffered (110)-DyScO$_{3}$ substrates. Theoretical predictions on the PbTiO$_{3}$-SrTiO$_{3}$ solid solution show an interesting phase transition, expected to give rise to enhanced piezoelectric properties, as a function of composition when the films are grown under strain on (110)-DyScO$_{3}$. A series of high quality epitaxial thin films has been grown with various Pb/Sr ratios. We have experimentally confirmed the predicted phase transition. Highly periodic domains with purely in-plane polarization have been observed by both X-ray diffraction and piezoresponse force microscopy. The piezoelectric properties have then been studied as a function of composition and of the lateral dimensions of nano-objects defined by Electron Beam Lithography.   

Direct observation of intrinsic localized modes as precursors to polar nanoregions in a relaxor ferroelectric

Displacive ferroelectric phase transitions can be understood in terms of a soft zone center phonon tending towards zero frequency as the material is cooled towards the transition. Relaxor ferroelectrics are less well understood but there is a growing consensus that dispersed polar nanoregions (PNRs), pinned by chemical inhomogeneities, are responsible for the behavior. Furthermore, it has been argued that PNRs form via soft localized phonon modes, modeled as intrinsic localized modes (ILMs), tending towards zero frequency as the material is cooled into the relaxor region, but these modes have never been observed directly. In this talk, neutron scattering measurements will be presented that reveal the existence of a dispersionless (localized) mode appearing near the Burns temperature in PMN-PT. The local mode softens and diminishes in intensity on cooling towards the relaxor region, ultimately vanishing as the PNRs form.   

Structure and dynamics analyses of Pb(Mg$_{1/3}$,Nb$_{2/3})$O$_{3}$-PbTiO$_{3}$

Relaxor ferroelectric materials are of importance in applications due to their giant piezoelectricity, anomalous dielectric response, and diffuse phase transitions. However, mechanisms of the anomalous physical properties are still ambiguous, especially local structure and dynamics. According to our recent molecular dynamics simulations using a rock salt random site B-cation arrangement, the relax local structure is analogous to the hydrogen bonded network in water. In this work, we present structure and dynamics obtained from Bond-Valence model atomistic molecular dynamics simulations with the random site model and fully disordered 0.75PMN-0.25PT using diffuse scattering and dynamic pair distribution function techniques and compare our results with the available experimental data.   

Debye Relaxations, Fano Resonances and Heterophase Oscillations in the Relaxor K$_{\mathrm{1-x}}$Li$_{\mathrm{x}}$TaO$_3$

Besides characteristic dielectric relaxations, relaxor ferroelectrics have also been shown to exhibit strong resonances. These resonances are related to the ubiquitous presence of polar nanodomains in relaxors in their ``paraelectric'' phase below a certain temperature T*. In the relaxor K$_{\mathrm{1-x}}$Li$_{\mathrm{x}}$TaO$_{3}$ (KLT), the dielectric spectrum reveals pairs of coupled resonances with a Fano-type line shape that evolves dramatically with temperature. At higher temperature, the line shape reflects the close interplay between relaxations and resonances. Near the phase transition, it reveals the existence of coherent heterophase fluctuations. KLT provides a good example of the multiscale dynamics (from nano to macro) that is intrinsic to relaxors.   

Finite-Temperature Properties of Ba(ZrTi)O3 Relaxors from First Principles

A first-principles-based technique is developed to investigate the properties of Ba(ZrTi)O3 relaxor ferroelectrics as a function of temperature. The use of this scheme provides answers to important, unresolved and/or controversial questions such as the following. What do the different critical temperatures usually found in relaxors correspond to? Do polar nanoregions really exist in relaxors? If yes, do they only form inside chemically ordered regions? Is it necessary that antiferroelectricity develop in order for the relaxor behavior to occur? Are random fields and random strains really the mechanisms responsible for relaxor behavior? If not, what are these mechanisms? These ab initio based calculations also lead to deep microscopic insight into relaxors.   

Resonant Ultrasonic Spectroscopy of O-18 and O-16 Strontium Titanate

We have carried out [J. F. Scott, M. A. Carpenter, E. K. H. Salje et al., Phys. Rev. Lett. 106, 105502 (2011); 108, xxxxxx (2012)] resonant ultrasonic studies of bulk strontium titanate. Below 50K both O-18 and O-16 isotope studies reveal asymmetric Fano-lineshapes due to interaction between acoustic phonon branches related to C44 near 400 kHz and a continuum background due to Sr disorder along [111] directions, originally determined by the NMR studies of Blinc et al. The inference is that the ferroelectric phase of O-18 SrTiO3 has a disordered triclinic ground-state structure; this is compatible with the neutron studies by Bartkowiak et al. at ANSTO and helps reconcile paradoxes in the Brillouin studies of Shigenari et al. and Takesada, Yagi et al. For O-16 isotopic SrTiO3 the data show that the Brillouin splitting below ca. 50K previously misinterpreted as second sound by Courtens et al. and Tagantsev et al. is simply the required splitting of modes that would be degenerate in the tetragonal phase. The new studies show that the ferroelastic domains in O-16 SrTiO3 are polar and compatible with the 2012 flexoelectric model of Morozovska et al.   

Neutron Diffuse Scattering in Pure and Ba-Doped Single Crystals of the Relaxor NBT

We report neutron diffuse scattering measurements on the lead-free relaxors Na$_{1/2}$Bi$_{1/2}$TiO$_3$ (NBT) and NBT doped with 5.6\% BaTiO$_3$, a composition that is located close to the morphotropic phase boundary. The diffuse scattering in NBT appears on cooling near 700 K, which coincides with the temperature at which the dielectric constant deviates from Curie-Weiss behavior. Strong, anisotropic diffuse scattering intensity is observed near the (100), (110), (200), (220), and (210) Bragg peaks. The reciprocal space distribution of the diffuse scattering is consistent with the presence of competing rhombohedral and tetragonal short-range structural correlations. Doping NBT with 5.6\% BaTiO$_3$ reduces the correlation length associated with the tetragonal order by a factor of 10 while simultaneously enhancing the piezoelectric properties.   

Effects of electric field on acoustic properties of 0.83Pb(Mg$_{1/3}$Nb$_{2/3})$-0.17PbTiO$_{3}$ single crystals studied by Brillouin light scattering

Relaxor-based ferroelectric Pb[(Mg$_{1/3}$Nb$_{2/3})_{1-x}$Ti$_{x}$]O$_{3}$ (PMN-xPT) single crystals have attracted great attention because of their exceptionally strong piezoelectric properties. This peculiar characteristic was attributed to the rotation of polarization directions and structural complexity. In this study, the phase transition behaviors of PMN-17PT single crystals have been investigated under an electric field applied along [001] by micro-Brillouin scattering. PMN-17PT single crystals were grown by the modified Bridgeman method. The two (001) surfaces were Au-coated to apply the electric field, and the coating was thin enough to allow the incident beam to transmit without much loss. The electric field of different values was applied to the sample along the [001] direction, and the Brillouin scattering spectrum was measured under both field-heating (FH) and field-cooling (FC) conditions. The electric field of 1kV/cm induced a new longitudinal acoustic (LA) mode component along with a broad Brillouin peak evolving continuously from the paraelectric phase during both FC and FH processes. This was attributed to the remnant polar nanoregions that were not aligned under the electric field due to quenched random fields. However, the splitting of the LA mode did not appear when the electric field was over 2kV/cm indicating a clear structural phase transition.   

Polarization Reversal in Ferroelectric Nanowires using Terahertz Pulses

Ferroelectric nanowires are very attractive for potential applications in nanodevices, nanosensors or ferroelectric computer memory, since they posses reversible polarization at the nanoscale. Here we report the possibility to remotely control the polarization direction in ferroelectric nanowires by the application of a small biased field in combination with a terahertz Gaussian-shaped pulse. Our study is carried out on Pb(Zr$_{0.4}$Ti$_{0.6}$)O$_3$ nanowires using classical molecular dynamics with first-principle-based effective Hamiltonian[1]. The conditions for which the polarization reversal in the nanowire can be achieved by the coupled effect of a biased field with the application of a terahertz pulse are investigated. In particular, we will report computational data on the polarization reversal by application of THz pulses of different amplitude, frequency and width. Furthermore the dependence of the polarization reversal on the temperature is considered.   

Session M22: Strongly Correlated Electron Theory II

Bond Disorder Induced Criticality of the Three-Color Ashkin-Teller Model

An intriguing result of statistical mechanics is that a first-order phase transition can be rounded by disorder coupled to energylike variables. In fact, even more intriguing is that the rounding may manifest itself as a critical point, quantum or classical. In general, it is not known, however, what universality classes, if any, such criticalities belong to. In order to shed light on this question we examine in detail the disordered three-color Ashkin-Teller model by Monte Carlo methods. Extensive analyses indicate that the critical exponents define a new universality class. We show that the rounding of the first-order transition of the pure model due to the impurities is manifested as criticality. However, the magnetization critical exponent, $\beta$, and the correlation length critical exponent, $\nu$, are found to vary with disorder and the four-spin coupling strength, and we conclusively rule out that the model belongs to the universality class of the two-dimensional Ising model.   

A multi-critical point of strongly interacting itinerant fermions with supersymmetry

A key challenge in theoretical condensed matter physics is the study of strongly interacting fermions, for which perturbative techniques do not work. In recent years a specific model has been put forward where exact results at intermediate densities can be obtained by incorporating supersymmetry. For 2D lattices the supersymmetric model exhibits superfrustration, a strong form of quantum charge frustration, characterized by an extensive ground state entropy. In 1D the model also shows a rich structure. In particular, we discuss the supersymmetric model on the square ladder and show that it describes a multi-critical point where an Ising and a KT transition coincide. The RG equations for the continuum theory reveal an intricate flow diagram with a marginal direction that preserves supersymmetry. We will argue that these results imply that there is a whole class of models with a U(1) and a Z2 symmetry, for which the multi-critical point has emergent supersymmetry.   

Two-dimensional Hubbard model on a honeycomb lattice

In the honeycomb lattice, a combination of nontrivial topology and electronic correlations drives a great variety of phenomena. We study the 2-dimensional fermionic Hubbard model on a honeycomb lattice using exact diagonalization method at various onsite interaction strength U values. By introducing holes in the model at different filling levels, we analyze the charge gap instability of the lattice which indicates the possibility the system going into a paired state. We further monitor the one-particle excitation spectrum and density of states at various k-points. We find that the electronic interaction introduces quasiparticle states around the Fermi level and the system can undergo a metal-insulator transition. /newline /newline The authors acknowledge the computing facilities provided by the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000) and the Center for Functional Nanomaterials, Brookhaven National Laboratory supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No.DE-AC02-98CH10886.   

Order and supersymmetry at high filling zero-energy states on the triangular lattice

We perform exact diagonalization studies in $d=2$ dimensions for the Fendley and Schoutens model of hard-core and nearest-neighbor excluding fermions that displays an exact non-relativistic supersymmetry. Using clusters of all possible shapes up to 46 sites, we systematically study the behavior of the ground state phase diagram as a function of filling. We focus on the highly degenerate zero-energy states found at fillings between $1/7$ and $\sim 1/5$. At the lower end of that interval, at filling $1/7$, we explicitly show that the ground states are gapped crystals. Consistent with previous suggestions, we find that the extensive entropy of zero states peaks at a filling of $\sim 0.178$. At the higher end of the interval, we find zero energy ground states at fillings above $1/5$, contrary to previous expectations; which display non-trivial amplitude degeneracies.   

Toward a unified description of spin incoherent behavior at zero and finite temperatures

While the basic theoretical understanding of spin-charge separation in one-dimension, known as ``Luttinger liquid theory'', has existed for some time, recently a previously unidentified regime of strongly interacting one-dimensional systems at finite temperature came to light: The ``spin-incoherent Luttinger liquid'' (SILL). This occurs when the temperature is larger than the characteristic spin energy scale. I will show that the spin-incoherent state can be written exactly as a generalization of Ogata and Shiba's factorized wave function in an enlarged Hilbert space, using the so-called ``thermo-field formalism.'' Interestingly, this wave-function can also describe the *ground-state* of other model Hamiltonians, such as t-J ladders, and the Kondo lattice. This allows us to develop a unified formalism to describe SILL physics both at zero, and finite temperatures.   

Duality of Weak and Strong Scatterer in Luttinger Liquid Coupled to Massless Bosons

We study electronic transport in a Luttinger liquid (LL) with an embedded impurity, which is either a weak scatterer (WS) or a weak link (WL), when interacting electrons are coupled to one-dimensional massless bosons (e.g., acoustic phonons). The additional coupling competes with Coulomb interaction changing scaling exponents of various correlation functions. The impurity strength $\lambda$ and the tunneling amplitude $t$ in the WS and WL limits scale at low energies $\varepsilon$ as: $\lambda(\varepsilon) \sim \lambda_0\, \varepsilon^{\Delta_{\mathrm{ws}} - 1}$ and $t(\varepsilon) \sim t_0\, \varepsilon^{\Delta_{\mathrm{wl}} - 1}$, correspondingly. We find that the duality relation between the scaling dimensions established for the standard LL, $\Delta_{\mathrm{ws}}\Delta_{\mathrm{wl}} = 1 $, holds in the presence of the additional coupling for an arbitrary fixed strength of boson scattering from the impurity. As a result, at low temperatures the system remains either an ideal insulator or an ideal metal, regardless of the scattering strength. However, in the case when electron and boson scattering from the impurity are correlated, the system has a rich phase diagram that includes a metal-insulator transition at some intermediate values of the scattering.   

Absence of Luttinger's Theorem

We show exactly with an $SU(N)$ interacting model that even if the ambiguity associated with the placement of the chemical potential, $\mu$, for a $T=0$ gapped system is removed by using the unique value $\mu(T\rightarrow 0)$, Luttinger's sum rule is violated. The failure stems from the non-existence of the Luttinger-Ward functional for a system in which the self-energy diverges. Since it is the existence of the Luttinger-Ward functional that is the basis for Luttinger's theorem which relates the charge density to sign changes of the single-particle Green function, no such theorem exists. Experimental data on the cuprates are presented which show a systematic deviation from the Luttinger count, implying a breakdown of the electron quasiparticle picture in strongly correlated electron matter.   

Non-Fermi Liquid behavior at the Orbital Ordering Quantum Critical Point in the Two-Orbital Model

The critical behavior of a two-orbital model with degenerate $d_{xz}$ and $d_{yz}$ orbitals is investigated by multidimensional bosonization. We find that the corresponding bosonic theory has an overdamped collective mode with dynamical exponent $z=3$, which appears to be a general feature of a two-orbital model and becomes the dominant fluctuation in the vicinity of the orbital-ordering quantum critical point. Since the very existence of this $z=3$ overdamped collective mode induces non-Fermi liquid behavior near the quantum critical point, we conclude that a two-orbital model generally has a sizable area in the phase diagram showing non-Fermi liquid behavior. Furthermore, we show that the bosonic theory resembles the continuous model near the $d$-wave Pomeranchuk instability, suggesting that orbital order in a two-orbital model is identical to nematic order in a continuous model. Our results can be applied to systems with degenerate $d_{xz}$ and $d_{yz}$ orbitals such as iron-based superconductors and bilayer strontium ruthenates Sr$_3$Ru$_2$O$_7$.   

Time-reversal symmetry breaking Pomeranchuk instabilities in hexagonal systems: emergence of the $\beta$ phase

We show how nematic order that breaks time reversal symmetry can be stabilized by longer-range repulsive interactions in a variety of hexagonal systems. For the triangular, honeycomb and Kagome lattices at the van Hove filling, we show how spinful fermions can enter the so called $\beta$ phase, in analogy to the B phase in superfluid $^3$He. This Pomeranchuk instability in the spin channel involves a splitting of the Fermi surface into two parts, with the spin direction winding in momentum space. This is possible for angular momentum $l=2$ nematics, since these form a doubly degenerate irreducible representation of the $C_{6v}$ point group symmetry of the lattices in question. We demonstrate how our results are exact in the weak coupling limit, although separate numerical studies have shown that these phases can persist at stronger coupling.   

Self consistent solution of the tJ model in the overdoped regime

Detailed results from a recent microscopic theory of extremely correlated Fermi liquids, applied to the t-J model in two dimensions, are presented. The theory is to second order in a parameter $\lambda$, and is valid in the overdoped regime of the tJ model. The solution reported here is from Ref [1], where relevant equations given in Ref [2] are self consistently solved for the square lattice. Thermodynamic variables and the resistivity are displayed at various densities and T for two sets of band parameters. The momentum distribution function and the renormalized electronic dispersion, its width and asymmetry are reported along principal directions of the zone. The optical conductivity is calculated. The electronic spectral function $A(k,\omega)$ probed in ARPES, is detailed with different elastic scattering parameters to account for the distinction between LASER and synchrotron ARPES. A high (binding) energy waterfall feature, sensitively dependent on the band hopping parameter $t'$ is noted.\\[4pt] [1] ``Extremely Correlated Fermi Liquids: Self consistent solution of the second order theory,'' D. Hansen and B. S. Shastry, arXiv:1211.0594 (2012). \\[0pt] [2] ``Extremely Correlated Fermi Liquids: The Formalism,'' B. S. Shastry, arXiv:1207.6826 (2012).   

ECFL in the limit of infinite dimensions

Novel techniques for strongly correlated matter are of great importance. Here we compare two recent and independent methods that show considerable promise, and have overlapping regimes of applicability. We evaluate in infinite dimensions the leading order (i.e. $O(\lambda^2)$) equations from the theory of Extremely Correlated Fermi Liquids of the tJ model and compare the resulting Greens functions with recent results from the dynamical mean field theory of the Hubbard model, valid at large U/t that are broadly in the same parameter range where the tJ model is valid. Using the Schwinger equations of motion of the tJ model, we also show exactly that in infinite dimensions a suitably defined Dysonian self energy for the tJ model is independent of the wave vector, while the two self energies of the ECFL theory $\Phi(\vec{k}, i \omega_n)$ and $\Psi(\vec{k}, i \omega_n)$ are respectively linear in $\varepsilon_{\vec{k}}$ and independent of $\vec{k}$ in a minimal description. In particular, we prove that in the minimal theory $\Psi(\vec{k},i\omega_n) = \Psi(i\omega_n)$ and $\Phi(\vec{k},i\omega_n)= \chi(i\omega_n)+ \varepsilon_{\vec{k}} \Psi(i\omega_n) $.   

Chiral Non-Fermi Liquids

We propose a renormalization group scheme which is suitable for theories with Fermi surface. Low energy modes near the Fermi surface are viewed as a collection of one dimensional fermions with a continuous flavor labelling the momentum along the Fermi surface. Based on this approach, we study a class of chiral metals where one patch of Fermi surface is coupled with a gapless boson in two dimensions. Depending on the dispersion of the boson, one obtains either non-Fermi liquid or Fermi liquid state. We provide a non-perturbative argument for the stability of the states, and compute the exact critical exponents. Finally, we propose a possible experimental realization of a chiral non-Fermi liquid state.   

Non-Fermi liquids in three dimensions

The shape of the fermi surface may have important effects in determining the relevance (in Renormalization group sense) of interactions for the underlying fermions. In our work, we show that for certain physically realizable fermi surfaces in three dimensions, the coupling of the fermions to critical bosons is relevant at the Gaussian fixed point. We find that such interactions may lead to a three dimensional non-Fermi liquid state. We calculate one-loop corrections to the electron self energy within a scheme of (3-$\epsilon$) perturbation in the spatial dimensions to understand the features of such a non-Fermi liquid state.   

Broken time-reversal symmetry phase in a 2D electron fluid by using higher dimensional bosonization

We explore a phase in two-dimensional electron fluids in which the time-reversal symmetry is broken spontaneously by using the method of higher dimensional bosonization. This phase breaks time-reversal and chiral symmetries but does not break space inversion and the combination of chiral and time-reversal symmetries. This phase exhibits non-quantized anomalous Hall effect in the absence of external magnetic fields which corresponds to the Berry curvature on the Fermi surface. In the mean-field limit we show that the fluid spontaneously transforms into the the time-reversal broken phase [1]. The properties deep within the phase is also studied by solving the semi-classical equation of the bosonized fields. [1] Kai Sun and Eduardo Fradkin, Phys. Rev. B {\bf 78}, 245122 (2008).   

There is more to $d$-electrons than Hubbard $U$ and Hund's rule $J$

Multi-band Hubbard models including all $d$-bands are central for the description of many interesting correlated materials, e.g., the Iron based High-T$_c$ materials. In this work we compare two prevailing spin and angular momentum rotationally invariant models for the local $dd$-interaction, the generalized Kanamori interaction, and the Slater-Condon atomic Coulomb interaction, and establish how the first can be mapped to a very special case of the former. Using the recently developed multi-band Gutzwiller approximation solver, we show that the partial localization of orbital moments in the intermediately correlated regime of the paramagnetic state, is poorly described by the Kanamori model containing only Hubbard and Hund's rule interactions. In fact, for some integer fillings it differs qualitatively compared to the Slater-Condon interaction.   

Session M23: Optical and Dielectric Properties

Temperature-Dependent Cathodoluminescence of Disordered SiO2 Layers

Optical coatings of disordered thin film SiO2/SiOx dielectric samples on reflective metal substrates exhibited electron-induced luminescence (cathodoluminescence) under electron beam irradiation. These experiments provided measurements of the absolute radiance and emission spectra as functions of incident electron energy, flux and power over a range of sample temperatures (\textless 40 K to \textgreater 300 K). The overall luminescent intensity increased linearly with increasing power, plateaued, then fell off approximately exponentially. Spectrometer data revealed four spectral bands. The structural defects associated with three of the four bands have been identified. Temperature dependence of the peak intensity and central position differs for the lower and higher energy bands. These results are interpreted with a model of the band structure of highly disordered trapped states within the band gap of SiO2, used to describe the excitation of electrons from the valence band to the conduction band and subsequent relaxation into trapped states. The cathodoluminescence model describes these experimental observations, providing a fundamental basis for understanding the dependence of cathodoluminescence on irradiation time and accumulated charge, incident flux and energy, and sample thickness and temperature.   

Mechanical properties of highly porous low-k dielectric nano-films: A Brillouin light scattering study

To reduce RC time delays in micro-electronic devices, the semiconductor industry has pursued low dielectric constant (k) hybrid organic-inorganic interconnect layers with controlled levels of porosity. However, increased porosity as well as reduced film thicknesses (\textless\ 100nm) could reduce mechanical and thermal stability thereby degrading device functionality. Such structural characteristics present limitations with traditional measurement techniques as nanoindentation to characterize the mechanical properties of these highly compact and porous structures. We report on Brillouin light scattering measurements to determine the independent elastic constants, and thus the mechanical properties, of dielectric films with thicknesses as low as 25 nm and porosity levels up to 45{\%}, the highest in the industry. The frequency dispersion and associated light scattering intensities of longitudinal and transverse acoustic standing mode type excitations were utilized to determine Poisson's Ratio ($\nu$) and Young's Modulus (E). Significant modifications were found in $\nu $ and E of these highly porous carbon-doped SiO$_{2}$(Si-O-C-H) and amorphous carbon(a-C:H) materials compared to traditional SiO$_{2}$ and non-porous low-k materials.   

Power and Charge Deposition and Electron Transport in Disordered SiO2 Layers Under Electron Bombardment

Power and charge deposition in multilayer dielectrics from electron bombardment is dependent on the flux and energy-dependent electron penetration depth of the electron beam. Using the Continuous Slow Down Approximation (CSDA), a composite analytical formula has been developed to approximate the electron range which can be related to the dose rate, deposited power and Radiation Induced Conductivity (RIC). Based on the constituent layer geometry and material, the deposited charge can also be inferred. Three separate pulsed electron beam experiments were conducted to measure charge deposition, power dependent cathodoluminescence and RIC. The power and charge deposition experiments measured the net surface potential, electrode currents and electron induced luminescence of disordered SiO2 multilayer dielectrics with a grounded or floating conductive middle layer, using beam energies from 200 eV to 25 keV at \textless 40 K to room temperature. These results showed that the power and charge deposition's dependence on electron beam flux and incident energy compare favorably with the model predictions. The RIC experiments measured electrode currents using disordered SiO2 layers from \textless 40 K to \textgreater 320 K with dose rates from 10-5 Gy/s to 10-1 Gy/s. The onset of RIC in the energy-dependant depth of the RIC region provides an explanation for observed retrograde charging.   

Structural evolution of nanoporous ultra-low k dielectrics under voltage stress

High speed interconnects in advanced integrated circuits require ultra-low-k dielectrics. Reduction of the dielectric constant is achieved via incorporation of nanopores in structures containing silicon, carbon, oxygen and hydrogen (SiCOH). We study nanoporous SiCOH films of k=2.5 and thicknesses of 40 - 400 nm. Leakage currents develop in the films under long-term voltage stress, eventually leading to breakdown and chip failure. Previous work* has shown the build-up of trap states as dielectric breakdown progresses. Using FTIR spectroscopy we have tracked the reorganization of the bonds in the SiCOH networks induced by voltage stress. Our results indicate that the cleavage of the Si-C and SiC-O bonds contribute toward increase in the density of bulk trapping states as breakdown is approached. AC conductance and capacitance measurements have also been carried out to describe interfacial and bulk traps and mechanisms. Comparison of breakdown properties of films with differing carbon content will also be presented to further delineate the role of carbon. *Atkin, J.M.; Shaw, T.M.; Liniger, E.; Laibowitz, R.B.; Heinz, T.F. Reliability Physics Symposium (IRPS), 2012 IEEE International   

Spectroscopic analysis of erbium doped laser-induced crystals for fiber-laser applications

Laser induced crystallization of glasses is a highly spatially selective process which could be used to produce crystalline-core optical fibers for fiber-laser applications. Toward this goal, single crystal lines were ``written'' in Er:LaBGeO$_{5}$ glass using a femtosecond pulsed laser. These structures were analyzed using micro-Raman and luminescence spectroscopy in order to determine their viability as waveguiding laser gain media. Two-dimensional scans reveal that the erbium fluorescence is inhomogeneous over the cross-section of the crystal and lacks spatial coordination with the Raman emission, implying a physical ion accumulation in addition to enhancement due to the crystal field. Additionally, erbium fluorescence spectra taken at low temperatures from polycrystals with varying concentrations of erbium were compared to those from the laser-induced crystal lines. Significant differences in the emission energies and intensity ratios of the erbium peaks were observed. These differences may be due to the presence of strain, grain boundaries, or charge resulting from the different crystallization processes used.   

Luminescence and Local Structure Correlation of Er-doped Glasses and Composites

Er-doped (0.05{\%} to 3{\%}) Ga2O3 containing silicate glasses and composites have been prepared by rapid coolong from the melt (glasses), followed by annealing at various temperatures from 800C to 1100C (composites). The Er luminescence has been measures and will be correlated to the llcal structural properties of the Er atoms as measured by x-ray absorption spectroscopy (XAS) at the MRCAT (Sector 10) beamline at the Advanced Photon Source. Preliminary analysis of the XAS data indicates that the Er is in an octahedral environment in both the glasses and composites. The glasses show no clustering of Er atoms which would lead to quenched lumineacence.   

Large change in dielectric constant of CaCu$_3$Ti$_4$O$_{12}$ under violet laser

This work reports the influence of light illumination on the dielectric constant of CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO) polycrystals which exhibit giant dielectric constant. When the CCTO samples were exposed to 405-nm laser light, the enhancement in capacitance as high as 22{\%} was observed for the first time, suggesting application of light-sensitive capacitance devices. To understand this change better microscopically, we also performed electronic-structure measurements using photoemission spectroscopy, and measured the electrical conductivity of the CCTO samples under different conditions of light exposure and oxygen partial pressure. All these measurements suggest that this large change is driven by oxygen vacancy induced by the irradiation.   

Giant dielectric constant in CaCu$_3$Ti$_4$O$_{12}$-MgB$_2$ composites near the percolation threshold

We have investigated the enhancement of the dielectric constant K in CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO)-MgB$_{2}$ composite near the percolation threshold. To optimize the dielectric properties of pure CCTO we have sintered the samples at variuos temperatures. We will present the results of the measurements of $K$ in a broad frequency for pure CCTO for the samples sintered at 1100$^{\circ}$C and 500$^{\circ}$C. Commercially available MgB$_{2}$ powder was mixed with different weight fractions of CCTO and the pressure of 1GPa was applied to form composite pellets. Near the percolation threshold P$_{\mathrm{C}}$, CCTO/MgB$_{2}$ composite system exhibit a dramatic increase of the dielectric constant K by several orders of magnitude, compared to pure CCTO. We will also discuss the magnetic field dependence of the capacitance of CCTO composite powders.   

Coupling of photonic, plasmonic and electric effects in metal nanostructures

Strong photon drag was observed in thin metal films and nanostructures, with the maximum of the effect at plasmon resonance conditions. To better understand mechanism of the effect and explore the possibility to control it with nanoscale geometry, we studied photoinduced currents in gold films and nanomesh structures in the dependence on the wavelength and period of nanostructure. We showed that nanostructuring of the surface lead to significant (50-fold) increase in the magnitude of the effect. Results are discussed in terms of coupling of optical, plasmonic and electric effects   

Plasmons for Coulomb Coupled Spherical Shells

We report calculations of the collective plasmon excitations for an electron gas confined to the surface of a spherical shell. The energy spectra of the plasmons and particle-hole modes are presented as functions of the radius of the shell as well as the angular momentum quantum number $L$. We compare results for the plasma excitations for a single shell, a pair of concentric shells as well as when two shells have their centers separated by a distance which exceeds the sum of the radii of the two shells. For the single shell and pair of concentric shells, the plasma modes are labelled by the angular momentum quantum number $L$ only. However, for the pair of non-concentric shells, the plasma modes are labelled by both $L$ and $M$, the projection of angular momentum on the $z$ axis. These results have been obtained in the random phase approximation (RPA).   

High Optical Performance and Practicality of Active Plasmonic devices based on Rhombohedral BiFeO$_3$

BiFeO$_3$ is a multiferroic oxide with perovskite type structure, which has been studied extensively for its ferroelectric and magnetic behavior. The magnetoelectric coupling could potentially provide new functionalities. We have studied the electronic and optical properties of Rhombohedral BiFeO$_3$, which we show to be a very promising candidate material to build active nanophotonic devices, in particular nanoplasmonic devices. It has a strong switching modulated optical properties and a large optical birefringence $\Delta $n arising from the combination of octahedral tilts, ferroelectricity and G-type antiferromagnetism in BiFeO$_3$. A prototype of a plasmonic resonator with a Rhombohedral BiFeO$_3$ thin film layer is used as an example and shows excellent switch and modulation responses. The proposed approach provides potential opportunities to develop high performance nanophotonic devices for optical communication. We find excellent switching and modulation responses. The use of Rhombohedral BiFeO$_3$ provides an effective way to actively control optical performance of plasmonic nanostructures.   

Taming the flow of light via active magneto-optical impurities

We demonstrate that the interplay of a magneto-optical layer sandwiched between two judiciously balanced gain and loss layers which are both birefringent with misaligned in-plane anisotropy, induces unidirectional electromagnetic modes. Embedding one such optically active non-reciprocal unit between a pair of birefringent Bragg reflectors, results in an exceptionally strong asymmetry in light transmission. Remarkably, such asymmetry persists regardless of the incident light polarization. This photonic architecture may be used as the building block for chip-scale non-reciprocal devices such as optical isolators and circulators.   

Gyro-active structures: Unidirectional Reflectionless Isolators and Perfect Absorbers

We propose a novel circuit architecture that consists of gyrotropic elements sandwiched between two judiciously balanced gain and loss constituents. These structures exhibit unique transport characteristics stemming from a generalized parity-time (${\cal P{\tilde T}}$)-symmetry. Some of these features include unidirectional reflection-less isolation and perfect absorption as well as asymmetric Anderson localization when disorder is introduced. Realizations as well as applications within the framework of electronic and photonic circuitry are discussed.   

Local heating of ZnO due to the surface plasmon excitation of Au nanoparticles

Temperature dependent $E_{2}$\textit{(high)} Raman active optical phonon mode was investigated to identify the local heating of the ZnO, due to the surface plasmon excitation of the Au nanoparticles. The variation of the linewidth (FWHM) of $E_{2}$\textit{(high)} mode for ZnO was investigated from room temperature to 450 $^{\circ}$C with 25 $^{\circ}$C steps under constant 532 nm laser excitation intensity of 2.6*10$^{5}$ W/m$^{2}$. Linewidth (FWHM) was increased with the temperature and it was fitted into the theoretical model originally developed by Menendez \textit{et al}, which contains both cubic and quadratic anharmonicities. After optimizing the cubic and quadratic anharmonic coupling constants, the fit was used to estimate the local temperatures of Au/ZnO, which were irradiated with different laser intensities. The estimated local temperature for Au/ZnO was 613 $^{\circ}$C at the laser intensity of 8.1*10$^{5}$ W/m$^{2}$. ZnO without Au nanoparticles didn't show any large temperature variation under the different laser intensities. This is a clear evidence for the heat generation of Au nanoparticles due to the surface plasmon excitation.   

Session M24: Electronic Structure Methods I

Role of electronic localization in the phosphorescence of iridium sensitizing dyes

In this talk we present a recent systematic study\footnote{B. Himmetoglu, A. Marchenko, I. Dabo and Matteo Cococcioni, J. Chem. Phys. {\bf 137}, 154309 (2012)} of three representative iridium dyes, namely, Ir(ppy)$_3$, FIrpic and PQIr, which are commonly used as sensitizers in organic optoelectronic devices. We show that electronic correlations play a crucial role in determining the excited state energies in these systems, due to localization of electrons on Ir $d$ orbitals in the ground state. Electronic localization is treated by employing hybrid functionals within time-dependent density functional theory (TDDFT) and with Hubbard model based corrections within the $\Delta$-SCF approach. The performance of both methods are studied in a comparative fashion and shown to be in good agreement with experiments (within a few tenths of an electron-volt in predicting singlet-triplet splittings and optical resonances). The Hubbard corrected functionals provide further insights on the charge-transfer character of the excited states. The gained insight allows us to comment on envisioned functionalization strategies to improve the performance of these systems.   

A Strategy for Finding a Reliable Starting Point for G$_0$W$_0$ Demonstrated for Molecules

Many-body perturbation theory in the G$_0$W$_0$ approximation is an increasingly popular tool for calculating electron removal energies and fundamental gaps for molecules and solids. However, the predictive power of G$_0$W$_0$ for molecules is limited by its sensitivity to the density functional theory (DFT) starting point. In this contribution, the starting point dependence of G$_0$W$_0$ is demonstrated for several small organic molecules. Analysis of the starting point dependence leads to the development of a non-empirical scheme that allows to find a consistent and reliable DFT starting point for G$_0$W$_0$ calculations by adapting the amount of Hartree-Fock-exchange in a hybrid DFT functional. The G$_0$W$_0$ spectra resulting from this {\it consistent starting point (CSP) scheme} reliably predict experimental photoelectron spectra over the full energy range. This is demonstrated for a test set of various typical organic semiconductor molecules.\\[4pt] [1] T. Korzdorfer and Noa Marom, Phys. Rev. B Rapid Communications {\bf 86}, 041110 (2012).   

A Benchmark of GW Methods for Azabenzenes: Is the GW Approximation Good Enough?

Many-body perturbation theory in the \textit{GW} approximation is a useful method for describing electronic properties associated with charged excitations. A hierarchy of \textit{GW} methods exists, starting from non-self-consistent $G_{\mathrm{0}}W_{\mathrm{0}}$, through partial self-consistency in the eigenvalues (ev-scGW) and in the Green's function (sc\textit{GW}$_{\mathrm{0}})$, to fully self-consistent GW (sc\textit{GW}). Here, we assess the performance of these methods for benzene, pyridine, and the diazines. The quasiparticle spectra are compared to photoemission spectroscopy (PES) experiments with respect to all measured particle removal energies and the ordering of the frontier orbitals. We find that the accuracy of the calculated spectra does not match the expectations based on their level of self-consistency. In particular, for certain starting points $G_{\mathrm{0}}W_{\mathrm{0}}$ and sc\textit{GW}$_{\mathrm{0}}$ provide spectra in better agreement with the PES than sc\textit{GW}.   

Local atomic energies from optimal atomic orbitals

Decomposing the energy of a condensed matter system into atomic contributions is of great use e.g. for understanding the physical origin of defect and surface energetics or for identifying chemically reactive regions in disordered systems. However, commonly employed energy calculations in the framework of density-functional theory (DFT) do not in general provide a natural decomposition into atoms. Here we propose a novel scheme to achieve this based on the recently introduced concept of atom-centered Quamols [1] that are variationally optimized to represent the electronic structure with a minimal basis set, which largely avoids local overcompleteness issues. The spillage resulting from the remaining small incompleteness is segmented according to a space separation derived from the Quamol atomic densities, maintaining the accuracy of the underlying DFT calculation. The total energy is then decomposed by combining this basis set with a local energy density treatment based on the ideas of Chetty and Martin [2]. We demonstrate the performance of our scheme by visualizing and analyzing the energy distribution at surfaces and in amorphous silicon.\\[4pt] [1] Lange, B et al., Phys. Rev. B 84, 085101, (2011)\\[0pt] [2] Chetty, N. and Martin, Richard M., Phys. Rev. B 45, 6074, (1992)   

Fast response function for finite and bulk systems

Many-body perturbation theory of bulk systems is often realized within reciprocal space, using plane-wave (PW) basis sets. PW basis is advantageous because of its elementary basis functions and simple convergence control. However, the number of functions in PW basis grows with third power of unit cell size, irrespective of actual number of atoms present in the unit cell. Moreover, PW basis gives rise to full matrices in tensor algebra due to space-filling nature of PW. An alternative to PW would be usage of localized basis functions. In this contribution, we show how a basis of \textit{dominant products} (DP) can be used to describe excitations in finite and bulk systems. We present calculations of absorption spectra and electron-energy loss spectra within time-dependent density functional theory, realized within DP basis. The usage of localized functions and iterative techniques allow to keep the complexity of the calculations rather low: the overall number of operations grows with third power of number of atoms in the unit cell.Moreover, we have recently shown that Hedin's $GW$ calculations can also be performed using DP basis with an order-$N^3$ scaling for finite systems. We are currently extending this $GW$ methodology to bulk systems.   

Density-Functional Theory Applied to Rare Earth Metals: Approaches Based on the Random-Phase Approximation

The description of the volume collapse exhibited by some \emph{rare earth} metals poses a great challenge to density-functional theory (DFT) since local/semilocal functionals (LDA/GGA) fail to produce the associated phase transitions. We approach this problem by treating all electrons at the same quantum mechanical level, using both hybrid functionals (e.g. PBE0 and HSE06) and exact-exchange plus correlation in the random-phase approximation (EX+cRPA). We also assess the performance of recently developed beyond RPA schemes (e.g. rPT2 [1]). The calculations are performed for cerium and praseodymium, that display a volume collapse, and neodymium, in which the volume collapse is absent. The isostructural $\alpha$-$\gamma$ phase transition in cerium is the most studied. The exact exchange contribution in PBE0 and HSE06 is crucial to produce two distinct solutions that can be associated with the $\alpha$ and $\gamma$ phases, but quantitative agreement with the extrapolated phase diagram requires EX+cRPA [2].\\[4pt] [1] Ren \emph{et al.}, J. Mater. Sci. \textbf{47}, 7447 (2012).\\[0pt] [2] M. Casadei {\it et al.}, Phys. Rev. Lett. \textbf{109}, 14642 (2012).   

Vibrational spectroscopy of liquid water from first principles simulations: Raman Spectra

Raman spectroscopy is an important probe of the structural and vibrational properties of aqueous solutions and of water at interfaces. While many experimental data are available for various systems, no results of ab initio computations have yet been reported for the Raman spectra of liquid water or solutions. We computed the Raman spectrum of water at ambient conditions using first principles molecular dynamics simulations, coupled to the calculation of polarizability within density functional perturbation theory. We used semi-local functionals, 64 molecule cells and the Qbox code. Our results are in satisfactory agreement with experiment. We provided an interpretation of the spectral features oberved at low frequency and within the stretching band by defining a polarizability of water molecules in the fluid. Coupling the calculation of Raman and IR spectra is in progress: such coupling will open the way to interpret advanced vibrational spectroscopy measurements, e.g. Sum Frequency Generation spectroscopy.   

The bond-breaking and bond-making puzzle: many-body perturbation versus density-functional theory

Diatomic molecules at dissociation provide a prototypical situation in which the ground-state cannot be described by a single Slater determinant. For the paradigmatic case of H$_2$-dissociation we compare state-of-the-art many-body perturbation theory in the $GW$ approximation and density-functional theory (DFT) in the exact-exchange plus random-phase approximation for the correlation energy (RPA). Results from the recently developed renormalized second-order perturbation theory (rPT2) are also reported. For an unbiased comparison and to prevent spurious starting point effects both RPA and $GW$ are iterated to {\it full} self-consistency (i.e. sc-RPA and sc-$GW$). Both include topologically identical diagrams for the exchange and correlation energy but are evaluated with a non-interacting Kohn-Sham and an interacting $GW$ Green function, respectively. This has profound consequences for the kinetic and the correlation energy. $GW$ and rPT2 are both accurate at equilibrium, but fail at dissociation, in contrast to sc-RPA. This failure demonstrates the need of including higher order correlation diagrams in sc-$GW$. Our results indicate that RPA-based DFT is a strong contender for a universally applicable electronic-structure theory. F. Caruso {\it et al.} arxiv.org/abs/1210.8300.   

Structural and Electronic Properties of the Solvated Chloride Ion from First Principles Simulations

First principles simulations of anions in aqueous solutions represent a challenging task both from a theoretical and computational standpoint, and only sporadic ab initio studies of their electronic properties have appeared in the literature. We carried out first principles molecular dynamics (MD) simulations of the chloride anion in liquid water with semi-local (PBE) and hybrid (PBE0) functionals, using the Qbox code. We found substantial differences in the orientation of the water molecules in the first anion solvation shell when using the two different levels of theory. Most importantly, the relative energies of the highest occupied level (HOMO) of the anion was found to be lower than the top of the valence band of water with PBE and the HOMO state is fairly delocalized, while it is higher with PBE0 and the corresponding state is localized on the anion. Although qualitative correct, the result obtained with PBE0 is only in fair agreement with experiment. It is only when using many body perturbation theory at the GW level and PBE0 trajectories that we could find qualitative and quantitative agreement with experiment [1]. Work supported by DOE-CMSN DE-SC0005180 and DOE-BES DE-SC0008938.\\[4pt] [1] P. Delahay, Acc. Chem. Res. 15, 40 (1982).   

Ab initio calculations of quasiparticle energies of solids, liquids and molecules using a spectral decomposition of the dielectric matrix

We recently developed a method for the calculation of quasiparticle energies within many body perturbation theory, at the $GW$ level, which avoids costly summations over empty electronic states and does not require the use of plasmon-pole models [1]. We present a comprehensive validation of this method, encompassing calculations of i) the vertical ionization energies of a set of over 80 molecules (containing from 14 to 424 valence electrons); ii) the relative position of energy levels of anions and water in hydrated sulfate and chloride clusters; iii) the band structure of a variety of semiconductors and (iv) the electronic properties of amorphous and liquid systems. The efficiency of our approach allowed us to compute quasiparticle energies of multiple configurations of liquid water, using samples with 64 molecules, selected over trajectories generated by ab initio molecular dynamics simulations. \\[4pt] [1] H. Viet Nguyen, T. Anh Pham, D. Rocca and G. Galli, Phys. Rev. B \textbf{85}, 081101(R) (2012); T. Anh Pham, H. Viet Nguyen, D. Rocca and G. Galli (submitted)   

Structural Stability Driven by the Spin-Orbit Coupling and the Superconductivity in simple-cubic Polonium

Polonium is the only element which has the simple-cubic (SC) structure in the periodic table. We have studied its structural stability based on the phonon dispersion calculations using the first-principles all-electron full-potential band method. We have demonstrated that the strong spin-orbit coupling (SOC) in SC-Po suppresses the Peierls instability and makes the SC structure stable. We have also discussed the structural chirality realized in beta-Po, as a consequence of the phonon instability. Further, we have investigated the possible superconductivity in SC-Po, and predicted that it becomes a superconductor with Tc $\sim$ 4 K at ambient pressure. The transverse soft phonon mode at q $\sim$ 2/3 R, which is greatly affected by the SOC, plays an important role both in the structural stability and the superconductivity in SC-Po. We have explored effects of the SOC and the volume variation on the phonon dispersions and superconducting properties of SC-Po.   

Parallelized electronic transport calculations in real space

We present a real-space method for first-principles nano-scale electronic transport calculations, using the non-equilibrium Green's function (NEGF) method and complex absorbing potentials (CAPs) to represent the effects of the semi-infinite leads. In real space, the electronic Hamiltonian from Density Functional Theory (DFT) is very sparse. As a result, the transport problem parallelizes naturally and can scale favorably with system size. We illustrate our method with calculations on several realistic test systems and find good agreement with a reference calculation.   

Surface chemistry in a full-potential QM/MM approach: making hybrids affordable

Nanostructured oxide surfaces are promising candidates for a wide range of energy and catalysis applications. When addressing corresponding functionalities through quantitative first-principles calculations, exploitation of the localized character of the chemical processes yields numerically most efficient approaches. To this end we augment the FHI-aims\footnote{V. Blum \textit{et al.}, Comput. Phys. Commun., \textbf{180}, 2175-2196 (2009)} package with a QM/MM\footnote{N. Bernstein \textit{et al.}, Rep. Prog. Phys., \textbf{72}, 026501 (2009)} functionality, in which the nanostructure and immediate oxide surrounding is described quantum mechanically, the long-range electrostatic interactions with the support are accounted for through a polarizable monopole field, and a shell of norm-conserving pseudopotentials correctly connects the two regions. We illustrate the accuracy and efficiency of the implementation with examples from the photo-catalytic water splitting context and specifically discuss the use of charged system states to address charge transfer processes.   

Spin-Orbit Coupling within the GW Approximation

We have developed and implemented an approach in which the effects of spin-orbit interactions to the quasiparticle band structure are incorporated within the GW approach, employing spinor wavefunctions computed at the density functional theory (DFT) level with fully relativistic pseudopotentials. Special consideration is given to the significance of the spin-dependent exchange-correlation potential. We compare these results to separate calculations where spin-orbit coupling is applied as a perturbation. We apply these methods to the properties of materials with heavy ion cores to determine the possible differences from the different treatments of spin-orbit coupling.   

Cumulant expansion treatment of phonon contributions to the electron spectral function

We present an approach for calculations of phonon contributions to the electron spectral function at finite temerature based on cumulant expansion techniques. Our approach is based on a many--pole representation of the Eliashberg function for the electron--phonon interaction, calculations of the dynamical matrix using ABINIT [1], and an Einstein self--energy model [2]. The code has been implemented as part of a plug--in to ABINIT for calculations of various phonon properties, and is applicable to complex structures with several atoms per unit cell. Results are given for a number of systems and compared to those obtained with the GW approximation.\\[4pt] [1] X. Gonze et al., Computational Materials Science \textbf{25}, 478 (2002). \\[0pt] [2] A. Eiguren and C. Ambrosch--Draxl, Phys. Rev. Lett. \textbf{101}, 036402 (2008).   

Session M25: Focus Session: Modeling of Rare Events I

Local Hyperdynamics

We present a new formulation of the hyperdynamics method in which the biasing effect is local, making it suitable for large systems. In standard hyperdynamics, the requirement that the bias potential be zero everywhere on the dividing surface bounding the state has the consequence that for large systems the boost factor decays to unity, regardless of the form of the bias potential. In the new method, the bias force on each atom is obtained by differentiating a local bias energy that depends only on the coordinates of atoms within a finite range D of this atom. This bias force is thus independent of the bias force in distant parts of the system, providing a method that gives a constant boost factor, independent of the system size. Although the resulting dynamics are no longer conservative, we show that for a homogeneous system (all atoms equivalent) using a simplifed bond-boost bias potential, the bias forces in any local region are equivalent to those in a system accelerated by a specific boost factor, except for additional error forces that balance in a time average. We also argue that even for inhomogeneous systems, the errors relative to an exactly accelerated dynamics should should decay roughly as 1/D. We demonstrate for some realistic atomistic systems that the method gives escape rates in excellent agreement with direct molecular dynamics simulations.   

A Local Superbasin Kinetic Monte Carlo Method

A ubiquitous problem in atomic-scale simulation of materials is the small-barrier problem, in which the free-energy landscape presents ``superbasins'' with low intra-basin energy barriers relative to the inter-basin barriers. Rare-event simulation methods, such as kinetic Monte Carlo (KMC) and accelerated molecular dynamics, are inefficient for such systems because considerable effort is spent simulating short-time, intra-basin motion without evolving the system significantly. We developed an adaptive local-superbasin KMC algorithm (LSKMC) for treating fast, intra-basin motion using a Master-equation / Markov-chain approach and long-time evolution using KMC. Our algorithm is designed to identify local superbasins in an on-the-fly search during conventional KMC, construct the rate matrix, compute the mean exit time and its distribution, obtain the probability to exit to each of the superbasin border (absorbing) states, and integrate superbasin exits with non-superbasin moves. We demonstrate various aspects of the method in several examples, which also highlight the efficiency of the method.   

Free energy calculation from umbrella sampling using Bayesian inference

Using simulations to obtain information about the free energy of a system far from its free energy minima requires biased sampling, for example using a series of harmonic umbrella confining potentials to scan over a range of collective variable values. One fundamental distinction between existing methods that use this approach is in what quantities are measured and how they are used: histograms of the system's probability distribution in WHAM, or gradients of the potential of mean force for umbrella integration (UI) and the single-sweep radial basis function (RBF) approach. Here we present a method that reconstructs the free energy from umbrella sampling data using Bayesian inference that effectively uses all available information from multiple umbrella windows. We show that for a single collective variable, our method can use histograms, gradients, or both, to match or outperform WHAM and UI in the accuracy of free energy for a given amount of total simulation time. In higher dimensions, our method can effectively use gradient information to reconstruct the multidimensional free energy surface. We test our method for the alanine polypeptide model system, and show that it is more accurate than a RBF reconstruction for sparse data, and more stable for abundant data.   

Characterization of the relation between energy landscape and the time evolution of complex materials using kinetic ART

In the last two decades, there has been a considerable interest in the development of accelerated numerical methods for sampling the energy landscape of complex materials. Many of these methods are based on the kinetic Monte Carlo (KMC) algorithm introduced 40 years ago. This is the case of kinetic ART, for example, which uses a very efficient transition-state searching method, ART nouveau, coupled with a topological tool, NAUTY, to offer an off-lattice KMC method with on-the-fly catalog building to study complex systems, such as ion-bombarded and amorphous materials, on timescales of a second or more. Looking at two systems, vacancy aggregation in Fe and energy relaxation in ion-bombarded c-Si, we characterize the changes in the energy landscape and the relation to its time evolution with kinetic ART and its correspondence with the well-known Bell-Evans-Polanyi principle used in chemistry.   

Atomistic simulations of melting and solidification using temperature accelerated molecular dynamics

A detailed understanding of melting/solidification mechanisms in metals remains obscure, though over the years many simulations and experiments have been performed for clarifying it. We have applied the enhanced-sampling method, Temperature-Accelerated Molecular Dynamics, to study the melting/solidification of FCC metals like copper, nickel under the constant temperature and pressure conditions. Free energy surfaces along Steinhardt order parameters and local density are obtained and minimum free energy path (MFEP) between the metastable states are calculated. An analysis of the atomic structure along the MFEP, reveals that an interplay between orientation ordering and positional ordering governs this phase transition.   

An Algorithm to Compute Statistics of Stochastic Paths on Complex Landscapes

Many systems in physics, chemistry, and biology can be modeled as a random walk on a network subject to a potential landscape. There is great interest in understanding the statistical properties of pathways on these landscapes, especially their times, lengths, and distributions in space. The complexity of the networks and landscapes arising in many models makes them difficult to solve by traditional analytical and computational tools. Moreover, standard methods do not always provide the most relevant information for characterizing these pathways. We develop an explicitly path-based formalism for studying these problems, which we implement using a numerical dynamic programming algorithm. It is especially well-suited to studying first-passage problems and rare transitions between metastable states. This method is valid for arbitrary networks and landscapes, as well as semi-Markovian processes with non-exponential waiting-time distributions. We explore this method on a variety of simple models including regular lattices, fractals, and protein sequence evolution.   

An Efficient Kernel Polynomial Method for Calculating Transition Rates in Large-Scale Materials

We present an efficient method for calculating transition rates in large-scale materials using harmonic transition state theory. In this method, we first reformulate the prefactor of the transition rates in terms of the density of states (DOS) of Hessian matrices. The DOS are then efficiently calculated with the kernel polynomial method. The scaling of our method is discussed in detail. We demonstrate our approach by calculating the prefactors for vacancy hopping and Frenkel pair formation in silver. Very good agreement between the KPM approach and exact diagonalization is observed.   

Reaching extended length-scales with temperature-accelerated dynamics

In temperature-accelerated dynamics (TAD) a high-temperature molecular dynamics (MD) simulation is used to accelerate the search for the next low-temperature activated event. While TAD has been quite successful in extending the time-scales of simulations of non-equilibrium processes, due to the fact that the computational work scales approximately as the cube of the number of atoms, until recently only simulations of relatively small systems have been carried out. Recently, we have shown that by combining spatial decomposition with our synchronous sublattice algorithm, significantly improved scaling is possible. However, in this approach the size of activated events is limited by the processor size while the dynamics is not exact. Here we discuss progress in developing an alternate approach in which high-temperature parallel MD along with localized saddle-point (LSAD) calculations, are used to carry out TAD simulations without restricting the size of activated events while keeping the dynamics ``exact'' within the context of harmonic transition-state theory. In tests of our LSAD method applied to Ag/Ag(100) annealing and Cu/Cu(100) growth simulations we find significantly improved scaling of TAD, while maintaining a negligibly small error in the energy barriers.   

Switching time distributions and scaling behavior in a bistable tunnel diode circuit with adjustable noise intensity

We report the measurement of first-passage time distributions associated with electrical current switching in a tunnel diode circuit that is driven by a noise generator with adjustable noise intensity $D$. The tunnel diode circuit is biased with a voltage $V_{\mathrm{f}}$ that is set in a range of bistability which terminates at the upper end in a saddle-node bifurcation at voltage $V_{\mathrm{th}}$. We employ a high bandwidth technique that permits measurement of stochastically-varying switching times over a very large dynamic range [1], with measured times ranging from 1 $\mu $s to several seconds. The dependence of both the form of the distribution and extracted mean switching time $\tau $ are also studied as a function of reduced voltage $V_{\mathrm{th}} - V_{\mathrm{f}}$ and $D$. Switching time distributions are generally found to possess exponential tails at long times, consistent with a picture of noise-induced escape via a single saddle point. Also, parameter regimes are identified in which the mean switching time scales as reduced voltage to the 3/2 power and linearly with inverse noise intensity. [1] Yu. Bomze, R. Hey, H. T. Grahn, and S. W. Teitsworth, Phys. Rev. Lett. \textbf{109}, 026801 (2012).   

Dependence of switching path distributions on relative noise intensities for a two-dimensional model of electrical conduction in a tunnel diode circuit

The incorporation of negative differential resistance elements such as tunnel diodes into electronic circuits often leads to bistability, i.e., distinct co-existing states of current for a given applied voltage. Such systems are generally far-from-equilibrium and non-gradient. We discuss a model of electrical conduction in a tunnel diode circuit in the form of a two-dimensional dynamical system, and use a geometric minimum action method (gMAM) [1] to study the dependence of the most probable escape paths (MPEPs) and associated actions on the ratio of the noise amplitudes associated with the two variables. We find that the MPEP follows the time-reversed path (i.e., a saddle-node trajectory) for a unique value of noise amplitude ratio; however, in general, MPEPs follow distinct paths that vary significantly as the noise amplitude ratio is varied. Additionally, we find good agreement between the computed MPEPs and actions and numerically generated switching path distributions and mean first-passage times, respectively. \\[4pt] [1] M. Heymann and E. Vanden-Eijnden, Phys. Rev. Lett. \textbf{100}, 140601 (2008).   

Kinetics of droplet wetting-mode transitions on grooved surfaces: Forward flux sampling

Liquid droplets on rough surfaces typically exhibit either the Cassie wetting mode, in which the droplet resides on top of the roughness, or the Wenzel mode, in which the droplet penetrates into the roughness. For a fixed surface topology and droplet size, one of these modes is the global free-energy minimum. However, the other state is often metastable and long-lived due a free-energy barrier that hinders the transition between the two wetting states. Metastable wetting states have been observed experimentally and we also observe them in molecular dynamics (MD) simulations of a droplet on a grooved surface. Using forward flux sampling, we study the kinetics of the Cassie-Wenzel transition. The global-minimum wetting states that emerge from our nanoscale MD approach are consistent with those predicted by a macroscopic model for the free energy. We find that the free-energy barrier for this transition depends on the droplet size and surface topology. A committor analysis indicates that the transition-state ensemble consists of droplets that are on the verge of initiating/breaking contact with the substrate below the grooves.   

Diffusion of small Ni and Cu clusters on Ni (111): Application of SLKMC-II*

We have examined the diffusion of small Ni and Cu islands (consisting of up to 10 atoms) on the Ni(111) surface using a self-learning kinetic Monte Carlo (SLKMC-II) [1] method with an improved pattern-recognition scheme that allows inclusion of both fcc and hcp sites in the simulations. In an SLKMC simulation [2] a database holds information about the local neighborhood of an atom and associated processes that is accumulated on-the-fly, as the simulation proceeds. The activation energy barriers for the identified diffusion processes were calculated using semi-empirical interaction potential based on the embedded-atom method. Although a variety of concerted, multi-atom and single-atom processes were automatically revealed in our simulations, we found that these small islands diffuse primarily via concerted motion. We report diffusion coefficients for each island size at several temperatures, and from the Arrhenius plot extract the size-dependent effective diffusion barrier for these islands. Our evaluation of the occurrence frequency of processes most responsible for the diffusion of island of a specific size reveal several that are not accessible in SLKMC-I [2] or in short time-scale MD simulations. We also provide results of extending SLKMC-II to examine epitaxial growth in these systems. [1] S. Islamuddin Shah, et al., J. Phys.: Condens. Matter 24, 354004 (2012). [2] O. Trushin, et al., Phys. Rev. B 72, 115401 (2005).   

Session M26: Semiconductor Qubits - RF Measurement and Hybridization

Circuit quantum electrodynamics with a spin qubit

Electron spins in quantum dots have been proposed as the building blocks of a quantum information processor. While both fast one and two qubit operations have been demonstrated, coupling distant spins remains a daunting challenge. In contrast, circuit quantum electrodynamics (cQED) has enabled superconducting qubits to be readily coupled over large distances via a superconducting microwave cavity. I will present our recent work aimed at integrating spin qubits with the cQED architecture.\footnote{K.D. Petersson et al., Nature 490, 380 (2012).} Our approach is to use spin qubits formed in strong spin-orbit materials such as InAs nanowires to enable a large effective coupling of the spin to the microwave cavity field. For an InAs nanowire double quantum dot coupled to the superconducting microwave cavity we achieve a charge-cavity coupling rate of $\sim 30$ MHz. Combining this large charge-cavity coupling rate with electrically driven spin qubit rotations we demonstrate that the cQED architecture can be used a sensitive probe of single spin dynamics. In another experiment, we can apply a source-drain bias to drive current through the double quantum dot and observe gain in the cavity transmission. We additionally measure photon emission from the cavity without any input field applied. Our results suggest that long-range spin coupling via superconducting microwave cavities is feasible and present new avenues for exploring quantum optics on a chip.   

Measuring the Charge Parity of an InAs Double Quantum Dot

We have fabricated tunable, few electron InAs nanowire double quantum dots (DQDs) which support rapid electrically driven single spin rotations.\footnote{M. D. Schroer, K. D. Petersson, M. Jung and J. R. Petta, Phys. Rev. Lett. \textbf{107}, 176811 (2011).} However, the measurement of nanowire DQDs presents an outstanding problem, typically relying on transport through the sample due to the lack of a local quantum point contact charge detector. We demonstrate a non-invasive charge sensing method based on a radio frequency measurement of the sample's complex admittance, which yields a fast and sensitive determination of the charge state.\footnote{M. Jung, M. D. Schroer, K. D. Petersson and J. R. Petta, Appl. Phys. Lett. \textbf{100}, 253508 (2012).} We show that this measurement is also sensitive to the spin state of the DQD, allowing a simple determination of the total charge parity in the sample.\footnote{M. D. Schroer, M. Jung, K. D. Petersson and J. R. Petta, Phys. Rev. Lett. \textbf{109}, 166804 (2012).} Radio frequency charge parity measurement may prove useful in high effective mass systems, such as Si/SiGe quantum dots, where the determination of the absolute charge number is not always feasible.   

Photon emission from a cavity-coupled double quantum dot

Circuit quantum electrodynamics (cQED) allows strong coupling between a microwave photon and a superconducting qubit. We recently demonstrated coupling of a double quantum dot (DQD) spin qubit to a high quality factor cavity in the cQED architecture, with a charge-cavity coupling rate of 30 MHz. Here we explore the same system, but with a finite source-drain bias applied across the DQD, which forces electrons to tunnel through the device. For specific experimental conditions, we observe gain in the cavity transmission. Moreover, in the absence of an input field, we directly measure photon emission from the cavity-coupled DQD. Our results are inconsistent with existing theoretical models, suggesting that contributions from phonons or cotunneling may be necessary to quantitatively describe the gain mechanism.   

Superconducting coplanar waveguide resonators for electron spin resonance applications

Superconducting coplanar waveguide (CPW) resonators are a promising alternative to conventional volume resonators for electron spin resonance (ESR) experiments where the sample volume and thus the number of spins is small. However, the magnetic fields required for ESR could present a problem for Nb superconducting resonators, which can be driven normal. Very thin Nb films (50 nm) and careful alignment of the resonators parallel to the magnetic field avoid driving the Nb normal, but flux trapping can still be an issue. Trapped flux reduces the resonator Q-factor, can lead to resonant frequency instability, and can lead to magnetic field inhomogeneities. At temperatures of 1.9 K and in a magnetic field 0.32 T, we have tested X-band resonators fabricated directly on the surface of a silicon sample. Q-factors in excess of 15,000 have been obtained. A thin layer of GE varnish applied directly to the resonator has been used to glue a sapphire wafer to its surface, and we still find Q-factors of 16,000 or more in the 0.32 T field. ESR applications of these resonators will be discussed.   

Microwave Measurements Electrons on Helium with Superconducting Coplanar Waveguide Resonators

Electrons on helium is a unique two-dimensional electron gas system formed at the interface of a quantum liquid (superfluid helium) and vacuum. The motional and spin states of single-electron quantum dots defined on such systems have been proposed for hybrid quantum computing [1,2]. Traditional AC transport experiments of electrons on helium are conducted at kilohertz frequencies. Here, we will present microwave measurements of electrons trapped in a 5GHz superconducting coplanar waveguide resonator with 1 MHz bandwidth. The effect of trapping parameters on the resonance, and experimental progress towards a single trapped electron regime will also be discussed.\\[4pt] [1] S. Lyon, Phys. Rev. A. 74, 5 (2006)\\[0pt] [2] D.I. Schuster, et al. Phys. Rev. Lett. 105, 040503 (2010)   

Photon mediated interaction between distant quantum dot circuits

Cavity QED allows one to study the interaction between light and matter at the most elementary level, by using for instance Rydberg atoms coupled to cavity photons. Recently, it has become possible to perform similar experiments on-chip, by using artificial two-level systems made from superconducting circuits instead of atoms. This circuit-QED offers unexplored potentialities, since other degrees of freedom than those of superconducting circuits could be used, and in particular, those of quantum dots. Such a hybrid circuit QED would allow one to study a large variety of situations not accessible with standard cavity QED, owing to the versatility of nanofabricated circuits. Here, we couple two quantum dot circuits to a single mode of the electromagnetic field in a microwave cavity. Our quantum dots are separated by 200 times their own size, with no direct tunnel and electrostatic couplings between them. We demonstrate their interaction mediated by the cavity photons. This could be used to scale up quantum bit architectures based on quantum dot circuits, and simulate on-chip phonon-mediated interactions between strongly correlated electrons.   

Phonon-Mediated Population Inversion in a Driven Double Quantum Dot

We examine phonon emission processes in a double quantum dot, configured as either a single or two-electron charge qubit and driven with resonant microwave excitation. Fast readout using a proximal rf quantum point contact (rf-QPC) enables charge sensing with high resolution and allows fine phonon-related features to be observed in microwave spectroscopy data. Spontaneous phonon emission is observed to produce level broadening and population inversion of a two-level system, a phenomena predicted theoretically but previously unreported. For the two-electron configuration, microwave transitions are shown to be spin-dependent, consistent with the well-understood mechanism of Pauli-blockade in double quantum dots.   

Dispersive Readout of a Few-Electron Double Quantum Dot with Fast rf Gate-Sensors

We report the dispersive charge-state readout of a double quantum dot in the few-electron regime using the $in$ $situ$ gate electrodes as sensitive detectors. We benchmark this gate-sensing technique against the well established quantum point contact (QPC) charge detector and find comparable performance with a bandwidth of $\sim$ 10 MHz and an equivalent charge sensitivity of $\sim$ 6.3 $\times$ $10^{-3}$ e/$\sqrt{\mathrm{Hz}}$. Dispersive gate-sensing alleviates the burden of separate charge detectors for quantum dot systems and promises to enable readout of qubits in scaled-up arrays.   

Spectroscopy of a GaAs Double Dot Qubit with Dispersive Readout

We report microwave spectroscopy of a GaAs double dot qubit device using the dispersive gate sensor (DGS) readout technique. In contrast to charge sensing methods based on quantum point contacts (QPCs) or single electron transistors (SETs), the DGS detection method senses the tunneling of charge between states that are near degenerate in energy. Microwave excitation applied to the surface gates enables this readout approach to resolve low energy spectroscopic features not apparent in transport or standard charge sensing measurements. We discuss the origin of these features and the use of this technique for characterizing semiconductor qubit systems.   

Radio frequency charge sensing in a Si double quantum dot device

Coherent spin manipulation has recently been demonstrated in a variety of silicon based devices.\footnote{B. M. Maune \emph{et al.}, Nature \textbf{481}, 344 (2012).}$^,$\footnote{J. J. Pla \emph{et al.}, Nature \textbf{489}, 541 (2012).} We fabricate accumulation mode double quantum dot devices and use radio frequency reflectometry to perform fast charge sensing in the few-electron regime. Our devices employ a nearby single quantum dot as a charge sensor. Charge transitions in the double dot result in a $\sim$60\% relative change in the charge sensor conductance when the sensor is operated in the Coulomb blockade regime, compared to a $\sim$1\% conductance change when the sensor is operated as a traditional quantum point contact. Further development of these techniques may enable us to perform single shot spin readout in a silicon quantum dot.   

Transport and Charge Manipulation in a Single Electron Silicon Double Quantum Dot

Silicon is one of the most promising candidates for ultra-coherent qubits due to its relatively early position in periodical table and the absence of nuclear spin in its naturally abundant isotope. Here we demonstrate a reliable recipe that enables us to reproducibly fabricate an accumulation mode few electron double quantum dot (DQD). We demonstrate tunable interdot tunnel coupling at single electron occupancy in the device. The charge state of the qubit is monitored by measuring the amplitude of the radio frequency signal that is reflected from a resonant circuit coupled to a charge sensor. By applying microwave radiation to the depletion gates, we probe the energy level structure of the DQD using photon assisted tunneling (PAT). We apply bursts of microwave radiation and monitor the dependence of the PAT peak height on the burst period to extract the charge relaxation time, T$_{1}$. By experimentally tuning the charge qubit Hamiltonian, we measure the tunnel coupling and detuning dependence of T$_{1}$.   

Progress towards microwave readout of a silicon double quantum dot

Microwave resonators coupled to quantum systems have been used for fast dispersive measurement in several different architectures in solid state and atomic physics. The electronic states of a semiconductor quantum dot represent a promising candidate for quantum information processing. Our work is geared toward developing a fast, non-demolition readout of a semiconductor qubit in silicon through coupling to a superconducting microwave resonator. We report progress on a novel design of a lateral   

Fabrication and measurement of an RF-QPC in an undoped Si/SiGe heterostructure

We perform radio-frequency reflectometry measurements on a quantum point contact fabricated in an undoped accumulation-mode Si/SiGe heterostructure. This device is a promising candidate for high-bandwidth charge sensing in Si/SiGe, and it provides the capability for fast qubit readout in this material. We show operation of the device with a well-defined resonance that can be modulated by a nearby gate. We will discuss design challenges that are particular to accumulation-mode structures and how they can be resolved.   

Session M27: Focus Session: Quantum Error Correction and Decoherence Control I

Dynamical quantum error correction: recent achievements and prospects

Precisely controlling the dynamics of real-world open quantum systems is a central challenge across quantum science and technology, with implications ranging from quantum physics and chemistry to fault-tolerant quantum computation. Dynamical quantum error correction strategies based on open-loop time-dependent modulation of the system dynamics provide a general perturbative framework for boosting physical-layer fidelities in the non-Markovian regime. I will describe recent progress in designing dynamical error correction schemes able to incorporate various system and control constraints encountered in realistic scenarios. In particular, I will show how to employ dynamical decoupling methods to achieve high-fidelity quantum storage for long times, while minimizing access latency and sequencing complexity, and how to synthesize dynamically corrected quantum gates for simultaneously canceling non-Markovian decoherence and control errors, while accommodating internal always-on dynamics and limited control. In the process, I will make contact with current qubit devices to the extent possible and point to remaining challenges and directions for further explorations.   

Quantum Control and Fault-tolerance

Quantum control (QC) and the methods of fault-tolerant quantum computing (FTQC) are two of the cornerstones on which the hope for a quantum computer rests. However QC methods do not generally scale well with the size of the system, and it is not known how their performance is hindered when integration with FTQC methods, especially considering these demand a large system size overhead, is attempted under realistic noise models. Here we study this problem using dynamical decoupling in the bang-bang limit as a toy model, with a non-Markovian noise where interactions decay with distance, and show that there exists a regime of the norms of the relevant Hamiltonians, in which dynamical decoupling protected gates provide an advantage over the bare gate implementation. This is a first step towards showing that QC protocols designed for a small set of qubits can be extended to larger sets without a significant loss of performance, as long as the noise model behaves reasonably well.   

Preserving electron spin coherence by dynamical decoupling based on Nitrogen-Vacancy center in diamond

To exploit the quantum coherence of electron spins in solids in future technologies such as quantum manipulating, it's first vital to overcome the problem of spin decoherence due to their coupling the noisy environment. Dynamical decoupling is a particularly promising strategy for combating decoherence. I will briefly introduce the roadmap for dynamical decoupling and show our experimental research on the field in detail. We first applied the optimal dynamical decoupling scheme [1] on electron spins of ensemble sample [2]. Based on the technology, the dynamical decoupling sequence was used to observe the anomalous coherence effect and of single electron spin based on nitrogen-vacancy defect center in diamond [3]. For application, combined the dynamical decoupling together with quantum metrology protocol, the phase estimation was enhanced [4]. Instead of pulsed model, continuous dynamical decoupling was realized in our experiment and applied to protect quantum gate [5]. The next step, we will apply multi flip pulses to enhance the magnetic field sensitivity of NV center towards to the micro-scale magnetic resonance and single molecular imaging. [1] G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007) [2] J. Du, et al., Nature 461, 1265 (2009) [3] P. Huang, et al., Nature Communications, 2, 570 (2011) [4] X. Rong, et al., Europhys. Lett. 95, 60005 (2011) [5] X. Xu, et al., Phys. Rev. Lett. 109, 070502 (2012)   

Improving quantum gate fidelities by using a qubit to measure microwave pulse distortions

We present a new method for determining pulse imperfections and improving the single-gate fidelity in a superconducting qubit. By applying consecutive positive and negative $\pi$ pulses, we amplify the qubit evolution due to microwave pulse distortions, which causes the qubit state to rotate around an axis perpendicular to the intended rotation axis. Measuring these rotations as a function of pulse period allows us to reconstruct the shape of the microwave pulse arriving at the sample. Using the extracted response to predistort the input signal, we are able to improve the pulse shapes and to reach an average single-qubit gate fidelity higher than $99.8\%$.   

Accurate quantum $Z$ rotations with less magic

We present quantum protocols for executing arbitrarily accurate $\pi/2^k$ rotations of a qubit about its $Z$ axis. Unlike reduced instruction set computing (RISC) protocols which use a two-step process of synthesizing high-fidelity ``magic'' states from which $T = Z(\pi/4)$ gates can be teleported and then compiling a sequence of adaptive stabilizer operations and $T$ gates to approximate $Z(\pi/2^k)$, our complex instruction set computing (CISC) protocol distills magic states for the $Z(\pi/2^k)$ gates directly. Replacing this two-step process with a single step results in substantial reductions in the number of gates needed. The key to our construction is a family of shortened quantum Reed-Muller codes of length $2^{k+2}-1$, whose distillation threshold shrinks with $k$ but is greater than 0.85\% for $k \leq 6$. AJL and CC were supported in part by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Fault-tolerant, nondestructive measurement of logical operators and quantum teleportation in large stabilizer codes

Fault-tolerant quantum computation seeks to perform large calculations by protecting quantum information against decoherence using quantum error-correcting codes. Such schemes have been widely studied, but the resources needed to actually perform a fault-tolerant computation are daunting. In principle, it may be possible to reduce this overhead by using large block codes with significantly higher rates. Logical gates can be done in such a scheme by teleporting the logical qubits between code blocks. Logical teleportation can be done fault-tolerantly by measuring a particular set of logical operators. This measurement involves preparing an entangled ancillary state and doing a transversal circuit between the codeword and the ancilla. We study this procedure, and show that a wide range of such measurement protocols exist. There is a trade-off between the size of of the ancilla and the robustness against errors; for a large codeword, it may be fruitful to use a larger ancilla that has greater error-correcting power.   

Quantum ``hyperbicycle'' low-stabilizer-weight finite-rate error correction codes

We construct a large family of finite-rate quantum error correcting codes (QECCs) which interpolate between the hypergraph-product [1] and generalized bicycle codes [2]. The construction allows for the lower and upper bounds on the distance which generally scale as a square root of the block size; in several important cases the two bounds coincide. The constructed QECCs include several new classes of codes with low stabilizer weights; they can offer an advantage compared to the toric codes. \\[0.2em] [1] J.-P. Tillich and G. Z\'emor, in Proc. IEEE Int. Symp. Inf. Th., 2009 (ISIT 2009), pp. 799-803.\\[0em] [2] D. MacKay, G. Mitchison, and P. McFadden, IEEE Trans. on Inf. Th., {\bf 50}, 2315 (2004).   

Error correction with quantum low-density parity check codes

We study quantum low-density parity check (LDPC) codes and their fault tolerance. We show that any family of quantum LDPC codes where each syndrome measurement involves a limited number of qubits, and each qubit is involved in a limited number of measurements (as well as any similarly-limited family of classical LDPC codes), where distance scales as a positive power of the number of physical qubits, has a finite error probability threshold. We conclude that for sufficiently large quantum computers, finite-rate quantum LDPC codes can offer an advantage over the toric codes. Error correction in the presence of errors in syndrome measurements is also addressed. We discuss possible realizations of decoders and their error thresholds, e.g. in relation to LDPC versions of the quantum hypergraph-product codes [1] and their generalizations [2].\\[4pt] [1] J.P. Tillich, G. Zemor, in Proc. IEEE Int. Symp. Inf. Theory (ISIT), 799 (2009). \newline [2] A. A. Kovalev and L. P. Pryadko, in Proc. IEEE Int. Symp. Inf. Theory (ISIT), 348 (2012).   

Relative performance of ancilla verification and decoding in the [[7,1,3]] Steane code

We present numerical simulation results comparing the logical error rates for the fault-tolerant [[7,1,3]] Steane code using standard ancilla verifications techniques vs. the newer method of ancilla decoding, as described in [1]. We simulate a realistic QEC procedure in which failed ancilla creation requires storing the data until a new ancilla can be created; we expect the decoding method, which avoids the need for such storage, to be advantageous when the failure probability is sufficiently high. For the [[7,1,3]] code, we analyze the effect of both different syndrome extraction techniques and of different classes of physical error (initialization, measurement, hold etc.) on the relative performance of these two methods.\\ $\bf 1$. David P. DiVincenzo and Panos Aliferis, Phy. Rev. Lett. $\bf 98$ 020501(2007).   

Quantum Error Correction with Mixed State Ancilla Qubits

It is commonly assumed that ancilla qubits must be in a pure state for successful quantum error correction. We show in our talk that they can initially be in any mixed state if the error operator acts simultaneously on all the physical qubits (fully correlated noise). In particular, they can be in the uniformly mixed state, which makes implementation of our scheme extremely cheap. We also note that 1-qubit gate operations can be implemented easily within the codeword. We experimentally demonstrated our scheme by using a liquid state NMR quantum computer. The encoded state has an interesting nature in terms of quantum discord, which is purely quantum correlations between the data qubit and the ancilla qubits.   

Session M28: Liquid Crystals II

Stable nematic droplets with handles

We use a simple method to generate nematic liquid crystal droplets with handles. The method relies on the viscous forces exerted by a flowing continuous phase above its yield stress over a liquid crystal which is extruded from an injection needle; the resultant jet is forced to close into a torus, due to the imposed rotation, and is stable against surface tension instabilities, due to the elasticity of the outer phase. We find that the ground state of these nematic liquid crystal toroidal droplets is defect free and exhibits twist, irrespective of the aspect ratio of the torus. By including the saddle-splay contribution to the elastic free energy density, we find that this state indeed corresponds to the lowest energy state. For droplets with additional handles, we find there are two surface defects or boojums per additional handle.   

Nanosecond electrooptics of nematic liquid crystals: induced orientational order and quenching of director fluctuations

We demonstrate a fast (1-100 ns) electrooptic response of a thermotropic nematic liquid crystal in a geometry when a strong electric field (\textgreater\ 10$^{\mathrm{8}}$~V/m) does not realign the director and influences only the orientational order and the spectrum of director fluctuations.   

Impact of Photo-Induced Surface Adsorption of Azo-Dyes on the Liquid Crystal Anchoring Conditions

Using optical techniques, we measured the anchoring conditions of azo-dye doped nematic liquid crystals on rubbed polyimide surfaces. Linearly polarized light induces the formation of a second easy axis on the polymer surface oriented toward the polarization direction of the pump laser beam. This additional easy axis is the result of photo-induced adsorption of the \textit{cis} isomer of the azo dye. An effective easy axis is the weighted average of the original easy axis and this new easy axis.   

Surface Nano pattering for aligning Chromonic liquid crystals

We present results on planar alignment of several Chromonic Liquid Crystals. We use a high aspect ratio nano pattern of electrically conductive ITO, which was fabricated by employing a new patterning technique that relies on a secondary sputtering phenomenon (SSP). This method is particularly useful in the case of aligning Chromonics which are considerably harder to align in comparison with conventional thermotropics. Berreman's theory was employed to study the alignment of the Liquid Crystals as a function of the anchoring energy which depend on the dimension of the ITO patterns.   

Differential Dynamic Microscopy for measuring viscoelastic ratios of Chromonic Liquid Crystals

Differential Dynamic Microscopy(DDM) enables one to access the scattering information from a sample by Fourier analyzing the real space images obtained from a light microscope. Thermal fluctuations of the director about the mean position allows one to study the viscoelastic properties of the nematic. Normally such measurements of the viscoelastic constants require time consuming and sensitive light scattering experiments. DDM enables us to extract the same data just by analyzing a real space movie a few seconds long using a high speed camera. We present results of viscoelastic measurements of Chromonic liquid crystal Sunset yellow using DDM measurements.   

Phase and Topological Behavior of Lyotropic Chromonic Liquid Crystals in Double Emulsions

Lyotropic chromonic liquid crystals, assembled by non-covalent interactions, have fascinating temperature- and concentration-dependent phase behavior. Using water-oil-water double emulsions, we are able control the inner droplet chromonic phase concentration by osmosis through the oil phase. We then study the configurations of the chromonic liquid crystal phases in droplets by varying the oil types, oil soluble surfactants, and inner droplet diameter. We employ polarization microscopy to observe resulting nematic and columnar phases of Sunset Yellow FCF, and we deduce the liquid crystal configuration of both phases within the droplets. Simulations based on Jones matrices confirm droplet appearance, and preliminary observations of chromonic liquid crystal shells in oil-water-oil double emulsions are reported.   

Homeotropic alignment of the lyotropic chromonic liquid crystal Sunset Yellow FCF using pi-pi stacking chemical interactions

We report on the homeotropic alignment of the lyotropic chromonic liquid crystal, Sunset Yellow FCF (SSY), using pi-pi stacking interactions between the SSY molecules and (1) thin parylene films or (2) a graphene monolayer. The nematic and columnar phases of SSY molecules arise via self-assembly in water into stacks through non-covalent attractions between the SSY molecules. Interestingly, we find that the same non-covalent interactions between SSY molecules and a parylene or graphene alignment layer lead to homeotropic anchoring of these stacks. The nematic phase of SSY is introduced between two glass substrates coated with parylene films or graphene monolayers, and homeotropic alignment of SSY is confirmed by polarized optical microscopy and conoscopy. Additionally, we observe and can explain the stripe domains that occur during cooling of the sample in this cell, and we consider possible novel applications for homeotropically aligned chromonic liquid crystals.   

Kinetics of Assembly and Dis-assembly of Structures Forming a Chromonic Liquid Crystal at Low Concentrations

The molecules of the near-IR absorbing dye IR-806 spontaneously assemble in water at very low concentrations, forming a chromonic liquid crystal phase at room temperature when the concentration is above 0.5 wt\%. The assembly process proceeds in two steps and results in a complex structure that orientationally orders in a liquid crystal phase. The kinetics of the assembly and dis-assembly of these complex structures can be followed through absorption measurements by rapidly mixing the initial sample with either a small fraction of salt solution (assembly) or a large fraction of water (dis-assembly). The kinetics of dis-assembly is exponential while the kinetics of assembly is non-exponential, both with rate constants depending on the starting and ending conditions, but falling in the 0.1-1.0 s$^{-1}$ range. While past equilibrium absorption measurements on IR-806 offer evidence for a threshold concentration for the assembly of these complex structures, the kinetics experiments show with certainty the existence of such a threshold. Similar experiments on Benzopurpurin 4B, another dye that forms a chromonic liquid crystal at low concentrations, reveal kinetics that are slower by two orders of magnitude and a threshold concentration for the assembly of complex structures.   

Free energy power expansion for orientationally ordered phases: energy and entropy

We propose a new approach for description of orientational phase transitions that utilizes the following specific features of the orientational energy $E$ and entropy $S$: (a) $S$ possesses an additional symmetry in comparison with $E$, being invariant under rotation of the molecular frame; and (b) $E$ contributes only to the second order terms because the pair molecular interaction is dominant. The approach is based on minimization of the scaled orientational free energy $\bar{F}=F/T=E/T-S$ instead of $F$ because $\bar{F}$ obeys the standard assumption of the Landau theory that only the second order terms are temperature dependent. We apply the approach to build a model for nematic phases in materials with non-polar parallelepiped-type molecules with symmetry$\ D_{2h}$. The presented model introduces complex OPs, generalizes the Landau-de Gennes (LdeG) theory and predicts the existence of a biaxial nematic phase for the forth order expansion of $\bar{F}$.   

Surface topography and rotational symmetry breaking

The surface electroclinic effect, which is a rotation of the molecular director in the substrate plane proportional to an electric field applied normal to the substrate, requires both a chiral environment and $C_{2}$ (or lower) rotational symmetry about the field. The two symmetries typically are created in tandem by manipulating the surface topography, a process that conflates their effects. Here we use a pair of rubbed polymer-coated substrates in a twist geometry to obtain our main result, viz., that the strengths of two symmetries, in this case the rub-induced breaking of $C_{\infty }$ rotational symmetry and chiral symmetry, can be separated and quantified. Experimentally we observe that the strength of the reduced rotational symmetry arising from the rub-induced scratches, which is proportional to the electroclinic response, scales linearly with the induced topographical rms roughness and increases with increasing rubbing strength of the polymer. Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.   

Probing Viscoelasticity of Cholesteric Liquid Crystals in a Twisting Cell

Viscoelastic properties of liquid crystals are typically studied either using Poiseuille flow, which can be produced by a pressure gradient in a capillary tube,\footnote{R. J. Atkin, ``Poiseuille Flow of Liquid Crystals of the Nematic Type, ARCHIVE FOR RATIONAL MECHANICS AND ANALYSIS, \textbf{38},~224-240 (1970)} or Couette flow, which can be generated by a shear between concentric cylinders.\footnote{CLADIS, P. E., {\&} S. TORZA, ``Stability of nematic liquid crystals in Couette flow''. \textit{Phys. Rev. Lett.} \textbf{35}, 1283-1286 (1975).} We use a different method in which we twist the liquid crystal sandwiched between two cylindrical glass plates, one of which can rotate about its center, the other of which is fixed. When the cell is twisted, there is a force proportional to the twist angle and the twist elastic constant, and inversely proportional to the pitch and sample thickness, normal to the substrates due to the change in pitch in the cholesteric liquid crystal (CLC). Measuring this force on various CLCs with known pitch we could obtain the twist elastic constants. In addition to the equilibrium force, we observed a transient force during the rotation, which is related to the flow of the material, thus allowing us to determine the Leslie viscosity component $\alpha_{1}$, which typically cannot be assessed by other methods. We expect this apparatus to be a useful tool to study the visco-elastic properties of liquid crystals.   

Chiral hierarchal self-assembly in Langmuir monolayers of diacetylenic lipids

A Langmuir monolayer made of chiral lipid molecules forms a hierarchal structure when compressed in the intermediate temperature range below the chain melting temperature. These structures are captured via Brewster angle microscopy. When the liquid monolayer is compressed, an optically anisotropic condensed phase nucleates in the form of long, thin claws. These claws pack closely to form stripes. This appears to be a new mechanism for forming stripes within Langmuir monolayers. In the lower temperature range these stripes arrange into spirals within overall circular domains, while near the chain melting transition the stripes arrange into target-structure. We attributed this transition to a change in boundary conditions at the core of the largest-scale circular domains.   

Session M29: Focus Session: Wet Granular Material: Capillary Aggregation to Shaping of Landscapes

Dynamics of failure in 2d granular packings

We explore the grain-scale interactions that precede large-scale deformations and mark the onset of mechanical failure in two-dimensional granular packings. The two-dimensionality of the system allows for direct observation of all particle dynamics during the compression of a pillar. The grains are cohesive, with an attraction governed by tunable capillary forces that are induced through an interstitial fluid. We are particularly interested in the initial deformation of the pillar. Here we characterize local structure and dynamics leading up to the first large-scale event. For our analysis, we focus on how local structure within the packing relates to local dynamics and eventually to large-scale deformation. Local structure and rearrangements are characterized by information from a Delaunay triangulation, and are compared with larger-scale deformations identified by spatial variations in the velocities of the particles. We explore the the effects of pillar size and cohesion strength on the dynamics in both ordered and disordered packings.   

Compaction dynamics of wet granular packings

The extremely slow compaction dynamics of wet granular assemblies has been studied experimentally. The cohesion, due to capillary bridges between neighboring grains, has been tuned using different liquids having specific surface tension values. The characteristic relaxation time for compaction $\tau $ grows strongly with cohesion. A kinetic model [1], based on a free volume kinetic equations and the presence of a capillary energy barrier (due to liquid bridges), is able to reproduce quantitatively the experimental curves. This model allows one to describe the cohesion in wet granular packing [2]. The influence of relative humidity (RH) on the extremely slow compaction dynamics of a granular assembly has also been investigated in the range $20\%-80\%$. Triboelectric and capillary condensation effects have been introduced in the kinetic model. Results confirm the existence of an optimal condition at $\rm{RH} \approx 45\%$ for minimizing cohesive interactions between glass beads [3]. References : [1] F.Ludewig, S.Dorbolo, T.Gilet, and N.Vandewalle, EPL 84, 44001 (2008) [2] J.E.Fiscina, G.Lumay, F.Ludewig and N.Vandewalle, Phys. Rev. Lett. 105, 048001 (2010) [3] N.Vandewalle, G.Lumay, F.Ludewig, J.E.Fiscina, Phys. Rev. E 85, 031309 (2012)   

Self-assembled granular towers

When some water is added to sand, cohesion among the grains is induced. In fact, only 1{\%} of liquid volume respect to the total pore space of the sand is necessary to built impressive sandcastles. Inspired on this experience, the mechanical properties of wet piles and sand columns have been widely studied during the last years. However, most of these studies only consider wet materials with less than 35{\%} of liquid volume. Here we report the spontaneous formation of granular towers produced when dry sand is poured on a highly wet sand bed: The impacting grains stick on the wet grains due to instantaneous liquid bridges created during the impact. The grains become wet by the capillary ascension of water and the process continues, giving rise to stable narrow sand towers. Actually, the towers can reach the maximum theoretical limit of stability predicted by previous models, only expected for low liquid volumes.   

Building towers, domes, and arches by self-organized solidifying flows

We demonstrate that a wide variety of delicate solid structures from slender towers to arches, and chiral pagodas can be created by simply pouring a mixture of grains and water on a liquid absorbing substrate [Phys. Rev. Lett. 107, 208304 (2011)]. The same suspension poured on a solid substrate would form a featureless puddle or a pile with an angle of repose. However, an absorbing substrate can quickly drain the liquid from the suspension, rapidly causing the solidification of the fluid into a mechanically stable structure. In a dripping regime, successive drops are observed to jam rapidly upon impact literally stacking on top of each other forming slender granular towers. In a jetting regime and using a moving substrate, the jet is found to bounce on and off the substrate forming regular arches. We will discuss the subtle interplay of the incoming flux of the granular suspension, the drainage efficiency of the substrate, and the mechanical properties of the solid structure. The drainage driven jamming of granular suspensions gives a new route to shape cohesive granular materials and, from a broader perspective, demonstrates the potential a solidifying fluid spreading on a substrate to create new morphologies harder to achieve by other techniques. Applications to surface patterning, rheology of dense suspension and mechanics of wet granular materials will be discussed.   

Capillary fracturing in granular media

The invasion of gas into liquid-saturated, deformable porous media occurs in many processes including gas venting, hydrocarbon recovery, and geologic CO$_2$ sequestration. While fracturing during gas invasion has been observed in several studies, the underlying mechanisms and macroscopic patterns remain poorly understood. Here, we experimentally investigate the fracturing mechanism and resulting patterns during the invasion of air into a thin bed of water-saturated glass beads. The control parameters are the air injection rate, the bead size, and the vertical confining stress applied to the top of the bed. We identify three invasion regimes: capillary fingering, viscous fingering, and ``capillary fracturing,'' where capillary forces overcome frictional resistance and induce the opening of fracture-like conduits. We show that the transitions between the regimes are governed by a modified capillary number and a fracturing number. We then extend the experiments to investigate the effect of wettability. Our analysis predicts the emergence of fracturing in fine-grained media under low confining stress, a phenomenon that likely plays a fundamental role in many natural systems.   

Diffusive evolution of experimental river channel networks

Braided rivers are complex systems in which a network of ephemeral, interacting channels continually migrate to create a rapidly changing landscape. We present results of a set of $\sim$ 1m-scale experiments of braided rivers forming over a bed of monodisperse glass beads. The experiments evolve from an initial flat bed, allowing us to study the approach to a steady state, with data in the form of repeat high-resolution topography scans. We find that, although channels migrate rapidly, they have stable, self-similar geometries organized to a critical Shields stress criterion. Above the individual channel scale, we find that we can directly describe many aspects of the system with a diffusional framework. The timescale to equilibrium slope, the timescale of decorrelation of the channel network, the rate at which downstream correlation lengthscales increase, and the dependence of the equilibrium slope on sediment flux can all be described with diffusivities that are consistent with a theoretical prediction. The emergent picture of our braided river system is one in which sediment transport drives the interaction of dynamic but equilibrium channels, which in turn act as elements of randomness that create diffusive behavior at the system scale.   

A converging granular flow driven by fluid drag

The dynamics of granular flows are known to depend on applied confining stresses and the need for the material to dilate when subjected to a shearing motion, as has been shown by studies on free-surface flows driven by gravity and confined flows driven by a moving boundary.~ Here, we present an experimental study at the 2 meter scale of a granular flow subject to a confining stress driven by fluid drag as is encountered in some petroleum recovery and geologic processes.~ Before the particles start to flow, a Darcy's law pressure gradient is generated by fluid flow.~ The onset of flow, or failure, is history dependent.~ That is, it depends on both the stress state and the particle concentration as a result of the history of deformation applied to the material.~~~ The particles begin to flow when the pressure gradient exceeds the friction due to the confining stress and the gradient of stress along the flow direction.~ The flow also depends on the ability of the granular material to dilate.~ We will show that converging flow conditions allow for this required dilation.~ Once the particles are flowing, the pressure gradient is proportional to the confining stress on the moving sand rather than on the fluid flow rate.   

Rainwater Channelization and Infiltration in Granular Media

We investigate the formation of fingered flow in dry granular media under simulated rainfall using a quasi-2D experimental set-up composed of a random close packing of mono-disperse glass beads. We determine effects of grain diameter and surface wetting properties on the formation and infiltration of water channels. For hydrophilic granular media, rainwater initially infiltrates a shallow top layer of soil creating a uniform horizontal wetting front before instabilities occur and grow to form water channels. For hydrophobic media, rainwater ponds on the soil surface rather than infiltrates and water channels may still occur at a later time when the hydraulic pressure of the ponding water exceeds the capillary repellency of the soil. We probe the kinetics of the fingering instabilities that serve as precursors for the growth and drainage of water channels. We also examine the effects of several different methods on improving rainwater channelization such as varying the level of pre-saturation, modifying the soil surface flatness, and adding superabsorbent hydrogel particles.   

Characterizing shear-flow-driven erosion of granular beds

The complex interactions between granular media and flowing fluid play a principal role in shaping landscapes via erosion. Despite a large body of work in granular materials and large scale topographical changes in granular beds due to fluid flow, the detailed physical mechanisms that underlie particle entrainment into a fluid flow from an erodible bed and the coupling between hydrodynamic shear and internal rearrangement remain poorly understood. To address these questions, we perform experimental studies of pulsed shear flow across granular beds. We characterize the fluid flow using particle tracking techniques and monitor changes in the structural properties of the granular packing and contour of the granular bed.   

River deltas: channelizing sandpiles with memory

River deltas are wedges of sediment that are built via the lateral migration of self-channelizing rivers, but the timescale of this process is prohibitively long to observe in nature. Here we present laboratory results that allow us to examine how channels form and fill space to create a delta. Flow collapses into a single channel whose dimensions adjust to threshold transport conditions for the imposed sediment load. This channelization causes localized shoreline growth until the slope drops below a threshold value for sediment transport. This leads to deposition within the channel, with an upstream-migrating step akin to a stopping front in granular flows, which causes widespread flooding and the selection of a new (steeper) channel path. This cycle is remarkably periodic; delta slope oscillates between two thresholds - entrainment and distrainment - analogous to static and dynamic angles of repose. Selection of a new flow path is inherently stochastic, but previously abandoned channels act as significant attractors for the flow. Once a critical density of flow paths has been established, the flow oscillates among the same 3-5 channels indefinitely. These dynamics result in self-similar (quasi-)radial growth of delta lobes, which can be described using a simple geometric model. Despite its simplicity, the experimental system agrees well with what can be measured from natural deltas Thus, temporal and spatial patterns of deltas appear to be a robust result of mass conservation and transport thresholds.   

Effect of moisture content on nest construction activity of fire ants

Large underground nests protect ants from severe weather and predators. Field observations have revealed that the soil wetness affects the nest building activity. In this work we use x-ray computed tomography to study the growth of fire ants nests as a function of soil moisture content. Because capillary cohesion in wet soils leads to the competition between tunnel stability and the labor-intensity of the excavation, we expect to find an optimal soil wetness, which allows the most effective nest construction. We prepared digging containers (3.8 cm diameter by 14.5 cm deep aluminum tubes) with 2 types of simulated soil (50 and 210 um glass particles). The prepared moisture content W varied from 0.01 to 0.18 by mass. Hundred ants were allowed to dig in the containers for 20 hours. Although, the ants were able to construct tunnels in all moisture levels, the maximum tunnel depth, H, was significantly affected by W. At moderate moisture content (W$=$0.1) H was at least twice greater than at the lowest moisture content (W$=$0.01) for all tested colonies (n$=$9) for both particle sizes. The increase in H mirrors the dependence of the soil cohesion on W and we therefore conclude that the tunnel stability is a key factor influencing the digging strategy of fire ants.   

Role of Surface Tension in Magnetorheological Adhesion

Magnetorheological (MR) fluids are colloidal suspensions of magnetizable particles that show an increment the yield stress and in the apparent viscosity in the presence of a magnetic field. It has been shown previously that MR fluids can be used for field-controlled static adhesion to non magnetic surfaces. Here we demonstrate the important role the surface tension plays in this adhesion effect (for a low viscosity carrier fluid) and that the adhesive property is not related to the field-dependent yield stress.   

Shear thickening oscillation in a dilatant fluid

We report experimental observations of the shear thickening oscillation; spontaneous macroscopic oscillation in the shear flow of severe shear thickening fluids. Using a phenomenological fluid dynamics model for dilatant fluids, we have been predicted theoretically that a dilatant fluid under constant shear stress oscillates due to the shear thickening property coupled with the fluid dynamics. However, such a macroscopic oscillation has never been reported in the literature. In this presentation, we report that strong vibrations of the frequency around 20 Hz is observed using a density-matched starch-water mixture, in the cylindrical shear flow of a few centimeters flow width. The oscillation behavior is consistent with the theoretical prediction.   

Session M30: Self-Assembly: Janus and other Colloids

Directed Self-Assembly of Colloidal Janus Matchsticks

The ability to assemble anisotropic colloidal building blocks into ordered configurations is scientifically and technologically important for developing new classes of soft materials. We are studying the fabrication and electric field driven assembly of end- and side-coated Janus rods. Specifically, we fabricate silica rods (L/D $=$ 2-4) functionalized with hydrophobic gold (Au) patches using a multistep process involving electric field alignment and crystallization, microcontact printing, and selective metallization. In the absence of an applied electric filed, the Janus matchsticks (end-coated rods) self-assemble into multi pods (e.g., bi-, tri- and tetrapods) of varying coordination number and patch angle in aqueous solution. By contrast, both Janus matchsticks and side-coated Janus rods form complex chains in applied AC electric fields of varying magnitude and frequency, whose configurations vary significantly from those formed by pure silica rods.   

Theory of crystallization and orientational ordering of spherical Janus colloids

Amphiphilic Janus particles have two chemically distinct surfaces, one hydrophobic (attractive) and the other hydrophilic (repulsive), resulting in orientationally anisotropic interparticle interactions. In contrast to homogeneous spherical particles, broken rotational symmetry can result in more exotic crystals that possess distinct orientational patterns, and also plastic crystals. We study the rich phase behavior of Janus colloids using a self-consistent phonon theory that includes coupled translational and rotational entropic and enthalpic contributions to the free energy. Ground states are identified based on the compatibility between the patch geometry of particles (e.g., patch coverage, number, shape) and the lattice symmetry. The coupled self-consistent equations for translational and rotational localization parameters are then solved for a given crystal symmetry, thermodynamic state, and patch orientational order, and their stability determined. For two-dimensional diblock AB Janus crystals, we predict the phase sequence of stripes, modulated stripes (zig-zag), and plastic crystals (rotator phases), which depends sensitively on particle chemical composition and pressure. We also study triblock Janus colloids, including the possibility a Kagome lattice.   

Modeling of tunable structural re-configuration of Janus colloidal particles

Colloidal particles can assemble into a myriad of structures by virtue of the many interaction forces available to them. Variable range attraction and repulsion and the recently explored non-isotropic character, exemplified by Janus particles, are examples of the versatility of colloidal particles as building blocks. A systematic approach to understand the assembly of Janus colloids, as a function of Janus balance and particle concentration is not yet available. In this work we study the phase behavior of Janus particles as a function of the strength of interaction, Janus balance and volume fraction of spherical particles. A secondary goal of this work is the assessment of re-configurability of the structures found. Our results show the range of stability of several structures, including a fluid phase of small clusters, bilayers and worm-like aggregates. We find the bilayer structures are very stable over a range of phase space and provide a good pre-cursor to hexagonally close-packed structures. These findings enable the understanding of the assembly process of Janus building blocks and provide a framework with which to study the kinetics of structure change.   

Janus Magnetic Rods, Ribbons, and Rings

Dipolar particles are fundamental building blocks in nature and technology but the roles of anisotropy are seldom explored in their assembly. Here, we fabricate colloidal silica rods coated on one hemicylinder with a thin magnetic layer to satisfy multiple criteria: nearly monodisperse, easily imaged, and magnetic interaction dominant over gravity. We confirm long-predicted features of dipolar assembly and stress the microstructural variety brought about by shape and chemical anisotropy, especially by borrowing knowledge learned from molecules. We describe analogies to liquid crystalline deformations with bend, splay and twist; an analogy to cis/trans isomerism in organic molecules, which in this system can be controllably and reversibly switched; and a field-switching methodology to direct single ribbons into not only single but also multiple rings that can subsequently undergo hierarchical self-assembly. Going beyond earlier investigations of phase behavior, we show that dynamic reconfigurability presents subtle materials issues and possibilities.   

Study of Aggregation of Janus Ellipsoids

We perform numerical simulations of a quasi-square well potential model of one-patch colloidal particles to investigate the collective structure of a system of Janus ellipsoids. We show that for Janus ellipsoids such that one half is an attractive patch, while the entire ellipsoid has a hardcore repulsion, the system organizes into a distribution of orientationally ordered micelles and vesicles. We analyze the cluster distribution at several temperatures and low densities and show that below certain temperatures the system is populated by stable clusters and depending on temperature and density the system is populated by either vesicles or micelle structures.   

Amphiphilic Janus cylinders at fluid-fluid interfaces

We study the configuration and assembly of amphiphilic Janus cylinders at fluid-fluid interfaces at the single- and two-particle levels using experimental and theoretical approaches. We observe that high aspect ratio Janus cylinders have two configurations -- upright and tilted orientation, whereas Janus cylinders with small aspect ratios adopt only the upright orientation. These configurations are confirmed by numerically calculating and minimizing the attachment energy of each Janus cylinder as a function of the orientation angle as well as the vertical displacement with respect to the interface. Unlike homogenous cylinders which show deterministic assembly behaviours at fluid-fluid interfaces, Janus cylinders exhibit a variety of assembly behaviours. We show the origin of such a diversity stems from the attractive capillary interactions between tilted Janus cylinders, which could be explained by the quasi-quadrupolar interface deformation that is caused by the wetting of the fluids on the particle surface. We will also describe our recent results involving the configuration and interactions of asymmetrically hydrophilic cylinders at an air-water interface.   

Thermodynamically Stable Pickering Emulsions Stabilized by Janus Dumbbells

Janus particles have two sides with different, often opposite, surface properties. Janus dumbbell is one type of Janus particles that consists of two partially fused spherical lobes. It is possible to independently control the geometry and surface wettability of Janus dumbbells. Janus dumbbells can also be produced in a large quantity, making them useful for practical applications such as emulsion stabilization. In this work, we calculate the free energy of emulsion formation using amphiphilic Janus dumbbells as solid surfactants. In contrast to kinetically stable emulsions stabilized by homogeneous particles, emulsion stabilized by Janus dumbbells can be thermodynamically stable. There also exists an optimal radius of droplets that can be stabilized by infinite or limited number of amphiphilic dumbbells in the continuous phase. We demonstrate that the optimal radius of dumbbell-stabilized droplets can be predicted based on the volume of the dispersed phase and the volume fraction of dumbbells in the continuous phase. We believe our calculation will provide guidelines for using Janus dumbbells as colloid surfactants to generate stable emulsions.   

Analytic Solutions and Numerical Simulation of Self-Assemble Magnetic Colloidal Structures

Self-assembled magnetic colloidal structures that lie at a fluid-air interface can have a wide range of behavior, from localized axisymmetric star-like objects to linear, snake-like ones. Modeling these structures requires both the extensive use of the Navier-Stokes Equations from an analytic standpoint as well as the ability to numerically solve and simulate them alongside Newton's Equations. Analytically, these equations are approximated by an asymptotic expansion with a small viscosity. Using those expressions, simulations are run on GPU's to utilize their parallel capability. The result is a remarkable, qualitative recapturing of the experimentally observed behavior, namely, the formation of both snakes and stars from a randomized initial condition.   

Dynamic phases in non-equilibrium magnetic colloids at liquid interfaces under in-plane magnetic field driving

Ensembles of interacting colloidal particles subject to an external periodic forcing often develop nontrivial collective behavior. We study emergent phenomena in magnetic colloidal ensembles suspended at a liquid-air interface and driven out of equilibrium by alternating magnetic fields. We use ferromagnetic colloidal micro-particles (so the magnetic moment is fixed in each particle and interactions between colloids is highly anisotropic and directional) suspended over a water-air interface and energized by alternating magnetic fields applied in-pane of the interface. Experiments reveal new types of dynamic self-assembled phases (in particular, ``wires,'' ``rotators'') emerging in such systems in a certain range of excitation parameters. Transition between different self-assembled phases with parameters of external driving magnetic field is observed. Molecular dynamic simulations captures some of the non-equilibrium self-assembled phases in our system.   

Strictly Polyhedral Colloids Challenged by Electric Field

We have succeeded in fabricating monodisperse polyhedral metal-organic framework (MOF) crystals. Here, the micron-sized rhombic dodecahedra are suspended in liquid as candidates for directed self-assembly. The application of AC electric field is found to produce assembly at various facets truncations, probably owing to induced dipole attraction, with linear chaining that we observe and analyze based on direct in-situ imaging. The facet-to-facet preference during assembly produces striking selectivity for these1D chains.   

Crystalline aggregates of magnetic colloidal particles

A colloidal system of magnetic and non-magnetic spheres confined to a 2D monolayer has been found to form a variety of structures, including Kagome, honeycomb, and square lattices, as well as various chain and ring configurations [1]. In these experiments, the layer of beads is immersed in a ferrofluid and placed in an external magnetic field and the different structures are obtained for different values of the relative concentrations of the bead types, the susceptibility of the ferrofluid, and the angle of the field with respect to the assembly plane. We study an approximate model for the potential energy of the system based on self-consistent solutions for the magnetic moments of point dipoles. We find that the model accounts well for the stability of the observed phases and we identify additional possible phases via a genetic algorithm that searches for crystal structures with up to ten atoms per unit cell. Further calculations suggest the possibility of creating materials with strong elastic responses to applied magnetic fields.\\[4pt] [1] K. S. Khalil, A. Sagastegui, Y. Li, M. A. Tahir, J. E. S. Socolar, B. J. Wiley, and B. B. Yellen, {\it Nat. Comm.}, {\bf 3}, Article number: 794 (2012).   

Using chaotic Faraday waves to create a two-dimensional pseudo-thermal bath for floating particles with tunable interaction potentials

Whether chaos in actively driven systems can be described by an effective temperature is an unresolved question in the study of nonlinear physics. We use chaotic Faraday waves to create a two-dimensional pseudo-thermal bath to investigate tunable interactions between floating particles. By vertically oscillating a liquid with an acceleration greater than $g$ we excite the Faraday instability and create surface waves. Increasing this acceleration above some critical value causes this instability to become chaotic with fluctuations over a broad range of length scales. Particles placed on the surface are buffeted by random excitations in analogy to Brownian motion. We can change the ``temperature'' of the pseudo-thermal bath by manipulating the driving frequency and amplitude, a feature of the system we verify using real-time tracking to follow the diffusive movement of a single particle. With an eye toward creating complex self-assembling systems we use this system to measure the tunable interaction potential in two-, three-, and many-particle systems and to probe the effects of particle size, shape, symmetry, and wetting properties.   

Twisted results on interior packing and surface energy for filament bundles

Twisted filament bundles are a common structural motif found in both natural and synthetic systems. Examples range from protein assemblies such as collagen and fibrin, to artificial structures such as carbon nanotube ropes and microfabricated materials. They are oftentimes found to self-assemble from fibers via various adhesive interactions, be they depletion, capillary, or other such forces. In these assemblies, twist frustrates the perfect crystalline order of the fibers, requiring the presence of defects in the packing. Through numerical simulations, we discover defect organizations ranging from dislocations and grain boundaries for low twist, to multi-five-fold disclination clusters for high twist. And furthermore, by developing and employing an analytical continuum model, we find that for sufficiently long fibers, twist reduces the surface energy of the assembly. Together, this suggests that the equilibrium lowest energy state of a filament bundle may be twisted regardless of any intrinsic chirality present in the system.   

Kinetics of Phase Separation in Binary Mixtures

We present numerical simulation results of the phase separation kinetics in three-dimensional symmetric binary fluid mixtures and binary solid mixtures. In the former system, our extensive molecular dynamics simulation is able to probe an extended period where the domain size grows linearly with time, leading to an unambiguous confirmation of the viscous hydrodynamic regime. On the other hand, for the binary solid mixture, we use Monte Carlo simulation with spin-exchange dynamics to verify the Lifshitz-Slyzov growth law. In spite of the differences in the growth mechanisms, the pair correlation functions and structure factors of the two systems overlap, indicating similarity in the morphologies during phase separation.   

Cooperative Symmetry Breaking from One to Three Dimensions in Multi-Component Double Emulsions

We follow the evolution of aqueous inner drops confined in a thin sheath of oil in the dimensional crossover from one to three dimensions using a fast camera and microfluidics. Surprisingly, inner drops interact cooperatively to pair with their next nearest neighbor to transform their linear configuration into a three dimensional composite sphere. The measured time scales of transforming these multi-component double emulsions are investigated as a function of number, size, and composition of inner drops. We model the dynamics to understand and predict how both folding and buckling occur in these complex microfluidic systems.   

Session M31: Polymer Melts and Solutions

Uniaxial Extension of Entangled Polymer Melts close to T$_{\mathrm{g}}$

Transient (nonlinear) responses of entangled polymers to startup deformation indicate a transition from the initial elastic deformation to irreversible deformation (flow) [1-3]. This yielding behavior varies with the applied rate: at a higher rate the entanglement network can be strained to a higher degree before its breakdown. In this work, we subject entangled melts such as polystyrene to startup uniaxial extension to show how yielding takes place as a function of temperature. The objective is to explore whether there would be any mechanical signature of emergence of any secondary structure as the glass transition temperature T$_{\mathrm{g}}$ is approached from above. \\[4pt] [1] S. Q. Wang, S. Ravindranath, Y. Wang and P. Boukany, \textit{J. Chem. Phys}. \textbf{127}, 064903 (2007).\\[0pt] [2] Y. Y. Wang and S. Q. Wang, \textit{J. Rheol}. \textbf{53}, 1389 (2009).\\[0pt] [3] S. Q. Wang, S. Ravindranath and P. E. Boukany, \textit{Macromolecules} \textbf{44}, 183 (2011).   

Non-Gaussian chain stretching in simple shear of branched polystyrene solutions

Entangled polymers with long chain branching (LCB) exhibit a higher apparent viscosity than the zero-rate viscosity upon startup uniaxial extension whereas polymers either of linear chains or with LCB only show a lower transient viscosity than the zero-rate viscosity envelope. We report for the first time that simple shear of well-entangled polystyrene solutions with LCB produces a higher transient viscosity than the zero-shear envelope. In presence of sufficient LCB, non-Gaussian stretching can even show up in simple shear, which was previously observed only in uniaxial extension. Moreover, LCB resists against a structural breakdown of the entanglement network, postponing the stress overshoot to an unprecedented high shear strain of 30 units when the backbone of the PS would be nearly straightened without retraction and resulting elastic recovery as high as 20 strain units.   

Tube diameter of oriented polymer melts

The tube diameter is a key material parameter controlling the flow behavior of polymer melts. The Lin-Noolandi ansatz successfully accounts for the dependence of the tube diameter on polymer density, chain stiffness and diluent concentration. We extend the Lin-Noolandi ansatz to polymer melts under uniform tension. We find that the tube diameter $a$ decreases as $F^{-1/2}$ when the pulling force $F$ exceeds the thermal tension $k_BT/a$, and approaches a limiting value for typical flexible polymers near full extension of about half the unperturbed value. Our prediction is compatible with assumptions made in the GLaMM model [1] for polymer rheology. We have directly verified the predicted force-dependence of tube diameter by using isoconfigurational ensemble averaging [2] to measure the tube diameter in simulations of oriented polymer melts. In the simulations, the chains are oriented by pulling on the ends of the chains, and topologically equilibrated by allowing the chains to occasionally cross.\\[4pt] [1] R. Graham, A. Likhtman, T. McLeish, and S. Milner, {\textit J. Rheo.}, 47(2003):1171;\\[0pt] [2] W. Bisbee, J. Qin, and S. Milner, {\textit Macromolecules}, 44(2011):8972)   

An Intriguing Empirical Rule for Estimating the First Normal Stress Difference from Steady Shear Viscosity Data for Concentrated Polymer Solutions and Melts

The Cox-Merz rule and Laun's rule are two empirical relations that allow the estimation of steady shear viscosity and first normal stress difference, respectively, using small amplitude oscillatory shear measurements. The validity of the Cox-Merz rule and Laun's rule imply an agreement between the linear viscoelastic response measured in small amplitude oscillatory shear and the nonlinear response measured in steady shear flow measurements. We show that by using a lesser known relationship also proposed by Cox and Merz, in conjunction with Laun's rule, a relationship between the rate-dependent steady shear viscosity and the first normal stress difference can be deduced. The new empirical relation enables \textit{a priori} estimation of the first normal stress difference using only the steady shear viscosity vs shear rate data. Comparison of the estimated first normal stress difference with the measured values for six different polymer solutions and melts show that the empirical rule provides values that are in reasonable agreement with measurements over a wide range of shear rates; thus deepening the intriguing connection between linear and nonlinear viscoelastic response of entangled polymeric materials.   

Assumptions in Entanglement models and Their Effect on Non-Linear Rheology Predictions

While tube and slip-link theories are able to describe shear flow stresses qualitatively, and in some cases quantitatively, elongational flow prediction remains elusive. Both the GLaMM tube theory and primitive chain network simulations overpredict the magnitude of stress. As a result, several groups have suggested making the friction chain-conformation dependent, giving an enhancement to stress relaxation in elongational deformations when chains are highly oriented. Here we take a different tack, and examine the effect of typical assumptions and approximations made in these theories by use of the discrete slip-link model. Since the model exists on a relatively detailed level of description, it allows examination of assumptions without resorting to crude approximations. We find that while some of these approximations indeed fail in elongational flows at high strains, the theory is still unable to predict data. What's more, unlike other predictions, this model underpredicts the stress, and would therefore not be in agreement with the assumption of conformation-dependent friction as currently hypothesized.   

Microscopic Theory of Entangled Polymer Melt Dynamics: Flexible Chains as Primitive-Path Random Walks and Super Coarse-Grained Needles

We qualitatively extend our recent microscopic dynamical theory for the transverse confinement potential and diffusion of infinitely thin rigid rods (PRL, 107, 078102 (2011)) to construct a first-principles theory of topologically entangled melts of flexible polymer chains (PRL, 109, 168306 (2012)). Polymer coils are treated as ideal random walks of self-consistently determined primitive-path (PP) steps, and chain uncrossability is included exactly at the binary collision level. A strongly anharmonic (tube) confinement potential for a primitive path segment is derived and favorably compared with recent simulations. A fundamental basis is derived for the Lin-Noolandi conjecture that relates the tube diameter to the invariant packing length, along with the reptation scaling laws for the diffusion constant and terminal relaxation time, including numerical prefactors. The relationship of the PP-level theory to two simpler models, the melt as a disconnected fluid of primitive-path steps, and a super coarse-graining that replaces the entire chain by a needle corresponding to its end-to-end vector, is examined. Remarkable connections between the different levels of coarse graining are discovered.   

Entanglement elasticity in polymer chain melts: microscopic calculation of the rubbery plateau modulus via intermolecular correlations

Textbook models of stress relaxation in melts of entangled polymer chains are built on the assumption that intra-molecular or backbone stresses are the dominant contribution to the system's total stress. Numerous simulations over the last two decades have challenged this assumption, but calculating the intermolecular or non-bonded contribution to the stress has proven a daunting theoretical task. Building on our recent progress in microscopically constructing the transverse confinement field of entangled rods (PRL 107, 078102 (2011)) and ideal coils (PRL 109, 168306 (2012)), we explicitly separate stress correlations into intra- and inter-molecular terms, and calculate the contribution of intermolecular stress correlations in the ``plateau'' region of stress relaxation. We derive, with no adjustable parameters, the characteristic relation $G_e \sim k_BT/p^3$ (where $p$ is the packing length) with a prefactor that agrees within a factor of two with experiment and simulation. This theoretical advance has major implications for the effect of nonlinear deformation, confinement, and chain orientational ordering on entanglement elasticity.   

Influence of Reversibly Associating Side Group Bond Strength on Viscoelastic Properties of Polymer Melts

Reversible hydrogen-bonding between side-groups of linear polymers can sharply influence a material's dynamic mechanical behavior, giving rise to valuable shape memory and self-healing properties. Here, we investigate how bond-strength affects the bulk rheological behavior of functional poly(n-butyl acrylate) (PBA) melts. A series of random copolymers containing three different reversibly bonding groups (aminopyridine, carboxylic acid, and ureidopyrimidinone) were synthesized to systematically vary the side-group hydrogen bond strength ($\sim$26, 40, 70 kJ/mol). The materials' volumetric hydrogen-bond energy densities can be tuned by adjusting the side-group composition. By comparing the viscoelastic behavior of materials containing an equivalent bond energy density, with different bonding groups, the efficacy and cooperativity of reversible binding can be directly examined. Melt rheology results are interpreted using a state-of-ease model that assumes continuous mechanical equilibrium between applied stress and resistive stresses of entropic origin arising from a network of reversible bonds.   

Correction of Doi-Edwards' Green function in harmonic potential and its implication for stress-optical rule

We derive a corrected Green's function for a polymer chain trapped in a two-dimensional anisotropic harmonic potential with a fixed boundary condition. This Green's function is a modified version of what Doi and Edwards first derived to describe the polymer chain trapped in the tube-like domain of surrounding entangled polymers [J. Chem. Soc. Farad. Trans. II 74 (1978) 1802]. In contradiction to the results found by Ianniruberto and Marrucci using the incorrect Green's function [J. Non-Newtonian Fluid Mech. 79 (1998) 225], we find that the stress-optical rule is violated for any tube potential either circular or elliptic. The violation is due to the presence of the virtual springs to trap the chain in the tube rather than the anisotropy of the confinement potential.   

Explaining the absence of high-frequency relaxation modes of polymers in dilute solutions

Using multi-scale modeling, including Molecular Dynamics and Brownian dynamics (BD) simulations, we explain the long-mysterious absence of high frequency modes in the dynamics of isolated polymer chains in good solvents, reported years ago by Schrag and coworkers. The relaxation spectrum we obtain for a chain of 30 monomers at atomistic resolution is, remarkably, a single exponential while that of a chain of 100 monomers is fit by only two modes. This result is surprising in view of the many relaxation modes present in melts of such chains, but agrees perfectly with experimental observations (Peterson et al. J. Polym. Sci.: Part B 2001). We also performed BD simulations in which the explicit solvent molecules are replaced by a viscous continuum. Although the local dynamics is suppressed with the addition of bending, torsion, side groups and excluded volume interactions (as suggested in Jain and Larson, Macromolecules 2008), none of the BD simulations predict a single exponential relaxation for a short chain. Our results indicate that the chain dynamics at small length scales (down to a few Kuhn steps) is significantly different from the predictions of models based on a continuum solvent, and finally help explain the experimental results of Schrag and coworkers.   

Simultaneous determination of the interaction parameter and topological scaling features of polymers in dilute solutions

The RPA equation using the Debye polymer chain scattering function has been widely used to model polymer blends of linear chains in the melt where it is safe to assume a Gaussian conformation. When chains display more complicated topologies or when chains are in dilute solution Gaussian scaling no longer applies. In some cases the Zimm double extrapolation has been used to determine the second virial coefficient and the interaction parameter under the assumption that the deviation of chain scaling from a random walk is acceptable in the low qRg region such as when light scattering is used. If it is of interest to explicitly determine the nature of chain scaling, related to topology or solvent quality, as well as to quantify the thermodynamic interaction, such as in studies of cyclic and branched chains, networks, or polymers in good solvents, there is no analytically valid scattering model for data analysis. We propose the coupling of the unified scattering function with the RPA equation to analytically model these effects. Nevertheless, some issues remain to be resolved with star polymers in particular, such as scattering from highly branched high molecular weight symmetric stars in good solvents where it appears that the Daoud-Cotton model may be appropriate but a colloidal scattering model may be more appropriate.   

Anisotropic Thermal Conduction in Polymers and its Molecular Origins

The strong coupling of mechanical and thermal effects in polymer flows have a significant impact on both the processing and final properties of the material. Simple molecular arguments suggest that Fourier's law must be generalized to allow for anisotropic thermal conductivity in polymers subjected to deformation. In our laboratory we have developed a novel application of the optical technique known as Forced Rayleigh Scattering to obtain quantitative measurements of components of the thermal diffusivity (conductivity) tensor in polymers subjected to deformations. We report measurements of anisotropic thermal diffusivity and stress in molten, cross-linked and solid polymers subjected to several types of flows. The deformed samples have significant anisotropy in polymer chain orientation that results in significant anisotropy in thermal conductivity. Stress and thermal conductivity data support the validity of the stress-thermal rule, which is analogous to the well-known stress-optic rule. We also report measurements on solid polymers with isotropic polymer chain orientation that are under stress, which display rather unexpected behavior. These measurements are used to develop an understanding of the molecular origins of anisotropic thermal conduction in polymeric material   

Molecular modeling simulations in phase stability of polyethylene solutions at elevated pressures

Molecular dynamics (MD) simulations using the OPLS-AA force field are conducted to compute pressure, molecular weight dependence of Hildebrand's solubility parameters (SP) and density of hexane and high-density polyethylene (HDPE) at high pressures from 100 to 3000 bar. The electrostatic energy contribution to the cohesive energy and density leads to increases in the SP with pressure for molecular mechanical models (MMM) with and without electrostatic terms. The Flory-Huggins interaction parameter (IP) predicted from the pressure dependence of SPs and molar volumes decreases upon increasing pressure, indicating that miscibility improves by raising pressure. This is consistent with the solution polymerization process for producing PE, where pressure-induced phase separation (PIPS) is used to separate the polymer from solution. Exclusion of electrostatic potentials in the MMM results in larger IPs while the decreasing trend remains intact with and without electrostatic forces. There is a pressure limit beyond which the IP has less sensitivity to pressure indicating that PE miscibility is not further affected. It is shown that pressure increases the chemical potential factor of the phase stability condition, stabilizing the solution. These results contribute to the fundamental understanding of PIPS, an important demixing process poorly understood when compared to thermally-induced phase separation.   

Miscibility of Polymers in Supercritical Solvents

In this work we make use of our ability to correlate underlying thermodynamic behavior with trends in miscibility to study mixtures of polymers and supercritical carbon dioxide (scCO$_{2})$. scCO$_{2}$ has garnered significant interest as a ``green'' solvent, and supercritical solvent in general, for its highly accessible critical point. Experimental cloud point investigations have determined the miscibility for a range of polymers in scCO$_{2}$. We have used a simple equation of state (EOS) to study a series of poly-(acrylates) in scCO$_{2}$ solvent. Although polymer/scCO$_{2}$ mixtures have been modeled with some success in the past, the ability of an EOS to make accurate predictions has yet to be demonstrated. Our mixture modeling procedure yields parameters from pure component experimental data. Then, by pinning the mixed interaction parameter to the experimental critical temperature (T$_{\mathrm{c}})$ for one mixture from the series, we predict the T$_{\mathrm{c}}$ shifts for the remaining members. In addition to discussing miscibility we draw insight via the trends revealed from the parameterization of the pure component data, alone.   

The effect of topology on the conformations of cyclic polymers in melts

The bond fluctuation method is used to simulate both non-concatenated entangled and interpenetrating melts of cyclic polymers. We find that the swelling of interpenetrating rings upon dilution follows the same laws as for linear chains. The knotting probability of cyclic polymers decays exponentially as function of the number of blobs per chain. A power law dependence $f_{n}\sim\phi R^{2}\sim\phi^{0.77}N$ for the average number $f_{n}$ of linked rings per cyclic polymer at concentrations larger than the overlap volume fraction of rings $\phi^{*}$ is determined from the simulation data. The fraction of non-concatenated cyclic polymers displays an exponential decay $P_{OO}\sim\exp(-f_{n})$, which indicates $f_{n}$ to provide the entropic effort for not forming concatenated conformations. These results lead to four different regimes for the conformations of cyclic polymers in melts separated by critical lengths $N_{OO}$, $N_{C}$ and $N^{*}$ that describe the onset of concatenation, the cross-over between weak and strong compression, and the cross-over to an overlap dominated concatenation contribution. The four characteristic exponents describing ring size in these regimes are 1/2, 2/5, 3/8, and 4/9 as confirmed by simulation data for the first three regimes.   

Session M32: Focus Session: Polymer Nanocomposites: Dynamics

Polymer Dynamics in Nanocomposites and under Confinement

In this talk I will present neutron spin echo investigations on polymers interacting attractively with nanoparticles or confining surfaces. Polyethylene-oxide (PEO) was filled with neat SiO$_{2}$ nanoparticles up to 15 vol{\%}. Investigating a short chain matrix we realised that a fraction of chains is adsorbed at the nanoparticle surface suppressing completely its translational diffusion. Nevertheless these adsorbed chains undergo an unchanged segmental dynamics seemingly forming a micelle like corona of chains connected with their OH-end groups. Changing to methylene terminated chains the picture changes drastically now showing a tightly adsorbed layer that however is not glassy as often assumed but undergoes pico second local dynamics. These results are corrobated and extended in studying the dynamics of Polydimethylsiloxane (PDMS) confined in nanoporous Alumina. There a partly anchored chain fraction is found that undergoes restricted Rouse motions with segmental mobilities as in the bulk phase. The size of this layer exceeds significantly the length scale of the directly adsorbed polymer, presenting a first direct microscopic evidence for the hypothetical interphase.   

Universal Scaling of Polymer Diffusion in Nanocomposites

The tracer diffusion of deuterated polystyrene (dPS) is measured in a polystyrene (PS) nanocomposite containing hard and soft silica nanoparticles (NPs). The soft NPs are grafted with a PS brush (87 kg/mol). The matrix for both NPs is PS (160 kg/mol). The diffusion coefficients for dPS (23 - 1,866 kg/mol) decrease as the hard and soft NP volume fractions increase. To accurately determine the interparticle distances (ID) relevant to each dPS (M) diffusing through the PS(160k):soft NP matrix, self consistent field theory and small angle neutron scattering studies were performed; both theory and experiment show that short dPS chains can deeply penetrate the brush, whereas longer dPS chains only penetrate the periphery and mainly remain in the matrix. The reduced diffusion coefficient (D/D$_{\mathrm{0}}$), plotted against the confinement parameter, namely ID relative to tracer size (2R$_{\mathrm{g}}$), collapses onto a master curve independent of NP type. These experiments demonstrate that polymer diffusion in nanocomposites is captured by the confinement parameter over an extremely wide range of ID/2R$_{\mathrm{g}}$ and, hopefully, motivate new models to capture the dynamics in confined (ID/2R$_{\mathrm{g}}$ \textless\ 10) regimes.   

Hopping Diffusion of Nanoparticles Subjected to Topological Constraints

We describe a novel hopping mechanism for diffusion of large non-sticky nanoparticles subjected to topological constraints in polymer solids (networks and gels) and entangled polymer liquids (melts and solutions). Probe particles with size larger than the mesh size of unentangled polymer networks (tube diameter of entangled polymer liquids) are trapped by the network (entanglement) cages at time scales longer than the relaxation time of the network (entanglement) strand. At long time scales, however, these particles can move further by hopping between neighboring confinement cages. This hopping is controlled by fluctuations of surrounding confinement cages, which could be large enough to allow particles to slip through. The terminal particle diffusion coefficient dominated by this hopping diffusion is appreciable for particles with size slightly larger than the network mesh size (tube diameter). Very large particles in polymer solids will be permanently trapped by local network cages, whereas they can still move in polymer liquids by waiting for entanglement cages to rearrange on the relaxation time scale of the liquids.   

The Role of Excluded Volume on the Reduction of Polymer Diffusion into Nanocomposites

An analytic model for the reduction of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chain diffuses. The distribution of cylinder diameters in the ensemble is predicted from statistical mechanical theories based on the packing of spheres. The reduction in polymer diffusion is accounted for by the truncation of the partition function for a random walk in a cylinder. For low loadings of spherical particles in nanocomposites, we show that this theory results in a master curve for the reduced diffusion coefficient. The theory, with no adjustable parameters, is in agreement with recent data for tracer diffusion measurements in polymer nanocomposites at low loading.   

Diffusivity and Transient Localization of Filler Particles in Polymer Melts and Crosslinked Systems

Building on recent progress in describing the microscopic equilibrium structure of polymer nanocomposites (PRISM theory), as well as the na\"{i}ve mode coupling and nonlinear Langevin equation approaches for predicting localization and activated barrier hopping, we have initiated the study of dynamical phenomena in nanocomposites at finite filler loading. A colloidal suspension perspective is adopted whereby the polymer dynamics are assumed to remain unperturbed by fillers. Both entangled polymer melts and crosslinked systems are studied. The long time behavior of a tagged nanoparticle (localization and diffusivity) is calculated for various melt (tube diameter, polymer radius of gyration) and nanoparticle (filler size and volume fraction, polymer-filler attraction strength) parameters. For transiently localized particles, a dynamic free energy is constructed and employed to compute the nanoparticle localization length, mean barrier hopping time, and self-diffusion constant. The influence of filler-filler interactions on the Stokes-Einstein violation phenomenon in entangled melts is established. In addition, the influence of nanocomposite statistical structure (e.g., in the depletion, steric stabilization, or bridging regimes) on slow dynamics and localization is investigated.   

Nanoparticle diffusion in dense polymer melts

The diffusion of nanoparticles in melts and solutions of polymers facilitates understanding of the viscoelastic behavior of the respective polymers and their composites. It also plays a vital role in determining the equilibrium structure and morphology of polymer nanocomposites and hence, their mechanical properties. In this work, we present the diffusion coefficients of non-sticky smooth spherical particles of different sizes (1-10 $\sigma$) in an athermal mixture of particles and polymers of different chain lengths ($N$ = 20 to 400) using molecular dynamics simulations. The diffusion of nanoparticles of size comparable to the polymer segment size ($\sigma$) is independent of chain length and hence, nanoparticles apparently feel only the local viscosity, as predicted by scaling theories. When the nanoparticle becomes larger than a segment (or alternately the correlation length in the melt), then, the diffusion coefficient decreases. This is due to the fact that the mobility of the particles is retarded either by a chain section of size equivalent to the particle size or by entanglement mesh size depending on the nanoparticle size. We also elucidate the role of chain entanglements on diffusion of nanoparticles.   

Entanglement-Controlled Subdiffusion of Nanoparticles within Concentrated Polymer Solutions

Microrheology techniques, in which colloids suspended in a complex fluid probe their mechanical environment, can provide unique information on the microscopic length scales characterizing the fluid's hierarchical structure. We describe x-ray photon correlation spectroscopy (XPCS) experiments tracking the motion of colloidal gold nanoparticles in solutions of high-molecular-weight polystyrene. The particle radius is tuned to be comparable to the length scales characterizing the entangled polymer mesh. Over displacements of nanometers to tens of nanometers, the particles undergo subdiffusive motion in which the particle mean-squared displacement grows as a power law in time, with power-law exponent, $\alpha < 1$, that depends on solution conditions. Scaling behavior of the nanoparticle mobility with respect to temperature and to polymer concentration and molecular weight indicates the subdiffusion results from the temporal evolution of the entanglement mesh in the immediate vicinity of the particles. The results thus provide a novel microscopic dynamical characterization of a key structural property of polymers and more broadly demonstrate the capability of XPCS-based microrheology to interrogate heterogeneous mechanical environments in nanostructured soft materials.   

Segmental Dynamics of Polymer Nanocomposites by Dielectric Relaxation Spectroscopy

The addition of nanoparticles dramatically affects the physical properties of polymer melts. The general agreement on this interaction mechanism is that the polymer-filler interface is the key region for the changes of properties. Previous studies have suggested the existence of a bound polymer layer in this interfacial region by various techniques. Here, we use Dielectric Relaxation Spectroscopy (DRS) to study the segmental relaxation of poly-2-vinylpyridine (P2VP) nanocomposites by the presence of silica nanoparticles (NPs), with sizes ranging from 14nm to 100nm. For nanocomposites with large amounts of surface area per unit volume (i.e., 14 nm NPs at high loadings) the segmental relaxation dispersion is broadened significantly, suggesting that the bound layer of P2VP is slower than the bulk P2VP, which is attributable to a restriction from solid surface of NPs. Additionally, the thickness of the bound polymer layer is estimated from the reduction in the magnitude of the segmental relaxation.   

Dissipative Particle Dynamics Simulations of Polymer Nanocomposites

We investigate the topological constraints (entanglements) in polymer - nanorod nanocomposites in comparison to polymer melts using dissipative particle dynamics (DPD) simulations. The nanorods have a radius smaller than the polymer radius of gyration. We observe an increase in the number of entanglements, corresponding to a 50{\%} decrease of the entanglement degree of polymerization in the case of 0.11 volume fraction of nanorods dispersed in the polymer matrix, in the nanocomposites as evidenced by larger contour lengths of the primitive paths. The end-to-end distance is essentially unchanged with the nanorod volume fraction for the range of concentrations that we have studied. An increase of the nanorod radius reduces the polymer - nanorod entanglements while the polymer -- polymer entanglements remain unaffected. Interaction between polymers and nanorods affects the dispersion of nanorods in the nanocomposites and also alters the primitive path.   

Polymer Chain Conformation in CNT/Polystyrene Nanocomposites by SANS

Polymer conformations are a critical factor that affects the performance of polymer nanocomposites. Using small angle neutron scattering, we probed chain conformations and confinement of polymers in both SWCNT/polystyrene ($R_{SWCNT}$ \textless\ $R_{g})$ and MWCNT/polystyrene ($R_{MWCNT}$ $\sim$ $R_{g})$ nanocomposites. Through contrast matching experiments, we optimize the dPS:hPS ratio (0.725:0.275) to minimize the scattering from CNTs. To fit the scattering data, we developed a fitting model that includes scattering from polymer chains, rod networks, and defects. We found that the rod network scattering increases as the CNT concentration increases (0.3wt{\%} - 10wt{\%}) in both SWCNT and MWCNT composites, and the rod network scattering is much higher for SWCNT due to the smaller mesh size. When the CNTs concentration is below 2wt{\%}, there is no significant change in $R_{g}$ for both SWCNT and MWCNT nanocomposites. Above 2wt{\%}, the $R_{g}$ for SWCNT nanocomposites increases monotonically as a function of CNT concentration ($\sim$ 30{\%} increase for 10wt{\%} SWCNT loading), while the $R_{g}$ for MWCNT is not affected.   

Microscopic theory for tube confinement and self-diffusivity of entangled needle liquids in presence of hard spherical obstacles

A microscopic theory for the motion of topologically entangled, non-rotating needles in presence of spatially fixed, hard sphere inclusions has been formulated. Exact two-body dynamical uncrossability constraints are imposed, and an effective Brownian evolution equation at two-needle level is self-consistently constructed. The needle transverse localization length (effective tube diameter) and long-time diffusivity are determined as a function of its length and concentration, the sphere diameter and volume fraction, and needle-sphere liquid pair structure. In contrast to single-component entangled needle liquids, the transverse and longitudinal diffusivity become coupled, and reptation is increasingly suppressed with sphere volume fraction in a manner that depends on the relative sphere-needle size. The slow dynamics also depends on needle concentration, reflecting a competition between inter-needle topological uncrossability constraints and needle-sphere excluded volume interactions. The effective tube diameter is a monotonically decreasing function of the sphere density, consistent with the suppression of polymer translational diffusion. Extension to treat entangled flexible chains, and comparison with recent simulations and experiments, are under study.   

Dynamics of nanoparticles in non-Newtonian aqueous dispersions

The transport properties of nanoparticles in soft complex media are relevant for polymer and hydrogel nanocomposites but are still poorly understood. We use single-particle tracking to measure the diffusional dynamics of nanoparticles in non-Newtonian aqueous polymer solutions, which also serve as models of viscoelastic porous media. We track the motion of polystyrene nanoparticles of diameter 400 nm in aqueous solutions of hydrolyzed polyacrylamide whose radii of gyration are comparable to the diameter of the nanoparticles over a wide range of dilute and semi-dilute concentrations. At all concentrations, the mean-square displacement (MSD) of nanoparticles at long times is linearly proportional to time, indicating diffusive motion. The viscosity extracted from the MSD systematically varies with polymer concentration but is smaller than the zero shear rate viscosity measured at each polymer concentration using bulk rheometry, indicating that the dynamics cannot be explained in the context of microrheology of viscous solutions.   

The impact of fragility on the properties of the glass formation of polymer nanoparticle composites

We investigate the effects of nanoparticles on glass formation in a model polymer melt by molecular dynamics simulations. The addition of nanoparticles allows us to change the relaxation time, glass transition temperature $T_g$, the fragility of glass formation in a controlled fashion. We show that the structural relaxation for different temperatures, concentrations, and polymer-NP concentrations can be expressed in terms of a simple universal function of the short-time Debye-Waller factor. We further examine how the stretching exponent $\beta$ and the degree of the breakdown of the Stokes-Einstein relation depend upon fragility, which we relate to the extent of cooperative motion.   

Session M33: Focus Session: Organic Electronics and Photonics - Excited State Dynamics for Photovoltaics

Time resolved energy transfer in polymers doped with heavy atom molecule

We used the technique of pump/probe transient photomodulation (PM) spectroscopy with high intensity and low repetition rate in the spectral range of 1.2 -- 2.5 eV and 100 fs time resolution for studying the excitons dynamics properties of solid state mixtures of few {\%} (X) heavy metal organic molecules in a pi-conjugated polymer host up to 2 ns. We found that the photobleaching (PB) spectrum contains two components; an instantaneous component due to the direct excitation of the heavy metal molecule guests, and a slower component due to energy transfer from the host polymer chains to the guest molecules. The PM spectrum also contains a built-up of a photoinduced absorption band at $\sim$1.5 eV that we assigned as due to excitons in the guest molecules, that has the same dynamics as that of the PB.   

Femtosecond optical study of chemically induced polaron states in polythiophene films

We performed femtosecond pump-probe measurements in poly(3-hexylthiophenes) (P3HT) doped with ionic liquid (OMIM/BF$_{\mathrm{4}})$. We fabricated an electrochemical cell with glass/ITO/P3HT (regioregular oriented film)/ionic liquid/ITO/glass structure. By applying the voltage between the electrodes, we electrochemically control the doping level. By increase of the applied voltage, the two polarized absorption peaks due to polaron states appear within the optical gap. The dynamics of the photoexcited states were studied by two-color femtosecond pump-probe measurements, in which the photon energies of the pump and probe pulses correspond to the $\pi $-$\pi $* transition and the polaron absorption band, respectively. At lower voltage, the increase of the near infrared absorption is observed, which is assigned to the photoinduced polaron absorption. On the other hand, at higher voltage, photoinduced bleaching is observed. The increase of the applied voltage reduces the lifetime of the excited states. These facts suggest that the photoexcitation in the chemically induced polaron states changes the electronic states and induces the new photoexcited species. The detailed origins of the new states are discussed by comparison with the femtosecond pump-probe spectra in PEDOT/PSS.   

Transient picosecond studies of pristine and DIO doped PTB7/PC71BM blend for photovoltaic applications

Recently there have been reports of a significant increase of power conversion efficiency in organic solar cells upon mixing the donor-acceptor blend with various additives. We studied the photoexcitation dynamics in thin films of pristine PTB7 (a low band-gap polymer), and pristine and doped PTB7/PCBM blend with DIO additives, using the pump/probe photoinduced absorption technique with probe in the mid-IR spectral range. We found that the photogenerated charges in DIO doped blend is more efficient than in the pristine blend. Specifically we found that the charge polarons in the DIO doped blend are photogenerated instantaneously, simultaneously with photogenerated singlet excitons. The excitons decay into charge-transfer (CT) excitons at the D-A boundaries within $\sim$1ps.The CT excitons may geminately recombine or dissociate into free charge polarons; where the dissociation time constant was found to be $\sim$450 ps.   

Managing thermal effects in z-scan measurements on PTCDA films

We study the two-photon absorption in micrometer thick polycrystalline PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) films using the open aperture z-scan technique. The films were grown by organic molecular beam deposition on Pyrex substrate and have been excited with 150 fs high repetition rate laser pulses at a wavelength of 820 nm. The pulses are focused onto the sample with a 10 x or a 20 x long distance microscope objective lens. The excitation intensities have been kept the same in both cases. To study the influence of sample heating the laser repetition rate has been varied from 4 MHz to 50 kHz by an acousto-optic pulse selector. At laser repetition rates larger than 200 kHz and 1 MHz for the 10 x and 20 x microscope lenses, respectively, we observe a reduction of the z-scan transmission dip. This reduction is attributed to a counteracting thermal effect due to film heating in the focus area. The reduced thermal effect using a 20 x microscope lens is attributed to faster thermal diffusion from the smaller focus area into the unexcited film. At lower repetition rates the z-scan dip is independent of the repetition rate and the two-photon absorption coefficient in PTCDA films was determined to be approximately 4 cm/GW.   

Excitonic Properties of Novel $\pi $-conjugated Polymers for Organic Electronics

We compare the photophysics of different derivatives of the $\pi $-conjugated polymer Poly(thienylenevinylene) (PTV) by photoluminescence (PL) and electro-absorption (EA) spectroscopy. The binding energy of the primary excitonic excited state is obtained from EA and is found to be related to the quantum efficiency for PL. In particular, both quantities are determined by the energies of the first optically allowed state and the first optically forbidden state above the ground state. In most PTV derivatives, the optically forbidden state lies below the optically allowed state and the PL is efficiently quenched by internal conversion. When the order of excited states is reversed, PL is observable with an efficiency that scales with the binding energy of the exciton as determined by EA. Thus, the chemical structure governs the interplay between $\pi $-conjugation delocalization and electron correlation that determines the ordering of excitonic states. This ordering then in turn dictates the effectiveness of a $\pi $-conjugated polymer for both emission and exciton dissociation and therefore dictates a material's suitability for either Organic Light-emitting Diodes or Organic Photovoltaic devices. This information then may be useful in the design of novel materials for application in these devices.   

External quantum efficiency exceeding 100{\%} in a singlet-exciton-fission-based solar cell

Singlet exciton fission can be used to split a molecular excited state in two. In solar cells, it promises to double the photocurrent from high energy photons, thereby breaking the single junction efficiency limit. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the peak external quantum efficiency is (109$+$/-1){\%} at $\lambda =$ 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160$+$/-10){\%}. Independent confirmation of the high internal efficiency is obtained by analysis of the magnetic field effect on photocurrent, which determines that the triplet yield approaches 200{\%} for pentacene films thicker than 5~nm. To our knowledge, this is the first solar cell to generate quantum efficiencies above 100{\%} in the visible spectrum. Alternative multiple exciton generation approaches have been demonstrated previously in the ultraviolet, where there is relatively little sunlight. Singlet exciton fission differs from these other mechanisms because spin conservation disallows the usual dominant loss process: a thermal relaxation of the high-energy exciton into a single low-energy exciton. Consequently, pentacene is efficient in the visible spectrum at $\lambda =$ 670 nm because only the collapse of the singlet exciton into \textit{two }triplets is spin-allowed.   

An Electric Field Stimulated Spin Crossover Transition in a Molecular Adsorbate

We have investigated the occupied and unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H$_2$B(pz)$_2)_2$(bipy)] complex (with H$_2$B(pz)$_2 =$ bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy $=$ 2,2'-bipyridine) by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). A bandgap of 2-3 eV is deduced from combined UPS and IPES measurements of the films on Au substrates. The shift of the unoccupied density of states seen in IPES is consistent with the thermally induced spin crossover transition for molecules deposited on the organic ferroelectric copolymer polyvinylidene fluoride with trifluoroethylene (PVDF-TrFE). Perhaps more significant is the fact that the spin crossover transition, and certainly the unoccupied electronic structure, is influenced by the ferroelectric polarization direction of PVDF-TrFE substrates at temperatures in the vicinity of the thermally driven spin crossover transition.   

Exploration of Excitonic States in Dilute Magnetic Organic Semiconductors

The electronic and excitonic properties of mixed dilute metal/metal-free phthalocyanine crystalline thin films are explored. The immediate focus is on molecular systems containing Cobalt and Copper phthalocyanines in ratios to the metal-free phthalocyanines ranging from 1:1 to 1:10. The molecular thin films samples are deposited using a novel hollow pen-writing technique\footnote{R. Headrick et al, APL 92 063302 (2008)} that produce millimeter sized grains with long range macroscopic order. Electronic and excitonic states are investigated using temperature dependent absorption/transmission and photoluminescence spectroscopy. All optical characterization indicates a very uniform mixing of the species is achieved in films without loss of long range order previously observed in individual species. At low temperatures, a novel high energy state is observed. Its intensity is directly related to the ratio of metal to metal-free Phthalocyanine. In addition, a unique linear dichroism mapping is performed on these thin film samples, giving insight into electronic states both close to and far from grain boundaries.   

Harvesting singlet fission for solar energy conversion: one versus two-electron transfer electron transfer from the quantum superposition state

Singlet fission (SF) is being explored to increase the efficiency of organic photovoltaics. A key question is how to effectively extract multiple electron-hole pairs from multiple excitons with the presence of other competing channels such as electron transfer from the singlet state. Recent experiments on the pentacene and tetracene show that a quantum superposition of the singlet (S$_{1}$) and multiexciton (ME) state is formed during SF. However, little is known about the kinetics of electron transfer from this quantum superposition. Here, we apply time-resolved photoemission spectroscopy to the tetracene/C$_{60}$ interface to probe one and two electron transfer from S$_{1}$ and ME states, respectively. Because of the relatively slow (~7 ps) SF in tetracene, both one- and two-electron transfer are allowed. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photo-excited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C$_{60}$. Kinetic analysis shows that ~60\% of the quantum superposition transfers one electron through the S$_{1}$ state to C$_{60}$ while ~40\% undergoes two-electron transfer through the ME state.   

Charge Transfer and Triplet States in High Efficiency OPV Materials and Devices

The advantage of using polymers and molecules in electronic devices, such as light-emitting diodes (LED), field-effect transistors (FET) and, more recently, solar cells (SC) is justified by the unique combination of high device performance and processing of the semiconductors used. Power conversion efficiency of nanostructured polymer SC is in the range of 10{\%} on lab scale, making them ready for up-scaling. Efficient charge carrier generation and recombination in SC are strongly related to dissociation of the primary singlet excitons. The dissociation (or charge transfer) process should be very efficient in photovoltaics. The mechanisms governing charge carrier generation, recombination and transport in SC based on the so-called bulk-heterojunctions, i.e. blends of two or more semiconductors with different electron affinities, appear to be very complex, as they imply the presence of the intermediate excited states, neutral and charged ones [1-3]. Charge transfer states, or polaron pairs, are the intermediate states between free electrons/holes and strongly bound excitons. Interestingly, the mostly efficient OLEDs to date are based on the so-called triplet emitters, which utilize the triplet-triplet annihilation process. In SC, recent investigations indicated that on illumination of the device active layer, not only mobile charges but also triplet states were formed [4]. With respect to triplets, it is unclear how these excited states are generated, via inter-system crossing or via back transfer of the electron from acceptor to donor. Triplet formation may be considered as charge carrier loss channel; however, the fusion of two triplets may lead to a formation of singlet excitons instead. In such case, a generation of charges by utilizing of the so far unused photons will be possible. The fundamental understanding of the processes involving the charge transfer and triplet states and their relation to nanoscale morphology and/or energetics of blends is essential for the optimization of the performance of molecular photovoltaic devices. I will present the state of the art in this field and discuss the mechanisms of polaron pair generation and recombination in the novel low band gap polymer-fullerene blends as well as in high-efficiency SC.\\[4pt] [1] C. Deibel, T. Strobel, V. Dyakonov, Phys. Rev. Lett. 103, 036402 (2009).\\[0pt] [2] C. Deibel, T. Strobel, and V. Dyakonov, Adv. Mater. 22, 4097 (2010).\\[0pt] [3] C. Deibel, and V. Dyakonov, Rep. Prog. Phys. 73, 096401 (2010).\\[0pt] [4] M. Liedtke, et al., JACS 133, 9088 (2011).   

Session M34: Thin Films of Block Copolymers and Hybrid Materials: Directed Assembly I

Inverse Solution for Directed Self-Assembly of Thin Film Cylindrical Morphology Block Copolymers

Using topographical templates, the directed self-assembly of thin film cylinder forming block copolymers has allowed for the fabrication of complex patterns with the sub-20nm length scales necessary for nanolithography. However, the templates for these circuit-like patterns have been developed from empirical methods that require either experimental examination of many input templates or time-consuming simulations over a wide parameter space. To address this problem, we have developed an inverse self-assembly algorithm that allows for the prediction of the template necessary to obtain a desired target pattern which includes bends, junctions, and terminals. The model system has been optimized for comparison with a cylindrical PDMS-PS block copolymer (45.5 kg/mol molecular weight and PDMS volume fraction 33.5{\%} ) under equilibrium neutral solvent annealing conditions. Example target structures are shown with the resulting predicted template found from the algorithm and compared with traditional simulation methods using those templates.   

Directed Assembly of Block Copolymer Ordering on Rough and Patterned Flexible Substrates

Directed self-assembly of block copolymer (BCP) thin film on flexible substrates has potential in fabrication of flexible electronic devices due to its nanometer scale pattern formation capability. We studied the BCP ordering properties of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films on a flexible substrate, where the PS-b-PMMA films are initially coated on a smooth poly(dimethylsiloxane) (PDMS) substrate, whose surface energy (SE) was tuned between (20-69) mJ/m2 by UV-ozone (UVO) exposure. This range of SE allows for controlled wettability and orientation of the BCP overlayer. Further, we replicated different patterned media and observed perpendicular lamellar BCP orientation and parallel cylindrical BCP orientation on patterned flexible PDMS in the wetting SE regime. Rough surface structures created by silica xerogels were replicated on PDMS. RMS roughness of the xerogels is tuned by controlling sol-gel catalyst concentration and aging time. Effect of the aspect ratio of the rough PDMS substrates on the orientation of BCP films was studied. Surface morphology of the BCP films was studied by optical microscopy and Atomic Force Microscope (AFM), while orientation of the film's interior was studied using Grazing-Incidence Small Angle X-ray Scattering (GISAXS)   

Consequences of Surface Neutralization in Thin Film Block Copolymers

Changes in boundary conditions have been found to induce novel physical phenomena in numerous systems. In this presentation, the consequences of surface neutralization to the structures of thin-film block copolymers were investigated using partially epoxidized poly(styrene-$b$-isoprene) (PS-PI) diblock copolymers. The thickness dependence of thin-film structures, prepared on non-preferential and preferential underlying brushes, were studied using scanning electron microscopy and atomic force microscopy. The PS-PI precursor, without epoxidation, exhibited parallel, layer-by-layer structures covered with one component, and the corresponding hole/island structures had step heights of one bulk lamellar periodicity (L$_{\mathrm{0}})$, consistent with previous studies. On the other hand, the thin films of epoxidized PS-PI showed perpendicular ordering independent of the thickness above non-preferential brushes, indicative of surface neutralization at both interfaces. The parallel lamellae of epoxidized PS-PI above preferential brushes were characterized as hole/island structures of 0.5 L$_{\mathrm{0}}$ step heights and the free surface wetting by both components of the diblock copolymers. The formation of the distinctive relief structures was attributed to the surface neutralization from the chemical modification.   

Block Copolymer Directed Assembly for Nanomaterials and Nanodevices

Block copolymer nanopatterning is a promising technology that can complement the inherent limitations of conventional photolithography. The spontaneous and parallel assembly of block copolymers may generate densely packed, periodic 10-nm-scale nanodomains in a scalable way. Furthermore, laterally ordered, device-oriented nanostructures are attainable by the directed self-assembly principles employing prepatterned substrates. In this presentation, the overview of my research achievements associated to block copolymer nanopatterning will be presented. My research group demonstrated the world-first successful integration of block copolymer nanopatterning with 193 nm ArF lithography. We also developed soft-graphoepitaxy, which generates highly aligned nanoscale metal and semiconductor nanostructures without any trace of structure-directing topographic pattern. Soft-graphoepitaxy could be further developed to ultralarge-area nanopatterning, where micrometer scale photoresist pattern can be completely transformed into large-area block copolymer nanopattern. My research group also developed various pattern transfer methods for block copolymer nanopatterning. Mussel-inspired block copolymer nanopatterning exploiting universal natural adhesive of mussel polydopamine enables the nanopatterning of low surface energy materials, such as gold, Teflon and graphene. Our recent transferrable and flexible nanopatterning employing chemically modified graphene films as pattern substrates makes it possible to apply block copolymer nanopatterning onto arbitrary nonplanar and flexible geometries and generates ideal three-dimensional assembly of carbon nanotubes and graphene.   

Self-annihilation of defects in block-copolymer thin films induced by corrugated substrates

Ultradense perfectly ordered structures with nanometric periodicity are of crucial importance for applications such as microelectronics, data storage media or meta-materials. Herein we demonstrate the use of a polymeric guiding pattern to control the self-assembly of block copolymers into highly-ordered 2D arrays. For this, a sinusoidal surface-relief grating was interferometrically inscribed onto an azobenzene containing copolymer sub-layer. A poly(styrene-$b$-ethylene oxide), PS-$b$-PEO, film was cast on top, resulting in cylinders with a 6-fold coordination. When film thickness reaches a critical value where the PS-$b$-PEO free-surface is smooth and no hint of the underlying sinusoidal pattern is apparent, a defect-free 2D-array of PS-$b$-PEO cylinders is observed over a large surface. Our results show that the surface deformation induced by the topological pattern controls the diffusion of defects and consequently their annihilation.   

Directed Self-assembly of High-Molecular-Weight Block Copolymer Films

The solvent-vapor annealing of block copolymer (BCP) films facilitates the mobility of highly entangled polymer chains, or the path-way barriers to the formation of well-ordered structures. In this study, the microdomain orientation of BCP films has been studied by in-situ grazing incidence small angle x-ray scattering (GISAXS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). We demonstrate the rapid evolution of a perpendicularly oriented lamellar morphology in high-molecular-weight (up to 1,000,000 g/mol) block copolymer films, to achieve topographically patterned BCP substrates.   

Directed Assembly of block copolymers on topologically complex surfaces: A self-consistent field theoretic study

The self-assembly of a lamella-forming diblock copolymer guided by topological complexity, namely, substrates composed of trenches with different heights and widths via self-consistent field theoretic simulations has been studied. In general, when the substrate is neutral to both blocks of the copolymer, the perpendicular lamella morphology is obtained. However, natural substrate usually has a preferred affinity to one of the blocks, and parallel lamella morphology is often obtained. Tuning the substrate roughness has proven useful in creating the perpendicular lamellar morphology. To this end, it has been shown that the perpendicular lamellae vertical to the trench direction is preferred when the trench size is relatively large. However, the orientation of the highly sought after perpendicular lamellar morphology can be changed by manipulation of the trench size, i.e., when the trench size is comparable to the natural periodic spacing of diblock copolymer, the perpendicular lamellae parallel to the trench direction is the preferred morphology. Overall this study clearly demonstrates the impact of this class of simulations in rational design of morphologies in thin multi-component polymeric films with application to technologies such as ultra-high-density magnetic recording media, metal nanostructures for metamaterials and plasmonic circuits, and sensors.   

Topcoat approaches for directed-assembly of copolymer films with blocks exhibiting differences in surface energy

Fabricating patterns with feature dimensions smaller than 10 nm scale using block copolymer lithography requires the use of materials with large Flory-Huggins interaction parameters. Because such block copolymers (BCPs) typically show the large differences in surface energy between the blocks, one block (with lower surface energy) tends to segregate to the free surface of films and precludes the assembly of the desired through-film perpendicularly oriented structures. Here we describe a generalizable strategy to overcome this limitation. By coating the BCP film with an additional layer, a topcoat, thermodynamically favorable boundary conditions at the top surface of the film can be engineered for directed self-assembly. The allowable properties of the topcoats depend on the interfacial energies of the layer with the blocks of the copolymer, and the block-block interfacial energy. The strategy is demonstrated experimentally by directing the assembly of polystyrene-block-poly-2-vinylpyridine (PS-$b$-P2VP) films on chemically nanopatterned substrates with different topcoat materials.   

Bad-solvent Induced Tunable Nanoscale Roughness in Polymer, Block Co-polymer and Carbon Thin Films

Nanoscale surface roughness of a material plays significant role in various applications such as adhesion, micro-/nano-electromechanical systems and antireflective coatings. We demonstrate here a novel and simple method for tuning nanoscale surface roughness of polymer coatings using a modified flow coater assembly. A dual-blade flow coating assembly was used to coat films of Poly(styrene) (PS), poly(methylmethacrylate) (PMMA) and PS-b-PMMA block co-polymer (BCP) dissolved in toluene on silicon substrates with a secondary blade flow coating a bad-solvent (water, ethanol) on top of the polymer film after a controlled delay. The bad-solvent and good--solvent miscibility and evaporation dynamics dictates the surface roughness/porosity in the polymer-liquid-air-interface. Combination of miscible ethanol-toluene solvents led to PS-chain formation of iterated function system (IFS) like fractal patterns with a root-mean-square (RMS) roughness $\sim$ 250 nm. However, PS films with much smaller roughness (\textless\ 20nm) were obtained for immiscible water and toluene solvents. The rough polymer coatings were also pyrolysed under optimized conditions to obtain carbon films with similar morphologies. Surface morphology and chemistry of the polymer and carbonized films were studied using AFM and XPS.   

Orientation of Microdomains in Cylinder-Forming PS-PHMA Thin Films

There is much interest in the study of self-assembled block copolymer thin films for uses in nanofabrication. For many applications control of the microdomain order is required. One method to achieve long-range orientational order in thin films is through the use of shear, which has been shown to orient block copolymer microdomains in the direction of the applied shear. A particular interest is shear-alignment of cylinder-forming poly(styrene)-poly(hexylmethacrylate) (PS-PHMA) thin films, which are effective masks for nanofabrication via reactive-ion etching. The present work examines the effects of changing PS block volume fraction, within the cylinder-forming region of the phase diagram, to both modulate the range of film thicknesses over which in-plane vs. out-of-plane cylinders are observed as well as improve the quality of in-plane alignment post-shear. Increasing the volume fraction of PS, away from the cylinder-sphere boundary, increased the range of film thicknesses over which the cylinders orient in-plane. The effects of the substrate wetting condition on cylinder orientation were also examined through grafting of PS and PHMA brushes to the substrate before deposition of the PS-PHMA film.   

Tunable-Morphology Block Copolymer Thin Films with Controlled Solvent Vapor Annealing for Lithographic Applications

Solvent annealing is an alternative to thermal annealing for improving long-range order and reducing defect density in block copolymer thin films. However, the fundamentals of block-copolymer self-assembly under solvent annealing conditions have yet to be studied in detail. We have developed a specialized hardware platform to perform solvent annealing experiments with active and precise control over solvent vapor saturation which allows us to quantitatively understand the structure-processing relationship during different stages of solvent annealing. Using polystyrene-b-polyethylene oxide/water/toluene as a model system and AFM, TEM and GISAXS characterization, we have found that a decrease in water vapor saturation during the post-annealing quenching step induces a change in domain spacing and reduction in long-range order. We have also found that by changing the water vapor saturation during steady-state annealing we are able to tune the domain spacing over a wide range and that this spacing remains after quenching. This controlled approach to solvent annealing affords considerable control over the morphology of annealed block copolymer thin films and a deeper understanding of the fundamentals of the process, making this technique more relevant to industrial applications.   

Ultrathin block copolymer films under shear

Ultrathin block copolymer films of 1-2 microdomains thick were investigated by means of a large scale coarse grained computer simulation, Cell Dynamics Simulation. Our simulation method allowed to computationally reach the size scale of experimental samples and to explain some recent experiments on sheared lamellae and cylindrical block copolymer morphologies. A detailed dynamical phase diagram, which covered parallel and perpendicular lamellae and cylinders, as well as perforated lamellae, was constructed. The crucial role of defects in orientation phase transitions and structure ordering and non-trivial defects dynamics was found. Our results provide detailed insights into how to use shear to control and manipulate block copolymer structure in thin films.   

Continuity and Network Morphologies of Lamellar Nanostructures Self-assembled in Block Copolymer Thin Films: Comparison of Processing by Thermal and Solvent Annealing

Self-assembled block copolymers in thin films have advantages for nanolithography including tunable and scalable feature sizes below 50 nm, parallel patterning over large areas, inexpensive material costs, and attractive processability. One process for inducing order in block copolymer thin films is solvent annealing, in which a film is swollen with solvent and domain ordering is induced as the solvent evaporates from the film. Solvent annealing is advantageous compared to thermal processing because it occurs rapidly and enables the use of polymer constituents that may be thermally unstable. Here the continuity of lamellar networks formed in thin films of poly(styrene-block-methyl methacrylate) with volume fractions of PMMA ranging from 0.45 to 0.55 will be shown to be favored in the block with a higher volume fraction. Network characteristics such as branch point density and end point density correlate with continuity, but at lower densities in solvent annealed than thermally annealed thin films of identical composition. Further comparison between thermal and solvent annealed morphologies sheds light on the mechanism through which ordering is achieved in solvent annealing and allows for additional control over the nanoscale features formed by block copolymers in thin films.   

Session M35: HTSC: Mainly X-ray Probes and Related Theory

Surface-enhanced charge-density-wave instability in underdoped Bi2201

Neutron and x-ray scattering experiments have provided mounting evidence for spin and charge ordering phenomena in underdoped cuprates, ranging from stripe correlations in Nd-LSCO to the recently discovered charge-density-waves in YBCO. Here we show that these electron-lattice instabilities also exhibit a previously unrecognized bulk-surface dichotomy. Surface-sensitive electronic and structural probes uncover a temperature-dependent evolution of the CuO$_2$ plane band dispersion and apparent Fermi pockets in underdoped Bi2201, which is associated with a strong temperature dependence of the incommensurate superstructure periodicity below 130 K. In stark contrast, the structural modulation revealed by bulk-sensitive probes is temperature independent. These findings point to a surface-enhanced incipient charge-density-wave instability, driven by Fermi surface nesting. This discovery is of critical importance in the interpretation of single-particle spectroscopy data and establishes the surface of cuprates as a rich playground for the study of electronically soft phases.   

Distinct Charge Orders in the Planes and Chains of Ortho-III-Ordered YBa2Cu3O6 identified by Resonant elas- tic x-ray scattering

Recently, charge density wave order with {\bf \it Q} = [0.3 0 $L$] and [0 0.3 $L$] was detected for the first time in underdoped YBCO using resonant soft x-ray scattering at the Cu $L_3$ absorption edge. Here, we explore the energy and polarization dependence of the resonant scattering intensity in detwinned YBa$_2$Cu$_3$O$_{6.75}$ with ortho-III oxygen ordering in the chain layer. We show that the ortho-III order results in a commensurate peak at $H$ = 0.33 whose energy and polarization dependence agrees with expectations for oxygen ordering in the chains. The [0.3 0 $L$] and [0 0.3 $L$] peaks, which result from a modulation of Cu 3$d_{x^2-y^2}$ states in the CuO$_2$ planes, are shown to be distinct and seemingly unrelated to the structure of the chain layer. Moreover, the energy dependence of the [0.3 0 $L$] and [0 0.3 $L$] scattering intensity is found to result from a spatial modulation of the energies of the Cu 2$p$ to 3$d_{x^2-y^2}$ transition, similar to stripe-ordered 214 cuprates.   

Microscopic theory of resonant soft x-ray scattering

We have developed a microscopic theory of resonant soft x-ray scattering (RSXS) that accounts for the delocalized character of valence electrons as well as excitonic and orthogonality catastrophe effects due to the core hole. We have derived a convenient and intuitive exact formula for RSXS intensities. Applying our formalism to the underdoped cuprates, we find that dynamic nesting in the band structure provides the most natural explanation for the two peaks observed in RSXS spectra. Our results give evidence for the existence of reasonably well-defined quasiparticles as far as 1.5 eV above the Fermi level in underdoped cuprates, and establish RSXS as a bulk-sensitive probe of electron quasiparticles.   

Determinant Quantum Monte Carlo Study of the Enhancement of d-wave Pairing by Charge Inhomogeneity

Striped phases, in which spin, charge, and pairing correlations vary inhomogeneously in the CuO$_2$ planes, are a known experimental feature of cuprate superconductors, and are also found in a variety of numerical treatments of the two dimensional Hubbard Hamiltonian. In this paper we use determinant Quantum Monte Carlo to show that if a stripe density pattern is imposed on the model, the $d$-wave pairing vertex is significantly enhanced. We attribute this enhancement to an increase in antiferromagnetic order which is caused by the appearance of more nearly half-filled regions when the doped holes are confined to the stripes. We also observe an enhanced $d$-wave pair correlation inside stripes reaching its maximum value when the $\pi$-phase shift in the magnetic order takes place. \\ Reference: Rubem Mondaini and Tao Ying and Thereza Paiva and Richard T. Scalettar, Phys. Rev. B 86, 184506 (2012).   

Energetics of superconductivity in the two dimensional Hubbard model

The energetics of the interplay between superconductivity and the pseudogap in high temperature superconductivity is examined using the eight-site dynamical cluster approximation to the two dimensional Hubbard model. Two regimes of superconductivity are found: a weak coupling/large doping regime in which the onset of superconductivity causes a reduction in potential energy and an increase in kinetic energy, and a strong coupling regime in which superconductivity is associated with an increase in potential energy and decrease in kinetic energy. The crossover between the two regimes is found to coincide with the boundary of the normal state pseudogap, providing further evidence of the unconventional nature of superconductivity in the pseudogap regime. However the absence, in the strongly correlated but non-superconducting state, of discernibly nonlinear response to an applied pairing field, suggests that resonating valence bond physics is not the origin of the kinetic-energy driven superconductivity.   

Superconconductivity and antiferromagnetism for the one-band Hubbard model of the cuprates including inter-plane hopping

While the overall features of the zero-temperature phase diagram of the cuprates are well described by the two-dimensional Hubbard model, the quest for a quantitative theory must include three-dimensional effects to account for differences between materials. To this end, using first-principles calculations [1,2], we obtain realistic parameters for the one-band Hubbard model that include hopping between planes. We then solve the resulting Hubbard Hamiltonian using the Variational Cluster Approximation [3] and Cellular-Dynamical Mean-Field Theory with an exact diagonalization impurity solver [4,5]. For single-layer materials, the effect of the inter-plane hopping is not sufficient to explain all the differences between the experimental phase diagrams for the various materials. We suggest other avenues of investigation. [1] Weber et al., Europhysics Lett. 100 37001 (2012) [2] Souza et al, Physical Review B 65 035109 (2001) [3] S\'{e}n\'{e}chal et al, Phys. Rev. Lett. 94 156404 (2005) [4] Caffarel and Krauth, Phys. Rev. Lett.72 1545-1548 (1994) [5] S\'{e}n\'{e}chal, Theoretical methods for Strongly Correlated Systems, eds: Mancini, Avella (Springer series, 2011)   

Spectral properties near the Mott transition in the two-dimensional Hubbard model

Single-particle excitations near the Mott transition in the two-dimensional (2D) Hubbard model are investigated by using cluster perturbation theory. The Mott transition is characterized by the loss of the spectral weight from the dispersing mode that leads continuously to the spin-wave excitation of the Mott insulator [1,2]. The origins of the dominant modes of the 2D Hubbard model near the Mott transition can be traced back to those of the one-dimensional Hubbard model. Various anomalous spectral features observed in cuprate high-temperature superconductors, such as the pseudogap, Fermi arc, flat band, doping-induced states, hole pockets, and spinon-like and holon-like branches, as well as giant kink and waterfall in the dispersion relation, are explained in a unified manner as properties near the Mott transition in a 2D system [1].\\[4pt] [1] M. Kohno, Phys. Rev. Lett. 108, 076401 (2012).\\[0pt] [2] M. Kohno, Phys. Rev. Lett. 105, 106402 (2010).   

A RIXS study on Spin and Charge Excitations in Electron-Doped Cuprates, NCCO

The phase diagram of the high-Tc cuprates is known to exhibit intriguing asymmetric doping evolution between the hole and electron-doping. ARPES and inelastic neutron scattering experiments have been extensively applied to study cuprates on both sides of the phase diagram, revealing a distinct Fermi surface evolution between the hole- and electron-doped cuprates, and the properties of low energy spin excitations. In this presentation, I will report high energy spin excitations and charge excitations of electron-deoped cuprates, Nd2-xCexCuO4, measured via resonant inelastic x-ray scattering (RIXS) at the Cu L-edge. The doping evolution of these excitations and their differences with those of the hole-doped cuprates will be discussed.   

Inelastic X-ray scattering measurement of electronic order in Bi2212

We present inelastic x-ray scattering measurements on superconducting Bi2212, showing evidence for a phonon anomaly associated with an underlying electronic density-wave state. We observe an broadening of the longitudinal acoustic phonon at a wavevector comparable to the antinodal nesting wavevector, near (1/4,1/4,0) in orthorhombic notation. An observed asymmetry between phonon creation and annihilation processes indicates breaking of time reversal and inversion symmetry as temperature is lowered. These measurements are consistent with prior work on single layer Bi2201, indicating universality of these features in the family of Bi-based high-Tc materials.   

Temperature and doping dependence of x-ray absorption spectral weight in YBa$_2$Cu$_3$O$_y$

The comprehensive study of the temperature dependent x-ray absorption spectroscopy (XAS) could be attributed to a dynamical spectral weight $\alpha$ in YBa$_2$Cu$_3$O$_y$ (YBCO). Large spectral weight changes with the temperature for both the Upper Hubbard band and the Zhang-Rice band due to dynamics of holes are experimentally found in the underdoped regime. These spectral weight changes become larger when the doping level $p$ goes deeper into the underdoped regime, but quickly vanishes as $p$ goes to the undoped limit. Our results clearly indicate that the pseudogap is related to the double occupancy and originates from bands in higher energies.   

Doping Evolution of Oxygen K-edge X-ray Absorption Spectra in Cuprate Superconductors

We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate the validity of the Zhang-Rice Singlet (ZRS) picture in overdosed cuprate superconductors. Using large-scale exact diagonalization of the three-orbital Hubbard model, we observe the effect of strong correlations manifesting in a dynamical spectral weight transfer from the upper Hubbard band to the ZRS band. The quantitative agreement between theory and experiment highlights an additional spectral weight reshuffling due to core-hole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the low-energy quasi-particles, but also demonstrate the continued relevance of the ZRS even in the overdosed region.   

Temperature and doping dependence of spectral features in determinant quantum Monte Carlo studies of the three-orbital Hubbard model of cuprate superconductors

Studying temperature and doping trends in strongly correlated materials is integral to understanding how their properties emerge and develop, and possibly can be tuned. To this end, determinant quantum Monte Carlo simulations are used to investigate spectral features in the three-orbital Hubbard model as applied to the cuprate superconductors. Spectral functions relevant to photoemission measurements are calculated and various spectral features, such as the indirect charge-transfer gap and Zhang-Rice singlet band, are shown to vary with doping and temperature. These orbitally resolved calculations help shed light on the applicability of the Zhang-Rice singlet picture at high hole doping levels. The density of states is also compared and contrasted with exact diagonalization studies as well as recent x-ray absorption spectroscopy measurements.   

Covalent magnetic form factor and neutron scattering in cuprates

We investigate the effect of covalent hybridization on magnetic excitations measured by the inelastic neutron scattering (INS) in the 1D cuprate Sr$_2$CuO$_3$ and the 2D La$_2$CuO$_4$. It has been previously shown that strong hybridization of Cu 3d states with O p states leads to the dramatic modification of the measured INS intensity, which is strongly suppressed, by factor 2.5-3, compared to the ionic spin model [1]. The result was obtained by comparing the measured intensity in a chain cuprate Sr$_2$CuO$_3$ with the dynamical spin structure factor predicted by the exact theory [2] of the model spin-1/2 Heisenberg Hamiltonian, which is typically used for cuprates. In the present follow-up study we extend these measurements so as to probe directly the wave vector dependence of the magnetic form factor, which is the Fourier transform of the magnetic electron's density, both in Sr$_2$CuO$_3$, and in the LSCO parent material, the two-dimensional La$_2$CuO$_4$. Our results yield a model-independent measurement of the magnetic form factor and provide an explanation for the suppressed magnetic intensity in La$_2$CuO$_4$ and other cuprates.\\[4pt] [1] A. Walters, \emph{et. al}, Nature Physics {\bf 5}, 867 (2009).\\[0pt] [2] J.-S. Caux, R. Hagemans, J. Stat. Mech., {\bf P12013} (2006)   

Angle and frequency dependence of the self-energy induced by boson fluctuation spectrum

We study the effects of the electron-boson coupling on the angle and frequency dependence of the self-energy. The spin susceptibility spectrum of the LSCO in superconducting state measured by the inelastic neutron scattering experiments has commensurate and incommensurate peaks. The energy scale of the self-energy induced by the commensurate peak is independent on the angle because of a small correlation length. On the other hand, that induced by the incommensurate peak depends on the angle because it has a large correlation length. The Eliashberg calculation using the measure spin fluctuation spectrum yields that the energy scale of the self-energy is larger along the anti-nodal direction than along the nodal direction. This result, however, is not consistent with the self-energy extracted from the ARPES analysis. Then we also considered the self-energy induced by Varma's loop current fluctuations. The results will be presented in comparison with the ARPES experiments.   

Session M36: Superconductivity: Josephson and Nanoscale Phenomena

Frequency-dependent admittance of a short superconducting weak link

We consider the electromagnetic response of a nanowire connecting two bulk superconductors. Andreev states appearing at a finite phase bias substantially affect the finite-frequency admittance of such wire junction. We evaluate the complex admittance analytically at arbitrary frequency and arbitrary, possibly non-equilibrium, occupation of Andreev levels. Special care is given to the limits of a single-channel contact and a disordered metallic weak link. We also evaluate the quasi-static fluctuations of admittance induced by fluctuations of the occupation factors of Andreev levels. In view of possible qubit applications, we compare properties of a weak link with those of a tunnel Josephson junction of the same normal conductance. Compared to the latter, weak link has smaller low-frequency dissipation. However, because of the deeper Andreev levels, quasi-static fluctuations of the complex admittance in a weak link are exponentially larger than in a tunnel junction. These fluctuations limit the applicability of nanowire junctions in superconducting qubits.   

Nonlocal transport in superconducting oxide nanostructures

We report nonlocal transport signatures in the superconducting state of nanostructures formed\footnote{J.P. Veazey, \textit{et al.}, arXiv:1210.3606 (2012).} at the LaAlO$_3$/SrTiO$_3$ interface using conductive AFM lithography. Nonlocal resistances (nonlocal voltage divided by current) are as large as 200 $\Omega$ when 2-10 $\mu$m separate the current-carrying segments from the voltage-sensing leads. The nonlocal resistance reverses sign at the local critical current of the superconducting state. Features observed in the nonlocal \textit{V-I} curves evolve with back gate voltage and magnetic field, and are correlated with the local four-terminal \textit{V-I} curves. We discuss how nonlocal and local transport effects in LaAlO$_3$/SrTiO$_3$ nanostructures may result from the electronic phase separation and superconducting inhomogeneity reported by others in planar structures\footnote{Ariando, \textit{et al.}, \textit{Nature Comm.} \textbf{2}, 188 (2011); J.A. Bert, \textit{et al.}, \textit{Nature Phys.} \textbf{7}, 767 (2011).}.   

Superconductivity in Centimeter Length Indium-Gallium Nanowires

In-doped Ga nanowires 150 nm in diameter and 6mm in length have been formed in silica nanocapillaries. X-ray fluorescence and diffraction measurements performed at the Advanced Photon Source have been used to characterize their chemical composition and crystal structure. Investigation of the low temperature transport properties of these wires reveals a two stage superconducting transition. Magnetoresistance measurements are suggestive of vortex trapping in the wire. The X-ray fluorescence measurements suggest phase separation in the capillaries into Ga nanodroplets and In-Ga eutectic wires. A model to explain the vortex trapping consistent with this observation is being developed.   

Investigating long-range proximity effect in ferromagnetic Ni and Ni-Fe nanowires

Singlet superconductors and ferromagnets entail incompatible spin orders severely limiting the range of the superconducting proximity effect in a ferromagnet ($\sim$ 1 nm). Contrary to this expectation, a very long-range proximity effect (LRPE, $\sim$ 600 nm) was found in crystalline ferromagnetic nanowires [Wang et al., Nat. Phys. 6, 389 (2010)]. Several mechanisms have been suggested to explain the LRPE, the most intriguing of which is the possibility of triplet superconductivity in the ferromagnet. We have conducted experiments to probe the mechanism of the LRPE. The LRPE persists in granular Ni nanowires, ruling out ballistic transport as a possible mechanism. Surface superconductivity in the oxide layer on the ferromagnetic nanowire is also ruled out based on critical current measurements. On changing the nature of the contacting electrodes, the range of the proximity effect is found to diminish significantly. This indicates that the nature of the interface between the superconductor and the ferromagnet is important as expected for triplet superconductivity. Tunneling measurements probing the superconducting gap in the ferromagnetic nanowire are underway.   

Critical current oscillations in superconducting Al strips

We have studied current-voltage characteristics as a function of temperature and magnetic field in superconducting aluminum strips with varying lengths and cross sections. We find that the critical current oscillates as a function of magnetic field and suggest that the effect depends on the relative energies of vortex configurations in the strips in different transport regimes.   

Ultralow Noise Microwave Amplifier Based on the Superconducting Low-inductance Undulatory Galvanometer

We have developed an ultralow noise microwave linear amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). The compact SLUG element is straightforward to model at microwave frequencies, allowing separate optimization of the SLUG element and the resonant input matching network. SLUG amplifiers incorporating high-Jc junctions have shown gains of order 15 dB in the frequency range from 3-10 GHz with instantaneous bandwidth up to several hundred MHz. Large-volume normal metal cooling fins have been integrated into the SLUG element to promote thermalization of hot electrons in the resistive shunts at millikelvin temperatures, and the amplifiers have achieved added system noise of one photon in the GHz frequency range. We discuss application of the SLUG amplifier to single shot dispersive readout of the transmon qubit.~   

Flux noise in SQUIDs: Effects of deposited surface films

Magnetic flux noise in SQUIDs and superconducting qubits with a spectral density $S_\Phi(f)$ scaling as $1/(f/1 Hz)^\alpha$ is understood to arise from the random reversal of spins localized at the surface of the superconducting film. We present experimental results showing the effects on $S_\Phi(f)$ of Au, SiNx, NbN, and Al2O3 films deposited on the upper surface of Nb and NbN dc SQUID loops. For each measurement, we fabricated six identical SQUIDs on a single chip and then capped the surface of either half or all the SQUID loops. Certain capping layers, such as Au, had no discernible effect on $S_\Phi(f)$ with regard to the magnitude, slope $\alpha$, and temperature dependence. On the other hand, some capping layers significantly reduced $S_\Phi(1 Hz)$---by a factor of about two in the case of SiNx. Furthermore, some layers significantly affected the value of $\alpha$ and the temperature dependence of both $S_\phi(1 Hz)$ and $\alpha$. These results further establish the importance of the role of the surface of the SQUID loop on its flux noise. We discuss implications for microscopic models of flux noise in light of these measurements.   

Geometry and temperature dependence of low-frequency flux noise in dc SQUIDs

Measurements on dc SQUIDs reveal a flux noise spectral density $S_\Phi(f) = A^2/(f/1~Hz)^\alpha$. An analytic model assuming non-interacting spins localized at the surface of the SQUID loop predicts that the mean square noise scales as R/W---the radius and width of the loop, respectively. However, there are no established theories for the scaling of $\alpha$ with geometry or the dependences of A and $\alpha$ on temperature T. To test the predicted geometric scaling of this model experimentally, we measured flux noise in ten SQUIDs with systematically varying geometries. We find that, at fixed T, $A^2$ scales approximately as R. From the measured values of A and $\alpha$, we estimate the mean square flux noise, which does not scale with R. As T is lowered, $\alpha$ increases significantly and in such a way that the spectra ``pivot'' about an approximately fixed frequency. This phenomenon implies that the mean square noise is temperature-dependent, an effect not predicted by the analytic model. We discuss our attempts to reconcile these discrepancies by considering the locking together of spins to form clusters.   

Niobium Nitride Thin Films and Multilayers for Superconducting Radio Frequency Cavities

Niobium nitride in thin film form has been considered for a number of applications including multi-layered coatings onto superconducting radio frequency cavities which have been proposed to overcome the fundamental accelerating gradient limit of $\sim$50 MV/m in niobium based accelerators [1]. In order to fulfill the latter application, the selected superconductor's thermodynamic critical field, H$_{\mathrm{C}}$, must be larger than that of niobium and separated from the Nb surface by an insulating layer in order to shield the Nb cavity from field penetration and thus allow higher field gradients. Thus, for the successful implementation of such multilayered stack it is important to consider not just the materials inherent properties but also how these properties may be affected in thin film geometry and also by the specific deposition techniques used. Here, we show the results of our correlated study of structure and superconducting properties in niobium nitride thin films and discuss the shielding exhibited in NbN/MgO/Nb multilayer samples beyond the lower critical field of Nb for the first time.\\[4pt] [1] A. Gurevich, Appl. Phys. Lett., \textbf{88}, 012511 (2006).   

Induced superconducting FFLO states in patterned island systems and in topological insulators

We explore the possibility of inducing the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting phase in 2D metal films by means of proximity coupling to patterned superconducting islands. We show that as a function of externally applied magnetic field, such a system not only renders the phase stable for a large region of parameter space but can also be tuned through different spatial ordering wavevectors associated with the FFLO order. We generalize these results to the surface states of 3D topological insulators and metallic surface states with Rashba coupling. We find that these FFLO states can be mapped into BCS states in which a uniform superconductor gap occurs in momentum space and can potentially be accessed in physical systems with relative ease.   

Evidence for synchronized Andreev reflections in NSN devices

In mesoscopic NSN devices, in which a short superconducting region separates two metallic electrodes, the Andreev reflection process may delocalize and couple electron- and hole-states on opposite sides of the superconductor. In addition to such nonlocal (or crossed) Andreev reflections, quasiparticles may also tunnel directly between the electrodes. We have studied nonlocal transport and current correlations in Cu/Al/Cu structures. We observe that the current correlations are symmetric in applied bias and show local minima when the applied voltages at the two contacts are equal in magnitude. This behavior matches theoretical predictions for devices with intermediate interface transparency in which the nonlocal exchange of quasiparticles triggers additional synchronized Andreev reflection events at the two interfaces.   

Josephson current and density of states in proximity circuits with s+- superconductors

We study the emergent proximity effect in mesoscopic circuits which involve conventional superconductor and unconventional pnictide superconductor separated by a diffusive normal or ferromagnetic wire. The focus is placed on revealing signatures of the proposed $s^{+-}$ state of pnictides from the proximity-induced density of states and Josephson current. We find analytically a universal result for the density of states which exhibits both a Thouless gap at low energies, and peculiar features near the superconducting gap edges at higher energies. The latter may be used to discriminate between $s^{+-}$ and $s^{++}$ symmetry scenarios in scanning tunneling spectroscopy experiments. We also calculate Josephson current-phase relationships for different junction configurations, which are found to display robust $0-\pi$ transitions for a wide range of parameters.   

Results of Switching Measurements in MgB$_2$ Josephson Heterojunctions: Search for Multiple Tunneling Channels and Leggett-Mode Signatures

Josephson tunnel junctions made of multi-gap and single-gap superconducting electrodes provide a useful system for understanding multiple gap superconductivity. Peaks in the differential conductance curve have been used to characterize the energy gaps of such multi-gap materials [e.g. Chen, K. et al., Nat. Commun. 3:619 (2012)]. Superconducting-to-normal switching data can also provide useful insights. While ramping the current from zero to the critical current, the washboard potential is tilted, thereby adjusting the resonant frequency of the potential well, and altering the energy level spacing. By exciting the junction with microwaves, resonant modes may be explored. We report results of conductance and switching experiments on MgB$_2$/I/Pb and MgB$_2$/I/Sn junctions, with and without microwaves, in a helium dilution refrigerator with a base temperature ~20mK. These results exhibit tunneling modes and resonances not observed in single-gap/single-gap junctions, including a peak in the escape rate that may be consistent with coupling to the Leggett mode.   

Evidence for Multi-photon transitions between energy levels in a large Current-Biased Magnesium Diboride Josephson Heterojunction

When photons are strongly coupled to a quantum system, multiphoton transitions can be observed between two energy levels when the quantum energy of the exciting radiation, multiplied by an integer, matches the level spacing. This phenomenon can be observed in Josephson junction qubits exposed to weak microwave radiation at very low temperatures. At microwave resonance, the transition probability of a junction from superconducting to normal state is enhanced and these are used to map multiphoton transitions. We report observation of single- and multi-photon transitions between ground and first excited states in current-biased MgB2 thin film junctions by applying RF with frequencies between 0.5 and 3 Ghz. These large (up to 0.2mm x 0.3 mm) junctions consist of an MgB2 electrode insulated by native oxide from a lead (Pb) or tin (Sn) counter-electrode, and have areas at least 600 times bigger than Nb junctions previously shown to exhibit multiphoton transitions. The data is consistent with theoretical models of junctions behaving in the quantum limit and show anharmonicity of the junction potential when biased near the critical current.   

In-gap States of Josephson Junction with Two-gap Superconductiors

We investigate the transport property of SIS junctions with two-gap superconductors. The effects of two superconducting condensates on critical current density is estimated by studying the microscopic structure of Josephson current density in a dual-mode tunnel junction with a narrow quasi-classical tunnel barrier. Following the suggestion by Golubov and coworkers [1], we use two Bloch functions to describe the condensates in the two-band superconductors. In this junction, the in-gap states which include the interband interference effect appear at the interfaces due to the discontinuity of the superconducting phase. Also, similar to a Josephson junction [2] involving one-gap and two-gap superconductors, novel broken time-reversal symmetry states are found. We estimate the effects of interband interference and broken time-reversal symmetry on the in-gap bound states and critical Josephson current density. \\[4pt] [1] A. A. Golubov \textit{et al.}, Phys. Rev. Lett. \textbf{103}, 3398 (2009). \\[0pt] [2] T. K. Ng and N. Nagaosa, EPL \textbf{87}, 17003 (2009).   

Session M37: Focus Session: Fe-based Superconductors: Novel Selenides

The nodal crisis in Iron based superconductivity

The recent observation of fully gapped high temperature superconductivity in an iron chalcogenide without a hole Fermi surface[1], combined with the observations that rule out a node-less d-wave state [2] constitute a ``nodal crisis'' in iron based superconductivity, for we do not understand how the underlying singlet state avoids the strong Coulomb interactions on the iron site without some kind of node within the superconducting condensate. In this work, we re-analyze the allowed symmetries of the superconducting condensate in the iron superconductors, taking into account both orbital symmetries between the $zx$ and $zy$ orbitals and the presence of two equivalent Fe sites per unit cell. We argue that the additional orbital degrees of freedom provide for a much richer class of pairing symmetries than normally considered. A particularly interesting possibility, is a p-wave, spin singlet, orbital triplet state that is a fully gapped iron analog of the B-phase of superfluid He-3. We will discuss this interesting possibility. \\ \hbox{[1]} Wang Qing-Yan et al, Chinese Phys. Lett. 29 037402 (2012).\\ \hbox{[2]} X.-P. Wang et al, Europhysics Letters 99, 67001 (2012).   

Electronic Structure and Superconductivity in Bilayer FeSe$\backslash$SrTiO$_3$ Films

We have carried out high resolution angle-resolved photoemission (ARPES) measurements on bilayer FeSe films grown on the SrTiO$_3$(001) substrate by the MBE method. Detailed doping evolution of the electronic structure has been investigated through an annealing process. Similar to the single-layer FeSe film, two phases are observed during the annealing process which coexist and compete. On the other hand, the bilayer FeSe film exhibits obviously different behaviors from that of the single layer FeSe film. Details of the experiment and their implications will be discussed.   

Spin fluctuations in alkali-metal iron selenide superconductors probed by inelastic neutron scattering

We employ inelastic neutron scattering (INS) on iron-based superconductors to study the spectrum of low-energy magnetic excitations. According to the most commonly accepted theory of the superconducting state, spin fluctuations could act as the bosonic ``glue'' that mediates Cooper pairing in Fe-based compounds, thus playing the role similar to that of phonons in the conventional BCS theory. The knowledge of the spin-fluctuation spectrum is therefore important for understanding the mechanisms that stabilize high transition temperatures in Fe-based superconductors. Our most recent results include observations of magnetic resonant modes and normal-state paramagnon excitations in alkali-metal iron selenide superconductors Rb$_x$Fe$_2$Se$_2$ and K$_x$Fe$_2$Se$_2$. These excitations were found at a wave vector that differs from the ones characterizing magnetic resonant modes in other iron-based superconductors, but appears to be universal for all alkali-metal iron selenide compounds independently of the alkali-metal element or the crystal-growth procedure. Using time-of-flight neutron spectroscopy, we also estimated the absolute spectral weight of the magnetic resonant mode, which exceeds that in the iron arsenides.   

Evidence of Chemical Phase Separation in K$_{0.65}$Fe$_{1.74}$Se$_{2}$

K$_{x}$Fe$_{2-y}$Se$_{2}$ has been widely investigated and many samples show a co--existence of superconductivity and antiferromagnetic properties. Recently the it was shown that this system shows a clear phase separation, however the nature of the two phases remained unclear. In the present work we report on a chemical phase separation in crystalline superconducting K$_{0.65}$Fe$_{1.74}$Se$_{2}$, investigated by means of magnetization experiments, scanning electron microscopy, electron backscatter diffraction, and energy--dispersive X--ray spectrometry. It is shown that the crystal consists of platelets oriented in $<$100$>$ with an approximated volume fraction of about 30\% in the surrounding $<$001$>$ oriented matrix. The platelets (the matrix) are depleted in K (Fe) and enriched in Fe (K). Chemical phase separation is demonstrated by a stable, antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ matrix, and K$_{x}$Fe$_{y}$Se$_{2}$ platelets inducing superconductivity. This time driven chemical spinoidal phase separation may therefore be responsible for several alternative properties measured in K$_{x}$Fe$_{2-y}$Se$_{2}$ samples as superconductivity and antiferromagnetism.   

Intrinsic crystal phase separation and detailed structural characterization in Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$ superconductor via high resolution diffraction

The discovery of high critical temperature superconductivity in complex metal cuprate pnictide and chalcogenide compounds is a major breakthrough in materials synthesis and in developing new concepts, compounds and technologies. The mechanisms of charge carrier density control are important as small changes in composition produce metal-insulator transitions and generate superconductivity at temperatures of up to 37K in chalcogenides. Reported materials are based on a square FeSe layer built from edge-sharing of FeSe$_4$ tetrahedra. Insertion of alkali metal cations between FeSe layers affords superconductivity in this system. We have grown Cs-intercalated FeSe samples that show superconductivity with different Tc between 10K and 28K by changing the iron and cesium concentration in the nominal composition Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$ (0.7 \textless\ x \textless\ 1.1, 0 \textless\ y \textless\ 0.7). These are two phase systems and only one phase is SC. The relationship between structural and superconducting properties will be discussed based on high-resolution X-ray diffraction and single-crystal X-ray measurements combined with magnetometry, heat capacity, and transport measurements.   

Phase separation and superconductivity in K$_{\mathrm{1-x}}$Fe$_{\mathrm{2-y}}$Se$_2$ single crystals under different thermal treatments

Single crystals with the starting composition of K$_{0.8}$Fe$_{2}$Se$_2$ have been thermally treated in three different ways: slow furnace cooling (SFC) from 1020 $^{\circ}$C, retreated for 2 hours at 250 $^{\circ}$C (S250) and 350 $^{\circ}$C (S350:) and followed by quenching. The DC magnetization measurements on them exhibit very different behavior: the SFC samples show a tiny diamagnetic signal, while the sample S350 shows a quite large Meissner shielding volume with the S250 in the middle. The resistive measurements on the sample S350 show zero resistance below 31 K with a sharp transition; while those from the sample SFC or S250 show much larger residual resistance together with a much wider transition. By using the SEM, we have successfully identified that, in SFC, the superconducting areas have relatively larger sizes (about one micrometer) and are widely separated; the superconducting area change into many thin but well connected networks in the sample S350, which construct a 3D spider-web. This explains both the magnetic shielding and the resistive transitions in the three samples. In addition, the superconducting area has a composition of about K$_{0.64}$Fe$_{1.8}$Se$_2$. We suggest that the thermodynamically stable phase for the superconducting state has probably one vacancy in every 10 Fe-sites.   

Nodeless superconducting gap in K$_x$Fe$_{2-y}$Se$_2$ and its evolution with doping probed from angle-resolved photoemission

The nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in K$_x$Fe$_{2-y}$Se$_2$, whether its pairing symmetry is s wave or nodeless d wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in K$_x$Fe$_{2-y}$Se$_2$, which favors the s-wave pairing symmetry [1-3]. At the same time, we will present some of the recent data on the evolution of the band structure and superconducting gap of iron chalcogenides K$_x$Fe$_{2-y}$Se$_2$ as a function of electron and hole doping.\\[4pt] [1] M. Xu, et.al, Phys Rev B 85 (22), 220504(R) (2012).\\[0pt] [2] F. Chen, et.al, Phys Rev X 1, 021020 (2011).\\[0pt] [3] Y. Zhang, et.al, Nature Mater. 10, 273 (2011).   

Terahertz spectroscopy on Rb$_{\mathrm{1-x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$

Single crystals of superconducting and non-superconducting Rb$_{\mathrm{1-x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ [1] have been investigated by terahertz time-domain transmission spectroscopy as a function of temperature. In the superconducting samples, we observe the signatures of the superconducting transition [2] and an isosbestic point in the temperature dependence of optical conductivity in the vicinity of 100 K, which could be related to the reported phase separation in these compounds. In the non-superconducting samples, the optical conductivity exhibits features which can be interpreted in terms of spin wave excitations in agreement with neutron experiments [3].\\[4pt] [1] V. Tsurkan et al. Phys. Rev. B \textbf{84}, 144520 (2011)\\[0pt] [2] A. Charnukha et al. Phys. Rev. B \textbf{85}, 100504 (2012)\\[0pt] [3] Miaoyin Wang et al. Nature Communications \textbf{2}, 580 (2011)   

High T$_{\mathrm{C}}$ superconductivity in single-layer FeSe films on SrTiO$_3$

The latest scanning tunneling spectroscopy and angle resolved photoemission spectroscopy of single-unit-cell FeSe films on SrTiO$_3$ show signatures of high temperature superconductivity with T$_{\mathrm{C}}$ \textgreater\ 55 K, the maximum value that has been stagnant since the discovery of the iron-based superconductors in 2008. Here we report a detailed transport study of the single-unit-cell FeSe film. Electrical transport measurements reveal a transition temperature of $\sim$ 50 K. The robust superconductivity is further confirmed by measuring Meissner effect. We show that the characteristics of the transition are consistent with a two-dimensional superconductor undergoing a Berezinskii-Kosterlitz-Thouless transition.   

Effective doping and suppression of Fermi surface reconstruction via Fe vacancy disorder in K$_x$Fe$_{2-y}$Se$_2$

We investigate[1] the effect of disordered vacancies on the normal-state electronic structure of the newly discovered alkali-intercalated iron selenide superconductors. To this end we use a recently developed Wannier function based method[2] to calculate from first principles the configuration-averaged spectral function $\langle A(k,\omega)\rangle$ of K$_{0.8}$Fe$_{1.6}$Se$_2$ with disordered Fe and K vacancies. We find that the disorder can suppress the expected Fermi surface reconstruction without completely destroying the Fermi surface. More interestingly, the disorder effect raises the chemical potential significantly, giving enlarged electron pockets similar to highly doped KFe$_2$Se$_2$, without adding carriers to the system. [1] T. Berlijn, P. J. Hirschfeld, and W. Ku, Phys. Rev. Lett. 109, 147003 (2012) [2] T. Berlijn, D. Volja and W. Ku, Phys. Rev. Lett. 106, 077005 (2011)   

Orbital-Selective Mott Phase in Multiorbital Models for Alkaline Iron Selenides

The degree of electron correlations is crucial for understanding the properties of both the normal and superconducting states of the iron-based superconductors. The superconductivity near an antiferromagnetic insulating phase in the newly discovered alkaline iron selenide superconductors suggests stronger electron correlations in these materials than in iron pnictides. To investigate the correlation effects in the alkaline iron selenides, we study the metal-to-Mott-insulator transition in multiorbital models for this system using a slave-spin mean-field method [1]. We show that when the Hund's coupling is beyond a threshold, this transition is via an intermediate orbital-selective Mott phase, in which the 3d xy orbital is Mott localized while the other 3d orbitals remains itinerant. We find that this phase is still stabilized over a range of carrier dopings, and has unique experimental signatures [2,3]. Our results lead to an overall phase diagram for the alkaline iron selenides, in which the orbital-selective Mott phase provides a natural link between the alkaline iron selenide superconductor and its parent Mott-insulating compound. [1] R. Yu and Q. Si, arXiv:1208.5547. [2] M. Yi et al., arXiv:1208.5192. [3] P. Gao et al., arXiv:1209.1340.   

Modeling local interface and impurity effects in phase separated iron chalcogenide superconductor K$_x$Fe$_{2-y}$Se$_2$

Superconductivity in iron chalcogenide superconductor KxFe$_{2-y}$Se$_2$ exists near a phase separated block antiferromagnetic state (BAFM) with magnetic moments of 3.3$\mu_B$/Fe. The nature of the superconducting state compared to other pnictide superconductors is unclear because the Fermi surface contains electron pockets only. This raises the fundamental question whether the superconducting phase is described by s- or d-wave gap symmetry. We study the magnetic state, the superconducting state as well as their interface in phase separated K$_x$Fe$_{2-y}$Se$_2$ using a real space extended Hubbard model. The model includes the effects of all five Fe d-orbitals and the superconducting pairing interaction is generated within the spin-fluctuation exchange mechanism. We propose the existence of signatures in the local density of states near the interface and impurities that could discriminate between the d-wave and s-wave superconducting gap symmetries. Further, we show how the interface between the superconductor and BAFM leads to novel features in the various mean fields, including e.g. a strong interface-enhanced orbital-ordering.   

Session M38: Energy Storage and Conversion

Adsorbed Methane Film Properties in Nanoporous Carbon Monoliths

Carbon briquetting can increase methane storage capacity by reducing the useless void volume resulting in a better packing density. It is a robust and efficient space-filling form for an adsorbed natural gas vehicle storage tank. To optimize methane storage capacity, we studied three fabrication process parameters: carbon-to-binder ratio, compaction temperature, and pyrolysis temperature. We found that carbon-to-binder ratio and pyrolysis temperature both have large influences on monolith uptakes. We have been able to optimize these parameters for high methane storage. All monolith uptakes (up to 260 bar) were measured by a custom-built, volumetric, reservoir-type instrument. The saturated film density and the film thickness was determined using linear extrapolation on the high pressure excess adsorption isotherms. The saturated film density was also determined using the monolayer Ono-Kondo model. Film densities ranged from ca. 0.32 g/cm$^{3}$ - 0.37 g/cm$^{3}$.The Ono-Kondo model also determines the binding energy of methane. Binding energies were also determined from isosteric heats calculated from the Clausius-Clapeyron equation and compared with the Ono-Kondo model method. Binding energies from Ono-Kondo were ca. 7.8 kJ/mol - 10 kJ/mol.   

Finite-Temperature Dihydrogen Adsorption/Desorption Thermodynamics on Metallo-Porphyrin Incorporated Graphene: Enthalpy versus Vibration

Gas adsorption is closely related to a variety of important physicochemical processes and technologies. Especially, hydrogen storage has been attracting much interest due to high energy density and the environmetally-friendly nature. Although a lot of theoretical studies have been carried out, the thermal vibration effect on hydrogen-sorbent interaction is relatively laking. Here we report the thermodynamics of H$_{\mathrm{2}}$ molecules adsorbed onto metallo-porphyrin-incoporated graphenes based on first-principles density-functional theory calculations. We found that the slow vibrations induced by weak binding tend to make the system more stable under finite temperature while the fast vibrations induced by strong binding disturb the adsorption. This tendency is expected to be universally found in various gas-sorbent systems.   

Hydrogen Storage Investigation on Nanotube, Graphene and Organo-metallic Complexes

New materials and methods for storing hydrogen at high gravimetric and volumetric densities are required because of the widely use of hydrogen for clean fuel. With exceptionally high surface areas, porous materials based on carbon have recently emerged as some of the most promising candidate materials. Here I reviewed our former work on hydrogen storage based on several kinds of organometallic Complexes. Maximum capacities of the hydrogen storage in organometallic compounds consisting of Co and Ni atoms bound to C$_{\mathrm{m}}$H$_{\mathrm{m}}$ ring were found 3.48 wt {\%} and 3.49 wt {\%}, respectively; for the structures having a transition metal (TM) Co and Ni inserted in C$_{\mathrm{m}}$H$_{\mathrm{m}}$ ring, the maximum number of H$_{2}$ molecule bound to the inserted-type CoC$_{\mathrm{m}}$H$_{\mathrm{m}}$ and NiC$_{\mathrm{m}}$H$_{\mathrm{m}}$ complexes is three, and the largest hydrogen storage density is 5.13 wt {\%} and 3.49 wt {\%} for CoC$_{4}$H$_{4}$ and NiC$_{4}$H$_{4}$, Meanwhile, the ionic (C$_{4}$H$_{4}^{+}$ and C$_{5}$H$_{5}^{+})$ improves the capability of hydrogen storage and makes all H$_{2}$ adsorbed to the charged compounds in molecular form. With the CH$_{3}$ ligand bound to the compounds, the adsorption energy of H$_{2}$ decreases to an ideal range, and stability of the compounds are improved. At last, the hydrogen adsorption properties on the complex structures TiRH$_{7}$Si$_{8}$O$_{12}$ are investigated, and the kinetic stability when H$_{2}$ was added to organometallic compounds is also discussed by analyzing HOMO-LUMO gaps. Here we also mentioned our results of hydrogen storage based on nanotubes and graphene.   

First-principles calculations of mass transport in magnesium borohydride

Mg(BH$_{4}$)$_2$ is a hydrogen storage material which can decompose to release hydrogen in the following reaction: Mg(BH$_4$)$_{2(\mathrm{solid})}$ $\rightarrow \frac{1}{6}$MgB$_{12}$H$_{12(\mathrm{solid})}$ + $\frac{5}{6}$MgH$_{2(\mathrm{solid})} + \frac{13}{6}$H$_{2(\mathrm{gas})}$ $\rightarrow$ MgH$_{2(\mathrm{solid})}$ + 2B$_{(\mathrm{solid})}$ + 4H$_{2(\mathrm{gas})}$. However, experiments show that hydrogen release only occurs at temperatures above 300 $^{\circ}$C, which severely limits applications in mobile storage. Using density-functional theory calculations, we systematically study bulk diffusion of defects in the reactant Mg(BH$_{4}$)$_2$ and products MgB$_{12}$H$_{12}$ and MgH$_{2}$ during the first step of the solid-state dehydrogenation reaction. The defect concentrations and concentration gradients are calculated for a variety of defects, including charged vacancies and interstitials. We find that neutral [BH$_3$] vacancies have the highest bulk concentration and concentration gradient in Mg(BH$_{4}$)$_2$. The diffusion mechanism of [BH$_3$] vacancy in Mg(BH$_{4}$)$_2$ is studied using the nudged elastic band method. Our results shows that the calculated diffusion barrier for [BH$_3$] vacancies is $\approx. 2$~eV, suggesting that slow mass transport limits the kinetics of hydrogen desorption.   

Effect of transition-metal additives on dehydrogenation kinetics of MgH$_2$

Using first-principles calculations based on hybrid density functional theory we study the (de)hydrogenation process in MgH$_2$, an important solid-state hydrogen storage material. This reaction proceeds through diffusion processes, mediated by native point defects such as vacancies and interstitials. Reducing the formation energy of relevant defects increases their concentrations, resulting in higher diffusion rates and an enhancement in kinetics. We investigate the formation energies of native point defects in MgH$_2$ and determine the position of the Fermi level in the band gap using the charge neutrality condition. The presence of transition-metal (TM) impurities (Ti, Fe, Co and Ni) causes the Fermi level to shift according to the position of the TM acceptor/donor levels in the band gap. This shift can bring down the formation energy of native defects. Our calculations predict that all of the TM additives, in either interstitial or substitutional configurations, may cause such a shift in the Fermi level and thus increase the concentration of the hydrogen vacancies that govern hydrogen diffusion. Our proposed mechanism explains the experimentally observed enhancement in the rate of dehydrogenation of MgH$_2$ upon addition of TM impurities.   

Low-Energy Polymeric Phases of Alanates

Low-energy structures of alanates are currently known to be described by patterns of isolated, nearly ideal tetrahedral [AlH$_4$] anions and metal cations. We discover that the novel polymeric motif recently proposed for LiAlH$_4$ plays a dominant role in a series of alanates, including LiAlH$_4$, NaAlH$_4$, KAlH$_4$, Mg(AlH$_4$)$_2$, Ca(AlH$_4$)$_2$ and Sr(AlH$_4$)$_2$. In particular, most of the low-energy structures discovered for the whole series are characterized by networks of corner-sharing [AlH$_6$] octahedra, forming wires and/or planes throughout the materials. Finally, for Mg(AlH$_4$)$_2$ and Sr(AlH$_4$)$_2$, we identify two polymeric phases to be lowest in energy at low temperatures.   

Stability of transition metals on the Mg-terminated $MgB_2$ (0001) surface and their effects on hydrogen dissociation

The re-hydrogenation of $MgB_2$ is a critical step in the reversibility of several key hydrogen storage reactions. Two main activated processes affect the kinetics of hydrogen absorption by $MgB_2$: the dissociation of the $H_2$ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processes need to have a low barrier. Using first-principles calculations, we investigate the dissociation of $H_2$ on the Mg-terminated $MgB_2$ (0001) surfaces. We investigate both ideal surfaces as well as surfaces with vacancies, and transition-metal-dopants (TM=Sc$\sim$Zn,Y$\sim$Cd,Au,Pt). Our calculations show that the late TMs more favorably substitute for the Mg atoms in the outermost layer of the Mg-terminated surface, rather than for those in the sub-layers. We find the dissociation barrier for $H_2$ on the clean Mg-terminated $MgB_2$ (0001) surface is 0.46eV. The TM dopants have only a small effect on dissociation barrier when they are incorporated into the sub-layers. However, when doped in the outermost layer, we find examples of dopants that significantly decrease the activation barrier for the dissociation of $H_2$. We also investigate the diffusivity of H in $MgB_2$ and find strong anisotropy in the diffusion pathways.   

Adsorbed Hydrogen Film Densities and Thicknesses Determined from Low-Temperature Hydrogen Sorption Experiments

Hydrogen storage through physisorption has shown tremendous promise. Advancement of our understanding about hydrogen behavior in confined pores can lead to a development of new storage materials. For example, isosteric heat is used to determine the quality of a sorbent. Yet, Clausius-Clapeyron isosteric heat calculations are typically performed on excess adsorption, which leads to unphysical results. Absolute adsorption should be used for these calculations. To determine absolute adsorption from excess adsorption, the volume of the adsorbed film is needed. We have built a Sievert type instrument capable of temperatures from 10 K to 300 K and pressures up to 200 bar. Using this instrument to measure low temperature ($<$ 77 K) and high pressure ($>$ 100 bar) isotherms, experimental film density and volume have been determined from the linear decrease in excess H$_{2}$ as a function of bulk gas density. Additionally, some materials have shown H$_{2}$ uptakes higher than what their surface area predicts. One hypothesis is N$_{2}$, the standard gas to determine surface areas, is sterically forbidden to go into pores that H$_{2}$ can. Sub-critical H$_{2}$ isotherms have been measured to determine surface area available to the H$_{2}$ and comparisons are made to N$_{2}$ surface area.   

Thermodynamics, kinetics, and catalytic effect of dehydrogenation from MgH2 stepped surfaces and nanocluster: a DFT study

We detail the results of a Density Functional Theory (DFT) based study of hydrogen desorption, including thermodynamics and kinetics with(out) catalytic dopants, on stepped (110) rutile and nanocluster MgH$_{2}$. We investigate competing configurations (optimal surface and nanoparticle configurations) using simulated annealing with additional converged results at 0 K, necessary for finding the low-energy, doped MgH$_{2}$ nanostructures. Thermodynamics of hydrogen desorption from unique dopant sites will be shown, as well as activation energies using the Nudged Elastic Band algorithm. To compare to experiment, both stepped structures and nanoclusters are required to understanding and predict the effects of ball milling. We demonstrate how these model systems relate to the intermediary sized structures typically seen in ball milling experiments.   

Measurements of Increased Enthalpies of Adsorption for Boron-Doped Activated Carbons

Boron-doping of activated carbons has been shown to increase the enthalpies of adsorption for hydrogen as compared to their respective undoped precursors ($>$10kJ/mol compared to ca. 5kJ/mol). This has brought significant interest to boron-doped carbons for their potential to improve hydrogen storage. Boron-doped activated carbons have been produced using a process involving the deposition of decaborane (B$_{10}$H$_{14}$) and high-temperature annealing resulting in boron contents up to 15\%. In this talk, we will present a systematic study of the effect that boron content has on the samples' structure, hydrogen sorption, and surface chemistry. Measurements have shown a significant increase in the areal hydrogen excess adsorption and binding energy. Experimental enthalpies of adsorption will be presented for comparison to theoretical predictions. Additionally, samples have been characterized by thermal gravimetric analysis, gas chromatography-mass spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. TGA and GC-MS results investigated the decomposition of the decaborane in the carbon. Boron-carbon bonds are shown in the FTIR and XPS spectra, indicating that boron has been incorporated into the carbon matrix.   

Bulk Diffusion via a ``kick-out'' method for Lithium in the decomposition reaction LiAlH4/Li3AlH6

In the pursuit to find a practical system for hydrogen storage, complex metal hydrides have long been considered as viable candidates due to their high hydrogen content. However, some of the challenges faced with these types of systems are poor thermodynamics or kinetics. The underlying mechanisms, and their limiting processes, for the decomposition of these materials need to be understood. From experimental work on the decomposition of hydrogen storage materials, it has been suggested that bulk diffusion of metal species is the bottleneck for hydrogen release. In this work is the dehydrogenation we investigated the system $ LiAlH_4 \longrightarrow LiAlH_6 $ with favorable hydrogen release (5.3 wt \%), at moderate temperatures. Using first-principles density functional theory we found the defects facilitating mass transport by calculating individual formation energies, highest concentrations, and activation barriers for defect mobility. The mass transport of Lithium is found to be mediated by a ``kick-out'' mechanism. The results are used to further our understanding of the fundamental mechanism of mass transport and evaluate the possibility of kinetics as the limiting process in this reaction.   

Gold Nanoparticle Enhancement for Polymer Electrolyte Membrane (PEM) Fuel Cell

PEM fuel cell is one of the most promising future alternative energy sources. However, its relatively low power output has prevented it from many practical applications. Marvrikakis et al have predicted that gold nanoparticles that are platelet shaped andhave direct contact to the substrate to be the perfect catalysts. In our experiment, hydrophobic, thiol-functionalized gold nanoparticles were synthesized through two-phase method developed by Brust et al. When particle solution is spread at the air water interface, EXAFS spectroscopy indicate that some of the gold atoms are removed, as the water displaces the hydrophobic thiol chains from the particle surface, resulting in platelet shaped particles. Furthermore, after these nanoparticles are spread on the surface of water in a Langmuir-Blodgett trough where surface pressure can be applied to compress them, they form LB film consisting of one or more monolayers. This LB film can then be deposited onto a solid surface, such as the Nafion membrane where the particle surface can make direct contact with electrodes and take effect. We also find that there is an optimal surface pressure for forming gold nanoparticles monolayer to achieve the highest enhancement of output power.   

Density Functional Theory Study of Oxygen Reduction Reaction Mechanism on Pt$_3$Ti(111) Surface

Density functional theory (DFT) calculations are performed to explain the ORR mechanism on Pt segregated Pt$_{3}$Ti(111) surface. The possible ORR mechanism is elucidated by calculating the activation energies of all ORR elementary reaction steps. Our preliminary results predict that the ORR proceeds via a H$_{2}$O$_{2}$ dissociation mechanism with coverage dependent kinetics. At high coverage, the rate determining step (RDS) is protonation of adsorbed O$_{2}$ to form OOH. The energy barrier for this process is 0.20 eV which is lower than the energy barrier for RDS on pure Pt(111) surface. These findings suggest that modified PtTi(111) surface has better ORR activity in comparison to pure Pt(111) surface. Furthermore, we have studied the corrosion behavior of Pt$_{3}$Ti(111) surface by evaluating the electrochemical potential shift for clean and oxygenated surface. The computations predict enhanced stability of Pt$_{3}$Ti(111) surface against surface Pt dissolution in comparison to Pt dissolution from pure Pt(111) surface.   

Neutron Scattering Studies of Destabilized Lithium Borohydride

One of the most promising materials for hydrogen storage is lithium borohydride, LiBH$_{\mathrm{4}}$, due to its high hydrogen mass fraction. However, applications require destabilization of the material in order to reduce the temperature and pressure required for hydrogen cycling. One possible avenue for destabilization has been via the use of mixed crystals, for example, LiI and LiBH$_{\mathrm{4}}$, in which the relatively large iodide anion expands the crystal lattice of bulk LiBH$_{\mathrm{4}}$. Here we present neutron scattering results comparing BH$_{\mathrm{4}}^{\mathrm{-}}$ anion reorientational dynamics for bulk LiBH$_{\mathrm{4}}$ and the destabilized LiI-LiBH$_{\mathrm{4}}$ system. Quasielastic neutron scattering spectroscopy shows that at temperatures below room temperature, the reorientational dynamics for hexagonal LiI-LiBH$_{\mathrm{4}}$ is very similar to that of the high-temperature (380 K and above) hexagonal phase of LiBH$_{\mathrm{4}}$ instead of its low-temperature orthorhombic phase, which exhibits different dynamics. This is consistent with the behavior found using NMR spectroscopy.   

Predicting hydrogen and methane adsorption in carbon nanopores for energy storage

There are increasing demands for alternate fuels for transportation, which requires safe, high energy density, lightweight storage materials. Experimental measurements and theoretical predictions show relatively low hydrogen storage capacities in various porous materials, limiting hydrogen as a viable alternative for automobiles. In this work, we use a continuum model based on van der Waals density functional (vdW-DF) calculations to elucidate the role that long-range interactions play in the hydrogen adsorption properties of model slit nanopores in carbon. The proper treatment of long-range interactions gives an optimal pore size for hydrogen storage of 8-9 {\AA} (larger than previously predicted). Remarkably, we find a peak hydrogen density close to that of liquid H$_{2}$ at ambient temperatures, in agreement with recent experimental results on pore-size dependent adsorption in nanoporous carbon. We then show that such nanopores would be better suited to storing methane, possibly providing an alternative to fill the gap between the capacity required by DOE goals and that attainable with current hydrogen storage technology.   

Session M39: Drops, Bubbles, and Interfacial Fluid Mechanics I

Condensed droplet jumping: Capillary to inertial energy transfer

When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump from the surface due to the release of excess surface energy. This behavior has been shown to follow a simple inertial-capillary scaling. However, questions remain regarding the nature of the energy conversion process linking the excess surface energy of the system before coalescence and the kinetic energy of the jumping droplet. Furthermore, the primary energy dissipation mechanisms limiting this jumping behavior remain relatively unexplored. In this work, we present new experimental data from a two-camera setup capturing the trajectory of jumping droplets on nanostructured surfaces with a characteristic surface roughness length scale on the order of 10 nm. Coupled with a model developed to capture the main details of the bridging flow during coalescence, our findings suggest that: 1. the excess surface energy available for jumping is a fraction of that suggested by simple scaling due to incomplete energy transfer, 2. internal viscous dissipation is not a limiting factor on the jumping process at droplet sizes on the order of 10 $\mu$m and 3. jumping performance is strongly affected by forces associated with the external flow and fields around the droplet. This work suggests bounds on the heat transfer performance of superhydrophobic condensation surfaces.   

How does an air film evolve into a bubble during drop impact?

When a liquid drop impacts on a solid substrate, a tiny air film is generally entrapped between the drop and the substrate and eventually evolves into a bubble by surface energy minimization. We investigated how air evolves into a bubble during drop impact using ultrafast x-ray phase-contrast imaging that enables us to track the detailed morphological changes of air with high temporal and spatial resolutions. We found that the evolution takes place through complicated three stages: inertial retraction of the air film, contraction of the top air surface into a toroidal bubble, and pinch-off of a daughter droplet inside the bubble. The collapse and the pinch-off can be explained by energy convergence that is associated with Ohnesorge number (Oh) regarding capillary waves and viscous damping. We measured a critical Oh number, Oh* $\sim$ 0.026 $\pm$ 0.003, above which the generation of the daughter droplet is suppressed. Interestingly we found that the bubble is detached favorably from wettable surfaces, which suggests a feasible way to eliminate bubbles for many applications by controlling surface wettability. The threshold angle for bubble detachment was measured as $\sim$ 40 $\pm$ 5$^{\circ}$ for water, which agrees with a geometrical estimation.   

Swirls and splashes: air vortices created by drop impact

A drop impacting a solid surface with sufficient velocity will splash and emit many small droplets. While liquid and substrate properties are clearly important for determining the splashing threshold, it has been shown that removing the ambient air suppresses splashing completely [1]. However, the mechanism underlying how the surrounding gas affects splashing remains unknown. As has been recently shown, there is no air beneath the liquid that could cause the splash [2] -- thus where does the air matter? We use modified Schlieren optics combined with high-speed video imaging to visualize the air vortices created by the rapid spreading of the drop after it hit the substrate. In the first moments after impact, these vortices remain bound to the spreading drop, creating a low-pressure zone that travels with the advancing lamella. At a later time, after the occurrence of the splash, the vortices detach from the drop. We discuss possible connections between the forces generated by the vortices on the liquid lamella and the initiation of a splash. [1] L. Xu, W. W. Zhang and S. R. Nagel, Phys. Rev. Lett. 94, 184505 (2005) [2] M. M. Driscoll and S. R. Nagel, Phys. Rev. Lett. 107, 154502 (2011)   

Stability of electrically charged toroidal droplets in a viscous liquid.

Droplets and bubbles are spherical due to surface tension. As a result, making non spherical droplets and understanding their evolution is a challenge. Nevertheless, we were able to develop a method to generate toroidal droplets in a viscous liquid and study their stability. Recently, we have extended this method to generate charged toroidal droplets suspended in an electrically insulating and highly viscous liquid, and have studied the evolution of these droplets subject to constant charge or constant voltage constraints. In this talk I will be presenting the initial results on the stability of charged toroidal droplets.   

Clapping wet hands: dynamics of a fluid curtain

Droplets splash around when a fluid volume is quickly compressed. This has been observed during common activities such as kids clapping with wet hands. The underlying mechanism involves a resting fluid volume being compressed vertically between two objects. This compression causes the fluid volume to be ejected radially, thereby generating fluid ligaments and droplets at a high speed. In this study, we designed and performed experiments to observe the process of ligament and drop formation while a fluid is squeezed. A thicker rim at the outer edge forms and moves after the squeezing, and then becomes unstable and breaks into smaller drops. We compared experimental measurements with theoretical models over three different stages; early squeezing, intermediate ejection, and later break-up of the fluid. We found that drop spacing set by the initial capillary instability does not change in the course of rim expansion; consequently final ejected droplets are very sparse compared to the size of the rim.   

Electric Charging Effects on Condensed Droplet Jumping

When condensed droplets coalesce on a superhydrophobic surface, the resulting droplet can jump due to the conversion of surface energy into kinetic energy. This frequent out-of-plane droplet jumping has the potential to enhance condensation heat transfer. Furthermore, for more than a century, researchers have shown that droplet-surface interactions can be dominated by electrostatic charge buildup. In this work, we studied droplet jumping dynamics on nanostructured copper oxide and carbon nanotube surfaces coated with tri-chloro silane and PFDA hydrophobic coatings, respectively. Through analysis of droplet trajectories and terminal velocities under various electric fields (0 -- 50 V/cm), we show that condensation on these surfaces having both conducting and insulating substrates results in a buildup of positive surface charge (H$^{\mathrm{+}})$ due to dissociated water ion adsorption on the superhydrophobic coating. Consequently, an accumulation of the opposite charge (OH$^{\mathrm{-}})$ occurs on the condensing droplet interface, which creates an attractive force between the jumping droplet and the condensing surface. Using this knowledge, we demonstrate a novel condensation mechanism whereby an external electric field is used to oppose the droplet-surface attraction, further enhancing the coalescing droplet jumping frequency and overall surface heat transfer.   

Dynamics of a Disturbed Sessile Drop Measured by Atomic Force Microscopy

A new method for studying the dynamics of a sessile drop by atomic force microscopy (AFM) is demonstrated. A hydrophobic microsphere (radius, r $\sim $ 20 - 30 $\mu $m) attached to an AFM cantilever is brought into contact with a sessile water drop. Immediately after the initial rise of the meniscus, the microsphere oscillates about a fixed average position while partially immersed in the liquid. The small (\textless\ 100 nm) oscillations of the interface are readily measured with AFM. The oscillations correspond to the resonance oscillation of the entire droplet. Although the microsphere volume is 6 orders of magnitude smaller than the drop, it excites the normal resonance modes of the liquid interface. Resonance oscillation frequencies were measured for drop volumes between 5 and 200 $\mu $L. The results for the two lowest normal modes are quantitatively consistent with continuum calculations for the natural frequency of hemispherical drops with no adjustable parameters. The method may enable sensitive measurements of volume, surface tension, and viscosity of small drops.   

How leaves survive falling raindrops

Plant surfaces found in nature often exhibit hydrophobic or hydrophilic wetting properties; a particular example is the surface of leaves. Most leaves are compliant enough to survive while being impacted by rain droplets. Here, we investigate this leaf-drop system exhibiting a unique system of coupled elasticity and drop dynamics. By replacing the leaf with a thin piezoelectric cantilever beam, we further measure and harvest this drop kinetic energy as a workable model for an energy-harvester from rain drops.   

The Vibrating Vapor Layer Beneath a Leidenfrost Drop

The levitation of a liquid drop above a hot surface is known as the Leidenfrost effect. Due to strong evaporation, a vapor layer forms beneath the drop that both levitates and thermally insulates the liquid, resulting in extremely long drop life times. The geometry of this vapor layer has been characterized using high-speed laser-light interference imaging [1], which showed spatial oscillations of the interface. Here we report the evolution of these oscillations using an algorithm we developed for identifying the interference fringes. From these fringes we extract the relative height profile of the vapor layer. We track the time evolution of the spatial-fluctuations and measure the absolute change in the average height of the drop over a time scale of seconds. Large, transient, azimuthal deformations to the bottom of the drop are correlated with the rapid escape of vapor and a change in height above the surface. We also observe and characterize a range of metastable star-like oscillations in the shape. \newline \newline [1] Burton et al., PRL 109, 074301 (2012).   

Coalescence of Two Drops Surrounded by an Outer Fluid

When two liquid drops make contact, a liquid bridge forms and then rapidly expands due to surface-tension forces that are divergent at the point where the drops first touch. This nonlinear process has received a lot of recent attention, especially for two liquid drops coalescing in vacuum or air. However, little is known about how the surrounding fluid influences the singularity when the two drops are surrounded by an external fluid with significant density or dynamic viscosity. We use a combination of high-speed imaging and an ultrafast electrical method to study coalescence in this regime. We find that even if the outer fluid is over 10 times more viscous than the fluid within the drops, the coalescence speed need not be affected, even near the singularity. In order to understand the nature of the flows in the surrounding fluid, we also study the limiting case of air bubbles coalescing inside a very viscous external liquid.   

Measurement of Bubble Size Distribution Based on Acoustic Propagation in Bubbly Medium

Acoustic properties are strongly affected by bubble size distribution in a bubbly medium. Measurement of the acoustic transmission becomes increasingly difficulty as the void fraction of the bubbly medium increases due to strong attenuation, while acoustic reflection can be measured more easily with increasing void fraction. The \textsc{ABS Acoustic Bubble Spectrometer}$^{\mathrm{\mbox{\textregistered }\copyright }}$, an instrument for bubble size measurement that is under development tries to take full advantage of the properties of acoustic propagation in bubbly media to extract bubble size distribution. Properties of both acoustic transmission and reflection in the bubbly medium from a range of short single-frequency bursts of acoustic waves at different frequencies are measured in an effort to deduce the bubble size distribution. With the combination of both acoustic transmission and reflection, assisted with validations from photography, the \textsc{ABS Acoustic Bubble Spectrometer}$^{\mathrm{\mbox{\textregistered }\copyright }}$ has the potential to measure bubble size distributions in a wider void fraction range.   

Experimental and Numerical Investigation of Pressure Wave Attenuation due to Bubbly Layers

In this work, the effects of dispersed microbubbles on a steep pressure wave and its attenuation are investigated both numerically and experimentally. Numerical simulations were carried out using a compressible Euler equation solver, where the liquid-gas mixture was modeled using direct numerical simulations involving discrete deforming bubbles. To reduce computational costs a 1D configuration is used and the bubbles are assumed distributed in layers and the initial pressure profile is selected similar to that of a one-dimensional shock tube problem. Experimentally, the pressure pulse was generated using a submerged spark electric discharge, which generates a large vapor bubble, while the microbubbles in the bubbly layer are generated using electrolysis. High speed movies were recorded in tandem with high fidelity pressure measurements. The dependence of pressure wave attenuation on the bubble radii, the void fraction, and the bubbly layer thickness were parametrically studied. It has been found that the pressure wave attenuation can be seen as due to waves reflecting and dispersing in the inter-bubble regions, with the energy absorbed by bubble volume oscillations and re-radiation. Layer thickness and small bubble sizes were also seen as having a strong effect on the attenuation with enhanced attenuation as the bubble size is reduced for the same void fraction.   

Bubble Augmented Propulsor Mixture Flow Simulation near Choked Flow Condition

The concept of waterjet thrust augmentation through bubble injection has been the subject of many patents and publications over the past several decades, and computational and experimental evidences of the augmentation of the jet thrust through bubble growth in the jet stream have been reported. Through our experimental studies, we have demonstrated net thrust augmentation as high as 70{\%}for air volume fractions as high as 50{\%}. However, in order to enable practical designs, an adequately validated modeling tool is required. In our previous numerical studies, we developed and validated a numerical code to simulate and predict the performance of a two-phase flow water jet propulsion system for low void fractions. In the present work, we extend the numerical method to handle higher void fractions to enable simulations for the high thrust augmentation conditions. At high void fractions, the speed of sound in the bubbly mixture decreases substantially and could be as low as 20 m/s, and the mixture velocity can approach the speed of sound in the medium. In this numerical study, we extend our numerical model, which is based on the two-way coupling between the mixture flow field and Lagrangian tracking of a large number of bubbles, to accommodate compressible flow regimes. Numerical methods used and the validation studies for various flow conditions in the bubble augmented propulsor will be presented.   

Dynamics of a Cylindrical Bubble between Two Parallel Plates for Biomedical Applications

Microbubbles have been shown to produce directional and targeted membrane poration of individual cells in microfluidic systems, which could be of use in ultrasound-mediated drug and gene delivery. To study and understand the mechanisms at play in such interactions, a full three- dimensional Boundary Element Method (BEM) has been developed to describe complex bubble deformations, jet formation, and bubble splitting. The present work aims at providing analytical validation for the three-dimensional BEM code, \textsc{3DynaFS}$^{\mathrm{\copyright }}$, when the dynamics of a bubble between two parallel plates is studied. The analytical equations of a cylindrical (2-D) bubble between two flat plates were derived without accounting for any shape deformation. Comparisons between the analytical model and the numerical model were carried out in scenarios where the shape of an expanding/collapsing bubble between two parallel plates is nearly cylindrical (large maximum equivalent bubble radius to plate gap ratio). Interestingly, both the analytical and the numerical methods predict a strong dependence of the bubble period on the plate size.   

Universality Results for Multi-phase Hele-Shaw Flows

Saffman-Taylor instability is a well known viscosity driven instability of an interface separating two immiscible fluids. We study linear stability of displacement processes in a Hele-Shaw cell involving an arbitrary number of immiscible fluid phases. This is a problem involving many interfaces. Universal stability results have been obtained for this multi-phase immiscible flow in the sense that the results hold for arbitrary number of interfaces. These stability results have been applied to design displacement processes that are considerably less unstable than the pure Saffman-Taylor case. In particular, we derive universal formula which gives specific values of the viscosities of the fluid layers corresponding to smallest unstable band. Other similar universal results will also be presented. The talk is based on the following paper.\\[4pt] [1] Prabir Daripa and Xueru Ding, ``Universal Stability Properties for Multi-Layer Hele-Shaw Flows and Application to Instability Control,'' {\it SIAM Journal of Applied Mathematics}, Vol 72, No. 5, pp. 1667-1685, 2012.   

Session M40: Surfaces, Interfaces, and Thin Film Reactions: Kinetics & Dynamics

Effects of Plasmon Excitation on Photocatalytic Activity of Ag/TiO$_{2}$ and Au/TiO$_{2}$ nanocomposites

Model composite photocatalysts consisting of undoped TiO$_{2}$ films and optically active Ag or Au nanoparticles (NP) were prepared and examined in order to address the role of plasmon excitation in their performance. The particles were either in direct contact or isolated by thin SiO$_{2}$ layer from TiO$_{2}$. We found, as measured for the reactions of methanol and ethylene oxidation in two different photoreactors, that composites show always enhanced (up to x100) activity compared to pure TiO$_{2}$. Interfacial charge transfer between TiO$_{2}$ and NPs plays major role for the enhancement. Plasmonic near-, far-field and thermal effects are present but do not dominate.   

In-situ coherent x-ray scattering from Ag (001) and Ag (111) surfaces in vacuum and gas-phase environments

We have been able to obtain X-ray photon correlation spectroscopy (XPCS) quality data from the Ag (001) and Ag (111) surfaces at two different locations along the specular scattering rod. We observe dynamic behavior related to temperature and gas-phase composition. We will present the methods of the XPCS analysis routines, as they have been adapted to this specific system, and the preliminary results for the dynamics, such as step edge motion, island growth, and surface phase transitions, of the Ag surface features in these different conditions. These dynamics are also q dependent and vary from slow at low q, to faster dynamics at positions near the Ag (001) anti-Bragg scattering position where the experimental sensitivity is sufficient to detect changes at a monolayer level. This indicates that the dynamics involved are occurring right at the surface and do not involve multiple layers. These results will then be compared to our recent similar measurements on the Au (001) surface [1].\\[4pt] [1] M.S. Pierce, V. Komanicky, A. Barbour, D.C. Hennessy, A. Sandy, and H. You, Physical Review B 86, 085410 (2012).   

Inhibition of Hydrogen Absorption in Pd by the Formation of a Pd-Ru Surface Alloy

Hydrogen absorption by palladium has been studied for decades due to the significant importance in a number of applications like production and storage of hydrogen and hydrogen sensors. Alloying Pd with just a 4{\%} of Ru drastically reduces the absorption properties of the Pd. The fcc crystal structure is preserved but the lattice constant is reduced slightly. In order to understand this phenomenon, we used three samples: a Pd foil, a Pd-Ru(4{\%}) alloy foil, and a Pd foil with a Pd-Ru surface alloy. The surface alloy was made evaporating 8 nm of Ru using an e-beam evaporation technique on top of Pd, followed with a heating the sample up to 700 $^{\circ}$C in a high vacuum system. We studied the changes in absorption properties of these samples using Thermal Program Desorption (TPD), resistance changes and grazing incidence X-ray Diffraction (GID).   

Pd/Ru surface alloys -- Creating a ``noble'' surface from reactive elements

We have studied the growth and reactivity of ruthenium thin films on palladium (111) substrates. To obtain smooth and well-ordered film surfaces, the films were annealed to 600 $^{\circ}$C. The surface structure, morphology, and chemical composition were investigated with LEED, STM, and AES. The experiments showed that annealed Ru film surfaces contain large concentrations of Pd. The reactivity of this alloy surface towards oxygen was then studied in oxygen gas adsorption experiments at room temperature, and compared to the oxidative properties of bulk Ru and bulk Pd. The surface alloy of the film turns out to be quite inert to oxygen adsorption at room temperature. STM experiments of oxygen adsorption at 112 K reveal that oxygen does adsorb at low temperature but it readily desorbs above 200 K. This surprise finding of a ``noble'' Pd/Ru surface alloy provides an interesting contrast to the surfaces of bulk ruthenium and palladium, which oxidize easily at room temperature. Research supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division   

Solid-State Diffusional Mixing in Cu Core/Ni Shell Nanoparticles

Cu core/Ni shell nanoparticles have been prepared in a polyol process using ethylene glycol as the solvent /reducing agent solution and Cu and Ni acetates as the metal sources. The more positive reduction potential of Cu(II) relative to Ni(II) lead to the formation of Cu core/Ni shell nanoparticles. The structural evolution of these core/shell diffusion couples was studied by high temperature x-ray diffraction measurements. Between room temperature and 350 $^{\circ}$C, the evolution in the diffraction pattern was only due to lattice expansion. At higher temperatures, the elemental Cu and Ni diffraction peaks began to merge until, at a temperature of 600 $^{\circ}$C only a single set of diffraction peaks remained, indicating the formation of a single homogeneous Cu-Ni alloy. These diffraction patterns have been decomposed into a set of 11 individual subpeaks corresponding to 9 intermediate Cu-Ni compositions in addition to subpeaks corresponding to pure Cu and Ni. The angular positions of each subpeak were fixed to the values appropriate for their composition and the best fit peak areas determined. These data were then used to reconstruct the radial composition profiles of the diffusion couples as a function of the reaction temperature and time.   

Investigation of Fe/CuO Interface by X-ray Photoelectron Spectroscopy

The Fe/CuO interfaces have been investigated by x-ray photoelectron spectroscopy. Thin films of iron were deposited on copper oxide substrates at room temperature. The spectral data show considerable reactivity at the interfaces. The spectral data have been compared with those of the oxidized iron and confirms the formation of the iron oxide at the interface. The interface is found to consist of a mixture of iron oxide and elemental copper. Presence of unreacted iron near the interface has been observed for thicknesses equal to or greater than 0.9 nm of the iron overlayer. The interface was also prepared by depositing 2.0 nm of iron on the copper oxide substrate under two different conditions. In one, the substrate temperature was kept constant during the deposition of the iron overlayer. In the other, post deposition annealing of the sample was performed. The iron overlayer was observed to be completely oxidized at the sample temperature of 450 $^{\circ}$C and the oxidation is independent of the processing conditions. The amount of elemental iron and iron oxide in the samples has been estimated by modeling the spectrum using the spectra of elemental iron and pure iron oxide. The investigation provides a new method of preparing sub-nano-oxide films of iron.   

Coherent X-ray Scattering Experiments of Pt (001) Surface Dynamics near Roughening Transition

We will present the results of a series of coherent x-ray scattering temperature dependent experiments from Pt (001) in high vacuum. The resulting speckled diffraction patterns are analyzed with x-ray photon correlation spectroscopy. We find that the hexagonally reconstructed Pt (001) surface exhibits orientational dynamics below 1640 K and a critical behavior as $T$ increases to $T_{\mathrm{R}} =$ 1834 K, near the roughening transition as proposed by Abernathy, et al. [Phys. Rev. Lett. \textbf{69}, 941 (1992)]. The inverse autocorrelation time constant $\tau ^{-1}$ of the surface diverges as $T$ approaches $T_{R}$. The average integrated intensity remains constant below $T_{\mathrm{R}}$ but drops suddenly over a narrow temperature range, indicating abrupt lifting of the hexagonal reconstruction with the roughening transition. This behavior is compared to that of Au (001), for which $\tau^{-1}$ approaches a finite value as the reconstruction lifts gradually over a wide temperature range.   

A DFT Study of the Interaction of Monometallic Pd$_{\mathrm{n}}$/Pt$_{\mathrm{n}}$ (n$=$1, 9) Clusters with $\gamma $-Al$_{2}$O$_{3}$(100) Surfaces

The reduction of carbon monoxide and hydrocarbon emissions in advanced low temperature combustion engines has become more difficult for the advanced combustion systems in transportation sector. Exploration of effect of interface formation on the electronic properties of the existing platinum group materials may provide insight for the new material development that rivals platinum. In order to address the effects of the interface on the electronic properties of small Pd$_{\mathrm{n}}$ and Pt$_{\mathrm{n}}$ clusters (n$=$1-9) with a $\gamma $-Al$_{2}$O$_{3}$(100) support we have performed density-functional-theory (DFT) computations. The preliminary results suggest that the most favorable Pd$_{9}$ binding geometry is characterized by four Pd atoms binding to both Al and O surface atoms. The average Pd-O bond length across the interface is $\sim$ 2.2 {\AA}, corroborating the formation of bonds. The preliminary analysis of the electronic density of states shows that the main electronic modifications occur at the Fermi energy, leading to an overall metallic behavior. We will discuss cluster size effects on the character of bonding across the interface, its stability, and electronic structure.   

Dynamics of tungsten and cobalt carbonyls on silica surfaces

Metal carbonyl species adsorbed on a substrate are the starting point for the electron beam induced deposition of metallic nanostructures. We employ first principles molecular dynamics simulations to investigate the dynamics of tungsten hexa- and pentacarbonyl as well as cobalt octacarbonyl precursor molecules on fully and partially hydroxylated silica substrates. We find that physisorbed carbonyls are quite mobile on a silica surface saturated with hydroxy groups, moving around half an Angstrom per picosecond. In contrast, chemisorbed ions like [W(CO)$_5$]$^-$ or [Co(CO)$_4$]$^-$ are more stable at room temperature. We determine the vibrational spectra which can provide signatures for experimentally distinguishing the form in which precursors cover a substrate.   

Effects of biaxial strain on diffusivity of low index tungsten surfaces

Detailed knowledge of diffusion behaviors is necessary toward fully understanding of damage of tungsten serving as reactor pressure vessels. Using first-principles calculations, we observed different diffusion scenarios on W(001) and W(110) surfaces with external biaxial strains. Hopping is the major diffusion mechanism on the W(110) surface under all kinds of loadings in the present work. On the other hand, the main mechanism on the W(001) surface transfers between the adatom hopping and the formation and movement of surface crowdions depending on biaxial strains. Our results also indicate high mobile and strong anisotropy of surface crowdions on both surfaces. The microscopic explanation is presented by analyzing the charge density. We have built up the diagram of diffusion on the W(001) surface. This diagram presents that not only the diffusion mechanism, but also the diffusion direction can be modulated by patterns of biaxial strains. These results are important to the future dynamical modeling and simulations. We have further performed kinetic Monte Carlo simulations and observed (1) the modulation of diffusion of single adatom on W(001) surface by strains and (2) the aggregation of multiple adatoms on W(110) surface.   

Determination of shift in electrodic reaction rates due to the presence of stress

An extension of Butler-Volmer formulation is proposed to determine the stress-induced changes in electrodic reaction rates. Gibbs-Duhem equation is used to determine the stress-dependent chemical potential and the corresponding change in the reaction rate. The scope of possible amplification or reduction in the reaction rates due to tensile and compressive stress fields is explored numerically. Though quantitative experimental validation remains to be pursued, behavioral agreement of the extended Butler-Volmer model with some observations made in the field of corrosive dissolution is established. Our numerical results also indicate that in addition to altering the speed of a reaction, a stress field can modify the shape of an anodic dissolution front. The effect of stress-generated surface patterns is also considered. It is well-established that a stress field can create surface patterns due to surface wrinkling or surface diffusion. We determine the possible significance of such patterns on the reaction rate, and identify the factors that may enhance their contribution to electrodic reaction rates.   

The model that takes the Marangoni effect into account for drying process of polymer solution coated on a flat substrate

We have proposed and modified a model of drying process of polymer solution coated on a flat substrate for flat polymer film fabrication supposing resist coating process in photolithography process. And we have clarified dependence of distribution of polymer molecules on a flat substrate on various parameters based on analysis of many numerical simulations of the model. Then we applied the model to thickness control of a thin film after drying through thermal management. Above model consists of two elements. One is vaporization at the gas-liquid interface. The other is the diffusion inside the liquid film on a substrate. The diffusion is divided into two kinds of diffusion, that is, diffusion of solvent with solutes due to gradient of the number density of particles per space and diffusion of diffusion of concentration of solution. Because it is assumed that coated solution film on a flat substrate is very thin and therefore both Rayleigh number and Marangoni number are small enough, it is thought that B\'enard convection or Marangoni convection does not occur and therefore it is sufficient to consider only above-mentioned two kinds of diffusion inside the liquid film. However it is thought that there is some sort of Marangoni effect regardless Marangoni convection does not occur. Therefore, in this study we add the Marangoni effect to the existing model. Then we evaluate effects of the Marangoni effect in the drying process through numerical simulation of the modified model.   

Mesoscopic Aligned and Cu-Coordinated Surface Linear Polymerization at Low Temperature

The on-surface synthesis of covalent organic aggregates and networks has received considerable attention. However, most of the polymerization reactions require high temperatures to overcome the activation barrier. We demonstrate a surface-coordinated linear polymerization, which occurred at 100 K and forms long chain that are well-organized into a ``circuit-board'' pattern on Cu(100) surface. This highly strained 1D conjugated polymer alters greatly the electronic structure compared to unperturbed polymer and it was investigated by electronic and vibrational spectroscopies, as well as \textit{ab initio} calculations. More importantly, the processes of polymerization and depolymerization can be controlled locally at the nanoscale by a using a charged metal tip. This work thus demonstrates the feasibility of accessing and controlling chain-growth polymerization at low temperature that may lead to the bottom-up construction of sophisticated architectures for molecular nano-devices.   

Surface reactivity/stability and hydration of calcium silicate phases

Recent studies on synthetic calcium silicate structures revealed important mechanisms to tune the reactivity of various cement phases. Interaction of water with dicalcium silicate (C2S-belite) and tricalcium silicate (C3S-alite), dominant phases in Portland Cement, are the most important and anticipated reactions. In this work, using first-principles calculations, a fundamental understanding of how water pressure affects the reactivity of C3S and C2S phases is provided. In order to understand the hydration of different phases, as a first step the surface energetics of all lower index orientations are calculated and the stability/reactivity of the surfaces are determined. Taking into account the most and least energetic surfaces of the C3S phase, detailed analyses are carried out in order to understand the induction period in hydration. Surface transformation from highly reactive C3S to low reactive C2S revealed that upon increasing the water pressure, the surface with C2S character becomes energetically more favorable. Reduction of the surface energy is more intense in the case of proton exchange of surface Ca atoms. Our calculations suggest that these processes are the most probable mechanisms underlying the rapid decrease in reactivity in alite hydration.   

Session M41: Theory of Quantum Gases in Low Dimensions

Adiabatic evolution of the Fulde-Ferrell-Larkin-Ovchinnikov state of imbalanced fermionic-atom superfluids in an optical lattice of coupled tubes

We study two-species imbalanced fermionic superfluids in an array of one-dimensional tubes that are coupled via particle tunneling between nearest neighbors. Incorporating the interplay of Cooper pairing, spin imbalance (or magnetization), and single-particle tunneling, we obtain imbalance profiles accompanied with oscillatory pairing reminiscent of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, and show that the magnetization of the system can undergo an incompressible-compressible transition by the tuning of the magnetic field as well as tunneling strength [Phys. Rev. A {\bf 85}, 051607 (2012)]. The system's phase diagram is well described by an effective extended Bose-Hubbard model. In addition, we discuss another viable process of pair tunneling that strongly affects the evolution of the FFLO profiles. With this new element, one can build a model describing the development of signatures characteristic of the incipience of the dimensional crossover and in partial agreement with preliminary experimental data.   

Asymptotic Limit of Momentum Distribution Functions in the Sudden Expansion of a Spin-imbalanced Fermi Gas in One Dimension

We study the sudden expansion of a spin-imbalanced Fermi gas in an optical lattice after quenching the trapping potential to zero [1], described by the attractive Hubbard model. Using time-dependent density matrix renormalization group simulations we demonstrate that the momentum distribution functions (MDFs) of majority and minority fermions become stationary after surprisingly short expansion times. We explain this via a quantum distillation mechanism [2] that results in a spatial separation of excess fermions and pairs, causing Fulde-Ferrell-Larkin-Ovchinnikov correlations to disappear rapidly. We further argue that the asymptotic form of the MDFs is determined by the integrals of motion of this integrable quantum system, namely the rapidities from the Bethe ansatz solution. We discuss the relevance of our results for the observation of Fulde-Ferrell-Larkin-Ovchinnikov correlations in 1D systems, related to recent experiments from Rice University [3].\\[4pt] [1] Bolech et al., Phys. Rev. Lett. 109, 110602 (2012)\\[0pt] [2] Heidrich-Meisner et al., Phys. Rev. A 80, 041603(R) (2009)\\[0pt] [3] Liao et al., Nature 467, 567 (2010)   

Superfluidity of Bosons in Kagome Lattices with Frustration

We consider spinless bosons in a Kagome lattice with nearest-neighbor hopping and on-site interaction, and the sign of hopping is inverted by insetting a $\pi$ flux in each triangle of Kagome lattice so that the lowest single particle band is perfectly flat. We show that in the high density limit, despite of the infinite degeneracy of the single particle ground states, interaction will select out the Bloch state at the K point of Brillouin zone for boson condensation at the lowest temperature. As temperature increases, the single boson superfluid order can be easily destroyed, while an exotic triple-boson paired superfluid order will remain. We establish that this trion superfluid exists in a broad temperature regime until the temperature is increased to the same order of hopping and then the system turns into normal phases. Finally we show that time of flight measurement of momentum distribution and its noise correlation can be used to distinguish these three phases.   

The Higgs amplitude mode in superfluids of Dirac fermions

Motivated by recent developments of cold atom experiments in optical lattices, we study collective modes of atomic Dirac fermions on the two-dimensional honeycomb lattice. The attractive fermion Hubbard model on the honeycomb lattice was found to exhibit the quantum phase transition at half-filling between a semimetal with massless Dirac fermion excitations and a simple s-wave superfluid phase.\footnote[1]{E. Zhao and A. Paramekanti, Phys. Rev. Lett. 97, 230404 (2006).} We calculate collective modes in superfluid phase as well as in normal phase in the vicinity of the quantum critical point within the generalized random phase approximation. We find evidence for a {\it undamped} gapful Higgs amplitude mode below the two-particle continuum, together with a gapless Anderson-Bogoliubov (AB) mode in superfluid phase. As approaching the quantum critical point from the superfluid side, the energy gap of the Higgs mode decreases and eventually the Higgs mode and AB mode become degenerate at the quantum critical point. In the normal phase, we find that these collective modes split into Cooperon and exciton excitations that are particle-particle and particle-hole bound states, respectively. We discuss possibilities of observing these collective modes in optical lattice experiments.   

Quantum phase-manipulation of a two-leg ladder in mixed dimensional Fermonic cold atoms

The recent realization of mixed dimensional cold atoms has attracted intense attentions from both experimentalists and theoreticians. Exotic phases arise due to correlation effects, and the systems can be engineered with quantum phase-tunable parameters. We investigate a two-species Fermi gas: one is confined in a two-leg ladder with on-site interactions, and the other is free in a two dimensional square lattice. By integrating out the two-dimensional gas, a long-range mediated interaction in the ladder is generated due to the on-site interspecies interactions. Using the renormalization group method, we show that the mediated interactions enhance the instability of charge density waves, and can be controlled by the filling in the two-dimensional gas. Parameterizing the phase diagrams with different quantities, we discuss the possible quantum phase-manipulation of a two-leg ladder in mixed dimensional Fermionic cold atoms.   

Quantum phases of cold fermions in mixed dimensions: 2D layer embedded in 3D gas

Recently two-species cold atoms in mixed dimensions have been realized experimentally, triggering lots of studies to explore new exotic phases in these systems. Inspired by this, we study the phase diagram of a mixed Fermi system, in which one species is confined in a two-dimensional square or triangular lattice with a correlation effect, and the other is free to move in three-dimensional space. By integrating out the free three-dimensional fermions, a long-range mediated interaction is generated in the two-dimensional lattice due to the interspecies interaction. We employ a functional renormalization group method to discover the possible phases, which may shed light to new exotic quantum phases created in ultracold atoms systems.   

Exact Self-Consistent Condensates in (Imbalanced) quasi-1D Superfluid Fermi Gases

Borrowing some techniques from high-energy physics, and in particular from the study of Nambu-Jona-Lasinio model in 1+1 dimensions [1,2], we present an analytic method to approach Eilenberger equation and the associated Bogoliubov-de Gennes equation for quasi-1D fermionic gases. The problem of finding self-consistent inhomogeneous condensates is reduced to solving a certain class of nonlinear Schr\"odinger equations, whose most general solitonic solution is indeed available. Previously known solutions can be retrieved by taking appropriate limits in the parameters. The applicability of the method extends to ring geometry and to population imbalanced Fermi gases [3,4]. In particular we show exactly that fermionic zero-modes are robust against imbalance. References: \newline [1] G. Basar and G. V. Dunne, Phys. Rev. Lett. 100, 200404 (2008) \newline [2] G. Basar and G. V. Dunne, Phys. Rev. D 78, 065022 (2008) \newline [3] R. Yoshii, S. Tsuchiya, G. Marmorini and M. Nitta, Phys. Rev. B 84, 024503 (2011) \newline [4] R. Yoshii, G. Marmorini and M. Nitta, J. Phys. Soc. Jpn. 81 (2012) 094704   

Tuning the Kosterlitz-Thouless transition to zero temperature in anisotropic boson systems

We study the two-dimensional Bose-Hubbard model with anisotropic hopping. Focusing on the effects of anisotropy on superfluid properties such as the helicity modulus and the normal-to-superfluid [Berezinskii-Kosterlitz-Thouless (BKT)] transition temperature, two different approaches are compared: large-scale quantumMonte Carlo simulations and the self-consistent harmonic approximation (SCHA). For the latter, two different formulations are considered, one applying near the isotropic limit and the other applying in the extremely anisotropic limit. Thus we find that the SCHA provides a reasonable description of superfluid properties of this system provided the appropriate type of formulation is employed. The accuracy of the SCHA in the extremely anisotropic limit, where the BKT transition temperature is tuned to zero (i.e., at a quantum critical point) and therefore quantum fluctuations play a dominant role, is particularly striking.   

Variational Matrix Product Ansatz for Interacting 1D Gases

Shortly after the advent of the density matrix renormalization group (DMRG) method, Ostlund and Rommer [PRL 75, 3537-3540 (1995)] have demonstrated that ground states of one-dimensional lattice systems obtained with the DMRG procedure can be written in terms of products of matrices and, remarkably, that those ground states can be obtained from variational methods without making any reference to DMRG. Since then, a lot of activity ensued and recently there was some additional success in going beyond lattice models and obtaining the ground state properties of interacting bosons in the continuum. We extend those findings and discuss systems of both interacting Bosons and Fermions in one-dimension.   

Revealing the breakdown of spin-charge separation in spin-imbalanced fermions in one dimension using quench dynamics

Recently, spin-imbalanced fermions in one dimension have attracted considerable attention both theoretically and experimentally. This system was successfully simulated using ultracold atoms in optical lattices. The phase diagram was measured and found to be in agreement with exact analytical calculations. It was also established theoretically that the spin-charge separation, an important property of Luttinger liquids, is absent. Low-energy bosonic excitations do not carry spin and charge separately due to the interaction between spin and charge degrees of freedom. Based on our numerical (time-dependent density matrix renormalization group method (t-DMRG)) and analytical calculations (Bethe Ansatz, Bosonization) on the Hubbard model, we propose quench experiments which not only reveal the breakdown of spin-charge separation but also make it possible to study the so called ``string'' bound states in this system.   

Exploration of the Exact Bose-Fermi Mixture Phase Diagram via Quantum Monte Carlo

Following unprecedented success studying Bose and Fermi gases in optical lattices, atomic physicists are becoming increasingly interested in Bose-Fermi mixtures. It has been suggested that mixtures possess a complex phase diagram, containing a number of intriguing phases, including two-particle superfluids, supersolids, and density waves. Nevertheless, much of this phase diagram remains unknown because of algorithmic limitations. In this work, we explore the exact phase diagram of Bose-Fermi mixtures at finite temperatures in the hopes of uncovering Bose-Fermi density waves using a novel Auxiliary Field Quantum Monte Carlo (AFQMC) technique. Our AFQMC method expresses the Bose-Fermi partition function as a determinant that can be sampled to obtain accurate results throughout the phase diagram. Based upon this determinant, we calculate several correlation functions to look for signatures of mixture density waves. For certain system sizes and in certain parameter regimes, we compare and contrast with Exact Diagonalization and Mean Field Theory. Here, we begin this study by focusing on one-dimensional systems; future work will be extended to multidimensional systems were AFQMC is expected to be the only method capable of studying them.   

Finite-size scaling of the chemical potential of bosonic quantum fluids

We study the finite-size scaling of the chemical potential of interacting bosonic quantum fluids using large-scale quantum Monte Carlo calculations. We consider realistic interactions for helium as well as short range repulsive interactions for bosons in one, two and three dimensions at finite temperatures. In one dimension, we compare our results to the scaling predicted by Luttinger liquid theory allowing for the identification of a parametric regime of validity for quantum linear hydrodynamics. In higher dimensions, grand canonical simulations of helium allow for the accurate computation of experimentally relevant quantities such as the chemical potential along the liquid-solid transition line at low temperatures.   

Session M42: Focus Session: Physics of Glasses and Viscous Liquids III

High-dimensional surprises neat the glass and the jamming transitions

The glass problem is notoriously hard and controversial. Even at the mean-field level, there is little agreement about how a fluid turns sluggish while exhibiting but unremarkable structural changes. It is clear, however, that the process involves self-caging, which provides an order parameter for the transition. It is also broadly assumed that this cage should have a Gaussian shape in the mean-field limit. Here we show that this ansatz does not hold, and explore its consequences. Non-Gaussian caging, for instance, persists all the way to the jamming limit of infinitely compressed hard spheres, which affects mechanical stability. We thus obtain new scaling relations, and establish clear mileposts for the emergence of a mean-field theory of jamming.   

Microscopic theories of the structure and glassy dynamics of ultra-dense hard sphere fluids

We construct a new thermodynamically self-consistent integral equation theory (IET) for the equilibrium metastable fluid structure of monodisperse hard spheres that incorporates key features of the jamming transition. A two Yukawa generalized mean spherical IET closure for the direct correlation function tail is employed to model the distinctive short and long range contributions for highly compressed fluids. The exact behavior of the contact value of the radial distribution function (RDF) and isothermal compressibility are enforced, as well as an approximate theory for the RDF contact derivative. Comparison of the theoretical results for the real and Fourier space structure with nonequilibrium jammed simulations reveals many similarities, but also differences as expected. The new structural theory is used as input into the nonlinear Langevin equation (NLE) theory of activated single particle dynamics to study the alpha relaxation time, and good agreement with recent experiments and simulations is found. We demonstrate it is crucial to accurately describe the very high wave vector Fourier space to reliably extract the dynamical predictions of NLE theory, and structural precursors of jamming play an important role in determining entropic barriers.   

Beyond the mode-coupling theory: a perturbative diagrammatic approach

We analyze corrections to the mode-coupling theory of the glass transition, focusing on the self-consistent equation for the non-ergodicity parameter. We use a diagrammatic formulation of the dynamics of interacting Brownian particles\footnote{G. Szamel, J. Chem. Phys. 127, 084515 (2007)}. Our approach builds upon an earlier identification of a divergent contribution to a four-point correlation function\footnote{G. Szamel, Phys. Rev. Lett. 101, 205701 (2008)}. We find that diagrams similar to those generating the divergence of the four-point function lead to divergent corrections to the mode-coupling theory's prediction for the long time limit of the irreducible memory function. We propose and investigate a new equation for the non-ergodicity parameter that self-consistently includes the diagrams leading to the divergent corrections.   

Shapes of dynamically heterogeneous regions in glassy fluids with attractive and repulsive interactions as revealed through anisotropic four-point correlation functions

We investigate the size and anisotropy of dynamically heterogeneous regions in glassy fluids with attractive and repulsive interactions. To this end we simulate a binary Lennard-Jones mixture and its Weeks-Chandler-Andersen truncation. We use a four-point correlation function $G_4(\vec{k},\vec{r};t)$, which depends on the angle between $\vec{k}$ and $\vec{r}$, and its associated structure factor $S_4(\vec{k},\vec{q};t)$, which depends on the angle $\theta$ between $\vec{k}$ and $\vec{q}$, to characterize the size and anisotropy of the dynamically correlated regions. In particular, $G_4(\vec{k},\vec{r};t)$ allows us to explore dynamic heterogeneities at shorter distances. In contrast, to investigate dynamic heterogeneities at longer distances we analyze the small $q$ behavior of $S_4(\vec{k},\vec{q};t)$ and obtain an anisotropic dynamic correlation length $\xi(\theta)$. We explore the dependence of dynamic heterogeneities at shorter and longer distances on the presence of attractive interactions.   

Reversible and Irreversible Behavior of Glass-forming Materials from the Standpoint of Hierarchical Dynamical Facilitation

Using molecular simulation and coarse-grained lattice models, we study the dynamics of glass-forming liquids above and below the glass transition temperature. In the supercooled regime, we study the structure, statistics, and dynamics of excitations responsible for structural relaxation for several atomistic models of glass-formers. Excitations (or soft spots) are detected in terms of persistent particle displacements. At supercooled conditions, we find that excitations are associated with correlated particle motions that are sparse and localized, and the statistics and dynamics of these excitations are facilitated and hierarchical. Excitations at one point in space facilitate the birth and death of excitations at neighboring locations, and space-time excitation structures are microcosms of heterogeneous dynamics at larger scales. Excitation-energy scales grow logarithmically with the characteristic size of the excitation, giving structural-relaxation times that can be predicted quantitatively from dynamics at short time scales. We demonstrate that these same physical principles govern the dynamics of glass-forming systems driven out-of-equilibrium by time-dependent protocols. For a system cooled and re-heated through the glass transition, non-equilibrium response functions, such as heat capacities, are notably asymmetric in time, and the response to melting a glass depends markedly on the cooling protocol by which the glass was formed. We introduce a quantitative description of this behavior based on the East model, with parameters determined from reversible transport data, that agrees well with irreversible differential scanning calorimetry. We find that the observed hysteresis and asymmetric response is a signature of an underlying dynamical transition between equilibrium melts with no trivial spatial correlations and non-equilibrium glasses with correlation lengths that are both large and dependent upon the rate at which the glass is prepared. The correlation length corresponds to the size of amorphous domains bounded by excitations that remain frozen on the observation time scale, thus forming stripes when viewed in space and time. We elucidate properties of the striped phase and show that glasses of this type, traditionally prepared through cooling, can be considered a finite-size realization of the inactive phase formed by the s-ensemble in the space-time thermodynamic limit.   

Dynamical Heterogeneity in Higher Dimensions: Kinetically Constrained Models

We use kinetically constrained models to investigate the dimensional dependence of dynamic heterogeneity in supercooled liquid systems. Higher dimensional generalizations of one dimensional East model and its variation with an embedded probe particle are used as a representative fragile liquid system. We first investigate how the breakdown of the Stokes-Einstein relation changes with the system dimensionality from $d=1$ up to $d=10$. The fractional scaling behavior $D\propto{\tau}^{-\xi}$ are observed, where $D$ and $\xi$ are the diffusion constant of the probe and the relaxation time of the liquid, respectively. The scaling exponent, $\xi$, decreases as the dimensionality increases. The decoupling between persistence and exchange times are also characterized as the dimensionality changes. Time and length scales of the dynamic heterogeneity are analyzed by calculating persistence functions and the dynamic susceptibility. Comparisons are made with respect to recent atomistic MD simulation results.   

Dynamics in a meta-basin and its relation to $\beta$ relaxation in glass-forming liquids

A clear interpretation of the short-time $\beta$ relaxation of glass-forming liquids in terms of dynamics in the potential energy landscape is not yet available. We have studied the relation between dynamics in a meta-basin of the potential energy landscape and $\beta$ relaxation in a well-known glass-forming liquid - the Kob-Andersen binary mixture. Meta-basins are determined from the series of inherent structures obtained by minimizing the potential energy, starting from configurations obtained from a constant-temperature molecular dynamics (MD) simulation. The eigenvalues and eigenvectors of the Hessian matrices of the inherent structures in a meta-basin are then used to calculate various dynamical quantities in the harmonic approximation. We find that the results of the harmonic calculation begin to deviate from those obtained from MD simulations at time scales substantially shorter than the $\beta$ relaxation time corresponding to the plateau in the mean-square displacement versus time plot. The agreement between the results of our analysis of the dynamics in a meta-basin and those of MD simulations is found to extend to longer times when anharmonic effects are included in the analysis. A detailed comparison between these two set of results will be presented.   

Universal Microstructure of Jammed Packings in Higher Dimensions

Jammed packings' mechanical properties depend sensitively on their detailed local structure. We simulate the structure of jammed packings of frictionless spheres over a range of spatial dimensions $d$=3-10 using a variety of preparation protocols for both hard and soft spheres. We provide a complete characterization of the pair correlation close to contact and of the force distribution. We find that even as the density for jamming depends strongly on the packing protocol there nevertheless exist universal scaling relationships that hold true for all jammed packings. These relationships connect the behavior of particles participating in the mechanical structure of the packing and particles that bear no force but are almost in contact.   

Ginzburg criterion for the glass transition

I will discuss the onset of slow relaxation in glassy systems by constructing a static replica field theory approach to the problem. At the mean field level, criticality in the four point correlation functions arises because of the presence of soft modes and I will present an effective replica field theory for these critical fluctuations. At the Gaussian level many physical quantities are obtained: the correlation length, the exponent parameter that controls the Mode-Coupling dynamical exponents for the two-point correlation functions, and the prefactor of the critical part of the four point correlation functions. A one-loop computation allows to identify the region in which the mean field Gaussian approximation is valid. The result is a Ginzburg criterion for the glass transition, which confirms that the upper critical dimension for the glass transition is d$=$8. Finally, I will present numerical results for hard spheres in dimension d ranging from 3 to 9 that support the analytical results.   

Session M43: Focus Session: Protein Misfolding and Aggregation II

In-Vivo Like Studies of the hIAPP Amyloid Precursors Using Dielectric Relaxation Spectroscopy

Recent studies show that the amyloid formation in Type II diabetic disease involves aggregation of monomers of the human islet amyloid polypeptide (hIAPP) into oligomers, protofibrils, and fibrils. Here we present data showing that Dielectric Relaxation Spectroscopy is a very sensitive technique to detect the hIAPP precursors. We measured the dielectric response of amyloidogenic hIAPP and non-amyloidogenic rIAPP as a function of frequency (10$^{-3}$ Hz to 10$^{7}$ Hz), temperature (193K to 283K), and incubation time (0-120 h). To mimic in-vivo like conditions, the proteins were measured in bovine serum albumin. Our results show that the dielectric signal of amyloidogenic hIAPP shifts towards the dielectric signal of the background, as predicted by theoretical calculations. No similar shift is observed for the non-amyloidogenic rIAPP. In addition, the dielectric signal of both the hIAPP and the rIAPP shows two relaxation processes over the measured temperature range. We used two Havrilik-Negami functions plus conductivity to fit the two relaxation processes we determined the relaxation time for both processes and calculated the corresponding activation energies.   

Structure and Thermodynamic Stability of Islet Amyloid Polypeptide Monomers and Small Aggregates

Human islet amyloid polypeptide (hIAPP, also known as human amylin) is associated with the development of type II diabetes. It is known to form amyloid fibrils that are found in pancreatic islets. Pramlintide, a synthetic analog of hIAPP with three proline substitutions, is not amyloidogenic and has been applied in amylin replacement treatments. In this work, we use molecular simulations with advanced sampling techniques to examine the effect of these proline substitutions on hIAPP monomer conformations. We find that all three proline substitutions are required to attenuate the formation of $\beta $-sheets encountered in amylin. Furthermore, we investigate the formation of hIAPP dimers and trimers, and investigate how that process is affected by the presence of various additives. Our simulations show that hIAPP can form a $\beta $-sheet at the N-terminus and the C-terminus independently, in agreement with experimental observations. Our results provide valuable insights into the mechanism of hIAPP early aggregation and the design of fibril formation inhibitors.   

Control the aggregation of model amyloid insulin protein under ac-electric fields

In vitro experiments have been widely used to characterize the misfolding/unfolding pathway characteristic of amylodogenic proteins. Conversion from natively folded amyloidogenic proteins to oligomers via nucleation is the accepted path to fibril formation upon heating over a certain lag time period. In an alternative engineering approach to manipulate and control protein aggregation, we have investigated the aggregation kinetics of insulin, a well-established amyloid model protein, under applied ac-electric fields of varied ac-frequency and voltage at room temperature. Using fluorescence correlation spectroscopy and fluorescence imaging, we have observed that the insulin aggregation can occur at much shortened lag time under applied ac-electric fields, when a critical ac-voltage is exceeded. The strong dependence of lag time on ac-frequency over a narrow range of 500 Hz-5 kHz indicates the effect of ac-electroosmosis on the diffusion controlled process of insulin nucleation. Yet, no difference of conformational structure is detected with insulin under applied ac-fields, suggesting the equivalence of ac-polarization to the conventional thermal activation process for insulin aggregation.   

A physical chemical approach to understanding cellular dysfunction in type II diabetes

The conversion of soluble protein into b-sheet rich amyloid fibers is the hallmark of a number of serious diseases. Precursors for many of these systems (e.g. Ab from Alzheimer's disease) reside in close association with a biological membranes. Membrane bilayers are reported to accelerate the rate of amyloid assembly. Furthermore, membrane permeabilization by amyloidogenic peptides can lead to toxicity. Given the b-sheet rich nature of mature amyloid, it is seemingly paradoxical that many precursors are either intrinsically b-helical, or transiently adopt an a-helical state upon association with membrane. We have investigated these phenomena in islet amyloid polypeptide (IAPP). IAPP is a 37-residue peptide hormone which forms amyloid fibers in individuals with type II diabetes. We report here the discovery of an oligomeric species that arises through stochastic nucleation on membranes, and results in disruption of the lipid bilayer. These species are stable, result in all-or-none leakage, and represent a definable protein/lipid phase that equilibrates over time. To characterize the reaction pathway of assembly, we apply an experimental design that includes ensemble and single particle evaluations \textit{in vitro} and correlate these with quantitative measures of cellular toxicity.   

Amyloid Aggregation and Membrane Disruption by Amyloid Proteins

Amyloidogenesis has been the focus of intense basic and clinical research, as an increasing number of amyloidogenic proteins have been linked to common and incurable degenerative diseases including Alzheimer's, type II diabetes, and Parkinson's. Recent studies suggest that the cell toxicity is mainly due to intermediates generated during the assembly process of amyloid fibers, which have been proposed to attack cells in a variety of ways. Disruption of cell membranes is believed to be one of the key components of amyloid toxicity. However, the mechanism by which this occurs is not fully understood. Our research in this area is focused on the investigation of the early events in the aggregation and membrane disruption of amyloid proteins, Islet amyloid polypeptide protein (IAPP, also known as amylin) and amyloid-beta peptide, on the molecular level. Structural insights into the mechanisms of membrane disruption by these amyloid proteins and the role of membrane components on the membrane disruption will be presented.\\[4pt] References:\\[0pt] [1] Sciacca et al., \textit{Biophys. J.} 2012, \textbf{103}, 702-10.\\[0pt] [2] Sciacca et al., \textit{Biochemistry}. 2012, \textbf{51}, 7676-84\\[0pt] [3] Brender et al., \textit{Acc. Chem. Res.} 2012, \textbf{45}, 454-62.\\[0pt] [4] Nanga et al., \textit{Biochim. Biophys. Acta} 2011, \textbf{1808}, 2337-42.\\[0pt] [5] Brender et al., \textit{Biophys J.} 2011, \textbf{100}, 685-92.   

New technology for 2D IR spectroscopy and its application to protein aggregation and drug binding

We are using 2D IR spectroscopy to study the aggregation and drug inhibition of proteins involved in common human diseases. It is extremely difficult to obtain precise structural information about drug inhibition of amyloid fibrillization, because it is very difficult to apply NMR spectroscopy and x-ray crystallography to these systems. As a result, there are very few molecular level details known about even the simplest inhibitors. We have studied a peptide inhibitor whose sequence was used to design an FDA approved drug, partially because this peptide has never before been observed to aggregate on its own. According to the sequence, we would expect that the C-terminal is responsible for inhibition, but in fact we found that the N-terminal was instead. In fact, we also observed that the complex formed between the inhibitor and amylin caused the inhibitor itself to form amyloid fibers. These surprising results were not previously observed, in part because the prior methods used to study inhibition was not sensitive to the specific structural fold of the fibers.   

Early-Stage Aggregation of Islet Amyloid Polypeptide on Membrane Surfaces Probed by Label-Free Chiral Sum Frequency Generation Spectroscopy

The aggregation of human islet amyloid polypeptide (hIAPP) into fibrils is associated with type II diabetes. It can be catalyzed by interactions with membranes. Recent studies have shown that cytotoxicity arises from the intermediates of aggregation instead of mature fibrils. However, the pathogenic mechanism is still unknown and it remains challenging to probe structures of the intermediates on membrane surfaces due to a lack of biophysical methods that are sensitive to both protein secondary structures and interfaces. Here, we used label-free chiral sum frequency generation spectroscopy (cSFG) to probe the intermediates. Recently, we have discovered cSFG provides highly specific peptide vibrational signatures that can distinguish protein secondary structures at interfaces. Using cSFG, we observed in situ and in real time the aggregation of hIAPP from disordered structures to $\alpha $-helices and then $\beta $-sheets on membrane surfaces. We also obtained the orientation of the $\beta $-sheet aggregates inserted into the membranes. We further studied the S20G mutant, which is linked to the early onset of type II diabetes among Asian populations. We compared the mutant with the wild-type hIAPP to evaluate the effect of S20G in the early-stage aggregation on membrane surfaces.   

Achiral and Chiral Sum Frequency Generation Spectroscopy of Peptides

In vibrational sum-frequency generation (SFG) spectroscopy, a resonant IR and a non-resonant visible laser pulse are applied to a sample, and a signal is detected at the sum frequency of the pulses. This signal is sensitive to the local environments of interfacial chromophores. For the \emph{psp} polarization combination (\emph{p}-polarized SF, \emph{s}-polarized visible, \emph{p}-polarized IR), the signal is selectively sensitive to chiral structures. Recently, it was found that peptide secondary structures could be distinguished by the presence of absence of \emph{psp} signals for the amide I and NH stretch modes. This finding has been exploited to track the aggregation of human islet amyloid polypeptide at a water/air interface. To facilitate the interpretation of these experiments in terms of detailed structures, we present here a mixed quantum/classical method for the computation of both achiral and chiral SFG spectra for the peptide amide I mode, based on classical molecular dynamics simulations. We then apply this method to model systems, and comment as to the importance of both intrinsic chirality (the presence of atomic chiral centers) and structural chirality (the presence of chiral secondary structure) to the strength of the \emph{psp} signal.   

Session M44: Focus Session: Translocation through Nanopores I

Stiff Filamentous Virus Translocations through Solid-State Nanopores

We present experimental results of filamentous virus translocations through a solid-state nanopore. A nanopore can easily detect fd virus due to its linear shape and high linear charge density. With a width of 6.6 nm, a monodisperse length of 880 nm, and a long persistence length of 2.2$\,\mu$m, fd is a model stiff polymer for testing theories of translocation dynamics. The distribution of measured ionic current blockade amplitudes indicates that fd virus does not fold during translocation. The mean fd translocation time was linearly proportional to the applied voltage in the range 75 mV to 500 mV. The dispersion in translocation times was much greater for fd virus than expected from Brownian motion or the conformation-dependent fluid drag. Possible explanations for the observed dispersion will be discussed in light of its dependence on voltage and the salt concentration. This work was supported by NSF Grant CBET0846505 and the Brown University IMNI.   

Biomolecular translocation through nanopores: from an anonymous polymer to realistic DNA

We have developed an efficient multiscale approach to treat biomolecular motion in a fluid solvent. This scheme has been applied to the problem of polymer translocation through a nanopore, an intensively studied subject due to its variety of applications with ultra-fast DNA sequencing being one of them. Our first results involve an anonymous polymer translocating in pure water. We habe obtained important insight into the statistics and dynamics of the process. The translocation time exponent compares well with the experimental values, while we were able to monitor multiconformational translocation, the signatures of which are also relevant to the experimental counterparts. As a next step, we have made our modelling more realistic by including electrokinetic effects, i.e. ions, as well as a realistic quantum-mechanically derived potential for double stranded DNA. We are now able to look more deeply into what happens in the pore. The ionic conductance and DNA blockade can be qualitatively and quantitatively observed and connected to the experiments. Finally, we also investigate the effect of pore geometry in the DNA translocation process.   

Effect of charge patterns along nanopores on the translocation kinetics of flexible polyelectrolytes

One of the major challenges in DNA sequencing with nanopore-based electrophoresis is to slow down the DNA translocation. In the present study, we investigate the effectiveness of charge patterns along the pore on translocation dynamics. We perform a coarse-grained, three-dimensional Langevin dynamics simulation of a uniformly charged flexible polyelectrolyte translocating under uniform external electric field through a patterned solid-state nanopore. We maintain the total charge along the pore to be constant, while varying its distribution by placing alternate charged and uncharged sections of different lengths along the pore length. We observe a translocation success ratio close to 100 percent due to the presence of an attractive section near the cis end of the pore in all studied patterns. Further, we observe a nonmonotonic dependence of the translocation time with the period of the pattern. The optimum period corresponding to the longest translocation time is independent of lengths of polyelectrolyte and pore within the range studied. Calculations of mean first passage time based on free energy are able to predict the optimum period of the pattern qualitatively.   

Nanopore Translocation Dynamics of star polymers

The translocation of polymers through a narrow channel or a nanopore has a significant impact on numerous biological systems and industrial process, examples including rapid DNA sequencing, controlling drug delivery, and designing nanopore sequencing device.We consider the dynamics of flow-induced translocation of star polymers through a nanopore in three dimensions by dissipative particle dynamics approach, focusing on the dependence of the translocation time on the polymer chain length. The scaling of the average translocation time $\tau $ \textit{vs.} the total length $N_{\mbox{tot}} $ of the star polymer with three arms, $\tau \sim N_{\mbox{tot}}^{1.09\pm 0.04}$, is obtained in our simulation. We establish that the overall translocation time, with the translocation probability $P_{i}^{\mbox{trans}} $ and the translocation time $\tau_{i}$ under different translocation paths. We demonstrate that the translocation time $\tau$ of star polymers through the nanopore increases with the increase of the total arm numbers, while $\tau$ decreases with increasing number the forward arms that are initially squeezed into the nanopore. Our findings may provide a valuable guidance for experimental studies on the conformational and dynamics behaviors of star polymer translocation for further applications.   

A Fluid Channel Coincident With Graphene Tunneling Leads for DNA sequencing

One of the strategies towards controlled DNA sequencing by electrical readout of individual bases has been to direct single-stranded DNA through a tunnel junction. For this method to be viable, the DNA must be severely constrained to minimize geometric factors. We present a method for creating fluid channels the size of tunnel junctions, with tunneling leads across them. The fluid channel is formed by Atomic Layer Deposition around a gold wire thinned by feedback-controlled electromigration. The channel itself is used as a mask to assist in defining the tunneling leads out of graphene by electroburning. The principal reasons for selecting graphene are its proven tunnelling sensing ability, stability, and exceeding thinness.   

Dynamics of polymer translocation through a nanopore under an applied external field

Polymer translocation is of considerable importance to many biological processes and is envisaged to be useful for rapid DNA sequencing. Using analytical techniques and Langevin dynamics simulations, we have investigated the following problems. (1) For polymer translocation into a long narrow channel driven by longitudinal flow,\footnote{K. Luo, and R. Metzler, J. Chem. Phys., {\bf 134}, 135102 (2011)} we find that the translocation time shows a linear scaling behavior with the chain length. (2) For polymer translocation through nanochannels with different lengths,\footnote{H. Yong, Y. Wang, S. Yuan, B. Xu, and K. Luo, Soft Matter, {\bf 8}, 2769 (2012)} we observe a minimum of translocation time as a function of the channel length. (3) We have examined polymer translocation into confined systems, such as a slit,\footnote{K. Luo, and R. Metzler, J. Chem. Phys., {\bf 133}, 075101 (2010)} a fluidic channel,\footnote{K. Luo, and R. Metzler, Phys. Rev. E, {\bf 82}, 021922 (2010)} nanocontainers with different shapes,\footnote{K. Zhang, and K. Luo, J. Chem. Phys., {\bf 136}, 185103 (2012); to be published} which shows different translocation dynamics compared with the translocation into an unconfined environment.\\[4pt] This work is supported by the ``Hundred Talents Program'' of CAS and the National Natural Science Foundation of China (Grant Nos. 21225421, 21074126, 21174140).   

Polymer Translocation Through a Nanopore from a Crosslinked Gel to Free Solution

We present results from a computer simulation study of DNA translocation through a nanopore in a membrane that separates a gel region from free solution. The gel is modeled by a square lattice of fixed poles such that the pore size is set by the lattice spacing. Starting with the DNA on the gel side, we examine how the gel pore size affects the dynamics of translocation. We find that due to entropic and frictional forces, the mean translocation time is affected by gel pore size. Since the spatial restrictions imposed by the gel limit the dynamics to one-dimensional motion on the cis side, variations in the width of the distribution of translocations times are also observed.   

Statistical Inference of DNA Translocation using Parallel Expectation Maximization

DNA translocation through a nanopore is an attractive candidate for a next-generation DNA sequencing platform, however the stochastic motion of the molecules within the pore, allowing both forward and backward movement, prevents easy inference of the true sequence from observed data. We model diffusion of an input DNA sequence through a nanopore as a biased random walk with noise, and describe an algorithm for efficient statistical reconstruction of the input sequence, given data consisting of a set of time series traces. The data is modeled as a Hidden Markov Model, and parallel expectation maximization is used to learn the most probable input sequence generating the observed traces. Bounds on inference accuracy are analyzed as a function of model parameters, including forward bias, error rate, and the number of traces. The number of traces is shown to have the strongest influence on algorithm performance, allowing for high inference accuracy even in extremely noisy environments. Incorrectly identified state transitions account for the majority of inference errors, and we introduce entropy-based metaheuristics for identifying and eliminating these errors. Inference is robust, fast, and scales to input sequences on the order of several kilobases.   

Thermophoretic Regulation of Molecular Flux through a Nanopore

Transport of ions, nucleic acids and other molecular species through pores in thin membranes is a process of fundamental importance to the biological function of a cell and practical applications in the field of molecular separation, filtering, and, recently, DNA sequencing. Various approaches to control the transport have been examined, including the effects of the geometry, charge and chemical functionalization of the nanopore surface. Thermophoresis in liquids, i.e. movement of molecules along a temperature gradient, was discovered more than a century ago and has already been employed in various applications, typically involving macroscopic systems. In this work, we explore the use of thermal gradients for regulation of nanoscale fluxes. Specifically, we use all-atom molecular dynamics simulations to examine the effect of thermal gradients on transport of ions, small organic solutes and long DNA molecules through solid-state nanopores. In our typical simulation, multiple thermostats are applied to different parts of the same simulation system, allowing steady-state temperature gradients to be established and the effective forces associated with the thermal gradients to be determined. The results of our simulations suggest that nanopore fluxes of molecular species can be regulated by means of thermal gradients. We expect our results to find applications in molecular separation and filtering technologies, nanofluidic electronics and nanopore sequencing of DNA.   

The effects of diffusion on an exonuclease/nanopore-based DNA sequencing engine

The electronic detection and characterization of individual polynucleotides driven through a single protein nanopore holds promise for rapid and low-cost DNA sequencing. A variation on reading DNA in a ticker tape fashion was recently proposed. The pore would electrically detect single nucleotides that are cleaved sequentially by an exonuclease enzyme in close proximity to one pore entrance. We will present the results of an analytical and computational study of this scheme. The analysis examines the effects of diffusive motion on the capture probability for each nucleotide. The capture probability increases with the applied transmembrane potential, but this is offset by the reduction in the residence time of each nucleotide in the pore. The theoretical results demonstrate that these two effects limit the capability of a cleavage-based nanopore sequencing engine. We will discuss these constraints and speculate on how the system could be improved.   

DNA translocation through graphene nanopores

Nanopores are versatile platform for studying structure and behaviour of individual biopolymers. In a nanopore device, an individual DNA molecule in aqueous solution is electrophoretically threaded through the nano-scale pore in a linear fashion. Resulting modulation of the ionic current through the nanopore is characteristic of the geometrical and chemical properties of the translocating molecule. It has been shown that a new class of nanopore fabricated in free-standing single-layer graphene membrane -- graphene nanopores -- have excellent predisposition to achieve sub-nanometre resolution in discerning features along the length of individual DNA molecules [1]. In this talk, we will demonstrate very high sensitivity of the graphene nanopore current on small variation of the diameter of translocating molecule, and we will examine the dynamics of the DNA molecule within the graphene nanopore. The implications of those results on prospects of physical DNA sequencing will be discussed. \\[4pt] [1] S. Garaj \textit{et al.}, Nature \underline{467}, 190-193 (2010).   

Ionosphere perturbation of single DNA molecules in a.c. electric fields

The collapse of DNA molecules under an a.c. electric field was recently established, but is little understood. We applied alternating electric fields (0 - 200 kV/cm) to fluorescently labeled $\lambda$-DNA confined in quasi 1-d nanochannels. DNA was dissolved in a buffer that contained anionic tracer dyes of varying mobilities. Under a.c. electric fields we obseved a depletion of the anionic fluorophores in the region occupied by the DNA, and enrichment in the regions directly flanking it. The critical field strength to induce expulsion of the fluorophores was above 60 kV/cm. We believe that double-sided isotachophoresis can model the ion distributions in our experiment. Furthermore, we will comment on the dynamics of fluorescent co-ions in the solution perturbed by the DNA by observing their dynamics.   

Session M45: Focus Session: Physics of the Cytoskeleton I

Filament turnover kinetics determine the mechanical relaxation of entangled F-actin solutions

The actin cytoskeleton of eukaryotic cells displays rich mechanical behaviors, which enable cells to efficiently transmit forces required for shape maintenance and tissue stability while also facilitating large shape changes required for morphogenic processes at longer time scales. The molecular processes that control mechanical relaxations of the actin cytoskeleton are poorly understood. Actin filament assembly kinetics are controlled in vivo by an assortment of regulatory proteins, which lead to a complete dissolution and re-formation of filaments on the timescale of seconds. How such ``turnover'' of filaments influences the mechanical properties of the actin cytoskeleton is less clear. To address this, we developed a system using purified actin regulatory proteins, including the severing protein ADF/cofilin and the formin nucleation/elongation factor mDia1, to tune filament turnover kinetics and measured the frequency-dependent shear modulus of entangled actin solutions via particle-tracking microrheology. We observe a tunable reduction in the terminal relaxation time when filament turnover is enhanced through severing, despite a constant mean filament length.   

Measuring actin dynamics during phagocytosis using photo-switchable fluorescence

Phagocytosis has traditionally been investigated in terms of the relevant biochemical signaling pathways. However, a growing number of studies investigating the physical aspects of phagocytosis have demonstrated that several distinct forces are exerted throughout particle ingestion. We use variations on FRAP (Fluorescence Recovery After Photobleaching) in combination with photo-switchable fluorescent protein to investigate actin dynamics as a phagocyte attempts to engulf its prey. The goal of our actin studies are to determine the recruitment and polymerization rate of actin in the forming phagosome and whether an organized \textit{contractile actin ring} is present and responsible for phagosome closure, as proposed in the literature. These experiments are ongoing and contribute to our long term effort of developing a physics based model of phagocytosis.   

Model of Yeast Actin Cable Distribution and Dynamics

The growth of fission yeast relies on the polymerization of actin filaments at the cell tips. These filaments are nucleated by formin proteins that localize at tip cortical sites. These actin filaments bundle to form actin cables that span the cell and guide the movement of vesicles toward the cell tips. Since fluorescence microscopy shows the structure and dynamics of actin cables, we are able to compare the results of the theoretical models of actin cables to experiment, thus enabling quantitative tests of the mechanisms of actin polymerization in cells. We used computer simulations to study the spatial and dynamical properties of actin cables. We simulated individual actin filaments as three-dimensional semiflexible polymer, composed of beads connected with springs. Formin polymerization was simulated as filament growth out of cortical sites located at cell tips. Actin filament severing by cofilin was simulated as filament turnover. We added attractive interactions between beads to simulate filament bundling by actin cross-linkers such as fimbrin. Comparison of the results of the model to prior experiments suggests that filament severing, nucleation and crosslinking are sufficient to describe the many features of actin cables. We found bundled and unbundled phases as cross-linking strength was varied and propose experiments to test the model predictions.   

Myosin II Dynamics during Embryo Morphogenesis

During embryonic morphogenesis, the myosin II motor protein generates forces that help to shape tissues, organs, and the overall body form. In one dramatic example in the \textit{Drosophila melanogaster} embryo, the epithelial tissue that will give rise to the body of the adult animal elongates more than two-fold along the head-to-tail axis in less than an hour. This elongation is accomplished primarily through directional rearrangements of cells within the plane of the tissue. Just prior to elongation, polarized assemblies of myosin II accumulate perpendicular to the elongation axis. The contractile forces generated by myosin activity orient cell movements along a common axis, promoting local cell rearrangements that contribute to global tissue elongation. The molecular and mechanical mechanisms by which myosin drives this massive change in embryo shape are poorly understood. To investigate these mechanisms, we generated a collection of transgenic flies expressing variants of myosin II with altered motor function and regulation. We found that variants that are predicted to have increased myosin activity cause defects in tissue elongation. Using biophysical approaches, we found that these myosin variants also have decreased turnover dynamics within cells. To explore the mechanisms by which molecular-level myosin dynamics are translated into tissue-level elongation, we are using time-lapse confocal imaging to observe cell movements in embryos with altered myosin activity. We are utilizing computational approaches to quantify the dynamics and directionality of myosin localization and cell rearrangements. These studies will help elucidate how myosin-generated forces control cell movements within tissues. \textit{This work is in collaboration with J. Zallen at the Sloan-Kettering Institute.}   

Model of Capping Protein and Arp2/3 Complex Turnover in the Lamellipodium Based on Single Molecule Statistics

Capping protein (CP) and Arp2/3 protein complex regulate actin polymerization near the leading edge of motile cells. Actin and regulatory proteins assemble near the leading edge of the cell, undergo retrograde flow, and dissociate into the cytoplasm as single subunits (monomers) or as part of multiple actin subunits (oligomers.) To better understand this cycle, we modeled the kinetics of actin CP and Arp2/3 complex near the leading edge using data from prior experiments [Miyoshi et al. JCB, 2006, 175:948]. We used the measured dissociation rates of Arp2/3 complex and CP in a Monte Carlo simulation that includes particles in association with filamentous and diffuse actin in the cytoplasm. A slowly diffusing cytoplasmic pool may account for a big fraction of CP, with diffusion coefficients as slow as 0.5 $\mu m^2/s$ [Smith et al. Biophys. J., 2011,101:1799]. Such slow diffusion coefficients are consistent with prior experiments by Kapustina et al. [Cytoskeleton, 2010, 67:525]. We also show that the single molecule data are consistent with experiments by Lai et al. [EMBO J., 2008, 28:986]. We discuss the implication of disassembly with actin oligomers and suggest experiments to distinguish among mechanisms that influence long range transport.   

Effect of surface topography on actin dynamics and receptor clustering in B cells

B cells are activated upon binding of the B cell receptor (BCR) with antigen on the surface of antigen presenting cells (APC). Activated B cells deform and spread on the surface of APCs which may comprise of complex membrane topologies. In order to model the diverse range of topographies that B cells may encounter, substrates fabricated with vertical ridges separated by gaps ranging from hundreds of nm to microns were coated with activating antigen to enable B cell spreading. Simultaneous imaging of actin and BCR shows that the organization of both depends profoundly on the ridge spacing. On smaller ridge spacing (\textless 2 microns), actin forms long filopodial structures that explore the substrate parallel to ridges while the BCR clusters accumulate linearly along the direction of the ridges with limited ability to escape these channels. Cells on larger ridge spacing (\textgreater 2 microns) exhibit central actin patches and peripheral actin waves and form semi-stable polymerization zones at ridges, while BCR distribution is more homogeneous. Our results indicate that surface topography may be a critical determinant of cytoskeletal dynamics and the spatiotemporal organization of signaling clusters.   

Biomimetic active emulsions capture cell dynamics and direct bio-inspired materials

The main biopolymers which make up the cellular cytoskeleton and provide cells with their shape are well understood, yet, how they organize into structures and set given cellular behavior remains unclear. We have reconstituted minimal networks of actin, a ubiquitous biopolymer, along with an associated motor protein myosin II to create biomimetic networks which replicate cell structure and actively contract when selectively provided with ATP. We emulsify these networks in 10-100 micron drops, provide a system to investigate strain-mediated protein interactions and network behavior in confined cell-similar volumes. These networks allow us to study strain-mediated protein-specific interactions in an actin network at a precision impossible in vivo. Using this system, we have identified strain-dependent behavior in actin cross linking proteins; mechanotransduction of signaling proteins in Filamin A, and unique catch-bond behavior in Alpha-actinin. This understanding of biopolymer self-organization to set cell mechanics, will help clarify how biology both generates and reacts to force; moreover this system provides a highly controlled platform for studying non-equilibrium materials, and creating microscopic building block for a entirely new class of active materials.   

Eukaryotic and Prokaryotic Cytoskeletons: Structure and Mechanics

The eukaryotic cytoskeleton is an assembly of filamentous proteins and a host of associated proteins that collectively serve functional needs ranging from spatial organization and transport to the production and transmission of forces. These systems can exhibit a wide variety of non-equilibrium, self-assembled phases depending on context and function. While much recent progress has been made in understanding the self-organization, rheology and nonlinear mechanical properties of such active systems, in this talk, we will concentrate on some emerging aspects of cytoskeletal physics that are promising. One such aspect is the influence of cytoskeletal network topology and its dynamics on both active and passive intracellular transport. Another aspect we will highlight is the interplay between chirality of filaments, their elasticity and their interactions with the membrane that can lead to novel conformational states with functional implications. Finally we will consider homologs of cytoskeletal proteins in bacteria, which are involved in templating cell growth, segregating genetic material and force production, which we will discuss with particular reference to contractile forces during cell division. These prokaryotic structures function in remarkably similar yet fascinatingly different ways from their eukaryotic counterparts and can enrich our understanding of cytoskeletal functioning as a whole.   

Cell shape can mediate the spatial organization of the bacterial cytoskeleton

The bacterial cytoskeleton guides the synthesis of cell wall and thus regulates cell shape. Since spatial patterning of the bacterial cytoskeleton is critical to the proper control of cell shape, it is important to ask how the cytoskeleton spatially self-organizes in the first place. In this work, we develop a quantitative model to account for the various spatial patterns adopted by bacterial cytoskeletal proteins, especially the orientation and length of cytoskeletal filaments such as FtsZ and MreB in rod-shaped cells. We show that the combined mechanical energy of membrane bending, membrane pinning, and filament bending of a membrane-attached cytoskeletal filament can be sufficient to prescribe orientation, e.g. circumferential for FtsZ or helical for MreB, with the accuracy of orientation increasing with the length of the cytoskeletal filament. Moreover, the mechanical energy can compete with the chemical energy of cytoskeletal polymerization to regulate filament length. Notably, we predict a conformational transition with increasing polymer length from smoothly curved to end-bent polymers. Finally, the mechanical energy also results in a mutual attraction among polymers on the same membrane, which could facilitate tight polymer spacing or bundling. The predictions of the model can be verified through genetic, microscopic, and microfluidic approaches.   

Elastic behavior of vimentin intermediate filament networks

A cell's response to mechanical stress is closely linked to the structure and elasticity of its cytoskeleton, which is comprised primarily of actin, microtubule, and intermediate filament (IF) networks. Vimentin is an IF found in mesenchymal cells that plays a role in anchoring organelles and contributes to overall cellular elasticity. Previous research has shown that vimentin networks behave like softly crosslinked gels in the presence of divalent cations. The linear elastic modulus, a measure of stiffness and resistance to elastic deformation, of the network is related to the degree of crosslinking, which is itself controlled by the cation concentration. Increasing the concentration of the divalent cations further results in the formation of bundles within the network, but this bundling behavior is not well understood. Here we investigate the response of in vitro reconstituted vimentin networks to applied shear in the presence of various divalent species to better understand their individual contributions to the network's elastic behavior.   

Biopolymer Networks: Simulations of Rigid Rods Connected by Wormlike Chains

The cytoskeleton of cells is a composite network of filaments ranging from stiff rod-like microtubules to semiflexible actin filaments that together play a crucial role in cell structure and mechanics. The collective dynamics of these cytoskeletal filaments with different mechanical properties are yet to be understood completely. To model such a strongly heterogeneous composite, we simulate networks of \textit{rigid} rods connected by \textit{flexible} linkers (wormlike chains). We extract elastic moduli by quasistatic deformations at varying filament densities and analyze the crossover between cross-link dominated and rod dominated regimes. In particular, we are interested in the asymptotic stress dependence of the \textit{differential modulus}. The simulations are accompanied by rheological experiments on networks of \textit{microtubules} (MTs) cross-linked by double-stranded \textit{DNA} of variable length (cf. talk Meenakshi Prabhune).   

Session M46: Novel Instrumentation and Measurements for Biomedical Research

Investigation of cell morphology for disease diagnostics via high content screening

Ninety percent of all cancer-related deaths are caused by metastatic disease, i.e. the spreading of a subset of cells from a primary tumor in an organ to distal sites in other organs. Understanding this progression from localized to metastatic disease is essential for further developing effective therapeutic and treatment strategies. However, despite research efforts, no distinct genetic, epigenetic, or proteomic signature of cancer metastasis has been identified so far. Metastasis is a physical event: through invasion and migration through the dense, tortuous stromal matrix, intravasation, shear forces of blood flow, successful re-attachment to blood vessel walls, migration, the colonization of a distal site, and, finally, reactivation following dormancy, metastatic cells may share precise physical properties. Cell morphology is the most direct physical property that can be measured. In this work, we develop a high throughput cell phenotyping process and investigate the morphological signature of primary tumor cells and liver metastatic pancreatic cancer cells.   

Multiplexing nano-electroporation for simultaneous transfection of multiple cells

Transfection of biomolecules into cells via electrophoresis across nanochannels, or nano-electroporation, is a recently developed technique shown to deliver precisely controlled dosages with low cell mortality rates. Such advantages are due to the nanochannels used for transfection, which distinguish this technique from bulk and micro-electroporation. Recent demonstrations of nano-electroporation rely on optical tweezers for cell localization, which restrict throughput to sequential electroporation of one cell at a time. In the current work, we overcome this drawback by advancing a multiplexed approach that integrates the nano-channel device with an array of magnetic traps remotely controlled by external magnetic fields. This setup enables multiple magnetically labeled cells to be manipulated in parallel, allowing for simultaneous electroporation of many cells with precisely controlled dosages. After transfection, the cells can be moved downstream for further analysis. Such a magnetically-actuated, remotely-controlled approach for loading of cells and subsequent removal of transfected cells has the potential to transform the current device into an automated platform for simultaneous dosage-controlled biomolecule delivery to large numbers of individual cells.   

Nanopore Mass Spectrometry

We report on the design, construction, and characterization of a nanopore-based ion source for mass spectrometry. Our goal is to field-extract ions directly from solution into the high vacuum to enable unit collection efficiency and temporal resolution of sequential ion emissions for DNA sequencing. The ion source features a capillary whose tip, measuring tens to hundreds of nanometers in inner diameter, is situated in the vacuum $\sim$ 1.5 cm away from an extractor electrode. The capillary was filled with conductive solution and voltage-biased relative to the extractor. Applied voltages of hundreds of volts extracted tens to hundreds of nA of current from the tip. A mass analysis of the extracted ions showed primarily singly charged clusters comprising the cation or anion solvated by several solvent molecules. Our interpretation of these results, based on the works of Taylor and of de la Mora, is that the applied electric stresses distort the fluid meniscus into a Taylor cone, where electric fields reach $\sim$ 1V/nm and induce significant ion evaporation. Accordingly, the abundances of extracted ionic clusters resemble a Boltzmann distribution.   

Coupled External Cavity Photonic Crystal Enhanced Fluorescence

In this work we report a fundamentally new approach to enhance fluorescence in which surface adsorbed fluorophore-tagged biomolecules are excited on a photonic crystal surface that functions as a narrow bandwidth and tunable mirror of an external cavity laser. This scheme leads to $\sim$10x increase in the electromagnetic enhancement factor compared to ordinary photonic crystal enhanced fluorescence. In our experiments, the cavity automatically tunes its lasing wavelength to the resonance wavelength of the photonic crystal, ensuring optimal on-resonance coupling even in the presence of variable device parameters and variations in the density of surface-adsorbed capture molecules. We achieve $\sim$10$^5$x improvement in the limit of detection of a fluorophore-tagged protein compared to its detection on an unpatterned glass substrate. The enhanced fluorescence signal and easy optical alignment make cavity-coupled photonic crystals a viable approach for further reducing detection limits of optically-excited light emitters that are used in biological assays.   

Confocal absorption microscopy of biomolecules and single cells from the visible to the ultraviolet spectral range

We present a versatile approach for absorption spectroscopy on the micron scale that combines a broadband white light source with a confocal microscope and a multichannel detector. The attenuation of the propagating light provides a mechanism for contrast that allows spectrally resolved measurements of biomolecules in minuscule quantities and of single live cells. UV absorption spectra of aromatic amino acids, proteins, and single stranded DNA oligomers (100 bases) in solution are measured with less than 10$^7$ molecules in the probe volume. We discuss applications to spectroscopically identify heterogeneities at the single cell level and to the label-free detection of nucleic acids.   

Reflectance spectrometry of placental vessels in cases of twin-twin transfusion syndrome: experiments and modeling

A stochastic photon transport model in multilayer skin tissue combined with reflectance spectroscopy measurements is used to study placental vessels in cases of twin-twin transfusion syndrome (TTTS). TTTS occurs in about 12{\%} of monozygotic (identical) twin pregnancies wherein flow within placental vessels linking the twins together becomes unbalanced, leading to dual mortality. Endoscopic laser ablation can halt the syndrome by occluding the anastomoses connecting the two fetuses. The objective of this study is to develop a technique to determine hemoglobin (Hb) content through spectral analysis of diffuse reflectance spectra of placental vessels to aid in identification of the anastomoses. Previous work by researchers at Brown University has shown that the reflectance spectra of the donor twin and recipient twin are considerably different in the wavelengths for Hb absorbance. This presentation will give preliminary results for a Monte Carlo model adapted to fit the physiology of the placenta that can be used to quantitative determine the Hb levels. The reflectance spectra of the vessels are simulated for different values of Hb as well oxygenation and water concentration with the vessel and placental mass. The preliminary results will be shown to be in good approximation with the prior experimental data. The combination of modeling with spectroscopic measurement will provide a new tool for detailed prenatal study.   

Microwave Spectrometry for the Assessment of the Structural Integrity and Restenosis Degree of Coronary Stents

Cardiovascular disease is the main cause of death worldwide. Coronary stents are one of the most important improvements to reduce deaths from cardiovascular disorders. Stents are prosthetic tube-shaped devices which are used to rehabilitate obstructed arteries. Despite their obvious advantages, reocclusion occurs in some cases arising from restenosis or structural distortions, so stented patients require chronic monitoring (involving invasive or ionizing procedures). We study microwave scattering spectra (between 2.0 - 18.0 GHz) of metallic stents in open air, showing that they behave like dipole antennas in terms of microwave scattering. They exhibit characteristic resonant frequencies in their microwave absorbance spectra that are univocally related to their length and diameter. This fact allows one to detect stent fractures or collapses. We also investigate the ``dielectric shift'' in the frequency of the resonances mentioned above due to the presence of different fluids along the stent lumen. This shift could give us information about the restenosis degree of implanted stents.   

Higher Resolution and Faster MRI of $^{31}$Phosphorus in Bone

Probing the internal composition of bone on the sub-100 $\mu$m length scale is important to study normal features and to look for signs of disease. However, few useful non-destructive techniques are available to evaluate changes in the bone mineral chemical structure and functional micro-architecture on the interior of bones. MRI would be an excellent candidate, but bone is a particularly challenging tissue to study given the relatively low water density, wider linewidths of its solid components leading to low spatial resolution, and the long imaging time compared to conventional $^1$H MRI. Our lab has recently made advances in obtaining high spatial resolution (sub-400 $\mu$m)$^3$ three-dimensional $^{31}$Phosphorus MRI of bone through use of the quadratic echo line-narrowing sequence (1). In this talk, we describe our current results using proton decoupling to push this technique even further towards the factor of 1000 increase in spatial resolution imposed by fundamental limits. We also discuss our work to speed up imaging through novel, faster reconstruction algorithms that can reconstruct the desired image from very sparse data sets. (1) M. Frey, et al. \textit{PNAS} \textbf{109}: 5190 (2012).   

Accelerated Acquisition of 2D NMR Spectra using Iterative Projections

Typically, in 2D NMR (or 2D MRI), only one ``row'' of the time-dependent (or k-dependent) signal is sampled $N$ times per $\sim T_1$ (spin-lattice relaxation time). Thus, filling a 2D Cartesian grid of $M \times N$ data points requires $M$ additional experiments, for a total spectral acquisition time $T_\textrm{acq} \approx M \times T_1$. Measuring fewer ``rows'' than required for Fourier reconstruction decreases $T_\textrm{acq}$, but this results in a low-quality spectrum (unless more complicated, computationally slower reconstruction techniques are used). Here, we show that a new approach to this problem, using iterative projections, can work on actual 2D NMR data. This approach is built upon the Fast Fourier Transform, so it can handle large data sets (2D, 3D, 4D). Moreover, this approach is expected to work even better in higher dimensions, yielding greater speed ups. Finally, we will discuss how the accelerated acquisition may also improve signal-to-noise and frequency resolution.   

Fast Spectral Reconstruction of Noisy, Sparse Time Domain Data through Iterative Projections

We discuss here an approach for reconstructing spectra from sparse time domain data, by way of iterated projections, and more specifically by alternating projections or by use of the difference map algorithm developed by Veit Elser. This is done in a purely deterministic way, by reformulating any a priori knowledge or constraints into projections, and then iterating. This method is extremely flexible, can be applied to a variety of different signals, and is robust enough to handle real data (with noise and artifacts). In this talk we explain the motivation behind this approach, the formulation of the specific projections, and various methods for handling noise. We will demonstrate the approach using 2D NMR spectra and will compare and contrast this approach with existing methods, such as Maximum Entropy reconstruction.   

Tether-free endoscopic biopsy with self-assembled micro-surgical tools

Feynman's futuristic vision of ``swallowing the surgeon'' or a truly non-invasive surgery relies on the invention and utilization of tetherless, stimuli-responsive and miniaturized surgical tools. We propose a step in this direction by the use of sub-millimeter scale, untethered, self-assembled endoscopic tools by designing and deploying microgrippers ($\mu $-grippers) for effective mucosal sampling from large surface-area organs and for tissue retrieval from hard to reach places in the body. Due to their small size, tether-free actuation, parallel fabrication and deployment, $\mu $-grippers can be dispersed in large numbers (hundreds or thousands) to collect tissue samples and allow statistical sampling of large mucosal areas. Monte Carlo simulations showed that using large number of biopsy tools increases the sampling coverage for screening procedures and hence the chance of detecting the malignant lesions. To establish the feasibility of using $\mu $-grippers for sampling large organs we used with ex-vivo colon and in-vivo esophagus models. Our results showed that it is possible to retrieve high quality tissue samples which are suitable for either conventional cytologic or genetic analyses by using $\mu $-grippers.   

Towards Truly Quiet MRI: animal MRI magnetic field gradients as a test platform for acoustic noise reduction

Clinical MRI acoustic noise, often substantially exceeding 100 dB, causes patient anxiety and discomfort and interferes with functional MRI (fMRI) and interventional MRI. MRI acoustic noise reduction is a long-standing and difficult technical challenge. The noise is basically caused by large Lorentz forces on gradient windings---surrounding the patient bore---situated in strong magnetic fields (1.5 T, 3 T or higher). Pulsed currents of 300 A or more are switched through the gradient windings in sub-milliseconds. Experimenting with hardware noise reduction on clinical scanners is difficult and expensive because of the large scale and weight of clinical scanner components (gradient windings $\sim$ 1000 kg) that require special handling equipment in large engineering test facilities. Our approach is to produce a Truly Quiet (\textless 70 dB) small-scale animal imager. Results serve as a test platform for acoustic noise reduction measures that can be implemented in clinical scanners. We have so far decreased noise in an animal scale imager from 108 dB to 71 dB, a 37 dB reduction. Our noise reduction measures include: a gradient container that can be evacuated; inflatable antivibration mounts to prevent transmission of vibrations from gradient winding to gradient container; vibration damping of wires going from gradient to the outside world via the gradient container; and a copper passive shield to prevent the generation of eddy currents in the metal cryostat inner bore, which in turn can vibrate and produce noise.   

Extracellular recording of \textit{Hirudo medicinalis} neurons using high density, nanocoax neurointerface array

We describe the development of a nanocoax-based neuroelectronic array with submicron pixelation with potential for recording and stimulation with high spatial and temporal resolution. Our device is composed of an array of nanoscale open-ended coaxial electrodes addressed in either a group or individual configuration. As a neuroelectronic interface, our device is characterized by noninvasive real-time coupling to the ganglion sac located along the main nerve cord of the \textit{Hirudo medicinalis}. This allows for extracellular recording of interneural synaptic activity, while also showing the capability of actuating precisely-localized stimulation (faradaic regime). We report on initial results from measurements of electrical signals associated with induced and spontaneous synapse firing in pre- and post-synaptic somata.   

Session M47: Invited Session: Imaging and Manipulating Multicellular Systems and Molecular Clusters

Imaging proteins, cells, and tissues dynamics during embryogenesis with two-photon light sheet microscopy

Light sheet microscopy has gained widespread recognition in recent years due to its distinct advantages for the 3-dimensional imaging of living biological samples. Light sheet microscopy, also known as selective plane illumination microscopy, uses a planar sheet of light to illuminate a sample, generating fluorescence over an optical section of the sample that is collected by a wide-field microscope camera oriented orthogonal to the light sheet. The orthogonal geometry between the illumination and detection pathways enables massive parallelization in both illumination and detection. Furthermore, it allows light illumination to be confined to essentially only the optical section that is being interrogated, minimizing undesired interaction of light with the biological sample. Because of these features, light sheet microscopy significantly outperforms standard imaging modalities in imaging speed, photodamage, and signal to noise in many imaging applications. We recently applied two-photon excitation to light sheet microscopy to improve its penetration depth, allowing long-term imaging of cells deep inside of live embryos. We present a comparison of two-photon light sheet microscopy with other conventional imaging modalities in live imaging of embryos to demonstrate its ability to simultaneously achieve high penetration depth, high acquisition speed, and low photodamage. We also present a selection of applications where two-photon light sheet microscopy is utilized to study the spatio-temporal organization and control of proteins, cells, and tissues during embryogenesis.   

Voltage imaging in vivo with a new class of rhodopsin-based indicators

Reliable, optical detection of single action potentials in an intact brain is one of the longest-standing challenges in neuroscience. We have recently shown that a number of microbial rhodopsins exhibit intrinsic fluorescence that is sensitive to transmembrane potential. One class of indicator, derived from Archaerhodopsin-3 (Arch), responds to voltage transients with a speed and sensitivity that enable near-perfect identification of single action potentials in cultured neurons [Nat Methods. (2011). 9:90-5]. We have extended the use of these indicators to an in vivo context through the application of advanced imaging techniques to the larval zebrafish. Using planar-illumination, spinning-disk confocal, and epifluorescence imaging modalities, we have successfully recorded electrical activity in a variety of fish structures, including the brain and heart, in a completely noninvasive manner. Transgenic lines expressing Arch variants in defined cells enable comprehensive measurements to be made from specific target populations. In parallel, we have also extended the capabilities of our indicators by improving their multiphoton excitability and overall brightness. Microbial rhodopsin-based voltage indicators now enable optical interrogation of complex neural circuits, and electrophysiology in systems for which electrode-based techniques are challenging.   

Spatially and temporally coordinated processes of cells at molecular to cellular scales

Our approach to engineer cellular environments is based on self-organizing spatial positioning of single signaling molecules attached to synthetic extracellular matrices, which offers the highest spatial resolution with respect to the position of single signaling molecules. This approach allows tuning tissue with respect to its most relevant properties, i.e., viscoelasticity, peptide composition, nanotopography and spatial nanopatterning of signaling molecule. Such materials are defined as ``nano-digital materials'' since they enable the counting of individual signaling molecules, separated by a biologically inert background. Within these materials, the regulation of cellular responses is based on a biologically inert background which does not initiate any cell activation, which is then patterned with specific signaling molecules such as peptide ligands in well defined nanoscopic geometries. This approach is very powerful, since it enables the testing of cellular responses to individual, specific signaling molecules and their spatial ordering. We found that integrin cluster have a functional packing density which is defined by an integrin-integrin spacing of approximately 68 nanometers. We have also developed methods which allows the light initiated activation of adhesion processes by switching the chemical composition of the extracellular matrix. This enabled us to identify the frequency of leader cell formation in collective cell migration as a matter of initial cell cluster pattern size and geometry. Moreover, ``nano-digital supports'' such as those described herein are clearly capable of involvement in such dynamic cellular processes as protein ordering at the cell's periphery which in turn leads to programming cell responses.   

From flexibility to cooperativity: multiscale modeling of cadherin-mediated cell adhesion

Cadherins constitute a large family of Ca2$+$-dependent adhesion molecules in the Inter-cellular junctions that play a pivotal role in the assembly of cells into specific three-dimensional tissues. Although the molecular mechanisms underlying cadherin-mediated cell adhesion are still not fully understood, it seems likely that both cis dimers that are formed by binding of extracellular domains of two cadherins on the same cell surface, and trans-dimers formed between cadherins on opposing cell surfaces, are critical to trigger the junction formation. Here we present a new multiscale computational strategy to model the process of junction formation based on the knowledge of cadherin molecular structures and its 3D binding affinities. The cell interfacial region is defined by a simplified system where each of two interacting membrane surfaces is represented as a two-dimensional lattice with each cadherin molecule treated as a randomly diffusing unit. The binding energy for a pair of interacting cadherins in this two-dimensional discrete system is obtained from 3D binding affinities through a renormalization process derived from statistical thermodynamics. The properties of individual cadherins used in the lattice model are based on molecular level simulations. Our results show that within the range of experimentally-measured binding affinities, cadherins condense into junctions driven by the coupling of cis and trans interactions. The key factor appears to be a loss of molecular flexibility during trans dimerization that increases the magnitude of lateral cis interactions. We have also developed stochastic dynamics to study the adhesion of multiple cells. Each cell in the system is described as a mechanical entity and adhesive properties between two cells are derived from the lattice model. The cellular simulations are used to study the specific problems of tissue morphogenesis and tumor metastasis. The consequent question and upcoming challenge is to understand the functional roles of cell adhesion in intracellular signal transduction.   

Adhesion and receptor clustering stabilizes lateral heterogeneity in cell plasma membranes

The thermodynamic properties of plasma membrane lipids play a vital role in many functions that initiate at the mammalian cell surface. Some functions are thought to occur, at least in part, because plasma membrane lipids have a tendency to separate into two distinct liquid phases, called liquid-ordered and liquid-disordered. We find that isolated cell plasma membranes are poised near a miscibility critical point separating these two liquid phases, and postulate that critical composition fluctuations provide the physical basis of functional membrane heterogeneity in intact cells. In this talk I will describe several possible mechanisms through which dynamic fluctuations can be stabilized in super-critical membranes, and will present some preliminary evidence suggesting that these structures can be visualized in intact cells using quantitative super-resolution fluorescence localization imaging.   

Session M48: Tutorial for Authors and Referees

Tutorial for Authors and Referees

Editors from Physical Review Letters and Physical Review will provide information and tips for our less experienced referees and authors. This session is aimed at anyone looking to submit to or review for any of the APS journals, as well as anyone who would like to learn more about the authoring and refereeing processes. Topics for discussion will include advice on how to write good manuscripts, similarities and differences in writing referee reports for PRL and PR, and other ways in which authors, referees, and editors can work together productively. Following a short presentation from the editors, there will be a m oderated discussion. A light breakfast of bagels, pastries, coffee and tea will be served.