Session W1: Invited Session: Superconductivity at High Pressure

Achieving higher \textit{T}$_{\mathrm{C}}$ superconductivity in dense cuprates, iron selenides, and hydrocarbons

Pressure plays an essential role in inducing or tuning superconductivity as well as shedding insight on the mechanism of superconductivity. There are much rich phase diagrams in unconventional superconductors under pressure. Finding ways to control the quantum coherence properties to have a higher critical temperature $T_{\mathrm{C}}$ than the material has remains a challenge. Here we will talk about our recent experimental efforts in achieving higher temperature superconductivity in cuprates, iron selenides, and hydrocarbons. We will show how to enhance remarkably $T_{\mathrm{C}}$ through the pressure tuning of competing electronic order in multilayer cuprates [1] and how to have superconductivity in two distinct regimes in iron selenides [2,3]. We will present a discovery of an enhancement of $T_{\mathrm{C}}$ at more than doubled ambient value in a highly compressed aromatic hydrocarbon [4]. Our results have important implications for designing and engineering superconductors with much higher $T_{\mathrm{C}}$s at ambient conditions.\\[4pt] [1] X. J. Chen, V. V. Struzhkin, Y. Yu, A. F. Goncharov, C. T. Lin, H. K. Mao, and R. J. Hemley, \textit{Nature} \textbf{466}, 950-953 (2010).\\[0pt] [2] L. L. Sun, X. J. Chen, J. Guo, P. W. Gao, H. D. Wang, M. H. Fang, X. L. Chen, G. F. Chen, Q. Wu, C. Zhang, D. C. Gu, X. L. Dong, K. Yang, A. G. Li, X. Dai, H. K. Mao, and Z. X. Zhao, \textit{Nature} \textbf{483}, 67-69 (2012) .\\[0pt] [3] X. J. Chen, Q. Huang, S. B. Wang, J. X. Zhu, W. Bao, M. H. Fang, J. B. Zhang, L. Y. Tang, Y. M. Xiao, P. Chaw, J. Shu, W. L. Mao, V. V. Struzhkin, R. J. Hemley, and H. K. Mao, unpublished.\\[0pt] [4] X. J. Chen, X. F. Wang, Z. X. Qin, H. Wu, Q. Z. Huang, T. Muramatsu, J. J. Ying, P. Cheng, Z. J. Xiang, X. H. Chen, W. G. Yang, V. V. Struzhkin, and H. K. Mao, unpublished.   

Elemental superconductivity at high pressure

Most of superconducting materials show a negative pressure effect in the superconducting critical temperature, $T_{c}$, however, some of simple elements show the positive effect. It has been already revealed that not a few elements that are not the superconductor at ambient pressure became superconductive under combination of low temperature and high pressure. Not only for searching higher $T_{c}$ but also for understanding the fundamental mechanism of ``superconductivity'' systematically, we have worked on pressure effect as well as pressure-induced superconductivity especially in simple elements. Here we report two characteristic results of the high-pressure phenomena including superconductivity in calcium (Ca) and lithium (Li). The $T_{c}$ of Ca increases with pressure and reaches 29 K, the highest $T_{c}$ in elements, at very high pressure above 200 GPa. The lightest metal element of Li exhibits relatively high $T_{c}$ at high pressure, however suddenly becomes semiconductor at 80 GPa. Recently we discovered the reentrance of the superconductivity in Li at around 120 GPa.   

NMR Studies of Novel Electronic Phases in Low Dimensional Molecular Solids at High Pressure and Low Temperature

Molecular superconductors are known for anisotropic electronic band structure, correlations, and a sensitivity to mechanical or chemical pressure which acts to control the relative strength of the respective kinetic and potential energies. Modest pressures, of order 1 GPa are commonly used to continuously tune from a Mott insulating ground state to a superconducting state, and NMR has been particularly successful in identifying the orders involved, and the nature of the excitations in the various phases encountered. The family of quasi-two dimensional systems $\kappa$-(BEDT-TTF)$_2$X ({\it e.g.}, X=Cu(NCS)$_2$, Cu[N(CN)$_2$]Cl) includes a line of first order phase transitions separating the Mott and superconducting phases, with the superconducting state exhibiting signatures for line nodes associated with an order parameter sign-change over the Fermi surface. The pressure/temperature phase diagram of the quasi-one dimensional materials (TMTSF)$_2$X, X=PF$_6$, ClO$_4$,...) includes more phases, as a consequence of effective 1/4-filling and a substantial density wave susceptibility. The SC ground state is singlet, and there is evidence for a sign-change of the order parameter over the Fermi surface. The high-conductivity normal state exhibits properties associated with two-dimensional spin fluctuations, with signatures in the relaxation rate, as well as transport that are reminiscent of behaviors observed in other correlated superconductors.   

Pressure effects in cuprate and iron-based superconductors studied by muon spin rotation

Pressure effect (PE) studies of physical parameters of solid state systems allow one to investigate the properties of a material as a function of tuned inter-atomic distances. Such studies are performed on the same material with well defined composition and microstructure which is often advantageous, since {\em e.g.} chemical tuning of material properties (chemical pressure) may give rise to a number of misleading experimental artefacts. Muon-spin rotation ($\mu$SR) is a powerful and highly sensitive tool for probing static and dynamic magnetic fields in solids on the atomic scale. In type-II superconductors the nanoscale variation of the local magnetic field in the vortex state can be detected by $\mu$SR from which the magnetic penetration depth (superfliud density) can be extracted. Furthermore, $\mu$SR is a unique microscopic technique to explore magnetic ordering phenomena and various magnetic phases in solids. At the Paul Scherrer Institute (PSI) a high-pressure set-up was realized which allows to perform $\mu$SR experiments at hydrostatic pressures up to 25 kbar and low temperatures ($\simeq 0.3$~K) [1]. Such experiments open a wide spectrum of new possibilities for investigating the superconducting and magnetic properties of novel materials, such as high-temperature superconductors and related magnetic materials. Here, we present some representative examples of such $\mu$SR pressure studies carried out at PSI: Iron-based superconductors turned out to exhibit a rich and complex phase diagram which strongly depends on pressure [2,3]. $\mu$SR pressure experiments have significantly contributed to a better understanding of these novel class of superconductors [1,2]. In a further $\mu$SR study the PE on the magnetic penetration depth in cuprate superconductors was investigated and found to exhibit an interesting relation to the observed isotope effect [4]. Very recently, we also investigated the PE on the magnetic penetration depth in the heavy fermion system CeCoIn$_{5}$, revealing a strong increase of the superfluid density with pressure [5].\\[4pt] [1] A. Maisuradze {\it et al.}, arXiv:1211.3584 (2012); M. Bendele {\it et al.}, Phys. Rev. B {\bf 85}, 064517 (2012). \\[0pt] [2] R. Khasanov {\it et al.}, Phys. Rev. Lett. {\bf 104}, 087004 (2010). \\[0pt] [3] M. Bendele {\it et al.}, Phys. Rev. Lett. {\bf 104}, 087003 (2010). \\[0pt] [4] A. Maisuradze {\it et al.}, Phys. Rev. B {\bf 84}, 184523 (2011). \\[0pt] [5] L. Howald {\it et al.}, submitted for publication.   

Pressure tuning of magnetic fluctuation and superconductivity in CeCoIn$_5$

One of the greatest challenges to Landau's Fermi liquid theory -- the standard theory of metals - is presented by complex materials with strong electronic correlations. The non-Fermi liquid transport and thermodynamic properties of these materials are often explained by the presence of strong quantum critical fluctuations associated with a quantum phase transition that happens at a quantum critical point (QCP). The heavy-fermion material CeCoIn$_{5}$ is a prototypical system for which its pronounced non-Fermi liquid behavior in the normal state and unconventional superconductivity are thought to arise from the proximity of this system to a QCP [1-5]. Previous experiments address the physics of this QCP by extrapolating results obtained in the normal state, i.e., there were no \textit{direct} probes of antiferromagnetism and quantum criticality in the superconducting state. This motivated us to study the transport in the mixed state, thus revealing the physics of antiferromagnetism and quantum criticality of the underlying normal state [6]. In this talk I will present the results obtained in these studies by measuring the vortex core dissipation under applied hydrostatic pressure ($P$). The vortex core resistivity increases sharply with decreasing magnetic field ($H)$ and temperature ($T)$ due to quasiparticle scattering on critical antiferromagnetic fluctuations. This behavior is greatly suppressed with increasing $P$. Using our experimental results, we obtained an explicit equation for the antiferromagnetic boundary inside the superconducting dome and constructed an $H-T-P$ phase diagram. This work provides direct evidence for a quantum critical line inside the superconducting phase and reveals the close relationship between quantum criticality, antiferromagnetism, and superconductivity.\\[4pt] In collaboration with T. Hu, H. Xiao, T. A. Sayles, M. Dzero, and M. B. Maple.\\[4pt] [1] V. A. Sidorov et al., Phys. Rev. Lett. 89, 157004 (2002). \newline [2] J. Paglione et al., Phys. Rev. Lett. 91, 246405 (2003). \newline [3] S. Singh et al., Phys. Rev. Lett. 98, 057001 (2007). \newline [4] S. Zaum et al., Phys. Rev. Lett. 106, 087003 (2011). \newline [5] F. Ronning et al., Phys. Rev. B 73, 064519 (2006). \newline [6] T. Hu et al., Phys. Rev. Lett. 108, 056401 (2012).   

Session W2: Invited Session: Theory of Interacting Topological Insulators

Simplified topological invariants for interacting insulators and superconductors

Topological invariants are precise mathematical tools characterizing the topological properties of topological insulators and superconductors. While many simple and powerful topological invariants for noninteracting insulators and superconductors have been well established, the topological invariants for interacting systems are much less investigated, despite of their great importance in studies of topological states in interacting systems. In this talk I will report some recent progress in the search of topological invariants for interacting systems. I will show that topological invariants defined in terms of zero frequency Green's function are precise and convenient tools for interacting topological insulators and superconductors. They have much simpler forms compared to earlier interacting topological invariants, and have the potential to facilitate discoveries of new topological insulators with strong electron-electron interaction.   

Interaction effects on 3D topological insulators and semi-metals

We discuss the effects of interactions on 3D Z2 topological insulators and related phases such as axion insulators, Weyl semi-metals and topological Mott insulators. Our analysis is motivated by the pyrochlore iridates but is of general scope. We begin by studying the effects of interactions on topological phases adiabatically connected to non-interacting Hamiltonians using both regular and dynamical mean field theories. Both the bulk and boundary topological signatures are analyzed. We then move to stronger interactions where a Mott transition from a topological insulator to a fractionalized topological Mott insulator can occur. We discuss the effects of gauge fluctuations on the transition and the resulting spin liquid.   

Topological Insulator Materials with Strong Interaction

All kinds of topological insulator materials have recently been discovered in two-and three-dimensional systems with strong spin-orbit coupling (SOC) hosting helical gapless edge or surface states consisting of odd number of Dirac fermion states inside the bulk band gap. Most of these discovered topological insulators have negligible interaction. Here we theoretically predict a new class of topological insulators with strong interaction. The typical examples are PuTe and AmN, with a simple rocksalt structure, which lie on the boundary between metals and insulators. We show that the interaction can effectively enhance SOC and drives a quantum phase transition to the topological insulator phase with a single Dirac cone on the surface (001). In addition, this kind of compounds has fully or partly filled f states, which could exhibit all kinds of magnetic phases, potentially leads to the discovery of intrinsic quantum anomalous Hall effect (QAHE) and topological magnetic insulators with dynamic axion field.   

Interacting topological phases and quantum anomalies

Since the quantum Hall effect, the notion of topological phases of matter has been extended to those that are well-defined (or: ``protected'') in the presence of a certain set of symmetries, and that exist in dimensions higher than two. In the (fractional) quantum Hall effects (and in ``chiral'' topological phases in general), Laughlin's thought experiment provides a key insight into their topological characterization; it shows a close connection between topological phases and {\it quantum anomalies}. Compared to genuine topological phases, symmetry protected topological phases are more fragile and less entangled states of matter, and hence for their characterization we need to sharpen our understanding on how topological properties of the systems manifest themselves in the form of a quantum anomaly. By taking various kinds of symmetry protected topological phases as an example, I will demonstrate that quantum anomalies serve as a useful tool to diagnose (and even define) topological properties of the systems. I will also discuss quantum anomalies play an essential role when developing descriptions of these topological phases in terms bulk and boundary (effective) theories.   

Braiding statistics approach to symmetry-protected topological phases

Symmetry-protected topological (SPT) phases can be thought of as generalizations of topological insulators. Just as topological insulators have robust gapless boundary modes protected by time reversal and charge conservation symmetry, SPT phases have boundary modes protected by more general symmetries. In this talk, I will describe a method for analyzing 2D SPT phases using braiding statistics. I will present this approach in the context of a simple example: a 2D Ising paramagnet with gapless edge modes protected by Ising symmetry. First, I will show that if the paramagnet is coupled to a $Z_2$ gauge field, the resulting $\pi$-flux excitations have different braiding statistics from that of a usual Ising paramagnet. This result provides a simple proof that the spin model belongs to a distinct quantum phase from a conventional paramagnet. Second, I will show that the $\pi$-flux braiding statistics directly imply the existence of protected edge modes. I will argue that this analysis can be generalized to any 2D SPT phase with unitary symmetries.\\[4pt] [1] M. Levin and Z.-C. Gu, Phys. Rev. B 86, 115109 (2012)   

Session W3: Invited Session: Quantum Foundations

The freedom of choice assumption and its implications

The assumption that the parameters of an experiment (e.g., those determining the basis of a quantum measurement) can be chosen freely is implicit to most considerations in physics. One may therefore ask whether it is possible to give a precise meaning to the notion of ``free choice'' and, if yes, study its implications. One natural approach towards defining free choice, considered already by Bell, is to specify a causal structure on the set of all physically relevant parameters and observables. A parameter may then be considered ``free'' if it is statistically independent of all other parameters and observations that do not lie in its causal future. Recently, it has been realized that the assumption of free choice, as defined above, has various interesting consequences. In particular, if defined relative to a causal structure compatible with relativity theory, free choice immediately implies completeness of quantum theory. This means that there cannot exist any additional (hidden) parameters that would improve the statistical predictions that quantum theory makes about the outcomes of future measurements. In this talk, I motivate and explain this definition of free choice and give an overview of the most important implications of the free choice assumption.   

Quantum correlations in Newtonian space and time: arbitrarily fast communication or nonlocality

Experimental violations of Bell inequalities using space-like separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, ``everything looks as if the two parties somehow communicate behind the scene.'' We investigate the assumption that they do so at a speed faster than light, though finite. Such an assumption doesn't respect the spirit of Einstein relativity. However, it is not crystal clear that such ``communication behind the scene'' would contradict relativity. Indeed, one could imagine that this communication remains for ever hidden to humans, i.e. that it could not be controlled by humans, only Nature exploits it to produce correlations that can't be explained by usual common causes. To define faster than light hidden communication requires a universal privileged reference frame in which this faster than light speed is defined. Again, such a universal privileged frame is not in the spirit of relativity, but it is also clearly not in contradiction: for example the reference frame in which the cosmic microwave background radiation is isotropic defines such a privileged frame. Hence, a priori, a hidden communication explanation is not more surprising than nonlocality. We prove that for \textit{any} finite speed, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication doesn't require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Consequently, all possible explanations of quantum correlations that satisfy the principle of continuity, which states that everything propagates gradually and continuously through space and time, or in other words, all combination of local common causes and direct causes that reproduce quantum correlations, lead to faster than light communication. Accordingly, either there is superluminal communication or the conclusion that Nature is nonlocal (i.e. discontinuous) is unavoidable [Nature Physics DOI: 10.1038/NPHYS2460 (2012); arXiv:1210.7308].   

Three-dimensionality of space and the quantum bit: an information-theoretic approach

It is sometimes pointed out as a curiosity that the state space of quantum two-level systems, i.e. the qubit, and actual physical space are both three-dimensional and Euclidean. In this talk, I report on joint work with Lluis Masanes [1], where we attempt an information-theoretic analysis of this relationship, by proving a particular mathematical result: suppose that physics takes place in d spatial dimensions, and that some events happen probabilistically (not assuming quantum theory in any way). Furthermore, suppose there are systems that behave in some sense as ``units of direction information,'' interacting continuously and reversibly in time. We prove that this uniquely determines spatial dimension d=3 and quantum theory on two qubits (that is, the complex Hilbert space formalism and unitary time evolution). Moreover, we prove that it allows observers to infer local spatial geometry from probability measurements. This applies and generalizes results obtained earlier with further collaborators [2,3]. \\[4pt] [1] M. P. Mueller and Ll. Masanes, Three-dimensionality of space and the quantum bit: how to derive both from information-theoretic postulates, arXiv:1206.0630\\[0pt] [2] G. de la Torre, Ll. Masanes, A. J. Short, and M. P. Mueller, Deriving quantum theory from its local structure and reversibility, Phys. Rev. Lett. 109, 090403 (2012)\\[0pt] [3] Ll. Masanes, M. P. Mueller, D. Perez-Garcia, and R. Augusiak, Entangling dynamics beyond quantum theory, arXiv:1111.4060   

Quantum correlations with indefinite causal order

Quantum mechanics differs from classical physics in that no definite values can be attributed to unobserved physical quantities. However, the notion of time and of causal order preserves such an objective status in the theory: all operations are assumed to be ordered such that every operation is either in the future, in the past or space-like separated from any other operation. Consequently, the correlations between operations respect definite causal order: they are either signalling correlations for the time-like or no-signalling correlations for the space-like separated operations. I will present a framework that assumes only that operations in local laboratories are described by quantum mechanics (i.e. are completely-positive maps), but relax the assumption that they are causally connected. Remarkably, we find situations where two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operations is before or after the other. The correlations between the operations are shown to enable performing a communication task (``causal game'') that is impossible if the operations are ordered according to a fixed background time. I will discuss experimental perspectives for observing such correlations in nature.   

Realism and the epistemic view of quantum states

The idea that quantum states reflect only an observers knowledge/beliefs/information about the world has a long history, with a wide variety of strong arguments having been proffered in its favour. The challenge for an advocate of this position, however, is to identify what we can deduce is ``really going on'' out there. There seem to three main paths proponents of the epistemic view have followed in trying to extract such a narrative from quantum theory. I will explain how the most naive such path--that quantum states can be associated with standard (probabilistic) uncertainty about some (arbitrary) real states of the world--is not tenable under some extremely mild assumptions about how any theory of reality must treat independent experiments. I will then overview the other two paths and what I see as the challenges they face.   

Session W4: Invited Session: Start-ups and Small Businesses: Success Stories and Tool Kits

Top 10 Steps to Business Success

What does it really take to build a successful technology based company? This fast paced and interactive discussion will highlite potential missteps as well as actions that increase the likelihood of success. Topics under consideration will include: how to begin, creating an organizational structure, creating a plan, selecting a name, financing, allocating resources as efficiently as possible, building a team, protecting intangible assets, strategic alliances, obtaining revenue and transitioning from startup to growth. The primary goal of this presentation is to help you identify value-creating practices as well as wasteful practices, while providing the general nuts and bolts required to move forward.   

Learnings from an Entrepreneur: How to Start a Consulting Practice

There are important basic learnings I have experienced in starting my own consulting practice over 7 years ago. These learnings will help you maximize your value, reduce competition and build your reputation and business income. I believe these can apply to many fields but certainly for the Life Sciences. A few of the basic I will cover are \begin{enumerate} \item Why do you want to start a consulting practice \item Qualifications/Specialty/Experience vs the Competition \item What is your target market vs the Competition \item Contracts/ constructing and costing for your target market \item Networking/Involvement in Professional Organizations \end{enumerate}   

The Road from University to Small Business

Scientists are trained to solve problems, persevere, and be innovative with a goal of improving the quality of life for others. Pursuit of science undergraduate and graduate degrees is often based on our desire to become a physician, solve a critical problem, follow in the footsteps of a family member, or satisfy an inquisitive mind. This aptitude uniquely suits us to be successful entrepreneurs who can change the world by developing high tech companies. Keys to being an entrepreneur include perseverance, innovation, and commitment. These qualities are cultivated during the rigorous process of obtaining a science degree which requires laboratory work, problem solving in team settings, innovation to answer exam questions that are sometimes abstract, and dedication to take electives and non-electives not always of one's choosing over the course of four to ten years. Taking a risk and starting a business using the core skills developed during science studies is the focus of this talk. The viewpoint is based on the path of one scientist who always dreamed of performing research and obtained a PhD in chemical engineering along the way to founding a biotechnology company.   

The Untapped Entrepreneurial Frontier: Transferring Innovation from the Laboratory to the Market

Technology transfer from federally funded research laboratories and universities into the private sector holds great promise, yet those promises are largely unfulfilled. In this session you will learn about the scope of technology transfer in our country, the barriers to successful commercialization of scientific innovations and suggestions for how the system can be fixed. You will also learn what inventors, entrepreneurs and investors can do to improve the chances of success.   

Identifying, Licensing, and Commercializing Technology: An Entrepreneur's View

A linguist by trade, Kris Appel left government service to pursue entrepreneurship. She knew she wanted to start a company, but she did not have a business idea. After researching various technologies available for commercialization, she began to focus on a prototype medical device at the University of Maryland Medical School, which had been developed to help stroke survivors recover their arm movement. The device was based upon emerging science into brain re-training, and was backed by very convincing clinical trials. Working closely with University researchers, she licensed the rights to the device, developed a commercial version, and launched it in 2009. Today the device is used around the globe, and has helped thousands of stroke and brain injury survivors improve their arm function and way of life. Kris will tell the story of the device, and how it got from idea to prototype to successful rehabilitation product.   

Session W5: Focus Session: Graphene: Transport and Optical Phenomena: Nanostructures

Large Scale Mesoscopic Transport in Nanostructured Graphene

We report the observation of strong 2D Anderson localization at the charge neutrality point (CNP) in nanostructured antidot graphene samples. A localization length of 2 micron is obtained through sample size scaling up to 10 micron. Localization length is seen to increase with applied magnetic field, in accurate agreement with the theoretical prediction of Ono [Prog. Theor. Phys. Suppl. 84, 138 (1985)]. Our observation is made possible by the very large dephasing length of 10 micon, owing to the opening of a Coulomb quasigap, observable below 25 K, that suppresses the inelastic electron-electron scatterings. Such a large dephasing length is further substantiated by the observation of a crossover from the mesoscopic transport (with exponential size scaling) to diffusive transport (with size independence) at 10 micron. Large scale mesoscopic transport may provide promising future to graphene nanoelectronic device applications.   

Ballistic transport in nanometer-scale suspended graphene

We study electron transport in suspended ultra-short graphene transistors. We fabricate narrow bowtie gold junctions on exfoliated graphene, and use oxygen plasma to etch away the graphene crystal except under the gold junctions. We then use a wet etch to remove the SiO$_{2}$ under the junctions and suspend the devices. Finally, we use a feedback-control electromigration procedure to break the gold junctions and expose sections of graphene which are $\sim$100 nm wide, and as short as $\sim$10 nm. Using low-temperature electron transport, we observe Fabry-Perot oscillations in the conductivity as a function of charge density, as expected for ballistic transport. The conductivity is asymmetric for electron and hole gate-doping, signaling charge doping from the gold contacts and the formation of p-n junctions. At temperatures below $\approx$ 1 Kelvin, a very strong hysteresis is observed in the gate-dependence of conductivity. We study these devices as a function of charge density, temperature, magnetic field and aspect ratio.   

The Effects of the Mean-Field Interaction on the Anderson Localization of Graphene Nanoribbons

A generalized tight-binding (TB) model,\footnote{Hancock {\em et al.} PRB {\textbf 81}, 245402 (2010).} which includes a mean-field Hubbard-{\em U} and up to 3rd nearest-neighbor hopping terms, is applied to edge-disordered zigzag graphene nanoribbons in order to study spin-transport within the Landauer-B\"utticker formalism. Edge-disorder is modeled by random perturbation of the on-site energy in the range $-E..E$ on all edge atoms, and the resulting Anderson localization lengths determined. We compared the Anderson localization lengths and spin-transport features obtained from the generalized model, an extended TB model (non-interacting) and the simplified TB model (1st nearest neighbor hopping only). Within the range $\pm E=$0.5~eV the Anderson localization length for a single spin was found to decrease by 86.4\% with the introduction of the Hubbard-$U$ in the generalized model compared to the non-interacting models, whereas the opposite spin remained unchanged across all model types. For the range $\pm E=$2.0~eV the Anderson localization length for both spin types decreased by 71.4\% and 76.2\% in the generalized model when compared to the extended TB model, and 76.5\% and 80.4\% when compared to the simplified TB model.   

Graphene-based spaser

We propose graphene-based surface plasmon amplification by stimulated emission of radiation (spaser) formed in the graphene nanoribbon located near a semiconductor quantum dot (QD). The population inversion of the two electron levels of the QD can be achieved by applying external electric current or laser pumping. If the frequency of the dipole plasmon resonance in a graphene nanoribbon comes in the resonance with the transition frequency for the QD, it is possible to excite plasmons and generate the coherent surface plasmon states in the graphene nanoribbon. Therefore, the oscillating dipole in the QD excites coherent surface plasmons in the graphene nanoribbon. By solving the system of equations for the number of coherent localized plasmons in a graphene-based spaser the optimal design, optimal width of graphene nanoribbon and optimal regime for the graphene-based spaser are found. The minimal size and minimal threshold pumping intensity for the graphene-based spaser are obtained. The advantage of using graphene for the spaser is discussed.   

Shearing graphene and its transmission properties

Graphene being the thinnest possible membrane is prone to deformations under slight external forcing or even under thermal fluctuations. Here, we take advantage of this proneness to deformations to manipulate transport properties of graphene ribbons. We do so by using the spontaneous pattern produced when a wide ribbon is subject to shear. The deformation of the ribbon produces pseudo-magnetic fields as well as scalar potentials, resulting in the modification of transmission properties without the need of an external gate potential. Our proposal is a concrete realization of a quantum device that takes full advantage of an elastic instability that spans from the nano to macro-scales.   

Optical Properties of Graphene Nanoribbons

The electrical transport properties of graphene nanoribbons have been extensively studied. However, the experimental investigation of the optical properties is still lacking. In this paper, we present the infrared (IR) absorption measurement of graphene nanoribbons with width down to 50 nm. The optical response is dominated by plasmonic resonances in the mid-IR when the incident light polarization is perpendicular to the ribbon axis. By varying the width of the ribbons, we were able to determine the plasmon dispersion in graphene. Meanwhile, we revealed the important role of surface polar phonons and graphene intrinsic optical phonons in the plasmon dispersion and damping. In conjunction with theoretical analysis, we found that graphene plasmons are severely damped through the emission of an optical phonon together with an intraband electron-hole pair. Our study paves the way for graphene applications in infrared photonics and opto-electronics.   

The role of the disorder range and electronic energy in the graphene nanoribbons perfect transmission

Numerical calculations based on the recursive Green's function method in the tight-binding approximation are performed to calculate the dimensionless conductance g in disordered graphene nanoribbons with Gaussian scatterers. The influence of the transition from short- to long-ranged disorder on g is studied as well as its effects on the formation of a perfectly conducting channel. We also investigate the dependence of electronic energy on the perfectly conducting channel. We propose and calculate a backscattering estimate in order to establish the connection between the perfectly conducting channel (with g = 1) and the amount of intervalley scattering.   

First-principles calculation of the heat transport properties of strained graphene nanoribbons

We use density-functional theory coupled with a nonequilibrium Green function's method to calculate the characteristics of ballistic thermal transport (P.S.E. Yeo, K.P. Loh, and C.K.Gan, Nanotechnology, 2012, accepted and to appear) of tensile-strained armchair (AGNR) and zigzag (ZGNR) edge graphene nanoribbons, with widths between $3$ and $50$~\AA. The optimized lateral lattice constants for AGNRs of different widths display a three-family behavior when the ribbons are arranged according to $N$ modulo 3, where $N$ represents the number of carbon atoms across the width of the ribbon. Two lowest-frequency out-of-plane acoustic modes play an important role in increasing the thermal conductance of AGNR-$N$ at low temperatures. At high temperatures the effect of tensile strain is to reduce the thermal conductance of AGNR-$N$ and ZGNR-$N$. These results could be explained by the changes in force constants in the in-plane and out-of-plane directions when strain is applied. This fundamental atomistic understanding of the heat transport in graphene nanoribbons suggests a route to controlling heat transport properties via strain at various temperatures.   

Optical selection rules in graphene quantum dots

We theoretically study the optical absorption of graphene quantum dots for different shapes, sizes and edge types. We calculate the single particle energy spectrum using the tight-binding Hamiltonian and the Dirac-Weyl equation and show that dots with zigzag edges exhibit a degenerate shell of zero energy states, in agreement with previous results. Using standard group theoretical tools, we obtain the optical selection rules for triangular and hexagonal quantum dots and discuss the role of light polarization on the absorption spectrum. Finally, we calculate the oscillator strengths and absorption spectra for different quantum dot sizes and identify the contribution of the zero energy states therein.   

Immense Weak Localization Effect in CVD Graphene

In this study, we report magnetoresistance (MR) measurements on graphene grown by chemical vapor deposition (CVD) on copper. CVD graphene is transferred onto SiO$_{2}$/Si substrate and Hall bar devices with Au/Ti contacts are fabricated by photo-lithography. Measurements show that the diagonal resistance R$_{xx}$ varies logarithmically vs. temperature and magnetic field, as expected for weak localization. The interesting aspect here in CVD graphene is that weak localization effect is immense compared to the typical observation in dirty metals. At zero magnetic field, R$_{xx}$ increased by about 7$\%$ with decreasing temperature from 110 K to 1.5 K. From the observed weak localization, we extract characteristics lifetimes and length scales, and compare the results with theoretical expections [1], and other weak localization work on CVD graphene [2,3].\\[4pt] [1] McCann, E. et al. Phys. Rev. Lett. 97, 2006, 146805.\\[0pt] [2] Miao, Z. et al. J. Phys.: Condens. Matter 24, 2012, 475304.\\[0pt] [3] Wang, W. et al. Carbon, 2012.   

Conductance Fluctuation and Superconducting-to-Normal State Switching Measurements of Superconducting Graphene Devices

We report on gate voltage dependent conductance fluctuations (CF) in superconducting graphene devices and compare measurements in the superconducting versus normal state at temperatures down to 20 mK. The CF arise from the averaged interference of charge carrier wave functions caused by scattering in the graphene. An enhancement in the magnitude of the average CF is expected when in the superconducting state due to Andreev reflections. We fabricate devices by contacting graphene with two parallel superconducting leads that are spaced a few hundred nanometers apart. The leads are a Pd/Al or Ti/Al bilayer with the thin Pd or Ti layer providing high transparency contact to graphene. Additionally, we report on our ongoing superconducting-to-normal state switching measurements in graphene Josephson junctions. The distribution of the stochastic switching current gives insight into the dynamics of the junction such as the phase particle escape mechanisms and dissipation processes. The use of graphene as the weak link allows novel control of the critical current, and thus the dynamics of the junction. By gathering switching data, we can study the modified Josephson washboard potential in these devices (J. G. Lambert, et al., IEEE Trans. in Appl. Supercond. 21, 734 (2011)).   

Conductance fluctuation and dimensional crossover in hydrogenated graphene systems

The conductance of mesoscopic disordered systems in the localized transport regime exhibits extremely large sample-to-sample fluctuations. Thus, their transport properties must be understood in terms of the conductance distribution function. Although the distribution functions show distinctive behavior depending on the dimension of system, previous studies have been mainly focused on one, two, and three dimensional systems individually. Here, we investigate the dimensional transition from two-dimensional (2D) graphene to quasi-one-dimensional (Q1D) graphene nanoribbons and discuss the effect of the dimensional crossover on the conductance fluctuation. As a model system, we consider hydrogenated graphene systems which have attracted much attention due to the observation of a metal-insulator transition. Adopting two different strategies to examine the crossover behavior of conductance between Q1D and 2D systems, we find that a transition from 2D to Q1D is attainable by reducing the sample width, while it is not possible by increasing the length of the 2D system. Our results provide fundamental insights into the dimensionality change not only in graphene, but also in general mesoscopic systems in the localized regime.   

Impurity state and variable range hopping conduction in graphene

The variable range hopping (VRH) theory is widely accepted as explaining the temperature dependence of the conductivity of doped semiconductors. However, as formulated for exponentially localized impurity states, it does not necessarily apply in the case of graphene with covalently attached impurities. We analyze the localization of impurity states in graphene using the nearest neighbor tight-binding model of an adatom-graphene system with Green's function perturbation methods. The impurity states in graphene are characterized as resonant states in the band continuum and both low energy approximations and numerical evaluation of the Green's functions indicate that the amplitude of the wave function decays as a power law with exponents depending on sublattice, direction, and the impurity species. We revisit the VRH theory in view of this result and find that considering only the overlap and energy difference of the impurity states, the conductivity obeys a power law of the temperature with an exponent related to the localization of the wave function. Other factors that were ignored in the original VRH are included due to the weaker temperature dependence, which contribute an additional exponent. We show that this relationship is in agreement with available experimental results.   

Gate-tuned two-channel Kondo screening in Graphene: Universal scaling of the nonlinear conductance

We study the nonlinear conductance through magnetic adatoms on Graphene. In particular, we address the finite-temperature crossover from a quantum critical to the two-channel Kondo regime expected to occur in doped Graphene. Based on the non-crossing approximation, We calculate both the linear and nonlinear conductance within the two-lead single-impurity Anderson model where the conduction electron density of states vanishes in a power-law fashion $ \propto |\omega-\mu_F|^r$ with $r=1$ near the Fermi energy, appropriately for Graphene. For given gate voltage, we study the universal crossover from a 2-channel Kondo (2CK) phase to a un-screened local momemt (LM) phase. We extract universal scaling functions governing charge transport through the adatom and discuss our results in the context of a recent scanning tunneling spectroscopy (STM) experiment on Co-doped Graphene.   

Session W6: Focus Session: Graphene on SiC: Synthesis and Properties

Spin transport in epitaxial graphene on SiC (0001)

Graphene has been identified as a promising material for future spintronics devices due to its low spin orbit coupling and long spin diffusion lengths, even at room temperature [1-2]. However, any device application requires the use of large-area graphene compatible with wafer-scale manufacturing methods, such as graphene grown epitaxially on SiC. We study spin transport in epitaxial graphene grown on SiC (0001) as a step toward future spintronics devices. A non-local spin valve signal of 200m$\Omega $ is observed at 77K, with a signal of 50m$\Omega $ resolved at 145K. Assuming a contact polarization of 10{\%} [1], the measured signal corresponds to a spin diffusion length of 130nm at T$=$77K. Hanle effect spin precession measurements are ongoing. [1] Tombros et al. \textbf{Nature} 448 571 (2007) [2] Maassen et al. \textbf{Nano Lett.} 12, 1498 (2012)   

Scanning tunneling microscopy/spectroscopy study of hydrogen intercalated epitaxial graphene on SiC(0001)

In this work, we studied the atomic structures and electronic properties of hydrogen intercalated epitaxial graphene on Si-face SiC(0001) using scanning tunneling microscopy/spectroscopy and density functional theory (DFT) calculations. Hydrogen intercalation was achieved by either annealing graphene/SiC(0001) in hydrogen gas at atmospheric pressure or in hydrogen plasma in ultrahigh vacuum. We found that while the as-grown graphene is n-type, the H-intercalated graphene is p-type, which can be attributed to the saturation of the Si dangling bonds at the interface by hydrogen atoms. These results and the origin of the p-type doping in hydrogen intercalated epitaxial graphene on SiC(0001) will be discussed at the meeting.   

Scanning Tunneling Microscopy and Spectroscopy of Quasi-freestanding Graphene on SiC

Epitaxial graphene on SiC(0001) is a promising approach for industrial-scale production of very high quality graphene. Recently, it has been demonstrated by angle-resolved photoelectron spectroscopy (Riedl et al., Phys. Rev. Lett 103, 246804 (2009)) that graphene can be prepared on SiC in almost undoped form by intercalating atomic hydrogen beneath the non-graphitic carbon-rich ``buffer layer.'' We present scanning tunneling microscopy and spectroscopy measurements of quasi-free-standing monolayer graphene on SiC(0001) obtained by atomic hydrogen intercalation. Small hydrogen-intercalated domains formed at the initial stages of quasi-free graphene nucleation exhibit a $\left( {\sqrt {\mbox{3}} \times \sqrt {\mbox{3}} } \right)\mbox{R}30$ corrugation due to the sub-surface hydrogen. Local image potential state spectroscopy on these domains is used to observe changes in local doping due to intercalation. These states show the energetic shift ($\approx $ 0.4 eV) with respect to the usual n-doped single-layer graphene on SiC(0001) that suggests that H-intercalated graphene is almost charge-neutral.   

Spin transport over long distance in epitaxial graphene grown on C-face SiC

Spintronics is a paradigm focusing on spin as the information vector and ranging from quantum information to zero-power non-volatile magnetism. Several spintronics evices (logic gates, spin FET, etc) are based on spin transport in a lateral channel between spin polarized contacts. However while spin is acclaimed for information storage, a paradox is that efficient spin transport as remained elusive. We will present magneto-transport experiments on epitaxial graphene multilayers on SiC showing very large spin signals and spin diffusion length in graphene in the 100$\mu$m range (as high as 285$\mu$m). In the best case, the spin transport efficiency of epitaxial graphene is found to be of 75\% of the ideal channel. Graphene, could turn out as a material of choice for large scale logic circuits and the transport/processing of spin information. Understanding the mechanism of the spin relaxation, improving the spin diffusion length and also testing various concepts of spin gates are the next challenges.\\[4pt] Collaborators: B. Dlubak, M.-B. Martin, A. Anane, C. Deranlot, R. Mattana, H. Jaffr\`es, F. Petroff, A. Fert, B. Servet, S. Xavier, M. Sprinkle, C. Berger, and W. de Heer, Unite Mixte de Physique CNRS/Thales, Palaiseau, France and Universit\'e de Paris-Sud, Orsay, France; Institut N\'eel, Grenoble, France and Georgia Tech, Atlanta, USA.\\[4pt] References:\\[0pt] Dlubak et al. Nature Phys 8 557 (2012)\\[0pt] Seneor et al. MRS Bulletin 37 1245 (2012)   

Imaging stacking faults in epitaxial graphene/buffer layer structures on SiC(0001)

In characterizing the structure of epitaxial graphene on SiC, the homogeneity of the number of monolayers (MLs) of graphene on the surface is important due to its substantial effect on graphene's electronic properties and, until recently, was not easily controlled. As the processing of samples continues to improve, other structural properties of the films and substrate (e.g., substrate morphology, step density, and grain area) have become important in the pursuit of improved electronic behavior. In this talk, imaging of rotational stacking faults in epitaxial graphene on SiC(0001) using low-energy electron microscopy (LEEM) is described. Using a pattern of fiducial marks on the SiC surface, we have correlated LEEM imaging of these stacking faults with micro-Raman imaging. Additionally, while stacking domains in $\ge $1ML graphene have been studied previously in LEEM [1-2], here we introduce first-principles calculations of low-energy electron reflectivity for various stacking arrangements of 1ML graphene/buffer- layer structures on SiC(0001), and compare these predictions to the reflectivity seen in LEEM.\\[4pt] [1] C. Virojanadara et al., Surface Science 603, L87 (2009).\\[0pt] [2] Hibinio et al., PRB 80, 085406 (2009).   

Control of epitaxial graphene growth by SiC-SiC capping

The growth of epitaxial graphene on the surfaces of silicon carbide is considered to be one of the most promising techniques for obtaining high quality large scale graphene for electronics applications. Although graphene grown on the C-face has high mobility, its growth under vacuum is too fast, not self limited and produces high concentration of crystalline defects. Therefore a precise control over the Si evaporation rate is required. We demonstrate a new method to reduce the growth rate and yield thin graphene layers with excellent thickness uniformity on the C-face of SiC in ultra high vacuum conditions. The sample is capped by another SiC substrate with a rectangular recess of about one micron depth on its surface which forms a partially open cavity between the surfaces. During the growth by high temperature annealing, silicon atoms sublimated from the capped sample are confined inside the cavity between the two substrates. The confined silicon vapor maintains a high partial pressure at the sample surface which significantly reduces the growth rate of graphene to an easily controllable range. We demonstrate that the growth rate linearly increases with the area of the cavity opening. We investigated the effect of Si confinement on the thickness and morphology of UHV grown epitaxial graphene on C-face SiC by Raman spectroscopy, atomic force microscopy, scanning electron microscopy and low energy electron diffraction.   

Growth and characterization of the graphene and its interface on the SiC (0001) face

The confinement controlled sublimation method [1] provides a method of producing high quality epitaxial graphene on silicon carbide by controlling the silicon evaporation rate through confinement. Here we present growth studies of the first few graphene layers on the silicon terminated face (SiC (0001)). Surface properties of the grown layers are characterized by Raman spectroscopy, AFM, EFM, ellipsometry, and LEED, along with resistivity measurements of the grown graphene. Together these characterization methods can provide information on the substrate step structure and doping of the first layers of graphene. The growth of the initial buffer layer from SiC, of graphene nanoribbons from the SiC substrate steps (e.g. sidewall growth [2, 3]), and of large-area graphene can be better understood for different growth conditions. Finally, we will present electronic transport data for these well characterized graphene layers. Ultimately, the right growth conditions provide control of the substrate steps and number of graphene layers grown, leading to quality epitaxial graphene devices. [1] PNAS 108, 16900 (2011) [2] Nature Nanotechnology 5, 727 (2010) [3] J. Phys. D: Appl. Phys. 45 154010 (2012)   

Studies of epitaxial graphene growth on vicinal silicon carbide

The growth of epitaxial graphene on SiC has been shown to begin at step edges. Therefore, control of the step-edge density and step bunching on the substrate is important for the production of large-area and high-quality graphene. Additionally, recent experiments [1] have exploited the nucleation of graphene at step edges to produce graphene nanoribbons. Here we study the kinetics of graphene growth as a function of SiC step morphology by using dimple-ground SiC samples. This method of sample preparation allows for the study of a continuous range of miscut angles, prepared under identical growth conditions. Samples are annealed inside a graphite furnace with the flux of silicon controlled via physical confinement and a controlled background pressure of argon or silane. The morphology and graphene coverage of the samples are characterized in situ with LEED and Auger spectroscopy and ex-situ by AFM, SEM, and Raman spectroscopy.\\[4pt] [1] M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu, C. Berger and W. A. de Heer, Nature Nano 5, 727 (2010).   

Correlating low-energy electron microscopy and micro-Raman imaging of epitaxial graphene on SiC

Several techniques exist for determining the number of graphene layers grown on SiC such as low-energy electron microscopy (LEEM) and Raman spectroscopy. The method which is arguably the most definitive for SiC-grown graphene isLEEM. Low-energy (0 -- 10 eV) electrons interfere with the graphene layers, yielding minima in the electron reflectivity vs. energy curve that can be used to determine the layer number.1 LEEM also provides the means of collecting selected-area diffraction on ?m-size surface regions (micro-LEED), giving access to further useful structural information. While Raman spectroscopy is also commonly used to determine graphene layer number on SiC substrates; such measurements have no definitive calibration for large-area graphene on SiC, unlike the case of exfoliated graphene on SiO2. In this talk, results of correlated LEEM/micro-Raman imaging of large-area, mono and multilayer graphene samples are presented. These initial findings show that LEEM can show the contrast between terrace regions and step edges at particular areas of monolayer-graphene surfaces. Micro-Raman imaging of these same locations show Raman shifts in the G' (2D) band. The influence of heterogeneities on electrical behavior of graphene will be discussed. Comparative studies of multilayer graphene are in progress, and will also be reported. 1. H. Hibino, et al., Phys. Rev. B 77, 075413 (2008). 2. L. I. Johansson, et al., Phys. Rev. B 84, 125405 (2011).   

Electronic and Magnetic Properties of Epitaxial Graphene Sidewall Nanoribbons

Confinement controlled sublimation growth of epitaxial graphene on silicon carbide has proven to be a viable method for the production of high quality graphene for use in nanoelectronics. However, patterning of bulk graphene using oxygen plasma leads to rough edges that cause electronic transport in nanostructures to be dominated by edge scattering through localization and quantum dot effects. To overcome this, we have developed a method to create graphene nanostructures directly during growth. For this the SiC substrate is etched to reveal sidewall facets that graphitize more readily than the SiC (0001) face. High temperature growth on such pre-patterned SiC yields graphene nanoribbons only a few tens of nanometers wide with well-controlled edges anchored to the SiC substrate. Here we present these growth techniques as well as experimental evidence showing that the resulting ribbons are metallic with unique spin transport properties.   

Electron flow in polycrystalline graphene on C-face SiC

Graphene films can be grown both on the Si and C faces of SiC (0001), and the films grown have strikingly different morphologies. Previously, we have used scanning tunneling potentiometry to characterize electron flow in epitaxial graphene grown on the Si face of SiC [1]. Here we will describe recent measurements on nanoscale electronic transport in graphene films grown on the C-face of SiC. In particular, C-face graphene has several topographical features such as pleats, ridges and carbon beads, which determine the quality of the material. We use scanning potentiometry to relate these topographical features to the electron transport in these films at the nanoscale, and discuss the relative impact of different sources of scattering in the epitaxial graphene. \\[4pt] [1] Ji, S.-H. et al. \textit{Nature Mat}. \textbf{2012}, 11, 114   

Electronic Structure of Self-Organized Graphene Nanostructures on SiC(0001)

Graphene nanostructures directly grown on SiC are appealing for their potential application to nanoscale electronic devices. We use different methods to control the step morphology of the SiC$(0001)$ surface in order to guide the growth of graphene, which initiates at step edges. ``Sidewall'' graphene nanoribbons can be formed on step bunches by limiting the graphene growth. We study such nanostructures via scanning tunneling spectroscopy (STS) in ultra-high vacuum. Significant features are observed in tunneling dI/dV spectra, which we interpret in terms of both strain and quantum confinement. Scanning tunneling microscopy (STM) reveals that the epitaxy between SiC and layer-zero (buffer-layer) graphene on nearby terraces determines the crystalline orientation of the sidewall nanoribbons on step-bunches. We also find a somewhat variable character to the insulating buffer layer, depending on growth conditions and air exposure.   

The metastable chemical gallery of the oxide of epitaxial graphene at room temperature

Insights in the chemistry of graphene oxide and its response to external stimuli are crucial to control its electronic and optical properties, thus enabling future applications of this material. Here, we present a combined experimental and density functional theory study concerning the compositional and structural properties of the oxide of epitaxial graphene (OeG) as a function of time[1, 2] and temperature. Our result indicates that OeG synthesized by oxidizing epitaxial graphene grown on SiC via the Hummers method is a metastable material whose structure and chemistry evolve with a notable degree at room temperature. XPS studies reveal, metastable OeG reaches a nearly stable reduced O/C ratio of ~0.37 with a featured relaxation time of a month. Initially the most enriched epoxide groups decrease with time while hydroxyl groups increase. In addition to this, further XPS study of OeG as a function of temperature shows heating above 120 C in air can abruptly deteriorate the OeG structure. Our calculations show that the availability of hydrogen atoms could be the key factor in tuning structural and chemical properties at relatively low temperatures. [1] S. Kim et al., Nature Materials 11, 544(2012). [2] Z. Wei et al., Science 328, 1373 (2010).   

Session W7: Focus Session: Carbon Nanotubes: Optical Properties

Tailoring of optoelectronic properties in nanotube-chromophore energy transfer complexes

The formation of nanotube-chromophore energy transfer complexes is of great interest for a number of applications, in particular for energy conversion. Certain chromophores can $\pi$ - $\pi$ stack on the nanotube wall: when they are radiatively excited an exciton is formed, which subsequently passes into the carbon nanotube. In the carbon nanotube it can radiatively recombine, emitting a photon characteristic for that nanotube's chirality, or, by applying a voltage, the exciton can be split into an electron and a hole, generating a photocurrent. We demonstrate that the chromophore may be incorporated directly into a surfactant molecule, which then serves two distinct purposes: constituting the photon collecting half of the energy transfer complex, and solubilizing said complexes (Ernst et al., Adv. Funct. Mat 2012). This approach results in temporally stable, biologically compatible solutions which are functional in a wide range of pHs. Alternatively, nanotubes suspended in surfactant micelles can be functionalized with dyes in organic media through micelle swelling. Both processes yield functional nanotube-chromophore complexes with tunable optoelectronic properties, paving the way for scalable optoelectronic devices.   

Optical transitions of small-diameter carbon nanotubes

The optical properties for most of carbon nanotubes have been well understood based on the band structure of graphene with some curvature effects. In small-diameter nanotubes, however, it is well known that the curvature drastically affects the electronic structures. Thus, to clarify the optical properties of these small-diameter tubes from first principles, we theoretically study all the small-diameter nanotubes including chiral ones using the density-functional theory, and predict the absorption and emission properties within the single-particle picture. It is found that the wavefunction that originates from M point in the hexagonal Brillouin zone of the graphene plays an key role to understand the optical properties of small-diameter nanotubes.   

Probing the Free Carrier Doping Effects in Individual Carbon Nanotubes by Optical Spectroscopy

The free carrier (electron or hole) doping in carbon nanotubes will shift their Fermi level, which has dramatically effects in the nanotube electrical transport properties. At the same time, the free carrier doping will also significantly modify the nanotube optical properties. Here we report the development of a new optical spectroscopy method to measure the field-induced change of optical transitions in individual semiconducting and metallic nanotubes. We will discuss the important role of electron-electron interactions to explain our results.   

Non-Adiabatic/Adiabatic Phase Transitions in Ultra-Clean Suspended Carbon Nanotubes

We have recently reported pronounced electron-phonon interactions in suspended, nearly defect-free metallic carbon nanotubes, observed through a Kohn anomaly of greater strength than theoretically predicted. This Kohn Anomaly is accompanied by a gate-induced modulation of the G band Raman intensity. In a systematic study of over 20 quasi-metallic carbon nanotubes devices, we have established a quantitative correlation between the strength of the non-adiabatic Kohn anomaly and the modulation of Raman intensity, indicating that the underlying cause that leads to both these effects is the same. We find that metallic nanotubes can switch between a regime in which the non-adiabatic Kohn anomaly is clearly observed and a regime where the non-adiabatic Kohn anomaly is absent, by varying temperature. In the non-adiabatic regime, an enhancement of the Raman intensity is observed under electrostatic gating. However, in the regime where the non-adiabatic Kohn anomaly is not observed, suppression of the Raman intensity with gating is observed.   

The Double Resonance Raman Behavior of the Carbon Nanotube 2-D Mode Observed in Samples Enriched in a Single Chirality

Access to carbon nanotube samples enriched in single chiralities allows the observation of new photophysical behaviors obscured or difficult to demonstrate in mixed-chirality ensembles. Recent examples include the observation of strongly asymmetric G-band excitation profiles [1] and the unambiguous demonstration of Raman interference effects [2]. Likewise, the complex response expected for the CNT 2-D mode has not yet been clearly defined because of similar limitations. We present results on the dispersive and resonance behaviors of the 2-D mode obtained from samples enriched in a single chirality. The response will be discussed in the context of the interplay of dispersive effects and resonance with the E11 and E22 transitions. The results will be compared to simulations that include all relevant electronic and phonon bands tied to the double-resonance process. 1. J.G. Duque, et. al., ACS Nano, 5, 5233 (2011). 2. J. G. Duque, et. al., Phys. Rev. Lett., 108, 117404 (2012).   

Observation and Spectroscopy of a Two-Electron Wigner Molecule in Ultra-Clean Carbon Nanotubes

Coulomb interactions can have a decisive effect on the ground state of electronic systems. The simplest system in which interactions can play an interesting role is that of two electrons on a string. In the presence of strong interactions the two electrons are predicted to form a Wigner molecule, separating to the ends of the string due to their mutual repulsion. This spatial structure is believed to be clearly imprinted on the energy spectrum, yet to date a direct measurement of such a spectrum in a controllable one-dimensional setting is still missing. Here we use an ultra-clean suspended carbon nanotube to realize this strongly-correlated system in a tunable potential. Using tunneling spectroscopy we measure the excitation spectra of two interacting carriers, electrons or holes. Seven quantum states are identified, characterized by their spin and isospin quantum numbers. These states are seen to fall into two distinctive multiplets according to their exchange symmetries. Interestingly, we find that the splitting between multiplets is quenched by an order of magnitude compared to the non-interacting value. This quenching is shown to be a direct manifestation of the formation of a strongly-interacting Wigner-molecule ground state.   

Raman Studies on Chirality Purified Nanotubes: the Chirality Dependence of the G Modes in Semiconducting and Metallic Carbon Nanotubes

We present results from resonant Raman experiments on nanotube samples which are highly enriched in particular chiralities (n,m). Our study includes 14 different semiconducting tube species and 5 different types of metallic armchair (n,n) carbon nanotubes. Results from G peak positions of semiconducting tubes show a significant dependence on tube diameter, chiral angle and family. Considering theoretical predictions we discuss the origin of these dependences with respect to rehybridization of the carbon orbitals, confinement, and electron-electron interactions.\footnote{H. Telg et al., ACS Nano 6, 904 (2012)} As all armchair nanotubes have the same chiral angle and family, results from these samples are restricted to a diameter dependence, which, however, strongly deviates from the diameter dependence of semiconducting tubes. This deviation has been predicted to be associated with non-adiabatic effects and the Kohn-anomaly in metallic carbon nanotubes. We discuss the contribution of these effects on the peak positions of armchair carbon nanotubes based on electro-chemical doping experiments.   

Surface-enhanced Raman scattering study using metal oxide nanowires grown by chemical vapor deposition

We present surface-enhanced Raman scattering (SERS) results using templates made of metal oxide nanowires such as IrO$_{2}$ and RuO$_{2\, }$that were grown by chemical vapor deposition. SERS has been attracting great attention due to its interesting optical behavior and great potential for applications such as chemical sensor, optoelectronic devices, etc. For promising applications utilizing SERS effect, however, there are crucial issues to be resolved. One is to find a way to systematically control `hot spots' of enhancement and the other is to fully understand the enhancement mechanism. In addition to the well-known two dominant mechanisms, i.e., electromagnetic enhancement mechanism and charge transfer mechanism, we observed that the enhancement greatly depends on the geometry of the nanowires that could suggest another mechanism for SERS. Our results were compared to the FDTD simulations. Our finding may lead us to a way to systematically create, or control hot spots for enhancement of light field using one dimensional nanostructures.   

Deformations and nanomechanical energy storage in twisted carbon nanotube ropes

We determine the deformation energetics and energy density of twisted carbon nanotube ropes that effectively constitute a torsional spring. Due to the unprecedented stiffness and resilience of constituent carbon nanotubes, a twisted nanotube rope becomes an efficient energy carrier. Using {\em ab initio} and parameterized density functional calculations, we identify structural changes in these systems and determine their elastic limits. The deformation energy of twisted nanotube ropes contains contributions associated not only with twisting, but also with stretching, bending and compression of individual nanotubes. We quantify these energy contributions and show that their relative role changes with the number of nanotubes in the rope. The calculated reversible nanomechanical energy storage capacity of carbon nanotube ropes surpasses that of advanced Li-ion batteries by up to a factor of ten.   

Probing Mechanical Resonances in Cantilevered Coiled Carbon Nanowires

Helically coiled carbon nanowires (CCNW) and nanotubes are promising elements for use in MEMS/NEMS devices and nanorobotics, as nano-inductors and sensors, and for impact protection (e.g. Bell \textit{et al.} 2007 IEEE International Conference, J. Appl. Phys. \textbf{100}, 064309 (2006)). Understanding and characterizing their mechanical resonance behavior is essential for the reliability in applications. In this study, we have electrically actuated an individual CCNW in a diving-board cantilever configuration inside a scanning electron microscope, and electrically detected its mechanical resonance modes. By oscillation at low frequency we confirmed the induced-charge actuation mechanism. Among the modes we observed, some appeared to have both axial and lateral components. We also observed closely spaced resonance modes which we attribute to the splitting of degenerate modes, consistent with our COMSOL simulations. We suggest that the helical morphology facilitates inter-mode coupling that results in the observed complex resonance behavior.   

A Novel Multidirectional, Non-Contact Strain-Sensing Nanocomposite

Single-walled carbon nanotubes (SWCNTs) have been successfully dispersed in a polymeric host resulting in the development of a novel strain-sensitive nanocomposite material with promise for scalability. Dubbed ``strain paint'' this new material when coated onto a surface becomes a smart-skin sensor that can detect strain through load transfer from the polymeric host to embedded SWCNTs. Strain is easily measured in a non-contact manner via laser excitation and detection of the unique near-infrared (NIR) fluorescence spectrum of semiconducting SWCNTs. When strained, each ($n,m)$ SWCNT type exhibits a predictable shift in its NIR fluorescence peak. SWCNTs with high intensity are easily detected in the bulk fluorescence spectrum of raw, unsorted SWCNTs embedded in the polymer. Thin films of the polymer/SWCNT nanocomposite were spin-coated onto substrates, strains typically up to 1{\%} were applied, and strain magnitudes were determined by resistive strain gauges bonded to the coating and substrate. Spectral shifts reveal a linear response to strain with little hysteresis. Two SWCNT types exhibiting opposite spectral shifts with strain were used to improve sensitivity. Strain along any direction is determined simply by adjusting the polarization of the excitation laser.   

Strong electromechanical coupling in ultra-short carbon nanotube quantum dots

We study electromechanical coupling in suspended single-wall carbon nanotubes using low-temperature electron transport. Using a feedback-controlled electromigration method $[1]$, we create gate-tuneable single quantum dots whose lengths range from tens of nm down to $\approx$ 3 nm. We observe current suppression of low bias stretching vibron sidebands due to the Franck-Condon blockade, and extract the electron-vibron coupling strength, $g$, both in the electron and hole doped regimes in the same devices. We observe strong $g$ and are exploring its dependence on mechanical strain in the tube. Due to a positive feedback mechanism between tunneling electrons and bending mode vibrations of the nanotubes, we observe bending mode frequencies up to the 100 GHz range $[2]$. The bending mode frequency is found to be tuneable by a factor of two by applying electrostatic strain. \\ $[1]$ J.O. Island \textit{et al}. Appl. Phys. Lett. \textbf{99}, 243106 (2011) \\ $[2]$ J.O. Island \textit{et al}. Nano. Lett. \textbf{12}, 4564 (2012)   

Origin of Compressive Strain Induced Electromechanical Oscillations in Multiwalled Carbon Nanotubes

We show by the application of compressive strain, the electrical conductance of multiwalled carbon nanotubes can be increased monotonically. The strain induces oscillations in electrical conductance, which can have potential applications in many electromechanical nanodevices. While the monotonic increase in the conduction is due to the intra-wall interaction of the nanotubes, the oscillations are caused by the transition from \textit{sp}$^{2}$ to \textit{sp}$^{3}$ hybridization of the carbon atoms, promoted by the interwall interaction. A series of experimental and theoretical analyses based on density functional tight binding method were performed to confirm this finding. These results opens up a possibility of enhancing the conductance of carbon nanotubes by controlling applied strains.   

Movement of solid iron nanocrystal through a constriction in the multiwall carbon nanotube

It has been known for some time that iron (and some other metals) can move inside multiwall carbon nanotube under the application of an external electrical current to the nanotube (B.C. Regan et al, Nature 428, 924 (2004)). Here we report on finding that a solid piece of iron nanocrystal can move through a constriction in the multiwall carbon nanotube that has a smaller cross-sectional area than the nanocrystal itself. Furthermore, we find that during this entire process the core of the nanocrystal remains solid and that the carbon in the nanotube does not chemically interact with iron. We performed kinetic Monte Carlo simulation based on a first principles density functional theory calculation which can reproduce this experimental finding. Additionally, we discuss the nature of the movement of the iron nanocrystal in our simulation and show why the nanocrystal is able to go through a constriction. Also, we compare the dependence of the nanocrystal speed on applied current with available experimental data. From this comparison we are able to estimate the experimental temperature and infer the magnitude of the electromigration force experienced by individual iron atoms for given applied external current.   

Session W8: Topological Insulators: Theory III

Modifying properties of Chern insulators by time dependent perturbations

We study the quantum dynamics of topological Chern insulators in the presence a time dependent perturbation. We show that and under proper drive conditions they can be turned in to trivial insulators or insulators with a higher Chern number. We discuss signatures of such states in the context of non-adiabatic Thouless pumping. We argue that this provides a way to tune the properties of topological systems.   

Band Splitting by Period Potential and Resultant Topological Quantum Numbers

When a Chern band is split into two subbands by breaking lattice translation symmetry that results in a doubled unit cell, the subbands have a set of Chern numbers whose sum has to be the same as the origin band. This, however, does not uniquely determine the Chern numbers of individual subbands. We show how the subbands Chern numbers are related to the structure of the original band, as well as the details of the periodic perturbation. We also generalize this one-to-two band splitting case to one-to-many splitting, as well as the case with time-reversal symmetry, where the Chern number is zero but the bands can carry Z2 topological quantum numbers.   

Using topological entanglement entropy to identify low energy effective field theories of fractional Chern Insulators

The physics of quantum interacting many-body systems allow for a wide variety of phases, whose properties are governed by low energy field theories. In this talk, we write down prototypical parton Chern insulating wave-functions with chern numbers 1,2,3, and 5 and determine their corresponding low energy effective field theory by computing their topological entanglement entropy. We also discuss non-universal aspects of the entanglement entropy including the effect of changing the mass on the corner terms and the slope of the area law.   

Adiabatic continuity between Hofstadter and Chern insulator states

We show that the topologically nontrivial bands of Chern insulators are adiabatic cousins of the Landau bands of Hofstadter lattices. We demonstrate adiabatic connection also between several familiar fractional quantum Hall states on Hofstadter lattices and the fractional Chern insulator states in partially filled Chern bands, which implies that they are in fact different manifestations of the same phase. This adiabatic path provides a way of generating many more fractional Chern insulator states and helps clarify that nonuniformity in the distribution of the Berry curvature is responsible for weakening or altogether destroying fractional topological states.   

Entanglement Entropy at Generalized RK Points of Quantum Dimer Models

We study the $n=2$ R\' enyi entanglement entropy of the triangular quantum dimer model via Monte Carlo sampling of Rokhsar-Kivelson(RK)-like ground state wavefunctions. Using the construction proposed by Kitaev and Preskill [Phys. Rev. Lett. 96, 110404 (2006)] and an adaptation of the Monte Carlo algorithm described in [Phys. Rev. Lett. 104, 157201 (2010)], we compute the topological entanglement entropy (TEE) at the RK point $\gamma = (1.001 \pm .003) \ln 2$ confirming earlier results. Additionally, we compute the TEE of the ground state of a generalized RK-like Hamiltonian and demonstrate the universality of TEE over a wide range of parameter values within a topologically ordered phase approaching a quantum phase transition. For systems sizes that are accessible numerically, we find that the quantization of TEE depends sensitively on correlations. We characterize corner contributions to the entanglement entropy and show that these are well described by shifts proportional to the number and types of corners in the bipartition.   

Thermal Instability of Edge States in a 1D Topological Insulator

The stability of topological phases of matter, also known as topological orders, against thermal noise has provided several surprising results in the context of topological codes used in topological quantum information. However, very little is known about the behavior of a topological insulator (TI) subjected to the disturbing thermal effect of its surrounding environment. This is of great relevance if we want to address key questions such as the robustness of TIs to thermal noise, existence of thermalization processes, use of TIs as platforms for quantum computation, etc. In this work, we have studied the dynamical thermal effects on the protected edge states of a TI when it is considered as an open quantum system in interaction with a noisy environment at a certain temperature $T$. Let us recall that stable edge states are a defining signature of topological insulators. Outstandingly, we find that the usual protection of edge states against quantum perturbations and randomness is lost in the case of thermal effects, despite the fermion-boson interaction with the thermal environment respects chiral symmetry, which is the symmetry responsible for the protection (robustness) of the edge states in this TI. We are able to compute decay rates for practical implementations. PRB (2012)   

Torsional Response, bulk-boundary correspondence, and Viscosity in Topological Insulators

We discuss the relationship between torsion and visco-elastic response of 2D time-reversal breaking topological insulators. We connect the bulk topological response to a new anomalies in the momentum current of the chiral edge theory that we have determined. We also discuss the implications for spectral flow and the emergence of a chiral-gravity type response theory.   

Effect of static charge fluctuations on the conduction along the edge of two-dimensional topological insulator

Static charge disorder may create electron puddles in the bulk of a material which nominally is in the insulating state. A single puddle -- quantum dot -- coupled to the helical edge of a two-dimensional topological insulator enhances the electron backscattering within the edge. The backscattering rate increases with the electron dwelling time in the dot. While remaining inelastic, the backscattering off a dot may be far more effective than the proposed earlier inelastic processes involving a local scatterer with no internal structure. We find the temperature dependence of the dot-induced correction to the universal conductance of the edge. In addition to the single-dot effect, we calculate the classical temperature-independent conductance correction caused by a weakly conducting bulk. We use our theory to assess the effect of static charge fluctuations in a heterostructure on the edge electron transport in a two-dimensional topological insulator.   

Backscattering Between Helical Edge States via Dynamic Nuclear Polarization

We describe how the non-equilibrium spin polarization of one dimensional helical edge states at the boundary of a two dimensional topological insulator can dynamically induce a polarization of nuclei via the hyperfine interaction. When combined with a spatially inhomogeneous Rashba coupling, the resulting steady state polarization of the nuclei produces backscattering between the topologically protected edge states leading to a reduction in the conductance which persists to zero temperature. We study these effects in both short and long edges, uncovering deviations from Ohmic transport at finite temperature and a current noise spectrum which may hold the fingerprints for experimental verification of the backscattering mechanism.   

Symmetries in the entanglement spectrum and topological phases protected by spatial discrete symmetries

We study topological phases protected by spacial (non-local) symmetries using the entanglement spectrum. Exploiting the structure of the entanglement Hamiltonian that can be formulated as the supersymmetric quantum mechanics, we study how a spacial symmetry constrains the entanglement spectrum when the bipartitioning is consistent with the spatial symmetry. Specific examples we took a look at include a reflection symmetric topological insulator composed of two Chern insulators with opposite chiralities in one and two spacial dimensions. For both topological insulators, the edge states in the physical energy spectrum can be gapped while the entangling boundary remains gapless.   

Interfacial Protection of Topological Surface States in Ultrathin Sb Films

Spin-polarized gapless surface states in topological insulators form chiral Dirac cones. When such materials are reduced to thin films, the Dirac states on the two faces of the film can overlap and couple by quantum tunneling, resulting in a thickness-dependent insulating gap at the Dirac point. Calculations for a freestanding Sb film with a thickness of four atomic bilayers yield a gap of 36 meV, yet angle-resolved photoemission measurements of a film grown on Si(111) reveal no gap formation. The surprisingly robust Dirac cone is explained by calculations in terms of interfacial interaction. Our work suggests that quantum tunneling, an intrinsic property dependent on the film thickness, and substrate bonding, an extrinsic factor amenable to interfacial engineering, can be effectively manipulated to achieve desired electronic and spintronic properties of topological thin films.   

Controlling topological insulating phases by tuning the coupling strength of Dirac fermions in chalcogenide ternary compounds

Chalcogenide ternary compounds such as Ge$_{2}$Sb$_{2}$Te$_{5}$ are considered as superlattice of topological insulating layers and band insulating layers. Using first-principles methods and a model Hamiltonian, we study the topological phases of the chalcogen compounds arising from the interactions of Dirac fermionic states existing at the interfaces between the topological insulating and band insulating layers. We particularly investigate the interactions of Dirac fermions upon varying the thickness of band insulating layers or upon introducing magnetic impurities in the layers. We observe a jump of Dirac cones from one time-reversal invariant momentum to another when the thickness is changed. We also discuss the degree of freedom in the spin helicity of the Dirac fermions and how it limits the topological phases.   

The space group classification of topological band insulators

The existing classification of topological band insulators(TBIs) departs from time-reversal symmetry, but the role of the crystal symmetries in the physics of these topological states remained elusive. I will discuss the classification of TBIs protected not only by time-reversal, but also by space group symmetries [1]. I find three broad classes of topological states: (a) $\Gamma$-states robust against general time-reversal invariant perturbations; (b) Translationally-active states protected from elastic scattering, but susceptible to topological crystalline disorder; (c) Valley topological insulators sensitive to the effects of non-topological and crystalline disorder. These three classes give rise to 18 different two-dimensional, and, at least 70 three-dimensional TBIs. I will show how some of these topological states can be realized in two dimensions when tight-binding M-B model, originally introduced for HgTe quantum wells, is generalized to include longer-range hoppings. Finally, experimental implications of our classification scheme with an emphasis on topological states in Sn-based materials will be discussed. \\[4pt] [1] R.-J. Slager, A. Mesaros, V. Juricic, and J. Zaanen, arXiv:1209.2610.   

Why is the bulk resistivity of topological insulators so small?

As-grown topological insulators (TIs) are typically heavily-doped $n$-type crystals. Compensation by acceptors is used to move the Fermi level to the middle of the band gap, but even then TIs have a frustratingly small bulk resistivity. We show that this small resistivity is the result of band bending by poorly screened fluctuations in the random Coulomb potential. Using numerical simulations of a completely compensated TI, we find that the bulk resistivity has an activation energy of just $0.15$ times the band gap, in good agreement with experimental data. At lower temperatures activated transport crosses over to variable range hopping with a relatively large localization length. \textbf{Reference:} B. Skinner, T. Chen, B. I. Shklovskii, \textit{Phys. Rev. Lett.} \textbf{109}, 176801 (2012).   

Session W9: Invited Session: Physics of Next Generation DNA Sequencing

Detection and interrogation of biomolecules via nanoscale probes: From fundamental physics to DNA sequencing

A rapid and low-cost method to sequence DNA would revolutionize personalized medicine [1], where genetic information is used to diagnose, treat, and prevent diseases. There is a longstanding interest in nanopores as a platform for rapid interrogation of single DNA molecules. I will discuss a sequencing protocol based on the measurement of transverse electronic currents during the translocation of single-stranded DNA through nanopores. Using molecular dynamics simulations coupled to quantum mechanical calculations of the tunneling current, I will show that the DNA nucleotides are predicted to have distinguishable electronic signatures in experimentally realizable systems. Several recent experiments support our theoretical predictions. In addition to their possible impact in medicine and biology, the above methods offer ideal test beds to study open scientific issues in the relatively unexplored area at the interface between solids, liquids, and biomolecules at the nanometer length scale [1]. \\[4pt] [1] M. Zwolak and M. Di Ventra, ``Physical Approaches to DNA Sequencing and Detection,'' Rev. Mod. Phys. 80, 141 (2008).   

Single Molecule Electrical Sequencing of DNA and RNA

Gating nanopore devices are composed of nanopores with embedded nanoelectrodes, and they are expected to be one of the core devices used to realize label-free, low-cost DNA sequencing, subsequently leading to {\$}1000-genome sequencing technologies. The operating principle of these nanodevices is based on identifying single base molecules of single DNA passing through a nanopore using a tunneling current between nanoelectrodes. We successfully identified single base molecules of DNA and RNA using tunneling currents. To make gating nanopore devices fit for practical use, core technologies should be integrated on one device chip. One core technology is the identification of single DNA and RNA composed of many base molecules using tunneling currents. We have succeeded in the single-molecule electrical sequencing of DNA and RNA formed by 3 and 7 base molecules, respectively, using a hybrid method of identifying single base molecules via a tunnelling current and random sequencing. A method that controls the speed of a single DNA passing through a nanopore is one core technology that determines the speed and accuracy of sequencing. We successfully developed a method that controls the translocation speed of a single DNA by three orders of magnitude using a voltage between nanoelectrodes.   

DNA Electronic Fingerprints by Local Spectroscopy on Graphene

Working and scalable alternatives to the conventional chemical methods of DNA sequencing that are based on electronic/ionic signatures would revolutionize the field of sequencing. The approach of a single molecule imaging and spectroscopy with unprecedented resolution, achieved by Scanning Tunneling Spectroscopy (STS) and nanopore electronics could enable this revolution. We use the data from our group [1] and others in applying this local scanning tunneling microscopy and illustrate possibilities of electronic sequencing of freeze dried deposits on graphene. We will present two types of calculated fingerprints: first in Local Density of States (LDOS) of DNA nucleotide bases (A,C,G,T) deposited on graphene[2]. Significant base-dependent features in the LDOS in an energy range within few eV of the Fermi level were found in our calculations. These features can serve as electronic fingerprints for the identification of individual bases in STS. In the second approach we present calculated base dependent electronic transverse conductance as DNA translocates through the graphene nanopore. Thus we argue that the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS.\\[4pt] [1] Yarotski DA, Kilina SV, Talin AA, Tretiak S, Prezhdo OV, Balatsky AV, Taylor AJ., ``Scanning tunneling microscopy of DNA-wrapped carbon nanotubes.'' Nano Lett. 2009 Jan;9(1):12-7\\[0pt] [2] Ahmed T, Kilina S, Das T, Haraldsen JT, Rehr JJ,~Balatsky~AV, ``Electronic Fingerprints of DNA Bases on Graphene,'' Nano Lett., 2012, v 12 ~~Issue: 2 ~~Pages: 927-931 ~~DOI: 10.1021/nl2039315   

Edge-functionalization aspects in DNA sequencing with graphene nano-electrodes

Slowing down DNA translocation and achieving single-nucleobase resolution are major issues for the realization of nanopore-based sequencing [see, e.g., our review in J Mater Sci 47, 7439 (2012)]. On the one hand, complex functionalization of nanopore-embedded gold electrodes with one [J Phys Chem C 112, 3456 (2008)] or two types of molecules [Appl Phys Lett 100, 023701 (2012)] might address both these issues simultaneously, but is difficult to implement in practice. On the other hand, the fabrication process of nano-gaps or -pores in graphene could readily introduce more simple edge-functionalization in the form of hydrogen atoms saturating the dangling bonds resulting from cutting the carbon network. --- A range of computational tools can be used to theoretically determine the electronic structure and quantum transport properties of individual nucleotides or short DNA strands in realistic models of nanopore-based sequencing device setups. In this manner, we were able to explore the effects of the temporary formation of weak H-bonds between hydrogenated graphene edges and suitable atomic sites in the nucleotides on the dynamical [Adv Funct Mater 21, 2674 (2011)] and static [Nano Lett 11, 1941 (2011)] properties of this system. Recently also more ambitious functionalization schemes for graphene edges [arXiv:1202.3040] as well as a promising bilayer graphene setup [arXiv:1206.4199] were investigated by us. Finally, there might be a particular appeal to use graphene edges terminated with nitrogen atoms, and we have studied some of the benefits that this type of edge-functionalization could offer for the purpose of DNA sequencing. --- Funding provided by the Swedish Research Council (VR), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), and the Carl Trygger Foundation for Scientific Research.   

Session W10: Invited Session: Many Body Physics in Quantum Gases

Magnetic correlations and density ordering in quantum gases

Quantum gases provide a unique avenue to study fundamental concepts in quantum many-body physics. In our research we go beyond the class of atomic many-body systems that are governed by the interplay between kinetic energy and contact interactions. Using a tunable geometry optical lattice, we create hexagonal, dimerized or anisotropic lattice structures [1]. This allows us to control the exchange energy in a repulsive two-component Fermi gas and study the formation of magnetic correlations. In a different approach, we place a Bose-Einstein condensate into a dynamic lattice potential created by the interaction of the atoms with the vacuum field of an optical cavity. This gives rise to long-range interactions, which result in a transition to a supersolid phase with a broken discrete symmetry, preceded by a mode softening [2]. In the talk I will introduce our experiments and discuss recent results.\\[4pt] [1]: L. Tarruell, D. Greif, T. Uehlinger, G. Jotzu, and T. Esslinger, Nature 483, 302--305 (2012).\\[0pt] [2]: R. Mottl, F. Brennecke, K. Baumann, R. Landig, T. Donner, and T. Esslinger, Science 336, 1570-1573 (2012).   

Dissipative quantum glasses in optical cavities

Strong light-matter interactions offer the prospects of quantum realizations of soft matter phases. We discuss how glassy phases of matter may appear with atomic ensembles in multi-mode optical cavities. Our computations show that some of these quantum optical glasses have no direct analogue in condensed matter realizations due to the photon-mediated long-range interactions and the nature of the driving and dissipation that occurs in the many-body cavity QED systems.   

Non-Equilibrium Dynamics of Ultra Cold Atoms and Effective Spin Models in Optical Cavities

There has been spectacular progress in exploring the properties of ultra cold atoms using light. Recent experiments [1] on Bose--Einstein condensates in optical cavities have reported a novel self-organization transition of the atom-light system. This coincides with the superradiance transition in an effective non-equilibrium Dicke model, describing two-level ``spins'' coupled to light. The light leaking out of the cavity provides valuable information on this hybrid matter-light system, and the time-dependent nature of the experiments demands consideration of the associated dynamics. We present a rich dynamical phase diagram [2,3], accessible by quench experiments, with distinct regimes of collective dynamics separated by non-equilibrium phase transitions. These findings open new directions to study the emergent dynamics and non-equilibrium phase transitions of quantum many body systems and effective spin models.\\[4pt] In collaboration with J. Keeling (University of St Andrews), J. Mayoh (University of Cambridge) and B. D. Simons (University of Cambridge).\\[4pt] [1] K. Baumann, C. Guerlin, F. Brennecke and T. Esslinger, ``Dicke Quantum Phase Transition with a Superfluid Gas in an Optical Cavity,'' Nature 464, 1301 (2010).\\[0pt] [2] J. Keeling, M. J. Bhaseen and B. D. Simons, ``Collective Dynamics of Bose--Einstein Condensates in Optical Cavities,'' Phys. Rev. Lett. 105, 043001 (2010).\\[0pt] [3] M. J. Bhaseen, J. Mayoh, B. D. Simons and J. Keeling, ``Dynamics of Nonequilibrium Dicke Models,'' Phys. Rev. A 85, 013817 (2012).   

Heavy Solitons in a Fermionic Superfluid

Topological excitations are found throughout nature, in proteins and DNA, as dislocations in crystals, as vortices and solitons in superfluids and superconductors, and generally in the wake of symmetry-breaking phase transitions. In fermionic systems, topological defects may provide bound states for fermions that often play a crucial role for the system's transport properties. Famous examples are Andreev bound states inside vortex cores, fractionally charged solitons in relativistic quantum field theory, and the spinless charged solitons responsible for the high conductivity of polymers. However, the free motion of topological defects in electronic systems is hindered by pinning at impurities. We have created long-lived solitons in a strongly interacting fermionic superfluid by imprinting a phase step into the superfluid wavefunction, and directly observed their oscillatory motion in the trapped superfluid. As the interactions are tuned from the regime of Bose-Einstein condensation (BEC) of tightly bound molecules towards the Bardeen-Cooper-Schrieffer (BCS) limit of long-range Cooper pairs, the effective mass of the solitons increases dramatically to more than 200 times their bare mass. This signals their filling with Andreev states and strong quantum fluctuations. For the unitary Fermi gas, the mass enhancement is more than fifty times larger than expectations from mean-field Bogoliubov-de Gennes theory. Our work paves the way towards the experimental study and control of Andreev bound states in ultracold atomic gases. In the presence of spin imbalance, the solitons created in our experiment represent one limit of the long sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of mobile Cooper pairs. \\[4pt] [1] Tarik Yefsah, Ariel T. Sommer, Mark J.H. Ku, Lawrence W. Cheuk, Wenjie Ji, Waseem S. Bakr, Martin W. Zwierlein, Heavy Solitons in a Fermionic Superfluid, preprint arXiv:1302.4736 (2013)   

Collective Dipole Oscillations of a Spin-Orbit Coupled Bose-Einstein Condensate

We present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. The dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region as a function of spin-orbit coupling parameters as well as the oscillation amplitude. The anharmonic properties beyond the effective-mass approximation are revealed, such as the amplitude-dependent frequency and finite oscillation frequency at a place with a divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of the spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility---a static physical quantity---can be measured via the dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates the magnetic nonzero-momentum condensate from the nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a benchmark for recently developed theoretical approaches.   

Session W11: Invited Session: Polymer Based Soft Materials: Industrial Applications

Tuning the Photoinduced Motion of Glassy Azobenzene Polymers and Networks

Continual innovation at the forefront of soft-matter, in areas such as liquid crystal networks, nano-composites and bio-molecules, is providing exciting opportunities to create smart materials systems that exhibit a controlled, reproducible and reversible modulation of physical properties. These material systems evoke the adaptivity of natural organisms, and inspire radical aerospace notions. A key example is photo-responsive polymers, which convert a light stimulus input into a mechanical output (work). Photoinduced conformational changes, such as within azobenzene, dictate molecular-level distortions that summate into a macroscopic strain, which often manifests as a shape change or motion. The transduction of the molecular-level response to a macroscale effect is regulated by mesoscopic features, such as chain packing, free volume, and local molecular order - factors which depend on chemical composition as well as the process history of the material. For example, physical aging increases the density of the glass, reduces local free volume, and thus decreases the minima in local conformation space which strongly influences the azobenzene photochemistry (trans-cis-trans isomerization). The subsequent change in the energy landscape of the system reduces the fraction of azobenzene able to undergo reconfiguration as well as increases the probability that those photoinduced conformations will relax back to the initial local environment. The result is a tuning of the magnitude of macroscopic strain and the ability to shift from shape fixing to shape recovery, respectively.   

Using Modeling to Design new Rheology Modifiers for Paints

Since their invention in 1970-s, hydrophobically ethoxylated urethanes (HEUR) have been actively used as rheology modifiers for paints. Thermodynamic and rheological behavior of HEUR molecules in aqueous solutions is now very well understood and is based on the concept of transient network (TN), where the association of hydrophobic groups into networks of flower micelles causes viscosity to increase dramatically as function of polymer concentration. The behavior of complex mixtures containing water, HEUR, and latex (``binder'') particles, however, is understood less well, even though it has utmost importance in the paint formulation design. In this talk, we discuss how the adsorption of HEUR chains onto latex particles results in formation of complex viscoelastic networks with temporary bridges between particles. We then utilize Self-Consistent Field Theory (SCFT) model to compute effective adsorption isotherms (thickener-on-latex) and develop a rheological theory describing steady-shear viscosity of such mixtures. The model is able to qualitatively describe many important features of the water/latex/HEUR mixtures, such as strong shear thinning. The proposed approach could potentially lead to the design of new HEUR structures with improved rheological performance.   

Starch Applications for Delivery Systems

Starch is one of the most abundant and economical renewable biopolymers in nature. Starch molecules are high molecular weight polymers of D-glucose linked by $\alpha $-(1,4) and $\alpha $-(1,6) glycosidic bonds, forming linear (amylose) and branched (amylopectin) structures. Octenyl succinic anhydride modified starches (OSA-starch) are designed by carefully choosing a proper starch source, path and degree of modification. This enables emulsion and micro-encapsulation delivery systems for oil based flavors, micronutrients, fragrance, and pharmaceutical actives. A large percentage of flavors are encapsulated by spray drying in today's industry due to its high throughput. However, spray drying encapsulation faces constant challenges with retention of volatile compounds, oxidation of sensitive compound, and manufacturing yield. Specialty OSA-starches were developed suitable for the complex dynamics in spray drying and to provide high encapsulation efficiency and high microcapsule quality. The OSA starch surface activity, low viscosity and film forming capability contribute to high volatile retention and low active oxidation. OSA starches exhibit superior performance, especially in high solids and high oil load encapsulations compared with other hydrocolloids.   

New Developments in Brominated and Halogen-Free Flame Retardants

With a broad portfolio of brominated, organophosphorus and inorganic flame retardants, ICL Industrial Products (ICL-IP) is engaged in the development of new flame retardants by exploiting the synergism between bromine based, phosphorus based and other halogen-free flame retardants. ICL-IP is also focusing on the development of polymeric and reactive flame retardants. This presentation will give examples of existing and new polymeric and reactive products for applications in thermoplastics, thermosets and polyurethane foam. This presentation will also show examples of phosphorus-bromine synergism allowing partial or complete elimination of antimony trioxide in many thermoplastics for electronic applications. New synergistic combinations of magnesium hydroxide with phosphorus and other halogen-free FRs will be presented.   

Session W12: Focus Session: Thermoelectrics Nanomaterials I

Thermoelectric performance of chemically exfoliated n-Bi$_2$Te$_3$

Bi$_{2}$Te$_{3}$ based thermoelectric (TE) devices are of interest because of their high thermoelectric figure of merit (ZT) near room temperature, and ability to be utilized in both refrigeration and power generation modes. Recently, nano-structuring has shown promise in improving the TE performance of $p$-type Bi$_{2}$Te$_{3}$, however $n$-type counterparts are still lagging in this respect. Here, we display high ZT values ($\sim$ 0.9) in exfoliated $n$-Bi$_{2}$Te$_{3}$ at elevated temperatures (400-- 500 K). The chemically exfoliated samples were prepared by an ultra-sonication technique with subsequent spark plasma sintering to obtain dense pellets. Our transport results showed improved compatibility and a shift in the ZT maximum towards a higher temperature ($\sim$ 430 K) than commercially available ingots. The experimental details and transport data will be discussed within the frame work of exfoliation-induced structural modifications.   

Room Temperature Thermoelectric Properties of Porous BiSbTe Thin Films

Bi$_{(2-x)}$Sb$_{x}$Te$_{3}$ is currently the best known room temperature p-type thermoelectric material, with a ZT value $\sim$ 0.75. We report synthesis of Bi$_{(2-x)}$Sb$_{x}$Te$_{3}$ thin films via pulsed laser deposition using a Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ target. We have investigated the effect of deposition parameters, including substrate, laser power and inert gas pressure, and annealing conditions on the microstructure, composition and thermoelectric properties of the films. We find a strong dependence of film characteristics on background pressure: The Sb content of the films increases with deposition pressure. Low pressure (1-2 mTorr) depositions yield highly conducting and amorphous films deficient in Te. In addition, we will present a comparison of the thermoelectric properties of porous and dense BiSbTe films, to evaluate film porosity as a means for increasing confinement and improving the thermoelectric power factor.   

Thermoelectric properties of electrolessly etched silicon nanowire arrays

Patterning silicon as nanowires with roughened sidewalls enhances the thermoelectric figure-of-merit ZT by order of magnitude compared to the bulk at 300 K [1]. The enhancement is mainly achieved by the remarkable reduction in the thermal conductivity below 5 W/mK at 300 K with only a negligible effect on the power factor of these nanowires. While the focus remained on understanding the implications of surface disorder on the thermal conductivity, the phonon transport effects on the Seebeck coefficient of these wires remains largely unexplored. We developed an electroless etching technique to generate nanowire arrays (NWAs) with controlled surface roughness, morphology, porosity and doping [2]. We conduct the simultaneous device-level measurements of the Seebeck coefficient and thermal conductivity of the NWAs using frequency domain techniques. We observe that nano-structuring quenches the phonon drag [3] in NWAs thereby reducing the Seebeck coefficient by $\sim$25{\%} compared to the bulk at degenerate doping levels. Further, we observe that the sidewall roughness greater than 3 nm roughness height lowers the thermal conductivity 75{\%} below the Casimir limit [4] with 10{\%} - 15{\%} increase in Seebeck coefficient. The porous NWAs show thermal conductivity close to the amorphous limit of Si with enhancement in the Seebeck coefficient primarily due to the carrier depletion. References: [1] A. I. Hochbaum et al, Nature 451, 163-167 (2008). [2] K. Balasundaram et. al., Nanotechnology 23, 305304 (2012). [3] C. Herring, Phys. Rev. 96, 1163 (1954). [4] H. G. B. Casimir, Physica 5, 495 (1938).   

Low temperature phonon boundary scattering in slightly rough Silicon nanowires

Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are promising candidates for thermoelectric applications. In particular, measurements on single silicon nanowires show a reduction in thermal conductivity below the Casimir limit. This reduction increases with surface roughness [4] but the trend and its connection to phonon boundary scattering are still elusive. Here, we measure the thermal conductivity of single silicon nanowires fabricated using metal-assisted chemical etching. High resolution TEM imaging shows crystalline wires with slightly rough surfaces. Their statistical correlation lengths (5-15 nm) and RMS heights (0.8-1.5 nm) are in a range where perturbation-based wave scattering theory is still applicable. We use the thermal conductivity data to extract the frequency dependence of phonon boundary scattering at low temperatures (10-40 K) and show agreement with multiple scattering theory. This work provides insight into enhancing the thermoelectric performance of nanostructures. 1-A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008). 2-A. J. Minnich et al, Energy Environ. Sci. 2, 466-479 (2009). 3-L. Shi, Nanoscale Microscale Thermophys. Eng. 16, 79--116 (2012). 4-J. Lim et al, Nano Lett. 12, 2475$-$2482 (2012).   

Thermal conductivity of disordered porous Silicon

Nanostructuring bulk materials is a promising approach for engineering high-efficiency thermoelectric devices thanks to its ability to decoupling the thermal and electrical transport. Among different approaches, porous Silicon has been attracting much attention due to its ability of strongly suppressing heat transport. Recent experimental works show that classical size effects of phonons can be further enhanced by having staggered pores, as opposed to the aligned pores case. Motivated by these results, we solve the phonon Boltzmann Transport Equation to compute heat transport across an arbitrary pores arrangement. The model has been discretized by means of the Discontinuous Galerkin method, which allows complex simulation domains. We focus on triangular, circle and square pores where the orientation is allowed to change stochastically. In order to compute the ZT, the electrical conductivity and the Seebeck coefficients are computed by means of diffusive theory. Our main finding is that pore disorder can play a crucial rule in optimizing thermoelectric materials. Indeed, in the special case of triangular pores we predict an increasing in ZT of up to ten times the value found for the aligned case.   

Strong suppression of near-surface thermal transport by metal-assisted chemical etching of Si

Recently, we reported that the thermal conductivity of Si nanowire arrays roughened by metal-assisted chemical etching (MAC-etch) is strongly correlated to both the magnitude of the roughness and a broadening of the one-phonon Raman linewidth. We hypothesized that microstructural disorder induced by the etching chemistry leads to changes in the Raman linewidth and reduced thermal conductivity. Here, we simplify the study of such effects by chemically roughening Si wafers instead of nanowires. We have studied the effects of various roughening procedures on the near-surface thermal transport properties using time-domain thermoreflectance. We find that the thermal conductance of the near-surface region is systematically reduced by the MAC-etch process, despite the expectation that pristine roughened surfaces should have increased conductance due to enhanced surface area. In addition, highly roughened surfaces show strong picosecond acoustic echoes with reflection coefficient indicative of a soft interface. These features are consistent with the presence of strong disorder or nanoporosity in the near-surface region created by the MAC-etch process.   

Evaluating Heat Dissipation in Si/SiGe Nanostructures using Raman Scattering

Bulk SiGe alloys and SiGe nanostructures exhibit relatively low thermal conductivity and have found applications in efficient thermoelectric devices. Practical measurements of thermal conductivity involve a sophisticated device design, which may not be applicable to sub-micrometer structures and devices. Raman scattering can be used to measure local temperature with a high accuracy, and it allows calculations of thermal conductivity. In this work, we present Raman data obtained for three sets of samples: partially-relaxed SiGe alloy layers with thickness close to 50 nm; planar Si/SiGe superlattices (SL) with $\sim$ 30{\%} Ge content; and three-dimensional (3D) Si/SiGe cluster multilayers with different Ge concentration and degrees of vertical self-ordering. Despite a high signal-to-noise ratio (better than 1000 to 1), quantitative analysis of Raman spectra requires proper baseline modeling and subtraction. By measuring multi-modal Stokes/anti-Stokes Raman signals and performing base line correction, we calculate local temperatures and develop a model of heat dissipation in the different SiGe and Si/SiGe nanostructures.   

Probing Large-Wavevector Phonons in Silicon Nanomembranes using X-ray Thermal Diffuse Scattering

Phonons play a critical role in determining physical properties of crystalline materials. Phonon dispersions can be modified via nanoscale engineering, by introducing boundaries separated by distances comparable to phonon wavelengths. In free-standing nanowires and sheets, theoretical and experimental investigations have been largely restricted to studying small-wavevector phonons lying within the central 1$\%$ of the Brillouin Zone. Large-wavevector phonons, important for transport in nanostructures, cannot be modeled using continuum physics, and are difficult to probe using conventional optical techniques. Synchrotron x-ray thermal diffuse scattering (TDS) collects information from the scattering of x-rays by phonons with wavevectors spanning the entire Brillouin zone. We adopt this technique to probe the dispersion of large-wavevector acoustic phonons in the nanoscale regime. TDS measurements were performed on silicon nanomembranes, from 315 nm thick sheets exhibiting bulk Si dispersions, to membranes as thin as 6 nm, where deviations from bulk-like behavior are observed. Systematic examinations of the variation of scattered intensity with crystallographic orientation, wavevector, and membrane thickness will be presented.   

Atomistic Monte Carlo simulations of heat transport in Si and SiGe nanostructured materials

Efficient thermoelectric energy conversion depends on the design of materials with low thermal conductivity and/or high electrical conductivity and Seebeck coefficient [1]. Semiconducting nanostructured materials are promising candidates to exhibit high thermoelectric efficiency, as they may have much lower thermal conductivity than their bulk counterparts [1]. Atomistic simulations capable of handling large samples and describing accurately phonon dispersions and lifetimes at the nanoscale could greatly advance our understanding of heat transport in such materials [2]. We will present an atomistic Monte Carlo method to solve the Boltzmann transport equation [3] that enables the computation of the thermal conductivity of large systems with both empirical and first principles Hamiltonians (e.g. up to several thousand atoms in the case of Tersoff potentials). We will demonstrate how this new approach allows one to rationalize trends in the thermal conductivity of a range of Si and SiGe based nanostructures, as a function of size, dimensionality and morphology [3]. [1] See e.g. A. J. Minnich et al. Energy Environ. Sci. 2, 466 (2009). [2] Y. He, I. Savic, D. Donadio, and G. Galli, accepted in Phys. Chem. Chem. Phys. [3] I. Savic, D. Donadio, F. Gygi, and G. Galli, submitted.   

Thermoelectric Power Factor Engineering of Low-Dimensional and Nanocomposite Si Nanostructures

By employing nanostructured materials the thermoelectric figure of merit ZT has been raised to unprecedented large values, with a present record of ZT$=$2.4. Even in traditionally poor thermoelectric materials such as Si, high ZT values were achieved. The improvement was a result of the drastic reduction in the thermal conductivity, which could be suppressed close or even below the amorphous limit. Since thermal conductivity reduction is reaching its limits, additional benefits resulting from electronic structure engineering have to be investigated. In this work we theoretically provide design directions for the thermoelectric power factor (comprising Seebeck coefficient and electrical conductivity) and thermal conductivity in nanostructured Si channels. We consider 1D nanowires, 2D ultra-thin layers, and nanocomposite Si-based materials. We employ semiclassical Boltzmann transport and use both atomistic and continuum calculations for the electronic and phononic structure of the materials. This study examines how length scale can be exploited as a degree of freedom in designing the nanoscale thermoelectric material properties.   

Enhanced Power Factor in Strained Silicon Nanomesh Thin Film

The power factor $S^{2}\sigma $ ($S$ is the Seebeck coefficient and $\sigma $ is the electrical conductivity) of n-type silicon thin films is increased by utilizing both tensile lattice strain and nanomesh structures. The tensile strained lattice in n-type silicon splits the six-fold degenerate conduction band, which results in reduced inter-valley scattering and consequently enhanced electron mobility. The nanomesh feature structure decreases the thermal conductivity due to increased phonon scattering. The nanomesh was patterned onto both strained and unstrained silicon on insulator (SOI) using reactive ion etching with self-assembled block copolymers as masks. The Seebeck coefficient and electrical conductivity measurements were then performed on both strained and unstrained nanomesh SOI in vacuum over a wide temperature range. Increases in $S$ and $\sigma $ were observed and an enhanced power factor was obtained.   

Studies on Seebeck Coefficient of Individual Bismuth Telluride Nanotube

We have studied on Seebeck coefficient (S) of freestanding individual Bismuth Telluride nanotubes using micro-fabricated thermoelectric workbench at the temperatures from 300 K to 25 K. The thermoelectric workbench is composed of three main elements: heater, thermocouple, and platinum pad. A polysilicon-gold thermocouple accurately measures the temperature, arising from the heat generated at the tips of the test sites from the polysilicon heater located 2 $\mu $m apart from the thermocouple. Platinum pads placed on top of the heater and thermocouple structures and electrically isolated from these constitute S measurement circuit. IPA solution containing Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ nanotubes was drop-cast on the workbench and the Ebeam Induced Deposition of platinum was used to improve the electrical and thermal contacts between nanotube and platinum pads. The inner and outer diameter of nanotube is 50 nm and 70 nm, respectively. The sign of obtained S was positive which is indicating the nanotube is p-type. And the magnitude was increased compared to the bulk and nanowire types. The measured S (364 $\mu $V/K) of nanotube at T $=$ 300 K is 1.65 times larger than that (220 $\mu $V/K) of bulk and 1.4 times larger than the previously reported value (260 $\mu $V/K) of nanowire.   

Signatures of 1D Electron Subbands in the Thermoelectric Properties of InAs Nanowire

We report electrical conductance and thermopower measurements on InAs nanowires synthesized by chemical vapor deposition. Gate modulation of the thermopower of individual InAs nanowires with diameter around 20nm is obtained over $T=$ 40 to 300K. At low temperatures (less than c.a.100K), oscillations in the thermopower and power factor concomitant with the stepwise conductance increases are observed as the gate voltage shifts the chemical potential of electrons in InAs nanowire through quasi-one-dimensional (1D) sub-bands. This work experimentally shows the possibility to modulate semiconductor nanowire's thermoelectric properties through the peaked 1D density of states in the diffusive transport regime, a long-sought goal in nanostructured thermoelectrics research. Moreover, we point out the importance of scattering (or disorder) induced energy level broadening in smearing out the 1D confinement enhanced thermoelectric power factor at practical temperatures (e.g. 300K). The authors acknowledge NSF CAREER Award (DMR-1151534), ACS PRF (48800-DNI10) and the NSF of China for support of this research.   

Thermoelectric effects in disordered branched nanowires

We shall develop formalism of thermal and electrical transport in $Si_{1-x}Ge_x$ and $BiTe$ nanowires. The key feature of those nanowires is the possibility of dendrimer type branching. The branching tree can be of size comparable to the short wavelength of phonons and by far smaller than the long wavelength of conducting electrons. Hence it is expected that the branching may suppress thermal and let alone electrical conductance. We demonstrate that the morphology of branches strongly affects the electronic conductance. The effect is important to the class of materials known as thermoelectrics. The small size of the branching region makes large temperature and electrical gradients. On the other hand the smallness of the region would allow the electrical transport being ballistic. As usual for the mesoscopic systems we have to solve macroscopic (temperature) and microscopic ((electric potential, current)) equations self-consistently. Electronic conductance is studied via NEGF formalism on the irreducible electron transfer graph. We also investigate the figure of merit $ZT$ as a measure of the suppressed electron conductance.   

Electrochemical Deposition of Lanthanum Telluride Thin Films and Nanowires

Tellurium alloys are characterized by their high performance thermoelectric properties and recent research has shown nanostructured tellurium alloys display even greater performance than bulk equivalents [1-2]. Increased thermoelectric efficiency of nanostructured materials have led to significant interests in developing thin film and nanowire structures. Here, we report on the first successful electrodeposition of lanthanum telluride thin films and nanowires. The electrodeposition of lanthanum telluride thin films is performed in ionic liquids at room temperature. The synthesis of nanowires involves electrodepositing lanthanum telluride arrays into anodic aluminum oxide (AAO) nanoporous membranes. These novel procedures can serve as an alternative means of simple, inexpensive and laboratory-environment friendly methods to synthesize nanostructured thermoelectric materials. The thermoelectric properties of thin films and nanowires will be presented to compare to current state-of-the-art thermoelectric materials. The morphologies and chemical compositions of the deposited films and nanowires are characterized using SEM and EDAX analysis. References: [1] D. M. Rowe, \textit{CRC Handbook of Thermoelectrics}, CRC Press (1995). [2] A. May \textit{et al.,} \textit{Phys. Rev. B} \textbf{78}, 125205 (2008).   

Session W13: Topological Insulators: Bi2Se3, Pure and Chemically Doped

Insulating behavior in ultra-low carrier density Bismuth Selenide single crystals

The topological insulator material Bi$_2$Se$_3$ is well known to suffer from a non-insulating bulk due to doping caused by selenium vacancies. We present results on the synthesis and characterization of pure undoped Bi$_2$Se$_3$ crystals that exhibit nonmetallic transport behavior over the entire measured temperature range, from room temperature down to at least 2 K. Measurements of longitudinal transport and Hall effect are used to characterize the transport temperature and magnetic field dependences, carrier sign and density, and sensitivity to air exposure.   

THz generation and the detection on the Dirac-cone surface states in topological insulator Bi$_2$Se$_3$

A terahertz (THz) wave is generated from the (001) surface of cleaved Bi$_2$Se$_3$ and Cu-doped Bi$_2$Se$_3$ single crystals, using femtosecond pulses of 800 nm. The generated THz power is strongly dependent on the carrier concentration of the crystals, which can be explained by considering the absorption of both surface and bulk states altogether. In particular, the Dirac-cone surface states in Bi$_2$Se$_3$ significantly affect the THz emission efficiency. Therefore, the THz radiation from topological insulators can be used to ascertain the existence and characteristics of the Dirac-cone surface states.   

Coexistence of Bulk and Surface Shubnikov-de Haas Oscillations in Bi$_{2}$Se$_{3}$

Topological insulator possesses insulating bulk state and spin-momentum interlocked conducting topological surface state. Among many materials, bismuth selenide (Bi$_{2}$Se$_{3})$ is an important candidate, which hosts a single Dirac cone in the surface energy spectrum. In electron transport measurements, 3-dimensional Shubnikov-de Haas (SdH) oscillations of bulk state were observed. Under a very high magnetic field, our rotating sample experimental results exhibit the coexistence of bulk and surface SdH oscillations: Hall bar shape device based on Bi$_{2}$Se$_{3}$ nano-plate was fabricated and studied at a dilution temperature with a tilted magnetic field up to 45 T. Three types of carrier, one of 3-dimensional and two of 2-dimensional, were identified by analyzing the angular dependence of SdH oscillations, which confirmed the coexistence of bulk carrier and band bending induced two-dimensional electron gas in transport experiment. The co-contributions to quantum oscillations indicated the independence of these states, without smearing out by scattering with each other, which may pave off the way for studying topological surface states with residual bulk carriers in Bi$_{2}$Se$_{3}$. The data analysis and experimental results are included in the presentation.   

Bulk versus surface contributions to the Shubnikov-de Haas Effect

Among the bulk materials that are considered as experimental realizations of topological insulators Bi$_{2}$Se$_{3}$ is of particular interest due to its large bulk band gap and surface states with a single Dirac cone. It has been recently shown that Bi$_{2}$Se$_{3}$ can become superconducting when Cu$^{\, }$intercalation is introduced (Hor, Y. S.; Williams, A. J. et al. \textit{Phys. Rev. Lett. }\textbf{2010}, 104, 057001). We report on transport measurements of cleaved flakes $\sim $1$-$100 $\mu $m thick of Cu intercalated Bi$_{2}$Se$_{2}$. Clear Shubnikov-de Haas oscillations are observed. We study the temperature and angular dependence of these oscillations together with the Hall coefficient at low temperatures for various Cu concentrations. We discuss possible contributions from bulk and the protected surface states to the various transport channels.   

Modification of topological insulator transport properties by electron beam irradiation

Topological insulators (TI) are predicted to present a variety of interesting surface transport phenomena. However, in TI devices, the metallic bulk conduction usually overwhelms the surface transport. In this work, we first fabricate TI devices based on our high bulk resistivity material ($\sim$5 $\Omega $.cm) Bi$_{\mathrm{x}}$Sb$_{\mathrm{2-x}}$Te$_{\mathrm{y}}$Se$_{\mathrm{3-y}}$ using ebeam lithography. Then we expose the devices with an electron beam to introduce disorders to localize the bulk carriers. The devices are $\sim$100-200 nm in thickness and the resistivity is weakly temperature dependent. Upon initial low-energy exposures, we find that the resistance of device decreases and reaches a saturation state as the dosage increases. We attribute this decrease in resistivity to an increased electron density in the devices. As we ramp up the energy of the electron beam, the resistance starts to increase, showing the effect of additional scattering. At low temperatures, the resistance rapidly increases in a diverging trend. At 4 K, the magnetoresistance starts to display oscillatory features that are likely the Shubnikov-de Haas oscillations from the surface states. We believe that the disorders introduced by the electron beam play an important role in modifying the transport of the bulk carriers. More detailed experimental results and discussions will be presented.   

Topological Spin-Polarized Electron Layer above the Surface of Ca-Terminated Bi$_2$Se$_3$

Spin-polarized gapless surface states on the boundary of topological insulators are of interest for spintronic applications. First-principles calculations show that adsorption of a Ca monolayer on films of the prototypical topological insulator, Bi$_2$Se$_3$, yields a substantial enhancement of the surface-state spin polarization, despite the low atomic mass of Ca and its weak spin-orbit coupling. Much of the topological surface electron distribution is transferred outside the Ca to form a polarized electron layer out in vacuum; this spatial separation from the substrate minimizes scattering by defects in Bi$_2$Se$_3$ and is very desirable for spin transport.   

Strong single-ion anisotropy and anisotropic interactions of magnetic adatoms induced by topological surface states

The nature of the magnetism brought about by Fe adatoms on the surface of the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ was examined in terms of density functional calculations. The Fe adatoms exhibit strong easy-axis magnetic anisotropy in the dilute adsorption limit due to the topological surface states (TSS). The spin exchange J between the Fe adatoms follows a Ruderman-Kittel-Kasuya-Yosida behavior with substantial anisotropy, and the Dzyaloshinskii-Moriya interaction between them is quite strong with \textbar D/J\textbar\ $\approx $ 0.3 under the mediation by the TSS, and can be further raised to 0.6 by an external electric field. The apparent single-ion anisotropy of a Fe adatom is indispensable in determining the spin orientation.   

The Fermi Surface of Highly Doped Bi$_{2}$Se$_{3}$ and the Implications for Superconductivity in CuBi$_{2}$Se$_{3}$

The 3D Fermi-surface (FS) mapping of Bi$_{2}$Se$_{3}$ for different samples with carrier-density ranging from $10^{17}$ to $10^{20}$ cm$^{-3}$ was made using Angle Resolved Photoemission Spectroscopy. While in the low carrier density samples a closed FS was observed, in high carrier density superconducting Cu$_{x}$Bi$_{2}$Se$_{3}$ samples the FS was found to be open. The open FS puts constraints on the possible order-parameters in this system.   

Scanning tunneling microscopy of defects and electronic fluctuations in Cu-doped Bi$_{2}$Se$_{3}$

We report scanning tunneling microscopy and spectroscopy studies of the topological insulator Cu$_{x}$Bi$_{2}$Se$_{3}$. We have identified five different atomic-resolution signatures of Cu dopant-related point defects and correlated several of them to density functional theory simulations of the defects. Most interestingly, by investigating the dI/dV images of the known Bi$_{Se}$ antisite defects as a function of bias, we show that local electronic structure can vary substantially over a length scale of 30nm, with amplitudes as large as $\pm$50meV. The strong fluctuations appear to be caused by a variety of defects and may have consequences for the topological surface state, as revealed by quasiparticle scattering studies. Correlation of quasiparticle scattering with the various defects indicates that the surface state is robust to backscattering, though detailed analysis shows that some defects are more effective in producing stationary scattering states than others.   

Probing the pairing symmetry of candidate topological superconductor CuxBi2Se3 via point contact spectroscopy

We perform point contact spectroscopy measurements on the candidate topological superconducting material, Cu$_{\mathrm{0.25}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, using a normal-metal gold tip or an s-wave superconductor niobium tip. For the Au- Cu$_{\mathrm{0.25}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ interface, we observe a marked zero-bias conductance peak in the point contact spectra on the superconducting area of the sample, indicative of unconventional superconducting pairing symmetry. The point contact spectra also exhibit a pronounced linear background, which we attribute to inelastic scattering at the tip-sample interface. We compare the background subtracted spectra to a single-band p-wave model. For the Nb- Cu$_{\mathrm{0.25}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ interface, we observe a two-gap-like feature in the spectra, corresponding to the superconducting gap of the niobium and the sample, respectively. In addition, we find that the spectra are highly dependent on the interface barrier strength, and exhibit non-monotonic temperature dependence at zero bias, possibly owing to the incompatibility of the pairing symmetries between the Nb tip and the sample. Our results signify the unconventional superconductivity in Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$.   

Quantum oscillations in topological superconductor candidate Cu$_{x}$Bi$_{2}$Se$_{3}$

In Cu$_{x}$Bi$_{2}$Se$_{3}$, a candidate to be a 3-dimensional topological superconductor, it is of high interest to study how its bulk electronic structure differs from Bi$_{2}$Se$_{3}$, since the nature of the emergent bulk superconducting order puts constraints on the possible surface state. The de Hass-van Alphen effects is observed on a single crystals of Cu$_{0.25}$Bi$_{2}$Se$_{3}$ using sensitive torque magnetometry. Our results show that the Cu doping in Bi$_{2}$Se$_{3}$ increases the carrier density and the effective mass, without increasing the scattering rate or decreasing the mean free path. In addition, the Fermi velocity remains the same in copper doped compound as that in Bi$_{2}$Se$_{3}$. These results imply that the insertion of Cu does not change the overall band structure and that conduction electrons in Cu doped Bi$_{2}$Se$_{3}$ sit in the linear Dirac-like band.   

Gapped Dirac surface states in In doped topological insulator Bi$_2$Se$_3$

Topological insulators host helical Dirac surface states which linearly disperse through bulk band gap. The unusual helical surface states are protected by time reversal symmetry, and therefore believed to be robust against disorders that do not break time reversal symmetry. It has been debated whether massive Dirac surface states (i.e. a gap at the Dirac point) are experimentally observed in doped topological insulators [1-3]. Herein, we report the observation of a spectroscopic gap of topological surface states in Bi$_{2-x}$In$_x$Se$_3$ using low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). The tunneling spectroscopic maps suggest that the interactions between In dopants effectively change the topological class of local band structure, resulting in a nanoscale mixture of topologically trivial and nontrivial states. This electronic inhomogeneity poses a nanoscale spatial confinement to the Dirac surface states so that the long wavelength surface states near the Dirac point are suppressed, i.e. a gap is opened at the Dirac point. \\[4pt] [1] Sato et al, Nat. Phys., 7, 840, (2011).\\[0pt] [2] Xu, et al., arXiv:1206.0278v1 (2012).\\[0pt] [3] Souma et al, PRL, 109, 186804 (2012).   

Topological phase transition in the (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ system investigated via STM

Transport and photoemission measurements on (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ have shown that the system transforms from a pure (x$=$0) topological insulator (TI) into a topologically trivial material (x \textgreater\ 0.07) through a topological phase transition. Indium (In) substitution for heavier Bismuth is expected to have a large effect on the electronic properties of TIs and is a very sensitive way to tune spin-orbit coupling while maintaining the same lattice structure. In this talk we present scanning tunneling microscopy measurements of the surface state and electronic structure of (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ single crystals over a wide range of In concentrations. We identify the local density signature of the In impurities and use these local measurements to determine the actual doping levels. Using spectroscopy and Fourier transform maps we then trace the evolution of the topological insulator into the trivial phase, thereby providing insights into the nanoscale evolution of this process.   

Topological-Metal to Band-Insulator Transition in (Bi$_{1-x}$In$_x)_2$Se$_3$ Thin Films

By combining transport and photoemission measurements on (Bi$_{1-x}$In$_x)_2$Se$_3$ thin films, we report that this system transforms from a topologically nontrivial metal into a topologically trivial band insulator through three quantum phase transitions. At $x\approx $3{\%}--7{\%}, there is a transition from a topologically nontrivial metal to a trivial metal. At $x\approx $15{\%}, the metal becomes a variable-range-hopping insulator. Finally, above $x\approx $25{\%}, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically tunable physics and devices with seamless gating or tunneling insulators.   

Theoretical study of topological phase transitions in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$

We use first-principles calculations to study the phase transition from a topological to a normal insulator with concentration $x$ in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ in the Bi$_2$Se$_3$ crystal structure. The spin-orbital coupling (SOC) strength is similar in In and Sb, which have similar atomic numbers, so that if the topological transitions in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ are purely driven by the decrease of SOC strength, we would expect to see similar critical concentrations $x_{\rm c}$ in the two systems. However, based on our preliminary calculations, $x_{\rm c}$ is much lower in (Bi$_{1-x}$In$_{x})_2$Se$_3$ than in (Bi$_{1-x}$Sb$_{x})_2$Se$_3$, indicating that different mechanisms control the behavior in the two cases. Specifically, in (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ we find that the phase transition is mostly dominated by the decrease of SOC. However, for (Bi$_{1-x}$In$_{x})_2$Se$_3$, the In $5s$ orbitals also play an important role, both in the phase-transition behavior and in determining the indirect bulk band gap. Finally, we discuss the accuracy of the energy-level position of the In $5s$ orbitals in (Bi$_{1-x}$In$_{x})_2$Se$_3$ as predicted by density-functional theory and more advanced methods.   

Session W14: Focus Session: Magnetic Domains

Observation and Control of exotic magnetic domain structures in ferromagnetic CeRu$_{2}$Ga$_{2}$B

The structure of magnetic domains in a single crystal of CeRu$_{2}$Ga$_{2}$B was investigated using low-temperature magnetic force microscopy (MFM) over a wide range of fields and temperatures. The low Curie temperature (T$_{C}\approx $16 K) allows for extensive tunability, revealing~a rich variety of magnetic states including branched stripes, bubble domains, and finger-like domains. In addition to~the higher~spatial resolution, the advantage of MFM over optical imaging techniques~is~the~ability~to~manipulate~magnetic domains. In particular, we are able to manipulate (move and destroy) individual circular domains~with~the MFM tip, which suggests that we observe unusual spherical `bubble' domains (as opposed to~cylindrical ones, with round terminations~at the surface). Our results~clarify the origins and illustrate the diversity of the domain structures in nearly ferromagnetic compounds.   

Distribution of non-adiabatic and adiabatic torques in domain wall systems

Spin torque and the subsequent motion of domain walls caused by coherent carrier transport[1] is an important aspect in the development of spintronic devices[2]. We model spin torque in N\'eel walls[3] in various configurations using a piecewise linear transfer-matrix method[4] and calculate the spin torque distribution[5] throughout the system. We find a large non-adiabatic component to the spin torque throughout the system, that oscillates with position in the wall, as if it would introduce an out-of-plane twist in the magnetization. This twisting effect is especially pronounced in a domain sandwiched between two domain walls, where the non-adiabatic torque increases almost linearly from a large negative value to a large positive value across the domain. We also note differences in the adiabatic torques across materials: ferromagnetic semiconductors have symmetry of the adiabatic torques around the wall center that is lost when considering a magnetic metal. Work supported by an ARO MURI. [1] M. Yamanouchi et al., Nature 428, 539 (2004) [2] S. Parkin et al., Science 320, 190 (2008) [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89, 098302 (2002) [4] E. Golovatski and M. Flatt\'e, Phys. Rev. B, 84, 115210 (2011) [5] J. Xiao et al., Phys. Rev. B, 73, 054428 (2006)   

Large scale magnetic domain wall fluctuations in ultrathin cobalt films

Controlling anisotropy through ion bombardment is a convenient method for manipulating domain walls in perpendicularly magnetized films. In ultrathin (\textless 1nm) cobalt deposited on platinum, exposure to 50eV argon ions reduces the perpendicular magnetic anisotropy until the magnetization lies in plane.~ Just before this in-plane transition, the domain wall energy and pinning strength are reduced such that zero-field Barkhausen-like domain wall jumps become observable at zero field and room temperature. The domain wall jumps are large enough (\textgreater 100nm) to be measured optically. In this work we use magneto-optic Kerr effect to measure how these fluctuations depend on the film thickness and applied magnetic field. Furthermore, we observe magnetostatic correlations between fluctuations in nearby domain walls.   

Voltage Control of Domain Wall Motion in Perpendicular Magnetic Anisotropy Materials

High-performance solid-state operation of a wide variety of spintronic devices requires efficient electrical control of domain walls (DWs). In this work we examine DW dynamics in ultrathin Co films under the influence of an electric field applied across a gadolinium oxide gate dielectric. By measuring the velocity scaling with temperature, driving field, and gate voltage, we verify domain expansion via thermally-activated creep dynamics. We show that an electric field linearly modulates the activation energy barrier $E_{\mathrm{A}}$ that governs DW creep, leading to an exponential dependence of DW velocity on gate voltage. As a consequence, significant voltage-induced velocity enhancement can be achieved in the low-velocity regime, but the efficiency is diminished at high velocities where $E_{\mathrm{A}}$ is correspondingly small. We overcome this limitation by engineering novel device structures with significantly larger voltage induced effects on magnetic anisotropy and demonstrate voltage modulation of the DW propagation field by hundreds of Oe. Implementation into magnetic nanowire devices allows us to engineer gate voltage controlled DW traps which are nonvolatile and robustly switchable for many cycles.   

Direct Imaging the Thermally Excited Magnon Driven Domain Wall Motion in Magnetic Insulators

Experimental demonstrations of domain wall (DW) motion induced by the thermally excited magnons in YIG are revealed by applying spatial/temporal resolved polar MOKE imaging in the presence of various temperature gradients. These results include: (1) the DW moves from the cold regime towards the hot regime (for both positive and negative temperature gradients); (2) a threshold temperature gradient (5 K/mm), $i$.$e$., a minimal temperature gradient required to induce DW motion; (3) the linear relation of the average DW velocity with the (positive/negative) temperature gradients. Our results suggest that DWs in insulating magnetic materials can be effectively manipulated by a magnonic STT simply by applying small temperature gradients. Further efforts are required to understand this exciting phenomenon, such as quantifying the thermally excited spin wave spin current $J_{m}$, resolving the reflection, and transmission of $J_{m}$ across the DW. Nevertheless, our observations demonstrate that, by incorporating thermal effect into DW engineering, insulating magnetic materials could potentially enable many devices for information processing and other applications in spin caloritronics.   

Mechanical manipulation of magnetic domains in continuous and patterned magnetostrictive FeGa thin films

The controlled and reversible switching of magnetic domains using static electric fields has been previously demonstrated via magneto-electric (ME) coupling in a multiferroic system [T. Brintlinger, Nano Lett. 10, 1219(2010)]. In these systems, enhanced magnetostriction allows for magnetic switching in response to an electrically induced deformation. Here we demonstrate the nature of magnetic switching using mechanical stress alone. Magnetostrictive iron-gallium (Fe$_{\mathrm{70}}$Ga$_{\mathrm{30}})$ thin films are deposited on flexible free-standing membranes, and patterned to square arrays. Using a mechanically manipulated tip a strain is directly applied to the film. We observe the resulting magnetization dynamics using Lorentz-force transmission electron microscopy (LTEM). The varied hysteretic behaviors under applied magnetic and strain fields will be presented for both continuous and patterned films.   

Magnon-induced motion of magnetic domain wall in a nanowire with non-uniaxial anisotropy

Magnons propagating along a nanowire may interact with a magnetic domain wall (DM) and shift the DW position. Often, the DW shift direction is opposite to the magnon propagation direction, which can be explained by the angular momentum conservation when the wire has only uniaxial anisotropy. We studied a nanowire with non-zero perpendicular anisotropy constant, in which the angular momentum conservation argument is broken. Additional to the term comes from the angular momentum conservation, we found new term in the DW shift which comes from the rotation of the DW plane during the magnon pass through the DW. The rotation direction gives DW shift in the opposite direction to the magnon propagation direction, and same for two types of transvers DW: head-to-head or tail-to-tail. The magnitude of this term can be comparable to that comes from the angular momentum conservation when the large perpendicular anisotropy and the small magnon wavelength compare to the DW width.   

Current-Induced Dynamics in Antiferromagnetic Metal: Domain Wall Dynamics and Spin Wave Excitation

When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the magnetization via spin transfer torque. However, corresponding theory is absent in antiferromagnetic (AFM) metals due to the absence of spin conservation. We solve this problem via effective gauge theory without the necessity of spin conservation. By identifying the adiabatic dynamics of conduction electrons as a non-Abelian gauge theory on degenerate band, we derive the AFM version of Landau-Lifshitz-Gilbert equation with current-induced dynamics from a microscopic point of view. Quite different from its FM counterpart, current-induced dynamics in AFM materials does not behave as a torque, but a driving force triggering second order derivative of local staggered order with respect to time. Its physical consequences are studied in two examples: 1. A domain wall is accelerated to a terminal velocity without a Walker's threshold; 2. A sufficiently large spin current will generate spin wave excitation.   

Domain wall propagation through spin wave emission

We theoretically study field-induced domain wall (DW) motion in an electrically insulating ferromagnet with hard- and easy-axis anisotropies. Different from the common wisdom, we prove that a DW in a dissipationless wire with a finite transverse magnetic anisotropy can propagate along the wire. The DW subjected to an external magnetic field undergoes a periodic transformation that excites SWs. The energy carried away must be compensated by the Zeeman energy that is released by DW propagation. Thus, a domain wall propagation mode through spin wave emission is revealed. The DW propagation locked into the known soliton velocity at low fields. In the presence of small damping, the usual Walker rigid-body propagation mode may become unstable for magnetic fields below the Walker breakdown.   

Enhanced controllability of domain-wall pinning by asymmetric control of domain-wall injection

Recently, using magnetostatic interactions via the magnetic-charge distributions, a few ideas to effectively and selectively manipulate the DW pinning without additional alterations to the nanowire have been suggested. Even though the DW pinning via the magnetostatic interaction is locally controlled, the pinning strength is insufficient to reliably manipulate the DW propagation in the real DW-mediated device. Here, it is experimentally studied that depinning fields of domain walls (DWs) under an interaction between magnetic charges distributed at a nanobar and at a notch can be enhanced by controlling injection fields for injecting DWs into the ferromagnetic nanowire with an asymmetrical nucleation pad. The DWs injected from the asymmetrical pad show an asymmetrical dependence of the injection field on the saturation angle and are pinned by the notch with the nanobar vertical to it. We have found that the shape of the pinning potential energy experienced by the DW depends on the magnetized direction of the nanobar and the level of that is lifted by the injection field leading to an increase in the depinning field with respect to the saturation angle. This is consistent with our estimation based on micromagnetic simulation.   

Domain Wall Trajectory Determined by its Fractional Topological Edge Defects

A domain wall in a ferromagnetic nanowire is composed of elementary topological bulk and edge defects with integer and fractional winding numbers, respectively. The spatial arrangement of the defects reflects the chiral internal structure of the domain wall. By breaking the symmetry across the width of the nanowire we show that we can control the formation of these topological defects and thereby can form domain walls of a given chirality with high fidelity. Utilizing this capability, we show that the fractional topological edge defects of the domain wall determine its trajectory in branched nanowire networks. Our results can account for the motion of domain walls in complex networks of magnetic nanowires such as ``Artificial Spin Ice'' systems, explaining the formation of Dirac strings, and may also lead to fault-tolerant domain wall based memory and logic devices.   

Dynamics of topological defects in a 2D magnetic stripe pattern

The magnetic stripe domain patterns formed in perpendicularly-magnetized ultrathin films are one example of pattern formation in 2D systems with short-range attractive (exchange) and long-range repulsive (dipole) interactions. Topological pattern defects (dislocations) play a key role in the evolution of the pattern. The magnetic susceptibility due to domain wall motion is very sensitive to the presence of the topological defects, and can be used to study their energetics and population dynamics. The total energy density of the domain pattern is altered by the contribution from the concentration of topological defects, changing the average domain density and magnetic ``stiffness'' in a characteristic way. These changes can be directly monitored in the magnetic susceptibility peak, where the peak location and shape can be related quantitatively to the defect concentration. These ideas are confirmed using recently published data for perpendicularly-magnetized Fe/ 2 ML Ni/W(110) films, and allows the extraction of the characteristic time scale, lifetime, and activation energy for the annihilation of topological defects. In addition, it is possible to quantify the proportion of the domain system energy density that is due to the topological defects.   

Topological defects and misfit strain in magnetic stripe domains of lateral multilayers with perpendicular magnetic anisotropy

Stripe domain patterns are characteristic of magnetic films with perpendicular magnetic anisotropy (PMA). In this work, PMA amorphous Nd-Co films have been nanostructured with a periodic thickness modulation that induces the lateral modulation of magnetic stripe periods and in-plane magnetization. Confinement effects of stripe domains within the nanostructured regions are combined with coupling effects between nearby elements through elastic interactions within the magnetic stripe pattern. The resulting ``lateral'' magnetic superlattice is the 2D equivalent of a strained superlattice controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy: misfit dislocations appear in the stripe pattern at the boundaries between nanostructured regions and, during magnetization reversal, a 2D variable angle grain boundary is observed within the magnetic stripe pattern. Beautiful patterns appear at the point of maximum misfit strain due to the decay of dislocations in the magnetic stripe pattern into 1/2 disclination pairs. The link between topological defects in the magnetic stripe patterns and domain walls for the in-plane magnetization component allow us to tailor the whole magnetization reversal process. [1] A.Hierro-Rodriguez et al, PRL 109(2012)117202   

Current-driven domain wall dynamics in ultrathin heavy-metal/ferromagnet/oxide submicron strips

Recent studies have reported efficient current-driven domain wall (DW) motion and magnetization switching in out-of-plane magnetized structures consisting of an ultrathin (\textless 1 nm) ferromagnetic Co layer embedded between a heavy-metal Pt underlayer and an oxide overlayer such as AlOx [1, 2] and GdOx [3]. These phenomena have been attributed to ``spin-orbit'' torques arising from the metal-oxide interface (Rashba effect) and in the heavy-metal underlayer (spin Hall effect). We investigate current-driven DW motion in submicron-wide strips of ultrathin Ta/CoFe/MgO and Pt/CoFe/MgO. DWs move in the direction of electron flow in Ta/CoFe/MgO, whereas they move against electron flow in Pt/CoFe/MgO. Measurements of the DW propagation field and velocity reveal large spin torque efficiencies exceeding 100 Oe/10$^{\mathrm{11}}$ A/m$^{\mathrm{2}}$ in both structures. Because the signs of the spin Hall angles of Ta and Pt are opposite, the spin Hall effect may partially explain such efficient current-driven DW motion whose directionality differs with the heavy-metal underlayer. [1] I.M. Miron et al, Nat. Mater. 10, 419 (2011). [2] L. Liu et al, arXiv:1110.6846 (2011). [3] S. Emori et al, Appl. Phys. Lett. 101, 042405 (2012)   

Session W15: Focus Session: Theory of Kagome Magnetism

Simplex SU(3) quantum antiferromagnets on the kagome and hyperkagome lattices

We investigate SU(3) ``simplex solid'' antiferromagnets on the kagome and hyperkagome lattices. The ground states of these systems are annihilated by certain local projectors acting on triples of sites, and are analogous to the valence bond solid wavefunctions constructed by Affleck, Kennedy, Lieb, and Tasaki. Using a coherent state representation, we map to a classical model of $CP^2$ spins with 3-spin interactions, which we analyze via single-spin Monte Carlo simulations and a cluster algorithm for the three-body interactions. We compute the static structure factor and short-range correlations encoded by the simplex solid wavefunction and rationalize the results in terms of the ``order by disorder'' mechanism.   

Structure of dynamical correlations developing on top of an entropically designed frustrated manifold

By combining monte carlo and spin dynamics simulations, we investigate the precessional dynamics of the classical kagome antiferromagnet through the calculation of the dynamical structure factor $S({\bf Q},t)$. Recently, evidences for spin wave like excitations in the two distinct low temperature regimes whose temperature ranges are given by the entropically driven onset of spin plane coplanarity at $T_0/J\approx 5\ 10^{-3}$ has been given. However, only a little is known about the longer time scales describing the fluctuations around the ground-state manifold. We give more insight about this relaxationnal dynamics and establish in particular the temperature and wave-vector dependence of the lifetime of locally ordered states. Although the infinite components spins model qualitatively accounts for the dynamical properties in the cooperative paramagnetic regime, we show at lower temperature that the entropic selection (i) leads to strongly different dynamical correlations for the in- plane and out-of-plane spin components below the transition, and (ii) almost suppresses the diffusive behaviour observed in the cooperative regime in favour of mainly propagative spin transfers.   

DMRG Study of the $S\ge 1$ quantum Heisenberg Antiferromagnet on a Kagome-like lattice without loops

The Kagome quantum Heisenberg antiferromagnet, for spin up to $S=1$ and perhaps $S=3/2$, is a prime candidate to realize a quantum spin liquid or valence bond crystal state, but theoretical or computational studies for $S>1/2$ are difficult and few. We consider instead the same interactions and $S\ge1$ on the Husimi Cactus, a graph of corner sharing triangles whose centers are vertices of a Bethe lattice, using a DMRG procedure tailored for tree graphs [1]. Since both lattices are locally identical, properties of the Kagome antiferromagnet dominated by nearest-neighbor spin correlations should also be exhibited on the Cactus, whereas loop-dependent effects will be absent on the loopless Cactus. Our study focuses on the possible transition(s) that must occur with increasing $S$ for the Cactus antiferromagnet. (It has a disordered valence bond state at $S=1/2$ but a 3-sublattice coplanar ordered state in the large $S$ limit [2]). We also investigate the phase diagram of the $S=1$ quantum XXZ model with on-site anisotropy, which we expect to have three-sublattice and valence-bond-crystal phases similar to the kagome case [3]. ([1] Changlani et al, arXiv:1208.1773 (2012), [2] Doucot and Simon, J. Phys. A 31, 5855 (1998), [3] Isakov and Kim, Phys. Rev. B 79, 094408 (2009))   

Wannier Permanents and Featureless Bosonic Mott Insulators on the 1/3 Filled Kagome Lattice

We study Bose-Hubbard models on tight-binding, non-Bravais lattices, with a filling of one boson per unit cell -- and thus fractional site filling. At integer filling of a unit cell, a fully symmetric insulating state is in principle allowed without triggering topological order. We demonstrate by explicit construction of a family of wavefunctions that such a featureless Mott insulating state exists at 1/3 filling on the kagome lattice. We construct Hamiltonians for which these wavefunctions are exact ground states. Such wavefunctions also yield 1/3 magnetization plateau states for spin models in an applied field. The featureless Mott states we discuss can be generalized to any lattice for which symmetric exponentially localized Wannier orbitals can be found at the requisite filling, and their wavefunction is given by the permanent over all Wannier orbitals.   

Spin $1/2$ Heisenberg antiferromanget on kagome: Z$_2$ spin liquid with fermionic spinons

Motivated by recent numerical and experimental studies of spin $1/2$ Heisenberg antiferromagnet on kagome, we formulate a many-body model for the fermionic spinons introduced in Phys. Rev. Lett. 103, 187203. The spinons experience strong onsite attraction. They also couple with a compact U$(1)$ gauge field. The ground state of the model is generically a Z$_2$ liquid. We calculate the edge of the two-spinon continuum, which can be measured in numerics and inelastic neutron scattering experiments.   

Symmetry-broken phases proximate to Z2 spin liquid on Kagome lattice

Recently, $Z_{2}$ spin liquid was proposed as the ground state of the Kagome quantum antiferromagnet [S. Yan, D.A. Huse, and S.R. White, {\it Science}, 332, 1173 (2011)]. We study proximate symmetry-broken phases that may appear on exiting the spin liquid phase, by tuning parameters such as further neighbor couplings. Given that the Dirac spin liquid is also a relatively low energy state, we consider models of $Z_{2}$ spin liquids that are proximate to it. Specifically we consider the s-wave paired state of an algebraic spin liquid on Kagome lattice, $Z_{2} [0,\pi]\beta$ state of Y.-M Lu, Y. Ran, and P.A. Lee, {\it Phys. Rev.} B 83, 224413 (2011)] and examine its relations with other competing states. This allows us to characterize the proximate magnetically ordered and VBS phases and criticality between them and the quantum spin liquid.   

A Phenomenological Theory for the $Z_2$ Spin-Liquid Phase of the $S=1/2$ Kagome Heisenberg Antiferromagnet

The spin-1/2 kagome Heisenberg antiferromagnet is one of the most promising candidate systems for a quantum spin liquid. However, the precise nature of its ground state is still being debated. Recent density-matrix renormalization group (DMRG) calculations show evidence for a possible $Z_2$ spin-liquid phase, the effective description of which is a $Z_2$ gauge theory [1,2]. In this work, we construct a minimal $Z_2$ gauge Hamiltonian encapsulating the DMRG phenomenology in the $S=0$ sector. We generalize Misguich's Hamiltonian [3] by including dynamical visons [4]. We show that our minimal model naturally produces the diamond resonance pattern observed in DMRG. Moreover, puzzling even-odd effects in kagome cylinders are easily explained by our model. We also predict the existence of edge spinons in certain cylindrical geometries.\\[4pt] [1] S. Yan, D. A. Huse, and S. R. White, Science \textbf{332}, 1173 (2011).\\[0pt] [2] S. Depenbrock, I. P. McCulloch, and U. Schollw\"ock, Phys. Rev. Lett. \textbf{109}, 067201 (2012).\\[0pt] [3] G. Misguich, D. Serban, and V. Pasquier, Phys. Rev. Lett. \textbf{89}, 137202 (2002).\\[0pt] [4] Y. Huh, M. Punk, and S. Sachdev, Phys. Rev. B \textbf{84}, 094419 (2011).   

Spinon-vison interactions in kagome-lattice spin liquids

Recent neutron-scattering measurements on the kagome-lattice antiferromagnet Herbertsmithite [1] suggest that the ground state is well-described by a spin liquid consisting of weakly correlated (i.e., non-dispersing) singlets. We consider how these observations can be accounted for within a Schwinger-boson mean-field theory, by including interactions between spinons (i.e., the spin-1/2 excitations of the Z$_2$ spin liquid) and the topological excitations known as visons. We compute the dynamic structure factor (which is measured in the experiments of Ref. [1]) as a function of a phenomenological spinon-vison coupling constant, and discuss how this coupling constant may be extracted from numerics. [1] T.H. Han et al., to appear.   

Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice

Ever since the experiments which founded the field of highly frustrated magnetism, the kagome Heisenberg antiferromagnet has been the archetypical setting for the study of fluctuation induced exotic ordering. To this day the nature of its classical low-temperature state has remained a mystery: the non-linear nature of the fluctuations around the exponentially numerous harmonically degenerate ground states has not permitted a controlled theory, while its complex energy landscape has precluded numerical simulations at low temperature. Here we present an efficient Monte Carlo algorithm which removes the latter obstacle. Our simulations detect a low-temperature regime in which correlations saturate at a remarkably small value. Feeding these results into an effective model and analyzing the results in the framework of an appropriate field theory implies the presence of long-range dipolar spin order with a tripled unit cell.   

Spin Correlations in Quantum Spin Liquids on the Kagome Lattice

The spin-1/2 Heisenberg kagome antiferromagnet, due to its highly frustrated nature, is considered a prime candidate to realize a spin-liquid ground state that breaks no symmetry and hosts fractionalized magnetic excitations. Recent numerical results indicate a close competition for the ground state between different spin-liquid states. We study spin correlations in competing phases, suggest possible experiments to distinguish different ground states, and discuss the application of these ideas to Herbertsmithite.   

p6 - Chiral Resonating Valence Bonds in the Kagome Antiferromagnet

The Kagome Heisenberg antiferromagnet is mapped onto an effective Hamiltonian on the star superlattice by Contractor Renormalization. Comparison of ground state energies on large lattices to Density Matrix Renormalization Group justifies truncation of effective interactions at range 3. Within our accuracy, magnetic and translational symmetries are not broken (i.e. a spin liquid ground state). However, we discover doublet spectral degeneracies which signal the onset of p6 - chirality symmetry breaking. This is understood by simple mean field analysis. Experimentally, the p6 chiral order parameter should split the optical phonons degeneracy near the zone center. Addition of weak next to nearest neighbor coupling is discussed.   

Normal Modes of Frustrated Spins on a Kagome Lattice

We study the normal modes of spins in a classical kagome antiferromagnetic Heisenberg model (KAHM), seeking evidence for the canonical and gauge-like zero modes predicted by the constrained spin model of Ref. [1]. We do so by splitting the degeneracy of the low energy configuration space through the introduction of Dzyaloshinski-Moriya (DM) interactions of strength D, performing a Monte-Carlo calculation to find the new ground state configuration, expanding the Hamiltonian to quadratic order about the minimum and diagonalizing the resulting problem to obtain the normal modes. We find that the resulting spectrum splits up into modes that scale with J, the strength of the Heisenberg interactions, and modes that scale with D and D$^{2}$/J. The latter two types of modes map directly into the canonical and gauge-like modes of the constrained spin model. In addition, we find clear evidence for ``edge modes,'' which involve the motion of the dangling triangles, in agreement with the conjecture of Ref. [1]. Our calculations shed much light on how the low energy spin dynamics of the classical KAHM behaves like a gauge theory. \\[4pt] [1] Michael J Lawler, Emergent Gauge Dynamics of Highly Frustrated Magnets, arXiv:1104.0721   

Monte Carlo Simulations of FCC Kagome Lattice: Competition Between Triangular Frustration and Cubic Anisotropy

The impact of an effective local cubic anisotropy [1] on the magnetic states of the Heisenberg model on the FCC kagome lattice are examined through classical Metropolis Monte Carlo simulations. Previous simulations revealed that the macroscopic degeneracy of the 2D kagome exchange-coupled co-planar spin system persists in the 3D case of ABC stacked layers [2] giving rise to a discontinuous (possibly order-by-disorder) phase transition. Local cubic anisotropy is shown to reduce this degeneracy by re-orienting the spins out of the co-planar configuration. In addition, the re-oriented states are shown to carry a uniform magnetic moment. The effect of anisotropy on the order of the phase transition will also be reported. These results are relevant to Ir-Mn alloys which have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in GMR and TMR spin valves [2]. \\ $[1]$ L. Szunyogh, B. Lazarovits, L. Udvardi, J. Jackson, and U. Nowak, Phys. Rev. B 79, 020403(R) (2009).\\ $[2]$ V. Hemmati, M.L. Plumer, J.P. Whitehead, and B.W. Southern, Phys. Rev. B 86, 104419 (2012).   

Session W16: Focus Session: Biomagnetics, Magneto-Optics, and Ultrafast Effects

GMAG PhD Dissertation Research Award Talk: Dynamic Magnetic Traps for Particle Self-Assembly and Lab-on-Chip Applications

Micro-patterned Permalloy thin films serve as an excellent means to architect the spatial profile of magnetic fields with the tunable, high gradients required to manipulate objects with weak induced magnetic moments. In this presentation, I will highlight two projects carried out during my PhD studies. These findings demonstrate the functionalities achieved through carefully designed patterns of different sizes and shapes (e.g. circular, triangular, octagonal profiles): (i) By tuning a precessing magnetic field in conjunction with such Permalloy patterns, microsphere (i.e. dipole) cluster structures ranging from closely packed to frustrated and to plum-pudding-like planar lattices are stabilized. Such self-assembly of components at the micro to nanometer range not only support a rich variety of physical phenomena, but also have applications, for example, as filters or force probes and field-tunable photonic crystals. (ii) Mobile magnetic trap arrays consisting of Permalloy disks have enabled rapid transport of magnetic beads or immunomagnetically labeled cells across surfaces. Integration of these arrays with microfluidic droplet technology allows separation of labeled cells and their subsequent encapsulation into picoliter-sized droplets. The droplets serve as isolated containers for individual cells to be probed without cross-contamination. The separation-encapsulation function could become a critical component in point-of-care single-cell analysis platforms.   

Magnetic multicomponent nanoparticles Cu$_{x}$Mn$_{1-x}$Fe$_{2}$O$_{4}$ for biomedical applications

Magnetic nanoparticles (NPs) are increasingly important in many biomedical applications, such as drug delivery, hyperthermia, and magnetic resonance imaging (MRI) contrast enhancement. In this multicomponent nanoparticles Cu$_{x}$Mn$_{1-x}$Fe$_{2}$O$_{4}$ (CuMnF), x$=$ 0, 0.6, 1, were prepared by hydrothermal synthesis, sol-gel and solid state methods. To build the most effective magnetic nanoparticle systems for various biomedical applications, particle characteristics including size, surface chemistry, magnetic properties and toxicity have to be fully investigated. In this work, effects of production methods of magnetic nanoparticles for the bio-medical applications are discussed. X-ray powder diffractometry (XRD), scanning electron microscopy (SEM) and vibrating scanning magnetometer (VSM) were used to characterize the structural, morphological and magnetic properties. The particle size of samples is measured by Malvern Instruments Zeta Sizer Nano-ZS instrument. The temperature dependence of field cooled (FC) magnetization of all Cu$_{x}$Mn$_{1-x}$Fe$_{2}$O$_{4}$ samples have been shown here. The data were recorded under 1k Oe and 100 Oe magnetic fields for different ratio.   

Detection of low-concentration superparamagnetic nanoparticles using a functional biosensor based on magneto-impedance technology

Improving the sensitivity of existing magnetic biosensors for detection of magnetic nanoparticles as biomarkers in biological systems is an important and challenging task. Here we demonstrate the possibility of combining the magneto-resistance (MR), magneto-reactance (MX), and magneto-impedance (MI) effects to develop a functional magnetic biosensor with tunable and enhanced sensitivity. A systematic study of the 7 nm Fe$_{\mathrm{3}}$O$_{\mathrm{4\thinspace }}$nanoparticle concentration dependence of MR, MX, and MI ratios of a soft ferromagnetic amorphous ribbon shows that these ratios first increase sharply with increase in particle concentration (0 - 124 nM) and then become unchanged for higher concentrations ($>$124 nM). This points to the sensitivity and limit of the detection of the biosensor. The MX-based biosensor shows the highest sensitivity. With this sensor, 2.1$\times $10$^{\mathrm{11}}$ 7 nm Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ nanoparticles can be detected over a detection area of 2.0$\times $10$^{\mathrm{5\thinspace }}\mu $m$^{\mathrm{2}}$, which is comparable to a SQUID biosensor that detects the presence of 1$\times $10$^{\mathrm{8\thinspace }}$11 nm Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ nanoparticles over a detection area of 6.8$\times $10$^{\mathrm{4\thinspace }}\mu $m$^{\mathrm{2}}$.   

Characterization of magnetic nanoparticles using Magnetic Hyperthermia System (MHS) for the application in cancer treatment

In this study, the heating profiles of various concentrations of three Fe$_{3}$O$_{4}$ magnetic nanoparticle systems were measured when the nanoparticles were exposed to alternating magnetic fields in a RF Magnetic Hyperthermia System. The Fe$_{3}$O$_{4}$ core nanoparticles of each system were approximately 10nm in diameter, but each system had different nanoparticle configurations and surface modifications. The heating profiles were used to investigate the dominant heating mechanism, the heat transfer into the surrounding fluid, and the overall effectiveness of each nanoparticle system for possible use in hyperthermia cancer treatments. Magnetization measurements showed that all samples were superparamagnetic in nature with almost zero retentivity and coercivity. For all samples, the saturation magnetization was observed to increase linearly with increasing concentration of Fe$_{3}$O$_{4}$. Five different concentrations of the three Fe$_{3}$O$_{4}$ nanoparticle samples were exposed to a 13.56 MHz alternating magnetic field with an amplitude of 4500 A/m, while the solution temperature was measured as a function of time using an optical fiber temperature probe. A correlation was observed between the heating rate, the initial susceptibility, and the type of surface modification of the Fe$_{3}$O$_{4}$ nanoparticles.   

Magnetization measurements of magnetic fluids

Magnetic fluids are used for damping in vehicle suspensions, as MRI contrast agents, heat transfer materials, and even in art installations. Most of these applications benefit from high quality magnetic characterization. Techniques for measuring magnetization ($M$) of materials, such as vibrating sample magnetometry (VSM), and superconducting quantum interference device (SQUID) magnetometry, are well-developed for small solid samples such as bulk crystals and thin films. This presentation discusses special issues that arise in measurement of fluid samples. First, the effects of the sample vessel must be taken into account. Often, the vessel must be vacuum-tight; care must be taken that the sealing process does not physically change the properties of the fluid. Then, the portion of the signal due to the sample vessel should be subtracted from the total, not a trivial subtraction as the sample vessel has a different geometry from the sample (in contrast to, e.g., a thin film sample and substrate). In addition, the sample must be centered, adding an additional degree of difficulty when the material is fluid and the center position may be a dynamic property. Our results show that incorrect centering can lead to not only incorrect values of $M$, but to a change in the shape of $M(H)$.   

Probing Brownian relaxation in water-glycerol mixtures using magnetic hyperthermia

Generation of heat by magnetic nanoparticles in the presence of an external oscillating magnetic field is known as magnetic hyperthermia (MHT). This heat is generated by two mechanisms: the Neel relaxation and Brownian relaxation. While the internal spin relaxation of the nanoparticles known as Neel relaxation is largely dependent on the magnetic properties of the nanoparticles, the physical motion of the particle or the Brownian relaxation is largely dependent on the viscous properties of the carrier liquid. The MHT properties of dextran coated iron oxide nanoparticles have been investigated at a frequency of 400KHz. To understand the influence of Brownian relaxation on heating, we probe the MHT properties of these ferrofluids in water-glycerol mixtures of varying viscosities. The heat generation is quantified using the specific absorption rate (SAR) and its maximum at a particular temperature is discussed with reference to the viscosity.   

Propagation of Electromagnetic Waves in 3D Opal-based Magnetophotonic Crystals

Opals, a class of self-organized 3D nanostructures, are typical representatives of photonic bandgap structures. The voids inside of the opal structure of close packed SiO$_{2}$ spheres can be infiltrated by a magnetic material, creating magnetically tunable magnetophotonic crystals with interesting and potentially useful properties at GHz and THz frequencies. The propagation of electromagnetic waves at microwave frequencies was investigated numerically in SiO$_{2}$ opal based magnetic nanostructures, using rigorous mathematical models to solve Maxwell's equations complemented by the Landau-Lifshitz equation with electrodynamic boundary conditions. The numerical approach is based on Galerkin's projection method using the decomposition algorithm on autonomous blocks with Floquet channels. The opal structure consists of SiO$_{2}$ nanospheres, with inter-sphere voids infiltrated with nanoparticles of Ni-Zn ferrites. Both the opal matrix and the ferrite are assumed to be lossy. A model, taking into account the real structure of the ferrite particles in the opal's voids was developed to simulate the measured FMR lineshape of the ferrite infiltrated opal. The numerical technique shows an excellent agreement when applied to model recent experimental data on similar ferrite opals.   

Ultrafast Magnetization Enhancement in Metallic Multilayers Driven by Superdiffusive Spin Current

We report on the surprising enhancement in the magnetization of iron in Ni:Fe based multilayer structures following the excitation by an ultrafast laser pulse. Few femtosecond extreme ultraviolet pulses from tabletop high harmonic generation, tuned to the M-edges of Ni and Fe, are used to probe the layer- and element- specific spin dynamics in multilayer structures of Ni/X/Fe, where X is Ru, Ta, W, or Si3N4. We find that both the Ni and Fe moments demagnetize on timescales of 100 fs when excited by an ultrafast optical pulse, for good spin scattering and insulating spacer layers consisting of Ta, W, and Si3N4. However, we also find that the Fe magnetization is enhanced by 16{\%} for Ru spacer layers of 1.7 nm thickness, when the magnetizations of the Fe/Ni layers are initially aligned parallel. Our observations can be explained by a laser-generated superdiffusive spin current between the Ni and Fe layers, whereby a substantial current of majority spins injected into the Fe layer enhances its magnetization. [1] [1] D. Rudolf et. al. Nat. Comm. 3, 1037 (2012)   

A microscopic model for ultrafast remagnetization dynamics

In this work, we provide a microscopic model for the ultrafast remagnetization of atomic moments already quenched above Stoner-Curie temperature by a strong laser fluence. Combining first principles density functional theory, atomistic spin dynamics utilizing the Landau-Lifshitz-Gilbert equation and a three temperature model, we show the temporal evolution of atomic moments as well as the macroscopic magnetization of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to picoseconds. Our simulations show [1] a variety of complex temporal behavior of the magnetic properties resulting from an interplay between electron, spin and lattice subsystems, which causes an intricate time evolution of the atomic moment, where longitudinal and transversal fluctuations result in a macro spin moment that evolves highly non-linearly.\\[4pt] [1] Raghuveer Chimata, Anders Bergman, Lars Bergqvist, Biplab Sanyal and Olle Eriksson, Phys. Rev. Lett. {\bf 109}, 157201 (2012).   

Current understanding of the laser-induced ultrafast (de)magnetization process

The laser-induced ultrafast (de)magnetization process in ferromagnets is complex. There are several theories available [1], but none of these is satisfactory. In this talk, we first review several theoretical formalism for femtomagnetism and point out strengths and weaknesses of each theory [2]. In particular, we address issues associated with comparing experimental and theoretical results, which have been very challenging. Our first-principles theory includes electron correlation and electron-phonon effects along with spin-orbit coupling in metals or rare-earth compounds. Some of the newest results are presented, which are expected to tremendously enhance our understanding of the overall (de)magnetization process [3].\\[4pt] [1] G. P. Zhang, G. Lefkidis, W. H\"ubner, and Yihua Bai, J. APPL. PHYS. {\bf 111}, 07C508 (2012).\\[0pt] [2] M. S. Si and G. P. Zhang, AIP ADVANCES {\bf 2}, 012158 (2012).\\[0pt] [3] G. P. Zhang, PHYSICAL REVIEW B {\bf 85}, 224407 (2012).   

Session W17: Focus Session: CMR Manganites

Paramagnetic Spin Fluctuations in Optimally Doped CMR Manganites La$_{0.7}A_{0.3}$MnO$_{3}$ ($A$ = Ca, Sr, Ba)

Hole doped perovskites of the form La$_{0.7}A_{0.3}$MnO$_{3}$ (where $A$ = Ca, Sr, or Ba) display colossal magnetoresistance at a combined ferromagnetic and metal-insulator transition. The spin fluctuation spectrum of these materials develops a quasielastic spin diffusive central component that dominates the spectrum near $T_{C}$. We report inelastic neutron scattering measurements that reveal an additional and unexpected component to the spin fluctuation spectrum, in the form of anisotropic ridges of surprisingly strong quasielastic scattering running along ($H$~0~0) and equivalent directions. Temperature and field dependent measurements show that this scattering is most pronounced at temperatures in the paramagnetic phase and is suppressed by applied magnetic fields exceeding 10 Tesla.   

Role of covalency in ``Charge Ordering'' perovskite ferrates

Transition-metal oxides (TMO) with the perovskite crystal structure exhibit strong electron--electron correlation effects and complex structural distortions. The balance of those factors determines the stability of charge ordered states in chemistries susceptible to valence instabilities. We use first-principles density functional calculations to investigate the role of symmetry-unique structural distortions on covalent bonding in the ``charge-ordered'' insulator CaFeO$_3$. We evaluate the electronic density distribution along the Fe--O bonds to assess the ground state stability by tracing the evolution in the oxygen environment, which appears as octahedral expansion/contractions and rotations. We show that nearly zero charge transfer occurs; the insulating phase results from a complex interplay of symmetry-lowering structural distortions and enhanced covalent interactions. Finally, we discuss possible routes to control the metal--insulator transition by fine-tuning the covalency.   

Magnetic, structural and magneto-resistance studies of doped LaMnO$_3$ bulk samples prepared by citrate combustion process

We present the structural and magnetic properties of polycrystalline samples of La$_{0.95}$A$_{0.05}$MnO$_{3}$ (where A = Na, Sr, Er, Dy and Ce) prepared by chemical citrate combustion method. Er substituted samples (La$_{1-x}$Er$_{x}$MnO$_{3}$ with x = 5, 10, 20 and 30\%) were also investigated, because their studies lack in the literature. The pervoskite structure was confirmed from X-ray diffraction and Raman spectra in these doped samples, excluding higher Er substituted samples (x $>$ 0.1). The grain sizes were around 2-3 micrometres for all of the sintered samples (at 1300$^{\circ}$C), whereas it was below 100 nm for the powders calcined at 600$^{\circ}$C, determined from the SEM images. Curie transition temperatures in those doped LMO bulk samples were found to be around 250K, which is higher than the ideal value ($\sim$140 K) for undoped samples. The increase in the Curie temperature can be related to non-stoichiometry and cation vacancies created due to higher/lower valence substitutions for trivalent La$^{3+}$ ions. The temperature dependence of resistivity also confirms the MIT transition in some of these samples.   

Theory of K-edge resonant inelastic x-ray scattering and its application for La$_{0.5}$Sr$_{1.5}$MnO$_{4}$

We present a formula based on tight-binding approach for the calculation of K-edge resonant inelastic x-ray scattering spectrum for transition metal oxides, by extending the previous result [K. H. Ahn, A. J. Fedro, and M. van Veenendaal, Phys. Rev. B 79, 045103 (2009).] to include explicit momentum dependence and a basis with multiple core hole sites. We apply this formula to layered charge, orbital, and spin ordered manganites, La$_{0.5}$Sr$_{1.5}$MnO$_{4}$. The K-edge RIXS spectrum is found not periodic with respect to the actual reciprocal lattice, but approximately periodic with respect to the reciprocal lattice for the hypothetical unit cell with one core hole site. With experimental strcuture and reasonable tight-binding parameters, we obtain good agreement with experimental data, in particular, with regards to the large variation of the intensity with momentum. We find that the screening in La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ is highly localized around the core hole site and demonstrate the potential of K-edge RIXS as a probe for the screening dynamics in materials.   

LSCO: Consistent agreement for electronic structure and experimental X-ray spectra

We have investigated magnetic properties of La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CoO$_{\mathrm{3}}$ (LSCO) as a function of Sr doping with X-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism(XMCD) at the O K edge and a first principles method. Experiment shows the peak of the oxygen XAS at beginning of the edge is increased with increasing Sr doping. The calculations, using supercells, are in good agreement with detailed XAS of the O K-edge as a function of doping. XMCD calculations reproduce the full experimental spectrum well, and show an increase of the magnetic moment on the oxygen with the number of Sr nearest neighbors. The calculations show that the hybridization involving Co d- and O-p electrons is the key factor for obtaining agreement with the changing XAS spectra as a function of doping. In this talk, we will discuss the XAS, XMCD results and the large external magnetic field effects on the ground state of LSCO(x$=$0).   

Crossover from Polaronic to Magnetically Phase-Separated Behavior in La$_{1-x}$Sr$_x$CoO$_3$

Dilute hole-doping in La$_{1-x}$Sr$_x$CoO$_3$ leads to the formation of ``spin-state polarons'' where a non-zero spin-state is stabilized on the nearest Co3+ ions surrounding a hole [1]. Here, we discuss the development of electronic/magnetic properties of this system from non-magnetic x=0, through the regime of spin-state polarons, and into the region where longer-range spin correlations and phase separation develop. We present magnetometry, transport, heat capacity, and small-angle neutron scattering (SANS) on single crystals. Magnetometry indicates a crossover with x from Langevin-like behavior (polaronic) to a state with a freezing temperature and finite coercivity. Fascinating correlations with this behavior are seen in transport measurements, the evolution from polaronic to clustered states being accompanied by a crossover from Mott variable range hopping to intercluster hopping. SANS data shows Lorentzian scattering from short-range ferromagnetic clusters first emerging around x = 0.03 with correlation lengths of order two unit cells. We argue that this system provides a unique opportunity to understand in detail the crossover from polaronic to truly phase-separated states.\\[4pt] [1] A. Podlesnyak et al., Phys. Rev. Lett. 101, 247603.   

Energy-loss magnetic circular dichroism measurements of ferromagnetic ordering in LaSrCoO$_3$

Experimental results show that tuning the ferromagnetism of LaSrCoO$_3$ can be achieved at various temperatures by doping bulk sample with smaller atoms or straining thin film sample. In this work, we will use atomic-resolution Z-contrast imaging, annular bright field (ABF) imaging and electron energy-loss spectroscopy in the aberration-corrected JEOL JEM-ARM200CF in combination with in-situ heating and cooling experiments to examine the magnetic and spin-state transitions in La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CoO$_3$ (x$=$0-0.3) between 80 K and 600 K. Using energy-loss magnetic circular dichroism method, we confirm the magnetic ordering transition at room temperature with increasing doping concentrations. Differences in the O K- and Co-L-edges will be utilized to determine the Co valence of the samples. A magnetic transition is observed in 5{\%} doped sample during in-situ cooling experiment to 95 K. Additionally, with increasing the doping concentration, a change in crystal structure is measured using ABF imaging, more specifically a distortion of the CoO$_6$ octahedra.   

Metastable low-spin character of Co$^{2+}$ and the control of spin state transition

We have studied different spin states of the octahedrally coordinated Co$^{2+}$ systems. For every tested systems, we found metastable character of low-spin phase and, interestingly, the energy differences between the high-spin and low-spin phases are similar regardless of the anion (X) type, Co$^{+2}$-X bond lengths and CoX$_{6}$ octahedron distortion. For CoCl$_{2}$ as a model system, we studied pressure-induced high-spin to low-spin state transition, which is governed by $J$/$\Delta_{CF}$ value ($J$: exchange parameter, $\Delta_{CF}$: crystal-field parameter). CoCl$_{2}$ shows sudden collapse of volume and spin moment at the point of spin state transition together with the insulator-to-metal transition. Unlike the other transition-metal oxides, which shows pressure-driven Mott-type transition, physics of CoCl$_{2}$ is determined mainly by $J$ and $\Delta_{CF}$, not by $U$ and $W$.   

Pressure Effect on the Structural Transition and Suppression of the High-Spin State in the Triple-Layered T'-La4Ni3O8

We have carried out a comprehensive high-pressure study on the triple-layer T'-La4Ni3O8 with a suite of experimental probes, including structure determination, magnetic, and transport properties up to 50 GPa. Consistent with a recent ab inito calculation [1], application of hydrostatic pressure suppresses an insulator-metal spin-state transition at Pc $\sim$ 6 GPa. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For P $>$ 20 GPa, the ambient T' structure transforms gradually to a T'-type structure, which involves a structural reconstruction from fluorite La-O2-La blocks under low pressures to rock-salt LaO-LaO blocks under high pressures. Absence of the metallic phase under pressure has been discussed in terms of local displacements of O2- ions in the fluorite block under pressure before a global T* phase is established [2]. Ref. [1] V. Pardo and W. E. Pickett, Phys. Rev. B 85, 045111 (2012). [2] J.-G. Cheng, et al. Phys. Rev. Lett. 108, 236403(2012).   

Lattice and transport properties of the misfit-layered oxide thermoelectric Ca$_3$Co$_4$O$_9$ from first principles

The misfit-layered oxide Ca$_3$Co$_4$O$_9$ (CCO) has recently been the subject of many experimental and some theoretical investigations due to its remarkable thermoelectric properties. CCO is composed of two incommensurate subsystems, a distorted rocksalt-type Ca$_2$CoO$_3$ layer sandwiched between hexagonal CoO$_2$ layers. Taking into account that the composition ratio between these subsystems is very close to the golden mean, which is the limit of the sequence of the ratios of consecutive Fibonacci numbers $F(n)$, we model CCO from first principles\footnote{A. Rebola, R. F. Klie, P. Zapol, and S. Ogut, Phys. Rev. B {\bf 85}, 155132 (2012)} by using rational approximants of composition [Ca$_2$CoO$_3$]$_{2F(n)}$[CoO$_2$]$_{2F(n+1)}$. In the present study, we use 3/2 and 5/3 rational approximants and PBE+U computations to calculate the {\em ab initio} phonon dispersion curves, related thermal properties, as well as {\em ab initio} electronic transport properties such as DC conductivity and thermopower within the relaxation time approximation by applying the Boltzmann transport theory. Results are compared with available experimental data and potential routes for increasing the thermopower of CCO are discussed.   

Magnetostructural transitions and metamagnetism induced by Ising spins in spinel-rock salt intergrowth Co$_{10}$Ge$_3$O$_{16}$

Co$_{10}$Ge$_3$O$_{16}$ crystallizes in an intergrowth structure featuring alternating layers of spinel and rock salt, making it related to GeCo$_2$O$_4$. Variable-temperature synchrotron X-ray powder diffraction, magnetometry, and heat capacity experiments reveal a magnetostructural transition at antiferromagnetic $T_N$ = 205 K. This rhombohedral-to-monoclinic transition involves a slight elongation of the CoO$_6$ octahedra. Curie-Weiss analysis suggests that the Co$^{2+}$, with $S$ = 3/2 and $L$ = 3, acts as a Kramer's doublet due to spin-orbit coupling. Below $T_N$, the Ising-like Co$^{2+}$ causes spin reorientation at high applied magnetic field that is first seen as an upward kink in $M$-$H$ near $H_C$ = 3.9 T. A ``butterfly'' loop emerges when $T <$ 150 K, with the transition causing hysteresis at high fields while linear and reversible behavior persists at low fields. $H_C$ decreases as temperature is lowered and the loops at positive and negative fields merge beneath $T$ = 20 K. The low-temperature behavior is complicated by a field-induced first-order transition that is observed in temperature-dependent measurements for $H >$ 1000 Oe. We discuss the $H$-$T$ phase diagram with reference to other measurements including neutron powder diffraction and high-field magnetometry.   

Anisotropy and Magnetostriction in Cobalt-Modified Magnetite: A Crystal Field Approach

The anisotropy and magnetostrictive properties of magnetite are altered by the introduction of cobalt ions into the spinel crystal lattice. 4{\%} of Co$^{2+}$ substituted for Fe$^{2+}$ changes both the sign and magnitude of magnetocrystalline anisotropy coefficient. Such strong dependence can be useful for tailoring the properties of cobalt-iron oxides for applications. This is especially important, considering that cobalt ferrite materials prepared for magnetostrictive, multiferroic and other related applications often deviate from targeted or stoichiometric compositions. In this study, magnetite has been systematically modified by substitution of cobalt. The changes in anisotropy and magnetostriction have been studied and can be explained using the single ion model. The agreement between the trend observed in this experimental investigation and previous theoretical studies is noteworthy. The variation in anisotropy and magnetostriction will be presented on the basis of two competing factors; the unquenched orbital angular momentum of Co$^{2+}$ and changes in the crystal field due to Co$^{2+}$ substitution.   

Thermal expansion study of anisotropic magnetolattice coupling and antiferromagnetic transition in CuO

Transition metal oxides have been the subject of intense research over the past few decades since they form the basic building block of many materials showing exotic properties such as high temperature superconductivity, spin and charge ordering, magnetoresistance, multiferroicity etc. Recently, Kimura et al. demonstrated an intriguing coupling between electric and magnetic dipole ordering in CuO, which opened a new route for finding materials exhibiting induced multiferroic behavior.\footnote{T. Kimura et al., Nature Mater. \textbf{7}, 291 (2008).} Here we present results on anisotropic thermal expansion in single crystalline CuO in the temperature range $5 < T < 350$ K. Our results demonstrate anisotropic magnetolattice coupling in CuO around the two known antiferromagnetic phase transitions at $T_{N1}$= 230 K and $T_{N2}$= 213 K. We also discuss the pressure dependence of $T_{N1}$ and critical behavior in CuO using the scaling of heat capacity and thermal expansion data.   

Ab initio study on magnetic coupling in A-site-ordered perovskite CaCu3B4O12 (B=Ti, Ge, Zr, and Sn)

Magnetism of A-site-ordered perovskites, CaCu$_3$Ti$_4$O$_{12}$, CaCu$_3$Ge$_4$O$_{12}$, CaCu$_3$Sn$_4$O$_{12}$, and CaCu$_3$Zr$_4$O$_{12}$, is comprehensively studied by ab initio electronic structure calculations. The magnetic exchange constants between Cu spins, $J_1$, $J_2$ and $J_3$, are estimated via an effective Heisenberg model, which reveals relative importance of $J_3$ despite its long interaction length. The ground-state magnetic order is reasonably explained by combination of relatively weak ferromagnetic super-exchange interaction ($J_{1}$ and $J_{2}$) and dominant super-exchange interaction ($J_{3}$) which can be tuned by replacement of the B-site element. We will also discuss the effect of A-site-cation replacement by comparing with the results of other A-site-ordered perovskite materials.   

Session W18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Dynamic and Nuclear Effects

THz Magneto-photoresponse of an InAs-based Quantum Point Contact Structure in the Region of Cyclotron Resonance

We have studied the THz magneto-photoresponse of a 2DEG in an InAs quantum well with an embedded Quantum Point Contact in the frequency/field region where electron cyclotron resonance (CR) dominates the response suing several lines from an optically pumped THz laser. The photoresponse near CR is manifested as an envelope of the amplitude of the Shubnikov-de Haas oscillations of the 2DEG with a peak near the CR field. Clear spin-splitting of the quantum oscillations is observed for B \textgreater\ 4, while the SdH oscillations do not show resolved spin-splitting up to 10 T. Data were simulated by a model of resonant carrier heating (due to CR), and from the simulations the carrier density, the CR effective mass, scattering times and the g-factor were obtained. We find a significantly enhanced g-factor, apparently due to many-electron exchange interaction effects. The g-factor determined from fitting spin-split Landau level peaks increases with magnetic field. Work at UB was supported by NSF DMR 1008138 and the Office of the Provost; work at the University of Cincinnati was supported by NSF ECCE 1028483.   

Terahertz excitation and control of spin photocurrents in a semiconductor nanostructure

Time dependent calculations of induced photocurrents are presented for ZnMnSe semiconductor nanostructures under the action of a static magnetic field of a few Tesla. The study shows the existence of spectral domains in the THz range for which the spin polarization in the photocurrent is strongly sensitive to static biases applied in the growth direction of the structures. For such photon frequencies, changing the bias is predicted to reverse the spin polarization quite effectively for specific absorption frequencies. This behavior suggests the possibility of conveniently simple mechanisms for switching and torque generation. The physics underlying these results is studied and understood in terms of the spin dependent profiles of the structures.   

Anomalous spin precession and spin Hall effect in semiconductor quantum wells

We study the contributions of the anomalous position operator to the spin-Hall effect in quasi two-dimensional semiconductor quantum wells with strong band structure spin-orbit interactions. The skew scattering and side-jump \textit{scattering} terms in the SHE vanish, but we identify two additional terms in the SHE due to the anomalous position operator. One term reflects the modification of the spin precession due to the action of the external electric field, which produces an effective magnetic field perpendicular to the plane of the quantum well. The other term reflects a similar modification of the spin precession due to the action of the electric field created by random impurities. We refer to these two effects collectively as \textit{anomalous spin precession}. In electron systems with weak momentum scattering, anomalous spin precession due to the external electric field equals 1/2 the side-jump SHE, while the additional impurity-dependent contribution depends on the form of the band structure SO coupling. For band structure SO linear in wave vector the two additional contributions cancel. For band structure SO cubic in wave vector external electric field contribution can be detected through its density dependence. In 2D hole systems both additional contributions vanish.   

Electrically generated nuclear spin polarization in In$_{.04}$Ga$_{.96}$As

The promises of lower power consumption and simple interfacing to magnetic storage has driven interest in the development of spintronics, in which devices could take advantage of electron spin as a means to store, move, and process data. Due to its long lifetime in moderate fields, nuclear polarization could serve as intermediate timescale data storage in both classical spintronic and quantum computation schemes. Here, we investigate the role of nuclear spins in materials with electrically driven spin polarization. The electron spin polarization generated by electrical current in a non-magnetic semiconductor is transferred via dynamic nuclear polarization to the nuclei. The resulting nuclear field is interrogated using Larmor magnetometry. We measure nuclear field as a function of current, applied magnetic field, and temperature. Polarization decay dynamics and the role of nuclei in devices are also discussed.   

Knight shift and quadrupolar relaxation measured by NMR in Fe/GaAs heterostructures

We report on all-electrical measurements of nuclear magnetic resonance (NMR) in epitaxial (100) Fe/GaAs heterostructures with a channel doping (Si) of $n =5\times 10^{16}$~cm$^{-3}$. By changing the electrical bias, measurements of NMR were performed as a function of spin accumulation. A Knight shift due to the presence of spin-polarized electrons is demonstrated under conditions of large (10-20\%) spin polarization. The effects of nuclear quadrupole moments are also investigated. Although GaAs is cubic, strain induced field gradients split the NMR line into quadrupole multiplets. We investigate the role of nuclear quadrupole relaxation as a function of temperature. Phonon induced quadrupolar relaxation is expected to increase strongly with temperature and be more pronounced for the As nuclei. We show that the evolution of the relative magnitude of the NMR peaks as a function of temperature agrees well with a model dominated by quadrupole relaxation. Supported by NSF DMR-0804244 and DMR-1104951.   

Long-lived electron spins in a modulation doped (100) GaAs quantum well

We have measured $T_1$ spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. $T_1$ lifetimes in excess of 1 microsecond were measured at 1.5 K and 5.5 T, exceeding the typical $T_2^*$ lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently-localized subsets of electrons.   

Spin-orbit ferromagnetic resonance

In conventional magnetic resonance techniques the magnitude and direction of the oscillatory magnetic field are (at least approximately) known. This oscillatory field is used to probe the properties of a spin ensemble. Here, I will describe experiments that do the inverse [1]. I will discuss how we use a magnetic resonance technique to map out the current-induced effective magnetic fields in the ferromagnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P). These current-induced fields have their origin in the spin-orbit interaction [2-4]. Effective magnetic fields are observed with symmetries which resemble the Dresselhaus and Rashba spin-orbit interactions and which depend on the diagonal and off-diagonal strain respectively. Ferromagnetic semiconductor materials of different strains, annealing conditions and concentrations are studied and the results compared with theoretical calculations. Our original study measured the rectification voltage coming from the product of the oscillatory magnetoresistance, during magnetisation precession, and the alternating current. More recently we have developed an impedance matching technique which enables us to extract microwave voltages from these high resistance (10 k$\Omega )$ samples [5]. In this way we measure the microwave voltage coming from the product of the oscillating magneto-resistance and a direct current. The direct current is observed to affect the magnetisation precession, indicating that anti-damping as well as field-like torques can originate from the spin-orbit interaction. \\[4pt] [1] D. Fang et al. Nat. Nano. 6, 413 (2011).\\[0pt] [2] A. Chernyshov et al. Nat. Phys. 5, 656 (2009).\\[0pt] [3] A. Manchon and S. Zhang Phys. Rev. B 79, 094422 (2009).\\[0pt] [4] I. Garate and A. H. MacDonald Phys. Rev. B 80, 134403 (2009).\\[0pt] [5] D. Fang et al. Appl. Phys. Lett. 101, 182402 (2012).   

Experimental demonstration of Scanned Spin-Precession Microscopy

We present the demonstration of a new spin-microscopy tool that relies on the precessional response of spins to the spatially heterogeneous field of a micromagnet. In this first experiment, we map the spin density within an optically pumped GaAs sample by recording the variations of a global spin-photoluminescence signal as a function of a micromagnetic probe's position (relative to the pump beam). The spin density map is then obtained by deconvolving the measured signal with an experimentally or theoretically determined response of the spins to their magnetic environment. The response function is sensitive to other important properties, such as spin lifetime and gyromagnetic ratio, and thus these properties can also imaged. Further, the technique can be employed in conjunction with both optical and electrical detection schemes. In the former case it can enhance the imaging resolution while for the latter it can enable imaging. Due to the magnetic nature of coupling between the probe and the spins, this technique has the potential to be material independent and enable subsurface imaging.   

Spin relaxation near the metal-insulator transition: dominance of the Dresselhaus spin-orbit coupling

We identify the Dresselhaus spin-orbit coupling as the source of the dominant spin-relaxation mechanism in the impurity band of a wide class of n-doped zincblende semiconductors. The Dresselhaus hopping terms are derived and incorporated into a tight-binding model of impurity sites, and they are shown to unexpectedly dominate the spin relaxation, leading to spin-relaxation times in good agreement with experimental values. This conclusion is drawn from two complementary approaches: an analytical diffusive-evolution calculation and a numerical finite-size scaling study of the spin relaxation time. Reference: G. A. Intronati, P. I. Tamborenea, D. Weinmann, and R. A. Jalabert, Phys. Rev. Lett. vol. 108, 016601 (2012).   

X-Ray Circular Dichroism Detected Spin Populations in N Doped (100) GaAs

We present the x-ray absorption and reflectivity of optically injected spin populations into highly doped n:GaAs. The spin population was excited in the GaAs using a circularly polarized laser at the band gap energy and detected using synchronous methods referenced to the x-ray repetition rate and laser chopping frequency. We observe x-ray circular dichroism along the Ga L$_{3}$ and L$_{2}$ edges two orders of magnitude larger than expected from LMTO band structure calculations. This observation is explained in the context of a surface related spin dependent non-equilibrium population immediately above and below the GaAs band-gap.   

Mott's scattering and Spin Hall Effect modeled by means of numerical solutions of the Schr\"{o}dinger equation

We have developed a code for numerical solution of non-stationary Schr\"{o}dinger equations based on the finite difference time-domain (FDTD) method. We model the 2 dimensional free electron gas system using perfectly matched layers for the open surrounding space. We study the effect of localized impurities on the time evolution of the electron wave function, thereby observing dephasing introduced by the impurities. Our numerical simulations show the de-coherence due to the impurities at moderate impurity densities and Anderson localization at high impurity densities. We implement the code for studying an effect of the spin orbit interaction in presence of the impurities. The clear picture of Mott's scattering gives rise to the Spin Hall Effect. Our results are important for the implementation of quantum computing, quantum communication, and spintronics.   

Diode Aided Geometrical Enhancement of Magnetoresistance in Semiconductors

Magnetoresistance (MR) reported in some non-magnetic semiconductors particularly silicon has triggered considerable interest owing to the large magnitude of the effect. Here we showed that MR in lightly doped n-Si can be significantly enhanced by introducing a diode in the device and proper design of the carrier path [1,2]. We designed an MR device whose room-temperature MR ratio reaching 30{\%} at 0.065T and 20000{\%} at 1.2T, respectively, approaching the performance of commercial MR devices. We also realized MR of over 2600{\%} in GaAs and Ge at 1.2T [2]. The MR mechanism of our devices is: The diode helps to establish a transition from low resistance state to high resistance state. In the transition region the small change in magnetic field cause a large change in MR. Because our MR device is based on a conventional Si/semiconductor platform, it should be possible to integrate it with existing Si/semiconductor devices and so aid the development of Si/semiconductor-based magneto-electronics leading to some multifunctional devices.\\[4pt] [1] Caihua Wan, Xiaozhong Zhang, et al.\textbf{,} Nature, \textbf{477}, 304 (2011).\\[0pt] [2] Xiaozhong Zhang, et al. Geometrical enhanced magnetoresistance in semiconductors (in submission)   

A spin-influenced hopping theory for transport in molecular semiconductors$\backslash $fs20

We investigate the influence of charge carrier's spin interaction on the hopping transport in molecular semiconductors. By considering the quenching of the spin correlation after the carrier's incoherent jump between molecules, we obtain the carrier's hopping rate that contains explicitly the contribution of carrier's spin interaction. As a consequence, the rate is modulated by applied magnetic field, leading to the magnetoresistance with a general feature of a Lorentzian-shape saturation at large fields and an ultrasmall-field component, which explains well the related experiments observed in organic semiconducting materials.   

Session W19: Holography and Higgs Physics in Condensed Matter

Spin and holographic metals

We examine the spin structure of the Green's function of the holographic metal, demonstrating that the excitations of the holographic metal are ``chiral,'' lacking the inversion symmetry of a conventional Fermi surface, with only one spin orientation for each point on the Fermi surface aligned parallel to the momentum. This implies that ferromagnetic spin fluctuations are absent from the holographic metal, leading to a complete absence of Pauli paramagnetism. The talk will discuss a possibility of going to a 3-dimensional holographic metal, where electrons should have both left- and right-handed chiralities.\\[4pt] [1] Phys. Rev. B 86, 125145 (2012)   

The Pauli exclusion principle in semi-local quantum criticality

A crucial consequence of the Pauli exclusion principle in weakly coupled systems is the presence of low energy degrees of freedom at finite momenta; a natural question is then to what extent does this aspect of Pauli exclusion persist at strong coupling, which may not even admit well-defined quasiparticles? We use holography to address this issue by studying the momentum space structure of low energy current-current correlation functions in finite density field theories exhibiting semi-local criticality. The semi-locally critical theories are characterized by an exponent $\eta$ that determines the low temperature scaling of entropy density to be $s \sim T^\eta$. Despite the fact that spatial momenta do not scale in semi-locally critical theories, we find that operator dimensions can have non-trivial momentum dependence, leading to novel momentum space structure. In particular, for $0 < \eta < 2$, we find sharp discontinuities in the transverse response functions at a non-zero $k_*$, reminiscent of Pauli exclusion-type dynamics. Finally, we comment on the $\eta=1$ geometry, which allowed for analytic expressions for correlation functions at finite temperature as well as interesting phenomenological properties and string theory embeddings.   

Superconducting Dome and Anisotropy in Holographic Striped Superconductor

Using gauge/gravity duality, we study the properties of a strongly coupled striped superconductor with unidirectional charge density wave order. By including the effects of fluctuations, we show that there is a regime in which this holographic model exhibits a superconducting dome. Furthermore, we study the anisotropy of the optical conductivity at temperature below the critical temperature and compare it with the experimental results in cuprate.   

Inhomogeneous Phases in Holographic Superfluids

We discuss inhomogeneous solutions of a gravitating system consisting of two $U(1)$ gauge fields and a real scalar field. One of the $U(1)$ gauge fields determines the chemical potential, whereas the other one corresponds to a magnetic field interacting with the spin in the boundary theory. We solve the field equations and find a second-order phase transition to an inhomogeneous phase at a critical temperature which we compute. Below the critical temperature, the equations are solved perturbatively, and a spatially dependent charge density is generated. This is compatible with the generation of a charge density wave in condensed matter systems.   

Compressible quantum phases from conformal field theories in 2+1 dimensions

Conformal field theories (CFTs) with a globally conserved U(1) charge $Q$ can be deformed into compressible phases by modifying their Hamiltonian, $H$, by a chemical potential $H \rightarrow H - \mu Q$. We study 2+1 dimensional CFTs upon which an explicit S duality mapping can be performed. We find that this construction leads naturally to compressible phases which are superfluids, solids, or non-Fermi liquids which are more appropriately called `Bose metals' in the present context. The Bose metal preserves all symmetries and has Fermi surfaces of gauge-charged fermions, even in cases where the parent CFT can be expressed solely by bosonic degrees of freedom. Monopole operators are identified as order parameters of the solid, and the product of their magnetic charge and $Q$ determines the area of the unit cell. We discuss implications for holographic theories on asymptotically AdS$_4$ spacetimes: S duality and monopole/dyon fields play important roles in this connection.   

Metal-Insulator Transition from Holography

The holographic correspondence allows theoretical control of certain phases of matter that do not admit a quasiparticle description. This approach has proved helpful for the description of quantum critical transport. I will present holographic results for transport away from particle-hole symmetry. This requires explicit inclusion of lattice effects to render the conductivity finite. I will show that the holographic system undergoes a metal-insulator transition as a function of the strength of the lattice. This results implies that holography is capable of describing localization physics in strongly interacting systems. I will present results for the optical conductivity, exibiting a transition from a metallic drude peak to Mott insulating behavior.   

Multipoint correlators of conformal field theories: implications for quantum critical transport

We relate three-point correlators between the stress-energy tensor and conserved currents of conformal field theories (CFTs) in 2+1 dimensions to observables of quantum critical transport. We first compute the correlators in the large-flavor-number expansion of conformal gauge theories and then do the computation using holography. In the holographic approach, the correlators are computed from an effective action on 3+1 dimensional anti-de Sitter space (AdS$_4$), and depend upon the co-efficient, $\gamma$, of a four-derivative term in the action. We find a precise match between the CFT and the holographic results, thus fixing the values of $\gamma$. The CFTs of free fermions and bosons take the values $\gamma=1/12,-1/12$ respectively, and so saturate the bound $|\gamma| \leq 1/12$ obtained earlier from the holographic theory; the correlator of the conserved gauge flux of U(1) gauge theories takes intermediate values of $\gamma$. The value of $\gamma$ also controls the frequency dependence of the conductivity, and other properties of quantum-critical transport at non-zero temperatures. Our results for the values of $\gamma$ lead to an appealing physical interpretation of particle-like or vortex-like transport near quantum phase transitions of interest in condensed matter physics.   

The quasi-normal modes of quantum criticality

We study the general features of charge transport of quantum critical points described by CFTs in 2+1D. We use an effective field theory on an asymptotically AdS spacetime, expanded to fourth order in spatial and temporal gradients. The presence of a horizon at non-zero temperatures implies that this theory has quasi-normal modes with complex frequencies. The quasi-normal modes determine the poles and zeros of the conductivity in the complex frequency plane, and so fully determine its behavior on the real frequency axis, at frequencies both smaller and larger than the temperature. We describe the role of particle-vortex or S-duality on the conductivity, specifically how it maps poles to zeros and vice versa. These analyses motivate two sum rules obeyed by the quantum critical conductivity. Finally, we compare our results with the analytic structure of the O(N) model in the large-N limit, and other CFTs.   

FFLO States in Holographic Superconductors

We discuss a novel mechanism to set up a gravity dual of FFLO states in strongly coupled superconductors. The gravitational theory utilizes two $U(1)$ gauge fields and a scalar field coupled to a charged AdS black hole. The first gauge field couples with the scalar sourcing a charge condensate below a critical temperature, and the second gauge field provides a coupling to spin in the boundary theory. The scalar is neutral under the second gauge field. By turning on an interaction between the Einstein tensor and the scalar, it is shown that, in the low temperature limit, an inhomogeneous solution possesses a higher critical temperature than the homogeneous case, giving rise to FFLO states.   

Study of Higgs mode near quantum critical points

We present a study of Higgs excitation mode in different quantum theories in 2 space dimensions. $O(N)$ theory and $CP(N)$ theory near the quantum critical points will be discussed for zero and finite temperature. Electron systems with fermi surfaces will be studied under this framework.   

Superconductivity in a model involving transverse gauge bosons

It has been known for some time that a system of fermions interacting with transverse gauge bosons does not behave like a Fermi liquid and provides a bona fide model for a non-Fermi liquid. Here we study superconductivity in this model Preliminary calculations show explicitly that a superconducting gap exists only for couplings greater than a threshold. It is hoped that a proper elucidation of this problem would lead to insights that may be useful in developing effective low energy theories of realistic physical problems, such as the normal state of high temperature superconductors, the state of half-filled quantum Hall systems, or the color superconductivity in the quark-gluon system, or even in the effects of disorder in a non-Fermi liquid system that could provide a new paradigm.   

Detection of Higgs mode in D-wave Superconductors

Higgs modes, which are collective excitations of the amplitude of the order parameter, have zero spin and no charge, do not couple directly to experimental probes. They are, however, linearly coupled to excitations which shake the ground state and therefore appear as poles or branch-cuts in their self-energy. In the superconducting state the Higgs modes can be distinguished from other excitations because they can only appear as satellites which steal all their spectral weight from excitations which promote superconductivity. This is an observable effect if such excitations and the Higgs modes are not too far separated in energy. We show that the Higgs mode in the $A_{1g}$ Raman scattering channel appears as a sharp resonance below $2 \Delta$ in the spectral weight of excitations responsible for superconductivity in Cuprates in a class of theories. Comparison is made with existing experiments and further experiments to confirm or rule out the idea are proposed.   

Unified Description of Nambu-Goldstone Bosons without Lorentz Invariance

We address the well-known problem that Nambu-Goldstone's theorem does not correctly predict the number of Nambu-Goldstone bosons in systems without Lorentz invariance. Using the effective Lagrangian approach, we provide a general prescription to predict the number of Nambu-Goldstone bosons and the form of their dispersion relation correctly. We trace the abnormalities in non-Lorentz invariant systems back to Nambu-Goldstone boson pairs becoming canonically conjugate--this reduces the number of Nambu-Goldstone bosons and changes the linear dispersions to quadratic. The generality of our construction clarifies the powerful approach of analyzing quantum many-body systems--including strongly coupled systems--by their symmetry breaking patterns. This will also aid our understanding of recent experiments and theoretical works on spinor BECs and lattices of topological defects. Reference: H. Watanabe, H. Murayama, PRL 108, 251602 (2012)   

Session W20: Focus Session: Electron, Ion, Exciton Transport in Nanostructures: Charge Transport in Functional Nanostructures

Two important physical models for resistance switching phenomena

Resistance switching (RS) phenomena refer to reversible resistance changes between two metastable resistance states driven by an external voltage. Recently, there has been a flurry of investigations into RS due to their inherent scientific interest and potentials for memory applications. In spite of extensive efforts, the basic mechanisms of RS still remain to be elucidated. One of the reasons is that RS usually occurs in very dirty materials, where defects should play important roles. In this talk, I will present two models for RS phenomena, which are material independent and can be used to make quantitative predictions. The first model is for unipolar RS, where the corresponding current-voltage ($I-V)$ curves are quite symmetric. We introduced a new kind of percolation model, called the random circuit breaker (RCB) network model, which allows reversible changes between two resistance states. This model can describe the formation of conducting channels due to dielectric breakdown and make quantitative predictions especially for scaling behaviors. We will show that collective behavior of conducting channels plays an important role in most aspects of unipolar RS, including the wide distribution of set and reset voltages, scaling behaviors, and large 1/$f$ noise. The second model is for bipolar RS, where the corresponding $I-V$ curves are quite asymmetric. We introduced a quantitative model which can describe motion of mobile defects under electric field. We will show that oxygen vacancy migration near the interface region could determine important features of bipolar RS, including two switching directions of $I-V$ curves. We also showed that important aspects of these two models can be combined successfully in a unified scheme by putting interface effects into the RCB network model.   

Fractal dynamics in chaotic quantum transport

Despite several experiments on chaotic quantum transport, corresponding ab initio quantum simulations have been out of reach so far. Here we carry out quantum transport calculations in real space and real time for a two-dimensional stadium cavity that shows chaotic dynamics. Applying a large set of magnetic fields yields a complete picture of the magnetoconductance that indicates fractal scaling on intermediate time scales. Two methods that originate from different fields of physics are used to analyze the scaling exponent and the fractal dimension. They lead to consistent results that, in turn, qualitatively agree with the previous experimental data.   

Schottky Barrier Transport for Multiphase Gallium Nitride Nanowire

Our group has shown that gallium nitride nanowires grown by catalyst-free vapor deposition at 850$^{\mathrm{o}}$C have multiple internal crystalline regions that may be zinc blende or wurtzite phase. Stability is enabled by one or more totally coherent (0001)/(111) internal interfaces. Cross-section HRTEM has further demonstrated that, while the transverse nanowire profile appears triangular, it is actually made up of two or more surface orientations corresponding to the multi-phase internal regions. We present results of a transport investigation of these multiphase nanowires within a nanoFET circuit architecture, focusing on injection from the contacts into the nanowires. Experimental results demonstrated that a variety of surface state derived Schottky barriers could be present at the contact-nanowire interfaces. Transport across the Schottky barriers was modeled using a combined thermionic emission-tunnelling approach, leading to information about barrier height, carrier concentrations, and expected temperature behavior. The experimental and theoretical results indicate that with optimal design taking surface and internal structures into account, high current densities can be supported.   

Non-adiabatic excitation and detection of coherent oscillations of single electrons

Surface acoustic waves (SAWs) are used to drive single-electron quantum dots along a complex depleted channel defined by various split gates. As the electron moves through this potential landscape at the SAW velocity (2800m/s), the evolution of the electron's wavefunction may be probed by detecting oscillations in the probability of tunnelling through a narrow barrier on one side of the channel. Coherent oscillations of the wavefunction are generated by non-adiabatic potential changes on a time-scale of tens of ps. We present here results of work in which this phenomenon is observed in two separate tunnelling regions, indicating a charge coherence time $> 500$ picoseconds. Additionally, we show that the initial state of the oscillations may be determined a significant distance from the tunnelling region through the use of suitably tuned gate voltages.   

Non-Radiative Energy Transfer Into Nanometer-Scale Thin Semiconducting Films

\noindent Non-radiative energy transfer (NRET) has gained a lot of attention recently due to its possible utility in new generations of light-emitting and photovoltaic devices. In this process, a ``donor'' species in an excited state transfers its excitation energy resonantly to an ``acceptor'' species. A classical realization of NRET is F\"{o}rster ET between two point-like species. Our interest is in ET between a small donor and an ultrathin acceptor layer. The layers can be realized as planar ensembles of molecules or QDs or as a thin crystalline semiconductor slab. We use two complementary approaches to study the effects of dielectric polarization in thin layers on ET rates: (1) The classical macroscopic electrodynamics treating the acceptor layer as a continuum of certain dielectric permittivity; (2) A direct modeling utilizing planar acceptor lattices, each of the acceptors treated as a polarizable point dipole. Comparison of the results allows us to establish salient qualitative features as well as to clarify the role of local-field factors. Of particular interest is our finding a broad region of the dielectric responses where ET into thinner films \textit{counter-intuitively} turns out to be more efficient than ET into thicker films.   

Quantum decrease of capacitance in a nanometer-sized tunnel junction

We have studied the capacitance of the tunnel junction defined by the tip and sample of a Scanning Tunnelling Microscope through the measurement of the electrostatic forces and impedance of the junction. A decrease of the capacitance when a tunnel current is present has shown to be a more general phenomenon as previously reported in other systems [1]. On another hand, an unexpected reduction of the capacitance is also observed when increasing the applied voltage above the work function energy of the electrodes to the Field Emission (FE) regime, and the decrease of capacitance due to a single FE-Resonance has been characterized. All these effects should be considered when doing measurements of the electronic characteristics of nanometer-sized electronic devices and have been neglected up to date.\\[4pt] [1] J.G. Hou, B. Wang, J. Yang, X.R. Wang, H.Q. Wang, Q. Zhu, and X. Xiao. Phys. Rev. Lett. 86, 5321 (2001)   

Detecting stray microwaves and nonequilibrium quasiparticles in thin films by single-electron tunneling

Superconducting thin films and tunnel junctions are the building blocks of many state-of-the-art technologies related to quantum information processing, microwave detection, and electronic amplification. These devices operate at millikelvin temperatures, and -- in a naive picture -- their fidelity metrics are expected to improve as the temperature is lowered. However, very often one finds in the experiment that the device performance levels off around 100--150 mK. In my presentation, I will address three common physical mechanisms that can cause such saturation: stray microwaves, nonequilibrium quasiparticles, and sub-gap quasiparticle states. The new experimental data I will present is based on a series of studies on quasiparticle transport in Coulomb-blockaded normal-insulator-superconductor tunnel junction devices. We have used a capacitively coupled SET electrometer to detect individual quasiparticle tunneling events in real time. We demonstrate the following record-low values for thin film aluminum: quasiparticle density $n_{\mathrm{qp}} < 0.033 / \mu\mathrm{m}^3$, normalized density of sub-gap quasiparticle states (Dynes parameter) $\gamma < 1.6 \times 10^{-7}$. I will also discuss some sample stage and chip designs that improve microwave shielding.   

Frequency Regimes of Kondo Dynamics in a Single-Electron Transistor

It has been theoretically predicted that the Kondo temperature, T$_K$, serves as the intrinsic timescale for the formation of Kondo correlations between conduction electrons and local spin moments. To probe this timescale, we have measured the time averaged differential conductance, $\langle$G$\rangle$=d$\langle$I$\rangle$/dV$_{ds}$, of a single electron transistor in the spin 1/2 Kondo regime in presence of an oscillating bias voltage, V(t)=V$_{ds}$+V$_{AC}$ sin(2$\pi$ft). We present the amplitude dependent conductance over select frequencies spanning several orders of magnitude below T$_K$ to twice T$_K$ (T$_K \sim$ 16GHz). At frequencies above T$_K$, we find good agreement with theory [Kaminski, et al. Phys. Rev. B 62, 8154 (2000)] in both the low (V$_{AC} \sim$ T$_K$/10) and high (V$_{AC} \sim$ 10T$_K$) amplitude regimes. The onset of non-adiabatic conductance behavior occurs well below prediction, f $\sim$ T$_K$, and becomes more apparent as the frequency nears T$_K$.   

Treatment of High Conductance Kondo Transport in Single Molecule Devices

A single molecule break junction device serves as a tunable model system for probing the many body Kondo state. There are predictions of universality across many realizations of the Kondo model in which the response of the system to different perturbations is characterized by a single emergent energy scale, $k_BT_K$. Comparisons between different experimental systems have shown issues with numerical consistency. With a new constrained analysis examining the response of conductance to temperature, bias, and magnetic field perturbations simultaneously, we show that these deviations from universality can be resolved by properly accounting for background, non-Kondo contributions to the conductance that are often neglected. We clearly demonstrate the importance of these non-Kondo conduction channels by examining transport in devices with total conductances exceeding the theoretical maximum due to Kondo-assisted tunneling alone.   

Constrained-DFT method for energy level alignment of metal-molecule interfaces

The electron transport properties of molecular junctions depend strongly on the alignment of the molecule's ionization potential (IP) and electron affinity (EA) with respect to the metal Fermi energy. It has been demonstrated experimentally\footnote{M. T. Greiner et al., Nature Mater. \textbf{11}, 76 (2011)} and theoretically \textbf{J. Neaton et al., Phys. Rev. Lett. \textbf{97}, 216405 (2006).} that the IP and the EA of molecules change when they are absorbed on a polarizable substrate, due to the formation of an image charge in the surface when an electron is either removed or added to the molecule. While within the GW approximation such a renormalization can be described, the energy levels of standard density functional theory (DFT) fail to capture it. However, DFT total energy differences between charged and neutral systems can usually describe IP and EA of molecules rather well. Here we therefore apply constrained DFT (CDFT) to calculate charge transfer energies between molecules and a metallic substrate in the weak coupling limit. We present CDFT results for the IP and EA of a benzene molecule as function of molecule-surface separation, and find good agreement with GW calculations. Within the CDFT approach we also evaluate the image plane height as function of separation.   

Vibrationally Induced Decoherence in Single-Molecule Junctions: The Role of Electron-Hole Pair Creation Processes

We investigate quantum interference effects and vibrationally induced decoherence in single-molecule junctions, employing nonequilibrium Green's function theory [1]. Molecular junctions often exhibit quasidegenerate electronic states that allow an electron to tunnel through the junction in different ways [2,3]. The respective outgoing wavefunctions interfere constructively or destructively, leading to an increase or decrease of the tunnel current, respectively. Interaction of the tunneling electrons with the vibrational degrees of freedom of the junction, however, gives 'which-path' information about the corresponding tunneling pathways because of the state-specific nature of electronic-vibrational coupling [2,3,4]. We demonstrate how this interplay between interference and vibrationally induced decoherence results in a strong temperature dependence of the current and highlight the role of electron-hole pair creation processes in this context [3,4]. To this end, we employ both generic models of single-molecule junctions as well as realistic models that are based on first-principles electronic structure calculations. [1] Phys. Rev. Lett. 102, 146801 (2009), [2] Phys. Rev. Lett. 107, 046802 (2011), [3] Phys. Rev. Lett. 109, 056801 (2012), [4] arXiv:1209.5619 (2012).   

Charge Transport in Azobenzene-Based Single-Molecule Junctions

The azobenzene class of molecules has become an archetype of molecular photoswitch research, due to their simple structure and the significant difference of the electronic system between their \textit{cis} and \textit{trans} isomers. However, a detailed understanding of the charge transport for the two isomers, when embedded in a junction with electrodes is still lacking. In order to clarify this issue, we investigate charge transport properties through single Azobenzene-ThioMethyl (AzoTM) molecules in a mechanically controlled break junction (MCBJ) system at 4.2 K. Single-molecule conductance, \textit{I}-\textit{V} characteristics, and IETS spectra of molecular junctions are measured and compared with first-principles transport calculations. Our studies elucidate the origin of a slightly higher conductance of junctions with \textit{cis} isomer and demonstrate that IETS spectra of \textit{cis} and \textit{trans} forms show distinct vibrational fingerprints that can be used for identifying the isomer.[1] \\ 1. Y. Kim, A. Garcia-Lekue, D. Sysoiev, T. Frederiksen, U. Groth, E. Scheer, Phys. Rev. Lett. (accepted).   

High bias shot noises measurement and electronic heating in STM style gold junctions at room temperature

Shot noise is a powerful tool in transport measurements, which encodes individual transmission channel's behavior; thus shot noise provides more information than solely conductance measurements. Using a STM-style gold break junction method, we can measure shot noise and conductance simultaneously at room temperature to study its bias-dependence and the distribution of noise and so on. Quantum suppression of shot noise remains very robust even at room temperature. The standard Landauer-Buttiker treatment of shot noise in nanoscale junctions at finite temperature assumes that the electronic temperature in the source and drain electrodes is unaffected by the applied bias. That is, the applied bias is assumed to shift the relative chemical potentials of the electrodes without broadening the electronic distributions. We perform noise measurements at biases as high as 0.5 V (an energy scale much larger than room temperature) and analyze the noise to determine if its bias dependence shows evidence of electronic heating. We will discuss the evolution of shot noise with bias voltage in detail and the role of electronic heating in this experiment.   

Session W21: Optoelectronics & Photonics

Development and Test of a Travelling Wave Tube mm-wave Source

We report on the fabrication and test of a Traveling Wave Tube (TWT) amplifier designed for operation over a 40 GHz bandwidth centered on 220 GHz, and producing 50 W output power. The TWT amplifier uses a slow wave structure with staggered interdigitated vanes within a waveguide [1]. Each vane is 110 micron wide situated inside a 770 micron wide waveguide, and was directly machined into copper using a 100 micron wide end mill. This structure slows radiation down to group velocity of 8.16 x 10$^{7}$ ms$^{-1}$ where the velocity matches the speed of electrons from a 20 keV source. The TWT uses a sheet electron beam of 7:1 aspect ratio and 400 A/cm$^2$ charge density stabilized by a Brillouin flow magnetic field provided by an external permanent magnet. RF vacuum windows were designed and built using brazed diamond windows, providing less than 1 dB insertion loss across the full 40 GHz bandwidth. Solid state preamplifiers have been developed which provide 20 dB gain and 50 mW output power over the full bandwidth to the input of the TWT. \\[4pt] [1] Y-M. Shin {\&} L.R. Barnett, \textit{Appl.Phys. Lett. 2008,} 92 pp. 091501.   

Velocity-matching dispersion maps for zincblende and chalcopyrite terahertz sources

Pulsed terahertz radiation has been shown to be a useful diagnostic in fundamental and applied science. A common method for generating pulsed THz is by optical rectification. (110)-cut ZnGeP$_{2}$ was previously demonstrated as an efficient source of broadband THz radiation for near-infrared pump pulses [1], while other orientations have been modeled to show equal or greater efficiency [2]. Here we explore and compare phase-matching in ZnGeP$_{2}$ to that in other commonly used near-infrared THz sources including GaAs and GaP. We experimentally demonstrate that the three most efficient orientations provide distinct phase-matching configurations and thus distinct phase-matched near-infrared and THz frequencies. Our calculations also show that thin ($\sim$100 micron) crystals of ZGP may be promising sources for phase-matched and broadband THz emission out to 9 THz for 850 nm pump pulses. \\[4pt] [1] J. D. Rowley, J. K. Pierce, A. T. Brant, L. E. Halliburton, N. C. Giles, P. G. Schunemann, A. D. Bristow, Opt. Lett. 37, 788 (2012)\\[0pt] [2] J. D. Rowley, J. K. Wahlstrand, K. T. Zawailski, P. G. Schunemann, N. C. Giles, A. D. Bristow, Opt. Express 20, 16968 (2012)   

Modeling ultra-broadband terahertz waveguide emitters through difference frequency generation using coupled mode theory

We use a coupled mode theory that adequately incorporates both terahertz (THz) and infrared (IR) losses, to model and design ultra-broadband terahertz waveguide emitters (0.1-15 THz) based on difference frequency generation of femtosecond IR optical pulses. We apply the theory to generic, symmetric, five-layer, metal/cladding/core waveguides using transfer matrix theory. Our expressions for the conversion efficiency and output THz power spectrum depend on the pump power, pulse width, beam waists, laser repetition rate, material optical properties, and waveguide dimensions. Using this approach we design waveguides whose active cores are composed of a poled guest-host electro-optic polymer composite DAPC, comprised of DCDHF-6-V chromophores embedded in an amorphous polycarbonate matrix host. The resulting bandwidths are greater than 15 THz and we obtain high nonlinear conversion efficiencies up to $1.2\times 10^{-4}W^{-1}$. Our results reveal that a perfectly phase-matched structure is not necessarily the one with the highest conversion efficiency. The highest efficiency is obtained by balancing both the modal phase-matching and modal effective loss effects.   

Intense Nanosecond-Pulsed Cavity-Dumped Laser Radiation at 1.04 THz

We report first results of intense far-infrared (FIR) nanosecond-pulsed laser radiation at 1.04 THz from a previously described\footnote{T.E. Wilson, \textit{Proc. Int. Conf. Lasers '91}, (STS Press, McLean, VA), 762-767 (1992), and references therein.} cavity-dumped, optically-pumped molecular gas laser. The gain medium, methyl fluoride, is pumped by the 9R20 line of a TEA CO$_2$ laser\footnote{Cornelius T. Gross et al., \textit{IEEE J. QE}, \textbf{QE-23}, 377-387 (1987).} with a pulse energy of 200 mJ. The THz laser pulses contain of 30 kW peak power in 5 nanosecond pulse widths at a pulse repetition rate of 10 Hz. The line width, measured by a scanning metal-mesh FIR Fabry-Perot interferometer, is 100 MHz. The novel THz laser is being used in experiments to resonantly excite coherent ns-pulsed 1.04 THz longitudinal acoustic phonons in silicon doping-superlattices.   

Tunable terahertz detectors on GaAs substrates

Despite considerable research activitities in terahertz science and technologies, there has not been much progress in terahertz detectors. At present, the sensitivity of room temperature detector does not exceed 10$^{-9}$ W/(Hz)$^{1/2}$ in terms of noise equivalent power. Also most detectors are not tunable, and their response time is slow. In order to make terahertz technology practical substantial improvements should be made on the detector. Earlier, throughout research collaboration with UCSB, we have demonstrated a terahertz detector based on a Metal-Semiconductor Field Effect Transitor (MESFET) technology, which enabled us to achieve a high speed, tunable terahertz detector. The detector was tunable over the frequency range from 0.1 THz to 1.4 THz with a sensitivity of 10$^{-8}$ W/(Hz)$^{1/2}$. Recently we have attempted to modify this earlier design in order to improve its sensitivity up to 10$^{-11}$ W/(Hz)$^{1/2}$ and the operating frequency range from 0.07 to 2.5 THz. We employed a GaAs/AlGaAs heterostructure substrate, and drastically modified the previous MESFET design.We will present our fabrication process and experimental results.   

Semiconductor-core optical fibers for terahertz waveguides

Waveguiding of terahertz (THz) radiation is important for imaging and communications applications. Simulations have been performed based on a fiber optic geometric waveguide with a poly-crystalline silicon core and silica cladding [1]. High-resistivity silicon has a flat dispersion over a 0.1 -- 3 THz range [2], making it viable for propagation of broadband picosecond pulses of THz radiation such as that produced by optical rectification [3]. Frequency-dependent mode indices are determined for 0.1 -- 0.3 mm diameter cores. The normalized frequency parameter V is also determined and a 140 micron core is selected as the low edge of diameters that can support a THz pulse. Finite-difference time-domain simulations are performed in two-dimensions to extract the propagation dynamics and the integrated intensity, from which transverse mode profiles and absorption lengths are extracted. It is found that for this core diameter the mode partially propagates in the cladding, such that the overall absorbance is only slightly less than in bulk polycrystalline silicon. [1] J. Ballato, T. Hawkins, P. Foy, R. Stolen, B. Kokuoz, M. Ellison, C. McMillen, J. Reppert, A. M. Rao, M. Daw, S. R. Sharma, R. Shori, O. Stafsudd, R. R. Rice, and D. R. Powers, Opt. Express 16, 18675-18683 (2008) [2] D. Grischkowsky, S{\o}ren Keiding, Martin van Exter, Ch. Fattinger, J. Opt. Soc. Am. B 7, 2006 (1990) [3] J. D. Rowley, J. K. Pierce, A. T. Brant, L. E. Halliburton, N. C. Giles, P. G. Schunemann, A. D. Bristow, Opt. Lett. 37, 788 (2012)   

Simulations of Brillouin Scattering in Optical Fibers

Brillouin scattering arises when a laser beam generates density variations in a medium via electrostriction. The density variations modulate the refractive index, resulting in a grating that Bragg scatters pump light into a Stokes beam. The Stokes wave is downshifted in frequency by the Doppler effect because the grating is moving at the speed of acoustic phonons. To conserve both energy and momentum, the Brillouin photons are backscattered. This back-reflected radiation is a major factor limiting the transmission of laser power in optical fibers for practical applications. It is mathematically described by a set of coupled partial differential equations. I will describe some of the known analytic solutions of these equations, as well as how to find numeric solutions using MATLAB.   

Metamaterial features for a pure dielectric fiber

We consider a solid cylindrical dielectric waveguide with an extremely thin coaxial cylindrical shell of higher refraction index inserted on it. We calculate the propagation parameters and the band structure of this fiber as function of the contrast index, and show that there exist propagating modes whose transverse distribution of amplitudes are both oscillating and evanescent. The oscillating modes exhibit the usual dispersion relation of a standard wave guide, whereas the evanescent modes gives rise to regions for which the group velocity almost vanishes and with propagation direction opposed to the Poynting vector, as seen in metamaterials.   

Measurements of Effective Schottky Barrier in Inverse Extraordinary Optoconductance Structures

Individually addressable optical sensors with dimensions as low as 250nm, fabricated from metal semiconductor hybrid structures (MSH) of AuTi-GaAs Schottky interfaces, display a transition from resistance decreasing with intensity in micron-scale sensors (Extraordinary Optoconductance, EOC) to resistance increasing with intensity in nano-scale sensors (Inverse Extraordinary Optoconductance I-EOC). I-EOC is attributed to a ballistic to diffusive crossover with the introduction of photo-induced carriers and gives rise to resistance changes of up to 9462{\%} in 250nm devices. We characterize the photo-dependence of the effective Schottky barrier in EOC/I-EOC structures by the open circuit voltage and reverse bias resistance. Under illumination by a 5 mW, 632.8 nm HeNe laser, the barrier is negligible and the Ti-GaAs interface becomes Ohmic. Comparing the behavior of two devices, one with leads exposed, another with leads covered by an opaque epoxy, the variation in Voc with the position of the laser can be attributed to a photovoltaic effect of the lead metal and bulk GaAs. The resistance is unaffected by the photovoltaic offset of the leads, as indicated by the radial symmetry of 2-D resistance maps obtained by rastering a laser across EOC/IEOC devices.   

Light Intensity Influence on the Effective Schottky Barrier Height in Extraordinary Optoconductance (EOC) Structures

Novel micro to nanoscale metal-semiconductor-hybrid (MSH) structures capable of room temperature light detection have been previously reported and classified as Extraordinary Optoconductance (EOC) devices. The devices are square stacked structures, with a Au-Ti shunt forming a Schottky-Interface with an n-doped Ga-As mesa. Resistance measurements were taken by a 4-point van-der Pauw method to remove contact and lead resistance and eliminate DC offsets. The device's resistance changes as light incident on the surface of the structure modifies the charge density within the body of the device. The change in charge density changes the effective Schottky Barrier height and shifts the measured 4 point resistance of the heterogeneous structure. We investigate the dependence of the effective Schottky Barrier height on the incident intensity of light by measuring the open circuit voltage under various intensities of optical perturbation at room temperature. The barrier height is negligible and the interface ohmic under HeNe laser 632.8 nm illumination at a power density of 636 mW/cm$^{\mathrm{2}}$, allowing the flow of current through the shunt. This device performance will be contrasted with that of an FET, where current does not propagate through the gate.   

Proposal for realization of one-way electromagnetic modes at the interface of two lossless metals

One-way electromagnetic waveguides are of special interest because of complete suppression of back-scattering by disorder. Such waveguides support a unique class of photonic modes that completely forbid propagation in the opposite direction. We show that a one-way electromagnetic waveguide can be realized at the interface of two dissimilar lossless metals in an external magnetic field parallel to the interface. Electromagnetic surface plasmon modes bound to the interface of the two metals and propagating parallel to it and normal to the direction of the external magnetic field, with the electric field polarized normal to the plane of the interface, support one-way electromagnetic propagation in a range of frequencies. Increasing the magnetic field increases the window of frequencies for one-way propagation. Adding damping reduces the range of frequencies. Details of the calculation and plots showing the dispersion relation will be presented.   

Pulse Shaping with Moire Volume Bragg Gratings

Optical pulses of various temporal profiles are required for many applications but their durations and shapes are available only in limited ranges for certain laser wavelengths. For generation of pulse durations around ten ps, we proposed to reflect short pulses from volume Bragg gratings (VBGs) with few millimeter thickness. In case of VBG reflection bandwidth much narrower than incident pulse spectral width the significant loss of power occurs but such approach can be acceptable if there are no other generation processes for required pulse duration. VBGs in photo-thermo-refractive glass developed in our group are characterized by wide transparency range, small absorption and high laser damage threshold. In comparison with fiber Bragg gratings VBGs have additional spatial transverse degrees of freedom which allow not only tuning the pulse carrier wavelength but also shaping of generated pulses. Recording of two gratings with slightly different periods in the same glass wafer provides VBG with moire fringe pattern. After skew cutting of specimen with thickness of moire semi-period the longitudinal modulation VBG profile will vary in transverse direction from sine semi-period to cosine one. Reflected pulse from VBG with apodized sine profile has temporal profile close to transform limited Gaussian one while pulse reflected from transverse part of moire VBG with cosine semi-period profile has zero dip in temporal profile. At intermediate position the flat-top pulse shape is achievable.   

Monolithic Single-Mode DFB Laser Array with Precise Wavelength Control for Optoelectronic Integration using an Equivalent Phase Shift Method

The integrated distributed feedback (DFB) laser array is a key component in photonic integrated circuits for wavelength-division multiplexing (WDM) system. However, it is difficult to precisely control the wavelength of individual lasers. When the rear facet of the laser is coated with a high-reflectivity mirror, a random phase change is introduced that shifts the lasing wavelength, making monolithic integration of a wavelength-controlled WDM array very difficult. To solve this problem, we propose a method to precisely control the lasing wavelength of DFB lasers over a wide range by introducing an equivalent phase shift in the cavity using sampled Bragg gratings, using wafer-scale optical lithography and requiring only coarse dimension control. The wavelength can be fine-tuned by applying different DC currents. It is shown that a WDM-DFB laser array with uniform wavelength spacing can be controlled accurately in this manner. Integrated arrays of single-mode DFB lasers for WDM systems can thus be fabricated in a low-cost manner without using low-throughput e-beam lithography, and is scalable for mass-manufacturing.   

Observation of Polarization Switching in Vertical-Cavity Surface-Emitting Lasers at Constant Injection Current

This study investigated the thermal characteristics of the polarization switching (PS) in vertical-cavity surface-emitting lasers (VCSELs) at constant injection current. The experiments were performed with a quasi-step function current experiment. A simplified temperature rate equation was used to simulate the experiment of the step function. The consistency of the experiments and simulations concludes that the thermal effect plays a major role in PS and PS's hysteresis. These results contribute to the understanding of the mechanism of VCSEL's polarization switching.   

Development of an image-analysis light-scattering technique

We describe the progress in developing a versatile image-analysis approach for a light-scattering experiment. Recent advances in image analysis algorithms, computational power, and CCD image capture has allowed for the complete digital recording of the scattering of coherent laser light by a wide variety of samples. This digital record can then yield both static and dynamic information about the scattering events. Our approach is described using a very simple and in-expensive experimental arrangement for liquid samples. Calibration experiments were performed on aqueous suspensions of latex spheres having 0.5 and 1.0 micrometer diameter for three concentrations of 2 X 10$^{-6}$, 1 X 10$^{-6}$, and 5 X 10$^{-7}$ {\%} w/w at room temperature. The resulting data span a wave-vector range of q $=$ 10$^{2}$ to 10$^{5}$ cm$^{-1}$ and time averages over 0.05 to 1200 sec. The static analysis yield particle sizes in good agreement with expectations and a simple dynamic analysis yields an estimate of the characteristic time scale of the particle dynamics. Further developments in image corrections (laser stability, vibration, curvature, etc.) as well as time auto-correlation analysis will also be discussed.   

Session W22: Transparent Conductors, Titania, and Other Oxides

The doping effect of Mn and Co ions in PbPdO$_{2}$

Spintronics is a promising field in which the spin of electrons along with the charge is used for data storage and data manipulation. For spintronics application a long mean free path with high spin polarization is required. In this sense, the magnetic gapless semiconductor is a promising material since it satisfies both conditions. Here we have studied PbPdO$_{2}$, which is predicted to be a gapless semiconductor, and its Co and Mn doping to be a spin gapless semiconductor. We have tried to tune its electrical and magnetic properties with magnetic ions such as Co and Mn, in order to achieve the magnetic gapless semiconductors for spintronics application. A drastic change in the magnetic properties has been observed when doped with magnetic ions. The Co doping induces a weak ferromagnetic exchange, while the Mn doping induces an antiferromagnetic exchange. To investigate the electronic structures of PbPdO$_{2}$ we have measured the valence band photoemission spectroscopy and X-ray absorption spectroscopy. The results show Mn$^{4+}$ and Co$^{3+}$ states for the Mn and Co doped PbPdO$_{2}$, respectively. This implies that the magnetic and electrical properties of PbPdO2 can be easily tuned by chemical doping, and it leads to possible applications for spintronics.   

P-type K-doping of BaSnO$_3$ and its pn junctions

We have recently reported high mobility in La-doped BaSnO3 (BSO), whose transparency and chemical stability promises large potential for scientific and technical applications. The doping possibility with p-type carrier will further enhance its utility in scientific and technical endeavors. For such purpose, we will present our work in p-type doping BSO by epitaxially growing K-doped BSO by pulsed laser ablation on SrTiO3 substrates. We have found that K replaces Ba from EPMA. Although K-doped BSO exhibited rather high resistivity at room temperature, its conductivity increased dramatically at high temperature and the conductivity decreased when small amount of oxygen was removed from the thin films, consistent with the behavior of p-type doped oxides. The carrier type of K-doped BSO will be further confirmed by direct high-temperature Hall measurement. We will report on the mobility of the K-doped BSO and the performance of pn junctions fabricated by using K- and La-doped BSO.   

Role of annealing temperature on microstructural and electro-optical properties of ITO films produced by sputtering

In this study, we investigate the effect of annealing temperature on electrical, optical and microstructural properties of indium tin oxide (ITO) films deposited onto Soda lime glass substrates by conventional direct current (DC) magnetron reactive sputtering technique at 100 watt using an ITO ceramic target (In$_{2}$O$_{3}$:SnO$_{2}$, 90:10 wt. {\%}) in argon atmosphere at room temperature. The films obtained are exposed to the calcination process at different temperature up to 700 $^{\circ}$ C. Resistivity, Hall Effect, X--ray diffractometer (XRD), ultra violet-visible spectrometer (UV--vis) and atomic force microscopy (AFM) measurements are performed to characterize the samples. Moreover, phase purity, surface morphology, optical and photocatalytic properties of the films are compared with each other. Furthermore, mobility, carrier density and conductivity characteristics of the samples prepared are carried out as function of temperature in the range of 80-300 K at the magnetic field of 0.550 T. The results obtained show that all the properties depend strongly on the annealing temperature and in fact the film annealed at 400 $^{\circ}$ C obtains the better optical properties due to the high refractive index while the film produced at 100 $^{\circ}$C exhibits much better photoactivity than the other films as a result of the large optical energy band gap.   

Transparent oxide semiconductors (Ba,La)SnO$_{3}$ with high mobility at room temperature

We present our discovery of (Ba,La)SnO$_{3}$ system exhibiting electrical mobility at 300 K of 200-320 cm$^{2}$V$^{-1}$s$^{-1}$ in a doping range from 1.0x10$^{19}$ to 4.0x10$^{20}$ cm$^{-3}$. Moreover, their conductivity values were as large as around 10$^{4}$ S/cm, being comparable to those of indium tin oxides. The system yet shows the optical gap around 3.33 eV and only slight increase of the in-gap states, maintaining visual transparency. Several unique physical properties of (Ba,La)SnO$_{3}$ are also discussed: a superior oxygen stability evidenced by persistent transport properties under high temperature environments, a small effective mass coming from the ideal Sn-O-Sn bonding in a cubic perovskite, small disorder effects due to doping away from the main conduction channels (SnO$_{6}$ octahedra network) and reduced carrier scattering due to the high dielectric constant. (Ba,La)SnO$_{3}$ thus holds great potential for realizing transparent, high power, high temperature functional devices.   

Small polarons and their interaction with donor centers in Titania

The use of TiO$_{2}$ in photocatalysis, photosensitized solar cells, and memristors strongly depends on the behavior of conduction-band electrons, prompting a more profound understanding of conduction mechanisms. The reported results for the behavior of excess electrons in TiO$_{2}$ are contradictory. High carrier mobilities, characteristic of delocalized electrons, have been observed in Hall measurements, whereas optical spectra indicate the presence of localized, small polarons. Using first-principles calculations based on a hybrid functional we study the formation of small polarons, comparing it to delocalized electrons in the conduction band of TiO$_{2}$. From the calculated configuration coordinate diagram and migration energy barriers, we discuss the coexistence of small polarons with delocalized electrons, and address how the observed behavior depends on the type of experiment being conducted. The interaction of small polarons with intrinsic defects such as the oxygen vacancy and donor impurities will also be discussed.   

Preparation of perpendicular oriented TiO2 films via hydrothermal method: phase selection and growth control

Either rutile or anatase vertical orientated TiO$_2$ array films were synthesized successfully on FTO (F: SnO$_2)$ substrate via hydrothermal method through controlling the concentration of Cl$^-$ and SO$_4^{2-}$. The density of nanorods can be adjusted by varying the volume ratio of ethanol/water, and the degree of orientation and crystallinity of TiO$_2$ nanofilms were enhanced with increasing dosage of ethanol. Meanwhile, completely dense anatase films with [004] oriented growth appear within a very narrow concentration window when adding sulfuric acid into precursor. Besides, other alcohols such as methanol, n-propanol and n-butyl were also used as solvent to examine the role of alcohol type during hydrothermal process for both two phase films. The growth rate and degree of perpendicular orientation declined as the alkyl length of solvents increases. Hydrogen sensing characteristics of dense films of both rutile and anatase phases showed that there was a remarkable improvement of sensitivity response over reported data. It was found that rutile films have higher sensitivity while anatase films have faster response.   

Visible Light Sensitization of TiO$_{2}$ Films by co-doping with Nitrogen and Carbon

Anatase phase of TiO$_{2}$ has a band gap of 3.20 eV. Therefore, only UV light can be absorbed from the solar spectrum. Introducing defect states narrows the band gap of TiO$_{2}$ semiconductor and enhances the visible light activity. In this study, the defect states in the band gap are created by nitrogen and carbon dopants. Reactive pulsed laser deposition technique is used to prepare nitrogen and carbon co-doped TiO$_{2}$ films. Total pressures of nitrogen and methane gases are kept at 100 mTorr. Two types of co-doped samples are investigated with partial pressures of 80 mTorr nitrogen with 20 mTorr methane and 20 mTorr nitrogen with 80 mTorr methane. Undoped, control, sample is also prepared under 100 mTorr oxygen gas. All films show polycrystalline anatase structure. Nitrogen dopant is calculated from XPS high resolution scans while carbon incorporation into TiO$_{2}$ lattice is supported by XRD and FESEM analyses. Also, direct relation between oxygen vacancies and nitrogen doping concentration is observed from XPS high resolution scans of N 1s and Ti 2p regions. Band gap is calculated using absorption coefficient obtained from UV-Vis diffuse reflection spectroscopy measurements. 80 mTorr nitrogen and 20 mTorr methane co-doped TiO$_{2}$ film has the lowest band gap among all with 2.17 eV which lies near the most intense peak in the visible part of the solar spectrum. Therefore, co-doping TiO$_{2}$ with nitrogen and carbon is a possible method for visible light sensitization.   

the effect of electron doping in TiO2 assessed by ARPES

The titanium oxide TiO2 has been object of extensive studies because of its suitability in many practical fields, ranging from photovoltaic applications, to catalysis, memristors, and others. As for many other transition metal oxides, great attention has been devoted to the impact on the electronic structure of different doping mechanisms, either extrinsic or due to the creation of oxygen vacancies. Here we report an angle-resolved photoemission (ARPES) work on TiO$_2$ single crystals and epitaxial films grown wIth the \textit{in situ} pulsed-laser-deposition (PLD) system available on beamline 7.0.1 at the Advanced Light Source. We show the evolution of the electronic structure as a function of the amount of oxygen vacancies induced by the photon beam.   

Thermodynamic Effects on Phase Stabilities and Structural Properties of TiO2 from the First-principles

Titanium dioxide (TiO2) is one of the most representative photocatalytic materials and much attention is focused on understanding and improvement of its photocatalytic activity. At the same time, TiO2 is known to be a highly polymorphic material and as many as eleven crystal phases have been identified so far. It is expected that TiO2 show various photocatalytic properties depending on crystal phases. However, relative stabilities of these identified phases are still controversial. In order to clarify the thermodynamic phase stabilities of TiO2, we obtain the free energies of its several representative phases, rutile, anatase, brookite, and TiO2-II within the framework of the density-functional theory using the pseudopotential method. We calculate both the static energy and the contribution of phonons to the free energy through the quasiharmonic approximation for each phase. It is found that treatment of semicore electrons in constructing the pseudopotential of the Ti atom significantly affects the relative phase stabilities. From the phase diagram obtained, we find that the anatase phase is the most stable at lower temperature and pressure. We also discuss the thermodynamic effects on structural properties such as thermal expansion.   

Properties of p-type ZnO Films co-doped with Lithium and Phosphorus

Thin films of ZnO co-doped with lithium and phosphorus were deposited on sapphire substrates by RF magnetron sputtering. The films were sequentially deposited from ZnO and Li$_{3}$PO$_{4}$ solid targets on the substrates maintained at 500 $^{\circ}$C. An undoped ZnO buffer layer was first deposited at a substrate temperature of 900 $^{\circ}$C for 2 hours. Post deposition annealing was carried using a rapid thermal processor in N$_{2}$ and O$_{2}$ at temperatures ranging from 400 $^{\circ}$C to 900 $^{\circ}$C for 3 min. Analyses performed using low temperature photoluminescence spectroscopy measurements reveal several luminescence peaks at 3.36, 3.353, 3.317, 3.11and 2.33 eV whose relative intensities vary with annealing environments and temperatures. We will discuss the origins of these luminescence peaks and their relevance to p-type doping of ZnO films. The x-ray diffraction 2$\theta $-scans for all the films showed a single peak at about 34.4$^{\circ}$ with FWHM of about 0.17$^{\circ}$. Hall Effect measurements revealed conductivities that change from p-type (with concentration up to about 1.3 x 10$^{17}$ cm$^{-3})$ to n-type (with concentration up to about 1.5 x 10$^{19}$ cm$^{-3})$ as the annealing temperature is increased to 900 $^{\circ}$C.   

Magnetic and Optical Properties of Co-doped ZnO Nanorods

Transition-metal doped ZnO is considered as an ideal system for carrying out research in the field of spintronics as well as optoelectronics as they can successfully combine magnetism and electronics in a single substance. ZnO is a wurtzite-type wide-bandgap semiconductor of the II-VI semiconductor group with band gap energy of 3.37 eV. Synthesis of undoped and Co-doped ZnO nanorods is carried out using aqueous solutions of Zn(NO$_{3})_{2}$.6H$_{2}$O, and Co(C$_{2}$H$_{3}$OO)$_{2}$.4 H$_{2}$O, using NH$_{4}$OH as hydrolytic catalyst by hydrothermal process. For optimizing the nanorod growth condition, parameters such as concentration, pH, synthesis time and temperature are varied. Optimum condition for the growth of pure zinc oxide nanorods is found 0.15 M pH 9, 6 hrs and 130$^{\circ}$C respectively. Structural, morphological, optical and magnetic properties are studied using XRD, Raman spectroscopy, SEM, UV-vis spectroscopy, PL spectroscopy and SQUID magnetometer.Detailed structural, optical, and magnetic properties will be discussed in this presentation. This work is supported by National Science Foundation (Award Number DMR-0907037).   

Effective Lifetimes of Atomic Layer Deposited Diffusion Barrier Films for Silver Artifacts

We investigated using atomic layer deposition (ALD) to create dense, transparent oxide diffusion barrier coatings to reduce the tarnishing rate for silver art objects. An elevated H$_{\mathrm{2}}$S aging chamber was used for accelerated aging to directly compare the effectiveness of 5-100nm Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ ALD thin films and nitrocellulose coatings, the current technique for silver preservation, at reducing the tarnishing rate of silver while minimally affecting the visual appearance of the silver. Reflectance spectroscopy and an integrated sphere spectrophotometer were used to measure the thickness of the tarnish layer and indicate the lifetimes of the ALD and nitrocellulose coatings. Electrochemical impedance spectroscopy (EIS) was used to determine the porosity and average pore size of ALD films. Failure mechanisms for the two types of films were observed, the ALD films failing in defects or pinholes in the films and the nitrocellulose failing due to non-uniform in coating thickness. Thin Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ ALD films were found to be more porous than thick ALD films, sufficient in protecting silver five times longer and effected the overall color change of the object less than micron thick nitrocellulose films.   

Impact of carbon and nitrogen on gate dielectrics in metal-oxide-semiconductor devices

Al$_2$O$_3$ and HfO$_2$ are used as alternative gate oxides in CMOS technology. Promising results have been achieved with Al$_2$O$_3$/III-V and HfO$_2$/Si MOS structures, which exhibit relatively low densities of interface states. However, the presence of charge traps and fixed-charge centers near the oxide/semiconductor interface still poses serious limitations in device performance. Native point defects are usually proposed as an explanation; unintentional incorporation of impurities in the gate dielectric during the deposition process has so far received less attention. Using first-principles calculations based on hybrid functionals we investigate the effects of carbon and nitrogen impurities in Al$_2$O$_3$ and HfO$_2$. By analyzing the position of the impurity levels with respect to the III-V and Si band edges, we determine if these impurities can act as charge traps or sources of fixed charge. Our results show that carbon can act as a charge trap and lead to leakage current through the gate dielectric. Nitrogen can act as a source of negative fixed charge, but may be effective in alleviating the problem of charge traps and fixed charges associated with Al, Hf, and O vacancies.   

Characterization of Er$^{+3}$:Y$_2$O$_3$ films made via Atomic Layer Deposition

Er$^{+3}$:Y$_2$O$_3$ thin films with spatially-controlled Er$^{+3}$ ion incorporation, were deposited on various substrates using Atomic Layer Deposition. By systematically varying the Erbium precursors used in the deposition of the films, a method to spatially control the Erbium has been realized. All films were polycrystalline as deposited and no appreciable change was detected after post-deposition annealing. Emission spectra for all precursors used show crystalline emission lines, similar to those grown via a melt process. Photoluminescent lifetimes up to 6.5ms have been recorded from these films, the largest to date in films deposited with Atomic Layer Deposition. Films have been characterized using XRD/GIXRD, UV-Vis spectroscopy, XAFS, RBS, HFS, SEM, TEM, and AFM. The results of these various measurements, and the influence on photoluminescent lifetime will be discussed.   

Session W23: Semiconductors: Theory & Spectra II

Line width resonance of the longitudinal optical phonon in GaAs:N

We extend resonant Raman scattering studies of Mascarenhas et al. [PRB68, 233201 (2003)] of GaAs$_{1-x}$N$_x$ to the ultra-dilute nitrogen doping concentrations, whereby we unambiguously resolve the line width resonances of the LO phonon. A discontinuity is observed in the LO phonon line width resonance energy as a function of concentration. With decreasing nitrogen concentration the $E_W$ line width resonance energy reduces by ca. 40 meV at $x=0.4\%$. This value corresponds to the concentration, at which the localized to delocalized transition manifests itself in the electro-reflectance signature line widths.   

Micro-Raman study of InAs/GaSb superlattices from front and cleaved edge

The InAs/GaSb superlattice (SL) has a ``broken-gap'' type II band alignment, with its effective bandgap being able to be tuned by changing the thickness of individual layers. Therefore, it is of great interest for mid- and far-IR detection. Because the SL does not have common cation or anion at the interface, there are two types of interfacial layers: InSb and GaAs, that impact the device performance. We investigate the SLs grown on either GaSb or InAs substrate and with difference interfacial treatment using confocal micro-Raman spectroscopy on both front surface and cleaved edge of the epilayer with polarization.   

Raman Investigation of p-type Amorphous Silicon Thin Films

Thin film layers of p-type a-Si:H of differing doping concentration and hydrogen dilution were investigated by Raman spectroscopy to determine their effect on short- and mid-range order. In this study, the TA and TO peaks were used to study the microstructure of the thin films. Our analyses reveal an interesting counter-balance relationship between the boron-doping and hydrogen-dilution growth parameters. Specifically, an increase in the hydrogen dilution ratio (H$_{\mathrm{2}}$/SiH$_{\mathrm{4}})$ was found to cause the increase in the short-range order, as evidenced by an increase in the TO frequency and a decrease in the FWHM of the spectral peak. However, an increase in the doping concentration resulted in a decrease in the short-range order, as evidenced by a decrease in the TO frequency and an increase in the FWHM of the spectral peak. These results will be correlated with Multiple Internal Reflection Infrared Spectroscopy, electrical transport and noise in a-Si:H thin films to determine the effects of doping and hydrogen on the transport mechanisms in a-Si:H.   

Scanning tunneling microscope study of La- and Sb-doped BaSnO$_3$ thin films

The La-doped BaSnO$_3$(BLSO) system was found to exhibit high electron mobility and high oxygen stability along with its transparency in visible spectrum. Additionally, we recently observed a significant difference in electron mobility values between BLSO and Sb-doped BSO (BSSO) epitaxial thin films. In order to elucidate the origin of the different mobility in BLSO and BSSO thin films, we have investigated a density of states (DOS) of BLSO and BSSO by scanning tunneling microscopy and spectroscopy. Our measurements were performed at 77 K in ultra-high vacuum of 2x10$^{-10}$ Torr. We will compare the DOS of the conduction band of BLSO with that of BSSO. Only in the conduction band of BSSO, we found a specific peak that can be identified as due to the localized Sb impurity states. Our results provide strong evidence for the strong influence of localized Sb impurity states on the electron mobility. We will explain our data by anisotropy of scattering on the Fermi surface by resorting to band structure calculations of BLSO and BSSO.   

Origin of Charge Separation in III-Nitride Nanowires under Strain

The structural and electronic properties of BN, AlN and GaN nanowires (NWs) under different strain condition are investigated using first-principles calculations. We found an anomaly of band gap change with respect to the applied external uniaxial strain. We show that this is due to the band crossing caused by the crystal field splitting at the top of the valance band. Due to the difference of the atomic relaxation at the core and surface regions of the NW, we show that electron and hole separation can be achieved when the compressive uniaxial strain exceeds the critical value $|\epsilon_c|$.   

Band offsets at GaN/ZnGeN$_2$ interfaces

The interfaces of GaN/ZnGeN$_2$ are of interest because of their close lattice match and hence suitability of GaN as substrate for ZnGeN$_2$ film growth. In the present work, the band offsets for various polar and non-polar interfaces between GaN and ZnGeN$_2$ are determined from full potential linear muffin-tin orbital (FP-LMTO) within the local density approximation (LDA). We determine the dipole potential formed at the interface from self-consistent supercell calculations and then add the difference between the bulk band-edges energy levels. Quasiparticle self-consistent GW corrections of the bulk band edges relative to the LDA edges are added. The strain state of the ZnGeN$_2$ is determined by assuming an unstrained GaN substrate with the ZnGeN$_2$ in-plane lattice constants matched to the substrate and the perpendicular lattice constant determined by minimizing the elastic strain energy using the known elastic constants. We find that the offset is type II, meaning staggered instead of straddled alignment, which is of interest for photovoltaic applications as holes and electrons would separate in different regions. The band offset depends slightly on interface direction. The orientation averaged valence band maximum of GaN is 0.86 eV lower than ZnGeN$_2$'s. The charge neutrality point alignment model is tested and found to give a significantly smaller band offset.   

Pentacene Derivatives: Electronic Structure and Spectra

The variation in composition and structure of the substituent groups of pentacene compounds promises a broad range of electronic structures and behaviors and provides a vast and alluring field of inquiry with avenues of exploration. These include the development of synthetic schema, the process of design for novel derivatives and, in order to identify those hypothesized compounds which demonstrate the desired behavior, the identification and refinement of computational tools that make accurate predictions about the electronic behavior of theoretical compounds. Two computational techniques and six pentacene derivatives are here examined. One technique was used to predict the vibrational spectra of the compounds, in order to both acquire data about the optical conductivity of the compounds and to establish a pool of theoretical data against which experimental data will be compared. The molecular orbital energy level diagram of the same six compounds was derived using a second approach, with the same goals of discerning between valid and invalid predictive schema by comparison with pending experimental data and between hypothesized compounds which show promise and those which present little potential for use in organic semiconductor technology.   

Many-body physics of intersubband polaritons

Intersubband polaritons are light-matter excitations originating from the strong coupling between an intersubband quantum well electronic transition and a microcavity photon mode. Up to now intersubband polaritons have been observed in a wide range of the electromagnetic spectrum from the mid-infrared to the THz regime. Due to their composite bosonic nature, the matter part of these excitations is responsible for a non-trivial dynamics of cavity polaritons. We studied how the Coulomb electron-electron interaction and the Pauli saturation of the electronic transitions affect the many-body physics of intersubband polaritons [1]. As a first application we calculated the efficiency of intersubband polariton-polariton scattering, paving the way to promising quantum non-linear optics especially in the THz regime.\\[4pt] [1] L. Nguyen-Th\^e, S. De Liberato, M. Bamba, C. Ciuti, submitted.   

Band-gap variations in polytypes of SiC: misleading parameter ``hexagonality''and proposal of new parameter

Silicon carbide (SiC) has been discovered in various polytypes. Each polytype is characterized by its stacking of atomic planes. The band gap varies substantially in each polytype from 2.40 eV to 3.33 eV in spite that the local atomic structures are identical to each other. Recently, we have clarified the mechanism of this intriguing property based on the density functional theory [1]. We have found that the Kohn-Sham orbital at the conduction-band bottom extends broadly in the internal space called channels, and thus floating in the matter. Therefore, important parameter describing the band-gap variations is the channel length, not ``hexagonality,'' which is thought to be important for the band-gap variations. \\[4pt] [1] Y.-I. Matsushita, S. Furuya, A. Oshiyama, PRL, 108, 246404 (2012).   

Band gap formation in tetrahedrally bonded I$_3$-V-VI$_4$ semiconductors-the role of V lone pairs

An interesting class of tetrahedrally coordinated ternary compounds have attracted considerable interest because of their potential as good thermoelectrics. These compounds, denoted as I$_3$-V-VI$_4$, contain three monovalent-I (Cu, Ag), one pentavalent-V (P, As, Sb, Bi), and four hexavalent-VI (S, Se, Te) atoms; and can be visualized as ternary derivatives of the II-VI zincblende or wurtzite semiconductors, obtained by starting from four unit cells of (II-VI) and replacing four type II atoms by three type I and one type V atoms. In trying to understand their electronic structures and transport properties, some fundamental questions arise: whether V atoms are indeed pentavalent and if not how do these compounds become semiconductors, what is the role of V lone pair electrons in the origin of band gaps, and what are the general characteristics of states near valence band maxima and conduction band minima. We will answer some of these questions using ab initio electronic structure calculations (density functional methods with both local and nonlocal exchange-correlation potentials). Some part of this work has been published in Dat et al, J. Phys.: Condens. Matter 24, 415502 (2012).   

NMR spectroscopy around filling factor three

We probe the spin signatures of a two-dimensional electron system, confined to a GaAs quantum well, around filling factor three ($\nu \sim 3$) using resistively detected nuclear magnetic resonance (RDNMR) spectroscopy at milliKelvin temperatures. Whereas the existence of spin textures, known as skyrmions, around filling factor one is well established, an understanding of the spin degrees of freedom for odd-integer states in higher Landau levels remains elusive. It is believed that for skyrmions to exist at $\nu \sim 3$, the Zeeman energy needs to be smaller than in the case of $\nu \sim 1$ [1]. We measured the spin-lattice relaxation time, $T_1$, which is sensitive to these spin textures as they trigger a rapid nuclear spin relaxation. Our $T_1$ measurements around $\nu = 3$ at 5 T find a small spin-lattice relaxation rate, suggesting the absence of skyrmions. In addition, our Knight shift measurements corroborate this interpretation. Furthermore, we report striking anomalies in the RDNMR spectral line shape and discuss their origin in conjunction with our findings. \\* $[1]$N. R. Cooper, Phys. Rev. B 55, R1934 (1997).   

Phonon-Assisted Auger Recombination in Gallium Arsenide and Gallium Nitride from First Principles

GaN and GaAs and their alloys are technologically important materials for solid-state optoelectronic devices such as light emitting diodes. The internal quantum efficiency of these devices, defined as the fraction of electron-hole pairs converted to photons, is limited by non-radiative loss mechanisms. Auger recombination is such a mechanism which decreases the efficiency at high current densities. In this process, the energy and momentum of an electron-hole pair is transferred to a third carrier. Numerically it is found that this process does not lead to relevant loss rates. However, if a phonon is emitted or absorbed at the same time, Auger loss rates increase by several orders of magnitude. We calculate the Auger recombination rate coefficients from first principles using density functional theory. Treating also the phonons from first principles allows us to analyze which modes and wave vectors contribute predominantly to Auger recombination and the non-radiative loss in these materials.   

Core level shift and charge transfer of Sr templates on Si(001) for epitaxial oxide growth: theoretical and experimental study

Sub-monolayer Sr templates are used as a transition layer in the epitaxial growth of perovskite oxides on semiconductors. However, a detailed understanding of how the template enables oxide growth on Si(001) is still lacking. Sr on Si(001) shows different structural and electronic properties as a function of Sr coverage. Using a combination of \textit{in situ }reflection high energy electron diffraction (RHEED) and \textit{in situ} x-ray photoelectron spectroscopy (XPS), we observed both the Si 2p and Sr 3d core levels shift toward higher binding energy as Sr coverage increases up to one half monolayer. In addition, increase of Sr coverage leads to unbuckling of the Si dimer atoms as evidenced by the merging of the up and down dimer core level components as Sr donates charge to the dimer atoms. The work function of Si also shifts with Sr coverage as observed using ultraviolet photoelectron spectroscopy (UPS).   

Inelastic electron and light scattering from the elementary electronic excitations in quantum wells

The most fundamental approach to an understanding of electronic, optical, and transport phenomena which the condensed matter physics offers is generally founded on two experiments: the inelastic electron scattering and the inelastic light scattering. This work embarks on providing a systematic framework for the theory of inelastic electron scattering and of inelastic light scattering from the electronic excitations in quantum wells. To this end, we start with the Kubo's correlation function to derive the generalized dielectric function, the inverse dielectric function, and the Dyson equation for the screened potential within the framework of Bohm-Pines' random-phase approximation. This is followed by a thorough development of the theory of inelastic electron scattering and of inelastic light scattering. After trying and testing the eigenfunctions, we compute the density of states, the Fermi energy, the full excitation spectrum made up of single-particle and collective (plasmon) excitations, the loss functions for the inelastic electron scattering, and the Raman intensity for the inelastic light scattering. It is found that HREELS can be a potential alternative of the overused Raman scattering for investigating collective excitations in such nanostructures.   

Session W24: Focus Session: Configuration interaction Quantum Monte Carlo techniques

Stochastic Coupled Cluster Theory

In an extension of the Full Configuration Interaction Monte Carlo method of Alavi et al.[1], I describe a stochastic algorithm to perform Coupled Cluster Theory[2] which represents excitation amplitudes as populations discrete excitation particles (excips) in the space of excitation operators (excitors). Re-expressing the Coupled Cluster equations as the dynamics of excips in this space, we show that a simple set of rules consisting of spawning, death, and annihilation steps suffice to evolve a distribution of in the space of excitors to sample the Coupled Cluster solution and correctly evaluate its energy. These rules are extremely simple to implement and not truncation-specific and thus this method can calculate solutions to an arbitrary level of truncation. I present results of CCSDTQ calculations on the neon atom with basis sets up to cc-pV6Z as well as calculations on the uniform electron gas beyond the capability of other present methods. \\[1pt] [1] GH Booth, AJW Thom, A Alavi, J. Chem. Phys. (2009) 131, 054106 \\[1pt] [2] AJW Thom, Phys. Rev. Lett. (2010) 105, 263004   

Full Configuration Interaction Quantum Monte Carlo and Diffusion Monte Carlo: A Comparative Study of the 3D Homogeneous Electron Gas

Full configuration interaction quantum Monte Carlo$^1$ (FCIQMC) and its initiator adaptation$^2$ allow for exact solutions to the Schr\"odinger equation to be obtained within a finite-basis wavefunction \textit{ansatz}. In this talk, we explore an application of FCIQMC to the homogeneous electron gas (HEG). In particular we use these exact finite-basis energies to compare with approximate quantum chemical calculations from the VASP code$^3$. After removing the basis set incompleteness error by extrapolation$^{4,5}$, we compare our energies with state-of-the-art diffusion Monte Carlo calculations from the CASINO package$^6$. Using a combined approach of the two quantum Monte Carlo methods, we present the highest-accuracy thermodynamic (infinite-particle) limit energies for the HEG achieved to date. $^1$ G. H. Booth, A. Thom, and A. Alavi, J. Chem. Phys. 131, 054106 (2009). $^2$ D. Cleland, G. H. Booth, and A. Alavi, J. Chem. Phys. 132, 041103 (2010). $^3$ www.vasp.at (2012). $^4$ J. J. Shepherd, A. Gr\"uneis, G. H. Booth, G. Kresse, and A. Alavi, Phys. Rev. B. 86, 035111 (2012). $^5$ J. J. Shepherd, G. H. Booth, and A. Alavi, J. Chem. Phys. 136, 244101 (2012). $^6$ R. Needs, M. Towler, N. Drummond, and P. L. R\'ios, J. Phys.: Condensed Matter 22, 023201 (2010).   

Development of multicomponent semistochastic quantum Monte Carlo method for variational solution of molecular Hamiltonian without invoking the Born-Oppenheimer approximation

We present the multicomponent extension of the semistochastic quantum Monte Carlo (mc-SQMC) method for treating electron-nuclear correlation in the molecular Hamiltonian. All particles in the molecule are treated quantum mechanically and the variational solution is obtained with the SQMC method. The key feature of this approach is that the BO and separation-rotor approximation are not assumed. The application of the SQMC method for multicomponent systems involves many formidable challenges and this talk will focus on strategies to address these challenges including, appropriate coordinate system for the molecular Hamiltonian, separation of the center of mass kinetic energy, construction of the 1-particle basis functions for electrons and nuclei, construction of the multicomponent CI space and evaluation of connected configurations needed during propagation step in the SQMC method. Results from mc-SQMC will be presented for H$_2$, He$_2$, and H$_2$O systems. The H$_2$ system has been extensively studied using various methods, such as QMC and PIMC, making it an ideal system to test and compare the mc-SQMC implementation. The impact of the BO approximation and vibration-rotation coupling will be discussed by comparing mc-SQMC results with reported values for the weakly bound He$_2$.   

Reduced Density Matrices in Full Configuration Interaction Quantum Monte Carlo

Reduced density matrices are a powerful construct in quantum chemistry, providing a compact representation of highly multi-determinantal wavefunctions, from which the expectation values of important physical properties can be extracted, including multipole moments, polarizabilities and nuclear forces$^{1,2}$. Full configuration interaction quantum Monte Carlo (FCIQMC)$^3$ and its initiator extension (\emph{i}-FCIQMC)$^4$ perform a stochastic propagation of signed walkers within a space of Slater determinants to achieve FCI-quality energies \emph{without} the need to store the complete wavefunction. We present here a method for a stochastic calculation of the 1- and 2-body reduced density matrices within the framework of (\emph{i})-FCIQMC, and apply this formulation to a range of archetypal molecular systems. Consideration is also given to the source and nature of systematic and stochastic error, and regimes to effectively alleviate these errors are discussed$^5$. $^1$ P.-O. L\"{o}wdin, Phys. Rev. 97, 1474 (1955). $^2$ C. A. Coulson, Rev. Mod. Phys. 32, 170 (1960). $^3$ G. H. Booth, A. Thom, and A. Alavi, J. Chem. Phys. 131, 054106 (2009). $^4$ D. Cleland, G. H. Booth, and A. Alavi, J. Chem. Phys. 132, 041103 (2010). $^5$ D. Cleland, PhD thesis, University of Cambridge, 2012.   

Evaluation of expectation values in full configuration interaction quantum Monte Carlo

The full configuration interaction quantum Monte Carlo (FCIQMC) method[1-3] provides access to the exact ground state energy. However, like diffusion Monte Carlo, it is hard to precisely calculate expectation values of operators which do not commute with the Hamiltonian due to the stochastic representation of the wavefunction. Following related work on diffusion Monte Carlo[4], we have formulated an approach to stochastically sample additional operators in FCIQMC by using the Hellmann-Feynman theorem and sampling pumped equations of motion coupled to the standard equation of motion used to evolve the wavefunction. Our approach requires only minor modifications to existing FCIQMC programs and can be used to evaluate expectation values of arbitrary operators. We will present example calculations on the Hubbard model and molecular systems. \\[1pt] [1] G.H. Booth, A.J.W. Thom, A. Alavi, J. Chem. Phys. 131, 054106 (2009). [2] D. Cleland, G.H. Booth, A. Alavi, J. Chem. Phys. 132, 041103 (2010). [3] J.S. Spencer, N.S. Blunt, W.M.C. Foulkes, J. Chem. Phys. 136, 054110 (2012). [4] R. Gaudoin, J.M. Pitarke, Phys. Rev. Lett. 99, 126406 (2007).   

Improvements and Applications of Semistochastic Quantum Monte Carlo

Fully stochastic quantum Monte Carlo (QMC) methods, such as the full configuration interaction quantum Monte Carlo (FCIQMC) [1,2] allow one to compute the ground state of a Hamiltonian in a far larger Hilbert space than is possible using deterministic iterative diagonalization techniques. However, QMC methods suffer from the sign problem and may have large statistical errors. Recently we have shown [3] that these problems can be greatly alleviated by using a semistochastic quantum Monte Carlo (SQMC) approach, wherein the iterative projector is applied deterministically for a small subset of the Hilbert space states and stochastically elsewhere. In addition, the initiator bias, which is introduced to tame the sign problem in FCIQMC, is often greatly reduced. We explore further improvements to SQMC and apply it to a subset of the G2 set of molecules [4]. [1] George Booth, Alex Thom, Ali Alavi. J Chem Phys 131, 050106, (2009). [2] Deidre Cleland, George Booth, and Ali Alavi. J Chem Phys 132, 041103 (2010). [3] F. R. Petruzielo, A. A. Holmes, Hitesh J. Changlani, M. P. Nightingale, and C. J. Umrigar. Phys Rev Lett (Accepted 5 Oct 2012). [4] L. A. Curtiss, K. Raghavachari, G. W. Trucks, and J. A. Pople, J Chem Phys 94, 7221 (1991).   

Investigating the Singlet-Triplet Gap in Tetramethyleneethane using Quantum Monte Carlo Techniques

Tetramethyleneethane (TME) is an organic molecule composed of two allyl subunits that is the simplest disjoint diradical. The ground state according to experimental and theoretical evidence is a singlet state with $^{1}$A symmetry.~\footnote{Clifford, E. P.; Wenthold, P. G.; Lineberger, W. C.; Ellison, G. B.; Wang, C. X.; Grabowski, J. J.; Vila, F.; Jordan, K. D. J. Chem. Soc., Perkin Trans. 2 1998, 1015.} Due to the near degeneracy of the frontier orbitals, however, this state is inherently two-configurational. As the molecule is twisted through torsional angles about the central C-C bond, we compute the singlet-triplet gap using quantum Monte Carlo (QMC). In its diffusion Monte Carlo (DMC) variant, QMC is an exact method for solving the Schr\"{o}dinger equation within the bounds of the fixed-node approximation.~\footnote{ J.B. Anderson, J. Chem. Phys. 63, 1499 (1975).} DMC calculations using a multi-configurational trial wave function produce the correct ordering of the singlet and triplet states. We also investigate an alternate approach, full configuration interaction quantum Monte Carlo (FCIQMC). We compare the FCIQMC singlet-triplet energy gap as a function of torsional angle with the different theoretical methods.   

Systematically improvable auxiliary-field quantum Monte Carlo for strongly correlated systems

The quest for an accurate and scalable many-body method for strongly correlated systems is still ongoing despite many years of efforts. The auxiliary-field quantum Monte Carlo (AFQMC) method is an exact many-body method, but it suffers from a sign problem that limits its usefulness. The phaseless AFQMC (ph-AFQMC) has been introduced\footnote{Zhang and Krakauer, Phys. Rev. Lett. 90, 136401 (2003)} to control the sign problem. In this work we employ the unconstrained (exact) AFQMC method on massively parallel supercomputers to systematically improve ph-AFQMC results. Applications to strongly correlated systems, including transition-metal compounds, will be presented.   

Acceleration of Self Healing Diffusion Monte Carlo for nearly degenerate eigenstates

The Self-Healing Diffusion-Monte-Carlo method (SHDMC) recurisvely applies an evolution operation for a finite imaginary time. SHDMC and finds the full configuration interaction coefficients of the many-body ground state by projecting out excited states. The convergence of the SHDMC, being a projection method, is dictated by the energy separation between the ground and excited states. In this talk we explore methods to accelerate the convergence of SHDMC for nearly degenerate states using the dynamical information of the excited states accumulated over the recursive iterations and to compute ground and excited states simultaneously.   

Excited states and spectral functions within full configuration interaction quantum Monte Carlo

Here we consider a modified propagator in order to obtain stable convergence to excited states within the full configuration interaction quantum Monte Carlo framework.\footnote{G. H. Booth and G. K.-L. Chan, ArXiv:1210.6643 (2012)} By working with a Gaussian propagator, the dominant eigenstate is one which is closest to an initial guess energy for the state. Issues with the speed of convergence compared to the ground state propagator are discussed, with results presented for pilot applications, and potential improvements for the algorithm considered.   

Quantum Monte Carlo Study of $\pi $-bonded Transition-metal Organometallic Sandwiches

Accurate quantum Monte Carlo (QMC) calculations enabled us to determine the structure, spin multiplicity, ionization energy, dissociation energy, and spin-dependent electronic gaps of neutral and positively charged vanadium-benzene and cobalt-benzene half-sandwich and sandwich systems. The most intriguing application of these systems is as spin filters. For this purpose we have used a multi-stage combination of techniques with consecutive elimination of systematic biases except for the fixed-node approximation in QMC. The-fixed node approximation was treated at different levels from quantum chemistry (CAS-SCF) to various DFT schemes such as GGA, meta-GGA, hybrid, double-hybrid and local-hybrid functionals. While QMC results indicate a very limited predictive power of mean field DFT methods for this class of systems, QMC results are quite stable with fixed-node approximation based on several classes of DFT orbitals. Our results significantly differ from the established picture based on previous less accurate calculations and point out the importance of high-level many-body methods for predictive calculations of similar transition metal-based organometallic systems.   

Symmetry in Auxiliary-Field Quatnum Monte Carlo Calculations

We discuss how symmetry properties can be preserved rigorously to improve the accuracy and efficiency in auxiliary-field quantum Monte Carlo (AFQMC) calculations. Using the Hubbard model as an example, we study the effect of symmetry in two aspects of ground-state AFQMC calculations, the Hubbard-Straonovich transformation and the form of the trial wave function. In unconstrained calculations, the implementation of symmetry often leads to shorter convergence time and much smaller statistical errors, thereby resulting in a substantial reduction of the sign problem and allowing exact calculations for larger and more strongly correlated systems. Moreover, certain excited states become possible to calculate which are otherwise beyond reach. In calculations with constraints,\footnote{S.~\ Zhang, J.~\ Carlson, and J.~\ Gubernatis, Phys. Rev. B {\bf 55}, 7464 (1997)}$^,$\footnote{C.~\ Change, S.~\ Zhang, Phys. Rev. B {\bf 78}, 165101 (2008)} it is shown that the use of symmetry can often reduce the systematic error significantly. Results are presented for the two-dimensional repulsive Hubbard model.   

Session W25: Classical Monte Carlo and Molecular Dynamics: Methods and Applications

Hard-Disk Equation of State: First-Order Liquid-Hexatic Transition in Two Dimensions with Three Simulation Methods

We report large-scale computer simulations of the hard-disk system at high densities in the region of the melting transition~[1]. Our simulations reproduce the equation of state, previously obtained using the event-chain Monte Carlo algorithm, with a massively parallel implementation of the local Monte Carlo method~[2] and with event-driven molecular dynamics. We analyze the relative performance of these simulation methods to sample configuration space and approach equilibrium. Phase coexistence is visualized for individual configurations via the local orientations, and positional correlation functions are computed. Our results confirm the first-order nature of the liquid-hexatic phase transition in hard disks. \\[4pt] [1] J.A. Anderson, M. Engel, S.C. Glotzer, M. Isobe, E.P. Bernard, W. Krauth, arXiv:1211.1645. \newline [2] J.A. Anderson, E. Jankowski, T.L. Grubb, M. Engel, S.C. Glotzer, arXiv:1211.1646.   

Massively parallel Monte Carlo for many-particle simulations on GPUs

Current trends in parallel processors call for the design of efficient massively parallel algorithms for scientific computing. Parallel algorithms for Monte Carlo simulations of thermodynamic ensembles of particles have received little attention because of the inherent serial nature of the statistical sampling. We present a massively parallel method that obeys detailed balance and implement it for a system of hard disks on the GPU.[1] We reproduce results of serial high-precision Monte Carlo runs to verify the method.[2] This is a good test case because the hard disk equation of state over the range where the liquid transforms into the solid is particularly sensitive to small deviations away from the balance conditions. On a GeForce GTX 680, our GPU implementation executes 95 times faster than on a single Intel Xeon E5540 CPU core, enabling 17 times better performance per dollar and cutting energy usage by a factor of 10. [1] J.A. Anderson, E. Jankowski, T. Grubb, M. Engel and S.C. Glotzer, arXiv:1211.1646. [2] J.A. Anderson, M. Engel, S.C. Glotzer, M. Isobe, E.P. Bernard and W. Krauth, arXiv:1211.1645.   

Generalized Bond Order Parameters to Characterize Transient Crystals

Higher order parameters in the hard disk fluid are computed to investigate the number, the life time and size of transient crystal nuclei in the pre-freezing phase. The methodology introduces further neighbor shells bond orientational order parameters and coarse-grains the correlation functions needed for the evaluation of the stress autocorrelation function for the viscosity. We successfully reproduce results by the previous collision method for the pair orientational correlation function, but some two orders of magnitude faster. This speed-up allows calculating the time dependent four body orientational correlation between two different pairs of particles as a function of their separation, needed to characterize the size of the transient crystals. The result is that the slow decay of the stress autocorrelation function near freezing is due to a large number of rather small crystal nuclei lasting long enough to lead to the molasses tail.   

Size-dependent Melting Behavior of Iron Nanoparticles by Replica Exchange Molecular Dynamics

Due to the finite size effect, nanoparticles own unique physical, chemical, and magnetic properties. Comparing with the bulk materials, the large surface/volume ratio of nanoparticles could lead to more complicate atomic and electronic behavior, thus the thermodynamical properties can be also very rich. In the last a few decades, as one of the fundamental problems in the nano science, the melting behavior of nanoparticles had been widely investigated by numerous experimental and theoretical studies. Using replica-exchange molecular dynamics method (REMD), we have investigated the size dependence of the melting behavior of iron nanoparticles. Comparing to the conventional molecular dynamics (MD), the REMD method is found to be very efficient to determine the melting point, by avoiding the superheating and undercooling phenomena. With accurate determination of the melting point, we find that the melting temperature does not follow linearly with the inverse of size. By incorporating the size dependent thickness of surface liquid layer which is observed in our simulation, we propose a revised liquid skin melting model to describe the size dependent melting temperature.   

Nanoindentation in Nanoporous Silica: Multimillion-Atom Molecular Dynamics Simulations

Nanoporous silica is widely used in catalysis, chromatography, anticorrosion coatings, desalination membranes, and as drug delivery vehicles because it is easy to tune the size of pores and their morphologies and to functionalize pore surfaces with a variety of molecular moieties. We have performed multimillion-atom molecular dynamics simulations to examine the structural properties and mechanical behavior of nanoporous silica at various densities. The simulations are based on experimentally validated force field for silica. We have examined the pore size distribution, and calculated roughness exponents of pores to characterize pore morphologies. We have determined the scaling of elastic moduli, hardness and fracture toughness with porosity of nanoporous silica through nanoindentation simulations. Our calculated value of hardness (10.6 GPa) for amorphous silica at normal density agrees very well with the experimental value (10 GPa) [1].\\[4pt] [1] K. Nomura, Y. Chen, R. K. Kalia, A. Nakano and P. Vashishta, Appl Phys Lett \textbf{99} (11), 111906 (2011).   

Automated generation of quantum-accurate classical interatomic potentials for metals and semiconductors

Molecular dynamics (MD) is a powerful condensed matter simulation tool for bridging between macroscopic continuum models and quantum models (QM) treating a few hundred atoms, but is limited by the accuracy of available interatomic potentials. Sound physical and chemical understanding of these interactions have resulted in a variety of concise potentials for certain systems, but it is difficult to extend them to new materials and properties. The growing availability of large QM data sets has made it possible to use more automated machine-learning approaches. Bart\'{o}k \emph{et~al.} demonstrated that the bispectrum of the local neighbor density provides good regression surrogates for QM models. We adopt a similar bispectrum representation within a linear regression scheme. We have produced potentials for silicon and tantalum, and we are currently extending the method to III-V compounds. Results will be presented demonstrating the accuracy of these potentials relative to the training data, as well as their ability to accurately predict material properties not explicitly included in the training data.   

Database Optimization for interatomic potential model

We develop a new algorithm for database optimization of interatomic potential models with Bayesian statistics. Conventional classical potential fitting schemes generates a best fit parameter set, but do not show inadequacies of the potential model nor give insight into viability of the fitting database. Our algorithm generates an ensemble of potential fits with Markov Chain Monte Carlo and make predictions based on Bayesian error estimation according to the ensemble. We consider a fitting database to be optimal when the sum of relative errors for all entries of the database is minimized. A specific objective function is proposed and an optimized database of the interatomic potential model can be obtained by modifying the relative importance (weights) of different structures in the database. We test the algorithm with a Lennard-Jones potential fitting of Ti, which shows specific limitations of this simple potential model. We also show that the derivative of the objective function with respect to weight determines whether a structure should be added to or removed from the database.   

A first-principles interatomic potential via perturbative theory

Here we propose a new approach for constructing a first-principles interatomic potential based upon a Taylor series expansion in clusters of the atomic displacements. While the number of clusters is very large in general, group theory can be used to generate a tractable number of clusters in materials with sufficiently high symmetry. A large dataset of perturbed structures which randomly samples the irreducible cluster phase space is constructed and computed in density functional theory. The cross validation score is then used to determine which clusters should be retained in the expansion. This method is then benchmarked on a one-dimensional atomic chain. Excellent agreement is achieved within a large range of atomic displacements in addition to large lattice strains. Additionally, one can recover the phonons as a function of strain. Further application of the method to two and three dimensional materials are also presented.   

A new type of interatomic potential for oxides and its applications to BiFeO$_3$ and PbTiO$_3$

Conventional first-principles methods are limited due to their intense computational cost. There is therefore still a strong need to develop accurate and efficient atomistic potential that could reproduce the full dynamical behaviors of metal oxides for large-scale finite-temperature simulations. We will present a new type of interatomic potential based on principles of bond-valence conservation and bond-valence vector conservation. The physical basis is justified quantum mechanically in the framework of a tight-bonding model, demonstrating that our model is formally equivalent to the bond-order potential (BOP), but is dramatically more efficient computationally. We will present an interatomic potential for BiFeO$_3$ and PbTiO$_3$, respectively. The validity of those model potentials is tested for both constant-volume and constant-pressure molecular dynamics simulations. The ferroelectric-to-paraelectric phase transitions of BiFeO$_3$ and PbTiO$_3$ are successfully reproduced. The calculated domain-wall energies using classical potentials are in satisfying agreement with DFT values. We conclude that our model potential is a promising type of force field that can have a broad application to a wide range of inorganic materials.   

A New Charge Model in The Valence Force Field Model for Phonon Calculations

The classical ball and spring Valence Force Field model is useful to determine the elastic relaxation of thousand-atom nanosystems. We have also used it to calculate the phonon spectra of nanosystems. However, we found that the conventional point charge model in the Valence Force Field model can cause artificial instability in nanostructures. In this talk, we will present a new charge model which represents the electron cloud feature of the Born charge in a real crystal. More specifically, we have two opposite-signed point charges assigned to each atom, one at its real position, another at a position determined by its neighbor atoms. This innovation allows both electrostatic charges and Born charges to be accurately represented while retaining extreme efficiency. This customized VFF method is developed to be fittable to the results of density functional theory (DFT) calculation. We will present the results of CdSe bulk, surface, and nanowire calculations and compare them with the equivalent ab-initio calculations, for both in their accuracies and their costs.   

Temperature-dependent classical phonons from efficient non-dynamical simulations

We describe a rigorous approach to the calculation of classic lattice-dynamical quantities from simulations that do not require an explicit consideration of the time evolution. We focus on the temperature-dependent vibrational spectrum. We start from the usual moment expansion of the relevant time correlation function (position-position or velocity-velocity) for a many-body system, and show that it can be conveniently split into one-body-like contributions by using a basis in which the low-order terms are diagonal. This allows us to compute the main spectral features (e.g., position {\em and} width of the phonon peaks) from thermal averages readily available from any statistical simulation. We demonstrate our method with an application to a model system that presents a structural transition and strongly temperature-dependent phonons. Our theory justifies and clarifies the status of previous heuristic schemes to estimate phonon frequencies in a computationally efficient way.   

Density and Spectral-Density Matrices in Atomistic-Scale Models

Density matrices for the states of atoms appear from the construction of a model referred to as the Fragment Hamiltonian (FH) model. The FH model is not dependent on construction of one-electron as a prelude to the atomistic level. Rather a density matrix of occupation numbers of the integer charge states is composed directly from a many-electron point of view to represent the state of each atom or fragment in a molecule or material. The properties of these density matrices comply with those general density matrices. Two particular properties are explored. One property that is unique to the FH model is that the coefficients of the occupancy density matrix can be transformed into functions of more familiar variables, such as net charge and ionicity that play a central role in regulating charge flow in a molecule or material. The second property is that the concept of a spectral density matrix can be defined as an extension of the occupancy density matrix and again is utilized in a manner that is analogous to the role of that concept in one-electron theories of electronic structure. The construction and functionalities of both density matrix concepts are illustrated through examples from idealized systems such as one-dimensional chains.   

Atomistic Simulation Studies of the Bulk Lithiated TiO$_2 $

TiO$_2$ has been confirmed as a safe anode material in lithium ion batteries due to its higher Li-insertion potential, (1.5V) in comparison with commercialised carbon anode materials. In the current study, amorphisation recrystallization method is used to produce bulk TiO$_2$ with a brookite structure and lithium is inserted at different concentrations. In accordance with pair distribution function experiments [1], it is found that lithiation tends to amorphise the structures. Simulated X-ray diffraction patterns are produced from such structures and compared with the experimental XRDs. Microstructures of TiO$_2$ are generated and are found to be highly twinned hence forming straight and zigzag tunnels. The microstructures of lithiated TiO$_2$ display limited twinning and tunnels with less pathways available for lithium transport. The microstructures are compared with those of nanostructural TiO$_2$ and suggestions for the preference of the latter in anodes are put forward. \\[4pt] [1] D. Dambournet, K. W. Chapman, M.V. Koudriachova, P.J. Chupas, I. Belharouak, and K. Amine, Inorg. Chem. 2011, 50, 5855--5857.   

Atomistic simulations studies of the bulk cobalt pentlandite (Co$_9$S$_8)$: Validation of the potential model

We investigate various forms of the cobalt pentlandite, Co$_{9}$S$_{8}$, at different temperatures, using classical atomistic simulation methods with the support of electronic structure calculations. The first interatomic potentials of Co$_{9}$S$_{8}$ based on the Born model, were derived with input data such as structure and elastic properties from experiments and electronic structure calculations respectively. The interatomic potentials were validated by running energy minimization and molecular dynamics calculations. The structure, elastic properties and phonon spectra corresponded well with those determined by electronic structure methods. The calculations further reproduced the complex high temperature transformation to high form pentlandite and the melting of Co$_{9}$S$_{8}$; as deduced from the crystal structure and radial distribution functions. The interatomic potentials can be used for studies of surfaces and nanostructures.   

Stable Algorithms for Modeling Thin-Film Epitaxial Growth

We search for stable time-stepping schemes for a phase-field model of thin film epitaxial growth. In particular, we consider a class of linear semi-implicit schemes which ensure the free energy decreases with time, a property called gradient stability. System dynamics slow at late times, so gradient stable schemes which allow adaptive time stepping are highly desirable. We perform a linear stability analysis and support it with numerical testing, revealing a region in parameter space of gradient stable semi-implicit schemes.   

Session W26: Focus Session: Semiconductor Qubits - Progress in Si

Robust few-electron quantum dot devices in nuclear spin engineered Si/SiGe

Spins in gate-defined quantum dots are currently discussed as one of the most promising scalable qubit architecture. Since the identification of the hyperfine interaction as a dominant spin qubit decoherence mechanism, Si/SiGe heterostructures have been receiving steadily increasing attention for realizing devices almost free of nuclear spin carrying isotopes. Building Si/SiGe heterostructures from material enriched in nuclear spin-free isotopes brings new perspectives of reaching a regime of further improved decoherence times compared to Si/SiGe of natural isotope composition. In such isotopically engineered heterostructures, the decoherence is predicted to no longer be governed by the hyperfine interaction with the nuclear spin bath, but solely by dipolar interactions. In the first part of my presentation I will review the development of two-dimensional electron systems in 28Si for spin qubit applications in my group and discuss few electron double quantum dot devices based on these heterostructures. Being able to avoid hyperfine-induced decoherence then brings a second major limitation for the realization of robust spin qubits into focus. Indeed, the manipulation of such qubits relies on Coulomb interactions, enabling electronic noise to cause decoherence. Charge traps in the heterostructure may contribute to decoherence through a fluctuation of charges or through dipolar interactions of the spin degree of freedom of the trap and the qubit. In the second part of my talk I will present our recent study of charge noise in modulation-doped Si/SiGe heterostructures and discuss device and heterostructure designs which efficiently suppress charge noise.   

\textit{In situ} isotopic enrichment and growth of $^{28}$Si for quantum information

Starting from natural abundance silane gas, we deposit $^{28}$Si films enriched \textit{in situ} to 99.9{\%} in support of solid state quantum information systems. Isotopically enriched materials such as $^{28}$Si are known to act as a ``solid state vacuum'' allowing for qubits with coherence (T$_{2})$ times of minutes. Quantum coherent devices rely on long T$_{2}$ times, but nuclear spin impurities are a major cause of decoherence. Isotopically enriching materials to eliminate stray nuclear spins (such as the 4.7{\%} $^{29}$Si in natural silicon) greatly improves coherence. Our objective is to produce silicon that is not only isotopically enriched, but chemically pure and defect free. We crack and ionize a natural abundance source gas, magnetically mass filter the ions in a beam line, and deposit the enriched material hyperthermal energies. In addition to our first $^{28}$Si samples assessed by SIMS to be enriched to \textgreater\ 99.9{\%}, we previously implanted $^{22}$Ne enriched at 99.4{\%} (9.2{\%} natural abundance) as proof of principle and have also grown $^{12}$C films enriched at \textgreater\ 99.996{\%} (98.9{\%} natural abundance). To our knowledge, no other effort is actively producing enriched solid silicon directly from natural abundance silane. Ongoing improvements are leading us towards our goal of $^{28}$Si enriched to \textgreater\ 99.99{\%} and epitaxial deposition.   

Coherence time of the nuclear spin of ionized phosphorus donors in $^{28}$Si at liquid He and room temperature

Remarkable coherence times have recently been reported for the nuclear spin of dilute neutral $^{31}$P in highly enriched $^{28}$Si [1]. For ionized $^{\mathrm{31}}$P, the removal of the hyperfine-coupled electron should result in a nuclear spin even more decoupled from the environment, and hence even longer coherence times at cryogenic temperatures. The coherence time of ionized $^{31}$P was recently observed in natural Si, and while the nuclear coherence time was indeed much longer than the electron coherence time measured in the same device, it was limited to 18 ms due to both the presence of $^{29}$Si as well as the readout mechanism being employed [2]. Here we report on coherence time measurements for ionized $^{31}$P in the same $^{28}$Si samples used for the previous [1] neutral donor study. In addition to the promise of longer cryogenic coherence times, the removal of the hyperfine-coupled electron should result in a profound change in the temperature dependence of T$_{2}$. For the neutral donor, the electron T$_{1}$ decreases very rapidly with increasing temperature, and even at 4.2 K the nuclear T$_{2}$ is limited by the electron T$_{1}$ [1]. This mechanism is absent for the ionized donor, and we will report on nuclear coherence time measurements for ionized $^{31}$P at room temperature.\\[4pt] [1] M. Steger et al., Science 336, 1280 (2012).\\[0pt] [2] L. Dreher et al., Phys. Rev. Lett. 108, 027602 (2012).   

Decoherence of Neutral $^{31}$P Donor Nuclear Spins by $^{29}$Si

NMR data from degenerately doped Si:P has suggested that the coherence of $^{31}$P nuclear spins can be limited to a few ms in natural Si by spectral diffusion from $^{29}$Si [1]. Here we report measurements of the nuclear spin coherence of neutral isolated $^{31}$P donors in lightly-doped ($\sim $10$^{15}$ /cm$^{3})$ Si with $^{\mathrm{29}}$Si concentrations from 1\% to 50\%. Pulsed ENDOR at X-band microwave frequency and a magnetic field of 0.35 T was used to measure the nuclear spins. The light doping and measurement temperature of 1.7K ensured that neither electron spin flips nor flip-flops limited the nuclear T$_{2}$. We find that the resulting echo intensity decays are nonexponential, and the time to reach 1/e is inversely proportional to the $^{29}$Si density. The nuclear decoherence time for natural silicon is found to be approximately 1 second, about 2000 times longer than donor electron spins in natural Si.\\[4pt] [1] G.P. Carver et al., Phys. Rev. B 3, 4285 (1971).   

Spin measurement in an undoped Si/SiGe double quantum dot incorporating a micromagnet

We present recent measurements on a double dot formed in an accumulation mode undoped Si/SiGe heterostructure. The double dot incorporates a proximal micromagnet to generate a stable magnetic field difference between the quantum dots. By measuring the ground state and excited state spectrum of this double dot as a function of in-plane magnetic field we identify the (1,1) and (2,0) charge degeneracy point. Using single-shot readout we measure transitions between the (2,0) singlet and the (1,1) triplet states. This method enables the identification of the crossing as a function of detuning between the (1,1) triplet states (both the first and second excited states) and the (2,0) singlet state. We also present data showing that this undoped device has good charge stability and can be measured with high frequency (up to 500MHz) voltage pulses.   

Valley-orbit hybrid states in Si quantum dots

The conduction band for electrons in layered Si nanostructures oriented along (001) has two low-lying valleys. Most theoretical treatments assume that these valleys are decoupled from the long-wavelength physics of electron confinement. In this work, we show that even a minimal amount of disorder (a single atomic step at the quantum well interface) is sufficient to mix valley states and electron orbitals, causing a significant distortion of the long-wavelength electron envelope. For physically realistic electric fields and dot sizes, this valley-orbit coupling impacts all electronic states in Si quantum dots, implying that one must always consider valley-orbit hybrid states, rather than distinct valley and orbital degrees of freedom. We discuss the ramifications of our results on silicon quantum dot qubits.   

A new mechanism for spin and valley relaxation in silicon quantum dots

We consider spin and valley relaxation in imperfect silicon quantum dots with 1 to 3 electrons. Phonons, spin-orbit coupling, and the electrostatic confining potential of the dot all play roles in both the functional dependence on key parameters (say magnetic field) and the quantitative magnitude of the relaxation rate. Level mixing in the dot allows for spin relaxation via phonons and also explains anti-crossing behavior of dot levels as a function of magnetic field. We show that valley state relaxation can be fast in realistic dots and that spin relaxation can be a few orders of magnitude longer. Our results compare favorably to recent experimental data including the power dependence on magnetic field, location of relaxation hot spots, and the magnitude of the relaxation rates themselves. Some of this work is in collaboration with A. Dzurak group at the University of New South Wales, Australia.   

Interactions and valley-orbit coupling in Si quantum dots

The valley-orbit coupling in a few-electron Si quantum dot is a function of its occupation number N, and for N \textgreater 1 is in principle renormalized by the electron-electron Coulomb interaction, which is known to be strong. We study the interaction renormalization of the valley-orbit coupling for 2 $\le $ N $\le $ 4, showing that, counterintuitively, interaction effects on the valley-orbit coupling are weak. For N $=$ 2 the renormalization is suppressed by valley interference, while for N $=$ 3 all renormalization terms are zero due to spinor overlaps, and for N $=$ 4 interaction renormalization terms cancel between different pairs of electrons. Experimental observations reveal no evidence of interaction effects on the valley-orbit coupling, consistent with these findings.   

Genetic Design of Enhanced Valley Splitting towards a Spin Qubit in Silicon

The quantum state of an electron in the Si conduction band holds exceptional promise for quantum computing, owing to its attractive spin coherence properties and adaptability to standard electronics. A paramount challenge is the orbital degeneracy of the lowest conduction band of Si, which is potentially a serious source of decoherence for spin qubits. Hence, isolating a single electron valley state by creating a sufficiently large valley splitting (VS) is a prerequisite for the realization of Si-based spin qubits. Previous explorations of Si quantum wells confined by Si-Ge alloy barriers led thus far to a limited VS of the order of 1 meV or smaller. Here we demonstrate, via an atomically resolved pseudopotential theory, that the monolayer ordering of Si-Ge barriers within reach of modern superlattice growth techniques can be harnessed to enhance the VS by up to one order of magnitude compared to disordered random alloy barriers. A biologically inspired genetic-algorithm search allowed us to identify magic atomic layer sequences of the superlattice barriers that isolate single electron valley state in Si with VS as large as $\sim$9 meV. These results may provide a roadmap for reliable spin-only quantum computing in Si.   

Impact of the valley degree of freedom on the control of donor electrons near a Si/SiO$_2$ interface

We analyze the valley composition of one electron bound to a shallow donor close to a Si/barrier interface as a function of an applied electric field within a multivalley effective mass model. Switching from low to high fields, the electron ground state is drawn from the donor site into the interface, leaving the donor partially ionized. Valley splitting at the interface occurs due to the valley-orbit coupling, $V_{vo}^I = |V_{vo}^I| e^{i \theta}$. At intermediate electric fields, close to a characteristic shuttling field, the electron states may constitute hybridized states with valley compositions different from the donor and the interface ground states. The full spectrum shows crossings and anticrossings as the field varies. The degree of level repulsion depends on the relative valley compositions, which vary with $|V_{vo}^I|$, $\theta$ and the interface-donor distance. We focus on the valley configurations of the states involved in the donor-interface tunneling process, given by the anticrossing of the three lowest levels. A sequence of two anticrossings takes place and the complex phase theta affects the symmetries of the eigenstates and level anticrossing gaps. Implications of our results on the practical manipulation of donor electrons in Si nanostructures are discussed.   

Localization of Si/SiO$_2$ Interface States: Properties and Physical Implications

Interface states (IS) form spontaneously at some semiconductor-barrier interfaces and they may improve or hinder electronic control and coherence for semiconductor-based qubits. Intrinsic Si/SiO$_2$ IS and its hybridization to the Si bulk states were recently investigated within tight binding (TB) models [1]. From the simplest model (1D), new insights emerge regarding the IS's energy and hybridization with the band states. In this work the 1D TB Hamiltonian is further explored, here within a Green's function formalism. The problem is solved exactly via a decimation technique based on renormalization group ideas [2]. The IS thus obtained are strictly related to the junction of two semi-infinite chains modeling the Si material and the SiO$_2$ barrier, excluding possible contributions from parameters (e.g. chain length) previously invoked [1]. We obtain the energy of IS as well as the exponential longer (shorter) localization lengths into the Si (barrier) material. The IS may be probed experimentally by an external electric field, which modulates the capacitance of the system, or by the spacing between the two lowest levels, related to the valley splitting [1].\\[4pt] [1] Saraiva et al, Phys. Rev. B 82, 245314 (2010).\\[0pt] [2] da Siva and Koiller, Solid State Commun. 40, 215 (1981)   

Session W27: Focus Session: Superconducting Qubits: Quantum Computing Architectures

Overview of a Quantum Annealing Processor

Quantum Adiabatic Evolution algorithms have been proposed as a potentially powerful set of methods to solve computationally hard problems.\footnote{E. Farhi, \emph{et al.}, SCIENCE \textbf{292}, pp. 472-476, 20 April 2001} One example of this approach is to find the ground state configuration of an Ising spin system with a transverse field using quantum annealing (QA).\footnote{T. Kadowaki and H. Nishimori, Phys. Rev. E,\textbf{58}(5), pp. 5355-5363, (1998)} I will present an overview of the architecture and operation of the D-Wave One, an end-to-end computing platform that performs QA by slowly decreasing the transverse field of a programmable Ising spin system. After a brief review of quantum annealing, I will describe how superconducting flux qubits are used to construct the programmable Ising spins.\footnote{R. Harris, \emph{et al.}, Phys. Rev. B, \textbf{82}, 024511 (2010)} I will then discuss some recent experiments performed to determine whether or not the processor behaves as intended. Toward this end, it is particularly useful to be able to measure the spectrum of single and multiple coupled qubits as they progress through the annealing algorithm.\footnote{A. J. Berkley, \emph{et al.}, \texttt{arXiv:1210.6310v1}} Finally, since the primary measure of the efficacy of such a machine is how well it solves problems, I will conclude with a discussion of system performance and scaling.   

Cross-Talk in Superconducting Transmon Quantum Computing Architecture

Superconducting transmon quantum computing test structures often exhibit significant undesired cross-talk. For experiments with only a handful of qubits this cross-talk can be quantified and understood [1], and therefore corrected. As quantum computing circuits become more complex, and thereby contain increasing numbers of qubits and resonators, it becomes more vital that the inadvertent coupling between these elements is minimized. The task of accurately controlling each single qubit to the level of precision required throughout the realization of a quantum algorithm is difficult by itself, but coupled with the need of nulling out leakage signals from neighboring qubits or resonators would quickly become impossible. We discuss an approach to solve this critical problem.\\[4pt] [1] ``Characterization of addressability by simultaneous randomized benchmarking,'' Jay M. Gambetta, et al., arXiv:1204.6308 [quant-ph].   

Implementation of a two-qubit Grover algorithm using superconducting qubits

High fidelity two-qubit gates have previously been demonstrated with fixed frequency superconducting qubits and employing the cross-resonance effect generating the qubit-qubit interaction in which qubit 1 is driven at the frequency of qubit 2. The drawback of previous implementations of the cross-resonance gate is the fact that single qubit gates on qubit 2 emerge when the qubits are multi-level systems instead of strictly two-level systems. As a result, two-qubit gates must be tuned up by careful timing or by explicitly applying single-qubit correction pulses. This is a cumbersome procedure and can add overall errors. Instead, we show a refocusing scheme which preserves the two-qubit interaction but eliminates the single-qubit gates. The total gate length is only increased by the duration of two single qubit pi-pulses which is a low overhead. When tuning up this composite pulse we show an implementation of a two-qubit Grover's algorithm without applying any correction pulses. The average success probability of the algorithm is consistent with fidelity metrics obtained by independent randomized bench-marking experiments (both single and two-qubit).   

Emulating a mesoscopic system using superconducting quantum circuits

We demonstrate an emulation of a mesoscopic system using superconducting quantum circuits. Taking advantage of our ReZQu-architectured quantum processor, we controllably splitted a microwave photon and manipulated the splitted photons before they recombined for detection. In this way, we were able to simulate the weak localization effect in mesoscopic systems - a coherent backscattering process due to quantum interference. The influence of the phase coherence was investigated by tuning the coherence time of the quantum circuit, which in turn mimics the temperature effect on the weak localization process. At the end, we demonstrated an effect resembling universal conductance fluctuations, which arises from the frequency beating between different coherent backscattering processes. The universality of the observed fluctuation was shown as the independence of the fluctuation amplitude on detailed experimental conditions.   

Speed limits for quantum gates in multiqubit solid-state systems

We derive speed limits for various unitary quantum operations in multiqubit systems under typical experimental conditions, using parameters and constraints that are commonly encountered with superconducting qubits. In particular we focus on two- and three-qubit gates. We find that simple methods for implementing two-qubit gates generally provide the fastest possible implementations of these gates. We also find that the three-qubit Toffoli gate time varies greatly depending on the type of interactions and the system's geometry, taking only slightly longer than a two-qubit controlled-NOT (CNOT) gate for a triangle geometry.   

Designing entangling microwave gates between fixed frequency superconducting circuits coupled by resonators

Many of the recent techniques for controlling superconducting quantum circuits are directly derived from the atomic theory of cavity QED, and the fixed frequency transmon provides a particularly close analogy to an ``artificial atom.'' However, even in this case new modelling techniques are required as we engineer parameter regimes that have been previously unexplored in atomic systems. In this talk we develop the Schrieffer-Wolff transformation as a means of adiabatically eliminating high-energy subspaces in order to derive effective entangling Hamiltonians. We can use this theory to explain many of the recent, experimentally demonstrated fixed frequency gates such as the cross-resonance gate and the two-photon 00 to 11 transition. In the case of the cross-resonance gate this more detailed model predicts spurious single qubit rotations, and their rates, which can then be removed through refocusing techniques.   

Hardware-efficient quantum memory protection

We propose a new method to autonomously correct for errors of a logical qubit induced by energy relaxation. This scheme encodes the logical qubit as a multi-component superposition of coherent states in a harmonic oscillator, more specifically a single cavity mode. The sequences of encoding, decoding and correction operations employ the non-linearity provided by a single physical qubit coupled to the cavity. We layout in detail how to implement these operations in a circuit QED architecture. This proposal directly addresses the task of building a hardware-efficient and technically realizable quantum memory.   

Engineered circuit QED with dense resonant modes

In circuit quantum electrodynamics even in the ultrastrong coupling regime, strong quasi-resonant interaction typically involves only one mode of the resonator as the mode spacing is comparable to the frequency of the mode. We are going to present an engineered hybrid transmission line consisting of a left-handed and a right-handed portion that has a low-frequency van-Hove singularity hence showing a dense mode spectrum at an experimentally accessible point. This gives rise to strong multi-mode coupling and can be utilized in multiple ways to create strongly correlated microwave photons.   

Observing the nonequilibrium dynamics of the quantum transverse-field Ising chain in circuit QED

Circuit QED architectures of superconducting artificial atoms and microwave resonators are currently moving towards multi-atom, multi-resonator setups with drastically enhanced coherence times, making them increasingly attractive candidates for quantum simulations of interesting interacting quantum many-body systems. Here we propose and analyze a circuit QED design that implements the quantum transverse-field Ising chain coupled to a microwave resonator for readout. Our setup can be used to study quench dynamics, the propagation of localized excitations, and other nonequilibrium features, in a field theory exhibiting a quantum phase transition, and based on a design that is feasible with current technology and could easily be extended to break the integrability of the system.   

Strongly-coupled Josephson junction array for simulation of frustrated one-dimensional spin models

We study the capacitance-coupled Josephson-junction array beyond the small-coupling limit. We find that, when the scale of the system is large, its Hamiltonian can be obtained without the small-coupling approximation and the system can be used to simulate strongly frustrated one-dimensional Ising spin problems. To engineer the system Hamiltonian for an ideal theoretical model, we apply a dynamical-decoupling technique to eliminate undesirable couplings in the system. Using a six-site junction array as an example, we numerically evaluate the system to show that it exhibits important characteristics of the frustrated spin model.   

Session W28: Focus Session: Soft-Matter, Biology, & Bioinspiration

Cavitation in trees monitored using simultaneously acoustics and optics

Under hydric stress, in dry weather conditions, the sap within trees may reach extreme negative pressures and cavitate: bubbles appear, which eventually causes an embolism in the circulation. It has been shown that cavitation is associated with short acoustic emissions, and they can be recorded in the ultrasound range. However the precise origin of each acoustic emission is still not clear. In particular, the acoustic emissions could be not only the consequence of cavitation, but also of the collapse of xylem conduits, or of fractures in the wood. Here we present an original set-up where we can simultaneously record (i) the acoustic emissions, (ii) the location of cavitation events, by imaging the sap channels under light transmission microscopy. We are then able to correlate the sounds to the visible changes in channels, such as the appearance of cavitation bubbles. We hope the results of the present study might help to better understand the acoustic signals emitted by trees, and to obtain further information in the evolution of wood under dry stress conditions.   

Reversible Rigidity Control Using Low Melting Temperature Alloys

Inspired by nature, materials able to achieve rapid rigidity changes have important applications for human body protection in military and many other areas. This talk presents the fabrication and design of soft-matter technologies that exhibit rapid reversible rigidity control. Fabricated with a masked deposition technique, the soft-matter composite contains liquid-phase and phase-changing metal alloys embedded in a soft and highly stretchable elastomer. The composite material can reversibly change its rigidity by three orders of magnitude and sustain large deformation.   

``Lock and key mechanism'' for ligand binding with adrenergic receptors and the arising mechanical effects on the cell membrane

Chemicals hitting the surface of cell aggregates are known to give arise to cyclic Adenosine Mono Phosphate (cAMP), a second messenger that transduces inside the cell the effects of species that cannot get through the cell membrane. Ligands bind to a specific receptor following the so called ``lock and key mechanism''; (beta)-adrenergic receptors are proteins embedded in the lipid bilayer characterized by seven transmembrane helices. Thinning and thickening in cell membranes may be initiated by conformational changes of some of three of the seven domains above. The cell response is linked to the coupling of chemical, conformational and mechanical effects. Part of the cAMP remains intracellular, whereas the remaining fractions migrates outside the cell due to membrane transporters. A new Helmholtz free energy, accounting for receptor and transporter densities, receptor conformation field and membrane elasticity is investigated. It is shown how the density of active receptors is directly related to the conformation field and it enters the resulting balance equation for the membrane stress. Balance laws for fluxes of transporters and receptors, coupled with the former because of the outgoing cAMP flux caused by the transporters, as well as for the diffusive powers must be supplied.   

Geometrical study of the deformations of a thin spherical shell inspired by pollen grains.

Various monocotyledon pollen grains have a geometric design. They are constituted by a stiff thin shell with an n-fold rotationally symmetric softer sector. The mechanic response of these inhomogeneous shells can be approximated as an open shell. Isometric modes are known to be energetically favorable for thin shells when they are possible. Although the literature for the complete sphere, for which these modes are impossible, is extensive, analyses of the deformation of open shells whose isometric deformations are not inhibited, are much more scarce. We focus on the isometric deformation of spheres with n-fold rotationally symmetric openings. The isometric deformation means that the surface remains a constant gaussian curvature surface. Using differential geometry, we obtained an integrable family of surfaces whose gaussian curvature remains approximatively constant. We performed both simulations by tethered mesh methods and experiments with cut ping-pong balls. We observe that first the shell surface deforms without any stretching and is very well described as a part of an approximative constant gaussian curvature surface whose singularities remain outside the shell surface and get closer to the shell surface as the load increases.   

Hysteresis in the creasing instability of hydrogels and elastomers

Soft polymers placed under compressive stress can undergo an elastic creasing instability in which sharp folds spontaneously form on the free surfaces. This process can play an important role in a variety of material failure modes, but has also been harnessed to fabricate dynamic chemical and topographic patterns. Creases have been found to form by nucleation and growth, which we show reflects the influence of surface energy as a barrier for both processes. Hysteresis in the loading and unloading cycles is an important aspect of this process, but has been reported to occur to different degrees in different material systems. Through variations in interfacial energy, we show that for a model elastomeric system, it is self-adhesion within the folding region rather than plastic deformation that gives rise to hysteresis.   

Extreme Mechanics of Growing Matter

Growth is a distinguishing feature of all living things. Unlike standard materials, living matter can autonomously respond to alterations in its environment. As a result of a continuous ultrastructural turnover and renewal of cells and extracellular matrix, living matter can undergo extreme changes in composition, size, and shape within the order of months, weeks, or days. While hard matter typically adapts by increasing its density to grow strong, soft matter adapts by increasing its volume to grow large. Here we provide a state-of-the-art review of growing matter, and compare existing mathematical models for growth and remodeling of living systems. Applications are plentiful ranging from plant growth to tumor growth, from asthma in the lungs to restenosis in the vasculature, from plastic to reconstructive surgery, and from skeletal muscle adaptation to heart failure. Using these examples, we discuss current challenges and potential future directions. We hope to initiate critical discussions around the biophysical modeling of growing matter as a powerful tool to better understand biological systems in health and disease.   

A micromechanical viscoelastic model for soft biological tissue

Understanding the viscoelastic behavior of soft collageneous tissue from micromechanical considerations is critical to the characterization of their physiological and pathological response. In this study, we propose to model biological tissue as an aggregate of unit cells (UC). Each UC represents two wavy parallel collagen fibrils cross-linked by intrafibrillar bridges. A fibril consists of two linear springs deforming axially, and interconnected by a linear torsional spring modeling the fibril bending rigidity. When an axial displacement is applied to the unit cell, the uncrimping and stretching of the fibrils cause the ground substance to shear and the intrafibrillar bridges to rotate. This model assumes that the time-dependent behavior of the UC is due to the viscous rotation of the bridges, which are modeled as Maxwell solids. The constitutive equation of the tissue is calculated from the orientation average of the constitutive equation of the unit cell weighted by the probability density function for unit cell distribution. The performance of the model to predict the creep response will be illustrated using the results of an inflation test performed on the human sclera.   

Spatially localized structure-function relations in the elastic properties of sheared articular cartilage

Contemporary developments in therapeutic tissue engineering have been enabled by basic research efforts in the field of biomechanics. Further integration of technology in medicine requires a deeper understanding of the mechanical properties of soft biological materials and the structural origins of their response under extreme stresses and strains. Drawing on the science generated by the ``Extreme Mechanics'' community, we present experimental results on the mechanical properties of articular cartilage, a hierarchically structured soft biomaterial found in the joints of mammalian long bones. Measurements of the spatially localized structure and mechanical properties will be compared with theoretical descriptions based on networks of deformed rods, poro-visco-elasticity, and standard continuum models. Discrepancies between experiment and theory will be highlighted, and suggestions for how models can be improved will be given.   

Highly Deformable Liquid Embedded Soft-Matter Capacitors and Inductors for Stretchable Electronics

We have developed a family of soft-matter capacitors and inductors that can be stretched to several times their natural length. These circuit elements are composed of microchannels of a liquid-phase Gallium-Indium-Tin alloy (Galinstan) embedded in a soft silicone elastomer (Ecoflex$^{\scriptsize{\textregistered}}$ 00-30). As the elastomer stretches, the embedded liquid channels deform, causing the capacitance and inductance to change monotonically. The relative changes in capacitance and inductance are experimentally measured as a function of stretch in three directions. The relationships found show potential for these devices to be used as strain sensors and tunable electronic filters. Additionally, theoretical predictions derived using finite elasticity kinematics are consistent with these experimentally found relationships.   

Delayed Fluid-Driven Fractures on Soft Gels

A droplet of surfactant spreading on a weak gel substrate ($\sim 10$ Pa) can produce fractures on the gel surface, which originate at the contact-line and propagate outwards in a star-burst pattern. Experiments show that the number of arms is controlled by the ratio of the surface tension differential to the gel's shear modulus. We interpret the number of fractures formed in the context of a linear elastic model arising from the uncompensated, Young-Dupre (out-of-plane) force acting at the contact-line. However, we also observe that there is an inherent variability in both the number of fractures formed and the delay for fractures to form. In the regime where single fractures form, we observe a range of delay values consistent with a thermally-activated process. The mean delay time is set by the modulus of gel substrate, decreasing for weaker substrates. In the regime where multiple fractures form, we observe that all fractures appear simultaneously and the long delays are suppressed.   

Soft-Matter Resistive Sensor for Measuring Shear and Pressure Stresses

Building on emerging paradigms in soft-matter electronics, we introduce liquid-phase electronic sensors that simultaneously measures elastic pressure and shear deformation. The sensors are com- posed of a sheet of elastomer that is embedded with fluidic channels containing eutectic Gallium- Indium (EGaIn), a metal alloy that is liquid at room temperature. Applying pressure or shear traction to the surface of the surrounding elastomer causes the elastomer to elastically deform and changes the geometry and electrical properties of the embedded liquid-phase circuit elements. We introduce analytic models that predict the electrical response of the sensor to prescribed surface tractions. These models are validated with both Finite Element Analysis (FEA) and experimental measurements.   

Manufacturing of Liquid-Embedded Elastomers for Stretchable Electronics

Future generations of robots, electronics, and assistive medical devices will include systems that are soft, elastically deformable, and may adapt their functionality in unstructured environments. This will require soft active materials for power circuits and sensing of deformation and contact pressure. As the demand for increased elasticity of electrical components heightens, the challenges for functionality revert to basic questions of fabrication, materials, and design. Several designs for soft sensory skins (including strain, pressure and curvature sensors) based on a liquid-embedded-elastomer approach have been developed. This talk will highlight new ``soft MEMS'' manufacturing techniques based on wetting behavior between gallium-indium alloys and elastomers with varying microtextured surface topography.   

Elastoswellability: Will it bend or will it buckle?

Soft mechanical structures such as biological tissues and gels exhibit motion, instabilities, and large morphological changes when subjected to external stimuli. Swelling is a robust approach for inducing structural change as it occurs naturally in humid environments and can be easily adapted for industrial design. Small volumes of fluid that interact favorably with a material can cause large, dramatic, and geometrically nonlinear deformations including beam bending, plate buckling, and surface wrinkling. In this talk we address an overarching question regarding swelling-induced deformations: will the structural change occur globally, or will it be confined to the material's surface? We introduce a materials and geometry defined transition point that describes a fluid-structure's characteristic ``elastoswellability'' lengthscale. By locally swelling unconstrained slender beams and plates with solvents of varying solubility, we identify a transition between local surface wrinkling and global structural bending.   

Session W29: Straight-Up Jamming

Dynamical Heterogeneity in a Granular System Near the Jamming Transition

We investigate dynamical heterogeneity in event driven simulations of a two-dimensional bidisperse granular fluid. We study the dynamic susceptibility $\chi_4(t)$ extracted from two different correlation functions $Q(t)$ and estimate the dynamic correlation length $\xi(t)$ obtained from the four-point structure factor $S_4(q,\tau_4)$, where $\tau_4$ is the time corresponding to the maximum of $\chi_4(t)$. We find that the dynamic correlation length grows as the volume fraction is increased to approach the jamming transition.   

Jamming in Emulsions and Elastic Tomography

Attempting to bridge the gap between the jamming of soft, athermal particles and soft colloids, we measure the elasticity of packings of $\sim 10\mu m$ droplets using light scattering and tomography. Droplets in this size range retain the soft, frictionless contacts of colloidal dispersions, yet are large enough to resist thermal agitation. Nearly buoyant droplets form disordered piles where the compression varies smoothly and slowly with depth. Using light scattered from different sections of the pile we measure the dependence of the shear modulus on pressure using Diffusing Wave Spectroscopy (DWS) microrheology. We find a shear modulus that is proportional to pressure down to loads corresponding to a $\sim 0.1\%$ compression. However, below a critical pressure, the shear modulus drops abruptly and the droplets exhibit what appears like glassy rearrangements: despite loads many orders of magnitude greater than $\frac{K_{B}T}{a^3}$.   

Jamming of 2D foams

We probe the jamming of 2D wet foams by lateral compression of a bidisperse foam monolayer sandwiched between a glass plate and a fluid surface. Boundaries and residual gravitational effects prevent the foam to be truly unjammed, obstructing the observation of a jammed/unjammed transition. Instead, we find a clear transition from a ``gravity jammed'' to a ``boundary jammed'' state, where the bulk modulus jumps from essentially zero to a finite value, in agreement with theory. In addition, we probe the nonaffine bubble motion, which becomes large near this transition.   

Stress distributions of jammed particle clusters and the maximum entropy principle

Using a simple model of frictionless bidisperse disks in two dimensions, we consider the distribution of stress on finite clusters of particles, within a statically jammed granular system at fixed global stress tensor. We compare our results against recent theories of the stress ensemble [1] and force network model [2] to investigate whether the distribution of stress is well described by a maximum entropy assumption. \\[4pt] [1] B. P. Tighe, A. R. T. van Eerd, and T. J. H. Vlugt, PRL 100, 238001 (2008);\\[0pt] [2] S. Henkes and B. Chakraborty, PRE 79, 061301 (2009)   

The link between the geometric and mechanical phase transitions at jamming

We have observed a phase transition in the geometrically defined network of nearest neighbors of sphere packings as a function of packing density. By creating packings in a range of spatial dimension, from $d=2$ to $d=9$, we have amassed evidence suggestive of an upper critical dimension for this transition of $d\leq 3$. However, as of yet we do not have a field theory to confirm this fact. It is suggestive that the geometric transition point coincides with the mechanical jamming point in all dimensions, raising the question of how the geometry of nearest-neighbors relates to the formation of contacts necessary for mechanical stability. We present an answer to this question based on the evolution of geometric constraints as mechanical jamming is approached. In addition, we find that many of the requirements for renormalization are met by the order parameters associated with the geometric phase transition. Taking cues from traditional condensed matter systems and networking theory we explore various renormalization group approaches to this phase transition.   

Sedimentary Deposition and the Kinetics of Jamming

We observe a dispersion of spheres sedimenting in a fluid until all grains form a packing. In a Newtonian fluid, the dispersion is roughly homogenous in space and time except at two well-defined interfaces: a dispersion-supernatant interface, and a jamming front below which grains form a jammed packing. This system is ideal for the study of jamming kinetics because the jamming front is stationary: it moves upwards with a constant speed and shape. To characterize the concentration profile at the front, we have utilized x-ray absorption to directly measure volume fraction as a function of height and time. To characterize the grain-scale dynamics across the front, we now utilize a light scattering technique, speckle-visibility spectroscopy, to directly measure fluctuations of the grain velocities as a function of height and time. In order to alter the kinetics of jamming in this model system, we perturb the hydrodynamic interactions between grains by using a viscoelastic fluid, and observe how the shape and speed of the jamming front changes.   

How far is the Jamming point street-lamp illuminating the real world?

The jamming of soft spheres at zero temperature has been extensively studied both numerically and theoretically, thus defining a well defined location, where a street lamp has been lit up. However it has been shown [1] that even model experiments on colloids are rather far away from the scaling regime illuminated by this lamp. Is it that the J-point has little to say about real system? We investigate the statics and the dynamics of the contact network of an horizontally shaken bi-disperse packing of photoelastic discs, close to jamming, we observe a remarkable dynamics of the contact network. It exhibits strong dynamical heterogeneities, which are maximum at a packing fraction $\phi^*$, distinct and smaller than the packing fraction $\phi^\dagger$, where the average number of contact per particle starts to increase. We demonstrate that the two cross-overs, one for the maximum dynamical heterogeneity, and the other for structural jamming, converge at point J in the zero mechanical excitation limit. Our grains are frictional and are far from thermal equilibrium. However we succeed in mapping these behaviors onto those observed for thermal soft spheres and demonstrate that some light of the J-point street-lamp reaches our granular universe. [1] Ikeda et al. arXiv.1209.2814(2012)   

Jamming of Cylindrical Grains in Featureless Vertical Channels

We study jamming of low aspect-ratio cylindrical Delrin grains falling through a featureless vertical channel. With a grain height less than the grain diameter, these grains resemble aspirin tablets, poker chips, or coins. Unidisperse grains are allowed to fall under the influence of gravity through a uniform channel of square cross-section where the channel width is greater than the grain size and constant along the length of the channel. Channel widths are chosen so that no combination of grain heights and diameters is equal to the channel width. Collections of grains sometimes form jams, stable structures in which the grains are supported by the channel walls and not by grains or walls beneath them. The probability of a jam occurring and the jam's strength are influenced by the grain dimensions and channel width. We will present experimental measurements of the jamming probability and jam strength and discuss the relationship of these results to other experiments and theories.   

Local strain field fluctuations in quasi-two-dimensional colloidal glasses

We investigate the local strain field fluctuations in a quasi-two-dimensional colloidal glass as a function of packing fraction as the jamming transition is approached. Using standard video microscopy and particle tracking techniques, we derive the best-fit affine strain tensor and the non-affinity for each particle in the sample; this information is obtained by analyzing the variations of local configurations around each particle due to thermal motion. The spatial and temporal distributions of this local deformation permit us to probe the mechanical properties of our colloidal systems. We study how these mechanical properties evolve as the systems approaches the jamming transition. Furthermore, we explore the connection between the mechanical heterogeneity and the onset of irreversible rearrangements.   

Unusual order in squeezed repulsive spheres

The soft spheres that we have been using for years to study jamming into disordered packings can make a range of surprising ordered structures at higher densities. Monodisperse repulsive harmonic disks in two dimensions form, apart form the triangular lattice everyone would expect, a square lattice and various non-bravais lattices that can be described as a triangular lattice with a basis. The latter class includes the honeycomb structure, a chiral structure, and a structure which is best described as a tiling of pentagons and triangles. The appearance of these structures, some of which have not been previously reported, is surprising because the potential between the disks only very weakly violates the condition of complete monotonicity which has been conjectured to guarantee the triangular lattice to be the ground state structure. I will discuss how these structures come about, how they are related to tiny periodic packings of hard spheres and in what ways the resulting structures might be useful.   

The role of curvature in the jamming of hard spheres on the surface of a spheroid

Using various packing protocols, we investigate numerically the jamming of spherical particles that are constrained to the surface of a larger, host spheroid. While jamming has been extensively investigated for different particle shapes and containers, the role played by curvature in the frustration that arises when spherical particles are adsorbed to curved interfaces is not yet well understood. Accordingly, we explore the dependence of the critical particle coverage fraction $\Phi$ required for jamming to occur upon the number and polydispersity of the smaller particles as well as the shape and relative size of the host spheroid. Along the way, we evaluate the relative efficiency of the numerical algorithms we employ in terms of their efficiency and their relevance to the physics of recent experiments in microfluidics and colloid deposition on curved surfaces.   

Thinking Inside the Box: The Optimal Filling of Shapes

We introduce a new spatial partitioning problem called filling[1,2], which combines aspects of traditional packing and covering problems from mathematical physics. Filling involves the optimal placement of overlapping objects lying entirely inside an arbitrary shape so as to cover the most interior volume. In n-dimensional space, if the objects are polydisperse n-balls, we show that solutions correspond to sets of maximal n-balls. We investigate the mathematical space of filling solutions and provide a heuristic for finding the optimal filling solutions for polygons filled with disks of varying radii. We consider the properties of ideal distributions of N disks as N approaches infinity. We discuss applications of filling to such problems as tumor irradiation, designing wave fronts and wireless networks, minimal information representations of complex shapes, and molecular modeling of nanoparticles and colloids. \\[4pt] [1] Phillips, Anderson, Huber, Glotzer, The Optimal Filling of Shapes, PRL 108, 198304, 2012\\[0pt] [2] Phillips, Anderson, Huber, Glotzer, Optimal Fillings - A new subdivision problem related to packing and covering, arXiv:1208.5752, 2012   

Jammed frictional tetrahedra are hyperstatic

We prepare packings of frictional tetrahedra with volume fractions $\phi$ ranging from 0.469 to 0.622 using three different experimental protocols under isobaric conditions. Analysis via X-ray micro-tomography reveals that the contact number Z grows with $\phi$, but does depend on the preparation protocol. While there exist four different types of contacts in tetrahedra packings, our analysis shows that the edge-to-face contacts contribute about 50\% of the total increase in Z. The number of constraints per particle C increases also with $\phi$ and even the loosest packings are strongly hyperstatic i.e. mechanically over-determined with C approximately twice the degrees of freedom each particle possesses.   

Packings and assemblies of hard convex polyhedra

Dense packings of hard polyhedra have been studied for centuries due to their mathematical aesthetic and more recently for their applications in fields such as granular matter, amorphous matter, and biology. The spontaneous organization of hard polyhedra under compression has only recently been addressed, demonstrating a plethora of assembled complex structures. The infinite pressure dense packings and the finite pressure, thermodynamically assembled structures for a given shape, however, are often different. In this talk we investigate connections between those two limits for convex polyhedra. We discuss the possibility of predicting one limit from the other, discuss some general rules, and link with previous works.   

Jamming of Ordered Vortex Lattice Domains

Jamming is mostly associated with granular materials, but is applicable in a variety of physical situations. Our results indicate that the vortex lattice (VL) in type-II superconductors can be used as a model system to study jamming. Previous small-angle neutron scattering (SANS) studies of the VL in MgB$_2$ with H $\parallel$ c found a triangular VL which undergoes a field-driven 30$^\circ$ reorientation transition, forming three distinct ground state phases. The low and high field phases have hexagonal VLs aligned with high symmetry directions in the crystal, whereas at intermediate fields the VL is marked by the presence of domains of vortices continuously rotating from one high symmetry direction to another. A high degree of metastability between the VL phases of MgB$_2$ has been observed [P. Das et al., Phys. Rev. Lett. 2012]. Our recent SANS measurements show that this cannot be understood based on the single domain free energy. We applied a transverse AC magnetic field to the sample and found the decrease in the metastable volume fraction depends logarithmically on the number of AC cycles, similar to some jamming scenarios. We propose that the origin for the VL metastability is a jamming of counter-rotated VL domains that prevents rotation to the equilibrium orientation.   

Session W30: Nonlinear Dynamics

Time Reversal Experiments in Chaotic Cavities

Wave focusing through a strongly scattering medium has been an intriguing topic in the fields of optics, acoustics and electromagnetics. By introducing the time reversal technique, prior knowledge about each transmission channel is no longer needed since the step of sending waves through the medium measures this information. Many approaches have been explored to achieve better focusing quality, which is influenced by several factors, such as the propagation loss. We present two methods to conduct time reversal experiments in ray-chaotic billiards or cavities. The first method uses a ray-tracing algorithm to calculate orbit information from knowledge of the cavity geometry. We then use this information to generate a synthetic signal, which is then sent into the cavity as if it's the time reversed signal in the traditional time-reversal scheme. This method tries to obtain channel information numerically but has limited accuracy due to the chaotic properties of the cavity. Another method is to utilize the transmission scattering parameter, obtained from the time domain response of the cavity between two ports. We amplify the time-reversed signal for each frequency channel in proportion to the loss it experiences during the transmission. The experimental results show that the amplitude of side lobes around the reconstructed signal is reduced significantly and the correlation between the reconstruction and the initial signal is improved from 0.8 to 0.98 in a low-mode density cavity.   

Statistical fluctuations in chains of chaotic electromagnetic enclosures

Today, the statistical analysis of complex electromagnetic cavities constitutes a very active field of research in applied electromagnetics and statistical physics. The Random Coupling Model (RCM) provides a framework for predicting the statistics of scattering of radiation in complicated enclosures. RCM makes use of results from random matrix theory (RMT) to model the mode spectrum of irregular cavities. Here, we show how to use the RCM to study the scenario of two (or more) three-dimensional cavities interconnected by apertures. We imagine exciting the first cavity of the so formed chain with a small antenna, and receiving a signal in the last cavity with a similar antenna. Recently, we derived the probability distribution of the power flowing through the cavity chain. A closed form solution of the trans-impedance between the two ports is derived, and its statistics discussed. Variations of cavity losses and aperture geometry are discussed within our statistical framework, for which distribution functions are generated by the Monte Carlo method. In the high-loss limit we are able to identify self- and cavity-cavity interaction terms. The extreme case of an irregular aperture connecting to an irregular cavity is also proposed and investigated.   

Finding equilibrium statistical mechanics in spatiotemporal chaos

Ruelle has argued that the extensivity of the complicated dynamics of spatiotemporal chaos is evidence that these systems can be viewed as a gas of weakly-interacting regions of a characteristic size. We have performed large-scale computational studies of spatiotemporal chaos in the 1D complex Ginzburg-Landau equation and have found that histograms of the number of maxima in the amplitude are well-described by an {\it equilibrium} Tonks gas (and variants) in the grand canonical ensemble. Furthermore, for small system sizes, the average number of particles in the Tonks gas (with particle sizes and temperatures determined from fits to the CGL histograms) exhibits oscillatory, decaying deviations from extensivity in agreement with the deviations in the fractal dimension found by Fishman and Egolf. This result not only supports Ruelle's picture but also suggests that the coarse-grained behavior of this far-from-equilibrium system might be understood using equilibrium statistical mechanics.   

Dynamic Scaling of Synchronization in Kuramoto-type Globally Coupled Oscillators

We investigate the dynamic scaling behavior of the phase synchronization order parameter in the framework of the original Kuramoto model with Gaussian natural frequecies near and at the critical value of the coupling strength. The temporal behavior has been never paid attention to in the earlier studies of synhronization and its transition nature including finite-size scaling (FSS), whereas the stationary critical behavior has been widely studied. We focus on the scaling behavior of the order parameter until the system reaches its steady state from various initial conditions in the context of the dynamic scaling form at criticality. It is found that dynamic scaling of synchronization can indicate the critical valule of the coupling strength and also estimate all critical exponents of the continuous synchronization transition, based on the scaling relation of the earlier suggested FSS theory. Moreover, we figure out that the dynamic scaling analysis is quite useful even though the system does not reach its steady state, provided that the system size is not too small. Finally, we argue how the generating method of natural frequecies and the thermal effect of phases affect dynamic scaling with the change of the dynamic exponent, which are numerically confirmed.   

Observation of Asymmetric Transport in Structures with Active Nonlinearities

A mechanism for asymmetric transport based on the interplay between the fundamental symmetries of parity (P) and time (T ) with nonlinearity is presented. We experimentally demonstrate and theoretically analyze the phenomenon using, as a reference system, a pair of coupled van der Pol oscillators, one with anharmonic gain and the other with the complementary time reversed anharmonic loss, connected to two transmission lines. An increase of the degree of the gain/loss strength or of the number of PT -symmetric nonlinear dimers in a chain, increases the non-reciprocality effect.   

Wave scattering from cavities with both regular and chaotic ray trajectories

The random plane wave hypothesis has been used to characterize fields inside chaotic cavities where all ray trajectories are chaotic and visit the available phase space uniformly. We consider incident and reflected waves in channels connecting to a chaotic cavity. From Random Matrix Theory, the impedance, obtained from the scattering matrix, for pure chaotic cavities can be described as a Lorentzian random variable with predictable mean and width. For some shapes of cavities, called mixed systems, some rays are chaotic and visit subregions of phase space ergodically, while some rays are regular staying on invariant troi. We generalize the previous chaotic cavity theory to mixed systems by separating the impedance into regular and chaotic parts. We test the theory by numerically solving for eigenmodes of the Helmholtz equation in a mushroom shaped cavity where there is a clear separation between regular and chaotic regions of phase space. We compare our theoretical predictions with numerical calculations for one-port and two-ports cases with different port positions.   

Testing the Predictions of Random Matrix Theory in Low Loss Wave Chaotic Scattering Systems

Wave chaos is a field where researchers apply random matrix theory (RMT) to predict the statistics of wave properties in complicated wave scattering systems. The RMT predictions have successfully demonstrated universality of the distributions of these wave properties, which only depend on the loss parameter of the system and the physical symmetry. Examination of these predictions in very low loss systems is interesting because extreme limits for the distribution functions and other predictions are encountered. Therefore, we use a wave-chaotic superconducting cavity to establish a low loss environment and test RMT predictions, including the statistics of the scattering (S) matrix and the impedance (Z) matrix, the universality (or lack thereof) of the Z- and S-variance ratios, and the statistics of the proper delay times of the Wigner-Smith time-delay matrix. We have applied an in-situ microwave calibration method (Thru-Reflection-Line method) to calibrate the cryostat system, and we also applied the random coupling model to remove the system-specific features. Our experimental results of different properties agree with the RMT predictions.   

Phase dynamics of coupled oscillators reconstructed from data

We present a technique for invariant reconstruction of the phase dynamics equations for coupled oscillators from data. The invariant description is achieved by means of a transformation of phase estimates (protophases) obtained from general scalar observables to genuine phases. Staring from the bivariate data, we obtain the coupling functions in terms of these phases. We discuss the importance of the protophase-to-phase transformation for characterization of strength and directionality of interaction. To illustrate the technique we analyse the cardio-respiratory interaction on healthy humans. Our invariant approach is confirmed by high similarity of the coupling functions obtained from different observables of the cardiac system. Next, we generalize the technique to cover the case of small networks of coupled periodic units. We use the partial norms of the reconstructed coupling functions to quantify directed coupling between the oscillators. We illustrate the method by different network motifs for three coupled oscillators. We also discuss nonlinear effects in coupling.   

The Universal $\alpha$-Family of Maps

We modified the way in which the Universal Map is obtained in the regular dynamics to derive the Universal $\alpha$-Family of Maps depending on a single parameter $\alpha > 0$ which is the order of the fractional derivative in the nonlinear fractional differential equation describing a system experiencing periodic kicks. We show that many well-known regular maps, like integer n- dimensional (area/volume preserving for $n>1$) quadratic maps (including for $n=1$ the Logistic Map which is not measure preserving) and n-dimensional (volume preserving for $n>2$) standard maps (including the non-measure preserving Circle Map and the area preserving Standard Map), can be considered as particular forms of the Universal $\alpha$-Family of Maps. In the case of the fractional $\alpha$ corresponding maps, which are maps with memory, demonstrate various types of attractors including cascade of bifurcation types trajectories. Maps with memory can be applied for modeling biological systems and circuit elements with memory.   

Synchronization Dynamics of Coupled Anharmonic Plasma Oscillators

The synchronization of two identical mutually driven coupled plasma oscillators modeled by anharmonic oscillators was investigated. Each plasma oscillator was described by a nonlinear differential equation of the form: The model employed the spring-type coupling. Numerical simulations, including Poincare sections, time series analysis, and bifurcation diagram were performed using the fourth-order Runge-Kutta scheme. The numerical value of the threshold coupling Kth was determined to be approximately 0.15.   

Nonlinear Time-Reversal in a Wave Chaotic System

Time reversal mirrors are particularly simple to implement in wave chaotic systems and form the basis for a new class of sensors [1-3]. The sensors make explicit use of time-reversal invariance and spatial reciprocity in a wave chaotic system to sensitively measure the presence of small perturbations to the system. The underlying ray chaos increases the sensitivity to small perturbations throughout the volume explored by the waves. We extend our time-reversal mirror to include a discrete element with a nonlinear dynamical response [4]. The initially injected pulse interacts with the nonlinear element, generating new frequency components originating at the element. By selectively filtering for and applying the time-reversal mirror to the new frequency components, we focus a brief-in-time excitation only onto the nonlinear element, without knowledge of its location. Furthermore, we demonstrate a model which captures the essential features of our time-reversal mirror, modeling the wave-chaotic system as a network of transmission lines arranged as a star graph, with the discrete nonlinearity modeled as a diode terminating a particular line. [1] Appl. Phys. Lett. 95, 114103 (2009) [2] J. Appl. Phys. 108, 114911 (2010) [3] Acta Physica Polonica A 112, 569 (2007) [4] arXiv:1207.1667   

Quantifying Transport in Chaotic Rayleigh-Benard Convection

The transport of a scalar species in a complex flow field is important in many areas of current interest such as the combustion of premixed gases, the dynamics of particles in the atmosphere and oceans, and the reaction of chemicals in a mixture. There has been significant progress in understanding transport in steady periodic flows such as a ring of vortices. In addition, transport in turbulent flow has an extensive literature. Here we focus on the transport of a scalar species in a three-dimensional time-dependent flow field given by the spiral defect chaos state of Rayleigh-Benard convection. We use a highly efficient and parallel spectral element approach to simultaneously evolve the Boussinesq equations and the reaction-advection-diffusion equation in large cylindrical domains with experimentally relevant boundary conditions. We explore the active and passive transport of a scalar species in a chaotic flow field to quantify the transport enhancement for a range of Lewis and Damkholer numbers.   

Quantifying Spatiotemporal Chaos in Rayleigh-Benard Convection: Using Numerics to Connect Theory and Experiment

Spatiotemporal chaos is a common and important feature of spatially-extended systems that are driven far-from-equilibrium. Many open questions remain regarding the high-dimensional chaotic dynamics that describe fluid systems for laboratory conditions. In this talk we explore the spiral defect chaos state of Rayleigh-Benard convection. Recent advances in computing algorithms and available supercomputing resources have made possible the computation of fundamentally important quantities of theoretical importance that are currently inaccessible to experiment. For example, the temporal variation of the spectrum of Lyapunov exponents, the spatial and temporal variation of the Lyapunov vectors, and the variation of the fractal dimension with system parameters. We use large-scale parallel numerical simulations to compute theoretically important diagnostics of spatiotemporal chaos, such as these, with particular interest in connecting these numerical results with experimentally accessible quantities that describe the pattern dynamics.   

Effect of Size Polydispersity on Diffusion Behaviors of Traces in Random Obstacle Matrices

Diffusion behavior on random obstacle matrices has been studied extensively for several decades to explain dynamic behaviors in disordered systems, such as dynamic arrest in colloidal glass phase and anomalous diffusion in crowded biological systems. We present the effect of size polydispersity of the obstacles on diffusion behavior in two-dimensional random obstacle matrices. We generate the random matrices by randomly locating non-overlapping hard disks in two-dimensional space, and consider the diffusion behavior of the tracers. We show that the diffusion behavior is sensitive to the size polydispersity of the obstacles even though their average sizes are the same. In addition, we locate the percolation threshold of void space, and find that diffusion constant D follows scaling relation $D\sim \left( {\varphi_{c} -\varphi } \right)^{\mu -\beta }$ regardless of the size polydispersity, where $\varphi $ and $\varphi_{c} $ is the area fraction of the obstacles and its value at percolation threshold, respectively. The value of the dynamic scaling constant $\mu $ is, however, not universal. We will also discuss briefly non-universal dynamic scaling exponents of two-dimensional random obstacle matrices.   

Colloidal Bandpass and Bandgap Filters

Thermally or deterministically-driven transport of objects through asymmetric potential energy landscapes (ratchet-based motion) is of considerable interest as models for biological transport and as methods for controlling the flow of information, material, and energy. Here, we provide a general framework for implementing a colloidal bandpass filter, in which particles of a specific size range can be selectively transported through a periodic lattice, whereas larger or smaller particles are dynamically trapped in closed-orbits. Our approach is based on quasi-static (adiabatic) transition in a tunable potential energy landscape composed of a multi-frequency magnetic field input signal with the static field of a spatially-periodic magnetization. By tuning the phase shifts between the input signal and the relative forcing coefficients, large-sized particles may experience no local energy barriers, medium-sized particles experience only one local energy barrier, and small-sized particles experience two local energy barriers. The odd symmetry present in this system can be used to nudge the medium-sized particles along an open pathway, whereas the large or small beads remain trapped in a closed-orbit, leading to a bandpass filter, and vice versa for a bandgap filter.   

Session W31: Focus Session: Understanding Fluctuation and Correlation Effects in Polymers

Recent Developments in Field-Theoretic Polymer Simulations

This presentation will address recent progress in methods and algorithms for conducting simulations of statistical field theory models of polymers and complex fluids beyond the mean-field approximation (as invoked, e.g., in self-consistent field theory). Topics to be discussed include regularization methods, improved stochastic integration algorithms for complex Langevin equations, techniques for locating phase boundaries, and systematic coarse-graining/renormalization techniques for multi-scale simulations. Early results on a promising ``coherent state'' formulation of polymer field theory will also be presented.   

Understanding Fluctuation/Correlation Effects on the Order-Disorder Transition of Symmetric Diblock Copolymers with a Density-Functional Theory

How fluctuations change the order-disorder transition (ODT) of symmetric diblock copolymers (DBC) is a classic yet unsolved problem in polymer physics.\footnote{\textit{L. Leibler}, \textbf{Macromolecules, 13}, 1602 (1980); \textit{G. H. Fredrickson and E. Helfand}, \textbf{J. Chem. Phys., 87}, 697 (1987). } Taking a model system of discrete Gaussian chains interacting with soft, finite-range repulsions as commonly used in dissipative-particle dynamics simulations we formulate a density-functional theory (DFT) based on the polymer integral equation theories,\footnote{\textit{D. Chandler and H. C. Andersen,}~\textbf{J. Chem. Phys.},~\textbf{57}, 1930 (1972);~\textit{K. S. Schweizer and J. G. Curro},~\textbf{Phys. Rev. Lett.},~\textbf{58},~246, (1987).} which includes the system fluctuations and correlations neglected by the mean-field theory (i.e., the widely applied self-consistent field theory) and can be reduced to the latter under the mean-spherical approximation. We then unambiguously reveal the fluctuation/correlation effects on the ODT of symmetric DBC by direct comparisons among the mean-field theory, DFT, and fast off-lattice Monte Carlo simulations,\footnote{\textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).} all using exactly the same model system (Hamiltonian) and thus without any parameter-fitting.   

Computational Investigation of Block Copolymer Surfactants for Stabilizing Fluctuation-Induced Polymeric Microemulsions

High molecular weight diblock copolymers introduced into a blend of immiscible homopolymers can act as a surfactant to suppress macroscopic two-fluid phase separation. With variation of block copolymer composition, the crossover between low-temperature ordering into microphase or macrophase separated states is marked by a mean-field isotropic Lifshitz multi-critical point. Strong fluctuations close to the Lifshitz point are observed[1,2] to suppress the low-temperature ordering; a microemulsion state emerges, with large, co-continuous domains of segregated fluid lacking any long-range order. We study this phenomenon with fully fluctuating field-theoretic simulations based on complex Langevin sampling, and we attempt to design new block polymer surfactants that can produce the microemulsion state with a wider composition tolerance. [1] Bates et al., PRL 79, 849 (1997) [2] Hillmyer et al., J Phys Chem B 103, 4814 (1999)   

Condensation of semiflexible polyelectrolytes in mixed solutions of mono- and multivalent salts

The salt-dependent condensation of highly charged polyelectrolytes in aqueous solution is a topic of great biological and industrial importance that has been widely studied over the past decades. It is well established that interaction with multivalent counterions leads to the formation of bundle-like aggregates for rigid polyelectrolytes and to collapsed structures or disordered aggregates for flexible polyelectrolytes. Here, we investigate the behavior of semiflexible chain molecules, where the electrostatically induced aggregation is impeded by the intrinsic bending stiffness of the polymer. Moreover, we study the competition between monovalent and multivalent counterions in mixed solutions and establish the threshold salt concentration required for condensation. Our findings are relevant for a range of biomedical problems, including the fabrication of nanoparticles for gene delivery [1] and the packaging of DNA by histones. \\[4pt] [1] X. Jiang \emph{et al.}, Adv. Mater., DOI: 10.1002/adma.201202932.   

Rattle, restrict, and release in entangled polymer solutions

The nature of entanglement release and chain fluctuation is studied in entangled solutions of high molecular weight PEG in water. Reporter fluorescent polystyrene particles of radius comparable to the entanglement length are suspended in solution and tracked individually with with nm resolution using epifluorescence microscopy. Thousands of single particle trajectories are analyzed to quantify caging and hopping dynamics. The cage relaxation time changes by orders of magnitude depending on the polymer concentration, but is faster than and therefore more accessible within experimentally accessible time scales, than for colloidal glasses.   

Direct imaging of fluctuations in a cross-linked biopolymer network

Cross-linked networks are ubiquitous in synthetic and biological polymer systems, such as rubbers and cytoskeletons. To model cross-linked networks, several theories have been developed on the basis of different assumptions as to fluctuations in the networks. Here we put these theories to direct test. This talk will describe direct single-molecule imaging of the dynamic fluctuations of junction points in a cross-linked semiflexible polymer (F-actin) network. The actin filaments are cross linked by biotin/avidin. The junction points are selectively labeled to allow nm spatial imaging resolution. The surprising results point to limitations of the prevailing network models.   

Effect of Fluctuation on Order-Disorder Transition in Polydisperse Block Copolymer Melts

We examine fluctuation effects on order-disorder transition (ODT) temperature in polydisperse block copolymer melts using single chain in mean field simulations. Diblock copolymer melts having monodisperse A blocks and polydisperse B blocks with symmetric composition on an average are examined. Increase in polydispersity at constant composition resulted in change in equilibrium morphology in accordance with the mean-field theory prediction. The dependence of ODT temperature on the strength of fluctuations as characterized by Ginzburg parameter is examined and scaling prediction for fluctuation induced shift in ODT is reported. Also, the qualitative shift in ODT as a function of increasing polydispersity in asymmetric copolymers is investigated.   

Directed polymer liquids addressed via the two-dimensional one-component plasma: Developing the framework

The distribution of \emph{small} density fluctuations in a directed polymer liquid is characterized by the equilibrium structure factor. By contrast, the distribution of \emph{large} density fluctuations embodies new information about the polymer state. Physically, large density fluctuations are closely related to particle inclusions, i.e., compact regions from which polymers are excluded. The highly correlated nature of directed polymer liquids complicates a single-chain approach to such issues and, instead, we invoke a quantum many-body technique to map the three-dimensional polymer system to a two-dimensional hard-core Bose fluid. Then, by using Chern-Simons field theory, we make the standard transformation of this Bose fluid into a system of non-interacting fermions that fill a single Landau level. The density distribution of these fermions is that of a classical two-dimensional one-component plasma (2DOCP), whose properties are well understood; we invoke them to obtain the entropy cost of particle inclusions in the polymer liquid. Along the way, we examine the validity of the various approximations that have been made.   

Directed polymer liquids addressed via the two-dimensional one-component plasma: Implications for the density profile

We consider the inclusion of one or more particles into a dense, three-dimensional liquid of long, directed polymers. The particles represent an excluded volume within the liquid which raises its free energy. As discussed in the accompanying talk, the statistical mechanics of such a polymer liquid can be described in terms of certain two-dimensional fluids of quantum particles and, hence, via an exactly solvable classical two-dimensional one-component plasma (2DOCP). The free energy cost of a particle inclusion is related to the probability of spontaneous formation of a large void within the quantum fluid or the plasma. We use these relationships to study the effect of particle inclusions in the polymer liquid, as well as large fluctuations of the liquid. We find that displaced polymers accumulate near the edge of the inclusion, in a manner similar to the accumulation of excess charge near the surface of a conductor. In addition, we are able to determine the equilibrium density profile for polymer liquids subject to more general constraints, e.g., ones that force some fixed number of polymers to pass through a ring.   

Disentangle Model Differences and Fluctuation Effects in DPD Simulations of Diblock Copolymers

In the widely used dissipative particle dynamics (DPD) simulations [Hoogerbrugge and Koelman, \textbf{Europhys. Lett. 19}, 155 (1992); Groot and Warren, \textbf{J. Chem. Phys. 107}, 4423 (1997)], polymers are commonly modeled as discrete Gaussian chains interacting with soft, finite-range repulsions. In the original DPD simulations of microphase separation of diblock copolymer melts by Groot and Madden [\textbf{J. Chem. Phys. 108}, 8713 (1998)], the simulation results were compared and found to be consistent with the phase diagram for the ``standard model'' of continuous Gaussian chains with Dirac ?-function interactions obtained from self-consistent field (SCF) calculations. Since SCF theory is a mean-field theory neglecting system fluctuations/correlations while DPD simulations fully incorporate such effects, the model differences are mixed with the fluctuation/correlation effects in their comparison. Here we report the SCF phase diagram for exactly the same model system as used in DPD simulations. Comparing our phase diagram with that for the standard model highlights the effects of chain discretization and finite-range interactions, while comparing our phase diagram with DPD simulation results reveal without any parameter-fitting the effects of fluctuations/correlations neglected in the SCF theory.   

Static Correlation Functions of Polymer Concentration Fluctuations in the Presence of an Interface

We study static correlation functions of polymer solutions using the Cahn-de Gennes square gradient theory of interfacial energies. Fluctuations are considered for good, theta, and poor solvents at repulsive and adsorbing surfaces as well as at the interface of phase separated solutions. We predict the existence of bound state fluctuations associated with an interface under certain conditions.   

Dynamical simulation of disordered micelles in a diblock copolymer melt with fluctuations

By including composition fluctuations in our dynamical simulation of the time-dependent Landau-Brazovskii model for a diblock copolymer melt, we find that disordered micelles form above the order-disorder transition to a BCC phase. At high-temperatures, the micelle number density is effectively zero, and the melt is disordered at the molecular level. As we lower the temperature, the micelle number density increases gradually and approaches the number density in the BCC phase. If we increase the strength of the fluctuations, the temperature range over which disordered micelles exist broadens, and the onset of BCC order is suppressed. We examine the dynamics of crystallization of disordered micelles into the BCC phase. By tracking trajectories, we also investigate the dynamical behaviour of individual micelles in an environment of disordered micelles.   

X-ray imaging of wetting ridge on a soft solid

Softness of solids affects a microscopic deformation, called a `wetting ridge', at a three-phase contact line. We present a direct visualization of wetting ridges by high-resolution x-ray imaging, which shows a spatial transition between elastic and fluidic wetting behaviors on a soft solid. The fluidic behavior that corresponds to Neumann's triangle occurs at the vicinity of the triple point while the elastic deformation at \textbar x\textbar \textless le (the elasto-capillary length). Real-time x-ray imaging clearly shows temporal variation of wetting ridge.   

Session W32: Focus Session: Micro/Nanofluidics I

Uncovering stem-cell heterogeneity in the microniche with label-free microfluidics

Better suited for large number of cells from bulk tissue, traditional cell-screening techniques, such as fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS), cannot easily screen stem or progenitor cells from minute populations found in their physiological niches. Furthermore, they rely upon irreversible antibody binding, potentially altering cell properties, including gene expression and regenerative capacity. We have developed a label-free, single-cell analysis microfluidic platform capable of quantifying cell-surface marker expression of functional organ stem cells directly isolated from their micro-anatomical niche. With this platform, we have screened single quiescent muscle stem (satellite) cells derived from single myofibers, and we have uncovered an important heterogeneity in the surface-marker expression of these cells. By sorting the screened cells with our microfluidic device, we have determined what this heterogeneity means in terms of muscle stem-cell functionality. For instance, we show that the levels of beta1-integrin can predict the differentiation capacity of quiescent satellite cells, and in contrast to recent literature, that some CXCR4$+$ cells are not myogenic. Our results provide the first direct demonstration of a microniche-specific variation in gene expression in stem cells of the same lineage. Overall, our label-free, single-cell analysis and cell-sorting platform could be extended to other systems involving rare-cell subsets.   

Designing artificial phagocyte that selectively ``ingests'' solutes

We use dissipative particle dynamics to design an active composite vesicle that can controllably and selectively ``ingest'' solutes from the surrounding fluid. The vesicle consists of a lipid membrane that envelops a stimuli-responsive microgel particle. When the microgel swells and increases in size due to an external stimulus, the lipid membrane breaks forming pores that expose a part of the microgel to the external solvent. Solutes initially dispersed in the solvent diffuse and bind to the uncovered surface of microgel particles. After the stimulus is removed and microgel deswells to its original size, the transmembrane pores close isolating the adsorbed solutes inside the vesicle. In our simulations, we formulate the criteria for the controlled pore opening and closing, and probe how this smart vesicle can be harnessed to ``ingest'' specific macromolecules. Our results will be useful for developing a new class of artificial phagocytes for targeted sampling in various biomedical applications.   

Transient Flow Induced by the Adsorption of Particles

When small particles, e.g., glass, flour, pollen, etc., come in contact with a fluid-liquid interface they disperse so quickly to form a monolayer on the interface that it appears explosive, especially on the surface of mobile liquids like water. This is a consequence of the fact that the adsorption of a particle in an interface causes a lateral flow which on the interface away from the particle. In this study we use the particle image velocimetry (PIV) technique to measure the transient three-dimensional flow that arises due to the adsorption of spherical particles. The PIV measurements show that the flow develops a fraction of a second after the adsorption of the particle and persists for several seconds. The fluid below the particle rises upwards and on the surface moves away from the particle. These latter PIV results are consistent with the surface velocity measurements performed in earlier studies. The strength of the induced flow, and the time duration for which the flow persists, both decrease with decreasing particle size. For a spherical particle the flow is axisymmetric about the vertical line passing through the center of the particle.   

Direct measurement of friction of a fluctuating contact line

What happens at a moving contact line, where one fluid displaces another (immiscible) fluid over a solid surface, is a fundamental issue in fluid dynamics. In this presentation, we report a direct measurement of the friction coefficient in the immediate vicinity of a fluctuating contact line using a micron-sized vertical glass fiber with one end glued to an atomic force microscope (AFM) cantilever beam and the other end touching a liquid-air interface. By measuring the broadening of the resonance peak of the cantilever system with varying liquid viscosity $\eta$, we obtain the friction coefficient $\xi_c$ associated with the contact line fluctuations on the glass fiber of diameter $d$ and find it has the universal form, $\xi_c= 0.8\pi d\eta$, independent of the contact angle. The result is further confirmed by using a soap film system whose bulk effect is negligibly small. This is the first time that the friction coefficient of a fluctuating contact line is measured. *Work supported by the Research Grants Council of Hong Kong SAR.   

Giant slip at liquid-liquid interfaces using a hydrophobic ball bearing

We suggest to build an interface where hydrophobic beads maintain a gas layer between two liquids. We show that this interface behaves as a liquid-liquid ball bearing under shear and exhibits giant slip. Such a metastable configuration reminds of pillar-based superhydrophobic surfaces, used to amplify liquid-solid slip. To the advantage of hydrophobic ball bearings, beads are able to roll, thereby reducing friction at the liquid-bead interface. However beads will always penetrate inside the liquid, inducing viscous dissipation and consequently decreasing slippage. The penetration depth being directly controlled by the wetting angle of the liquid at the bead surface, the latter is expected to have a strong influence on the efficiency of the liquid/liquid bearing. We start by quantifying analytically the influence of the wetting angle on liquid/liquid slip in this system. We then confirm the obtained scaling law by means of Molecular Dynamics (MD) simulations. Liquid-liquid bearings open new pathways for micro and nanofluidics. One major direction could be to build fluidic channels without walls, where different liquids in contact could flow independently while maintaining an extremely low interfacial friction, and preventing mixing by diffusion between the different channels.   

A Study of the Concentration Dependent Water Diffusivity in Polymer using Magnetic Resonance Imaging

Hydrogel allows solvent molecules to migrate in and out of the polymer network, often in response to various environmental stimuli such as temperature and pH, resulting in significant volumetric change. Kinetics of penetrants in polymeric network determines time dependent behavior of hydrogel. However, swelling deformation resulting from the solvent uptake in turn significantly changes diffusivity of solvent, and this strong coupling makes it challenging to study dynamic behavior of hydrogels. Here we study concentration dependent diffusivity of water in poly(ethylene glycol) diacrylate (PEGDA) hydrogel using magnetic resonance imaging (MRI). Projection micro-stereolithography is used to fabricate gel samples in which a gradient of water volume fraction occurs. In situ measurement using MRI provides quantitative relationship between diffusivity and volume fraction of water in the gel. This result will help better understand interstitial diffusion behavior of solvent in polymers, which has great implication in board areas such as soft matter mechanics, drug delivery, and tissue engineering.   

Closing the loop in the boundary layer: water slippage, interfacial viscosity and wettability

Understanding and manipulating fluids at the nanoscale is a matter of growing scientific and technological interest. Here, we present experiments showing that the interfacial viscosity of water depends drastically on the wetting properties of the confining surfaces. By using an atomic force microscope (AFM), we have measured the lateral viscous force experienced in water by a nano-size AFM tip while it is sheared in parallel to a smooth solid surface, as a function of the tip-surface distance. The viscous force curves, FL(d), have been measured for five surfaces with various wettabilities. In particular, the experiments indicate that in water lower forces are required to shear a tip very close to a slippery non-wetting surface, yielding to a lower effective viscosity. A modified form of the Newtonian definition of viscosity, which includes slippage, is used to successfully predict the measured shear forces in the boundary layer as a function of surface wettability, and slippage. We prove that this effect is general and can be applied in different contexts such as in explaining the relationship between dissipation and surface wettability for a nano-tip vibrating in proximity of a surface in water.   

Linear Behavior of the Kapitza Jump at Liquid/Solid Interfaces

At macroscale dimensions, it is normally assumed that two distinct materials maintain equal temperature across the surface of contact. Even in the presence of a thermal flux across the interface, the contacting boundary is assumed to maintain thermal equilibrium so long as the interfacial resistance is negligible in comparison to that of the bulk. This has long been assumed an excellent approximation for liquid/solid (L/S) interfaces since liquids conform in shape even to roughened surfaces. Recent molecular dynamics simulations of nanoscale films, however, have revealed the existence of intrinsic temperature jumps at L/S interfaces. While previous studies have shown how stronger interaction potentials between the liquid and solid will diminish temperature jumps, they cannot be altogether eliminated due to commensurability mismatch. Here we show how the magnitude of the thermal jump also is controlled by the applied thermal flux. This finding suggests that temperature jumps across an L/S interface are not simply a local effect due to density mismatch. These jumps are also controlled by the actual rate of heat transfer, underscoring the importance of thermal resistance effects in nanoscale hydrodynamic systems. We thank P. Thompson for assistance in optimizing the simulations.   

Near-wall Brownian motion of anisotropic particles

Anisotropic microscopic objects are ubiquitous such as biological cells, filamentous macromolecules, as well as synthesized nanomaterial. Near interfaces, the thermal motion of these objects is strongly constrained due to the hydrodynamic interactions, impacting the overall behavior of the biophysical and colloidal systems. Thus, understanding this wall-effect is a key to describe many surface-related problems. Unlike the well-studied case of spheres, however, both its experimental and theoretical studies have been elusive due to the intrinsic complexity of the system. Here we present a comprehensive experimental and computational study of the Brownian motion of silicon nanowires tethered on a substrate. A uniquely developed interference method enables the direct visualization of its microscopic rotations in three dimensions with high angular and temporal resolutions. The quantitative measurement at short time scales revealed the anisotropic reduction in their rotational diffusivities as a function of the inclined angles, resulting in the decrease more than 40-80 {\%} at long time scales. We then developed a numerical model from a string-of-beads idealization, which implicitly simulates the complex hydrodynamic interaction and showed excellent agreement with the experimental observations. Our study provides insights into the fundamental diffusive processes, useful for understanding the anisotropic behavior of anisotropic macromolecules near interfaces. The demonstrated methods offers a systematic approach for studying the interfacial rheology of various anisotropic objects.   

Enhancing microscale particle deposition using actuated synthetic cilia

We use three dimensional simulations to examine deposition of diffusive nanoscopic particles suspended in a viscous fluid onto the walls of a microchannel containing an array of actuated synthetic cilia. We model the cilia as elastic filaments attached to the channel walls and actuated by an external periodic force. We use a lattice Boltzmann model coupled with a lattice spring model to simulate the system and investigate the effects of the oscillating cilia on the rate of particle deposition. We consider the effects of variation of cilia properties and spacing, as well as the frequency and amplitude of the applied force on the deposition of particles with different diffusivity. Our findings are useful in understanding how active microscopic structures can be harnessed to design microfluidic devices and surfaces with controllable transport properties.   

Statics and dynamics of polymer droplets on topographically structured substrates

Using Molecular Dynamics simulations of a polymer liquid flowing past flat and patterned surfaces, we investigate the influence of corrugation, wettability and pressure on slippage and friction at the solid-liquid interface. We devise a computational method to compute the interface potential that does not rely on grandcanonical simulation techniques and quantitatively compare droplet profiles obtained in simulations with the predictions of a thin-film equation using the independently determined interface potential. For substrates structured by one-dimensional, rectangular grooves, we observe a gradual crossover between the Wenzel state, where the liquid fills the grooves, and the Cassie state, where the corrugation supports the liquid and the grooves are filled with vapor. Using two independent flow set-ups, we characterize the near-surface flow by the slip length and the position, at which viscous and frictional stresses are balanced according to Navier's partial slip boundary condition. This hydrodynamic boundary position depends on the pressure inside the channel and may be located above the corrugated surface. In the Cassie state, we observe that the edges of the corrugation contribute to the friction.   

The Effect of Polarization on Structure, Dynamics and Electric Double Layer for Interfacial Water near Charged Graphene

A solid surface perturbs water for up to 10-20 {\AA}. Quantifying the structural and dynamics properties of water within this interfacial layer remains crucial for a number of applications, including lab-on-chip and micro- and nano-fluidic devices, and also for designing efficient electric double layers capacitor. As graphene is finding wide applications in the energy sector (batteries and capacitors) we revisited the graphene-water interface. Because at the air-water interface it is known that accounting for the polarization of water and ions is required to properly describe the ions distribution, we conducted a parametric study in which we varied the polarization of carbon atoms on charged graphene. The polarization is described implementing a classic Drude oscillator, which is consistent with the model implemented to describe water and ions. External electric fields are represented by uniform charge distributions on the carbon atoms. The results are quantified in terms of structure and dynamics of interfacial water, as well as of structure of the electric double layer. Comparison with accurate experimental observations is provided.   

Session W33: Focus Session: Organic Electronics and Photonics - Organic Photovoltaics II - Efficiency, Stability, and Interfaces

Organic Solar Cell Efficiency Limitations and Pathways to Overcoming Them

Organic solar cell device efficiencies are often limited by a reduced external quantum efficiency, particularly for low band gap materials used in either single- or double-junction devices. This can be attributed to loss mechanisms occurring either at the device-physics scale, in the form of carrier recombination, or at the molecular donor-acceptor scale, in the form of incomplete photo-carrier generation and/or geminate recombination. Here mechanisms at both scales are addressed, utilizing drift-diffusion models of device operation and kinetic models of photo-carrier production. At the device-physics level, the negative impact of dark carriers, commonly derived from defect states in the organic semiconductor, is demonstrated. For dark carrier densities above a typical threshold of $10^{16}$ cm$^{-3}$, depletion at one of the electrodes leads to a field-free region of the device and substantial carrier recombination. At the molecular level, the fundamental impact of the molecular reorganization energy $\lambda$ on device efficiency is considered through use of a Marcus Theory-based kinetic model. It is shown that substantial gains in efficiency, to values approaching 20\%, are possible for hypothetical materials in which $\lambda$ has been reduced to approximately 0.3 eV. Finally, measurements of molecular alignment at interfaces are presented, and implications on the above two mechanisms are explored.   

Tailored exciton diffusion in organic photovoltaic cells for enhanced power conversion efficiency

Organic photovoltaic cells (OPVs) have the potential to become a low-cost source of renewable energy due to their compatibility with high throughput processing techniques and the demonstration of power conversion efficiencies exceeding 10{\%}. In the simplest planar heterojunction OPVs, photoconversion is limited by a short exciton diffusion length (L$_{\mathrm{D}})$ that restricts migration to the dissociating electron donor-acceptor (D-A) interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternate approach that seeks to directly engineer L$_{\mathrm{D}}$ by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride (SubPc) into a wide energy gap host material, we optimize the degree of interaction between donor molecules and observe a nearly 50{\%} increase in L$_{\mathrm{D}}$. Using this approach, we construct planar heterojunction OPVs with a power conversion efficiency of 4.4{\%}, \textgreater 30{\%} larger than the case of optimized devices containing an undiluted donor layer. It is worth noting that this efficiency also rivals those realized in optimized, bulk heterojunction OPVs based on SubPc and C$_{\mathrm{60}}$. The underlying correlation between L$_{\mathrm{D}}$ and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.   

\textgreater 1.0{\%} solar cell derived from carbon nanotube excitons

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising photoabsorbers for photovoltaics due to their strong optical absorptivity, tunable NIR bandgaps, fast charge transport, and solution processability.~We have previously shown that electrons can be extracted from photogenerated excitons in s-SWCNTS by C$_{60}$ with internal quantum efficiency (QE) over 90{\%}. Here, we demonstrate s-SWCNT/C$_{60}$~heterojunction devices with over 1.0{\%} AM1.5G power conversion efficiency for the first time. We implemented highly monochiral (7,5) s-SWCNTs to optimize exciton diffusivity and tailored the device stack to tune the spectral response. External QE of over 35{\%} and 20{\%} are achieved at the~$E_{11}$~bandgap of the s-SWCNTs at 1055 nm and the~$E_{22}$~transition at 655 nm. More than 50{\%} of the AM1.5G photoresponse is derived from the s-SWCNTs, substantially exceeding previous s-SWCNT hybrid devices in which the photoresponse has mostly originated from the organic phase. This work will lead to solar cells based on s-SWCNT photoabsorbers with higher responsivity across the solar spectrum by tailoring the s-SWCNT film morphology and blending them directly with acceptors.   

Towards high performance inverted polymer solar cells

Bulk heterojunction polymer solar cells that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based electronics. Despite the envisioned advantages and recent technology advances, so far the performance of polymer solar cells is still inferior to inorganic counterparts in terms of the efficiency and stability. There are many factors limiting the performance of polymer solar cells. Among them, the optical and electronic properties of materials in the active layer, device architecture and elimination of PEDOT:PSS are the most determining factors in the overall performance of polymer solar cells. In this presentation, I will present how we approach high performance of polymer solar cells. For example, by developing novel materials, fabrication polymer photovoltaic cells with an inverted device structure and elimination of PEDOT:PSS, we were able to observe over 8.4{\%} power conversion efficiency from inverted polymer solar cells.   

The Science of Making Organic Solar Cells Stable

As organic PV efficiencies exceed 10{\%}, the science of stabilization and lifetime gains importance. We seek the origin of the exponential decrease, or ``burn-in,'' of OPV device efficiency in the first 200 hours of operation. First, we examine an efficient polymer, PCDTBT, and demonstrate a 6.2 year lifetime. For standard PCDTBT devices, burn-in is not caused by reactions at the transport layers; rather, it is caused by photochemical traps. We hypothesize that impurities could play a role. The effect of impurities is investigated in another polymer, PBDTTPD, with an 8.3{\%} PCE. For PBDTTPD we find that degradation correlates to the presence of small, organic impurities. We stabilize PBDTTPD, without diminishing performance, by purifying it further. We also investigate the fullerene's role in degradation using photobleaching experiments, and find that photoactive layer stability correlates with the fullerene's electron affinity. From our conclusions, we outline strategies for improving OPV device stability.   

Influence of MoO$_{\mathrm{x}}$ interlayer on the maximum achievable open-circuit voltage in organic photovoltaic cells

Transition metal oxides including molybdenum oxide (MoO$_{\mathrm{x}})$ are characterized by large work functions and deep energy levels relative to the organic semiconductors used in photovoltaic cells (OPVs). These materials have been used in OPVs as interlayers between the indium-tin-oxide anode and the active layers to increase the open-circuit voltage (V$_{\mathrm{OC}})$ and power conversion efficiency. We examine the role of MoO$_{\mathrm{x}}$ in determining the maximum achievable V$_{\mathrm{OC}}$ in planar heterojunction OPVs based on the donor-acceptor pairing of boron subphthalocyanine chloride (SubPc) and C$_{\mathrm{60}}$. While causing minor changes in V$_{\mathrm{OC}}$ at room temperature, the inclusion of MoO$_{\mathrm{x}}$ significantly changes the temperature dependence of V$_{\mathrm{OC}}$. Devices containing no interlayer show a maximum V$_{\mathrm{OC\thinspace }}$of 1.2 V, while devices containing MoO$_{\mathrm{x}}$ show no saturation in V$_{\mathrm{OC}}$, reaching a value of \textgreater 1.4 V at 110 K. We propose that the MoO$_{\mathrm{x}}$-SubPc interface forms a dissociating Schottky junction that provides an additional contribution to V$_{\mathrm{OC}}$ at low temperature. Separate measurements of photoluminescence confirm that excitons in SubPc can be quenched by MoO$_{\mathrm{x}}$. Charge transfer at this interface is by hole extraction from SubPc to MoO$_{\mathrm{x}}$, and this mechanism favors donors with a deep highest occupied molecular orbital (HOMO) energy level. Consistent with this expectation, the temperature dependence of V$_{\mathrm{OC}}$ for devices constructed using a donor with a shallower HOMO level, e.g. copper phthalocyanine, is independent of the presence of MoO$_{\mathrm{x}}$.   

The effect of interfaces on charge transport and recombination in polymeric solar cells

Charge-carrier transport and recombination in hybrid TiO2/P3HT:PCBM bulk-heterojunction solar cells (BHSCs) have been measured using photo-CELIV. We have fabricated hybrid devices in the form of indium tin oxide/titanium dioxide/P3HT:PCBM/Cu) to clarify the impact of the TiO$_{2}$/P3HT:PCBM interface on the charge transport using the charge extraction by linearly increasing voltage (CELIV) technique. We found that a large equilibrium charge reservoir is accumulated at negative offsets at the TiO$_{2}$/P3HT:PCBM interface leading to space charge limited extraction current (SCLC) transients. We show analytically the SCLC transient response and compare the experimental data to calculated SCLC in a linearly increasing voltage. The theoretical calculations indicate that the large charge reservoir at negative offset voltages is due to thermally generated charges combined with poor hole extraction at the ITO/TiO$_{2}$ contact, due to the hole blocking character of TiO2. In this presentation we will discuss how interfaces, both metal-organic but also organic-organic interfaces affect charge carrier transport and recombination measurements.   

Effect of interfacial modification of organophosphonate-based self-assembled monolayers on the performance of inverted hybrid ZnO:P3HT photovoltaic devices

Hybrid organic-inorganic photovoltaics have not lived up to their promise because of our poor handle of the exciton dissociation interface. Interfacial modification based on self-assembled monolayer (SAM) adsorption provides a way of improving device performance. Here, we provide the first examples of a stepwise functionalization methodology that allows binding of phosphonic acid derivatives to ZnO nanowire arrays with minimal surface degradation and etching. We examined different adsorption methods; SAM adsorption via tethering-by-aggregation-and-growth (T-BAG) yields the most robust surface-bound monolayers. Poly(3-hexylthiophene), P3HT, infiltrated in surface modified ZnO nanowire arrays yielded functional hybrid solar cells with power conversion efficiencies as high as 2.1{\%} due to improvements in both the short-circuit current density (Jsc) and the open-circuit voltage (Voc). The increase in Jsc can be attributed to enhanced charge transfer with surface passivation of ZnO, while the increase in Voc is attributed to the interfacial dipole introduced and improved P3HT wettability on ZnO with SAM adsorption.   

Anomalous charge storage exponents of organic bulk heterojunction solar cells.

Organic bulk heterojunction (BHJ) devices are increasingly being researched for low cost solar energy conversion. The efficiency of such solar cells is dictated by various recombination processes involved. While it is well known that the ideality factor and hence the charge storage exponents of conventional PN junction diodes are influenced by the recombination processes, the same aspects are not so well understood for organic solar cells. While dark currents of such devices typically show an ideality factor of 1 (after correcting for shunt resistance effects, if any), surprisingly, a wide range of charge storage exponents for such devices are reported in literature alluding to apparent concentration dependence for bi-molecular recombination rates. In this manuscript we critically analyze the role of bi-molecular recombination processes on charge storage exponents of organic solar cells. Our results indicate that the charge storage exponents are fundamentally influenced by the electrostatics and recombination processes and can be correlated to the dark current ideality factors. We believe that our findings are novel, and advance the state-of the art understanding on various recombination processes that dictate the performance limits of organic solar cells.   

Physical Processes in Organic Photovoltaic Devices Tuned by Ionic Double Layer Doping

We have recently found that Organic Photovoltaic (OPV) performance can be improved by creating p-i-n structures via doping by double layer charging. We have designed a hybrid device; an OPV attached to a supercapacitor via a common transparent carbon nanotube (CNT) electrode. We've demonstrated that photoexcitation of this hybrid results in double layer capacitive doping of the upper organic layers in the OPV and the CNT electrode. This device can also be viewed as an electrochemically gated CNT/OPV which is ionically reconfigurable either upon photoexcitation or upon application of a voltage bias to the gate electrode. We have demonstrated a two fold increase in the short circuit current and filling factor of our initial test device; an inverted P3HT:PCBM bulk heterojunction cell attached to an electrochemical microcell with a CNT anode laminated on top of the OPV functioning as a common anode. The physical processes in this ionically tuned OPV are discussed in terms of better ohmic contact with CNT electrode and formation of p-i junction in P3HT chains which contribute to better separation of photogenerated carriers and their improved collection. Optical studies of the bleaching effects in both in CNT and in P3HT independently confirm the DLC ionic doping.   

Correlation between magneto-photocurrent and power conversion efficiency in organic solar cells

In order to investigate the effect of spin 1/2 radical on the photocurrent in organic solar cells, we studied magneto-photocurrent (MPC) and power conversion efficiency (PCE) of ``standard'' P3HT:PCBM (1.2:1) device at various Galvinoxyl radical wt{\%}. The MPC measurements were performed to understand the increase in $J$sc and hence PCE of the OPV device with Galvinoxyl wt{\%}. We found that the MPC reduction with Galvinoxyl wt{\%} follows the same trend as that of the PCE enhancement. We propose that MPC in OPV blends is due to spin-mixing mechanism related with the manifold of the charge transfer (CT) state at the donor-acceptor interfaces. Our results thus demonstrate that the Galvinoxyl spin 1/2 radical additives act as spin flip initiator within this exciton manifold. This process is unraveled via MPC of the doped devices. Supported by the NSF-MRSEC program at the UoU.   

Session W34: Thin Films, Surfaces and Interfaces II

Photo-crosslinkable polymers for fabrication of photonic multilayer sensors

We have used photo-crosslinkable polymers to fabricate photonic multilayer sensors. Benzophenone is utilized as a covalently incorporated pendent photo-crosslinker, providing a convenient means of fabricating multilayer films by sequential spin-coating and crosslinking processes. Colorimetric temperature sensors were designed from thermally-responsive, low-refractive index poly($N$-isopropylacrylamide) (PNIPAM) and high-refractive index poly(para-methyl styrene) (P$p$MS). Copolymer chemistries and layer thicknesses were selected to provide robust multilayer sensors which show color changes across nearly the full visible spectrum due to changes in temperature of the hydrated film stack. We have characterized the uniformity and interfacial broadening within the multilayers, the kinetics of swelling and de-swelling, and the reversibility over multiple hydration/dehydration cycles. We also describe how the approach can be extended to alternative sensor designs through the ability to tailor each layer independently, as well as to additional stimuli by selecting alternative copolymer chemistries.   

Diffusion of single molecules on surface tethered polymer brushes

The interaction between polymer molecules and brush surfaces in aqueous media is a multi-dimensional problem; the polymer competes with the solvent for surface sites, and the resultant molecular conformation controls its diffusion properties. The diffusion coefficients of single fluorescence-labeled poly(ethylene glycol) (PEG) molecules on surface-immobilized PEG brushes are measured by fluorescence correlation spectroscopy, and are shown to slow down by nearly 10 times when grafting density increased from 0.11 to 0.42 chain per nm\textasciicircum 2. This diffusion dynamics can be explained by Stokes-Einstein treatment of the surface-adsorbed polymer. Subsequently, we prepared a series of surface-grown poly(oligo(ethylene glycol) methacrylate) (POEGMA) brushes with varying grafting density. Diffusion coefficients of three types of fluorescence-labeled polymer (PEG, POEGMA, PGMA) on the POEGMA brushes were quantitatively measured. It was found that diffusion coefficient of PEG changed substantially over those POEGMA samples, with POEGMA to a small degree, and PGMA not affected. The data indicates that not only grafting density of polymer brushes, but also intermolecular interaction could affect the transport of macromolecules on polymer brushes.   

Molecular Dynamics Simulations of Tension Amplification in Tethered Bottle-brushes

Bottle-brush polymers are grafted comb polymers in which the density of side chains grafted to the polymer backbone is large enough that steric repulsions between the side chains force the backbone to stretch and preclude it from forming random-coil configurations. Tethering one end of the bottle-brush backbone to a solid substrate restricts the conformations of the side chains near the surface and leads to side-chain repulsion that induces significant amplification of the tension along the polymer backbone. Depending on the grafting density on the substrate, the density of side chains and the length of the side chains, this tension amplification may be large enough break bonds along the bottle-brush backbone, especially at the site of the link with the substrate, where the tension is maximized. We have performed coarse-grained molecular dynamics simulations to understand the interplay between the factors affecting backbone tension amplification and whether the amplification effects can be controlled in such a way as to predict, for a given architecture and surface coverage, the maximum allowable packing of chains on the substrate surface prior to tethering failure.   

Fast Lattice Monte Carlo Simulations of Grafted Homopolymers under Compression

Fast lattice Monte Carlo (FLMC) simulations [Q. Wang, Soft Matter 5, 4564 (2009); 6, 6206 (2010)] with multiple occupancy of lattice sites and Kronecker $\delta$-function interactions give orders of magnitude faster/better sampling of configuration space for many-chain systems than conventional lattice MC simulations with the self- and mutual- avoiding walk and nearest-neighbor interactions. Using FLMC simulations with the novel Wang-Landau-Optimized-Ensemble sampling, we have studied homopolymers end-grafted on an impenetrable and flat substrate under the compression by another impenetrable and flat surface. Comparing various quantities (including chain dimensions, internal energy, Helmholtz free energy, and pressure) obtained from FLMC simulations with predictions from the corresponding lattice self-consistent field (LSCF) calculations, both using the same model system (Hamiltonian) and thus without any parameter-fitting, we unambiguously quantify the effects of system fluctuations and correlations neglected in LSCF theory. In particular, we find LSCF theory underestimates the pressure for compression of mushrooms in the athermal and $\theta$-solvents and for compression of brushes in the $\theta$-solvent, but overestimates it for compression of brushes in the athermal solvent.   

Low voltage switching of crease patterns on gel surfaces with topographically patterned microelectrodes

Exercising precise control over surface instability patterns on soft hydrogels is of significant interest for applications in biological and biomedical contexts. Here, we show that patterns of surface creases can be successfully programmed on thin hydrogel layers by applying a direct current electric voltage to underlying micro-patterned electrodes. We characterize the dependence of the critical switching voltage on the swelling of the gel layer and the geometry of the electrode array, as well as the depth of creases as a function of applied voltage and the switching kinetics. We also show that introducing topographically structured electrodes lowers the critical voltage slightly, and provides better control over crease shape. To better understand the mechanism of electrically-triggered creasing, we have developed an \textit{in situ} strain mapping technique based on bleaching of markers within the gel layer.   

Tunable Surface Properties from Bioinspired Comb Copolymers

A modular polymer system which incorporates multiple functionalities simultaneously while keeping an identical backbone chemistry is a useful tool in determining necessary functionalities for marine antifouling properties. We have investigated the surface properties and antifouling behavior of polypeptoids, a class of non-natural biomimetic polymers based on an N-substituted glycine backbone, that combine many of the advantageous properties of bulk polymers with those of synthetically produced proteins, including controllable chain shape, sequence, and self-assembled structure. Using thiol-ene click chemistry, thiol functionalized amphiphilic peptoid sequences consisting of hydrophilic methoxyethyl and hydrophobic heptafluorobutyl side chains were attached to polystyrene-block-poly(ethylene oxide-stat-allyl glycidyl ether), creating comb-shaped molecules. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) was used to study the surface characteristics as a function of peptoid length and composition. Only 20{\%} of fluorinated groups in the peptoid were sufficient for promoting surface display of the otherwise hydrophilic PEO/peptoid comb block. Antifouling experiments with spores of the green algae Ulva indicated an influence of sequence.   

Microwave-Assisted Surface-Initiated Free Radical Polymerization

We investigate microwave ($\mu $w) irradiation as a heat source for surface-initiated (SI) free-radical polymerization (FRP). First, we consider the possibility of SI controlled radical polymerization (CRP) without chemical additives, based on local heating due to microwave absorption by the substrate. A simple model is developed to predict the temperature gradient at the interface between a microwave absorbing substrate and a nonabsorbing medium. Stochastic simulations are then applied to predict the molecular weight distribution for polymerizations with decoupled kinetics of initiation, propagation, and termination due to the temperature gradient. The simulations shed light on experimental requirements for $\mu $w-induced SI-CRP, as well as general conditions for any successful CRP. Secondly, we consider whether $\mu $w irradiation may increase throughput of SI-FRP, affording either faster brush growth, thicker brushes, or both, compared with conventional heating (CH) (e.g. by an oil bath). Experimental results of $\mu $w SI-FRP are compared against CH on silicon wafers, quartz slides, particles, and in bulk. Reproducibility of heating for silicon wafers is found to depend on orientation relative to the incident irradiation.   

Free Volume Model of Enhanced Mobility at a Free Surface

Experiments on polymer thin films during the past two decades have revealed a number of intriguing properties as they approach the glass transition. In addition to dynamic heterogeneity, which is also characteristic of the bulk, there is a substantial body of evidence for enhanced mobility at and near a free surface, leading to local suppression of the glass transition temperature. We have developed a simple kinetic lattice model of free volume and mobility transport in a near-glassy fluid. The model qualitatively exhibits hallmarks of the glass transition in bulk fluids, e.g. power-law growth of the cooperative length scale of glassified material, and slowing global dynamics on approach to a ``kinetic arrest'' transition. In this talk we discuss how introducing a free surface into the model yields a gradient of mobility, the depth of which depends on proximity to the bulk glass transition.   

Quantification of tip-sample forces on and below resonance in tapping mode atomic force microscopy

There has been a recent resurgence of interest in multi-frequency tapping mode AFM techniques, in which quantifying the tip-sample force is crucial. In particular, knowledge of the magnitude of tip-sample force may be essential in understanding the nature of contrast in imaging soft materials such as block copolymers or novel complex macromolecular architectures. This presentation will focus on the quantitative understanding of the dependence of average tip-sample forces on imaging conditions such as set-point ratio and operating frequency. First, the derivation of an analytical expression for the average tip-sample force will be presented. Its predictions will be then shown to be in excellent agreement with the results of numerical simulations using a single degree of freedom, driven damped harmonic oscillator model of tapping mode AFM.   

Grazing Resonant Soft X-ray Scattering: A New Way to See Inside Mesoscale Thin Films

Thin film structures are becoming increasingly important in energy and engineering applications as functional films and specifically as thin film electronics. Often the most important structures in these thin films are the interfaces between different materials. The internal structure of thin film complex systems, particularly interfacial structure, has been difficult and often impossible to characterize with traditional characterization techniques. Existing methods either lack materials contrast necessary to distinguish different components, lack penetrating power to see structure beneath the film surface, require special sample preparation which may change important features, or are too local a probe to get statistically meaningful information. This talk highlights a new technique, Grazing Resonant Soft X-ray Scattering (GR-SOXS), capable of probing buried structures in thin film systems. GR-SOXS uses varying energy x-rays near the 1S core electron absorption peak of carbon to scatter from thin polymer films at a grazing angle. Using simulations of the electric field propagation and scattering contrast of different features in model systems as a guide, Scattered X-rays from different structures within the film can be disentangled, elucidating their internal structure.   

Marangoni-Driven Topographic Patterning of Polymer Thin Films

When exposed to UV light polystyrene (PS) undergoes partial dehydrogenation of its polymer backbone, raising its surface energy. By exposing a PS thin film to UV light through a photomask, a surface energy pattern can be programmed in to the polymer film. Upon heating the film to a liquid state without the mask present, the polymer flows from the unexposed (relatively low surface energy) to exposed (relatively high surface energy) regions of the film. The driving force for this phenomenon is the Marangoni Effect, familiar to most in the `tears' or `legs' in a glass of wine, which describes convective mass transfer due to surface energy gradients. This flow results in three-dimensional topography reflective of the photomask used in the patterning step, which can be preserved indefinitely by quenching the film below its glass transition temperature. In this talk, this process, a preliminary model, and its kinetics will be described.   

Polymer Thin Films and Interfaces; a Layer-by-Layer Approach

In this talk we discuss new ways to model polymer films and interfaces, including properties such as density and concentration gradients, interfacial tension, and surface enrichment. We build on recent work where we developed a very simple equation of state approach for polymer thin films, and successfully applied it to determine thermodynamic properties and even to make predictions for the thickness-dependent depression of the thin film glass transition temperature. In that very simplified mean field model, the film properties across the entire interface region were treated as a ``whole sample'' average. Here, we take the next step, and develop a layer-by-layer equation of state model wherein details of the interface region are captured by allowing properties to vary from one discretized layer (within which properties are uniform) to the next. The model can be solved by imposing hydrostatic equilibrium in each layer, which then leads to predictions for the corresponding density gradient and other key interface properties.   

Melting of Linear Alkanes between Swollen Elastomers and Solid Substrates

We have measured the melting and freezing behavior of linear alkanes confined between a poly(dimethylsiloxane) (PDMS) elastomer and a solid sapphire substrate. For shorter alkanes (15 and 17 carbons) the interfacial layer has a higher melting temperature (T$_m$) than the majority of the alkane crystals inside the swollen PDMS elastomer. For longer alkanes (19, 21, and 22 carbons), a large depression in T$_m$ was observed and the crystallization takes place outside the contact region first and then proceeds to the PDMS-sapphire boundary. In heating, the sapphire/alkane interface shows a pre-melting layer (or melts first) before the melting of a thicker alkane layer next to the sapphire surface. The observation of this unusual depression of T$_m$ of the interfacial layers was unexpected and these findings have important implications in understanding friction and adhesion of soft elastomeric materials.   

Tacticity Effects on the Local Conformation and Interfacial Properties of poly (methyl methacrylate) at the Liquid-Vapor Interface

The orientation of functional groups of poly (methyl methacrylate) (PMMA) play a key role in understanding functionalities like wettability, aggregation and solvent interaction. We have studied the orientation of different functional groups such as the $\alpha $-methyl, ester methyl, methylene and carbonyl groups of the PMMA chain through all atom Molecular Dynamics (MD) simulations for different chain lengths of the polymer. Through orientational correlation, and number density computations we are able to establish the identity and extent of groups coming to the surface. Surface tensions are computed to validate our PMMA model. Analysis has been carried out for all three tacticities-atactic, syndiotactic, and isotactic. Sum Frequency Generation (SFG) spectroscopy also provides insight into the orientation of various groups at the liquidvapor interface. Characterization of the SFG peaks is the point of some debate and MD simulations aim to aid in the understanding of local ordering.   

Session W35: Novel Superconductors II

Evolution of superconductivity and magnetic order in LaRu$_3$Si$_2$ by rare earth and transition metal substitutions.

The recent discovery of high temperature superconductivity in iron based materials has renewed interest to condensed matter physics. Although its mechanism is not yet settled completely, it should have a close relationship with the electron correlations. The compound LaRu$_{3}$Si$_{2}$ shows superconductivity with a transition temperature $T_{\mathrm{c}}=$ 7.8 K. Recent study indicates that electron correlations play a significant role for superconductivity in this Kagome lattice of Ru and the Ru band dominates at the Fermi level, similar to Fe-band in iron-superconductors. Superconductivity in LaRu$_{3}$Si$_{2}$ has been found robust against the local paramagnetic moment. We will present our study on the evolution of superconductivity and magnetic order in LaRu$_{3}$Si$_{2}$ due to substitutions of Tm, a J$=$6 (J is the total angular momentum) ion with a maximum ordered moment of 7 $\mu_{\mathrm{B}}$, and transition metals by measuring magnetic, transport and Neutron scattering properties.   

Possible Pressure Driven Quantum Critical Point in CaCo$_{2}$P$_{2}$

We performed electrical resistivity measurements under pressures up to a maximum of $\approx $ 5 GPa for the d-electron antiferromagnet CaCo$_{2}$P$_{2}$,$_{\, }$where we find that the N\'{e}el temperature (T$_{N} \quad =$ 106 K) is rapidly suppressed towards zero near 1.4 GPa. In the vicinity of the suppressed magnetic state, the Fermi liquid coefficient of the electrical resistivity A increases abruptly, suggesting a divergence in the effective mass of the charge carrier quasiparticles. In addition, we find that the residual resistivity $\rho_{0}$ increases abruptly at 1.4 GPa. For P \textgreater 1.4 GPa, we also observe a broad hump in $\partial \rho $/$\partial $T at a temperature T*, which increases with increasing P. We will compare these measurements to expectations for prototypical f-electron quantum critical point (QCP) systems (e.g., CeRhIn$_{5}$ and CeRh$_{2}$Si$_{2})$ and the iron arsenide high temperature superconductors (e.g., CaFe$_{2}$As$_{2}$, SrFe$_{2}$As$_{2}$, and BaFe$_{2}$As$_{2})$ and discuss implications for studying a possible d-electron QCP in the absence of superconductivity.   

Universal scaling relations in exotic superconductors

Universal scaling relations are of tremendous importance in science, as they reveal fundamental laws of nature. Several such scaling rations have recently been proposed for superconductors, however, they are not really universal in a sense that some important families of superconductors appear to fail the scaling, or obey the scaling with different scaling pre-factors. In particular, a large group of materials called organic (or molecular) superconductors are a notable example. In this paper we show that such apparent violations are largely due to the fact that the required experimental parameters were collected on different samples, with different experimental techniques. When experimental data is taken on the same sample, using a single experimental technique, organic superconductors, as well as all other studied superconductors, do in fact follow universal scaling relations.   

Anomalous thermodynamic power laws in nodal superconductors

Unconventional superconductors are frequently identified by the observation of power law behaviour on low temperature thermodynamic properties such as specific heat. These power laws generally derive from the linear spectrum near points or lines of zeros, or nodes, in the superconducting energy gap on the Fermi surface. Here we show that, in addition to the usual point and line nodes, a much wider class of different nodal types can occur. Some of these new types of nodes typically occur when there are transitions between different types of gap node topology, for example when point or line nodes first appear as a function of some physical parameter. We derive anomalous, non-integer thermodynamic power laws associated with these new nodal types and predict their occurrence in iron pnictide superconductors and in the noncentrosymmetric system Li$_{2}$Pd$_{3-x}$Pt$_{x}$B.   

Anomalous angular dependence of the upper critical induction of orthorhombic ferromagnetic superconductors with completely broken $p$-wave symmetry

We calculate the angular dependence of the upper critical field, $H_{c2}(\theta,\phi,T)$, for an orthorhombic ferromagnetic superconductor with a general ellipsoidal Fermi surface with effective masses $m_{1}$, $m_{2}$, and $m_{3}$, in which we have $p$-wave parallel-spin pairing that is locked onto the $z$-axis direction. We report anomalous angular dependence of $H_{c2}$ for fixed $3 [Preview Abstract]

Triplet Nodeless Superconductivity Scenario in the Quasi-One-Dimensional Layered Conductor Li$_{0.9}$Mo$_6$O$_{17}$

We solve a theoretical problem about the upper critical magnetic field, parallel to a conducting axis of a layered quasi-one-dimensional superconductor. In particular, we consider the case, where triplet superconducting order parameter is not sensitive to the Pauli destructive effects against superconductivity and has no zeros on two quasi-one-dimensional pieces of the Fermi surface. We demonstrate [1] that in this case the orbital destructive effects against superconductivity can destroy superconducting state at magnetic fields much higher than the so-called Clogston-Chandrasekhar paramagnetic limit. Comparison of our theoretical results with the very recent experimental data [2] is in favor of a triplet superconducting pairing in the layered quasi-one-dimensional superconductor Li$_{0.9}$Mo$_6$O$_{17}$.\\[4pt] [1] A.G. Lebed and O. Sepper, Phys. Rev. Lett., submitted.\\[0pt] [2] J.-F. Mercure et al., Phys. Rev. Lett. \textbf{108}, 187003 (2012).   

Ferromagnetism in CuFeSb: Evidence of competing magnetic interactions in Fe-based superconductors

In this talk, we will report a new layered iron-pnictide compound CuFeSb [1]. This material shares similar layered tetragonal structure with iron-based superconductors, with Fe square planar sheets forming from the edge-sharing iron antimony tetrahedral network. CuFeSb differs remarkably from Fe-based superconductors in the height of anion Z$_{anion}$ from the Fe plane; Z$_{Sb}$ for CuFeSb is $\sim$1.84 {\AA}, much larger than Z$_{As}$ (1.31-1.51 {\AA}) in FeAs compounds and Z$_{Te}$ ($\sim$1.77 {\AA}) in Fe$_{1+y}$Te. In contrast with the metallic antiferromagneticor superconducting state of iron pnictides and chalcogenides under current studies, CuFeSb exhibits a metallic, ferromagnetic state with $T_{c} =$ 375 K. This finding provide strong experimental evidence for the competition between antiferromagnetic and ferromagneticcorrelations in layered Fe-based superconductors, and that the nature of magnetic coupling within the Fe plane is indeed dependent on the height of anion as predicted in theories [2,3].\\[4pt] [1] B. Qian\textit{ et al.}, Phys. Rev. B \textbf{85}, 144427 (2012).\\[0pt] [2] C.-Y. Moon,\textit{ et a.l}, Phys. Rev. Lett \textbf{104}, 057003 (2010).\\[0pt] [3] W.-G. Yin, \textit{et al.}, Phys. Rev. Lett \textbf{105}, 107004 (2010).   

Selective $d$-band Participation in Magnetic and Electronic Behavior of Spin-Ladder Iron-chalcogenides

The mechanism of superconductivity in the iron-based superconductors, particularly the role of magnetism and band nesting, remains controversial. The iron-based superconductors share many properties with the high-$T_c$ cuprates, including two-dimensional layers and proximity to magnetic order. Using reduced dimensionality, as exemplified by the ``spin ladder'' cuprates, we attempt to understand the electronic and magnetic behavior of the $A$Fe$_2X_3$ ($A$= alkali or alkali earth, $X$ = chalcogenide) family of materials. These compounds have $2\times \infty$ double-chains (``ladders'') of edge-sharing Fe$X_4$ tetrahedra, cutouts of the full two-dimensional Fe$_2X_2$ layers of the iron-based superconductors which provide a platform from which to understand the interplay of structure, magnetism, and electronic behavior. The unique properties of these compounds is exemplified by both the inability of DFT programs recapitulate either the underlying physical properties or the dramatic transition from block to stripe magnetic order in Ba$_{1-x}$K$_x$Fe$_2$Se$_3$ that coincides with a change from magnetic to non-magnetic behavior of one $d$- orbital-derived band. I will also present the influence of pressure and chemical doping on metallic and/or superconducting behavior.   

Physical properties of Kx(Ni,Fe)2-ySe2 single crystal alloys1

We report physical properties and ground state phase diagram of Kx(Fe,Ni)2-ySe2 single crystal alloy series. The ground state evolves from a heavy-Fermion-like metal KxNi2-ySe2 (I4/mmm) to a phase separated superconducting KxFe2-ySe2 (I4/m and I4/mmm space groups). Intermediate alloys show rich variety of ground states including semiconducting magnetic spin glass as Ni is replaced by Fe. We will address magnetic, thermodynamic, electronic and thermal transport properties and their connection to relevant structural parameters. 1Work at Brookhaven is supported by the U.S. DOE under Contract No. DE-AC02-98CH10886 and in part by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. DOE, Office for Basic Energy Science (H. L. and C. P). Work at the National High Magnetic Field Laboratory is supported by the DOE NNSA DEFG52-10NA29659 (D.G.), by the NSF Cooperative Agreement No. DMR-0654118, and by the state of Florida.   

Physical and magnetic properties of LaFe$_{0.6}$Sb$_{2}$

Currently, there is a tremendous effort to grow and characterize new iron pnictide materials with the hopes of discovering the next set of novel high temperature superconductors. The previous research has been focused on iron phosphides and arsenides, with relatively little work being done on the next heavier pnictogen, antimony. In this work, single crystals of the layered iron pnictide LaFe$_{0.6}$Sb$_{2}$ have been grown with the ZrCuSi$_{2}$ structure with vacancies on the Fe sites as determined via x-ray diffraction and energy-dispersive x-ray spectroscopy. The DC magnetization, resistivity, and heat capacity have been measured in a range of temperatures between 300 K and 0.5 K. The susceptibility is small and shows very little anisotropy; there is a maximum at 265 K and we see no Curie-Weiss-like behavior from room temperature down to 1.8 K. This material is a good metal whose resistivity decreases by a factor of 1.4 from 300 K to 0.5 K and we see Fermi liquid-like behavior from 7 K to 20 K. Although there is no evidence of bulk superconductivity down to 0.5 K in this undoped material, a large Sommerfeld coefficient of 50 mJ/(mol Fe) K$^{2}$ suggests that this metal is very strongly correlated.   

Quasi-two-dimensional non-collinear magnetism in the Mott insulator Sr$_2$F$_2$Fe$_2$OS$_2$

We study the magnetism of Sr$_2$F$_2$Fe$_2$OS$_2$ through neutron powder diffraction and thermodynamic and transport measurement. Quasi-two-dimensional magnetic order develops below $T_{\rm N}$=106K with an in-plane correlation length exceeding 310 {\AA} and an out-of-plane correlation length of only 17(3) {\AA}. The data are well described by a two-k structure with k$_1$=(1/2,0,1/2) and k$_2$=(0,1/2,1/2). The ordered moment is 3.3(1) $\mu_B$ oriented along the in-plane components of k. This structure is composed of orthogonal AFM chains intersecting at super-exchange mediating O sites. The Density Function Theory ( by Liang L.Zhao, Jiakui K. Wang, etc.) also leads to this structure and a narrower Fe 3d band than for the iron pnictides from which electronic correlations produce a Mott insulator.   

Functional interfaces in La$_{2/3}$Ca$_{1/3}$MnO$_{3}$/ YBa$_{2}$Cu$_{3}$O$_{7-x}$ heterostructures

Interfaces have emerged as one of the focal points of current condensed matter science. In complex, correlated oxides, heterointerfaces provide a powerful route to create and manipulate the charge, spin, orbital, and lattice degrees of freedom. In this study, epitaxial bilayers of ferromagnetic of La$_{2/3}$Ca$_{1/3}$MnO$_{3}$(LCMO) and superconducting YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) with two distinct interfaces have been fabricated to understand the effects of these two distinct interfaces. X-ray absorption near edge spectroscopy (XANES) was applied to characterize the interfaces and also provided direct evidence of the charges transfer at these interfaces. The studies of the macroscopic properties, such as the transport and magnetic properties, established the connection between macroscopic properties and the interface structures. This present study opens new venue to design the functional interfaces.   

Superconducting interface in cuprate p-n heterostructures

In this explorative work, we combined two kinds of non-superconducting cuprates : over-doped ${\rm Pr}_{2-x}{\rm Ce}_x{\rm CuO}_4$ and under-doped ${\rm La}_{2-x}{\rm Sr} _x{\rm CuO}_4$ in the same p-n heterostructures in order to generate new behaviors through the interplay between the two materials. We will show that a thin superconducting layer ($<10$ nm) arise at the interface between these two compounds. We will discuss its actual location, its unexpected occurrence and its origin which is partly compatible with a charge transfer scenario that takes place in similar p-p cuprate heterostructures [1,2].\\[4pt] [1] A. Gozar \textit{et al.}, Nature 456, 782 (2008)\\[0pt] [2] G. Logvenov \textit{et al.}, Science 326, 699 (2009)   

Electron Doping by Charge Transfer at LaFeO$_3$/Sm$_2$CuO$_4$ Epitaxial Interfaces

We examine the interfacial charge transfer in epitaxial heterostructures formed between Mott insulating Sm$_2$CuO$_4$ (SCO) and charge transfer insulator LaFeO$_3$ (LFO) in LFO/SCO superlattices. High resolution EELS measurements at the O-K edge have provided evidence for 0.09$+$/-0.01 extra electrons in the SCO d- band as revealed by a reduction of the Cu oxidation state. The transfer of electrons from LFO to SCO is further supported by the spectroscopic signature of Cu$^{1+}$ as obtained from XAS measurements. Transport measurements have evidenced a metallic state at the interface between two nominally insulating materials. Dielectric spectroscopy measurements have allowed ascribing the metallic state to the LFO/SCO interfaces, consistent with DC measurements. When lowering the temperature a metal to insulator transition occurs at 120 K, indicating, in accordance with the phase diagram, an insufficient doping level to enter a superconducting state.   

Session W36: Focus Session: Fe-based Superconductors: Synthesis and Characterization

Preparation and characterization of annealed single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ at and near optimally doped, 0.07$\le $x$\le $0.095

Using self flux single crystal growth and long term annealing in the presence of an As vapor source, we report resistivity, magnetic susceptibility and specific heat characterization of optimized samples at and near to optimally doped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. The ultimate achievable T$_{c}$ in 122 BaFe$_{2}$As$_{2}$ doped on the Fe layers will be discussed, along with the variation with composition on a very fine scale of the linear T term in the resistivity and the discontinuity in the specific heat, $\Delta $C/T$_{c}$, on both the overdoped and underdoped (coexistent with magnetism) sides of optimally doped.   

Properties of epitaxial Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ thin films on different substrates

We have grown epitaxial, optimally-doped superconducting Ba(Fe$_{0.92}$Co$_{0.08})_{2}$As$_{2}$ films on SrTiO$_{3}$, (La, Sr)(Al, Ta)O$_{3}$ and LaAlO$_{3}$ substrates, which have a range of lattice mismatch, and studied the strain effect on the structural and transport properties of the films. We found that the superconducting transition temperature increased as the c lattice constant decreased and a lattice constant increased. The thickness dependence of the superconducting transition temperature was studied, which was related to the strain and strain relaxation. A zero-resistance $T_{c}$ of 21.7 K was obtained in the 120 nm-thick Ba(Fe$_{0.92}$Co$_{0.08})_{2}$As$_{2}$ film on SrTiO$_{3}$ substrate.   

Combined effects of annealing/quenching and transition metal substitution on physical properties of CaFe$_{2}$As$_{2}$

Our previous work on CaFe$_{2}$As$_{2}$ single crystals grown out of FeAs flux has shown that a process of annealing and quenching can be used as an additional control parameter which can tune the ground state of CaFe$_{2}$As$_{2}$ systematically, in a manner similar to applied pressure. With combined effect of annealing/quenching and transition metal substitution, CaFe$_{2}$As$_{2}$ system offers ready access to the salient low-temperature states associated with Fe-based superconductors: antiferromagnetic/orthorhombic, superconducting, and nonmagnetic/collapsed tetragonal. In this talk we will present systematic studies of the combined effects of annealing/quenching and chemical substitution with various transition metals (Co, Ni, Rh) on the physical properties of CaFe$_{2}$As$_{2}$ and construct phase diagrams for different substitution levels and different annealing/quenching temperatures.   

Superparamagnetism and interfacial superconductivity in rare earth Pr-doped Ca122

To better understand the origin of the non-bulk superconductivity with an unusually high onset-T$_{c}$ (49 K) and its superconducting behavior in the rare earth Pr-doped Ca122 [(Ca$_{1-x}$Pr$_{x})$Fe$_{2}$As$_{2}$], detailed chemical analyses and magnetization measurements on both the as-synthesized and annealed single crystals were carried out. A small but non-negligible As-deficiency and superparamagnetic clusters (SPCs) were detected in the superconducting as-synthesized crystals, suggesting that the SPCs originate from the As vacancies. The magnetic moment of the SPC were found to be insensitive to the doping level x, while the SPC density (n) is zero for x \textless 0.05 in the non-superconducting region and increases monotonically with x for x \textgreater 0.1 in the superconducting region. The superconducting volume fraction (f) was shown to be very closely related with n. Noticeable inter-cluster interactions, from antiferromagnetic for x \textless 0.05 (non -SC region) to weakly ferromagnetic for x \textgreater 0.1 (SC region) were found, suggesting that the defects are ordered. Systematically annealing the crystals over 500-920${^\circ}$ simultaneously suppress both n and f. Therefore, we propose that the ordered vacancies, and the associated interfaces, are responsible for the rather high onset-T$_{c}$.   

Evidence of Interface-Enhanced T$_{c}$ in Rare-Earth Doped Ca122

Nonbulk superconductivity with an onset-T$_{c}$ up to 49 K has been observed in single crystalline rare-earth doped CaFe$_{2}$As$_{2}$ [(Ca$_{1-x}$,RE$_{x})$122] recently. Such a T$_{c}$ is more than $\sim$ 20 K higher than any known compounds that consist of one or more of the Ca, RE, Fe and As elements at ambient or under high pressures. The unusually high onset-T$_{c}$ has therefore been attributed to interface effect. We have made systematic magnetic, transport, calorimetric and structural studies. They show: a chemically homogeneity of $\Delta $x \textless\ 0.005 over a 1$\mu$m; less than 5 {\%} of a bulk superconducting volume fraction; a doping-insensitive onset-T$_{c}$ in samples with or without the ``collapsed phase'', varying from $\sim$ 42 K for RE $=$ Nd to 49 K for RE $=$ Pr with a doping sensitive superconducting volume fraction, suggesting that the high onset-T$_{c}$ cannot be due to chemical doping or the effect of the ``collapsed phase''; an unusually high magnetic anisotropy up to 200, in contrast to the value of 4 from the sample geometric anisotropy, suggesting that the superconducting body has a very high aspect ratio; several steps in the magnetic susceptibilities along both the c- and ab-directions in the field range between 10$^{-3}$ to 10$^{+3}$ Oe, demonstrating the sample consisting of Josephson-Coupled superconducting islands imbedded with nano-scale interfaces; and the presence of superparamagnetic clusters associated with minute As-vacancies, consistent with theoretical calculations. The present studies therefore present the strongest evidence for interface-enhanced T$_{c}$ to date.   

Films of Iron-Chalcogenide Superconductors and Applications

Iron chalcogenides are of great interest for both basic physics and applications. Although their superconducting transition temperatures are typically lower than those of iron pnictides, iron chalcogenides exhibit lower anisotropies with very high upper critical field slopes near the superconducting transition temperatures. In this presentation, I will discuss recent progress in the superconducting thin films and coated conductors of iron chalcogenides. The very high upper critical fields and critical current densities of these films suggest that they are prospective candidates for high field and energy applications. - Reference: Qiang Li, Weidong Si, and Ivo Dimitrov, ``Films of Iron-Chalcogenide Superconductors,'' Rep. Prog. Phys. \textbf{74} 124510 (2011)   

Plused Laser Deposition growth of iron chalcogenide with tunable structural and physical properties

Since the discovery of iron superconductor in 2008, plenty of spectroscopic experimental and theoretical works have been done to explore the mechanism of superconductivity. In parallel, much effort is devoted to thin film growth with the aim to fabricate high quality samples with tunable structural and physical properties,as well as for the development of new functional devices. Here we apply the Plused Laser Deposition (PLD) method to obtain iron chalcogenide superconductor thin films. By adjusting the growth parameters and procedure, we can modulate the structure and properties of the thin films. One of the main results is the enhancement of Tc. We are constructing a new system combining ARPES and PLD for in-situ measurements which will surely shed interesting light on the mechanism of superconductivity.   

Effects of Oxygen Annealing in Fe(Te,Se) Single Crystals

Iron-chalcogenide superconductor Fe(Te,Se) has the simplest structure among all iron-based superconductors. Yet, its superconducting properties except for $T_{\mathrm{c}}$ are not very much reproducible. This is partly due to the fact that the as-grown crystals of Fe(Te,Se) is not superconducting, and post-annealing is important to induce superconductivity. We found that the annealing in a controlled oxygen atmosphere is very important to induce superconductivity in this system. Upon annealing in oxygen atmosphere, the content of excess iron in the crystal decreases. We will demonstrate the dynamics of the oxygen annealing process by changing the annealing time and temperature. We also compare the effect of different annealing conditions, such as vacuum annealing, with that of oxygen annealing. Finally, physical properties of well-characterized Fe(Te,Se) crystals are discussed together with the vortex physics in this system.   

The mechanism of alcoholic beverage induced superconductivity in Fe-chalcogenide compounds

We have clarified the mechanism of alcoholic beverage induced superconductivity in Fe-chalcogenide compounds. Previously we reported that the bulk superconductivity in Fe-based compounds Fe(Te, Se) and Fe(Te, S) is achieved by heating in alcoholic beverages [1,2]. However, the exact mechanism of how they act to enhance the superconductivity in the compounds remains unsolved. To understand the effect of alcoholic beverage treatment, we investigated the mechanism using a technology of metabolomic analysis [3]. We found that weak acid in alcoholic beverages has the ability to deintercalate the excess Fe, which is not in favor of superconductivity. In this presentation, we will discuss the systematic mechanism to induce superconductivity in Fe-chalcogenide compounds. [1] K. Deguchi et al., Supercond. Sci. Technol. 24 (2011) 055008. [2] K. Deguchi et al., arXiv: 1210.5889. [3] K. Deguchi et al., Supercond. Sci. Technol. 25 (2012) 084025.   

Improved growth of Ln1111 superconducting crystals from NaAs/KAs flux

Single crystals of the LnFeAsO (Ln1111, Ln $=$ Pr, Nd, and Sm) family with lateral dimensions up to 1 mm were grown from NaAs and KAs flux using the cubic anvil high-pressure and high-temperature technique. The crystals become superconducting when O is partially substituted by F (PrFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ and NdFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}})$ or when Fe is substituted by Co (SmFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$AsO). In SmFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$AsO the maximum $T_{\mathrm{c\thinspace }}$is 16.3 K for x $=$ 0.8. From transport and magnetic measurements we estimate the critical fields and their anisotropy, and we find these superconducting properties to be quite comparable to the ones in SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ with a much higher $T_{\mathrm{c}}$ of $\approx $ 50 K. The magnetically measured critical current densities are as high as 10$^{\mathrm{9}}$ A/m$^{\mathrm{2}}$ at 2 K up to 7 T, with indications of the usual ``fish tail'' effect.   

Synthesis methods and character of iron-based mixed-anion superconductor with suppression of the amorphous FeAs impurity phase

To obtain the high superconducting properties of polycrystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$, we investigated the following three synthesis methods: a high pressure synthesis, a low temperature synthesis with gradual cooling and a metal added synthesis. Generally, polycrystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ is composed of superconducting grains and a little amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between the superconducting grains. Therefore, we suggest that the superconducting current is prevented by the amorphous areas. In fact, although the single crystal of this material shows a large critical current density of 10$^{\mathrm{6}}$ A/cm$^{\mathrm{2}}$, polycrystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ shows a significant depression of critical current density due to this grain boundary blocking effect. To obtain a high global critical current density, it is important to investigate how to remove the amorphous FeAs. It is found that the impurity phase of amorphous FeAs is decreased by using the above three synthesis methods.   

Hydrostatic and chemical pressure tuning of CeFeAs$_{1-x}$P$_x$O single crystals: The intriguing interaction between 3$d$- and 4$f$-correlations

We present a combined P-substitution and hydrostatic pressure study on CeFeAs$_{1-x}$P$_{x}$O single crystals in order to investigate the peculiar relationship of the local moment magnetism of Ce, the ordering of itinerant Fe moments, and their connection with the occurrence of superconductivity [1,2]. Our results evidence a close relationship between the weakening of Fe magnetism and the change from antiferromagnetic to ferromagnetic ordering of Ce moments at $p^*=1.95$ GPa in CeFeAs$_{0.78}$P$_{0.22}$O. The absence of superconductivity in CeFeAs$_{0.78}$P$_{0.22}$O and the presence of a narrow and strongly pressure sensitive superconducting phase in CeFeAs$_{0.70}$P$_{0.30}$O and CeFeAs$_{0.65}$P$_{0.35}$O indicate the detrimental effect of the Ce magnetism on superconductivity in P-substituted CeFeAsO.\\[4pt] [1] A. Jesche, T. F\"orster, J. Spehling, M. Nicklas, M. de Souza, R. Gumeniuk, H. Luetkens, T. Goltz, C. Krellner, M. Lang, J. Sichelschmidt, H.-H. Klauss, and C. Geibel, Phys. Rev. B 86, 020501(R) (2012).\\[0pt] [2] K. Mydeen, E. Lengyel, A. Jesche, C. Geibel, and M. Nicklas, Phys. Rev. B 86, 134523 (2012).   

Raman spectroscopic analysis for grain boundary of Superconducting polycrystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$

The observation of grain boundary structures is essential technique to fabricate high-T$_{\mathrm{c}}$ superconducting wires. Spatial crystal distribution analysis for grain boundary of superconducting polycrystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ is demonstrated by Raman Spectroscopy. Polycrystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ samples were synthesized using two-step solid state reaction described elsewhere [New J. Phys.\textbf{12}, 033005 (2010)]. Samples' surface and their structures were checked by microscopic optical measurement and electron beam backscattering diffraction (EBSD) analysis. The Raman spectroscopy was performed at the range from 150 cm$^{-1}$ to 500 cm$^{-1}$. F contents (x) were 0, 0.019, 0.037, 0.045, 0.069, 0.075. Although our several spectra are similar to which had been reported [Hadjiev, et al, Phys. Rev. B. \textbf{77}, 220505 (2008)], our results indicate that grain boundary structures are mixtures of small single crystalline SmFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ and amorphous-FeAs. Details of the Raman spectra will be presented at the conference.   

Synthesis and characterization of whisker crystals of iron-based superconductor

Single-crystal superconducting whiskers of Ca$_{10}$(Pt$_4$As$_8)$(Fe$_{1.8}$Pt$_{0.2}$As$_2)_5$ were grown in a Ta capsule in an evacuated quartz tube by a flux method [J. Li, \textit{et al.} J. Am. Chem. Soc. 134, 4068$-$4071 (2012)]. This technique can be potentially useful for growth of other whiskers containing toxic elements, although the growth mechanism is not understood well. The Ca$_{10}$(Pt$_4$As$_8)$(Fe$_{1.8}$Pt$_{0.2}$As$_2)_5$ whiskers were confirmed to have excellent crystallinity with $T_{\mathrm{c}}$ of 33 K, $\mu_{\mathrm{0}}H_{\mathrm{c2}}$ of 52.8 T, and $J_{\mathrm{c}}$ of 6.0 $\times$ 10$^5$ A/cm$^2$ (at 26 K). The $T_{\mathrm{c}}$ value is comparable with that of the bulk material. Since cuprate high-$T_{\mathrm{c}}$ superconducting whiskers are fragile ceramics, the present intermetallic superconducting whiskers with high-$T_{\mathrm{c}}$ have better opportunities for device applications. In addition, we studied the Ca$_{10}$(Pt$_4$As$_8)$(Fe$_{2-x}$Pt$_{x}$As$_2)_5$ superconducting whiskers consisting of several grains. With current tunneling across the grain boundaries, current-voltage characteristics show the behavior of Josephson tunnel junction effect with pronounced hysteresis. In this talk, we review the growth of the superconducting whiskers and shows progress of studies of the Josephson junction using the whiskers.   

Annealing and doping effects of Fe-based superconductors with thick perovskite-type blocking layers

After the discovery of superconductivity in LaFeAs(O,F), several types of Fe-based superconductors were developed. In particular, iron-based superconductors having extremely thick perovskite-type blocking layers, such as (Fe2As2)(Ca5(Sc,Ti)4Oy) and (Fe2As2)(Ca4(Mg,Ti)3Oy) were discovered[1]. Interlayer Fe-Fe distances of these compounds are from 2 to 3 nm, which are much longer than other iron-based superconductors. Antiferromagnetic ordering or structural transition is not observed in these compounds, and superconducting transitions appear without intentional carrier doping. In this study, we have investigated carrier doping and annealing effect of these compounds. Relationship between crystal structure, chemical compositions and physical properties will be discussed. [1] H. Ogino et al., Appl. Phys. Express 3 (2010) 063103   

Session W37: Focus Session: Fe-based Superconductors: General Theory

Gap symmetry and nodal structure of iron-based superconductors

We first analyze the gap symmetry in iron chalcogenides with only electron pockets. Previously, two competing alternatives for the gap symmetry were considered. In the first scenario the order parameter has opposite sign on two pockets which gives rise to a $d$-wave symmetry. In the second scenario the order parameter has a constant sign, resulting in an $s$-wave symmetric state. Experimentally, the $d$-wave is excluded by ARPES, while $s$-wave scenario is inconsistent with the spin resonance as seen by neutrons. We present the third alternative agreeing with both ARPES and neutron scattering. In contrast to the earlier theories we suggest that the pairing of electrons at different pockets is equally or more important than the usual intra-pocket pairing. The inter-pocket pair momentum $(\pi,\pi)$ is supplied by the lattice via the inter-pocket hybridization processes. When the hybridization amplitude exceeds the threshold set by the pocket ellipticity the system is brought into an $s^{\pm}$ state. In this state both intra- and inter-pockets pair condensates are present. We argue that $s^{\pm}$ state is consistent with experiments. We next argue that the hybridization is crucial for the nodal structure of iron pnictides. In these superconductors with both electron and hole pockets the hybridization causes the nodal lines to form a closed nodal loops. This is consistent with ARPES, penetration depth and specific heat measurements.   

S+iS superconductivity in hole-doped Fe-pnictides

The extended s-wave (s+-) symmetry proposed for Fe-pnictides requires flipping of the phase of the superconducting order parameter (the gap) on at least two pockets. In optimally doped BaK-122, the phase is flipped between the hole and electron gaps-but have the same phase on the hole gaps (e.g., both are +). But in the strongly hole doped sample only hole pockets remain, and ARPES experiments were interpreted as evidence for s+- symmetry. This requires flipping of the phase on a pair of hole pockets (one gap is + and another is -). We address this issue of ++ to +- transition of the hole gaps as doping is changed. We find that such a transition occurs via an intermediate phase of s+is type, in which time reversal symmetry is broken (TRSB state). The ++ and +- states are two end points of the s+is state. We show that TRSB state emerges at a single point at $T_c$, but the parameter range over which it exists widens as we go down in temperature down to $T=0$. We investigate the structure of collective phase and amplitude gap fluctuations in the TRSB state and analyze the sensitivity of this state to the angular anisotropy of the interaction. We find that anisotropy-driven accidental gap nodes can survive in s+is state, unlike in s+id state (proposed for electron doped pnictides).   

Enhancement of the London penetration depth in pnictides at the onset of SDW order under superconducting dome

Recent measurements of the doping dependence of the London penetration depth $\lambda(x)$ in clean samples of isovalent BaFe$_2$(As$_{1-x}$P$_x$)$_2$ at $T \ll T_c$ [Hashimoto et al., Science 336, 1554 (2012)] revealed a sharp peak in $\lambda(x)$ near optimal doping $x=0.3$. This observation points to the existence of the quantum critical point beneath the superconducting dome. We show that quantum magnetic fluctuations, associated with the emerging spin-density-wave order give rise to the observed feature. The effect comes from the dynamic renormalization of the effective mass $m^*$, which is related to $\lambda$ as $\lambda \propto \sqrt{m^*}$. We show that the effective mass has a maximum at the onset of the spin-density-wave order. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at $T=0$.   

Prediction for fingerprints of bosonic modes through self-energy effects in LiFeAs

The role of bosonic modes has been of great interest in the research of Fe-pnictides. We aim at identifying fingerprints of specific bosonic modes in the spectral properties of the multi-orbital superconductor LiFeAs. For this, we contrast the lowest order contributions to the self energy of Bogoliubov quasiparticles from two bosonic modes: antiferromagnetic(AF) fluctuation and $E_g$ phonon. Focusing on the largest hole pocket in LiFeAs, we find that $E_g$ phonon leads to an almost completely isotropic self energy. In contrast, AF mode leads to a pronounced angle dependent self energy. We predict signatures of such self-energy in ARPES and quasiparticle interference measured by spectroscopic imaging STM.   

Quantum Monte Carlo study of a dominant $s$-wave pairing symmetry in iron-based superconductors

We perform a systematic quantum Monte Carlo study of the pairing correlation in the $S_4$ symmetric microscopic model for iron-based superconductors. It is found that the pairing with an extensive $s$-wave symmetry robustly dominates over other pairings at low temperature in reasonable parameter region regardless of the change of Fermi surface topologies. The pairing susceptibility, the effective pairing interaction and the $(\pi,0)$ antiferromagnetic correlation strongly increase as the on-site Coulomb interaction increases, indicating the importance of the effect of electron-electron correlation. Our non-biased numerical results provide a unified understanding of superconducting mechanism in iron-pnictides and iron-chalcogenides and demonstrate that the superconductivity is driven by strong electron-electron correlation effects.   

Magnetic and superconductivity structures near the twin boundaries in low doped Fe-pnictides

The spatial distributions of the magnetic, superconducting (SC) and charge orders near twin boundaries (TBs) in slightly electron-doped Ba(Ca)(FeAs)$_2$ superconductors are investigated. Two different types of TBs, which respectively correspond to the 90-degree lattice rotation and asymmetrically placement of As atoms, are considered. We find that the domain walls, which spatially separate different magnetic regions, can be formed under a relatively small Coulomb interaction due to the existence of TBs. We show that the SC is enhanced on the TBs of the first type, while on the TBs of the second type, the SC is always suppressed.   

Oriented gap opening in the magnetically ordered state of Iron-pnicitides: an impact of intrinsic unit cell doubling on the Fe square lattice by As atoms

We show that the complicated band reconstruction near Fermi surfaces in the magnetically ordered state of iron-pnictides observed by angle-resolved photoemission spectroscopies (ARPES) can be understood in a meanfield level if the intrinsic unit cell doubling due to As atoms is properly considered as shown in the recently constructed S4 microscopic effective model. The (0,pi) or (pi,0) col-linear antiferromagnetic (C-AFM) order does not open gaps between two points at Fermi surfaces linked by the ordered wave vector but forces a band reconstruction involving four points in unfolded Brillouin zone (BZ) and gives rise to small pockets or hot spots. The S4 symmetry naturally chooses a staggered orbital order over a ferro-orbital order to coexist with the C-AFM order. These results strongly suggest that the kinematics based on the S4 symmetry captures the essential low energy physics of iron-based superconductors.   

Local Quantum Criticality in an Iron-Pnictide Tetrahedron

The iron-based superconductors display a close experimental relationship between the Tc values and the tetrahedral bond angle of the As-Fe-As layer, with optimal Tc clustering close to the ideal tetrahedron geometry. This motivates a study of the local physics of an Fe atom within an As tetrahedron, and we find a strong interplay between spin and orbital degrees of freedom. The d-orbitals are crystal field split, and the lower eg orbitals have an SU(2) $\times$ SU(2) symmetry with both a spin and orbital Kondo interaction. The spin Kondo coupling is strongly reduced by the Hund's coupling; hence the system flows to an over-screened orbital Kondo state. A perturbative RG analysis of the strong-coupling fixed point is done using a Majorana fermion representation of the SU(2) $\times$ SU(2) symmetry. The low-temperature physics, and the possibility of a Marginal Fermi Liquid ground state, is carefully studied using the CTQMC method, taking into account the effect of Hund's coupling on the Kondo physics.   

Cooper Pair Formation from Quantum Magnetism in Iron-Pnictide High-Tc Superconductors

We study how spin fluctuations mediate the formation of Cooper pairs in iron-pnictide high-$T_c$ superconductors via a Schwinger-boson-slave-fermion analysis of a two-orbital $t$-$J$ model for a square lattice of iron atoms that includes magnetic frustration and Hund's Rule coupling. The starting point is a hidden half-metal state across the two-orbitals that recovers correct nested Fermi surfaces at a quantum-critical transition with a commensurate spin density wave (cSDW) metal [1]. A mean-field approximation indicates that hidden spinwaves at zero 2D momentum [2] result in an s-wave Cooper-pair instability on the hole Fermi surface pockets centered at 2D momentum $(0,0)$. Proximity to the quantum-critical transition results, additionally, in a simultaneous s-wave Cooper-pair instability on the electron Fermi surface pockets centered at 2D momenta $(\pi,0)$ and $(0,\pi)$, but with a sign change. This mean-field prediction will be checked by extracting the amplitude for such $s_{+-}$ pairing from exact numerical diagonalizations of the two-orbital $t$-$J$ model over the $4\times 4$ lattice with two holes.\\[4pt] [1] J. Rodriguez, M. Araujo \& P. Sacramento, Phys. Rev. B 84, 224504 (2011).\\[0pt] [2] J. Rodriguez, Phys. Rev. B 82, 014505 (2010).   

Anisotropic Superconducting Gap in a Multiorbital t-J$_1$-J$_2$ Model for Iron Pnictides

We study the anisotropy of the superconducting gaps in the iron pnictides within a five-orbital t-J$_1$-J$_2$ model. We show that the interplay between the multiorbital nature and the magnetic frustration can give rise to an anisotropic superconducting gap with the $A_{1g}$ pairing symmetry. We have also calculated the dynamical spin susceptibility in the superconducting state, and find that the anisotropic gap structure affects the spin dynamics by showing two resonance peaks. We further discuss the connections between our results and recent ARPES and inelastic neutron scattering measurements.   

Detecting pairing symmetry in Fe-based superconductors: Solitons and proximity patch

We suggest a mechanism which promotes the existence of a phase soliton -- topological defect formed in the relative phase of superconducting gaps of a two-band superconductor with $s_{+-}$ type of pairing. This mechanism exploits the proximity effect with a conventional $s$-wave superconductor which favors the alignment of the phases of the two-band superconductor which, in the case of $s_{+-}$ pairing, are $\pi$-shifted in the absence of proximity. In the case of a strong proximity such effect can be used to reduce soliton's energy below the energy of a soliton-free state thus making the soliton thermodynamically stable. Based on this observation we consider an experimental setup, applicable both for stable and metastable solitons, which can be used to distinguish between $s_{+-}$ and $s_{++}$ types of pairing in the iron-based multiband superconductors.   

Intersoliton forces and magnetic response of three band superconductors with broken time reversal symmetry

The recent discovery of iron pnictide superconductors has resulted in a rapidly growing interest in multiband models with more than two bands. The three-band Ginzburg-Landau model does in part of the parameter space exhibit broken time reversal symmetry and degenerate ground states. As was shown in Phys. Rev. Lett. 107, 197001 (2011) these systems possess topological defects in the form of bound states of fractional vortices that are different from ordinary vortices, and lack rotational symmetry. We discuss intersoliton forces, and show that they exhibit a strong orientational dependence and thus can results in nontrivial structures appearing in an applied external field. Such structures can be detected by surface magnetic probes such as scanning SQUID, magnetic force microscopy etc.   

Chiral $CP^2$ skyrmions in three-band superconductors and layered superconducting structures

Recently discovered iron-based superconductors and well as multilayer structures involving $s_{\pm}$ superconductors can exhibit a spontaneous breaking of the time reversal symmetry. This raises the question of experimental manifestations of this additional broken symmetry. We demonstrate that it can result in formation of experimentally detectable nontrivial flux-carrying excitations which are topologically different conventional vortices. This new kind of solitons can provide an experimental signature of the breaking of time reversal symmetry.   

Session W38: Focus Session: Novel Photophysics and Transport in NanoPV III

Embedded metal nanopatterns for near-field scattering-enhanced optical absorption

Simulations of metal nanopatterns embedded in a thin photovoltaic (PV) absorber show significantly enhanced absorbance within the semiconductor, with a more than 300\% increase for $\lambda$ = 800 nm. Integrating with AM1.5 solar irradiation, this yields a 70\% increase in simulated short circuit current density and thus power conversion efficiency (single junction $\eta$ = 13\%) in a 60 nm amorphous silicon film. Embedding such metal patterns inside an absorber maximally utilizes enhanced electric fields that result from intense, spatially organized, near-field scattering in the vicinity of the pattern. Appropriately configured (i.e., with a thin insulating coating), this optical metamedium architecture may be useful for increasing PV efficiency in thin film solar cells, including offering prospects for realistic ultrathin hot electron cells.   

Improved electrical response of photovoltaic devices by photonic structuring

We describe the use of dispersion engineered photonic materials to develop a new photovoltaic technology that can achieve much higher efficiencies than traditional devices through the modification of spontaneous emission. The limiting efficiency of photovoltaic energy conversion was determined by Shockley and Queisser using the theory of detailed balance, which described the balance between absorption and emission of photons. However, when the solar cell is formed from a photonic crystal or a similar material is placed on top of a solar cell, both the absorption and emission of photons is modified, a fact not considered in the original formalism. Here we show that photonic crystal structuring can improve the cell efficiency by either effectively modifying the semiconductor bandgap energy or reducing the spontaneous emission within the device, leading to higher carrier concentrations and hence higher open circuit voltages.   

Optical absorption of nanoporous silicon: quasiparticle band gaps and absorption spectra

Silicon is an earth-abundant material of great importance in semiconductors electronics, but its photovoltaic applications are limited by the low absorption coefficient in the visible due to its indirect band gap. One strategy to improve the absorbance is to perforate silicon with nanoscale pores, which introduce carrier scattering that enables optical transitions across the indirect gap. We used density functional and many-body perturbation theory in the GW approximation to investigate the electronic and optical properties of porous silicon for various pore sizes, spacings, and orientations. Our calculations include up to 400 atoms in the unit cell. We will discuss the connection of the band-gap value and absorption coefficient to the underlying nanopore geometry. The absorption coefficient in the visible range is found to be optimal for appropriately chosen nanopore size, spacing, and orientation. Our work allows us to predict porous-silicon structures that may have optimal performance in photovoltaic applications.   

Absorption enhancement in amorphous silicon thin films via plasmonic resonances in nickel silicide nanoparticles

Silicon is a near ideal material for photovoltaics due to its low cost, abundance, and well documented optical properties. The sole detriment of Si in photovoltaics is poor absorption in the infrared. Nanoparticle surface plasmon resonances are predicted to increase absorption by scattering to angles greater than the critical angle for total internal reflection (16$^{\circ}$ for a Si/air interface), trapping the light in the film. Experiments confirm that nickel silicide nanoparticles embedded in amorphous silicon increases absorption significantly in the infrared. However, it remains to be seen if electron-hole pair generation is increased in the solar cell, or whether the light is absorbed by the nanoparticles themselves. The nature of the absorption is explored by a study of the surface plasmon resonances through electron energy loss spectrometry and scanning transmission electron microscopy experiments, as well as first principles density functional theory calculations. Initial experimental results do not show strong plasmon resonances on the nanoparticle surfaces. Calculations of the optical properties of the nickel silicide particles in amorphous silicon are performed to understand why this resonance is suppressed.   

Progress Developing Hybrid Silicon Quantum Dot/Amorphous Silicon Thin Films for Photovoltaics Application

Quantum confined (QC) nanostructures exhibit novel, size tunable, quantum mechanical phenomena and their use in solar cell architectures may yield significant efficiency gains. We demonstrate QC hybrid silicon nanocrystal(nc-Si:H) -- hydrogenated amorphous silicon (a-Si:H) structures, which can potentially serve as photo-stable, thin film silicon, solar cell materials and provide higher open-circuit voltage compared to conventional materials. We deposit a/nc-Si:H films sequentially, where nc-Si:H and a-Si:H are grown layer-by-layer using separate plasma reactors in a common deposition chamber. X-ray diffraction, Raman spectroscopy, and electron microscopy results confirm the nanoparticles are the appropriate size to achieve QC (3-7nm). Photoluminescence spectroscopy reveals the QC. Co-planar electrical probe experiments investigate carrier transport in a/nc-Si:H, which could be limited by defects accompanying plasma interruption in the sequential deposition process. Defect spectroscopies, such as electron paramagnetic resonance and photothermal deflection spectroscopy are used to study this relationship. These studies reveal material quality limitations to be addressed for realizing film silicon materials that harvest QC to enhance PV device performance.   

Computational spectroscopy of nanocomposites

Most of the first principles calculations of the opto-electronic properties of nanoparticles appeared in the literature were conducted using structural models of isolated particles. However experiments are carried out on nanocomposites, e.g. nanoparticles in solution or embedded in solid matrices. Recent ab initio studies [1,2] pointed at the importance of taking into account interactions between nanoparticles and the environment surrounding them, in order to provide sensible predictions of their electronic properties, as well as interpretation of experiments. Here we report calculations of the relative position of energy levels of Si nanoparticles embedded in amorphous matrices, as obtained using many body perturbation theory, at the GW level. Our calculations were carried out using a newly developed method to obtain quasi particle energies, based on the spectral decomposition of the dielectric matrix [3].\\ $[1]$ T.S.Li, F.Gygi and G.Galli \textit{Phys. Rev. Lett.} 107, 206805 (2011)\\ $[2]$ M.Govoni, I.Marri and S.Ossicini \textit{Nature Photonics} 6, 672 (2012)\\ $[3]$ H-V.Nguyen, T.A.Pham, D.Rocca and G.Galli \textit{Phys. Rev. B} 85, 081101(R) (2012)   

Complementary transport channels in Si-ZnS nanocomposites: first principles simulations

In solar energy conversion devices, nanoparticles (NPs) are often embedded in solid matrices, either crystalline or amorphous. At present a detailed understanding of the influence exerted by the embedding matrix on the absorption of sunlight by the nanoparticle, and the role of the nanoparticle-matrix interface remain elusive. We investigated Si NPs embedded in ZnS, a system that was used as a charge transport layer in recent experiments. A realistic model of the NP-matrix interface was created from ab-initio molecular dynamics simulations. We found that this nanocomposite exhibits complementary transport channels, where electron transport occurs by hopping between NPs and hole transport through the ZnS-matrix. In analogy to Si NPs embedded in SiO2 [1] we found a strong gap reduction and corresponding red-shifted optical absorption, caused by chemical shifts at the NP-matrix interface. \\[4pt] [1] T. Li, F. Gygi, G. Galli, Phys. Rev. Lett. 107, 206805 (2011)   

Quantum Monte Carlo Characterization of Excited States and Energy-Level Alignment of Oligomer/Quantum-Dot Interfaces

Charge separation of excitons in materials is one of the most important physical processes to utilize the solar energy in diverse devices including solar cells and photo-catalysts. Heterogeneous interfaces with the so-called type-II character are often employed to infer the interfacial charge transfer in this context. As a simple criterion for designing such an interface, the energy alignment of the quasi-particle states together with the exciton binding energy of electron-donating materials is often discussed in the literature. However, an accurate description of the effect of exciton binding at the interface has not been investigated extensively. Although density functional theory (DFT) is a powerful method to investigate various electronic properties of materials, incomplete description of many-body interactions can lead to an incorrect interpretation of the energy level alignment. While Many-Body Perturbation Theory and Quantum Monte Carlo are promising in this context, much more work is necessary to assess how well these methods perform in practice. In this talk, we will discuss our preliminary results using diffusion Quantum Monte Carlo to calculate the excited states and energy-level alignment at an Oligomer/Quantum-Dot interface -- a system that is often discussed in context of solar energy conversion.   

Binding mechanism of CdSe quantum dots to carbon nanotubes/graphene

Decorating carbon nanotube or graphene with CdSe quantum dots (QDs) is one approach to creating next generation high efficiency photovoltaics. We have used first principles methods to calculate the binding mechanisms of oleic acid (OA) to CdSe QDs as well as how -COOH functional groups can link the QD to graphene. In both cases, the strongest binding involves the terminating double-bonded oxygen atom in the -COOH group covalently bonding to a surface Cd atom while the hydrogen (from the OH part of the -COOH) aligns to make a weak hydrogen-like bond to a neighboring surface Se. We find a strong defect enhanced binding of the QD to graphene via -COOH: when the -COOH links the QD to a defect site on the graphene, the binding energy of the complex is ~ 0.5 eV larger than when a -COOH links the QD to a pristine graphene region. These results are consistent with available edge X-ray absorption fine structure (EXAFS) data and also rationalize the growth procedure by which ultrasonication of the OA functionalized QDs leads to the replacement of some QD-OA bonds by QD-COOH-graphene bonds, which strongly link the QDs to the graphene surface.   

ZnO Transistor Interfaces Sensitized with Photo Donor Molecules

A better understanding of the physics at interfaces between semiconducting oxides and monolayers of covalently bonded organic molecules is relevant to important applications such as inexpensive chemical sensors and improved dye-sensitized solar cells. We use field-effect transistor (FET) structures in which electrical measurements are made before and after functionalizing the surface of ZnO nanocrystalline films, which form the channel of the FET, with organic dye molecules based on rhenium-bipyridine complexes that act as electron donors during illumination with monochromatic light. Measurements of the charge transfer as a function of light intensity and dye coverage give the ratio between the rates of charge transfer and recombination between the dyes and the ZnO, an important parameter to maximize to further improve the efficiency of solar cells based on donor functionalized oxides.   

Session W39: Computational Fluid Dynamics

Transient Non-Newtonian Screw Flow

The influence of axial flow on the transient response of the pseudoplastic rotating flow is carried out. The fluid is assumed to follow the Carreau-Bird model and mixed boundary conditions are imposed. The four-dimensional low-order dynamical system, resulted from Galerkin projection of the conservation of mass and momentum equations, includes additional nonlinear terms in the velocity components originated from the shear-dependent viscosity. In absence of axial flow the base flow loses its radial flow stability to the vortex structure at a lower critical Taylor number, as the pseudoplasticity increases. The emergence of the vortices corresponds to the onset of a supercritical bifurcation which is also seen in the flow of a linear fluid. However, unlike the Newtonian case, pseudoplastic Taylor vortices lose their stability as the Taylor number reaches a second critical number corresponding to the onset of a Hopf bifurcation. Existence of an axial flow, manifested by a pressure gradient appears to further advance each critical point on the bifurcation diagram. In addition to the simulation of spiral flow, the proposed formulation allows the axial flow to be independent of the main rotating flow. Complete transient flow field together with viscosity maps are also presented.   

Indeterminism in Classical Dynamics of Particle Motion

We show that ``God plays dice'' not only in quantum mechanics but also in the classical dynamics of particles advected by turbulent fluids. With a fixed deterministic flow velocity and an exactly known initial position, the particle motion is nevertheless completely unpredictable! In analogy with spontaneous magnetization in ferromagnets which persists as external field is taken to zero, the particle trajectories in turbulent flow remain random as external noise vanishes. The necessary ingredient is a rough advecting field with a power-law energy spectrum extending to smaller scales as noise is taken to zero. The physical mechanism of ``spontaneous stochasticity'' is the explosive dispersion of particle pairs proposed by L. F. Richardson in 1926, so the phenomenon should be observable in laboratory and natural turbulent flows. We present here the first empirical corroboration of these effects in high Reynolds-number numerical simulations of hydrodynamic and magnetohydrodynamic fluid turbulence. Since power-law spectra are seen in many other systems in condensed matter, geophysics and astrophysics, the phenomenon should occur rather widely. Fast reconnection in solar flares and other astrophysical systems can be explained by spontaneous stochasticity of magnetic field-line motion   

Higher Order Thermal Lattice Boltzmann Model

Lattice Boltzmann method (LBM) modelling of thermal flows, compressible and micro flows requires an accurate velocity space discretization. The sub optimality of Gauss-Hermite quadrature in this regard is well known [1]. Most of the thermal LBM in the past have suffered from instability due to lack of proper H-theorem and accuracy [2]. Motivated from these issues, the present work develops along the two works [3] and [4] and imposes an eighth higher order moment to get correct thermal physics. We show that this can be done by adding just 6 more velocities to D3Q27 model and obtain a ``multi-speed on lattice thermal LBM'' with 33 velocities in 3D and ${\cal{O}}(u^4)$ and ${\cal{O}}(T^4)$ accurate $f_{i}^{\rm eq}$ with a consistent H-theorem and inherent numerical stability. Simulations for Rayleigh-Bernard as well as velocity and temperature slip in micro flows matches with analytical results. Lid driven cavity set up for grid convergence is studied. Finally, a novel data structure is developed for HPC.\\[4pt] [1] X. Shan and X. He, Phys. Rev. Lett. 80, 65 (1998).\\[0pt] [2] G. McNamara, A. Garcia, and B. Alder, J. Stat. Phys. 81, 395 (1995).\\[0pt] [3] S. Chikatamarla and I. Karlin, Phys. Rev. E 79, 046701 (2009).\\[0pt] [4] W. Yudistiawan et al. Phys. Rev. E 82, 046701 (2010)   

ODTLES: Simulations of wall-bounded turbulent flows with small-scale resolution

The numerical simulation of turbulent flows is difficult because of their broad range of scales of motion and because they include a large variety of small-scale processes, such as friction near a wall, diffusion at an interface, multiphase couplings, and chemical reactions. Traditional approaches to model these flows are limited in breadth and accuracy because they filter out information from small-scale processes. An alternative method that circumvents this problem is ODTLES. This method resolves, not models, small-scale phenomena in a computationally affordable way, in comparison with full three-dimensional resolution, through the use of a lattice-work of one-dimensional (1D) domains, where flow properties are time-advanced with 1D stochastic simulations. This talk will discuss the methodology behind ODTLES and results for incompressible wall-bounded turbulence.   

Shock Formation and Disintegration in Fluids with Non-Convex Equations of State

We consider the steady, two-dimensional, inviscid, high-speed, flow around thin turbine blade profiles with special attention given to fluids having a non-convex equation of state; such fluids are commonly known as Bethe-Zel'dovich-Thompson (BZT) fluids. We show that the essential flow physics can be described by an inviscid Burgers equation having quartic nonlinearity rather than the quadratic nonlinearity of perfect gases. In order to illustrate the flow behavior, a fifth-order WENO (weighted essentially non-oscillatory) numerical scheme is employed. New results of interest include the formation of oblique expansion shocks, shock-splitting induced by the interaction of a single shock with Mach waves, the capture of shock-fan combinations, and the collision of oblique compression and expansion shocks.   

Polarized Turbulence on the 3-sphere

We have simulated He II superfluid turbulence on a 3-sphere, using the Hopf vector field $(-y,x,-w,z)$ as the driving velocity. This vector field lies along parallel great circles of the 3-sphere. It has a uniform magnitude, is divergence-free, and is analogous to a uniform driving velocity in periodic boundaries (a flat 3-torus), with the important exception that it has a non-zero curl tangent to the field itself. The resultant system is an interesting modification of rotating counterflow turbulence, which produces a state of polarized turbulence for driving velocities above a critical velocity $V_{DG}$. The average polarization of the vortex tangent field on the 3-sphere is 0.8-0.95, significantly higher than rotating counterflow. We also found a vortex reconnection rate proportional to $L^{1.6}$, in contrast to homogeneous turbulence, which yields exponents of 5/2 or 2, depending on the importance of the local velocity term and on the turbulence state. A reduced exponent is consistent with predictions and previous simulations of polarized turbulence, but the degree of reduction is remarkable. Development of this polarized turbulence state is still under investigation.   

A Spectral Adaptive Mesh Refinement Method for the Burgers equation

Adaptive mesh refinement (AMR) is a powerful technique in computational fluid dynamics (CFD). Many CFD problems have a wide range of scales which vary with time and space. In order to resolve all the scales numerically, high grid resolutions are required. The smaller the scales the higher the resolutions should be. However, small scales are usually formed in a small portion of the domain or in a special period of time. AMR is an efficient method to solve these types of problems, allowing high grid resolutions where and when they are needed and minimizing memory and CPU time. Here we formulate a spectral version of AMR in order to accelerate simulations of a 1D model for isotropic homogenous turbulence, the Burgers equation, as a first test of this method. Using pseudo spectral methods, we applied AMR in Fourier space. The spectral AMR (SAMR) method we present here is applied to the Burgers equation and the results are compared with the results obtained using standard solution methods performed using a fine mesh.   

Multiscale simulation of electroosmotic flows

We develop an efficient hybrid multiscale method for simulating nano-scale electroosmotic flow based on spatial ``domain decomposition'' [1]. Molecular dynamics (MD) is used in the near wall region where atomistic details are important. A multigrid Particle-Particle Particle-Mesh (PPPM) method [2] is used to calculate the long-range Coulombic interaction between charged ions. Continuum (incompressible Navier-Stokes) equations for the solvent are solved in the bulk region, reducing the computational cost substantially. A discrete description of ions is retained in the continuum region because of the low density of ions and the long-range of electrostatic interactions. Langevin dynamics is used to model the Brownian motion of these ions in the implicit solvent. The fully atomistic and continuum descriptions are coupled through ``constrained dynamics'' [1] in an overlap region. Continuity of flux of both charged and solvent particles is ensured. The scheme is implemented in channel flow simulations with and without wall roughness. Results are compared with pure MD simulations. \\[4pt] [1] X. Nie, S. Chen, W. E, and M. O. Robbins, J. Fluid Mech., 500:55-64, 2004.\\[0pt] [2] J. Liu, M. Wang, S. Chen, and M. O. Robbins, J. Comput. Phys., 229:7834-7847, 2010.   

Chaos Synchronization in Navier-Stokes Turbulence

Chaos synchronization (CS) has been studied for some time now (Pecora \& Carroll 1990), for systems with only a few degrees of freedom as well as for systems described by partial differential equations (Boccaletti et al 2002). CS in general is said to be present in coupled dynamical systems when a specific property of each system has the same time evolution for all, even though the evolution itself is chaotic. The Navier-Stokes (NS) equations describe the velocity for a wide range of fluids, and their solutions are usually called turbulent if fluctuation amplitudes decrease as a power of their wavenumber. There have been some studies of CS for continuous systems (Kocarev et al 1997), but CS for NS turbulence seems not to have been investigated so far. We focus on the synchronization of the small scales of a turbulent flow for which the time history of large scales is prescribed. Our DNS results show that high-wavenumbers in turbulence are fully slaved to modes with wavenumbers up to a critical fraction of the Kolmogorov dissipation wavenumber. The motivation for our work is to study deeply sub-Kolmogorov scales in fully developed turbulence (Schumacher 2007), which we found to be recoverable even at very high Reynolds number from simulations with moderate resolutions.   

Multiscale Modeling of Cavitating Bubbly Flows

Modeling of cavitating bubbly flows is challenging due to the wide range of characteristic lengths of the physics at play: from micrometers (e.g., bubble nuclei radius) to meters (e.g., propeller diameter or sheet cavity length). To address this, we present here a multiscale approach which integrates a Discrete Bubble Model for dispersed microbubbles and a level set N-S solver for macro cavities, along with a mesoscale transition model to bridge the two. This approach was implemented in 3DYNAFS$^{\copyright}$ and used to simulate sheet-to-cloud cavitation over a hydrofoil. The hybrid model captures well the full cavitation process starting from free field nuclei and nucleation from solid surfaces. In low pressure region of the foil small nuclei are seen to grow large and eventually merge to form a large scale sheet cavity. A reentrant jet forms under the cavity, travels upstream, and breaks it, resulting in a bubble cloud of a large amount of microbubbles as the broken pockets shrink and travel downstream. This is in good agreement with experimental observations based of sheet lengths and frequency of lift force oscillation.   

Surface cooling mechanism of fire suppression by aqueous foam

We investigate the ability of room-temperature foam to directly cool the surface of a liquid fuel pool at burning conditions and to reduce the fuel vapor pressure. We solve an unsteady, one-dimensional heat conduction equation using the finite element method to predict the temperature within an aqueous foam layer above a liquid fuel (heptane) layer. The sharp gradients in temperature and thermal properties at the foam-fuel interface are treated approximately inside of a thin interfacial layer above the fuel surface. We predict a rapid, significant reduction in the fuel surface temperature due to the large initial temperature gradient and the foam thermal diffusivity. The predicted surface cooling leads to a significant decrease in the fuel vapor pressure in less than a second. The mechanisms of fire suppression by aqueous foams are not well understood and the model predictions show that direct surface cooling could provide an important contribution to fire suppression. Experiments are in progress to quantify the surface cooling effect on heptane pool fire suppression.   

Formation of Kinneyia via shear-induced instabilities in microbial mats

Kinneyia are a class of microbially mediated sedimentary fossils. Characterised by clearly defined ripple structures, Kinneyia are generally found in areas that were formally littoral habitats and covered by microbial mats. To date there has been no conclusive explanation as to the processes involved in the formation of these fossils. Microbial mats behave like viscoelastic fluids. We propose that the key mechanism involved in the formation of Kinneyia is a Kelvin-Helmholtz instability induced in a viscoelastic film under flowing water. A ripple corrugation is spontaneously induced in the film and grows in amplitude over time. Theoretical predictions show that the ripple instability has a wavelength proportional to the thickness of the film. Experiments carried out using viscoelastic films confirm this prediction. The ripple pattern that forms has a wavelength roughly three times the thickness of the film. This behaviour is independent of the viscosity of the film and the flow conditions. Well-ordered patterns form, with both honeycomb-like and parallel ridges being observed, depending on the flow speed. These patterns correspond well with those found in Kinneyia fossils, with similar morphologies, wavelengths and amplitudes being observed.   

Optimal Concentrations in Transport Networks

Biological and man-made systems rely on effective transport networks for distribution of material and energy. Mass flow in these networks is determined by the flow rate and the concentration of material. While the most concentrated solution offers the greatest potential for mass flow, impedance grows with concentration and thus makes it the most difficult to transport. The concentration at which mass flow is optimal depends on specific physical and physiological properties of the system. We derive a simple model which is able to predict optimal concentrations observed in blood flows, sugar transport in plants, and nectar feeding animals. Our model predicts that the viscosity at the optimal concentration $\mu_{\mathrm{opt}}=2^{n}\mu_0$ is an integer power of two times the viscosity of the pure carrier medium $\mu_0$. We show how the observed powers $1\leq n\leq 6$ agree well with theory and discuss how $n$ depends on biological constraints imposed on the transport process. The model provides a universal framework for studying flows impeded by concentration and provides hints of how to optimize engineered flow systems, such as congestion in traffic flows.   

Resonating Vector Strength: How to Find Periodicity in a Time Sequence

For a given periodic stimulus with angular frequency $\omega_{\circ} = 2\pi/T_{\circ}$ we find responses as events at times $\{t_{1}, t_{2},\ldots, t_{n} \}$ located on the real axis $R$. How periodic are they? And do they repeat in ``some'' sense in accordance with the stimulus period $T_{\circ}$? The question and the answer are at least as old as a classical paper of von Mises dating back to 1918. The key idea is simply this. We map the events $t_{j}$ onto the unit circle or torus through $t_{j} \mapsto \exp (i \omega t_{j})$ and consider their center of gravity, $\rho(\omega)$, a complex number in the unit disk. Its absolute value $|\rho(\omega_{\circ})|$ with $\omega := \omega_{\circ}$ is what von Mises studied and is now called the vector strength. We prove that the nearer $|\rho(\omega_{\circ})|$ is to $1$ the more periodic the events $t_{j}$ are w.r.t. $T_{\circ}$. Furthermore, we also show why it is useful to study $\rho(\omega)$ as a function of $\omega$ so as to obtain a `resonating' vector strength, an idea strongly deviating from the classical characteristic function.   

Session W40: Quantum Information in AMO Physics

Nonlinear Optics Quantum Computing and Quantum Simulation with Circuit-QED

One approach to quantum information processing is to use photons as quantum bits and rely on linear optical elements for most operations. However, some optical nonlinearity is necessary to enable universal quantum computing. Here, we suggest a circuit-QED approach to photon-based quantum processors and quantum simulators in the microwave regime, including a deterministic two-photon interaction. Our specific example uses a hybrid quantum system comprising a LC resonator coupled to a superconducting flux qubit to implement a nonlinear coupling. Compared to the self-Kerr nonlinearity, we find that our approach has improved tolerance to noise in the qubit while maintaining fast operation. We also envision using a similar resonator and fluxonium qubit system to create higher order photon nonlinearities, which is a generalization of effective two-photon interactions and opens the range of potential Hamiltonians that can be efficiently simulated.   

Two Electromagnetically Induced Transparency Windows and Cross-Phase Modulation with Four-Level Superconducting Artificial Atoms

Superconducting circuit quantum electrodynamics (SCQED) employs microwave transmission lines coupled to artificial atoms, which are typical two-level and recently three-level for electromagnetically induced transparency (EIT). We propose SCQED with a four-level tripod-configuration artificial atom to enable cross-phase modulation between two traveling-wave microwave fields. Our master-equation analysis for three driving fields (``signal,'' ``probe'' and ``coupling'') demonstrates the existence of two distinct EIT transparency windows in the spectral-response profile as a function of coupling and weak fields strength. We provide the first theoretical analysis of this unexpected second window and show its advantages over the known first EIT window. Specifically we show that this second EIT window provides both the signal and probe fields with identical response functions provided that their Rabi frequencies and detunings are the same. Exploiting the second window with judiciously chosen external flux and energy detuning result in low absorption, excellent group velocity matching, and high nonlinearity, thereby enabling strong cross-phase modulation for SCQED.   

Nonlocal Interferometry Using Macroscopic Coherent States and Weak Nonlinearities

Bell's inequality has been violated numerous times in microscopic systems with the use of nonlocal interferometry. Described here will be an extension of the Franson interferometer to the macroscopic case of coherent states entangled in phase. The entanglement is generated using weak nonlinearities, and the entanglement is probed using single photons and homodyne detection. Without loss the predicted nonlocal interference visibility of the interferometer is unity, and the inclusion of atomic absorption allows for a large number of photons to be absorbed with only a small reduction in the visibility. This interferometer can be extended in a straightforward manner to a quantum key distribution scheme using the Ekert protocol to insure security. A method for the extension of the entanglement distance using entanglement swapping is described. This nonlocal interferometer may therefore be of practical use in quantum communications in addition to being of fundamental interest.   

Efficacy of weak measurement reversal for stochastic amplitude damping

A recent experiment demonstrated the restoration of entanglement in a photonic system using weak measurement reversal [S. Kim et al., Nature Physics 8, 117 (2012)]. Here, we analyze the statistical properties of entanglement for pairs and triples of entangled qubits subject to stochastic amplitude damping followed by restoration. After the random disturbance, the state is restored by applying a static weak measurement reversal. We then show that the fidelity of the restored state, and therefore its entanglement, can be restored with high success, despite the statistical fluctuations of the disturbance. In particular, we show that the variance of the entanglement of the restored states is substantially reduced, independent of the strength of the disturbance. We conclude with a proposed experimental implementation.   

Nonlinear waveguide arrays and disorder

Waveguide arrays with quadratic nonlinearity has been studied recently. We investigated the waveguide arrays with quadratic nonlinearity and explored the possibility of generating broadband continuous-variable entanglement in such structures. We propose an integrated approach toward continuous-variable entanglement based on integrated waveguide quantum circuits, which are compact and relatively more stable. We further continued our work on waveguide arrays by studying a hybrid system which contains a combination of linear and nonlinear waveguides. We assume that all the waveguides except the central one are assumed to be linear. The central waveguide is assumed to have $\chi^{(2)}$ nonlinearity. We assume that the central waveguide is pumped through a coherent light. The coupling between the waveguide is achieved by the evanescent overlap of the guided modes. For all the other waveguides in the array the light propagates in the linear regime. We also study the effect of disorder which can be introduced by varying the distance between the waveguides. We are particularly interested in investigating the effect of disorder and quadratic non-linearity in the waveguide array system.   

Squeezing of Spin Waves in a Three-Dimensional Atomic Ensemble

Spin squeezed states (SSS) have generated considerable interest for their potential applications in quantum metrology and quantum information processing. Many protocols for generating SSS in atomic gases rely on the Faraday interaction that creates entanglement between atoms through the coupling of the collective spin of the ensemble to polarization modes of an optical field. Most descriptions of this process rely on an idealized one-dimensional plane wave model of light-matter interactions that is not appropriate for describing a real system consisting of a cigar-shaped cold atomic cloud in dipole trap interacting with a probe laser beam. We provide a first principles three-dimensional model of squeezing via a quantum nondemolition measurement of the collective magnetization for an ensemble of atoms with hyperfine spin $f$. The model includes spin waves, diffraction, paraxial modes, and optical pumping, derived by a full master equation description. Including dissipative dynamics, we find the optimal ensemble geometry and input Gaussian beam parameters for generating spin squeezing. We also study the effect of enhancing the atom-light interface using internal hyperfine control of atoms with large spin $f$.   

Selection of semiconductor quantum dots for multi-qubit encoding using an optical microcavity

Controlling the quantum states of excitons or spin-polarized carriers in semiconductor quantum dots (QDs) has been the focus of a considerable research effort in recent years due to the promise of using this approach to develop a solid state quantum computing architecture. In such experiments, the need to isolate the optical response of a single QD represents a formidable challenge, one that is greatest for QDs with emission wavelengths compatible with existing telecommunications infrastructure due to the lower quantum efficiency of the associated detectors. Encoding qubits in ensembles of QDs would greatly facilitate quantum state readout due to the larger optical signals involved, however the spread of optical transition energies limits the fidelity of the control process. Here we report time-resolved differential transmission experiments on QDs in a dielectric Bragg stack optical microcavity. Our results indicate that the angle dependent transmission resonance of the cavity allows for the separate excitation and detection of distinct subsets of QDs in the ensemble differentiated by their optical transition energies. These findings demonstrate the feasibility of developing a scalable computing architecture based on multi-qubit encoding using semiconductor QDs.   

Quantum plasmonics of a metal nanoparticle array for on-chip nanophotonic network

With the advancement of nanofabrication techniques, metallic nanoparticles have been attracting significant attention due to their novel capabilities offering the prospects of miniaturization, scalability, and strong coherent coupling to single-emitters that conventional photonics cannot achieve. In this work, we investigate an array of metal nanoparticles for on-chip quantum networking, quantum computation and communication on scales far below the diffraction limit. For this purpose, we first consider the transfer of quantum states, including single qubits as plasmonic wave packets, and explore the interference of single plasmons associated with the quantum properties of the plasmon excitation. In addition, we study dipole induced reflection effects in the plasmonic setting. The results seem promising for quantum control applications such as single-photon switching and slow light in the nanoscale. We also propose a scheme of entanglement generation between distant emitters embedded in the array of metal nanoparticles. The techniques introduced in this work may assist in the further theoretical and experimental studies of plasmonic nanostructures for quantum control applications and probing nanoscale optical phenomena.   

Dynamic Hole Trapping Effect in an InAs/AlGaAs quantum dot molecule

It is well established that the charge and spin configurations of single electrons or holes are promising candidates for next generation computational and logic devices. Quantum Dots Molecules (QDMs) are attractive components for confining and manipulating single charges because the discrete energy levels, charge interactions and spin properties can be tailored with size and composition. The strong confinement QDMs causes overlap of wavefunctions and results in different Coulomb interactions and unique energy levels for different numbers of charges and even for distinct spatial distributions of the same total charge. Quantitative measurements of the Coulomb interactions are important in order to understand charge and spin interactions and design structures for device applications. We present a new phenomenon discovered during optical spectroscopy of a QDM with an AlGaAs barrier between two QDs. AlGaAs barrier allows an extra hole to be trapped in a metastable state of the higher energy QD due to the higher barrier potential. This ?dynamic trapped hole? occurs only under certain electric field conditions and perturbs the Coulomb interactions of the other charges present in the QDM. We propose a model of the kinetic pathways that leads to this dynamic hole trapping effect. We compare the energy of states with and without the extra hole in order to understand many body Coulomb interactions that perturb states energies. We then discuss the challenges and opportunities this effect provides for future devices.   

Ideal Multipole Ion Traps from Planar Ring Electrodes

We present designs for multipole ion traps based on a set of planar, annular, concentric electrodes which require only rf potentials to confine ions. We illustrate the desirable properties of the traps by considering a few simple cases of confined ions. We predict that mm-scale surface traps may have trap depths as high as tens of electron volts, or micromotion amplitudes in a 2-D ion crystal as low as tens of nanometers, given realistic experimental parameters. We also discuss applications to quantum information science, frequency metrology, and cold ion-atom collisions.   

Resolved sideband spectra of calcium ions in a Penning trap

I report on recent work at Imperial College London, with laser cooled calcium-40 ion Coulomb crystals in Penning traps. Penning traps provide a number of advantages over the more common radiofrequency (RF) trap; namely the ability to trap 3-dimensional, micromotion-free ion Coulomb crystals, and the ability to produce deep traps while maintaining a large ion-electrode surface distance. While these factors should permit lower heating rates than in typical RF traps, very little research has been conducted into the behavior and control of small Coulomb crystals in Penning traps due to the experimental challenges involved. We have spent several years developing techniques to overcome these obstacles, and are now making rapid progress towards the sub-Doppler cooling and coherent control of small ion crystals. We have already observed high resolution optical spectra showing sidebands due to radial and axial motions, giving estimated temperatures close to the Doppler limit.   

Rapid ion cooling by controlled collision

I propose a method to cool trapped ions by controlled collisions. Motional excitation of a hot ion is transferred to a coolant ion due to Coulomb interaction when they are brought to proximity. The whole process can be conducted diabatically, involving only a few oscillation periods of the harmonic trap. Our proposal is useful for rapid recooling of ion qubits during quantum computation and fast cooling of an ion whose mass is significantly different from the coolant ion.   

Session W41: Bose Gauge Fields

3D quaternionic condensation and spin textures with Hopf invariants from synthetic spin-orbit coupling

We study unconventional condensations of two-component bosons in a harmonic trap subject to the 3D $\vec{\sigma}\cdot \vec{p}$-type spin-orbit (SO) coupling. The topology of condensate wavefunctions manifests in the quaternionic representation. The spatial distributions of the $S^3$ quaternionic phase exhibit 3D skyrmion configurations, while those of the $S^2$ spin orientation possess non-zero Hopf invariants. As increasing SO coupling strength, spin textures evolve from concentric distributions to lattice structures at weak interactions. Strong interactions change condensates into spin-polarized plane-wave states, or, superpositions of two plane-waves exhibiting helical spin spirals.   

Rashba Spin-Orbit Coupled Bose-Einstein Condensates with Magnetic Dipole-Dipole Interactions

In this talk we consider the effect of Rashba spin-orbit coupling on a quasi-two dimensional Bose-Einstein Condensate with dipolar interactions. The interplay of the spin-orbit coupling, which favors textured and vortex-antivortex lattice ground states, and the dipole-dipole interaction, which introduces non-local spin-exchange processes and a strongly geometry-dependent interaction character, leads to a variety of novel ground states including combinations of spin and purely motional vortices. With the assistance of a numerical Bogoliubov-de Gennes analysis, we map the relevant phase boundaries, thereby characterizing the rich ground-state phase diagram of this system.   

Exotic Quantum States of Rashba Bosons

The recently discovered spin-orbit coupled boson systems are remarkable for their capacity to explore physics that may not be revealed in any other way. The spin-orbit couplings, which can be artificially engineered in cold-atom experiments, in many instances lead to single-particle dispersion relations exhibiting multiple minima or even degenerate manifold of minimal energy states. It is entirely the effect of collisions (i.e. boson-boson interactions) which lifts this degeneracy and leads to an amazing variety of completely new quantum many-body states. This talk describes a theoretical discovery of a novel phase of matter that realizes for Rashba spin-orbit coupled bosons, where, at low densities, bosons essentially redress themselves and behave as fermions. This state is a composite fermion state with a Chern-Simons gauge field and filling factor one.   

Exotic Quantum Spin Models in Spin-Orbit-Coupled Mott Insulators

We study cold atoms in an optical lattice with synthetic spin-orbit coupling in the Mott-insulator regime. We calculate the parameters of the corresponding tight-binding model using Peierls substitution and ``localized Wannier states method'' and derive the low-energy spin Hamiltonian for bosons and fermions. The spin Hamiltonian is a combination of Heisenberg model, quantum compass model and Dzyaloshinskii-Moriya interactions and it has a rich classical phase diagram with collinear, spiral and vortex phases. We discuss the state of the art of experiments to realize and detect magnetic orderings in strongly correlated optical lattices.   

The Fate of Bose-Einstein Condensate in the Presence of Spin-orbit Coupling

We show that spin-orbit coupling can destroy a Bose-Einstein condensate. For non-interacting bosons, some types of spin-orbit coupling destroy a condensate at any finite temperature or even at the ground state, due to the drastic change of single-particle Density of States at low energies. Whereas interaction stabilizes the condensate at zero temperature, condensate depletion is significantly enhanced by spin-orbit coupling. Particularly, thermal depletion becomes divergent when both interaction and spin-orbit coupling become isotropic, leading to the disappearance of a three-dimensional condensate at any finite temperature.   

Emergence of Topological and Strongly Correlated Ground States in Rashba Spin-Orbit Coupled Bose Gases

We theoretically study an interacting few-body system of two-component Bose gases with isotropic Rashba spin-orbit coupling in a 2D isotropic harmonic trap. We show that the Hamiltonian is gauge-equivalent to particles subject to a pure non-abelian vector potential preserving time-reversal symmetry. We use Exact Diagonalization scheme to obtain the low-energy states of the system with large Rashba spin-orbit coupling strength for a range of interatomic interaction strengths. At small particle numbers, we observe that the bosons condense to an array of topological ground states that have $n+1/2$ -quantum angular momentum vortex configuration, with $n = 0, 1, 2, 3$. At relatively large particle numbers, we observe two distinct regimes: (a) at weak interaction strengths (mean-field regime), we observe ground states with topological and symmetry properties that are also obtained via mean-field theory computations. (b) at intermediate to strong interaction strengths (beyond mean-field regime), we report the emergence of strongly correlated ground states. We analyze ground state properties using various techniques: energy spectrum, density distribution, pair-correlation function, conditional wavefunction, entanglement spectrum, and entanglement entropy.   

Many-body ground states for bosons with Rashba spin-orbit coupling

The ground state of $N$ non-interacting bosons with a Rashba dispersion is macroscopically degenerate. It is of fundamental interest---and also relevant to current experiments in cold atomic gases with synthetic spin-orbit coupling---to determine whether a unique ground state is stabilized by interactions and what the properties of such a state might be. Motivated by exact solutions for the two-body problem, we construct many-body bosonic wave functions that saturate the kinetic energy and minimize the interaction energy, and compare with other recently proposed trial ground states.   

Flat-band engineering of interactions in spin-orbit coupled optical lattices

The recent experimental realization of spin-orbit coupled ultra cold atomic gases established a new platform to investigate many-body states of matter. In this talk we show that for such a system in optical lattices we can tune the spin-orbit coupling to achieve a flat energy band. We then model this system with a tight-binding Hamiltonian and further project the Hamiltonian to the Hilbert subspace of the lowest flat band. We will also discuss the important effect of interactions in such a projected flat-band system.   

Bosons on the Kagome lattice with artificial gauge fields

We investigate bosons on the Kagome lattice subject to artificial gauge fields such that no net flux is applied on a unit cell [1]. This allows for example the existence of quantized and non-quantized anomalous Hall effects on the Kagome lattice [2]. If two layers or two-component bosons are introduced, the topological phase is robust to inter-species interactions of moderate strength. We study the conditions under which the total density degree of freedom undergoes a Mott transition, while the pseudo-spin, or charge difference between layers, is in a superfluid phase with topological properties. Similar results can be obtained for two-component bosons on the honeycomb lattice. Such systems could work as a template for the realization of interacting topological phases in cold atom or cavity QED systems. \\[4pt] [1] Jens Koch, Andrew A. Houck, Karyn Le Hur, and S. M. Girvin, Phys. Rev. A 82, 043811 (2010).\\[0pt] [2] Alexandru Petrescu, Andrew A. Houck and Karyn Le Hur, Phys. Rev. A 86, 053804 (2012).   

A spin Hall effect in a quantum gas

The spin Hall effect is a phenomenom that couples spin current to particle current via spin-orbit coupling. The effect may be used to develop useful devices for spintronics, which may have advantages over corresponding conventional electronic devices. In addition, the spin-Hall effect is intimately related to certain types of topological insulators. Spin-orbit coupling in an ultracold bosonic sample of $^{87}$Rb has been demonstrated. We now use this spin-orbit coupling to produce a spin Hall effect in a bosonic sample, the first demonstration of the effect in an ultracold atom system.   

Experiments on BECs with Synthetic Gauge Fields and Spin Orbit Coupling

We report experiments on $^{87}$Rb BECs subject to synthetic gauge fields and spin orbit interactions created by optical Raman fields that couple different hyperfine spin and momentum states. We have reproduced several recently shown results of the effects of such synthetic gauge potentials by characterizing the quasimomentum of the dressed states. We have also observed a spin Hall-like effect on our BECs in a spatially inhomogeneous synthetic spin orbit coupling. We create BECs with equal populations of $|F=1,m_F=-1>$ and $|F=1,m_F=0>$, representing a pseudo spin 1/2 system, and launch them into a common mode oscillation within an optical dipole trap. When an inhomogeneous spin orbit coupling Raman field is applied, they exhibit an anticorrelated transverse oscillation, manifesting in cyclotron motions of opposite chirality. Measurements of such a spin dependent transport versus the intensity and detuning of the Raman coupling and versus the position of the BEC are also presented with discussions of possible interpretations.   

Measuring the Berry Curvature of Optical Lattices

New schemes propose how artificial gauge fields may be imprinted on ultracold atomic gases in optical lattices, allowing experiments to access strongly correlated phenomena. In particular, fractional quantum Hall physics may be explored in systems where the lowest energy band resembles a Landau level, as in the proposed ``optical flux lattices". These energy bands have a nonzero Chern number and are topologically nontrivial. The physical properties of such a band are encoded not only in its energy spectrum over the Brillouin zone (the ``bandstructure" in the usual sense) but also importantly, in its Berry curvature. When the Berry curvature is nonzero, it can have many important physical consequences; for example it can modify the semiclassical dynamics of a wave packet undergoing Bloch oscillations. We will explain how experimentalists may turn such physical consequences into new tools to determine the topological properties of a band. We will discuss how Berry curvature effects may be observed in ultracold gases and give examples in systems relevant to future experiments. [H. M. Price and N. R. Cooper, Phys. Rev. A 85, 033620 (2012) ]   

Controllable Transport of Ultra-Cold Atoms in 1D Optical Lattices with Uniform Peierls Substitution

We show that the recently developed optical lattices with Peierls substitution (PRL 108, 225303 and 225304 (2012)) -- which can be modeled as a lattice with a complex tunneling coefficient -- may be used to induce quantum transport of ultra-cold atoms. In particular, we show that by ramping up the phase of the complex tunneling coefficient in a spatially uniform fashion, a finite quasi steady-state current (QSSC) ensues from the exact dynamics of non-interacting fermions. The direction and magnitude of the current can be controlled by the overall phase difference but not the details of the ramp. The entanglement entropy does not increase when the QSSC lasts. Due to different spin statistics, condensed non-interacting bosons do not support a finite QSSC under the same setup. We also find that an approximate form of the QSSC survives when perturbative effects from interactions, weak harmonic background traps, and temperature are present, which suggests that our findings should be observable with available experimental capabilities. Our study could be useful in developing novel devices in the thriving field of atomtronics.   

Phase Transitions and Collective Modes in Spin-Orbit Coupled Bose-Einstein Condensates

Recent experiments on trapped bosonic atomic gases interacting with Raman lasers have realized an artificial spin-orbit coupling (SOC) among two dressed spin states of bosons. The phase diagram of this system, as a function of the interaction parameters, strength of SOC, and the densities of the two species of bosons, possesses regimes of mixed superfluid (featuring two interpenetrating dressed-state condensates), and phase separation (between regions of single dressed-state condensate). We present our results on the Bogoliubov sound velocity in the mixed phase, and propose that it can be used as a probe of the spatially-varying density (i.e. stripe order) of the mixed phase as well as of the phase transition to the phase separation regime. The effects of the trapping potential are also discussed.   

Phase-modulated superfluids of bosons in spin-orbit coupled optical lattice

We study the phase diagram of spin-orbit coupled ultra-cold bosons in a square lattice using the Gutzwiller method. In the superfluid regime, we show that the interplay between spin independent and spin-dependent tunnelings may give rise to a few different types of phase-modulated superfluids. The transitions between different superfluids are found to be the first-order. We investigate the rich~periodic structure of the phases of the superfluids, which may be directly probed using the spin structure factor. Different types of superfluids may also possess different excitation spectra.   

Session W42: Focus Session: Supercooled and Nanoconfined Water IV

Freezing of supercooled water nanodroplets

All three states of water play important roles in nature, from thermostating the atmosphere to providing reactive surfaces environments. The rates at which transitions between the phases occur, the degree to which pure liquid water can be supercooled, and the solid phases that form are all fundamentally interesting questions with strong atmospheric relevance. We have followed and characterized the nucleation, growth, and subsequent freezing of pure water droplets formed in a supersonic nozzle apparatus using both Small Angle X-ray Scattering (SAXS) and Fourier Transform Infrared Spectroscopy (FTIR). Because the droplets have radii $r$ between 3 nm and 6 nm, and the cooling rates are on the order of 5E5 K/s, liquid water only begins to freeze below approximately 215 K. These temperatures are well below the homogeneous freezing limit for bulk water. The experiments show the expected decrease in freezing temperature with decreasing droplet size, or alternatively, with increasing droplet internal pressure.   

Probing no man's land: ice nucleation at the nanoscale

Nucleation is a stochastic process. At a given thermodynamic condition, nucleation events occur at a frequency that scales with the volume of the system. Therefore at the nanoscale, {\em e.g.}, in nano droplets, one may expect to obtain supercooled liquids below the bulk homogeneous nucleation temperature. However it is not clear to what extent would nucleation in nano droplet be connected with bulk water. In this talk, I will discuss the insight gained from our recent molecular simulations on ice nucleation at nanoscale. In particular, the study provides direct computational evidence for size-dependent ice nucleation rate within supercooled water nano droplets. Using a thermodynamic model based on classical nucleation theory, I will show that it is the Laplace pressure induced by the curved liquid vapor interface present in droplets that is responsible for the suppression of ice crystallization. Consistent with this model, our simulations show that the nucleation rates found for droplets are similar to those of liquid water subject to a pressure of the order of the Laplace pressure within droplets. The findings thus provide a link between supercooled bulk water and nano droplet through ice nucleation rate. In addition, the findings also support the hypothesis of surface crystallization of ice in microscopic water droplets in clouds.   

Experimental Observation of Bulk Liquid Water Structure in ``No Man's Land''

Experiments on pure bulk water below about 235 K have so far been difficult: water crystallization occurs very rapidly below the homogeneous nucleation temperature of 232 K and above 160 K, leading to a ``no man's land'' devoid of experimental results regarding the structure. Here, we demonstrate a new, general experimental approach to study the structure of liquid states at supercooled conditions below their limit of homogeneous nucleation. We use femtosecond x-ray pulses generated by the LCLS x-ray laser to probe evaporatively cooled droplets of supercooled bulk water and find experimental evidence for the existence of metastable bulk liquid water down to temperatures of 223 K in the previously largely unexplored ``no man's land''.   

Glass softening, crystallization, and vaporization of nano-aggregates of Amorphous Solid Water: Fast Scanning Calorimetry studies

Despite intense efforts, complete understanding of relationships between various condensed phases of water remains an elusive goal. In particular, the molecular kinetics and phase transitions of water in confining geometries (e.g., nano-scale films) are of special interest due to the relevance to environmental and biological processes. With the objective of gaining insights into fundamental distinctions in physical and chemical properties of confined water, we have developed an experimental approach which relies on rapid (10$^5$~K/s) heating of nanoscale films of Amorphous Solid Water (ASW) prepared by vapor deposition in vacuum at cryogenic temperatures. With recent advances, the approach, Fast Scanning Calorimetry (FSC), facilitates studies of glass softening, crystallization, and vaporization of ASW films with thicknesses down to two nanometers. Unlike bulk samples, the thermograms of ultrathin ASW films show two endotherms at 40 and 10 K below the onset temperatures of crystallization. We will report the conclusion of our analysis of the FSC thermograms of nanoscale ASW aggregates, and discuss the implications of these studies for developing better models of molecular kinetics of water in confining geometries.   

Temperature dependence of the Oxygen-Oxygen separations in water from high energy x-ray diffraction

We have used state of the art, high energy x-ray diffraction to obtain detailed measurements of the Oxygen-Oxygen (O-O) pair distribution function (g(r)) of liquid water between -20 and 92 degrees Celsius. These measurements show ordinary linear behavior of the first O-O distance, over the full temperature range, even through the density maximum. Conversely we do see interesting, non-linear behavior in the O-O distribution at higher separations distances, particularly around the 4.5{\AA} peak. Another interesting feature of these measurements is the presence of a temperature-independent crossover point in the running O-O coordination number at the location of the first minimum in r$^{2}$[g(r)-1], which defines the end of the first shell. At this 3.4(1){\AA} distance the O-O coordination number is 4.5(2) at all the temperatures studied. We believe this work offers important insight into some of the unusual physical properties of water, and provides a valuable validation point for the many Molecular dynamics models of liquid water.   

The structure of ice crystallized from supercooled water

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. Traditionally ice was thought to exist in two well-crystalline forms: stable hexagonal ice and metastable cubic ice. It has recently been shown, using X-ray diffraction data, that ice which crystallizes homogeneously and heterogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I (ice I$_{sd})$. This result is consistent with a number of computational studies of the crystallization of water. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder, which raises the question of whether cubic ice exists. New data will be presented which shows significant stacking disorder (or stacking faults on the order of 1 in every 100 layers of ice I$_{h})$ in droplets which froze heterogeneously as warm as 257 K. The identification of stacking-disordered ice from heterogeneous ice nucleation supports the hypothesis that the structure of ice that initially crystallises from supercooled water is stacking-disordered ice I, independent of nucleation mechanism, but this ice can relax to the stable hexagonal phase subject to the kinetics of recrystallization. The formation and persistence of stacking disordered ice in the Earth's atmosphere will also be discussed.   

Investigation of water-graphite interaction using molecular beam technique.

We have investigated water scattering from a graphite surface using the molecular beam technique. The time-of-flight and angular distributions of the scattered molecules were measured at the incident energy lower than 100 meV with the surface temperature of 300 K. As the incident energy decreases from 35 to 130 meV, adsorption-desorption component increases in the time-of-flight distributions. At the incident energy of 35 meV, the angular flux distribution deviates from lobular pattern and approaches to cosine distribution. The final energy of the scattered molecules at the incident energy of 35 meV becomes less dependent on the scattering angle than at the incident energy of 130 meV. These results confirm that the reduction of the incident energy from 130 to 35 meV enhances the accommodation of water molecule to graphite surface.   

Session W43: Chemical Physics of Graphene and Other Crystals

Mullite Ceramics at Extreme Conditions

Mullite is perhaps one of the most important phases in both traditional and advanced ceramics and thus one of the most widely studied ceramic phases. Even though the thermo-elastic behavior of mullites have been studied extensively (spectroscopy, diffraction, dilatometry, theoretical simulations), there are only few studies into the effects of pressure on mullites. This work aims at filling this gap by examining the role of oxygen vacancies on the mechanical stability and on the bulk modulus of mullite-type structures.   

Chemical structure of multilayer oxidized epitaxial graphene

In this work, density functional theory (DFT) calculations are used to interpret new X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscope (AFM) measurements of the oxide of epitaxial graphene. This layered carbon material is obtained by Hummers oxidation of 6- to 17-layer graphene films grown epitaxially at high temperature on a silicon carbide substrate. The extensive DFT calculations carried out to solve the inverse problem posed by the aforementioned measurements show that a most plausible molecular structure for the oxide of epitaxial graphene consists of mildly oxidized graphene layers covalently bridged by short polyoxymethylene chains. Possible chemical reactions leading to this form of graphene derivative are discussed.   

Study on Hydrogen Interaction with Graphene, Graphene Hydroxide, and Lithiated Graphene

Neutron vibrational spectroscopy, together with adsorption isotherm measurements, has been employed to investigate interaction of hydrogen with graphene, hydroxylated graphene, and lithium incorporated graphene. The adsorption studies of hydrogen on these materials indicate varying degrees of hydrogen storage capacity. Graphene is found to have significantly higher hydrogen uptake than graphite and graphite oxide. Neutron vibrational spectroscopy provides direct information concerning hydrogen dynamics including the occurrence of the rotational mode at 119 cm-1; slightly below the free rotor position observed for H2 rotation on graphite. We have also explored how the interaction of hydrogen changes when hydroxyl groups are attached onto the graphene surface and when lithium is incorporated into graphene. The outcome of these studies will also be discussed.   

Superpermeability of water through graphene based membranes

Permeation through nanometre-pore materials has been attracting unwavering interest due to fundamental differences in governing mechanisms at macroscopic and molecular scales, the importance of water permeation in living systems, and relevance for filtration and separation techniques. One of the most spectacular findings in this field was the observation that carbon nanotubes and other hydrophobic nanocapillaries allow anomalously fast permeation of gases and liquids and, in particular, of water. In this contribution we show that membranes made from graphene oxide which are impermeable to liquids, vapours and gases, including helium, but allow unimpeded permeation of water (H$_{2}$O permeates through the membranes at least 10$^{10}$ times faster than He). We attribute these seemingly incompatible observations to a nearly frictionless flow of a monolayer of water through two dimensional capillaries formed by closely spaced graphene sheets. The flow is driven by a large capillary-like pressure and normally limited only by evaporation from the wetted surface of the membranes. The permeation can be stopped by either reducing graphene oxide or inducing a reversible drying transition in low humidity, which narrow nanocapillaries in both cases. I will also give an overview of our latest results on ion permeation through these membranes.   

Quantification of crumpling in sheet-like nanostructures

Two-dimensional sheet-like nanostructures have garnered significant scientific interest in recent decade, particularly due to their inherent high specific surface areas (SSAs). Such large SSAs also result in an intrinsic tendency to crumple or fold based on surface interactions under ambient conditions. An understanding of the topological details of such structures has revealed various qualitative features driven by thermodynamics and interfacial chemistry. A scaling model based methodology will be presented which can be utilized to do quantitative analysis using small angle scattering data. A wide range of materials like graphene oxide, membrane layers as well exfoliated sheets of molybdenum oxide and tungsten oxide have been investigated to understand how such quantification may yield a general classification of such materials based on crumpling behavior.   

Concavity effects on the optical properties of aromatic hydrocarbons

We address the modifications on the ground and excited state properties of polycyclic aromatic hydrocarbons (PAHs) induced by variations of concavity and $\pi$-connectivity. We study three series of PAHs, inspired by experimentally feasible systems, from hydrogen-saturated graphene flakes to concave ``buckybowls'' related to the formation of fullerene C$_{60}$ and carbon nanotube caps. We work within the framework of Hartree-Fock-based semiempirical methods (AM1 and ZINDO/S), and our results are supported by a generally good agreement with the available data. We see clearly that the interplay between concavity and $\pi$-connectivity shifts the bright optical lines to higher energies, and introduces symmetry-forbidden dark excitations at low energy [1]. These features can be the basis for designing optical properties of novel curved aromatic molecules.\\[4pt] [1] C. Cocchi et al. submitted (2012).   

Giant Fullerenes for Target Specific Drug Delivery

Carbon nano-structures, such as giant fullerenes, have a great potential for biological and medical applications. Most of the previous research is dedicated to investigate the use of fullerenes as vehicles for carrying medication which is chemisorbed on the outside surface of the fullerenes. In contrast, using fullerenes as an enclosure was largely abandoned due to the high strength of the carbon-carbon bonds which has been perceived to prevent the rupturing of the fullerene to release their cargo. We performed atomistic computations based on classical force fields that will address this perception. Specifically we explore the physics and chemistry of OH functionalized carbon based giant fullerenes with diameters from 0.72 nm (60 atoms) to 5.7 nm (3840 atoms). The preliminary results show that OH functionalization on these fullerenes is not only viable but also provides a pH sensitive release mechanism. Furthermore our current results show that carbon-carbon bonds can be broken in low energy biological environments in the presence of a flow induced strain field. These insights may have implications for target specific drug delivery in general and cancer treatment in particular.   

Photo-Electron Injection into TiO2: Quantum Dot vs. Graphene

We presented a detailed comparison on the similalaries and differences of the ultrafast photoinduced electron transfer (ET) from two kinds of donor species, namely PbSe quantum dot (QD) and graphene, into the acceptor TiO$_{2}$ surface via \textit{ab initio} time domain density functional theory simulations. The main diffrences stem from the size and dimensionality of the donor species and donor-acceptor bonding characteristics. For exmaple, the QD is localized species and composed by heavy atoms that connected to TiO$_{2}$ surface via chemical bonds. In contrast, the graphene layer is delocalized two-dimensional object that attached to TiO$_{2}$ substrate by van der Waals interaction and partial chemical bonds. The ET mechanism depends on the dimensionality of the donor and donor-acceptor chemical bonding. The injection from the localized donor states of the QD is dominatly adiabatic. In contrast, the injection from the two-dimensional graphene into TiO$_{2}$ exhibits prominently nonadiabatic (NA) component. The NA mechanism is efficient for the graphene/TiO$_{2}$ composites because it is delocalized over two dimensions and is able to couple with a dense manifold of delocalized TiO$_{2}$ conduction band states and weak coupling as well. The high density of acceptor states in this case favors the NA mechanism.   

Lattice dynamics of cubic CaSiO$_3$ perovskite at high temperatures and pressures

Cubic CaSiO$_{3}$-perovskite is a minor but important phase of the Earth's lower mantle. It is a mechanically unstable phase at low temperatures but it is stabilized at lower mantle temperatures. We have investigated its vibrational properties at high pressures and temperatures of the lower mantle. We have projected ionic velocities from ab initio molecular dynamics trajectories onto vibrational normal modes and computed the mode-mode correlation function from which we extract phonon frequencies and life times at finite temperatures. These correlations clearly indicate that normal modes with imaginary frequencies at 0 K are stabilized with increasing temperature. To overcome the finite size effect inherent in molecular dynamics simulations, a renormalized second-order force constant matrix in real space is constructed from the phonon frequencies at finite temperature and the phonon polarization vectors. Phonon dispersions and vibrational density of states are then determined by Fourier interpolation using the renormalized force matrix. These temperature dependent dispersions allow us to investigate thermodynamics and thermal elastic properties at lower mantle conditions.   

\textit{In situ} neutron diffraction study of SII CO deuterohydrate clathrate

SII CO clathrate has been successfully synthesized at $\sim$ 100 bar and 252 K. During the synthesis process, SI CO clathrate was formed first as an intermediate phase and then transformed to SII clathrate. Structural parameters of SII CO clathrate at temperatures from 25 K to 260 K have been determined from Rietveld analysis of neutron diffraction data. With decreasing temperature, the decrease of lattice parameter can be described by a two-order polynomial thermal expansion equation. The molecular lengths of CO in the small and large cages decrease linearly with decreasing temperature. There is one CO molecule in each small cage, whereas two CO molecules occupy in each large cage. CO molecules are not localized at the cage centers. Rather, they exhibit disordered distributions in both small and large cages, while the CO in small cage shows a donut shape nuclear distributions around the cage center, the CO in large cage delocalized from the cage center and more disordered with increasing temperature.   

Study of growth mechanism and atomic structure of Au-Pd core-shell nanocube by Cs-corrected scanning transmission electron microscopy

Au-Pd core-shell nanocubes of controlled sizes from 14 nm to 30 nm were synthesized using seed mediated growth process. The Pd shell layers were controlled from some monolayers to 10 nm. The stepwise growth mechanism from nucleation and growth of Au nanoparticles to final core-shell nanocube was studied by using conventional transmission electron microscopy (TEM) and Cs-corrected scanning transmission electron microscopy (STEM). It was found that the nanocubes grew from octahedral Au seeds due to fast growth along \textless 111\textgreater\ directions and concavity occurred because of high reduction rate of ascorbic acid (AA). The concave nanocube showed a change in strain-release mechanism as the Pd shell grew from a few layers to a 30 nm nanocube. Shockley partial dislocations (SPD), stacking faults (SF) and edge dislocations were found to be the mechanism to release the mismatch strain. The smallest size nanocube with HIFs will be suitable in order to maximize the catalytic activity per unit weight and mass specific activity.   

Study of B1 (NaCl-type) to B2 (CsCl-type) pressure-induced structural phase transition in BaS, BaSe and BaTe using first-principles computations

We have studied the pressure-induced phase transitions from NaCl-type (B1) to CsCl-type (B2) structure in BaS, BaSe and BaTe by using {\it ab initio} density functional theory computations in the local density approximation. The Buerger and WTM\footnote{M. Watanabe {\it et. al}, Acta Crystallogr., Sect. A {\bf 33}, 294 (1977).} mechanisms were explored by mapping the enthalpy contours in two and four dimensional configuration space for the two mechanisms, respectively. Transition pressures for BaS, BaSe and BaTe were determined to be 5.5 GPa, 4.9 GPa and 3.4 GPa, respectively. From these configuration space landscapes, a low enthalpy barrier path was constructed for the transitions to proceed at three different pressures. We obtained barriers of 0.18, 0.16 and 0.15 eV/pair (17.4, 15.4 and 14.5 kJ/mol) for the Buerger mechanism and 0.13, 0.13 and 0.12 eV/pair (12.5, 12.5 and 11.6 kJ/mol) for the WTM mechanism at the transition pressures for BaS, BaSe and BaTe, respectively, indicating that the WTM mechanism is slightly more favorable in these compounds. We describe the difference of the two mechanisms by differences in their symmetry and atomic coordination.   

The quasi-Bragg law, transforming the icoshedral diffraction pattern onto a hierarchic structure

Previously, we have demonstrated [1]: 1) The golden section $\tau $ is as fundamental to the icosahedral structure (length /edge) as $\pi $ is to the sphere (circumference /diameter). 2) The diffraction series are in restricted Fibonacci order because the ratio of adjacent terms $f_{n}/f_{n-1}$ does not vary, but is the constant $\tau $. The series is therefore geometric. 3) The matrix fcc Al is an approximant for i-Al$_{6}$Mn. 4) A three dimensional stereographic projection and a quasi-Bragg law are derived, correctly representing the diffraction series in powers of $\tau $ [2], without redundancy. 5) By the normal conventions of electron microscopy, the diffraction patterns are completely indexed in three dimensions. Now we describe significant consequences: 1) The diffraction pattern intensities near all main axes are correctly simulated, and all atoms are located on a specimen image. 2) The quasi-Bragg law has a special metric that we have measured. Atomic locations are consistently calculated for the first time. 3) Whereas the Bragg law transforms a crystal lattice into a reciprocal lattice in diffraction space, the quasi-Bragg law transforms a geometric diffraction pattern into a hierarchic structure. 4) Hyperspatial indexation [3] is superceded.\\[4pt] [1] Bourdillon, A.J., APS conference, Louis Obispo, Nov. 2-3 2012.\\[0pt] [2] Bourdillon, A. J.,\textit{ Sol. State Comm.} \textbf{2009}, 149, 1221-1225.\\[0pt] [3] Duneau, M., and Katz, A., \textit{Phys Rev Lett} \textbf{54}, 2688-2691   

Session W44: Focus Session: The Physics of Behavior

Dimentionality and behavior of swimming Zebrafish: ``The EigenFish''

How simple is the underlying control mechanism for the complex locomotion of vertebrates? To answer this question, we study the swimming behavior of zebrafish larvae. A dimensionality reduction method (singular value decomposition), in analogy to previous studies of worms, is used to analyze swimming movies of fish. That way, the animals can directly provide us with a minimal set of shapes to describe their motion, rather than us imposing arbitrary coordinates. We show that two low imensional attractors (an ellipse and a distorted ellipse) embedded in a threedimensional space of motion coordinates are sufficient to describe \textgreater\ 95{\%} of the locomotion. We also show that scoots and R-turns, previously thought to be independent behaviors based on qualitative studies, are in fact just extremes of a~continuous family of motions bounded by~the two attractors.   

Controlling neural activity in \textit{Caenorhabditis elegans} to evoke chemotactic behavior

Animals locate and track chemoattractive gradients in the environment to find food. With its simple nervous system, \textit{Caenorhabditis elegans} is a good model system in which to understand how the dynamics of neural activity control this search behavior. To understand how the activity in its interneurons coordinate different motor programs to lead the animal to food, here we used optogenetics and new optical tools to manipulate neural activity directly in freely moving animals to evoke chemotactic behavior. By deducing the classes of activity patterns triggered during chemotaxis and exciting individual neurons with these patterns, we identified interneurons that control the essential locomotory programs for this behavior. Notably, we discovered that controlling the dynamics of activity in just one interneuron pair was sufficient to force the animal to locate, turn towards and track virtual light gradients.   

Quantification of Nociceptive Escape Response in \textit{C.elegans}

Animals cannot rank and communicate their pain consciously. Thus in pain studies on animal models, one must infer the pain level from high precision experimental characterization of behavior. This is not trivial since behaviors are very complex and multidimensional. Here we explore the feasibility of \textit{C.elegans} as a model for pain transduction. The nematode has a robust neurally mediated noxious escape response, which we show to be partially decoupled from other sensory behaviors. We develop a nociceptive behavioral response assay that allows us to apply controlled levels of pain by locally heating worms with an IR laser. The worms' motions are captured by machine vision programming with high spatiotemporal resolution. The resulting behavioral quantification allows us to build a statistical model for inference of the experienced pain level from the behavioral response. Based on the measured nociceptive escape of over 400 worms, we conclude that none of the simple characteristics of the response are reliable indicators of the laser pulse strength. Nonetheless, a more reliable statistical inference of the pain stimulus level from the measured behavior is possible based on a complexity-controlled regression model that takes into account the entire worm behavioral output.   

Discovery of stereotypy through behavioral space embedding

Most experiments in the neurobiology of behavior rely upon the concept that animals frequently engage in stereotyped movements -- behaviors that an animal performs often and with great similarly. While these actions are often the basis for mapping neural circuits and understanding the effects of genetic manipulations, stereotypy is usually defined in an ad hoc manner, thereby limiting the sensitivity and repeatability of subsequent analyses. Moreover, the underlying assumption that an animal's behavior can be described in terms of discrete states typically remains unverified. In this talk, I will describe our novel method for the identification and characterization of stereotyped behaviors. Using the fruit fly \emph{Drosophila melanogaster} as a model organism, we show that it is possible to start from raw videos of a freely-behaving animal and statistically isolate stereotyped movements. Our method achieves this through leveraging ideas from statistical physics, non-linear dynamics, and information theory. The rigorous behavioral metrics resulting from this technique allow us to explore questions in animal behavior ranging from speciation, to aging, to the control of locomotion, thus providing further insight in the interplay between genes, neurons, and behavior.   

Swarming in disordered environments

The emergence of collective motion over a wide range of length scales in biology has inspired research in a multitude of disciplines. Possessing only local information, a group of moving individuals can form crowds, swarms or flocks which can traverse the entire system forming a self organized co-moving collective. An important question that arises is: how do these groups deal with environmental disorder? It is rare that perfectly connected homogeneous environments exist in nature and more often biological environments are intrinsically spatially disordered. We investigate the effects of intrinsic disorder or \textit{topological noise} on the formation of collective motion by studying interacting agents on a $2d$ percolated lattice with bond occupation probability $p$. We find that the existence of collective motion depends critically on $p$ and disappears completely for rather small amounts of disorder. Furthermore, we show that repulsive forces between agents within the swarm can rescue collective motion even for large amounts of topological disorder, suggesting that nearest neighbor alignment alone is not enough for swarms to navigate a disordered environment.   

Physical limits to gradient sensing by swimming cells

The chemotactic motion of cells relies on their ability to infer the location of a chemical source from the random arrival of molecules at chemical receptors on the cell surface. Small organisms like bacterial cells generally employ a temporal sensing mechanism to measure spatial gradients in concentration. For example, the bacterium Escherichia coli compares concentrations in time as it swims, and modulates its swimming behavior accordingly to swim up the concentration gradient. Slightly larger eukaryotic cells are able to directly sense spatial gradients of chemicals across their surface. Previous studies have demonstrated that the physical process of diffusion sets a fundamental limit to the accuracy with which cells can sense spatial gradients. However, most of these studies neglect the intrinsic coupling between the sensory task and the behavioral response of swimming. The swimming cell stirs the surrounding fluid, which in turn affects the arrival location of molecules at the cell surface, and hence the inferred spatial gradient. By considering the appropriate advection-diffusion equation for the arrival of molecules at the cell surface, we determine the fundamental physical limit to the accuracy of direct gradient sensing by swimming cells.   

Measurement of Behavioral Evolution in Bacterial Populations

A curious aspect of bacterial behavior under stress is the induction of filamentation: the anomalous growth of certain bacteria in which cells continue to elongate but do not divide into progeny. We show that {\em E.coli} under the influence of the genotoxic antibiotic ciprofloxacin have robust filamentous growth, which provides individual bacteria a mesoscopic niche for evolution until resistant progeny can bud off and propagate. Hence, filamentation is a form of genomic amplification where even a single, isolated bacteria can have access to multiple genomes. We propose a model that predicts that the first arrival time of the normal sized progeny should follow a Gompertz distribution with the mean first arrival time proportional to the elongation rate of filament. These predictions agree with our experimental measurements. Finally, we suggest bacterial filament growth and budding has many similarities to tumor growth and metastasis and can serve as a simpler model to study those complicated processes.   

Environmental engineering simplifies subterranean locomotion control

We hypothesize that ants engineer habitats which reduce locomotion control requirements. We studied tunnel construction, and locomotion, in fire ants ({\em Solenopsis invicta}, body length $L = 0.35 \pm 0.05$). In their daily life, ants forage for food above ground and return resources to the nest. This steady-state tunnel traffic enables high-throughput biomechanics studies of tunnel climbing. In a laboratory experiment we challenged fire ants to climb through 8 cm long glass tunnels (D = 0.1 - 0.9 cm) that separated a nest from an open arena with food and water. During ascending and descending climbs we induced falls by a motion-activated rapid, short, downward translation of the tunnels. Normalized tunnel diameter ($D/L$) determined the ability of ants to rapidly recover from perturbations. Fall arrest probability was unity for small $D/L$, and zero for large $D/L$. The transition from successful to unsuccessful arrest occurred at $D/L = 1.4 \pm 0.3$. Through X-Ray computed tomography study we show that the diameter of ant-excavated tunnels is independent of soil-moisture content (studied from 1-20\%) and particle size (50-595 $\mu m$ diameter), and has a mean value of $D/L = 1.06 \pm 0.23$. Thus fire ants construct tunnels of diameter near the onset of fall instability.   

Mosh pits and Circle pits: Collective motion at heavy metal concerts

Heavy metal concerts present an extreme environment in which large crowds ($\sim 10^2- 10^5$) of humans experience very loud music ($\sim130\rm{dB}$) in sync with bright, flashing lights, often while intoxicated. In this setting, we find two types of collective motion: mosh pits, in which participants collide with each other randomly in a manner resembling an ideal gas, and circle pits, in which participants run collectively in a circle forming a vortex of people. We model these two collective behaviors using a flocking model and find qualitative and quantitative agreement with the behaviors found in videos of metal concerts. Futhermore, we find a phase diagram showing the transition from a mosh pit to a circle pit as well as a predicted third phase, lane formation.   

Lift-off dynamics in a simple jumping robot

Jumping is an important behavior utilized by animals to escape predation, hunt, reach higher ground, and as a primary mode of locomotion. Many mathematical and physical robot models use numerous parameters and multi-link legs to accurately model jumping dynamics. However, a simple robot model can reveal important principles of high performance jumping. We study vertical jumping in a simple robot comprising an actuated mass-spring arrangement. The actuator frequency and phase are systematically varied to find optimal performance. Optimal jumps occur above and below (but not at) the robot's resonant frequency f$_{0}$. Two distinct jumping modes emerge: a simple jump which is optimal above f$_{0}$ is achievable with a squat maneuver, and a peculiar stutter jump which is optimal below f$_{0}$ is generated with a counter-movement. A simple dynamical model reveals how optimal lift-off results from non-resonant transient dynamics. An expanded explanation of this work is provided at http://crablab.gatech.edu/pages/jumpingrobot/index.html   

Session W45: Physics of Bacteria

Gene Location and DNA Density Determine Transcription Factor Distributions in \textit{E. coli}

The diffusion coefficient of the prototypical transcription factor LacI within living \textit{Escherichia coli} has been measured directly by in vivo tracking to be D~$=$~0.4 $\mu$m$^2$/s. At this rate, simple models of diffusion lead to the expectation that LacI and other proteins will rapidly homogenize throughout the cell. We have tested this expectation of spatial homogeneity by single molecule visualization of LacI molecules non-specifically bound to DNA in fixed cells. Contrary to expectation, we find that the distribution depends on the spatial location of its encoding gene. We demonstrate that the spatial distribution of LacI is also determined by the local state of DNA compaction, and that \textit{E. coli} can dynamically redistribute proteins by modifying the state of its nucleoid. Finally, we show that LacI inhomogeneity increases the strength with which targets located proximally to the LacI gene are regulated. We propose a model for intranucleoid diffusion which can reconcile these results with previous measurements of LacI diffusion.   

A model for the condensation of the bacterial chromosome by the partitioning protein ParB

The molecular machinery responsible for faithful segregation of the chromosome in bacteria such as \textit{Caulobacter crescentus} and \textit{Bacillus} \textit{subtilis} includes the ParABS a.k.a. Spo0J/Soj partitioning system. In \textit{Caulobacter}, prior to division, hundreds of ParB proteins bind to the DNA near the origin of replication, and localize to one pole of the cell. Subsequently, the ParB-DNA complex is translocated to the far pole by the binding and retraction of the ParA spindle-like apparatus. Remarkably, the localization of ParB proteins to specific regions of the chromosome appears to be controlled by only a few centromeric \textit{parS} binding sites. Although lateral interactions between DNA-bound ParB are likely to be important for their localization, the long-range order of ParB domains on the chromosome appears to be inconsistent with a picture in which protein-protein interactions are limited to neighboring DNA-bound proteins. We developed a coarse-grained Brownian dynamics model that allows for lateral and 3D protein-protein interactions among bound ParB proteins. Our model shows how such interactions can condense and organize the DNA spatially, and can control the localization and the long-range order of the DNA-bound proteins.   

Effect of an Antimicrobial Compound on Different Processes within the Oscillation of Min Proteins in E. coli Bacterial Cells

A key step in the life of a bacterium is its division into two daughter cells of equal size. This process is carefully controlled and regulated so that equal partitioning of the cellular machinery is obtained. In E. coli, this regulation is accomplished, in part, by the Min protein system. The Min proteins undergo an oscillation between the poles of rod-shaped E. coli bacteria. We use high magnification, time-resolved total internal reflection fluorescence microscopy to characterize the temporal distributions of different processes within the oscillation: the MinD-MinE interaction time, the residence time for membrane bound MinD, and the recruitment time for MinD to be observed at the opposite pole. We also characterize the change in each of these processes in the presence of the antimicrobial compound polymyxin B (PMB). We show that the times corresponding to the removal of MinD from one pole and the recruitment of MinD at the opposite pole are correlated. We explain this correlation through the existence of a concentration threshold. The effect of PMB on the concentration threshold is used to identify which process within the oscillation is most affected.   

Mechanism for longitudinal growth of rod-shaped bacteria

The peptidoglycan (PG) cell wall along with MreB proteins are major determinants of shape in rod-shaped bacteria. However the mechanism guiding the growth of this elastic network of cross-linked PG (sacculus) that maintains the integrity and shape of the rod-shaped cell remains elusive. We propose that the known anisotropic elasticity and anisotropic loading, due to the shape and turgor pressure, of the sacculus is sufficient to direct small gaps in the sacculus to elongate around the cell, and that subsequent repair leads to longitudinal growth without radial growth. We computationally show in our anisotropically stressed anisotropic elasticity model small gaps can extend stably in the circumferential direction for the known elasticity of the sacculus. We suggest that MreB patches that normally propagate circumferentially [1], are associated with these gaps and are steered with this common mechanism. This basic picture is unchanged in Gram positive and Gram negative bacteria. We also show that small changes of elastic properties can in fact lead to bi-stable propagation of gaps, both longitudinal and circumferential, that can explain the bi-stability in patch movement observed in $\Delta mbl \Delta mreb$ mutants.\\[4pt] [1] J. Dom\'{i}nguez-Escobar {\em et al.}, Science   

Modeling of storage-based heterocyst commitment and patterning in cyanobacteria

When deprived of fixed nitrogen, filamentous cyanobacteria differentiate nitrogen-fixing heterocyst cells in a regular, one-dimensional pattern. Many genes have been identified that contribute to heterocyst selection, but the selection process is still not well understood. By including fixed-nitrogen storage in a computational model of nitrogen dynamics, growth, and heterocyst differentiation with lateral inhibition along the filament we can explain the stochastic timing of heterocyst commitment. Notably, the only stochastic element of our model is growth rate randomness sufficient to achieve a natural population structure of cell lengths. Our computational model qualitatively reproduces many measurements associated with heterocyst differentiation including both initial and steady state heterocyst patterns. Our model shows that a fixed storage percentage, together with variability in cell length, can produce a strong implicit cell cycle effect on heterocyst commitment which favors the commitment of shorter cells.   

Low-temperature STM studies of electronic properties of microbial nanowires

\textit{Geobacter sulfurreducens} expresses pili that act as electrically conductive nanowires. These microbial nanowires transport metabolically generated electrons outside the cell body to electron acceptors in the organism's environment. We have performed scanning tunneling microscopy and spectroscopy on these pili in an endeavor to elucidate the mechanism of conductivity. In particular, we will discuss spectroscopy curves acquired at a temperature of 77 K.   

Direct observation and quantification of extracellular long-range electron flow in anaerobic bacteria

Some anaerobic microorganisms are capable of transporting electrons outside their cell to distant electron acceptors such as metals, minerals or partner species. Previous studies have focused primarily on transport over short distances (\textless\ 1 $\mu $m) via diffusion of molecular intermediates, or alternatively via tunneling or thermally-activated hopping across biomolecules. However, we have found that \textit{Geobacter sulfurreducens} can transport electrons over long distances (\textgreater\ 10 $\mu $m) using pili filaments that show organic metal-like conductivity [1]. Pili also enable direct exchange of electrons among syntrophic \textit{Geobacter} co-cultures [2]. In order to establish the physical principles underlying this remarkable electron transport, we have employed a novel scanning probe microscopy-based method to perform quantitative measurements of electron flow at a single cell level under physiological conditions. Using this nanoscopic approach, we have directly observed the propagation and distribution of injected electrons in individual native bacterial extracellular proteins. Our direct measurements demonstrate unambiguously for the first time that the pili of \textit{G. sulfurreducens} are a novel class of electronically functional proteins that can sustain electron flow in a surprising manner that has not been observed previously in any other natural protein.\\[4pt] [1] \textit{Nature Nanotechnology}, 6, 573 (2011)\\[0pt] [2] \textit{Science}, 330, 1413 (2010)   

Biofilm formation and surface exploration behavior of P. aeruginosa

Despite extensive studies, the early stages of biofilm formation are not fully understood. Recent work on the opportunistic pathogen Pseudomonas aeruginosa has shown that these bacteria deposit the exopolysaccharide Psl as they move across a surface, which in turn attracts repeat visits of bacteria to the sites of deposition. Using a massively parallel cell-tracking algorithm combined with fluorescent Psl staining and computer simulations, we show that this behavior results in a surface visit distribution that can be approximated by a power law. The steepness of this Zipf's Law is a measure of the hierarchical nature of bacterial surface visits, and is (among other parameters) a function of both Psl secretion rate and sensitivity of the bacteria to Psl. We characterize the bacterial distributions using various computational techniques to quantitatively analyze the effect of Psl on microcolony organization and to identify the key stages of microcolony growth.   

Pel promotes symmetric, short-ranged surface attachment in P. aeruginosa

Bacterial biofilms are surface mounted, multicellular communities of interacting bacteria that are often associated with chronic infections that resist antibiotics and damage host tissue. Bacteria in a biofilm are bound in a matrix of polymeric materials that adhere the bacteria to the surface, give the system spatial structure, and cluster the bacteria near each other. The opportunistic human pathogen \emph{Pseudomonas aeruginosa} is widely studied as a model biofilm-forming organism. The polymeric matrix of \emph{P. aeruginosa} strain PAO1 biofilms is dominated by two bacteria-produced extracellular polymers, Pel and Psl. We use both optical and atomic force microscopy to examine the roles of these polymers in very early biofilm development, in the hours after initial surface attachment. In agreement with other researchers, we find that Psl mediates strong attachment to a glass surface. Unexpectedly, we find that Pel promotes symmetric attachment, in the form of the rod-shaped bacteria lying flat on the surface, independently of permanent attachment to the surface. Further, the presence of Pel makes adhesion forces more short-ranged than they are with Psl alone. We suggest that these effects may result through synergistic interactions of Pel and Psl in the polymeric matrix.   

Surface-attachment sequence in Vibrio Cholerae

Vibrio cholerae is a gram-negative bacterium that causes the human disease cholera. It is found natively in brackish costal waters in temperate climates, where it attaches to the surfaces of a variety of different aquatic life. V. cholerae has a single polar flagellum making it highly motile, as well as a number of different pili types, enabling it to attach to both biotic and abiotic surfaces. Using in-house built tracking software we track all surface-attaching bacteria from high-speed movies to examine the early-time attachment profile of v. cholerae onto a smooth glass surface. Similar to previous work,\footnote{Lauga, E., DiLuzio, W. R., Whitesides, G. M., Stone, H. A. Biophys. J. 90, 400 (2006).} we observe right-handed circular swimming trajectories near surfaces; however, in addition we see a host of distinct motility mechanisms that enable rapid exploration of the surface before forming a more permanent attachment. Using isogenic mutants we show that the motility mechanisms observed are due to a complex combination of hydrodynamics and pili-surface interactions.   

Large scale surface migration of \textit{P. aeruginosa }at early stages of biofilm formation

\textit{Pseudomonas aeruginosa} is a commonly-studied bacterium which can form biofilms, surface-bound aggregates which display increased resistance to various forms of stress, including a greatly enhanced antibiotic resistance. In the early stages of biofilm formation, free-swimming planktonic cells attach to the surface and form microcolonies, expressing a variety of adhesins and transitioning from reversible to irreversible attachment. By using particle tracking algorithms, we can in principle examine the full motility and division history of all cells in a microcolony. Here, we study the effects of the \textit{pel} polysaccharides in microcolony formation by investigating how \textit{pel} impacts the initial stages of biofilm formation by the \textit{P. aeruginosa} PA14 strain. Specifically, we quantify the phenotypic effects of \textit{pel} on initial attachment, microcolony formation, and biofilm morphology.   

Quantifying the Dynamics of Bacterial Crowd Surfing

Type IV pili (TFP) are thin (several nanometers in diameter) adhesive protein filaments that can be extended and retracted by certain classes of Gram-negative bacteria including \textit{P. aeruginosa} PAO1 [1]. The motion of bacteria on surfaces by TFP is referred to as twitching motility because of its jerky nature, and it leads to complex, collective motion of large numbers of cells [2]. When non-motile mutants of \textit{P. aeruginosa} cells, which do not have pili and therefore cannot twitch, are mixed with motile, wild type cells, we observed the non-motile cells being carried along (``crowd surfing'') by the moving wild type cells. Crowd surfing extends to other non-motile species as well as inert particles and can lead to unexpected transport of non-motile, pathogenic bacterial cells, with direct implications for the spread of bacterial infections. We have developed a protocol for tracking and analyzing the trajectories of moving bacterial cells. Using a custom built, temperature and humidity controlled environmental chamber, we characterize the crowd surfing phenomenon under different environmental conditions. [1] Burrows, L.L. (2005) Mol. Microbiol. 57(4): 878-888. [2] Semmler, A.B., Whitchurch, C.B., Mattick, J.S. (1999). Microbiology 145: 2863-2873.   

The effect of flagellar motor-rotor complexes on twitching motility in \textit{P. aeruginosa}

\textit{P. aeruginosa} is an opportunistic bacterium responsible for a broad range of biofilm infections. In order for biofilms to form, \textit{P. aeruginosa} uses different types of surface motility. In the current understanding, flagella are used for swarming motility and type IV pili are used for twitching motility. The flagellum also plays important roles in initial surface attachment and in shaping the architectures of mature biofilms. Here we examine how flagella and pili interact during surface motility, by using cell tracking techniques. We show that the pili driven twitching motility of \textit{P. aeruginosa} can be affected by the motor-rotor complexes of the flagellar system.   

Biofilm streamer formation in a microfluidic porous media mimic

Biofilm formation in porous media is of significant importance in many environmental and industrial processes such as bioremediation, oil recovery, and wastewater treatment. Among different biological and environmental factors, hydrodynamics is considered an important determinant of the dynamics of biofilm formation. In the present study, we fabricated a microfluidic porous media mimic and investigated how fluid flow influences the formation of filamentous structures, known as streamers, between porous media structures. Streamers are viscoelastic materials composed of extracellular polymeric substances (EPS) and bacterial cells, and these filamentous structures are typically tethered at either one of both ends to surfaces. We studied evolution of streamers in different flow rates and identified a tangible link between hydrodynamic conditions and development of these filamentous structures. Our results show that hydrodynamic conditions not only determine the limit of the streamers formation, but also influence both temporal evolution and spatial organization of biofilm streamers.   

Observation of Spontaneous Circulation in a Confined Bacterial Suspension

The individual swimming behavior of many microorganisms is often well described by a run-and-tumble model. However steric and fluid interactions with other cells and boundaries can strongly affect this behavior. At high concentrations, rod-like bacteria are known to exhibit self-organization reminiscent of nematic liquid crytal ordering, except with polar alignment. Depending on the experimental conditions different large scale patterns can arise such as vortices, jets, plumes and swarms. We use the model organism \textit{Bacillus subtilis} to study the effect of a quasi-2D confinement on their large scale organization. Bacteria are concentrated in flattened drops surrounded by oil. Using fluorescent microsphere tracers and particle image velocimetry, we measure the flow of the cells and of the suspending fluid inside and outside of the drop. For drop diameters ranging from 10 to 100 $\mu$m and 20 $\mu$m in height, the suspension displays spontaneous circulation in the form of a single vortex, which, for the largest drops, significantly exceeds the size of swirls in the unconfined system. Moreover we observe a striking backward flow close to the boundary. We compare these results with a theoretical analysis to gain insights into the assembly and stability of such patterns.   

Session W46: Focus Session: Advances in Scanned Probe Microscopy III: Novel Approaches and Ultrasensitive Detection

Probing single molecules with the STM in the frequency and time domains

We have constructed a scanning tunneling microscope (STM) and combined it with a tunable femtosecond laser (210 nm to 1040 nm) to probe single molecules with simultaneous spatial and temporal resolutions. Employing the RF lock-in amplifier to measure the laser-induced tunneling current that is directly synchronized with the high repetition rate of the laser ($\sim$80 MHz), time resolved measurement of single molecules with atomic scale resolution can be achieved by varying the time delay between pairs of laser pulses in the two-pulse correlation or two-color pump-probe configuration. A femtosecond laser system with widely tunable wavelength enables resonant excitation of single molecules that are partially decoupled electronically from the underlying metallic substrate by a thin oxide or additionally atomic or molecular layers. The experimental arrangement allows measurement of molecular lifetimes by two-photon photoemission spectroscopy and microscopy.   

High Resolution Single Molecule Vibrational Spectroscopy with the STM

Inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope (STM) has been regarded as the ultimate tool to identify and characterize single molecules adsorbed on solid surfaces with atomic spatial resolution. With the improvement of energy resolution obtained at $\sim$ 600 mK, STM-IETS is able to resolve the lowest vibrational energies and reveal subtle interactions between the molecule and its environment which were previously not possible at higher temperatures. Here we demonstrate the capability of sub-Kelvin STM on detecting the influence of the tip as well as the anisotropy of the reconstructed Au(110) surface on the low energy hindered vibrational motions of single adsorbed CO molecule. Single molecule vibrational spectroscopy at $\sim$ 600 mK with atomic scale spatial resolution opens new possibilities to probe molecular interactions with high spectral sub-THz resolution.   

Measuring infrared absorption of molecular adsorbates at the submonolayer level by scanning tunneling microscopy-based IR spectroscopy (IR-STM)

Here we present a simple, effective technique whereby a scanning tunneling microscope (STM) can achieve vibrational spectroscopy of molecular adsorbates at the submonolayer level through the use of a tunable infrared (IR) laser source. By using the STM as a detector to probe the IR molecular response, the technique takes advantage of the high spectral resolution inherent to IR measurements while avoiding the typical difficulties related to optical detection. This technique also allows sub-nm scale spatial mapping of surface structure under the same experimental conditions that the STM-IR absorption spectra are acquired (sub-nm spatial resolution for specific IR spectral features has not yet been achieved). Using this technique we have obtained IR absorption spectra of higher diamondoid molecules, specifically [121]tetramantane and [123]tetramantane, deposited on a Au(111) surface. The significant differences between the IR-STM spectra obtained for these two molecular isomers show the power of this new technique to differentiate chemical structures.   

Imaging the Electron-Phonon Interaction on the Atomic Scale

New STM-based spectroscopic imaging technique, direct real-space imaging of electron-phonon interaction parameter $\lambda $, was demonstrated using the combination of STM and inelastic electron tunneling spectroscopy (IETS) for thin Pb islands epitaxially grown on 7x7 reconstructed Si(111). We found that $\lambda $ increases when the electron scattering at the Pb/Si(111) interface is diffuse and decreases when the electron scattering becomes specular. We show that the effect is driven by transverse redistribution of the electron density inside a quantum well. Reference: Igor Altfeder, K. A. Matveev, A. A. Voevodin, ``Imaging the Electron-Phonon Interaction on the Atomic Scale'', Physical Review Letters 109, 166402 (2012).   

Vibrational and electronic properties of small molecules on metal surfaces

Research of manipulating chemical bonds in a single molecule has been extremely active in recent years. Using a newly built milli-Kelvin scanning tunneling microscope, we can now resolve vibrational spectroscopic features down to a few tenths meV. Synergistic density functional calculations allow correct interpretation for each vibrational mode and provide links between experimental observations to the change of individual chemical bonds. In particular, we explored the effect of tunneling gap distance on different vibrational energies, by moving the tip toward the molecules, so as to shed some light for selective bond dissociation and formation. Here we discuss our results of the atomic structure, vibrational and electronic properties of several small molecules such as CO on the anisotropic Au(110) surface and C2H2 on the Cu(001) square lattice. Calculated vibrational frequencies, using the generalized gradient approximation or the non-local van der Waals density functional, are in good agreement with experimental results. \textbf{Acknowledgement. }Work was supported by the National Science Foundation under CHE-0802913 and computing time at XSEDE.   

Design and Implementation of a 4K Cryocooler-Based Scanning Tunneling Microscope

Low temperature, ultra-high vacuum scanning tunneling microscopes (STMs) have proved to be excellent tools for the study of electronic properties of complex materials. Unfortunately, with the continuing increase in liquid helium prices, already a dominant cost for operating these systems, their use is becoming exceedingly expensive. Here we describe the design and implementation of a STM cooled by a Cryomech PT407 Remote Motor Cryorefrigerator, allowing us to reach helium temperatures using a closed thermodynamic cycle with zero cryogen waste. Unfortunately, this refrigeration technique is not ultra-high vacuum (UHV) compatible and introduces vibrations. To tackle these problems, we separately house the cryocooler in a high-vacuum (HV) chamber. This provides both a UHV environment for the STM and mechanical isolation to minimize vibrations reaching the instrument. However, it makes for more challenging thermal connections. This last difficulty we solve by introducing a novel coaxial thermal feedthrough between the HV and UHV chambers.   

Spin dynamics of atoms and magnetic nanostructures on surfaces

Scanning tunneling microscopy is a powerful tool for studying the electronic and magnetic properties of magnetic nanostructures on surfaces. Over the last decade, inelastic tunneling spectroscopy has been used to probe discrete energy levels of quantum spin systems. These states can often be described as solutions of simple spin Hamiltonians. In spin excitation spectroscopy, a spin system is kicked from the ground into excited spin states at discrete energy increments. In this talk we will focus on the dynamics of quantum spin systems on surfaces. STM can measure tunnel currents in the range of pico amps with millisecond time resolution. This time resolution is well matched to observing transition between spin states of artificial magnetic nanostructures on surfaces that can be built and measured with STM. We will highlight an example of extended, artificial antiferromagnets on a Cu2N surface (Science 2012). Smaller magnetic clusters relax much faster but their dynamics can be measured with pump probe techniques. A pump voltage pulse drives the spin system into excited states and a subsequent probe pulse measures the resulting population of spin states. An exponential decay back to the ground state is observed when averaging over many pump-probe cycles (Science 2010). We will show results down to nanosecond time resolution with an ultimate limit set by modern electronics at about 100 pico seconds. Individual atoms on Cu2N relax their spin states even faster. Hence, another technique is employed to determine spin relaxation times: small tunnel currents always leave the spin system in the ground state while high currents can create non-equilibrium distributions of spin states. This approach relies on some modeling but allows time domain measurements down to about 1 pico second (Nature Physics 2010). Transition metal atoms on metal surfaces relax even faster, on time scales of about 100 femtoseconds. This fast relaxation manifests itself as a measurable lifetime broadening of spin excitation spectra. Combining these approaches allows measurements of spin relaxation times over about 16 orders of magnitude for spins on surfaces -- while maintaining the atomic scale spatial resolution of STM!   

A versatile variable field module for Asylum Cypher scanning probe system

Atomic force microscopy (AFM) has become one of the most widely used techniques for measuring and manipulating various characteristics of materials at the nanoscale. However, there are very limited option for the characterization of field dependence properties. In this work, we demonstrate a versatile variable field module (VFM) with magnetic field up to 1800 Oe for the Asylum Research Cypher system. The magnetic field is changed by adjusting the distance between a rare earth magnet and the AFM probe. A built-in Hall sensor makes it possible to perform in-situ measurements of the field. Rotating the magnet makes it possible to do angular field dependent measurements. The capability of the VFM system is demonstrated by degaussing a floppy disk media with increasing magnetic field. The written bits are erased at about 800 Oe. Angular dependence measurements clearly show the evolution of magnetic domain structures. A completely reversible magnetic force microscopy (MFM) phase contrast is observed when the magnetic field is rotated by 180$^{\circ}$. Further demonstration of successful magnetic switching of CoFe$_{2}$O$_{4}$ pillars in CoFe$_{2}$O$_{4}$-BiFeO$_{3}$ nanocomposites will be presented and field dependent MFM and piezoresponse force microscopy (PFM) will be discussed.   

Magnetoelectric Force Microscopy for visualizing cross-coupled domains

Intensive studies have been focused on magnetoelectric (ME) effect ever since Dzyaloshinskii and Astrov's seminal works on linear ME effect in Cr$_{2}$O$_{3}$. The measurements of the components of ME tensor are of great importance in technical applications and in fundamental science (e.g. determining magnetic point groups). For bulk ME measurements, it is necessary to obtain a single domain state by the ME annealing (i.e. applying magnetic and electric fields simultaneously) of the specimen through its transition temperature. However, the ME domain structure has never been directly observed due to the weakness of the ME effect in most magnetoelectrics. To address this critical issue, we have developed a nanoscale imaging technique, namely, the Magnetoelectric Force Microscopy (MeFM), to directly detect local ME response based on magnetic force microscopy with \textit{in-situ} high voltages. Preliminary results of visualizing ME domains will be presented to demonstrate the feasibility of the MeFM technique.   

Background-free Piezoresponse Force Microscopy with high sensitivity

Piezoresponse Force Microscopy (PFM) detects small mechanical deformation of a specimen by applying an AC voltage between a conductive AFM tip (as a top electrode) and the bottom electrode. It is widely used for visualizing ferroelectric domain patterns with high lateral resolution. In nominal or commercial setups, the PFM signal is contaminated by the so-called ``system-inherent background'' with a complex frequency spectrum which consists of many cross-talk resonances with peak amplitude over 10 pm/V [1]. The presence of the system-inherent background will severely distort the PFM contrast (especially the phase signal) and the domain pattern in PFM images of ferroelectrics with weak piezoelectric response ($<$1 pm/V). Although the system-inherent background can be subtracted out by proper calibration using a non-piezoelectric material (e.g. glass slides), it is highly desirable to eliminate it directly from PFM setup for background-free measurements. Here we demonstrate that the system-inherent background can be eliminated using carefully designed electric wiring of PFM setup. Results of background-free PFM detection with excellent sensitivity($\le$0.1 pm/V) will be presented. \\[4pt] [1] Jungk et al, Appl. Phys. Lett. 89 163507 (2006).   

On-line Scanned Probe Microscopy Transparently Integrated with DualBeam SEM/FIB Systems

A multifunctional scanning probe microscope (SPM) will be described that transparently integrates with a DualBeam SEM/FIB System. This is done without perturbing any of the capabilities of the Dual Beam in terms of detectors, gas injectors, analyzers etc while allowing for a completely exposed probe tip to be imaged online even with immersion objectives at working distances as short as 4 mm. In addition, the completely free motion of the rotation axis of the stage is maintained with the probe tip at the eucentric point, this makes it possible to orient the sample in any direction on any structure The X and Y scan range of the atomic force microscopic (AFM) imaging achieves 35 microns with rough motion over 10 millimeters. This permits the SPM to tilt into position perpendicular to the SEM or FIB or under an angle for rapid and accurate placement of the probe tip at or on structures such as biopolymeric materials that are nanometric in X, Y and Z extent. Thus, not only can a structure's nanometric height be accurately profiled but this can be accomplished with the on-line excellence of SEM for X, Y metrology. Furthermore, electron and ion beam sensitive samples can be imaged and characterized by AFM at high resolution.   

Massively Multiplexed Cantilever-free Scanning Probe Lithography

Cantilever-free scanning probe lithography has emerged as a low-cost technique for rapidly patterning nanoscale materials. In this architecture, an array of probes is fabricated on a soft backing layer that provides mechanical compliance to each probe while an underlying hard surface maintains the structural integrity of the array. One drawback of this technique is that each probe in the array acts simultaneously and thus generates a copy of the same pattern. Here, we discuss recent efforts to incorporate heaters into these probe arrays so that when a given heater is activated, the thermal expansion of the elastomer actuates a single tip. We find thermal actuation to be powerful enough to actuate individual tips over 4 $\mu $m with minimal crosstalk, fast enough to actuate on relevant time scales (20 ms), and scalable by virtue of being electrically addressable. Furthermore, tuning the individual heaters allows for variability in the arrays to be compensated for precisely, resulting in high quality nanopatterning. The addition of tunable actuators transforms cantilever-free scanning probe lithography into a technique capable of true desktop nanofabrication.   

Tuning the Spring Constant of Cantilever-free Probe Arrays

The versatility of atomic force microscope (AFM) based techniques such as scanning probe lithography is due in part to the utilization of a cantilever that can be fabricated to match a desired application. In contrast, cantilever-free scanning probe lithography utilizes a low cost array of probes on a compliant backing layer that allows for high throughput nanofabrication but lacks the tailorability afforded by the cantilever in traditional AFM. Here, we present a method to measure and tune the spring constant of probes in a cantilever-free array by adjusting the mechanical properties of the underlying elastomeric layer. Using this technique, we are able to fabricate large-area silicon probe arrays with spring constants that can be tuned in the range from 7 to 150 N/m. This technique offers an advantage in that the spring constant depends linearly on the geometry of the probe, which is in contrast to traditional cantilever-based lithography where the spring constant varies as the cube of the beam width and thickness. To illustrate the benefit of utilizing a probe array with a lower spring constant, we pattern a block copolymer on a delicate 50 nm thick silicon nitride window.   

Debye screening length of electrolytic solutions from capacitive force measurements using atomic force microscopy

We present a method to obtain the Debye screening length of a dilute electrolytic solution by measuring the capacitve force using atomic force microscopy (AFM). A small AC bias voltage of frequency $\omega$ was applied between an AFM cantilever and conducting substrate in an electrolytic solution and the resulting capacitive force between them was measured from the cantilever oscillations. The $2\omega$ component of the oscillating force was used to obtain the capacitance gradient between the AFM cantilever tip and substrate as a function of tip-sample distance $z$. An analytic expression relating tip-sample distance $z$ and capacitance gradient between AFM tip and conducting substrate in an electrolytic solution was derived using the solution of the linearized Poisson-Boltzmann equation. We find that the analytic expression fits well with the experimental data for dilute KCl-water solutions. The fit parameters were further used to calculate the Debye screening length of the electrolytic solution.   

Session W47: Invited Session: The Spread of Cancer and the Tumor Microenvironment

Modeling invasion of brain tissue by glioblastoma cells: ECM alignment and motility

A key stage in the development of highly malignant brain tumors (Glioblastoma Multiforme) is invasion of normal brain tissue by motile cells moving through a crowded, complex environment. Evidence from \emph{in vitro} experiments suggests the cell motion is accompanied by considerable deformation and alignment of the extra-cellular matrix (ECM) of the brain. In the case of breast cancer, alignment effects of this sort have been seen \emph{in vivo}. We have modeled features of this system including stress confinement in the non-linear elasticity of the ECM and contact guidance of the cell motion.   

The interplay between invasion and proliferation in tumor cell navigation

Tumor cells can employ different cellular and molecular modes of invasion. The two main phenotypic mechanisms are: 1. \textit{Amoeboid} (or ``path finder'') cells that can squeeze through small gaps in the ECM (extracellular matrix). 2. \textit{Mesenchymal} (or ``path generator'') cells that are more rigid and can decompose the ECM to pass through. In addition there is interplay between energy directed to more rapid motility vs. energy used for proliferation. Understanding the relative contributions of these distinct mechanisms and the balance between motility and proliferation to the efficiency of metastatic cancer migration is fundamental to the therapeutic targeting of cancer. We present a conceptual and modeling framework for the analysis and assessment of the success rate, time-to-target, and survival probability of amoeboid vs. mesenchymal modes. Similarly, we contrast invasion with and without proliferation. We treat the complex ECM geometry as a maze and employ semi-realistic modeling of cell motility. Our approach includes metabolic and timing degrees of freedom. The theoretical studies were compared with experimental efforts of cell navigation in specially designed microfluidic devices.   

The Interplay between Signaling and Metabolism in Breast Cancer Cell Motility and Metastasis

The initiation and growth of tumor metastases require tumor cells go through a transition between collective-to-individual cell migration. Understanding the molecular, cellular and physical mechanisms of these different migration modes is limited. We focus on the tumor cell migration induced by Hepatocyte Growth Factor / Scatter Factor (HGF/SF) - Met-signaling, a master regulator of cell motility in normal and malignant processes. Met has been implicated in tumorigenesis and metastasis and several Met targeting agents have been introduced into the clinic, and are currently in all phases of clinical trials Our analysis demonstrates that Met signaling dramatically alter the morpho-kinetic dynamics of collective migration of tumor cells. It induce a ``wave'' of increasing velocities that propagates back from the leading edge, increases cells' orientation and cooperation capabilities. In parallel Met signaling induces amoeboid cell motility that increased cell individuality. The decision making regarding the motility mode is dependent on the extent of activation of unique signal and metabolic cues. We present a combination of molecular imaging, conceptual and modeling framework for the analysis and assessment of the collective mesenchymal to epithelial versus amoeboid motility. Combined together our analysis can contribute to the understanding of metastasis and personalizing anti Met targeted therapy.   

The interplay between cell motility and tissue architecture

Glandular tissue form arboreal networks comprised of acini and tubes. Loss of structure is concomitant with the in vivo pathologic state. \textit{In vitro} models have been shown to recapitulate the functional units of the mammary gland and other organs. Despite our much improved understanding gleaned from both in vitro and in vivo interrogation, the mechanisms by which cells are able to achieve the correct tissue organization remain elusive. How do single mammary epithelial cells form polarized acini when cultured in a surrogate basement membrane gel but not on 2D surfaces? Simply put, how does a cell know which way is up? Why do malignant breast cells show a differential response in that they form non-polarized aggregates? Recently, it was determined that non-malignant cells undergo multiple rotations to establish acini while tumor cells are randomly motile during tumor formation. Can it be that a tumor cell has simply lost its way.