Session Z1: Coherent Optical Manipulation of Electron and Nuclear Spin in Artificial Atomic and Molecular Systems in Solids

Ultrafast optical spin echo for electron spins in semiconductors

Spin-based quantum computing and magnetic resonance techniques rely on the ability to measure the coherence time, T2, of a spin system. We report on the experimental implementation of all-optical spin echo to determine the T2 time of a semiconductor electron-spin system. We use three ultrafast optical pulses to rotate spins an arbitrary angle and measure an echo signal as the time between pulses is lengthened. Unlike previous spin-echo techniques using microwaves, ultrafast optical pulses allow clean T2 measurements of systems with dephasing times (T2*) fast in comparison to the timescale for microwave control. This demonstration provides a step toward ultrafast optical dynamic decoupling of spin-based qubits.   

Increasing the electron spin coherence time by coherent optical control of the nuclear spin fluctuations

A single electron spin plays a central role for spin-based quantum information science and electronic devices. One crucial requirement for the future success is to have a long quantum coherence time. It has been demonstrated that in III-V materials, the electron spin coherence time deteriorates rapidly due to the hyperfine coupling with the nuclear environment. Here, we report the increase of the electron spin coherence time by optical controlled suppression of nuclear spin fluctuations through coherent dark-state spectroscopy. The experiment is performed in a single negatively charged InAs self assembled quantum dot (SAQD). The dynamic nuclear spin polarization manifests itself as a hysteresis in the probe absorption spectrum and in the spectral position of the dark state as a function of the frequency scanning direction of the probe field. We demonstrated that the nuclear field can be locked to the maximum trion excitation by observing a flat-top of the trion absorption lineshape, and the switching of the nuclei from unstable to stable configurations by fixing the laser frequencies and monitoring the coherent optical response as a function of time. The optically controlled locking of the nuclear field leads to an enhancement of the electron spin coherence time, which is measured through dark state spectroscopy. The suppression of the nuclear field fluctuations result from a hole spin assisted dynamic nuclear spin polarization feed-back process. We further demonstrated the electron spin coherence enhancement by a three-beam measurement, where two-pump beams lock the nuclear field and the third probe measures the coherence time through the dark state. The inferred spin coherence time is increased by nearly 3 orders of magnitude compared to its thermal value. Our work lays the groundwork for the reproducible preparation of the nuclear spin environment for repetitive control and measurement of a single spin with minimal statistical broadening.   

Tunable spin interactions in self-assembled semiconductor quantum dot molecules

Carrier spins in coupled semiconductor quantum dots have been proposed as logic elements for quantum information processing. The spatial localization of spins in quantum dots has two obvious advantages: common dephasing mechanisms are suppressed and the spins can be spatially selected using a focused laser beam. Here we describe a very important but less obvious advantage. In a cluster of tunnel-coupled dots, Coulomb interactions are substantial, and state energies are very sensitive to the position of each carrier. Through the Pauli principle, small shifts in position can be used to induce large changes in spin energies. This provides a high degree of flexibility and is particularly useful in an optically controlled system, where exciton dipoles produce additional energy shifts. We have developed two-spin quantum dot molecules where one or more electrons or holes can tunnel between two quantum dots [1]. Combining applied magnetic and electric fields, kinetic spin exchange [2] and Zeeman splittings can be used to generate new spin mixings that are not easily obtained in single quantum dots. The mixing results from small asymmetric exchange interactions and produces optical selection rules that can be used for spin initialization, rotation, and measurement [3]. Tunable exchange energies also provide an important level of control over the two-spin resident carrier states that are a model for two-qubit gates and spin entanglement in a semiconductor system. \\[4pt] [1] E. Stinaff, et al., 311, 636 (2006). \\[0pt] [2] M. Scheibner, et al., Phys. Rev. B 75, 245318 (2007). \\[0pt] [3] D. Kim, et al., Phys. Rev. Lett. 101, 236804 (2008).   

Session Z2: Plasmonic Nanogaps: From Single Molecule Sensing to Light Manipulation and Beyond

Emission and propagation properties of surface plasmons on metal nanowires

Manipulating light on the nanometer scale is a challenging topic not only from a fundamental point of view, but also for applications aiming at the design of miniature optical devices. Nanoplasmonics is a rapidly emerging branch of photonics, which offers variable means to manipulate light using surface plasmon excitations on metal nanostructures. Here we report our recent studies about emission and propagation properties of surface plasmons on metal nanostructures. For the propagating properties, we found the propagating plasmons can remotely excite surface enhanced Raman scattering at a few molecules level, and excite the excitons of quantum dots directly. For the emission properties, we observed that light from the end of a silver nanowire, following excitation of plasmons at the other end of the wire, is emitted in a cone of angles peaking at nominally 45-60 degrees from the nanowire axis, with virtually no light emitted along the direction of the nanowire. This surprising characteristic can be explained in a simple picture invoking Fabry- P\'erot resonances of the forward and back-propagating plasmons on the nanowire.   

Quantum description of plasmons in strongly coupled metallic nanostructures

The plasmonic couplings between closely positioned metallic nanoparticles can induce extraordinary large electric field enhancements in the junctions between the particles of relevance for surface enhanced spectroscopies such as SERS.[1] Such plasmonic couplings can also lead to plasmonic interference and coherence effects that manifest themselves as narrow Fano resonances in the optical spectra with extraordinary sensitivities to their dielectric environment.[2] Until very recently, the modeling of the plasmonic response of closely coupled metallic nanoparticles has been made using classical approaches neglecting quantum mechanical effects such as electron tunneling between the particles and screening due to the finite electron density in the junction. In this talk we will present a fully quantum mechanical investigation of the plasmonic response of two coupled metallic nanoparticles as a function of interparticle separation.[3] We identify three distinct regimes of interaction. In the classical regime for separations larger than 1 nm, the nanoparticles remain neutral and the plasmonic response is well described using classical theory. In the cross-over regime for separations between 0.5 and 1nm, electrons begin to tunnel between the nanoparticles and a reduction of the plasmonic couplings and field enhancements result. In the conductive regime for separations smaller than 0.5nm, a large conductive overlap is established between the two particles and a blue-shifted Charge Transfer Plasmon (CTP) emerges.[4] The CTP is a collective plasmon mode which both includes a polarization of the electron distribution of each individual nanoparticle and a significant electron current between the two particles. [1] F. Le et al., ACS Nano 2(2008)707-718 [3] N.A. Mirin, K. Bao, and P. Nordlander, J. Phys. Chem. A 113 (2009)4028-4034 [3] J. Zuloaga, E. Prodan, and P. Nordlander, Nano Lett. 9(2009) 887-891 [4] J.B. Lassiter et al., Nano Lett. 8(2008)1812-1816   

The nano-gap and the emitting molecule: Control of polarization and spectral shape

The realization of single-molecule surface-enhanced Raman scattering (SERS) from molecules positioned within nano-gaps between metallic nanopraticles has opened up exciting opportunities for studying plasmonic fields and their effects on quantum emitters. We recently showed that constructs made of pairs of nanoparticles with an individual molecules bridging their gap can be systematically formed and studied [1]. By changing the size of the particles, we were able to tune the position of the plasmon resonance spectrum, so that the overlap with different parts of the molecular Raman spectrum changed, leading to significant modulation of its shape. More intricate control over molecular properties can be achieved if a third particle is added to the contstruct. It was found that by breaking the dimer symmetry, a third particle can couple strongly to the emitted Raman field and modulate its polarization in a wavelength-dependent fashion [2]. This surprising experimental result was backed up by a series of Generalized Mie calculations, showing the effect of the distance of the third particle, its size and position [3]. Interestingly, the refractive index of the surrounding medium serves as another control parameter that allows changing the coupling between the particles and modulating the polarization of emitted light. \\[4pt] [1] Dadosh T, et al. (2009) Plasmonic Control of the Shape of the Raman Spectrum of a Single Molecule in a Silver Nanoparticle Dimer. Acs Nano 3:1988-1994. \\[0pt] [2] Shegai T, et al. (2008) Managing light polarization via plasmon-molecule interactions within an asymmetric metal nanoparticle trimer. Proc Natl Acad Sci USA 105:16448-16453. \\[0pt] [3] Li ZP, Shegai T, Haran G, Xu HX (2009) Multiple-Particle Nanoantennas for Enormous Enhancement and Polarization Control of Light Emission. Acs Nano 3:637-642.   

Accurate tuning of the electronic coupling and emergent magnetic properties of metal nanoparticle dimers from the linear to nonlinear dielectric-response regime

Closely-packed nanoparticle aggregates have sparked great interest in recent years due to their potential applications in new functional materials and devices at the nanoscale. Experimentally, it has been demonstrated that the properties of such aggregates crucially depend on the electronic coupling between nanoparticles. This coupling originates from the overlap of electronic wavefunctions between neighboring particles, thus requiring a full quantum-mechanical treatment. In this talk, I will discuss the tuning of the electronic coupling via particle separation and external electric field, as well as its effect on the dielectric and magnetic properties of a nanoparticle dimer system [1,2]. Using atomistic real-space first-principles calculation, we find that there is an optimal separation at which the static polarizability reaches its maximal value. Such a peak structure is completely missing in the classical electromagnetic theory, and is associated with the ``bond''-breaking process between the two nanoparticles. In some systems, the electronic coupling can be strong enough to give rise to a net magnetic moment of the dimer, even though the isolated nanoparticles are nonmagnetic. Furthermore, we show that the electronic coupling can be tuned by a modest electric field, resulting in an electric-field tunable magnetic moment. We discuss these results in the context of spin-polarized molecular transport, nanoscale multiferroics, and nanoplasmonics. [1] K. Zhao, M. C. Troparevsky, D. Xiao, A. G. Eguiluz, Z. Y. Zhang, Phys. Rev. Lett. 102, 186804 (2009). [2] M. C. Troparevsky, K. Zhao, D. Xiao, Z. Y. Zhang, and A. G. Eguiluz, ``Tuning the Electronic Coupling and Magnetic Moment of a Metal Nanoparticle Dimer in the Nonlinear Dielectric-Response Regime'', to be published in Nano Lett.   

High harmonic generation by surface plasmon resonance: Design of plasmonic devices and their applications

Seung-Woo Kim has been researching femtosecond ultrafast optics for ultraprecision manufacturing technologies including EUV and X-ray generation. Recently, he and his colleagues achieved a novel method of high-harmonic generation by exploiting the local field enhancement in the nanogap induced by resonant plasmons within a metallic nanostructure consisting of bow-tie shaped gold elements on a sapphire substrate. Plasmonic gold elements enhance the pulse intensity enough to induce high harmonic generation with no extra cavities at all. By injection of argon and xenon gas jets onto bow-tie nanostructures, high harmonics up to 21st (38 nm) order were produced while the incident laser intensity remained only 10$^{11}$ Wcm$^{-2}$. Other nanostructures such as tapered cones are now being investigated to construct laptop-sized coherent EUV sources for advanced lithography and high resolution imaging applications.   

Session Z3: Response of Magnetism to Electric Fields and Light

Domains in multiferroics with magnetically induced ferroelectricity

Two types of multiferroics are distinguished. In the split-order-parameter multiferroics magnetic and ferroelectric order evolve independently while in the joint-order-parameter multiferroics the emergence of the spontaneous polarization is a direct consequence of the magnetic order. The latter type is particularly interesting because of the inherent giant magnetoelectric effects. In the joint-order-parameter multiferroics any magnetoelectric interaction is, at its root, an interaction of its magnetic and ferroelectric domains. Yet, very little is known about the topology of these domains. In my talk I will discuss the domain topology and its magnetoelectric manipulation in a variety of joint-order-parameter multiferroics: MnWO$_{4}$, RMn$_{2}$O$_{5}$, RMnO$_{3}$, CuO, CuCrO$_{2}$. Domains are resolved by optical second harmonic generation. Two types of unusual and fundamentally different domains will be distinguished: (i) hybrid-multiferroic domains in which hallmarks of magnetic and ferroelectric domains are inseparably entangled; (ii) incommensurate translation domains whose walls correspond to discontinuities in the incommensurate magnetic wave vector.   

Femtosecond response of exchange biased bilayers

Ultrafast heating by femtosecond laser pulses can decouple the ferromagnetic and antiferromagnetic layers in exchange biased bilayers, and induce a precession of the magnetization if a reverse magnetic field is applied. In Ni/FeF$_{2}$ bilayers, however, the ultrafast excitation produces novel magnetization dynamics that have not been observed before. An unexpected precession of the magnetization is initiated by a weak excitation, which does not decouple the layers, in reverse magnetic fields that exceed the exchange bias. The precession results from an abrupt change, as a function of the temperature, of the favorable orientation of frustrated spins at the interface. Another remarkable response is obtained when the laser heats the interface above the blocking temperature. The precession is then accompanied by reversal of the exchange bias. The reversal can be induced by a single excitation pulse, and shows that the antiferromagnet is also strongly affected by the optical perturbation. This non-trivial response cannot be extrapolated from the known slow dynamics of the bilayers, and provides important information on the physics of the interlayer coupling.   

Electric field modulation of magnetism in multiferroics

Multiferroics with coexistent ferroelectric and magnetic orders can provide an interesting laboratory to test unprecedented magnetoelectric responses and their possible applications. One such example is the dynamical and/or resonant coupling between the magnetic and electric dipoles in a solid. Here, as the examples of electric field modulation of magnetism in multiferroics, (1) the multiferroic domain wall dynamics and (2) the electric-dipole active magnetic responses are discussed with the review of recent experimental observations.   

Electrical control of exchange coupling in disordered multiferroics

The revival of the magnetoelectric (ME) effect [1] has vitally been boosted by recent intensified research on multiferroic materials [2], which promise to maximise the ME efficiency. While the primordial bilinear ME effect requires stringent symmetry properties, higher order ME effects are less demanding. In particular the biquadratic ME effect has recently attracted growing interest, $e.g$. in ferroelectromagnetic RE manganites, where it is related to the magnetocapacitance or magnetodielectric effect. In disordered systems with broken translational symmetry it is even dominating, while ME effects of lower order may be absent. In type I multiferroics, where magnetic and electric ordering have different origins, it controls the exchange interaction via quadratic spin-lattice interaction. This has been realized in the magnetic relaxor Pb(Fe$_{0.5}$Nb$_{0.5})$O$_{3}$, in quantum paraelectric EuTiO$_{3}$ and in the magnetoelectric multiglass Sr$_{0.98}$Mn$_{0.02}$TiO$_{3}$ [3]. We have measured nonlinear ME$_{E}$ effects in these `disordered multiferroics' using SQUID susceptometry [4] and interpret the \textit{EH}$^{2}$- and $E^{2}H^{2}$-type magnetoelectric (ME) effect in terms of electric field or polarization controlled exchange coupling. \\[4pt] [1] M. Fiebig, \textit{J. Phys}.\textit{ D: Appl. Phys.} \textbf{38}, R123 (2005) \\[0pt] [2] H. Schmid, \textit{Ferroelectrics }\textbf{221}, 9 (1999) \\[0pt] [3] V. V. Shvartsman \textit{et al.}, \textit{Phys. Rev. Lett}. \textbf{101}, 165704 (2008) \\[0pt] [4] P. Borisov \textit{et al., Rev. Sci. Instrum.} \textbf{78}, 106105 (2007)   

Session Z4: Plasmonics Applications

Plasmonics for Photovoltaics

Photovoltaics is transcending its former status as an elegant yet expensive boutique energy technology, and is developing the potential to significantly impact energy supply. Reaching this ultimate goal requires a reduction in the cost per Watt of generated electricity, which motivates both increased conversion efficiency and reduction in material utilization. Both are facilitated by enhancing the optical absorption in solar cell active layers. I will describe a plasmonic photovoltaic design approach in which metallic nanostructures can couple sunlight into guided modes of thin absorber films, enhancing photoabsorption and photocurrent. Recent progress has enabled quantitative understanding of enhanced absorption in plasmonic absorber structures. Full-field electromagnetic simulations are used to calculate spatially and spectrally-resolved photocurrents in plasmonic photovoltaic devices, which can be compared quantitatively with measured solar cell spectral response. Semi-analytic multiple scattering models also yield insights about scattering into guided and free space modes, and losses from parasitic metallic absorption. Experimentally we have observed short-circuit current and efficiency enhancements under AM1.5G solar irradiation for thin GaAs plasmonic solar cells. We will also discuss recent results for enhancement of absorption and photocurrent in thin film amorphous Si solar cells, which feature nanoscale plasmonic structures fabricated by nanoimprint lithography that outperform previously-designed light trapping structures for amorphous silicon cells. Finally, I will describe optical design of light scattering structures that are capable of exceeding previously anticipated absorption limits. Attention to fundamental light-matter interaction physics enables design of solar cells whose absorption surpasses `classical' light trapping limits for planar textured sheet absorbers, enabling new thin solar cell designs.   

Active plasmonics and nano-scale laser light sources

Active plasmonics is a fascinating emerging research area presenting the opportunity to match the length scale of light with those of molecular, solid state and atomic electron wave-functions for the first time. The natural mismatch of visible and infrared light-matter interactions is about three orders of magnitude leaving the majority inherently weak and slow. However, by squeezing light and matter into the same nano-scale volume the interaction not only becomes stronger and faster, weaker effects that were once difficult to detect are dramatically enhanced and more accessible to application. I will discuss how both optical confinement and the available degrees of freedom in plasmonic and meta-material systems give them the unique ability to drastically enhance naturally weak physical effects such as spontaneous emission, stimulated emission and optical non-linearity. I will then use plasmonic amplification as an example of how this physics can be applied. Our recent progress on the realization of laser action of sub-wavelength surface plasmons neatly illustrates how active plasmonics can help us efficiently excite highly localized optical fields, sustain them indefinitely and restore the coherence that is typically destroyed by plasmonic losses. I will summarize by discussing potential applications of plasmonic lasers, loss compensation in plasmonics and lasing of localized surface plasmons.   

Plasmonics for Nanowaveguides, Nanoantennas, and Imaging

There has been significant interest and development in the field of plasmonic optics in recent years due to the numerous breakthroughs in the areas of nanotechnology, nanooptics and exciting potentials for merging of nanooptics and nanoelectronics. By combining the plasmonic phenomena with the notion of metamaterials, in my group we have been developing the concept of `metactronics' in which properly arranged collections of plasmonic and non-plasmonic nanostructures can provide a platform for tailoring and manipulating optical signals at the nanoscale. This paradigm can provide `circuits with light at the nanoscale', bringing some of the concepts of microwaves, such as waveguiding, antennas, and imaging into the nanoscale optics, thus providing nanostructures that function as nanowaveguides, nanoantennas, etc. In my group, we have been extensively exploring these topics and some of their exciting potential applications. In this talk, we will give an overview of our recent results on this topic.   

Biomedical Plasmonics

The near infrared region of the optical spectrum provides a window into the human body that can be exploited for diagnostics and therapeutics, offering an opportunity to merge these concepts. We have shown that the strong light-absorbing and light-scattering properties of noble metal nanoparticles can be controlled by manipulating their shape: in a core-shell geometry, the metallic shell layer can be easily tuned to this spectral region. This `nanoshell' geometry has proven to be ideal for enhancing both diagnostic and therapeutic modalities for cancer. Nanoshells can serve as light scattering beacons, strong enhancers of fluorescent markers for optical tomography, and impart a highly effective, targeted therapeutic response via their unparalleled light-to-heat conversion properties. This latter effect has been used to induce cell death and tumor remission in animals at greater than 90{\%} efficacy, and is currently in clinical trials. This nanoparticle platform can be combined with MRI contrast agents for the enhancement of dual imaging modalities, and also shows promise as a light-controlled nonviral vector for intracellular gene delivery.   

Plasmonics for data storage and photo-catalytic chemical reactor

Applications of plasmonic effects for data storage and photo-catalytic chemical reactor will be discussed in this talk. Plasmonic near-field optical and thermal interactions are considered to be the novel methods to achieve ultrahigh capacity and density for data storage. The localized and enhanced electromagnetic field of plasmonic nanostructures provides ultrahigh spatial resolution for achieving nano recording marks size. The readout of nano recording marks closely relies on the plasmonic coupling effect as well. Responses of the local plasmonic nano-structures of the nano thin films are found to be the key of the nano storage. Similarly, local electromagnetic interactions of various plasmonic nanostructures for the photo-catalytic chemical process are useful as well. Measurement and analysis of the photo-catalytic process happened in the plasmonic photo-chemical reactors clearly demonstrate better efficiency of some photo-catalytic chemical process such as the decomposition of the Methyl Orange to carbon dioxide and water. Interesting and promising applications of the plasmonic nanostructures on data storage and photo-catalytic chemical reactor are demonstrated.   

Session Z9: QHE: Quantum Computing

Double quantum dots at the edge of Abelian and non-Abelian factional quantum Hall sates

We theoretically study two quantum dots tunnel coupled to the edge of Abelian and non-Abelian fractional quantum Hall states. We find a number of interesting low-energy fixed points which are a function of inter-dot coupling and separation. We study the stability and structure of the fixed points under various perturbations.   

Density dependence of the 5/2 energy gap

In this talk, we will present results from our recent experiments examining the spin-polarization of the 5/2 state by investigating the competition between the Coulomb and Zeeman energies utilizing a HIGFET (heterojunction insulated-gated field-effect transistor) device. Rather than tuning their ratio in a fixed density specimen by tilt, we keep the $B$-field perpendicular to the 2D electron gas and vary its density. This approach is equivalent to tilting the sample, but it cannot cause a tilted-field induced phase transition. The HIGFET device has a peak electron mobility of $12 \times 10^6$ cm$^2$/Vs, more than a factor of two increase compared to the one used in an earlier study. We observed that in the density range of $1.2 - 3.6 \times 10^{11}$ cm$^{-2}$ the 5/2 state was activated. Therefore, a true 5/2 energy gap was obtained. It increases with increasing electron density. We have fitted the density dependence with various theoretical models and will discuss its implications on the spin polarization of the 5/2 state.   

Activation energies for the $\nu $=5/2 Fractional Quantum Hall Effect at 10 Tesla

We reported on the low-temperature magnetotransport in a high-purity (mobility $\sim $ 1$\times $10$^{7}$cm$^{2}$/Vs) modulation-doped GaAs/AlGaAs quantum well with a high electron density (6$\times $10$^{11}$ cm$^{-2})$. A quantized $\nu $=5/2 Hall plateau is observed at B $\sim $ 10 T, with an activation gap $\Delta _{5/2} \sim $ 125$\pm $10 mK; the plateau can persist up to $\sim $ 25$^{o}$ tilt-field. We determined the activation energies $\Delta $ and quasi-gap energies $\Delta ^{quasi}$ for the $\nu $=5/2, 7/3, and 8/3 fractional quantum Hall states in tilted-magnetic field ($\theta )$. The $\Delta _{5/2} $, $\Delta _{7/3} $ and the $\Delta _{5/2}^{quasi} $, $\Delta _{7/3}^{quasi} $are found to decrease in $\theta $. We will present the systematic data and discuss their implications on the spin-polarization of $\nu $=5/2 states observed at 10 T.\\[4pt] [1] R. Willett, Phys. Rev. Lett. \textbf{59}, 1776 (1987).\\[0pt] [2] W. Pan et al, Solid State Commun. \textbf{119}, 641 (2001).   

Breaking of Particle-Hole Symmetry by Landau Level Mixing and the $\nu=5/2$ Quantized Hall State

The nature of the $nu$=5/2 quantum Hall state has been a puzzle for several decades. Based on a large body of numerical work, the community has been slowly converging to the conclusion that the 5/2 state is the same phase of matter as described by the Moore-Read wavefunction. However, two recent papers [1,2] point out that in fact two inequivalent possibilities still remain--the Moore-Read wavefunction, and its particle-hole conjugate, the so-called Anti-Pfaffian, which are distinct topological phases. In the absence of Landau-level mixing (an approximation used in all prior numerical works) these two possibilities cannot be distinguished. In the current work, we perform numerical studies aimed to determine if the fractional quantum Hall state observed at filling $\nu$=5/2 is the Moore-Read wavefunction or the Anti-Pfaffian. Using a truncated Hilbert space approach we find that for realistic interactions, including Landau-level mixing, the Moore-Read state is strongly favored. We also confirm that the ground state remains polarized even in the absence of Zeeman energy when Landau level mixing is allowed. [1] M. Levin, B. I. Halperin and B. Rosenow, Phys. Rev. Lett. 99, 236806 (2007). [2] S.-S. Lee, S. Ryu, C. Nayak and M. P. A. Fisher, Phys. Rev. Lett. 99, 236807 (2007).   

Local Compressibility of the Fractional Quantum Hall State at Filling Factor 5/2

Understanding the ground state and excitations of the quantum Hall state at filling factor $5/2$ is a subject of great interest due to the possibility of realizing non-abelian braiding statistics. All previous experimental probes of the state have relied on transport, and have therefore only accessed the chiral edges of the system. Here we present the first thermodynamic measurements of bulk properties at $\nu=5/2$. By measuring the local compressibility, $\left( \frac{\partial \mu}{\partial n} \right)^{-1}$, of states in the second Landau level, we can monitor the charging of individual localized states in the bulk. Comparing charging spectra at $\nu = 7/3$ and $\nu = 5/2$, we are able to extract the ratio of local charges in the bulk at these filling factors. Averaged over several disorder configurations and samples, we find this ratio to be $4/3$, suggesting that the local charges for these states are $e^*_{7/3}=e/3$ and $e^*_{5/2}=e/4$. Further, by integrating $\frac{\partial \mu}{\partial n}$, we obtain a value for the thermodynamic gap $\Delta_{5/2}$ without relying on activated transport.   

Clustering in quantum Hall wavefunctions and conformal field theory amplitudes

We consider lowest Landau level wavefunctions for bosons subjected to a magnetic field in the plane. We study the zero-energy eigenstates of a projection Hamiltonian which forbids three particles to come together with relative angular momentum less than six and, in addition, forbids one of two linearly-independent states of relative angular momentum six. The counting of edge excitations of this Hamiltonian agrees with the character formula for the N=1 superconformal Kac vacuum module at generic central charge c. This Hamiltonian is expected to be gapless for all c. For particular c, we try to ``improve'' the Hamiltonian by adding additional terms (related to singular vectors in the modules), so as to obtain a rational theory. We consider specifically states whose wavefunctions are related to the M(8,3) and tricritical Ising CFTs.   

Alternating e/4 and e/2 period interference oscillations as evidence for filling factor 5/2 non-Abelian quasiparticles

It is a theoretical conjecture that 5/2 fractional quantum Hall state charge e/4 excitations may obey exotic non-Abelian statistics. In edge state interference these purported non-Abelian quasiparticles should display period e/4 Aharonov-Bohm oscillations if the interfering quasiparticle encircles an even number of localized e/4 charges, but suppression of oscillations if an odd number is encircled. To test this hypothesis, here we perform swept area interference measurements at 5/2. We observe an alternating pattern of e/4 and e/2 period oscillations in resistance. This aperiodic alternation is consistent with proposed non-Abelian properties: the e/4 oscillations occur for encircling an even number of localized quasiparticles, e/2 oscillations are expressed when encircling an odd number. Aperiodic alternation corresponds to the expected area sweep sampling the localized quasiparticles. Importantly, adding localized quasiparticles to the encircled area by changing magnetic field induces interchange of the e/4 and e/2 oscillation periods, specifically consistent with non-Abelian e/4 quasiparticles.   

Transport in line junctions of $\nu=5/2$ quantum Hall liquids

We calculate the tunneling current through long line junctions of a $\nu=5/2$ quantum Hall liquid and i) another $\nu=5/2$ liquid, ii) an integer quantum Hall liquid and iii) a quantum wire. Momentum resolved tunneling provides information about the number, propagation directions and other features of the edge modes and thus helps distinguish several competing models of the 5/2 state. We investigate transport properties for two proposed Abelian states: $K=8$ state and 331 state, and four possible non-Abelian states: Pfaffian, edge-reconstructed Pfaffian, and two versions of the anti-Pfaffian state. We also show that the non-equilibrated anti-Pfaffian state has a different resistance from other proposed states in the bar geometry.   

Exact Solution for Bulk-Edge Coupling in the Non-Abelian $\nu=5/2$ Quantum Hall Interferometer

It has been predicted that the phase sensitive part of the current through a non-abelian $\nu = 5/2$ quantum Hall Fabry-Perot interferometer will depend on the number of localized charged $e/4$ quasiparticles (QPs) inside the interferometer cell. In the limit where all QPs are far from the edge, the leading contribution to the interference current is predicted to be absent if the number of enclosed QPs is odd and present otherwise, as a consequence of the non-abelian QP statistics. Here, we consider a localized QP which is close enough to the boundary so that it can exchange a Majorana fermion with the edge via a tunneling process. We derive an exact solution for the dependence of the interference current on the coupling strength for this tunneling process, and confirm a previous prediction that for sufficiently strong coupling, the localized QP is effectively incorporated in the edge and no longer affects the interference pattern. We confirm that the dimensionless coupling strength can be tuned by the source-drain voltage, and we find that there is a universal shift in the interference phase as a function of coupling strength.   

A numerical study of the disorder effect on the 5/2 fractional quantum hall system

We study the 5/2 fractional quantum hall (FQH) system in the presence of random disorder using exact diagonalization in the torus geometry. We examine the low-lying spectra with different unit cells and find a persistent and robust spectral gap characterizing the incompressible Pfaffian-like states. This gap narrows with increasing disorder strength. The structure factor also exhibits robust characteristics of a uniform quantum hall liquid. The topologically invariant chern number has been calculated to determine the mobility gap of the FQH states, which is used to compare with the experimentally measured activation gap. The mobility gap tends to collapse as the disorder grows, suggesting a disorder-driven transition from the FQH states to the insulator. We further demonstrate the characteristic features of the ground state of the 5/2 FQH system and Pfaffian states using reduced density matrices.   

The entanglement gap and a new principle of adiabatic continuity

We give a complete definition of the entanglement gap separating low-energy, topological levels, from high-energy, generic ones, in the ``entanglement spectrum'' of Fractional Quantum Hall (FQH) states. By removing the magnetic length inherent in the FQH problem - a procedure which we call taking the ``conformal limit,'' we find that the entanglement spectrum of an incompressible ground-state of a generic (i.e. Coulomb) lowest Landau Level Hamiltonian re-arranges into a low-(entanglement) energy part separated by a full gap from the high energy entanglement levels. As previously observed, the counting of these levels starts off as the counting of modes of the edge theory of the FQH state, but quickly develops finite-size effects which we show can also serve as a fingerprint of the FQH state. As the sphere manifold where the FQH resides grows, the level spacing of the states at the same angular momentum goes to zero, suggestive of the presence of relativistic gapless edge-states. We use the entanglement spectrum in the conformal limit basis to investigate whether two states are topologically connected, by using the adiabatic continuity of the low entanglement energy levels.   

General trial wave functions for a three body interaction

The Pfaffian wave function, which is a candidate for the 5/2 FQHE state, is the exact ground state of a short range three body model interaction, but little is known about the solutions of this model at other filling factors. Our starting point is the observation that the Pfaffian can be obtained by fully anti-symmetrizing a bilayer wave function of Halperin. A more general class of composite fermion wave functions for bilayer systems was constructed by Scarola and Jain. We find that, upon full antisymmetrization, these wave function provide a decent approximation to the low energy solutions of the three body model interaction at filling factors other than 1/2. The charged and neutral excitations of the full state are naturally constructed by creating excitations in one or both ``layers.'' We also investigate how well the ground and excited state wave functions work for the Coulomb interaction, both in the lowest and the second Landau levels. Systems with up to 18 particles are studied by a combination of exact diagonalization and Monte Carlo method.   

Superconducting Order Parameter of the Even-denominator Fractional Quantum Hall Effect

Usually formulated in terms of a trial wave function called the Moore-Read Pfaffian wave function, current leading theories attribute the origin of the 5/2 FQHE to the formation of a new superconducting state. The nature of superconductivity in the 5/2 FQHE is particularly puzzling in the sense that this state apparently coexists with strong magnetic field, which poses an interesting dilemma since the Meissner effect is the most important defining property of superconductivity. To overcome this dilemma, it is crucial to understand what it is that actually forms the superconducting condensate, if any. Here, we develop a numerically exact method to create a Cooper pair in terms of the true (elementary) quasi- particle of the system (identified with composite fermion) and explicitly compute the superconducting order parameter as a function of real space coordinate instead of usual momentum. As results, in addition to direct evidence for superconductivity, we obtain quantitative predictions for superconducting coherence length. Based on our calculation, we propose an experimental setup for demonstrating the 5/2 FQHE counterpart of the Josephson effect and thus, if successful, conclusively proving the existence of superconductivity in the 5/2 FQHE.   

A special relationship between non-unitary non-Abelian and unitary Abelian quantum Hall states

We point out a special relationship between quantum Hall states connected with non-unitary conformal field theories (CFTs) and those connected with unitary bosonic CFTs. They are related via boundary insertions as encoded in their root configurations. We discuss the cases of Gaffnian and Haldane-Rezayi non-unitary theories (i.e. quantum Hall states) which, via boundary insertions, can be transmuted into abelian bosonic unitary theories. The construction mimics a global change of parameters in the phase space of the electron system. We also discuss the cases of permanent non-unitary theory and Pfaffian unitary theory which are resistant to this mechanism described in M.V. Milovanovic, Th. Jolicoeur, and I. Vidanovic, Phys. Rev. B 80, 155324 (2009).   

Session Z10: Physics of Physiological Systems

Quantitative Analysis of \textit{Dictyostelium Discoideum} Chemotaxis

We used microfluidic tools to expose \textit{Dictyostelium discoideum} to stationary spatial gradients of the chemoattractant cyclic adenosine 3',5' monophosphate (cAMP). At a cAMP gradient of $10^{-2}$ ${\rm nM}/\mu{\rm m}$, the chemotactic velocity reached a plateau, which continued for gradients up to 1 ${\rm nM}/\mu{\rm m}$. Our measurements agree with [Song at al, Eur. J. Cell Biol., 85(10):981]. We also found that the chemotactic velocity was highly correlated with the cell's polarization. We present a model based on a generalized Langevin equation that provides good agreement with the measured data.   

Memristive model of amoeba learning

Recently, it was shown that the amoeba-like cell Physarum polycephalum when exposed to a pattern of periodic environmental changes learns and adapts its behavior in anticipation of the next stimulus to come. Here we show that such behavior can be mapped into the response of a simple electronic circuit consisting of a LC contour and a memory-resistor (a memristor) to a train of voltage pulses that mimic environment changes [1]. We also discuss a possible biological origin of the memristive behavior in the cell. These biological memory features are likely to occur in other unicellular as well as multicellular organisms, albeit in different forms. Therefore, the above memristive circuit model, which has learning properties, is useful to better understand the origins of primitive intelligence. [1] Yu. V. Pershin, S. La Fontaine, and M. Di Ventra, Phys. Rev. E 80, 021926 (2009)   

Dynamics of asexual reproduction in flatworms

Planarians (flatworms) are one of the simplest bilaterally symmetric organisms and famous for their extraordinary regenerative capabilities. One can cut a worm in 100 pieces and after a few weeks one obtains 100 new worms that have reconstructed their entire body, including a central nervous system. This amazing regenerative capability is due to a population of stem cells distributed throughout the planarian body. These stem cells do not only allow the worms to heal without scarring after wounding, they also allow for asexual reproduction: Planarians can split themselves in two, and then regenerate the missing body parts within about a week. Naively, one would think that this kind of asexual reproduction could be captured by simple models that describe cell growth in bacteria or other lower organisms. However, we find that there is much more to the story by monitoring $>$15 generations of many individuals, as well as the long-term behavior ($>$ 9 months) of worm populations under different environmental conditions, such as population density, temperature, and feeding frequency. Surprisingly, we observe that reproduction decreases with increasing food supply, opposite to the relationship between food and reproduction in other asexually reproducing organisms (e.g. bacteria, yeast), and causing obese worms. Finally, our data allows us to address the question of aging in an organism that is thought to be ``forever young''.   

\textit{E. coli} as a biological model for cancer cells

Uninhibited growth and invasion of healthy tissue characterize cancer. We co-cultured two strains of \textit{E. coli} bacteria in a microfabricated environment to model cancer. During starvation, growth-advantage-in-stationary-phase, or GASP, cells grew to a higher population than wild-type cells. GASP cells also displaced wild-type cells from nutrient-rich chambers. When we repeated the experiment with medium depleted by wild-type cells, the peak GASP population density increased 54\%, and the ``invasion,'' or displacement of wild-type cells from nutrient-rich chambers, occurred 5 hours earlier. We mathematically modeled both this increase in GASP population and this acceleration of spatial invasion by assuming that GASP cells consume detritus secreted by wild-type cells. Our experimental and model results corroborate recent caution against using tumor starvation as a cancer therapy.   

Hybrid Cellular Continuum Simulations of Heterogeneity in Tumor Growth

We will discuss simulations of pre-angiogenic tumor growth using a class of hybrid cellular-continuum models. A lattice site can be occupied either by a cell of a specific tumor cell population or consist of extracellular matrix. The local concentrations of oxygen is described by continuum reaction-diffusion equations. Dynamic linked lists of cells are evolved in time and contain information on cell type, position, age, concentration of oxygen at cell site. When cells proliferate via mitosis or differentiate, new cells are added to the list, if mutation occurs the cell types are altered, and if the cell dies via apoptosis the cells are removed from the linked list. The motion of individual cells consist of random walks subject to caging and chemotaxis away from regions of low oxygen concentration. We will describe the heterogenous spatial segregation of different cell types in the tumor, the development of necrotic cores as well as micronecrotic regions, and the effects of externally applied drugs on cell populations and overall tumor shape.   

A Molecular Model of Plant Cell Morphogenesis: The Case of Polar Growth in Pollen Tubes

The growth of plant, fungal, and bacterial cells depends critically on two processes: the deposition of new wall material at the cell surface and the mechanical deformation of this material by forces developed within the cell. To understand how these two processes contribute to cell growth, we have undertaken an experimental and theoretical investigation of polar morphogenesis in pollen tubes. The pollen tube is an ideal model system for the study of polar growth because of its rich phenomenology and its ease of experimental manipulation. We formulated an experimentally-motivated model of pollen tube morphogenesis that incorporates 1) the microscopic architecture and rheology of the polymeric wall, 2) the dynamics of intracellular calcium, a key morphogen in pollen tubes, and 3) the exocytosis of wall material. These processes constitute a feedback loop that controls growth. Our model shows two regimes corresponding to observed steady and pulsatile growth in pollen tubes. The model accounts for the frequency, amplitude and waveform of pulsatile cells, and the scaling relationships between these variables. By solving the dynamical system on a three-dimensional thin-shell geometry we can also explain the surface expansion pattern and morphology of steady-growing cells.   

Experimental Evidence of Strong Anomalous Diffusion in Living Cells

We show that transport of polymeric particles within living cancer cells exhibits strongly anomalous diffusion. Particle motion demonstrated super-diffusion, indicating active cellular transport of particles likely due to molecular motors. We also calculated a range of time-dependent displacement moments and extracted scaling exponents\textit{ $\lambda $}($q)$ for each moment order $q$. Those were non-linear with $q$, indicating non-scale-invariant motion. Also, \textit{$\lambda $}($q)$/$q$ was non-decreasing, fulfilling conditions for strong anomalous diffusion, presented here experimentally for the first time. Specifically, \textit{$\lambda $}($q)$ exhibited bi-linearity, with slopes of $\sim $0.6 and $\sim $0.8 at low and high $q$-values. That bi-linearity indicates that particle motion is composed of sub-diffusive regimes separated by active flights; those were sub-ballistic and not separable using a directionality criterion. We suggest that sub-ballistic flights are associated with the small particles used in this work (100-200 nm); those diffuse through the cytoplasm while being actively transported. Results are discussed in terms of particle interactions with their microenvironment and its dynamics.   

Damage and fluctuations in optimal transport networks

Leaf venation is a pervasive example of a complex biological transport network that is necessary for the survival of land plants. Transport networks optimized for efficiency have been shown to be trees, i.e. loopless. However, dicotyledon leaf venation has a large number of functional closed loops. Inspired by leaf venation, we study two possible reasons for the existence of a high density of loops in biological transport networks: resilience to damage and fluctuations in load (transpiration rate across the leaf blade). We consider optimizing functionals that account for these two criteria, and examine the topology and transport properties of the resulting networks.   

Terminating Ventricular Fibrillation Using Pulsed Far-Field Stimulation in Whole Rabbit Hearts

During life-threatening cardiac fibrillation, chaotic spatio-temporal dynamics is mediated by vortex-like rotating waves. Current defibrillation strategies rely on global control through high-energy shocks, which may have severe side-effects including traumatic pain and tissue damage. Far-field antifibrillation pacing terminates fibrillation using a train of low-energy electric pulses [1,2]. Using optical mapping in isolated rabbit heart preparations, we evaluate the efficiency and robustness of this approach. We found that a series of pulses at low energies ($<$ 2.0 J) is sufficient to extinguish ventricular fibrillation with a success rate of 95{\%}. We will discuss the physical mechanisms involved.\\[4pt] [1] F.H. Fenton et al, Circulation 120 467-476 (2009).\\[0pt] [2] A. Pumir et al., Phys. Rev. Lett. 99, 208101 (2007).   

Propagation Dynamics of Cardiac Action Potentials on a Ring

We study the effects of the underlying ion-channel dynamics on the morphology and propagation of cardiac action potentials (AP) by investigating the stability of small perturbations to a steady-state AP rigidly propagating along a fiber of cardiac cells arranged in a ring. The Fox-McHarg-Gilmour model (FMGM) is used to describe the ion-channel dynamics, and a standard gap-junction term is used to couple neighboring cells. We compute the eigenvalues and eigenmodes of the infinitesimal evolution matrix in the moving frame and, along with numerous stable modes, find several unstable eigenmodes. Their frequencies are half-integer multiples of the fundamental frequency of action potential repetition, and represent alternans modes of increasing degree of discordance. Our results for a fiber of cells described by a physiologically realistic ion-channel model (FMGM) agree with earlier simulations for a single cardiac cell and for spiral waves in 2D cardiac tissue. The analysis provides the basis for developing more efficient electrical stimulation protocols for controlling alternans.   

Phase singularities in cardiac electrical activity

Abstract theory of topological spaces has its analogy in biological systems, one of which is the heart. The heart is an excitable medium that can be represented as a set of excitable elements (cardiomyocytes) that behave similarly to hourglasses. Excitable element needs external stimuli to be excited and after finite time goes back to its initial state, so its natural topological space is a ring. Topological space set (phases) can be simple set as \textit{``rest,'' ``excited,'' ``refractory,'' ``relatively refractory'', }but it can be as continuous as a set of angles on a 2$\pi $ circle. In topological spaces topological charge is defined by: \[ W=\frac{1}{2\pi }\oint\limits_l {d\theta } (l) \] where $l$ is the integration path and \textit{d$\theta $} is the change in phase. Non zero topological charge is called phase singularity of mapping. Practical application of topological charge analysis is a powerful method to quantify electrical dynamics during ventricular fibrillation (VF). Particularly by means of phase singularity detection it is possible to track wave breaks which relate to anatomical and electrophysiological heterogeneities.   

The role of resonant ear canal thermal noise pressure on the eardrum in helping to determine auditory thresholds

The influence of thermal pressure fluctuations on the tympanic membrane has been re-examined as a possible contributing determinant of the threshold of human hearing over the range of audible frequencies. The early approximate calculation of Sivian and White [1] is shown to result in higher values of thermal noise pressure on the tympanium of a model meatus than the result obtained by directly calculating the noise pressure from thermally excited resonant ear canal modes. \\[4pt] [1] L.J. Sivian and S.D. White, ``Minimum audible sound fields,'' J. Acoust. Soc. Am. \textbf{4}, 288-321 (1933).   

Characterization of chaotic dynamics in the human menstrual cycle

The human menstrual cycle exhibits much unexplained variability, which is typically dismissed as random variation. Given the many delayed nonlinear feedbacks in the reproductive endocrine system, however, the menstrual cycle might well be a nonlinear dynamical system in a chaotic trajectory, and that this instead accounts for the observed variability. Here, we test this hypothesis by performing a time series analysis on data for 7438 menstrual cycles from 38 women in the 20-40 year age range, using the database maintained by the Tremin Research Program on Women's Health. Using phase space reconstruction techniques with a maximum embedding dimension of 6, we find appropriate scaling behavior in the correlation sums for this data, indicating low dimensional deterministic dynamics. A correlation dimension of $\cong $2.6 is measured in this scaling regime, and this result is confirmed by recalculation using the Takens estimator. These results may be interpreted as offering an approximation to the fractal dimension of a strange attractor governing the chaotic dynamics of the menstrual cycle.   

Low Velocity Waves Inside and Outside of Plants

I have been reporting organizing waves in plants for many years. In 1989 I reported wave travel between plants. The waves travel at near 25 m/s horizontally through air on earth. Recently I built my own transmitters and receivers and found that the waves will penetrate mountains. Monitoring plants suggest that there is constant communication between plants with the cacophony peaking during the summer months. The location of the sun has a direct influence. I hypothesize that the observed waves are waves in dark matter as well as the other media involved. Apparently dark matter not only interacts with gravity but has much to do with the organization of nature. The velocities of waves in plants peak vertically. For example in Ponderosa pine the ratio of the vertical to horizontal velocity is 3/1 making a tall spindly tree. In apple the ratio is 4/3 making a nearly round tree. The velocity anisotropy may suggest that dark matter interacts differently with respect to the gravity direction. The penetrating qualities of the waves may provide useful communication. There appears to be a rather large velocity distribution, however, when the waves travel far through dense matter.   

Session Z11: Lipid Bilayers II

Stress Induced Domain Formation in Multilamellar Lipid Bilayers

Domain formation in lipid mixtures due to phase separation of the components is a well-known phenomenon that has been studied in mono- and bi-molecular lipid configurations. We report same phenomenon, however, in multilamellar configurations consisting of thousands of lamellae where the domain pattern in each layer is interestingly aligned with the other lamellae. In this process, both dehydration and hydration of lipid cake can act as the driving force to separate two phases of liquid ordered and liquid disordered. In a controlled experiment with a stack lipid saturated with water, mechanical perturbation can induce domain formation too. Series of experiments of this kind reaches us to the conclusion that any sort of stress in special condition may cause domain formation. We use a combination of microscopy tools including AFM, fluorescence confocal and bright-field microscopy to determine the influence of interaction between the line tension and key elastic properties of the lipid bilayers. As a particular interest we studied the dynamics of the domain pattern formation and the interactions between the domains such as long-term fusion.   

Antibiotics and the mechanics of cellular bulging in gram-negative bacteria

For most bacteria, the cell wall, consisting of a cross-linked polymer network, is the primary stress-bearing structure. Due to the high osmotic pressure difference across the cell membrane, the presence of the cell wall is essential for cell stability. Recent experiments have addressed the effect of cell-wall defects induced by antibiotics such as vancomycin, and find that in Gram-negative bacteria, antibiotics can lead to pronounced bulging of the cell membrane and eventually to lysis. Here we address the mechanics of bulging and its relationship to cell-wall defects. We estimate the critical defect size for bulging and discuss the biological implications of our results. We also discuss the relevance of our physical model to blebbing and vesiculation in eukaryotic cells.   

Compositional Heterogeneity in Ternary Models for the Cell Membrane

Ternary models for the cell membrane comprised of cholesterol (Chol) plus high and low melting temperature lipids exhibit rich phase behavior as a function of temperature and composition. Of particular interest is a region of coexisting disordered and ordered fluid phases that is thought to indicate how lipids organize to promote protein function in the cell membrane. We have used fluorescence resonance energy transfer to investigate the ternary mixtures DOPC(1,2-dioleoyl-sn-glycero-3-phosphocholine)/bSM (porcine brainsphingomyelin)/Chol and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine)/bSM/Chol at high compositional resolution. We confirmed liquid coexistence for DOPC/bSM/Chol at 15 and 25C that melts by 35C, but in contrast to previous studies we detected no fluid-phase compositional heterogeneity for POPC/bSM/Chol from 5-35C. If domains exist, they must be smaller than the approximately 5 nm sensitivity provided by the fluorescent lipid analogs employed. We propose electron spin resonance and x-ray scattering for measuring whether liquid-phase compositional heterogeneity occurs for POPC/bSM/Chol. Understanding POPC/bSM/Chol phase behavior will provide a framework for investigating peptide/lipid interactions in a biologically relevant lipid mixture.   

Field-theoretic model of membrane/protein assemblies

We developed a coarse-grained field-theoretic model of an assembling membrane/protein system that includes immovable cylindrical transmembrane protein(s) and assembling membrane species including neutral and charged lipids, counterions, and water. This field-theoretic model is able to capture the molecular architecture of lipids that constitute the membrane. Our study focused on the following aspects: i) membrane thickness fluctuation due to hydrophobic mismatch, ii) the corresponding lipid tail stretching near the protein, iii) ion and cationic lipid distributions at various levels of protein head charge, iv) protein tilting, v) membrane-mediated protein-protein interactions, and vi) in-plane packing structures of proteins. Our simulations were used to evaluate free energies and structure that enabled the quantitative analysis of features such as lipid tail stretching and membrane/protein structural stability.   

Effect of biocompatible polymers on the structural integrity of lipid bilayers under external stimuli

Cell membrane dysfunction due to loss of structural integrity is the pathology of tissue death in trauma and common diseases. It is now established that certain biocompatible polymers, such as Poloxamer 188, Poloxamine 1107 and polyethylene glycol (PEG), are effective in sealing of injured cell membranes, and able to prevent acute necrosis. Despite these broad applications of these polymers for human health, the fundamental mechanisms by which these polymers interact with cell membranes are still under debate. Here, the effects of a group of biocompatible polymers on phospholipid membrane integrity under osmotic and oxidative stress were explored using giant unilamellar vesicles as model cell membranes. Our results suggest that the adsorption of the polymers on the membrane surface is responsible for the cell membrane resealing process due to its capability of slowing down the surface hydration dynamics.   

Towards general design rules for membrane active antimicrobials

Membrane active antimicrobials are short amphipathic peptides that selectively disrupt and lyse bacterial cell membranes. While it is believed that the combination of peptide hydrophobicity and cationic charge is essential for function, the detailed molecular mechanism of selective membrane permeation remains unclear. We use synchrotron small angle x-ray scattering (SAXS) to investigate the interaction of model bacterial and eukaryotic cell membranes with archetypes from each of the three defensin subfamilies found in mammals. The relationship between membrane composition and peptide induced changes in membrane curvature and topology is examined. By comparing the membrane rearrangement and corresponding phase behavior induced by these different peptides we will discuss the importance of amino acid composition and placement on antimicrobial peptide design.   

pH-dependent Differential Scanning Calorimetry and Dynamic Light Scattering Studies of 21:0 PC and 18:0 PS Lipid Binary System

Large unilamallar vesicle has been a model system to study many membrane functions. High Tg lipid systems offer many potential biomedical applications in lipid-based delivery applications. While the optimized vesicle functionalities are achieved by Polyethylene Glycol (PEG) polymer, modified PEG and other functional molecule incorporation, however, the host binary lipid system plays the pivotal role in pH-dependent phase transition based lipid vehicular methods. We have investigated a lipid binary system composed of 21:0 PC (1,2-dihenarachidoyl-sn-glycero-3-phosphocholine) and 18:0 PS(1,2-distearoyl-sn-glycero-3-phospho-L-serine). Preliminary studies implementing differential scanning calorimetry shows pH plays key role in temperature shift and thermotropic phase behavior of the binary system. While dynamic light scattering study shows lipid vesicle size is almost independent of pH changes. We will also present pH-dependent thermodynamic parameters to correlate underlying molecular mechanism in relevant pH-range.   

Quantitative Imaging of Membrane Shape Transformation and Pearling

Experiments show, in areas from vesicle budding, to pearling and even stochastic fluctuation of shape, the ubiquity of non-spherical shape in phospholipid assemblies. Here we focus on pearling and the massive stochastic fluctuations which precede it when nanoparticles induce this transformation by adsorption to the inner leaflet of a giant unilamellar vesicle (GUV). Novel methods to quantify non-spherical contours in movies with massive numbers of frames allow us to imagine membrane fluctuations frame-by-frame, even in the case of low signal-to-noise.   

Criticality in Plasma Membranes

Recent work in giant plasma membrane vesicles (GPMVs) isolated from living cells demonstrated that they can be tuned with a single parameter (temperature) to criticality, not far from in vivo temperatures [1,2]. Criticality requires the fine-tuning of two parameters suggesting important biological function, and its presence resolves many of the paradoxes associated with putative lipid rafts. Here we present a minimal model of membrane inhomogeneities. We incorporate criticality using a conserved order parameter Ising model coupled to a simple actin cytoskeleton interacting through fields which act as point-like pinning sites. Using this model we make a host of experimentally testable predictions that are in line with recent published findings. At physiological temperatures we find inhomogeneities in the form of critical fluctuations with a length scale of roughly 20nm. Individual constituents making up these liquid domains are mobile, though they diffuse anomalously, but the correlated regions themselves can last as long as the cytoskeleton persists. We explain this by considering the effective long ranged interaction mediated by the Ising order parameter. In general we find Ising criticality organizes and spatially segregates membrane components by providing a channel for interaction over large distances. [1] Veatch et al., ACS Chem Biol. 2008 3(5):287-93 [2] Honerkamp-Smith, Veatch, and Keller, Biochim Biophys Acta. 2008 (in press)   

Topological Transitions in Mitochondrial Membranes controlled by Apoptotic Proteins

The Bcl-2 family comprises pro-apoptotic proteins, capable of permeabilizing the mitochondrial membrane, and anti-apoptotic members interacting in an antagonistic fashion to regulate programmed cell death (apoptosis). They offer potential therapeutic targets to re-engage cellular suicide in tumor cells but the extensive network of implicated protein-protein interactions has impeded full understanding of the decision pathway. We show, using synchrotron x-ray diffraction, that pro-apoptotic proteins interact with mitochondrial-like model membranes to generate saddle-splay (negative Gaussian) curvature topologically required for pore formation, while anti-apoptotic proteins can deactivate curvature generation by molecules drastically different from Bcl-2 family members and offer evidence for membrane-curvature mediated interactions general enough to affect very disparate systems.   

The buckling transition of ionic shells and electrostatics

Can one design the morphology of a shell with diverse symmetries by coassembling oppositely charged molecules? We present the results of numerical simulations of a model for an ionic shell at different stoichiometric ratios. The tendency of electrostatic interactions to organize a system of charges (globally electroneutral) along flat planes, competes with the curved geometry of the shell. An ``electrostatic buckling'' instability ensues, and at low-temperatures a variety of shapes arise, beyond the icosahedral one typical of large viruses, large fullerenes, and catanionic-anionic vesicles. We study also the effects of temperature, different dielectric environments, and screening salt.   

Facilitation of Electron Transfer in the Presence of Mitochondria-Targeting Molecule SS31

Electron transfer (ET) processes in mitochondria are very important for the production of adenosine triphosphate (ATP), the common source of the chemical energy. The inability to transfer electrons efficiently in mitochondrial ET chain plays a major role in age associated diseases, including diabetes and cancer. In this work, we used the time dependent absorption and photoluminescence spectroscopy to study the electron transfer kinetics along the ET chain of mitochondria. Our spectroscopic results suggest that SS31, a small peptide molecule targeting to the mitochondrial inner membrane, can facilitate electron transfer and increase ATP production. We show that SS31 targets cytochrome c to both increase the availability of state and also potentially reduce the energy barrier required to reduce cytochrome c.   

Mean-field overcharging of macromolecules via charge or surface modulation

It is well known that multivalent counterions can at times overcharge macromolecules in electrolyte solutions. In this talk, I will discuss a simple mean-field mechanism that can allow for this effect: modulation of source charge and/or surface geometry can induce additional charge condensation sometimes resulting in overcharging. The qualitative features of this mechanism will be related to experimental observations. In addition, an experimental method by which one may be able test for modulation effects will be discussed.   

Session Z12: Disordered and Glassy Systems II

Microscopic statistical dynamical theory of correlated motion in glassy fluids and suspensions

The naive mode coupling theory and the stochastic nonlinear Langevin equation theory of single particle activated glassy dynamics has been extended to treat the correlated motion of two tagged particles in a dense fluid. Starting with a generalized Langevin equation deduced using projection and mode coupling approximations, we derive an effective nonequilibrium free energy surface for the stochastic motion of the tagged degrees of freedom. The dynamical free energy surface involves contributions from an ideal entropic term, the potential of mean force, and a many particle `caging' term that is explicitly dependent on the relative separation between the particles. The theory allows the study of space-time dynamic heterogeneity effects, including the length scale beyond which single particle motion of two tagged particles becomes independent and how the emergence of irreversible rearrangements affects the equilibrium pair structure relaxation. Numerical results for hard sphere fluids and colloidal suspensions will be presented.   

Improved model for the transit entropy of monatomic liquids

In the original formulation of vibration-transit (V-T) theory for monatomic liquid dynamics, the transit contribution to entropy was taken to be a universal constant, calibrated to the constant-volume entropy of melting. This implied that the transit contribution to energy vanishes, which is incorrect. Here we develop a new formulation that corrects this deficiency. The theory contains two nuclear motion contributions: (a) the dominant vibrational contribution $S_{vib}(T/\theta_0)$, where $T$ is temperature and $\theta_0$ is the vibrational characteristic temperature, and (b) the transit contribution $S_{tr}(T/\theta_{tr})$, where $\theta_{tr}$ is a scaling temperature for each liquid. The appearance of a common functional form of $S_{tr}$ for all the liquids studied is deduced from the experimental data, when analyzed via the V-T formula. The theoretical entropy of melting is derived, in a single formula applying to normal and anomalous melting alike. An \textit{ab initio} calculation of $\theta_0$ for Na and Cu, based on density functional theory, provides verification of our analysis and V-T theory. In view of the present results, techniques currently being applied in \textit{ab initio} simulations of liquid properties can be employed to advantage in the further testing and development of V-T theory.   

Free energy landscape theory of glass transition

I first present a free energy landscape (FEL) description of statistical mechanics, which is an exact reformulation of statistical mechanics and can be applied to non-equilibrium systems. Then, I discuss thermodynamic and dynamic properties of the vitrification process on the basis of the FEL formalism. I show that thermodynamic and dynamic anomalies at the glass transition, including the cooling rate dependence, can be understood in a unified manner which has not been achieved by any other theories of the glass transition. Namely, I show that the vitrification is a transition from annealed to quenched averages in the FEL and that the fast beta, the JG and the slow alpha relaxations are attributed to stochastic dynamics within a basin of FEL, jumping motion among locally connected basins and diffusive dynamics over barriers of the FEL.   

Observation of the Disorder-Induced Crystal-to-Glass Transition

The role of frustration and quenched disorder in driving the transformation of a crystal into a glass is investigated in quasi-two-dimensional binary colloidal suspensions. Frustration is induced by added smaller particles. The crystal-glass transition is measured to differ from the liquid-glass transition in quantitative and qualitative ways. The crystal-glass transition bears structural signatures similar to those of the crystal-fluid transition: at the transition point, the persistence of orientational order decreases sharply from quasi-long-range to short-range, and the orientational order susceptibility exhibits a maximum. The crystal-glass transition also features a sharp variation in particle dynamics: at the transition point, dynamic heterogeneity grows rapidly, and a dynamic correlation length-scale increases abruptly.   

Characteristic length scale of the inhomogeneous mode-coupling theory: beyond scaling predictions

The inhomogenous mode-coupling theory of Biroli \textit{et al.}\ [Phys. Rev. Lett. \textbf{97}, 195701 (2006)] allows for the identification of a characteristic length scale that diverges as the mode-coupling transition is approached. We numerically investigate this length scale as a function of time, wave-vector, and distance from the transition by examining the small $\mathbf{q}$ expansion of the dynamic susceptibility $\xi_{\mathbf{q}}(\mathbf{k};t)$ defined by Biroli \textit{et al.} We confirm the scaling predictions of Biroli \textit{et al.}. In addition, we show that the characteristic length is in qualitative agreement with simulations where the length scale is obtained from four-point correlation functions. Finally, we show that the length scale has virtually no $k$ dependence and thus it is well defined. The $k$-independence of the length contrasts with the very strong $k$ dependence of $q\to 0$ limit of the dynamic susceptibility.   

On the Molecular Structure of Ge$_{x}$Sb$_{x}$Se$_{1-2x}$ glasses

The Ge$_{x}$Sb$_{x}$Se$_{100-2x }$ternary is isovalent to the phase-change material, Ge$_{x}$Sb$_{x}$Te$_{100-2x }$, except the Selenides can be prepared as bulk alloy glasses while the Tellurides exist only as amorphous thin-films. Here we report on the Selenides synthesized over a wide composition range, 0 $<$ x $<$ 25{\%}, and examined in modulated-DSC, Raman scattering and molar volume experiments. The enthalpy of relaxation at T$_{g}$ shows the opening of a reversibility window or Intermediate Phase (IP) in the 13{\%} $<$ x $<$ 18{\%} range, or 2.40 $<$ \textbf{\textit{r }}$<$ 2.54 mean coordination number range, where \textbf{\textit{r }}= 2 + 3x. FT- Raman studies reveal frequency of the CS mode of GeSe$_{4}$ tetrahedra to steadily blue-shift with increasing x as networks stiffen. New vibrational modes are observed near 150 cm$^{-1}$ and near 220 cm$^{-1}$ at x $>$ 18.18{\%}, the chemical threshold, and are thought to result from homopolar bonds. Ab-initio cluster calculations place pyramidal SbSe$_{3}$ units and ethylene-like Sb$_{2}$Se$_{2}$ units to reveal Raman activity near 215 cm$^{-1}$ and 228 cm$^{-1}$ respectively. Evolution of glass structure with composition x will be discussed.   

Relating the Dynamics of Supercooled Liquids to the Sensitivity of Modes to Small Perturbations

We propose an alternate method to relate the structural and dynamical properties of a model supercooled binary Lennard-Jones (BLJ) liquid approaching the glass transition. Our proposal builds on methods from random matrix theory and transport theory of disordered systems, where it was shown that the diffusivity/localization can be probed by an appropriate statistical analysis of the eigenvalues and eigenvectors of the Hamiltonian function. Specifically, we examine the viability of connecting the diffusion constant of the BLJ liquid to: (i) the mean level velocities (MLV) of eigenmodes, (ii) the variance of MLVs, and, (iii) the participation number of the eigenmodes.   

Direct evidence of enhanced surface mobility in molecular glass forming system 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene

We have performed nanoparticle embedding studies on the organic glass forming system 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB). Films are prepared by vapor deposition onto a Si substrate held at a temperature near T$_{g}$ -- 50K (T$_{g}$ = 347K) and subsequently annealed. The surfaces of the films are covered with 20 nm diameter gold nanoparticles. Atomic force microscopy is used to track the apparent height of specific nanoparticles as a function of time elapsed at embedding temperatures of 323K, 333K, and 343K. The experiments reveal direct evidence for surface mobility at temperatures below the bulk glass transition. In addition to changes in the apparent heights of the nanoparticles, there is clear evidence that material surrounding the nanoparticles is being drawn up to engulf the nanoparticles; something not observed in polymeric films. These results directly establish the presence of enhanced surface mobility in molecular glass forming systems.   

On the Elastic behavior of Sodium Borate Glasses

Alkali Borates are industrial glasses and their physical properties are of general interest. We have made a special effort to synthesize dry (Na$_{2}$O)$_{x}$(B$_{2}$O$_{3})_{100-x}$ glasses over a wide composition range, 0 $<$ x $<$ 70{\%}, and have examined them in modulated-DSC, Raman scattering, FTIR, and molar volume experiments. The enthalpy of relaxation at T$_{g}$ shows a global minimum in the 20{\%} $<$ x $<$ 40{\%} range, which we identify with the rigid but stress-free Intermediate Phase (IP). The Boroxyl ring vibrational mode near 808 cm$^{-1}$ in B$_{2}$O$_{3}$, steadily softens by about 4 cm$^{-1}$ as the soda content increases to about 20{\%}. A vibrational mode of mixed rings\footnote{Kamitsos et al., Jour. Mol. Struct 247, 1 (1996).} (containing 3-fold and 4-fold B) is also observed near 775 cm$^{-1}$ at low x, and it also steadily softens by nearly 10 cm$^{-1}$ as x increases in the 20{\%} $<$ x $<$ 40{\%} soda range (IP). We are examining the underlying optical elasticity power-laws to ascertain the nature of the elastic phases. IR reflectance experiments provide the 4-fold coordinated B fraction to increase from 0.17 near x = 20{\%} to 0.44 near x = 40{\%} in broad agreement with NMR results. Evolution of physical properties of these glasses with soda content will be reviewed.   

Aging of the generalized density susceptibility in a strong glass

We investigate dynamical heterogenities in a strong glass below the glass transition temperature. Our model is produced by molecular dynamics simulations of an amorphous silica system, where the atoms interact via the BKS potential. We quantify the heterogenous dynamics by measuring the four-point generalized dynamic susceptibility, i.e., the volume integral of the spatial correlations. We study this quantity as a function of the waiting time and as a function of the global intermediate scattering function. We test for universality by comparing the fluctuations in this model to those of fragile glasses which consist of either small molecules or polymers.   

Glass transition and dynamic scaling in soft repulsive particles: a mode-coupling theory study

We combine the hypernetted chain approximation with the mode-coupling theory to analyze structure and dynamics of dense systems consisting of soft repulsive particles (harmonic spheres). We investigate the phase diagram for a broad range of temperatures and volume fractions. We find that in the vicinity of the T=0 mode-coupling transition for hard spheres, the dynamics obey a power-law form of dynamic scaling. We find that the critical MCT exponent describing the divergence of the relaxation time at the mode-coupling transition decreases with increasing volume fraction.   

Time reparametrization symmetry in a short-range p-spin model

We explore the existence of time reparametrization symmetry in the p-spin model. We follow closely the approach previously used to prove the presence of this symmetry in the Edwards-Anderson model. Using the Martin-Siggia-Rose generating functional, we analytically probe the long-time dynamics.We introduce a cut-off in the time difference $\tau_{0}\leq t-t'$ and perform a Renormalization Group analysis where we systematically integrate over short-time scale fluctuations. We find that the RG flow converges to a fixed point that is invariant under reparametrizations of the time variable. This continuous symmetry is broken in the glass state and we argue that this gives rise to the presence of Goldstone modes. We expect the Goldstone modes to determine the properties of fluctuations in the glass state.   

Brillouin scattering study of glass-transition dynamics in glycerol at pressures up to 60 kbar

Isothermal pressurization data for glycerol, a prototypic intermediate glass-forming system, will be presented. Brillouin scattering studies were performed at constant temperature to pressures as high as 60 kbar. An equal-angle forward scattering geometry is used for which the pressure dependence of the refractive index is not required to convert Brillouin frequency shift data to sound velocities. Through a careful optical setup acoustic mode frequencies and linewidths are measured, and from data analysis methods that include convolution with the instrument function, both pressure-dependent sound velocities and true linewidths are extracted from these data. Further analysis allow us to model the relaxation time of the glass-forming system as a function of pressure and to calculate the equation of state for this important system to previously unexplored regions of the pressure-temperature phase space.   

Triangular Relations in Structural Glasses

Structural glasses exhibit the phenomenon of dynamical heterogeneity: different regions of the system present different dynamical behavior. To study this phenomenon, we analyze simulations of four models of structural glasses performed in the aging regime. We compute the triangular relations of the local and global two-time correlation functions, i.e., the mathematical relationships among correlators calculated for the time pairs (t1,t2), (t2,t3) and (t1,t3) with t1$>$ t2$>$ t3. We plot the triangular relations of the global and local correlations together to compare their behavior. We find that the probability distribution of local correlations is concentrated along the curve representing the global correlations. Our results provide evidence of time reparametrization invariance and also point toward universality in the aging.   

Session Z13: Statistical and Nonlinear Physics II

Prediction, Retrodiction, and the Amount of Information Stored in the Present

We introduce an ambidextrous view of stochastic dynamical systems, comparing their forward-time and reverse-time representations and then integrating them into a single time-symmetric representation. The perspective is useful theoretically, computationally, and conceptually. Mathematically, we prove that the excess entropy---a familiar measure of organization in complex systems---is the mutual information not only between the past and future, but also between the predictive and retrodictive causal states. Practically, we exploit the connection between prediction and retrodiction to directly calculate the excess entropy. Conceptually, these lead one to discover new system measures for stochastic dynamical systems: crypticity (information accessibility) and causal irreversibility. Ultimately, we introduce a time-symmetric representation that unifies all of these quantities, compressing the two directional representations into one. The resulting compression offers a new conception of the amount of information stored in the present.   

Information Accessibility and Cryptic Processes

We give a systematic expansion of the \emph{crypticity}--a recently introduced measure of the inaccessibility of a stationary process's internal state information. This leads to a hierarchy of \emph{k-cryptic} processes and allows us to identify finite-state processes that have infinite cryptic order--the internal state information is present across arbitrarily long, observed sequences. The crypticity expansion is exact in both the finite- and infinite-order cases. It turns out that k-crypticity is complementary to the Markovian finite-order property that describes state information in processes. One application of these results is an efficient expansion of the \emph{excess entropy}--the mutual information between a process's infinite past and infinite future--that is finite and exact for finite-order cryptic processes.   

New integro-differential diffusion equation for continuous time random walk

We present a new integro-differential diffusion equation for continuous time random walk that is valid for a generic waiting time probability density function. Using this equation we also study diffusion behaviors for a couple of specific waiting time probability density functions such as exponential, and a combination of power law and generalized Mittag-Leffler function. We show that for the case of the exponential waiting time probability density function a normal diffusion is generated and the probability density function is Gaussian distribution. In the case of the combination of a power-law and generalized Mittag-Leffler waiting probability density function we obtain the subdiffusive behavior for all the time regions from small to large times, and probability density function is non-Gaussian distribution.   

Universal and non-universal properties of wave chaotic scattering systems

The application of random matrix theory to scattering requires introduction of system-specific information. Here, we show that the average impedance matrix, which characterizes such system-specific properties, can be semiclassically calculated in terms of ray trajectories between ports [1]. We compare theoretical predictions with experimental results for a microwave billiard, demonstrating that the theory successfully uncovered universal statistics of wave-chaotic scattering systems [2]. These results should be broadly useful in nuclear scattering, atomic physics, quantum transport in condensed matter systems, electromagnetics, acoustics, geophysics, etc. [1] James A. Hart, T. M. Antonsen, E. Ott, "\textbf{The effect of short ray trajectories on the scattering statistics of wave chaotic systems}," \underline {Phys. Rev. E }\underline {\textbf{80}}\underline {, 041109~(2009)}. [2] Jen-Hao Yeh, \textit{et al}., \underline {arXiv:0909.2674}.   

Towards random matrix model of breaking the time-reversal invariance of elastic waves in chaotic cavities by feedback

We developed a random matrix model to describe the statistics of resonances in an acoustic cavity with broken time-reversal invariance. Time-reversal invariance braking is achieved by connecting an amplified feedback loop between two transducers on the surface of the cavity. The model is based on approach [1] that describes time- reversal properties of the cavity without a feedback loop. Statistics of eigenvalues (nearest neighbor resonance spacing distributions and spectral rigidity) has been calculated and compared to the statistics obtained from our experimental data. Experiments have been performed on aluminum block of chaotic shape confining ultrasound waves. [1] Carsten Draeger and Mathias Fink, One-channel time- reversal in chaotic cavities: Theoretical limits, Journal of Acoustical Society of America, vol. 105, Nr. 2, pp. 611-617 (1999)   

Sensing Small Changes in a Wave Chaotic Scattering System

We had demonstrated a new remote sensor scheme by applying the wave mechanical concept of fidelity loss to classical waves. The sensor makes explicit use of time- reversal invariance and spatial reciprocity in a wave chaotic system to sensitively and remotely measure the presence of small perturbations to the system [1]. The loss of fidelity is measured through a classical wave-analog of the Loschmidt echo by employing a single-channel time-reversal mirror to rebroadcast a probe signal into the perturbed system. We now compare and contrast the detection power and computational efficiency of our sensor with other techniques such as correlation and/or mutual information of probing signals. We also introduce the use of exponential amplification of the probe signal to partially overcome the effects of propagation losses. It is demonstrated that exponential amplification can be used to vary the spatial range of sensitivity to perturbations, and the extent to which the spatial range of the sensors can be varied. Experimental results are presented for the acoustic version of the sensing techniques under study. \\[4pt] [1] B. T. Taddese, \textit{et al}., Appl. Phys. Lett. 95, 114103 (2009) (http://link.aip.org/link/?APPLAB/95/114103/1)   

Simulations of fractal electronic circuits

Many natural structures make use of fractal geometry's inherent properties, which can include very high surface area to volume ratios, connectivity, and dispersion. Recent research and technological applications have begun to leverage these same properties in artificial structures including antenna and capacitor designs~[1]. Here we present DC electrical simulations as a first step toward circuits in which the components themselves have fractal character. Our results show that such `fractal circuits' produce complicated differential resistance curves (in response to a simple electrostatic gating scheme) that is unique to the underlying fractal geometry. Finally, we will discuss potential applications of these devices as well as candidate systems for fractal circuit fabrication. \\[4pt] [1] Cohen, N. L. \emph{Communications Quarterly} Summer, 9 (1995).; Samavati, H., Hajimiri, A., Shahani, A. R., et al. \emph{IEEE J Sol St Circ} 33 2035 - 2041 (1998).   

Simple Autonomous Chaotic Circuits

Over the last several decades, numerous electronic circuits exhibiting chaos have been proposed. Non-autonomous circuits with as few as two components have been developed. However, the operation of such circuits relies on the non-ideal behavior of the devices used, and therefore the circuit equations can be quite complex. In this paper, we present two simple autonomous chaotic circuits using only opamps and linear passive components. The circuits each use one opamp as a comparator, to provide a signum nonlinearity. The chaotic behavior is robust, and independent of nonlinearities in the passive components. Moreover, the circuit equations are among the algebraically simplest chaotic systems yet constructed.   

$\cal{PT}$ -symmetry Wave Chaos

We study a new class of chaotic systems with dynamical localization, where gain/loss processes break the hermiticity, while allowing for parity-time ${\cal PT}$ symmetry. For a value $\gamma_{\cal PT}$ of the gain/loss parameter the spectrum undergoes a spontaneous phase transition from real (exact phase) to complex values (broken phase). We develop a one parameter scaling theory for $\gamma_{\cal PT}$, and show that chaos assists the exact ${\cal PT}$-phase. Our results will have applications to the design of optical elements with ${\cal PT}$-symmetry.   

Optical synthetic materials with local Parity-Time symmetry

We discuss the eigenvalue and eigenvector properties of a class of optical synthetic materials that are described by effective non-hermitian Hamiltonians with Parity-Time symmetry. The building blocks of such systems are coupled dimers with judiciously tailored internal structure such that one element of the dimer incorporates losses while the other balanced these losses with a gain. We show that these systems have a robust exact PT-phase (i.e. parameter regime of the gain/loss coefficient where the spectrum is real), even if the inter-dimer and intra-dimer couplings are random. We further analyze the beam dynamics in such optical lattices and show non-reciprocal diffraction beam evolution.   

A new test for missing levles using the $\Delta_3(L)$ statistic

The $\Delta_3(L)$ statistic of Random Matrix Theory is a measure of how a spectrum deviates from the equidistant harmonic oscillator spectrum. While it is usually used as a signature of quantum chaos, in this work it is used to gauge the incompleteness of an experimental spectrum. Two approaches are presented. In the first, the $\Delta_3(L)$ statistic extracted from the experimental data is compared to randomly depleted spectra in numerical simulations. The second approach depends on the fact that $\Delta_3(L)$ is the mean value of a quantity that is evaluated many times over the spectrum. These values are not statistically independent, and their distribution is non trivial. In this second approach this distribution of numbers (whose average is $\Delta_3(L)$) is parametrized, and a maximum likelihood method is then developed as a tool to detect missing levels.   

Coupled parallel totally asymmetric exclusion processes with extended particles

A complex system consisting of coupled parallel totally asymmetric processes (TASEP) with extended particles is investigated theoretically. Stationary-state properties and phase diagrams are obtained using several approximate methods. We find that the maximum-current phase is very well described by the Tonks gas lattice-based treatment as in the case of the extended-particle single-lane TASEP. However, although the probability balance treatment used in that case for the low and high-density phases yields acceptable results for the low-density phase, it completely fails for the high-density phase in our system. We show that this discrepancy is a result of the coupling and demonstrate that a simple ansatz derived from the single-lane single-particle TASEP restores consistency to the theory. We validate all theoretical predictions via extensive Monte-Carlo simulations: agreement between them and theory is mostly excellent except for the low density/maximum-current phase transition which the theory consistently underestimates. It is shown this disagreement is a result of very slow current saturation with increased exit rate in the relevant region of the phase diagram.   

Newtonian trajectories: a powerful tool for solving quantum dynamics

Since Ehrenfest's theorem, the role and importance of the classical paths in quantum dynamics have been examined by several means, and nowadays stochastic versions of the classical equation of motion are being investigated. Along this line, we show that the classical equations of motion provide a solution to quantum dynamics, if appropriately incorporated in the Wigner distribution function, exactly reformulated in a type of Boltzmann equation. Also the quantum-mechanical features of thermal equilibrium are studied in this framework. Even fermions and bosons can be treated on the basis of classical paths, provided the initial distribution function is constructed in agreement with the identical-particle statistics.   

Session Z14: Graphene: Adsorbates and Defects

Electronic and transport properties of heme-b adsorbed on graphene nanoribbons

Heme-B is the prosthetic group of several hemoproteins, such as several peroxidases and hemoglobin. It contains an Iron atom that can be oxidized or reduced depending on its environment. Changes in Iron oxidation state enable diverse biological functions like oxygen transport and electron transfer. In this work, we present a quantum density functional study on the adsorption of a heme-b group in graphene. The effects of heme-b adsorption on the graphene electronic structure will be shown. These changes have clear effects on the quantum transport properties of graphene, which coupled with the affinity of heme-b group to molecules like oxygen, carbon dioxide and carbon monoxide, could help to develop new graphene based amperometric biosensors.   

Alkali and halogen adsorption on graphene using ab initio calculation

A seamless sp2 graphene sheet prevents the penetration of atoms through the sheet, yet allows the penetration of electrons. Thus, a suspended single sheet graphene forms a geometrical constraint by separating the surrounding vacuum into upper half and lower half spaces. Alkali and halogen atoms, each constrained to one of the spaces, are forced to interact electrostatically via charge transfer through the sheet. We have found that under this constraint, a K atom can break the Cl-Cl bond of a chlorine molecule and form a new class of inter-atomic interaction, with strong long-ranged Coulomb effects on the band-structure. This investigation has now been extended to other alkali atoms (Na, Rb) and halogens (Br, I) to determine the generality of these new physical effects.   

Ab-initio study of hydrogen atom pairs adsorption on potassium doped graphene

The effects of the interactions of hydrogen (H) atoms on graphene (G) with potassium (K) pre-adsorbed, were predicted by means of first-principles calculations. The results were obtained with the pseudopotentials method and the generalized gradient approximation for the exchange-correlation potential. The structural parameters, bonding, electronic structure and magnetic properties of two H atoms on potassium doped graphene (2H-K/G) system are calculated by molecular dynamics. We found an important charge transfer from the K atom towards the G surface when an H atom was adsorbed, producing a chemical bonding transition from sp$^{2}$ to sp$^{3}$ in the bonded carbon atom. The binding energy per H atom was greater in the 2H-K/G system than both H-K/G and a H atom on the single G systems (H/G). The present results suggest that the hydrogen adsorption on graphene layer could be modulated by the pre-adsorption of potassium.   

Characterization and Control of the Barrier for Hydrogen Adsorption on Graphene

We study the chemisorption of atomic hydrogen on graphene. The barrier for chemisorption is about 0.21 eV. We explain the evolution of the partial density of states of the hydrogen and carbon atoms in graphene upon adsorption with a simple three-site Hubbard model. The barrier results from the competition between the pi-bonding between carbon atoms and the covalent bonding with hydrogen. With knowledge of the principles at play, we propose adjustments to the graphene plane which lower or eliminate this adsorption barrier. With a reduced barrier, formation of graphane can be facilitated, and applications for selective hydrogen desorption to create channels are discussed.   

The Effect of Cluster Formation on Graphene Mobility

The transport properties of graphene are strongly influenced by the presence of impurities on the surface. Additionally, the structure of the impurities, whether in the form of clusters or isolated adatoms, has an effect on scattering. Using molecular beam epitaxy, small amounts of gold impurities are introduced to the graphene surface. When deposited at low temperatures, the resulting decrease in mobility and a shift in Dirac point is consistent with scattering from point-like charged impurities. To investigate the effect of the formation of clusters, the temperature is slowly raised to room temperature while transport properties are monitored. For a fixed amount of gold impurities, it is discovered that the formation of clusters significantly enhances the mobility and causes the Dirac point to shift back toward zero.   

Local density of states and scanning tunneling currents in graphene

Graphene consists of an atom-thick layer of carbon atoms arranged in a honeycomb lattice, and its low-energy electronic excitations are well described as massless Dirac fermions with spin half and an additional pseudospin degree of freedom. We study local properties of graphene with isolated impurities (diagonal and non-diagonal impurity potential) such as the local electronic spectra and real-space and k-space local density of state (LDOS) maps. Using a multimode description for an scanning tunneling microscope (STM) tip, we calculate STM currents and find that strong resonances in the LDOS at finite energies lead to the presence of steps in the STM current and suppression of the Fano factor. [Ref: N. M. R. Peres, L. Yang, and S.-W. Tsai, New J. Phys. 11, 095007, (2009)]   

Ab-initio study of NH3 and NH adsorption over graphene

Currently, solid state sensors for detecting chemicals are an intensive area of research and development. Solid state sensors have advantages over conventional systems such as decrease the size and cost, broadening the range of applications. In this work we study the possibility of using graphene for the detection of NH3 and NH, through structural and electronic changes induced in graphene by adsorption of both molecules. The results are obtained with the seudopotential LCAO and GGA approximation for the exchange correlation potential. Study reveals that nitrogen in the NH and NH3 molecules have affinity for the surface of graphene. Analysis of charge transfer is performed to analyze the adsorption process. The molecules have preferential sites of adsorption for NH3, which are the interstitial sites of the hexagonal lattice, the links between carbon atoms, and over the carbon atoms. In the case of NH (nitrene) these sites correspond to the spaces between links, and over carbon atoms. Also, adsorption of both molecules produces major distortions in the network of graphene, which are analyzed.   

Physisorption of nucleobases on graphene: the role of van der Waals interactions

The physisorption of the nucleobases adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) on graphene is studied within the generalized gradient approximation (GGA) of the density functional theory (DFT) with the inclusion of van der Waals interaction (vdW) based on the London dispersion equation. We find that the inclusion of the latter interaction increases the binding energy by about 0.5eV (from an almost zero value) and moves these nucleobases by about 0.5{\AA} toward the graphene, as compared to the results obtained with regular DFT-GGA. The binding energies of nucleobases on graphene are found to be in the following order: G$>$A$>$T$>$C$>$U, with a dispersion of about 200meV. Details of the dynamics (diffusion barriers) and adsorption characteristics of these nucleobases on graphene will be presented as well as the description of their electronic structure and nature of the bonding with the substrate.   

Absence of a supercritical regime induced by short-range impurity scattering in gapped graphene

We show that the changes in the electronic density of states (DOS) in graphene induced by impurity scattering with short-range potentials are completely different from those caused by the long-range Coulomb potential. The spectral weight of the state that eventually disappears into the valence band (as the strength of scattering increases) does not transform into a resonance state. Therefore no unusual screening effects related to a redistribution of the density of states in the valence band are observed. The states induced by the short-range impurities in graphene, therefore, have distinctively different properties compared with the long-range potential case. These properties, in fact, closely resemble the case of a short-range single impurity in other bipartite lattices, such as the square, body centered cubic, and simple cubic lattices.   

Fullerenes, Zero-modes, and Self-adjoint Extensions

The continuum Dirac hamiltonian provides a good account of the low-energy eigenstates on an infinite sheet of graphene. It is also known that that geoemetric defects, such as the pentagons that occur in fullerenes, may be modelled by a spin connection and a non-abelian gauge field. We show here that to understand the bound states localized in the vicinity of a pair of pentagons one must, in addition to the gauge flux, consider the effect of the short-range lattice disruption near the defect. Although the defect appears as a point object to low wavelength excitations, there is additional information contained in non-trivial boundary conditions that need to be imposed at this point to ensure self-adjointness of the hamiltonian. We demonstrate that by adjusting this boundary conditions one may analytically match the results of a numerical tight-binding calculation. To appear in Journal of Physics A. Archive number: arXiv:0909.1569v1   

Patterning Effects on electronic Properties of Hydrogenated Graphene Superlattices

Recently, it was found that the absorption of hydrogen atoms on graphene can modify the electronic properties greatly. This finding not only opens another possibility for tuning the electronic properties of graphene, but also allows us to pattern hydrogenated graphene to obtain the desired electronic properties. Here, we report results of density-functional based tight-binding calculations of patterning effects such as pattern edge, pattern shape (triangular and hexagonal shape) and type (zigzag and armchair), and lattice pattern (triangle and square) and size on electronic properties of hydrogenated graphene superlattices. It is found that electronic properties of hydrogenated graphene superlattices are sensitive to the pattern edge. Superlattices with zigzag edge, have very small bandgaps or are metallic. Supperlattices with armchair edge, exhibit much larger bandgaps whose magnitude is dependant on the pattern shape, lattice, and pattern size. In addition, electronic properties of a quantum dot, formed by removing a lattice from the two-dimensional hydrogenated graphene superlattices, will also be discussed.   

Factors Contributing to Size Selection of Metal Nanoparticles on Graphene

We examine layer number dependence in the size of metal nanoparticles grown on single and multilayer graphene. Graphene offers a smooth inert substrate for nanoparticles, in particular for particles grown in situ. Upon annealing, the particles forming on thin layers are smaller. A theory based on balance between self-repulsive dipole interaction and surface tension is presented. We test this theory by examining size distributions for various metals.   

Resonant Impurity band induced by point defects in graphene

In this talk, we shall present our theory of point defects on graphene. In particular, we shall pointed out that point defects on graphene are strongly correlated and can not be treated as independent scatters. For large on-site defect potential and finite quasi-particle lifetime, we show that defects induce an impurity band with density of state characterized by the Wigner semi-circle law. By including long-range Coulomb interaction, we show that depending on, quasi-particle lifetime and defect density, the impurity band may support ferromagnetism. Furthermore, the impurity band can enhance the conductivity of graphene to the order of 4e$^2$/h, in consistent with experimental observations.   

Doping a graphene sheet with impurities

It has been predicted theoretically and achieved experimentally that a graphene sheet can bond strongly with hydrogen atoms. The resulted hydrocarbon or graphane is a semiconductor with a large band gap around 3.5 eV. In the paper I discuss the density functional calculation that shows how the pristine graphene is attached to lithium, creating instead a metal. In the case of graphane, the calculation also predicts that defects and doping with transition-metal impurities can greatly enhance the conduction and generate high magnetic moments. These properties offer promising application of doped graphane as a nanoelectronic device.   

Session Z15: Structural and Electronic Properties of Metals II

Bonding in boron: building high-pressure phases from boron sheets

We present the results of a study of the high pressure phase diagram of elemental boron, using full-potential density functional calculations. We show that at high pressures (P > 100 GPa) boron crystallizes in quasi-layered bulk phases, characterized by in-plane multicenter bonds and out-of-plane unidimensional sigma bonds. These structures are all metallic, in contrast to the low-pressure icosahedral ones, which are semiconducting. We show that the structure and bonding of layered bulk phases can be easily described in terms of single puckered boron sheets [1]. Our results bridge the gap between boron nanostructures and bulk phases.\\[4pt] [1] Kunstmann et al., Phys. Rev. B 74, 035413 (2006).   

Phonon dispersion relations in cerium under pressure across the volume-collapse gamma-alpha transition

Cerium is a rare-earth metal with many of its physical and chemical properties governed by the complex behavior of its 4f electron in contributing to the bonding of the crystal structure. Specifically, its gamma-alpha volume-collapse transition under pressure is the only known solid-solid transition in an element with the phase boundary ending at a critical point, and the detailed mechanism is still not fully understood. We report recent inelastic x-ray scattering measurements of phonon dispersion relations of cerium at room temperature as a function of pressure up to 25 kbar. The phonon dispersion relations show abrupt changes across the gamma-alpha transition at $\sim $7 kbar and continue to evolve at higher pressures. Various thermodynamic quantities associated with the transition are deduced. These results will be discussed in light of existing theories and models.   

Nonlinear Surface Plasmon Polariton Propagation and Third Harmonic Generation at a Planar Metal/Dielectric Interface

Surface plasmon polaritons (SPPs) are electromagnetic (EM) waves guided at the interface between a metal and a dielectric. The confinement of EM fields to practically two dimensions leads to extremely high energy densities at the interface. We consider here the interaction of such optical fields with noble metals possessing large third order hyperpolarizabilities. We present analytical solutions to the nonlinear Maxwell equations with third order optical susceptibility. Surface electric dipoles induced at the metal/dielectric interface induce a third harmonic EM wave whose frequency lies above the plasma edge of the metal. The induced field thus becomes a leaky mode, guided at the air-side of the interface while freely propagating into the metal. Various necessary approximations, their limitations and implications are also discussed.   

Measurements of higher-order moments of the persistent current in normal metal rings

A normal metal ring exhibits a persistent electrical current provided its circumference is less than the electron's phase coherence length and its temperature is less than the Thouless temperature. The amplitude of the persistent current is a random function of the ring's disorder configuration. To date, theory and experiments have focused on measuring the variance of the distribution from which the persistent current amplitude is drawn. We have measured persistent currents in arrays of normal-metal rings over a wide range of magnetic fields, or equivalently, over many independent realizations of the disorder potential. This data allows us to produce a histogram of the current amplitude and determine its higher order moments. We find these higher order moments are consistent with a Gaussian distribution for the persistent currents. We also directly confirm that the amplitudes of different harmonics of the currents' Aharonov-Bohm oscillations are uncorrelated.   

Anisotropic Rashba splitting of Au(110) surface states

We investigate the surface Rashba effect subject to reduced in-plane symmetry. Based on a \textbf{K.p} perturbation theory, we give a detailed microscopic description of the Anisotropic Rashba Splitting (ARS). Furthermore, we show that this ARS can not be explained within the standard theoretical picture of the Rashba effect assuming a purely normal-to-surface variation of the crystal potential. The new microscopic expression for the Rashba Hamiltonian is explicitelly supported by fully relativistic first principles calculations for the case of unreconstructed Au(110) surface.   

Strain-induced metal-hydrogen interactions across the first transition series -- An \textit{ab initio} study of hydrogen embrittlement

The attractive interaction between hydrogen and distorted regions of the host matrix underlies all the currently discussed mechanisms of hydrogen-induced embrittlement of metals, such as hydrogen enhanced local plasticity (HELP), hydrogen enhanced decohesion (HEDE) and stress-induced hydride formation. In this study we investigate these interactions systematically by determining heat of solutions, H-H binding energies within the metal matrix, as well as phase diagrams as a function of the lattice strain and the H chemical potential across the first transition series (3d elements) using Density Functional Theory (DFT) calculations. The results will be interpreted in terms of the likely embrittlement mechanisms of these metals.   

Ab initio determination of the magnetic free energy contribution of metallic systems

An accurate prediction of the free energy is the basis to compute phase diagrams, finite temperature materials parameters, or kinetic barriers and is thus fundamental in computational materials design. One of the most challenging contributions - but crucial for many engineering materials - is the magnetic entropy. The most popular ab initio approach for the latter is the use of an effective Heisenberg model solved using classical Monte Carlo (cMC) approaches and neglecting quantum effects. We discuss the impact of the latter based on extensive model calculations where Quantum MC calculations are available. An empirical rescaling scheme is derived allowing to considerably improve the cMC. The method is applicable to strong ferromagnetic systems with magnetic frustration is absent or weak. The application and performance of the new approach is demonstrated for pure Fe.   

Tuning Magnetic Interactions in a Two Dimensional Matrix

The layered dichalcogenides can be used as a matrix for incorporating and manipulating dopants in dimensionally constrained manner. The crystal structure of the dichalcogenides is formed of two-dimensional strongly bound layers separated by a van der Waals gap. Dopants can be incorporated between the layers as intercalants through a variety of methods to form semi-ordered phases. These intercalants have a strong impact on the electronic and magnetic properties of the overall system and can be used to tune existing or induce new phase transitions in the pure parent compounds. For magnetic intercalants, RKKY interactions, which have a strong dependence on the ion-ion spacing, appear to determine the overall magnetic character of the system. Herein, we discuss how Coulomb interactions between intercalated magnetic and non-magnetic ions can be used to influence the spacing between magnetic species and drastically alter the overall magnetic properties of the system.   

Kapitza Resistance at the Solid-Liquid Interface

We study the thermal boundary resistance between the electrons in a metal and the phonons in a liquid. The theory includes transverse modes in the fluid that can carry diffusive heat from the interface into the bulk of the liquid. The theory can be an extension over the acoustic mismatch theory for Kapitza resistance.   

Quantifying Uncertainty in Materials Properties from Microstructure Variability

Most materials are inherently inhomogeneous. This means that their internal structure varies from point to point on the microscale; from region to region on the macroscale; from part to part; and throughout time as they age. Because a material's microstructure often controls its properties, the variability in structure leads to uncertainty about the material's properties and performance. We will discuss the concept of a statistics-based treatment of the process-structure-properties-performance relationships in engineering materials, and describe both experiments and computational simulations designed to quantify the statistics underlying structure-properties relationships in poly-silicon MEMS devices, stainless steel welds, and alpha-brass with engineered defects.   

Low-energy behavior near the semi-Dirac point

The recent discovery that a three unit cell slab of VO$_2$ confined to 2D dispersion within insulating TiO$_2$ slabs leads to point Fermi surfaces [PRL 102, 166803 (2009)] has opened up a new class of electronic behavior. It shows four symmetry related point Fermi surfaces along the (1,1) directions in the 2D Brillouin zone. The dispersion away from this point is however different from graphene and was unanticipated: disperson is linear along one direction but quadratic in the perpendicular direction. This semi-Dirac behavior has extreme anisotropy, from massless to massive. The dispersion depends on the Fermi velocity $v_F$ and the effective mass $m^*$, but finally scales to a single semi-Dirac system with energy scale $m^* v_F^2$. We have extended the study of the low energy behavior of this system [PRL 103, 016402 (2009)], showing that the Hall coefficient reduces to the conventional expression $\frac{1}{nec}$ ($n$ is the carrier concentration) as also the case for graphene, but does not hold generally. The small-q electronic response and plasmon frequency, and its very strong anisotropy, will be presented and analyzed.   

Construction of Chiral Metamaterial with U-Shaped Resonator Assembly

Chiral structure can be applied to construct metamaterial with negative refractive index (NRI). In an assembly of double-layered metallic U-shaped resonators with two resonant frequencies $\omega _{H}$ and $\omega _{L}$, the effective induced electric and magnetic dipoles, which are contributed by the specific surface current distributions, are collinear at the same frequency. Consequently, for left circularly polarized light, NRI occurs at \textit{$\omega $}$_{H}$ , whereas for right circularly polarized light it occurs at \textit{$\omega $}$_{L}$. Our design provides a new example to apply chiral structures to tune electromagnetic properties, and could be enlightening in exploring chiral metamaterials.   

Field-Biased Molecular Simulation Technique for Polyelectrolytes

External fields can be used to impose density profiles in inhomogeneous fluids and interfacial phenomena1. In this study an electric field has been imposed on 1372 hard spheres through 20 negative point charges and 20 positive charges. Also, the effect of partial charges was investigated on a polyelectrolyte with implicit and explicit solvent. Long-range interactions are considered through particle-mesh Ewald summation and its pairwise alternatives. It has been found that it is not necessary to update the Coulombic interactions after each time-step. Energy is conserved even after many numbers of time-steps. Therefore, the computation time for the long-range interaction is less than the discontinuous molecular dynamic (DMD) and/or discontinuous Monte Carlo components. This means that the forced-biased discontinuous molecular simulation method is viable for future studies of confined fluids containing interface and ionic liquids as performed with a field-biased conventional molecular dynamic method by Wardle et al. Finally, the effect of the biased method on dihedral angle is investigated.   

Point defects in In$_2$O$_3$

Point defects in $In_2O_3$ were studied using first principles, density functional theory within the local density /local spin density approximation. New results will be reported about the conductivity and magnetism, arising from these point defects. Various systematic corrections were performed to check the validity of the results.   

Session Z16: Focus Session: Organic Electronics and Photonics: Fundamentals

Near-infrared photoresponse in single walled carbon nanotube/polymer composite films

We present a near-infrared photoresponse study of single-walled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (SWCNT/P3HT-b-PS) composite films for different loading ratios of SWCNT in the polymer matrix. Compared to the pure SWCNT film, the photoresponse [(light current -- dark current)/dark current] is much larger in the SWCNT/polymer composite films. The photoresponse is up to 157{\%} when SWCNTs are embedded in P3HT-b-PS while for a pure SWCNT film it is only 40{\%}. We also show that the photocurrent strongly depends on the position of the laser spot with maximum photocurrent occurring at the metal--film interface. We explain the photoresponse due to exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode - SWCNT interface   

Single molecule photoluminescence excitation spectroscopy of polyfluorene

Polyfluorene is a remarkable conjugated polymer with a uniquely rich polymorphism [1]. Because of this characteristic it can be considered as a model playground to understand structure-property relationships in conjugated polymers. Here, by low temperature single polymer chain photoluminescence excitation spectroscopy [2], we look at the spectral characteristics of the absorbing and emitting chromophores on a chain. These experiments are performed on both the $\beta $-phase and the glassy disordered structure, elucidating the role of chain polymorphism on conformational relaxation and energy transfer. Moreover, we compare results on multichromophoric polymers with short oligomers, where a single chromophore is responsible for the optical response. These experiments illuminate directly the emergence of chromophores in conjugated polymers through delocalization: how a pi-electron system evolves from a localized molecular (oligomeric) unit into a delocalized species. \\[4pt] [1] Da Como Nano Lett. \\[0pt] [2] Walter PRL 2009   

Quadratic Electro-Optic Effect in Doped Nonconjugated Conductive Polymer 1,4-Trans-Polyisoprene, an Organic Quantum Dot System

Thin, optically uniform films of 1,4-trans-polyisoprene have been prepared on quartz substrate from a toluene solution. These films have been characterized using FTIR and optical absorption spectroscopy before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at $\sim $ 4.2 eV due to radical cation and the other at $\sim $ 3.2 eV due to charge-transfer. Doping leads to a reduction of the intensity of =C-H bending vibration-band due to formation of radical cations upon charge-transfer. Quadratic electro-optic measurements have been made using field-induced birefringence method at 633 nm. A modulation depth of $\sim $ 0.13{\%} has been observed for an applied field of 1.1 V/$\mu $m for a 0.37 $\mu $m thick film. The modulation depth had a quadratic dependence on applied field. The Kerr coefficient as measured is exceptionally large and has been attributed to the subnanometer size metallic domains (quantum dots) formed upon doping and charge-transfer.   

Spin-orbit coupling and spin relaxation rate in singly charged pi-conjugated polymer chains

In inorganic semiconductor spintronics the spin-diffusion length is usually limited by spin-orbit coupling. Here we examine the effect of spin-orbit coupling in organic spintronics. We consider singly charged pi-conjugated polymer chains. We show that the diagonal matrix elements for spin-orbit coupling are zero. Even the off-diagonal matrix elements are zero or negligibly small unless a twisted, non-planar polymer chain is considered. We calculate these matrix elements as a function of twist-angle using tight-binding wavefunctions. We show that time reversal symmetry prevents spin-orbit induced spin-precession and propose a phonon-assisted spin-flip process.   

Polarizability, susceptibility, and dielectric constant of nano-scale molecular films: a microscopic view

We explore theoretically the size-dependence of the polarizability, susceptibility, and dielectric constant of nano-scale molecular layers. This is achieved by comparing first principles calculations based on density functional theory to phenomenological modeling based on polarizable dipolar arrays, for a model system of organized monolayers comprised of oligophenyl chains. We show that molecular packing density is the single most important factor controlling the bulk limit of all three quantities as well as the rate at which they are approached. Finally, we show that the polarization does not reach its ``bulk'' limit, as determined from the Clausius-Mossotti (CM) model, but the susceptibility and dielectric constant do converge to the correct bulk limit. However, whereas the CM model describes the dielectric constant well at low lateral densities, finite size effects of the monomer units cause it to be increasingly inaccurate at high lateral densities.   

Tuning vibrations in single-molecule junctions: inelastic electron tunneling spectroscopy of an alkanedithiol

We study pentanedithiol molecular junctions formed by means of the break-junction technique with a scanning tunneling microscope at low temperatures. Using inelastic electron tunneling spectroscopy and first-principles calculations, the response of the junction to elastic deformation is examined. We show that this procedure makes a detailed characterization of the molecular junction possible. In particular, our results show unequivocally that tunneling takes place through just a single molecule.   

Towards molecular electronics with scalable nanopore junctions

We have fabricated and measured nanoscale molecular junctions. Each device consists of a shallow pore in an oxide layer, with a self-assembled monolayer (SAM) on a gold surface at the bottom. The use of a conductive polymer as a top-contact avoids previously noted issues of metal diffusion into contacted SAMs. Larger pores are more likely to contain monolayer defects and dislocations, thus nanometer-scale control over the pore size allows us to investigate transport through the SAM as a function of defect density. The planar geometry and use of robust materials in the device allows for additional molecular synthesis after monolayer formation. For example, we use ``click'' chemistry to alter the functionality of SAMs of azide-terminated alkanethiols. The use of mixed monolayers to substantially dilute the number of conducting molecules in a 50-nanometer diameter pore allows us to observe few to single-molecule transport behavior.   

Photophysics of MEH-PPV film under High Hydrostatic Pressure: The Role of Charge-Transfer Excitons

We report theoretical calculations for interacting pairs of PPV oligomers that explain the pressure-dependent experimental observations on MEH-PPV films by E.~Olejnik {\it et al}. We use the Pariser-Parr-Pople (PPP) model Hamiltonian for single chains. We assume that pressure decreases the intermolecular distance in the ordered phase of this two-phase material, and simulate pressure effects by assuming distance-dependent interchain Coulomb interactions and electron hopping. Our calculations show that the photophysics of the ordered phase is dominated by a charge-transfer exciton, which is a quantum-mechanical superposition of the covalent delocalized exciton state and the ionic polaron-pair state. We are able to explain the pressure-induced (i) quenching of the photoluminescence and its redshift, (ii) the redshift of the cw triplet photoinduced absorption, (iii) the appearance of ps PA bands at $\sim$ 0.35 eV and 0.9 eV and their strong blueshifts. Detailed comparisons between experiments and theory are made.   

Optical Probes of MEH-PPV films at High Hydrostatic Pressure

We investigate the primary and long-lived photoexcitations in $\pi$-conjugated polymer films with increased interchain coupling by studying the photophysics of substituted PPV derivative thin films, namely 2-methoxy-5-(2'-ethylhexyloxy) [MEH-PPV] at high hydrostatic pressure, P up to 120 kbar in a diamond anvil cell, using both ultrafast transient mid- and near-IR spectroscopies with 0.1 ps resolution, and cw optical techniques (photo induced absorption (PA) and photoluminescence (PL) in a broad spectral range from 0.2 to 2.2 eV). With increasing P the cw PL band weakens, broadens, and red-shifts by $\sim$ 2 meV/kbar; whereas the triplet PA red shifts to a lesser extent. The ultrafast PA band of the singlet exciton at $\sim$ 0.95 eV at ambient splits, blue shifts and acquires a much longer decay component. A second, weak PA band at $\sim$ 0.33 eV at ambient, dramatically blue-shifts ($\sim$ 3 meV/kbar) and substantially intensifies with P. These pressure-induced effects are discussed considering the interplay of two phases in the MEH-PPV film: a disordered phase with large PL efficiency, and PA that does not change much with P; and a less emissive ordered phase that increases with P, where the interchain coupling substantially increases with P.   

Self Assembled Dipole Monolayers on CNTs: Effect on Transport and Charge Collection

We propose a method of quickly and dramatically increasing the conductivity of carbon nanotubes via growth of a self assembled monolayer (SAM) of fluoroalkyl trichlorosilane dipoles following the method demonstrated with organic semiconductors in [1,2]. Growth of a SAM on carbon nanotubes results in a strong p-type doping which improves the conductivity by a factor of two or more. Additionally, this doping is nonvolatile and persists in high vacuum and inert atmospheres. Improvements to conductivity are most dramatic in the case of predominantly semi-conducting, single walled carbon nanotubes (SWCNT) due to the remarkable introduction of about 1.2e14 holes/sq. cm, but this method is also an effective means to improve metallic, multi-walled carbon nanotubes (MWCNT). We will demonstrate improvement of transport and charge collection properties of both SWCNTs and MWCNTs by these SAM coatings in FETs and also in organic photovoltaic solar cells and in OLEDs. [1] M. F. Calhoun et al. Nature Materials 7, 84 - 89 (2008). [2] C. Y. Kao et al. Adv. Func. Mater. 19, 1 (2009).   

Efficient electron and hole injection in organic transistors with carbon nanotube electrodes

Single Wall Carbon Nanotubes (SWCNTs) are of great interest as electrode materials in Organic Field Effect Transistors (OFETs) since they are easy to process and stable in ambient contitions. Thanks to their field emission properties, SWCNTs electrodes, in principle, are able to inject both electrons and holes into organics with low injection barriers, promoting tunneling injection. We well present recent result on the electrical properties of p-type and n-type OFETs using hairy SWCNTs electrodes, where the CNTs are attached on the substrate by means of metallic Ti contact pads. Devices with SWCNTs electrodes show improved injection characteristics compared with those using conventional metallic electrodes both for and p-type (pentacene) and n-type (fullerene) materials.   

Octadecanethiol Island Formation on Single Crystal Zinc Oxide Surfaces

Organic photovoltaic devices, containing ZnO nanorod electron acceptor arrays intercalated with organic polymers, could lead to low-cost solar cells. Surface modifications of ZnO with octadecanethiol (ODT) monolayers have been shown to improve charge transfer in such devices. The present work is an effort to understand these monolayers through studies of ODT on single crystals of ZnO with well-defined oxygen-terminated or zinc-terminated surfaces. Both bare and ODT- functionalized surfaces were characterized with atomic force microscopy, Fourier transform infrared spectroscopy, x-ray photoemission spectroscopy, and water contact angle measurements. ODT seemed to form islands of multilayers on zinc-terminated surfaces and islands of monolayers on oxygen- terminated surfaces. While ODT was expected to preferentially bond along defects and terraces on oxygen-terminated surfaces, this was not observed. ODT was also expected to more effectively bond to the zinc-terminated surface, which was observed. This work was supported by the National Science Foundation Division of Materials Research DMR-0606054, DMR-0907409, and the Renewable Energy Materials Research Science and Engineering Center at the Colorado School of Mines.   

Mechanical control of morphology in fluorene oligomers: First principles calculations

Mechanically induced strain can have a significant impact on the optical properties of conjugated polymers. We use \textit{ab-initio} computational methods to investigate tensile stretching and compression of polyfluorene oligomers. We show that strain can result in changes in backbone morphology which relate to shifts in transition energies in a non-trivial manner. In particular, compression of oligomers results in two distinct morphologies which shift the signal in opposite directions, despite an equal distance between terminal atoms. We also consider the application of strain through adsorption on a silicon substrate. Extension or compression can be induced through mismatch between the lattice of the substrate and the size of the repeat unit of the molecule.   

Large Single Grain Thin Film by Hollow Capillary Method

Using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-Pentacene), we have made solution processed thin film transistor by hollow capillary method. The grain size is 1mm wide and more than 10mm long along with the writing direction routinely, which cover the channels of entire device. Thickness of the highly uniform and continuous film can be varied from 10nm to 100nm by tuning the concentration and speed of substrate. We also show the Evidence of two preferred growth orientation along with the writing direction. The field effect mobility shows highly orientation-dependence.   

Exploring photomechanical switching capability and self-assembly of individual molecules on semiconductor surfaces

Surface-bound photoactive organic molecules reveal substantially different photomechanical switching properties compared to when they are in solution-based environments. Metal surfaces, for example, often reduce photomechanical activity due to molecule-substrate interactions. Semiconductor surfaces are expected to induce different molecular switching behavior due to the presence of a band gap, potentially resulting in longer excited-state lifetimes and enhanced control of photomechanical properties. Here we report our exploration of single-molecule-resolved self-assembly and photomechanical switching capability of azobenzene derivatives on semiconducting GaAs(110) using variable temperature scanning tunneling microscopy.   

Session Z17: Charged and Ion-Containing Polymers II

Segmental dynamics and cross-linking in ion containing polymers

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

The interplay of ion crosslinking, free ion content, and polymer mobility in PEO-based single-ion conductors

We use molecular dynamics simulation to study ion clustering and dynamics in ion containing polymers. This PEO based single-ion conducting ionomer serves as a model system for understanding cation transport in solid state polymer electrolytes (SPEs). Although small-angle x-ray scattering does not show an ionomer peak, we observer various cation-anion complexes in the simulation, suggesting ionomer backbones are crosslinked through ion complexes. These crosslinks reduce the adjacent PEO mobility resulting in a symmetric mobility gradient along the PEO chain. We vary the cation-anion interaction in the simulation to observe the interplay of cation-anion association, polymer mobility and cation motion. Cation-anion association controls the number of free ions, which is important in ionic conductivity when these materials are used as SPEs. Polymer mobility controls how fast the free ions are able to move through the SPE. High conductivity requires both a high free ion content and fast polymer motion. To understand the connection between the two, we ``tune'' the force field in order to manipulate the free ion content and observe the influence on PEO dynamics.   

Conductivity and Water Content in Asymmetrical Sulfonated Block Copolymers

We have determined the morphology, proton conductivity and water uptake of asymmetric sulfonated poly(styrene-$b$-methylbutylene) (PSS-PMB) membranes equilibrated with 98{\%} relative humidity (RH) air. To our surprise we found that the conductivity of low molecular weight PSS-PMB samples decreased slowly and irreversibly when the temperature of the membrane (and air) was increased. In contrast, high molecular weight PSS-PMB samples with the same asymmetry decreased more rapidly in response to a temperature change. In addition the factor by which the conductivity decreased was significantly higher in the case of the low molecular weight PSS-PMB. This puzzle was resolved by in-situ small angle neutron scattering which enabled determination of the morphological response of the samples to changes in temperature at RH=98{\%}. The morphology-conductivity relationship in the equilibrated state gives insight into factors that govern charge transport in these systems.   

Ionic Conductivity Trends with Molecular Weight in PEO and PEO-Based Solid Polymer Electrolytes

Poly(ethylene oxide) based polymer electrolytes with lithium bis(trifluoromethane)sulfonamide (LiTFSI) salt remain one of the most promising class of solid polymer electrolyte for rechargeable lithium metal batteries. Among those, poly(styrene-b-ethyleneoxide) (SEO) doped with LiTFSI has been shown to exhibit acceptable levels of conductivity while possessing a sufficiently high modulus to suppress the growth of dendrites. The purpose of this study is to explore the molecular weight dependence on conductivity for the PEO/LiTFSI system to which previous studies have alluded, but never quantified, and contrast this with the observed molecular weight dependence of SEO reported in previous work. Conductivities were measured using AC impedance spectroscopy over a broad range of temperatures and molecular weights beyond those reported in the literature.   

Electrospinning of an Alkaline Polymer Electrolyte

The polymer electrolyte membrane is a key component of the low temperature fuel cell to block fuel and electron crossover, while enabling ions to pass and complete the half-cell reactions. Proton exchange membranes (PEMs) are anion-containing polymers, such as Nafion, which offer proton conduction pathways. Alkaline polymer electrolytes utilize hydroxyl anions as charge carriers and are currently being researched as an alternative to PEMs because they may offer the use of inexpensive metal catalysts. However, hydroxyl anion in an alkaline electrolyte has relatively low mobility compared to that of protons in an acid electrolyte; hence a high concentration of OH$^{-}$ is required to obtain high ionic conductivity. Here, we report the use of an electrospinning process to prepare nonwoven membranes. Polysulfones are first functionalized with varied ionic content of quaternary ammonium functional groups and then are electrospun to get alkaline electrolyte mat. The morphology at various ionic content, mechanical property, and in-plane conductivity of resulting films will be discussed and compared to solvent-cast films of the same material.   

Thermal Properties of Poly(allylamine hydrochloride)/Poly(acrylic acid) Layer-by-Layer Assemblies

Layer-by-layer (LbL) assemblies are promising for global energy and health applications, but their materials properties are not well understood. LbL assemblies are created from the alternate adsorption of oppositely charged species from solution to a substrate. Particularly, little is known about the thermal properties of LbL assemblies because the supporting substrate impedes characterization. It is not initially clear if electrostatic LbL assemblies possess a glass transition temperature, if they are rubbery or glassy, or if their heat capacity is comparable to their homopolymer constituents. Here, we isolate large areas of LbL assemblies from a low-energy substrate, which facilitates thermal characterization via modulated differential scanning calorimetry (MDSC) and thermal gravimetric analysis (TGA). LbL assemblies of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were deposited onto hydrophobic substrates, and subsequently isolated. Results highlight that PAH/PAA LbL films are glassy, and have low mobility because of the high density of ion pair crosslinks. The techniques presented here are general, and can be applied to any LbL film.   

Mean Field Theory of Aggregation and Symmetry-Breaking in Ionomer Melts

Solutions and melts of charged polymers are studied with Single Chain Mean Field Theory, which preserves intramolecular correlations. System parameters include temperature, chain length, and monomer density. The thermodynamics of the equilibrium system are calculated, including electrostatic energy and polymer and counterion entropy. Phenomenology of interest include the association-dissociation transition of counterions and cooperative conformational and morphological transitions which are expected to dominate the macroscale temperature variations. Theoretical results are compared to simulation findings of low temperature condensation of chains to form ordered sheets. This study provides a basis for future stochastic models of the temporal evolution of such large-scale structures, with immediate relevance to measurable dynamic properties.   

Hierarchical Structure of Poly(ethylene) Based Ionomers

The effect of chain architecture (linear vs branched), acid placement (precise vs random), acid content, neutralization extent, and crystallinity on the hierarchical structure of poly(ethylene-acrylic acid) ionomers was investigated via X-ray scattering and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). HAADF STEM reveals randomly dispersed, spherical ionic aggregates in all materials. At temperatures where the ionomers are fully amorphous, the scattering at intermediate angle arises from interaggregate interference and can be described by the Kinning-Thomas model. If the acid groups are placed every 21$^{st}$ carbon, the materials are semicrystalline at room temperature and contributions from acid layers associated with crystallites are convoluted with interaggregate scattering. The ionic aggregates have diameters of $\sim $ 1 nm for all samples; however, the number density of aggregates is strongly dependent on the acid content but weakly dependent on the extent of neutralization.   

Interdiffusion of long alcohols into thin ionomer films; In situ Neutron Reflectivity study

Transport of solvents and ions within ionic polymers controls their many current and potential applications from energy related to drug delivery systems. The transport is determined by the phase structure and the interaction of the diffusing species with the polymers, coupled with interfacial effects. The current work presents the kinetics of penetration of long chain alcohols diffusing into rigid ionomer thin films formed by a rigid polyphenylene sulfonated ionomer, using \textit{in situ} neutron reflectivity. The penetration of deuterated n-octanol and n-hexanol into $\sim $20nm thick films was followed as a function of time for different sulfonation levels of the polymer. As for shorter molecules, the diffusion process consists of two stages, a relatively fast one in which the film thickness increases linearly with time followed by a slow phase in which structural changes take place. With increasing sulfonation levels, the diffusion first increases and then decreases; a trend that is attributed to hydrophilic/hydrophobic balance.   

Anisotropic Proton Conduction in Aligned Block Copolymer Electrolyte Membranes at Equilibrium with Humid Air

The effect of alignment of proton-conducting domains in hydrated poly(styrenesulfonate-b-methylbutylene) copolymer films on conductivity was studied by impedance spectroscopy. Pressing isotropic samples obtained by casting results in lamellae aligned in the plane of the film. Application of electric fields and flow fields on the isotropic samples results in lamellae aligned perpendicular to the plane of the film. The alignment of lamellae, quantified by a combination of 2D SAXS, birefringence, and TEM, was much better in the pressed samples than in the field-aligned samples. Conductivity was measured in the plane of the film and normal to the plane of the film. Only the pressed sample showed highly anisotropic proton conduction with the ratio of 75. In this case, the parallel conductivity increased by 30\% after alignment, relative to that obtained from the as-cast samples. The conductivity ratio obtained from after electric field and shear field alignment were 1.2 and 1.4, respectively, in spite of partial alignment of the domains, and the increase in the perpendicular conductivity after alignment was less than 20 percent.   

Ab-initio study of polypyrrole as a pervaporation membrane

The affinities between polypyrrole, water, ethanol and a sulfonate-carrying ion were calculated from first principles. All interactions were demonstrably hydrogen bonds between the oxygen from the sulfonate groups and the hydrogen in the hydroxyl (for water and ethanol) and amine groups (for polypyrrole). Each sulfonate group was shown to form three hydrogen bonds, with any of the three other types of molecules, allowing the ion complex to bind to multiple polypyrrole chains, water, and ethanol molecules simultaneously. The energies indicated a higher affinity between the ion and poypyrrole, second highest between water and the ion, and the lowest for ethanol and the ion. A high affinity of the ion to the polymer backbone is desirable to prevent leeching. The ion was found to have a higher affinity to water molecules than to ethanol, confirming the system's selectivity in separating water from ethanol.   

Dynamics of Sulfonated Polystyrene Ionomers by Dielectric Relaxation Spectroscopy

Broadband dielectric spectroscopy was used to investigate the dynamics of sulfonated polystyrene (SPS) ionomers, in both the acid and neutralized form. This study seeks to elucidate the role of counter ion type (Zn, Na, and Cs), degree of sulfonation (9 and 6{\%}), and ion cluster morphology on the relaxation phenomena of SPS. Degree of neutralization and ion type have been found to significantly impact the breadth and time scale of the segmental relaxation process. High temperature relaxation processes, tentatively proposed to arise from Maxwell-Wagner-Sillars interfacial polarization and a hydrogen bonding relaxation, have also been identified. Bands in the sulfonate stretching region of FTIR spectra reveal information about ion coordination in the local aggregate environment. A combination of scanning transmission electron microscopy imaging and X-ray scattering confirmed the presence of homogeneously distributed, nearly monodisperse spherical ionic aggregates in the polymer matrix.   

A Polarizable Potential for Poly(ethylene oxide) ~in Aqueous Solution

We have developed a quantum chemistry-based polarizable potential for poly(ethylene oxide) (PEO) in aqueous solution based on the APPLE{\&}P{\textregistered} polarizable ether and SWM4-DP polarizable water model. Ether-water interactions were parameterized to reproduce the binding energy of water with 1,2-dimethoxyethane (DME) determined from high-level quantum chemistry calculations. Simulations of DME/water and PEO/water solutions at room temperature using the new polarizable potential yielded thermodynamic and transport properties in better agreement with experiment than previously published polarizable and non-polarizable potentials. The predicted miscibility of PEO and water as a function of temperature was found to be strongly correlated with the predicted free energy of solvation of DME in water for the various force fields investigated. Simulations of PEO/water solutions confirm the ability of the new potential to capture, at least qualitatively, the LCST behavior of these solutions   

Charging and Screening in Nonpolar Solutions of Nonionizable Surfactants

Nonpolar liquids do not easily accommodate electric charges, but surfactant additives are often found to dramatically increase the solution conductivity and promote surface charging of suspended colloid particles. Such surfactant-mediated electrostatic effects have been associated with equilibrium charge fluctuations among reverse surfactant micelles and in some cases with the statistically rare ionization of individual surfactant molecules. Here we present experimental evidence that even surfactants without any ionizable group can mediate charging and charge screening in nonpolar oils, and that they can do so at surfactant concentrations well below the critical micelle concentration (cmc). Precision conductometry, light scattering, and Karl-Fischer titration of sorbitan oleate solutions in hexane, paired with electrophoretic mobility measurements on suspended polymer particles, reveal a distinctly electrostatic action of the surfactant. We interpret our observations in terms of a charge fluctuation model and argue that the observed charging processes are likely facilitated, but not limited, by the presence of ionizable impurities.   

Session Z18: New Experimental, Theoretical, and Computational Methods in Polymer and Soft Matter Physics

Computational modeling of red blood cells: A symplectic integration algorithm

Red blood cells can undergo shape transformations that impact the rheological properties of blood. Computational models have to account for the deformability and red blood cells are often modeled as elastically deformable objects. We present a symplectic integration algorithm for deformable objects. The surface is represented by a set of marker points obtained by surface triangulation, along with a set of fiber vectors that describe the orientation of the material plane. The various elastic energies are formulated in terms of these variables and the equations of motion are obtained by exact differentiation of a discretized Hamiltonian. The integration algorithm preserves the Hamiltonian structure and leads to highly accurate energy conservation, hence he method is expected to be more stable than conventional finite element methods. We apply the algorithm to simulate the shape dynamics of red blood cells.   

Testing Theory Against Experiment and Simulation for Chain Fluids: Can Lattice Compete with Continuum?

We present new results from an in-depth four-way comparison which contrasts the performance of analogous lattice and continuum integral equation theories, with both being held accountable to recently obtained Monte Carlo simulation results and real experimental data. The success of the modeling methods is compared in terms of both experimentally accessible physical properties (e.g., \textit{PVT} surfaces and coexistence boundaries), as well as the more fundamental underlying quantities, such as free energies, and model internal energies. Without fitting to any mixture data, we find good to excellent predictive ability for mixing behavior, even for the simplest lattice-based approach. Our results lead us to propose the most crucial elements required in constructing simple, yet effective, theories.   

Efficient calculation of electrostatic interactions within dynamic dielectric environments

Many biological and soft-matter systems exhibit self-assembly driven by electrostatic interactions. The resulting structures are often dense aggregates, and consequently the electrostatic interactions are significantly modified by dielectric inhomogeneities. This effect is typically ignored in computer simulations, as it greatly complicates the evaluation of energies and forces. Pioneering simulations have demonstrated that fixed dielectric inhomogeneities play a crucial role in biological processes such as water permeation through nanopores [Allen \emph{et al.}, J. Chem.\ Phys.\ {\bf 119}, 3905 (2003)], but the effects of \emph{dynamic} inhomogeneities remain relatively unexplored. We introduce a new method for the efficient calculation of electrostatic interactions within dynamic dielectric environments, and show preliminary simulation results for aqueous suspensions of synthetic colloids.   

Theory of spinodal decomposition assisted polymer crystallization in a binary polyolefin mixture

Recent experiments by Han et. al. have observed a new kind of coupling process for polymer crystallization in a binary polyolefin blend system, which originates from the fluctuation growth of a two-component phase separating system in the unstable spinodal region. A strong coupling was observed between the concentration fluctuation liquid-liquid phase separation and the nucleation of crystallization, which resulted in significant changes in the crystallization kinetics. In this paper, we propose a possible mechanism which can explain these experimental observations. The spinodal decomposition in the unstable region causes the spontaneous growth of domains of the two constituent polyolefins. It is proposed that these domains then present an interface on which heterogeneous nucleation of the crystallizable component can take place with a much reduced energy barrier. Combining the theories of heterogeneous nucleation and spinodal decomposition kinetics, we present an anaytic calculation of the nucleation rate as a function of the spinodal decomposition time. The analytic formula is found to correspond well with the experimental results for the late stage of spinodal decomposition kinetics. More detailed experiments are required to verify our prediction for the nucleation rate for early times.   

Field theoretic simulations in the Gibbs ensemble

Measuring the properties of phases in equilibrium and the calculation of phase diagrams is one of the most common applications of field theoretic simulations. To obtain properties of both phases from one simulation (such as the concentration of a species in each phase), it is necessary to perform simulations large enough so that the interface between the two phases does not affect the estimate of the bulk properties, which is computationally very demanding. Alternatively, one can perform a sweep through parameter space searching for multiple state points that meet the criteria for equilibrium, which is again computationally expensive. In this talk, I will describe how we have adapted the Gibbs ensemble to field theoretic simulations, where two simulation boxes are kept in chemical and mechanical equilibrium by allowing the boxes to exchange both particles and volume. By maintaining a constant total number of particles and total volume, such a simulation can efficiently simulate two bulk phases in equilibrium in the canonical ensemble, allowing a reliable estimate of the properties of the two phases. Our method will be demonstrated in both the mean-field limit and in simulations that fully sample the fluctuations of the field theory.   

Highly constrained polymer dynamics with an enhanced bond-fluctuation model

We introduce a generalization to the bond-fluctuation model for simulating polymer dynamics in a highly constrained environment. The technique is applied to the quenching of self-attracting polymer chains which demonstrates a three-fold collapse. Both the extent and dynamics of the collapse are greatly enhanced by using the generalized bond-fluctuation model where the bond length $l = \sqrt{8}$ (in units of the lattice spacing) is explicitly utilized. We also show that lattice effects in dense melts ($\phi > 0.5$) are alleviated with this enhancement. Efficiency is maintained by implementing a simple check to prevent phantom chain dynamics.   

Molecular Dynamics Simulations of Polymer Surfaces

Due to the increased use of polymer based materials in aerospace adhesive applications, the issues of molecular structure effects and contamination at interfaces have become critical. Computational modeling is being developed to study these systems, with the dual goals of elucidating mechanisms of degradation and developing insights into key structure-property relationships both of which contribute to enabling the prediction of adhesive bond failure. Atomistically detailed models of polymer surfaces, interfaces and bulk structures are constructed and analyzed for this purpose. These models are built with a controlled distribution and content of water molecules to assess the effect of moisture ingress on relevant properties of interest. The models are analyzed and compared to study the effect of moisture on surface and interfacial properties which may influence adhesive bonding characteristics.   

HOOMD-blue, general-purpose many-body dynamics on the GPU

We present HOOMD-blue, a new, open source code for performing molecular dynamics and related many-body dynamics simulations on graphics processing units (GPUs). All calculations are fully implemented on the GPU, enabling large performance speedups over traditional CPUs. On typical benchmarks, HOOMD-blue is about 60 times faster on a current generation GPU compared to running on a single CPU core. Next generation chips are due for release in early 2010 and are expected to nearly double performance. Efficient execution is achieved without any lack of generality and thus a wide variety of capabilities are present in the code, including standard bond, pair, angle, dihedral and improper potentials, along with the common NPT, NVE, NVT, and Brownian dynamics integration routines. The code is object-oriented, well documented, and easy to modify. We are constantly adding new features and looking for new developers to contribute to this fast maturing, open-source code [1]. In this talk, we present an overview of HOOMD-blue and give examples of its current and planned capabilities and speed over traditional CPU-based codes. [1] Find HOOMD-blue online at: http://codeblue.umich.edu/hoomd-blue/   

Quantifying and Minimizing Lattice Anisotropy

We have quantified the anisotropy of various lattice models used in polymer simulations based on two quantities: the Fourier transform of the normalized Boltzmann factor of allowable bonds on a lattice (which is the central quantity for describing lattice chain conformations), and the bulk lamellar period at the mean-field order-disorder transition of symmetric diblock copolymers on a lattice (which is pertinent to the study of microphase separation). This allowed us to compare the anisotropy of different lattices and to design new lattice models that minimize the quantified anisotropy.   

Quantitative test of polymer field theories by fast lattice Monte Carlo simulations

Recently, one of us proposed the so-called fast lattice Monte Carlo (FLMC) simulations,\footnote{\textit{Q. Wang}, \textbf{Soft Matter, 5}, 4564 (2009).} where, instead of the self- and mutual-avoiding walks used in conventional lattice Monte Carlo simulations, multiple occupancy of lattice sites is allowed with a proper Boltzmann weight. FLMC simulations give orders of magnitude faster/better sampling of configurational space for multi-chain polymeric systems, and further allow stringent test, without any parameter-fitting, of the widely used polymer field theories. Taking homopolymer solutions and brushes as examples, we formulate the field theories (the self-consistent field theory, Gaussian-fluctuation theory, and the self-consistent Hartree approximation) on the same lattice and with the same Hamiltonian as used in FLMC simulations. Direct comparisons between the simulations and theories therefore unambiguously and quantitatively reveal the consequences of approximations used in the latter.   

Cracks in Ductile Polymers Using Cohesive Zone Modeling

Ductile polymer fracture is studied by using a relatively new technique in which cohesive elements are placed between elastic solid elements, along the mesh boundaries. Polymer chain elongation is described using cohesive model parameters that are calibrated to simulate the conical crack observed in a single fiber fragmentation experiment that uses a ductile polyester matrix. This approach limits the crack trajectory to align with the mesh, thus severely limiting the accuracy. We propose a new crack trajectory method to describe polymer chain elongation by incorporating both normal and shear traction contributions in a strictly cohesive zone model approach. Our formulation shows that local polymer chain orientation depends on the ratio of mode I and mode II stiffness penalty parameters and tractions. The corresponding stress state reaches a critical value that is represented by a material parameter. The new crack tip extends to a location where the critical stress is reached at a maximum distance from the existing crack tip. Implementation is performed by adding the proposed crack trajectory method to an extended finite element code (X-FEM) with cohesive element modeling.   

What exactly is measured by passive microbead rheology?

The dynamic modulus $G^*$ of a viscoelastic medium is often measured by following the trajectory of a small bead subject to Brownian motion in a method called ``passive microbead rheology". In the pioneering manuscript that introduced the idea [T. G. Mason and D. A. Weitz, Phys. Rev. Lett. 74, 1250 (1995)], this equivalence between the autocorrelation function and $G^*$ was assumed via the generalized Stokes-Einstein relation (GSER). We show here that this expression does not satisfy the correct initial condition. Also, earlier derivations of the GSER use an initial condition that freezes the bead in space until measurements begin, which is not typical for experiments. We use here an analytic solution of the forces on a sphere undergoing arbitrary displacement in an arbitrary viscoelastic medium combined with the fluctuation-dissipation theorem to derive what is actually measured in the microbead rheology experiment. We find that a convolution of $G^*$ is indeed measured in bead-displacement statistics, which is similar to GSER but obeys the correct initial conditions. The result includes inertial effects, and allows for the presence of an optical trap, allowing a more general technique to extract the dynamic modulus from microrheology.   

Measuring colloid interactions and dynamics with digital holographic microscopy and multi-particle scattering theory

We describe an \textit{in situ}, nonperturbative optical technique for measuring the pair potential between two colloidal particles in bulk suspension. We image clusters of colloidal spheres at or near contact with digital holographic microscopy and fit the resulting holograms to the exact numerical solution for electromagnetic scattering from the clusters. We measure the depletion interaction between two 1 $\mu$m polystyrene spheres in a bulk suspension by studying the thermal fluctuations of their $\sim$ 50 nm separation distance with $\sim$ 5 nm spatial resolution. Our method does not require the use of optical tweezers and thus may be useful for studying interactions between colloids that are too small or too nearly index-matched to be optically trapped. We also use our methods for recording and fitting holograms to simultaneously measure the 3D translational and rotational Brownian motion of sphere clusters.   

Probing the Physics of Organic-Organic Heterojunctions using Laterally defined Organic Field Effect Diodes

Scanning Probe Microscopy has been used extensively to probe the physics of metal/organic semiconductor contacts. However, a similar level of knowledge for Organic/Organic interfaces has been elusive because of the buried nature of the junction. We have invented a novel lithographic technique to fabricate a lateral heterojunction diode, the characteristics of which can be tuned by use of third terminal in a thin film transistor configuration. Kelvin Probe microscopy reveals the built in potential at the junction and its modulation by use of the third gate terminal. Changes in potential are consistent with changes in rectification behavior as evidenced by I-V plots. ~We also report the use of such a junction to investigate the energetics of doped organic interfaces, including the density of states profile on each side of the junction.   

Positron Annihilation Spectroscopy as a Novel Interfacial Probe for Thin Polymeric Films and Nano-Composites

Positron annihilation spectroscopy (PAS) has been developed as a novel probe to characterize the sub-nanometer defect, free volume, profile from the surface, interfaces, and to the bulk in polymeric materials when a variable mono-energy slow positron beam is used. Free-volume hole sizes, fractions, and distributions are measurable as a function of depth at the high precision. PAS has been successfully used to study the interfacial properties of polymeric nanocomposites at different chemical bonding. In nano-scale thin polymeric films, such as in PS/SiO$_{2}$, and PU/ZnO, significant variations of T$_{g}$ as a function of depth and of wt{\%} oxide are observed. Variations of T$_{g}$ are dependent on strong or weak interactions between polymers and nano-scale oxides surfaces.   

Session Z19: Focus Session: Polymer Brushes

Design and applications of functionalized polymer brushes

Response to stimuli is one of the major life processes, by which living systems interact with the external environment. Advances in nanotechnology have focused on designing ``responsive to stimuli'' or ``smart'' materials that mimic many processes found in living systems. The talk addresses our recent results on the synthesis, study, interesting applications and prospects of functionalized polymer brushes for the fabrication of smart responsive surfaces, sensors with various transduction mechanisms, micro/nanoactuators, and electrochemical gating devices. We also use surface modification of nanoparticles with polymer brushes for a new intriguing opportunity to turn on and off and tune interactions between nanoparticles, allowing control of the directed self-assembly with external stimuli/signals. The stimuli responsive polymeric and hybrid systems demonstrate strong advantages for the fabrication of robust multifunctional and multiresponsive materials and nanodevices.   

Mixed Polymer Brushes: A Tool for Nano-lithography?

Self-consistent field theory (SCFT) simulations are presented that examine the suitability of mixed polymer brushes as a nano- lithography tool by adapting lateral confinement methods that have proved effective in enhancing the in-plane order of self- assembled block copolymer films . In the present context, however, we explore a type of ``chemical'' (rather than ``topological'') confinement in which a ``pure'' polymer brush of either A or B homopolymer is used to laterally confine the mixed A/B brush into a region of prescribed shape. SCFT simulations of such confined mixed brushes show that the introduction of a pure polymer brush alongside the mixed brush region directs the microdomains to align with the interface between the two regions. Results are also presented that demonstrate the possibility of forming features with multiple sizes and pitches in precise locations within a film by modulating the grafting densities of one or both mixed brush components.   

Mechanical Properties of Polystyrene Brush Films

Fifteen years ago, Fredrickson et al. [\textit{Macromolecules} \textbf{1992,} 25, 2882-2889] predicted that a molten polymer brush possesses a shear modulus that would cause the surface structure to deviate from that of a liquid. Their predictions, though broadly used, have been largely unchecked. Here, we present experimental data on polystyrene PS brushes that validate Fredrickson et al.'s predictions. Measurement obtained by following the time evolution of the surface structure of a brush shows that the shear modulus of the brush is established prior to the onset of the terminal flow regime, suggesting that the entropic elasticity in the brush chains needed to bring about the solid behavior of the brush is operative already in the rubbery regime.   

Diblock-copolymer brush tethered to a nanoparticle

We model a brush of diblock copolymers grafted to a spherical nanoparticle, using self-consistent field theory (SCFT). This is achieved with a computationally fast algorithm that combines a pseudo-spectral method for solving the diffusion equation in spherical-polar coordinates and Anderson mixing to iterate the self-consistent field equations. The self-assembling block-copolymer films coat the nanoparticles with a variety of periodic surface patterns, which can be tuned by changing the various system parameters. Results are compared to previous SCFT calculations on flat surfaces, and it is seen that the curvature and finite surface area of the nanoparticle shifts the phase boundaries and introduces additional phase transitions. These patterned nanoparticle coatings are expected to have promising future technological applications.   

Adsorption of end-adsorbing homopolymers A and B from solution onto a solid interface

Determination of the composition and spatial organization in multicomponent monolayers formed by the adsorption of polymeric ligands from solution is important in variety of applications. Prior studies on adsorption of end-functionalized homopolymers show that the final surface coverage depends on the molecular weight, the solution concentration, adsorption energy of the terminal group and degree of polydispersity of the polymer chains. This study addresses the thermodynamics of adsorption of end-adsorbing homopolymers A and B onto a flat surface from solution. Specifically, we examine the role of solvent selectivity, incompatibility between the A and B segments, and the degree of bidispersity on the structure of the adsorbed layers. The relevance of our findings to synthesis of multivalent nanoparticles (nanoparticles with two or more types of ligands attached to their surfaces) by immersion of the nanoparticles into a solution containing the various ligands will also be presented.   

Grafted Peptides for the Control of Interfacial Properties

Peptide or protein polymers that are used to control interfacial properties are usually prepared by solid-state synthesis and then adsorbed to an interface. Such a method results in a low yield and places restrictions on polymer structure, because the peptide must be designed to adsorb, as well as to provide the interfacial control. The method of grafting peptides from surfaces is an alternate method that is potentially very useful because the peptide is covalently linked, and the sequence limitations related to adsorption are removed. To demonstrate this technique, we have used solid-phase peptide synthesis to graft a 15-residue peptide, EKEKEKEKEKEKEGG, containing a zwitterionic sequence of alternating lysine and glutamic acid residues from the surface of an aminosilanized silicon wafer by placing the silicon wafer within a commercial microwave peptide synthesizer. We confirmed the presence of this peptide layer on the surface by X-ray photoelectron spectroscopy (XPS) and ellipsometry. Atomic force microscopy (AFM) was then used to study the forces between the peptide-modified surface and a borosilicate glass sphere as a function of solution pH. We will also discuss the use of grafted peptides to control the stability of colloidal suspensions.   

Strong Polyelectrolyte Brushes: A self-consistent field theory study

We investigated the properties of a polyelectrolyte brush system by means of self-consistent field theory. We considered the case of a polyelectrolyte system grafted to both a similarly and oppositely charged surface, in the presence of counter-ions. The properties of the system are described, in the weakly charged limit, by saddle point equations, that couple a modified diffusion equation to the one-dimensional Poisson-Boltzmann equation, describing the electrostatic field in the system. A systematic, numerical study of this set of equations is presented and comparison is made with previous studies. Possible extensions to different grafting geometries are suggested and throughout discussed.   

Protein adsorption at polyethylene oxide brushes of various surface coverage

The adsorption of proteins onto surfaces enables the unwanted formation of Bio-films that are detrimental to a wide range of applications as diverse as artificial implants and the hulls of ocean liners. A surface that exhibits excellent protein resistant behavior is polyethylene oxide brushes (PEO). The amount of adsorbed protein at a brush surface is related to the grafting density and molecular weight of the PEO chains. However it has not yet been proven experimentally where the proteins adsorb and therefore why the brush offers resistance to adsorption. There are three suspected modes of adsorption; ``primary'', at the substrate, ``secondary'', at the edge of the brush and ``tertiary'', within the brush. Recently theoretical work by Katira et al has proposed a random sequential model, explaining the adsorption of proteins at brushes. In this theory the proteins adsorb at random surface sites, which are not covered by the brush. As the surface coverage of the chains increases, the number of available adsorption sites decreases. This theory is in agreement with experimental work carried out in our group.   

Surface Dynamics of Untethered Chains on top of Chemically Identical Polymer Brushes

The dynamics of the surface height fluctuations on untethered chains spun cast on chemically identical homopolymer brushes are studied using X-ray photon correlation spectroscopy (XPCS) and neutron reflectivity (NR). These dynamics are found to be well described by theory of overdamped capillary waves. Underlying brush layer alters dramatically the dynamics of the film surface of PS chains atop the brush. Surface dynamic behavior strongly depends on the interpenetration of free chains into the brush layer which is dictated by the thickness of untethered chains, thickness and grafting density of the brush layer. Increase of underlying brush thickness or decrease of brush grafting density, increases the interpenetration of free chains into the brush layer and increases relaxation time constants. The interpenetration depth decreases with increasing molecular weight of untethered chains.   

Molecular Dynamics Simulations of Fractionation of Molecular Brushes During Spreading on Substrates

We have performed coarse-grained molecular dynamics simulations of motion of brush-like molecules in a matrix of linear chains in contact with a substrate under Poiseuille flow conditions. Our simulations show that short brush molecules move faster than the long ones resulting in fractionation of the brush molecules according to their molecular weight in the spreading polymeric films. The simulation data are in a good qualitative agreement with the predictions of the theoretical model which relates the brush velocity with the ratio of the friction coefficients between brush and substrate and between substrate and linear chains and brush geometric characteristics (length of the brush molecule and its width). Computer simulations and theoretical model provide an explanation of the brush fractionation observed during spreading of the mixtures of brush-like and linear macromolecules on a solid substrate.   

Surface Diffusion of Single Molecules on Poly(N-isopropylacrylamide) Brush Surfaces

How molecules and macromolecules diffuse at polymer surface remain inadequately understood. In this talk, we present a recent single-molecule spectroscopy study of fluorescent probes molecules at surface-tethered polymer brush surfaces. We focus on the dynamics of fluorescent molecules, Rhodamine110 (RG110) on thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) brush thin films as a model system of tunable molecule-polymer surface interactions by varying temperature (T) across the LCST of PNIPAM brushes. The diffusion coefficient, D of RG110, measured by fluorescence correlation spectroscopy (FCS) at a single molecule resolution, decreases as increasing the solution T across the LCST of PNIPAM brush thin films, due to the enhanced hydrophobic interaction at the molecule-polymer interfaces. However, it is surprising to observe the faster diffusion of RG110 on the self-assembled monolayer of octadecyltriethoxylsilane (OTE) than that on the PNIPAM brush surface, despite the stronger RG110 interaction with OTE than PNIPAM. Additionally, we observe that reduced PNIPAM brush thickness lead to further slowing down the diffusion dynamics of RG110 on PNIPAM brush surfaces at constant T. We thus speculate that the retarded diffusion process of small molecules on soft polymer brush surfaces, in comparison to the faster diffusion on hard surfaces, is a result of the coupling between molecule surface diffusion and the relaxation of wiggling polymer brush chains at the interface.   

Structure of End-Grafted Polymer Brushes -- A Molecular Dynamics Study

Molecular dynamics simulations of a polar polymer brush in a polar solvent are presented using a coarse-grained approach. Chain extension is heavily influenced by temperature as expected. Chains extend far from the surface at high temperature, while surface adsorption at a weakly attractive surface dominates at low temperature. Increasing grafting density leads to greater chain extension due to excluded volume effects under all conditions, consistent with previous scaling analysis. Polymer depletion regions are found near the surface even at very high grafting densities indicating a chain orientation normal to the surface close to the grafting points. Radial distribution functions reveal that the grafting pattern does not affect the overall brush configuration beyond the first five monomers of each chain as long as the surface is homogeneously covered.   

First principles Modelling of Brush formation of linear oligomers on Al Surfaces

We study the role of chemical detail in short-linear parafins and olefins in the formation of polymer brushes on an Aluminum substrate. The presence of unsaturated bonds near the end of the chain greatly enhances the brush formation on the Al surface. In addition we examine the resulting change in slip boundary conditions for flow over the surface. Our simulations are based on a molecular dynamics force field we have developed using force matching to density functional theory with careful attention to correct reproduction of lateral as well as normal forces.   

Session Z20: Carbon Nanotubes: Fundamentals and Applications

Tuning Circular Carbon Nanotubes into Regular Polygons via Selective Hydrogenation

Using density functional theory approach, we study the selective hydrogenation of single walled carbon nanotubes (SWNTs). We confirmed that fully hydrogenated SWNT (FH-SWNT) is energetically more favorable than partially hydrogenated ones. Previous studies have revealed polygonization of bare carbon nanotubes (CNTs) through plasticity and buckling. Different from those traditional mechanical methods, we found that the curvature energy of a FH-SWNT can be significantly relaxed by the breaking of its cylindrical symmetry through a chemical pathway with low energetic compensation. Flipping a few rows of H over the FH-SWNT wall significantly reduces the tube curvature energy and leads to more stable configurations with polygonal (triangle, square etc) cross-sections.   

Synthesis, Characterization and Adaptability of Carbon Nanotube-Based Solid Lubricants

We report on the experimental investigations of the tribological properties of carbon nanotube (CNT) based composites. Two different CNT composites were obtained by electro-depositing molybdenum disulfide (MoS$_{2})$ and silver (Ag) on vertically aligned assemblies of CNTs. Both of the CNT based composites, CNT-MoS$_{2,}$ as well as CNT-Ag, showed substantially lower values of friction coefficients and wear rates than traditional thin films of MoS$_{2,}$ and Ag. The adaptabilities of these composites under humid and non-humid conditions for CNT-MoS$_{2,}$ and high temperature cycling in the case of CNT-Ag composites, were also tested and will be presented. Our results indicate that the CNT-MoS$_{2}$ composites were able to sustain their lubricating properties under humid/non-humid cycling, whereas the CNT-Ag composites showed degradation of their frictional properties under high temperature cycling.   

Phase behavior of SWNT-superacid solutions and fabrication of aligned macrostructures

Single-wall carbon nanotubes (SWNTs) are carbon based molecules which possess very high aspect ratio, high persistence length and behave as rigid rods when dispersed in a liquid phase. Superacids (oleum, chlorosulfonic acid etc) have been shown as one of the most effective solvents for dispersing and dissolving SWNTs. The SWNT-superacid systems exhibit a very rich phase behavior with well defined isotropic, bi-phasic and liquid crystalline phases, and controlled by factors like the SWNT concentration, SWNT length and strength of the acid (solvent). We report the fabrication of SWNT macrostructures with high degree of alignment by self assembly and exploiting the phase behavior of SWNT-superacid solutions. Phase transitions were induced in SWNT-superacid solutions in a controlled manner by gradually changing the strength of the acid solvent. This resulted in the precipitation of SWNT flakes and fibrils which exhibit a high degree of alignment. Detailed characterization of these macrostructures was performed using Raman spectroscopy and polarized optical microscopy. The above method presents a self-assembly based route for fabrication of aligned SWNT based structures from SWNT-superacid systems.   

An ab initio realization of the transport in molecular junctions with armchair carbon-nanotube leads

We perform an ab initio study for the system of molecular junctions with armchair carbon-nanotube leads. The transport behavior of the different 2-polyene junction cases shows remarkable agreement with the expected interference effect we predict with tight binding model which greatly simplified and omitted realistic parameters. Moreover, the slight disagreements between the ab initio study and the analytic study could be well understood with the parameter tests in the tight binding model.   

Nematic Anchoring on Carbon Nanotubes

A$^{ }$dilute suspension of carbon nanotubes (CNTs) in a nematic liquid$^{ }$crystal (LC) does not disturb the LC director. Due to$^{ }$a strong LC-CNT anchoring energy and structural symmetry matching, CNT$^{ }$long axis follows the director field, possessing enhanced dielectric anisotropy$^{ }$of the LC media. This strong anchoring energy stabilizes local \textit{pseudonematic} domains, resulting in nonzero dielectric anisotropy in the isotropic$^{ }$phase. These anisotropic domains respond to external electric fields and$^{ }$show intrinsic frequency response. The presence of these domains makes$^{ }$the isotropic phase electric field-responsive, giving rise to a large$^{ }$dielectric hysteresis effect.   

A Multi-scale Approach to Nano-Composite Nickel-Based Materials

The behavior of nano-composite materials that are formed by incorporating aligned carbon nanotubes (CNTs) into a bulk nickel matrix have been considered. Mechanical properties of these novel materials have been predicted and strain-stress relationships have been investigated by atomistic calculations with interactions derived from the modified embedded-atom method (MEAM). The mechanical stability has been assessed, with consideration of Young's modulus both within and beyond the small elastic deformation regime. Comparisons have been made between Ni/CNTs, the pure FCC nickel matrix, and pristine CNTs. Both single-walled and multi-walled nanotube systems have been considered.   

Physical Properties of Carbon Nanotube Sheets Dry-Drawn from Tall MWCNT Forests

Highly aligned MWCNT forests can by grown by catalytic CVD process in a dry-spinable highly oriented form which allows to draw CNT sheets and twist spin yarns [1,2]. However the sheet resistance of such transparent CNT sheets with average height of 300 um is quite high: 500-700 Ohm/sq. Motivation of our study is to grow taller forests with optimal interbundle connectivity [3] which may result in lower sheet resistance of CNT sheets and higher optical transparency by optimized control of CCVD conditions. We have succeeded to grow tall CNT forest with h=1 $\mu$m and resulting sheet resistance about 200 Ohm/sq. To find the correlation between properties of CNT forests and CNT sheets we conducted SEM analysis combined with Raman, AFM and small-angle X-ray scattering. This study shows how the number, geometry, and mechanical strength of interconnects between bundles are related to the physical properties of CNT sheets. \\[4pt] [1] M.Zhang, S.Fang et al., Science, V.309 (2005) 1215 \\[0pt] [2] M.Zhang, K.Atkinson, R.Baughman, Science, V.306 (2004) 1358 \\[0pt] [3] A.Kuznetsov, A.Fonseca et al., Adv.Mat., (submitted)   

Isosteric heat of adsorption and uptake of gases in open and closed individual carbon nanotubes

We compute by the method of Grand Canonical Monte Carlo the adsorption of argon, methane and hydrogen in the interior and exterior of a single carbon nanotube. The isosteric heat of adsorption is calculated, and the steps observed in the computed adsorption isotherms are interpreted as the formation of cylindrical layers. Our simulations are compared with novel experimental results obtained recently for adsorption in individual carbon nanotubes.   

Characterisation of Carbon Nano-Materials with the Confocal Raman AFM

Carbon is known to exist in a number of allotropes which range from single crystalline diamond - the hardest of all known materials, to the soft, mainly amorphous graphite. The recently discovered carbon nano- materials such as nanotubes and graphene gain more attention in the field of material science due to their light weight, unique electrical and optical properties and mechanical strength. The implementation of such carbon nano-materials into electrical devices or as fillers in polymeric matrixes requires characterization techniques suited for the nanometer range. Due to the unique optical properties of carbon nano-materials, Raman microscopy can be used to characterize single walled carbon nanotubes (SWCNT) with a diameter far below the optical resolution limit in the nano-meter range. Furthermore, the distribution of such nanotubes in polymeric matrixes can be visualized structurally as well as chemometrically. Raman spectroscopy is a well-suited tool to characterize graphene with its unique optical properties, similar to those of CNTs, as a two-dimensional model system. The correlation of spectral data with the number of grapheme layers can be achieved by combining Raman microscopy with AFM.   

Characterization of the junction between single-walled carbon nanotube films and silicon substrates

We experimentally characterize the junction between single-walled carbon nanotube (CNT) films and both $n$-type and $p$-type Si substrates. We prepare CNT films by vacuum filtration, transfer them onto Si substrates and pattern them by photolithography and reactive ion etching. We also fabricate control samples, in which the CNT film is replaced with a Ti/Au metal stack for comparison. We characterize these devices by dark and photo $I-V$ and $C-V$ measurements at various temperatures. Our dark $I-V$ measurements reveal that the CNT film forms a Schottky contact with an average barrier height of 0.44 $\pm $ 0.03 eV and 0.6 $\pm $ 0.02 eV on $p$-type and $n$-type Si, respectively. Average ideality factors and series resistances (normalized with area) are also extracted for both the $n$- and $p$-type CNT film-Si junctions. Furthermore, photocurrent measurements at reverse bias result in responsivity and normalized photo to dark current ratio of 0.197 A/W and 7.11 $\times $ 10$^{4}$ mW$^{-1}$ respectively at a bias of 3 volts. $C-V$ measurements verify the barrier heights extracted from $I-V $measurements. These results extract the important parameters of CNT film-Si junctions and facilitate the application of CNT films as Schottky electrodes in conventional semiconductor electronic and optoelectronic devices.   

Session Z21: Focus Session: Graphene: Growth

Carbon growth on noble metal films

Noble metals offer an attractive combination of properties as substrates for graphene growth, including low carbon solubility and minimal environmental reactivity. Regardless of noble metal commonalities, both experimental and \textit{ab initio} results show significant variations in their structural relationship with epitaxial graphene films, including carbon-metal bond lengths and the orientation of carbon films relative to the substrate [Giovannetti, \textit{et al}., PRL \textbf{101}, 026803 (2008); Loginova, \textit{et al}., New J. Phys. \textbf{11}, 063046 (2009)]. Despite the possible richness of this experimental landscape, insufficient experimental work has been done to ascertain any systematic trends in the dependence of carbon film structure on surface chemistry. In this work we report on the form and properties of carbon deposited on noble metals under UHV conditions. Carbon morphologies on gold are found which grow both up and down the step edge gradient, suggesting a dendritic precipitation mode from the adatom gas on the surface of the substrate.   

Scanning Tunneling Microscopy of Graphene on Single Crystal Copper Surface

Graphene is a monolayer of carbon atoms tightly packed into a nearly ideal two-dimensional hexagonal lattice. Graphene is a promising electronic material because of its distinctive band structure and physical properties. Large-area synthesis of high-quality graphene is one of the main obstacles towards fabricating graphene devices. Recently, large area graphene with high electrical quality has been realized on copper foils. Copper has demonstrated its advantage in fabricating high-quality uniform graphene monolayer. In this talk, we will present our studies of graphene on single crystal copper surface by variable temperature scanning tunneling microscopy and spectroscopy. We studied the bonding configurations between copper and carbon, as well as the atomic-scale electronic structure of the graphene on the copper surface. Our results provide valuable information for understanding the growth mechanism and the electronic quality of graphene on copper.   

Thermoelectric properties of CVD grown large area graphene

The thermoelectric power (TEP) of CVD (Chemical Vapor Deposition) grown large area graphene transferred onto a Si/SiO$_{2}$ substrate was measured by simply attaching two miniature thermocouples and a resistive heater. Availability of such large area graphene facilitates straight forward TEP measurement without the use of any microfabrication processes. All investigated graphene samples showed a positive TEP $\sim $ + 30 $\mu $V/K in ambient conditions and saturated at a negative value as low as $\sim $ -75 $\mu $V/K after vacuum-annealing at 500 K in a vacuum of $\sim $10$^{-7}$ Torr. The observed p-type behavior under ambient conditions is attributed to the oxygen doping, while the n-type behavior under degassed conditions is due to electron doping from SiO$_{2}$ surface states. It was observed that the sign of the TEP switched from negative to positive for the degassed graphene when exposed to acceptor gases. Conversely, the TEP of vacuum-annealed graphene exposed to the donor gases became even more negative than the TEP of vacuum-annealed sample.   

Formation of graphene-like carbon layers on TiO2(110) and Al2O3 and their relevance for protecting EUV mirrors

Extreme ultraviolet lithography (EUVL) uses short-wavelength photons (13.5 nm) to increase patterning density in IC manufacturing. Because EUV photons are strongly absorbed, multilayer reflective optics in high vacuum must be used. An active area of research is the development of a capping layer that prevents the carbon contamination and oxidation of EUV mirrors. Here, two model capping layers, a single crystal rutile TiO$_{2}$(110) surface and an ultrathin Al$_{2}$O$_{3}$ film grown on NiAl(110), were employed to investigate the fundamentals of adsorption and photo-induced oxidation processes. Using catechol (C$_{6}$H$_{6}$O$_{2})$ as a model contaminant, the adsorption and uv-induced (248 nm) removal in various gaseous atmospheres were investigated using x-ray photoelectron spectroscopy (XPS){\&}scanning tunneling microscopy (STM). When catechol overlayer is heated to 400$^{\circ}$C, some of the molecules desorb and the XPS signature of the remaining C suggests a graphene-like overlayer. Repeated adsorption/flash cycles result in a layer with self-limited thickness of 1.6 monolayers that is inert against further adsorption. Using such a layer for protecting EUV mirrors is being explored.   

Graphene on Ir(111) surface: interplay between chemical bonding and van der Waals

Graphene is an interesting new material, which consists of carbon atoms forming a hexagonal lattice. Within graphene, carbon atoms are connected by strong chemical bonds but when graphene sheets bind to something else different binding mechanisms take place. For example, when graphene sheets bind among themselves forming graphite, bonding between them is exclusively of van der Waals type i.e. there is no formation of chemical bonds and sheets are only physisorbed one ontop of each other. A graphene sheet on top of Ir(111) surface is experimentally studied by means of STM and photoemission. In standard DFT calculations this system is not described correctly due to great importance of van der Waals binding. Employing the newly developed vdW-DF functional we have calculated this system and have shown that besides pure van der Waals binding additional chemical interaction takes place giving rise to interesting phenomena (anti-corrugation) to be observed in STM images.   

Chemical vapor deposition growth of patterned graphene on copper

Graphene possesses unique electronic properties and application potentials. However, the synthesis of high-quality, single-layer graphene on large scale remains challenging. Mechanical exfoliation from graphite crystals yields graphene of the highest quality but in an uncontrolled and non-scalable way. Epitaxial growth on SiC has made significant advances in large-scale synthesis, although the cost is relatively high. Very recently, chemical vapor deposition (CVD) is used to grow graphene on Ni and Cu surfaces and has also produced large-area graphene of reasonably high quality. Cracks and ripples, however, present considerable challenges to the CVD growth and transfer process. We report the CVD growth of single-layer graphene on patterned, micron-size copper templates. Raman spectra of the films show low D-band and relatively narrow 2D peak, suggesting high quality. We present and discuss the transport properties of graphene films transferred onto an insulating substrate.   

Synthesis of large area single- and bilayer graphene on Ni (111) by chemical vapor deposition

Graphene has been reported as a promising material due to the fascinating electronic properties of ideal two-dimensional carbon. A lot of efforts have been made on the synthesis of graphene on Ni but achieving large graphene domains with uniform thickness remains a challenge. In this talk we will present our method of single- and bilayer graphene synthesis over large area, as well as micro Raman study of obtained graphene. The graphene synthesis was achieved by using Ni (111) as substrates and a scalable technique via chemical vapor deposition. The formation of the graphene layers were confirmed by micro Raman analysis. Furthermore, we obtained the information of number of layers of as-grown graphene over a large area by micro Raman map. A clear comparison of the layers between graphene synthesized on Ni (111) substrates and polycrystalline Ni films was given by Raman spectra: Within about the same size map ($\sim $40um*40um), graphene grown on Ni (111) has a much higher percentage of single- and bilayer graphene. Our results demonstrate that Ni (111) substrates have a great advantage over polycrystalline Ni film on the synthesis of large area, single- and bilayer graphene.   

CVD graphene films and its application in organic photovoltaic cells

In this work, CVD of graphene was used as a simple, scalable and cost-efficient method to prepare single and few-layer graphene films over large areas. CVD-G was characterized by Raman spectroscopy and TEM. Back-gated thin-film transistors were used to evaluate transport properties of the synthesized films. In addition, CVD graphene films were transferred to transparent substrates for photovoltaic cell fabrication. Solar cells obtained from the synthesized graphene films showed comparable performance to those fabricated with the standard indium tin oxide film (ITO) and showed superior performance under bending conditions due to the high flexibility of graphene. CVD Graphene constitutes a significant advance towards the production of transparent conductive films of graphene at large scale and has great implications for future graphene-related electronic devices.   

Electronic properties of CVD graphene grown on copper

We report the electronic properties of graphene grown by chemical vapor deposition (CVD) on copper foils at ambient pressure. Large size graphene films (4 inch*4 inch) are synthesized and transferred to SiO2/Si substrate. Raman mapping demonstrates that the films consist primarily of monolayer graphene (up to $\sim $90{\%} area coverage). Low temperature transport measurements are performed on devices made from such CVD graphene. The ``half-integer'' quantum Hall effect, which is the hall-mark of mono-layer graphene, has been observed in these devices. We also observe the ambipolar field effect and weak localization, which allow us to extract carrier mobility $\sim $3000cm$^2$/Vs and phase coherence length $\sim $300nm at 1.5K.   

Contrast Behavior of Carbon Adatom Diffusion and Nucleation in the Initial Stage of Graphene Epitaxial Growth on Stepped Metal Surfaces

Using first-principles calculations within density functional theory, we study the energetics and kinetics of carbon adatom diffusion and nucleation on three stepped metal surfaces: Ir(111), Ru(0001) and Cu(111). We find that on the flat surfaces, two carbon atoms repel each other on Ir(111) and Ru(0001), while they prefer to form a dimer on Cu(111). Moreover, the step edges on Ir and Ru surfaces cannot effectively trap single carbon adatoms either, whereas it is strongly favorable to form carbon dimers at the step edges. The different behaviors are attributed to the competition between C-C bonding and different types of C-metal bonding, and the picture is generalized to other C/metal systems with predicted results. These findings provide an insight into the understanding of experimentally observed carbon nucleation in the initial stage of graphene epitaxial growth on metal surfaces.   

Reduced-defect growth of large area graphene by CVD on Au and Cu foils

A single layer or few-layers graphene can be formed by dissolving hydrocarbon gas on the metal surfaces at high temperatures. So called, this chemical vapor deposition (CVD) method has several advantages in graphene production such as large area, controlled layers, and low cost growth. However, it is known that the quality of CVD grown graphene is not so good compared to that of a detached graphene sample from HOPG. Especially, defects were detected at 1350 (1/cm) of Raman spectroscopy in the graphene samples grown on Au and Cu surfaces, which directly might be related to the electron mobility. As a matter of fact, there are many factors for graphene quality determination in the CVD growth process. Those are growth temperature, gas composition and flow rate, and cooling rate, etc. Surprisingly we found that defects can be reduced significantly by rapid and prompt pumping hydrogen gas after the growth under the optimum growth conditions. We believe that residue atomic hydrogen after the growth might be attached on a graphene surface and leads to deformation of honeycomb graphene structure. Recent experiments have confirmed that this kind of hydrogen absorption tilts graphene structure and makes graphene insulating.   

Catalyst-assisted thermal CVD syntheses of large area uniform mono- and few-layer graphene

Catalysts with carbon solubilities varying from low to intermediate solubilities were utilized to synthesize large area uniform mono- and few-layer graphene employing catalyst-assisted thermal CVD at both ambient and low pressures. Our results demonstrate that both APCVD and LPCVD from a low carbon solubility catalyst resulted in uniform growth of monolayer graphene. Uniform growth of multi-layer graphene was observed while employing catalysts with intermediate carbon solubility in a LPCVD process that is in contrast to previous reports using APCVD process. In an APCVD process (for low solubility catalysts such as Copper), the graphene synthesis at high temperatures ($\sim $1000 $^{\circ}$C) proceeds in mass-transport limited regime, and in a LPCVD process, proceeds in a surface-reaction limited regime. The role of kinetic factors in graphene growth using APCVD and LPCVD processes would be discussed with reference to aforementioned results.   

Graphene-covered iron layers on Ni(111): structural and electronic properties

Recently, Dekdov et al. [Appl. Phys. Lett. 93, 022509 (2008)] reported the intercalation of atomic Fe layers between graphene and a Ni surface. In the present work, we report new experimental and theoretical results on this novel nanostructure. Fe intercalation was produced by repeatedly evaporating monolayers of $^{57}$Fe on previously prepared graphene/Ni(111), and post-annealing at 320 C. In situ M\"ossbauer spectra are consistent with a single hyperfine magnetic field for the one-Fe-monolayer system, while the two- monolayer system presented two field values. By using low energy electron diffraction (LEED) the strucuture of both graphene/Ni(111) and graphene/Fe/Ni(111) was investigated. The model that best fits the experimental LEED curves corresponds to one C atom on top of Ni or Fe and the other C atom on a fcc hollow-site, consistent with the most stable systems in the first-principles calculations. Regarding the calculated electronic structure of the studied systems, the graphene/Ni structure presents a bandgap of 0.06 eV for the minority-spin electronic states near the original graphene Fermi point. The inclusion of Fe layers modify the magnitude, and even the existence, of such bandgap. For instance, in the case of the one-Fe-monolayer system, the bandgap increases to 0.63 eV.   

One-dimensional extended defects in epitaxial graphene with metallic properties

Extended one dimensional line defects with metallic electronic properties are described. These defects have been formed in epitaxial graphene on Ni(111) surfaces and are the consequence of domain boundaries between graphene-sheets occupying different registry relative to the nickel substrate. The domain boundary forms a reconstructed line defect with a repeat unit of one octagon and a pair of pentagons. All the atoms in the defect are sp$^{2}$ hybridized three-fold coordinated carbon and thus do not exhibit any dangling bonds. DFT calculations indicate that these defect lines exhibit similar flat band states at the Fermi-level as zigzag-edge states in nanoribbons. STM-imaging indicates a bright contrast surrounding these defects, which we attribute to the decaying wave function of the defect states and its associated self-doping effect in the surrounding graphene sheet. This makes this extended defect a metallic wire embedded in a perfect graphene lattice.   

Session Z22: Graphene Optical Properties and Imaging

Photon interactions with graphene electrons: consequences for the optical conductivity

We calculate the lowest-order amplitude for interaction between 3D photons and 2D graphene electrons. Using Fermi's Golden Rule, we find the thermal corrections to the graphene's zero-temperature opacity of $\pi\alpha$. Equating the power absorbed to the Joule heating rate gives an expression for the optical conductivity. We find an optical conductivity that agrees with previous results in the limit of zero temperature, but gives a finite value at zero frequency and finite temperature.   

Photoresponse in reduced graphene oxide thin films

We examine photo-response and positional dependent photocurrent generation in chemically reduced graphene oxide (RGO) thin film under near infrared illumination. We have observed that the photocurrent depends strongly on the position of laser spot with maximum photocurrent occurring at the metal-film interface. A slow time constant ($\sim $2.8 seconds) was observed and the photocurrent exhibits a linear dependence on the incident laser intensity. In light of our observations, the positional sensitive photocurrent generation is explained as originating from the diffusion of photo-excited carriers around the Schottky barriers at the RGO thin film electrode junctions.   

Why is the optical transparency of graphene determined by the fine structure constant?

The observed $97.7\%$ optical transparency of graphene [R.R. Nair, et al, Science {\bf 320}, 1308 (2008)] has been linked to the value $1/137$ of the fine structure constant, by using results for noninteracting Dirac fermions. The agreement in three significant figures requires an explanation for the apparent unimportance of the Coulomb interaction. Using arguments based on Ward identities, the leading corrections to the optical conductivity due to the Coulomb interactions are correctly computed (resolving a theoretical dispute) and shown to amount to only $1$-$2\%$, corresponding to $0.03$-$0.04\%$ in the transparency.   

Far infrared spectroscopy of graphene

The electronic properties of graphene are described by massless Dirac electrons, and their DC conductivity and Hall conductivity have attracted great attention. However, the high frequency conductivity (AC conductivity) of graphene is little known. Here we perform far-infrared spectroscopy on large area graphene sample to probe its AC conductivity response. The response can be largely described by the Drude model, which yields direct information on the electron density and scattering rate in the graphene samples. We will discuss the comparison of our experimental results to theoretical predictions.   

GW-Bethe-Salpeter study of the optical properties of graphane

Recently, hydrogenated graphene (i.e., graphane) has been synthesized experimentally. Interesting properties such as reversible hydrogenation and transforming from a metal into an insulator have been observed. According to a recent study [Lebegue et. al., Phys. Rev. B 79, 245117 (2009)], the band gap of graphene is open up from 0 to $\sim $5 eV through the hydrogenation of graphene to graphane. In this talk, we will present results of a first-principles study of the optical properties of graphane using the GW-Bethe-Salpeter equation approach.   

Optical study of electron-phonon coupling in multilayer graphene with different stacking order

The optical conductivity spectra of mechanically exfoliated multi-layer graphene samples were explored in the infrared range. In samples from three to six layers in thickness, two distinct types of spectra were observed for different samples with precisely the same number of layers, which can be attributed to the optical absorption of multi-layer graphene samples with Bernal stacking (ABAB series) and rhombohedral stacking (ABC series). Furthermore, the G-mode phonon exhibits a lineshape characteristic of a Fano resonance due to strong electron-phonon coupling. The width and lineshape of the phonons are strongly modified by the interband electronic transition as the layer number increases. The intensity of the phonons in samples with rhombohedral stacking is much higher than those in samples with Bernal stacking. We will discuss the new aspects of electron-phonon coupling in multi-layer graphene revealed by this work.   

Infrared imaging of power dissipation in graphene field effect transistors

We have employed thermal infrared microscopy to image temperature distributions in monolayer and bilayer graphene transistors under high bias. The hot spot position is sensitive to device electrostatics, corresponding to the location of minimum charge density in unipolar transport, and to that of charge neutrality in ambipolar operation. The hot spot shape carries information of spatial variations in charge density of devices. By comparison with a self-consistent electrical-thermal model, the imaged temperature profiles are correlated with power dissipation, carrier distributions, and electric fields within such devices, providing rich insight into their operation and energy relaxation physics. For instance, the combined approach reveals that low-field mobility is limited by impurity scattering, while velocity saturation is set by substrate phonon scattering in our samples. These results also open up the possibility of thermal imaging as a more general non-invasive tool for examining transport and energy dissipation in novel devices.   

Light Emission from Graphene Induced by Femtosecond Laser Pulses

Since graphene has no band gap, light emission is not expected from relaxed carriers. On the other hand, the strong optical absorption in graphene over a wide spectral range suggests the possibility of hot luminescence from non-equilibrium carriers. Here we report the observation of light emission from monolayer graphene induced by excitation with ultrashort (30-fs) laser pulses. We observe emission throughout the visible spectrum, extending to a photon energy of 3.5 eV in the near UV. In contrast to conventional hot luminescence processes, however, we find strong light emission at photon energies \textit{exceeding }that of the pump laser at 1.5 eV. In addition to detailed measurements of the emission spectra and their dependence on pump fluence, we have performed ultrafast time-domain correlation technique in which light emission is measured as a function of the temporal separation between a pair of femtosecond excitation pulses. A dominant relaxation time of a few 10's of fs is observed. The origin of this unusual light emission process and its relation to the underlying carrier dynamics in graphene will be discussed.   

Anomalous Dirac charge dynamics in multilayer graphene at high optical transition energies

Almost all discussion of the optical absorption, the interlayer interaction and relationship between them in multilayer graphene is almost conclusively decided~ taking into account only the charge dynamic in states at K point in the hexagonal Brillouin zone while other states are relatively ignored. The lack of charge dynamic information at states beyond the Dirac cone may give us inadequate knowledge of their interaction with the lower states. Here, we present an optical conductivity study on graphene as function of layers using a combination of the dc-conductivity, optical ellipsometry, and vacuum ultraviolet (VUV) reflectance to reveal the electronic band structure and the novel interlayer interaction of graphene in pi and sigma bands over a broad energy range from 0 to 35 eV.   

Observation of coherent excitation of the interlayer shearing mode in graphite and multilayer graphene

Raman spectroscopy is one of the key methods for the characterization of single and multilayer graphene. In the bulk limit, the lateral motion of adjacent graphene planes gives rise to a Raman active low-frequency mode, the so-called interlayer shearing mode. Coherent excitation of this mode has been observed by femtosecond time-resolved reflectivity [1]. For the case of few-layer graphene, related modes are predicted to be present and to exhibit different properties as a function of layer thickness [2]. Here we report the observation of coherent oscillation of such shearing mode phonons in multilayer graphene. The experiments are performed on mechanically exfoliated graphene samples using femtosecond laser excitation pulses and time-delayed femtosecond probe pulses in a transient reflectivity measurement. The coherent shearing-mode phonons exhibit a period of 800 fs, with a lifetime exceeding 10 ps. We will discuss the characteristics of shearing mode phonons as a function of the thickness of multilayer graphene. [1] T. Mishina et al., Phys. Rev. B 62, 2908 (2000) [2] S. K. Saha et al., Phys Rev. B 78, 165421 (2008)   

Nonlinear photoluminescence from graphene

Upon femtosecond laser irradiation, a bright, broadband nonlinear photoluminescence (PL) is observed from graphene at frequencies well above the excitation frequency. Analyses show that it arises from radiative recombination of a broad distribution of non-equilibrium electrons and holes, generated by rapid scattering between photo-excited carriers within tens of femtoseconds after the optical excitation. Its highly unusual characteristics come from the unique electronic and structural properties of graphene.   

Infrared Transmission of Chemically Reduced Graphene Oxide from 0.2-200THz

Single-layer graphene oxide can be chemically reduced and deposited from solution to form conducting films of graphene flakes in an inexpensive, versatile and scalable process. However, chemically reduced graphene oxide (CRGO) films produced to date have low DC conductivities (10$^{2 }$ -- 10$^{4}$S/m) compared to pristine graphite, likely due to poor electrical transport between flakes and from structural disorder. We prepared thin, free-standing CRGO films by reduction of graphene oxide in hydrazine, solution deposition and substrate removal. Film properties are similar to previous reports[1], with DC conductivity $\sim $1700 S/m, and ordered domain sizes of $\sim $15nm determined from Raman spectroscopy. THz and IR measurements of 400nm thick films show $T\sim $0.8 for $f<$1THz smoothly decreasing to $T\sim $0.05 at $f$=100THz, consistent with a $\sim $70-fold increase in conductivity over this frequency range. We will compare our results to a model of the films as a network of weakly linked conductive particles. Future work will investigate carrier scattering rates and lifetimes in this material.\\[4pt][1] Sungjin Park, Rodney S. Ruoff. Nature Nanotechnology 2009; \textbf{4}: 217.   

Ultrafast Spectroscopy of Hot Carriers in Graphite

We studied the ultrafast dynamics of photogenerated hot carriers in graphite single crystal by using transient pump-probe photoreflectivity (PR) spectroscopy with $\sim $100 fs resolution. Using two different laser systems;visible/near-IR {\&} mid IR range with pump excitation photon energies at 1.55 {\&} 3.1 eV, our transient PR spectrum covers a broad spectral range from 0.2 -- 2.4 eV. Surprisingly, we found that the transient PR spectrum resembles the cw thermo-modulation spectrum that was measured and explained previously by the well-known band structure of graphite; and contains several zero-crossings modulated reflectivity that are determined by the van-Hove singularities in the band structure at the K point of the Brillouin zone. We interpret the transient PR spectrum as due to hot carriers in the various valence and conduction bands of graphite. The decay dynamics can be fit with a bi-exponential decay with two processes that are interpreted as: sub-picosecond Auger recombination following hot plasma with well defined electronic temperature; and a longer process of hot plasma cooling to the lattice temperature by emitting strongly coupled phonons.   

THz Time Domain Spectroscopy Studies of a Graphite Film and Perforated Graphite Hole Arrays

We studied the optical properties of highly ordered pyrolytic graphite (HOPG) films in the 0.1 to 0.5 THz spectral range using the technique of time domain THz spectroscopy. HOPG has a low free carrier density, and highly anisotropic conductivity tensor, having much higher in-plane conductivity compared to that of the out-of-plane. First we studied a thin graphite film for obtaining the spectra of both real and imaginary components of the dielectric constant, from which we obtained the free carrier relaxation time. Subsequently we fabricated and investigated hole arrays in an otherwise opaque graphite film, which consist of \textit{periodic} hole array (``plasmonic lattice''), and corresponding \textit{random} hole array. For the periodic hole array we found that the transmission spectrum is modulated with several resonance/anti-resonances that correspond with the reciprocal vectors in the Fourier space; whereas a broad transmission band of which peak depends on the hole diameter characterizes the random hole arrays.   

Optical Properties of Suspended and Substrate Graphene

Graphene, a two-dimensional material made purely of carbon atoms arranged in a hexagonal lattice has attracted the attention of scientific community since it was first produced in 2004. Due to the peculiarity in its band structure and various striking characteristics (eg. high electrical conductivity, mechanical robustness, large thermal conductivity, tunable carrier type and mobility etc.) this has become significant both technologically as well as for fundamental research. Both experimental and theoretical investigations have been taking place to study its various properties viz. transport, electronic, thermal and optical properties. In this work, optical properties of suspended monolayer-graphene and monolayer-graphene deposited on dielectric substrates are studied by calculating the optical quantities such as coefficient of reflection and reflected polarization analytically with the help of Maxwell's equations for the respective systems. Behavior of above mentioned optical quantities with respect to various parameters are studied to compare the two systems. This study can be used to obtain the conductivity tensor of graphene with its anisotropic behavior obtained from the azimuthal angle dependence of the optical quantities. The substrate-graphene is also interesting due to the observation of Brewster's phenomena with Brewster's angle varying with respect to the azimuthal angle (an oscillation with a period of 180 degrees).   

Session Z24: Oxides and other Insulators

A Synchrotron Investigation Of The Electronic Structure Of Lanthanide Zirconates

abstract- The lanthanide zirconates are of interest for use in inert matrix fuels and nuclear wasteforms. For use in these applications, the material's structure must be resistant to radiation damage and its thermal, thermodynamic and mechanical properties must be known. The rare earth zirconates are interesting model systems to explore such problems. In such materials the f-electrons may play a localized valence decisive role in determining their thermo-mechanical properties. We have undertaken a synthesis of the full range of the lanthanide zirconate series using solid state techniques. We have performed X-ray photoemission spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) with synchrotron radiation on a selection of the series, in conjunction with a density functional theory (DFT) determination of the electronic structure. -   

A General Mechanism for Negative Capacitance Phenomena

Negative capacitance (NC) is a relatively unknown, yet common, phenomenon that is found in a wide variety of materials and devices spanning the major branches of science. The microscopic mechanisms governing NC in these systems are, naturally, as varied as the materials themselves. However, they do share several common features. NC arises in the presence of a \textit{dc} bias, while the materials themselves are nonlinear and possess strong dispersion. The current study focuses on NC in an electrorheological fluid composed of urea coated Ba$_{0.8}$(Rb)$_{0.4}$TiO(C$_{2}$O$_{4})_{2}$ nanoparticles dispersed in silicone oil. The NC of the fluid is plasma-like in nature and related to the nonlinearity of the fluid's conductivity. A general mechanism, describing the NC of the fluid as well as other materials, has been developed by exploiting the common features associated with NC. The mechanism demonstrates that NC arises from \textit{dc}/\textit{ac} signal mixing across a nonlinear conductor.   

Electronic structure and linear response properties of the crystal KIO$_3$: A first-principle density-functional theory study

The electronic band structure and both the partial and total density of states of the ABO$_3$ type crystal KIO$_3$ are investigated based on the first principles pseudopotential plane wave approach within density functional theory formalism. The structural optimization was performed in terms of generalized gradient approximation. And the optical properties and the imaginary parts of the frequency-dependent dielectric function of the material are also calculated. The acquired calculational results of the material are in good agreement with available experimental data.   

Experimental Demonstration of Memory Capacitance and Memory Resistance in VO$_{2}$ devices

Memristors are a special case of non-linear resistors which store information about the history of applied voltage in their instantaneous resistance value. These memory-resistors have attracted considerable attention for their possible uses in information storage and neuromorphic circuits. The same circuit principles behind memory-resistance have been extended to postulate that memory-capacitance and memory-inductance phenomena are also likely to exist$^{1}$. In this talk, we discuss experimental results from a Vanadium-Dioxide device which exhibits memory-capacitance$^{2}$. The nanoscale phase-separation phenomena which underlie this memory-capacitance suggest similar effects may exist in a variety of materials.\\[4pt] [1]. M. Di Ventra et.al. Proc. IEEE 97, 1717 (2009) \newline [2]. T.Driscoll et.al. Science. 325, 1518 (2009)   

First Principles Study on Ta$_{25 }$ Low- and High-Temperature Phases

Low- and high-temperature phases of Tantalum pentoxide (Ta$_{2}$O$_{5})$ have been studied by density functional method. Our calculations have been carried out using the projector-augmented wave method and a plane wave basis set. Tantalum pen-oxide, Ta$_{2}$O$_{5}$ is considered as a potential alternative to SiO$_{2}$ because of its high breakdown voltage, its high dielectric constant, and its excellent step coverage characteristics. It is also a dielectric material for optical coating application that is important to high precision instrumentation. We have studied structure, electronic properties, and phonon spectra, as well as elastic modulii, including bulk modulus, Young's modulus and Poisson's ratio. Four different isomorphs will be presented. Furthermore, SiO$_{2}$-doped Ta$_{2}$O$_{5}$, which is used as mirror coatings in current interferometric gravitational wave detectors, has also been investigated. Our results help to understand the properties of this material in different phases.   

Electronic structure of PbTiO3-based ferroelectric materials determined from LDA+U study

We perform a systematic theoretical study of the electronic structure of PbTiO$_3$ and related ferroelectric materials, using LDA+$U$ calculations. The effective on-site correlation terms $U$ for the localized electronic states of PbTiO$_3$ are determined with a linear-response method. Compared to the DFT- LDA method (where the band gap values are greatly underestimated), band gap values for PbTiO$_3$ obtained by LDA+$U$ calculations are much closer to the experimental results. Special focus has been paid to solid-state solutions formed by doping PbTiO$_3$ with a $d^8$ cation (Ni, Pd, Pt) and accompanying O vacancy, which have recently been proposed. We re-examine the dopant states of these materials, and also recalculate their electronic structures using the LDA+$U$ method. The improvement achieved with the LDA+$U$ method and comparison with experimental results will be presented.   

Ferroelectric-Ceramic Nanoparticle Composite Materials Synthisized via Physical Vapor Deposition

This study documents an inquiry into the preparation of ferroelectric polymer-ceramic nanoparticle composite films via physical vapor deposition. The combination of these types of materials has developed an interest in the material science community due to their incredible potential as a dielectric material for capacitor applications. Through the course of this particular investigation, polyvinylidene fluoride (PVDF) containing nanoparticles of titanium dioxide (TiO$_{2}$) were synthesized inside a vacuum. In this study, various parameters of physical vapor deposition were investigated so as to better manipulate the composition of the PVDF/TiO$_{2}$. The resultant composition and distribution of the films were analyzed using x-ray photoelectron spectroscopy (XPS), scanning force microscopy (SFM), and energy dispersive x-ray analysis (EDX).   

Structural and dielectric properties of monodisperse TiO$_{2}$-paraffin core-shell nanoparticles

Core-shell nanoparticles made of oxides having high dielectric constant and organic materials with large breakdown field are attractive candidates for higher-energy-density capacitors. In the present study, monodispersed TiO$_{2 }$nanoparticles were produced using a cluster-deposition method and subsequently coated with uniform paraffin nanoshells using an \textit{in-situ} thermal evaporation to form core-shell structure. The thickness of the paraffin nanoshells was varied by controlling the evaporation temperature of paraffin. The dielectric properties of TiO$_{2}$-paraffin core-shell nanoparticles show an enhanced effective dielectric constant with a decrease in the thickness of the nanoshells and also, reveal a minimum dielectric dispersion and low dielectric losses in the frequency range of 100 Hz -- 1MHz, which are highly desirable for potential device applications.   

Solid-liquid phase coexistence of alkali nitrates from molecular dynamics simulations

Alkali nitrate eutectic mixtures are finding application as industrial heat transfer fluids in concentrated solar power generation systems. An important property for such applications is the melting point, or phase coexistence temperature. We have computed melting points for lithium, sodium and potassium nitrate from molecular dynamics simulations using a recently developed method [1-3], which uses thermodynamic integration to compute the free energy difference between the solid and liquid phases. The computed melting point for NaNO3 was within 15K of its experimental value, while for LiNO3 and KNO3, the computed melting points were within 100K of the experimental values [4]. We are currently extending the approach to calculate melting temperatures for binary mixtures of lithium and sodium nitrate.\\[4pt] [1] D. M. Eike, J. F. Brennecke, and E. J. Maginn, J. Chem. Phys. 122, 014115 (2005).\\[0pt] [2] D. M. Eike and E. J. Maginn, J. Chem. Phys. 124, 164503 (2006).\\[0pt] [3] S. Jayaraman and E. J. Maginn, J. Chem. Phys. 127, 214504 (2007).\\[0pt] [4] S. Jayaraman, A. P. Thompson, O. A. von Lilienfeld, and E. J. Maginn, I\&EC Res., Accepted (2009).   

Measuring Strain field of Multi-Component Material Systems Using X-Ray Bragg- Surface Diffraction

We investigated the strain field of the $\beta $-FeSi$_{2}$ semiconductor on a Si(001) substrate, where FeSi in a grain form coexists with $\beta$-FeSi$_{2}$ during the growth of $\beta $-FeSi$_{2}$. The lattice-parameter variations of silicon, $\beta $-FeSi$_{2}$, FeSi and the grain boundary were detected using x-ray Bragg-Surface Diffraction (BSD). With the penetration depth calculated by the dynamical theory of x-ray diffraction, the strain field versus depth of Si-substrate near the interface is determined with the resolution of 0.002 {\AA}. The largest strain detected is about 0.4{\%} up to 8$\sim$12 {\AA} below the interfaces.   

Negative Thermal Expansion and Other Elastic properties of a Class of Solids

We consider the thermal expansion, change of sound velocity with pressure, and the Poisson ratio of lattices which have rigid units (with very large stiffness to change in bond-length and to bond-angle variations) connected to other such units with springs with largestiffness for bond-stretching compared to that required for bond-angle variation at the rigid unit-spring connection. The hierarchy of force constants leads to a negative thermal expansion coefficient over a large range of temperature as well as allows calculation of elastic properties, in particular the Gruneisen constant and the Poisson ratio. We find, consistent with experiments, that crystals with negative thermal expansion coefficients also have sound-velocities which go down with temperature. We also find a very weakly negative Poisson ratio for such solids.   

Bi-Stable Thermal Actuators

Conventionally, most MEMS devices are constructed and remain within the plane of the wafer. Presented here are devices that have been released from a silicon wafer and generate a profile that can be electrically manipulated above the wafer surface. The advantage of this profile is that the devices can be employed in a gaseous or liquid flow parallel to the wafer surface to capture trace elements or gaseous vapor of interest for either liquid or gas analysis. Additionally, the selection of particular thin films and their thicknesses can benefit from differing thin film stresses to control the shape and profile of the released structures. Electrical manipulation of the devices by varying voltage application can alter the shape of the devices due to thermal heating and differences in the thermal coefficients of expansion of the selected materials. Actuation of the devices can enable possibilities of motion of the device (i.e. walking) or transfer of items from device to device. Presented herein is the fabrication process, the relationship of the shape of the devices based upon the fabricated pattern, thin film metal selection, film thicknesses, with SEM images of the electrically manipulated devices.   

Session Z25: Theoretical Methods and Applications

Rate description of Fokker-Planck processes with time-periodic parameters

The large time dynamics of periodically driven Fokker-Planck processes possessing several metastable states is investigated. At weak noise the rare transitions between these metastable states can be represented as a discrete Markov process characterized by time dependent rates. At large times the full Fokker-Planck process is completely specified by the transition probabilities of this discrete process and by two types of functions associated to each metastable state: so-called state specific probability densities and localizing functions. The localizing functions assign to the continuous states of the original Fokker-Planck process probabilities for the metastable states. The state specific probabilities allocate a time dependent probability density of continuous states to each metastable state. We specify these functions by their equations of motion and illustrate and validate the presented theory for a periodically forced bistable Brownian oscillator in a wide range of driving frequencies. P. Talkner, J. Luczka, Phys. Rev. E, 69, 046109 (2004). C. Kim, P. Talkner, E.K. Lee, P. Hanggi, Chem. Phys. (2009), doi:10.1016/j.chemphys.2009.10.027; arXiv:0908.1730.   

\textit{Ab initio} study of anharmonic vibrations for polymers

Energies of optically active $k=0$ phonons in extended systems of one-dimensional periodicity (polyethylene) are computed by taking account of the anharmonicity in the potential energy surfaces (PES) and the resulting phonon-phonon couplings explicitly. The electronic part of the calculations is based on Gaussian-basis-set crystalline orbital theory at the coupled-cluster singles and doubles, second-order M{\o}ller--Plesset perturbation (MP2), and Hartree--Fock levels, providing one-, two-, and three-dimensional slices of the PES, respectively, which are in turn expanded in the fourth-order Taylor series of normal coordinates. For the vibrational part, we employ the vibrational self-consistent-field, vibrational MP2 and vibrational truncated configuration-interaction (VCI) methods within the $\Gamma $ approximation that amounts to including only $k=0$ phonons. It is shown that inclusion of both electron correlation and anharmonicity is essential in achieving good agreement between computed and observed frequencies of optical phonons in polyethylene. The VCI calculations also identify quantitatively the frequency separation and intensity ratio of the Fermi doublets in the vibrational spectrum of polyethylene.   

A statistical estimation of the precision of reweighting-based simulations

Reweighting a sampled configuration plays a central role in decreasing the computational cost of a simulation involving ergodic and semi-ergodic systems and is the base of all biased sampling methods. A very simple application of this method is reweighting a primary ensemble, usually generated by a computationally cheap Hamiltonian, to a target ensemble, typically a more expensive Hamiltonian. However, the precision can be strongly affected by the distribution of the Hamiltonians' difference in each bin of conformation distribution. Theoretically, one should sample forever to have a complete distribution of energy in the ensemble, but sampling in energy space is much faster than sampling in conformation space. Using this idea and taking advantage of the Gaussian nature of the energy distribution, we study a way to make a good estimate of error in reweighting histogram procedures before running a long simulation on the system. We are applying these ideas on two polypeptide chains, ala 10 and fs21, to see how they work practically.   

Applications of the Projective Dynamics method to stochastically driven systems

A dynamic system which can be interpreted as continuously evolving along one coordinate can be discretized by dividing this coordinate into non-overlapping intervals, which cover the entire domain. We further impose the (sufficient) condition that only motion between adjacent intervals are permitted. A generalization of the Projective Dynamics method [1] then ensures that correct mean first passage times to an absorbing interval can be obtained by having correct transition rates. Furthermore the theoretical framework shows that the intervals can be chosen in an arbitrarily way, while keeping the above minor condition. Thus we project the dynamic system onto a master equation with the same mean first passage time. We present applications demonstrating that this procedure is in general applicable to a wide range of problems. We illustrate the application of the Projective Dynamics method to Brownian motion of particles in one and two dimensional smooth or rough energy landscapes. We also apply the method to the folding process of small linear polymer chains (with two types of atoms) subject to Brownian motion. We compare results of the mean first passage time obtained from the Projective Dynamics method with those of direct measurements. [1]Phys. Rev. Lett. 80, 3384(1998)]   

Understanding many-electron fermionic ground states as bosonic excited states

We consider the novel question of, ``Which state in the spectrum of a system of distinguishable particles is the ground state of the same system made up of indistinguishable fermions?'' We discuss how this can be approximated for many-electron systems and illustrate its importance in our new semiclassical method correlated Thomas-Fermi (CTF).   

Displaced path integral formulation for the momentum distribution of quantum particles

The proton momentum distribution, accessible by deep inelastic neutron scattering, is a very sensitive probe of the potential of mean force experienced by the protons in hydrogen-bonded systems. In this work we introduce a novel estimator for the end-to-end distribution of the Feynman paths, i.e. the Fourier transform of the momentum distribution. In this formulation, free particle and environmental contributions factorize. Moreover, the environmental contribution has a natural analogy to a free energy surface in statistical mechanics, facilitating the interpretation of experiments. The new formulation is not only conceptually but also computationally advantageous, because the displaced path distribution can be sampled accurately with thermodynamic integration techniques. We illustrate the method with applications to one-dimensional model systems and to an empirical water model.   

A Comparative Study of Mixed Quantum-Classical and Semiclassical Methods for an Electronic Spectroscopy Benchmark Model

We performed a comparative study of the reliability of different mixed quantum-classical and semiclassical approaches to calculating equilibrium one- and two-dimensional electronic spectra. These approaches include the popular second-order Cumulant expansion approximation, the Mixed Quantum-Classical Liouville method, and the Forward-Backward Semiclassical method. The comparison was performed within the framework of a benchmark system that can distinguish between these methods and for which the exact quantum results can be obtained [1]. More recently, this work has been extended to cases where the system is initially prepared in a non-equilibrium state, in order to study electronic pump-vibrational probe type spectroscopies.   

Effective interactions between pH-responsive particles

The DLVO potential is commonly used to describe the pair interactions between charged macroions in solution. It neglects effects due to responsive surface groups, by which the charge and surface entropy can fluctuate. Here, within the framework of density functional theory, we calculate the pair potential between reactive macromolecules. We compare with results found by molecular dynamics simulations of the restricted primitive model, including a short ranged binding potential between the ions and the macroions. Thereby, we extend DLVO-theory significantly, and even find conditions for a like-charge attraction and opposite-charge repulsion.   

The relation between collapsed states of a polymer chain and compact Lennard-Jones clusters

We present computational studies of compact states of small linear polymer chains consisting of hydrophobic and polar monomers. At sufficiently low temperatures, the polymer chain undergoes a transition from a fully extended to a compact state, and the dynamics of the collapse process is studied with a particular emphasis on the distribution of the end-to-end distances of the polymer. We show that in certain parameter regimes the distribution of end-to-end distances of the polymer chain near the ground state resembles the distribution of the pairwise distances of compact Lennard-Jones (non-polymeric) clusters.   

How long does it take to measure eternity? Solving problems in hours that would take brute-force simulations the age of the universe

We present two new rare event techniques that we use to determine transition times in stochastic systems. We call the first technique forward flux sampling in time (FFST). This algorithm is similar to forward flux sampling (FFS) but doesn't suffer from the main flaw of FFS: a choice between either a slow initial flux calculation or an approximate solution for the transition time. The second algorithm, called the barrier method, is significantly more efficient than FFS and FFST, especially for transitions with long-lived meta-stable states. These algorithms can be applied to any problem that can be written as a Markov (memory-free) process. The barrier method is useful in relatively low dimensional problems. We present results comparing these new algorithms with FFS on 1D exactly solvable systems.   

Tunable Fano resonance in a ferromagnetic diatomic molecular transistor

We investigate electron transport through a diatomic molecule parallelly coupled to ferromagnetic source and drain contacts. We utilize a model Hamiltonian involving a Hubbard term in which the contacts are modeled using recently developed complex source and sink potentials. The zero bias transmission spectrum for a symmetrically coupled system as a function of the Fermi energy acquires a Fano lineshape as the Hubbard interaction is turned on. For large values of U, the Fano lineshape broadens and shifts to higher energy values disappearing eventually. Meanwhile, the Breit-Wigner resonance located at the bonding resonance in the noninteracting limit survives but its position is shifted twice the coupling between the atoms in the molecule in the infinite U limit and its linewidth is reduced to half. We attribute this behaviour to the unavailability of one of the transmission channels.   

Session Z26: Superconducting Qubits: New States and Effects

Quantum Interference Induced by Landau-Zener Transition in Strongly Driven Flux Qubits

We irradiate superconducting flux qubits with strong microwaves. Quantum interference patterns corresponding to the population transitions between discrete macroscopic quantum states were observed. The interference patterns, which depend on the microwave frequency and power, are complicated because of the short decoherence time. An analytical model based on Landau-Zener transition is developed to quantitatively describe the interference patterns. This work is partially supported by NSFC (10704034, 10725415), the State Key Program for Basic Research of China (2006CB921801).   

Coherent Population Trapping in a Superconducting Phase Qubit

The phenomenon of Coherent Population Trapping (CPT) of an atom (or solid state ``artificial atom''), and the associated effect of Electromagnetically Induced Transparency (EIT), are clear demonstrations of quantum interference due to coherence in multi-level quantum systems. We report observation of CPT in a superconducting phase qubit by simultaneously driving two coherent transitions in a $\Lambda$-type configuration, utilizing the three lowest lying levels of a local minimum of the phase qubit. We observe $\sim 60$\% suppression of excited state population under conditions of two-photon resonance, where EIT and CPT are expected to occur. We present data and matching theoretical simulations showing the development of CPT in time. We also used the observed time dependence of the excited state population to characterize quantum dephasing times of the system, as predicted in [1]. [1] K.V. Murali, Z. Dutton, W.D. Oliver, D.S. Crankshaw, and T.P.Orlando, Phys. Rev. Lett. {\bf 93}, 087003 (2004).   

Generation of three-qubit entangled states using superconducting phase qubits

Entanglement is one of the crucial resources necessary for quantum computation. For three qubits, there are two fundamentally different types of entanglement, typified by the states $\left|\mathrm{GHZ}\right\rangle = \left|000\right\rangle + \left|111\right\rangle$ and $\left|\mathrm{W}\right\rangle = \left|100\right\rangle + \left|010\right\rangle + \left|001\right\rangle$. Using three capacitively-coupled phase qubits, we have implemented protocols designed for fast single-step generation of these states. The resulting states were characterized with quantum state tomography and compared with entanglement witnesses that identify true multi-partite entanglement.   

Bistability of qubit chains coupled to a superconducting resonator

When a quantum many-body system is coupled to a cavity, the cavity not only can be used to probe the quantum phase transition but can also induce novel effects in the many-body system. In this work, we study the bistable effect in a chain of superconducting qubits coupled to a superconducting resonator cavity. The qubits are connected to their nearest neighbors capacitively and form a transverse Ising model. Using a semiclassical approach to treat the resonator in the bad cavity limit, we show that a bistable regime exists where the ground state of the transverse Ising model can be in either the paramagnetic state or the ferromagnetic state as the driving of the resonator increases. In a full quantum calculation including the resonator damping and the qubit decoherence, the photon distribution of the resonator shows bimodular behavior which agrees well the bistable solutions in the semiclassical approach.   

Autler-Townes effect in a superconducting three-level system

When a three-level quantum system is irradiated by an intense coupling field resonant with one of the three possible transitions, the absorption peak of an additional probe field involving the remaining level is split. This process is known in quantum optics as the Autler-Townes effect. We observe these phenomena in a superconducting Josephson phase qubit, which can be considered an ``artificial atom'' with a multilevel quantum structure. The spectroscopy peaks can be explained reasonably well by a simple three-level Hamiltonian model. Simulation of a more complete model (including dissipation, higher levels, and cross-coupling) provides excellent agreement with all the experimental data.   

Landau-Zener-Stueckelberg interferometry with low- and high-frequency driving

The problem of a periodically driven two-level system cannot be solved exactly. The rotating-wave approximation (RWA) is the most common approximation used to analyze this problem. I will discuss an alternative approximation that applies in the case of very strong driving, where the RWA is generally invalid. The dynamics is approximated by a sequence of Landau-Zener transitions that can interfere constructively or destructively, depending on the Stueckelberg phase accumulated between transitions. It turns out that the resonance conditions are qualitatively different for the cases of low- and high-frequency driving. I will discuss the two respective limits. I will also show that our theoretical results describe recent experiments on Landau-Zener-Stuckelberg interferometry with superconducting qubits [S.N. Shevchenko, S. Ashhab, and F. Nori, arXiv:0911.1917].   

The size of superposition states in flux qubits

Flux qubits, small superconducting loops interrupted by Josephson junctions, are successful realizations of quantum coherence for macroscopic variables. Superconductivity in these loops is carried by $\sim 10^6$ -- $10^{10}$ electrons, which has been interpreted as suggesting that coherent superpositions of such current states are macroscopic superpositions as exemplified in the extreme case by Schr\"odinger's 1935 gedanken experiment. We provide a full microscopic analysis of such qubits, from which the macroscopic quantum description can be derived. This reveals that the number of microscopic constituents participating in superposition states for experimentally accessible flux qubits is surprisingly but not trivially small. The combination of this relatively small size with large differences between macroscopic observables in the two branches is seen to result from the Fermi statistics of the electrons and from the large disparity between the values of superfluid and Fermi velocity in these systems.   

Time-domain observation of macroscopic quantum coherence

Thirty years ago, A. J. Leggett proposed that a superconducting loop interrupted by a Josephson tunnel junction might display a coherent oscillation between trapping and detrapping of a single flux quantum. This phenomenon of reversible quantum tunneling between two classically separable states of identical energy, known as Macroscopic Quantum Coherence (MQC), is regarded crucial for precise tests of whether macroscopic systems such as circuits fully obey quantum mechanics. We report time-domain observation of MQC oscillations at sub-GHz frequency and quality factor larger than 500. Two major innovations have been introduced to achieve this result: (i) the loop inductance is 10,000 larger than in previous experiments, allowing the junction to enter the charging regime and (ii) a novel microwave cavity-assisted readout scheme free of Purcell effect. Contrary to expectations, we find that the MQC transition could be the basis of a superconducting qubit of improved coherence and readout fidelity.   

Electromagnetically-induced transparency combined with lasing without inversion in superconducting qubits

By strongly driving a transition of a three-level atom one dresses the atomic states with the external field resulting in an Autler-Townes energy level splitting. The absorption and dispersion properties of the medium can then be controlled optically in order to realize effects such as electromagnetically-induced transparency (EIT) and lasing without inversion (LWI). In atomic systems, these two effects are however usually not realized together. Away from their symmetry point, both the flux qubit and the fluxonium form a $\Delta$-configuration where transitions between any two of the lowest three states are allowed. When driven by two resonant fields, we show that the system exhibits a transparency frequency window sandwiched between an absorption band (EIT) on one side and an amplification band (LWI) on the other. Finally, we discuss a possible implementation and measurement scheme using the flux qubit or the fluxonium charge qubit.   

Measurements of Microwave Single Photon Correlations: Theory

Superconducting circuit implementations of cavity QED have enabled the exploration of various regimes of light-matter interaction. In this work, we present theoretical aspects of the observation of quantum properties of the field emitted from a cavity without access to non-linear/single-photon detectors (which have not been demonstrated reliably in the microwave regime). In particular, we focus on how to perform the measurement of optical coherence functions in pulsed circuit QED experiments using field quadrature measurements of the outputs of a two-sided cavity. We illustrate how the standard Hanbury Brown and Twiss setup can be replaced with the monitoring of these cavity outputs, while still allowing for the calculation of arbitrary first and second order correlation functions. Moreover, we illustrate how the significant noise contributions from thermal fields, amplifiers and mixers can be accounted and compensated for.   

Measurements of Microwave Single Photon Correlations: Experiment

Circuit QED allows for excellent control and measurement of the quantum mechanical properties of qubits, photons and their interactions. As a result, circuit QED is an ideal testbed to investigate the quantum nature of light. In our experiments we prepare a complete family of zero and one photon superposition states in a high quality on-chip resonator by controlling single qubit Rabi and qubit-cavity vacuum Rabi oscillations. By detecting the emitted radiation at both outputs of a symetric two-sided cavity, we are able to perform time-resolved measurements of the cavity field quadratures, photon number and the first-order correlation function. We characterize the prepared field states and also show that we are able to cool a small thermal background field present in the cavity to below its thermal equilibrium value. Furthermore we suggest that any correlation which can be expressed in terms of the cavity field operators can be measured by using beam splitters, homodyne detection and efficient real time signal processing.   

State tomography of a three-level superconducting quantum circuit

Coherent control of higher than two-dimensional quantum systems can considerably improve present techniques for quantum information processing. In particular, superconducting quantum circuits can be operated in a regime with closely spaced energy levels, where arbitrary superposition states can be prepared by applying appropriately shaped microwave pulses at different frequencies. We employ dispersive read-out [1] to discriminate the population of upper energy levels of superconducting transmon circuits coupled to a coplanar microwave resonator. This allows us to determine the dynamics in the restricted two-dimensional qubit subspace and assess the population transfer to the third level. Finally, we fully characterize arbitrary three-dimensional qutrit states by a complete tomographic measurement.\\[4pt] [1] R. Bianchetti \emph{et al.}, Phys.~Rev.~A {\bf 80}, 043840 (2009).   

Tomography of a superconducting phase qutrit

Benchmarking the fidelity of quantum state preparation and evolution is vital for further advances in quantum engineering. State and process tomography are normally used for such benchmarking of individual qubit and coupled qubit systems. We extend this procedure to a superconducting phase circuit operating with three levels (qutrit), and measure the 3X3 density matrix for a set of arbitrary prepared states and their evolution. We quantify the diagonal and off diagonal decays due to relaxation and decoherence and compare to simulation.   

Tomographic reconstruction of the Wigner function of an itinerant microwave field

In an increasing number of experiments, dispersive coupling is successfully used to encode the state of nanomechanical resonators or superconducting qubits onto the state of a microwave field. However most the information is lost due to the poor quantum efficiency of the best commercially available microwave amplifiers. To circumvent this limitation our lab has been developing quantum limited Josephson Parametric Amplifiers (JPAs). In this talk we will present an application of the JPA leading to a dramatic increase of the performance of the Quantum State Tomography. It has enabled us to reconstruct the Wigner function of a squeezed state of the microwave field. We will discuss the achieved degree of squeezing and the quantum efficiency of the state tomography, from the perspective of using these squeezed states as building blocks for quantum information experiments. Indeed these states, which are highly non-classical and are easily generated by JPAs, form EPR like states when combined together and thus are the basis of a complete quantum information processing strategy, known as the continuous variables quantum information.   

Rapid Adiabatic Passage for robust quantum gates

Rapid adiabatic passage has been suggested as tool to coherently transfer population between two capacitively coupled phase qubits. Together with single qubit rotations, rapid adiabatic passage can be used to generate universal logic gates for quantum computing. In this talk we will describe our experimental effort to use rapid adiabatic passage to transfer an excitation between two phase qubits capacitively coupled to a coplanar waveguide.   

Session Z27: Surfaces

\textit{Ab initio} Thermodynamic Approach to Screen Good Solid Sorbents for CO$_{2}$ Capture

CO$_{2}$ is the major product from coal combustion and released into the air to cause global climate warming. Current technologies for capturing CO$_{2}$including solvent-based (amines) and CaO-based materials are still too energy intensive. Solid materials have been proposed for capturing CO$_{2}$ through a reversible chemical transformation at low cost. By combining DFT with phonon lattice dynamics, the thermodynamic properties of solid materials are obtained and used for computing the thermodynamic reaction equilibrium properties of CO$_{2}$ absorption/desorption cycle based on chemical potential and heat of reaction analysis cycle. Based on our calculated thermodynamic properties of reactions for each solid capturing CO$_{2}$ varying with T and P, only those solids, which result lower energy cost in the capture and regeneration process and could work at desired conditions, will be selected as promised candidates of CO$_{2}$ sorbents and further be sent for experimental validations. Here, we first report our screening results on alkali and alkaline earth metal oxides, hydroxides and carbonates/bicarbonates and compare with available thermodynamic data, then, report the predicted good candidates of CO$_{2}$ sorbents from vast of mixing and substituted/doped solids which thermodynamic data are usually not available.   

Kinetic Monte Carlo Simulations of a Lattice-Gas Model of Pulsed Electrodeposition with Diffusion

We have studied the effect of diffusion during the desorption phase in pulsed electrodeposition in a square lattice-gas model using Kinetic Monte Carlo simulations. In cases without diffusion, the desorption rate increases when the size distribution of the droplets is dominated by smaller clusters at the beginning of the desorption process. For a particular initial size distribution, the presence of diffusion increases the desorption rate. As we decrease the average initial droplet size, the increase in the desorption rate becomes less pronounced. By analyzing the size distributions at different times during the desorption process we found that the dynamics of the size distribution when diffusion is present follows the same pattern as the dynamics of size distributions without diffusion, only with a difference in magnitude. Therefore, the effect of diffusion on the desorption rate also decreases when the size distribution of the droplets is dominated by smaller clusters.   

Electronic relaxation of a photoexcitation of AgSi(111):H surface

A combination of time dependent density matrix and {\it ab initio} electronic structure methods provide details of the relaxation pathways of photo-induced charge redistribution at nanostructured semiconductor surfaces, giving their changes in energy and space over time. They are applied to a Ag cluster on a Si(111) surface, initially photoexcited by a short pulse, and show that the Ag cluster adds surface-localized states that enhance electron transfer. Population density distributions in energy and in space, for valence and conduction bands, explore the energy band landscape of a Si slab, with various relaxation pathways ending up in a charge-separated state, with a hole in the Si slab and an electron in the adsorbed Ag cluster. Calculated electronic relaxation times for Si(111):H are of the same order as experimental values for similar semiconductor systems. We have also noticed that average non-adiabatic coupling and transition dipoles have similar dependence on numbers of orbitals involved in transition. Results from a reduced density matrix propagation with Hamiltonian and rates parametrized from {\it ab initio} electronic structure calculations give new insight on electronic dynamics at nanostructured surfaces.   

Inelastic Helium Atom Scattering from the Commensurate Monolayer Solid H$_2$/NaCl(001)

A calculation of inelastic low energy helium atomic scattering by a monolayer with one-phonon creation\footnote{F. Y. Hansen and L. W. Bruch, J. Chem. Phys. {\bf 127}, 204708 (2007)} has been adapted to treat the case where the monolayer is the dilated quantum solid H$_2$/NaCl(001). The interactions He-H$_2$ and He-NaCl are rather well known inputs, but the dilated solid presents the most corrugated surface yet treated in such calculations. Progress in performing calculations for the condition of the inelastic scattering experiments\footnote{F. Trager and J. P. Toennies, J. Phys. Chem. B {\bf 108}, 14710 (2004)} will be described. Compared to earlier work, there is remarkable sensitivity to the number of fourier components used to represent the corrugation, the number of coupled channels, and the width of the wave packet.   

Heat transfer studies at solid/gas interfaces using time-resolved ellipsometry

Heat transfer from a solid surface to gas is studied by pump laser pulses which impulsively increase the temperature of Au metal film. Transient changes of refractive index in the nearby fluid phase are monitored by off-null ellipsometry using time-delayed probe pulses with 100 fs resolution. The initial psecond rise of signals shows how acoustic waves are created and reveals energy exchange mechanisms at solid-gas and solid-liquids interfaces.   

Mapping the spin states of surface deposited Fe(II) SCO Compounds by STM

We describe a novel method for analyzing the spin states of surface deposited Fe(II) spin crossover (SCO) compounds. The talk focuses on the investigation of $[FeII(L)_{2}](BF_{4})_{2}$ (L=2,6-di(1H-pyrazol-1-yl)-4-(thiocyanatomethyl)pyridine)$^1$ and the comparison to a high spin compound with a similar coordination motif. Single molecules and small clusters were investigated on HOPG. We were able to show a strong current contrast for the different spin states using the CITS technique. Changes of the spin state from high- to low-spin state and vice versa were observed at room temperature. Switching was statistically distributed, indicating a widening of the spin transition compared to the bulk state.$^{2}$ \newline $^{1}$ M. Haryono, et al., Eur. J. Inorg. Chem. 2009, 2136. \newline $^{2}$ M.S. Alam, et al., Angew. Chem. (2009) (accepted).   

First-principles investigations for the catalytic dissociation and oxidation of methane on the Cu surfaces

The catalytic reactions of dissociation and oxidation of methane on the copper surfaces play a key role in, for example, the development of high-performance solid oxide fuel cells. We used first-principles quantum theory and large-scale parallel calculations to investigate the atomic-scale mechanism of the catalytic chemical reactions. We report the calculated results, which provide fundamental information and understanding about the atomic-scale dynamics and electronic structures pertinent to the reactions and specifically the catalytic role of the Cu(100) and Cu(111) surfaces. We also report comparison of our results with available experimental data and previous theoretical investigations.   

Electronic properties of methyl and hydrogen terminated Si(111) surfaces

Functionalized Si(111) surfaces have many applications in photo-electrochemistry, and some of those (e.g. the use of Si rods as photo-cathodes in solar cell applications) require the development of chemical protection strategies so as to prevent uncontrolled oxidation. Recently [1] a full methylation of Si(111) has been achieved experimentally, which constiutes a promising means to protect Si(111) from oxidation. However, the apparently simple atomic structure of this surface is still under debate. In particular, low temperature STM images appear to yield a pattern in disagreement with structural, first principles optimizations. We have carried out a series of ab-initio calculations of both the structural and electronic properties of the CH3-Si(111) aimed at interpreting STM and STS measurements. A comparison between results obtained at the DFT-GGA level and by using GW calculations will be presented and compared with the corresponding ones for the H-Si(111). This work supported by grant NSF-CHE-0802907 [1]H. Yu et al., Appl. Phys. Lett. 88, 152111, (2006)   

Tetracosane (C$_{24}$H$_{50})$ trilayers physisorbed onto the basal plane of graphite: perpendicular patches

Results of explicit - hydrogen Molecular Dynamics computer simulations of tetracosane (C$_{24}$H$_{50}$, or C24) trilayers deposited on a graphite substrate in the temperature range 100 K $\le \quad T \quad \le $ 550 K are presented. The third layer is perpendicular to the alkane underlayers as well as to the graphite substrate. Diffusion thakes place predominantly at the bottom of the patch through a ratcheting mechanism that is coupled to dihedral (torsional) defects. In the low - temperature solid the patch exhibits a dome - like shape and, with increasing temperature rolling of the interior molecules couple to the collapse of the patch into a droplet - like shape and, ultimately a liquid C24 patch atop the graphite layer results. Structural, thermodynamic and bond - orientational distributions and parameters are utilized in understanding the temperature evolution of the system and results are compared to those under the United Atom approximation.   

Theoretical Investigation of TI-Doped Zirconia Surfaces

We use density functional theory calculations with Hubbard corrections (DFT+U) to investigate the electronic and redox properties of Ti-substituted zirconia (111) surfaces. It is found that titanium dopants are more likely to segregate at the surface than to migrate to the zirconia bulk. The formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. If an O vacancy is created around an isolated Ti dopant, a Ti$^{4+} \quad \to $ Ti$^{2+}$ reduction takes place, while if the vacancy is created in the vicinity of a pair of dopants, each Ti atom adopts a 3+ oxidation state, with additional decrease in the vacancy formation energy. We investigate in detail the relevant distribution of dopants and vacancies in the system, and discuss the implications of our results for some applications of zirconia-based ceramics.   

Excess Charge Density at the Air-Electrolyte Solution Interface

Understanding the differential adsorption of ions at the interface of the electrolyte solution is very important because it is closely related, not only to the fundamental aspects of biological systems, but also to many industrial applications. We have measured the excess interfacial negative charge density at the air-electrolyte solution interfaces by using resonant second harmonic generation of oppositely charged probe molecules. The excess charge density increased with the square root of the bulk electrolyte concentration. A new adsorption model which includes the electrostatic interaction between adsorbed molecules is proposed to explain the measured adsorption isotherm and it is in good agreement with the experimental results.   

Polarization dependence of carbon dioxide dissociation on palladium supported on ferroelectrics

Using density functional theory (DFT) calculations, we investigate the effect of polarization direction on the catalytic activity of palladium supported on ferroelectrics. It is calculated how the energy barriers of the carbon dioxide (CO$_2$) dissociation process into carbon monoxide (CO) and oxygen (O) are changed for positively and negatively polarized ferroelectric lithium niobate (LiNbO$_3$) surfaces. In this study, the LiNbO$_3$ surfaces are passivated by ions, which is thormodynamically favored for a wide range of chemical potentials. Multiple possible CO$_2$ dissociation paths and their energy barriers are presented. We also perform a detailed analysis of electronic structure to explain differences in the reaction process on the two surfaces.   

Ethanol adsorption on transition-metal surfaces: A DFT investigation

The development of low-cost and long-term stability catalyst compounds for the production of hydrogen from ethanol is one of the main problems to be solved for large scale use of direct- ethanol fuel cells. Steam reforming, which is one of the main routes to obtain hydrogen from ethanol, as well as ethanol oxidation, are critically dependent on the choice of the catalyst devices. Therefore, an atom-level understanding of the interaction of ethanol with catalysts systems is on the first problems to be addressed. In this talk, we will report first- principles calculations based on density functional theory for the adsorption of ethanol on close-packed transition-metal surfaces at the limit of low-coverage. In particular, we will report the following properties, namely, adsorption energy, work function changes, and structural parameters for a large number of substrates, which will be used to build up a simple picture to describe the interaction of ethanol with transition-metal surfaces. This work is supported by FAPESP.   

Electron-beam irradiation of supported DPPC monolayer films -- an XPS study

Chemical changes in phospholipid (DPPC -- 1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayer films deposited on gold are studied in an X-ray photoelectron spectroscopy experiment. DPPC films have been irradiated by a monoenergetic electron beam in the energy range from 5 to 200 eV. The shifts in the binding energy of C 1s, O 1s, P 2p, and N 1s electrons, as well as the change in the intensity of corresponding photoelectron peaks, were observed before and after electron beam irradiation. We show that the electrons with energy between 20 and 100 eV have the largest effect on DPPC, mostly stripping off the protons from the tails and breaking the COO- bond in the head of the molecule, but also releasing methyl group from the choline group (N-(CH$_{3})_{3})$. The least effect of electron irradiation is shown on the P 2p band, regardless of the incident energy, which may be linked to the orientation of the DPPC molecules and additional intramolecular bonding.   

Nitrogen Adsorption on Graphite: Defying Physisorption

The adsorption of a nitrogen molecule at the graphite surface can be considered a paradigm of molecular physisorption [1]. The binding of N$_2$ can be phenomenologically described in terms of a competition between quadrupole--quadrupole and van der Waals dispersion energies. Of particular interest is the relative stability of the so-called ``in-plane'', ``out-of-plane'' and ``pin-wheel'' monolayer structures, in which the nitrogen molecules alternate between parallel and perpendicular configurations on the surface. By combining state-of-the-art electronic structure methods, such as dispersion-corrected density-functional theory and M{\o}ller-Plesset second-order perturbation theory along with high-level coupled cluster [CCSD(T)] calculations, we are able to gain quantitative insight into the adsorption mechanism of N$_2$@graphite and achieve very good agreement with experimental desorption enthalpy. We challenge the commonly held view of a closed-shell adsorbed N$_2$ molecule, finding a noticeable charge-density polarization for nitrogen in a perpendicular configuration on the surface. We map out the N$_2$@graphite potential energy surface as a function of sliding and orientation and discuss the influence of quantum zero-point energy for different adsorption sites. [1] D. Marx and H. Wiechert, Adv. Chem. Phys. {\bf 95}, 213 (1996).   

Session Z28: Clusters

Why Boron clusters are Planar?

The origin of the unusual stability of planar and quasi-planar B12 and B13$^{+}$ clusters is explored. Our results demonstrate that in B$_{12}$ and B$_{13}^{+}$ clusters a 6$_{\pi }$-6$_{\sigma -delo}$-6$_{\sigma -3ring}$ trifurcation leads to the triple aromaticity, which is unique to these clusters. Most importantly, the H-L gaps of these clusters are strongly dependent on the strength of the interaction between the inner- and the outer-rings, which make up these clusters. Furthermore, the similarities and the differences between B$_{12}$ and other stable boron species, B$_{10}$ and B$_{14}$ clusters are also discussed. The implication of the current analysis is discussed with respect to Carbon, Silicon and Aluminum clusters.   

Adsorption of CO and O2 molecules on supported small Au clusters

We investigate the catalytic activity of metal-oxide/metal supported tiny gold clusters towards the carbon monoxide oxidation by means of density functional theory calculations. Our focus lies on cluster-size effects, the influence of different support materials and co-adsorption of other molecules, e.g. water. In agreement with experimental data we could explain, why Au ad-atoms and dimers on MgO do not show any catalytic activity towards CO oxidation and why a Langmuir-Hinshelwood reaction mechanism via co-adsorption is possible for the trimer and tetramer. Furthermore we thoroughly studied the influence of spin-orbit coupling, a hitherto widely neglected effect in these systems, on the adsorption of gold clusters on the surface and of small molecules on the cluster/surface system. Last but not least clusters consisting of an odd number of gold atoms carry a spin moment from one unpaired 6s electron. We studied its coupling to the moments of a magnetic metal beneath a thin supporting metal-oxide layer.   

Hydrogen production from methanol on transition metals: A study of thermodynamics and kinetics on subnanometer clusters

The mechanistic studies of Pd-based catalysts and its interaction with methanol have attracted huge attention because of the possibility of using methanol as an on-board source of hydrogen for fuel cells. Stabilizing subnanometer metal clusters is a challenging process that has exhibited novel catalytic properties for various industrially important reactions such as production of hydrogen from hydrogen-rich molecules. One such reaction is methanol decomposition that was modeled by applying DFT methods on metal clusters. The thermodynamics and kinetics of three decomposition routes involving C-O, C-H and O-H scission were investigated; activation energy barriers were determined with the nudged elastic band method on Pd clusters with a comparison to Co and Cu clusters. A detailed analysis of the PES for methanol decomposition shows C-O activation to be the least favorable step on all three metal clusters. However we find activation to be $\sim$0.30 eV smaller on Co cluster. In addition, estimated thermodynamical data for a large number of transition metals has been generated from linear correlations constructed from the binding energies of Pd, Cu and Co to broaden our understanding of the role such metal clusters can play as catalyst for such reactions.   

Intrinsic nano-transformation of Al$_{55}$ clusters below the melting temperature

A recent series of experiments [1] have shown diverse melting behaviors in size-selected Al nanoclusters (Al$_{n})$. In particular, Al$_{55}$ is a magic cluster that serves as a boundary for abrupt change of melting points when $n$ is around 55. Here, resulting from first-principles molecular dynamics simulations of Al$_{55}$ clusters, we reveal a new dynamic melting state that has both solid and liquid characteristics. In thermal fluctuations near the melting point, the low-energy tetrahedral Al$_{55}$ survives through rapid, collective surface transformations --- such as parity conversions and intervened row hopping --- without losing its structural orders. The emergence of the collective motions is due to efficient thermal excitation of soft phonon modes at nanoscale. A series of spontaneous surface reconfigurations result in a mixture or effective flow of surface atoms as is random color shuffling of a Rubik's cube. This novel ``lattice-liquid'' state will provide useful insights into understanding stability and functionality of nano systems near or below melting temperatures. [1] G. A. Breaux et al., Phys. Rev. Lett. \textbf{94}, 173401 (2005).   

Theoretical Studies of the Stability and Electronic Properties of Pd$_{n}$, and Pd$_{n}$O$_{2}$ (1$\le $n$\le $13) Clusters

First principles electronic structure studies on the ground state geometry, electronic structure and magnetic moment of Pd$_{n}$(1$\le $n$\le $13) clusters have been carried out using a gradient corrected density functional approach. The clusters are found to be magnetic with a moment per atom that varies with cluster size. In particular, Pd$_{13}$ is shown to have a two layers structure that can be looked upon as a fragment of the bulk and has a spin magnetic moment of 6 Bohr magnetons. The calculated magnetic moments are compared with available data from Stern Gerlach experiments. We also study the effect of adding an O$_{2}$ molecule on the electronic and magnetic properties by carrying out corresponding studies on Pd$_{n}$O$_{2}$ (1$\le $n$\le $13) clusters. Our findings on the strength of binding of oxygen will be compared with recent experiments on the oxidation of palladium clusters by oxygen.   

A density functional theory investigation of the 3d, 4d, and 5d metals 13-atom clusters

In this work, we report a first-principles study based on density functional theory calculations of the atomic structure, binding energies, coordination numbers, average bond lengths, magnetic properties, and vibrational frequencies of 3d, 4d, and 5d metal clusters (30 elements) containing 13 atoms, M13. A set of lower energy local minimum structures were obtained by combining high-temperature first-principles molecular dynamics simulations with geometric optimizations at zero temperature for different spin configurations. The ground state structures for the M13 clusters show very clear features: (i) Compact icosahedral-like forms at the beginning of each metal series, (ii) more opened structures such as hexagonal bilayer-like and double cubic layer in the middle of each metal series, and (iii) structures with increasing average coordination number, e.g., icosahedron (Hg), for elements with valence d-states having more than half-occupation, at the end of each series. The magnetic exchange interactions play an important role for particular systems such as Fe, Cr, and Mn. Most of the properties can be explained by the occupation of the bonding and anti-bonding states   

The Origin of ``Magic-Number'' Stability and Chiral Selectivity for Serine Clusters in Gas Phase

Serine ``magic-number'' clusters have attracted substantial experimental and theoretical interest since their discovery. They have been implicated in one possible mechanism leading to the origin of homochirality, as certain clusters exhibit remarkable chiral selectivity. We aim to develop a ``structural landscape'' for these clusters over a range of relevant cluster sizes, enantiomeric compositions, and ionizing charge states using theoretical tools of statistical and quantum mechanics. In this work, we search for low-lying stationary points and global minima of the potential energy landscape via a combined annealing, replica exchange and basin-hopping molecular dynamics approach in a modified AMBER forcefield. These structures are used as inputs for further DFT-based optimization and energy decomposition analysis. It is shown that the behavior and stability of these systems is due to major structural rearrangements as a function of size and charge. Further, the experimentally observed chiral selectivity may be understood in part by the unique network of hydrogen bonds facilitated by the serine hydroxyl side chain. The influence of a further kinetic mechanism is not ruled out by the current results and is discussed.   

Effect of Charge and Composition on the Structural Fluxionality and Stability of Nine Atom Tin-Bismuth Zintl Analogues

Synergistic studies of bismuth doped tin clusters combining photoelectron spectra with first principles theoretical investigations establish that highly charged Zintl ions, observed in the condensed phase, can be stabilized as isolated gas phase clusters through atomic substitution that preserves the overall electron count but reduces the net charge. Mass spectrometry studies reveal that Sn$_{8}$Bi$^{-}$, Sn$_{7}$Bi$_{2}^{-}$, and Sn$_{6}$Bi$_{3}^{-}$ exhibit higher abundances than neighboring species, and photoelectron spectroscopy show that all of these heteroatomic gas phase species have high adiabatic electron detachment energies. Sn$_{6}$Bi$_{3}^{-}$ is found to be a particularly stable cluster, having a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap. Theoretical calculations demonstrate that Sn$_{8}$Bi$^{-}$, Sn$_{7}$Bi$_{2}^{-}$, and Sn$_{6}$Bi$_{3}^{-}$ are deltahedral clusters and isoelectronic to the Zintl polyatomic clusters Sn$_{9}^{2-}$, Sn$_{9}^{3-}$ and Sn$_{9}^{4-}$, respectively. However, the fluxionality reported for tin-Zintl clusters is suppressed by substituting Sn atoms with Bi atoms in Sn$_{8}$Bi$^{-}$ and Sn$_{6}$Bi$_{3}^{-}$. The similarities between bismuth doped deltahedral tin clusters and deltahedral Zintl polyanions, suggest these gas phase Zintl clusters (GPZC) may find use for building blocks of cluster assembled materials.   

Activity and Selectivity of Size-Selected Sub-nm to Nanometer Size Silver Clusters in the Selective Oxidation of Propylene

The activity and selectivity of Ag nanoparticles in propene oxidation will be discussed and compared with the performance of Ag3 clusters. The experimental studies are based on 1) chemically uniform support fabrication, 2) size-selected cluster deposition, 3) electron microscopy of nanoclusters, and 4) in situ synchrotron X-ray characterization of clusters under working conditions, combined with mass spectroscopy analysis of reaction products.   

On the Oxidation of Palladium clusters supported on alumina/NiAl(110)

Palladium nanostructures supported on alumina substrates have been widely studied as catalysts for several combustion processes. Following recent experimental results [1], we have performed electronic structure density functional calculations in order to study the geometry, stability and electronic properties of small Pd clusters supported on alumina/NiAl(110). Using an accurate model of such surface, the properties of selected cluster sizes have been determined, as well as the effect of the addition of oxygen. A carefully analysis of the results as well as a comparison with the experimental data will be presented. \\[4pt] [1] T.Wu et al, Surface Science 603, 2764 (2009).   

Not so loosely bound: temperature dependent vibrational fingerprints of Au$_N$Kr$_M$ clusters

In order to interpret the vibrational spectra of \textit{neutral} Au$_N$Kr$_M$ clusters, as measured in a Multiple Photon Dissociation Far-IR experiment (P. Gruene \emph{et al.}, Science \textbf{321}, 674 (2008)), we calculated their finite temperature vibrational spectra, by means of all electron density functional theory, including the Tkatchenko-Scheffler van der Waals correction. We surprisingly find that Kr forms weak chemical bonds (binding energy around 0.2 eV per Kr atom) with 1- to 3-fold coordinated Au atoms belonging to small Au$_N$ clusters ($N \leq 4$). Such Au$_N$Kr$_M$ clusters have a vibrational spectrum which is different from the related Au$_N$ clusters. For bigger Au$_N$ clusters, Kr physisorbs to the clusters, forming a complex whose vibrational spectrum is practically identical to the spectrum of the bare cluster. Anharmonicities affect the spectrum by changing (with respect to the harmonic spectrum) the relative intensity of the peaks and by showing new peaks, due to interactions among eigenmodes.   

Signature of the Superatom to Superhalogen Behavior of Au$_{n}$(BO$_{2})_{m}$ clusters

We report the discovery of a new class of clusters consisting of Au$_{n}$(BO$_{2})_{m}$ which formed during the oxygenation of gold clusters when boron nitride was used as insulation in the pulsed arc cluster ion source (PACIS). Using DFT based calculations, we trace the origin of these species to be due to the unusual stability of the BO$_{2}$ moiety as well as shed light on their formation process. PES measurements and the corresponding DFT calculations further reveal some rather remarkable properties of Au$_{n}$(BO$_{2})_{m}$ clusters such as large HOMO-LUMO gaps in the range of 3.00 eV -- 3.95 eV and electron affinities substantially larger than that of F, the most electronegative element in the periodic table. In addition, some of the most predominant features of the electronic structure of the bare Au clusters, namely odd-even alternation in the electron affinity, are preserved in the Au$_{n}$(BO$_{2})$ species. The synergy between theory and experiment illustrates that Au$_{n}$(BO$_{2})_{m}$ clusters, behave as superatoms and superhalogens, opening the door for the synthesis of a new class of cluster-assembled materials.   

Ab Initio Study of Atomic and Molecular Adsorption to Pt Clusters on Graphenes

In recent years, Pt nanostructures have received a great deal of attention because of their exceptional chemical/electrochemical reaction properties with high catalytic efficiency. Particularly, a number of studies have focused on their versatile catalytic properties depending on their sizes, shapes, substrate and/or alloy compositions {\it etc}. Motivated by those studies, we investigated the adsorption of several important adsorbates (H, O, and CO) to Pt$_x-$graphene complexes (x = 1 or 13) with the use of {\it ab initio} density functional methods. Our calculations of Pt adsorption on various graphene defects demonstrate that Pt {\it d} band profiles remarkably vary depending on the type of graphene defects and the corresponding adsorption strength of the adsorbates is also substantially affected. Also for Pt$_ {13}-$graphene complexes, the overall adsorption strength of H, O, and CO is remarkably changed depending on graphene defects, which indicates the variation of their catalytic behavior. The role of graphene defects for the interaction between Pt and the adsorbates will be presented.   

Magnetic Superatoms

The electronic states in metal clusters are grouped in shells much in the same way as in atoms. Filling of the electronic shells leads to stable species called magic numbers. This has led to the proposition that selected stable metal clusters can mimic chemical properties of atoms in the periodic table and can be classified as superatoms. So far the work on superatoms has focused on non-magnetic species. Here, we propose an extension of the superatom concept to magnetic species by invoking systems that have both localized and delocalized electronic states. Here, the localized electrons stabilize spin magnetic moments while filled nearly free electron shells lead to stable superatoms. We demonstrate it for an isolated VCs$_{8}$ and a ligated MnAu$_{24}$(SH)$_{18}$ motifs that are shown to be magnetic superatoms. The magnetic superatoms assemblies offer prospect of tunable molecular electronic devices, as the coupling can be altered by applying fields.   

Session Z29: A Potpourri of AMO and Quantum Information

Towards Hybrid Quantum Information Processing with Electrons on Helium

Electrons on helium is a unique system in which a two-dimensional electron gas is formed at the interface of a quantum liquid (superfluid helium) and vacuum. As outlined in our recent proposal [1], single-electron quantum dots on helium can be built using submerged electrostatic gates and the lateral motion of the electron can be coupled to the electromagnetic field in a superconducting resonator by integrating the quantum dot into a circuit QED architecture [2]. Energy can be exchanged coherently between motional states and individual photons at an estimated Rabi frequency of $g/2\pi\sim 20$ MHz while motional and spin coherence times exceed 20 $\mu$s for charge and 1 s for spin with a spin-photon coupling as high as 1 MHz [1,3], making the system attractive for quantum information processing. Here, I will present recent experimental progress towards trapping and detecting single electrons on helium with a high-finesse superconducting cavity.\\[4pt] [1] D.I. Schuster, et al. in preparation (2009) \newline [2] A. Wallraff, et. al. Nature 431, 162 (2004) \newline [3] S. A. Lyon, Phys. Rev. A 74, 052338 (2006)   

Electron occupancy of micro-structured helium-filled channels

The spins of electrons floating on the surface of superfluid helium have been suggested to be promising qubits. High charge transfer efficiency of electrons in a narrow channel clocked with underlying gates, has been previously reported.\footnote{G. Sabouret, F.R. Bradbury, S. Shankar, J.A. Bert, S.A. Lyon, Appl. Phys. Lett. \textbf{92}, 082104 (2008).} We have fabricated similar devices with an array of parallel channels and small gaps between the underlying gates. These channels are filled with superfluid helium by capillary action, onto which electrons are photoemitted. Electrons are initially trapped by a gate (``door''), so that they capacitively couple to a sense gate which is the input of a cold HEMT preamplifier. An oscillatory potential applied to a third gate moves electrons on and off the sense gate to allow lock-in detection. Electrons are allowed to escape the sensing region by slowly ramping down the door barrier. Features in the electron occupancy signal correlate with the oscillatory drive voltage and preamp gain, and show evidence of discrete occupancy as the channels depopulate.   

Exploiting silicon chip technology for control of electrons on superfluid helium

Electrons on the surface of superfluid helium have extremely high mobilities and long predicted spin coherence times, making them ideal mobile qubits. Previous work has shown that electrons localized in helium filled channels can be reliably transported between multiple underlying gates. Silicon chips have been designed, fabricated, and post processed by reactive ion etching to leverage the large scale integration capabilities of silicon technology. These chips, which serve as substrates for the electrons on helium research, utilize silicon CMOS for on-chip signal amplification and multiplexing and the uppermost metal layers for defining the helium channels and applying electrical potentials for moving the electrons. We will discuss experimental results for on-chip circuitry and clocked electron transport along etched channels.   

Spin and Orbital Rotation of Electrons and Photons via Spin-Orbit Interaction

We show that when an electron or photon propagates in a cylindrically symmetric waveguide, its spin angular momentum (SAM) and its orbital angular momentum (OAM) interact. Remarkably, we find that the dynamics resulting from this spin- orbit interaction are quantitatively described by a single expression applying to both electrons and photons. This leads to the prediction of several novel rotational effects: the spatial or time evolution of either particle's spin/polarization vector is controlled by its OAM quantum number, or conversely, its spatial wavefunction is controlled by its SAM. We show that the common origin of these effects in electrons and photons is a universal geometric phase. We demonstrate how these phenomena can be used to reversibly transfer entanglement between the SAM and OAM degrees of freedom of two-particle states.   

Positronium Cooling in Porous Silica Measured via Doppler Spectroscopy

We have measured the kinetic energy of positronium (Ps) atoms emitted into vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the line width of the Ps 13S-23P transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, \textit{Ex }= 42 $\pm $ 3 meV, that is consistent with conversion of the confinement energy of Ps in the 2.7 nm diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures less than 100K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.   

Cooling Atoms with a Moving One-Way Barrier

We demonstrate the use of a moving optical one-way barrier for cooling a collection of atoms. In our experiment, rubidium atoms begin in a far-detuned dipole trap consisting of a single focused Gaussian beam. Two laser beams transversely cross the trap; one provides a repulsive (attractive) potential for atoms in the upper (lower) ground state, and the other pumps atoms into the upper ground state on one side of the first beam, forming a one-way barrier. The optical one-way barrier is adiabatically swept along the longitudinal axis of the trap. At each point, the barrier traps atoms near their turning point, where they have less kinetic energy. As the barrier sweeps, the atoms do not regain their kinetic energy, and are eventually left at the trap focus with less kinetic energy than before. We experimentally study the effectiveness of barrier-cooling, focusing on how experimental limitations affect the cooling limit.   

Recombination of N bosons near threshold

We derive a generalized cross section for scattering events involving an arbitrary number of particles, and apply our result to the recombination of N bosons. We obtain a semi-analytical formula that encapsulates the overall Wigner threshold scaling as well as resonant enhancements due to the presence of N-body states near threshold. For the case N=4, we obtain quantitative results for the event rate that exhibit resonant enhancement due to known universal 4-boson states tied to Efimov physics.   

Universality in Three- and Four-Body Bound States of Ultracold Atoms

The universal regime of Efimov few-body physics occurs when the strength of the interparticle interaction is much larger than the effective range of the two-body potential. By exploiting a broad Feshbach resonance in the $|1,1\rangle$ hyperfine state of $^7$Li, we can tune the interactions well into the universal regime. The rate of atom loss from our optical trap increases by 9 orders of magnitude from the weakly interacting regime to the strongly interacting regime, allowing unprecedented access to universal physics. We find evidence for two universally connected Efimov trimers in addition to their associated four-body bound states. Intimately related to the Efimov trimers, two tetramer states exist for each trimer, and no additional parameters are required to describe their binding energies. A total of eleven features in the three- and four-body inelastic loss spectra are discovered. The relative locations of these features on either side of the Feshbach resonance agree with universal theory, whereas a systematic deviation from universality is found when comparing features across the resonance. Science.1182840 (2009).   

Nonlinear optical response and ionization of a metal tip plasmon in ultrafast strong fields

The nonlinear response of a silver tip plasmon is investigated by simultaneously measuring its optical response and field induced ionization current. The measurements rely on interferometric cross-correlation of frequency-modulated optical pulse trains. The method allows for a quantitative analysis of the plasmon nonlinear optical susceptibilities, and a unique interpretation of the ionization process as field induced tunneling. Nonlinear optical mixing up to the 4th harmonic of the Ti:Sapphire fundamental is observed by detecting electron current, demonstrating an electron pulse train of 600 attosecond period. The electron pulse train results from the detachment of the strongly modulated plasmon tail.   

Velocity Map Imaging Spectroscopy of the Lanthanide Negative Ions

The technique of Velocity Map Imaging Spectroscopy (VMIS) is being adapted for the study of the heavy negative ions of the Lanthanide series. The VMIS technique will allow us to determine structural properties of these negative ions as well as yield angular distribution information for photon-negative ion interactions. An overview of the experimental apparatus as well as preliminary data on the cerium negative ion Ce$^-$ will be presented.   

Spectral theory for molecules and materials: Pauli revisited

Implementations of new theoretical methods are reported for {\it ab initio} chemical structure calculations of molecules and materials based on an atomic spectral-product representation of aggregate electronic degrees of freedom. In this approach, the Pauli principle is enforced subsequent to construction of the Hamiltonian representative matrix in the basis, greatly simplifying its evaluation. It is shown that atomic pair-interaction calculations, which can be performed once and for all and retained for repeated applications, are sufficient to determine the electronic eigenstates and chemical structures of arbitrary chemical aggregates. The spectral-product representation is seen to span the totally antisymmetric representation of the aggregate electron symmetric group once and only once, but to also span other non-Pauli representations in which the desired Pauli solutions of the Schr\"odinger equation are generally embedded. Progress in isolating totally antisymmetric solutions is described employing the antisymmetrizer matrix constructed in the spectral-product basis. The Pauli subspace of the full spectral-product Hilbert space is isolated in this manner, and the corresponding physical block of the aggregate Hamiltonian matrix determined. Illustrative calculations of ground- and excited-state potential energy surfaces in simple molecules exhibit convergence to corresponding totally antisymmetric results.   

An ``Anatomic approach" to study the Casimir effect

The Casimir effect, in its simplest definition, is a quantum mechanical force between two objects placed in vacuum. In recent years the Casimir force has been the object of an exponentially growing attention both from theorists and experimentalists. A new generation of experiments paved the way for new challenges and spotted some shadows in the comparison to theory. Here we are going to isolate different contributions to the Casimir interaction and perform a detailed study to shine new light on this phenomenon. As an example, the contributions of Foucault (eddy current) modes will be discussed in different configurations. This ``anatomic approach'' allows to clearly put into evidence special features and to explain unusual behaviors. This brings new physical understanding on the undergoing physical mechanisms and suggests new ways to engineer the Casimir effect.   

Electromagnetic Energy, Absorption, and Casimir Forces

The derivation of Casimir forces between dielectrics can be simplified by ignoring absorption, calculating energy changes due to displacements of the dielectrics, and only then admitting absorption by allowing permittivities to be complex. As a first step towards a better understanding of this situation we consider in this paper the model of a dielectric as a collection of oscillators, each of which is coupled to a reservoir giving rise to damping and Langevin forces on the oscillators and a noise polarization acting as a source of a fluctuating electromagnetic (EM) field in the dielectric. The model leads naturally to expressions for the quantized EM fields that are consistent with those obtained by different approaches, and also results in a fluctuation-dissipation relation between the noise polarization and the imaginary part of the permittivity. Our main result is the derivation of an expression for the QED energy density of a uniform dispersive, absorbing media in thermal equilibrium. We also show how the fluctuation-dissipation theorem ensures a detailed balance of energy exchange between the (absorbing) medium, the reservoir and the EM field in thermal equilibrium.   

Quantum Phases of Atom-Molecule Mixtures of Fermionic Atoms

Cold atom experiments have realized a variety of multicomponent quantum mixtures, including Bose-Fermi atomic mixtures. Mixtures of fermionic atoms and diatomic molecules, which are boson, have also been obtained by tuning of the interactions with external fields [1]. We study many-body correlations in such a system where the molecules are weakly bound and therefore pairs of fermionic atoms easily convert into and dissociate from the bound molecule state. This exchange mediates a long-range interaction between the fermions. We consider a simple many-body Hamiltonian that includes the destruction of fermionic atom pairs to form single bosonic molecules and vice versa [2]. We employ a functional renomalization-group approach and calculate the renormalized frequency-dependent interaction vertices and fermion self-energies. We find an instability from the disordered quantum liquid phase to a BCS phase and calculate the energy scale for the transition. The unusual frequency-dependence of this mediated interaction leads to strong renormalization of the self-energy, and also affects the couplings in the BCS channel. [1] M. Greiner, C. A. Regal, J. T. Stewart, and D. S. Jin, Phys. Rev. Lett. {\bf 94}, 110401 (2005) [2] E. Timmermans, K. Furuya, P. W. Milonni, and A. K. Kerman, Phys. Lett. A {\bf 285}, 228 (2001)   

Session Z30: Optoelectronic Devices and Applications

Near-Field Generation and Detection of Surface Plasmon Polaritons on Silver Nanowires

Chemically-grown silver nanowires are highly crystalline and excellent waveguides for surface plasmon polaritions (SPPs). As their radii approaches the nanowire limit, their SPP modes become highly confined, but it becomes increasingly difficult to scatter light into and out of these modes. We demonstrate nanowire junction-based techniques for generating and detecting SPPs in the near field, thereby circumventing the need for scattering. For near-field SPP generation, we use a silver nanowire as both an SPP waveguide and an electrode for an electroluminescent Schottky junction. For near-field SPP detection, the silver nanowire doubles as a local gate for a photosensitive nanowire. We discuss the mechanism of SPP generation and detection, including a gain mechanism in the detector and a memory effect in the emitter, which is related to filamentary current paths.   

Experimental demonstration of TE-excited surface plasmon polariton wave

Plasmonics, as the most rapidly developing subject in nanophotonics and nano-scale optoelectronics, is finding ample applications ranging from bio-imaging, sensing, solar cell to chip scale optoelectronic integration. However, the inherent polarization feature of surface plasmon polariton (SPP) dictates that it can only be excited by incident light with TM polarization, thus limiting the excitation efficiency to 50{\%} at most if the incident light is unpolarized. Here, we propose a novel plasmonic nanostructure that can overcome this inherent limitation for SPP excitation. The proposed structure supports highly efficient SPP-TE coupling, due to an excited hybrid mode inside the plasmonic structure. This unique TE-excited SPP was successfully verified both in numerical simulation and in experiment using the Kretschmann configuration as a sharp dip was identified in the reflection spectrum, consistent with our theoretical prediction. Furthermore, we show that SPPs could be simultaneously excited with both TE and TM polarization and thus the excitation efficiency could approach 75{\%}.   

Plasmonic Biosensors based on Multi-Layered Metallodielectric Nanostructures

Nanoplasmonics found many applications in diverse topics of optics such as biosensing, solar cells, etc. Studies built up so far is focused on 2D nanostructures, however expanding into the third dimension will provide higher degrees of freedom in the design space. Third dimension is mostly avoided because of fabrication related issues and therefore novel approaches are required. In this work, we have investigated the hybrid multi-layered metallic structures. Transmission spectra provided the conventional extraordinary optical transmission peaks and in addition the newly found modes, which are observed due to the coupling of nanohole and nanoparticle layers. In this talk, we will present the effects of these plasmonic and photonic interactions between nanostructure layers. The newly found mode is explained as the fundamental Fabry-Perot mode of the nanocavity. Numerical analysis shows that the field pattern overlap in the dielectric is superior to any other mode, therefore making this resonance highly sensitive to refractive index changes. Also we will present new results from another coupled 3D structure, multi-layered nanohole arrays.   

Simulation and Testing of Type-II Strained-Layer Superlattices for Low Temperature Thermophotovoltaic Cells.

The focus of this paper is the characterization of a novel, low band gap, long-wavelength, Thermophotovoltaic (TPV) cell design. These cells are based on type-II strained layer superlattice (SLS) structures where the effective bandgap is adjustable and a function of the thickness of the individual layers, creating minibands. Additionally, the type-II nature of the SLS causes the charge carriers to be spatially separated, which minimizes Auger recombination, allowing for the creation and operation of lower temperature TPV cells. The simulation was done using nextnano, while the testing was done on a custom-designed, cryogenic, high vacuum thermal simulator, specifically developed for characterizing low temperature TPV. These cells have the ability to extract energy from long wavelength photons, which will enable devices to harvest energy from more common sources than previously possible. Through this work, energy harvesting could occur at body temperature and below. Here, we characterize several TPV samples with wavelengths up to 10 microns. The capability to extract energy from longer wavelengths opens up new possibilities for TPVs, such as cooling microprocessors and other low temperature applications, or enabling devices like wireless sensors and biological implants to power themselves using heat from their ambient surroundings.   

Enhancing Thermophotovoltaics: 2D photonic crystals and Surface Plasmon Resonance to Increase the efficiency of GaSb

For many years researchers have attempted to efficiently harvest waste heat via thermophotovoltics (TPVs). The low quantum efficiency (QE; i.e. the probability that a photon will be absorbed) in most cells is probably the biggest limiting factor in achieving an economically viable device and directly affects the conversion efficiency (CE; i.e. the probability that a photon will be converted into a carrier that is collected). In many cases, top of the line TPV cells might only have a CE of 20 percent. Recent advances have enabled the creation of novel structures to enhance the absorption and the conversion of the incident thermal photons. In particular, photonic crystals (PhC) and surface plasmon (SP) interface enhancements have been shown to increase the efficiency of photon to current conversions for infrared photodetectors. Here, we report on the enhancement of photon conversion by integration of PhC and SP structures into the TPV cells. Photonic crystals consisting of rods of either air or dielectric surface-passivation material are placed into the base semiconductor TPV cells to increase duration of thermal photon absorption, resulting in significantly enhanced QE and CE. The ability to harvest waste heat for energy will help make many processes more energy efficient, a critical component in ushering the USA into an era of energy independence.   

A nanoscale Inverse-Extraordianry Optoconductance (I-EOC) efficient room temperature photodetector

We present here a new nanoscale efficient photon sensor based on a new form of extraordinary optoconductance phenomenon, (EOC), in nanoscopic metal-semiconductor hybrid structures (MSH) at room temperature. Our macroscopic devices (dimension $>$ 500 nm) exhibit a normal EOC in which the effective resistance decreases with increased illumination intensity, whereas nanoscopic structures (dimension $<$ 500 nm) of the same geometric design exhibit an inverse and much larger response in which the effective resistance increases with illumination intensity. This inverse EOC (I-EOC) effect is driven by the cross-over from ballistic to diffusive transport of the photo-induced carriers. We observe at room temperature a maximum I-EOC of $9460\%$ for a 250 nm device under 633 nm illumination corresponding to a specific detectivity of $D^* = 3.2\times 10^{11}$ cmHz$^{1/2}$/W with a dynamic response of 40 dB making this sensor technologically competitive for a wide range of nanophotonic applications.   

Non-Symmorphic and Quasi-periodic PhoXonic Crystals

PhoXonic(X=n,t) Crystals allow for the manipulation of elastic and electromagnetic wave propagation. This has led to an abundance of novel effects such as negative refraction, artificial birefringence and complete band-gaps. The key to these effects lies in the design of the artificial structure of the medium. A rational approach towards this task may be adopted by choosing the correct symmetry; hence both the dispersion relations and the normal modes can be controlled and interpreted in the symmetry framework. By continuous deforming a structure from i) The periodic approximant of a quasi-periodic structure to its maximal subgroup and ii) a starting symmorphic plane group into a related non-symmorphic plane group, we demonstrate control over where to open up complete in-plane (TM) and out-of-plane gaps (TE) for 2D phononic(photonic) systems and enforce artificial degeneracy of certain bands through ``sticking'' along certain directions. We can also selectively enhance curvatures for certain bands, providing a handle for ``mode-engineering.'' We also identify features of the dispersion relations that are i)invariant to deformations preserving discrete space group symmetry and ii)invariant to deformations preserving topology. All these features of the band structure become transparent within the symmetrical framework, pointing a rational approach towards designing phoXonic devices.   

Seed Layer Dependence of ZnO Nanorod Growth

ZnO is a wide band gap semiconductor for optoelectronic applications such as solar cells, transparent conducting electrodes, and chemicals sensors. In past decades, significant progress has been achieved in controlled growth of ZnO nanorods and nanotubes. In this study we investigate the optimization of the growth properties such as orientation, diameter and shape of ZnO nanorods grown by a low temperature, chemical bath deposition technique. Our group fabricated nanorods on a glass substrate with a seed layer of ZnO deposited by RF and DC sputtering in a formamide solution bath (5{\%}v/v) with zinc metal foil at 65\r{ }C for 24 hours. Scanning electron microscopy (SEM) images of ZnO nanorods reveal that the orientation and size of nanorods grown on various seed layers depend greatly on the initial seed layer of (doped) ZnO. Our research investigates the substrate dependence by experimenting with multiple seed layer deposition methods such as DC and RF coating, yielding both doped and undoped ZnO seed layers. The dependence on growth parameters, such as the concentration of formamide solution and heating methods, will be also characterized.   

Directed Growth of ZnO Nanobridge Sensors using Carbonized Photoresist

Metal oxide nanowires (NWs) are a natural candidate for high sensitivity sensor applications due to their inherently high surface-to-volume ratio. However, developmental challenges still remain for wafer-scale methods to align and integrate NWs to lithographically-defined contacts.~ Recently, selective growth of ZnO NWs was achieved without a metal catalyst using lithographically-patterned carbonized photoresist (C-PR), but electrical measurements were not reported. We have used C-PR to construct directly-integrated ZnO nanobridge devices. PR was spun onto SiO2 coated Si samples, then patterned and carbonized in a reducing atmosphere. Vapor-solid transport was used to grow nanowires between C-PR pads to form nanobridge devices. Current-voltage measurements revealed a Schottky contact between the C-PR and NWs.~ Operation of these nanobridge devices as bottom gate (Si substrate) modulated transistors, UV sensors (up to two orders of magnitude current increase), and gas / humidity sensors is demonstrated.   

Improved Open Circuit Voltage in Hybrid Photovoltaics by Surface Modification of ZnO with Mercurochrome

Inorganic--organic PV cells such as ZnO/P3HT provide an attractive alternative to conventional silicon solar cells due to their solution based, low-temperature fabrication, and scalability in manufacturing. ZnO/P3HT heterojunctions however suffer poor PV performance compared to all-organic cells which use PCBM as the electron acceptor. One pathway for improving performance in these hybrid devices replies on surface modification of ZnO with electron acceptors such as C60 to aid in charge transfer from the electron donating polymer to ZnO. In this study ZnO sol-gel films are modified with mercurochrome resulting in a decrease in ZnO work function as measured by Kelvin probe and concurrently an increase in open circuit voltage (Voc). Additionally, EQE measurements show that part of the current in the modified cells results from absorption by mercurochrome. Potential mechanisms for the increased Voc in these modified hybrid cells will be discussed.   

Light-induced binding of metal nanoparticles via surface plasmons

Recently, nanomachines based on the interaction of nanosize objects with nanostructrued surfaces have attracted much attention. In this work, we study theoretically the light-induced binding forces between a metallic nanosphere and a planar structure, and also between nanoparticles in a diatomic plamonic chain of shelled and unshelled metallic nanoparticles placed alternatively. These forces are calculated by Bergman-Milton spectral representation and multiple image methods within the long wavelength limit. When we tune the incident frequency to the surface plasmon resonant frequency, a stable local minimum in the potential energy is found. It signifies a binding between nanoparticles (nanostructures), which indicates a possible stable structure of the metallic clusters. Such binding is caused by the excitation of collective plasmon modes, which depends on the interparticle distances. This study has potential applications in plasmonic waveguides and colloidal metallic clusters on the nanoscales.   

Session Z31: Quantum Optics and Quantum Many-body Physics in Optical Lattices

Linked and knotted beams of light

Maxwell's equations allow for curious solutions having linked and knotted field lines. A particularly striking solution is one characterized by the property that {\it all} electric and magnetic field lines are closed loops with {\it any } two electric(magnetic) field lines linked. These little known solutions, are based on the Hopf fibration and have a remarkably simple representation in terms of self-dual Chandrasekhar-Kendall curl eigenstates. I will discuss their structure, time evolution, physical properties and how they may be physically realizable.   

Interference of Photons from a Weak Laser and a Quantum Dot

We demonstrate two-photon interference from two unsynchronized sources operating via different physical processes [1]. One source is spontaneous emission from the X$^{-}$ state of an electrically-driven InAs/GaAs single quantum dot with $\mu $eV linewidth, the other stimulated emission from a laser with a neV linewidth. We mix the emission from these sources on a balanced non-polarising beam splitter and measure correlations in the photons that exit using Si-avalanche photodiodes and a time-correlated counting card. By periodically switching the polarisation state of the weak laser we simultaneously measure the correlation for parallel and orthogonally polarised sources, corresponding to maximum and minimum degrees of interference. When the two sources have the same intensity, a reduction in the correlation function at time zero for the case of parallel photon sources clearly indicates this interference effect. To quantify the degree of interference, we develop a theory that predicts the correlation function. Data and experiment are then compared for a range of intensity ratios. Based on this analysis we infer a wave-function overlap of 91{\%}, which is remarkable given the fundamental differences between the two sources. \textbf{[}1] Bennett A. J \textit{et al} Nature Physics, \textbf{5}, 715--717 (2009).   

Photon blockade in circuit quantum electrodynamics

Strong photon-photon interactions arise in a cavity strongly coupled to an atom or qubit, resulting in blockaded transmission[1].~ In such a system, the resonant frequency of the cavity shifts with the presence of a single photon due to the strong number-dependent nature of the cavity nonlinearity.~ Here, we investigate the photon blockade regime in superconducting circuits with integrated transmon qubits.~ To maximize the nonlinear effects, both the cavity $Q$ and qubit-cavity coupling are made extremely large by design, with $Q$ exceeding 100,000.~ Cavity transmission is characterized using a microwave generator with a controllable output bandwidth.~ Measurements of transmitted power and spectra versus incident center frequency and bandwidth are presented.~ [1] K.M. Birnbaum et al., Nature, 436, 87 (2005).   

Photons as phonons: Flexible crystals of atoms and light in multimode cavities

Condensed-matter phenomena involving the emergence and dynamics of crystal lattices can be realized using ultracold atoms confined in multimode optical cavities~[1]. The atoms locally crystallize at the antinodes of one of the cavity modes, provided the cavity is transversely pumped by a laser of sufficient intensity. The mode into which the atoms crystallize is likely to vary across the cavity, giving rise to dynamical dislocations, frustration, and possibly glassiness. The crystallization transition is a nonequilibrium quantum phase transition, involving a condensation of cavity photons that is analogous to the condensation of phonons during crystallization in solids. This photon-phonon analogy goes further: the dynamics of atoms in multimode cavities is governed by polaron-like dissipative effects. We discuss various possible kinds of quantum ordering in atom-cavity settings, their imprint on the light emitted from the cavity, and prospects for their experimental realization. [1] S. Gopalakrishnan, B.L. Lev, and P.M. Goldbart, Nature Physics 5, 845-850 (2009).   

Dynamical properties of Coupled Cavity Arrays and the Bose Hubbard Model

We study a system of cavity arrays coupled by photons. It can be described by a model based on the Jaynes-Cummings Hamiltonian. It resembles the Bose Hubbard model, which describes recent experiments on cold atoms in optical traps. Dynamical properties like the dynamical structure factor have recently been observed there using Bragg spectroscopy or lattice modulation. Employing an exact QMC algorithm, we calculate excitation spectra of both coupled cavities and the Bose Hubbard model. We examine the Mott insulator to superfluid phase transition and monitor single-particle excitations and polariton-density excitations. We study both the phase transition with fixed polariton density and the transition with fixed chemical potential. Finite tempererature and detuning effects are discussed. The excitation spectra of coupled cavities and the Bose Hubbard model turn out to closely resemble each other. Bose Hubbard physics can therefore be investigated in coupled cavities.   

Polaritons and Pairing Phenomena in Bose-Hubbard Mixtures

Motivated by recent experiments on cold atomic gases in ultra high finesse optical cavities, we consider the two-band Bose-Hubbard model coupled to quantum light. Photoexcitation promotes carriers between the bands and we study the interplay between Mott insulating behaviour and superfluidity. The model displays a $U(1)\times U(1)$ symmetry which supports the coexistence of Mott insulating and superfluid phases,and yields a rich phase diagram with multicritical points. This symmetry is shared by several other problems of current experimental interest, including two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover for atom-molecule mixtures induced by a Feshbach resonance. We corroborate our findings by numerical simulations. [M. J. Bhaseen, M. Hohenadler, A. O. Silver, and B. D. Simons, Phys. Rev. Lett. \textbf{102}, 135301 (2009)]   

Pure Mott phases in confined ultra-cold atomic systems

We propose a novel scheme for confining atoms to optical lattices by engineering a spatially-inhomogeneous hopping matrix element in the Hubbard-model (HM) description, a situation we term off-diagonal confinement (ODC). We show, via an exact numerical solution of the boson HM with ODC, that this scheme possesses distinct advantages over the conventional method of confining atoms using an additional trapping potential, including the presence of incompressible Mott phases at commensurate filling and a phase diagram that is similar to the uniform HM. The experimental implementation of ODC will thus allow a more faithful realization of correlated phases of interest in cold atom experiments.   

Dynamical modulation of Optical lattices studied by a numerical investigation on response theory

The response to the dynamical modulation of optical lattice potentials is studied by Determinantal Quantum Monte Carlo combined with the Maximum Entropy method. We simulate three and two-dimensional repulsive fermionic Hubbard models within the strong coupling regime and near half-filling for a wide range of temperatures. We discuss the relation between the first and second order response \% to the dynamical modulation and the dynamical generated double occupancy and the relevance of bond order excitations near half-filling.   

Superflow instabilities of atomic fermion superfluids in an optical lattice

We study the superfluid phase of the one-band attractive Hubbard model as a prototype of a strongly correlated fermionic superfluid on a lattice. We characterize its collective mode and compute the sound velocity and ``roton'' gap within a generalized random phase approximation (GRPA). At strong coupling, we perform a spin wave analysis of the appropriate pseudospin model, with our GRPA results matching onto the spin wave results. With our two-pronged understanding of the collective mode, we examine breakdown of superfluidity due to imposed supercurrent. We find several mechanisms of superflow breakdown - depairing, Landau or dynamical instabilities. The most interesting is a charge modulation dynamical instability distinct from those previously studied in Bose superfluids. The associated charge order can be of two types: (i) a commensurate checkerboard modulation driven by softening of the roton mode at the Brillouin zone corner, or (ii) an incommensurate modulation arising from flow-induced finite-momentum pairing of Bogoliubov quasiparticles. We map out a dynamical phase diagram showing critical flow momentum of the leading instability, and point out implications for experiments on cold atom superfluids in an optical lattice.   

Unconventional Bose-Einstein condensation with Rashba coupling in optical lattices

We study the effect of Rashba spin-orbit coupling on the ground state properties of ultracold bosonic atoms in optical lattices. The Rashba coupling in the center-of-mass of the bosons is generated by spatially varying external laser fields which couple to the internal degrees of freedom of the atoms. As a result of the spin-orbit coupling, the ground state of the system acquires a finite quasi-momentum $\vec{k}_0$, which spontaneously breaks time-reversal symmetry. The Gross- Pitaevskii many-body ground state, the current density and the pseudo-spin density distributions are calculated in the high- particle-density superfluid regime, and time-of-flight calculations are carried out. In the low-particle-density regime, the phase diagram is computed showing the effect of the coupling on the Mott insulator-to-superfluid transition using a modified Bose-Hubbard model. We supplement this with the computation of the ground state of the system with a superimposed harmonic trap using a Gutzwiller ansatz, which shows the effect of the Rashba coupling on the wedding-cake structure of the system.   

Hubbard Model study of Off Diagonally Confined fermions in a 2D Optical Lattice

We report Quantum Monte Carlo simulations of a Hubbard Hamiltonian which incorporates a proposed new method for confining atoms in an optical lattice employing an inhomogeneous array of hopping matrix elements which trap atoms by going to zero at the lattice edges. This has been termed ``Off Diagonal Confinement (ODC)'' [1] to distinguish it from the more conventional use of a parabolic trap coupling to (diagonal) density operators. It has the advantage of producing systems which, while still being inhomogeneous, are entirely in the Mott phase, and allow simulations which are free of the sign problem at low temperatures. We analyze the effects of using ODC traps on the local density, density fluctuation, spin, and pairing correlation functions. Finally, we will discuss the advantages and importance of this new confinement technique for modeling correlated systems. Research supported by the Department of Energy, Office of Science SCIDAC program, DOE-DE-FC0206ER25793. [1] V.G. Rousseau {\em et al.}, arXiv:0909.3543   

Spontaneous interlayer superfluidity in bilayer systems of cold polar molecules

Quantum degenerate cold-atom gases provide a remarkable opportunity to study strongly interacting systems. Recent experimental progress in producing ultracold polar molecules with a net electric dipole moment opens up new possibilities to realize novel quantum phases governed by the long-range and anisotropic dipole-dipole interactions. In this work we predict the existence of experimentally observable novel broken-symmetry states with spontaneous interlayer coherence in cold polar molecules. These exotic states appear due to strong repulsive interlayer interactions and exhibit properties of superfluids, ferromagnets and excitonic condensates.   

Possible critical behavior driven by the confining potential in optical lattices with ultra-cold fermions

A recent paper [V.L. Campo {\it et al.}, {\it Phys. Rev. Lett.} {\bf 99}, 240403 (2007)] has proposed a two-parameter scaling method to determine the phase diagram of the fermionic Hubbard model from optical lattice experiments. Motivated by this proposal, we investigate in more detail the behavior of the ground-state energy per site as a function of trap size ($L$) and confining potential ($V(x)=t\vert x/L \vert^\alpha$) in the one-dimensional case. Using the BALDA-DFT method, we find signatures of critical behavior as $\alpha \to \infty$.   

Session Z32: Interactions and Thin Films

Long-range repulsive interaction induced Cs superlattices on graphene/SiC

The adsorption behavior of Cs on graphenes formed on 6H-SiC(0001) substrate has been investigated by low-temperature scanning tunneling microscopy. At low coverages ($<$0.032 nm$^{-2})$, individual Cs atoms absorb preferentially on distinct sites of the mori\'e pattern, which is formed by the first carbon buffer layer and underlying SiC substrate. At higher coverages ($>$0.33 nm$^{-2})$, short-range ordered structures are presented. Specially, when the coverage is appropriate, Cs atoms can spontaneously form two hexagonal superlattices with a lattice constant of 1.86 nm and 3.24 nm, respectively. By analyzing the coverage-dependent Cs-Cs interatom distance distributions, a long-range repulsive electrostatic interaction between Cs atoms is revealed. The occurrence of Cs superlattices results from the inhomogeneous surface potential on the few layer graphene and electrostatic repulsion between Cs atoms.   

Simulation tactic on dynamic cluster surface interaction

The wave function of time dependent Schrodinger Equation for few surface atoms is simulated to visualize the dynamics of external interaction detail in three dimensions. Instead of finding the solution of continuous variation, step consequence assembled from independent stable moments is used to approach a desired precision as close as possible. The function is plotted in a serious setting of time interval. The graphic distribution shows a similarity as the orbit structures during stable moments. In the computation, the singularity is artificially and logically avoided. Other factors such as spin are not yet considered. For modeling the dynamic interaction, external features such as particles or charges or radiation are integrated as an adjustable environment in the calculation route. Under these stimulations, the change of distribution can be seen and the effects of interaction are imitated roughly. Based on the obtained results, more subjects mainly on the exchanges between the surface states and the incoming factors are under investigated.   

Substrate mediated smooth growth of para-sexiphenyl on graphene

We report on the layer-by-layer growthof lying para-sexiphenyl (6P) molecules on metal supported graphene flakes. The formation of multilayers has been monitored in situ by means of LEEM. $\mu $-LEED has been used to reveal a bulk-like structure of the submonolayer, monolayer and multilayer regime. Graphene is a flexible, highly conductive and transparent electrode material, making it a promising technological substrate for organic semiconductors. 6P is a blue light emitting molecule with a high charge carrier mobility. The combination of an established deposition technique with the unique properties of organic semiconductors and graphene is an enabler for future flexible and cost efficient devices based on small conjugated molecules\textbf{.}   

Investigation of Stability of Single Mn Monolayers on and in w-GaN(000-1)

There has been much interest in dilute magnetic semiconductors involving GaN. Recently, it has become of interest to consider the possible advantages of delta-doped magnetic layers, rather than a random alloy. Here we investigate experimentally the growth of single Mn monolayers on top of GaN as well as the re-deposition of GaN on the Mn monolayer, using a combination of N-plasma molecular beam epitaxy and reflection high energy electron diffraction (RHEED). The single Mn monolayers form a novel rt3 x rt3 R-30deg structure.[1] Upon nitrogen plasma exposure, this periodicity is removed as seen in RHEED. However, even after heating to as high as 700 C, Auger electron spectroscopy shows very little change in the Mn peak intensity. Furthermore, a rapid reduction of Mn Auger electron peak intensity after only 3-5 GaN bilayers of redeposition is seen, showing that Mn is fully covered by the subsequent GaN layers. Therefore the Mn monlayer appears to be quite stable within the GaN (000-1) surface. This work has been supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No.0730257). Equipment support from ONR is also acknowledged. [1] Chinchore et al., Applied Physics Letters \textbf{93(18)}, 181908 (2008).   

Growth and Investigation of Mn delta-doped superlattices in w-GaN(000-1)

It is of great interest to form novel spintronic systems involving magnetic layers in semiconductor hosts. Recently, the possibility to form delta-doped magnetic layers in wurtzite GaN has been postulated theoretically [1]. In this work, we deposit single Mn monolayers on w-GaN(000-1) followed by a thin spacer layer of GaN, and then this process is repeated many times in order to form a superlattice of the form Mn/GaN/Mn/GaN/Mn? Samples having different GaN interlayer spacings and repetitions of 50 to 100 have been grown. Reflection high energy electron diffraction data acquired during deposition indicates good quality growth for many repetitions. Magnetic property measurements are currently in progress, and results will be presented. This work has been supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No.0730257). Equipment support from ONR is also acknowledged. \\[4pt] [1] Cui X.Y. et al, JAP 106, 043711 (2009)   

Out-of-plane nesting of spin spiral in ultrathin Fe/Cu(001) films revealed by SX-ARPES

We investigate the origin of the spin spiral (SS) state in epitaxial ultrathin Fe films on Cu(001) using soft x-ray (SX)-ARPES. Fe/Cu(001) films exhibit a SS, in contrast to the ferromagnetic bulk bcc Fe. We study the in-plane and out-of-plane Fermi surfaces (FSs) of the SS in 8 monolayer Fe/Cu(001) films. It was found that the SS is due to nested regions confined to out-of-plane FSs, which are drastically modified compared to in-plane FSs. From precise reciprocal space maps along $k_z$ in successive Brillouin zones, we identify the associated real space compressive strain of 1.5$\pm$0.5\% along {\it c}-axis. An autocorrelation analysis quantified the incommensurate ordering vector $\mathbf{q}$=(2$\pi/a$) (0,0,$\sim$0.86), favoring a SS. These results are consistent with magneto-optic Kerr effect [1] and surface x-ray diffraction experiments [2] on the Fe/Cu(001) films, and suggest the importance of in-plane and out-of-plane FS mapping for ultrathin films. [1] D.~Qian, {\it et al.}, Phys.~Rev.~Lett.~{\bf 87}, 227204 (2001). [2] H.~L.~Meyerheim, {\it et al.}, Phys.~Rev.~B {\bf 71}, 035409 (2005).   

Effective mass anomalies in strained silicon thin films

The most fundamental shape of nanostructures may be a slab or thin film. Semiconductor slabs sandwiched insulators are a most basic model for the channel region of modern devices such as multi-gate and SOI (silicon on insulator) MOSFET. The aim of this presentation is to make systematic investigation of the shape effect on the electronic structures in the semiconductor slabs using a density functional pseudoptentital method. Hydrogen terminated silicon thin films are used as a model of the slabs sandwiched by insulators. Adopted parameters are biaxial strain and crystal direction, as well as the thickness of the film. Among the calculated results, a remarkable feature is that the longitudinal effective mass component of the conduction band reveals anomaly on certain parameter lines in the $<110>$ and $<111>$ confinement cases. This anomaly is due to the confinement effect and lowering of the crystal symmetry by the strain. It is found that the confinement effect is semi-quantitatively explained by an extension of simple zero-point energy model using the first-principles k.p perturbation calculation. [1] J. Yamauchi, IEEE Electron Device Letters vol.29 186 (2008); J. Comp. Theor. Nanoscience (in press).   

Surface electronic structure of single-crystalline zirconium diboride thin films

Single-crystalline thin films of zirconium diboride (ZrB$_{2})$ with a simple crystal structure consisting of alternating hexagonal close-packed Zr and honeycomb B layers have been epitaxially grown on Si(111) by chemical vapor epitaxy. Oxide layers formed upon exposure to air can be removed by heating in ultra-high vacuum resulting in oxide-free and atomically-flat surfaces making the ZrB$_{2}$ films ideal for the epitaxial growth of heterostructures in other setups. The electronic structure of the as obtained ZrB$_{2}$(0001)-(2$\times $2) surface has been studied using angle-resolved ultraviolet photoelectron spectroscopy. Along the $\bar {\Gamma }\bar {M}$ direction two parabolic features in the vicinity of the Fermi level are clearly resolved. While the dispersion of these Zr-derived surface states is similar to those observed at (1x1) single crystal surfaces and calculated dispersion curves for a Zr-terminated slab model, a pronounced intensity change at the zone boundary is a strong indication of a back-folding of electronic bands into the reduced Brillouin zone. The origin of the (2$\times $2) reconstruction is likely the presence of Si atoms on the surface. A flat band at 0.25 eV is accordingly assigned to localized Si-derived states   

Elastic properties of polycrystalline aluminum and silver films down to 6 mK

Many mechanical resonators, from nanoscale beams to gravitational wave detectors, are coated with polycrystalline or amorphous films, and it is important to understand the contribution of the film to the elastic properties of the composite structure. We have made elastic measurements on high purity micron-thick polycrystalline aluminum and silver films with the double paddle resonator technique, using a single crystal silicon substrate with internal friction $Q^{-1}\approx 2\times10^{-8}$ below a few kelvin. We observed large $Q^{-1} \approx 10^{-4}$ in both films, indicating that these films can contribute substantially to the damping of mechanical resonators, even at very low temperatures. In aluminum, we also observed remarkable agreement between the relative change in sound speed $\delta v/v_0$ and $Q^{-1}$ of the aluminum and the predictions of the tunneling model for an amorphous superconductor well below $T_c$. This agreement might be due to tunneling of dislocation kinks in a modulated kink-Peierls potential. However, previous measurements on polycrystalline bulk metal and films have shown that other glassy properties such as the thermal conductivity and heat capacity are not in agreement with the tunneling model predictions.   

Effects of crystallinity in resistance switching behavior of epitaxial NiO films

We fabricated epitaxial NiO films (epi-NiO) on (100) SrRuO$_3$ (SRO) films at room temperature (NiO-RT), 500 $^{\circ}$C (NiO-500), and 700 $^{\circ}$C (NiO-700). Crystallinity of epi-NiO was characterized by X-ray diffraction spectra, which indicates that NiO grown at a higher temperature shows a better crystallinity. I-V properties and associated resistance switching (RS) are investigated by using Pt and SRO as top and bottom electrodes; NiO-RT and NiO-500 exhibit bipolar RS, while the RS phenomenon is not observed in NiO-700. Temperature dependence of initial I- V curves shows that pristine Pt/NiO-500 and Pt/NiO-700 are in an insulating and a metallic state, respectively. The Pt/epi-NiO interfaces are further investigated by transmission electron microscopy and its results will be presented. Our experimental results suggest that crystallinity of epi-NiO is a key parameter for bistability of oxygen states at the Pt/epi-NiO interfaces, which results in distinctive I-V characteristics and associated RS behavior. The implication of our work on the microscopic origin of general switching behavior in NiO will be discussed.   

Structural and Electronic Properties of Mn$_{x}$Ga$_{1-x }$Monolayers on Wurzite GaN(0001) Surface

Ferromagnetic (FM) metal/semiconductor bilayers are of great interest due to their importance in novel spintronics applications, such as spin injection and spin light-emitting diodes$^{[1]}$. It has been reported$^{[2]}$ that $\delta $-MnGa, a FM alloy with T$_{C}$ higher than room temperature (RT), can be grown epitaxial on top of w-GaN(0001) with sharp interface and controllable magnetism. Using molecular beam epitaxy, we deposit up to 3 monolayers (ML's) of Mn onto w-GaN(0001) ``1x1'' surface, which forms Mn$_{x}$Ga$_{1-x}$ with x varying from 0 to $\sim $ 0.6. Mn-induced surface reconstructions and formation of Mn$_{x}$Ga$_{1-x }$crystalline phases are observed by reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy as well as \textit{in-situ} RT-STM. The data suggests large-period reconstructions upon deposition of $<$ 0.25ML Mn and quick formation of $\delta $-MnGa at $\sim $1 ML of Mn. Structural and electronic properties at representative stages will be presented, as well as possible magnetic properties of MnGa ML's. This work has been supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No.0304314). Equipment support from ONR is also acknowledged. [1] S.A.Wolf \textit{et al}, Science \textbf{294}, 1488 (2001) [2] E.Lu \textit{et al}, Phys.Rev.Lett. \textbf{97}, 146101 (2006) K.K.Wang \textit{et al, M}ater.Res.Soc.Symp.Proc.1118-K06-06 (2009)   

Structural Characterization of $Y_{2}O_{3}$ Films Grown on Sapphire by MBE

Yttrium oxide is a hard, thermally stable, transparent oxide, making it an excellent host for rare earth ions used in solid state laser waveguides. The structural quality of the these thin films can be expected to affect the emission cross section of the active ions and losses in the waveguide due to absorption and scatter. $Y_{2}O_{3}$ films were deposited on A, M, R and C plane sapphire substrates by molecular beam epitaxy. X-ray diffraction and transmission electron microscopy measurements were performed in order to determine the structure of the films. The films exhibited a textured mosaic structure (111) oriented with all substrates. The grains were found to be twinned with strong in plane orientations on A,R and C plane substrates but randomly oriented on M-plane. Grain size and tilt were dependent on the orientation of the sapphire substrate. R-plane substrates gave the highest quality films with larger grains and less mosaic tilt. This is likely due to better lattice matching between the substrate and film.   

Session Z33: Open Quantum Systems and Decoherence

Quantum Darwinism in hazy environments

Quantum Darwinism provides an information-theoretic framework for the emergence of the classical world from the quantum substrate. It recognizes that we - the observers - acquire our information about the ``systems of interest'' indirectly from their imprints on the environment. Objectivity, a key property of the classical world, arises via the proliferation of redundant information into the environment where many observers can then intercept it and independently determine the state of the system. After a general introduction to this framework, we demonstrate how non-ideal initial states of the environment (e.g., mixed states) affect its ability to act as a communication channel for information about the system. The environment's capacity for transmitting information is directly related to its ability to increase its entropy. Therefore, environments that remain nearly invariant under the Hamiltonian dynamics, such as very mixed states, have a diminished ability to transmit information. However, despite this, the environment almost always redundantly transmits information about the system.   

Critical dynamics of decoherence

Quantum decoherence is the key to how the classical world emerges from the quantum substrate. Its understanding is also essential for creation of nanoscale devices that need long-time quantum coherence for their operation (e.g., quantum computers). Here we study for the first time how the non-equilibrium quench of the environment affects decoherence of the quantum system. Namely, we investigate decoherence of a central spin-1/2 surrounded by the environment that is driven through a quantum critical point. The quantum Ising model in the transverse field serves as the environment. Thus, we combine extensive studies of decoherence with rapidly growing field of dynamics of quantum phase transitions. We show that coherence of the central spin undergoes rapid decay that encodes critical exponents of the environment as well as exhibits certain periodicities that allow for identification of the central spin -- environment coupling and ground state fidelity. Our discussion is based on a remarkably simple analytical expression verified through numerical simulations.   

Limits of quantum speedup in photosynthetic light harvesting

It has been suggested that excitation transport in photosynthetic light harvesting complexes features speedups analogous to those found in quantum algorithms. Here we compare dynamics in these systems to quantum walks to elucidate the limits of such quantum speedups. For the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria, we show that while there is indeed speedup at short times, this is short lived (70 fs) despite longer lived (ps) quantum coherence. Remarkably, this time scale is independent of the details of the decoherence model. More generally, we show that the distinguishing features of light-harvesting complexes limit quantum speedup and cause even diffusive transport to be slowed. These results suggest that quantum coherent effects in biological systems are optimized for efficiency and robustness rather than for achieving the more elusive goal of quantum speedup.   

Decoherence and Disentanglement of Two Intereacting Qubits in the Presence of Random Telegraphic Noise

We have studied the dissipative dynamics of a pair of quits coupled via the exchange interaction in the presence of random telegraphic noise. We use a recently developed transfer-matrix formalism that is suitable for computing the temporal evolution of a quantum system affected by a classical stochastic process. For bipartite systems, the concurrence provides a measure of the entanglement between two qubits. We have calculated the concurrence as a function of the qubit working point, noise coupling strength, fluctuator rates and exchange interaction strengths. Sudden death of entanglement and its revival are seen to depend on various factors. We show that for certain cases, the exchange interaction between the qubits can be used to significantly slow down the decoherence process and maintain entanglement over much longer periods of time. This could be particularly promising for quantum computing applications.   

Two-Qubit Disentanglement and Decoherence from Classical Random Telegraph Noise

We consider the two-qubit disentanglement due to classical random telegraph noise where the qubits do not interact and have tunable working points. Using a new mathematical method that is suited to treat all working points, we show that entanglement sudden death and revival are dependent on several factors, such as qubit working point, noise coupling strength and initial state entanglement. For extended Werner states, the concurrence is related to the difference of two functions: one is related to dephasing and the other longitudinal relaxation.~ A physical interpretation based on a generalized Bloch sphere representation is given.   

Resonant regimes in the Fock-space coherence of non-equilibrium quantum dots

We investigate theoretically the real-time evolution of the coherence between discrete quantum states differing in particle number in the sequential transport regime. We find that such a Fock-space coherence can be established when at least one transport channel is available within a quantum-confined structure, and that the evolution of this coherence is decoupled from that of the occupation probabilities of the states, even in the presence of boson-mediated relaxation. Through a systematic analysis, we find quantum interference patterns producing highly resonant regimes where the Fock-space decoherence times are extended significantly, while no resonant regimes are found in the Hilbert-space coherence between states with equal particle numbers. We conclude that the dominant parameters yielding the resonances are the coupling anisotropy to different transport channels as well as the symmetry of the confining barriers.   

Phonon-induced decoherence in donor-based charge qubits

Solid-state based nanostructures have become in recent years promising candidates for the experimental realization of devices in which quantum information can be processed. In this work we study a particular kind of charge qubits in which the information is stored in the lowest orbital states of an electron shared by a pair dopant ions embedded in a silicon crystal. In particular, we investigate the phonon-driven decoherence and its dependence on temperature. The influence of the inter-ion distance on the decoherence process will also be discussed.   

Decoherence in Quantum Magnets: Theory and Experiment on T$_{2}$

The individual properties of molecular magnets are controlled by chemistry rather than nanoengineering, and are highly tunable. This makes them ideal candidates for solid-state qubits. However decoherence in many solid-state systems is anomalously high, and their advantages cannot be exploited until decoherence is understood and suppressed. In molecular magnets decoherence is caused by coupling to the nuclear spin bath, to phonons, and to each other via dipole-dipole and exchange interactions. Here we study decoherence in 2 different crystals of Fe8 nanomolecules, in several field orientations, both theoretically and experimentally. The experimental results for the decoherence time T$_{2}$ agree with the existing theory (Morello et al., Phys Rev Lett 97, 207206 (2006)). To our knowledge this is the first time that experimental decoherence rates agree with theory in magnetic systems.   

Decoherence Suppression of a Solid State Qubit by Uncollapsing

We show that the qubit decoherence due to zero-temperature energy relaxation can be almost completely suppressed by using the quantum uncollapsing procedure. To protect a qubit state, a partial quantum measurement moves it towards the ground state, where it is kept during the storage period, while the second partial measurement restores the initial state. This procedure preferentially selects the cases without energy decay events. Stronger decoherence suppression requires smaller selection probability; a desired point in this trade-off can be chosen by varying the measurement strength. The experimental verification of the uncollapsing procedure has already been performed and using our model we can explain the reported fidelity of this process. The decoherence suppression experiment can be realized in a straightforward way using the superconducting phase qubit, with a minor modification to the experiment that has already been performed.   

Non-Markovian evolution equation of two-time correlation functions of system operators: beyond the quantum regression theorem

Two-time (multiple-time) correlation functions (CF's) are important quantities that can provide more significant information about the system, which the single-time expectation values cannot. In the Markovian open systems, an extremely useful procedure to calculate the two-time (multiple-time) CF's of the system operators is the quantum regression theorem (QRT) that gives a direct relation between the time evolution equation of the single-time expectation values and that of their corresponding two-time (multiple-time) CF's. For the non-Markovian case, it is known that the QRT is not valid in general. Here we present, valid to second order in the system-environment interaction Hamiltonian, a non-Markovian evolution equation of two-time CF's of the system operators at finite environment temperatures for both Hermitian and non-Hermitian system coupling operators and for any initially separable system-environment state (pure or mixed). We then apply the non-Markovian evolution equation to a simple problem of a two-level system coupled to a bosonic environment, as well as to a problem of an exactly solvable pure-dephasing model. The presented evolution equation that generalizes the QRT to the non-Markovian finite-temperature case will have applications in many different branches of physics.   

Decoherence Induced Spontaneous Symmetry Breaking

We study time dependence of exchange symmetry properties of Bell states when two qubits interact with local baths having identical parameters. In case of classical noise, we consider a decoherence Hamiltonian which is invariant under swapping the first and second qubits. We find that as the system evolves in time, two of the three symmetric Bell states preserve their qubit exchange symmetry with unit probability, whereas the symmetry of the remaining state survives with a maximum probability of $ 0.5 $ at the asymptotic limit. Next, we examine the exchange symmetry properties of the same states under local, quantum mechanical noise which is modeled by two identical spin baths. Results turn out to be very similar to the classical case. We identify decoherence as the main mechanism leading to breaking of qubit exchange symmetry. [1] G. Karpat and Z. Gedik, Optics Communications \textbf{282}, 4460 (2009).   

Linear Assignment Maps for Correlated System-Environment States

An assignment map is a mathematical operator that describes initial system-environment states for open quantum systems. We reexamine the notion of assignments, introduced by Pechukas, and show the conditions assignments can account for correlations between the system and the environment, concluding that assignment maps can be made linear at the expense of positivity or consistency is more reasonable. We study the role of other conditions, such as consistency and positivity of the map, and show the effects of relaxing these. Finally, we establish a connection between the violation of positivity of linear assignments and the no-broadcasting theorem.   

Session Z34: Focus Session: Frustrated and Low-D Magnetism -- Quantum Magnetism III

Hall Effect of Spin Waves in Frustrated Magnets

Spin transport phenomena have been attracting much interest in connection with applications to spintronics as well as their fundamental relation with the notion of topologically-induced spin currents. The spin Hall effect in semiconductors and metals, which has been studied extensively, allows for topological interpretation in terms of the Berry phase, and in this sense, it has an origin similar to that of the intrinsic anomalous Hall effect of charge currents. On the other hand, it was shown by several authors that the topological Berry phase is also raised by spin textures, and the Hall effect occurs in itinerant electron systems coupled with a nontrivial spin texture. Here, we propose a possible analogue of this phenomenon for localized spin systems with no charge degrees of freedom. In this scenario, a longitudinal magnetic field gradient induces a spin Hall current carried by spin wave excitations; i.e. Hall effect of spin waves involving no charge degrees of freedom. Our argument is based on a semiclassical analysis of spin dynamics taking into account topological Berry-phase effects. We also present a realistic miscroscopic model of a frustrated magnet with non-coplanar order which exhibits the Hall effect of spin waves.   

Spin chirality and thermal Hall effect in quantum magnets

We theoretically study the thermal Hall effect in insulating quantum magnets [1]. In contrast to itinerant magnets, there are no charge degrees of freedom in the localized spin systems and hence the heat current is totally carried by charge-neutral objects such as magnons and spinons. We consider the effect of the coupling between the scalar chirality and external magnetic fields or the effect of the Dzyaloshinskii-Moriya interaction which is related to the vector spin chirality, and find two distinct classes of thermal Hall responses. For ordered magnets, the intrinsic thermal Hall effect for magnons arises if the lattice geometry and the magnetic order satisfy certain conditions. A TKNN-type formula for the thermal Hall conductivity is also obtained. For a spin liquid, the thermal Hall effect for spinons due to the ``Lorentz force'' is expected if the spinons are deconfined. These results offer a new experimental method to study the ground state and low energy excitations in quantum magnets using thermal transport measurements. [1] H. Katsura, N. Nagaosa, and P. A. Lee, arXiv:0904.3427[cond-mat.str-el].   

Entanglement spectrum of a topological phase in one dimension

We propose a scheme to classify gapped phases of one dimensional systems in terms of properties of the entanglement spectrum. We show that the Haldane phase of $S=1$ chains is characterized by a double degeneracy of the entanglement spectrum which is protected by any one of the following three symmetries: (i) the dihedral group of $\pi$-rotations about $x,y$ and $z$ axes; (ii) time-reversal symmetry $S^{x,y,z} \rightarrow - S^{x,y,z}$; (iii) link inversion symmetry. The degeneracy cannot be lifted unless either a phase boundary to another, ``topologically trivial'', phase is crossed, or the symmetry is broken. Physically, the degeneracy of the entanglement spectrum can be observed by adiabatically weakening a bond to zero, which leaves the two disconnected halves of the system in a finitely entangled state.   

Spin Liquid Phase in the Hubbard Model on the Honeycomb Lattice

Using projective (${T=0}$) quantum Monte Carlo simulations, we investigate the ground-state properties of the half-filled Hubbard model on the honeycomb lattice. We provide evidence for a gapped phase separating the weak-coupling semi-metal and the antiferromagnetically ordered phase at strong coupling. Exploring quasi-particle and spin excitation gaps, flux quantization as well as probing for various correlation functions, we conclude that in this intermediate interaction region the system exhibits no long-range magnetic or bond-order nor superconductivity. Several proposals on novel phases in related models have been put forward, whereas our simulations establish a spin liquid - even in the absence of magnetic frustration.   

Spin Bose-Metal phase in a spin-1/2 model with ring exchange on ladders

I will discuss recent developments in the study of a 2D quantum phase of strongly correlated spins, the Spin Bose-Metal (SBM), a spin liquid characterized by gapless excitations residing on surfaces in momentum space (i.e. ``Bose surfaces''). Thus far, significant progress has been made by considering a triangular lattice Heisenberg model with a four-site ring exchange term on a 2-leg strip (see [1]), where quasi-1D signatures of the parent 2D phase can be detected (i.e. ``Bose points''). The ladder systems have provided a fruitful scaffolding for the implementation of the quasi-exact numerical method DMRG, as well as a theoretical approach via slave particles and Bosonization. To test the theory numerically, variational Monte Carlo (VMC) is employed with Gutzwiller projected products of Slater determinants as a direct comparison with DMRG results. Here, I will present new results for 3- and 4-leg ladders as we continue to drive towards two dimensions where this phase is potentially relevant in the study of organic Mott insulators near the metal-insulator transition. Indeed, we will offer evidence that the phase diagram of the 4-leg triangular ladder contains a Spin Bose-Metal phase. [1] D. N. Sheng \emph{et al}., Phys. Rev. B {\bf 79}, 205112 (2009).   

$d$-wave correlated Bose liquid phases on multi-leg ladders with ring exchange

We discuss recent progress on the study of ladder descendants of a novel two-dimensional quantum phase of bosons moving on the square lattice which is characterized by singular surfaces in momentum space, namely the $d$-wave correlated Bose liquid (DBL). Using a combination of numerics (e.g., density matrix renormalization group, variational Monte Carlo, and exact diagonalization) and analytics (e.g., bosonization of a compact U(1) lattice gauge theory) we explore the existence and stability of ladder analogs of the DBL on $N$-leg ladders, with $N\ge3$, in the context of a model of itinerant hard-core bosons with frustrating four-site ring exchange. As in the case of $N=2$, see [1], we find numerical evidence for various strong-coupling DBL phases which can rather remarkably be understood within a slave-fermion picture in which the boson wave function is written as a product of two Slater determinants. The additional features and difficulties associated with taking $N>2$ will be addressed. The boson ring model we consider has potential physical realizations in the contexts of low-dimensional frustrated quantum magnets and in ultracold quantum gases. [1] D. N. Sheng \emph{et al.}, Phys. Rev. B {\bf 78}, 054520 (2008).   

Frustrated Orbital Exchange Models in p-band Mott Insulators

We investigate the general structure of orbital exchange physics in Mott-insulating states of p-orbital systems. Orbital orders occur in both the triangular and kagome lattices. In contrast, orbital exchange in the honeycomb lattice is frustrated as described by a novel quantum 120$^{\circ}$ model. Its classical ground states are mapped into configurations of the fully packed loop model with an extra U(1) rotation degree of freedom. Quantum orbital fluctuations select a six-site plaquette ground state ordering pattern in the semiclassical limit from the ``order from disorder'' mechanism. This effect arises from the appearance of a zero energy flat band of orbital excitations.   

A quantum liquid with deconfined fractional excitations in three dimensions

Excitations which carry ``fractional'' quantum numbers are known to exist in one dimension in polyacetylene, and in two dimensions, in the fractional quantum Hall effect. Fractional excitations have also been invoked to explain the breakdown of the conventional theory of metals in a wide range of three-dimensional materials. However the existence of fractional excitations in three dimensions remains highly controversial. Here we report direct numerical evidence for the existence of an extended quantum liquid phase supporting fractional excitations in a concrete, three-dimensional microscopic model --- the quantum dimer model on a diamond lattice [1]. We demonstrate explicitly that the energy cost of separating fractional monomer excitations vanishes in this liquid phase, and that its energy spectrum matches that of the Coulomb phase in $(3+1)$ dimensional quantum electrodynamics [2,3]. \\[4pt] [1] O. Sikora {\it et al.} arXiv:0901.1057v3 --- to appear in Phys. Rev. Lett. \\[0pt] [2] R. Moessner and S.L. Sondhi, Phys. Rev. B {\bf 68}, 184512 (2003).\\[0pt] [3] D.L. Bergman {\it et al.} Phys. Rev. B {\bf 73}, 134402 (2006).   

projective construction of spin nematic states in S=1/2 frustrated ferromagnets

An $SU(2)$ slave-boson formulation of bond-type spin nematic orders is developed in the context of quantum frustrated ferromagnets, where the spin nematic states are described as the resonating spin-triplet valence bond (RVB) states. Namely, the $d$-vector of the spin-triplet pairing ansatz plays the role of the so-called `director' in the spin nematic states. The low-energy excitations around such bond-type spin quadrupolar orders generally comprise the gauge boson, massless goldstone bosons, spinon individual excitations and their composites. Using the projective symmetry-group arguments, we will argue how to identify the number of massless gauge bosons. Applying this formulation, we will next enumerate possible `mixed' RVB ansatzes in the $S=\frac{1}{2}$ $J_1$-$J_2$ Heisenberg model on the square lattice ($J_1$ ferromagnetic nearest neighbor and $J_2$ antiferromagnetic next nearest neighbor), and argue their stability against gauge fluctuations. As a result, we found two stable ansatzes in the intermediate coupling region, $J_1:J_2=1:0.4$. One is the $Z_2$ `Balian-Werthamer (BW)' state stabilized by the Higgs mechanism. The other is the $SU(2)$ `chiral $p$-wave' state, where the massless gauge fluctuations are controlled by the Chern-Simon mechanism. Especially, the former $Z_2$ state exhibits the same spatial configuration of the spin quadrupolar moment as found in the previous exact diagonalization studies.   

Competing quantum paramagnetic ground states of the Heisenberg antiferromagnet on the star lattice

We investigate various competing paramagnetic ground states of the Heisenberg antiferromagnet on the two dimensional star lattice which exhibits geometric frustration. Using slave particle mean field theory combined with a projective symmetry group analysis, we examine a variety of candidate spin liquid states on this lattice, including chiral spin liquids, spin liquids with Fermi surfaces of spinons, and nematic spin liquids which break lattice rotational symmetry. Motivated by connection to large-N SU(N) theory as well as numerical exact diagonalization studies, we also examine various valence bond solid (VBS) states on this lattice. Based on a study of energetics using Gutzwiller projected states, we find that a fully gapped spin liquid state is the lowest energy spin liquid candidate for this model. We also find, from a study of energetics using Gutzwiller projected wave functions and bond operator approaches, that this spin liquid is unstable towards two different VBS states --- a VBS state which respects all the Hamitonian symmetries and a VBS state which exhibits $\sqrt{3}\times\sqrt{3}$ order --- depending on the ratio of the Heisenberg exchange couplings on the two inequivalent bonds of the lattice. We compute the triplon dispersion in both VBS states within the bond operator approach and discuss possible implications of our work for future experiments on candidate materials.   

Fermionic mean field theory for arbitrary spin and Spin-1 algebraic spin liquid on triangle lattice

We generalized the fermionic representation for Heisenberg model with spin-1/2 to arbitrary spin. The particle-hole symmetry for spin-1/2 Hilbert space is absent for $S\geq1$. We find a Lagrangian for Heisenberg model with spin-1 or spin-3/2 with restored particle-hole symmetry and study the corresponding mean fields. The excitation spectrum is gapped for the former and gapless for the latter, which is consistent with Haldane's conjecture. We also study the magnetic insulator $\mathrm{NiGa_2S_4}$ by applying the fermionic mean field theory to the spin-1 $J_1$-$J_3$-$K$ model $H=\sum_{\langle i,j\rangle} \left[J_1\mathbf S_i\cdot\mathbf S_j+K(\mathbf S_i\cdot\mathbf S_j)^2\right]+ J_3\sum_{[ i,j']}\mathbf S_i\cdot \mathbf S_{j'}$. We find two spin liquid solutions with gapless spinon excitations, which has never been discussed in literature to our knowledge. We assume that the ground state is in one of these spin liquid phases, then the gapless excitation spectrum explains the $T^2$ law of the specific heat at low temperature. The susceptibility calculated from the mean field theory is linear in T at low temperature. We attribute the experimentally observed nonzero susceptibility at zero temperature and the partially spin freezing below $T_f$ to the defects such as impurities or surface effects.   

Functional renormalization group study of charge and spin orders of the extended Hubbard model in frustrated lattices

In order to study for novel spin and charge orders in strongly correlated electron systems in frustrated lattices,we investigated extended Hubbard model in 2 dimensional (2D) checkerboard and triangular lattices using the functional renormalization group method(fRG) with an improved algorithm [1]. In this method, both the quantum effect and the geometrical frustration effect at finite temperature are taken into account properly. Non-monotonic temperature dependence of the spin susceptibility was observed both in the models . In a 2D isotropic triangular lattice at half-filling, divergence of the particle-particle channel vertex functions was observed in a region of the intermediate value of the on-site Coulomb interaction. We have also investigated the extended Hubbard model with long-range Coulomb interactions. A possibility of the ferromagnetic order and calculations with including the self-energy correction will be introduced .[1] H. Takashima, R. Arita, K. Kuroki, and H. Aoki, J. Phys.: Conf. Ser, \textbf{150, }052261 (2009)   

Phase diagram of the SU(N) Hubbard-Heisenberg model on the honeycomb lattice

We present a projective ${T=0}$ quantum Monte-Carlo study of the ground-state properties of the the SU($N$) Hubbard Heisenberg model on hexagonal lattices up to ${18 \times 18}$ unit cells. The phase diagram is mapped out for increasing many body correlations from the large-$N$ limit to SU(2) for even $N$. It is shown that for all SU($N$) symmetric realizations, the model undergoes a quantum phase transition from a semimetal to an insulator for large values of $J/t$. While for ${N \ge 4}$ the insulating state is a valence bond crystal the SU(2) Hubbard Heisenberg model exhibits a gapped short range resonating valence bond phase separating the semimetal and an antiferromagnetic insulator. At SU(2) we reproduce the intermediate spin liquid phase found recently in the SU(2) Hubbard model on the hexagonal lattice.   

Quantum phase transition of the Hubbard model on a honeycomb lattice

We consider the Hubbard model on a honeycomb lattice at zero temperature. Within the cellular dynamical mean-field theory we study the quantum phase transition in the system. The antiferromagnetic transition, which is driven by the increase of the local interaction, is demonstrated by the staggered magnetization. We also examine the spectral properties of the system. The results are discussed in comparison with earlier works.   

Session Z35: Focus Session: Spins in Semiconductors -- DMS: II-VI and Group IV

Photophysical Properties of Colloidal Mn(II)-Doped CdSe Nanoparticles: Exchange Fields, Exciton Storage, and Light-Induced Spontaneous Magnetization

An attractive approach to controlling spin effects in semiconductor nanostructures for applications in electronics is to use light to generate, manipulate, or read out spins. The main focus of this presentation will be on the recent demonstration of spontaneous photoinduced polarization of Mn(II) spins in doped colloidal CdSe quantum dots, an effect due to the formation of excitonic magnetic polarons. Photoexcitation generates large dopant-carrier exchange fields, enhanced by strong spatial confinement, that lead to giant Zeeman splittings of the semiconductor band structure in the absence of applied magnetic fields. These internal exchange fields allow spontaneous magnetic saturation of the Mn(II) spins to be achieved at zero external magnetic field up to ca. 50 K, and photomagnetic effects are observed all the way up to room temperature. The factors that allow this fascinating effect to be observed in colloidal Mn(II)-doped CdSe nanoparticles will be discussed. Relevant Publications: 1) Beaulac, Schneider, Archer, Bacher, and Gamelin. Science, 325, 973 (2009) 2) Beaulac, Archer, Ochsenbein, and Gamelin, Adv. Funct. Mat., 18, 3873 (2008)   

Polarized Magneto-Photoluminescence from Mn-doped ZnSe/CdSe Core/Shell Nanocrystals

We study the low temperature magneto-optical properties of Mn-doped ZnSe/CdSe core/shell nanocrystals using magnetic circular dichroism (MCD) and circularly polarized luminescence (PL) as a function of magnetic field. MCD studies reveal giant field- and temperature-dependent Zeeman splittings of the band-edge exciton, demonstrating a strong \textit{sp-d} exchange coupling of electrons and holes to the embedded paramagnetic Mn atoms [1]. Magneto-PL studies surprisingly reveal a strongly circularly polarized PL from internal Mn transitions at $\sim $2.15 eV with applied magnetic fields, which follows the same field- and temperature-dependent (Brillouin-like) magnetization of the Mn spins. Notably, the intensity of the right- and left-circularly polarized Mn PL increases and decreases with applied field, respectively, in strong contrast to similar studies in bulk ZnMnSe and in ZnCdMnSe quantum wells. We discuss the effects of strong quantum confinement on coupling between spin-polarized excitons and the local Mn spins. [1] D. A. Bussian \textit{et al.}, Nature Materials \textbf{8}, 35 (2009).   

Magnetism and Carrier Spin Polarization in Mn-doped CdSe Quantum Dots

We report the magnetic and magneto-optical properties of Mn2+ doped CdSe nanoparticles synthesized by hot colloidal solution method. Magnetic hysteresis measurements on a particle ensemble show that they are paramagnetic at room temperature, and become ferromagnetic below about 50 K. The coercivity reaches to about 0.4 Tesla at 6K. The carrier spin polarization has been investigated by circularly polarized photoluminescence. Positive circular polarization of the PL of 30{\%} at 7K has been observed. This is due to the excitonic Zeeman splitting resulting from the strong sp-d exchange interactions between the carriers and Mn dopants. The circular polarization has been investigated as a function of applied magnetic field in the 7-100 K temperature range.   

Magneto-PL Measurements of (Zn,Mn)Se Nanowires and Residual Nanostructures

Magnetic semiconductor nanowires (NWs) potentially provide model systems for studying spin polarized 1D Fermi liquids [Nano Letters {\bf 9}, 3142 (2009)]. Here, we report low temperature magneto-photoluminescence measurements of (Zn,Mn)Se NWs grown using a two-stage vapor-liquid-solid process [e.g. Appl. Phys. Lett {\bf 93}, 143106 (2008)] that yields defect-free NWs in the 1D regime. Far-field photoluminescence (PL) measurements of as-grown samples show near band edge emission as well as a broad defect band convolved with Mn emission. In a magnetic field, we observe significant Zeeman shifts in the band edge luminescence at low temperatures, indicating the presence of strong sp-d exchange. Micro-PL measurements of as-grown samples and dispersed NWs map out the magnetic field variation of spatially resolved spectra at submicron length scales, revealing spatially localized emitters with both spectrally sharp features near the band edge as well as broad defect PL. We observe blinking and spectral diffusion in the sharper spectral features. We attribute the observed PL spectra to both (Zn,Mn)Se NWs and residual (Zn,Mn)Se nano-crystallites nucleated during the NW growth process. Supported by NSF.   

Optical Studies of Single Zn$_{1-x}$Mn$_x$Se Nanowires Synthesized by Chemical Vapor Deposition

Magnetic semiconductor nano-filaments provide interesting model systems for fundamental studies of quasi-one-dimensional spintronics [1]. Here, we report the growth and characterization of single crystal Zn$_{1-x}$Mn$_x$Se nanowires (NWs) and nanobelts fabricated on Si and quartz substrates via the vapor-solid-liquid mechanism during chemical vapor deposition. We obtain NWs that are tens of $\mu$m in length, with diameters in the range 40 nm$-$100 nm. The Mn concentration can be varied over the range $0 \leq x \leq 0.5$ by controlling the substrate temperature. We carry out Raman and photoluminescence (PL) measurements on single NWs supported over the holes of a transmission electron microscope (TEM) grid. This allows us to directly correlate optical properties with structural characteristics of the NWs obtained using TEM. Room temperature micro-Raman measurements on single NWs probe the phonon modes, while low temperature PL spectra show clear evidence for the substitutional incorporation of Mn into the ZnSe lattice. This work is supported by NSF-MRSEC.   

Magnetic Polarons in Anisotropic Quantum Dots

Tunability of confinement in magnetically-doped quantum dots (QDs) allows to tailor magnetism to an extent not available in bulk semiconductors. Versatile control of magnetic ordering, along with piezomagnetism, has been predicted even at a fixed number of carriers [1]. Recent experiments on colloidal QDs revealed strongly bound magnetic polarons (MPs) [2]. Previous studies of MPs in bulk semiconductors showed that the mean-field theory predicts a spurious magnetic phase transition, which is removed by taking into account spin fluctuations [3]. Here we present our theoretical results for MPs forming in QDs with pronounced magnetic anisotropy, which influences the spin fluctuations. We apply our findings to explain some peculiarities of the magnetic behavior of type-II ZnSe/(Zn,Mn)Te QDs, where magnetic polarons are found to persist to at least 200K [4]. Supported by ONR, AFOSR, and NSF-ECCS CAREER. \\[4pt] [1] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. 101, 207202 (2008); I. Zutic and A. G. Petukhov, Nature Mater.4, 623 (2009). \\[0pt] [2] R. Beaulac et al., Science 325, 973 (2009). \\[0pt] [3] T. Dietl and J. Spalek, Phys. Rev. Lett. 48, 355 (1982). \\[0pt] [4] I. R. Sellers, R. Oszwaldowski, et al., preprint; I. R. Sellers et al., Phys. Rev. Lett. 100, 136405 (2008).   

Magnetic Polarons in type-II (Zn,Mn)Te columnar quantum dots

We present the results of a time-resolved photoluminescence (TRPL) study of type-II (Zn,Mn)Te/ZnSe quantum dots. The sample consists of 5 layers of (Zn,Mn)Te QDs separated by ZnSe spacers. We observe magnetic ordering in these QDs through measurements of the peak energy (E) of the PL vs. time. The large red shift with time ($\sim $ 40 meV at low temperatures) is attributed to the formation of magnetic polarons (MPs) in the QDs, induced by the exchange interaction between the spins of photoexcited holes and those of Mn within the QDs. The MPs are detected at temperatures up to $\sim $ 200 K, with a binding energy that is very weakly dependent on temperature. We find two separate time scales for all temperatures. The shorter time (0.7 ns) is assigned to the MP formation while the origin of the longer time (11 ns) is not well understood [1]. The TRPL measurements on a control ZnTe QD sample showed a much different behavior (initial blue shift of E over about 20 ns followed by a very gradual red shift). We also present magnetization measurements of these two samples. [1] I. R. Sellers et al. pre-print.   

Single Fluorine Impurities in ZnSe: Magnetospectroscopy and Spin Qubit Applications

We report on the optical detection and investigation of single donors in ZnSe. By isolating the donors in quantum well mesas, we are able to probe them individually using magnetospectroscopy in both Voigt and Faraday geometry. The structure of interest is the electron bound to a F$^{19}$ neutral donor, which has been proposed as a strong candidate for a semiconductor-based qubit. The donor electron is optically accessible through the bound exciton transition, allowing the possibility of ultrafast optical spin control and detection. We present our recent spectroscopic and $g^2(0)$ experimental results and discuss their spin qubit applications.   

Diluted magnetic semiconductor quantum wells: disorder and electron-electron interaction

Using an equation of motion approach for the current-current response function, we develop a theory of electron transport in diluted magnetic semiconductor (DMS) quantum wells that treats disorder and electron-electron interaction on the same footing. A first principle treatment of disorder is implemented which goes beyond the simple relaxation time approximation. Interactions within the electron liquid including correlation effects and collective excitations are accounted for through the methods of time-dependent density functional theory. We present results for transport and optical properties as well as charge and spin collective modes in DMS quantum wells and discuss the influence of charge and spin disorder and electronic many-body effects.   

Chemical trend of exchange coupling in II-VI diluted magnetic semiconductors

We present an ab-initio study of the magnetic couplings in Mn- and Co-doped II-VI DMS ZnA (A=O,S,Se,Te). We show the necessity of taking into account the strong electron correlation on the transition metal (TM) 3d level to reproduce correctly the experimental chemical trend. Within the LSDA+U (local spin density approximation with a Hubbard-type correction to TM 3d electrons), we find (i) the d-d exchange couplings between nearest-neighbor magnetic ions to be antiferromagnetic (AFM) of the order of -1 meV and (ii) the sp-d exchange constants between magnetic ions and conduction (valence) band electrons (holes) N$\alpha $ (N$\beta )$ to be FM (AFM) of the order of 0.1 eV (-1 eV). In ZnMnO and ZnCoO, the strong p-d hybridisation leads to the presence of a bound state above the valence band, the failure of the commonly-used Larson perturbation theory formulae for p-d and d-d exchange interactions [1] and prevents high-Tc ferromagnetism [2]. \newline [1] B. Larson \textit{et al. }, PRB 37, 4137 (1988) \newline [2] T. Chanier \textit{et al.} , PRB 79, 205204 (2009)   

Studies of the (013) HgTe/Cd$_x$Hg$_{1-x}$Te heterostructure in tilted high magnetic fields

Properties of 2D carriers in the symmetrically doped Cd$_x$Hg$_ {1-x}$Te/HgTe/Cd$_x$Hg$_{1-x}$Te heterostructure with the quantum well thickness of 20 nm, carrier density n = 1.6 $\times$10$^{11}$\,cm$^{-2}$, and mobility $\mu$ = 28 $m^2/Vs$ were studied in tilted magnetic fields of up to 18 T at temperature 0.6 K. The heterostructure was grown on a (013) surface of a GaAs wafer as it was expected that the quantum well quality might be better than if created on a customary (001) surface. Coincidence of quantum levels in the range of SdH oscillations at $\nu$ = 4 and 6 was observed at the tilt angle values of about 67, 78, and 83$^0$. Thus, m*g*/m$_0 $=0.786 and if the effective mass m*/m$_0$ = 0.024, the effective g-factor g*=33 in agreement with the value obtained on the (001) oriented HgTe/Cd$_x$Hg$_{1-x}$Te wells. However, in the quantum Hall regime at $\nu$ = 3 maximum of the magnetoresistance does not occur at the corresponding critical angle of 78$^0$.   

Amorphous Magnetic Ge(1-x)Mn(x) Thin Films Grown by MBE

We explored the properties of Mn doped magnetic Group IV semiconductors with the ultimate goal of providing a new structure for logic switches that have extremely low bit switching energy. Precipitate-free amorphous Ge(1-x)Mn(x) thin films have been prepared by co-depositing Ge and Mn on SiO(2)/Si using a Molecular Beam Epitaxy (MBE) system. We varied the growth temperature and Mn doping concentration (2.8{\%}, 10.9{\%} and 21.3{\%}) in order to achieve the optimal magnetic properties. The ferromagnetic saturation moments were found to increase with Mn concentration with a maximum of 0.7 Bohr magnetron per Mn in the as-grown samples. Similar to MBE-grown single crystalline and implanted amorphous GeMn, two magnetic transition temperatures around 15 K and 200 K were observed in these amorphous MBE-grown samples. The Anomalous Hall Effect (AHE) persisted up to 200 K and disappeared together with the magnetism, which confirmed the strong correlation between the magnetization and transport properties and indicated the presence of substitutional Mn ions dispersed in the Ge host. In addition, negative magnetoresistance (MR) was detected from 5K to room temperature.   

Modulated spinodal decomposition in (Ge,Mn) films grown on GaAs(001)

The field of ferromagnetic semiconductors evolves very fast nowadays for their potential use in spintronic devices. Up to now, efforts have mainly focused on Diluted Magnetic Semiconductors but Curie temperatures in these materials still remain modest. One possible route to increase at least locally transition temperatures is to use spinodal decomposition leading to transition metal-rich high T$_{C}$ nanostructures. We focus here on (Ge,Mn) considered as a model system for spinodal decomposition and compatible with Si-based microelectronics. While the growth of (Ge,Mn) films on Ge substrates leads systematically to Mn-rich self-assembled nanocolumns exhibiting high T$_{C}$, we demonstrate the fine control of spinodal decomposition in (Ge,Mn) films grown on GaAs. Using different surface preparations, we clearly identify the role of surface morphology and impurity diffusion from the substrate (Ga or As) on the nanocolumns growth and the electrical properties (MR and AHE).   

Many-body effects in the cyclotron resonance of few-electron quantum dots doped with a single magnetic impurity

The magneto-optical absorption spectrum of a II-VI cadmium telluride based quantum dot containing few electrons ($N_e=1\div5$) doped with a single magnetic impurity ($Mn^{2+}$) is studied in the presence of a magnetic field. The strongly correlated electrons interact with the magnetic ion (Mn-ion) through the spin-spin exchange interaction which 1) competes with the Zeeman splitting energies leading to the existence of different magnetic phases, 2) results in the coupling of the electron center-of-mass motion with the relative motions leading to significant changes in the cyclotron resonance spectrum as compared to the case without a Mn-ion. At the ferromagnetic-antiferromagnetic transition: 1) the ground-state energy exhibits a cusp, 2) the cyclotron resonance energies exhibit a shift, 3) the oscillator strengths are discontinuous, and 4) the number of allowed transitions increases. The cyclotron resonance spectra are obtained which are quantitative and qualitative different for different $N_e$ due to the breakdown of Kohn's theorem. The results are dependent on the position of the Mn-ion inside the quantum dot. \begin{flushleft} \small{Nga T. T. Nguyen and F. M. Peeters, Phys. Rev. B \textbf{78}, 045321 (2008); \textbf{78}, 245311 (2008); \textbf{80}, 115335 (2009).} \end{flushleft}   

Session Z36: Focus Session: Bulk Properties of Complex Oxides -- 4d and 5d Systems

Giant Magneto-electric Effect in the J$_{eff}$ = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Mott Insulator Sr$_{2}$IrO$_{4}$

Our magnetic, electrical, and thermal measurements on single-crystals of the J$_{eff}$ = 1/2 Mott insulator, Sr$_{2}$IrO$_{4}$, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state [1] whose signatures are: (1) a strongly enhanced electric permittivity that peaks near a newly observed magnetic anomaly at 100 K, and (2) a large ($\sim $100{\%}) magneto-dielectric shift that occurs near a metamagnetic transition. The GME hinges on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted. \\[4pt] [1] S. Chikara, et al, Phys. Rev. B 80, 140407 (R) (2009)   

Evolution of Physical Properties with Mn Content in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ Single Crystals

We have systematically studied the doping dependence of physical properties of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7 }$with 0.0$\le $x$\le $1.0. Although the undoped Sr$_{3}$Ru$_{2}$O$_{7}$ is a paramagnetic metal, partial substitution of Ru by Mn results not only in metal-insulating transition (T$_{MIT})$ but also in complex magnetic ordering at T$_{M}$. Interestingly, the difference between T$_{M}$ and T$_{MIT}$ (T$_{M}<$T$_{MIT})$ increases with increasing doping level x. The correlation between these two transitions will be reported.   

Continuous metal-insulator transition at 410 K of the antiferromagnetic perovskite NaOsO$_3$

Newly synthesized perovskite NaOsO$_3$ shows a Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low temperature limit is absent, indicating that the band gap fully opens. $In situ$ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role in the gap opening. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This research was supported in part by the WPI Initiative on Materials Nanoarchitectonics from MEXT, Japan, and the Grants-in-Aid for Scientific Research (20360012) from JSPS.   

Pressure-Induced Insulating State in Ba$_{1-x}$RE$_{x}$IrO$_{3}$ (RE = Gd, Eu) Single Crystals

BaIrO$_{3}$ is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure ($\le $ 12.1 kbar) readily restores an insulating state characterized by a three-order-of-magnitude increase of resistivity. A pressure-induced insulating state is not common, and has never been observed in 5d-electron materials. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO$_{3}$.   

Unusual interlayer magnetic coupling in quasi 2-D heavy-mass nearly ferromagnetic state of (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$

Perovskite ruthenates exhibit a wide range of complex magnetic ground states. In this talk we focus on an unusual heavy-mass, nearly ferromagnetic state with an extremely large Wilson ratio (Z. Qu \textit{et al.}, Phys. Rev. B \textbf{78} R180407 (2008)). Despite considerable FM correlations, this state never develops long-range FM order, instead freezing into a cluster-spin-glass (CSG) state. We have further investigated this magnetic state through in-plane angular dependence of magnetoresistivity and magnetization on (Sr$_{0.62}$Ca$_{0.38})_{3}$Ru$_{2}$O$_{7}$. The in-plane magnetoresistivity $\rho_{ab}(\phi)$ at high magnetic fields reveals a change in anisotropy symmetry from 2-fold to 4-fold at the frozen temperature $T_{f}$ of the CSG phase, whereas inter-planar magnetoresistivity $\rho_{c}(\phi)$ at high fields only exhibits 4-fold symmetry. For low magnetic fields, both $\rho_{ab}$ and $\rho_{c}$ only exhibit anisotropy below $T_{f}$, also with 4-fold symmetry. Angle-dependent magnetization data reveal that at high field the anisotropy exhibits 8-fold symmetry for $T > T_{f}$. However, for $T < T_{f}$, an additional asymmetric 2-fold anisotropy develops. These results may indicate non-traditional interlayer magnetic coupling, one possible scenario involving perpendicular spin stacking between alternate layers.   

Ferromagnetism in perovskites Sr$_{1-x}$Pb$_{x}$RuO$_{3}$ (0 $\le $ x $\le $ 1)

Orthorhombic SrRuO$_{3}$ is a well-known metallic ferromagnet with T$_{c}$ $\approx $ 165 K.$^{1}$ We have demonstrated that a complete solid solution of the orthorhombic perovskites Sr$_{1-x}$Pb$_{x}$RuO$_{3}$ can be made with high-pressure synthesis. Whereas the ground state for the whole series remains metallic, the Pb substitution reduces the Curie temperature and eventually ferromagnetic phase is totally suppressed at x $\approx $ 0.5. Moreover, an unusual low-temperature phase with the Imma space group is developed through a first-order transition below T$_{t} \quad \approx $ 90 K in the orthorhombic PbRuO$_{3}$. Based on physical properties measurements on a small grain of the high-pressure product PbRuO$_{3}$, we have shown that the transition at T$_{t}$ is a metal-metal transition. T$_{t}$ is suppressed below 10 K under high pressure 3GPa as verified by the structural study with a diamond anvil cell with synchrotron radiation. Suppression of ferromagnetism in this system can be attributed to the hybridization of the Pb$^{2+}$ 6s electrons with the Ru 4d electrons that broadens the Ru 4d band. $^{1}$ G. Cao, et al., Phys. Rev. B \textbf{56}, 321 (1997). $^{2 }$S. A. J. Kimber, et al., Phys. Rev. Lett. \textbf{102}, 046409 (2009). $^{3 }$J.-G. Cheng, et al., Phys. Rev. B, in press.   

Intermediate-Valence Behavior in the Transition-Metal Oxide CaCu$_3$Ru$_4$O$_{12}$

A detailed study of the electronic properties of strongly correlated CCRO will be presented. Along with transport, $\chi(T)$ and specific heat, we focus on NMR/NQR. CCRO is a metallic system with perovskite structure showing non-Fermi-liquid behavior below 2${\,}$K which is inferred from $C_p/{\,}T\propto -ln(T)$ and a deviation of $1/{\,}T_1 (T)$ from the Korringa law at the copper site [PRB \textbf{78}, 165126 (2008)]. On closer inspection, a volume change could be detected by neutron diffraction which comes along with a corresponding anomaly in the specific heat around 150${\,}$K. Furthermore, the $^{99}$Ru Knight shift shows a cross-over between paramagnetic behavior of localized moments at high $T$ and itinerant band states at low $T$, respectively. Complementary density-functional calculations (LDA+DFT) relate these phenomena to the ruthenium \emph{d}-electron number. We conclude that dynamic charge fluctuations originating from the strong electronic correlations are present in CCRO and give rise to the intermediate valence of the ruthenium ions [PRB \textbf{80}, 121101(R) (2009)].   

Surface state van Hove singularity effect to bulk electron-bosonic mode coupling in Sr$_{2}$RuO$_{4}$

Discovery of spin triplet superconductivity in Sr$_{2}$RuO$_{4}$ brought attention to the electronic structure studies on the system, especially by using angle resolved photoemission (ARPES). There are three bands that cross the Fermi level. The system is particularly interesting because these bands have different orbitals with different characters such as dimensionality. Along the way, it was found that there are surface states due to RuO$_{6}$ octahedral rotation on the surface layer. RuO$_{6}$ octahedral rotation results in a dramatic change in the surface electronic structure such as van Hove singularity. By controlling temperatures, we can traverse or kill the van Hove singularity in the surface electronic states. Through comparison of measured bulk electronic structures at different temperatures, we elucidate the effect of surface state van Hove singularity to bulk electron-bosonic mode coupling. We will discuss the importance of the surface states in interpretation of bulk properties measured by ARPES.   

Electronic density of states of Ca3Ru2O7 measured by tunneling spectroscopy

The ruthenates are perhaps one of the most diverse of group of materials known up to date. These compounds exhibit a wide array of behaviors ranging from the exotic p-wave superconductivity in Sr$_{2}$RuO$_{4}$, to the itinerant ferromagnetism in SrRuO$_{3}$, and the Mott-insulating behavior in Ca$_{2}$RuO$_{4}$. One of the most intriguing compounds belonging to this group is Ca$_{3}$Ru$_{2}$O$_{7}$ which is known to undergo an antiferromagnetic ordering at 56K and an insulating transition at 48K. We have prepared Ca$_{3}$Ru$_{2}$O$_{7}$/Al$_{2}$O$_{3}$/Ag planar junctions and used tunneling spectroscopy to study the density of states of Ca$_{3}$Ru$_{2}$O$_{7}$ from room temperature to 4.2K. Distinguish gap structure can be observed in all spectra at temperatures below 48K. The tunneling spectra behave very differently and the gap has dissimilar values along the three crystal axes directions. We have also studied the effect of magnetic field on these spectra by applying an external field up to 7T along the a-axis direction. In this presentation we will summarize our results and discuss their implications in relation to the competition between different interactions in the ground state of this material. This research was supported by NSF grants DMR-0800367, DMR-0856234 and EPS-0814194.   

Experimental studies of structure-property relationship in nine- and four-layer BaRuO$_{3}$

BaRuO$_3$ is a fascinating material as it possesses four different crystalline forms that possess rather different physical properties. We report results of our low-temperature magnetotransport measurements on single crystals of nine-layer rhombohedra (9R) and four-layer hexagonal (4H) BaRuO$_3$, two of the four different crystalline structures adopted by this material. Structurally a very short Ru-Ru distance was found in both 4H and 9R BaRuO$_3$, leading to metal-metal bonding, while 9R BaRuO$_3$ features three but 4H BaRuO$_3$ two face-sharing RuO$_6$ octahedra. For both 9R and 4H BaRuO$_3$, the magnetoresistance was found to become significant only below 30 K. More importantly, the magnetoresistance of 9R BaRuO$_3$ was found to be negative, while that of 4H BaRuO$_3$ positive. We suggest that the difference in the sign of magnetoresistance is associated with the difference in the two crystalline forms and electronic states of BaRuO$_3$, with only 9R BaRuO$_3$ featuring the previously proposed energy gap on certain part of the Fermi surface. Specific heat measurements are also being pursued to seek for additional insight into the physics of BaRuO$_3$. The work is supported by DOD ARO and NSF.   

Unusual Fermi surface reconstruction driven by moderate magnetic fields in Sr$_{4}$Ru$_{3}$O$_{10}$

Trilayered ruthenate Sr$_{4}$Ru$_{3}$O$_{10}$ exhibits puzzling magnetic properties. For field applied along the c-axis it exhibits typical itinerant ferromagnetic (FM) behavior, while moderate field (2-3T) applied within the \textit{ab}-plane can induce a metamagnetic (MM) transition [1]. Such coexistence of ferromagnetism and metamagnetism has been shown to be associated with a multiple band effect; FM bands derived from the $d_{xy}$ orbital coexists with MM bands from the $d_{xz,yz}$ orbitals [2,3]. In this talk, we report on Hall effect studies of this compound. Our results reveal that Fermi surface of Sr$_{4}$Ru$_{3}$O$_{10}$ changes dramatically through the MM transition. This Fermi surface change leads the Hall resistivity \textit{$\rho $}$_{xy}$ to strongly deviate from the scaling relation with magnetization, i.e. \textit{$\rho $}$_{xy}$ = $R_{0}H$ + 4$\pi R_{s}M$, which holds at fields well below and above the MM transition field. Such a significant change of the FS is unexpected for typical itinerant MM transitions.\\[4pt] [1] Cao \textit{et al}., Phys. Rev. B 68, 174409 (2003)\\[0pt] [2] J. Jo \textit{et al}., Phys. Rev. B 75, 094413 (2007)\\[0pt] [3] Fobes \textit{et al}., unpublished.   

Influence of spin-orbit coupling on the metamagnetic transition in Sr$_3$Ru$_2$O$_7$

Clean single crystals of Sr$_3$Ru$_2$O$_7$ undergo a metamagnetic transition at low temperatures. This transition shows a strong anisotropy in the applied field direction with a critical field $H_c$ ranging from $5.1$T for the case of $H\perp c$ to almost $8$T for $H\parallel c$. In addition, studies on ultra-pure samples revealed a bifurcation of the metamagnetic line for fields in $c$-direction and it is argued that a nematic phase emerges between the magnetization jumps. The aim of this study is to explain the field anisotropy of these phenomena. Based on a microscopic tight-binding model, we introduce the metamagnetic transition by means of a van Hove singularity scenario. We show that the rotation of the O-octahedra around the $c$-axis expected for this material introduces a staggered Rashba-like spin-orbit coupling within the planes and naturally leads to an anisotropy of the magnetic response. We describe the low-temperature phase as a nematic state favored by forward scattering processes. The spin-orbit coupling shows an influence on both, the critical field $H_c$ and the occurence of the nematic phase.   

XPS and ARPES study of the metal-insulator transition in Mn-substituted Sr$_3$Ru$_2$O$_7$

We have studied the metal-insulator transition in Mn-substituted Sr$_3$Ru$_2$O$_7$ by core-level x-ray photoemission (XPS) and angle-resolved photoemission spectroscopy (ARPES). In XPS, both the surface- and bulk-sensitive spectra show a two-peak structure, corresponding to the well screened and the unscreened excitations. The intensity of the well-screened peak is suppressed upon increasing the concentration of Mn, reflecting a metal-to-insulator transition induced by Mn impurities. In ARPES, changes in Fermi surface topology and band dispersions are observed as the system crosses over from a metal to a - possibly Mott - insulator. We observed a variation and enhancement of the Fermi-surface nesting upon Mn substitution, which might be connected to the emergence of the magnetic superstructure revealed by our resonant elastic soft x-ray scattering results [1].\\[4pt] [1] M.A. Hossain {\it et al.}, arXiv:0906.0035 (2009).   

Heavy d-Electron Quasiparticle Interference and Real-space Electronic Structure of $Sr_{3}Ru_{2}O_{7}$

The intriguing idea that strongly interacting electrons can generate electronic liquid crystalline phases is already a decade old, but these systems still represent an unexplored frontier of condensed matter physics. One reason is that visualization of the many-body quantum states generated by the strong interactions, and of the resulting nematic electronic phases, has not been achieved. $Sr_{3}Ru_{2}O_{7}$ possesses (i) a very strongly renormalized ``heavy'' d-electron Fermi liquid , and (ii) exhibits a field-induced transition to an electronic nematic phase. We present the first observation of scattering interference of heavy d-electron quasiparticles at individual Ti dopant atoms substituted in $Sr_{3}(Ru_{1-x}Ti_{x})_{2}O_{7}$, and the associated discovery that it derives from a band formed from the Ru $d_{xz}, d_{yz}$ orbitals recently postulated to be behind the formation of the nematic state. Simultaneously, we achieve direct sub-atomic imaging of r-space electronic structure which can be associated with these same d-orbitals within the many-body state.   

Magnetic Superstructure and Metal-Insulator Transition in Mn-Substituted Sr$_3$Ru$_2$O$_7$

We present a temperature-dependent resonant elastic soft x-ray scattering (REXS) study of the metal-insulator transition in Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$, performed at both Ru and Mn $L$-edges. Resonant magnetic superstructure reflections, which indicate an incipient instability of the parent compound, are detected below the transition. Based on modelling of the REXS intensity from randomly distributed Mn impurities, we establish the inhomogeneous nature of the metal-insulator transition, with an effective percolation threshold corresponding to an anomalously low $x\sim 0.05$ Mn substitution. In collaboration with A.G. Cruz Gonzalez, J.D. Denlinger (Berkeley Lab), I. Zegkinoglou, M.W. Haverkort (MPI, Stuttgart), I.S. Elfimov, D.G. Hawthorn (UBC), R. Mathieu, S. Satow, H. Takagi (Tokyo), H.-H. Wu and C. Sch\"{u}\ss ler-Langeheine (Cologne).   

Session Z37: Focus Session: Nanomagnetism -- Nanoparticles II

Progress toward Synthesis and Characterization of Rare-Earth Nanoparticles

Magnetic nanoparticles exhibit interesting phenomena, such as enhanced magnetization and reduced magnetic ordering temperature (i.e. superparamagnetism), which has technical applications in industry, including magnetic storage, magnetic imaging, and magnetic refrigeration. We used the inverse micelle technique to synthesize Gd and Nd nanoparticles given its potential to control the cluster size, amount of aggregation, and prevent oxidation of the rare-earth elements. Gd and Nd were reduced by NaBH$_{4}$ from the chloride salt. The produced clusters were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The results from the XRD show that the majority of the peaks match those of the surfactant, DDAB. No peaks of Gd were observed due to excess surfactant or amorphous clusters. However, the results from the SEM and EDX indicate the presence of Gd and Nd in our clusters microscopically, and current synthesized samples contain impurities. We are using liquid-liquid extraction method to purify the sample, and the results will be discussed.   

Shape-dependency of magnetic properties of FePt nanostructures

The fabrication and magnetic properties of size- and morphology- controlled magnetic nanostructures have attracted much interest owing to their potential application in ultrahigh density magnetic recording media. Chemically ordered binary alloy L10- FePt has emerged as an ideal candidate for information storage as result of its ultrahigh uniaxial magnetocrystalline anisotropy and good chemical stability. The role of shape anisotropy in the magnetization reversal of FePt nanostructures remains as a fundamental issue to be clarified for both scientific and technological purposes. We have used wet chemistry to fabricate nanocrystals of different shapes and measured their magnetic properties using a SQUID magnetometer. Spheres have an average diameter of 3 nm, while ellipsoids are of average dimension of 3.5 nm by 2.5 nm. Rods have average diameter of 2.5 nm and length of 50-70 nm. The magnetic properties of the as-grown nanocrystals were dramatically enhanced after annealing. One can engineer the Curie temperature and coercive field of the nanostructures by tuning their shapes. Preliminary theoretical simulations enable us to qualitatively explain the dissimilar magnetic properties of these colloids.   

Elemental Specific Study on FeCo-Au nanoparticles

Core-shell type nanoparticles are a topic of interest due to its diverse magnetic properties such as coercivity enhancement\footnote{J. Nogu\'{e}s et al., Phys. Rep. 422, 65 (2005)} and exchange bias.\footnote{O. Iglesias, et al., J. Nanosci. Nanotechnol. 8, 2761 (2008)} At the core-shell interface, the orbital magnetism is expected to change due to the symmetry broken, which could be substantial on the overall magnetic performance in nanoparticle system given its large surface to volume ratio. Here we present elemental specific and chemical selective XMCD study on high magnetic moment FeCo-Au core-shell nanoparticles, which are synthesized from a one-step gas condensation sputtering method. It is found the orbital magnetic moment of Fe and Co response differently when both elements are subject to the same broken symmetry. The spin and orbital magnetic moment on Fe and Co are determined according to XMCD sum rule, respectively. Giant orbital magnetic moment is observed on Co L edge while no significant change is found on Fe case. This finding hints at the necessary modification of Slater-Pauling curve of systems in nanoscale, pointing at a new direction on searching high materials with high magnetic moment.   

Magnetic nano-structures for the manipulation of individual nano-scale particles in bio-compatible environments

The manipulation of geometrically constrained magnetic domain walls (DWs) in nanoscale magnetic strips has attracted much interest recently, with proposals for prospective memory and logic devices. Here we demonstrate that the high controllability of the motion of geometrically constrained DWs allows for the manipulation of individual nanoparticles carrying proteins or cells in solution on a chip with the active control of position at the nanometer scale. Our approach exploits the fact that magnetic nanoparticles in suspension can be captured by a DW, whose position can be manipulated with nanometer scale accuracy in specifically designed magnetic nanowire structures. We hereby show that the precise control over DW nucleation, displacement, and annihilation processes in such nanostructures allows for the capture, transport and release of magnetic nanoparticles. Although this application of nano-magnetism to bio-technology and nanomedicine is still in its infancy, it already reveals its huge potential as one example of the synergetic combination of nano-physics, nano-chemistry and nano-biotechnology.   

Iron et al.: Incorporation of Manganese in the Crystal Lattice of Magnetosome Magnetite

Incorporation of foreign metal into the crystal matrix of the magnetotactic bacterial magnetite has been attempted worldwide. Recently, presence of small amounts of cobalt and manganese in magnetosome magnetite crystals in cultured and uncultured magnetotactic bacteria, respectively, was reported. Magnetization of the uncultured cells and their magnetosomes were not determined, while only marginal changes in the magnetic properties of the cultured cobalt-grown cells and their magnetosomes were observed, however no evidence of incorporation of these metals into the crystalline lattice was presented. We grew cells of a magnetotactic bacterium, \textit{Magnetospirillum gryphiswaldense }strain MSR-1, in the presence of manganese, ruthenium, zinc and vanadium, of which only manganese was incorporated within the magnetosome magnetite crystals. For the first time we demonstrate that the magnetic properties of magnetite crystals of magnetotactic bacteria can be significantly altered by the incorporation of metal ions, other than iron, in the crystal structure, as signaled by a major shift in the Verwey transition.   

Controlling Brownian motion of magnetic microspheres by magnetic wire traps

Microspheres with embedded superparamagnetic particles are being widely used in biomedical research to selectively influence biological entities. As most of these applications are in fluid-based suspensions, random Brownian movements of the microspheres have important consequences on their targeted behavior. We demonstrate a technique based on microscale ferromagnetic wires patterned on a silicon platform, where when the external field (H) augments the domain-wall field from the designed wires, the microspheres in the fluid can be tightly confined (trapped) within an area remote from the wires. Upon weakening the trap by tuning H, the amplitude or range of the Brownian motion steadily increases until an abrupt onset of large random fluctuations is reached. These results, which demonstrate control on the random walk of fluid-borne magnetic microspheres through non-contact forces, are well reproduced through simulations.   

Magnetic Polymer Nanocomposites with Tunable Microwave Properties

Due to the multifunctionality, polymer nanocomposites (PNCs) have potential applications for electromagnetic interference shielding, tunable electromagnetic devices and flexible electronics. We report on synthesis, magnetic and RF characterization of polymer films loaded with varying concentrations of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles. The nanoparticles (5 $\pm $ 1 nm) were synthesized by chemical co-precipitation. Structural properties were characterized by XRD and TEM. Nanoparticles were dispersed through a solution method in a low-loss microwave polymer from the Rogers Corporation. No aggregation was observed and particles remain well dispersed throughout the volume of the polymer matrix. Magnetic measurements using a Physical Property Measurement System revealed characteristic features of superparamagnetism at room temperature and blocking at low temperature. Microwave transmission/reflection studies on the PNCs were done using a microstrip resonator technique, and strong tunability in the microwave absorption was observed.   

Unique Separation of Core-Shell Nanoparticle Components using Resonant X-Ray Scattering

Nanoparticle-based devices, fluids, and biomedical applications are at the research forefront due to an unprecedented ability to control growth and uniformity. In particular, it is now possible to produce bulk quantities of monodisperse nanoparticles comprised of distinctive chemical layers. Characterizing these internal structures with conventional techniques such as TEM, however, remains a challenge. Here we demonstrate using Fe-oxide and Mn-oxide core-shell nanoparticles, that multiple-energy resonant small angle x-ray scattering (SAXS) can be effectively utilized to uniquely and unambiguously separate the scattering contributions from the Fe oxide and the Mn oxide regions without any \textit{a priori} knowledge of the internal structure. This technique reveals that the nanoparticles have a monodispersity less than 10{\%} and a total spherical radius of 4.3 nm that is divided into a distinctive Fe oxide core of radius 1.5 nm and a Mn oxide impregnated shell covering the outermost 2.8 nm. Although especially well suited for determining core-shell nanoparticle morphology, this novel approach is applicable for resolving the diffraction contributions from any layered system such as multilayered ultra-thin films.   

Soft Magnetic Nanocomposites Assembled by Fe/Al$_{2}$O$_{3}$ Core-Shell Nanoparticles with Tunable High-Frequency Property

High-frequency soft magnetic films synthesized at room temperature (RT) are significant to the growing demand for improvement of next-generation microelectronic devices. For working in the gigahertz range, it is a challenge to develop uniaxial anisotropic films with high saturation magnetization, small coercivity and large resistivity. Accordingly, new Fe/Al$_{2}$O$_{3}$ core-shell cluster-assembled nanocomposites are created by employing novel energetic cluster impact. By applying potentials up to 20 kV to tilted Si substrates, in-plane uniaxial anisotropy is induced and tailored at RT, which is interpreted by the uniaxial shape anisotropy of the ellipsoidal nanoparticles and the alignment of the nanoparticle assembly. Moreover, the Fe/Al$_{2}$O$_{3}$ core-shell ratio is adjusted to control the excellent magnetic softness and ultra-high resistivity. Consequently, the Si-integration compatible nanocomposite films demonstrate tunable magnetic dynamic properties up to 8.5 GHz, measured by a shorted transmission-line perturbation method.   

Synthesis and Magnetic Properties of Cobalt Carbide Nanoparticles

Co-based alloys and compounds have wide applications in traditional and advanced materials. Co$_{n}$C (n=1-6) thin film and bulk materials have drawn much attention and been studied experimentally though less report can be found in studies of their magnetic properties. We report synthesis and characterization of cobalt carbide (Co$_{3}$C and Co$_{2}$C) nanoparticles by a one-pot polyol reduction process. Tetraethylene glycol was used as both the solvent and reducing agent, and polyvinylpyrrolidone (PVP) as the surfactant. It is found that the size, structure and magnetic properties of the product can be controlled by adjusting the reaction parameters, such as the heating rate, the concentration of NaOH and PVP. PVP plays an important role in controlling the particle size, and therefore magnetic properties. By changing the concentration of PVP, the particle size can be adjusted from 1 micrometer to 20 nm and the coercivity reaches to the maximum value of 3.2 kOe at room temperature when the size was reduced to 20 nm. Thermomagnetic measurements showed that the Curie temperature of the cobalt carbide nanoparticles is around 500 K. Decomposition of the carbides was observed at about 700 K at which the cobalt carbides decomposed into pure Co during measurement.   

Femtosecond functionalization of magnetic 2- and 3-center nanostructures

We present an \emph{ab initio} theory of ultrafast nanologic elements based on optical $\Lambda$-processes [1]. Using high-level quantum chemistry we show that in 2- and 3-magnetic-center structures containing Fe, Co and Ni as active centers both spin flips and spin transfers are possible within a hundred femtoseconds. Spin transfer can be resolved by a sufficiently large shift of the vibrational stretch frequency of a CO marker [2]. From 3-magnetic-center clusters we are able to construct OR, XOR (CNOT), and AND gates [3]. Thus multicenter magnetic clusters allow to exploit spin dynamics for full-fledged logic functionalization.\\[4pt] [1] G. Lefkidis, G. P. Zhang, and W. H\"{u}bner, PRL (2009, in press)\\[0pt] [2] C. Li, T. Hartenstein, G. Lefkidis and W. H\"{u}bner {\bf 79}, 180413(R) (2009)\\[0pt] [3] W. H\"{u}bner, S. Kersten and G. Lefkidis PRB {\bf 79}, 184431 (2009)   

Surface Induced Magnetic Switching in Nanoparticles

We show that the magnetic structure of nanoparticles with competing exchange interactions, i.e. having ferromagnetic exchange coupling between nearest neighbors, J01, and antiferromagnetic one between second nearest neighbors, J02, is very sensitive to the ratio of these exchanges, R=-J02/J01. The magnetic structurein ground state changes as a function of R from ferromagnetic to non-collinear, and to antiferromagnetic. This change occurs in a very narrow window of R. The moderate modification of the surface exchange parameters of such nanoparticle may lead to a substantial change in the temperature dependence of its total magnetic moment. Using Monte Carlo simulations we show that the ``ordering'' temperature of nanoparticles of 3-4nm in diameter can be varied by about 25{\%} with the change of nearest neighbor exchange by only 25{\%}. Thus, if the surface exchange is modified by the external stimuli in core shell nanoparticles, the magnetic moment of the nanoparticle can be switched from nearly zero to about half of its maximum value. We discuss the modification of surface exchange in core-shell nanoparticles with core of iron oxide and shell made of photochromic materials as azobenzene.   

Structural and Magnetic Properties of As-Prepared and Annealed Ni/Cu Core/Shell Nanoparticles

Air stable Ni-core Cu-shell nanoparticles with diameters between about 100 and 300 nm have been synthesized via a one-pot polyol synthesis. Structural and chemical analysis shows the particles to be essentially free of metallic oxides and copper rich (Cu$_{59}$Ni$_{41})$, while room temperature magnetic measurements indicate a similar composition. The freely-flowing powder was compacted into disks under moderate pressure and a series of these samples were were annealed by scanning in a DSC to progressively higher maximum temperatures under a constant flow of forming gas. A DSC scan to a maximum temperature of 250 \r{ }C results in a large drop in the coercivity (208 Oe to 133 Oe) and relatively little change in the high-field magnetization ($\sim $20 emu/g of sample) and XRD-determined lattice parameter (0.3625(1) nm to 0.36169(5) nm). The high-field (H=9T) magnetization remains relatively unchanged near 20 emu/g for samples similarly scanned up to 400 \r{ }C, while samples scanned to higher temperatures have high-field magnetizations and coercivities that drop to a final value of 2.8 emu/g and 76 Oe upon scanning to 600 \r{ }C.   

Session Z39: Focus Session: Iron Based Superconductors: Spectroscopy II

Evolution of Fermi surface nesting of BaFe2(As{1-x}Px)2 revealed by de Haas-van Alphen effect

The iron-pnictide superconductors are a new class of materials with unique superconducting and magnetic properties. Many theoretical frameworks describing these materials rely heavily on the nature of the size and topology of the Fermi surface. The classic method ofdetermining the Fermi surface is by looking at oscillations in the magnetization as a function of field. These oscillations, known as the de Haas-van Alphen effect, is extremely powerful in that it can determine the full three-dimensional topology of the FS, in addition to the quasiparticle renormalization to the effective mass. In the present study we measure the Fermi surface of the superconducting P-doped BaFe2As2. We are able to reveal the curvature of the electron pockets and the size and topology of a corresponding hole pocket, revealing a dramatic enhancement of the nesting for superconducting compounds, in contrast to the non-superconducting compounds.   

De Haas-van Alphen Oscillations in KFe$_2$As$_2$

In order to clarify pairing mechanisms and symmetries of the new high-$T_c$ superconductivity in the FeAs compounds, it is necessary to know their Fermi surfaces. We report on de Haas-van Alphen effect in KFe$_2$As$_2$, which is an end member of the high-$T_c$ binary alloy (Ba, K)Fe$_2$As$_2$. It shows no magnetic or structural phase transition down to low temperatures and becomes superconducting below about 3 K. We have observed many dHvA frequencies and their angular dependences are basically 1/cos$\theta$, where $\theta$ is the angle between the $c$ axis and the magnetic field. At the moment, our analysis indicates that three quasi-two-dimensional FS cylinders have been observed and that they occupy about 1, 8, and 12\% of the Brillouin zone, respectively. The effective masses of electrons are fairly heavy, ranging from 6 to 9 times the free electron mass for $B \parallel c$. This seems consistent with previously reported $T^2$ dependence of $\rho$ with a large $A$ coefficient [1] and large Sommerfeld coefficient of the specific heat [2]. [1] T. Terashima \textit{et al}., JPSJ 78, 063702 (2009). [2] H. Fukazawa \textit{et al}., JPSJ 78, 083712 (2009).   

Angle-dependent Magnetoresistance Oscillations in KFe$_{2}$As$_{2}$

We report the results of angular-dependent magnetoresistance oscillations (AMRO) in the Fe-Pnictide superconductor KFe$_{2}$As$_{2}$. The two series of AMRO structures are observed, suggesting the existence of two quasi-two-dimensional Fermi surfaces (Q2D-FSs). The obtained FS cross sectional areas correspond to 11 and 15{\%} of the first Brillouin zone, and our results indicate that the cross sections of the Q2D-FSs are rounded square. The diagonal axes of the rounded squares are parallel to the a-axis in both FSs. These results are essentially consistent with the recent quantum oscillations and photoemission spectroscopy measurements.   

ARPES studies of FeAs-based compounds

With critical temperatures and 2$\Delta $/k$_{B}T_{c}$ ratios comparable to those of cuprates, the new iron-based superconductors are believed to be the host of an unconventional pairing mechanism. Since these superconductors are multi-band materials, a deep understanding of their electronic properties and of the paring mechanism necessitates a good knowledge of their electronic structure in momentum space, particularly in the vicinity of the Fermi level. Owing to its momentum resolution capability, angle-resolved photoemission spectroscopy (ARPES) is a very powerful tool to characterize precisely the electronic states lying close to the Fermi level, which trigger the electronic behavior of crystals. In this talk, I present recent ARPES results obtained on the so-called $122$ class of materials over a wide range of doping. I show the evolution of the multi-band Fermi surface and the superconducting gap with doping and emphasize on the importance of interband scattering. In particular, I reveal that the occurence of high-temperature superconductivity seems related to ``near-nesting'' of $\Gamma $-centered holelike and M-centered electronlike Fermi surface pockets.   

Observation of Dirac Cone Electronic Dispersion in BaFe$_{2}$As$_{2}$

As with cuprates, it is widely believed that high-$T_{c}$ superconductivity in pnictides emerges by tuning interactions already present in the parent compounds and it is thus imperative to understand their electronic structure. We performed an angle-resolved photoemission spectroscopy study of BaFe$_{2}$As$_{2}$, which is the parent compound of the so-called $122$ phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. Moreover, the bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description.   

Fermi surface and superconducting gap of FeTe$_{1-x}$Se$_{x}$ superconductor studied by high-resolution ARPES

The origin of superconductivity in FeTe$_{1-x}$Se$_{x}$ superconductor is a subject of intensive debate, since the parent compound FeTe shows considerably different electronic and magnetic properties compared to FeAs-based families. To clarify the superconducting mechanism, an experimental investigation of the low-energy electronic structure is of particular importance. Here, we report our recent high-resolution ARPES results on FeTe$_{1-x}$Se$_{x}$ superconductor, and demonstrate several universalities in the electronic states between FeTe$_{1-x}$Se$_{x}$ and FeAs-based superconductors.   

Fermi surface dichotomy of superconducting gap and pseudogap in underdoped pnictides

A systematic angle-resolved photoemission spectroscopy (ARPES) study has been performed on the hole-doped 122-phase (Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2})$ in the underdoped (UD) region. We observe that the superconducting (SC) gap of the UD pnictides scales linearly with the transition temperature, and a distinct pseudogap develops upon underdoping and coexists with the SC gap. Remarkably, this pseudogap occurs mainly on the FS sheets that are connected by the AF wave vector, where the SC pairing is stronger as well. The observed dichotomic behaviour of the pseudogap and the SC gap on different FS sheets in the UD pnictides shares many similarities with those observed in the UD copper oxide superconductors, providing a unifying picture for both families of high- temperature superconductors.   

ARPES Study on the Electronic Structure of FeTe

Among the iron-based superconductors, iron chalcogenides FeSe$_{x}$Te$_{1-x}$ (T$_{c}\sim $20K) are special for their structural simplicity. FeTe, the parent compound for iron chalcogenides, though without superconducting transition, shows a unique antiferromagnetic order below tetragonal-orthorhombic structural phase transition temperature. Here we present recent ARPES results on this material, including measurements on electronic band structure and Fermi surface topology. We discovered strong k$_{z}$ dispersion of the Fermi surface and observed electronic band evolution through phase transition. The comparison of iron chalcogenides and other iron-based superconductor families helps us identify the governing physics in this new family of superconductors.   

Photoemission in Ferropnictides

High resolution angle resolved photo emission spectroscopy yields the most direct and most detailed information on the electronic structure of solids. This opens the oppertunity to really compare theoretical band structures with experiment. However, the method is surface sensitive. For the ferropnictides highly resolved data are available, which are re-evaluated on the basis of density functional calculations.   

Ab initio study of de Haas van Alphen effect in LaFe2P2 and CeFe2P2

The use of the maximally localized Wannier functions (MLWF) scheme (Marzari {\&} Vanderbilt, \textit{Phys. Rev. B}, 56, 12847, 1997) to interpolate the Hamiltonian on a very dense k-point grid allows an accurate description of the Fermi surface (FS) of a metal. It is then possible to calculate sections of a FS with great precision. These areas are related to de Haas van Alphen (dHvA) frequencies which can be measured experimentally. In this work, we study LaFe2P2 and CeFe2P2, both crystals in the pnictide family. Results of dHvA frequencies for different functionals are compared directly to experimental data. More specifically, we will present the effects of including a Hubbard U term in the calculations in order to take into account strong correlation on the Fe d orbitals. We will also consider another approach to deal with these orbitals by adding a certain amount of exact exchange to the functional. In this case, we used a PBE0 functional which adds 25{\%} of exact exchange (M. Ernzerhof and G.E. Scuseria, \textit{J. Chem. Phys.}, 110, 5029, 1999).   

Quantum oscillation experiments on iron pnictides LaFe$_{2}$P$_{2}$ and CeFe$_{2}$P$_{2}$

We investigated the quantum oscillations of the non-superconducting iron pnictides LaFe$_{2}$P$_{2}$ and CeFe$_{2}$P$_{2}$. Those compounds are isostructural to the high-temperature superconductor (Ba$_{1-x}$K$_{x}$)Fe$_{2}$As$_{2}$ [M. Rotter et al., Phys. Rev. Lett. 101, 107006 (2008)]. Measurements have been carried out using a torque cantilever in fields up to 35 T. Angular-dependent observations of the extremal Fermi surface areas shows a good agreement with our density functional theory calculation using GGA+U functional, which has been used to constructed the Fermi surfaces. We found significant differences between the Fermi surfaces of the two compounds, with LaFe$_{2}$P$_{2}$ showing a much more three dimensional Fermi surface.   

Fermi surface and electronic structure in iron-based superconductors from Angle-Resolved Photoemission Spectroscopy

We will report our high resolution Angle-Resolved Photoemission measurements on iron-based superconductors. Multiple Fermi pockets are well resolved around the $\Gamma $(0, 0) and M($\pi $, $\pi )$ point. In addition, detailed temperature dependent electronic structures will be shown both above and below magnetic/structural transition (T$_{MS})$.   

ARPES study of the new iron-based superconductor Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$

A new FeAs-based compound Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$ was discovered recently to show superconductivity at a relatively high temperature (Tc $\sim $ 37K). We will present ARPES results of band structure and Fermi surface of Sr$_{4}$V$_{2}$O$_{6}$Fe$_{2}$As$_{2}$, and discuss implications to its superconductivity.   

Session Z40: Superconductivity: Spectroscopy (Neutron, Optical and others)

Evolving Picture of Striped Superconductivity in La$_{2-x}$Ba$_x$CuO$_4$

We have previously presented experimental evidence for two-dimensional superconductivity in La$_{1.875}$Ba$_{0.125}$CuO$_4$ coexisting with charge and spin stripe order for $T < 40$ K. A pair-density-wave (PDW) state has been proposed to explain the dynamical layer decoupling; however, recent photoemission measurements indicate that a $d$-wave gap develops on the nodal arc in the same temperature range. We show that our various experimental results are consistent with the onset of PDW correlations together with the charge-stripe order at 53 K, followed by the development of uniform $d$-wave superconductivity. We can rule out layer decoupling due to a charge-density-wave gap.   

Effect of covalent bonding on magnetism and the missing neutron intensity in cuprates

We report a detailed survey of magnetic excitations in the one- dimensional cuprate Sr2CuO3 using inelastic neutron scattering (INS). We show that although the overall shape of the experimental dynamical spin structure factor is well described by the exact theory [1] of the model spin-1/2 nearest-neighbour Heisenberg Hamiltonian typically used for cuprates, the magnetic intensity is strongly suppressed, by factor 2.5-3, compared to the ionic spin model. We further show that this dramatic modification results from strong hybridization of Cu 3d states with O p states, showing that the ionic picture of localized 3d Heisenberg spin magnetism is markedly inadequate. Our findings provide natural explanation for the puzzle of the missing INS magnetic intensity in cuprates and have profound implications for understanding current and future experimental data on these materials [2]. We observe no corrections to spin excitations spectral weight resulting from electron itineracy. [1] J.-S. Caux and R. Hagemans, J. Stat. Mech., P12013 (2006). [2] A. Walters, T. Perring, A. Savici, G. Gu, C. Lee, W. Ku, J.-S. Caux and I. A. Zaliznyak, Nature Physics 5 (2009).   

Bosonic spectral density of optimally and overdoped LSCO superconductors from optical spectroscopy

Optical spectroscopy on single crystals of optimally doped La$_{2- x}$Sr$_{x}$CuO$_{4}$ (x=0.17) show two bosonic peaks at ~50 meV and ~18 meV at low temperatures (30K) as reported by Hwang et al[1]. The bosonic spectrum is acquired through the Eliashberg formalism by inverting the measured optical spectra, and shows a remarkable similarity to the spin excitation spectrum achieved through inelastic neutron scattering results. The optical study is extended into the overdoped region (x=0.22) for which detailed neutron scattering results suggest a suppression of the strong 50 meV response and a characteristic shift from 18 meV to 10 meV of the low energy response. [1] J. Hwang, E. Schachinger, J.P. Carbotte, F. Gao, D.B. Tanner, T. Timusk, Phys. Rev. Letters 100, 137005 (2008)   

Scaling analysis of the static and dynamic critical exponents in Pr$_{2-x}$Ce$_x$CuO$_4$ films as a function of doping

We investigate the static and dynamic critical exponents of the electron-doped superconductor Pr$_{2-x}$Ce$_x$CuO$_4$. Our results are based on current vs.\ voltage measurements in zero-field of the normal-superconducting phase transition in Pr$_{2-x}$Ce$_x$CuO$_4$ films as a function of doping. We find that these materials posses an unusually small critical regime ($\sim$25mK) that gives rise to mean-field behavior at the phase transitions and a static critical exponent of about $\nu\sim$0.5 for all dopings. This is quite unexpected when compared to the critical behavior seen in well-known hole-doped superconductor YBa$_2$Cu$_3$O$_7$, where $\nu\sim$2/3. In addition, mean-field behavior is also exhibited in the dynamic critical exponent ($z$). We find that Pr$_{2-x}$Ce$_x$CuO$_4$ behaves not like other cuprate superconductors, but similarly to conventional superconductors in this regard. Only as transition width decreases to zero does the dynamic critical exponent ($z$) approach the value found in YBa$_2$Cu$_3$O$_7$.   

Coexistence of weak ferromagnetism and superconductivity in rutheno-cuprate RuSr$_2$Eu$_{1.5}$Ce$_{0.5}$Cu$_2$O$_{10}$

We address the question of possible coexistence between weak ferromagnetism (W-FM) and superconductivity (SC) in the Ru-1222 rutheno-cuprate layered structure using element-specific x-ray magnetic circular dichroism (XMCD) and x-ray absorption fine structure (XAFS) measurements. XMCD probes Ru magnetization independently from the paramagnetic contributions of rare-earth ions and XAFS is ideally suited for detection of nano-sized impurities that may go undetected in diffraction measurements. We report the presence of a significant zero-field FM component (0.21 $\mu\rm{_B}$/Ru) associated with Ru ions in the Ru-1222 lattice. The results, together with bulk susceptibility and resistivity measurements, imply by necessity coexistence of W-FM and SC at the atomic level in this rutheno-cuprate structure [see N. M. Souza-Neto {\it et al.}, Phys. Rev. B {\bf 80}, R140414 (2009).]   

Quasiparticle scattering from a double vortex scatterer in d-wave superconductors

The low energy quasiparticle excitations of a d-wave superconductor are massless Dirac fermions. In the presence of a magnetic field, the scattering of quasiparticles from vortices receives both a superflow contribution, due to interaction with the superflow circulating about each vortex, as well as a Berry phase contribution, due to the Berry phase acquired upon circling a vortex. Calculating the cross section for quasiparticle scattering from a double vortex provides a clean way of isolating and studying the two effects. We do so by making use of elliptical coordinates, a natural setting for studying this two-center problem. With proper gauge choice, the Berry phase contribution takes the form of a branch cut between vortex centers, providing a boundary condition for the spinor wave function across the line segment joining the foci of the elliptical coordinate system. We solve the quantum scattering of Dirac quasiparticles in elliptical coordinates. Our approach is to separate the free Dirac equation in elliptical coordinates. The separated angular and radial functions turn out to be the solutions of angular and modified Whittaker-Hill's equations. We summarize the technique to expand incident plane wave spinor in terms of Whittaker-Hill functions. We also present the asymptotic form of the separated solutions in order to setup an analytical formula for differential cross section.   

Broken rotational symmetry in the pseudogap phase of a high-Tc superconductor

The nature of the pseudogap phase is a central problem in the quest to understand high-$T_c$ cuprate superconductors. A fundamental question is what symmetries are broken when that phase sets in below a temperature $T^*$. Here we report the observation of a large in-plane anisotropy of the Nernst effect in YBa$_2$Cu$_3$O$_y$ that sets in precisely at $T^*$, throughout the doping phase diagram [1]. We show that the CuO chains of the orthorhombic lattice are not responsible for this anisotropy, which is therefore an intrinsic property of the CuO$_2$ planes. We conclude that the pseudogap phase is an electronic state which strongly breaks four-fold rotational symmetry.\\[4pt] [1] R. Daou et al., arXiv:0909.4430   

Constraints on Models of Electrical Transport in Optimally Doped La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4}$ from Precise Measurements of Radiation-Induced Defect Resistance

Recent studies of normal-state magnetotransport in overdoped cuprate superconductors have shed much light on charge carrier transport, showing that both linear T and T$^2$ scattering rates are distributed around the Fermi surface. Unfortunately, the most discerning magnetotransport probes are not easily applied for the most interesting, optimally-doped cuprates. We have been able to characterize anisotropic scattering in La$_{\rm 1.83}$Sr$_{\rm 0.17}$CuO$_{\rm 4}$ by using a mostly overlooked but powerful resource---measuring the temperature dependence of the defect scattering resistance. When different regions of the Fermi surface contribute to conduction with different temperature dependences, then the gradual degradation of each contribution via added scattering alters the balance in a characteristic way that reveals much about how transport varies around the Fermi surface. Careful new measurements and a new analysis show how both T and T$^2$ scattering rates coexist as separate parallel conductance channels in La$_{\rm 1.83}$Sr$_{\rm 0.17}$CuO$_{\rm 4}$.   

Probing magnons with RIXS

Resonant Inelastic X-ray Scattering (RIXS) at the copper L and M edge can probe single spin-flips, which makes it possible to probe the dispersion of magnetic excitations (for instance magnons) of cuprates such as the high Tc superconductors [1]. The cross section factors into a local, atomic spin flip scattering amplitude and a momentum dependent factor describing the final state excitation. Recently, the single magnon dispersion has been measured and found to coincide with earlier neutron measurements [2]. For the cuprates, these results put RIXS as a technique on the same footing as neutron scattering, opening a new window for experiments on this class of materials. \newline \newline [1] L.J.P. Ament, G. Ghiringhelli, M. Moretti Sala, L. Braicovich, and J. van den Brink, PRL {\bf 103}, 117003 (2009) \newline [2] L. Braicovich, J. van den Brink, V. Bisogni, M. Moretti Sala, L.J.P. Ament, N.B. Brookes, G.M. De Luca, M. Salluzzo, T. Schmitt, and G. Ghiringhelli, arXiv:0911.0621   

Direct observation of bulk Fermi surface at higher Brillouin zones in a heavily hole-doped cuprate

We have observed the bulk Fermi surface (FS) in an overdoped ($x$=0.3) single crystal of La$_{2-x}$Sr$_x$CuO$_4$ by using Compton scattering. A 2-D momentum density reconstruction [1] from measured Compton profiles, yields a clear FS signature in a higher Brillouin zone centered at p=(1.5,1.5) a.u. The quantitative agreement with density functional theory (DFT) calculations [2] and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. We have also measured the 2-D angular correlation of positron annihilation radiation (2D-ACAR) [3] and noticed a similar quantitative agreement with the DFT simulations. However, 2D-ACAR does not give a clear signature of the FS in the extended momentum space in both theory and experiment. Work supported in part by the US DOE.\\ \mbox{[1]} Y. Tanaka \textit{et al.}, Phys. Rev. B {\bf 63}, 045120 (2001).\\ \mbox{[2]} S. Sahrakorpi \textit{et al.}, Phys. Rev. Lett. {\bf 95}, 157601 (2005).\\ \mbox{[3]} L. C. Smedskjaer \textit{et al.}, J. Phys. Chem. Solids {\bf 52}, 1541 (1991).   

Dynamical structure factor computations in extended momentum space in electron doped cuprates

We report first principles computations of the dynamical structure factor $S(q,\omega)$ in the electron doped cuprates Nd$_{2-x}$Ce$_{x}$CuO$_{2}$ as a function of energy $\omega$ and momentum $q$ extended over several Brillouin zones. We show the efficacy of obtaining $S(q,\omega)$ through the use of simple products of real space Green functions and fast Fourier transform (FFT). We also calculate the susceptibility $\chi(q,\omega)$ efficiently by recalling that $S(q,\omega)$ is proportional to the imaginary part of the susceptibility $\chi(q,\omega)$. The present work is useful in going beyond the standard LDA-based modeling of various highly resolved spectroscopies.[1-4] We will provide some illustrative examples. Work supported by the US DOE.\\ \mbox{[1]} R. S. Markiewicz \textit{et al.}, Phys. Rev. Lett. {\bf 96}, 107005 (2006).\\ \mbox{[2]} S. Sahrakorpi \textit{et al.}, Phys. Rev. Lett. {\bf 95}, 157601 (2005).\\ \mbox{[3]} G. Stutz \textit{et al.}, Phys. Rev. B {\bf 60}, 7099 (1999).\\ \mbox{[4]} J. Nieminen \textit{et al.}, Phys. Rev. Lett. {\bf 102}, 037001 (2009).   

Fermi Liquid Description of X-ray Absorption Spectra in Overdoped LSCO

We show that a paramagnetic self-energy correction [1] to the real-space Green's function code FEFF9 [2] can provide a good description of the x-ray absorption spectra (XAS) of cuprate system such as La$_{\rm (2-x)}$Sr$_{\rm (x)}$CuO$_{\rm 4}$ (LSCO). This self energy includes coupling to both charge and magnetic excitations. We also find good agreement with recent XAS results of Peet et al. [3] in the over-doped regime of LSCO. We have also investigated various prescriptions for including core-hole effects. We infer that at low doping, the system behaves as an anti-ferromagnetic insulator, while Fermi liquid physics is recovered at high doping. \\ \mbox{[1]}Tanmoy Das, R.S. Markiewicz, and A. Bansil, Phys.Rev. B {\bf 77}, 134516 (2008). \\ \mbox{[2]} J. J. Rehr et al., Comptes Rendus Physique {\bf 10}, 548 (2009).\\ \mbox{[3]} D.C. Peets \textit{et al.}, Phys. Rev. Lett. {\bf 103}, 087402 (2009).   

Quantitative determination of penetration depth using magnetic force microscopy

Extracting quantitative information from magnetic force microscopy has long been considered a difficult problem. We present a method to extract a numerical value of the penetration depth utilizing detailed knowledge of the MFM tip properties. Modeling the vortex field as that of a magnetic monopole we use the experimental data on a Nb film to find a penetration depth that is in good agreement with SQUID magnetometer measurements. We discuss the influence of tip geometry on the extracted values of the penetration depth and explore the differences between the simple model of the vortex field and a detailed calculation in the London model.   

Fermi surfaces and quantum oscillations in underdoped high-T$_{c}$ superconductors YBa$_{2}$Cu$_{3}$O$_{6.5 }$and YBa$_{2}$Cu$_{4}$O$_{8}$

We study the underdoped high-T$_{c}$ superconductors YBa$_{2}$Cu$_{3}$O$_{6.5}$ and YBa$_{2}$Cu$_{4}$O$_{8}$ using first-principles pseudopotential methods with additional Coulomb interactions at Cu atoms and obtain Fermi-surface pocket areas in close agreement with measured Shubnikov-de Haas and de Haas-van Alphen oscillations. With antiferromagnetic order in CuO$_{2}$ planes, stable in the calculations, small hole pockets are formed near so-called Fermi-arc positions, reproducing the low-frequency oscillations. A large electron pocket, necessary for the negative Hall coefficient, is formed in YBa$_{2}$Cu$_{3}$O$_{6.5}$, giving rise to the high-frequency oscillation as well. Effective masses and specific heats are also calculated and compared with measurements. Our results highlight the crucial role of magnetic order in the electronic structure of underdoped high-T$_{c}$ superconductors. This work was supported by the KRF Grant No. KRF-2007-314-C00075, the KOSEF Grant No. R01-2007-000-20922-0, NSF Grant No. DMR07-05941, and DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2008-S02-0004), NSF through TeraGrid resources, and DOE NERSC.   

Temperature-dependent spectral weight transfer in YBa$_2$Cu$_3$O$_x$ probed by x-ray absorption spectroscopy

The x-ray absorption spectroscopy was utilized to critically examine the temperature dependency of the spectral weight in YBa$_2$Cu$_3$O$_x$. Large excess spectral weight for the Zhang- Rice singlet due to dynamics of holes is found with its doping dependence showing similar doom-like shape as that for $Tc$. Furthermore, appreciable spectral weight transfer from the upper Hubbard band to Zhang-Rice singlet was observed as the temperature acrosses the onset temperature for the pseudogap. The observed spectral weight transfer follows the change of the pseudogap, indicating a strong link between pseudogap and the upper Hubbard band.   

Session Z42: Many Body II

Test of Variational Procedures for Electronic Structure Studies by Comparison of Results for Energies of Atoms with Experiment and Results from Bruckner-Goldstone Many-Body Perturbation Theory --Neon Atom

During the latter half of the last century, great advances were made, through the Bruckner Goldstone Diagrammatic Many Body Perturbation Theory (BGMBPT), in accurate quantitative understanding of atomic properties. These investigations have provided a wealth of data which can now be used to test the accuracy of variational procedures in use currently for investigations of electronic structures and properties of multicenter systems like molecules and solid state systems. In the present talk, we shall consider neon atom where an earlier BGMBPT investigation [1] has provided excellent agreement with experiment for the total energy including correlation contributions. We have focused for this comparison on the Gaussian basis set based, first-principles Hartree-Fock procedure combined with Many Body Perturbation Theory, and the B3LYP procedure using DFT based exchange and correlation potentials, for neon. Results of our investigations will be presented and discussed.\\[4pt] [1] Taesul Lee, N.C. Dutta and T.P. Das, Phys. Rev. A4,1410(1971)   

Calculation of Correlation Functions using the Momentum Average Approximation

The Momentum Average (MA) approximation has been successfully applied to a growing number of Hamiltonians involving electron-phonon (el-ph) coupling since its discovery only a few years ago. This analytical non-perturbative approximation is exact in both the zero bandwidth and zero el-ph coupling limits, and by summing all of diagrams in the full diagrammatical expansion of the self-energy, albeit with approximations made on each of them, it gives highly accurate results over the entire parameter space. In this work we explore another significant generalization of the approximation by using MA to calculate correlation functions, where the optical conductivity of the Holstein polaron is used as a specific example. A comparison of the MA results against available numerical data again displays a high degree of accuracy for very minimal computational effort. Based on our previous generalizations of MA to systems with momentum-dependent el-ph couplings, we argue that MA could be used to study the linear response of an even broader class of problems.   

Exact solution to the random matrix problems from the symmetries of integrable systems

The problem with random non-Hermitian Hamiltonian (the Hatano-Nelson problem) arises in context of theory of depinning of the flux lines from extended defects in type II superconductors subject to a tilted external magnetic field. It is also of great interest in the context of random matrix theories. We employ a novel method, based on the inverse scattering/spectral transform, to obtain an exact analytic solution to the matrix version of the Hatano-Nelson problem. The idea is to use the exact connection between the linear (spectral or scattering problem) and exactly integrable nonlinear systems. This allows us to evaluate exactly the average over the ensemble of random Hamiltonians and to calculate the complex eigenvalue distributions for the random non-Hermitian matrices, the localization length, Lyapunov exponents. Applications of the method to other quantum and classical systems with random Hamiltonians both discrete and continuous, are discussed.   

First-Principles Study of Nuclear Quadruple Interaction of $^{19}$F* and Binding in Solid Fluorine

We have studied the binding energy (BE) and nuclear quadrupule interaction (NQI) parameters for the $^{19}$F* excited nuclear state in solid fluorine as part of our investigation [1] of the properties of solid halogens using the first principles Hartree-Fock Cluster procedure combined with many-body perturbation theory (MBPT), implemented by the Gaussian set of programs. Our results show that Van der Waals interaction obtained from intermolecular electron correlation effects has dominant influence on the BE but negligible effect on the NQI parameters. For the latter, ourcalculated e$^{2}$qQ is 119.0MHz using for Q(19F*), the value of 0.072 *10$^{-28}$m2 [2], and $\eta $, the asymmetry parameter, is essentially zero. The influence of rotational vibrational effects on e$^{2}$qQ is being investigated using a first-principles procedure [3] to bridge the small remaining difference with experiment (127.2 MHz) for e$^{2}$qQ [4]. [1] M.M. Aryal et al., Hyperfine Interact, 176, 51 (2007). [2] K.C.Mishra et al.,Phys. Rev.B25, 3389(1982). [3] N. Sahoo et al. Phys. Rev. Lett. 50, 913(1983) [4] H. Barfuss et al., Phys. Lett. 90A, 33(1982)   

Ab initio Calculations of X-ray Spectra: Comparison with Accurate Measurements

A number of advances in the theory of x-ray absorption (XAS) have been developed with the aim of achieving a parameter-free treatment of the key many-body effects.\footnote{J. J. Rehr et al., Comptes Rendus Physique, {\bf 10}, 548 (2009)} These include a GW many-pole self-energy model, {\it ab initio} Debye-Waller factors, and an RPA screened core-hole. These developments have been implemented in the real-space multiple-scattering code FEFF9.0, and applied to calculations of x-ray absorption spectra and electron energy loss spectra as well as a variety of other core level spectroscopies. Calculations span a broad spectrum from the visible to x-ray energies. Results for a number of materials are compared with previous theories and with accurate experimental data.   

Ab initio investigation of magnetic transport properties by Wannier interpolation

An efficient ab initio approach for the study of magnetic transport properties is developed based on the Boltzmann equation with the Wannier interpolation scheme. Using this method, we can investigate magnetoresistance [1], low field Hall coefficient, anomalous Hall effect, orbit magnetization, cyclotron motion and the effective mass, etc. As a typical application of this method, we present the band-resolved electric conductivities of MgB$_{2}$ under finite magnetic fields, multiband characters for the individual bands are revealed. Combined with experimental result, fully band resolved scattering rate for each band was obtained for MgB$_{2}$. It seems that the scattering from el-ph coupling or impurities affects the $\pi _{1}$ band more weakly [2]. \\[4pt] [1] Yi Liu, Hai-Jun Zhang, and Yugui Yao, Phys. Rev. B 79, 245123 (2009); \\[0pt] [2] H. Yang, et al. Phys. Rev. Lett 101, 067001 (2008).   

Dynamical Mean-Field Theory approach to study the magnetic properties of nanostructures

We extend the Nanoscale Dynamical Mean-Field Theory (NDMFT) approach [1] to study the magnetic properties of nanosystems. It is shown that the NDMFT solution becomes more accurate when the number of atoms in the cluster and their coordination number increase. We applied this method to study the magnetic properties of small cobalt clusters. The dependence of the cluster magnetization on the geometry, temperature and Coulomb repulsion energy was analyzed. We estimate the value of the Coulomb repulsion energy parameter, which is necessary to reproduce the experimental results in the case of different clusters. We compare our results with other approximations including GGA and GGA+U. In particular, we show that the last approximation in general overestimates the role of correlation effects. [1] S. Florens, Phys. Rev. Lett. 99, 046402 (2007)   

First-principles based calculation of phonon spectra and related properties in disordered alloys

The study of lattice vibrations in the presence of substitutionally disordered alloys is one of the most fascinating areas of condensed matter physics. A huge array of.experimental data is available for over nearly half a century awaiting interpretation of the microscopic understanding of the various kinds of disorder that play a role in the lattice dynamical properties of these alloys. The theoretical calculations, on the other hand, were limited due to the lack of a suitable tool which can address both diagonal(mass) and off-diagonal(force-constant) disorder in these systems. This problem has been alleviated recently with the advent of the `Itinerant Coherent Potential Approximation'. In this work, we propose a first-principles based methodology to compute the phonon spectra and properties derivable from them. The method is a combination of first-principles density functional perturbation theory, the transferable force constant approach by Van De Walle et al (Rev. Mod. Phys. 74, 11(2002)) and Itinerant Coherent Potential Approximation. We present results for the phonon spectra and elastic constants of disordered FePd alloys. .