Session L1: Basic Research Needs for Superconductivity

Superconductivity: Challenges and Opportunities

As part of its effort to define transformational opportunities for fundamental research in energy security, the Department of Energy's Office of Basic Energy Sciences held a workshop on Basic Research Needs for Superconductivity. The workshop identified a number of materials grand challenges and priority research directions for transforming the power grid to meet the needs of the 21st century. The prospect of moving from materials by serendipity to materials by design and of advancing the frontiers of epitaxial science to yield higher performing nano-structured architectures are two of the these challenges that could impact superconductivity research specifically and materials research more broadly. In this talk we highlight recent technical successes that motivate and illustrate these opportunities. We also discuss the science that might be necessary to accomplish these goals in the hopes of nucleating further community input and engagement. In collaboration with Wai Kwok, Argonne National Laboratory.   

Approaches to New Superconducting Materials

Over the last twenty years a large set of new superconductors with extraordinary properties have been discovered and studied. They are all by any measure complex materials, involving several elements arranged in complex crystal structures. Even the normal states of these materials exhibit highly correlated behavior, and the superconducting states are equally unusual, with complex order parameter symmetries, exotic vortex behavior and strong dependence on carrier doping. In the future, new complex materials that are chemically stable compounds will certainly continue to be found, at least some of which may result from rational searches. There is also an opportunity to create new materials in which the molecular, electronic, spin and phonon structure that sets the stage for the emergence of a superconducting state is defined artificially. Such ``meta-materials'' allow for the factors that are important for an emergent state to be included in new ways, and they also provide a test bed for predictive theory. Oxides are particularly well suited for this kind of work, since heterojunctions between different phases can be formed in many cases with little disorder. In other correlated systems, some new and useful properties not found in naturally occurring compounds have been engineered this way by researchers in several labs. Many heterojunction issues governing the resulting electronic and magnetic structure remain to be systematically studied, including state line-up, charge transfer, interface composition and bond energies to name a few.   

Structure and Dynamics of Vortex Matter

The DOE Basic Energy Sciences Workshop on Basic Research Needs for Superconductivity identified grand challenges and research priorities for \textit{discovery} and \textit{use inspired} basic research to transform the US power grid to meet the needs of the 21$^{st}$ century. Vortex matter research is central to this endeavor and helps support both fundamental and applied research. The science of vortex matter embodies the fundamental mysteries of vortex-vortex interactions in an inhomogeneous and anisotropic matrix. Understanding the complex phase diagrams and the dynamic responses that result from these competing effects is an outstanding challenge. Simultaneously, the prospect of controlling these interactions opens new horizons for basic research such as the development of a microscopic theory for vortex dynamics, exploration of vortex nucleation at magnetic and superconducting interfaces and designs for pinning a vortex liquid at high temperatures. This presentation will highlight ways in which nanotechnology based methodologies, dynamic vortex creep phenomena and powerful computer simulations play a role in enhancing our understanding of next-generation and new classes of superconductors.   

Understanding of Mechanisms for Design of Advanced Superconductors

A recent DOE panel considered the future of research in superconducting materials and made a number of recommendations for priority research directions ({\it http://www.er.doe.gov/bes/reports/files/SC\_rpt.pdf}), two of which will be discussed. These items, under the rubric of {\it Enabling Superconductivity}, emphasize that {\it Finding the Mechanisms} is essential for furthering the field, and that once understood, the prospect of {\it Superconductors by Design} becomes a viable line of research. Establishing the mechanism in the high temperature superconducting cuprates continues to attract substantial efforts, with no consensus near. In several superconductors, including some discovered in the past decade or so, having T$_c$ around or above 20 K [(Ba,K)BiO$_3$; Li$_x$HfNCl; PuCoGa$_5$] the mechanism is in question. On the more positive side, there are several cases established in the past six years, beginning with MgB$_2$ and extending to elemental metals under pressure (Li, Y, Ca), where the familiar electron-phonon mechanism has provided unexpectedly high T$_c$ and thereby stimulated enthusiasm and optimism into this area of superconductivity research. The clear understanding of this mechanism (at least in many respects) provides a path for improvements in superconducting materials.   

Transforming the Grid with Superconductivity

The electric power grid in the United States faces critical challenges: overloading caused by years of limited investment and steady load growth, bottlenecks in power corridors into urban centers, voltage instability leading to brownouts and blackouts, growing fault currents in large urban and suburban areas, as well as the need for increased efficiency. Power equipment based on high temperature superconductors (HTS) offers solutions to these challenges: high capacity, non-interfering HTS cables addressing power bottlenecks, HTS fault current limiters controlling fault currents, HTS synchronous condensers and novel controllability features of HTS cables which address stability issues, HTS transformers and generators with increased efficiency. A variety of commercial-level demonstrations make the impact of HTS power equipment imminent.   

Session L2: Organic and Molecular Bistability and Memory Devices

Organic electrical bistable devices and applications as electronic digital memory

Recently, organic electrical bistability has attracted considerable attention due to its potential applications as digital memory devices. In this presentation, we will present our recent study on organic electrical bistability phenomena and the application as nonvolatile memory devices (NVM). The bistability was discovered when a thin layer of metallic nano-particles introduced between two organic layers as the active cell, which interposed between two electrodes. We attribute this bistaility to the charge transfer and trap in the metal nano-particles. A further material engineering by dispersing metal nano-particles and organic electron donor within polymer films as the active cell, it forms the polymer-based memory devices. When the metal nanoparticles are integrated with tobacco mosaic virus as the active cell, it forms a virus-based (or bio-based) memory device. Mechanism studies on the polymer and bio-based memory device reveal that charge-storage in the metal nanoparticles plays an important role in the device operation. Details of the mechanism study and the memory device performance will be discussed in this presentation.   

Evaluation of switchable organic devices for nonvolatile memory applications

Many organic electronic devices exhibit switching behavior and have therefore been proposed as the basis for a nonvolatile memory technology. In particular, bistable resistive elements, in which a high or low current state is selected by application of a specific voltage, may be used as the elements of a crosspoint memory array. This architecture places very stringent requirements on the electrical response of the individual devices, in terms of on-state current density, switching and retention times, cycling endurance, rectification and size-scaling. In this talk, I will describe the progress that we and others have made towards satisfying these requirements. In many cases, the mechanisms responsible for conduction and switching are not fully understood. In some devices, it has been shown that current flows in a few highly localized regions. These so-called ``filaments'' are not necessarily metallic bridges between the electrodes, but may be associated with chains of nanoparticles introduced into the organic matrix either deliberately or accidentally. Coulomb blockade effects can then explain the switching behavior observed in some devices. \newline \newline This work was done in collaboration with L. D. Bozano, M. Beinhoff, K. R. Carter, V. R. Deline, B. W. Kean, G. M. McClelland, D. C. Miller, P. M. Rice, J. R. Salem, and S. A. Swanson.   

Controlling nanostructure in organic films to achieve high photovoltaic efficiency

We discuss the materials and device structures used to attain high efficiency organic solar cells based on small molecular weight organic thin films. The influence of structural morphology introduced using both vacuum thermal evaporation and organic vapor phase deposition in so-called bulk heterojunction and mixed molecular heterojunction cells is described. Furthermore, we describe the growth of all-organic nanostructures by organic vapor phase deposition to achieve very high solar energy conversion efficiencies. Many of these approaches have potential for resulting in solar power conversion efficiencies $>$10{\%}. In addition, materials and strategies for increasing the open circuit voltage, and to extend the sensitivity of organic solar cells out into the near infrared spectral region are discussed.   

Large-Scale Molecular and Nanoelectronic Circuits \& Associated Opportunities

According to the International Technology Roadmap for Semiconductors (ITRS), by year 2020 it is expected that the most closely spaced metallic wires within a DRAM circuit will be patterned at a pitch of about 30 nm, implying that the conductors themselves will be of a width of around 15 nm. However, virtually every aspect of achieving this technology is considered to be `red,' meaning that there is no known solution. Nevertheless, ultra-high density (semiconductor and metallic) nanowire circuitry, fabricated at 2020 dimensions and beyond, would be expected to provide a host of value traditional (logic \& memory) and nontraditional (sensing, thermoelectrics, etc.) functions. In this talk we will discuss the fabrication and testing of large scale circuitry ($>$ 10$^5$ devices) aimed a these various applications. This will include a 160,000 bit memory circuit that is no larger than a white blood cell, high-performance, ultra-dense \& energy efficient logic circuitry, nanowire sensing arrays, and high-performance silicon-based thermoelectric devices. We will also discuss how these circuits may be fabricated on a host of substrates, including plastic.   

Session L3: Novel Phenomena in Granular Systems with Complex Interactions

Thresholds and Dynamics for Oscillating Granular Layers

The onset and dynamics of flow in shallow horizontally oscillating granular layers are studied and compared to the behavior of avalanches. The variation with depth of the starting acceleration for the oscillating layer matches (approximately) the corresponding variation of the tangent of the starting angle for avalanches in the same container at low frequencies, but deviates as the frequency is increased. However, the threshold behavior depends significantly on the measurement protocol. Just above threshold, the motion decays with time as the material re-organizes over a minute or so, causing the apparent threshold to increase. Once excited, the rheology of the material is found to vary in time during the cycle in surprising ways. If the maximum inertial force (proportional to the container acceleration amplitude) is slightly higher than that required to produce flow, the flow velocity grows as soon as the inertial force exceeds zero in each cycle, but jamming occurs long before the inertial force returns to zero. At higher acceleration, the motion is fluid- like over the entire cycle. However, the fraction of the cycle during which the layer is mobile is typically far higher than what one would predict from static considerations or the behavior of the inclined layer. Finally, we consider the flow profiles as a function of both the transverse distance across the cell at the free surface, and also as a function of the vertical coordinate in the boundary layer near the sidewall. These profiles have time-dependent shapes, and are therefore significantly different from profiles previously measured for avalanche flows.   

Particle Shape and Dynamics of Granular Matter: Swarming to Swirling

We will discuss a series of experiments performed with granular rods, dimers, and flexible chains on a vibrated plate to illustrate the effect of particle shape on self-organization. A non-spherical shape is shown to lead to not only states which resemble nematic and smectic phases but also causes novel dynamics [1]. The ratchet mechanism which leads to vortex motion in a collection of rods on a vibrated plate and drift motion in a bouncing dimer will be discussed [2, 3]. The friction at the point of contact between particle and the substrate, and the coupling about the center of mass of a non-spherical is proposed to lead to observed motion. Exploiting this mechanism we construct mechanical self-propelled particles (SPP) using rods with asymmetric mass distributions. We then investigate the SSP number fluctuations, flow fields, and orientation order inside a container as a function of number density and excitation, and compare their statistics with recent models of active nematic particles and living cells.\\ 1. ``Vortices in vibrated granular rods," D.L. Blair, T. Neicu, and A. Kudrolli, Phys. Rev. E 67, 031303 (2003).\\ 2. ``Anisotropy driven dynamics in vibrated granular rods," D. Volfson, A. Kudrolli, and L.S. Tsimring, Phys. Rev. E 70, 051312 (2004).\\ 3. ``Dynamics of a bouncing dimer," S. Dorbolo, D. Volfson, L. Tsimring, and A. Kudrolli, Phys. Rev. Lett. 95, 044101 (2005).   

Externally driven magnetic granular layers at a liquid/air interface: self-organization, flows and magnetic order

Collective dynamics and pattern formation in ensembles of magnetic microparticles suspended at the liquid/air interface and subjected to an alternating magnetic field are studied. Experiments reveal a new type of nontrivially ordered dynamic self-assembled structures (``snakes'') emerging in such systems in a certain range of field magnitudes and frequencies. These remarkable structures are directly related to surface waves in the liquid generated by the collective response of magnetic microparticles to the alternating magnetic field. In addition, a large-scale vortex flows are induced in the vicinity of the dynamic structures. Some features of the self-localized snake structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density. Self-assembled snakes have a complex magnetic order: the segments of the snake exhibit long-range antiferromagnetic ordering mediated by the surface wave, while each segment is composed of ferromagnetically aligned chains of microparticles. A phenomenological model describing magnetic behavior of the magnetic snakes in external magnetic fields is proposed.   

Nonlinear dynamics of semiflexible magnetic filaments in an external ac magnetic field

Chains of magnetic particles exist in nature (magnetotactic bacteria, magnetic colloids) and can be created artificially by linking magnetic particles with some polymer (PAA,DNA). Theoretical description of magnetic filaments is based on models of semiflexible polymers extended by incorporation of the effects of body torques due to long-range magnetic interactions. On the basis of these models different phenomena are described - buckling due to body torques, self-propulsion in an AC field, tumbling in the shear flow, orientation of ferromagnetic filaments in the direction perpendicular to an AC field, liquid flow excited by oscillating in an AC field tips of magnetic filaments floating on the surface of the liquid and others. Connection of equilibrium shapes of magnetic filaments with solutions of elastica problem is established. Different regimes of magnetic response of the suspension of magnetic filaments are analyzed by taking into account the thermal noise.   

Nematic Ordering in a Population of Growing and Dividing Rod-like Cells

Morphogenesis is one of the most important themes in biology, and it is also central to nonequilibrium physics. The fundamental issue is to understand how local interactions of elementary components lead to collective behavior and the formation of a highly organized system. In nature this self-organization is found on many different scales, from single cells to schools of fish and herds of animals. Collective behavior leads to significant selective advantages for living organisms. At low density, communication among cells occurs mainly due to chemotaxis, the mechanical response of cell to the gradients of chemicals emitted by other cells. At higher densities, steric exclusion effects may strongly affect their collective behavior. In this work we focus on the mechanical interaction among non-motile bacteria in engineered biofilms. These biofilms are formed by growing two-dimensional bacterial colonies in a highly controlled microfluidic environment. We combine experimental observations and analysis with discrete-element molecular dynamics simulations and theoretical modeling to provide mesoscopic description of the biofilm growth. Our results reveal how cell growth and colony expansion trigger the formation of the orientational (nematic) order in the biofilms.   

Session L4: DCMP / GQI Prize Session

Quantum Measurement with the Josephson Bifurcation Amplifier

The Josephson tunnel junction is a unique dipolar circuit element which can be both non-linear and non-dissipative. This combination makes it well suited to measuring quantum systems since non-linearity enables fast, sensitive detection while the absence of dissipation reduces loss of coherence. When the junction is driven close to a bifurcation point with a sufficiently intense microwave drive, then two metastable states exist which differ in oscillation amplitude and phase. The junction remains confined to a single well of its sinusoidal potential in both of these states and no DC voltage is generated. The oscillation state of the junction can be determined by measuring either the reflected or transmitted AC microwave drive signal. The transition between these dynamical states is a sensitive function of the junction critical current. Therefore, the critical current serves as the input variable of the amplifier and can be modulated by the application of a magnetic flux, electric charge, or a superconducting phase. The bifurcation amplifier has been successfully used for the state readout of superconducting qubits, and has many potential applications including the coherent detection of magnetic nanostructures such as single molecule magnets.   

Prize for Research at an Undergraduate Institution Talk: A Discrete Wigner Function

For a quantum particle moving in one dimension, the Wigner function represents the particle's quantum state as a real function on the two-dimensional phase space. Though the Wigner function typically takes negative values and can therefore not be interpreted as a probability distribution, its integral along any axis in phase space---even a skew axis---is in fact the probability distribution of an observable associated with that axis. A number of authors have developed generalizations of the Wigner function that apply to discrete quantum systems, but such generalizations are often problematic when the state-space dimension is even. Here we present a discrete Wigner function that shares with the continuous Wigner function the ``tomographic'' property described above, and is well suited to describe a system of binary quantum objects. We discuss potential applications to quantum computation and quantum cryptography.   

Atoms in a Cavity: A Source of Narrowband Photon Pairs

Coupling atoms to an optical cavity can significantly enhance the directionality of photon emission from atoms. Using such an atoms-cavity system, we have created a high-brightness source of narrowband, identical-photon pairs. The source was applied to two experiments: interferometry and entanglement. Biphoton interferometry holds promise to demonstrate precision beyond the shot noise limit, although the measured interference fringe visibility of 0.84 $\pm$ 0.04 only translated to a shot noise limited phase uncertainty. Polarization-time entangled photon pairs were also directly generated via an adjusted optical pumping scheme for the atoms.   

Low-temperature infrared spectroscopy of H$_2$ in solid C$_{60}$

Diffuse reflectance infrared spectroscopy was used to probe the quantum dynamics of H$_2$ trapped in a C$_{60}$ lattice. Because free H$_2$ is infrared inactive, features of the infrared spectra are induced solely through interactions with the host material and as such provide detailed information about the potential at the binding site. The design and construction of a cryogenic apparatus allowed the extension of previous room temperature measurements to temperatures as low as 10 K at pressures as high as 100 atm. The low temperature spectra contained much sharper peaks and a rich fine structure, enabling more precise determination of the details of the C$_{60}$-H$_{2}$ interaction potential. These studies of H$_2$ in C$_{60}$ inform hydrogen storage materials research in a broader context, as illustrated by the diffuse reflectance spectra of H$_2$ in MOF-5.   

Maximum entropy-principle approach to quantum storage in strongly correlated systems

We shall investigate whether it is possible to generate QUBITS and/or QUTRITS starting with a modified version of Hubbard-Anderson Hamiltonian pertinent to describe magnetic properties of strongly correlated systems, particularly manganites. For this purpose, we shall derive the expressions for expectation values of a set of relevant operators starting with the Shanon entropy and using maximum entropy principle. It also allows us to derive Weiss relation that relates the spin-projection at a site to the interaction of that site with the rest of the medium. In the presence of an internal or applied magnetic field, or both, the absolute minima of free energy for spin projection in z-direction is +1, 0 and -1 for a triplet pair of fermion at three different temperatures, which are identified as QUBITS (in case one does not distinguish between $\pm 1$ projections) or QUTRITS.   

Session L5: Spin Manipulation in Semiconductors and Metals for Spintronics

Coulomb Interaction in the Spin-Hall Effect

The spin-Hall effect is the generation of a steady spin current perpendicular to an externally imposed d.c. electric field. The effect is driven by spin-orbit interactions but its details are influenced by several processes like electron-impurity scattering, electron-electron scattering, and spin precession. In this talk I describe our recent work on the role of electron- electron scattering in the spin Hall effect in an n-type [110] GaAs quantum well, where spin precession is absent. We have studied the spin Hall conductivity (SHC) by a combination of the Boltzmann equation and the Kubo formula for the spin current [1],[2]. The two main contributions to the SHC -- ``skew scattering'' (SS) and ``side-jump'' (SJ) -- respond very differently to the inclusion of Coulomb interactions. The SS contribution is significantly reduced by the spin Coulomb drag -- the Coulomb friction between electrons of opposite spin orientations. At the same time, the SJ contribution remains completely unaffected by Coulomb scattering. The different behaviors of the SS and SJ contributions result in a Coulomb- induced reduction of the spin accumulation at the edges of a spin Hall bar, even when the spin current is zero. We have also pointed out that the relative size of the SJ and SS contributions depends on mobility and we have proposed an experiment to distinguish between the two [2]. [1] E. M. Hankiewicz and G. Vignale Phys. Rev. B 73, 115339 (2006) [2] E. M. Hankiewicz, G. Vignale and M. E. Flatt\'e cond- mat/0603144 (PRL in press)   

Electrically-Induced Polarization and the Spin Hall Effect in Semiconductors at Room Temperature

The capability to generate and manipulate spin polarization through the spin-orbit interaction inspires growing interest in all-electrical techniques to exploit electron spins for applications in semiconductor spintronics. Experiments show spin polarization can be electrically generated by current- induced spin polarization from internal magnetic fields in the bulk of a conducting channel, or accumulation of spin polarization near sample edges due to transverse spin currents generated by the spin Hall. These spin currents can drive spin accumulation over micron length scales in semiconductor arms transverse to a conducting channel \footnote{V. Sih, W. H. Lau, R. C. Myers, V. R. Horowitz, A. C. Gossard and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 096605 (2006).}. More recently, we investigate electrical generation of spin polarization in n-ZnSe epilayers using Kerr rotation spectroscopy\footnote{N. P. Stern, S. Ghosh, G. Xiang, M. Zhu, N. Samarth, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 126603 (2006)}. The internal magnetic field is studied and found to only be measurable in strained layers, likely due to the weak spin-orbit interaction in ZnSe. Despite this, unstrained n-ZnSe layers exhibit both in-plane bulk current-induced spin polarization and an out-of-plane spin accumulation of opposite sign on opposite edges of a conducting channel indicative of the spin Hall effect. The spin Hall conductivity is estimated according to a spin accumulation model and is found to be consistent with the extrinsic spin- dependent scattering mechanism. Both the current-induced spin polarization and the spin Hall effect are robust to room temperature in ZnSe. These results suggest the potential for practical utilization of electrically generated spin polarization in room temperature semiconductor devices.   

Spin Transport and Scattering in Ferromagnetic Semiconductor Heterostructures

A fundamental understanding of the transport and scattering of spin-polarized carriers in semiconductors is central to the development of semiconductor spintronics. We describe recent work that probes the spin-dependent transport of holes in heterostructures derived from the ferromagnetic semiconductor (Ga,Mn)As. In tensile-strained (Ga,Mn)As/(In,Ga)As heterostructures with perpendicular magnetic anisotropy, we observe a longitudinal magnetoresistance that is {\it antisymmetric} in magnetic field and attributed to slowly propagating magnetic domain walls [1]. This is confirmed both by a simple calculation and by measuring patterned submicron channels designed to trap single domain walls. In (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayer heterostructures, we demonstrate an all-semiconductor spin-valve effect, despite short spin-diffusion and elastic scattering lengths in the spacer layer [2]. Magnetoresistance (MR) measurements carried out in the current-in-plane geometry reveal positive MR peaks when the two ferromagnetic layers are magnetized orthogonal to each other. Measurements with different post-growth annealing conditions and spacer layer thickness show that the positive MR originates in a noncollinear spin valve effect due to spin-dependent scattering at interfaces. \newline \newline [1] G. Xiang, A. W. Holleitner, B. L. Sheu, F. M. Mendoza, O. Maksimov, M. B. Stone, P. Schiffer, D. D. Awschalom, N. Samarth, Phys. Rev. B {\bf 71}, 241307(R) (2005).\newline [2] G. Xiang, M. Zhu, B. L. Sheu, P. Schiffer, N. Samarth, cond-mat/0607580.   

Electrical detection of spin transport in lateral ferromagnet-semiconductor devices

A fully electrical scheme of spin injection, transport, and detection in a single ferromagnet-semiconductor structure has been a long-standing goal in the field of spintronics. In this talk, we report on an experimental demonstration of such a scheme. The devices are fabricated from epitaxial Fe/GaAs (100) heterostructures with highly doped GaAs as a Schottky tunnel barrier. A set of closely spaced Fe contacts on the top of an n-GaAs channel are used as spin injectors and detectors. Reference electrodes are placed at the far ends of the channel, allowing for non-local spin detection [1]. The electro-chemical potential of the detector is sensitive to the relative magnetizations of the injector and detector. In spin-valve measurements, a magnetic field is applied along the Fe easy axis to switch the relative magnetizations of injector and detector from parallel to antiparallel, resulting in a voltage jump that is proportional to the non-equilibrium spin polarization in the channel. A more rigorous test of electrical spin detection is the observation of the Hanle effect, in which an out-of-plane magnetic field is used to modulate and dephase the spin polarization in the channel. The magnitudes of the observed Hanle curves agree with the results of the spin-valve measurements. The dependence of the Hanle curves on temperature and contact separation is studied in detail and is consistent with a drift-diffusion model incorporating spin precession and relaxation. The spin polarization generated by spin injection (reverse bias at the injector) or spin accumulation (forward bias at the injector) is measured using the magneto-optical Kerr effect and is found to be in good agreement with the spin-dependent non-local voltage. Both the transport and optical measurements show a non-linear relationship between the bias voltage at the injector and the spin polarization in the channel. [1] M. Johnson and R. H. Silsbee, Phys. Rev. Lett. \textbf{55}, 1790 (1985).   

Current-induced domain wall motion in ferromagnetic semiconductors

Low magnetization ($\sim $0.05 T) and high spin-polarization in ferromagnetism of transition metal-doped GaAs allow us to explore a number of spin-dependent phenomena not readily accessible in metal ferromagnets. Spin-polarized current induced domain wall (DW) motion in (Ga,Mn)As [1, 2] reveals rich physics resulting from the interaction between spin-polarized electrons and localized spins inside a magnetic DW. By using a 30 nm thick (Ga,Mn)As layer ($x_{Mn}$ = 0.045) with perpendicular magnetic anisotropy, we have measured by magneto-optical Kerr microscopy a wide range of velocity-current density curves in the sample temperature range of 97 -- 107 K. Two regimes are found in the current density dependence of the DW velocity. At high-current densities ($>$ 2 x 10$^{5}$ A/cm$^{2})$, the domain wall velocity is approximately a linear function of the current density above a threshold current density. This result will be compared to the recent theories of DW motion. At low-current densities, the functional form of the velocity-current curves follow an empirical scaling law, obtained by modifying the one for magnetic-field induced creep. This shows that current-induced DW creep is present. We have also determined the intrinsic resistance of the DW in a similar configuration [3]. \begin{enumerate} \item M. Yamanouchi, D. Chiba, F. Matsukura, and H. Ohno, Nature \textbf{428}, 539 (2004). \item M. Yamanouchi, D. Chiba, F. Matsukura, T. Dietl and H. Ohno, Phys. Rev. Lett. \textbf{96}, 096601 (2006). \item D. Chiba, M. Yamanouchi, F. Matsukura, T. Dietl, and H. Ohno, Phys. Rev. Lett. \textbf{96}, 096602 (2006). \end{enumerate}   

Session L6: Minorities in Medical Physics

Building a new Radiation Oncology Department

This presentation will consist of a description of some of the steps followed to organize a new radiation oncology department in a medium size community hospital. In here the duties and involvement of the medical physicist at the different stages of the project are explained. Also some of the new techniques such as Intensity Modulated Radiation Therapy, Image Guided Therapy with Cone Beam CT and Respiratory Gating now implemented in the new department will be briefly presented along with our personal experience.   

Methods to Differentiate Radiation Necrosis and Recurrent Disease in Gliomas

Given the difficulty in differentiating Radiation Induced Necrosis (RIN) and recurrent disease in glioma patients using conventional techniques (CT scans, MRI scans), researchers have looked for different imaging modalities. Among these different modalities are Diffusion Weighted Magnetic Resonance Imaging (DWMRI) and Magnetic Resonance Spectroscopy (MRS). In DWMRI, an {\em{Apparent Diffusion Coefficient}} (ADC) is calculated for a Region Of Interest (ROI), and then monitored over time (longitudinally). In the brain, different anatomical features can complicate the interpretation of ADCs. In particular, the density and spatial variation of the cerebral spinal fluid filled fissures known as {\em{sulci}} can influence how a change in an ADC is explained. We have used the covariance of pixel intensity in T1 weighted MRI scans to study how intra-patient and inter-patient sulci density varies, and will present these results. MRS uses the shift in the MR signal due to the local chemical environment to determine the concentration of brain metabolites like choline and creatin. The ratio of metabolites such as these has been shown to have the power to discriminate between RIN and recurrent disease in glioma patients. At our institution, we have initiated a protocol whereby we will use DWMRI and MRS to study how best to utilize these complimentary forms of imaging.   

A journey into medical physics as viewed by a physicist

The world of physics is usually linked to a large variety of subjects spanning from astrophysics, nuclear/high energy physics, materials and optical sciences, plasma physics etc. Lesser is known about the exciting world of medical physics that includes radiation therapy physics, medical diagnostic and imaging physics, nuclear medicine physics, and medical radiation safety. These physicists are typically based in hospital departments of radiation oncology or radiology, and provide technical support for patient diagnosis and treatment in a clinical environment. This talk will focus on providing a bridge between selected areas of physics and their medical applications. The journey will first start from our understanding of high energy beam production and transport beamlines for external beam treatment of diseases (e.g., electron, gamma, X-ray and proton machines) as they relate to accelerator physics. We will then embrace the world of nuclear/high energy physics where detectors development provide a unique tool for understanding low energy beam distribution emitted from radioactive sources used in Brachytherapy treatment modality. Because the ultimate goal of radiation based therapy is its killing power on tumor cells, the next topic will be microdosimetry where responses of biological systems can be studied via electromagnetic systems. Finally, the impact on the imaging world will be embraced using tools heavily used in plasma physics, fluid mechanics and Monte Carlo simulations. These various scientific areas provide unique opportunities for faculty and students at universities, as well as for staff from research centers and laboratories to contribute in this field. We will conclude with the educational training related to medical physics programs.   

Protons -- The Future of Radiation Therapy?

Cancer is the 2$^{nd}$ highest cause of death in the United States. The challenges of controlling this disease remain more difficult as the population lives longer. Proton therapy offers another choice in the management of cancer care. Proton therapy has existed since the late 1950s and the first hospital based center in the United States opened in 1990. Since that time four hospital based proton centers are treating patients with other centers either under construction or under consideration. This talk will focus on an introduction to proton therapy: it's medical advantages over current treatment modalities, accelerators and beam delivery systems, applications to clinical radiation oncology and the future outlook for proton therapy.   

The Roles and Responsibilities of a Medical Physicist.

Since the discovery of x-rays in 1895, they have played a very important role in medicine. As the use and need for x-rays and radioactive material in medicine has progressed, the role of the medical physicist has also expanded. Imaging in the diagnosis of disease continues to change as new modalities e.g. magnetic resonance imaging become available and as the modalities improve with advances in technology. Similarly, the treatment of disease with radiation has improved with new developments. The role of the medical physicist will be discussed especially as it relates to the changes in technology.   

Session L7: Physics Education Award - PSSC

The PSSC project in perspective

The talk will consist of three parts. (1) A tribute to J.R. Zacharias and F.L. Friedman, who would have received the award had they been alive today, (2) A brief description of the characteristics of the project, and (3) some lessons for future projects in the area of development of a physics curriculum.   

Jerrold Zacharias and PSSC

In 1956 with NSF support Jerrold Zacharias, a notable professor of physics at MIT, called into existence the Physical Sciences Study Committee and launched the largest effort ever to change and improve the teaching of physics in American high schools. Zacharias had a talent for eliciting bold ideas from the best physicists of his day and then inspiring them to put their ideas into action. PSSC was just one of many instances when he did this. He was a member of that cohort of physicists whose accomplishments in World War II empowered them with confidence and authority that they applied with great effect in the early years of the Cold War.\footnote{Charles H. Holbrow, Scientists, Security, and Lessons from the Cold War, \textit{Physics Today } \textbf{59}(7), 39-44 (2006).} More than passive agents of the government, they influenced it to respond to various crises with broader vision and higher idealism than were associated with conventional views of defense. PSSC exemplifies their ability to spin the straw of Cold War tension and fear into the gold of major educational reform. I will describe some memorable aspects of the man and his times, and how he and Francis Friedman shaped the early efforts of PSSC.\footnote{\textit {PSSC Remembered} -- An AAPT Online Publication at http://www.aapt-doorway.org.}   

Twenty Eight Years with PSSC and Still Counting

When this instructor arrived at Christian Brothers High School in Memphis, Tennessee twenty eight years ago, PSSC Physics had already been in use from 1962. Now with the Seventh Edition it is still being used to teach some ninety students per year. A brief history, some reminiscences, and a description of the current program are provided, illustrated with samples of current student work.   

Beginnings of PSSC, Film Experiences, Later Thoughts

As an MIT student and faculty member I worked with Jerrold Zacharias and Francis Friedman, and was in on the genesis, planning and early evolution of what became the PSSC program. Zach's experiences with high school students and the views on education of Edwin Land (inventor of polaroid and founder of the Polaroid Co.) were important in forming the program. I was involved in the PSSC movies and had roles in about a dozen films as principal or advisor or builder of apparatus. I will tell about this aided by excerpts from memos, speeches and films. I will also describe how this has led to my view of what needs to be done in education now.   

Session L8: Superconductivity Theory: Strongly Correlated Systems

Superconductivity in Cuprates through Loop Current Fluctuations

The quantum-critical fluctuations of the loop-current order parameter discovered [1] in underdoped cuprates has been derived [2] recently to be of the phenomenological form proposed to produce the marginal fermi-liquid [3]properties in the normal state. The coupling function of these fluctuations to fermions is calculated and an effective particle-particle scattering through exchange of such fluctuations is generated. Partial wave decomposition of this scattering shows attractive interaction in the d-wave pairing channel. The coupling constant and the cut-off of the fluctuations is used to estimate the order of magnitude of $T_c$. Variation of $T_c$ with hole density is also discussed. [1] C.M. Varma, Phys. Rev. {\bf B73}, 155113 (2006); B. Fauque et al., Phys. Rev. Lett, {\bf 96}, 197001 (2006); A. Kaminski, et al., Nature {\bf 416}, 610 (2002). [2] Vivek Aji and C. M. Varma, cond-mat/0610646. [3] C.M. Varma, et al. Phys. Rev. Lett., {\bf 63}, 1996 (1989).   

Superfluid Stiffness, Nodal Quasiparticles and Quantum Phase Fluctuations in Underdoped Cuprates

We study the low temperature superfluid stiffness $\rho_s(T;x)$ as a function of hole doping $x$ and temperature $T$ for strongly correlated d-wave superconductors. Using Gutzwiller projected wavefunctions and renormalized mean-field theory (RMFT), we calculate $\rho_s(0;x)$ and show that it scales with the quasiparticle spectral weight $Z$. These analytical results are in excellent agreement with earlier variational Monte Carlo studies [1]. We next show that self-consistent inclusion of the zero point motion of phase fluctuations leads to further suppression of $\rho_s(0;x) $, which now vanishes below a doping level of approximately $5\%$. To determine the $T$-dependence of $\rho_s$ we calculate the current carried by nodal quasiparticles (QP) within RMFT and show that the effective charge of the nodal QP is given by $Z m^\ast/m$. Our analytic formula for the effective charge is in excellent agreement with numerical Monte Carlo results of Nave {\it et al.}~[2]. We will conclude by comparing our results with experiments on underdoped cuprates. \\ \noindent [1] A. Paremakanti, M. Randeria and N. Trivedi, Phys. Rev. Lett. {\bf 87}, 217002 (2001) \\ \noindent [2] C. P. Nave, D. A. Ivanov and P. A. Lee, Phys. Rev. B. {\bf 73}, 104502 (2006)   

Phenomenological theory of the underdoped phase of a high-T$_c$ superconductor

We model the Fermi surface of the cuprates by one-dimensional nested parts near $(0,\pi)$ and $(\pi,0)$ and unnested parts near the zone diagonals. Fermions in the nested regions form 1D spin liquids, and develop spectral gaps below some $\sim T^*$, but superconducting order is prevented by 1D phase fluctuations. We show that the Josephson coupling between order parameters at $(0,\pi)$ and $(\pi,0)$ locks their relative phase at a crossover scale $T^{**}< T^*$. Below $T^{**}$, the system response becomes two-dimensional, and the system displays Nernst effect. The remaining total phase gets locked at $T_c < T^{**}$, at which the system develops a (quasi-) long-range superconducting order.   

Competing ferromagnetism in high temperature copper oxide superconductors

While much attention has been paid to the underdoped regime of the hole-doped cuprates because of its proximity to a complex Mott insulating phase, little attention has been paid to the overdoped regime. Experiments are beginning to reveal that the phenomenology of the overdoped regime is just as puzzling. For example, the electrons appear to form a Fermi liquid, but this interpretation is problematic; any trace of Mott phenomena, as signified by incommensurate antiferromagnetic fluctuations, is absent, and the uniform spin susceptibility shows a ferromagnetic upturn.Here we show and justify that many of these puzzles can be resolved if we assume that competing ferromagnetic fluctuations are simultaneously present with superconductivity, and the termination of the superconducting dome in the overdoped regime marks a quantum critical point beyond which there should be a genuine ferromagnetic phase at zero temperature. We propose new experiments, and make new predictions, to test our theory and suggest that effort must be mounted to elucidate the nature of the overdoped regime, if the problem of high temperature superconductivity is to be solved.   

Theory of Infrared Hall Conductivity of Electron-doped Cuprates

It has been proposed by several experiments that the electron-doped cuprate Pr$_{2-x}$Ce$_x$CuO$_{4+\delta}$ undergoes a quantum phase transition to an antiferromegnetic state for the doping x smaller than 0.16. Here, we investigate the infrared Hall conductance of the electron-doped cuprates in the commensurate spin density wave state, using the linear response theory. The qualitative agreement between our results and the available experimental data provides strong evidence in favor of the spin density wave scenario and suggests that the magnitude of the gap is large, while quantitative discrepancies point towards additional physics which may be related to scattering of carriers off spin fluctuations. We also discuss the Hall conductivity sum rule and its connection to the experiments.   

Strong correlations lead to protected low energy excitations in disordered d-wave superconductors

We show that strong correlations play a vital role in protecting low energy excitations in disordered high temperature superconductors. The impurity-induced low-energy density of states (DOS) is greatly reduced in the strongly correlated superconductor compared to d-wave Bogoliubov-deGennes theory which ignores strong correlations. The gapless nodal quasiparticles, and the resulting `V' in the low-energy DOS, are much more robust against disorder compared to the large-gap antinodal excitations. We discuss the relevance of our results to angle-resolved photoemission and scanning tunneling spectroscopy experiments. Reference: A. Garg, M. Randeria, and N. Trivedi, cond-mat/0609666   

Electron-phonon renormalization in Cuprates.

Electron-phonon (e-ph) renormalization effects in a model cuprate system, CaCuO2, are studied by employing density functional theory based methods. Whereas calculations based on the local spin density approximation (LSDA) predicts negligible e-ph coupling effects of the half-breathing Cu-O bond stretching mode, the inclusion of a screened on-site Coulomb interaction (U) in the LSDA+U calculations greatly enhances the e-ph coupling strength of this mode. The full breathing mode, on the other hand, shows a much weaker e-ph renormalization effect. Enhanced oxygen-p character of the top valence states, together with the (local) antiferromagnetic spin ordering, seems to be responsible for a strong e-ph coupling of the half-breathing mode in the LSDA+U calculations.   

``Underlying Fermi surface'' and violation of Luttinger count in strongly correlated superconductors

The question of determining the ``underlying Fermi surface'' (FS) that is gapped out by superconductivity (SC) is of great importance in strongly correlated systems, particularly in view of angle-resolved photoemission (ARPES) experiments. We explore various definitions for the FS in the T=0 SC state using the zero-energy Green's function, the excitation spectrum and the momentum distribution. We examine (i) the d-wave SC in high Tc cuprates, and (ii) the s-wave superfluid in the BCS-BEC crossover. In each case we show [1] that the various definitions agree, to a large extent, but all of them violate the Luttinger sum rule and do not enclose the total electron density. We discuss the important role of chemical potential renormalization and incoherent spectral weight in this violation. We show that the magnitude of the violation scales like $(\Delta/E_f)^2$, and its sign correlates with the electron-like or hole-like topology of the FS. These results are in good agreement with ARPES data on LSSCO [2]. \\ \noindent [1] R. Sensarma, M. Randeria, N. Trivedi, cond-mat/0607006. \\ \noindent [2] T. Yoshida {\it et al.}, cond-mat/0510608.   

High Energy features in the photoenission spectra of cuprates

We calculate the real part of the self-energy of fermions scattering off the quantum critical fluctuations derived for cuprates. At the upper cut-off of the quantum critical fluctuation spectra a logarithmic divergence in the real part of the self-energy occurs. The position of the peak of the one-particle spectral function is pinned near this divergence. This explains the recent high energy features observed in angle-resolved photoemission in cuprates superconductors.   

The nature of the two energy scales in underdoped superconducting cuprates

Raman and ARPES experiments have demonstrated that in superconducting underdoped cuprates nodal and antinodal regions are characterized by two energy scales instead of the one expected in BCS. Using the Yang, Rice and Zhang (YRZ) model, in which pseudogap and superconductivity compete below a critical doping, we find that the antinodal Raman pair-breaking peak shifts to higher frequency with underdoping, follows the antinodal ARPES gap and is closely connected with the pseudogap. Its intensity decreases due to the competition between pseudogap and superconductivity. The nodal scale follows the doping dependence of the superconducting order parameter (cond-mat/0611154).   

``String excitations'' of a hole in a quantum antiferromagnet and ARPES data

Recently, high resolution angle-resolved photoelectron spectra (ARPES) from cuprates have been reported where an anomalous high-energy dispersion was identified. We suggest that these ARPES results reveal the internal structure of the hole quasiparticle in quantum antiferromagnets and more importantly it is evidence for the existence of ``string-excitations'' which validate early predictions based on the $t-J$ and related models. The following features of the ARPES results are all in agreement with predictions without adjusting any parameters: (a) the energy-momentum dispersion of the string-excitations, (b) the manner in which the spectral weight is transfered to higher energy string excitations, and (c) the vanishing of the quasiparticle spectral weight near the $\Gamma$ point.   

Doping dependence of quasi-particle gaps at low hole doping in the Hubbard model

Using the variational cluster approach we investigate the doping dependence of the pseudogap and the superconducting gap in the $t$-$t^\prime$-$U$ Hubbard model at low hole doping and zero temperature. The self energy of the system is calculated on a well suitable reference system for the investigated doping range and provides well defined quasi particles in the nodal region. We show that the pseudogap in the paramagnetic regime decreases with increasing hole doping, whereas the superconducting gap in the superconducting solution shows the opposite doping dependence for low hole doping. Furthermore our calculations suggest that the superconduting pseudogap in the antinodal region can be seen as sort of superposition of the paramagnetic pseudogap and the superconducting gap as measured near the nodal region. Thus, we claim that the occurence of two distinct energy gaps recently found in experiments can naturally be explained by the single-band Hubbard model.   

Dynamic mean field theory of the Gutzwiller-projected BCS Hamiltonian: phase fluctuation and pseudogap

One of the most prominent problems in high temperature superconductivity is the nature of the pseudogap phase in the underdoped regime and its relationship to phase fluctuations. In this context, the Gutzwiller-projected BCS Hamiltonian is a useful model especially suited for the study of high temperature superconductivity in the underdoped regime due to the fact that there is an exact mapping to the Heisenberg model at half filling and a close connection to the t-J model at low doping in general. To be concrete, we have developed a dynamic mean field theory of the d-wave BCS Hamiltonian with on-site repulsion U. The large U limit corresponds to the Gutzwiller- projected BCS Hamiltonian. Effects of the phase fluctuations are studied as a function of on-site repulsion U and doping x.   

Dynamical Cluster Approximation results for the effect of long range hoppings on $T_c$ in Cuprates

The Dynamical Cluster Approximation along with the Quantum Monte Carlo (QMC) algorithm are employed to study the effect of long-range hoppings on the superconducting critical temperature of Cuprates. A two-dimensional $t-t'-t''-$U Hamiltonian describes the physics of copper oxide planes in this model. We perform calculations on $4$-site and $16$-site clusters. The results show a weak dependence of the maximum $T_c$ on the long-range hoppings. We see a suppression of $T_c$ due to $t'$ in the hole-doped systems. $t'$ increases the critical doping (the doping beyond which the superconducting phase disappears) in the hole-doped regime, but this doping value is decreased by including $t''$.   

Impurity Induced Kondo-like Screening in Cuprates

We study the magnetic response of \(t-t'-J\) model to a single nonmagnetic impurity using slave boson mean field theory, with restricted Bogoliubov-de-Gennes(BDG) method which allows us to deal with the strong correlations and reduction of order parameters around the impurity self-consistently. The temperature dependence of the paramagnetic susceptibility \(\chi\) follows a Kondo-like form \(1/(T+\Theta)\), where the screening temperature \(\Theta\) increases with increasing doping. Both this form and the magnitude of \(\chi\) are consistent with NMR experiments in the normal state of Zn doped YBCO.   

Session L9: Superconducting Fluctuations

Direct test of pairing fluctuations in the pseudogap phase of an underdoped cuprate

In underdoped cuprates, many experiments have provided evidence for the presence of a gap-like structure in the electronic excitations spectrum, in a region above the critical temperature and below a characteristic temperature T*. The origin of this so-called pseudogap is still hardly debated and the answer to this question turns out to be essential for the understanding of high-T$_{c}$ superconductivity. One doesn't know if the pseudogap is related to superconductivity or to an order in competition. In the former case, it has been suggested that superconducting pairing fluctuations may be responsible for the partial suppression of electronic excitations. This remains to be tested experimentally, but most of the probes used to investigate the pseudogap are not sensitive to pairs and therefore cannot provide such a test. Here, we report for the first time on a direct test of pairing fluctuations in the pseudogap regime using a Josephson-like experiment. Our results shows that fluctuations survive only in a restricted range of temperature close to T$_{c}$ (T-T$_{c}<$15K), and therefore cannot be responsible for the opening of the pseudogap at high temperature.   

Evidence for Quantum Criticality (QC) and Universal Field-Induced Quantum Fluctuations (QF) in Cuprate Superconductivity (SC)

We present experimental evidence for universal field-induced QF among cuprate superconductors as the result of their proximity to QC and the coexistence of SC and competing orders. We employ various experimental techniques to derive the in-plane magnetic irreversibility field in hole- and electron-type cuprate superconductors of varying doping levels and numbers of CuO$_{2}$ layers per unit cell, and we find strong suppression of the extrapolated zero-temperature in-plane irreversibility field relative to the paramagnetic field in all cuprates, suggesting universal field-induced QF. The irreversibility fields follow a universal dependence on a parameter that combines the effect of the doping level, electronic anisotropy, and charge imbalance in multi-layer samples.   

Magnetic field effect on the superconducivity fluctuation of cuprates, LSCO and LCCO measured by microwave broadband technique

Understanding of the electronic phase diagram is essential to clarify the mechanism of high-$T_c$ superconductivity (SC) of cuprates. Previously, we studied the SC fluctuation of hole doped LSCO by microwave broadband technique, and found that there was a sharp crossover from the 2D-XY (BKT) behavior to the 3D-XY behavior by changing the doping. However, behaviors in the overdoped region and the effect of disorder were remained to be seen as future issues. To answer these, we investigated the effect of magnetic field on the SC fluctuation on LSCO with various carrier concentrations. For underdoped samples, the BKT behavior observed in zero-field experiemnts sruvived. However, the divergence of correlation length was found to be supressed by the externally applied magnetic field. In contrast, for optimally doped samples, the range of the 3D-XY behavior became narrowed definitely under finite magnetic field. These provide strong support for our previous conclusion that there is a sharp change in the SC nature around at the optimum doping. The data in the overdoped LSCO and in electron doped cuprate LCCO will also be presented in a comparative manner.   

Superconducting fluctuations in underdoped $La_{2-x}Sr_xCuO_4$ thin films

Underdoped $La_{2-x}Sr_xCuO_4$ thin films resistivity was measured under high pulsed magnetic fields (50 T) in order to suppress superconductivity and extract the paraconductivity, or the conductivity due to superconducting fluctuations. Quite surprisingly, this paraconductivity is consistent \textit{without any adjustable parameter} with a Gaussian model for the fluctuations, where both amplitude and phase of the order parameter fluctuate, as calculated by Aslamazov and Larkin (AL). This tends to indicate that the pairs responsible for the transition at $T_C$ are not preformed, as that would rather lead to Kosterlitz-Thouless type fluctuations. At higher temperature, the paraconductivity departs from AL behavior and follows a power law in 1/T. At intermediate magnetic fields, the possibility of a quantum superconductor/insulator phase transition is investigated, as a plateau in the resistance versus temperature is observed under perpendicular magnetic field for all underdoped films.   

Determination of the dynamical scaling exponent in the superconducting to normal metal phase transition

In the high Tc superconductors, measurements of fluctuation effects reveal interesting behavior. Thermodynamic measurements have been done to investigate scaling behavior, to obtain critical exponents and to test the universality of the transition and the 3D XY model. Transport measurements of critical fluctuations, such as the AC conductivity, are less explored, and a wide range of critical exponents have been reported. We have investigated critical fluctuations in the microwave conductivity of $\mathrm{YBa_{2}Cu_{3}O_{7-\delta}}$ films. Our improved temperature stability and conductivity calibration(10 MHz to 50 GHz) allow us to take high quality data at small temperature intervals(50mK). This improves the conventional data analysis method and allows a new method of extracting exponents to be developed. With these two methods, we determined consistent values of $T_c$ and the critical exponent using eight different samples.   

Superconducting fluctuations and disorder in high-Tc cuprates

The determination of critical fields and of the superconducting fluctuations in the cuprates are still highly debated questions, as both extremely high field and reduced $T_{c}$ cuprates are required to attempt to reach the normal state regime. We have studied in fields up to 60T the variation of the transverse magnetoresistance (MR) of underdoped YBCO$_{6.6}$ crystals either pure or with $T_{c}$ reduced down to 3.5K by electron irradiation [1]. We show that the normal state MR is restored above a threshold field $H_{c}^{\prime }(T)$, which is found to vanish at $T_{c}^{\prime}>>T_{c}$. This allows us to evidence a $(H,T)$ range where superconductivity survives at least as fluctuations. When $T_{c}$ is decreased by disorder, we found that the fluctuation range expands significantly as $T_{c}^{\prime}$ is slightly depressed. This $T_{c}^{\prime}$ behaves similarly versus defect content as the onset temperature $T_{\nu}$ of the Nernst signals measured on the same samples [2] which indicates that the $T_{c}$ decrease is partly due to the loss of the phase coherence. We found that $T_{c}^{\prime}$, $H_{c}^{\prime}(T)$ and $T_{\nu}$ which can be related to pair formation are depressed, although moderately, by the introduction of defects in contrast to the pseudogap temperature which is known to be insensitive to disorder, showing that these energy scales are not related. [1] F. Rullier-Albenque et al, cond-mat 0610838 [2] F. Rullier-Albenque et al., Phys. Rev. Lett. \textbf{96}, 067002 (2006)   

Fluctuations and Glassy Behavior in the 2D Superconductor-Insulator Transition in Granular Bismuth

The superconductor-insulator transition has been investigated in granular, amorphous bismuth films. The system's dynamics have been investigated at various levels of disorder by incrementing film thickness and measuring voltage fluctuations. In insulating films in which local superconductivity was not evident, the first power spectra had 1/f$^{2}$ frequency dependences. In films that exhibited local superconductivity, the spectra had weaker frequency dependences. In a film with low enough disorder, the resistance had a very weak temperature dependence below 1.5 K and non-ergodic behavior and strong fluctuations were observed below 400 mK. The variations of the first and second power spectra with disorder and temperature will be discussed.   

A Theory of the Quantum Metal to Superconductor Transition In Highly Conducting Films

Treating the inhomogeneous solution of the BCS mean-field equations as the saddle point of an effective quantum action, we derive the theory of the superconductor to metal transition in films under the conditions in which the critical resistance is small compared to the quantum of resistance. The present results are applicable to the magnetic field driven transition in MoGe films. It is also applicable to the transition in zero field in a weakly coupled d-wave superconductor, which may in turn be a useful caricature of a cuprate high temperature superconductor.   

The approach to a superconductor-to-Bose-insulator transition in disordered films

We study the superconductor-insulator transition in the limit of strongly disordered films of indium oxide. It was observed previously that the insulating phase is strengthened as the disorder increases, creating a strong barrier to pair-breaking in the vicinity of the critical point. We find that for the strongest insulators, the critical resistance is approximately the universal resistance for pairs, RQ = h/4e2 and the scaling of both the linear and non-linear resistance is consistent with the quantum percolation solution to the dirty boson model. We combine these results with previous data and note separate branches corresponding to strong and weak disorder. The strong disorder branch suggests a true dirty boson superconductor-insulator transition.   

Observation of Pairing Correlations in Strongly Localized Amorphous Films

We have measured the Superconductor to Insulator Transition (SIT) as a function of thickness at dilution refrigerator temperatures in ultrathin Bi/Sb films perforated with a regular honeycomb array of holes separated by 100 nm. The presence of these perforations profoundly influences the character of the transition. In particular, on the insulating side of the SIT, the resistance as a function of temperature, R(T), rises monotonically and becomes activated below ~1K. Closer to the SIT, a minimum develops in the R(T) suggestive of strong superconducting fluctuations and the onset of Cooper pairing. Simultaneously, the perpendicular field magnetoresistance begins to oscillate with a period that corresponds to the superconducting flux quantum. Yet thicker films exhibit a relatively broad R(T) transition toward a zero resistance state. This behavior constitutes direct evidence that the superconducting ground state of this amorphous film system emerges from an insulating state containing localized Cooper pairs. This work has been supported by the NSF through DMR-0203608, and DMR-0605797, AFRL, and ONR.   

Nonequilibrium Voltage Fluctuations in Aluminum Wires

We present measurements of the nonequilibrium voltage fluctuations across current biased superconductive aluminum wires in the vicinity of $T_c$. Above $T_c$ these voltage fluctuations are due to superconductive fluctuations which persist on the time scale of the Ginzburg time. Below $T_c$ we believe they are due to the thermal activated phase slips. The frequency dependence of the fluctuations suggests the observation of the ac Josephson effect above $T_c$.   

Fluctuations in Two-Dimensional Superconducting NbN Nanobridges and Nanostructures Meanders

We have observed fluctuations, manifested as sub-nanosecond to nanosecond transient, millivolt-amplitude voltage pulses, generated in two-dimensional NbN nanobridges, as well as in extended superconducting meander nanostructures, designed for single photon counting. Both nanobridges and nano-stripe meanders were biased at currents close to the critical current and measured in a range of temperatures from 1.5 to 8 K. During the tests, the devices were blocked from all incoming radiation by a metallic enclosure and shielded from any external magnetic fields. We attribute the observed spontaneous voltage pulses to the Kosterlitz-Thouless-type fluctuations, where the high enough applied bias current reduces the binding energy of vortex-antivortex pairs and, subsequently, thermal fluctuations break them apart causing the order parameter to momentarily reduce to zero, which in turn causes a transient voltage pulse. The duration of the voltage pulses depended on the device geometry (with the high-kinetic inductance meander structures having longer, nanosecond, pulses) while their rate was directly related to the biasing current as well as temperature.   

Interaction of vortices in thin superconducting films and Berezinskii-Kosterlitz-Thouless transition

The precondition for the BKT transition in thin superconducting films, the logarithmic intervortex interaction, is satisfied at distances short relative to $\Lambda=2\lambda^2/d$, $\lambda$ is the London penetration depth of the bulk material and $d$ is the film thickness. For this reason, the search for the transition has been conducted in samples of the size $L<\Lambda$. It is argued below that film edges turn the interaction into near exponential (short-range) thus making the BKT transition impossible. If however the substrate is superconducting and separated from the film by an insulated layer, the logarithmic intervortex interaction is recovered and the BKT transition should be observable.   

Density of States, Entropy, and the Superconducting Pomeranchuk Effect in Pauli-Limited Al Films

We present low temperature tunneling density of states measurements of Pauli-limited Al films in which the Zeeman and orbital contributions to the critical field are comparable. We show that films in the thickness range of 6-7 nm exhibit a reentrant parallel critical field transition which is associated with a high entropy superconducting phase, similar to the high entropy solid phase of $^3$He responsible for the Pomeranchuk effect. This phase is characterized by an excess of states near the Fermi energy so long as the parallel critical field transition remains second order. Theoretical fits to the zero bias tunneling conductance are in good agreement with the data well below the transition but theory deviates significantly near the transition. The discrepancy is a consequence of the emergence of $e$-$e$ interaction correlations as one enters the normal state.   

Session L10: Heavy Fermions in the 115 Compounds

Incoherent Non-Fermi Liquid Scattering in a Kondo Lattice

The effect of Kondo lattice dilution was investigated in the heavy-fermion superconductor CeCoIn$_5$ to study the evolution of unconventional superconductivity and non-Fermi liquid properties. A systematic substitution of both non-magnetic (full or empty $f$-shell) and large, stable $f$-moment rare earth impurities into high-quality single-crystal specimens of Ce$_{1-x}$R$_x$CoIn$_5$ (where R=Y, Pr, Gd, Er and Lu) has revealed two contrasting features. First, both superconducting electron pair-breaking and the suppression of Kondo coherence proceed in a manner which is insensitive to the magnetic state of the dopant atom, suggesting spin-independent disorder is the dominant perturbation in both phenomena. In contrast, the evolution of the non-Fermi liquid properties with substitution shows a striking sensitivity to the dopant atom's f-moment configuration.   

Effects of self-irradiation on local crystal structure and 5$f$ localization in PuCoGa$_5$

X-ray absorption fine-structure (XAFS) measurements demonstrate the structural and electronic changes involved in destroying superconductivity in PuCoGa$_5$ due to self-irradiation damage. In particular, the Pu $L_\textrm{III}$-edge data indicate a more localized $f$-orbital relative to the itinerant paramagnet UCoGa$_5$, potentially increasing with radiation damage. Moreover, the local crystal structure in aged material is disordered much more strongly than expected, consistent with all atoms within a damage cascade displaced from their equilibrium positions.   

Superconductivity due to cooperative two-channel Kondo effect

We discuss the application of the co-operative two-channel Kondo model to describe the low temperature properties of the Ce and Pu- based 115's. Based on the crystal field multiplet structure for Ce and Pu ions in tetragonal symmetry, we employ the group theory to justify the validity of the two-channel Kondo physics. Additional screening channel contributes to destabilization of the Fermi liquid and leads to development of the heavy Fermi surface. The fluctuations between the zero modes for the Kondo singlets leads to divergent composite pair susceptibility.   

Coexistence of antiferromagnetism and superconductivity in a model for CeRhIn$_5$

The finite temperature phase diagram of CeRhIn$_5$ as a function of presure and magnetic field has three main highlights: 1) the competitive coexistence of metallic antiferromagnetism and superconductivity, 2) the abrupt dissapearance of antiferromagnetism when the Neel and superconducting temperatures become equal at a critical pressure P$_c$ and 3) the reentrance of the antiferromagnetic phase in a range of pressures larger than P$_c$ when a magnetic field is applied. Based on first-principles band structure calculations, we propose a quasi-two-dimensional model of interacting electrons, which reproduces, at the mean-field level, the central features of the phase diagram. We also discuss the divergence of the cyclotron mass observed in dHvA oscilations and the amount entropy released at the ordering temperatures.   

Quantum Criticality in Cd doped CeMIn$_{5}$.

In pure CeCoIn$_{5}$ the application of magnetic field has revealed a field tuned quantum critical point surprisingly coincident with the superconducting H$_{c2}$. The application of pressure suppressed the antiferromagnetic fluctuations in the system and pushed the quantum critical field inside the superconducting dome. By substituting Cd for In it was shown that one can quickly induce the antiferromagnetic state. With predominantly thermodynamic probes, such as specific heat, we investigate how the appearance of long range antiferromagnetic order induced by Cd doping influences the field tuned quantum critical behavior. This will also be contrasted with the behavior found for the antiferromagnetic quantum critical point seen in CeRhIn$_{5}$ under pressure.   

Multiple vortex phases in the heavy fermion superconductor CeCoIn$_{5}$

We report the entire HT-phase diagram of the vortex lattice (VL) in CeCoIn$_{5}$ for fields applied along the crystallographic $c$-axis. At the upper critical field $H_{c2}$ of about 5~T and 50~mK, we observe a distorted hexagonal flux lattice, which first gives way to a rhombic lattice at 4.3~T and then to a square lattice at 3.3~T, before entering the previously reported flux lattices at lower fields. The distorted hexagonal phase extends to lower fields and higher temperatures than the $H$-$T$-phase space that was previously assigned to a inhomogeneous FFLO state by magnetization and NMR measurements. Surprisingly, we also observed an increase of the flux lattice form factor as a function of increasing field in the rhombic phase, in constrast to the flux lattice in most superconductors. We will discuss the relevance of these results for the presence of the FFLO and magnetic phases.   

Nature of a possible FFLO state in CeCoIn$_5$ as revealed by NMR.

We report low temperature $^{115}$In nuclear magnetic resonance (NMR) measurements of the heavy-fermion superconductor CeCoIn$_5$ in high magnetic fields. We will discuss the implications of our findings for the nature of a possible inhomogeneous superconducting state, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in this compound.   

Superconductivity and magnetism in heavy-fermion systems

Recent experiments on CeCoIn$_5$ have observed coexisting superconductivity and magnetic order under an applied magnetic field near the Pauli limit. Motivated by such observations, we study the interplay between pairing and magnetism in the SU(N) Kondo lattice model, believed to provide an accurate description at low temperatures of such heavy-fermion systems. We present results for the phase diagram as a function of temperature and magnetic field.   

Effect of pressure on hole-doped CeCoIn$_{5}$ and CeRhIn$_{5}$.

With one less p-electron than In, Cd or Hg adds one hole per substituted In in CeMIn$_{5}$ (M=Co,Rh, Ir). Progressive Cd/Hg substitutions tune the ground state of superconducting CeCoIn$_{5}$ to one of coexisting magnetism and superconductivity and eventually to antiferromagnetic only. This systematic evolution of states in doped CeCoIn$_{5}$ is reversed accurately by applied pressure and maps onto the temperature-pressure phase diagram of pure CeRhIn$_{5}$. These observations, together with the response of Cd-substituted CeRhIn$_{5}$ to applied pressure, show that ground states of CeMIn$_{5}$ are controlled by fine details of electronic structure and that disorder on the In site and different ionic radii of Cd and Hg have an insignificant effect.   

Magnetic and electrical transport properties of Ce$_{1-x}$Nd$_{x}$CoIn$_{5}$

Single crystals of Ce$_{1-x}$Nd$_{x}$CoIn$_{5}$ (0$\le $x$\le $1) were grown by molten metallic flux technique. Synchrotron powder X-ray diffraction confirms phase purity and smooth evolution of the lattice parameters with increased Nd concentration. Evolution of the ground state in this alloy series between heavy fermion superconducting for x = 0 (CeCoIn$_{5})$ and antiferromagnetic for x = 1 (NdCoIn$_{5})$ will be presented.   

In-plane torque measurements on CeCoIn$_5$ single crystals

In-plane torque measurements were performed on single crystals of CeCoIn$_5$ in the mixed state in order to determine the symmetry of the superconducting gap. The reversible part of the mixed state torque shows a four fold symmetry. The sign of the four fold symmetry is positive. The amplitude of the in-plane torque first increases with increasing magnetic field H, and then decreases with further increasing field until it vanishes towards H$_{c2}$. Sharp irreversible peaks are present in the irreversible torque at 45, 135, 225, and 315$^{0}$. These experimental results imply d$_{xy}$ symmetry of the superconducting order parameter. We also performed in-plane torque measurements in the normal state. Another four fold symmetry is found which has a different origin than the one in the mixed state. The amplitude of the normal state torque has an $H^{4}$ dependence and it shows no saturation up to 14 T. This later four fold symmetry could be related with the symmetry of the Fermi surface.   

Flux Growth of Heavy Fermion LiV$_2$O$_4$ Single Crystals

The spinel-structure compound ${\rm LiV_2O_4}$ is a rare $d$- electron heavy fermion. Measurements on single crystals are needed to clarify the mechanism for the heavy fermion behavior. In addition, it is known that small concentrations ($< 1$ mol\%) of magnetic defects in the structure strongly affect the properties, and measurements on single crystals containing magnetic defects would help to understand the latter behaviors. Herein, we report growth at 950--1030~$^\circ$C of 1~mm$^3$ size octahedron-shaped ${\rm LiV_2O_4}$ single crystals using a self- flux technique. The magnetic susceptibility of the as-grown crystals shows a Curie-like upturn at low temperatures arising from $\approx 0.5$ mol\% magnetic defects within the spinel structure. After annealing at 700~$^\circ$C, the Curie-like upturn (and magnetic defects) disappeared in some crystals, thus revealing the known intrinsic nearly temperature-independent behavior below $\sim 20$~K\@. Preliminary heat capacity measurements on as-grown crystals containing magnetic defects showed a high linear specific heat coefficient $\gamma$ = 450 mJ/ (mole K${^2}$) at 1.8~K\@. Additional electronic tranport, magnetic and thermal measurements on both as-grown and annealed crystals will be presented.   

Session L11: Metal-Insulator Transition I

Flow diagram of the metal-insulator transition in two dimensions

Recently, a two-parameter scaling theory comprehensively describing the metal-insulator transition in 2D was developed by two of us [1]. Here, we report experimental verification of the basis of this theory. We demonstrate, for the first time, that as a result of the interplay between the electron-electron interactions and disorder, both the resistance and the interactions become scale (temperature) dependent. We show that not only the resistance but also the interaction amplitude exhibits a fan-like spread as the MIT is crossed. We use these data to construct a resistance-interaction flow diagram of the MIT that clearly reveals a quantum critical point, as predicted in Ref.[1]. The metallic side of this diagram is accurately described by the theory without any fitting parameters. In particular, the temperature dependence of the resistance, which is non-monotonic, passes through a maximum when the interaction amplitude reaches a certain value $\gamma_2\approx0.45$ that is in remarkable agreement with the calculated one.\\ $[1]$ A. Punnoose and A.~M. Finkel'stein, \textit{Science} \textbf{310}, 289-291 (2005).   

Quantum Coulomb glasses and electron assisted hopping

In Anderson insulators where the single particle localization length is much larger than the mean distance between electrons, Coulomb interactions drive the electrons into a strongly correlated quantum glass phase. In the limit of large localization length, the resulting quantum Coulomb glass can be studied analytically. The theory predicts many almost degenerate quantum states with a spectrum of gapless collective excitations in each of them. The latter can acts as a bath with which individual electrons can exchange energy. This is a crucial ingredient for activated transport, the collective modes of the quantum glass providing a natural mechanism for electron-assisted hopping conductance. In particular, for 2D systems we predict a weakly temperature dependent pre-exponential factor of order $e^2/h$ for variable range hopping, as has been reported in many recent experiments.   

Multifractality and Conformal Invariance at 2D Metal-Insulator Transition in the Spin-Orbit Symmetry Class

We study the multifractality of critical wave functions at boundaries and corners at the Anderson metal-insulator transition for noninteracting electrons in the two-dimensional (2D) spin-orbit (symplectic) universality class. We find that the multifractal exponents near a boundary are different from those in the bulk. The exponents at a corner are found to be directly related to those at a straight boundary through a relation arising from conformal invariance. This provides direct numerical evidence for conformal invariance at the 2D spin-orbit metal-insulator transition. We also show that the presence of boundaries modifies the multifractality of the whole sample even in the thermodynamic limit.   

Localization of interacting fermions at high temperature

We suggest that if a localized phase at nonzero temperature $T>0 $ exists for strongly disordered and weakly interacting electrons, as recently argued, it will also occur when both disorder and interactions are strong and $T$ is very high. We show that in this high-$T$ regime the localization transition may be studied numerically through exact diagonalization of small systems. We obtain spectra for one-dimensional lattice models of interacting spinless fermions in a random potential. As expected, the spectral statistics of finite-size samples cross over from those of orthogonal random matrices in the diffusive regime at weak random potential to Poisson statistics in the localized regime at strong randomness. However, these data show deviations from simple one-parameter finite-size scaling: the apparent mobility edge ``drifts'' as the system's size is increased. Based on spectral statistics alone, we have thus been unable to make a strong numerical case for the presence of a many-body localized phase at nonzero $T$.   

Out-of-Equilibrium Dynamics of a Strongly Correlated Electron System in Two Dimensions

Slow, nonexponential relaxations of conductivity $\sigma (t)$ have been studied in a strongly disordered two-dimensional electron system (2DES) in Si MOSFETs in the vicinity of the metal-insulator transition (MIT). The 2DES is excited far from equilibrium by a rapid change of carrier density $n_s$ at low temperatures $T$. The dramatic and precise dependence of $\sigma(t)$ on $n_s$ and $T$ shows that (a) the equilibration time diverges exponentially as $T\rightarrow 0$, suggesting a glass transition at $T_g=0$, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics~[1]. The scaling of $\sigma(t,T)$ is also consistent with $T_g=0$. These results support conclusions based on earlier noise measurements~[2] that, in a 2DES in Si, the glass transition occurs in the metallic phase as a precursor to the MIT.\\ \noindent [1] J. Jaroszy\'nski and D. Popovi\'c, Phys. Rev. Lett. {\bf 96}, 037403 (2006).\\ \noindent [2] S. Bogdanovich and D. Popovi\'c, Phys. Rev. Lett. {\bf 88}, 236401 (2002); J. Jaroszy\'nski, D. Popovi\'c, and T. M. Klapwijk, Phys. Rev. Lett. {\bf 92}, 226403 (2004).   

Electric field gating near the metal-insulator transition using ionic liquid dielectrics

Ionic liquids (ILs) are highly polar low-melting-temperature binary salts typically comprising nitrogen-containing organic cations and inorganic anions. Since there is no solvent, ILs are distinctly different from aqueous, organic, gel or polymer electrolytes. Using either coplanar or overlay gate configurations in which the IL is the gate dielectric, we demonstrate room temperature field-induced resistance changes on the order of a factor of 10$^{4}$ for thin conducting InOx films. There is a large asymmetry manifested by the significantly larger changes in impedance for negative gate voltage $V_{g}$ (electron depletion) compared to positive $V_{g}$ (electron enhancement). The pronounced frequency dependence over the range 10$^{-2}$--10$^{6}$ Hz, due to the low ionic mobilities in the dielectric fluid, is modeled well by a simple RC circuit from which an effective areal gate capacitance can be derived. The induced surface charge densities and field-effect mobilities noticeably exceed those that can be achieved on similar films using AlOx dielectrics. In addition, the charge state can be frozen in by reducing the temperature below the glass transition ($\sim $250K) of the IL, thus providing an opportunity for electric field tuning of metal-insulator transitions in a variety of novel thin-film systems.   

Phase diagram of amorphous Ta thin films in B-T-disorder space

We have studied the effect of temperature (T) and perpendicular magnetic fields (B) on the transport properties in amorphous Ta thin films. In the zero T limit, the films exhibit superconducting, metallic, and insulating phases with increasing B. Each phase can be identified by distinct nonlinear current-voltage (I-V) characteristics: the I-V curves in the superconducting phase are characterized by a hysteresis, in the metallic phase the differential resistance (dV/dI) increases with increasing I, while in the insulating phase dV/dI decreases with increasing I [1]. As demonstrated for the superconducting and metallic phase, these nonlinear transports arise from a non-thermal origin [2]. In order to understand the effect of B, T, and disorder on the electronic states and the nature of the resulting ground states, we construct a B-T-disorder space ``phase diagram''. Disorder is controlled by film thickness. The resulting phase diagram shows that the superconducting phase is completely surrounded by the metallic phase; in the zero temperature limit (B-disorder plane) a B-induced direct superconductor-insulator transition is not allowed, while a superconductor-metal-insulator or metal-insulator transition are possible depending on the degree of disorder in our 2D system. [1] Y. Qin et al., Phys. Rev. B 73, 100505(R) (2006). [2] Y. Seo et al., Phys. Rev. Lett. 97, 057005 (2006).   

First-order metal-insulator transition and structural phase transition: analysis of coherent phonons observed by femtosecond pulse laser in VO$_{2}$

It has been well-known that VO$_{2}$ undergoes both a structural phase transition (SPT) (electron-phonon interaction) from monoclinic (insulator phase) to tetragonal (metal phase) and of a discontinuous first-order metal-insulator transition (MIT) (Jump) (electron-electron interaction) near 68$^{o}$C. Peierls transiton and Mott transition in VO$_{2}$ remain controversial. We have investigated a relation of the MIT and the SPT in VO$_{2}$ by observing coherent phonons using a laser with a femtosecond pulse width (10$\sim $20 ft). A coherent phonon indicating a metal phase is measured after MIT. This indicates that the SPT does not affect the MIT. This is confirmed by a micro-Raman scattering experiment and XRD. The speed of the first-order MIT is interpreted as about 100 femtosecond. This is different from a well-known analysis in which the SPT and the MIT simultaneously occur. (References on the MIT: New J. Phys. 6 (1994) 52 (http://www.njp.org), Appl. Phys. Lett. 86 (2005) 242101, Physica B 369 (2005) 76; cond-mat/0607577; cond-mat/0608085; cond-mat/0609033)   

Spin-Valley Phase Diagram of the 2D Metal-Insulator Transition

It has been recognized that the spin degree of the freedom plays a crucial role in the controversial metal-insulator transition problem in 2D carrier systems. Here, we directly probe the role of another discrete electronic degree of freedom, namely the valley polarization. Using symmetry breaking strain to tune the valley occupation of a 2D electron system in an AlAs quantum well, together with an applied in-plane magnetic field to tune the spin polarization, we map out a spin-valley phase diagram for the 2D metal-insulator transition. The insulating phase occurs in the quadrant where the system is both spin- and valley- polarized. This observation establishes the equivalent roles of spin and valley degrees of freedom in the 2D metal-insulator transition.   

Doping variation of orbitally-induced anisotropy in electronic structure of the perovskite-type vanadium oxides

Recently, the perovskite-type vanadium oxide LaVO$_{3}$ has been attracting much attention. In this system, the anisotropic charge dynamics due to the one-dimensional orbital exchange interaction is observed. In addttion, the filling control insulator-metal transition (FC-IMT) concomitant with the orbital ordering-disordering transition can be achieved in the hole doped system La$_{1-x}$Sr$_{x}$VO$_{3}$[1]. In this study, the variation of anisotropic charge dynamics in the course of FC-IMT in the perovskite-type vanadium oxide has been investigated by measurements of optical conductivity spectra with focus on the role of t$_{2g}$-orbital degree of freedom. The orbitally-induced anisotropic feature of the Mott-gap excitation as well as of the doping-induced mid-infrared excitation is suppressed with increasing the hole concentration, and instead the isotropic and incoherent dynamics of the doped hole dominates over the low-energy excitation near and above the IMT point. \newline \newline [1] S.Miyasaka et al., Phys.Rev.Lett. 85,5388(2000)   

Screening of disorder by the Hubbard interaction near a metal-insulator transition in two dimensions

We present a determinant quantum Monte Carlo study of the metal-insulator transition in the Hubbard model on a square lattice with random site disorder. We show that beyond a critical value of the Hubbard interaction U, the Anderson insulator can undergo a phase transition to a two-dimensional metal. It is also shown that a further increase of the Hubbard interaction can lead to a decrease in conductivity, in direct analogy with the superfluid to Bose-glass transition in the bosonic Hubbard model. We point out that screening of disorder by the Hubbard interaction is not enough to explain the metal-insulator transition in the two-dimensional disordered Hubbard model.   

The metal to insulator transition in manganites - evidence for changes in the kinetic energy up to 24 eV

The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.8}$Ca$_{0.2}$MnO$_3$ was investigated by a combination of dc conductivity, ellipsometry, and VUV-reflectance measurements covering an energy range from 0 to 24 eV. By performing a stabilized Kramer-Kronig transformation, we obtain the optical conductivity as a function of temperature around the metal to insulator transition. Our main findings are that changes in the kinetic energy exceed energies of more than 22 eV. In the spectral range between 0 and 24 eV the spectral weight is conserved within a fraction of 3/1000. The pronounced redistribution of the spectral weight between low and high energies has important ramifications for the construction and down-folding of effective low-energy Hamiltonians. We discuss the importance of local interactions to the electronic bandstructure such as the Coulomb onsite and Jahn-Teller effects.   

Metal-Insulator Transition and Coulomb Gap: A Real-Space Dynamical Mean-Field Study of the Anderson-Hubbard Model

The interplay between disorder and electron interactions in the two-dimensional paramagnetic Anderson-Hubbard model is studied by real-space dynamical mean-field theory (DMFT) with a Hubbard- I solver. At half-filling, the Mott gap evolves into a Coulomb- like gap with power law energy dependence $|E - E_F|$, suggesting a Mott insulator to Anderson insulator transition as a function of disorder. Away from half filling for strong interactions and disorder, we find a negative density of states (DOS) anomaly at the Fermi level that is distinct from the Mott gap. Far from half-filling, we obtain a positive DOS anomaly at the Fermi level. While this positive anomaly is consistent with paramagnetic mean-field calculations, the negative anomaly near half filling is a feature unique to strong correlation physics.   

Jastrow theory of the Mott transition in bosonic Hubbard models

We show that the Mott transition occurring in bosonic Hubbard models can be successfully described by a simple variational wave function that contains all important long-wavelength correlations. Within this approach, a smooth metal-insulator transition is made possible by means of a long-range Jastrow correlation term that binds in real space density fluctuations. We find that the Mott transition has similar properties in two and three dimensions but differs in one dimension. We argue that our description of the Mott transition in terms of a binding-unbinding transition is of general validity and could also be applied to realistic electronic systems.   

Session L12: Focus Session: Magnetic Semiconducting Oxides

Challenges in the Synthesis of Diluted Magnetic Semiconducting Oxides

In the rapidly advancing field of Spintronics, the quest for an above room temperature diluted magnetic semiconductor (DMS) has been thwarted by the lack of a conventional semiconductor with above room temperature ferromagnetism (FM). The recent observation of high temperature FM in numerous oxides has created a flurry of publications with controversial results. I will address the origin of this controversy and trace it to the fact that unlike their predecessors, the manganites, which had significantly large magnetization signals, we are now dealing with samples of weaker magnetization and the measurements are vulnerable to extraneous effects. With the example of Co doped TiO2, I will show that the substitutional incorporation of the magnetic ion in the host lattice is a process dependent phenomenon and Co incorporation and the lattice crystallization have opposing behavior with temperature. The role of TEM-EELS and XAS in distinguishing between intrinsic vs. extrinsic effects will be delineated. There are process regimes in which a homogeneously doped TiO2 DMS system does exist while in the rest of the region one obtains a super paramagnetic system with Co clusters embedded in a TiO2 host. Results from anomalous Hall and field effect studies will be discussed and other magnetically doped oxide host systems will also be covered.   

Room temperature ferromagnetism in Fe-doped TiO$_{2}$ films is unrelated to magnetic ordering of iron.

TiO$_{2-\delta }$ films doped with x = 1 -- 5 at.{\%} $^{57}$Fe produced in oxidizing or reducing conditions by pulsed laser deposition on sapphire substrates exhibit ferromagnetic behaviour at room temperature. Conversion electron M\"{o}ssbauer spectra show that most of the iron in the oxidized film (10$^{2}$ Pa) is present as paramagnetic Fe$^{3+}$ or Fe$^{2+ }$whereas in the reduced films (10$^{-3}$ Pa) most of the iron is present as well-crystallized iron metal. The moment per iron atom for 1{\%} and 5{\%} films significantly exceeds the value of 2.2 $\mu _{B}$ for iron metal and in the 1{\%} oxidized film it is as much as 6.9 $\mu _{B}$ per iron atom. Films where the dopant ions are not magnetically ordered possess some of the largest ferromagnetic moments. Saturation moments are in the range 2-20.10$^{-8}$ Am$^{2}$ or 90-900 $\mu _{B}$ nm$^{-2}$ of substrate area. The iron in these oxidized films is in Fe$^{3+}$ or Fe$^{2+}$ state, but it is \textit{not magnetically ordered} and cannot therefore contribute to the observed ferromagnetic moments. The isomer shifts indicate octahedral oxygen co-ordination for both ions. We conclude that Fe-doped TiO$_{2}$ cannot be regarded as dilute magnetic semiconductor. The magnetism of this, and may be that of many other dilute magnetic oxides might be explained in terms of oxygen-based electronic defects, or orbital currents which do not involve the dopant directly.   

Room temperature ferromagnetism in Mn and Fe-doped indium-tin oxide films

Following the reports of high temperature ferromagnetism in n-type Mn-doped Indium tin oxide (ITO) thin films, we have undertaken a systematic investigation of the magnetic and transport properties of ITO thin films doped with all the 3d transition metal ions. ITO films were grown on c-cut sapphire substrate by pulsed laser deposition. The X-ray diffraction patterns reveal that they are oriented mainly in (111) direction of cubic bixbyite structure. Undoped ITO films are diamagnetic. Room temperature ferromagnetism is observed in Fe and Mn-doped thin films of varying concentrations, when deposition temperatures are greater than 600\r{ }C. The largest magnetic moments were found in 2.5{\%} Mn-doped and 7.5{\%} Fe-doped ITO films. The Mn-doped films are anhysteretic, while the Fe-doped films exhibit a hysteresis with a coercivity of 30 mT and a moment which increases with concentration. However, in the Mn-doped samples, we see a higher moment for the lower concentrations. None of the other doped ITO films were found to be magnetic, ruling out the possibility of cluster based magnetism. Conversion electron M\"{o}ssbauer spectra of the ferromagnetic iron-doped films show the presence of magnetite in quantities sufficient to explain the magnetization. No such secondary phase is found for Mn.   

Room Temperature Ferromagnetism in Mn-implanted CVD-Grown ZnO Films and Nanostructures

We have characterized the chemical, compositional, and magnetic properties of Mn-ion implanted epitaxial ZnO films and single crystal nanostructures grown by MOCVD as candidate room temperature diluted magnetic semiconductors. X-ray absorption spectroscopy (SXAS) and EXAFS show that the as Mn-implanted films contain isolated Mn$^{2+}$ ions substitutional for Zn. Upon annealing the distribution of Mn ions changes becomes locally enriched with a substantial fraction of the nearest cation neighbors being Mn. SQUID magnetometry shows that as-implanted films are ferromagnetic at 5K with a saturation magnetization of $\sim $ 0.2 \textit{$\mu $}$_{B}$/Mn-ion. The annealed films have an Ms that is $\sim $ 1 \textit{$\mu $}$_{B}$/Mn-ion and are ferromagnetic at room temperature. Elemental analysis of the nanorods in the transmission electron microscope shows that the Mn concentration is relatively uniform perpendicular to the axis of the structure, but has a higher concentration near the tip than at the base.   

A comparative RIXS study on Co2+ systems

We use RIXS at the ALS BL8 to investigate systems in which Co is preferentially in the Co2+ state. The systems include Co:ZnO, Co2O3, Co doped in polypyrrole, Co-phthalocyanine films, and CoO. For all these systems we report on the XAS and RIXS data at the Co2p edge. We separate the inelastic Raman losses due to d-d excitations from valence band induced excitations. We identify and quantify the relative contributions of the d7 HS and LS states and d8L charge transfer states.   

V, Nb and Ta doping of anatase TiO$_{2}$: from a dilute magnetic semiconductor to a transparent conducting oxide

We have investigated by means of first-principle supercell calculations the effects of doping anatase TiO$_{2}$ by V, Nb and Ta. We find: ({\em i}) V doping makes TiO$_{2}$:V ferromagnetic. A single V impurity has a magnetic moment of 1.0 $\mu_{B}$/V atom with an electronic configuration $a_1^2\, t_{1+}^3\, t_{1-}^3\, t_{2+}^1\, t_{2-}^0\, e_+^{0}\, e_-^0$. The ferromagnetic interaction between two V impurities is found to extend to more than fifth neighbors, with calculated ferromagnetic stabilization energy raging from 124 meV at the first neighbor to 27 meV at the fifth neighbor. ({\em ii}) Nb and Ta doping of TiO$_{2}$ makes the system conductive, but not magnetic. The calculated equilibrium free-electron concentration ($n_{e}$) at $T = 1000$K for Ti-rich--O-poor growth conditions is $2.7 \times10^{21}$ and $5.9 \times10^{21}$ cm$^{-3}$ for Nb and Ta doping respectively, whereas pure TiO$_{2}$ is calculated to have a electron density of only $1.8 \times10^{18}$ cm$^{-3}$ due to intrinsic defects. Thus, Nb and Ta doping of TiO$_{2}$ enhance dramatically the electron concentration and hence are good transparent conductor oxides.   

Ferromagnetism and n-type conductivity in Zn$_{1-x}$Fe$_{x}$O

Room temperature ferromagnetism in Zn$_{1-x}$Fe$_{x}$O can be obtained by precipitation of ZnFe$_{2}$O$_{4}$ impurity phase (with the Curie temperature of 440 K) after low-oxygen-pressure synthesis.* This impurity can be controlled by changing the synthesis temperature, which makes this material promising for spintronic applications. We have studied this material by magnetic, transport and thermoelectric experiments. The electrical resistivity shows a semiconducting behavior with $\rho \sim $0.4 $\Omega $cm at 300 K, much lower than Mn- and Co-substituted ZnO. Hall effect measurements show n-type conductivity with mobility $\sim $1-10 cm$^{2}$/Vs. The n-type conductivity is independent of the presence of ferromagnetic impurities. A high negative Seebeck (-300 $\mu $V/K at 300 K) would make this material suitable for thermoelectric applications if its resistivity could be further reduced. *S. Kolesnik et al., J. Appl. Phys. \textbf{95}, 2582 (2004). Supported by NSF (DMR-0302617) and the U.S. Department of Education.   

Anisotropy Lock-in model for the magnetism of (Zn,Co)O

An explanation for the magnetism of diluted magnetic oxides remains elusive to the present day. Super-exchange is short ranged and leads to anti-ferromagnetic interaction, while double exchange needs unrealistically large charge densities to give room temperature ferromagnetism. Moreover there is growing experimental evidence that free-charges alone are not sufficient for the magnetism, which in turn is driven by intrinsic defects. Supported by density functional theory and Monte Carlo simulations we will present a coherent and complete picture of the magnetism in (Zn,Co)O. We will argue that Co clustering is essential for the magnetism and that a wurtzite CoO phase, difficult to detect by X-ray, is responsible for most of the magnetic signal. In this picture, strongly compensated CoO clusters with random anisotropy fields mimic the hysteresis loops often observed experimentally and attributed to long-range ferromagnetic interaction.   

Enhanced Magnetism in Fe-Doped TiO$_{2}$ Anatase Nanorods

In addition to the applied interest concerning dilute magnetic semiconductors, ferromagnetism in $d^{0}$ oxides is of fundamental interest in understanding the interaction character between magnetic impurities in insulating systems. We report here simultaneous ferromagnetism and enhanced Pauli Paramagnetism in TiO$_{2}$ anatase nanorods doped with nominal 0.5 at{\%} Fe, synthesized by a hydrothermal route followed by low-temperature heating in air. The resultant nanorods are $\sim $ 20 nm in width and several hundreds nanometers in length. Transmission electron microscopy (TEM) reveals that the Fe concentration ranges from 0.3 at{\%} - 0.8 at{\%} within the nanorods. No evidence of pure iron nanoparticles in the sample is detected with TEM or with synchrotron diffraction. SQUID magnetometry performed in the temperature range 10 K $\le $ T $\le $ 800 K in fields up to 1 T show clear ferromagnetism at low fields that transitions to paramagnetic behavior at higher fields. Decomposition of demagnetization curves reveals that the nanorods possess Pauli paramagnetism that is over 100 times larger than that of pure bulk anatase TiO$_{2}$ as well as ferromagnetism that persists to temperatures slightly above 800 K. It is hypothesized that the Pauli paramagnetism originates from anatase regions with lower Fe doping, while the ferromagnetism originates from regions of higher Fe doping, suggesting a percolative mechanism for ferromagnetism.   

Room temperature ferromagnetism in undoped TiO$_{2}$ films

We have prepared thin films of undoped TiO$_{2}$ having rutile and anatase structures, using both spin coating and sputter deposition techniques, on sapphire and quartz substrates. The structural characteristics of the films have been investigated using Raman spectroscopy and transmission electron microscopy (TEM). We found that the annealing condition strongly influences the magnetic properties of the films. When annealed in high vacuum, all films demonstrate room temperature ferromagnetism (FM) whereas air annealed samples show insignificant FM. The ferromagnetic moment in vacuum-annealed samples stored under ambient conditions was not stable, but decayed on a time scale of hours. The sample magnetization was found to depend on the film thickness; the saturation magnetic moment was observed to decrease with increasing film thickness. These results suggest that FM in TiO$_{2-\delta }$ films is mediated by surface oxygen defects. The details of Raman and TEM studies will be presented and the appearance of FM on vacuum annealing will be discussed.   

Preparation of Dilute Magnetic Oxide Thin Films by Reactive-Biased Target-Ion Beam-Sputter Deposition

We have used reactive-biased target-ion beam-sputter deposition to prepare Co$_{x}$Ti$_{1-x}$O$_{2 }$thin films on LaAlO$_{3}$ (100) and SrTiO$_{3 }$(100) substrates for \textit{0.005$<$x$<$0.06}. The influence of the growth parameters on the microstructure, magnetic and transport properties of Co$_{x}$Ti$_{1-x}$O$_{2 }$was systematically investigated. Both pure anatase phase and mixed anatase/rutile phases of TiO$_{2}$ films were obtained by varying the growth conditions and subsequently demonstrated different magnetic and transport properties. All samples show a curie temperature higher than 300 K. The pure anatase Co$_{x}$Ti$_{1-x}$O$_{2 }$thin films have saturated magnetic moments of 1$\sim $2 $\mu _{B}$/Co at 10 K. The presence of rutile phase seems to greatly enhance the moments at lower temperatures.   

Observation of ferromagnetic behaviors in doped and undoped TiO$_{2}$ and ZnO

Transition-metal doping has been widely used to produce ferromagnetic oxides such as TiO$_{2}$ {\&} ZnO for use as a diluted magnetic semiconductor. Recently, however, undoped samples were also found to be ferromagnetic. We have studied the ferromagnetic behaviors of ZnO and TiO$_{2}$ single crystals and powders annealed both in vacuum and in flowing oxygen at various temperatures. In order to understand the observed ferromagnetism, we have used X-ray photoemission and electron spin resonance spectrometry to characterize the possible valance states or chemical bonding variation to study the roles that oxygen vacancies may have played in the occurrence of ferromagnetism. Comparisons on the doped and pure samples will be discussed.* Also with NSYSU .   

Electronic structure of Gd-doped GaN: vacancy-stabilized ferromagnetism

The study of dilute-doped magnetic semiconductors has attracted significant interest recently, especially that of Gd-doped GaN. While experimental evidence for a colossal moment/Gd have been found in the low doping regime, we will focus on the stabilization of ferromagnetism over antiferromagnetism with much better understood 7--8 $\mu_B$/Gd. Electronic structure calculations have been performed using the LAPW method within the WIEN2k code. Supercell calculations have shown that, in the absence of Ga or N vacancies, the large distance between Gd atoms leads to antiferromagnetism by the superexchange mechanism. Ga and N defects allows for extra electrons or holes to mediate the magnetism between the Gd atoms and stabilizes the ferromagnetic state. The degree of vacancies necessary to stabilize ferromagnetism has been approximated within a percolation model. This work is supported by the Air Force Office of Scientific Research.   

Session L13: Focus Session: Interfacial Ordering

Atomic resolution spectroscopic imaging of electronic phenomena in oxide interfaces

Electron energy loss spectroscopy in the STEM is a powerful tool to study the structure, chemistry and electronic properties of oxides with atomic resolution, in real space. In perovskites the O 2p bands and the transition metal 3d bands are very close to the Fermi level so the electronic properties can be probed by studying the fine structure on the O K edge and the transition metal L edge. Column-by-column EELS reveals direct information about the unique phenomena going on in oxide interfaces. For example, in superconducting/ferromagnetic YBa$_{2}$Cu$_{3}$O$_{7}$/ La$_{0.3}$Ca$_{0.7}$MnO$_{3}$ superlattices significant transfer of electrons from the manganite into the superconductor is found over nanometric length scales. But quite different phenomena occur in other manganite interfaces such as LaMnO$_{3}$/SrTiO$_{3}$. In this talk both experiments and first principles calculations with simulations of the ELNES will be discussed. Sponsored by the Office of Basic Energy Sciences, Div of Materials Sciences and Engineering, US DOE under contract DE-AC05-00OR22725 with ORNL managed by UT-Battelle LLC, by the ORNL LDRD Program and by the ORNL-ORISE Postdoctoral Program.   

Magnetic and electronic properties of complex oxide interfaces

Interfaces between two complex materials based on perovskite oxides can have novel physical properties. We studied the magnetic and electronic properties of La$_{0.67}$Ca$_{0.33} $MnO$_3$/YBa$_2$Cu$_3$O$_7$ (LCMO/YBCO) superlattices using first-principles density-functional theory (DFT). The energetics of several types of magnetic (spin) configurations of Mn ions near the LCMO/YBCO interface have been calculated using the DFT approach. Their magnetic and electronic properties have been explored and compared to the properties of bulk materials. These results are compared to recent experimental observation of suppressed magnetization at the LCMO/YBCO interface. We will also discuss the possibility of charge transfer across the interface, as suggested by recent experimental results from local electron energy-loss spectroscopy (EELS). This research was sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC, and by the McMinn Endowment at Vanderbilt University.   

Modified doping at cuprate / lanthanum manganite interfaces

Oxide heterostructures allow combining materials with similar structure but with very different ground states, which may compete at the interface to yield novel behaviors and functionalities. We explore the YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO) / La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) interface in thin film heterostructures. For x=0.3 the manganite is ferromagnetic which causes a strong depression of the superconductivity at the YBCO side. There is also a depression of the ferromagnetic moment at the interface suggesting electron transfer from the manganite into the YBCO. This is confirmed from superlattices alternating YBCO and LaMnO$_{3}$ (LMO), an A- type AF insulator. While for thin LMO layers ($<$ 6 unit cells) there is little effect on YBCO superconductivity, thicker LMO layers result in reduced Tc values and induced ferromagnetism at the interface, thus providing a firm indication of charge transfer. The occurrence of charge transfer over length scales much longer than the Thomas Fermi screening length (1 nm) is a novel behavior which, we hope, will stimulate future theoretical studies. Work supported by CICYT MAT2005 06024 C02-02.   

Interaction between magnetism and superconductivity in $La_{0.7}Ca_{0.3}MnO_3$/$YBa_2Cu_3O_{7-\delta}$ multilayers

Angular dependent resistivity measurements were performed on $La_{0.7}Ca_{0.3}MnO_3$/$YBa_2Cu_3O_{7-\delta}$ (LCMO/YBCO) heterostructures below and above the superconducting transition temperature $T_c \approx$ 90 K in different applied magnetic field. Besides the conventional intrinsic anisotropic magnetoresistance (AMR) present above $T_c$, we observe another anisotropic magnetoresistance, which only arises below $T_c$ and increases significantly with decreasing temperature. Also, the proximity-induced resistance, which appears in the LCMO layer, displays a spectacular increase at $T_c$ and then decreases significantly with decreasing temperature, persisting down to the lowest measured T of 72 K. This anomalous AMR and the proximity-induced resistance in the LCMO layer could be due to the triplet component of the superconducting condensation which penetrates into the ferromagnet over a long distance.   

Magnetic anisotropy and vortex dynamics in LCMO/YBCO heterostructures

Interplay of ferromagnetism and superconductivity in heterostructures of highly spin polarized CMR oxides and cuprate superconductors, is of topical interest. We have used a sensitive radio-frequency (RF) resonant method based on a tunnel-diode oscillator (TDO) to simultaneously probe the dynamic magnetic susceptibility and the vortex penetration depth in well characterized sputtered bi-layers (LCMO/YBCO) and tri-layers (LCMO/YBCO/LCMO), grown on STO substrates with the thickness of LCMO and YBCO being 40 u.c. and 15 u.c., respectively. Transverse susceptibility in the normal state shows distinct peaks associated with the anisotropy fields in LCMO. In the superconducting state, complex coupled response is observed with the region just below T$_{c}$ dominated by flux flow in a vortex liquid state. Experimental results with various field orientations are reported and analyzed in the context of proximity effect, spin diffusion, flux penetration and dissipation in the presence of geometrical barriers. Overall, our work demonstrates the effectiveness of RF experiments in probing the magnetization and vortex dynamics in these systems.   

Magnetism at the interface between ferromagnetic and superconducting oxides.

Atomically controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone. Modern synthesis methods have yielded high-quality heterostructures of oxide materials with competing order parameters. Although magnetic correlations at the interface are expected to be important in determining the macroscopic properties of such nanosystems, a quantitative determination of the interfacial magnetic structure in oxides has thus far been very limited. Here we examine superlattices composed of the half-metallic ferromagnet La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ and the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7}$ by core-level absorption spectroscopy with circularly polarized x-rays and by diffuse neutron reflectometry. The resulting data yield microscopic insight into the interplay of spin and orbital degrees of freedom at the interface. The data also reveal an extensive rearrangement of the magnetic domain structure at the superconducting transition temperature. The combination of techniques establishes an incisive probe of the interplay between competing electronic order parameters in oxide heterostructures. J. Chakhalian, J. W. Freeland, G. Srajer, J. Strempfer, G. Khaliullin, J.C. Cezar, T. Charlton, R. Dalgliesh, C. Bernhard, G. Cristiani, H.-U. Habermeier and B. Keimer, ``Magnetism at the interface between ferromagnetic and superconducting oxides,'' Nature Physics, v.2 , 244 (2006).   

Interface Magnetic Order in LaFeO$_3$/LaCrO$_3$ and LaFeO$_3$/La$_{1-\delta}$MnO$_3$ Superlattices

Creation of sharp interfaces between strongly correlated electron systems can result in novel states at the boundary. Here we present our work using element-resolved x-ray probes to study the magnetic order in LaFeO$_3$(LFO)/LaCrO$_3$(LCO) and LaFeO$_3$/La$_{1-\delta}$MnO$_3$ superlattices. Using pulsed laser deposition with RHEED control, (111) and (100) oriented ultra-thin superlattices were grown with layer thicknesses of 1 to 9 unit cells. In the bulk LaFeO$_3$ and LaCrO$_3$ are antiferromagnetic while La$_{1-\delta}$MnO$_3$ is ferromagnetic. At the interface of (111) oriented LaFeO$_3$/La$_{1-\delta}$MnO$_3$ superlattices we find clear sign of large net magnetic moment on both Mn and Fe even at moderate fields. For the LFO/LCO case, the (111) case displays small net mangetic moment in both layers while the (100) orientated samples shows no clear sign of net magnetic moment even at fields up to 5T. Research sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.   

Doping the interface of Mott-Insulator heterostructures

Recent rapid progress in techniques for layer-by-layer growth of transition metal oxides is making new types of heterostructures available. Previous studies have demonstrated interesting charge transfer and band bending effects near interfaces between Mott insulators and band insulators [1] and between polar and non-polar insulators [2]. We propose [3] interesting effects at several different classes of heterojunctions between ABO3 perovskites based on a single-band Hubbard model studied with several different approximate treatments of electron-electron interactions. Some potentially interesting material combinations will be discussed. \newline \newline [1] A. Ohtomo, etc., Nature 419, 378 (2002). \newline [2] N. Nakagawa, etc., Nature Materials 5, 204 (2006). \newline [3] W.-C. Lee and A.H. MacDonald, Phys. Rev. B 74, 075106 (2006) and work in preparation.   

Polar Kerr effect of epitaxial magnetite thin films in the visible and near infrared spectral region

Magnetite thin films are of interest for spin polarized injection and magneto-electric devices. The polar magneto-optical Kerr effect (MOKE) and optical absorption were measured on epitaxial films over the visible and near infrared spectral region at room temperature. Magnetite thin films on magnesium oxide, strontium titanate, barium titanate and spinel substrates were deposited by molecular beam epitaxy using molecular oxygen and iron. A complex MOKE spectrum was measured over the spectral range of 1.5 to 3.0 eV. A negative transition was observed at 1.6 eV and a positive transition at 2.7 eV. These were previously attributed to intervalence charge transfer and intersublattice charge tranfer transitions. The detailed spectrum was dependent upon the substrate suggesting strain may be playing a role. Using MOKE magnetometry the coercive field was measured. The coercive field of the epitaxial film varied from 370 gauss for films deposited on MgO to 620 gauss for films deposited on barium titanate.   

Epitaxial CoFe$_2$O$_4$(111)-based multilayers for spin filter applications

Efficient spin filtering at room temperature has high potential for ultra sensitive detectors and spin injection into semiconductors, leading to the growth of spin-based devices. We investigate the interaction of spin filter CoFe$_2$O$_4$(111) epitaxial tunnel barriers with Co and Fe$_3$O$_4$ electrodes in light of their possible application at room temperature. The question of the exchange coupling that often prohibits the independent switching between a magnetic tunnel barrier and its magnetic electrode is addressed, as is the difference between an oxide/metal and oxide/oxide system. Our study of the magnetic reversal in the CoFe$_2$O$_4$/Co and CoFe$_2$O$_4$/Fe$_3$O$_4$ bilayers, supported by a detailed structural and chemical analysis of the samples and their interfaces, clearly evidences the effect of a metallic or an oxide interface. An unusual exchange spring magnet behavior arises in the case of the CoFe$_2$O$_4$/Fe$_3$O$_4$ samples due to the superexchange interactions found in these ferrimagnetic oxides. This unique exchange phenomenon at the oxide-oxide interface ultimately leads to a barrier/electrode system that switches independently without the necessity of a non-magnetic spacer.   

Electron leakage and double-exchange ferromagnetism at a prototype metal-insulator interface: CaRuO$_3$/CaMnO$_3$

Density-functional studies of the electronic structure of a prototype interface between a paramagnetic metal and an antiferromagnetic insulator (CaRuO$_3$/ CaMnO$_3$) reveal how magnetism near the interface can be modified by the leaked electrons from the metallic to the insulating side. These electrons mediate a ferromagnetic interaction between the interface Mn moments via Anderson-Hasegawa double-exchange, which competes with the already existing antiferromagnetic superexchange, resulting in an interfacial ferromagnetic layer. Electron penetration beyond the first layer is insufficient to alter the bulk antiferromagnetism. We argue that a canted state in the first layer is possible, consistent with earlier magnetic measurements on this system.   

First principles calculations of interfacial magnetism in CrO2-SnO2 rutile junctions.

Rutile oxides possess a wide range of interesting properties including the half metallic behavior of CrO$_{2}$ which has been shown both theoretically and experimentally to have essentially 100{\%} spin polarization. For this reason, CrO$_{2}$ has attracted interest as an electrode material for fabrication of spin-valve structures and tunnel junctions with extremely high magnetoresistance ratios. SnO$_{2}$ is considered to be an ideal candidate for such magnetic tunnel junctions since it is an insulator with the same rutile crystal structure as CrO$_{2}$. Understanding of the atomic configuration and magnetic structure at the interfaces is important for obtaining high magnetoresistance ratios because of possible mixing of Cr and Sn interfacial atoms. We report first-principles studies of the role of the magnetic structure for different interfacial intermixing configurations between Cr and Sn atoms for supercells in the (100) and (110) directions. The calculations were performed using the Vienna ab-initio simulation program (VASP) within the Generalized Gradient Approximation (GGA) to Density Functional Theory (DFT).   

Session L14: Focus Session: Patterned and High Anisotropy Films for Data Storage

Computational study of local meta-magnetic states in manganese doped silicon nano-crystals

Using a real-space electronic structure theory with ordinary pseudo-potentials, we show that manganese doped at the center of the nano-crystals (diameters 1-5 nm) and near the surface has local meta-magnetic states which differ from their respective ground states by few tens to hundreds of meV in energy but larger than $K_{B}T$ at room temperature so they are not switchable easily at room temperature. We discuss origin of such meta-magnetic states and argue about a possible switching between these meta-magnetic states for potential information storage applications   

Thickness dependence of magnetization switching of sub-100 nm magnetic nanodots.

Vortex and single domain states are found in sub-100 nm Fe planar nanodots, fabricated by electron-beam evaporation using self-ordered porous alumina masks.[1] The thickness dependence of the magnetization reversal is studied by in-plane and out-of-plane SQUID measurements, and the thickness is verified using low-angle X-ray diffraction. The hysteresis loops of 53 nm and 66nm diameter dots changes with the dot thickness. This is attributed to the magnetization reversal occurring via a combination of in-plane and out-of-plane single domain and vortex states, as obtained from micromagnetic simulation. The 66 nm diameter and thickness above 50 nm dots are expected to develop a funnel-like configuration with two in-plane vortices of opposite chiralities as an out-of-plane field is applied. [1] C.-P. Li \textit{et al}., J. Appl. Phys. \textbf{100}, 074318 (2006).   

Imaging Magnetic Nanostructures via Resonant Soft X-Ray Spectro Holography

We will present how to exploit the coherence and tunable polarization of soft X-ray synchrotron radiation for imaging magnetic nanostructures via Fourier Transform Holography. This new lensless imaging technique is based on the direct Fourier inversion of a holographically formed soft x-ray interference pattern. Our implementation is particularly simple and is based on placing the sample behind a lithographically manufactured mask with a micron-sized sample aperture and a nano-sized reference hole. By exploiting the magnetic dichroism in resonance at the L$_{3}$ edges of the magnetic transition metals, images of magnetic nanostructures have been obtained with a spatial resolution of 50 nm. Different examples will be presented. The technique is transferable to a wide variety of specimen, appears scalable to diffraction-limited resolution (about 2 nm), and is well suited for ultra-fast single-shot imaging with future X-ray free electron laser sources.   

Patterned Magnetic Media: Recording Properties and Fabrication Issues

As the onset of thermal instability with decreasing grain size makes the extension of conventional sputtered granular media to higher data recording densities increasingly difficult, the use of nanopatterning techniques to create well-ordered arrays of isolated, highly uniform magnetic islands may enable a new generation of recording media extendible to densities of 1 terabit per square inch and beyond. A workable patterned media recording system requires a highly uniform magnetic island array, both in terms of dimensional parameters (island size, shape, and placement tolerance within the array) and materials properties (magnetic moment and switching field). Selectively writing individual tracks of islands without affecting neighboring islands requires a write transducer with sufficiently high field gradient and peak field. The island size range of interest (15-25 nm diameter islands on 20-40 nm array periodicity) makes fabrication of patterned media particularly challenging. One strategy for media fabrication is to create a high resolution master pattern via e-beam lithography and/or self-assembly, and to replicate this pattern over thousands of media samples using UV-cure nanoimprint lithography. The imprinted pattern can serve as an etch mask for patterning either the media substrate or magnetic layer. Trenches between islands may be filled to create a smooth surface suitable for flying a read/write head over the media surface at a spacing of a few nm. Although a variety of magnetic materials may be used, multilayer Co-Pt or Co-Pd are preferred based on their perpendicular anisotropy, moment, switching field, and strong coupling between grains (necessary to ensure that islands switch as a single unit). Depending on the fabrication method used, magnetic material in the trenches between islands can generate unwanted magnetic flux which generates noise in the readback signal. Island nonuniformity (both dimensional and magnetic) also contributes to increased errors in writing and increased noise in readback. Write errors may be generated via imperfect synchronization of the switching of the write field as the write transducer passes over the media. Tight tolerance control is required both for write synchronization and positional tracking of the head over the island array.   

Origins of switching field distributions in nanopatterned Co/Pd multilayers

We studied the reversal properties of perpendicularly magnetized nanodot arrays down to 50 nm in diameter. When continuous films undergo nanopatterning the phenomenon of a switching field distribution (SFD) becomes significant whereby the reversal fields from nanostructure-to-nanostructure vary. In applications such as patterned storage media, a broad SFD is undesirable since all nanostructures will not reliably switch at the same applied field. The origin of this phenomenon has been attributed primarily to grain boundary variations within a nanostructure as well as lithographic variations that occur during processing. While these two factors will certainly contribute to a larger SFD, we find that the primary origin of a SFD is an intrinsic material property of the continuous film. We will present our results of nanostructured Co/Pd exchange coupled multilayers. By changing the material properties using various seed layers and growth conditions, we were able to reduce the SFD to below 5 {\%} of the average switching field. By studying both polycrystalline and epitaxial multilayers we isolated the effects of grain boundary variation since epitaxial nanostructures are all single grain with identical orientation.   

Intrinsic switching field distribution in perpendicular recording media: numerical study of the $\Delta H(M, \Delta M)$ method

We present a numerical study of the $\Delta H(M,\Delta M)$ method and its ability to accurately determine intrinsic switching field distributions in interacting granular magnetic materials such as perpendicular recording media. In particular, we study how this methodology fails for large ferromagnetic inter-granular interactions, at which point strongly correlated magnetization reversal sets in that cannot be properly represented by the $\Delta H(M,\Delta M)$ method, which is based on a mean-field approximation. In this study, we use a 2-dimensional array of square hysterons that have a distribution of intrinsic switching fields and ferromagnetic next-neighbor interactions $J$. We find the $\Delta H(M,\Delta M)$ method to be very accurate for small $J$ values, while substantial errors develop once the effective exchange field becomes comparable with the intrinsic switching field distribution width, corroborating earlier results from micromagnetic simulations. This failure is correlated with deviations from data set redundancy, which is a key property of the mean-field approximation. Thus, the $\Delta H(M,\Delta M)$ method fails in a well defined and quantifiable manner that can be easily assessed from the data sets alone.   

Optimizing Graded Recording Media

A key limitation of recording densities arises from the fact that maintaining high thermal stability requires high anisotropies, whereas writeability requires low anisotropies: yielding contradictory requirements. Recently, Victora and Shen proposed that the recording density of perpendicular media is increased in exchange spring media: a structure with a soft and a hard layer. A decrease in coercivity up to a factor of 4 has been predicted. Very recently Suess considered a tri-layer system, reporting further increase. In the present work, we report optimizing media where the anisotropy is a continuous function of the thickness. We performed extensive finite element simulations and optimized the media performance by minimizing the coercivity, while maintaining a high energy barrier against thermal decay and the squareness of the hysteresis loop. Simple analytic estimates suggest that a quadratic thickness dependence is optimal. We explore the role of anisotropy convexity, a hard capping layer, and the exchange interaction. This graded anisotropy media decouples minimizing the coercivity while maximizing the barrier height, promising efficient new ways to optimizing recording media.   

Figures of Merit for Magnetic Recording Media

Since the first nucleation-field calculations for hard-soft nanostructures with multilayered [1] and arbitrary [2] geometries, exchange-spring magnets have attracted much attention in various areas of magnetism, including magnetic recording. Ultrahigh storage densities correspond to the strong-coupling limit, realized on small length scales and described by volume-averaged anisotropies. Second-order perturbation theory yields finite-size corrections that describe a partial decoupling of the phases. Since soft phases reduce the nucleation field, nanostructuring can be used to reduce the coercivity $H_{c}$ while maintaining the energy barrier $E_{B}$. However, the ratio $E_{B}$/$H_{c}$ is an ill-defined figure of merit, because the comparison with the Stoner-Wohlfarth model requires the introduction of a particle volume, as contrasted to an area. By using elongated particles with a continuous anisotropy gradient, it is possible to reduce the coercivity by a factor scaling as the bit size divided by the domain-wall width of the hard phase. However, with decreasing bit size this effect becomes less pronounced. In the strong-coupling limit, thermal stability yields a maximum storage density of order \textit{$\gamma $}/$k_{B}T$, where \textit{$\gamma $} is the domain-wall energy of the hard phase. - This research is supported by NSF MRSEC, INSIC, and NCMN. [1] S. Nieber and H. Kronm\"{u}ller, phys. stat. sol. (b) \textbf{153}, 367 (1989). [2] R. Skomski and J. M. D. Coey, Phys. Rev. B \textbf{48}, 15812 (1993).   

Experimental Study of Perpendicular Exchange Spring Media

We have investigated the magnetic reversal and recording properties of perpendicular exchange spring media. These structures, which combine a soft and a hard layer material$^{1}$, have recently been proposed as suitable candidates for advanced perpendicular magnetic recording$^{2}$. Previous studies$^{3}$ have also shown that the magnetization reversal can be tuned by means of a suitable coupling layer. In our study, we have investigated structures that consist of two magnetic layers having different H$_{K}$-values and being separated by a coupling layer of adjustable thickness. Similar to the results of our previous work on longitudinal exchange spring media$^{4}$, we find that there is an optimum coupling layer thickness, at which the magnetic reversal field is minimized. We also observe in our magnetometry experiments that the most robust parameter to quantify this improved magnetization reversal is the closure field H$_{S}$. The anticipated writability improvements for exchange spring media with optimal interlayer coupling strength are corroborated by detailed recording studies. [1] E.E. Fullerton et al., Phys. Rev. \textbf{B} \textbf{58}, 12193 (1998); [2] R. Victora et al.$, $IEEE Trans. MAG \textbf{41}, 537 (2005); [3] K.C. Schuermann et al., J Appl. Phys. \textbf{99}, 08Q904 (2006); [4] N. Supper et al., IEEE Trans. MAG \textbf{41}, 3238 (2005)   

Texture development and magnetic properties of Ru-doped FePt films

L1$_{0}$ ordered FePt films are promising candidates for ultra-high density recording media due to their high magneto-crystalline anisotropy when grown with (001) texture. In this paper, the effects of Ru doping on the FePt L1$_{0}$ phase formation and development of (001) texture are studied systematically. Ru doping is realized by preparing Fe/Pt/Ru multilayers by magnetron sputtering on SiO$_{2}$ substrates, with subsequent annealing at 650 $^{o}$C for 5 minutes in forming gas or hydrogen gas. It appears that hydrogen gas annealing leads to improved (001) texture. For small Ru alloying (less than 5 at. {\%}), the L1$_{0}$ texture and degree of chemical ordering remain the same. X-ray diffraction analysis shows that the (001) and (002) peaks shift slightly to larger angles, indicating that the Ru is dissolved in the FePt L1$_{0}$ phase. Increasing the Ru concentration beyond 5 at. {\%} resulted in an increasing (111) texture and a steady decrease of both coercivity and saturation magnetization. The effects of Ru on the magnetization and the magnitude of the coercivity have been studied. The mechanism by which Ru doping influences the texture development also will be presented in this work.   

Real-time thermal annealing studies in FePt thin films and nanostructures

$L$1$_{0}$-ordered FePt thin films and nanostructures have been heavily studied for ultrahigh-density recording applications taking advantage of the very large perpendicular magnetic anisotropy this phase exhibits ($\sim $10$^{7}$ erg/cc). A high degree of $L$1$_{0}$ order can be achieved by optimizing deposition conditions and/or performing annealing treatments. Here, we report on recent real-time thermal annealing studies of Fe-implanted Pt thin films that begin to exhibit chemical ordering upon annealing at $\sim $400$^{o}$ C. Clusters of Fe are implanted onto a Pt thin film using the Toledo Heavy Ion Accelerator (THIA) in which their size and penetration depth can be tailored by modifying the implantation conditions. These annealing studies were partially performed at the MHATT/XOR beam line (Sector 7) at the Advanced Photon Source at Argonne National Laboratory.   

Exchanged-Coupled FePt Cluster Films

L1$_{0}$ structure FePt films with (001) texture have attracted much attention for potential application in high-density perpendicular recording. In an effort to fine-tune their magnetic properties, novel structures of continuous FePt coupled to a FePt nano-composite layer have been investigated. The FePt layer, called the Continuous Layer (CL), was magnetron sputtered from a Pt target partially covered with Fe chips. The nano-composite layer uses carbon as a matrix and was made by two different methods: magnetron sputtered multi-layers of FePt and C, and cluster deposited $\sim $5 nm FePt particles with the C sputtered on top. All films were deposited on thermally oxidized Si substrates and processing was done for 300 seconds at 600$^{o}$C in an Ar with 5{\%} H$_{2 }$environment. Characterization was done with SQUID magnetometry and XRD. It was found that for the sputtered bilayer films, fixing the thickness of the nano-composite layer (5, 10, 15, or 20 nm) and varying the CL from 2 to 14 nm gave an increase in the coercivity of the film. The films with cluster deposited FePt particles showed exchange coupling after annealing and a decrease in coercivity over the pure CL. Models of these systems will be discussed in the talk. This work was supported by NSF-MRSEC, NCMN, DOE, INSIC and NRI.   

Session L15: Magnetism Experiment and Theory

Precise measurements of dynamic susceptibility near Curie temperature

We report tunnel diode resonator measurements of the 10 MHz dynamic magnetic susceptibility in the vicinity of the Curie temperature on a variety of ferromagnetic compounds with $T_{c}$ ranging from 4.5 K for CeVSb3 to 325 K for LaMn2Ge2. The outstanding sensitivity of the technique allows for a detection of the magnetic signal at pico-emu level. A sharp peak in susceptibility in the critical region is rapidly suppressed by applying relatively weak DC magnetic fields. Measurements are compared to specific heat, resistivity, and DC magnetization data. Results of scaling analysis in the critical fluctuations region are presented.   

Anomalous magnetoresistance and quadratic Hall effect in a time-reversal symmetry breaking state without net magnetization

Recently, the authors proposed a magnetic structure that breaks time reversal symmetry in the absence of net magnetization as an explanation for the high pressure ``partially ordered'' state of MnSi. The structure has a magnetic octupolar moment, but no net dipole moment. Based on symmetry, we demonstrate that this leads to anomalous magneto-transport properties: a magnetoresistivity which is linear and a Hall conductance which is quadratic in the applied magnetic field. Field cooling procedures for obtaining single domain samples are also discussed. The anomalous effects are elaborated in the case of three geometries chosen to produce experimentally unambiguous signals of this unusual magnetic state; e.g., it is predicted that a field in z-direction induces an anisotropy in the x-y plane. Another geometry leads to a Hall voltage which is parallel to the magnetic field.   

Structure and magnetic properties on Y$_{0.26}$Sr$_{0.72}$CoO$_{3-\delta }$ single crystal

We studied structural and magnetic properties on a single crystal of Y$_{0.33}$Sr$_{0.67}$CoO$_{3-\delta}$ in nominal composition. Single crystal was grown by the floating-zone method with radiation heating. Both electron-probe microanalysis (EPMA) and inductively coupled plasma (ICP) spectroscopy indicated the obtained single crystal was Y$_{0.26}$Sr$_{0.72}$CoO$_{3-\delta}$ in composition. The iodometric titration gives the oxygen content of 2.95. These results show that almost all Co ions are in trivalent state. The dc magnetization increases remarkably to reach a maximum at 301 K, then falls abruptly with decreasing temperature after both cooling and zero-field cooling. These behaviors were interpreted as a successive transition in which with decreasing temperature there is a transition from paramagnetic to ferromagnetic and eventually the antiferromagnetic properties appear at low temperatures. This should be associated with a successive spin state transition of Co ions [1, 2]. The detailed structural study and magnetic anisotropy will be reported and discussed with respect to a possible spin state change of Co ions. [1] Y. F. Zhang et al., accepted by Phys Rev. B for publication. [2] Y. F. Zhang et al., published in JMMM.   

Magnetic ordering in PrBCO by MaxEnt Muon-Spin Research

Muon-Spin Research ($\mu $SR) is used to probe the magnetic ordering of PrBa2Cu3O7. The $\mu $SR PrBCO data are analyzed using the Maximum Entropy (ME) technique, a spectral analysis tool more sensitive than Fourier transformation. [1] At low temperature and zero applied field, muons are localized and their Larmor spin-precessions map the internal magnetic fields. For temperatures well below room temperature, ME-$\mu $SR analysis yields two unique frequencies ($\sim $1.8 MHz and $\sim $2.4 MHz) corresponding to two different magnetic field regions (13 mT and 18 mT ) probed by the muon. Previous studies [2, 3] using Fourier analysis and curve fitting have shown only one broad frequency signal. We seek to confirm our new improved findings through dipole field search calculations and ME-$\mu $SR simulations at these near-zero $\mu $SR frequencies. [1] JC Lee \textit{et al,} J Appl Phys 95 (2004) 6906; AIP/APS www: Virtual J Applications of Superconductivity, June 2004 V6 Iss11; S. Alves \textit{et al,} Phys Rev \textit{Rapid Comm }B49 (1994) 12396. [2] WK Dawson \textit{et al,} J Appl Phys 69 (1991) 5385. [3] DW Cooke \textit{et al}, Phys Rev B41 (1990) 4801.   

Observation of Magnetic Excitations in Na$_3$RuO$_4$ using Inelastic Neutron Scattering

We report results on magnetic excitations observed in polycrystalline sodium ruthenate (Na$_3$RuO$_4$) in an inelastic neutron scattering study. Previous work has suggested that this material consists of relatively isolated tetramers of S=3/2 Ru(V) ions, and a Heisenberg antiferromagnet Hamiltonian was proposed. We compare predictions for the neutron inelastic structure factor in this model with our observations, and suggest future studies that might clarify apparent discrepancies between this model and our results.   

High Frequency EPR studies of an antiferromagnetic supramolecular grid

The magnetic anisotropy of an antiferromagnetic Mn(II)-[3 x 3] grid was studied by High Frequency Electron Paramagnetic Resonance(HFEPR). The ground state level crossing was observed from various temperature dependent studies. Frequency dependent studies illustrate a breaking of the $\Delta $S~=~0 EPR selection rule due to the strong mixing induced by the comparable exchange and magneto-anisotropy energy scales within the grid. In addition, the field-orientation dependence of the energy gap between the ground and first excited state was studied. All of these studies have contributed to a better understanding of this supramolecular grid, which represents a promising candidate to observe the long sought after quantum tunneling of the N\'{e}el vector.   

Latent heat and magnetocaloric properties of MnAs, CaMn$_{2}$Sb$_{2}$ and Mn-based compounds

The intense search for new magnetic materials for magnetic refrigeration has brought out several aspects of the magnetocaloric effect (MCE) behavior and interest in the underlined physics. We investigated the behavior of the magnetocaloric effect of materials such as MnAs and alloys, CaMn$_{2}$Sb$_{2}$ and Mn-based compound which have been a fruitful playground for competing interactions that lead to a variety of magnetic ordering such as spin liquid behavior, glass phase, etc. The MCE of those systems We extracted The MCE of those systems from heat capacity data and magnetization data using Maxwell's relations and we also investigated the latent heat involved in magnetic and structural phase transitions using a differential scanner calorimeter. Our results can discussed using a mean field approach in a quenched (or frozen) disorder parameters frame. In the present work, the MCE will be present as characterization tool for materials which exhibt unusual magetic ordering phases.   

Coexistence of a long-range and short-range ordered state in MnV$_2$O$_4$

We report $^{51}$V zero-field NMR of manganese vanadate spinel, MnV$_2$O$_4$ together with both ac and dc magnetization measurements. The zero-field NMR spectrum consists of multiple lines distributed from 240 MHz to 320 MHz. Its temperature dependence reveals that the ground state is formed by the delicate balance between long-range ferrimagnetic order and second short-range order, which causes reentrant-spin-glass-like behavior. The unusual ground state is ascribed to the competition between Mn$^{2+}$ and V$^{3+}$ exchange couplings and the orbital ordering of the V site induced by the structural phase transition.   

Internal Magnetostatic Potentials of Magnetization-Graded Hexagonal Ferrites

Compositionally graded ferromagnetic material offer novel functional properties that have promising device applications. Our investigations of the internal magnetic field induced by a spatially varying magnetization in a compositionally graded ferromagnet will be presented. We discuss results on a hexagonal ferrite sample, with a variation in saturation magnetization of 30 emu/g over a sample thickness of 2.5 mm. The DC magnetization shows a small anisotropy depending on the direction of the externally applied magnetic field relative to the grading direction. This contribution from a grading induced magnetic field is more pronounced in AC susceptibility measurements, which show small gradient dependent shift in M$\prime $ and M$\prime \prime $. We find a shift in magnetic properties corresponding to an internal magnetic field of 30 Oe, which is significantly lower than the predicted value of approximately 1900 Oe. The reasons for this difference and the implications of domains as the possible origin for the small H$_{int}$ will be will be discussed.   

Magneto-Optical Approach in Characterization on Ferrites in Millimeter Waves

Complex magnetic permeability and dielectric permittivity characteristics of yttrium iron garnets (YIG), aluminum and gadolinium substituted YIG and nickel ferrite materials in broad band millimeter wave frequency range have been obtained. The measurements have been done using free-space quasi-optical millimeter wave spectrometer in magnetic fields up to 1 T. A set of backward wave oscillators have been used as sources of high power coherent radiation, tunable in 30 -- 120 GHz frequency range. Magneto-optical approach has been successfully employed for the separation of dielectric and magnetic effects and simultaneous determination of complex dielectric permittivity and magnetic permeability of ferrite materials. Frequency dependences of real and imaginary parts of magnetic permeability of ferrites have been obtained. Strong frequency dependence of complex magnetic permeability in external magnetic field has been found for these ferrites materials.   

Optical Probe of Magnetization Dynamics in SrRuO3 Thin Films

We report all-optical measurements of ferromagnetic resonance (FMR) in the oxide ferromagnet SrRuO3 using the time-resolved magneto-optic Kerr effect. A Kerr signal at the FMR frequency is triggered by a laser-induced change in the direction of the magnetocrystalline anisotropy. We measure the subsequent precession of the magnetization vector as a function of temperature and laser intensity. We perform measurements in very thin films, with thicknesses from 2-10 nm, and show that the cooling of the film is limited by the boundary resistance between the film and substrate. This cooling of the SRO film via the substrate varies linearly with the film thickness, allowing us to separate thermal effects from the recovery kinetics.   

Density functional studies of magnetic behavior of layered Ag(II) fluorides

We present an extensive study of the structural and magnetic properties of layered Ag(II) fluoride Cs$_{2}$AgF$_{4}$ by using density functional theory within the local spin density approximation. We find that this material is well described as a two-dimensional ferromagnetism. Our investigations of a number of ground state properties are in good agreement with reported experiments. We also predict Jahn-Teller distortions in the low pressure phase and find that elastic anisotropy and the electric field created by the displacement of F ions does not modify the orbital order and origin of ferromagnetism.   

Electronic Structure, Chemical Bonding and Magnetic Properties in the Intermetallic Series Sc$_{2}$Fe(Ru$_{1-x}$Rh$_{x}$)$_{5}$B$_{2}$ from First Principles

First-principles, density-functional studies of the electronic structure, chemical bonding, ground-state magnetic ordering and exchange-interaction parameters have been performed for the entire Sc$_{2}$Fe(Ru$_{1-x}$Rh$_{x})_{5}$B$_{2}$ series of magnetic compounds. The results indicate that their magnetic properties depend in an extremely sensitive way on the degree of band filling and bandwidth. Continuous substitution of Ru by Rh changes the ground state from an antiferromagnet to a ferromagnet, as well as increases the effective spin moment caused by filling the bands with five additional electrons per formula unit together with a narrowing of the 4d band. The correlations between the character of the chemical bonding and the resulting exchange couplings are discussed. Trends for the macroscopic ordering temperatures are correctly reproduced.   

Session L16: Correlated Electrons: General I

Super-exchange in transition-metal oxides

Using contemporary tight-binding theory and parameters[1]. Anderson's perturbation approach [2] gives a qualitatively correct energy difference (a factor 2.3 too high) between ferromagnetic and antiferromagnetic configurations for MnO, It corresponds to a Heisenberg model with $J_2/J_1 = 11/7$. Perturbation theory fails as the energy denominator gets smaller for FeO and CoO, and changes sign for NiO. Use of the special- points method to treat exchange-split bands gives smaller values not well characterized by a $J_1$ and $J_2$. Carrying it out self-consistently reorders the NiO levels and leads to still smaller energy differences near experiment for all four oxides, as estimated from the experimental N\'eel temperature TN , The theory predicts a variation with pressure corresponding to $(d/ TN)\partial TN/\partial d = -12.2$ for MnO , near experiment, dropping to -9.1 for NiO. The theory is applicable also to the paramagnetic susceptibility. \newline \newline [1] Walter A. Harrison, Elementary Electronic Structure, World Scientific (Singapore, 1999), revised edition (2004). \newline [2] P. W. Anderson, Phys. Rev. 115, 2 (1959).   

Electronic specific heat enhancement in the half-metallic ferromagnet $Cro_2$ explained by Fermi Liquid Theory

Available data on the electronic specific heat of the half-metallic ferromagnet (HMF) $CrO_2$, show that the obtained experimental values are systematically greater than the corresponding theoretical ones calculated through various band theory methods. This discrepancy is due to the presence of many-electron correlation effects (spin fluctuations, strong electron-magnon scattering) which are not taken into account in the band theory calculations. A renormalization of the band theory results is therefore needed to account for the observed enhancement in the value of the specific heat. A microscopic many-electron approach has been proposed and explains the referred enhancement in terms of non-quasiparticle effects. It has been argued that Fermi liquid theory is not sufficient to provide the appropriate renormalization able to explain the observed enhancement in the electronic specific heat of HMFs. Contrary to this statement, we have shown that the introduction of a spin-dependent density of states, in the framework of the Fermi liquid theory for spin polarized systems, gives place to a renormalization which, indeed, provides a reasonable account of the observed enhancement in the electronic specific heat of the HMF $CrO_2$.   

Effect of Magnetic Field on the Induced Magnetic Moment System Pr$_{3}$In.

Pr$_{3}$In is a singlet-triplet system similar to the classic induced moment system Pr$_{3}$Tl, with identical crystal structure. Both materials order magnetically at similar temperatures (T$_{C}$=11.6 K for Pr$_{3}$Tl and T$_{N}$=11.4 K for Pr$_{3}$In). The magnetic order is antiferromagnetic (AF) in Pr$_{3}$In as opposed to ferromagnetic in Pr$_{3}$Tl. The exchange interaction between Pr sites causes admixture of the crystal field triplet excited state into the singlet ground state, resulting in induced moment magnetic order below T$_{N}$. Application of a magnetic field can change the energies of the singlet and triplet in such a manner as to alter the admixture. We have measured magnetization, magnetoresistance, specific heat and magneto-caloric effect in the range 0 to 15 T. We observed a phase transition below 11 K and at magnetic field of order 1.9 T. At present, whether this is a spin rearrangement or a spin polarized phase remains an open question. It would be surprising for the 1.9 T transition to be to a spin polarized state, given that T$_{N}$ is around 11.4 K. In addition, at high fields, we observe a strong reduction of the specific heat as the AF interactions are suppressed and the system reverts to a crystal-field-only behavior.   

de Haas-van Alphen study of the Fermi Surface of Ce$_{x}$La$_{1-x}$B$_{6}$ as a function of composition: the evolution of field-dependent quasiparticle effective masses

The de Haas-van Alphen effect has been studied in single crystals of Ce$_{x}$La$_{1-x}$B$_{6 }$ (0 $\le \quad x \quad \le $ 0.075) using pulsed magnetic fields of up to 60 T and temperatures 0.38 K $\le \quad T \quad \le $ 4.0~K. The low-field effective mass grows smoothly with increasing $x. $Moreover, for $x \quad >$ 0, the effective mass becomes a function of magnetic field, decreasing as the field rises. These results may be fitted using the extended Lifshitz-Kosevich formalism due to Wasserman, the decrease in mass reflecting the suppression of spin fluctuations by the field. The data also show that a previously-observed effect, attributed to complete spin polarization of one of the Fermi-surface sheets for $x \ge $ 0.05, is in fact an artifact of the field-dependent mass, ignored in previous works.   

Angle-resolved Resonant Inelastic X-ray Scattering in NaV$_2$O$_5$

Angle-dependent resonant inelastic x-ray scattering spectrum at the V-$L_3$ edge is analyzed to determine the origin of the V-$dd$ peak in NaV2O5 [1]. Experiment shows that as the incident photon polarization is rotated from the $b$ to $c$ axis, the V-$dd$ peak grows relative to the V-d/O-p peak and its energy loss becomes larger [2]. Our first-principles calculations demonstrate that such growth must involve both the unoccupied dxy and dxz/dyz bands. Neither the dxz/dyz nor dxy excitation alone can reproduce the ratio change. For the $bc$ scan, the light first samples the dxy orbital and then the dxz/dyz orbital. Slightly detuning the incident energy away from the resonant edge reveals that the dxy band is slightly lower in energy and much narrower than the dxz/dyz band. The results suggest that our previous analysis of the correlation splitting of the dxy band is valid [3]. [1] G. P. Zhang, T. A. Callcott, G. T. Woods, {\it et al} Phys. Rev. Lett. {\bf 88}, 077401 (2002); [2] G. P. Zhang {\it et al.}, Phys. Rev. B {\bf 65}, 165107 (2002); [3] G. P. Zhang and T. A. Callcott, Phys. Rev. B {\bf 73}, 125102 (2006).   

Self-organized Electronic Extended van Hove Singularity as Electron- lattice Dynamic Confinement Effect

A mechanism of self-organized one-dimensionality in correlated electron systems is proposed. It is found that unidirectional dynamic confinement of electron motion by quantum lattice vibrations may cause transition into ordered state with extended electronic van Hove singularities. This may explain observed enhancement of the ordering instability in the anti-nodal regions of the ``Fermi surface'' in the under- and optimally doped high-T$_{c}$ cuprates. It is shown that ordered electrons in the anti-nodal regions bind with quantum lattice vibrations that obey ``selection rules'': $Q/\Omega =z_s /g$, where$_{ }z_s $ are zeros of the Bessel function $J_0 (z)$, and Q is amplitude of lattice vibration with frequency $\Omega $, g is electron-lattice coupling strength in the force units. Confining potential of these vibrations creates one-dimensional ``nesting'' of the Floquet indices of electronic states, provoking electronic ordering transition. The transition is destructed by external magnetic field with the Larmour frequency $\Omega _L \ge \Delta ^2/\hbar t_\bot $, here $\Delta $ is ordered electrons energy-gap; $t_\bot /t_\parallel \ll 1$ are bare hopping integrals of the anisotropic electron tight-binding model.   

ARPES kink from strong electron correlations

Recent ARPES experiments have found a `kink' in the energy dispersion ($\omega$ vs $k$) which has been attributed to electron-phonon interactions. In this study, we compute the energy dispersion defining the maximum in spectral function using the 2D Hubbard model to see if such kinks can be explained simply from strong correlations. To treat the strong correlations, we employ the cellular dynamical mean-field theory method and compute $\omega$ vs $k$ for various hole dopings, temperatures and ratios of $U/t$. The computed dispersions show `kinks' similar to those seen in ARPES experiments. The energy scale for the kink is $t^2/U$ and arises from local spin correlations.   

Unique ground state in Ce$_{3}$Au$_{3}$Sb$_{4}$.

Ce$_{3}$Au$_{3}$Sb$_{4 }$has diverging specific heat coefficient at low temperature in a semiconducting state, a property which can be approached differently within the Kondo and band insulator viewpoints. Sample sensitivity here presents difficulty for determining the intrinsic behavior of this system. We will present pressure and powder neutron scattering experiment data in addition to its basic physical properties in order to discuss the underlying physics. This work has been supported by NSF-DMR-0503360.   

Strongly correlated electron behavior in RFe$_2$Zn$_{20}$ (R=rare earth elements, Zr, Hf)

GdFe$_2$Zn$_{20}$ has a remarkably high ferromagnetic ordering temperature ($T_C$=86K), which can be explained as a result of submerging large local moments into a nearly ferromagnetic Fermi liquid YFe$_2$Zn$_{20}$. Thermodynamic and transport properties of pseudoquaternary compounds Y$_{1-x}$Gd$_x$Fe$_2 $Zn$_{20}$ show ferromagnetic ground state for $x>0.02$, and reveal the polarization of correlated electrons related to the concentration of Gd. Comparing with YFe$_2$Zn$_{20}$, stronger itinerant electron magnetism was observed in ScFe$_2$Zn$_{20}$, ZrFe$_2$Zn$_{20}$ and HfFe$_2$Zn$_{20}$, whose properties place them even closer to the Stoner limit than YFe$_2$Zn$_{20}$.   

Magnetic Structure and Crystal Field Potential of PrOs$_4$As$_{12}$

Neutron powder diffraction and elastic neutron scattering have been used to determine the magnetic structure of the Filled Skutterudite compound PrOs$_4$As$_{12}$. The system becomes antiferromagnetically ordered with a Neel temperature ($T_N$) at 2.28K, which has A-type magnetic structure with spins lying along the doubled axis of the magnetic unit cell. The crystal field potential of PrOs$_4$As$_{12}$ has been studied by inelastic neutron scattering (INS). The ground state in the $T_h$ point group symmetry is determined to be a $\Gamma_5$ triplet. This is confirmed by Zeeman effect exhibited at low temperatures under high magnetic fields.   

Unusual transport properties in the orbitally-ordered system Lu$_{2}$V$_{2}$O$_{7}$

DC susceptibility ($\chi )$, AC and DC resistivity ($\rho )$, specific heat ($C_{p})$, and thermoconductivity ($k)$ measurements on single crystalline Lu$_{2}$V$_{2}$O$_{7}$ with the pyrochlore structure reveal two transitions: (1) a short-range magnetic ordering transition at $T_{s}$ = 175 K, which is identified by the slope change of 1/ $\chi $ and 1/$k$, an anomaly in the AC resistivity, and a change in the activation energy (2) an orbital ordering transition at $T_{o}$ = 70 K, which is confirmed by the sharp transition on $\chi $, $k$, and $C_{p}$. At $T_{o}$, the resistivity shows an unusual insulator-metal transition which will be discussed in relation to the orbital ordering transition.   

Magnetic and Magnetoelastic Properties of Substituted Cobalt Ferrites

We report recent results on a family of compounds based on cobalt ferrite with various chemical additions that can be used to dramatically alter the properties. These have high magnetostriction, high sensitivity of magnetic induction to applied stress and are chemically very stable, making them attractive for use in magnetoelastic sensors. For practical applications a family of materials is needed. The magnetic properties, magnetoelastic response, and temperature dependences can be controlled by selecting the chemical composition and adjusting the site occupancies of cations. A series of Mn-, Cr-, and Ga-substituted cobalt ferrite compounds, CoMn$_{x}$Fe$_{2-x}$O$_{4}$, CoCr$_{x}$Fe$_{2-x}$O$_{4}$, and CoGa$_{x}$Fe$_{2-x}$O$_{4 }$(where x=0.0 to 0.8) have recently been investigated and these showed dramatic changes in properties including reductions of over 350K in Curie temperature. Another significant result was that the effects of the substituted contents (x) on magnetic and magnetoelastic properties were significantly different for each substituted cation due to the differences in cation site occupancies of the elements within the spinel crystal structure.   

Momentum dependent light scattering in insulating cuprates

We investigate the problem of inelastic x-ray scattering in the spin$-\frac{1}{2}$ Heisenberg model on the square lattice. We first derive a momentum dependent scattering operator for the $A_{1g}$ and $B_{1g}$ polarization geometries. On the basis of a spin-wave analysis, including magnon-magnon interactions and exact-diagonalizations, we determine the qualitative shape of the spectra. We argue that our results may be relevant to interpret inelastic x-ray scattering experiments in the antiferromagnetic state of copper oxide materials.   

Session L17: Rheology, Transport, and Processing

A new molecular theory beyond tube model to describe cohesive breakdown in nonlinear flow of entangled polymers.

When an entangled polymer is subjected to shear or extensional flow at a rate of deformation (RD) much greater than its dominant relaxation rate (dRR), it may not flow homogenously all the way to the limiting strain of (RD/dRR) before it suffers cohesive failure. What keeps the chains entangled is an essential question to answer before an appropriate theory of polymer flow can be established. Unlike the tube model that assumes presence of an infinitely high energy barrier preventing escape of chain entanglement, our theory [1] recognizes a finite barrier height given by kT(M/Me) for a polymer whose number of entanglements per chain is (M/Me). A second essential ingredient is to realize that a sufficiently high level of elastic force can be generated per chain by the externally imposed flow. This elastic force can overcome the entanglement (cohesive) force as a rate-activation process, leading to the onset condition for the cohesive breakup either during flow or upon cessation of flow. Flow produces frictional inter-chain interactions among all entangling chains. These interactions also resist constitutive disintegration, delaying the onset of cohesive collapse to a larger strain. A higher level of cohesive strength results from the very flow deformation that could eventually produce enough internal (elastic) forces to destroy the cohesive structure made of chain entanglement. [1] \textit{Phys. Rev. Lett. }\textbf{97}, 187801 (2006).   

Image Correlation Spectroscopy of Actin Networks

We analyze fluctuations of entangled and cross-linked networks of fluorescently-labeled actin filaments using fourier space image correlation spectroscopy. Images from a fast confocal microscope are fourier-transformed, and the autocorrelation function for each wave vector is separately computed, producing the equivalent of the intermediate scattering function. We find that for entangled networks the long time decay of the long wavelenth modes is diffusive, possibly due to to filament reptation, but that the short-time behavior is more complicated   

Shape instabilities in absorbed polymer condensates

Self-assembly in polymeric liquids results in morphological structures which show ordering on a range of length scales. Two examples of this phenomena are the structures which result from a homopolymer in a poor solvent and a polyelectrolyte in a poor solvent. We specifically consider the scenario of imaging such condensates via techniques such as Atomic Force Microscopy or Surface Force Apparatus, where the condensate strongly absorbs to the surface. We demonstrate that the real-space, Self-Consistent Field method is an ideal numerical tool in predicting equilibrium morphologies. New structures are predicted, which are supported by explicit free energy calculations.   

Large-scale diffusion in thick photopolymer systems

The development of index change in millimeter-thick photopolymers designed for holography and optical devices has been studied on the micron scale using Bragg diffraction. These studies have revealed the importance of the relative diffusion rate of small molecules to the local polymerization rate but are limited to scales of less than about one micron. To probe the role of diffusion on larger scales, we introduce a form of direct-write lithography using multiple mutually-incoherent foci. This enables measurement of the development and relaxation time-constants over millimeter scales. These large-scale diffusion currents will impact applications in optical data storage, integrated optics, lenslet arrays and other large-scale exposures of these diffusion-limited photopolymers.   

Mechanisms for achieving high energy density in PVDF: a first-principles investigation

It is known that copolymers of vinylidene fluoride (VDF) with about 50 -- 80 {\%} VDF fraction favor the polar $\beta $-phase, and these copolymers exhibit a paraelectric phase transition below the melting point. However, a larger concentration of VDF prefers a non-polar $\alpha $-phase. We have used first-principles calculations to determine the stable phases of chloro-tri-fluoroethylene (CTFE)-VDF mixtures. Our results show that a phase transition occurs in this system as a function of the electric field, leading to a very high energy density in P(VDF-CTFE)-based capacitors. Our results for polarization, dielectric constant, and energy density are in excellent agreement with earlier experiments [1] and provide a microscopic explanation for the formation of high energy density phases in P(VDF---CTFE) and similar polymer mixtures. [1] B. Chu et al., Science 313, 334 (2006).   

Interfacial Density Profiles of Poly(methyl Methacrylate) with Liquids

Density profiles of a perdeuterated poly(methyl methacrylate) (dPMMA) film in water and hexane, which were `non-solvents' for dPMMA, along the direction normal to the interface were examined by neutron reflectivity. Interfaces of dPMMA with liquids were diffused in comparison with the dPMMA/air interface; the interfacial width with water was thicker than that with hexane. Interestingly, in water, the dPMMA film was composed of the strongly swollen layer and the interior region, which also contained water, in addition to the diffused layer. In contrast, such a strongly swollen layer was not observed at all in hexane.   

Initiated Chemical Vapor Deposition of Poly(methyl methacrylate)

We are exploring a way to process Poly(methyl methacrylate) into thin films using initiated Chemical Vapor Deposition (iCVD) technique. A unique iCVD reactor was designed and several experimental parameters such as substrate temperature, hot-zone temperature, monomer/initiator molar ratio, hot-zone/substrate distance and reactor pressure were adjusted to achieve micron-thick, uniform films. Resulting films were investigated by GPC, optical microscopy, and white light interferometry. The deposition rate was about 1 micron /hr. Computational fluid dynamics software Fluent was used to understand and simulate gas flow inside the reactor chamber and to optimize the deposition process.   

Molecular Dynamics Simulations of Nanomolding Process

The process of nanomolding hydrophobic monomers and polymers is studied by molecular dynamics simulations. A thin film with a monomer density of 0.524 $\sigma ^{-3}$ consisting of monomers or polymer chains with different degrees of polymerization is prepared by NVT-ensemble simulations. The mold is created by pressing the substrate with attached spherical nanoparticles, representing a master, into thin film. To fix the mold structure the film is crosslinked at different crosslinking densities. The nanoparticle pattern is recovered by molding a similar thin film into the crosslinked mold. The quality of the molding process is evaluated by calculating the eigenvalues of the radius of gyration tensor of the molded nanoparticles as a function of the crosslinking density, degree of polymerization and Lennard-Jones interaction parameters.   

Morphology development in electrospun nanofibers

The present article presents the modeling and simulation of the kinetics of electro- spinning process in conjunction with the spatio-temporal evolution of fiber morphology driven by phase separation. The spinning process has been modeled based on an array of beads connected by Maxwell's elements in cylindrical coordinates to describe the viscous retracting force counter-balanced by the Columbic forces representing the repulsive electrostatic charges. The dynamics of phase separation in the unstable region of a polymer solution has been calculated based on the Cahn-Hilliard time evolution equation. The simulation based on the coupling of these two processes has revealed the formation of porous voids, concentration bands perpendicular to the spinline (similar to banded textures) and along the spinline leading to splitting of the electro-spun fiber into nanofibrillar strands.   

Production of bi-component core-sheath nanofibers using Chitosan and Polyethylene oxide

There has been a renewed interest to develop fibers at nanometer scale due to the large number of potential biomedical uses such as tissue engineering, drug delivery and wound care applications. Chitosan is a naturally occurring polysaccharide obtained from crustaceans. Its antibacterial properties have been acknowledged. Our effort has been to develop core-sheath nanofibers using chitosan, and poly (ethylene oxide) (PEO), another bio-compatible polymer. The critical properties and parameters such as feed rate, electric field, distance between needle and grounded collector and their consequences on morphology are discussed. Chitosan/PEO solutions have been characterized by surface tension, molecular weight and viscosity which are crucial factors to achieve core-sheath geometry. Tensile and conductive properties of these core-sheath nanofibers have been investigated which could be important for them to be used in wound scaffolds and cell-culture respectively.   

Coarse Grained Modelling of Nanotube Stabilization by PEO Adsorption and Grafting

Coarse grained, implicit solvent models have been developed to represent the interaction between two infinite (periodic) single-walled carbon nanotubes and poly(ethylene oxide) in an aqueous environment. The polymer is modelled at a monomeric level of granularity, while the nanotubes are represented as cylindrical effective fields anchored to infinite, periodic lines. This coarse-grained model has been utilized to determine the potential of mean force between two nanotubes with either freely adsorbing or grafted polymer association models. The similarities and differences in relative stabilization of the nanotubes due to the polymer presence and association method is discussed.   

Session L18: Metal Clusters

Size-Selected Cluster Based Catalysts: Physical and Chemical Properties Studied by GISAXS, .Mass Spectrometry and UV-VIS Spectroscopy

Properties of highly stable cluster-based model nanocatalysts are studied. Examples on size-selected clusters Au$_{n}$ (n=7-10), Ag$_{n}$ (n=15-19), Pt$_{n}$ (n=8-10) and 1-3 nm particles supported on thin oxide film coated flat and mesoporous supports address cluster stability under realistic reaction conditions, selective stabilization of particles and their reactivity. In situ GISAXS allows for correlation of catalyst performance with its size and shape of the catalyst. Pt-cluster based catalyst supported on mesoporous membranes were tested in a commercial tester in oxidative dehydrogenation of propane and exhibited excellent propane conversion and superb selectivity towards propene production at moderate temperatures, 400-550 $^{o}$C with and without SnO promoter. Au and Ag catalysts were tested in ethylene and propylene oxidation, showing an onset of the reactivity between 160-200 $^{o}$C.   

Computational studies of small neutral vanadium oxide clusters and their reactions with sulfur dioxide

Vanadium oxide is a catalytic system that plays an important role in the conversion of SO$_{2}$ to SO$_{3}$. Density functional theory at the BPW91/LANL2DZ level is employed to obtain structures of VO$_{y}$ (y=1,{\ldots},5), V$_{2}$O$_{y}$ (y=2,{\ldots},7), V$_{3}$O$_{y}$ (y=4,{\ldots},9), V$_{4}$O$_{y}$ (y=7,{\ldots},12) and their complexes with SO$_{2}$. BPW91/LANL2DZ is insufficient to describe properly relative V-O and S-O bond strengths of vanadium and sulfur oxides. Calibration of theoretical results with experimental data is necessary to compute enthalpies of reactions between V$_{x}$O$_{y}$ and SO$_{2}$. Theoretical results indicate SO$_{2}$ to SO conversion occurs for oxygen-deficient clusters and SO$_{2}$ to SO$_{3}$ conversion occurs for oxygen-rich clusters. Subsequent experimental studies confirm the presence of SO in the molecular beam as well as the presence of V$_{x}$O$_{y}$ complexes with SO$_{2}$. Some possible mechanisms for SO$_{3}$ formation and catalyst regeneration for solids are also suggested.   

Unbiased studies on structural and electronic properties of gold clusters with up to 58 atoms

Isolated neutral Au$_N$ clusters are studied using a parameterized density-functional tight-binding method combined with genetic algorithms for $N$ from 2 up to 58. Various descriptors are used in analysing the results, including stability, shape, and similarity functions, as well as radial distances of the atoms and the orbital energies, all as functions of $N$. Based on a harmonic approximation, also the heat capacity of the Au clusters are studied as a function of temperature.   

Quantized Ferromagnetism in Free Cobalt and Iron Clusters

The magnetic moments $\mu$$_{N}$ for cobalt clusters \textit{Co}$_{N}$ (20$\le N\le $200) measured in a cryogenic molecular beam are found to be quantized both in the ground state: $\mu _{N}\sim $2$N$µ$_{B}$ and in the metastable excited state: $\mu _{N}^{\ast }\sim N\mu _{B}$ in contrast with the bulk where it is fractional: $\mu _{N=\infty }$=1.7$N\mu _{B}$. For $N$=30, the ionization potentials of the excited state is about 0.1 eV lower than of the ground state while this difference diminishes with increasing size, which implies that the two states become degenerate at large sizes. The evolution from localized moments in small clusters to itinerant moments in the bulk appears to be related to the closing of this energy gap which results in a fluctuating ground state. These effects can be understood in terms of the Falicov-Kimball model. Two states are also observed in iron clusters, with $\mu _{N}\sim $3$N\mu _{B}$ for \textit{Fe}$_{N}$, and $\mu _{N}^{\ast }$ $\sim N\mu _{B}$ for \textit{Fe}$_{N}^{\ast }$ (20$\le N\le $150).   

On the Stability of Sodium-Tin Zintl Ions in Gas phase experiments

A synergistic effort combining negative ion photoelectron spectroscopy of Na$_{n}$Sn$_{m}^{-}$ clusters along with the first principles electronic structure studies has been used to demonstrate that Zintl ions found in solutions also exist as stable species in free clusters. The theoretical investigations are carried out within a gradient corrected density functional approach. Our studies on Na$_{n}$Sn$_{4}^{-}$ clusters where n=0-4 and NaSn$_{m}^{-}$ clusters where m=4-7 show that Na$_{3}$Sn$_{4}^{-}$ is a very stable cluster marked by a distorted tetrahedral tin core and can be regarded as (Na+)$_{4}$(Sn$_{4})^{-4}$ gas phase analogue of the Na:Sn tetrahedral Zintl phase. In addition, the NaSn$_{5}^{-}$ cluster is shown to be the most abundant species in the mass spectrum in the NaSn$_{m}^{-}$ series and its stability can be reconciled with Sn$_{5}^{2-}$ Zintl ions. The existence of stable Zintl ions in the gas phase can provide an alternate approach to look for possible Zintl phases.   

On the stability and vibrational properties of super-polyiodides

We had earlier shown that a new class of polyhalides can be formed by combining the Al$_{13}$ super-halogen with the conventional halogen, I. Experimental reactivity studies demonstrate that the new super-polyhalides, Al$_{13}$I$_{x}^{-}$, exhibit pronounced stability for even numbers of I atoms. Theoretical investigations probing the geometry and the electronic structure reveal that the enhanced stability is associated with pairs of I atoms occupying the on-top sites around the Al$_{13}^{-}$ core. We had also demonstrated another series, Al$_{14}$I$_{x}^{-}$, that exhibits stability for odd numbers of I atoms. In this work we have examined the vibrational properties of the new super-polyiodides using gradient corrected density functionals. It is shown that the low frequency modes involve motion of the central Al and that the geometrical progressions with high iodine coverage can be understood in terms of these vibrations.   

New Assemblies Combining Super-halogens and Super-alkalis

An Al$_{13}$ cluster has been shown to exhibit behaviors reminiscent of halogen atoms with an electron affinity comparable to a Cl atom while molecular units like K$_{3}$O, called superalkalis, are known to have low ionization potentials. We have carried out first principles electronic structure calculations to examine the stability and the electronic properties of compound clusters formed by combining super halogens with superalkalis. An Al$_{13}$K$_{3}$O unit is shown to be a strongly bound ionic molecule that can be assembled into stable cluster superatom assemblies of composition (Al$_{13}$K$_{3}$O)$_{n}$. It will be shown that the individual clusters maintain their identity during the growth. The nature of the super-assemblies and their electronic properties will be highlighted.   

Nano-Assemblies using Designer Clusters

It is shown that a new procedure that combines studies on clusters in gas phase, theoretical investigation of the stability patterns, and the directed assembly in solutions can enable synthesis of nano-assemblies where the building blocks are designer clusters identified in gas phase. As a demonstration of its viability, we first examine As$_{7}$K$_{3}$ as a potential building block through gas phase molecular beam experiments starting from a dispersed mixture of bulk arsenic and potassium. Combining the experimental results with first principles electronic structure calculations, we identify As$_{7}$K$_{3}$ species as a uniquely stable Zintl entity that could affect self-assembly. Through directed assembly, we report success in synthesizing and characterizing a lattice of analogous super-cluster assembled material crystallized from the liquid phase. Electronic structure calculations on the nanoassembled material indicate that it is a wide band gap semiconductor.   

Optical and magnetic excitations in small transition-metal clusters using TDDFT

Magnetic properties of transition-metal clusters have been the subject of intensive study in the last decades both theoretical and experimentaly. In particular, the importance of noncollinear effects and spin-orbit coupling in those systems has recently gained great interest. In this work we use time dependent density functional theory (TDDFT) to study optical and magnetic excitations of small transition-metal clusters. In particular, we investigate the role of non-collinear magnetism and spin-orbit coupling in such phenomena. We present some results concerning the linear response calculations and show how noncollinear effects and spin-orbit coupling modify the optical and/or magnetic spectra (including the natural magnetic dichroism spectroscopy). We will discuss further line of research that we are conducting to understand the size dependence magnetic response of this clusters for potential technological applications. The calculations are done within a real-time real-space TDDFT framework using the Octopus code.   

Methanol Formation from Carbon Monoxide and Hydrogen on Neutral Nb$_{8}$ Clusters in the Gas Phase

Reactions of neutral V$_{n}$, Nb$_{n}$, and Ta$_{n}$ metal clusters ($n \le $ 11) with (CO + H$_{2})$/He mixed gases and CH$_{3}$OH/He in a flow tube reactor (P $\sim $ 14 Torr) are studied by time of flight mass spectroscopy. Metal clusters are generated by 532 nm laser ablation and reactants and products are ionized by low fluence ($\sim $200 $\mu $J/cm$^{2})$ 193 nm excimer laser light. Nb$_{n}$ clusters exhibit strong size dependent reactivity in reactions both with CO + H$_{2}$ and CH$_{3}$OH compared with V$_{n}$ and Ta$_{n }$ clusters. A remarkably strong mass peak Nb$_{8}$COH$_{4}$ is observed in the reaction of Nb$_{n}$ clusters with the mixed gases CO + H$_{2}$ at various concentration of H$_{2}$. This suggests a stable, low energy CH$_{3}$OH structure may form on an Nb$_{8}$ cluster. Methanol formation is not found on other Nb$_{n}$ ($n \ne $ 8), V$_{n}$, and Ta$_{n}$ clusters. In reactions of CH$_{3}$OH with metal clusters $M_{n }(M $= V, Nb, Ta, $n$ = 3-11), molecularly adsorbed products ($M_{n}$CH$_{3}$OH) are only observed on Nb$_{8}$ and Nb$_{10}$, whereas dehydrogenated products ($M_{n}$CO) are observed for all other clusters. This observation supports the suggestion that CH$_{3}$OH can be formed on Nb$_{8}$ in the reaction of Nb$_{n}$ with CO + H$_{2}$. Reaction mechanisms are discussed based on the experimental results in this work and those in the literature. Theoretical calculations are carried out to confirm our experimental results and suggested reaction mechanisms.   

Potential for Strong Pairing and High Transition Temperatures in Metallic Nanoclusters

Studies of atomic clusters containing tens or hundreds of atoms have gained much interest in recent decades because of their potential to bridge the gap between isolated atoms and bulk systems. Notable results include the observation of a shell structure$^{1}$ similar to that found in electronic shells of single atoms. Theoretical calculations$^{2}$ show that certain levels within this shell structure allow for strong Cooper pairing. These calculations also show that the particular shell levels, which are realistically attainable, have high density of states in the HOS and LUS levels and could show substantially higher values of the superconducting transition temperature $T_{C}$ than are observed in the bulk material. At temperatures near $T_{C}$, the onset of strong pairing can be experimentally observed by an increase in the minimum excitation energy of the particular cluster. Our group will first look for this energy increase in Al clusters at around 90K, the predicted $T_{C}$ for Al clusters of interest. Here we present a progress report on Al and describe future work. \newline \newline $^{1}$ W. Knight, K. Clemenger, W. de Heer, W. Saunders, M. Chou, and M. Cohen, Phys. Rev. Lett. {\bf 52}, 2141 (1984). \newline $^{2}$V. Z.Kresin and Y. N. Ovchinnikov, Phys. Rev. B {\bf 74}, 024514 (2006).   

Electron pairing in pure Niobium and Niobium alloy clusters

Electrons in pure niobium and in niobium alloy clusters are ferroelectric at low temperatures. The ferroelectric effect is enhanced for niobium clusters doped with non-magnetic metals and reduced when doped with magnetic atoms. The effect is enhanced (reduced) for clusters with an even (odd) total number of valence electrons. For specific alloy clusters the ferroelectric state persists up to room temperature. Ferroelectricity in these clusters and superconductivity in the corresponding bulk appear to be related, with similar transitions temperatures and similar responses to specific impurities. The spontaneous polarization of a ground state involving a Cooper pair explains the observations.   

Session L19: Frontiers in Electronic Structure Theory II

Many-Body Perturbation Theory and Density-Functional based approaches: successful combinations

Today, in the framework of solid state physics two main ab initio approaches are used to describe ground- and excited state properties of condensed matter: on one side, static ground state density functional theory (DFT) and its time-dependent extension (TDDFT) for the description of excited states; on the other side, Many-Boby Perturbation Theory (MBPT), most often used in Hedin s GW approximation [1] for the electron self-energy, or the Bethe-Salpeter equation for the calculation of response functions. Both approaches have led to breakthroughs, but suffer from different shortcomings: MBPT has a relative conceptual clarity and therefore allows one to find good approximations, but calculations are in general numerically very demanding. DFT-based approaches are in principle computationally more efficient, but a generally reliable and at the same time efficient description of exchange-correlation effects within TDDFT is difficult to obtain. In recent years a major effort has therefore been made in order to combine MBPT and TDDFT, searching for a formulation that would keep the advantages of both approaches (see e.g. [2,3]). In this talk we will discuss different ways to derive a linear response exchange-correlation kernel for TDDFT from MBPT. The strength of various approximations, that have been shown to reproduce continuum and bound excitons for a wide range of materials, as well as possible problems will be outlined, and the computational efficiency of the method examined. The question of how to use such a combination of MBPT and TDDFT in order to obtain vertex corrections to the self-energy [4] will also be addressed. \newline \newline [1] L. Hedin, Phys. Rev. 139, A796 (1965). \newline [2]F. Sottile, V. Olevano, and L. Reining, Phys. Rev. Lett. 91, 056402 (2003) \newline [3] S. Botti, F. Sottile, N. Vast, V. Olevano, L. Reining, H.-C. Weissker, A. Rubio, G. Onida, R. Del Sole, R.W. Godby, Phys. Rev. 69, 155112, (2004). \newline [4] F. Bruneval, F. Sottile, V. Olevano, R. Del Sole and L. Reining, Phys. Rev. Lett. 94, 186402, (2005).   

Using Constrained DFT to Define a Diabatic Configuration Space

We show that several of the well-known shortcomings of approximate density functionals for treating electron transfer (ET) can be overcome by applying physically motivated constraints to the electron density. We summarize our implementation of this constrained density functional theory (CDFT) and present several illustrative applications that demonstrate the strengths of the new formalism: 1) CDFT allows charge transfer excitations to be treated accurately within a ground state formalism, including the long range -1/r interaction between the electron and the hole 2) One directly obtains diabatic states, which can be unambiguously associated with Marcus theory parameters and 3) Long-standing ground state electronic structure problems -- such as the prediction of exchange couplings and certain reaction barrier heights -- can be treated accurately in a rigorous fashion.   

Ab initio study of near-edge x-ray absorption fine structure of hexagonal ice and liquid water

We report first-principles calculations of near-edge x-ray absorption fine structure (NEXAFS) spectra of hexagonal ice and liquid water. Our work is motivated by the importance of accurately modeling NEXAFS spectra, which provide sensitive information on local molecular structures. In particular, we find a systematic improvement in the agreement of the calculated spectra with the experiment, by including excitonic effects, final state and self-interaction corrections. We correlate the calculated corrections to the degree of localization of the excited states.   

Going beyond the Tamm-Dancoff approximation in the Bethe-Salpeter approach to the optical properties of solids

The solution of the Bethe-Salpeter equation (BSE) has turned out to be the method of choice for the ab-initio calculation of optical properties of semiconductors and insulators which is capable of correctly accounting for excitonic effects. Commonly, however, the coupling between the resonant and anti-resonant excitations is neglected, referred to as the Tamm-Dancoff approximation (TDA). This is well justified in many cases, in particular, for the working horses of theoretical solid state physics, such as bulk Si and GaAs. Here, we report on a first-principles investigation of the optical properties of organic semiconductors which are highly anisotropic systems. We find that the TDA no longer holds in such low-dimensional systems, where the exciton binding energies are no longer small compared to the band gaps. Going beyond the TDA leads to an increase of the exciton binding energy in the order of several tenths of an eV thereby considerably improving the agreement with experiment.   

Electron-atom scattering using time-dependent density-functional theory

We present a method to obtain single-channel elastic electron-atom scattering phase shifts from time-dependent density functional theory (TDDFT). The system is placed in a spherical box, and TDDFT is used to calculate its discrete spectrum, from which phase shifts are deduced. The influence of ground state Kohn-Sham potentials and exchange-correlation kernels on the results are discussed.   

Investigation of Bonding in the BF$_{3}$-H$_{2}$O Complex

The catalytic properties of BF$_{3}$ involving its complexes with different classes of molecules is of great current interest. As a typical system of complexes involving the B-O bond we have studied the BF$_{3}$- H$_{2}$O system using first-principle Hartree-Fock-Roothaan procedure combined with many-body perturbation theory to include Van der Waals (VDW) interaction between BF$_{3}$ and H$_{2}$O molecules. From our results, the VDW contribution to the binding energy of the BF$_{3}$-H$_{2}$O complex comes out as 34.5{\%} of the covalency, close to the 36.4{\%} result from our earlier investigations on BF$_{3}$-NH$_{3}$ . The absolute values for the covalency and VDW contributions are both about 35{\%} of the BF$_{3}$-NH$_{3 }$result. Physical implications of these results will be discussed.   

van der Waals coefficients in DFT: a simple approximation for the polarizability

Long range van der Waals interaction plays a crucial role in many systems. Density functional Theory (DFT) within Local Density and Generalized Gradient Approximations for exchange-correlation energy is known to fail in describing properly this interaction, while direct calculations based on the exact Adiabatic Connection Formula are computationally impracticable, except for few simple systems. A simple and computationally fast scheme to calculate imaginary-frequency-dependent polarizability, hence asymptotic van der Waals interaction, within density functional theory is considered. The van der Waals coefficients for a large number of closed-shell ions and several molecules are calculated and compare well with available values obtained by more refined first-principle calculations. The success in these test cases shows the potential of the approximation in capturing the essence of long range correlations and may give useful information for constructing a functional which naturally includes van der Waals interaction in DFT.   

First-Principles Study of the Nature of Binding in BF$_{3}$ Molecular Solids

The binding of BF$_{3}$ molecules in solid BF$_{3}$ is studied by the Hartree-Fock Cluster Procedure, with Van der Waals interaction between the BF$_{3}$ molecules included by the many-body perturbation theory procedure. The binding appears to be the result of strong cancellation between one-electron effects, represented by the covalent interaction between neighboring molecules combined with the coulomb interaction between the effective charges on the boron and fluorines in each of the neutral BF$_{3}$ molecules and the many-body correlation effect between electrons on neighboring molecules leading to the Van der Waals interaction, the latter being the determining factor for the binding. Quantitative results will be presented for the binding energy in this solid state system which represents a class of molecular solids for which the neutral molecular units have substantial effective charges of different signs on the constituent atoms.$_{ }$   

Optimized orbitals with second order opposite-spin correlation

Despite tremendous progress, the most ubiquitous electronic structure methods, based on density functional theory (DFT), that can be applied to molecules ranging well over 100 atoms, exhibit failures for molecules with strong correlations, some types of radicals, and systems where dispersion interactions are important. At the same time, the most accurate electronic structure methods, based on coupled cluster theory, remain too computationally demanding to enable the routine treatment of molecules containing more than about 20 atoms. I will discuss a new self-consistent approach that correctly and inexpensively recovers dispersion interactions, without either excessive spin-contamination for radicals (as plagues traditional unrestricted Hartree-Fock-based methods), or the difficulties of self-interaction that can affect DFT calculations of radicals. This approach yields optimized Breuckner-type orbitals. Its performance for relative energies, structures, and frequencies will be assessed, both for closed shell molecules, radicals, as well as some cases which exhibit pathological failures at both the DFT and MP2 levels of theory.   

On the dynamics of the spin-boson model: variational principle versus adiabatic approximation

The time-dependent variational principle is applied to the spin-boson model. We use two different trial functions that exhibit various degrees of separation between the bosonic dynamics and the electronic dynamics. The equations of motion obtained for these two trial functions are shown to be equivalent with the equations of motions obtained with two different adiabatic approximations of the dynamics.   

Session L20: History of Physics; Cosmic Microwave Background

The Missing Part in the Story of Spin: What is the Spin Content of Stern-Gerlach?

Explaining the complex structure of atomic spectra was a determining factor in the development of the old quantum theory and it contributed significantly to the invention of quantum mechanics in the 1920s. Eventually it also led to the introduction of an additional degree of freedom for the electron and to the spin model of Goudsmit and Uhlenbeck. All along, information on the Stern-Gerlach effect, which is widely interpreted today as a manifestation of spin, was available. It did not seem to influence the invention or the acceptance of spin. We examine the connection between spin and Stern-Gerlach and review the lack of mutual influence in the publication record. We conclude by suggesting possible reasons for the absence of the Stern-Gerlach effect in the story of spin.   

The Entangled Histories of Physics and Computation

The history of physics and computation intertwine in a fascinating manner that is relevant to the field of quantum computation. This talk focuses of the interconnections between both by examining their rhyming philosophies, recurrent characters and common themes. Leibniz not only was one of the lead figures of calculus, but also left his footprint in physics and invented the concept of a universal computational language. This last idea was further developed by Boole, Russell, Hilbert and G\"odel. Physicists such as Boltzmann and Maxwell also established the foundation of the field of information theory later developed by Shannon. The war efforts of von Neumann and Turing can be juxtaposed to the Manhattan Project. Professional and personal connections of these characters to the development of physics will be emphasized. Recently, new cryptographic developments lead to a reexamination of the fundamentals of quantum mechanics, while quantum computation is discovering a new perspective on the nature of information itself.   

Einstein's Jury: Trial by Telescope

While Einstein's theory of relativity ultimately laid the foundation for modern studies of the universe, it took a long time to be accepted. Between 1905 and 1930, relativity was poorly understood and Einstein worked hard to try to make it more accessible to scientists and scientifically literate laypeople. Its acceptance was largely due to the astronomy community, which undertook precise measurements to test Einstein's astronomical predictions. The well-known 1919 British eclipse expeditions that made Einstein famous did not convince most scientists to accept relativity. The 1920s saw numerous attempts to measure light-bending, as well as solar line displacements and even ether-drift. How astronomers approached the ``Einstein problem'' in these early years before and after the First World War, and how the public reacted to what they reported, helped to shape attitudes we hold today about Einstein and his ideas.   

Forty lost years of Coherent States

In search to satisfy the correspondence principle Schr\"odinger in 1926 introduced the minimum uncertainty state. Almost forty years later in 1963 Glauber put these states to use in what now know as quantum theory of optics. He also gave them the name we know them by, coherent states. And soon after Sudarshan completed Glauber's unfinished work in achieving the theory of quantum optics. Crucial mathematical work was done in these forty years to able Glauber to consider these states. I will discuss why Glauber was attracted to these states. I will talk about what it was that Schr\"odinger was after, and why they were forgotten for almost forty years.   

Reception of the Kaluza Theory in Britain, 1921-1958

The Kaluza five-dimensional unified thoery was part of a programme to geometrise physics largely abandoned in the wake of the successes of quantum mechanics. However, a small group of British physicists continued to work on the subject throughout the middle decades of the 20th Century.   

On the origins of the Raman Effect

I explore the events that led to the discovery of the Raman effect by C.V. Raman and K.S. Krishnan at Calcutta in 1928. I also argue that, although the Raman effect was generally seen as providing strong evidence for the quantum nature of light, Raman himself was a staunch supporter of the classical wave theory of light. This work is part of a larger project, which seeks to understand the role of Raman scattering in the experimental verification of the quantum dispersion theory of Hendrik A. Kramers, which formed a bridge between Bohr and Sommerfeld's old quantum theory and Heisenberg's matrix mechanics.   

COBE and the Absolute Assignment of the CMB to the Earth.

The FIRAS instrument on COBE initially reported a CMB temperature of 2.730+/-0.001 K (1$\sigma )$. At the same time, using the 1st derivative, FIRAS reported a CMB temperature of 2.717+/-0.003 K (1$\sigma )$. These two values are significantly different at the 99{\%} confidence interval. In order to remove this significance, NASA lowered the absolute value of the CMB by changing the calibration on the external calibrator long after launch. It also raised the error bars on the second value. However, the observed difference in the CMB temperature measured by these two methods may well constitute evidence that the CMB monopole arises from the Earth. It should be assumed that a second, much weaker, microwave field exists both at L2 (the WMAP position) and at the COBE position. Motion through this much weaker field is responsible for the dipole observed. The value of the CMB temperature obtained by the 1st derivative is sensitive to motion. It is also sensitive to the complicating effect of the weak field also present at L2 when sampling the CMB temperature using FIRAS. The presence of a second weak field at L2 and the Earth is required in order for COBE to be able to resolve this situation. The PLANCK satellite should soon reveal that that CMB monopole does not exist at L2.   

Session L21: Computational Fluid Dynamics and Related Methods

Spatial Grand Canonical Monte Carlo Algorithms for Fluid Simulation

Strict detailed balance is essentially unnecessary for Markov chain Monte Carlo simulations to converge to equilibrium. Recently, we proposed a general Monte Carlo algorithm based on sequential updating that only satisfies the weaker balance condition. We have shown analytically that the new algorithm identifies the correct equilibrium distribution of states. Analysis of the diagonal elements of the transition matrices shows that the new algorithm is more mobile than the conventional Metropolis algorithm. Monte Carlo simulations of the Ising model and the lattice gas show that the new algorithm reduces autocorrelation time and thus improves the statistical quality of sampling. By exploiting the equivalence of the Ising model and the lattice gas, we demonstrate that the new method can also be applied to continuum systems, such as Lennard-Jones, in the grand canonical ensemble. Potential applications of the new algorithm are simulations of 1st and 2nd order phase transitions. Any massively parallel Monte Carlo simulation based on spatial decomposition involves simultaneous moves of atoms/molecules on multiple CPUs. Parallel Monte Carlo simulations based on spatial decomposition suffer from loss of precision due to the periodic switching of active domains. On the other hand, our algorithms are intrinsically sequential, and can be parallelized easily without compromising precision.   

Solving Parabolic Equations on a Random Grid Using a Generalized Finite Difference Method

A novel, generalized finite differencing scheme for solving parabolic initial value PDEs on a random grid will be described and results from its application to the diffusion equation will be presented. For a given number of points, $N,$ in a computational star, we parameterize the ($N-$6)-dimensional space of all possible approximations to the Laplacian that are accurate to first order. We have generalized von Neumann stability analysis to the random grid and we use simulated annealing to search parameter space to find a Laplacian that gives stable time evolution of the system. The creation of a stable Laplacian is moderately computationally intensive, but using it to evolve the PDE in time is of the same order as standard finite difference schemes on regular grids. We will present simulations using a Gaussian initial profile on a 10,000 point, annealed random grid in 2D. We also show that the same Laplacian also allows stable time evolution of the stiff, nonlinear, Cahn-Hilliard equation on the same grid. .   

A multiscale algorithm for pulled fronts in reaction-diffusion

I will present a numerical scheme for simulating front propagation in the discrete $A{\rightarrow}2A$ reaction-diffusion problem. The FKPP equation describes the continuum approximation to the particle model. However it has been suggested that the dynamics of the particle model converges very slowly to the continuum dynamics. As such, it is infeasible to probe the approach to continuum with a purely particle-based method. Instead, we have created a hybrid model which properly treats important fluctuations where needed.   

Numerical Model for Hydrovolcanic Explosions.

A hydrovolcanic explosion is generated by the interaction of hot magma with ground water. It is called Surtseyan after the 1963 explosive eruption off Iceland. The water flashes to steam and expands explosively. Liquid water becomes water gas at constant volume and generates pressures of about 3GPa. The Krakatoa hydrovolcanic explosion was modeled using the full Navier-Stokes AMR Eulerian compressible hydrodynamic code called SAGE [1] which includes the high pressure physics of explosions. The water in the hydrovolcanic explosion was described as liquid water heated by magma to 1100 K. The high temperature water is treated as an explosive with the hot liquid water going to water gas. The BKW [2] steady state detonation state has a peak pressure of 8.9 GPa, a propagation velocity of 5900 meters/sec and the water is compressed to 1.33 g/cc. \newline \newline [1] Numerical Modeling of Water Waves, Second Edition, Charles L. Mader, CRC Press 2004. \newline [2] Numerical Modeling of Explosions and Propellants, Charles L. Mader, CRC Press 1998.   

Efficient Nonlinear Atomization Model for Thin 3D Free Liquid Films

Reviewed is a nonlinear reduced-dimension thin-film model developed by the author and aimed at the prediction of spray formation from thin films such as those found in gas-turbine engines (e.g., prefilming air-blast atomizers), heavy-fuel-oil burners (e.g., rotary-cup atomizers) and in the paint industry (e.g., flat-fan atomizers). Various implementations of the model focusing on different model-aspects, i.e., effect of film geometry, surface tension, liquid viscosity, coupling with surrounding gas-phase flow, influence of long-range intermolecular forces during film rupture are reviewed together with a validation of the nonlinear wave propagation characteristics predicted by the model for inviscid planar films using a two-dimensional vortex- method. An extension and generalization of the current nonlinear film model for implementation into a commercial flow- solver is outlined.   

Discrete Bubble Modeling for Cavitation Bubbles

\textsc{Dynaflow, Inc.} has conducted extensive studies on non-spherical bubble dynamics and interactions with solid and free boundaries, vortical flow structures, and other bubbles. From these studies, emerged a simplified Surface Averaged Pressure (SAP) spherical bubble dynamics model and a Lagrangian bubble tracking scheme. In this SAP scheme, the pressure and velocity of the surrounding flow field are averaged on the bubble surface, and then used for the bubble motion and volume dynamics calculations. This model is implemented using the Fluent User Defined Function (UDF) as Discrete Bubble Model (DBM). The Bubble dynamics portion can be solved using an incompressible liquid modified Rayleigh-Plesset equation or a compressible liquid modified Gilmore equation. The Discrete Bubble Model is a very suitable tool for the studies on cavitation inception of foils and turbo machinery, bubble nuclei effects, noise from the bubbles, and can be used in many practical problems in industrial and naval applications associated with flows in pipes, jets, pumps, propellers, ships, and the ocean. Applications to propeller cavitation, wake signatures of waterjet propelled ships, bubble-wake interactions, modeling of cavitating jets, and bubble entrainments around a ship will be presented.   

Experimental and Numerical Studies on Multibubble Interaction

The behavior of multiple interacting bubbles can vary substantially from that of single bubble dynamics. We have conducted experiments and developed different levels of numerical tools for studying multiple bubble dynamics effects. In the experiments, multiple bubbles were generated simultaneously by spark and visualized using high speed video camera. The experimental results were then used to validate the numerical simulations obtained by 3\textsc{DynaFS}$^{\copyright }$\textsc{ }and\textsc{ PhantomCloud}$^{\copyright }$. 3\textsc{DynaFS}$^{\copyright }$\textsc{ }uses the boundary element method, which is capable of predicting non-spherical bubbled deformation as well as multibubble interaction. \textsc{PhantomCloud}$^{\copyright }$ is based on our earlier asymptotic expansion studies in which all bubbles are replaced with sources/sinks and dipoles whose intensities are determined by the underlying flow field and the presence of the other bubbles and their dynamics. The dynamics of each bubble is then recovered by a modified Rayleigh-Plesset equation and a center of the bubble equation of motion. In this approach, bubbles, which are punctual singularities, can interpenetrate without resulting in code failure, thus the ``phantom'' naming.   

Very-high temperature molecular dynamics of dense plasmas.

Finite temperature Orbital Free DFT coupled consistently with moleculer dynamics is applied to the hot-dense plasma regime up to 1000~eV and 100~$\mathrm{g\,cm^{-3}}$. Results obtained on dense iron and boron plasmas are compared with all-electron Quantum MD and effective classical theories like OCP and Yukawa OCP. A prescription on ionization for the classical model is made through the structural properties. Emphasis is also done on a comparison between Kubo-Greenwood and Ziman theories on dc conductivity in the very dense regime.   

Particle Beam Waist Location in Plasma Wakefield Acceleration

The role of beam waist location in interactions between a plasma and a particle beam is not yet fully understood. Nonlinear effects within the plasma make an analysis of such interactions difficult. I present five simulations in which I vary the waist location of a beam of ultra-relativistic electrons propagating through one meter of self-ionized lithium plasma. The simulation parameters are chosen to model the recent experiment 167 at the Stanford Linear Accelerator, relevant to the design of future plasma wakefield accelerating afterburners. I find that beams focused near the point of entry into the plasma propagate further into the plasma and accelerates witness particles to a greater maximum energy before disintegrating. These results could indicate that ion channel formation is dependent on the drive beam waist location and that the plasma accelerating medium can have an observable effect on the focusing of the drive beam.   

Distance of closest approach of two hard ellipses

The distance of closest approach of hard particles is a key parameter in their interaction and plays an important role in the resulting phase behavior. The distance of closest approach of the centers of hard spheres in 3-D or of hard circles in 2-D is the diameter. For non-spherical particles, the distance depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed(1,2) for use in computer simulations, no analytic solutions have been obtained for ellipsoids in 3-D, or, until now, for ellipses in 2-D. We have succeeded in deriving an analytic expression for the distance of closest approach of the centers of two arbitrary hard ellipses as function of their orientation relative to the line joining their centers. We describe our method for solving this problem, illustrate our result, and discuss its usefulness in modeling and for simulating systems of anisometric particles such as liquid crystals.   

Discontinuous Molecular Dynamics for Rigid Bodies: Applications.

Event-driven molecular dynamics simulations are carried out on two rigid body systems which differ in the symmetry of their molecular mass distributions. First, simulations of methane in which the molecules interact via discontinuous potentials are compared with simulations in which the molecules interact through standard continuous Lennard-Jones potentials. It is shown that under similar conditions of temperature and pressure, the rigid discontinuous molecular dynamics method reproduces the essential dynamical and structural features found in continuous-potential simulations at both gas and liquid densities. Moreover, the discontinuous molecular dynamics approach is demonstrated to be between 3 to 100 times more efficient than the standard molecular dynamics method, depending on the specific conditions of the simulation. The rigid discontinuous molecular dynamics method is also applied to a discontinuous-potential model of a liquid composed of rigid benzene molecules, and equilibrium and dynamical properties are shown to be in qualitative agreement with more detailed continuous-potential models of benzene. The few qualitative differences in the angular dynamics of the two models are related to the relatively crude treatment of variations in the repulsive interactions as one benzene molecule rotates by another.\newline *This work was supported by grants from the National Sciences and Engineering Research Council of Canada (NSERC).   

Wave Phenomena during Lattice Boltzmann Simulation of Interdiffusion for Species having Unequal Masses

We discuss wave propagation phenomena in a Lattice Boltzmann (LB) model of a binary diffusion couple for species having unequal masses. LB simulations reveal oscillations in the position of the global center of mass of the couple as it moves toward the center of the couple from its initial position located toward the more massive species. These oscillations are related to waves in the total number density and barycentric velocity. Waves are generated at the initial discontinuity in composition and propagate toward the ends of the couple, from which they reflect. For a small difference in the masses of the diffusing species, we use a perturbation expansion to obtain driven wave equations for the total number density and the barycentric velocity. For sufficiently long samples these waves have a negligible effect on the composition versus distance profiles during interdiffusion; however, for microfluidic devices whose length is comparable to the diffusivity divided by the wave speed, the tails of the composition profiles get cut off. Periodic boundary conditions (PBC) were used in the direction perpendicular to the axis of the diffusion couple. If these PBC are replaced by no-slip walls, the waves are heavily damped.   

Potential-based Reduced Newton Algorithm for Nonlinear Multiphase Flow in Porous Media

We present a phase-based potential ordering for the finite volume discretization of the multiphase porous media flow equations. This ordering is an extension of the Cascade ordering introduced by Appleyard and Cheshire. The extension is valid for both two-phase and three-phase flow, and it can handle countercurrent flow due to gravity and/or capillarity. We show how this ordering can be used to reduce the nonlinear algebraic system that arises from the fully-implicit method (FIM) into one with only pressure dependence. The potential-based reduced Newton algorithm is then obtained by applying Newton's method to this reduced-order system. Numerical evidence shows that our potential-based reduced Newton solver is able to converge for time steps that are much larger than what the standard Newton's method can handle. In addition, when standard Newton does converge, the reduced Newton algorithm also converges, and often at a faster rate than standard Newton. Applications of the potential ordering to linear preconditioning will also be discussed.   

Session L22: Focus Session: Structure and Dynamics of Complex Networks

Functional Aspects of Biological Networks

We discuss biological networks with respect to 1) relative positioning and importance of high degree nodes, 2) function and signaling, 3) logic and dynamics of regulation. Visually the soft modularity of many real world networks can be characterized in terms of number of high and low degrees nodes positioned relative to each other in a landscape analogue with mountains (high-degree nodes) and valleys (low-degree nodes). In these terms biological networks looks like rugged landscapes with separated peaks, hub proteins, which each are roughly as essential as any of the individual proteins on the periphery of the hub. Within each sup-domain of a molecular network one can often identify dynamical feedback mechanisms that falls into combinations of positive and negative feedback circuits. We will illustrate this with examples taken from phage regulation and bacterial uptake and regulation of small molecules. In particular we find that a double negative regulation often are replaced by a single positive link in unrelated organisms with same functional requirements. Overall we argue that network topology primarily reflects functional constraints. References: S. Maslov and K. Sneppen. ``Computational architecture of the yeast regulatory network." Phys. Biol. 2:94 (2005) A. Trusina et al. ``Functional alignment of regulatory networks: A study of temerate phages". Plos Computational Biology 1:7 (2005). J.B. Axelsen et al. ``Degree Landscapes in Scale-Free Networks" physics/0512075 (2005). A. Trusina et al. ``Hierarchy and Anti-Hierarchy in Real and Scale Free networks." PRL 92:178702 (2004) S. Semsey et al. ``Genetic Regulation of Fluxes: Iron Homeostasis of Escherichia coli". (2006) q-bio.MN/0609042   

Optimal transport on complex networks

Transport optimization is a problem encountered in connection to many different types of complex networks, including biological networks, social networks, and a variety of natural and human-made transport and communication networks. We present results for transport optimization on random, scale-free, as well as geometric networks, obtained using a novel heuristic algorithm. Some of the results have been published in Phys.Rev.\ E {\bf 74}, 046106 (2006). Our algorithm balances transport by adjusting traffic routing at every iteration to minimize the betweenness of the busiest node on the network. This is done with the least possible lengthening of the paths passing through that node. Our results are compared with those obtained using classical shortest path routing, as well as previously proposed network transport optimization algorithms. We show that routing produced by our algorithm enables networks to sustain significantly higher traffic without jamming than in the case of any previously proposed routing optimization algorithm. Furthermore, we show that, in spite of unavoidable lengthening of the average path, the small-world character of network routing is preserved.   

Model framework for describing the dynamics of evolving networks

We present a model framework for describing the dynamics of evolving networks. In this framework the addition of edges is stochastically governed by some important intrinsic and structural properties of network vertices through an attractiveness function. We discuss the solution of the inverse problem: determining the attractiveness function from the network evolution data. We also present a number of example applications: the description of the US patent citation network using vertex degree, patent age and patent category variables, and we show how the time-dependent version of the method can be used to find and describe important changes in the internal dynamics. We also compare our results to scientific citation networks.   

Transport on weighted Networks: when correlations are independent of degree

Most real-world networks are weighted graphs with the weight of the edges reflecting the relative importance of the connections. In this work, we study non degree dependent correlations between edge weights, generalizing thus the correlations beyond the degree dependent case. We find that two measures, the disparity and the range, defined below, are able to discriminate between the different types of correlated networks. We also study the effect of weight correlations on the transport properties of the graphs. We find that positive correlations dramatically improve transport. The classic case of degree dependent weight correlations relates to our graphs with positive weight correlations.   

Limited Percolation on Complex Networks

We study the stability of network communication under removal of $q=1-p$ links when communication between nodes is possible only through a subset of the paths connecting them. We find a new percolation transition $\tilde{p}$ below which only a fractal fraction of nodes $N^{\gamma}$ can communicate, where $\gamma$ is a function of the accepted communication paths. Above $\tilde{p}$, order $N$ nodes can communicate. The results may be useful for the design of communication networks and immunization strategies.   

Criticality in finite dynamical networks

It has been shown analytically and experimentally that both random boolean and random threshold networks show a transition from ordered to chaotic dynamics at a critical average connectivity $K_c$ in the thermodynamical limit [1]. By looking at the statistical distributions of damage spreading (damage sizes), we go beyond this extensively studied mean-field approximation. We study the scaling properties of damage size distributions as a function of system size $N$ and initial perturbation size $d(t=0)$. We present numerical evidence that another characteristic point, $K_d$ exists for finite system sizes, where the expectation value of damage spreading in the network is independent of the system size $N$. Further, the probability to obtain critical networks is investigated for a given system size and average connectivity $k$. Our results suggest that, for finite size dynamical networks, phase space structure is very complex and may not exhibit a sharp order-disorder transition. Finally, we discuss the implications of our findings for evolutionary processes and learning applied to networks which solve specific computational tasks. [1] Derrida, B. and Pomeau, Y. (1986), Europhys. Lett., 1, 45-49   

Rich-club ordering in complex networks

Uncovering the hidden regularities and organizational principles of networks arising in physical systems ranging from the molecular level to the scale of large communication infrastructures is the key issue for the understanding of their fabric and dynamical properties. The ``rich-club'' phenomenon refers to the tendency of nodes with high centrality, the dominant elements of the system, to form tightly interconnected communities and it is one of the crucial properties accounting for the formation of dominant communities in both computer and social sciences. The talk will provide the analytical expression and the correct null models for the measurement of the rich-club ordering and its relation with the function and dynamics of networks in examples drawn from the biological, social and technological domains.   

Scaling properties of critical random Boolean networks

Until a few years ago, it was believed that random Boolean networks at the critical point (i.e., Kauffman networks) have a square-root relationship between the mean number of length of attractors and the system size N (i.e. the number of nodes). In the meantime, it became known that in fact the mean number and the mean length of attractors increase faster than any power law with increasing N. This talk gives an intuitive understanding of why this is the case. We investigate mainly analytically the scaling behavior of the number of nodes that are not frozen on all attractors, and of the number of relevant nodes, i.e. the nodes that determine the number and length of attractors. From the results it becomes clear that the relevant nodes form of the order of log(N) components, most of which have the form of simple loops. From this in turn attractor numbers and lengths follow by simple combinatorics. References: V. Kaufman, T. Mihalev, B. Drossel, PRE 72, 046124 (2005). B. Drossel, T. Mihalev, F. Greil, PRL94, 088701 (2005). B. Drossel, PRE72, 016110 (2005).   

Exhaustive percolation in binary avalanches

We introduce the concept of binary avalanches as a generalization of commonly investigated site or bond percolation processes. Binary avalanches are capable of displaying a behavior that we call exhaustive percolation, where the fraction of nodes that are not affected by an avalanche approaches zero in the large system limit. We present a numerical example of exhaustive percolation on a directed lattice and analytical results for directed random networks. For random networks, the transition to exhaustive percolation is second order with scaling properties different from ordinary percolation. Our numerical calculations indicate that the exhaustive percolation transition on the directed lattice is also second order. Our analytic results for random networks improve the understanding of dynamical properties in random Boolean networks.   

Heterogeneity in community structure and renormalization in scale-free networks

While many real-world networks contain community structures within them, their structural feature has not been fully understood yet. Here we study heterogeneities of community structures such as their sizes, cohesiveness, modularity, and renormalized degree, finding that there exist nontrivial power-law relationships between them, based on real-world networks. We show that such obtained relationships provide the condition of scale invariance of the degree distribution under renormalization. Also we show that the load or betweenness centrality of communities depends on such structural properties of communities.   

Price of Anarchy on Complex Networks

We present an optimization problem of decentralized transportation networks, where latency depends linearly on congestion of a link. The system shows that a collection of individual optimization of the flow does not always meet the most optimized outcome that is conventionally assumed. We suggest that the Price of Anarchy, the ratio of two optimums, can quantify such discrepancy and accordingly regarded as an index of inefficiency of the system. We also measure the Price of Anarchy of model networks with various underlying structures, and a simplified Boston road network. Our numerical result confirms the existence of the Price of Anarchy in the networks. Finally, we find Braess's paradox is not just a pedagogical example, but inefficiency that can counterintuitively take place in real.   

Session L23: High Pressure IV

On the high pressure behavior of body-centered cubic iron

The high pressure--high temperature behavior of iron is of current interest for geophysical reason, i.e., the earth's core is believed to be iron at high p and T. The value of the pressure at which the rigid lattice of body-centered-cubic ferromagnetic iron goes unstable was recalculated by newer methods. We give some thermodinamic arguments in support of our procedure, which minimizes the Gibbs free energy at constant pressure rather than internal energy at constant volume, and then finds elastic constants as second derivatives of G with respect to strains. The calculations used WIEN2k band-structure program and a minimum path program that makes a series of jumps in structure based on the local slope and curvature of G at a point in structure space. Errors are pointed out in several recent papers that found values different than ours, mainly due to neglect of the pressure correction required when elastic constants are calculated in a system under finite pressure by differentiation of the energy rather than Gibbs free energy.   

Rheology of iron under conditions of the Earth's inner core

It is well established that the solid Earth's inner core (IC) consists of an iron-rich alloy. However, low rigidity of the IC (Poisson ratio of about 0.44) remains enigmatic. Both measured at low temperature elastic properties of hexagonal (hcp) iron phase as well as the calculated properties of the various hypothetic iron phases at high pressure (above 3 Mbar) and high temperature (from 5000 to 8000 K) are inconsistent with seismological observations. The velocity of shear waves propagation in the IC is considerably lower than the measured/calculated shear velocity of iron phases. We performed ab initio as well as classical molecular dynamics (MD) simulations of iron polycrystals, grown from melt as well as obtained by the Voronoi construction. We demosntrate, that the account for grain boundaries and/or for various structural inhomogeneities allow to bring the calculated data in close agreement with the experimental seismic data.   

Recovery Studies of Shocked Iron Single Crystals

Time resolved, \textit{in-situ }X-Ray diffraction measurements indicate that the bcc-hcp transition in single crystal iron occurs at about 13 GPa. These results also show that the high pressure phase is a polycrystal with two variants. Further studies on the recovered specimens using transmission electron microscopy show that these shocked samples surprisingly reverse transform from a high pressure polycrystal to the original single crystal structure upon release. These results will be discussed in the context of the time resolved data and theoretically based transformation pathways.   

Multi-scale modeling of ferromagnetism in bcc Fe as a function of pressure and temperature

We investigate the magnetic properties of bcc Fe as functions of pressure and temperature using multi-scale modeling techniques. We employ a first-principles fitted tight-binding total-energy model in the generalized-gradient approximation to examine bcc Fe at numerous ferromagnetic, antiferromagnetic and spin spiral states, and fit the tight-binding data to a generalized Heisenberg Hamiltonian which includes both the on-site and local exchange energy to describe the magnetic energy for any arbitrary magnetic configuration. We obtain the Curie temperature, magnetization curve, and other finite-temperature magnetic properties through extensive Monte Carlo simulations, which have been further applied to examine the influence of the magnetic fluctuations on the free energy and thermal equation of state properties of bcc Fe at high temperatures. This work was supported by US Department of Energy ASCI/ASAP subcontract to Caltech, Grant DOE W-7405-ENG-48 (to REC).   

Ellipsometry Measurements of Shock-Induced Phase Transitions

{\em In situ} measurements of crystal structures and phase transitions during dynamic high-pressure experiments are complex, thus knowledge of high-pressure high-temperature phase diagrams for many materials is limited. Since typical Hugoniot EOS and sound speed experiments do not provide this information, we have developed an ellipsometric technique which allows the real-time measurement of optical constants. Coupling measured optical properties with calculations allows one to infer structural information complimentary to techniques such as x-ray diffraction. We present dynamic ellipsometry measurements of shock-induced solid-solid ($\alpha-Fe\to \epsilon-Fe$) and solid-liquid ($\beta \to liquid -Sn$) phase transitions. In addition, the time-resolution of such dynamic phenomena suggests that information on the kinetics of phase transitions as well as deformation/relaxation can be obtained. We will also discuss our efforts to incorporate multiple wavelengths into ellipsometry measurements of dynamically compressed materials.   

Symmetry breaking in a dense liquid: Why sodium melts at room temperature.

The melting curve of sodium measured in [1] exhibits unusual features under pressure : the melting temperature, Tm, reaches a maximum around 30 GPa followed by a sharp decline from 1000 K to 300 K in the pressure range from 30 to 120 GPa. In this study, the structural and electronic properties of molten sodium are studied using first principles theory. With increasing pressure, liquid sodium initially evolves by assuming a more compact local structure, which accounts for the maximum of Tm at 30 GPas. However, at pressure around 65 gigapascals a transition to a lower coordinated structure takes place, driven by the opening of a pseudogap at the Fermi level. Remarkably, the broken symmetry liquid phase emerges at rather elevated temperatures and above the stability region of a closed packed free electron-like metal. The theory explains the measured drop of the sodium melting temperature, down to 300 kelvin at 105 GPas. [1] Gregoryantz et al., Phys. Rev. Lett. 94, 185502 (2005).   

The breakdown of a simple-metal paradigm at high pressures

The light alkalis at one bar and room temperature are considered the paradigms of 'simple-metal' behavior. They adopt cubic structures and their valence bands are free electron-like. Under normal conditions this has been well accounted for by pseudopotential theory. It is a common expectation that the light alkalis might even be more free electron-like at higher densities, as impelled by pressure. Advances in diamond anvil cell methods have yielded new insights in the behavior of the alkalis at megabar pressures, presenting a considerable challenge to the above paradigm. Under pressure, the light alkalis adopt non-simple structures. Initial studies by Neaton and Ashcroft [Letters to Nature, Vol. 400, 141 (1999)] on lithium suggested that with increasing pressure the valence bands first broaden, but then start narrowing substantially. Corresponding to this, the valence charge density is localizing in the interstitial spaces of the lattice and the core bands are acquiring significant width. Our work focuses on showing that this behavior may be fairly general and can be explained at the one electron level as an emerging breakdown of the weak pseudopotential hypothesis.   

Melting of Sodium Under Pressure

The bcc, fcc, and liquid phases of sodium are investigated with density functional molecular dynamic (DFT-MD) simulations. We focus on the behavior of the melting curve at high pressure. Diamond anvil cell experiments have determined a melting line with a negative slope at pressures above 33GPa [1]. In the bcc phase, the melting temperature drops from around 1000K to 700K. It decreases even further to 300K in the fcc phase. We have performed simulations for sodium in a range from zero to 100 GPa, temperatures ranging from 300K to 2000K. Equations of State (EOS) for the bcc, fcc and liquid phase are obtained. To investigate the underlying principles of melting in sodium, we study ionic and electronic structure of solid and fluid. Using our EOS, we reproduce positive and negative slopes of the melting line in the proposed pressure regions for the bcc as well as for the fcc phase. [1] E. Gregorianz, O. Degtyarewa, M. Somayazulu, R.J. Hemley, H. Mao, Phys. Rev. Lett. 94, 185502 (2005)   

An Investigation of s-d promotion at high pressure with the Projector Augmented Wave method

The PAW(1) method for ab initio density functional calculations combines advantages of both pseudopotential (PP) and all-electron (AE) methods. The PAW method provides accuracy comparable to AE methods, core-sensitive calculations, and ab initio molecular dynamics with large systems. The requirement for high accuracy in the determination of s-d promotion pressures in transition metals serves as a proving ground for the accuracy of the PAW method. We present PAW, PP, and AE APW+lo results for the case of Molybdenum, for which there is significant disagreement among recent ab initio predictions above 600 GPa. 1. P.E. Blochl, Physical Review B, 50, 17953 (1994)   

Quasi-Isentropic Compression of Ta Using Graded Density Impactors

Recent advances in the fabrication of graded density impactors have enabled the production of smooth, continuous quasi-isentropes for gas gun experiments. Using these impactors, we have performed experiments on Ta in which the sample is initially shocked to 66 GPa on the Hugoniot and then quasi-isentropically compressed to over 1 Mbar. We will present the results of lagrangian analysis of the data and compare with previous Hugoniot measurements as well as the calculated isentrope of Ta. We will also discuss potential sources of error in both the data and analysis and their effect on the measured quasi-isentrope.   

A multi-scale, atomistic-based strength model for tantalum and molybdenum

For the description of bcc tantalum and molybdenum strength at the continuum level, we have combined several extensive sets of quantum-based, atomistic calculations into a new parameterization of the Steinberg-Lund (SL) and mechanical threshold stress (MTS) strength models. This model is then used to simulate recent gas-gun shock experiments. The atomistic calculations that determine the parameters of these model were derived from two disparate methods but both based on quantum-based model generalized pseudopotential theory (MGPT) for the ion-ion interactions. In one method, Green's function boundary conditions are used to relax dynamically the boundary forces in molecular dynamics simulations of the kink-pair activation enthalpy and Peierls stress for $(a/2)<111>$ screw dislocations. The other method combines MGPT Monte Carlo simulations with full potential linear muffin-tin orbital (FP-LMTO) method of density functional theory to determine the temperature and pressure dependence of the shear modulus. We discuss the new parameterization of the models and hydrodynamic simulation results.   

Osmium under high pressure: a fully relativistic first-principles study of the structural and electronic properties.

Recently, there has been much interest in the high-pressure properties of Os after that the first bulk modulus measurement was made only four years ago. It is important to mention that to date, the phase diagram of Os is unknown. In the present work, we have studied the structural and electronic properties of Os using the full-potential LAPW method and the GGA for the exchange-correlation functional. The calculations were performed including the spin-orbit coupling which is important for heavy metals like Os. The total-energy as a function of the cell volume was computed assuming the hcp, fcc, and $\omega $ structures, for compressions up to 65{\%} of the equilibrium volume. In contradiction with the previous non-relativistic LDA-calculation, we find that Os in the hcp phase have lower energy than the fcc and $\omega $ structures. The hcp structure remains stable for pressures up to 400 GPa and not structural transition to the fcc or $\omega $ phase was found. Nevertheless, from the analysis of the band structure, we find an electronic topological transition induced by pressure at the high-symmetry point L, where three bands cross the Fermi level upon compression.   

Difference Frequency Generation Measurements of Phase Transitions in Gallium

We recently measured the vibrational excitation spectra of solid and liquid gallium with ultrafast terahertz difference frequency generation (DFG) spectroscopy. The two phases had clearly different DFG spectra, with a 250 cm$^{-1}$ phonon feature visible in the solid phase and a 50 cm$^{-1}$ excitation feature seen in the liquid phase. Prospects for using this technique to measure phase transitions of shocked systems \textit{in situ} will be discussed.   

First The motion and mobility of screw and edge dislocation in bcc tantalum

Strength in bcc metals is, surprisingly, not well understood; it is thought to be dominated at low temperature by the motion of 1/2$<$111$>$ screw dislocations. The motion of these screw dislocations is thought to be controlled by nucleation and propagation of kinks along the dislocation line, which can at high stress result in the formation of debris (vacancies, interstitials, loops, etc) in the dislocation wake. We studied the motion of screw and edge dislocations in bcc tantalum by performing large-scale molecular-dynamics simulations using both Finnis-Sinclair potentials, and model-generalized-pseudopotential-theory (MGPT) potentials [1]. We present here simulation predictions for dislocation motion, mobility, and debris formation with respect to pressure, temperature, and strain rate. [1] J. A. Moriarty, Phys. Rev. B 42, 1609 (1990).; J. A. Moriarty, Phys. Rev. B 49, 12431 (1994).   

Material Strength on Quasi-isentropes

We have recently performed experiments to study strength properties of aluminum on quasi-isentropes. The aluminum samples were initially shocked to a fixed state on the Hugoniot, then quasi-isentropically compressed and released isentropically. In these experiments, the strain rates on compression and release isentropes are nearly equivalent. We will discuss the details of the experiments and data and error analysis in deriving strength of aluminum. Recent advances in the functionally graded density impactor technology have made it possible for us to carry out these experiments with significantly reduced uncertainties. We will discuss these advances including reproducibility and planarity of the impactors. Methods to characterize these advances will be discussed. [1] Work performed under the auspices of the U.S. DOE at the University of California/Lawrence Livermore National Laboratory under contract W-7405-ENG-48.   

Session L24: John H. Dillon Award Symposium

Swell Gels to Dumbbell Micelles: Construction of Materials and Nanostructure with Self-assembly

Bionanotechnology, the emerging field of using biomolecular and biotechnological tools for nanostructure or nanotecnology development, provides exceptional opportunity in the design of new materials. Self-assembly of molecules is an attractive materials construction strategy due to its simplicity in application. By considering peptidic or charged synthetic polymer molecules in the bottom-up materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. Several molecular systems will be discussed. Synthetic block copolymers with charged corona blocks can be assembled in dilute solution containing multivalent organic counterions to produce micelle structures such as toroids. These ring-like micelles are similar to the toroidal bundling of charged semiflexible biopolymers like DNA in the presence of multivalent counterions. Micelle structure can be tuned between toroids, cylinders, and disks simply by using different concentrations or molecular volumes of organic counterion. In addition, these charged blocks can consist of amino acids as monomers producing block copolypeptides. In addition to the above attributes, block copolypeptides provide the control of block secondary structure to further control self-assembly. Design strategies based on small (less than 24 amino acids) beta-hairpin peptides will be discussed. Self-assembly of the peptides is predicated on an intramolecular folding event caused by desired solution properties. Importantly, the intramolecular folding event impart a molecular-level mechanism for environmental responsiveness at the material level (e.g. infinite change in viscosity of a solution to a gel with changes in pH, ionic strength, temperature).   

Morphological Characterization of Silicone Hydrogels

Silicone hydrogel materials are used in the latest generation of extended wear soft contact lenses. To ensure comfort and eye health, these materials must simultaneously exhibit high oxygen permeability and high water permeability / hydrophilicity. The materials achieve these opposing requirements based on bicontinuous composite of nanoscale domains of oxygen permeable (silicones) and hydrophilic (water soluble polymer) materials. The microphase separated morphology of silicone hydrogel contact lens materials was imaged using field emission gun scanning transmission electron microscopy (FEGSTEM), and atomic force microscopy (AFM). Additional morphological information was provided by small angle X-ray scattering (SAXS). These results all indicate a nanophase separated structure of silicone rich (oxygen permeable) and carbon rich (water soluble polymer) domains separated on a length scale of about 10 nm.   

The Application of Specular X-ray Reflectivity to Characterize Patterned Surface

Specular $x$-ray reflectivity (SXR) has been used extensively for thin film characterization and depth profiling. Recently its application has been extended to quantify nanoscale patterns made of photo lithographic polymers. SXR results complement small angle $x$-ray scattering measurements by providing details information in the cross section of features on nanoscale surface patterns. This talk is to focus on the limit of applying SXR as an effective surface pattern metrology. Polymeric line gratings with periodicities or pitch ranging from 200 nm to 16 $\mu $m were chosen as test samples and it is expected that as the pitch size reaches beyond the coherent length of x-ray SXR will no longer be applicable. The optics of the SXR instrument dictates the coherent length of the x-ray; it provides a coherent length of a few micrometers in the longitudinal direction and sub-micrometers along the lateral direction on the reflection plane and a few nanometers along the lateral direction perpendicular to the reflection plane. SXR measurements were made at various azimuthal angles between the incident $x$-ray beam and the line grating, with 0\r{ } being the incident beam parallel to the line grating and 90\r{ } being perpendicular to each other. For periodicities less than 900 nm, the perpendicular and parallel measurements yield comparable SXR results which can be quantitatively analyzed using a one-dimensional model invoking effective medium approximation (EMA), i.e. SXR measures the lateral average electron density of the surface pattern. For periodicities 900 nm and greater, EMA breaks down and the SXR results can not be analyzed using any one-dimensional model. Work is on-going to determine the nature of transition between the EMA applicable region and the inapplicable region. The effect of surface patterns irregularity on this SXR application is another topic of current study.   

Collaborative Investigations of Supramolecular Polymer Assembly Processes.

It is a pleasure to participate in this symposium, honoring Darrin J. Pochan's awarding of the John H. Dillon Medal for advancing our understanding of the physics of assembly and chain conformation of synthetic polypeptides. Assemblies of polypeptides, polysaccharides and polymers of nucleic acids are, of course, complex natural systems that form the bases of life. Over the past three years, we have worked together as a highly interdisciplinary team of investigators, to investigate the self assembly behaviors and resulting morphologies for synthetic amphiphilic block copolymer systems. This presentation will highlight the findings from these collaborative studies, including the importance of the block copolymer composition and topology and the significance of the assembly conditions.   

Self-Generated Fields and Morphogenesis in Polymer Crystallization

Thermal, compositional and stress fields are created during the crystallization of polymers from the melt. The roles of the thermal and compositional fields are dictated by their Peclet number for crystallization: Pe = $\Lambda $V/D, where $\Lambda $ is a characteristic length for the process, V is the velocity of the crystallization front, and D is the mass or thermal diffusivity. Pe is a measure of the ratio of the distance a molecule \textit{must} move in the process to the distance it \textit{can} move. If Pe is significantly greater than 1, the growth interface must restructure itself, to lower the Peclet number toward unity. Reviewed in this talk are numerical and analytical studies of the effects of compositional fields on morphological development during spherulite growth and the effect of thermal fields during high-speed spinning.   

Propagating Waves of Self-Assembly in Organosilane Monolayers.

Wavefronts associated with reaction diffusion and self-assembly processes are ubiquitous in the natural world. While it often claimed that this type of self-sustaining front propagation is well described by mean field `reaction diffusion' or `phase field' models, respectively, it has recently become appreciated from simulations that fluctuation effects can lead to appreciable deviations from the classical mean field theory (MFT) of this type of front propagation. The present work addresses the existence of fluctuation effects in the particular case of the frontal self-assembly of the organosilane self-assembled monolayers on silica-coated surfaces. By following the progress of this self-assembly process via near-edge x-ray absorption fine structure spectroscopy (NEXAFS), we find that these layers organize from the edge of the wafer as a propagating planar wavefront with a well-defined velocity, c. In accordance with two-dimensional simulations of this type of front propagation that include fluctuation effects, we find that the interfacial widths w(t) of these self-assembly fronts exhibit a power-law broadening in time rather than the constant width predicted by mean field theory. Moreover, the observed exponent values accord rather well with previous simulation estimates. This study confirms that fluctuation effects can cause interfacial broadening in autocatalytic front propagation, as found in earlier computations.   

Receptor-Ligand Interactions and Adsorption at the Oil Water Interface

Liquid/liquid interfaces are excellent models for studying adsorption processes, because equilibration at these interfaces occurs more readily than at liquid/solid interfaces. In addition, the interfacial tension can be measured directly, and can be used to probe adsorption processes and molecular binding events that take place at the interface. In this investigation we use drop shape analysis to study receptor/ligand interactions at the chloroform/water interface. A pendant chloroform droplet is suspended in water. When hydrophobically-modified polyethylene glycol is added to the chloroform droplet, segregation of this molecule to the interface introduces a barrier to protein adsorption from the aqueous phase. Avidin adsorbs irreversibly to the oil water interface when the terminus of the PEG molecule is functionalized with biotin. By changing the volume of the chloroform drop (and hence the interfacial area) we obtain pressure/area isotherms at fixed avidin coverage. Adhesion of these functionalized interfaces to other surfaces can be quantified by bringing the pendant drop into contact with another surface or interface, and measuring the contact angle.   

Microstructure foundations of high carrier mobility in polymers.

The microstructure of organic semiconductor films can impact charge carrier mobility because it defines the persistence and quality of $\backslash $pi bond overlap in the source-drain plane. Important aspects of microstructure include the intermolecular packing arrangement within crystals, the surface-relative crystal orientation, and the overall crystal size and connectivity. We combine complementary microstructure measurements including polarized absorption spectroscopies, scanning probe techniques, and X-ray diffraction to investigate the microstructure details of polymer semiconductors for organic thin film transistors (OTFTs). Here, we demonstrate this approach by solving the packing arrangement of a polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTTs), with hole mobility of (0.2 to 0.6) cm2/Vs. NEXAFS combined with FTIR spectroscopy reveals nearly all-trans side chains that strongly tilt. With the XRD lamellar spacing, we show that vertically adjacent layers interdigitate. A general consideration of side chain configuration reveals a striking signature packing motif that sets high performance polymers such as pBTTT apart from the lower performance poly(3-alkylthiophenes).   

Phase Separated Polymer Systems on Surfaces and Some Applications in Super-Hydrophobicity

The study of the influence of temperature and confinement on surface segregation in thin films of deuterated polybutadiene and polyisoprene near the critical point for phase separation by the neutron reflectivity measurements will be discussed. The results show that polyisoprene enriches at the air and silicon interfaces in both the 1- and 2-phase regions. A transition between in-plane and surface-directed (layered) phase separation is observed with increasing film thickness. We will then describe some application with the use of phase separation of polymer blends on the surface. We demonstrate that a superhydrophobic surface can be facilely created by a simple casting process under environmental atmosphere by exploiting the different solubility of the two common polymers in the solvent. Furthermore, we will report a very attractive procedure to prepare a super-hydrophobic polymeric surface with controllable sliding angle (SA) from a single component commodity polymer, isotactic polypropylene (iPP), without further modification with low-surface-energy component under ambient atmosphere. The process we used is simply subject the iPP/decalin solution to a shear field and than lower the temperature to form a crystallized network structure.   

Multicompartment micelles from ABC copolymers

We have prepared a variety of novel compartmentalized micelles from ABC terpolymers in water. The three blocks were chosen to be mutually incompatible, with one being water-soluble (polyethylene oxide), one lipophilic (poly ethyl ethylene), and one both hydrophobic and lipophobic (poly perfluoropropylene oxide). The resulting morphologies were characterized by cryogenic transmission electron microscopy. The various structures can be understood, at least qualitatively, in terms of the relative interfacial tensions and block lengths. Mixing copolymers and mixing solvents (THF and water) can also be used to tune micellar morphology.   

Gold Nanoparticle Liquids and Dispersions: Structure and Phase Stability

By minimizing the volume fraction of the organic corona necessary to generate a net long-range repulsive pair-potential (and thus maximizing inorganic volume fraction), neat nanoparticle assembles may exhibit liquid-like behavior, which affords intriguing possibilities for numerous applications including solvent-less inks for micro-fabrication and compliant electrodes. For example, noble metal nanoparticle fluids have extended the life-cycle of an RF MEMS switch simulator by six-orders of magnitude relative to SAM surfaces and by one hundred times relative to uncoated gold switches by limiting switch failure by adhesion and shorting mechanisms. The thickness of the ionic-liquid corona, comprised of mercaptoethane sulfonate and a mixture of alkyl and PEG quaternary ammonium, is 1.4-1.5 nm, measured by small angle neutron scattering. Evidence of reversible de-aggregation of close-packed assembles of gold nanoparticles is observed around 1-4 wt{\%} in aromatic solvents, and at much lower concentrations in alkanes by small angle x-ray scattering. Thus, in addition to potential technological utility, these functionalized gold nanoparticles are providing experimental avenues to measure the phase stability of nanoparticle dispersions over a wide range of compositions.   

Thin Film Composites of Block Copolymers and Bio-Nanoparticles

Thin film composites of block copolymer and bio-nanoparticle were fabricated through two-step process; adsorption of bio- nanoparticles on polymer film and subsequent annealing under solvent vapor. The humidity of the annealing chamber influenced the dispersion of bio-nanoparticles and the final morphology of the composites. Under high humidity condition, ferritins were dispersed and selectively localized at PEO cylinders of poly (styrene-b-ethylene oxide), P(S-b-EO), while the bio- nanoparticles were aggregated at low humidity. When one component of a block copolymer was charged positively, as in poly(styrene-b-N-methyl-4-vinylpyridinium iodide), P(S-b-4VPQ), the loading of bio-nanoparticles increased significantly. When the loading was low, the morphology was the same as P(S-b-EO) case. However, at high loading, ferritin particles were segregated and formed a continuous boundary around the grains of microphase separated block copolymers. As a result, a 2- dimensional hierarchical structure, where block copolymer chains microphase separated inside of discrete patches surrounded by bio-nanoparticles, was generated. This process was also applicable to anisotropic bio-nanoparticles (e.g. Tobacco Mosaic Virus).   

Structure and Rheology of Shear-Banding Wormlike Micellar Solutions

Measurements of the micellar alignment, flow kinematics, and microstructure are presented for three WLM solutions. A special SANS flow cell enables the first direct measurements of the microstructure and micellar alignment in each individual band. These gap resolved 1-2 plane experiments demonstrate that the degree and orientation of segmental alignment of the micelles by the shear flow correlate with the measured shear viscosity. Combining the SANS measurements with flow-light scattering measurements shows that shear induces strong concentration fluctuations in the high shear band. These results show two distinctly different types of shear banding that is related to the underlying equilibrium phase behavior. The results help elucidate the mechanism driving shear banding in WLMs.   

Session L25: Organic Spintronic Materials and Nano-Spintronic Materials

Understanding electronic properties at organic/silicon interfaces from first principles

Organic/inorganic interfaces often possess properties that are significantly different from those of the organic molecules and the inorganic substrate that comprise them, due to both inter- molecular and molecule-substrate interactions. In this talk, I show how we explore such electronic effects using first principles calculations of prototypical silicon/organic interfaces. I focus on dipole depolarization effects, demonstrated for benzene derivatives on Si(111), and on interface-induced gap states, demonstrated on alkyl chains on Si (111). By comparing the results to both experiment and phenomenological models, we rationalize these effects and predict their manifestation in a range of organic electronic structures and devices.   

Ferromagnetism in a Porphyrin-based Organic Semiconductor

Current efforts in growing supramolecular quasi two-dimensional magnetic organic semiconductors, such as porphyrin-based or bimetallic oxalates materials, have not been followed by close theoretical studies of their magnetic properties. Interplay between experimental and theoretical approaches is needed to increase their ferromagnetic transition temperatures, which are still quite low. Our aim is to contribute to the theoretical effort by studying a simplified model of a two-dimensional array of magnetic ions embedded in a porphyrin matrix. Since the distance between the local moments is very large their magnetic couplings are mediated by the metal-like extended pi-orbitals. Therefore, our approach is based on a Double-Exchange Hamiltonian with effective hopping between magnetic ions derived from Huckel model. We solve this model using the Dynamical Mean Field Approximation (DMFA) including several ions on the local site. In order to predict the optimal magnetic properties, we calculate the ferromagnetic transition temperature, magnetization and susceptibility for a range of parameters.   

Self-Assembly of Magnetic Molecules on GaN(000\underline 1)

Self-assembled clusters of TBrPP-Co molecules are formed on a freshly grown nitrogen polar GaN (000\underline {1}) surface. The structural and electronic properties of the molecular clusters are then studied by using a scanning tunneling microscopy and spectroscopy at low-temperature (4.6 K) under an ultra-high-vacuum condition. The TBrPP-Co molecule has a spin-active cobalt atom caged at the center of porphyrin unit and four bromo-phenyl groups are attached to its four corners. On GaN(000\underline {1}), the molecules bind the surface through the bromo-phenyl units and form a saddle conformation, in which the central part of the molecule is bent by lifting the two pyrrole units of the porphyrin macrocycle. Within the self-assembled molecular clusters on this surface, the molecules are aligned either parallel or 90 degree rotated to each other. This molecule-substrate system may be useful for spintronic applications. This work is supported by NSF-NIRT grant number DMR 0304314.   

Atomic and Electronic Structure of a Novel Two-Dimensional Molecular Magnet System

Molecular magnet systems show much promise to replace standard metals and metal oxide systems in a broad range of application due to the comparative simplicity of processing. Understanding the coupling of the spins in these systems is important to determine their full range of applicability. We have studied the local atomic and electronic structure of a recently synthesized Mn carboxylate system which forms two-dimensional interconnected rings. To understand the spin interactions, the local atomic and electronic structure about the Mn sites was investigated by x-ray absorption spectroscopy and high resolution x-ray emission spectroscopy. The valence and spin configuration are described. Comparisons are made between the coupling of Mn sites via the oxygen atom with standard magnetic oxide systems such as the manganties.   

Growth and electronic structure of tetracyanoethylene on noble metals studied by scanning tunneling microscopy

Tetracyanoethylene (TCNE, C$_{2}$(CN)$_{4})$ is a $\pi $-electron acceptor with a very strong electron affinity that easily forms charge-transfer complexes with other organic molecules and metals. We have performed STM and STS of isolated TCNE molecules and ordered sub-monolayer coverages on noble-metal surfaces in order to study the competition between intermolecule and molecule-substrate interactions, and the impact this might have on film-growth and electronic structure. HOMO and LUMO peaks were observed for single TCNE molecules on Ag and Au substrates using STS, but not for Cu substrates which react more strongly with TCNE. The spatial distribution of the TCNE HOMO, as observed in dI/dV maps, fits well with DFT calculations and shows that TCNE is in a negatively charged state on these metal substrates. dI/dV maps of ordered TCNE arrays indicate that neighboring TCNE molecules interact strongly with each other in some cases.   

Strong electron-phonon interaction in e-e correlated molecular systems

Molecular systems (molecules) with strong electron-phonon interaction are described in terms of the Green functions. In the case of the strong e-ph interaction a general scheme that includes of the Dyson equations for the electron and phonon Green functions, is not productive. Hence, the different methodology is developed where the unitary transformation (that included both electron and phonon subsystems) is used. In this case the Dyson equation for the electron Green function is not valid any longer. Different approximations are proposed. The developed approach is extremely important for electron transfer reactions without single electron transfer assumption. It can be also used in the transport in molecular junction devices.   

Electron Spin Relaxation in Hole Polaron States of Conjugated Porphyrin Arrays

It has been previously demonstrated that stable hole-polaron states may be produced in a family of highly $\pi $-conjugated (porphinato)Zn(II) in which the monomeric units are bridged by ethyne linkages. Furthermore, EPR results verify that hole delocalization or incoherent hopping occur over substantial distances ($\sim $ 7.5 nm) along a single conjugated backbone. The electron spin relaxation times in traditional conducting materials are on the order of picoseconds. Preliminary data gleaned from progressive microwave saturation will show that electron spin relaxation times in these materials are on the order of 1$\mu $s at 298 K in both solution and in film architectures and moreover are relatively insensitive to oligomer length with distances spanning 1.4 to 7.5 nm. Since hopping rates have been observed to be on the order of 10$^{+7}$ Hz, it is possible then for spin memory of the hole polaron to be retained during its migratory process.   

Spin states and their relaxation in transition-metallorganic self-assembled molecules

The coexistence of spins localized on transition-metal ions and mobile charges on the $\pi $-conjugated ligands in transition-metallorganic self-assembled molecules (TMSAMs) makes these molecules attractive for molecular spintronic devices and quantum computing. We present our theoretical results on the spin states localized on the transition-metal ion in a TMSAM using both the first-principles approaches and the ligand-field theory. Then we construct a spin Hamiltonian to calculate spin lifetimes and identify the dominant spin relaxation mechanisms in the molecule. We also discuss the relation between the spin states on the transition-metal ion and the charge transport along the $\pi $-conjugated ligand in the molecule.   

Low Temperature STM Investigation of Molecular Kondo Effect

We investigate site-dependent Kondo effect on TBrPP-Co molecules on a Cu(111) surface at 4.6 K using scanning tunneling microscopy and spectroscopy [1]. The TBrPP-Co molecule has a spin-active cobalt atom caged at the center of porphyrin unit and four bromo-phenyl groups are attached to its four corners. On Cu(111), the molecules can anchor on the surface with either planar or saddle conformation [2]. For the current study, we choose only planar type molecules, in which the porphyrin unit is lying parallel to the surface and the molecule binds through the surface via four bromo-phenyl units as well as central porphyrin unit. The Kondo temperature of 170 K is measured above the cobalt atom location, i.e. at the center of the molecule. The observed Kondo effect is caused by spin-electron coupling between the cobalt atom of the molecule and the free electrons from the surface [2,3]. We find that the Kondo effect observed here is contributed from both surface and bulk states of Cu(111). This work is supported by the US Department of Energy Basic Energy Sciences grant no. DE-FG02-02ER46012. [1] G. Perera, V. Iancu, Luis G.G.V. Dias da Silva, S.E. Ulloa, and S.-W. Hla. Submitted. [2] V. Iancu, A. Deshpande, and S.-W. Hla, Nano Lett. 6 (2006) 820-823. [3] V. Iancu, A. Deshpande, and S.-W. Hla, Phys. Rev. Lett. (2006) in press.   

Session L26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections III

Exploring conical intersections through high resolution photofragment translational spectroscopy

High resolution measurements of the kinetic energies of H atom fragments formed during UV photolysis of gas phase imidazole, [1,2] pyrrole, [3] phenol [4] and thiophenol molecules show that: (i) X-H (X = N, O, S) bond fission is an important non-radiative decay process from the $^{1}\pi \sigma $* excited states in each of these molecules, and (ii) that the respective co-fragments (imidazolyl, pyrrolyl, phenoxyl and thiophenoxyl) are formed in very limited sub-sets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photo-induced evolution from parent molecule to ultimate fragments. \newline \newline [1] M.N.R. Ashfold, B. Cronin, A.L. Devine, R.N. Dixon and M.G.D. Nix, \textit{Science }(2006), \textbf{312}, 1637. \newline [2] A.L. Devine, B. Cronin, M.G.D. Nix and M.N.R. Ashfold, \textit{J. Chem. Phys.} (in press). \newline [3] B. Cronin, M.G.D. Nix, R.H. Qadiri and M.N.R. Ashfold, \textit{Phys. Chem. Chem. Phys.} (2004), 6, 5031. \newline [4] M.G.D. Nix, A.L. Devine, B. Cronin, R.N. Dixon and M.N.R. Ashfold, \textit{J. Chem. Phys.} (2006), \textbf{125}, 133318.   

Dynamic Stark Control of Photochemical Processes

A technique for controlling the outcome of photochemical reactions using the dynamic Stark effect due to a strong, nonresonant infrared field is demonstrated numerically and experimentally. A precisely timed infrared laser pulse is used to reversibly modify a potential energy barrier during a chemical reaction without inducing any real electronic transitions. Dynamic Stark control (DSC) is experimentally demonstrated during the nonadiabatic photodissociation of IBr. Significant modification of reaction channel probabilities are observed. The DSC process is nonperturbative, insensitive to laser frequency, and affects all polarizable molecules, suggesting broad applicability.   

Ab initio design of laser pulses to control molecular motion

Our recent attempts to design laser pulses entirely theoretically, in a quantitative and accurate manner, so as to fully understand the underlying mechanisms active in the control process will be outlined. We have developed a new Born-Oppenheimer like separation called the electric-nuclear Born-Oppenheimer (ENBO) approximation. In this approximation variations of both the nuclear geometry and of the external electric field are assumed to be slow compared with the speed at which the electronic degrees of freedom respond to these changes. This assumption permits the generation of a potential energy surface that depends not only on the relative geometry of the nuclei, but also on the electric field strength and on the orientation of the molecule with respect to the electric field. The range of validity of the ENBO approximation is discussed. Optimal control theory is used along with the ENBO approximation to design laser pulses for exciting vibrational and rotational motion in H$_{2}$ and CO molecules. Progress on other applications, including controlling photodissociation processes, isotope separation, stabilization of molecular Bose-Einstein condensates as well as applications to biological molecules also be presented. *Support acknowledged from EPSRC.   

Probing NO$_{2}$ close to the $\tilde {A}{ }^2B_2 /\tilde {X}{ }^2A_1 $ conical intersection by time-resolved imaging spectroscopy

Time-resolved imaging spectroscopy (TRIS) is emerging as a versatile technique with which to study the non-adiabatic coupling of vibrational and electronic degrees of freedom in molecules. The electronic predissociation of NO$_{2}$ in the near UV proceeds by internal conversion between the $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states and is a benchmark example of such barrierless reactions. We have applied time-resolved imaging to measure the time-evolution, angular and kinetic energy distributions of NO$^{+}$, NO$_{2}^{+}$ and photo electrons produced in pump-probe experiments using harmonics from a regeneratively amplified self-mode locked Ti:sapphire laser. Oscillations in the slow NO$^{+}$ and photoelectron signals are observed and are interpreted as measuring the energy level density of the coupled $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states close to the conical intersection. By using an optical pulse shaper we are able to manipulate the spectrum of the $\sim $400 nm excitation to create pulse sequences with which we can exert partial control over the coupling between the $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states.   

The Jahn Teller and pseudo-Jahn Teller effect in the dark \~{A} state of the nitrate radical NO$_{3}$

Despite its apparent simple molecular structure, the lowest electronic states of the nitrate radical NO$_{3}$ remain poorly understood. In particular, the three lowest states of the radical provide a benchmark for testing models of the Jahn-Teller (JT) and pseudo-JT effects. The dark \~{A} state of NO$_{3}$ undergoes strong JT distortion, suggesting that models with only linear and quadratic vibronic couplings are inadequate. We present cavity ringdown (CRD) and integrated cavity output (ICOS) spectra of the forbidden $\mathop A\limits^\sim { }^2E"\leftarrow \mathop X\limits^\sim { }^2A_2 '$ transition (preliminary report in Deev, et. al. \textit{J.Chem. Phys}., 2005. 122:224305) and compare them to a simulation based on a model Hamiltonian developed by Koppel, Domcke and Cederbaum that incorporates both JT and PJT couplings. New insights into the pseudo-JT effect among the lowest states are gained by examination of intensity-borrowing mechanisms for the observed vibronic bands.   

Nonadiabatic Electronic and Rotational Energy Partitioning in F + H$_{2}$O $\to$ HF + OH

OH product state distributions from F + H$_{2}$O $\to $ HF + OH have been carried out at a COM collision energy of 6(2) kcal/mol. These measurements complement earlier work on the dueterated version of the system (F + D$_{2}$O $\to $ DF + OD) where extensive non-adiabatic interactions led to a population of spin-orbit excited OD products despite energetically inaccessible barriers on all but the ground electronic surface. In the present F + H$_{2}$O measurements, the branching ratio is, within error bars, the same as in the deuterated case: 69(1){\%} of the molecules are found in the ground spin-orbit state, and 31(1){\%} are found in the excited (nonadiabatic) state. In contrast to this isotopomer-independent electronic branching ratio, the rotational distributions for this system are distinctly different from the deuterated case. A detailed analysis of the rotational distributions for the title reaction leads to an estimate of the vibrational distribution of the unobserved HF fragment (v=2:v=1) of 3:1. The fact that isotoperization dramatically changes the rotational distributions while leaving electronic distributions unchanged sheds light on the important question of how and where nonadiabatic transitions take place in this four-atom system.   

Quantum State Resolved Reactive Scattering Near Conical Intersections: $F(^2P)+HCl\to HF(v,J)+Cl(^2P)$ and $F(^2P)+H_2 O\to HF(v,J)+OH(^2\Pi )$ via High Resolution IR Spectroscopy on Nascent HF Product

State resolved reaction dynamics of the reactions$F({ }^2P)+HCl\to HF(v,J)+Cl({ }^2P)$and $F({ }^2P)+H_2 O\to HF(v,J)+OH({ }^2\Pi )$have been studied under rigorous single collision conditions in crossed molecular jets via IR absorbance of the HF product. Supersonic jet collision energies exceed the ground electronic state barrier height predicted by ab initio (DW-MCSCF) calculations, but can not overcome the larger barriers on excited state surfaces. The experimental results reveal highly vibrationally inverted nascent HF populations containing significant population above the average energy available to products for both of the title reactions. Such excited products may be formed by the tail of the collision energy distribution, but may also be favored by the extra $\sim $1.1 kcal/mol available for reaction with spin-orbit excited fluorine, previously observed in other systems. F+HCl product rotational distributions are found to be particularly non-statistical and are poorly modeled by single surface QCT.   

Optical control of dynamics in a simple chemical reaction: The cyclohexadiene ring-opening reaction

UV excitation of 1,3-cyclohexadiene (CHD) results in an optically induced ring-opening reaction to form 1,3,5-cis-hexatriene (ZHT). The initial excited state wave packet accelerates away from the Frank-Condon region and is funneled through a conical intersection onto the 2A excited state where the nuclear ring-opening reaction occurs. Return to the ground state proceeds within a few hundred femtoseconds through two or more conical intersections between the 1A and 2A potential energy surfaces. Recent studies have demonstrated that multiphoton excitation of CHD can be used to influence the photochemical yield of ZHT. The multidimensional search of the control space for optimal pulses identified both the quadratic and cubic phase parameters of the pulse as important control parameters. These results are discussed in terms of potential physical models.   

Matching-pursuit/split-operator-Fourier-transform simulations of excited-state nonadiabatic quantum dynamics in pyrazine.

A simple approach for numerically exact simulations of nonadiabatic quantum dynamics in multidimensional systems is introduced and applied to the description of the photoabsorption spectroscopy of pyrazine. The propagation scheme generalizes the recently developed matching-pursuit/split-operator-Fourier-transform (MP/SOFT) method [Y. Wu and V. S. Batista, J. Chem. Phys. 121, 1676 (2004)]. The time-evolution operator is applied, as defined by the Trotter expansion to second order accuracy, in dynamically adaptive coherent-state expansions. These representations are obtained by combining the matching-pursuit algorithm with a gradient-based optimization method. The accuracy and efficiency of the resulting computational approach are demonstrated in calculations of time-dependent survival amplitudes and photoabsorption cross sections, using a model Hamiltonian that allows for direct comparisons with benchmark calculations. Simulations in full-dimensional potential energy surfaces involve the propagation of a 24-dimensional wave packet to describe the S1 /S2 interconversion of pyrazine after after S0-S2 photoexcitation. The reported results show that the generalized MP/SOFT method is a practical and accurate approach to model nonadiabatic reaction dynamics in polyatomic systems.   

Femtosecond pump -- shaped-dump -- probe quantum control

We present a three pulse pump--shaped-dump--probe scheme for femtosecond spectroscopy. The objective is a reversion of regular control schemes for optimal excitation in which the pump pulse is shaped. Instead, we seek optimal de-excitation with a shaped dump pulse. Besides variation of the time delay between pump and dump pulses, the versatility of a femtosecond pulse shaper furthermore allows to record systematic fitness landscapes as a function of selected pulse parameters, providing additional information on wave-packet evolution and the potential energy surfaces of the system under study. Since the dump pulse is independent from the pump pulse, the pump-- shaped-dump--probe scheme facilitates control of molecular systems away from the initial Franck-Condon window in regions of the potential-energy landscape where the decisive reaction step occurs, e.g. near conical intersections. Experimental results on the retinal photoisomerization reaction in bacteriorhodopsin and exemplary model calculations demonstrate the potential of this new scheme.   

Session L27: Glassy Systems

Pressure Dependence of the Glass-Transition Temperature for Intermediate and Fragile Glass-Forming Systems

The glass transition temperature, T$_{g}$, is determined over the pressure range 0 to 80 kbar using a diamond anvil cell (DAC). Two methods are used: i) the onset or disappearance of pressure gradients indicated by ruby fluorescence, and ii) a new method in which T$_{g}$(P) is determined by significant changes in slope in the P-T curve during pseudo-isobaric temperature ramps. This slope change accompanies the significant change in volume expansion coefficient between the highly viscous, metastable, supercooled liquid state and the solid glassy state. Good agreement is found in the T$_{g}$(P) curve using the two methods. While the second method does not allow for quantitative determinations of the volume expansion coefficients of these systems, qualitative results can be obtained. It is found, e.g., that differences in the volume expansion coefficient upon crossing the glass transition are much greater for low-pressure fluids than for the much denser fluids at high pressure. Data will be presented for glycerol, an intermediate strength glass-forming system, as well as the fragile glass former salol.   

Pressure Raman experiments on Ge$_{x}$As$_{x}$Se$_{1-2x}$ glasses$^{\ast }$

It is known$^{1}$ that variations in the non-reversing enthalpy associated with glass transitions, $\Delta $H$_{nr}$(x), display a global minimum ($\sim $0) in the 0.09 $<$ x $<$ 0.16 range and the term increases at x $>$ 0.16 and at x $<$ 0.09 in the titled glasses. In this reversibility window, glasses are thought to in the Intermediate phase and form stress-free networks. Since the size of As, Ge and Se are nearly the same, Raman pressure experiments using a DAC provide a useful way to check the stress-free nature of glasses in the window$^{2}$. Preliminary results at x = 0.11, and 0.14, compositions in the reversibility window, reveal Raman frequency of the symmetric stretch of Ge(Se$_{1/2})_{4}$ tetrahedra to blue-shift linearly with external pressure (P) once P$>$0. At x = 0.18, a composition in the stressed-rigid phase, a blue-shift of the mode is also observed but only once P exceeds a threshold (P$_{c})$ value of 14 kbar. The present finding of a finite value of P$_{c}$ at x = 0.18, but its vanishing at x = 0.11 and 0.14, is quite similar to a previous one in binary Ge$_{x}$Se$_{1-x}$ glasses$^{2}$. We are now examining other glass compositions in the present ternary. $^{\ast }$ Supported by NSF grant DMR 04-56472 $^{1}$ T. Qu et al. Mater. Res. Soc. Symp. Proc. \textbf{754}, 111 (2003). $^{2}$ F. Wang et al. Phys. Rev. B, \textbf{71}, 174201 (2005).   

Two Simple Models of Monoatomic Glass Formers

Glass formation in one component systems remains a challenge for computer simulations, and therefore most studies to date have been done on binary mixtures. Here we explore the origin of resistance to crystallization of single component systems for two examples: modified silicon potential (Stillinger-Weber model) and Lenard-Jones ellipsoids (Gay-Berne model of liquid crystals). To produce glass formers these potentials were tuned by optimization of the parameter of three-body interaction (for the former) and aspect ratio (for the later). The kinetic properties and the potential energy landscapes of both models are studied.   

Tunable structures and properties in vitreous silica

We studied the structures and properties of vitreous silica samples prepared by specific high pressure processing routes, using molecular dynamics simulations based on a charge-transfer three-body potential. Our study shows that the ability of the glass to undergo irreversible densification is inherently connected to its anomalous thermo-mechanical properties, such as the minimum in the bulk modulus at $\sim $2-3 GPa and the negative thermal expansion while under pressure. These behaviors can be tuned by controlling the pressure under which the initial glass was quenched. By preparing silica glass in ways that eliminates anomalous thermo-mechanical behaviors, e.g., by quenching a melt under pressure, the propensity of the glass to undergo irreversible densification can be eradicated. Such ``pressure-treated'' silica glass is less susceptible to radiation damage and can potentially increase the lifetime of many optical components.   

Molecular origin of the giant conductivity enhancement in (Ag$_{2}$S)$_{x}$(As$_{2}$S$_{3})_{1-x}$ glasses

The solid electrolyte additive Ag$_{2}$S is found to homogeneously alloy with base As$_{2}$S$_{3}$ glass at low concentrations ( x $<$ 6 {\%}, single: T$_{g}$ = T$_{g}^{high} \quad \sim $ 210C$_{ })$, but it rapidly segregates as a Ag-rich glass phase at medium concentrations ( 6{\%} $<$ x $<$ 20{\%}, bimodal : T$_{g}^{high}$ and T$_{g} \quad ^{low} \quad \sim $ 170C ), and becomes the principal glass phase populated at higher x $>$ 35 {\%} (single: T$_{g}^{low})$ as revealed by modulated calorimetric measurements. The stoichiometry of the Ag-rich (T$_{g}^{low}$ phase) is suggested to be near AgAs$_{3}$S$_{7}$ at x $\sim $ 25{\%} but becomes closer to that of Smithite (AgAsS$_{2})$ at x $>$ 40{\%}, as revealed by Raman scattering. In the 9{\%} $<$ x $<$ 14{\%} composition range, one observes, in calorimetric experiments, the opening of a reversibility window, and a pronounced increase in the fractional population, R(x) of the Ag-rich glass phase, both of which correlate well with the 5-orders of magnitude increase in electrical conductivity$^{1,2}$ across this compositional interval. In the same interval molar volumes on our samples show a local plateau. These observations suggest a new interpretation of the giant electrical conductivity enhancement observed at x $>$ 15{\%} in the present electrolyte glass system. $^{1}$ E.A. Kazakova and Z.U.Borisova, Fiz. Khim.Stekla \textbf{6}, 424(1980).   

Enhanced Icosahedral Order in Supercooled Liquid Iron

As part of a study of metallic glass-forming ability, we perform first-principles molecular dynamics simulations of supercooled liquid Iron. Analyzing the results according to the icosahedral order parameter $\hat {W}_6 $, we find that Iron exhibits enhanced icosahedral order compared with supercooled Copper and compared with dense random packing. Voronoi analysis confirms the enhanced order is in the form of 13-atom icosahedral clusters as well as characteristic Frank-Kasper type disclinated icosahedra. Upon further cooling the sample crystallizes to a BCC lattice, with the icosahedra clustering to form a novel point defect.   

Full Recovery of Electron Damage in Glass at Ambient Temperatures

An unusually complete recovery of the electron beam induced damage in a CaO-Al$_{2}$O$_{3}$-SiO$_{2}$ glass was discovered. Nanoscale measurements carried out in a scanning transmission electron microscope show that the Ca ions migrate about 10 nm away during irradiation and return during recovery. Oxygen atoms are trapped largely as molecular oxygen and do not migrate. Electron energy loss measurements demonstrate that for glass to return completely to the original compositional and structural state the following processes must take place: First, no mass loss should occur. Thus the irradiation time should be less than the time necessary for significant mass-loss to occur. Second, diffusion must be sufficient at the ambient temperature for atoms to migrate back to suitable bonding sites. Third, the role of oxygen is critical: unless oxygen is available for recombination with the displaced atoms then recovery is incomplete. Finally, the observation that the system recovers so completely (structurally, as well as compositionally) after such a substantial perturbation is evidence that the initial state of the glass must be a very stable thermodynamic minimum [1]. [1] K.A. Mkhoyan et al., Phys. Rev. Lett. \textbf{96}, 205506 (2006).   

Glass Transition and Structure in the Phase Field Crystal Model

The dynamics of the glass transition and structure of the disordered phase are studied using the Phase Field Crystal (PFC) model in two and three dimensions. It is shown that a kinetically driven glass transition is produced in 3D for sufficiently large cooling rates. Analysis of free energy barriers indicates that the glass phase is more accessible from the liquid than the crystalline phase, but will not be stable for long times unless a critical cooling rate is exceeded. Below the critical cooling rate an equilibrium BCC structure is established. A Vogel-Fulcher type divergence in the density autocorrelation function is produced as the glass transition temperature is approached, signifying fragile glass forming behavior. As well the structure factor of the glass phase shows the characteristic split second peak. Notable differences between results in 2D and 3D will be discussed, as well as results for pure and binary systems.   

Ion-conduction and rigidity/flexibility of glasses

The (AgI)$_{x}$(AgPO$_{3})_{1-x }$ solid electrolyte glass system has been examined extensively although a consensus on the increase of electrical conductivity with x data has been elusive. Here we show that the variability of the data is likely due to water contamination. Our work is on specifically prepared \textit{dry} samples which display glass transition temperatures T$_{g}$(x) that are at least 50\r{ }to 100\r{ }C higher than those reported hitherto. In Raman scattering the frequency of the P-O$_{t}$ bonds in PO$_{4}$ tetrahedra of long chains is found to systematically red-shift with increasing x, and to display thresholds near x= x$_{c}$(1) =0.095(3)(stress-transition) and x =x$_{c}$(2) = 0.379(5)(rigidity transition). Calorimetric measurements show a reversibility window in the 0.09 $<$ x $<$ 0.38 range. Room temperature electrical conductivity, $\sigma $(x), increases with x to display thresholds near x$_{c}$(1) and x$_{c}$(2), and a logarithmic increase at x$>$ x$_{c}$(2) with a power-law $\mu $ = 1.78(10) that is in good agreement with theoretical predictions$^{1}$. Properties of flexibility and rigidity of backbones commonplace in covalent systems$^{2}$ is a concept that extends to solid electrolyte glasses as well. \newline $^{1}$Richard Zallen, Physics of Amorphous Solids \newline $^{2}$ P. Boolchand et al. Phil. Mag 85, 3823 (2005)   

Locations of metal ions in the new glasses in the alumina-calcia-monazite (LaPO$_{4})$ system

The new group of glasses synthesized from calcium aluminate (CaAl$_{2}$O$_{4})$ or C12A7 (CaO)$_{12}$(Al$_{2}$O$_{3})_{7}$ with varying fractions of La-monazite (LaPO$_{4})$ has been characterized by electron microscopy, $^{31}$P and $^{27}$Al NMR, Raman scattering and chemical methods. These techniques have yielded information concerning the environments of the metal ions Al and La in the glasses. A substantial negative shift of the principal $^{31}$P NMR line at all monazite fractions, along with Raman spectra showing that PO$_{4}$ groups do not share bridging oxygens, places La within the second coordination shell surrounding P. P-Al TRAPDOR double NMR experiments show that aluminum and phosphorus are also closely coordinated, accounting for a second, more negatively shifted line in the $^{31}$P single resonance spectra. Models for the structures of these glasses have been constructed for a range of monazite contents, and will be presented.   

X-ray and Neutron Scattering Studies of Methyl Iodide Confined in GelTech$_{\copyright}$ Glass

X-ray diffraction and neutron scattering studies of methyl iodide confined in 200 {\AA} GelTech$_{\copyright }$ glass have revealed a never before observed intermediate solid phase of methyl iodide. Bulk methyl iodide has one phase transition below room temperature, at 207 K where it transitions from a liquid to an orthorhombic solid. Neutron scattering studies of the diffusion of methyl iodide confined in the 200 {\AA} pores show a transition from a liquid to a solid at 203 K. X-ray diffraction measurements support this finding and identify the transition as one from a liquid to the never before observed amorphous solid. The amorphous solid remains down to 168 K upon cooling at which point a second transition appears from the amorphous solid to an orthorhombic solid, which upon indexing is identical to the bulk.   

Nonlinear susceptibility of the dipolar glass

Glassy behavior is seen in various systems, among which are structural glasses, electron glasses, spin glasses and electric dipolar glasses. The latter two seem to share the same effective Hamiltonian. However, experimentally some fundmental differences are seen between these two systems, most notably the divergence of the nonlinear susceptibility in the spin glass system and its absence in the electric dipolar glass. Here we discuss the similarities and differences between these two systems leading to the above mentioned phenomenon.   

Conductivity thresholds and glass structure in (K$_{2}$O)$_{x}$(GeO$_{2}$)$_{1-x}$ glasses

There are reports of conductivity thresholds with glass composition in solid electrolyte glasses. In the titled glass system, a seven order of magnitude increase in conductivity\footnote{Jain et al JNCS 222, 361 (1997).} occurs at x $>$ 0.10. The origin of the observation remains an open question. In titled glasses, we show that glass structure probed by the elastic behavior of its backbone shows two thresholds, a stress transition near x = 0.04 and a rigidity transition near x = 0.09. These elastic thresholds emerge from the reversibility window\footnote{S. Chakravarty et al. J.C.M.P 17,L1-7 (2005).} observed in calorimetric measurements, and in Raman scattering experiments that show scattering strength of the 520 cm$^{-1}$ mode of 3-member rings to show a global maximum in the reversibility window. The pronounced increase of conductivity apparently occurs when backbones become flexible at x $>$ 0.09, permitting K$^{+}$ ions to freely diffuse. The correlation between the electrical, thermal and optical properties of the present solid electrolyte glasses may well be a generic feature of these materials.   

Session L28: Focus Session: Carbon Nanotube Optics IV

Probing Non-equilibrium Phonon Dynamics in Carbon Nanotubes by Time-Resolved Raman Spectroscopy

In this paper we present a direct determination by femtosecond pump-probe laser spectroscopy of the lifetime of zone-center optical phonons in semiconducting single-walled carbon nanotubes. The non-equilibrium phonon population was created by the rapid relaxation following ultrafast optical excitation of the E$_{22}$ transition of a suspension of isolated HipCO nanotubes. As a probe of the phonon population, we made use of antiStokes Raman scattering from G mode. From the variation of the Raman signal with pump-probe delay, we deduced a phonon lifetime around 1 ps. The relation between the measured population lifetime and Raman linewidth will be considered, as will be the implication of this result for the existence of non-equilibrium phonon population in nanotubes carrying high current densities.   

One Dimensional Exciton Diffusion on Semiconducting Nanotubes using Time Resolved Photoabsorption Spectroscopy

We extend our recently reported analysis of the population relaxation of optically excited states on semiconducting carbon nanotubes to study the spectral shifts of their photoabsorption spectra. Highly excited tubes show a long time $1/\sqrt t $ decay of their photobleaching spectra which is well described by a one dimensional diffusion limited two body population relaxation. We find that the absorption spectra also show time-dependent spectral shifts with respect to the ground state absorption spectra. The spectral shifts are of order 10 nm and history dependent: two tubes prepared from different initial excitation densities but evolving to the same instantaneous excitation density show different lineshapes and spectral shifts. These features are analyzed by a model for the distribution of exciton separations produced in a diffusing population. The model provides an excellent parameter free description of the lineshape, and gives an estimate of the experimental exciton diffusion constant.   

Tracing exciton formation and relaxation in (6,5)- enriched single walled carbon nanotubes with sub-10 fs resolution

We perform pump and probe spectroscopy on (6,5) enriched single walled carbon nanotubes using broadband visible pulses of 7 fs duration. Apart from the direct photogeneration of the E22 exciton, we find a delayed channel which is operative at higher pump intensities during the first 20 fs after photoexcitation. It results in i) a saturation of the maximum population of the E22 exciton and ii) a strong retardation of the relaxation kinetics of E22 into E11, that cannot be accounted for by considering regeneration of E22 states by annihilation of E11 states. We suggest free carrier recombination as origin of the delayed E22 formation channel. The G mode oscillation of the nanotubes is traced via coherent oscillations as function of probe wavelength. It exhibits an abrupt phase jump at the maximum of the E22 absorption band, clearly demonstrating the oscillation of the E22 transition energy exerted by the G mode vibrational distorsion. Mapping the oscillatory amplitudes against probe wavelength allows us to separate oscillations in the ground state from those in the excited state.   

Time-Domain Ab Initio Studies of Phonon-Induced Relaxation of Electronic Excitations in Carbon Nanotubes

Electron-phonon interactions in carbon nanotubes (CNT) determine response times of optical switches and logic gates, the extent of heating and energy loss in CNT wires and field-effect transistors, and even a mechanism for CNT superconductivity. Numerous time-resolved experiments have revealed intriguing features of the electron-phonon relaxation in CNTs in response to external stimuli. We report the ab initio studies of the relaxation performed in real-time, directly mimicking the experimental data. The results reveal a number of unexpected features of the relaxation processes, including the differences between the intraband relaxation and electron-hole recombination, the photoexcitation energy dependence of the relaxation, the importance of defects, the dependence on the excitation intensity, and a detailed role of active phonon modes. \newline \newline [1] B. F. Habenicht, C. F. Craig, O. V. Prezhdo, ``Electron and hole relaxation dynamics in a semiconducting carbon nanotube'', \textit{Phys. Rev. Lett.} \textbf{96} 187401 (2006); \textit{Virtual J. Nanoscale Sci. {\&} Tech,} May 29, 2006; \textit{Virtual J. of Ultrafast Science}, June 2006   

Exciton-Polariton Dynamics in Carbon Nanotubes

This work addresses theoretically the nonlinear response of phonon-coupled excitons[1] in carbon nanotubes to an external electromagnetic field. The photon Green's function approach developed recently to quantize the electromagnetic field in the presence of quasi-1D absorbing bodies[2],[3] is being used to study the dynamics of phonon-coupled excitonic states interacting with the surface photonic modes excited by the external electromagnetic field in semiconductor carbon nanotubes. The formation of the new elementary excitations, exciton-polaritons, representing the eigen states of the full photon-matter Hamiltonian has been studied for small-diameter nanotubes under strong exciton-photon coupling. Time-resolved simulations have been performed of the coherent exciton- polariton dynamics with the exciton-phonon interactions taken into account. The criteria for the coherent control of the excitonic states population in optically excited carbon nanotubes have been formulated. \newline [1]F.Plentz et al, Phys. Rev. Lett. 95, 247401 (2005). \newline [2]I.V.Bondarev and Ph.Lambin, Phys. Rev. B 72, 035451 (2005). \newline [3]I.V.Bondarev and Ph.Lambin, in: Trends in Nanotubes Reasearch (NovaScience, New York, 2006), p.139.   

Photoinduced transient mid-infrared absorption in single-walled carbon nanotubes

We have performed optical pump - mid-infrared (MIR) probe spectroscopy on single-walled carbon nanotubes (SWNTs). The second excitonic absorption band ($E_{22})$ of (6,5) SWNTs was resonantly excited and the resulting photoinduced absorption was monitored in the MIR range (3.5 -- 5.5 $\mu $m) in a time range up to several hundred ps. Carrageenan films containing individualized CoMoCAT SWNTs formed on sapphire substrates were used for the measurement. This sample is optically transparent in the $\sim $3.5 -- 6 $\mu $m region, where the transition of $E_{11}$ excitons from the lowest dark state (1$g)$ to the second bright state (2$u)$ is expected to be observed. Our preliminary data shows the existence of photoinduced absorption in the investigated range. The origin of the observed transient absorption will be discussed.   

The Graphenic Bicontinuum Provides a Unified Analytical Treatment of Lattice Dynamics in Carbon Nanostructures.

A two-field bi-continuum model for the vibrational dynamics of graphene and carbon nanotubes describes a wealth of phenomena absent in a traditional continuum, including optical phonons, the high wave-vector nonlinearity of the acoustic branches, and even the hexagonal graphenic Brillouin zone. Since it includes all the degrees of freedom of the honeycomb lattice, the model provides a complete description of important electromechanical effects such as strain-induced gap opening or gap-induced phonon softening. The bi-continuum provides a unified framework for understanding and extending a previously disparate accumulation of analytical and computational results on deformations and vibrations in carbon nanostructures.   

ARPES study of the electronic dynamics from graphene to graphite

We report unique electronic information about the low energy electronic dynamics of atomically-thin graphene and bulk graphite by using high-resolution angle-resolved photoemission spectroscopy (ARPES). I will discuss the evolution of the electronic structure from single layer graphene to graphite and the dynamic of the Dirac quasiparticles as a function of energy, momentum, temperature and sample thickness.~ I will also discuss some very interesting features near the Fermi energy E$_{F}$ and address the effects of disorder on the low energy excitations. These findings from the electronic side can provide insight on the intriguing physics in graphene and graphite, as well as other carbon-based materials. References: [1] S. Y. Zhou \textit{et al.} Nature Phys. \textbf{2}, 595 (2006) [2] S. Y. Zhou \textit{et al.} Annals of Physics \textbf{321}, 1730 (2006) [3] E. Rollings, G.-H. Gweon, S.Y. Zhou \textit{et al}. J. Phys. Chem. Solids \textbf{67}, 2172 (2006) [4] S. Y. Zhou \textit{et al.} Phys. Rev. B \textbf{71}, 161403(R) (2005)   

Electron-phonon renormalization effects in the ARPES spectra of doped graphene: a first principles approach

Recent experimental investigations of the hole excitations in graphene [1], bilayer graphene [2] and graphite [3] by angular resolved photoemission indicated the occurrence of kink structures in the band dispersions and in the lifetime of hole excitations. These kinks have been attributed to electron-phonon coupling effects. In this work we calculate the effect of electron-phonon scattering on the angular resolved photoemission spectra (ARPES) of graphene as a function of doping. We use electron-phonon coupling parameters derived from density functional theory calculations. We compare our results for the quasiparticle dispersion and for the lifetime of the electrons and holes with those obtained from ARPES.\\ ${\rm [1]}$ A. Bostwick {\it et al.}, cond-mat/0609660 \\ ${\rm [2]}$ T. Ohta {\it et al.}, Science {\bf 313}, 951 (2006) \\ ${\rm [3]}$ S. Zhou {\it et al.} cond-mat/0609028   

Anisotropic electron-phonon coupling in doped graphene

The effects of doping single layer graphene are investigated by mapping the valence band in the vicinity of EF using angle-resolved photoemission spectroscopy (ARPES). The carrier concentration was varied from 0.04 -- 1.05 electrons per unit cell with the deposition of Ca and K at low temperatures. As the doping increases there is an enhancement of the electron-phonon coupling along certain high symmetry directions. Changes in electron-phonon coupling parameter, lambda, shows that the systems goes through a transition from the weak-coupling regime to the strong-coupling regime.   

Thermo-Plasma Polariton within Scaling Theory of Single-Layer Graphene

Electrodynamics of single-layer graphene is studied in the scaling regime. At any finite temperature, there is a weakly damped collective thermo-plasma polariton mode whose dispersion and wavelength dependent damping is determined analytically. The electric and magnetic fields associated with this mode decay exponentially in the direction perpendicular to the graphene layer, but unlike the surface plasma polariton modes of metals, the decay length and the mode frequency are strongly temperature dependent. This may lead to new ways of generation and manipulation of these modes.   

Theory of resonant multiphonon Raman scattering in graphene monolayers

The Raman spectrum of graphene consists of distinct narrow peaks corresponding to different optical phonon branches as well as their overtones [1]. We show how the relative intensities of the overtone peaks encode information about relative strengths of different inelastic scattering processes electrons are subject to. In particular, assuming that the most important processes are electron-phonon and electron-electron scattering, it is shown that one can deduce their relative interaction strengths from the Raman spectra. [1] A. C. Ferrari et al., Phys. Rev. Lett. 97, 187401 (2006); A. Gupta et al., cond-mat/0606593; D. Graf et al., cond-mat/0607562.   

Extracting optical properties of individual or few layers of graphite oxide sheets on surfaces by developing simple optical approaches

An optical method for extracting optical properties of individual or few layers of graphite oxide sheets is presented. The substrate consists of a dielectric layer of controlled thickness on semiconducting silicon. The intensity ratio between reflected light from the material and the substrate can be optimized through choice of the optical properties and the thickness of the dielectric layer; analysis of the reflection of an incident light beam demonstrates this, and confocal microscopy images obtained on different thickness dielectric layers verifies the analysis. By comparing the measured and predicted intensity ratios of single layers of graphite oxide the optical properties before and after thermal treatment were obtained. The use of a designed substrate in terms of the thickness and optical properties of a dielectric layer on silicon, could find use for optically characterizing exceptionally thin platelets and also thin biological materials which might otherwise not be discerned through ``standard'' optical microscopy.   

Session L29: Colloids III

Clustering instabilities in lattice gas models with isotropic repulsive interactions

In previous work we have shown that liquid crystalline order arises in systems of particles with purely repulsive, spherically symmetric pair interactions. We have observed a variety of liquid crystalline phases, as well as rich crystalline and quasicrystalline polymorphism, in simulations of two and three dimensional systems of particles with isotropic pair potentials consisting of an impenetrable hard core plus an isotropic penetrable, repulsive soft shoulder. We have further explored the clustering instabilities in the model by using mean field theory and Monte Carlo simulations of lattice gas models with an isotropic soft-shoulder repulsion that extends out many lattice spacings. The lattice gas model maps exactly onto an Ising model with antiferromagnetic interactions. At low temperatures this repulsive soft shoulder leads to the development of structure on the length scale of the repulsion. Mean field theory predicts both layered and solid structures in the temperature/magnetic field (chemical potential) plane. Monte Carlo simulations display liquid phases with short range patterns, layered incommensurate phases with quasi-long-range order, long- range ordered layered solids, hexagonal micellar solids with quasi- long-range ordering of micelles and long-range ordered hexagonal phases.   

Structures Formed by Small Numbers of Colloidal Particles Bound to a Spherical Interface

We study the behavior of micron sized colloidal particles adsorbed on the interface of spherical droplets not much larger than the colloids. We compare the structures formed by interfacially-bound particles at low particle number to predicted geometries such as the proposed solutions to the Thomson and Tammes problems. The predicted geometries depend critically on the interactions between particles in this low particle number regime. Because all particles on a droplet must be tracked simultaneously, such colloidosome systems have not yet been explored experimentally due to the limited time and z-resolution of confocal and bright-field microscopy. To overcome such limits, we use digital holographic microscopy to locate all particles within a volume of roughly 100$\times $100$\times $50 $\mu $m$^{3}$ at speeds of up to 500 frames per second. The experimental setup and reconstruction algorithms will be discussed along with our results.   

Mechanisms of Size and Shape Selection and Control in Self-Assembly of Colloid Particles Synthesized from Nanosize Crystalline Precursors

The importance of well-defined dispersions of particles of different shapes, ranging in sizes from nanometer to colloidal, has been widely recognized in applications and in basic studies of advanced materials. Our program endeavors to advance understanding of formation of uniform particles of simple and composite structure, with focus on synthesis involving self-assembly of nanosize particles and their new unique properties for dimensions smaller than the typical submicron-size colloid scales. Presently, there is convincing experimental evidence that many monodispersed colloids of various shapes, obtained by precipitation in solutions, are formed by aggregation of such nanocrystalline subunits. Our group's theoretical explanation of this process expands the classical model of formation of uniform particles, by LaMer, and offers an interesting link between nanosize and micrometer size particles.   

Orientational Order of Chain Forming Ferroelectric Nano Particles in Heptane .

Previous computational work [1] has shown that under the appropriate conditions, dipolar spheres aggregate and form chains. In this report, we study nano-sized ferroelectric BaTiO$_{3 }$particles dispersed in heptane. We demonstrate dependence of the particles organization in the colloid \textit{vs.} particles size and concentration. When the particles are large ($>$40 nm) they sediment to the bottom of the solution; smaller particles ($\sim $10-15 nm) form gels or networks that do not sediment. Probing particle organization by means of freeze fracture electron microscopy reveals that at small sizes ferroelectric particles form a network of chains of particles that have local nematic like order. We compare our observations with the described in literature predictions. [1] J. Weis, D. Levesque Phys. Rev. Lett. 71, 2729 (1993).   

Specific and Reversible Assembly of DNA Coated Colloids

We aim to create a new class of materials that self-assemble and self-replicate. Biotin-terminated DNA strands are attached to neutravidin coated polystyrene particles via the well established avidin-biotin coupling mechanism. The DNA is composed of a 61-base strand and a 50-base complementary strand, leaving a single sticky end of 11 bases to interact with its complement attached to another particle. Complementary particles mixed together aggregate into fractal structures. Increasing the temperature leads to dehybridization of the DNA strands and disaggregation of the particles. A typical cycle of aggregation, disaggregation, and reaggregation, as investigated by videomicroscopy, takes $\sim $ 20 minutes and has been repeated more than a dozen times. Our melting curves are sharp and show a strong dependence with buffer concentration. In a highly ionic environment, the aggregation is well described by a diffusion limited process and slows down considerably as the aggregation temperature approaches the melting temperature. We show how these materials are promising for creating new self-replicating structures.   

Search for Optical Binding with Shape Phase Holographic Optical Trapping

Light scattered by an illuminated particle should repel that particle's neighbors through radiation pressure. Nearly two decades ago, Burns, Fournier and Golovchenko (BFG) proposed that the coherent superposition of scattered fields can lead to an attractive interparticle interaction, which they called optical binding. Their pioneering experimental observation has generated considerable interest, most of which has focused on developing the theory for the effect. Accurate measurements of the optical binding force in the BFG geometry have been lacking, however. The need to quantify optical binding forces is particularly acute for colloidal interaction measurements on linear optical traps. We present a new method to directly measure optical binding forces between colloidal spheres that exploits the ability of shape-phase holography to create linear optical traps with accurately specified intensity and phase profiles. Our ability to control the trap's phase profile makes possible precise discrimination between intensity- and field-dependent interactions, i.e. between radiation pressure and optical binding. The same novel technique that allows us to project holographic line traps also can be used to project two- and three-dimensionally structured ring traps, novel Bessel-beam traps, which we also will describe.   

Segregation of Defects at Grain Boundaries.

Interstitial impurity segregation at grain boundaries plays an important role in materials properties such as cohesion, grain growth kinetics, and transport. Unfortunately, direct measurement of grain boundary composition is difficult in bulk crystals and polycrystals. In this contribution we directly study impurity segregation at grain boundaries using a model colloidal crystal. The polycrystals are made of temperature-sensitive micron size NIPA microgel particles [1]. We add 100-200 nm fluorescent polystyrene particles to this system to model interstitial impurities. The impurities are then tracked using video microscopy close to and far from the grain boundaries. We find that impurities hop from one position to another and diffuse anisotropically when far from the grain boundaries, and they diffuse isotropically in the grain boundaries. Upon increasing the temperature, the packing volume fraction of NIPA particles decreases and grain boundaries start to melt. We also explored the effects of the segregated impurities on grain boundary melting. [1] A. M. Alsayed, M. F. Islam, J. Zhang, P. J. Collings, A. G. Yodh, Science 309, 1207 (2005). This work was supported by grants from NSF (DMR-0505048 and MRSEC DMR05-20020) and NASA (NAG8-2172).   

Realizing Colloidal Artificial Ice on Arrays of Optical Traps

We demonstrate how a colloidal version of artificial ice can be realized on optical trap lattices. Using numerical simulations, we show that this system obeys the ice rules and that for strong colloid-colloid interactions, an ordered ground state appears. We show that the ice rule ordering can occur for systems with as few as twenty-four traps and that the ordering transition can be observed at constant temperature by varying the barrier strength of the traps.   

Shear Events in Colloidal Glasses

We analyze shear events that occur in sheared amorphous colloidal suspensions. We use the three-dimensional particle positions determined by confocal microscopy to determine irreversible local rearrangements that give rise to high local strain. These shear regions show a long-range strain field characteristic of dipolar strain events. Large displacements of only one or a few particles in the shear event core are enough to stabilize the new configuration and lead to permanent deformation. We will elucidate the interplay between thermal fluctuations and local strain that drives the nucleation of these shear regions.   

Hydrodynamic Forces in the Lubrication Regime: A Molecular Dynamics Study

We report on classical molecular dynamics simulations of large spheres moving toward a flat substrate and large spheres moving toward each other. The simulations are designed to investigate hydrodynamics at the molecular scale. We show a new decomposition approach appropriate for force microscopy measurements, and extract the static and dynamic components of the total force from approaching- and receding-force curves that are obtained from simulations or experiments. The dynamic force is evaluated for a range of sphere sizes and approach velocities, with different fluids and as well as with different surface characteristics - smoothness, roughness, and compliance. A comparison with hydrodynamic predictions for the dynamic force is made for these various cases.   

Colloid-Polymer Demixing in the Protein Limit: A Simulation Study

Mixtures of hard colloidal particles and nonadsorbing polymers can exhibit entropy-driven demixing into colloid-rich and colloid-poor phases. The classic Asakura-Oosawa-Vrij (AOV) model idealizes the polymers as effective spheres that are mutually noninteracting but impenetrable to the colloids. Here the AOV model is adapted to the protein limit by assuming the polymers to be (1) penetrable to the smaller colloids (or nanoparticles) and (2) polydisperse in size (radius of gyration). Using Gibbs ensemble Monte Carlo simulation, we explore the influence of the colloid-polymer penetration energy profile on the demixing instability and polymer size distribution. Structural and thermodynamic properties (radial distribution functions, osmotic pressures, and demixing phase diagrams) are computed and compared with predictions of density-functional theory.\footnote{M. Schmidt and M. Fuchs, {\it J. Chem. Phys.} {\bf 117}, 6308 (2002).}   

Colloidal Electrostatic Interactions Measured on Holographic Line Traps

We measure the electrostatic colloidal interaction between two colloidal particles diffusing in water along a quasi-1D potential that we generated by shape-phase holography. Interparticle potential measurements are affected in principle by light-induced contributions generated by the confining potential. We present both a measurement of such effect and a method to correct for it without the need for an independent measurement. Fast and accurate measurements on a line tweezer have the potential to become a standard method for assessing locally both equilibrium and out-of-equilibrium processes.   

Phase Behavior of Charged Colloids: Closed versus Donnan Equilibrium$^1$

The influence of chemical boundary conditions on thermodynamic properties of deionized charge-stabilized colloidal supensions is analyzed. Effective electrostatic interactions and phase behavior are shown to depend fundamentally on whether a suspension is confined to a closed (electroneutral) cell or is in Donnan equilibrium with a microion reservoir, {\it e.g.}, electrolyte solution. Linear-response theory$^2$ predicts that at low ionic strength closed suspensions of highly charged macroions and monovalent microions can phase separate, while microion exchange with a reservoir stabilizes the fluid phase. \\[1ex] $^1$~Supported by National Science Foundation grant DMR-0204020. \\[0.5ex] $^2$~A.R.~Denton, {\it Phys. Rev.} E {\bf 73}, 41407 (2006).   

Epitaxial Growth of Thin Colloidal Films in the Presence of a Depletant

We describe the epitaxial growth of thin films comprised of hard-sphere colloidal particles sedimenting in the presence of a depletant polymer. The depletant polymer induces an effective attraction between microspheres, causing them to nucleate islands that grow and coalesce with one another. In addition, we use photolithography to control the morphology of the substrate. This allows us to investigate the effects of the underlying substrate structure on the epitaxial growth process. Using confocal microscopy, we image and track the colloidal particles as they diffuse, aggregate and rearrange their configurations during deposition. Island density and degree of layer-by-layer growth are determined as functions of the deposition rate and depletant concentration. The ease with which we are able to image deposition in real time and the similarity of our results to those obtained in atomic deposition experiments suggest that our system will allow us to model various processes that occur in atomic thin film epitaxial growth.   

Patterning Colloidal Films via Evaporative Lithography

We investigate evaporative lithography as a route for patterning colloidal films during drying. Specifically, films composed of mixtures of silica microspheres and polystyrene nanoparticles are patterned by placing a mask above the film surface to induce periodic variations between regions of free and hindered evaporation. Fluorescence and confocal microscopy, coupled with surface profilometry measurements, reveal that particles segregate laterally within the drying film, as fluid and entrained particles migrate towards regions of higher evaporative flux. The colloidal films exhibit remarkable pattern formation that can be regulated by carefully tuning the initial suspension composition, separation distance between the mask and underlying film, and mask geometry.   

Session L30: Micro and Nano Fluidics

Microfluidic droplets and electric fields

Manipulating droplets through mazes of microchannels is a challenge faced by digital microfluidics (i.e microfluidics based on droplets). In this domain, using electric fields is an option. This option is justified by the fact that producing large electric fields in miniaturized systems is feasible, and dielectric contrasts between dispersed and continuous phases are typically large. Examples of devices reported in the literature are droplet guides, droplet mergers. In the present paper, we extend this approach by reporting two novel examples of droplet manipulations that exploit the action on an electric field in a microfluidic system: one is the control of droplet emission frequencies and the other is the inhibition of droplet breakup. Throughout the work, we analyze in some detail the various aspects of the action of the electric field. The experiments are performed in PDMS microfluidic systems using hexadecane and water for the continuous and dispersed phases respectively.   

Chemotaxis in Microfluidic Devices: What does a cell see?

The use of microfluidic devices is increasingly popular in the study of chemotaxis due to the exceptional control of the flow field and the concentration profiles on the length scale of individual cells. One aspect often forgotten is that the cells are attached to the inner surfaces of the microfluidic channel. The flow field is perturbed and distorted as the fluid is flowing around/over the cells. Depending on the flow speed and dynamics (steady flow - increasing flow - decreasing flow) the cell membrane is not exposed to the ``nominal'' concentration profiles, but may see a very different signal. The underlying physics will be discussed and ``optimal'' flow conditions will be identified.   

Active microfluidic mixing based on transverse electro-osmotic flows

As with their macroscale counterparts, laminar fluid mixing becomes a very important, albeit inherently difficult step at the microscale. Micromixers based on electro-osmotic flow (EOF) rely on either a modification of microchannel geometries or a modification of the $\zeta $-potential of the microchannel surfaces to enhance fluid mixing. Here we present a new method of achieving chaotic advection in microchannels by applying an electric field perpendicular to the mean flow direction driven by a pressure gradient in a planar rectangular microchannel. EOF on microchannel surfaces in a direction orthogonal to the main channel axis is generated via an electric field produced by integrated electrodes at the corners of a microchannel. By using serial combinations of different mixing cycles, we show that complete mixing can occur in straight microchannels of length scales on the order of a millimeter. Computational fluid dynamics (CFD) is used to characterize and optimize the mixing efficiency of the system and to compare with experimental measurements.   

Microfluidic bubble logic and applications

We present a novel all-fluidic logic family operating at low Reynolds numbers in newtonian fluids. A bubble in a microfluidic channel represnets a bit. Nonlinearities are introduced in an otherwise linear, reversible flow by bubble-bubble interactions. This allows us to simultaneously perform chemistry and process control without external control elements. A toggle flip-flop, AND/OR/NOT gates, ring oscillator and an electro-bubble modulator will be presented. Applications in high-throughput screening and combinatorial chemistry will be highlighted.   

Using patterned surfaces to sort elastic microcapsules

For both biological cells and synthetic microcapsules, mechanical stiffness is a key parameter since it can reveal the presence of disease in the former case and the quality of the fabricated product in the latter case. To date, however, assessing the mechanical properties of such micron scale particles in an efficient, cost-effective means remains a critical challenge. By developing a three-dimensional computational model of fluid-filled, elastic spheres rolling on substrates patterned with diagonal stripes, we demonstrate a useful method for separating cells or microcapules by their compliance. In particular, we examine the fluid-driven motion of these capsules over a hard adhesive surface that contains soft stripes or a weakly adhesive surface that contains ``sticky'' stripes. As a result of their inherently different interactions with the heterogeneous substrate, particles with dissimilar stiffness are dispersed to distinct lateral locations on the surface. Since mechanically and chemically patterned surfaces can be readily fabricated through soft lithography and can easily be incorporated into microfluidic devices, our results point to a facile method for carrying out continuous ``on the fly'' separation processes.   

Lab-on-a-chip Single Particle Dielectrophoretic Traps

Cell-patterning and cell-manipulation in micro-environments are fundamental to biological and biomedical applications, for example, spectroscopic cytology based cancer detection. Dielectrophoresis (DEP) traps with transparent centers for stabilized cell and particle optofluidic intracavity spectroscopy (OFIS) were fabricated by patterning 10 $\mu $m wide, planar gold electrodes on glass substrates. The capturing strength of DEP traps was quantified based on the minimum AC voltage required to capture and hold varying diameter polystyrene or was it some other material, e.g. silica or PMMA microspheres in water as a function of frequency required under a constant flowrate of 20 $\mu$m/s. The maximum required trapping voltage in the negative DEP regime of $f$ = 1 kHz to 40 MHz was 5.0 VAC. The use of AC fields effectively suppressed hydrolysis. New geometries of DEP traps are being explored on the basis of 3-D electrostatic field simulations.   

Hybrid CMOS / Microfluidic Systems for Cell Manipulation with Dielectrophoresis

A hybrid CMOS/microfluidic chip combines the biocompatibility of microfluidics with the built-in logic, programmability, and sensitivity of CMOS integrated circuits (ICs)$^{1}$ We have designed a CMOS IC for moving individual cells using dielectrophoresis (DEP). The IC was built in a commercial foundry and we subsequently fabricated a microfluidic chamber on the top surface. The chip consists of a 1.4 by 2.8mm array of over 32,000 individually addressable 11x11 micron pixels. An RF voltage of 5V at 10MHz can be applied to each pixel with respect to the conductive lid of the microfluidic chamber, producing a localized electric field that can trap a cell. By shifting the location of energized pixels, the array can trap and move cells along programmable paths through the microfluidic chamber. We show the design, fabrication, and testing of the hybrid chip. Bringing together the biocompatibility of microfluidics and the power of CMOS chips, hybrid CMOS / microfluidic systems are an exciting technology for biomedical research. Thanks to NSEC NSF grant PHY-0117795 and the NCI MIT-Harvard CCNE. [1] H Lee, Y Liu, RM Westervelt, D Ham, IEEE JSSC 41, 6, pp. 1471-1480, 2006   

Optical Chromatography of Bacterial Spores

The technique of optical chromatography uses a laser mildly focused against fluid flow in a microfluidic channel to trap microscopic particles. Particles in the channel near the focal point of the laser are drawn toward the beam axis and then accelerated via optical pressure against the fluid flow, reaching an equilibrium point when the optical and fluidic forces on the particle are balanced. This equilibrium point may occur at differing distances from the focal point for microscopic particles with differing properties, such as size, shape, morphology, and refractive index. Thus, identification and separation of particles may be achieved in the system. Optical chromatography may be used as a detection technique for biological particles of interest, either directly or as a means of concentrating and filtering a sample. Of particular interest would be reliable methods for detection of {\em Bacillus anthracis}, a common weaponized biological agent. In this work we present optical chromatography experiments on bacterial spores which may be environmentally present with {\em B. anthracis} spores and interfere with detection.   

``Nanonails'' -- a Simple Geometrical Approach to ``Superlyophobic'' Surfaces

Modern nanofabrication techniques allow creation of a wide range of sophisticated surface topographies that strongly enhance wetting properties of solids. Such surfaces serve as a basis for so-called superhydrophilic and superhydrophobic materials that demonstrate a range of remarkable properties. In both of these cases the topography acts to ``amplify'' the type of wetting behavior, which is already determined by the surface energies of the liquids and solids involved. In this work we propose and experimentally demonstrate a unique three-dimensional nano-scale geometry that dramatically extends the influence of topography on the wetting properties of the substrate. Using this approach we are able to transform ordinary Teflon-like fluoropolymer surfaces, which are readily wetted by the majority of common low-surface tension liquids into nanostructured substrates with profound superlyophobic behavior. The resulting surfaces are essentially non-wetting and support highly mobile liquid droplets with contact angles close to 150\r{ } for a wide variety of liquids with surface tensions ranging from 72.0 mN/m (water) to 21.8 mN/m (ethanol). The proposed approach provides a simple, material-independent method for creating practically useful superlyophobic surfaces.   

Double Emulsions through Wettability Control in PDMS Microfluidic Devices

Hydrodynamic Flow Focusing allows for the well-controlled production of monodisperse double and multiple emulsions. While this method of emulsification is well described for glass capillary devices, it has not yet been developed for PDMS devices that are readily accessible using soft-lithography. The reason is the difficulty of spatially controlling the wetting behavior of PDMS microchannels. We will present a novel technique of photopatterning that allows for the production of double emulsions in PDMS devices. Moreover, owing to an optimized setup, smaller droplets may be made down to a size range that was not accessible using the conventional approaches.   

Atomistic simulation study of charge inversion in silica nanochannels

Recent experiments report charge inversion, i.e. interfacial charges attracting couterions in excess of their own nominal charge, in divalent ionic solutions near charged silicon oxide interfaces. We have conducted a series of atomistic molecular dynamics simulations in order to investigate the mechanism of charge inversion in these systems. We studied both CaCl$_2$ and MgCl$_2$ solutions near an amorphous silica substrate which had a charge density of $\sim 1/50${\AA}$^2$. Our simulation results give a detailed description of the structure of the ions and water near the silica interface. Finally, we show that our simulations are in remarkable agreement with the experimental results.   

Ion solvation in confined water: A first-principles molecular dynamics investigation

The importance of water in many areas of science has motivated an enormous number of experimental and theoretical investigations. While the properties of bulk liquid water have been relatively well characterized, much less is known about the properties of water when it is confined in a nanoscale environment. We have carried out a series of first-principles molecular dynamics simulations in order to examine how the solvation properties of simple ions are modified upon nanoscale confinement. These simulations include the aqueous solvation of cations contained within a carbon nanotube. By comparing to the properties of ions in bulk liquid water, the dynamical and structural characteristics of confined ion solvation will be discussed in detail. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48.   

Measurement of the growth rate of the breakup instability of a propane nanobridge using molecular dynamics simulations

Understanding the instability of a nanojet or a nanobridge is of importance for the design of nanoscale fluid devices. Examination and determination of the whole growth rate curve of the instability in these nanostructures is a theoretical challenge. Using large scale molecular dynamics (MD) simulations at 185K we determined the growth rate curve of a nanoscale liquid propane nanobridge of a 0.3 micron length and a 6-8 nm diamater; the system consistes of ~340,000 particles. We analyzed, using a discrete spatial Fourier transform, the time evolution of small sinusoidal perturbations of various wavelengths applied to the fluid nanobridge. The large length-to-diameter ratio of our systems allows us to achieve suffcient wavelength resolution. The results of 100 independent simulations were averaged to reduce fluctuation noise. The results were compared with both Rayleigh's and Chandrasekhar's theories and we conclude that the latter is a better fit to our data.   

Instability in extensional microflow of aqueous gel

Microfluidic devices are typically characterized by laminar flows, often leading to diffusion limited mixing. Recently it has been demonstrated that the addition of polymer to fluids can lead to elastic instabilities and, under some conditions, turbulence at arbitrarily low Reynolds numbers in mechanically driven flows [1]. We investigated electroosmotic driven extensional flow of an aqueous polymer gel. Microchannels with 100 micron width and 20 micron depth with the characteristic ``D'' chemical etch cross section were formed in glass. A Y-channel geometry with two input channels and a single output created extensional flow at the channel intersection. Instabilities where observed in the extensional region by fluorescently tagging one input stream. Instabilities were characterized by 1/f spectra in laser induced fluorescent brightness profiles. Due to the simple geometry of extensional flow and the importance of electroosmotic flows for integrated applications and in scaling, this is of interest for device applications. [1] A. Groisman and V. Steinberg, Nature 405, 53-55, 2000.   

Interactions between micro droplets and a flowing soap film

When a jet of micron sized water droplets impact on a thin freely suspended soap film, craters of various sizes are created in the film. Depending on the velocity of the jet and the thickness of the film, a fraction of the particles is able to penetrate through the film without breaking it while others merge with the film. The statistical nature of penetration suggests that the energy barrier for passage is a fluctuating quantity but the cause of such fluctuation is not understood. Using a high-speed video camera, the interaction between the droplet and the film is investigated for various conditions. Aside from its fundamental interest, the technique is potentially useful for generating predetermined number of vortices in the fluid and for depositing precisely passive scalar quantities, such as dyes, into two-dimensional turbulence in the flowing film.   

Session L31: Carbon Nanotubes: Superconductivity

Carbon Nanotube Superconducting Quantum Interference Device.

We report on the study of a superconducting quantum interference device (SQUID) with Josephson junctions made of portions of metallic single-walled carbon nanotube [1]. Quantum confinement in each nanotube junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with a lateral electrostatic gate. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting to change the hybridization of the QD states with the superconducting contacts [2]. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of a 6nA supercurrent current with magnetic flux is achieved when both QD junctions are in the ``on'' or ``off'' state. Futhermore, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled $\pi $-junctions. This allow to verify that the sign of the current-phase relation across a proximity coupled Qdot can be reversed with a gate voltage. Noise studies shows that the noise figure of the nanotube SQUID together with the size of the junction should allow the detection of a single molecule magnet. [1] J-P. Cleuziou et al. Nature Nanotec., \textbf{1}, 53, (2006). [2] J-P. Cleuziou et al. cond-mat/0610622.   

Nonequilibrium giant loop currents and orbital magnetism in carbon nanotubes

Recent experiments have shown that carbon nanotubes can have large orbital magnetic moments ($\sim 10\mu_B$). Although isolated carbon nanotubes in equilibrium in external magnetic fields have been theoretically studied, nonequilibrium transport of nanotubes attached to electrodes has yet to be established. Based on Keldysh formalism, we analyze currents flowing in carbon nanotubes attached to electrodes with finite bias voltages. We show that large magnetic moments are generated from giant loop currents circulating around the tube, which makes carbon nanotubes ``molecular solenoids''. While this is an example of the quantum loop current when incident electrons are resonant to degenerate levels of molecules as proposed by Nakanishi and Tsukada [Surf. Sci. {\bf 438}, 305 (1999)], a speciality of the nanotubes is that they have inherent doubly-degenerate states (propagating clockwise and anticlockwise around the tube). We have further identified the full conditions for large loop currents that include the position of the electrodes and the chirality of the tube. The current-voltage characteristic and effects of external magnetic fields are also discussed.   

Zero-bias anomaly and possible superconductivity in single-walled carbon nanotubes

We report measurements of field-effect transistors made of isolated single-walled carbon nanotubes contacted by superconducting electrodes. For large negative gate voltage, we find a dip in the low-bias differential resistance. Remarkably, this dip persists well above the superconducting transition temperature of the electrodes, indicating that it is {\em not} caused by superconducting proximity effect from the electrodes. This conclusion is supported by measurements on carbon nanotubes contacted by normal electrodes showing similar features. One possible explanation is superconductivity in the nanotubes, occurring when the gate voltage shifts the Fermi energy into van Hove singularities of the electronic density of states.   

Interplay between induced superconductivity and Luttinger liquid behavior in carbon nanotubes

Superconductors and Luttinger Liquids (LL) are two prototypical strongly correlated electron systems. In a Josephson junction where the normal metal is a LL rather than a Fermi Liquid, the Cooper pairs are expected to decay as a power law the distance between the superconductors. Here we experimentally investigate the interplay between LL and superconductivity by coupling individual single-walled carbon nanotubes to superconducting leads. Low bias conductance peaks induced by proximity effects are observed, and induced superconductivity in nanotubes are examined as a function of inter-electrode spacing. Latest experimental results will be discussed in terms of various theoretical models.   

Double-gap proximity effect in nanotubes

We study the properties of a single-walled metallic carbon nanotube placed on a superconducting substrate. Given that the nanotube possesses two bands in its excitation spectrum, we find a novel proximity effect which allows the existence of a ``double superconducting gap.'' We show that there is a critical experimentally-accessible interaction strength in the nanotube at which this proximity effect transitions from being suppressed to being enhanced. We also analyze the effect of possible phase fluctuations within the substrate on the induced superconductivity. We discuss the consequences of these features on the single-particle tunneling density-of-states of the nanotube.   

Pressure-Induced Metal-Insulator Transition in Single-Walled Carbon Nanotubes

The resistance of single-walled carbon nanotube (SWNT) bundles was studied under combined extreme conditions of high pressure (up to 10 GPa), low temperature (down to 2 K) and strong magnetic field (up to 12 T). A pressure-induced metal-insulator transition was found to occur at $\sim $ 1.5 GPa, across which the temperature and field-dependent functional forms of the resistance changes dramatically. The transition pressure of $\sim $ 1.5 GPa correlates closely with the structural phase transition of SWNT under pressure. In the insulator phase, the magnetoresistance of the samples shows typical behaviors of two-dimensional electron weak localization, presumably reflecting the coherent hopping processes of the electrons in the collectively flattened plane of the SWNTs bundles.   

Thermoelectric transport in the vicinity of a superconductor-metal quantum phase transition in nanowires

We consider a model of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism, e.g. a magnetic field in the wire direction or disorder in an unconventional superconductor. The critical theory contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. In a large-N limit, we calculate the universal dependence of the electrical and thermal conductivity on both pair breaking strength and temperature. We find that the conductivity has a non-monotonic temperature dependence on the metallic side of the transition and that the Wiedemann-Franz law is obeyed at low temperatures. In the quantum critical region, we study the dynamics of a two-component order parameter field via the Langevin equation formalism and compare with the large-N result.   

Superconductivity of nanowires in contact with bulk metals

A counter-intuitive anti-proximity effect (APE) was recently reported for Zn nanowires in contact with two superconducting bulk electrodes (PRL \textbf{95}, 076802 (2005)). It was observed that the Zn nanowires were superconducting when the bulk electrodes were normal. When bulk electrodes were superconducting, superconductivity in Zn nanowires appeared to be partially or fully suppressed. However, the resistance of the extrinsic contacts between the Zn nanowires and the bulk electrodes has raised questions about these experiments. To address this issue, we have fabricated Sn, Pb, and Zn single nanowires of various diameters and lengths in contact with a number of different bulk materials using an in-situ contact method develop by our group (APL \textbf{84}, 6996 (2004)) which eliminates any extrinsic contact resistance. Transport properties of the nanowires have been measured using a Physical Property Measurement System (PPMS). We have found that long ($\sim $60$\mu $m) nanowires of Sn and Pb demonstrate superconductivity as expected with either superconducting or normal bulk electrodes. However, short ($<$10$\mu $m) Sn and Pb nanowires demonstrate superconductivity only when the bulk electrodes are superconducting, such as Sn and Pb. Other samples with similar structures are being studied and will be used to clarify these results. We will discuss these results in the context of the proximity effect.   

Magnetization drops in arrays of superconductive multi-walled carbon nanotubes

Superconductivity in CNT, which is an ideal one-dimensional (1D) conductor, is attracting significant attention, because it allows one to study how Cooper pairs can be generated and behave in 1D space within a ballistic charge transport regime. This study also reveals interplay between superconductive phase and phases of 1D quantum phenomena, which tend to prevent superconductivity from its appearance (e.g., Tomonaga-Luttinger liquid states and Pierls transition). We have recently reported finding of superconductivity with the highest Tc of 12 K for abrupt resistance drops in arrays of MWCNTs by entirely end-bonding those by gold electrode [1]. Here, I will report finding of magnetization drops with the highest Tc of 18K, which is greater than the above-mentioned Tc of 12K, in the arrays of MWCNTs. Because only the samples with showing resistance drops can exhibit this magnetization drips, we conclude that this is attributed to Meissner effect. Based on this observation, we clarify that contribution of graphite structure of a MWNT is a dominant mechanism for the present Meissner effect rather than influence of curvature. We also reveal contribution of intertube coupling in an array of MWCNTs. [1] I.Takesue, J.Haruyama. et al., Phys.Rev.Lett. 96, 057001 (2006)   

Very Unusual Magnetic Properties in Multi-walled Carbon Mats

We report magnetic measurements up to 1100 K on a multi-walled carbon nanotube mat sample using a Quantum Design vibrating sample magnetometer. In an ultra-low field ($H$ = $-$0.02 Oe), we find a very large paramagnetic susceptibility (up to 12.7$\%$ of 1/4$\pi$) at 1100 K and a very large diamagnetic susceptibility (at least 8.4$\%$ of $-$1/4$\pi$) at 482 K. A small magnetic field (2.1 Oe) completely suppresses the diamagnetic susceptibility at 482 K and reduces the paramagnetic susceptibility at 1100 K by a factor of over 20. We rule out explanations based on magnetic contaminants, instrument artifacts, and the orbital diamagnetism. The magnetic data are inconsistent with any known physical phenomena except for granular superconductivity. The present results suggest the existence of an unknown new physical phenomenon or superconductivity with an ultra-high transition temperature.   

Lifetime of a one-dimensional fermion

Interaction between fermions in one dimension is usually accounted for within the exactly solvable Tomonaga-Luttinger model. The crucial simplification made in this model is the linearization of the fermionic spectrum. That simplification leads to an infinite lifetime of a fermion at the mass shell, i.e., the corresponding Green function $G(\varepsilon,\xi_k)$ diverges at $\varepsilon=\xi_k$. We find that inclusion of the curvature of electron spectrum, $\xi_k=v_Fk+k^2/2m$, yields a finite decay rate of a fermion, $1/\tau(\xi_k)\propto \theta(k)k^8/m^3$; here for definiteness we consider right-moving particles, and $k$ is measured from the Fermi wave vector. The found finite lifetime allows one to assess the limitations of the Luttinger liquid paradigm.   

Electron Transport and Tunneling in Single Walled Carbon Nanotube Devices

Carbon nanotubes remain a fertile ground for the exploration of interacting one-dimensional (1D) physics and Tomonaga-Luttinger liquid theory. Much is still unknown about the factors that influence the transport and tunneling properties of interacting 1D systems such as nanotubes. We report on experiments that use techniques such as multiple contacts on long nanotubes and tunable tunnel barriers to determine how the manifestations of electron-electron interactions, such as the zero-bias anomaly, depend on the length and defect strength in nanotubes.   

Luttinger liquid parameters of carbon nanotubes from first-principles calculations

Electron interactions in carbon nanotubes are responsible for particle correlations which manifest themselves in a power-law suppression of the density of states observed in tunneling transport. The Tomonaga-Luttinger model, which describes the behavior of 1d metals at low energy, links the power law exponents to the microscopic parameters, interaction strength and Fermi velocity. These exponents have been measured in recent experiments [1]. Motivated by these findings, we employ density functional theory methods to estimate charge compressibility and Fermi velocity, and thereby obtain the Tomonaga-Luttinger model parameters in the charge sector [2]. Our calculations are in quantitative agreement with experimental results and previous RPA calculations [3]. [1] M. Bockrath et al., Nature 397, 598 (1999); Z. Yao et al., Nature 402, 273 (1999); H. Ishii et al. Nature 426, 540 (2003). [2] B. Kozinsky et al. (to be published). [3] R. Egger, A. O. Gogolin, Phys. Rev. Lett. 79, 5082 (1997).   

The Study of Electron-Electron Interactions in Semi-Conducting Carbon Nanotubes Using a Numerical Renormalization Group

We present a non-perturbative, numerical renormalization group (NRG) based technique for the study of the spectrum of semi-conducting single-walled carbon nanotubes in the presence of electron-electron interactions. This technique permits a full many-body treatment of the system. As our starting point, we model a single walled semi-conducting carbon nanotube as four gapped Dirac fermions in the presence of interactions. Focusing on a poorly screened carbon nanotube where the interactions are strongest in the forward scattering direction, the nanotube can be equivalently modeled as four Luttinger liquids coupled together with a quadratic gap term. The NRG based technique then is readily able to determine non-perturbatively the effects of the gap term upon the four Luttinger liquids. Using this approach we are able to obtain results for both the excitonic and single particle spectra of the nanotube.   

Observation of the Kondo effect in a carbon nanotube with asymmetric Schottky barriers

A CVD carbon nanotube sample measured at low temperature and high magnetic field was observed to show a substantial increase in differential conductivity ($\Delta $I/$\Delta $V) near zero voltage and a pronounced asymmetry with bias voltage that appear only below temperatures of 3.0K. The magnetic field peaks show complicated shifts and possible splits. The simplest interpretation that satisfies the data is that of a metallic tube whose Schottky barriers to the external contacts are asymmetric. The observation of a Kondo effect that varies with bias polarity dramatically illustrates the impact of coupling to the external leads.   

Session L32: Focus Session: BEC/BCS Crossover

Large N expansion for superfluid Fermi gases at unitarity

We study an s-wave resonant Fermi gas near the unitarity point. We treat this problem by generalizing the Fermi gas to a model with $2N$ hyperfine states (with Sp($2N$) symmetry). We show that for $N=\infty$, the model can be solved exactly by the BEC-BCS mean field solution. In order to address the physically relevant problem ($N=1$), we perform a systematic $1/N$ loop expansion around the BEC-BCS solution. For $N=1$, we obtain a variety of thermodynamic quantities, including the energy, the pairing gap, and the upper critical polarization. We compare our results to experimental data and other theoretical approaches.   

Four-fermion problem in hyperspherical coordinates

The four-particle system is the simplest few-body system which contains the fundamental physics involved in ultracold fermionic gases. We solve the quantum four-body problem in the adiabatic hyperspherical representation. Our approach yields a set of coupled-channel equations which can in turn be solved for all elastic and inelastic processes. These rates are expected to play an important role in the lifetime of molecules in ultracold fermi gases. This provides insights into the nature of ultracold fermi systems and the physics of the BCS-BEC cross-over.   

Correlation effects in the BCS/BEC crossover.

We use imaginary-time propagation to find zero-temperature ground states in the BCS/BEC crossover. A cumulant expansion allows us to systematically include higher-order interactions between bosons and fermions. In particular, we calculate the Hartree term across the resonance and show how to correctly describe the dimer-dimer scattering on the BEC side.   

Pairing and Superfluid Properties of Dilute Fermion Gases at Unitarity

We study the pairing and superfluid properties of a dilute gas of fermions in 3-dimensions with attractive interactions tuned to the unitarity point [1]. The finite temperature, non- perturbative, Restricted Path Integral Monte Carlo (R-PIMC) method is used for our simulations and tested against previous ground-state Quantum Monte Carlo calculations. From the growth of the density correlations for unequal spins, we identify the pseudogap crossover temperature scale T*~0.70 Ef below which pairing correlations develop. We estimate the critical temperature for condensation Tc~0.24 Ef from a finite size scaling analysis of the superfluid density. The pseudogap phase is characterised by the spin susceptibility and compressibility. We will also present results for unequal populations of fermions. \newline \newline [1] V. Akkineni, N. Trivedi, D.M. Ceperley, cond-mat/0608154   

Exact Relations for A Strongly-Correlated Fermi Gas With Large Scattering Length

A 2-component Fermi gas with a large and tunable scattering length $a$ is considered. If the interfermionic forces have a range much shorter than the average interparticle spacing, the characteristic de Broglie wavelength, and $\mid\! a\!\mid$, the system is in a universal regime in which the interaction is described by a single parameter, $a$. We show that the energy, the momentum distribution, the pressure, the change of energy during a real-time ramp of the scattering length, and the energy spectrum of such a Fermi gas satisfy a few simple \emph{exact} relations. The importance of the $C/k^4$ tails of the momentum distributions at large $k$ is stressed. Implications of these results for experiments on ultracold atomic Fermi gases near Feshbach resonances are discussed.   

Dilute Bose and Fermi gases with large generalized scattering lengths

Dilute weakly-interacting Bose and Fermi gases can be described to a very good approximation by a single atomic physics parameter, the $s$-wave scattering length. Utilizing broad Feshbach resonances, strongly-interacting two-component Fermi gases with infinitely large interspecies scattering lengths can now be studied experimentally. In this so-called unitary regime, the only remaining energy scale is the energy $E_{FG}$ of the non-interacting Fermi gas, and it has been shown that the energy of the Fermi gas becomes about $0.44 E_{FG}$. We investigate Bose and Fermi gases with non-vanishing angular momentum using the lowest order constrained variational method. In particular, we focus on the regime where the generalized scattering length becomes infinite. For example, we show that the energy of $d$-wave interacting fermions depends not only on $E_{FG}$ but additionally on an energy scale set by the range of the underlying two-body potential.   

Luttinger theorem and Fermi liquid behavior close to a Feshbach resonance

Based on the results obtained in a previous paper \thanks{cond-mat/0505306}, we derive the thermodynamic properties of a Fermi gas, deep into the quantum degenerate regime and provide a useful test for the validity of Luttinger theorem. We show that, if Luttinger theorem holds, a first order phase transition has to occur in the normal phase as a function of the interaction strength, U, as a consequence of a jump occurring in the compressibility, spin susceptibility and specific heat. The signature of the transition is given by the presence of a non-zero latent heat. We also show that a volume change occurs at finite temperatures from the BEC to the BCS side of the Feshbach resonance, in the normal phase. The transition has an end point close to the BCS critical temperature. Thus, observation of these properties will require suppression of the superfluid phase. Also we demonstrate that a paramagnetic system in equilibrium, close to a diverging scattering length, expels any applied magnetic field and as a consequence, there is no Clogston limit in the in the superfluid phase.   

Collisional hydrodynamic mode frequencies in the BCS-BEC crossover near unitarity

In the collisional region at finite temperatures (produced by the large value of the $s$-wave scattering length), the collective modes of a superfluid Fermi gas are expected to be described by the Landau two-fluid hydrodynamic equations. These equations predict two types of modes: an in-phase oscillation of the normal and superfluid components as well as an out-of-phase oscillation. We prove that at unitarity and at all temperatures, the in- phase breathing mode solution of the two-fluid equations has a frequency identical to that calculated at $T=0$ by Cozzini and Stringari. This temperature-independence has been verified in recent experiments by Thomas and coworkers. For the special case of an isotropic trap, we find the temperature-independent frequency $\omega = 2\omega_0$, a result predicted to be valid under all conditions at unitarity by Castin. We also discuss the more interesting finite-$T$ out-of--phase (the analogue of second sound) breathing mode frequency given by the Landau-two- fluid equations at unitarity.   

Suppression of $T_c$ by Medium Effects from Dilute to Dense Regime: A Crossing-symmetric Approach

We study medium effects on superfluid transition temperature of 1- and 2-component strongly correlated Fermi systems. A crossing-symmetric approach allows us to explore this across dilute and dense regimes within a single framework. We include many-body effects such as density, spin-density, and current fluctuations. Pairing interactions are deduced from scattering amplitudes in the pairing channel. For 2-component systems, we find the known factor-of-2 suppression in $T_c$ to be robust across both regimes, except near the unitarity limit, where the suppresion is more pronounced. For the 1-component case, the suppresion can be greater, and not universal across the regimes. We discuss possible physical causes for the $T_c$ suppression.   

Quantum fluctuations in the superfluid state of the BCS-BEC crossover

While the Leggett-BCS mean-field approach gives a reasonable `zeroth order' description of the superfluid ground state in the BCS-BEC crossover, there are many ways in which it is inadequate. In addition to quantitative discrepancies with quantum Monte Carlo and experimental results at unitarity and with exact results for dimer-dimer scattering in the BEC limit, the mean field theory also misses the qualitative effects of quantum depletion of the condensate in a strongly interacting Fermi system. To address these concerns, we include the effects of zero-point motion of collective excitations and of the pair continuum in calculating various ground state properties. We implement this RPA in a functional integral formalism which ensures that the feedback of the collective modes on the saddle point respects Goldstone's theorem. We will present results on the ground state energy, gap, compressibility and collective mode frequency as a function of $1/k_f a_s$.   

The universal phase diagram of fermionic quantum liquids near the unitarity limit

We consider several models of particles with short-range attractive interactions whose universal properties are controlled by an unstable renormalization-group fixed point at zero density and temperature. The fixed point corresponds to the Feshbach resonance, and relevant perturbations are the detuning of the resonance, and parameters that control the particle densities. Some critical exponents are determined exactly as expansions about two and four spatial dimensions. The existence of a renormalization-group fixed point implies a universal phase diagram as a function of density, temperature, population imbalance, and detuning. We study this phase diagram in the context of BEC-BCS crossover of s-wave paired fermions. We develop a 1/N expansion, based upon models with Sp(2N) symmetry, and use it to systematically analyze the universal properties of interacting fermions near the unitarity limit. This approach overcomes several limitations of the expansions about two and four dimensions, and allows a well controlled exploration of the full phase diagram of imbalanced fermion populations in the experimentally relevant three-dimensional space.   

Properties and dynamics of four particles in a trap in the BCS-BEC crossover.

The Hamiltonian of two spin up and two spin down fermions in a trap is calculated using a correlated gaussian basis in the vicinity of the BCS-BEC crossover. From the spectrum, key properties of the few-body system are deduced as a function of the 2-body scattering length. After a diabatization procedure, the wavefunctions are used to evolve in time an initial configuration, mimicking molecule formation experiments with Fermi gases in the BCS-BEC crossover. The dynamics are successfully modeled as a sequence of Landau-Zener transitions. Finally, atom-molecule coherent quantum beats in this system are studied and a ramping scheme is proposed for experimental investigation.   

Describing the degenerate Fermi in a renormalized hyperspherical treatment

We describe the degenerate Fermi gas with zero-range density-dependent renormalized interactions (eprint cond-mat/0610848) in an isotropic trap using a variational hyperspherical approach. This method reduces the complex many body Hamiltonian to a simple one-dimensional effective potential in a collective coordinate, the hyperradius, which can be thought of as the rms size of the gas. Exploring the behavior of the effective potential in the unitarity region where the two-body scattering length becomes very large and negative produces interesting effects. The low energy collective excitation frequency of a two spin component gas approaches the noninteracting frequency, as has been seen in hydrodynamic treatments. For larger numbers of spin components an interesting dynamical instability develops.   

Session L33: Focus Session: Superconducting Qubits III

Novel quantum transport in superconducting phase-qubit arrays.

The dramatic increase of coherence times in superconducting phase qubit experiments allows the exploration of multi-qubit quantum dynamics. In this talk I theoretically explore the controlled propagation of excitations in capacitively-coupled phase qubits. By exploiting the tunability and flexible topology of phase-qubit arrays, this artificial solid can demonstrate novel quantum transport effects such as perfect state transfer. These ideas are confirmed by multi-qubit, multi-level simulations including the effects of long-range couplings, disorder, and decoherence.   

Josephson Phase Qubits with Hydrogenated Amorphous Silicon Dielectric

The lifetime of Josephson phase qubits is limited by the presence of two-level defect states in the dielectric material of the qubit. Improvements in the loss tangents of dielectric materials have resulted in substantial gains in qubit lifetime by reducing the number of such defects. Measurements of the loss tangent of hydrogenated amorphous silicon indicate at least a five-fold decrease in the loss tangent compared to our current SiN dielectric, making a-Si:H a promising candidate dielectric for use in phase qubits. We discuss the incorporation of a-Si:H dielectric into our qubit fabrication process, and present measurements of the energy decay and dephasing lifetimes of qubits made with this material.   

Reducing defects in Josephson phase qubits -- interdigitated capacitors and microbridges

Josephson phase qubits have recently demonstrated increased coherence times, setting them as a serious option for scalable quantum computing. This has been made possible by identifying dielectric two-level defect states in the Josephson junction and in any additional capacitance in the circuit as a major source of decoherence. We show that by fabricating an external, high quality interdigitated capacitor the lifetime of the qubit is increased to at least half a microsecond. Further reduction in decoherence is expected by completely removing the dielectric of the tunnel junction and replacing it with a superconducting microbridge. Some preliminary results for MBE grown Rhenium microbridge qubits will be presented.   

Precise measurements of single gate errors in Josephson phase qubits

As Josephson phase qubits continue to improve, the accuracy of qubit manipulations become increasingly important. We have built a new generation of control electronics to generate microwave pulses with an accurate Gaussian-envelope for single qubit logic gates, using a modular and card-rack design to promote scalability. To understand the tradeoff between accuracy and speed, we plan to present an experiment that measures gate errors versus the width of the applied pulses. We intend to more precisely measure the accuracy of single qubit rotations using multi-pulse sequences, similar to that already done for ion-trap qubits.   

Attempt to Violate the CHSH Bell Inequality in Josephson Phase Qubits

The violation of Bell's inequality is the primary argument against the possible existence of a hidden-variable-theory as an alternative to quantum mechanics. It also often serves as a convincing demonstration that a given system behaves in a truly non-classical way. There have been many proposals of different classically binding inequalities that quantum mechanics can violate. The most widely accepted forms follow closely along a correlation measurement proposed by Clauser, Horne, Shimony and Holt (CHSH) in 1969. Here we present our attempt to implement the CHSH Bell test using Josephson phase qubits. The nature of this experiment places high demands –-- compared to the current state of the art in solid state qubits –-- on qubit performance measures such as the energy relaxation time T1, the decoherence time T2, measurement fidelities, and the quality of single and two qubit operations. We will examine these demands and position our past and current qubit designs against them.   

Quantum kinetics of a Josephson phase qubit continuously monitored for escape.

Inspired by recent experiment [1] on partial measurement of a Josephson phase qubit, we consider evolution of a qubit in a metastable potential being continuosly monitored for escape. Assuming that the continuous measurement may induce incoherence both in the tunneling reservoir and in the tunneling matrix elements but not in the qubit itself, we discuss the conditions for the qubit to retain coherence. We argue that qubit state remains pure as long as the tunneling event is never reverted, that is, the tunneling from the reservoir back to qubit state is suppressed. Such a suppression may happen due to the choice of system parameters (e.g., for nearly continuous spectrum in the tunneling reservoir), or dynamically due to the properties of coherent or incoherent evolution in the reservoir. We illustrate these scenarios by numerical simulations and with an analytical model where the exact solution of the master equation gives no decoherence of the qubit over a finite time interval. \medskip\par\noindent [1] N. Katz {\it et al.}, Science {\bf 312}, 1498 (2006).   

Crossover of phase qubit dynamics in presence of negative-result weak measurement

Coherent dynamics of a superconducting phase qubit is considered in the presence of both unitary evolution due to microwave driving and continuous non-unitary collapse due to negative-result measurement. In the case of a relatively weak driving, the qubit dynamics is dominated by the non-unitary evolution, and the qubit state tends to an asymptotically stable point on the Bloch sphere. This dynamics can be clearly distinguished from conventional decoherence by tracking the state purity and the measurement invariant (``murity''). When the microwave driving strength exceeds certain critical value, the dynamics changes to non-decaying oscillations: any initial state returns exactly to itself periodically in spite of non-unitary dynamics. The predictions can be verified using a modification of a recent experiment.   

Temperature Dependence of Rabi Oscillations in Phase Qubits

Using the experimental setup in Erlangen, we compared aluminum-based phase qubits with SiN$_x$ shunting capacitors made at UCSB with similarly designed circuits fabricated at HYPRES foundry using a standard niobium-based fabrication process with SiO$_2$ insulation. Measured decoherence times are about 100 ns and 5 ns, respectively. In both types of circuits, energy relaxation time $T_1$ scales inversely proportional to the area of the qubit junction, which agrees with earlier data. Rabi oscillations remain visible up to the temperature $T$ of about 400 mK (UCSB) and 800 mK (HYPRES), where the energy level separation becomes comparable with $k_{\rm B}T$. The current pulse readout in the upper temperature range is dominated by thermal escape rather then tunneling. Temperature dependence data for the decoherence time and oscillations contrast will be presented and discussed.   

A dc-SQUID phase qubit

A current and flux biased dc-SQUID behaves as a quantum particle trapped in a cubic-quadratic potential well. Resonant transitions between the ground and first excited states are induced by the application of microwave current or flux pulses. Measurement is performed by an adiabatic nanosecond flux pulse which projects the excited levels of the quantum particle onto the voltage state of the SQUID. Rabi-like coherent oscillations have been observed with a decay time $\tau \simeq 20$ ns [PRL 93, 187003]. The dominant source of this decoherence was thermal current fluctuations [PRB 73, 180502]. We propose operation of this circuit as a qubit at an optimal point where it is insensitive to these current fluctuations to first order. Preliminary measurements show an increase in $\tau$ by a factor of 5.   

Quantum Behavior of the dc SQUID phase qubit.

We analyze the quantum behavior of a SQUID phase qubit in which one junction acts as a qubit while the other filters out any external low frequency bias current noise. We solve Schr\"{o}dinger's equation for the two dimensional Hamiltonian of the system at zero temperature assuming no dissipation. We obtain the states and from these the energy levels, tunneling rates, and expectation values of the currents in the junctions. We use these results to show how this design isolates the system from noise without affecting the essential nature of the qubit. This work is funded by the NSA, NSF Grant EIA 0323261, and the Center for Superconductivity Research.   

Strong Field Effects in Rabi Oscillations of the dc SQUID Phase Qubit

In the phase qubit, Rabi oscillations between the two lowest metastable zero-voltage states can be driven with a microwave current. At the high microwave powers needed to perform fast single-qubit operations, multilevel and multiphoton effects lead to an ac Stark shift of the resonant drive frequency and modification of the Rabi frequencies. We have observed these effects in an asymmetric Nb/AlOx/Nb dc SQUID at 25 mK, where one junction (with a roughly 20 $\mu$A critical current) behaves as a phase qubit and the other provides isolation from the bias line. We found quantitative agreement between experimental results and theoretical predictions obtained with a three-level multiphoton analysis.   

Evidence of Microstates in dc SQUID Phase Qubits

We report experimental results consistent with external quantum systems coupling to a Josephson junction phase qubit. When the energy level spacing for the qubit is made equal to that of the fixed external system the coupling lifts the degeneracy. By applying microwaves to excite transitions in the qubit, we are able to map out the splittings in the spectrum due to the coupling. This effect has been seen in both an Al/AlOx/Al and a Nb/AlOx/Nb dc SQUID phase qubit. This work is supported by the NSA, NSF Grant EIA 0323261, and the Center for Superconductivity Research.   

In situ variation of the coupling of a dc SQUID phase qubit to its bias leads

In dc SQUID phase qubit[1], one junction (Al/Al$_2$O$_3$/Al or Nb/Al$_2$O$_3$/Nb) acts as an ideal phase qubit and the rest of the SQUID which includes a second junction acts as an inductive isolation network. The Josephson inductance of the isolation junction was varied by changing its bias current, allowing in situ control of the coupling between the qubit junction and the leads. Measurements of the tunneling escape rate showed excess tunneling events due to high-frequency noise exciting the qubit junction out of the ground state $|0\rangle$. The impedance of the isolation junction becomes infinite at its resonance frequency where the isolation fails and the isolation network lets noise in to the qubit junction. Analysis of the data taken at 80 mK reveals that excess tunneling was largest when the $|0 \rangle$ to $|1\rangle$ resonance frequency (~10 to 15 GHz) of the isolation junction equaled the $|0\rangle$ to $|2\rangle$ or the $|1\rangle$ to $|3\rangle$ transition frequency of the qubit junction. [1] J. M. Martinis, et al. Phys. Rev. Lett. 89, 117901 (2002)   

Improving Superconducting Phase Qubits with Low-Loss Vacuum-Gap Capacitors

Significant progress has been made in eliminating sources of decoherence in superconducting qubits by carefully selecting, manipulating and engineering materials used in fabrication. Dielectrics in and around a qubit remain a major source of decoherence. By decreasing the size of a Josephson junction (JJ) one can reduce the number of decoherence-causing spurious two level systems. However, in order to maintain a typical phase qubit operation frequency, one has to shunt the JJ with a capacitor. We have fabricated structurally robust parallel plate capacitors in which lossy dielectrics are replaced by vacuum. Our LC oscillator measurements show that the loss tangent of the vacuum-gap capacitor is significantly lower than that of SiO2 and SiNx capacitors. Vacuum-gap capacitor fabrication has been integrated with phase qubit fabrication. We also show that our vacuum-gap technology can be used to fabricate on-chip wiring crossovers without dielectrics and vacuum suspended qubit junctions.   

Optimization of Silicon Nitride Films For Use in Phase Qubits

The lifetime (coherence time) of superconducting phase qubits is currently severely limited by lossy materials used in standard fabrication techniques. In particular, the insulator material - typically Silicon Nitride - used to isolate and physically separate different layers of the qubit is of interest. We have conducted a fractional factorial design experiment to optimize SiNx loss properties with respect to several deposition parameters in an Electron Cyclotron Resonance (ECR) Plasma-Enhanced Chemical Vapor Deposition (PECVD) reactor. Our experimental design included a three-level, four-parameter matrix with N2/SiH4 ratio, microwave power, rf power, and pressure as the parameters. The test-bed for these films is a low temperature microwave LC resonator circuit in which the various insulator films are used as the dielectric between a parallel plate capacitor and the Q (Quality Factor) of the circuit gives the relevant loss information for qubit operations.   

Session L34: Focus Session: Virus-Inspired Supramolecular Structures

Size regulation of ss RNA viruses

Under the right circumstances, single-stranded RNA viruses self assemble spontaneously from aqueous solutions containing the subunit proteins and genome molecules. While a monodisperse size distribution is common for most icosasedral viruses, the size of the spherical viral shells can vary from one type of virus to another. We study the effect of genome length, genome concentration and protein concentration on the size of spherical viral capsids in the absence of spontaneous curvature and bending energy. We find that based on the size of genome, it could be advantageous to have relatively small spherical shells with higher curvature rather than bigger and thus flatter shells. Furthermore, we find that the small ratio of genome to protein concentration could, quite interestingly, result in larger spherical shells. Experimental data on the encapsidation of model genome supports these findings.   

Synthesis and properties of virus-like particles

The principles underlying self-assembly of virus-like particles (VLP), which are composed of an icosahedral virus protein coat encapsulating a nanoparticle core are discussed. Such VLPs have potential practical utility as biomedical imaging and sensing tools, as novel functional materials, and as experimental models for molecular self-assembly of quasi-spherical molecular cages. Moreover, we show that, as a consequence of their regular protein surface, VLPs readily form three-dimensional crystals having optical properties influenced by multipolar plasmonic coupling.   

Soft modes near the buckling transition of icosahedral shells

Closed shells comprised of pentamers and hexamers may be smooth and nearly spherical, or sharply faceted and icosahedral, depending on the elastic constants of the shell. We interpret the transition from smooth to faceted as a soft-mode transition. Our analysis is based on the phonon spectrum of a simplified mass-and-spring model of the shell. In contrast to the case of a disclinated planar network, where the transition is sharply defined, the mean curvature of the sphere smooths the transition rather like a magnetic field smears out a ferromagnetic phase transition. We define susceptibilities of the transition as the response to forces applied at vertices, edges and faces of an icosahedron. At the soft-mode transition the vertex susceptibility is largest, but as the shell becomes faceted the edge and face susceptibilities greatly exceed the vertex susceptibility.   

A Precise Packing Sequence for Self-Assembled Convex Structures

We present molecular simulations of the self-assembly of cone-shaped particles with patchy, attractive interactions[1,2]. Upon cooling from random initial conditions, we find that the cones self assemble into clusters and that clusters comprised of particular numbers of cones have a unique and precisely packed structure that is robust over a range of cone angles. These precise clusters form precise packing sequence that for small sizes is identical to that observed in evaporation-driven assembly of colloidal spheres. This sequence is reproduced and extended in simulations of two simple models of spheres self-assembling from random initial conditions subject to convexity constraints, and contains six of the most common virus capsid structures obtained in vivo including large chiral clusters, and a cluster that may correspond to several non- icosahedral, spherical virus capsid structures obtained in vivo. For prolate spheroidal convexity conditions, we demonstrate the formation of several prolate virus structures from self-assembling hard spheres[3]. \newline [1] Chen T, Zhang ZL, Glotzer SC, PNAS, in press (http://xxx.lanl.gov/pdf/cond-mat/ 0608592) [2] Chen T, Zhang ZL, Glotzer SC, http://xxx.lanl.gov/pdf/cond-mat/0608613 [3] Chen T, Glotzer SC http://xxx.lanl.gov/pdf/q-bio.BM/0608040   

Charge profiles inside sigle-stranded viruses

Many single stranded viruses pack their genome using flexible peptide arms of capsid proteins. Genome binding may then be mapped onto interaction between polyelectrolyte and charged brush. In this talk we pursue an electrostatic model of genome binding and address several questions on charge profiles inside capsids.   

Buckling and Mechanical Failure of Viral Shells

We present a combined theoretical and experimental study of the structural failure of viral shells under mechanical stress due to indentation by atomic force microscopy. Modeling the indentation of icosahedral viruses with two-dimensional continuum shell elasticity theory, we find that the fivefold-symmetric disclinations precipitate geometric ``buckling'' instabilities, leading to structural collapse at indentation loads that are significantly lower than those which buckle perfectly spherical shells. Coincident with these instabilities, discontinuities in the force-indentation curve appear when the so-called F\"oppl-von K\'arm\'an (FvK) number exceeds a critical value. A nano-indentation study of a viral shell subject to a soft-mode instability, where the stiffness of the shell decreases with increasing pH, confirms the predicted onset of failure as a function of the FvK number.   

Using The Interfaces In Self-Assembled Protein Cage Architectures For Materials Synthesis

The self-assembled architectures of viral capsids have been used as models for understanding processes of encapsulation of both hard and soft materials. We have explored modifications to the exterior and interior interfaces of viral (and other protein cage architectures) while maintaining the assembly of stable icosahedral capsid particles. This has allowed us to utilize the high symmetry of the viral capsid to engineer unique functionality for highly ordered multivalent presentation for controlled nucleation of hard inorganic materials and packaging of soft organic materials. Of particular interest is the nature of the hard-soft interface in these systems. Through the incorporation of peptides derived from phage display we can direct the nucleation and growth of specific inorganic phases, constrained within the protein cage architecture. The coupled synthesis of cage-constrained ferrimagnetic and antiferromagnetic nanoparticles results in formation of stable composites that exhibit unique exchange bias magnetic coupling. To understand the role of the protein in directing inorganic materials synthesis, we have probed the protein-mineral interface using genetic and chemical modifications, spatially controlled inorganic synthesis, high-resolution transmission electron microscopy, and cryo-electron microscopy and image reconstruction. The role of protein interfaces in these assembled protein cage architectures has been explored to understand and exploit packaging of a wide range of materials as diverse as nucleic acids, drugs, and inorganic nano-materials.   

Direct measurement of the elastic properties of the Wiseana Iridovirus (WIV) capsid using Brillioun Spectroscopy

Viral capsids are of great interest for their potential as templates or scaffolds to direct the growth of secondary structures for various sensing, energy harvesting, and photonic devices. However, due to their size (10's-100's nms) and complex structure (symmetrically repeating protein subunits); the mechanical properties of viruses and viral films has yet to be directly measured. We measured the phononic spectra of virus capsids assembled on silicon substrates using Brillioun Light Scattering at different scattering wave vectors. The phononic spectrum provides a direct measurement of the mechanical properties of individual viruses as well as that of the collective assemblage. The spectra are analyzed to understand the origins of both the propagating phonons as well as those that remain localized within individual viruses. Understanding the mechanical properties of the viruses is critical for the reliable utilization of viral technologies, as well as contributing to the understanding of the impact of capsid flexibility and rigidity on cellular infection by viruses.   

Viral Capsid Assembly in a crowded environment

While many experimental and all theoretical studies of viral capsid assembly dynamics focus on assembly in dilute solution, viruses replicate in the cell, which presents a crowded environment composed of numerous confining sub-volumes. We examine the effects of crowding and confinement on the formation of T1 capsidlike objects by using Newtonian dynamics simulations[1]. Subunits have excluded volume and asymmetric pairwise bonding interactions between complementary sides [1] and are confined to a three-dimensional box. We address the effects of finite system size on assembly dynamics by varying the system size with a fixed volume fraction of capsid subunits, and by varying the system size with a fixed number of subunits. In both cases, we find a non-monotonic variation in capsid formation times as the system dimensions become comparable with the size of a capsid. By analyzing assembly mechanisms, we probe the nature of assembly in crowded and confined environments. This work is supported by NSF DMR-0403997. \newline [1] M. Hagan and D. Chandler, Biophys. J. v 91, 2006   

Microrheology of Viscoelastic Shells: Applications to Viral Capsids

We study the microrheology of nanoparticles shells [cite: Dinsmore et al. Science 298, 1006 (2002)] and viral capsids by computing the fluctuation spectrum of a viscoelastic spherical shell that is permeable to the surrounding solvent. We determine analytically the overdamped dynamics of the shear, bend, and compression modes of the shell coupled to the solvent both inside and outside the sphere in the zero Reynolds number limit. In this talk we identify fundamental length and time scales in the system, and compute the thermal correlation function of displacements of antipodal points on the sphere. We describe how such an antipodal correlation function, which should be measurable in new AFM-based microrheology experiments, can probe the viscoelasticity of these synthetic and biological shells constructed of nanoparticles. We then discuss some of the remaining challenges in interpreting these measurements.   

Session L35: Metalloproteins: Theory and Experiment

Hartree - Fock study of the Heme Unit of deoxy-hemoglobin for Hyperfine Interactions and Vibrational Properties.

The electronic structure of the Heme Unit of deoxy- Hemoglobin has been studied by the Hartree- Fock - Roothaan procedure for understanding the hyperfine interaction properties of the $^{57m}$Fe nucleus and vibrational properties associated with Fe and proximal imidazole. Results will be presented for the $^{57m}$Fe nucleus, including the isomer shift in Mossbauer spectroscopy, magnetic hyperfine and nuclear quadrupole interactions and for the Fe-N$_{\varepsilon }$ vibrational frequency. Comparisons will be made with available experimental data and possible further investigations will be discussed.   

DFT Studies of NO Activation of Heme Proteins.

Many important cardiovascular and neural system processes are triggered by activation of the enzyme soluble guanylate cyclase (sGC), a sensor of NO widely thought to be activated through binding of NO to heme. Our understanding of these processes will remain incomplete without knowing why NO activates sGC more effectively than other diatomic ligands. We report DFT calculations on various porphyrins and heme protein active sites to test the hypothesis that activation of sGC is associated with disruption of the Fe-histidine bond to the protein. We demonstrate that NO binding significantly weakens this bond. Also, comparing the predicted vibrational spectra of these compounds with nuclear resonance vibrational spectroscopy (NRVS) measurements allows us to identify the Fe- histidine stretching mode, a reaction coordinate for histidine dissociation in NO-ligated heme proteins. Comparison of 5-coordinate and 6-coordinate NO and CO compounds provides additional tests of the hypothesis.   

Ab-Initio-Based Approach to Study Complete Metalloproteins: Divide and Conquer Geometry Optimization of Nitric-Oxide Reductase

The direct application of ab-initio methods (Hartree-Fock or density functional theory) to study complete biomolecules has been impossible due to the huge computational cost of fully quantum mechanical calculations. As an initial step towards overcoming this problem, we implemented an ab-initio-based method to predict geometric structures of large metalloproteins using the principle of ``divide and conquer.'' The method has been applied to small test systems showing satisfactory agreement with all-atom ab initio calculations. We have successfully applied the divide and conquer approach to partially optimize the geometry of a ligand-enzyme system, namely NO binding to nitric-oxide reductases (NOR, P450nor). NOR is a metalloenzyme that catalyzes the reduction of NO to N$_2$O. To compare our results with all atom calculations we studied a biochemically relevant subsystem (375 atoms) of the ligand-enzyme complex. The deviation between the divide and conquer geometry and the all atom partial geometry optimization is minor, on order of 10$^-1$ {\AA} for bond lengths. The computational cost of the method is moderately expensive making its application to large (bio) molecules plausible. Supported by NSF CAREER Award CHE-0349189 (JHR).   

Ab-Initio Based Computation of Rate Constants for Spin Forbidden Metalloprotein-Substrate Reactions

Some chemical and biochemical reactions are non-adiabatic processes whereby the total spin angular momentum, before and after the reaction, is not conserved. These are named spin- forbidden reactions. The application of ab-initio methods, such as spin density functional theory (SDFT), to the prediction of rate constants is a challenging task of fundamental and practical importance. We apply non-adiabatic transition state theory (NA-TST) in conjuntion with SDFT to predict the rate constant of the spin- forbidden recombination of carbon monoxide with iron tetracarbonyl. To model the surface hopping probability between singlet and triplet states, the Landau-Zener formalism is used. The lowest energy point for singlet-triplet crossing, known as minimum energy crossing point (MECP), was located and used to compute, in a semi-quantum approach, reaction rate constants at 300 K. The predicted rates are in very good agreement with experiment. In addition, we present results for the spin- forbidden ligand binding reactions of iron-containing heme proteins such as myoglobin.   

Dependence of Localized Electronic Structure on Ligand Configuration in the [2Fe] Hydrogenase Catalytic Core$^{\ast}$

The [FeFe] hydrogenase enzyme is found in a variety of organisms, including Archaea, Eubacteria, and green algae$^{1,2}$, and crystallographically determined atomic position data is available for two examples. The biologically unusual catalytic H-cluster, responsible for proton reduction to H$_{2}$ \textit{in vivo}, is conserved in the known structures and includes two \textit{bis}-thiolato bridged iron ions with extensive cyano- and carbonyl ligation. To address the configurational specificity of the diatomic ligand ligation, density functional theoretical calculations were done on [2Fe] core models of the active center, with varying CO and CN$^{-}$ ligation patterns. Bonding in each complex has been characterized within the Natural Bond Orbital formalism. The effect of ligand configuration on bonding and charge distribution as well as Kohn-Sham orbital structure will be presented. [1] M. Forestier, P. King, L. Zhang, M. Posewitz, S. Schwarzer, T. Happe, M.L. Ghirardi, and M. Seibert, Eur. J. Biochem. \textbf{270, }2750 (2003). [2] Posewitz, M.C., P.W. King, S.L. Smolinski, R.D. Smith, II, A.R. Ginley, M.L. Ghirardi, and M. Seibert, Biochem. Soc. Trans. \textbf{33, }102 (2005). \newline $^{\ast}$This work was supported by the US DOE-SC-BES Hydrogen Fuels Initiative, and done in collaboration with the NREL Chemical and Biosciences Center.   

Ab Initio Computation of Spin Orbit Coupling Effects on Magnetic Properties of Iron-Containing Complexes and Proteins

Zero-Field Splittings (ZFS) in metalloproteins and other metal complexes arise from the combined action of crystalline fields acting on the metal valence electrons and spin-orbit coupling (SOC), a relativistic effect. The ab-initio calculation of ZFS parameters of metal-containing (bio)molecules is a challenging computational problem of practical relevance to metalloenzyme biochemistry, inorganic chemistry, and molecular-based bio- nanotechnology. We have implemented a methodology which treats the nonrelativistic electronic structure of magnetic (bio) molecules within the framework of spin density functional theory (SDFT) and adds the relativistic effects of SOC via perturbation theory (PT). This combined SDFT-PT approach allowed us to compute the ZFS parameters of iron-containing complexes and non-heme iron proteins with a good degree of accuracy. We also developed a semiquantitative approach to elucidate the physico-chemical origin of the magnitudes of ZFS parameters. We present results for biochemically relevant iron complexes and for nitric oxide-containing non-heme iron proteins, such as isopenicillin N synthase, which have unusually large ZFS. The computed ZFS parameters are in good agreement with experiment. Supported by NSF CAREER Award CHE- 0349189 (JHR).   

F\"{o}rster-type mechanism of the redox-driven proton pump

We propose a model to describe an electronically-driven proton pump in the \textit{cytochrome c oxidase} (\textit{CcO}). We examine the situation when the electron transport between the two sites embedded into the inner membrane of the mitochondrion occurs in parallel with the proton transfer from the protonable site that is close to the negative (inner) side of the membrane to the other protonable site located nearby the positive (outer) surface of the membrane. In addition to the conventional electron and proton tunnelings between the sites, the Coulomb interaction between electrons and protons localized on the corresponding sites leads to so-called F\"{o}rster transfer, i.e. to the process when the simultaneous electron and proton tunnelings are accompanied by the resonant energy transfer between the electrons and protons. Our calculations based on reasonable parameters have demonstrated that the F\"{o}rster process facilitates the proton pump at physiological temperatures. We have examined the effects of an electron voltage build-up, external temperature, and molecular electrostatics driving the electron and proton energies to the resonant conditions, and have shown that these parameters can control the proton pump operation.   

Investigation of copper(II) binding to the protein precursor of Non-Amyloid-Beta Component of Alzheimer Disease Amyloid Plaque

The Non-Amyloid-Beta Component Precursor (NACP) is a natively unfolded synaptic protein that is implicated in Alzheimers and Parkinsons diseases. Its aggregation into fibrillar structures is accelerated by the binding of copper(II). Experimental studies suggest that the dominant copper binding site is located at the histidine residue in NACP. Based on this evidence we assembled a model fragment of the binding site and used DFT to analyze the conformational details of the most probable binding motifs. We investigated the overall conformational effects with classical MD by constraining the copper binding site to the most energetically favorable geometry obtained from the DFT calculations. These results are compared and contrasted with those of the unbound NACP.   

Cooperative binding modes of Cu(II) in prion protein

The misfolding of the prion protein, PrP, is responsible for a group of neurodegenerative diseases including mad cow disease and Creutzfeldt-Jakob disease. It is known that the PrP can efficiently bind copper ions; four high-affinity binding sites located in the octarepeat region of PrP are now well known. Recent experiments suggest that at low copper concentrations new binding modes, in which one copper ion is shared between two or more binding sites, are possible. Using our hybrid Thomas-Fermi/DFT computational scheme, which is well suited for simulations of biomolecules in solution, we investigate the geometries and energetics of two, three and four binding sites cooperatively binding one copper ion. These geometries are then used as inputs for classical molecular dynamics simulations. We find that copper binding affects the secondary structure of the PrP and that it stabilizes the unstructured (unfolded) part of the protein.   

Studies of myoglobin dynamics by dielectric relaxation spectroscopy

Proteins are dynamic molecules and their motions are intimately linked to the fluctuations of their solvent environment. In this work we studied the protein-solvent interactions by measuring the dielectric response of horse myoglobin (Mb) in glycerol/H$_2$O mixtures over a frequency range of 40Hz-110MHz. Two relaxation processes were observed at temperatures above 220K. The high frequency process corresponds to the $\alpha$-fluctuations of the glycerol/H$_2$O solvent and its rates were found to increase slightly at the presence of the Mb protein. The low frequency process, slower by roughly four orders of magnitude, is relevant to Mb motions and absent for the samples without Mb. The temperature dependence of the two processes can be approximated with the same Vogel-Tammann-Fulcher temperature dependence. Preliminary analyses suggest that the Mb-related process is associated with the conformational fluctuations of the whole Mb protein. Such fluctuations require the coordinated motions of surrounding solvent molecules and are thus an example of protein slaving to the solvent fluctuations.   

Vibrational Characterization of Myoglobin Compound II

Compound II intermediates are essential to oxygen activation by heme proteins. The protonation status of the Fe$^{IV}$ oxo fragment is controversial. EXAFS and Raman spectroscopy have long suggested an Fe$^{IV}$=O group, but recent crystal structures show a long Fe-O distance more consistent with a protonated Fe-OH. We use nuclear resonance vibrational spectroscopy (NRVS) to probe the motion of $^{57}$Fe in compound II of horse heart myoglobin(Mb II). Although the NRVS signal is weaker than expected, we clearly identify the Fe-O stretch at 805 cm$^{-1}$, in addition to previously unobserved in-plane Fe vibrations near 360 cm$^{-1}$. Cryogenic Raman measurement on isotopically labeled Mb II reveals that the kinetic energy distribution (KED) of the Fe-O stretch is localized on the Fe-O fragment, with no significant involvement from the putative proton. Comparison with DFT vibrational predictions provide further insight into the character of the observed normal modes. We conclude that the oxo group is not protonated in Mb II.   

Investigations of the 40cm$^{-1}$ mode in hexacoordinated ferric heme systems

The 40cm$^{-1}$ mode dominates the low frequency spectra of most hexacoordinated ferric heme systems investigated to date. For a better understanding and assignment of this mode we have measured the FCS excitation profile of cyanide bound myoglobin, which shows this feature particularly well. We observe a very interesting behavior of the initial phase and the amplitude of this mode which do not fit within the existent theoretical models. The experimental results could be explained if we postulate the existence of a fast non-radiative transition between the nuclear excited and the ground states. There are also arguments that support the existence of a charge transfer band that underlies the Soret band.   

Novel photo-protection mechanism in strongly coupled chlorophyll complexes: triplet excitons in chlorosomes and in artificial chlorophyll aggregates.

Bacteriochlorophyll (BChl) and chlorophyll (Chl) molecules are known to produce highly toxic singlet oxygen due to energy transfer from their excited triplet states to oxygen molecules. The monomeric (B)Chl molecules in a solution photo-degrade within minutes under sunlight. In (B)Chl pigment-protein complexes of photosynthesis, a carotenoid is typically positioned within a distance of 4 {\AA} of individual (B)Chl or antenna arrays, allowing rapid triplet energy transfer from (B)Chl to the carotenoid. Our time resolved and steady state optical experiments reveal that strongly coupled BChl arrays of pigments are inherently protected due to the formation of triplet excitonic states. According to model simulations, the energy of the triplet exciton is substantially lower than that of the triplet state of an individual BChl, dropping below that of singlet oxygen, and blocking the triplet energy transfer to both carotenoid and to oxygen. This effect is observed experimentally in photosynthetic chlorosomes and in artificial BChl complexes.   

Transport dynamics in membranes of photosynthetic purple bacteria

Photo-Syntethic Unit (PSU) of purple bacteria is conformed by three basic constituents: Light Harvesting Complex 2 (LH2) antenna complexes, where chromophores are distributed in a ring in close contact with caroteniods with a function of collecting light; LH1s, ring shaped structures of chromophores which harvest and funnel excitations to the Reaction Centre (RC), where phtosynthesis takes place. Studies concerning a single PSU have been capable of reproducing experimental transfer times, but incapable of explaining the fact that architecture LH2-LH1-RC of phototosynthetic membranes changes as light intensity conditions vary. The organization of antenna complexes in the membranes that support PSU seems to have its own functionality. A hopping model where excitations are transferred within a membrane is used, and populations of RC, LH1 and LH2 are investigated. Different statistics concerning arrival times of excitations that excite a single PSU are considered and compared with the global model where coordinates of a great portion of a membrane are included. The model permits in a classical basis to understand which parameters make photosynthesis in purple bateria efficient and reliable.   

Session L36: Panel Discussion with APS Journal Editors

Panel Discussion with APS Journal Editors

Editors from \textit{Physical Review A, B, E,} and \textit{Letters} will respond to questions and comments (general, not manuscript specific) from the audience. The panel will address current issues facing the journals, including possible enhancements of both content and delivery. It also will discuss long-standing issues related to growth in submissions, such as ways to maintain and improve the efficiency of the selection process and the quality of published articles. The editors will seek opinions on these and other issues, and look forward to a productive exchange.   

Session L38: Focus Session: Advances in Scanned Probe Microscopy I: Low Temperatures, Manipulation,and Optical Methods I

An ultrahigh vacuum, variable temperature scanning tunneling microscope

We will discuss the design and operation of an ultrahigh vacuum, variable temperature (2 K -- 300 K) scanning tunneling microscope (STM) system. The STM has been designed to minimize tip-sample displacements with thermal variation, allowing the tracking of single atoms over a wide range of temperatures. We will first describe STM details such as sample holder and capacitive position sensor design, as well as tip shielding to reduce scanner cross-talk. We will then discuss elements of the support system, including a low-temperature sample storage area to allow quick sample exchanges, a variable temperature cleaver and a novel counterweight system for quickly and safely lifting and lowering the experimental dewar. Finally, we will point out some common problems found in STM systems and show how we diagnosed and solved these problems.   

Scanning tunneling microscopy in high magnetic fields below 1 Kelvin

We have developed a scanning tunneling microscope (STM) which operates in a novel range of experimental parameters: ultra-high vacuum, low temperatures and high magnetic fields. Such operating conditions make the Zeeman energy for a typical magnetic system significantly larger than the thermal energy and hence one can resolve spin excitations in individual magnetic systems. In order to achieve temperatures below 4K we employ a pumped 3-He reservoir where we liquefy the 3-He with Joule Thomson expansion (without the use of a pumped 4-He reservoir). We can routinely operate the STM at 0.6K in magnetic fields up to 7T. It turned out to be surprisingly difficult to vibrationally decouple the STM from the high magnetic field, which was achieved only after investigating the low-temperature magnetic properties of all the components of the STM. This machine has been used for about 5 years to study atomic-scale magnetic systems and some examples will be discussed.   

Construction of a sub-Kelvin ultra-high vacuum scanning tunneling microscope in high magnetic field

A sub-Kelvin ultra-high vacuum (UHV) scanning tunneling microscope (STM) in high magnetic field has been built. The Besocke type scanner is mounted to the He3 pot of a bottom loading UHV compatible helium- 3 cryostat with a 9 Tesla superconducting magnet. The helium-4 reservoirs for the non-bakeable NbTi superconducting magnet and the UHV space are thermally separated in order to achieve UHV condition without overheating the magnet. A two-chamber UHV system creates reliable environment for tip and sample preparation, and surface imaging and characterization. Various atoms and molecules can be deposited at room or low temperatures. The STM system has the unique capability to probe matter at very low temperatures, in high magnetic fields, under ultrahigh vacuum conditions, and with spatial resolution below one nanometer.   

Design of a 20 mK/15 T STM system

We describe the design of a versatile ultra-high vacuum (UHV) STM system capable of ultra low temperatures ($\sim $20 mK) and high magnetic fields (15 T). A bakeable UHV dilution refrigerator (DR) was designed adopting a Joule-Thomson He3 condenser for low-noise closed-cycle operation, while maintaining the option of a traditionally pumped 1 K pot. The entire STM module can be transferred from an upper room temperature chamber, where the sample and tip are easily exchanged, into the DR in UHV. The sample holder has five isolated electrical contacts which are also used for kinematic mounting of the sample. This allows 4 probe electrical measurements to be performed simultaneously with STM measurements for microscopic transport studies. To achieve a stable environment, we use 3 stages of vibration isolation with a combination of active and passive feedback loops. Current progress will be discussed in relation to design goals.   

Atomistic constructions using a scanning tunneling microscope.

We demonstrate an atomic scale construction scheme, which is performed at an area as small as a few tens of nanometer square. In this atomic scale construction site, all the basic building blocks, single atoms, are extracted locally from the substrate using a scanning-tunneling-microscope tip. These extracted atoms are then precisely positioned on the surface to form desired structures. After the completion of the construction, the remaining debris are removed and the undesired holes near the construction site are filled with atoms/clusters to tidy up the area. This entire construction scheme closely resembles our real world construction process and can be considered as its atomic scale analog. This work is supported by NSF grant DMR-0304314 and US-DOE grant DE-FG02-02ER46012.   

Scanning Tunneling Microscope Manipulation of $\beta$-Carotene on Au(111) at 4.6 K

The properties of isolated and clustered $\beta $-carotene molecules adsorbed on a Au(111) surface were investigated using a low temperature scanning tunneling microscope (STM) at 4.6 K in an ultra-high-vacuum condition. A sub-monolayer coverage of all trans- $\beta $-carotene molecules were deposited on Au(111) via thermal evaporation using a custom-built Knudson cell. On Au(111), the $\beta $-carotene molecules can be found as a form of a cluster, as well as, isolated single molecules. Furthermore, the $\beta $-carotene molecules can have both trans and cis conformations on this surface. In order to probe the mechanical stability of the molecules and molecular clusters, we employ STM manipulation procedures. Lateral manipulations of the molecules across the surface with the STM-tip reveal that the molecules are rather stable. Furthermore, the STM manipulation experiments on $\beta $-carotene clusters often result in lateral displacement of the entire cluster indicating strong interactions between the neighboring molecules within the cluster, but a weak molecule-surface interaction. Moreover, by injecting the tunneling electrons into the molecules, the rotation of a cis $\beta $-carotene has been able to induce in a controlled manner on the surface. This work is supported by the US Department of Energy, Basic Energy Sciences grant number DE-FG02-02ER46012.   

Vertical Atom Manipulation on GaN(000$\overline 1 $) Surface at Low Temperature

We report single atom manipulation on a GaN(000$\overline 1 )$ surface at 4.6 K by using a low temperature scanning tunneling microscope (STM) tip. The nitrogen polar Ga rich GaN samples are grown on sapphire substrate by using r.f. N-plasma molecular beam epitaxy (MBE). Low temperature STM images of GaN (000$\overline 1 )$ surface reveal a novel reconstruction with a basis of 12 x 12 unit cell. For the manipulation experiment, the STM tip is first coated with Ga atom by using a controlled tip-sample contact. Using a vertical manipulation technique with the STM-tip, individual Ga atom from the tip is transferred to the GaN (000$\overline 1 )$ surface on one atom-at-a-time basis. The successful atom deposition is conformed by subsequent STM imaging. Here, the controlled STM tip-sample contact plays a crucial role in an atom deposition process. This procedure allows construction of nanostructures on a MBE grown semiconductor surface with atomic scale precision. This work is financially supported by a NSF-NIRT grant no. DMR-0304314.   

Design and Construction of a UHV-LT-STM for Tip-Enhanced Optics.

The combination of optical techniques and scanning tunneling microscopy (STM) provides insight into a diverse set of physical processes including surface chemistry, surface-photon interactions, and spin scattering in semiconductors. We have designed and built a novel, cryogenic temperature, ultrahigh vacuum STM which utilizes a maneuverable high numeric aperture lens in proximity to the tunnel junction. The microscope currently operates at a base temperature of 12.5 K with 10 pm tip stability. Our initial efforts are focused on studies of photo-chemical reactions and chemical identification by tip-enhanced Raman spectroscopy (TERS). Chemically-etched Ag tips are optimized for field enhancement with characterization by scanning electron microscopy and collection of the plasmon emission from the tip. The optical setup for TERS has been tested utilizing the surface-enhanced Raman signal from the laser dye R6G. The field enhancement of metallic nanostructures can be tuned with atomic manipulation for single molecule spectroscopy and near-field microscopy. http://www.physics.ohio-state.edu/$\sim $jgupta   

Stress Imaging in Indented Si Wafers by Confocal Raman Microscopy

Controlling stress and strain, and consequently, carrier mobility in semiconductor devices is one of the main goals of recent electronic industry. On the other hand, fracture propagation is commonly related to performance degradation in microelectronic and microelectromechanical (MEMS) devices. As miniaturization reaches submicron scales, characterization tools with improved resolution and capable to detect buried surfaces is required. In this work we present confocal Raman imaging in Si wafers to analyze stress and fracture by means of hyperspectral measurements (typically 128x128 spectra). We analyzed indented Si wafers presenting wide range of plastic deformation and fractures. Wide scans (up to 150x150 $\mu $m$^{2})$ as well as high-resolution scans depict the stress distribution around indented regions and side fractures. Some of the samples were covered with 8 nm of Ti deposited in LN$_{2}$ temperature. In these samples we acquired hyperspectral images in subsurface conditions and detected possible influences of thermal budged in the stress distribution. We also demonstrate depth sensitivity in a vertical scan. Images suggest 0.3 $\mu $m resolution.   

Tip Enhanced Raman Scattering of Strained Silicon with Single and Multiple Probe Scanned Probe Microscopes.

Raman spectroscopy is an effective tool for the identification and analysis of molecular components of complex materials. The spatial resolution of Raman spectroscopy is limited by the wavelength of the light. One approach to overcome this drawback is Surface Enhanced Raman Scattering (SERS). This technique uses nanometric interactions between metal structures and surfaces to effect enhancement of the Raman signals. An important mechanism for enhancement originates from an electrostatic lightning rod effect due to the excitation of localized surface plasmon resonances. This is accomplished in a scanned probe microscopy context by employing an ultra-sharp metalized tip that is brought into a focused laser spot on the sample surface thereby enhancing the Raman signal. In this technique also known as Tip Enhanced Raman Scattering (TERS) the electrical field is locally enhanced near the sharp metalized tip. Rastering the sample should then allow for Raman imaging with nanometric resolution. Within this context it will be shown that multiple probe scanned probe microscopes have considerable potential in such tip enhanced applications.   

High Efficiency Surface Plasmon Enhanced Near-field Scanning Optical Microscope Probe Development.

We present results from the development of novel, high throughput, near-field scanning optical microscope (NSOM) probes based on excitation of surface plasmons. The probe consists of an opaque noble metal film with a bullseye grating cavity on the input surface, and a sharp metal post on the output surface. The post is centered inside the inner grating ring and surrounded by a sub-wavelength ring aperture. The grating structure couples incident photons into surface plasmon waves. The transmission efficiency is enhanced for wavelengths where the plasmon is resonant with the cavity. Topographic and optical resolutions are determined by the sharpness of the metal post. This design is anticipated to provide the high spatial resolution of an apertureless NSOM combined with the experimental convenience of an aperture NSOM. Experimental and computational results from test structures will be presented. This material is based on work supported by the National Science Foundation under Grant No. DMI-0522281.   

A Silicon MEMS Probe Integrated with Light Emitting Nanoparticles on Tip for Near-field Scanning Optical Microscopy

We have built a nanoscale light emitting diode (LED) on a silicon MEMS probe for near-field scanning optical microsopy (NSOM). The LED was made of semiconductor nanoparticles electrostatically trapped between a pair of silicon electrodes located on the tip. The probe was microfabricated on a Silicon-on-Insulator (SOI) wafer. The facing electrodes were made by cutting a lithographically patterned device layer using a focused ion beam (FIB). When the voltage was applied, the nanoparticles were polarized and attracted to the gap along the electric field gradient. Basic parameters of a nanoparticle-trapped LED were measured. The probe was attached to a tuning fork and mechanically oscillated. The resonant frequency of the tuning fork was originally about 100KHz and was dampened to 93.0KHz with the probe attached. As the tip approaches the surface of the sample, a drag force acting on the tip changes the oscillating amplitude; measured as a voltage signal from the fork, which in feedback allows the tip to be positioned in the near-field, roughly 5-10nm from the surface. Successful fabrication of the light emitting NSOM probe leads to integrated ``light-source free'' optical scanning arrays suitable for novel applications in nanomaterial characterization and biology.   

Spectroscopic near-field microscopy using frequency combs in the mid-infrared

\newcommand{\wn}{cm\textsuperscript{-1}} We introduce a new concept of spectroscopic scattering-type near- field optical microscopy that records 200 \wn \ broad infrared spectra at each pixel during scanning. Two coherent beams with harmonic frequency-comb spectra are employed, one for illuminating the scanning tip, the other as reference for multi- heterodyne detection of the scattered light. Our implementation yields amplitude and phase spectra centered at 950 \wn (this band can be tuned between 700 and 1400 \wn). A new technique of background suppression is introduced which is enabled by the short, 10 $\mu$s ``snapshot'' acquisition of infrared spectra which allows time-resolving the tapping motion. Thus we demonstrate broad-band mid-infrared near-field imaging that is essentially free of background artefacts.\\ (1) A. Schliesser, M. Brehm, F. Keilmann \& D. W. van der Weide \emph{Frequency-comb infrared spectrometer for rapid, remote chemical sensing} Optics Express, 13, 9029-9038 (2005)\\ (2) M. Brehm, A. Schliesser \& F. Keilmann \emph{Spectroscopic near-field microscopy using frequency combs in the mid-infrared} Optics Express, 14 ,11222-11233 (2006)   

Element specific imaging by STM combined with synchrotron radiation light

Atomically resolved imaging with a capability of elemental identification is one of the ultimate goals in the development of microscopy. Using scanning tunneling microscopy (STM), which provides us atomically resolved surface images, many attempts have been performed for elementally contrasted images. However, since STM basically probes electronic states near the Fermi energy, it is difficult to obtain definite ``fingerprints'' of elements. Here, we report on a new method to obtain elemental information by STM combined with synchrotron radiation light. We found that, by exciting core electrons with the soft-X-ray irradiation and detecting emitted electrons with the STM probe tip, we can obtain X-ray absorption spectra bearing elemental information of the sample. From the photo-induced current measured during the tip scanning over the surface, element specific images were obtained. An estimated spatial resolution of the chemical imaging is less than 20 nm, better than that achieved by photoemission electron microscopy.   

Waveguide Characterization Using Shear Force Scanning Optical Microscopy.

Waveguide characterization is an essential task in the development of photonic integrated circuits for a variety of applications, including biosensors and next generation optical interconnects. A shear force SOM (scanning optical microscope) is being developed for characterizing waveguide evanescent fields as well as scattered light in both the near field and the far field. These methods correspond to photon scanning tunneling microscopy, proximity scanning optical microscopy, and scatter imaging, respectively. Shear force feedback eliminates noise due to scattered light introduced by a second light source required for conventional optical feedback systems based on reflective cantilevers. Additionally, the shear force feedback configuration simplifies raster scanning of the probe rather than the sample allowing easier coupling to multiple waveguides on the sample. The distribution of scattered light intensity can be correlated with features in the evanescent fields that may prove useful for waveguide sensors.   

Session L39: Focus Session: Hydrogen Storage II

NMR Studies of the Li-Mg-N-H Phases.

Solid state NMR including magic-angle-spinning (MAS) and cross-polarization (CP) MAS experiments have been used to characterize various amide and imide phases containing Li and/or Mg. MAS-NMR spectra for the $^{1}$H, $^{6}$Li, $^{7}$Li, and $^{15}$N nuclei have been obtained to improve understanding on formation, processing, and degradation behavior. Only limited information could be obtained from the proton and $^{7}$Li MAS-NMR spectra to due large dipolar interactions and small chemical shifts. However, more success was obtained from the $^{6}$Li and $^{15}$N nuclei although their very long spin-lattice relaxation times did impact signal acquisition times. For example, three distinct $^{6}$Li peaks were resolved from LiNH$_{2}$ phases that were clearly separated from the LiH secondary phase in these samples. While the $^{15}$N spectra for LiNH$_{2}$ phase in isotopically enriched samples exhibited only a single peak at least three distinct $^{15}$N peaks were observed from the similarly enriched Mg amide samples. These differences will be related to crystal structures. The NMR spectra also revealed very little motion in these hydrides upon to nearly 500 K.   

Electronic Structure and Energetics of the Quaternary Hydride \textbf{Li}$_{4}$\textbf{BN}$_{3}$\textbf{H}$_{10}$

Li$_{4}$BN$_{3}$H$_{10}$ has been synthesized recently from LiNH$_{2}$/LiBH$_{4}$ mixtures and its crystal structure determined. We have calculated the electronic structure of this complex hydride and investigated its thermodynamic stability and decomposition energetics. We find that its enthalpy of formation is --708 kJ/mole with respect to the elemental constituents and --6 kJ/mole relative to a 3:1 molar LiNH$_{2}$/LiBH$_{4}$ mixture, in qualitative agreement with experiment. Reaction enthalpies computed for several decomposition pathways suggest \begin{center} Li$_{4}$BN$_{3}$H$_{10} \quad \to $ Li$_{3}$BN$_{2}+\textstyle{1 \over 2}$Li$_{2}$NH + $\textstyle{1 \over 2}$NH$_{3}$ + 4H$_{2}$ \end{center} as the likely dehydriding route.   

Tetragonal I4$_{1}$/amd Crystal Structure of Li$_{3}$BN$_{2}$ from Dehydrogenated Li-B-N-H

We have determined the crystal structure of Li$_{3}$BN$_{2}$ formed by dehydrogenation of LiB$_{0.33}$N$_{0.67}$H$_{2.67}$ from powder x-ray diffraction (XRD) data using the Rietveld method. XRD measurements indicate unambiguously that this Li$_{3}$BN$_{2}$ polymorph is distinct from any of the previously reported Li$_{3}$BN$_{2}$ phases. We find a body-centered tetragonal I4$_{1}$/amd structure (space group No. 141 in the \textit{International Tables for X-ray Crystallography}) with a = 6.60 {\AA} and c = 10.35 {\AA}. The structure features tightly coordinated, nearly linear N--B--N units with 1.3 - 1.4 {\AA} B--N bond lengths suggesting covalently bonded (BN$_{2})^{-3}$ anions. In situ temperature-dependent XRD showed that the body-centered tetragonal Li$_{3}$BN$_{2}$ phase was present both at elevated temperature during dehydrogenation and after cooling to room temperature. We also describe the results of first principles theoretical modeling of the body-centered tetragonal Li$_{3}$BN$_{2}$ polymorph as well as the tetragonal P4$_{2}$2$_{1}$2 and monoclinic P2$_{1}$/c Li$_{3}$BN$_{2}$ structures previously reported. We obtained excellent agreement between the theoretically calculated I4$_{1}$/amd Li$_{3}$BN$_{2}$ lattice constants and atomic positions and those obtained experimentally from XRD. The approximate enthalpy of formation of the I4$_{1}$/amd Li$_{3}$BN$_{2}$ phase is $\Delta {\rm H}$ = --495 kJ/mole-formula unit.   

Ab-initio Investigations of Li and Mg Amide-Imide Systems for Hydrogen Storage

Reversible hydrogen storage in light-element materials has been recognized as one of the most practical approaches for on-board application. Lithium nitride Li$_3$N can reversibly store large amount of hydrogen in the two-step reversible reaction composed of lithium amide LiNH$_2$ and imide Li$_2$NH[1]. Quite recently, in an effort to reach further performance, several types of magnesium substitutions in Li-N-H system have been investigated. For instance, Leng $et$ $al$. have examined a composite material made by ball milling of 3:8 molar mixture of magnesium amide Mg(NH$_2$)$_2$ and lithium hydride LiH[2]. The hydrogenating and dehydrogenating reaction mechanism and fundamental properties of these hydrides still remain as a matter to be investigated. In particular, crystal structures of some metel imides such as Li$_2$NH, MgNH and Li$_2$Mg(NH)$_2$ are not fully determined yet. In this paper, we discuss structural stability and heats of formation of these hydrides from first-principles calculations based on the all-electron FLAPW method. [1] P. Chen Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature 420, 302 (2002). [2] H. Y. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe and H. Fujii, J. Phys. Chem. B 108, 8763 (2004).   

First-principles investigation of the Li-Mg-N-H system

The Li-Mg-N-H system has been identified as a promising hydrogen storage material due to its moderate operation conditions as well as the high capacity and reversibility. The Li-Mg mixed imide is reported to have disordered cation or cation-vacancy arrangements at room temperature and above. We present our first-principles investigation to study the crystal structure of Li$_2$Mg(NH)$_2$ using total energy calculations within the density functional theory. A series of ordered low-energy configurations are identified. Specific local orderings are found in the cation-vacancy arrangement, shedding light on the experimental disordered structure models. A possible ordered phase at lower temperature is proposed based on our calculation. Furthermore, the reaction energetics and phase stability involved in this system are discussed.   

Ab-initio kinetics and thermodynamics studies of ammonia-borane for hydrogen storage

Ammonia-borane (BH$_{3}$NH$_{3})$ is a promising chemical hydrogen storage material given its high gravimetry and volumetric properties. However, the ammonia-borane (AB) thermal hydrogen release is not very efficient, being mainly limited by the kinetics of hydrogenation. Using ab initio calculations, we have investigated the thermodynamics and kinetics of hydrogen release on AB by calculating the free energies of the H$_{2}$ release reactions for different possible decomposition products. Our results indicate that AB regeneration through the ammonia-borane polymeric and borazine-cyclotriborazane cycles is very unlikely due to the strong exothermic character of the reactions. The kinetics of hydrogen release is further investigated with the recently developed metadynamics method. This method allows us to calculate the multidimensional free energy surface of hydrogen release on AB. Our simulations reveal the atomistic mechanism of hydrogenation and provide the free energies barriers and transition states involved in inter and intramolecule H$_{2}$ release on AB.   

pardInvestigation of the Direct Hydrogenation of Aluminum to Alane in Supercritical Fluids

Alane, AlH$_{3}$ has many of the properties that are requisite for materials to be considered viable for onboard hydrogen storage applications. Most notibly, it contains 10.1 wt{\%} hydrogen and undergoes dehydrogenation at appreciable rates at temperatures below 100$^{\circ}$C. However, the very low, $\ge $ 6 kJ/mol, enthalpy of dehydrogenation of AlH$_{3}$ prohibits subsequent re-hydrogenation through standard gas-solid techniques except at very high pressures or very low temperatures. The extremely low solubility of gaseous H$_{2}$ in conventional organic solvents also vitiates a solution-based approach. Re-hydrogenation of Al using a supercritical fluid potentially offers a workable approach since the fluid can act as a solvent, at the same time remaining completely miscible with permanent gases like hydrogen. Recently, it has been found that mixtures of NaH and Al can be hydrogenated to sodium alanate, NaAlH$_{4}$ under modest pressures and temperatures in supercritical fluids. We have now extended these studies to the hydrogenation of Al to AlH$_{3}$. The results of these studies and experimental details will be reported.   

Stability Studies of Aluminum Hydride

Aluminum hydride has attracted research attention recently as a promising hydrogen storage material due to its high gravimetric, volumetric storage capacity and very low enthalpy. AlH3 forms several phases, all of which are sensitive to moisture. In this study, the discharge kinetics of a stabilized form of alpha aluminum hydride newly synthesized was evaluated. Its desorption kinetics were measured in the temperature range of 60-120$^{\circ}$C at one atmosphere of hydrogen pressure. The material was stable at ambient temperature and no significant dehydrogenation was observed at 60$^{\circ}$C after 70 hours. Approximately 10 wt\% hydrogen was rapidly (quantify in wt\%/min.) released at 100$^{\circ}$C with no additional catalization. The activation energy for desorption was measured at 97.0 KJ/mole H2. The surface and bulk characterization methods Auger, SEM, XRD, and solid state NMR were used to investigate the mechanism of stabilization.   

Atomic Simulations of Alane Phase Transformations and Dehydrogenation Mechanisms

Density functional theory atomic ground state, molecular dynamics, and direct method lattice dynamic simulations were used to mechanistically probe phase transformations between the various crystallographically refined $\alpha $, $\alpha $'$_{, }\beta $, and $\gamma $ AlH$_{3}$ phases. Lattice dynamic predictions of the AlH$_{3}$ structures provided an ideal test case for systematically accessing the accuracy of the vibrational thermodynamic property contributions with the harmonic approximation. The predicted transformation pathways involved coordinated tilting and rotation mechanisms, similar to that observed in perovskite structures. Further simulations were conducted to elucidate the mechanism for $\alpha $ AlH$_{3}$ phase decomposition to the Al and H$_{2}$ products and to identify probable barriers to reversible rehydrogenation.   

Trapped H$_{2}$ in AlH$_{3}$

Trapped molecular hydrogen has been discussed for years in H-storage systems such as NaAlH$_{4}$. Here we report proton NMR and neutron vibration spectroscopy (NVS) evidence for H$_{2}$ in AlH$_{3}$ samples. In static sample NMR, a sharp line appears on top of the broad AlH$_{3}$ solid signal. MAS further sharpens this line and identifies it as H$_{2}$ by its chemical shift. Upon cooling, the line broadens and disappears near 20K, confirming the H$_{2}$ identification. NVS reports energy-gain peaks at the H$_{2}$ rotational energy (J=1 to 0).   

Inelastic Neutron Scattering Investigation of Ti-doped NaAlH$_{4}$

Complex hydrides (i.e. alanates (AlH$_{4})^{-}$ or borates (BH$_{4})^{-})$ are widely investigated as hydrogen storage materials. They have lower formation energy than simple metal hydrides and usually higher hydrogen to metal ratio. However, kinetics and performance still represent the main challenge for the actual application of these materials as hydrogen storage materials. The use of transition metal dopants such as Ti, Fe, Zr can improve the hydrogen exchange capability and hydrogen storage capability of a complex metal hydride significantly. However, a satisfactory explanation how and why certain dopants work best with certain complex metal hydrides has not yet been given. We choose sodium aluminium hydride NaAlH$_{4}$ doped with various amount of titanium (precursor: TiCl$_{4})$ for our research on the mechanism of doping. Incoherent inelastic neutron spectroscopy is a well-suited tool to look at hydride (H$^{-})$ in the material and the changes of the hydride in the material upon addition of dopants. Possible changes in the lattice of the ``host material'' NaAlH$_{4}$ are observed by X-Ray diffraction.   

Hydrogen-related defects and the role of Ti in NaAlH4

Titanium-doped sodium alanate is a promising storage material; however, the mechanism of the enhancement in (de)hydrogenation rates induced by Ti has remained unresolved. We performed a comprehensive investigation of hydrogen vacancies and interstitials, which play an important role in the (de)hydrogenation processes. Interestingly, these highly mobile defects cause large rearrangements of the surrounding lattice, and they are always charged; their formation energy therefore depends on the Fermi level. Our investigations show that the Ti-induced modification of the Fermi level increases the defect concentrations, thus explaining the improved kinetics. These novel insights may prompt a reexamination of the role of transition-metal impurities in alanates and related materials, and lead to the design of storage materials with improved characteristics.   

Dehydrogenation of NaAlH$_{4}$ from First-principles Molecular Dynamics

Key among the materials challenges facing a hydrogen economy is the discovery of lightweight materials for reversible hydrogen storage in the solid state. Although chemical and metal hydrides have been under intense investigation, progress has been curtailed by a lack of understanding of the reaction paths for hydrogenation and dehydrogenation in such materials. We present here our first-principles molecular dynamics results for NaAlH$_{4}$, one of the promising and most extensively studied candidates for hydrogen storage. We analyze proton transport in the presence of a variety of low-energy defects and surfaces, and we discuss possible candidates for the key mechanisms of dehydrogenation in the material. The results are presented in terms of the structural phase transition to $\alpha$-Na$_{3}$AlH$_{6}$.   

Thermodynamic properties of LiAlH4 from first-principle calculations

The potential hydrogen-storage materials of LiAlH4, and Li3AlH6 have been studied by using density functional theory (at GGA level), and harmonic phonon approximation. The thermodynamic properties of these materials have been studied in detail. We found that the decomposition of LiAlH4 is not reversible, which may indicate that the direct synthesis of LiAlH4 may be not possible. The calculations indicate that Li3AlH6 can be used as a hydrogen-storage material under certain conditions. In addition, the phase diagram of these materials will be presented.   

Hydrogen adsorption studies in micro-size cobalt dots

Hydrogen desorption curves were obtained from a sample composed of square arrangement of Co dots with average diameter of 4.4 microns, separated by a distance of 11.6 microns. A macroscopic sample of Co dots grown on a 2.5x2.5 cm Si substrate was made by standard lithographic techniques and used in these experiments. Thermal programmed desorption (TPD) was performed under ultra-high vacuum conditions. Hydrogen TPD curves were obtained from a 1x1 cm sample of Co dots, Co films and Co foils for comparison. The hydrogen TPD curves peaked at 425 K and have decreasing intensity from the Co foils to the Co dots and to the Co films. A desorption energy of 27 Kcal/mol was obtained for the Co dots suggesting that hydrogen is adsorbed on an hcp or fcc hollow site of the Co dot crystalline structure.   

Session L40: Properties of Semiconductor Nitrides

Measurement of the Enthalpy and Free Energy of GaN

Direct measurement of the thermochemical properties of GaN has proven difficult, and differences in reported values remain. We present a technique for direct measurement of the free energy of formation of GaN by finding the partial pressures of ammonia gas in hydrogen for spontaneous formation and elimination of a GaN film on a liquid Ga surface in the temperature range 750-1050 C. These data were used to calculate the enthalpy of formation of GaN. The measured values are compared to those obtained by other means, e.g. calorimetry or direct reaction of N$_{2}$ and molten Ga.   

First-principles study of the thermodynamics of InGaN alloys

We present the most rigorous density-functional study [1] to date of the thermodynamics of In$_x$Ga$_{1-x}$N alloys. These systems have attracted considerable theoretical and experimental attention due to its enormous potential for high-power, high-frequency, and high-temperature optoelectronic applications. Theoretical calculations of the pseudo-binary phase diagram require an accurate description of the interactions between the constituent atoms, a good representation of a random alloy, the inclusion of the effect of lattice vibrations, and a consideration of the configurational entropy. We have used accurate density-functional theory to calculate the heat of formation of the alloy (represented by special quasi-random structures), as well as the phonon spectra for the lattice vibrational contribution to the free energy. We find that the wurtzite structure is always more stable than the zinc-blende structure for all temperatures and compositions investigated, in agreement with experiment. We find that the lattice vibrations lead to a reduction of the critical temperature by more than 20\%, leading to a temperature of 1654 K and 1771 K for the wurtzite and zinc-blende structures, respectively. The lattice vibrations also change the shape of the binodal and spinodal curves. This result suggests that quaternary alloy additions may increase the vibrational contribution to the stability of the disordered phase. Our predicted phase diagrams have been used to interpret several key experiment measurements on MOCVD In$_x$Ga$_{1-x}$N films. The importance of In$_x$Ga$_{1-x}$N alloys has also prompted the study of the surface thermodynamics of InN [2]. New results will be presented. \newline [1] C. K. Gan, Y. P. Feng, and D. J. Srolovitz, Phys. Rev. B, {\bf 73} 235214 (2006). \newline [2] C. K. Gan and D. J. Srolovitz, Phys. Rev. B., {\bf 74} 115319 (2006).   

Theoretical study of thermodynamic and electronic properties of the Zincblende In$_{x}$Ga$_{1-x}$N alloys

Semiconductor alloys often show distinct atomic-scale microstructures such as long-range, or short-range order, clustering and phase- separation. Such microstructures directly affect the electronic properties. To establish how the atomic microstructure in InGaN zinc-blende alloys affects the electronic structure we (1) Calculate the equilibrium alloy phase-diagram both for bulk (``free-floating'') alloy as well as for the epitaxial alloy using the mixed-basis cluster expansion (MBCE) approach. The MBCE Hamiltonians are evaluated by a number of total-energy inputs from First- principle LDA calculations. Given the Cluster-expansion, we calculate the miscibility gaps, and short range order through Monte Carlo simulations. (2) Calculate the electronic properties of the ensuing microstructure using a supercell approach with atoms placed where the thermodynamic calculation dictates, and the electronic properties are obtained from plane-wave empirical pseudopotential approach.   

Thermoelectrical properties of InGaN

III-nitride semiconductors offer tremendous scope for the enhancement of the thermoelectric figure of merit (\textit{ZT}) through the use of the bandgap engneering, alloying and nanostructure manipulation. Although III-nitride materials have been extensively studied for visible and ultraviolet light emitters and detectors and high power transistors during the past decade, very little work has been done with respect to their applications for thermopower technology. The \textit{ZT }value of a thermoelectrical material can be enhanced by decreasing the thermal conductivity. In this study, we have employed the 3-Omega method to characterize the thermal conductivities of III-nitride nano-structures. It was found that the incorporation of indium in GaN significantly reduced the thermal conductivity. A systematic study has been carried out on the dependence of the thermal conductivity of InGaN on the In-content. Various growth schemes for introducing scattering centers and InGaN/GaN supperlattices for phonon-blocking were carried out with the aim of further reducing the thermal conductivity and enhancing the \textit{ZT} value. The results of these studies will be reported in this talk.   

Local electronic and optical behavior of ELO $a$-plane GaN

Conductive atomic force microscopy (CAFM) and near-field optical microscopy (NSOM) were used to study $a$-plane GaN films grown via epitaxial lateral overgrowth (ELO). The ELO films were prepared by metal organic chemical vapor deposition on a patterned SiO$_{2}$ layer with 4-$\mu $m wide windows, which was deposited on a GaN template grown on $r$-plane sapphire. The window regions of the coalesced ELO films appear as depressions with a high density of surface pits. At reverse bias below 12~V, very low uniform conduction (2~pA) is seen in the window regions. Above 20~V, a lower-quality sample shows localized sites inside the window regions with significant leakage, indicating a correlation between the presence of surface pits and leakage sites. Room temperature NSOM studies also suggest a greater density of surface terminated dislocations in the window regions, while wing regions explicitly show enhanced optical quality of the overgrown GaN. The combination of CAFM and NSOM data therefore indicates a correlation between the presence of surface pits, localized reverse-bias current leakage, and low PL intensity in the window regions.   

Electrical properties of Si-implanted Al$_{x}$Ga$_{1-x}$N with high Al mole fraction

Ion implanted AlGaN has not been well investigated, in particular AlGaN with high Al mole fraction, compared to the research of ion-implanted GaN. Therefore, a systematic electrical activation study of Si-implanted AlGaN with Al mole fraction from 0.1 to 0.5 has been made as a function of ion dose and anneal temperature. The AlGaN wafers were grown on sapphire substrates by migration enhanced metal organic vapor deposition. Silicon ions were implanted at 200 keV with doses from 1x10$^{14}$ to 1x10$^{15}$ cm$^{-2}$ at room temperature. The samples were proximity cap annealed from 1100 to 1350 $^{o}$C with a 500 {\AA} AlN cap in a nitrogen environment. The results of Hall Effect measurements show that electrical activation efficiencies of 77{\%} and 53{\%} were obtained respectively for the Al$_{0.4}$Ga$_{0.6}$N and Al$_{0.5}$Ga$_{0.5}$N implanted with a dose of 1x10$^{15}$ cm$^{-2}$ and annealed at 1350 for 20 min. The Al$_{0.1}$Ga$_{0.9}$N samples exhibited a mobility of 100 cm$^{2}$/V\textbullet s, while the Al$_{0.5}$Ga$_{0.5}$N showed a mobility of half that value. It has been found that the samples with higher Al mole fraction generally require a higher anneal temperature and/or longer anneal time for a better electrical activation. Also, the mobility of the sample was found to decrease as the Al mole fraction increases.   

Hydrostatic Pressure Study of GaN-Based FETs

Because of their large breakdown voltage and high saturation velocity, GaN-based field effect transistors (FETs) are best suited for applications that require high power and high frequency, but they face serious reliability problems. In these FETs the charge in the channel results from spontaneous and piezoelectric polarization, so any stress produced during device operation can change the charge in the channel and affect the device's electrical performance. With the aim of better understanding the role of stress, we have studied the current-voltage characteristics of GaN FETs under hydrostatic pressure up to 10 kbar. N-channel FETs were fabricated from GaN-Al$_{0.28}$Ga$_{0.72}$N structures grown on SiC substrates. The source-drain current was measured for voltage up to 15V, at various gate voltages, from turn-on (typically -3V) up to 1V. We have observed that for a given source-drain voltage the current increased monotonically with pressure. At 10 kbar, the increase in the saturation current relative to 1 atm was found to be device-dependent, but typically between 5{\%} and 15{\%}. These results, along with the pressure effects on the devices' intrinsic short term degradation and on strongly degraded GaN FETs devices, allow us to speculate about the possible origin of device degradation.   

Concentration dependence of the E$_{ +}$ transition in dilute GaAs$_{1-x}$N$_{x}$ alloy.

We investigate dilute isoelectronic doping in molecular-beam epitaxially-grown GaAs using low-temperature micro-photoluminescence to measure the above-bandgap transition energies of $E_{+}$ and the spin-orbit transition, $E_{SO}$. In the case of dilute nitrides, GaAs$_{1-x}$N$_{x}$, we examine the functional shape of the concentration dependence of E$_{+}(x)$ in the low-$x$ limit, with $x$ as low as 0.04 {\%}. Comparison with the concentration dependence of the E$_{0}$ bandgap gives compelling evidence against the picture of bandgap reduction via repulsion of the conduction band minimum from the impurity level acting alone.   

Symmetry of the N-H$_{2}$ complex in GaAsN

The III-N-V alloys have attracted much recent attention because of a large reduction of the band-gap energy that occurs for N concentrations of a few percent. The hydrogenation of these alloys eliminates the effect of N [1]. A previous IR study of the vibrational properties of GaAsN:H showed that the dominant N- and H-containing defect has two weakly coupled N-H modes [2], a finding that is inconsistent with an H$_{2}$* structure proposed in earlier studies. Recent theory [3-5] suggests a defect with C$_{1h}$ symmetry. While uniaxial stress results do not provide vibrational splittings sufficiently large to reveal the symmetry of the N-H$_{2}$ center, new IR absorption experiments performed for the H-wagging modes of GaAsN:H yield results that are consistent with the vibrational properties predicted for the C$_{1h}$ structure [3]. This work is supported by NSF Grant DMR 0403641. \newline [1] A. Polimeni \textit{et al.}, Phys. Rev. B \textbf{63}, 201204(R) (2001). \newline [2] F. Jiang \textit{et al.}, Phys. Rev. B \textbf{69}, 041309(R) (2004). \newline [3] W. B. Fowler \textit{et al.}, Phys. Rev. B \textbf{72}, 035208 (2005). \newline [4] M.-H. Du \textit{et al.}, Phys. Rev. B \textbf{72}, 073202 (2005). \newline [5] G. Ciatto, Phys. Rev. B \textbf{71}, 201301 (2005).   

Phonon spectra of the high-frequency modes in dilute nitride random GaN$_x$As$_{1-x}$ alloy

The broadening of the highly localized isolated vibrational mode in GaNAs has been studied using the force constants between nearest-neighbor and second-nearest neighbor nitrogen pairs obtained from density functional theory supercell calculations. The phonon spectra of the high-frequency modes associated with substitutional nitrogen atoms in the random alloy are calculated for different nitrogen concentrations.   

Simulation of time-resolved photoluminescence in the strongly disordered dilute nitride GaAsN

We calculate the time-resolved photoluminescence spectra at finite temperatures in the dilute nitride GaAsN by direct numerical solution of the rate equation for the electron distribution. Electron energy levels and wave functions are calculated numerically in a supercell geometry with a strong random alloy potential acting on the electron. The rate equation includes phonon-assisted transitions between states, radiative and non-radiative recombination. Results are compared to experiments.   

Photoluminescence blinking of InGaN Single Quantum Well: a study on time correlation

We studied the photoluminescence of Indium Gallium Nitride (InGaN) single quantum well crystals and found that in particular conditions it becomes unstable (optical blinking). This peculiar optical phenomenon is confined to few hundreds nanometers domains and is presumably related to the presence of dislocations or impurities in the quantum well. Moreover, time evolution of these blinking domains is not regular, but characterized by a random-like behavior with a certain degree of auto-correlation. It was found that distant domains do not blink independently, but are partially time-correlated in couples or groups. This suggests the existence of preferred carriers paths within the crystal; these pathways are not detectable by other means, but reveal themselves only through blinking. We believe this phenomenon is of interest to better understand fundamental carrier recombination processes and may help in the effort to realize novel and more efficient optical devices.   

Anomalous Beating Pattern in Wurtzite Al$_{x}$Ga$_{1-x}$N/GaN Heterostructures

We have confirmed the $k$-dependent spin splitting in wurtzite Al$_{x}$Ga$_{1-x}$N/GaN heterostructures. Anomalous beating pattern in Shubnikov-de Haas measurements arises from the interference of Rashba and Dresselhaus spin-orbit interactions. The dominant mechanism for the $k$-dependent spin splitting at high value k is attributed to Dresselhaus term which is enhanced by the $\Delta _{C1}-\Delta _{C3}$ coupling of wurtzite band folding effect.   

Femtosecond time-resolved photoreflectance of InN thin films.

The kinetics of the nonequilibrium photoexcited carriers in high-quality InN is investigated using femtosecond time-resolved pump-probe reflectivity measurements at room temperature. We observed that both of the hot-carriers relaxation times and carrier recombination times decrease with increasing photoexcited carrier density. We attribute the hot-carriers relaxation times anomaly is caused by the impact-ionization effect. And the carrier density-dependent recombination times can be explained by the Auger recombination (AR). The AR rate was found to have a quadratic rather than a cubic dependence on carrier density. The experimental results allowed the coefficients for impact-ionization, AR and a defect capture time in InN to be estimated as 2.3x10$^{-9}$cm$^{3}$/s, 2.5x10$^{-10}$cm$^{3}$/s and 535 ps, respectively.   

Session L41: Ripples, Grain Boundaries, & Interfaces

Gas cluster ion irradiation in formation of nano-ripple structures on silicon surfaces and its applications to producing III-nitride nanorods

Gas cluster ion beams (GCIB) have been used to fabricate nano-ripple structures on Si substrates. In this work, using (Ar)$_{n}^{+}$ clusters at 30 kV acceleration, where n$\approx $3,000, we have observed nano-ripple formations on the silicon surface after GCIB bombardment. The wavelength, amplitude and the dimensions of the ripples were studied in an effort to characterize the morphology as a function of angle of incidence, crystallographic orientations of the substrate, and the ion dosages. The underlying physics of ripple formation will be discussed and fabrication of nanorods on rippled (111)-Si substrates using III-nitrides, such as GaN, InGaN, and InAlN, will be presented.   

Studies of Ripple Formation on Si Surfaces During Ar$^{+}$ Ion Bombardment

A study of formation of ripples on Si surfaces during bombardment with Ar$^{+}$ ions is reported. Real-time grazing incidence small-angle x-ray scattering (GISAXS) measurements are performed at the National Synchrotron Light Source. Si (100) samples are bombarded by Ar$^{+}$ ions from a PHI sputter gun at off-normal incidence. The formation of ripple structures is monitored in real time. The effects of different ion energies and high temperatures on the formation of these ripples are studied. A separate study on the smoothening of ripples by ion bombardment at normal incidence is also performed. The real time smoothening of these ripples is monitored using GISAXS during ion bombardment of the surface at room temperature and at higher temperatures. The effects of ion energy and substrate temperature on the smoothening of ripples are discussed.   

Vertical Asymmetry and the Ripple Rotation Transition in the Epitaxial Growth and Erosion on (110) Crystal Surfaces

We theoretically elucidate the ripple rotation transition on Ag(110) crystal surface experimentally studied by de Mongeot and co-workers, as the transition between the Rectangular Rippled states (checker-board structures of alternating rectangular pyramids and pits). We show that the experimental diffraction patterns can be understood only by invoking the vertical growth asymmetry. Interestingly, we find that the ripple rotation transition occurs over an extended parameter range: The transition point where the qualitative change of the near in-phase patterns occurs turns out to be different from the one where the change of the out-of phase patterns occurs. In the proximity of the former transition point we find the four-lobe near in-phase diffraction pattern with the four peaks along the principal axed of the (110) surface, in accord with the experiments on Ag(110). Moreover, on the two sides of the extended ripple rotation transition, we find two novel interfacial states induced by the vertical asymmetry.   

Light scattering as a technique to probe grain boundary melting

Near phase equilibrium, bulk properties of polycrystalline materials are strongly influenced by an interlinked network of grain interfaces. While numerical simulations and theory have supported the idea of disorder along \textit{grain boundaries}, direct experimental access to the interface between two crystals of any material, in thermodynamic equilibrium, has been limited. The transparency, birefringency, and melting temperature of polycrystalline ice lend it to experimental probing. By scattering the collimated light of a 2.3 mW He-Ne laser off of the boundary of a hexagonal ice bi-crystal, prepared within a controlled ice growth cell, the boundary is directly explored. Reflected light intensity is measured as a function of the thermodynamic variables: temperature and impurity concentration. C-axis orientation can be determined by a systematic analysis of extinction angles for individual crystals held between cross polarizors. Assuming the index of refraction for bulk water, for any melted layer, a change in reflected signal strength of greater than 10{\%}, for a 1.5 nm melt layer, is expected. Results are compared with a recent theoretical study of impurity driven grain boundary melting [1], which found evidence for a wetted liquid layer separating individual grains within polycrystalline ice. [1] Benatov, L. and J.S. Wettlaufer, 2004. Abrupt grain boundary melting in ice. \textit{Phys. Rev. E}, 70, 061606.   

Adatoms, Grain Boundaries, and Thin Film Growth Stress

Atomistic simulations will be presented revealing fundamental stress generation mechanisms during later stages of thin film growth when substrate coverage is complete and the film is thickening. Typically, films exhibit texture with grain boundaries intersecting the surface. In situ growth stress experiments reveal compressive stress generation during this stage; if growth is interrupted, experiments show a relaxation in compressive stress with a characteristic rate. An existing theory proposes that, during deposition, adatoms incorporate into grain boundaries, generating compressive stress. However, debate exists regarding this mechanism and data observed in growth interrupt experiments. Simulations will demonstrate that adatoms are strongly attracted to grain boundaries and readily incorporate into them. Furthermore, adatom incorporation generates compressive stress in accord with experiments. A model is presented establishing a quantitative link between adatom incorporation into grain boundaries and the resultant stress generated.   

Grain-boundary grooving and agglomeration of alloy thin films: phase-field simulations

A common failure mode in polycrystalline thin films is grain- boundary grooving through the thickness of the film. This can bring the surface in contact with the substrate, leading to film agglomeration. Although grain-boundary grooving has received a great deal of attention over the past half-century, the extant models are too idealized to be useful to predict agglomeration in most technologically interesting materials, such as multicomponent alloy films. We relax many of the assumptions made in the classical analysis, thereby finding unprecedented richness in the problem. Our approach employs a phase-field model for grain-boundary grooving and agglomeration of polycrystalline alloy thin films. In particular, we study the effects of slow-diffusing species on grooving rate. As the groove grows, the slow species becomes concentrated near the groove tip so that further grooving is limited by the rate at which it diffuses away from the tip. At early times the dominant diffusion path is along the boundary, while at late times it is parallel to the substrate. This change in path strongly affects the time- dependence of grain boundary grooving and increases the time to agglomeration. The present model provides a tool for agglomeration-resistant thin film alloy design.   

Enhanced growth instability of strained film on curved substrate

We report linear stability analysis of a strained film on a curved surface. We show that the growth of a strained film is inherently less stable on a curved substrate than on a flat substrate. For small surface undulation, the lowest strain energy stat4e is for the film surface to adopt the same wavelength as the substrate surface in an anti-phase configuration. Our analysis provides some qualitative understanding of directed self-assembly of quantum dots on patterned substrates.   

Si-Ge Intermixing and the 5x5-(111) Surface Reconstruction

We present \textit{ab initio} results of the energetics and structure of a Si(111)-5x5 slab with two bilayers of Ge(5x5) adsorbed on it. We explore the aspects of this Si-Ge surface structure with those of an analogous pure Ge(111)-5x5 slab and we note many important differences such as the larger corrugation of the adatom heights in the former compared with the latter. \textit{Ab initio} results are also obtained for the effects of Si-Ge intermixing at the Si-Ge interface. These shed light on this transport mechanism as a way of lowering the strain.   

Ti as an Interface Stabilizer for Fe/Al and Al/Fe Interfaces

The use of ultra-thin metal interlayers to stabilize metal-metal interfaces and to limit interdiffusion has drawn much attention over the past few years, driven by a variety of technological applications. Earlier we reported that using a Ti monolayer as an interlayer stopped diffusion at the Fe/Al(001) interface. These findings encouraged us to explore the use of interlayer structures for thin films of technological interest deposited on Si wafers at room temperature using RF sputtering. AlFe and FeAl metal layers, with and without a Ti stabilizing interlayer, were studied using Rutherford backscattering (RBS) and X-ray reflectivity (XRR). Analysis revealed that FeAl and AlFe films without a Ti interlayer on SiO2/Si wafers showed considerable Fe-Al intermixing, especially when the Fe layer was deposited on top of the Al layer. With a Ti interlayer present at the interface both AlFe and FeAl interfaces exhibited less interdiffusion.   

Impurity Segregation at the Si/SiO$_{2}$ Interface

It is a widely known fact that impurities tend to segregate at interfaces between two materials. Here we report first-principles density-functional calculations and Z-contrast scanning transmission electron microscopy and demonstrate that impurities may either segregate or avoid the Si-SiO$_{2}$ interface, depending on their chemical identity and which side of the interface they originate from. Segregation mechanisms can be very different depending on the chemical nature of the impurity. In particular, we show that in the ``alternate dielectric'' Si-SiO$_{2}$-HfO$_{2}$ structure, which is highly promising for cutting-edge Si-based metal-oxide-semiconductor FET, individual Hf atoms, which enter the SiO$_{2}$ interlayer during high-temperature annealing, avoid the interface: both Z-contrast imaging and theory find that individual Hf atoms stay away from the nominal interface by about 2.5 to 3 {\AA}. In contrast, theory finds that Hf as a substitutional impurity in Si can reach the interface and in fact segregates in a single plane at substitutional sites at the nominal interface plane. This behavior is in contradiction to other dopant impurities such as As or P, which segregate at the interface only in the form of substitutional dimers in which the impurities achieve threefold coordination. (AFOSR/DOE in part)   

Temperature Induced Modifications of SiC Interfaces studied by High Resolution Electron Energy Loss Spectroscopy

High resolution electron energy loss spectroscopy (HREELS) is a fascinating tool for studying electronic and vibrational properties in the near surface regime. For SiC, a wide band gap semiconductor suited for several applications, the surface and interface chemical reactivity needs to be thoroughly understood. In addition to atmospheric adsorbates, C- and Si-terminated cub- and hex-SiC, changes in carrier concentration profiles and band bendings can be monitored by comparing HREELS-data with dielectric theory. There, the surface state density related to the reconstruction type and surface composition is important together with the substrate temperature. For oxygen on 6H-SiC (0001), we observed for the first time new vibrational modes linked to distinct Si-O-Si vibrations, namely its asymmetric- and symmetric stretching vibrations and wagging motion. The energy and intensity of the asymmetric stretching frequency is analogous to the initial stage oxidation of Si surfaces.   

Measurement of Young's modulus of thin films using modal characterization.

We present a method for experimentally measuring Young's moduli of thin films using micro-electro-mechanical systems (MEMS). Properties of thin films often differ from those in bulk material and, moreover, depend on deposition method. In this work, we describe the results of measurements of Young's modulus of titanium (Ti) thin film deposited by sputtering. We deposit Ti on microcantilever structures with specified dimensions and known material properties and then experimentally measure the frequency shifts for the first several resonant modes. Using the acquired resonant frequencies in finite element analysis, we obtain the Young's modulus of the deposited Ti film. Our results show that Young's modulus of Ti film deposited by sputtering varies with thickness from $\sim $60 GPa for 35 nm film to $\sim $73 GPa for 100 nm film. This approach is fast and independent of material or deposition method. It is especially valuable in determining effective Young's moduli of composite and nanostructured thin film materials with complex relationships between their composition and their properties.   

Session L42: Excitations at Surfaces

Low-Energy Acoustic Collective Excitations on Metal Surfaces

Sound-like longitudinal plasma waves where thought to only exist in layered systems where spatially separated 2D electron plasmas are realized. Due to their low energy and linear dispersion such waves were proposed as possible candidates to mediate the attractive interaction leading to the formation of Cooper pairs in high TC superconductors. A new type of collective excitation mode on metal surfaces has been found. In contrast to the usual surface plasmon, it has an acoustic dispersion. For Be(0001) the mode was observed using EELS. Detailed ab-initio calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface state band with the underlying 3D continuum in the same region of space. While it exists up to high energies for Be(0001), the mode as such has a very general character, for low energies it is expected to exist on many surfaces, profoundly affecting their electron and phonon dynamics. 1. V. M. Silkin et. al., Phys. Rev. B 72, 115435 (2005)   

Raman and infrared study of synthetic Maya pigments as a function of heating time and dye concentration

Maya Blue is a famous indigo-based pigment produced by the ancient Mayas. Samples for the present work are made by a synthetic route, and demonstrate similar chemical stability as the ancient Maya Blue samples. Since no direct proof exists that the indigo chemically binds to the inorganic palygorskite lattice, there is still controversy on the resting place of the indigo molecules; i.e$.$ are they in the channels of palygorskite, on the surface, or both. Our analysis by FT-Raman and FT-IR spectroscopy proves the partial elimination of the selection rules for the centrosymmetric indigo, and shows the disappearance of the indigo N-H bonding, as the organic molecules incorporate into palygorskite material. Infrared data confirm the loss of zeolitic water and a partial removal of structural water after the heating process. Evidence of bonding between cationic aluminum and indigo through nitrogen is revealed by FT-Raman measurements. The oxygen carbonyl is also believed to interact with the metal.   

Infrared Spectroscopy of the Elastically-Strained Silicon Nanomembrane Bonding Interface

We investigate the bonding interface between elastically strained silicon nanomembranes (SiNMs) and new host substrates with Fourier transform infrared spectroscopy in efforts to elucidate its chemical structure. We create SiNMs by heteroepitaxial growth of straining layers on the template layer of silicon-on-insulator (SOI) and then releasing the membrane sandwich by etching away the buried oxide. We then bond the SiNM to a new substrate, in the present case oxidized Si. Because the SiNM is only nanometers thick, the bonding interface contributes greatly to the membrane's structural and electrical properties. We probe the buried interface by attenuated total reflection via the evanescent wave from a germanium prism in intimate contact with the SiNM, which penetrates through the SiNM to interact with the interface bonds. In efforts to understand bond strength and interface electronic states, we probe the influence of different cleaning procedures, gas treatments, and annealing steps.   

Dynamical Properties of Surface-mounted Dipolar Molecular Rotators

We use dielectric relaxation spectroscopy (DRS) to study the rotational dynamics of dipolar molecules mounted on fused SiO$_2$ surfaces. Each ``molecular rotor'' consists of three parts: 1) a mounting group for attachment to the substrate, 2) a rotating group having a permanent dipole moment, and 3) an axis connecting the rotor to the attachment group. Attachment is facilitated either by covalent bonding through reaction of silane groups with surface hydroxyls or by van der Waals interactions. Fused SiO$_2$ substrates are patterned with interdigitated electrode Au capacitors ($C \simeq$ 1 pF), and rotor molecule dynamics are characterized by measurement of the capacitance $C$ and loss tangent $\tan \delta \equiv \mathrm{Re}\{Z\}/\mathrm{Im}\{Z\}$. We employ a ratio-transformer bridge technique to measure these quantities, with sensitivities in $C$ and $\tan \delta$ of 1 aF and 1 ppm, respectively. A unique aspect of this work is the experimental apparatus, which allows us to prepare sub-monolayer films, determine coverage via two independent methods (DRS and XPS), and study molecule rotational motion, in-situ in ultra-high vacuum. Results will be presented on the kinetics of rotor adsorption/desorption, barrier height and asymmetry of the rotational potential of the molecules, and the effects of varying rotor coverages and adventitious H$_2$O.   

Optical and opto-electronic based velocity and topographic measurements of a laser-ablated thin-metal layer on glass

We report on our ability to resolve the velocity and spatial profile of ablatively launched metal with nano-scale precision. We utilize a nanosecond laser pulse to launch a thin layer of titanium metal from a glass surface. Subsequently, we use optical and opto-electronic devices to diagnose the velocity and topography of the launched metal. Our Photonic Doppler Velocimeter (PDV) utilizes the heterodyne principle that allows us to track multiple velocity components. Our topographer incorporates a Shack-Hartmann interferometer to provide details of the deformation of the surface as it is launched. We compare the experimental data to simulations to provide a feedback loop to improve our theoretical models. We also discuss possibilities to extend the sensitivity of the PDV system to provide a compact diagnostic with a broad range of capabilities.   

Surface Plasmon Assisted Kondo Resonances on a Metallic Nanowire

We propose an experiment to measure the Kondo effect for magnetic atoms adsorbed on the surface of a metallic nanowire. In addition to the traditional $sp$-$d$ hybridization, by introducing the strong electromagnetic field of the localized surface plasmon on the nanowire, we show that it is possible to observe additional $sp$-$d$ electron transfer processes assisted by surface plasmons. Due to the good surface-to-volume ratio of the nanowire, the Kondo resonances could be observed as multiple anti-resonances in the differential conductance versus bias voltage curve   

Positronium physisorption at quartz surfaces

The possibility of having positronium (Ps) physisorbed at a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps$_2$ molecule on a material host. Experimental evidence for physisorbed Ps at the surface of quartz was reported some years ago,\footnote{Sferlazzo, Berko, Canter, Phys. Rev. B {\bf 3}, 6067 (1985).} but firm theoretical support for such a conclusion was lacking. With the FLAPW method\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B {\bf 24}, 864 (1981).} we calculated the electronic structure and dielectric function of $\alpha$-quartz and obtained the interaction potential with a Ps atom on its surface. We show that there is indeed a bound state with an energy of $\sim0.19$ eV, which is reasonably close to the experimental estimates of $0.14$ - $0.17$ eV. A brief energy analysis in terms of the Langmuir-Hinshelwood mechanism further shows that the formation of a Ps$_2$ molecule at quartz surface would be possible.   

Molecule-resolved structural and electronic properties of diamondoid self-assembled monolayers

Diamondoids represent an exciting new direction in the field of nanoscale carbon. While theoretically known to be stable, diamondoids have been experimentally inaccessible due to synthesis roadblocks and lack of natural sources, until recently purified from crude oil. We investigate the local structural and electronic properties of self-assembled monolayers formed from thiol-functionalized diamondoids. Using an ultra-high vacuum scanning tunneling microscope, we observe these monolayers to be robust and stable up to room temperature. Topographic data demonstrate well-ordered lattices for all molecules studied (adamantane through tetramantane), with lattice constants and angles that vary with polymantane order. Tunneling spectroscopy reveals a conductance gap in the energy spectra of each molecule, which we compare to calculated HOMO-LUMO gaps and band alignments. The hierarchical nature of these molecules, and the ability to functionalize them with specific atomic and molecular end groups, provide a new set of customizable molecular nanomaterials.   

X-Ray Photoemission Analysis of Chemically Treated CdZnTe Semiconductor Surfaces

Device-grade Cd$_{(1-x)}$Zn$_{x}$Te was subjected to various chemical treatments commonly used in device fabrication to determine the resulting microscopic surface composition/morphology and the effect on contact formation. Br-MeOH (2{\%} Br), N$_{2}$H$_{4}$, NH$_{4}$F/H$_{2}$O$_{2}$, and (NH$_{4})_{2}$S solutions were used to modify the surface chemistry of the Cd$_{(1-x)}$Zn$_{x}$Te crystals. Scanning electron microscopy was used to evaluate the resultant surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and Zn 2p, Cd 3d, Te 3d, O 1s core lines were used to evaluate the chemistry of the chemically treated surfaces. Metal overlayers were then deposited on these chemically treated surfaces and the I-V characteristics were measured. The measurements were correlated to understand the effect of interface chemistry on the electronic structure at these interfaces with the goal of optimizing the metal/Cd$_{(1-x)}$Zn$_{x}$Te Schottky barrier for radiation detector devices. This work was performed under the auspices of the U.S. Dept. of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.   

On the role of multiple hot electron processes in STM-induced atom motion on surfaces

The phenomenon of adsorbed atom excitation/manipulation induced by hot-electrons delivered to the surface by an STM tip is related to desorption induced by multiple electron transition (DIMET) processes in which femtosecond laser pulses excite substrate electrons, creating a flux of hot electrons incident upon the surface from within rather than externally as occurs with the STM. While the sources of the hot electrons differ, the individual inelastic electron scattering processes giving rise to atomic motion over activation barriers are identical. This facilitates an adaptation of DIMET theory [1] to multiple-electron STM surface processing. Recent relevant experiments: i) STM-induced non-local hot electron dissociation of dimethyldisulfide on Au (111) by Maksymovych and Yates; ii) ``below-one-electron-threshold'' excited motion of single Co atoms on Cu (111) by Stroscio and coworkers will be considered in the light of a DIMET-based theory that focuses on special aspects of multiple excitation processes. [1] J. W. Gadzuk, Chem. Phys. Vol.251, 87 (2000).   

Probing the mechanism of infrared resonant desorption of hydrogen from Si(111): anharmonicity and energy pooling

Desorption of hydrogen from a Si(111) surface by resonant infrared excitation of the Si--H vibrational stretch mode requires vibrational ladder climbing of a Si--H bond to a high level leading to associative desorption. We report recent experiments probing the mechanism of ladder climbing. H$_2$ desorption is observed when the excitation linewidths are narrower than the anharmonicity of the Si--H bond, favoring energy pooling over multiphoton absorption. The resonance width of H$_2$ desorption with an excitation linewidth of 8.7~cm$^{-1}$ is measured to be 39~cm$^{-1}$, opening a new opportunity for site--selective modification on the Si(111) surface. The desorption yield decreases when the sample temperature increases, consistent with an energy pooling process.   

Probing Electron-Hole Pair Production in Ultrathin Film Schottky Diode Devices using Hyperthermal Energy Ion Beams.

We are investigating the interactions of hyperthermal energy ions with ultrathin film Schottky diode devices, probing the role of ion-surface impact events and charge transfer on electron-hole pair production. Specifically, we measure currents that arise from electron-hole pair production at a diode surface. To date, these currents have been explored only for thermal energy gas-surface impacts, where they are called ``chemicurrents''. Using a UHV beamline to produce well-collimated monoenergetic noble gas and alkali-metal beams from 10 eV to 10 keV, we have the unique flexibility to probe our in-house designed diode devices with a wide range of incident species, energies, and charge states. Preliminary results are presented and discussed in the context of basic gas-surface energy transfer processes.   

Characterization of the Unoccupied Electronic Structure of Alkali Atoms on Noble Metal Surfaces by Time- and Angle-Resolved Two-Photon Photoemission

We perform angle-resolved two-photon photoemission (2PPE) spectral measurements on alkali atom (Li - Cs) covered noble metal (Cu(111) and Ag(111)) surfaces. The progressive evolution of 2PPE spectra with the alkali atom coverage is measured in the 0 - 0.1 monolayer range. We report on the dependence of 2PPE spectra on the alkali atom coverage, the photoemission angle, and the excitation laser polarization. The spectral measurements provide new information on the nature of chemisorption of alkali atoms on noble metals, as well as the photoinduced charge transfer excitation of alkali atoms. A general model for the image charge interactions at metal surfaces reproduces the experimental electronic structure quantitatively.   

Phononic and nonlocality contributions to Second Harmonic Generation in NiO.

The experimentally observed second harmonic signal in centrosymmetric NiO can be explained with symmetry lowering mechanisms: (i) signal from the surface, (ii) spin-orbit coupling, (iii) nonlocalities, and (iv) lattice distortions [1]. First the intragap energy levels of both the (001) surface and the bulk of NiO are obtained with highly correlated quantum chemistry methods: single excitation configuration-interaction, and multiconfigurational complete active space, optimizing each d-level separately. Then the second-order susceptibility tensor is calculated beyond the electric-dipole approximation. The effects of spin-orbit coupling are studied, and a detailed analysis of the effects of the inclusion of nonlocalities from magnetic dipoles and electric quadrupoles is performed. Finally the effects of phonons in the bulk of NiO within the frozen phonon approximation are included, and the second order susceptibility tensor is computed both in a time resolved and time averaged manner [2]. \newline \newline [1] G. Lefkidis and W. H\"{u}bner, Phys. Rev. Lett. 95, 77401 (2005). \newline [2] G. Lefkidis and W. H\"{u}bner, Phys. Rev. B 74, 155106 (2006).   

Session L43: Optical Properties of Quantum Dots and Nanowires

Magneto-Optical Studies of PbSe Colloidal Nanostructures

PbSe is an unusual semiconductor material with a direct band gap at the L point of 150 meV at 4 K. The band structure at this symmetry point is four-fold degenerate for both electrons and light holes, and conduction and valence bands possess similar effective masses and g-factors. Since both masses are relatively small, quantum confinement effects are easily achieved by reducing the nanostructure size to dimensions of the order of the large exciton Bohr radius, $a_B$=46 nm. We synthesized high quality PbSe nanocrystals and characterized them using transmission electron microscopy and optical methods. We probed the g-factor and fine structure of excitons in undoped PbSe quantum dots using optically detected magnetic resonance (ODMR) at 24 GHz and polarized photoluminescence in a magnetic field. The ODMR reveals that the g-factor is large for electron and holes (g=7.6) compared to other semiconductor nanocrystal systems. The photoluminescence polarization increases linearly with increasing magnetic fields up to 6 T, indicating that the fine-structure splitting is rather small.   

Multiple exciton generation in films of chemically treated lead chalcogenide quantum dots

Multiple exciton generation (MEG) is a unique process, which allows nanocrystals to produce several electron-hole pairs if the excitation energy is high compared to the bandgap of the material. Although the exact process that occurs in MEG is still under debate, the existence of the phenomenon is proven for quantum dots (QDs) in solution. This unique process leads to a desire to fabricate photovoltaic devices, among other things, which would benefit from the enhancement in photocurrent produced at short wavelengths. The fabrication of devices is problematic because the QDs must be close enough for charge transport to occur, yet remain confined for the process of MEG. In our work, we discuss how the exciton dynamics including MEG are affected by assembling the QDs into neat ordered arrays using transient absorption spectroscopy. The inter-QD distance is varied by treating the as-prepared array in dilute solutions of short-chained amines.   

Surface Termination Effects on Zinc Oxide Quantum Dots.

We investigate the effects of surface terminations on the optical properties of 2-6 nm ZnO quantum dots.~ Nanocrystals were grown by wet chemical synthesis in a short-chain alcohol solvent from zinc acetate and sodium hydroxide.~ Quenching of particle growth with various capping agents is necessary to maintain and enhance the unique characteristics of the nanocrystals.~ We reproduce results of previous work and expand on characterization of naked and surface terminated ZnO quantum dots.~ The nanoparticle properties were investigated by UV absorption spectrophotometry, photoluminescence, infrared spectroscopy, scanning electron microscopy , and atomic force microscopy techniques.   

Emission of quantum dots (QDs) and isoelectronic bound excitons (IBEs) from submonolayer ZnTe on ZnSe and dependence on thickness of ZnTe

Zn-Se-Te systems have been of great interest for both lighting applications and their unique optical properties. It is known that the PL of the dilute alloys [1] or quantum wells [2] is usually due to IBEs. ZnTe/ZnSe QDs have been grwon with full monolayer coverage of ZnTe on ZnSe using Volmer-Weber growth [3]. We have shown [4] the existence of type-II QDs in ZnSeTe multilayers grown by migration enhanced epitaxy with sub-monolayer quantities of ZnTe. The multilayers were grown using three Zn-Te deposition cycles sandwiched between nominally pure ZnSe barriers. Here, we report ZnTe/ZnSe QDs grown by a similar method, but with only one ZnTe deposition cycle. It is interesting that at T = 10K the PL emission related to these QDs does not shift upon varying the excitation intensity, and attributed to IBEs; however, a large energy shift is observed at T = 80K, suggesting formation of type-II QDs. The presence of type-II QDs is also supported by magneto-PL measurements. \newline [1] Permogorov and Reznitsky, J. Lumin. \textbf{52}, 201 (1992). \newline [2] Suzuki et al., J. Crystal Growth \textbf{184/185}, 882 (1998). \newline [3] Yang et al., JAP \textbf{97}, 033514 (2005). \newline [4] Kuskovsky, et al., Phys. Stat. Sol. (b) \textbf{241}, 527 (2004); Gu et al., Phys. Rev. B \textbf{71}, 045340 (2005).   

\textit{Ab-initio} studies of semiconductor quantum dots

Quantum dots (QDs) in the form of semiconductor nanocrystals have considerable potential as cell markers/disease trackers in medical physics due to their favorable light-emitting properties and long lifetimes in the cellular environment. Quantum confinement is believed to be responsible for the optical properties of semiconductor QDs (for instance, the direct band gap transition in H or C terminated Si dots is allowed); size and surface functionalization both affect the degree of quantum confinement in the QD, and hence its electronic and optical properties. We present \textit{ab-initio} computational studies of Si and Ge based nanocrystals made using density functional theory (DFT); in particular, we obtain optical absorption spectra by application of Lanczos algorithms to the central equations of linear-response time-dependent DFT [1]. We examine how the atomic geometries, electronic structure and optical absorption spectra are affected by the nanocrystal size and surface functionalization. [1] B. Walker, A. M. Saitta, R. Gebauer, and S. Baroni, \textit{Phys. Rev. Lett.}, \textbf{96}, 113001, (2006).   

Optimizing Solar Conversion Efficiency Using Semiconductor Nanocrystals

The increasing need for large quantities of CO$_{2}$-free energy sources may be partially met through the development of novel highly-efficient solar photoconversion approaches. The efficient generation of multiple excitons following absorption of a single photon in semiconductor nanocrystals (NCs) represents one possible route towards inexpensive, high-efficiency solar cells. The multiple exciton state, which has been observed for absorption of a single photon with energy above twice the NC effective bandgap in various colloidal semiconductors, can be detected through the Auger recombination signature in the decay of the charge carrier population. Exciton population decays can be measured by studying the time-dependence of either intra- or inter-band photoinduced absorption, or via the decay of photoluminescence for emissive samples. We report on work in our laboratory to study multiple exciton generation, and its potential for solar energy conversion applications, in various lead-salt as well as Si and Ge NCs.   

First Principles Computation of Optical Response in Silicon Nanostructures

In the next few years, the typical size of finFET devices used in the microchip industry is expected to be of the order of a few nanometers. This poses formidable challenges, including for optical metrology, i.e. for the development of appropriate tools and techniques based on optical response, to monitor and validate the growth of silicon nanostructures. Current optical metrology tools are based on the assumption that in Si finFETs the dielectric response is piece-wise constant and equal to the bulk value. Such an assumption is expected to break down for sizes smaller then 10 nm, where the dielectric response of Si nanostructures may substantially deviate from that of the bulk. We present an analysis of the dielectric properties of Si slabs, spheres and rods as a function of size and shape, based on first principles, Density Functional Theory calculations. In particular, we discuss the relative influence of quantum confinement and surface effects, and propose a way to monitor dielectric properties changes at the nanoscale, based on the definition of local dielectric response functions.   

Optical properties of doping effects in Si quantum dots

Si quantum dots (QDs) have many applications such as in high-efficient solar cells and light emitting diodes. Understanding the doping properties in Si QDs are very important for both theory and experiment. Using first-principles methods, we have systematically calculated the defect formation energies and transition energy levels of group-III and group-V impurities doped in Si QDs as functions of the QD size. The general chemical trends found in the QDs are similar to those found in bulk Si. We show that defect formation energy and transition energy level increase when the size of the QD decreases, thus doping in small Si QDs becomes more difficult. We explain the general chemical trends and the variation as a function of QDs size in terms of the atomic eigenvalues and quantum confinement effects. We also calculate the absorption spectrum of size-dependent Si QDs and quantum rods by large-scale ``charge patching method''. We show that the band gap and optical transitions of Si nanocrystals can be tailored continuously by size or shape. These results provide guidelines for future device designs that require the knowledge of the size/shape-dependence of the nanocrystal's electronic and optical properties.   

Study of Charged Exciton in Silicon Quantum Dot.

The trion system, a bound state of two electrons and a hole or two holes and one electron, has received particular attention in the past few years due to the possible practical applications, ranging from single photon emitter which can be used in quantum cryptography, ulta-high density memory devices, mobile light emitters and probe for semiconductor material. Moreover the trion has been proposed as a key element in the fabrication of quantum gates allowing fast spin flip of an electron in a charged quantum dot. In this study we compute the lowest energies of a trion system for two Silicon quantum dots of radius 1.09 and 1.36 nm respectively by using the configuration interaction method. We first obtain the electron and hole energies states using the tight binding method and then we construct a basis set of trion states with good spin quantum number and diagonalize the trion Hamiltonian. We find that the spin three half state is more bound then the one-half and we confirmed that the binding energy decreases as the size of the system increases. We get that the trion binding energy of the dot with radius 1.09 nm is 0.242 eV and 0.227 eV for the spin three-half and one-half respectively, while that of the dot with radius 1.36 nm is 0.295 eV for the spin three-half state and 0.261 eV for the spin one half-state.   

Internal transitions of singlet and triplet charged excitons in quantum dots

We theoretically studied the spectra of internal transitions of spin-singlet and spin-triplet charged excitons X$^{- }$ in quantum dots in finite magnetic fields. The lateral confinement in quantum dots was modeled by a parabolic potential. Both equal and different oscillator lengths for electrons and holes were considered. The states of charged excitons were found using the expansion in Fock-Darwin in-plane states and in size-quantization levels in quantum wells of different widths. We performed systematic studies of the spectra dependencies on the quantum well width, on the regime of lateral confinement, and on the magnetic field strength. In the absence of lateral confinement, the spectra of internal transitions are governed by an exact optical selection rule that follows from the rotational symmetry and magnetic translations. The selection rule prohibits, in particular, a triplet bound-to-bound inter-Landau level transition with energies below the electron cyclotron resonance. When the magnetic translational symmetry is broken by the lateral confinement, the previously prohibited transitions become allowed and acquire finite oscillator strengths. We compare our theoretical results with the existing experimental data on internal excitonic transitions in interface fluctuation quantum dots in GaAs QWs.   

Influence of the substrate orientation on the electronic and optical properties of InAs/GaAs quantum dots

Variation of the electronic and optical properties of InAs/GaAs quantum dots (QD) as a function of the substrate orientation is studied in the framework of 3D eight-band k.p model. The QD transition energies are obtained for high index surfaces [11k], where k = 1, 2, 3 and are compared with [001]. We show that the QD size in the growth direction determines the degree of the influence of the substrate orientation, whereas the influence of the shape is of secondary importance. The effects of an external magnetic field applied parallel and perpendicular to the QD growth direction are analyzed taking into account the Zeeman effect and employing the gauge invariant discretization scheme. The available experimental data are successfully described by one of the optically active exciton states of the lowest lying exciton quartet. We also discuss the experimentally observed negative exciton diamagnetic shift for small values of the magnetic field: (1) for samples grown on a (001) substrate and magnetic field applied perpendicular to the growth direction, and (2) for samples grown on a (311) substrate and magnetic field applied parallel to the growth direction.   

Self-assembled (In,Ga)As Quantum Posts on GaAs.

We demonstrate a method for the MBE growth of height controlled (In,Ga)As quantum posts (QPs). Its main axis is along the growth direction. they are dislocation free and have a dimensions $\approx $ 20nm and 40nm. From EDX measurements, The Ga$_{.55}$In$_{.45}$As QPs are embedded laterally in a Ga$_{.9}$In$_{.1}$As layer. We have computed the electron and hole levels using an 8 bands, stain dependent, \underline {k.p} effective mass model. Comparison of calculated single particle electron and hole levels with the observed micro-PL spectra single QPs indicates that electrons are spread over the entire QP whereas holes are localized near the seed QD. * This research is sponsored through an NSF-NIRT grant No. CCF 0507295. HJK acknowledges support by the Alexander von Humboldt Foundation.   

Measurements of the bandgap of wurtzite InAs$_{1-x}$P$_x$ nanowires using photocurrent spectroscopy

We report measurements of the bandgap of InAs$_{1-x}$P$_x$ nanowires with wurtzite crystal structure as a function of the composition. The bandgap was measured using photocurrent spectroscopy (performed at 5 K) on single InAs nanowires with a centrally placed InAs$_{1-x}$P$_x$ segment, contacted at the InAs ends. The nanowires were grown with chemical beam epitaxy (CBE). The measured bandgap was larger than the bandgap of zincblende InAs$_{1-x}$P$_x$ by about 120 meV over the measured composition range, $0.15 [Preview Abstract]

Magnetoplasmon excitations in Rashba spintronic quantum wires: Maxons, rotons, and negative energy dispersion

We investigate the plasmon excitations in a quasi-one-dimensional electron gas (Q1DEG) in the presence of a perpendicular magnetic field ($B$) and spin-orbit interaction (SOI) induced by the Rashba effect. The problem involves three length scales: $\ell_0=\sqrt{\hbar/m^*\omega_0}$, $\ell_c=\sqrt{\hbar/m^*\omega_c}$, and $\ell_s=\hbar^2/(2m^*\alpha)$ associated with, respectively, the confining potential, the magnetic field, and the Rashba SOI. The resulting Schrodinger-like equations satisfied by the wave function $\phi_{\uparrow \downarrow}$ are two coupled equations, which cannot be solved in an explicit analytical form. However, the limit of a strong magnetic field ($\ell_c \ll \ell_0$) and $k\ell_0\ll 1$ helps solve this set of coupled equations exactly. We derive and discuss the dispersion relations for charge-density excitations within the framework of Bohm-Pines' RPA. The intrasubband and intersabband magnetoplasmons (MPs)in a Q1DEG are characterized by, respectively, the negative energy dispersion with increasing $B$ and the magnetoroton excitations . Here we scrutinize the effect of the SOI on these characteristics in depth. We observe that the SOI modifies drastically the behavior of both the intrasubband and intersubband MPs in the LW limit and renders them relatively more prone to the Landau damping in the SW limit. We discuss the dependence of the MPs energy on the propagation vector, the magnetic field, the 1D charge-density, and the Rashba parameter characterizing the SOI.   

Session L44: Focus Session: Nanoscale Transport - Molecules II

Image potential and molecular conductance

The image potential plays an important role in condensed matter physics including tunneling phenomena at surfaces, but its role in molecular conductance has not been thoroughly investigated. We discuss the influence of the image potential on molecular conductance. It is known that the predominant mechanism of conductance in relatively long organic molecules is resonant tunneling,, i.e. the current between electrodes is due to resonant transfer of electrons via the resonant levels of the negative molecular ion (electron states) or/and the levels of positive molecular ion (hole states). Both the energies and wave functions of these resonant states are influenced by the dynamic image potential of the tunneling electron due to the presence of both electrodes. The physics of the image potential is governed by the relative balance between the plasmon energy of electrodes and the band width of the molecular resonant states. In particular, the image potential may lower the resonant electronic levels by as much as 1 eV or raise the resonant hole levels by the same amount. We will discuss interesting phenomena associated with image potential including possible diode effects in symmetric molecules.   

Real-space pseudopotential method for charge and spin transport properties of nanoscale junctions

We present an {\sl ab initio} method for the electronic transport of nano-scale junctions under finite bias. Our method is based on density functional theory using real space pseudopotentials. The scattering wave function is obtained by solving a set of linear equations with a sparse coefficient matrix. Our method does not require a matrix inversion. We apply the method to Na or Mg atomic point contacts coupled to two planar electrodes, and good agreement with previous work is obtained. We also extend this study and examine spin-dependent transport in select magnetic atomic point contacts, where trends in magnetoresistence are examined as a function of junction bias, magnetic moment, and electronic coupling.   

Ab initio investigation of rotaxane-based molecular switches

A parallel crossbar architecture based on a bistable [2]rotaxane has been achieved experimentally [1]. The rotaxane molecule contains two recognition sites and a macrocyclic ring. Depending on the applied bias, the ring is expected to move from one site to the other and the molecule switches from a high to a low conductance state. We investigate the energetics, forces and quantum transport properties of rotaxane structures using a massively parallel real-space multigrid method. Several stable and metastable configurations are identified and investigated as a function of the applied bias and the current flowing through the molecule. To account for the effects of the current and the open boundary conditions, we use a linear-scaling non-equilibrium Green's function method in a basis of optimized localized orbitals. We discuss current-induced charge redistribution, forces as a function of applied bias, interactions between the ring and the recognition sites, and the I/V characteristics. 1. Luo et al, Chem. Phys. Chem. 3, 519 (2002).   

Is Density Functional Theory adequate for quantum transport?

Density functional calculations for the electronic conductance of single molecules attached to leads are now common. I'll examine the methodology from a rigorous point of view, discussing where it can be expected to work, and where it should fail. When molecules are weakly coupled to leads, local and gradient-corrected approximations fail, as the Kohn-Sham levels are misaligned. In the weak bias regime, XC corrections to the current are missed by the standard methodology. Finally, I will compare and contrast several new methodologies that go beyond the present standard approach of applying the Landauer formula to ground-state DFT. \newline \newline {\em Self-interaction errors in density functional calculations of electronictransport}, C. Toher, A. Filippetti, S. Sanvito, and K. Burke, Phys. Rev. Lett. {\bf 95}, 146402 (2005) \newline {\em The Dramatic Role of the Exchange-Correlation Potential in ab initio Electron Transport Calculations}, S-H. Ke, H.U. Baranger, and W. Yang, cond-mat/0609367. \newline {\em Zero-bias molecular electronics: Exchange-correlation corrections to Landauer's formula}, M. Koentopp, K. Burke, and F. Evers, Phys. Rev. B Rapid Comm., {\bf 73}, 121403 (2006). \newline {\em Density Functional Theory of the Electrical Conductivity of Molecular Devices}, K. Burke, Roberto Car, and Ralph Gebauer, Phys. Rev. Lett. {\bf 94}, 146803 (2005). \newline {\em Density functional calculations of nanoscale conductance}, Connie Chang, Max Koentopp, Kieron Burke, and Roberto Car, in prep.   

Ab initio calculation of transmission and I-V curve for $\pi $-stacked polythiophene layers sandwiched between gold electrodes.

We have applied an implementation of recently developed [Faleev et. al. PRB 71, 195422 (2005)] non-equilibrium Green's Function method in framework of the tight-binding LMTO approach in its atomic sphere approximation to calculate the transmission function and I-V curves of $\pi $-stacked polythiophene layers sandwiched between Au(111) electrodes. Our approach is a fully \textit{ab initio} all-electron approach that treats the central region and electrodes on equal footing. To the best of our knowledge, this is first application of an \textit{ab initio} approach to calculation of transport properties of multiple polymer layers arranged \textit{parallel} to the metal surface, as opposed to previously studied systems of a small molecule or oligomer attached at both ends to the electrodes. We found that for a number of layers L $>$ 1, an increasingly pronounced dip in the transmission function is formed at energies from E$_{F}$ to E$_{F}$ + 0.5 eV, reflecting the semiconductor nature of a polythiophene multilayer film. The zero-bias conductance of the film exhibits large-L asymptotic behavior $\sigma \quad \approx $ G$_{0 }$exp(-1.2(L-5)), starting with L $\approx $ 6 that can be seen as a thickness of the thin polythiophene film at which a metal-semiconductor transition occurs. For L = 1, the current depends linearly on applied voltage, while at L $>$ 1, current is non-linear, reflecting strong bias and energy dependence of the transmission function.   

First Principles Simulation of STM Image and Spectroscopy

In this talk, we shall present a framework for simulating STM images and transport spectroscopy based on density functional theory (DFT) carried out within nonequilibrium Green's function approach (NEGF). In our model, the STM tip and the sample are treated together on equal footing within the self-consistent NEGF-DFT formalism, in contrast to the usual practice where electronic structure of the tip and sample are calculated separately. The NEGF-DFT formalism allows one to do STM simulation whether the STM tip is far (weak coupling) or close (strong coupling) to the sample. The main implementation issues of this STM simulation tool will be discussed and several examples will be given.   

Electron correlations in transport through molecular junctions: Coulomb blockade and hysteresis in the I-V characteristics of a model system.

Electron-electron interaction effects can play a very important role in explaining the mechanism of charge transport in molecular junctions. We use a simple tight-binding model to describe the leads and the electron-ion interaction inside the molecule. The electron-electron interaction inside the molecule is treated at the Hartree-Fock level. We study the model as a function of the number of sites in the molecule and the alignment of molecular energy levels relative to the average chemical potential in the leads. This model captures important phenomena such as the Coulomb blockade. We find that depending on the gate voltage and applied bias, there can be more than one Hartree-Fock steady-state solution for the system, which may give rise to a hysteresis in the I-V characteristics.   

Quantum quasi-steady states in current transport

We investigate quasi-steady state solutions to transport in quantum systems by finding states which at some time minimize the change in density throughout all space and have a given current density flowing from one part of the system to another [1]. Contrary to classical dynamics, in a quantum mechanical system there are many states with a given energy and particle number which satisfy this minimization criterion. Taking as an example spinless fermions on a one-dimensional lattice, we explicitly show the phase space of a class of quasi-steady states. We also discuss the possibility of coherent and incoherent mixing of these steady state solutions leading to a new type of noise in quantum transport. [1] M. Di Ventra and T.N. Todorov J. Phys. Cond. Matt. {\bf 16}, 8025 (2004).   

DC conductance of long molecular chains

Inspired by the work of Kamenev and Kohn, we obtained a general and formally exact expression for the 2-terminal dc conductance of linear molecular structures, within the framework of Time Dependent Current-Density Functional Theory. In this talk we focus on the adiabatic conductance. For linear molecular chains, both insulating and metallic, we derive exact and asymptotic analytic expressions for the conductance. For insulating chains, for example, not only do we get the exponentially decaying factors, but also the coefficients in front of these factors, which are highly dependent on the contacts. The results are based on the analytic structure of the bands and a compact expression for the Green's functions. Applications will also be presented.   

Nonequilibrium Electron Transport in Mott Insulators

We study the nonequilibrium transport properties of a Mott insulator using a recently developed time-dependent DMRG procedure to study conductances [1]. As a setup, we use a Hubbard chain connected to two ideal leads. We find a simple functional form of the I-V characteristics, and a universal functional dependence of the current on the electric field and the Mott gap. A mechanism of transport is described based on these results. The properties of the conducting phase induced by a strong electric field are also studied. We compare these properties to those of the doped phase. We also compare the Mott insulator to the band insulator case and discuss the similarities and differences [2]. \newline \newline [1] K. A. Al-Hassanieh, A. E. Feiguin, J. A. Riera, C. A. Busser, and E. Dagotto, Phys. Rev. B 73, 195304 (2006). K. A. Al-Hassanieh, A. E. Feiguin, F. Heidrich-Meisner, I. Gonzalez, M. J. Rozenberg, and E. Dagotto, preprint.   

Conductance, surface traps and passivation in doped Silicon Nanowires

By means of {\it ab initio} total energy and conductance calculations within the Landauer Formalism we investigate the structural, electronic and transport properties of doped silicon nanowires (SiNWs). We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Impurities located in the core of the wire induce a strong resonant backscattering at the impurity bound state energies. Surface dangling bond defects have hardly any direct effect on conductance. Upon surface trapping, impurities become transparent to transport, as they are both electrically inactive and do not induce any resonant backscattering. \begin{itemize} \item{}M. V. Fern\'andez-Serra, Ch. Adessi and Xavier Blase, Phys. Rev. Lett. {\bf 96}, 166805 (2006). \item{}M. V. Fern\'andez-Serra, Ch. Adessi and Xavier Blase, NanoLetters. (In press) {\bf 12}, (2006) \end{itemize}   

Low-Temperature Nonequilibrium Transport through Multi-pathway Single-Molecule-Transistor Systems as Which-way Effect Detector

We theoretically investigate the electronic transport through an AB-ring with a quantum dot (QD) embeded and a double-quantum- dot (DQD) interferometer. The QDs in both structures are coupled to a vibrational mode by electron- phonon interaction. With the help of Non-equilibrium Green function formalism and an improved independent boson model approximation, low-temperature bias-induced phonon-assisted sidebands are resolved. Electrons can tunnel via the phonon sidebands, pure electronic levels, or directly from one lead to the other. For DQD interferometer, electrons tunneling via two dots must emit the same number of the identical phonons to interfere with each other. Emitting different numbers of phonons would make the two pathways distinguishable, thus destroys the coherence between two pathways. Therefore this system could act as an intrinsic which-way effect detector in energy space. For the AB-ring model, the which-way effect does not apply since the direct lead-lead tunneling is assumed to be independent of energy. As a result, all phonon side peaks show flux-dependent Fano lineshapes.   

Observation of tribo-induced melting of an asperity contact with STM-QCM

Heating, energy dissipation and melting at the interface of a nanoscale sliding contact can trigger a broad range of interfacial physical and chemical processes. A fundamental understanding of these processes is largely lacking, however, because they occur at buried interfaces that are extremely difficult to characterize during sliding. In the arena of nanotechnology, heating, energy dissipation and conversion issues are integral to successful device design, construction and operation. Particularly compelling is the fact that any frictional heat produced is available to destroy device function: If any portion melts, the nanoscale device itself is destroyed. We report here on our use of the unique capabilities resulting from combining a scanning tunneling microscope (STM) and a Quartz Crystal Microbalance (QCM) to characterize the melting of a buried sliding interface between a tungsten tip and an indium overlayer. An analytic heat transport model supports the conclusion of melting arising from tip-surface sliding. These combined results constitute the first compelling evidence for the observation of tribo-induced melting of an asperity contact.