Session W1: ARPES in High Tc Superconductors

Laser ARPES, the sudden approximation, and quasiparticle-like peaks in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$. The low energy increases bulk-sensitivity, reduces background, and improves resolution. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink are found to be robust to a possible breakdown of the sudden approximation. We observe spectral peaks which are sharp on the scale of their binding energy - the clearest evidence yet for quasiparticles in the normal state. The very sharp spectral peaks and high statistics enables detailed investigations of the temperature and energy dependences of the lineshapes, giving critical insights into the nature of the scattering mechanisms in these materials. We thank collaborations with J. D. Koralek, J.F. Douglas, N.C. Plumb, Z. Sun, A.V. Fedorov, M. Murnane, H. Kapteyn, S. Cundiff, Y. Aiura, K. Oka, and H. Eisaki   

What is the ``glue'' for high temperature superconductivity?

It has been 20 years since the high-temperature superconductivity (HTSC) was discovered in La$_{2-x} $Ba$_x$CuO$_4$, by Bednorz and M\"{u}ller. Since then, many different HTSC compounds, all containing copper-oxide planes, were synthesized and HTSC became one of the most studied problems in science. However, the mechanism of HTSC still remains unknown. Recent advancements in Angle-Resolved Photoemission Spectroscopy (ARPES) have enabled a direct probing of effects of interactions between electrons and different bosonic excitations in a system, raising the expectations that a ``pairing boson'' responsible for HTSC could finally be identified. Here, we will present the study of single-particle excitations in two different cuprates: Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, a high-temperature superconductor with $T_C=91$ K and La$_{1.875}$Ba$_{0.125}$CuO$_4$, a system with suppressed superconductivity ($T_C\le 2.5$ K) due to the spin/charge ordering. The extracted self-energies and single-particle gaps will be compared and possible coupling mechanisms will be discussed. This work was supported by the DOE under contract number DE- AC02-98CH10886.   

Dispersion Anomalies and High Frequency Optical Conductivity in Cuprate Superconductors.

We argue that the shape of the dispersion along the nodal and antinodal directions in the cuprates can be understood as a consequence of the interaction of the electrons with collective spin excitations. In the normal state, the dispersion displays a crossover at an energy where the decay into spin fluctuations becomes relevant. In the superconducting state, the antinodal dispersion is strongly affected by the $(\pi,\pi)$ spin resonance and displays an $S-$shape whose magnitude scales with the resonance intensity. For nodal fermions, relevant spin excitations do not have resonance behavior, rather they are better characterized as a gapped continuum. As a consequence, the $S-$shape becomes a kink, and superconductivity does not affect the dispersion as strongly. We also analyzed recent infrared conductivity data in the normal state. We find that the high frequency behavior, which has been suggested as evidence for quantum critical scaling, is well described by the same interaction with overdamped collective modes. From explicit calculations, we find a frequency exponent for the modulus of the conductivity, and a phase angle, in good agreement with experiment.   

ARPES Investigation of Quasiparticle Renormalization in Cuprates

We investigated by Angle-Resolved-Photoemission-Spectroscopy the renormalization of the bands in Bi2212 as a function of doping, including underdoped (T$_{c}$ =85K), optimally doped (T$_{c}$ = 94K), and overdoped (T$_{c}$ = 65K) samples. We identified the sharp energy scale seen in the superconducting state with the B$_{1g}$ bond buckling mode, the out-of-phase vibrations of the in-plane oxygen. More recently, we compare doping dependent data to a theoretical calculation involving the identified$_{ }$modes, including both temperature and momentum dependence. This comparison brings insights to the doping induced spectral changes in the overdoped regime, in connection to some recent discussions. We will also present complimentary pressure dependent Raman scattering and X-Ray diffraction data showing how the B$_{1g}$ mode renormalizes with metallization of the insulating parent compound of Bi2212. The pressure variable allows a continuous assessment of the electron-phonon coupling lambda, based on both phonon frequency and lineshape, across the phase diagram. Finally, we discuss ways to test whether these mode couplings have a direct bearing on superconductivity.   

ARPES Study of Nodal Quasiparticles Using Low-Energy Tunable Photons

Low-energy quasiparticle excitations govern the thermodynamic properties of a superconductor both in the zero-field and vortex-mixed states. For a $d$-wave superconductor, nodal quasiparticles are crucial excitations starting from zero energy. So far, however, the nodal quasiparticle dynamics of high-Tc cuprates has been controversial. For example, it has been reported by an angle-resolved-photoemission (ARPES) experiment that the marginal-Fermi-liquid behavior persists into the superconducting state without appreciable change in the scattering rate, while microwave conductivity increases upon the superconducting transition. Here, we show a new ARPES result that solves the controversies with unprecedented momentum-resolution. Low-energy tunable photons have enabled us to resolve a small nodal bilayer splitting clearly, and to reveal the detailed temperature- and energy-dependence of the scattering rate, indicating the behaviors unique to the nodal quasiparticles. Due to the opening of the $d$-wave gap, the nodal scattering rate is remarkably suppressed, and shows a linear energy dependence. The difference in the energy-linear term between the bilayer-resolved scattering rates hints the nature of impurities involved. This work was done in collaboration with T. Yamasaki, T. Kamo, K. Yamazaki, H. Anzai, M. Arita, H. Namatame, M. Taniguchi, \textit{Grad.~Sch.~of Science and Hiroshima Synchrotron Radiation Center, Hiroshima Univ.}, A. Fujimori, \textit{Dept.~of Complexity Science and Engineering, Univ.~of Tokyo}, Z.-X. Shen, \textit{Dept.~of Physics, Applied Physics and SSRL, Stanford Univ.}, M. Ishikado, K. Fujita, and S. Uchida, \textit{Dept.~of Physics, Univ.~of Tokyo}.   

Session W2: Imaging Charge and Spin and Semiconductors

Imaging Transport Resonances in the Quantum Hall Effect

We image charge transport in the quantum Hall effect using a charge accumulation microscope. Scanning a charge sensitive tip just above the surface of a very high mobility AlGaAs/GaAs heterostructure, we measure the charging underneath the tip that results from applying an ac voltage to the 2D electron system (2DES). Applying a dc bias voltage to the tip induces a highly resistive ring-shaped incompressible strip (IS) in the 2D electron system (2DES) that moves along with the tip. This IS acts as a barrier that prevents charging of the region under the tip. At certain tip positions, short-range disorder in the 2DES creates a quantum dot island inside the IS that enables breaching of the IS barrier by means of resonant tunneling through the island. The images that result show striking ring shapes that directly reflect the shape of the IS. Within the ring shaped features, we also observe striations that arise from Coulomb Blockade of the quantum dot island. Varying the magnetic field, the tunneling resistance of the IS varies significantly, and takes on drastically different values at different filling factors. Measuring this tunneling resistance provides a unique {\em microscopic} probe of energy gaps in the quantum Hall system. To better understand the origin of the transport resonances, we have completed a series of simulations that show that the native disorder from remote ionized donors can create islands in the IS. Comparing the simulations with the experimental images provides a direct view of the disorder potential of a very high mobility 2DES. The experiments and simulations reveal the potential importance of single-electron resonant tunneling to bulk transport in the quantum Hall effect.   

Terahertz Imaging of cyclotron emission from quantum Hall conductors

Microscopy of extremely weak terahertz (THz) waves via photon-counting method is reported. A quantum-dot photon detector [1] is incorporated into a scanning terahertz microscope [2]. By using a quantum Hall detector [3] as well, measurements cover the intensity dynamic range more than five orders of magnitude. The minimum intensity reaches as lo as 10\^{}-21$^{ }$watt (one photon per one second). Applying the measurement system to the study of semiconductor quantum Hall (QH) devices, we image cyclotron radiation emitted by non-equilibrium electrons generated in QH electron systems. Owing to the unprecedented sensitivity, a variety of new features of electron kinetics are unveiled [4]. It is stressed that the present approach is in marked contrast to the THz- wave applications recently discussed extensively in a wide variety of fields including clinic, security, and environment. In the vast majority of those applications, room-temperature operation is implicit. The intensity of treated THz radiation is hence well beyond the level of 300K black body radiation (roughly 10\^{}-7 watts or 10\^{}14 photons/s per square centimeter in a 1/10 relative band width). From the scientific viewpoint, however, detecting extremely weak THz waves from an object without external illumination such as applied in the present work is of strong importance, because the microscopic kinetics of an object can be probed only in such a passive method. Besides semiconductor electric devices studied here, we will also discuss possible applications of the present method for molecular dynamics, micro thermography, and cell activities.. \newline \newline [1] S. Komiyama et al., Nature 403, 405 (2000). \newline [2] K. Ikushima et al.,. Rev. Sci. Instrum. 74, 4209 (2003). \newline [3] Y.Kawano et al., J. Appl. Phys. 89, 4037 (2001). \newline [4] K.Ikushima et al., Phys. Rev. Lett. 93, 146804 (2004).   

Imaging coherent electron flow in a two-dimensional electron gas

Images of electron flow through a two-dimensional electron gas can be obtained at liquid He temperatures using scanning probe microscopy. Near a quantum point contact (QPC), the images show angular lobe patterns characteristic of the wavefunctions in the QPC. At distances greater than one micron from the QPC, narrow branches of electron flow are observed due to the cumulative effect of small angle scattering. All of the images are decorated by interference fringes spaced by half the Fermi wavelength demonstrating that the flow is coherent. To determine the origin of the interference fringes, an imaging interferometer is created by adding a circular reflecting gate. The strength and position of the interference fringes can then be controlled by the voltage on this reflecting gate. Using the interferometer, we show that the interference fringes are due to backscattering to the QPC. Both experiments and theory demonstrate that the interference signal is robust against thermal averaging.   

Imaging Magnetic Focusing in a Two-Dimensional Electron Gas

Using a liquid-He cooled scanning probe microscope (SPM), we have directly imaged cyclotron orbits of electrons in a two-dimensional electron gas (2DEG) traveling between two side-by-side quantum point contacts (QPCs). The images show magnetic focusing when the spacing between the QPCs is an integer multiple of twice the cyclotron radius. An image is created by deflecting electrons away from their original trajectories using a capacitively coupled SPM tip, and recording the change in conductance as the tip is raster scanned above the surface.~ The cyclotron orbits are clearly visualized, as well as fringes that demonstrate the coherent nature of the flow.~ Classical and quantum simulations show how electrons are deflected by the tip to produce the image. With an applied magnetic field, the simulated images of magnetic focusing agree very well with the measured images. The simulations also show the effect of small angle scattering due to the ionized donor atoms. Fully quantum simulations show that interference fringes can be produced. Imaging and understanding the motion of electrons in magnetic fields is useful for the development of devices for spintronics and quantum information processing.   

STM and AFM; Which is Better for Surface Structural Analysis? Non- contact AFM Studies on Ge/Si(105) Surface

Scanning tunneling microscopy (STM) has been utilized to determine surface atomic structure with its highly resolved images. Probing surface electronic states near the Fermi energy (E$_{F})$, STM images, however, do not necessarily represent the atomic structure of surfaces. It has been believed that atomic force microscopy (AFM) provides us surface topographic images without being disturbed by the electronic states. In order to prove the surpassing performance, we performed noncontact (nc) AFM on the Ge/Si(105) surface [1], which is a facet plane of the ?hut? clusters formed on Ge-deposited Si(001) surface. It is found that STM images taken on the surface, either filled- or empty-state images, do not show all surface atoms because of the electronic effect; some surface atoms have dangling bond states below E$_{F}$ and other surface atoms have states above E$_{F}$. [2]. In a nc-AFM image, on the other hand, all surface atoms having a dangling bond are observed [3], directly representing an atomic structure of the surface. Electronic information can also be obtained in AFM by using a Kelvin-probe method. From atomically resolved potential profile we obtained, charge transfer among the dangling bond states is directly demonstrated. These results clearly demonstrate that highly-resolved nc-AFM with a Kelvin-probe method is an ideal tool for analysis of atomic structures and electronic properties of surfaces. This work was done in collaboration with T. Eguchi, K. Akiyama, T. An, and M. Ono, ISSP, Univ. Tokyo and JST, Y. Fujikawa and T. Sakurai, IMR. Tohoku Univ. T. Hashimoto, AIST, Y. Morikawa, ISIR, Osaka Univ. K. Terakura, Hokkaido Univ., and M.G. Lagally, University of Wisconsin-Madison. \newline \newline [1] T. Eguchi et al., Phys. Rev. Lett. 93, 266102 (2004). \newline [2] Y. Fujikawa et al., Phys. Rev. Lett. 88, 176101 (2002). \newline [3] T. Eguchi and Y. Hasegawa, Phys. Rev. Lett. 89, 256105 (2002)   

Session W3: Topological Aspects of Electron Transport in Solids

Berry Phase and Dissipationless Currents in Solids

It is now recognized that the electronic band structures in solids are characterized by nontrivial quantum topological nature associated with Berry phase. This situation is analogous to the quantum Hall system under the strong magnetic field, but occurs in almost every material even without the magnetic field. Anomalous Hall effect in ferromagnets is a representative example, where the anomalous velocity induced by Berry phase leads to the transverse motion of the electrons to the applied electric field. In paramagnetic materials, on the other hand, the Kramers degeneracy makes the Berry connection non-Abelian. The spin dependent anomalous velocity leads to the spin Hall effect in semiconductors such as GaAs. These charge and spin currents are distinct from the usual transport current since it is not due to the deviation from the equilibrium electron distribution in momentum space, but is driven by the anomalous velocity of all the occupied states in equilibrium. Therefore it is essentially dissipationless. However, in real situation, the disorder effect and contact to the leads introduces the dissipation. This aspect is discussed in detail using the Keldysh formalism for each problem. Ideas of anomalous Hall insulator and spin Hall insulator are also proposed to avoid this dissipation. Applications to optical phenomena are also discussed. This work has been done in collaboration with Z. Fang, S. Murakami, K. Ohgushi, M.Onoda, S.Onoda, K. Sawada, R.Shindou, N. Sugimoto, K. Terakura, and S.C.Zhang.   

Fermi Liquid Berry Phase Theory of the Anomalous Hall Effect

Charged Fermi liquids with broken time-reversal symmetry have an intrinsic anomalous Hall effect that derives from the Berry phases accumulated by accelerated quasiparticles that move on the Fermi surface. The intrinsic Hall conductivity is given by a new fundamental geometric Fermi liquid formula that can be regarded as the derivative with respect to magnetic flux density of the Luttinger fomula relating the density of mobile charge carriers to the k-space volume enclosed by the Fermi surface. This formula can be derived by an integration-by-parts of the Karplus-Luttinger free-electron band-structure formula to yield a topological (QHE) part plus a geometrical part expressed completely at the Fermi surface, and which has a natural generalization to interacting Fermi liquid quasiparticles (QP's). The QP Berry phases are properties of the {\it eigenstates} of the (exact) single-particle Green's function at the Fermi surface, which is a Hermitian matrix with Bloch-state eigenvectors; the Berry phases derive from the variation on the Fermi surface of the spatially-periodic factor of the QP Bloch state that characterizes how the total QP amplitude is distributed among the different electronic orbitals in the unit cell. In the case of 3D ferromagnetic metals, the Berry phases derive from the interplay of exchange splitting with spin-orbit coupling (both must be present). Remarkably, the new formula also applies to Fermi-liquid analogs such as the 2D composite fermion (CF) fluid in the half-filled lowest Landau level: in this case, the QP is a bound electron+vortex composite and not a Bloch state. This QP structure varies on the CF Fermi surface in a way that exactly gives the expected result $\sigma^{xy}$ = $e^2/2h$, unaffected by any Fermi surface anisotropy, thus explaining how a quantized value of $\sigma^{xy}$ persists even though the CF Fermi liquid is {\it not} an incompressible FQHE state. The geometric anomalous Hall effect formula suggests a more intrinsic geometric description of the Fermi surface, where the Fermi vector ${\boldmath k}_F({\bf s})$ is only one of a number of properties that vary on a curved $(D-1)$-dimensional Fermi surface manifold parametrized by curvilinear coordinates ${\bf s}$; other properties include the Berry curvature field ${\cal F}({\bf s})$, quasiparticle mean free path $\ell({\bf s})$,{\it etc}. The new formula also naturally takes into account non-trivial (multiply-connected) Fermi surface topology and open orbits.   

The Quantum Spin Hall Effect

We show that the intrinsic spin orbit interaction in a single plane of graphene converts the ideal two dimensional semi metallic groundstate of graphene into a quantum spin Hall (QSH) state [1]. This novel electronic phase shares many similarities with the quantum Hall effect. It has a bulk excitation gap, but supports the transport of spin and charge in gapless ``spin filtered" edge states on the sample boundary. We show that the QSH phase is associated with a $Z_2$ topological invariant, which distinguishes it from an ordinary insulator [2]. The $Z_2$ classification, which is defined for any time reversal invariant Hamiltonian with a bulk excitation gap, is analogous to the Chern number classification of the quantum Hall effect. We argue that the QSH phase is topologically stable with respect to weak interactions and disorder. The QSH phase exhibits a finite (though not quantized) dissipationless spin Hall conductance even in the presence of weak disorder, providing a new direction for realizing dissipationless spin transport.\\ \\ 1. C.L. Kane and E.J. Mele, Phys. Rev. Lett. {\bf 95}, 226801 (2005). \\ 2. C.L. Kane and E.J. Mele, Phys. Rev. Lett. {\bf 95}, 146802 (2005).   

Spintronics, propagating mode, (quantum) spin transport, and new electron liquids

The field of spintronics deals with the physics of the electron-spin in semiconductors, metals, insulators and other materials. Among these, the systems which are characterized by strong spin-orbit coupling hold a special place and exhibit a plethora of new phenomena. In classical transport in semiconductors, a propagating spin-charge collective mode appears that is qualitatively different from the diffusive charge transport. The propagating spin-charge mode is the hallmark of spin-orbit coupled systems with a Fermi surface. The Boltzmann transport equations are qualitatively different from the diffusive normal behavior. Three dimensional bulk transport will be analyzed for the first time. In semiconductors without spin-orbit coupling, an orbital Hall effect (similar to the spin-Hall effect) is present in which electrons selectively occupy different orbitals depending on their direction of motion. In quantum transport, a spatially varying spin-orbit coupling is equivalent to a Landau level problem in which electrons of opposite spin feel opposite magnetic fields, thereby exhibiting a quantum Spin-Hall effect. Time-reversal symmetry is unbroken. In these spin Hall insulators, the quantum spin transport takes place through edge states that cross the bulk gap and the Fermi level. The electron liquid on the edges is a helical liquid, in which spin is correlated with chirality, and represents a new class of one-dimensional liquids different from the Luttinger spinless, spinful or chiral liquids. I will also briefly discuss the possibility of three-dimensional quantization in systems with spin-orbit coupling. New experiments are needed and proposed to verify these predictions.   

Session W4: Carbon Nanotube Dispersions

From Networks to Nematics -- Carbon Nanotubes as Soft Matter

Carbon nanotubes represent just one example of an emerging paradigm in condensed matter physics and materials science: traditionally ``hard'' materials appearing in new ``soft'' applications and environments. In part, this trend being is being fueled by the desire to exploit solution and fluid-based approaches, such as self assembly and flow processing, in an effort to streamline the engineering and commercialization of new materials and applications. In this talk I will review our recent work on dispersing, aligning and manipulating carbon nanotubes in complex fluids and polymer melts. Due to the large aspect ratios and strong attractive interaction potentials intrinsic to such materials, a number of scientific and technical challenges become immediately apparent. In particular, I will focus on the subtle interplay of rheological influences, such as externally applied shear and elongation stresses, with the inherent ``stickiness'' of carbon nanotube suspensions and melts, where the latter typically favors the formation of disordered networks or ``gels'' over the more desirable liquid-crystalline order. For simple shear, the strength of the applied stress is found to be a critical factor in dictating carbon nanotube morphology, which varies from a quiescent network to macroscopic aggregates to a fully dispersed, flow-aligned (para)nematic state. Although we find remarkably low loading thresholds for elastic percolation, our results highlight a fundamental dilemma for the engineering of conducting carbon nanotube polymer composites; dispersion stability will often be achieved at the expense of electrical conductivity.   

Single-walled carbon nanotubes in strong acids: controlling solubility and the liquid crystal phase.

Single Walled Nanotubes (SWNTs have remarkable electrical, thermal, and mechanical properties. Neat, well-aligned SWNT fibers and sheets could be the ultimate building blocks of strong, ultra-light multifunctional materials for aerospace applications, and could yield electromechanical actuators and sensors with unprecedented performance. After the achievement of scalable production of SWNTs, the difficulty of processing pristine SWNTs by liquid-phase methods has been the single most important roadblock to manufacturing macroscopic materials composed solely of SWNTs. Here we show that SWNTs dissolve at high concentration in acids; the SWNTs are stabilized because acids protonate their sidewalls, balancing wall-wall van der Waals forces. Acid strength controls the phase behaviour. At low concentration, SWNTs in acids dissolve as individual tubes which behave as Brownian rods. At higher concentration, SWNTs form a highly unusual nematic liquid phase consisting of spaghetti-like self assembled supermolecular strands of mobile, solvated tubes in equilibrium with a dilute isotropic phase. At even higher concentration, the spaghetti strands self-assemble into a polydomain nematic liquid crystal, where the domains are entangled with each other. Under anhydrous condition, the liquid crystalline phase can be processed into continuous highly aligned fibers of pure SWNTs without the aid of surfactants or polymers. By using a new fluorescent staining technique, we measure the rotational diffusivity and persistence length of SWNTs suspended in water with the aid of surfactants, and show that SWNTs behave as Brownian rods.   

Phase Behavior of Carbon Nanotube Suspensions

We study the phase behavior of nanotube suspensions stabilized by surfactants or amphiphilic polymers. The control of the composition of the solutions allows the interaction potential between the nanotubes to be finely tuned. As a consequence, it is possible to quantitatively analyze important phenomena such as percolation or liquid crystalline phase transitions. In particular, we describe how the percolation of rod-like particles is quantitatively decreased in the presence of attractive interactions (1). We show that rod-like particles respond much more strongly than spheres to attractive interactions; strengthening thereby the technological interest of carbon nanotubes to achieve low percolation thresholds for electrostatic dissipation or electromagnetic shielding. By contrast, carbon nanotubes which experience repulsive interactions can spontaneously order and form liquid crystalline solutions (2). Aligning and packing nanotubes is a major challenge to obtain macroscopic materials with improved properties. We will briefly discuss at the end of the presentation, our latest results concerning the fabrication of fibers aligned nanotubes (3). In particular, we will present new treatments of these fibers which lead to unusual mechanical properties and shape memory effects with giant stress recovery (4). \begin{enumerate} \item B. Vigolo, C. Coulon, M. Maugey, C. Zakri, P. Poulin, \textbf{Science} \textbf{2005}. \item S. Badaire, C. Zakri, M. Maugey, A. Derr\'{e}, J. Barisci, G. Wallace, P. Poulin, \textbf{Adv. Mat.} \textbf{2005}. \item P. Miaudet, M. Maugey, A. Derr\'{e}, V. Pichot, P. Launois, P. Poulin, C. Zakri, \textbf{Nanoletters} \textbf{2005}. \item P. Miaudet, A. Derr\'{e}, M. Maugey, C. Zakri, P. Poulin, in preparation. \end{enumerate}   

Carbon Nanotube Suspensions: some underlying issues

Entropy of mixing of rigid particles in a suspending medium is determined on a per-particle basis and thus, for a given weight fraction will decrease with increasing particle size. In the case of carbon nanotubes, the entropy contribution to mixing will thus be small compared with the interparticle forces which comprise the enthalpic energy contribution to any thermodynamic equilibrium. These forces will generally be short range with the exception of electrostatic forces in the cases that the particles carry a charge. The ability to form carbon nanotube suspensions depends on both the chemical affinity between the medium and the tubes and, it appears, the size of the medium molecules. Surface treatments of the nanotubes have been developed both using covalently attached functional groups and surfactants, and each strategy has been successfully applied to both multi and single wall CNTs. Because carbon nanotubes are long, thin, rigid and comparatively straight, they have been shown to self assemble into liquid crystalline phases showing all the attributes of conventional systems. The relationship between such CNT systems and the conventional ‘Flory’ phase diagram will be described, as will the exploitation of these phase equilibria to fractionate nanotubes on the basis of mesogenicity. The use of liquid crystalline phases as a basis for the processing of carbon nanotubes into aligned assemblies such as fibres will also be outlined.   

Processes for Dispersing Single Wall Carbon Nanotubes in Polymers and How to Determine Their Spatial and Alignment Distributions.

We are currently designing and making polymer nanocomposites with single wall carbon nanotubes (SWNT) to obtain improved mechanical properties, electrical conductivity, flammability, and thermal conductivity. Our coagulation method is broadly applicable to nanocomposites using readily-soluble thermoplastics such as polystyrene and poly (methyl methacrylate). A variation of this method has been developed for nanotube/polymer composites based on high-density polyethylene. Nylon-based composites are prepared using our adaptation of an \textit{in situ }interfacial polymerization method. Epoxy-based SWNT composites are prepared using either shear mixing with multi-step thermal treatments or an infiltration method using a freestanding SWNT network. Recent studies have explored the use of solid-state shear pulverization to disperse SWNT. Rheology is employed to evaluate the spatial distribution of SWNT based on the onset of solid-like behavior associated with the formation of a SWNT network. Morphological tools including various microscopy methods, x-ray scattering and Raman imaging, are used to evaluate both dispersion and alignment. The resulting spatial and alignment distributions of SWNT depend on the composite fabrication method and any subsequent processing, such as melt fiber spinning to effectively align SWNT. Examples will be given detailing the importance of (1) SWNT dispersion on flammability and thermal conductivity and (2) SWNT alignment on electrical conductivity.   

Session W5: A Century of Critical Phenomena

The Richard T. Cox Lecture: Liquid State as an Occasional Result of Competing Interactions

Now it is even strange to think that in the early 50-ies the second order transitions and the liquid -- vapor critical point were considered as different fields of physics. In the USSR this lack of understanding (as everything in the USSR) had also a political dimension. Being a graduate of Kharkov University (domain of L.Landau) I was inclined to work in a framework of Landau-theory of the critical point. Having carefully analysed the published experimental data I discovered that the scattering of the data in the vicinities of both critical points and phase transitions was much higher than the implemented equipment had allowed [1]. For me it was a true sign of wrong conditions of measurements. As a result I had adjusted my experiment to the specific condition of the critical point. We worked together with the group of students of Kharkov University who had shared my enthusiasm. When we were already on a halfway of our own measurements I was deeply impressed by the excellent result of M.J.Buckingham and W.M. Fairbank on the $\lambda $-point of Helium [2]. Their achievement had turned our own measurements into a sort of experimen-tum crucis: should one expect a singularity also in the critical point or shouldn't? Experimental data on isochoric heat capacity near the critical point looked really similar (but not identical) to the singularity near the $\lambda $-transition. Both found their common ground in lattice models of different kinds [3]. The scaling concept was suggested to explain the universal features of both phenomena originated from developing fluctuations [4]. Our work was noticed first by C.Domb and M.Fisher in England. Michael was especially persistent in his demands that the Sovjet authorities would allow us a free communication. He produced a sort of frustration in their bureaurocratic heads. But it was great to feel not to be condemned for an eternal isolation in the USSR. All this development (now international) has opened way to express the properties of all liquids (including mixtures) in the vicinities of the singular points by the universal functions of reduced coordinates [5]. But the very existence of the critical point (and the liquid state itself) is in fact not an universal property of matter [6]. The freezing is depen-dent on a symmetry of packing and on a form of a potential well. It means the lower limit of the liquid state cannot be universal. However, if the freezing is somehow avoided the metastable critical point may be achieved instead [7]. And the universal features of the critical phenomena may be observed there again. Literature: [1] A. Voronel, M. Gitterman, Zh. Exp. Teor. Fiz. 39, 1162 (1960). M.Bagatsky, A.Voronel, V.Gusak., Zh. Exp. Teor. Fiz. 43, 728 (1962). See also a review: A. Voronel ``Thermal measurements and Critical Phenomena in Liquids.'' in PHASE TRANSITIONS AND CRITICAL PHENOMENA, vol. 5B, ed. by C.DOMB {\&} M.S.GREEN, 1976, Academic Press, London, New York, San Francisco. [2] M.J.Buckingham, W.M.Fairbank in 111,60, ``PROGRESS IN LOW TEMPERATURE PHYSICS''(ed. by C.J.Gorter) North-Holland Pub.Co., Amsterdam, 1961. [3] M.E.Fisher,''The Nature of Critical Points'', University of Colorado Press, Boulder, 1965; [4] A.Patashinsky,V.Pokrovsky, Sov.Phys.JETP,23,292,(1966); L.P.Kadanov, Physics, 2,263, (1966) [5] M.E.Fisher, Phys.Rev.,176, 257, (1968); M.A.Anisimov, A.V.Voronel, E.E.Gorodetsky, Zh.Exp.Teor.Fiz.,60,1117, (1971) [6] H.J.Hagen,D.Frenkel,H.Lekkerkerker, Nature, 365, 425, (1993); D.Frenkel, Physica, A 263, 26, (1999). G.Vliegenthardt, H.Lekkerkerker, Physica, A 263, 378, (1999). [7] O.Mishima,H.E.Stanley, Nature, 392, 164, (1998).   

The Role of Experiment in Our Understanding of Critical Phenomena

Progress in physics usually is maximized when there is close contact between experimentalists and theorists. This talk will review some of the interactions between experiment and theory that have occurred in the field of critical phenomena since the discovery of the critical point by Andrews in 1869. These interactions appear to have been very fruitful in the 19th century, but seem to have been lacking to some extent during the first half of the 20th century. In more recent times experimental results again have had a more profound influence, but with the advent of the renormalization-group theory they mostly served the important purpose of confirming theoretical predictions. In the present century the emphasis of theory has largely moved on to other fields because the problem of critical phenomena is considered by many to be ``solved''; but some experimentalists are still exploring new frontiers at the boundary between critical phenomena and other areas of condensed-matter physics that hopefully will re-attract the attention of the theoretical community.   

Some Fruits of Genius: Lars Onsager and the Ising Model

The story of the exact solution of the two-dimensional Ising model by Lars Onsager in the 1940's will be sketched and some of the striking developments following from it, especially for the behavior of fluctuating interfaces, will be recounted.   

Session W6: New Applications of Silicon in Photonics and Biomedicine

TBD

This abstract has not been submitted electronically.   

Prospects of Mid Infrared Silicon Raman Laser

Mid wave infrared (MWIR) lasers in the wavelength range of 2-5$\mu $m form an important tool for free space communications, bio-chemical detection and certain medical applications. Most organic chemicals and biological agents have unique signatures in the MWIR and can be detected using these lasers. The strong water absorption peak at 2.9$\mu $m renders such a laser attractive for surgery and dentistry. Solid state lasers comprising OPO-based nonlinear frequency converters and Raman lasers have been the popular choice for these applications. However, the low damage threshold, poor thermal conductivity and high cost limit the commercial availability of these sources. The recent demonstration of the first silicon Raman laser in 2004 combined with excellent transmission of silicon in the mid-IR suggests that silicon should be considered as a MWIR Raman crystal. In the near IR, where current silicon Raman lasers operate, free carriers that are generated by two photon absorption (TPA) create severe losses. TPA vanishes in the MWIR regime ($\lambda \quad >$ 2.25$\mu $m), hence eliminating the main problem with silicon Raman lasers. This combined with (i) the unsurpassed quality of commercial silicon crystals, (ii) the low cost and wide availability of the material, (iii) extremely high optical damage threshold of 1-4 GW/cm2 (depending on the crystal resistivity), and (iv) excellent thermal conductivity renders silicon a very attractive Raman crystal. Moreover, integrated waveguide and resonator technologies can lead to device miniaturization. This talk discusses the MWIR silicon laser and its applications.   

Nonlinear Optics in Silicon - Applications in Optical Communication Systems

Silicon photonics is quickly becoming an important and active research area, primarily because of the desire to leverage existing silicon fabrication technology and the potential for integration with conventional silicon electronic components. In this talk, I will discuss nonlinear optical effects in silicon, and ways in which they can be employed in optical telecommunication systems. The nonlinear effect that we have been exploring is two-photon absorption: a process in which two photons are simultaneously absorbed in a silicon photodiode to generate a single electron-hole pair. Unlike many other nonlinear processes, two-photon absorption does not require phase matching, and can occur over a very broad wavelength range with an ultrafast (fs) response time. In silicon, two-photon absorption can be observed at wavelengths from 1100 to 2200 nm, a range that spans the entire spectrum presently used in fiber telecommunications. For years, two-photon absorption was regarded as a deleterious effect in nonlinear optics, because it consumes the optical signal that was meant to produce a nonlinear phase shift. More recently, researchers have found ways to exploit two-photon absorption effects for optical signal processing. For example, if the electrical carriers produced by two-photon absorption are collected by an en external electrical bias circuit, the resulting photocurrent can be directly used in a number of nonlinear processing functions including optical autocorrelation, cross-correlation, quality monitoring, demultiplexing, optical sampling, and clock recovery. In this presentation, I will review the recent applications of two-photon absorption in communication systems, and describe ongoing research being conducted at the University of Maryland. In particular, we have found an explanation for the polarization dependence that is often observed in two-photon absorption, and we have developed a new way to overcome this dependence. As an example of how two-photon absorption can be used in a real communication system, we have demonstrated an 80 Gb/s optical clock recovery system based upon two-photon absorption in a silicon photodiode, and we deployed the system in a 1000 km fiber transmission experiment.   

Session W7: Physics of Cell Elasticity, Interactions and Tissue Formation

Nucleation and growth of cell contacts

Living cells develop adhesive contacts with their environments. We present experiments which are dedicated to measure geometric and density evolutions of these contacts in living cells. We focus on two contacts: focal contacts, formed between a cell and the extracellular matrix, and adherens junctions, assembled between neighbouring cells. Both include: (i) a transmembraneous protein dictating the given adhesive junction assembly, (ii) a specific protein complex with tens of different components, (iii) an actin cytoskeletal structure. Our experiments involve the observations of fluorescently labelled proteins of these contacts in living cells, local force application, force measurements, and optical development such as evanescent wave excitation. These adhesive cellular entities are usually described as a result of activation of signalling events. However cell adhesive contacts can be seen as discrete \textit{particles }aggregates, which undergo nucleation and growth like in a first order phase transition. We will show that self-assembly processes are indeed imposing contacts shapes and dynamics. Far from being two antagonistic ways of describing cells dynamics, signalling pathways and cell self-assembly complement each other to dictate contacts shapes. In addition, eventhough focal contacts and adherens junctions involve different proteins, we will show that they share common features such as mechanosensitivity. Via these contacts, cells behave as climbers seeking to probe the resistance of their environment in order to reinforce appropriately specific adhesive areas.   

Cell morphologies depend on substrate rigidity.

Extracellular matrices and intracellular cytoskeletons are composed mainly of open meshworks formed by semi-flexible polymers linked by accessory proteins in networks with specific geometries. The viscoelastic properties of such networks often differ strongly from those of flexible polymer gels and are characterized by relatively large elastic moduli for low volume fractions, increased rigidity at increasing strains, and in some cases, negative normal stresses when deformed in simple shear. Cell structure and function depend on the stiffness of the surfaces on which cells adhere as well as on the type of adhesion complex by which the cell binds its extracellular ligand. Most cell types, including fibroblasts and endothelial cells, switch from round to spread morphology as stiffness is increased between 1000 and 10,000 Pa. In contrast, other cells types such as neutrophils do not require rigid substrates in order to spread, and neurons extend processes better on soft (50 Pa) materials than on stiffer gels. Differences in rigidity sensing and response can be exploited to design soft matrices optimized for growth and differentiation of specific cell types.   

Physics of adhesion and elasticity of biological cells

Forces exerted by adherent cells are important for many physiological processes such as wound healing and tissue formation. By pulling on their environment, cells sense rigidity gradients, boundaries and strains induced by the presence of other cells. Many cell types respond to these signals by actively adjusting the magnitude and direction of the adhesions that connect cells to surfaces or to each other. These adhesions are formed from membrane-bound integrin proteins and other cytoplasmic proteins that form condensed domains that grow in the direction of externally applied or internal, cytoskeletal forces. We present a model for the adsorption of adhesion proteins from the cell interior to the adhesion site and the resulting, force-sensitive anisotropic growth. The theory couples the mechanical forces to the non- linear adsorption dynamics and predicts the growth velocities of the back and front of the adhesion in qualitative agreement with experiment. The adhesion forces generated by a collection of cells in a tissue significantly alter the overall elastic response of the system. We model an ensemble of cells by an extension of the treatment of dielectric response of polar molecules to elastic interactions. By introducing the elastic analogy of the dielectric constant of the medium, we are able to predict the average cell polarization, their orientational order, and the effective material constants.   

Cell mechanics and human disease states

This presentation will provide summary of our very recent studies exploring the effects of biochemical factors, influenced by foreign organisms or \textit{in vivo} processes, on intracellular structural reorganization, single-cell mechanical response and motility of a population of cells in the context of two human diseases: malaria induced by \textit{Plasmodium falciparum }merozoites that invade red blood cells, and gastrointestinal cancer metastasis involving epithelial cells. In both cases, particular attention will be devoted to systematic changes induced in specific molecular species in response to controlled alterations in disease state. The role of critical proteins in influencing the mechanical response of human red bloods during the intra-erythrocytic development of \textit{P. falciparum} merozoites has also been assessed quantitatively using specific protein knock-out experiments by recourse to gene inactivation methods. Single-cell mechanical response characterization entails such tools as optical tweezers and mechanical plate stretchers whereas cell motility assays and cell-population biorheology characterization involves microfluidic channels. The experimental studies are accompanied by three-dimensional computational simulations at the continuum and mesoscopic scales of cell deformation. An outcome of such combined experimental and computational biophysical studies is the realization of how chemical factors influence single-cell mechanical response, cytoadherence, the biorheology of a large population of cells through microchannels representative of \textit{in vivo} conditions, and the onset and progression of disease states.   

Session W8: Focus Session: Nonlinear Electrokinetics

Electro-Convective and Non-Equilibrium Electro-Osmotic Instability of Electric Conduction from an Electrolyte Solution into a Charge Selective Solid

Electro-convection is reviewed as a mechanism of mixing in the diffusion layer of a strong electrolyte adjacent to a charge-selective solid, such as an ion exchange (electrodialysis) membrane or an electrode. Two types of electro-convection in strong electrolytes may be distinguished: bulk electro-convection , due to the action of the electric field upon the residual space charge of a quasi-electro-neutral bulk solution, and convection induced by electro-osmotic slip, due to electric forces acting in the thin electric double layer of either quasi-equilibrium or non-equilibrium type near the solid/liquid interface. According to recent studies, the latter appears to be the likely source of mixing in the diffusion layer, leading to `over-limiting' conductance in electrodialysis. Electro-convection near a uniform charge selective solid/liquid interface sets on as a result of hydrodynamic instability of one-dimensional steady state electric conduction through such an interface. We discuss instabilities of this kind appearing in the full electro-convective and limiting non-equilibrium electro-osmotic formulations. The short- and long-wave aspects of these instabilities are discussed along with the wave-number selection principles and possible sources of low frequency excess electric noise experimentally observed in these systems.   

Nonlinear Surface Transport in the Thin Double-Layer Limit

At high applied electric fields, ionic transport within the double layer plays a significant role in the overall response of electrokinetic systems. It is well-known that surface transport processes, including surface electromigration, surface diffusion and surface advection, may impact the strength of electrokinetic phenomena by affecting both the zeta-potential and the magnitude of the tangential electric field. Therefore, it is important to include these effects when formulating the effective boundary conditions for the equations that govern electrokinetic flow outside of the double layer. In this talk, we discuss the application of a general formulation of ``surface conservation laws'' for diffuse boundary layers to derive effective boundary conditions that capture the physics of electrokinetic surface transport. Previous analyses (\textit{e.g.} Deryagin \& Dukhin 1969) are only valid for weak applied fields and are based on a linearization of the concentration and potential about a reference solution, but our results are fully nonlinear and hold at large applied fields as long as the double layer is sufficiently thin. We compare our nonlinear surface transport theory with existing linear analogues and apply it to the canonical problem of induced-charge electro-osmosis around a metal sphere or cylinder in a strong DC field.   

Experiments on Nonlinear Electrokinetic Pumps in Microfluidics

Nonlinear electrokinetic pumps are attractive in the development of portable and flexible microfluidic analysis systems, since they operate without moving parts using low (battery powered) alternating potentials. Since the discovery of AC electro-osmosis (ACEO) in the late 1990s, there has been much work in designing and building two-dimensional, periodic micro-electrode geometries, which exploit broken symmetry to rectify AC forcing and produce steaming flow over a surface. Building on this work, we exploit more general principles of induced-charge electro-osmosis (ICEO) in three-dimensional electrode geometries to enhance pumping in microfluidic devices. Our fabrication efforts are guided by theoretical analysis and simulations using the standard low-voltage theory, which, in some cases, predict flow rates faster than existing planar ACEO pumps by an order of magnitude (for the same voltage and feature size). We test various microfabricated pump geometries in a microfluidic loop following the methodology of Studer et al (2004). We are also measuring the strong effect of solution chemistry (e.g. ion valence and concentration) on ICEO flow to guide further developments in the theory of nonlinear electrokinetics.   

Microfluidic pumps based on Induced Charge Electroosmosis and flow Field Effect Transistor phenomena

We are developing AC electrokinetic micropumps to drive electrically conductive biological fluids in microchannels. These pumps work on the principles of Induced Charge Electroosmosis (ICEO) and flow Field Effect Transistor (flowFET) phenomena. AC (as well as DC) electric fields can induce electrical double layer on a polarizable surface and in turn, cause fluid motion by moving this layer. However, when the polarizable surface is electrically floating, symmetric vortices are observed on the surface. Symmetry of this flow leads to zero net pumping. In order to achieve pumping, we apply a second AC signal to the polarizable surface. When the magnitude of this second AC signal is different from the floating potential of the surface, unidirectional flow is observed i.e., pumping is achieved by modulating the induced zeta potential of the surface. Pumping caused by modulation of zeta potential is also known as flowFET phenomenon and has been shown to work with DC electric fields by other community members. We show a detailed study of flowFET with AC electric fields.   

Nonlinear studies of AC electrokinetic micropumps

Recent experiments have demonstrated that AC electrokinetic micropumps permit integrable, local, and fast pumping (velocities $\sim$~mm/s) with low driving voltage of a few volts only. However, they also displayed many quantitative and qualitative discrepancies with existing theories. We therefore extend the latter theories to account for three experimentally relevant effects: $(i)$ vertical confinement of the pumping channel, $(ii)$ Faradaic currents from electrochemical reactions at the electrodes, and $(iii)$ nonlinear surface capacitance of the Debye layer. We report here that these effects indeed affect the pump performance in a way that we can rationalize by physical arguments.   

Electric field driven motion of flexible polyelectrolytes

Our work aims to study dielectrophoresis of biomolecules in micro/nano-fluidics using molecular dynamics (MD) simulations. Our model combines electrohydrodynamics with molecular theories for the macromolecule polarization caused by the distortion of the counterion cloud. Unlike most available MD studies of polyelectrolytes, solvent atoms are explicitly represented in our model, so that hydrodynamic interactions are included naturally with no ad hoc assumption. The polyelectrolyte is modeled as a negatively charged bead-spring chain. The charges interact through the Coulomb potential and other molecular interactions are included via Lennard-Jones potentials. We study the transport properties of flexible polyelectrolytes suspended in a solvent, with or without added salt, under the action of DC or AC electric fields. MD data provide the information needed to compute the dipole moments of the molecule and the surrounding double layer, which are required for understanding the dielectrophoretic behavior of these molecules in nanoscopic channels.   

Anomalous ionic transport at nanometer-scale electrodes

We probe the transport of charged species in high ionic strength solutions on the scale of a few nanometers by monitoring electrochemical reactions at correspondingly sized electrodes. The electrical current through the nanoelectrode provides a direct measure of the flux of ions across the diffuse double layer. Because both the concentration gradient and the electric field are localized in the immediate vicinity of the nanoelectrode, this provides very local information. Furthermore, when the electrode dimensions are in the order of a few nanometers, very steep concentration gradients are achieved. To carry out these experiments, we have developed a method for fabricating nanoelectrodes with a well-defined size and geometry. A pore is first drilled in an insulating membrane with a focused electron beam and it is then filled with a noble metal yielding conically shaped, convex electrodes with radii as small as 2 nm. We observe pronounced non-linearities of ion flux versus concentration when transport is localized within a region smaller than 10 nm in size. We numerically calculate the predicted ion flux using the Stern-Poisson-Nernst-Planck formalism. We show that our observations cannot be explained using this widely applied mean-field description of ionic transport.   

Nonlinear Electroosmosis and Biomolecule Electrokinetic Trapping Induced by Ion Selective Nanofluidic Channels

This paper describes a nanofluidic device that can concentrate dilute biomolecule by electrokinetic trapping mechanism. This device has nanofluidic channels with a depth down to 40 nm, therefore, having significant Debye layer overlap. Depending on the strength of the applied potential across the nanochannel, one can observe phenomena such as concentration polarization; charge depletion and nonlinear electrokinetic flow in the adjacent microfluidic channel using fluorescent microscopy. By manipulating the electric field, the device can generate an extended space charge region, maintained for several hours, within a microchannel as a mean to collect and trap biomolecules. Our studies demonstrate such device can achieve up to 10 million fold sample preconcentration within 30 minutes. Besides, if applied a higher potential, a much faster chaotic flow can be seen in the microchannel adjacent to nanochannels. This kind of nonlinear electrokinetic flow is often called the electroosmosis of the second kind or induced-charge electroosmosis in electrode and ion exchange membrane studies. The presented device can be used as either a preconcentrator or an injector to other separation and detection systems preferred its performance and integrabilty. Also, it is an ideal experimental platform for studying such nonlinear electrokinetic effects, by directly tracking molecules \textit{in situ}.   

Modeling of Selectively Permeale Vesicle Membrane in Electrolytes: An Energetic Variational Formulation

We introduce a self consistent coupled system to model the deformation of selectively permeable vesicle membranes in electrolytes via an energetic variational formulation. The equations governing a diffuse charge system and the evolution of the vesicle membrane are coupled with the momentum equations of the electrolyte through the Lorentz force along with the induced elastic forces due to the membrane bending energy. The force coupling and charge flux selectivity are consequences of the energetic balance and competition of kinetic, electric and membrane elastic bending energy. We will also present some numerical simulation results of the system.   

Reversible Transitions on Electrically-Tunable Nanostructured Superhydrophobic Surfaces

Recently demonstrated electrically tunable nanostructured superhydrophobic surfaces provide a promising new way of manipulating liquids at both micro and macro scales. Dynamic control over the interaction of liquids with the solid substrate is of great interest to many research areas ranging from biology and chemistry to physics and nanotechnology. In this work the reversibility of the electrically induced superhydrophobic -- hydrophilic transition on nanostructured surfaces is addressed. Recently demonstrated approach based on momentarily induction of film boiling in a very thin layer of liquid adjacent to the solid-liquid interface is discussed. The dependence of the hydrophilic -- superhydrophobic transition on the topography of the nanostructured layer, as well as on the energy and duration of the electrical pulse is investigated. Several emerging applications of these surfaces, including lab-on-a-chip, chemical microreactor, and on-chip power sources are discussed.   

Nonlinear alternating current susceptibilities of rotating microparticles in electrorheological fluids

A perturbation approach [1] has been employed to investigate the nonlinear alternating current (AC) responses of the rotating microparticles in electrorheological (ER) fluids under AC or direct current electric fields. The shear flow of ER fluids exerts a torque on the particles and leads to the rotational motion of the particles about their centers [2]. We show that the dynamic effects can play a significant role in the AC responses. Our results can be conveniently interpreted in the dielectric dispersion spectral representation [3], thus offering a convenient method to determine the relaxation time and the rotation velocity of the ER particles by measuring the nonlinear AC responses. \newline \newline [1] G. Q. Gu and K. W. Yu, Phys. Rev. B 46, 4502 (1992); K. W. Yu, P. M. Hui, and D. Stroud, Phys. Rev. B 47, 14150 (1993). \newline [2] Jones T. K. Wan, K. W. Yu, and G. Q. Gu, Phys. Rev. E 62, 6846 (2000). \newline [3] Jun Lei, Jones T. K. Wan, K. W. Yu, and Hong Sun, Phys. Rev. E 64, 012903 (2001).   

Electrophoresis in bounded doamins

Electrophoretic motion is usually described by a linear model, based upon the smallness of the applied field relative to the equilibrium field in the screening Debye layer surrounding the particle. This model, in turn, leads to the Smoluchowski's slip condition and eventually results in mobility relations. The mobility concept is valid provided the external electric force is neglected --- a procedure superficially supported by the net electric neutrality of the combined particle-layer system. In this talk, however, I will show that this force does not vanish in general. Accordingly, a consistent scheme is formulated for analyzing the nonlinear motion of a particle in an applied field. This motion is illustrated in two contexts: rotation of non-spherical particles, and drift of a spherical particle away from a planar wall. I will also describe an analogy to incompressible and inviscid fluid motion. This analogy enables for experimental verification of three-dimensional potential flow solutions.   

How accurate is the Poisson-Boltzmann theory for monovalent ions near highly charged interfaces?

Monovalent ion distributions next to highly charged interfaces were determined by synchrotron surface X-ray sensitive techniques. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the $L_3$ Cs$^+$ resonance, we provide, for the first time, spatial counterion distributions (Cs$^+$) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H$_3 $O$^+$ at the interface leads to proton-transfer back to the phosphate group by a high contact-potential, whereas high salt concentrations lower the contact-potential resulting in proton- release and increased surface charge-density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.   

Session W9: X-ray, Light, and Particle Scattering and Diffraction

Strain maps with ppm resolution for single crystal wafers obtained from x-ray rocking curve maps

A double crystal (+, -) x-ray technique has been used to obtain separate maps of strain and tilt across single crystal samples of high crystalline perfection. Rocking curves were obtained for each pixel of a CCD detector and from these data angular shifts of the rocking curve center were mapped. By using data for two azimuthal rotations, that is, by combining data from two diffraction conditions separated by 180\r{ } rotation around the diffraction vector, we obtained separately the tilt and the strain. Data for diamonds has been obtained to demonstrate the technique in the case of a symmetric reflection[1]. Extensions of the method to asymmetric reflections in order to also extract strains parallel to the surface[2] will be discussed. Also a correction for wavelength dispersion in the case of different d-spacings for first and second crystals will be discussed. This work was supported by DOE Basic Energy Sciences-Materials Science, under contract No. W-31-109-ENG-38 and by NSF under contract No. EAR-0421020. [1] A.T. Macrander et al., Applied Physics Letters, 87, 194113 (2003). [2] V. Swaminathan and A.T. Macrander, ``Materials Aspects of GaAs and InP Based Structures'', Prentice Hall, 1991, ISBN 0-13-346826-7.   

Is Resonant X-ray Scattering Sensitive to the Electronic Structure of the CDW State?

The strong ``white-line'' observed at the Ta L$_{III}$ x-ray absorption edge (9.881KeV) in 1$T$-TaS$_{2}$ indicates resonance with the 2p $\to $ 5d atomic transition. Theories of the charge density wave (CDW) state in TaS$_{2}$ highlight the special role played by the 5d states of the thirteen Ta atoms in each unit cell of the super-lattice in forming the reconstructed conduction bands of the CDW state. By measuring the resonant diffraction at the CDW satellites, we combine this additional periodicity with the resonant scattering to amplify and isolate the x-ray signal from the CDW. Then, by studying the behavior as the system goes through the incommensurate-commensurate transition, we probe the sensitivity of this resonant x-ray scattering technique to changes in the electronic structure near the Fermi surface.   

Diffraction by Distorted Object -- a Unified Description of Coherent X-ray Diffraction and Imaging.

It is well-known that a diffracted or scattered wave by an object can be simply described by a Fourier transform of the electron density distribution of the object. This, in principle, is true only in the so-called far-field regime. In the near-field regime, evaluations of wave field amplitudes become more complicated and Fresnel diffraction and imaging effects have to be taken into account. In this paper, we present a unified diffraction theory that is valid in both far-field and near-field regime. Using the concept of a `phase-chirped' distorted object, where Fresnel-zone construction is embedded on an original object, we show that the Fourier transform of this distorted object can be used to evaluate Fresnel coherent diffraction images or phase-contrast images from objects. This approach is valid continuously from the near-field to the far-field regimes. In addition, the distorted-object approach extends the applicability of Fourier-based iterative phasing algorithm that is already established for far-field diffraction into the near-field holographic regime where phase retrieval had been difficult in high-resolution structural imaging of noncrystalline specimens. Imaging in near-field also possesses the advantage that it can eliminate twin image ambiguity that may exist in far-field diffraction.   

Recovering Ancient Inscriptions by X-ray Fluorescence Imaging

For many ancient cultures including those of the Mediterranean, carved stone inscriptions provide our most detailed historical record. Over the ages the surfaces of many of these inscriptions have been eroded so that the original text can no longer be distinguished. A method that allowed at least partial recovery of this lost text would provide a major breakthrough for the study of these cultures. The scope of analytical techniques that can be applied to stone tablets is limited by their large size and weight. We have applied X-ray fluorescence imaging to study the text of ancient stone inscriptions [1]. This method allows the concentrations of trace elements, including those introduced during inscription and painting, to be measured and mapped. The images created in this way correspond exactly to the published text of the inscription, both when traces of letters are visible with the naked eye and when they are barely detectable. [1] J. Powers et al., Zeitschrift f\"{u}r Papyrologie und Epigraphik 152: 221-227 (2005).   

Site specific valence band structure of SrTiO$_{3}$ determined with X-ray standing waves

Structure and chemical composition define the properties of materials, notably band structure and electronic characteristics of solids. Ab initio calculations deliver frequently reliably predictions, which are, however, difficult to verify, in particular in view of the direct relationship between atomic and electronic structure. Combining the x-ray standing wave (XSW) technique with X-ray photoelectron spectroscopy (XPS) is a unique tool in this sense. It is used here to identify unambiguously parts of the valence band of SrTiO$_{3}$, which can be assigned to Sr, Ti, or O-sites of the lattice. The XSW/XPS measurements were performed in UHV at the ID32 insertion device beamline at the ESRF using a (001) oriented, atomically clean SrTiO$_{3}$ crystal. Traversing the (111) and (112) Bragg reflections and recording the valence band for different standing wave positions within the lattice unit cell, the site specific contributions could directly be identified. Obtained experimental results are in very good agreement with theory, by utilizing as adjustable parameters the X-ray absorption cross sections of the valence electrons.   

CMR Manganite Sensors for Total Energy Measurements of the Linear Coherent Light Source Pulsed X-ray Laser

We are developing CMR manganite thin film bolometric sensors for total energy measurements of the Linear Coherent Light Source (LCLS) pulsed free electron x-ray laser (FEL). This application requires the sensor array to be fabricated on a low Z substrate capable of withstanding the pulse impact of 2 mJ in $\sim $ 200 femtoseconds, without the thermal expansion exceeding the yield strength, when the back side of the substrate is irradiated. Si is a potential candidate for meeting this requirement though its stability for long term exposure is a concern that needs to be tested. Optimal operating temperature of the sensor is estimated to be $\sim $ 100 K-200 K, based on finite element simulations of the temperature evolution in the sensor pixel. Our initial work has identified Nd $_{1-x}$ Sr$_{x}$ MnO$_{3}$ as the manganite material suitable for the LCLS sensor. We will present our materials development efforts towards LCLS sensor design as well as simulations of the sensor response.   

Verification and Application of a New Analysis Method for X-ray Diffraction Microscopy

X-ray diffraction microscopy has been used to determine microstructure maps of bulk polycrystalline material. Data are collected at the Advanced Photon Source beamline 1-ID using line focused 50keV x-rays. Diffracted beams are imaged with a CCD camera and are tracked through space so that orientation, point-of-origin, and shape of individual grains are encoded. Analysis uses a computer simulation of the measurement and sample to generate calculated diffraction patterns; orientations of sample space area elements are adjusted to obtain a match to the data. We illustrate data and analysis using a thin silicon wafer sample. We then show images of several layers of an aluminum polycrystal. The ability to obtain such images in a non-destructive way opens the possibility of measurements of the response to external stimuli of ensembles of individual grains. Our analysis is amenable to inclusion of complex scattering rules such as will be needed to study defected materials.   

Nanometer Focusing X-rays With Multiple Kinoform Lenses

It has been suggested that for refractive optics operating at photon energies of order 10 keV, that the resolution is limited to the wavelength divided by the critical angle. Using a compound kinoform lens consisting of individually optimized kinoform lenses, we investigate the possibility of exceeding this limit. Single-dimensional, kinoform lens stacks in deep-etched silicon have been fabricated that in principle can exceed the critical angle limit. These optics have been tested and the results will be presented. Additionally we present calculations that show that the resolution of radially-symmetric kinoform lenses is limited only by x-ray wavelength.   

Comparison of polycapillary and curved crystal optics for convergent beam powder x-ray diffraction

Comparisons were made of diffracted ring width, ring uniformity, system resolution and diffracted beam intensity for convergent beam powder diffraction using two different types of x-rays optics, doubly curved crystal optics$^{2}$ and polycapillary x-ray optics.$^{3,4}$ Measurements were made using very low power microfocus sources for small inorganic and organic standard samples. Detailed source and optics characterizations were performed to develop comparisons with theoretical calculations. Resolution and intensity were in good agreement with those obtained from simple geometrical calculations. \newline \newline $^{2}$Z. W. Chen, N. Mail, F.Z. Wei, C. A. MacDonald, W. M. Gibson ``Total reflection x-ray fluorescence with low power sources coupled to doubly curved crystal optics,'' Spectrochimica. Acta. B, 60 (4), pp.471-8, 2005. \newline $^{3}$C.A. MacDonald and W.M. Gibson, ``Applications and Advances In Polycapillary Optics'', X-ray Spectrometry, \textbf{\textit{32 (}}\textit{3), 2003, pp 258-268.} \newline $^{4}$C.A. MacDonald, S.M. Owens, and W.M. Gibson, ``Polycapillary X-Ray Optics for Microdiffraction,'' Journal of Applied Crystallography, \textbf{\textit{32}}\textit{, pp160-7, 1999.}   

Light diffraction from a metallic bigrating.

The diffraction of $\pi $- and $\sigma $- polarized electromagnetic plane waves from metallic bigratings is studied theoretically. The reduced Rayleigh equations are solved using a perturbation approach. The diffracted amplitudes are calculated until second order on the surface height profile. Numerical results of the diffraction orders and Near-Field are obtained using both, two-dimensional sinusoidal and semicircular profiles.   

Electron structure factor: a unique quantity in probing material's properties.

Electron diffraction is very sensitive to valence charge distribution compared with x-ray at small scattering angles due to the near cancellation of the scattering from the positively charged nucleus and the negatively charged electrons. Thus, small changes in electron density can lead to considerable variations in the scattering amplitude. The well known divergence at small scattering vector for Coulomb scattering leads to strong measurable scattered intensities. However, the advantage of the accurate measurement using quantitative electron diffraction has not been well appreciated. We propose here that the accurate measured electron structure factor can be a unique quantity in probing properties of materials. We demonstrate this by examining the sensitivity of electron structure factor to valence charge distribution, chemical composition variations, and charge / orbital orderings in many functional materials. We also show that the accurately measured low-order electron structure factors can be used to test first principles theories, especially to optimize exchange-correlation functionals.   

$^{3}$\textbf{He neutron spin filters for polarized neutron scattering}.

Polarized neutron scattering (PNS) is a powerful tool that probes the magnetic structures in a wide variety of magnetic materials. Polarized $^{3}$He gas, produced by optical pumping, can be used to polarize or analyze neutron beams because of the strong spin dependence of the neutron absorption cross section for $^{3}$He. Polarized $^{3}$He neutron spin filters (NSF) have been of great interest in PNS community due to recent significant improvement of their performance. Here I will discuss successful applications using $^{3}$He NSFs in polarized neutron reflectometry (PNR) and triple-axis spectrometry (TAS). In PNR, a $^{3}$He NSF in conjunction with a position-sensitive detector allows for efficient polarization analysis of off-specular scattering over a broad range of reciprocal space. In TAS, a $^{3}$He NSF in combination with a double focusing pyrolytic graphite monochromator provides greater versatility and higher intensity compared to a Heusler polarizer. Finally I will present the results from patterned magnetically-coupled thin films in PNR and our first ``proof-of-principle'' experiment in TAS, both of which were performed using $^{3}$He NSF(s) at the NIST Center for Neutron Research.   

A low and hyperthemal energy UHV ion beamline for surface scattering spectroscopies

We are using a differentially pumped beamline to provide well- collimated, monoenergetic beams of noble gas and alkali-metal ions that range in energy from $<$10eV to 10keV. These ion beams are scattered from a surface (e.g. Cu(001)) to study charge transfer effects, energy loss, and the excitation of surface phonons and excitons. The ion beam is focused into a UHV scattering chamber that possesses capabilities for studying and characterizing samples using LEED, Auger spectroscopy, and a Kelvin probe for work function measurements. Recent additions to this setup include replacing diffusion pumps with turbo pumps as well as the addition of a fast entry load-lock sample exchange system. Our current research is focused on developing a source to produce an ion beam of C$_{60} $ as well as studying charge transfer and energy loss effects at the low and hyperthermal energy range. Also, we are investigating chemicurrents associated with Schottky diodes in this energy regime.   

Session W10: Focus Session: Surfaces and Interfaces in Electronic Materials IV

Synthesizing Metal Nanowires that Detect Molecules

Noble metal nanowires have attributes including strength, ductility, and chemical stability, that make them attractive candidates for chemical sensing applications. However, in contrast to semiconductor nanowires, the conductivity of metal nanowires is not expected to be responsive to ``charge gating'' induced by the presence at the surface of the nanowire of bound ions. Consequently the properties of metal nanowires for chemical sensing have not been explored. We have developed a new method for preparing arrays of noble metal nanowires that involves the electrodeposition of metals (palladium, silver, platinum and gold) onto stepped graphite surfaces. Under the conditions employed for nanowire growth, metal is deposited selectively at step edges on the graphite surface leading to the formation of polycrystalline nanowires that are up to 1 mm in length and 30-500 nm in diameter. These nanowires adhere weakly to the graphite surface, and arrays of hundreds of wires may be transferred onto glass surfaces using a simple embedding process. These transferred nanowires can form the basis for chemical sensors in which the resistance of the nanowire array is modulated by molecules that chemisorb at the surfaces of these metals. Two examples involve palladium nanowires in the presence of hydrogen, and silver nanowires in the presence of amines. For both of these systems, the changes in resistance ($\Delta $R/R$_{o})$ can be 1000{\%} or more, but the mechanism response for the resistance changes are completely different. What is the origin of these enormous and unexpected resistance changes? In this talk, we focus attention on this issue and we discuss the prospects for developing practical chemical sensors based on these novel mechanisms.   

Novel Metallic Surface Arrays for SERS and Surface Forces Experiments

Nanofabrication techniques based on FIB (dual-beam focused ion beam lithography) and utilizing a novel solid masking scheme have produced extended arrays of coinage metal nanostructures on muscovite mica and semiconductor wafers. First, the FIB process is used to drill holes in the mask with various alternative shapes, from circles to squares to triangles. Their size is variable from $<$100nm to many microns, and their spacing and arrangement are also easily varied. These novel structures were used for two emerging applications. First, we demonstrate nanostructured ``forest'' patterns, arranged perpendicular to the solid, with a high aspect ratio in height to cross-section. Alternatively, these new structures were embedded into hollow spaces within the solid substrate, producing a \textit{physically flat} yet \textit{chemically rough} surface capable of electronic field enhancement. We demonstrate the ability of these new structures to enhance a Raman (SERS) signal with applications to nano-plasmonics.   

Scattering T-matrix theory for surface enhanced Raman scattering in clusters of nanoscale metal particles

Very large enhancements up to 14 orders of magnitude in the Raman cross-section from a molecule adsorbed on a single cluster of a few nanaoscale metal particles has been reported recently. The enhancement is believed mainly due to the enhanced electric field because of the excitation of the localized surface plasmon modes. We have developed a Green's function theory using scattering t-matrix approach in the wave-vector space to solve the Maxwell equations for the enhanced field near a metal particle cluster. The large enhancement in the field is due to the multiple scattering of the local modes of the individual metal particles that has been included exactly. We have considered clusters of different shape and size, for example, clusters of two, three, or four spherical particles forming a liner chain, a triangle or a square. Examples of clusters formed on the glass and metal plates are also discussed. We find the enhancement in the Raman cross section can reach up to 10 orders of magnitude for silver particle clusters and is in a broad frequency range. The results for gold particle clusters are also presented.   

Ordered Pore Arrays in Arrays in Alumina: Fabrication and Application Issues

Ordered two-dimensional arrays of nanopores in alumina have become a popular model system in nanotechnology because of its ease of fabrication and versatility in terms of geometrical parameters and of using it as template for a variety of materials. The presentation will cover the original two-step approach by Masuda published in Science in 1995 for selfordered pore arrays without long range order as well as nanoimprint and interference lithography approaches for long range ordered pore arrays. Guided self-assembly will also be covered. Metal filling of the pores by electrochemical deposition methods for magnetic storage applications and wetting of the pores by polymers which allows the fabrication of complex tube structures will be discussed. A novel method to get segmented nanotubes consisting e.g. of various gold and nickel segments will be described. Finally, the potential of atomic layer deposition in combination with porous alumina will be touched upon.   

Fabrication of and application of anodic alumina film with custom-designed nanochannel arrays

Among the strategies for growing one-dimensional straight nanostructure such as nanorods and nanowires, a viable approach is to grow the materials into templates with aligned nanochannels. Recently, porous anodic aluminum oxide (AAO) film has become an attractive template material for its self-aligned array of nanochannels. In this work, we have demonstrated, for the first time, a focused ion beam (FIB) direct-write lithographic method for selectively closing part of the channels of an ordered array on an AAO film creates a custom-designed nanochannels array. The initial ordered arrays are fabricated by FIB lithographic guiding techniques while the closure of the nanochannels within certain area is achieved by FIB bombardment of the AAO film. Besides, arrays of Ag-nanoparticles grown on anodic alumina nanochannels with precisely tunable gaps (5-25 nm) are exploited for surface-enhanced Raman spectroscopy. The enhancement becomes significant for gaps below 10 nm and turns dramatic when gaps reach an unprecedented value of 5 nm. The results are quantitatively consistent with theories based on collectively coupled surface plasmon.   

Novel Solid State Fabrication Techniques

We have electrochemically fabricated high dielectric coatings and nanowires in porous membranes. TEM images showed the nanowires to contain grains of single crystallinity. I-V characteristics of dielectric coatings have been investigated to optimize resistivity for minimum thickness. We will report on transport properties of structures constructed using these novel components. The feasibility of incorporating these electrochemically prepared solid state structures in nanoimprinted pentacene thin-film transistors will be evaluated.   

Confined self-organization of a lattice of surface magic-cluster and its structure determination

The ability to create an ensemble of nanostructures with specific size, shape, and arrangement on particular positions in space is one of the most important issue in the exploration nanoscience and realization nanotechnology. We have been exploring methods to set an initial structure of a substrate surface, which provides desirable constrains to self-organization process and lead to the formation of arrays of nanostructures with identical size and structure. A two dimensional lattice of Ga surface-magic-clusters (SMC), i. e. clusters exhibiting enhanced stability at certain sizes on a particular surface, has created by using the Si(111)-7x7 surface as a confining template. The structure of the individual SMC is determined by a combination of STM, density-functional calculations, and dynamic low energy electron diffraction. The diffraction method is applicable because the SMCs have identical size/structure and form an ordered array with the exact translational symmetry. The unprecedented detailed structure information provided by the diffraction measurement is consistent with direct microscopic imaging and theoretical calculations.   

Reversible adsorption of Au nanoparticles on SiO$_{2}$/Si: an \textit{in situ} ATR-IR study

Adsorption and desorption of Au nanoparticles (AuNP) on the (aminopropyl)triethoxysilan (APTES) treated SiO$_{2}$/Si surface was monitored by \textit{in situ} attenuated total reflection (ATR) infrared spectroscopy in combination with a liquid flow cell. With increasing the AuNP coverage at the surface, the absorption by water vibration was increased due to surface enhanced infrared absorption (SEIRA). Repulsive electrostatic forces between the incoming AuNP and the already adsorbed AuNP layer lead to saturation at submonolayer coverage of the surface. We clarified that the adsorption process can be described very well by a diffusion limited first-order Langmuir-kinetics model. Furthermore, we show that the AuNPs desorb from the surface when they are exposed to the solution of aminoethanethiol (AET).   

Session W11: Focus Session: Aerosols, Clusters, Droplets: Physics and Chemistry of Nanoobjects VI: Nanocatalysis, Supported Clusters

Chemical Functionalities in the non-scalable size-regime

The reactivity and optical properties of nanoscale systems are mainly dominated by quantum-size effects that govern the electronic spectra of clusters, by the structural dynamical fluxionality of clusters, as well as by impurity-doping effects. In this talk these fundamental and unique cluster properties will be illustrated by specific examples obtained from molecular beam experiments in the gas phase and experiments on size-selected clusters on surfaces. Where possible, concepts for their understanding are given. Specifically, in the first part of the talk new results on the optical properties of small gold clusters on amorphous silica will be presented, where Cavity Ringdown Spectroscopy is used to measure optical transitions of clusters at surfaces with extremely high sensitivity. In the second part of the talk a new approach for obtaining thermodynamic properties of chemical reactions by using micromechanical devices is introduced and an overview of results on the catalysis of gold clusters is presented. By combining these experimental data with ab-initio calculations, a picture of the peculiar catalytic behavior of gold is now emerging.   

Structure and Reactivity of M$_{x}$S$_{y}^{+}$ (M= Mo,W) Clusters with CO in the Gas Phase: an Experimental and DFT Study

We have recently constructed a cluster deposition apparatus which employs a magnetron sputtering source for generating gas-phase cation clusters of pure metals and metallic compounds. Of particular interest are clusters of the transition metal sulfides, M$_{x}$S$_{y}^{+}$ (M = Mo, W), which are known in their bulk form to be active catalysts for a wide range of heterogeneous reactions. The work reported here examines the gas-phase reactivity of small transition metal sulfide clusters as a first step towards investigations of model catalysts prepared by size-selected deposition. Specifically, we have used density functional theory (DFT) along with mass spectroscopy and gas-phase collision studies to examine the structure and stability of small sulfide clusters, M$_{x}$S$_{y}^{+}$ (x/y = 2/6, 3/7, 4/6, 5/7, 6/8, 7/10, 8/12). The number of metal sites was probed through the formation of adducts with CO, which was introduced into a hexapole collision cell. Calculated binding energy curves for the addition of CO onto available metal sites were compared with experiment to give insight as to which geometry for the bare clusters fit best. Results will be presented for the calculated structures and stabilities of the prominent clusters as well as their adducts.   

Oxidation of CO on various Fe$_{2}$O$_{3 }$ surfaces: A Theoretical Study

Recent experiments indicate that Fe$_{2}$O$_{3}$ nanoparticles can oxidize CO to CO$_{2}$ in the absence or presence of O$_{2}$. Depending on the size and experimental conditions, a Fe$_{2}$O$_{3}$ nanoparticle can have different faces that correspond to bulk surfaces of various orientations; which in turn can affect the catalytic activity of the nanoparticle.~ Hence,~an understanding of the reaction pathways, transition barriers, and the feasibility of CO oxidation on bulk surfaces of different orientations are critical to optimize the selection of nanoparticles. Theoretical investigations of the oxidation of CO on various Fe$_{2}$O$_{3}$ surfaces using gradient corrected density functional approach has been carried out. BPW91 functional form and double numeric basis set (DNP), as implemented in Dmol3 code are employed here. Different Fe$_{2}$O$_{3}$ (corundum) faces/surfaces are modeled by a cluster where the edge atoms are saturated by H atoms to simulate the effect of the infinite surface. Results corresponding to the reaction of CO with (100) and oxygen terminated (0001) surfaces at various surface sites; oxidation of CO, both in the presence as well as absence of oxygen, will be presented and discussed.   

Efficient Low-Temperature Oxidation of Carbon-Cluster Anions by SO$_{2}$

Carbon-cluster anions, C$_{N}^{-}$, are very reactive toward SO$_{2}$ (sticking probability of 0.012 $\pm $ 0.005 for C$_{27}^{-}$ at 25 $^{o}$C), in contrast to their inertness toward other common atmospheric gases and pollutants. In flow-reactor experiments at ambient temperature and near atmospheric pressure, primary adsorption of SO$_{2}$ by the carbon cluster anions, N = 4 -- 60, yields C$_{N}$SO$_{2}^{-}$ or C$_{N-1}$S$^{-}$. The inferred elimination of neutral CO$_{2}$ is also detected as meta-stable decay in collision-induced dissociation. At higher temperatures, the reaction of SO$_{2}$ with nascent carbon clusters yields C$_{N-1}$SO$^{-}$ as well as undetected CO. Such carbon clusters are formed in sooting flames and may act as nuclei for the formation of primary soot particles, and serve as models for the local structural features of active soot particle sites for black-carbon soot. The facile generation of reactive carbon-sulfide and --sulfinate units may therefore have implications for understanding the health and environmental effects attributed to the coincidence of soot and SO$_{2}$.   

Oxidation of magnesia-supported Pd-clusters leads to the ultimate limit of epitaxy with a catalytic function

Oxide-supported transition metal clusters and nanoparticles have recently attracted significant attention due to their important role as components of model-catalysts, sensors, solar-cells and magnetic recording devices. For small clusters, functionality and structure are closely interrelated. However, knowledge of the structure of the bare cluster is insufficient since the interaction with the chemical environment might cause drastic structural changes. Here we show by ab initio simulations based on the density functional theory that the reaction with molecular oxygen transforms small, non-crystalline, magnesia-supported Pd-clusters to crystalline Pd$_{x}$O$_{y}$ nano-oxide clusters that are in epitaxy with the underlying support [1]. Restructuring of the Pd backbone is controlled by the electrostatic interaction with magnesia leading to a strong reduction of the O$_{2 }$dissociation barrier. The supported Pd$_{x}$O$_{y}$ clusters are likely to serve as Mars-van-Krevelen oxygen reservoirs in catalytic oxidation reactions as observed previously for PdO overlayers and demonstrated here for the oxidation of CO molecules. [1] B.Huber, P.Koskinen, H.H\"{a}kkinen, M.Moseler, Nature Materials, advanced online publication 4. Dec. 2005   

Size effects on catalytic activity of supported metal clusters

Size-selected cluster deposition is used to prepare and study model catalysts with size-selected gold and iridium clusters supported on single crystal oxide supports. Chemistry is found to be strongly size-dependent and a combination of ion scattering and xray photoemission is used to probe the origins of the effects.   

The origin of catalytic activity of supported noble-metal nanoparticles

Supported Au nanoparticles $<$5 nm show a sharp rise in the low-temperature catalytic oxidation of CO while the reverse occurs for Pt nanoparticles. Subsequent experimental and theoretical investigations focused on Au nanoparticles and reached conflicting conclusions, attributing the enhanced Au activity to particular nanoscale features such as perimeter sites and low-coordination atoms, or to a particular bilayer structures, independent of particle size. Here we report atomically-resolved Z-contrast images of on TiO$_{2}$- supported Au nanoparticles and theoretical results on an ensemble of rutile- and anatase- supported Au and Pt nanoparticles. We show that high catalytic activity requires that (i) reaction barriers are small, (ii) reaction barriers are smaller than desorption energies of reacting molecules. As nanoparticle size is reduced, attached molecules induce reconstruction of Au clusters, resulting in looser Au-Au bonding and higher desorption energies and smaller reaction barriers for the attached molecules. Pt clusters get tighter by attached molecules, resulting in larger reaction barriers. The bilayer gold structures (M. S. Chen and D. W. Goodman, Science \textbf{306}, 252 (2004)) are extremely active because local reconstruction significantly reduces coordination of Au atoms. This work was supported in part by DOE Grant DE-FC02-01CH11085 and by DOE Division of Chemical Sciences under contract No. DE-AC05-00OR22725 with ORNL.   

Supported Gold and Platinum Clusters: Stability under Vacuum and Hydrogen at Elevated Temperatures; Optical Properties

The Achilles heal of supported clusters remains their low stability at elevated temperatures or when exposed to reactive gases. In this paper, the stability of Au$_{n}$ and Pt$_{n}$ clusters (n=6-10) supported on SiO$_{2}$, Al$_{2}$O$_{3}$ {\&} TiO$_{2}$ films is addressed. The clusters were heated in vacuum and in H$_{2}$ atmosphere, their stability monitored by synchrotron grazing incidence small angle X-ray scattering. Pt clusters supported on Al$_{2}$O$_{3}$ did not undergo sintering in vacuum and when exposed to hydrogen during a lengthy heat treatment reaching 400C; Au clusters on SiO$_{2}$ remained stable up to 350C. These temperatures are considerably higher than those characteristic for the onset of the catalytic activity of these clusters. Results on heat-induced structural isomerization of clusters will be shown. Single-particle UV-VIS spectra of Au-particles obtained by dark-field microscopy will be presented as well.   

Clusters at Surfaces Studied with Low-Temperature STM and UPS

We study the electronic structure of cluster/surface systems and the nature of charge transfer processes between the cluster and the surface. STM/STS and UPS on size selected large clusters in contact with a surface will be combined with photoemission on the same clusters in the gas phase. For metal islands (Au, Pb) on different surfaces (HOPG, Au(111) and Pb(111)) we observed significant energetic shifts in UPS if the islands were decoupled from the surface by a thick rare gas layer and different materials for the substrate and the islands were used [1]. In addition we measure mass spectra of size selected Ag clusters with a cluster machine consisting of a magnetron sputter gas aggregation source, a differential pumping stage with a cryo pump and a high transmission infinite range mass selector. In current experiments we extend these studies to the deposition of mass selected clusters on rare gas layers and different substrate systems. For these samples low-temperature STM and UPS will be compared with photoemission on the same clusters in a free cluster beam. [1] T. Irawan, D. Boecker, F. Ghaleh, C. Yin, B. v. Issendorff and H. H\"{o}vel, Appl. Phys. A (published online Sept. 2005)   

Size-Selected Au$_{n}$ and Ag$_{n}$ Nanoclusters on Rutlie TiO$_{2}$(110)-1x1 Surfaces Probed by UHV-STM.

Catalysis of the oxidation of CO and small olefins by Au$_{n}$ and Ag$_{n}$ nanoclusters on oxide supports is known to be strongly dependent on the size of the cluster and its interaction with the oxide surface. In our group we have probed this size dependence by depositing size-selected clusters of Ag$_{n}^{+}$ and Au$_{n}^{+}$(n = 1-7) from the gas phase onto single crystal rutile TiO$_{2}$ (110) (1x1) surfaces at room temperature under soft-landing ($<$ 2 eV/atom) conditions. We analyze the clusters on the surface using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and compare the resulting structures with theory. In the case of Au$_{n}^{+ }$, Ag$^{+ }$and Ag$_{2}^{+}$ clusters deposited under soft-landing conditions we observe large, sintered clusters indicating high mobility for these species on the surface. For Au$_{n}^{+}$ (n $\ge $ 2) and Ag$_{n}^{+}$ (n $\ge $ 3) clusters deposited under soft-landing conditions, however, we observe a high density of intact clusters bound to the surface and no sintered clusters indicating that these species have very limited mobility on the surface. For the intact clusters we can also observe the binding site and geometry of the cluster in the STM image and compare these with structures calculated using density functional theory (DFT) as well as with structures observed in the gas phase.   

Angle-resolved photoemission of Au clusters on graphite: quantized surface states on cluster facets

We present an experimental study for the electronic properties of metal clusters on surfaces. For the specific case of the confined Shockley surface state on the top (111) facets of gold clusters on graphite [1] we were able to detect the quantized electronic structure with two independent experimental techniques, scanning tunneling spectroscopy (STS) and ultraviolet photoelectron spectroscopy (UPS). Here we present new UPS data and their analysis which shows a quantitative agreement if we compare the density of states, extracted from the STS spectra by averaging over the cluster size distribution, with the UPS spectra using a deconvolution to compensate the dynamic final state effect [2] which leads to a systematic asymmetric broadening of all valance band UPS features [3]. \newline [1] I. Barke, H. H\"{o}vel, Phys. Rev. Lett. 90, 166801 (2003). \newline [2] H. H\"{o}vel, B. Grimm, M. Pollmann, B. Reihl, Phys. Rev. Lett. 81, 4608 (1998). \newline [3] H. H\"{o}vel, I. Barke, Prog. Surf. Sci., submitted for publication.   

Session W12: Molecules on Surfaces

Influence of interfacial structure on the charge transfer between adsorbed C$_{60}$ and Cu(111)

C$_{60}$ adsorption on metal surfaces typically incurs different amount of charge transfer from the substrate to the molecules. The charge transfer amount has never been found to approach that of optimally doped fulleride, e.g., K$_{3}$C$_{60}$, in which three electrons occupy the C$_{60}$ LUMO states. Here we demonstrate that C$_{60}$ adsorbed on Cu(111) render a nearly optimally doped fullerene film. The critical factor to produce such strong charge transfer is the identification of interfacial reconstruction in which a C$_{60}$ most probably resides in a monolayer pit consisting of seven removed Cu atoms. A direct comparison of low temperature (77 K, 4 K) STS on C$_{60}$ regions with and without interfacial reconstruction reveals drastic differences in charge transfer amount. Our STM/STS results are also consistent with a recent photoemission study [1] showing the optimal doping characteristic of the same system. This study thus demonstrates the importance of interfacial structure, which is often based on assumption, on prominent properties of molecular thin films. \newline [1] C. M. Cheng, K. D. Tsuei, unpublished.   

Coverage dependent supramolecular structures: 2D phases of C$_{60}$:ACA monolayers on Ag(111)

The dependence of supramolecular structures on fractional molecular coverage in a 2-component adlayer has been investigated using scanning tunneling microscopy. A series of acridine-9-carboxylic acid (ACA) surface structures emerges sequentially when deposited on Ag(111) at room temperature. At low molecular coverage ($\theta <0.4ML)$, ACA forms a two-dimensional gas phase. Ordered ACA structures appear with increased coverage: firstly a chain structure composed of ACA molecules linked by O--H$\cdot \cdot \cdot $N hydrogen bonds ($\theta >0.4ML)$, then a dimer structure composed of ACA dimers linked by carboxyl-carboxyl hydrogen bonds ($\theta \sim 1.0ML)$. The structures of the C$_{60}$:ACA binary system depend on the coverage of pre-deposited ACA. When the initial ACA coverage is between 0.4 ML and 0.8 ML, subsequent C$_{60}$ deposition results in a hexagonal cooperative structure with C$_{60}$ period nearly three times as large as the normal C$_{60}$ 2-D packing of 1 nm, and exists in enantiopure domains. A C$_{60}$ quasi-chain structure is formed when the initial ACA coverage is above 0.8 ML. Parallel C$_{60}$ chains are separated in space by the ACA dimer structure. Chemically reasonable molecular packing model are presented based on the observed STM images.   

Step fluctuations on Ag(111) surfaces with C$_{60 }$

STM has been used to characterize fluctuation properties of segments of step edges partly covered by C$_{60}$ on Ag(111) at room temperature. The distribution of C$_{60}$ at step edges exhibits a step orientation dependence: low-symmetry step edges are more favorable for C$_{60}$ binding. The temporal correlation functions of step segments between C$_{60}$-covered step regions scale as a power law, with an average exponent of 0.23$\pm $0.02, indicating that fluctuations of these ``confined'' steps are consistent with step-edge diffusion limited fluctuations. Parameters extracted from temporal correlation and autocorrelation analysis consistently indicate that close-packed steps have smaller fluctuation magnitude and higher step mobility than low-symmetry steps. The measured system sizes of step segments with different lengths show at most a weak step-length dependence. Fluctuation features thus yield the surprising conclusion that C$_{60}$ molecules are not acting as pinning points that constrain mass transport along the step edges.   

Self-intermixed patterns of perylene derivatives

Self-assembled systems are in the focus of nanotechnology research because of their potential use in the ``bottom-up'' creation of functional supramolecular structures. Potential applications of such systems include several functional groups. Therefore, the intermixing of different molecular compounds will become a key issue. In our approach we made use of H-bonding to form well-ordered intermixed patterns of two different perylene derivatives - PTCDA and DPDI. In an UHV-setup thin films of DPDI and PTCDA were prepared by evaporation on Ag(111). The sample characterization was carried out with a homebuilt STM. For a ratio of 1:1 and a total coverage of about one monolayer, an ordered intermixed pattern was observed. Each PTCDA molecule is interacting via H-bonding with four neighbouring DPDI molecules and vice versa. Furthermore, different intermixed patterns were observed depending on the ratio of the molecules and on the total coverage.   

Directed self-assembly of virus particles at nanoscale chemical templates

Because viruses can be site-specifically engineered to present catalytic, electronic, and optical moieties, they are attractive as building blocks for hierarchical nanostructures. We report results using scanned probe nanolithography to direct virus organization into 1D and 2D patterns and \textit{in situ} AFM investigations of organization dynamics as pattern geometry, inter-viral potential, virus flux, and virus-pattern interaction are varied. Cowpea Mosaic Virus was modified to present surface sites with histidine (His) or cysteine (Cys) groups. Flat gold substrates were patterned with 10-100nm features of alkyl thiols terminated by Ni-NTA or meleimide groups to reversibly and irreversibly bind to the Hys and Cys groups, respectively. We show how assembly kinetics, degree of ordering and cluster-size distribution at these templates depend on the control parameters and present a physical picture of virus assembly at templates that incorporates growth dynamics of small-molecule epitaxial systems and condensation dynamics of colloidal systems. This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.   

Enhanced Chemisorption of Cu(hfac)$_{2}$ on Parylene Surface by N$_{2}$ Plasma Treatment

The metallization of polymers has been intensively studied due to its wide industrial applications. We report a study of interfacial interaction of metalorganic Cu(hfac)$_{2}$ with the Parylene surface. Parylene is a low k dielectric polymer prepared by a chemical vapor deposition technique. The as-deposited Parylene surface is shown to be hydrophobic with a measured water droplet contact angle $\sim $72\r{ }. However, after the N$_{2}$ plasma treatment, the water droplet contact angle decreases to $\sim $40\r{ } due to the formation of oxygen and nitrogen functional groups on the surface, as observed by x-ray photoelectron spectroscopy (XPS). These functional groups improve Cu(hfac)$_{2}$ chemisorption on the plasma treated Parylene surface. Further studies by XPS show that chemisorption of Cu(hfac)$_{2}$ is self-limiting up to 20 sec of Cu(hfac)$_{2}$ precursor exposure time. The enhancement of chemisorption of metalorganic precursors on the polymer surface is an important step for chemical vapor deposition or atomic layer deposition of metal. $^{a }$Supported by Thai govt. fellowship (SP) and SRC (JSJ).   

Surface Dynamics in thin films of a small organic glass-former

Enhanced dynamics of molecules near the free surface of thin supported films have been measured using neutron reflectivity. Thin films of the small molecule glass former tris-naphthylbenzene were vapor deposited with subnanometer initial surface roughness, allowing diffusion rates to be measured between isotopically labeled layers. Measured dynamics suggest a mobile surface layer of about 4 nm with a significant decrease in Tg. This can be compared with recent measurements in polymeric systems which find thick active layers and Tg shifts up to 30 K.   

Topography and Wetting of Dotriacontane Films on Graphite Surfaces

We have used Atomic Force Microscopy (AFM) in the noncontact mode and synchrotron x-ray diffraction to investigate the structure, morphology, and wetting of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited from a heptane solution onto highly-oriented pyrolytic graphite (HOPG). Consistent with previous neutron diffraction measurements,$^{2}$ the x-ray patterns indicate one to two layers immediately adjacent to the HOPG surface in which the molecules are oriented with their long axis parallel to the interface. Above these parallel layers, the AFM images show a partial layer of C32 molecules oriented with their long axis perpendicular to the surface. Upon heating above room temperature, we observe the area occupied by the perpendicular monolayer first to increase and then to decrease. Just above the bulk melting point, the perpendicular monolayer dewets the underlying parallel layers as we have found for C32 films adsorbed on a SiO$_{2 }$substrate. $^{2}$K. W. Herwig, B. Matthies, and H. Taub, Phys. Rev. Lett. \textbf{75}, 3154 (1995).   

Ellipsometric Measurements of Dotriacontane Films Adsorbed on Au(111) Surfaces

We have conducted ellipsometric and stray light intensity measurements on dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films adsorbed on Au(111) substrates in air as a function of temperature in order to determine their optical thickness and surface roughness. The C32 films were deposited from a heptane ($n$-C$_{7}$H$_{16})$ solution onto the gold surface. Our large, atomically flat gold substrates were produced by the method reported by Hegner \textit{et al}.$^{2}$ in which gold films grown on mica are glued onto Si(100) wafers. For films of 25 {\AA} thickness, our ellipsometry measurements show a decrease of about 75{\%} in the height of the monolayer substep compared to the same film adsorbed on SiO$_{2 }$substrates.$^{3}$ This substep is believed to be contributed by a monolayer phase in which the molecules are oriented with their long axis perpendicular to the surface. The substep decrease may be interpreted as reduction in the number of molecules in this phase or possibly a tilting of the molecules. $^{2 }$M. Hegner \textit{et al.}, Surf. Sci. \textbf{291}, 39 (1993). $^{3}$U.G. Volkmann \textit{et al}., J. Chem. Phys. \textbf{116}, 2107 (2002).   

Comparison of Thickness and Morphology of Dotriacontane Films on SiO$_{2}$/Si Surfaces Vapor-deposited in High Vacuum with those Deposited from Solution

We have used Very High Resolution Ellipsometry (VHRE) and Atomic Force Microscopy in the noncontact mode to compare the thickness and morphology of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited by two different methods on Si(100) wafers coated with their native oxide. During deposition, the substrate temperature was held below the bulk melting point of C32. As monitored by VHRE, the film thickness of different samples was found to be in the range 20 {\AA} to 400 {\AA}. Films deposited by physical vapor deposition from a Knudsen cell in high vacuum are optically smooth and homogeneous, while deposition by dip-coating from a heptane solution also results in optically smooth but less homogeneous layers. Heating/cooling cycles were performed on these two sample types while conducting VHRE and stray light intensity measurements in order to compare the wetting behavior and surface roughness of C32 as a function of film thickness on both hydrophilic and hydrophobic SiO$_{2}$/Si surfaces.   

Controlling growth kinetics and morphology of crystal surfaces through biomolecular interactions

The complex shapes and hierarchical designs of biomineralized nanostructures arise from biomolecular controls over crystallization. One prevailing view is that mineral-associated macromolecules are responsible for nucleating and stabilizing non-equilibrium polymorphs and morphologies through interactions at crystal surfaces. Here we report results of \textit{in situ} AFM and molecular modeling investigations of calcite growth in the presence of acidic amino acids, polypeptides, and proteins associated with biomineral formation. We show how the sterochemical relationship between modifier and crystal lattice lead to step-specific interactions and how those interactions account for the changes in kinetics and morphology. We analyze the results in terms of classic physical models of crystal growth and epitaxy and show that there are important deviations from those classic models that stem, in part, from the low kink density of steps on calcite. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.   

Mixing in a Solid Monolayer: SF$_{6}$ + C$_{2}$F$_{6}$ on Graphite

We have studied mixing of SF$_{6}$ and C$_{2}$F$_{6}$ in a solid monolayer film on graphite by admitting one gas to the cell to a pressure between monolayer condensation and saturation, then slowly admitting the other gas. The total coverage is monitored by ellipsometry and the condition of the individual adsorbates is determined by infrared absorption spectra. In particular, shifts in absorption lines due to dynamic dipole coupling indicate the local environment of molecules of each species. At 75 and 80 K there is no detectable solubility of C$_{2}$F$_{6 }$in a dense SF$_{6}$ monolayer, then a layer of nearly pure C$_{2}$F$_{6 }$condenses on top. At 86 K there is limited solubility of C$_{2}$F$_{6}$ in dense SF$_{6}$, followed by appearance of a second C$_{2}$F$_{6}$--rich phase in the monolayer. At 90 K and above, C$_{2}$F$_{6}$ appears to replace SF$_{6}$ by continuous substitution; thus there appears to be a 2-D solid consolute critical point near 90 K. If SF$_{6}$ is added to a higher-temperature, low-density C$_{2}$F$_{6}$ layer, the C$_{2}$F$_{6}$ molecules are rapidly compressed from flat or tilted orientation to axes perpendicular to the substrate, then are continuously diluted.   

Energetic Study of the Flip-flop Motion of P(VDF-TrFE)

Ferroelectric copolymer P(VDF-TrFE) has attracted significant attention in material science and technology due to its excellent electromechanical properties and easy manipulation of the individual monomer or monomer clusters resulting in the change of the such properties. Theoretical aspect of the switching dynamics of P(VDF-TrFE) has been investigated by using the density function theory and compared with experimental results. Through calculation, we find a simple flipping of the individual monomer with lower energy that involves no change of the bond length and the bond angle of --C--C--, --C--H--, and \mbox{--C--F--.} These consist with the experimental results obtained with STM. We compare the STM image before and after the flipping of the monomers. Except the observation of the lattice shift at the boundary, we find no other structure distortion and no change in the inter- and intra- chain spacing.   

Exploiting Photo-induced Reactions in Polymeric Thin Films to Create Hierarchically Ordered, Defect-free Materials

Computer simulations reveal how photo-induced chemical reactions in polymeric thin films can be exploited to create long-range order in materials whose features range from the sub-micron to the nanoscale. The process is initiated by shining a spatially uniform light on a 2D photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures, which resemble the phases of microphase-separated diblock copolymers. We then add a non-reactive homopolymer C, which is immiscible with both A and B. This component localizes in regions that are irradiated with a higher intensity light and one can effectively write a pattern of C onto the AB film. Rastering over the ternary blend with the collimated light now leads to hierarchically ordered patterns of A, B and C. The findings point to a facile, non-intrusive process for manufacturing high quality polymeric devices in a low-cost, efficient manner.   

Session W16: Semiconductor Applications

Extraordinary Electrical Conductance in GaAs-In Hybrid Structures

Following the demonstration of extraordinary magnetoresistance (EMR)in semiconductor-metal hybrids\footnote{ S.A. Solin et al., Science {\bf289},1530 (2000).}, it has been realized that EMR is but one example of a general class of EXX phenomena that can be geometrically enhanced by the judicious choice of sample geometry. Two other EXX phenomena reported recently are extraordinary piezoconductance, EPC, and extraordinary Optoconductance, EOC. Here we address a fourth EXX phenomena, extraordinary electrical conductance, EEC. We develop a new design concept for an EEC sensor, which is a van der Pauw plate structure of Si-doped GaAs with a non-magnetic metallic shunt on top so that the external E field is perpendicular to the interface. EEC arises from the current redistribution between the shunt and GaAs when an external E field lowers the Schottky barrier at the interface. This allows more electrons to tunnel through and results in a larger conductance. We compare the response of a sample with a Schottky barrier to an unshunted sample and to a shunted sample with an Ohmic contact. The conductance of each sample was measured as a function of temperature, bias current and external perturbing field. In addition, we will compare the EEC structure to the Schottky diode structure to illustrate the advantages of an EEC sensor for static charge imaging.   

High-power mid-infrared interband cascade lasers

We have grown and fabricated interband cascade lasers (ICLs) with ``W'' active regions. The ICL structures were etched into 140-$\mu $m-wide ridges, with 100-$\mu $m-wide metal strips deposited in the middle, and operated epitaxial-side-up. Initial devices displayed lasing thresholds as low as 12 A/cm$^{2}$ at 78 K, series resistance as low as 0.21 m$\Omega \cdot $cm$^{2}$, and a voltage efficiency of 96{\%}. Cavity length studies on a series of ICLs with 5, 10, and 15 stages determined that the internal losses at 78 K were 16, 27, and 37 cm$^{-1}$, respectively, while the internal efficiencies were $\approx $ 80{\%} in all cases. Pulsed operating temperatures as high as 300 K were obtained, and a 5-stage device with 0.5 mm cavity length had a wallplug efficiency per facet of $\approx $ 20{\%} without facet coatings. A 3-mm-long laser with high-reflection (95{\%}) and anti-reflection (5{\%}) coatings produced $>$1.1 W of cw output power at 78 K.   

Single-Mode Distributed-Feedback ``W'' Diode and Interband Cascade Lasers in the Mid-Infrared

To obtain spectrally pure output, we have fabricated narrow index--guided ridges with lateral distributed-feedback (DFB) line gratings on both ``W'' diode and interband cascade lasers. The ``W'' diode structure containing a GaSb $p$-SCH etch-stop layer was chemically etched into a 5 $\mu $m ridge and a first-order DFB grating constructed on both side walls of the ridge. For the interband cascade laser, a first-order top DFB grating was patterned on top of a chemically-etched ridge that was 15 $\mu $m wide. The low-loss DFB mode was roughly resonant with the gain peak at $T$ = 165 K for the ``W'' diode and at $T$ = 110 K for the interband cascade laser. The sidemode suppression ratios were definitely $>$ 20 dB for both devices, and all of the features above 30 dB appeared to result from instrument noise rather than actual parasitic modes. Just beyond the stop band on the long-wavelength side of the ``W'' diode DFB, a series of longitudinal modes became apparent at $>$ 30 dB suppression. For the narrow-ridge waveguide DFB devices, the temperature ranges over which single-mode lasing were successfully obtained were 140-162 K for the ``W'' diode, for which $\lambda $ = 3.195 $\sim $ 3.202 $\mu $m (0.29 nm/K), and 110-125 K for the interband cascade laser, for which $\lambda $ = 3.452 $\sim $ 3.456 $\mu $m (0.27 nm/K).   

High Resolution 2D dopant profiling of FinFET structures and Silicon-based Devices using Scanning Probe Microscopies

The ability to perform dopant/junction profiling with high spatial resolution is critical for development of future generation devices such as FinFET structures. Among various forms of scanning probe microscopy, scanning tunneling microscopy (STM) has demonstrated direct atomic imaging of dopant atoms on GaAs (110) surfaces. More recently, scanning thermoelectric microscopy (SThEM) (H.K. Lyeo et al \textit{Science} v.303 p816 (2004)) has been applied to profile GaAs $p-n$ junction with unprecedented spatial resolution. The key challenge to successfully apply these techniques to silicon-based devices is to prepare a surface that is both chemically and electronically passivated. Here we present our progress toward this goal. We present STM and SThEM studies on Si $p-n$ junction devices including FinFET structures. We also present in-depth profiling of fin structures using scanning capacitance (SCM) and conductive atomic force microscopy (C-AFM) (Khajetoorians \textit{et al} APL (submission)).   

Photoelectron Multipliers Based On Avalanche Pn-I-Pn Structures

We present a new optoelectronic device, which consists of a multilayered semiconductor structure, where the necessary conditions for the creation of photoelectrons are met, such that it will enable sequential avalanche multiplication of electrons and holes inside two depletion slabs created around the p-n junctions of a reverse biased pn-i-pn structure [1]. The mathematical model and computer simulations of this Semiconductor Photo-electron Multiplier (SPM) for different semiconductor materials are presented. Its performance is evaluated and later on compared with conventional devices. [1] K.A.Lukin, H.A.Cerdeira, A.A.Colavita, Current Oscillations in Avalanche Particle Detectors with pnipn-Structure. IEEE Transactions on Electron Devices. 43(3), 1996, 473-478.   

Electron relaxation through multiphonon processes in a double quantum dot

We theoretically study the relaxation of the electron orbital states of a double quantum dot system due to two-phonon processes. In particular, we calculate how the relaxation rates depend on the separation distance between the quantum dots, the strength of quantum dot confinement, and the lattice temperature. Our results show interesting crossovers in the relative strength of different scattering channels as temperature is varied. Furthermore, although at low temperatures two-phonon processes are much weaker compared to one-phonon processes in relaxing electron orbital states, at room temperature they are as important as one-phonon processes.   

Dark current generation in a confined and depleted region of silicon

Dark current generation degrades the performance of silicon solid-state imagers. The present study examines whether a depleted region with defects supplies dark current to a photodiode when it is separated by a neutral region. Thermally-excited electron-hole pairs appear in the depleted region, which is confined by infinite potential barriers in two dimensions and a triangular potential well in the third. The triangular well has ground states of 0.036 and 0.058 eV for electric fields of 0.1 and 0.2 MV/cm, respectively. Lateral dimensions of 0.1 to 1.0 micrometers lead to high electron densities, which quench further dark current generation before excited states are occupied. As a result, the electrons must diffuse against the strong electric field and are unlikely to reach the photodiode. The same potential barrier accelerates the holes and creates electron-hole pairs by impact ionization. The electron would be generated near the top of the potential barrier. However, the probability that the hole acquires sufficient energy is 0.00001 for 0.2 MV/cm and 0.0028 for 0.3 MV/cm, based on J. S. Marsland in Solid-State Electronics 30, 125 (1987). If the defect is a gold atom at 55 C, this leads to 0.0056 and 1.6 electrons/s.   

Measuring Ionization and Athermal Phonons: Detectors of the Cyrogenic Dark Matter Search

The Cryogenic Dark Matter Search (CDMS) is a search for weakly-interacting massive particles (WIMPS) in the halo of our galaxy. WIMPs are a favored solution to the dark matter problem in cosmology and particle physics. We will describe how CDMS measures simultaneously the number of charge carriers and the energy in athermal phonons created by particle interactions in Ge and Si crystals at a temperature of 50 mK. Together these distinct signals create a signature response for each event, allowing candidate WIMP interactions with nuclei to be discriminated from electromagnetic radioactive background which interacts with electrons. Combining this method with additional information contained in the athermal phonon signal shape, CDMS has achieved a sensitivity roughly ten times better than any other experiment in the world. These techniques introduce a number of unique challenges. Bias levels must remain at only a few volts, else the secondary phonons emitted by the drifted carriers dominate the original phonon signal. The neutralization of charge-trapping sites, even at concentrations of only $\sim10^{10}~ cm^{-3}$, is of primary importance to the performance of charge collection. We present the methods of crystal neutralization, subsequent characterization, and representative phenomena encountered in practice.   

Resistive all boron carbide neutron detectors

Semiconducting boron carbide is a promising material for true solid-state neutron detection [1]. An all boron carbide (BC) layer was deposited on sapphire (Al$_{2}$O$_{3})$ with sputtered Chrome/Gold electrical contacts. Resistance vs. temperature measurements indicate a T$^{-3/2}$ dependence and a band gap of $\sim $ 0.17eV. X-ray diffraction measurements confirm the similarities in crystal structure of the films grown on Al$_{2}$O$_{3}$ and Si. Detection area ranged from 0.25mm$^{2}$ to 1mm$^{2}$ and the thickness of the films ranged from 280nm to 600nm. Neutron detection measurements show no sharp spectral peaks but a long high energy tail which increased in counts as the reactor power was increased, in agreement with both monte carlo simulations and simple model calculations [2]. The low thermal neutron capture cross section of Al and O ensures that the entire neutron signal observed is from the resistive boron carbide layer, thus demonstrating the fabrication of an all boron carbide neutron detector. We show plots as a function of reactor power and thickness. [1] B.W. Robertson, S. Adenwalla, A. Harken, et al., \textit{Appl. Phys. Lett.} \textbf{80}, 3644 (2002). [2] C. Lundstedt, A. Harken, E. Day, B. W. Robertson, S. Adenwalla, submitted to NIM.   

What should neutron spectra from boron carbide devices look like?

GEANT4 (Geometry ANd Tracking) monte carlo modeling was performed on boron based neutron detectors [1]. Two different detector geometries were used. Geometry 1 consisted of a boron carbide (BC) layer placed on a Silicon (Si) layer in a cylindrical design with thermal neutrons of energy 0.025eV incident on the BC face. Geometry 2 was a rudimentary calorimeter made by sandwiching a moderator material between two BC/Si layers. The energy deposition spectra for the BC/Si device of various BC layer thicknesses for geometry 1 are presented as well as the spectra for geometry 2. In geometry 2, by changing the moderator material and thickness, higher energy neutrons may be detected, due to thermaization of neutrons in the moderator material. We show results for incident neutrons ranging in energy from 0.025eV to 2.5MeV. [1] C. Lundstedt, A. Harken, E. Day, B. W. Robertson, S. Adenwalla, submitted to NIM.   

Neutron detection characteristics of semiconducting boron carbide

The all boron carbide semiconducting neutron detector is sought because is could potentially yield the most useful and efficient of all thermal neutron detectors. We report on experiments to obtain data using alpha particle and neutron capture measurements. The results are analyzed in relation to our measurements of the dielectric properties and to initial charge transport considerations. The neutron capture results are compared with our modeling of the ideal neutron detector behavior calculated for an all boron carbide semiconductor device.   

Alpha-Energy-Deposition-Profiling of Radioisotope $p-i-n$ Diodes for Power Generation

The high energy density and long half-life of certain alpha-emitting radioisotopes enables viable and long-lived power supplies to be fabricated on the micro-scale. A design incorporating an InGaP $p-i-n$ photovoltaic (PV) device that directly converts the kinetic energy of the alpha-particles into electricity represents both a scalable and efficient microsystem design. To better understand the relationship between the alpha-energy-deposition-profile (ADEP) and the maximum power conversion efficiency for this device structure, we have performed two systematic studies. In these studies, I-V characteristics for the InGaP PV device under alpha-flux are measured as a function of alpha source distance, and as function of aluminum film thickness (10 nm to 10 $\mu $m) which is deposited onto the surface of the PV device. Both techniques will alter the ADEP in relation to the active region of the PV device. These experimental results are compared to a theoretical model which utilizes Monte Carlo simulations and numerical calculations to determine the ADEP for the same device configuration. The understanding gained from this analysis has direct implications towards the fabrication of radioisotope microbatteries with structural characteristics that enable optimal power conversion efficiencies to be achieved.   

Analysis of microwave-frequency field patterns in an externally-driven Single-Electron Transistor

We report a numerical study of electromagnetic field patterns that emerge in a Single-Electron Transistor (SET) device driven by a microwave-frequency signal. In an SET, an electronic droplet (quantum dot) containing a few electrons is connected to two macroscopic conductors via tunnel barriers, and DC current measurements are used to investigate the quantum properties of the lead-dot system. Our goal is to develop a method for a well-controlled excitation of few-electron devices with microwaves. Such capability is needed for investigating the intrinsic time scales of Kondo-correlated electrons, not accessible in static experiments. We study realistic model geometry of an SET defined lithographically on a semiconductor heterostructure such as GaAs/AlGaAs. We find that at frequencies $\sim $ 10 GHz and above, the microwave voltages across the sub-micron features of the SET can be drastically different from those applied to the large-scale pads and depend in a complex and sensitive way on the excitation frequency, thus presenting a challenge for dynamic transport experiments with SETs. We discuss possible strategies for resolving the problem.   

The effect of negative electron affinity emitter materials on space charge mitigation of vacuum thermionic energy conversion devices

Vacuum thermionic energy conversion (TEC) devices provide a way to convert heat directly into electrical work. The negative space charge effect has been an effect that significantly degrades the performance of these devices, requiring small interelectrode spacings for reasonable performance. Recently, Nitrogen doped, Hydrogen terminated, ultra-nanocrystalline diamond films have been investigated as possible candidates for low operating temperature, low work function emitter materials. Furthermore, these materials exhibit a so-called negative electron affinity (NEA) where the vacuum level lies below the conduction band minimum of the material. As a result of this NEA property, the distribution of thermionically emitted electrons will have some nonzero minimum initial velocity. A model was developed to determine the effect that the NEA property of these types of emitters have on mitigation of the space charge effect. The model shows that a TEC with an NEA emitter material will have comparable performance with a non-NEA emitter TEC with a smaller gap. Thus, it is possible to use NEA emitters to relax the requirement of a small gap distance.   

Session W18: Focus Session: Carbon Nanotubes: Electronic and Optical Properties III

Phonon sideband in optical spectra of C13 carbon nanotubes

Photoluminescence (PL) of single-walled carbon nanotubes (SWNTs) has been widely used for the measurement of chirality distribution of single-walled carbon nanotubes (SWNTs). However, there are unassigned peaks in the 2-D photoluminescence map plotted as a function of emission and excitation photon energy. In order to clarify the origin of these peaks, we have studied photoluminescence (PL) and resonant Raman scattering of single-walled carbon nanotubes (SWNTs) consisting of carbon-13 (SW$^{13}$CNTs) synthesized from a small amount of isotopically modified ethanol [1]. There was almost no change in the Raman spectra shape for SW$^{13}$CNTs except for a downshift of the Raman shift frequency by the square-root of the mass ratio 12/13. By comparing photoluminescence excitation (PLE) spectra of SW$^{13}$CNTs and normal SWNTs, the excitonic phonon sideband due to strong exciton-phonon interaction was clearly identified with the expected isotope shift [2]. The PLE line shape and energy difference from E$_{ii}$ are remarkably similar to the excitonic phonon sideband predicted by Perebeinos \textit{et al.} for (7, 5) and (6, 5) nanotubes. In addition to these phonon sideband features, we also found low-intensity `pure electronic' features whose origin has never been elucidated. In order to examine these `pure electronic' features, polarized PLE measurements on individually-dispersed SWNTs aligned in a gelatin-based thin film. By comparing optical transitions of SWNTs for incident light parallel or perpendicularly polarized to the nanotube axis, we have attributed these features to excitation by perpendicularly polarized light to the nanotube axis. The measured absorption energies of perpendicularly polarized light are compared with cross-polarized version of the Kataura plot. References: [1] S. Maruyama and Y. Miyauchi, AIP Conf. Proc. 786, 2005, 100-105. [2] Y. Miyauchi and S. Maruyama, cond-mat/0508232.   

Raman Studies of Exciton-Phonon Coupling in Single-Walled Carbon Nanotubes

Significant chirality-dependent effects on nanotube Raman intensities have recently been observed whose origins lie in the chirality dependence of exciton-phonon coupling. We present resonance Raman excitation data that demonstrates this dependence in radial breathing mode (RBM) intensities for both E11 and E22 excitation. For E22 excitation, intensities for (n- m)mod 3 = -1 chiralities are significantly more intense than for (n-m)mod 3 = +1, with more complex behavior in E11 excitation. We discuss the results in terms of a new theoretical analysis of exciton-phonon coupling that accurately describes the observations with simple intuitive analytical expressions. Relative intensities can be easily predicted using a newly introduced parameter that is also able to explain a number of anomalies in the observations. We also present the first direct comparison of E11 vs. E22 intensities for a number of chiralities. This comparison yields the ratio of the decay rates for the excited and ground excitonic states serving as intermediate states in the Raman process under E22 and E11 excitations, respectively.   

Resonance Raman Spectroscopy of $1.2 $< d_t < 2.0$\,nm Diameter Single Wall Carbon Nanotubes in the $E_{33}^S$ and $E_{44}^S$ Optical Range

This work uses Resonant Raman Spectroscopy with excitation laser energy from 1.26 to 2.98 eV to measure the $E_{33}^S$ and $E_{44}^S$ optical transition energies, for single wall carbon nanotubes (SWNTs) with diameters in the range$1.2 < d_t < 2.0 $\,nm.We identify the families of $(2n+m) =$ constant and analyse the radial breathing mode (RBM) frequencies, the $E_{33} ^S$ and $E_{44}^S$ energies and intensities as a function of $(n,m)$. The excitonic effects are weaker in the $E_{33}^S$ and $E_{44}^S$, the energies being blue-shifted when compared with earlier predictions. We also study the relation between the RBM intensity and tube chirality, clearly showing that the RBM Raman spectrum is less intense for armchair tubes.   

Raman spectroscopy and imaging of surface and suspended carbon nanotubes

Freely suspended single walled carbon nanotubes show enhanced photoluminescence and Raman signals compared with those from nanotubes on surfaces. We prepared suspended carbon nanotubes by chemical vapor deposition on lithographically patterned substrates. The nanotubes were of the order of 100 microns in length with suspended segments ranging from 1 to 20 microns. Individual nanotubes and bundles were characterized with both Raman spectroscopy and scanning electron microscopy. Raman signals from suspended and non-suspended segments of the same nanotube were detected with up to a tenfold signal enhancement observed for the suspended segments. The effect of suspension is clearly illustrated in spatially resolved confocal images of nanotubes extending over many microns.   

Effects of collective excitations on the G-band and RBM modes in the Raman spectra of metallic unfilled and filled carbon nanotubes

The Raman spectra of a single-wall carbon nanotube (SWNT) consist of three types of modes; (i) the high frequency G-mode arising out of tangential oscillations of carbon atoms, (ii) D-mode due to the defects in the nanotube and (iii) the low frequency radial breathing mode (RBM) resulting out of radial oscillations of the carbon atoms. In this paper we theoretically investigate the effects of collective oscillations of electrons (plasmons) on the G and RBM modes in the Raman spectra of a filled and unfilled metallic SWNT. Inclusion of plasmon and the filling (rattler) atom produces four peaks in the Raman spectra in general. The positions and relative strengths of the Raman peaks [1] depend upon phonon frequencies of the nanotube and that of the filling atoms, the plasmon frequency, the strength of the electron-phonon interaction, strength of the interactions between the nanotube phonons and rattler phonon and radius of the nanotube [2]. Usually the intensity of the G-mode is higher than that of RBM. For heavier filling atoms the frequency of the rattler phonon is lower in value, which may broaden the peak to such an extent that it may disappear in the background spectrum altogether. 1.S.M. Bose et al., Physica B \textbf{351}, 129 (2004) 2. S.M. Bose, S.Gayen and S. Behera, Phys. Rev. B \textbf{72}, 153402 (2005).   

Chirality dependence of Raman intensity of single wall cabon nanotubes

We present calculated Raman intensity of radial breathing modes (RBM), and other first and second order Raman signals as a function of (n,m) with exciton wavefunctions. Because of strong k dependent electron-phonon and electron-photon matrix elements, the Raman intensity shows (2n+m) family pattern. Within the extended tight binding calculation, we make exciton Kataura-plot for RBM. The Raman intensity is enhanced by localized wavefunction of the bright exciton which decreases with increasing energy and diameter. We will further discuss disorder induced D-band Raman intensity with some experimental results.   

Carbon Nanotube Population Analysis from Raman and Photoluminescence Intensities

Large efforts are now being directed to developing synthesis or manipulation processes able to generate single-wall carbon nanotubes (SWNT) with well-defined geometric structure, i.e. $(n,m)$ indices. The $(n,m)$ population in a SWNT sample can be obtained from intensity analysis of photoluminescence excitation (PLE) and Resonance Raman spectroscopy (RRS) experiments, after the information is corrected to account for the $(n,m)$ dependence of the RRS and PLE efficiency. In the absence of standard single-wall carbon nanotube samples with well-known (n,m) population, we provide both a photoluminescence excitation (PLE) and resonance Raman scattering (RRS) analysis that together can be used to check the calculations for PLE and RRS efficiency. We show that available models describe well the chirality dependence of the intensity ratio, confirming the differences between type 1 and type 2 semiconducting tubes and the existence of a node in the radial breathing mode intensity for type 2 carbon nanotubes with chiral angles between 20$^{\circ}$ and 25$^{\circ}$. The method is used to characterize SWNT samples grown by the CoMoCAT, HiPco and alcohol-CVD processes.   

Optical studies of finite length effects in short DNA-wrapped carbon nanotubes

In this study, a systematic resonance Raman study was carried out on samples of DNA-wrapped SWNTs with average lengths between 50 and 100nm using multiple laser excitation energies. The different Raman features have been studied in detail as a function of nanotube length and laser excitation energies. The ratio of the D-band to G-band intensities has been found to increase with decreasing average SWNT length and decreasing laser excitation energy. As the nanotubes becomes much shorter than 1/4 wavelength of light, distinct finite length effects are also observed in overtone and intermediate frequency modes between 400 and 1500cm$^{-1}$. The MIT authors acknowledge supports under the Dupont-MIT Alliance, and NSF Grant DMR 04-05538.   

($n$,$m)$-dependent environmental effect on photoluminescence of single-walled carbon nanotubes

The photoluminescence (PL) map was measured for 20 chiralities of single-walled carbon nanotubes (SWNTs) suspended in air, and the $E_{11}$ and $E_{22}$ were compared to the results reported for SDS-wrapped SWNTs [1]. The $E_{11}$ and $E_{22}$ are mostly blueshifted by a few tens of meV, except for $E_{22}$ of type-II near zigzag SWNTs which show a redshift. The energy shifts of $E_{11}$ and $E_{22}$ from those of SDS-wrapped SWNTs, $\Delta E_{11}$ and $\Delta E_{22}$, show clear dependence on the chirality, in particular on the chiral angle rather than the diameter. $E_{11}$ and $E_{22}$ show different dependences on the chiral angle between type-I and type-II SWNTs. In the case of type-I SWNTs, $\Delta E_{11}$ is lager for the larger chiral angle whereas $\Delta E_{22}$ is smaller for the larger chiral angle. In contrast, type-II SWNTs shows the opposite dependences. The difference between type-I and type-II disappears for the SWNTs with the chirality near armchair. The chiral angle dependence of environmental effect can be explained by difference in effective mass. [1] R. B. Weisman \textit{et al}. \textit{Nano Lett. }\textbf{3 }1235(2003).   

Raman Scattering from few-layer Graphene Films

Few layer-graphene sheet ($n$GL's) films, where n is the number of graphene layers, are new two-dimensional sp$^{2}$ carbon systems that have been shown to produce exciting Fractional Quantum Hall phenomena. We report here on the first Raman scattering (RS) results of $n$GLs. $n$GLs with lateral dimensions of $\sim $1-3 $\mu $m were prepared by chemical delamination of graphite flake or HOPG and then transferred from solution onto substrates (mica, pyrex,In/pyrex and Au/pyrex). RS spectra have been collected on $n$GL's with n=1, 2, 3 and compared with the graphite. Graphite exhibits two E$_{2g}$ interlayer modes at 42 cm$^{-1}$ and 1582 cm$^{-1}$. The Raman spectra of (n=1-3) $n$GLs were found to exhibit peaks at 1350 cm$^{-1}$ and 1620 cm$^{-1}$, i.e., near frequencies associated with high phonon density of states. The high frequency E$_{2g}$ band is found to split into two bands when the $n$GL is supported on metallic substrates (In,Au). In both these cases, we observe bands at 1583 cm$^{-1}$, $\sim $1592 cm$^{-1}$ rather than one band at 1581 cm$^{-1 }$when the nGL is on insulating pyrex. The splitting of the interlayer band when on metallic substrates is identified with charge transfer between the nGL and the substrate. The phonon density of states scattering observed does not appear to be due to disorder in the basal plane.   

Resonance Raman Study of Linear Carbon Chains Formed by the Heat Treatment of Double-Wall Carbon Nanotubes

The Raman spectra of carbon nanotubes exhibit weak features in the spectral range between 1600 and 2000 cm$^{-1}$ that are ascribed to a second-order Raman process. However, the observation of unusual and strong spectral features around 1850\,cm$^{-1}$ have been reported recently in the Raman spectra of carbon nanotube systems, and have been ascribed to the vibration of one-dimensional chains of carbon atoms. We study the resonance behavior of the unusual Raman feature known as the coalescence-inducing mode (CIM), observed at $\sim$1850\,cm$^{-1}$, in samples of double-wall carbon nanotubes annealed at high temperatures. Resonance Raman spectra taken with many different laser energies show that the intensity of the CIM band exhibits a maximum around 2.20 eV. By comparing the experimental results with first principles calculations for the vibrational frequency and the energy gap, we propose that the CIM feature is associated with short carbon chains with an odd number of atoms, interconnecting the nanotube surfaces.   

Raman Scattering Study of the Thermal Conversion of SWNTs into Graphitic NanoRibbons.

Thermal processing of purified bundled SWNTs in vacuum at high temperatures has been found to lead to a series of important structural transformations: SWNT coalescence (1400 \r{ }C-1600 \r{ }C), formation of MWNTs ($\sim $1600-1800 \r{ }C) and the formation of a new filamentary allotrope, i.e., Graphitic Nanoribbons (GNRs). HRTEM indicates that GNRs are collapsed MWNTS. ARC and HiPCO SWNTs go through the first two transformations, but only ARC material transforms to GNRs. At the highest temperatures, the ARC material is almost completely transformed into GNRs. Here we present the results of Raman scattering experiments on these carbon filaments after high temperature heat treatment (HTT) for $\sim $ 6 hr. For coalesced SWNTs, the structural order in the tube walls is sufficient to observe new low frequency radial (R) Raman modes ($\sim $100 cm$^{-1})$ identified with the diameter-doubled tubes. We can conclude that small diameter tubes (d$<$ 1.4 nm) are preferentially lost in the range HTT$\sim $1600-1800 \r{ }C. After HTT$\sim $1800 \r{ }C, the formation of MWNTs occurs via massive bond rearrangement of coalesced SWNTs, and this transformation is observed in Raman as a broadening of the high frequency bands and a loss of R-band intensity. A few isolated SWNTs with d $\sim $ 1.3-1.5 nm were found to survive HTT$\sim $2000 \r{ }C, but not HTT$\sim $2200 \r{ }C.   

Coherent phonon oscillations from micelle-suspended single-walled carbon nanotubes

Time-domain oscillations were generated via degenerate pump- probe spectroscopy from individual single-walled carbon nanotubes (SWNTs) dispersed in aqueous media using ultrafast excitation pulses from a Ti:sapphire laser over the range of 710- 860 nm. Fast Fourier transform of such oscillations reveals the observation of coherent phonons (CP) corresponding to the radial breathing mode (RBMs) of 16 distinct (n,m) SWNTs. Comparison to Resonance Raman scattering (RRS) experiments indicates excellent agreement with observed RBMs, with significantly narrower linewidths seen for CP. Additionally, different RBM intensity behavior is observed within 2n+m families compared to RRS. Finally, we have directly observed two-peak maxima, separated by tens of meV, in the excitation profile for a given RBM. A possible origin of this two-peak structure is discussed. This technique represents a novel method for (n,m) characterization as well as electronic structure probing.   

Session W19: Focus Session: Semiconductor Spin Transport: Noise/Theory

Shot noise and s-o coherent control of entangled and spin polarized electrons.

We extend our previous work on shot noise for entangled and spin polarized electrons in a beam-splitter geometry with spin-orbit (s-o) interaction in the incoming leads. Besides accounting for both the Dresselhaus and the Rashba spin-orbit terms, we present general formulas for the shot noise of singlet and triplets states derived within the scattering approach. We determine the full scattering matrix of the system for the case of leads with two orbital channels coupled via a weak s-o interaction inducing channel anti-crossings. We show that this interband coupling gives rise to an additional modulation angle which allows for further coherent control of the electrons. We also derive explicit shot noise formulas for a variety of correlated pairs (e.g., Bell states) and lead spin polarizations. Interestingly, the singlet and $\backslash $textit{\{}each{\}} of the triplets defined along the quantization axis perpendicular to lead 1 and in the plane of the beam splitter display distinctive shot noise for injection energies near the channel anti-crossings -- one can tell apart all the triplets through noise measurements. Finally, we find that backscattering within lead 1 reduces the visibility of the noise oscillations. This work was supported by NCCR Nanoscale Science, EU-Spintronics, CNPq, Swiss NSF, DARPA, ARO, and ONR (to appear in PRB).   

Shot Noise of a Quantum Point Contact

We report detailed simultaneous measurements of shot noise and DC transport in a quantum point contact (QPC) as a function of source-drain bias, gate voltage and in-plane magnetic field. The magnetic field evolution of the 0.7 feature in both conductance and noise is clearly visible and is compared to a simple model, giving good quantitative agreement.   

Theory of spin noise spectroscopy of itinerant fermions

We study the noise of spin magnetization in a region within a system of fermions in different regimes of temperature (degenerate/non-degenerate) and disorder (ballistic/diffusive), with and without spin-flip processes included, and with no particle-particle interactions. We obtain a single spectral line, analyze the dependence of its width and spectral weight on the parameters of the system and the size of the region of interest, and discuss conditions for the maximal effect. We compare our results to the recent measurements of spin noise of alkali-gas vapor and of conduction electrons in a GaAs epilayer.   

Spin-resolved shot noise in multichannel spin-orbit coupled quantum wires

The characterization of spin-dependent transport via shot noise has recently attracted considerable attention in semiconductor spintronics. The key quantity that makes it possible to obtain the shot noise of both spin currents and spin-polarized charge currents is the correlation function between the spin-resolved charge currents. We extend the Landauer-B\"uttiker scattering formalism to obtain the spin-resolved shot noise for arbitrary polarization of the injected current and apply it to two-probe multichannel quantum wire with the Rashba spin-orbit interaction. We find that the Fano factor of the charge shot noise is non-zero even for ballistic transport and increases with the strength of the Rashba spin-orbit coupling due to enhancement of the backscattering at the lead-wire interface. In disordered quantum wires this effect enhances the Fano factor beyond the standard $F=1/3$ shot noise suppression.   

Valley-Kondo Effect in Transport Through a Silicon Quantum Dot

The Anderson model is applied to transport through a silicon quantum dot with infinite Coulomb interaction U with the valley degeneracy taken into account. At zero temperature, we study the conductance in the Kondo regime as a function of applied magnetic field, using the variational method developed by Gunnarsson and Sch\"{o}nhammer. The conductance peaks show a characteristic evolution due to the interplay of Zeeman splitting and valley splitting.   

Time-dependent multiple scattering approach for a single finger-gate in a Rashba-type quantum channel .

We consider a Rashba-type quantum channel (RQC) consisting of one AC-biased finger-gates (FG) that orient perpendicularly and located above the RQC. Such an AC-biased FG gives rise to a local time-modulation in the Rashba coupling parameter, and generates a dc spin current (SC). A static potential is located inside or outside the FG in the RQC and the backscattering effect is studied. We use analytical time-dependent multiple scattering approach to treat the effect of the SC suppression due to a static potential in the RQC.   

Spin-current detector by a periodic array of quantum dots with Rashba Hamiltonian

Electron diffraction by a periodic array of quantum dots is studied using the Rashba Hamiltonian and a plane wave-based Green function method. A transverse charge current is found due to the asymmetric spatial diffraction of a spin-polarized electron injection. The spin-polarization of the forward electon current is maintained after passing through the array. Such an array can be used as a non-magnetic spin-current detector. Detailed results in terms of the parameters relevant to experimental design, such as the size of the quantum dots, strength of the spin-orbit coupling, are presented.   

Effect of Coulomb interaction on the spin-galvanic mode in a two dimensional electron gas with Rashba spin-orbit interaction.

Recently a new propagating mode of coupled charge and spin oscillations was predicted in a two dimensional electron gas with a sufficiently strong Rashba interaction. We show [cond-mat/0511534] that Coulomb interaction qualitatively modifies the spectrum and increases the characteristic wavelength of this mode by orders of magnitude, but does not suppress it. An absorption experiment that can conclusively detect the presence or absence of such a propagating mode is proposed.   

Quantum coherence oscillations in InSb and InAs

Quantum oscillation phenomena in parallel arrays of loops have been investigated in InSb/AlInSb and InAs/AlGaSb heterostructures, notable for their strong spin-orbit interaction. The arrays consist of parallel lines of hexagonal lattice cells, forming linear concatenations of loops. From the h/2e periodicity, the dominance of Altshuler-Aronov-Spivak (AAS) oscillations is deduced. Measurement of the temperature dependence of the oscillations enables the extraction of spin and phase coherence lengths in InSb and InAs. The spin coherence lengths show a weak drop with increasing temperature, akin to the mobility mean free path behavior, and consistent with a dominant Elliott-Yafet related spin relaxation mechanism in both heterostructures. The phase coherence lengths follow a power law without observed saturation at the lowest temperatures. NSF DMR-0094055 (JJH), DMR-0080054, DMR-0209371 (MBS).   

Rashba spin-orbit and lateral local confinement effects in quasi-two-dimensional electronic systems

We consider the effects of lateral confinement on a two dimensional electron gas in the presence of different types of spin-orbit coupling as applied to the electrons and holes in $GaAs$ heterostructures. Both the linear and cubic Rashba spin-orbit coupling mechanisms have been studied. We show that confinement leads to interesting spin polarization effects that can in principle be observed by transport measurements across a quantum point contact.   

Response Functions and Collective Oscillations of a Two Dimensional Electron Gas in the Presence of Rashba Spin-Orbit Coupling

Various response functions and the spectrum of the collective excitations of a two dimensional electron liquid in the presence of Rashba type spin-orbit coupling have been studied within time dependent mean field formalism. Of particular interest are the results concerning the in-plane and out-of- plane spin susceptibility of the paramagnetic phase and the corresponding spin excitations. As an by product of this analysis we have derived an exact analytical expression for the static density response function which corrects formulas previously appeared in the literature. Approximate analytical expressions have also be derived for the low frequency, long wave length dependence of the same function.   

Mean field phase diagram of a two dimensional electron liquid with Rashba spin-orbit

By a combination of analytic an numerical techniques we have mapped out the mean field phase diagram of a two dimensional electron liquid in the presence of Rashba or Dresselhaus spin-orbit. Although inhomogeneous solutions can be found that minimize the total energy, we have carried out a systematic study of the spatially homogeneous phases in the $(r_s, \alpha)$ diagram (with $r_s$ the density parameter and $\alpha$ the strength of the spin-orbit coupling). The scenario is intriguing for a number of broken symmetry states have been unraveled that can be characterized by suitable momentum space occupation numbers $n_{\bf k}$ and local spin quantization axes $\hat{s}_{\bf k}$. While at high densities the system is as expected paramagnetic, at lower densities (or larger $\alpha$ values) markedly different ferromagnetic phases exist with spontaneous polarization oriented perpendicular or parallel to the plane of motion that are characterized by non trivial spin textures in momentum space. Of particular interest is a phase transition between an isotropic paramagnetic state to an anisotropic ferromagnetic one that occurs in the large $\alpha$ limit. The relation between the various phase transitions and the differential instabilities signaled by the in plane and out of plane spin susceptibilities will be discussed.   

Cubic Dresselhaus Spin Orbit Coupling in Small Quantum Dots

Due to the suppression of linear spin-orbit effects in small quantum dots in two-dimensional electron systems, the cubic Dresselhaus spin-orbit coupling can play a significant role in such phenomena as the variance of conductance through a dot. We characterize the different spin-orbit coupling terms by the strength of the anti-crossings they induce in the eigenstates of a closed quantum dot as an in-plane magnetic field is increased, and we perform numerical simulations in a chaotic billiard model to estimate the RMS anti-crossing energy. We investigate the conditions under which the cubic Dresselhaus effects may be measurable and significant for realizable dot configurations.   

g-factor anisotropy in p-type GaAs/AlGaAs quantum point contacts

In this work we report the influence of lateral confinement on the effective Lande \textbf{g}-factor for holes in GaAs. For 2D hole gas grown along crystallographic direction other than the high symmetry [100] or [111], mixing of heavy and light hole subbands leads to anisotropic g-factor depending on the direction of the in-plane magnetic field.Further lateral confinement of the holes into 1D channel modifies g-factor depending on the strength and direction of the confining potential. We investigate \textbf{g}-factor in quantum point contacts (QPC) fabricated on 2D hole gas on [$\bar {3}11$]A GaAs by AFM lithography using the local anodic oxidation technique. Several QPCs are oriented along the major [$\bar {2}33$] or [$\bar {1}10$] axis. In-plane \textbf{B} is predominantly acting on the spin part of the Hamiltonian and the magnitude of the Zeeman splitting can be obtained from energy level spectroscopy combined with the critical fields at which half-integer steps in the conductance (in units of $\frac{2e^2}{h})$ appear. The \textbf{g}-factor is found to be strongly modified compared to the values reported for a 2D hole gas and has strong dependence on the quantized momentum normal to the current flow (number of filled energy levels in the point contact). Thus the g-factor can be modulated electrostatically, providing an extra degree of control of spintronic devices.   

The effect of interactions on the geometrical structure of the Fermi surface in systems with spin-orbit interactions

The spectrum of many electronic systems contains band degeneracies, which can be thought of as monopoles in momentum space leading to a non-trivial topological structure of the Fermi surface. This structure is characterized by two quantities: curvature and metric, which, being gauge invariant, are in principle observable in experiment. In electronic systems with spin-orbit interactions, the Berry's curvature determines the so-called spin Hall conductivity, which may be related to observable spin accumulation near the edges. We consider Rashba, Dresselhaus, and Luttinger models and study the effect of interactions on the topology of the Fermi surface in these systems. We find that interactions renormalize the spin-orbit couplings but in certain cases do not change the Berry's phase structure. This suggests that the non-trivial geometry of the Fermi surface is a true Fermi liquid property.   

Session W20: Focus Session: Multiferroics IV

Electronic Mechanism for the Coexistence of Ferroelectricity and Ferromagnetism

We study the strong coupling limit of a two-band Hubbard Hamiltonian that also includes an inter-orbital on-site repulsive interaction $U_{ab}$. When the two bands have opposite parity and are quarter filled, we prove that the ground state is simultaneously ferromagnetic and ferroelectric for infinite intra-orbital Coulomb interactions $U_{aa}$ and $U_ {bb}$. We also show that this coexistence leads to a singular magnetoelectric effect.   

``Lorentz force" acting on a ray of light in multiferroics

We theoretically propose that optical analogue of a Lorentz force acting on a ray of light is realized in multiferroic materials showing an optical magneto-electric effect. The toroidal moment ${\vec T} = \sum_j {\vec r}_j \times {\vec S}_j$ plays a role of ``vector potential" while its rotation corresponds to a ``magnetic field" for photons. Hence the light is subject to the Lorentz force when propagating through the domain wall region of the ferromagnetic or ferroelectric order. Realistic estimate on the magnitude of this effect is given.   

Enhanced optical magnetoelectric effect in a patterned polar ferrimagnet

A simple method to dramatically enhance the optical magnetoelectric (ME) effect, i.e., nonreciprocal directional birefringence, is proposed and demonstrated for a polar ferrimagnet GaFeO$_3$ as a typical example. We patterned a simple grating with a pitch of 4 $\mu$m on a surface of GaFeO$_3 $ crystal and used the diffracted light as a probe. Optical ME modulation signal for Bragg spot of the order $n=1$ becomes gigantic in the photon energy 1--4 eV and reaches 1--2\% of the bare diffracted light intensity in a magnetic field of 500 Oe. This is amplified by more than three orders of magnitude compared to that for the reflection of bulk GaFeO$_3$. Fabricating a photonic crystal will make it possible to lead a new route for the practical use of the optical ME effect.   

Magnetoelectric effects in multiferroics

Magneto-electric phenomena were investigated in two different multiferroic systems: The strong coupling of dielectric and magnetic properties and the simultaneous occurrence of long-range magnetic and ferroelectric order are discussed for rare earth manganites and sulfo spinels. A phase diagram of Eu$_{1-x}$Y$_{x}$MnO$_{3}$ is established, which recovers the main features of the well-known magneto-electric phase diagram for the pure rare earth manganites RMnO$_{3}$. Here a variety of magnetic and electric phases emerge with varying rare earth ions R. As function of temperature and external magnetic field, also Y doped EuMnO$_{3 }$compounds undergo a sequence of different magnetic and polar phase transitions for varying effective ionic radii of the rare earth ions. Special attention is paid to the occurrence of fundamentally new hybrid spin-electromagnetic excitations, which we name electromagnons and are characterized as spin waves that can be excited by an ac electric field. These excitations are identified in Eu$_{1-x}$Y$_{x}$MnO$_{3 }$with x = 0.2, in GdMnO$_{3}$, and in TbMnO$_{3}$. Specifically in GdMnO$_{3}$ the electromagnons can easily be suppressed by external magnetic fields and allow tuning the index of refraction by moderate fields. In the second part we discuss the simultaneous appearance of colossal magneto-resistance (CMR) and colossal magneto-capacitance (CMC) effects in chromium sulfo spinels. In CdCr$_{2}$S$_{4}$ ferromagnetism of localized Cr spins evolves at 85 K, while polar order is established below 130 K. The onset of ferroelectric order is neither accompanied by the occurrence of soft modes nor by structural changes which break the inversion symmetry of the high-temperature cubic phase. HgCr$_{2}$S$_{4}$ becomes ferroelectric close to 70 K while a complex antiferromagnetic order is found below 25 K. CMR and CMC effects are specifically strong in the mercury compound, as moderate magnetic fields of only 0.1 T induce ferromagnetism at much higher temperatures. We speculate that the occurrence of ferroelectricity in these multiferroic compounds is rather of electronic than of ionic origin.   

A neutron scattering study on a ferrimagnetic magnetocapacitance system Mn$_{3}$O$_{4}$

The low-temperature phase of the tetragonal Mn$_{3}$O$_{4}$ has long been known as a ferrimagnet with the Yafet-Kittel structure. The long-range ferrimagnetic order first develops at 41.2 K, upon cooling, and a commensurate cell-doubling magnetic order occurs along the $b$-axis at 32.7 K. Recently magnetocapacitance behaviors were observed in Mn$_{3}$O$_{4}$. We present our high-resolution neutron scattering data to show that Mn$_{3}$O$_{4}$ undergoes an additional lattice distortion around 25 K. Peak broadening of selected reflections suggests that the crystal structure becomes pseudo-orthorhombic at low temperatures. Relation between the lattice distortion and magnetism will also be discussed.   

Magnetodielectric consequences of phase separation in the colossal magnetoresistance manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$

We have studied the low-frequency dielectric properties of the phase-separated manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ as a function of applied magnetic field in the low temperature phase-separated state. The dielectric constant is strongly field dependent and also depends on the magnetic field history of the sample. The dielectric behavior appears to be associated with the hopping of polaronic charge carriers, and we can derive the field dependent hopping energy barrier from the frequency dependence of the dielectric constant. This analysis allows us to associate the metal-insulator transition observed in this material with the field-induced suppression of the polaron activation energy. Reference: \textit{Phys. Rev. B} \textbf{72}, 144429 (2005). Research was supported by the NSF and DOE.   

Dielectric anomalies in CoCr$_2$O$_4$

We investigate the magnetic,dielectric, and thermodynamic properties of CoCr$_2$O$_4$ polycrystalline samples. AC susceptibility and specific heat measurements show the existence of two distinct magnetic transitions in this material. Neutron scattering experiments confirm a ferrimagnetic ordering transition at T$_c$=95 K and a transition to a spiral magnetic phase below T$_N$~25 K. We observe a significant dielectric anomaly coincident with the onset to long-range spiral magnetic order, and a separate feature with significant thermal hysteresis above T=50 K. We associate this higher temperature dielectric anomaly with short-range spiral magnetic order, and discuss these results in the context of utilizing magnetodielectric couplings to capacitively probe short range magnetic structures.   

Mossbauer Spectroscopy Investigation of Substituted Cobalt Ferrites (CoM$_{x}$Fe$_{2-x}$O$_{4}$, where M = Mn or Cr, and x = 0.0-0.8)

In order to enable applications of substituted cobalt ferrites for strain sensing, magnetostrictive actuating, and ``multiferroic'' composites, more basic knowledge is needed concerning how cation substitution affects the atomic level environments, distributions, and interactions of the cations. In this study, the local environments of the Fe atoms in two series of substituted cobalt ferrites (CoM$_{x}$Fe$_{2-x}$O$_{4}$, where M = Mn or Cr, and x = 0.0-0.8) have been investigated using Mossbauer spectroscopy. Results of both series show two distinct six-line hyperfine patterns, indicating Fe in A (tetrahedral) and B (octahedral) sites. They can be identified by isomer shift and hyperfine distribution width. Both series show some similar behavior: with increasing substitution, magnetic hyperfine field decreases and hyperfine field distribution width increases for both A and B sites. B-site hyperfine fields and distribution widths are more affected than A. All of these effects are more pronounced for Cr-substitution than for Mn. Results are consistent with a model of Mn or Cr ions substituting into B-sites and displacing Co ions onto A sites. It would appear that Cr has an even stronger B-site preference than Mn, and displaces more of the Co to the A sites.   

Preparation and properties of single phase PbTi$_{1-x}$Mn$_{x}$O$_{3}$ perovskites at high Mn concentrations.

Recent observation of multiferroic properties in the PbTi$_{0.5}$Fe$_{0.5}$O$_{3}$ perovskite material raises questions about the electronic and structural driving forces causing the coupling between ferroelectric and ferromagnetic properties. It is known that the Jahn-Teller distortion of oxygen octahedra due to d orbital occupancy inhibits formation of the ferroelectric double well potential in ABO$_{3}$ perovskites. Thus, the presence of a ferroelectric distortion in d$^{n}$ magnetic transition metal perovskite oxide is an unexpected result. We report observation of tetragonal structure (XRD) in a similar PbTi$_{1-x}$Mn$_{x}$O$_{3}$ system. The material is prepared using the sol-gel method with various Mn concentrations. In this study, we access a range of concentrations starting from a relatively high value of x=0.1 in order to introduce considerable amount of magnetic sites into the system. Reduction in the tetragonal ratio (c/a) is observed with increase in x as expected. Further characterization of the material involves magnetoelectric measurements and X-ray Absorption Spectroscopy. Preliminary results are discussed.   

Colossal magnetocapacitance and scale-invariant dielectric response in mixed phase manganites

We are studying thin-film capacitors utilizing (La$_{0.5}$Pr$_{0.5})_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LPCMO) as the base electrode, AlOx as the dielectric and Al as the counter-electrode. The LPCMO films exhibit \textit{colossal magnetoresistance} (CMR). Likewise, the capacitance changes by three orders of magnitude in the region of the resistance drop. These \textit{colossal magnetocapacitance} (CMC) effects are related to magnetic field induced changes in the relative extent of coexisting ferromagnetic metal and charge ordered insulating phases. The widths of the hysteresis loops, in capacitance and resistance, are about the same, but the center of the capacitance loop is shifted 20~K below the center of the resistance loop. When the LPCMO resistance is at a maximum (low capacitance) the electrode comprises filamentary conductors threading an insulating medium. In this region log-log Cole-Cole plots reveal an intrinsic dielectric response in which the data plotted as a function of frequency ($\omega )$ collapse onto single straight lines, implying scale-invariance over a wide range of $\omega $, magnetic field and temperature.   

Epitaxial thin films of novel multiferroic double perovskites.

Recently multiferroic materials have attracted great interest. However, relatively a few pure multiferroic compounds are currently known. Here we show the exploration of design of multiferroic properties in double perovskites by combining the ferrolectrisity driven by the Bi lone pairs and selectively choosing the 3d transition metals following Goodenough-Kanamori's rules to bring in ferromagnetism. We present growth issues in stabilizing the single phase, epitaxial thin films of new double perovskite multiferroic systems such as Bi$_{2}$NiMnO$_{6}$, Bi$_{2}$FeCrO$_{6 }$and La$_{2}$NiMnO$_{6 }$by pulsed laser deposition. Targets of these compositions were synthesized by solid state method with 15{\%} of excess Bi in the composition to compensate the volatility of Bi during the deposition. We also present the synthesis of Bi$_{2}$FeCrO$_{6 }$by growing a superlattice structures from individual targets of Bi FeO$_{3}$ and BiCrO$_{3}$. In the cubic double perovskites, cations show rock salt kind of ordering in the (111) direction and hence growing these films on STO (111) substrates has an advantage. We present the growth, structural and multiferroic properties in these double perovskite thin films..   

First-principles exploration of multiferroic oxides with double-perovskite structure

Multiferroics have attracted much attention recently because of their novel properties. There are a few known as ferromagnetic and ferroelectric materials, particularly with perovskite-type crystal structure. Ferroelectrics should be insulating and likely ionic. Furthermore, it is widely recognized that covalent bonds between the cation and anion orbitals are crucial to realize atomic displacements to a noncentrosymmetric structure. As for magnetism, most of magnetic perovskite oxides usually have an antiferromagnetic order (mostly frustrating) due to a superexchange coupling. According to the Kanamori-Goodenough rule for the superexchange coupling, certain combinations of the transition-metals ions ($d^3$-$d^5$ and $d^3$-$d^8$ configurations) may possibly give a ferromagnetic coupling by the 180$^{\circ}$ superexchange mechanism. In this study, we explore possible co-existence of spontaneous electric polarization and ferromagnetic ordering from first principles, by focusing bismuth double-perovskite oxides Bi$_2$$BB'$O$_6$ ($B, B'$ = 3$d$ ions) as target materials. Ferromagnetic and ferrimagnetic solutions are obtained for cubic Bi$_2$MnNiO$_6$, Bi$_2$CrFeO$_6$ and Bi$_2$CrCuO$_6$ with nearly gapped electronic structure. Quite recently, Bi$_2$MnNiO$_6$ has been successfully synthesized by a high-pressure technique and revealed multiferroic properties. Possible multiferroic properties of Bi$_2$MnNiO$_6$ with the observed monoclinic structure are investigated in detail.   

Magnetoelectric gyrator

As well-known [1], an ideal gyrator would be an unusual device with respect to other network elements. It would have the unique properties of (i) anti-reciprocity, and (ii) being capable of acting like an impedance inverter. Here, for the first time, we report the design and study of such an ideal gyrator. Our ideal gyrator consists of a trilayer composite of Terfenol-D/PZT/Terfenol-D operated in a L-L mode at its electromechanical resonance (f$\approx $80kHz). Measurements have shown that magnetoelectric (ME) susceptibility of our composite is comparable with its permeability and permittivity, and that the gyration coefficient achieves a value 0.9. In addition, we have observed a 180o phase-shift between an input current and an output voltage, or vice versa, and proved that our ME laminate behaves as an impedance inverter. We believe that our gyrator may enable resolutions to numerous important and complex network problems. [1] B.D.H. Tellegen, Phillips Research Reports 3, 81 (1948). The work was supported by grants from the Office of Naval Research.   

Session W21: Liquid Crystals III

Dielectric Dispersion Effects in Liquid Crystals.

As the switching speed in practical LC devices is pushed from the currently common 10 ms to sub-millisecond levels, it is important to take into account the effects associated with the finite rate with which the electric displacement changes in the external electric field. We discuss two important general consequences of the dielectric relaxation phenomenon: (1) Non-local time relationship between the electric displacement and the electric field [1]. In a quickly changing electric field, orientation of the liquid crystal depends not only on the instantaneous value of the electric field, but also on the previous values of the field and previous orientations of the material. (2) Dielectric heating. [1] Y. Yin, S.V. Shiyanovskii, A.B. Golovin, and O. D. Lavrentovich, \textit{Phys. Rev. Lett.} \textbf{95}, 087801 (2005) .   

Using micro-focus synchrotron X-ray diffraction to probe textured liquid crystal samples

16 KeV X-rays from a bend magnet source at Sector 20 of the Advanced Photon Source were micro-focused by Kirkpatrick-Baez mirrors to a 14$\mu $m x 14$\mu $m cross-section and used in conjunction with an in-situ polarizing optical microscope to measure the diffraction from select areas in textured liquid crystal samples between thin glass plates. The technique will be described and its utility illustrated by three examples: (1) measuring the orientational deformation of smectic-A liquid crystal layers under the bend strain imposed by an AFM-scribed polymer alignment film, (2) mapping the concentration dependence of the liquid crystal phases exhibited by suspensions of short DNA oligomers of 6 to 16 base-pairs, and (3) selecting local monodomain regions from a globally unaligned conducting porphyrin-derivative sample for structural determination of its liquid crystal phases.   

Impurity Induced Cross-over from Continuous to First-order Nematic-to-Smectic A Phase Transitions in a Liquid Crystal.

We used adiabatic scanning calorimetry (ASC) to study the impact of adding small amounts of cyclohexane on the N-SmA transition of the liquid crystal octylcyanobiphenyl (8CB). The transition remains continuous upto a mole fraction of cyclohexane near 0.05, where at a tricritical point the transition becomes first- order with latent heats increasing with mole fraction of cyclohexane. Along the continuous part of the N-SmA transition line the effective specific heat capacity critical exponent increases from 0.31 for 8CB to 0.50 at the tricritical point. Ongoing experiments with other non-mesogenic impurities will also be reported.   

Phase morphology of a disk-sphere dyad molecule.

A disk-sphere dyad molecule was synthesized by attaching a discotic triphenylene molecule to a spherical polyhedral oligomeric silsesquioxane (POSS) molecule via esterification reaction. The self-assembly behavior of the dyad molecule was studied by differential scanning calorimetry, polarized light microscopy, X-ray diffraction (XRD), and transmission electron microscope. Two-dimensional (2D) XRD results showed the dyad molecules self-assembled into a lamellar structure, which composed of a crystalline POSS layer and a discotic-nematic triphenylene double-layer. The POSS layer consisted four layers of ABCA-stacked spherical molecules. The liquid crystalline triphenylene molecules were parallel and staggered in the double-layer. Computer simulation of the XRD intensity confirmed the proposed structural model. Compared with that of the POSS crystal in bulk (melting point at ca 220 $^{o}$C), the melting temperature of POSS crystal was dramatically decreased to 67 $^{o}$C, possibly due to effects of the asymmetry molecular shape and plasticization of the discotic triphenylene moieties between POSS layers.   

Excitation-Enhanced Optical Reorientation in Pure Liquid Crystalline Materials

Electronic excitation with polarized light necessarily creates complementary orientational anisotropies from the excited and ground-state molecules. If the intermolecular interaction with the surrounding experienced by the excited-state molecules is different from that experienced by the ground-state molecules, a net excitation-induced orientational anisotropy will develop, enhancing the molecular reorientation provided directly by the optical field. This effect is analogous to that observed in dye-doped liquid crystals (LC) when dye molecules are excited. We report here the study of the effect in a pure isotropic LC medium. We use an optical pump-probe method to observe the excitation-induced reorientational dynamics. As the system relaxes back from picosecond pulse excitation, an increase in the orientational anisotropy of the ground-state molecules is observed, signifying the enhanced optical reorientation due to the state-dependent intermolecular interaction. The observed dynamics is well predicted by a mean-field model describing the intermolecular interaction between LC molecules. This work was supported by NSF   

Effect of disorder on a nematic-smectic A phase transition

Using X-ray scattering, we studied the nematic to smectic A phase transition of the liquid crystal butyloxybenzilidene-octylaniline (4O.8) confined in an aerosil gel. The aerosil particles introduce quenched randomness in the system by providing pinning centers to the liquid crystal molecules. We find that the introduced disorder destroys the long range nature of the phase transition, and that the transition becomes similar to a transition in a finite-size system. Finite low temperature correlation lengths of the ordered moments are measured and the order parameter follows a power law behavior with respect to the reduced temperature in a limited temperature range. We also show evidence for a shift of the effective order parameter critical exponent $\beta$ with increasing disorder.   

Observation of polarization current accompanying smectic A electroclinic reorientation

We have been studying the liquid crystalline material W530, and report observations of polarization current of the field-induced molecular reorientation in the SmA phase. W530 exhibits the following phase diagram on cooling: isotropic -- SmA -- uncharacterized Sm'X' -- metastable SmC -- crystal. The temperature range of the SmA and SmX phases is $\sim $50\r{ }C, and x-ray diffraction (XRD) shows very little layer spacing change throughout the width of these two phases, while the SmC fractional layer compression is $\sim $5{\%}. The SmX is nearly identical in appearance to the SmA phase under depolarized light microscopy (DPLM). However, when measuring polarization current while cooling from SmA to SmX, two polarization peaks appear throughout the range of the SmX phase. By adapting the Langevin model for deVries SmA, we are able to explain the two polarization peaks. Through a combination of DPLM cone angle and birefringence measurements, dielectric spectroscopy measurements, aligned sample and powder XRD experiments, and freely suspended film observations, we are able to show that the previously uncharacterized phase is a deVries SmA. Work supported by NSF MRSEC Grant DMR-0213918.   

Defects in liquid crystal nematic shells

We generate water/liquid crystal (LC)/water double emulsions via recent micro-capillary fluidic devices [A. S. Utada, et.al. Science 308, 537 (2005)]. The resultant objects are stabilized against coalescence by using surfactants or adequate polymers; these also fix the boundary conditions for the director field n. We use 4-pentyl-4-cyanobiphenyl (5CB) and impose tangential boundary conditions at both water/LC interfaces by having polyvinyl alcohol (PVA) dispersed in the inner and outer water phases. We confirm recent predictions [D. R. Nelson, NanoLetters 2, 1125 (2002)] and find that four strength s=+1/2 defects are present; this is in contrast to the two s=+1 defect bipolar configuration observed for bulk spheres [A. Fernandez-Nieves, et.al. Phys. Rev. Lett. 92, 105503 (2004)]. However, these defects do not lie in the vertices of a tetrahedron but are pushed towards each other until certain equilibration distance is reached. In addition to the four defect shells, we observe shells with two s=+1 defects and even with three defects, a s=+1 and two s=+1/2. We argue these configurations arise from nematic bulk distortions that become important as the shell thickness increases. Finally, by adding a different surfactant, sodium dodecyl sulphate (SDS), to the outer phase, we can change the director boundary conditions at the outermost interface from parallel to homeotropic, to induce coalescing of the two pair of defects in the four defect shell configuration to yield two defect bipolar shells.   

Water as a Wetting Agent for Liquid Crystal Films

The dewetting of nCB liquid crystals from silicon wafer surfaces was first observed in 1999 [1] and has since grown into a subject of great fascination. The dewetting behavior occurs within a narrow coexistence region just below the nematic-to-isotropic phase transition temperature. When a wetted film is brought within this coexistence region, the film splits into two film thicknesses that are in apparent equilibrium with each other. A tentative but highly controversial explanation for this phase diagram has been proposed van Effenterre [2] in terms of mean field forces acting within the film. In our laboratory, we have undertaken a high-resolution measurement of the shape of this dewetting region for 5CB on silicon in search of evidence for the existence of fluctuation-induced forces that affect the thickness of these films. We have found, to our surprise, that ambient humidity affects the wetting behavior. Based on preliminary evidence taken thus far, water appears to act as a wetting agent that promotes the wetting of 5CB on silicon. We will present measurements showing how water affects the two-film thickness coexistence region. [1] F Vandenbrouck et al., Phys. Rev. Lett. 82, 2693 (1999). [2] D. Van Effenterre et al., Phys. Rev. Lett. 87, 125701 (2001).   

Surface morphology of SiO deposited substrates and alignment of nematic LC*

Glass substrates with thin film of SiO are known to align nematic liquid crystals homogeneously for oblique deposition. X-ray reflectivity was employed to probe the surface morphology of approximately 150{\AA} thick SiO films deposited at different landing angles. The interfacial roughness and morphological anisotropy was determined along the two orthogonal in-plane directions and the average electron density profile of the film calculated. The results show that the homogeneous and planar aligning films consists of SiO film with different roughness anisotropy and film thickness. The results will be discussed in light of previous reflectivity and AFM results on SiO [1] and other [2] surfaces. [1]. R. Barberi, Giocondo, G.V. Sayko, AK. Zvezdin, Phys. Lett. \textbf{A213}, 293 (1996). [2]. S. Kumar, J.-H. Kim, and Y. Shi, Phys. Rev. Lett. \textbf{94}, 077803 (2005).   

Crystal Nucleation behavior near gas-liquid spinodal line

The complex problem of crystal nucleation is currently at stage. Using molecular dynamics simulations, we study the crystal nucleation behavior of colloids modeled by hard-core particles with narrow square well attractive potential. For this system the liquid gas critical point lies below the gas-crystal equilibrium line. We investigate how the nucleation rate depends on the pressure and density, in particular in the vicinity of the liquid-gas spinodal. We find that there is a correlation between nucleation rate and spinodal line. We interprete our results using classical nucleation theory.   

Entropy driven formation of a chiral liquid crystalline phase of helical rods

We study the liquid crystalline phase behavior of a concentrated suspension of helical flagella isolated from Salmonella typhimurium. With increasing concentration, a suspension of helical flagella undergo an entropy driven first order phase transition to a liquid crystalline state having a novel chiral symmetry. Flagella are prepared with different polymorphic states, some of which have a pronounced helical character while others assume a rod-like shape. We show that the static phase behavior and dynamics of chiral helices are very different when compared to simpler achiral hard rods.   

Dynamics of Director Fluctuations in Confined and Filled Liquid Crystals.

Dynamic light scattering was applied to study the influence of randomness as well of boundary conditions (planar-axial and homeotropic-radial) and layer thickness (at nanoscale) of 5CB and 8CB confined to random porous matrices, to cylindrical pores and filled with Aerosil particles (hydrophilic and hydrophobic) on phase transitions and relaxation of director orientational fluctuations. For confined 8CB in the nematic phase two well-defined relaxation processes were for confined liquid crystals. The first process is associated with bulk-like nematic director fluctuations. The second relaxation process (with relaxation time slower than the first one) is most likely due to the fluctuations in layers nearest the wall surface. We found that for homeotropic boundary conditions of confined liquid crystal, the pore wall-liquid crystal interactions influence on the properties of the surface layer is stronger than in the case of axial orientation, particularly, and the influence of boundary conditions on N-Sm-A phase transition in confined 8CB is stronger than on isotropic- nematic phase transition. The separation between the first and the second (slow) process is clearer for thinner layers and the amplitude of slow process is greater for thinner layers. This suggests that the slow process is surface related relaxation. This relaxation was observed in filled liquid crystals as well.   

Micro-focus synhrotron X-ray diffraction study of novel mesomorphic porphyrin derivatives

The mesophase structures of three novel mesomorphic porphyrin derivatives were examined using polarized optical microscopy and microfocus synchrotron X-ray diffraction at various temperatures using a beam with a 14 $\mu $m$\times $14$\mu $m cross-section at the bending magnet beamline of Sector 20 at the Advanced Photon Source. The x rays were diffracted from microscopic monodomains in thin glass cells while simultaneously observing the optical textures. The results confirmed a hexagonal arrangement of discotic columns in the liquid crystalline phase. At a lower temperature, highly ordered plastic crystal phase was obtained. The results of the microdiffraction experiment and promising properties of these compounds as a carrier transporting material will be presented.   

Stochastic Rotation Dynamics: generalizations and applications for non-ideal fluids, binary mixtures and colloids

A particle-based algorithm for the coarse-grained modeling of a fluctuating Solvent, namely Stochastic Rotation Dynamics (SRD), was recently introduced by Malevanets and Kapral[1]. This algorithm describes a fluid with an ideal gas equation of state and has been successfully applied to study polymers, colloids, and vesicles in flow. Here, we present generalizations of SRD for modeling fluids with non-trivial equations of state[2]. In particular, we show how to model a simple liquid with a non-ideal equation of state by incorporating excluded volume effects. We show the thermodynamic consistency of the model by independently measuring the pressure, density fluctuations and the speed of sound and compare with analytical results. This idea is extended to model binary mixtures with a miscibility gap; and the phase diagram of such a mixture will be presented. Furthermore, colloids are included in the SRD solvent and results for colloidal suspensions driven by external forces will be shown. [1] A. Malevanets, R. Kapral, J. Chem. Phys. 110, 8605 (1999). [2] T. Ihle, E. Tuzel, D. M. Kroll, cond-mat/0509631.   

Session W22: Focus Session: Magnetic Nanoparticles II

The Structure and Magnetic Properties of Nanoparticles and Their Arrays

The physics of magnetic nanoparticles and arrays is a very important topic of current research. Many questions remain. How does the structure of a single nanoparticle influence its magnetic properties? For instance, core/shell interactions can increase the coercivity and lead to exchange biasing, causing ferromagnetic rather than superparamagnetic behavior. At what thickness does the oxide shell begin to behave like an antiferromagnet? How do uncompensated surface spins affect the magnetic behavior? In addition, novel nanoparticle structures can lead to interesting physical behavior. In a bio-inspired approach, we are synthesizing \textit{highly} monodisperse oxide nanoparticles inside of protein cages. For these systems, magnetocrystalline anisotropy plays an important role; the surface anisotropy term becomes large, reducing the total particle moment. However, we find that the encapsulating protein shell reduces the surface anisotropy and increases the particle moment. Furthermore, we have synthesized mixed phase gamma-Fe$_{2}$O$_{3}$/CoO nanoparticles with large exchange biasing. Further questions arise for nanoparticle arrays. Dipole interactions modify the collective magnetic behavior. What are the strength and orientation of these interactions, and how do they depend on particle size, spacing, \textit{and} array ordering. Recent experiments have shown the importance of array order in determining the collective magnetic properties. The physics of magnetic nanoparticles is rich and complex, and depends upon both the structure of the individual particles and their assemblies. By using synchrotron based magnetic circular dichroism, small angle X-ray scattering and neutron scattering, we have been able to quantify many aspects of both nanoparticle and array structures. A quantitative understanding of these structural relationships has led to a better understanding of their magnetic behavior.   

Particle Size Control of Polyethylene Glycol Coated Fe Nanoparticles

Recent interest in Fe nanoparticles with high magnetization is driven by their potential use in biomedical applications such as targeted drug delivery, MRI contrast enhancement and hyperthermia treatment of cancer. This study looks at the use of a polyethylene glycol (PEG) solution to mediate the particle size and therefore control the coercivity of the resulting nanoparticles. Iron nanoparticles were synthesized using an aqueous sodium borohydride reduction of ferrous chloride by a simultaneous introduction of reagents in a Y- junction. The resulting product was collected in a vessel containing a 15 mg/ml carboxyl terminated polyethylene glycol (cPEG) in ethyl alcohol solution located under the Y junction. By varying the length of tubing below the Y junction, the particle size was varied from 5-25 nm. X-ray diffraction data indicates the presence of either amorphous Fe-B or crystalline alpha Fe, depending on the molar ratio of reagents. Magnetic measurements indicate the particles are ferromagnetic with values of coercivity ranging from 200-500 Oe and a saturation magnetization in range of 70-110 emu/g. The XRD shows that the particles are not affected by the polymer coating.   

Using Single Nanoparticle Devices to Investigate Nano-junction GMR and TMR

High-pressure sputtering / cluster-beam deposited single nanoparticle devices were fabricated to study giant magneto- resistance (GMR) and tunneling magneto-resistance (TMR) of nano- junctions. The junction used to investigate GMR was formed by the point contact of a nearly spherical $\sim$20 nm diameter ferromagnetic (FM) particle on a FM thin film deposited on a silicon substrate. The particles were covered with a thick Al$_ {2}$O$_{3}$ dielectric layer. The tunneling device is identical, except for an additional Al$_{2}$O$_{3}$ tunnel barrier sandwiched between the FM particle and film. Using a focused ion beam (FIB), small apertures were milled in the dielectric layer to expose individual particles and metallic contacts were subsequently deposited. Other contacts were made directly to the underlying FM film creating a simple two- contact measurement geometry. Temperature dependent TMR and GMR are presented for isolated particles that are independent of proximity and ensemble effects. This work is supported by the ACS Petroleum Research Fund.   

High Frequency Properties of Magnetodielectric Composites Consisting of Oriented Fe-based Flakes Embedded in a Polymeric Matrix

Magnetodielectric composites containing small ferromagnetic inclusions in a continuous dielectric matrix could be useful high frequency materials if relatively large and similar values of the permeability ($\mu )$ and permittivity ($\varepsilon )$ could be obtained. This potential, however, can not be fully achieved for spherical inclusions because their demagnetizing fields severely limit the effective permeability of the composite. Therefore, we have prepared and studied a series of magnetodielectric composites containing oriented Fe-based flakes. The flakes were produced by a mechanical deformation technique and were typically several 100 $\mu $m wide and several $\mu $m thick. These flakes were mixed with a styrene based liquid resin and aligned in an applied magnetic field prior to polymerizing the resin. X-ray diffraction and hysteresis loop measurements confirm a significant degree of alignment. Permeability and permittivity measurements indicate that values of $\mu $ and $\varepsilon $ in excess of 20 can be achieved in these samples with small losses when the loading fraction of the Fe flakes approaches 50{\%}.   

Fabrication of Ordered Mesoporous Silica with Encapsulated Iron Oxide Particles using Ferritin-Doped Block Copolymer Templates

Recently, two-dimensional arrays of iron oxide clusters were fabricated by dip-coating a silica substrate into an aqueous solution. Here we report the encapsulation of ferritin in 3D mesoporous silica structures by the replication of block copolymer templates in supercritical CO$_{2}$. In our approach, preparation of the highly ordered, doped template via spincasting and microphase separation and silica network formation occur in discreet steps. A solution of an amphiphilic PEO-PPO-PEO triblock copolymer (Pluronic) template, horse spleen ferritin and a low concentration of PTSA acid was prepared and spin-coated onto a Si wafer. Upon drying the block copolymer microphase separates resulting in partitioning of the acid catalyst and ferritin to the hydrophilic domain. The polymer template was then exposed to a solution of supercritical carbon dioxide and tetraethyl orthosilicate (TEOS) at 125 bar and 40$^{o}$C. Equilibrium limited CO$_{2}$ sorption in the block copolymer template resulted in modest dialation of the microphase segregated structure. Under these conditions, the precursor was readily infused into the copolymer and reacted within the hydrophilic domain containing the acid catalyst. The resultant film was calcined in air at 400$^{o}$C for 6 hours producing a well-ordered iron oxide-doped mesoporous silica film. TEM and XRD revealed crystalline iron oxide structures within the mesoporous silica supports. Magnetic properties were analyzed using a superconducting quantum intereference device (SQUID).   

Synthesis of Barium hexaferrite nanoparticles for functional multilayers

Magnetic barium ferrite (BaFe$_{12}$O$_{19 }$or BaM) nanoparticles were synthesized by a two system microemulsion process. X-ray diffraction of these nanoparticles confirmed the presence of a dominant hexagonal BaM phase. The magnetic characterization of the nanoparticles was performed using a Physical Properties Measurement System (PPMS). The M-H hysteresis of the BaM, at 5K and 300K, displays a saturation magnetization of $\sim $ 68 emu/g, 48 emu/g and large coercivities of $\sim $ 2300 Oe, 3100 Oe respectively, consistent with bulk BaM. The zero field cooled (ZFC) and field cooled (FC) curves illustrate that superparamagnetism was not present in the BaM below 300K. These particles will be used to prepare multilayers of ferroelectric and ferromagnetic films by depositing on a ferroelectric polymer (polyvinylidene fluoride) matrix using the Langmuir-Blodgett technique. The functional properties of these multilayers will be discussed. Work supported by NSF grant {\#}CTS-0408933 and NSF Integrated Interdisciplinary Nanoscience REU DMR 0243997.   

Theory of Brillouin Light Scattering from Ferromagnetic Nanospheres

We develop the theory of Brillouin light scattering (BLS) from spin waves in ferromagnetic nanospheres, within a framework that incorporates the spatial variation of the optical fields within the sphere. Through use of our recent theory [1] of exchange dipole spin wave modes of the sphere, we develop a method which properly normalizes the eigenvectors. We then describe the BLS spectrum associated with the first few dipole/exchange spin wave modes with emphasis on their relative intensity. We also discuss the stokes/anti stokes ratio. \newline \newline References: \newline [1] Rodrigo Arias, Ping Chu and D. L. Mills, Phys. Rev.B71, 224410 (2005).   

Self Assembled CoFe$_2$O$_4$ Nanoparticles within Block Copolymer Films: Structural and Magnetic Properties

Nanosize CoFe$_2$O$_4$ particles have been synthesized by self-assembly within diblock co-polymers, through a room-temperature templating strategy, amenable to large scale fabrication. XRD, TEM, SQUID and Mossbauer studies are combined in order to explore the morphological, structural, micromagnetic and interfacial characteristics of this nanocomposite system. TEM micrographs indicate low polydispersity, with particle size of 9.6 nm diam. Low temperature Mossbauer studies predict average sub lattice saturation hyperfine magnetic fields H (A) =501 kOe and H [B] = 527 kOe, respectively, for the tetrahedral and octahedral iron coordination sites of the ferrite spinel structure. Superparamagnetic relaxation processes, analyzed within a cubic magnetic anisotropy model, give a magnetic anisotropy density K = $3.23 x 10^5$J/m$^3$, while SQUID magnetometry predicts a saturation coercivity of 6.1 kOe. Deviations from bulk CoFe$_2$O$_4$ and unsupported CoFe$_2$O$_4$ nanoparticles are discussed in terms of finite-size effects and interfacial interactions.   

Magnetization of iron clusters from first-principles calculations

The magnetic moment of Fe clusters as function of number of atoms has been observed to show a slow decrease from the isolated atom value (4 Bohr magnetons) to its bulk value of 2.2 Bohr magnetons per atom. In addition, a series of peaks has been observed, for which the causes are not yet fully understood (see Billas, Chatelain, and de Heer, Science, 1994). We analyze the dependence of total magnetic moment, local magnetic moment, cohesive energy and other physical quantities in iron clusters Fe$_n$ ( $1 < n < 250$ ), and compare these results with available experimental data. We use a real-space method, pseudopotentials and first-principles DFT to obtain the properties of the cluster in its ground state. Calculations are done using the PARSEC code ( www.ices.utexas.edu/parsec ). We also discuss some of the recently developed capabilities of PARSEC.   

Chern-number spin Hamiltonians for magnetic nanoclusters by ab-initio methods

Combining field-theory methods and ab-initio calculations, we construct an effective Hamiltonian with a single giant-spin degree of freedom, capable of describing the low-energy spin dynamics of ferromagnetic metal nanoclusters consisting of up to a few tens of atoms. In our procedure, the magnetic moment direction of the Kohn-sham SDFT wave-function is constrained by means of a penalty functional, allowing us to explore the entire parameter space of directions, and to extract the magnetic anisotropy energy and the Berry curvature functionals. The average of the Berry curvature over all magnetization directions is a Chern number, a topological invariant that can only take on values equal to multiples of half-integers, which represents the dimension of the Hilbert space of the effective spin system. The spin Hamiltonian is obtained by quantizing the classical anisotropy-energy functional, after a change of variables which yields a constant Berry curvature. We illustrate this procedure by explicitly constructing the Hamiltonian for dimers and trimers of Co and Cr, whose spin dynamics has been recently investigated experimentally by STM methods.   

The coupling of magnetic and dielectric properties in magnetic nanoparticles

The low frequency dielectric properties of \textit{$\gamma $}-Fe$_{2}$O$_{3}$ and MnFe$_{2}$O$_{4}$ magnetic nanoparticles have been investigated. These samples showed frequency dependent dielectric anomalies near their respective magnetic blocking temperatures suggesting a coupling between the magnetic and dielectric properties of the systems. In addition, the samples exhibited considerable magnetocapacitance above the magnetic blocking temperature. The magnetic field induced change in the dielectric constant was shown to be proportional to the square of the magnetization, suggesting that the dielectric properties of these systems are strongly connected to the distribution of magnetic moments in the samples. The results will be discussed in the framework of a theory explaining how magnetodielectric effects can arise from magnetoresistance in a Maxwell-Wagner capacitance model.   

A new magnetization-reversal strategy for Stoner particles

A new strategy is proposed aimed at substantially reducing the minimal magnetization switching field for a Stoner particle. Unlike the normal method of applying a static magnetic field which must be larger than the magnetic anisotropy, a much weaker field, proportional to the damping constant in the weak damping regime, can be used to switch the magnetization from one state to another if the field is along the motion of the magnetization. The concept is to constantly supply energy to the particle from the time-dependent magnetic field to allow the particle to climb over the potential barrier between the initial and the target states.   

Structural, Magnetic and Dynamical Properties of Dipolar Nanoparticles

We investigated the structural, magnetic and collective properties of dipolar nanoparticles. The dynamic of the systems is determined by differential equations for the translational and rotational degrees of freedom, which are studied using molecular dynamics. The interaction potential of the particles consists of both an anisotropic dipolar interaction and an isotropic hard-sphere potential. Dependent on the temperature and external magnetic field, the system is found to be in different states. These states can be characterized by their respective structural ordering, that is closely related to the magnetic and energetic properties of the assembly of particles. In the ground state the particles arrange themselves in closed rings due to the anisotropic nature of the interaction. Besides this structure also the formation of metastable chains and network-like structures can be observed. Thermal excitations lead to a destabilization while the influence of an external magnetic field depends on its relative orientation with respect to the structures. In this work the phase diagrams in two and three dimensions of the various structures are determined as a function of temperature and external field.   

Session W23: Focus Session: MAG.THY IV / ab initio Studies

Ab initio Study of Mirages and Magnetic Interactions in Quantum Corrals

We present the state of the art ab initio studies of mirages and magnetic interactions in quantum corrals. Our results demonstrate that quantum corrals could permit to manipulate the exchange interaction between magnetic adatoms on metal surfaces at large distances. We show that the spin-polarization of surface-state electrons can be projected to a remote location by quantum states of corrals. Our study gives a clear evidence that the 'spin-polarization transfer' takes place in a mirage experiment of Manoharan et al.,[2]. We find that the spin-polarization of surface-state electrons on transition metal surfaces[3] can be manipulated by quantum corrals. Our results reveal that an atomic motion in quantum corrals could be strongly affected by the quantum confinement of surface-state electrons. 1. V. S. Stepanyuk, L. Niebergall, W. Hergert, P. Bruno, Phys. Rev. Lett. 94, 187201 (2005). 2. H.C. Manoharan, C.P. Lutz, D.M. Eigler, Nature 403, 512 (2000). 3.L. Diekh\"{o}ner, M.A. Schneider, A.N. Baranov, V.S. Stepanyuk, P. Bruno, K. Kern Phys. Rev. Lett. 90, 236801 (2003).   

Noncollinear magnetism in antiferromagnetic manganese chalcogenides

Metastable zincblende compounds of transition-metal pnictides and chalcogenides have recently become the subject of much attention due to their unique properties exhibiting combinations of magnetism and semiconductivity. Here we investigate magnetism in the antiferromagnetic (AFM) transition-metal chalcogenides, namely MnSe and MnTe, by using the FLAPW method.\footnote{Wimmer, {\it et al.}, PRB 24, 864(1981)} Assuming a collinear magnetic structure, we demonstrate that the AFM structure consisting of alternating Mn (001) spin-up and spin-down planes is favored over the ferromagnetic state, since the majority-spin $d$-bands are completely filled and so achieve the half-filling state that leads to the superexchange interaction. However, with FLAPW calculations that now treat full noncollinear magnetism,\footnote{Nakamura, {\it et al.}, PRB 65, 12402 (2002); 67, 14420 (2003)} we find that the lowest energy state is a noncollinear AFM structure --- the so-called AFM type III structure --- which relaxes frustration in the AFM Mn moment alignment on the fcc sublattice, a result that agrees with neutron experiments.\footnote{Samarth, {\it et al.}, PRB 44, R4701 (1991)} *Supported by NSF MRSEC through the NU MRC.   

Embedded Clustering and Metastable Magnetism in Transition-Metal doped III-Nitrides

From extensive density-functional theory calculations [1] we find that Cr atoms in GaN prefer to form embedded clusters, occupying Ga sites [2]. Significantly, for larger than 2-Cr- atom clusters, states containing antiferromagnetic coupling with net spin in the range 0.06-1.47 $\mu_{\rm B}$/Cr are favored. Similar behavior is found for Mn:GaN, and Cr:AlN and Mn:AlN. We show that various configurations may coexist leading to a strong dependence of the magnetic properties on the growth conditions. This elucidates many puzzling observations such as the 5 (20-30) times lower value of the measured magnetic moment on Cr (Mn) as compared to the theoretically predicted one for the isolated dopants. In addition to the expected ground high spin (HS) states for isolated Mn and Fe in GaN (4 $\mu_{\rm B}$/Mn and 5 $\mu_{\rm B}$/Fe), metastable low spin (LS) states (0 $\mu_{\rm B}$/Mn and 1 $\mu_{\rm B}$/Fe) are found. The transition between the HS and LS states corresponds to an intra-ionic electron transfer between the $t_2$ and $e$ orbitals, accompanied by a spin-flip process.\\ $[1]$ B. Delley, J. Jchem. Phys. {\bf 113}, 7756 (2000).\\ $[2]$ X.Y. Cui, $et\, al.$, Phys. Rev. Lett. Dec. 2005.   

Optimized Effective Potential Method for Non-Collinear Magnetism

A description of non-collinear magnetism in the framework of spin-density functional theory is presented for an exact exchange energy functional which depends explicitly on two-component spinor orbitals. The equations or the effective Kohn-Sham scalar potential and magnetic field are derived within the optimized effective potential framework. We have implemented this formalism within the full-potential linearized augmented planewave method, with an unconstrained magnetization density. Our calculations for Co and Fe show that the overestimation of moments seen in previous work was an artifact of the decoupled equations used. We further demonstrate, with the example of a magnetically frustrated Cr monolayer, how intra-atomic non-collinearity may be underestimated by local functionals.   

Spin susceptibility calculation based on the QP self-consistent GW method

Recently we have developed the quasi-particle self-consistent $GW$ method (QPsc$GW$) based on the full-potential LMTO method. The method is designed to determine the best independent-particle picture. The most significant impact is that QPsc$GW$ covers rather wide-range of materials, including semiconductor and transition metal oxides with acceptable accuracy [1,2]. In contrast to LDA$+U$, QPsc$GW$ can provide reasonable description of the $d$ band position relative to the $sp$ band without any free parameters. The $d$ band position critically affects the exchange coupling between magnetic ions. We are now developing the method to calculate the dynamic spin susceptibility based on the QPsc$GW$ method. We will show results for elemental transition metals and zincblende(ZB)-type of materials including magnetic ions. We have found that the ferromagnetic phase of ZB-MnAs is stable, contrary to the LDA result. In addition, we will show analysis of the exchange-coupling between magnetic ions. [1] Mark van Schilfgaarde, Takao Kotani, and Sergey V. Faleev, cond-mat/0510408 [2] Sergey V. Faleev, Mark van Schilfgaarde, and Takao Kotani, PRL93, 126406 (2004)   

A band theory for magnetic cuprates based on self-interaction free local density approximation

The pseudo-SIC approach is based on an approximate form of self-interaction corrected (SIC) Kohn-Sham Equations. We overview the functionalities of this method applied to cuprates, which are prototypes of difficult materials for standard local-spin density functional theories such as LSDA (or even GGA). Indeed, theories based on local exchange-correlation potentials fail to predict the correct spin-polarized ground-state solution expected for the low-magnetization state (S=1/2) of the Cu(I) ions, thus describing these systems as metallic and nonmagnetic. Here we present our results for a series of relevant cases, including CuO, Cu$_2$O, CuGeO$_3$, and YBa$_2$Cu$_3$O$_{6+x}$, showing that the pseudo-SIC is capable to correct the gross failures of LSDA, restoring the expected S=1/2 electronic ground state and an overall satisfying description of the chemistry and the electronic and magnetic properties of these systems. Furthermore, since the pseudo-SIC is designed to work for metals as well as for insulators we can approach the challenging task of studying by first-principles the insulating-metal transition in doped Mott insulators. We will consider the example of Mn-doped CuO, where Mn-doping induces a simultaneous insulating-to-metal and antiferromagnetic-to-ferromagnetic phase transition.   

Interplay of Vacancy Defects and Magnetism in Carbon Structures

Magnetic properties of diamond and graphite with vacancy defects have been studied using spin-polarized plane-wave basis density functional theory. Various scenarios of vacancy defects are investigated in these two allotropic configurations. The calculation shows that the vacancy defect concentration and nearby bonding structure is critical to determine the induced magnetism. The total magnetism start to decrease after vacancy accumulation reach the interacting configuration, in both diamond and graphite. We also shows that foreign species like nitrogen close to the vacancy is able to further enhance the magnetic moment in graphite.   

The effect of disorder and short-range correlations on ferromagnetism in dilute magnetic semiconductors

We use the Dynamical Cluster Approximation (DCA) and double exchange model, coupling spin one-half holes to magnetic impurities, to study the ferromagnetic transition in semiconductors doped with transition metal magnetic ions. Our approach includes the effect of local dynamics as well as short-range correlations between the magnetic impurities. We systematically incorporate the effect of disorder in the impurity positional configurations with a new algorithm, based on the DCA, specific to dilute systems. This new algorithm serves as a replacement for the Traveling Cluster Approximation and Coherent Potential Approximation. We focus on the appearance of the impurity band and the development of the magnetization for a range of coupling strengths and hole and impurity concentrations. In addition, we discuss the effect of impurity clustering on the hole mobility and the ferromagnetic transition temperature. We conclude that the successful design of spintronic nanostructures based on ferromagnetic semiconductors must include an understanding and careful analysis of disorder and spatial correlations.   

Spatial correlations, spin-orbit coupling, and ferromagnetism in Ga(Mn)As

The self-consistent Dynamical Cluster Approximation (DCA) is used to study the effect of strong spin-orbit coupling in models of GaMnAs. Both heavy and light carrier bands, degenerate at the $\Gamma$-point, are included using the spherical approximation. Local dynamics as well as short-range spatial correlations are studied using the DCA, adapted for impurity systems in the dilute limit. The critical temperature for ferromagnetism is obtained for different arrangements of magnetic impurities and a range of coupling strengths and carrier concentrations. These calculations clearly demonstrate the suppression of the ferromagnetic transition temperature when one accounts for spatial correlations between impurities and the reduction in saturation magnetization due to the strong spin-orbit coupling.   

Structural, electronic and magnetic properties of Mn-doped GaAs(110) surface

First principles total-energy pseudopotential calculations have been performed to investigate structural, electronic---including scanning tunneling microscopy (STM) images---and magnetic properties of the (110) cross-sectional surface of Mn-doped GaAs. We have considered configurations with Mn in interstitial positions in the uppermost surface layers with Mn surrounded by As (Int$_{As}$) or Ga (Int$_{Ga}$) atoms. The presence of Mn on the GaAs(110) surface originates strong local distortion in the underlying crystal lattice, with variations of interatomic distances up to 8\%. In both cases, Int$_{As}$ and Int$_{Ga}$, the surface electronic structure is half-metallic (or \emph{nearly} half metallic) with details strongly dependent on the local Mn environment. The atoms surrounding the Mn impurity show an induced polarization resulting in a ferromagnetic Mn--As and antiferromagnetic Mn--Ga configuration respectively in the two cases. The simulation of the STM images show very different patterns of the impurity region in the two cases, suggesting that they could be easily discerned by STM analysis. We have also simulated STM images of Mn interstitials pairs on surface. The comparison of the simulated images with recent experimental cross-sectional STM images of Mn $\delta$-doped GaAs is discussed.   

Structures and magnetic properties of Cr-doped GaN nanotubes

The electronic and magnetic properties of Cr-doped GaN nanotubes are investigated theoretically from first principles using the generalized gradient approximation (GGA) as well as LSDA+U method. We have shown that GaN single wall nanotube, which was generated from GaN wurtzite crystal undergoes large structural relaxation and resemble the structure of carbon (9,0) single wall nanotubes. In addition, it is stable at room temperature. Cr-doped GaN single wall and multi-wall nanotubes are ferromagnetic with each Cr atom carrying a magnetic moment of about 2.67 \textit{$\mu $}$_{B. }$ This ferromagnetic coupling is mediated by the neighboring N atoms which are weakly polarized and carry a magnetic moment of -0.18 \textit{$\mu $}$_{B}$. These results are not sensitive to the tube diameter, Cr concentration, and the level of correlation. Thus, Cr doped GaN nanotubes may be a robust system for applications in spintronics.   

Ferromagnetism in Mn doped GaN Nanowires

Using density functional theory and generalized gradient approximation for exchange and correlation potential we show that the magnetic coupling of Mn atoms in the nanowires, unlike that in the thin film, is ferromagnetic in spite of the thickness of the wire and the contraction of the Mn-Mn and Mn-N bond distances. This ferromagnetic coupling, brought about due to the confinement of electrons in the radial direction and the curvature of the Mn-doped GaN nanowires' surface, is mediated by N as is evidenced from the overlap between Mn 3$d$ and N 2$p $states. The Mn atoms prefer to occupy the nearest neighbor positions on the outer surface of the wire and carry a magnetic moment ranging from 0.56 to 3.5 \textit{$\mu $}$_{B}$/atom depending on the thickness of the wire. Calculations of the anisotropic energy show that the magnetic moment orients preferably along the [10$\overline 1 $0] direction while the wire axis points along the [0001] direction. The flexibility of both controlling the magnetic coupling and the magnetic moment by choosing the dimensionality and the size of the wire may be useful in practical applications. The results are in agreement with the recent experimental data which show that Mn-doped GaN nanowire can be ferromagnetic without the presence of other defects.   

First-principles calculation of Mn Atoms on the CuN/Cu(100) Surface

The electronic structure is calculated using GGA+U for one and two Mn atoms on a single CuN layer coated on the Cu(100) surface. This unique insulator-metal junction surface prevents the Mn spins from being screened by the conduction electrons and at the same time allows experimentalists to pass tunneling electrons through the Mn atoms to flip their spins. Our spin-density analysis shows that Mn atoms in such a surface preserve their atomic spins S=5/2. This result agrees with a recent STM measurement on such systems. Electron-density change and surface relaxation due to the Mn atoms are also analyzed.   

Session W24: Focus Session: Lithography

Nanoimprint Lithography: Process Induced Stresses and Pattern Stability

Nanoimprint lithography is emerging as an economical technique for fabricating polymeric nanostructures. Features as small as 10 nm in a hard master or mold can be faithfully replicated by imprinting this master into a polymer film. At elevated temperatures and pressures, the molten polymer fills the nanoscale cavities of the mold. When the film is cooled to the vitreous state and the mold removed, freestanding polymeric nanostructures remain. In this presentation we illustrate that the NIL process induces large degrees of residual stress into these structures. Upon heating imprinted nanostructures to just above the glass transition temperature of the polymer, a physical relaxation of the nanostructure shape occurs. The features shrink in height and broaden in width with increased annealing time. However, this decay or slumping of the imprinted pattern is not driven by a simple viscous flow. High molecular mass polymer patterns slump faster than their low molecular mass analogs, contrary to the viscosity changes. Rather, the high viscosity resins generate greater shear stresses along the mold interfaces that lead to extensional flow of the polymer in the fill directions of the patterns. This traps residual stresses in the nanostructures when they are cooled into the glassy state. We quantify this slumping process using X-ray scattering and reflectivity techniques for a range of polymers and pattern sizes and explore potential relations with the glass transition of the polymer within the nanostructure.   

Nanometer-scale control of the crystallization of oligomers and polymers.

The ability to control the position and local orientation of organic crystals at the nanometer scale paves the way to the fabrication of hybrid nano-devices displaying better properties. Here, we present two ways to control the assembly of organic chain compounds into nanometric crystals of defined location or orientation. We first show how the location of crystals of model oligomers can be directed by chemical nano-templates [1]. The templates are obtained by combining electron-beam lithography with the deposition of self-assembled monolayers [2]. These surfaces can then be used to control a variety of assembly processes [3], such as the crystallization of model alkane-1-ol oligomers in solution. By using directing maps with the appropriate chemical inks, nano-squares, nano-corrals and nano-lines of organic crystals are rapidly and massively grown at pre-defined locations, at least down to 60 nm. At this scale, confinement effects mediated by van der Waals forces become prominent, providing a unique handle to design crystal growth. Then, we show how the nucleation and orientation of polymer crystals can be controlled by nano-imprint lithography [4]. The combination of confinement, and of preferential nucleation at the vertical walls of the nano-molds probably arising from partial chain orientation due to the polymer flow during embossing, results in local control over the 3D orientation of the crystals. We demonstrate that crystals may be guided through complex geometries, and investigate the case of systems where conflicting instructions are delivered to the crystallizing chains. \newline \newline References: \newline [1] J. Plain et al., submitted. \newline [2] A. Pallandre et al., Nano Letters 2004, 4, 365. \newline [3] A. Pallandre et al., J. Am. Chem. Soc. 2005, 127, 4320; F.A. Denis et al., Small 2005, 1, 984; A. Pallandre et al., Adv. Mater., in press. \newline [4] Zhijun Hu et al., Nano Letters 2005, 5, 1738.   

UV Polarizer Fabricated by Diblock Copolymer Lithography

Transmission UV polarizers are desired for next-generation semiconductor device fabrication using ArF or F$_{2}$ excimer laser lithography. Controlling polarization is essential especially for high numerical aperture (NA) immersion lithography processes. The polarizer requirements are thickness less than 1 mm and low absorption of the light used for the exposure. A wire grid polarizer is ideal for this purpose but it requires wires with a pitch less than quarter of wavelength of the light. A cylinder-forming polystyrene-polyhexylmethacrylate diblock copolymer (PS-PHMA, 21-64 kg/mol) was used as a mask for fabrication because its cylinders macroscopically align by simple application of shear stress, and the PHMA domains etch faster than PS by reactive-ion etching (RIE), providing sufficient contrast for pattern transfer. The diblock was spin-coated on a UV transparent fused silica substrate and shear-aligned. The stripe pattern was transferred by RIE onto the substrate by a multilayer technique to enhance the pattern height, then a metal was deposited by evaporation. Finally, the remaining polymer was lifted off to complete the wire grid, having a 33nm pitch (16.5nm line and space). The UV light polarization characteristics of these grids will be presented.   

Simple Analytic Model for Nanowire Array Polarizers

Cylinder-forming diblock copolymers can be used to pattern nanowire arrays on a transparent substrate to be used as a polarizer, as described by Koji Asakawa in a complementary talk at this meeting. With a 33nm period, these wire arrays can polarize UV radiation, which is of great interest in lithography, astronomy and other areas. One can gain an analytical understanding of such an array made of non-perfectly conducting material of finite thickness using a model with an appropriately averaged complex dielectric function in an infinite wavelength approximation. This analysis predicts that the grid can go from an E-polarizer to an H-polarizer as the wavelength decreases below a cross-over wavelength, and experimental data confirm this cross-over. The overall response of the polarizing grid depends primarily on the plasma frequency of the metal used and the volume fraction of the nanowires, as well as the grid thickness. A numerical approach is also used to confirm the analytical model and assess departure from infinite wavelength effects. For our array dimensions, the polarization is only slightly different from this approximation for wavelengths down to 150nm.   

Robust Nanopatterns from Self-Assembly of a Diblock Copolymer and an Inorganic Precursor

Nanoscopic patterns from self-assembled block copolymer thin films have been recognized as a promising route to sub-lithographic patterns on substrates. Line patterns from lamellar phase of block copolymers are particularly attractive as they can be used as an etch mask for transferring patterns into substrates. A few organic block copolymers have been studied for generating line patterns by controlling the orientation of lamellar microdomains. The organic nature of the block copolymers, however, often gives poor thermal stability and etching contrast, which limits potential applications. Indeed robust nanostructures of sub-lithographic length scales are highly desirable to comply with common nanofabrication processes. Here we report a simple method to create robust nanoscopic line patterns on surfaces from self-assembly of mixtures of a diblock copolymer and an inorganic precursor. The organic diblock copolymer directs the structure of the inorganic precursor and can be removed by thermal treatment. By tuning the interfacial energy at two interfaces, normally oriented lamellar patterns of approximately 20nm half-pitch and 40nm thick were obtained. Results on transferring patterns to substrate will be reported as well.   

Fabrication of inorganic photonic crystals from interference lithography

We have fabricated 3D FCC-like microstructure using multi-beam interference pattern. This polymeric structure was used as a sacrificial template. Silica was deposited into the pores by alternating exposure to water and silicon tetrachloride vapors under atmospheric pressure and at room temperature. This inorganic structure can provide a platform for the deposition of high refractive index materials such as silicon, germanium, and titania. We investigate the photonic bandgap property of this structure as a function of refractive index as well as filling ratio. Using a two-parameter level-set approach, we find that the FCC-like structure has multiple complete photonic bandgaps at 2-3 and 7-8 bands, respectively, while the bandgap width is sensitive to the morphology of coated-structure. Our calculation results suggest that the complete-filled structure possessed a wider photonic bandgap between 2 and 3 bands than the incompletely-coated core-shell structure.   

Effects on Low Voltage Electron Beam Lithography

To examine the practical limits and problems of low voltage operation we have studied e-beam lithography in the low (few keV) to ultra-low ($<$500eV) energy range, employing commonly used resists such as PMMA and compared the results to those from conventional high voltage processing. We have directly imaged, exposed and developed resist profiles as well as deposited metal after liftoff, using scanning electron microscopy, and compared to our advanced Monte Carlo simulations which incorporate elastic, inelastic, fast secondary electrons, and plasmon contributions. The results show that the exposed profiles and resolutions experimentally achieved at low energy can only be matched by simulations which include a significant FSE and plasmon contributions to the energy PSF. With an optimized resist thickness proximity effects are greatly reduced and process latitude is improved.   

Hierarchical Organization of Nanoparticle Composites through Nano-Imprinting

Formation of highly ordered and morphologically controlled nanoparticle/polymeric assemblies is highly desirable in a variety of applications including optoelectronics, sensing, photonics and catalysis. Structure-guiding polymer matrices, such as block copolymers, have demonstrated to be an effective means for controlling the composite morphology as well as localizing particles in nanoscale domains. Yet, fabrication of polymer/nanoparticle composites with precise control over final morphology and particle location is still a major challenge. In this paper, we will show the use of nano-imprint lithography to pattern polystyrene/gold composites at different length scales (20 nm to 1micron). Imprint lithography (the technique of physically pressing and deforming a polymer layer for patterning purposes) is a low cost and high throughput alternative to conventional patterning. PS coated gold nanoparticles were synthesized following Brust's method and subsequently dispersed in PS matrices of varying molecular weights. Nano- imprinting was conducted under different temperature and pressure conditions. TEM, SEM, AFM and UV-Vis techniques were used to characterize these composites.   

Mesoscale Simulation of the Lithography Process

Process simulation is an important tool for the semiconductor industry. Optimization of the microlithography process is extremely expensive since exposure tools that cost in excess of 20 million dollars, which could be dedicated to manufacturing, must be used in the optimization experiments. There exist several commercial simulation packages that work efficiently and accurately all of which are based on parameterized continuum models. Continuing improvements in materials and equipment for microlithography have now provided the ability to print circuit elements with minimum dimensions approaching the size of the molecules that comprise the photoresist. As a result, stochastic and molecular scale effects such as line edge roughness have become an increasing concern and may limit continued progress in this industry. These effects can not be captured by continuum models. Hence, we have worked to develop a mesoscale simulation of the process. The simulator is based upon with discretization of the photoresist film into cells on a three dimensional lattice and a Monte Carlo approach. The entire process has now been simulated in this way. New models for reactive transport, polymer dissolution, etc. have been incorporated in this simulator. Progress will be reported.   

New Directions in 3-D Multiphoton Lithography.

Multiphoton absorption polymerization (MAP) is a promising technique for the lithographic fabrication of 3-D microdevices. However, this technique also has two major shortcomings that have so far precluded its use in the mass production of devices. First, MAP is an inherently serial technique, and structures must be created on a voxel-by-voxel basis. Second, the fabrication of many desirable 3-D devices requires incorporation of materials other than polymers. We will discuss our recent progress in attacking both of these problems. We have developed soft-lithographic techniques that allow for the creation of complex 3-D structures that can include closed loops based on master structures created using MAP. We have also developed a technique that allows for the selective deposition of materials that include metals, metal oxides, and biomolecules on desired regions of 3-D structures fabricated with MAP. We demonstrate the use of the latter technique in the creation of functional microinductors.   

High efficient LEDs having columnar structure surface fabricated by block copolymer lithography

Recently, the internal quantum efficiency of LEDs has improved, but the external efficiency remains low due to the high refractive index of semiconductors. To extract more light, a columnar structure with sub-micron period was fabricated on the LED surface by block copolymer lithography. Since the desired pattern is relatively larger than the block copolymer microdomains, a super-high molecular weight block copolymer was used. The polystyrene (PS) - polymethyl methacrylate (PMMA) diblock copolymer was used in this study since the PMMA has a much faster etch rate than the PS by reactive-ion etching (RIE). The PMMA was removed by RIE, and the gallium phosphide (GaP) substrate was etched by chlorine-based inductively coupled plasma RIE using the remaining PS dots as a mask. The optical extraction efficiency of the patterned substrates improved 2.6 times compared to unprocessed flat GaP substrates; the pillars' height was 450 nm, diameter was 100 nm, and pitch was 150 nm. We also prototyped a real LED and increased light emission volume 1.8 times compared with conventional LED at the same energy consumption.   

Session W25: Gels and Networks

Tunable and Reversible Swelling of a p(tBA)-$b$-p(HEMA-\textit{co}-DMAEMA) Block Copolymer

Hydroxyethyl methacrylate (HEMA) and dimethylaminoethyl methacrylate (DMAEMA) have been investigated as precursor materials for pH-responsive hydrogels. DMAEMA in these hydrogel systems allows for pH-tunability, as it is reversibly protonated below its pKa (7.5). In this work, we present the design of a nano-structured hydrogel diblock copolymer whose major block consists of a statistical copolymer of p(HEMA-\textit{co}-DMAEMA) (30.5 kg/mol) polymerized at the azeotropic composition (71 mol{\%} HEMA), with a poly(tert-butyl acrylate), p(tBA), (12.1 kg/mol) minor block. The resulting diblock copolymer is narrow in molecular weight distribution (PDI = 1.24) and spontaneously self-assembles to form hexagonally-packed p(tBA) cylinders (R = 9.5 nm) within a p(HEMA-\textit{co}-DMAEMA) matrix in the solid state. When swollen in an aqueous medium, hydrophobic p(tBA) cylinders serve as physical cross-links. We monitor the extents of swelling by quantifying changes in the characteristic (10) spacing of the hexagonal lattice by SAXS. Swelling is tunable and reversible with changes in pH; we observe 35{\%} and 21{\%} swelling relative to the dry state at pH 5 and 8.5, respectively.   

Highly Responsive Self-Assembled Gels from Triblock Copolymers in Liquid Crystal Solvent

Triblock copolymers having random coil endblocks and a side-group liquid crystalline polymer (SGLCP) midblock self-assemble in small-molecule liquid crystal (LC) solvent to form highly responsive gels. In these block copolymers, the LC solvent switches from being strongly selective toward the SGLCP block below its isotropic-nematic transition (T$<$T$_{NI})$ to being a good solvent for both blocks in the isotropic phase. In the nematic phase, the LC solvent is a poor solvent for the polystyrene (PS) endblocks, driving them to physically associate to form the network crosslinks. In the isotropic phase, at dilute polymer concentrations, the PS endblocks are solubilized to yield a solution of free polymer chains. Synergistic coupling between polymer and solvent results in a gel with novel properties; the orientational order of the nematic LC solvent imparts electo-optic and mechano-optic properties that are forbidden by symmetry in isotropic gels, and the polymer network provides memory via long-time relaxation processes that do not exist in the bulk LC. Gels can be aligned into a single-crystal monodomain by applied shear, electric fields, magnetic fields, or surface effects, and the alignment state is preserved by the network's elastic restoring force. Insights into the structure and dynamics of the gels are gained by rheometry, small-angle neutron scattering, and electro-optical switching experiments.   

Nanoparticle-reinforced associative PLA-PEO-PLA hydrogels

Hydrogels of poly(lactide)-poly (ethylene glycol)-poly (lactide) have potential applications in drug delivery and tissue engineering. Control over the structure and rheology of the gels is of fundamental importance for the use of this polymer in medical applications. We have performed a complete rheological and structural characterization of these hydrogels using dynamic mechanical rheology, SANS, and USAXS. These polymers form very stiff hydrogels and the structure and properties of these materials can be substantially modified by varying the crystallinity or degree of polymerization (DP) of the hydrophobic PLA block. We have also created reinforced hydrogels with enhanced mechanical properties by addition of laponite nanoparticles. Our recent studies show that the elasticity of the PLA-PEO-PLA hydrogels can be enhanced by orders of magnitude by addition of small amounts of laponite particles to the hydrogels. It is expected that the triblock copolymer micelles adsorb on the surface of the laponite particles to form additional junctions in the hydrogels leading to enhancement in their elasticity. We verify this hypothesis using DLS and SANS techniques.   

Controlling the Self-Assembly of ABCBA Pentablock Copolymer Gels in Water Solution by the Hydrophobic Effect

We characterize the phases of a system of non-ionic pentablock copolymers with an ABCBA structure in water solution, where the A and C blocks are hydrophobic and the B blocks are hydrophilic. Coarse-grained simulations are performed using molecular dynamics with the solvent modeled implicitly, and the interaction potential includes a parameter that controls the quality of the solvent. In a good solvent, spherical micelles form and assemble into a swollen gel. We examine the aggregation number, gyration radii, micelle superstructures and percolation at various concentrations for this phase. As the B blocks become less hydrophilic, which occurs for increasing temperature, the micelles move close to one another and expel water. There is a gradual phase transition from spherical micelles to cylindrical worm-like micelles. We model the further expelling of water by increasing the concentration of polymers in the simulation and find that a lamellar phase forms. We compare our simulations with experimental results on recently synthesized modified Pluronic systems.   

PEO Hydrogels Prepared by End-linking with PAMAM Dendrimers

End-linking is a preferred synthetic technique for preparing polymer networks and gels for fundamental structure/property studies. End-functionalized telechelic linear polymers are joined to a multifunctional crosslinker to form a network in which the molar mass of the polymer chains between chemical crosslink points is known. Although end-linked elastomers prepared in bulk have been well-studied over the preceding decade, much remains to be learned about how the presence of a good solvent affects the equilibrium swelling and modulus of end-linked gels. We prepared well-defined hydrogels in a good solvent (water) by linking epoxide end-functionalized, linear poly(ethylene oxide) (PEO) to the amine endgroups of poly(amidoamine) (PAMAM) dendrimers of generations 0, 2, and 4. Dendrimers can serve as well-defined macromolecular crosslink junctions because they can have nearly monodisperse numbers of reactive endgroups. We have characterized how reaction conditions such as junction functionality, polymer concentration at preparation, ratio of crosslinker endgroups to precursor endgroups, and precursor molar mass affect gelation and equilibrium swelling. We will discuss the somewhat surprising observation of ``superabsorbent'' behavior in selected PAMAM- PEO gels.   

Probe diffusion in polymer solutions and hydrogels using fluorescence correlation spectroscopy

We apply fluorescence correlation spectroscopy (FCS) to measure the diffusion of small fluorescent probes (TAMRA, Mw = 430 Da; dextran, Mw = 10 kDa) in poly(vinyl alcohol) (PVA) solutions and hydrogels. PVA is a linear, neutral, biocompatible polymer, whose hydrogels have many biotechnology applications, such as drug-delivery devices and tissue scaffolds. The FCS measurements indicate that the probe diffusion decreases when the polymer solution is cross-linked. Further, the more the polymer chains are cross-linked, the slower the particles diffuse. These results suggest that the cross-link density, which is often ignored in the analysis of probe diffusion data in gels, must be taken into account. Remarkably, we find that the apparent diffusion time and the elastic modulus of the gels show a linear correlation.   

Large-strain deformation and fracture of tough hydrogels

Highly-swollen, chemically-crosslinked hydrogels generally behave in a very brittle manner, fracturing suddenly after a small amount of reversible deformation. Because of their importance as biomaterials, it is useful to control and augment the resistance to fracture of these materials. Tougher, stronger hydrogels are emerging, and it is important to understand the structural origins of strength in these relatively robust, highly-swollen, polymer systems. We have investigated the rheological, mechanical and fracture properties of tough hydrogels, using novel testing techniques and focusing on the high-strain compression and tension behavior. Results from large-strain and fracture experiments were correlated to the chemical structure of the hydrogels. Because we believe that the mechanical properties of these tough hydrogels are due to the presence of dissipative mechanisms at the molecular level, we have explored several methods of synthesis to create these materials.   

Rheological behavior of Slide Ring Gels.

Slide ring gels were synthesized by chemically crosslinking, sparsely populated $\alpha $-cyclodextrin ($\alpha $-CD) present on the polyrotaxanes consisting of $\alpha $-CD and polyethylene glycol (PEG). [1] Unlike physically or chemically crosslinked gels, slide ring gels are topological gels where crosslinks can slide along the chain. [2] We investigate the rheological behavior of these gels swollen in water and compare their viscoelastic properties to those of physical and chemical gels. We also study the equilibrium swelling behavior of these gels. [1] Okumura and Ito, Adv. Mater. 2001, 13, 485 [2] C. Zhao et al, J. Phys. Cond. Mat. 2005, 17, S2841   

Mechanical and swelling properties of PDMS interpenetrating polymer networks

Poly(dimethylsiloxane) (PDMS) interpenetrating networks (IPNs) of a large and a small molar mass PDMS were prepared. Six series of IPNs were obtained by first tetra-functionally end-linking long vinyl-terminated PDMS neat or in a 50 per cent solution with unreactive PDMS chains. These networks were then dried and swollen with short reactive telechelic PDMS that were subsequently end-linked. We found that the correlation between modulus (E) and equilibrium swelling (Q) in toluene of the PDMS IPNs obeys a scaling relation identical to that of normal uni-modal PDMS networks. The results of the toughness of the networks represented by the energy required to rupture them were analyzed in terms of a recent model by Okumura (Europhysics Letters 67(3), 470, 2004). A modified version of this model that assumes each component of the double network to be subjected to an equal stress gives a good representation of the data.   

Generation of Oriented Buckling Patterns by Modulation of Local Elastic Moduli

Wrinkling patterns based on elastic instabilities are interesting due to the spontaneous formation of relief structures that consists of a dominant periodicity. While a wide variety of soft materials has been utilized to generate surface buckling patterns, alignment of these structures has only been demonstrated previously through pre-defined topographic patterns. In this contribution, we introduce a new methodology to producing aligned, or patterned, surface wrinkles through the manipulation of the local stress distributions. We define the specific regions of local differences in the elastic moduli of a poly(dimethyl siloxane) (PDMS) elastomer by selective oxidation of the PDMS surface into a silicate thin film. Subsequent swelling with a photopolymerizable monomer provides the buckling stress necessary for the formation of aligned surface wrinkles. We show that geometric confinement of the oxidized regions coupled with an osmotic stress controls the formation and orientation of the wrinkling structures.   

Melting Point Depression of Small Molecules in Cross-linked and Uncross-linked Polyisoprene: Deviations from Flory-Huggins Theory

Thermoporosimetry (TPY) is becoming increasingly used to study nano-scale heterogeneity in polymers. The starting point for TPY is the Gibbs-Thomson (GT) relation between melting point and inverse crystal size. In the case of polymers, the Flory-Huggins (FH) model predicts that there is a depression of melting point due to the mixing of the polymer and the solvent molecules, and this needs to be taken into account. The first step in analysis of heterogeneity size using TPY and the GT equation requires that there be quantitative agreement between FH and the melting points in the uncross-linked rubber. We find that both benzene and hexadecane exhibit excessive melting point depressions in uncross-linked polyisoprene. This may imply that the uncross-linked polymer is divided into `nanoheterogeneities.' We further find that the heat of fusion decreases as polymer concentration increases for the benzene, but not for the hexadecane. To our knowledge this is the first systematic investigation of the validity of melting of small organics in un-cross-linked polymers using the FH expressions.   

Elastic Fluctuations and Rubber Elasticity

A coarse-grained phenomenological model is constructed to describe both phonon fluctuations and elastic heterogeneities in rubbery materials. It is a nonlocal, spatially heterogeneous generalization of the classical model of rubber elasticity, and with a tunable repulsion interaction. This model can also be derived from the Vulcanization theory. The residual stress and the non-affine deformation field, as well as their correlations, are calculated perturbatively, to the leading order of quenched randomness. It is explicitly shown that the interplay between the repulsive interaction and quenched randomness induces non- affine deformation. The spatial correlations of the non- affine deformation field and residual stress exhibit power-law scaling, with no characteristic length scale. We also calculate the contributions to the elastic free energy from both thermal and quenched fluctuations for arbitrary deformation. We find that they naturally explain the universal features in the Mooney-Rivlin plot of the stress-strain curve for rubbery materials. The (disorder averaged) thermal fluctuation of monomers is shown to depend on deformation, and becomes anisotropic upon shear deformation, as long as the repulsive interaction is finite.   

Developing a lattice spring model to simulate the behavior of polymer gels

A basic feature of responsive polymer gels is an inherent coupling of multiple physicochemical processes with a finite deformation of the material. We have developed a new, computationally efficient approach - the gel lattice spring model (gLSM) - which allows us to model responsive gels that undergo relatively large deformations in 2D. We start by writing an equation for the energy of the deformed gel in terms of the invariants of the strain tensor. We introduce the representative, rectangular-shaped unit element of the system, obtain an approximation of the total gel energy as a function of the coordinates (nodes) of this element, and derive the equations for the forces acting on the nodes. In accordance with the two-fluid model of gel dynamics, we assume a purely relaxational dynamics by taking the velocity of a node to be proportional to the force acting on that site. Using this gLSM, we simulate the structural evolution of a swelling gel in 2D, and the propagation of the swelling-deswelling waves through a rectangular chemo-responsive gel undergoing the Belousov-Zhabotinsky reaction.   

MD simulations of chemically reacting networks: analysis of permanent set

The Independent Network Model (INM) has proven to be a useful tool for understanding the development of permanent set in strained elastomers. Our previous work showed the applicability of the INM to our simulations of polymer systems crosslinking in strained states. This study looks at the INM applied to theoretical models incorporating entanglement effects, including Flory's constrained junction model and more recent tube models. The effect of entanglements has been treated as a separate network formed at gelation, with additional curing treated as traditional phantom contributions. Theoretical predictions are compared with large-scale molecular dynamics simulations.   

A Gaussian Slip-Link Model for Polymer Single and Double Networks

In this study, we developed Schieber's slip-link model for lightly cross-linked polymers assuming the equilibration of deformed Gaussian chains. Our simulation consists of two steps: preparation and deformation. In the preparation step, cross-links and slip-links are assumed to be distributed randomly along the chain, but with independent statistical parameters: the average number of Kuhn steps between entanglements, $N_{e}$, and the average number of Kuhn steps between cross-links, $N_{c}$. In the second step, the cross-links and slip-links are deformed affinely, but since the chain can slide through the slip-links, its deformation is non-affine. The stress tensor can be determined as a function of deformation using Brownian dynamics as a sort of Monte Carlo algorithm. The Mooney plot of our simulation result has good agreement with most experimental data for uniaxial elongation deformation for cross-linked NR, PDMS, and PBd. The model is used to predict values for the Mooney plot parameters ($C_{1}$ and $C_{2}$) as a function of $N_{e}$ and the $N_{c}/N_{e}$ ratio. The $C_{2}/C_{1}$ ratio is found to be strongly dependent on $ N_{c}/N_{e}$, but weakly dependent on $N_{e}$. This observation provides a new way of predicting the cross-link density and separating it from the entanglement density and for systems of known $N_{e}$ and $N_{c}$, the model requires no adjustable parameters. We are also developing our model for double network polymers in order to demonstrate different applications for the model.   

Session W26: Focus Session: Biological Photophysics

Direct Observation of Thymine Dimer Repair in DNA by Photolyase

Departments of Physics, Chemistry, and Biochemistry, Programs of Biophysics, Chemical Physics, and Biochemistry, The Ohio State University, Columbus, 191 West Woodruff Avenue, OH 43210. Photolyase uses light energy to split ultraviolet-induced cyclobutane pyrimidine dimers in damaged DNA, but its molecular mechanism has never been directly revealed. We report here the direct mapping of catalytic processes through femtosecond synchronization of the enzymatic dynamics with the repair function. We observed direct electron transfer from the excited flavin cofactor to the dimer in 170 ps and back electron transfer from the repaired thymines in 560 ps. Both reactions are strongly modulated by active-site solvation to achieve maximum repair efficiency. These results show that the photocycle of DNA repair by photolyase is through a radical mechanism and completed on subnanosecond time scale at the dynamic active site with no net electron change in redox states of the flavin cofactor.   

Near-infrared femtosecond laser assisted cell membrane permeabilization

The controlled delivery of membrane impermeable molecules into single living cells (micro-injection) is important for a variety of applications such genomics, proteomics or drug screening and testing. Recently, it has been demonstrated that opto-injection with tightly focused ($\sim $300nm) femtosecond (fs) laser pulses at near-infrared (nir) wavelengths (700-1100nm) has the potential to create highly localized transient pores in single living cells with high cell survival rates and transfection efficiency. We have investigated the creation of transient pores in single living BAEC cells by focused fs nir laser pulses dependent on the incident laser intensity by dye uptake studies. Our experimental data agree very well with the experimentally and theoretically determined thresholds for laser-induced plasma formation and LIB. We observe that pore creation is observed for laser intensities of 4.0x10$^{12}$W/cm$^{2}$ and higher. For laser intensities above 3.3x10$^{13}$W/cm$^{2}$ BAEC cells are irreversibly destroyed. Within these two limits the pore size increases logarithmically with increasing laser intensity. This functional dependence is explained by considering the Gaussian intensity distribution across the laser focal spot. The physical understanding of the relationship between pore size and laser intensity allows the control of the number of molecules delivered into a cell per unit time through the control of the pore size.   

Biological Photophysics with Small Photons: Terahertz measurements of the protein dynamical Transition

Protein tertiary structural vibrations lay in the terahertz frequency range. These motions are associated with the large scale conformational motions necessary for function. Previously we have shown protein terahertz dielectric response is sensitive to protein conformation, hydration, oxidation state, ligand binding and folding. In this talk I will focus on our measurements of the 200 K dynamical transition and how these results fit into the slaved solvent model. Further I will discuss how these measurements along with hydration dependence measurements demonstrate the water beyond the first solvation shell continues to deviate from bulk water behavior. This work was supported by ACS grant PRF 39554-AC6, NSF CAREER grant PHY-0349256 and NSF IGERT grant DGE0114330.   

DFT calculation of photo-induced charge transfer in organic molecule

We propose a method for obtaining the charge transfer time for a chromophore-donor-acceptor system from density functional theory. Our calculations are done on a porphyrin-carotene-C$_{60}$ molecular triad. The geometry of the large molecular triad was optimized at the all-electron level using GGA. We are considering single electron excitations, the energies of which are obtained using a newly developed approach. The electronic dipolar transition probabilities are calculated from Einstein's A and B coefficients. However, in real systems the polarization effects play an important role in the transfer process since the charge separated states can possess huge dipole moments. The stabilization of the large dipole state can be calculated from the classical dipole-dipole interaction and the polarizability of the surrounding medium. The stabilization of the dipole states is an important aspect which dominates the charge transfer process and therefore the rise time. The efficiency of the molecule as a solar energy converter will also be discussed.   

Structural colours in blue-banded bee

Periodic, micro-textured biological materials are ubiquitous in nature. Electromagnetic waves at different frequencies are selectively reflected by such materials. This phenomenon is the origin of structural colours observed in variety of insects. In this work, we analyze the mechanisms that lead to the bluish-green colour of the blue-banded bee feathers. The reflection spectrum of the blue-banded bee feather was calculated by the transfer matrix method (TMM). The reflection peaks found are compatible within the experimental data. In addition to Bragg scattering, guided resonance has been observed in our theoretical calculation, which leads to a novel understanding of the structural colours in blue-banded bees.   

Wavelength-Dependent Conformational Changes of Collagen in Mid-IR Ablation

Single pulses of the Mark-III free electron laser have been used to ablate porcine corneas at a fluence of 240 J/cm$^{2}$ and wavelengths of 2.77 and 6.45 $\mu $m. As previously characterized, the non-volatile ablation debris shows evidence of wavelength-dependent collagen fragmentation. We have measured micro-Raman spectra of the debris and the ablation crater to determine if any wavelength-dependent conformational changes have taken place. Comparison of the spectra from two different wavelengths shows that a 938 cm$^{-1}$ Raman band -- assignable to the peptide C$_{C=O}$-C$_{\alpha }$ stretch of collagen -- loses substantial intensity during 6.45-$\mu $m ablation, but not in 2.77-$\mu $m ablation. This intensity decrease may be associated with a reduction of collagen triple-helix structure. Other spectral techniques yield mixed results; signatures for the loss of triple-helix structure are evident in UV-CD and some aspects of $^{13}$C-NMR spectra, but not in FTIR spectra. Raman measurements on thermally-treated corneal slices display similar changes at high temperatures, suggesting that higher protein temperatures are reached during ablation at 6.45 $\mu $m when compared to 2.77 $\mu $m. These observations suggest that any pre-vaporization loss of protein structural integrity includes not only collagen fragmentation, but also a loss of collagen triple-helix structure.   

Novel protection mechanisms against singlet oxygen formation by the Chl $a$ molecule in the cytochrome $b_{6}f$ complex of oxygenic photosynthesis

A Chl molecule is known to produce highly toxic singlet oxygen under light illumination as a result of energy transfer from its triplet excited state to oxygen. To prevent that, a carotenoid is typically positioned close to a Chl molecule ($\sim $4 $\AA$) in Chl containing proteins to ensure rapid triplet-triplet energy transfer from Chl to carotenoid. Surprisingly, the X-ray structures of the cytochrome $b_{6}f$ complex show that the $\beta$ -carotene is much too far from the only Chl $a$ found in this complex to provide effective protection by the usual triplet-triplet energy transfer mechanism. Our optical femtosecond time resolved experiments on diluted samples as well as on the single crystals of the $b_{6}f$ complex suggest that the Chl $a$ is protected by two novel mechanisms: (i) the yield of the Chl $a$ triplet state formation is reduced through electron-transfer exchange with the nearby amino acid residues, and (ii) a long distance triplet energy transfer to carotenoid mediated by a third mobile molecule (NSF MCB- 0516939, NIH GM-38323).   

Electrostatic Steering of Functional Dynamics in GFP

Distinctive photodynamic properties of the green fluorescent protein (GFP) from the jellyfish \emph{A. victoria} result from charge transfer processes involving the autocatalytically generated chromophore. We investigate structural changes in response to chromophore photoionization at cryogenic temperatures, using both X-ray crystallography and polarized infrared measurements on oriented single crystals. These measurements identify conformational changes of Gln 69, Cys 70, and an associated H-bonded cluster of internal water molecules in the chromophore environment. These structural changes take place at 100 K, far below the ``dynamical transition'' traditionally regarded as enabling functional protein motions. This contrasts with the prevailing view that the rigid interior of the GFP $\beta$-barrel sterically inhibits nonradiative processes. Instead, we propose that rapid rearrangements of the chromophore environment enhance the fluorescence quantum yield by stabilizing the abruptly altered charge distribution in the radiative state. We suggest that the conformational response to charge transfer influences two fundamental and useful spectroscopic properties of GFP---the large frequency separation between excitation and emission and the efficient fluorescence.   

Terahertz Dielectric Response of Photoactive Yellow Protein (PYP): Influence of Conformational-Vibrational State during Photocycle and Hydration Effects

Protein conformational change alters flexibility and conformational-vibrational modes that occur on a picosecond or sub-picosecond time scale. Terahertz dielectric measurements are sensitive to protein flexibility as they directly probe the density of states of these vibrational modes. Using terahertz time-domain spectroscopy, we measured the dielectric response of PYP thin films as a function of resting and photointermediate state. The absorbance increases smoothly as a function of frequency while the index of refraction exhibits no frequency dependence. A sharp transition in the dielectric response of the ground state is observed at 86{\%} relative humidity (r.h.), corresponding to the point where the protein film has lost $\sim $50 water molecules relative to a 100{\%} r.h. environment. Similar transitions observed for hen egg white lysozyme and cytochrome c correspond to the filling point of the first hydration shell.   

Novel Photo-Protecticon Mechanisms in Chlorosomes from Green Sulfur Bacterium \textit{Chlorobium Tepidum}

Chlorosome is the largest known photosynthetic light-harvesting antenna complex that incorporates thousands of bacteriochlorophtylls (BChl) and carotenoids (Car) in a closely packed quasi-regular structure. BChl is known to produce highly toxic singlet oxygen as the result of energy transfer from their excited triplet states to oxygen molecules. It has been proposed that that the carotenoids in chlorosome serve both light harvesting and photo-protection functions, transferring light excitations to nearby BChl and simultaneously quenching excited triplet state of BChl. However, experiments indicate that photoprotective role of carotenoids in chlorosomes is not as extensive as expected---photo-degradation of carotenoid-free mutants occurs only twice faster than photo-degradation of native complexes. An additional non-conventional photoprotection mechanism must exist in chlorosomes. The possible nature of this mechanism is discussed based on optical kinetic measurements of BChl and Car excited states.   

Hydration Dependence of Energy Relaxation Time for Cytochrome C

Hydration plays a critical role in protein dynamics. Here we consider the effects of hydration on energy relaxation for an electronically excited heme protein cytochrome c. We measure the hydration dependence of energy relaxation time of cytochrome C films after photoexcitation in the Soret regionusing two-color pump/probe time resolved transmission measurements. Thin films were prepared from cytochrome C/ Trizma buffer solutions and mounted in a hydration controlled cell. We used 400nm ($\sim $3 mW) to pump the B band and 800 nm ($\sim $1 mW) to probe the III band. The III band corresponds to the charge-transfer transition between heme $\pi $ and iron d orbital, and is assigned to the ground electronic state of the heme. Therefore this band can be used to probe the ground state population. Three separate dynamic components were observed: a very fast transient $\tau _{1} \quad \sim $ 200 fs; a several hundred femtosecond component ($\tau _{2})$; and a recovery of the ground state absorption($\tau _{3})$. We find $\tau _{3}$ apparently decreases with decreasing hydration while $\tau _{1}$ and $\tau _{2}$ are independent of hydration.   

Session W28: Focus Session: Ordered Optoelectronic Organics

The phase diagram of the organic charge transfer salts (TMTTF)$_2$X

We report on NMR spectroscopy and relaxation measurements probing the temperature/pressure phase diagram of the quasi-one-dimensional organic conductor (TMTTF)$_2$SbF$_6$. This material undergoes a charge ordering (CO) phase transition at $T=156K$, and the ground state is antiferromagnetic (AF) at ambient pressure. Our experiments show that the AF ground state is supressed sharply by applying hydrostatic pressure and a new phase appears at higher pressure. At intermediate pressures, magnetic ordering is suppressed to temperatures below the minimum measured so far, $T=2K$. The nature of the high pressure phase is not yet established; there are features consistent with both spin-Peierls (SP), and antiferromagnetic ordering. We discuss the possible role of counterion disorder in producing an inhomogenous system consistent with our observations, and the implications for a general phase diagram for the TMTTF/TMTSF family of organic conductors.   

Intermolecular bonding in conjugated polymers: The effect on aggregate morphology.

Intermolecular interactions play an important role in the mechanical and optical properties of conjugated polymer films and solutions. However, the nature of such inter-polymer interactions is poorly understood. We present a calculation showing that in the tight-binding approximation, conjugated polymers that approach each other on the angstrom length scale will form weak, covalent-like bonds. These bonds then drive the formation of aggregate structures. The morphology of the aggregates formed by the polymers depends on the net binding strength of the interaction. At low bond strength, the polymers form loose aggregates, while strong bonds lead to the formation of tight bundles with an effective persistence length one order of magnitude greater than the free polymer.   

Effects of polymer side chains on the self-assembling of conjugated polymer in thin film

Conjugated polymers are inherently semi-conducting and optically active materials, with immense potential applications in organic electro-optical devices. The chemical structure of the polymer including the rigidity of the backbone and the nature of substituents affect their association as well as their electro-optical response. The following work reports the effects of different side chains on the structure and fluorescence of highly conjugated polymer, poly(para phenyleneethynylene) (PPE). When substituted by long polylactide side chains they self-assemble into wires with fingerprint-like arrangement, casting from chloroform solutions on oxidized silicon wafer. With increasing content of poor solvent, the dimension of the structures increased and then crystallized area appeared, as showed in AFM studies. The introducing of the long flexible polymer side chains has significantly reduced the stacking between rigid backbones. This in tern results in a frequency shift in their fluoresces response, indication changes in the electronic levels. Direct measurements of the electronic levels using ATM are currently in progress.   

Chain morphologies in semi-crystalline polyfluorene: evidence from Raman scattering

Organic semiconductors, such as short-chain oligomers and long- chain polymers, are now a core constituent in numerous organic and organic-inorganic hybrid technologies. Polyfluorenes (PF) have emerged as attractive alternatives to other polymers due to their strong blue emission and high charge carrier mobilities. Almost all PF derivatives utilize side-chain substitution that improves solution processing and confers new functionality. There are many ambiguities regarding structure- property relationships in various side-chain substituted PFs. Di-octyl substituted PF (PF8) is known for its mesomorphic behavior, multitude of crystalline phases, and several conformational isomers that depend on the torsional angle between monomer units. In particular, the $\beta$ phase that represents a more planar backbone conformation dominates the optical emission although it appears as a minority constituent. We present Raman scattering studies of PF8 as a function of thermal cycling, which establishes a connection between the conformational isomers and the side and main chain morphology. Density-functional calculations of the vibrational spectra of single chain oligomers in conjunction with the experimental results demonstrate the incompatibility of the $\beta$ phase with the overall $\alpha$ crystalline phase in PF8.   

Structural Changes in Phthalocyanine Thin Films from Analyte Vapor Exposure

Organic phthalocyanine thin films were fabricated with varied thicknesses and varied grain size structure. These films act as gas sensors as detected via conduction measurements due to analyte vapors. We have observed structural changes with high-resolution x-ray diffraction due to exposure to different analyte vapors. In thin films with small grains ($\sim $30nm diameter), low-angle irreversible changes and high-angle reversible variations were observed. We associate the irreversible behavior with current drift observed in transport measurements. In contrast, reversible variations of the first Bragg peak of the phthalocyanine are compared to the sensing changes in conductivity from exposure to analyte vapors.   

Electronic Structure of Potassium-doped Magnesium Phthalocyanine measured using Soft X-ray Spectroscopies.

We report a synchrotron radiation-excited resonant soft x-ray emission spectroscopy (XES) study of the electronic structure of magnesium phthalocyanine (MgPc) doped with potassium. XES measures directly the element specific partial density of states (PDOS) in solids. The electronic structure near the Fermi level in organic systems can be accurately measured by using this non-ionizing spectroscopy. The MgPc films were grown \textit{in-situ} by using a custom designed ultra-high vacuum organic molecular beam deposition system, and transferred under vacuum to the spectrometer system. As with our earlier study of Cu-Pc and vanadium oxide phthalocyanine (VO-Pc), the K-doped MgPc films were discovered to be highly susceptible to synchrotron radiation beam damage. We successfully circumvented this effect by continuous translation of the films during measurement. We find that the measured C $2p$ PDOS for K-doped MgPc differs from that of pure MgPc, and will discuss the possible origins of these results. Supported in part by ACS Petroleum Research Fund, and by the NSF. Experiments were performed at the NSLS.   

Polaron transport in triphenylene-based discotic liquid crystals

We report on the investigation of charge carrier transport in the columnar phase of the triphenylene-based discotic liquid crystal, hexapentyloxytriphenylene, by the time-of-flight technique. The hole mobility was found to be temperature and electric field dependent with a maximum value of 2$\times $10$^{-3}$ cm$^{2}$/Vs. Its temperature dependence is described by a T$^{-n}$ power law, with an electric field dependent n varying from 2.5 to 4.5 corresponding to electric field values from 5$\times $10$^{4}$ V/cm to 5$\times $10$^{3}$ V/cm, respectively. The drift velocity of charge carriers is a linear function of the electric field for small fields below 5$\times $10$^{4 }$V/cm and tends to saturation at strong fields. These results are interpreted in the framework of correlated polaron motion as described by the non-adiabatic low-temperature limit of Holstein's small polaron theory. The applicability of the Holstein polaron model to triphenylene-based materials showing thermally activated or temperature-independent carrier mobilities will be discussed.   

Atomic Force Microscope-Based Surface Potential and Surface Photovoltage Studies of Porphyrin Nanorod Thin Films

We have performed atomic force microscope-based surface potential and surface photovoltage measurements on porphyrin grown as nanorods about 5 nm in diameter and 1 micron long. These nanorods have been shown to have peculiar photoconducting properties in that the photoconductivity grows under light illumination for up to 1 hour. In addition, when a current is flowed through the nanorods, they become ``trained.'' That is after the light is turned off and the ends of the nanorod are short circuited together, a small current will flow opposite to the direction of the original photocurrent. The material exhibits nanoscale potential fluctuations as well as selective surface potential sensitivity to light. These offer some insight to the material's novel properties.   

Holographic liquid crystal photonic materials stabilized with monoacrylate LC monomer.

Active structured optical materials such as dynamically tunable photonic crystals have potential technological applications in imaging and communications. Structured liquid crystal materials are especially promising for their observed high performance and wide range of dynamic response. In addition, they provide multiple routes of response, including electro-optic and photo-optic. Reverse mode holographic polymer dispersed liquid crystals (HPDLCs) are typically fabricated by multiple laser beam exposure of an LC photo-sensitive syrup containing diacrylate liquid crystal (LC) monomer. The function of the diacrylate monomer is to form the basis of a highly cross-linked network which stabilizes regions with high polymer content. The use of monoacrylate functional liquid crystals in this application has the potential of tuning the stabilizing effect of the polymer network by changing the crosslink density. This affects the switching time and the contrast between stabilized and unstabilized regions of the patterned structure. These parameters are particularly important to photo-switchable patterned photonic structures containing azobenzene derivatives. The effects on electro-optic and photo-optic properties of reverse mode HPDLCs containg of different monoacrlyate functional liquid crystal fractions in the stabilizing patterned polymer network will be presented.   

Electrooptic Properties of Holographic Polymer-Stabilized Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLCs) have attracted significant attention for uses in photonic and electrooptic devices, such as photonic crystals, light switches, and display applications. These materials selectively reflect circularly polarized light due to the existence of a macroscopic helical structure. Application of an electric field reorients the LC from a planar geometry to a homeotropic alignment, eliminating the reflection notch. This LC reorientation to the clear state is rapid with the application of an electric field. After the electric field is removed, the return to the cholesteric orientation is compounded by the long lifetime of a highly scattering focal conic state. To avoid this undesired prolonged focal conic lifetime, a small concentration of monomer can be polymerized, which acts as a memory for the rapid return into the cholesteric state. In this research, we examine the effect of holographically patterning the polymer stabilization. Reflection grating patterning is studied and varying bilayer spacings are examined. This research shows the possibility of minimal notch broadening, with no apparent chirp in the reflection notch. Additionally, we examine the possibility of incorporating a narrow notch Bragg reflection at a differing wavelength than the CLC broad reflection notch, within the same device.   

High Refractive Index Poly(thiophene) for Organic 3-D Photonic Crystals with a Complete Photonic Band Gap

Photonic crystals (PC) with a complete 3-D photonic band gap (PBG) require materials with sufficient refractive index ($n)$ contrast to be in specific 3-D periodic structures on the length scale of light. Currently, only inorganics have an adequate $n$ to open a complete 3-D PBG. Poly(thiophene) (PT), a sulfur containing conjugated polymer, is predicted to have a sufficient $n$, but this has not been realized. By optimizing the electropolymerization of PT including reaction rates, temperatures, additives, and reactant concentrations, high quality PT films with an adequately high $n$ can be synthesized. Using a density differential colloidal crystallization technique, which allows the crystallization process to approach thermodynamic equilibrium, high quality templates were produced. A nano-mechanical annealing technique was developed to enable the further perfection of the entropy driven structures. The next step is to combine these to fabricate an organic 3-D PC with a complete PBG.   

Thermopower of Pentacene Thin-film Transistors

The mechanism of conduction in organic thin-film transistors remains poorly understood, though it is generally thought that conduction occurs via hopping in a disordered band tail. Thermopower in principle can give additional information about the density of states D(E), and the dependence of the mobility on D(E); such information can be used to discriminate between various conduction mechanisms. We report for the first time measurements of the thermopower of pentacene thin films on SiO$_{2}$/Si in a field-effect transistor configuration as a function of charge carrier density and temperature. This work was supported by the Laboratory for Physical Sciences.   

In-Situ Measurements of Organic Electronic Devices Fabricated via Transfer Printing on Flexible Substrates

Transfer printing was used to fabricate high quality organic thin-film transistors (TFT) on flexible substrates.~ The model system of a pentacene (Pn) TFT with 600 nm thick poly(methyl methacrylate) dielectric layer and gold electrodes on a polyethylene terephthalate substrate has shown a mobility (adjusted for contact resistance) of 0.237 cm$^{2}$/Vs, on/off ratio of 10$^{5}$ and threshold voltage of -7 V.~ To optimize the transfer printing parameters of the Pn semiconductor layer, mobility and contact resistance were studied as a function of printing temperature and pressure.~ The best TFT devices resulted from printing at 120 $^{o}$C and 600 psi.~ A detailed study of the effect of transfer printing on the device properties was performed via in-situ measurements of drain current (ID) as a function of both drain (VD) and gate (VG) voltages.~ Details of the in-situ measurements while transfer printing the Pn layer will be presented and discussed. *Work supported by the Laboratory for Physical Sciences, College Park, MD and ARDA.   

Session W29: Biological Networks and System Biology

Self-Organization of Networks Via Synchrony-Dependent Plasticity

We employ an adaptive parameter control technique based on a previously developed measure that detects phase/lag synchrony in the system to dynamically modify the structure of a network of non-identical, weakly coupled R\"{o}ssler oscillators. Two processes are simulated: adaptation, under which the initially different properties (such as frequency) of the units converge, and aggregation, in which coupling between units is altered and clusters of interconnected elements are formed based on the temporal correlations. We show that adaptation speed depends on connectivity and topology, with more global connections resulting in greater temporal order and faster convergence of adaptation. We find that aggregation leads to unidirectional clusters, and that asymmetric aggregation (with differing rates for increasing or decreasing coupling strength) has an optimum ratio of rates to make denser clusters that maintain their selectivity. Combining adaptation and aggregation results in clusters of identical oscillators with bi-directional coupling. An optimum ratio of process rates results in stable coupling between the units. Change from this ratio may result in annihilation of the network for slow aggregation, or more numerous, denser, and more transient clusters for faster aggregation.   

Phase reduction analysis of coupled neural oscillators: application to epileptic seizure dynamics

Epileptic seizures are generally held to be result from excess and synchronized neural activity. To investigate how seizures initiate, we develop a model of a neocortical network based on a model suggested by Wilson [1]. We simulate the effect of the potassium channel blocker 4-aminopyridine, which is often used in experiments to induce epileptic seizures, by decreasing the conductance of the potassium channels (g$_{K})$ in neurons in our model. We applied phase reduction to the Wilson model to study how g$_{K}$ in the model affects the stability of the phase difference. At a normal value of g$_{K}$, the stable phase difference is small, but the neurons are not exactly in phase. At low g$_{K}$, in-phase and out-of-phase firing patterns become simultaneously stable. We constructed a network of 20 by 20 neurons. By decreasing g$_{K }$to zero, a dramatic increase in the amplitude of mean field was observed. This is due to the fact that in-phase firing becomes stable at low g$_{K}$. The pattern was similar to local field potential in 4-aminopyridine induced seizures. Therefore, it was concluded that the neural activity in drug-induced seizure may be caused by a bifurcation in stable phase differences between neurons. [1] Wilson H.R., J. Theor. Biol. (1999) 200, 375-388 [2] Ermentrout, G.B. and Kopell, N., SIAM J. Math. Anal. (1984), 215-237   

Effect of Delays and Network Topology in Spatiotemporal Pattern Formation

Synchronization between connected neurons is believed to play a role in the processing of information within the brain. This implies a temporal ordering in the discharge of their electrical signals but since the axons have a finite length over which a signal must traverse, information relating to a particular process and emanating from different neurons reaches a target neuron after a time delay. We investigate the effects of delays on the formation of temporally ordered states in a model network with SWN topology. We show that incorporation of two different types of delay, length independent and length dependent, lead to dramatically different properties of the network. In the first case, the formation of global random connections leads to an increase in temporal ordering, while in the second case locally ordered clusters are annihilated and form a disordered state.   

Inhibitory Synaptic Coupling and Spatiotemporal Synchrony in a Neural Model

We study the behavior of an array of neurons, connected by excitatory and inhibitory synapses, when the relative proportion of such connections is varied. The cells, described by the Huber-Braun model [1], exhibit different bursting states as parameters such as temperature and coupling strength are tuned. In a recent paper [2], stochastic phase synchronization was studied in this model, using gap-junction type connections. Here, we extend this work to more realistic synaptic connectivities, to investigate the connection between bursting and synchronization, which has been implicated in the triggering of pathological processes such as epilepsy, since synchronous firing in neuronal populations is viewed as a hallmark of seizures. We also present evidence suggesting that noise could be responsible for transitions between various types of field potential oscillations, reminiscent of the transitions between spike-and-wave firing and low voltage fast activity observed in the epileptic cortex. [1] H. A. Braun, M. T. Huber, M. Dewald, K. Sch\"{a}fer, and K. Voigt. Computer simulations of neuronal signal transduction: the role of nonlinear dynamics and noise. Intl. J. Bifurcation and Chaos 8(5): 881-889, 1998. [2] S. Bahar. Burst-enhanced synchronization in an array of noisy coupled neurons. Fluctuation and Noise Letters 4(1):L87-L96, 2004.   

Attentional modulation of stimulus competition in a large scale model of the visual pathway

Neurons in cortical area V4 are sensitive to shape and have large receptive fields. In a typical visual scene there are multiple objects in the V4 cell's receptive field, only a few of which may be behaviorally relevant. The visual system is capable of selecting relevant objects by increasing the neural response to them and reducing the response to non-relevant objects. Neuronal synchrony may play an important role in this process. Using a large-scale network model of the visual pathway, we study the emergence of shape selectivity in V4, the competition between different objects for control of the firing rate of individual V4 neurons, the attentional modulation of this stimulus competition and the role of synchrony.   

Specificity, promiscuity, and the structure of complex information processing networks

Both the top-down designs of engineered systems and the bottom-up serendipities of biological evolution must negotiate tradeoffs between specificity and control: overly specific interactions between components can make systems brittle and unevolvable, while more generic interactions can require elaborate control in order to aggregate specificity from distributed pieces. Complex information processing systems reveal network organizations that navigate this landscape of constraints: regulatory and signaling networks in cells involve the coordination of molecular interactions that are surprisingly promiscuous, and object-oriented design in software systems emphasizes the polymorphic composition of objects of minimal necessary specificity [C.R. Myers, Phys Rev E 68, 046116 (2003)]. Models of information processing arising both in systems biology and engineered computation are explored to better understand how particular network organizations can coordinate the activity of promiscuous components to achieve robust and evolvable function.   

Modeling of signal transduction in bacterial quorum-sensing

Several species of bacteria are able to coordinate gene regulation in response to population density, a process known as ``quorum-sensing''. Quorum-sensing bacteria produce, secrete, and detect signal molecules called autoinducers. For several species of bacteria in the {\it Vibrio} genus, recent results have shown that the external autoinducer concentrations control the expression of regulatory small RNA(s) which are critical to the process of quorum-sensing. We present a theoretical analysis of the network which relates the rate of small RNA expression to the external autoinducer concentrations. We relate the results from our modeling to previous experimental observations and suggest new experiments based on testable predictions of the model.   

Sloppiness is universal in systems biology: making predictions nonetheless

Quantitative models of complex biological systems often possess dozens of unknown parameters. We argue that such systems are universally ``sloppy''; their behaviors are orders of magnitude more sensitive to moves in some directions in parameter space than others. To establish this, we survey models from the literature and show that their ``complete and perfect data'' Fisher Information Matrices possess eigenvalues typically spanning a range of more than $10^6$. Sloppiness implies that collectively fitting model parameters to even the best experimental data will tightly constrain only a few parameter combinations, perhaps suggesting the necessity of a difficult experimental program to measure each individual parameter. An example demonstrates, however, that a collective fit to a modest amount of real data may tightly constrain model behavior even though it only poorly constrains many parameter combinations. Low uncertainty predictions can thus be made without knowledge of precise values for individual parameters.   

Inference and analysis of gene-regulatory networks in the bacterium B.subtilis

We present the methods and results of a two-stage modeling process that generates candidate gene-regulatory networks of the bacterium B.subtilis from experimentally obtained, yet mathematically underdetermined microchip array data. By employing a computational, linear correlative procedure to generate these networks, and by analyzing the networks from a graph theoretical perspective, we are able to verify the biological viability of our simulated networks, and we demonstrate that our networks' graph theoretical properties are remarkably similar to those of other, more well-studied biological systems. We test the robustness of the inference process first by introducing noise into the experimental data, and then by comparing the graph theoretical properties of the resulting perturbed networks to those of the original networks.   

Sensitivity-based approach to optimal experimental design in a receptor trafficking and down regulation model

We apply the ideas of optimal experimental design to systems biology models: minimizing a design criterion based on the average variance of predictions, we suggest new experiments that need to be performed to optimally test a given biological hypothesis. The estimated variance in predictions is derived from the sensitivities of protein and chemical species in the model to changes in reaction rates. The sensitivities also allow us to determine which interactions in the biological network dominate the system behavior. To test the design principles, we have developed a differential equation model incorporating the processes of endocytosis, recycling and degradation of activated epidermal growth factor (EGF) receptor in a mammalian cell line. Recent experimental work has discovered mutant proteins that cause receptor accumulation and a prolonged growth signal. Our model is optimized to fit this mutant experimental data and wild type data for a variety of experimental conditions. Of biological interest is the effect on surface and internalized receptor levels after the overexpression or inactivation of regulator proteins in the network: the optimal design method allows us to fine tune the conditions to best predict the behavior of these unknown components of the system.   

Stochasticity in the Expression of LamB and its Affect on $\lambda $ phage Infection

$\lambda $ phage binds to \textit{E. Coli's} lamB protein and injects its DNA into the cell. The phage quickly replicates and after a latent period the bacteria bursts, emitting mature phages. We developed a mathematical model based on the known physical events that occur when a $\lambda $ phage infects an \textit{E.Coli} cell. The results of these models predict that the bacteria and phage populations become extinct unless the parameters of the model are very finely tuned, which is untrue in the nature. The lamB protein is part of the maltose regulon and can be repressed to minimal levels when grown in the absence of inducer. Therefore, a cell that is not expressing any lamB protein at that moment is resistant against phage infection. We studied the dynamic relationship between $\lambda $ phage and \textit{E. Coli} when the concentration of phage greatly outnumbers the concentration of bacteria. We study how the stochasticity of the expression of lamB affects the percentage of cells that the $\lambda $ phage infects. We show that even in the case when the maltose regulon is fully induced a percentage of cells continue to persist against phage infection.   

Control of lineage stability and its role in resolving cell fates

We synthesize experimental data from recent studies to construct a computational model for the gene regulatory network that governs the development of immune cells and use it to explain several surprising results. At the heart of the model is a cross-antagonism between the macrophage-promoting factor Egr and the neutrophil-promoting factor Gfi. This module is capable of giving rise to both graded and bistable responses. Increasing the concentrations of these factors forces the system into the bistable regime in which cells can decide stochastically between fates. This bistable switch can be used to explain cell reprogramming experiments in which a gene associated with one cell fate is induced in progenitors of another. In one such experiment, C/EBP$\alpha $, a neutrophil promoting factor, was induced in B cell progenitors which then differentiated to macrophages. Our model shows that if C/EBP$\alpha $ is induced early, it can induce differentiation to a neutrophil. In B cell progenitors, however, the bistable switch is already in a macrophage promoting state. Thus, expression of C/EBP$\alpha $ cannot activate the neutrophil pathway, but it can repress the B cell pathway and promote macrophage differentiation.   

Role of finite-size fragments in analysis of DNA replication

In higher organisms, DNA replicates simultaneously from many origins. Recent in- vitro experiments have yielded large amounts of data on the state of replication of DNA fragments. From measurements of the time dependence of the average size of replicated and non-replicated domains, one can estimate the rate of initiation of DNA replication origins, as well as the average rate at which DNA bases are copied. One problem in making such estimates is that, in the experiments, the DNA is broken up into small fragments, whose finite size can bias downwards the measured averages. Here, we present a systematic way of accounting for this bias by deriving theoretical relationships between the original domain-length distributions and fragment-domain length distributions. We also derive unbiased average-domain-length estimators that yield accurate results even in cases where the replicated (or non-replicated) domains are larger than the average DNA fragment. Then we apply these estimators to previously obtained experimental data to extract improved estimates of replication kinetics parameters.   

Stochastic dynamics of macromolecular-assembly networks.

The formation and regulation of macromolecular complexes provides the backbone of most cellular processes, including gene regulation and signal transduction. The inherent complexity of assembling macromolecular structures makes current computational methods strongly limited for understanding how the physical interactions between cellular components give rise to systemic properties of cells. Here we present a stochastic approach to study the dynamics of networks formed by macromolecular complexes in terms of the molecular interactions of their components [1]. Exploiting key thermodynamic concepts, this approach makes it possible to both estimate reaction rates and incorporate the resulting assembly dynamics into the stochastic kinetics of cellular networks. As prototype systems, we consider the \textit{lac} operon and phage $\lambda $ induction switches, which rely on the formation of DNA loops by proteins [2] and on the integration of these protein-DNA complexes into intracellular networks. This cross-scale approach offers an effective starting point to move forward from network diagrams, such as those of protein-protein and DNA-protein interaction networks, to the actual dynamics of cellular processes. [1] L. Saiz and J.M.G. Vilar, submitted (2005). [2] J.M.G. Vilar and L. Saiz, Current Opinion in Genetics {\&} Development, 15, 136-144 (2005).   

Selective advantage for sexual reproduction

We develop a simplified model for sexual replication within the quasispecies formalism. We assume that the genomes of the replicating organisms are two-chromosomed and diploid, and that the fitness is determined by the number of chromosomes that are identical to a given master sequence. We also assume that there is a cost to sexual replication, given by a characteristic time $ \tau_{seek} $ during which haploid cells seek out a mate with which to recombine. If the mating strategy is such that only viable haploids can mate, then when $ \tau_{seek} = 0 $, it is possible to show that sexual replication will always outcompete asexual replication. However, as $ \tau_{seek} $ increases, sexual replication only becomes advantageous at progressively higher mutation rates. Once the time cost for sex reaches a critical threshold, the selective advantage for sexual replication disappears entirely. The results of this talk suggest that sexual replication is not advantageous in small populations per se, but rather in populations with low replication rates. In this regime, the cost for sex is sufficiently low that the selective advantage obtained through recombination leads to the dominance of the strategy. In fact, at a given replication rate and for a fixed environment volume, sexual replication is selected for in high populations because of the reduced time spent finding a reproductive partner.   

Session W30: Focus Session: Biopolymers at Interfaces

Studies in Biological-Materials Interfaces.

The control of the physicochemical properties of surfaces in contact with biological systems represents a fundamental issue in many applications ranging from coatings to biotechnology and microelectronics. In particular, advances in biotechnology depend on the ability to fashion materials with precise control of feature size and functionality. This presentation focuses on issues of specific and non-specific binding and strategies being developed to control both. Examples of specific binding that enable investigation of cell function will be presented. The broader issue of non-specific binding and how it relates to fouling release will also be discussed in terms of surface structure. Both polar and non-polar surfaces have been investigated and each type shows promise for release specific biological systems. The identification of a ``universal'' surface for release of all biological systems remains elusive.   

Universality Classes and Unusual Thermodynamics of Unbinding Transitions of Semi-flexible Polymers Confined to a Surface

We theoretically address unbinding of semi-flexible polymers from long line-like attractive potential wells of various forms. These transition phenomena are seen in recent experiments with DNA adsorbed on microstructured supported cationic lipid membranes, and they provide a new way to stretch single DNA molecules [Hochrein, Leierseder, Golubovic, and Raedler, 2005]. For simple attractive potential wells (``rectangular wells'') the transition is of the second order. Heat capacity divergence however has a non-standard from, C$\sim $1/[$\vert $Tc - T$\vert $ log($\vert $Tc - T$\vert )$], marked by a logarithmic correction related to the fact that the probability to find the polymer within the well region vanishes as $\sim $1/ log($\vert $Tc-T$\vert )$ at the transition point. On the other hand, for attractive potential wells having a hard wall potential added on one side, the transition becomes a non-standard hybrid between the first and second order phase transitions: the probability to find the polymer within the well approaches a non-zero value as the transition is approached and then it discontinuously drops to zero (producing a latent heat consumption). However, interestingly, in addition to the latent heat consumption, an unusual heat capacity divergence (of the form C$\sim $1/$\vert $Tc - T$\vert$$^{1/2}$) also occurs as the polymer unbinding point is approached.   

Polymer confinement and bacterial gliding motility

Cyanobacteria and myxobacteria use slime secretion for gliding motility over surfaces. In cyanobacteria the slime is extruded from the nozzle-like pores of 14-16 nm outer diameter and approximately 7nm inner diameter located near the septa that separate the cells of a filament. The pores are inclined at an angle of 30-40 degrees relative to the cell axes, and are oppositely directed on both sides of the septum. Such pore orientation provides directionality for the slime secretion as well as cell motion. To understand the mechanism of gliding motion and its relation to slime polymerization, we have performed molecular dynamics simulations of a molecular nozzle with growing inside polymer chains. These simulations show that the compression of polymer chains inside the nozzle is a driving force for its propulsion. There is a linear relationship between the average nozzle velocity and the chain polymerization rate with a proportionality coefficient dependent on the geometric characteristics of the nozzle such as its length and friction coefficient. This minimal model of the molecular engine was used to explain the gliding motion of cyanobacteria and myxobacteria over surfaces.   

Direct Observation of Biaxial Confinement of a Semi-flexible Filament in a Channel

We have studied the biaxial confinement of a semi-flexible filament in a channel by in situ video fluorescence microscopy*. As the channel width decreases, F-actin undergoes a transition from a 2D random regime to a 1D biaxially confined regime, leading to an increased effective persistence length. A theoretical calculation shows that the tangent-tangent correlation function in the confined regime shows a minimum, then reaches to a constant at long distances, indicating that confinement induces long-range order in a semi-flexible filament. The location of the minimum of the experimental correlation function is consistent with our theoretical calculation. This work was supported by KISTEP I-03-064, KISTEP IMT-2000-B3-2, MOHW 0405-MN01-0604-0007, NSF DMR 00-80034, 05-03347, 02-03755, 01-29804, NSF CTS-0404444, ONR N00014-05-1-0540, and DOE W-7405-ENG-36. M.C.Choi acknowledges partial support from the Korea Research Foundation Grant KRF-2005-214-C00202. *M.C.Choi et al., \textit{Macromolecules}, 38, 9882 (2005)   

Conformation of Lysozymes Confined to nano Particles

Confinement of bio-molecules while retaining their activity is a key to many applications. The main challenge lies in the fact that when protein molecules interact with other particles they often lose their tertiary structure, resulting in irreversible reduction of their biological activity. The interfacial interactions of including direct chemical interactions, morphological factors, as well as adsorption under shear and hydrodynamic characteristics of flow next to the interfaces are among the factors that control the configuration. Using lysozyme as a model protein, the effects of physical absorption as a function of the topography of the confining surface from flat to curve with controlled roughness will be discussed. Atomic Force Microscopy together with small angle neutron studies correlated with measurements of retaining the degree of helicity in the system, provide a new insight into the factors that affect the conformational changes in protein upon confinement. While the chemical nature of the surface is an important parameter the topography of the surface determine many characteristics from the amount of absorption to distribution as well as the desoption of the protein.   

Interaction forces and surface morphology of microtubule-associated protein tau

The microtubule-associated protein tau exists in six isoforms due to alternative mRNA splicing and is localized in the axons of neuronal cells. These isoforms differ by the inclusion of 3 or 4 microtubule-binding imperfect repeat regions (31 aa each) at the C-terminal end of the protein or by 0, 1, or 2 N-terminal end inserts (29 aa each). Using a surface forces apparatus (SFA), we have measured the interaction forces as a function of distance between two symmetric layers of tau protein (all six isoforms) adsorbed onto mica. By comparing the interaction forces between the different isoforms , it is clear that the tau protein forms a brush-like layer on the mica surface which swells upon increasing ionic strength. Additionally, we have looked at an asymmetric system with one surface of tau opposite bare mica. In the asymmetric system of tau and an opposing mica surface, there is a $>$10$^{2}$ increase in the magnitude of this adhesive force suggesting that the tau-mica interaction is much more adhesive than the tau-tau interaction. These data clearly show that tau adsorbed onto mica acts as a spacer and due to the cross-bridging between the mica surfaces, provides a strong adhesion, as has been observed in vitro with microtubules.   

Surface Plasmon Resonance Studies of Polysaccharide Self-Assembly on Cellulose

Wood is a multiphase material consisting of cellulose crystals embedded within a non-crystalline hetereopolysaccharide (hemicellulose) and lignin rich phase. The hierarchial arrangement of these three chief components in wood produces excellent properties like resistance to fracture and toughness. Through the study of polysaccharide self-assembly onto a model cellulose surface, further insight into the interactions between hemicelluloses and cellulose can be gained. In our study, we synthesized pullulan cinnamates with different degrees of substitution of cinnamoyl groups as a model for a hemicellulose with lignin-like moieties. Surface plasmon resonance measurements probe the self-assembly behavior of pullulan and pullulan cinnamate onto a cellulose coated gold surface. Our results suggest that pullulan does not adsorb onto the model cellulose surface, whereas pullulan cinnamate does. These preliminary results signify the important role that lignin-like substituents play on hemicellulose self-assembly onto cellulose surfaces.   

Assembly artificial proteins and conjugated porphyrins for biomolecular materials

It is non-trivia to incorporate both the electron donor and acceptor in a controlled manner into amphiphilic 4-helix bundle peptides. Extended pi-electronÊsystems have been designed and tailored, with appropriate donors, acceptors and constituents, exhibit selected light-induced electron transport and/or proton translocation over large distances. We studied the binding between a series of conjugated porphyrins and the designed amphiphilic 4-helix bundles peptides at selected locations. Incorporation of the conjugated porphyrins into the 4-helix bundle did not interfere the protein secondary structure or the 4-helix bundle formation. The amphiphilic protein/cofactor complexes have good thermal stability.The artificial protein Langmuir monolayers, both the apo- and holo-form, can be oriented vectorially at the air/water interface upon compression. GID show a glass-like inter-bundle positional ordering in the monolayer plane. We will discuss the efforts on re-designing the artificial proteins to incorporate them into these nanoporous templates made from diblock copolymers .   

Structural Transitions of F-actin Polyelectrolyte Bundles in the Presence of Strongly Size-mismatched Cations

In the presence of multivalent cations, the polyelectrolyte F-actin exhibits the phenomenon of `like-charge attraction'. Simple divalent ions cause F-actin to form close-packed bundles with an interstitial 1-D density wave of ions along the length of the bundle. Lysozyme, a nonavalent (+9) cationic globular protein (45{\AA}x25{\AA}x25{\AA}) causes F-actin to form similar bundles, with a larger inter-actin distance and an incommensurate 1-D column of close-packed lysozyme along the three-fold tunnel within the bundle. Using genetically engineered lysozyme with different charges, we examine the competition of these cationic agents and their effect on F-actin bundle structure.   

Defect Induced Morphologies of Biopolymer Bundles

Bundles of stiff biopolymers, such as actin and microtubules, form important structural elements in the cell, including filopodia, microvili, cilia and contractile rings. These structures perform specific functions that rely crucially on their mechanical properties, which in turn depend on the internal organization of the bundles. Recent experiments on microtubule bundle formation in the presence of multivalent counterions [D. J. Needleman, \textit{et. al}, \textit{Proc. Natl. Acad. Sci.}, \textbf{101} 16099 (2004)] have observed that the bundles adopt static curved configurations whose wavelengths are several orders of magnitude less than their persistence length. In this talk, we show that these severe distortions can be explained by the presence of edge dislocation and twist defects, indicating that these defects could play a significant role \textit{in vivo}.   

Polyamine Induced Bundling of F-actin

To better understand the mechanism of F-actin bundle formation, we have measured the phase boundary between isotropic F-actin and F-actin bundles as a function of polyamine concentration. F-actin was incubated with spermine or spermidine overnight, and the samples were spun at low speeds to separate bundles from unbundled F-actin. The relative amounts of actin in the pellet and supernatant were determined via gel electrophoresis. With this approach, we have mapped the phase boundary between bundled F-actin and isotropic unbundled F-actin for two F-actin/polyamine linker systems. Surprisingly, the dependence of bundle formation on actin concentration is small to non-existent. At the actin concentrations we studied, actin tends to form bundles at or above a single linker concentration. In order to understand the interactions holding F-actin together in bundles, we used NMR to determine where the polyamines were with respect to the bundled and unbundled phases of actin. Surprisingly, the spermine and spermidine did not segregate with the bundled actin indicating that they do not bind to the actin strongly even though their addition to F-actin solutions induces bundle formation.   

The fluctuating-rod limit of semiflexible biopolymers

We study the mechanical properties of semiflexible polymers, such as DNA and actin, in the ``fluctuating rod'' limit. This limit is attained when the contour length of the polymer is comparable to its persistence length, or when thermal fluctuations have been smoothed out by a large applied force. In this limit, we compute the exact average end-to-end distance and shape of the polymer for boundary conditions that correspond to different single molecule stretching experiments. We consider both the case of a force applied at one end of the polymer, when the tension is uniform along the chain, and the case of an applied field, when the tension increases linearly. For the latter case, we derive the force-extension relation valid for a wide range of electric field strengths, which may be used to extract the effective charge density of actin in solution. We also show that the experimental condition of axis-clamping by a laser tweezer gives rise to a measurable effect on its force-extension properties. This calculation underscores the importance of taking the entropic effects of the boundary conditions into account in single molecule experiments. This work is supported by NSF DMR-0403997. JK is a Cottrell Scholar of Research Corporation.   

Statistical and Mechanical Properties of Semiflexible Polymers in an External Field

Semiflexible polymers such as the double-stranded DNA, are well described by the worm-like chain model originally proposed by Kratky and Porod (Rec. Trav. Chim. 68, 1106, (1949)). Recent work has focused on understanding statistical properties such as their end-to-end distribution function (J.Chem.Phys 121, 6064 (2004), PRE 71, 031803 (2005)) and their mechanical properties in response to a stretching force or the electric field (PRE 72, 041918 (2005)). The problem becomes very complicated unless the long-chain or rod-like-chain approximations for the persistence length are made. Self-avoidance effects are always neglected even for long chain in two dimensions and for confined polymers where these effects could become important. We make use of the Bond Fluctuation Algorithm (Macromolecules 21, 2819(1988)) to study the behavior of semiflexible polymers for all persistence lengths and investigate the relationship of their shape to the persistence length, the chain length and the external field. We will compare our results for the extension of a polymer under a constant stretching with analytical results in weak and strong force limit (PRE 72, 041918 (2005)). This work has been supported by NSF-DMR 0403997.   

Session W31: Nanotubes: Devices

High On-Currents in Doped Schottky-Barrier Nanotube Transistors

For many contact metals, the short channel single-walled carbon nanotube field-effect transistor (SWNT-FET) has been understood as a ballistic Schottky barrier-FET (SB-FET), in which high on- currents may be realized with thin gate dielectrics through narrowing of the SB by the gate field [1]. Recently Ohmic contacts to nanotubes have been achieved through the use of high work function metals; such devices show high on-currents and near-ideal subthreshold swings [2]. Here we demonstrate that SWNTs in ambient on SiO$_{2}$ are p-doped. Doped SB-SWNT- FETs exhibit high on-currents due to thinning of the SB by doping, but retain the poor subthreshold behavior of SB-FETs. Dopants in SWNT-FETs can be removed by applying up to 50 V drain bias in vacuum, corresponding to dissipated power of $>$ 1 mW. Undoped devices exhibit much lower on-currents, and intrinsic, ambipolar behavior with symmetric SB. This work is supported by National Science Foundation under Grant No. 0102950. [1] S. Heinze, et al., Phys. Rev. Lett. 89, 106801 (2002). [2] A. Javey, et al., Nature 424, 654 (2003).   

Temperature and Carrier-Density Dependence of 1/f Noise in Single-walled Carbon Nanotube Transistors

Field-effect transistors (FETs) have been fabricated from individual semiconducting single-walled carbon nanotubes (SWNTs) grown by chemical vapor deposition on SiO$_{2}$/Si substrates and contacted by metal (Cr/Au) electrodes. We have measured the low-frequency anomalous noise (1/f noise) in such SWNT-FETs as a function of temperature and charge carrier density. This material is based upon work supported by the National Science Foundation under Grant No. 0102950 and the Center for Superconductivity Research.   

Optical switching of functionalized carbon nanotube transistors

Carbon Nanotube (CNT) transistors can emit or detect photons at wavelengths defined by the CNT chirality. To extend their capabilities in optoelectronics, it is important to be able to tune this wavelength independently of the CNT structure. A way to achieve such a goal is to chemically functionalize the CNT. In the present study, we demonstrate that drastic photo-induced modifications of the electrical characteristics of self-assembled CNT transistors functionalized by photoactive polymers can be achieved. We show that the polymer film acts as a wavelength dependent 'optical gate', which is much more efficient than a conventional electrostatic gate and can induce changes in conductance exceeding four orders of magnitude. The switching mechanism involves the creation and separation of photo-excited charges in the polymer, the spatial distribution and relaxation rates of which are studied taking advantage of the very high charge sensitivity of the CNT transistor.   

Large Area Aligned Arrays of SWNTs for High Performance Thin Film Transistors.

This talk will emphasize a convenient method for generating large scale, horizontally aligned arrays of pristine, single walled carbon nanotubes (SWNTs).~ The approach uses guided growth, by chemical vapor deposition (CVD), of SWNTs on Y-cut single crystal quartz substrates.~ Studies of the growth reveal important relationships between the density and alignment of these tubes, the CVD conditions and the morphology of the quartz.~ Electrodes and dielectrics patterned on top of these arrays yield thin film transistors (TFTs) that use the SWNTs as effective thin film semiconductors.~ Channel length scaling of device mobility, on current and off current provide insights into the transport characteristics. Combining the aligned arrays with random networks, which are grown simultaneously through the use of patterned catalysts, yields `all-tube' based devices.~ The ability to build high performance devices of this type suggests significant promise for large scale aligned arrays of SWNT in electronics, sensors and other applications.   

Field-Effect Transistors Assembled From Functionalized Carbon Nanotubes

We have fabricated field effect transistors from carbon nanotubes using a novel selective placement scheme. We use carbon nanotubes that are covalently bound to molecules containing hydroxamic acid functionality. The functionalized nanotubes bind strongly to basic metal oxide surfaces, but not to silicon dioxide. Upon annealing, the functionalization is removed, restoring the electronic properties of the nanotubes. The devices we have fabricated show high ON current (about 1 uA) and an ON/OFF ratio of more than 1e6.   

Template-directed Self-assembly of Carbon Nanotube Field-Effect Transistors

We pattern self-assembled monolayers (SAMs) of organic molecules to control the interactions between carbon nanotubes and inorganic surfaces. Deposition of the SAMs forms a template that directs the placement and alignment of nanotubes on lithographically defined electrodes to create field-effect transistors (FETs). Our assembly process is highly scalable and we demonstrate parallel fabrication of five FETs on a single substrate. These FETs exhibit large ``on'' currents of $\sim $1$\mu $A with ``on/off'' ratios as high as 10$^{6}$. Furthermore, our devices exhibit novel functionality by operating hysteresis-free without passivation of the nanotube or electrode surfaces. These features may lead to enhanced performance for delicate sensing applications utilizing these devices. We discuss the electrical characteristics of these FETs and contrast them with other state-of-the-art devices and assembly strategies. This work has been supported by NSF NIRT grant ECS-0210332.   

Gas Sensitivity of Carbon Nanotube Devices

We have measured the gas sensitivity of field effect transistors made from individual single-walled carbon nanotubes in an ultra high vacuum environment. We exposed nanotube devices to varying partial pressures of oxygen and argon. We will compare the results to existing theoretical calculations for oxygen sensitivity of carbon nanotubes and discuss the ultimate gas sensitivity for these devices.   

Single Walled Carbon Nanotube-based Aqueous Sensors

Single walled carbon nanotube (SWNT) field effect transistors (FETs) have been utilized as chemical specific sensors by incorporating a sensitizing agent into the nanotube sidewalls. Here we report the non-covalent sidewall functionalization of SWNT FETs through the adsorption of macro-organic molecules. The modified SWNT FETs recognize changes in pH and oxidation states through a change in current flow across the devices. These uniformly dispersed nanotubes, grown directly on the FET substrate prior to electrode deposition, enhance the available tube surface area for molecular adsorption, and thus enhance the signal sensitivity.   

Atomic Nanotube Welders.

We demonstrate that the incorporation of boron between double walled carbon nanotubes (DWNTs) during thermal annealing results in covalent nanotube ``Y'' junctions, DWNT coalescence and the formation of flattened multi-walled carbon nanotubes (MWNTs). The processes occur via the merging of adjacent tubes which is triggered by B interstitial atoms. In order to demonstrate the unique welder properties of B in the process, we have carried out \textit{AM1} molecular dynamics simulations at high temperatures and \textit{ab-initio }calculations. We observe that B atom interstitials between DWNTs are responsible for the rapid establishment of covalent connections between neighboring tubes (polymerization). Once B is in the lattice, tube faceting (polygonization) starts to occur, and the electronic properties are expected to change dramatically.   

Quantized conductance observed in reversible atomic contacts fabricated by template electroplating using an on-membrane anode

We report a new template electroplating method for fabricating reversible atomic contacts between a long nanowire and a macroscopic contact pad. In comparison to a typical template method using a standing-alone anode, we directly evaporate the anode on one of the porous membrane surfaces. Single nanowires, upon emerging from the pores, make reversible atomic contacts with the on-membrane anode via a self-terminating mechanism. Quantized conductance steps have been observed in a controlled fashion during deposition and dissolution. This method can potentially be applied for the controlled fabrication and integration of nanowires, point contacts, and nanosized interconnects in template-based nanofabrication.   

Structural and Transport Properties of Dielectrophoretically Assembled Interconnects

Dielectrophoresis was used to form $\sim $140nm diameter interconnects composed of gold nanorods between targeted points in a circuit. Cleanroom-based lithographic procedures were used to produce identical arrays of electrodes, improving the sample-to-sample reproducibility of the interconnect-conductances to $\sim $10{\%}. Transmission electron microscopy and low temperature conductivity analyses indicate that the Coulomb Blockade associated with the individual nanorods is the primary conductance-limiting feature. To further improve the reproducibility of the structural and transport properties of dielectrophoretic interconnects, we investigate submicron wire formation in aqueous solutions of indium acetate. Our preliminary data show that single crystal wires with submicron diameters may be fabricated from such solutions.   

DNA-Functionalized Carbon Nanotubes for Chemical Sensing

We demonstrate nanoscale sensors based on single-stranded DNA (ss-DNA) as the chemical recognition site and single-walled carbon nanotube field effect transistors (swCN-FETs) as the electronic readout component. SwCN-FETs functionalized with ss-DNA respond to gaseous analytes that do not cause a detectable current change in bare swCN-FETs. The response differs in sign and magnitude depending on the type of analyte and the DNA base sequence. The sensors maintain a constant response through at least 50 air-analyte cycles, and have response and recovery times on the scale of seconds. Furthermore, ss-DNA is found to chemically gate swCN-FET. The analytes used are found to interact with both the nanotube and the substrate. This sensor is promising for electronic olfaction systems consist of coupled sensor arrays and an odor recognition algorithm. Applications range from homeland security to disease diagnosis.   

Electrical transport behavior of all - carbon nanotube - based three terminal junctions

In this study, we propose a process for suspended \textit{in situ} lateral growth of all - carbon nanotube(CNT) based junction and report on the high current capacity of the CNT junction, especially, its current (I) response characteristics with and after UV. Furthermore, the analogy between current suppression and reversible switching of capacitor was studied by capacitance(C) spectroscopy. The designed diluted magnetic impurity doped oxide film was adopted as catalyst for the fabrication of all - CNT-based junction. 40 suspended junctions was tested and normally, one junction produced current of a few uA/1um at room temperature. The suspended CNT with the same electrode materials is expected to operate with ambipolarity. The nearly same low barrier height for the hole and electron conduction was estimated via I-T(K) measurements. Also, the surface of CNT was easily cleaned by low intensity UV treatment, resulting in a highly conductive channel that showed high current carrying behavior. Our result can be applied to develop a practical, accessible system for forming reproducible nanoelectronic junctions as well as to accelerate the realization of all low dimensional molecular devices.   

Energy Conversion Efficiency in Nanotube Optoelectronics

We present theoretical performance estimates of nanotube optoelectronic devices under bias. Current-voltage characteristics of illuminated nanotube {\it p}-{\it n} junctions are calculated using a self-consistent non-equilibrium Green's function approach. Energy conversion rates in the tens of percent range are predicted for incident photon energies near the band gap energy. In addition, the energy conversion rate increases as the diameter of the nanotube is reduced, even though the quantum efficiency shows little dependence on nanotube radius. These results indicate that the quantum efficiency is not a limiting factor for use of nanotubes in optoelectronics.   

A Novel Nanotube-on-Insulator (NOI) Approach toward Single-Walled Carbon Nanotube Devices

We present a novel nanotube-on-insulator (NOI) approach to produce high-yield nanotube devices based on aligned single-walled carbon nanotubes. First, we managed to grow aligned nanotube arrays with controlled density on crystalline, insulating sapphire substrates, which bear analogy to industry-adopted silicon-on-insulator substrates. Based on the nanotube arrays, we demonstrated registration-free fabrication of both top-gated and polymer-electrolyte-gated field-effect transistors with minimized parasitic capacitance. In addition, we have successfully developed a way to transfer these aligned nanotube arrays to flexible substrates. Our approach has great potential for high-density, large-scale integrated systems based on carbon nanotubes for both micro- and flexible electronics.   

Session W32: Glassy and Amorphous Systems

First-principles investigation of pressure-induced amorphization in zeolites

Crystalline zeolites can be transformed into amorphous structures by application of pressure, without ever forming a liquid. Upon release of the applied pressure, some zeolites transform back to their crystalline structure while others do not. Thus, zeolites are very interesting from the point of view of the theories trying to explain {\sl reversible} amorphization by pressure [Cohen et al., JNCS 307-310, 602 (2002)]. On the other hand, recent studies of the amorphization process in zeolites have led to the identification of co-existing phases of the same composition but markedly different densities and degrees of disorder [Greaves et al., Nat. Mats. 2, 622 (2003)]. Further, it has been argued that upon application of pressure (or, equivalently, temperature) zeolites render a low-entropy, low-density, amorphous phase that could constitute a new type of glass, with physical properties that might differ considerably from those of {\sl typical} glasses obtained by slow cooling from the melt. In this talk we will report a first-principles investigation of the structural changes induced by pressure in zeolites. More precisely, we will show results for three zeolites with the so-called LTA structure (Na-ZK4, Na-A, and an idealized SiO$_2$ system with the ZK4 structure). We will discuss the implications regarding the various amorphous phases experimentally found and the reversibility of the amorphization.   

Glass transition and viscosity of simple glasses and liquids

The theoretical understanding of liquids and glasses at an atomistic level lags well behind that of crystalline materials, even though they are important in many fields including biology and the medical sciences.~ We present a simple microscopic model for the glass transition based on topological fluctuations in the bonding network. The model makes predictions for important parameters of the glassy state, such as the glass transition temperature, $T_{g}$, and the liquid fragility coefficient, $m$, based on microscopic variables. Excellent agreement with a number of experimental observations from metallic glasses is demonstrated. A key to this success is to focus on the dependence on Poisson's ratio, following the work of Novikov and Sokolov,$^{1}$ that characterizes the interaction between local density and shear fluctuations. To our knowledge, this is the first model to~predict $T_{g}$ and $m$ quantitatively from microscopic variables. It presents a simple conceptual framework that should provide the basis for a more general microscopic understanding of liquids and glasses, including molecular systems. \newline 1. V. N. Novikov and A. P. Sokolov, \textit{Nature}, \textbf{431}, 961-963 (2004).   

Temperature induced density anomaly in Te rich liquid Germanium Tellurides : p versus sp3 bonding ?

The density anomaly of liquid Ge$_{0.15}$Te$_{0.85}$ measured between 633K and 733K is investigated with ab initio Molecular Dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp3 hybridization of the Ge bonding is observed. As a consequence the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases.   

Giant photocontraction effects in obliquely-deposited chalcogenide glass thin-films*

Ge$_{x}$Se$_{1-x}$ thin-films at several obliqueness angles $\alpha $ (= 0, 20, 45, 60, 80) and compositions x ( = 0.15, 0.20, 0.23, 0.25 and 0.33) were vapor-deposited, and examined in Raman scattering and SEM measurements both in the pristine and illuminated state. The films, placed in an inert ambient, were exposed to Hg lamp radiation, and photo-contraction of the films established using a profilometer. Raman scattering of the pristine and exposed films were studied as a function of depth using a confocal microscope attachment. Our results show (i) Raman scattering of the normally deposited ($\alpha $ = 0) films in the pristine state are similar to those of corresponding bulk glasses, (ii) obliquely deposited films at x = 1/3 reveal Raman lineshapes that change qualitatively with $\alpha $, suggestive of nanoscale phase separation of the films, while those at x = 0.23 show Raman lineshapes that are largely independent of $\alpha $, (iii) the photocontraction effect maximizes in the 0.20$<$ x $<$ 0.25 range, confirming the earlier finding (ref1) (iv) light illumination partially undoes effects associated with nanoscale phase separation. Possible interpretation of these results in relation to origin of photocontraction effects will be presented. *Supported by NSF grant DMR 04-56472. 1.Bhanwar Singh, S. Rajagopalan, P. K. Bhat, D. K. Pandya and K. L. Chopra, Solid State Communications, Vol. 29, pp. 167-169 (1979)   

Non-perturbative Renormalization Group Study of A Kinetically Constrained Model for Glasses

We study a dynamic field theory, which is based on the dynamic heterogeneity, with the non-perturbative renormalization group (NRG) method. Dynamic heterogeneity has been observed both experimentally, and in numerical simulations. It may play an important role in the physics of glass transition. Dynamic heterogeneity means that the slow dynamics of glass formers is dominated by the spatial fluctuations. And the length scales of dynamically correlated regions increase when the system approaches the glass transition, together with the increase of time scale. This is similar to the conventional dynamic critical phenomena. People have derived a dynamical field theory for a kinetically constrained model based on the Fredrickson-Anderson (FA) model, which puts the dynamic heterogeneity at its core. Here we study this field theory with the NRG method. The NRG method is not restricted to small parameters, and thus can be applied to more general cases. The critical exponents are calculated, and the phase structure is given. Especially the 1D spatial dimension case, which is difficult for the usual perturbative calculation, is also studied. Other possible models are also mentioned.   

Self-organization of glass networks in a model with equilibration

Recent experimental results suggest the existence of an intermediate phase in covalent glasses attributed to a self-organization of the glass network minimizing its internal stress. While a number of models have been proposed recently to explain this phenomenon, a full understanding of the network self-organization and the intermediate phase is lacking. We modify a previously studied model,$^(*)$ in which a network is grown in a way that keeps it stressless, by allowing continuous equilibration as the mean coordination is increased while still avoiding stress. In our model, an unusual intermediate phase appears, in which both rigid and floppy networks have a chance to occur, a result similar to that obtained by Barr\'{e} {\it et al.} for less realistic Bethe lattices. We discuss various structural properties of the resulting self-organized networks, as well as some results for the entropy cost of self-organization. \newline \vskip 2pt \noindent $^(*)$ M.F. Thorpe, D.J. Jacobs, M.V. Chubynsky, and J.C. Phillips, {\it J. Non-Cryst. Solids} {\bf 266-269}, 859 (2000)   

The Impact of Liquid Structure and Long Range Diffusion on Glass Formation and Nanoscale Devitrification

Recently, synchrotron X-ray diffraction measurements of electrostatically levitated samples revealed a growing icosahedral order in many undercooled metallic liquids and alloys. In a TiZrNi liquid, this icosahedral order catalyzed the nucleation of a metastable icosahedral phase, instead of the stable C14 tetrahedral Laves phase, confirming a half-century-old hypothesis made by Frank that connects the crystal nucleation barrier with the local structure of the liquid. This is one example of a growing number of cases of multiple order parameter coupling in nucleation, which are not readily described within the framework of the commonly used classical theory of nucleation. The implications for glass formation, nanoscale devitrification and the role of microalloying of such coupled nucleation processes and of a new coupled flux kinetic model for nucleation are discussed.   

Thermally Activated Processes in Polymer Glasses

A derivation is given for the Vogel-Fulcher-Tammann thermal activation law for the glassy state of a bulk polymer. Our microscopic considerations involve the entropy of closed polymer molecular chains (i.e. polymer closed strings). For thin film polymer glasses, one obtains open polymer strings in that the boundary surfaces serve as possible string endpoint locations. The Vogel-Fulcher-Tammann thermal activation law thereby holds true for bulk polymer glass but does not hold true in the ultra thin film limit of polymer glass.   

Transient nucleation in a Zr-Ti-Cu-Ni-Al metallic glass

Upon annealing Zr-Ti-Cu-Ni-Al bulk metallic glasses (BMG) crystallize to a nano-quasicrystal/amorphous composite. To probe the nucleation processes that underlie this microstructure formation we have determined the nucleation rate as a function of temperature, employing the two-step annealing method commonly used in silicate glasses. Samples were first annealed at temperatures where the nucleation rates were high, but the growth rates were low to produce a population of nuclei. These were subsequently grown to an observable size for transmission electron microscopy study by annealing at a higher temperature where the nucleation rate is small. We present the first quantitative time-dependent nucleation data obtained for a metallic glass by this method. The data are analyzed in terms of the classical theory of nucleation and an extended kinetic model that includes the coupling between the interfacial attachment and long-range diffusion fluxes.   

The role of Ti in the formation of Zr-Ti-Cu-Ni-Al glasses

It has been widely reported that glass formation improves in Zr$_{62}$Cu$_{20}$Ni$_{8}$Al$_{10 }$alloys when small amounts of Ti are substituted for Zr. Glasses containing greater than 3 at.{\%} Ti crystallize to a metastable icosahedral phase, suggesting that Ti enhances icosahedral short range order (ISRO) in the liquid/glass, making crystallization more difficult during cooling. Based on \textit{in-situ} high-energy synchrotron diffraction studies of electrostatically levitated (ESL) supercooled liquids and rapidly quenched amorphous alloys, we demonstrate ISRO in all cases irrespective of the Ti concentration. Further, our ESL solidification studies show that Ti inhibits surface crystallization, but does not improve glass formation.   

High-Energy X-ray Diffraction Study of Liquid Structure of Zr-based Binary Alloys

High-energy (E = 113 keV) synchrotron x-ray diffraction experiments were performed for metallic glass-forming Zr-Cu alloys in the liquid state at high temperatures. Accurate structure information of highly reactive melts has been obtained by applying conical nozzle levitation technique as a containerless method. The total structure factor extracted for the liquid Zr$_{50}$Cu$_{50}$ alloy near its melting temperature shows a particular shoulder at the high-$Q$ side on the second peak as well as the liquid Zr$_{70}$Cu$_{30}$ alloy. This feature of structure factor is similar to those of structure factors observed in deeply undercooled metallic liquids or metallic glasses, in which local icosahedral short range ordering was found to exist. With the use of reverse Monte Carlo simulation analysis, it was demonstrated that short-range ordered clusters exist even in the equilibrium liquid state of Zr$_{70}$Cu$_{30}$ alloy.   

Elasticity and conductivity thresholds in solid electrolyte glasses

The solid electrolyte glass, AgI, possesses a low mean coordination number [1], and when alloyed in the base oxide glass, AgPO$_{3}$, steadily lowers the connectivity of the alloyed glass, (AgI)x(AgPO3)1-x, as reflected in reduction of glass transition temperatures T$_{g}$(x). Non-reversing enthalpy associated with T$_{g}$s vanish in the 0.10 $<$ x $<$ 0.35 range, the reversibility window, which we identify with the Intermediate elastic phase [2]. Glasses at x $<$ 0.10 belong to the Stressed-Rigid while those at x $>$ 0.35 to the Floppy elastic phase. Electrical conductivity, $\sigma $(x), reveal a mild increase near x = 0.10 as glasses become unstressed, and a pronounced increase near x = 0.35, when glasses become floppy. The correlation between $\sigma $(x) and the elastic phases opens a new paradigm in understanding electrical transport in glasses. Chains of PO$_{4}$ tetrahedra ( Q$^{2})$ present in the pristine oxide are steadily cut and eventually transformed into rings as networks become less connected with increasing x, as revealed by Raman and P$^{31}$ NMR measurements. \newline [1] P. Boolchand and W.J.Bresser, Nature \textbf{410}, 1070 (2001). \newline [2] S.Chakravarty, D.Georgiev, P.Boolchand {\&} M.M.Micoulant, JPCM \textbf{17}, L1-L7 (2005).   

Chemical alloying induced collapse of reversibility windows in ternary As-S-I glasses*

Thermally reversing windows represent glass compositions across which glass transitions are thermally reversing in character. These windows have been observed in several chalcogenide glasses, and are identified$^{1}$ with self-organized phases of glassy networks. Upon alloying halogen (iodine) in base chalcogenide glasses (Ge-Se, Ge-S), the reversibility windows collapse$^{2}$ about the mean-field rigidity transition. We attempt to understand this behavior better. We have now synthesized ternary glass compositions of the type, (AsI$_{3})_{x}$(As$_{0.30}$S$_{0.70})_{1-x}$ and (AsI$_{3})_{y}$ (As$_{2}$S$_{3})_{1-y}$ over wide composition ranges of x and y, and have examined them systematically in Raman scattering and MDSC experiments. Along with earlier results$^{3}$ on binary As$_{z}$S$_{1-z}$ glasses, the present results permit mapping the reversibility window over the glass forming range of the present As-S-I ternary. The results show the window region to be of nearly triangular shape, with a base extending in the 0.20 $<$ z $<$ 0.27 range and a vertex located near y = 0.28. A possible interpretation of the results will be presented. * Supported by NSF grant DMR-04 -56472 1. P.Boolchand et al.Phil. Mag \underline {85},3823 (2005). 2. Y. Wang et al. Phys. Rev. Lett. \underline {87}, 18, 5503 (2001) 3. D.G. Georgiev, Ph.D. Thesis , Univ. of Cincinnati (2003) unpublished   

Glass structure and electrical conductivity in (As$_2$S$_3$ $_{1-x}$ (Ag$_{2}$S)$_{x}$

We have synthesized titled glasses in the 0 $<$ x $<$ 0.16 range, and have examined them in modulated DSC experiments. The starting materials, As$_{2}$S$_{3}$ and Ag$_{2}$S lumps, were reacted in evacuated fused quartz tubings, and glasses synthesized by water-quench of homogenized melts. Thermal measurements used a TA instruments model 2920 operated at 3\r{ }C/min scan rate and 1\r{ }C/100s modulation rate. Preliminary results reveal a single glass transition in the 0 $<$ x $<$ 0.05 range, which steadily decrease from a value of 210\r{ }C at x = 0 to 182\r{ } C near x = 0.05. In contrast, bimodal glass transitions are observed at x $>$ 0.09, with one T$_{g}$(1) near 167\r{ }C and the second, T$_{g}$(2) near 186\r{ }C, and with the endotherm associated with T$_{g}$(1) steadily increasing with x. Non-reversing enthalpies associated with T$_{g}$s are found to steadily decrease in the 0 $<$ x $<$ 0.09 range, to nearly vanish in the 0.10 $<$ x $<$ 0.12 range and to increase thereafter ( x $>$ 0.12).These findings suggest that glasses at low x ( $<$ 0.09) are Stressed- rigid, those at x $>$ 0.12 Floppy while those in between in the Intermediate phase$^{1}$. The present results correlate well with earlier$^{2}$ electrical conductivity results in suggesting the possibility of an elastic origin to the conductivity thresholds in solid electrolyte glasses. 1. P. Boolchand, D.Georgiev and B. Goodman, J.Opto {\&} Adm. Mater. 3, 703 (2001). 2. E.A. Kazakova and Z.U.Borisova, Fiz. Khim.Stekla \textbf{6}, 424(1980).   

New glasses in the alumina-calcia-monazite (LaPO$_{4})$ system: structural evidence from NMR, Raman scattering and thermal properties

A new group of glasses has been synthesized from the well-known compounds CaAl$_{2}$O$_{4}$ and (CaO)$_{12}$(Al$_{2}$O$_{3})_{7}$, melted together with monazite (LaPO$_{4})$ in compositions containing 2 to $>$75{\%} of the latter. Raman and $^{31}$P NMR spectra in the solid state show that PO$_{4}$ groups do not share bridging oxygens, i.e. that the materials are orthophosphates. Thermal properties and $^{27}$Al NMR in both liquid and solid states indicate the presence of a strong aluminate network, based upon AlO$_{4}$ tetrahedra sharing corners. P/Al TRAPDOR NMR measurements show that P and Al are in close proximity in the glasses, most likely sharing P-O-Al linkages. However, clear and unambiguous signatures of the aluminate network are still sought. Models for the structures of these glasses, drawn from experiment, will be presented.   

Session W33: Nonequilibrium and Templated Assembly

Designing elastic sheets to self-assemble in a viscous environment

A recent work by Boncheva et al. (Proc. Nat. Acad. Sci. 2005 102: 3924-3929) has raised some basic issues about designable self-assembly within the context of planar elastic sheets which fold into 3D structures under magnetic forces. While being agitated in water, millimeter-scale structures were shown to fold with varying success depending on the locations of magnets on the sheets. Our work considers how to design such structures, an understanding of which will be necessary when moving this process to the micron scale. Among the important parameters are the geometry of the flat sheet, the configurations of the magnets, and the ratios of magnetic to elastic forces. We consider this problem using a numerical model of an elastic sheet, and restrict to the simpler case of electrostatic forces in a quasi-static limit. We identify a simple algorithm for choosing configurations of electrostatic charges, and select ratios of charge strength to elastic energy using physical arguments. We then demonstrate our algorithm on dynamical foldings of a sphere and more general geometries, in the overdamped viscous regime. We also give an asymptotic formula for the elastic energy in the thin-plate limit.   

Dynamic self-assembly of magnetic particles on the fluid interface: surface wave assisted effective magnetic exchange

Novel dynamic self-assembled multi-segment magnetic structures (``snakes'') induced by a vertical alternating magnetic field in an ensemble of magnetic particles suspended on a liquid/air interface are reported. We demonstrate that these structures are directly related to surface waves in the liquid generated by the collective response of magnetic microparticles to the alternating magnetic field. The segments of magnetic ``snake'' exhibit long-range antiferromagnetic ordering mediated by the surface waves, while each segment is composed of ferromagnetically aligned chains of microparticles. To describe observed magnetic behavior of the generated structures we propose a simple phenomenological model where the effect of surface waves is replaced by an effective exchange interaction. In the framework of the proposed model the effective exchange constants corresponding to different regimes of magnetic driving were extracted from the experimental data.   

Demonstration of shape selectivity in depletion-induced colloidal aggregation

We have developed a set of monodisperse, non-spherical colloids using photolithography in order to elucidate fundamental questions related to the role of shape in defining colloidal phase behaviour and, eventually, to build new microstructured materials. Our goal is to use depletion and DLVO forces to induce specific and directional interactions during the aggregation process of these non-spherical colloids. We will first describe the development and basic characterization of these particles, including index of refraction, zeta potential, polydispersity, and surface roughness. We will then present an initial state diagram of depletion-induced structure, and provide mechanistic insight into the role of specific characteristics of the particles in defining this behaviour. We will finally discuss theoretical calculations of the expected interactions and the possibility of generalizing the results to other colloidal systems.   

Pattern formation and liquid crystallinity in evaporating drops of gold nanorods.

The drying drops of colloidal rods on glass provide a coffee ring type stain accompanied by formation of highly birefringent deposit, suggesting lyotropic liquid crystalline phase forms prior to deposit formation. Further, Liesegang ring like patterns which have concentric deposits, appear on drying, only under specific conditions, and in optical microscope show similar birefringent bands. Similar experiments done on carbon coated copper TEM grids and observed under TEM, show smectic and nematic-like phases as well as create vortex-like assembly of nanorods reminiscent of defect structures in liquid crystals, and are likewise explained in terms of lowest energy configuration of twist configuration. In the present, work we compliment the rich experimental observations, by proposing mechanisms that explain formation of concentric rings, as well as other complex patterns using both the existing framework based on coffee ring stain models and on basis the observed liquid crystalline phase behavior.   

Molecular Detection with Self-Assembled Gold Nanoparticle Wires

Recently, we have reported the creation of gold nanowires by evaporation-driven vertical colloidal deposition (VCD) of gold nanoparticles. Subsequently, we have noted systematic changes in conductivity associated with post-deposition annealing. Here we describe the change in room temperature conductivity of gold nanoparticle wires after exposure to thiol-derivatized molecules. A self-assembled gold nanoparticle wire is immersed into a solution of octadecanethiol (ODT) dissolved in ethanol. An ODT molecule comprises a sulfur atom and a short hydrocarbon chain. The sulfur atoms form metallic bonds with the conduction electrons on the surface of the gold nanoparticles, and the high surface to volume ratio of the wire allows us to see a marked jump in resistance. We have seen roughly a 10 percent increase in the resistance of previously annealed wires when immersed in 2 millimolar ODT solution. Further experiments include measurement of resistance as a function of thiol-concentration and as a function of wire preparation prior to immersion.   

Orientational Order of Molecular Assemblies on Inorganic Crystals

Surfactant micelles form oriented arrays on crystalline substrates such as HOPG (Highly Ordered Pyrolytic Graphite) although registration is unexpected since the template unit cell is small compared to the size of a rod-like micelle. In addition, with atomic force microscopy, we show that orientational ordering is a dynamic, multi-molecule process. Interaction energy calculations based on molecular simulations reveal that orientational energy differences on a molecular scale are too small to explain matters. However, treating the cooperative processes as a balance between van der Waals torque on a large, rod-like micellar assembly and Brownian motion shows that orientation is favored. Our study provides a physical insight on regulation of self-assembly structures at small length scale.   

Vesicle-Templated Supramolecular Assembly of Alginate Nanogels

In this work, large uni- and multilamellar dipalmitoyl phosphatidylcholine (DPPC) liposomes (800-900 nm in diameter) were used as templates for the formation of alginate gels. DPPC liposomes encapsulating sodium alginate were prepared in a 15 mM NaCl buffer solution by the solvent injection method, followed by several freeze/thaw cycles to achieve higher encapsulation efficiency and larger vesicle size. Purified liposomes were placed in a 10 mM CaCl2 buffer solution and permeabilized by heating and cooling over the phase transition temperature (Tm) of DPPC. The increased membrane permeability at the Tm allowed calcium ions from the surrounding buffer solution to traverse the membrane to the interior region and subsequently crosslink the encapsulated alginate. Removal of the lipid by detergent resulted in nanogels that were similar in size (800-900 nm in diameter) to the template liposome, as characterized by multi-angle and dynamic light scattering techniques. In the future these nanogels may be useful for single-molecule encapsulation or controlled release applications.   

Self-Assembly of Two Dimensional Hard Rod Fluids in the Presence of Surface Barriers

Hard rods are interesting building blocks for assembly as they organize themselves into different phases depending on their aspect ratio and concentration. Much work has been done to develop an understanding of the various phases exhibited by bulk, hard-rod fluids in two and three dimensions. For applications in nanoelectronics, it is desirable to be able to assemble nanowires into various structures conducive to nanocircuitry. In this work, we use Monte Carlo simulations to examine the phase behavior of two different systems involving two-dimensional, colloidal nanowires. In the first study, we probe the influence of regularly spaced surface barriers on the ordering of two-dimensional hard rods. By varying the spacing between the barriers and their size relative to the rods, we demonstrate that a number of interesting phases can be achieved, indicating that surface barriers can effectively tune the alignment of the rods. In a second study, we probe the hard-sphere templated assembly of colloidal nanowires. We demonstrate that a number of interesting and potentially beneficial phases occur as the rod length relative to the sphere diameter, rod aspect ratio, and concentration is varied.   

Multipolar interaction model for microtubule self-assembly

Tubulin protein monomers (m = 50 kDa, d = 4-5 nm) are known to self-assemble into biologically significant structures called microtubules. Calculations for microtubule models using the full crystallographic structure of tubulin are prohibitive. As a substitute, we investigate a simpler multipolar interaction model of tubulin which can capture important features of microtubules. We present energy-minimization calculations showing that a four point-charge rectangular model reproduces the 0.93 nm staggering of observed microtubules. We then attempt to validate these static Coulomb calculations with molecular dynamics (MD) using NAMD (from the Theoretical and Computational Biophysics group at the University of Illinois). These simulations include electrostatic interactions, stiff bonded interactions, a Lennard-Jones potential and Langevin damping. The results of the MD simulations are strongly dependent on each of these influences. We find stable filaments of tubulin using the multipole model in MD simulations. When these filaments are combined into a realistic microtubule in MD, an energy minimum is found which supports a stable tube. The study encompasses fraying of tube ends, staggering angle, and ring stability for tubulin and microtubules based on our simple, four charge multipolar model.   

Thermoreversible Vesicle-to-Micelle Transitions in Surfactant-Salt Mixtures

Mixtures of the cationic surfactant, CTAB and the organic compound, 5-methyl salicylic acid (5mS) spontaneously self-assemble into unilamellar vesicles at room temperature. Upon heating, these vesicles undergo a thermoreversible transition to wormlike micelles. This phase transition results in a 1000-fold increase in the solution viscosity with increasing temperature. Small-angle neutron scattering (SANS) measurements show that the phase transition from vesicles to micelles is a continuous one, with the vesicles and micelles co-existing over a range of temperatures. A mechanism for the above phase transition is proposed, based on the desorption of bound aromatic counterions from the vesicle as a function of temperature.   

Self-assembly models for lipid mixtures

Solutions of mixed long and short (detergent-like) phospholipids referred to as ``bicelle'' mixtures in the literature, are known to form a variety of different morphologies based on their total lipid composition and temperature in a complex phase diagram. Some of these morphologies have been found to orient in a magnetic field, and consequently bicelle mixtures are widely used to study the structure of soluble as well as membrane embedded proteins using NMR. In this work, we report on the low temperature phase of the DMPC and DHPC bicelle mixture, where there is agreement on the discoid structures but where molecular packing models are still being contested. The most widely accepted packing arrangement, first proposed by Vold and Prosser had the lipids completely segregated in the disk: DHPC in the rim and DMPC in the disk. Using data from small angle neutron scattering (SANS) experiments, we show how radius of the planar domain of the disks is governed by the effective molar ratio $q_{eff}$ of lipids in aggregate and not the molar ratio $q (q$ = [DMPC]/[DHPC] ) as has been understood previously. We propose a new quantitative (packing) model and show that in this self assembly scheme, $q_{eff }$is the real determinant of disk sizes. Based on $q_{eff }$, a master equation can then scale the radii of disks from mixtures with varying $q$ and total lipid concentration.   

End-to-End Adhesion of Short Duplex DNA Oligomers

The classic model for the formation of liquid crystal phases of rod shaped objects was presented by Onsager, who showed that hard rods of length L and diameter D form a nematic phase when volume fraction is f $>$ fc = 4D/L. This criterion is obeyed reasonably well for rod-shaped nucleosomal [150 base pair (bp)] B-DNA duplexes (L = 50nm, D = 2nm). Recently we found, however, that very short duplex B-DNA oligomers, 6bp -- 20bp (2 to 6nm) in length, form similar nematic and columnar LC phases, even though their L/D ratio is almost 1 and f $<<$ fc. We attribute these phases to intermolecular interaction which provides an end-to-end adhesion force between these short oligomers to form extended anisotropic ``living polymers.'' The theory of the formation of such anisotropic aggregates will be reviewed and applied to the DNA observations. Work supported by NSF MRSEC Grant DMR 0213918 and NSF Grant 0072989.   

Modeling of fractal intermediates in the self-assembly of silicatein filaments

Silicateins are proteins with catalytic, structure-directing activity that are responsible for silica biosynthesis in certain sponges, Self-assembly of the silicatein monomers and oligomers was previously shown experimentally (Murr and Morse 2005) to form fibrous structures through the formation of diffusion limited, fractally patterned aggregates on the path to filament formation. We present a diffusion-limited aggregation (DLA) based model that is capable of capturing the basic properties of this self-assembly process. The Silicatein oligomer is modeled with three sites of attachment. Rules of attachment are specified that allow for specific interaction between these sites when oligomers are in proximity. The process differs from a DLA process in the following: 1) The process of aggregation is continued dynamically, i.e. the growing structures are spatially distributed and keep diffusing as they are growing 2) The molecules are oriented. Thus rotational diffusion is important. 3) The attachment can happen at more than 1 site and the strength of the active sites can be varied. We show that the self-assembled structures have a good level of similarity with the in-vitro experimental results. We quantify this by comparing the fractal dimension of the experimental data and the model output.   

Structure of Porous Columns Self-assembled from Dendritic Dipeptides

Synthetic pores are an important step in the development of biomimetic materials incorporating features such as trans-membrane channels, gene delivery, protein folding, and selective encapsulation. We have used small-angle xray scattering to study helical pores self-assembled from dendritic dipeptides. The main features of the supramolecular assembly are computed by least-squares fitting the parameters of a simplified structural model to the x-ray diffraction data. This work reports the supramolecular assembly temperature stability and conformational changes of the 3-dimensional packing as a function of dipeptide structure and stereochemistry. The results provide a methodology to design the synthetic pores in order to control the pore size and separation at the \AA\, level, according to the desired function.   

X-ray Study of the Electrical Double Layer at the Oil - Water Interface

Our understanding of the structure of the insulator/electrolyte solution interface is of fundamental importance in describing electrochemical processes in systems involving membranes, absorbers, catalysts, surfactants, or surfaces of other dielectrics. Due to the specific interaction of the solvent with the insulator, a heterogeneous highly polarized region or an electrical double layer forms at the boundary between bulk phases. We studied the spatial structure of the transition region between n-hexane (insulator) and silica sol (electrolyte) solution by x-ray scattering. The structure factor of the interface and the angular dependence of the grazing incidence small-angle scattering can be explained by the interfacial model, which agrees with the theory of the electrical double layer, shows the separation of positive and negative charge, and consists of three layers, i.e., a thin layer of Na$^{+}$, a monolayer of nanocolloidal particles as the part of the diffuse layer, and a low-density layer sandwiched between them.   

Session W34: Fluid Structure & Properties

Scaling fields and the nature of liquid-gas asymmetry in fluids

Fisher and coworkers [Phys. Rev. Lett. \textbf{85}, 696 (2000); Phys. Rev. E \textbf{67}, 061506 (2003).] recently suggested that in fluids the two theoretical scaling fields, commonly known as ``ordering'' and ``thermal'', are mixtures of three physical fields, namely, chemical potential, temperature, and pressure. We have examined experimental consequences of this formulation (``complete scaling'') with regard to the asymmetry of vapor-liquid coexistence in real fluids. By analyzing the coexisting curves of various fluids, we have shown that the vapor-liquid asymmetry originates from two different sources: one from mixing of chemical potential and pressure into the thermal field and another one from mixing of pressure into the ordering field. The first source is attributed to a correlation between entropy and density, whereas the second source is associated with the excluded volume. Real fluids can be mapped into the symmetric lattice-gas (Ising-like) model by a redefinition of the order parameter that can be now expressed through a combination of density, entropy, and molar volume. We have also demonstrated which molecular parameters of fluids control these two sources of vapor-liquid asymmetry.   

NMR characterization of complex fluids by diffusion -- relaxation distribution functions

Many natural fluids are complex mixtures of different types of molecules. As an example, the molecular composition of crude oils typically consists of molecules with a number of carbon atoms that range between one to over 100. In addition to the diverse size, the constituent molecules can be classified into different chemical classes, such as saturates, aromatics, resins and asphaltenes. It is well known that measurements of diffusion and NMR relaxation times can give information on molecular size. We demonstrate that two-dimensional diffusion -- relaxation time distribution functions, f(D,T2), can provide a more unique fingerprint of complex fluids with information on both chain length distribution and chemical composition. The new approach is illustrated with results for different crude oils. The experiments were conducted at a Larmor frequency of 5 MHz and temperatures between 10 C and 58 C. The measurements show a strong correlation between the distributions of diffusion coefficients and relaxation times that are sample specific. The diffusion - relaxation correlation function provides information on the correlation between the rotational and the translational diffusion coefficients of each component of the fluid.   

Structural Change of the Mixtures of Ionic Liquid and Water Studied by Infrared Absorption Spectroscopy

Infrared absorption spectra of the mixtures of ionic liquid and water (1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF$_{4})$ with varying concentrations were obtained by Attenuated Total Reflection (ATR) method. Investigation of the spectra in the OH-stretch vibration range indicated the structural change of the water with the change in the concentration. At very low concentration of water, two peaks around 3600cm$^{-1}$ were assigned to the monomeric form of water molecules weakly hydrogen bonded to the BF$_{4}^{-}$ anions. With the increase in the water concentration, the broad feature at $\sim $3460cm$^{-1}$ corresponding to the bulk water took over the above monomeric peaks, which gradually redshifted with the increased water concentration. In the range from 2800 to 3200cm$^{-1}$ for the various CH-stretch vibration modes in the cation, the peaks in this ranged blueshifted with the increase in the water concentration. This blueshift was as much as $\sim $7cm$^{-1}$ for the CH$_{3}$ vibration modes of butyl chain while it hardly changed for the modes for the CH attached to the imidazolium core, suggesting varying degree of interactions between the carbon-bonded hydrogen and the water molecules.   

Weighted density functional theory for water

We report a weighted density functional theory for water that correctly describes bulk properties of water as well as perturbations at large and small length scales. Calculation of the free energy of solvation for hard sphere solutes of different sizes verifies that this functional gives a simple description of the hydrophobic effects in water. Use of this functional within a joint-density functional theory framework allows a rigorous replacement of molecular water with a continuum in Kohn-Sham calculations of systems in equilibrium with a solution.   

Small-angle neutron scattering study of pH dependence of the liquid structure factor of concentrated solutions of eye lens gamma-B crystallin

We are evaluating the pH dependence of the liquid structure of aqueous solutions of the eye lens protein, gammaB crystallin, near its critical point for liquid-liquid phase separation, to help evaluate the influence of protein charge on the phase separation. We have obtained small-angle neutron scattering data from gammaB crystallin solutions at pH 6.4, 7.1 and 7.4 in a 0.1 M sodium phosphate buffer, and at pH 4.5 in a 0.020M sodium acetate buffer, all in D2O. Protein concentrations ranged from 6 to 260 mg protein/ml solution and the scattering vector magnitude (q) ranged from 0.004 to 0.45 inverse Angstroms. At pH 6.4 to 7.4 liquid structure factors vs. concentration and temperature near the cloud point for liquid-liquid phase separation are well represented, in general, by the Baxter sticky sphere model. In contrast, at pH 4.5, concentrated gammaB shows a very different liquid structure indicating highly repulsive interprotein interactions, consistent with both high net protein charge and reduced screening.   

Phase behavior and mixing-demixing transitions in binary liquid mixtures with spherical and non-spherical interactions

We have carried out extensive equilibrium molecular dynamics simulations to study the temperature versus density phase diagrams and the mixing-demixing transition line in fluid equimolar binary mixtures modeled by: (i) Lennard-Jones, (ii) Stock-Mayer, and (iii) Gay- Berne molecular interactions. These studies are performed as function of miscibility parameter, $\alpha = \epsilon_{AB}/ \epsilon_{AA}$, where $\epsilon_{AA} = \epsilon_{BB}$ and $\epsilon_{AB}$ stand for the parameters related to the attractive part of the intermolecular interactions for similar and dissimilar particles, respectively. When the miscibility of the Lennard-Jones mixture varies in the range $0 < \alpha < 1$, a continuous critical line of consolute points $T_{\rm cons}(\rho)$, appears. This line intersects the liquid-vapor coexistence curve at different positions depending on the values of $\alpha$, yielding mainly three different topologies for the phase diagrams. These results are in qualitative agreement to those found previously for square well and hard-core Yukawa binary mixtures. We also carry out a detailed study of the liquid-liquid interfacial and liquid-vapor surface tensions, as function of temperature and miscibility as well as its relationship to the topologies of the phase diagrams. Similar studies and analysis are also performed for Stock-Mayer and Gay-Berne binary mixtures.   

Confined Fluids: the Time Variable in the Force-Distance Profile

Hitherto-overlooked time dependence is known to play a prominent role in determining the friction of confined fluids. In this study, for the first time we introduce the time variable into measuring force-distance profiles of several simple alkane fluids. The existence of near-surface layered structures in confined fluids is generally manifested as oscillatory forces in force-distance profiles obtained using surface forces apparatus (SFA) and atomic force miscroscopy (AFM) experiments. While it is generally agreed that the rate of the experiment should be slow enough to achieve a quasi-static state, it is less clear what the appropriate rate should be. In this study, while maintaining the experimental time scale uniformly slow enough to avoid trivial hydrodyanamically-induced surface deformations, we demonstrate time dependence in the measured force-distance profile. The role of time scale on the actual structure of the confined fluid will be discussed.   

Spectroscopic Observation of Fluid Molecular Alignment in a Molecularly-Thin Confined Geometry

For the first time, we present data of molecular alignment of a linear chain (1,3-dicyanopropane) under confinement. Confinement was produced between two mica surfaces within a surface forces apparatus (SFA) and measurements employed confocal Raman spectroscopy. We focused on the CH$_{2}$ symmetric stretch vibrations and CN triple bond stretch vibrations. A polar plot of Raman band intensity as a function of incident light polarization allows us to determine the orientation and order parameter of alignment. It is confirmed that alignment can be achieved in molecularly-thin films. The decrease of alignment as the film thickness increases will be mentioned. Also, the effect of shear on molecular alignment will be discussed.   

Active Microrheology of Dense Colloidal Suspensions

We investigate the active microrheology of a colloidal suspension using laser tweezers. The experimental system described here is composed of a hard sphere suspension of fluorescent, index-matched poly(methyl methacrylate) particles seeded with a low concentration of index-mismatched melamine probes. The probe particles are held in an optical trap and subjected to a uniform flow, enabling measurements of the suspension microrheology. Additionally, confocal microscopy is used to obtain non-equilibrium microstructural information. An anisotropic pair distribution function, with a dense region at the leading surface of the probe and a wake trailing it, is observed as the P\'{e}clet number increases to much greater than unity. This structural transition gives rise to a shear thinning regime in the measured microviscosity. The results are in qualitative agreement with recent simulation [I. C. Carpen and J. F. Brady, J. Rheol. 49, 1483-1502 (2005)], and demonstrate the non-linear microrheology of colloidal suspensions.   

Nonlinear microrheology of wormlike micelle solutions using magnetic nanowire probes

Using ferromagnetic Ni nanowires we investigate the local mechanical properties of wormlike micelle solutions composed of equimolar concentrations of the surfactant cetylpyridinium chloride (CPCl) and sodium salicylate (NaSal). Rotating the nanowires with external magnetic fields, we access both linear and nonlinear regimes of the fluid's rheology. The linear viscosity at low rotation rates is strongly temperature dependent as expected from mechanical rheometry experiments. At high rotation rates the viscosity exhibits pronounced shear thinning that is independent of temperature. The onset of the nonlinear response is characterized by a hysteretic shear thickening that is strongly dependent on temperature, but has no counterpart in the macroscopic rheometry. Further, the nonlinear regime coincides with a transient, anisotropic shear-induced state in the fluid that generates a torque on the wire, causing it to tip out of the plane of rotation when the field is removed.   

Myelin figures: an Elastic Instability?

Myelin figures form when certain lamellar phase surfactants swell upon exposure to water. The formation of these myelins, which are tubular structures composed of multiple bilayers of surfactant, is puzzling because it represents the formation of a higher bending-energy configuration from a lower bending-energy initial state. We show that single myelins can be produced in isolation and require a driving force to form and grow; they retract into their parent structure when the driving is removed. We present a model, consistent with our experimental observations, where the formation of myelins is due to an elastic instability of the lamellar phase under internal stress. We propose an experiment to test of this model in comparison to other models, such as that of Huang et al.[1] \newline \newline [1]J.-R. Huang, L.-N. Zou, and T. A. Witten, Eur. Phys. J. E (2005). DOI: 10.1140/epje/e2005-00035-8.   

Charge fluctuations and correlations in finite electrolytes

Charge fluctuations, $\langle Q^{2}_{\Lambda}\rangle$, for the 1:1 equisize hard-sphere electrolyte with the diameter $a$ are computed via grand canonical Monte Carlo simulations, where $Q_{\Lambda}$ is the total charge inside a subvolume $\Lambda$ contained in a simulation box of dimensions $L\times L\times L$ with periodic boundary conditions. The charge fluctuations increase like the surface area $|\partial\Lambda|$ as $\Lambda$ increases, even for small system sizes $L\leq 12a$. For slabs of dimensions $L\times L\times \lambda L$ with $0 < \lambda < 1$, the scaled charge fluctuations, $\langle Q^{2}_{\Lambda}\rangle/|\partial\Lambda|$, approach the thermodynamic limits exponentially fast. The extrapolations to $L\rightarrow\infty$ then yield the Lebowitz length, $\xi_{\mbox{\scriptsize L}}(T,\rho)$, where densities $\rho\alt 3\rho_c$ and temperatures $T\agt T_c$ have been studied. An exact asymptotic expression is obtained for $\langle Q^2_\Lambda \rangle$. This enables one to compute the charge correlation length $\xi_{Z}(T,\rho)$ precisely. The results for $\xi_Z(T,\rho)$ agree with Debye-H\"{u}ckel-type theories at low densities, but show deviations as the density increases. Charge oscillations at higher densities are also observed, as anticipated theoretically. \newline \noindent [1] Y. C. Kim, E. Luijten, and M. E. Fisher, Phys. Rev. Lett. {\bf 95}, 145701 (2005).   

Solid or Liquid ? -- Kinetically induced solidification in a simple nanoconfined liquid

For many years there has been a controversy regarding the supposed solidification of simple liquids when they are confined to a few nanometer film thickness. By using a novel, ultra-small amplitude Atomic Force Microscopy (AFM) technique, we have found that solidification in these systems seems to be due to a kinetic effect and does not occur in thermodynamic equilibrium. In particular, we studied OMCTS confined between a flat silicon surface and a silicon tip and found that at very low approach speeds ($<$= 0.3 {\AA}/sec) the confined fluid remains liquid-like with no change in mechanical relaxation time from the bulk, although ordering is observed in the stiffness and damping of the film. However, when approaching the tip slightly faster at or above 6 {\AA}/sec, the liquid suddenly changes properties dramatically. In the ordered regime, damping is greatly reduced and the mechanical relaxation times show large peaks, indicating an elastic, solid-like response. This result suggests that the observed solidification is a non-equilibrium effect induced at very long time scales.   

Investigation of Liquid Transport/Diffusion through a Nanopore Driven by a Constant Pressure/Chemical Potential Difference.

Fluid transport/diffusion through a nanopore in a membrane was investigated by using a novel molecular dynamics approach proposed in this study. The advantages of this method, relative to dual-control-volume grand-canonical molecular dynamics (DCV-GCMD), are that it eliminates disruptions to the system dynamics that normally created by inserting or deleting particles from control volumes, and that it functions well for dense systems as the number of particles in the studied system remain fixed. Using this method, we examined liquid argon transport/diffusion through a nanopore by performing non-equilibrium molecular dynamics (NEMD) simulations under different back-pressures/chemical potentials. The MD code was validated firstly by comparison with published experimental data, and NEMD results of the present method show that constant pressure/chemical potential difference across the membrane was readily achieved. The soundness of classical Navier-Stokes (NS) solutions for these nanochannel flows was also checked by direct comparison between the NS predictions and results from the proposed NEMD method. The density distributions along the nanopore for both methods were found to be significantly different, but the velocity profile had a similar pattern, although some difference between them exists.   

Phase behavior in binary fluid mixtures with spherical and non-spherical interactions

We have carried out extensive MD simulations to study the T vs. $\rho$ phase diagram and the mix-demix transition in fluid binary mixtures with (1) Lennard-Jones, (2) Stock-Mayer and (3) Gay-Berne molecular interactions. This analysis is performed in terms of the miscibility parameter, $\alpha=\epsilon_{AB}/\epsilon_{AA}$, with $\epsilon_{AA}=\epsilon_{BB}$. When the miscibility of the mixture is in the range $0<\alpha<1$, a continuous critical line of consolute points appears. This line interscts the LV coexistence curve at different positions depending on the value of $\alpha$, yielding mainly three different topologies for the phase diagrams. We also carried out a detailed study of the interfacial properties as function of $T$ and $\alpha$.   

Session W36: Focus Session: Optical Properties of Nanostructures of Si & GaAs

Excitonic Effects and the Optical Properties of Silicon Nanowires

Semiconductor nanowires have potential applications in many fields such as optoelectronics, photovoltaic cells, and especially device miniaturization. The excited-state properties are of critical importance in the design of these functional devices. The low dimensionality and reduced size tend to strengthen the effective Coulomb interaction in these nanostructures. In this study, we focus on the correlated electron-hole states in semiconductor nanowires and the influence of this excitonic effect on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire of a diameter of 1.2 nm. Using plane waves and pseudopotentials, the quasiparticle states are calculated within the many-body perturbation theory with the so-called GW approximation. It is found that the fundamental gap depends on both the orientation and size of the wire, and that the gap increases as the diameter decreases in an inverse quadratic fashion [1]. The electron-hole interaction is then evaluated by the Bethe-Salpeter equation (BSE). The enhanced Coulomb interaction in this confined geometry not only gives rise to an unusually large exciton binding energy of more than 1 eV (compared to a value of 14.7 meV in silicon bulk), but also significantly modifies the relative strength of the absorption peaks. The characteristics of these exciton states will be discussed.\break [1] ``Quantum Confinement and Electronic Properties of Silicon Nanowires,'' X. Zhao, C. M. Wei, L. Yang, and M. Y. Chou, Phys. Rev. Lett. 92, 236805 (2004).   

Electronic structures and optical properties of silicon nanowires

Recent optical spectroscopic\footnote{Holmes, Johnston, Doty, and Korgel, Science 287, 1471 (2000)} and theoretical/computational studies\footnote{Zhao, Wei, Yang, and Chou, Phys. Rev. Lett. 92, 236805 (2004)} challenge the previous consensus on the nature of the optical properties of Si nanowires (SiNW). Here, we present results of precise theoretical FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB 24, 864 (1981)} determinations of the electronic structures and optical properties of (001) and (111) one nm SiNW. The electronic states at the gaps demonstrate a strong orientation dependent parabolic character in the Brillouin zone and a clear entanglement in real space between 1D and 2D dimensions of the wire. The local symmetry imposed by quantum confinement quenches the transitions around the gap, yielding an optically inactive direct gap. The observed (001) photoluminescence is attributed to a transition rooted in an Si$_8$ ring. The optical structure in the experimental range is well reproduced by our first-principles calculations that include the screened exchange-LDA correction to the well-known failure of the LDA. Our predictions about the anisotropy and orientation dependent optical absorption are easily verified experimentally. Work supported by DARPA B529527//W-7405-Eng-48.   

Comparison of the optical properties of bare and capped GaAs nanowires

We study the optical properties of single bare GaAs nanowires and GaAs/AlGaAs core/shell nanowires fabricated by the VLS technique. SEM and AFM images show that the wires are uniform in length (3-4$\mu $m long) and needle-shaped. Low temperature photoluminescence measurements of individual nanowires indicate that the quantum efficiency of the core/shell nanowires is significantly larger compared to the uncapped nanowires. We believe that the reason for the low emission efficiency of the uncapped nanowires is the significant influence of the surface-related non-radiative trap states. We anticipate that time-resolved measurements will show a significant increase of the recombination lifetime in the core/shell nanowires compared to the uncapped nanowires due to the passivation of the surface-related states by the AlGaAs shell. We acknowledge the support of the NSF through grants 0071797, 0216374, and the Australian Research Council.   

Linear optical absorption in silicon and GaAs nanocrystals

The linear optical spectrum of Si and GaAs nanocrystals containing up to 100 atoms is calculated and discussed. We use two first-principles theories: time-dependent density-functional theory in the local adiabatic approximation (TDLDA), and the many-body solution of the Bethe-Salpeter equation (BSE). Both theories predict a strong blue shift in the energy-resolved polarizability and in the absorption cross section, which is characteristic of confined systems in the nanoscale. When many-body effects are included, in the framework of the BSE, the low-energy range of the spectrum is modified in two ways: the energy of excitation lines increases by almost 1 eV, as a result of self-energy and screening corrections; and the oscillator strength of those lines is enhanced. In bulk semiconductors, many-body effects are known to produce exciton lines and enhanced absorption in the low-energy range of the spectrum. Although the size of the nanocrystals studied is much smaller than the radius of Wannier excitons, the enhancement in oscillator strength is suggested to have the same source as excitonic effects in bulk samples.   

Light from Silicon-Based Nanostructures

Si-nanocrystals (Si-nc) embedded in SiO$_{2}$ glass matrices shows undoubtedly efficient room temperature light emission under optical pumping and sizable optical gain and light amplification have been demonstrated [1]. However, the presence of an insulating SiO$_{2}$ matrix prevents the fabrication of reliable and efficient electrically-driven devices and the efficiency of light emission is severely curtailed by the slow radiative lifetime of Si-nc. An alternative possibility is offered by the nucleation of Si-nc in dielectric hosts with smaller band-gaps. In this talk we will show our results on light-emitting Si-rich silicon nitride films (SRN) and photonic structures obtained by Plasma Enhanced Chemical Vapor Deposition (PE-CVD) followed by low temperature (500-900\r{ }C) thermal annealing[2]. The optical properties of SRN$_{ }$films are studied by micro-Raman and photoluminescence spectroscopy and demonstrate the presence of small Si-clusters with nanosecond recombination time and negligible emission thermal quenching. The electrical transport properties of SRN films are also investigated and efficient charge injection at low bias voltages is demonstrated. Additionally, we show that SRN matrices are suitable for efficient energy sensitization of Er ions emitting at 1.54 $\mu $m. The light emission mechanism in SRN nanostructures is studied by DFT-LDA \textit{first principles} calculations showing that, largely Stokes-shifted, nanosecond-fast and efficient light emission in PE-CVD deposited SRN samples originates from strongly localized excitons transitions at the surface of small Si-nc ($\sim $ 1-2 nm) embedded in Silicon nitride[3]. Additionally, we show that the presence of bridging nitrogen groups at the surface of small Si nanocrystals can explain the origin of the experimentally measured Stokes-shift and the nanosecond relaxation times[3]. \begin{enumerate} \item L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, F. Priolo ``\textit{Optical gain in Si nanocrystals}'', Nature 408, 440, 23 November 2000. \item L. Dal Negro, J.H. Yi, V. Nguyen, Y. Yi, J. Michel, L.C. Kimerling, ``\textit{Spectrally enhanced light emission from aperiodic photonic structures}'', Appl. Phys. Lett., \textbf{86}, 261905, (2005) \item L. Dal Negro, J. H. Yi, L. C. Kimerling, S. Hamel, A. Williamson, G. Galli, \textit{Light Emission from Silicon-rich Nitride Nanostructures, }Appl. Phys. Lett., submitted 2005 \end{enumerate}   

Optical characterizations of one-dimensional wetting layers in InGaAs/GaAs quantum dot chains

We report spectroscopic evidence for the formation of 1D wetting layers (WLs) during the Stransky-Krastanov (SK) growth of multi-layered InGaAs/GaAs quantum dot (QD) chains. The wire-like features of these 1D WLs were demonstrated by their 1D density of states as well as the anisotropic absorption and emission properties. Two groups of QD's were found sitting on these 1D and the traditional 2D WLs, respectively, with size-dependent polarization anisotropies of $\sim $6{\%}-25{\%} due to their elongated shapes. The previously-unexplored new SK growth mode of 1D WLs could be potentially tailored by varying the In content and barrier thickness to yield QD's and 1D WLs with expected energy level separations. This may lead to the efficient carrier transfer between QD's on top of the same 1D WL for quantum technology applications that require quantum information transfer between different nanostructures of controlled positioning.   

Electronic and optical fine structure of GaAs nanocrystals: the role of d orbitals in a tight-binding approach

Electronic structure and optical spectra of GaAs nanocrystals for a wide range of sizes are studied by using both $sp^3s^*$ and $sp^3s^*d^5$ nearest-neighbor tight-binding models. Our results show that the inclusion of $d$ orbitals into a minimal basis set is necessary for a proper description of the lowest electron states, especially in the strong confinement regime. For dot sizes below 2.5 nm, the ground electron state is primarily built of L-point bulk band-states, giving the nanocrystals indirect-gap character. Simpler $sp^3s^*$ models yield an incorrect description of electron states made from bulk band- states away from the Brillouin zone center. In contrast, $sp^3s^*d^5$ models are able to provide a consistent picture of the main optical features in agreement with experiments.   

Neutral and charged excitons in single GaAs-based interface quantum dots

We report circularly polarized photoluminescence (PL) in high magnetic fields ($<$ 28 T) and optically detected resonance (ODR) experiments of interface fluctuation quantum dots (IFQDs) in narrow GaAs/AlGaAs quantum wells (QWs) doped in the barriers with donors to allow creation of both neutral and negatively charged excitons. In the narrowest QW the diamagnetic shift of the trion is smaller than that of the neutral exciton. This is attributed to the larger spatial extent of the trion wavefunction in these ensembles of weakly confined QDs. Along with a careful study of the excitonic Zeeman splitting and complemented by a comparison of ensemble and single dot ODR measurements, this signature can be used to assign the narrow spectral lines observed in single dot PL studies as neutral and charged excitons. The PL of the trion is found to increase under resonant irradiation with far-infrared laser light, opposite to the behavior observed for wide QWs. Lateral carrier redistribution is believed to be the dominant mechanism that gives rise to the ODR signal in QWs with monolayer well width fluctuations.\\ Work supported in part by NSF-DMR \#0203560.   

Nanostructured solar cells

In this work we investigate the use of nanofabrication technique to improve the overall efficiency of silicon solar cells. The efficiency and the durability of silicon solar cells largely depends on the quality of the anti-reflectivecoating. In this work, the change in the index of refraction on the surface of a substrate can be controlled by the amount of porosity, which is well known in effective medium theory. Also by changing the thickness of the porous layer, the medium can be fine tuned to a specific wavelength as an AR coating. We fabricate the nanoporous layer by using a self-assembled P(S-b-MMA) coating as a mask to etch into the silicon substrate using reactive ion etching. The use of different molecular weight diblock copolymer and different etching time allow us to tune the index of refraction. FT-IR and variable angle ellipsometry provide information about the transmission and reflection properties along with the index of refraction and the thickness of the coating. The investigation of the efficiencies are performed by comparing the I-V plots of conventional and nanostructured cells. Additional research is underway in order to apply this technology to other types of substrates.   

Effects of high magnetic fields on the scattering rates of GaAs/AlGaAs and GaInAs/AlInAs quantum cascade lasers

Using magneto-spectroscopy, we investigate the influence of a strong magnetic field on the intersubband scattering rates in MIR GaAs/AlGaAs and GaInAs/AlInAs quantum cascade lasers (QCLs). In our experiments, we measured light-current, voltage-current and laser emission spectra as a function of magnetic field up to 40T with the magnetic field perpendicular to the 2DEG. We observed strong oscillations in the intensity and threshold current. From these, the magnetic field dependences of the intersubband lifetime of both structures were derived and compared to their calculated dependence of electron-LO phonon scattering rates.   

Ultra-low threshold quantum dot microdisk laser

Ultra-low threshold lasers have applications in low-power communications. These lasers are also of fundamental interest, where a full understanding of lasing based on a few discrete emitters is evolving. This is especially true in solid-state systems, for instance those with a quantum dot (QD) gain medium, where a typical spectrum of discrete emission lines observed at lower pump power is often highly modified under higher pump powers. Here we discuss a microcavity laser containing a dilute QD gain medium that has an ultra low, sub-microwatt CW lasing threshold. The structure is based on a high-quality factor microdisk cavity of GaAs with a low density of InAs-based QDs embedded in the microdisk. We estimate 250 QDs in the 1.8 $\mu $m diameter microdisk under investigation. Of these QDs approximately 60 are spatially located within the modal region of the disk, which extends inwards approximately 250 nm from the disk edge. Only a small portion of these QDs couple to the narrow cavity modes, which have a free spectral range of 45 nm and an initial linewidth of 0.06-0.07 nm. Linewidth narrowing and lifetime reduction with increasing pump are both observed. Despite the small number of QDs it is unlikely from our estimates the system lases from a single QD state.   

Session W37: Nanoscale Conductance Theory II

Variable-range cotunneling and non-Ohmic transport in a chain of one-dimensional quantum dots

A 1D wire with a finite density of strong random impurities is modeled as a chain of weakly coupled quantum dots. The resistance of such a system is shown to exhibit a rich dependence on bias voltage $V$ and temperature $T$ due to the interplay of Coulomb blockade, Luttinger-liquid, and disorder effects. At low $T$ and $V$ electrons propagate through the wire by means of thermal activation and a multiple cotunneling. In this regime the resistance is limited by the ``breaks'': randomly occurring clusters of dots with a special length distribution pattern that inhibits the transport no matter how the activation and tunneling are combined. As $T$ or $V$ increases, the breaks become shorter and less resistive. The resistance can exhibit a (stretched) exponential and a quasi power-law dependence on $T$ and $V$ depending on the position at the $T$-$V$ diagram. Unlike the case of a single impurity the effect of $T$ and $eV$ is not symmetric. The Ohmic resistance of a macroscopic wire is always dictated by breaks not single impurities. Our results imply that the power-laws reported in several recent transport measurements of one-dimensional systems may reflect not only intrinsic Luttinger parameters but also impurity distribution statistics.   

Pair tunneling through single molecules

By a polaronic energy shift, the effective charging energy of molecules can become negative, favoring ground states with even numbers of electrons. Here, we show that charge transport through such molecules near ground-state degeneracies is dominated by tunneling of electron pairs which coexists with (featureless) single-electron cotunneling. Due to the restricted phase space for pair tunneling, the current-voltage characteristics exhibits striking differences from the conventional Coulomb blockade. In asymmetric junctions, pair tunneling can be used for gate-controlled current rectification and switching. We find that pair tunneling also has interesting consequences for the shot noise.   

Spin Related Effects in Transport Properties of "Open" Quantum Dots

We study the interaction corrections to the transport coefficients in open quantum dots (i.e. dots connected to leads of large conductance $G \gg e^2/\pi\hbar$), via a quantum kinetic equation approach. The effects of all the channels of the universal (in the Random Matrix Theory sense) interaction Hamiltonian are accounted for at one loop approximation. For the electrical conductance we find that even though the magnitude of the triplet channel interaction is smaller than the charging energy, the differential conductance at small bias is greatly affected by this interaction. Furthermore, the application of a magnetic field can significantly change the conductance due to the Zeeman splitting, producing finite bias anomalies. For the thermal conductance we find that the Wiedemann-Franz law is violated by the interaction corrections, and we investigated the effect of magnetic field on the Lorentz ratio for contacts of finite reflection. The charge and triplet channel corrections to the electrical and thermal conductance vanish for reflectionless contacts. In the latter case the temperature and magnetic field dependence of the conductance is determined by the Maki-Thompson correction in the Cooper channel.   

Numerical studies of the dynamics of interacting electrons confined in nanostructures

At low temperatures electrons have long phase-relaxation time. They tunnel coherently through nanostructures in a wave-like manner, which leads to various interference effects. We presently have adequate knowledge about the transport phenomena that can be described using single-electron models. The transport in the presence of interactions is, however, still a subject of intensive research. More refined theoretical tools are required to tackle problems such as that of the transport through systems of coupled quantum dots in the Kondo regime. We present our studies using complementary methods: the quantum Monte Carlo, the variational method and the numerical renormalization group. We show the phase diagram of the triple quantum dot and explain the various regimes of enhanced conductance.   

Non-equilibrium conductance of a three-terminal quantum dot in the Kondo regime: Perturbative Renormalization Group

Motivated by recent experiments, we consider a single-electron transistor in the Kondo regime which is coupled to three leads in the presence of large bias voltages. Such a steady-state non-equilibrium system is to a large extent governed by a decoherence rate induced by the current through the dot. As the two-terminal conductance turns out to be rather insensitive to the decoherence rate, we study the conductance in a three-terminal device using perturbative renormalization group and calculate the characteristic splitting of the Kondo resonance. The interplay between potential biases and anisotropy in coupling to the three leads determines the decoherence rate and the conditions for strong coupling.   

Dissipative quantum phase transition in a single electron transistor

We study the transport properties of a single electron transistor (SET) with highly resistive gate electrodes, and show that the SET displays a quantum phase transition analogous to the famous dissipative phase transition studied by Leggett. At temperature $T=0$, the charge on the central island of a conventional SET changes smoothly as a function of gate voltage, dueto quantum fluctuations. However, sufficiently-strong dissipation, $R_g>R_C$, can freeze out charge fluctuations on the island even at the degeneracy point, causing the charge on the island to change in sharp steps as a function of gate voltage. For $R_g [Preview Abstract]

The Interplay of Spin and Charge Channels in Zero Dimensional Systems

We study the interplay of charge and spin (zero-mode) channels in quantum dots. The latter affects the former in the form of a distinct signature on the differential conductance. We also obtain both longitudinal and transverse spin susceptibilities. All these observables, underlain by spin fluctuations, become accentuated as one approaches the Stoner instability. The non-perturbative effects of zero-mode interaction are described in terms of the propagation of gauge bosons associated with charge (U(1)) and spin (SU(2)) fluctuations in the dot, while transverse spin fluctuations are analyzed perturbatively.   

The 0.7 anomaly in quantum point contacts: a scattering approach

The conductance steps observed in the electronic transport through quantum point contacts (QPCs) became a paradigm of the Landauer conductance formula. For this reason, the ubiquitous experimental observation of the 0.7 anomaly in the first conductance step of QPCs, that defied the single-particle scenario, raised a lot of attention. The most successful theoretical explanation of this transport feature is in terms of Kondo physics: It builds on an Anderson-like model, whose parameters, namely, the resonance position, its couplings to the reservoirs and the charging energy are adjusted to give meaningful results. Starting from a scattering approach, that uses the Feshbach projection formalism, we construct a single-particle basis that allows us to directly calculate the resonance position and its coupling to left and right reservoirs. We then include an electron-eletron interaction term and proceed as standard. This approach unveils a novel interpretation for the underlying physics of the 0.7 anomaly.   

Electron transport in the presence of a magnetic field and the absence of translational invariance

Recent experimental techniques in 2DEGs using scanning probe microscope tips allow one to spatially image electron flow directly (see also the talk by Kathy Aidala). These developments motivate theoretical consideration of (localized) magnetic edge states in position space. For the case of a parabolic confinement, a semi-analytic expression of the Green function is given. The underlying physics differs from a conventional edge-state model by the absence of translational invariance. It is also possible to derive a semiclassical interpretation of the current density, which provides additional physical insight into the nature of transport in position space. For additional information, see also http://people.deas.harvard.edu/$\sim$tkramer   

Divergent beams of nonlocally entangled electrons emitted from NS structures

We propose the use of normal and Andreev resonances in normal-superconducting structures to generate divergent beams of nonlocally entangled electrons. Resonant levels are tuned to selectively transmit electrons with specific values of the perpendicular energy, thus fixing the magnitude of the exit angle. When the normal metal is a ballistic two-dimensional electron gas, the proposed scheme guarantees arbitrarily large spatial separation of the entangled electron beams emitted from a finite interface. We perform a quantitative study of the linear and nonlinear transport properties of some suitable structures, taking into account the large mismatch in effective masses and Fermi wavelengths. Numerical estimates confirm the feasibility of the proposed beam separation method.   

Non-equilibrium Entanglement and Noise in Coupled Double Quantum Dots

We study charge entanglement in two capacitively-coupled double quantum dots in thermal equilibrium and under stationary non-equilibrium transport conditions. In the transport regime, the entanglement exhibits a clear switching threshold and various limits due to suppression of tunneling by Quantum Zeno localisation or by an interaction induced energy gap. We also calculate quantum noise spectra and discuss current cross-correlations as an indicator of the entanglement in transport experiments.   

Conductance Fano lineshapes for Kondo impurities on surfaces: A numerical renormalization group description.

Scanning tunneling microscopy (STM) measurements of Kondo impurities on metallic surfaces has been an active field in recent years. For a flat density-of-states (DoS) near the Fermi energy in the host metal, the low-bias STM conductance acquires the characteristic Fano lineshape, with width proportional to the Kondo temperature $T_K$. In this work, we study how this picture is modified when a \textit{structured} DoS (non-flat) is considered. A variety of physical effects can introduce peak/dips in the DoS, including the presence of a second impurity, hybridization between surface and bulk conduction states, and a magnetic impurity embedded in a molecule. Using numerical renormalization group techniques, we calculate the low-temperature conductance for this system. The zero-bias dip in the Fano conductance is modified by the presence of resonances or anti-resonances in the DoS near $E_F$. In particular, for DoS with pseudogaps and impurities in the mixed-valence regime, zero-bias Fano-like dips appear {\em even when no Kondo state has developed}, but governed by energy scales much larger than $T_K$. We further show that measurements of the scattering phase could be used as an additional probe into the Kondo regime. Supported by NFS-NIRT.   

Effects of the electron-phonon interaction on the electron transport in low-dimensional disordered semiconductor structures

We investigate the effects of the interference between electron-phonon scattering and elastic electron scattering in heterostructures and nanotubes. Interference strongly enhances the effective electron-phonon coupling in semiconductor structures and strengthens the electron-phonon relaxation [1]. Employing the quantum transport equitation, we calculate the interference contribution to the electrical conductivity and phonon drag thermopower. Our results show that the interference term follows to the logT-law and dominates in the temperature dependence of the conductivity. Phonon drag is also enhanced due to disorder. [1] A. Sergeev et al., Phys. Rev. Lett., 94, 136602 (2005).   

Mesoscopic and nanoscopic physics of molecular-scale electronics

Going from the mesoscopic regime of quantum semiconductor device to nanoscopic molecular device, the dominant or first-order transport mechanism remains quantum mechanical coherent transport due to the small size. A large part of the theoretical efforts in nanoelectronics is thus to recast the accumulated knowledge about mesoscopic physics into forms that are suitable for evaluating quantum transport phenomena at the atomic-scale. On the other hand, electrical conduction is intrinsically a dynamical phenomenon. Since the different degrees of freedom (electronic, mechanical, phonon{\ldots}) in the nanostructures can be strongly coupled to each other and to their nano-environment, the measured electrical signal is often the result of complex dynamic coupling processes without requiring ensemble average. New theoretical principles and computational techniques may be needed to unravel the rich physics involved in molecular-scale transport. In this talk, we discuss our efforts in moving from mesoscopic theory to nanoscopic theory of molecular-scale electronics.   

Phonon Broadening of Spectral Lines in Scanning Tunneling Spectroscopy

The observation and interpretation of spectral lines associated with quasi-localized states in condensed matter systems has provided a rich source of information pertaining to the transient coupling of these states to their dynamic environment.$^{1}$ While polaron/Franck-Condon models in which a transient localized potential excites the ambient phonon system have formed the basis for phonon broadening in a wide variety of core level spectroscopies, Sunjic and Lucas have put forth an elegantly simple solution to the problem in terms of driven harmonic oscillators which easily incorporates the time scales for both the switching on and the decay of the localized potential.$^{2}$ In recent STS studies of thin NaCl films on Cu substrates, Repp et al. have observed Gaussian-broadened lines that are signatures of bound electrons at Cl vacancies (F-centers).$^{3}$ These resonance tunneling line shapes are here analyzed within the context of the SL model, properly accounting for lifetime effects due to both tip-to-vacancy and also vacancy-to-substrate tunneling, thus enabling determination of the actual electron-phonon interaction. $^{1}$J.W.Gadzuk, PRB \textbf{44}, 13466 (1991). $^{2}$M.Sunjic and A.A.Lucas, CPL \textbf{42}, 462 (1976). $^{3}$J.Repp et al., PRL \textbf{95}, 225503 (2005).   

Session W38: Flux Pinning and Critical Currents

Theory for the Dependence on Thickness Shown by the Critical Current vs. Magnetic Field in Films of PLD-YBCO

The theoretical consequences of the proposal that the vortex lattice induced by perpendicular magnetic field in films of PLD-YBCO is in a thermodynamic Bose glass state are explored. Attention is focused on the high-field regime at the extreme type-II limit, in which case only a small fraction of the vortex lines are localized at the dislocations that thread the film along the c axis, and in which case the pinning of the vortex lattice is collective. The critical current density along the film is predicted to follow an inverse square-root power law as a function of external magnetic field in the collective-pinning regime. It gives a fair account of the critical current density at kG magnetic fields in films of PLD-YBCO that are microns thick, at liquid nitrogen temperature. It fails, however, for much thinner films at lower temperature. This failure is corrected by including the effect of point pins along the interstitial vortex lines that lie in between the correlated pins. They contribute an inverse dependence on film thickness to the critical current density in magnetic field oriented near the c axis.   

The growth mechanism of pinning-effective nanostructures embedded in YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) superconducting thin film

Following up on previous success in modifying the pulsed-laser-deposition (PLD) film processing to introduce self-assembled pinning defects in films of various high-temperature superconductors (HTSC), specifically the case of self-assembled columnar arrays of oxide nanodots in YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin film [\textit{Superconductor Science and Technology} \textbf{18}, 1533 (2005)] using a nanodot-doped YBCO target, a careful and systematic examination of the growth mechanism is yet to be conducted on this and similar systems. This study examines (1) how the oxide nanodots retain their character during the deposition and (2) how the nanodots both influence, and are influenced by, the local potential-energy landscape that promotes spontaneous assembly into coherent stacks. This is done by growing the film subject to slight variations in the processing parameters which may influence the nature of the heterogeneity of any given layer in the film. Particular attention is paid to the influence of varying laser-pulse frequency which determines the time duration by which the potential-energy landscape of a pulsed layer is consolidated in time for the next pulsed layer. The mechanism is also tested for a system in which the embedded nanostructures are made from a non-oxide material (gold). Results consist of microstructure (cross-sectional HRTEM, XRD, surface SEM), transport properties (critical temperature and critical current), and magnetic susceptibility.   

Magnetic coupling between vortices in superconductors and adjacent magnetic layers

We presented a study of magnetic coupling between vortices in superconductors and adjacent magnetic layers in two systems: superconductor/magnetic multilayers and HTS films on magnetic substrates. The flux motion in superconductor and the behavior of magnetic layer (or magnetic substrate) were captured by quantitative Magneto-optical imaging (MOI) technique with an external magnetic field applied perpendicular to the sample surface and varied along a whole hysterisis loop cycle. Bulk dc SQUID magnetization, ac loss, and direct transport measurements were performed to compliment the MOI studies. It was found that magnetic substrate has limited effect on transport properties of HTS films, although some enhancement of $J_{c}$ was observed near $T_{c}$ in the multilayers due to the magnetic coupling. However, magnetic substrate did result in significant reduction of the ac losses.   

Time resolved magneto-optical imaging of ac currents in YBCO conductor.

The use of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) in ac applications, whether with applied ac currents or alternating magnetic fields, is strictly related to the availability of long-length ac-tolerant YBCO coated conductor. These ac applications, such as fully superconducting generators and motors or transformers, may operate with ac currents in a range of frequencies from tens of Hz up to a thousand Hz. We present a newly developed time-resolved magneto-optical imaging (MOI) technique for studying high-temperature superconductors (HTS) with applied alternating currents (AC) in the frequency range 30 -- 1000 Hz. The evolution of the magnetic flux density distribution in YBCO thin films and coated conductors is studied as a function of the phase of the applied AC current. Time- and spatially-resolved images of the magnetic flux profiles are presented for a detailed series of values of the phase. A quantitative analysis of the data allows us to calculate the current density profiles at different phases. We observe for the first time that the maxima of the AC current density is shifted from the edges further inside the sample which may be caused by the higher self-induced field in that region.   

Scanning Hall probe microscopy of AC losses in YBCO coated conductors

Magnetic imaging of current-induced vortex movement in superconducting films yields detailed information about dissipation and the path of an applied current. In our cryogenic scanning Hall probe microscope, a micro-Hall probe is rastered near the sample surface with submicron resolution and centimeter scan range. Hall probe time traces taken at each point are assembled into movies of the flux penetration as a function of time over a cycle of AC sample current. We image coated conductors---films of the high-temperature superconductor YBCO grown on metal tapes which give rise to grain boundaries. We then isolate the effects of the boundaries by imaging YBCO grown on bicrystal substrates that induce a single boundary at various angles to the current path. Current density, electric field, and dissipation distributions are reconstucted from the images.   

Twins, Their Microstructure and Correlation to Critical Current Densities in Superconducting Melt-Textured Grown Y-Ba-Cu-O

Refinement of twin microstructure in melt-textured grown (MTG) Y-Ba-Cu-O (YBCO) is engineered by annealing at different temperatures. This isothermal method exploits the temperature dependence of twin-boundary-energy. The twin boundary energy is obtained by two independent methods: (1) twin spacing and (2) twin-tip shape. The twin boundary energy measurement is accomplished with transmission electron microscopy. Refinement of twin spacing with increasing temperature is confirmed. Twin spacing decreases from 190 nm to 54 nm as the temperature increases from 450 to 680$^{o}$C. Critical current measurements support that a sample with a higher density twin density often results in a higher pinning-force i.e. 3.4 x 10$^{8}$ N/m$^{3}$ versa 1.2 x 10$^{8}$ N/m$^{3}$.   

Scanning Hall Probe Microscope and Imaging of Vortex Penetration into Nb

We report on the construction of a scanning Hall probe microscope with 100 nm lateral resolution and a large scan range, which exceeds 40 $\mu$m at 4.2 K. The microscope is based on the beetle design and operates between room temperature and 1.5 K. The DSP-based control electronics achieves a high (100 kHz) sampling rate and a low noise. The system is capable of simultaneous tunneling and Hall signal acquisition. The Hall sensor for measuring local magnetic fields is fabricated on a GaAs heterostructure through standard EBL and wet etching. It has an active area of 300 nm x 300 nm and a sensistivity of ~ 0.2 $\Omega$/Gauss. We will present (1) a description of the microscope, and (2) progress on imaging the penetration of vortices and the growth of vortex dendritic patterns into thin Nb films \footnote{ Altshuler E. \textit{et al.}, Rev. Mod. Phys.\textbf{76}, 471 (April 2004)} and grid arrays \footnote{Hallen H. D. \textit{et al.}, Solid State Commumications \textbf{99} (9), 651-654 (SEP 1996)}.   

Vortex Pinning in a YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Thin Film

Vortices, and the nanoscale structures that pin them, are important both fundamentally and for the development of high temperature superconductor technologies. We use a home-built magnetic force microscope (MFM) to determine the depinning forces required to move individual vortices in a 200nm thick YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ film. Our results indicate a wide distribution of depinning forces for different vortices. A comparison between distributions at various temperatures is underway, qualitatively showing a decrease of forces with increasing temperature.   

Anomalous Slow Relaxation of the Magnetization in Y$_{1-x}$Pr$_{x}$Ba$_2$Cu$_3$0$_{7-\delta}$

We have investigated the time $t$ evolution of the irreversible magnetization $M_{irr}$ in a series of single crystals of Y$_{1-x}$Pr$_{x}$Ba$_2$Cu$_3$0$_{7-\delta}$, $x=0.13$ ($T_{c}=82$ K), $x=0.34$ (($T_{c}=50$ K), and $x=0.47$ (($T_{c}=34$ K), all displaying a second peak in magnetization. In all cases, $M_{irr}(t)$ follows the well known law of relaxation, $M_{irr}(t)\propto[\mu k_{B}T/U_{o}ln(t/t_{0})]^{-1/\mu}$. For fields/temperatures lower than the corresponding values of the second magnetization peak, $\mu$ is anomalously large, $2.5\leq\mu\leq4$, in contrast with theoretical predictions which gives a maximum value of 2.5, and with data reported for other cuprates. These large $\mu$ values in the vortex glass state, which give rise to a low relaxation rate, occur for all Pr doping and could be due to the presence of Pr ions. In contrast, at fields/temperatures above the second magnetization peak, $\mu$ decreases to values expected from theories of plastic vortex liquid and validated by most experimental reports.   

Effective Action for Vortex Dynamics in Clean d-wave Superconductors

We describe influence of gapless nodal quasiparticles on vortex dynamics in clean two-dimensional d-wave superconductors. At zero temperature, the guasiparticles give rise to a finite renormalization of vortex mass, as well as a universal sub-Ohmic damping of vortex motion. Slow vortex motion is dissipated only at finite temperatures, or when some perturbation, such as disorder, creates a finite quasiparticle density of states at the gap nodes. These results are obtained by a non-perturbative derivation of the effective vortex action, where the quasiparticles are integrated out exactly in a continuum functional formalism. Fortunately, an uncontrolled perturbative analysis reaches the same conclusions, and all findings are reflected in a simple scaling argument where the gapless Dirac quasiparticles are regarded as a quantum-critical system. Our results appear to differ from those of the semiclassical theory, which obtains singular corrections to a vortex mass appearing in transport equations.   

Flux pinning and Critical current density in La$_{2-x}$Sr$_{x}$CuO$_{4+d }$

We have studied the magnetic characteristics of the critical states in a series of samples of the type La$_{2-x}$Sr$_{x}$CuO$_{4+d}$ that is doped with both Sr and excess O incorporated using electrochemistry. These samples spontaneously phase separate and show both a superconducting phase with T$_{C}$ near 40 K and a magnetic phase with T$_{M}$ near 40 K. Our previous studies established that the superconducting phase is similar to an optimally doped sample while the magnetic phase is consistent with the static spin density wave reported for x=1/8 Sr or Ba doped samples. Magnetization data at various temperatures showed large reversibility in all the samples. The critical current densities J$_{C}$(0) values were at least an order of magnitude smaller than that of the reported values for YBa$_{2}$Cu$_{3}$O$_{7-d }$and La$_{2-x}$Sr$_{x}$CuO$_{4}$. At higher fields J$_{C}$(H) was smaller indicating the existence of weak flux pinning in the system. Based on our magnetization data we conclude that the vortex lattice pinning is different from non-phase separated cuprates. This work was partially supported by the US-DOE through contract DE-FG02-00ER45801 and the Cottrell Scholar Program of the Research Corporation.   

Magnetic Pinning in Nb and YBCO Thin Films by Co/Pt Multilayers with Perpendicular Magnetic Anisotropy

Magnetic pinning of vortices has the advantage over intrinsic pinning in that the superconducting critical current can be reversibly tuned by the magnetic field (H). Magnetic pinning by Co/Pt multilayers with perpendicular magnetic anisotropy has been studied in two ferromagnetic/superconducting bilayers of Nb and YBCO with different superconducting properties (e.g. penetration depth $\lambda )$. Magnetic force microscopy reveals similar magnetization (M) reversal process in the two cases, both exhibiting a large density of narrow residual domains but with different domain width w at the final reversal stage. However, the magnetic pinning, revealed by the M-H loop shape in the superconducting state, is different. The Nb film exhibits an enhancement of M with the strongest effect during the final reversal stage, while the YBCO film shows a suppression of M in the vicinity of central M peak and an enhancement of M in large magnetic fields. These different behaviors are related to the different $\lambda $/w ratio in the two cases.   

Flux penetration in a ferromagnetic/superconducting bilayer utilizing perpendicular magnetic anisotropy

The Hall sensor array is a useful tool for measuring local magnetic fields. An array of miniature Hall sensors has been used to study the flux penetration in a ferromagnetic/superconducting (F/S) bilayer consisting of Nb as the S layer and Co/Pt multilayer with perpendicular magnetic anisotropy as the F layer, separated by an amorphous Si layer to avoid proximity effect. The F layer is first premagnetized to different magnetization reversal stages to obtain various magnetic domain patterns. The effect of these domain patterns on the flux behavior in the S layer is then studied at various temperatures in the superconducting state. We have observed that, in addition to the vortex pinning enhancement, some domain patterns strongly increase the first penetration field and induce large thermomagnetic instabilities (flux jumps), which are not detectable by magnetometry. We also discuss the profiles of the flux distribution across these F/S bilayers.   

Session W39: Superconductivity-Josephson Junctions and Qubits

Finite size scaling analysis of the helicity modulus and the inverse dielectric constant in two capacitvely coupled Ultrasmall 2D Josephson Junction Arrays

We have carried out a finite size scaling analysis of the helicity modulus $\Upsilon_{i}$ and the inverse dielectric constant $\epsilon_{i}$, $(i=1,2)$ of two capacitively coupled Josephson junction arrays with charging energy, $E_c$, and Josephson coupling energy, $E_J$. The arrays are coupled via the capacitance, $C_{\rm inter}$, at each site of the lattices. The parameter that measures the importance of quantum fluctuations in the i-th array is, $\alpha_i\equiv \frac{E_{{c}_i}}{E_{J_i}}$. We have considered the interplay between vortex and charge dominated individual array phases by means of extensive path integral Monte Carlo simulations. It has been found that this system develops a {\it reentrant transition} in $\Upsilon(T,\alpha)$, at low temperatures, when one of the arrays is in the semiclassical limit (i.e. $\alpha_{1}=0.5 $) and the quantum array has $2.0 \leq\alpha_{2} \leq 2.5$, for $C_{{\rm inter}}= 0.26087, 0.52174, 0.78261, 1.04348$ and $1.30435$. Similar behavior was obtained for larger values of $\alpha_{2} =4.0$ with $C_{{\rm inter}}=1.04348$ and 1.30435.   

RF critical current of Josephson junction

The Josephson junction is the only radio-frequency electrical element which can be both non-dissipative and non-linear at low temperatures. While the stability of the junction dynamics in presence of a DC drive has been extensively studied, the microwave drive case is relatively poorly understood, at least experimentally. It is explored by driving an increasing AC current through a Josephson junction which is effectively biased by an AC voltage generator in series with a finite linear imbedding impedance $Z\left( \omega \right)$. For small signal amplitude, the junction behaves as a linear inductor. For higher signal amplitudes, we show that there exists a critical current $I_{c}^{RF}$ beyond which the dynamics of the junction changes qualitatively as a result of its non-linear characteristic. This AC critical current depends strongly on the biasing impedance. We provide a detailed stability diagram from experimental measurements and show that it obeys the simple theory of nonlinear resonance.   

Parametric amplification with the Cavity Josephson Amplifier

Several types of amplifiers are approaching the quantum limit, namely, the SQUID, the RF-SET (radio-frequency single electron transistor) and the QCP (quantum point contact). We investigate a new amplifier which harnesses the nonlinearity of a Josephson junction for parametric amplification. It consists of a Josephson junction placed in a high-quality on-chip superconducting cavity, pumped by microwave radiation. The high level of control over the environment provides a system which is well described by the simplest nonlinear oscillator formalism with no adjustable parameters. such that theoretical predictions can be compared with experimental results. The planar geometry of the device can accommodate operation over a wide range of frequencies, opening the possibility of a quantum limited amplifier for practical use. We present preliminary results on the performance of the amplifier and discuss the possibility of observing quantum noise squeezing.   

Josephson current through a molecular transistor in a dissipative environment

We study the Josephson coupling between two superconductors through a single correlated molecular level, including Coulomb interaction on the level and coupling to a bosonic environment. All calculations are done to the lowest, i.e., the fourth, order in the tunneling coupling and we find a suppression of the supercurrent due to the combined effect of the Coulomb interaction and the coupling to environmental degrees of freedom. Both analytic and numerical results are presented.   

Vortex doping into finite-sized superconducting Pb networks.

Superconducting finite-sized Pb square networks with 2x2, 3x3, 5x5 and 10x10 square holes have been fabricated by electron beam lithography of photoresist layer and a lift-off process after depositing Pb film on the resist patterns. The application of magnetic field corresponds to the particle (vortex) doping into networks. Vortex image observations were carried out by a SQUID microscope to compare with the theoretical predictions. We found the exactly reversed pattern between the vortex doping x and the anitivortex doping x into the fully occupied network (x=1/4). The Ginzburg-Landau calculations show that there are several vortex configurations with almost equivalent free energy. The complete coincidence of the two patterns might be due to residual randomness caused in the fabrication processes.   

Nano-mechanical-resonator induced synchronization in Josephson junction arrays

We show that a serial array of N critical-current disordered, underdamped, Josephson junctions coupled piezoelectrically to a nanomechanical (NEM) oscillator results in phase locking (synchronization) of the junctions. We find a semi-classical solution to the coupled differential equations generated by Heisenberg operator equations, based on a Hamiltonian including the following effects: charging and Josephson energies of the junctions, junction dissipation, effect of a dc bias current, and an undamped simple harmonic oscillator representing the NEM. Synchronization of the array is signaled by a step in the current- voltage (I-V) curve. Stability analysis reveals that the phase-locked junctions are neutrally stable at the bottom and top of the step. We calculate an analytic expression for the location of the resonance step in the I-V curve. We also find it is possible to set a desired number $N_{a} \quad \le N$ of junctions on the resonance step, with $N_{a}$ --$N $junctions in zero-voltage state.   

Effect of Microwaves on the Current-Phase-Relation of diffusive SNS Junctions

We investigate the current-phase-relation (CPR) of long diffusive superconductor - normal metal - superconductor (SNS) Josephson junctions under microwave irradiation. The samples consist of narrow Ag bridges with a length between 300 and 500 nm inserted into a Nb loop by shadow evaporation on top of a mesoscopic Hall cross. Our Hall-sensors are based on high mobility GaAs/AlGaAs- heterostructures. They directly detect the magnetic response of the loop to an external magnetic field, from which the full CPR can be reconstructed. The measurements are done in the high-temperature regime $E_{Th} [Preview Abstract]

Generation of Microwave Single Photons and Homodyne Tomography on a Chip

We show that flux-based qubits can be coupled to superconductive resonators by means of a quantum-optical Raman excitation scheme and utilized for the deterministic generation of propagating microwave single photons. We introduce also a microwave quantum homodyning technique that enables the detection of single photons and other weak signals, and full state reconstruction via quantum tomography, realizing linear optics on a chip. These generation and detection protocols are building blocks for the advent of quantum information processing in the field of circuit QED (M. Mariantoni {\em et al.} cond-mat/0509737) . We discuss further applications of these ideas to create multipartite nonclassical states of the electromagnetic field.   

Crossover from single electron counting to Cooper pair counting

We present experimental studies of charge transport in a one-dimensional array of Josephson junctions using a single charge counting device based upon a radio-frequency single-electron transistor$^1$. We observe a crossover from time-correlated tunneling of single electrons to Cooper pairs as a function of an applied magnetic field. At relatively high magnetic field, single electron transport dominates and the frequency is given by f=I/e. As the magnetic field is lowered the frequency gradually shifts to f=I/2e, indicating tunneling of Cooper pairs. $^1$Jonas Bylander, Tim Duty and Per Delsing, Nature 434 (2005) 285.   

1/f Noise in Josephson Junctions

A major obstacle to the realization of Josephson junction qubits is decoherence due to noise. Our goal is to understand the microscopic mechanisms which lead to the 1/f critical current noise spectrum at low temperatures. One possible source of critical current fluctuations is the presence of defects such as two level systems in the insulating junction barrier. We present a model of the critical current noise spectrum and compare it with recent experiments.   

Decoherence in Josephson Vortex Quantum Bits

We investigated decoherence of a Josephson vortex quantum bit (qubit) in dissipative and noisy environment. As the Josephson vortex qubit is fabricated by using a long Josephson junction (LJJ), we use the perturbed sine-Gordon equation to describe the phase dynamics representing a two-state system and estimate the effects of quasiparticle dissipation and weakly fluctuating critical and bias currents on the relaxation time $T_1$ and on the dephasing time $T_\phi$. We show that the critical current fluctuation does not contribute to dephasing of the qubit in the lowest order approximation. Modeling the weak current variation from magnetic field fluctuations in the LJJ by using the Gaussian colored noise with long correlation time, we show that the time $T_2$ is limited by the low frequency current noise at very low temperatures. Also, we show that a ultra-long coherence time may be obtained from the Josephson vortex qubit by using experimentally accessible value of physical parameters.   

Phase-space theory for nonlinear detectors of superconducting qubits

Superconducting circuits are envisioned as quantum bits and demonstrate quantum-coherent features i.e. Rabi oscillations and Ramsey fringes. The detector (e.g. a superconducting quantum interference device, SQUID) can itself be described by a Hamiltonian and treated quantum-mechanically.This allows more insights into the measurement process. Several experimental groups have recently realized good detectors with strong coupling to the measured system, where nonlinear dynamics plays a significant role. Motivated by the recent experiment [1], we study a nonlinear detector where the qubit couples to the square amplitude of a driven oscillator, which can be used for dispersive detection. We use a complex-environment approach treating the qubit and the oscillator exactly, expressing their full Floquet-state master equations in phase space. We investigate the backaction of the environment on the measured qubit and explore the resolution of measurement. We discuss the possibility for using the squeezing capability of the nonlinear interaction for beating the standard quantum limit and emphasize the resulting role of non-Gaussian and non-Markovian effects in the backaction including significant non-exponential shape of the coherence decay. \\ {[}1{]} A. Lupascu et al. PRL 93 177006 (2004)   

Quasiparticle Poisoning in a Cooper Pair Box caused by a measuring SET

We have developed a model to calculate the average charge on a Cooper pair box in the presence of quasiparticle poisoning. The model uses a master equation approach to find the probabilities for the box to be in the even or odd state. The transition rates between the two states are calculated assuming a fixed number of non-equilibrium quasiparticles in the leads and island of the box. We fabricated Al/AlOx/Al devices with a Cooper pair box capacitively coupled to an SET and measured the charge on the box for SET bias currents ranging from about 1 pA to 1 nA. We find good agreement between the theory and measurements in the temperature range from 60 mK to 300 mK. For large SET bias the poisoning in the Cooper pair box increases and the charge staircase develops additional features. Our model is capable of qualitatively reproducing the features induced by the measuring SET.   

Cavity Josephson Bifurcation Amplifier: a microwave readout for a superconducting qubit

A Josephson junction, embedded in a microwave circuit that displays a resonance, and driven near the resonance frequency by a sinusoidal signal with adequate amplitude, can adopt one of two dynamical metastable states. The transition between the two states can be triggered by a small variation in the environment of the junction. This switching phenomenon naturally lends itself to the readout of a superconducting quantum bit. We are approaching the problem of mapping the two states of a qubit onto the two dynamical states of the Josephson junction by placing it in an on-chip coplanar waveguide superconducting cavity. We present the characterization of the cavity Josephson bifurcation amplifier (CJBA) and show that it follows theoretical predictions over a wide range of operating frequencies and bandwidth. This architecture provides a calculable RF environment which can be readily optimized. We also discuss a multi-resonator chip geometry that would implement the multiplexed readout of more than 10 qubits.   

Quantum dynamics and leakage of the superconducting flux qubit

We study the quantum dynamics of the superconducting flux qubit of Mooij et al, which consists on a SQUID with 3 Josephson junctions. We simulate the corresponding time-dependent Schrodinger equation and also eigenfunctions and eigenvalues are computed. We calculate the dynamical evolution of the qubit device after a pulse in magnetic field is applied from the operational point at half flux quantum. The system is started in the lowest energy level and we calculate the leakage into the non-computational basis after the pulse is applied, computing the change in the average population of the two lowest energy levels. The leakage is analyzed for different pulse intensities. Two different regimes are found for weak and strong pulses. We discuss the relationship of the response to strong pulses with the quantum chaoticity of the spectrum of high energy levels outside the computational space.   

Session W40: Quantum Communication, Cryptography and Computation

Quantum Communication via Frequency Upconversion

We describe a method for efficiently and coherently converting photonic qubits from one frequency to another for quantum communication. The conversion is done using quasi-phase-matched up-conversion in a Periodically Poled Lithium Niobate (PPLN) crystal. We have observed 99\%-efficient and 95\%-coherent conversion which allows faithful conversion of ``flying'' qubits to ``stationary'' qubits for use in quantum communication. We have also used up-conversion to prepare photons in arbitrary superpositions of widely separated frequency states, enlarging the accessible Hilbert space for communication of quantum states. Finally, we have seen 56\%-efficient detection of 1550-nm photons using up-conversion to the visible and silicon Avalanche Photodiodes (APD), which would enhance the performance of quantum communication protocols (e.g., BB84) based on infrared (IR) photons over what is achievable with conventional IR single-photon detectors.   

Relativistic Quantum Cryptography

We present results from a relativistic quantum cryptography system which uses photon storage to avoid bit sifting, in principle doubling the useful key rate. Bob stores the photon he receives from Alice in an optical delay line until she sends him the classical basis information, allowing him to measure every photon in the correct basis. Accounting for loss in our 489-ns storage cavity, we achieve a 66\% increase in the BB84 key rate. The same system could be used for even greater gains in either the six-state protocol or cryptography using a larger Hilbert space. We show that the security of this protocol is equivalent to standard BB84: assuming the quantum and classical signals are space-like separated, no eavesdropper bound by special relativity can access both simultaneously.   

Quantum Cryptography in Existing Telecommunications Infrastructure

Quantum cryptography has shown the potential for ultra-secure communications. However, all systems demonstrated to date operate at speeds that make them impractical for performing continuous one-time-pad encryption of today's broadband communications. By adapting clock and data recovery techniques from modern telecommunications engineering practice, and by designing and implementing expeditious error correction and privacy amplification algorithms, we have demonstrated error-corrected and privacy-amplified key rates up to 1.0 Mbps over a free-space link with a 1.25 Gbps clock. Using new detectors with improved timing resolution, careful wavelength selection and an increased clock speed, we expect to quadruple the transmission rate over a 1.5 km free-space link. We have identified scalable solutions for delivering sustained one-time-pad encryption at 10 Mbps, thus making it possible to integrate quantum cryptography with first-generation Ethernet protocols.   

Alternative Design for Quantum Cryptographic Entangling Probe

An alternative design is given for an optimized quantum cryptographic entangling probe for attacking the BB84 protocol of quantum key distribution [1], [2]. The initial state of the probe has a simpler analytical dependence on the set error rate to be induced by the probe than in the earlier design. The new device yields the same maximum information to the probe for a full range of induced error rates. As in the earlier design, the probe contains a single CNOT gate which produces the optimum entanglement between the BB84 signal states and the correlated probe states. \newline \newline [1] H. E. Brandt, Phys. Rev. A \textbf{71}, 042312 (2005). \newline [2] H. E. Brandt, ``Design for a quantum cryptographic entangling probe,'' to appear in J. Mod. Optics (2005).   

$\pi/3$ Phase-Shift Quantum Searching

Quantum searching normally consists of an alternate sequence of selective inversion and diffusion operations. The algorithm has been extensively studied and is well understood. However, there was a surprising result that was discovered last year. According to this, if we change the selective inversions to $\pi/3$ phase shifts and adjust the sign of the phase shift in a prescribed manner, we obtain an algorithm that converges monotonically towards the solution [1]. This is in contrast to the well-known search algorithm that has an oscillatory character. This leads to a number of new and interesting applications. For example, if we consider a situation where the probability of getting a target state for a random item, is $1-\epsilon$ (with $\epsilon$ unknown), then the probability of getting a target state after a single query in the new algorithm, can be increased to $1-\epsilon^3$, classically this can be increased to only $1-\epsilon^2$. The performance of the new algorithm has recently been proved to be optimal. Another important application of this technique is in correction of systematic errors [2]. \newline References - \newline (1) L.K. Grover (2005), Fixed-point quantum search, Phys. Rev. Letters, Oct. 3, 2005. \newline (2) B.W. Reichardt and L.K. Grover, Quantum error correction of systematic errors using a quantum search framework, Phys. Rev. A, Oct. 25, 2005   

Relativistic Connection of Continuous and Discrete Quantum Walks

Quantum algorithms, based on a quantum-mechanical generalization of random walks, have been shown to be very effective at solving local search problems. These quantum walks come in two very different forms (discrete and continuous-time) with surprisingly similar properties. An open problem has been to identify just what makes these two walks so similar. In this talk I present the analytical connection of these two walks, by way of an analogy with properties of the Dirac equation, including entanglement, zitterbewegung, and most importantly, relativistic wave-packet spreading.   

Mixing and Decoherence in Continuous-Time Quantum Walks

We present analytical results showing that decoherence can be useful for speed-up of mixing in a continuous-time quantum walks on finite cycles. Our treatment of continuous-time quantum walks includes a continuous monitoring of all vertices that induces the decoherence process. We identify the dynamics of the probability distribution and observe how mixing times undergo the transition from quantum to classical behavior as our decoherence parameter grows from zero to infinity. Our results show that, for small rates of decoherence, the mixing time improves linearly with decoherence, whereas for large rates of decoherence, the mixing time deteriorates linearly towards the classical limit. In the intermediate region of decoherence rates, our numerical calculations confirm the existence of a unique optimal rate for which the mixing time is minimized.   

Decoherence by Correlated Noise and Quantum Error Correction

We study the decoherence of a quantum computer in an environment which is inherently non-Markovian and spatially correlated. We first derive the non-unitary time evolution of the computer and environment in the presence of a stabilizer error correction code. Our results demonstrate that effects of long-range correlation can be systematically reduced by suitable changes in the error correction codes. The new element that we discuss is that the periodic measurements in the QEC method separate the environmental modes into high and low frequencies. This natural ``new'' scale can then be used to better engineer quantum codes. As an example of this general discussion, we study decoherence in a quantum memory protected by Steane's three qubit code. The memory interacts with a bosonic enviroment through the spin-boson Hamiltonian. We calculate explicitly the long-range correlations in this case and demonstrate that a simple change in Steane's code reduces their effect.   

Topological Quantum Computing with Only One Mobile Quasiparticle

In a topological quantum computer, universal quantum computation is performed by dragging quasiparticle excitations of certain two dimensional systems around each other to form braids of their world lines in 2+1 dimensional space-time. We show that any such quantum computation that can be done by braiding $n$ identical quasiparticles can also be done by moving a single quasiparticle around $n-1$ other identical quasiparticles whose positions remain fixed. This result may greatly reduce the technological challenge of realizing topological quantum computation.   

CNOT for Fibonacci anyons with only one mobile quasiparticle

Certain two-dimensional systems with non-abelian quasiparticle excitations can be used for topological quantum computation (TQC). In TQC qubits are encoded using 3 or 4 quasiparticles and quantum gates are carried out by braiding quasiparticle world lines. We focus on the problem of finding explicit braiding patterns that yield a universal set of quantum gates, using Fibonacci anyons --- quasiparticles which are thought to exist in an experimentally observed fractional quantum Hall state at filling fraction $\nu = 12/5$. In previous work\footnote{N.E. Bonesteel, L. Hormozi, G. Zikos, and S. H. Simon, Phys. Rev. Lett. {\bf 95}, 140503 (2005).} we have shown how to construct arbitrary controlled rotation gates (which together with single qubit gates provide a universal set of quantum gates) by moving a pair of quasiparticles from the control qubit around the quasiparticles in the target qubit while keeping the latter at fixed positions. In this talk we show how to take advantage of one of the structural properties of Fibonacci anyons (namely the fusion matrix) to construct a certain class of two-qubit gates (including CNOT) with only {\it one} mobile quasiparticle --- therefore reducing the number of braiding operations by a factor of two.   

Quantum Phase Transitions and Typical Case, Polynomial Time Solution of Randomly Generated NP-Complete Problems via Adiabatic Quantum Computation

We argue theoretically that adiabatic quantum computation using only polynomial resources can solve almost all members of a nontrivial randomly generated set of NP-complete problem instances, namely the problem of finding the ground states of spin glasses on 3D cubic lattices having independent, identically Gaussian-distributed couplings. The argument uses the droplet model of quantum spin glasses, particularly its prediction that the paramagnet-spin glass transition is unstable to even infinitesimal longitudinal fields. We then review the ongoing debate as to how well the droplet model describes 3D spin glasses and note that those inclined to view the intractability of NP-complete problems as a guiding physical intuition could take the results presented here as justifying greater suspicion toward the droplet model. Finally, due to this uncertainty as well as uncertainty in regard to the typical case classical complexity of this random NP-complete problem, we outline work using rigorous mean-field methods on a NP-complete problem whose typical-case classical complexity on random instances is better established, namely MAX CLIQUE on random graphs.   

Adiabatic Quantum Computing in systems with constant inter-qubit couplings

We propose an approach suitable for solving NP-complete problems via adiabatic quantum computation with an architecture based on a lattice of interacting spins (qubits) driven by locally adjustable magnetic fields. Interactions between qubits are assumed constant and instance-independent, programming is done only by changing local magnetic fields. Implementations using qubits coupled by magnetic-dipole, electric-dipole and exchange interactions are discussed.   

Quantum Phase Transition and complexity of adiabatic quantum algorithm for Constraint Satisfaction problem

We study the dynamics of adiabatic quantum computation (AQC) for solving the problem of satisfiability of randomly chosen clauses, each with 3 Boolean variables (3sat). We map this problem to that of a diluted long-range spin glass in traverse magnetic field and derive a self-consistent equation for the order parameter. We show the existence of the first-order quantum phase transition and investigate analytically and numerically the phase diagram on the plane: strength of the transverse field $\Gamma$ vs the ratio $\gamma$=M/N of a number of clauses M to a number of variables N. We show that the phase transition line approaches $\Gamma$=0 at the point of a classical replica symmetry breaking transition $\gamma_{RSB}$. We discuss the implications of the quantum phase transition for the complexity of the AQC for the 3sat.   

Computation in Finitary Quantum Processes

We introduce quantum finite-state generators as a first step toward a computational description of quantum dynamical processes. We developed their mathematical foundations, establishing probability conservation, reversibility, and consistency with quantum mechanical laws, and connect the class to the existing theory of finite-state recognizers and generators. These computational models allow for a quantitative description of quantum languages generated by quantum dynamical systems. Their descriptive power is explored via several example quantum dynamical systems.   

Some Thoughts Regarding Practical Quantum Computing

Quantum computing has become an important area of research in computer science because of its potential to provide more efficient algorithmic solutions to certain problems than are possible with classical computing. The ability of performing parallel operations over an exponentially large computational space has proved to be the main advantage of the quantum computing model. In this regard, we are particularly interested in the potential applications of quantum computers to enhance real software systems of interest to the defense, industrial, scientific and financial communities. However, while much has been written in popular and scientific literature about the benefits of the quantum computational model, several of the problems associated to the practical implementation of real-life complex software systems in quantum computers are often ignored. In this presentation we will argue that practical quantum computation is not as straightforward as commonly advertised, even if the technological problems associated to the manufacturing and engineering of large-scale quantum registers were solved overnight. We will discuss some of the frequently overlooked difficulties that plague quantum computing in the areas of memories, I/O, addressing schemes, compilers, oracles, approximate information copying, logical debugging, error correction and fault-tolerant computing protocols.   

Session W41: Cold Fusion

Cold Fusion – A 17 Year Retrospective

Seventeen years after the APS voted to refute the reality of Cold Fusion in Baltimore, it is appropriate to consider what has changed. Who was right? We will review the current state of knowledge from the perspective of what we know now compared to what we knew then. Discussion will be made of various avenues of research that have followed from the original Fleischmann-Pons proposal: some failed, some unresolved and some successful.   

Recent Developments in Cold Fusion / Condensed Matter Nuclear Science

Krivit is recognized internationally as an expert on the subject matter of cold fusion / condensed matter nuclear science. He is the editor of $New$ $Energy$ $Times$, the leading source of information for the field of cold fusion. He is the author of the 2005 book, $The$ $Rebirth$ $of$ $Cold$ $Fusion$ and founder of New Energy Institute, an independent nonprofit public benefit corporation dedicated to accelerating the progress of new, sustainable and environmentally friendly energy sources.   

Role of Finite Size in Triggering Excess Heat: Why Nanoscale PdD Crystals Turn on Faster

Two persistent questions have been: 1. Why is a finite triggering time required after the near full-loading condition (PdD$_x$, 0.85 \approx < x \rightarrow1$) before the Excess Heat effect\footnote{ C.G. Beaudette, \underline{Excess Heat: Why Cold Fusion Research Prevailed.} (Oak Grove Press, Bristol, ME, 2002)} is observed? 2. Is it possible to identify physical properties of the materials and/or crystals that are used that might be playing a role in the length of the interval of time associated with this phenomenon? In the talk, through a generalization\footnote{ S.R. Chubb, ``Role of Broken Gauge Symmetry on Conduction of Charged and Neutral Particles in Finite Lattices,'' submitted to Proc Roy. Soc Series A (2005).} of conventional energy band theory, as it applies to infinitely-repeating, periodic lattices to situations involving finite lattices, I have been able to address both questions. In particular, the tunneling time depends on crystal size. Crystals with dimensions $\approx <$6 nm, which have tunneling times $\approx$ microseconds, either can not provide enough momentum to initiate d+d$\rightarrow ^4$He reactions or conduct ion charge so rapidly that collisions occur. Crystals with dimensions $\approx$ 60nm create heat and load rapidly ($\approx$ 3 ms). But crystals with dimensions $>\approx$60 microns have tunneling times that are longer than a month.   

Resolving the Laughlin Paradox

For paired Bloch electrons in a metal not subject to Pauli exclusion, the 2-electron Hamiltonian has the form\\ \\ $H=$$-\hbar ^2\over{4m_e}$$\Delta$$_c_m+(2e) U_l_a_t_t_i_c_e(r_c_m,N_c_e_l_l) +$$e^2\over{( N_c_e_l_l r_1_2)}$$-$$\hbar ^2\over{3m_e}$$\Delta$$_1_2$, \\ \\ where $r_c_m = r_1 +r_2, r_1_2 = r_1 - r_2$, and $r_1$ and $r_2$ are position vectors in configuration space, involving independent Bravais vectors $R_1$ and $R_2$ , such that $R_1 - R_2 = R_1_2$ is an independent Bravais lattice vector, and N$_c_e_l_l$ is the number of mutually shared potential wells over which the 2 electrons are coherently partitioned with entangled local density maxima. At large N$_c_e_l_l$, the magnitude of term 3 $<<$ the magnitude of term 1. When coordinate exchange symmetry is satisfied and energy minimized, term 3 cancels term 1 at $r_1_2= 0$, eliminating the singularity in the wave equation, thereby resolving Laughlin's paradox\footnote{R.B. Laughlin, ``A Different Universe'', (Basic Books, Cambridge MA, 2005) pp. 84-85.}   

Dynamics of Non-linear Soft X-Ray Emission from a Plasma Discharge-Driven Hydride Target

A high current discharge apparatus with a pulsed power supply has been constructed and successfully demonstrated an intense soft x-ray ( $>$ 600 eV) emission during bombardment by a 300 V deuterium plsma discharge. Emission is delayed until $\sim$1/2 ms into the msec voltage pulse\footnote{G, Miley, et al., Trans. ANS, Washington, DC (Nov. 2005)}. Both electron and ion Bremsstrahlung have been ruled out as significant contibutions to the emission. A possible mechanism to explain this highly nonlinear x-ray emission is collective generation of soft x-ray quanta induced by a coherent D-diffusion process near the cathode's surface. This combined with continuous high current deuteron bombardment results in the penetration of recoil deuterons into the inner electron shell of the cathode material, generating x-ray emission.   

Control of Tardive Thermal Power

Previously, calorimetric improvements including thermal power analysis, dual ohmic controls, noise measurement and time-integration of multi-ring calorimetric systems with waveform reconstruction has led to the development of Phusor$^{TM}$ devices providing undeniable proof of excess heat in palladium heavy water (Pt/$D_2O$/Pd; ~0.5 cm$^3$, peak excess power ratios of 2.30 $^{+/-}$ 0.84; 1). We now report improved control of tardive thermal power (TTP) which develops long after the termination of electric input power. From an engineering perspective, this is important because the effective excess power generated is further greatly increased (up to an additional $\sim 410\%$ beyond that obtained without tardive thermal power operation); and because this improved means of operation can be coupled into over-unity motors and other work-producing systems. In addition, these systems have revealed further insight into the kinetics of the desired condensed matter reactions.   

Progress in Excess of Power Experiments with Electrochemical Loading of Deuterium in Palladium

A research activity has been carried out, during the last three years, in the field of triggering anomalous heat effects in palladium deuteride. An enhancement of the excess of power reproducibility in deuterated palladium was obtained by using HeNe laser irradiation during electrochemical loading. A preliminary correlation between excess of energy and helium-4 concentration increasing above the background was found. The continuation of the experimental program confirmed that laser triggering produces an interesting gain of reproducibility. An upgrade of the experimental set-up has been realized.   

Cavitation Foil Damage

We have developed a much improved cavitation system for sonofusion, compared to our initial systems. The new system is a low mass 1.6 MHz unit that produces 40 watts of excess heat with an acoustic input power of 17 watts. The increase in frequency (to 1.6 MHz from 40 KHz) increases the heat, improves the performance, shows reproducible results, and indicates durability. The calorimetry is a simple in flow through system. The difference between output and input temperature (T$_o$$_u$$_t$ - T$_i$$_n$) at steady- state, times the flow gives the power (calories/s) output of the sonofusion reactor. The energy density of this system is of the order of commercial energy suppliers.   

Isoperibolic Calorimetry Applied To The Pt/D$_2$O Blank System

Doubts have often been expressed about the precision and accuracy of isoperibolic calorimeters where the heat transfer is controlled by radiation across the vacuum gap of the Dewar cells. Therefore, experiments were conducted on blank systems consisting of Pt cathodes polarized in 0.1 M LiOD/D$_2$O. Both the differential and intergral heat transfer coefficients were evaluated, and the latter based on backward integration of the data sets should be used for accurate evaluations of the experimental data. The heat transfer coefficients obtained are in agreement with values given by the product of the Stefan-Boltzmann coefficient and the radiant surface area. It is shown that the precision of this calorimetry is better than 99.99 percent while the accuracy is close to this figure. This high precision and accuracy allows the determination of the rate of enthalpy generation due to the reduction of oxygen electrogenerated in the cell. This rate was 0.0011 W for oxygen reduction whereas the input enthalpy to the cell was about 0.8 W for these experiments.   

New Mechanism of Low Energy Nuclear Reactions Using Superlow

We proposed a new mechanism of LENR (low energy nuclear reactions)\footnote{ F.A. Gareev, I.E. Zhidkova, E-print arXiv Nucl-th/0511092 v1 30 Nov 2005.}: cooperative processes in the whole system - nuclei+atoms+condensed matter can occur at smaller threshold than the corresponding ones assoiciated with free constituents. The cooperative processes can be induced and enhanced by (``superlow energy'') external fields. The excess heat is the emission of internal energy, and transmutations from LENR are the result of redistribution of the internal energy of the whole system. A review of possible stimulation mechanisms of LENR is presented. We have concluded that transmutation of nuclei at low energies and excess heat are possible in the framework of the known fundamental physical laws: The universal resonance synchronization principle\footnote{ F.A. Gareev, In: FPB-98, Novosibirsk, June 1998, p.92; F.A.Gareev, G.F. Gareeva, in: Novosibirsk, July 2000, p.161.}, and, based on it, different enhancement mechanisms of reaction rates are responsible for these processes\footnote{ F.A. Gareev, I.E. Zhidkova and Yu.L. Ratis, Preprint JINR P4-2004-68, Dubna, 2004.}$^,$\footnote{F.A. Gareev, I.E. Zhidkova, E-print arXiv Nucl-th/0505021 9 May 2005}. The excitation and ionization of atoms may play the role of a trigger for LENR.   

Comments on Summary of Condensed Matter Nuclear Science

Research involving investigations of the production of tritium in electrolytic cells was a topic that was recommended by the Energy Research Advisory Board(ERAB) report of the U. S. Department of Energy (DOE) in November, 1989. Fifteen years later, the evolution of related research has proven that this was an important recommendation. In the talk, a selective resonant tunneling model is used to attempt to explain the initial discoveries of tritium production. Deuterium flux might play a key role for solving the problem of reproducibility. A further investigation is suggested, based on this model.   

Excess heat observed during electrolysis of deuterated phosphoric acid with palladium electrodes and a solid state electrolyte in deuterium gas

We start with the hypothesis that the production of excess heat is occurring at the recombination H+H$\rightarrow$H$_2$ gas. If the pressure of hydrogen at the time of recombination is high enough, nuclear reactions can occur. In the case of hydrogen H+H$\rightarrow$D+e$^+$ and in the case of deuterium D+D$\rightarrow$He$^{-4}$. The high pressure can be obtained using Nernst's law, the potential between a hydrogen electrode and the cathode is given by E=E$_o$+RTln($P\over{P_o}$). There are two sources for the potential: the electrochemical potential which is a characteristic of the metal in the presence of the metal ions, and on the other side,the over-potential for the formation of the hydrogen molecules. In this study we use palladium anodes and cathodes, but the cathode is covered with a thin film of a metal having either a low chemical potential or a high over-voltage for hydrogen formation. When deuterium molecules form at the surface of the electrode, very high pressures can be produced during a very short period of time during which possible nuclear reactions can happen. We show that excess heat is observed with clean palladium foils, and more excess heat is produced when the cathode is covered by a thin, metallic film, constructed using one of many possible metals.   

Creating an International Scientific Society as an Act of Scientific Rebellion

When a new science is born, it is often necessary to unite dispersed groups of researchers all over the world. In this talk, I intend to describe the process of constituting and managing a new international scientific society covering such diverse issues as: 1.) Rationale; 2.) Initial feelers; 3.) Achieving consensus in the international community; 4.) Choice of jurisdicition; 5.) Corporate format; 6.) Establishing international pre-eminence in law; 7.) Reducing expenses; 8.) Tax minimization; 9.) Decision making; 10.) Democracy \& transparency; 11.) Raising funds; 12.) Rewarding excellence; 13.) Online publishing; 14.) Organizing meetings. These issues will be covered with reference to the history of the International Society of Condensed Matter Nuclear Science, which will celebrate its second birthday in March 2006. It currently has nearly 200 members from 23 different countries.   

Session W42: Focus Session: Simulations of Matter at Extreme Conditions III

Ab Initio Studies of High Pressure States of Crystalline Nitromethane.

We have calculated the mechanical compression curve for solid nitromethane with the ab-initio periodic structure code CRYSTAL using both Hartree-Fock and Density Functional Methods. In addition, calculations with both 6-21G and 6-31G** basis sets were performed and the effect of basis set superposition error was estimated using the counterpoise method. In each calculation the internal atomic coordinates and the crystal lattice parameters were relaxed at constant unit cell volume to the minimum energy configuration. The 6-31G** basis set was optimized by scaling the outer valence and polarization orbitals. It was found that Hartree-Fock calculations with a 6-21G basis set, uncorrected for basis set superposition error, gave the best agreement with experiment. These results may be due to the cancellation of basis set superposition error with dispersion force errors. While this result may be accidental, it appears that it extends to a number of other energetic organic molecular crystals, including beta HMX, PETN, and 1,1-diamino-2,2-dinitroethylene.   

Atomistic Studies of Plastic Deformation and Dissipation in Crystalline HMX

We are using large scale molecular dynamics simulations of crystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) to better understand the dominant fundamental mechanisms of inelastic deformation and other dissipative processes in anisotropic organic molecular crystals. A fully flexible force field (Smith, G. D. and Bharadwaj, R. K.;\textit{ J. Phys. Chem. B} 1999, 103, 3570) used in numerous preceding studies is used without modification in the present work. Our results, based on the results of simulations containing 25,000-250,000 molecules, indicate a large degree of directional anisotropy in response to compression, for both quasi-static and shock loading. Plastic deformation is observed for some loading directions whereas solid-solid phase transitions are observed for others. The emphasis of the present talk will be identifying and characterizing detailed molecular mechanisms and rate dependencies in those cases for which dislocation-induced plasticity occurs.   

First-principles Study of Shock Compressed Carbon

The phase diagram of carbon at high pressures and temperatures is of scientific interest to material science, geology and astrophysics. Major issues include the liquid-liquid phase transition, the melting curve of graphite and diamond, the nature of the liquid state and the nature of carbon in the interior of Uranus and Neptune. Strong shock waves generated by lasers, and even nuclear explosions have been used to study carbon at these extreme conditions. Because it is often difficult to replicate these shock-wave experiments, first-principles electronic structure calculations can play a prominent role in verifying, guiding, and interpreting these experiments. We report DFT results for the diamond Hugoniot.   

Molecular dynamics simulation of shock compression of silicon

Shock compression of condensed matter is a fascinating scientific field that provides an excellent opportunity to probe the fundamental physics and chemistry of matter at extreme pressures and temperatures. In spite of substantial theoretical and experimental efforts, a full understanding of shock-induced elastic and plastic responses and polymorphic phase transitions is still far from complete. These phenomena often occur at the nanometer size and picosecond time scales, which makes molecular dynamics simulations an ideal tool for exploring nanoscale mechanisms of shock induced processes such as chemical reactions and phase transitions. We report the results of a molecular dynamics simulation of shock wave propagation in silicon in the [100], [110], and [111] directions obtained using a classical interatomic potential. Several regimes of materials response are classified as a function of shock wave intensity and crystalline orientation of shock wave propagation using calculated shock Hugoniot. The shock induced chemistry and shock wave splitting are discussed in relation to recent experimental results [1] that indicate an anomalous elastic response of the lattice at high compression ratios. [1] A. Loveridge-Smith, Phys. Rev. Let. \textbf{86}, 2349 (2001).   

Molecular Dynamics Studies of Dynamical High-Pressure Phase Transitions in Rare-Gas Solids

The phase diagram of pair potential models of rare-gases was studied with respect to the effect of the choice of potential on the nature of the phase diagram. In particular the existence of a high-pressure bcc phase is shown to be potential sensitive. We show using molecular dynamics that the fcc-bcc phase transition cannot be reproduced with the Lennard-Jones (12-6) pair potential, though it is reproduced with the Buckingham pair potential. We propose a simple analytical technique, based on the Einstein theory of a harmonic solid, for predicting an fcc-bcc phase transition in a given system. Using the atomic volume and the pair potential as input, we were able to predict the transition temperature. These findings agree with an earlier work by A. B. Belonoshko et al., Phys. Rev. Lett. 87, 165505 (2001). Additionally, shock wave simulations of several model systems were conducted. The structure of shock wave in this model was examined as a function of shock strength and the existence of a dynamic phase transition was explored.   

Interfacial instabilities and structure during high velocity sliding

Interfacial sliding under high pressure loading at high velocities (0 $<$ v $<$ 1 km/s) results in a variety of mesosc ale phenomena at extreme strain rates. For ductile metal interfacial pairs, these include nano- and mesoscale dynami c strucutral transitions, local melting and amorphization, material mixing, and localization of plastic deformation. We illustrate these phenomena with large scale NonEquilibrium Molecular Dynamics (NEMD) simulations for Cu/Ag, Ta/Al, and Al/Al interfaces. These suggest universal behavior in sliding velocity for the frictional force and a scaling form for the frictional force vs. velocity at high velocities which will be discussed.   

Simulations of Rapid Solidification in Metals at High Pressure

Although computer simulation has played a central role in the study of nucleation and growth since the earliest molecular dynamics simulations almost 50 years ago, confusion surrounding the effect of finite size on such simulations have limited their applicability. Modeling molten tantalum in systems ranging from 64,000 to 131,072,000 atoms on the BlueGene/L computer, I will discuss the first atomistic simulations of solidification that demonstrate independence from finite size effects during the entire nucleation and growth process, up to the onset of coarsening. Using both our new results and historical data, we show that the observed maximal grain sizes for systems smaller than about 8,000,000 atoms are functions of the simulation size, following the predictions of finite size scaling theory. For larger simulations, a crossover from finite size scaling to more physical size-independent behavior is observed.   

Properties of molten sodium under pressure from first principles theory.

Recent measurements of the melting curve of sodium [1] have found a sharp decline in the melting temperatures from 1000 to 300 K in the pressure range from 30 to 120 GPa. In this study, we investigate the stability and structural properties of solid and liquid sodium at high pressure and temperature using first principles molecular dynamics. The experimental melting curve is reproduced from 0 to 120 GPa. The local structure of the liquid is found to be strongly correlated to the multiple finite temperature crystalline phases of sodium. Based on a quantitative analysis of the structural and electronic properties of the solid and liquid phases, we propose an explanation for the unusual melting curve and a new perspective on the phase diagram of sodium. [1] Gregoryantz et al., Phys. Rev. Lett. 94, 185502 (2005).   

Melting and phase stability of high-density beryllium

First-principles Molecular Dynamics calculations have been performed to determine the liquid vs. solid phase boundary for beryllium up to 250 GPa. Shock Hugoniot curves have been calculated for both solid and liquid beryllium in this range of pressures and temperatures to determine the shock melting onset conditions and pressure range of liquid-solid coexistence. The results of these simulations also provide insights on the problem of relative stability of various crystalline forms of beryllium at high temperature. 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.   

Ab initio simulation of intense short-pulse laser irradiation of metals and semi-conductors

The effect of intense ultra-laser irradiation on crystal stability is not completely elucidated. Ultrashort laser pulses heat electrons to a very high temperature and leave the lattice relatively cool since the heat capacity of electrons is much smaller than that of lattice. This non-equilibrium system can be described as two subequilibrium systems : the hot electrons and a cold lattice. We studied the effect of this intense electronic excitations on the interatomic forces and the possible melting of the underlying lattice for a semi-conductor (Si) and two metals (Al and Au). We used {\it ab initio} linear response to compute the phonon spectrum in the Density Functional Theory framework for several electronic temperatures ranging from 1 to 6 eV. We found that semi-conductors and metals behave in an opposite ways when increasing electronic temperature. Phonon instability appears in silicon at a electronic temperature of $1.5\;\rm{eV}$ inducing the melting of the lattice. Gold samples become more stable. The Debye temperature was deduced from the phonon spectrum and using the Linderman criterion, we showed that gold undergoes a sharp increase of its melting temperature under intense laser irradiation. The same effect is observed for aluminium.   

Nonequilibrium Dynamics of Ultracold Neutral Plasmas

In a number of recent experiments ultracold plasmas (UNPs) have been produced by photoionizing laser-cooled atomic ensembles [1]. Their very low initial kinetic energies suggest that they are created deeply in the strongly correlated regime. Moreover, UNPs are produced far from equilibrium, leading to a complex relaxation dynamics. We present a hybrid-molecular dynamics approach [2], to describe the long-time plasma evolution while fully taking into account the strongly correlated character of the ionic motion. We demonstrate that the method yields an accurate description of recent measurements [2,3] and allows to address problems beyond present experimental capabilities [3]. It turns out that under the conditions in UNPs the commonly applied Bogoliubov assumption about a hierarchy of relaxation timescale becomes invalid, resulting in an unusual relaxation dynamics connected with a wave-like temperature evolution and an ultimate relaxation to a non-equilibrium undercorrelated state.\\ (1) Y.C. Chen et al., Phys. Rev. Lett. 93, 265003 (2004).\\ (2) T. Pohl, T. Pattard and J.M. Rost, Phys. Rev. A 70, 033416 (2004).\\ (3) T. Pohl, T. Pattard and J.M. Rost, Phys. Rev. Lett. 94, 205003 (2005); Phys. Rev. Lett. 92, 205003 (2004).   

Shock-Induced Polarization in Distilled Water

This study is aimed at developing a theoretical model to describe shock-induced polarization in water. The model is based on the notion that polar water molecules tend to align in the shock front due to inertial and stress forces. Analytical formulas for calculation of the shock-induced polarization charge, potential generated by this charge, and accompanied polarization current produced by the shock wave are derived. A comparison with experimental curves for polarization currents suggests that two factors contribute into the measured polarization signal: change of the polarization charge once the wave front enters the sample and change of the sample capacity while the front is progressing across the sample. Good agreement with experimental data on polarization in distilled water leads us to believe that the results obtained bring about a better understanding of mechanisms of shock induced polarization in liquids containing polar molecules.   

Quantum Dynamics of Energy Transfer under Shock Conditions

Classical molecular dynamics (MD) simulations predict efficient energy transfer from translational to vibrational modes near shock fronts in molecular solids. The validity of the classical description of collisional energy transfer under shock conditions has not been tested for extended systems. In this research effort, quantum mechanical (QM) simulations are used to study energy transfer in a system consisting of three collinear diatomic molecules and a stationary wall. A fast-moving projectile diatom collides with its neighbor initiating a collision cascade. The multiplicity of collisions precludes \textit{a priori} prediction of the detailed collision dynamics. The time dependence of the six-degrees-of-freedom wave function is determined using QM time-dependent wave packet methods. Intra- and inter-molecular interactions are described using nearest-neighbor potentials. Probabilities for vibrational excitation and bond rearrangement are predicted as a function of the collision energy of the projectile for differing interaction potentials and atomic masses.   

Session W43: Focus Session: Cold Atoms in Optical Lattices

Macroscopic Quantum Tunneling and Entangled States in Bose-Einstein Condensates

We use a multi-band Hubbard model to study beyond-mean-field effects in macroscopic quantum tunneling of excited states in Bose-Einstein condensates. Our goal is to determine straightforward observables such as the oscillation frequency of kink-like structures between two wells or the propagation speed of such structures on a lattice. As a preliminary step, we present some surprising new results for entangled states of N bosons in two wells.   

Disorder-induced enhancement of phase coherence in trapped bosons on optical lattices

Using numerical methods, we have investigated the effects of disorder on a system of interacting bosons trapped in a one-dimensional optical lattice. Our results show that there is a unique effect at small to moderate strengths of disorder if there is a Mott plateau at the center of the trap in the ordered system - long range phase coherence actually {\it increases} as a result of disorder. The localization effects due to correlation and disorder compete against each other which results in a partial delocalization of the particles in the Mott region leading to increased coherence. Eventually, at large disorder strengths, the phase coherence decreases. In the absence of a Mott plateau at the center, this effect is absent and the phase coherence decreases for all disorder strenghts. Further analysis of the uniform (no trap) system shows that the disordered states belong to the Bose glass phase.   

Cold Atom Optical Lattices as Quantum Analog Simulators for Aperiodic One-Dimensional Localization Without Disorder

Cold atom optical lattices allow for the study of quantum localization and mobility edges in a disorder-free environment. We predict the existence of an Anderson-like insulator with sharp mobility edges in a one-dimensional nearly-periodic optical lattice. We show that the mobility edge manifests itself as the early onset of pinning in center of mass dipole oscillations in the presence of a magnetic trap which should be observable in optical lattices. This work is supported by NSA-LPS and ARO-ARDA.   

Measuring correlation functions in interacting systems of cold atoms

I will discuss two approaches to measuring correlation functions in experiments with cold atoms. The first approach is based on analyzing atom shot noise in the time of flight experiments. I will compare this approach to Hanburry-Brown-Twiss experiments and show that it can be used to probe novel quantum states of cold atoms including paired states of fermions and magnetically ordered states in optical lattices. The second approach relies on interference experiments between extended condensates. I will show that the interference pattern contains information about correlation functions within individual condensates and that the full distribution of the fringe contrast provides information about high order correlation functions. I will discuss possible applications of this method to study Luttinger liquid behavior in one dimensional systems and probe Kosterlitz-Thouless transition in two dimensional condensates.   

Feshbach resonances in optical lattices

In the last few years there has been much excitement in the field of ultracold atomic gases. In a large amount this is due to the use of so-called Feshbach resonances and, in addition, the use of an optical lattice for the atoms. Recently, the first steps have been made to experimentally combine these techniques, which can both be used to tune the interactions between the atoms. Motivated by these developments, we show that the physics of these systems is described by a generalized Hubbard model for which the microscopic parameters are determined by the details of the lattice and the experimentally known parameters of the Feshbach resonance in the absence of the optical lattice. As a particular application we also discuss the phasediagrams of a Bose gas and a Bose-Fermi mixture near a Feshbach resonance in an optical lattice.   

Ramping Fermions in Optical Lattices across a Feshbach resonance

We study the properties of ultracold Fermi gases in a three-dimensional optical lattice when crossing a Feshbach resonance. By using a zero-temperature formalism, we show that three-body processes are enhanced in a lattice system in comparison to the continuum case. This poses one possible explanation for the short molecule lifetimes found when decreasing the magnetic field across a Feshbach resonance. Effects of finite temperatures on the molecule formation rates are also discussed by computing the fraction of double-occupied sites. Our results show that current experiments are performed at temperatures considerably higher than expected: lower temperatures are required for fermionic systems to be used as quantum simulators. In addition, by relating the double occupancy of the lattice to the temperature, we provide a means for thermometry in fermionic lattice systems, previously not accessible experimentally. The effects of ramping a filled lowest band across a Feshbach resonance when increasing the magnetic field are also discussed: fermions are lifted into higher bands due to entanglement of Bloch states. Our results are in good agreement with recent experiments.   

Competing phases in Bose-Fermi mixtures of ultracold atoms in optical lattices

We study mixtures of ultracold bosonic and fermionic atoms, confined to a two-dimensional lattice, with a numerical functional renormalization group (RG) method. The method is an extension of the RG approach to interacting fermions\footnote{R. Shankar, Rev. Mod. Phys. 66, 129 (1994).} which also takes into account couplings of the fermions to bosonic modes.\footnote{S.-W. Tsai, A. H. Castro Neto, R. Shankar, and D. K. Campbell, Phys. Rev. B 72, 054531 (2005).} We obtain the phase diagram of the system for the limit of large bosonic phonon velocity in comparison to the Fermi velocity. The renormalization group method provides the value of the gaps of the various phases, as well as the subdominant orders and the short range fluctuations.   

Vidal’s simulation method applied to two coupled 1D lattices

Recently, a method was developed employing matrix product states to simulate the quantum dynamics of a one dimensional lattice system using an adaptive time stepping technique [G. Vidal, Phys. Rev. Lett. 91, 147902 (1993); Phys. Rev. Lett. 93, 040502 (1994)]. We use this approach to simulate the dynamics of bosons loaded into a double-well optical lattice geometry relevant to recent experiments at NIST [I. Spielman et al., Bull. Am. Phys. Soc. (2005)]. We study a pair of coupled 1D lattices, which can be mapped into a single 1D lattice with next-nearest neighbor interactions.   

The speed of sound in a Bose-Einstein condensate in optical lattices

We have studied the speed of sound of a Bose-Einstein condensate in optical lattices both analytically and numerically. We find that in the one-dimensional case, the speed of sound falls monotonically with increasing lattice strength. However, the trends are different in two and three dimensional cases. In these two cases, when the interaction is strong, the speed of sound also decreases monotonically with increasing lattice strength. But when the interaction is weak, the sound speed first increases then decreases when the lattice strengh increases.   

Instability of a superfluid Bose gas induced by a locked thermal gas in an optical lattice

We use a dissipative Gross-Petaevskii equation derived from the Bose-Hubbard Hamiltonian to study the effect of the thermal component on the stability of a current-carrying superfluid state of a Bose gas in an optical lattice potential. We explicitly show that the superfluid state becomes unstable at certain quasi-momentum of the condensate due to a thermal component which is locked by an optical lattice potential. It is shown that this instability coincides with the Landau instability derived from the GP equation.   

Imaging of diverging correlations close to a quantum phase transition in optical lattices

We suggest real space determination of diverging space-time correlations close to a quantum phase transition from Bose Mott insulator to superfluid in optical lattices. The method relies on interference of either the released cloud or the outcoupled atomic beam with some reference Bose-Einstein condensate. Upon approaching the transition from the Mott phase, the resulting interference pattern represents a set of uncorrelated domains, with a typical size determined by the correlation length $\xi$ as long as it is smaller than a system size $L$. Repetition of the measurements in a progression of $L$ for $\xi > L$ provides crucial information on the critical behavior in the context of the finite size scaling approach. The Hanbury Brown \& Twiss type measurements allow extracting the average spatial correlator, which is insensitive to the expansion time and decaying on distances $\approx \xi$ (for $\xi < L$). The non- destructive scheme employing two outcoupling pulses separated by some time $\tau$ can probe both the spatial and time correlations.   

Attractive Bosons in Optical Lattices

We study the theory of attractive bosons in an optical lattice with a hard-core constraint, limiting on-site occupations to 0, 1, or 2 particles per site. Our goal is to investigate the Boson pairing phase transition. We describe how an off-resonant Raman photoassociation transition [C. Ryu, et. al. Cond-mat/0508201] may be used to generate this model. We explore the properties of this system through a mean-field theory that allows short-range correlations. We write a wavefunction that describes both atomic and molecular superfluid phases, and study properties of the system near the phase transition, including the structure of vortices.   

The repulsive interacting bosons in an one dimensional moving lattice ring

We investigate the properties of the ground state of the repulsive interacting bosons in an one dimensional moving lattice ring, and reveal that the superfluid density of the system is a periodic function of the velocity of the lattice. In the weakly interacting limit, the Umklapp process of the mutual scattering of bosons in the moving lattice provide the generation mechanism for the vortices. In the strongly interacting limit, the moving lattice can cause transitions between the Mott insulator and the different superfluid phases carrying vortex with different winding number.   

Session W45: Spin Structure in Magnetic Materials

Magnetism in the diluted induced moment system (La,Pr)$_{6}$Ni$_{2}$Si$_{3}$

Pr$_{6}$Ni$_{2}$Si$_{3}$ crystallizes in a complex hexagonal structure with two distinct Pr sites with no point symmetry, thus the 2J+1=9 ground state levels are CEF-split into singlets. Single-crystal magnetization and specific heat measurements suggest that this splitting is large compared to the ordering temperature. As CEF-induced singlets are nonmagnetic, any ordered magnetic moment is induced by magnetic interactions. Pr$_{6}$Ni$_{2}$Si$_{3}$ orders with a ferromagnetic component parallel to the c-axis at $\sim $40K, whereas La$_{6}$Ni$_{2}$Si$_{3}$ appears to be nonmagnetic. Upon substitution of Pr by La the ordering temperature decreases more rapidly than predicted by a rule of mixtures. For substitutions of more than 50{\%} La, no ordering is observed above 5 K. For 50{\%} La, both AC and DC magnetization measurements suggest superparamagnetic or spin glass type behavior. This behavior will be discussed in terms of clusters of Pr ions with size-dependent moment and anisotropy.   

Ferromagnetism in Ba$_2$NaOsO$_6$

Due to the extended nature of 5d orbitals, magnetism in systems of 5d electrons is uncommon. Here we present results of structural, thermodynamic and optical reflectivity experiments on single crystals of the novel magnetic material Ba$_2$NaOsO$_6 $. The material has a double perovskite structure, space group Fm-3m, with full occupancy of all sites. The osmium ions have a 7$^+$ valence, corresponding to a 5d$^1$ electron configuration. The effective moment at high temperatures is 1.10 $\mu_B$, somewhat less than the spin-only value due to spin-orbit coupling, with no apparent anisotropy. The Weiss temperatures are -11.0 and -12.4 $\pm$ 0.4 K for fields orinted along the [100] and [111] directions respectively. At 6.8 K a sharp anomaly in the heat capacity indicates the onset of long range magnetic order. The ordered state is characterized by a small ferromagnetic moment of just 0.2 $\mu_B$ per formula unit, with only a slight anisotropy, indicative of a helical magnetic structure. Infra red reflectivitity measurements confirm that the material is an insulator.   

Magnetic properties of (R, R', R",...)Ni$_2$Ge$_2$ solid solutions

Anisotropic magnetic properties of single crystals of (R, R', R",...)Ni$_2$Ge$_2$ (R = rare earth) solid solutions will be presented. Whereas the magnetic ordering temperatures and the paramagnetic $\Theta$'s broadly follow de Gennes scaling there are some systematic deviations from this oversimplfied trend. In this talk we will examine these deviations and also discuss the apparent absence of a spin-glass state induced by random magnetic anisotropy for the most highly mixed samples.   

Temperature dependence of magnetization of hcp Gd

Careful magnetization measurements of high purity Gd single crystal have been performed. Our results confirm a strong deviation from the Bloch law for the magnetization at low temperatures, and we demonstrate that this effect is even stronger than had been previously measured on less pure Gd samples. We have analyzed the physical nature of this deviation qualitatively and quantitatively using known theoretical models and the newly obtained results. However, no fully satisfactory agreement between the new experimental data and existing theories has been reached.   

Magnetic phase separation in electron-doped Bi1-xCaxMnO3 systems

The manganite system Bi1-xCaxMnO3 possesses intriguing properties in the low bismuth doping region. In this electron doped region (0.6$<$x$<$1), a ferromagnetic (FM) moment of $\sim $1.2 Bohr magnetons per Mn site is found for x$\sim $0.875. The magnetic moment per Mn site maintains a value $\sim $1/3 the theoretical limit even in fields a high as 60 T. The physical origin of this high moment region is not well understood. Various models including canted ferromagnetism and ferromagnetic clusters hosted by an antiferromagnetic background have been proposed. In our previous work, we have conducted small-angle neutron scattering (SANS) on Bi0.125Ca0.875MnO3 polycrystalline samples as has revealed existence of FM clusters embedded in an AFM background. New 55Mn NMR results give more evidence supporting of this heterogeneous phase model: resonance signals from both AFM and FM phases were identified. More progress from multiple-temperature Bi-L3 edge XAFS measurements will be presented as well. This work is supported by NSF DMR-0209243 and NSF DMR-0512196.   

Physical Properties of Single Crystal EuIn$_{2}$P$_{2}$ and EuGa$_{2}$P$_{2}$

Single crystals of EuIn$_{2}$P$_{2}$ and EuGa$_{2}$P$_{2}$ have been grown by a metal flux method.$^{ }$The EuIn$_{2}$P$_{2}$ material crystallizes in a new hexagonal structure type and orders magnetically at 24 K. The magnetic ordering is anisotropic suggesting a possible canted ferromagnetic magnetic structure. The temperature dependent resistivity data indicate semi-metallic behavior. Negative colossal magnetoresistance is observed at the ordering temperature. The gallium metal analogue, EuGa$_{2}$P$_{2}$, crystallizes in a related monoclinic structure and magnetically orders at a slightly higher temperature. Magnetization, resistivity and specific heat data are presented for both compounds.   

Structural and magnetic properties of single layered manganite Pr0.5Ca1.5MnO4

High resolution neutron powder diffraction and elastic neutron scattering have been used to determine the lattice and magnetic structure of the single layer manganite Pr0.5Ca1.5MnO4. The system becomes charge/orbital ordered (CO-OO) near 300K and antiferromagnetically ordered with a Neel temperature ($T_N$) near 125K, which has CE-type (checkerboard like) structure in the Mn-O plane. At temperatures above $T_N$ but below $T_{CO-OO} $, we discovered an anomalous lattice response around 160K. We discuss the microscopic origin of this lattice distortion and its association with competing CO-OO and antiferromagnetic states.   

Magnetic structure of the Kondo lattice compound CeZn$_{0.6}$Sb$_2$

The new Kondo lattice compound CeZn$_{0.6}$Sb$_2$ has a tetragonal structure with space group P4/nmm and shows ferromagnetic behavior below 2.5 K. The Curie-Weiss temperature is 22 K along the tetragonal $ab$ plane, indicating ferromagnetic interactions in the plane. Along the $c$ axis, however, the Curie-Weiss temperature is -145 K, suggesting antiferromagnetic exchange interaction in this direction [1]. We determined the magnetic structure of CeZn$_{0.6}$Sb$_2$ using single crystal neutron diffraction. ($h$,0,$l$) and ($h$,$h$,$l$) scattering planes were investigated. We found CeZn$_{0.6}$Sb$_2$ orders ferromagnetically at T$_C$=2.5 K. The magnetic structure is collinear with a low temperature ordered Ce moment of 1.3 (1) $\mu_B$ that lies in the $ab$ plane. In addition, we measured the order parameter of the ferromagnetic transition. [1] Studies of the ferromagnetic Kondo lattice system of single crystal CeZnSb$_2$, H. Lee, S. Nakatsuji, Y. Chen, W. Bao, R. Macaluso, J. Chan, T. Park, B. Carter, P. Klavins, J. Thompson, Z. Fisk, BAPS, Session L41, 2005.   

APRES Study of Spin Spiral States in TlCo$_{2}$Se$_{2}$

The electronic structure of TlCo$_{2}$Se$_{2}$ has been measured using high resolution angle resolved photoemission spectroscopy (ARPES). TlCo$_{2}$Se$_{2}$ is thought to be a spin spiral system, but experimental evidence from the electronic structure supporting the spin spiral states has been lacking. The original indication of a spin spiral state came from neutron powder diffraction experiments, and was confirmed in a subsequent neutron diffraction study of single crystals. However, it was not possible to distinguish between the helical structure and the sine-modulated moments since the intensity of the magnetic pattern satellites from neutron diffraction was very low. We used high resolution ARPES to study the electronic structure of single crystals of TlCo$_{2}$Se$_{2}$ cleaved in ultra high vacuum. Our experimental results clearly show the existence of band crossings near the Fermi level that would support spin spiral states, and reveals the predicted quasi-two dimensional electronic structure. The Boston University program is supported in part by the Department of Energy under DE-FG02-98ER45680.   

Magnetic Ordering in Copper Pyrazine Perchlorate, a S=1/2 2D Quantum Heisenberg Antiferromagnet

Copper pyrazine perchlorate, Cu(Pz)$_{2}$(ClO$_{4})_{2}$, consists of antiferromagnetic layers of Cu$^{2+}$ ions bridged by neutral pyrazine molecules. The exchange strength within the layers is J/k$_{B}$ = 17.5 K; excellent isolation between layers is provided by the bulky perchlorate ions. Specific heat studies and muon spin resonance measurements [1] show the ordering temperature to be 4.25 K, corresponding [2] to an interlayer/intralayer exchange ratio of 1 x 10$^{-3}$. The specific heat data show no characteristic anomaly at T$_{N}$, only a broad contribution attributable to the short-range order within the layers. This result is consistent with recent theoretical predictions [3]. The excellent isolation results in the observation of field-induced XY-behavior in the magnetic susceptibility, as predicted elsewhere [4]. 1. J. Manson \textit{et al}, unpublished results. 2. C. Yasuda \textit{et al}, \textit{Phys. Rev. Lett.} \textbf{94}, 217201 (2005). 3. P. Sengupta \textit{et al}, \textit{Phys. Rev. B}, \textbf{68}, 094423 (2003). 4. A. Cuccoli \textit{et al}, \textit{Phys. Rev. B}, \textbf{68}, 060402 (2003).   

Magnetic field dependence of the order parameter in weakly ordered quasi-1D antiferromagnets

We report neutron diffraction study of the antiferomagnetic order in two isostructural quasi-one-dimensional S=1 Heisenberg antiferromagnets, CsNiCl3 and RbNiCl3, in magnetic fields up to 15 T. These materials present model systems of Haldane spin chains with very similar in-chain exchange interaction, but coupled by the inter-chain exchange of different strengths. In both cases the inter-chain coupling is super-critical, so that the Haldane gap is suppressed and weak antiferromagnetic order appears below 4.84 K in CsNiCl3 and 11.1 K in RbNiCl3. In zero field the ordered magnetic moments in CsNiCl3 and RbNiCl3 are approximately 0.9 and 1.2 Bohr magnetons, respectively. We find that the antiferromagnetic order is enhanced by application of the magnetic field, as it is expected both for coupled Haldane chains and from the spin-wave theory arguments. However, we show that the spin-wave theory can not reproduce the observed behavior quantitatively.   

Neutron Diffraction Study of A-site Size and Variance Effect on the Spin and Orbital Ordering in RVO$_{3}$ Perovskites

RVO$_{3}$ members all exhibit an intriguing sequence of orbital and magnetic orderings below a Too and a T$_{N}$, respectively. Since only t-electrons are active in this system, they have been a prototype to study cooperative orbital ordering among the $\pi $-bonding t orbitals in the absence of $\sigma $-bonding e electrons and the relationship of this orbital order to long-range magnetic ordering. We have studied the A-site size and variance effect on the spin and orbital ordering in single phase (YLa)VO3 and (YLaLu)VO3 systems by magnetization, specific heat, thermal conductivity, and nenutron diffraction measurements. The results show that both A-site size and variance stabilize G-type magnetically, C-type orbitally ordered state before an electronic phase segregation takes place. The results will be presented with special emphasis on the neutron diffraction experiments for both single crystal and polycrystalline samples.   

Soft X-ray absorption Spectroscopic Investigation on Electronic Evolutions in SrFe$_{1-x}$Mo$_x$O$_{3\pm\delta}$ (0 $\le x \le$ 1)

A double perovskite Sr$_2$FeMoO$_6$, which an alternative mixture of SrFeO$_3$ and SrMoO$_3$, is a half-metallic ferrimagnet with a high critical temperature $T_C$ $\sim$ 400K, although a certain amount of anti-site disorder diminishes the half-metallicity in a real system. SrFeO$_3$ and SrMoO$_3$, which have the ionic states of Fe$^{4+}$ and Mo$^{4+} $, are known to be an antiferromagnetic insulator and a non- magnetic metal, respectively. However, as they forms the double perovskite, the ionic states primarily form Fe$^{3+}$ (3$d^5_ {\uparrow}$) and Mo$^{5+}$ (4$d^1_{\downarrow}$) , and then the Fe 3$d$-Mo4$d$ hybridization makes a down spin band at the Fermi level. Hence the real ground state becomes a state with Fe$^{2+} $-Fe$^{3+}$ and Mo$^{5+}$-Mo$^{6+}$ mixed valences. It means that the valences of Fe and Mo can vary by two, i.e. from Fe$^ {4+}$ to Fe$^{2+}$ and from Mo$^{4+}$ to Mo$^{6+}$ in SrFe$_{1-x} $Mo$_x$O$_{3\pm\delta}$ (0$\le x \le$1), respectively. Here we presents the structural, electrical, and magnetic phase diagram and electronic evolutions in SrFe$_{1-x}$Mo$_x$O$_{3\pm\delta}$ (0$\le x \le$1).   

Electronic bandstructure of the NiMnSb(001) surface above and below the Fermi energy

The predicted 100\% spin polarization at the Fermi energy ($E_F$), together with the high Curie temperature (750K) of the half-Heusler alloy NiMnSb makes it interesting for technological applications. However, experimentally only 50\% surface polarization has been observed. Knowledge of the electronic structure of NiMnSb is key to improving our understanding of this material. We report on surface bandstructure measurements for the (001) surface of single crystal NiMnSb. The experimental techniques used are angle resolved ultraviolet photoemission spectroscopy (ARUPS)and inverse photoemission spectroscopy (ARIPES) to probe the bandstructure both below and above $E_F$. ARUPS and ARIPES are used quasi-simultaneously in a multi- chamber UHV-system to ensure well defined and equal sample preparation conditions. We see both non-dispersive (ARIPES) and dispersive (ARUPS) structures in the spectra, which stem from d-like bulk states. With our normal emission UPS data, we resolve the conflict in the various contradictory reports in the literature about the position of spectral features for both polycrystal and single-crystal samples. Finally, through comparison with theoretical calculations we have identified a surface state candidate.   

Contact-less measurements of Shubnikov-de Haas oscillations below N$\acute{e}$el temperature in single crystals SmAgSb$_{2}$

Oscillations of a skin depth with magnetic field were measured in single crystals SmAgSb$_{2}$ by using radio-frequency resonant technique. Comparison with directly measured de Haas -- van Alphen and Shubnikov -- de Haas oscillations revealed additional details in the frequency spectra, probably due to high sensitivity of the measurements $\Delta \rho_{min} \approx 20\;{\rm{p}}\Omega \cdot {\rm{cm}} $. The temperature evolution of the frequency spectra was obtained. The correlation of the observed oscillations with calculated Fermi surface and possible influence of antiferromagnetic ordering are discussed.   

Session W46: Compound Semiconductor Defects and Dopants

Silicon-interstitials-based Benchmarking of DFT Exchange-correlation Potentials

Diffusion Monte Carlo (DMC) benchmarks DFT functionals: LDA, GGA, and HSE [1]. Extensive DFT studies on single-, di-, and tri-interstitials [2] provide stable structures and converged energies. For single-interstitial formation energies, our DMC results confirm earlier work [3], with 1.5 and 1.0 eV underpredictions for LDA and GGA, respectively. We continue to observe this trend in most di- and tri-interstitials. Additionally, we find HSE reproduces DMC results for single-interstitals. Preliminary analysis indicates that large LDA and GGA discrepancies with DMC occur for highly distorted defect configurations. \begin{itemize} \item[{[1]}] J.~Heyd \emph{et al.}, J.Chem.Phys. \textbf{118}, 8207 (2003). \item[{[2]}] D.~A.~Richie \emph{et al.}, Phys. Rev. Lett. \textbf{92}, 45501 (2004). \item[{[3]}] W.~-K.~Leung \emph{et al.}, Phys. Rev. Lett. \textbf{83}, 2351 (1999). \end{itemize}   

Random doping and oxide roughness induced fluctuations in nanoscale semiconductor devices

Random doping and oxide roughness induced fluctuations in nanoscale semiconductor devices are analyzed by using self-consistent Poisson-Schr\"{o}dinger computations. A very fast and robust technique based on linearization of the transport equations is presented for the computation of fluctuations of various parameters (such as threshold voltages, terminal currents, and cutoff frequencies) of the semiconductor device. This technique is computationally much more efficient than the traditional Monte-Carlo approach and yields information on the sensitivity of device parameters fluctuations to the locations of doping and oxide thickness fluctuations. Hence, it can be used in the design of fluctuation resistant structures of semiconductor devices. Sample simulation results obtained by using the linearization technique are reported for MOSFET devices with channel lengths under 25 nm and compared with results obtained by using the Monte-Carlo technique.   

Thermodynamics of semiconductor doping and stoichiometry from first-principles methods

The theoretical investigation of semiconductor doping involves assessment of opposing physical effects: While \textit{extrinsic} doping can shift the Fermi level in the desired direction, such shifts disturb the balance between \textit{intrinsic} defects (vacancies, interstitials, etc.) leading to the creation of ``killer-defects'' and, in some materials, to considerable deviation from ideal stoichiometry. We have developed a self-consistent procedure which uses first- principles calculated formation enthalpies of impurities and intrinsic defects, to predict the correlation between doping, stoichiometry, and equilibrium carrier density. With this method, we explore the deviation from ideal stoichiometry and the electrical properties of the photovoltaic materials CuInSe$_{2}$ and CuGaSe$_{2}$, sampling the entire space of thermodynamical variables. We find a systematic correlation between stoichiometry and conductivity type, which in case of CuInSe$_{2}$ spans the whole range from $p$- to $n$-type. Application to the case of N-doping of ZnO, identifies the narrow window for growth conditions that lead to $p$-type doping by N$_{O}$ acceptors.   

Stoichiometry Driven Impurity Configurations in Compound Semiconductors

Precise stoichiometry and departures therefrom in the composition of the tetrahedrally coordinated compound semiconductors allow impurity incorporation in more than one configuration. Ultra-high resolution infrared spectroscopy of CdTe:O at low temperatures reveals a unique pair of sharp lines, a non-degenerate $\nu _{1}$ = 1096.78 cm$^{-1}$ and a doubly degenerate $\nu _{2}$ = 1108.35 cm$^{-1}$ at 5 K, associated with the local vibrational modes of O$_{Te}$ in a (O$_{Te}$ -- V$_{Cd})$ complex in crystals grown with (CdTe + CdO + excess Te) or (CdTe + TeO$_{2})$ which enhances the occurrence of Cd vacancy (V$_{Cd})$; in contrast, a single, triply degenerate sharp line at $\nu _{0}$ = 349.79 cm$^{-1}$ observed at 5 K occurs in CdTe grown with (CdTe + CdO + excess Cd) in which the appearance of V$_{Cd}$ is inhibited. In the former, oxygen, O$_{Te}$, is bonded to three nearest neighbor Cd's with a nearby V$_{Cd}$. The latter corresponds to O$_{Te}$ attached to all the four nearest neighbor Cd cations. With increasing temperature, $\nu _{1}$ and $\nu _{2}$ approach each other and behave as a single triply degenerate line at $\nu _{0}^{\ast }$ for temperature T $\ge $ T$^{\ast } \quad \sim $ 300 K; the uniaxial (C$_{3v})$ symmetry of (O$_{Te}$ -- V$_{Cd})$ transforms to T$_{d}$ symmetry at T$^{\ast }$, acquired due to an increasing rate of bond switching among the four possible O$_{Te}$ -- V$_{Cd}$ directions as T approaches T$^{\ast }$.   

\textit{Ab initio }studies of defects in CdTe and HgTe with symmetrized basis

We have performed \textit{ab initio} pseudopotential calculations of the total energies and atomic realxations of neutral and charged Cd vacancies in CdTe and Hg vacancies in HgTe. Our method takes advantage of the high point symmetry of the system, which enables us to use large supercells containing up to 64 atoms per unit cell. Supercells of 8, 16, 32, 54, and 64 atoms are employed and spin-orbit interactions are included. Considering only symmetric relaxations of neighboring atoms in CdTe system of a 64-atom supercell, we find that first-neighbor atoms around a Cd vacancy move toward the vacancy with a 9{\%} contraction in bond length and second-neighbor atoms move toward the vacancy with a 2.5{\%} lowering contraction in inter-atomic distance. The relaxation lowers the total energy by about 0.2eV. Similarly in HgTe, the contractions are 7{\%} and 1.5{\%}, respectively for first-neighbor and second-neighbor atoms. Three defect levels (with symmetry$\Gamma _{6}$,$\Gamma _{7}$, and$\Gamma _{8})$ are found for both CdTe and HgTe systems and one of the levels is located in the band gap while two of them are buried under the valence band maximum. Separate calculations are carried out using a full potential linearized augmented Slater-type orbital (LASTO) method and give consistent results. Self-energy corrections due to many-body effect are also estimated with the GW approach.   

Dislocation filtering by buffer layer interfaces in InSb/Al$_{x}$In$_{1-x}$Sb heterostructures grown on GaAs (001) substrates

Recent efforts have been devoted to the development of InSb-based transport devices, including mesoscopic magnetoresistors and field-effect transistors, on GaAs (001) substrates. The small effective mass of electrons in InSb leads to a high mobility at room temperature. Remotely-doped InSb quantum wells offer the additional advantage of a conducting layer that is close to the surface. One key to maximizing the performance of such devices is the reduction of dislocation and micro-twin densities induced by the large lattice mismatch between InSb/Al$_{x}$In$_{1-x}$Sb and GaAs. We investigated the dislocation filtering effects of the interfaces formed between an Al$_{y}$In$_{1-y}$Sb interlayer and an Al$_{x}$In$_{1-x}$Sb matrix layer with y$>$x. Transmission electron microscopy analysis shows that the interlayer interfaces filter out threading dislocations. We improve this dislocation filtering by optimizing the interlayer thickness, the number of interlayers, and the growth conditions.   

Thermodynamic model for the structure of the 90 degree partial dislocation in diamond cubic semiconductors.

Recent studies of the 90 degree partial dislocation in diamond cubic semiconductors indicate that the structure of the core is not homogeneous, but rather is a combination of reconstructions and low energy structural excitations, such as kinks and anti-phase defects. As a result, direct investigation of the macroscopic properties of dislocation core by \textit{ab initio} methods is unfeasible. A model is presented that maps the complicated structure of a dislocation core onto a one-dimensional spin lattice. At each lattice site two spins are present, one to represent the reconstructed bonds, and the other kink structures. The model is sufficiently complex to allow expression of the essential nature of the structural excitations along a dislocation line. This Ising-like model can be investigated within a Monte Carlo framework.   

Strain relaxation and crystal quality in compositionally graded GaAsSb/GaAs metamorphic buffer layers

We have compared linearly graded, step graded, and constant composition layers of GaAs(1-x)Sb(x)/GaAs grown by Molecular Beam Epitaxy to determine which grading schemes result in the highest crystalline quality, while relaxing the lattice parameter most effectively. The incorporation rates used throughout the experiment for Ga and As were kept constant at 0.96 and 1.11 ML/s respectively. The Sb incorporation rate was varied from 0 to 0.63 ML/s to obtain a final composition of the topmost layers of x=0.5. The real-time stress evolution was obtained using an in situ multi-beam optical stress sensor. In our experiments, aggressive grading of the Sb flux results in decreased Sb incorporation at low x, a higher residual stress, and a bifurcation in the tilt of the sample. Less aggressive grading increases results in more uniform incorporation and lower residual stress, but the tilt remains. The tilt may be reduced by incorporating large steps in the grading, and completely eliminated when a constant composition layer of GaAs(0.5)Sb(0.5) is deposited directly on GaAs. The defect density of constant composition layers is somewhat higher than linearly graded layers with the same thickness and final composition. However, increasing the thickness of the layer reduces the defect density.   

Mutual Passivation of Donors and Isovalent Nitrogen in GaAs

Using large supercell total energy and band structure calculations, we have studied the mutual passivation mechanism of isovalent N and shallow donors in GaAs. We find that all the donor impurities, Si$_{Ga}$, Ge$_{Ga}$, S$_{As}$, and Se$_{As}$, bind to N in GaAs:N, which has a large N-induced band gap reduction relative to GaAs. For group-IV impurity such as Si, the formation of the nearest-neighbor Si$_{Ga}$-N$_{As}$ defect complex creates a deep donor level below the conduction band minimum (CBM). The coupling between this defect level with the CBM pushes the CBM upwards, thus restoring the GaAs band gap; the lowering of the defect level relative to the isolated Si$_ {Ga}$ shallow donor level is responsible for the increased electrical resistivity. Therefore, Si and N mutually passivate each other's electrical and optical activities in GaAs. For group-VI shallow donors such as S, the Coulomb binding between S$_{As}$ and N$_{As}$ does not form a direct bond and a deep level inside the gap; thus, no mutual passivation exists in the GaAs:(S+N) system. We also explained the difference between the mutal passivation of Si and N and the mutal passivation of H and N in GaAs. Our study provides a deep understanding of the mutual passivation mechanism and explained some of the recent puzzling experimental observations.   

Effect of indium on the localized vibrational mode of nitrogen in GaN$_x$As$_{1-x}$

The effect of the substitution of nearest-neighbor gallium atoms by indium (In-N-Ga, In-N-In) on the frequency of the localized vibrational mode of substitutional nitrogen in the dilute nitride, GaN$_x$As$_{1-x}$, has been studied within first-principles density functional theory, using a supercell approach. The splitting of the highly localized triply-degenerate mode into singly- and doubly-degenerate modes is obtained and compared with available Raman and FTIR spectroscopy measurements. The results are in good agreement with the experimental values.   

Effects of edge and screw dislocations on optical properties of Wurtzite GaN

The wide bandgap and high temperature stability of GaN makes it a desirable material for applications such as blue light-emitting diodes, blue lasers, and high-power transistors. Despite these advantages, the large lattice mismatch in most epitaxial GaN leads to a high density of dislocations, on the order of 10$^{9}$cm$^{-2}$ for edge dislocations and 10$^{8}$cm$^{-2}$ for screw dislocations in WZ GaN. Edge dislocations are electron acceptors and take on a negative charge. Open-core screw dislocations are essentially voids, or nanopipes, in the material. The presence of these defects, plus the strain field associated with each dislocation type, change the density of states and reduce the PL intensity in typical epitaxial GaN device layers. In the present work, the effects of edge and screw dislocations in WZ GaN have been studied computationally as a function of dislocation density. Spectral properties are determined by solving a 6x6 multiband kp Hamiltonian in three-dimensions using a real-space finite element method. Results compare favorably to available experimental data.   

Vacancy induced localized states in graphene

We show, analytically, that vacancies in an half-filled honeycomb lattice induce the formation of quasi-localized electronic states. If particle-hole symmetry is broken, these states become resonances close to the Fermi level. We also calculate numerically the electronic density of states for a finite density of vacancies, and discuss the issue of electronic localization in these systems. Our results have also relevance for the problem of disorder in d-wave superconductor.   

Low temperature photoluminescence in the strongly disordered dilute nitride GaAsN.

Photoluminescence (PL) due to the radiative recombination of excitons is used to study the structural disorder of alloys. We calculate the PL spectra at finite temperatures in the dilute nitride GaAsN. The shape of PL spectra is determined by the exciton occupation under steady-state optical excitation. Exciton energy levels and wave functions are calculated numerically in a supercell geometry with a strong random alloy potential acting on the electron. The distribution of electrons in states of the random potential is found by solving a kinetic equation, including phonon-assisted transitions between states and radiative recombination. Results are compared to the recent experiments.   

Session W47: Quantum Transport in Semiconductors: Theory and Experiment

Wigner approach to quantum transport in graded semiconductors

Graded electron bandstructures have long being used to beneficially influence the performance and functionality of electronic devices. In this work, we have developed a consistent fully quantum description of electron transport in terms of the Wigner distribution function, making use of the symmetric and hermitian effective-mass-like single band Hamiltonian that can be unambiguously constructed for graded systems. The generalized Wigner equation includes contributions that, in the quasiclassical limit, can be interpreted as directly corresponding to the drift and diffusion terms, but, unlike the homogeneous materials, the velocity operator is coordinate dependent and the electron is subject to the influence of the k-dependent quasielectric fields originating from both the inhomogeneous potential profile and composition-dependent modulation of the quasiparticle inertia. The approach is useful for the analysis of a broad class of transport phenomena in graded systems, where quantum effects are important, but a full quantum treatment would be prohibitively costly.   

Variational studies of quantum liquid crystal phases of 2DEG

The ground state of a nematic phase of the 2DEG at filling fraction$\nu =1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2$is studied. The pair distribution function and interaction energy are calculated for a wavefunction having the Jastrow form for the correlation part of$\prod\limits_{i [Preview Abstract]

Photoconductivity in AC-driven modulated two dimensional electron gas

We study the photoconductivity of two-dimensional electron system in a perpendicular homogeneous magnetic field, under the influence of periodic modulation potential and microwave irradiation. The model includes the microwave and Landau contributions in a non-perturbative exact way, the periodic potential is treated perturbatively. The Landau-Floquet states provide a convenient base with respect to which the lattice potential becomes time-dependent, inducing transitions between the Landau-Floquet levels. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. The total resistivity exhibits strong oscillations, leading to negative resistance states as the electron mobility and the intensity of the microwave power increases. It is proposed that, depending on the geometry, negative conductance sates or negative resistance states may be observed in lateral superlattices fabricated in $GaAs/AlGa As$ heterostructures.   

Charge transfer between a superconductor and a hopping insulator

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron -- Cooper pair conversion process, \textit{time reversal reflection}, where electrons tunnel into superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. This process is an analog to conventional Andreev show that the time reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.   

Simulating the interaction of a scanning probe with the quantum Hall liquid

Motivated by recent experiments [1], we have developed a simulation of the interaction of a metallic scanning probe with a 2D electron system (2DES) in the quantum Hall regime. The simulation is based on an electrostatic relaxation method, modified to include the non-linear screening of the 2D electron system at high magnetic fields. Using 2D simulations with cylindrical symmetry that allow us to account for the exact shape of the tip, we predict the diameter and width of ring shaped incompressible strips (ISs) induced by DC tip biases. Extending these results to 3 dimensions, we incorporate the effect of the disorder on the shape of the IS, and predict the formation of quantum dot islands observed in [1]. Comparison of the simulation results with experimental data provides a direct and quantitative view of the disorder of a very high mobility 2DES. [1] G. A. Steele, R. C. Ashoori, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 95, 136804 (2005)   

Quantum dynamics for a dissipative quantum harmonic oscillator as a model for a NEMS frequency control resonator

For the simplest model of dissipation, that is, a linear oscillator coupled to an infinite number of degrees of freedom to form a dissipative bath, it is found that the Heisenberg equation of motion for the oscillator displacement takes the form of a Langevin equation with a memory dependent dissipation [Ford, Lewis, O'Connell; Phys. Rev. A 37, 004419 (1988)]. When Fourier analyzed, this leads to a complex susceptibility which gives rise to a generalized frequency dependent ``quality factor'' which relates to the dissipative environment. We explore the limits of resonator integrity, especially with regard to insights afforded by the dependence of quality factor and other observables on the microscopic connection between resonator material parameters and circuit performance.   

Kohn localization in the quantum Hall regime

A two-dimensional electron fluid in the quantum Hall regime shows both quantized transverse conductivity and vanishing longitudinal conductivity: the latter property characterizes insulators. According to Kohn's theory of the insulating state, electron localization---defined in an appropriate sense---is the {\it cause} for the insulating behavior in any insulator. I show that a quantum Hall ``insulator'' is no exception; furthermore both quantization of the transverse conductivity and vanishing of the longitudinal one stem here from the same elegant formalism. Since 1999 onwards, the theory of the insulating state has been reformulated in term of a ``localization tensor'' which provides a measure of electron localization. This tensor is an intensive property, geometric in nature, having the dimensions of a squared length; it characterizes the ground wavefunction as a whole, {\it not} the individual states. It is finite in any insulator and divergent in any metal. A fluctuation-dissipation theorem relates this ground-state property to the system conductivity. So far, the theory has only addressed systems with time-reversal symmetry, in which case the localization tensor is real. I show that in absence of such symmetry the localization tensor is naturally endowed with an imaginary part, proportional to transverse dc conductivity, and quantized in two-dimensional systems. Therefore electron localization is the {\it common cause} for both vanishing of the dc conductivity and quantization of the transverse one in quantum Hall fluids.   

The mass of the electron in Shubnikov-de Haas effect:Spin-charge locking

At low temperatures, the integration over the Fermi distribution leads to x/$\sin$h x type expression which is called the Dingle's formula. The spin symmetry is found to modify this formula which determines the oscillation amplitude of resistivity as a function of magnetic field. The theory introduces the effective charge so that the cyclotron frequency gets fractionalized resulting into m/$\nu_{\pm}$. At a certain magnetic field 1.5m is found instead of m. The Shubnikov-de Haas effect uses quantization of Landau levels but not the flux quantization. Hence we find that there is a ``quantized S-dH effect'' which measures the m/h$^2$. We determine that when fractional values of the filling factor are taken into account, the mass of the electron, equal to the band mass is obtained. \newline 1. K. N. Shrivastava, Phys. Lett. A113, 435(1986). \newline 2. K. N. Shrivastava, Phys. Lett. A 326,469(2004). \newline 3. K. N. Shrivastava, Introduction to quantum Hall effect, Nova Sci. Pub. N.Y. (2002).   

Conductance reduction without shot noise in quantum wires

Shot noise can only be avoided in conductors without backscattering of conduction electrons. Such conductors without backscattering and a twofold spin-degeneracy have a minimal (nonzero) conductance of 2 $e^{2}/$h in the case of weak interactions. In recent experiments, however, also conductors with a reduced conductance of 1.4 $e^{2}/h$ have shown a clear tendency of noise suppression in zero magnetic field. It has been argued, that these experiments point to a lifted spin-degeneracy in these wires, spin-polarizing their conductance electrons. In this talk I will describe a model of an interacting quantum wire that is able to reproduce the transport behavior observed in these experiments qualitatively: that of the ``Coulomb Tonks gas'' of impenetrable electrons. It can be realized in ultra-thin wires, such as carbon nanotubes. We have studied transport through a finite-length Coulomb Tonks gas connected to bulk leads in various exactly solvable limits, both in and out of equilibrium. While we find a reduction of the conductance of such a wire to $e^{2}/h$ in all cases, the current in the wire does not exhibit any fluctuations at zero temperature. Most importantly, our model demonstrates that such noise suppression does not require a spin-polarization.   

Low-temperature transport in high quality strained Ge channels in SiGe

Presently, the mobility of holes in strained germanium achieved so far exceeds 100000 cm$^{2}$/Vs at a carrier density of $\sim $8*10$^{11}$cm$^{-2}$. For lower carrier density, the highest reported mobilities are roughly proportional to the carrier density. Accessing the upper left corner in the density-mobility diagram remains a challenge. Background charges, inhomogeneous strain distribution and growth defects are the main difficulties of growing strained Ge channels. We have fabricated high quality Ge channels and measured the transport parameters at temperatures down to 0.4 K. We discuss the influence of growth problems on the mobility as a whole and how these mechanisms may influence the magnetic field dependence of the sheet resistance. We explore the effects of different doping geometries, in particular backside doping and symmetrical doping on electrical transport, including the effects of dopant segregation. Our quantitative analysis shows that local charged impurities dominate the scattering rate. It also shows the effect of too low substrate temperature, leading to point defects whose existence can be detected by a pure transport measurement.   

Electronic transport properties of amorphous Sb$_2$Te$_3$ and Ge$_2$Sb$_2$Te$_5$ films

The electrical conductivity, Seebeck coefficient, and Hall coefficient of amorphous Sb$_2$Te$_3$ and Ge$_2$Sb$_2$Te$_5$ films have been measured as functions of temperature from room temperature down to as low as 200~K. The electrical conductivities manifest an Arrhenius behavior with a larger pre-exponential factor. In Sb$_2$Te$_3$ the energy characterizing the p-type Seebeck coefficient's temperature dependence, about 0.10~eV, is considerably smaller than the activation energy of the electrical conductivity, about 0.28~eV. In addition, the heat-of-transport constant of the Seebeck coefficient is much larger than that of conventional semiconductors. The Hall mobility is low (near 0.1~cm$^2$/V-sec at room temperature), anomalously signed (n-type), and increases with rising temperature with an activation energy of about 0.05~eV. These results are consistent with the charge carriers being hole-like small polarons that move by thermally assisted hopping. Ge$_2 $Sb$_2$Te$_5$ also has low mobility (0.7~cm$^2$/V-sec) and a high conductivity activation energy (0.41~eV), but Seebeck data is indicative of multi-band transport.   

Magnetic and transport properties of Fe$_{1-x}$Co$_{x}$Sb$_{2}$

Anisotropic magnetic and electronic transport measurements were carried out on large single crystals of Fe$_{1-x}$Co$_{x}$Sb$_{2}$, grown by self flux method, in the temperature range 1.8-350K for 0$\le $x$\le $1. The diamagnetic semiconducting state of FeSb$_{2}$ evolved into metallic by substitution of Fe with Co for x$<$0.5. With further doping there was a structural transformation from orthorhombic Pnnm structure of FeSb$_{2}$ to monoclinic P21/c structure of CoSb$_{2}$. Large magnetoresistance and anisotropy in electronic transport were observed.   

Transport Properties of Ag Nanoparticles in Carbon Matrix Prepared with a Cluster Gun

The use of ``cluster guns'' with in-situ processing capabilities has been found to be suitable for the fabrication of nanoparticles in a wide range of materials, avoiding external annealing and possible surface oxidation of the nanoparticles[1, 2]. In this study, we have used our cluster gun to fabricate Ag nanoparticles and embed them in a C matrix formed by conventional sputtering. With the increased amount of Ag, the transport properties of thin films show a gradual transition from a semiconductor-like behavior to a metallic one. At cryogenic temperatures, the magnetoresistance (MR) is generally negative at low fields and becomes positive at high fields. The field at which the MR changes sign increases with increased temperature. At higher temperatures (around 20 K), only negative MR is observed. For the samples with semiconductor-like behavior, the temperature dependence of resistance follows the relation $R=R_0 \exp \left[ {\left( {T_0 /T} \right)^{1/2}} \right]$ in the temperature range from 5 to 50 K. We are investigating the origin of this behavior and those results will be reported.   

Controlling the Inherent Magnetoresistance in thin InSb epilayers on GaAs (001)

There is great advantage to controlling the magnetoresistance (MR) in high mobility semiconductors for a number of applications which require thin active surface layers. Previously we have produced n type thin epilayers of InSb with the highest reported mobility\footnote{T. Zhang et al. Appl. Phys Lett. {\bf84}, 4463 (2004).} and we have used these epilayers to explore novel geometries that enhance the high field MR.\footnote{W.R. Branford et al., Appl. Phys Lett. {\bf86}, 202116 (2005).} Here we show that by virtue of the inherent inhomogeneity in the growth direction, thin InSb epilayers can be designed to have significant MR without external geometric manipulation. The observations can be explained using a transport model that describes the electrical properties of the layers including contributions from conduction and impurity bands.\footnote{J.J. Harris et al., Semicond. Sci. Tech. {\bf19}, 1406 (2004).} We will explore using the model, the possibility of maximizing or minimizing the inherent MR in these layers and we show experimentally how to create thin high mobility layers where the inherent MR is significantly reduced or enhanced without compromising the layer mobility.\footnote{T Zhang et al., Semicond. Sci. Tech., in press.}   

Magnetocapacitance of Semiconductors with Nonmagnetic and Magnetic Impurities

Positive magnetoresistance in semiconductors has been studied by previous investigators and found to have an exponential dependence on magnetic field in the regime of hopping conduction and a power law dependence at higher temperatures, due to band carriers. To our knowledge, little experimental study has been performed on the magnetocapacitance of semiconductors outside of the low temperature regime, where phenomena such as the quantum Hall effect have been studied. Here we report on the magnetocapacitance of lightly doped ($\rho $ ~$>$~1 $\Omega $-cm) n- and p-type silicon, using both Schottky and oxide barriers to form capacitor structures. The frequency-dependent negative magnetocapacitance can be as large as 30{\%} at 50K and decreases to a few percent at room temperature. We attribute this effect to a field-induced localization of shallow donor impurity wavefunctions in directions transverse to the applied magnetic field. The effect can only be observed if the measurement frequency ($\sim $1MHz) is comparable to or greater than the field-dependent transition rate between impurity sites. We will also contrast the differences in the magnetocapacitance effect for diluted magnetic semiconductors such as GaCrN and GaMnAs.