Session U1: Spin Liquids and Superconductivity near the Mott Transition

Spin Liquid States in the Hubbard Model: Implications for Organics

We formulate a U(1) gauge theory of the Hubbard model in the slave-rotor representation. From this formalism it is argued that spin liquid phases may exist near the Mott transition in the Hubbard model on triangular and honeycomb lattices at half filling. The organic compound $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ is a good candidate for the spin liquid state on a triangular lattice. We predict a highly unusual temperature dependence for the thermal conductivity of this material.   

Spin liquid and superconductivity in two-dimensional organic charge transfer salts

We introduce and analyze a variational wave function for quasi two-dimensional organic salts containing strong local and nonlocal correlation effects. We find an unconventional superconducting ground state for intermediate charge carrier interaction, sandwiched between a conventional metal at weak coupling and a spin liquid at larger coupling. Most remarkably, the excitation spectrum is dramatically renormalized and is found to be the driving force for the formation of the unusual superconducting state.   

Anomalous superconductivity near the Mott transition

High-temperature superconductivity appears near an antiferromagnetic Mott insulating phase and a normal phase with a pseudogap. It was suggested early on by Anderson that the strong-coupling limit of the Hubbard model should contain the main physics. It is only recently that we have begun to have access to sufficiently accurate algorithms and powerful enough computers to begin to extract the main features of the phase diagram of high-temperature superconductors from the Hubbard model in a nearly quantitative manner. In this talk, the zero temperature phase diagram of the two-dimensional Hubbard model is discussed based on several ``quantum cluster'' approaches, mainly Variational Cluster Perturbation Theory [1] and Cellular Dynamical Mean Field Theory [2], that shall be introduced. The overall ground state phase diagram of the high-temperature superconductors as well as the asymmetric one-particle excitation spectra for both hole- and electron-doping are reproduced. The d-wave order parameter is found to assume a dome shape as a function of doping and to scale like the magnetic exchange coupling J for U comparable to the bandwidth. We stress the features of superconductivity that are non-BCS like due to the proximity to the Mott insulator. In stark contrast with BCS theory, the superconducting gap can decrease monotonically at the same time as the d-wave order parameter increases away from half-filling. Also, d-wave superconductivity is driven by a lowering of kinetic energy instead of potential energy, in conformity with experiments on cuprates. The pseudogap [3-5] and results of other approaches will also be briefly touched upon. \newline [1] David S\'{e}n\'{e}chal, P.-L. Lavertu, M.-A. Marois, and A.- M.S. Tremblay, Phys. Rev. Lett. \textbf{94}, 156404 (2005). \newline [2] S. S. Kancharla, M. Civelli, M. Capone, B. Kyung, D. Senechal, G. Kotliar, A.-M.S. Tremblay, cond-mat/0508205. \newline [3] B. Kyung, S.S. Kancharla, D. S\'{e}n\'{e}chal, A.-M.S. Tremblay, M. Civelli, and G. Kotliar cond-mat/0502565 \newline [4] B. Kyung, V. Hankevych, A.-M. Dar\'{e} et A.-M.S. Tremblay, Phys. Rev. Lett. \textbf{93}, 147004 (2004). \newline [5] A.-M.S. Tremblay, B. Kyung and David S\'{e}n\'{e}chal, cond-mat/0511334   

The origin of the pseudogap in the high Tc superconductors

The origin of the pseudogap is one of the most important questions in high Tc superconductors. The idea of circulating currents as being responsible for the pseudogap will be considered and various current paths through the Cu-O plane examined in the light of recent neutron scattering experiments.   

Polarized Neutron Diffraction to discover symmetry breaking in pseudogap region of Y(123)-Cuprate

\vspace {0.5 cm} One of the leading issues in high-$T_C$ superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YBa$_2$Cu$_3$O$_{6+x}$ system$^*$. The observed magnetic order preserves translational symmetry as proposed for orbital moments in the circulating current theory of the pseudogap state (see C.M. Varma, at http://fr.arxiv.org/abs/cond-mat/0507214). To date, it is the first direct evidence of an hidden order parameter characterizing the pseudogap phase in high-$T_C$ cuprates. \\ $^*$ B. Fauqu\'e, Y.~Sidis, V.~Hinkov, S.~Pailh\`{e}s, C.T. Lin, X. Chaud and P.~Bourges, at http://fr.arxiv.org/abs/cond- mat/0509210.   

Session U2: Quantum Magnets in High Magnetic Fields

Spinons, Solitons, and Breathers in Quasi-One-Dimensional Magnets

By scattering neutrons from coordination polymer magnets, we contrast the effects of a uniform and a staggered magnetic field on the quantum critical state of a spin-1/2 chain. In a partially magnetized state of copper pyrazine dinitrate (CuPzN) we find bounded spectral continua indicating that neutrons scatter from spin-1/2 quasi-particle pairs [1]. The complex boundaries including an incommensurate soft spot result from a field induced shift in the Fermi points for these quasi-particles. The measurements indicate that the magnetized state of CuPzN remains quantum critical. Copper benzoate [2] and CuCl$_{2}^{.}$2(dimethylsulfoxide) (CDC) [3] differ from CuPzN in that there are two spins per unit cell along the spin chain. Rather than continuous spectra, we find resolution limited gapped excitations when these materials are subject to high fields. So with two spins per unit cell, an applied field can drive the spin-1/2 chain away from criticality. The explanation for this effect was provided by Affleck and Oshikawa. The alternating coordination environment induces a transverse staggered field and spinon binding. The quantum sine-Gordon model is the relevant low energy field theory and it predicts soliton and breather excitations at specific energies and wave vectors that we compare to the experiments. We shall also compare a complete measurement of the dynamic spin correlation function for CDC in a field to exact diagonalization results for a spin-1/2 chain with a staggered and uniform magnetic field [4]. \newline \newline [1] M. B. Stone, D. H. Reich, C. Broholm, K. Lefmann, C. Rischel, C. P. Landee, and M. M. Turnbull, Phys. Rev. Lett. \textbf{91}, 037205 (2003). \newline [2] M. Kenzelmann, Y. Chien, C. Broholm, D. H. Reich, and Y. Qiu,~ Phys. Rev. Lett. \textbf{93}, 017204 (2004). \newline [3] D. C. Dender, P. R. Hammar, Daniel H. Reich, C. Broholm, and G. Aeppli, Phys. Rev. Lett. \textbf{79}, 1750 (1997). \newline [4] M. Kenzelmann, C. D. Batista, Y. Chen, C. Broholm, D. H. Reich, S. Park, and Y. Qiu, Phys. Rev. B \textbf{71}, 094411 (2005).   

Ordering and Excitations in the Field-Induced Magnetic Phase of Cs$_{3}$Cr$_{2}$Br$_{9}$

Cs$_{3}$Cr$_{2}$Br$_{9}$ is an interesting example of interacting spin-dimer system. As in other isotropic antiferromagnets such as Haldane or alternating chains and ladders, the ground state in zero field is a total spin singlet separated from the excited triplet by an energy gap. In a magnetic field $H$, a phase transition occurs at a critical field $H_{c1}$, where the gap to the lowest component of the Zeeman-split triplet closes. Above $H_{c1}$, field-induced magnetic order (FIMO) for spin components perpendicular to $H$ is induced by inter-dimer or inter-chain couplings. The FIMO transition may be considered as a Bose-Einstein Condensation. Cs$_{3}$Cr$_{2}$Br$_{9}$ differs from other dimer systems currently studied ($e$.$g$. PHCC, TlCuCl$_{3})$ in two main ways: each Cr$^{3+}$ ion of the dimer has spin 3/2 rather than 1/2 for Cu-based systems and the arrangement of the dimers is hexagonal. This gives rise to anisotropy and frustration in a 3D lattice, respectively. The possibility of studying the magnetic ordering and the spin dynamics in a FIMO with sufficient detail to bring out features of frustration and anisotropy motivated the present neutron scattering study in Cs$_{3}$Cr$_{2}$Br$_{9}$*. Two field orientations have been exploited, perpendicular and parallel to the easy axis \textbf{c} (direction of the dimers). First, I present the diffraction study: the FIMO displays large hysteresis incommensurability, showing the importance of frustration. The impact of anisotropy is seen in the magnetic structure, whose nature strongly depends on the field direction. Second, I focus on spin dynamics: it quantifies the presence of anisotropy and shows its crucial role on the energy gap at $H_{c1}$, which is measurably open or not, depending on whether $H$ is perpendicular or parallel to \textbf{c}. Third, an explanation is proposed for the large value of the gap at higher field: it involves the mixing of higher order states (\textit{extended}-FIMO), reflected by the absence of magnetization plateaus. Comparison with the sister Cs$_{3}$Cr$_{2}$Cl$_{9}$ compound provides a test of this hypothesis. *B. Grenier \textit{et al.,} Phys. Rev. Lett. \textbf{92}, 177202 (2004)   

Quantum phase transitions in integer spin chains

High field inelastic neutron scattering experiments on the $S$=1 bond-alternating 1D antiferromagnet NTENP, the anisotropic $S=1$-chain Haldane-gap compound NDMAP and the isotropic ``composite'' Haldane spin chain IPA-CuCl$_3$ [T. Masuda {\it et al.}, cond-mat/0506382] reveal key differences in the spin dynamics of these distinct types of quantum spin liquids. In modest applied fields the spectra of NDMAP [A. Zheludev {\it et al.}, Phys. Rev. Lett. {\bf 88}, 077206 (2002)] and IPA-CuCl$_3$ feature three sharp stable gap excitations. In contrast, in NTENP the highest mode is anomalously weak at $H=0$ and rapidly broadens and vanishes when the field is turned on. Above the critical field of 1D Bose condensation of magnons and long-range ordering NDMAP retains a triplet of massive long-lived excitations [A. Zheludev {\it et al.}, Phys. Rev. B $\bf{68}$, 134438 (2003)]. In IPA-CuCl$_3$ only two sharp gap excitations persist, with possibly an additional gapless mode. In NTENP only one sharp excitation branch is observed in this regime [Hagiwara {\it et al.}, Phys. Rev. Lett. {\bf 94}, 177202 (2005)], but there is new evidence of low-lying excitation continua. Work at ORNL was carried out under DOE Contract No. DE-AC05-00OR22725.   

High-field ESR and thermodynamic studies of uniform and bond-alternating $S$=1 spin chains

Recently, field-induced phenomena in quantum spin systems have attracted considerable interest. Gapped one-dimensional (1D) spin systems with a spin value$ S$=1 subject to an external magnetic field strong enough to close the gap ($H_{c})$ are driven into a new phase. Spin excitations in this field-induced phase have been studied by experiments on a uniform S=1 antiferromagnetic spin chain Ni(C$_{5}$H$_{14}$N$_{2})_{2}$N$_{3}$(PF$_{6})$, alias NDMAP and a bond-alternating one Ni(C$_{9}$H$_{24}$N$_{4})$NO$_{2}$(ClO$_{4})$, alias NTENP. We performed high-field and multi-frequency ESR experiments at 1.5 K on these compounds and observed gapped excitations above $H_{c}$. Two or three excitation modes were observed depending on the field direction in NDMAP and only one excitation in NTENP. These results are consistent with those obtained by inelastic neutron scattering experiments in a magnetic field. Both compounds exhibit the long-range order (LRO) at a magnetic field above $H_{c}$ and a low temperature. Observed gapped excitations are very different from those expected from a conventional spin-wave theory in the LRO state. For NDMAP, observed branches satisfactorily agree with those analyzed by a phenomenological field theory. The difference of observed gapped excitations between NDMAP and NTENP can be explained by an interaction with a low-lying two magnon continuum at q=$\pi $ that is present in a bond-alternating chain but absent in a uniform one. When an antiferromagnetic spin chain with$ S$=1 has an XY or Heisenberg symmetry, the phase above $H_{c}$ is critical and its low-energy physics is described by a Tomonaga-Luttinger liquid (TLL), which is characterized by a gapless $k$-linear energy dispersion with an incommensurate $k_{0}$ and a spin correlation having an algebraic decay. NTENP has nearly an XY symmetry and a linear temperature($T)$ dependence of the specific heat ($C_{mag})$ was observed for the magnetic field parallel to the chain above $H_{c}$ in a temperature region above that of the LRO state. The ratio $C_{mag}$/$T$ increases as the magnetic field approaches $H_{c}$ from above and is in good agreement with the prediction of the $c$=1 conformal field theory, providing a conclusive evidence for a TLL in a gapped quasi-1D antiferromagnet.   

Anisotropic Haldane-gap chains in a magnetic field

We consider quasi one dimensional spin-1 Heisenberg chains with crystal field anisotropy in a uniform magnetic field. We determine the dynamical structure factor in various limits and obtain a fairly complete qualitative picture of how it changes with the applied field. In particular, we discuss how the width of the higher energy single magnon modes depends on the field. We consider the effects of a weak interchain coupling. We discuss the relevance of our results for neutron scattering experiments on the quasi-1D Haldane-gap compound NDMAP.   

Session U3: Nanomechanical Architecture of Strained Thin Films

Precise semiconductor nanotubes and nanocorrugated quantum systems: concept, fabrication and properties

Physics and technology of several new classes of nanostructures, namely, variously shaped semiconductor, metal, dielectric and hybrid nanoshells, are overviewed. Previously, we discoved that ultrathin epitaxial heterofilms (down to two monolayers in thickness in the case of InGaAs/GaAs) can be controllably released from substrates and rolled up under the action of internal stresses into various cylindrical micro- and nanotubes, scrolls, rings, helices, etc. [1]. In this way, nanotubes with minimum diameter of 2-nm can be obtained. The fabricated nanoshells offer much promise as building blocks for nanoelectronic and nanomechanic devices, their fabrication technology being fully compatible with the well-established integrated-circuit technology [2]. Experimental and theoretical results concerning the quantum processes in the fabricated micro - and nanoshells are reported, including ballistic and tunnel transport in bent waveguides, magnetotransport, bending-induced formation of deep quantum wells and quantum dots molecules [3]. New results on the formation of spatially periodic nanostructures, nanocorrugated systems, shells with 1-nm minimum radius of curvature, building blocks for nanodevices and new nanocomposite materials are described. The present report outlines the cornerstone stages in the development of this fabrication technology for semiconductor and metal nanoobjects, including: directional rolling of films, super-critical drying of nanoshells, passivation of electron states in them, etc. Benefits offered by the new approach in the creation of 3D ordered nanoobject arrays, as well as challenges met in the development of the original nano- and molecular technology are discussed. \begin{enumerate} \item V.Ya. Prinz et al., Physica E 6, 828 (2000). \item V.Ya. Prinz, Physica E 23 260; 24, 54 (2004). \item V. M. Osadchii and V. Ya. Prinz, Phys. Rev. B 72, 033313 (2005). \end{enumerate}   

Nanomechanical Architecture of Strained Bi-layer Thin Films: From Design Principles to Fabrication

Controlled and consistent fabrication of different classes and shapes of nanostructures (as opposed to simply stochastic self-assembly) will be a requirement if nanotechnology expects to achieve its promised impact on society. We illustrate by both theory and computation the design principles of an emerging nanofabrication approach based on the \textit{nanomechanical architecture} of strained bi-layer thin films, which are further confirmed by experiments through fabrication of a variety of nanostructures, including nanotubes, nanorings, nanodrills, and nanocoils. This approach demonstrates the possibility of fabricating nanostructures with an unprecedented level of control over their size, geometry, and uniformity, based on \textit{a priori} designs. It possesses also an unparallel level of versatility for making nanostructures with combinations of different materials. By combined multi-scale modeling and simulations from first-principles calculation, to molecular dynamics simulation, and to continuum mechanics modeling, we demonstrate how mechanical bending of nanoscale thin films differs from that of macroscopic thin films. For example, we show that surface stress will even play a more dominant role than misfit strain in bending a film that is down a few monolayers thick. *This work is supported by DOE and NSF.   

Nonochannel networks, light emission and waveguidung of micro- and nanotubes, and ultra-compact coils

Quite generally, thin solid films can be partially released from a substrate surface by selective underetching and form into various 3D micro- and nano-objects [1-3]. Here, we show that such released layers form into complex nanochannel networks, which can be fluid-filled and emptied within fractions of a second. Furthermore, we demonstrate that single material layers roll-up into micro- and nanotubes. In particular, we show that all-Si tubes can be fabricated. Quantum emitters such as InAs/GaAs quantum dot heterostructures are integrated into the wall of rolled-up microtubes, and we study the emission and the waveguiding properties of such ``quantum dots in a tube'' [4]. Finally, metal/semiconductor bilayers are rolled up into microtubes. This technique opens the way to realize and integrate ultra-compact coils, transformers and capacitors on a single chip [5]. I am grateful to my collaborators Y. Mei, R. Songmuang, C. Mendach, C. Deneke, D. Thurmer, F. Cavallo, and A. Rastelli (all Max-Planck-Institut fuer Festkoerperforschung Stuttgart, Germany) \newline \newline [1] O. G. Schmidt and K. Eberl, Nature 410, 168 (2001) \newline [2] O. G. Schmidt et al., Advanced Materials 13, 756 (2001) \newline [3] V. Ya. Prinz et al., Physica E 6, 828 (2000) \newline [4] S. Mendach et al., Appl. Phys. Lett. (submitted) \newline [5] O. G. Schmidt et al., IEEE J. Selected Topics Quantum Electronics 8, 1025 (2002)   

Fabrication and Applications of Tubular Semiconductor Membranes

We present transport measurements on curved semiconductor membranes. The aim is to investigate geometric potentials in low dimensional electron systems. We have conducted first studies on topography dependant electron transport in complete tubes, using built in strain between lattice mismatched semiconductors. We will discuss the processing details in SiGe and InGaAs strained layers. Initial studies reveal two regimes of electron transport which are probed by a varying perpendicular magnetic field. At low magnetic field, we see an increase in electron scattering along curved regions due to an increase in electron scattering. At high magnetic field, we find a linear increase in resistance of the curved region as compared to planar regions. Finally, we will give an outlook into possible applications in nano-electromechanical systems.   

Session U4: Lithography

Step and Flash Imprint Lithography

Step and Flash Imprint Lithography has been recognized as a potentially low cost, high resolution patterning technique. Most of the published development work has been directed toward tool design and processing techniques. This work will be reviewed. There remains a tremendous opportunity and need to develop new materials for specific SFIL applications. An overview of relevant materials-related development work for SFIL lithographic applications will be presented. Material requirements for SFIL patterning for the sub-50 nm integrated circuit regime are discussed along with proposed new imprint applications, such as imprintable dielectrics that are targeted for use as on chip insulation layers.   

Directed assembly of block copolymer containing materials on chemically nanopatterned substrates: a platform for two and three-dimensional nanofabrication

Directed assembly often refers to fabrication strategies that involve the organization of one or more materials on substrates through specific interactions with patterned activated regions. Based on engineered interfacial interactions between lithographically-defined chemically nanopatterned substrates and block copolymer thin films, the domain structure of the films can be directed to assembly into defect free periodic and non-regular structures over large areas, with each structure registered with the underlying substrate. Advantages of integrating self-assembling materials into the lithographic process, particularly for the fabrication of nanoelectronic devices, include sub 1 nm control over feature dimensions, reduced line edge roughness, and the opportunity to scale the approach to pattern at dimensions of 10 nm and below. Exciting opportunities exist to extend the use of self-assembling materials in conjunction with two-dimensionally (2D) patterned activate surfaces for the fabrication complex three-dimensional (3D) materials. Arrays of functional nanoparticles, for example, can be directed to assemble using block copolymer/particle nanocomposites or in a hierarchical process using chemically functional polymers followed by in situ particle synthesis. 3D bicontinuous morphologies in which the two phases of the assembly are readily addressable, a geometrically complex structure, can be created using materials that normally form lamellae and directing them to assemble on chemically patterned surfaces consisting of square arrays of spots. The principal concept of this work is that high value added 3D structures can be created from simple 2D templates, retaining the lithographic properties of perfection and registration for applications where input and output connections to the structures are required.   

Probing the 3-Dimensional Structure of Nanomanufactured Materials using CD-SAXS

The realization of routine nanofabrication will demand new measurement platforms capable of probing the size, shape, internal morphology, and chemical uniformity of structures ranging from nanometers to 100's of nanometers in size. Traditional microscopies such as scanning electron microscopy and atomic force microscopy are often limited to exposed surfaces and are challenged to probe internal morphologies and structures with complex 3-dimensional shapes. We have developed a platform for non-destructive characterization of repeating nanostructures or nanostructured materials applicable to a wide range of sizes (5 to 500 nm) and materials (polymers, ceramics, and metals). Critical Dimension Small Angle X-ray Scattering (CD-SAXS) utilizes a relatively high energy, collimated x-ray beam to probe the dimensions, shape, and homogeneity of nanostructures fabricated on substrates such as silicon or quartz with sub-nm precision. CD-SAXS is capable of non-destructive measurements in real time during fabrication, providing insight into a wide range of fabrication methods. We demonstrate the wide ranging capabilities of CD-SAXS using recent data from structures created with photolithography, nanoimprint, and self-assembly. Patterns are characterized in terms of their average width, height, sidewall angle, and chemical uniformity. In addition, the distribution in orientation is quantified for self-organized systems, providing insights into the factors controlling defects. Finally, the technique is demonstrated for complex systems involving pattern directed self-assembly, such as in nanoimprinted block copolymers. In these systems, confinement between a mold and substrate prevent conventional imaging during fabrication. Real time data are used to elucidate the evolution of nanometer scale structures within 100 nm scale cavities.   

Characterization of Materials for Nanoscale Lithography

Current state-of-the-art semiconductor devices are fabricated at dimensions below 100 nm and industry planning anticipates that devices at the 20 nm scale will be in production a decade from now. The sizes of the component molecules of typical polymeric photoresists are of this same magnitude, and due to this convergence of scales and intrinsic materials limitations, the formation of high fidelity relief images at these dimensions will be a significant challenge. We summarize here the materials issues that must be addressed to enable the practical application of nanoscale photolithography, and describe instrumentation and methods we have developed that allow their suitability for such use to be assessed by characterizing basic materials properties.   

Session U5: Low Temperature Physics, A Historical Perspective

Low Temperature Physics at Yale in the late 30's through the early 50's

The low temperature program at Yale was initiated by C. T. Lane (1904-1991) in the fall of 1937 when he was appointed to the teaching staff as an instructor in the department of Physics. Following his doctorate from McGill in 1929 he investigated the magnetic susceptibilities of ``soft'' metals supported by the National Research Council of Canada, the Commissioners of the 1851 Exhibition and a Sterling Fellowship at Yale. Arranged by Louis McKeehan, with {\$}5000 from the new George Sheffield research fund, he started the construction of a Kapitza type helium liquefier. The machine was largely completed in the fall of 1939, yet liquid helium was not made until early December 1940 due to the need for extensive on line purification of the gas. Returning in 1945 from war research, Lane and Henry A. Fairbank (Ph.D 1944) continued the metals work along with new thrusts into Second Sound , properties of helium$^{ }$three impurities in liquid helium and starting in the 50's on rotating He II. In 1933 both Lane and Onsager were awarded Sterling Fellowships, which initiated a stimulating experimental- theoretical exchange continuing until they both retired. The best-known example was the rediscovery at Yale of the deHaas-van Alphen effect, previously observed only in bismuth, in zinc; where upon Onsager and his students provided new insights into our understanding of the Fermi surface of metals. With the development of new instrumentation one observed vast changes in experimental style during this period. The evolution of the production of liquid helium from Lane's device though the Collins machine to the commodity business of today now makes experiments of huge size and importance possible.   

Rotating Superfluids

Rotation of a fluid, particularly studying phenomena affected by Coriolis forces, plays a significant role in nearly all branches of fluid dynamics. Quantum fluids are no exception, as evidenced by remarkable devices such as ``Rota'' in Helsinki. This talk concerns the early days of rotating superfluids, starting long before superfluid helium-3 appeared on the scene. I will attempt to describe some of the early experiments, how the apparatus was designed, and what the experiments revealed. There has been so much activity in this area, I will discuss mostly experiments in my own fields of interest. Time will not permit an exhaustive review of this fascinating subject.   

Fritz London's Legacy at Duke University

When 3He became available in small quantities after WWII Fritz London, Professor at Duke University since 1939, became very interested in its properties in the liquid and solid phases, as contrasted with those of 4He. His influence and that of his colleague Walter Gordy led to the appointment of William Fairbank in 1952, who was able to verify experimentally the prediction on the Fermi degeneracy of liquid 3He below 1K, a few weeks before London's death in 1954. With his students and associates, Fairbank carried out a number of important experiments which became classics, several of which will be described. At Duke he also started planning other experiments inspired by London's predictions. After W. Fairbank's departure for Stanford in 1959, further research on liquid and solid 3He and 3He-4He mixtures was carried out by his successors at Duke University and some of the results in the sixties will be briefly described.   

Liquid Helium 3 and Solid Helium at Yale and Beyond

Many of the foundations of low temperature physics in the latter half of the twentieth century were built at Yale University under the leadership of Professor Cecil T. Lane who came to Yale in 1932 and Henry A. Fairbank who obtained his Ph.D. at Yale in 1944 under Lane's guidance. This discussion will mainly treat the contributions of Henry Fairbank and his students during the period between 1954 and 1963, when Henry Fairbank left Yale to become chairman of the Physics Dept. at Duke University. Following World War II small amounts of helium three became available to low temperature experimenters. Henry Fairbank’s graduate students were provided with the opportunity to investigate second sound in dilute and later concentrated mixtures of helium three in superfluid helium four. These measurements showed strong effects of the phase separation in helium 3 - helium 4 mixtures previously discovered in the laboratory of William Fairbank (a student of Lane and a brother of Henry Fairbank). As more helium three became available, studies of pure helium three were performed, including measurements of the thermal conductivity, the density and the specific heat. Early evidence for the melting curve minimum was found. The main emphasis in this work was to search for Fermi liquid behavior. Much of the later work in this area was performed by the group of John Wheatley at the University of Illinois. In studies of solid helium four at Yale, a surprising observation was made. Hitherto it had been thought that hcp was the stable phase throughout the low temperature part of the phase diagram. It was found via ultrasound experiments that a small silver of bcc solid existed at the lowest pressures. While this author was a graduate student at Yale, Henry Fairbank pointed out to him the possibility of cooling helium three via adiabatic compression from the liquid into the solid phase. (Pomeranchuk Cooling). A brief discussion is given of the use of this technique in the discovery of superfluid helium 3 by Osheroff, Richardson and the author at Cornell.   

Panel Discussion: Memories and Comments

\\   

Session U6: Strong Electronic Correlation in Solids: Applications of the LDA+U method

LDA+U Based Studies of Electronic, Vibrational and Spectroscopic Properties of Solids

The LDA+U method is a physically motivated approach that attempts to incorporate the effects of important orbital-specific local Coulomb interactions in strongly correlated electron systems while retaining the simplicity of local density approximation (LDA) calculations for real materials. In this talk, we discuss several applications of this method within the ab initio pseudopotential planewave framework. For transition metal oxides, the appropriate inclusion of the effects of onsite Coulomb U significantly alters their electronic structure leading to better agreement with experiment for quantities such as the nature of the electronic state, structural parameters, magnetic moments, phonon frequencies, etc. We have also studied the effects of doping on the electronic, magnetic, and structural properties of NaxCoO2. Undoped CoO2 is a metal with a high density of states at the Fermi level within LSDA, but a charge transfer insulator within LSDA+U. It is found that, due to a strong interaction between the doped electrons and the other Co d electrons, the calculated electronic structure is sensitively depended on the doping level. Finally, we discuss the use of LDA+U results as a starting mean-field solution for calculation of the electron self energy and quasiparticle excitations within the GW approximation.   

LDA+$U$ applied to oxide and nitride wide-band-gap semiconductors

Nitride and oxide semiconductors have important technological applications, but the theoretical understanding of their properties is hampered by the shortcomings of density functional theory (DFT) in the local density approximation (LDA). In particular, DFT-LDA underestimates the binding energy of the semicore $d$ states, leading to poor descriptions of quantities such as band offsets and deformation potentials. In this work we calculate the electronic and structural properties of wurtzite MgO, ZnO, and CdO, and discuss their similarities and dissimilarities with the corresponding nitrides AlN, GaN, and InN. We treat the semicore $d$ states of Zn, Cd, Ga, and In explicitly as valence states in a pseudopotential framework, and improve the description of electron-electron interactions in these narrow bands by including an on-site Coulomb interaction through the LDA+$U$ method. We propose a novel approach to calculate the parameter $U$, based on first-principles calculations for atoms. The approach is general and could be extended to other semiconductors and insulators where semicore $d$ states play a fundamental role in the description of electronic and structural properties. The LDA+$U$ approach systematically improves the LDA band gap by indirectly acting on both the valence-band maximum and conduction-band minimum. We investigate the effects of the on-site Coulomb interaction on lattice parameters, band structure, absolute deformation potentials, and band lineups. Finally we discuss how results based on LDA and LDA+$U$ can be used to calculate defect transition levels and formation energies that can be directly compared with experiment.   

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

Realistic approach to the electronic structure of complex materials which contains correlated d- or f- electrons will be discussed. The density functional theory within the local spin density approximation have been highly successful for electronic structure calculations and zero temperature magnetic properties of non-correlated systems. We investigate some failures of the LDA-scheme for the charge, spin and orbital ordering in transition metal compounds. General formulation of the LDA+U method which takes into account local Coulomb correlations for the d-shell of transition metals ions in the crystal within the mean-field approximation will be presented. The LDA+U scheme describe well the antiferromagnetic Mott insulators, rare-earth and actinide systems. Electronic structure, spin and orbital moments and lattice distortions of transition-metal compounds are investigated in the framework of rotationally invariant LDA+U method. Starting from conventional LDA+U scheme the different ways to go beyond the mean-field approximation which includes in effects of the spin- and charge-fluctuations will be analyzed. Dynamical mean field theory (DMFT) in combination with the first-principle LDA scheme (LDA+DMFT) is a good starting point for calculation of the quasiparticle spectrum for metallic transition metal systems. Recent progress in analysis of the metal-insulator transition for complex transition metal oxides and calculations of the spectral function for itinerant magnetic systems will be discussed.   

Disproportionation, Metal-Insulator Transition, and Critical Interaction Strength in Na$_{1/2}$CoO$_2$

Spontaneous breaking of symmetry is one of the key concepts of solid state physics related to phase transitions. Charge/spin density wave, or charge/spin ordering if the propagation vector is commensurate, are notorious examples of broken symmetry. The charge disproportionation in Na$_{0.5}$CoO$_2$ is the main theme of the present work. The results of LDA+U calculations will be presented, exhibiting a charge disproportionation transition at U$\approx$3eV. Na$_x$CoO$_2$ attracted considerable attention mainly due to superconductivity of its hydrated form Na$_{0.3}$CoO$_2$.1.3H$_2$O [1]. Besides the superconductivity Na$_x$CoO$_2$ exhibits several intriguing properties throughout its phase diagram, such crossover from Pauli-like to Curie-Weiss susceptibility at x=0.5, spin-density wave around x=0.7 or several phase transitions for x=0.5 including metal-insulator transition, charge ordering and magnetic ordering [2]. The Na$_x$CoO$_2$ lattice consists of triangular CoO$_2$ layers separated by Na layer. The mobility of Na ions and fractional occupation of Na sublattice provides an additional complication. Using LDA+U functional within FPLO [3] bandstructure method we have performed series of supercell calculations allowing for breaking of the symmetry between different Co sites. We have found that for large enough, but physically realistic, values of the on-site Coulomb interaction U the Co sublattice disproportionates into sites with formal valencies Co$^{4+}$ and Co$^{3+}$. We have found that at the same time a gap opens in the excitation spectrum. Details of the bandstructure across the transition as well as the driving forces of the transition in the LDA+U mean field picture will be discussed. \newline \newline [1] K. Takada {\it et al.}, Nature (London) {\bf 422}, 53 (2003).\newline [2] M. L. Foo {\it et al.}, Phys. Rev. Lett. {\bf 92}, 247001 (2004).\newline [3] K. Koepernik and H. Eschrig, Phys. Rev. B {\bf 59}, 1743 (1999).   

A consistent, linear-response approach to LDA+U

Hubbard U-correction to LDA or GGA has proven very effective in describing several strongly-correlated systems for which these approximations to DFT otherwise fail. \newline Constrained DFT or semiempirical approaches have been often used to compute the Hubbard U. I introduce here an alternative scheme to evaluate the effective electronic interaction in a fully consistent way. \newline This approach is based on the linear response of the system under consideration to a potential shift acting on the localized orbitals of the correlated sites. Using the occupations of these orbitals as the relevant electronic degrees of freedom we compute the on-site electronic coupling as the difference between the inverse of the bare and of the fully-interacting response matrices. The U computed in this way thus corresponds to the effective, atomically-averaged kernel of the Hartree-exchange-correlation interaction, in agreement with the second quantization expression of the "+U" energy functional. In this way the strength of the "+U" correction is evaluated from the same DFT scheme we aim to correct so that LDA+U becomes a consistent non-parametric method, with no need for semiempirical evaluations of the effective coupling. \newline \newline With this approach we successfully studied the structural, electronic, chemical and electrochemical properties of several transition-metal compounds. Examples will include minerals in the Earth's interior$^{1}$, cathode materials for next-generation lithium batteries$^{2}$ and metal-organic complexes $^{3}$. \newline \newline 1) M. Cococcioni and S. de Gironcoli, PRB (2005). \newline \newline 2) F. Zhou, M. Cococcioni, A. C. Marianetti, D. Morgan and G. Ceder, PRB (2004). \newline \newline 3) H. J. Kulik, M. Cococcioni, D. Scherlis and N. Marzari, submitted to PRL.   

Session U7: Nanoscale Pattern Generation and Lithography

Extreme Ultraviolet Lithography

//   

Maskless Electron-beam and Optical Lithography

Mask-based lithography is ideal for high-volume manufacturing because it enables enormous data transfer rates. In manufacturing, the high cost of masks and lithography tools can be amortized over large numbers of products. However, for low-volume manufacturing, research and the exploration of novel applications of lithography, maskless lithography systems have significant cost and convenience advantages. Scanning-electron-beam lithography (SEBL) systems are widely used in research and some low-volume manufacturing. They suffer from well known problems of pattern-placement accuracy, slow writing speed and, in some cases, substrate damage. Strategies for circumventing these problems will be described. A maskless optical-lithography system, called zone-plate-array lithography (ZPAL), has recently been demonstrated that achieves high throughput by the parallel operation of 1000 diffractive-optical lenses [www.lumarray.com]. The performance of ZPAL will be described and compared to SEBL. Also, novel nonlinear strategies for pushing the resolution of ZPAL to feature sizes comparable to those achieved by SEBL will be described.   

Ion Beam Patterning at the Nanometer Scale

Due to the absence of diffraction limitations, the extensive available process parameter space, and the prospects for one-shot imposition of a projection-reduced master mask pattern, ion beam patterning appears to offer a viable path to large-scale manufacturing of devices and systems based on nanoscale features, while offering robustness, flexibility, high quality of image definition and high throughput. We will review a variety of process variables, and the strategies by which they can be optimized for a specific application, in terms of resolution of the smallest features, minimal proximity effects, minimal edge effects, minimal statistical noise, high dimensional stability and pattern registration, and minimal effects on underlying layers. We use SRIM and other simulations of ion interactions to model the effects of ion species, energy, fluence and beam current density, and their impact on the choice of mask structure and type of photoresist where appropriate. We consider the application of the ions to pattern photoresist layers, or to locally modify the topography of polymer films, or to locally activate surfaces for selective adsorption. We also consider options for in-situ growth of 3D nanoscale features. Direct modification of the interfaces of thin film structures, and local ballistic disordering will also be discussed. Experimental demonstrations of low energy ion beam patterning with $<$40 nm resolution will include contact mask patterning of thin films of various polymers, and patterning of high-anisotropy magnetic multilayers for high storage density disk drive applications.   

Atomic Image Projection Electron Beam Lithography

While we are approaching to the nanotechnology era, as was proposed by Richard Feynman in 1959, our main concern still lies in how one can controllably manufacture and utilize nanometer scale features. The top-down approaches, most notably, lithography based techniques still have the problem of throughput although it has been successfully demonstrate to make features with the size less than 10 nm. The bottom-up approaches, either utilizing chemical vapor deposition process to make carbon nanotube or wet-chemical process to make size controllable quantum dots and rods, still have the limitation of extending it to many different types of materials and also delivering them on a wafer size substrate to make nanodevices. In this talk, we will propose a novel electron beam lithography technique to make nanometer scale features. The novelty of this process lies in the fact that one can utilize the crystalline lattice image commonly observed by the high resolution transmission electron microscopy as an ultimate mask to generate nanometer scale patterns. Using this technique, we demonstrate that down to 45 nm pitch size can be resolved on hydrogen silsesquioxine (HSQ) e-beam resist material. The patterns are formed on Si substarte with the dot size of about 30 nm and the line size of about 25 nm. This technique can be extend to define less than 10 nm size features only if the suitable resist is developed.   

Diblock Copolymers for Nanoscale Patterning

As the size scale of device features becomes increasingly smaller, conventional lithographic processes become increasingly more difficult and expensive, especially at a minimum feature size of less than 50 nm. Consequently, to achieve higher density circuits, storage devices or displays, it is evident that alternative routes need to be developed to circumvent both cost and manufacturing issues. An ideal process would be compatible with existing technological processes/manufacturing techniques and these strategies, together with novel materials, could allow significant advances to be made in meeting both short-term and long-term demands for higher density and faster devices. The self-assembly of block copolymers (BCP), two polymer chains covalently linked together at one end, provides a robust solution to these challenges. As thin films, immiscible BCP self-assemble into a range of highly-ordered morphologies where with size scale of the features is limited to the size of the polymers chains and are, therefore, nanoscopic in size. While self-assembly alone is sufficient for a number of applications in fabricating advanced microelectronics, directed self-orienting self-assembly processes are also required to produce complex devices with the required density and addressability of elements to meet future demands. By combining tailored self-assembly processes, a bottom-up approach, with micro-fabrication processes, a top-down approach, the ever-present thirst of the consumer for faster, better and cheaper devices can be met in very simple, yet robust, ways.   

Session U8: Granular Materials

Statistical and dynamical properties of a vibrated granular polymer

We investigate the structure and dynamics of granular polymers on a vibrated bed to test the applicability of models of self-avoiding random walks. The granular polymer is composed of a chain of hollow 3~mm steel beads connected by flexible links, and moves on a 30 cm diameter flat circular bed which is roughened by gluing a layer of 1 mm steel beads in order to give the chain random kicks in the vertical and horizontal directions. High speed digital imaging is used to track the position of the particles to a fraction of the bead diameter using a centroid technique. Using the identified bead positions, we analyze the motion of the center of mass over a time interval $\Delta t$, and its standard deviation as a function of chain length $L$. The standard deviation is consistent with a scaling of $\sqrt{\Delta t / L}$. The chain end-to-end distance scales as $L^\nu$, with $\nu\approx 3/4$ as for self-avoiding walks. The evolution of the scattering functions and the effect of the size of the container on the observed scaling will be also discussed.   

Power Spectra of Force Fluctuations in Granular Materials Under Shear

We measure the time-varying forces at the bottom surface of a granular system sheared at the top. The shear is applied by rotating a roughened piston while maintaining a constant, uniaxial compressive force. We report on the force autocorrelation and the corresponding power spectrum $S$ of the variation of force on individual grains at the bottom surface. These forces are obtained from video tracking of imprints in a pressure-sensitive birefringent layer across the bottom surface. Averaging over concentric annuli we find power-law behavior $S \sim 1/f^{\alpha}$ over several orders of magnitude in each annulus. The power law exponents $\alpha$ appear to be correlated with the in-plane shear strain rate. In our system friction with the stationary side walls introduces a radial gradient in the shear rate, which is maximum at the outer edge and zero at the center. The corresponding power law exponents suggest strict $1/f$ noise ($\alpha = 1$) at the outer, shearing edge and an increasing index as one approaches the center and the shear rate vanishes.   

Self-diffusion of particles in gas-driven granular layers with periodic flow modulation

We study particles self-diffusion in gas-driven granular layers by high-speed fluorescent video-microscopy. We show that periodic flow modulation results in an enhancement of the particle's diffusion. The diffusion enhancement, which in turn is an indication of more efficient fluidization of the granular layer, is associated with the onset of disordered sub-harmonic patterns. Our measurements provide a sensitive characterization method of the fluidization properties of particulate/gas systems.   

Free cooling of the one-dimensional wet granular gas

In the present work we consider a one-dimensional gas of hard balls covered with a thin liquid film. A liquid bridge, formed at each collision, is responsible for the hysteretic and dissipative interaction. Each rupture of a liquid bridge requires a fixed amount of energy, and thus determines a threshold of relative velocities below which the two colliding particles form a bounded state loosing their relative kinetic energy. We aim to study the cluster formation process in the free cooling system. Macroscopic laws of energy dissipation and cluster growth are studied in this model on the basis of numerical simulations supported by a scaling-like system of equations. We show that the sticky gas regime is an attracting asymptotic limit of the wet granular gas and does not dependent on the liquid bridges strength. The next neighbor velocities correlations play the key role in the establishing of this regime.   

A Theory of Stochastic Plasticity in Dense Granular Flow

There have been many attempts to derive continuum models for dense granular flow, but a general theory is still lacking, which can describe different flow conditions, such as gravity-driven silo drainage and forced shear cells. Here, we start with Mohr-Coulomb plasticity for quasi-2d granular materials to calculate stresses and slip planes, but we propose a simple ``stochastic flow rule'' to replace the principle of co-axiality in classical plasticity. This formulation takes into account two crucial features of granular materials -- discreteness and randomness at the scale of a continuum element -- via diffusing ``spots'' which cause chain-like cooperative particle displacements, as in recent simulations of silo drainage. We postulate that spots perform random walks along slip lines, biased by body forces (gravity) and local fluidization (switch from static to dynamic friction). Stochastic plasticity allows a natural description of dense granular flows in silos and shear cells within a single theory, rooted in classical mechanics.   

The Solitary Wave Collision Problem in Granular Alignments

Any impulse travels as a solitary wave in an alignment of spherical elastic grains where the system grains are barely in contact. These solitary waves are about 7 grain diameters wide. Their speeds depend upon the maximum displacement amplitudes associated with these waves. We focus on the dynamical problem associated with the collision of two identical and opposite propagating solitary waves. Interface and grain center collisions reveal markedly different dynamics. Solitary wave collisions lead to the destruction of the original waves and the subsequent creation of new smaller waves along with ``baby" or secondary solitary waves. In the absence of dissipation, these granular systems point towards the existence of a generalized equilibrium phase that involves Maxwellian distribution of velocities with no depedence on initial conditions but one that violates the equipartition theorem.   

Toward Zero Surface Tension Limit: Granular Fingering Instability in a Radial Hele-Shaw Cell

Because of the absence of cohesive forces between grains, dry granular material can, in many respects, be thought of as a fluid with zero surface tension. In the zero surface-tension limit, viscous fingering is known to possess singular behavior. We have studied the viscous fingering instability in such a granular ``fluid.'' In our experiment, we use a conventional radial Hele-Shaw cell consisting of two parallel glass plates separated by a gap. Gas with controlled pressures is blown through a hole at the center of one glass plate and displaces the surrounding dry granular material. We have systematically studied the fingering pattern as a function of gas pressure, gap thickness, and grain size. Two stages are observed during pattern growth. In the first stage, we find fluid-like fingering. However, as opposed to normal fluids, the pattern is more ramified at low pressure. In the second stage, we find several new behaviors in the system such as merging and pinching off of fingers and the existence of satellite bubbles.   

Force fluctuations in collisional and frictional granular flows

We make measurements of the force delivered to the wall in 2D and 3D flow geometries to explore the difference between collisional and frictional flows, and between flow geometries with and without velocity gradients in the flow direction. The distribution of force fluctuations has an exponential tail at large force in collisional flows, but falls off slower than an exponential in frictional flows. We do not see a clear signature in the force distribution of the approach to jamming and therefore the connection to force distributions in quasistatic flows remains to be understood. However, the temporal characteristics of the force fluctuations do show the approach to jamming. As reported earlier, the distribution of collision times tends to a power law in collisional flows. Similarly, the power spectrum of forces in frictional flows develops power-law behaviour at low frequencies as jamming is approached. Supported by NSF DMR 0305396 and NSF MRSEC DMR 0213695   

Thermal collapse of a granular gas under gravity

Free cooling of a gas of inelastically colliding hard spheres is a central paradigm of the kinetic theory of granular gases. At zero gravity the temperature of a freely cooling homogeneous granular gas follows a power law in time. How does gravity affect the cooling? We consider a semi-infinite layer of granular gas bounded from below by an elastic wall. An initially isothermal dilute granular gas is prepared in the state of hydrostatic equilibrium with barometric density distribution. We combine molecular dynamics simulations, a numerical solution of granular hydrodynamic equations and an analytic theory to show that the cooling gas undergoes thermal collapse: it condenses on the bottom of the container and cools down to zero temperature in a finite time $t_c$ as $T\sim (t_c-t)^2$. The cooling scenraio is determined by the interplay between the collisional energy loss and heat conduction, while the collapse time $t_c$ is much longer than the typical free fall time of the grains if the inelasticity of the particle collisions is small. The hydrodynamic description is found to be in excellent agreement with molecular dynamics simulations until very close to $t_c$.   

Two particle contact lifetime distribution in gravity driven granular flow

The distribution of two particle contact life times for gravity driven granular flow down an inclined plane are determined from large-scale, three-dimensional discrete element simulations. Results are presented for both cohesive and non-cohesive particles for Hertzian and Hookean contact forces. The distribution of lifetimes is analyzed as a function of height from the surface for different strength $k_n$ of the normal force, coefficient of restitution $e_n$ and coefficient of friction $\mu$. In addition a generalized form of the Bagnold constitutive relation in which the shear stress depends on a sum of terms that are linear and quadratic in the shear rate is proposed for cohesive granular flows. The linear term represents a new mode of momentum transport made possible through the long lived contacts in the network while the quadratic term represents the usual Bagnold contribution from short time scale collisions. For non-cohesive grains, the strength of the linear term disappears as strength of the normal interaction $k_n$ increased.   

Particle collisions in a granular gas

We report a study of particle collisions in a 2D granular system vibrated in a vertical plane. We have previously studied this experimental system in a variety of contexts. With improved image analysis algorithms, we are able to locate particles with enough precision to allow detailed tracking of the collision process, when the particles are close to each other. This allows us to better study the role of the vertical walls in the collision process and to place a limit on the dissipation by mechanisms other than inelastic collisions. We report the distribution of collision parameters and comment on violations of molecular chaos resulting from the inelasticity of the system.   

Boltzmann's \textit{stosszahlansatz }generalized for granular contact forces

Is there a valid way to generalize Boltzmann's \textit{stosszahlansatz }(molecular chaos), the assumption that colliding molecules are not statistically correlated before the collision takes place, to the case of granular contact forces in a static packing? In thermal statistical mechanics the assumption produces a transport equation that obtains the density of single particle states and the Maxwell Boltzmann distribution. The problem in generalizing this to granular contact forces is that we must maintain the spatial symmetries of granular packing ensembles, which is not trivial. The essential trick is to sum the density of states over all particle exchanges, which destroys multi particle state information but maintains the distribution of single particle states. This summation transforms the equations into a generalized form of boson statistics. I will show that, in the summation, the first shell approximation of the fabric is transformed into the properly symmetric version of Boltzmann's \textit{stosszahlansatz}. This produces a transport equation that obtains the density of single particle states and hence the distribution of granular contact forces. Granular simulation data will also be presented to validate the theory.   

Dynamical Heterogeneity close to the Jamming Transition in a Sheared Granular Material

The dynamics of a bi-dimensional dense granular packing under cyclic shear is experimentally investigated close to the jamming transition. Measurement of multi-point correlation functions are produced. The self-intermediate scattering function, displaying slower than exponential relaxation, suggests dynamic heterogeneity. Further analysis of four point correlation functions reveal that the grain relaxations are strongly correlated and spatially heterogeneous, especially at the time scale of the collective rearrangements. Finally, a dynamical correlation length is extracted from spatio-temporal pattern of mobility. Our experimental results open the way to a systematic study of dynamic correlation functions in granular materials.   

The Decorated Tapered Chain as a Granular Shock Absorber

A 1$D$ alignment of progressively shrinking spherical grains (a tapered chain) turns out to be an excellent impulse absorber with rich nonlinear dynamical behavior. Here we discuss a tapered chain with interstitial grains between every sphere of the original tapered chain and demonstrate analytically (using the hard sphere approximation), numerically and experimentally that the shock absorption ability of the ``decorated" system is far superior to that of the system without the interstitial grains.   

Shock Absorption by Small, Scalable, Tapered Granular Chains

Making shock proof layers is an outstanding challenge. Elastic spheres are known to repel softer than springs when gently squeezed but develop strong repulsion upon compression and the forces between adjacent spheres lead to \textit{ballistic-like} energy transfer between them. Here we demonstrate for the first time that a \textit{small alignment} of progressively shrinking spheres of a strong, light-mass material, placed horizontally in an appropriate casing,$^{ }$can absorb $\sim $ 80{\%} ($\sim $90{\%}) of the incident force (energy) pulse. The system can be scaled down in size. Effects of varying the size, radius shrinkage and restitutive losses are shown via computed ``dynamical phase diagrams.''   

Session U9: Scanning Probe Microscopy

An \textit{in-situ} Study of Martensitic Transformation in Shape Memory Alloys using PEEM

The thermally-induced martensitic transformation in a polycrystalline CuZnAl and NiTi thin film shape memory alloy (SMA) was probed using photoemission electron microscopy (PEEM). Ultra-violet photoelectron spectroscopy (UPS) measurements indicate that the apparent surface work function changes reversibly during transformation, presumably due to the contrasting electronic structures of the martensitic and austenitic phases. \textit{In situ} PEEM images provide information on the spatial distribution of these phases and the microstructural evolution during transformation. The evolution of the photoemission intensities obtained from PEEM images during transformation can provide quantitative information on fractional percentages of austenite and martensite phases as the transformation proceeds. PEEM offers considerable potential for improving our understanding of martensitic transformations in shape memory alloys in real time.   

Investigation of ferroelectric materials with scanning microwave microscope

By using scanning microwave microscope (SMM), we investigated dielectric properties of ferroelectric materials in high frequency regime (1.5GHz). Our SMM had the capability to measure a complex dielectric constant of the samples from the shift of resonant frequency (fr) and Q value of the probing resonator. In order to obtain non-linear dielectric constants of the ferroelectric samples, we applied oscillating electric field perpendicular to the sample and measured the 1$^{st}$ order derivative of the resonant frequency of the resonator (dfr/dE) with respect to the applied field. In this way we could image the ferroelectric domain and the domain boundary structure of the triglycine sulfate single crystal using the dfr/dE and the fr signal, respectively. Moreover we observed the ferroelectric responses from the tunable dielectric Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ thin film under the additional DC voltage bias to the film.   

Plasmon-based Enhanced NSOM Spectroscopy.

Surface enhanced Raman spectroscopy is a well established technique for enhancing the Raman signal of a particular sample, allowing for spectroscopy of far lower quantities of the molecule of interest than other procedures allow. This enhancement is mainly caused by the enhancement of the incident electric field by exciting the plasmon resonance of the surface. By attaching metal nanoparticles on an NSOM probe, we demonstrate that the plasmon-based enhancement can come from the probe itself instead of the surface, resulting in a powerful tool for the chemical analysis at the nanometer scale.   

Dielectrophoretic Force Microscopy

Dielectrophoretic force microscopy, a novel scanning probe microscopy technique in which a tip-sample dielectrophoretic force is incorporated into the feedback mechanism of a standard atomic force microscope, is shown to allow for the facile noncontact imaging of the dieelctric properties of systems in aqueous media. By tuning the ac frequency, dielectric spectroscopy can be performed at solid/liquid interfaces with high spatial resolution. In studies of cells, the frequency-dependent dielectrophoretic force is sensitive to biologically relevant electrical properties, including local membrane capacitance and ion mobility. Additionally, the presence of a dielectrophoretic force reduces the mechanical tip-sample contact forces that frequently hinder microscopy studies of soft, deformable systems. Consequently, dielectrophoretic force microscopy is well suited for \textit{in situ} scanning probe microscopy studies of biological systems.   

Spectral density of fluctuations for a driven, nonlinear micromechanical oscillator at kinetic phase transition

We measure the spectral densities of fluctuations of an underdamped nonlinear micromechanical torsional oscillator. By applying a sufficiently large periodic driving force, two stable dynamical states occur within a particular range of drive frequency. White noise is injected into the driving force allowing the system to overcome the activation barrier and to switch between the two states. While the system predominately resides in one of the two states for most excitation frequencies, a narrow range of frequencies exist where the population levels are approximately equal and the system is at a `kinetic phase transition' that bears resemblance to the phase transition of thermal equilibrium systems. By examining the power spectral densities of the measured oscillation amplitude, the fluctuation characteristics of the system can be studied. At the `kinetic phase transition' a supernarrow peak, centered at the excitation frequency, arises as a result of noise-induced transitions between the two dynamic states. Smaller, secondary peaks associated with fluctuations about the two attractors are also examined. Its dependence on noise and excitation frequency is shown to be distinct from that of the supernarrow peak.   

Nonlinear coupling of nano mechanical resonators to Josephson quantum circuits

We study a technique to couple the position operator of a nano mechanical resonator to a SQUID device by modulating its magnetic flux bias. By tuning the magnetic field properly, either linear or quadratic couplings can be realized, with a discretely adjustable coupling strength. This allowes us to realize nonlinear dynamics on the nano mechanical resonator by coupling it to a Josephson quantum circuits. As an example we show how squeezing of the nano mechanical resonator can be realized with this technique. We also propose a simple method to measure the uncertainty in the position of the nano mechanical resonator without quantum state tomography.   

Nanomanipulation with dynamic AFM

Nanomanipulation [1] is one of the most important current issues in dynamic AFM (DAFM). Following the first vertical manipulation on Si(111)-(7x7) [1] a lateral manipulation was documented by interchange manipulation of Sn and Ge adatoms on the Ge(111)- c(2x8) surface [2]. However, the atomistic details and nature of these processes remain unclear. In order to shed light on these experiments we have performed DFT simulations on two model systems: (1) anionic antisite defect on the InP(110) surface [3], and (2) the Sn-covered Ge(111)-c(2x8) surface. In (1) the P defect atom moves vertically in a double well potential with two minima, which opens the possibility to vertically manipulate the defect atom from one minimum into the other. We will addresses issues such as whether the experiments can be performed in both attractive and repulsive interaction regimes and whether the basic atomistic mechanism is related to lowering of the barriers by the presence of the tip, or by a purely mechanical effect where the atom is pushed over a barrier. In (2) we will show how presence of the tip can affect the charge transfer processes between the different dangling bonds and hence induce atomic manipulation. [1] N. Oyabu et al., Phys. Rev. Lett. 90, 176102 (2003) [2] Y. Sugimoto et al., Nature Mater. 4, 156 (2005); N. Oyabu et al., Nanotechnology 16, S112 (2005). [3] P. Dieska, I. Stich, R. Perez, Phys. Rev. Lett. 95 126103 (2005)   

Assembly of Nanoparticle-Attached AFM Tips for Nano-Optical Applications

The well-defined geometry and chemical properties of the end of atomic force microscopy tips are critical components for various tip-enhanced nano-optical applications such as nanoscale Raman and FRET imaging. However, conventional AFM tip fabrication method often results in a large variation of tip shapes and chemical properties. Recent nanotechnology allows us to synthesize `nanoparticles' (e.g. Au, Ag, CdSe, etc). We developed a method to mass-produce `AFM tips with well-defined geometry and chemical properties' by assembling a single nanoparticle at the end of the tip via self-assembly strategy. In this way, only the end part of the tip is functionalized with organic molecules which attract nanoparticles in the solution. When the functionalized tip is placed in the nanoparticle solution, nanoparticles are selectively assembled only onto the end of the tip. We assembled a nanoparticle (e.g. 50nm diameter Au nanoparticle) at the end of the tip and demonstrated AFM imaging using these tips. Our method allows us to assemble nanoparticles at the end of the tip, and it can be scaled up for large scale assembly.   

Investigation of Electrical Behaviors of Nanostructures through Scanning-Probe Microscopy

A scanning-probe microscope with two electrically-isolated electrodes fabricated on one probe is used to locally investigate electrical behavior of nanostructures. The split-tip probe, which we have recently developed, is optimized for light coupling into a particular region of a nanostructure while non-contact measurements are simultaneously made between the two electrodes. The capacitance is influenced by the presence of a conducting region on the surface beneath the electrodes. The capacitance coupled or scanning conductivity mode allows rapid characterization of large numbers of molecules so that molecules of interest can be identified for further study. Finite element models aid in the quantification and understanding of the data.   

Scanning Tunneling Potentiometry for Nanoscale Transport Studies

We have developed a scanning tunneling potentiometry (STP) system for study of electrical transport on nanometer length scales. A novel biasing scheme is used to achieve electrochemical potential resolution at the theoretical limits of this measurement - the thermal noise of the tunnel junction. We apply this technique to several materials in order to explore the capabilities of the instrument. These include thin films of Au, the ``bad metal'' SrRuO$_{3}$ and amorphous indium oxide. Homogeneity of transport in these systems is discussed. Work supported initially by the AFOSR and currently by the NSF.   

Alpha Control - A new Concept in SPM Control \newline

Controlling modern Scanning Probe Microscopes demands highly sophisticated electronics. While flexibility and powerful computing power is of great importance in facilitating the variety of measurement modes, extremely low noise is also a necessity. Accordingly, modern SPM Controller designs are based on digital electronics to overcome the drawbacks of analog designs. While todays SPM controllers are based on DSPs or Microprocessors and often still incorporate analog parts, we are now introducing a completely new approach: Using a Field Programmable Gate Array (FPGA) to implement the digital control tasks allows unrivalled data processing speed by computing all tasks in parallel within a single chip. Time consuming task switching between data acquisition, digital filtering, scanning and the computing of feedback signals can be completely avoided. Together with a star topology to avoid any bus limitations in accessing the variety of ADCs and DACs, this design guarantees for the first time an entirely deterministic timing capability in the nanosecond regime for all tasks. This becomes especially useful for any external experiments which must be synchronized with the scan or for high speed scans that require not only closed loop control of the scanner, but also dynamic correction of the scan movement. Delicate samples additionally benefit from extremely high sample rates, allowing highly resolved signals and low noise levels.   

Theory of Q-Controlled Dynamic Force Microscopy in Liquids

The so-called Q-control method allows the active modification of the effective cantilever damping in dynamic force microscopy (DFM) by increasing or decreasing the Q-value of the cantilever. This feature has been used in recent years in numerous experimental studies to improve the apparent imaging capabilities of DFM in liquids. However, it is striking that an in-depth analytic description that would allow a rigorous theoretical explanation of the various features of Q-controlled dynamic force microscopy (QC-DFM) is still missing. Here, we present an analysis of QC-DFM based on the analytical solution of the equation of motion considering a model tip-sample interaction force. Explicit formulas allowing for the calculation of relevant parameters such as amplitude, surface deformation, and maximum forces during an individual oscillation cycle are given. It is found that higher effective Q-factors assist in reducing the maximum tip-sample forces. This helps suppressing unwanted deformations of the sample surface, leading to the reported enhanced image quality. Finally, the results are discussed in relation to the situation in air.   

Simulation of contact and non-contact AFM images of H-terminated Si(100) surface with a CH3 impurity

Using a density-functional-based tight-binding method, we have investigated whether atomic force microscope (AFM) images with atomic resolution can be obtained for hydrogen-terminated silicon (100) 1x1 surface including a methyl. We have simulated contact mode images of this surface using a silicon tip with and without a hydrogen atom at the apex. For the silicon tip without hydrogen at the apex, we obtained good images with anisotropic spots reflecting the symmetry of a methyl for large tip-sample distance. For the silicon tip with hydrogen at the apex, we found that better images with atomic resolution, showing internal hydrogen and carbon atoms of a methyl, are expected if the forces can be measured precisely. We have also examined non-contact mode images. Although a force line profile of non-contact mode is smoother than one of contact mode, their difference is not so large.   

Session U10: Focus Session: Surfaces and Interfaces in Electronic Materials II

Nanoscale Patterning of Electrochemically Deposited Metallic Features on Si, Ge, InP and GaAs Surfaces

Nanostructured materials continue to be the focus of intense research due to their promise of innumerable practical applications as well as advancing the fundamental understanding of these intriguing materials. In particular, the need for metallic features of increasingly smaller size regimes has imposed stringent demands upon chemists to produce a variety of highly functional materials with reduced dimensions.While much effort has been expended towards the synthesis of nanoscale structures, one of the most challenging aspects for the nanoscale materials community is the question of how to `wire in' these functional elements with the real world. In this talk, we will describe recent work towards the synthesis and nanoscale patterning of metallic structures on semiconductor surfaces such as silicon, germanium, gallium arsenide and indium phosphide. Through simple and efficient galvanic displacement reactions on these interfaces, complex metal nanostructures form spontaneously, and can be patterning via self-assembling soft block copolymer materials. The self-assembled materials direct transport of reagents to the semiconductor so that the reaction takes place in a spatially defined manner, with precise control over the quantity of reagent delivered. Even mixtures of reagents can be 'sorted out' by these interfaces to produce nanoscale ($\sim $10 nm) domains of different chemical functionalities, simultaneously. We will describe these and related approaches towards precise patterning of semiconductor surfaces, entirely via wet-chemical processes that are compatible with existing fabrication strategies.   

Scanning Tunneling Microscopy Study of Molecular Structure: Controlled Monolayer Formation on Graphite at the Liquid-solid Interface

The self-assembly of heptadecanoic acid \textbf{1} and racemic 2-bromoheptadecanoic acid \textbf{2} mixtures on the basal plane of a graphite surface has been studied using scanning tunneling microscopy at the liquid-solid interface. The domain structure varies as a function of the ratio of coadsorbed molecules. At lower concentration of acid \textbf{2}, heptadecanoic acid controls the surface structure by forming a template with fixed lamellar axis-molecular axis angle and domains with alternating R- and S-enantiomer molecular rows. Increasing the concentration of acid \textbf{2} leads to the segregation of chiral domains. The inter-correlation between heptadecanoic acid and 2-bromoheptadecanoic acid determines the 2D chiral configuration in the mixed monolayer. A model based on energetically favorable molecular conformations is proposed and will be discussed.   

Static and Dynamic Aspects of Surfactant Surface Aggregates studied by AFM

Using AFM, we show that surfactants form micellar aggregates of varying morphology, depending on the surface structure. While all previous studies were limited to atomically flat substrates, we achieve imaging the micelles on rough gold. By gradually annealing these surfaces, we show the influence of roughness on the aggregate structures. For crystalline gold (111), aligned, hemi-cylindrical micelles that recognize the symmetry axes of the gold lattice are found. With increasing roughness, the degree of organization of the aggregates decreases. We also show that the micellar pattern on HOPG and gold(111) surfaces changes with time and responds to perturbations in a self-healing way. Our results suggest that this organization happens at the molecular scale. Theoretical analysis for HOPG, however, show that the micelle orientation cannot be explained on the molecular level, but the anisotropic van der Waals interaction between micelles and HOPG has to be considered as well [1]. \newline \newline [1] Saville, D. A.; Chun, J.; Li, J.-L.; Schniepp, H. C.; Car, R.; Aksay, I. A., accepted by Physical Review Letters.   

Accelerated Molecular Dynamics Simulation of Alkane Desorption

Thermal desorption has been the focus of much surface science research. Studies of alkanes on graphite$^{1}$ and gold$^{2}$ have shown prefactors that are constant with alkane chain length but vary by over six orders of magnitude. Other studies on magnesium oxide$^{3}$ and gold$^{4}$ show a prefactor that increases with increasing chain length. We have developed an all-atom model to study alkane desorption from graphite. Transition state theory is used to obtain rate constants from the simulation. Accelerated MD is used to extend the desorption simulation to experimentally relevant temperatures. Our results show a prefactor that increases with increasing chain length. We predict that it will become constant as internal conformational changes occur significantly. We examine the effect of desorption environment through varying the alkane surface coverage. 1. K.R. Paserba and A.J. Gellman, \textit{J. Chem. Phys. }\textbf{115}, 6737 (2001). 2. S.M. Wetterer et al., \textit{J. Phys. Chem.} \textbf{102}, 9266 (1998). 3. S.L. Tait et al., \textit{J. Chem. Phys. }\textbf{122}, 164707 (2005). 4. K.A. Fichthorn and R.A. Miron, \textit{Phys. Rev. Lett. }\textbf{89}, 196103 (2002).   

Atomic force microscopy and fluorescence correlation spectroscopy studies of interfacial fluids.

We have studied the dynamic structure of thin ($\sim $ few nm) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of $\sim $ 1 nm, which is consistent with the molecular dimension as well as previous x-ray reflectivity measurements. However, it fails to detect any stiffness oscillation, indicating that molecules are layered weakly near the solid-liquid interface. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measure the translational diffusion of the probe molecule. The autocorrelation function cannot be fitted with a single diffusion coefficient indicating that the dynamics may vary in different layers.   

A Single Molecule View of Bi-stilbene Photoisomerization Using Scanning Tunneling Microscopy

The advent of scanning tunnelling microscopy (STM) has permitted a detailed atomic view of organic molecules adsorbed on solid surfaces. With the use of the STM, we present an unprecedented direct single-molecule perspective on the cis-trans photoisomerization of stilbene molecules within ordered-monolayers physisorbed on the Ag/Ge(111) surface. The STM view of the molecular structure transformation upon irradiation provides a direct evidence for the generally accepted one-bond-flip mechanism proposed for the photoisomerization process. We also find that the surface environment produces a profound effect on the reaction mechanism. The reaction is observed to proceed mainly through pairs of co-isomerizing molecules situated at domain boundaries. To explain these observations, we propose a mechanism whereby excitation migrates to the domain boundary and the reaction occurs through a biexciton reaction pathway.   

A combined theoretical and experimental study on the structure of Methylthiolates on the Au(111) surface

Self-assembled monolayers (SAMs) of sulphur containing organic molecules on gold have received enormous attention due to the central role these interfaces play in molecular electronic devices, biosensors, surface coatings and nanolithography. Despite their interest the atomic structure of Methylthiolates on Au(111) surfaces, the simplest SAM in this class, is not fully understood. Here we address this problem with a combined theoretical and experimental study. We show that an asymmetric bridge (quasi on-top) site fits both the X-Ray and Photoelectron Diffraction data better than either the symmetric bridge site or on-top site. To understand this phenomenon we have performed molecular dynamics simulations employing density functional theory within the generalized gradient approximation. We show that at high temperatures the presence of vacancies and gold adatoms tends to favour the quasi on-top site, in spite of the fact that the symmetric bridge site is the lowest energy site at $T=0$.   

Spectroscopic STM study of the binding configuration of benzene molecules on the Si(111)7x7 surface

We have used a home-built UHV STM to study the bonding configuration of benzene molecules chemisorbed onto clean silicon. Our compact STM head is based on the symmetrical, Besocke design. In our design, thermal drift is eliminated to first order, by using the correct combination of materials with known thermal expansion coefficients.$^{ 1}$ This STM head is also capable of being positioned inside of a liquid helium cryostat currently under construction in our lab. Our long-term goal is to use spectroscopic dI/dV imaging to focus on spatial variations of the LDOS and gain valuable information not generally available in topographic imaging.$^{2}$ In particular we will present room-temperature results on the role of restatoms in the binding of benzene molecules to adjacent adatoms. Results on the spatial position and direction of C-C double bonds will also be presented. 1)Stipe \textit{etal}, \textit{Rev. Sci. Instr}. \textbf{70}, 137 (1999). 2)Hamers \textit{etal}, \textit{Phys.Rev.Lett.} \textbf{56}, 1972 (1986).   

STM study of adsorption and dissociation of trichloroethylene molecules on the Si(111)7x7 surface.

We have performed, for the first time, STM studies of the adsorption of trichloroethylene (TCE) on clean silicon. The results were taken with a home-built UHV STM operating at room temperature. Our STM is capable of obtaining both topographic as well spectroscopic images. One of the products of the dissociation has been determined to be single chlorine atoms bound to Si adatoms as evidenced by topographic imaging and tip induced diffusion.$^{1}$ The other product of the dissociation is believed to be a vinyl group attached to an adatom as proposed in a study using EELS and TDS.$^{2}$ Results on the binding site preference (ex. corner vs. middle adatoms) for the Cl and vinyl group will also be presented as well as conclusions about the diffusion of Cl on Si. 1)Nakamura \textit{etal}, \textit{Surf. Sci}. \textbf{487}, 127 (2001). 2)He \textit{etal, Surf. Sci}. \textbf{583}, 179 (2005).   

Decompositional, incommensurate growth of Ferrocene molecules on a Au(111) surface

We have investigated in depth the first layer growth of ferrocene molecules on a Au(111) surface with a low temperature scanning tunneling microscope. Ferrocene molecules adsorb dissociatively and form a two layer structure after being decomposed into fragments. The toplayer unit cell is composed of two tilted cyclopentadienyl rings, while the first layer consists of the remaining fragments. Surprisingly a fourfold symmetry is observed for the top layer while the first layer displays threefold symmetry elements. It is this symmetry mismatch which induces an incommensurability between these layers in all except one surface direction. The toplayer is weakly bonded allowing for an antiferromagnetic ordering as shown by density functional theory calculations.   

Session U11: Focus Session: Aerosols, Clusters, Droplets: Physics and Chemistry of Nanoobjects IV: Metal Clusters I

Infrared Spectroscopy of Metal Ion Complexes: Models for Metal Ligand Interactions and Solvation

Weakly bound complexes of the form M$^{+}$-L$_{x}$ (M=Fe, Ni, Co, etc.; L=CO$_{2}$, C$_{2}$H$_{2}$, H$_{2}$O, benzene, N$_{2})$ are prepared in supersonic molecular beams by laser vaporization in a pulsed-nozzle cluster source. These species are mass analyzed and size-selected in a reflectron time-of-flight mass spectrometer. Clusters are photodissociated at infrared wavelengths with a Nd:YAG pumped infrared optical parametric oscillator/amplifier (OPO/OPA) laser or with a tunable infrared free-electron laser. M$^{+}$-(CO$_{2})_{x}$ complexes absorb near the free CO$_{2}$ asymmetric stretch near 2349 cm$^{-1}$ but with an interesting size dependent variation in the resonances. Small clusters have blue-shifted resonances, while larger complexes have additional bands due to surface CO$_{2}$ molecules not attached to the metal. M$^{+}$(C$_{2}$H$_{2})_{n}$ complexes absorb near the C-H stretches in acetylene, but resonances in metal complexes are red-shifted with repect to the isolated molecule. Ni$^{+}$ and Co$^{+}$ complexes with acetylene undergo intracluster cyclization reactions to form cyclobutadiene. Transition metal water complexes are studied in the O-H stretch region, and partial rotational structure can be measured. M$^{+}$(benzene) and M$^{+}$(benzene)$_{2}$ ions (M=V, Ti, Al) represent half-sandwich and sandwich species, whose spectra are measured near the free benzene modes. These new IR spectra and their assignments will be discussed as well as other new IR spectra for similar complexes.   

Adaptive Tempering Monte Carlo Optimization of Calcium Clusters.

The global minimum energy structures of calcium clusters with 15 to 34 atoms were obtained by the Adaptive Tempering Monte Carlo (ATMC) method. The cluster binding energy was obtained within a tight binding approach with parameters reported in previous work[1]. The ATMC optimization process is fast and drives the system across configuration space very effectively reaching the global minimum within a small number of tempering events. The structure of six cluster sizes 15, 16, 18, 21, 23 and 25 corresponding to the global minimum has not been reported in the literature for any other metals. Three clusters Ca$_{15}$, Ca$_{21}$ and Ca$_{23}$ are relatively more stable than the others in this size range. Melting of these clusters are further studied with the weighted histogram analysis method and the free energy profile is predicted. The melting transition is monitored with a novel structural order parameter that reflects the mobility of surface atoms, their bonding order and bonding directionality. [1] X. Dong, G. M. Wang, E. Blaisten-Barojas, Phys. Rev. B 70, 205409 (2004).   

Structures and ligand binding energies of size seleceted gold and silver clusters:~ Approach to the bulk

Gold and silver clusters are formed by laser ablation, mass selected, and either reacted with ethene or subjected to ion mobility measurement.~ Structures are assigned by two methods.~ In the first sequential ligand binding energies are measured and correlated with possible structures.~ In the second experimental cross sections are correlated with cross sections from model structures.~ Both anionic and cationic clusters are measured in the size range 3 to 13.~ For the anions calculated and experimental detachment energies are also used as structural diagnostics.~ The various data are compared with bulk values and approach to the bulk assessed.   

A comparison of the electronic structure and optical plasmons in Cs$_{x}$ clusters, Cs$_{x}$ shells and C$_{60}$ coated with a Cs$_{x}$ shell

We present calculations of the electronic structure and collective excitations in Cs clusters, Cs shells and C$_{60}$ coated with a shell of Cs atoms. The ground state properties of these systems are described using the Local Density Approximation and the electronic excitations by the Random Phase Approximation. The jellium shell approximation underlying our calculations correctly predicts the magic numbers. The optical excitation spectra in Cs clusters and Cs coated C$_{60}$ are found to be in agreement with available experimental data.   

Signatures of Random Matrix Theory in the Discrete Energy Spectra of Shaped Disordered Metallic Clusters

It has been predicted that the distribution of the discrete energy levels of disordered metallic clusters should follow random matrix theory. It has been possible to study distributions of energy levels for different shaped metallic clusters using a low temperature scanning tunneling microscope. Depending on the degree of ``shape'' disorder, the statistics either follow Wigner-Dyson statistics, a mixed state, or Poisson-like statistics for the distribution of energy levels. We will present a summary of results on Pb clusters grown by a buffer layered assisted growth technique and in addition show how it is possible to use scanning tunneling spectroscopy to image a quantity proportional to the square of the amplitude of the eigenfunctions for quantum confined systems. These images resemble images acquired in microwave cavity experiments for classically chaotic and nonchaotic systems. This work was supported by the Department of Energy under grant DE-FG02-02ER46004.   

Structure in Binary Nanodroplets

Recent SANS measurements of core-shell structure in binary nanodroplets ($\sim $ 9 nm) have stimulated our research on the structure of droplets of this size. [Wyslouzil, et. al., Phys. Chem. Chem. Phys. \textbf{8}, xxx, (2006)] By structure, we mean the spatial distribution of chemical species within the droplet. Based on recent work by Cordeiro and Pakula [J. Phys. Chem. \textbf{109}, 4152 (2005)], we developed an efficient Lattice Monte Carlo (LMC) method to simulate binary droplets containing 5000 to 10000 particles. Simulations of nanodroplets of various compositions were made to study phenomena such as species segregation and phase separation. Depending on the relative strengths of the intermolecular interactions, various interesting structures were found. Droplets may be fairly well-mixed, strongly segregated core-shell structures, or even highly segregated nonspherical shapes resembling partially disassembled Russian dolls. We explored the temperature dependence of the droplet structures and observed that the reversible change between the core-shell and Russian doll structures could be viewed as a wetting---dewetting transition. The transition temperature was determined for a specific system.   

Source for a Temperature-Controlled Metal Cluster Beam

Metal clusters can be produced easily by laser vaporization of a sample into an inert cooling gas. We have used a pulsed Nd:YAG laser to evaporate cobalt from a rotating rod into a 20cm-long narrow pipe filled with helium gas, injected by a pulsed gas valve. The outgoing part of the pipe (15cm long) is attached to a helium refrigerator and an electrical heater, which allow us to control the pipe's temperature over the range from 60K to room temperature. If the gas-cluster mixture stays in the pipe long enough before supersonic expansion, it reaches thermal equilibrium with the pipe.   

Modeling the Melting of Free and Supported Metal Clusters

The growth rate and mechanism of one-dimensional structures, such as carbon nanotubes and zinc-oxide nanorods, is expected to be significantly affected by the phase of catalytic metal particle. It is therefore important to understand the structure and dynamics of these particles in their solid and liquid phases, and to know how their melting points depend on cluster size and substrate adhesion. Results from molecular dynamics studies on the structural and dynamic changes during melting of free and supported iron clusters, ranging from 150 to 10 000 atoms, will be presented. We will also present a method to determine effective diameters of supported metal clusters, so that the melting point dependence on cluster size can be predicted in a physically meaningful way by the same analytic model used for free clusters.   

Geometric and electronic structure of mixed metal-semiconductor clusters from global optimization.-

In addition to pure metal and semiconductor clusters, hybrid species that contain both types of constituents occur at the metal-semiconductor interface. Thus, clusters of the form Cu(x)Si(y) were detected by mass spectrometry [1]. In this contribution, the geometric and energetic features of Me(m)Si(7-m) (Me=Cu and Li) clusters are discussed. The choice of these systems is motivated by the structural similarity of the pure Si(7), Li(7), and Cu(7) systems which all stabilize in D(5h) symmetry. On the other hand, Li and Cu, representing the alkali group (IA) and the noble metal group (IB) of the periodic system, are expected to display strongly differing behavior when integrated into a Si(n) cluster, resulting in different ground state geometries for the cases Me = Li and Me = Cu. Addressing this problem by means of geometry optimization requires, in view of the large number of possible atomic permutations for Me(m)Si(7-m) with 0 $<$ m $<$ 7, the use of a global search algorithm. Equilibrium geometries are obtained by simulated annealing within the Nose' thermostat frame. It is observed that Cu(m)Si(7-m) clusters with m $<$ 6 tend towards ground state geometries derived from the D(5h) prototype. For Li(m)Si(7-m), the Li(m) subsystem is found to adsorb on the framework of the Si(7-m) dianion. [1] J.J. Scherer, J.B. Pau, C.P. Collier, A. O'Keefe, and R.J. Saykally, J. Chem. Phys. 103, 9187 (1995).   

Unbiased search of minimal energy nanocluster structures

A new strategy to find global minima is applied to the structure of metallic clusters. It consists in implementing a conformational space annealing (CSA) unbiased search in combination with many body phenomenological potential techniques to create a data bank of putative minima. Next, the clusters in this data bank are examined by first principle methods to obtain the minimum energy cluster. The scheme is successfully applied to magic number 13 atom clusters of rhodium, palladium and silver. Global minimum energy cluster structures not previously reported are found through our procedure.   

Session U12: Focus Session: Electrochemical and Related Growth

Physical Origin of Long-Range-Order in Lateral Development of Crystallites: A New Lateral Growth Mode

The basic picture of hetroepitaxial growth can be summarized as formation of islands on a foreign substrate first, followed by horizontal expansion of the islands on the substrate. Previously little attention has been paid to how the interfacial tensions affect the horizontal expansion of a crystalline island over the substrate, which has recently been found to affect the physical property of thin film significantly. For example, tilting of crystallographic orientation has frequently been observed in epitaxial layers, and our understanding of such effect is very limited. Recently Wang and his colleagues studied lateral growth of NH$_{4}$Cl crystallite on a foreign substrate mediated by successive nucleation. With state-of-the-art structural and morphological characterization methods, they observed that the crystallographic orientation is consecutively rotated, leading to periodic structures on the surface of crystallite aggregate. They demonstrated that this unusual effect is related to the asymmetric surface/interface tensions in the early stage of nucleation, and should be enlightening for a class of thin film growth where nucleation plays a dominate role. Mu Wang, D.-W. Li, D.-J. Shu, P. Bennema, et.al., Phys. Rev. Lett., \textbf{94}, 125505 (2005) D. W. Li, Mu Wang, P. Liu, et.al., J. Phys. Chem. \textbf{B107}, 96-101, 2003, X. Y. Liu, Mu Wang, D. W. Li, et al., J. Cryst. Growth \textbf{208}, 687-695 (2000) Mu Wang, X.Y. Liu, C. Strom, et al., Phys. Rev. Lett. \textbf{80}, 3089 (1998)   

Steering induced growth anisotropy as a probe for long range interaction

Grazing incidence homo-epitaxy of 0.5 ML on Cu(001) leads to anisotropic structures as determined with high-resolution LEED. This is the result of attractive forces between the surface and the incoming particle. The trajectory of an incoming particle changes so dramatically that a large deposition flux enhancement on protruding structures results [1]. Trajectory calculations based on an attractive Lennard-Jones potential were combined with a kMC simulation that treats the surface diffusions processes in order to investigate the evolution of the observed anisotropy. Modifications of this potential at short range distances only slightly influence the anisotropy, while modifications at long range has a significant influence on the anisotropy as observed during sub-monolayer growth. This enables to probe the long range interaction. The experimental feasibility of the detailed probing will be discussed. [1] S. van Dijken, L.C. Jorritsma and B. Poelsema, Phys. Rev. Lett. 82 4038 (1999)   

Analysis of Chemical Reactions between Radical Growth Precursors Adsorbed on Plasma-Deposited Silicon Thin-Film Surfaces

The dominant precursor in the plasma deposition of hydrogenated amorphous silicon (a-Si:H) thin films is the SiH$_{3}$ radical. In this presentation, we report results of first-principles density functional theory calculations on the crystalline Si(001)-(2$\times $1):H surface and molecular-dynamics simulations on a-Si:H surfaces for the interactions between SiH$_{3}$ radicals adsorbed on Si thin-film surfaces. The analysis reveals that two SiH$_{3}$ radicals may either form disilane (Si$_{2}$H$_{6})$ that desorbs from the surface or undergo a disproportionation reaction producing an SiH$_{2}$ radical that is incorporated in the film and a silane molecule that is released in the gas phase. The corresponding activation barriers depend on the local atomic coordination of the surface Si atoms; Si$_{2}$H$_{6}$ formation is barrierless if both radicals are bonded to overcoordinated surface Si atoms and exhibits barriers in excess of 1 eV for two chemisorbed SiH$_{3}$ radicals.   

Controlling the periodicity of the Si(112)nx1-Ga surface via tuning of the chemical potential

We show that the chemical potential, an important parameter in the initial stages of (hetero-) epitaxial semiconductor growth, can be tuned for the Ga atoms on the Si(112)nx1-Ga surface. As a result the periodicity of the surface can be controlled in the range of n=5 to n=6. STM shows that meandering vacancy lines determine the local size of the unit cell. Large scale statistics of the unit cell size extracted from STM images show that the average periodicity n is not an integer, but lies somewhere in between 5 and 6. These findings are confirmed by a careful analysis of new LEED data, which show a range of periodicities in between 5x1 and 6x1 depending on the surface preparation conditions. The extracted periodicities are consistent with periodicities extracted from Fourier Transform STM images. Thus, changes of the chemical potential of the Ga atoms on the surface can be easily monitored in situe by extracting the average surface periodicity from LEED images.   

Nanolithographic Write, Read and Erase via Reversible Nanotemplated Nanostructure Electrodeposition on Alkanethiol Modified Au(111) in an Aqueous Solution

A Write, Read and Erase nanolithographic method, combining \textit{in-situ} electrodeposition of metal nanostructures with atomic force microscopy (AFM) nanoshaving of a 1-hexadecanethiol (HDT) self-assembled monolayer (SAM) on Au(111) in an aqueous solution, is reported. The AFM tip defines the local positioning of nanotemplates via the irreversible removal HDT molecules. Nanotemplates with lateral dimensions as narrow as 25 nm are created. The electroactive nanotemplates determine the size, shape and position of the metal nanostructures. The potential applied to the substrate controls the amount of metal deposited and the kinetics of deposition. Metal nanostructures can be reversibly and repeatedly electrodeposited and stripped out of the nanotemplates by applying appropriate potentials.   

Growth of Electrodeposited Ag Nanowires in Anionic Surfactant Nanotemplates on Au(111)

Ordered molecular systems should provide templates of molecular dimensions as demonstrated by the growth of silver nanostructures in the potential induced nanotemplates of SDS (sodium dodecyl sulfate). Electrochemical STM (Scanning Tunneling Microscopy) results suggest that SDS molecules form hemicylinders on the Au(111) surface in 0.1M HClO$_{4}$ solution. The hydrophilic sulfate groups self-assemble to face to the aqueous interface while the hydrophobic backbone adopts a tail to tail configuration. The SDS hemicylinders structures are stable over the potential range of -0.1 V$_{SCE}$ to 0.4 V$_{SCE}$. Silver electrodeposition takes place near the hydrophilic sulfate head, and leads to the formation of nanowires that grow in the same direction as the SDS hemicylinders. Ag nanowires are typically less than 2 nm wide.   

A New Phase of the Au(111) Surface in Electrolyte Revealed by STM and Asymmetric Potential Pulse Perturbation.

Asymmetric potential pulse perturbations were combined with STM to separately the (22$\times \sqrt 3 ) \quad \leftrightarrow $ (1$\times $1) phase transition from the dynamics of the dynamics of nanoscale island growth and dissolution at Au(111)/0.1M HClO$_{4}$ interfaces. In the course of these experiments a new surface phase, characterized by a gas of highly mobile Au adatoms on the surface, the absence of islands and a paucity of reconstruction stripes, was observed. This phase coexists with a low density of reconstructed stripes in ``holes''. This new phase is an intermediate state and can only be observed over a potential range range from 0.3 V$_{SCE}$ to 0.45V$_{SCE}$, after a potential pulse lifting the reconstruction. In addition, and contrary to previous reports, the(1$\times $1) $\diamondsuit $ (22$\times \sqrt 3 )$ reconstruction process can be fast.   

Observation of surface layering in a nonmetallic liquid

Non-monotonic density profiles (layers) have previously been observed at the free surfaces of many metallic liquids, but not in isotropic dielectric liquids. Whether the presence of an electron gas is necessary for surface layering has been the subject of debate. Until recently, MD simulations have suggested that layering at free liquid interface may be a generic phenomenon and is not limited to the metallic liquids$^{1}$. The theories predict that if normal liquids can be cooled down to temperatures low enough, layering structure should be observed experimentally. However, this is difficult for most molecular liquids because these liquids freeze well above the temperature necessary for observing the layering structure. By studying the surface structure of liquid TEHOS (tetrakis(2-ethylhexoxy)silane), which combines relatively low freezing point and high boiling point compared to that of most molecular liquids, we have observed the evidence of layering at the free interface of liquid TEHOS using x-ray reflectivity. When cooled to T/T$_{c} \quad \approx $0.25 (well above the bulk freezing point, Tc is the critical temperature of TEHOS), the surface roughness drops sharply and density oscillations appear near the surface. Lateral ordering of the surface layers is liquid-like, just as at liquid metal surfaces. 1. E. Chac\'{o}n and P. Tarazona, Phys. Rev. Lett.\textbf{ 91} 166103-1 (2003)   

Chemical Electrode Modification for Charge Injection in Organic Thin Film Transistors

The nature of the interface between an organic material and an inorganic electrode (metal or semiconductor) is critical to the performance of organic electronic and optoelectronic devises. To improve the electrical contact between gold electrodes and pentacene thin film transistors prepared by nanotransfer printing [1], the effect of coating the gold electrodes with self-assembled monolayers (SAMs) of organic molecules with electro-withdrawing groups is being explored. The first experiments have been done with commercially-available molecules and oligo(phenylene ethylene) derivatives have been synthesized for further investigation. The transport and noise characteristics of pentacene TFTs fabricated using the different coating groups will be presented. [1] D.R. Hines et al, Appl. Phys. Lett. 86, 163101 (2005).   

Effects of Stress and Void-Void Interactions on Current-Driven Void Surface Evolution in Metallic Thin Films

We report results of electromigration- and stress-induced migration and morphological evolution of voids in metallic thin films based on self-consistent numerical simulations. The analysis reveals the complex nature of void-void interactions and their implications for the evolution of metallic thin-film electrical resistance, providing interpretation for experimental measurements in interconnect lines. Interestingly, for two voids migrating in the same direction under certain conditions, we find that a smaller void does not always approach and coalesce with a larger one, while a larger void may approach and coalesce with a smaller one. In addition, we find that under certain electromechanical conditions, biaxially applied mechanical stress can cause substantial retardation of void motion, as measured by the constant speed of electromigration-induced translation of morphologically stable voids. This effect suggests the possibility for complete inhibition of current-driven void motion under stress.   

Characterization and Control of Microstructure in Combinatorially Prepared Aluminum-Silicon Thin Film Nanocomposites

In this presentation, we describe the application of thin film combinatorial methods to systematically control the microstructure of Al$_{x}$Si$_{(1-x)}$ alloys through variations in composition and growth temperature. Libraries of compositionally graded films have been sputter deposited onto silicon substrates. The microstructure was investigated using x-ray diffraction while atomic force microscopy techniques were employed to obtain surface morphology and phase distribution. We will also report the results of the mechanical properties we have investigated on these films.   

Mechanical Properties of Electrophoretically-Deposited CdSe Nanocrystal Films

Approaches to measuring and then minimizing the strain in electrophoretically deposited CdSe nanocrystal films are investigated. The films are seen to fracture above a critical thickness which vaies with nanocrystal size. Cracking and delamination have been studied by SEM and AFM and are attributed to the high strain energy in the film. Raman microprobe scattering and EDX mapping show the strain distribution in the nanocrystal films. The Young's modulus measured by nanoindentation is in good agreement with the parameters obtained from Raman scattering. The deposition conditions have been varied to minimize this strain, which is thought to be due to the evaporation of residual hexane solvent after electrophoretic deposition. In situ observations confirm this assumption about the origin of film strain. Thermogravimetric analysis and differential scanning calorimetry measurements provide the chemical composition of CdSe nanocrystals. The CdSe nanocrystal films become mechanically stronger and more resistant to chemical dissolution after being treated by different cross-linker molecules. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award No. DMR-0213574 and by the NYSTAR.   

Characterization of electromigration in semiconductor device interconnects using microscopic techniques

Electromigration is an important failure mechanism which affects the functionality and lifetime of integrated circuits. The addition of relatively small amounts of copper has been previously shown to improve device interconnect lifetimes. Through the use of Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopic (EDS) capabilities we have measured the copper concentration as a function of position along an interconnect after several accelerated stress time periods. We observe a migration of copper atoms from the cathode to the anode side of the interconnect as a function of stressing time. In some cases, a pileup of copper near the middle of the interconnect indicates a blocking of copper diffusion and creates a site for interconnect failure. Metal pileup (hillocks) and depletion (voids) are observed by Atomic Force Microscopy (AFM). We also observe a correlation between relative reflectance using optical microscopy and roughness (observed by AFM) of an interconnect line.   

Session U16: Nanotechnology: Applications and Measurements

Development of Carbon Nanotube Based Isotropic X-ray Source for Cone-Beam Tomography Imaging

We have developed a carbon nanotube based microfocus X-ray source with an isotropic focal spot. Two focusing electrodes were implemented in the design with one electrode harnessing the divergence of field-emitted electrons from gate and the other focusing electrons onto the anode. Isotropic X-ray focal spot was achieved by utilizing an elliptical cathode that forms elliptical electron probe on the anode after electrostatic focusing. Based on the design method, an x-ray source with an isotropic focal spot of 65 $\mu$m in diameter was experimentally demonstrated in X-ray projection images. This type of X-ray source sees wide applications in cone-beam tomography imaging studies.   

Stationary scanning x-ray source based on carbon nanotube field emitters

Carbon nanotube is an ideal field emitter thanks to its large aspect ratio and small diameter. Based on its field emission property, we have developed a stationary scanning x-ray source, which can generate a scanning x-ray beam to image an object from multiple projection angles without mechanical motion. The key component of the device is a gated carbon nanotube field emission cathode with an array of electron emitting pixels that are individually addressable via a metal-oxide-semiconductor field effect transistor-based electronic circuit. The characteristics of this x-ray source are measured and its imaging capability is demonstrated. The device can potentially lead to a fast data acquisition rate for laminography and tomosynthesis.   

A New Thermionic Cathode Using Oxide Coated Carbon Nanotubes

We have demonstrated a new type of thermionic cathode utilizing carbon nanotubes that exhibited superior electron emission properties. A field enhancement factor as high as 2000 was observed and thermionic electron emission current at least an order of magnitude higher than the emission from a conventional oxide cathode was obtained. This cathode combines the low work function of the oxide coating with a high field enhancement factor introduced by carbon nanotubes and we have demonstrated that it can be used as a highly efficient electron source. The cathode was fabricated by sputter deposition of a thin film of oxide materials on aligned carbon nanotubes, which were grown on a tungsten substrate with plasma enhanced chemical vapor deposition.   

Selective Adsorption and Alignment Phenomena of ZnO Nanorods on Molecular-Patterned Substrates for Large-Scale Integrated Device Fabrication

ZnO nanorods have been utilized for various device applications such as field effect transistor, UV sensor, etc. However, a major stumbling block holding back their practical applications is a lack of mass-production method of such devices. Since ZnO nanorods are first synthesized in solution, one has to pick up and assemble individual nanorods onto substrate for device fabrication, which is not an easy task. We studied the selective assembly and alignment phenomena of ZnO nanorods on molecule-patterned solid substrates. When the molecule-patterned substrate is placed in the solution of ZnO, ZnO nanorods are selectively adsorbed onto negatively charged surface region. Furthermore, we found the adsorbed nanorods slide on the substrate resulting in aligned nanorod structures. This presentation will discuss the systematic study of ZnO nanorod assembly process on patterned substrates and applications of this method for large-scale assembly of ZnO nanorod-based integrated devices.   

Atom selective force measurement with STM

Scanning tunneling microscope (STM) manipulation and spectroscopy is used to determine the strength of interactions necessary to manipulate individual silver and bromine atoms on a Ag(111) surface at 4.6 K. In order to distinguish between the two types of atoms, we use local atom extraction procedures: bromine atoms are extracted from individual molecules of cobalt porphyrin (5,10,15,20-Tetrakis-(4-bromophenyl)-porphyrin-Co(II)), which are deposited prior to this experiment, by selectively breaking the C-Br bonds with the STM tip. The individual silver atoms are extracted from the native Ag(111) surface by a controlled tip-crash procedure. Then, we laterally manipulate these two atoms using the same STM-tip along the close packed rows of the Ag(111) surface. The tip-height signals during manipulation are recorded as a function of the tip-atom distance, which include the force information necessary to move a halogen atom, bromine, and a metallic atom, silver, on this surface. This work is financially supported by US-DOE grant, DE-FG02-02ER46012.   

Nanofabrication Based on Nanoporous Membranes

We describe nanofabrication methods to produce nanopore array templates in aluminum oxide and titanium dixode films. The method is based on anodization of thin films of aluminum and titanium under \textit{dc} conditions in an acid. We also describe non-lithographic means of transferring the pore pattern from such nanoporous membranes onto a generic substrate. This is based on reactive ion etching through the nanoporous template grown directly on the substrate. In our demonstration, a thin alumina template consisting of a hexagonal array of pores $\sim $50nm in diameter is first deposited on the substrate. The pores reach within 10-20 nm of aluminum, which is protected by an alumina barrier layer. By controlling reactive ion etching conditions, we demonstrate highly anisotropic etching through the aluminum layer, barrier alumina layer and into the substrate. The 50nm pore layer is thus directly transferred to create nanoporous and nanopillar arrays of a variety of materials such as Al, Si, GaN, GaAs, etc. Such nanoporous, nanopillar arrays will be useful in a variety of applications involving biosensors, optoelectronic and spintronic devices.   

Resonant Operation of Nanoelectromechanical Systems in a Viscous Fluid

Up to date, most work on nanoelectromechanical systems (NEMS) has been done in high vacuum. Yet, many applications may require fluidic NEMS operation. Here, we present measurements of the quality ($Q)$ factor and resonance frequency in nanomechanical doubly-clamped beam resonators as a function of surrounding gas pressure --- from high vacuum to atmospheric conditions. Atmospheric $Q$s obtained are $\sim $10$^{2}$. The experimental results also suggest that viscous effects become less severe in high frequency devices.   

Experimental Measurement of Elastic Contact Diameter

Manipulation of nano-objects, as well as further development of the MEMS technology, needs an understanding and control of surface forces, including friction and adhesion forces, which depend on contact area in the nanoscale. Meaningful measurements of surface forces and correct interpretation of the force data require knowledge of contact area. Due to experimental difficulties of contact area measurements, a common practice today in surface force research using atomic force microscopy (AFM) is to compute the contact area using the contact mechanics theories. In this talk, I will present a method of experimental determination of contact diameter, or contact width, between a diamond tip and a flat silicon surface. The experiments used diamond tips with their surface geometry determined by AFM imaging. The measured elastic contact widths for diamond tips with a spherical shape, under controlled magnitudes of force in the surface normal direction, agree with the Hertzian contact mechanism. The technique has also been used successfully to obtain contact widths for non-spherical tip geometry.   

Preparation of different protected bimetallic nanoelectrodes with 30nm gapwidth and access window

Reproducible fabrication of 30 nm metallic nanogaps on silicon chips and their electrochemical characterization are presented. The fabrication of the chip is a combination of an optical lithography step and two electron-beam (e-beam) steps. An optimized adhesion layer/metal layer combination (Ti/Pt/Au) and an adopted two layer e-beam resist are used. Specifically the chip has been covered with different protection layers, except of an access window located on top of the nanogaps, calibration electrodes and contact pads, respectively (Fig.1). After characterising the gaps and of the protection layer by cyclical voltammetry in 0.1 M H$_2$SO$_4$ aqueous electrolyte, the deposition of Cu onto the nanogaps will be presented. Fig.1: Different Nanoelectrode Strcutures with access window on top covered by SiO$_2$/Si$_3$N$_4$/SiO$_2$ used as protection layer.   

Nanoscale metal thermometry using a radiofrequency single electron transistor

We report on the development of single electron transistors for thermometric readout of nanoscale normal metal volumes. Due to the weak electron-phonon interaction at low temperatures, the electron gas in a normal metal can be heated to a temperature significantly greater than that of the surrounding lattice.  Below 100 mK, the electron-phonon coupling time is on the order of microseconds to milliseconds, making direct measurements of the electron temperature's time dependence possible.   Achieving sensitive and high frequency readout of this system is of critical importance for applications in nanocalorimetry and nanobolometry.  We will describe the use of a radiofrequency single electron transistor to time-resolve the temperature of the electron gas in a submicron scale normal metal volume.   

Electrothermal Tuning of Nanomechanical Resonators

A highly effective electrothermal tuning method has been demonstrated for Al-SiC nanomechanical resonators. Doubly clamped beam devices are actuated and read out using a magnetomotive technique under moderate vacuum. DC current applied to a beam heats the structure and shifts the resonance frequency downward. Frequency shifts of 10 percent are easily achievable, and the thermal time constant of these structures is in the $\mu $s range. The initial frequency and frequency tunability are studied for beams of varying Al thickness, and the device performance can be accurately modeled using simple mechanical and thermal models. Because of the different mechanical properties of SiC and Al, both the initial frequency and the frequency tunability can be modified by varying the Al layer thickness. This approach has the potential to become an important tool for effective frequency tuning in deployable SiC-based NEMS devices and systems for applications that would benefit from SiC as the structural material.   

Resonant absorption in micrometer and nanometer absorbing particles

Resonance effects can occur in laser absorption by micrometer and nanometer sized particles when a train of pulses is used. The pressure generated by the train of pulses may be significantly different than the pressure generated by a single pulse with the same total energy. For pulsed lasers with a gap duration between pulses that is an integer multiple of the characteristic oscillation time of the absorber, constructive interference occurs and the pressure generated inside the absorber is approximately the same as that generated by a single laser pulse. For pulsed lasers with a gap duration between pulses that is not an integer multiple of the characteristic oscillation time, destructive interference occurs and the pressure is significantly decreased. We present numerical computations comparing this effect in two model systems: 1 micrometer melanosome and a 100 nm gold absorber. The resonance effects have implications for both damage thresholds and therapeutic applications of laser radiation.   

Optical Trapping and Integration of Semiconductor Nanowire Assemblies in Water

The use of nanowires in scientific, biomedical, and microelectronic applications is greatly restricted due to a lack of methods to assemble nanowires into complex heterostructures with high spatial and angular precision. Here we show that an infrared single-beam optical trap can be used to individually trap, transfer, and assemble high-aspect-ratio semiconductor nanowires into arbitrary structures in a fluid environment. Nanowires with diameters as small as 20 nm and aspect ratios of above 100 can be trapped and transported in three dimensions, enabling the construction of nanowire architectures which may function as active photonic devices. Moreover, nanowire structures can now be assembled in physiological environments, offering novel forms of chemical, mechanical, and optical stimulation of living cells.   

Low temperature internal friction peak in Boron doped nanocrystalline diamond

Recent measurements of the low-temperature internal friction ($Q^{-1}$) of nanocrystalline diamond films have revealed that these films have a broad but distinct internal friction peak at approximately 2K. In contrast to the off-peak baseline low-temperature $Q^{-1}$ of these films, which show no measureable variation over a factor of 4 span in amplitude, the $Q^{-1}$ at the peak decreases by as much as 60\% when the measurement amplitude is increased by a factor of 4. The similarity of this peak with a low-temperature peak previously observed in boron-doped silicon led to the possibility that the peak is the result of boron contamination of the diamond films. To further investigate this, diamond films with varying degrees boron doping were grown and measured between room temperature and 400 mK. The films are typically 0.5 $\mu$m thick and are grown on silicon double paddle oscillator substrates, which have an extremely low internal friction background and enable highly sensitive measurements of the mechanical properties of thin films. Preliminary results show an upwards shift in temperature of the peak with increasing boron levels.   

Microwave Dielectric Resonance and Negative Permittivity Behavior in Al$_{2}$O$_{3}$-CuO-Cu Nanocomposites

The frequency-dependent microwave (0.1-18 GHz) complex permittivity of nanocomposites based on the Al$_{2}$O$_{3}$/CuO/Cu system is investigated. The composites are formed by solution infusion of copper precursors into a porous Al$_{2}$O$_{3}$ matrix, followed by thermal decomposition to copper oxides and localized formation of CuAl$_{2}$O$_{4}$ spinels, and finally partial reduction by H$_{2}$ firing. The final material has a complicated microstructure and exhibits strong amplitude, relatively narrowband dielectric resonance in the microwave regime at intermediate concentrations ($\sim $15-18{\%} by volume) of Cu. The resonances are superficially similar in structure to plasmon and Reststrahlen resonances typically seen in conductors at far-infrared to optical frequencies, but occurring at much lower frequencies in the composites. This is in contrast to the usual broadband induced-polarization dielectric relaxations observed in standard composites. Large concentrations of copper cause negative permittivity behavior below 6 GHz. Permittivity data, SEM micrographs, and possible explanations will be presented.   

Session U17: Physics and Imaging in Medicine

Careers in Medical Physics and the American Association of Physicists in Medicine

The American Association of Physicists in Medicine (AAPM), a member society of the AIP is the largest professional society of medical physicists in the world with nearly 5700 members. Members operate in medical centers, university and community hospitals, research laboratories, industry, and private practice. Medical physics specialties include radiation therapy physics, medical diagnostic and imaging physics, nuclear medicine physics, and medical radiation safety. The majority of AAPM members is based in hospital departments of radiation oncology or radiology and provide technical support for patient diagnosis and treatment in a clinical environment. Job functions include support of clinical care, calibration and quality assurance of medical devices such as linear accelerators for cancer therapy, CT, PET, MRI, and other diagnostic imaging devices, research, and teaching. Pathways into a career in medical physics require an advanced degree in medical physics, physics, engineering, or closely related field, plus clinical training in one or more medical physics specialties (radiation therapy physics, imaging physics, or radiation safety). Most clinically based medical physicists also obtain certification from the American Board of Radiology, and some states require licensure as well.   

International Standardization of the Clinical Dosimetry of Beta Radiation Brachytherapy Sources: Progress of an ISO Standard

In 2004 a new work item proposal (NWIP) was accepted by the International Organization for Standardization (ISO) Technical Committee 85 (TC85 -- Nuclear Energy), Subcommittee 2 (Radiation Protection) for the development of a standard for the clinical dosimetry of beta radiation sources used for brachytherapy. To develop this standard, a new Working Group (WG 22 - Ionizing Radiation Dosimetry and Protocols in Medical Applications) was formed. The standard is based on the work of an ad-hoc working group initiated by the Dosimetry task group of the Deutsches Insitiut f\"{u}r Normung (DIN). Initially the work was geared mainly towards the needs of intravascular brachytherapy, but with the decline of this application, more focus has been placed on the challenges of accurate dosimetry for the concave eye plaques used to treat ocular melanoma. Guidance is given for dosimetry formalisms, reference data to be used, calibrations, measurement methods, modeling, uncertainty determinations, treatment planning and reporting, and clinical quality control. The document is currently undergoing review by the ISO member bodies for acceptance as a Committee Draft (CD) with publication of the final standard expected by 2007. There are opportunities for other ISO standards for medical dosimetry within the framework of WG22.   

Scientific Programs and Funding Opportunities at the National Institute of Biomedical Imaging and Bioengineering

The mission of the National Institute of Biomedical Imaging and Bioengineering (NIBIB) is to improve human health by promoting the development and translation of emerging technologies in biomedical imaging and bioengineering. To this end, NIBIB supports a coordinated agenda of research programs in advanced imaging technologies and engineering methods that enable fundamental biomedical discoveries across a broad spectrum of biological processes, disorders, and diseases and have significant potential for direct medical application. These research programs dramatically advance the Nation's healthcare by improving the detection, management and, ultimately, the prevention of disease. The research promoted and supported by NIBIB also is strongly synergistic with other NIH Institutes and Centers as well as across government agencies. This presentation will provide an overview of the scientific programs and funding opportunities supported by NIBIB, highlighting those that are of particular important to the field of medical physics.   

Medical Physics Graduate Program At An HBCU

The Physics Department at Hampton University houses the first Medical Physics graduate program at a minority institution, and the first in the state of Virginia. Jointly established with the Eastern Virginia Medical School, the program requires students to take standard physics courses in addition to medical physics classes and clinical rotations performed at local hospitals. The associated medical physics research primarily focuses on detectors development for absolute 3D dose distribution measurements (with accuracy better than $\pm $100 microns), characterization of the uniformity or non-uniformity of Brachytherapy sources, and extraction of the 2D and 3D in-vivo dose maps for real time dose monitoring. Recent novel fundamental studies on the energy dependence of cancer cells to address, among others, mono-energetic Brachytherapy source treatments, reaction mechanisms associated with cancer cell destruction, and cancer genome identification have been launched. Each of the research conducted is strongly coupled to dedicated Geant4 Monte Carlo simulations. After presenting this unique medical physics program, we will review results obtained from its research group.   

Session U18: Focus Session: Carbon Nanotubes: Transport III

Electron Phonon Coupling Effects in Nanotubes

Electron-phonon coupling (EPC) is a key physical parameter in nanotubes. Here we discuss its effects on phonon dispersions, Raman spectra and electron transport. The main EPC effect on the phonon dispersions is the presence of Kohn anomalies. These are distinct features of the phonon dispersion in metallic systems, associated to the presence of a Fermi surface [1]. Graphite has two Kohn anomalies for the Gamma-E$_ {2g}$ and K-A'$_{1}$ optical modes [2]. Their strength is proportional to the EPC square [2]. Kohn anomalies are enhanced in metallic nanotubes due to their reduced dimensionality, but absent in semiconducting nanotubes [2,3]. At 0 K all metallic nanotubes are not stable and undergo a Peierls distortion. We show that the Peierls distortion temperature decreases exponentially with the tube diameter [3]. For nanotubes generally used in experiments, with diameters larger than 0.8 nm, we find that this temperature is smaller than 10$^{-8}$ K [3]. We then show that EPC is the major source of broadening for the Raman G and G$^{-}$ peaks in graphite and metallic nanotubes [3]. The EPC explains the difference in the Raman spectra of metallic and semiconducting nanotubes and their dependence on tube diameter [3]. We dismiss the common assignment of the G$^{-}$ peak in metallic nanotubes to a Fano resonance between phonons and plasmons. We assign the G$^ {+}$ and G$^{-}$ peaks to TO (circumferential) and LO (axial) modes, the opposite of what often done. We then present five independent approaches to directly measure the optical phonons EPC in graphite and nanotubes from their phonon dispersions and Raman spectra. This allows us to quantify the EPC effects on high field electron transport in nanotubes. High field measurements show that electron scattering by optical phonons breaks the ballistic behavior. From our EPCs we derive a simple formula for the electron mean free path for optical phonon scattering in high-field transport [4]. The comparison with the scattering lengths fitted from experimental I-V curves shows that hot phonons are created during high-bias transport [4]. Their effective temperature is thousands K and sets the ultimate limit of ballistic transport [4]. \begin{enumerate} \item W. Kohn, Phys. Rev. Lett. \textbf{2}, 393 (1959) \item S. Piscanec et al. Phys. Rev. Lett. \textbf{93}, 185503 (2004) \item M. Lazzeri et al. cond-mat/0508700; S. Piscanec et al. Phys. Rev. B submitted (2005) \item M. Lazzeri et al. Phys. Rev. Lett. \textbf{95}, 236802 (2005) \end{enumerate}   

Self-Heating and Non-Equilibrium Optical Phonons in Suspended Carbon Nanotubes

Understanding of current-limited transport in single-walled carbon nanotubes (SWNTs) is vital to many potential nanotube applications. In this talk I will discuss the high bias electrical transport characteristics of well-contacted suspended SWNTs in various environments. Negative differential conductance at low bias (below 0.4V) appears as a result of extreme self-heating and the formation of non-equilibrium optical phonons. Various gas and molecular solid environments lead to the reduction or elimination of the non-equilibrium phenomenon. Finally I will discuss the ways in which we can use the data to directly and indirectly measure the nanotube's intrinsic properties and temperature.   

Direct measurements of electron-phonon coupling of radial breathing modes in carbon nanotubes and their chirality dependence

A method for direct measurement of electron-phonon coupling matrix elements, M$_{e-ph}$, is proposed and demonstrated experimentally by correlating resonant Raman excitation profiles of the first and second harmonics of the radial breathing mode. M$_{e-ph}$ values are quantitatively determined for individual carbon nanotubes (CNT) excited in small ropes suspended in air. The results show that the matrix elements satisfy S. V. Goupalov and coworker’s empirical tight binding theory calculation$^{1}$ with quantitative values that show a smaller electron-phonon coupling than reported from ab initio calculations$^{2}$ for isolated carbon nanotubes. We find that resonant excitation profile broadening $\eta$ for CNTs in small ropes show a correlation with chiral angles that appears to be unchanged from isolated carbon nanotubes. 1 S. V. Goupalov, Satishkumar B. C., and S. K. Doorn, Pre- print (2005). 2 M. Machon, S. Reich, H. Telg et al., Phys. Rev. B 71 (3) (2005).   

Nonequilibrium phonon occupation in carbon nanotube quantum dots

We present a formalism for electron transport through a coulomb blockaded quantum dot strongly coupled with vibrations and weakly with leads and the thermal environment. By calculating the joint electron-phonon probability distribution, we show that recently observed anomalous conductivity through single-walled carbon nanotube (SWCNT) quantum dots arises from `hot' phonons that are generated by the current at a faster rate than their extraction rate by the surrounding. We explain semi-quantitative details of the experiment and predict a nontrivial temperature dependence of the phonon population arising from a subtle interplay between phonon emission and absorption rates at specific bias voltage values.   

Phonon scattering in Carbon Nanotube Field Effect Transistors -- an NEGF Treatment.

We examine the influence of phonon scattering on DC current transport in carbon nanotube field-effect transistors using the non-equilibrium Green's function (NEGF) formalism. Both optical and acoustic phonon modes are considered, and electron-phonon interaction is modeled through a scattering self-energy. Intra-valley scattering due to longitudinal optical (LO) and radial breathing mode (RBM) phonons is examined. Zone-boundary phonon eigenmodes that mediate inter-valley scattering are found to be a mixture of fundamental polarizations such as LO/TA and to couple strongest to the electrons. The effect of phonon scattering on the current vs. voltage characteristic of a filed-effect transistor is found to have distinct gate voltage (Vg) dependence. High-energy optical phonons can significantly degrade the ON-current (large Vg) while their effect is negligible in the OFF-state (low Vg). On the other hand, low-energy phonons (acoustic and RBM) can considerably affect the current transport for all gate biases. Their influence is enhanced at low Vg due to the one-dimensional density of states.   

Transport properties of suspended carbon nanotubes

The study of suspended doubly clamped carbon nanotubes allows for the observation of many novel phenomena due to the intimate coupling of the mechanical and electrical degree of freedom, e.g. high frequency quantum limited displacement sensing, phonon adsorption and emission spectroscopy and quantized frequency tuning. We use a high frequency mixing technique originally develloped by Sazonova et al. to monitor the high frequency properties of suspended carbon nanotubes. Our setup allows for measurements from DC up to 4 GHz from room temperature down to 300 mK.   

Electrical Switching in Metallic Carbon Nanotubes

We present first-principles calculations of quantum transport which show that the resistance of metallic carbon nanotubes can be changed dramatically with homogeneous transverse electric fields if the nanotubes have impurities or defects. The change of the resistance is predicted to range over more than two orders of magnitude with experimentally attainable electric fields. This novel property has its origin that backscattering of conduction electrons by impurities or defects in the nanotubes is strongly dependent on the strength and/or direction of the applied electric fields. We expect that this property will open a path to new device applications of metallic carbon nanotubes. Ref.) Young-Woo Son {\em et al.}, Phys. Rev. Lett. {\bf 95}, 216602 (2005).   

High Temperature Conductivity and Reactivity of Carbon Nanotube Electronic Circuits

At sufficiently high temperatures, carbon nanotubes (CNTs) begin to react with their immediate environment. For example, adsorbates first desorb, then the carbon may react with connective electrodes, and ultimately Stone-Wales defects become mobile and can be annealed. These reactions are conventionally studied by thermogravimetric analysis (TGA), but they can also profoundly effect the conductance of the nanotubes. We have measured the four probe impedance and transimpedance of individual metallic and semiconducting nanotube devices from room temperature to 1200 K in ultra-high vacuum. When the nanotubes are initially heated from room temperature, the conductance increases as adsorbates are desorbed. On subsequent heating, the device resistance is linearly dependant on temperature over the range 300 to 900 K. Above 900 K the temperature dependence becomes more complex as chemical reactions change the nanotube and as optical phonon modes become thermally populated. This electronic characterization agrees with and complements TGA of bulk, purified CNTs.   

On Current Carrying Capacity of Single Wall Semiconducting Carbon Nanotubes

The current carrying capacity of single wall semiconducting carbon nanotubes (CNTs) in the presence of phonon scattering and band-to-band tunneling is studied by self-consistently solving Poisson and Schr\"{o}dinger equation using the non-equilibrium Green's function formalism. We show that the current delivery capacity of semiconducting CNTs strongly depends on the bias regime and is drastically different from metallic CNTs. A long metallic single wall CNT carries a saturation current of $\sim $25$\mu $A due to optical phonon (OP) scattering. In contrast, a semiconducting CNT can deliver a current well above 25$\mu $A in ambipolar transport regime even when the channel length is much longer than the OP scattering mean free path (mfp). When the channel length is short (comparable to the OP scattering mfp), a semiconducting CNT biased in unipolar transport regime can deliver a current larger than 25$\mu $A. Biasing the CNT in ambipolar transport regime, however, further doubles the current. The different current carrying capacity in the ambipolar transport regime is due to nearly uncoupled dissipative transport through both the lowest conduction and valence subbands in the channel.   

A comparison of measured electron-phonon and electron-photon coupling strengths in isolated and small ropes of single wall carbon nanotubes

Resonant Raman scattering excitation profiles and photoluminescence (PL) are measured for isolated carbon nanotubes (CNT) and small ropes suspended in air. Most of the measured CNTs do not exhibit PL and are believed to be in small ropes. The radial breathing mode electron-phonon coupling, M$_ {e-ph}$, are measured, and values for the isolated CNT are in good quantitative agreement with ab initio calculations. The matrix elements M$_{e-ph}$ and electron-photon coupling, M$_{e- op}$, for a CNT in a small rope are 1.7 times and 1.4-2.7 times weaker than in an isolated CNT. The reduced e-phonon coupling in small ropes is correlated with a smaller RRSE broadening $\eta$, compared to the value (45meV) obtained from an isolated CNT. Despite the reduced values of M$_{e-ph}$ and $\eta$, M$_{e- ph}$ in small ropes still display the same chiral dependence predicted for isolated CNTs.   

Anharmonic decay of the Radial Breathing Mode in Suspended Single-walled Nanotubes

The growth of isolated single-walled nanotubes (SWNTs) suspended over trenches in Si substrates makes it possible to study the Raman lineshapes of individual tubes. High-resolution room temperature resonant micro-Raman spectra from a number of suspended SWNTs exhibit very narrow radial breathing modes (RBMs), with full-width at half maximum (FWHM) values ranging from 1.3-2.5 cm$^{-1}$. These values are much smaller than previously reported in the literature. The observed FWHM is \textit{not} a smooth function of the tube's radius. We note that the two-phonon density of states (2DOS) for the anharmonic decay of the RBM phonon shows many singularities whose energies depend both on the tube's radius \textit{and} chirality. Therefore, tubes with very similar RBM frequencies, and similar radii, could have different linewidths because of a different 2DOS. The observed linewidths increase with increasing incident laser power, as expected if the origin of the linewidth is anharmonic. We analyze the RBM linewidth in terms of down-conversion and up-conversion third-order anharmonic contributions. A comparison of the temperature dependence of both FWHM and peak frequency suggest that up-conversion processes are important, as found previously for low-frequency optical phonons in semiconductors.   

Phonon anomalies in the resonance Raman spectra of graphite and single-wall carbon nanotubes

Phonon dispersion relations for a graphene sheet and single-wall carbon nanotubes (SWNTs) are calculated within the extended tight-binding model that has recently been shown to accurately predict the optical transition energies in small-diameter SWNTs. Anomalies in the dispersion relations are found at certain high- symmetry points of the reciprocal lattice and these anomalies are attributed to the strong electron-phonon coupling. These anomalies are very sensitive to changes to electron and lattice temperatures, electron doping, mechanical stress, SWNT diameter, and SWNT metallicity. Resonance Raman measurements of doping and strain induced shifts of the phonon frequencies in SWNTs are in qualitative agreement with the present calculations. The MIT authors acknowledge support under NSF Grant DMR 04-05538.   

Phonon stiffening in semiconducting single-walled carbon nanotubes under n-type doping

The doping dependence of the high-frequency Raman-active modes in single-walled semiconducting carbon nanotubes is studied by density functional theory. We find that the $A_{1g}$ longitudinal mode in $(3*n+1,0)$ zigzag tubes shows a small anomalous upshift, followed by a large downshift under electron doping. This doping-induced stiffening of the $A_{1g}$ mode is related to the large anharmonicity of the mode. Connections are made to recent experiments in the group of P. C. Eklund.   

Session U19: Focus Session: Semiconductor Spin Nanostructures for Quantum Computing

Electron spin decoherence by interacting nuclear spins in quantum dot I: Quantum theory

We present a quantum theory to the electron spin decoherence by a nuclear pair-correlation method for the electron-nuclear spin dynamics under a strong magnetic field and low temperature. The theory incorporates the electron nuclear hyperfine interaction, the intrinsic nuclear interactions, and the nuclear coupling mediated by the hyperfine interaction with the electron in question. Results for both single electron spin free-induction decay (FID) and ensemble electron spin echo will be discussed. Single spin FID is affected by both the intrinsic and the hyperfine-mediated nuclear interactions, with the dominance determined by the dot size and external field. The spin echo eliminates the hyperfine-mediated decoherence but only reduces the decoherence by the intrinsic nuclear interactions. Thus, the decoherence times for FID and spin echo are significantly different. Electron spin decoherence is explained in terms of the quantum entanglement with the pair-flip excitations in the nuclear spin environment. This work was supported by NSF DMR- 0403465, NSA/ARO, and DARPA/AFOSR.   

Electron spin decoherence by interacting nuclear spins in quantum dot II: Coherent control

Due to the hyperfine interaction, the nuclear spins in a quantum dot, driven by nuclear spin pair-wise flip-flops, evolve in different pathways in the Hilbert space for different electron spin states, resulting in the electron-nuclei entanglement and hence the electron spin decoherence. When the electron spin is flipped by a pulse, the nuclear spin states for different electron spin states swap their pathways, and could intersect in the Hilbert space, which disentangles the electron and the nuclei and hence restores the electron spin coherence. The coherence restoration by disentanglement and the conventional spin echo in ensemble dynamics are fundamentally different and generally occur at different time. Pulse sequences can be applied to force the disentanglement to coincide with the spin echo, making the coherence recovery observable in ensemble dynamics. This work was supported by NSF DMR-0403465, NSA/ARO, and DARPA/AFOSR.   

Analytical Solution of Electron Spin Decoherence Through Hyperfine Interaction in a Quantum Dot

We analytically solve the {\it Non-Markovian} single electron spin dynamics due to hyperfine interaction with surrounding nuclei in a quantum dot. We use the equation-of-motion method assisted with a large field expansion, and find that virtual nuclear spin flip-flops mediated by the electron contribute significantly to a complete decoherence of transverse electron spin correlation function. Our results show that a 90\% nuclear polarization can enhance the electron spin $T_2$ time by almost two orders of magnitude. In the long time limit, the electron spin correlation function has a non-exponential $1/t^2$ decay in the presence of both polarized and unpolarized nuclei.   

Quantum dot dynamical Quantum dot dynamical nuclear spin polarization in the

Hyperfine interaction in quantum dots (QD) is qualitatively different than in atoms: coupling of a single electron spin to the otherwise well isolated QD nuclear spins plays a key role in spin-based solid-state quantum information processing. Dynamical nuclear spin polarization (DNSP) is observed by resonant optical pumping of single self-assembled QDs in gated structures that allow deterministic charging with a single excess electron or hole. In the absence of external magnetic fields, the optically polarized electron spin induces an effective inhomogeneous magnetic (Knight) field which determines the direction along which the mesoscopic ensemble of nuclear spins could polarize and enables nuclear spin cooling by surpassing depolarization induced by nuclear dipolar interactions. Due to the effective magnetic (Overhauser) field induced by the polarized nuclei, photoluminescence of these charged trion transitions exhibit spin splitting $\approx 15 \mu eV$ which can be determined by high-spectral-resolution ($<1 \mu eV$) spectroscopy based on a scanning Fabry-Perot interferometer. Our experiments constitute a first step towards a quantum measurement of the Overhauser field, which is in turn predicted to suppress electron spin decoherence in QDs.   

Dynamical nuclear spin polarization in a double quantum dot

The hyperfine interaction between an electron spin confined in a semiconductor quantum dot and the nuclear spins in the surrounding lattice has been identified as one of the main sources for decoherence in low temperature GaAs quantum dots. Recent experiments in gated double dot systems [1] have attempted to utilize the degeneracy point between the two-electron singlet and polarized triplet states to polarize the nuclear spins, thereby reducing their decoherence effects on the electron spins. Here we analyze the dynamics of the system of two electrons and a nuclear spin bath subject to the hyperfine interaction. We consider the effective spin Hamiltonian for the two-electron system, and represent the nuclear spins in the basis of their collective states. The nuclear polarization rates are evaluated for various initial conditions of the nuclear spin system, and optimal conditions for efficient polarization are discussed. [1] J. R. Petta, A. C. Johnson, J. M. Taylor, E. A. Laird, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, A. C. Gossard, Science 309, 2180 (2005).   

Coherent Control of Entangled Spin Pairs in a CdMnTe Quantum Well.

We used ultrafast light pulses to control the spin state of electrons bound to donors in a CdMnTe quantum well. Previously, we reported on the observation of up to three harmonics of the bound electron spin flip transition indicating that at least three bound electron sites were entangled(J.M. Bao, A.V. Bragas, J.K. Furdyna, R. Merlin, Phys. Rev. B 71 045314 (2005)). Using a pulse shaper, we are now able to suppress all coherent oscillations, but the signal of the first spin flip overtone. Therefore, only entangled electron pairs remain oscillating; all non-entangled donor bound electrons have been restored to their ground state. The quantum state of the remaining entangled electron spins is closely related to the Bell state. This technique holds promise for quantum computing applications.   

Electron-spin quantum computation: Three- and four-body interactions and other implementation challenges

Several leading quantum computer proposals are based upon electron spins. While these designs do potentially satisfy the DiVincenzo criteria, subtle implementation challenges have been uncovered that need to be addressed if these designs are to be realized successfully. In this talk, we start by pointing out that, when several spins are engaged mutually in pairwise interactions, a change can arise in those interactions. In the case of three spins, the quantitative strengths of the interactions can change. For four or more spins, qualitatively new terms can arise in the Hamiltonian, including four-body interactions. Other implementation challenges are also considered, including the difficulty of performing strong projective measurements on solid state qubits (weak measurements are generally more natural to implement but their behavior is more subtle). These issues will need to be handled in quantum computer realizations, either as a source of error to be overcome or as new physics to be exploited.   

Transport of Quantum Information Using Spin Wires

One-dimensional antiferromagnetically coupled spin systems have properties that make them useful as conduits for quantum information$_{ }$(PRL \textbf{90}, 047901 (2003)). Here we investigate possible mechanisms for using engineered spin chains as ``spin wires'' that can faithfully transport qubits. An analysis of the spin chains is carried out through numerical diagonalization of the effective spin Hamiltonian. We find that dimerized chains with a defect can support a highly localized qubit. We also demonstrate how it may be possible to propagate these kinks rapidly through a large system, thus providing a mechanism for producing ``flying'' spin qubits in the solid state.   

GHz Optical Spin Transceiver

The ability to measure spin coherence in semiconductor nanostructures is important for determining the feasibility of spin-based quantum computing architectures. Quantum dots are often referred to as 'solid-state atoms' because of their sharp absorption and emission lines that resemble those of single atoms. Electron spins localized on these quantum dots may be useful for storing quantum information, but their small optical cross section makes detection challenging. In order to take advantage of resonant enhancement of spin detection using the magneto-optical Kerr effect, we have developed a GHz Optical Spin Transceiver (GHOST) which uses a cw optical probe to measure Kerr signals in the time domain with 5 GHz bandwidth. Initial results will be presented for a test sample consisting of n-doped GaAs.   

Photocurrent spectroscopy of self--assembled quantum dots.

Quantum dots systems had been envisioned as possible candidates for building the hardware of quantum computers. They provide the necessary localization of electrons on length scales comparable to the electron Fermi wavelength and also exhibit distinct discrete energy spectra due to quantum confinement. Nevertheless, the characterization of optical and coherence properties of single quantum dots, especially at wavelengths larger than 1100 nm, is challenging due to the small SNR in these systems and the lack of high quantum efficiency detectors at these wavelengths. To circumvent these challenges, we use photocurrent measurements as a probe of the absorption spectra of quantum dots systems embedded in Schottky diode structures. The use of spectrally narrow laser sources allows the exciton absorption spectra of single quantum dots to be characterized as a function of temperature and magnetic field.   

Spin dynamics in coupled core/shell quantum dots

Colloidal nanoparticles provide a flexible system for studying individual quantum-confined electrons and holes. By layering different semiconducting materials in a single nanoparticle, we can create a low bandgap (CdSe) core and surrounding shell, separated by a high bandgap (ZnS) barrier. We have studied spin dynamics in such colloidal heterostructures using two-color time-resolved Faraday rotation (TRFR). By tuning the excitation energy, electron spins can be initialized into different states either in the core or the shell of the nanoparticle. The resulting spin dynamics show a g-factor (spin splitting) that depends on the size of the core or the shell. This g-factor dependence, as well as the spectroscopic dependence of the Faraday effect, allow electron spins in the core or the shell to be addressed independently.   

Spin Manipulation in lateral quantum dots under time-dependent confinement.

Single spin manipulations are a critical component to potential realizations of spintronic devices and quantum computers in lateral quantum dots. In this work, we demonstrate a new method for creating spin excitations in lateral quantum dots which uses the interplay between the spin-orbit interaction and a time-dependent lateral confining potential. For an asymmetric dot in the presence of an in-plane magnetic field, the spin quantization axis can be tilted away from the applied magnetic field due to the Rashba spin-orbit coupling, with the degree of tilting depending parametrically upon the confinement potential. By making small modulations to the confinement potential at a frequency given roughly by the Zeeman splitting, efficient spin excitations can be performed. We have performed theoretical and numerical calculations which demonstrate that Rabi frequencies on the order of tens of megahertz can be achieved for experimentally accessible parameters. Extensions to spin excitations in multi-electron quantum dots will also be presented.   

Effect of photon-assisted tunneling through the leads on spin current polarization in ac-driven quantum dot molecules

A new scheme for realizing spin pumping and spin filtering has been recently proposed using an ac-driven double quantum dot in the Coulomb blockade regime. Using a master equation approach for the density matrix, it was shown$^{1}$ that the spin polarization of the current through the system can be controlled by tuning the parameters (amplitude and frequency) of the ac-field. In the present work we extend our previous model to include photon-assisted tunneling through the contact barriers. This introduces new features in the current due to absorption and emission processes affecting the spin polarization of the current. We discuss these new features, their dependence on the ac-field parameters and the effects on the robustness of the proposed device as a spin pump and spin filter. In particular, we find that the spin filtering property depends strongly on the intensity of the applied ac field. The effect of photoassisted cotunneling on the spin current polarization will also be discussed.$^{ 1}$E.Cota, R. Aguado and G. Platero, Phys. Rev. Lett. \textbf{94}, 107202 (2005)   

Session U20: Focus Session: Metal-Insulator Transition and Electron Phonon Coupling in Perovskites

'CMR' Manganites: Strongly or Weakly Correlated?

A newly developed ``semiclassical impurity solver'' [1] is used to perform dynamical mean field calculations of the kinetic energy, optical conductivity and magnetic transition temperature of the two-orbital double-exchange model for colossal magnetoresistance manganites, including the full Kanamori ($U-U'-J$) interactions for the $e_g$ multiplet as well as the $e_g-t_{2g}$ Hunds coupling $J_H$. The effective correlation strength is shown to be weak in the ferromagnetic ground state, while in the high temperature paramagnetic state the multiplet interactions block many of the possible final states, leading to an effectively strongly correlated situation characterized by a large effective $J_H$.. \newline [1] S. Okamoto et. al., Phys. Rev. {\bf B71} 235113 (2005).   

Resistivity peak in the one-band double-exchange model with cooperative phonons

We present the results of Monte Carlo simulations of a single-band double-exchange model with cooperative phonons, both with and without quenched disorder, at a carrier density $n$=0.3. Both in two and three dimensions, the simulations reveal a peak in the resistivity at $T_{\rm Curie}$, in agreement with previous studies by other groups. The peak gets destroyed upon the application of an external field, resulting in a large magnetoresistance. The results are in good agreement with experiments involving CMR manganites. Studies at other densities are also presented, and an intuitive picture for the presence of the resistivity peak is discussed.   

Exchange Striction and Heat Conduction in Ca$_{1-y}$Sr$_y$MnO$_3$ ($0\leq y\leq 0.5$)

CaMnO$_3$, a G-type antiferromagnet with orthorhombic structure, exhibits a substantial enhancement of its thermal conductivity$^{a,b}$ ($\kappa$) for $T [Preview Abstract]

Strong current dependence of resistivity in CaMnO$_3$

The perovskite manganite CaMnO$_3$ (CMO) has a G-type antiferromagnetic ground state with N\'eel temperature $T_N$=125K. Prior transport measurements in the magnetic and paramagnetic phases$^{a}$ establish that CMO is a $n$-type semiconductor with $n\sim 10^{18}-10^{19} {\rm cm}^{-3}$ (from native defects like oxygen vacancies) and modestly heavy (large- polaron) mass, $m^{\ast}\sim 10m_0$. Here we report transport measurements on single crystal and polycrystalline CMO which reveal a strong current dependence of the resistivity ($\rho$) at low temperatures where $\rho >10^6\ \Omega\ {\rm cm}$ and impurity-band conduction predominates. For example, at 30~K, $\rho$ decreases by an order of magnitude for small current densities ($J<100\ \mu$A/cm$^2$), indicating that the effect is not associated with Joule heating. The possible role of spin-polarized hopping in this phenomenon will be discussed.\hfill \vskip .05in \noindent $^a$ J. L. Cohn, C. Chiorescu, and J. J. Neumeier, Phys. Rev. B {\bf 72}, 024422 (2005).   

On the nature of the pressure-induced insulator-to-metal transition in LaMnO$_3$

Since the discovery of colossal magnetoresistance, manganites have been intensively studied. We focus on the pressure induced insulator-to-metal (IM) transition which was found experimentally~[1] in the undoped parent compound LaMnO$_3$ with configuration $t^3_{2g}e^1_g$. This transition occurs at room temperature (above $T_N$=140 K) and at a hydrostatic pressure of 32 GPa where the Jahn-Teller distortion appears to be completely suppressed~[1]. The IM transition thus seems to be a bandwidth-driven Mott-Hubbard transition of the $e_g$ electrons. We employ the local density approximation combined with static and dynamical mean-field theories (LDA+$U$ and LDA+DMFT) and conclude that the IM transition observed at 32 GPa in paramagnetic LaMnO$_3$ at room temperature is {\it not} a Mott transition, but is caused by the overlap of the majority-spin $e_g$ bands, orbitally polarized by the Coulomb repulsion. Crucial are also the bandwidth reduction of $\sim0.6$ and $2/3$ arising from, respectively, the gap generated by the crystal-field splitting and the random, spatially uncorrelated direction of the $t_{2g}$ spin at room temperature. [1] I. Loa {\it et al}., Phys. Rev. Lett. {\bf 87}, 125501 (2001).   

Double-exchange driven metal-insulating transition in Mn-doped CuO

Doping antiferromagnetic CuO with Mn causes a uncommon metal-insulating transition where the low-temperature ($T < T_c$=80 K) phase is ferromagnetic, with a large but metallic-like resistivity, while the high-temperature phase is paramagnetic and insulating, but with a resistivity typical of Mott insulators in the hopping conducting regime. Applying a first-principles, self-interaction corrected local spin density approach, we are able to understand and rationalize this puzzling behavior: each doping Mn in CuO acts as a single donor, inducing a double-exchange driven metallic regime and a Mn-Mn ferromagnetic allignement. Nicely, here double-exchange can also work at rather low Mn concentrations since carriers can freely flow within the CuO (x,y) planes and only need the Mn assistance to move through the c axis. In the antiferromagnetic phase the system is insulating, but a polaron hopping conductivity through a few meV-wide Coulomb gap is envisaged. This scenario depicts the intriguing possibility of designing double-exchange driven ferromagnetic cuprates.   

Computational Studies of Magnetoresistance in Double-Exchange-Based Models for Manganites

Double-exchange-based models for manganites are studied using Monte Carlo techniques, with both exact-diagonalization and polynomial-expansion of the fermionic sector. One and two- orbital models are investigated, with and without phonons. We focus our analysis on the study of the resistance (R) vs. temperature (T) using the Landauer formalism. Highlights of investigations by our group and others are: (1) R vs. T curves, parametric with magnetic fields, that closely resemble experimental data for the case of one orbital and considering cooperative phonons; and (2) an insulator to bad metal transition induced by quenched disorder for the two-orbital model. In spite of these positive results, we remark that this is just the beginning of a frontal attack to the manganite problem using realistic models and efficient algorithms that scale linearly with the lattice volume. Future directions and open problems will also be discussed.   

Neutron Scattering Study of Electron- Phonon Coupling in La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ Perovskite

La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta }$ compounds are reported to have an unusual magneto-structural transition at low temperatures. Below $\sim $210$K$, it is proposed that charge disproportionation occurs according to 2Fe$^{4+}=>$Fe$^{3+}$+Fe$^{5+}$, thereby creating different valence on the iron sites. The different iron valences order along the body diagonal [111]$_{c, }$resulting in a change in crystal structure from rhombohedral to orthorhombic and antiferromagnetic ordering. Inelastic neutron scattering was used to determine the effect of simultaneous charge and magnetic ordering on the phonon and spin wave excitations. We find that the high frequency oxygen phonons ($\sim $80 meV) soften above the transition by several meV. Spin wave excitations appear below the transition with a characteristic energy of 50 meV at the Brillouin zone boundary. The result and relationship between the charge ordering and the electron-phonon interaction are discussed.   

Magnetic polarons in a one-dimensional antiferromagnetic chain

We present several results concerning magnetic polarons in a double-exchange system. We use a simplified model consisting on an antiferromagnetic (AF) chain doped with donor impurities. First, we study the ``back-effect'' in the AF background of a conduction electron bound to its donor impurity. We show that a region with extended spin distortions appear in the AF structure, similar to a domain wall and stabilizing the magnetic polaron. Second, we discuss the effect of these distortions in a system doped with a finite density of donor impurities. We show that from a critical density a non-trivial energy dependence on conduction electrons density appears, which can be interpreted as an attractive interaction between magnetic polarons. Third, we analyze temperature evolution of such a system of magnetic polarons. We show that they remain stable up to rather high temperatures, larger than the N\'eel temperature of the undoped chain. Our results may be relevant to understand the physics of low-doped manganites.   

Zener Polarons Ordering Variants Induced by A-Site Ordering in Half-Doped Manganites

Zener Polaron (ZP) ordering [1] provides a still polemic [2] and elusive interpretation of the charge ordering (CO) phenomenon in A site disordered half doped (A$_{1/2}$Ca$_{1/2}$) MnO$_3$, which is classically pictured by the Goodenough model (GM) of Mn$^{3+}$ and Mn$^{4+}$ CO [3,4]. ZP ordering considers instead the ordering of pre-formed ferromagnetic Mn pairs sharing an charge and keeping Mn in a Mn$^{+3.5}$ valence state. The recently synthesized A site cation ordered ABaMn$_2 $O$_6$ were shown to not present the generic magnetic CE state found of (A$_{1/2}$Ca$_{1/2}$)MnO$_3$ [5]. We present our magnetic structure determination of YBaMn$_2$O$_6$: the non- collinear magnetic order obtained unexpectedly reveals ferromagnetic plaquettes of four Mn attributable to larger 4-Mn ZPs, whose presence additionally fits very well the effective paramagnetic moments inferred from susceptibility measurements. The results unambiguously reveal the possible existence of ZP ordering variant in charge ordered manganites. [1] A. Daoud-Aladine et al., Phys. Rev. Lett. 89, 097205 (2002) [2] S. Grenier et al., Phys. Rev. B 69, 134419 (2004) [3] J. B. Goodenough, Phys. Rev. 100, 564 (1955) [4] P.G. Radaelli et al., Phys. Rev. B, 55, 3015 (1997) [5] T. Arima et al., Phys. Rev. B 66, 140408 (2002)   

Evolution of the local Jahn-Teller distortion across the phase diagram of La$\bf _{1-x}$Ca$\bf _x$MnO$\bf _3$ ($0 \le x \le 0.5$)

We report on the most comprehensive study to date of the {\it local} Jahn-Teller (JT) distortion across the phase diagram of the colossal magnetoresistive (CMR) La$\bf _{1-x}$Ca$\bf _x$MnO$\bf _3$ ($0 \le x \le 0.5$). The local structure has been measured, using the neutron powder diffraction based atomic pair distribution function (PDF) approach, over the wide temperature and Ca-doping range. These results are compared to the conventional crystallographic results obtained by Rietveld analysis. The results will be compared with both homogeneous and inhomogeneous models of the electronic structure. The magnitude of the {\it local} JT distortion is quantified over the entire phase diagram. In agreement with earlier work, we see the local JT distortion disappear in the metallic phase. However, in contrast with some earlier studies, we show that in the insulating phases the magnitude of the JT distortion decreases with increasing doping, becoming constant at higher doping. This new result should be incorporated in theoretical models of CMR manganites.   

Structural and Transport Study of La$_{0.9}$MnO$_{3}$ Under Pressure.

The detailed transport and structural properties of the self-doped system La$_{0.9}$MnO$_{3}$ have been studied under hydrostatic pressures extending to 6GPa and 11GPa, respectively.~ Like the doped manganite systems previously studied by our group, in the La deficiency (x=0.9) system a maximum shift of the peak resistivity is obtained at around 3.4GPa.~~ A crossover from metallic to insulating behavior is observed above this optimum pressure.~ The x-ray diffraction study results show that the self-doped system is compatible with a single phase of monoclinic space group up to 11 GPa. The detailed changes in the lattice parameters, volume, bond distances and bond angles have been obtained.~Comparisons with pressure dependent measurements on the doped manganites will be made.~ This work was supported by NSF DMR-0209243 and DMR-0512196.   

A quantum phase transition in the monoxides of the first transition series

The monoxides of the 3d metals (MO; M= Ti to Ni) provide an isostructural series (cubic, Fm $\bar{3}$m) in which to study the transition from metallic paramagnetism (TiO) to insulating antiferromagnetism (MnO, FeO, CoO, NiO). But the transition is not smooth, and while CrO has eluded synthesis over the years, the intrinsic properties of VO are under discussion. Here we present strong experimental and computational evidence that VO is a strongly correlated metal with Non-Fermi Liquid low temperature thermodynamics, a pseudogap in the density of states and an unusually strong spin-lattice coupling. All these properties are interpreted as signatures of the proximity to a magnetic quantum phase transition. Interestingly, TiOx displays a superconducting transition with a dome-shape dependence of the superconducting critical temperature with doping in x (0.8$<$x$<$1.2). The analogies with the phase diagram of the High Tc cuprates and their structural and electronic simplicity makes 3d monoxides ideal candidates to make progress in the understanding of correlated electron systems.   

Session U21: Liquid Crystals I

Aggregation in Two Dye Systems That Form Chromonic Liquid Crystals

X-ray scattering and various optical techniques are utilized to study the aggregation process and aggregate structure for two water-soluble dyes that form chromonic liquid crystal phases. The x-ray measurements indicate that the molecules stack in columns with a cross-section approximately equal to the area of a single molecule. The optical measurements point to an aggregation process that occurs at all concentrations, with the distribution of aggregate size shifting to larger and larger aggregates as the concentration is increased. A simple theory based on the law of mass action and an isodesmic aggregation process is in excellent agreement with the experimental data, yielding a value for the ``bond energy'' between the molecules in an aggregate.   

Orientational order of an ideal rodlike nematic: Rewriting the theory of nematic liquid crystals?

Order in rodlike nematic liquid crystals (LCs) represents a rich field described by myriad theories and studied using various analytical methods. We have made deuterium (D) NMR observations on the labeled para-quinquephenyl LC, which closely approximates a rigid rod. To investigate this high-melting nematic (range 390 - 427 deg. C), we have fabricated a high-efficiency oven on an NMR probehead using fumed silica tiles and utilizing only ambient air cooling. Observations on p-quinquephenyl clearly and drastically deviate from the Maier-Saupe theory and all other molecular theories of nematics, thus indicating the necessity for more a complete theory (e.g., including microscopic director fluctuations) to describe nematic order. We measure the complete order tensor for this LC using combinations of quadrupole and dipolar NMR coupling constants and different D label positions. We will discuss progress on refinements to nematic order theory, relations to NMR measurements, and fits using the phenomenological Landau-deGennes theory.   

Interlayer interactions in ferroelectric liquid crystals

We have recently drawn attention to a physical mechanism that can lead to an aligning interaction between distant layers in a ferroelectric smectic-C$^*$ liquid crystal. This effect arises because the amplitude of thermal fluctuations in layer shape is sensitive to correlations in $c$-director orientation in layers that are not nearest neighbors. This makes the entropy of the system dependent on the relative alignment of the $c$-director in all the smectic layers. In earlier treatments of this problem, a mean-field approximation was made in order to obtain an order-of-magnitude estimate of the strength of the interlayer interaction. While this was sufficient to demonstrate the significance of the mechanism, it led to an overestimate of the overall strength of the interaction because it included a self-energy term related to the anisotropy of a single layer. We have now remedied this shortcoming by evaluating in more detail the interlayer interaction due to layer shape fluctuations. We find that the rate at which the interaction decays as a function of interlayer distance does not follow any simple power-law form, but depends on a number of material parameters.   

The B4 phase: layer curvature driven by frustrated intralayer packing

We combine freeze fracture transmission electron microscopy, atomic force microscopy, and x-ray diffraction to show that the B4 phase is a smectic phase with highly curved layers (mean radius $\sim $ 4 layer spacings). The layer structure of the phase is a TGB-like phase made up of parallel arrays of multiple burgers vector screw dislocations (grain boundaries) giving 45 degree rotations across the grain boundaries. Models of the layer structure are based on periodic arrays of grain boundaries, each described by Scherk's first surface, and yield key features of the observed structures. This layered structure is dominated by saddle splay and we propose that the energy cost of defects required to make such a structure is offset by an energy gain of the layer curvature. We show that analysis of the wide angle x-ray diffraction of this phase indicates that layer curvature relieves the intralayer frustration produced by the packing of bent-core molecules. This work is supported by a NSF GRF and by NSF MRSEC Grant DMR0213918.   

Polarization-Enhanced Interaction between Islands on Freely-Suspended Smectic C* Liquid Crystal Films

Smectic liquid crystals can be made to form freely suspended films, two-dimensional systems locally quantized in thickness by an integral number of smectic layers, on which islands, circular regions of greater thickness than the surrounding film area, can be generated. In smectic C films, each such island is accompanied by a topological defect pair, an s = +1 topological defect inside and an s = -1 defect nearby on the background film. The distortions of the in-plane orientational order of the smectic C director field result in elastic interactions between the islands, with a short-range repulsion and a long-range dipolar attraction governing their stability and leading to their organization in chain-like structures with an equilibrium island separation. We have directly measured the repulsive and attractive forces between smectic C* islands using multiple optical traps and have compared the results quantitatively with theory. We find that the interactions between islands are much smaller in the racemic smectic C case than in the chiral smectic C*, an effect we attribute to long- range coulombic forces arising from polarization charges.   

Riverbottom texture: Patterns of compressional stress in an SSFLC cell

We have been studying the texture of remnant compressional stress in a bookshelf aligned SmA phase of the Displaytech mixture MDW8068. MDW8068 exhibits isotropic - nematic - SmA - SmC phases, and throughout the range of the SmA phase the layers show significant expansion on cooling. This layer expansion causes layer compression, which is relieved by dislocation formation and surface depinning events throughout the cell. The resulting SmA has essentially perfect alignment, but with a pattern of remnant stress that can be visualized near the SmA - SmC transition because of the divergent tilt susceptibility and resulting compression-induced tilt near the SmA - SmC transition. Low dislocation density areas are the areas of greatest layer compression, implying that the edge dislocations relieve the compressive stress. Temperature cycling shows the texture is set near the N - SmA transition, though x-ray diffraction data shows that the layer expansion occurs through the entire range of the SmA. X-ray diffraction from oriented samples has been done which shows that the texture is a result of competition between smectic ordering and surface pinning. Work supported by ED GAAN Fellowships P200A030179 and P200A000839, and NSF MRSEC Grant DMR-0213918.   

Random Lasing in Multidomain Cholesteric Liquid Crystals

A conventional laser consists of a pumped amplifying medium and an optical cavity to provide feedback for light amplification. In disordered materials, light can be trapped by multiple scattering processes and, if a gain medium is added, random lasing can occur. This random laser source does not require a regular cavity, but instead depends on multiple scattering in a random medium. Random lasers have attracted considerable attention recently because of their low cost and ease of construction. We present recent results of our random lasing experiments in dye-doped multidomain cholesteric liquid crystals, with submicron pitch, where the highly reflective cholesteric domains are the scattering elements. We discuss the underlying physics, compare the performance of these systems with others, consider the effects of temperature on the emission spectrum, and suggest possible applications.   

Wavelength Hopping in Cholesteric Liquid Crystal Lasers

Due to their birefringence and periodic structure, cholesteric liquid crystals (CLCs) in the helical cholesteric phase are one-dimensional photonic band gap materials. Gain enhancement and distributed feedback effects give rise to low threshold mirrorless lasing at the band edges. Since the wavelength at the band edge is proportional to the cholesteric pitch, which is a smooth function of temperature, one would expect the lasing wavelength to vary smoothly with temperature. Observations show, however, that the lasing wavelength does not depend smoothly on temperature, but instead exhibits periodic jumps between regions of smooth monotonic behavior. We have determined the reflection band dynamics, observed multiple lasing peaks at the hopping wavelength, and see evidence of hysteresis on measuring the reflection band and lasing peaks at different heating rates. We compare our observations with theoretical models, and propose an explanation for the observed dynamics.   

Poisson-bracket formulation of the dynamics of polar liquid crystals

We develop the dynamical theory of polar liquid crystals with local $C_{\infty v}$-symmetry using the general Poisson-bracket formalism. We obtain dynamical equations for the slow macroscopic fields that govern the dynamics in both the polarized and the isotropic phases. Starting from a microscopic definition of an alignment vector proportional to the polarization, we obtain Poisson bracket relations for the director field. The hydrodynamic equations differ from those of nematic liquid crystals ($D_{\infty h}$) in that they contain terms violating the ${\bf{n}} \rightarrow -{\bf{n}}$ symmetry. We find that the $\mathcal{Z} _2$-odd terms induce a general splay instability of a uniform polarized state in a range of parameters.   

Finite Element Elastodynamics Studies of Shape Evolution in Liquid Crystal Elastomers

Liquid crystal elastomers change shape under heating/cooling, applied fields, or optical illumination, with induced strains up to 400{\%}. We present a novel finite element elastodynamics technique to model dynamics of shape change in these materials, with explicit coupling between nematic order and elastic strain. Without added dissipation, the elastodynamics algorithm conserves the sum of kinetic and potential energy to one part in 10$^{6}$, even for large strains and rotations. In initial studies, we model shape evolution during a transition from the isotropic phase to nematic and back again, and model the induced curvature of an elastomer strip under local optical illumination. This method allows modeling of complex geometries and dynamic perturbations, and can serve as a bridge between fundamental soft condensed matter theory and engineering design.   

Temperature Dependence of Acousto-Optic Effect in a Nematic Liquid Crystal Cell

The acousto-optic effect occurs in a nematic liquid crystal cell when an incident ultrasonic wave causes a rotation of the director. This effect is observable as a change in the optical transmission through a cell, and has been exploited as a means of nondestructive imaging. The sensitivity and speed of this rearrangement are dependent on the viscosity of the liquid crystal material. Because of this, the effect is sensitive to the temperature. In this work we investigate quantitatively how the acousto-optic response is affected by the temperature of the liquid crystal cell. We present the results of studies of changes to the acoustic sensitivity of the cells and changes of their dynamic responses to the introduction of the ultrasonic wave.   

Elliptic Phases: A Study of the Nonlinear Elasticity of Twist-

The twist-grain boundary phase in smectic-A liquid crystals, constructed from rotating walls of parallel screw dislocations, is a prime example of a stable, ordered configuration of defects. In smectics, nonlinearities in the strains strongly affect the energetics and interactions between defects, thus complicating their analysis. By exploiting the properties of Jacobi elliptic functions, we construct a triply-periodic surface locally composed of screw dislocations, called Schnerk's surface, which has the structure of a series of ninety degree twist-grain boundaries. This is a candidate structure for the recently observed large-angle twist-grain boundary phases. Because of the analytic tractability of our construction, we compute that the grain boundaries interact exponentially at long distances through both the compression and bending energies, and that there is a preferred grain boundary spacing.   

Calorimetric study of aligned liquid crystal + aerosil

A high-resolution ac-calorimetric study was performed on magnetically aligned colloidal dispersions of 8CB and aerosil spanning the weakly first-order \textit{I}-\textit{N} and second order \textit{N}-Sm\textit{A} phase transitions. Stable aligned samples were prepared by repeated cycling between the isotropic and nematic phase in the presence of a $2$~T magnetic field. Zero-field measurements were carried out on six aligned conjugate density samples ranging from $0.03$ to $0.15$~g~cm$^{-3}$ (mass of aerosil per volume of liquid crystal). For comparison, two unaligned samples from the same batch ($0.05$ and $0.13$~g~cm$^{-3}$) were also studied. The unaligned samples reproduce very closely previous studies on this system. The magnetically aligned samples, exhibits lower transition temperatures for the same aerosil density sample and a shift to higher aerosil density of the non-monotonic $T_c$ evolution. The clear differences between aligned and unaligned sample indicate the ``memory'' of the magnetic field even after heating deep into the isotropic phase. The origin of this ``memory'' remains unexplained.   

Disordering Effects in Smectic -- Aerosil Gels*

We studied quenched disorder effects on the 12CB liquid crystal upon dispersion of silica nano particles (type A-300): hydrophilic silica spheres of diameter 7nm and surface area S = 300 m$^{2}$g$^{-1}$, with hydroxyl groups covering the surface. The LC-aerosil dispersions form a gel if aerosil density exceeds the percolation threshold. For low densities of aerosil dispersions and in cooling the sample, the LC director in the void volume is parallel and follows the external NMR field; a well defined and stable LC configuration forms. When a complete silica network forms and the sample orientation in the field is changed, a few silica links are broken by the field, re-aligning only a few Sm layers; the aerosil locks in the LC configuration which follows a P$_{2}$ (Cos$\Theta )$ dependence. In contrast, if the dispersion is cooled from isotropic phase outside the field, the spectra in the Sm phase is a powder pattern. The field anneals the aerosil-induced random disorder up to a certain density beyond which, disordering effects dominate; for aerosil densities greater than $\rho _{S} \quad \approx $ 0.055~g/cm$^{3}$ spectral patterns are consistent with an isotropic distribution of smectic domains. The quenching of the 12CB Sm-A phase at $\rho _{S} \quad \approx $ 0.055~g/cm$^{3}$, is one order of magnitude less than that in 8CB [1]. The aerosil induced disorder, quantified by the percent of LC molecules in a powder pattern, depends linearly on the density. [1] T. Jin and D. Finotello, \textit{Phys. Rev}. E 69, 041704 (2004); \textit{Phys. Rev Lett}. 86, 818 (2001). *Supported by NSF-INT 03-06851, OBR B-7844 and B-7845.   

The Effect of Aerosil Network on Smectic A-Reentrant Nematic Liquid Crystal

We report on a high resolution x-ray scattering study of aerosil dispersion effects on nematic-smectic A and smectic A-reentrant nematic phase transitions in 6OCB (hexyloxycyanobiphenyl) and 8OCB (octyloxycyanobiphenyl) liquid crystal mixtures. Dispersed aerosil particles introduce quenched randomness to the liquid crystal phases, which destroys the long range smectic order [1]. The experiment was conducted on mixtures with different 6OCB:8OCB concentrations and aerosil densities. The parabolic smectic A phase boundary is found to be slightly distorted in the presence of the aerosil network, with shifted transition and median (T$_{M})$ temperatures. Above T$_{M}$, the order parameter, susceptibility and parallel correlation lengths for the thermal and random parts of the structure factor show behaviors similar to those observed in non-reentrant nematic-smectic A second order phase transitions [2]. At T$_{M}$, where the order parameter has its maximum value, the scattering peaks are only defined by the random part of the structure factor. The smectic order parameter decreases with a further decrease in temperature, while the susceptibility and thermal correlation length increasingly show nematic-like behavior. Finally, at the lowest temperature, the mixtures are found in the reentrant nematic phase. [1] P.S. Clegg et. al. PRE 67,021703 (2003) [2] S. Larochelle et. al. in preparation   

Session U22: Focus Session: Magnetic Tunneling I

Tunneling spin polarization in planar tunnel junctions: measurements using NbN superconducting electrodes and evidence for Kondo-assisted tunneling

The fundamental origin of tunneling magnetoresistance in magnetic tunnel junctions (MTJs) is the spin-polarized tunneling current, which can be measured directly using superconducting tunneling spectroscopy (STS). The STS technique was first developed by Meservey and Tedrow using aluminum superconducting electrodes. Al has been widely used because of its low spin orbit scattering. However, measurements must be made at low temperatures ($<$0.4 K) because of the low superconducting transition temperature of Al. Here, we demonstrate that superconducting electrodes formed from NbN can be used to measure tunneling spin polarization (TSP) at higher temperatures up to $\sim $1.2K. The tunneling magnetoresistance and polarization of the tunneling current in MTJs is highly sensitive to the detailed structure of the tunneling barrier. Using MgO tunnel barriers we find TSP values as high as 90{\%} at 0.25K. The TMR is, however, depressed by insertion of ultra thin layers of both non-magnetic and magnetic metals in the middle of the MgO barrier. For ultra-thin, discontinuous magnetic layers of CoFe, we find evidence of Kondo assisted tunneling, from increased conductance at low temperatures ($<$50K) and bias voltage ($<$20 mV). Over the same temperature and bias voltage regimes the tunneling magnetoresistance is strongly depressed. We present other evidence of Kondo resonance including the logarithmic temperature dependence of the zero bias conductance peak. We infer the Kondo temperature from both the spectra width of this conductance peak as well as the temperature dependence of the TMR depression. The Kondo temperature is sensitive to the thickness of the inserted CoFe layer and decreases with increased CoFe thickness. * performed in collaboration with S-H. Yang, C. Kaiser, and S. Parkin.   

High sensitivity of tunneling spin polarization to chemical bonding of transition metal ferromagnetic alloys at interface with insulating barrier

We report that the tunneling spin polarization (TSP) is found to be strongly influenced by the amount of oxygen used in the deposition of the tunnel barrier itself that chemical bonding at the interface between Al$_{2}$O$_{3}$ and ferromagnetic Co and Co-Pt alloys. For reactive sputter (RS) deposition of alumina using an argon-oxygen gas mixture with a low concentration of oxygen ($\sim $0.1 mTorr), much lower TSP values are found than when the alumina barrier is formed by post-plasma oxidation (PO) with $\sim $100mTorr oxygen of Al layers. X-ray absorption spectroscopy (XAS) has been used to characterize the chemical bonding at the Co or Co-Pt/Al$_{2}$O$_{3}$ interface. These studies show that Co-O bonds are much more formed for the barrier fromed by PO of Al than for that formed by RS deposition. We attribute the changes in TSP to changes in the relative tunneling probabilities from Co and Pt which are strongly influenced by oxygen bond formation.$^{1}$ $^{1}$C. Kaiser, S. van Dijken, S.-H. Yang, H. Yang, and S. S. P. Parkin, Phys. Rev. Lett. 94, 247203 (2005).   

Anomalous magnetic behaviors in AlO$_x$/Co$_{84}$Fe$_{16}$ Tunneling Magnetoresistance (TMR) systems induced by the interfacial oxidations

Due to the practical application of the magnetic tunnel junction as magnetic memory cells and sensors, tunneling magnetoresistance (TMR) in the magnetic tunnel junction (MTJ) has been very actively studied, and extensive effort has been exerted in order to improve the physical properties of MTJs. As a result, it is well recognized that the performance of the MTJs strongly depends on the environment at the interface between the magnetic layer and the insulating oxide layer. In spite of such critical importance, the interface environment and its effects on the magnetic behaviors have not been well understood yet. Here we present anomalous magnetic behaviors observed in AlO$_x$/Co$_{84}$Fe$_{16}$ magnetic tunnel junction structures prepared in various oxidation processes. We examined the magnetic behaviors using the magneto-optical Kerr effect (MOKE) measurements in vacuum. The system turns out to exhibit a strong temperature dependence of the magnetic coersive field, and the behavior consistently varies with the oxidation process. The interface environments and the origin of the anomalous behaviors will be also discussed based on x-ray reflectivity and diffraction results.   

Tunneling magnetoresistance in MgO-based magnetic tunneling junction with (001) oriented FeCo electrode fabricated by sputtering deposition.

MgO-based Magnetic Tunneling Junction (MTJ) is now one of the most promising structures for spintronics applications due to its very large Tunneling Magnetoresistance (TMR). However, the high TMR is generally achieved only after the annealing at high temperature that promotes the crystallization of MgO. We fabricated FeCo/MgO/FeCo MTJs on both oriented and non-oriented buffer layers by DC and RF sputtering depositions at room temperature. MgO barrier layer was formed by reactive sputtering method following a very thin Mg deposition. Without annealing, 70\% TMR ratio was observed for MTJ with oriented buffer layers, whereas 40\% TMR ratio was observed with non-oriented buffer layers. This indicates that an oriented buffer layer can enhance the TMR ratio even without high temperature annealing. We also studied the dependence of TMR on the Mg layer thickness, MgO barrier thickness, and annealing conditions.   

Effect of interface states on spin-dependent tunneling in Fe/MgO/Fe tunnel junctions

The electronic structure and spin-dependent tunneling in epitaxial Fe/MgO/Fe(001) tunnel junctions are studied using first-principles calculations.$^1$ For small MgO barrier thickness the minority-spin resonant bands at the two interfaces make a significant contribution to the tunneling conductance for the antiparallel magnetization, whereas these bands are, in practice, mismatched by disorder and/or small applied bias for the parallel magnetization. This explains the experimentally observed decrease in tunneling magnetoresistance (TMR) for thin MgO barriers. We predict that a monolayer of Ag epitaxially deposited at the interface between Fe and MgO suppresses tunneling through the interface band and may thus be used to enhance the TMR for thin barriers. [1] K. D. Belashchenko, J. Velev, and E. Y. Tsymbal, Phys. Rev. B \textbf{72}, 140404(R) (2005).   

The atomic and electronic structure of the FeCoB/MgO interface

Magnetic tunnel junctions (MTJs) have recently aroused much interest due to their potential applications as random access memories and magnetic field sensors. MTJs consist of a thin insulating layer separating two ferromagnetic electrodes. Very recently FeCoB/MgO/FeCoB MTJs have shown promising results. Upon annealing, the amorphous FeCoB electrodes crystallize in a bcc structure epitaxial to the MgO(001) surface. Many groups have observed a significant increase in TMR ratios (higher than 300{\%} at room temperature [1]) after annealing. It is clear that the crystallization of the electrodes plays an important role in this increase. It is not clear, however, what happens to B after annealing and what role it plays in enhancing TMR. We present results of first-principles total energy calculations that suggest that it is energetically favorable for B to reside at the crystalline FeCoB/MgO interface rather than remain in the bulk of the FeCoB electrode. We also find that the presence of B at the interface significantly weakens bonding between the FeCoB electrode and the MgO barrier. We are investigating the presence of resonant states[2] at the FeCoB/MgO interface and will discuss the effects of interfacial B on them. [1] J. Hayakawa \textit{et al}., 2005 MMM Conference. [2] Belashchenko \textit{et al}., \textit{Phys. Rev. B}~\textbf{72}, R140404 (2005).   

Structural and magnetic properties of Fe/MgO/Ge(100) heterostructures and Fe/MgO/Fe(100) magnetic tunnel junctions

MgO is one of the most attractive materials for spintronic devices due to a novel spin filtering effect that dramatically increases spin polarization. We utilize molecule beam epitaxy (MBE) to synthesize ferromagnet (FM)/MgO/semiconductor heterostructures for efficient spin injection and detection in Si, Ge and GaAs lateral devices and MgO-based magnetic tunnel junctions (MTJs) for MRAM applications. Initial studies have focused on optimizing the growth of Fe/MgO/Ge(100) heterostructures and Fe/MgO/Fe MTJs and investigating their magnetic properties. Results indicate high-quality layer-by-layer growth with roughness at the atomic scale. \textit{In situ} reflection high energy electron diffraction (RHEED) is utilized for investigating surface roughness during growth. We observe streaky RHEED patterns and intensity oscillations for the homoepitaxial growth of Ge on Ge(100) at 370$^{o}$C, which indicates an atomically flat Ge buffer layer. Epitaxial MgO layers grown on top of the Ge buffer at room temperature also showed streaky RHEED patterns and atomic force microscopy (AFM) images revealed the rms roughness to be 0.2 nm ($\sim $1 atomic layer) for a 3 nm thick MgO film. High remanance and small coercive field have been observed in epitaxial Fe (5nm)/MgO(3nm)/Ge(100) heterostructures via magneto-optic Kerr effect (MOKE) illustrating good magnetic properties.   

XPS studies of MgO based magnetic tunnel junction structures

The very high tunneling magnetoresistance (TMR) obtained in MgO magnetic tunnel junctions (MTJ)$^{(1,2)}$ motivates the investigation of the electronic properties of the MgO barrier layer and the study of the ferromagnetic metal - MgO interface chemistry. Such large TMR values are predicted by theory due to the high degree of order apparent in the barrier and electrode materials. However, as grown ultra-thin MgO films generally contain defects that can influence electron transport properties through the creation of low energy states within the bulk MgO band-gap. We will report the results of x-ray photoelectron spectroscopy (XPS) studies of (001) textured ultra-thin MgO layers that are prepared by RF magnetron sputtering and electron beam evaporation on ordered ferromagnetic electrodes and in ordered MTJ structures with and without post growth vacuum annealing. XPS spectra for both MgO deposition techniques clearly indicate a surface oxygen species that is likely bound by defects in the oxide$^{(3)}$ in half-formed junctions and improvements in MgO quality after counter electrode deposition. We will discuss our results regarding the chemical properties of the oxide and its interfaces directed towards possibly providing guidance to engineer improved MgO MTJ devices. [1] S.S.P. Parkin et. al., Nature Materials, \textbf{3}, 862 (2004). [2] S. Yuasa et. al., Nature Materials, \textbf{3}, 868 (2004). [3] E. Tan et. al. , Phys. Rev. B. , \textbf{71}, 161401 (2005).   

Interlayer Exchange Coupling in Fe$\vert $MgO$\vert $Fe Magnetic Tunnel Junctions

Fully epitaxial Fe$\vert $MgO$\vert $Fe(001) films with wedge-shaped MgO layer were prepared on single-crystal MgO(001) substrates using MBE technique [1]. Structure of the films is Fe-free-layer(15, 20, 30 nm)/MgO(0.3-1.8 nm)/Fe-pinned-layer(10 nm)/Ir-Mn. The interlayer exchange coupling (IEC) energy was obtained at room temperature from a unidirectional shift of the Kerr hysteresis loop. The IEC was found to be antiferromagnetic for small MgO thickness but changed sign at 8.5{\AA}. In order to explain this behavior we performed \textit{ab-initio} calculations of IEC in Fe$\vert $MgO$\vert $Fe(001) MTJs with and without oxygen vacancies in MgO. Our results show that without O vacancies the IEC is ferromagnetic and decays exponentially with MgO thickness. However, in the presence of O vacancies the IEC is antiferromagnetic for thin barriers and changes sign with increasing barrier thickness. This behavior is consistent with our experimental observations and is explained by the resonance contribution to the IEC due to localized defect states [2]. [1] S. Yuasa \textit{et al}., Nature Mater. \textbf{3}, 868 (2004). [2] M. Y. Zhuravlev \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 026806 (2005).   

Inelastic electron tunneling spectroscopy of MgO$_{x}$F$_{y}$ barriers

Recent development in TMR junction with MgO barrier attracts a great deal of attention. It is reported that the junctions with MgO barrier exhibit higher TMR with lower RA value. Combined with the spin-transfer switching that has been demonstrated, the future MRAM architecture will to incorporate the MgO barrier TMR junctions. The~ device parameters for MRAM will require the RA value of about 100 $\Omega -\mu $m$^{2}$, corresponding to about 1 nm thick MgO barrier layer. In order to understand the electrical properties of MgO barrier, we have fabricated Mg/MgO/Mg tunneling junctions as the function of oxidation time of the Mg metal layer. These Mg/MgO/Mg cross-strip junctions are deposited using stencil masks without a vacuum break, and the size of junction area is about 130 $\mu $m by 160 $\mu $m. When measuring d$^{2}$I/dV$^{2}$-V, namely the inelastic tunneling spectroscopy, we observed the peaks correponding to MgO bonds, indicating that the MgO barrier is a stable and good insulator. Using the IETS measurement technique, we will present the interface properties between the ferromagnetic electrode and the MgO barrier layer.~Moreover, we will report on our MgO$_{x}$F$_{y}$ tunnel barrier made by our fluorine-doping method.   

Spin torque and spin current in magnetic tunnel junctions

In recent years, current-induced spin torque [1] has attracted strong interest both because it may advance our understanding of fundamental physics and because it may have useful applications. We present a study of non-equilibrium spin currents and the corresponding spin torques in magnetic tunnel junctions with non-collinear moments. Calculations are based on the Keldysh formalism in which the non-equilibrium Green functions are calculated within a tight-binding model using the technique of Caroli et al. [2]. The properties of spin torque and spin currents are studied as a function of applied bias, barrier thickness and lattice structure type. In addition, the exchange coupling between ferromagnetic layers in magnetic tunnel junctions is investigated via its relation to the current induced spin torque. [1] J. C. Slonczewski, J. Magn. Magn. Mat. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996); E. Myers, D. Ralph, J. Katine, R. Louie, R. Buhrman, Science, 285, 867 (1999). [2] C. Caroli, R. Combescot, P. Nozieres, D. Saint-James, J. Phys. C: Solid St. Phys., 4, 916 (1971).   

Shot noise in Cr-doped and undoped Co/Al$_{2}$O$_{3}$/Py magnetic tunnel junctions

We have found that shot noise in Co(80{\AA}) /Al$_{2}$O$_{3}$(14{\AA})/Py(100{\AA}) magnetic tunnel junctions (MTJs) is reduced with respect to Poisonian value. The Fano factor, obtained at frequencies (100$<$f$<$1000Hz), temperatures (T$<$10K) and biases (below 150 meV) where the shot noise dominates, varies between F $\approx $0.8 and 0.65, indicating correlated electron tunnelling. Doping of the insulating barrier with Cr inclusions suppresses the conductivity and tunnelling magnetoresistance, and restores the Fano factor to a value corresponding to uncorrelated transport (F$\approx $1). These results indicate an enhanced cross-correlation between electrons due to trapping or spin- flip assisted tunnelling in the undopped MTJs, and a possible Coulomb blockade in the Cr doped MTJs.   

Session U23: Frustration in 2D

Spin Frustration, Magnetic Susceptibility and Excitations in Spatially Anisotropic Triangular-Lattice Antiferromagnets

We calculate the temperature dependent magnetic susceptibility and excitation spectra for the spatially anisotropic triangular- lattice Heisenberg model. We show that suitably scaled plots of magnetic susceptibility provide a direct measure of frustration in the system and allow one to infer the exchange parameters from the susceptibility data. We find that the organic material $\kappa-(BEDT-TTF)_2Cu_2(CN)_3$ is very well described by the isotropic triangular lattice model. We also find that the excitation spectra of the model shows various anomalies in the Neel and Spiral phases.   

Anomalous renormalization of excitation spectra of the triangular-lattice antiferromagnet

We use series expansions to calculate the excitation spectra of the spin-1/2 triangular-lattice Heisenberg antiferromagnet above the 3-sublattice ordered phase. We find that the spectra are renormalized downwards with respect to linear spin-wave theory. This is in sharp contrast to the square-lattice antiferromagnet, where the spectral frequencies are renormalized upwards due to quantum fluctuations. The triangular-lattice spectra show sharp downward renormalization at special wavevectors, which (a) can be interpreted as evidence of high-energy spinons and (b) provides an explanation for rapid loss of antiferromagnetic correlations with temperature, consistent with earlier high-temperature series expansion studies.   

Pairing of doped holes in the Shastry-Sutherland lattice

We investigate the magnetic and pairing correlations in a hole doped Shastry-Sutherland lattice. We use a variational wave function that reproduces the valence bond ground state of the un-doped material, relevant to SrCu2(BO3)2. For finite doping we study the disappearance of the valence bond crystal and the emergence of pairing of doped holes.   

In Gap Excitations and Triplet Lifetime Broadening in the Dilute Singlet Ground State System SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$

We have carried out high resolution time-of-flight neutron scattering measurements on a new high quality single crystal of SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$ with $x$ = 0.1. These studies revealed the presence of new excitations within the singlet-triplet gap of this quasi-two dimensional, dilute, singlet ground state system. These new excitations showed little or no shift in energy with increasing applied magnetic field. In addition, we observe substantial broadening of the three triplet excitations in the dilute single crystal, as compared with pure SrCu$_{2}$(BO$_{3})_{2}$.$^{1}$ The triplet excitations in doped SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$ therefore possess finite lifetimes at low temperatures in the range that can be measured with cold neutron spectroscopy. We have also calculated the dynamical spin structure factor using the zero temperature Lanczos method, and solving a Shastry-Sutherland model at zero and finite doping for different strengths of external magnetic field. This theory reproduces all the qualitative features observed in the experiments on SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$. $^{1}$ B.D. Gaulin \textit{et al.}, Phys. Rev. Lett., \textbf{93}, 267202, 2004.   

Infrared studies of a quantum magnet SrCu$_2$(BO$_3$)$_2$

We will report results of our infrared studies of SrCu$_2$(BO$_3 $)$_2$, a two-dimensional spin system with a disordered ground state even at very low temperatures, and a spin gap of about 24 cm$^{-1}$ (3 meV). This material has recently attracted attention because of a possibility that doping may lead to a superconductivity mediated by antiferromagnetic fluctuations, possibly similar to high-T$_c$ cuprates. Using polarized light we have probed both crystallographic directions over a broad range of frequencies (from about 30 cm$^{-1}$ to 20,000 cm$^{-1} $) and temperatures (from 4.2 K to 300 K). The results reveal significant differences between the ab-plane and c-axis directions. We will discuss these findings in relation with the resonance effects observed in inelastic light scattering experiments from collective magnetic excitations.   

Magnetization plateaus for Cs$_{2}$CuBr$_{4}$

Cs$_{2}$CuBr$_{4}$ is a new two-dimensional spin-1/2 system, where 1/3- and 2/3-plateaus have been observed in external magnetic fields. The magnetic behaviors of the material are well explained by a two-dimensional antiferromagnetic Heisenberg model on a distorted triangular lattice. In the model, there are two types of interactions $J_{1}$ and $J_{2}$, where $J_{1}$ chains are coupled with inter chain interactions $J_{2}$. Using an exact diagonalization method, we investigated magnetic properties, especially magnetization curve. In the magnetization, 1/3-plateau appears for 0.7 $\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} J_{2 }$/$J_{1 }\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} $1.3. At the plateau, three-fold degenerate ground state, up-up-down structure, is realized. Our results indicate that the material has a more frustrated character $J_{2 }$/$J_{1 }\approx $0.7 than what has been expected from a classical theory $J_{2 }$/$J_{1 }$= 0.47. The magnetic properties at 2/3-plateau will also be discussed.   

Dynamical properties of the anisotropic triangular quantum antiferromagnet with Dzyaloshinskii-Moriya interaction

We present a detailed study of the anisotropic triangular quantum antiferromagent with Dzyaloshinskii-Moriya (DM) interaction building on earlier work by Veillette, James and Essler [Phys.\ Rev.\ B, {\bf 72}, 134429 (2005)]. The DM interaction generates an easy-plane anisotropy and opens a gap in the spin-wave spectrum at the incommensurate ordering wave vector $\vec Q$. Our calculation utilizes the Holstein-Primakoff representation of spins and goes beyond linear spin wave theory by taking into account magnon-magnon interactions in a $1/S$ expansion. We calculate renormalized dispersion relations for the magnons to order $1/S$ for different values of the DM interaction and pay particular attention to the interesting case of zero DM interaction. The dynamical structure factor is calculated to order $1/S$. It is found that, compared to linear spin wave theory, a significant fraction of the scattering intensity is shifted to higher energies. We compare our findings with the recent neutron scattering data measured on the frustrated quantum antiferromagnet $Cs_2 Cu Cl_4$, [R. Coldea, {\it et. al.}, Phys.\ Rev.\ B, {\bf 68}, 134424, (2003)].   

Spiral magnetic order on an anisotropic triangular lattice in the presence of Dzyaloshinskii-Moriya interaction

We consider the ground state energy, magnetization and the energy spectrum of the two-dimensional antiferromagnets on the triangular lattice in the presence of anisotropy in the exchange couplings, and Dzyaloshinskii-Moriya (DM) interaction $D$. $J$ and $J^{\prime}$ are meant to be the couplings along the chain direction and zigzag bonds respectively. Assuming that in the wide range of those parameters the system has the spiral Neel ordering, we consider the role of quantum fluctuations within the framework of a standard $1/S$ expansion. We show that DM interaction considerably suppresses fluctuations and seems to play an important role in stabilizing the spiral Neel ordering in this frustrated system. We discuss our theoretical results within the context of the recent experimental measurements in the frustrated quantum magnet $Cs_2 Cu Cl_4$, [R. Coldea, et. al., Phys. Rev. B, {\bf 68}, 134424, (2003)], and the recent theoretical structure factor calculations by M. Y. Veillette, et. al., Phys. Rev. B {\bf 72}, 134429 (2005)   

A quasi-1d approach to the triangular antiferromagnet Cs$_2$CuCl$_4$

We discuss the low-temperature properties of a spin-1/2 triangular Heisenberg antiferromagnet in a field, including the effects of Dyaloshinskii-Moriya interactions, as believed appropriate to Cs$_2$CuCl$_4$\footnote{R. Coldea et al., Phys. Rev. Lett. 88, 137203 (2002).}. Our treatment is based upon a view of the problem as a system of weakly-coupled spin chains. Analytic results are obtained through a combination of renormalization group, chain mean-field, and bosonization methods. Using various exactly-known properties of the individual Heisenberg chains, we calculate numerous physical properties, including various phase boundaries in longitudinal and transverse fields. We compare our results to published experimental data.   

Simultaneous breaking of lattice symmetry and spin frustration in triangular lattice antiferromagnet $\rm CuFeO_2$

We use high resolution synchrotron X-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet (TLA) $\rm CuFeO_2$. We show that the occurrence of the two magnetic transitions, at $14$~K and $11$~K, respectively is accompanied simultaneously by a second-and first- order structural phase transitions from a hexagonal structure to a monoclinic form. This is the first observation of two successive spin-driven structural transitions directly coupled with incommensurate and commensurate magnetic orderings in frustrated TLA systems. \newline \newline This work is supported by the U. S. NSF DMR-0453804 and DOE Nos. DE-FG02-05ER46202 and DE-AC05-00OR22725 with UT/Battelle LLC. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38.   

Neutron scattering studies on Kagome bilayers -- Spin fluctuations in a broad dynamic range

The kagome bilayer compounds Ba2Sn2ZnCr7xGa10-7xO22 and SrCr9xGa12-9xO19 are localized spin (S=3/2) systems with strong antiferromagnetic exchange (J/k $\approx $ 50 K) that show no long-range order down to mK temperatures. This remarkable behaviour originates from the geometrical frustration. We have examined these systems using polarized neutron diffraction, inelastic neutron scattering (INS) and neutron spin-echo (NSE) spectroscopy for characterizing the magnetic correlations in a very broad energy range. INS data reveals the dynamic nature of the broad diffuse scattering that characterises the quasi-elastic magnetic response above the spin freezing. New aspects on the freezing phenomenon and the ground state properties are revealed using NSE.   

Hall constant on triangular lattice ladder systems

To make the transition from a one-dimensional to a planar system one can connect $n$ 1D-chains to yield $n$-leg ladders. For square lattices, this approach has already led to quite interesting and unexpected results (Dagotto et al, Prelov\v sek et al). In this work, frustration is introduced through triangular geometry, achieved by adding additional bonds to the square lattice ladders. We study the response of such $t$-$J$ ladders to an applied magnetic field and investigate the Hall constant $R_H$ as function of temperature, interaction strength, doping, and frequency. These systems complement toroidal or spherical systems since these allow the limit $\partial/\partial B$ to be taken more accurately which leaves the geometric frustration fully preserved. We investigate the crossover from a degenerate Fermi system to a high T regime where $R_H$ grows indefinitely ($R_H\propto T$) with temperature.   

Position-Space Renormalization-Group Treatment of the Triangular Ising Antiferromagnet with Quenched Disorder

We apply the Niemeijer-van Leeuwen cluster approximation [1] to frustrated Ising models on a triangular lattice. The homogeneous Ising antiferromagnet is fully frustrated and shows no ordered phase. Frustration can be relieved via the addition of quenched randomness through either dilution or the introduction of ferromagnetic bonds. The result is a rich phase diagram with different types of ordering depending upon the details of the quenched disorder. [2] Using a binning procedure to retain the full distribution of interactions under rescaling [3], we are able to calculate the phase diagram of this system, with each phase having its own characteristic attractor. This model system provides a two-dimensional example of the impacts of tunable frustration on short- and long-range order. 1. T. Niemeijer and J.M.J. van Leeuwen, Phys. Rev. Lett. \textbf{31}, 1411 (1973); Physica (Utr.) \textbf{71}, 17 (1974). 2. G. S. Grest and E.G. Gabl, Phys. Rev. Lett. \textbf{43}, 1183 (1979); H. Kaya and A.N. Berker, Phys. Rev. E \textbf{62}, 1469 (2000).; M. Robinson, M.S. Thesis, University of Maine (2003). 3. E. Hartford and S. McKay, J. Appl. Phys. \textbf{70}, 6068 (1991); E. Hartford, Ph.D. Thesis, University of Maine (1994); A. Falicov, A.N. Berker, and S.R. McKay, Phys. Rev. B \textbf{51}, 8266 (1995).   

Session U24: Liquid-Crystalline Polymers

Patterns and Defects in Nematic Elastomers

Nematic elastomers are materials that combine the orientational properties of nematic liquid crystals with the elastic properties of rubber. Ideal nematic elastomers, formed via a spontaneous symmetry breaking transition from the isotropic rubber state, exhibit soft elasticity in which one of the five elastic moduli of a uniaxial elastic medium vanishes. Monodomain samples crosslinked under imposed strain exhibit semi-soft elasticity in which that elastic modulus is small but nonzero. Applying linear stability analysis to the semi-soft elastic energy, we investigate two phenomena observed in experiments on nematic elastomers: (1) the formation, in experiments by Bob Meyer at Brandeis, of periodic modulations of the nematic director and elastic displacement (stripes) in cells subjected to a normal electric field in which the direction of stripe normals is at an oblique angle to the original nematic director and (2) the formation of +1 disclination defects at the surface of nanotube gel films [Islam, M. F., Nobili, M., Ye, Fangfu , Lubensky, T. C. and Yodh, A. G. , \textit{Phys. Rev. Lett }. \textbf{95}, 148301/1-4 (2005)].   

In situ synchrotron studies of structure development during injection molding of liquid crystalline polymers

As in all polymer materials, the effect of polymer processing on the underlying molecular structure has a profound effect on the properties of liquid crystalline polymer products. While in situ scattering techniques have proven powerful for studying complex polymer structure during comparatively simple shearing or channel flows, their application to processing flows has largely been limited to in situ x-ray scattering/diffraction studies of structure development during fiber spinning. Here we report a new experiment in which a lab-scale injection molding machine has been modified to allow real-time, in situ measurements of molecular orientation development and subsequent crystallization during injection molding. The experiment requires high x-ray energy to reduce absorption in the aluminum mold wall, and high flux and a fast area detectors to achieve the necessary resolution to track time-dependent changes in fluid structure during mold filling. Hence it is ideally suited to the capabilites of the Advanced Photon Source. We report measurements injection molding of a commercial liquid crystalline copolyester (Vectra A) as a function of position in the mold and various process variables.   

Liquid crystalline pattern formation in drying droplets of biopolymers

When a droplet of DNA in water dries out, a ring-like deposit is observed along the perimeter, similar to the stains in spilled drops of coffee. However, the dried ring of DNA is a self-similar birefringent pattern composed of extended molecules. We examine dynamics of the pattern formation at the droplet's rim. This gives us an insight into the underlining physics. During the major part of drying process the contact line is pinned so that DNA molecules are brought to the perimeter and extended by the radial capillary flow. Lyotropic nematic phase is formed in which highly concentrated DNA aligns along the triple line to minimize elastic energy. When the contact angle becomes small, the contact line starts to retract and the radial dilative stress causes buckling distortions at the rim which then propagate deep into the elastic liquid- crystalline medium and give rise to the pattern.   

Orientational dynamics of dipolar or magnetized rigid nematic liquid crystal polymers and suspensions in imposed flow and external fields

In this talk, I will present a scheme for computing the exact solution of the Smoluchowski equation under imposed flow and electric or magnetic external fields for rigid nematic liquid crystal polymers. This includes a suite of mathematical theorems that reveal the intrinsic relationship among the lower order moments of the pdf and the flow as well as the external field. Then, a simple mathematical transformation links the problem to some target model problems for which the solutions can be readily obtained using the existing numerical tools. The orientational dynamics is then studied from the exact solutions of the Smoluchowski equation.   

Spacer length controlled lamello-columnar to oblique-columnar mesophase transition in liquid crystalline DNA - discotic cationic lipid complexes

A series of asymmetric triphenylene imidazolium salts with different spacer lengths (C5, C8, and C11) were synthesized and their ionic complexes with double-strand DNA were prepared in aqueous solution. The molecular composition of the complexes was determined by FTIR analysis. The liquid crystalline morphology was characterized by polarized light microscopy, X-ray diffraction (XRD), and transmission electron microscope. 2D XRD results indicated an oblique columnar phase for the complex with a short spacer length of C5, while lamello-columnar phases for those with longer spacer lengths (C8 and C11). Thin film circular dichroism results showed the disappearing of any helical conformation in the DNA in all the complexes. Instead, the complexation between single-strand RNA and discotic cationic lipids did not show columnar morphology; therefore, the columnar liquid crystalline morphology in the DNA-discotic cationic lipid complexes was attributed to the DNA double-strand chain rigidity.   

Binary phase diagrams of liquid crystal/polymer systems exhibiting crystal, smectic, and nematic transitions

Current theories describing the phase diagrams of nematic-nematic mixtures and polymer-nematic liquid crystal mixtures are based on the combination of classical Flory-Huggins (FH) theory of isotropic mixing and the Maier-Saupe (MS) free energy of nematic ordering. This combined FH-MS theory was extended by Kyu and Chiu to polymer-smectic-A systems by incorporating McMillan free energy of the nematic-smectic-A transition. We have developed a generalized model for an LC system undergoing crystallization, extending the Maier-Saupe-McMillan theory in conjunction with the Landau-Brazowiskii model of weak first order phase transition such as solidification. The generalized FH-extended MSM free energy is then minimized with respect to all order parameters and the phase diagram is constructed by balancing the chemical potentials of corresponding phases. This form of the free energy could be valuable in the time evolution studies of phase transitions in polymer/liquid crystal mixtures.   

Interfacial Characteristics of Semi Fluorinated Polymeric and Polymeric Liquid Crystalline Surfaces

The interfacial interactions of semi fluorinated perfluorocyclobutane (PFCB) liquid crystalline, and semi crystalline polymers have been investigated. The interfacial characteristics are critical in any of their applications in which a well define stable interface is required from LCD technology to wave-guides and lithography. The inherent segregation between fluorine rich and hydrogen rich segments results in induced liquid crystallinity within the polymers themselves, even though these groups are small. The segregation to the interfaces controls the capability of the surface energy of the system. In addition of interfacial tension measurements, we used their alignment of small LC molecules using of 4, 4'-octylcycanobiphenyl (8CB) as a model system to further explore their interfacial characteristics. The correlation between surface energies, dynamics of the interface and their effects of the orientation of small LC will be discussed.   

The Origin of Supra-Molecular Columnar Structures from Symmetrically Tapered Bisamides

A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CnPhBA) were synthesized to study the effect of alkyl chain length on supra-molecular structures. Phase transitions were studied with DSC, PLM, WAXD, IR, SAED, solid-state $^{13}$C NMR and computer simulations. All of the CnPhBAs formed a highly-ordered oblique columnar ($\Phi _{OK})$ phase and a low-ordered oblique columnar ($\Phi _{OB})$ phase. The two main driving forces to form these supra-molecular structures were identified: One is the $H$-bond between N-H and C=O groups, and the other is the micro-phase separation of the bisamide cores and the alkyl chains. With increasing alkyl length, the isotropization temperature decreased, while the alkyl chain disordering temperature increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing alkyl length. However, the alkyl length did not have a significant influence on the packing along the columnar axis.   

Crystallization, the gel point and DQ NMR in PDMS networks

In the framework of classical models, the presence of constraints that reduce the mobility of the chains should lead to a reduction in the crystallization rate of polymer melts and result in a lower degree of crystallinity at a given cooling rate. An increasing number of experimental observations which seem to contradict the basic premises of the classical picture have been reported in the last couple of years. In particular, recent experiments findings suggest that in some cases crystallization from a crosslinked melt is more efficient than that from the non crosslinked analogue. In this work we report on a detailed study carried out in order to examine the relation between crystallization and the crosslinked network parameters. The effect of precursor molecular weight, crosslinker functionality and degree of crosslinking were examined. The thermal characteristics of the system were obtained by DSC. These were complemented by DQ NMR (dynamic order parameter) and rheological measurements. The use of DQ NMR as means to determine the gel point is also discussed..   

Phase Separation of Model Segmented Poly(Carbonate Urethanes)

The present paper focuses on the phase separated morphology and segment demixing of model poly(carbonate urethanes) [PCU] with hard segment contents ranging from 30 -- 65{\%} and soft segments composed of 1,6 poly(hexamethylene carbonate) [MW = 1K]. Hard segments were formed from 4,4'-methylenediphenyl diisocyanate and 1,4 butanediol. This family of materials represents a recent approach in the development of polyurethanes with improved long-term biostability, and is under clinical investigation in a number of biomedical devices. Only a single glass transition temperature was observed for each copolymer, increasing in temperature with increasing hard segment content. However, loss spectra from dynamic mechanical analysis showed clear evidence of two mixed phases. The results of small-angle X-ray scattering and tapping mode AFM experiments were consistent with these observations and will be discussed. Finally, these results will be compared with initial findings on phase separation in another family of polyurethane copolymers of current interest as blood-contact materials in biomedical devices having mixed poly(dimethylsiloxane) -- poly(hexamethyleneoxide) soft segments.   

How Does Rubber Crystallizes Upon Application of Strain

Strain induced crystallization of natural and synthetic rubbers is one of the most interesting phenomena in the field of rubber science and technology. Most of the research in the field was done using X-ray diffraction techniques. However up to this day there are no reports of direct observation of strain induced crystallization of rubber. This presentation reports first atomic force micrographs of high cis polybutadiene rubber surface at various strains. The crystallization of the rubber was confirmed by X-ray diffraction and crystallite orientation is discussed.   

Melt-processable high acrylonitrile copolymers

High acrylonitrile homo- and copolymers (PAN) are unique because of chemical, ultraviolet, and corrosion resistances. Historically, because of the atypical processing and thermal behavior of solution-processable PAN, consensus regarding the actual microstructure and paracrystalline order was elusive -- it has been described as `two-dimensional liquid crystalline-like structure with many defects.' New, sequence-structured copolymers rendered PAN for the first time melt-processable before degradation and are providing new insight into the solid state conformation. Solution $^{13}$C nuclear magnetic resonance was used to compare the new and historical comonomer sequence lengths. Optimal processing conditions were obtained using capillary rheometry as a function of dwell time and melt temperature. A filament extrusion investigation was conducted and wide angle x-ray diffraction, differential scanning calorimetry thermograms and mechanical properties of the filaments were used to characterize the nonequilibrium melt transitions and paracrystal morphology as a function of processing parameters.   

Session U25: Focus Session: Oligoacene Semiconductors

Preparation and properties of substituted acenes for organic electronics: pentacene through heptacene

Acenes (such as pentacene and tetracene) and functionalized acenes (such as rubrene) have demonstrated remarkable electronic properties in thin film devices and in single crystals. Our research effort is directed toward the synthesis of new classes of acene-based compounds that are stable, soluble, and that can be used to probe the relationships between solid-state order and electronic properties. Careful consideration of the relationship between molecular substituent and crystal packing has led to the development of new, soluble materials with thin-film properties comparable to their unfunctionalized counterparts. Functionalization of the central ring of pentacene can lead to a change in solid-state order from the classic herringbone arrangement to the less-common face-to-face interactions, while functionalization of the outermost rings of pentacene leads to subtle variations in the herringbone arrangement. Similar approaches can be taken with heteroacenes, again leading to solid-state interactions that favor strong interactions between the aromatic pi-clouds. Furthermore, the added stability gained through functionalization has allowed for the first time the preparation and study of derivatives of higher acenes such as hexacene and heptacene. The detailed study of the optical and electronic properties of a variety of functionalized acenes has led to the development of new materials for use in photovolatics, light-emitting diodes and thin-film transistors. We are grateful to the Office of Naval Research for support of this research.   

Conducting AFM and 2D GIXD Studies on Pentacene Thin Films

2D GIXD, TM-, and C-AFM analyses of pentacene films support the idea that the morphology of ultrathin layers plays a crucial role in determining mobility in OTFT. While 60-nm-thick pentacene films exhibited similar terrace-like multilayer structure with the long axis of pentacene perpendicularly oriented as determined from TM-AFM and 2D GIXD, its charge mobility in an OTFT was quite different, depending on the types of hydrophobic SAM surface treatment. This difference is related to the morphological difference of the first pentacene layer ``buried'' under the terrace-like multilayers. We found that the faceted islands on HMDS showed larger current flow than the dendritic islands on OTS using C- AFM. This trend in C-AFM current images correlated well with the charge carrier mobility measured in OTFTs. Such faceted morphology represents single crystal-like pentacene islands, which have fewer internal crystal defects and higher current flow than the dendritic islands.   

Hall Effect Measurements in Organic Single-Crystal FETs

We have observed classical Hall effect in the single-crystal rubrene OFETs [1]. The mobility determined from the Hall measurements (mu{\_}H) represents intrinsic, i.e. trap independent mobility of the charge carriers. At high temperatures, Hall mobility coincides with the longitudinal mobility determined from the standard FET measurements. In the investigated temperature range T = 170-300 K, mu{\_}H monotonically increases with decreasing T, while the longitudinal mobility first increases at high T (intrinsic regime) and then decreases at low T (non-intrinsic regime), consistent with the previous observations [2, 3]. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, n{\_}H, coincides with the density n calculated using the gate-channel capacitance, and becomes smaller than n in the trap-dominated regime. The Hall data strongly support a band-like nature of the charge carrier transport in this system. 1. Podzorov, et al., Phys. Rev. Lett 95, 226601 (2005); 2. V. C. Sundar, et al., Science 303, 1644 (2004); 3. V. Podzorov, et al., Phys. Rev. Lett. 93, 086602 (2004);   

Hall effect in organic single-crystal field-effect transistors

Hall effect is detected in organic field-effect transistors at room temperature, using appropriately shaped rubrene (C$_{42} $H$_{28}$) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The observation of the normal Hall effect means that the accumulated surface charge is well extended in space over molecules, so that the external magnetic field can provide a transverse electromotive force. The result is consistent with band-like transport of the surface carriers rather than consecutive hopping processes occurring in response to the source-drain voltage in the organic transistors.   

Control of Channel Conductivity of Rubrene Single Crystal Field Effect Transistors.

Carrier mobility higher than 1 cm$^{2}$/Vs has been measured in numerous organic single crystal FETs, making them interesting for microelectronic applications. The understanding why some organic pi-electron systems show high mobility and others, very similar molecules, show much lower mobility is crucial for design of efficient and robust organic semiconductor devices. It seems that transistor properties measured on FETs are extrinsic properties limited by technology used for transistor fabrication. However, to evaluate the applicability of organic semiconductors, intrinsic properties need to be assessed. We have carried out a program to purify and grow low defect density single crystals and fabricate FETs on their surfaces. Using graphite as electrodes and parylene as an insulator we measured maximal mobility in rubrene of 13 cm$^{2}$/Vs and significant anisotropy of transport properties. To control the transistor properties, we chemically modify the channel area and measured the conductivity of transistor channels before covering it with dielectrics and gate electrode. We found that the channel area of rubrene is very sensitive on reduction and oxidation and that the transistor properties may be modified by performing chemical reactions on the crystal surfaces before finishing transistor structure..   

Thermal Expansion and Molecular Motion in Rubrene and Tetracene

The closely related molecules rubrene (tetra-phenyl-tetracene) and tetracene are model systems for organic semiconductor materials. Very high carrier mobilities have been observed in rubrene crystals. Single crystals of rubrene and tetracene, produced by vapor phase growth at elevated temperatures, show clear differences in their quality. The thermal expansion/contraction coefficients for crystals of the two molecules have been measured using X-ray diffraction. The triclinic symmetry of tetracene crystals is reflected in the strongly anisotropic thermal expansion observed. In the case of rubrene, relatively small thermal expansion coefficients are found. Libration/translation values obtained using the rigid molecule model will be presented for both systems.   

A Systematic Study of Metal Contacts on Single Crystalline Rubrene

The performance of semiconductor devices is critically dependent on the metal- semiconductor heterojunction. In organics, despite the technological importance of such interfaces, little is known about the fundamental mechanisms that govern their performance in real devices. We have studied a series of metal contacts on rubrene single crystals and find systematic dependence of the transport barrier on the metal workfunction. These data provide insight into surface states that strongly influence the contact resistance. In the process of the study of metal-rubrene heterojunctions, we have realized an efficient single-crystal rubrene diode employing a hole-blocking metal contact.   

Negative Thermal Expansion in Pentacene

The molecules in organic semiconductors, such as the oligoacenes, are held together by weak van-der-Waals (v-d-W) forces. As a result, one observes low dissociation temperatures, and large thermal expansion coefficients. Surprisingly, we have found a negative expansion coefficient in particular crystal directions. We have performed a complete X-ray structure analysis of pentacene and tetracene single crystals in a temperature range from 100--380 K. The anisotropic thermal parameters are analyzed in terms of librations and translations of the rigid molecules. Interestingly, we find upon increasing temperature a near-zero thermal expansion along $a$ in tetracene, and a distinct contraction in pentacene. Upon close inspection of the full expansivity tensor and the thermal parameters we find a consistent explanation assuming that the v-d-W forces tend to minimize the relative shift along the long axis of adjacent molecules. This is further supported by the observation of an unusually large thermal expansion perpendicular to the layers, and the indications in our data of a distinctly anharmonic potential for the sliding motion along the long axis. These v-d-W forces leading to negative thermal expansion are expected to be larger in longer molecules and also to be the driving force behind the well known polymorphism of pentacene.   

Charge transport in single-crystals of pentacene studied with temperature dependent THz time-domain spectroscopy.

We study the charge dynamics and transport mechanisms in single crystals of pentacene by investigating the frequency dependent complex conductivity using THz time-domain spectroscopy. Such measurements on this material show generation of THz radiation from the sample as well as ultra-fast charge decay. Our measurements have been corrected for these effects, giving us the ability to see the true frequency dependent behaviour of the conductivity. Two different pump energies were used (266 and 400 nm) in the experiments. In the low temperature regime the mobile charges give a different response when being photo-excited at different wavelength. This disparity in charge transfer will be discussed and compared to theoretical models.   

Optical emission and vibrational modes of uniform pentacene monolayers (*)

Pentacene monolayers are probed by photoluminescence and resonant Raman spectroscopies below 10K. Monolayers grown on polymeric substrate of poly-alpha-methyl-styrene (PAMS) exhibit high uniformity within micron size clusters. These films show sharp exciton luminescence bands, and the energy of the exciton optical emission displays a red-shift as the average film thickness increases. The large resonance enhancements of Raman scattering intensities enable the measurements of low-lying (40- 200cm-1) optical lattice vibrations from these monolayers. These experiments demonstrate that luminescence and resonant Raman scattering from single pentacene monolayers are venues for probing 2D properties, studies of interface effects, and thin film characterization. (*) Supported primarily by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award No. CHE-0117752 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR), and by a research grant of the W. M. Keck Foundation.   

Sensitivity of 1/f Noise to Chemical Constituents in Pentacene Thin Film Transistors

This study systematically investigates the sensitivity threshold of 1/f noise for use as a device diagnostic tool, with pentacene thin-film transistors (TFTs) as a model. When pentacene in TFTs was mixed with an incremental series of the oxidative impurity 6,13-pentacenequinone (PQ), 1/f noise power rose proportionate to increasing impurities with a preliminary sensitivity threshold of $\geq$ 0.6 $\%$ PQ, coupled with a decreasing hole mobility. The result and further theoretical interpretation can supplement current quality assessments and help better understand the innate deficiencies in organic electronics, thus potentially improving their quality.   

Interface and Contact Formation in Pentacene Monolayer Transistors

Understanding charge transport in the accumulation layer of organic field effect transistors is crucial to improving their performance. Our in-situ electrical measurements during the deposition of pentacene onto a bottom-contact transistor structure with a silicon dioxide gate dielectric allowed us to study the formation of a transistor channel at the single- molecular-layer scale. At pentacene coverages near a percolation threshold the monolayer-high islands come into contact and current begins to flow through the channel. Using pauses in the deposition we have extracted transistor characteristic parameters with well-defined submonolayer island structures. Van der Pauw sheet resistance measurements show that the small field effect mobilities of monolayer transistors are associated with the formation of contacts rather than with the mobility of carriers within the semiconductor layer. Both near edge x-ray absorption fine structure measurements and atomic force microscopy show that the morphology and crystal structure of the pentacene layers changed as the total amount deposited onto the sample increased through the few-monolayer regime.   

Growth and structure of pentacene thin films

During the last few years, there has been an explosion of interests in exploring organic thin-films of small conjugated molecules, like pentacene (C22H14), as active semiconductor devices. The full potential of organic thin-film semiconductor devices for applications will not be realized until the growth of highly ordered organic thin films can be optimized and well controlled. Thin films of pentacene are known to crystallize in different polymorphs, which have layered structures characterized by their different interlayer spacings, d(001). How to grow highly ordered single phase molecular thin films remains a challenging subject. Motivated by this, we have investigated the growth of pentacene thin films on hydrogen-terminated Si (111) and clean Si (111) surfaces in an effort to gain a comprehensive understanding of the molecule-substrate interactions and explore strategies to suppress the formation of polymorphism and therefore to grow highly ordered pentacene thin films. Electron diffraction is used to investigate the in-plane molecular structure. The crystallographic results are correlated with growth. The detailed structural analysis results will be presented.   

Electron and hole transport in organic single crystals

Transport measurements reported in literature often classify the semiconducting behavior in organic thin films and single crystals to be whether p-type or n-type. Moreover, the majority of polyaromatic hydrocarbons such as pentacene in combination with technically relevant contact materials such as Au are found to preferentially show hole conductivity. In this presentation, we will discuss these key aspects of the charge carrier transport in case of highly-ordered organic crystals, the latter grown by diferent methods from previously purified material. By time-of-flight spectroscopy it will be demonstrated for various compounds that the chemical purity rather than the growth conditions determine the resulting semiconducting behavior. E.g. anthracene single crystals grown by sublimation provide as good electron as hole mobilities of about 1$\ cm^{2}$/Vs if the material is purified by zone- refinement. For crystals made of sublimation purified anthracene, no electron transport could be detected. Finally, comparing various structural motifs we will discuss possible concepts of molecular design enabling stability against photo- oxidation and allowing for the formation of highly-ordered thin films.   

Oxygen and water defect formation processes in pentacene.

Organic thin-film devices have emerged as promising candidates for novel electronic applications. Unlike traditional solid-state devices, the weak intermolecular non-covalent bonding of organic thin films leads to flexibility and possible pattern formation. The same mechanism, however, is responsible for the appearance of a variety of defects that may interfere with film growth and are potentially detrimental to the desired transport properties. Here we use first-principles calculations to study defect formation processes in the prototypical system of pentacene. In particular, we report on defect configurations of oxygen and water molecules in bulk pentacene and ultra-thin films on Si-based substrates. The results show that several stable configurations of such defects exist. Their presence has a direct bearing on growth processes and transport properties through strong covalent bonding and induced molecular distortions in their vicinity. This work was supported in part by DOE Grant DEFG0203ER46096.   

Session U26: Focus Session: Cytoskeletal Dynamics

Isotropic, nematic and polarized states in active motor-filament solutions

We characterize the phase diagram of interacting polar biofilaments and motor proteins in terms of experimentally accessible parameters. The active filament solution is described by a set of hydrodynamic equations. These in turn are obtained by coarse-graining the Smoluchowski equation for rods coupled by active crosslinkers that mediate the exchange of forces among the filaments. We find that motor activity and the polarity of motor clusters play a key role in the formation of homogeneous isotropic, nematic and polarized states. We also investigate the stability of such homogeneous states against spatially varying fluctuations in the hydrodynamic fields. Motor-induced bundling can destabilize each homogeneous state at high filament and motor density, albeit via different mechanisms (diffusive versus oscillatory). Our analysis suggests that spatially inhomogeneous oscillatory structures, such as vortices, can be formed in the polarized state.   

Rheology of active polymer solutions

In vitro solutions of biopolymers and associated motor proteins have been used to probe cytoskeletal dynamics under controlled conditions. Experiments have shown that motor-induced filament sliding competes with the slow Brownian polymer dynamics, driving the organization of the polymer network into complex patterns and altering its rheological and mechanical properties. Starting from a semi-microscopic model of the motor-mediated interaction among filaments, we have obtained continuum equations for the coarse-grained fields (filament and motor densities, polarization, alignment tensor) describing the response of the active solution to an externally imposed flow. After deriving an expression for the active contribution to the stress tensor, we have evaluated the linear viscoelastic response of a dilute solution to a shear stress. In the isotropic state motor activity strongly enhances the viscoelasticity of the system, especially near the transition to an orientationally ordered state. Activity also increases the high-frequency shear modulus of the solution. -- MCM was support by the National Science Foundation, grants DMR-0305407 and DMR-0219292. TBL acknowledges the support of the Royal Society.   

Thermally Controlling the Polymeric Cytoskeleton in Living Cells

Cell structure is controlled to a large degree by the cytoskeleton, which is an intracellular polymer network. This cytoskeleton is critical as it strongly influences many cellular functions such as motility, organelle transport, mechanotransduction and mitosis. In our studies, we controlled the thermal environment of living cells and after applying an increase in temperature of only 5 $^{o}$C, we observed a change in the polymer network as the actin filaments depolymerized. Interestingly, when we then lowered the temperature, the actin repolymerized indicating a reversible phase that is controlled by the thermal environment. We characterized the presence of F-actin and G-actin for these phases through analyzing the intensity from immunofluorescent studies for these proteins. The F-actin concentration decreased when increasing the temperature from the initial state and then increased when decreasing the temperature. Although the cell is known to be affected by heat shock responses, this is not a function of just the polymers as they do not exhibit these polymerization characteristics when we probed them as single filaments in vitro. These studies suggest that the cell has distinct phases or patterns while maintaining a reversible equilibrium due to the thermal environment for these networked polymers.   

How to detect single cancer cell?

Cell mechanics is now considered as a critical parameter closely related to cell functions. Moreover, we know that cytoplasm of cancer cells compared to normal cells is disorganized; therefore this should be directly translated to their mechanical properties. Here we show that we are able to distinguish cancer cells from normal cells in a cell mixture through their mechanical properties. Advantage of our method is that we measure single cell mechanical response where any cells, cancer or normal, are attached on a surface \textit{in situ}. When imaged in the mixture of cells, through usual microscopic imaging, cancer and normal cells did not show obvious differences and could not be identified with certitude unless using specific biochemical markers. In a mixture of cancer and normal cells, mechanical measurements were done randomly on six different cells. The relative modulus gave a bimodal distribution. These moduli were compared to the known modulus obtained from normal or cancer cell and assigned to each group of cells very precisely. This method, which is directly related to the intrinsic cytoskeleton cell mechanical properties, is a sensitive and reliable tool to detect cancer cells from a culture or a tissue at a single cell level.   

The Effects of Chronological Age on the Cellular Mechanics of Human Dermal Fibroblasts

It is often observed that older people display diminished wound healing abilities. Understanding of this phenomenon is important for many in vivo applications of tissue engineering. In this study, the cell mechanics of dermal fibroblasts from 25, 40 and 84 years old female subjects were compared. These cells were cultured on functionalized hyaluronic acid hydrogel substrates which emulated physiological conditions in dermal tissue. The deformation of the substrate caused by cellular traction forces was detected by tracing the displacement of fluorescent beads embedded in the substrate using Digital Image Speckle Correlation. Then cellular traction forces were quantitatively determined by Finite Element Method in a linear elastic model with a high spatial resolution. These results were correlated with auxiliary measurements of substrate modulus, cell modulus and migration. We found that with increasing age, the magnitude of the cellular traction forces diminished. Similarly, the ability of the cells to adapt to changes in the mechanical properties of their environment and migrate was also impaired. The interrelationship between these factors and wound healing will be discussed. This work is supported by NSF- MRSEC program.   

Interplay between crosslinkers and dynamic molecular motor-induced instabilities in the moderation of biopolymer organization

Structure and function of biological cells rely on the highly-dynamic self-organization of protein filaments to an intracellular cytoskeleton responsive to mechanical and chemical stimuli. While dissolving these complex cellular structures through Brownian motion is inherently slow (tens of minutes), changes in the activity of the molecular motor myosin II cause rapid order-disorder transitions within 1-2 minutes in reconstituted cytoskeletal actin networks. When motor-induced filament sliding decreases, actin network structure rapidly and reversibly self-organizes into various assemblies triggered by a nonlinear instability. Modulation of static crosslinker concentrations allow for a wide phase space of order ranging from nematics to compact asters \& dense packing of motor-filament clusters. The observed isothermal transitions between disorder and self-organization illustrate that molecular motors can substantially contribute to dynamic cellular organization.   

The mechanics of cell protrusion

The protrusion of the cell edge is the first step in a cycle of molecular processes that drive cell movements during development, immune responses, wound healing and many other physiological functions. It is also the earliest pathological event observed during metastasis of cancer. Textbook models associate protrusion with the assembly of an actin polymer network subadjacent to the cell plasma membrane. However, for this process to be transformed into edge advancement, polymerization-induced forces need to be balanced by adhesion complexes that link the actin network to the extracellular domain. Also, the effectiveness of network assembly in mediating forward movement of the cell edge depends on how contraction forces pull the network in the cell front retrogradly towards the cell center. Thus, what is observed in a microscope as cell protrusion reflects the kinematic output of at least three space- and time-modulated mechanisms of force generation. The coordination of these machineries is thought to be regulated by a complex network of mechano-chemical signals. Our goal is to establish the contributions of each those mechanisms and their control by reconstructing the spatiotemporal distribution of intracellular forces via inverse dynamics and molecular intervention with the relevant signalling pathways. To this end, we have developed quantitative Fluorescent Speckle Microscopy (qFSM) which provides high-resolution spatiotemporal measurements of actin network deformation and material properties in migrating cells. In addition, qFSM delivers maps of cytoskeleton assembly and disassembly, so that we can infer the plasticity of the material in situ. Together, this data allows us to deduce intracellular force distributions from the constitutive laws of strain and stress in the actin polymer network. Using this approach we discovered that unperturbed cells protrude in a dynamic steady state where periodic patterns of network assembly, adhesion formation, and cytoskeleton transport are tightly connected to protrusion waves. We exploited the sub-cellular heterogeneity of these patterns to identify the causality and timing between dynamic events in the actin network, leading towards a first integral view of the mechano-chemical process interaction in the protrusion machinery.   

Tensile Force Generation by Actin-Myosin Networks

Tensile force generation by the actin-myosin system is a crucial factor in many cellular processes, including the function of the contractile ring in cytokinesis. Calculations of such tensile forces have often been based on specific one-dimensional models of the structure based on parallel overlapping filaments, sometimes in sarcomere-like structures. However, the detailed arrangement of the actin filaments is not known in general, and it is likely to be disordered. For this reason we have developed a general theory of force generation by myosin in actin networks, based on treating the myosin motors as external forces in a viscoelastic medium. The analysis is based on two ingredients: the strain field of a force dipole in a homogeneous medium, and a correction for the inhomogeneity of the actin network. We obtain a simple expression for the tensile stress induced by the myosin motors in terms of the density of motors and the average actin filament length. This formula is used to relate the force that can be generated by a contractile ring to the actin network structure.   

Simulation of Actin-Polymerization Near Moving Surface

An important component of the cellular cytoskeleton is F-actin, a biopolymer whose self-assembly is key to the process of cell crawling. The polymerization and branching of F-actin near the cell membrane is known to drive cell crawling, but the precise mechanism by which these processes lead to the generation of a mechanical force is still controversial. We have constructed a Brownian dynamics simulation of F-actin polymerizing near a surface, which includes all known important processes, including polymerization, depolymerization, branching, severing and capping. Using this model, we study the dynamics of the moving surface in conjunction with the morphology of the resulting actin network.   

The Fysics of Filopodia (or The Physics of Philopodia)

Cell motility is driven by the dynamic reorganization of the cellular cytoskeleton which is composed of actin. Monomeric actin assembles into filaments that grow, shrink, branch and bundle. Branching generates new filaments that form a mesh-like structure that protrudes outward allowing the cell to move somewhere. But how does it know where to move? It has been proposed that filopodia serve as scouts for the cell. Filopodia are bundles of actin filaments that extend out ahead of the rest of the cell to probe its upcoming environment. Recent in vitro experiments [Vignjevic {\it et al.}, J. Ce ll Bio. {\bf 160}, 951 (2003)] determine the minimal ingredients required for such a process. We model these experiments analytically and via Monte Carlo simulations to estimate the typical bundle size and length. We also estimate the size of the mesh-like structure from which the filopodia emerge and explain the observed nonmonotonicity of this size as a function of capping protein concentration, which inhibits filament growth.   

Lipid-Protein Nanotubes with Open or Closed Ends, Microtubules Bundles and Inverted Tubulin Nanotubes

We describe synchrotron x-ray diffraction, electron microscopy, and optical imaging data of the self-assembly of microtubules (MTs) with various cationic agents. We established the conditions under which cationic liposomes can coat MTs and form lipid-protein nanotubes (LPNs). The LPNs exhibit a rather remarkable architecture with the cylindrical lipid bilayer sandwiched between a MT and outer tubulin oligomers forming rings or spirals. By controlling the cationic lipid/tubulin stoichiometry it is possible to switch between two states of nanotubes with either open ends or closed ends with lipid caps, a process which forms the basis for controlled chemical and drug encapsulation and release (Raviv et al, PNAS, 2005). Multivalent (3+,4+ and 5+) cations can form three dimensional MT bundles that in some cases become tubulin based inverted nanotubules. Divalent cations form two dimensional MT necklaces (Needleman et al, PNAS, 2004).   

The mechanics of cell crawling over a flat surface

The chemotaxis of different strains of the amoeba \textit{Dictyostelium Dicoideum} when exposed to a wide range of concentrations and gradients of chemoattractant has been studied experimentally. First, the time evolution of the velocity as well as the shape of the cell have been measured from microscopy images for a large number of individuals. Secondly, the force that the amoebas exert over the substrate in order to propel themselves has also been measured. Some insights into the physical mechanism by which cells crawl over the surface are obtained by comparing the time evolution of those magnitudes for the different strains under study.   

Micromechanical Properties of Endothelial Cell Cytoskeleton

Atherosclerotic plaques occur in regions of arterial curvature, where there is blood re-circulation and physiologically low shear stress conditions. This phenotype may be related to flow-induced shear stress on the monolayer of endothelial cells that make up the endothelium. When endothelial cells in static culture are exposed to laminar flow, they respond by rearranging their cytoskeleton, and aligning their actin filaments in the direction of flow. The changes in cytoskeletal structures induced by flow are different from region to region of the same cell. We employ the optical tweezers technique to obtain very local mechanical properties of endothelial cell cytoskeleton. We used endocytosed polystyrene beads, as well as intrinsic granular structures, as probes for our measurements. Endothelial cells were also treated with Cytochalasin B and Nocadozole, which are drugs that de-polymerize actin filaments and microtubules respectively, to measure the visco-elastic moduli, and obtain the contribution of each cytoskeletal structure in the cells' micro-mechanical properties.   

Session U27: Computational Methods: Monte Carlo/Molecular Dynamics I

Density-Matrix Based Fixed-Node and Fixed-Phase Approximation for Quantum Monte Carlo

We have generalized the fixed-node and fixed-phase approximations to use density matrices. For a given trial density matrix, we generate a quantum Monte Carlo algrithm that minimizes the free energy, subject to the nodal or phase restriction. This method has enabled us to perform efficient fermion path-integral simulations at all temperatures. In the T=0 limit the algorithm simulates two copies of the system that do not interact, but which are entangled through the nodal constraint. We illustrate the advantages of this density matrix formalism in applications to semiconductor nanostructures and small molecules. We are currently investigating the use of the Kubo formula and other linear response theories within the fixed-node approximation. (For simulation codes, preprints, and other information, see http://phy.asu.edu/shumway.)   

Worm algorithm for continuous-space Path Integral Monte Carlo simulations

We present a new approach to Path Integral Monte Carlo (PIMC) simulations based on the ``worm" algorithm, originally developed for lattice models,\footnote{N. V. Prokof'ev, B. V. Svistunov, and I. S. Tupitsyn, Phys. Lett. {\bf 238}, 253 (1998)} and recently extended to continuous-space many-body systems.\footnote{M. Boninsegni, N. Prokof'ev and B. Svistunov, cond-mat/0510214} The scheme allows for efficient computation of thermodynamic properties, including winding numbers and off-diagonal correlations, for systems of much greater size than that accessible to conventional PIMC. We present results for the superfluid transition of Helium-four in two and three dimensions. Using systems comprising several thousand particles, a very accurate determination of the superfluid transition temperature is feasible.   

Green's function analysis of path integral Monte Carlo molecular simulations

We demonstrate the direct determination of molecular properties from path integral Monte Carlo simulations. By sampling Matsubura Green's functions, we have calculated several linear response properties of the hydrogen molecule (H$_2$) directly from quantum Monte Carlo. We show that the vibration frequency of H$_2$ as calculated directly from the phonon temperature Green's function is in very good agreement with the calculated Born-Oppenheimer potential energy surface. We have also obtained the polarizability from the polarization correlation function, and we are looking at Raman spectra. For the high-accuracy simulations needed in chemical physics, we have developed new, fast and accurate techniques for the tabulation of Coulomb density matrices. This work motivates future path integral Monte Carlo simulations on larger molecules and could also be immediately useful in simulations of hydrogen storage materials.   

Energy Optimization of Many-Body Wave Functions: Application to Silicon Interstitial Defects

Energy minimization [1], as opposed to the standard variance minimization [2], of the Jastrow factor results not only in lower variational Monte Carlo (VMC) energies but also in lower diffusion Monte Carlo (DMC) energies for systems that employ a nonlocal pseudopotential. We apply this approach to solids: single-interstitials in silicon. Allowing the Jastrow for the defect atom(s) to differ from that for bulk atoms lowers the VMC energy but not the DMC energy, indicating the pseudopotential locality error is small. DMC energies from 8 and 64 atom cells (plus interstitial) computed with energy-optimized trial wave functions estimate a 0.2 eV finite-size error in the formation energy. Cubic spline and Lagrange polynomial representations of orbitals have comparable efficiency in memory usage, run time and accuracy. [1] C. J. Umrigar and C. Filippi, Phys. Rev. Lett. 94, 150201 (2005). [2] C. J. Umrigar, K. G. Wilson and J. W. Wilkins, Phys. Rev. Lett. 60, 1719 (1988).   

Study of Atoms and Molecules with Auxiliary-Field Quantum Monte Carlo

We study the ground-state properties of second-row atoms and molecules using the phaseless auxiliary-field quantum Monte Carlo (AF QMC) method.\footnote{S. Zhang and H. Krakauer, Phys. Rev. Lett. \textbf{90}, 136401 (2003)} This method projects the many-body ground state from a trial wave function by means of random walks in the Slater-determinant space. We use a single Slater-determinant trial wave function obtained from density-functional theory (DFT) or Hartree-Fock (HF) calculations. The calculations were done with a plane-wave basis and supercells with periodic boundary condition. We investigate the finite-size effects and the accuracy of pseudopotentials within DFT, HF, and AF QMC frameworks. Pseudopotentials generated from both LDA (OPIUM\footnote{\texttt{http://opium.sourceforge.net}}) and HF\footnote{I. Ovcharenko, A. Aspuru-Guzik, and W. A. Lester, J. Chem. Phys. \textbf{114}, 7790 (2001)} are employed. We find that the many-body QMC calculations show a greater sensitivity to the accuracy of the pseudopotentials. With reliable pseudopotentials, the ionization potentials and dissociation energies obtained using AF QMC are in excellent agreement with the experimental results.   

Auxiliary field quantum Monte Carlo study of transition metal and post-d group atoms and molecules

We applied the phaseless auxiliary field quantum Monte Carlo [1] to the study of several transition metal and post-d atoms and molecules. The transition metal study includes both all-electron and pseudopotential calculations, while the post-d group elements are studied using the consistent correlated basis which employs a small core relativistic pseudopotential [2]. The obtained electron affinities, dissociation energies, and equilibrium geometries compare favorably with experiment and with coupled cluster results. [1] S. Zhang and H. Krakauer, Phys. Rev. Lett. {\bf 90}, 136401 (2003). [2] Kirk A. Peterson, J. Chem. Phys. {\bf 119}, 11099 (2003); Kirk A. Peterson {\emph et al.}, J. Chem. Phys. {\bf 119}, 11113 (2003)   

Accuracy of the pseudopotential and fixed-node approximations for C$_2$, Si$_2$ and defects in crystalline Si

Quantum Monte Carlo calculates binding energies and atomic structures for molecules and defect energies in solids. Accurate QMC calculations require the control of the pseudopotential and the fixed-node approximation. The calculated binding energies and bond lengths for the Si and C dimer and the energies of defects in crystalline Si test the accuracy of a range of pseudopotentials and optimized trial-wave functions. For the Si dimer and defects in crystalline Si different pseudopotentials provide similar results. The results for the Si dimer are comparable with experiments with HF pseudopotentials being most accurate. While a single determinant wave functions is sufficient for the Si dimer, the C dimer requires an optimized multi-determinant trial-wave function to achieve experimental accuracy.   

Structure of fermion nodes and nodal cells for QMC wave functions

We study nodes of fermionic ground state wave functions. For $2D$ and higher we analytically prove that spin-polarized, noninteracting fermions in a harmonic well have two nodal cells for arbitrary system size. The result extends to other noninteracting/mean-field models such as fermions on a sphere, in a periodic box or in Hartree-Fock atomic states. Spin-unpolarized noninteracting states have multiple nodal cells, however, interactions and many-body correlations generally relax the multiple cells to the minimal number of two. This is again analytically proved, with some restrictions, for general interactions in $2D$ and higher-dimensional harmonic fermions of arbitrary size using the Bardeen-Cooper-Schrieffer variational wave function. We discuss implications and limits of the proofs for more complicated systems. The results offer an elegant and unifying framework for several previously conjectured or numerically investigated ideas and open exciting perspectives for studies of many-body effects which are beyond the usual fixed-node quantum Monte Carlo limits.   

Multi-pfaffian pairing wave functions for quantum Monte Carlo

We investigate the limits of accuracy of trial wave function for quantum Monte Carlo based on pfaffian functional form with singlet and triplet pairing. Using a set of first row atoms and molecules we find that this wave function provides very consistent and systematic behaviour in recovering the correlation energies on the level of 95\% . In order to get beyond this limit we have explored the possibilities of multi-pfaffian pairing wave functions. We show that small number of pfaffians recovers another large fraction of the missing correlation energy comparable to the larger-scale configuration iteraction wave functions. The trade-offs between the size of the underlying optimization problem and amounts correlation energy recovered will be discussed.   

Backflow transformations in inhomogeneous systems

The quality of trial wave-functions, and of their nodal surface in particular, determines the accuracy of the results obtained within the Fixed-Node Diffusion Monte Carlo (DMC) method. Backflow transformations have been proven capable of improving the nodal surface of Slater-Jastrow (SJ) wave-functions in homogeneous systems. In this work we will present the extension of backflow to inhomogeneous systems, along with DMC results for atoms, molecules and solids which show the improved accuracy of this form of trial wave-function. We will also discuss the advantages of using electron-by-electron algorithms to enhance the computational efficiency of QMC with backflow wave-functions.   

The equation of state of diamond from quantum Monte Carlo calculations

We describe variational and diffusion quantum Monte Carlo (VMC and DMC) calculations that have been performed to evaluate the elastic properties of diamond up to pressures of about 500 GPa. We have used a smooth, norm-conserving, Hartree-Fock carbon pseudopotential in our work. Our trial wave functions were of Slater-Jastrow form, containing orbitals generated in plane-wave DFT-GGA calculations, which were re-expanded in a blip-function basis set. We propose a new scheme for determining the cutoff lengths that occur in our Jastrow factor. Using a 512-electron simulation cell, and fitting a Vinet equation of state to our energy-volume data, we have calculated the equilibrium lattice constant (A, in Angstrom), bulk modulus (B, in GPa), and pressure derivative of the bulk modulus (B') to be (A,B,B')=(3.547, 4.83, 3.43) within VMC and (3.563, 4.52, 3.61) within DMC, as compared with the experimental values of (A,B,B')=(3.567, 4.4-4.5, 3.0-4.0).   

Improved estimators for quantum Monte Carlo calculation of spherically averaged intracule densities

System-averaged pair densities or ``intracule densities'' are important for qualitative and quantitative descriptions of electron correlation~[1] In quantum Monte Carlo (QMC) simulations, spherically averaged intracule densities are usually calculated by means of the traditional histogram technique (i.e., by counting the number of times two electrons are found at a certain distance) that is very noisy at short electron-electron distances. We will show how previously-used improved estimators for the on-top pair density~[2,3] can be generalized to the case of non-vanishing electron-electron distances, as an application of the ``zero-variance'' procedure~[4]. The obtained estimators lead to noise several orders of magnitude smaller than the histogram technique, allowing unprecedented fast and accurate calculations of intracule densities in QMC. Illustrative calculations on simple atomic systems will be given. [1] J. M. Mercero, E. Valderrama and J. M. Ugalde, in ``NATO-ASI Series in Metal-Ligand Interaction in Molecular-, Nano-, Micro, and Macro-systems in Complex Environments'', Ed.: N. Russo, D. R. Salahub and M. Witko, Kluwer Academic Publishres, Dordrecht (2003). [2] P. Langfelder, S. M. Rothstein and J. Vrbik, J. Chem. Phys. {\bf 107}, 8525 (1997). [3] A. Sarsa, F. J. G\'alvez and E. Buend\'ia, J. Chem. Phys. {\bf 109}, 7075 (1998). [4] R. Assaraf and M. Caffarel, Phys. Rev. Lett. {\bf 83}, 4682 (1999).   

Accurate energy differences with Quantum Monte Carlo

Computation of accurate energy differences is of primary importance in the study of transformations as those occurring in solid to solid phase transitions or chemical reactions. In stochastic quantum simulations this can be done efficiently, employing correlated sampling techniques whereby fluctuations cancel with each other leading to results with a much smaller statistical error. Although correlated sampling is very effective for variational Monte Carlo such is not the case for diffusion Monte Carlo where branching and different nodal structures force the introduction of uncontrolled approximations. Here we describe the use of reptation Monte Carlo as a method that maintains a single path for both systems and leads to energies which are exact within the fixed node approximation. We show how to combine umbrella sampling with coordinate transformations to give a simple and efficient algorithm to compute small energy differences. Application to dissociation reaction paths and weakly bound systems are presented.   

Session U28: Focus Session: Biological Hydrodynamics II

Fluidic control over cell proliferation and chemotaxis

Microscopic flows are almost always stable and laminar that allows precise control of chemical environment in micro-channels. We describe design and operation of several microfluidic devices, in which various types of environments are created for different experimental assays with live cells. In a microfluidic chemostat, colonies of non-adherent bacterial and yeast cells are trapped in micro-chambers with walls permeable for chemicals. Fast chemical exchange between the chambers and nearby flow-through channels creates essentially chemostatic medium conditions in the chambers and leads to exponential growth of the colonies up to very high cell densities. Another microfluidic device allows creation of linear concentration profiles of a pheromone ($\alpha$-factor) across channels with non-adherent yeast cells, without exposure of the cells to flow or other mechanical perturbation. The concentration profile remains stable for hours enabling studies of chemotropic response of the cells to the pheromone gradient. A third type of the microfluidic devices is used to study chemotaxis of human neutrophils exposed to gradients of a chemoattractant (fMLP). The devices generate concentration profiles of various shapes, with adjustable steepness and mean concentration. The ``gradient'' of the chemoattractant can be imposed and reversed within less than a second, allowing repeated quantitative experiments.   

The Conformation of Clathrin Triskelia in Solution

A principal component in the protein coat of certain post-golgi and endocytic vesicles is clathrin, a three-legged heteropolymer (known as a triskelion) that assembles into polyhedral cages principally made up of pentagonal and hexagonal faces. In vitro, this assembly depends on the pH, with cages forming more readily at low pH and less readily at high pH. We have developed procedures, based on static and dynamic light scattering, to determine the radius of gyration, R$_{g}$, and hydrodynamic radius, R$_{H,}$ of isolated triskelia, under conditions where cage assembly occurs. Calculations based on rigid molecular bead models of a clathrin triskelion show that the measured values can be accounted for by bending of the legs and a puckering at the vertex. We also show that the values of R$_{g}$ and R$_{H}$ measured for clathrin triskelia in solution are qualitatively consistent with the conformation of clathrin in a ``D$_{6}$ barrel'' cage assembly measured by cryoEM tomography.   

A macromolecular model for the endothelial surface layer

The endothelial surface layer (ESL) is a micron-scale macromolecular lining of the luminal side of blood vessels composed of proteoglycans, glycoproteins, polysaccharides and associated plasma proteins all in dynamic equilibrium. It has numerous physiological roles including the regulation of blood flow and microvascular permeability, and active participation in mechanotransduction and stress regulation, coagulation, cell adhesion, and inflammatory response. The dynamic structure and the mechanical properties of the ESL are crucial for many of its physiological properties. We present a topological model for the ESL composed of three basic macromolecular elements: branched proteoglycans, linear polysaccharide chains, and small plasma proteins. The model was studied using non-equilibrium molecular dynamics simulations and compared with scaling theories for associating tethered polymers. We discuss the observed dynamical and mechanical properties of the ESL captured by this model, and the possible physical insight it provides into the physiological behavior of the ESL.   

Simulation of metachronal wave in a model of pulmonary cilia

A simulation of the formation of metachronal waves in carpets of pulmonary cilia is presented. The cilia move in a two-layer fluid model. The fluid layer adjacent to the cilia base is purely viscous while the tips of the cilia move through a viscoelastic fluid. An overlapping fixed-moving grid formulation is employed to capture the effect of the cilia on the surrounding fluid. The 9+2 internal microtubule structure of an individual cilium is modeled using large-deflection, curved, finite-element beams. Realistic models of the forces exerted by dynein molecules are extracted from measurements of observed cilia shapes. The possibility of formation of metachronal waves under different assumptions of boundary conditions is investigated and shown to be dependent on the surrounding geometry.   

On the Evolution of Voltage Gated Ion Channels

This talk summarizes some ideas, calculations and data analysis/collection surrounding the structure and evolution of ion channels, in particular voltage gated sodium channels. The great advantage of ion channels is that they are individual proteins whose function has long been known and is readily inferred through voltage measurements. Their evolution can be tracked through the growing data base of sequences. Kinetic data is readily available, showing important differences between nearly identical channels. I will discuss our efforts to collate available functional data on voltage gated sodium channels into an 'ion channel property space' . We then use this dataset to infer underlying kinetic models, and to create evolutionary trees based on the function of the channels. Finally, I will discuss our endeavors to how ion channels evolved to be the way they are: Examples of questions we would like to answer include: to what extent do design principles dictate the details of the kinetic schemes of ion channels, such as (a) the symmetry of the sodium and potassium channels (or lack thereof), as reflected in their kinetic schemes ; (b) the coupling of sodium channel kinetics to potassium channel kinetics; or (c) activation/inactivation of the channels themselves.   

Optimization of Anguilliform Swimming

Anguilliform swimming is investigated by 3D computer simulations coupling the dynamics of an undulating eel-like body with the surrounding viscous fluid flow. The body is self-propelled and, in contrast to previous computational studies of swimming, the motion pattern is not prescribed a priori but obtained by an evolutionary optimization procedure. Two different objective functions are used to characterize swimming efficiency and maximum swimming velocity with limited input power. The found optimal motion patterns represent two distinct swimming modes corresponding to migration, and burst swimming, respectively. The results support the hypothesis from observations of real animals that eels can modify their motion pattern generating wakes that reflect their propulsive mode. Unsteady drag and thrust production of the swimming body are thoroughly analyzed by recording the instantaneous fluid forces acting on partitions of the body surface.   

Flow measurement in an in-vitro model of a single human alveolus

The alveolus is the smallest and most important unit in the acinar region of the human lung. It is responsible for gas exchange between the lungs and the blood. A complete knowledge of the airflow pattern in the acinar region is necessary to predict the transport and deposition of inhaled aerosol particles. Such knowledge will benefit the pharmaceutical community in its effort to deliver therapeutic aerosols for lung-specific as well as system-wide ailments. In addition, it can also help to assess the health effects of the toxic aerosols in the environment. We have constructed an in-vitro model of a single spherical alveolus on a circular tube. The alveolus is capable of expanding and contracting in phase with the oscillatory flow through the tube. Realistic breathing conditions are reproduced by matching Reynolds and Womersley numbers. Experimental methods such as particle imaging velocimetry and laser induced fluorescence are used to study the resulting flow patterns. In particular, recirculating flow within the alveolus, and the fluid exchange between the alveolar duct and the alveolus are important for better understanding the flow in the acinar region.   

Transport and collective dynamics in suspensions of swimming particles

Direct simulations of large populations of hydrodynamically interacting swimming particles at low Reynolds number are performed. Hydrodynamic coupling between the swimmers leads to large-scale coherent vortex motions in the flow that are consistent with experimental observations. At low concentrations, swimmers propelled from behind (like spermatazoa) strongly migrate toward solid surfaces in agreement with simple theoretical considerations; at higher concentrations this localization is disrupted by the large-scale coherent motions. Correspondingly, at large concentrations the swimmers move with velocities several times larger than they could achieve in isolation.   

Computational Modeling of Microfluidic Rapid-Mixing Device Used in Infrared Micro-Spectroscopy

Microfluidic rapid-mixing device is employed to chemically trigger biological functions of proteins for time-resolved Fourier transform Infrared (FTIR) micro-spectroscopic study. There are two criteria for the optimal design of such devices: (i) minimizing the mixing time (thus better time-resolution) and (ii) minimizing the consumption of protein samples. Computational modeling has been performed on the Poiseuille and diffusional flow patterns. Due to the low-Reynolds number of microfluidic flow in study, finite-difference methods for the Navier-Stokes and advection-diffusion equations are employed in our computational modeling. The viscosity of the fluids is related to the pressure gradient required to achieve maximum velocity according to Pouisulle flow. Several mixing channels are modeled in order to determine the optimal dimensions for our microchip according to the design criteria. We will report our computational modeling results and their relevance to the optimal design of rapid-mixing devices used in time-resolved infrared micro-spectroscopy.   

Session U29: Focus Session: Nonequilibrium Fluctuation in Biomolecules and Artificial Nanodevices

DNA's Liaison with RNA Polymerase – Physical Consequences of a Twisted Relationship

RNA polymerase is the molecular motor that performs the fundamental process of transcription. Besides being the key- protagonist of gene regulation it is one of the most powerful nano-mechanical force generators known inside the cell. The fact that polymerase strictly tracks only one of DNA's strands together with DNA's helical geometry induces a force-to-torque transmission, with several important biological consequences like the ``twin supercoil domain'' effect and remote torsional interaction of genes. In the first part of the talk we theoretically explore the mechanisms of non-equilibrium transport of twist generated by a moving polymerase. We show that these equations are intrinsically non-linear in the crowded cellular environment and lead to peculiar effects like self-confinement of torsional strain by generation of alternative DNA structures like cruciforms. We demonstrate how the asymmetric conformational properties of DNA lead to a ``torsional diode'' effect, i.e. a rectification of polymerase-generated twist currents of different signs. In the second part we explore the possibility of exploiting the polymerase as a powerful workhorse for nanomechanical devices. We propose simple and easy to assemble arrangements of DNA templates interconnected by strand-hybridization that when transcribed by the polymerase linearly contract by tenfold. We show that the typical forces generated by such ``DNA stress fibers'' are in the piconewton range. We discuss their kinetics of contraction and relaxation and draw parallels to natural muscle fiber design.   

Enhanced Fano factor in a molecular transistor coupled to phonons and Luttinger-liquid leads

We study how the electron-phonon coupling {\it and} intra-lead electron interaction affect the transport properties of a molecular quantum dot coupled to leads. We consider the effects on the steady state current and DC noise for both equilibrated and unequilibrated on-dot phonons. The density matrix formalism is applied in the high temperature approximation and the resulting semi-classical rate equation is numerically solved for various strengths of electron-electron interactions in the leads and electron-phonon coupling. We have found that the Fano factor, which measures the noise to current ratio, is enhanced as the intralead electron interaction is increased, while both the current and its noise are smeared out and suppressed due to the interaction. Interestingly, the Fano factor exhibits super-poissonian behaviour as the electron-phonon coupling becomes greater than order one.   

Multi-scale dynamics and relaxation of a tethered membrane in a solvent by Monte Carlo simulations

A tethered membrane modeled by a flexible sheet dissipates entropy as it wrinkles and crumples. Nodes of a coarse grained membrane are connected via multiple pathways for dynamical modes to propagate. We consider a sheet with nodes connected by fluctuating bonds on a cubic lattice. The empty lattice sites constitute an effective solvent medium via node-solvent interaction. Each node execute its stochastic motion with the Metropolis algorithm subject to bond fluctuations, excluded volume constraints, and interaction energy. Dynamics and conformation of the sheet are examined at a low and a high temperature with attractive and repulsive node-node interactions for the contrast in an attractive solvent medium. Variations of the mean square displacement of the center node of the sheet and that of its center of mass with the time steps are examined in detail which show different power-law motion from short to long time regimes. Relaxation of the gyration radius and scaling of its asymptotic value with the molecular weight are examined.   

Ion distribution inside a nanopore in the presence of a polyelectrolyte

Experimental studies of the translocation of DNA through nanopores rely on measurements of the ionic current. In order to understand the behavior of this current, we employ molecular dynamics simulations to study the ion distribution within a nanopore in the presence of a polyelectrolyte. We characterize the ion distribution in terms of radial density profiles around the polyelectrolyte. Several factors affecting the ion distribution are studied, including the role of chain flexibility, salt concentration, nanopore size and its polarizability. Our study also provides information on the dynamics of the ions inside the pore. The combination of static and dynamic information is used to explain experimental observations.   

Measuring the direction of coupling between biological oscillators

The electroreceptor system of the paddle fish comprises two self-sustained noisy oscillators: one oscillator resides in the sensory epithelium and is coupled through excitatory synapse with another oscillator residing in the afferent neuron terminal. We test recently developed algorithms for estimating the directionality of their coupling from experimental recordings of spontaneous and stimulated activity. These experimental bivariate time series are structurally different: while the signal from the epithelial oscillations is represented by a continuous stochastic process, the neuron oscillations are represented by a stochastic point process. We show that the tested algorithms detect reliably directionality of coupling both in experimental and simulated data and can be used for physiologically relevant short segments of data.   

Applications of the ratchet effect at nano- and mesoscopic scales

I will discuss the application the ratchet effect in superconducting vortex dynamics and interacting colloidal systems. We have shown theoretically how in superconductors patterned on sub-micron or nanometer scale with various pinning potentials a DC vortex transport and vortex manipulation can be achieved with an external AC drive. I will discuss several applications of the vortex ratchet effect as well as a series of experiments aimed at the detection and investigation of the vortex ratchet transport. For colloidal systems, we have recently shown that a rich variety of dynamic phases can be realized for mono- and bidisperse mixtures of interacting colloids under the influence of a symmetric flashing periodic substrate. With the addition of dc or ac drives, phase locking, jamming, and new types of ratchet effects occur. In some regimes we find that the addition of a non-ratcheting species increases the velocity of the ratcheting particles. We show that these effects occur due to the collective interactions of the colloids.   

On-Chip Integration of Cell-Free Gene Expression

We present a synthetic approach for the study of gene networks \textit{in vitro} which is complementary to traditional \textit{in vivo} methodologies. We have developed a technology for submicron integration of functional genes and on-chip protein synthesis using a cell-free transcription/translation system. The interaction between genes is facilitated by diffusion of on-chip gene expression products from `source' genes towards `acceptor' genes. Our technology is simple and inexpensive and can serve as an improved platform for a wide variety of protein and DNA biochip applications.   

``Burnt Bridge'' Mechanism of Molecular Motor Motion

Motivated by a biased diffusion of molecular motors with the bias dependent on the state of the substrate, we investigate a random walk on a one-dimensional lattice that contains weak links (called ``bridges'') which are affected by the walker. Namely, a bridge is destroyed with probability $p$ when the walker crosses it; the walker is not allowed to cross it again and this leads to a directed motion. The velocity of the walker is determined analytically for equidistant bridges. The special case of $p=1$ is more tractable --- both the velocity and the diffusion constant are calculated for uncorrelated locations of bridges, including periodic and random distributions.   

Session U30: Focus Session: Mechanical Properties: Deformation, Rupture and Failure

Shock Wave Theory for Rupture of Rubber

The rupture of rubber differs from conventional fracture. It is supersonic, and the speed is determined by strain levels ahead of the tip rather than total strain energy as for ordinary cracks. Dissipation plays a very important role in allowing the propagation of ruptures, and the back edges of ruptures must toughen as they contract, or the rupture is unstable. In this talk I will review the experimental evidence for these claims. I will present several levels of theoretical description of the phenomenon: first, a numerical procedure called mesoscopic particle modeling, which is capable of incorporating large extensions, dynamics, and bond rupture; second, a simple continuum model that can be solved analytically, and which reproduces several features of elementary shock physics; and third, an analytically solvable discrete model that accurately reproduces numerical and experimental results, and explains the scaling laws that underly this new failure mode. Rupture speeds compare well with experiments, although opening angles of the rupture are not captured especially well. Some additional interesting topics that may be encountered along the way include the question of how to model sound dissipation in disordered solids, and a numerical instability that is suggestive of the phenomenon of strain crystallization.   

Tuning the Adhesion of Soft Elastomers with Topographic Patterns

Nature (e.g. gecko and jumping spider) utilizes surface patterns to control adhesion. The primary mechanism of adhesion for these systems can be sufficiently described by linear elastic fracture mechanics theory and material-defined length scales. Based upon these natural inspirations, similar mechanisms can be used to control the adhesion of elastic polymers. For viscoelastic polymers, patterns tune adhesion through additional mechanisms that have not been previously observed. Here, we illustrate the effects of topographic patterns in tuning the adhesion for soft, elastic or viscoelastic, elastomers. Contact adhesion tests based on Johnson, Kendall and Roberts (JKR) theory are used to characterize the adhesion of patterned poly(dimethyl siloxane) as well as poly(n-butyl acrylate) elastomers. We demonstrate that patterns can be utilized to control the adhesion of these polymers by: 1) controlling the balance of initiation and propagation for local separation process, 2) controlling the local crack velocity to alter the global viscoelastic response, and 3) altering the local separation mode through modification of a polymer layer's lateral confinement.   

Finite Element Calculations Using a New Constitutive Model for the Chemical Aging of Rubber

We have developed a constitutive model for rubber networks undergoing simultaneous crosslinking and scission reactions. This model is a modification of the independent network hypothesis that includes the coupling between strain history and chemical reactions. This coupling occurs because formation of networks in the strained state is greatly affected by the networks that were already present. Even when early networks scission, the overall material response shows some memory of the initial networks (i.e., some later stage networks act as earlier stage networks). We account for this effect using stress transfer functions. The model has been tested on microscopic molecular dynamics simulations. We will present results using this constitutive model in finite element calculations showing the large effect that the coupling of strain history and chemical reaction has.   

Effect of chain bridging on mechanical properties of lamellae-forming block copolymers

We report studies on solid-state mechanical properties of lamellae-forming block copolymers composed of poly(cyclohexylethylene) (C) and poly(ethylene) (E). Specifically, we have investigated the effect of bridging conformations in the semicrystalline E block. We studied CEC, ECEC, and ECECE architectures and found that tensile properties of C/E block copolymers are extremely sensitive to the fraction of ``soft'' E chains tethered between glassy C domains. While the CEC polymer has a strain-to-failure of $\sim $300{\%}, the ECEC and ECECE polymers fail at $\sim $1{\%} strain. By employing ECEC/CEC and ECECE/CEC blends, we have come up with a molecular parameter that describes a sharp brittle-to-ductile transition and captures the tensile properties of a broad range of C/E block copolymer architectures having equal sized E blocks. In another set of experiments, increasing the ``middle-to-loose'' E block length ratio was found to toughen the ECECE block copolymers. We propose that these effects are related to a critical concentration of bridged E chains that governs the failure mechanisms in glassy-semicrystalline block copolymers.   

Probing the Contact and Sliding of Elastomer/Polymer Interfaces

In this study, we have designed a novel approach to couple interface sensitive infrared-visible sum frequency generation (SFG) spectroscopy with adhesion and friction experiments. This provides a direct probe of the interfacial structure in terms of orientation and density of molecules during contact and sliding which is important in understanding the molecular origin of adhesion and friction. Here, we show that the friction forces between poly(dimethyl siloxane) (PDMS) lens and glassy poly(styrene) (PS) are $\approx $4 times higher than PDMS sliding on surfaces of crystalline alkyl side chain comb polymers. This cannot be explained by the differences in adhesion energy or hysteresis. The in-situ SFG measurements indicate local interdigitation during contact, which is evident from the decrease in the number of oriented phenyl groups at the interface. The local penetration is unexpected at room temperature (T$_{R})$ that is much below the T$_{g}$ of PS. For comparison, we have also studied poly(n-butyl methacrylate) and poly(n-propyl methacrylate) having T$_{g}$ above and below T$_{R}$, respectively. Both of these polymers show similar adhesion and friction forces as PS. The SFG results indicate that local changes in interfacial structure affect friction, regardless of the bulk T$_{g. }$ These results also show that the adhesion energy and hysteresis are not sufficient to predict friction, which makes the characterization of the molecular structure during contact and sliding essential.   

Mesoscopic Random Lattice Models of Rupture in Rubber

In an earlier work, Marder illustrated how rupture in rubber differs from conventional fracture. Dissipation and toughening of the back edges of ruptures are critical for the propagation of stable ruptures. In this earlier work, mesoscopic models were arrived at by approximating the Mooney-Rivlin theory of rubber by a finite difference scheme on a triangular lattice. From this perspective, qualitatively the lattice sites are considered to be crosslinkers and the bonds are polymers. We extend this work by considering the crosslinkers to be randomly distributed throughout the material rather than being ordered. For both random and ordered lattices, without rupture, there are many different ways to construct free energy functionals that reproduce the continuum theory. However, not all of the constructions are numerically stable. We explore the physical consequences of the disorder and the physical interpretations of the observed numerical instabilities.   

Annealing History Dependence of Young's Modulus in Thin Polymer Films Using an Axi-symmetric Peel Test Apparatus

We present a study of chain relaxation in thin spincast films above the glass transition temperature. We employ a novel axi-symmetric peel test, which uses the deformation of a thin spincast polymer film brought into contact with a flat substrate. The use of a thin membrane minimises uncertainty in the contact radius while the use of spincast films provides very smooth surfaces by means of a very simple method. The experimental profile of the deformed membrane shows good agreement with the expected logarithmic profile. While this agreement allows measurement of the Young's modulus and solid-solid work of adhesion in thin films, this study will focus on the dependence of the Young's modulus on the annealing history in thin films. The thermal history dependence shows that for short annealing times Young's modulus is larger than expected, suggesting that the chains are oriented during spincasting. For longer times, Young's modulus reaches literature values.   

Contributions to the Adhesion of Glassy Polymers from Radical Recombination and Segmental Interpenetration at Elevated Temperatures

We study two examples of adhesive interactions between glassy polymers that occur when the polymers are heated to elevated temperatures. First set is the adhesion between thin films of poly(phenylene oxide) (PPO). The samples are brought into contact at an elevated temperature and cooled to room temperature prior to measuring the fracture energy by using the contact mechanics approach based on JKR (Johnson, Kendall and Roberts) theory. Very little adhesion is observed at temperatures below about 130C, which enables us to perform a second set of experiments, where adhesion is due to radical recombination across the interface. Polystyrene (PS) is synthesized by anionic polymerization, and terminated by an end-capped nitroxide radical group. Nitroxide mediated polymerization is used to attach a poly (\textit{tert-}butyl acrylate) (tBA) block to the PS chains. These block copolymers are added to the PPO films that are brought into contact with one another. Nitroxide radicals become uncapped at elevated temperatures, with subsequent radical recombination providing another potential mechanism for the formation of bonds across the interface.   

Adhesion and Spatial Distribution of Water in the Presence of Moisture: Surface Chemistry Affects.

Many polymer adhesive bonds experience a complete loss of adhesion above a critical threshold relative humidity value. The adhesion loss from water exposure does not generally correlate with moisture solubility of the polymer; instead the surface layer of the polymer appears to be the controlling factor in the adhesion. Here, the adhesion and spatial distribution of water of a series of PMMA-Al adhesive samples in the presence of moisture is measured with neutron reflectivity, while the adhesive strength of the joint is measured using the shaft-loaded blister test. The role of changing surface chemistry was examined to determine their effect on this interfacial moisture content. The loss of adhesive strength upon exposure to moisture correlates directly with the interfacial water content. Surface modification methods that decrease the interfacial water content are used to tune the adhesive strength in moist environments. Minimization of the interfacial water concentration does not however result in the best adhesion in moist environments as interplay between the dry adhesion and water content exists.   

Quantifying Ballistic Armor Performance: A Minimally Invasive Approach

Theoretical and non-dimensional analyses suggest a critical link between the performance of ballistic resistant armor and the fundamental mechanical properties of the polymeric materials that comprise them. Therefore, a test methodology that quantifies these properties without compromising an armored vest that is exposed to the industry standard V-50 ballistic performance test is needed. Currently, there is considerable speculation about the impact that competing degradation mechanisms (e.g., mechanical, humidity, ultraviolet) may have on ballistic resistant armor. We report on the use of a new test methodology that quantifies the mechanical properties of ballistic fibers and how each proposed degradation mechanism may impact a vest's ballistic performance.   

Measurement of the Viscoelastic Bulk Modulus

A new piston-cylinder type pressurizable dilatometer has been developed to measure the time-dependent bulk modulus of viscoelastic materials. The PVT behavior and the glass transition temperature of a polystyrene have been measured as a function of pressure. Preliminary measurements of the time-dependent bulk modulus have also been made. The isothermal bulk modulus is important because its magnitude is directly related to isotropic residual stress development in curing thermoset-reinforced composites; in addition, by comparing the bulk and shear responses, proposed differences in the molecular origins can be explored.   

Influence of Physical Aging on the Mechanical Properties of a Random Polypropylene-Polyethylene Copolymer

Spontaneous aging of a random polypropylene-polyethylene copolymer was studied at 50 $^{o}$C above its glass transition temperature using differential scanning calorimetry, wide (WAXS) and small (SAXS) angle X-ray scattering, and dynamic mechanical relaxometry. Both the melting temperature and melting enthalpy of the copolymer increased with physical aging time at room temperature suggesting increased crystallinity. WAXS measurements also indicated an increase in crystallinity along with coexistence of $\gamma$ and $\alpha$ forms for highly aged samples. Dynamic mechanical measurements showed that the shear storage modulus increased uniformly at all frequencies of measurement from 0.1 Hz to 20 Hz. Furthermore a direct correlation was observed between the storage modulus and the melting enthalpy of the copolymer. Conversely, the change in shear loss modulus was frequency dependent, with larger changes at 20 Hz than at 0.1 Hz. This suggests that there are changes in the relaxation time spectra with aging. The aging process will be explored in more detail using both the SAXS and stress relaxation data.   

Fracture versus cavitation in probe-tack geometry: theory and experiments

We perform traction experiments on viscous liquids highly confined between parallel plates, a geometry known as the probe-tack test in the adhesion community. Direct observation during the experiment coupled to force measurement shows the existence of several mechanisms for releasing the stress. Bubble nucleation and instantaneous growth had been observed in a previous work. Upon increasing further the traction velocity or the viscosity, the bubble growth is progressively delayed. At even higher velocities, fractures at the interface between the plate and the liquid are observed before the bubbles have grown to their full size. We present a theoretical model that describes these regimes, using a Maxwell fluid as a model for the actual fluid. We present the resulting phase diagram for the different force peak regimes. It remarkably accounts for the data. Our results show that in addition to cavitation, interfacial fractures, commonly thought to be characteristic of soft viscoelastic solids like adhesives are already encountered in \emph{liquid} materials.   

Session U31: Nanotubes, Experiment

Controlled Dielectrophoretic Positioning of Carbon Nanotubes

Single-walled carbon nanotubes have been dielectrophoretically aligned between micropatterned electrodes. Use of a limiting resistor enables control over the number of carbon nanotubes deposited in the electrode gap. Further, the electric field between micropatterned electrodes can be perturbed by arrays of metal nanostructures. Simulating electric fields in the presence of metal objects allows us to design electrodes with arrays of metal dots for the precise positioning of nanotubes. Complex network structures can be fabricated using carefully placed metal nanostructures and also by varying the electrode geometry. Crossed-junction nanotube structures have been controllably fabricated by optimization of the electrode geometry, applied electric field, and load resistor. The dielectrophoretically aligned nanotube structures work as functional field-effect transistors. Several approaches to improving the contact resistances will be discussed. The work is supported by the NSEC program of the National Science Foundation under NSF Award Number CHE-0117752 and by NYSTAR.   

Tuning the electrical and mechanical properties of carbon nanotubes interfaced with silicon surfaces using the UHV-STM

Nanoscale patterning of the Si(100)-2x1:H surface with the UHV-STM [1] is used to selectively modify the Si substrate acting as a semiconducting support for isolated single-walled carbon nanotubes (SWNTs) deposited via dry contact transfer (DCT) [2]. By desorbing H at the SWNT-Si interface, we can mechanically stabilize SWNTs that initially were only weakly coupled to the Si surface and thus highly sensitive to STM tip induced perturbations. Moreover, on the degenerately doped n-type H-Si(100) surface, the presence of negatively charged Si dangling ponds patterned in close proximity to a semiconducting SWNT decreases the magnitude of the substrate voltage required for the onset of filled states conduction through the SWNT. Our results suggest new opportunities for engineering -- on the sub-nm scale -- both the mechanical and electronic properties of SWNTs integrated with semiconductor platforms. [1] J.W. Lyding et al., APL 64, 2010 (1994). [2] P.M. Albrecht and J.W. Lyding, APL 83, 5029 (2003).   

Effect of doping on electro-optical properties of thin carbon nanotube membranes

Nanotube membranes can prove to be a practical alternative to the current transparent conductors, such as ITO, that are in touch screen displays and photovoltaic devices. ITO with a transmittance of 80{\%} in the visible spectrum has a resistance of 10ohms/square. Similar transmittance could be obtained only with very thin membranes with nanotube loadings below the percolation threshold and would have very high resistance. Doping of membranes changes transmittance at the S11 and S22 transitions of semiconducting nanotubes, but does not change the transmission in the visible spectrum, while decreasing the resistance. Membranes of different thicknesses have been produced and characterized. Post-production doping of the membranes has been achieved and the change in resistance and in transmission spectrum has been examined. We will discuss effect of donor and acceptor dopants on the conductivity and transmittance of the nanotube membranes.   

Environmental Manipulation of the Electronic Structure of Suspended Carbon Nanotubes

We use tunable resonant Raman spectroscopy to study the effect of changing the dielectric environment on the electronic structure of single wall carbon nanotubes (CNTs) suspended over trenches. The 1D nature of CNTs is responsible for weak intrinsic screening and large Coulomb interactions are anticipated. Two-photon absorption experiments have determined the presence of excitons with large binding energies.[1,2] Therefore, modulation of the surrounding dielectric constant significantly alters the strength of the Coulomb interactions and leads to changes in the exciton binding energy and band-gap renormalization which should be evident in the resulting spectra.[3] Until recently, CNTs were primarily studied in bulk, in suspensions, or coated in surfactants. We measure resonance excitation profiles (REPs) from individual suspended CNTs where the intensity of the Raman peak is plotted vs. excitation energy. We vary the humidity and monitor the shift of the REP peak of the radial breathing mode and the G-Band. We thereby directly measure the relative shift of the renormalized band-edge and exciton binding energy as a function of dielectric constant. [1] Wang, Science, 308, 838 (2005). [2] Maultzsch, arXiv, 0505150 (2005). [3] Perebeinos, PRL, 92, 257402 (2004).   

Capacitance measurements of individual carbon nanotubes

We present measurements of the capacitance of individual single walled carbon nanotubes. The nanotubes were grown from ethylene at 700$^{\circ}$C using evaporated iron nanoclusters as the catalyst. Electrical contacts and local top gates were patterned using optical lithography and liftoff. The top gate consists of a thin oxide film ($\sim $15 nm, different oxides have been used) covering the nanotube with metal on top. The capacitance was measured between the nanotube and the top gate using a commercially available capacitance bridge. We also measure the transport through the tube and correlate the transport and capacitance measurements. For semiconducting tubes, we measure the difference in capacitance between the conducting state and the state where the charge carriers in the tube are depleted. The measured capacitance per unit length of the nanotube is in reasonable agreement with the geometric capacitance of a metal wire embedded in oxide near a conducting plane.   

Electrical Characterization of Y-junction Carbon Nanotubes of Fish-bone Structure

Y-junction carbon nanotubes (YCNTs) of fish-bone structure, synthesized by the pyrolysis of methane over cobalt supported on magnesium oxide [1], have been characterized by means of electrical transport measurements. We report both 2- and 4-probe I-V characteristics of YCNTs down to $T$ = 3 K and up to $B$ = 8 T. At 3 K, we found that change in magnetoresistance was about 0.5 {\%} at 8 T, perhaps due to piezoeffect of YCNTs. Also, we report the piezoresistivity of YCNTs directly obtained by an \textit{in situ} STM incorporated into TEM. This piezoresistivity appeared to be substantially bigger than that of straight fish-bone CNTs. In addition, we fabricated some FET-shaped samples with YCNTs whose all 3 branches were contacted by Ti/Au electrodes. We found that there is no rectifying behavior in the fish-bone junctions unlike in similar YCNTs reported in [2]. [1] W. Z. Li, J. G. Wen, Z. F. Ren, Appl. Phys. Lett. \textbf{79}, 1879 (2001). [2] P. R. Bandaru, C. Daraio, S. Jin, A. M. Rao, Nature Mats. \textbf{4}, 663 (2005).   

Resonant Raman spectroscopy analysis of single wall carbon nanotubes treated with high density plasma of different gases.

Single wall carbon nanotubes (SWNTs) have been plasma treated with different gases (Ar, O$_{2}$ and Ar and H$_{2}$ gases mixtures) using an inductively coupled RF plasma system (IC-RFP). The gas pressure was varied from 50mtorr to 315mtorr at 50W plasma power. Microscopic plasma parameters including ion density (n$_{i})$ and electron temperature (T$_{e})$ (thermal energy of electrons) were been determined using a double Langmiur probe in the plasma. Treated SWNTs was been characterized using resonance Raman spectroscopy at 515nm and 633nm laser excitation. It was observed that there was a considerable increase of the D to G-band ratio of treated SWNTs with increasing gas pressures and also that the Breit-Wigner-Fano band (G$_{BWF})$ to G$^{+}$-band ratio was been considerably increased. Further, at 515nm laser excitation the frequency up-shift of the G-band for Ar {\&}(5{\%}) H$_{ 2}$ (gas mixture) plasma treated SWNTs was higher at all pressures than those of other gases.   

Measurements of 1/f Noise in Carbon Nanotube Devices

We have measured the low frequency noise of field effect transistors made from individual single-walled carbon nanotubes in an ultra high vacuum environment. We will compare Hooge’s constants measured in oxygen, argon, air, and ultra high vacuum, and propose a possible solution for reducing noise in nanotube devices. Furthermore, the utility of noise amplitude in carbon nanotube devices for chemical specific sensing will be discussed.   

Temperature dependence of mean free length of electrons in single walled carbon nanotubes

We have measured how single walled carbon nanotube resistance scales with channel length. Multiple two-terminal devices of varying source-drain separation are fabricated on isolated ultra-long ($>$1 mm) SWNTs grown by chemical vapor deposition. Pd electrodes provide low resistance contacts to the SWNTs. The resistance of SWNT devices are investigated in length scales ranging from 100 nm to 200 $\mu $m, from which the 1-dimensional resistivity is extracted. The temperature dependence of the electron mean free path obtained from the resistivity values indicate that in the majority of metallic SWNT devices the electron transport is ballistic up to $\sim $ 500 nm at room temperature and $\sim $ 10 $\mu $m at 1.6K.   

Field Enhanced Thermionic Electron Emission from Oxide Coated Carbon Nanotubes

We have created a novel nanostructure by coating carbon nanotubes with a thin functional oxide layer. The structure was fabricated by sputter deposition of a thin film of oxide materials on aligned carbon nanotubes, which were grown on a tungsten substrate with plasma enhanced chemical vapor deposition. This structure combines the low work function of the oxide coating with a high field enhancement factor introduced by carbon nanotubes and we have demonstrated that it can be used as a highly efficient electron source. A field enhancement factor as high as 2000 was observed and thermionic electron emission current at least an order of magnitude higher than the emission from a conventional oxide cathode was obtained.   

Low-temperature conductive tip scanning measurements of single walled carbon nanotubes.

We have built a low-temperature atomic force microscope (AFM) that fits inside a 38 mm bore cryostat. The scanning probe is attached to a quartz tuning fork, and a frequency shift is used as the feedback signal. By using a conductive tip we can locally tunnel into single walled carbon nanotubes grown on a non-conducting (SiO2) substrate. The nanotubes are contacted by a metal grid *electrode* evaporated on top of the sample. The tip is used as a second, movable contact. We measure the nanotube conduction as a function of the tip position and the gate voltage.   

Local Density of States in Nanoscale Systems Measured by Electrostatic Force Microscopy

We use Electrostatic Force Microscopy (EFM) to measure the local density of states (LDOS) in carbon nanotubes and semiconducting nanowires. A voltage biased EFM cantilever, driven at its resonant frequency is used to perturb the local charge density in these systems. The recorded change in the oscillation phase is proportional to the LDOS of the sample. We monitor the phase change as a function of both the tip voltage and cantilever oscillation amplitude for a fixed cantilever position above the sample. We also show that this is a general electrostatic method that can be used to measure the band gap and LDOS of both conducting and insulating nanoscale systems with no need for electrical contacts.   

Nanotorsional Actuator Devices Built on Individual Singlewall Carbon Nanotubes

Nanoelecromechnical devices have been fabricated comprising an individual singlewall carbon nanotube as a torsional spring for a fully suspended, lithographed metal platform. The torsional properties of the structure were measured through electrostatic deflections. We discuss the mechanical properties of the oscillator and the electrical response of the nanotube during deflections.   

Session U33: Dynamics and Systems Far From Equilibrium

Critical behavior and Griffiths effects in the disordered contact process

We study the nonequilibrium phase transition in the one-dimensional contact process with quenched spatial disorder by means of large-scale Monte-Carlo simulations for times up to $10^9$ and system sizes up to $10^7$ sites. In agreement with recent predictions of an infinite-randomness fixed point, our simulations demonstrate activated (exponential) dynamical scaling at the critical point. The critical behavior turns out to be universal, even for weak disorder. However, the approach to this asymptotic behavior is extremely slow, with crossover times of the order of $10^4$ or larger. In the Griffiths region between the clean and the dirty critical points, we find power-law dynamical behavior with continuously varying exponents. We discuss the generality of our findings and relate them to a broader theory of rare region effects at phase transitions with quenched disorder.   

Monte Carlo Studies of Phase Separation in Compressible 2-dim Ising Models

Using high resolution Monte Carlo simulations, we study time-dependent domain growth in compressible 2-dim ferromagnetic ($s=1/2$) Ising models with continuous spin positions and spin-exchange moves [1]. Spins interact with slightly modified Lennard-Jones potentials, and we consider a model with no lattice mismatch and one with 4\% mismatch. For comparison, we repeat calculations for the rigid Ising model [2]. For all models, large systems ($512^2$) and long times ($10^ 6$~MCS) are examined over multiple runs, and the growth exponent is measured in the asymptotic scaling regime. For the rigid model and the compressible model with no lattice mismatch, the growth exponent is consistent with the theoretically expected value of $1/3$ [1] for Model B type growth. However, we find that non-zero lattice mismatch has a significant and unexpected effect on the growth behavior.\\ \\ Supported by the NSF.\\ \\ $[1]$ D.P. Landau and K. Binder, {\em A Guide to Monte Carlo Simulations in Statistical Physics}, second ed. (Cambridge University Press, New York, 2005).\\ $[2]$ J. Amar, F. Sullivan, and R.D. Mountain, Phys.\ Rev.\ B 37, 196 (1988).   

Rate of Entropy Extraction in Compressible Turbulence.

The rate of change of entropy is measured for a system of particles floating on the surface of a fluid maintained in a turbulent steady state. This rate of entropy ${\dot S}$ equals the time integral of the two point temporal velocity divergence correlation function with a negative prefactor. The measurements satisfactorily agree with the sum of Lyapunov exponents (Kolmogorov-Sinai entropy rate) measured from previous simulations, as expected of dynamical systems that are very chaotic (Sinai-Ruelle-Bowen statistics).   

Non-constant nucleation rate in a system in apparent metastable equilibrium

The distribution of nucleation times for the two-dimensional Ising model with nearest-neighbor and with long-range interactions is simulated using the Metropolis algorithm. The distribution is exponential at long times as would be expected if the nucleation rate is a constant, but is suppressed at earlier times even after the mean magnetization is apparently in metastable equilibrium. We explain this discrepancy by investigating the relaxation behavior of the clusters whose size is comparable to the nucleating droplet.   

4:1 Resonance phenomena in the forced Belousov-Zhabotinsky chemical reaction

The oscillatory Belousov-Zhabotinsky (BZ) reaction has been successfully used to study generic aspects of resonance in spatially extended systems parametrically forced with pulses of light. Experiments have reproduced Arnol'd tongues and pattern forming behavior predicted by reaction diffusion models and amplitude equations. We use the BZ reaction to experimentally demonstrate a transition in the 4:1 resonance regime from patterns of pi/2 fronts to patterns of pi fronts. The transition matches the theoretical predictions. Above a certain driving strength, traveling pi/2 fronts become unstable and a new stable pattern of stationary pi fronts emerges.   

Mode locking in quasiperiodic structures

AC driven extended systems, such as charge density waves or arrays of Josephson junctions, exhibit mode locking or giant Shapiro steps. This mode locking is seen experimentally as plateaus in a generalized velocity or current as a function of drive parameter. In conventional mode locking, the frequency of the response is a rational multiple of the frequency of the AC drive. For a model, we use a sandpile automaton model with local nonlinear update rules. When random quenched disorder is present in the automaton, a Devil's staircase with mode locking at all rational numbers has been previously seen. We investigate the use of quasiperiodic structures in place of the disordered structures. We find the novel phenomena of mode locking where the currents are a quasiperiodic multiple of the drive frequency. These quasiperiodic steps turn out to be stable to thermal fluctuations. Application of this model to Josephson junction arrays and structured colloidal systems will be presented.   

Periodic time dependent problems in nonequilibrium quantum statistical mechanics

The usual Kadanoff-Baym or Keldysh formulation of nonequilibrium quantum statistical mechanics can be reformulated for steady state problems in terms of a nonequilibrium density matrix of the form $\exp (-(H - Y)/k_B T)$, where $H$ is the hamiltonian and $Y$ contains the information about how the system is driven out of equilibrium.$^*$ This approach has now been used to solve exactly solvable models as well as in approximate techniques. Here we show that for a periodic time dependent hamiltonian there is a similar formulation in terms of a nonequilibrium density matrix, where the density matrix acts in one higher dimension than in the original problem. Thus, a time dependent nonequilibrum problem is mapped onto a time independent nonequilibrium problem in one higher dimension. This is true for interacting as well as noninteracting problems. The approach is illustrated by applying it to some exactly solvable time dependent nonequilibrium problems such as tunneling through a resonant level where the level and/or the voltage applied are time dependent.\\ $*$ S. Hershfield, PRL {\bf 70}, 2134 (1993).   

On Rolling Loaded Dice

When an unfair die is tossed, what are the factors that determine the side upon which it lands? Sir Hermann Bondi (see European Journal of Physics 14, pp. 136-140) asked a related theoretical question in 1993 with the intention of determining the theoretical probability of a coin landing on its edge. He notes that the center of mass, the coefficients of restitution and friction, and the radius of gyration all play a role, perhaps. A simple model assumes that the probability of landing on a particular side is proportional to the solid angle subtended from the center of mass, but this model predicts too few base landings for tall cylinders, and too many rolling landings for squatty cylinders. Here we propose a thermodynamic modification of this model which qualitatively improves the match between experiment and theory by introducing an effective ``temperature'' parameter. We apply the model to several different geometrical shapes where the landing odds are not even, including right circular cylinders, rectangular prisms, hemispheres and semi-cylinders. We obtain, perhaps unreasonably, somewhat promising results.   

Broad spectrum period adding chaos in a transistor circuit.

Period adding chaos, in which a driven system makes transitions such as period 2-chaos-period 3-chaos-period 4, is well known. In most cases, however, the frequency of the chaotic signal is close to the frequencies of the periodic signals. I have done expriments with a simple circuit in which the chaos has a very broad power spectrum, covering 6 orders of magnitude. I have confirmed this broad band feature in numerical simulations of the circuit. These experiments have technological implications, because they show that a narrow band high frequency signal could produce broad band interference in even simple circuits.   

Solving the Problem of Excessive Time Delay in Attractor Reconstruction

We recently showed that the seemingly separate problems of finding a proper time delay and then finding a proper embedding dimension for attractor reconstruction are really the same problem which can be solved with a mathematical statistic faithful to the Takens reconstruction theorem. This approach also deals well with disparate time scales in data, and optimally choosing time series to use from a multivariate data set. However, the problem of when a time delay is too long for a chaotic time series remains. We introduce a new statistic that resolves this issue. The statistic is based on the mathematical observation that long time delays will result in data points that do not adequately populate the dynamical system's manifold. We present results with models and data that show we can predict when we have used excessively long time delays in attractor reconstruction.   

Stochastic Loewner evolution driven by L\'evy processes

Standard stochastic Loewner evolution (SLE) is driven by a continuous Brownian motion, which then produces a continuous fractal trace. If jumps are added to the driving function, the trace branches. We consider a generalized SLE driven by a superposition of a Brownian motion and a stable L\'evy process. The situation is defined by the usual SLE parameter, $\kappa$, as well as $\alpha$ which defines the shape of the stable L\'evy distribution. The resulting behavior is characterized by two descriptors: $p$, the probability that the trace self- intersects, and $\tilde{p}$, the probability that it will approach arbitrarily close to doing so. These descriptors are shown to change qualitatively and singularly at critical values of $\kappa$ and $\alpha$. These transitions occur as $\kappa$ passes through four (a well-known result) and as $\alpha$ passes through one (a new result). Numerical simulations are then used to explore the associated touching and near-touching events.   

Positional Order and Diffusion Processes in Particle Systems

In particle systems, a relation between the positional order parameter $\Psi$ and the mean square displacement $M$ is derived to be $\Psi \sim \exp(-v{K}^2 M/2d)$ with a reciprocal vector $v{K}$ and the dimension of the system $d$. On the basis of the equiation, the behavior of $\Psi$ is found to be $\Psi \sim \exp(-v{K}^2 D t)$ when the system involves normal diffusion with a diffusion constant $D$. While the behavior in two-dimensional solid is predicted to be $M \sim \ln t$, numerical simulations shows a linear diffusion $M \sim t$. This can be explained by a swapping diffusion process which allows particles to diffuse without destroying the positional order.   

Brownian dynamics of colloids in tilted periodic potential

We have studied the Brownian movements of micron-sized colloidal spheres in the presence of a periodic potential and the potential associated with gravity (which together form a so-called tilted-washboard potential). The optical potential is generated by the interference of two argon laser beams that are tightly focused through a common objective lens to form an in-plane standing wave in the vicinity of a substrate surface. As the intensity of standing wave is increased, the escape time of a particle trapped in a given potential well to the next lower one increases super-exponentially. More generally we have measured the time dependence of the probability density distribution of a colloidal particle as a function of the amplitude of the standing wave. The experimental data have been compared with a simulation based on the numerical integration of the Smoluchowski equation in the presence of the optical and gravitational potentials.   

Session U35: Nanowires

Dielectrophoretic assembly of reversible and irreversible metal nanowire networks and vertically-aligned arrays

We demonstrate dielectrophoretic control of metallic nanowires in liquid suspensions. By varying parameters including the magnitude and frequency of the applied electric field, the liquid suspending the nanotubes, the nanowire metal, and the flow conditions, we can generate sparse or dense networks, multiply branching or predominately end-to-end networks, and vertically aligned nanowires standing on top of metal electrodes. The networks can be made reversibly or irreversibly. These results demonstrate the applicability of dielectrophoresis in aligning and positioning nanowires, either in the plane of the substrate or perpendicular to it, thereby suggesting a simple and versatile strategy for fabricating a range of integrated devices composed of nanowires. For example, sparse end-to-end networks are promising for individual electronic devices, dense branching networks take advantage of the large surface-to-volume ratio of nanowires for use as sensors, vertically-aligned arrays of nanowires may be used as vertical interconnects in damascene integration of microelectronic devices or in controlling the flow of fluid or light in microfluidic or nanophotonic devices, etc.   

The Dielectrophoretically Guided Growth of Submicron, Near Single Crystal Indium Wires

Dielectrophoresis was used to direct the growth of crystalline indium wires between lithographic electrodes immersed in solutions of indium acetate. Determination of the conditions that suppress side branching on these structures has enabled the fabrication of arbitrarily long needle-shaped wires with diameters between 367nm and 556nm. Electron diffraction studies indicate that these wires are crystalline indium, that the unbranched wire segments are single-crystal domains, and that the predominant growth-direction is near $\langle $110$\rangle $. This outcome constitutes a critical step towards the use of simply prepared aqueous mixtures as a convenient means of controlling the composition of submicron, crystalline wires.   

Growth Characteristics of Dielectrophoretically Fabricated Single Crystal Wires

We investigate the mechanism underlying the dielectrophoretically guided growth of single-crystal metallic wires in aqueous solutions of metal-salts. The weak dependence of the mass deposition rate on the growth velocity suggests that the growth mechanism is consistent with microscopic solvability theory. We have also observed interesting dependencies of the growth velocities and the radii of these wires on the frequency of the dielectrophoretic voltage, and will report on our progress towards understanding these phenomena. The dependence of the wire's radius on the frequency is of potential technological interest in that it provides a sensitive means of controlling the submicron wire diameter.   

Self-assembly of Eu$_{2}$O$_{3}$ nanoneedles and nanocrystals

Growth of anisotropic nanostructures starting from isotropic nanostructures is observed in Eu$_{2}$O$_{3}$ system. Anisotropic structures, like nanoneedles and nanospindles, are grown from the concentrated solution of Eu$_{2}$O$_{3}$ nanocrystals. This process involved the stepwise thermal growth of nanocrystals into ordered, high aspect ratio, one dimensional nanoneedles and the subsequent assembly of said nanoneedles into larger, oriented bundles (nanospindles). The Eu$_{2}$O$_{3}$ nanocrystals were synthesized following a room temperature, colloidal chemistry procedure, adapted from the synthesis of G. Wakefield \textit{et al}.$^{1}$ We present the results of this self-assembly phenomenon using 4-nm Eu$_{2}$O$_{3}$ nanocrystals. High resolution transmission electron microscopy was employed to characterize the approximate shape, size distribution, and crystallinity of the nanostructures. Absorption and photoluminescence measurements were performed to investigate what effect the size and shape of materials has on optical properties. 1. G. Wakefield, H. A. Keron, P. J. Dobson, and J. L. Hutchison, J. of Coll. Interf. Sci. \textbf{215}, 179, 1999.   

Three-dimensional nanoscale composition mapping of semiconductor nanowires.

The composition of a single InAs nanowire was mapped in three dimensions with single-atom sensitivity and sub-nanometer resolution using local-electrode atom probe (LEAP) tomography. Arrays of epitaxial InAs nanowires were grown by chemical vapor deposition on GaAs substrates. Nanowires with diameters of 10-20 nm were analyzed over lengths of hundreds of nanometers. Three-dimensional reconstructions of the atoms in the nanowire showed hexagonal faceting, indicating that the LEAP analysis accurately reproduces the cross-section and shape of the nanowires. The Au catalyst particle sitting atop a nanowire was also analyzed; tomographic slices across the nanowire diameter, when displayed in 0.5 nm increments along the growth axis, revealed an extremely abrupt catalyst-nanowire interface that is also very flat. Despite the abruptness of the catalyst-nanowire interface, individual gold atoms were detected within the nanowire at a concentration of 100 ppm. These results indicate that LEAP microscopy can be used to (1) image buried nanowire interfaces in three dimensions and (2) analyze the concentration and distribution of dopant and impurity atoms in nanowires.   

Ligand Functionality to Control Morphology, Solubility, and Assembly Behavior of CdSe Nanorods

By varying the length of the carbon chain of the ligand, while keeping other reaction parameters the same, CdSe nanorods with different diameters and lengths, branched nanorods, and even nano-arrows have been synthesized. Since all the ligands used are phosphonic acids with the same binding group, the length of the carbon chain of the ligand can dramatically change the size and morphology of the nanocrystals. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been used to investigate the chemical composition of the nanorods. CdSe nanorod thin films have been made in hexane by electrophoretic deposition. The number and length of the ligands can dramatically change solubility, electrophoretic mobility of the nanorods, and quality of the nanorod film. We shall discuss mechanisms for ligand control of the nanocrystal structure. This work is supported by the MRSEC program of the National Science Foundation under Award No. DMR-0213574 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR).   

Twinning Superlattices in Semiconducting Nanowires

We report the first observation of quasi-periodic twinning superlattices (TSLs) in semiconducting nanowires. The periodicity of the superlattice appears to be controlled by $\Delta $T=T$_{m }$- T, i.e., the degree of undercooling of the liquid phase in contact with the solid phase during VLS growth, where T$_{m}$ is the melting point of the solid phase. We present results from two III-V systems (GaP, InP) in which the superlattice is generated by the periodic 180\r{ } flipping of the $<$112$>$ direction relative to the $<$111$>$ growth direction of the nanowire. We suspect that our observations mean that a TSL structure can be grown in many compound semiconducting nanowire systems. Control of the superlattice period should allow significant design possibilities for thermoelectric, electronic and electro-optic applications. This work was supported, in part, by NSF-NIRT DMR-0304178   

First Principles Studies of the Structural and Opto-Electronic Properties of Silicon Nanowires

We report the results of first principles studies of the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires with [001], [011], and [111] growth directions and diameters ranging from 1 to 3 nm. We show that the growth direction, diameter, and surface structure all have a significant effect on the structural stability, electronic band gap, band structure, and band edge effective masses of the nanowires. The band gap is found to decrease with increasing diameter and to be further reduced by surface reconstruction. The band gap is direct for [001] and [011] NWs with diameters smaller than 3nm and [111] NWs with diameters smaller than 2nm. While the electron and hole effective masses are found to depend on NW size for [001] and [111] NWs, they are almost independent of size for [011] NWs. Finally, we use FEFF calculations to predict the EXAFS spectra produced by the relaxed atomic structure of the NWs and show that these spectra can serve as a tool for detection of surface reconstructions of the NWs.   

Ab initio investigation into the stability and electronic properties of GaN-nanowires

Recent reports of successful fabrication of high quality gallium nanostructures such as quantum dots, nanocrystallites and nanowires, eg. [1], open the door to their possible role as important nanoscale building blocks for future optoelectronic, high-temperature/power and spintronic device applications. In the present work we perform \textit{ab initio} calculations, using the DMol$^{3}$ [2] and SIESTA [3] codes, for wurtzite GaN nanowires. We have examined nanowires in the [0001], $[10\bar {1}0]$, and $[11\bar {2}0]$ directions, and investigated the stability, electronic and atomic structures as a function of nanowire radius. We found that only nanowires in the [0001] direction are stable, and that wires in the other directions can be stabilised by saturating dangling bonds with hydrogen. We have also investigated the properties of key point defects and dopants. [1] J. C. Johnson, \textit{et al.} Nature Materials \textbf{1}, 106 (2002). [2] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); \textit{ibid} \textbf{113}, 7756 (2000). [3] J.M. Soler, \textit{et al}. J. Phys.: Condens. Matter. \textbf{14}, 2745 (2002).   

Au wetting of GaAs(111) studied on the atomic scale

Because of the potential importance of semiconductor nanowires in future devices, a full understanding of their growth mechanism is desired. Au is commonly used to catalyze III-V nanowire growth, however the exact role of the Au in the growth process of the resulting wire is not known. This makes atomic scale studies of the Au/GaAs interface system well motivated. Here we report on the interaction of thin Au films and Au aerosol nanoparticles with the GaAs(111)B surface on the atomic scale using STM, LEED,LEEM, PEEM and ab-initio DFT calculations. We show that after deposition of Au either as a thin film or as aerosol nanoparticles and subsequent annealing to 450-600\r{ }C, a well ordered ($\surd $3x$\surd $3)R30$^{o}$ structure is formed. A structural model with a Au atom on every third threefold hollow hcp site of the Ga lattice is proposed based on theoretical calculations and experimental data.   

Session U36: Focus Session: Optical Properties of Nano-Dots, Holes, and Wires

Geometrical scale invariance of the enhanced transmission spectra of subwavelength hole arrays

There is at present a lack of consensus on the relative strength of contributions from surface plasmon polaritons (SPPs) and composite diffracted evanescent waves (CDEWs) to the mechanism responsible for enhanced transmission through subwavelength hole arrays. For regular square hole arrays with small open area fraction $d^{2}/a^{2}$ ($a$ = lattice constant and $d^{2}$ = square hole area), both theories predict that the wavelength $\lambda =\lambda _{m}$ at which maximum transmission occurs scales linearly with $a$. The two interpretations diverge however when the open area fraction increases and the distance between two adjacent hole edges decreases. We test these ideas by comparing transmission spectra of sets of arrays where each set has a fixed open area fraction but is scaled to different sizes by changing $a$. We find in our preliminary data that for a factor of 10 change in $a$, from 1 $\mu $m to 10 $\mu $m, the transmission peaks, when plotted as a function of $\lambda $/$a$, collapse onto the same scaling curve. This collapse is independent of hole size $d$ and is in good agreement with SPP theory when the wavelength-dependent refractive indices of the substrates (quartz and ZnSe) are taken into account.   

Polarization and hole-shape dependence of the transmission of sub-wavelength hole arrays

We have measured the infrared and visible transmission of arrays of holes in silver films, to study the effects of hole shape and hole spacing as a function of the polarization of the light. The anomalous transmission enhancement of sub-wavelength hole arrays in metal films has been attributed to surface plasmon polaritons (SPPs) but this picture is not enough to explain the dependence of hole shape on the transmission due to the long wavelength approximation. We have measured the transmission of arrays of square holes, rectangular holes, and slits in a silver film. We studied the effect of different hole shapes on the enhanced transmission as a function of the polarization angle of the light and found a strong dependence on the hole shape and the polarization angle. In addition, transmission spectra of an array of square holes on a rectangular grid will be presented.   

Tunneling, dipole interactions and coherent Rabi oscillations in quantum dot molecules.

Quantum dot molecules (QDMs) - coupled quantum dot systems - have proved to be a promising optoelectronic circuit element for future implementation of quantum computation at the nanoscale. Here we investigate theoretically the coherent manipulation between exciton states in a single QDM. In particular, we study Rabi oscillations induced via strong laser pumping and their dependence on the interdot quantum coupling strength, including particle interdot tunneling and Coulomb interactions. The dynamics of the system is extracted by solving a quantum master equation using a multilevel density matrix that considers direct and indirect excitons in a rotating wave approximation. Possible decoherence mechanisms, such as coupling to wetting layer states and non-radiative recombination, are incorporated into the master equation using a Lindblad formulation. Careful control of the interdot coupling strength, laser detuning, and intensity, results in different population level dynamics. These are found to be critical for the entanglement between exciton states and the ultimate realization of Bell states for potential quantum information processing.   

Microscopic Models of Hybrid Nanocrystal Superstructures: photonic properties

We investigate the optical properties of hybrid superstructures composed of metal and semiconductor nanoparticles (NPs), and bio-linkers/polymers. Our study is inspired by recent experiments on bio-conjugated semiconductor-metal NP complexes and their potential applications as sensors. Metal NPs can quench semiconductor NP photoluminescence (PL). However, a plasmon enhancement can be achieved by organizing many Au NPs into a spherical or cylindrical shell around a CdTe NP. We compute electromagnetic fields induced in NP superstructures using a multipole expansion approach to describe the optical response of these complexes. Enhancement of CdTe emission can result from plasmon mediated enhancement of the excitation (Ag structures) or enhancement of the emission process (Au structures). The resultant optical response comes from a complex interplay of this enhancement and quenching and determines the potential applications of these superstructures.   

Spectroscopy of Charged Quantum Dot Molecules

Spins of single charges in quantum dots are attractive for many quantum information and spintronic proposals. Scalable quantum information applications require the ability to entangle and operate on multiple spins in coupled quantum dots (CQDs). To further the understanding of these systems, we present detailed spectroscopic studies of InAs CQDs with control of the discrete electron or hole charging of the system. The optical spectrum reveals a pattern of energy anticrossings and crossings in the photoluminescence as a function of applied electric field. These features can be understood as a superposition of charge and spin configurations of the two dots and represent clear signatures of quantum mechanical coupling. The molecular resonance leading to these anticrossings is achieved at different electric fields for the optically excited (trion) states and the ground (hole) states allowing for the possibility of using the excited states for optically induced coupling of the qubits.   

Multi-Excitonic Quantum Dot Molecules

With the ability to create coupled pairs of quantum dots, the next step towards the realization of semiconductor based quantum information processing devices can be taken. However, so far little knowledge has been gained on these artificial molecules. Our photoluminescence experiments on single InAs/GaAs quantum dot molecules provide the systematics of coupled quantum dots by delineating the spectroscopic features of several key charge configurations in such quantum systems, including X, X$^{+}$,X$^{2+}$, XX, XX$^{+ }$(with X being the neutral exciton). We extract general rules which determine the formation of molecular states of coupled quantum dots. These include the fact that quantum dot molecules provide the possibility to realize various spin configurations and to switch the electron hole exchange interaction on and off by shifting charges inside the molecule. This knowledge will be valuable in developing implementations for quantum information processing.   

Spin interactions in InAs quantum dots

Fine structure splittings in optical spectra of self-assembled InAs quantum dots (QDs) generally arise from spin interactions between particles confined in the dots. We present experimental studies of the fine structure that arises from multiple charges confined in a single dot [1] or in molecular orbitals of coupled pairs of dots. To probe the underlying spin interactions we inject particles with a known spin orientation (by using polarized light to perform photoluminescence excitation spectroscopy experiments) or use a magnetic field to orient and/or mix the spin states. We develop a model of the spin interactions that aids in the development of quantum information processing applications based on controllable interactions between spins confined to QDs. [1] Polarized Fine Structure in the Photoluminescence Excitation Spectrum of a Negatively Charged Quantum Dot, Phys. Rev. Lett. 95, 177403 (2005)   

Theory of Charged Quantum Dot Molecules

Recent optical spectroscopy of excitonic molecules in coupled quantum dots (CQDs) tuned by electric field reveal a richer diversity in spectral line patterns than in their single quantum dot counterparts. We developed a theoretical model that allows us to classify energies and intensities of various PL transitions. In this approach the electric field induced resonance tunneling of the electron and hole states occurs at different biases due to the inherent asymmetry of CQDs. The truncated many-body basis configurations for each molecule are constructed from antisymmetrized products of single-particle states, where the electron occupies only one ground state level in single QD and the hole can occupy two lowest levels of CQD system. The Coulomb interaction between particles is treated with perturbation theory. As a result the observed PL spectral lines can be described with a small number of parameters. The theoretical predictions account well for recent experiments.   

Ultrafast dynamics of surface plasmon polaritons in subwavelength nanohole array on metallic film

The ultrafast dynamics of surface plasmon polaritons (SPP) photogenerated on the surfaces of an Al film perforated with 2D subwavelength hole array ($\sim$300 nm lattice constant) was studied by the pump-probe correlation spectroscopy. Following an instantaneous rise at the onset of the impinging pulse, the transient differential transmission exhibits a fast rise with characteristic time constant of $\sim$300 fs reaching a plateau at $\sim$2 ps, followed by a slower decay with characteristic time of $\sim$40 ps. The observed dynamics can be explained by a fast energy transfer in the Al film from the electron gas to the lattice, with subsequent cooling of the Al film by heat transfer to the glass substrate. The fast dynamics is accompanied by a blue shift of the SPP band due to the increase in the Al lattice temperature. The obtained fast lattice temperature rise is caused by the strong electron-phonon interaction in Al, which makes the electron-lattice energy transfer rate comparable to the rate of nonequilibrium electrons thermalization via electron-electron interactions. A theoretical model based on the Boltzmann equation for nonequilibrium electron gas interacting with quasi-equilibrium phonons was developed, and is in good agreement with the data.   

Internal and external polarization memory loss in single quantum dots

Exciton spin relaxation counts among the most basic features of quantum dot (QD) ground-state dynamics and is intimately connected to the ubiquitous fine-structure doublet anisotropy. Numerous resonant measurements on QD ensembles support a spin relaxation frozen on the exciton lifetime, in agreement with theoretical expectations. Recent investigations, however, question this breakdown based on strictly non-resonant experiments on single QDs, pointing to possible variations among QDs. By using non-linear resonant control of single QDs we examine spin relaxation under different environments and excitation conditions. Data from dots with different dipole moments reveals two distinctive channels for polarization memory loss: (i) an external pathway due to carrier escape and capture to and from the wetting layer that is responsible for memory loss increasing with intensity; and (ii) an internal loss channel, independent of external excitation, due to intrinsic spin relaxation. The values obtained for the latter rule out a universal freezing of exciton spin relaxation in self-assembled QDs.   

Polarized Mid-Infrared Surface Emission from InAs Quantum Dots

Polarized mid-infrared surface electroluminescence from self-assembled InAs quantum dots has been observed at 77K. A graded AlGaAs injector is used to inject electrons into excited states in the quantum dot layer. A superlattice electron filter prevents direct electron tunneling out of the quantum dot excited states, increasing the probability of optical transitions to lower energy dot states. Two mid-infrared peaks are seen in the electrically pumped surface emission spectra of the device, one at 100meV, the other at 170meV. The emission peaks are orthogonally polarized within the growth plane, indicating photon emission from intersublevel electron transitions within anisotropically shaped quantums dots. This work suggests the feasibility of using quantum dot mid-infrared emission to study both the morphology of, and intersublevel transitions within, self-assembled quantum dots.   

Optical properties of semiconductor-metal nanocrystal molecules: Exciton-plasmon interactions

Motivated by recent experiments on bio-conjugated semiconductor-metal hybrid nanocrystal superstructures, we develop a theory to describe a system composed of a semiconductor quantum dot (QD) and a metal nanoparticle (NP) in the presence of external electric fields. The interaction between exciton (in QD) and plasmon (in NP) leads to interesting optical properties. We explore both the linear regime (for weak external field) and the non-linear regime (for strong external field). The interference between the external field and the induced internal field results in strong enhancement of energy absorption (compared with the energy absorption of QD in the absence of a metal NP) and also leads to an asymmetric peak and valley in the total energy absorption (Fano-like shape). We also consider Rayleigh scattering which also reveals this type of behavior. Our theory is useful for understanding present experimental results and can give guidance for future experiments, which may have important applications.   

The Optical Properties of Aluminum Oxide Templated Nanostructures

The optical properties of aluminum oxide templated nanostructures has been investigated with the specific goal of using the associated structures for enhanced transmission of light, and Surface Enhanced Raman Spectroscopy (SERS). We will present fabrication and characterization data for three different nanostructures based on the synthesis of anodic aluminum oxide (AAO) templates. These structures include self-assembled metal coated masks with sub-wavelength apertures for the enhanced transmission of light, striped Au/Ag nanowire arrays and nano-textured aluminum surfaces for SERS studies. Characterization using electron and atomic force microscopies, as well as absorbance, reflectance and Raman spectroscopy will be presented.   

First-Principles Optical Cross-Sections of Ultrathin ZnO Nanowires

One-dimensional nanostructures such as inorganic nanowires and nanotubes represent potential materials for key components of future electronic, optoelectronic, and nanoelectromechanical systems. They will also serve as important model systems to demonstrate quantum-size effects in nanostructured materials. We examine the electronic and optical properties of ZnO nanowires with different geometrical configurations within a first-principles, all-electron self-consistent local density functional (LDF) approach. Orientations along different growth directions are taken into account, with the preferred $\pm$ [0001] direction as the primary focus. The ultrathin nanowires considered here range in diameter from approximately 0.50 nm to 3 nm. We discuss trends in electronic properties and resulting optical properties as a function of nanowire axis orientation and diameter. This work was supported by the US Office of Naval Research, the DoD HPCMO CHSSI program through the Naval Research Laboratory, and the NSF IGERT program.   

Session U37: Focus Session: Nanowire and Nanodot Quantum Devices

Quantum Coherence and Time Dependent Conductance Fluctuations in Dilute Magnetic Semiconductors

Low temperature electrical properties of ferromagnetic nanowires are influenced by the interplay between disorder, quantum coherence, and magnetic correlations. Quantum coherence corrections to the conductance are of particular interest, and can be difficult to characterize experimentally. Time dependent universal conductance fluctuation (TD-UCF) at low temperatures provides a means of assessing these effects. Samples were fabricated by standard electron beam lithography and ion etching technique using In$_{1-x}$Mn$_{x}$As quantum well samples grown by off-equilibrium molecular beam epitaxy. Initial measurements of temperature and field-dependent TD-UCF in these devices are presented and compared with permalloy nanowire data.   

Microwave-enhanced decoherance in one-dimensional metal wires

We report the effect of monochromatic microwave (MW) radiation on the weak localization corrections to the conductivity of quasi-one-dimensional silver wires. Due to the improved electron cooling in the wires, the MW-induced dephasing was observed without a concomitant overheating of electrons over wide ranges of the MW power $P_{MW}$ and frequency $f$. The observed dependences of the conductivity and MW-induced dephasing rate on $P_{MW}$ and $f$ are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky.\footnote[1]{B.L. Altshuler, A.G. Aronov, and D.E. Khmelnitsky, Sol. State Communi. \textbf{39}, 619 (1981).} Our results suggest that the saturation of dephasing time, often observed at $T \leq$ 0.1 K, may be caused by an insufficient screening of the sample from the external microwave noise.   

Toward Single-Walled Nanotube Aharonov-Bohm interferometers

Single-walled carbon nanotubes (SWNTs), which have micron-scale phase coherent lengths at low temperatures, are grown so that they cross themselves, producing a loop that may act as an Aharonov-Bohm interferometer. In order to determine electron pathways at the junction, we performed scanned gate microscopy (SGM) using an Atomic Force Microscopy(AFM) tip as a local gate. If a SGM signal is observed when the tip is over any particular segment it indicates current flow in that segment. Surprisingly, one semiconducting tube showed that most electrons tunnel into the other segment at the junction without flowing through the loop. For other samples, however, current flow was observed in the loop. Taken together, this suggests the possibility of controlling tunneling probabilities at the junction. Moreover, for metallic or small bandgap nanotubes, we reproducibly observe an unusual conductance peak near zero gate voltage, obtaining nearly identical behavior from devices made from the same ~100 micron long nanotube. The results and interpretations of our ongoing experiments will be discussed.   

Quantum Transport of Semiconductor Nanowires Coupled to Superconductors

We report the electronic transport properties of the first superconducting transistors based on semiconductor nanowires (ref. Y.-J. Doh et al., Science 309, 272 (2005)). These nanoscale superconductor-semiconductor devices enable the fabrication of new nanoscale superconducting electronic circuits and at the same time provide new opportunities for the study of fundamental quantum transport phenomena. Indium arsenide (InAs) semiconductor nanowires individually contacted by two aluminum-based superconductor electrodes yield surprisingly low contact resistances. Below 1 kelvin, the high transparency of the contact gives rise to proximity-induced supercondcutivity and a resistance-free current (supercurrent) can flow through the nanowire from one superconducting contact to another. The supercurrent can be switched on and off by a gate voltage acting on the electron density in the nanowire. A variation in gate voltage induces universal fluctuations in the normal-state conductance, which are clearly correlated to critical current fluctuations. The alternating-current Josephson effect gives rise to Shapiro steps in the voltage- current characteristic under microwave irradiation. For indium phosphide (InP) nanowire devices, however, Coulomb blockade effect dominates the electrical transport, which can be modeled as a quantum dot weakly coupled to superconductors. As a result of BCS (Bardeen-Cooper-Schriffer) singularity of density of states, a negative differential conductance is observed in the superconducting state. By applying high magnetic field, Zeeman splitting is observed and g-factor of InP nanowire is estimated to be 1.5.   

Four-probe measurements of individual metallic and superconducting nanowires

We have performed 4-probe measurements for a number of individual Au, Ag, Sn and Zn nanowires. The resistivity of these nanowires is always larger than their bulk values, and varies with the crystallinity of the nanowires. Single crystalline Sn nanowires have a resistivity similar to that of bulk Sn, while polycrystalline Au nanowires shows a resistivity twenty to thirty times higher than that of bulk Au. For Sn nanowires with diameter from 280 nm to 70 nm, the superconducting transition temperature $T_{c}$ remains the same as, or slightly lower than the $T_{c}$ of bulk Sn, 3.7 K. The critical current density of these Sn nanowires is measured to be in the order of 105 A/cm2 at low temperature. The critical fields of the nanowires, as expected, are much higher than that of Sn films. A finite resistance, which increases with increasing excitation current, is observed in all the samples. The origins of this finite resistance will be discussed. We have also obtained evidence that the four metallic leads assembled on each single nanowire may have remarkable effects on the transport properties of the nanowire.   

Modifying Mesoscopic 1/f Noise Via Surface Chemistry.

Attempts to extrinsically control the 1/f noise related to Universal Conductance Fluctuation Theory in quasi-one dimensional Au wires were made using self-assembled monolayer (SAM) molecules. Measurements before and after the deposition of the SAM molecule comparing the noise power amplitude and the phase coherence of the devices via the weak localization magnetoresistance and noise power amplitude versus magnetic field were performed. The resulting data were used to determine if the 1/f noise is approaching the so-called saturated limit as the system is lowered from 14 to 2 K. The results may also shed light on the microscopic details of the two level systems responsible for time-dependent conductance fluctuations in normal metals. Preliminary results will be reported.   

Coulomb Blockade Imaging of Few-Electron Quantum Dots in a Magnetic Field

One-electron quantum dots are important candidates for quantum information processing. We have developed a technique to image electrons inside a quantum dot in the Coulomb blockade regime, using a scanning probe microscope (SPM) at liquid He temperatures (1). We have used this technique to image the last electron in the dot in a strong perpendicular magnetic field.dots are formed in a two-dimensional electron gas in a GaAs/AlGaAs heterostructure by surface gates. Images are obtained by recording the dot conductance while scanning the SPM tip above the dot. SPM Images show a ring of increased conductance about the center of the dot, corresponding to a Coulomb blockade peak in the dot conductance.observe changes in the shape and the size of the conductance rings with magnetic field. This is due to a combination of energy shifts and orbital changes of the electrons in the quantum dot. \newline (1) P. Fallahi \textit{et al,} Nano Letters 5, 223 (2005)   

Kondo and Superconducting Proximity Effect in Semiconductor Nanostructures

We have fabricated a unique device containing three quantum dots in a GaAs/AlGaAs heterostructure containing a two-dimensional electron gas using lithographically patterned gates and an etched trench in the center of the ring. By only energizing certain gates, this device allows us to study electron transport through a single dot, a double dot, or a triple dot ring. We can determine the absolute number of electrons in a quantum dot using a nearby charge sensor and find that we are able to tune a single dot to the one and two electron regime. We find several sharp peaks in the differential conductance, occurring at both zero and finite source-drain bias, for the one and two electron quantum dot. At zero source-drain bias, the temperature and magnetic field dependence of the conductance is consistent with a standard Kondo resonance. We attribute the peaks at finite-bias to a Kondo effect through excited states of the quantum dot. We also present recent observations of supercurrents in Ge/Si 1D nanowire heterostructures.   

Transition to Scarred States Probed by a Single Electron Spectrometer

We examine the use of a novel mesoscopic spectrometer to image electron dynamics in a large, open lateral quantum dot in a perpendicular magnetic field $B_{z} $. The spectrometer is comprised of a small single-electron quantum dot weakly coupled to the large dot via a tunnel junction; the current through the dots is measured at finite bias. By varying the energy level of the small dot, the local density of states, $g(r_{o},E)$ is measured at the tunnel junction. An unexpected interference effect persists at energies 1 meV below $E_{F}$. Using semiclassical and quantum approaches, we show these interference bands in $(E, B_{z})$ are due to a dominant, diamond-shaped periodic orbit in the large dot. As $B_{z}$ is varied, the orbit changes to unstable: the dominant state becomes a scar. This transition is marked by an abrupt change in the area of the orbit, and hence in the spacing $\Delta B_{z}$ of the bands. In a billiard model of the system, this coincides with a pitchfork bifurcation of the orbit. The existence of the bands depends strongly on whether the orbit has appreciable magnitude at the tunnel junction, belying the local nature of the measurement. In this way the tunnel junction, coupled to the small dot, acts like a fixed STM tip embedded in the 2DEG.   

Magneto-transport studies in mesoscopic InAs 2DEG devices

We report on magneto-transport studies in lithographically patterned InAs 2DEG devices. Electron transport between adjacent current injection and extraction channels was studied as a function of temperature and magnetic field. The mean free path in the quantum well at RT and 5 K in our devices is approximately 280 nm and 980 nm respectively. The spacing between the current tabs in our devices range from 1000 nm to 300 nm and typical tab widths range from 300 nm to 100 nm. Low temperature measurements reveal contributions of ballistic transport in our devices; fluctuations in magneto-resistance are observed at distinct values of the perpendicularly applied magnetic field. Said fluctuations will be discussed in the context of ballistic electron focusing trajectories and possible contributions from the quantum Hall effect.   

Asymmetry of Nonlinear Transport and Electron Interactions in Quantum Dots

The magnetic field symmetry of conductance beyond the linear source-drain bias regime in open chaotic GaAs quantum dots is experimentally investigated using gate-controlled shape distortion to gather ensemble statistics. We measure a conductance component $g_{B-}$ antisymmetric in perpendicular magnetic field $B$ which is of the form $\tilde g=\alpha V B$ for source-drain bias voltages $V$ smaller than the quantum dot level spacing $\Delta$ and for $B$ smaller than a flux quantum through the device area. Interestingly, according to recent theories, $\tilde g$ of this form vanishes in absence of electron interactions and $\alpha$ is proportional to the electron interaction strength. $g_{B-}$ shows mesoscopic fluctuations with shape gate and with $B$ and $V$ on a scale of the flux quantum through the dot area and quantum dot level spacing $\Delta$, respectively. As anticipated by theories, the average coefficient $\alpha$ measured over an ensemble of dot shapes vanishes. The standard deviation of $\alpha$ is used to characterize the strength of electron interactions. We discuss the dependence of the typical $\alpha$ on the number of modes in the quantum-point-contact leads, compare our experiment with theories and discuss related issues of electron equilibration, decoherence and thermal smearing in the quantum dot. This work was partially supported by DARPA QuIST, ARO/ARDA and by the NSEC program of the NSF.   

Two Electron Singlet Triplet Spectroscopy

We present measurements of few electron quantum dots formed by lateral depletion of a GaAs/AlGaAs 2D electron gas by surface gates. The two electron regime, on which we focus here, is characterized by singlet and triplet states which are relevant for quantum computation proposals. These two states are revealed in electronic transport through the dot in various ways: sequential tunneling, inelastic cotunneling as well as by an additional mode of transport we ascribe to sequential tunneling activated by inelastic cotunneling. These various signatures provide independent ways to measure the singlet-triplet energy splitting J over large ranges of gate voltages. We present the temperature, magnetic field and tunnel-coupling dependence of these transport features, which are in good agreement with recent theory. Further, we observe signatures of spin-blockade that becomes visible for source-drain voltages exceeding the triplet energy. This work was partially supported by the ARO (W911NF-05-1-0062), by the NSEC program of the NSF (PHY-0117795) and by NSF (DMR-0353209).   

Superpoissonian noise with positive current correlations

We report shot noise cross correlation measurements in a beam splitter configuration. We fabricated our devices in a two dimensional electron gas in a GaAs/AlGaAs heterostructure using a splitting gate technology. Electrons tunneling across tunnel barriers are incident on a beam splitter and are scattered into two different channels. Shot noise cross correlation between the two electrical currents is measured as a function of both the transmission coefficient of the beam splitter and the Fano factor of the tunnel barriers. Due to the Fermi statistics of electrons, such a measurement usually yields a negative correlation. However, in some barriers under certain circumstances, a positive correlation has also been observed. A correspondence between the Fano factor of the tunnel barriers and the cross correlation has been established. For example, positive cross correlation is always associated with barriers exhibiting superpoissonian shot noise (with a Fano factor greater than one). Studies on the frequency dependence of shot noise suggest that the observed positive cross correlation can be related to the dynamics of localized states in the tunnel barriers.   

Session U38: 1-D Superconductors and Organics

High-Tc superconductivity in entirely end-bonded carbon nanotubes

One-dimensional (1D) systems face some obstructions that may prevent the emergence of superconductivity(SC), e.g., a Tomonaga-Luttinger liquid (TLL) and Peierls transition. A carbon nanotube (CN) is one of the best candidates for investigating a possibility of 1D SC and its interplay with such obstructions. Only two groups have experimentally reported SC in ropes of single-walled CNs (SWNTs) and very thin SWNTs [1] to date. In addition, those interplay with 1D phenomena have never been clarified. Some theoretical papers also predicted strong correlation between TLL states and SC for SWNT ropes and importance of electron-phonon interaction for thin SWNTs [2]. Here, we report that entirely end-bonded multi-walled CNs (MWNTs) can show SC with the T$_{c}$ as high as 12K [3] (about 50-times larger than T$_{c}$ in former of [1]). We find that emergence of this SC and its interplay with TLL states are highly sensitive to junction structures of Au electrode/MWNTs. Only MWNTs with optimal numbers of electrically activated shells realized by the entire end-bonding can allow the SC due to intershell effects. \textbf{Refs.} \textbf{1.}M. Kociak, et al., PRL 86, 2416 (2001); Z. K. Tang, et al., Science 292, 2462 (2001), \textbf{2}.J.Gonzalez, PRL 88, 076403 (2002); R.Barnett, et al., PRB 71, 035429 (2005), \textbf{3}.J.Haruyama et al., PRL Accepted   

Superconductivity of granular Bi nanowires fabricated by electrochemical deposition at ambient condition

Rhombohedral bulk Bi is a semimetal which is not superconducting down to low temperatures under ambient pressure. Amorphous Bi films and bulk Bi subjected to high pressure can be superconducting. We report here the observation of superconductivity of Bi nanowires (the diameter ranges from 40 to 100 nm) fabricated by electrochemical deposition at room temperature under atmospheric pressure. The superconducting transition temperature Tc, depending on the sample morphology, can be either of 3.7 K, 7.2 K or 8.3 K, which correspond exactly to the Tc's reported for the three high pressure Bi phases (II, III and V). However, structural studies showed that these superconducting Bi nanowires showed granular morphology consisting of rhombohedral Bi particles oriented along the [001] direction. Because the superconducting Bi wires did not show any detectable diamagnetic signature, the observed superconductivity might be related to the interfacial structures of the granular wires. Further studies are in progress. \textit{This work is supported by }\textit{ the Center for Nanoscale Science (Penn State MRSEC) funded by NSF under grant DMR-0213623.}   

Enhancing superconductivity: Magnetic impurities and their quenching by magnetic fields

Magnetic fields and magnetic impurities are each known to suppress superconductivity. However, it has recently been found theoretically that in superconducting films with magnetic impurities the critical temperature can be raised by applying a magnetic field ($H$) [1]. Here, we extend the Eilenberger-Usadel formalism and use it to investigate this interplay of magnetic fields and magnetic impurities. Hence, we are able to compute the critical current ($J_c$) of a thin superconducting wire, in addition to its critical temperature ($T_c$). We find three regimes of wire parameters. In one, both $T_c$ and $J_c$ simply decrease with $H$; in a second, both $T_c$ and $J_c$ first rise and then fall with $H$; and in a third, $T_c$ simply decreases with $H$ but, at sufficiently low temperatures, $J_c$ first rises and then falls [2]. Our results are consistent with recent experiments on thin superconducting wires [3]. \newline [1] M.\ Kharitonov and M.\ Feigel'man JETP Lett. {\bf 82}, 473 (2005). \newline [2] T.-C.\ Wei et al.\ cond-mat/0510476. \newline [3] A.\ Rogachev et al.\ (manuscript in preparation).   

Anomalous behavior of the critical current in superconducting MoGe nanowires exposed to high magnetic fields.

At low temperatures the critical current of superconducting MoGe nanowires with diameters 6-10 nm shows an unusual initial growth with increasing magnetic field, and reaches a maximum at the field 3-5 T. The non-monotonic behavior is present both in parallel and perpendicular field orientations and disappears at high temperatures. We suggest that the effect is caused by magnetic impurities, which suppress superfluid density in the nanowire at low fields but, due to partial polarization in the applied magnetic field, become less efficient pair-breakers in high fields. We compare our data with the microscopic theory that considers this competition of the reduced depairing by localized spins and the increasing depairing by the orbital effects [1]. The theory reproduces all experiential observations and suggests that magnetic impurities reside on a surface of a wire. [1] T.-C. Wei, D. Pekker, A. Rogachev, A. Bezryadin, and P.M. Goldbart, cond-mat/0510476.   

Confined Vortices in NbSe$_2$ Nanowires

Superconducting NbSe$_{2}$ nanowires have been studied with electrical transport. The cross-sectional dimensions are smaller than the London penetration depth, and signatures of confined magnetic vortices have been observed. The critical current shows non-monotonic behavior as the external magnetic field is increased, including periodic features corresponding to matching fields. In the vicinity of the critical current, we observe several peaks in the differential resistance as a function of bias current, consistent with the plastic flow of vortices. These observations are discussed in the context of theoretical London model studies and experiments in thin-film superconductors and bulk NbSe$_{2}$.   

Growth Mechanism of EuBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Whiskers and Their superconducting properties for Intrinsic Josephson Junction Applications

We have grown Eu-123 single-crystal whiskers by annealing of precursor pellets containing Ca and Te. Microstructural and compositional analysis was performed on the longitudinal cross-section and the area of origin of a whisker to elucidated some aspects of the growth mechanism from bulk precursor. Sub-micron sized junctions fabricated by focus ion-beam etching on Eu-123 whiskers showed clear multi-branch with hysteresis structure in the I-V curve, which suggest excellent crystalline quality both of as-grown and high pressure annealed whiskers. We have also doped Er and Tm having a smaller ionic radius in single crystalline whiskers of (Eu,R)-123 (R= Er, Tm) and observed that Eu-rich whiskers, which require higher temperature to be grown, are more susceptible to oxygen deficiencies and structural instabilities. Our results shows that the carrier doping can be systematically controlled from highly underdoped to slightly overdoped range by suitable choice of average ionic radius of rare-earth elements in as-grown whiskers. The critical current density J$_{c}$ of a Eu-123 whisker (T$_{c}$=45K) was estimated to be 1.43 $\times$ 10$^{5}$ A/cm$^{2}$ at 4.2K , twice as much as observed for Y-123 having a similar T$_{c}$.   

Evidence for Current-Driven Phase Slip Lines in Submicron High-$T_c$ Superconducting Wires

We present superconducting current-voltage characteristics of submicron YBa$_2$Cu$_3$O$_{7-\delta}$ wires. The submicron-wide wires were fabricated using a chemical-free technique based on selective epitaxial growth. Pulsed current-biased and voltage-biased measurements were made between 4.2 K and $T_c$ and as a function of an applied magnetic field. The current-voltage characteristics exhibit distinctive behaviour suggesting that the current-driven or voltage-driven transition of our submicron high-$T_c$ wires into the resistive state is through the formation of phase slip lines.   

Quantum phase slips in the presence of finite-range disorder

To study the effect of disorder on quantum phase slips (QPS) in superconducting nanowires, we consider the plasmon-only model where disorder can be incorporated into a first-principles instanton calculation. We consider weak but general finite-range disorder and compute the formfactor in the QPS rate associated with momentum transfer. We find that the system maps onto dissipative quantum mechanics, with the dissipative coefficient controlled by the wave (plasmon) impedance $Z$ of the wire and with a superconductor-insulator transition at $Z_{\rm c}=6.5$ kOhm. The usual Ohmic resistivity of the wire at the transition point is non-universal. Its value depends on both the strength and the correlation length of disorder, and in the considered regime is much smaller than the normal-state resistivity. We argue that the system will remain in the same universality class after resistive effects at the QPS core are taken into account.   

Magnetization controlled superconductivity in a Pb film on a perpendicular array of ferromagnetic Co nanowires

We report the studies of superconductivity in a Pb film on a perpendicular array of ferromagnetic Co nanowires. We first evaporate a Pb film of 300nm in thickness on a 60$\mu $m-thick porous Al$_{2}$O$_{3}$ membrane as the cathode for electroplating. Cobalt nanowires 100nm in diameter were electrochemically deposited in the pores starting from the Pb film. Scanning electron microscopy images showed uniform distribution of cobalt nanowires. The magnetization of the individual Co nanowires should be oriented perpendicular to the Pb film due to the high aspect ratio of the Co nanowires. We have observed significant difference in the superconducting behavior of the Pb film between zero-field cooled experiments and field-cooled experiments. In field-cooled experiments, the samples are cooled from room temperature to 20 K in fields of 1-5 T applied along the direction of the Co nanowires. This field aligns the magnetization of the Co nanowires, and as a result, enhances the superconducting transition temperature by 1.5-2.0 K in comparison to zero-field cooled experiments in which case the magnetization of the nanowires is not aligned. These experimental data support recent theory by I. F. Lyuksyutov et al. [PRL \textbf{81}, 2344(1998)].   

Current carrying edges in unconventional superconductors

While the bulk of a p or d wave superconductor does not carry any current, a current can run along the edges of a sample even in the absence of a magnetic field, unlike the case of a s wave superconductor. Such a current is often quantized and is independent of the magnitude of the gap. However, the existence of such a current violates Bloch's theorem. Here, we examine, by computation and analytic calculation, the question of the edge current in p and d wave superconductors in various geometries and the connection with the Bloch argument. The computation of the edge current also allows us to shed some light on the angular momentum paradox in the A phase of Helium 3.   

The coherence conundrum in BEDT-TTF superconductors; how does interlayer transport die as temperature rises?

Recent attention has focused on ``bad metals'', systems which appear to be Fermi liquids at low temperatures ($T)$, but whose conductivity falls below the minimum metallic limit as $T$ rises. A key question concerns the coherence of the electron orbitals, and whether, as suggested by Anderson and others, it is destroyed by thermal fluctuations as $T$ rises. To address this, we have studied magnetic-field-orientation-dependent transport in the organic superconductor (BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ at temperatures of up to 45 K in magnetic fields of up to 45 T. This material was chosen because its Fermi surface (FS) is well characterized by experiment. We find that the angle-dependent magnetoresistance oscillations (AMROs) due to orbits on the quasi-one-dimensional (Q1D) and Q2D FS sections are suppressed by rising $T$, with a $T$ dependence suggesting phonon scattering. The coherence peak in the resistivity seen in exactly in-plane fields, and other signatures of a 3D FS, remain to values of $T$ that exceed the proposed Anderson criterion for incoherent transport by a factor of order 80! The implications of these data for currently-held ideas about bandstructure will be discussed.   

Interplane penetration depth in $\kappa $-(ET)$_{2}$Cu[N(CN)$_{2}$]Br

We report measurements of the interplane penetration depth $\lambda _{\bot }$(T)$_{ }$ in the organic superconductor $\kappa $- (ET)$_{2}$Cu[N(CN)$_{2}$]Br\textbf{ (}T$_{C}$ = 11.9 K). At low temperatures, the superfluid density $\rho _{\bot }$= [$\lambda _{\bot }$(0)/ $\lambda _{\bot }$(T)]$^{2} \quad \propto $ 1-AT$^{N}$ with N = 1.3 -- 1.5, close to the exponent measured for the in-plane superfluid density. This result adds support to a d-wave picture, but with transport between planes more coherent than is observed in similarly anisotropic copper oxide superconductors.   

London penetration depth in fully deuterated $\kappa $-(ET)$_{2}$Cu[N(CN)$_{2}$]Br

We report measurements of the London penetration depth, $\lambda $, for different magnetic field and crystal orientations in fully deuterated $\kappa $ (ET)$_{2}$Cu[N(CN)$_{2}$]Br\textbf{, }an organic superconductor with T$_{C}$ = 11.9 K. $\lambda $ increases dramatically with deuteration and develops a strong magnetic field dependence. The superfluid density exhibits a power law temperature dependence indicative of a nodal order parameter. We discuss possible connections to nanoscale antiferromagnetic domains.   

Pauli limiting in the superconductor $\kappa$-(ET)$_2$Cu(NCS)$_2$ under pressure

We have strong evidence that the organic superconductor $\kappa$-(ET)$_2$Cu(NCS) $_2$ is Pauli limited when it is compressed with a small ($< 2$~ kbar) amount of pressure. A superconductor is considered Pauli limited when the magnetic energy, $ \mu_bH$, overcomes the binding energy of the Cooper pairs to destroy superconductivity. In most situations superconductivity is destroyed by the formation of vortices. At ambient pressure, $\kappa$-(ET)$_2$Cu(NCS)$_2$ shows some behavior that is reminiscent of a Fulde Ferrell Larkin Ovchinnikov state, although no direct evidence of a transition into this state has been measured. Through a series of penetration depth measurements using a tunnel diode oscillator under pressure, we can show the evolution of the ambient pressure state to the clearly Pauli limited state at 1.75 kbar. We will also discuss the design of our new plastic, gas charged pressure cell that has allowed us to make these measurements in dc and pulsed magnetic fields. We acknowledge support from the DOE \#ER46214 and the NSF \#DMR- SGER-0331272.   

Session U39: Superconducting Proximity Effect-S/N and S/F

Proximity effect and Josephson current in clean strong/weak/strong superconducting tri-layers

Recent measurements of the Josephson critical current through LSCO/under-doped LSCO/LSCO thin films showed an unusually large proximity effect. Using the Bogoliubov-de Gennes (BdG) equations for a tight binding Hamiltonian we describe the proximity effect in weak links between a superconductor with critical temperature $T_c$ and one with critical temperature $T_c'$, where $T_c>T_c'$. The weak link (N') is considered to be a superconductor above it's critical temperature and the superconducting regions can have either s-wave or d-wave symmetry. We observe that the proximity effect is enhanced due to the presence of superconducting correlations in the weak link. The dc Josephson current is also calculated, and we observe a non-zero value for temperatures greater than $T_c'$ for sizes of the weak links that are greater than the conventional coherence length. This effect alone is unable to explain the experimental results, instead, we also consider pockets of superconductivity in the weak link.   

Superconducting Proximity Effect in Semiconductor Films - Experiment

Interface transparency and device topology together determine the information regarding the superconducting proximity effect that can be obtained from transport measurements. We have introduced a new three terminal device design and use junctions formed entirely in-situ between niobium(S) and a thin heavily doped InGaAs epitaxial layer(N). The junction design allows us to separately extract the junction conductance and the sheet resistance of the InGaAs from the two terminal and three terminal voltage readings at low bias currents. We see evidence for both fluctuating and phase-stiff superconductivity (SC) in the normal material. At temperatures below, but close to Tc of the niobium, SC fluctuations cause the spreading resistance, Rs, on the normal side of the junction to drop. At lower temperatures, phase-stiff SC emerges in the InGaAs, effectively stealing volume from the normal region. This makes Rs appear to increase as the SC order sets in. The specific junction conductance, Gc, rises to values much greater than the normal value. We propose this is caused by the N-S boundary moving into the semiconductor.   

Superconducting Proximity Effect in Semiconductor Films - Device Theory

A new three terminal device architecture is introduced and analyzed for studying the superconducting proximity effect. It consists of a narrow superconducting injector line that injects current into a thin normal film. The current is extracted from one side of the injector line by a superconducting drain electrode that is many normal state coherence lengths ``downstream'' of the injector. A third voltage tap is provided on the other or ``upstream'' side of the injector. We present a theory showing how measurements made in various voltage sensing configurations can be combined to provide enough information to extract the two dimensional sheet resistance of the normal metal under the superconductor, as well as the specific contact conductance between the superconducting and normal parts of the device. This theory has been used to characterize the proximity effect in thin heavily doped InGaAs layers. A transition from fluctuating to phase stiff pair correlations in the normal layer has been observed at temperatures below Tc of the superconductor.   

Coexistence of, and Competition between, Magnetism and Superconductivity

Magnetism and superconductivity are competitive types of order in correlated electron systems. However, when a magnetic material is brought in contact with a superconducting material to build a junction, a coexistence region exists near the boundary that can modify the properties of the heterostructure in a qualitative way. In the case of a ferromagnet in contact with a singlet superconductor, the importance of triplet pairing correlations in the interface region recently became the focus of research. Such triplet correlations have unusual properties. They are typically odd in frequency for the case of a diffusive material. For clean materials in addition a triplet component even in frequency but odd in momentum is present. We have predicted that such triplet correlations can lead to an unusual indirect Josephson effect in a superconductor/half-metal/superconductor structure. In the case of a long half-metal such a Josephson effect is solely due to equal-pair triplet superconducting correlations. The triplet supercurrent is converted into a singlet current in the interface regions of the structure. Although theoretically predicted, a direct experimental verification of the presence of triplet correlations in ferromagnet/superconductor hybrid structures is difficult. In addition to the above effect we propose to use the torque on a ferromagnet/superconductor/ferromagnet trilayer in an external magnetic field as a probe of the presence of triplet correlations in the superconducting phase.   

Magnetization-dependent T$_{c }$ shift in F/S/F trilayers with strong ferromagnets

Hybrid systems combining ferromagnetic (F) and superconducting (S) metals in contact exhibit a wide range of fascinating behaviors. Several experimental groups have used weak ferromagnetic alloys in F/S experiments to enhance the penetration of Cooper pairs into the ferromagnet. In an F/S/F trilayer structure, a difference in the critical temperature T$_{c}$, based on the mutual orientation of the outer ferromagnets, has been reported [1] in CuNi/Nb/CuNi. Systems with strong ferromagnets, on the other hand, present new challenges, due to the very different density of states and Fermi velocity for the two different spin bands. Using the strong ferromagnets Ni and NiFe (Permalloy) in F/S/F exchange-biased spin valves [2], we observe that the T$_{c}$ for the parallel (P) orientation is lower than that of the anti-parallel (AP) case, i.e. T$_{c}$ (P) $<$ T$_{c}$ (AP). These results are consistent with theoretical expectations, but opposite to recent experimental work from another group where an inverse spin switch has been reported in a NiFe/Nb/NiFe structure [3]. [1] J. Y. Gu et al, Phys. Rev. Lett. 89, 267001 (2002). [2] I. C. Moraru et al., submitted for publication (2005). [3] A. Yu. Rusanov et al., cond-mat/0509156 (2005).   

Tunneling Studies of Superconductor/Strong Ferromagnet Bilayers

Thin-film heterostructures composed of superconductors and ferromagnets have recently received much interest, as they provide a unique opportunity to study the proximity effect between superconductivity and magnetism. We report systematic tunneling density of states (DOS) measurements on superconductor (Nb) /strong ferromagnet (CoFe, Ni) bilayers made with high quality aluminum-oxide planar tunnel junctions as a function of ferromagnetic thickness, $d_F$. In CoFe, we find that as $d_F$ increases, the superconducting DOS exhibits a scaling behavior in which the deviations from the normal-state conductance have a universal shape, which decreases exponentially in amplitude. The decay length, $d_1$, is approximately $0.4$~nm. We do not see oscillations in the DOS as a function of $d_F$, as one would expect from predictions based on the Usadel equations using reasonable parameters, although an oscillation in $T_c(d_F)$ has been seen in the same materials. Measurements on Nb/Ni bilayers will also be presented. This work is supported by AFOSR, DOE, and KOSEF through CSCMR.   

Interplay between superconductivity and ferromagnetism in tunneling

We study tunneling currents in a model consisting of ferromagnetic spin-triplet superconductors with magnetization in an arbitrary direction separated by a thin insulating layer. A novel effect is found with both ferromagnetic and superconducting phases entering in the expressions for the single- and two-particle tunneling currents in both spin and charge sector. This interplay between ferromagnetism and superconductivity is present when unconventional Cooper pairs with parallell spin pairing are allowed to form.   

Tunneling in Dilute Al-Mn Alloys: Observation of Resonant Scattering and Implications for High-Temperature Superconductors

We report on the observation of superconducting energy gap suppression by resonant scattering. Tunneling measurements of dilute Al-Mn alloys demonstrate the absence of density-of-states smearing that accompanies pair breaking and verify the detailed predictions of the Kaiser resonant scattering theory. These materials represent model systems for quasi-particle scattering and interference phenomena in the high- temperature superconductors.   

Andreev reflection spectroscopy of the heavy-fermion superconductor CeCoIn$_{5}$ along different crystallographic orientations

We have performed Andreev reflection spectroscopy on single crystals of the heavy-fermion superconductor CeCoIn$_{5}$. Conductance spectra obtained along both (001) and (110) crystallographic orientations exhibit similar features including asymmetry in the background conductance, the magnitude of zero-bias conductance enhancement (12 - 13{\%}) and the energy scale for the conductance enhancement ($\sim $1 meV). Analysis of the (001) junction data taken at the lowest temperature (400 mK) using an extended Blonder-Tinkham-Klapwijk model gives 2\textit{$\Delta $/k}$_{B}T_{c}$ = 4.64 [1,2]. The failure to account for the full temperature dependence of the data sets requires further theoretical investigations to account for the magnitude of the Andreev signal, including the possibility of two-fluid behavior. Features in the (110) data may provide the first spectroscopic evidence for $d_{x2-y2}$ superconducting order parameter symmetry [2,3]. [1] W. K. Park \textit{et al}., Phys. Rev. B \textbf{72}, 052509 (2005). [2] W. K. Park \textit{et al}., cond-mat/0507353. [3] W. K. Park and L. H. Greene, cond-mat/0507489. This work was supported by the U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the Center for Microanalysis of Materials at UIUC.   

Elastic Cotunnelling and Crossed Andreev Reflection in Normal-Superconductor Nanostructures

Transport experiments were made on normal-superconductor-normal systems where the separation of the normal elements is less than a superconducting coherence length. For this geometry two coherent, nonlocal effects have been predicted. In elastic cotunnelling electrons from one normal element can tunnel to the other through a virtual state in the superconducting gap. In crossed Andreev reflection one electron from each spatially separated normal element join to enter the superconductor as a Cooper pair. We present evidence of these nonlocal effects and show that their spatial dependence agrees with theory.   

Anomalous Proximity Effect in Nb/Al/CoFe Trilayers

We have fabricated Nb/Al/CoFe(Ni, Cu$_{40}$Ni$_{60})$ trilayers to study the interaction between superconductivity and ferromagnetism. Increasing the thickness of Al in S/N/F trilayer, we observed that Tc values of S/N/F trilayers increase sharply close to the Tc of S/N bilayer until the Al thickness reaches 3 nm. As Al thickness increases from 3 nm to 180 nm, Tc value of S/N/F decreases again, following those of the S/N data. In order to fit the Tc data of Nb/Al/CoFe trilayers as a function of Al thickness in a conventional Usadel formalism, we had to use a large $\gamma _b ^{F}$(= R$_{b}$A/$\rho _{f}\xi _{f})$ value of about 4, which seems unphysically large. In order to examine the role of Al/CoFe interface, we fabricated Nb/Cu(2 nm)/Al(2 nm)/CoFe and Nb/Au(2 nm)/Al(2 nm)/CoFe and compared them with Nb/Al(4 nm)/CoFe. The Tc of the double N layer system showed lower value than the Tc of the single Al layer system, although the three systems shared the same Al/CoFe interfaces. Our data suggests the large $\gamma _b ^{F}$ value in a conventional Usadel picture is not sufficient and rather indicates towards the unique role of Al instead of the Al/CoFe interface. We will discuss other possibilities such as the triplet superconductivity in order to explain our experimental findings.   

The Role of Inelastic Scattering in Intermediate Spin Polarized Normal Metal/Superconductor Point Contacts

Charge transport in ferromagnetic normal metal/superconductor point contacts is constrained by both the limited minority spin population, which reduces the probability of the Andreev reflection process, and by quasiparticle finite-lifetime effects, i.e., inelastic scattering, which influences the probability of ordinary electron transport. For the case of intermediate polarization 0.30 $\le $ P $\le $ 0.60, where 0 $\le $ P $\le $ 1.0, these processes can play equally important roles. We present results for normalized conductance at zero bias, as a function of temperature, and for conductance as a function of voltage, at P = 0.40, parametrically, for the entire range of inelastic scattering. Experimental results for point contacts will be presented.   

Magnetically Induced Superconductor-Metal-Insulator Transition in Thin Tantalum Films

Homogeneously disordered superconducting thin tantalum films are found to exhibit a metallic behavior in the limit of zero temperature when the superconductivity is suppressed by weak magnetic fields. The metallic behavior is characterized by an apparent saturation of sample resistance to a finite value, which can be order of magnitude smaller than the normal state resistance. This implies that the metallic state exists as a separate phase rather than a point in phase diagram. Such a metallic behavior is in strong contrast to the traditional belief that the electronic state of a 2D superconducting film can either be superconducting or insulating. We present details of transport characteristics in the magnetically induced metallic and insulating phase. We also discuss the influence of disorder, represented by normal conducting sheet resistance, on the metallic behavior and the superconductor-metal-insulator phase transition.   

Magnetic Field-Induced Metallic Behavior in Superconducting Tantalum Films

We present the results of electronic transport measurements on superconducting thin tantalum films. The films are grown by dc sputtering on Si substrates. No sign of crystalline ordering is found from X- ray diffraction studies, particularly for films with thickness less than $\sim $ 5nm, indicating that the structure of the films is amorphous. The superconducting transition temperatures are found to continuously decrease with decreasing film thickness, which is characteristic of homogeneously disordered superconducting films. At zero magnetic field, a direct superconductor-insulator transition is observed at a critical thickness $\sim $ 3 nm. At this thickness the normal conducting sheet resistance is close to the quantum resistance, $h$/4$e^{2}$. When the superconductivity is suppressed by applying magnetic fields, however, the system exhibits an unexpected metallic behavior in the limit of zero temperature. The metallic behavior is characterized by a drop in resistance followed by an apparent saturation to a finite value as the temperature is reduced. We observe qualitatively different nonlinear voltage-current characteristics across the ``superconductor- metal'' boundary, and also the ``metal-insulator'' boundary.   

Session U40: Focus Session: Pathways to Practical Quantum Computing III

Ion traps and cold atoms for quantum computers

Atoms can be used to store and manipulate quantum information. In particular, their internal state can be considered to form a register, and they can also be manipulated using laser light. In the case of trapped ions, the Coulomb force gives the required interaction to perform two-qubit gates. For neutral atoms, cold collisions can be used for that purpose. During the last years there has been an extraordinary experimental progress with those systems, and it is now possible to perform simple quantum information tasks with them. In this talk I will review several proposals for implementing quantum computers and quantum simulators using trapped ions and neutral atoms in optical lattices, and I will report on the latest experimental advances. Then, I will consider two particular aspects of those systems: (i) the possibility of simulating spin and bosonic systems with trapped ions; (ii) the possibility of performing quantum computations with neutral atoms without addressing them and in the presence of defects.   

Ion trap quantum computing with transverse phonon modes

We propose a scheme to use the transverse modes to implement conditional phase gates on two trapped ions immersed in a large linear crystal of ions, without the sideband addressing. Comparing with the conventional approach using the longitudinal modes, with the cost that the laser power is slightly stronger, the proposed gate operation can be well inside Lamb-Dicke region and the gate infidelity due to the fluctuation of the effective Rabi frequency as well as the fundamental limits of the cooling procedure are approximately two orders smaller.   

Robust quantum memory using magnetic-field-independent atomic qubits

Scalable quantum information processing requires physical systems capable of reliably storing coherent superpositions for times over which quantum error correction can be implemented. We experimentally demonstrate a robust quantum memory using a magnetic-field-independent hyperfine transition in $^{9}$Be$^{+} $ atomic ion qubits at a field B = 0.01194 T. Qubit superpositions are created and analyzed with two-photon stimulated-Raman transitions. We observe the single physical qubit memory coherence time to be greater than 10 seconds, an improvement of approximately five orders of magnitude from previous experiments. The probability of memory error for this qubit during the measurement period (the longest timescale in our system) is approximately 1.4~$\times$~$10^{-5}$ which is below fault-tolerance threshold for common quantum error correcting codes.   

Quantum logic in Group-II neutral atoms via nuclear-exchange interactions

The spin exchange-interaction generates an entangling quantum-logic gate, the square-root of SWAP, at the heart protocols employing single electron quantum dots. This is typically accompanied by strong Coulomb interactions and commensurate decoherence due to strong coupling of charge degrees of freedom to the noisy environment. We propose a protocol utilizing a \textit{nuclear-exchange} interaction that occurs through ultracold collisions of identical spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Group-II \textit{neutral} atoms. A natural advantage is gained by storing the quantum information in nuclear spin states with long coherence times. Unlike NMR protocols based on weak magnetic dipole-dipole interaction, the nuclear exchange interaction stems from strong s-wave scattering of electrons. Nuclear exchange is ensured by the Fermi symmetry of the overall wave function. We have studied this protocol in the context of $^{171}$Yb atoms trapped in far-off resonance optical dipole traps. Using quantum control analysis, high-fidelity operation is possible through controlled collisions in dynamically varied double-well trapping potentials.   

Quantum state reconstruction via continuous measurement

We present a new protocol for quantum state reconstruction based on weak continuous measurement of an ensemble average. This procedure applies the techniques of quantum control theory and quantum measurement theory to achieve a more efficient reconstruction than those performed using standard projective measurement techniques. This efficiency allows reconstruction of a quantum state using an single emsemble with minimal quantum backaction, setting the stage for state-based feedback control. An experimental demonstration of the technique will be presented in the context of reconstruction of the spin state of the F=3 hyperfine ground-state manifold of Cs-133 using continuous polarization spectroscopy.   

Quantum state control of atoms in microscopic optical traps

We present recent progress in loading and manipulation of neutral atoms in microscopic optical traps. Single Rb atoms are loaded into far off resonant optical traps from a background vapor of cold atoms. Tightly focused optical beams are used to perform two-photon stimulated Raman rotations between hyperfine qubit states. We demonstrate qubit rotations at a rate of 1.4 MHz, 1 ms coherence time, and individual site addressing with crosstalk at the level of $10^{-3}$. These results are a significant step towards quantum computing using optically trapped neutral atoms. We discuss work in progress aimed at observing strong, angle independent dipole-dipole interactions for fast two-qubit gates using microwave dressing of Rydberg states. We demonstrate two-photon coherent excitation of Rydberg levels by a $5s_{1/2} - 5p_{3/2} - nd_ {5/2}$ sequence. The possibility of dipole-dipole interactions without angular zeroes will be important for gates, as well as for coupling to mesoscopic qubits to enable transmission of quantum states.   

Stochastic One-Way Quantum Computing with Ultracold Atoms in Optical Lattices

The one-way model of quantum computation has the advantage over conventional approaches of allowing all entanglement to be prepared in a single initial step prior to any logical operations, generating the so-called cluster state. One of the most promising experimental approaches to the formation of such a highly entangled resource employs a gas of ultracold atoms confined in an optical lattice. Starting with a Mott insulator state of pseudospin-1/2 bosons at unit filling, an Ising-type interaction can be induced by allowing weak nearest-neighbor tunneling, resulting in the formation of a cluster state. An alternate approach is to prepare each spin state in its own sublattice, and induce collisional phase shifts by varying the laser polarizations. In either case, however, there is a systematic phase error which is likely to arise, resulting in the formation of imperfect cluster states. We will present various approaches to one-way quantum computation using imperfect cluster states, and show that the algorithms are necessarily stochastic if the error syndrome is not known.   

Generalized Coherent States via Markovian Decoherence

Coherent states were introduced in the early days of quantum physics as 'quasiclassical' quantum states of an isolated quantum system. The decoherence program defines 'quasiclassical' (or 'pointer') states as states which are most stable in the presence of a coupling with the environment. Pointer states may be identified through the extremization of a 'predictability' functional on the Hilbert space. It has been known for some time that for the harmonic oscillator both concepts coincide under very generic conditions. Coherent states have been extended in the 70s to generalized coherent states. Recently, this approach has served as the basis to define generalized entanglement and conditions for quantum complexity. Here, we investigate the stability of generalized coherent states under Markovian open-system dynamics. In particular, we identify conditions under which generalized coherent states emerge as pointer states for systems described by algebras more general that the standard oscillator algebra. We present a streamlined method to find pointer states in the weak-coupling approximation, and discuss conditions for this approximation to be valid. We find that generalized coherent states and pointer states coincide under more restrictive conditions than the canonical, harmonic-oscillator coherent states. Finally, we address the connection of generalized coherent states to noiseless subspaces and subsystems.   

Generation of Werner states via collective decay of coherently driven atoms

We demonstrate deterministic generation of Werner states as a steady state of the collective decay dynamics of a pair of neutral atom coupled to a leaky cavity and strong coherent drive. We also show how the scheme can be extended to generate $2N$-particle analogue of the bipartite Werner states.   

Two-Qubit Quantum Computing using Pulsed ESR of N@C$_{60}$

N@C$_{60}$ is a fullerene molecule containing an atom of nitrogen. The low-temperature decoherence time, $T_{2}$, can be increased to 215 $\mu $s, which is attractive for quantum information processing applications. The electronic and nuclear spins of the nitrogen atom are good quantum numbers in a strong magnetic field, coupled by the hyperfine interaction. Pulsed ENDOR (electron nuclear double resonance) can be used to initialize, manipulate and measure this two-qubit system. We used dynamic nuclear polarization (DNP) to prepare an initial state in which the nuclear and electronic spins were aligned with the applied field.   

Optimal control of logical operations in the presence of decoherence: A two-spin model

We study the feasibility of optimal control of logical operations in a simple model system composed of two interacting spins. In our model, one spin serves as a qubit and its evolution is controlled by a time-dependent external field. The other (uncontrolled) spin serves as an effective environment, coupling to which is a source of decoherence. The aim of control is to generate a target unitary operation for the qubit in the presence of the environmentally-induced decoherence. Given a target unitary operation $G$ for the system, the fidelity of the actual transformation achieved is maximized with respect to the electric field $\epsilon(t)$ using two techniques, optimal control theory (OCT) and ``pre-design'' methods, which are well-developed in the field of nuclear magnetic resonance. The primary goal of this work is to illustrate the importance of OCT in designing logical operations, especially in the presence of environmental coupling, and the inadequacy of pre-designed gates in such situations.   

Rapid State-Reduction of Quantum Systems Using Feedback Control

Many potential applications of quantum devices, particularly in information processing, require quantum systems to be prepared in pure states. Due to environmental noise quantum systems often exist naturally in mixed states, and as a result a process of cooling or measurement must be used to purify them. In this work we consider the use of measurement for this purpose. The speed with which a measurement can purify, or reduce, the state of a quantum system is determined by the interaction between the system and measuring device, and places a limit on the speed of state-preparation. Here we consider using feedback control during the measurement to increase the rate of state-reduction. It was shown in [1] that for a single qubit this rate could be increased by a factor of 2. Here we show that for higher dimensional systems feedback control can provide a much larger speed-up. In particular, we show that for a measurement of an observable with $N$ equally spaced eigenvalues, there exists a feedback algorithm which will increase the rate of state-reduction by a factor proportional to $N$. References: 1. K. Jacobs, Phys. Rev. A \textbf{67}, 030301(R) (2003). 2. J. Combes and K. Jacobs, Phys. Rev. Lett. (in press).   

Quantum Zeno stabilization in weak continuous measurement of two qubits

We have studied quantum coherent oscillations of two qubits under continuous measurement by a symmetrically coupled mesoscopic detector. The analysis is based on a Bayesian formalism that is applicable to individual quantum systems. Measurement continuously collapses the two-qubit system to one of the sub-spaces of the Bell basis. For a detector with linear response this corresponds to measurement of the total spin of the qubits. In the other extreme of purely quadratic response the operator $\sigma_y^1 \sigma_y^2 +\sigma_z^1\sigma_z^2$ is measured. In both cases, collapse naturally leads to spontaneous entanglement which can be identified by measurement of the power spectrum and/or the average current of the detector. Asymmetry between the two qubits results in evolution between the different measurement subspaces. However, when the qubits are even weakly coupled to the detector, a kind of quantum Zeno effect cancels the gradual evolution and replaces it with rare, abrupt switching events. We obtain the asymptotic switching rates for these events and confirm them with numerical simulations. We show how such switching affects the observable power spectrum on different time scales.   

Session U41: Dielectrics: Response Properties

Enhanced performance in nanotemplated dielectric structures

The dielectric properties of nanoscale metallic inclusions in insulating media are anticipated to be significantly enhanced (1,2). We have prepared such nanocomposites via electrochemical deposition of metal (gold) in polycarbonate template membranes. We have characterized the properties of these via frequency-dependent capacitance measurements, and compare our results with a theory of enhanced $\varepsilon $ ($\omega )$ at the nanoscale (2). \newline \newline - 1. J.Xu and C.P.Wong, Proceedings of the Ninth International Symposium on Advanced Packaging Materials: Process, Properties and Interfaces, Atlanta, GA, 24 March 2004 (IEEE, New York, 2004,) p.158 \newline - 2. T. Kempa, D. Carnahan etc., Dielectric media based on isolated metallic nanostructures, J.Appl.Phys, 98, 34310 (2005)   

Low temperature specific heat and Brillioun scattering in nano-oscillator arrays

We consider a large, free-standing array of coupled, planar oscillators each several hundred nanometers on a side fabricated from a single layer of dielectric. In particular, we predict the low temperature heat capacity and Brillouin scattered cross section based upon a numerical calculation of the density of states (DOS) for this nano-structured array. The DOS, which is interesting in its own right, is found to have an average value nearly independent of frequency and a number of gaps of varying depths. The predictions suggest that it should be possible to use low temperature measurements of Brillouin cross section and/or the specific heat to observe the quantum statistics obeyed by various rigid body modes of the array, some of which involve the center of mass motion of a large number of atoms. As such, these measurements would result in a considerable extension of the domain in which quantum mechanics has been tested.   

Determining the quantum phase coherence time of a NEMS resonator

Recently steps have been made toward characterizing macroscopic quantum behavior in nanoelectromechanical devices (NEMS), particularly resonators with large frequencies and high Q factors. While the quantum phase coherence time as well as energy relaxation time of NEMS resonators are believed to be long, this cannot be tested directly using standard techniques. Using formalism typically found in quantum computation, we propose a procedure for directly measuring both phase coherence and energy relaxation times of NEMs resonators by coupling them to anharmonic Josephson junction devices. We hope that using this proposed method experiments will verify current models of decoherence in harmonic oscillator systems.   

Tunable Electronic Energy Transfer in Layered Inorganic Solids Codoped with Tb$^{3+}$ and Eu$^{3+}$

Ln[M(CN)$_2$]$_3$ systems (Ln=trivalent rare earth, M=Au,Ag) have a layered structure consisting of alternating layers of the M(CN)$_2^-$ ions and Ln$^{3+}$ ions. Our past studies of tunable energy transfer have focused on transfer from Au(CN) $_2^-$ or Ag(CN)$_2^-$ donor ions to a variety of rare earth ions, including Tb$^{3+}$ and Eu$^{3+}$. Most recently, we have characterized systems with mixed metal donors, such as La[Ag$_ {.5}$Au$_{.5}$(CN)$_2$]$_3$, La[Ag$_{.75}$Au$_{.25}$(CN)$_2$] $_3$, and La[Ag$_{.9}$Au$_{.1}$(CN)$_2$] $_3$. We have found that these systems exhibit ``tunability'' of emission energy due to the variation of the donor emission associated with varying the Ag/Au ratio. Tunability also occurs with changing temperature and excitation wavelength. Also, the steady-state luminescence spectra of these compounds reveals that they are strongly luminescent at room temperature, in contrast to the corresponding La[Ag(CN)$_2$]$_3$ and La[Au(CN) $_2$]$_3$ pure metal systems. Results will be presented from a new series of samples incorporating both Tb$^{3+}$ and Eu$^{3+}$ as acceptors. The tunability of the donor's emission wavelength allows for changes in the spectral overlap with each of the two donors. Preliminary results show variation in the rare earth (acceptor) emission with changing temperature and excitation wavelength, indicating the possibility of tuning the energy transfer off of one acceptor and onto the other.   

EPR and ENDOR of two axial Fe$^{3+}$ centers in stoichiometric lithium tantalate

The determination of structures of centers in oxide crystals created by impurity ions, including those of transition metals and rare- earth elements, is one of the most important tasks in defect study. The iron ions play a key-role in the photovoltaic and photorefractive effects, holographic records and many other physical properties of lithium tantalate. The axial Fe$^{3+}$ center, Fe1 with the crystal field parameter b$_{2}^ {0}$ $\approx $ 0.313 1/cm is well studied in congruent lithium tantalate crystals. Using the EPR we have discovered and investigated a new axial Fe$^{3+}$ center, Fe2 in stoichiometric samples prepared by vapor transport equilibrium treatment. The crystal field parameter of the Fe2 center b$_{2}^{0} \quad \approx $ 0.205 1/cm is significantly smaller than for Fe1. The ENDOR measurements have shown that hyperfine interactions of the Fe$^{3+}$ electrons with the surrounding Li nuclei for Fe2 are stronger than for Fe1. Therefore, the conclusion was made that in the case of Fe2 center the iron ion substitutes for Ta and has Li nuclei in the nearest neighborhood, whereas in the case of Fe1 it substitutes for Li, has Ta nuclei as nearest neighbors and Li nuclei in the second shell only.   

Near infrared emission properties of Nd doped Potassium Lead Halides

The incorporation of rare earth (RE) ions into host materials with low maximum phonon energies provides opportunities for improved RE infrared emission properties. In this work, we evaluated the IR emission from Nd doped potassium lead halides, namely Nd: KPb$_{2}$Cl$_{5}$ and Nd: KPb$_{2}$Br$_{5}$, for possible applications in IR lasers and optical communications. Both halides are nearly non-hygroscopic and have low maximum phonon energies, which reduces non-radiative decay rates through multi-phonon relaxations. Following optical excitation at 800nm, near IR emission bands were observed from the $^{4}$F$_{5/2}$ and $^{4}$F$_{3/2}$ excited states of Nd$^{3+}$. The $^{4}$F$_{5/2}$ level was strongly quenched through non-radiative processes in Nd: KPb$_{2}$Cl$_{5}$. On the contrary, for Nd: KPb$_{2}$Br$_{5}$ the $^{4}$F$_{5/2}$ was highly radiative with an emission efficiency of $\sim $50{\%}. More detailed results of the material synthesis, purification, steady-state and time-resolved emission spectroscopy will be presented at the conference.   

Non-resonant Inelastic X-Ray Scattering and Energy-Resolved Wannier Function Investigation of Local Excitations in Transition Metal Monoxides NiO and CoO

Non-resonant inelastic x-ray scattering (NIXS) and energy- resolved Wannier function analysis have been used to probe the strongly correlated electronic structure of NiO and CoO. NIXS measurements of the dynamical structure factor s(q,w) as a function of momentum transfer q and frequency w have shown that dipole-forbidden, d-d excitations appear within the optical gap for large wavevectors (q $>$ 2/A), become the dominant structure in the loss spectra for q $>$ 3/A, and reach a maximum at q $\sim$ 7/A. In contrast to the loss-spectra observed in resonant-probe studies of NiO and CoO, non-resonant spectra show only two excitations that are highly anisotropic - strongest in the [111] direction and weakest (or missing) in the [001] direction. Energy-resolved Wannier function analyses of vertex matrix elements within LDA+U demonstrate that the anisotropy provides a sensitive measure of electronic symmetry-breaking in these atomic-like d-d excitations as a result of point-group symmetry selection rules.   

Self-consistent linear density response within the LDA+U method: Application to transition-metal oxides(*)

We formulate a scheme to calculate self-consistently the dynamical linear density-response function based on correlated LDA+U theory. The orbital dependent V$_{U}$ term in the Kohn- Sham potential, leads to an additional self-consistent condition in the density fluctuations. The end result is a density response function which includes electron-hole interactions (that is, it goes beyond the random-phase approximation). We assess the performance of our scheme by calculating the electron-hole excitation spectrum of prototype transition metal oxides for arbitrary momentum transfers. (*) DOE-CMSN PCSCS collaboration. (1) Supported by NSF ITR-DMR 0219332. (2) Managed by UT-Battelle for the U.S. DOE under contract DE- AC05-00OR22725. (3) Supported by DOE Grant DE-FG03-01ER45876   

Determining the Critical Dose Threshold of Electron-Induced Electron Yield for Minimally Charged Highly Insulating Materials

When incident energetic electrons interact with a material, they excite electrons within the material to escape energies. The electron emission is quantified as the ratio of emitted electrons to incident particle flux, termed electron yield. Measuring the electron yield of insulators is difficult due to dynamic surface charge accumulation which directly affects landing energies and the potential barrier that emitted electrons must overcome. Our recent measurements of highly insulating materials have demonstrated significant changes in total yield curves and yield decay curves for very small electron doses equivalent to a trapped charge density of $<$10$^{10}$ electrons /cm$^{3}$. The Chung-Everhart theory provides a basic model for the behavior of the electron emission spectra which we relate to yield decay curves as charge is allowed to accumulate. Yield measurements as a function of dose for polyimide (Kapton$^{TM})$ and microcrystalline SiO$_{2}$ will be presented. We use our data and model to address the question of whether there is a minimal dose threshold at which the accumulated charge no longer affects the yield.   

Comparison of Methods for Determining Crossover Energies in Insulators

When a material is irradiated with energetic particles electrons can be emitted from the material. For electron-induced emission, the number of electrons that leave a particular material depends on the incident energy of the electrons, among other things. There are two critical energies where the ratio of emitted electrons to incident electrons crosses unity, called crossover energies. Measurements of the absolute total yield, secondary electron emission spectra, and sample and detector currents are used for a variety of methods to determine first and second crossover energies of both conductors and insulators. Precision is discussed for the following methods: i) Total Yield Curve, ii) Backscattered-to-Secondary Yield Ratio, iii) Mirror Potential, iv) Emission Spectral Shift, and v) Sample Null Current. Also, theoretical models for the Emission Spectral shift and Sample Null Current methods will be discussed. This work was funded by the NASA Space Environments and Effects Program, a Willard L. Eccles Undergraduate Research Fellowship, and a USU Undergraduate Research and Creative Opportunities award.   

Piezoelectric Coupling in Non-piezoelectric Materials due to Nonlocal Size Effects at the Nanoscale: Fundamental Solutions, Embedded Inclusions and Piezoelectric Composites without Electromechanical Constituents

In a piezoelectric material an applied \textit{uniform} strain can induce an electric polarization (or vice-versa). Crystallographic considerations restrict this technologically important property to non-centrosymmetric systems. It has been shown both mathematically and physically, that a \textit{non-uniform strain} can potentially break the inversion symmetry and induce polarization in non-piezoelectric materials. The coupling between strain gradients and polarization; and strain and polarization gradients, is investigated in this work. Based on a field theoretic framework accounting for this phenomena, we (i) develop the fundamental solutions (Green's functions) for the governing equations (ii) solve the general embedded inclusion problem with explicit results for the spherical and cylindrical inclusion shape and, (iii) Illustrate using the simple example of a bilaminate how an apparently piezoelectric composite may be created without using constituent piezoelectric materials.   

Ionic-Mode Contributions to the Refractive Index of Glasses

The refractive index of materials transparent in the visible is commonly much smaller in the infrared than at shorter wavelengths because low-frequency ionic polarization lags the \textbf{E} field of higher-frequency light. This is often described by an IR Sellmeier term in empirical index fits. However, this separation of ionic and electronic contributions is not unique. A unique separation is given by Taylor expansion of the K-K relations in the region of transparency that yields a Laurent series in photon energy squared as the IR contribution. The coefficients are odd moments of the IR extinction coefficient. We studied this for vitreous silica and Corning ULE glass (92.5 {\%} SiO$_ {2}$ + 7.5{\%} TiO$_{2})$. While the oscillator strength of the IR modes is four orders of magnitude less than that of the electronic transitions, the IR contribution to the index is comparable to the electronic contribution in the IR. In our examples, IR terms are sufficiently negative to bring the total index well below unity (but greater than zero) between 7 to 9 $\mu $m.   

Enhanced fluorescence in rare earth doped sol-gel glasses containing Al$^{3+}$

Sol-gel synthesis is a low temperature method for preparing rare earth (RE) doped glass. Aluminum is often used as a co-dopant because it increases fluorescence yield from RE's. There is a tendency for RE ions to form clusters in the sol-gel preparation, facilitating inter-ion interactions and fluorescence quenching from cross relaxation. It is generally believed that Al prevents clustering of RE ions, but recent work questions this long-established role of Al. We report on spectroscopic investigations of energy transfer in Tb- and Eu- doped glasses that probe the effect of Al co-doping. Pulsed laser excitation of $^{5}$D$_{3}$ fluorescence is used to measure Tb-Tb cross-relaxation rates. In materials containing Al, cross-relaxation occurs in concentrations much lower than 0.1{\%}Tb, indicating that RE clustering persists in glasses with Al. Line narrowing experiments in dilute Eu glasses confirm that ions remain clustered in co-doped samples. Our results point toward a model of aluminum rich regions in the glass that attract and confine RE ions. Reduced association with OH$^{-}$ in the confined regions causes RE fluorescence enhancement.   

Site-specific modification of oxide nanoclusters: Towards atomic-scale surface structuring

Atomic emission from MgO and CaO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. Using selective 4.66 eV laser excitation of nanocrystalline thin films and nanocube metal oxide samples we have recorded a unique pattern of hyperthermal atomic desorption. Not only neutral O-atoms, but neutral Mg-atoms, with hyper-thermal kinetic energies in the range of 0.1--0.4 eV are readily observed. Our \textit{ab initio} calculations suggest that metal atom emission is induced predominantly by electron trapping at surface 3-coordinated metal sites followed by electronic excitation at these sites- an `electron plus an exciton' mechanism. The proposed elementary mechanism involves both sequential excitation and localization of excitons as well as electrons and holes at 3-coodinated surface sites. This mechanism differs from all previously suggested mechanisms for desorption induced by electronic transitions. This desorption process serves as an example of atomic scale modification of a nanostructured metal oxide using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material.   

Optical Spectroscopy of Low-k Dielectric Films

Low-k dielectric materials based on porous carbon-doped oxides are widely used in the microelectronics industry. Despite their importance, relatively little is known about their spectroscopic properties. In this paper we report results of two classes of optical spectroscopy measurements, absorption spectroscopy and photocurrent spectroscopy. Optical absorption spectroscopy has been performed on various thin-film low-k materials. These measurements show the presence of strong optical absorption in the ultraviolet and yield the effective band gap of the medium. Photocurrent spectroscopy has been performed on films of low-k material deposited on both Si and metallic substrates using a transparent counter-electrode. A well-defined spectral dependence of the photocurrent efficiency is observed. The data provide information on the band offsets of the low-k materials, parameters that play a crucial role in models of electrical conduction.   

Session U42: Focus Session: Simulations of Matter at Extreme Conditions II

Condensed Matter and its Orderings: The Pressure Variable.

Advances in experimental high pressure condensed matter physics have led to near order-of-magnitude static isothermal compressions (high densities are also realizable in dynamic compressions). In sufficiently light systems the new realms of density have associated zero-point effects, which are substantial. Also generally anticipated are significant changes in electronic structure (band widths not always increasing with density, however) and effective state dependent interparticle potentials. Propitious use of the pressure variable can elucidate the many-body problem in incisive ways through new orderings including structural, magnetic and especially superconducting. Significant challenges to theory include the appearance at higher densities of exceedingly complex structures in systems hitherto regarded as 'simple'. Invasion of the valence electron domain by core space appears to impel an interesting clash of length scales. Not unrelated to the rise of quantum effects is the possibility (and even observation) of depression of melting points in low mass systems. Hydrogen, a tenacious insulator (but now a decreasingly reluctant alkali) remains a candidate for significant superconductivity in a metallic state, one which may also be manifested as a quantum liquid. In combination with other light elements further orderings are also predicted, but at pressures less than anticipated for pure hydrogen itself. For the future, this area of experimental investigation appears to be ideally matched to advanced electronic structure and simulation techniques.   

The Nature of the Hydrogen Plasma Phase Transition

We present details of a study of pure hydrogen fluid at high pressure. Using the Coupled Electron-Ion Monte Carlo (CEIMC) method [1,2], a quantum Monte Carlo scheme capable of accurately simulating systems at low temperature, we study the nature of the plasma phase transition (PPT): the mechanism by which a molecular to non-molecular transformation occurs under increasing pressure. We find no evidence for a first-order PPT. The CEIMC method centers on exploring the nuclear configuration space (classically or with quantum path integrals) using a modified Metropolis algorithm. Configurational energy differences are computed within the Born-Oppenheimer (BO) approximation using accurate ground-state quantum Monte Carlo techniques. \\ \\ 1. D. Ceperley, M. Dewing and C. Pierleoni, in Bridging Time Scales: Molecular Simulations for the Next Decade, eds. P. Nielaba {\it et al}, Springer-Verlag, pgs. 473-500 (2002). \\ 2. C. Pierleoni, D. M. Ceperley and M. Holzmann, Phys. Rev. Lett. {\bf 93}, 146402 (2004)   

Developments in the path integral Monte Carlo method for simulating fluids under extreme conditions

We summarize a number of improvements we have developed for the quantum simulation of fluids under extreme conditions with path integral Monte Carlo (PIMC). PIMC provides way to combine fully-correlated quantum effects with thermal fluctuations in a natural formalism by sampling the many-body thermal density matrix. These developments include the construction of accurate pseudohamiltonians and their incorporation into PIMC, computation of high-accuracy pair density matrices, improved optimization of the long/short-range breakup, a fast embedded band-structure calculation for the fermion nodal restriction, Brillouin-zone integration through twist-averaged boundary conditions, and coupled PIMC/Langevin dynamics. We present preliminary results for the simulation of sodium near its liquid/vapor critical point.   

Vibron Dynamics of Hydrogen at High Pressures and Temperatures

There is currently great interest in the behavior of molecular hydrogen at high pressures and temperatures. The van Kranendonk theory of vibrons in solid hydrogen has been used previously to provide a description of the Raman response as a function of pressure and para-ortho concentrations at low temperature. Here we apply the same model to very different environments, namely to the solid at high P-T conditions, and, with less justification, to the dense fluid. The effect of temperature is presumed to be to renormalize the hopping parameter. Within our model of the vibrons and approximate harmonic lattice dynamics, a $1/R^6$ dependence of the hopping parameter on intermolecular distance, $R$, gets averaged over fluctuations in the interatomic distance, and the average increases with temperature. Preliminary results using configurations obtained from hybrid path integral molecular dynamics calculations with empirical potentials suggest that there is very little change in the Raman peak upon melting at high pressure, in agreement with previous high P-T measurements.   

A systematic search method for finding new high-pressure phases of polymeric nitrogen

A recent discovery of single-bonded polymeric form of nitrogen in diamond anvil cell high pressure experiments [Nature Mat. \textbf{3}, 558 (2004)] opens a new promising direction in the development of high energy density materials. Besides the cubic gauche phase of polynitrogen stabilized in experiment, other yet unidentified metastable phases could emerge under certain experimental conditions. We present a systematic search method for finding metastable phases of single bonded nitrogen based on a set of Peierls distortions of a given reference structure. Using the most basic reference structure, a simple cubic unit cell, our approach not only reproduces all the single-bonded nitrogen phases reported so far, but also reveals many new metastable structures with promising properties. The equations of state of the structures calculated at the first-principles level are studied over a broad range of pressures up to 300 GPa. The stability of the structures is analyzed using directly calculated phonon spectra. This approach can be extended using more complex reference structures and relaxing the constraint of a pure single bonded phase.   

Complexity and Pressure Induced Fermi Surface Deformation in Lithium

Recently reported structural complexity and high temperature superconducting transition in lithium under pressure has increased the interest in light alkalis, otherwise considered as simple and well known systems under normal conditions. In this work we present an analysis of the pressure induced Fermi surface deformation in lithium and its relation to the observed complexity. According to our calculations, the Fermi surface becomes increasingly anisotropic with pressure and at around 8 GPa it contacts the Brillouin zone boundary, which preludes the bcc to fcc phase transition. Furthermore, at around 30 GPa, besides the increasing necks in the Fermi surface along the fcc $\Gamma $L direction, it develops an extended and well defined nesting in the $\Gamma $W direction, which enhances the electronic response for the nesting momentum and induces an strong phonon softening along the $\Gamma $K. The increasing electron-phonon coupling associated to this softening, besides preluding the transition to complex structures, also provides a better understanding of the observed superconducting transition in lithium at around the same pressure range.   

Prediction of a superionic phase of hydrogen fluoride (HF) at high temperature and pressure

We report first principles simulations of hydrogen fluoride. Ab initio molecular dynamics simulations of HF were conducted at densities of 1.8 -- 4.0 g/cc along the 900 K isotherm. At experimentally observable conditions, we find a transition to a superionic phase, in which the fluorine ions exhibit a stable lattice and the hydrogen ions exhibit rapid diffusion. This phase is similar to the recently reported superionic phase in water, in that there is a symmetrization of the hydrogen bond, and we observe a transient partially covalent network at pressures greater than 66 GPa. In addition, we describe a mechanism for hydrogen diffusion through the fluorine sub- lattice. Our results provide evidence that superionic solids are prevalent in solids that manifest low temperature symmetric hydrogen bonding. The pressures needed to induce superionic diffusion in HF are significantly lower than what is required for other known superionic hydrides, and thus will permit much more extensive experimental studies of this exotic phase.   

High-pressure and high-temperature phases of nitrous oxide

The phase diagram of nitrous oxide (N$_{2}$O) is investigated up to 50 GPa and 1000 K using first principles theory. The calculated stability and properties of numerous crystalline structures are compared with experimental results. We identify the structure of phase II of N$_{2}$O. On the basis of its stability with respect to orthorhombic deformations, an explanation for measured Raman spectra is provided. Similarly to CO$_{2 }$[1], crystalline structures with bent molecules are found to be extremely unfavorable energetically. \newline \newline [1] Bonev et al., Phys. Rev. Lett. 91, 065501 (2003).   

Mechanical strength and coordination defects in compressed silica glass

Contrary to ordinary solids, which are normally known to harden by compression, the mechanical strength of compressed SiO$_{2}$ glass shows a minimum around 10 GPa. Around this pressure, the compression of silica glass undergoes a change from purely elastic to plastic, leading to the recovery of a densified amorphous polymorph. The compressibility of silica glass is also anomalous, with a maximum at about 2-4 GPa. Despite the large pressure difference between the onset of the two anomalies, microscopic theories have traditionally attempted to explain both anomalies with the pressure induced appearance of coordination defects. Such models are seriously questioned however by the lack of evidence for coordination defects below 10 GPa, in Raman and NMR experiments. Here we show, using an improved interatomic potential for SiO$_{2}$, that a correct description of the pressure-induced appearance of five-fold coordination defects in silica glass is crucial to address the above phenomenology and to obtain a theoretical model consistent with experiments.   

USPEX - Predicting crystal structures of new phases

We have developed an very efficient and reliable method for crystal structure prediction [1], merging an evolutionary algorithm, based on local optimization and spatial heredity, with \textit{ab initio} total-energy calculations. This method allows one to predict the most stable crystal structure and a large number of robust metastable structures for a given compound at any $P-T$ condition, without requiring experimental input. The success rate is extremely high -- USPEX succeeded in all of the 25 tests performed so far, including ionic, covalent, metallic, and molecular structures with up to 20 atoms per unit cell. Using this methodology we have succeeded in predicting hitherto unknown structures [2]. Implementation of the algorithm, several applications and physical reasons for its success will be discussed. [1] Glass C.W, Oganov A.R., Hansen N. (2005). USPEX: a universal structure prediction algorithm. \textit{In prep.} [2] Oganov A.R., Glass C.W., Ono S. (2005). High-pressure phases of CaCO$_{3}$: crystal structure prediction and experiment. \textit{Earth Planet. Sci. Lett, in.press.}   

Session U43: Focus Session: Novel Phases in Low Dimensional Quantum Gases

Low-Dimensional Fermi Gases

Optical lattices are a powerful tool to create novel many-body quantum systems with ultracold atoms. They allow to study the role of interactions in the system in reduced dimensions. We have observed two-particle bound states of atoms confined in a one-dimensional matter waveguide. These bound states exist irrespective of the sign of the scattering length, contrary to the situation in free space. The strongly interacting one- dimensional Fermi gas which we create in an optical lattice represents a realization of a tunable Luttinger liquid. In a spin-polarized Fermi gas interacting via a p-wave Feshbach resonance the strong confinement allows us to restrict the asymptotic scattering states. When aligning the spins along (or perpendicular to) the axis of motion in a 1D gas, scattering into channels with the angular momentum projection of |m |=1 (or m=0) can be completely suppressed.   

Empirical manifestations of integrability in cold quantum gases

Integrable quantum many-body systems traditionally belong to the domain of mathematical physics, with little or no connection to experiments. However, the experiments on confined quantum-degenerate gases has recently yielded faithful realizations of a number of integrable systems, thus making them phenomenologicalily relevant. We show that the presence of few-body conserved quantities in a quantum system leads to dramatic, initial-state-dependent discrepancy between the state of the system after relaxation and the predictions of thermodynamics. Using the newly introduced concept of constrained thermal equilibrium we study quantitatively the effects of the memory of the initial conditions. As objects of study we choose bosons in one-dimensional optical lattices in the deep Mott regime and spin-$0$ Bose gases confined to waveguides, both of which have been experimentally realized already. We suggest momentum distribution and chemical composition as the simplest experimental observables sensitive to the effects of integrability. Overall, we argue that the kinetic and thermodynamic properties of integrable quantum gases are so different from the usual, that they well-qualify for a new state of quantum matter.   

Noise and counting statistics in one-dimensional insulators

We discuss the correlation properties of current carrying states of one-dimensional insulators, which could be realized by applying an impulse to atoms loaded onto an optical lattice. While the equilibrium noise has a gapped spectrum, the quantum uncertainty encoded in the amplitudes for the Zener process gives a zero frequency contribution out of equilibrium. We derive a general expression for the generating function of the full counting statistics and find that the particle transport obeys binomial statistics. Finally, we discuss the extent to which the technique employed in a recent experiment (Phys. Rev. Lett. 95, 090404 (2005)) can be considered an ideal measurement of counting statistics.   

Dynamics of reduced dimension Bose gases in optical lattices

We use deep optical lattices to tightly confine cold atoms in reduced dimensions. By applying shallower optical lattices in the weakly confined direction, we realize well-characterized one- and two- dimensional Bose-atom lattice gases. Transport dynamics is studied by observing motion of the atom cloud through the lattice. For a 1D quantum degenerate Bose gas, we report the observation of strongly damped dipole oscillations in a combined harmonic and optical lattice potential. Damping is significant for very shallow axial lattices (0.25 photon recoil energies), and increases dramatically with increasing lattice depth, such that the gas becomes nearly immobile for times an order of magnitude longer than the single-particle tunneling time. Surprisingly, we see no broadening of the atomic quasimomentum distribution after damped motion.   

The superfluid-Mott insulator transition in 2D

Ultra-cold atoms in optical lattices have been exploited to study the Mott-insulator transition in 1, 2, and 3 dimensions; here focus on the 2D Mott-insulator transition. Initially Bose-condensed rubidium atoms are loaded into a 3D optical lattice with an average occupancy of one atom per-site. By making one lattice much deeper in one direction than the remaining two, we construct an ensemble of 2D lattice systems. These 2D systems exhibit a superfluid-insulator transition as the lattice depth is increased. In this talk I present new measurements that show that even when the conventional signature of long-range order (namely diffraction) disappears, the system is not a perfect insulator -- partially responding to an impulsive force.   

Crystalline phases of bosons in rotating traps: Tonks-Girardeau gas on a ring.

We analyze the systems of strongly repelling bosons in two-dimensional harmonic and ring-shaped traps as a function of the rotational frequency of the trap for neutral atoms (and of an applied magnetic field for charged bosons). Our two-step approach consists of breaking the rotational symmetry at the Hartree-Fock level and of subsequent symmetry restoration via projection techniques, \footnote{Phys. Rev. Lett.{\bf 93}, 230405 (2004)} thus taking into account correlations beyond the Gross-Pitaevskii (GP) solution. The bosons localize and form crystalline patterns both for a repulsive contact potential and a Coulomb interaction, as revealed via conditional probability distribution (CPD) analysis. This behavior of the bosons in the ring-shaped traps in the strong repulsion limit is similar to the behavior of fermions and is a manifestation of the fermionization phenomenon. We present calculations for the ground state energies as a function of the rotational frequency (or the strength of the magnetic field) and as a function of the repulsion strength.   

Ordering and Entropy Production Across Qauntum Phase Transitions

We consider the transverse field Ising spin chain swept through a quantum critical point from the disordered to the ordered phase (and vice versa) and present exact results on the ordering and entropy production. Prepared in the ground state of the initial Hamiltonian, the system evolves to a state characterized by a non-equilibrium distribution of excitations of the final Hamiltonian. We show that the evolved system, while described by a pure many-body state, possesses finite entropy if considered ``locally.'' The notion of local entropy is defined by coarse-graining in momentum space, and is linked to the properties of the system of Kibble-Zureck domain walls. Exact results obtained for the spin correlation functions are presented and used to elucidate the relationship with the Kibble-Zureck theory of critical dynamics. Possible manifestations in ultracold atoms trapped in optical lattices will be discussed.   

Spin 1/2 fermions on spin-dependent optical lattices

We study the phase diagram of one-dimensional two-component (i.e. pseudo-``spin''-1/2) ultracold atomic Fermi gas. The two atom species can have different hopping or mass. A very rich phase diagram for equal densities of the species is found, containing Mott insulators with various quasi-long-range-order, superfluids and perhaps phase separation. We also discuss coupling such 1D systems together and the experimental signatures of the phases. In particular, we compute the ``spin''-structure factor at small momentum, which should reveal a ``spin'' gap.   

Quartetting and pairing instabilities in one dimensional spin 3/2 fermionic systems

Novel competing orders are found in spin 3/2 cold atomic systems in one-dimensional optical traps and lattices. In particular, the quartetting phase, a four-fermion counterpart of Cooper pairing, exists in a large portion of the phase diagram. The transition between the quartetting and singlet Cooper pairing phases is controlled by an Ising symmetry breaking in one of the spin channels. The singlet Cooper pairing phase also survives in the purely repulsive interaction regime. In addition, various charge and bond ordered phases are identified at commensurate fillings in lattice systems.   

Spinfull bosons in an optical lattice

We have studied the phase diagram of spinfull bosons in a one-dimensional optical lattice, using DMRG. Correlation functions and the dimerization are calculated. We also present results for energy gap to excited states and magnetization energies. We confirm the expected phase diagram with a dimerized phase in the insulating regions with an odd density and on-site singlets in the other insulating systems.   

Session U44: Models of Strongly Correlated Electrons

First-principles calculations of magnetic transition temperatures

We introduce a method to evaluate magnetic transition temperatures of strongly correlated systems. It is based on a combination of dynamical mean field theory, the local density functional theory, and employs ``magnetic force theorem'' for evaluating exchange constants. The method automatically predicts at a given temperature whether the system is ordered or disordered magnetically. We illustrate the approach on several systems, and discuss its accuracy in comparison with the experiment. The effect of electron-electron correlations on these predictions will be discussed.   

Electron-phonon-coupling-driven pairing symmetry transition in a ladder

We address the effects of electron-phonon coupling in the electron-electron interacting ladder using the recently developed functional renormalization group method, in which the full retardation effects can be taken into account impartially\footnote{S.-W. Tsai, A.H. Castro Neto, R. Shankar, D.K. Campbell, Phys. Rev. B 72, 054531 (2005)}. We study the doped Holstein-Hubbard ladder as a typical example and show that there is a transition between s-wave and d-wave pairing as a function of electron-phonon coupling and doping level. This contrasts with recent results from a two-step renormalization group, which suggest that the electron-phonon coupling only contributes in a subdominant fashion and that the spin-gapped pairing phase always has d-wave symmetry, unchanged from the doped Hubbard ladder without electron-phonon interaction\footnote{Alexander Seidel, Hsiu-Hau Lin, Dung-Hai Lee, Phys. Rev. B 71, 220501 (2005)}.   

Dynamical Mean Field Study of the Extended Hubbard Model

The competition between on-site $U$ and intersite $V$ repulsion in the extended Hubbard model drives ground state phase transitions between spin density wave (sdw) and charge density wave (cdw) phases. While it was originally thought that in one dimension the sdw-cdw transition was first order at strong coupling and second order at weak coupling, it is now known that for small $U$ and $V$ a bond ordered wave phase intervenes. Here we present studies of the extended Hubbard model using the dynamical cluster approximation. We study the influence of $V$ on the CDW and SDW transition temperatures, $T_c$ and $T_N$. We find $T_N$ is almost unmodified in the sdw region, even if $J=4t^2/(U-V)$ is strongly modified. We also study the effect of V on the pairing interaction and the pseudogap.   

Exact transformation for spin-charge separation of spin half fermions

We demonstrate an exact local transformation which maps a purely Fermionic manybody system to a system of spinfull Bosons and spinless Fermions, demonstrating a possible path to a non-Fermi liquid state. We apply this to the half-filled Hubbard model and show how the transformation maps the ordinary spin half Fermionic degrees of freedom exactly and without introducing Hilbert space constraints to a charge-like ``quasicharge'' fermion and a spin-like ``quasispin'' Boson while preserving all the symmetries of the model. We present approximate solutions with localized charge which emerge naturally from the Hubbard model in this form. Our results strongly suggest that charge tends to remain localized for large values of the Hubbard U. The results suggest that checkerboard patterns are natural patterns that result for the strongly interacting Hubbard model away from half filling.   

Dynamical Cluster Approximation Study of Ferromagnetism in the Periodic Anderson Model

The ferromagnetic phase in the strong coupling limit of the Periodic Anderson Model (PAM) is not fully understood. Previous studies using the Dynamical Mean Field Approximation (DMFA) (Tahvildar-Zadeh {\it et al.} (PRB {\bf 55}, R3332 (1997)) pointed to the importance of a ferromagnetic mechanism other than RKKY at conduction-band fillings near a quarter. This mechanism is related to the formation of a charge-density wave of the conduction electrons. However, it is questionable whether or not this effect persists when non-local correlations are incorporated into the theory. We try to answer this question by performing parametric studies on the phase diagram, the RKKY coupling, and the charge and spin susceptibilities for a three-dimensional system using the Dynamical Cluster Approximation (DCA).   

Phase Diagrams of Hubbard Models with Small Unit Cells

We present a controlled approach to the low temperature phase diagram of highly inhomogeneous Hubbard models in the limit of small coupling, $t'$, between clusters. We apply this to the dimerized and checkerboard models with any strength of $U$. The dimerized model is found to behave like a doped semiconductor, with a Fermi-liquid groundstate with parameters ({\it e.g.} the effective mass) which are smooth, and unspectacular functions of $U$. By contrast, the checkerboard model has a Fermi liquid phase at large $U > U_c = 4.67$, a d-wave superconducting state with a full gap for $U_c > U > 0$, and a narrow strip of an intermediate d-wave superconducting phase with gapless ``nodal'' quasiparticles for $|U - U_c| < {\cal O}(t^\prime)$.   

Effects of interplay between electron-electron and electron-phonon interactions in two-dimensional systems

We study the two-dimensional Hubbard model in the presence of electron-phonon interaction which is integrated into an effective electron-electron coupling producing a retardation effect [S.-W. Tsai {\it et al} Phys. Rev. B {\bf 72}, 054531 (2005)]. We work in the context of the functional renormalization group method [R. Shankar, Rev. Mod. Phys. {\bf 66}, 129 (1994)] to one loop accuracy, where self-energy corrections are included, and investigate the effect that isotropic and anisotropic phonons have near van Hove band fillings. This approach conveniently takes into consideration the effect of phonons at every step of the renormalization group method. We focus on the two-patch and many-patch schemes, where the Fermi surface is subdivided into two or multiple patches that label the electrons involved in each interaction process and produce a phase diagram for the different instabilities associated with the Hubbard model. We also depart from half-filling and investigate various specific cases of momentum transfer as well.   

Nearest-Neighbor Repulsion and Competing Charge and Spin Order in the Extended Hubbard Model.

We generalize the Two-Particle Self-Consistent (TPSC) approach to study the extended Hubbard model where the nearest-neighbor interaction $V$ is present in addition to the local interaction $U$. Our results are in good agreement with available Quantum Monte-Carlo results over the whole range of density $n$ up to intermediate coupling. As a function of $U, V$ and $n$ we observe different kinds of charge and spin orders, like commensurate/incommensurate charge and spin density wave, phase separation, and ferromagnetic order. For attractive $V$ superconductivity could exist in the regions where the other types of charge and spin orders do not dominate. Ref.: B. Davoudi and A.-M.S. Tremblay, cond-mat/0509707   

Quantum Monte Carlo Study of the Triangular Hubbard Model

We study the Hubbard model on a triangular lattice using Determinant Quantum Monte Carlo and the Dynamic Cluster Approximation. We compare the spin, charge and pairing response functions obtained with the two methods as a function of spatial lattice and cluster size, and also compute the one particle spectrum. We examine the possibility of charge ordering at one third filling driven by the avoidance of magnetic frustration.   

Theory of Cu K-edge Resonant Inelastic X-ray Scattering in Cuprates.

Resonant inelastic x-ray scattering (RIXS) has received much attention as a powerful technique to investigate elementary excitations in strongly correlated electron systems. In particular, the momentum-dependent spectra of Cu K-edge RIXS in high-Tc cuprates have been obtained by several experimental groups. The knowledge of these excitations across the gap as well as single-particle excitations is of importance for understanding the electronic properties in cuprates. We demonstrate theoretically momentum dependences of the RIXS in insulating and doped cuprates. The RIXS spectra are calculated by using the exact diagonalization techniques on small clusters in two-dimensional Hubbard models with 1s-core bands. In the insulating case, we find the anisotropic momentum dependence in the RIXS spectrum. The dependence is explained by the particle-hole excitations in which the antiferromagnetic correlation of the ground state plays a crucial role. Upon hole-doping, the spectrum from the lower Hubbard band to the upper Hubbard band becomes broad and less momentum dependent. This is in contrast to the case of electron-doping, where the momentum dependence of the spectrum in the undoped system remains, except that along the $<$1,0$>$ direction. The difference in the spectra between hole- and electron-doped systems follows the carrier-dependence of short-range AF spin correlations.   

Fermi Surface of the Half Heusler Compounds Ce$_{1-x}$La$_{x}$BiPt

We report on the Fermi surface in the correlated half-Heusler compound Ce$_{1-x}$La$_{x}$BiPt. In CeBiPt we find a field-induced change of the electronic band structure as discovered by electrical-transport measurements in pulsed magnetic fields. For magnetic fields above $\sim$25~T, the charge-carrier concentration determined from Hall-effect measurements increases nearly 30\%, whereas the Shubnikov--de Haas (SdH) signal disappears at the same field. In the non-$4f$ compound LaBiPt the Fermi surface remains unaffected, suggesting that these features are intimately related to the Ce 4$f$ electrons. Electronic band-structure calculations point to a $4f$-polarization-induced change of the Fermi-surface topology. In order to test this hypothesis, we have measured the (SdH) signal in a Ce$_{0.95}$La$_{0.05}$BiPt sample with a low La concentration.   

Charge Physics of the Falicov-Kimball Model

The charge-ordered phases of the one-dimensional Falicov-Kimball model are examined using a non-perturbative representation of the itinerant electrons. An effective model allows us to understand the competition between phase separation and long-range order. We construct a ground-state phase diagram for partial band-filling. The addition of an on-site hybridization potential is found to significantly alter the form of the phase diagram, with the appearance of mixed-valence phenomena.   

Application of LDA+DMFT to systems near volume collapse transition

The physical origin of the volume collapse transition in Cerium and related materials will be addressed. Using recently developed self-consistent LDA+DMFT method, we will show that the Kondo Volume collapse model, involving both the f and spd electrons, describes the optical data better than a Mott transition picture. We predict the full temperature dependence of the optical spectra and find the development of a hybridization pseudogap in the vicinity of the collapse transition.   

Functional renormalization group analysis of the one-dimensional half-filled Holstein-Hubbard model

The one-dimensional half-filled Holstein-Hubbard model (HHM) is studied by the newly developed electron-phonon coupled functional renormalization group (ep-FRG) [1]. The ep-FRG enables us to study the electron-phonon coupled system in an unbiased manner by taking account of the scatterings at different energy scales and momenta systematically. Previous studies of the half-filled HHM showed that there is a direct transition between the charge-gapped spin-density wave (SDW) phase and the spin-charge-gapped charge-density wave (CDW) phase. Recently, it has been proposed that there is an intermediate spin-gapped metallic phase with dominant superconducting (SC) pairing correlation between SDW phase and CDW phase [2]. Our ep-FRG results show that the dominant correlation in this intermediate phase is not SC pairing. \vskip 0.3cm \noindent [1] S.-W. Tsai, A. H. Castro Neto, R. Shankar, and D. K. Campbell, Phys. Rev. B 72, 054531 (2005). \vskip 0.1cm \noindent [2] R. T. Clay and R. P. Hardikar, Phys. Rev. Lett. 95, 096401 (2005).   

Antiferromagnetism and hot spots in CeIn3

Enormous mass enhancement at ``hot spots'' on the Fermi surface (FS) of the antiferromagnetic CeIn$_3$ has been reported at strong magnetic field near its antiferromagnetic quantum critical point [T. Ebihara et al., Phys. Rev. Lett. 93, 246401 (2004)]. The effect was ascribed to anomalous spin fluctuations at these spots owing to peculiar strong many-body interactions. The ``hot spots'' lie at the positions on FS same as in non-magnetic LaIn$_3$ where the narrow necks are protruded, thus, hinting on their possible relation. Assuming that in paramagnetic phase CeIn$_3$ has similar spectrum, we study the influence of the antiferromagnetic ordering (AFM) on the energy spectrum of CeIn$_3$ and show that its FS undergoes a topological change at the onset of AFM. The necks at the ``hot spots'' are truncated by the AFM, thus restoring the almost spherical d-part of the FS of CeIn$_3$. Applied field suppresses the AFM and restores the necks on the FS (so-called 2.5-order phase transition) leading to logarithmic divergence of the dHvA effective mass when the electron trajectory passes near or through the restored necks. This effect fully explains the observed dHvA mass enhancement in the ``hot spots'' in the frameworks of one-particle approximation and leads to the predictions concerning the spin-dependence of the effective electron mass.   

Session U45: Structural, Surface and other Phase Transitions

Effect of external strain on the order-disorder phase transition of the Si(001) surface

The Si(001) surface exhibits the phase transition from c(4$\times $2) to the (2$\times $1) structure at about 200 K[1, 2]. This is an order-disorder phase transition with respect to the buckling of the dimmer: the c(4$\times $2) structure results from an antiferromagnetic ordering of the buckled-dimmer and the (2$\times $1) structure is attributed to the time average of the flip-flop motion of the buckled-dimers. Externally applied tensile strain along the $<$110$>$ direction on the Si(001) surface is found to induce the flip-flop motion of the buckled dimmer below the critical temperature. This motion occurs cooperatively to form the disordered domain of the (2$\times $1) structure. Then the shape of the ordered domain as well as the size change with the strain. These results can be interpreted by the spontaneous shape instability originated from the strain relaxation energy. References [1] J. Ihm, D.H. Lee, J.D. Joannopoulos and J.J. Xiong, Phys. Rev. Lett. \textbf{51}, 1872(1983). [2] T. Tabata, T. Aruga and Y. Murata, Surf. Sci. \textbf{179}, L63(1987).   

Faceting and defaceting phase transitions of Pd/W(111)

We have studied the faceting and the defaceting phase transitions of the Pd/W(111) surface. Our studies show that for creating the largest facets, the best annealing temperature is right below the defaceting transition temperature, confirming the prediction by Oleksy (Surf. Sci. 549 (2004) 246). As we vary the programmed heating/cooling rate from 1/8 to 8 K/s, the paths of faceting transitions show normal retardation effect and shift to lower temperatures as the cooling rate increased. Surprisingly, the paths of defaceting transitions show negligible dependence on the heating rate. Detailed studies of this peculiarity lead us to propose that the defaceting transition is initiated at places subject to loss of too much Pd due to thermal desorption. As such loss can more readily be replenished at places near any one of the Pd 3-d islands, we propose that the rate independent path of defaceting transition is the consequence of a temperature dependent balance between the loss and the supply of Pd. Such balance should depend on the density of the Pd islands. Indeed, we find the paths of defaceting transition can be shifted to lower temperature by reducing the density of the Pd 3-d islands.   

Ultrafast lattice dynamics of FeRh

FeRh undergoes magnetic and structural phase transitions at $\sim 100^{\circ}$ C where a transition from antiferromagnetic to ferromagnetic orders occurs upon heating. Commensurate with this magnetic transition is a $\sim$1\% expansion in the lattice parameter. Recent optical measurements have shown that the magnetic transition can be quite fast, i.e., on the picosecond or sub-picosecond time scales [1,2]. We have used ultrafast x-ray diffraction techniques at the Advanced Photon Source to probe the speed of the corresponding structural transition. An epitaxial FeRh thin film on a MgO(001) substrate was driven through the phase transition by ultrafast laser excitation, and the response of the lattice was directly observed via picosecond-time-resolved x-ray diffraction. The temporal evolution of the FeRh lattice is reported as a function of laser fluence. [1] J.-U.\ Thiele \textit{et al}., Appl.\ Phys.\ Lett.\ 85, 2857 (2004). [2] G.\ Ju \textit{et al}., Phys.\ Rev.\ Lett.\ 93, 192301 (2004).   

Phase stability and Jahn-Teller distortion in doped lithiated manganese oxides: A LSDA+U study

We discuss how the rhombohedral phase of lithiated manganese oxide can be stabilized by doping with various impurities. Our study is based on LSDA+U calculations as implemented in the VASP code. We have considered rhombohedral and monoclinic phases using a supercell of 16 atoms. Our results are based on total energy calculations for 25{\%} dopant concentration and pure lithiated manganese oxide. Several dopants such as Co, Fe, Ni, Mg and Zn are considered. We find that oxidation state of the dopant plays an important role in suppressing the Jahn-Teller distortion. Divalent impurities are found to be most effective. The effect of including U in the calculation is discussed.   

Crystal Fields and the $ \gamma \rightarrow \alpha$ transition in Ce

In the $\gamma \rightarrow \alpha$ transition of Cerium, the material undergoes an isostructural change at which the volume changes by 15\% and the magnetic character changes. Recently, the transition has been described in terms of a balance between the free energy of the magnetic moments and the characteristic energy scale of the $\alpha$ phase. The field-temperature dependence of the phase diagram has been predicted, and was confirmed by experiment. Inelastic neutron scattering experiments on the $\gamma$-phase of Ce have shown indications of crystal field splittings, and similar experiments have determined the energy scale of the $\alpha$ phase. We shall examine the effects of the crystalline field splittings within the framework of NCA calculations on the single-impurity Anderson model, and examine their consequence for the phase diagram.   

Bifurcation Techniques for Structural Phase Transitions

A new technique for studying structural phase transitions in crystals has been developed which uses bifurcation theory to investigate a material's free energy landscape. In this method a material's behavior is numerically interrogated by beginning with its high temperature structure and mapping out the equilibrium branch corresponding to the distortions of the crystal structure that occur due to changes in parameters such as temperature or stress. The investigation of this equilibrium branch is continued into unstable regions of the material's free energy landscape, i.e., regions which are physically unobservable. In these unstable regions the equilibrium branch bifurcates, or splits, and leads to other stable regions corresponding to different crystal structure branches (phases), thus revealing the links and interactions between the various phases of the material. Often, unexpected stable phases are identified in this way. It is common to encounter non-generic bifurcation points, where a single equilibrium branch splits into many (instead of two) new equilibrium branches. In these complex situations, the current bifurcation method is guaranteed to systematically identify all of the new branches. To illustrate the method, an atomistic model for shape memory alloys is investigated and a commonly observed hysteretic transformation is identified between a cubic $B2$ (austenite) structure and an orthorhombic $B19$ (martensite) structure.   

Imaging the evolution of a glassy magnetic transition in a disordered ferromagnetic manganite

An intriguing glass-like transition in (La,Pr,Ca)MnO$_3$ is, for the first time, imaged using a variable-temperature magnetic force microscope. Images showing the temperature and magnetic- field evolution of the local magnetic structure illustrate the microscopic origin of the bifurcation of magnetic susceptibility, which is a ubiquitous phenomenon in heavily-disordered ferromagnets, and traditionally considered as a signature of a ``cluster glass transition.'' The observed avalanche-type behavior reveals the collective nature of the glassy transition in the manganites, where ferromagnetic and antiferromagnetic phases are intricately mixed.   

Quantum Relaxation in a Proton Glass

Rb$_{1-x}$(NH$_{4}$)$_{x}$H$_{2}$PO$_{4}$ is a dipolar structural glass with spatial frustration from the mixture of ferroelectric RDP and antiferroelectric ADP. We measure the ac dielectric susceptibility of RADP:72 and RADP:35 over 7 decades of frequency for 0.3 $<$ T $<$ 300 K. The relaxation is quantitatively similar for both concentrations at low temperatures, pointing to a local mechanism. We correlate the dielectric susceptibility with the potential energy landscape derived from neutron Compton scattering experiments and solve for the tunneling parameters of the protons, finding correlated rearrangements of the hydrogen network. By analogy with vortex tunneling in high-Tc superconductors, we relate the logarithmic decay of the polarization to the quantum mechanical action.   

Switching between one and two dimensions: Pb induced chain structures on Si(557)

The conductivity of epitaxially grown Pb-structures on Si(557) has been measured. Different characteristic transport mechanisms have been found: For coverages above the percolation limit(0.6ML) up to 3ML the electronic transport in the annealed Pb-films is activated. Furthermore, the uniaxial symmetry of the Si(557) surface is reflected directly in a higher conductance in the parallel direction compared to the direction perpendicular to the steps. For coverages higher than 3ML a metallic behavior is found for both directions, i.e. the conductance decreases with increasing temperature. In contrast, already one ML, but annealed to 640K, leads to the formation of atomic wires, as seen by STM, with an extremely high and quasi one-dimensional surface state conductance along the wire direction. At a critical temperature of T$_c$=78K, the system switches from low to high conductance anisotropy, with a metal-insulator transition in the direction perpendicular to the chain structure, while in the direction along the chains conductance with a (1/T + const.) temperature dependence was found. STM has shown further, that the 1D/2D transition is associated with an order-disorder phase transition of a 10- fold superperiodicity along the Pb chains.   

Spontaneous polarization in one-dimensional Pb(ZrTi)O$_3$ nanowires

Formation of spontaneous polarization in one-dimensional structures is the key phenomenon that reveals collective behaviors in systems of reduced dimension, but has remained unsolved for decades. Here we report {\it ab initio} studies on finite-temperature structural properties of infinite-length nanowires of Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ solid solution. Whereas existing studies have ruled out the possibility of phase transition in 1D chains, our atomistic simulations demonstrate an unambiguous otherwise. We show that phase transitions in 1D wires occur on a remarkable macroscopic length scale, but not necessarily on an infinite length scale as assumed in the general theories of 1D phase transition. Such phase transitions are chracterized by large longitudinal $d_{33}$, $\chi _{33}$ responses and a large $c/a$ strain. The long rang ordering in PZT nanowires is explained by use of depolarizing effects associated with finite thickness of wires. Our results suggest no fundamental constraint that limits the use of ferroelectric nanowires and nanotubes arising from the absence of spontaneous ordering.   

Computational investigation of wetting and prewetting phase behavior

Fluids in the presence of one or more surfaces exhibit a rich variety of phase transitions that are absent in bulk fluids. Even the simplest of systems display a broad range of phase behavior. In this presentation, we describe our recent efforts aimed towards obtaining a better understanding of surface phase behavior through the use of molecular simulation. The first part of the presentation will be used to provide an overview of transition-matrix based Monte Carlo algorithms that enable one to efficiently locate and characterize phase transitions. Results will then be presented that describe how the wetting behavior of a model substrate-fluid system evolves with temperature and the relative strength of the substrate-fluid interaction. Simulation results will be compared with density functional theory calculations. Finally, we will describe a series of calculations that enable us to estimate the boundary tension along the prewetting saturation line. This quantity is related to the line tension associated with the formation of liquid droplets on a solid substrate. The magnitude of this tension has been the subject of debate recently, with experimental values spanning several orders of magnitude.   

Magnetic properties of weak itinerant ferromagnetic Ni-V alloys

Magnetization measurements of a serie of Ni-V alloys at high magnetic fields and low temperatures will be presented. Characteristic exponents observed in the low field susceptibility and spontaneous magnetization can be extracted and traced upon dilution. Especially the changes around a critical V-concentration of 1/9 will be discussed probing and comparing the validity of itinerant models and significance of spin fluctuations and magnetic cluster contributions.   

Scaling Behavior of Classical Wave Transport in Mesoscopic Media at the Localization Transition

The propagation of classical wave in disordered media at the Anderson localization transition is studied. Our results show that the scaling behavior of wave transport depends on the sample's geometry. It is found that the averaged static diffusion constant $D(L)$ scales like $\ln L/L$ in cubes or slabs, and the corresponding transmission follows $\left\langle {T(L)} \right\rangle \propto \ln L/L^2$. This is in contrast to the scaling behavior of $D(L)\propto 1/L$ and $\left\langle {T(L)} \right\rangle \propto 1/L^2$ obtained previously for electrons or spherical samples. For wave dynamics, we solve the Bethe-Salpeter equation in a disordered slab with the recurrent scattering incorporated in a self-consistent manner. All of the static and dynamic transport quantities studied are found to follow the new scaling behavior of $D(L)$.   

Optical Near-Field Based Nanoscale Rapid Melting and Crystallization of Amorphous Silicon Thin Films

Nanostructuring of thin films is gaining widespread importance owing to ever-increasing applications in a variety of fields. The current study details nanosecond laser-based rapid melting and crystallization of thin amorphous silicon (a-Si) films at the nanoscale. Two different near-field processing schemes were employed. In the first scheme, local field enhancement in the near-field of a SPM probe tip irradiated with nanosecond laser pulses was utilized. As a second approach, the laser beam was spatially confined by a cantilevered near field scanning microscope tip (NSOM) fiber tip. Details of various modification regimes produced as a result of the rapid a-Si melting and crystallization transformations that critically depend on the input laser fluence are presented. At one extreme corresponding to relatively high laser fluence, ablated area surrounded by a narrow melt region was observed. At the other extreme, where the incident laser energy density is much lower, single nanostructures with a lateral dimension of $\sim$90 nm were defined. The ability to induce nucleation and produce single semiconductor nanostructures in a controlled fashion may be crucial in the field of nano-opto-electronics.   

Explosive Crystallization of Amorphous Semiconductor Films in the Presence of Melting

Explosive crystallization (EC) of thin amorphous solid films of germanium is investigated theoretically and experimentally. EC regime characterized by a propagating melting layer between the amorphous and the crystalline phases is considered. Laser-induced, linear EC fronts, uniformly propagating over large distances are achieved in films with various thicknesses deposited on quartz substrate. Depending on the front speed, the film thickness and the substrate temperature, different types of morphology of the resulting crystal phase are observed: columnar, scalloped and mixed. A theory of EC in the presence of melting is developed. The EC front propagation speed is calculated as a function of the substrate temperature and the film thickness; it is found to be in a good agreement with experiments. Linear stability analysis of a uniformly propagating planar EC front is performed. It is shown that for the parameter values where the columnar crystalline structure was observed the front is unstable with respect to a fingering instability similar to the Mullins-Sekerka instability of a solidification front in an undercooled melt. Nonlinear evolution of this instability is simulated numerically and is shown to exhibit a structure similar to the columnar one.   

Session U46: Optical Properties of Semiconductors: Excitons and Phonons

Laser induced trapping of excitons in coupled quantum wells

Optical trapping and manipulation of neutral particles plays a major role in single particle studies in physics, chemistry, and biology [1]. An exciting recent outgrowth of the technique has been the experimental implementation of atom Bose- Einstein Condensation [2,3]. In this contribution, we report proposal and demonstration of laser induced trapping for a new system - a cold gas of excitons in coupled quantum wells. We report trapping a cold gas of excitons in laser induced traps and on the formation of a highly degenerate Bose gas of excitons in the trap. [1] A.~Ashkin, {\it IEEE Journal on Selected Items in Quantum Electronics\/} {\bf 6}, 841 (2000). [2] E.~A. Cornell, C.~E. Wieman, {\it Rev. Mod. Phys.\/} {\bf 74}, 875 (2002). [3] W.~Ketterle, {\it Rev. Mod. Phys.\/} {\bf 74}, 1131 (2002).   

Spontaneous First-order Optical Coherence in Cold Exciton Gases in Coupled Quantum Wells

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells[1]. A strong enhancement of spontaneous first-order optical coherence was observed at low temperatures below a few Kelvin where the thermal de Broglie wavelength becomes comparable to the interparticle separation and the exciton gas becomes nonclassical. The onset of spontaneous first-order optical coherence was found to be correlated with macroscopic spatial ordering in the exciton system.[1] L.V. Butov, A.C. Gossard, D.S. Chemla, {\it Nature} {\bf 418}, 751 (2002).   

Theory of coherent population trapping and electromagnetically induced transparency in quantum wells and dots

Recently, there has been important experimental progress in quantum coherent phenomena in quantum-wells and dots, opening up possibilities for observations of quantum optical effects previously observed in atomic systems. In particular, coherent population trapping (CPT), electromagnetically-induced transparency (EIT), and slow light can occur in systems with sufficiently long ground state coherence, optically connected to an excited level, forming a Lambda-like system. We find that a quantum well or ensemble of dots, in Voigt geometry and illuminated by a bi-chromatic circularly polarized laser, can exhibit CPT and EIT. In this scheme, the electron spin provides the long lived ground states and a trion excitation acts as the excited level. By including optical and g-factor inhomogenous broadening, dephasing due to nuclear hyperfine interaction, and coupling to the both trion excitations, we derive a compact set of criteria for observations of these effects. We compare with experiments to date and discuss future prospects.   

Stimulated Emission of Terahertz Radiation from Internal ExcitonTransitions in Cu$_{2}$O

Excitons are among the most fundamental optical excitation modes in semiconductors. Resonant infrared pulses have been used to sensitively probe absorptive transitions between hydrogen-like bound pair states [1,2]. We report the first observation of the reverse quantum process: stimulated emission of electromagnetic radiation from intra-excitonic transitions [3]. Broadband terahertz pulses monitor the far-infrared electromagnetic response of Cu$_{2}$O after ultrafast resonant photogeneration of 3$p$ excitons. Stimulated emission from the 3$p$ to the energetically lower 2$s$ bound level occurs at a photon energy of 6.6 meV, with a cross section of $\sim $10$^{-14}$ cm$^{2}$. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency. Our results demonstrate a new fundamental process of THz quantum optics and highlight analogies and differences between excitonic and atomic systems. [1] R. A. Kaindl et al., Nature \textbf{423}, 734 (2003). [2] M. Kubouchi et al., Phys. Rev. Lett. \textbf{94}, 016403 (2005). [3] R. Huber et al., Phys. Rev. Lett., to appear.   

Ultrafast Raman Studies of Electron Transient Transport in InN Thick Film Grown on GaN

GaN, AlN, InN and their alloys have long been considered as very promising materials for device applications. Semiconductor alloys such as $In_x Ga_{1-x} N$ have been successfully used in the fabrication of blue-green light emitting diodes and laser diodes. Recently, growth of high quality InN as well as $In_x Ga_{1-x} N$ have been demonstrated. In Particular, progress in the manufacturing of very high quality, single-crystal InN thin films has opened up a new challenging research avenue in the III-nitride semiconductors. InN together with its alloys of GaN and AlN enable the operation of light emitting diodes and diode lasers ranging in spectral wavelength from infrared all the way down to deep ultraviolet. It has also been predicted that InN has the lowest electron effective mass among all the III-nitride semiconductors. As a result, very high electron mobility and very large saturation velocity are expected. In this paper, we report experimental results of electron transient transport on InN thick film grown on GaN. Electron drift velocity as large as $7.5x10^7cm/\sec $ has been found when the sample is excited by an ultrafast laser pulse with pulse width about 600 femtoseconds. Our findings demonstrate that InN has great potential for use in the ultrafast electronic devices.   

Exciton-mediated one phonon resonant Raman scattering from 1-dimensional systems

The Kramer's-Heisenberg approach is well developed for the theory of one phonon resonant Raman scattering (OPRRS) based on both intermediate states of free electrons and correlated electron-hole pairs in 3, 2 and 0-dimensional (D) systems. But to our knowledge, a theory of OPRRS incorporating excitonic effects has not yet been developed for quantum confined 1D system. In this talk we present a generic expression for the resonant Raman scattering cross section from a 1D system explicitly accounting for excitonic effects. We show how the theory is useful for analyzing the Raman resonance excitation profile lineshapes for of a variety of 1D systems. We apply this formalism to simple model systems to the similarities and differences between the free electron and correlated electron-hole 1D theory and also compare with the 3D excitonic theory.   

Wurtzite Effects on Spin Splitting of GaN

We report the theoretical study of the wurtzite effects on spin-splitting of GaN within a third-neighbor Linear Combination of Atomic Orbitals (LCAO) model. For wurtzite structure, there are two intrinsic wurtzite effects which are band-folding effect and structure inversion asymmetry. The band-folding effect generates two conduction bands ($\Delta _{C1 }$and $\Delta _{C3})$, in which the $p$-wave probability and, consequently, the spin-splitting energy have abrupt changes when $k_{z}$ increases toward the anti-crossing zone. The wurtzite effects, in addition to Rashba and Dresselhaus effect, give significant contributions to the large spin-splitting in GaN/AlN quantum wells.   

Ab initio calculations of excitons in wurtzite-type semiconductors.

The optical absorption spectra are calculated with inclusion of electron-hole correlations for GaN, ZnO, and AlN, all in the wurtzite structure. Quasi-particle states are approximated by local-density-functional calculations with gaps corrected by a scissors operator, and the final spectra are obtained by solving the Bethe-Salpeter Equation . The results for ZnO depend sensitively on the energetic positions of the Zn-3d states. These are corrected by means of LDA+U. The excitons originating from the valence-band maximum are labeled A, B, and C, but their symmetry type, and thus dependence on the polarization of the light, are related to the specific values of the spin-orbit and crystal field splittings, SOC and CFS. The SOC is positive in GaN and AlN, negative in ZnO. The CFS is positive in GaN and ZnO, but negative in AlN. The sensitivity of the excitonic states to structural parameters is discussed, and in one case, AlN, we examine the validity of Elliott's model, the effective hydrogen-atom model.   

Pump-Probe studies of Carrier Dynamics in bulk ZnO and ZnO epilayers and Nanorods

ZnO-based devices are potentially useful as short wavelength emitters and in spintronics applications, yet little is known about the ultrafast relaxation properties of ZnO. We have performed time-resolved differential reflectivity (TRDR) measurements of bulk ZnO, ZnO epilayers and nanorods as a function of temperature and excitation wavelength. Bi-exponential decays of the A and B exciton states are observed with fast ($\sim $ps scale) and slower ($\sim $ 50-100 ps scale) components, which depend strongly on excitation wavelength. We find that decay times can be correlated with relaxation channels in the band structure. In addition to their bi-exponential nature, the relaxation times we observe on ZnO epilayers and nanorods are shorter than high quality bulk ZnO, indicating a higher density of defects and impurity states in these samples.   

Rate Equation Model for Carrier and Exciton Dynamics in ZnO

There has been a renewed interest in ZnO materials for possible applications to short wavelength optical devices including blue lasers owing to its wide band-gap (3.37) and large exciton binding energy (approx. 60 meV). Recently, there have been several experimentalstudies of the dynamics of photoexcited carriers in bulk ZnO as well as epitaxial films and nanorods. In this talk, we report on theoretical calculations of the exciton and photoexcited carrier dynamics based on a multi-state, coupled rate equation model. We compare our theoretical results with recent tunable time resolved reflectivity measurements performed at the National High Magnetic Field Laboratory that study the relaxation dynamics when pumping and probing near the A and B excitons. In addition to solving the coupled rate equations, we also discuss the role of diffusion as well as phase space filling (non-linear rate equations) on the experimental results.   

Direct Observation of the Strength of Plasmon-Longitudinal Optical Phonon Interaction in n-type GaAs

The screening of longitudinal optical phonons by plasmons is investigated by time-resolved visible pump-mid infrared probe transmission measurements in a series of light to highly doped n-type GaAs wafers. The reduced relaxation of photogenerated carriers is strongly correlated to the coupling between longitudinal optical phonons and background plasmons as suggested by the variation of the phonon strength over the doping range. Our results show that at low photogeneration ($<$ 10$^{16 }$cm$^{-3})$ the critical doping density at which the strength of the coupling between LO phonons and plasmons decreases significantly is on the order of N$_{c }\sim $ 1x10$^{18}$cm$^{-3}$. The lack of LO phonons that participate in relaxation of carriers due to the hybridization of the longitudinal modes above this doping level, can either result in adverse effects in the spectrum of diode lasers and semiconductor electronic devices or enhance photonic device performance due to longer minority carrier recombination times.   

THz radiation from coherent acoustic phonon waves in strained GaN-based heterostructures

We present experimental results and discuss the generation mechanism of newly found THz radiation in GaN/InGaN based light emitting diode (LED) structures. These structures show strong coherent acoustic phonon oscillations under ultra-short optical excitation and we discuss the role these coherent phonons play in the generation of the THz signal. To better understand the role of piezoelectricity on the generation of the acoustic phonons and THz radiation, an external field was applied to compensate the built-in piezoelectric field. The coherent oscillatory behavior of the differential reflectivity spectra was reduced and finally become independent of the increasing applied voltage. However, with reverse bias, the THz emission from these structures was found to increase with increasing reverse voltage and excitation energy, slightly distinct from the trend of the photocurrent. The frequency of the THz emission is related to the transit time of the acoustic phonons between the AlGaN layers. The bias and wavelength dependence of the THz generation suggests that wavefunctions of confined carriers at the AlGaN/GaN and AlGaN/InGaN interfaces, are modulated by a temporally-changing potential shape associated with the piezoelectric field of the lattice and are responsible for the THz radiation.   

Theory of carrier dynamics and coherent phonons in piezoelectric semiconductor heterostructures

We model generation and propagation of coherent acoustic phonons in time resolved reflectivity experiments on InGaN/GaN multi quantum wells embedded in a pin diode structure. Carriers are created in the InGaN wells by ultrafast pumping below the GaN band gap. The electronic states in the multiquantum well structure are obtained in an effective mass model and the generation and subsequent relaxation of photogenerated carriers in the well are treated in a Boltzmann equation formalism. Coherent acoustic phonons are generated in the quantum well via a strong piezoelectric electron-phonon interaction with photogenerated carriers. These propagate into the structure at the LA sound speed modifying the optical properties and giving rise to an oscillatory differential reflectivity signal. We also study the THz radiation emitted by the photoexcited carriers and phonons. In addition to studying the multiquantum well structure, we also study chirped superlattices were the well widths increase with distance and investigate the possibility of selectively exciting carriers in a given well to coherently control the response.   

Studies of Coherent Acoustic Phonons in CdMnTe Single Crystals

We have demonstrated generation and detection of coherent acoustic phonons (CAPs) in Cd$_{0.91}$Mn$_{0.09}$Te (CdMnTe) single crystals using a femtosecond pump-probe spectroscopy technique. The Thomsen model based on propagation of a strain pulse in a crystalline lattice accounted very well the observed dependences of the frequency and the dephasing time of our CAP oscillations on the optical probe beam wave-vector. The CAP oscillation frequency was found to be dispersionless with the speed of sound equal to 3579 m/s. The comparison studies, performed using the pump beam with the photon energy well above the CdMnTe energy gap and the sub-gap probe beam demonstrated that in our crystals the measured dephasing time of CAP oscillations was limited by the absorption depth of the probe light rather than the intrinsic decay time of the coherent phonons. The latter value was estimated to be at least in the nanosecond range. Optically-induced electronic stress was determined to be the main generation mechanism of CAPs in CdMnTe.   

Dynamical interfacial-electric-field-induced electro-optics in multilayer semiconductors

Multilayer semiconductors with the thickness of layers of a few tens of nanometers are common materials for designing novel multifunctional electronic and opto-electronic devices. Once the materials are subjected to ultrafast laser light, the dynamical interfacial electric fields between adjacent layers is created as a result of charge separation at the interfaces within the carrier thermalization process. This dynamical electric field affects the ultrafast optical properties of the materials additionally to that of the bleaching effect (phase space filling-Pauli blocking). We report the first application of pump--probe technique allowing the interfacial-field-induced both electro-optical refractive-index change and the second harmonic generation to be monitored simultaneously. The pump-probe spectroscopy of GaAs/GaSb/InAs multilayers reveals predominantly the electro-optical nature. The interfacial fields contribute to the variety of electro-optical effects allowing the temporal and spatial resolution in carrier dynamics to be reached by monitoring responses resulted from different order nonlinear polarizations. The absorption bleaching is a secondary effect appearing with much smaller magnitude.