Session N1: Superconducting Qubits

Coherent Dynamics of a flux-qubit coupled to a harmonic oscillator

Superconducting circuits containing Josephson junctions are promising candidates for the implementation of solid-state quantum bits or qubits. Complex single-qubit operations have already been reported, as well as the realization of a two-qubit gate. Coupling a qubit to a harmonic oscillator is interesting from a fundamental point of view to generate and study non-classical states of the oscillator, and is relevant in quantum information as well, since coupling many qubits via a harmonic oscillator has been proposed. The strong coupling between a charge qubit and a superconducting coplanar waveguide resonator has recently been observed [1]. Here we present measurements demonstrating the coupling of a flux-qubit to the plasma mode of the DC Squid which at the same time is used to measure the qubit state [2]. This coupling is manifested by the appearance of two side-band resonances around the bare qubit peak. By performing two-pulse experiments, we show that these additional resonances are indeed excitations of the coupled system. We also observe Rabi oscillations between the coupled $\vert $qubit,Squid$>$ states, thus demonstrating entanglement between the states of two superconducting circuits. We use the qubit to measure intrinsic properties of the plasma mode : temperature, relaxation time. Conversely, we also measure the qubit state via the plasma mode. We finally investigate the influence of the qubit-plasma mode coupling on the qubit quantum coherence. These results indicate that complex manipulation of entangled states similar to cavity quantum electrodynamics or trapped-ions experiments is within reach with superconducting circuits. [1] A. Wallraff et al., Nature \textbf{431}, 162 (2004) [2] I. Chiorescu et al., Nature \textbf{431}, 159 (2004) .   

Continuous Impedance measurement of a Superconducting Flux Qubit

We implement the radio-frequency technique for characterization of superconducting qubits and for measuring their states. In the framework of this method, the qubit is inductively coupled to a high-quality tank circuit. We show that this technique is a powerful tool to study a response of externally controlled superconducting qubit to different types of excitations. Conclusive information about qubits is obtained from the read out of the tank properties. We also show that the tank circuit can be effectively used to monitor an adiabatic evolution of the superconducting flux qubits. By making use the radio- frequency technique the qubit's state can be determined, moreover such kind of measurements belong to the class of quantum nondemolition measurement.   

Circuit Quantum Electrodynamics: A New Architecture for Superconducting Quantum Computation

I will describe recent experiments in which the strong coupling limit of cavity quantum electrodynamics has been realized for the first time using superconducting circuits [1]. In our approach, we use a Cooper-pair box as an artificial atom, which is coupled to a one-dimensional cavity formed by a transmission line resonator. In the case when the Cooper-pair box qubit is tuned into resonance with the cavity, we observe the vacuum Rabi splitting of the cavity mode, indicating that the strong coupling regime is attained, and coherent superpositions between the qubit and a single photon are generated. When the qubit is detuned from the cavity resonance frequency, we perform high-fidelity dispersive quantum non-demolition readout of the qubit state. Using this readout technique, we have characterized the qubit properties spectroscopically, performed Rabi oscillations of the qubit, and attained coherence times greater than 500 ns, indicating that this architecture is extremely attractive for quantum computing and control [2].\par [1] A. Wallraff, D. I. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf Nature (London) \textbf{431}, 162 (2004)\par [2] A. Blais, R.-S. Huang, A. Wallraff, S. M. Girvin and R. J. Schoelkopf Phys. Rev. A \textbf{69}, 062320 (2004)\par   

Single-Shot State Measurement of Coupled Phase Qubits

A major reason that superconducting Josephson tunnel junctions are promising quantum bit (qubit) candidates for a quantum computer is their potential for scalability using conventional integrated-circuit technology. We have taken a first step toward implementing multi-qubit systems by fabricating pairs of capacitively coupled Josephson phase qubits with individual control and measurement circuitry. Spectroscopic measurements verify that the two qubits become coupled when they are tuned into resonance with each other. Furthermore, with the benefit of a fast state measurement technique, we are able to perform simultaneous single-shot measurements of the two qubits individually. The success of these experiments relies on an understanding of classical measurement crosstalk, where the readout of one qubit may cause unwanted transitions in the second qubit. Because the crosstalk does not occur instantaneously, it can be largely avoided by timing the separate qubit measurements to be coincident. Time-domain experiments reveal antiphase oscillations between the two-qubit basis states $|01\rangle$ and $|10\rangle$, with the frequency of oscillation determined by the engineered capacitive coupling strength between the two qubits. These results are consistent with quantum mechanical entanglement of the two qubits, and they open the possibility for the characterization of multi-qubit gates and elementary quantum algorithms.   

Superconducting Flux Qubits

This abstract was not received electronically.   

Session N2: Spin and Charge in Mott Systems

Cluster Dynamical Mean Field Analysis of the Mott transition

I will present recent results on the evolution from an anomalous metallic phase to a Mott insulator within the two dimensional Hubbard model, using a cluster extension of dynamical mean field theory. In particular, the density-driven Mott metal-insulator transition is approached in a non-uniform way in different regions of the momentum space. This gives rise to a breakup of the Fermi surface and to the formation of hot and cold regions, whose position depends on the hole or electron like nature of the carriers in the system.   

Orbital-selective Mott transitions in the degenerate Hubbard model

Strongly correlated electron systems with multi-orbital bands possess various unusual properties. Here we address how the multi-orbital nature affects the Mott transition and under which conditions consecutive orbital-selective Mott transitions may occur upon growing electron correlations. We study a model of an extended two-orbital Hubbard model (including onsite intra-orbital repulsion $U$, onsite inter-orbital repulsion $U'$ and Hund coupling $J_H$ with $ U=U'+2J_H $) with distinct hopping matrix elements for the two orbitals. By combining dynamical mean-field theory with exact diagonalization, the stability of itinerant quasi-particle states in each band is examined. There is a single Mott transition, simultaneously for both orbitals, in the absence of the Hund coupling, when the electron interaction is gradually increased. Once the Hund coupling is introduced, the Mott transitions splits into two, such that an intermediate region appears where the system consists of localized electronic degrees of freedom and of itinerant, though strongly renormalized, electrons. We also discuss the finite-temperature properties by means of Quantum Monte Carlo simulation. This allows us to elucidate the behavior of spin and orbital fluctuations in the vicinity of the Mott transition by analyzing the spin, charge and orbital susceptibilities as well as the one-particle spectral function. Moreover we study the effect of hybridization between the orbitals which leads to some essential modifications of the properties compared to the case of non-hybridized orbitals.   

Effects of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition

The Mott-Hubbard metal-insulator transition (MIT) is one of the most important problems in correlated electron systems. In the past decade, much progress has been made on examining a particle-hole symmetric form of the transition in the Hubbard model with dynamical mean field theory (DMFT) where it was found that the electronic self energy develops a pole at the transition. However, since most real materials are not at half filling one would like to examine the particle-hole asymmetric MIT. Here we analyze this problem using Falicov-Kimball (or simplified Hubbard) model. It is believed to describe correlated electron behavior and MIT in materials that can be fit into a binary alloy picture. Unlike the Hubbard model, which has a metal-insulator transition only at half filling, the Falicov-Kimball model exhibits a MIT for asymmetric particle densities. An example of the system that fits this picture is Ta$_x$N, which exhibits the MIT away from half filling at $x=0.6$. We find that away from half filling a number of features change when the noninteracting density of states has a finite bandwidth. First, we compare the nature of Mott-Hubbard transition at zero temperature in the Falicov-Kimball model for the lattices with finite and infinite bandwidths within the DMFT. We derive simple formulas for the critical interaction strength $U$ for both the development of a pole in the self energy and for the opening of a gap in the single-particle density of states. While the critical $U$ values are the same at half filling on both lattices, and for arbitrary filling on infinite bandwidth lattice, they are different for the particle-hole asymmetric cases on the finite bandwidth lattice. We discuss what role the development of the pole has on the physical properties of the MIT and the consequences these results have for the MIT in real materials. As an illustration we calculate a number of thermal transport properties and show how they are influenced by the bandwidth and the MIT for different fillings. [1] D.O.Demchenko, A.V.Joura, J.K.Freericks, Phys.Rev.Lett. {\bf 92}, 216401 (2004).   

Ferromagnetism in vanadium oxide nanotubes spin-tuned by electron/hole doping

An intrinsic feature of Mott insulators is that doping, i.e., changing the particle density, can dramatically affect their properties. I will report on our discovery of charge-doping controlled ferromagnetism at room temperature in self-assembled vanadium oxide nanotubes. By adding either electrons or holes, the initially spin-frustrated nanotubes develop a nearly identical nonlinear ferromagnetic spin response, demonstrating a novel unexpected electron-hole complementarity in the nanotube structures. The underlying picture is that, on doping, the Fermi level is swept through the Mott gap in this multiband strongly correlated system, removing the frustration responsible for the spin-gap. Itinerant carriers under spin control are produced in one vanadium Hubbard band which strongly interacts with other more localized vanadium spins. These findings show a path to new spin-aligned nanoscale building blocks, where the Fermi level sweep can be accomplished by applied voltage.   

Prominent Metal Phase Quasi-Particle Peak and High Temperature Mott-Hubbard Gap Filling in Photoemission Spectra of (V$_{1-x}$Cr$_x$)$_2$O$_3$

Two new findings [1,2] have been made in photoemission spectra of the paradigm Mott-Hubbard transition system (V$_{1-x}$Cr$_x$)$_2$O$_3$. First [1], in the paramagnetic metal phase of V$_2$O$_3$ there is a prominent quasi-particle peak at the Fermi energy, of amplitude larger than the rest of the V 3d spectrum by a factor approaching two. The peak is qualitatively much like that found in spectral calculations that combine dynamic mean field theory (DMFT) and band theory in the local density approximation (LDA), but shows important differences quantitatively. Second [2], in the paramagnetic insulating phase of (V$_{0.972}$Cr$_{0.028}$)$_2$O$_3$, spectra taken in ultra high vacuum up to the unusually high temperature (T) of 800K reveal a property unique to the Mott-Hubbard insulator. With increasing T the MH gap is filled by spectral weight transfer, in qualitative agreement with high-T LDA+DMFT theory. [1] S.-K. Mo {\em et al}, Phys. Rev. Lett. {\bf 90} 186403 (2003). [2] S.-K. Mo {\em et al}, Phys. Rev. Lett. {\bf 93} 076404 (2004).   

Session N3: Electronic and Spin Effects in Low-Dimensional and Nanoscale Systems

Controlling the Spin State of Individual Cobalt Adatoms and Molecules

The spin of individual magnetic atoms and molecules at surfaces is of fundamental interest and may play an important role in future atomic-scale technologies, among them classical and quantum computation. In this talk we demonstrate the ability to manipulate the spin state and associated magnetic properties of individual cobalt adatoms by the controlled attachment of molecular ligands. The spin state of the cobalt adatoms and complexes is determined via the Kondo resonance by low-temperature scanning tunneling spectroscopy. Spatial Kondo resonance mapping is also introduced as novel imaging tool to localize spin centers in magnetic molecules with atomic precision.   

Quantum Confinement by Schottky Barriers and its Consequences

Atomically uniform Pb and Ag films have been successfully grown on Si(111) and Ge(111), respectively, despite a large lattice mismatch in each case. The resulting Schottky barrier at the interface confines the electrons in the film to form quantum well states or subbands. The electronic structure of the film including the ground state wave function can be significantly different from the bulk case, leading to substantial variations in physical properties as a function of film thickness. These variations generally follow a damped oscillatory curve riding on an approximately $1 \mathord{\left/ {\vphantom {1 {N^x}}} \right. \kern-\nulldelimiterspace} {N^x}$ baseline function, with the exponent $x$ often close to unity. The oscillatory behavior is similar to the shell effect associated with the periodic property variations of elements in the period table. This talk discusses the basic electronic structure of thin metal films as measured by angle-resolved photoemission and the connections to physical properties including the surface energy, thermal stability, density of states, electron-phonon coupling, etc. Quantum size effects can also affect morphological evolution during film growth and heat treatment. The Schottky barrier can be modified by the use of interfactants, and experimental results will be presented to illustrate the utility of this method for quantum control and engineering. In collaboration with M. Upton, D. Ricci, P. Czoschke, L. Basile, S. J. Tang, Hawoong Hong, J. J. Paggel, D.-A. Luh, and T. Miller.   

Extracting surface phonon properties from the electronic spectral function

Angle resolved photoemission measurements have revealed an enhancement in the electron-phonon coupling (EPC) for two- dimensional surface states and quantum-well states in thin films. A recent theoretical advancement by J. Shi [1] has developed a method for the direct extraction of the momentum dependent Eliashberg function from the high-resolution photoemission data. The origin of the enhanced EPC at surfaces and interfaces will be explored as well as schemes to tune the EPC by modification of the surface electronic and vibrational properties. The implications of EPC on physical properties will be discussed, including the lifetime of electronic states near the Fermi energy. \\ \\1. Junren Shi et al., Phys. Rev. Lett. 92, 186401 (2004). \\ \\The work at UT supported by NSF- DMR 0105232. Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725.   

Session N4: Polymer Entanglement and Elasticity

Predicting The Tube Diameter For Polymer Melts and Solutions

A simple conjecture, relating the tube diameter to a characteristic length called the packing length, works well for all flexible entangled polymer melts. This is a remarkable result, because the tube diameter represents the confining effect of uncrossability of the chains, whereas the packing length is determined only by a chain's bulkiness and flexibility. I extend this conjecture to solutions: first for theta solvents, where it is shown to be equivalent to the Colby-Rubinstein scaling picture, and then for good solvents. In the latter case, it turns out that the number of blobs per entanglement strand is not a constant as had been previously assumed, but depends on the ratio of the packing length to the thermal blob size. Finally, I suggest that the packing length can be related to the Gauss winding number density, thus providing a topological basis for the conjecture.   

A primitive path analysis of entangled polymer melts and networks

Computer simulations provide unprecedented access to the microscopic structure and dynamics of polymeric systems. In particular, they are an ideal tool to study and analyze topological constraints on the dynamics of entangled polymer chains which can slide past but not through each other. We (i) show how the microscopic foundation of the tube model can be established by analyzing the topological state of polymeric liquids in terms of primitive paths, (ii) provide a unified view on loosely and tightly entangled systems, (iii) present an extension of the tube model to polymer networks which is (iv) shown to describe the microscopic and macroscopic response to strain of randomly end-linked and randomly cross-linked networks and (v) discuss the interpretation of scattering experiments addressing these issues.   

Convective constraint release, chain stretch and hopping tubes: details matter.

This talk will discuss constitutive equations, for entangled polymer melts and solutions, which incorporate the effects of convective constraint release (CCR) and chain stretch. The model for CCR is based on the conjecture that constraint release events produce local hops of the tube, giving rise to a dynamical equation similar to the Rouse model. Two recent articles [R.S. Graham et al, J. Rheol. 47, 1171-1200 (2003) and D.J. Read, J. Rheol. 48, 349-377 (2004)] have used this idea to derive, and solve, a PDE for the tube motion. The first article presents the more detailed model, including all known relaxation processes; it has been used to predict neutron scattering from melts in non-linear flow as well as melt and solution rheology. The second article uses an ``infinite tube" limit of the equations to examine in detail the CCR process and the coupling to chain stretch. It shows that ``details matter" – the equations appear to be highly sensitive to the nature of the tube on deformation, and in particular to the lower lengthscale cutoff to the CCR process. This talk will highlight these issues, and present a computer simulation scheme that can be used to further investigate the problem.   

Yield-like flow transition in entangled polymers: what do we understand about non-Newtonian polymer flow behavior?

In this talk, we discuss the latest results from our experimental studies of flow behavior of entangled polymers, in the context of the prevailing physical picture [1] prior to this work [2]. The model entangled polymers under study were 1,4-polybutadiene melts and their solutions. Flow behavior of these PBD samples was examined under various experimental conditions where shear flow was imposed by applying either a constant torque or a constant velocity on one of the two surfaces in a cone-plate shear cell (commonly known as controlled-stress or controlled-rate measurements respectively), and small or large step-strain was applied by a sudden displacement of one of the two surfaces in the same cell. The flow responses were found to be drastically different under these different conditions. When the applied shear stress was of a comparable magnitude to the elastic plateau modulus of the entangled solutions, a sharp yield-like constitutive transition was observed, revealing a discontinuous relationship between the shear rate and the shear stress, which was not anticipated according to the understanding prior to these experimental results. Such a discontinuity does not manifest itself in controlled-rate experiment and therefore has not been seen outside our lab. The implications of these results will be discussed to project our future efforts and activities. [1] Bent, J. et al, Science, 301, 1691 (2003); Graham, R. S et al, J. Rheol, 47, 1171 (2003). [2] Tapadia, P.; Wang, S. Q., Phys Rev. Lett., 91, 198301 (2003); Tapadia, P.; Wang, S. Q., Macromolecules, 37, 9083 (2004).   

Entanglements and Elasticity in Polymer Networks

We develop and solve a molecular model for nonlinear elasticity of entangled polymer networks, called non-affine slip-tube model. This model combines and generalizes several successful ideas introduced over the years in the field of rubber elasticity. Each chain passes through a sequence of slip-links. The topological constraints imposed by neighboring network chains on a given one are represented by the confining potential acting on the slip-links. This topological potential restricts fluctuations of the network chains to the non-affinely deformed confining tube and changes upon network deformation. Network chains are allowed to fluctuate and redistribute their length along the contour of their confining tubes. The dependence of the stress \textit{$\sigma $} on the elongation \textit{$\lambda $} is usually represented in the form of the Mooney stress $f^{\ast }$\textit{(1/$\lambda )=\sigma $/($\lambda -$1/$\lambda $}$^{2})$. We find a simple expression for the Mooney stress $f^{\ast }$\textit{(1/$\lambda )$=G}$_{c}+G_{e}$\textit{/(0.74$\lambda $+0.61$\lambda $}$^{-1/2}-0.35)$ where $G_{c}$ and $G_{e}$ are phantom and entangled network moduli. This allows analyzing the experimental data in the form of the universal plot and to obtain the two moduli $G_{c}$ and $G_{e}$ related to the densities of the cross-links and entanglements of the individual networks. The predictions of our new model are in good agreement with experimental data for uniaxially deformed polybutadiene, polydimethylsiloxane, and natural rubber networks. We generalize our non-affine slip-tube model to describe swelling of entangled networks in both good and theta solvents and find that non-affine effects due to entanglements decrease upon swelling. We also calculate the dependence of stress on strain in uniaxially deformed swollen networks.   

Session N5: Applications of THz Radiation

Characteristics and Applications of High Intensity Coherent THz Pulses from Linear Accelerators

Fifteen years have passed since coherent synchrotron radiation (CSR) from relativistic electrons was first observed[1,2]. Since then, CSR has served as a tool for characterizing electron bunch shapes[3] and has been proposed as a new type of millimeter wave source[4]. But until recently, the source characteristics (spectral range reaching 1 THz, waveform shape, energy per pulse, etc.) have not shown significant advantages over THz generators based on ultra-fast lasers. The present generation of photo-injected linear accelerators are now capable of producing sub-picosecond bunches with approximately 1nC of charge (or more), and the coherent radiation they emit has qualities that readily surpass what is available from other source types[5]. This presentation will describe characteristics of the coherent THz pulses produced as transition radiation from the Source Development Lab linac[6] at the National Synchrotron Light Source. Consistent with calculations, pulses can now be produced with energy approaching 100 microjoules (which is 2 orders of magnitude higher than from non-accelerator methods) and with spectral content reaching 2 THz. Other facilities (e.g., at Jefferson Lab) operate at multi-MHz repetition rates such that the average power is also very high. The E-field of a propagating THz pulse can be coherently detected and imaged using the electro-optic effect in ZnTe[7]. When focused, the transient E-field for such a pulse can exceed 1 MV/cm and should be sufficient for studying non-linear effects in solids, critical currents in superconductors, and ultra-fast magnetization in thin films. [1] T. Nakazato et al., \textit{Phys. Rev. Lett.} \textbf{63}, 1245 (1989). [2] H. Happek et al., \textit{Phys. Rev. Lett.} \textbf{67}, 2962 (1991). [3] R. Lai et al., \textit{Phys. Rev. }E\textbf{50}, R4294 (1994). [4] T. Takahashi et al., \textit{Rev. Sci. Instrum}., \textbf{69}, 3770 (1998). [5] G.L. Carr et al, \textit{Nature} \textbf{420}, 153 (2002). [6] X.J. Wang and X.Y. Chang, \textit{Nucl. Instr. {\&} Meth}. A \textbf{507}, 310 (2003). [7] Q. Wu et al., \textit{Appl. Phys. Lett.} \textbf{68}, 3224 (1996).   

Submillimeter wave spectroscopy of biological macromolecules

The recently emergence of submillimeter-wave or terahertz (THz) spectroscopy of biological molecules has demonstrated the capability to detect low-frequency internal molecular vibrations involving the weakest hydrogen bonds of the DNA base pairs and/or non-bonded interactions. These multiple bonds, although having only $\sim $ 5{\%} of the strength of covalent bonds, stabilize the structure of bio-polymers, by holding the two strands of the DNA double helix together, or polypeptides together in different secondary structure conformations. There will be a review of THz-frequency transmission (absorption) results for biological materials obtained from Fourier Transform Infrared (FTIR) spectroscopy during the last few years$^{1,2}$. Multiple resonances, due to low frequency vibrational modes within biological macromolecules, have been unambiguously demonstrated in qualitative agreement with theoretical prediction, thereby confirming the fundamental physical nature of observed resonance features. The discovery of resonance character of interaction between THz radiation and biological materials opens many possible applications for THz spectroscopy technique in biological sensing and biomedicine using multiple resonances as distinctive spectral fingerprints. However, many issues still require investigation. Kinetics of interactions with radiation at THz has not been studied and vibrational lifetimes have not been measured directly as a function of frequency. The strength of resonant modes of bio-molecules in aqueous environment and strong dependence of spectra on molecular orientation need explanation. Vibrational modes have not been assigned to specific motions within molecules. THz spectroscopy of bio-polymers makes it only in first steps. 1. T. Globus, D. Woolard, M. Bykhovskaia, B. Gelmont, L. Werbos, A. Samuels. International Journal of High Speed Electronics and Systems (IJHSES), \textbf{13}, No. 4, 903-936 (2003). 2. T. Globus, T. Khromova, D. Woolard and B. Gelmont. Proceedings of SPIE Vol. 5268-2, 10-18 (2004)   

Homeland Security, Medical, Pharmaceutical and Non-destructive Testing Applications of Terahertz Radiation

The terahertz region of the electromagnetic spectrum (300GHz-10THz) spans the region between radio and light. Recent advances in terahertz source, detector and systems technology are enabling new applications across a number of fields, based on both terahertz imaging and spectroscopy. This paper reviews our recent work on the development of practical systems and applications in security screening for the detection of explosives and non-metallic weapons; in medical imaging for cancer detection; as well as applications in non-destructive testing and the pharmaceutical industry.   

Non-Destructive Evaluation (NDE) Applications of THz Radiation

The technology and applications of time domain terahertz (THz) imaging to non-destructive evaluation (NDE) will be discussed. THz imaging has shown great promise in 2 and 3 dimensional non-contact inspection of non-conductive materials such as plastics, foam, composites, ceramics, paper, wood and glass. THz imaging employs safe low power non-ionizing electromagnetic pulses, with lateral resolution $<$ 200 um, and depth resolution $<$ 50 um. THz pulses can be analyzed spectroscopically to reveal chemical content. Recently, highly integrated turn-key THz imaging systems have been introduced commercially. We will demonstrate the detection of voids and disbonds intentionally incorporated within the sprayed on foam insulation of a space shuttle external tank mock-up segments. An industrially hardened THz scanning system which has been deployed to scan the space shuttle tank with small remote transceiver will be described. Additional terahertz security imaging applications for the detection of weapons and explosives will also be discussed, as well as the application of terahertz sensors for high speed industrial process monitoring and quality control.   

Terahertz Imaging and Security Applications

Imaging at millimeter-wave and terahertz frequencies could vastly improve the security of personnel checkpoints, because of the penetration through clothing and spatial resolution available in this spectral range. Since 9/11, the social need for improved checkpoint screening has been obvious and great. However, although efforts to develop such imagers had been underway for many years before that, practical low-cost systems, analogous to IR uncooled imagers, still don't exist. An emphasis on purely passive imaging places very stringent sensitivity requirements on such imagers. A number of long-term efforts, which I briefly mention, are underway to improve the sensitivity of such passive imagers. However, most of the emphasis in our program is on active imaging. With this approach, much simpler and lower-cost detectors, such as (uncooled) antenna-coupled microbolometers can be used, at the expense of incorporating slightly more complex optics and illumination components. I discuss several tradeoffs presented in the design of active imaging systems for the 100 to 1000 GHz frequency range, describe how we have addressed them in the design of a scanning, 95 GHz, bolometer-based imager for concealed weapons detection that is nearing completion, and describe how the system architecture can be modified to scale the operating frequency to the 650 GHz atmospheric window. \newline \newline Co-authors: Arttu Luukanen and Aaron Miller   

Session N6: Recent Work on Strongly-coupled Fermi Gases II

What do we know about the state of cold fermions in the unitary regime?

A gas of interacting fermions is in the unitary regime if the average separation between particles is large compared to their size, but small compared to their scattering length. Until recently the only physical realization was perhaps in neutron stars, but now experiments with trapped atoms have brought these unique systems into the laboratory. Such gases are strongly interacting many-body systems. From the theoretical point of view the properties of a fermion system in the unitary regime are remarkable, often being referred to as universal. Such a system is at the crossroad between a fermion and a boson superfluids, or what is called the BCS to BEC crossover. From the experimental point of view the fact that basically all parameters of such a system can be tunned essentially at will make these systems unique in physics, realizing for the first time many thought experiments envisioned by theorists over the years. The theorists are at the verge at describing fully and extremely accurately from first principles the properties of these strongly interacting many fermion systems. Many of the experimental results obtained so far are consistent with the theoretical expectation that these systems are superfluid. However, the existence of a superflow as such has not been put in evidence in experiments, yet! In this talk I shall review the basic experimental results obtained so far and confront them with our most detailed theoretical description available at this time.   

Scattering properties of weakly-bound dimers of Fermi atoms

We discuss the behavior of weakly bound bosonic dimers formed in a two-component Fermi gas with a large positive scattering length for the interspecies interaction. We present a theoretical approach for solving a few-body scattering problem and describe the physics of dimer-dimer elastic and inelastic scattering. We explain why these diatomic molecules, while in the highest ro-vibrational level, are characterized by remarkable collisional stability. \newline \newline Co-authors are Christophe Salomon, LKB, Ecole Normale Superieure, Paris, France; Georgy Shlyapnikov, LPTMS, University of South Paris, Orsay, France.   

Quantum Monte Carlo study of a Fermi gas in the BCS-BEC crossover

We calculate the equation of state of a two-component Fermi gas with attractive short-range interspecies interactions using the fixed-node diffusion Monte Carlo method. The interaction strength is varied over a wide range by tuning the value $a$ of the $s$-wave scattering length of the two-body potential. For $a>0$ and $a$ smaller than the inverse Fermi wavevector our results show a molecular regime with repulsive interactions well described by the dimer-dimer scattering length $a_m=0.6 a$. The momentum distribution of atoms, the pair correlation functions and the condensate fraction of pairs are discussed as a function of the interaction strength.   

Observation of the pairing gap in a strongly interacting Fermi gas

We study fermionic pairing in an ultracold two-component gas of lithium-6 atoms by observing an energy gap in the radio-frequency excitation spectra. With control of the two-body interactions through a Feshbach resonance, we demonstrate the dependence of the pairing gap on coupling strength, temperature, and Fermi energy. The appearance of an energy gap with moderate evaporative cooling suggests that our full evaporation brings the strongly interacting system deep into a superfluid state.   

Spectroscopic studies of superfluid atomic Fermi gases

We consider trapped atomic Fermi gases with Feshbach-resonance enhanced interactions in temperatures below and above the superfluid critical temperature. We analyze the spectrum of RF(or laser)-excitations for transitions that transfer atoms out of the superfluid state, and compare the results with recent experiments, both at the unitarity regime where a pseudogap is likely and away from it. We also analyze other spectroscopic signatures of pairing, e.g. in optical lattices.   

Session N7: Einstein and Friends II

Einstein and Boltzmann

In 1916 Einstein published a remarkable paper entitled ``On the Quantum Theory of Radiation''\footnote{A. Einstein ``On the Quantum theory of Radiation,'' Phys. Zeitschrift 18 (1917) 121. First printed in Mitteilungender Physikalischen Gesellschaft Zurich. No 18, 1916. Translated into English in Van der Waerden ``Sources of Quantum Mechanics'' (North Holland 1967) pp. 63-77.} in which he obtained Planck's formula for black-body radiation by introducing a new statistical hypothesis for the emmision and absorption of electromagneic radiation based on discrete bundles of energy and momentum which are now called photons. Einstein radiation theory replaced Maxwell's classical theory by a stochastic process which, when properly interpreted, also gives well known statistics of massless particles with even spin.$^{2}$ This quantum distribution, however, was not discovered by Einstein but was communicated to him by Bose in 1924. Like Boltzmann's classical counterpart, Einstein's statistical theory leads to an irreversible approach to thermal equilibrium, but because this violates time reversal, Einstein theory can not be regarded as a fundamental theory of physical process.\footnote{M. Nauenberg ``The evolution of radiation towards thermal equilibrium: A soluble model which illustrates the foundations of statistical mechanics,'' American Journal of Physics 72 (2004) 313} Apparently Einstein and his contemporaries were unaware of this problem, and even today this problem is ignored in contemporary discussions of Einstein's treatment of the black-body spectrum.   

Einstein, Bohr \& Born: Scientific Friendships and their Vagaries

Einstein, Bohr and Born, individually and also as members of a generation, founded modern physics as we know it today. We shall explore some of the threads that connected their life and work across several decades, the personal closeness but also the disagreements so vital to scientific discourse.   

Einstein and Planck

As an editor of the Annalen der Physik, Max Planck published Einstein's early papers on thermodynamics and on special relativity, which Planck probably was the first major physicist to appreciate. They respected one another not only as physicists but also, for their inspired creation of world pictures, as artists. Planck helped to establish Einstein in a sinecure at the center of German physics, Berlin. Despite their differences in scientific style, social life, politics, and religion, they became fast friends. Their mutual admiration survived World War I, during which Einstein advocated pacifism and Planck signed the infamous Manifesto of the 93 Intellectuals supporting the German invasion of Belgium. It also survived the Weimar Republic, which Einstein favored and Planck disliked. Physics drew them together, as both opposed the Copenhagen Interpretation; so did common decency, as Planck helped to protect Einstein from anti-semitic attacks. Their friendship did not survive the Nazis. As a standing secretary of the Berlin Academy, Planck had to advise Einstein to resign from it before his colleagues, outraged at his criticism of the new Germany from the safety of California, expelled him. Einstein never forgave his old friend and former fellow artist for not protesting publicly against his expulsion and denigration, and other enormities of National Socialism. .   

Einstein and Ehrenfest

After Paul Ehrenfest's untimely death, Albert Einstein wrote about their first meeting more than twenty years earlier. ``Within a few hours we were true friends – as though our dreams and aspirations were meant for each other.'' In fact, this warm friendship with a fellow theoretical physicist of his own age was unique in Einstein's life. I shall try to characterize it in this talk.   

Session N9: Geometrically Frustrated Magnets I

Incommensurate magnetic order from a geometrically frustrated spinel CdCr$_2$O$_4$

In an ideal pyrochlore lattice, antiferromagnetic spins are three-dimensionally frustrated and do not order down to zero temperature. It is often found in real life, however, that a long-range order appears by a symmetry-breaking transition at an ordering temperature much lower than Curie-Weiss temperature. ZnCr$_2$O$_4$ is a well-known example showing a long-range commensurate magnetic order that is coincident with a cubic-to- tetragonal distortion with $c < a$. In this study, we have observed a closely-related compound CdCr$_2$O$_4$, which has a larger A-site ion. Interestingly, the cubic-to-tetragonal distortion occurred with $c > a$, and the coincident magnetic order turned out to be incommensurate. This indicates that the nonmagnetic A-site ions play a ciritical role in determining the ground state properties of the chromate spinels. We present the model for the spin structure, and discuss the possible mechanisms that can lead to the incommensurate order.   

Probing spin correlations with phonons in the strongly frustrated magnet ZnCr$_2$O$_4$

Geometrically frustrated magnets can resist magnetic ordering and remain in a strongly correlated paramagnetic state well below the Curie-Weiss temperature. The spin-lattice coupling can play an important role in relieving the frustration in these system. In ZnCr$_2$O$_4$, an excellent realization of the Heisenberg antiferromagnet on the ``pyrochlore'' network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at $T_c=12.5$~K. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ZnCr$_2$O$_4$ tracks the nearest-neighbor spin correlations above $T_c$. Below $T_c$, the phonon triplet splits into a singlet and a doublet, separated by 11~cm$^{-1}$. This splitting is directly proportional to the spin-Peierls order parameter. We also observed a number of weak absorption bands, arising below $T_c$, which indicates doubling of the Brillouin zone at the structural/magnetic phase transition.   

Nonuniform spin-Peierls distortion in the ``pyrochlore'' antiferromagnet ZnCr$_2$O$_4$

The cubic spinel ZnCr$_2$O$_4$ is a nice realization of the Heisenberg antiferromagnet on the ``pyrochlore lattice'' with spins $S = 3/2$. A high degree of geometrical frustration creates a strongly correlated paramagnetic regime at low temperatures where spins remain liquid and move in a highly coordinated fashion. The frustration is eventually lifted through a spin-Peierls-like transition at 12.5 K [1]. A basic physical picture of the transition has been developed by Yamashita and Ueda [2] and Tchernyshyov, Moessner and Sondhi [3] for the cases of a uniform lattice distortion. However, accumulating experimental evidence points to the dominance of a nonuniform lattice distortion that enlarges the structural unit cell [4]. The lattice modes most likely involved in the transition are the four phonon doublets with the wavevectors $\langle \frac{1}{2} \frac{1}{2} \frac{1}{2} \rangle$. We discuss the symmetry aspects of the appropriate spin-Peierls order parameter and ways of measuring it. [1] S.-H. Lee {\em et al.}, Phys. Rev. Lett. {\bf 84,} 3718 (2000). [2] Y. Yamashita and K. Ueda, Phys. Rev. Lett. {\bf 85,} 4960 (2000). [3] O. Tchernyshyov, R. Moessner, and S. L. Sondhi, Phys. Rev. B {\bf 66,} 064403 (2002). [4] H. Ueda {\em et al.}, Bull. Amer. Phys. Soc. {\bf 48,} 826 (2003).   

Geometrical Frustration in Rare-Earth Face-Centered Cubic Crystals

In a 3-dimensional solid the simplest form of magnetic frustration is spins on a tetrahedron with antiferromagnetic coupling. A face-centered cubic ($fcc$) lattice is a simple example of a network of edge-sharing tetrahedra; however, most $fcc$ compounds exhibit well-defined magnetic order, dominated by next-neighbor ($nn$) and next-nearest-neighbor ($nnn$) interactions. To minimize the effects of $nnn$ interactions and maximize frustration, the network of edge-sharing tetrahedra has to be divided into sub-networks of corner-sharing tetrahedra, as is realized in pyrochlore and spinel structures. A further example of a $fcc$-lattice split in two sub-networks of corner-sharing tetrahedra are the intermetallic ternaries $RE$InCu$_4$ ($RE$ = heavy rare-earth). Here the rare-earth ions occupy a $fcc$-lattice, where half of the tetrahedra are filled with an In-ions and the other half with a Cu-tetrahedron. The extent of frustration in these systems is determined by the magnetic moment of the rare-earth ion and second by their separation distance, which can be tuned with chemical substitution, e.g. Ni for Cu. We present measurements of electrical resistivity, magnetic susceptibility and specific heat on single crystals of the title compounds with the trivalent rare-earth ions Gd, Dy, Ho and Er, demonstrating geometrical frustration of their spin and orbital angular momentum.   

Field induced magnetic ordering and spin excitations in the frustrated pyrochlore Antiferromagnet Tb$_{2}$Ti$_{2}$O$_{7}$

Tb$_{2}$Ti$_{2}$O$_{7}$ is a rare earth titanate which is characterized by antiferromagnetically-coupled Tb moments on the geometrically frustrated pyrochlore lattice. However unlike other members of this family, Tb$_{2}$Ti$_{2}$O$_{7}$ displays no static magnetic order at temperatures down to 70mK$^{1}$. Time-of-flight neutron scattering was performed on a single crystal of Tb$_{2}$Ti$_{2}$O$_{7}$ at the NIST Center for Neutron Research. Applied magnetic fields of up to 8.5T, directed along the $<$110$>$ direction, induced two magnetically ordered states at T=1 K. This applied field clearly splits the degenerate excited state doublet, and induces a dispersive collective spin excitation which appears to go to zero energy at the 001 and, possibly, 003 zone centres. These results require continuous spin degrees of freedom to be relevant to the Tb moments. Modeling of the ordered phases will also be presented. $^{1}$Gardner \textit{et al}., Phys. Rev. Lett. \textbf{82},1012 (1999)   

AC susceptibility studies of the cooperative paramagnet Tb$_{2}$Ti$_{2}$O$_{7}$

The cooperative paramagnet Tb$_{2}$Ti$_{2}$O$_{7}$ has generated much interest in the frustrated magnetism community due to the lack of ordering down to temperatures well below its Curie-Weiss temperature which is $\sim$10 K. Its pyrochlore magnetic lattice and highly anisotropic g-factor suggest a similarity to spin ice (i.e. Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$), however, the first excited crystal field level is of the same order as the exchange interaction. This presents a novel system where crystal field degeneracy should have an effect as a magnetic field is applied. We present ac susceptibility studies on single and polycrystalline samples over broad ranges of temperature, static magnetic field, and frequency. We find two peaks emerge in the complex susceptibility as a function of temperature in the presence of a static magnetic field, and we discuss the data in terms of the effects of single ion and cooperative behavior. This research is supported by the National Science Foundation.   

Neutron studies of Yb$_2$Ti$_2$O$_7$

Polarised neutron scattering experiments, including neutron spin echo has been performed on ytterbium titanate. Ytterbium titanate is an insulator that crystallizes into the cubic oxide pyrochlore structure with a lattice parameter of approximately 10.03 {\AA}. Earlier studies have shown this sample to have an effective paramagnetic moment of 3.0 $\mu_B$. During these investigations it was found that the system undergoes a phase transition at 240 mK, however it is not as simple as that reported previously in the literature. We can conclusively rule out the presence of a static, ferromagnetic state and confirm that the majority of the spin system remains dynamic below this transition.   

Evidence for a strong coupling transition in Y$_2$Ru$_2$O$_7$

Y$_2$Ru$_2$O$_7$ adopts the pyrochlore structure with Ru occupying the octahedral sites. Magnetization and specific heat measurements indicate that there is a magnetic phase transition at T= 78 K. Diffraction experiments have shown that the Ru-sublattice orders in a long range ordered q=0 structure where the total spin vanishes on each tetrahedron. We have performedneutron inelastic scattering experiments in order to investigate this magnetic phase transition. These experiments show what I would call a strong coupling transition within this material. There are rearrangements in the excitation spectrum at T= 78 K to energies that correspond to room temperature without an apparent change in the wave vector dependence. Light scattering experiments indicate that magneto-elastic effects are important in allowing this phase transition to proceed. From previous work on spinel’s it has been suggested that this type of transition should be considered the 3 D analogue of a spin-peierls transition in a cooperative spin system [1]. This research was funded by the U.S. Department of Energy, under Grant No. DE-FG02-02ER45983, and by the National Science Foundation, under Grant No. NSF-DMR-0103858. [1]S.-H. Lee et al., Phys. Rev. Lett. 84, 3718 (2000).   

Effective Low-Energy Hamiltonian for the Frustrated Tb$_2$Ti$_2$O$_7$ Pyrochlore Antiferromagnet

The antiferromagnetic pyrochlore Tb$_2$Ti$_2$O$_7$ presents a challenging puzzle to experimentalists and theorists studying frustrated magnets. Results from muon spin resonance and neutron scattering experiments for Tb$_2$Ti$_2$O$_7$ reveal a paramagnetic structure down to $50$mK despite an antiferromagnetic Curie-Weiss temperature, $\theta_{\rm CW}=-20 $K. Crystal field calculations show that the Tb$^{3+}$ ion in Tb$_2$Ti$_2$O$_7$ is a ground state doublet with local $\langle 111 \rangle$ anisotropy and is separated from the first excited doublet state by a gap of 20K. To understand the quantum nature of Tb$_2$Ti$_2$O$_7$, we apply the Rayleigh- Schrodinger method to map the four state problem with exchange and dipole-dipole interactions onto an effective Hamiltonian with two states per ion. We give some properties of this effective hamiltonian and compare the neutron scattering results with the experimental data.   

Magnetic Orders and Fluctuations in the Dipolar Pyrochlore Antiferromagnet

While the classical Heisenberg antiferromagnet on the pyrochlore lattice does not order, we will discuss, from a theoretical standpoint, possible magnetic phases induced by the dipole-dipole interactions. Such interactions play a role in systems such as Gd$_2$Ti$_2$O$_7$ or Gd$_2$Sn$_2$O$_7$ in stabilizing exotic forms of magnetic order, a subject of current debate. We will also argue that the external magnetic field induces multiple transitions, one of which is associated with no obvious broken symmetry, but can be characterized by a disorder parameter. Finally, Monte-Carlo simulations and Landau-Ginzburg expansion show that the dipolar Heisenberg model exhibits a fluctuation-induced first-order transition, thanks to the frustration and a continuous set of soft modes.   

Ice: a strongly correlated proton system

We discuss the problem of proton motion in Hydrogen bond materials with special focus on ice. We show that phenomenological models proposed in the past for the study of ice can be recast in terms of microscopic models in close relationship to the ones used to study the physics of Mott-Hubbard insulators. We discuss the physics of the paramagnetic phase of ice at 1/4 filling (neutral ice) and its mapping to a transverse field Ising model and also to a gauge theory in two and three dimensions. We show that H3O+ and HO- ions can be either on a confined or deconfined phase. We obtain the phase diagram of the problem as a function of temperature T and proton hopping energy t and find that there are two phases: an ordered insulating phase which results from an order-by-disorder mechanism induced by quantum fluctuations, and a disordered incoherent metal phase (or plasma). We also discuss the effects in the proton motion introduced by the lattice vibrations (phonons) and its effect on the phase diagram. Finally, we suggest that the transition from ice Ih to ice XI observed experimentally in doped ice is the confining-deconfining transition of our phase diagram.   

Bulk Properties and Neutron Scattering studies of LiVO2

LiVO2 crystallizes in the space group R-3m with V$^{3+}$ (S = 1,$t_{2g}^2 )$ ions forming a two-dimensional triangular lattice. Magnetically, the system changes from a high temperature Curie-Weiss paramagnetic state to a low-temperature non-magnetic state at T $\sim $ 500 K. It has been proposed [1] that this phase transition is associated with a peculiar frustration-related orbital ordering resulting in trimers of V$^{3+}$ ions forming a spin-singlet ground state. Single crystals of LiVO2 with typical size approximately 3 $\times $ 2 $\times $ 0.1 mm$^{3}$ have been grown by the flux method. These crystals have been studied by electron diffraction, susceptibility, and specific heat measurements. The results are largely consistent with the V$^{3+}$ trimer model picture. We also report recent powder and single crystal inelastic neutron scattering studies of the magnetic excitations in LiVO$_{2}$. [1] H. F. Pen \textit{et al}, Phys. Rev. Lett. \textbf{78} (1997) 1323.   

A Study of the Magnetic Properties of the Frustrated Spinels GeNi2O4 and GeCo2O4

The spinels GeNi$_{2}$O$_{4}$ and GeCo$_{2}$O$_{4}$, in which the spin-1 Ni$^{2+}$ or spin-3/2 Co$^{2+}$ ions are located on the vertices of a lattice of corner-sharing tetrahedra, exhibit interesting magnetic and structural properties. GeNi$_{2}$O$_{4}$ has a double N\'{e}el transition (T$_{N1}$ = 12.13 K and T$_{N2}$ = 11.46 K), but the crystal structure remains cubic in the N\'{e}el state. In contrast, GeCo$_{2}$O$_{4 }$has a single N\'{e}el transition (T$_{N}$ = 20.6 K) that coincides closely with a cubic to tetragonal structural phase transition, below which $c$/$a >$ 1. In the past we have used magnetic susceptibility, heat capacity, synchrotron x-ray, and neutron powder diffraction to study these materials. In this talk we will describe selected recent results of elastic and inelastic neutron scattering measurements of polycrystalline and single crystalline samples, the latter grown by the floating zone technique at AIST.   

Heat Capacity and Neutron Scattering Studies on Geometrically Frustrated Ho Double Perovskites

We have studied the magnetic properties of the frustrated double perovskites A$_{2}$HoSbO$_{6}$ (A = Sr or Ba) by performing heat capacity measurements in applied magnetic field and inelastic neutron scattering. The Ho nuclear contribution to the heat capacity displays a unique field dependence that can be explained by the slowing down of the Ho electron spins in an applied field. The lowest magnetic level appears to be a doublet. Inelastic neutron scattering has conclusively determined the energy of these low lying crystalline electric field levels.   

A large-N approach to two-dimensional frustrated ising antiferromagnets with transverse external and exchange fields

A large-N approach is developed to treat the problem of frustrated ising antiferromagnets on two-dimensional lattices-- in particular the triangular and kagom\'e. Quantum dynamics is introduced in this model by either a transverse external field, or an exchange field that may be ferromagnetic or anti- ferromagnetic. Special emphasis is given to the Kagom\'e XXZ model in which disordered phases are found for both ferromagnetic and anti-ferromagnetic transverse exchange.   

Session N10: Focus Session: Spin Transport and Dynamics in Quantum Dots

g tensor modulation resonance and single-spin manipulation in semiconductor quantum dots

We explore how electric fields can be used to drive single spin resonance in quantum dots without AC magnetic fields. We calculate the g tensor for a single electron in a semiconductor quantum dot as a function of electric field along the growth direction of the dot. The calculations are based on an eight-band envelope-function formalism[1]. The growth-direction g factor is relatively insensitive to this electric field, but for InAs/GaAs dots with transition energies around 1.2 eV the in-plane g factor changes by 20\% for an electric field of 150kV/cm. For a DC magnetic field oriented at 45 degrees to the growth direction the spin precession axis for an electron changes by 6 degrees from zero electric field to 150 kV/cm. Thus an AC pseudo-magnetic field almost 10\% the size of the DC magnetic field can be generated. This is sufficient to drive g-tensor modulation resonance[2] in the dot and perform single-spin manipulation. 1. C. E. Pryor and M. E. Flatt\'e, cond-mat/0410678. 2. Y. Kato, et al., Science 299, 1201 (2003).   

Land\'e $g$ factors and orbital angular momentum quenching in semiconductor quantum dots

We present calculations of g-factors for nanocrystal and self-assembled quantum dots. We find that in addition to the effects of dot geometry and strain, quantization quenches the orbital angular momentum of the dot states, pushing the electron $g$ factor towards $2$ even when all the semiconductor constituents of the dot have negative $g$ factors. This leads to trends in the dot's electron $g$ factors that are the opposite of those expected from the effective $g$ factors of the dot and barrier material. Both electron and hole $g$ factors are strongly dependent on the magnetic field orientation; hole $g$ factors for InAs/GaAs quatum dots have large positive values along the growth direction and small negative values in-plane. The approximate shape of a quantum dot can be determined from measurements of this $g$ factor asymmetry. This work was supported by DARPA/ARO DAAD19-01-1-0490.   

Spin Dynamics of Charged Colloidal Quantum Dots

Colloidal semiconductor quantum dots are promising structures for controlling spin phenomena because of their highly size- tunable physical properties, ease of manufacture, and nanosecond-scale spin lifetimes at room temperature. Recent experiments have succeeded in controlling the charging of the lowest electronic state of colloidal quantum dots \footnote{C. Wang, B. L. Wehrenberg, C. Y. Woo, and P. Guyot-Sionnest, \textit{J. Phys. Chem B} \textbf{108}, 9027 (2004).}. Here we use time-resolved Faraday rotation measurements in the Voigt geometry to investigate the spin dynamics of colloidal CdSe quantum dot films in both a charged and uncharged state at room temperature. The charging of the film is controlled by applying a voltage in an electrochemical cell and is confirmed by absorbance measurements. Significant changes in the spin precession are observed upon charging, reflecting the voltage- controlled electron occupation of the quantum dot states and filling of surface states.   

Spin Dynamics and Energy Levels in Quantum Shells

In a zero-dimensional analogue to planar quantum wells, nanoparticle heterostructures known as quantum-dot quantum wells (QDQWs) allow for the study of single quantum-confined electrons in an engineered potential energy landscape. We have characterized colloidal CdS/CdSe/CdS QDQWs using two-color time-resolved Faraday rotation (TRFR). The spin dynamics show that the electron g-factor is tunable with quantum well width and the transverse spin lifetime of several nanoseconds is robust up to room temperature. As a function of probe energy, the amplitude of the TRFR signal shows pronounced resonances, which allow one to identify individual exciton transitions. While the TRFR data are inconsistent with the conduction and valence band level scheme of spherical QDQWs, a model in which broken spherical symmetry is taken into account captures the essential features.   

Spin Dynamics in InAs/GaAs Quantum Structures

Arrays of InAs SAQD's with narrow size distributions are being developed for applications in optoelectronics and quantum information. Here we describe initial measurements of spin dynamics from a wafer with a varying dot-density. We measured T$_{2}^{\ast }$ using Time Resolved Kerr Rotation (TRKR) for a wavelength resonant with the 2D InAs wetting layer in a region of the sample where the Stranski-Krastanow strain mediated quantum dots have not formed and found a g-factor of 0.42 and the lifetime to be 125 ps. We attribute this relatively short lifetime, compared to the recombination time of $\sim $1 ns, to the inhomogeneity of the wetting layer. However, when performing the same measurement in a region of the sample where the dots were present the electron lifetime decreased by an order of magnitude to 12 ps. The reduction in lifetime is attributed to the exciton created in the wetting layer being captured by the dot. Weak signals were observed when resonant with the dots. Work supported in part by ONR, NSA/ARDA, and DARPA/SPINS. JW is an NRC/NRL Postdoctoral Research Associate.   

Electron spin relaxation by hyperfine interaction in a double quantum dot

We use a pulsed-gate technique to measure singlet-triplet relaxation in a GaAs/AlGaAs few-electron double quantum dot at low magnetic field. Electrostatic pulses are applied to probe the time dynamics of the (1,1) to (0,2) charge state transition, while average dot occupation is measured by nearby quantum point contact charge sensors. In the (0,2) configuration only a spin singlet is allowed, blocking the transition from (1,1) if a triplet state is initially formed, but relaxation is strongly enhanced near zero magnetic field. We attribute this enhancement to different nuclear environments in each dot, and extract an average effective Overhauser field of ~3 mT. This implies a spin dephasing time of ~30 ns in this system.   

Orbital Kondo Effect and Spin Polarization in Carbon Nanotubes

In this talk I will review our recent experiments on low-temperature electronic transport in carbon nanotube (CNT) quantum dots (QDs). I will focus, in particular, on strongly coupled quantum dots exhibiting Kondo effect. By means of a magnetic field we are able to modify the energy spectrum of CNT QDs, such that we can tune two orbital states with equal spin polarization into degeneracy. This purely orbital degeneracy enables the observation of an orbital Kondo effect and shows that carbon nanotubes can potentially act as low-impedance spin filters. At zero magnetic field, the four-fold degeneracy of the nanotube spectrum allows to observe a strong Kondo effect, with new transport properties in the non-linear regime and described theoretically by a so-called SU(4) symmetry.   

Spin coherence in CdS quantum dots

Spin coherence in CdS quantum dots (QDs) in a glass matrix has been investigated. Time-resolved differential transmission experiments were performed to measure the decay of the degree of circular (linear) polarization DCP (DLP). We show that due to the nearly spherical shape of our QDs the properties of DCP and DLP are considerably different compared to the most often investigated self-assembled QDs that are of pyramidal shape. Namely, we observed a decay of DCP with two distinct time components (300 fs and 10 ns at 300 K) and a strong dependence of the initial values of DCP on the laser wavelength. Our theoretical analysis of the experiments implies that the slow component in the observed decay of DCP is dominated by intralevel exciton transitions with electron spin flip, which are driven by the electron--hole exchange interaction and assisted by two LO phonons. We show that two-phonon processes significantly contribute also to exciton transitions without electron spin flip, which lead to the appearance of a fast component in the decay of DCP. This work was supported by the Grant agency of the Czech Republic (grant 202/03/P150), by the Ministry of Education of the Czech Republic (project 1K03022), by IUAP and FWO-V projects G.0274.01N, G.0435.03, the WOG WO.025.99N (Belgium).   

SU(4) Kondo effect in carbon nanotube quantum dots

We investigate theoretically the non-equilibrium transport properties of carbon nanotube quantum dots. Owing to the two-dimensional band structure of graphene, a double orbital degeneracy plays the role of a pseudo-spin, which is entangled with the spin. Quantum fluctuations between these four degrees of freedom result in an SU(4) Kondo effect at low temperatures. This exotic Kondo effect manifests as a four-peak splitting in the non-linear conductance when an axial magnetic field is applied [1]. Recent transport experiments in carbon nanotube quantum dots [2] clearly support our theoretical findings. [1] M. S. Choi, R. Lopez and R. Aguado, cond-mat/0411665 (2004). [2] P. Jarillo-Herrero, J. Kong, H. S. J. van der Zant, C. Dekker, L. P. Kouwenhoven and S. De Franceschi, to be published (2004).   

Spin polarization of current passing through a double level quantum dot in magnetic field

The spin polarization of the current passing through a GaAs quantum dot is found experimentally by Potok \textit{et. al}.$^{1}$ to be always polarized in the same direction as external field when varying the gate voltage. This is in disagreement with theories that assume single energy level on the quantum dot. We investigate the problem by considering a model of double level quantum dot with strong exchange coupling between the electrons on the two levels. A generalized 1/N expansion method was used to construct the approximate ground state, spectral function and then the current. Our results are compared with experimental result by Potok \textit{et.al}.. \newline \newline $^1$R. M. Potok, J. A. Folk, C. M. Marcus, V. Umansky, M. Hanson, and A. C. Gossard, Phys. Rev. Lett. \textbf{91}, 016802(2003)   

Auger recombination of excitons in semimagnetic quantum dot structure in a magnetic field

We present the results of magnetoluminescence study of ZnSe:CdMnSe quantum dots samples in a magnetic field up to 11 T both in the Faraday and Voigt geometries at liquid He temperatures and various levels of laser excitation. We found that the intensity of the quantum dot photoluminescence strongly increases (up to two orders of magnitude) in the Faraday geometry and only slightly ($\sim$ 1.5 times) in the Voigt geometry within the range of B=0-11 T . We explain the strong increase of the photoluminescence in the Faraday geometry within the frame of the spin-dependent Auger recombination of excitons through Mn ions. We relate the observed anisotropy of the quantum dot emission with the high anisotropy of the hole spins in QDs. We present a theoretical model which allows us to obtain selection rules for the Auger transition and thoroughly explains experimental results. The selections rules allow to explain characteristic fitures in single quantum dot spectra.   

Single exciton spectroscopy in a single semimagnetic quantum dot

Motivated by recent experiments of single spin detection [1,2] and optically induced magnetization [3], the problem of a photoexcited (II,Mn)VI Diluted Magnetic Semiconductor quantum dot is studied. The Hamiltonian of N spins exchange-coupled to an electron hole pair is solved both exactly (for N$\sigma$ 7) and in mean field approximation, using numerical diagonalization. The Hamiltonian involves as well the electron-hole pair exchange, the spin orbit interaction of the holes and the Mn-Mn antiferromagnetic exchange. The ground state and spin wave excitation are obtained both with and without the exciton present in the dot. Using linear response theory in the light-matter coupling, the absorption and emission spectrum are calculated and compared with those of the experiments. The optical signatures of the exciton induced spin order are discussed. Prospects for the optical detection of the number of Mn spins in a single are commented. [1] G. Bacher et al. Phys. Rev. Lett. {\bf 89}, 127201 (2002) [2] L. Besombes, et al. Phys. Rev. Lett. {\bf 93}, 207403 (2004) [3] S. Mackowski et al., Appl. Phys. Lett. {\bf 84}, 3337 (2004)   

Magnetically-controlled impurities in quantum wires with strong Rashba coupling

We investigate the effect of strong spin-orbit interaction on the electronic transport through non-magnetic impurities in one-dimensional systems. When a perpendicular magnetic field is applied, the electron spin polarization becomes momentum-dependent and spin-flip scattering appears, to first order in the applied field, in addition to the usual potential scattering. By tuning the Fermi level and the Rashba coupling, it is possible to suppress the potential scattering and the spin-flip scattering will dominate at low temperatures. As a result, the resistance of the wire will strongly depend on the magnetic field.   

Oscillatory spin-flip rates and anisotropic g-factor in quantum dots

We study phonon-induced electron spin relaxation rates in parabolic quantum dots (QDs) as function of in-plane and perpendicular magnetic fields as well as of QD lateral and vertical sizes. Rashba and Dresselhaus spin-orbit (SO) couplings are included via exact diagonalization of the model. Deformation and piezoelectric couplings for acoustic phonons are considered, and we show how the former (latter) yields the dominant mechanism in a narrow (wide) gap material. We also report an oscillatory spin-flip rate between QD Zeeman sublevels. In the minima of such rates, quite large spin relaxation times can be obtained in properly designed QDs. The rich interplay between external magnetic fields and intrinsic SO interactions is studied, where two distinct phases are visible in the spectrum of GaAs QDs in perpendicular fields if their vertical width is narrow. We also discuss the QD g-factor strong anisotropy and show how even a sign change can be induced for large magnetic field [1]. Good agreement with available experimental findings is obtained. [1] C. F. Destefani and Sergio E. Ulloa, cond-mat/0411071.   

Session N12: Vortices in Superconductors IV

Studies of quantum fluctuations and competing orders on vortex dynamics in cuprate superconductors

The existence of competing orders (CO) and the proximity to quantum criticality (QC) in cuprate superconductors create unconventional low energy excitations and significant quantum fluctuations (QF) which can alter the low temperature vortex dynamics of cuprates. We report studies on the effect of QF and CO on vortex dynamics in cuprates at low temperatures, focusing on the four-layer, hole-doped HgBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{x}$ (Hg-1234). Hg-1234 has two underdoped inner layers that are anti-ferromagnetic and two optimally doped outer layers that are superconducting. Vortex phase diagrams, derived from 3$^{rd}$ harmonic AC hall probe and high-field DC cantilever magnetization measurements, allow comparison of Hg-1234 with other cuprates such as YBa$_{2}$Cu$_{3}$O$_{7-x}$ and La$_{0.1}$Sr$_{0.9}$CuO$_{2}$. Comparison plots of the ab-plane reduced fields (normalized by the paramagnetic field, H$_{para})$, h$_{irr.}$(t)=H$_{irr.}$(t)/H$_{para }$and h$_{C2}$(t)=H$_{C2}$(t)/H$_{para }$ versus reduced temperature, t, demonstrate that QF and CO indeed affect Hg-1234 more than other cuprates, with Hg-1234 having the smallest extrapolated value of h*$\equiv $ h$_{irr.}$(0)$\approx $0.12, indicating its closest proximity to QC.   

Single Superconducting Vortex Depinning Force Measured with a Magnetic Force Microscope

The pinning of vortices plays an important role in superconductor applications, allowing transport currents to flow without dissipation. Measurements of vortex pinning to date have mostly been bulk measurements, yielding average properties without specific information about individual vortex pinning sites or strengths. We demonstrate the use of a magnetic force microscope to measure the depinning forces of single vortices in a superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ film.   

Super-Hard Superconductivity

We present the magnetic response of Type-II superconductivity in the extreme pinning limit, where screening currents within an order of magnitude of the Ginzburg-Landau depairing critical current density develop upon the application of a magnetic field. We show that this ``super-hard'' limit is well approximated in highly disordered, cold drawn, Nb wire whose magnetization response is characterized by a cascade of Meissner-like phases, each terminated by a catastrophic collapse of the magnetization. Direct magneto-optic measurements of the flux penetration depth in the virgin magnetization branch are in excellent agreement with the exponential model in which $J_c(B)=J_{co}\exp(-B/B_o)$, where $J_{co}\sim5\rm{x}10^6$ A/cm$^2$ for Nb. The implications for the fundamental limiting hardness of a superconductor will be discussed.   

Experimental Evidence for Giant Vortex States in a Mesoscopic Superconducting Disk

Response of a mesoscopic superconducting disk to perpendicular magnetic fields is studied by using the multiple-small-tunnel- junction method, in which transport properties of several small tunnel junctions attached to the disk are measured simultaneously. This allows us for the first experimental distinction between the giant vortex states and multivortex states. Moreover, we experimentally find magnetic field induced rearrangement and combination of vortices. The experimental results are well reproduced in numerical results based on the nonlinear Ginzburg-Landau theory.   

Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNi$_{2}$B$_{2}$C: A penetration depth study

The coexistence of weak ferromagnetism and superconductivity in ErNi$_{2}$B$ _{2}$C suggests the possibility of a spontaneous vortex phase (SVP) in which vortices appear in the absence of an external field. We report evidence for the long-sought SVP from the in-plane magnetic penetration depth $\Delta \lambda (T)$ of high-quality single crystals of ErNi$_{2}$B$_{2}$C, using a high-precision tunnel-diode based, self-inductive technique at 21~MHz.. In addition to a slight depression of superconductivity at the N\'{e}el temperature $T_{N}$ = 6.0~K and at the weak ferromagnetic onset at $T_{WFM}$=2.3~K, $\Delta \lambda (T)$ rises to a maximum at $T_{m}=0.45$ K before dropping sharply down to $\sim $0.1~K. We assign the 0.45 K-maximum to the proliferation of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing $\Delta \lambda (T)$ as the vortex density increases, and its subsequent decrease below $T_{m}$ as defect pinning suppresses vortex hopping.   

Finite-size Intrinsic Josephson Junctions under Layer Parallel Magnetic Field

Oscillations of Josephson vortex flow resistance have been studied in finite-size Bi-2212 intrinsic Josephson junctions as a function of magnetic field applied parallel to the junctions. The lengths of junctions fabricated are around one micrometer so as to enhance pinning effects of the Josephson vortex lattice to the junction edges and thus to enhance the formation of rectangular vortex lattice which would lead an \textit{in-phase} mode of the junctions as a possible candidate for the THz generator application. The observed Josephson vortex flow resistance showed a periodic oscillation with a period ($H_{P})$ corresponding to one flux quantum enters per junction, namely, corresponding to the rectangular vortex lattice. The peaks in the oscillations were found at the fields $H$=\textit{nH}$_{P}$, here n shows an integer number. Therefore, the Josephson vortex lattice flow speed is maximum when the outermost vortex rows geometrically match to the edges of the junction. Contrary to this, minimum of flow speed, namely pinning of the vortex lattice, was observed at $H$=($n$+1/2)$H_{P}$. The pinning configuration will be discussed based on the strong edge effects in finite-size intrinsic Josephson junctions.   

Anisotropic vortex structure in tilted magnetic fields in the spin triplett superconductor Sr$_{2}$RuO$_{4}$

Using a $\mu$SQUID microscope we imaged magnetic flux above the ab surface in the unconventional anisotropic superconductor Sr$_{2}$RuO$_{4}$, at temperatures between 0.4 K and 1.3 K and magnetic fields between 0 to 70 gauss at various angles. We observed vortex chains as well as coexistence of vortices and chains for tilted fields. The distance between the chains varies as 1/B. The mass anisotropy expressed as the ratio of the penetration depth ($\lambda_{c}/\lambda_{ab}$) is about 20 for Sr$_{2}$RuO$_{4}$, situating Sr$_{2}$RuO$_{4}$ in respect of anisotropy between YBCO and BSCCO. We'll discuss the different origins for vortex chain formation for these three superconductors. In the case of Sr$_{2}$RuO$_{4}$ we can successfully describe the ordering of the flux into vortex chains using Ginzburg-Landau theory.   

Phase Diagram for a Hiden Competing Order in the Mixed State of YBa$_2$Cu$_4$O$_8$ CRYSTALS UP TO 150 kG

Torque measurements on YBa$_2$Cu$_4$O$_8$ crystals show multiple peaks as a function of $\theta_{ca}$ between the c axis and the magnetic field $H$ in the c-a plane. We propose that the first peaks at $\theta_{ca} \simeq 80$ and 100 degrees arise from a possible long-range spiral spin density wave through the matching pinning between vortices and spins. We construct a contour map for a competing order parameter in YBa$_2$Cu$_4$O$_8$ which starts at a finite $H$ and is located far from Hc2 line. The regime of the proposed spin density wave goes to higher temperatures when field increases. The first peak does not appear when the field direction is scanned in the c-b plane. The NMR line width of the c-axis aligned YBa$_2$Cu$_4 $O$_8$ powders in H perpendicular c shows broadening at temperatures below 30 K. These features might be intrinsic in the cleanest underdoped cuprate.   

AC current rectification in Nb films with or without symmetrical Nb/Ni periodic pinning arrays in perpendicular magnetic field

Rectification of AC current has been observed in plain superconducting Nb films and in Nb/Ni films with symmetric periodic pinning centers. The rectified DC voltage appears for various sample geometries (cross or strip) both along and transverse to the alternating current direction, is nearly anti-symmetric with perpendicular magnetic field and strongly dependent on temperature below T$_{c}$. Analyses of the data at different temperatures, drive frequencies from 100kHz to 150MHz and at the different sample sides [1] shows that not far below Tc the rectification phenomena can be understood in terms of generation of electric fields due to local excess of critical current. Further below T$_{c}$ anisotropic pinning effects could also contribute to the rectification. [1] F.G.Aliev, et al., Cond. Mat.405656. Supported by Comunidad Autonoma de Madrid -CAM/07N/0050/2002   

Vortex Dynamics in Confined Systems Studied by Scanning

We report on a study of vortex arrangements in mesoscopic superconductors using scanning tunneling microscopy and spectroscopy. Using NbSe$_{2}$ single crystals and conventional ion beam lithography we defined different mesoscopic systems that confine vortex motion. The spatial confinement of vortices introduces novel collective behavior that strongly depends on the size of the superconductor, vortex density, and temperature. We will compare the experimental results with existing theoretical models based on Ginzburg-Landau theory.   

Vortex-glass transitions in Low-Tc superconductors

We have measured I-V characteristics in the mixed state of superconducting plain Nb thin films, Nb thin films with artificial periodic arrays of magnetic pinning centers, and Nb/Cu superlattices. Using a scaling analysis of I-V characteristics, we have found clear evidence of a vortex-glass transition in those Low-Tc systems. Such glass transitions belong to the same universality than those earlier observed in High-Tc systems, since similar critical exponents have been found. We have studied the effect of the presence of periodic pining centers on the glass transition, the effect of the artificial anisotropy in the dimensionality on the glass transition of Nb/Cu superlattices, as compared to the behavior of Nb plain films. Among the most remarkable results, a dimensional crossover in the glass transition is found, from a three-dimensional VG transition in Nb films to a two-dimensional one in Nb/Cu superlattices.   

AC-Susceptibility and Ultrasonic Attenuation Measurements of Vortex Dynamics in the Vicinity of the Peak Effect in V-Ti Alloys - Multicriticality Revisited

In-situ SANS and ac-susceptibility measurements have provided evidence for a first-order Bragg glass transition into a disordered vortex state in a Nb single crystal. This transition manifests itself in the peak effect (PE) in the critical current density, widely believed to be associated with the sudden softening of the vortex lattice. Subsequent studies mapping the full phase diagram in the same sample have suggested the existence of four distinct phase boundaries meeting at a single multicritical point (MCP). The natures of the transition lines combined with simple thermodynamic requirements suggest that the MCP is a bicritical point. This would rule out either the bulk transition line T$_{c2}$(T) or the surface superconducting transition H$_{c3}$(T) as being related to the MCP. Mutual inductance magnetic ac-susceptibility and ultrasonic attenuation measurements in V-21at.{\%}Ti have unequivocally established the presence of a PE in this alloy. The H-T phase diagram for this sample will be presented and vortex dynamics in the vicinity of the PE will be discussed. We are indebted to Prof. Shapira of Tufts University for providing us with the sample. This work was supported by the NSF under Grant No. DMR-0406626.   

Specific Heat Measurements at the Bragg Glass Disordering Transition

We will report specific-heat measurements on a Nb single crystal, in which previous neutron scattering studies showed that the peak effect (PE) coincides with an order-disorder transition of the vortex lattice. The PE transition shows metastability and is believed to be first order. In addition, the PE transition line has been uncovered to merge with the low-field continuous transition H$_{c2}$(T) line at a multicritical point. Thermodynamic considerations imply that this is a bicritical point. Measurements of specific heat and latent heat throughout the H-T phase diagram are essential for understanding the nature and relevance of the observed transition lines and the different phases of vortex matter in the presence of quenched disorder. In specific, the measurements will elucidate the nature of the disordered phase, vortex liquid or vortex glass, for bulk type-II superconductors. The measurements are performed in a home-built calorimeter cryostat. The cryostat allows for continuous-heating temperature measurements in vacuum with a heat-leak regulating stage included between the sample and the helium bath.   

Effect of the misalignment between the applied and internal magnetic fields on the critical currents in tilted superconducting thin films

The analysis of critical current density ($J_c$) as a function of the orientation of the applied magnetic field (\textbf{H}) provides a very effective way to identify and discriminate the various pinning mechanisms in high temperature superconductors. In thin and/or anisotropic superconductors, the vortices may decrease their free energy by tilting their orientation with respect to \textbf{H}, therefore producing a misalignment between the applied and internal magnetic fields. We present $J_c$ angular studies in films with the crystallographic axis tilted with respect to the sample surface. This results in a rich behavior, characterized by an asymmetric angular dependence and a shift of the $J_c$ $ab$-planes maxima position as function of the magnetic field. We show under which conditions the misalignment must be taken into account and quantify the magnitude of the shift.   

NMR spin-echo measurement of driven vortex motion in the superconducting phase of the electron-doped HTSC Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$.

We report modulation of the $^{63}$Cu spin echo as a function of arrival time in the superconducting phase of a single crystal of Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$. It occurs when the sample is immersed in liquid He, but not in He vapor. Our interpretation is that the RF pulses that form the spin echo excite an ultrasonic oscillation in the NMR coil which is transmitted to the sample via the He liquid. It generates an oscillatory motion of the vortex lattice, which causes the oscillation of the local magnetic field oscillation responsible for the spin echo modulation (near 20 kHz). Two features that support this interpretation are: (1) Adding mass to the NMR coil changes the spin echo oscillation frequency, and (2) the echo modulation disappears when the sample leaves the superconducting phase because the magnetic field is rotated away from the $a-b$ plane. We thank L. Bulaevskii for important comments and the NSF for support from Grants DMR-0334869 (WGC) and DMR-0203806 (SEB).   

Session N13: Devices and Applications I

The Influence of Dielectric Imperfections on the Optical Properties of 1D and 2D Photonic Crystal Structures

Both 1D and 2D photonic crystals have become extremely useful tools in the optics industry due to the presence of wavelength-tunable photonic band gaps. However, little is known about the optical effects of dielectric imperfections, such as interfacial roughness, in the geometry of these structures. We have employed a Finite Difference Time Domain (FDTD) code to explore this problem and gain further insight into the effect of such imperfections on the optical properties of both 1D and 2D photonic crystal geometries. Imperfections that have been explored include: interfacial roughness and surface scratches in 1D photonic crystals, and aspherical holes in 2D photonic crystal structures. We present the effects of these imperfections on the reflectivity of the photonic crystal structures for wavelengths corresponding to the perfect structures' photonic band gap. We also provide a parameterized fit to quantify the results and aid in tolerance estimations.   

The effect of the Crystal Geometry and Photorefraction to Electro-optic field sensors

For the development of electro-optic (EO) field sensors we have investigated the EO response of several Lithium Niobate and Strontium Barium Niobate crystals. The EO responsivity is greatly affected by the spatial variation of the refractive index induced by the photorefractive effects. For a Lithium Niobate crystal, with which the photorefractive effect is negligibly small, the detector output signals could be accurately modeled by assuming a beam of coherent phase. For a strongly photorefractive Strontium Barium Niobate crystals, we need to modify the model using a distribution function for the phase of polarization to reconcile our model with the data. Such distributions were evident through spatial-temporal instabilities of the detector signals. While the modeled distribution of the polarization phase which fits the data corresponds to refractive index variations of only 10$^{-5}$, the impact on the EO detector sensitivity can be devastating. We will discuss our detailed experimental results on the EO and photorefractive materials.   

Strain Induced Switching of Magnetostrictive Dot Arrays

For further miniaturization of magnetostrictive TMR strain sensors [1], it is necessary to analyze the switching properties of the magnetostrictive free layers under mechanical strain. Therefore, we have combined MEMS and thin-film technologies in order to fabricate highly magnetostrictive FeCo and amorphous CoFeSiB micro-/ nano-dot arrays on 0.5-1$\mu $m thick Si3N4-membranes with diameters of 50 to 300$\mu $m. By applying variable pressure to the membrane the dot arrays were exposed to compressive or tensile strain. MFM as well as MOKE measurements have been applied to resolve the micromagnetic structures and the corresponding hysteresis loops for different levels of applied mechanical strain. For 1$\mu $m CoFeSiB dots we observed a multi-domain state under stress free condition, whereas for 0.01{\%} of tensile strain a single domain behavior has been observed with an alignment of the magnetization parallel to the strain direction. By changing to compressive strain the domains are rotated by 90\r{ } leading to a magnetization perpendicular to the strain direction as expected for positive magnetostrictive materials. [1] M. L\"{o}hndorf et al. APL 81(2), 313 (2002)   

Minimizing 1/f Noise in Magnetic Sensors with a MEMS Flux Concentrator

The performance of magnetic sensors at frequencies on the order of 1 Hz is generally limited by $1/f $noise and the fact that the signal is small relative to the DC background. We have developed a new device, the MEMS flux concentrator, that solves these problems. Often flux concentrators (soft magnetic materials) are placed around magnetic sensor to increase the field. In the new device, the flux concentrators are placed on MEMS flaps that are driven to oscillate in the plane of the sensor, by electrostatic comb drives, at a frequency of about 15 kHz. The two MEMS flaps are connected by Si springs so that there is an 180$^{\circ}$ out of phase normal mode. If the amplitude of the motion is 12 microns, the amplitude of the magnetic field at the position of the sensor varies by a factor of about two. At 15 kHz, the sensor is operating in a region where the 1/f noise is often reduced by several orders of magnitude. Spin valves were employed as the magnetic sensor. SOI wafers were used in the fabrication. Because the Q of the mechanical resonance is 30, only microwatts of power are required to drive the motion.   

Anti-Stokes solid-state random laser

We report on the first demonstration of anti-Stokes solid-state laser operating in the regime, when only one pumping photon is required to excite one electron to the upper laser level. The anti-Stokes stimulated emission, which photon energy was almost fifty percent larger than the energy of the pumping photon, was realized in GaAs \textit{random laser}. Stimulated emission in random lasers is supported by feedback provided by scattering. In the experiment, highly scattering GaAs powder was pumped with $\sim $5 ns pulses of optical parametric oscillator tunable between 920 and 1300 nm. The random laser emission with the maximum at $\approx $885 nm (which corresponds to the edge of the absorption band in GaAs) has been observed when the pumping energy exceeded some critical threshold level. In some measurements, two distinctively different slopes have been found in the input-output curve. At 1100 nm pumping, the stimulated emission threshold scaled with the diameter of the excited spot d as $\sim $d$^{x}$, with x$\le $2. This behavior is typical to one-photon pumping of anti-Stokes GaAs random laser, while at two-photon pumping, the expected range of x is 3$<$x$<$4. The longest pumping wavelength, at which the stimulated emission in GaAs random laser has been obtained, was equal to 1300 nm. This implies that in an ideal medium without loss the cooling effect per photon can be as high as 46{\%}.   

Suppression of spatial hole-burning in a standing wave solid-state laser with a degenerate resonator

We numerically and experimentally demonstrated that the spatial hole burning in a solid-state laser with a standing wave resonator can be suppressed in use of a tightly-focused pumping beam. The laser can self- adjust its mode waist to match the small pump volume when it is operated in a degenerate cavity configuration, so that variation of the gain profile along the laser crystal can be minimized via the gain saturation.   

Recovering semiconductor lasers from coherence collapse by orthogonal-polarization optical feedback

A coherent optical feedback (COF) greater than around -30 dB will generally conduct a single-mode semiconductor laser to the coherence collapse that present a multimode oscillation and high-level intensity noise in the laser's output. This research employed orthogonal-polarization optical feedback (OPF) to recover semiconductor lasers from coherence collapse, induced by strong COF, to the solitary single-mode state. Experimentally, under a COF as strong as -14 dB, an OPF of -29 dB could recover the laser's primitive single-mode state from multimode. Moreover, a pre-fed OPF of around -25 dB provided the semiconductor laser with a resistivity against up to -19 dB COF. These results will significantly improve the performance of semiconductor lasers in many applications and provide a new method to investigate the coherence collapse.   

Nuclear Magnetization by Rotating Magnetic Fields Detected with a Superconducting Quantum Interference Device

We demonstrate that, in the absence of any static magnetic field, protons in a liquid sample can be polarized along the $z$-direction by application of a magnetic field rotating in the $xy$-plane. By detecting spin precession in 3 $\mu$T with a low-$T_{\mathrm{c}}$ Superconducting QUantum Interference Device, we observed that a rotating field can induce nuclear polarization comparable to that from a static field of the same magnitude. The spin-lattice relaxation times of Cr-doped methanol samples in the frame rotating at 10 kHz were the same as those in the laboratory frame within the error of the experiment. This experiment provides a direct test of the modified Bloch equation that includes spin relaxation in the instantaneous field when strong oscillating fields are present. A field rotating at several kHz is capable of polarizing only materials with short correlation times of spin fluctuation ($\tau_c \ll$ 1 ms) such as liquid. Therefore, use of such fields to prepolarize the sample enables high-resolution liquid-state nuclear magnetic resonance experiments even in the presence of strongly magnetic solid material near the sample. Supported by USDOE.   

Magnetic resonance elastography detected with a SQUID in microtesla magnetic fields

We have used a SQUID-based microtesla magnetic resonance imaging (MRI) system to perform magnetic resonance elastography (MRE) experiments in a measurement field of 132 microtesla. Magnetic resonance elastography is based on MRI and measures three-dimensional displacement and strain fields in a sample. With appropriate data processing this allows for a quantitative map of the physical response of a material to an applied deformation. In the past, MRE experiments using conventional (1.5 tesla and above) MRI systems have demonstrated that MRE may be used as a non-invasive method for measuring stiffness of human tissues, which may aid in the detection and diagnosis of breast cancer and other cancers. Our MRE experiment consists of applying a small axial deformation to a cylindrical sample of 0.5\% agarose gel. For samples approximately 30 mm in height, we were able to measure displacements on the order of 500 micrometers. Supported by USDOE.   

Correcting Concomitant Gradient Distortion in Microtesla Magnetic Resonance Imaging

Progress in ultra-low field magnetic resonance imaging (MRI) using an untuned gradiometer coupled to a Superconducting Quantum Interference Device (SQUID) has resulted in three-dimensional images with an in-plane resolution of 2 mm. Protons in samples up to 80 mm in size were prepolarized in a 100 mT field, manipulated by $\sim $100 $\mu $T/m gradients for image encoding, and detected by the SQUID in the $\sim $65 $\mu $T precession field. Maxwell's equations prohibit a unidirectional magnetic field gradient. While the additional concomitant gradients can be neglected in high-field MRI, they distort high-resolution images of large samples taken in microtesla precession fields. We propose two methods to mitigate such distortion: raising the precession field during image encoding, and software post-processing. Both approaches are demonstrated using computer simulations and MRI images. Simulations show that the combination of these techniques can correct the concomitant gradient distortion present in a 4-mm resolution image of an object the size of a human brain with a precession field of 50 $\mu $T. Supported by USDOE.   

Mixing with the radiofrequency single-electron transistor

By configuring a radio-frequency single-electron transistor as a mixer, we have demonstrated good charge sensitivity and large bandwidth around a tunable center frequency. Our implementation greatly increases the spectral range achievable for sensitive broadband charge measurements with this device, limited only by the RC time of the transistor's center island. In the current demonstration operating at 300 mK, a 16 MHz resonance bandwidth has been shown to be tunable to 1.2 GHz with an unoptimized charge sensitivity of $5 \times 10^{-3} e$/Hz$^{1/2}$.   

Quantum Steering of Electron Wave in 70nm InAs Y-Branch Switches

We report quantum steering of electron wavefunction in gated Y-branch switches (YBS). In this quantum regime, the coherent transport characteristics are drastically different from their classical counterparts. The practical difficulty in realizing YBS has been in fabricating nanometer-scale electron waveguides without depleting electrons in narrow channels. We overcome this hurdle by using InAs because it has zero lateral depletion width. We first fabricated cross-junctions for bend resistance measurements to verify the relatively long elastic mean free path of our devices. In addition, in order to verify the gating effect, we characterized quantum point contact devices made on the same wafer and observed a series of quantum conductance plateaus. Finally, we measured conductances through two drains of YBS, and observed oscillatory transconductances that are always out of phase. Our observation not only verifies the quantum steering of electron wave functions in a semiconductor waveguide, but it also opens up possibilities for further studies of quantum switches in multiple-terminal, nanometer-scale structure for information processing.   

Session N14: Focus Session: Multifunctional Oxides II

Molecular beam epitaxy for advanced gate stack materials and processes

The material requirements for future CMOS generations - as given by the ITRS roadmap - are very challenging. This includes a high K dielectric without a low K interfacial layer, a high mobility channel and the appropriate metal gate. With the help of two projects INVEST and ET4US, we are building up a molecular beam epitaxy (MBE) infrastructure to grow this material set on large area wafers that can be further processed into small scale devices. In the INVEST project, we have developed an MBE system for the growth of complex oxides on semiconductors. The system follows the overall design of a production tool and is equipped with an RF atomic oxygen source, effusion cells, e-beam evaporators and a differential pumping stage. The oxide growth process starts with desorbing the initial surface oxide on the Si wafers in ultra-high vacuum and high temperature to create a clean reconstructed 2x1 surface. Using the atomic oxygen it is possible to oxidize the surface in a well controlled manner at low temperature and to grow very thin and dense SiO$_{x}$ layers, followed by the growth of 2-6 nm amorphous high K dielectrics. The process parameters permit to tune the interface layer from a SiO$_{x}$ rich to a silicide rich interface with a significant impact on the capacitance and the leakage. Initial focus is on developing an optimized growth recipe for high quality amorphous HfO$_{2}$ and LaHfO$_{3.5}$ films. This recipe was subsequently used to make wafers for a transistor batch that gave us the first N short channel MBE MOSFET's (100 nm) using an etched gate process flow. Some highlights of the first batch for 3nm HfO$_{2}$ MOSFET are a high mobility ($>$ 270 cm$^{2}$/Vs) with a corresponding low leakage current of 2 mA/cm$^{2})$. While there were some process issues for LaHfO$_{3.5}$, the 3 nm MOSFET showed very low leakage currents below 10$^{-6}$ A/cm$^{2}$. Interestingly all the LaHFO$_{3.5}$ MOSFETs showed very low threshold voltage instabilities. In collaboration with C. Marchiori, M. Sousa, A.Guiller, H. Siegwart, D. Caimi, J. Fompeyrine, D. J Webb, C. Rossel, R. Germann of IBM Research GmbH Zurich Switzerland; L. Pantisano, M. Claes, T. Conard, M. Demand, W. Deweerd, S. DeGendt, M. Heyns, M. Houssa, M. Aoulaiche, G. Lujan, L. Ragnarsson, E. Rohr, T. Schram of IMEC Leuven Belgium; J. Hooker, Z.M Rittersma, Y. Furukawa of Philips Research Leuven Belgium and J. W. Seo of EPFL Lausanne Switzerland.   

Interface Electrostatics and the Schottky Barrier for Alkaline Earth Oxides on Silicon

In our recent work relating to Schottky Barrier Height measurements for alkaline earth oxides on silicon we reported on Coulomb Buffer effects for oxide/semiconductor junctions. The Coulomb Buffer effects arise from both interface chemistry and dielectric phenomena associated with ionic polarizability in an interface phase. In this talk we will address the latter of these issues with both XPS and capacitance measurements of the electronic structure of alkaline earth oxide/silicon heterostructures. We find systematic variations of the barrier height that are unique to a crystalline/crystalline interface in a MOS capacitor. This observation suggests a coupling between the dielectric constant of the oxide and the charge density that is localized around the silicon atoms in the interface phase.   

Magnetoelectric interactions in single crystal ferrite-piezoelectric bilayers

The nature of low-frequency (10$^{-2}$ - 10$^{4}$ Hz) magnetoelectric (ME) coupling has been investigated in bilayers of single crystal Ni-Zn ferrites and polycrystalline lead zirconate titanate or single crystal lead magnesium niobate-lead titanate. Important observations are as follows. (i) The ME coupling in the bilayers is found to be stronger than in polycrystalline multilayers [1]. (ii) Zn substitution in ferrite is found to enhance the strength of ME interactions. (iii) ME voltage coefficients show significant variation with the orientation of the bias magnetic field. (iv) Data analysis using our model reveals that superior magneto-mechanical coupling in the ferrites is the cause of strong ME interactions [2]. 1. G. Srinivasan, E. T. Rasmussen, and R. Hayes \textit{Phys. Rev. B}. \textbf{67}, 014418 (2003). 2. M. I. Bichurin, V. M. Petrov and G. Srinivasan$.$ \textit{Phys. Rev. B}. \textbf{68}, 054402 (2003). - supported by grants from the the National Science Foundation (DMR-0302254), Russian Ministry of Education (Å02-3.4-278), and the Universities of Russia Foundation (UNR 01.01.026).   

Properties of a ferromagnetic semiconductor: epitaxial Co-doped SnO2 films

Room-temperature ferromagnetic semiconductors are sought for future spintronic applications. Co-doped SnO2 has been shown to be a dilute ferromagnetic material that exhibits a giant magnetic moment at room temperature [1]. Here we characterize thin Co-doped SnO2 films, epitaxially grown on r-cut alpha alumina by oxygen plasma assisted molecular beam epitaxy [2]. The films exhibit the rutile structure of the SnO2 host material with a (101) orientation with the Co dopants in a Co2+ high spin state. XPD establishes that these dopants occupy Sn sites and are not forming a secondary phase in the SnO2 matrix. Angle-resolved UPS has been employed to characterize the valance band of Co-doped and pure SnO2. Further physical and magnetic properties will also be discussed. [1] S.B. Ogale, et al, Phys. Rev. Let., 91, 7, 077205 (2003). [2] M. Batzill, et al, submitted to Thin Solid Films.   

Probing Enhanced Ferroelectricity in Strained SrTiO3 and BaTiO3 Epitaxial Films using optical Second Harmonic Generation

This talk will present real-time second harmonic generation (SHG) experiments used for \textit{in-situ} probing of ferroelectric domain reversal and phase transitions in \textit{strained} SrTiO$_{3}$ and BaTiO$_{3}$ epitaxial thin films grown commensurately on scandate substrates such as GdScO$_{3}$ and DyScO$_{3}$. The Curie temperature, T$_{c}$ shifts by hundred of degrees because of the compressive strains (up to -1.5{\%}) imparted to these films. Using SHG we find that Tc shifts to $\sim $27C in SrTiO$_{3}$ (which normally is not ferroelectric at any temperature) and to $\sim $650C for BaTiO$_ {3}$ thin films on DyScO$_{3}$ as compared to T$_{c}$ of 120C in bulk crystals. Studies on real-time dynamics of domain reversal under external fields in these strained films beyond their normal Curie temperatures where domains are not expected at all would also be presented.   

Anisotropic epitaxial strain effect on the charge-order phase of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$

Strain effect in charge and orbital ordered state has been investigated for Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films deposited on (100), (110), and (111)-oriented SrTiO$_{3}$ (STO) substrates. Films on STO (001) and (111) substrates have monotonous temperature dependence for magnetic and transport properties showing no first-order phase transition. On the other hand, films on STO (110) show a clear ferromagnetic-antiferromagnetic and metal-insulator transition due to the onset of the charge and orbital order. Optical transmission spectra for the films on STO (110) show anisotry between the in-plane two axes. From the result, the orbital order plane of the film on STO (110) is deduced to be (100) or (010) plane, which lies out of the film surface. The reason for the difference in the magnetic and transport properties among the films on different substrate orientations, and why the clear metal-insulator transition occurs only on (110) substrates will be discussed.   

Magnetoelectric effect in nanostructured multiferroic ferrite-ferroelectric composite films

Recently the multiferroic coupling of self assembled pillars of nanostructured CoFe$_{2}$O$_{4}$(CFO) embedded in BaTiO$_{3}$(BTO) matrix on SrRuO$_{3}$(SRO)/SrTiO$_{3}$(STO) substrate are reported. Thus, if the BTO-CFO nanocomposites can have a strong coupling between the order parameters, such composite structures will work as a new paradigm for the multiferroic research. The magnetoelectic effects are the coupling behaviors of two ferroic properties. The composite films of BTO-CFO and Pb(Zr$_{0.52}$Ti$_{0.48})$O$_{3}$-NiFe$_{2}$O$_{4 }$on SRO/STO and Nb-STO substrates were fabricated using pulsed laser deposition technique. The simultaneous growths of ferroelectric and ferrite phases were confirmed by the x-ray diffraction measurements. The ferroelectric and ferromagnetic properties are measured by TF analyzer and SQUID magnetometer, respectively. We will study the temperature dependence of dielectric properties varying external magnetic fields. We will also measure the transverse, $\alpha _{31}$, and longitudinal, $\alpha _{33}$, magnetoelectric voltage coefficient.   

SrTiO3 single crystal field-effect transistor with an amorphous CaHfO3 gate insulator

It becomes more and more important to understand the electronic properties of interfaces in transition-metal oxides from a viewpoint of utilizing such materials in devices; tunneling magnetoresistance (TMR) junctions, resistance random access memory (RRAM), or field-effect transistors (FET). SrTiO$_{3}$ is a wide-gap semiconductor and a good model system for studying the electronic structure of various oxides with similar crystal structures. We have fabricated a field-effect transistor composed of SrTiO$_{3}$ (100) single crystal as a channel and an amorphous CaHfO$_{3}$ layer as a gate insulator. The amorphous CaHfO$_{3}$ gate insulator layer, grown by pulsed laser deposition, was atomically flat and had an average breakdown field of 5 MV/cm. All electrode and channel patterning was done with simple contact masks. The device showed prominent n-type transistor operation, a field-effect mobility of 0.4 to 0.5~cm$^{2}$ / V s, and an on-to-off channel current ratio of $\sim $ 10$^{5}$ at room temperature. However, an improvement of these transistor properties was not observed at low temperatures. The device performance was limited by the electric structure of the interface.   

Hydrogen-induced defect generation in amorphous SiO$_{2}$

Dielectric breakdown of thin silicon oxides (SiO$_{2})$ of metal-oxide semiconductor field-effect transistors is a long-standing problem. The breakdown is generally recognized to be due to the accumulation of defects in the oxide. An amorphous SiO$_{2}$ (a-SiO$_{2})$ has been known to have very few defects, thanks to its strong and flexible network of Si-O bonds. Then, why are defects created in this good insulator? The hydrogen related chemistry model has the potential to explain dielectric breakdown, but it is still unclear how hydrogen could generate the energy required to break Si-O bonds. Using the quartz structure, molecular dynamics simulations have been executed with Reax Force Field (ReaxFF) method that is fitted to give first principle energy profiles. In these simulations we found that at elevated temperatures a meta-stable structure of hydrogen attached to Si- O bond is formed, which we will refer to as the Attached Radical (Att{\_} Rad). Att{\_}Rad has a degenerated half-filled electronic level, so will be good candidate for breakdown precursor. We will discuss its structure and energetics, comparing to the state with hydrogen at interstitial site. Using ReaxFF, we also managed to obtain a defectless a-SiO$_{2}$ structure and found that the Att{\_}Rad state is significantly more stable in a-SiO$_{2}$ compared to quartz, indicating that this state might initiate dielectric breakdown in Si/a-SiO$_{2}$ interfaces.   

High temperature oxidation studies of filtered arc deposited CrAlN nanolayered coatings on steel plates

The requirements of low cost and high-temperature corrosion resistance for interconnect plates in solid oxide fuel cell stacks has directed attention to the use of metal plates with oxidation resistant coatings. We have investigated the performance of steel plates with nanolayered coatings consisting of CrAlN. The coatings were deposited using large area filtered arc deposition technology,[1] and annealed for up to 25 hours in air at 800 $^{\circ}$C. The composition, structure and morphology of the coated plates were characterized using RBS, NRA, and AFM techniques. The oxidation rate was reduced by a factor of about 40 relative to the uncoated steel plates. [1] Vladimir I. Gorokhovsky, Rabi Bhattacharya and Deepak G. Bhat, Surface and Coating Technology, \textbf{140} (2) 2001, pp. 82-92.   

Session N15: Focus Session: Theory of Nanostructures and Nanowires

Radiative recombination of charged excitons and multiexcitons in CdSe quantum dots

Radiative recombination of neutral and charged biexcitons has been recently observed in CdSe nanocrystals using time-resolved, femtosecond spectroscopy. Here we report semi-empirical pseudopotential calculations of charged exciton and multiexciton emission spectra of CdSe nanocrystals. We studied the mono-exciton X (one electron, one hole, 1e-1h), the charged excitons X$^{- }$(2e-1h) and X$^{+}$ (1e-2h), and the charged biexcitons XX$^{- }$(3e-2h) and XX$^{+}$ (2e-3h). For a 3.9 nm-diameter CdSe nanocrystal, we found that the emission peak for the X$^{-}$ recombination overlaps with that of X (at 2.16 eV), while the X$^{+}$ emission peak is slightly blue-shifted (by 0.02 eV). We also found that the main peaks in the XX$^{-}$ and XX$^{+}$ emission spectra are significantly blue-shifted with respect to the exciton peak X (by 0.04 and 0.05 eV, respectively) because of inter-particle interactions. In the case of XX$^{-}$, we observe an additional peak of lower intensity at 2.50 eV originating from the recombination of a 1p electron state with a partially occupied 1p hole state. This work was supported by the US DOE Office of Science LAB3-17 initiative.   

Shape, charge, and alloy fluctuation effects on optical properties of million-atom InGaAs/GaAs dots$^{*}$

Single-dot spectroscopy makes it possible to probe in detail dot-to-dot changes in optical properties of self-assembled In$_{1-x}$Ga$_{x}$As/GaAs dots. An atomistic pseudopotential method combined with the configuration interaction approach reveal the role of shape, charge, and alloy fluctuations on the electronic structure, polarization of optical transitions, and excitonic radiative lifetimes of In$_{0.6}$Ga$_{0.4}$As/GaAs quantum dots. Several features emerge. (i) {\em Height fluctuations.} Recombination energies and excitonic binding of $X^0$ (neutral exciton), $X^{-}$, $X^+$, and $XX$ (biexciton) change significantly with height, but not with randomness. (ii) {\em Charge fluctuations.} The lowest transitions of $X^-$ and $X^+$ are naturally unpolarized, whereas those of $X^0$ and $XX$ are polarized in plane. Thus, charge fluctuations affect polarization of an ensemble of dots. (iii) {\em Alloy fluctuations.} Different random realizations (configurations) of the same alloy dot lead to radically different polarizations of transitions in $X^0$ and $XX$. In the light of our simulations, we discuss dramatic changes in polarization recently observed in InAs/GaAs single dots.\newline $^{*}\,$Supported by DOE-SC-BES-DMS   

Radiative lifetime of excitons in CdSe quantum dots

Recent experimental measurements have shown that the low-temperature (T $\sim$ 2K) recombination lifetime of excitons in CdSe nanocrystal quantum dots is relatively short, of the order of 10$^{-6}$ s for quantum dots in the 2-4 nm size range. These results are surprising, since the lowest excitonic state of CdSe quantum dots is optically ``dark,'' and the next, ``bright'' state is several meV higher in energy, so it is not thermally populated at low temperature. Using a semi-empirical pseudopotential approach, we have investigated the exciton radiative lifetime of CdSe quantum dots as a function of size and temperature. We find that indeed, in the case of fully passivated CdSe nanocrystals the low-temperature lifetime is at least three orders of magnitude longer than the experimental value. However, we also find that the presence of surface states, such as dangling-bond states, mixes the dark and bright exciton states, dramatically reducing the dark exciton lifetime, and bringing it in agreement with experimental data. We conclude that surface states are the controlling factor of dark-exciton lifetimes in colloidal CdSe dots.   

Predictive simulations of semiconductor nanostructures

\textit{Ab-initio} simulations are playing an increasingly important role in understanding matter at the nanoscale and in predicting with controllable, quantitative accuracy the novel and complex properties of nanomaterials. A microscopic, fundamental understanding of nanoscale phenomena is very much in demand, as experimental investigations are sometimes controversial and usually they cannot be explained on the basis of simple models. In this talk, \textit{ab-initio} molecular dynamics simulations and quantum monte carlo calculations of semiconductor nanoparticles will be presented, with focus on electronic and optical properties and on the microscopic structure of surfaces at the nanoscale. The characterization of nanoscale surfaces and interfaces is of paramount importance to predict the function of nanomaterials, and eventually their assembly into macroscopic solids, and it is still very challenging from an experimental standpoint, due to the lack of appropriate imaging techniques. The presentation will focus on Si, Ge, SiC nanoparticles and nanodiamond, and in addition we will discuss several results for II-VI dots and rods. (*) Work done in collaboration with G.Cicero, E.Draeger, J.Grossman, F.Gygi, D.Prendergast, A.Puzder, J.-Y.Raty, F.Reboredo, E.Schwegler, A.Williamson This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48   

Intrinsic Surface States in Semiconductor Nanocrystals: HgS Quantum Dots

Confined states in typical nanocrystals are localized to the dot interior. Surface states are extrinsic states localized at unsaturated dangling bonds or surface defects. We show that intrinsic surface states occur in nanocrystals made from negative gap semiconductors such as HgS. We use atomistic tight-binding theory which allows explicit atomic models for the surfaces. We consider spherical HgS nanocrystals with saturated dangling bonds and diameters up the bulk limit. In small HgS dots, the lowest conduction band states are cation-derived and the band-edge valence states are anion-derived, as for finite-gap dots. In bigger HgS dots, valence states and higher conduction band states evolve toward their bulk limits. However, the lowest conduction band state has high density at the surface and slowly decays into the dot. Band mixing is critical for this state. It has mixed cation and anion character and is partly s- and light-hole-like. As the dot size increases, this conduction state crosses the valence band edge, reaching a limit inside the bulk negative gap for very large dots. In this limit, the state is localized to the surface. The optical response of Hgs dots is discussed to identify signatures for intrinsic surface states.   

First principles comparison of alkyl terminated Silicon dots with Silicon-Carbide dots.

Using ab-initio methods, we have studied different quantum dots that could be synthesized using the three elements: Si, C and H. In particular, we have compared hydrogen and alkyl passivated Silicon dots with Silicon-Carbide dots. We find that in Si clusters with reconstructed (100) facets a complete alkyl passivation is possible, but steric repulsions prevents full passivation of Si dots with unreconstructed surfaces. In addition, our calculations show that the stability of alkyl passivated Si clusters depends on the length of the carbon chains. Alkyl passivation weakly affects optical gaps of Si quantum dots, while it substantially affects ionization potentials and electron affinities. We also investigate theoretically the possibility to fabricate silicon-carbide quantum dots passivated with H. We find that the optical properties of this type of dots would be weakly dependent on size but strongly influenced by the structure of the surface, which in turn depends on the growth conditions. We discuss the conditions where quantum confinement could be observed in SiC quantum dots. Our results suggest that depending on the experimental conditions either alkyl terminated Si dots or SiC dots could be form being optical gaps strongly dependent on the core and surface structure.   

All-electron and pseudo-potential studies of structural and electronic properties of Si chains and nanowires

Recent experiments\footnote{Y. Wu, et.al., Nature 430, 61 (2004); and references therein} invoke Si nanowires as promising materials for nanoscale electronic and optical devices. We carried out electronic structure calculations of silicon chains and nanowires, by using both the full-potential linearized augmented plane wave (FLAPW) method\footnote{E.Wimmer, H.Krakauer, M.Weinert, AJ Freeman, PRB 24, 864 (1981)} and the pseudopotential plane wave method. We studied two sets of H-terminated one nanometer silicon wires, one oriented along (001) and the other along(111); both show direct band gaps, with the (111) oriented wires showing a smaller gap ($\sim$2.1 eV) than (001) ($\sim$2.5 eV). This trend differs from that reported in the literature \footnote{F. Buda, et.al., PRL 69, 1272 (1992); A. M. Saitta, et.al., PRB 53, 1446 (1996)}, but it is the same in both our all-electron and well converged pseudopotential calculations. We also found that structural relaxations induce different effects on the band structure of differently oriented wires; the band gap change is nearly 0.2 eV between the ideal and relaxed models for (001) while it is negligible for (111) wires.   

One-dimensional hole gas in Ge/Si nanowire heterostructures

Two-dimensional (2D) electron and hole gas systems have played a central role in condensed-matter physics research, as well as high performance electrical and optical devices. In this talk, I will discuss a one-dimensional (1D) hole gas system based on a germanium/silicon core/shell nanowire heterostructure. At room temperature, hole accumulation in the intrinsic germanium channel was observed due to the valence band offset at the Ge/Si interface. At low temperatures, conductance quantization at values close to that expected of a ballistic conductor was observed, and was attributed to the long mean free path in the hole gas and confinement of the hole gas in the radial direction. These effects showed little temperature dependence and suggested that transport in these small diameter nanowires is ballistic even at room temperature. The demonstration of a 1D hole gas in a flexible nanowire heterostructure opens up a number of possibilities for investigating quantum phenomena in low-dimensional systems, as well as applications in both conventional and quantum computing schemes.   

Nanowire Photonic Circuit Elements

We report an approach for guiding and manipulating light on sub-wavelength scales using active nanowire waveguides and devices. Semiconducting nanowire structures are distinct from conventional transparent dielectric waveguides since absorption and emission occur for modes with near band edge energies. Quantitative studies show that light propagation in nanowire structures takes place with only moderate losses through sharp and even acute angle bends. The losses measured are compared to those reported recently for photonic crystal structures and plasmon waveguides. Furthermore, a straightforward nanowire based electro-optic modulator (EOM) was demonstrated. The EOM has been shown to yield a substantial field-dependent modulation of the intensity of light propagating through these active waveguides. In addition, an efficient electrically-based injection of light into guided modes of the active nanowire waveguides will be described. Progress towards and challenges for electrical and optical manipulation of light in nanowire waveguides for logic will be discussed.   

ZnO Nanowires: Building Blocks for Nanoscale Electronics, Optoelectronics and Chemical Sensors

Chemical vapor deposition synthesized single crystalline ZnO nanowires are configured as n-channel field effect transistors and their electrical transport properties are studied. It is observed that electron concentration and mobility in nanowires can be modified by varying the synthesis conditions. Photoluminescence and photoconductivity of individual nanowires are investigated. These nanowire field effect transistors demonstrate a broadband and polarization dependent photo- response. Due to the small diameter and tunable electron concentration, ZnO nanowire transistors are implemented as highly sensitive, gate refreshable chemical sensors with potential selectivity. These results open up the future applications of ZnO nanowires as one of the promising materials for nanoscale electronics, optoelectronics and chemical sensing devices.   

Linear and Nonlinear Optical Properties of GaN Nanoclusters

The linear and nonlinear optical (NLO) properties of III-V binary semiconductors have been a subject of active research since the late 1960s. Recent advancements in (a) experimental techniques to fabricate/produce stable nanometer-size binary atomic clusters composed of group III and group V elements and (b) techniques and tools to probe response properties of nano-scale objects, have attracted a great deal of attention in the linear and NLO properties of III-V nanoclusters due to their potential applications in future technologies. An important issue in a bottom-up approach to fabricating nanoclusters for future technological applications is an understanding of the evolution of response properties with cluster size. In order to develop such an understanding, we have undertaken a systematic study of the electronic and geometrical structures and the optical properties of III-V nanoclusters by first-principles \textit{ab initio} time-dependent Hartree-Fock calculations. In this talk, we present the results of our first-principles quantum mechanical studies of the electronic structure, stability, and linear and NLO properties of Ga$_{m}$N$_{n}$ atomic clusters, with values of $m$ and $n$ ranging between $1 $and $17$. Our calculated results suggest that the linear and NLO properties both exhibit strong dependence on the cluster size and shape. However, the size-dependence is more pronounced for the NLO properties than that for the linear optical properties.   

Session N16: Focus Session: Optical Properties of Subwavelength Apertures and Nanoparticle Arrays

Theoretical and experimental study of surface plasmon enhanced transmission of subwavelength apertures

We present a study of surface plasmon (SP) enhanced transmission through subwavelength apertures in Au films on glass. Samples consisting of 100-500nm linear apertures flanked by periodic corrugations were prepared using e-beam lithography with subsequent ion milling. Transmission through these structures were studied experimentally and modeled numerically. Geometric parameters were varied in the numerical simulation which used a 2D Green's function approach and a frequency dependent Au dielectric function. Transmission with flanking corrugations was significantly enhanced related to an isolated aperture, at the wavelengths corresponding to SP resonance in the structure in agreement with the literature. Periodic corrugation also affected the spatial dependence of the transmitted field. Transmission through these structures was spectrally and spatially mapped using near-field scanning optical microscopy which revealed field intensity distributions consistent with those observed in the simulations. The authors acknowledge financial support from the NSF.   

Transmission properties of an array of sub-wavelength holes in a metal film

It is known that the intensity of light transmitted through an array of holes can be surprisingly high at certain wavelengths, even when the holes are of sub-wavelength scales. The enhanced transmission is attributed to a coupling of surface plasmons on the two sides of the film. We have studied the systematics of this transmission of hole arrays in silver films. We have observed a scaling of the transmission wavelength with the hole spacing, and found that the effect can be observed at long wavelengths. We have also studied the effect of the angle of incidence of the light and found a very strong dependence of the transmission on this angle. In addition, significant polarization dependence is observed. Finally, the effect of the dielectric media on the surface-plasmon-enhanced transmittance will be addressed.   

Enhancement of Light Transmission through Thin-Film Bull’s Eye Structures

Previous researchers have shown that light transmitted through a single hole in a silver film can be strongly enhanced by the presence of periodic concentric groove structures (bull's eye) on the film. We present a systematic study of the wavelength dependence (0.3 -- 2.0 $\mu $m) as measured by a Zeiss microscope photometer of the transmission of bull's eye structures with varying dimensions. A focused ion beam microscope is used to drill successively larger holes on the same structure thus facilitating the identification of transmission emanating from the hole compared to transmission associated with evanescent waves coupled through the opaque portions of the structure. The intensity and spectral distribution of the transmitted light is correlated with variation of sample parameters including the film thickness (40 -- 100 nm), groove periodicity (2.0- 4.0 $\mu $m), hole diameter (0.5 -- 10 $\mu $m) and phase difference between the entrance and exit groves. The relevance of surface plasmons to the enhanced transmission and the predictions of theory will be discussed.   

Diffracted Evanescent Wave Model for Enhanced, Suppressed and Directional Transmission through Subwavelength Apertures

The transmission spectrum of an array of subwavelength apertures in a metal film displays a set of peaks related to the periodicity. Extraordinary transmission efficiencies at these positions have been claimed and associated with discrete grating-coupling conditions that excite surface-plasmon polaritons (SPPs). In this talk, we re-evaluate the magnitude and origin of the effect by proper normalization of the as-collected transmission spectrum of the array to that of the corresponding isolated hole. The normalized spectrum then reveals a sequence of both enhancements and suppressions of modest and similar magnitude (less than a factor of ten). This continuous modulation is inconsistent with an SPP-based interpretation, but rather suggests an underlying mechanism based on interference. A subwavelength aperture couples inefficiently to a specific surface mode such as an SPP because it diffracts light into a continuum of evanescent surface waves with a large distribution of in-plane k-vectors. We show however that these components sum to form an effective surface wave which is coherent over a short range and phase-shifted with respect to the source. We confirm the presence of this composite diffracted evanescent wave (CDEW) via interferometric experiments involving pairs of subwavelength apertures. We propose a new model for the anomalous transmission of hole arrays in which enhancement and suppression result from the interference of light directly incident upon (or emerging from) a given hole with CDEWs launched by neighboring holes. This model successfully predicts equivalent effects in non-metallic systems. In addition, it accounts for the salient optical properties of single apertures surrounded by surface corrugations, such as efficient, low-divergence beaming.   

Optical transmission through metallic bilayers with subwavelength apertures

The optical transmission through a periodical array of subwavelength apertures in a metal film can be strongly enhanced by resonance of the incident light with surface plasmon polaritons on the metal surfaces. The excitation of surface plasmons is accompanied by a dramatic enhancement of the local electromagnetic field on the metal surfaces. We have fabricated subwavelength structures consisting of two layers of metal. The metal layers are positioned sufficiently close to each other such that the evanescent fields of the surface plasmons generated in the first layer excite surface plasmons in the second layer. In some cases the two metal layers are laterally displaced such that no direct line of sight exists through the structure. Nevertheless, the transmission through a number of these devices remains remarkably high at resonance, comparable to the single layer value. We will discuss the dependence of the optical transmission on various sample parameters, including metal layer thickness, separation, lateral shift and incident angle of light.   

Nonlinear and Active Optics in Nanolenses

We consider surface plasmon amplification by stimulated emission of radiation (SPASER) and second harmonic generation (SHG) in an effective nanolens. Such a nanolens is an aggregate of several nanospheres with progressively decreasing radii and separations .....[1]. It has a ``hottest spot'' of highly enhanced local fields between the smallest spheres. We show that such a system surrounded by semiconductor quantum dots is also an efficient SPASER that generates dark eigenmodes with gigantic, temporarily coherent local fields .()..()[2]. We also consider SHG in the nanolenses where we show that highly enhanced SHG local fields are generated at the nanofocus. Numerical data are presented for silver spheres .[1] K. Li, M. I. Stockman, and D. J. Bergman, \textit{Self-Similar Chain of Metal Nanospheres as an Efficient Nanolens}, Phys. Rev. Lett. \textbf{91}, 227402-1-4 (2003). [2] K. Li, X. Li, M. I. Stockman, and D. J. Bergman, \textit{Surface Plasmon Amplification by Stimulated Emission in Nanolenses}, Phys. Rev. B (2005 (In Print)).   

Optical studies of plasmon resonances in Ag nanoparticle arrays.

The optical (UV and visible) response of Ag nanoparticle arrays is studied in reflection measurements. The behavior (position and width) of the plasmon resonance is investigated as a function of size and shape of the nanoparticles. While the resonant frequencies can be understood in terms of the Mie type resonant response of ellipsoidal particles the widths of the resonances are controlled by radiation damping and is very sensitive to particle size. A model based on the Maxwell-Garnett effective medium theory is developed to comprehend the behavior of the resonance.   

Harmonic generation from metal nanoparticle arrays

Metal nanoparticle arrays have optical coherence properties that are predicted to have interesting consequences for optical harmonic generation. We have prepared planar arrays of non-spherically symmetric silver-nanoparticle clusters using a combination of focused ion-beam lithography and pulsed laser deposition. The 150-fs amplified pulses of a Ti:sapphire laser were incident on the array, which was mounted on the rotatable sample stage of a dark-field confocal microscope; this arrangement permits the elimination of virtually all optical background so that the scattered harmonic signal from the array is easily detectable. We describe experiments designed to test a recent theoretical prediction [1] to the effect that the nanoparticle array should produce phase-matched second harmonics like those generated by bulk media without a center of inversion symmetry, and that the harmonic- generation efficiency should scale inversely as the square of the nanoparticle size.[1] N. I. Zheludev and , J. Opt. A: Pure Appl. Opt. 6 (2004) 26-28.   

Electromagnetic energy transport through metallic nanoparticle arrays

We investigate electromagnetic (EM) energy transport through arrays consisting of Au spheres, 50 nm in diameter, with the Finite Difference Time Domain (FDTD) method. We assume the Drude model for the dielectric response of the Au nanospheres. The Drude model is incorporated into the FDTD technique with the introduction of an appropriate time-dependent polarization current. This method is known as Auxillary Differential Equation (ADE) method [1]. In order to test the validity of our numerical findings, we first focus on the EM excitations on a single metallic nanoparticle [2]. We compare for the latter case the FDTD results with analytical calculations following Mie theory. Over all we found reasonably good agreement between the two. Our numerical results indicate EM energy transport through the Au nanochain. Nonetheless, a careful analysis of the field profiles suggests that nearest-neighbor tight-binding like models fail to describe certain aspects of the observed EM energy transport through the nanochain. [1] ``Computational Electrodynamics'' A. Taflove, S. C. Hagness, Artech House, Boston (2000). [2] ``Electromagnetic excitations on a single metallic nanoparticle'', S. Foteinopoulou, J. P. Vigneron and C. Vandenbem, unpublished.   

Size-dependent optical properties of VO2 nanoparticle arrays

Arrays of vanadium oxide nanoparticles with long-range order have been fabricated by pulsed laser deposition in an arbitrary pattern defined by focused ion-beam lithography. Interaction of light with the nanoparticles is controlled by the geometrical arrangement as well as by the differing optical properties displayed by the metallic and semiconducting phases of VO$_{2}$. Contrary to previous VO$_{2}$ studies, we observe that the optical contrast between the semiconducting and metallic phases is dramatically enhanced in the visible region, presenting size-dependent optical resonances and size dependent transition temperatures. The collective optical response as a function of temperature presents an enhanced scattering state during the evolving phase transition. The effects appear to arise because of the underlying VO$_{2}$ mesoscale optical properties, the heterogeneous nucleation behind the phase transition and the incoherent coupling between the nanoparticles undergoing an order-disorder-order transition. Arrays such as this open up new opportunities to study surface plasmon interactions for nanoparticles in close proximity, with the added advantage that the interaction can be switched on by the thermally driven metal-semiconductor phase transition in VO$_{2}$. This research was supported by the NSF-NIRT program (DMR0210785).   

Long-Range Transmission of Evanescent Waves

Metal/dielectric periodic multilayer structures are shown to exhibit a new type of photonic transmission bands corresponding to resonant tunneling of evanescent waves. We show that evanescent fields can propagate over long distances as Bloch waves. This occurs by means of surface wave resonant coupling with electron plasma oscillations. Consistent with this, we also find that plane wave transmission across a metal/dielectric periodic multilayer structure oscillates periodically with increasing number of metal/dielectric pairs. The presence of the Bloch evanescent states provides a channel for optical tunneling and the long-range transmission of the evanescent waves. The structures considered here can be fabricated with existing nanotechnology.   

The Physical Basis of Enhanced Transmission Through Small Apertures in Metallic Films

While the coupling of electromagnetic radiation through small apertures in a conducting screen is well understood based on the work of Bethe and, more rigorously, through an eigenmode expansion in the intervening waveguide, there has been recent interest in enhanced transmission phenomena. Greater coupling through the hole than might be expected based on the theory of apertures in perfectly conducting screens has been found. Studies with physical metals have focused on surface plasmon waves on the top and bottom surfaces of the metal film. We present numerical results for two-dimensional apertures in silver films with various geometries. The transmission is a function of the aperture width and film thickness variables. An analytical model is provided that explains the effect for silver and other metals.   

FDTD modeling of the optical fields produced by nanoarrays of coaxial structures on gold films

Extraordinary optical transmission has been observed for nanoarrays of apertures in thin metallic films, originally attributed to coupling with surface plasmons (SP) [1]. More recently Baida et al. [2] have suggested that even larger enhancements can occur with nanoarrays of subwavelength coaxial structures at wavelengths much longer than those of the SP resonances. We employ the NRL HASP (FDTD) code to simulate the electromagnetic fields, in the 500-1500 nm wavelength range, produced by nanoarrays of silica coaxial cylinders (or rings) embedded in a thin gold film. We analyze the transmission spectrum as a function of ring geometry, film thickness, and periodicity. We contrast the results obtained from isolated rings and cylinders with those from arrays to assess the roles of SP and the resonances of the isolated structures in accounting for any field enhancements. We discuss the mechanisms of the field propagation in the real metal (versus an ideal metal) that may account for enhancements. [1] T. Ebbesen, H. Lezec, H. Ghaemi, T. Thio, and P. Wolff, Nature \textbf{391}, 667 (1998). [2] F. I. Baida and D. Van Labeke, Phys. Rev. B \textbf{67}, 155314 (2003).   

Session N17: Research on the Learning and Teaching of Physics

LivePhoto Physics Video Analysis Homework

The LivePhoto Physics project is creating homework problems that require students to analyze videos of physical phenomena on their own computers. Some parts of the assignments resemble laboratory activities, some resemble context-rich problems, and some are more like traditional homework problems. After pilot testing with calculus-based introductory students at RIT, the project is beginning the beta testing phase. Sample assignments will be shown during this talk and the status of the testing will be discussed.   

Identifying Student Difficulties with Control of Variables Reasoning

Emerging standards for the science learning of precollege students can be regarded as a statement of what constitutes science literacy.$^{1}$ These standards emphasize basic concepts such as mass, volume and density, and fundamental process skills such as proportional reasoning, the interpretation of graphs and other representations, and the control of variables in the design of experiments. At Western Washington University, the liberal arts physics course is a general university requirement and for many students one of the only physical science course taken between high school and college graduation. Thus the pre-course understandings of these students can be taken as a measure of the level of science literacy attained in precollege education. An effort is underway at Western Washington University to examine what students know and are able to do both before and after course instruction. Preliminary results indicate that in many cases students have serious conceptual and reasoning difficulties with the material. An example that involves the interpretation of experimental results in deciding whether a particular variable influences ($i.e., $affects) or determines ($i.e., $predicts) a given result will be discussed. Evidence from written questions will be presented to identify specific student difficulties.$^{1}$See, for example, Project 2061, American Association for the Advancement of Science. 1990. \textit{Science for All Americans.}New York, NY: Oxford University Press.   

Student difficulties with the concept of work in introductory physics

In order to apply the principle of energy conservation correctly, students need to be able to calculate the work done on a deformable system. The distinction between calculating work on a non-deformable system and on a deformable system is one that is only rarely made in introductory texts and lectures. At the University of Washington, the Physics Education Group has been developing research-based tutorials to supplement traditional instruction in textbooks, lectures, and labs. We will discuss how students frequently misapply the definitions of work that they are taught for non-deformable systems and ways in which this affects instruction on energy conservation. Results from student pretests, post-tests, and individual demonstration interviews will be presented.   

Student conceptual understanding of energy

Nearly all physics courses include the concept of energy, and many choose energy as a unifying theme for the course. In this talk, we examine conceptual understanding of energy among student who have completed a diverse array of courses, including a lecture course for non-science majors, an inquiry-based course for pre-service K-8 teachers, and lecture and lab courses for science and engineering majors. We will examine student conceptions of gravitational potential energy and the motion of bodies in a gravitational field and illustrate common incorrect predictions. In addition, we will present results from a series of questions probing student characterization of energy as a material substance. Implications for the development of a model of energy conservation will be discussed.   

Research on Student Learning of Rigid Body Dynamics

Most students find it difficult to believe that the effect of a force on the center-of-mass acceleration of a rigid body does not depend on how that force affects the rotational motion of that body. In the Physics Education Group at the University of Washington, we are continuing to develop a tutorial to help students with this issue. The results of some post-tests suggest that student learning is improving. However, on some other post-tests, little or no improvement is seen. We will discuss possible reasons for this apparent discrepancy and describe how the results of our research suggest modifications for instruction in this and other areas of the introductory course.   

Student Models of Electric Current and Electric Potential in Activity-Based Physics

With a three-year FIPSE grant, it has been possible at the University of Nebraska at Kearney (UNK) to develop and implement activity-based introductory physics at the algebra level. It has generally been recognized that students enter physics classes with misconceptions about current and potential difference in simple series and parallel circuits. Many of these misconceptions persist after instruction. Pretest and posttest responses on the ``Electric Circuit Concept Test'' (ECCT) are analyzed to determine the models that students use. Responses are divided into expert model (correct answer), one or more student models (approximately equally common incorrect answers), and null model (all other answers) categories. A description of each student model is also given. Changes in the use of these models are used to identify persistent and non-persistent misconceptions.   

Examining student understanding of fundamental concepts in electric circuits

As part of an ongoing investigation, the Physics Education Group at the University of Washington is continuing to examine student understanding of fundamental concepts in electric circuits. Several new research questions have been designed and administered to a variety of different populations, including undergraduates in introductory calculus-based courses, preservice teachers, and inservice K-12 teachers. In particular, we have been examining the relationship between the ability of students to incorporate an electrical element into a complete circuit and their understanding of the requirements for the element’s internal construction. The results reinforce our findings from previous investigations that many students lack a functional understanding of a complete circuit. The insights we have gained from this research will be discussed in the context of specific examples.   

Student Understanding of Reflection from a Plane Mirror

In this talk we explore students' pre-instruction knowledge of conceptual and procedural pieces of knowledge that we believe are prerequisite to one's ability to generate correct light ray diagrams. We do so within the domain of image formation by a plane mirror. The research population is students in an algebra-based, introductory physics course at a medium-sized, urban, public university.   

Student understanding of entropy and the second law of thermodynamics in an introductory physics course

We are investigating students' thinking regarding entropy and the second law of thermodynamics in a calculus-based general physics course. Most students enrolled in the class have had previous exposure to thermodynamics in chemistry courses or in high-school physics, and so many of them have specific ideas about these concepts even before instruction begins. To explore these ideas we administered a series of free-response pretest questions during the first week of class, before any instruction on thermodynamics had taken place. The questions probed student conceptions about entropy and its relationship with other thermodynamic properties. We will present an analysis of these data, as well as follow-up interview data that shed additional light on students' thinking.   

Students' reasoning regarding entropy and the second law of thermodynamics in an upper-level thermal physics course

We have been extending our investigation of student learning in thermal physics to the upper-level course targeted primarily at junior and senior physics majors. We are monitoring the progress of the students as they attempt to unify the macroscopic and microscopic/statistical viewpoints into a coherent understanding of thermal physics concepts. We will report on initial results of this work regarding the development of students' understanding of entropy and the second law of thermodynamics.   

The Future of Physics in the Undergraduate Education of Biologists: Beyond the Algebra Based Course

The success of quantitative and computational methods of research in the biological sciences has incited calls for change in the undergraduate biological sciences curriculum. This reevaluation of the biology curriculum presents physicists with an opportunity to rethink and rebuild service courses such as the introductory algebra based physics course. Beyond the one-year introductory course, some of the more ambitious curricular reforms include calls for a third semester of physics for students who plan on doing biomedical research. This talk will briefly explore the open question of how we can best serve the evolving needs of our colleagues in biology by considering the calls for change in the biology curriculum such as BIO 2010 and reviewing the current state of the introductory physics course for biologists. In addition, this talk will review the successes and failures of research based courses such as the introductory calculus-based physics course for biologists at Cal State San Marcos.   

A Framework for Understanding Physics Instruction in Secondary and Undergraduate Courses

As physics curricula are revised to implement techniques widely understood to be effective, instructors and students may react negatively to the changes in methods. Based on observations, interviews and document analysis in three very different physics courses, a general framework for physics instruction is proposed. Understanding how both traditional and reformed curricula are implemented in high school and undergraduate physics classes can reveal key discontinuities between the intended and actual effects.   

Will the No Child Left Behind Act Promote Direct Instruction of Science?

Education research in physics at the high school and undergraduate level strongly suggests that interactive engagement enhances students' conceptual understanding much more than traditional Direct Science Instruction (DSI). Similar conclusions can be drawn from K-8 science-education research. Nevertheless, DSI predominates in CA because of the DSI- orientation of the CA State Board of Education and Curriculum Commission [1]. Likewise the U.S. Dept. of Education's (USDE's): (a) DSI-orientation as demonstrated by its recent national-education summit showcasing of the research of Klahr and Nigam [2]; and (b) science achievement testing starting in 2007; threatens to promote DSI nationwide. It might be hoped that NRC's expert science education committees will steer the USDE away from promoting DSI, the antithesis of the NRC's own recommendations for inquiry methods. [1] R.R. Hake. ``Direct Science Instruction Suffers a Setback in California - Or Does It?" (2004), http://www.physics.indiana.edu/$\sim $hake/DirInstSetback-041104f.pdf. [2] Klahr, D. {\&} M. Nigam. 2004. ``The equivalence of learning paths in early science instruction: effects of direct instruction and discovery learning" (2004), http://www.psy.cmu.edu/faculty/klahr/papers.html   

Session N18: Focus Session: Wide Band Gap Semiconductors III

Semiconductor-Dielectric Interfaces: Structure, Defects and Mobility

The semiconductor-dielectric interface is the key to a successful MOSFET technology and has played the essential role in the silicon revolution. Wide-band gap materials have presented a challenge to achieve the same degree of interface perfection as silicon, although considerable progress is underway. The SiC/SiO$_{2}$ interface is of particular scientific interest in this development because of its close relationship to silicon, both in processing and structure. Silicon carbide itself provides an intriguing scientific platform for understanding such materials structures due to the availability of many poly-types with different band-gaps, access to different crystal faces-polar and non-polar-, and with a fabrication process that is similar to the well-studied silicon structure. This talk will review recent work in understanding in the oxidation process in SiC. We describe the nature of the (heavily) defected intrinsic interface, the use of chemical modification to reduce the interface defect density and the understanding of these processes that emerges employing varying band-gap and crystal-face, within the same bulk material. Systematic use of chemical modification and processing, combined with a careful analysis of interfacial structure, results in significant progress in reducing defects and increasing charge and inversion layer mobility. Collaborators: S. Dhar, J. Williams, S. Pantelides, L. Porter, J. Cooper Supported by DARPA Contract No. N00014-02-1-0628 and ONR DEPSCoR Grant N00014-01-1-0616.   

III-Nitrides on Ferroelectric Lithium Niobate: Impact of the Electrostatic Boundary Condition

Lithium niobate (LN) is a promising substrate for GaN high electron mobility transistors (HEMTs) for ``smart'' integrated electronics with optical modulators in fiber optical systems. Recent efforts\footnote{W. A. Doolittle, G. Namkoong, A. Carver, W. Henderson, and A. Brown, Proc. of Mat. Res. Soc. Fall, Boston MA, Dec. 2-6, Vol 743, L1.4, (2002)}\footnote{S. M. Madison, W. Henderson, K. M. Patel, G. Namkoong, K.-K. Lee, S. E. Ralph, and W. A. Doolittle, 2004 Dig. IEEE Lasers and Electro-Optic Soc. (LEOS) Sum. Top. Meetings, November 2004} have demonstrated GaN high electron mobility transistors using LN substrates and that the strong polarization discontinuity between the LN/GaN layers can be used to control the polarity of the GaN, including the demonstration of periodically poled GaN on periodically poled LN. Herein, the origin within the substrate, the fundamental nature and strength of the polarization discontinuity for determining the polarity of the GaN epitaxial layers are described. Kelvin force microscopy, x-ray diffraction and chemical etching studies show a strong correlation between LN inversion domain density resulting from non-uniform lose of Li$_{2}$0 from LN when heated in vacuum. This affects the polarization of III-Nitride films grown on these multi-domain LN substrates, strongly influencing the channel mobility due to polar scattering. Methods for reducing the inversion domain density in GaN/LN will be described and correlated with the HEMT channel mobility.   

Integration of Functional Perovskites with (0001) GaN

Hybrid structures in which the functional properties of oxides can be exploited in combination with semiconductors offer exciting opportunities for devices. Perovskite oxides exhibit a wide range of functional properties motivating their integration with (0001) GaN. We used two basic criteria to select functional perovskite oxides for integration with GaN: thermodynamic stability and lattice match. Using the NIST-ICDD Crystal Data database of about 150,000 inorganic compounds, a comprehensive lattice match search was performed between all orientations of all known oxide perovskites and the (0001) face of GaN. The best lattice match was for a \textit {$\sigma $}$_{3}$ boundary between the (111) pseudocubic perovskite plane and the (0001) plane of GaN. We also performed extensive thermodynamic stability calculations between all binary oxides and GaN. Our analysis led us to believe that (111) ~SrTiO$_{3}$ would be a good buffer layer to grow on (0001)~GaN, from which the transition to one of many functional perovskite oxides could be made. Extensive attempts to integrate epitaxial (111)~SrTiO$_{3}$ on (0001) GaN were, however, unsuccessful. As the surface of GaN often contains a thin Ga wetting layer, we calculated the thermodynamic stability of Ga with all binary oxides. In contrast to the thermodynamic stability of SrO in contact with GaN, SrO is found to be thermodynamically \textit{unstable} in contact with Ga while TiO$_{2}$ is stable in contact with Ga. This is consistent with our experimental observations during the deposition of SrTiO$_{3}$ on GaN.   

Polarization Effects on Heterojunction Band Offset Measurements of Oxide-GaN Interfaces

XPS has been used to determine the band offsets of both polar oxide-polar nitride and non-polar oxide-polar nitride heterojunctions for both N-face and Ga-face GaN. In the case of polar oxide-polar nitride heterojunctions (specifically, ZnO-GaN), it is expected that the spontaneous polarizations will align across the interface. ZnO epitaxial layers were prepared by MBE on unintentially doped GaN films grown on sapphire substrates. The valence band heterojunction band offset was deduced from the difference in the core level positions where the valence band of each material was determined from prior XPS and UPS measurements of clean GaN layers and bulk ZnO. We found the ZnO valence band to be positioned below the GaN valence band at 1.85 and 1.05 for the Ga-face and N-face heterojunctions, respectively. Assuming Eg = 3.4 for GaN and Eg = 3.3 eV for ZnO, the Ga- and N-face conduction band offset are 1.95 and 1.15 eV, respectively. The interface dipole for each configuration is deduced based on the deviation from the electron affinity model, and the band offsets are analyzed in terms of the interface bonding and polarity. It is found that polarization effects play an significant role in the band alignment and interface electronic structure.   

Effect of various surface treatments on optical and morphological characteristics of homoepitaxially overgrown GaN layers and device structures

Effect of ex-situ treatment on morphological and optical quality of homoepitaxially grown GaN layers as function of layer thickness is reported. Treatments include use of organic solvents, Hydrochloric and Hydrofluoric acids. The properties of the grown GaN on template layers were observed to vary significantly due to different surface treatments. For un-treated (surface blown dry with UHP N$_{2})$ samples morphology of the surface becomes smooth with RMS roughness of 0.6 nm shortly after nucleation and becomes as smooth as the initial surface (RMS=0.3nm) after 1 $\mu $m of overgrowth. The surface treated with HCl, on the other hand, shows increase in roughness initially and then recovers after the growth of 1 $\mu $m while the HF treated surface does not recover up to 1 $\mu $m of growth. In addition to the morphological changes, photoluminescence measurements are also presented which characterize the difference in optical properties of the layers as a function of layer thickness for different surface treatments. We will also present the effect of various treatments on performance of violet light emitting diodes.   

Reconstructions and adsorbates on polar and nonpolar GaN surfaces

Recently, exciting progress has been made in the ability to grow GaN in nonpolar orientations. We report on a systematic study of the reconstructed GaN $a$ (1120), $m$ (1010) and c (0001) planes. Using first- principles calculations, based on density-functional theory within the local density approximation (LDA), we examine the structural and electronic dissimilarities between the polar and the nonpolar surfaces. Adatom energetics and doping issues have been also investigated for the different surfaces considered. We propose a simple way to overcome the band-gap underestimation problem inherent in density-functional theory. New features will be presented, in light of the available experimental data.   

Self-heating study of an AlGaN/GaN-based high electron mobility transistor using visible and ultraviolet micro-Raman scattering

We report micro-Raman studies of self-heating in an AlGaN/GaN heterostructure field effect transistor using both visible (488.0 nm) and ultraviolet (363.8 nm) excitations. The $<$ 100 nm optical penetration depth of the UV light allows us to measure temperature rise ($\Delta T$) in the two-dimensional electron gas (2DEG) region of the device between source and drain, while visible light gives us the average $\Delta T$ in the GaN layer and that of the SiC substrate at the same lateral position. Combined, we depth profile the self-heating in the device. Measured $\Delta T$ in the 2DEG is consistently over twice the average GaN-layer value. Simulations are performed to describe the electrical behavior of the device. The results of electrical simulations are used for thermal simulations to describe the thermal behavior of the device. The presence of a hotspot, located at the edge of the gate in the 2DEG on drain side was observed. The measured temperature rise is related to the growth of the hotspot. Excellent agreement between experimental results and simulations is produced over the wide range of operating conditions.   

Air-Stable Field-Enhanced III-Nitride Photocathodes

We report on recent investigations of Si delta-doping by molecular beam epitaxy (MBE) near the surface of p-type GaN films to attain high efficiency photocathodes for use in intensified ultraviolet imagers. These delta-layers are prepared to achieve effective negative electron affinity (NEA) without the use of low work function metal coatings, such as cesium, which are suitable only in ultra-high vacuum environments. Hall measurements, secondary ion mass spectrometry (SIMS), and capacitance-voltage (C-V) depth profiling reveal highly confined delta-layers with activated carrier densities in excess of 10$^{14 }$cm$^{-2}$ as close as 2 nm from the semiconductor surface. When the delta-layer is biased relative to the bulk, a large field-enhancement of the photoelectron yield is observed. In addition to UV spectroscopic quantum efficiency data, we will present total electron yield measurements for these photocathodes under electron-beam bombardment at various incident energies.   

Isostructural phase transitions in GaN/ScN and InN/ScN superlattices

We predict the existence of pressure-induced isostructural phase transitions (IPTs) in GaN/ScN and InN/ScN superlattices from first principles. The IPTs in these superlattices are anomalous in the sense that they are associated with trivial order parameters and generate a dramatic change in many physical quantities. Furthermore, the {\it order} of the phase transition is found to be dependent on the superlattice period and on the non-transition-metal cation. We also reveal the reason behind, and consequences of, these unusual dependencies and IPTs.   

Atomic and Electronic Structure of Polar Nitride/Oxide Interfaces: h-GaN(0001) and c-GaN(111) on MgO(111)

Polarity can play an important role in atomic and electronic structures of surfaces and interfaces. In this work we show that MgO(111) surface polarity can be used as a parameter for controlled growth of both the hexagonal and the energetically less favorable cubic phase of GaN by electron- cyclotron resonance (ECR) plasma-assisted molecular beam epitaxy (MBE). The growth of cubic (111) (or hexagonal (0001)) GaN is achieved when N (or Ga) is first deposited on the polar MgO(111)-(1$\times $1) surface. High resolution transmission electron microscopy (HRTEM) and density functional theory (DFT) studies indicate that the cubic GaN(111)/MgO(111) interface structure is determined by Mg-O-N-Ga stacking, with each N atom bonded to O at top site. This specific atomic arrangement at the interface allows cubic stacking to more effectively screen the substrate and film electric dipole moment then the hexagonal stacking.   

Defect engineering in Si substrate for strain reduction at GaN/Si interface

We report on a novel method that could potentially reduce the high dislocation density and cracks caused by lattice and thermal mismatch strain at the GaN/Si interface. In this method nitrogen ion implantation at an energy of 75 keV and various doses is employed to cause defects in the substrate allowing higher freedom for realignment of AlN nuclei at the interface. Although a great number of misfit dislocations are formed at the interface due to lattice mismatch, screw dislocations are mostly formed at the grain boundaries. We will report on optical and morphological measurements on over 50 GaN/Si samples, performed to elucidate the potential for this technique. Our preliminary results indicate high crack reduction of the GaN layer to crack distances of 120 $\mu $m in 2$\mu $m thick GaN layers for samples undergoing ion implantation and defect annealing scheme. Photoluminescence measurements at room temperature show an increase in bandedge to yellow luminescence ratio indicative of higher quality of the layers.   

Session N19: Transport and Microwave Conductivity

Applications of R-Matrix Theory to Solid State Devices

R-matrix theory is a computationally efficient method for solving quantum collision problems. First introduced in nuclear physics and later applied in atomic and molecular physics, R- matrix theory is also a useful tool for calculating transport properties of solid-state devices. We have improved upon the existing implementations of R-matrix theory in device physics by inroducing boundary conditions that dramatically speed convergence. Moreover, we have extended the R-matrix formalism to scattering systems with very complicated, non-spherical device geometries. This new formalism, which we call ``the R-matrix connection formula,'' can be used to calculate the transport properties of practical solid-state devices. As an application, we calculate the bend resistance of InSb-based four-terminal devices. We compare our results with experimental data from a group at University of Oklahoma. In these experiments, the bend resistance was measured in a four-terminal device with an applied perpendicular magnetic field. A negative bend resistance was measured at zero magnetic field. This work is supported by NSF PHY-0354858, NSF MRSEC DMR- 0080054, and NSF EPS-9720651.   

First-principles calculation of alloy scattering of n-type carriers in SiGe

Starting from a virtual crystal approximation (VCA) for the band structure of the Si$_{1-x}$Ge$_x$ alloy host, calculated in first-principles density functional theory, we find the scattering matrix for intra-valley and inter- valley n-type carrier scattering by a Si or Ge substitutional atom in a lattice of VCA atoms. The scattering matrix is calculated from energy splitting in large supercells (containing up to 127 VCA host atoms and one Si or Ge atom) of degenerate levels corresponding to the L, X and other points in the Brillouin zone. Atomic relaxation is found to have a substantial effect on the scattering matrix elements. Using supercells containing more than one Si or Ge atom, we test the accuracy of the approximation, in which each Si or Ge atom is considered to contribute independently to the carrier scattering matrix. The carrier mobility is calculated from the scattering rate using the Boltzmann transport equation. The results are compared to experiment.   

First-principles calculation of mobilities in nano-MOSFETs

As metal-oxide-semiconductor field-effect transistors enter the nanoscale regime, the usual approximations made in mobility calculations fail to account for observations because wave penetration in the gate oxide becomes significant and the effective-mass approximation breaks down. We introduce a novel method for first-principles calculations of carrier mobilities in ultrathin silicon-on-insulator channels. The method is based on density functional theory and Green's functions. The silicon-oxide interface is treated at the atomic-scale, so that all wave functions extend on both sides of the interface. Interface roughness is included in terms of deviations from an abrupt interface (e.g. suboxide bonds, oxide protrusions) acting as scattering centers. Scattering from impurities (e.g. dopants, nitrogen, hydrogen) is also included. A dynamical approach to optical phonon scattering has been developed, including phonon-plasmon interactions. Initial results reveal the importance of the atomic scale in controlling the effects of interface roughness.   

High electric-field quantum transport for Bloch electrons in a single band scattering from a random distribution of impurities

The quantum Boltzmann equation for a Bloch electron in a single band under the influence of a homogeneous and inhomogeneous electric field subject to scattering from a random, spatially inhomogeneous distribution of impurities will be presented. The analysis assumes the use of a single band effective Hamiltonian to describe the Bloch dynamics, and makes use of the vector potential to define the homogeneous electric field. After developing an {\em interaction picture} transformed Liouville equation for the Bloch electron based on the Wigner function, and then taking the limit of slowly varying inhomogeneous electric field and slowly varying scatterer density distribution, a novel quantum generalization of the Boltzmann equation is obtained which includes a collision term with impurity-related intra-collisional field effects correct to second order in the impurity potential, and a drift term which includes the total force based on the homogeneous and inhomogeneous fields.   

Quantum and transport mobilities in an AlGaAs/GaAs parabolic quantum well structure

We study quantum and transport mobilities in a parabolic quantum well structure when one or more subbands are occupied. We developed an original analytical method to extract the quantum mobility from the multiply-occupied subband transport characteristics at low temperature. We tune the carrier density and hence the subband structure of the parabolic quantum well over a wide range by illumination with a red light-emitting diode. In order to obtain the quantum mobilities, Fourier transforms of the first differential of the experimental magnetoresistance traces are taken, and fitted by a conductivity tensor model in the same magnetic field range. We find that both the quantum and transport mobilities increase non-linearly with increasing carrier density for both the first- and second- subbands, and conclude that the intersubband scattering is predominantly large-angle.   

Studies of Zero-resistance States by Dichromatic Microwaves

$^1$Dept. of Physics, University of Utah, Salt Lake City, UT 84112, $^2$School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, $^3$Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 --- We have explored experimentally dichromatic (frequencies $\omega _1$ and $\omega _2$) photoresistance of a two-dimensional electron system in the regimes of microwave-induced resistance oscillations and zero-resistance states. We have found that dichromatic resistance is closely replicated by a linear superposition of $\omega _1$ and $\omega _2$ components, provided that both monochromatic resistances are positive. In contrast, if a zero-resistance state is to be formed by one of the frequencies, such superposition relation becomes invalid. More specifically, dichromatic resistance is suppressed in this regime. This finding can be explained by taking into account the absolute negative resistance and the formation of domains, as suggested by current theoretical models.   

Influence of a Parallel Magnetic Field on Microwave-Induced Oscillatory and Zero-Resistance States

We have studied experimentally the influence of a parallel magnetic field on microwave-induced resistance oscillations and the subsequent zero-resistance states (ZRS) previously discovered in a high-quality two-dimensional electron gas. Our experiments were performed in a frequency range from 25 to 150 GHz and at temperatures from 0.5 to 8 K; samples were Hall bars of GaAs/Al$_{x}$Ga$_{1-x}$As heterojunctions and quantum wells having low temperature mobility as high as 2 x 10$^{7}$ cm$^{2}$/Vs. A two-axis superconducting magnet was employed to facilitate the experiment. We have observed pronounced suppression of oscillations/ZRS by a parallel magnetic field (B$_{//})$. In contrast, resistance peaks associated with magnetoplasmon resonance become stronger in B$_{//}$. We discuss possible explanations for the observations.   

Experimental examination of current instabilities in the irradiated 2DEG

RF radiation imposed on a high quality heterostructure can result in a series of oscillations periodic in $\omega $/$\omega _{c}$ with $\omega $ the radiation frequency and $\omega _{c}$ the cyclotron frequency, using bare GaAs electron mass [1]. Subsequently it was observed [2,3] that in high mobility samples the minima can form apparent zeroes. These findings are consistent with micro- and macroscopic theoretical pictures of radiation induced transport and current instabilities due to local negative resistivities.[4] Using simple dipole configurations with radiation up to 20GHz frequency we have examined the magneto-resistance oscillations in ultra-high mobility samples, focusing on indications of current instabilities and the fundamental origin of the oscillations. We find that under radiation, voltages are observed from internal to external contacts in the absence of applied driving currents, with a distinction from simple rectification at the principal oscillation minima. Further measurements of magneto-resistance have been carried out using multiple dipoles to apply different radiation frequencies simultaneously. These results are reviewed in consideration of both the origin of the magneto-resistive oscillations and the current instabilities. [1] M.A. Zudov, et al, Phys. Rev. B. 64, 201311 (2001). [2] R.G. Mani, et al; Nature 420, 646 (2002). [3] M.A. Zudov, et al, Phys. Rev. Lett. 90, 046807 (2003). [4] A.V. Andreev, et al, Phys. Rev. Lett. 91, 056803 (2003).   

Microwave photoexcited magnetoresistance in the high mobility GaAs/AlGaAs system

The microwave-photoexcited high mobility GaAs/AlGaAs two dimensional electron system exhibits an oscillatory magnetoresistance with vanishing resistance in the vicinity of magnetic fields B = [4/(4j+1)]B$_{f }$where B$_{f }$= 2$\pi $ f m$^{\ast }$/e and f is the radiation frequency. Here, we report experimental results examining microwave induced phenomena in regimes where (a) the Landau level spacing, which is of the order of or smaller than the photon energy, exceeds both a broadening parameter defined from the transport time and k$_{B}$T, and (b) where the Landau level spacing significantly exceeds the photon energy.   

Steady States of the Inhomogenous Microwave Irradiated Quantum Hall Gas

To explain the observation of Zero-Resistance states (ZRS) in Microwave irradiated Quantum Hall gases[1], it has been proposed[2] that under appropriate conditions the sample will break into domains of photogenerated fields. In the absence of disorder induced pinning, motion of domain walls results in a ZRS state. In order to treat the effects of long wavelength disorder, we construct a Lyapunov functional for systems with uniform Hall conductivity. We use it to derive stability conditions on the domain structure and to compute the conductance. We show that weak white noise disorder does not destroy the ZRS although it produces current fluctuations. In contrast, separable and correlated disorder pin the domain walls, and produce a finite conductance and a photovoltage as demonstrated by one dimensional, and simple two dimensional, potentials.\\ 1. R.G. Mani et.al. Nature, 420, 646 (2002); M.A. Zudov et.al., Phys. Rev. Lett. 90, 046807 (2003).\\ 2. A.V. Andreev, I.L. Aleiner, and A.J. Millis, PRL 91, 056803 (2003).\\   

Numerical studies of domain patterns in inhomogeneous microwave-irradiated quantum Hall gas

Experimental observations of a zero conductance state in microwave-irradiated quantum Hall systems have been explained by a model which postulates the existence of domains with a non-zero dc electric field. [1] We have carried out numerical calculations to study the effects of long-range disorder on these domains and on the resulting conductivity. If the Hall conductivity is constant throughout the sample, then one can construct a Lyapunov functional, and domain wall patterns can be obtained by looking for a potential configuration which minimizes the functional. We have studied a range of examples and find that that long-range disorder can pin the domain walls, giving the state a nonzero conductance. For a spatially varying Hall conductivity, numerical calculations are more difficult, but results will be presented for simple cases. [1] A.V. Andreev, I.L. Aleiner, and A.J.Millis, PRL 91, 056803 (2003).   

Photoconductivity oscillations in a 2D electron gas and the mobility threshold for zero resistance states.

We present a model for the photoconductivity of a two dimensional electron system subjected to a magnetic field. The model includes the microwave and Landau contributions in a non-perturbative exact way, impurity scattering effects are treated perturbatively. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. We study the effects of both short-range and long-range disorder on the photoconductivity. Our calculation yields a magnetoresistance oscillatory behavior with the correct period and phase. It is found that, in agreement with experiment, negative dissipation can only be induced in very high mobility samples, an expression for the mobility threshold is provided. We analyze the dependence of the results on the microwave power and polarization. For high-intensity radiation multi-photon processes take place predicting new negative-resistance states centered at $ \omega / \omega_c=1/2$, and $ \omega / \omega_c= 3/2$. \begin{itemize} \item M. Torres, A. Kunold, {\em cond-mat/0407468, \/} {\em cond-mat/0409588. \/} \end{itemize}   

Transport properties of 2DEG with spin-orbit coupling in a strong magnetic field

We derive a Boltzmann equation for a disordered 2DEG with a spin-orbit coupling in a strong magnetic field (large Hall angle). This equation allows us to describe relations between spin magnetization and dc electric current. We also investigate effects of microwave radiation on the magnetization and conductivity. Particularly, we discuss the beats in the microwave-induced oscillatory part of the dc conductivity originating due to the spin orbit coupling.   

Quantum and classical surface acoustic wave induced magnetoresistance oscillations in a 2D electron gas

We study theoretically the geometrical and temporal commensurability oscillations induced in the resistivity of 2D electrons in a perpendicular magnetic field by surface acoustic waves (SAWs). We show that there is a positive anisotropic dynamical classical contribution and an isotropic non-equilibrium quantum contribution to the resistivity. We describe how the commensurability oscillations modulate the resonances in the SAW-induced resistivity at multiples of the cyclotron frequency. We study the effects of both short-range and long-range disorder on the resistivity corrections for both the classical and quantum non-equilibrium cases. We predict that the quantum correction will give rise to zero-resistance states with associated geometrical commensurability oscillations at large SAW amplitude for sufficiently large inelastic scattering times. These zero resistance states are qualitatively similar to those observed under microwave illumination, and their nature depends crucially on whether the disorder is short- or long-range. Finally, we discuss the implications of our results for current and future experiments on two dimensional electron gases.   

Microwave Photoconductivity of a High-Mobility 2D Hole Gas

We have measured millimeter wave (frequency from 25 to 150 GHz) photoconductivity in a high-quality 2D hole gas (2DHG), and found characteristically different responses as compared to those from a 2D electron gas. The 2DHGs are provided by Si-doped, (311) GaAs/Al$_{x}$Ga$_{1-x}$As quantum wells (QWs), or C-doped, (001) GaAs/Al$_{x}$Ga$_{1-x}$As QWs, having a range of hole sheet density from 1 to 4.5 x 10$^{11}$/cm$^{2}$, and low temperature (0.3 K) mobility from 2 to 6 x 10$^{5}$ cm$^{2}$/Vs. Magnetoresistivity, photoconductivity, as well as differential photoconductivity were measured using a low-frequency lock-in method at temperatures from 0.5 K to 1 K. Typically, differential photoconductivity data show a single peak corresponding to cyclotron resonance of 2D holes. In separate experiments we have measured the cyclotron absorption, from which we determined effective mass and scattering times of the 2DHGs. Our analysis shows that it is important to take into account the band structures of 2DHGs to understand the photoconductivity.   

Session N20: Focus Session: Ferroelectrics

Beyond PZT: Novel Perovskite Alloys and More

The formation of a morphotropic phase boundary (MPB) is crucial for obtaining good piezoelectric performances in PZT and perovskite relaxors. In these systems the MPB occurs between tetragonal and rhombohedral phases. These phases are driven by mostly A-site ferroelectric instabilities but, as we will show, it is the energetics of the B-site displacement that tips the balance between rhombohedral and tetragonal ground states. We have analyzed several perovskitic compounds and classified them according to four different classes: (1) stable cubic, e.g. BaZrO$_3$, (2) B-site active, e.g. BaTiO$_3$, (3) purely A-site active, e.g. PbZrO$_3$, and (4) cooperative systems, e.g. PbTiO$_3$. From the analysis of the interplay between structural instabilities we derived strategies to design novel compounds for piezoelectric applications in the class of scandates and niobates.   

Theoretical prediction of new high-performance lead-free ferroelectrics

We predict the occurence of large ferroelectric polarization and piezoelectricity in the hypothetical perovskite-structure oxides, bismuth aluminate (BiAlO$_{3}$) and bismuth gallate (BiGaO$_{3}$), using density functional theory within the local density approximation. We show that BiGaO$_{3}$ will have a similar structure to PbTiO$_{3}$, although with much stronger tetragonal distortion and therefore improved ferroelectric properties. Likewise, BiAlO$_{3}$ shares structural characteristics with antiferrodistortive PbZrO$_{3}$, but it is also a ferroelectric with large polarization. Therefore we propose the Bi(Al,Ga)O$_{3}$ system as a replacement for the widely used piezoelectric sensor, Pb(Zr,Ti)O$_{3}$ (PZT) that will avoid the environmental toxicity problems of lead-based compounds. Finally we show that, in both BiAlO$_{3}$ and BiGaO$_{3}$, the large distortions from the prototypical cubic structure are driven by the stereochemical activity of the Bi lone pair.   

Magneto-electric coupling in hexagonal RMnO3

The hexagonal RMnO3 exhibit much higher magnetic and ferroelectric ordering temperatures, T(N) = 75 K and T(FE) = 930 K than the orthorhombic RMnO3 with an incommensurate antiferromagnetic ordering below 40K. However, the coupling between the magnetic and electric order is very weak. We have investigated the origin of the electric order by high temperature x-ray diffraction using high energy synchrotron radiation. We discuss the change in symmetry at the ferro-electric ordering temperature, which is a few hundred degrees below the tripling of the unit cell. Additionally, we have used magneto-capacitance measurements to study the coupling between magnetic and electric order. We report results on hexagonal RMnO3 and also substitutions on the rare earth- and the Mn-site.   

High-pressure Raman scattering and x-ray diffraction study of relaxor ferroelectric 0.96Pb(Zn1/3Nb2/3)O3-0.04PbTiO3

High-pressure Raman scattering and x-ray diffraction of lead zinc niobate-lead titanate (PZN-4{\%}PT) solid solutions were measured from ambient to 13 GPa. All of the Raman peaks are broad due to the disorder of Zn$^{2+}$ and Nb$^{5+ }$on the B site of the perovskite structure. No obvious soft-phonon-mode feature was observed. High-pressure x-ray diffraction was used to determine the bulk modulus and also revealed diffuse scattering near the Bragg peaks. The diffuse x-ray scattering decreases on compression and disappears at 5 GPa. Changes in both the Raman spectra and the diffuse scattering reflect suppression of the local distortion in the material on compression. The results can be understood in terms continuous changes in the local potential surfaces with increasing pressure. Pressure causes the ferroelectric well-depths to decrease, which causes a continuous transition from ferroelectric to relaxor to paraelectric.   

BaTiO$_3$ and Quantum Monte Carlo: Lattice Constants

Transition metal oxides are well-known to be difficult to describe using standard first principles techniques like Density Functional Theory. For example, straightforward application of generalized gradient and local density approximations make errors of about 3\% on the primitive cell volume of BaTiO$_3$ in the tetragonal phase, one of the simplest ferroelectrics. Since the ferroelectric properties are extremely sensitive on lattice constants and structural parameters one has to either adjust the functionals or set the lattice constant to experiment with a resulting error in phase transition temperature. We show progress on calculations using Quantum Monte Carlo (QMC) to predict the static properties of BaTiO$_3$, using correlated sampling to overcome the statistical error on these extremely small energy differences. Preliminary results indicate that QMC is able to predict the tetragonal volume to less than 1\%(the error bars) without experimental input, as well as describe the band gap with a good agreement with experiment.   

Topological Nature of Polarization and Charge Pumping in Ferroelectrics

Dielectric properties of insulators have been one of the most important subjects in condensed matter physics. Recently, the quantum theory of electric polarization in insulators has been developed in terms of the Berry phase, which fully takes account of the covalency. However, the intuitive picture is hidden in sophisticated first-principle band calculations. Here, we provide a clear and comprehensive view on the quantum theory of polarization for two-band models: In the space of external parameters such as displacements of atomic or molecular orbitals, pressure, and electric field, a vector field representing an infinitesimal change of polarization due to that of the parameters is dominantly characterized by a string. This string is a trajectory of the band crossing points and fully describes the singularity of the vector field. The total flux of the string and thus the charge pumped during a cyclic adiabatic change of the external parameters are quantized to an integer multiplied by the electronic charge. Applications of this picture to various organic ferroelectrics and BaTiO$_3$ are discussed.   

Temperature dependent electrical properties of the epitaxial junction between Nb:SrTiO$_{3}$ and magnetite (Fe$_{3}$O$_{4}$)

Epitaxial films of magnetite (Fe$_{3}$O$_{4})$ were grown on single crystal (001) Nb:SrTiO$_{3}$ substrates by pulsed laser deposition. The films were characterized by x-ray diffraction, Rutherford backscattering-ion channeling spectrometry, SQUID, and four probe resistivity measurements. The growth conditions were optimized to achieve good crystallinity as well as the expected transport and magnetic characteristics of the Verwey transition at 120 K. Such junctions were then examined for the temperature dependent current-voltage (I-V) characteristics, which exhibited a non-linear behavior and an interesting non-monotonic temperature dependence. The data were also recorded in magnetic field up to 5 Tesla. These data were analyzed within the framework of a band description of transport across the interface between dissimilar semiconducting oxides and the fitting parameters were extracted. The temperature evolution of these parameters showed systematic trends, with interesting changes near the Verwey transition. These data are analyzed based on the electronic density of states$^{1}$ and the nature of transport$^{2,3}$ in magnetite. 1. Z. Zhang et al. Phys. Rev. B 44, 13319 (1991), 2. D. Ihle et al. J. Phys. C : Solid State Phys. 19, 5239 (1986), 3. S. B. Ogale et al. Phys. Rev. B 57, 7823 (1998).   

Polarization enhancement and heterointerfacial coupling in artificial perovskite superlattices

Bi- and tri-color superlattices comprised of BaTiO$_3$, SrTiO$_3$, and CaTiO$_3$ with compositionally-abrupt interfaces have been grown on atomically-flat SrRuO$_3$ bottom electrodes on (001) SrTiO$_3$ single crystals by pulsed laser deposition. These superlattices provide additional freedom in tuning the average lattice parameter and the structure’s physical properties. We found that locally asymmetric heterointerfaces (TiO$_6$- octahedra bound by different A-site cations) play a crucial role in the polarization enhancement due to elastic and electrostatic couplings at the interfaces. Such subtle effects, especially in tri-color superlattices, can be attributed to the broken inversion symmetry, although the effects are sometimes weak. A strong polarization enhancement is achieved by the proper balancing between two competing requirements: the ferroelectric layers must thick enough to contain a sufficient amount of non-interfacial TiO6 octahedra, but thin enough to remain fully strained. For a superlattice, this produces a maximum polarization as much as 50\% higher than that of a BaTiO$_3$ single film. Research sponsored by the U.S. Department of Energy under contract with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, as part of a BES NSET initiative.   

Phonons in SrTiO$_3$ under finite electric fields

It has been nearly 40 years since Worlock and Fleury discovered by means of Raman scattering that {\it under finite electric fields} the transverse-optic (TO) phonons in SrTiO$_3$ exhibit a striking increase in frequency---by more than 400\% of its zero-field value---when a very small field of 12 kV/cm is applied [1]. Despite its obvious importance, this giant field- induced shift of phonon frequency has not thus far been independently investigated via the density-functional theory (DFT). Here, we propose an approach within the DFT theory to determine the phonon structure of infinite solids under finite electric fields. We applied this approach to SrTiO$_3$ and found a giant shift in TO frequencies, in accordance with the observations [1]. Our calculations further predict other unusual properties that occur under finite electric fields, that is, an anomalous piezoelectric response and large dielectric tunability. (This work was supported by ONR). [1] J.M. Worlock and P.A. Fleury, Phys. Rev. Lett. {\bf 19}, 1176 (1967).   

Low-Temperature Transitions in Nanograin Barium Titanate

Nanograin ferroelectrics suffer from diffraction limit that causes line broadening making it difficult to ascertain phase transitions. This, however, may be overcome by analyzing the temperature dependence of line shapes. We investigated the successive low-temperature transitions from tetragonal to orthorhombic to rhombohedral symmetries in nanograin BaTiO$_{3}$ ceramics using high resolution diffraction and Raman local probe. The existence of orthorhombic and rhombohedral symmetries over a coherent length of the order of grain size, as small as 50 nm, was established by high-resolution diffraction. The coexistence of different symmetries and reduced distortions over different length scales was also evident from both diffraction and Raman local probe. These results explain the broad and rather weak dielectric anomalies associated with these ceramics, which will become important for future dielectric applications in ultrathin multilayer electronic components.   

Pressure-induced anomalous enhancement of piezoelectricity and polarization rotation via an unexpected monoclinic phase of PbTiO$_3$

Pressure-induced phase transitions and piezoelectricity of PbTiO$_3$ were studied using the {\it ab initio\ } density functional perturbation theory (ABINIT4.3.3). A tetragonal ($P4mm$) to monoclinic ($Pm$) phase transition occurs at 9 GPa, while this monoclinic phase transforms into paraelectric cubic phase at 22 GPa. The monoclinic ($Pm$) phase acts as the pressure-induced structural bridge between the tetragonal ($P4mm$) and rhombohedral ($R3m$) phases since its polarization rotates continuously in the pseudocubic ($\bar{1}$10) plane from the [001] towards the [111] pseudocubic directions. Under hydrostatic pressure, the enthalpy ($H = E + PV$) difference between the tetragonal and rhombohedral phases becomes tiny, and the polarization rotation via the monoclinic phase is possible, which results in huge enhancement of the piezelectric constant $e_{15}$, very similar to relaxor PZT. At 9 GPa, the spontaneous polarization is roughly half of that at 0 GPa, while the piezoelectric coefficient $d_{15}$ is comparable in magnitude to $d_{33}$ of relaxors PMN-PT and PZN-PT.   

Structural study of thin-film PbTiO$_3$-CoFe$_2$O$_4$ composition spreads

Multiferroic materials exhibiting magnetoelectric (ME) effects are of great interest for novel devices. In multiphase systems, the ME effect arises from the elastic interaction of ferromagnetic and ferroelectric phases. Recently we studied properties and structures of multiferroic PbTiO$_{3}$ (PTO) and CoFe$_{2}$O$_{4}$ (CFO) thin films produced by a composition-spread technique. The compositional spread was achieved by PLD of a superlattice of pure PTO and CFO. Here we report results of cross-sectional TEM studies, which can be summarized as following: a. For all compositions the microstuctures yielded pseudo-binary two phases equilibrium of PTO and CFO, and the phases are epitaxial to each other and to a MgO substrate; b. Morphology of the phases strongly depends on a thickness of the deposited superlattice; c.The most pronounced ME effect was found for a pancake-like morphology; d.Diffusional re-arrangement of layers during the deposition results in the solubility of CFO in PTO, which drastically reduces ferroelectric Curie temperature of PTO.   

Session N21: Focus Session: Single Molecule Nanobiology

Single molecule studies in spatially restricted fluid systems

We have used simple nanofluidic devices to isolate individual active biomolecules in solution in order to observe there identity and activity. We have employed metallic apertures a few tens of nanometers in diameter to confine a region of optical excitation to a volume on the order of 10$^{-20}$ liters, which allows for the observation of single molecule binding activity at meaningful rates and concentrations. Small fluid channels have also been used to isolate individual optically detected molecules. Temporal observation of either the driven or diffusive motion of molecules through the restricted observation volume provides information about the identity of the molecule and can also be used to detect specific chemical binding events. Single specific molecular binding events can be observed by optically observing the spectral characteristics of labeled molecules. In this talk we will describe several approaches and applications of the single molecule studies in microfabricated fluidic systems.   

The Physics of Nanoconfined DNA

Nanotechnology has the potential to revolutionize biology by making possible the construction of chip-based devices with nanoscale features that can not only detect and separate single DNA molecules by size but also--it is hoped in the future--actually sequence at the single molecule level. Understanding the physics of nanoconfined DNA is important for the future design of such devices. Here we present measurements of the static properties and Brownian dynamics of single DNA molecules confined in nanochannels using fluorescence microscopy techniques (end labeling and staining of the entire molecule using intercalating dyes). We study the effect of varying the degree of nanoconfinement, using nanochannels with widths ranging from 10 to 500nm. The nanochannels are fabricated using interference lithography and imprinting techniques. We also present scaling arguments and Monte Carlo simulations on the problem of confined semiflexible polymers and discuss how these results can be interpreted in the context of our experimental work.   

Real-time restriction mapping of DNA stretched in nanofluidic devices

We present real-time sequence-specific restriction mapping of single DNA molecules stretched in nanofabricated channels. In these channels, DNA is linearized and extended to up to 3/4 of its contour length, permitting attribution of the cutting sites to specific regions in the genetic code. We will present real-time restriction of genomic viral DNA with the enzymes Sma I, Sac I, Kpn I. We are able to determine cutting sites and can quantify the cutting rates at different genomic locations. Complete digestion can be achieved within less than 10 seconds. Our device operates in a quasi-continous mode, which we achieved by controlling the concetration of the necessary co-factor Mg$^{2+}$ throughout the mixed micro- and nanofluidic device. DNA was observed using fluorescence micrcoscopy and intercalating DNA stains.   

A DNA catalyst for speeding up a single-molecule DNA nanomotor

A drawback of single-molecule DNA-based nanomotors is the slow cycling speed. Previously, a motor based on the cyclic folding and unfolding of a DNA single strand was described by Tan et al. The DNA motor strand M is a 17-base sequence that folds into a chair-type quadruplex structure in the presence of potassium ions. A fuel strand A complementary to M is added. Hybridization of M with A unfolds the chair structure. Next, addition of a restoration fuel strand B de-hybridizes the double strand and restores M to its chair configuration, completing the cycle. We have found that the bottleneck for this cycle is the tendency for B to also fold into the chair structure. By introducing a short catalyst strand C which inhibits this premature folding, we have achieved a doubling of the speed of the motor. The catalyst shows robust behavior over several cycles.   

Single-molecule studies of biological molecules

Structural heterogeniety, i.e., the existence of multiple nearly-degenerate conformational substates of molecules, plays a key role in biological machinery. Therefore, in order to understand many biological processes, it is necessary to examine processes at the single molecule level. This talk will describe AFM imaging of processes involved in turning on genes (1), methods for chemically-identifying single proteins (2) and methods for wiring single molecules into electrical circuits (3). .1. H. Wang\textit{ et al.}, \textit{Biophys. J.} \textbf{87}, 1964--1971 (2004). 2. C. Stroh\textit{ et al.}, \textit{Proc. Natl. Acad. Sci. (USA)} \textbf{101}, 12503--12507 (2004). 3. X. D. Cui\textit{ et al.}, \textit{Science} \textbf{294}, 571 (2001).   

Single molecule studies of the mechanical stability of packed DNA

Biological organisms must compactly store and yet efficiently read the huge amounts of genetic information contained in their DNA. In the cell nucleus, DNA is highly compact as compared to naked DNA. The primary packing unit, the nucleosome, consists of roughly two turns of DNA wrapped around a core histone octamer. The mechanical stability of nucleosomes determines the accessibility of DNA to the cellular machinery that must decode it. We will discuss our recent progress towards understanding the mechanical stability of nucleosomes using single-molecule studies.   

Single Molecule Manipulation and Spectroscopy of Chlorophyll-a from Spinach

Chlorophyll-a, a molecule produced from `Spinach', adsorbed on a Au(111) surface has been investigated by using an ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) at liquid helium temperatures. Studies are carried out both on isolated single molecules and on self-assembled molecular layers. The tunneling I-V and dI-dV spectroscopy of chlorophyll-a elucidate electronic properties of single molecule, such as the HOMO-LOMO gap and molecular orbital states. Mechanical stability of the chlorophyll-a is examined by using STM lateral manipulation (1,2). Here, the STM tip is placed just a few angstrom separation from the molecule to increase the tip-molecule interaction. Then the tip is laterally scanned across the surface resulting in pulling of the molecule. The detailed molecule movement is directly monitored through the corresponding STM-tip height signals. Our results reveal that the spinach molecule is a promising candidate for environmental friendly nano-device applications. (1). S.-W. Hla, K.-H. Rieder, Ann. Rev. Phys. Chem. \textbf{54} (2003) 307-330. (2). S.-W. Hla, et al. Phys. Rev. Lett. \textbf{93} (2004), 208302. This work is financially supported by the US-DOE grant DE-FG02-02ER46012.   

Twirling DNA Rings - Swimming Nanomotors Ready for a Kickstart

We propose a rotary DNA nanomachine that shows a continuous rotation with a frequency of 10$^{2}$ -10$^{4}$Hz. The device consists of a minicircle with the DNA sequence chosen appropriately to achieve anisotropic elastic features generating a ratchet potential. The motor can be externally driven via the ratchet effect through periodic temperature oscillations. As a result the ring self-propels through the fluid like a molecular ``ring of smoke" with a speed up to microns per second. Hydrodynamic interactions open the possibility of self-organized collective ratchet behavior in semi-dilute solutions of twirling DNA rings.   

Biomotor-based single molecular biosensor

We present our development of novel engineering devices based on biomolecular motors. Adenosine triphosphate synthase (ATPase) is the only rotary motor in nature. The ATPase motor molecules have been genetically engineered to facilitate the immobilization of the motor to Ni surface and to attach additional nanofabricated rods or biomolecules to the $\gamma $ subunit of the biomotor. These properties enable the possibility of utilizating ATPase in the fabrication of novel engineeting devices and systems. This approach may enable the creation of a new class of sensors, mechanical force transducers, and actuators. Here we report our work on constructing an ATPase--nanobar based hybrid system for nanoscale molecular biosensing. We demonstrated avidin-coated Au nanobar can be attached to biotinylated $\gamma $ unit of the ATPase motor through biotin-avidin interaction and we observed single rotational assay by bright-field imaging.   

Session N22: High Magnetic Fields/Electronic and Photonic Devices

A new high magnetic field capability at the NHMFL at Los Alamos National Laboratory: 300 tesla Single Turn System

We report on the development of a unique system designed to study actinide materials in ultra-high magnetic fields. The energy scales of much of the interesting physics in $f$-electron materials in particular the actinides, demand ultra-high magnetic fields to adequately perturb interactions. We have developed an apparatus to generate magnetic fields approaching 300 tesla; while leaving the hazardous sample intact. The single turn system design is based on a brute force technique to drive a very large electrical current (up to $\sim$3.8 MA) through a simple solenoid before it vaporizes and explodes. Such systems have been pioneered by Herlach and Miura and have proven to be an effective condensed matter research tool. In each shot Lorentz forces and Joule heating destroy the coil, while the sample is untouched, thus allowing for multiple field traces on a single specimen. A distinctive feature of the Los Alamos system is the ability to safely study actinide specimens, this and other design features will be presented as well as experimental data.   

A photonic bandgap resonator to facilitate GHz frequency conductivity experiments in millisecond duration pulsed magnetic fields.

We report the details of a novel, all-dielectric, microwave resonator measurement system for used in pulsed magnets. In dynamic magnetic field environments, the large rate of change of flux places strict constraints upon the use of metallic components, i.e. avoiding unwanted eddy current heating and destructive magnetic forces and torques. Our solution to this problem is to use dielectric photonic bandgap structures to confine the radiation, producing a high Q-factor resonator. We are thus able to attain sufficient sensitivity to perform a wide range of experiments, such as the measurement of quantum oscillations and electron spin resonance in correlated electron systems. These GHz-frequency techniques are not only an exciting addition to the National High Magnetic Field Laboratory's pulsed field User Program, but also mark an important milestone in the development of instrumentation for dynamic ultra-high $B/T$ environments such as the 300 Tesla single-turn magnet facility currently under construction at the NHMFL in Los Alamos.   

Contactless Thermal Diffusivity Measurements for Pulsed Magnetic Fields

Most calorimetric techniques require long time constants. and are limited to DC magnets. We are developing calorimetric measurements on short time scales for pulsed field experiments. Specific heat ($C$) measurements have been performed in high fields (60 T) by Jaime et al.~(Nature, 405 (2000) 160); however, these measurements have been limited to a unique long-pulse magnet. Recently, Kim et al.(to be submitted) have successfully measured thermal conductivity~($\kappa$) in a short pulse magnet; however, the $3-\omega$ technique utilized by Kim et al. requires a time consuming deposition processes. In an effort to bring simple-to-use high field calorimetric measurements to the larger condensed-matter community, we are adapting existing contact-less conductivity techniques to measure the thermal diffusivity ($\frac{C}{\kappa}$) in pulsed magnetic fields. An amplitude modulated rf heater excites surface currents in a sample, which in turn drives small oscillatory variations in the surface temperature and are detected via changes in the sample's skin depth. The amplitude of the surface temperature variations depend upon the thermal diffusivity and the frequency at which the rf heater is modulated. By varying the modulation frequency of the rf heater, the thermal diffusivity can be deduced. As a first step toward performing these measurements in pulsed fields, initial measurements in a dc magnet will be presented.   

One Tera Sample-per-Second Single-Shot Digitizer

The ability to digitize wideband electrical waveform is urgently needed in state-of-the-art instruments. The sampling rate of a state-of-the-art system is currently about 20 GSa/s with $\sim $5 ENOB (Effective Number Of Bits). Here, we demonstrate a single-shot digitizer with a record 1 TSa/s sampling rate. This is accomplished by using a photonic time stretch preprocessor which slows down the electrical waveform before it is captured by an electronic digitizer. In the experiment, a 48 GHz tone is digitized in real time at 1 TSa/s. Over a 10 GHz bandwidth centered at 48 GHz, the average SNR within the 1.1 ns time aperture is 22.7 dB corresponding to 3.5 ENOB. Measurements at other input frequencies resulted in up to 4.2 ENOB. While the intrinsic bandwidth of digitizer is 200 GHz, it is presently limited to 80 GHz due to component limitations. To the best of our knowledge, this is by far, the fastest single-shot digitizer ever demonstrated.   

Non-Gaussian sonar clutter models

A new method for interpreting sonar signals in the presence of clutter is presented based on probability distributions following the formalism of the ideal gas. Likelihood detectors, used to evaluate the presence or absence of a desired target, are extremely sensitive to the tails of these distributions. The structure of the tails are highly sensitive to the underlying clutter statistics, implying that detection of a target is extremely sensitive to the underlying physics. We present simulations of non-Gaussian clutter and targets and explore the sensitivity of detection to these physical models and their parameters.   

Cryogenic RF filters with zero DC resistance

We present a design for cryogenic RF filters with zero DC resistance, based on wires with a superconducting core and a resistive sheath. The superconducting core allows low frequency currents to pass with negligible dissipation. Signals above the cutoff frequency are dissipated in the resistive part due to their small skin depth. The filters consist of twisted pairs shielded with copper tape [1]. Above approximately 1 GHz, the attenuation is exponential in $\sqrt{\omega}$, as typical for skin depth based RF filters. This mimics the exponential quantum cutoff above $f = k_BT/h \approx 200 MHz$ in a 10 mK black body spectrum. By using additional capacitors of 10 nF per line, an attenuation of at least 45 dB above 15 MHz, (the timescale relevant for dephasing in metals) can be obtained. Thus one single stage at mixing chamber temperature in a dilution refrigerator is sufficient to attenuate room temperature black body radiation to levels corresponding to 10 mK above about 15 MHz. \\ \\ 1. Lafe Spietz, John Teufel, and R. J. Schoelkopf. Submitted to RSI Sep 20, 2004   

The nonlinear effect of resistive inhomogeneities on van der Pauw measurements

The \textit{resistive weighting function} [D. W. Koon and C. J. Knickerbocker, Rev. Sci. Instrum. 63, 207 (1992)] quantifies the effect of small local inhomogeneities on van der Pauw resistivity measurements, but assumes such effects to be linear. This talk will describe deviations from linearity for a square van der Pauw geometry, modeled using a 5 x 5 grid network of discrete resistors and introducing both positive and negative perturbations to local resistors, covering nearly two orders of magnitude in -$\Delta \rho $/$\rho $ or -$\Delta \sigma $/$\sigma $. While there is a relatively modest quadratic nonlinearity for inhomogeneities of decreasing conductivity, the nonlinear term for inhomogeneities of decreasing resistivity is approximately cubic and can exceed the linear term.   

Piezo-electric Sample Rotators optimised for Low Temperature Experiments

The ability to rotate samples at ultra-low temperatures is of great importance to hall effect and quantum oscillation measurements but has traditionally been hampered in millikelvin experiments by friction in and heat leak through mechanical transmission components. Using recent developments in piezo-electric drives we have now been able to develop a range of completely electronically driven rotation devices that make rotation in high field on millikelvin ADR systems significantly easier than previously possible. Measurements to be presented will show that unlike mechanical devices these rotators feature very low backlash and low power dissipation. Together with multiple axis rotation and direct positional readout, these devices are now rugged enough to be made commercially available for the first time as interchangeable cryogenic components.   

Parallel detection of molecular and atomic ions with a delta-doped CCD at the focal plane of a miniature mass spectrometer

A delta-doped back-illuminated charge-coupled device (CCD) was used for the simultaneous detection of low-energy atomic and molecular ions at the focal plane of a miniature mass spectrometer (MMS). MMS is a JPL-developed instrument based on a focal plane double sector mass analyzer (Mattauch-Herzog geometry). Delta-doped, back-illuminated CCD technology enables high efficiency detection of low energy ions and molecules by eliminating the dead layer usually associated with solid-state detectors. The combination of delta-doped CCD and MMS enables high-speed, precision mass spectrometry of ions and molecules. Compounds studied include benzene, toluene, methylene chloride, and iron pentacarbonyl. Spectral images were captured using integration times from 5 to 120 seconds. Results are two-dimensional images of the spectral output for each molecule. Signal intensity and position were compared to NIST spectra and calculated ion trajectories to verify the identification of molecular peaks and mass to charge ratios.   

Direct detection of electrons in the 0.1-20 keV energy range using a delta-doped high purity silicon p-i-n diode array

We have demonstrated the direct detection of 0.1-20 keV electrons using a boron delta-doped high purity silicon p-i-n diode array. Full depletion allows the high-gain detection of incident electrons. Delta-doping enables the detection of low-energy electrons with high efficiency, and also allows the determination of device gain as a function of the incident energy over a wide energy range. Using a low-temperature process developed in our laboratory, we formed a thin electrode on the back surface of the pin diode arrays to enable full depletion and transparency to shallow-penetrating ionizing radiation. The electrode consists of a 1.5 nm boron delta layer grown by molecular beam epitaxy. In this talk, we will discuss the device structure, processing, and characterization methods used to demonstrate the direct detection of low-energy electrons. We will also discuss the use of this detector for making more accurate measurements of the silicon quantum yield for low-energy electrons.   

Alternate Biased Blocked Impurity Band Detectors

Silicon Blocked Impurity Band (BIB) detectors are state-of-the-art devices to detect light in the near to mid IR range (5-40$\mu $m). Extension of BIB wavelength coverage using either Ge or GaAs has been proposed and attempted, but not yet realized due to material growth challenges. BIBs are normally biased to collect free carriers at the blocking layer contact, from a depletion region that begins at the blocking/active layer interface (standard biasing). We propose and describe an alternate bias approach in which the depletion is initiated from the other contact. Numerical simulations, using a finite difference model, will be presented which show electric field, carrier and all current component distributions. The modeling has been applied to GaAs and Ge, as well as Si BIB detectors. Applying the alternate bias in the simulations demonstrates higher signal currents while allowing thicker blocking layers. Alternate biasing avoids significant voltage drop across the blocking layer, which generally limits blocker thickness in conventional BIB devices. The use of thicker blocking layers in alternate bias provides an option for easier fabrication of new far IR BIB detectors.   

Propagation properties and mass sensitivity of SAW on an AlN/a-plane Sapphire structure

AlN thin films were grown on a-plane sapphire substrates by a plasma enhanced RF sputtering system. The x-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED) techniques were combined studied the structural properties of the films. It is found that high quality (0001)AlN was epitaxially grown on a-plane sapphire. SAW delayline devices operating from 170 MHz up to 1 GHz were fabricated with propagation direction varying in a 15 deg. interval. Besides the excitation of Rayleigh typed SAW (R-SAW) mode at all angles, a shear-horizontal (SH) mode also emerged from the system at particular propagation directions. The measured phase velocities of the SH waves are 6091-6119 m/s, which is very close to the slow shear velocity of a-plane sapphire at about 6100 m/s. This SH mode is most likely the pseudosurface wave with a leaky nature into the sapphire substrate. The phase velocity dispersion with respect to the relative AlN film thickness and azimuth angle is presented. The device electro-mechanical coupling constant was determined to be 0.001-0.003 by the impedance measurements. Mass sensitivity of the two modes was studied by the deposition of highly elastic polymeric overlayers. The measured mass sensitivity is 95 Hz/ng cm-2 and 82 Hz/ng cm-2 for the R-SAW and SH-SAW, respectively.   

Session N23: Focus Session: Methods of Statistical Physics in Population Dynamics and Epidemiology

Statistical physics applied to ecology

Understanding an ecosystem is a formidable many-body problem. One has an interacting system, made up of individuals of various species with imperfectly known interactions, mainly governed by chance and characterized by a wide range of spatial and temporal scales. For example, in tropical forests across the globe, ecologists have been able to measure certain quantities such as the relative species abundance distribution, the species area relationship, and beta diversity, the probability that two trees separated by a given distance belong to the same species. In order to make progress, it is important to distill what one hopes are the essential ingredients of an ecosystem and incorporate them in tractable models whose predictions can then be compared with the observed data. Such an interplay between empirical data and theory is useful for the formulation of realistic models of ecosystems. A summary of recent work along these lines will be presented. Co-author: Amos Maritan Collaborators: John Damuth, Fangliang He, Steve Hubbell, Andrea Rinaldo, Igor Volkov and Tommaso Zillio   

Phase Transitions and Fluctuations in Lattice Predator-Prey Models with Site Restrictions

Studying the effects of spatial constraints and stochastic fluctuations on a class of predator-prey models with two species defined on a lattice it has been shown that the celebrated Lotka-Volterra's mean-field rate equation picture is invalidated. In this contribution, we report how site occupation constraints, modeling locally limited resources and the range of the interaction between species can lead to the emergence of an active-to-absorbing phase transition or to a first order phase transition. In particular, ecologically motivated models with nearest and next-nearest neighbor interactions are discussed and shown to display both an absorbing and an active steady state. In the latter case, where predators and prey coexist, the classical limit cycles or centers are replaced by either nodes or foci, leading to damped oscillatory behavior of the densities of predators and prey in the thermodynamic limit and to stationary configuration displaying complex spatiotemporal patterns. We discuss the validity of the analytic approach against numerical simulations and the subtle role played by the fluctuations and by the degree of ``stirring'' of the system.   

Absorbing multicultural states in the Axelrod model

We determine the ultimate fate of a limit of the Axelrod model that consists of a population of leftists, centrists, and rightists. In an elemental interaction between agents, a centrist and a leftist can both become centrists or both become leftists with equal rates (similarly for a centrist and a rightist), but leftists and rightists do not interact. This interaction is applied repeatedly until the system can no longer evolve. The constraint between extremists can lead to a frustrated final state where the system consists of only leftists and rightists. In the mean field limit, we can view the evolution of the system as the motion of a random walk in the 3-dimensional space whose coordinates correspond to the density of each species. We find the exact final state probabilities and the time to reach consensus by solving for the first-passage probability of the random walk to the corresponding absorbing boundaries. The extension to a larger number of states will be discussed. This approach is a first step towards the analytic solution of Axelrod-like models.   

Nucleation and Spread of an Invasive Allele

We analyze a prototypical discrete spatial model for the spread of an invasive allele when individuals compete preemptively for common limiting resources. Initially, the population is genetically monomorphic with the resident allele at high density. The invasive allele is introduced through rare, but recurrent, mutation. The mutant allele is the better competitor (has an individual-level advantage) but its spread is limited by the local availability of resources. We find that each successful introduction of the mutant leads to strong spatial clustering. Spatial patterns in simulation resemble nucleation and subsequent growth, articulately described by Avrami's law in sufficiently large systems\footnote{G. Korniss and T. Caraco, J. Theor. Biol. (in press, 2004); http://www.rpi.edu/~korniss/Research/JTB04.pdf}.   

Coupling ecological and evolutionary dynamics in a stochastic model of multiple-gene interactions

In this talk we discuss the ``genome template model'' (GTM) which we have recently introduced in order to connect fluctuations in a genotypically heterogeneous population to evolutionary processes such as adaptation and selection. This connection is explicitly made by modeling reproduction and mortality as polygenic traits, coupled, within each individual, to an underlying genome. We will highlight two properties of the GTM: i) high fitness ``ridges'' in genotype space which are a direct consequence of gene interactions, and ii) evolution of spatial polymorphism on environmental gradients.   

Extinction Times for Birth-Death Processes: Continuum Asymptotics and the failure of the Fokker-Planck Approximation

We consider extinction times for a class of birth-death processes commonly found in applications where there is a control parameter defining a threshold. Below the threshold, the population quickly becomes extinct; above, it persists for a long time. We give an exact expression for the mean time to extinction in the discrete case and derive its asymptotic expansion for large values of the population scale. We present results below the threshold, at the threshold, and above the threshold, observing that the Fokker-Planck approximation is valid only quite near the threshold. We compare the asymptotic results to exact numerical evaluations for the Susceptible-Infected-Susceptible (SIS) epidemic model. This is an interesting example of the delicate relationship between discrete and continuum treatments of the same problem.   

Infectious diseases in space and time: noise and nonlinearity in epidemiological dynamics

I illustrate the impact of noise and nonlinearity on the spatio-temporal dynamics and evolution of epidemics using mathematical models and analyses of detailed epidemiological data from childhood infections, such as measles.   

Pandemic Diseases and the Aviation Network – SARS, a case study

We investigate the mechanisms of the worldwide spread of infectious diseases in a modern world in which humans travel on all scales. We introduce a probabilistic model which accounts for the worldwide spread of infectious diseases on the global aviation network. The analysis indicates that a forecast of the geographical spread of an epidemic is indeed possible, provided that local dynamical parameters of the disease such as the basic reproduction number are known. The model consists of local stochastic infection dynamics and stochastic transport of individuals on the worldwide aviation network which takes into account over 95% of the entire the national and international civil aviation traffic. Our simulations of the SARS outbreak are in surprisingly good agreement with published case reports. Despite the fact that the system is stochastic with a high number of degrees of freedom the outcome of a single simulation exhibits only a small magnitude of variability. We show that this is due to the strong heterogeneity of the network ranging from a few two over 25,000 passengers between nodes of the network. Thus, we propose that our model can be employed to predict the worldwide spread of future pandemic diseases and to identify endangered regions in advance. Based on the connectivity of the aviation network we evaluate the performance of different control strategies and show that a quick and focused reaction is essential to inhibit the global spread of infectious diseases.   

Respiratory-borne Disease Outbreaks in Populations: Contact Networks and the Spread of Disease

A large class of infectious diseases spread through direct person-to-person contact. Traditional ``compartmental'' modeling in epidemiology assumes that in population groups every individual has an equal chance of spreading the disease to every other. The patterns of these contacts, however, tend to be highly heterogeneous. Explicit models of the patterns of contact among individuals in a community, contact network models, underlie a powerful approach to predicting and controlling the spread of such infectious disease and provide detailed and valuable insight into the fate and control of an outbreak. We use contact network epidemiology to predict the impact of various control policies for both a mildly contagious disease such as SARS and a more highly contagious disease such as smallpox. We demonstrate how integrating these tools into public health decision-making should facilitate more rational strategies for managing newly emerging diseases, bioterrorism and pandemic influenza in situations where empirical data are not yet available to guide decision making.   

Dynamical Epidemic Suppression Using Stochastic Prediction and Control

We consider the effects of noise on a model of epidemic outbreaks, where the outbreaks appear randomly. Using a constructive transition approach that predicts large outbreaks prior to their occurrence, we derive an adaptive control scheme that prevents large outbreaks from occurring. The theory is applicable to a wide range of stochastic processes with underlying deterministic structure.   

Chaotic desynchronization of multistrain diseases

Dengue fever, a multi-strain disease, has four distinct co-existing serotypes (strains). The serotypes interact by antibody-dependent enhancement (ADE), in which infection with a single serotype is asymptomatic, but contact with a second serotype leads to serious illness accompanied by greater infectivity. It has been observed from serotype data that outbreaks of the four serotypes occur asynchronously (Nisalak et al., Am. J. Trop. Med. Hyg. 68: 192). We developed a compartmental model and did bifurcation analysis for multiple serotypes with ADE. Both autonomous and seasonally driven versions were studied. For sufficiently small ADE, we find that the number of infectives of each serotype synchronizes, with outbreaks occurring in phase. However, when the ADE increases past a threshold, the system becomes chaotic, and infectives of each serotype desynchronize.   

Session N24: Focus Session: Friction, Fracture, and Deformation IV

Probing electronic friction in patterned silicon pn junctions

Phononic and electronic excitations are two of the fundamental processes that give rise to friction forces between sliding bodies in close proximity. We demonstrate that electronic contributions in friction can be quite significant, and even tunable for semiconducting samples, where the free carrier density in the vicinity of the contact can be modified electronically. In our experiments one of the solids is a Si(100) sample patterned with p and n regions that provide large differences in the density of charge carriers. The pattern consists of an array of 2 $\mu $m wide strips of highly doped p-type material in a nearly intrinsic n-type substrate. The other solid is a conductive Atomic Force Microscope tip. By varying the bias between tip and Si sample, charge depletion or strong accumulation could be induced in the n and p regions, which produces significant differences in the friction force. We attribute the increase in friction force following charge accumulation to energy dissipation by electrons. This result demonstrates not only the importance of electronic contributions to friction, but also the capability to electronically control friction with potential applications to nanoscale devices with moving parts.   

Simultaneous lateral force and STM imaging of Si (111) -7x7 surface using sub-Angstrom oscillation amplitude AFM

Lateral forces play an important role in friction studies as well as atomic manipulation. We present the design and performance of an nc-AFM which is capable of measuring lateral forces simultaneously with tunneling current. The microscope employs a sensitive fiber interferometer for high resolution force measurements. Home-made Tungsten cantilevers with typical stiffness of about 150 N/m is dithered in lateral directions respect to the sample with sub-{\AA}ngstrom oscillation amplitudes (A$_{0 }$=0.25 {\AA}) at a frequency, well below the resonance frequency and the changes in lateral oscillation amplitudes are recorded using a lock-in amplifier. In addition, the microscope can simultaneously be operated as STM. By changing the tunneling current and bringing the tip closer to the surface, we investigate the lateral forces during STM imaging. The lateral force images will be presented as a function of tunnel current (relative tip-sample distance) on Si(111) (7$\times $7) surface.   

Energy loss from repulsive contact to non-contact

Force microscopy experiments under ultrahigh-vacuum conditions are performed at separations from repulsive contact up to separations of 200nm. Energy loss at rather large separations is primarily related to the application of electrostatic fields. The relationship of adsorbates and non-contact friction is investigated. The transition to the repulsive contact is studied by the use of torsional oscillations. In the regime of repulsive contact, the important role of instabilities is confirmed.   

Molecular Level Investigations of Interfacial Friction of Polymer Brush Surfaces

The development of synthetic polymer lubricants to mimic joint lubrication within the human body will be presented. Unlike most industrial applications involving oils and greases, lubrication of these joints is accomplished in an aqueous environment. Fundamentally, water is a poor lubricant in most settings due to the weak pressure dependence of its viscosity, yet the contacting surfaces of skeletal joints function with low friction throughout a lifetime. Motivated by the molecular structure of materials making up joint surfaces, interfacial friction between polymer brush surfaces under aqueous environments has been probed with an array of molecularly sensitive surface analytical techniques including atomic force microscopy. The brush surfaces, comprised of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG), have been generated through the spontaneous adsorption of polymer from solution onto oxide substrates and sodium borosilicate surfaces (AFM tip). The character of the polymer films has been investigated in-situ with the quartz crystal microbalance (QCM) and atomic force microscope (AFM) and ex-situ with ellipsometry and X-ray photoelectron spectroscopy (XPS). The interfacial friction measurements have been carried out on polymer-coated substrates with bare or polymer-coated, microsphere-attached tips in over a range of solution conditions. It was found that the adsorption of polymer on oxides strikingly reduced the interfacial friction, resulting in ultra-low friction under certain conditions. By using a series of PLL-g-PEG polymers differing from each other in PEG side-chain length and grafting ratio, we observed that frictional properties of polymer-coated interfaces strongly depend on the architecture of PLL-g-PEG. Polymer-film formation and the influence of polymer architecture will be reviewed while the role of solvent and manifestation of ultra-low friction will be discussed in detail.   

Tribological properties of self-assembled monolayers in humid environments

Microelectromechanical systems (MEMS) are a rapidly growing area of technology. Due to the large surface area to volume ratio in MEMS, surface forces including friction and adhesion are tribological limitations that affect their performance. Self-assembled monolayer (SAM) coatings, which have high hydrophobicity, low surface energies and compact packing structures, are good canditates for MEMS lubrication. Large scale molecular dynamics simulations are used to study the frictional and adhesive behavior of hydrocarbon and fluorocarbon SAMs coatings on amorphous silica in the presence of water. The systems consist of SAMS with a chain length of 11 carbons physisorbed to the amorphous silica substrate with the water placed between the SAMs and the substrate. Humidity has no observed effect on the maximum adhesion of either of the SAMs. The coefficient of friction decreases with increasing water, which is in agreement with what is observed experimentally. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Tip-based simulations of nanotribology of self-assembled monolayers

Friction and adhesion simulations are generally performed with opposing flat-plate geometries, ignoring the effects of load-dependent contact areas arising from curved probe tips. While some previous tip/substrate simulations do exist, they tend to either use multi-timestep approximations or unrealistically sharp tips. We present the results of true dynamical nanotribological simulations of alkylsilane self-assembled monolayers (SAMs) with realistic tip/substrate geometries. Tips matching experimental dimensions ($\sim$~30 nm radius of curvature) were cut out of an amorphous silica substrate (a-SiO$_2$) and either coated with SAMs or annealed for uncoated tips. The adhesion and friction of the tip in contact with a SAM-coated amorphous a-SiO$_2$ substrate were studied with massively parallel molecular dynamics simulations. The effects of load-dependent contact areas are compared to previous simulations with flat plate geometries, and to atomic force microscopy measurments. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

Understanding the frictional response of organic monolayer coatings using Atomic Force Microscopy

Friction and wear are yet to be fundamentally understood, yet they can be major limiting factors for applications including microelectromechanical systems (MEMS). We use atomic force microscopy to determine frictional constitutive relations for nanoscale contacts designed to represent the asperities in MEMS. Quantitative measurements of friction and contact stiffness are performed using SiO$_{2}$- and organic monolayer-functionalized tips on organic monolayer-functionalized silicon. Using octadecyltrichrolosilane, octadecene, and perfluorinated monolayers, we find that friction depends on the type of molecule, its packing density, and the surface attachment chemistry. We also find that fluorination increases friction, as in MEMS, and that molecular transfer to the SiO$_{2}$ tip causes large variation in the measurements. With monolayer-coated tips, this variation, as well as the overall friction and adhesion, are significantly reduced.   

Capillary condensation and Ice formation at room temperature in nano-friction experiments

We report several direct observations of manifestations of capillary condensation in atomic-scale friction experiments. We have used a dedicated high-resolution friction force microscope to investigate the forces between a tungsten tip and a graphite surface under ambient conditions at a range of relative humidities. The velocity dependence of the friction shows a variety of new effects. We observe high friction and pronounced stick-slip instabilities with periods differing from those on graphite at very low scan velocities and moderate humidities. On the other hand, we see smooth sliding with strongly velocity dependent friction at higher humidities. We show that all aspects of the observed behavior can be interpreted in terms of capillary condensation of water, melting-freezing transitions and visco-elastic effects.   

Nanoscopic friction as a probe of local phase transitions

Water gas-liquid phase transitions have been investigated by measuring nanoscale friction forces between an atomic force microscope tip and a glass surface while varying the relative humidity, the scanning velocity and the temperature. We observe that it is possible to obtain the same friction versus velocity curves by fixing the sample temperature and varying the buffer humidity or by fixing the buffer humidity and varying the sample temperature. This behavior can be understood by introducing the concept of local humidity at the glass surface, which depends on the temperature. By using the well known macroscopic relationship between relative humidity and temperature we can fully explain our experimental results. This finding suggests that the water gas-liquid phase diagram is the same at the macroscopic scale as well as at the nanoscopic scale at a solid-gas interface. Furthermore, friction data for varying the scanning velocity provide mean nucleation times for capillary bridges formation. These times were found to alter from 3.5 ms up to 0.6 ms for temperatures ranging from 299 K up to 332 K. Natural logarithms of nucleation times plotted against inverse of experimental temperatures produce an Arrhenius plot and give a nucleation energy of 7.8$\times$10$^{-20}$ J for a nano-sized capillary bridge formation, in agreement with recent theoretical models. Our study provides the first direct experimental evidence of the thermally activated condensation of capillary bridges at the nanoscale.   

Superconductivity Dependent Friction

In order to gain a fundamental understanding of friction, one must understand, at the molecular level, how the energy associated with the work to overcome friction is converted to heat. One of the simplest possible geometries in which friction can occur, and thus be studied, is that of a fluid or crystalline monolayer adsorbed on an atomically flat surface. This geometry is experimentally accessible to experiments with a Quartz Crystal Microbalance (QCM), to numerical simulation techniques, and to analytic theory. A prior QCM experiment [1] sought to explore the nature of electronic contributions to friction by measuring the friction associated with nitrogen monolayers sliding on Pb substrates, that had been exposed to air, as the temperature passed through the superconducting transition at 7.2K. The work inspired a number of subsequent theoretical and experimental efforts, which yielded contradictory results. We have repeated these measurements on Pb substrates that were prepared \textit{in situ }for nitrogen and water films. We have observed the functional form of the rapid but smooth change in friction between nitrogen slipping on Pb near T$_{c}$. We have also observed a wider temperature range of frictional effects as compared to bulk changes in resistivity. We present these results and compare them to previous observations of superconductivity-dependent friction. [1] A. Dayo, W. Alnasrallah and J. Krim, Phys. Rev. Lett. vol 80, 1690 (1998); Work funded by NSF.   

Origin for Static Friction at Atomic Level Studied with Molecular Dynamics Simulations

Static friction has been always an interesting topic because of its ubiquitous presence in the sliding. With Molecular Dynamics simulations, we studied the static friction behavior for commensurate and incommensurate Al$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ interfaces, and flat and rough Al/Al interfaces. It is found that incommensurate Al$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ interface has lower static friction than commensurate Al$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ interface and roughness on the surface increases the static friction drastically. The relation between interfacial adhesion and friction has been investigated. Simulation results reveal that the origin of static friction is to overcome the potential barriers at the interface along the sliding distance. Static friction is determined by the amplitude of potential barrier of interfacial interaction rather than the absolute value of interfacial adhesion. The relation of static friction and kinetic friction are also discussed.   

Theory of Lubrication Due to Collective Pinning

In collective pinning theory, the problem of two three dimensional solids in contact is at its critical dimension. This implies that when the disordered forces acting between the two solids at the interface are relatively strong, the force of static friction should be large, but at smaller values of these forces, the system switches over to a regime of weak static friction. This provides a mechanism for the reduction of friction in boundary lubrication. It was shown previously that small lubricant molecules reduce static friction by filling in atomic depth holes in the surface and thus allowing the force pushing the surfaces together to be supported by more points of contact, which can switch the interface from the strong to weak static friction regime. Here it will be shown that lubricant molecules which are large compared to atomic dimensions can also put the interface in the weak pinning limit because molecules attached to high points on the surfaces can be easily compressed, allowing the load to be spread over more points of contact, and hence putting the interface in the weak pinning regime.   

A Model for Static and Dry Friction

It will be shown that the Muser-Robbins (MR) model, consisting of mobile molecules trapped between two incommensurate crystalline solids, exhibits many of the qualitative features of friction between macroscopic solids, such as the result that the static friction is greater than the kinetic friction, stick-slip motion and a force of static friction which increases as a function of the time that the two solids are in contact and stationary. At zero temperature, the kinetic friction is highly sensitive to the direction of sliding, but this sensitivity decreases markedly as the temperature rises. At low temperatures (with the surfaces stationary for a relatively long time), the model gives a static friction approximately 3 times larger than the kinetic friction for sufficiently slow velocities, but this ratio decreases steadily as the temperature is increased.   

Lack of Pinning for Rigid Sliding Monolayers in Microbalance

Recent work on the dynamics of monolayers on a metallic substrate attached to a quartz oscillator has provided interesting data on kinetic friction at the microscopic level. Sliding of the film relative to the substrate is often observed even though theoretical estimates seem to predict that the extremely small inertial forces should not be sufficient to make the film slide. It is shown here that if the defect potentials have a range of a little more than an atomic spacing, the net forces on the film due to the defects are likely to be quite small due to cancellations of the forces exerted by the defect on the atoms in the range of its potential. Thus, the net pinning force on the film is much smaller than it would be if each defect only acted on one atom at a time. It will be shown that this reduction of the pinning force due to the defects is quite significant and is able to account for the fact that films adsorbed on the quartz crystal are able to slide, even under the weak inertial forces provided by the quartz crystal's oscillations.   

Session N25: Focus Session: Novel and Complex Oxides: Cobaltites and Manganites

Anisotropic Magnetic and Magnetotransport Properties of $EuBaCo_{2}O_{5.5}$ Single Crystals

Magnetization, resistivity, and magnetoresistance were measured on detwinned EuBaCo$_{2}$O$_{5.5}$ single crystals over a wide range of dc magnetic fields (up to 33 T) and temperature (from 4 K to 300 K). EuBaCo$_{2}$O$_{5.5}$ has a layered structure (along the c-axis) with all the Co-ions in a trivalent state. The isothermal magnetization increases gradually until a critical field, where it undergoes a transition towards saturation for H $\vert \vert $ a-axis, while no similar transition is observed for H along b or c-axes. The critical field increases linearly with decreasing temperature, reaching 25 T at 4 K. From the M(H) data, a saturation moment of approximately 0.8$\mu _{B}$/Co is determined. Coinciding with the field induced transition in the magnetization, the isothermal resistivity shows a steep decrease for H $\vert \vert $ a-axis. The correlations between the magnetic order and the large negative MR will be discussed. We are grateful to the late Jack E. Crow who strongly influenced this work. *This work was carried out at the NHMFL, which is partially supported by the National Science Foundation through Cooperative Agreement No. DMR-0084173 and the State of Florida.   

Three Dimensional Magnetism in Na$_x$CoO$_2$

Recent neutron studies reveal an underlying A-type antiferromagnetic order and a surprisingly three dimensional magnetism in Na$_x$CoO$_2$ for x$>$0.7. We look carefully at interplanar hopping in this compound, comparing supercell and virtual crystal approximation (VCA) calculations, and find that the formation of an sp$^2$ hybrid on the Na ion plays an important role in the magnetic coupling between Co ions in different layers. The specific ($2b$ vs. $2d$) position of Na ions can change the hopping integral, and therefore the superexchange, between planes indicating that Na ordering may be related to magnetic order. By reformulating a linear spin wave model to account for more than one interplanar neighbor, we show that the isotropy of magnetic interactions is due not to isotropic exchange constants but rather to the ability of each Co ion to interact with seven other Co ions in the next plane.   

Antiferromagnetic Spin Waves and Pr Crystal Field Excitations in Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$

Neutron scattering investigations reveal three interesting features of antiferromagnetism in the doped manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$. The intensity of the (0.5 0 0.5) antiferromagnetic (AF) Bragg peak shows that the AF domains exist between 150 and 190 K in the ferromagnetic state. The spin wave dispersion of the Mn sub-lattice measured at 20 K in the wave vector range of (0.5 0 0.5) to (2 0 2) along the AF coupling direction could be well described by the Heisenberg model with nearest neighbor exchange interactions and single-ion anisotropy. The AF coupling and the single ion anisotropy energy of Pr$_{0.5}$Sr$_{0.5}$MnO$_{3 }$ are comparable to those found in LaMnO$_{3}$, suggesting the same of order of magnitude of the gap in the dispersion at the zone center. The ferromagnetic coupling of Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ is smaller by a factor of 3 as compared to that of LaMnO$_{3 }$ resulting in a smaller amplitude of the spin waves in the former. Pr crystalline field (CF) excitations in the AF state are found to be different from those in the ferromagnetic state suggesting the renormalization of at least one CF excitation of Pr due to an interaction with the spin waves of Mn near the zone boundary. Funded by DOE.   

Doped Cobaltites: Phase Separation, Intergranular Giant Magnetoresistance, and Glassy Transport

We have used magnetometry, transport, Nuclear Magnetic Resonance (NMR), Small Angle Neutron Scattering (SANS), and Transmission Electron Microscopy (TEM) to investigate magnetoelectronic phase separation in La$_{1-x}$Sr$_{x}$CoO$_{3}$. This material shows a crossover from a glassy phase at low doping to ferromagnetism (F) above x = 0.18, as well as a simultaneous transition from insulator to metal. NMR confirms magnetic phase inhomogeneity with low spin non-magnetic, glassy, and F regions coexisting spatially. SANS reveals 25 {\AA} F clusters forming in a matrix of non-F insulator at low doping, eventually leading to a percolation transition to long-range F order at x $>$ 0.18. In single crystals, this formation of isolated clusters leads to a hysteretic negative MagnetoResistance (MR) at low temperatures, which has field, temperature, and doping dependencies consistent with an intergranular Giant MagnetoResistance (GMR) effect. We argue that this system is a naturally forming analog to the artificial structures fabricated by depositing nanoscale F particles in a metallic or insulating matrix, i.e. this material displays an intergranular GMR effect without the deliberate introduction of chemical interfaces. The formation of nanoscopic F clusters also gives rise to glassy transport phenomena that are reminiscent of relaxor ferroelectrics. The transport properties show a bifurcation of field cooled and zero field cooled temperature traces, slow response to changes in magnetic fields, and, most notably, a ``waiting time'' effect that can be observed directly in the resistivity. \textbf{Acknowledgements:} ACS Petroleum Research Fund, UMN NSF MRSEC. \textbf{Co-Authors:} J. Wu, J. Lynn, C. Glinka, J. Burley, H. Zheng, J. Mitchell, W. Moulton, M. Hoch, P. Kuhns, A. Reyes, C. Perrey, N. Munoz, R. Thompson and B. Carter.   

Electron-hole excitations in CaMnO$_{3}$ and LaMnO$_{3}$(*)

The electron-hole excitations in CaMnO$_{3}$ and LaMnO$_{3}$ are investigated via ab initio techniques (time-dependent density-functional theory). The ground state is described within the LDA+U “correlated-band structure” method. The electron dynamics is handled within the random-phase approximation (RPA). The loss spectrum in both materials is dominated by a striking “collective” excitation; the same is directly related to the underlying electronic structure, as its energy is a signature of the relative location of the upper and lower Hubbard bands. The physics of the dynamical screening (the spectral weight of the leading excitation, its remarkable dependence on wave vector (both on the magnitude and direction of q, etc.) is controlled by d-d transitions and the microscopic crystal local fields. Our predictions can be readily verified via measurements of the dynamical structure factor with inelastic scattering of hard x-rays --providing a direct test of the quality of the LDA+U ground state and the RPA dynamics. In fact, there is qualitative agreement with a recent investigation for LaMnO$_{3}$ with resonant x-ray scattering (**). (*) Research supported by NSF Grant ITR-DMR 0219332 (**) S.Grenier et al., cond-mat/0407326.   

Doping Evolution of the Electronic Structure of Bilayer Colossal Magnetoresistive Manganites

Manganites have been the subject of great current interest not only because they exhibit the colossal magnetoresistance (CMR) effect, but also because they display a wide variety of magnetic properties and undergo several phase transitions from paramagnetic insulator (PMI) to ferromagnetic metals (FMM). Here we present a detailed momentum, doping and temperature dependent study of the electronic properties of bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, by means of angle resolved photoemission spectroscopy. Differences and similarities between different doping are discussed. In particular we will address how the electronic structure evolves from the PMI to the FMM phase as well as the emergence of the CMR phase from the doping evolution.   

Extracting d-orbital information from Magnetic Compton experiments in bilayer manganites

Magnetic Compton profiles (MCPs) have been measured for the colossal magnetoresistance double layer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ along various crystallographic directions over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations of the magnetic momentum density and the MCPs. The usefulness of the so called $B({\bf r})$ function, obtained by a one-dimensional Fourier transform of the MCP, is emphasized [1]. In particular, the form of $B({\bf r})$ for momentum transfer along the [110] direction is found to contain a prominent dip at around 1 a.u., whose depth is shown to provide a sensitive measure of the population of $e_g$ electrons of $d_{x^2-y^2}$ symmetry in the system. Work supported in part by the USDOE.\\ $\mbox{[1]}$ Yinwan Li, P. A. Montano, J.F. Mitchell, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk and A. Bansil, Phys. Rev. Lett. {\bf 93}, 207206 (2004).   

Orbital effects in cobaltites by neutron scattering

The orbital degree of freedom can play a central role in the physics of transition metal perovskite oxides because of its intricate coupling with other degrees of freedom such as spin, charge and lattice. In this talk the case of La$_{1-x}$Sr$_{x}$CoO$_{3}$ will be presented. Using elastic and inelastic neutron scattering, we investigated the thermal evolution of the local atomic structure and lattice dynamics in the pure sample and with the addition of charge carriers as the system crosses over from a paramagnetic insulator to a ferromagnetic metal. In LaCoO$_{3}$, the thermal activation of the Co ions from a nonmagnetic ground state to an intermediate spin state gives rise to orbital degeneracy. This leads to Jahn-Teller distortions that are dynamical in nature. Doping stabilizes the intermediate spin configuration of the Co ions in the paramagnetic insulating phase. Evidence for local static Jahn-Teller distortions is observed but without long-range ordering. The size of the JT lattice is proportional to the amount of charge. However, with cooling to the metallic phase, static JT distortions disappear for x $\le $ 30 {\%}, the percolation limit. This coincides with narrowing of two modes at $\hbar \omega =22\,and\,24\,meV$ in the phonon spectrum in which we argue is due to localized dynamical JT fluctuations$^{1}$. The implications of the orbital effects to the structural and magnetic properties will be discussed. $^{1}$D. Louca and J. L. Sarrao, Phys. Rev. Lett. \textbf{91,} 155501 (2003).   

Direct crystallographic evidence of charge ordering in the novel double perovskite mixed-valence (NaMn3)Mn4O12

By means of high-resolution synchrotron X-ray powder diffraction measurements, we studied in detail the temperature-dependent crystal structure of the mixed-valence manganese oxide, (NaMn$_{3})$Mn$_{4}$O$_{12}$. At 176 K, we observed a static ordering of the Jahn-Teller distortion of the Mn$^{3+}$O$_{6}$ octahedra that drives a cubic-monoclinic structural transition concomitant to the Mn$^{3+}$-Mn$^{4+}$ charge ordering of the octahedral B-sites of the double-perovskite structure AA'$_{3}$B$_{4}$O$_{12}$. This transition is followed by a CE-type magnetic ordering of these sites at 125K and by an independent antiferromagnetic ordering of the Mn A' sites at 90 K. Remarkably, both neutron [1] and X-ray data show that the charge ordering is intrinsic to the low symmetry phase, resulting in the setting up of two distinct MnO$_{6}$ octahedra with very different average Mn-O distances. A bond valence sum analysis shows that these two Mn sites exactly correspond to 3+ and 4+ formal valence states. This direct evidence of charge disproportionation has never been reported in half-doped manganites, where charge order has been believed to occur only from controversial analysis of structural modulations. Ref: [1] A. Prodi \textit{et al., Nature Materials} \textbf{3}, 48 (2004).   

On the Microstructure of the Charge Density Wave Observed in La1-xCaxMnO3

We have used low temperature (90~K) transmission electron microscopy to investigate the `charge ordering' modulation in the mixed valent manganite, La$_{1{\-}x}$Ca$_{x}$MnO$_{3}$. It has been stated that Mn$^{3+}$ and Mn$^{4+}$ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the \textbf{a*} axis of the crystal but need not be exactly collinear. A typical grain of size 0.5~$\mu $m in La$_{0.48}$Ca$_{0.52}$MnO$_{3}$ had a wavevector which varied on a scale of tens of nanometres with an average of $<$\textbf{q}$>$~=~0.450\textbf{a}$*$ and a standard deviation \textit{$\Delta $q}~=~0.004$a*$ in its magnitude and \textit{$\Delta \theta $}~=~0.56\r{ } in its direction at 90~K. The magnitude of the wavevector in this composition fell by 20{\%} as the temperature was increased from 90~K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30~K above the `charge ordering temperature'.   

Structural and magnetic properties of A-site ordered manganites RBaMn$_2$O$_6$ (R=Pr, Nd, Pr$_{1/2}$Nd$_{1/2}$)

Temperature and magnetic-field dependent structural and physical properties of A-site ordered manganites RBaMn$_{2}$O$_{6}$ (R = Pr, Nd, Pr$_{1/2}$Nd$_{1/2})$ were studied using high-resolution high-energy X-ray powder diffraction and magnetic and transport measurements. The ferromagnetic (FM) to antiferromagnetic (AF) phase transitions of all three materials are accompanied by first-order structural changes. Both the FM and AF phases of PrBaMn$_{2}$O$_{6}$ and Pr$_{1/2}$Nd$_{1/2}$BaMn$_{2}$O$_{6}$ have tetragonal structures, though the FM phase of the latter shows significant broadening of the (200) peak, suggesting a slight in-plane orthorhombic distortion. NdBaMn$_{2}$O$_{6}$ is tetragonal in the AF phase and orthorhombic in the FM phase. The FM-AF transition temperature T$_{c}$ increases with decreasing R$^{3+}$ ionic radius, while decreases with applied magnetic fields. The T$_{c}$ can be shifted by 15$\sim $25 K for H=6 T. 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 and work at NIU by NSF- DMR-0302617.   

Session N26: Computational Nanoscience III

Simulations of semiconductor nanoparticles

We review theoretical works concerning the optical properties of II-VI, III-V and group-IV semiconductor nanocrystals. We present simulations based on semi-empirical tight binding method. In the case of Si nanocrystals, we discuss why the time decay of the photoluminescence is characterized by stretched exponentials. The usual explanation is that excitations can migrate between neighbour nanocrystals but we discuss experimental results in which it cannot be the case. We show that stretched exponentials can be explained quantitatively by intrinsic recombination in an ensemble of isolated nanocrystals made of an indirect gap material. Optical properties of PbSe nanocrystals will be discussed in the second part of the talk. We show that the cubic lattice of the material leads to unusual properties compared to zinc-blende semiconductors. Interband and intraband optical transistions are calculated and compared to experiments. Finally we present results concerning the energy transfer between neighbor nanocrystals by Foerster-type transitions. Results for III-V and Si nanocrystals will be compared.   

Microscopic dielectric function in semiconductor quantum dots

Dielectric function and screening effects within a quantum dot is of paramount importance in describing the optical properties and energy levels in a quantum dot. Previously it was believed that the dielectric function inside a quantum dot decreases compared to the bulk due to the increase of the band gap. Recently[1], using macroscopic electric fields and response analysis, it was suggested that the reduction of dielectric constant in a quantum dot is due to surface bond breaking, not due to the opening of the band gap. We have investigated this issue by studying the microscopic response function using plane wave ab initio calculations. Indeed, we found that the microscopic response function f(r1,r2) is identical to the bulk value when both r1 and r2 are within the quantum dot. We have provided a model which allows one to accurately approximate the quantum dot microscopic dielectric function f(r1,r2) from its bulk values without doing explicit calculations. The model also produces accurately the overall dielectric constant reduction for a quantum dot compared to its bulk value. [1] C. Delerue, M. Lannoo, G. Allan, Phys. Rev. B 68, 115411 (2003).   

Computation of the influence of scanning probe microscope (SPM) on quantum dot eigenstates and 2DEG potential

We calculate the electronic structure of GaAs-AlGaAs two-dimensional electron gas (2DEG) devices, such as quantum dots and quantum point contacts (QPCs) in the presence of a tip of a scanning probe microscope at some distance above the surface. The calculation employs standard density functional theory with exchange and correlation treated in the local density approximation. The position and voltage on the tip are varied and the conditions for depletion of the 2DEG are shown to compare favorably to experiment [1]. We show that the size of the depletion region created (by a negative tip voltage) is unexpectedly small due to focusing of the potential lines by the higher dielectric. We study the interaction of the tip with an isolated quantum dot that contains one or two electrons. The raster pattern of the \textit{difference }between single particle energies reveals that the tip distorts the shape of the confining potential and suggests that excited state properties, if they can be measured experimentally, can contribute to the resolution of spatial information. [1] M.A. Topinka, R.M. Westervelt, E.J. Heller, ``http://meso.deas.harvard.edu/papers/Topinka, PT 56 12 (2003)'' (Imaging Electron Flow), Physics Today \textbf{56}, 12 (2003).   

Quasi-ballistic and diffusive electron transport in inhomogeneous semiconductor nanostructures

We study nonequilibrium electron transport in inhomogeneous, nondegenerate semiconductor nanostructures using a computational approach based on the self-consistent, direct solution of the semiclassical, steady-state Boltzmann transport equation and the Poisson equation. We show that, in general, large applied and built-in fields in these systems give rise to strongly out-of-equilibrium electron velocity distributions that display interesting structure in the high-energy tail of the distribution, caused by the interplay between quasi-ballistic and diffusive contributions to the electron transport. The observed characteristics have a strong spatial dependence, related to the large inhomogeneous electric field variations in these systems, as well as a strong dependence on temperature and the detailed nature of the electron scattering where we find that the impact of a phonon threshold-energy scattering mechanism on the nonequilibrium distribution is considerable.   

Thermodynamics and kinetics of group-IV nanoparticles under pressure

The kinetics and thermodynamics of phase transformations in group-IV nanoparticles during a shock compression are studied with full first-principles molecular dynamics simulations. A novel electronic-enthalpy functional is introduced to describe accurately and efficiently finite-size quantum systems under pressure. Significant differences are found in the structural response of carbon, silicon and germanium nanoparticles, depending on size, composition, and surface structure. The presence of trapped metastable amorphous configurations for Si and bigger Ge nanoparticles highlights the importance of kinetics effects in the phase transformation. It also demonstrates the possibility of using nanoparticles to study bonding rearrangements and structural transformations which are not accessible to the bulk counterparts.   

Effect of anharmonicity of the inter-atomic potential on the built-in strain in epitaxial quantum dot structures

It is demonstrated that the anharmonicity of the inter-atomic potential is important in covered nanostructures. Compared to the strain distribution found with the standard Keating model, corrections of over 100 meV are found in electronic band offsets, resulting in values significantly closer to the experimental data. The anharmonicity correction coefficients for Si, Ge, AlAs, GaAs, InAs, AlSb, GaSb, and InSb are presented. The simulated lattice constant profiles and deformation of the cleaved surface are shown to be in a good agreement with the data observed by XSTM measurements for InAs/GaAs and GaSb/GaAs quantum dot structures. The anharmonicity corrections can be performed without a significant increase of the computational cost, since the model remains limited to the nearest neighbor interactions. Simulations of strained systems containing up to 30 million atoms are demonstrated. This work was performed while OLL held a National Research Council Research Associateship Award at Jet Propulsion Laboratory.   

Electronic properties of phosphorous $\delta$-doped silicon

We present a comprehensive theoretical study of phosphorous $\delta$-doped silicon (a candidate material for quantum computation) with doping density up to $3.4 \times 10^ {14} cm^{-2}$ (which corresponds to 1/2 monolayer of doping). A microscopic model based on empirical pseudopotentials and planar Wannier orbital basis is used to calculate the delta- doped system. 1000 monolayers of silicon is included to minimize the boundary confinement effect. Self-consistent potential as well as the exchange-correlation effects due to the doping electrons have been taken into account. It is shown that the 2D band structure of the delta-doped system can be reasonably approximated by an effective mass model over a large range of doping density. However, for ultra-high doping ($> 2 \times 10^{14} cm^{-2}$), which reaches the experiment limit, the band-structure is significantly deformed, as a result of the strong confinement from the V-shape self- consistent potential. The Fermi level (relative to the conduction band minimum) as a function of the doping density is studied and its implication on the transport properties will be discussed.   

Hydrogenated silicon fullerenes and endohedral dopings

Empty cage fullerene structure of Si$_{20}$ can be stabilized by hydrogen capping in equi-atomic concentration [1]. We study hydrogenated silicon fullerenes Si$_n$H$_n$, ($n$ = 14-28) using {\it ab initio} ultrasoft pseudopotential method and generalized gradient approximation for the exchange-correlation energy. It is found that Si$_{20}$H$_{20}$ has the optimal size. The empty space in the cages can be filled with atoms and this allows the formation of endohedral silicon fullerenes. The interaction of the guest atom with the cage is weak as compared to metal encapsulated silicon clusters [2]. Our results show that doping can be used to manipulate the highest occupied-lowest unoccupied molecular orbital gaps of these fullerenes and prepare species with large magnetic moments and varied optical properties. Guest atoms with closed electronic shell configurations generally occupy the center of the cage while open shell atoms tend to drift towards the wall of the cage. [1] V. Kumar and Y. Kawazoe, Phys. Rev. Lett. 90, 055502 (2003). [2] V. Kumar and Y. Kawazoe, Phys. Rev. Lett. 87, 045503 (2001).   

Optical properties of CdTe and CdSe nanocrystals

The electronic structure of CdTe and CdSe nanocrystals has been calculated as a function of size using the tight-binding approximation with an sp3d5s* basis including spin-orbit coupling and taking into account d core atomic functions on the cation atoms. Two particle electron-hole interaction is also incorporated in the calculation of the photoemission decay rate. For CdSe, we compare the results for the two zinc blende and wurtzite atomic structures. Comparison is made with recent experimental results.   

Stability of DX center in semiconductor quantum dots

Semiconductor quantum dot(QD) are of great current interest for applications because the physical properties of QD such as the band gap can be tailored by size or shape. On the other hand the application of semiconductors as novel electronic devices depend critically on its doping properties. Although defect properties have been extensively studied in the past for bulk, very few studies have been done for QD. For example, it is known that DX$^ {-}$ center in Si doped GaAs is unstable in bulk, however, it is not clear whether it is stable the case in GaAs QD. Using first– principles band structure method, we study how the size of QD affects the stability and transition energy levels of DX center of GaAs:Si. We find that although Si DX center is unstable in bulk GaAs, when the dot size is small enough, it is stabilized. The critical size of QD is around 3nm of diameter. The stabilization is due to the strong quantum confinement effect, the conduction band edge of QD increases. The formation energy of the tetrahedral coordinated Si$_{Ga}^{-}$ also increases because the occupied shallow defect level is mostly CBM-like. On the other hand, the DX$^{-}$ defect level contains significant amount of non-CBM characters, so the increase of formation energy of the DX$^{-}$ center is less than the shallow Si$_{Ga}^ {-}$ defect. Our studies show that defect in QD could be significantly different from the bulk.   

Interaction of Magic Gold Cluster with Si60 Cage

Both Au clusters and Si clusters individually are important subjects in chemistry, physics, and materials science. It is very interesting to study the interactions between these two technologically important systems. In this paper, first-principles studies are performed on Au$_{12}$W@Si$_{60}$ by using projector-augmented wave (PAW) method and generalized gradient approximation for the exchange-correlation energy. The geometry, electronic structure, orbital hybridization, and charge transfer are discussed. It has been found that the magic Au$_{12}$W cluster actively interacts with Si, and Si$_{60}$ cage structure can be stabilized. Meanwhile the metal cluster is dissociated when encapsulated in Si60 cage, and charge is transferred from Si cage to the metal atoms. The present study suggests that due to the special properties of Au itself, the magic gold clusters have both energetic stability and chemical activity and can be used to design novel nano structures and nano devices.   

Fixed-Phase Path Integral Monte Carlo Simulations in Quantum Dots in Magnetic Fields

We have developed a fixed-phase approximation for path integral Monte Carlo (PIMC) simulations. With the fixed phase approximation, the difficulties created by phases in path integrals for magnetic systems are managed in a practical way. We first demonstrate the method on electrons in a 2-D parabolic dot in a magnetic field. The PIMC method allows us to extend the simulation to a realistic 3D model of an InGaAs/GaAs lens-shaped self-assembled dot, so we can study the deviations from an idealized parabolic model. We then apply the method to study the magnetic field dependence of biexciton binding in the different dot models.   

Ab-Initio Calculations of the Structural Properties of CdSe Nanorods and their Interaction with Organic Ligands

First principles electronic structure simulations are used to study the atomistic detail of the interaction between organic surfactant molecules and the surfaces of CdSe semiconductor nanoparticles.[1] These calculations provide insights into the relaxed atomic geometry of organics bound to semiconductor surfaces at the nanoscale as well as the electronic charge transfer between surface atoms and the organics. We calculate the binding energy of phosphine oxide, phosphonic and carboxylic acids and amine ligands to a range of CdSe nanoparticle facets. The calculated relative binding strengths of ligands to different facets support the hypothesis that these binding energies control the relative growth rates of different facets, and therefore the resulting geometry of the nanorods. The calculated relaxed atomic geometries of CdSe nanorods with a range of diameters and growth directions are then compared with the results of EXAFS measurements of recently synthesized nanorods to determine their atomic structure and surface relaxations. [1] A. Puzder, A.J. Williamson, N. Zaitseva, G. Galli, L. Manna and A.P. Alivisatos, Nano Lett. (2004).   

Session N27: Focus Session: Carbon Nanotubes: Functionalization I

Thermodynamic properties Ar films on the surface of a bundle of carbon nanotubes

We employ canonical Monte Carlo simulations to explore the properties of an Argon film adsorbed on the external surface of a bundle of carbon nanotubes. The study is concerned primarily with three properties: specific heat, differential heat of adsorption, and Ar-Ar correlation functions. These measurable functions exhibit information about the dependence of film structure on coverage and temperature. Our results are intended to stimulate further experimental studies of this system and analogous systems involving other gases on nanotube bundles. One of the more interesting general results is that the specific heat is typically larger than might have been expected. Particularly remarkable outcome from the correlation function studies include the reduced longitudinal correlations in the groove and striped phases as T rises above 60 K. These results would be amenable to testing by diffraction experiments.   

Neon adsorption isotherms on carbon nanohorns

We will present results of neon adsorbed on unpurified carbon nanohorns. The nanohorn sample was obtained from Nanocraft, Inc. Our adsorption isotherm measurements were conducted at temperatures between 19 and 27 K. We determined the specific surface area for our sample, and studied adsorption at different coverages. Results for isosteric heat will also be presented. This work is supported by the NSF through grant DMR-0089713.   

Adsorption of Xenon on Hipco Single Walled Carbon Nanotubes

We have measured the adsorption of Xenon on purified HiPco SWCNTs for coverages in the first layer. We wanted to compare the results on this substrate to those we had obtained on lower purity arc-discharge produced nanotubes. In order to obtain an estimate for the binding energy. We measured six low-coverage isotherms for temperatures between 220K and 260K. We determined a value of 272 meV for the binding energy; this value is lower, by about 4{\%} than the value we had reported on arc discharge nanotubes$^{1}$. It is 1.67 times greater than the value for this quantity on planar graphite. We have measured five full isotherms at 150K, 155K, 160K, 165K, and 175K and have used these data to obtain the coverage dependence of the isosteric heat. The experimental values will be compared with computer simulation results for this quantity that have been conducted using different models for bundles$^{2}$. 1 A. J. Zambano, S. Talapatra, and A. D. Migone Physics Review B, 64, 2001. 2 Wei Shi, and J. Karl Johnson Physical Review Letters 91, 2003. * The present study was supported by the National Science Foundation through Grant {\#} DMR-0089713   

Collisional-Induced Resistivity of Carbon Nanotubes

A single-walled carbon nanotube (SWNT) is often mentioned as one of the strongest materials known. In tension along the tube axis, this statement is correct. However, the tube is ``soft'' in the radial direction, i.e., deformation or squash modes which give rise to an oscillating elliptical cross section have freq's in the range 20-30 cm$^{-1}$. Here, we present results of an \textit{in situ} electrical transport study (thermoelectric power (S) and resistivity ($\rho )$ ) of bundled SWNTs exposed to a series of gases (He, Ar,Ne,Kr,Xe;CH$_{4}$,N$_{2})$. Unusually strong and remarkably systematic changes in these transport properties are observed as the nanotubes undergo collisions with these atomic and molecular gases. At fixed pressure and temperature, the changes in the transport parameters, i.e., $\Delta $S and $\Delta \rho $, are observed experimentally to exhibit an $\sim $ M$^{1/3}$ behavior. At fixed temperature, $\Delta $S and $\Delta \rho $ saturate in the range 0.3-0.5 atm,, with the saturation pressure depending on M. Results of molecular dynamics that simulate the gas-nanotube collision show that the maximum deformation of the tube wall and the radial kinetic energy transfer to the tube wall also exhibit this M$^{1/3}$ behavior. It appears that the transient deformation or ``dent'' caused by the collisions may provide new scattering mechanism for itinerant electrons in the tube walls. These dents ring as the fundamental ``squash'' mode of the tube wall. The pressure p$_{sat}$ at which $\Delta $S and $\Delta \rho $ can be shown to be consistent with the tube pressure at which co-existing dents first begin to overlap.   

Adsorption of Tetraflouromethane on HiPco Purified SWNTs

We have studied the adsorption behavior of tetraflouromethane, CF4, on purified, single-walled HiPco nanotubes. Isotherms were performed between 100K and 125K. We find that there are two substeps in the first layer data for CF4 on the SWNTs; results will be compared to previous measurements on this system. Results for the isosteric heat as a function of coverage will also be presented. Long waiting times are necessary to ensure that equilibrium is reached in these experiments. This research supported by National Science Foundation grant {\#} DMR-0089713.   

Adsorption of polar molecules on carbon nanotubes in transverse electric fields

Experiments in our laboratory show that capacitance measurements can be used for chemical sensing with arrays of single-wall carbon nanotubes (NTs). Molecular adsorption on NTs is affected by electrostatic gating that creates intense surface electric fields. The a.c. capacitance amplitude is found to be related to the intrinsic adsorbate dipole moment. To understand the details of the processes involved, \textit{ab-initio} calculations of molecular adsorption on graphene and on NTs with electric fields normal to the surfaces have been made. We study the dependence of adsorption energy, adsorption-induced polarizability and charge transfer on the field intensity and direction in the range 10$^{4}$-10$^{6}$ V/cm. We identify three groups of adsorbed molecules: (1) those with the dipole moment normal to the NT for which the observed capacitance depends $\sim $ linearly on dipole moment (e.g. C$_{3}$H$_{6}$O, CH$_{3}$Cl, NH$_{3}$, DMMP, CNH); (2) those with dipole moments parallel to the NT and that have little effect on the capacitance (e.g. C$_{6}$H$_{5}$Cl, C$_{6}$H$_{4}$Cl$_{2})$; (3) non-polar molecules (e.g. CH$_{4})$, polarized by the intense electric fields and by adsorption. The adsorption-induced dipole moment plays an important role in the total polarization. The results are consistent with the experiment, in particular with infrared spectroscopy data.   

Adsorption of Aromatic Compounds on Carbon Nanotubes

The functionalization of carbon nanotubes (CNTs) by molecular adsorption is of scientific interest and also of importance in potential applications as sensors. We have studied theoretically the interactions between CNTs and organic aromatic molecules that are derived from benzene by addition of different functional groups (e.g. $CH_{3}$, $OH$, $NO_{2}$). We perform density functional ${\it ab}$ ${\it initio}$ calculations based on the plane-wave supercell method using the generalized gradient approximation. We explore the possible configurations of bonding to both zigzag and armchair CNTs. Two types of minimum energy configurations are distinguished: i) those where the benzene ring is parallel to the CNT surface and the coupling is dominated by $\pi-\pi$ interactions; ii) those where the functional groups of the molecules arrange normal to the CNT surface and bind stronger to the CNT. We discuss quantities related to experimental observables , such as adsorption energy, bonding, and changes induced on the CNT electronic structure.   

Theoretical Study of Side Wall Ozonation of Single-Wall Carbon Nanotubes

Oxidation of single-wall carbon nanotubes (SWCNTs) by ozone has been utilized extensively, for example, in the elimination of amorphous carbon in purification processes, for opening closed tips and sidewalls of nanotubes to maximize the rate of metal adsorption and hydrogen uptake, as well as to facilitate functionalization for achieving solubility. In this work, we present a density functional theory study, to investigate the adsorption mechanisms of O$_{3}$ on the sidewall of metallic C(5,5), and semiconducting C(10,0) SWCNTs, in the presence and absence of Stone-Wales defects with different morphologies. The energetics, electronic structures, and Raman modes shifts, upon adsorption, as compared to pristine tubes, will be discussed in detail.   

Effects of Ion Beam Irradiation on Thermal Oxidation of Single Walled Carbon Nanotubes

The properties of carbon nanotubes (CNTs) are closely dependent on their structures, and therefore may be tailored by controllably introducing defects in the nanotube systems. In this work, we investigate the effects of energetic ions (H$_{2}$, He and Ne) on the thermal stability of single wall nanotubes (SWNTs) against oxidation in air. SWNTs were irradiated with ions of energy in MeV to various doses in the range of 10$^{13 }$- 10 $^{16}$ cm$^{-2}$. Thermogravimetric analysis was used to determine the loss of CNT masses within 300-700 $^{o}$C as a result of oxidation processes. As opposed to the case of pristine SWNTs, the temperature (T$_{m})$ corresponding to maximum oxidation rate was increased by about 25 $^{o}$C for the SWNTs implanted with Hydrogen dose of 10$^{15}$ cm$^{-2}$, while He and Ne ion implantation resulting in decrease in T$_{m}$. The activation energies for thermal oxidation under various conditions were also extracted from TGA data, with values ranging from 1.13 eV (for pristine SWNTs) to 1.37 eV, depending on ion doses and species. Raman spectroscopy was used to determine the characteristics of the G band (C-C stretching mode) and D band (disorder induced mode) in CNTs. The work suggests that the bonding in CNTs could be strengthened or weakened depending on the amount of ion-beam- induced defects, leading to the enhanced or reduced thermal stability of CNTs against oxidation.   

Mechanism for bias-assisted mass transport of indium on carbon nanotube surfaces

We have preformed \emph{ab initio} pseudopotential density functional calculations to study the adsorption and diffusion of indium atoms on graphite-like and carbon nanotube surfaces. The adsorption energy was calculated as a function of In coverage, and it is shown that, for low surface densities, In becomes positively charged by donating one electron to the surface. This explains the experimental evidence that In deposited on carbon nanotubes migrates towards the cathode under an applied voltage. The effects of nanotube surface curvature on In adsorption are shown to be small. Based on the calculated energy barrier between two neighboring adsorption sites and the calculated vibrational frequencies of the adsorbate, the hopping rate for In adsorbed on graphene is estimated. Finally, In adsorption is shown to be stronger near a Stone-Wales defect, which could be linked to the nucleation of In nanocrystals. This work was supported by National Science Foundation Grant No. DMR04-39768 and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S.Department of Energy under Contract No. DE-AC03-76SF00098. Computational resources have been provided by NPACI and NERSC.   

Morphological Evolution of Metal Nanoparticles in Metal-Carbon Nanotube Composites

Metal nanoparticle-carbon nanotube composites are new emerging nanomaterials with a variety of potential technological applications such as fuel cells, sensors, and nanocatalsyts. Currently, fabrication of these nanocomposite materials proceeds via trial and error due to the lack of fundamental understanding of their growth mechanisms. We propose a model to elucidate the morphological evolution of metal nanoparticles grown on surfaces of carbon nanotubes. The model is based on a novel concept, namely, bending-strain-induced self-organization of nanoparticles on curved surfaces. In the framework of continuum theory of elasticity, a criterion is derived to predict the size and shape of metal nanoparticles. Applications of the criterion to different metals show good agreement with experimental results. Our model is expected to be very fundamental. It has the potential to have important applications to understanding and controlling nanomaterials growth on any substrates with curved surfaces.   

C60-Incorporated Carbon Nanohorns: Control of Filling and Releasing

Single-wall carbon nanohorn (SWNH) has structures similar to single wall carbon nanotube, and nanometer-scaled holes were opened through the walls by heating in oxygen (SWNHox). Recently, we have succeeded in a large-scale preparation of C$_{60}$-incorporated SWNHox (C$_{60}$@SWNHox) at room temperature in liquid phase by our new method of ``nano-precipitation.'' In this report, we show that incorporation of C$_{60}$ inside SWNHox could be confirmed by TEM observation, Raman spectrum measurements, and X-ray diffraction measurements. Quantity of C$_{60}$ incorporated inside SWNHox was estimated from thermogravimetric analysis. The release rates of C$_{60}$ from C$_{60}$@SWNHox in solutions were able to be clarified by UV/Vis absorption measurements. Through these investigations, we found that the filling quantities and release rates of C$_{60}$ were able to be controlled easily for SWNHs. These are advantages of large-diameters of the tubes to which C$_{60}$ molecules are bound moderately.   

Session N28: Focus Session: Mechanical Properties of Metals

Effect of alloying on screw dislocation structure in Mo: atomistic modelling approach with ab-initio parametrization

The plastic deformation in bcc metals is realized by the motion of screw dislocations with a complex star-like non-planar core. In this case, the direct investigation of the solute effect by first principles electronic structure calculations is a challenging problem for which we follow a combined approach that includes atomistic dislocation modelling with {\it ab-initio} parametrization of interatomic interactions. The screw dislocation core structure in Mo alloys is described within the model of atomic row displacements along a dislocation line with the interatomic row potential estimated from total energy full-potential linear muffin-tin orbital (FLMTO) calculations with the generalized gradient approximation (GGA) for the exchange-correlation potential. We demonstrate (1) that the solute effect on the dislocation structure is different for ``hard'' and ``easy'' cores and (2) that the softener addition in a ``hard'' core gives rise to a structural transformation into a configuration with a lower energy through an intermediate state. The softener solute is shown to disturb locally the three-fold symmetry of the dislocation core and the dislocation structure tends to the split planar core.   

Dislocation structure and mechanical behavior of Rh$_3$X L1$_2$ intermetallic alloys: combined ab-initio-Peierls-Nabarro model approach

Alloys based on Pt-group metals are promising materials for ultra-high temperature applications. Among them, Rh-based alloys are attractive due to a combination of high melting point, strength and superior oxidation resistance. Unfortunately, there is no information about dislocation properties and mechanisms driving their mechanical behavior. We analyzed the structure and mobility of dislocations in Rh$_{3}$X, where X = Ti, Zr, Hf, V, Nb, Ta, within the modified Peierls-Nabarro model with generalized stacking fault energetics calculated using the FLAPW method\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB {\bf 24}, 864 (1981)}. Superdislocations with type I core structure (APB-bounded) are preferred in Rh$_{3}$Ti and Rh$_{3}$Ta, whereas superdislocations with type II core (SISF-bounded) are predicted in Rh$_{3}$V and Rh$_{3}$Nb. An unusual superdislocation core structure (SISF-bounded type II$^\prime$ with different sequence of Shockley partials), resulting from the unstable APB energy, was found in Rh$_{3}$Hf and Rh$_{3}$Zr. Based on our analysis of dislocation structure and mobility, we provide predictions of temperature yield stress behavior of Rh-based intermetallics, and show that their dislocation properties are closely connected with features of the electronic structure and the instability of the L1$_{2}$ phase with respect to D0$_{19}$ and D0$_{24}$.   

Continuum Theory of Dislocations: Cell Structure Formation

Line-like topological defects inside metals are called dislocations. These dislocations in late stages of hardening form patterns called \textit{cell structures}. We are developing a mesoscale theory for the formation of cell structures that systematically derives the order parameter fields and evolution laws from the conserved topological Burgers vector density or the Nye dislocation tensor. (In classical plasticity theories, describing scales large compared to these cells, one normally bypasses the complicated motions of the dislocations by supplying yield surface and plastic hardening function in order to determine the evolution of state variables.) Using Landau approach and a closure approximation, an evolution equation for the dislocation density tensor is obtained by employing simple symmetry arguments and the constraint that the elastic energy must decrease with time at fixed stress. The evolution laws lead to singularity formation at finite times, which we expect will be related to the formation of cell walls. Implementation of finite difference simulations using the upwind scheme and the results in one and higher dimensions will be discussed.   

Intrinsic solid-solution softening in BCC Mo from dislocation-solute interactions

Solid solution softening observed in the group VA and group VIA transition metals has traditionally been attributed to either extrinsic effects---such as interstitial scavanging---or intrinsic effects---direct solute/dislocation interaction. We invesitgate intrinsic mechanisms using first principles methods. First, density functional theory calculates directly the interaction of Re, Hf, Os, W, Ir and Pt solutes with a straight $<111>$ screw dislocation in Mo. The local strain field associated with the dislocation core is self-consistently coupled to the long-range elastic field using the recently developed lattice-Green function boundary-condition method. The construction of simple interaction models from the {\it ab initio} data allows the extension of chemically accurate calculations to physically relevant length scales. We contrast the direct interaction energies with size- and modulus-misfits of solutes using the work of Fleischer. The misfits alone are unable to explain the presence of both softening and hardening, requiring the more complete treatment provided by {\it ab initio} methods.   

How a NiAl alloy changes its stoichiometry -- special role of bulk dislocations

We are studying how a NiAl alloy changes its composition when exposed to an Al flux. As Al atoms deposited on the surface diffuse into the bulk of the Ni-rich crystal, Ni atoms are displaced to the surface, where they combine with Al to form new alloy crystal. We directly observe this crystal growth by watching atomic steps advance using low-energy electron microscopy (LEEM). We find that bulk dislocations play a special role in the mass transport between the surface and the bulk. Al deposition causes the points at which bulk dislocations terminate on the (110) surface to move linearly across the surface. The dislocations provide a channel for fast mass exchange between the surface and the bulk; as they move, new crystal is left in their wake. We will discuss the relationship between the dislocation motion and the crystal equilibration. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04-94AL85000.   

Dislocation Mobility and Cross-slip in Copper - A Molecular Dynamics Study

The dynamic properties of dislocations constitute one of the basic building blocks of any theory of plasticity. Experiments are not able yet to follow in detail the microscopic dynamic properties of the dislocation, such as dislocation motion or cross-slip, while atomistic simulations may serve as a powerful tool. Using molecular dynamics (MD) methods the dynamic properties of screw dislocations had been studied in detail for Cu, both as a function of the temperature and the applied stress. Upon applying a glide stress on the dislocation a transition from inertial to viscous motion with a stress dependent terminal velocity is observed. The experimentally observed stress dependence of the terminal velocity is reproduced quantitatively by our results [1]. Upon applying a narrowing stress on the dislocation, in a dislocation dipole structure, cross-slip occurred and the cross-slip rate in the calculations was found to be temperature and stress dependent, as expected. From these calculations the cross-slip mechanism was identified and the activation energy and volume was calculated. [1] D. Mordehai et. al. Phys. Rev. B, 67 024112 (2003)   

Solute Hardening in Al-Mg: Molecular Dynamics Simulations of Single Dislocations and One-Dimensional Potential Energy Model.

Magnesium is used as a substitutional alloying agent to improve the formability, and other properties, of aluminum in alloys such as 5xxx aluminum. Serrated flow (Portevin-Le Chatelier effect) in these alloys limit their usefulness in certain automotive applications. These serrated flow effects are believed to be dependent on Mg diffusion. In order to establish both a baseline for and a suitable model in which to study the effect of diffusion on dislocation mobility in Al-Mg alloys we have performed molecular dynamics simulations the motion of a single dislocation in Al with randomly distributed 2.5 and 5.0 at{\%} Mg. For a suitable length of dislocation, on the order of the Labusch length, we compare pinning and de-pinning of the dislocation in the molecular dynamics with a model in which a straight dislocation interacts with single Mg atoms, the small Mg-Mg interactions being ignored. We report on the results of the molecular dynamics simulations and the validation of the one-dimensional energy map model.   

Study of slip bands formation in single crystal aluminum during uniaxial deformation using laser-induced photoemission technique

We report the application of the photostimulated electron emission (PSE) technique to study the slip bands formation from single crystal aluminum (99.995{\%}) during uniaxial tensile deformation. A 248-nm excimer laser (5-eV photon energy) was used as light source and the deformation was conducted with a tensile stage in ultra high vacuum working at strain rate ranging from 1$\times $10$^{-3}$ to 1$\times $10$^{-4}$ s$^{-1}$. We show that photoelectron intensities are sensitive to changes in surface morphology accompanying deformation, including slip line and band formation. In all single crystal aluminum deformed at different strain rate, the PSE intensity increases linearly with strain. Time-resolved PSE measurements show step-like increases in intensity consistent with the heterogeneous nucleation and growth of slip bands during tensile deformation. The \textit{in situ} PSE data strongly supports a recently developed dislocation dynamics model based on a percolation process. Real-time stress versus strain curves further support this model. Characterization of slip bands on the deformed surfaces was examined by atomic force microscopy (AFM).   

Statistical models for the microstructural evolution in irradiated metals and alloys

The macroscopic mechanical properties of metals are intimately related to their microstructural features and their spatiotemporal evolution. We discuss a simplified statistical model for the dynamics of point defects in bcc metals that is solved through kinetic Monte Carlo (kMC) and rate equations. Self-interstitial atoms and vacancies can be produced in abundance upon irradiation with energetic particles, but they subsequently anneal due to recombination and absorption at sinks such as dislocations and grain boundaries. The model reveals a sequence of kinetic regimes that lead to a final steady state and allows us to study the size distribution of voids that form when vacancies aggregate into cluster. Here we focus on random alloys, where the point defect diffusivities are modified due to the presence of multiple exchange frenquencies. In addition, complex dealloying processes occur at sinks if the alloy components diffuse preferentially through one diffusion mechanism (self-interstitial or vacany exchange) only. We illuminate these effects with a generic kMC/rate equation model for binary alloys.   

Effect of impurities on the electronic structure and stability of the A15 phase of chromium

The cubic A15 phase is well-known as an important metastable phase for {\it bcc} transition metals: it was observed in thin films and as ultra-fine particles in Cr, Mo and W alloys and its formation in O and N atmospheres is found to be more preferable compared to the {\it bcc} structure. We present first-principles FLAPW\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB {\bf 24}, 864 (1981)} results on the electronic structure, elastic constants and stability of the A15 phase in Cr and discuss the stabilizing role of light impurities. At equilibrium, the total energy of the A15 structure is only 3 mRy higher than for {\it bcc} Cr, and the calculated A15 elastic moduli do not demonstrate any shear instability. The formation of stacking faulted (twin-related) structures and A15-based phases of Cr alloyed with substitutional Re, Fe, Ni and interstitial/substitutional O, N, C was investigated taking into account full structural optimization, and the most preferable structures were found. We discuss the effect of precipitates of such binary and ternary phases on the mechanical properties of Cr and suggest that the mechanism of the ``rhenium effect'' -- namely, the improvement of ductility and strength of {\it bcc} metals upon alloying with Re -- is connected with the presence of A15-type close-packed particles.   

Deformation and stress relaxation near edges in single crystal beta-NiAl during thermal oxidation

Using micro-fluorescence and optical microscopy we have investigated the deformation and stresses that develop in the vicinity of edges (i.e. the intersection of two crystallographic faces) in single crystal beta-NiAl as it is thermally oxidized at temperatures in the range 1100-1450 Celsius. We find that the edges, initially with a radius of curvature of 2 microns, develop a significant rounding. The radius of curvature of this rounding appears to be constant at temperatures above 1250 Celsius, suggesting that the rounding takes place below this temperature. Stresses in the oxide scale show a very large decrease close to the edges and the distances over which this decrease occurs is comparable to the rounding discussed above. Data for both the deformation and stress are presented for the following pair of crystal orientations: (001) and (011), (110) and (1-10), (111) and (1-10).   

Effects of high density electric currents on material processing

High density electric currents are common in integrated circuits and have recently been utilized as a parameter in material processing. discussion of the versatile material processing/synthesis techniques of spark plasma sintering and field activated pressure assisted synthesis. A variety of materials produced by these techniques including metals, nano-composites and intermetallics as well as possible applications of these materials as structural and functional materials are presented. The effectiveness of the electric currents are demonstrated by increased processing efficiency as well as manifested in the enhanced electrical and mechanical properties of the materials produced. In addition results from experiments performed with the aim of elucidating the current enhancing mechanism are introduced. These results show current enhanced reactivity and mass transport kinetics and help interpret the processing/synthesis findings.   

Temperature dependence of the lattice misfit in $\gamma/\gamma'$ superalloys: role of thermal expansion and composition changes

The magnitude of the lattice misfit $\delta$ between the $\gamma$ and $\gamma'$ phases is one of the key parameters determining the mechanical behavior, microstructure morphology and stability of $\gamma/\gamma'$ high temperature superalloys. The relative importance of two contributions to the temperature dependence $\delta(T)$ are under intense investigation, namely: (i) the difference in thermal expansion of the two phases, and (ii) the redistribution of alloying component between $\gamma$ and $\gamma'$ with the increase of temperature. We explore the role of both contributions for the Ni-Al and Ir-Nb $\gamma/\gamma'$ two-phase alloys based on \textit{ab initio} full-potential total energy and phonon spectra calculations. We demonstrate that the redistribution of the major alloy components (Al into Ni and Ni into Ni$_3$Al) gives the main contribution to $\delta(T)$ for Ni/Ni$_3$Al at $T>$600 K. For the Ir/Ir$_3$Nb system, the alloy component redistribution starts to contribute to $\delta(T)$ only at extremely high temperatures ($>$2000 K). The amplitude of these contributions can be determined by considering the shape of the $\gamma$--$\gamma'$ gap on phase diagrams. This conclusion is important for alloy design as it allows one to establish a simple relation between the alloy phase diagram and the temperature dependence $\delta(T)$.   

Anisotropic plasticity in NiAl alloy under dynamical loading

We use molecular dynamics with a first principles-based interatomic potential to characterize the orientational dependence of shock-induced plasticity in NiAl B2 alloy. For all directions studied plasticity starts with the nucleation of superpartial loops encircling 1/2$<$111$>$ slip but the subsequent events exhibit marked anisotropy. For shocks in the [110] direction we find an intricate pattern of $<$111$>${\{}110{\}} and $<$100$>${\{}110{\}} slip with the plastic wave moving at the shock velocity. In the case of [111] shocks plastic deformation is dominated by $<$100$>${\{}110{\}} slip that forms when trailing superpartials nucleate inside the initial 1/2$<$111$>$ loops. For shocks in the [100] direction (the hard direction) much stronger shocks [(uniaxial stress almost twice larger than for [110] and [111]] are required before plastic deformation is observed; we find almost simultaneous, nucleation of multiple 1/2$<$111$>$ superpartials, leading to frequent intersections that severely limit their mobility and even lead to local amorphization. In the [100] and [111] shocks we find an elastic precursor separating the leading shock front and the plastic wave.   

Vibrational Properties of point defects and dislocations in metals

We calculate the vibrational spectra of vacancies, interestitials as well as dislocations in the fcc copper and bcc molybdenum described by interatomic potentials of the embedded-atom type. An in-depth study of the low-lying localized vibrational modes caused by the defects is presented. The concept of local atom-projected entropy within the harmonic approximation is introduced, and in this way the defect-indcued change in the thermodynamics of these metals is analyzed.   

Session N29: Block Copolymers II

The melt, lyotropic and aqueous phase behavior of poly(ethylene oxide)-poly(butadiene) block copolymers

The melt, lyotropic and micellar phase behavior of poly(ethylene oxide)-poly(butadiene) (PEO-PB) block copolymers was studied using small-angle x-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). A series of sample solutions with varying copolymer concentration were investigated for a set of block copolymers spanning a range of composition and molecular weight. The melt phase behavior was consistent with the self-consistent mean-field calculations for diblock copolymers. A sequence of lyotropic liquid crystalline morphologies was documented upon gradual variation of water in block copolymer solutions. Swelling of the hydrophilic domain with addition of water drives phase transitions from cubic $\to $ hcp $\to $ lamellar $\to $ inverse hcp $\to $ inverse cubic $\to $ micellar. The results establish that the molecular composition and the extent of swelling are the two main parameters governing the self-assembly of lyotropic phases. Despite the thermotropic nature of the PEO-water interactions, the phase behavior of PEO-PB block copolymers was primarily lyotropic. The evolution of micellar phases depends upon the lyotropic morphology of copolymer solutions prior to the complete saturation of hydrophilic block. The findings are discussed in context of various factors that govern the self-assembly behavior in different regimes.   

Visualizing worm micelle dynamics and phase transitions of a charged diblock copolymer in water

Assemblies of block copolymer amphiphiles are sometimes viewed as glassy, frozen, or static colloids, especially in strongly segregating solutions. Here we visualize by fluorescence microscopy and AFM the dynamics and transitions of single cylindrical micelles and vesicles composed of a charged diblock copolymer in water. In mapping the salt- and pH-dependent phase diagrams of a near-symmetric diblock of polyacrylic acid--polybutadiene, low pH and high salt (NaCl, CaCl)$_{2}$ neutralize and screen the charged corona sufficiently to foster membrane formation and generate vesicles. Decreased salt and neutral pH increases intra-coronal repulsion and drives a transition to multi-branched cylinders and highly stable, but fluid and flexible worm micelles. Ca$^{2+}$ both stiffens cylinders and stabilizes them relative to spheres. Further increase of intra-coronal repulsion generates spherical micelles by fragmentation and pinch-off at the ends of worms. Both transition kinetics and phase diagrams indicate divalent cation is about 5-10 fold more effective than monovalent in stabilizing all non-spherical morphologies.   

Platelet self-assembly of a tetrablock copolymer in pure water

An amphiphilic tetrablock copolymer was synthesized via anionic polymerization, selective hydrogenation and sulfonation to create an A-B-C-A polymer where the hydrophilic ends are poly(styrene sulfonate) and the middle blocks are incompatible poly (methyl butylene) and poly (ethyl ethylene). The aggregation behavior of these polymers in water was studied using dynamic and static light scattering as well as light and electron microscopy. Both scattering and direct imaging experiments are consistent with monodisperse (s = 0.14) monolayer platelets with radii of 147 nm at 45 °C, while at temperatures below 38 °C we find coexistence of the platelets with micelles.   

Effect of the Soluble Block Size on Spherical Diblock Polymer Micelles

In order to understand the effect that the soluble block has on the equilibrium size and shape of polymer micelles, we have characterized spherical micelles formed from two series of polystyrene-b-polyisoprene in dilute solutions of the selective solvent, heptane. We show that varying the soluble (PI) block from 10-100K when the insoluble (PS) block is kept constant (20K and 40K) changes the CMC by over an order of magnitude for both series and the aggregation number by an order of magnitude for the 20K series and a factor of 3 for the 40K series. We have also studied the effect that temperature has on the CMC of the two series. The results are found to be in good agreement with recently developed theory.   

Phase Behavior and Local Dynamics of Concentrated Triblock Copolymer Micelles

We investigate the phase behavior and local dynamics of aqueous solutions of the triblock copolymer polyethylene oxide (PEO) - polypropylene oxide (PPO) - polyethylene oxide (Pluronic F108 and F68) by neutron scattering. In solution the polymer self-associates into spherical micelles with PPO cores and corona of solvated PEO. For sufficiently high concentration the Pluronic evolves on heating from fully solvated polymer to a micellar liquid phase then to a micellar crystal phase. The temperature range of the micellar liquid region increases with decreasing chain length, a feature we attribute to fluctuation effects as predicted by recent theory. The local micellar dynamics probed in neutron spin echo display fast and slow modes that depend systematically on concentration and temperature as the liquid-crystal phase boundary is traversed. However, these dynamics are surprisingly insensitive to phase and macroscopic rheology. Contrast matching the PEO corona to the solvent reveals that the slow mode corresponds to translational motion of the core while the fast mode is related to motion of the corona.   

Multicompartment Micelles from ABC Star Terpolymers

We have synthesized a series of ABC star terpolymers with three mutually immiscible polymeric components: a fluorocarbon, a hydrocarbon, and a hydrophilic segment. We have observed a new class of multicompartment micellar structures in dilute aqueous solutions by cryogenic transmission electron microscopy. The star architecture enforces interfacial contacts among the three components. The incompatible fluorocarbon and hydrocarbon form separate disk-like micellar cores that are protected from the water by the hydrophilic segment. The flat micelle cores are due to the strongly unfavorable interaction between segment pairs. The structures that emerge depend on the relative lengths of the segments and can be tuned from discrete multicompartment micelles to extended wormlike micelles with segmented cores.   

Brownian Dynamics Simulation of Multiblock Copolymers in Selective Solvents

In this talk we will present results of Brownian dynamics simulation on triblock (ABA, BAB), penta-block (ABABA, BABAB) and hepta-blocks (ABABABA or BABABAB) in selective solvents for the A blocks to study the effect of varying the number of blocks, solvent selectivity, concentration and temperature on the association behavior of multi-blocks. Structure factors and percolation analysis of the clusters was used to characterize the resulting structure. At a concentration of 20{\%} we obtained micellar clusters arranged in a BCC lattice, in agreement with scattering experiments. The ratio of number of loops to bridges decreases as the number of blocks in the copolymer increases, as does the polydispersity. The size of the clusters was larger when the outermost block was in the poor solvent condition. Our results imply that as the number of blocks increases it is favorable to form bridges instead of loops. This leads to a larger number of smaller clusters with more bridges.   

Structure and Properties of PBO-PEO Diblock Copolymer Modified Epoxy

Amphiphilic diblock copolymers poly(n-butylene oxide)-b-poly(ethylene oxide) (PBO-PEO) of various compositions were synthesized and studied as modifiers for epoxy resins. In blends of PBO-PEO, epoxy resin, and curing agent, the copolymers formed well-defined microstructures that persisted upon curing of the epoxy. The resulting morphologies were vesicles, wormlike micelles, and spherical micelles (in order of increasing size of PEO block), as well as transitional morphologies. Addition of 5{\%} by weight of these block copolymers improved the fracture toughness of the epoxy remarkably (by as much as nineteen times) with relatively small decrease in the elastic modulus. The highest level of toughness was measured in a system containing branched wormlike micelles. Close examination of the fracture surfaces of these composites suggests that while all the dispersed morphologies played a similar role to inclusions in particle-toughened thermosets, crack deflection toughening contributed to the significantly higher levels of toughness in the wormlike micelle systems.   

Complex Micelle Morphologies Constructed by Charged Block Copolymer Self-assembly

The combination of electrostatic interactions and block conformational control, when combined with traditional block copolymer parameters such as relative block length, architecture, and amphiphilicity, provides the opportunity to build novel self-assembled structures. Two sets of molecules will be discussed. The first is a synthetic, linear triblock copolymer with two hydrophobic blocks connected to a charged hydrophilic block. Assembling these molecules in water/organic solvent mixtures containing multivalent organic counterions produces biomimetic micelles such as toroids and disks. These nanostructures can be chosen with the correct choice of counterion and solvent conditions. The second molecule is a linear diblock copolypeptide containing a rigid rod alpha helical hydrophobic block and a random coil, charged hydrophilic block. The self-assembly is defined by the helicity of the hydrophobic block. By controlling the kinetics of assembly one can form interconnected hydrogels, twisted fibrils, or hexagonal plates, all anchored by hydrophobic cores of alpha helical peptide. Transmission electron microscopy, neutron scattering, atomic force microscopy, circular and infrared spectroscopy, and light scattering results will be presented.   

The Effect of Counterion Valency and Solvent Properties on Charged Amphiphilic Triblock Copolymer Assembly into Disks, Cylinders, or Spheres

Amphiphilic triblock copolymer with one charged/ion-containing block, polystyrene-b-polymethacrylate-b-polyacrylic acid (PS-PMA-PAA), was studied in a water/THF cosolvent system. By adding an amine-based divalent counterion, polymeric disc micelles were formed. Discs or cylinders can be purposefully formed by choosing the type and amount of the divalent counterions in the system. In comparison, systems without counterion or with monovalent amine counterions will also be discussed. In addition to the counterion effect, the charges and the ionization along the PAA chain can also be tuned by changing the water ratio in the solvent system. This PAA tunability through counterions and solvent content allows one to direclty tune the micellar structure formed during assembly. The system was studied using dynamic light scattering (DLS), transmission electron microscopy (TEM), cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering.   

Separation of PS-PMMA block copolymers from PS precursors via selective adsorption on nanoprous silica

We report a simple adsorption-based separation method using nanoporous silica in solution via controlling solvent quality to remove polystyrene (PS) homopolymers from polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymers. In particular, the solvent quality is controlled by employing binary mixed solvents of THF (good solvent) and isooctane (nonsolvent for both PS and PMMA). The aim of this work is to qualitatively study the competitive adsorption between PS and PS-PMMA and to provide a correlative understanding of polymer adsorption in nanopores with interaction chromatography techniques. In addition, the quantitative understanding of polymer adsorption is further employed to develop a simple polymer separation scheme for manipulating polymer adsorption via solvent quality. In particular, concentration changes of PS and PS-PMMA in the supernatant solution have been quantitatively measured for the adsorption studies using solvent gradient interaction chromatography techniques. We found that the PS-PMMA (43k-32k) selectively adsorb over PS (43k) precursors at the THF composition window between 42 {\%} and 55{\%} in THF/IO (v/v) mixed solvents. For THF/IO solvents with composition higher than 60 {\%} THF, polymers did not adsorb to the nanoporous silica due to the good solvent quality.   

Influence of Grain Boundaries on the Deformation Behavior of Block Copolymers: In Situ SAXS Tensile Deformation and Simulation of Bicrystals

The evolution of the microdomain structure of block copolymers (BCP) during tensile deformation have been studied using transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS). Most previous studies have been conducted on isotropic, polygranular materials, where the role of grain boundaries on the deformation is largely unknown. In order to develop a fundamental understanding of boundary defects on the deformation, we have utilized model bicrystals. Such ideal grain boundaries are made by first producing a near-single crystal BCP sample using the roll casting process and then cutting appropriate pieces and adhering these together to yield bicrystal BCP specimens with various types of tilt or twist boundaries. The reciprocal space patterns obtained dynamically using SAXS and step-scanning the small cross-section beam across the boundary region after each increment of applied strain provide detailed insight into the structural evolution of the microdomain across the grain boundaries. A theoretical study using finite element analysis of the deformation of each grain was also performed to compare with the experimental work.   

Grain Growth Kinetics of AnBn Star Block Copolymers in Supercritical Carbon Dioxide

Using a series of lamellae-forming AnBn (n = 1, 2, 4, and 16) miktoarm star block copolymers, the effect of the number of arms on the grain growth kinetics has been investigated by annealing in supercritical carbon dioxide (CO2). Across this series all materials have the same A and B block molecular weight and all have the equal number (n) of each type of arm. The grain growth was monitored in real space by transmission electron microscopy, followed by subsequent micrograph image analysis. It was found that supercritical CO2 could be used to promote the grain growth of these AnBn star block copolymers at relatively low temperatures. Also, the molecular architecture was found to have a significant impact on the grain growth kinetics. The grain growth of these AnBn stars annealed in supercritical CO2 was then compared to a previously completed grain growth study of the same materials under simple thermal annealing. It was found that the grain growth kinetics for the AnBn stars with n = 2, 4, and 16 were similar for both supercritical CO2 and thermal annealing. However, the grain growth of the diblock (AnBn with n = 1) was dramatically enhanced in supercritical CO2 relative to thermal annealing.   

A Mesoscopic Archimedean Tiling Having a New Complexity in ABC Star-shaped Block Terpolymers

The Archimedean tiling ($3^2.4.3.4$) is a regular but complex polygonal tessellation of equilateral triangles and squares. We have found the tiling in a melt of an ABC star-shaped polymer alloy molecule composed of polyisoprene, polystyrene and poly(2-vinylpyridine). The circumstance of a molecule splits into multiple sites and consequently two microdomains with different sizes and shapes are formed for one component. This complexity is the first observation in polymeric alloy systems and can lead to a new type of mesoscale self-organization.   

Nanotransforming Assemblies

Degradable polymeric materials with hydrolysable backbones have attracted much attention because they break down to non-toxic metabolites. They are the key solutions to many environmental problems, and are particularly useful for various biomedical applications. Much work has been focused on degradable polymers and their co-polymers as bulk, or films and monolayers.$^{2}$ Only limited work has explored the degradable amphiphilic copolymer self-assemblies (spherical micelles, worm micelles and vesicles) in solutions, which are quite important for soft-material engineering. Mostly spherical micelles, and in rare cases, vesicles, have been reported made from copolymers with degradable polyester, typically polylactide or polycaprolactone, as the hydrophobic block, connected to biocompatible, stealthy poly (ethylene oxide) as hydrophilic block. Morphological change of such spherical micelles induced by degradation is subtle, and the degradation kinetics and mechanism in assemblies, which can be quite different from that in bulk or film, are not well understood. Here we will describe the phase transformations of worm micelles and vesicles as they degrade and also highlight how these polymeric self-assemblies interact with lipid membranes.   

Mesophase formation of block copolymer in cylindrical nanopore

We investigate the influence of the confinement on the mesophase formation of diblock copolymer caged in a cylindrical pore in which the surface of the pore preferentially attracts one of the blocks. Using cell dynamics simulation, we construct phase maps as a function of the composition of diblock copolymer ($f)$ and the pore diameter ($D)$ relative to the period at bulk ($L_{o})$. Depending on $f$ and $D$/$L_{o}$, we observe a variety of confinement-induced mesophases ranging from a simple dartboard-like structure to more complicated structures involving various forms of helices or doughnuts. We also find that the creation of a new central domain of the layered structure in the cylindrical pore occurs at a critical value of $D$/$L_{o}$ larger than the critical value for the layered structure under the flat confinement, indicating that the block copolymers under curved confinement afford to be more stretched than that under flat confinement. These results are compared with experimental observations.   

Session N30: Polymer - Inorganic Composites II

Control of the Dynamic Behavior of The Particle-Copolymer Nanocomposites

Nanocomposites of linear copolymers and various kinds of particles have long been of great interest to researchers because of their attractive industrial applications. The microphase separation of copolymers can be used to template the particles into regular structure to gain needed optical/magnetical/electrical properties. On the other hand, clustering of the particles can be utilized to influence the morphology of the polymer to improve the properties of the polymer matrix. Theoretical studies on these phenomena have focused mostly on the cases where one of the self-organizing process dominates the behavior of the system. In this computatoinal study, we will focus more on the interplay/competition of two different self assembly processes. Our model combines the cell dynamical equations for the diblock polymers and Langevin dynamics for particles interacting with various potentials. We study particles with different magnetic/electrical properties. The aggregation behavior of these particles can be controlled through external fields and consequently make the behavior of the whole composite controllable through the interplay of the two competing self assembly processes. The results of these studies can potentially pose new avenues to the fabrication and application of the particle-polymer nanocomposites.   

Network formation in sheared polymer nanocomposites

We use Molecular Dynamics simulations to study the effects of shear on polymer nanocomposites. Our studies show the formation of a transient network at concentrations exceeding 5\% by vol of filler particles. The structure of this network under shear is investigated. We find at lower shear rates the network significantly enhances the viscosity of the polymer, but at higher shear rates it contributes to an acceleration in shear thinning, resulting in a viscosity that is close to the orginal viscosity of the unfilled polymer system. Our results also show an unexpected effect of fillers on chain orientation under shear. We find that even before a transient network exists in the system, small concentration of filler particles induce a large orientation effect on the polymer chains. Possible implications of this effect will be discussed.   

Phase Separation Dynamics of Polymer Blend Films Containing Polymer-Grafted Nanoparticles

Polymer blends containing nanoparticles (NP) are important in advanced technologies including opto-electronic and biosensor devices. Upon adding methyl-terminated silica NP's [22nm (NP$_{A})$] at dilute concentrations, PMMA:SAN (50:50) films (650nm) undergo early, intermediate and late stages of morphology development, similar to a PMMA:SAN film (Wang {\&} Composto, JCP (2000)). NP's partition into the PMMA-rich phase, and slow down the kinetics of domain growth. This result is consistent with a coalescence model that predicts $\xi \quad \sim $ (1 / $\eta )^{1/3}$ t$^{1/3}$, where $\xi $ and $\eta $ are the correlation length and PMMA viscosity, respectively (Chung \textit{et al}., EPL (2004)). Although the bulk $\eta $ agrees with this model, a microscopic understanding of the phase separation mechanism requires knowledge of polymer-NP and NP-NP interactions. To address this issue, well-characterized silica NP's (15 nm) with densely grafted PMMA [M$_{w}$ = 1.8K (NP$_{B})$ and 21K (NP$_{C})$] are employed as non-interacting fillers in the PMMA-rich phase. The impact of PMMA-grafted NP on the phase separation dynamics in films, as well as the rheology of PMMA/NP composites, is investigated. Specifically, phase separation was slowest for NP$_{B}$ relative to films containing NP$_{A}$ and NP$_{C}$. These studies show that wetting and domain coarsening in polymer blend films can be controlled by the judicial addition of surface modified NP.   

Thermally Induced Lateral Motion of $\alpha$-Zirconium Phosphate Layers Intercalated with Hexadecylamines

Well-defined intercalated structure, either interdigitated layers or bilayers, of hexadecylamines (HDAs) in a confined space of a highly-functionalized layered material, $\alpha$- zirconium phosphate ($\alpha$-ZrP), was prepared and these two distinct intercalated structures can serve as model systems to investigate the interaction of the two monolayers whose amphiphilic tails are adjacent to each other. Acidic functional groups (-POH) on the $\alpha$-ZrP are in well-ordered array and the number of functional group is quite high (i.e., cationic exchange capacity (CEC) = 664 mmole/100 g, area per one charge site = 0.24 nm$^{2}$) enough to realize the bilayers (i.e., discrete two monolayers) of HDAs within the $\alpha$-ZrP interlayer. We employed the two-step intercalation mechanism for the preparation of well- ordered interdigitated layers as well as the bilayers of alkyl chains attached to both sides of the $\alpha$-ZrP intergallery. An intriguing lateral motion of the $\alpha$-ZrP sheets was observed with in-situ SAXS measurements for the interdigitated layer during heating and cooling cycle and verified with TEM. This lateral motion is believed to be due to the transition from the tilted to the untilted conformation of the interdigitated HDA chains and this transition is found to be thermally reversible.   

Molecular Dynamics Simulations of Poly(dimethylsiloxane) - Silica Interfaces

Molecular Dynamics simulations using new quantum chemistry based interactions between Poly(dimethylsiloxane) (PDMS) and silica surfaces were conducted to explore the chain behavior near the interface between 300 and 500 K. The effects of surface chemistry particularly the presence of hydroxyl and trimethyl-silyl groups at the interface were examined. The PDMS chain dynamics, as measured by the mean squared displacement of backbone atoms, were strongly affected by the surface chemistry and intermolecular interactions. The PDMS structure near the interface depended upon the type of surface group on the silica surface and its concentration. The significance of intermolecular forces such as hydrogen bonding, electrostatics etc. will be discussed.   

First Observation of an ``Anomalous Mullins Effect'' in Silica Filled PDMS

We proposed a predictive model to explain the Mullins Effect (stress softening) and a new phenomenon we refer to as the ``Anomalous Mullins Effect'' in silica filled polydimethysiloxane (PDMS). The mechanism we propose is based on surface interaction between polymer chains and filler particles. The ``Anomalous Mullins Effect'' is the dependence of stress ``softening'' on the direction of a second pull relative to an initial strain axis. We will present experimental data to support this model. Atomic force microscopy (AFM) phase imaging was used to characterize filler size and distribution. A tensile stage was used to measure stress-strain properties using model samples with various filler content. Samples were not pulled to break in order to study stress softening as a function of elongation and second strain direction. As predicted by our model, we observed a clear Mullins Effect only when the second strain axis was parallel to the initial one but not when it was perpendicular. No change was seen in mechanical behavior over 26 weeks or with heat-treatment.   

A physical mechanism for the Mullins Effect in silica-filled polydimethylsiloxane

The Mullins Effect pertains to the reduction in tensile stress, or ``softening,'' that is observed between the first and subsequent extensions of filled polymer materials. First reported by W. L. Holt in 1938 and later studied in detail by L. Mullins, it is considered by many to be a major unsolved mystery of polymer physics. We propose a physical mechanism to explain this effect that is based on surface interactions between polymer chains and filler particles. Its predictions are consistent with most experimental results including the integrated strain energy and the shape of the tensile stress/strain curve. The proposed mechanism also predicts that stress softening should \underline {not} occur if a previously strained sample is stretched at right angles to the original strain axis. This effect, which we are calling ``The Anomalous Mullins Effect,'' has now been confirmed experimentally. We will present a description of the mechanism.   

SANS studies of polymer chain conformation in the presence of nanofillers

We will present the results from recent and ongoing SANS experiments on polystyrene nanocomposites containing silica nanofillers. Recent simulation results for the role of nanofiller on chain dimensions have been controversial. Nanocomposite samples corresponding to combinations of different weight percentages of the nanoparticles (highest being 5) and five matrix polymer molecular weight (highest being 550k g/mol) have been studied. We will report on chain dimensions and how they are altered by the presence of filler.   

Nanosphere Embedment into Polymer Surfaces: A Viscoelastic Contact Mechanics Analysis

Teichroeb and Forrest (\textit{Phys. Rev. Lett.}, \textbf{91}, 1, 016104-1(2003)) image gold nanosphere embedment into a polystyrene surface and imply the existence of a liquid surface layer. We use a viscoelastic contact mechanics model of their results to give a contrary interpretation. The surface interactions between gold and polystyrene and the indentation depth determine the loads on the nanospheres. Using bulk properties, quantitative agreement between the model and the data is obtained, implying little or no, depression in the glass temperature or existence of a liquid layer at the polystyrene surface. An important aspect of the present analysis is that it is the first to solve the problem for the time dependent Poisson's ratio. The fact that this varies from 0.33 to 0.5 upon traversing the glass transition (time) flattens the nanosphere embedment profile with increasing time.   

Influence of Nanoparticles on the Miscibility in Binary Polymer Blends – A Simple Theory

We propose a simple theory describing the influence of nanoparticles on thermodynamics of binary polymer mixture. In particular, we consider the case in which nanoparticles preferentially segregate into one of the polymeric components. Depending on the particle radius \textit{Rp }and the polymer degree of polymerization $N$, addition of nanoparticles can either promote or hinder mixing of the polymers. We calculate how the addition of nanoparticles shifts the spinodal of the polymer blend. Results are compared with recent experimental data of Nesterov and Lipatov, and satisfactory qualitative agreement is found.   

Surfaces of Fluoroelastomer Nanocomposites

Stiffening or reinforcement of elastomer with a second hard particle phase to produce a networked or crosslinked composite is common in applications of high-performance elastomers. The average size of reinforcing particle is frequently in the range of a few tenths to several microns, the shape from spheres to cylinders of high aspect ratio, and the particle concentration can be as high as about 50{\%} by weight partly because of ease of dispersing a small number of large particles. One of the main problems with micro-filled fluoroelastomer surfaces is the continuous removal of large particles by abrasion and wear resulting in large pits or surface defects. Furthermore, these large pits can lead to a roughened surface. The aim of this work is to investigate the influence of nano-particles versus micro-particles on the surface defect size and density of filled fluoroelastomer. We applied scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), and surface roughness measurement to examine the surfaces of paper abraded fluoroelastomer nanocomposites. Qualitatively, SEM images show the surface defect size or density of nanocomposites is generally reduced, as compared to that of fluoroelastomer microcomposites. On a somewhat larger scale, it is found that the surface roughness (Ra) of paper abraded nanocomposites can be controlled to less than 0.2 to 0.3 microns.   

Bulk and Interfacial Behavior of Nano\-particle/Polymer Blends

We have investigated a model athermal system consisting of polystyrene (PS) nanoparticles (NPs) in PS melts. Neutron scattering shows that the chain dimensions expand in the presence of the NPs. We investigate this result theoretically using self-consistent PRISM theory, and also find an expansion in chain dimensions as a function of NP volume fraction. Recently it has been shown that nanoparticles can suppress dewetting in thin polymer films, a counterintuitive result since particles usually induce dewetting. Neutron reflectivity measurements have shown that the NPs phase separate to the surface, so one proposed mechanism for the inhibition of dewetting is that this segregation changes the surface energies. We calculate the density profiles for dilute NPs in polymer melts near a substrate using classical density functional theory, which shows that the NPs do indeed segregate to the surface. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE- AC04-94AL85000.   

Interactions between nano-particles in solutions of adsorbing polymers

Adsorption of polymer on colloidal surfaces plays an important role in many practical applications like bridging flocculation, steric stabilization, protein crystallization and polymer nanocomposites. In the present talk, we present a multiscale approach that examines the phase behavior of colloidal suspensions in adsorbing polymeric solutions. We use generalized McMillan Mayer theory to coarse-grain the polymeric component. This leads to a framework wherein the effective interactions between the colloids can be computed quite accurately using polymer self-consistent field theory. By solving the self-consistent field theory equations numerically in bispherical coordinates we account for possible size disparities between the colloids and polymers. Here, we present our results on the effective interactions and phase behavior of colloids in solutions of reversibly and irreversibly adsorbing polymers. Our results suggest that specific interactions between polymers and colloids can lead to significant changes in both the interactions and the resulting phase behavior of the system. Further, we extend the self-consistent framework to characterize the structure of adsorbed polymer layers in terms of conformational statistics of bridges, loops and tails. We present a comparison between polymer adsorption on planar surface and that on a spherical particle and our results show a strong dependence of structure of adsorbed layer on surface curvature   

Models of the viscoelasticity of polymer nanocomposites

A family of models is developed to represent the viscoelasticity of polymers filled with nanoparticles. This includes discrete modeling and simulation, as well as rheological molecular modeling. Discrete coarse-grained models are used at various spatial and temporal scales to inform the rheological models as well as to validate their output. The main objective is to capture the mechanisms relevant for the chain dynamics such as chain-filler interactions, entanglements, tube fluctuations and constraint release. These are incorporated in the constitutive model whose predictions are then compared with relevant published experimental data.   

Magnetic Investigations of Titanium Doped Gamma Iron Oxides Dispersed in Polymers

Titanium doped gamma iron oxide nanoparticles were prepared by laser pyrolisis. X-Ray studies indicated the presence of $\gamma $ Fe$_{2}$O$_{3}$ and $\beta $ FeO(OH). Small amounts of $\gamma $ titanium maghemite were observed. The average particle size is 5 nm (Transmission Electron Microscopy). Magnetic nanoparticles were dispersed by sonicating a solution of styrene-isoprene-styrene blockcopolymer. The solvent was evaporated by heating at 75 $^{0}$C for 24 hours. The as obtained films were studied by SQUID and ferromagnetic resonance (FMR). The temperature dependence of the magnetization and of hysteresis loops in the temperature range 4 K -- 300 K is reported. The temperature dependence of FMR line parameters in then range 100 K to 450 K is reported. A weak matrix effect within the glass transition range of the polymeric matrix has been observed.   

Nucleation and Growth in Poly(L-lactic acid)/clay nanocomposites

We have investigated the crystallization behavior of semicrystalline poly (L-lactic acid) (PLLA) upon addition of organically modified montmorillonite clay. The real-time crystallization was probed by Polarized Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR) techniques. The exfoliation-adsorption technique was employed to fabricate the nanocomposites from solution. Crystallization studies were performed on cast nanocomposite films, which were isothermally recrystallized at different temperatures from the quiescent melt. The radial spherulite growth rate measurements and isothermal bulk crystallization kinetics indicate that the silicate layers, in the case of the fully miscible organic modifier, did not act as a nucleating agent. However, the less miscible clay acted as a good nucleating agent and significantly decreased the spherulite sizes. Interestingly, spherulite growth rates were significantly increased by the addition of organoclays, being the highest in the fully exfoliated case. Despite the increase in spherulite growth rate, the overall bulk crystallization rate was retarded in the exfoliated nanocomposites. The bulk crystallization rate was increased in the intercalated case in which clay acted as a good nucleating agent. In-situ FTIR studies revealed a valuable insight into the chain configurations, which are in good accordance with the DSC and POM experiments   

Session N32: Focus Session: Superconductivity: Theory and Computations

Chiral symmetry breaking in QED$_3$ in presence of irrelevant interactions: a renormalization group study

Motivated by recent theoretical approaches to high temperature superconductivity, we study dynamical mass generation in three dimensional quantum electrodynamics (QED$_3$) in presence of irrelevant four-fermion quartic terms. The problem is reformulated in terms of the renormalization group flows of certain four-fermion couplings and charge, and then studied in the limit of large number of fermion flavors $N$. We find that the critical number of fermions $N_c$ below which the mass becomes dynamically generated depends continuously on a weak chiral-symmetry-breaking interaction. One-loop calculation in our gauge-invariant approach yields $N_{c0} = 6$ in pure QED$_3 $. We also find that chiral-symmetry-preserving mass cannot become dynamically generated in pure QED$_3$.   

Thermal melting of density wave order on the square lattice

We present a theory of thermal fluctuations which melt a commensurate $p \times p$ density wave ordered state on the square lattice. A phase diagram is constructed which will act as a springboard to a variety of interesting phases and phase transitions. The commensurate lock-in solid can in general melt to either an incommensurate floating solid or by a second order phase transition to an anisotropic (striped) floating state with $p$-periodic order along one direction and incommensurate quasi long range order in the other direction. In either case, this transition will be accompanied by the proliferation of domain walls, with the adjacent state being distinguished by the sign of the domain wall interaction energy. The fully disordered high temperature state can be reached from the floating solid by a second order transition mediated by dislocations. For $p = 4$, and at special commensurate densities, the $p \times p$ commensurate state can melt directly into the disordered state via a self-dual critical point with non-universal exponents.   

Solution for the dynamics of the BCS and central spin problems

We develop an explicit description of a time-dependent response of fermionic condensates to perturbations. The dynamics of Cooper pairs at times shorter than the energy relaxation time can be described by the BCS model. We obtain a general explicit solution for the dynamics of the BCS model. We also solve a closely related dynamical problem - the central spin model, which describes a localized spin coupled to a ``spin bath''. A typical dynamics of the BCS and central spin models is quasi-periodic with a large number of frequencies and stable under small perturbations.   

Spin gap phase in the strong coupling limit of the hole-doped 2D Emery model

We investigate the strong coupling limit of the three-band Emery model of a CuO plane. Starting from the nematic phase of this model [1], an effective Hamiltonian is constructed to leading order in the strong coupling expansion and for a range of hole doping. We show that the effective Hamiltonian we found is equivalent to a Kondo lattice model which can be solved exactly. We find that this model exhibits a quantum phase transition from a spin gapless phase to a spin gap phase as a function of doping. We will discuss the implications of these results for the phase diagram. [1] E. Fradkin, S.A. Kivelson and T.H. Geballe, Phys. Rev. B 69, 144505 (2004)   

Theories that try to describe pairing fluctuations above T$_c$: a critical evaluation

Over the past couple of decades many attempts have been made to understand the pairing instability crossover from weak coupling to strong coupling, particularly at finite temperature. While the physics is {\it qualitatively} understood, there is still dispute over what theoretical framework accurately represents this physics. We offer our two cents worth by examining the strong coupling limit in a weak coupling framework, and find one prescription, used extensively by Levin and coworkers, and based on a modified proposal dating back to Kadanoff and Martin, works remarkably well.   

Gauge Theory of Pairing and Spin Fluctuations Near the Quantum Critical Point

We have solved the spin Fermion (periodic Kondo) model for the superconductor transition temperature $T_c $ and for the electron energy gap function $\phi $ as $T\to T_c $. We find for realistic parameters$W$, the electron band width, $N_{B} \left( \omega \right)$, the Boson density of states and $J_q ,$ the Kondo exhange interaction, that $T_c =1.14{\kern 1pt}\,\omega _s \;\,e-\frac{(1+\lambda _Z )}{\lambda _\phi }$ where $\lambda _Z $ is the normal state renormalization constant and $\lambda _\phi $ is the pairing interaction strength. We find $T_c $ is exponentially higher for $\ell =1$ (p-wave), $S=1$ (spin triplet) pairing than for $s$- wave pairing $S=0$. We note $\lambda _Z =0$ for $p$-wave pairing due to the odd parity of the relevant. For realistic parameters the solution of Eliashberg's equation for $T_c $ predicts $T_c \tilde {-}\;5\times 10^5\,^0K$ with $H_{c2} \sim 10^8T$ and$j_c \sim 10^8\,Amps/cm^2$. When $T_c $ and $\phi $ are simultaneously maximized, with respect to $N_B \left( \omega \right)$ and $J_q $ considerably high $T_c ,H_{c2} ,j_c $ values are predicted, namely $T_c $ of order $5\times 10^8\,^0K \quad H_{c2} \sim 10^{13}T$ and $j_c \sim 10^{13}\,Amps/cm^2$There values are predicted to exist in systems such as the Heusler alloys, e.g. for $Au_2 \left( {Mn_{2-x} \;A\ell _x } \right)$ for $x\tilde {-}0.1-0.5.$   

Problem of Gauge Invariance in the BCS theory: Revisited

It is well known that the derivation of the Meissner effect in the BCS theory is not gauge invariant. We reconsider this fundamental problem from a few different aspects. First, we point out that the gauge invariance is basically one particle property, although we are dealing with the many body BCS wavefunction. Second, if the field operators are transformed due to the gauge change, it is trivial to show the gauge invariance. Third, if the BCS state is changed instead according to the gauge transformation, we need to pair the states which include the effects of the perturbation. In fact, this point was realized by Bogoliubov, Blatt, and Ambegaokar and Kadanoff. However, their investigations were only formal and didn't pursue the implications of this generalized pairing on the electrodynamics of the BCS superconductors. We show that the generalized pairing leads to a gauge invariant derivation of the Meissner effect. We also discuss the resulting refinement of the electrodynamics of the BCS superconductors.   

4D-XY quantum criticality in a doped Mott insulator

We argue that the low energy, long wavelength physics of the underdoped cuprate superconductors is governed by a 4D-XY quantum critical point situated at the $T=0$ terminus of the $T_c(x)$ line. We derive the effective action for a d-wave superconductor in the vicinity of this point and show that much of the experimentally observed phenomenology follows from it. Most notably our theory explains the puzzling features of the recent magnetic penetration depth data on YBCO crystals, such as the apparent absence of the classical thermal fluctuation regime and the observed violations of Uemura scaling.   

Statistics of Cooper's pairs

It is well known that the Cooper pair~(pairon) operators may not be considered either as the Bose operators nor as the Fermi operators. The analysis of trilinear commutation relations for the pairon operators reveals that they correspond to the modified parafermi statistics of rank p=1. Two different expressions for the Cooper pair number operator are presented. We demonstrate that the calculations with a Hamiltonian expressed via pairon operators is more convenient using the commutation properties of these operators without presenting them as a product of fermion operators. This allows to study problems in which the interactions between Cooper's pairs are also included. The problem with two interacting Cooper's pairs is resolved and its generalization in the case of large systems is discussed$^{1}$. $^{1}$ I.G. Kaplan, O. Navarro, and J.A. S\'{a}nchez, Physica C (in Press). Key Words: Cooper's pairs interaction, commutation relations, parastatistics   

Theory of magic dopings in high Tc superconductors

Based on an effective bosonic model, we predict checkerboard- type ordering of the Cooper pairs at magic rational doping fractions $(2m+1)/2^n$, where m and n are integers. At the magic doping fraction $x = (2m+1)/2^n$, the charge unit-cell is $2^ {(n+1)/2}a \times 2^{(n+1)/2}a$, pointing along the original CuO bond direction when $n$ is odd, and along the diagonal direction when $n$ is even. It is generally expected that the charge ordering tendencies are stronger at higher levels of the hierarchy, with smaller n. Recently, a tendency towards charge ordering at particular rational hole doping fractions, 1/16, 3/32, 1/8, and 3/16, is reported in transport measurements of $La_{2-x}Sr_xCuO_4$ samples[1]. This observation is most consistent with our predicted hierarchy of charge ordering of hole-pairs.   

Some Comparisons of RVB Theories and High Tc Cu-oxides

We report extensive comparisons of the doping dependences of physical properties of the RVB theories and the high Tc Cu- oxides. We study an extended t-J model with the second and third-nearest-neighboring hopping terms by using the renormalized mean field theory and the variational Monte Carlo and the power Lanczos methods. We calculate the quantites related to the nodal quasiparticle physics as well as the thermal and spectral properties of the model and compare them to the available experiments   

Antiferromagnetism and charged vortices in high-Tc superconductors

We present the results of a detailed mean-field study of the effect of the long-range Coulomb interaction on charge accumulation in antiferromagnetic vortices in high-$T_c$ superconductors. We have found that antiferromagnetism is associated with an accumulation of charge in the vortex core, even in the presence of the long-range Coulomb interaction and that the local density of states in the vortex core, as described by a simple theory of competing $d$SC and AFM orders, is in excellent qualitative agreement with experimental data. We also touch on the manifestation of $\Pi$-triplet pairing in the presence of coexisting $d$SC and AFM order, and the intriguing appearance of one-dimensional stripe-like ordering.   

Maximization of a Superconductor's Transition Temperature in a Boson - Fermion Model by Variation of the Boson Density of States and the Boson - Fermion Coupling Constants or for Spin Vector or Scalar Couplings Respective

We have solved the problem of maximizing a superconductors transition temperature $T_c $ by varying the Boson (spin fluctuation or phonon) density of states $N_B \left( \omega \right)$ and coupling constants, $J_q $ or$g_{kq\lambda } $. We find that $T_c \sim 10^9\;{ }^0K$ can be obtained, for example in Heusler alloys, such as$Au_2 \left( {Mn_{2-x} \;A\ell _x } \right)$, with$x\sim 0.1-0.5$. Also values of $H_{c2} \sim 10^{13}T$ and$j_c \sim 10^{13}\,Amps/cm^2$are predicted. Additionally results for the tunneling density of states$N_T \left( {eV} \right)$, the arpes cross section,${d\Delta } \mathord{\left/ {\vphantom {{d\Delta } {d\Omega d\omega }}} \right. \kern-\nulldelimiterspace} {d\Omega d\omega }$, $e.m.,{\kern 1pt}\,I.R.$ and transport coefficients arising from these models will be presented. Also we will present a discussion of the 36 Leggett modes which our theory predicts to exist, whose energies are in the optical frequency range$\left[ {\omega \sim 1-3eV} \right]$, versus the microwave frequency range for superfluid $^{3}$He-A phase.   

Exact summation of vertex corrections to the penetration depth in $d$-wave superconductors

In high-purity YBCO single crystals, the impurity scattering may be dominated by slowly varying potentials due to dopant oxygen atoms. A d-wave superconductor presents an unusual situation in which such extended disorder potentials are essentially unable to change the charge current carried by nodal quasiparticles. We find that the inclusion of the important vertex corrections leads to a remarkably simple relationship between the normal fluid density and the quasiparticle density of states in the disordered system. This result is extremely general and allows the interpretation of the temperature dependence of the penetration depth in a model-independent fashion.   

Slave-boson approach to the $t$-$t'$-$t''$-$U$ model applied to electron-doped cuprates

By applying Hartree-Fock (HF) mean-field theory to the $t$-$t'$-$t''$-$U$ model, Kusko {\it et al.} have studied the Fermi surface evolution with doping in the antiferromagnetic (AF) phase for electron-doped cuprates. Although they reached consistent results with ARPES data, a doping-dependent effective $U$ was adopted, specifically, $U$ drops from $6t$ at $x=0$ to $3t$ at $x=0.15$. The strong doping dependence of $U$ and its small value at $x=0.15$ are both in disagreement with the analysis on the optical conductivity [A. J. Millis {\it et al.}, cond-mat/0411172]. In view that the HF theory often overestimates the AF order, we re-study the model analytically by Kotliar-Ruckenstein slave-boson approach which improves the consideration of fluctuations. A quicker decreasing of the staggered magnetization (and AF gap) with increasing doping than in HF theory is obtained, thus the ARPES results are possibly reproduced even under a doping-independent constant $U$. We have further considered superconductivity and its interplay with antiferromagnetism by introducing an attractive intersite $V$.   

Session N33: Quantum Fluid and Solids III

Liquid helium in disorder and boson localization

Neutron scattering measurements of the excitations of liquid $^4$He confined in three porous media focusing on temperatures around the superfluid-normal fluid critical temperature $T_c$ are presented and discussed. The three porous media are Vycor ($T_c = 2.05$ K at SVP), 44 {\AA} pore diameter gelsil ($T_c = 1.92$ K at SVP) and 25 {\AA} pore diameter gelsil ($T_c \approx 1.0$ K at SVP)~${^{[1,2]}}$. In all these media, liquid $^4$He supports well-defined phonon-roton excitations above $T_c$, in the "normal" phase (up $T_\lambda = 2.17$ K at SVP). Since well-defined excitations are associated with Bose-Einstein condensation (BEC), this suggests that there is BEC in the "normal" phase. Also, since there is no superflow, this BEC is apparently localized in the media separated by regions of normal fluid. In this picture, the superfluid-normal transition in disorder is associated with an extended-localized BEC crossover with localized BEC remaining above $T_c$~${^{[3]}}$. \\${^{[1]}}$F. Albergamo et al., Phys. Rev. B 69, 014514 (2004) \\${^{[2]}}$J. V. Pearce et al., Phys. Rev. Lett. 93, 145303 (2004)) \\ ${^{[3]}}$H. R. Glyde et al., Phys. Rev. Lett. 84, 2646 (2000)   

Excitations of metastable superfluid $^4$He at pressures up to 40 bars

We have performed neutron scattering measurements~${^{[1]}}$ of the fundamental excitations of liquid $^4$He confined in 44 {\AA} pore diameter gelsil glass at pressures up to 40 bars in the wave vector range 0.4 $$ 1.6 {\AA}$^{-1}$, especially the rotons, are observed up to complete solidification of all the liquid at $p\sim$ 40 bars, where the roton disappears. Simultaneous neutron diffraction measurements show the existence of Bragg peaks at and above 35.1 bars, indicating co-existence of liquid and solid in the pores at pressures $35 \leq p \leq$ 40 bars. These measurements are presented in conjunction with a measurement of the phase diagram of this system using ultrasound. %$\lesssim P\lesssim$ \vspace{.5cm} \\ ${^{[1]}}$J. V. Pearce et al. Phys. Rev. Lett. 93, 145303 (2004)   

$^4$He Films Adsorbed in Porous MCM--41 Ceramic: Crossover to One-dimensional Superfluidity

The superfluid density of $^4$He films adsorbed in MCM-41 ceramic having long \hbox{40-\AA}-diameter cylindrical pores is studied with a torsion oscillator technique. Finite-size Kosterlitz-Thouless transitions are observed in the data, with the broadening of the transitions increasing considerably as the film thickness decreases. The data are in good agreement with the Machta-Guyer theory of the KT transition in a cylindrical channel, but fits to the broadening require an increasing vortex core size. In the thinnest films a crossover to the one-dimensional behavior predicted theoretically is observed, where the vortex core radius limits at the cylinder radius and the $T = 0$ superfluid density approaches zero.   

Structure of He-4 adsorbed on single-wall carbon nanotube bundles

The structure and dynamics of $^4He$ adsorbed on single-wall carbon nanotubes (SWNT) has for a long time been the subject of intensive theoretical investigation. Here we present the first experimental measurements of the structure of $^4He$ adsorbed on SWNTs, obtained using neutron diffraction techniques. The structure of this highly quantum system has been measured as a function of the $^4He$ coverage, up to one monolayer, and appears to be qualitatively similar to that of more classical systems. By combining the current data with our existing measurements of the heat capacity and isosteric heat of adsorption, we are able to identify the contributions from the linear chains in the outer groove sites, and the two- dimensional patches on the curved outer walls. In conjunction with molecular dynamics simulations, we are able to draw some conclusions about the population mechanism of $^4He$ on SWNTs based on direct observations of the adsorbate structure.   

Ellipsometery to Probe prewetting and superfluid transitions of thin liquid helium films on rubidium

Helium films adsorbed on intermediate binding strength substrates, like rubidium, exhibit markedly different behavior than on more common strongly binding substrates, or weask substrates like cesium. Previous experiments on Rb using a quartz crystal microbalance(QCM) have shown that the prewetting and superfluid transitions occur at virtually the same chemical potential. The superfluid transition seems to be coupled to the prewetting transition and hysteretic, which would be an apparent disagreement with Kosterlitz-Thouless paradigm. By using a modulated null Ellipsometer in conjunction with the QCM we are able to simultaneously measure both the total coverage and the normal component of the helium film. This allows us to separate the prewetting and superfluid transitions and document how the binding potential of the substrate may lead to deviations from a standard Kosterlitz-Thouless transition.   

Simultaneous Measurement of Casimir Film Thinning and Superfluid Density in $^4$He Films Near the $\lambda$-Transition

Experiments are being undertaken to measure simultaneously the Casimir film thinning effect and the superfluidity density of $^4$He films near $T_\lambda$. A silicon substrate with nanometer-scale roughness will be employed to minimize any effects of capillary condensation. The superfluid density and superfluid onset point will be monitored by third-sound propagation at resonant frequencies below 5 Hz. Metallic capacitor plates for both the thickness measurement and third sound detection will be evaporated on the back sides of the silicon wafers to avoid perturbing the film being measured. High-resolution thermometry will be employed to minimize heat input from temperature regulation, which can lead to fountain pressures and nonuniform film thickness. A precise location of the KT transition onset point relative to the film thinning may help to resolve existing discrepancies between theory and experiment.   

Superfluidity and Capillary Condensation in Porous Materials

We have studied superfluid onset and capillary condensation in liquid helium in porous gold using the quartz crystal microbalance (QCM) technique. Au/Ag alloy films were sputtered onto QCM surfaces, and used to form porous gold substrates of various thicknesses and pore size. Helium isotherms on these samples show that at low temperature where the superfluid transition occurs prior to capillary condensation, superfluid onset has the conventional features of a KT transition, including mass decoupling and dissipation. At higher temperatures, the superfluid transition occurs within the capillary condensation hysteresis region where the film thickness is a multivalued function of the chemical potential. In this regime, the only signature of superfluid onset is a peak in the dissipation; there is no discernable mass unloading. This behavior persists even at temperatures near Tlambda, where onset occurs when the pores are full of liquid. To determine how universal this behavior is, we have attempted to make a porous material with uniform pore size by electrochemical anodization of aluminum films on a QCM. We will present preliminary isotherms on these porous alumina films.   

Capillary Condensation of Helium in Aerogels of Different Porosity

When fluids are adsorbed in small pores, capillary forces are large and usually result in hysteresis between adsorption isotherms taken during filling and emptying. However, previous measurements with fluids in high porosity silica aerogels showed non-hysteretic behavior which could be interpreted as an equilibrium liquid-vapor coexistence curve. This curve was much narrower than in bulk fluid and the critical temperature, T$_c$, was suppressed. We have made direct capacitive pressure-density measurements near the critical point of helium confined in aerogels with porosities between 95\% and 98\%. We see hysteresis in isotherms in both aerogels and, although the aerogels fill over very narrow pressure ranges, we never see a true two-phase coexistence region. The hysteresis loops shrink and eventually disappear at about 5.155 K (in the 95\% porosity aerogel) and 5.175 K (in the 98\% porosity aerogel). We compare the shapes of the hysteresis loops in the two aerogels and how they evolve near the critical point. This work was supported by a grant from NSERC.   

Measurements of $^3$He magnetiztion on ZYX graphite using SQUID NMR

$^3$He films on ZYX graphite are more nearly ideal 2D systems than is the case with $^3$He on more commonly used grafoil substrates, given the larger platelet size (100-200 nm) and smaller spread angle ($\sim$5 degrees) of ZYX. We have begun a study of these films using our SQUID NMR techniques. We are currently surveying a range of coverage from a little less than first layer completion to greater than third layer promotion in order to map out differences between the magnetism of $^3$He on ZYX and grafoil. The primary objective of this study is to sort out the size effects on the apparent finite temperature ordering of these films in the ferromagnetic coverage region. The small surface area of ZYX graphite (1-1.5 m$^2$/g) poses an experimental challenge. We will present details of our wide bandwidth two-stage SQUID system used in these experiments.   

Longitudinal and transverse NMR study of superfluid $^3$He in aerogel

The comparison of longitudinal and transverse resonances in the superfluid phases of $^3$He was the most decisive confirmation of the superfluid order parameters and of Leggett's theoretical predictions. Recently, the study of the impact of impurities on superfluid $^3$He has been made possible by using low-density aerogel as a filler in the liquid. Predictions of an altered superfluid order parameter in aerogel can be tested by comparing longitudinal and transverse resonances. Previously, longitudinal resonance has been measured using conventional NMR techniques with a high Q coil. One characteristic of the longitudinal NMR line is its short effective T$_2$ making conventional pulsed NMR techniques difficult. We expect that the presence of aerogel would make T$_2$ even shorter. SQUID NMR offers wide bandwidth at low frequencies. We will discuss our efforts to observe longitudinal resonance in both bulk superfluid and aerogel using SQUID techniques.   

Impurity Scattering in Superfluid $^3$He: A New Phase

We present continuous-wave NMR measurements of superfluid $^3$He confined to two low-density silica aerogels, with porosities of 99.3\% and 98.6\%. We find within the aerogel two superfluid phases of differing symmetries separated by a first-order transition. The transition between these two phases is broadened by strong interfacial pinning, even near $T_c$. This should preclude determination of the equilibrium transition temperature between the two phases, but we have developed a method for determining this temperature, despite the pinning. We find that the thermodynamic transition temperature versus sample pressure within the aerogel differs greatly from the bulk, yet is independent of aerogel density. We conclude that the presence of aerogel has stabilized a new phase, not known to exist previously.   

Bose-Einstein Condensation and atomic kinetic energies in liquid ${^3}$He-${^4}$He

We present Deep Inelastic Neutron Scattering (DINS) measurements of mixtures of liquid ${^3}$He-${^4}$He in both the superfluid and normal phases. The measurements were performed on the MARI time-of-flight spectrometer at the ISIS pulsed spallation neutron source, at wavevectors $26\le$ Q $\le29$ \AA$^{-1}$ for four different $^{3}$He concentrations $x$. From the data, we extract both the condensate fraction $n_0$ and the single particle kinetic energies $\langle K_3\rangle$ and $\langle K_4\rangle$ of each isotope. We find a relative increase in $n_0$ from 7.25\% for the bulk data ($x=0\%$) to $\sim$12\% for $x=20\%$, in agreement with theoretical calculations but less than that found in the only other DINS measurement of $n_0$. The measured values of $\langle K_3\rangle$ ($\sim$11 K for $x=10\%$) fall in the range of previous DINS measurements. Surprisingly, $\langle K_4\rangle$ is found to be somewhat independent of $x$, in contrast to most calculations and measurements.   

Structure in the Condensate Occupation of Trapped Hard Sphere Bosons

We have performed detailed variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) calculations of the ground state and condensate properties for small numbers $(1 < N < 100)$ of hard sphere bosons in a harmonic trap. Condensate properties are obtained by evaluating the eigenvectors (Natural Orbitals) and eigenvalues (Occupation numbers) of the one body density matrix. For macroscopic systems with weak interactions, $na^3 \ll 1$, the OBDM description of the condensate in terms of a single large eigenvalue with large occupation and the ``condensate wave-function" of Gross-Pitaevskii (GP) theory are equivalent. Unlike GP theory, however, condensate properties obtained within the OBDM formulation are a property of the full ground state wave-function of the many-body Hamiltonian and are equally valid for any number of particles and interaction strength. We find that in the mesoscopic regime, $N < 30$ \& $na^3 > 10^{-3}$, the $N$ dependence of the fraction of particles in the condensate orbital $n_0(N)$ is not a simple monotonically decreasing function of $N$. Quite remarkably, we find that there are instead ``magic" particle numbers for which the condensate experiences anomalous depletion. This structure in the condensate fraction will be described fully and possible implications for experiment will be discussed.   

Structure Factor of Superfluid He II about Dispersion Minimum

The SHM-RSB/$\Delta_b$-dynamic scheme of He II has within the framework of the well established condensed matter theory facilitated the predictions of superfluidity, critical velocity, circulation quantization, and other key properties of He II in overall good agreement with experiments[1]. In relevance to neutron scattering this scheme leads to that at the dispersion minimum $(q_b, \Delta_s)$, the structure factor of He II contains an elastic and inelastic component: $S(q)=S^{{\rm el}} (q)+ S^{{\rm inel}}(q)$. Here $S^{{\rm el}}(q)=\frac{N}{2\pi \hbar}e^{-2W}[1+\int g(R) e^{i {\bf q}_b\cdot {\bf R}} d R] f_0 \delta(\omega-0) $ probes the instantaneous configuration of the disordered superfluid atoms and is a broad function. $S^{{\rm inel}}(q)=\frac{1}{2\pi\hbar N}\delta(q-q_b) f_b \delta(\omega- \frac{\Delta_s}{\hbar}) $ is due to scattering by the excitations of superfluid bond $\Delta_b$, at an energy cost $\Delta_s$. (Definitions for other variables are given e.g. in [1]2004b.) $\Delta_b$ has its origin in many-quantum-atom correlation and thus has a well defined value through this many- atoms averaging operation. Accordingly $S^{{\rm inel}}(q)$ is a sharp function; it is related to the dynamic structure factor here as: $S^{\rm inel}_b(q)|_{q=q_b}= S_b(q,\omega)|_ {\omega=\frac{\Delta_s}{\hbar}}$.\\[0cm] [1] J.X. Zheng- Johansson and P-I. Johansson, in "New Developments in Superconductivity Research", R.W. Stevens Editor, Nova Science, 2003, ISBN 1-59033-862-6; "The Microscopic Theory of Superfluid He II", Nova Science, 2004a, ISBN 1-59033-974-6; arXi:cond- mat/0410442, 2004b; arXi:cond-mat/0410485, 2004c.   

Magnetism of a multielectron bubble in liquid helium

Multielectron bubbles are cavities in liquid helium containing electrons. A typical N=10000 electron bubble is forced open by the Coulomb repulsion of the electrons, balanced by the surface tension of the helium, leading to a typical radius of 1 micron. The electrons in the bubble form a spherical two-dimensional electron gas (S2DEG): they collect in a nanometer thin layer anchored to the in inner surface of the bubble. We investigate the properties of this S2DEG, both for weak and strong magnetic fields. Already at a few tens of gauss, typical multielectron bubbles enter the strong-field regime where the single-particle energy levels arrange themselves in Landau bands of doubly degenerate levels in stead of highly-degenerate Landau levels.   

Session N34: Dynamics in Condensed Phase II

Multiple probes of heterogeneous protein kinetics

Transition state theories break down for very fast folding proteins, where substantial populations exist along the reaction coordinate on a nanosecond to microsecond timescale. For such proteins, different spectroscopic probes yield different dynamics, i.e. there is no well-defined rate coefficient or set of rate coefficients. I will discuss fast relaxation experiments for several such proteins, as well as modeling by Langevin dynamics and molecular dynamics simulations, which now connect with experiment on the 0.1-10 microsecond time scale.   

Picosecond Studies of Enzyme Mechanism in B12 Dependent Glutamase Mutase

Adenosylcobalamin-dependent (coenzyme B12, AdoCbl) enzymes catalyze a variety of chemically difficult reactions that proceed by mechanisms involving organic radicals. Radicals are initially generated by homolysis of the cobalt-carbon bond to generate an adenosyl radical and a cob(II)alamin radical. In the present study time-resolved spectroscopic measurements spanning the time range from 10 fs to 10 ns are used to investigate the kinetics of homolysis and recombination for adenosylcobalamin bound in the active site of glutamate mutase. These are the first such direct measurements on an adenosylcobalamin dependent enzyme. A short-lived intermediate is formed prior to formation of the cob(II)alamin radical. This intermediate was not observed upon photolysis of adenosylcobalamin in free solution. The intrinsic rate constant for geminate recombination for adenosylcobalamin bound to glutamate mutase is only slightly smaller than the rate constant measured in free solution, suggesting the protein does not greatly perturb the stability of the cobalt-carbon bond upon binding the coenzyme.   

Nonequilibrium fluctuations of a single biomolecule

In recent years it has been realized that equilibrium information is subtly encoded in the fluctuations experienced by a system that is driven away from equilibrium. The key to decoding this information is a simple statistical reweighting procedure involving the external work performed in driving the system out of equilibrium. I will discuss the theoretical background of these results, as well as their applicability to the analysis of single- biomolecule pulling experiments, and to the numerical estimation of free energy differences.   

Photoinduced vibrational coherence transfer in molecular aggregates

At short times faster than the time of dephasing a strong photoinduced excitation in an electron-phonon molecular structure induces evolution of the vibrational subsystem that depends on the electronic evolution. The equilibrium position and oscillation frequency of the mean nuclear coordinate depend on which electronic state keeps the major part of the population. This effect is described theoretically at a simple analytic level by applying the quantized Hamiltonian dynamics (QHD) formalism [J. Chem. Phys. {\bf 120} 11209 (2004)] to the electronic and vibrational degrees of freedom of a model molecular aggregate, as motivated by recent experimental data in a bacteriochlorophyll aggregate [J. Phys. Chem. B {\bf 103} 2297 (1999)]. The ultrafast pump-probe signal is considered. The vibronic wavepacket driven by electronic energy transfer sequentially visits different excited states. The probe signal oscillates with the nuclear configuration at the frequency that is modulated depending on the curvature of the current potential energy surface. This modulation of the frequency of the probe signal known as coherence transfer is described within simple analytic and numerical models by the QHD method that can be easily extended to many degrees of freedom.   

New 2D IR techniques for studying the structures and dynamics of biomolecules

Two-dimensional infrared spectroscopy is proving to be a powerful technique for studying biomolecular structures and their rapid dynamics. We report investigations into the equilibrium structures of a series of DNA oligomers using 2D IR spectroscopy, where we have found vibrational modes that are delocalized over stacked bases and coupled to adjacent strands through hydrogen bonded basepairs. We also report new advances in time-resolved pulse sequences optimized for extracting vibrational dynamics and the coupling between vibrational modes of biomolecules.   

Two-dimensional infrared spectroscopy of the thermal unfolding of proteins

Steady-state and transient conformational changes upon the thermal unfolding of ubiquitin were investigated with femtosecond infrared spectroscopy of the amide I vibrations. Equilibrium temperature-dependent 2D IR spectroscopy reveals the unfolding of the $\beta $-sheet of ubiquitin through the loss of cross peaks formed between transitions arising from vibrations of the $\beta $-sheet. Transient unfolding following a nanosecond temperature jump is monitored with vibrational echo spectroscopy, a projection of the 2D IR spectrum. While the equilibrium study follows a simple two-state unfolding, the transient experiments observe complex relaxation behavior that differs for various spectral components and spans time scales from nanoseconds to milliseconds. By modeling the amide I vibrations of ubiquitin, this observation is explained as unfolding of the less stable strands \textit{III-V} of the $\beta $-sheet prior to unfolding of the hairpin that forms part of the hydrophobic core.   

Response of Biomolecules to Ultrafast Laser Pulses

Using two complementary techniques -- semiclassical electron-radiation-ion dynamics (SERID) and time- dependent density functional theory (TDDFT) -- we are studying the response of various biologically relevant molecules to femtosecond-scale laser pulses. Our simulations provide microscopic information on mechanisms for photoisomerization [1] and other molecular transformations [2] and on spectroscopic identification of pathogens with schemes like FAST CARS [3]. The coupled dynamics of electrons and nuclei is determined by solving the time-dependent Schr\"{o}dinger equation and using Ehrenfest's theorem, with a 30 attosecond time step. Results will be shown for molecules including stilbene, benzene, and dipicloninc acid. [1] Y. Dou and R. E. Allen, Chemical Physics Letters 378, 323 (2003).2] B. Torralva and R. E. Allen, Journal of Modern Optics 49, 593 - 625 (2002).3] M. O. Scully et al., Proc. Nat. Acad. Sci. 99, 10994 (2002).   

Thermal Fluctuations and Charge-Transfer Dynamics in 2-Aminopurine-Labeled DNA

We present results from picosecond fluorescence experiments and hybrid TDDFT/molecular dynamics simulations that examine the roles of rapid fluctuations in base-stacking interactions on both charge transfer and electronic energy transfer dynamics in single-stranded DNA trimers containing a central 2AP. Direct excitation of the 2AP $\pi -\pi $* state shows that in these highly flexible systems, the 2AP$\to $X charge transfer process occurs on timescales ranging from $<$50 ps to several ns. The dependence of the fluorescence decays on temperature and viscosity suggest that those trimers that are optimally stacked undergo rapid CT leading to a nonequilibrium ``hole'' in the conformational distribution. Diffusion of bases back into a stacked conformation plays a critical role in determining the long-time decays. Energy transfer (A$\to $2AP) in these systems occurs on the timescale of $\sim $1 picosecond. The energy transfer efficiency shows little dependence on solution viscosity over the range of 1-15 cP, suggesting that the large amplitude structural fluctuations that are important in the CT dynamics are ``frozen out'' on the EET timescale. These results are supported by TDDFT/MD simulations of the dynamic electronic coupling between 2AP and its flanking bases. Using a modified Forster approach, which incorporates the full multipole-multipole coupling between bases, we examine the sensitivity of the EET rates to conformational fluctuations and compute the ensemble-averaged (steady-state) transfer efficiency.   

Effect of electrostatic interactions on DNA melting observed using microcantilevers

Mechanical detection for biochemical reactions through the use of microcantilevers is an emerging technique that can be used to measure the biophysical properties of macromolecules.$^{1}$ By optically monitoring the bending of micocantilevers, we can measure the surface stress exerted on the cantilever as a DNA complex undergoes melting. With the microcantilevers, we are able to explore the stability of DNA under a variety of solution conditions. Differences in the lengths and intermolecular interactions between single and double stranded DNA are highlighted by variations in cantilever deflection. Additional parameters such as long-range electrostatic interactions between nucleic acids and ions affect the surface stress on a cantilever. Higher monovalent ion concentrations screen this interaction which results in higher stability of DNA. In our study, we evaluate the stability of short linear DNA complexes from 10-20 nucleotides at varying salt concentrations. We show that this technique is a useful probe of DNA melting dynamics, which allows us to better understand the stability of DNA complexes. Reference: \begin{enumerate} \item Wu, et al. ``Origin of Nanomechanical Cantilever Motion Generated from Biomolecular Interactions,'' Proc. National Acad. Science, Vol. 98, pp. 1560-1564 (2001). \end{enumerate}   

Session N35: Nanophotonic Materials, Nonlinear Optics and Spectroscopy III

Sub-cellular and Multi-cellular Signaling Mechanisms Revealed by Quantitative Laser Microscopies

Newly developed instrumentation and optical probes allows us to image quantitatively dynamic processes within ever more complicated biological systems. Using methods such as fluorescence recovery after photobleaching (FRAP) and F\"{o}rster resonance energy transfer (FRET) of GFPs fused to the glucose sensing enzyme glucokinase (GK), we have discovered that the location and activity of beta cell GK is acutely regulated by insulin. These findings provide a mechanism whereby the glucose sensing ability of the beta cell is tightly coupled to insulin signaling. We have also measured pancreatic $\beta $-cell metabolism during glucose stimulation by quantitative two-photon NAD(P)H imaging. We have developed methods to delineate quantitatively the NAD(P)H signals from the cytoplasm and mitochondria, and show that the metabolic response of these two compartments are differentially stimulated by glucose and other metabolites. Absolute levels of NAD(P)H were determined using two-photon excited fluorescence lifetime imaging (FLIM). These findings elucidate the relative contributions of glycolytic and citric acid cycle metabolism in normal and diabetic cells.   

Multimodal nonlinear imaging and manipulation of biomaterials

Multiphoton microscopy is quickly becoming a multimodal image contrast tool. Initially, multiphoton microscopy was essentially the domain of two-photon absorption fluorescence microscopy, but with the discovery of third- harmonic microscopy and the renaissance of second-harmonic microscopy, it is becoming increasingly interesting to simultaneously exploit all of these contrast mechanisms (and more!). As the image content explodes, so too does the ability to manipulate materials within the image using these same lasers. Multicontrast, three-dimensional images and manipulation will be presented in this talk.   

Experimental and Theoretical Investigation of Conformational Dynamics and Dynamic Disorder of Single Enzyme Molecules

We investigate conformational fluctuation and dynamic disorder of proteins by conducting single-molecule electron-transfer and enzymatic turnover experiments. Our findings lead to new insights on how enzymes work.   

Tunable resonance hyper-Raman spectroscopy of nonlinear optical chromophores

Two-photon-resonant hyper-Raman spectra have been obtained for several dipolar and octupolar donor-acceptor substituted conjugated organic chromophores that have large first hyperpolarizabilities. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 710 to 950 nm. The hyper-Raman spectra are compared to the linear resonance Raman spectra measured at the laser second harmonic. Excitation into regions that appear to contain more than one electronic transition gives rise to different intensity patterns in the resonance Raman and hyper-Raman spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. Hyper-Raman excitation profiles have been measured on an absolute scale by using the hyper-Rayleigh from acetonitrile as an external standard, and simulated using time-dependent wavepacket propagation techniques to obtain the geometric parameters and one- and two-photon transition strengths for both states. Hyper-Raman is a promising technique for probing electronic transitions that are both one- and two-photon allowed and for examining the origins of molecular first hyperpolarizabilities.   

Surface Enhanced Raman Scattering from a molecule adsorbed on a single cluster of nano metal particles

Very large enhancement up to 14 orders of magnitude in the Raman cross section from a molecule adsorbed on a single cluster of a few nano metal particles has been reported recently. The enhancement is believed mainly due to the enhanced electric field because of the excitation of the localized surface modes. We have developed a diagrammatic Green's function theory in the wave-vector space to solve the Maxwell equation for the enhanced electric field near a spherical metal particle cluster. The large enhancement in the field is due to the multiple scattering of the localized modes of the individual metal particles that has been included exactly. The advantage of working in the wave-vector space is that one does not need the use of complicated translational addition theorem required in the real space as used in earlier calculations. Therefore our approach can be easily extended to any shape or size of the metal particle cluster. We find the enhancement in the Raman cross section can reach up to 10 orders of magnitude for silver particle cluster. The enhancement is in a broad frequency range and is below the Mie resonance of the single metal sphere. The results for gold particle cluster are also presented.   

SERS of DNA bases with carbohydrate stabilized silver and gold nanoparticles

The phenomenon of SERS using silver and gold nanoparticles has boosted single molecule spectroscopy research in recent years. Among the various techniques available for synthesizing nanoparticles, generation of carbohydrate stabilized nanoparticles has two advantages: 1) carbohydrate, a biologically benign medium, ensures non- degradation of probe molecules and 2) its gelation property facilitates easy film formation for on-chip bio-sensor applications. We studied the effect of carbohydrate stabilized silver and gold nanoparticles on SERS of DNA bases. Films of probe molecules with and without nanoparticles were casted on a silicon wafer. Comparing with the known Raman scattering cross-section of silicon, relative SERS scattering cross- sections of DNA bases are obtained. The dependence of relative strengths of SERS of DNA bases on the excitation wavelength will be discussed. This work was supported by a grant (RB03-080) from the University of Missouri Research Board, Department of Physics and Radiology.   

Profiling the Near field of Nanoshells Using Surface Enhanced Raman Spectroscopy

There is tremendous interest in the enhancement of electromagnetic fields near metal surfaces. The spatial extent of the near field as a function of distance from the metal surface is of particular interest for applications such as surface enhanced Raman spectroscopy. By using specially designed molecular scaffolds with Raman-active constituents, we measure the profile of this fringing field at a nanoshell surface. Nanoshells are colloidal particles composed of a silica core covered by a gold shell, which exhibit a tunable plasmon resonance; close to this resonance there is a strong enhancement of the electromagnetic near field. The molecular scaffolds consist of polyadenine DNA strands as tethers with a terminal fluorescein molecule. By varying the length of the DNA strand, the fluorescein molecule is placed at controlled distances from the nanoshell surface. Both the DNA scaffold and the terminal fluorescein molecule provide us with independent SERS Stokes modes whose relative intensities permit us to map the average spatial decay length of the near field of the nanoparticle at its plasmon resonance.   

Single molecule sensitivity in near field tip enhanced Raman scattering

The local-field enhancement at a sharp metallic tip in combination with resonance Raman spectroscopy provides an optical scanning probe method with ultrahigh spatial resolution. Here we report on achieving sensitivity down to the single molecule level. Illuminating the apex of a Au wire tip at variable tip-sample distances down to nm proximity results in a strong field confinement and near field coupling. This provides a highly localized light source and a controlled degree of field enhancement for Raman scattering. In the tip-scattered resonance Raman response of malachite green and rhodamine 6G molecules spectral line narrowing compared to the ensemble average and spectral diffusion are seen. Temporal fluctuations of spectral position and relative peak intensities as well as transient line splitting in time series of sequentially recorded spectra are observed. The results illustrate that single molecule Raman spectroscopy can be achieved in scattering-type near-field microscopy. This approach provides the degrees of freedom necessary for a systematic investigation and understanding of the underlying mechanisms of surface-enhanced Raman spectroscopy.   

Manipulating Surface Plasmons at Nanoscales for Enhancing Raman Scattering

The excitation of surface plasmon resonances (SP) of noble metal nanoparticles and nanostructures results in extraordinary scattering at resonant wavelengths and local field enhancement at certain nano ``hotspots.'' Especially, when molecules are attached to these nano hotspots, the Raman signals of these molecules can be significantly enhanced, a phenomenon called surface-enhanced Raman scattering (SERS). Recent experiments revealed that at certain conditions, the Raman enhancement factor can reach 12 orders, allowing for single molecule detection. While large scale practical applications of SERS for biomolecular sensing have been prohibited by the poor reproducibility and controllability of SERS active substrates, this paper will report our recent efforts on manipulating surface plasmons on nanostructures such as nanoparticle arrays (1D and 2D), and nanotip arrays. Both experimental and numerical data will be reported on surface plasmons and SERS on these nanostructures.   

Session N36: Focus Session: Granular Liquids and Gases II

Structure formation in electromagnetically driven magnetic granular media

We report structure formation in submonolayers of magnetic microparticles subjected to periodic electrostatic and magnetic excitations. Depending on the excitation parameters, rich variety of structures: clusters, rings, chains, and networks can be generated in the system. The growth dynamics and shapes of the structures are strongly dependent on the amplitude and frequency of the external magnetic field. It was found that for pure ac magnetic driving at low densities of particles, the low-frequency magnetic excitation favors clusters while high frequency excitation favors chains and net-like structures. An abrupt phase transition from chains to a network phase with the frequency of external ac magnetic filed was observed for a high density of particles.   

Nematic, Smectic and Possibly Tetratic Steady States in Agitated Monolayers of Rods

We present experimental results on the nonequilibrium phase diagram and dynamics of a vertically vibrated monolayer of rodlike particles (diameter d from 0.5 to 1 mm) lying horizontally in a quasi-2d cell of height $<$ 2d. With increasing area fraction, rods with aspect ratio $\sim$12 form nematics and, possibly, translationally ordered phases. Rods with aspect ratio $\sim$5 form striped phases instead. While these results agree with thermal equilibrium simulations [Bates and Frenkel, (2000), Lagomarsino et al. (2003), Khandkar and Barma (unpublished)], some clear nonequilibrium signatures are observed, including global, systematic rotation of the ordered phase in response to weak asymmetries in the sample cell. To find nematics and smectics it was crucial to taper the tips of the rods, without which only tetratic correlations were seen. We will present comparisons with the theory of active nematics [EPL 62 (2003) 196-202], and discuss the possibility of long-ranged nematic and quasi-long-ranged smectic order in these nonequilibrium 2d systems.   

Bouncing dimer

A dimer composed of centimetric beads have been excited on a vibrated plate. The motion of the beads have been recorded. Four excited modes have been observed for accelerations of the plate below the gravity. By tuning the amplitude of the vibration of the plate, the dimer changes from one energetic mode to another. These transitions are discrete and depend on the initial conditions. Moreover the first excited mode has a novel horizontal drift in which one end of the dimer stays on the plate during most of the cycle, while the other end bounces in phase with the plate. The speed and direction of the drift depend on the aspect ratio of the dimer.   

First-order phase transition in a 2D granular fluid

We experimentally examine first-order freezing/melting phase transition in a non-equilibrium system --- a vertically oscillated two-dimensional isobaric granular fluid. The steady state transition occurs between a gas and a crystal and is characterized by discontinuous change in both density and temperature. The phase transition also shows rate dependent hysteresis. The hysteresis disappears for sufficiently slow heating rate producing a reversible transition characterized by single curve as in a first order phase transition in ordinary fluids. We also study the effect of pressure variation and number of particles on the phase transition and hysteresis. We further probe the system in the vicinity of the transition point to study the coexistence between the low temperature crystal and a high temperature gas and subsequent transition between the two states. The results obtained provide better understanding of the specifics of phase transition in granular fluids.   

Phase diagram of vibrated granular media confined between two parallel plates

We present the results from simulations and experiments of vibrated granular media which are confined between two parallel plates. Depending on the density and gap spacing, we see solid phases with hexagonal or square symmetry, or zig-zag `buckled' phases. This phase behavior is remarkably similar to the phase behavior of similarly confined hard-sphere colloidal suspensions in equilibrium. In the case of colloids, the phase diagram is determined through entropy maximization and depends only on the gap between the confining plates and the density of colloidal particles. For the granular system, however, there are modifications of the phase diagram which are caused by the presence of forcing and dissipation in the system. In particular, we find that the solid-liquid coexistence region in the granular system is much larger than in the equilibrium system. This difference is a direct consequence of the lack of equipartition between the coexisting phases.   

Measurements of Grain Motion in a Bubbling Fluidized Granular bed

The fluctuating motions of grains in a bubbling gas-fluidized granular bed are measured using NMR. Most experimental results, including ours, indicate that a gas-fluidized granular bed is only truly fluid in the bubbling state which occurs at high gas flow rates. In this state large grain-free voids (bubbles) rise through the bed and activate motion at smaller scales throughout the rest of the bed, which remains dense. Thus the fluidized bed is an inherently complex, multi-scale granular flow state. Using NMR we are able to measure the distributions of horizontal and vertical grain motions over time scales from one to several hundred milliseconds, and also to probe correlations of the motion over successive time intervals. As a function of observation time crossovers are observed that appear to delineate grain-scale diffusion as well as coherent and stochastic convection of the granular fluid.   

Energy Cascades in Granular Gases

A new class of stationary states in granular gases where energy is transfered from large velocity scales to small velocity scales is found. These steady-states exist for arbitrary collision rules and arbitrary dimension. Their signature is a velocity distribution $f(v)$ with an algebraic high-energy tail, $f(v)\sim v^{-\sigma}$. The exponent $\sigma$ is obtained analytically and it varies continuously with the spatial dimension, the homogeneity index characterizing the collision rate, and the restitution coefficient. These stationary states are realized in numerical simulations in which energy is injected into the system by infrequently boosting particles to high velocities. It is proposed that these states may be realized experimentally in driven granular systems.   

Stress and Velocity Correlations in Granular Flow

We report on studies of gravity-driven, dense, granular flow via simulations of two-dimensional, inelastic, bidisperse hard disks in a vertical tube geometry. We have previously reported (Europhys. Lett. \textbf{66}, 277 (2004)) the formation of linear chain-like structures of particles undergoing frequent collisions. In order to understand the effect of these transient structures on the long-wavelength behavior of the system, we have analyzed the flow in terms of coarse-grained velocity and stress fields, and their two-point correlation functions. We find that spatial correlation of the stress increases only modestly as the flow rate decreases, yet this leads to a marked increase in the spatial correlation of the velocity. This reinforces the idea that a small fraction of the particles can play an important role in the kinematics of flow. We present data on the change in the length scale of correlations as jamming is approached. We further analyze the flow in terms of four-point correlation functions of the stress analogous to those used to characterize dynamical heterogeneities in supercooled liquids.   

Velocity Distributions of Granular Gases with Long-Range Interactions

We study velocity statistics of electrostatically driven granular gases with long-range interactions. Our experiments involve anisotropic dipole forces between particles due to either magnetic or hydrodynamic interactions. Generally, the velocity distribution is non-Maxwellian, and its high-energy tail has a stretched exponential form $P(v)\sim \exp\left(-|v|^\xi\right)$. We find a simple exponential tail, $\xi=1$, for long-range dipole interactions, whereas $\xi=3/2$ for short-range, hard-core interactions. This behavior is consistent with kinetic theory of driven dissipative particles. We conclude that velocity statistics of dissipative gases are sensitive to the form of the particle interaction.   

Granular Jets: The Surprising Role of Air Pressure

We report on the symmetric, focused jet formed by a solid sphere impacting a loosely packed, dry granular powder. Similar jets have been observed in liquids and studied in detail. However, it is surprising that such a jet can form in sand, where there is little to no attractive potential between the individual particles and no surface tension. A model of the jet formation has been proposed \footnote{S. Thoroddsen and A. Q. Shen. Phys. Fluids {\bf 13}, 4 (1996).} \footnote{D. Lohse et. al., Phys. Rev. Lett. {\bf 93}, 198003 (2004). } in which the jet is the result of the uniform collapse of the void left by the impacting sphere due to 'hydrostatic' pressure. Experimentally, we find that there is a very strong {\it air} pressure dependance in the dynamics of the jet. Instead of a single jet, we observe two jets: a small, thin jet that is not strongly affected by the air pressure followed by a second jet whose size and velocity depend strongly on the air pressure. This air pressure dependance indicates that hydrostatic models based solely on void collapse may not completely capture nature of the phenomena.   

Impact induced splash and spill in a quasi-confided granular medium

Dissipation of the energy of impact in a granular medium and its effects has been a subject of considerable scientific for quite some time. In this work we have explored and analyzed the splash and spill effects caused by the impact of a ball dropped from a height into a granular medium in a open container. Three different granular media, namely rice, mustard seeds, and cream of wheat were used. The amount of spilled-over granular matter was measured as a function of the ball-drop height. Digital pictures of the splash process were also recorded. The quantity of spilled granular matter varies linearly with the impact energy. However additional step like structures are also noted. Specifically, a distinct and large jump is seen in the spilled quantity at a specific impact energy in the case of mustard seeds, which also exhibit obvious charging effects and repulsion. Although the parameters such as mass per grain and packing density for the case of mustard seeds are intermediate between those for rice and cream of wheat, the spill quantity for comparable impact energy is considerably higher. These data will be presented and discussed.   

Session N37: Focus Session: Microfluidic Physics II: Electrokinetics

Electrokinetic Microfluidic Systems

Microfabrication technology has enabled the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically-controlled microchip systems with no moving parts. Electrokinetics involves the interaction of solid surfaces, ionic solutions, and electric fields. Electric fields can be used to generate bulk fluid motion (electroosmosis) and to separate charged species (electrophoresis). Microfabrication technology has enabled the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically-controlled microsystems with no moving parts. This seminar reviews progress at Stanford including methods for sample stacking in capillary electrophoresis assays and fundamental studies of electrokinetic flow instabilities. Field amplified sample stacking (FASS) leverages conductivity gradients as a robust method of increasing sample concentration prior to electrophoretic separation. A major challenge to achieving robust, high-efficiency FASS is the role of electrokinetic instabilities (EKI) generated by a coupling of electric fields and ionic conductivity gradients. This coupling results in electric body forces in the bulk liquid that can generate instabilities. Suppression and/or control of electrokinetic flow instabilities is critical as they dramatically increase dispersion rates and thereby limit stacking efficiency. We have identified the key physical mechanisms in EKI; developed generalized models for electrokinetic systems; and validated the models with experiments. We have applied this understanding to the development of chip systems that achieve signal increases of more than 20,000 fold using FASS. This stacking ratio is over 200 times larger than previous on-chip FASS devices.   

AC electric field driven microfluidic control and mixing

The AC electrokinetic movement of fluids has its origin in two different physical mechanisms: AC electroosmosis, the interaction of the Electrical Double Layer induced on microelectrodes by an applied potential and the generated electric field; and electrothermal Electro-hydrodynamics, the interaction of an electric field with gradients in polarisability of the fluid produced by non-uniform heating. Both mechanisms are dependent on a range of factors: applied voltage, signal frequency, fluid properties and the use of AC electric fields requires significantly less voltage ($\sim $10V) than DC electrokinetics, therefore presenting a range of different applications in microfluidic systems. This paper presents results of the use of AC electrokinetics for a range of applications in pumping, mixing and microfluidic control. Patterned microelectrode structures were used for rapid, switchable mixing of multiple fluid streams in microchannels, enhancing diffusive mixing. The mixing occurred over a short distance in the microchannel and could be switched on and off rapidly. Also presented is the use of AC electrokinetics for the local modification of streamlines and deflection of fluid streams in microchannels.   

Microfluidic pumps based on AC electro-osmosis: non-lienear effects

We report on experiments demonstrating the possibility to pump electrolyte solutions in closed microchannel loops using asymmetric arrays of micro-electrodes addressed with AC voltages. Velocities of a few mm/s can be obtained with voltages of a few Volts in this integrable pumping scheme. The variation of pumping velocity with frequency, amplitude and ionic strength demonstrate the occurrence of complex mechanisms, far beyond the quasi-linear models present in the literature. In particular a reversal of the pumping direction at high frequencies is reported, which could be of practical use.   

Travelling Wave Electro-Osmosis: Nonlinear Double Layer Analysis and Application To Pumping of Liquid

Steady motion of aqueous solutions can be produced through ac electro-osmosis, due to the coupling between ac fields and the induced charge at the double layer close to microelectrodes. Continuous unidirectional fluid motion can be obtained when the solution is placed on top of an array of microelectrodes subjected to a travelling wave potential. In this paper we consider a simple model, consisting of a single mode travelling wave, and its extension to an square wave signal. To describe the double layer we use the nonlinear Gouy-Chapman theory. A numerical solution is obtained for this model, and the results are compared with experiments.   

Microfluidic experiments demonstrating induced-charge electro-osmosis

Motivated by recent work on AC electro-osmosis, a general theory of ``induced-charge electro-osmosis'' (ICEO) has been developed, and a variety of ICEO-based pumping and mixing strategies for micro-fluidics have been proposed, using both DC and AC applied voltages. As in the electrophoresis of metal colloids (studied in the Russian literature), ICEO slip of a liquid electrolyte occurs at polarizable (metal or dielectric) surfaces in response to applied electric fields. Due to the nonlinear coupling of the applied field and its nonuniform and time-dependent induced surface charge, the ICEO slip velocity depends on the field amplitude squared, and thus it provides electrohydrodynamic rectification of AC forcing, especially in asymmetric geometries.Although many theoretical predictions have been made, here we provide clear experimental demonstrations of steady ICEO flows near metal structures in polymer microchannels. We investigate the effect of AC frequency, applied voltage, and geometry, and find reasonable agreement with theoretical predictions, allowing for Stern-layer capacitance.   

Impact of double-layer charging dynamics on induced-charge electro-osmotic flows

Induced charge electro-osmotic (ICEO) flows depend crucially on a combination of diffuse layer charge build up and a non-trivial tangential electric field at the electrode surface. Moreover, since time-dependent fields are commonly used to drive ICEO flows, charging dynamics play a critical role in determining the magnitude and direction of the resulting fluid flow. Unfortunately, at strong applied fields, the traditional model of the electrochemical cell as a linear RC circuit breaks down and the impact of bulk diffusion on the charging cannot be ignored. To gain a deeper understanding of the dynamics of diffuse layer charging, we consider the following simple problem: What is the response of a metallic sphere in an electrolyte solution to a suddenly applied uniform electric field. Even in the weak-field limit, we find that there is a non-trivial temporal and spatial dependence to the charge build up at the surface of the sphere, which may impact transient fluid flows. At strong fields, we find that surface conduction begins to be important as the diffuse layer builds up sufficient charge to induce surface diffusion and electromigration.   

Simulations of induced-charge electro-osmosis in microfluidic devices

Theories of nonlinear electrokinetic phenomena generally assume a uniform, neutral bulk electroylte in contact with a polarizable thin double layer near a metal or dielectric surface, which acts as a "capacitor skin". Induced-charge electro-osmosis (ICEO) is the general effect of nonlinear electro-osmotic slip, when an applied electric field acts on its own induced (diffuse) double-layer charge. In most theoretical and experimental work, ICEO has been studied in very simple geometries, such as colloidal spheres and planar, periodic micro-electrode arrays. Here we use finite-element simulations to predict how more complicated geometries of polarizable surfaces and/or electrodes yield flow profiles with subtle dependence on the amplitude and frequency of the applied voltage. We also consider how the simple model equations break down, due to surface conduction, bulk diffusion, and concentration polarization, for large applied voltages (as in most experiments).   

The Response of a Colloidal Microparticle near an Electrode to an AC Electric Field

We monitored the elevation of single colloidal polystyrene microparticles near an electrode in response to an oscillating electric field. The media were HNO$_{3}$, NaHCO$_{3}$, and KOH, and the frequency band was10 kHz. At low frequencies, large oscillations at the driving frequency with small superimposed Brownian excursions were observed. At high frequencies deterministic oscillations in elevation were negligible compared to Brownian fluctuations, which allowed direct transformation of data into potential energy profiles. The ac field drew the particle closer on average to the electrode in KOH solutions (compared to the no-field average elevation) and the field pushed the particle farther from the electrode in NaHCO$_{3}$. In HNO$_{3}$ a reversal of average height was observed at a frequency of 300 Hz at 1.7 kV/m with the particle being drawn closer to the electrode at low frequencies, and being pushed away at higher frequencies. Analysis of the data at a high frequency (10 kHz) revealed a net force that was attractive in KOH, and repulsive in HNO$_{3}$. This net force scaled with E$^{2}\omega ^{-1}$, where E is the amplitude and $\omega $ is the frequency.   

`Designer' porous channels for electrokinetic injection and microfluidic manipulation

Microfabrication techniques allow effective `porous' media in microchannels to be designed with specified properties. In this talk, we present a general and intuitive framework for such systems. For electrokinetic phenomena, specifying the `pore' geometry is akin to effectively determining the dielectric constant. Pressure-driven systems, on the other hand, are even richer, since an effective permeability and volume fraction can be independently controlled. Furthermore, anisotropy can be deliberately designed into the channel properties, opening a range of possibilities for microfluidic applications. We present simple, intuitive examples to highlight the basic effect, and demonstrate how such ideas can be used for applications of practical interest, such as using electrokinetic injection to form sharp sample plugs for high-resolution separations. Both theoretical and experimental results will be presented.   

Patterning electrohydrodynamic flows with conductive obstacles in microfluidic channels

Flow patterns with both recirculating and unidirectional characteristics are useful for controlled mixing and pumping within microfluidic devices. We have developed a fabrication process that converts injection-molded polymer chips into devices that demonstrate induced-charge electroosmosis (ICEO) effects (1,2) in AC fields. Polymeric insulating posts are coated with metal to produce a nonuniform zeta potential under an applied electric field. Induced flows are analyzed by particle image velocimetry. Stable, recirculating flow patterns are discussed, along with their potential to produce well-characterized and reversible streamlines for on-chip mixing in chemical separation and synthesis devices. Asymmetric conductive features can bias the flow direction, generating unidirectional pumping in an AC field. This pumping approach will be discussed in comparison with DC electrokinetic pumps we have studied. 1) M. Z. Bazant and T. M. Squires, Phys. Rev. Lett. 92, 066101/1-4 (2004). 2) T. M. Squires and M. Z. Bazant, J. Fluid Mech. 509, 217 (2004).   

Control of electroosmotic flow in a nanofluidic channel using grafted polymer chain

Electroosmotic flow (EOF) refers to fluid flow past a surface induced by an external electric field. It initially arises near a solid-fluid boundary due to the net charge density in the Debye layer, but the bulk of the fluid is dragged into a uniform flow by viscosity. The phenomenon is ubiquitous in DNA capillary electrophoresis, and is bound to play a critical role in emerging nanopore technologies. However, ways in which it may be controlled or quenched rest mostly on empirical evidence. The most common approach consists in coating the inner capillary surface with adsorbed or grafted polymer chains, but the definite mechanism by which this modulates the EOF remains elusive. We report on large-scale Molecular Dynamics computer simulations of EOF in a nanoscale cylindrical capillary, and discuss the impact of grafted polymers chains on the properties of EOF. We present data for the velocity of the generated flow field as a function of the polymer brush density and the size of individual grafted polymers, and compare our results with theoretical scaling laws derived in the thin Debye layer limit.   

Electroosmosis through a Bottleneck: Formation of Eddies and Theory for Arbitrary Debye Lengths

Although using an applied electrical field to drive flows becomes desirable as channels become smaller, most discussions of electroosmosis treat the case of thin Debye layers. Here electroosmotic flow (EOF) through a constricted cylinder is presented for arbitrary Debye lengths $\kappa^{-1}$ using a perturbation approach. The varying diameter of the cylinder produces radially and axially varying effective electric fields, as well as an induced pressure gradient. We predict the existence of eddies for certain constricted geometries and propose the possibility of electrokinetic trapping in these regions. Eddies can be found both in the center of the channel and along the perimeter, and the presence of the eddies is a consequence of the induced pressure gradient that accompanies electrically driven flow into a narrow constriction. An experimental system is also presented in which we observe regions of recirculation in EOF eddies in the small Debye length limit.   

Electrokinetics for control of on-chip chemical reactions.

It is well known that electrokinetics affords precise control over flow and species transport in microfluidic systems through simple manipulation of externally applied electric potentials. In this work it is demonstrated how electrokinetic effects can be extended to provide simultaneous control over on-chip chemical reactions through manipulation of the local thermal (ohmic/joule heating), shear (electroosmosis) and electrical (electrophoresis) energies at the reaction site. The coupling of the electrical, flow and ``whole-chip'' thermal effects in both the fluidic and substrate domains are investigated through extensive finite element simulations and experimentally validated using microscale fluorescence thermometry. The simulations reveal changes in viscosity and local conductivity on the order of 50{\%} induced by changes in the fluidic geometry. General chip design guidelines for maximizing or minimizing these effects will also be discussed. The degree of precision available and clinical utility of the technique is demonstrated through the detection of a single base pair mutation (single nucleotide polymorphism) in a DNA microarray integrated into a PDMS/glass microfluidic chip.   

Session N38: Phase Transitions in Itinerant Magnets

Influence of the Verwey transition on the magnon dispersion of magnetite

Inelastic neutron scattering measurements of the magnon spectrum of magnetite (Fe$_{3}$O$_{4})$ were performed above and below the metal-insulator (Verwey) transition. Above the Verwey transition, the magnon dispersion behaves as expected for a classical Heisenberg ferrimagnet. Below T$_{V}$, a large gap (8 meV) forms in the middle of the acoustic magnon branch at \textbf{q}=(0,0,1/2) and E=43 meV. This wavevector corresponds to the main superlattice reflection of the low symmetry monoclinic structure that exists below T$_{V}$. It is plausible that the splitting is related to charge ordering occurring on the Fe spinel B-sites in the insulating phase. We examined this possibility by using Heisenberg models with large unit cells (up to 96 magnetic sites) to calculate the magnon dynamics when the superexchange is modified to reflect crystallographic symmetry lowering due to either atomic distortions or charge ordering. Neither of these models predicts the spin wave gap. Other physics is likely at play, such as strong magneto-elastic coupling, which may further complicate our understanding of the Verwey problem.   

Double Exchange Model for Magnetic Hexaborides

A microscopic theory for rare-earth ferromagnetic hexaborides, such as Eu(1-x)Ca(x)B6, is proposed on the basis of the double-exchange Hamiltonian. In these systems, the reduced carrier concentrations place the Fermi level near the mobility edge, introduced in the spectral density by the disordered spin background. We show that the transport properties such as Hall effect, magnetoresistance, frequency dependent conductivity, and DC resistivity can be quantitatively described within the model. We also make specific predictions for the behavior of the Curie temperature, Tc, as a function of the plasma frequency.   

Magnetic domains in itinerant metamagnets

Experimental results of the metamagnetism in Sr$_2$Ru$_2$O$_7$ give strong evidence for a new type of quantum critical behavior, namely a quantum critical end point [S. A. Grigera {\it et. al.}, Science {\bf 294}, 329 (2001)]. Our study shows that this behavior can be caused by band structure effects, e.g. by the vicinity of a van Hove singularity. Based on a mean field analysis, a phase diagram for the multi-layer ruthenates is developed as a function of magnetic field and band filling, showing the presence of a quantum critical end point which terminates a first-order phase boundary. At the end point, the Fermi level of the majority-spin band is precisely located at the Van Hove singularity. Thus, the system is rather susceptible to disorder effects. We investigate the possible appearance of Condon-domain like structures in a metamagnetic system. Condon-domains are the result of the coexistence of two magnetic phases at a first-order phase transition and may influence the motion of carriers via domain-wall scattering. We discuss the physical consequences and propose new test experiments.   

Ultra-sharp jumps of magnetoresistivity in triple-layered ruthenate Sr$_4$Ru$_3$O$_{10}$

Sr$_4$Ru$_3$O$_{10}$ is the triple-layered member in the layered perovskite Ruddlesden-Popper series Sr$_{n+1}$Ru$_n$O$_{3n+1}$ with $n$ = 3. The magnetic properties of this compound are very anisotropic: it shows ferromagnetic behavior for $H$//$c$ and a metamagnetic transition for $H$// $ab$. We have performed systematic electronic transport property measurements under the field configuration $H$//$ab$ using high quality Sr$_4$Ru$_3$O$_{10}$ single crystals grown by a floating- zone method. We have observed very strong evidence for an inhomogeneous electronic state near the metamagnetic transition. The system phase separates into paramagnetic and ferromagnetic phases near or within the transition range. This phase separation process, together with the critical fluctuations occurring near the metamagnetic transition, results in very unusual transport properties: (1) the magnetoresistivity exhibits ultra-sharp jumps (width $<$ 1G) on the down sweep cycle of magnetic field, (2) The resistivity shows an non- metallic temperature dependence below 5K in the up-sweep cycle of field, and a drop in the down-sweep cycle.   

Tunneling magnetoresistance studies of Sr$_3$Ru$_2$O$_7$

Recent work has supported the existence a new type of field- tuned quantum phase transition (QPT) in the double layered ruthenate Sr$_3$Ru$_2$O$_7$. To further probe the physical properties near this QPT, we have performed planar tunneling measurements on Sr$_3$Ru$_2$O$_7$ single crystals. Our previously reported work revealed an unusual oscillation in tunneling magnetoresistance. We here report further characterization of this new phenomenon, showing that the oscillation has a systematic dependence on the tunnel barrier, temperature, and the field orientation. The oscillation pattern is identical even for different barrier materials (such as Al$_2 $O$_3$ and SiO), but is only prominent when the junction resistance is between roughly 15$\Omega$ and 1k$\Omega$. The oscillation shows a field orientation dependence for $H$//$c$ and $H$//$ab$, both in its pattern and its temperature dependence. The oscillation frequency for $H$//$ab$ appears to be smaller than that for $H$//$c$. We discuss possible origins of this unusual oscillation phenomenon in light of recent bulk measurements on Sr$_3$Ru$_2$O$_7$.   

ARPES measurements of the 3-Dimensional Fermi Surface of LaRu$_2$Si$_2$

LaRu$_2$Si$_2$ is important as the $f^0$ reference compound for heavy fermion systems CeRu$_2$Si$_2$ and CeRu$_2$Ge$_2$ which are hallmarks for the agreement between dHvA experiments and a renormalized-LDA description of the heavy mass quasiparticle Fermi surface (FS). Recent advancements in angle resolved photoemission (ARPES) capabilities including angular resolution and automation allow the measurement of finer electronic structure detail as well as the mapping of larger regions of k-space. These experimental improvements combined with photon energy dependent excitation are used to probe k$_{\perp}$-variations in the LaRu$_2$Si$_2$ FS topology. Normal emission band structure maps newly reveal closed FS contours of small Z-centered hole pockets, thereby refining the value of the crystal inner potential. Wide-angle FS maps at three different photon energies also show clear signatures of three-dimensionality and incomplete k$_{\perp}$-broadening, including (i) new evidence for an LDA-predicted narrow electron pocket at the P-point and (ii) an electron-like connectivity between FS contours centered around $\Gamma$ and the famous large hole-like 'pillow' pocket centered on Z.   

Thermal Expansion and Magnetostriction of the Ising Antiferromagnet TbNi2Ge2

TbNi$_{2}$Ge$_{2}$ has been shown to be an extremely anisotropic, axial, Ising-like antiferromagnet. In zero field it enters incommensurate and commensurate antiferromagnetic states at 16.7 K and 9.6 K respectively; six additional metamagnetic phases have been observed in applied fields at 2 K. In this talk we present preliminary measurements of the thermal expansion and magnetostriction of this material along its c-axis from room temperature to 2 K and in magnetic fields (H$\vert \vert $c) to 14 T. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences, US DOE and was partially supported by the NSF under DMR-0305397.   

Unusual Spectral Weight Transfer in Temperature-dependent Optical Spectra of the Pyrochlore Nd$_{2}$Mo$_{2-x}$Ti$_{x}$O$_{7}$

We investigated the optical conductivity spectra of the pyrochlore Nd$_{2}$Mo$_{2-x}$Ti$_{x}$O$_{7}$ single crystals of $x$ = 0.0, 0.1, and 0.3. Recently optical spectra of a ferromagnetic metal Nd$_{2}$Mo$_{2}$O$_{7}$ and a spin glass insulator Y$_{2}$Mo$_{2}$O$_{7}$ were understood in terms of the Orbitally Degenerate Hubbard Model (ODHM). According to the model, the ferromagnetic correlation of nearest neighbors gives the lowest energy optical transition and the metallic conductivity of Nd$_{2}$Mo$_{2}$O$_{7}$. Because Ti ions have no $d$-electron, the Ti substitution should decrease both of the carrier density and the magnetic moment of the system. The optical spectra of Ti-substituted samples showed strong decrease in the transition located below 1.0 eV. In the meanwhile, the temperature dependent optical spectra revealed an unusual spectral weight transfer. That is, the total spectral weight below the charge transfer energy from O 2$p$ to Mo 4$d$ states increased as temperature increased. And this unusual spectral behavior became more conspicuous as $x$ increased. Based on the ODHM, possible origins of the large spectral change and the relation of the magnetic and the electronic structures will be discussed.   

Understanding Local Structure and Magnetically-Driven Phase Transitions in LiMO$_2$(M=V, Co)

We report the variable temperature optical properties of high-quality, nearly stoichiometric LiVO$_2$ through the magnetic phase transition to probe the role of the lattice in this process. In contrast to the symmetry-consistent spectra of LiCoO$_2$, LiVO$_2$ shows extra vibrational structure in both low and high temperature phases. These extra peaks can not be accounted for within a traditional symmetry analysis, suggesting that the local structure and bulk structure are different. Leading phase transition mechanisms focus on trimer formation at low temperature phase as well as orbital ordering processes. Considering the Jahn-Teller local distortions in LiVO$_2$, a different picture of the magnetically-driven transition seems to emerge.   

Spin structure probed by small-angle neutron scattering in Bi$_{0.125}$Ca$_{0.875}$MnO$_3$

The manganite system Bi$_{1-x}$Ca$_x$MnO$_3$ possesses intriguing properties in the high calcium doping region. In this electron doped region (0.6$<$x$<$1), a ferromagnetic (FM) moment of $\sim $1.2 Bohr magnetons per Mn site is found for x $\sim $0.875. The magnetic moment per Mn site maintains a value $\sim $1/3 the theoretical limit even in fields a high as 60 T. The physical origin of this high moment region is not well understood. Various models including canted ferromagnetism and ferromagnetic clusters hosted by an antiferromagnetic background have been proposed. In order to understand the nature of magnetism in this system we have conducted small-angle neutron scattering (SANS) on Bi$_{0.125}$Ca$_{0.875}$MnO$_3$ polycrystalline samples. Both temperature and magnetic field dependent measurements were performed. Nontrivial spin structure was revealed in this system: cluster-like spin structure forms at temperatures above Tc. With a reduction in temperature, the clusters begin to be correlated and grow in size (and changing in shape) as Tc is approached. When an external magnetic field is applied, the clusters grow and the correlation is enhanced. The high moment suggests, that the spins inside the clusters are gradually aligned at temperature is educed or a magnetic filed is applied. This work is supported by NSF DMR-0209243 and NSF INT-0233316.   

Evidence of Phase Separation on the Surfaces of La0.8MnO3-? Films

Magnetic, transport, surface, and structural studies have been conducted on La$_{0.8}$MnO$_{3}$ films of thickness varying from 52 to 4127{\AA}. Bulk magnetization measurements reveal that maximum T$_{c}$ is obtained by 450{\AA}, though, maximum saturation moment per manganese is not attained until the thickest films. Thinner films have a reduced T$_{c}$. Synchrotron x-ray diffraction measurements on the films exhibit relaxation of the lattice and the onset of additional structural phases with thickness. XMCD versus temperature measurements, which measures magnetization of the top 50{\AA} of the films, gives strong correlation of transition temperatures for some of the films. Differing behavior at the surface however leads to some interesting insights. Surface magnetization is reduced with thickness beyond 450{\AA} as measured at 100K. Additionally, multiple transition temperatures are measured at the surface for one of the films, indicative of some startling phase separation. Additional measurements on local, surface, and nano-structure as well as transport characteristics will be conducted in evaluating this surface behavior in comparison to the bulk film characteristics.   

Microscopic theory of multipole ordering in NpO$_2$

It has been a longstanding problem in physics of actinide compounds to determine the order parameter of the low-temperature ordered phase of NpO$_2$. Recently, several experimental facts have been found to be reconciled by assuming octupole ordering. To understand the origin of the octupole ordering, we construct an $f$-electron model on an fcc lattice based on a $j$-$j$ coupling scheme, and derive an effective multipole-interaction model. By analyzing the effective model numerically, we determine the interactions relevant to the ground state. Then, we apply mean field theory to the simplified model including only these interactions, and find that the longitudinal triple-$q$ $\Gamma_{5u}$ octupole order is realized in our model by the combined effects of multipole interactions and anisotropy of the $\Gamma_{5u}$ moment. We will discuss a possible relation between the present results and experimental observations for NpO$_2$.   

Phonon-stabilized electron distributions in uranium

Contrary to prevailing thinking, recent phonon measurements indicate that thermal electronic excitations may cause a thermodynamically significant softening of phonons in several actinides. A corollary of this effect is that these electronic excitations should be stabilized to higher energies by the entropy generated by phonon softening. Evidence in photoemission spectra will be presented that confirms this for alpha-uranium. While the overall electronic structure is unaffected by heating, electron distributions near the Fermi edge show a large temperature-dependent enhancement in good agreement with phonon-stabilized electron distributions predicted from phonon softening data.   

Photoelectron spectroscopy of cubic actinide compounds

Photoelectron spectroscopy (PES) was applied in investigating the electronic structure of single crystals of USb, NpSb, PuSb, UTe, NpTe and PuTe. Angular-resolved studies were perfomed on U compounds. The photoemission spectral features found within the valence band suggest all six materials contain comparable amounts of 5$f$ and conduction character. Some interesting and unexpected momentum dependent effects are observed in the angular-resolved studies of USb. In PuTe, we confirm the presence of a strong three-peak structure near E$_{F}$, which has been interpreted as the signature of an intermediate valence state in similar materials. Hybridization of the 5$f$ electrons with the conduction band is found within the series and the level of localization is shown to increase from Te to Sb. A surprising correlation between the binding energy of the peak bearing most of the 5$f$ spectral weight and the magnetic moment is discovered within the series, for which some explanations are suggested.   

Localized magnetic excitation in the hybridization gap of YbAl$_{3}$

YbAl$_{3}$ is an intermediate valence (IV) compound which enters the coherent Fermi liquid phase below T$_{coh}$ = 50K.$^{[1]}$ We have recently measured the magnetic scattering on the MAPS spectrometer at ISIS using high-quality single crystals. For T $<$ 50K, the data can be fit as the sum of a background of nonmagnetic scattering and a pair of peaks at E$_{1}$ = 50meV and E$_{2}$ = 33meV which, in the extended zone scheme, scale with Q as the 4f form factor, as expected for magnetic scattering. The scattering near 50meV exhibits a peak in intensity near Q = (1.2, 0.5, 0.5) which also disperses somewhat with Q. Such Q-dependence is as expected for interband scattering across the hybridization gap in IV compounds. The scattering near 33meV, however, is independent of Q in both intensity and position and hence is the result of a spatially localized excitation. The energy of this excitation coincides with a deep minimum in the optical conductivity$^{[2]}$, and hence the excitation energy lies in the middle of the hybridization gap. Both the magnetic excitation and the deep minimum in the conductivity gradually disappear above 50K, indicating that they are properties of the renormalized ground state. 1 A. L. Cornelius, et al, Phys. Rev. Lett. 88 (2002) 117201. 2. H. Okamura et al, Journ. Phys. Soc. Japan 73 (2004) 2045.   

Session N39: Structural Phase Transitions

Coherent optical and acoustic phonon generation correlated to the charge ordering phase transition in La1-xCaxMnO3

We have observed coherent optical and acoustic phonon generation, which are strongly coupled to the charge ordering (CO) transition in La$_{1-x}$Ca$_{x}$MnO$_{3 }$(x= 0.5, 0.58) using femtosecond optical pump-probe spectroscopy. Coherent optical phonons, observed at low temperatures, suddenly disappear above the charge ordering temperature T$_{CO}$. We attribute the sudden onset of coherent optical phonons to their enhanced coupling to the photoexcited charge carriers in CO phase. The oscillation frequency for coherent acoustic phonon depends on the probe wavelength, which is consistent with the propagating strain pulse mechanism. The dramatic change of lattice constants across the charge ordering transition explains the overall temperature dependence of the coherent acoustic phonon amplitude.   

Optically Induced Lattice Dynamics of hexagonal manganite using Ultrafast X-ray Diffraction

We have studied the picosecond lattice dynamics of optically pumped hexagonal manganite LuMnO3 using ultrafast x-ray diffraction. The results show a shift and broadening of the diffraction curve due to the stimulated lattice expansion. To understand the transient response of the lattice, the measured time- and angle-resolved diffraction curves are compared with a theoretical calculation based on dynamical diffraction theory modified for the hexagonal crystal structure of LuMnO3. Our simulations reveal that a large coupling coefficient between the a-b plane and the c-axis (c13) is required to the data. We compare this result to our previous coherent phonon studies of LuMnO3 using optical pump-probe spectroscopy.   

Electron interaction effects on Jahn-Teller instability in LaMnO$_3$

We investigate the origin of the orbital ordering in LaMnO$_3$ using the LDA+U method and Wannier functions analysis. We find that electron-electron interaction dramatically facilitates the distortion of the MnO$_6$ octahedra and effectively enhances electron-phonon interaction. In addition, electron-electron interaction plays a significant role in determining real lattice distortion and orbital ordering pattern, beyond the conventional Jahn-Teller picture. Our conclusion agrees with existing experimental data and could be directly verified by future measurements, e.g., soft X-ray scattering.   

Pseudo-spin of orbital-ordered hybridized $e_g$-states in manganites

The physics of orbital-ordered $e_g$-states in manganites can be conveniently described with quantum pseudo-spin. In order to properly account for the strong hybridization in real materials, energy-resolved Wannier functions are constructed from first-principles to rigorously define the pseudo-spin in LaMnO$_3$ and MnF$_3$. Our quantitative results show that the orientation of the pseudo-spin (mixing of the hybridized $e_g$- states) deviates significantly from what is expected with the lattice distortion, revealing the important role of electron- electron interaction (super-exchange) that competes with the conventional Jahn-Teller effect in determining the orientation. This conclusion can be experimentally verified ($e.g.$: soft X- ray or NMR), and enables further understanding directly accessible with future measurements.   

Photo Induced Structure in Spin Crossover Complex

The spin crossover complex, Fe(phen)$_{2}$(NCS)$_{2}$, is widely recognized for the temperature and photo-induced spin-state transition of the Fe$^{2+}$ ions. With the increase of temperature, the Fe$^{2+}$ ions show the transition from the low-spin (LS: S=0) to the high-spin (HS: S=2) state at Tc=180K. In addition, a photo-excitation by a green light induces a similar spin-state transition at low temperature and the HS state is trapped after the irradiation. Our purpose is the structural analysis on the charge density level of the photo-induced phase using synchrotron radiation powder diffraction. The powder diffraction data of the complex were measured at 92K under 532nm laser irradiation by using Large Debye-Scherrer Camera installed at BL02B2, SPring-8. The imaging plate was used as a detector to collect whole powder patterns simultaneously. The N$_{2}$ gas flow type system was used for the low-temperature measurements. The wavelength of the incident X-ray was 1.0Å. The photo-induced diffraction data with a high-counting statistics were successfully obtained. We have analyzed the powder diffraction data of photo-induced state by the MEM/Rietveld method. The difference of bonding nature between the photo-exited HS state and temperature-induced HS state has been revealed.   

X-ray structural and magnetic studies of the magnetic-martensitic transition in Gd$_5$Si$_{0.5}$Ge$_{3.5}$

Intermetallic Gd$_5$Si$_{0.5}$Ge$_{3.5}$ undergoes a martensitic phase transition between two different orthorhombic polymorphs. This transition, which involves large shear displacements of atoms, expands the unit cell volume on warming and causes the dissapearance of ferromagnetic ordering. We report Gd $L_3$ and Ge $K$- edges XMCD and XAFS measurements on this compound as a function of temperature and magnetic field. The results indicate this transition appears to occur through an intermediate phase. This phase affects the magnetic ordering and can be transformed into the low temperature martensitic phase by the application of a magnetic field.   

Local Structure of La$_{2-x}$Sr$_x$CuO$_4$ by Pulsed Neutron PDF Analysis

La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) has orthorhombic symmetry at $x$ = 0, and orthorhombicity decreases continuously with $x$ up to $x$ = 0.21, where a structural transition to tetragonal symmetry occurs. But the high-energy oxygen LO phonon softening appears to occur suddenly near $x$ = 0.06 where superconductivity sets in. Through the high-resolution pulsed neutron pair-density function (PDF) analysis we show that the medium-range structure changes significantly near this composition. The PDF was determined using the NPDF of LANSCE, LANL. Because of the high Q resolution of this spectrometer the PDF can be determined up to 30 $nm$, connecting the local structure smoothly to the average structure. We found that the PDF peaks in the short-range up to 1 $nm$ change little with $x$, and those in the long-range over 3 $nm$ follow the average structure. However, in the intermediate range from 1 to 3 $nm$ some PDF peaks change non-linearly around $x$ = 0.06, and appear to correspond to the insulator-superconductor transition. Possible implications of these changes in relation to nano-scale charge inhomogeneity will be discussed.   

Suppression of the gamma-alpha structural phase transition in Ce0.8La0.1Th0.1 by large magnetic fields

The $\gamma -\alpha $ transition in Ce$_{0.8}$La$_{0.1}$Th$_{0.1 }$is measured as a function of applied magnetic field using both resistivity and magnetization. The $\gamma -\alpha $ transition temperature decreases with increasing magnetic field, reaching zero temperature at approximately 56 T. The magnetic-field dependence of the transition of the transition temperature may be fitted using a model that invokes the field and temperature dependence of the entropy of the 4f-electron moments of the $\gamma $-phase, suggesting that the volume collapse in cerium and its alloys is primarily driven by entropic considerations. We thank DOE-LDRD-DR20030084, NSF-DMR-0433560, DOE-DE-FG03-03NA00066 and the State of Florida for support of this research.   

Three-Dimensional Dynamic Loading Simulations of Stress-Strain Response of Shape Memory Materials

We present 3-D simulations of the microstructure and mechanical response of shape memory materials. The simulations are based on a nonlinear elastic free energy for a cubic to tetragonal transition in terms of the appropriate strain fields. The dynamics is simulated by force balance equations for the displacement fields with a damping term derived from a dissipational function. This approach ensures that the elastic compatibility relations, which play a crucial role in determining the microstructure of shape memory alloys, are naturally satisfied. Stress-strain properties in the pseudoelastic as well as the shape memory regime are investigated using three dimensional strain loading simulations that take into account the microstructural evolution during deformation. The role played by the microstructural evolution on the strain-rate dependence of the stress-strain properties is also demonstrated.   

Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn

AuZn undergoes a shape-memory transition at 67~K. An abrupt change in the quantum oscillations is observed at this temperature, indicating a disintegration of the Fermi surface associated with the transition. The de Haas-van Alphen effect is measured above and below the transition, at temperatures up to 100~K, and is in reasonable agreement with band-structure calculations in both the high and low-temperature phases. The measurements are strongly suggestive of an inherent, bulk phase separation at low temperatures. In addition, a Dingle analysis reveals a sharp change in the scattering mechanism at a certain cyclotron radius. Both these observations are indicative of the characteristic microstructure that drives the shape-memory effect.   

Bilayer sliding mechanism for the zincblende to rocksalt transition in SiC

We have theoretically investigated the mechanism of the pressure-induced reconstructive zincblende-to-rocksalt phase transition in SiC. Starting with an extensive survey of 925 possible transition pathways, we found that those with the lowest enthalpy barriers all have a common mechanism: bilayer sliding of (111) planes.   

The charge-density wave mechanism in the 2H transition metal dichalcogenides

During recent years there has been a renewed interest in the transition metal dichalcogenides (TMDs) due to angle resolved photoemission spectroscopy (ARPES) studies. For instance, a saddle band in 2H-TaSe2 was measured around 10 meV below the Fermi energy, extending over large regions of the Fermi surface [1]. Despite the recent progress, however, the mechanism of the charge-density wave (CDW) has remained elusive and controversial for these materials. In another experiment, an intriguing shift of the peak of the quasiparticle self-energy as a function of temperature was observed in 2H-TaSe2 [2]. In this talk, we will address these recent experiments. Using the available ARPES data, we obtain a model for the band structure of these materials. With this model, we argue that the CDW is driven by Fermi surface nesting, but due to the high level of degeneracies at the zone boundaries, the Fermi surface is not gapped in the conventional way even for the commensurate CDW phase. By considering coupling to an optical phonon, we provide an explanation for the shift in the quasi-particle self-energy. Finally, we ways to extract information of the Fermi surface from STM experiments. [1] R. Liu et al., Phys. Rev. Lett. 80, 5762 (1998). [2] T. Valla et al. Phys. Rev. Lett 85, 4759 (2000).   

Understanding Hydrogen Bonding and Low-Energy Magnetic Excitations in VOHPO$_4$$\cdot$$\frac{1}{2}$H$_2$O

We report the variable temperature vibrational properties of the S=1/2, quasi-one-dimensional quantum Heisenberg antiferromagnet VOHPO$_4$$\cdot$$\frac{1}{2}$H$_2$O. Vibrational splitting points toward a weak local symmetry breaking near 180 K, and the low-temperature redshift of V-O and H-O related modes demonstrates enhanced low-temperature hydrogen bonding. Due to spin-orbit interaction, the singlet to triplet gap also appears in the infrared response. We compare this value to those obtained via magnetic susceptibility, electron-spin resonance, and neutron scattering, and we point out the existence of a spectral feature that supports weak interaction between traditional``isolated V-V dimers.'' Both magnon dispersion calculations and the experimental data suggest $\alpha$=J$^\prime$/J is $\sim$ 7\%.   

Inelastic light scattering investigation of the pyrochlore superconductor Cd2Re2O7

The Structural phase transitions of the pyrochlore superconductor Cd$_{2}$Re$_{2}$O$_{7}$, T$_{c} \quad \approx $ 1.5 K, are investigated by Raman light scattering. The cubic to tetragonal transition at 200 K is characterized by the gradual appearance of a broad phonon mode originating from motion of the oxygen ions that form the apices of the ReO$_{6}$ octahedra. In contrast, the rapid growth of well-defined low frequency modes below the 120 K transition indicates that it is driven by ordering of the Cd ions within the channel voids of the distorted pyrochlore. A physical model describing the consecutive phase transitions in terms of the interplay between the Re-O and Cd-O networks will be presented.   

Session N40: Focus Session: Morphology and Evolution at Surfaces: Ion Beams and Instabilities

Smoothening Mechanism for GaAs(100) Surfaces during Plasma Etching

Scanning probe microscopy and modeling have advanced the understanding of surface morphology evolution during thermal processing (deposition) and ion bombardment (sputtering). Ion-enhanced plasma etching, where the morphology is determined by the interplay between chemical and ion effects, is less advanced. We have demonstrated the transition from crystallographic to smooth morphology as ion energy increases during etching of GaAs(100) with BCl$_{3}$-Cl$_{2}$ gases. With negligible ion energy, the surface develops $<$110$>$ ridges and {\{}111{\}} facets, as expected from chemistry. With ions at \textit{ca.} 27 eV, ridges and facets are reduced, and the surfaces become smooth (RMS roughness $<$0.5 nm) at ion energy above 100 eV. This transition was simulated using Kinetic Monte Carlo methods, in which morphology is correlated with the relative etch rates at specific types of lattice sites. The simulation results suggest that ion bombardment increases the etch rate at ``ridge'' sites relative to other sites, and enables smooth surface etching, primarily by step flow.   

Sputter Erosion of Ni(111) studied using Low Energy Electron Microscopy

Sputter erosion of Ni (111) by 1 keV Ar$^{+ }$ions in the temperature range between 400 and 750K has been investigated using low energy electron microscopy (LEEM). Characteristic step profiles are found as a function of sputtering temperatures and durations. Below 500K sputtering leads to ragged steps and small islands. As the sputtering temperature is increased a regular sequence of the surface morphology changes with the sputtering temperatures. Sine-like waves occur at intermediate temperatures , which evolve into sharp peaked ripples resembling interfacial structures reported for the Mullins-Sekerka instability of driven solidification interfaces. This behavior is very similar to our earlier observations on Pd (111) and Pt (111). At high sputtering temperatures, the steps become smoother with only weak periodic structure. Distinctly, erosion of Ni (111) gives rise to spiral structures at screw dislocations exhibiting well-defined crystallographic features. At longer sputtering times with $\sim $ 120 ML removed, the star-like structures form. This research is supported by DOE grants DEFG02-02ER46011 and DEFG02-91-ER45439 through the Center for Microanalysis of Materials, University of Illinois.   

Optical Studies of the Morphologies of Metal Surfaces during Ion-erosion and Thermal Annealing

Ar- and Ne-ion sputtering and thermal annealing of Nb(110) and Cu(111) are studied at different temperatures using the oblique-incidence reflectivity technique (OI-RD). It is found that a step-flow or 2-D removal sets in at about 1073 K for Nb and at 700 K for Cu. In the case of the Cu(111) surface, the ratio of the 3-D/2-D-removal transition temperature to the melting temperature agrees with previously predicted values for fcc(111) metal surfaces. Moreover, using a moderately stepped Ni(111), it is demonstrated for the first time that the OI-RD technique can be used to monitor directly the average slope of a surface morphology during ion erosion.   

Real-Time X-ray Studies of Si Surface Morphology Evolution during Ar+ Ion Bombardment

A systematic study of Si surface evolution during normal- incidence Ar+ ion bombardment is reported. Real-time grazing incidence small- angle x-ray scattering (GISAXS) measurements were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. \textit{Ex- situ} atomic force microscopy was also used to provide real-space information. Si (100) samples were bombarded at ion energies ranging from 300 to 1000 eV. For normal-incidence sputtering at room temperature, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (``dot-like structures'') coarsened with time but approached a limiting value of 25-40 nm at all energies examined. These correlations eventually saturate. The surface roughness development then becomes dominated by the growth of the larger length-scale corrugations, causing kinetic roughening. To study the temperature dependence of the surface evolution, Si (100) samples were bombarded with 500 eV ions at temperatures ranging from 25 -- 700 C. There is a transition with increasing temperature from an amorphized surface to a crystalline surface. At high temperatures, the nanoscale correlations coarsen rapidly and are significantly longer in wavelength than the ``dot'' correlations observed at lower sputter erosion temperatures. No saturation is observed during the time of observation. This work is supported by NSF-DMR and DOE-BES. .   

Nanopore Sculpting with Low Energy Ion Beam of Noble Gases

Experiments show that 3keV Helium, Neon, Argon, Krypton, and Xenon ion beams can be used to controllably ``sculpt'' nanoscale features in silicon nitride films using a feedback controlled ion beam sculpting apparatus. Here we report nanopore ion beam sculpting effects that depend on the inert gas ion species. We demonstrate that: (1) all the noble gas ion beams enable single nanometer control of structural dimensions in nanopores; (2) every ion species above shows similar ion beam flux dependence of nanopore formation, (3) the thickness of nanopores sculpted with different inert gas ion beam is deferent. Computer simulations (with SRIM and TRIM) and an ``adatom'' surface diffusion model are employed to explain the dynamics of nanoscale dimension change by competing sputtering and surface mass transport processes induced by different ion beam irradiation. These experiments and theoretical work reveal the surface atomic transport phenomena in a quantitative way that allows the extraction of parameters such as the adatom surface diffusion coefficients and average travel distances.   

In situ studies of capillary filling in nano-porous alumina

Manipulation of matter on nano-scale requires thorough understanding of fundamental properties of materials defined by surface and interfacial phenomena, rather than bulk structure. We report studies of capillary filling in nano-porous alumina with a liquid solvent (perfluoromethyl-cyclohexane) as a function of the offset in chemical potential from liquid-vapor coexistence. Nano-porous alumina samples contains nearly hexagonally ordered, geometrically aligned cylindrical pores of $\sim $15nm diameter and aspect ratio of 1:4,000. Small-angle x-ray scattering measurements make it possible to observe formation of nano-scale wetting film at pore walls and to characterize the film thickness as the chemical potential is varied. Our data indicates a gradual thickening of a liquid film thickness on the capillary walls followed by a nearly discontinuous jump as pores get entirely filled.   

Step bunching kinetics of vicinal silicon (111) surface : A method for nanopatterning

A vicinal Si(111) surface shows a uniform atomic step array above the 7x7 reconstruction temperature. Below the reconstruction temperature, the surface phase separate into step-bunched regions and large (111) terraces. In this experiment, the evolution of the surface morphology during the step bunching is studied using \textit{in-situ} real time x-ray scattering measurement. The period of the nano-scale `step-bunched and terrace' structure at various quench temperatures below the 7x7 transition is obtained as a function of time by grazing incident small angle scattering measurement. The evolution of the surface roughness, measured by x-ray reflectivity, shows that the bunching occurs in two distinct steps. The nano-scale patterns are also examined by \textit{ex-situ} atomic force microscopy. We shall also discuss how to control the size of the step-bunched regions and the separation between them by adjusting kinetic parameters such as the step-bunching temperature and the bunching time.   

Self-Assembly, Dynamics, and Stability of Oxide Nano-Rods on the NiAl(110) Surface

We report the formation of parallel oxide rods upon exposing a clean NiAl(110) surface to oxygen at high temperatures (850- 1350 K). The rods are several microns long, several nanometers wide, and composed vertically of 2-{\AA}-thick atomic layers. We investigate how they grow and measure their thermodynamic stability by following their assembly and decomposition in real- time with low-energy electron microscopy (LEEM). At a fixed temperature and O2 pressure, the rods elongate along their axes at a constant rate. The temperature dependence of this rate yields an activation energy for growth of 1.2 $\pm$ 0.1 eV. The layered nature of the rods leads to sharp changes in their rates of elongation due to their tendency to gain (lose) atomic layers as they descend (climb) atomic steps on the surface. Thermodynamic measurements indicate that alumina in the form of nano-rods on NiAl(110) is far less stable than alumina that forms upon oxidation of aluminum surfaces. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04- 94AL85000.   

Intrinsic vacancy induced nanoscale wire structure in heteroepitaxial Ga$_2$Se$_3$/Si(001)

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga$_2$Se$_3$) on the lattice matched substrate, arsenic-terminated Si(001). This anisotropic, nanowire structure is attributed to surface coalescence of the intrinsic vacancies in $\beta$-Ga$_2$Se$_3$, a defected zinc-blende semiconductor with every third Ga site vacant. Scanning tunneling microscopy of Ga$_2$Se$_3$ films reveals nanoscale, wire-like structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect bulk $\beta$-Ga$_2$Se$_3$, which contains ordered $\langle 110\rangle $ arrays of Ga vacancies. The semiconducting, passivated wires form perpendicular to the underlying As-dimer rows of the Si(001):As substrate, and continue to lie in the same direction, with bilayer height differences, as the film grows thicker.   

Fractal-Mound Growth of Pentacene Thin Films: A Novel Morphology

We investigated the growth mechanism of pentacene film formation on SiO$_{2}$ substrate with a combination of atomic force microscopy measurements and numerical modeling. It is found that the submonolayer islands are diffusion-limited aggregates (DLA). With increased coverage, the Schwoebel barrier effect disrupts the desired epitaxial growth, leading to mound growth. The terraces of the growing mounds have a fractal dimension of 1.6, indicating a lateral DLA shape. This novel growth morphology thus combines horizontal DLA-like growth with vertical mound growth.   

A Quantitative Understanding of Ge Hut Formation on Si(001) Surface from First-principles

Ge growth on Si(001) is characterized by layer-by-layer growth, followed by 3D island formation with a distinct initial island shape of ``hut'' bounded by (105) facets. Despite of extensive studies over the last decade, our understanding of hut formation remains qualitative that it forms by strain relaxation overcoming the cost of increased surface energy but the values of strain energies and surface energies are unknown. We have carried out extensive first-principles calculations to determine, respectively, the surface energies and stresses of Ge/Si (001) and (105) surfaces and their strain dependence as a function of Ge coverage. Using these as inputs to continuum theory, we provide a quantitative analysis of Ge hut formation on Si(001).   

Time Dependent Surface Morphology During Pulsed Laser Deposition of SrTiO$_{3}$

The time evolution of the SrTiO$_{3}$ surface morphology has been studied using time-resolved surface x-ray diffraction during pulsed laser deposition. Measurements made at the $(00\frac{1}{2})$ anti-Bragg position on the crystal truncation rod (CTR) show the intensity oscillations associated with layer by layer growth, and the high count rate available provides information on surface kinetics at the sub-millisecond time scale. Diffuse scattering from the islands measured around the CTR oscillates out of phase with the intensity on the rod demonstrating the growth and filling in of islands through the layer by layer growth cycle. Measurements of the diffuse intensity peak position, which provide a measure of the island size, show the island size varying with both coverage and substrate temperature. The variation of island size with substrate temperature will be discussed in relation to surface mobility and the short and long time evolution of surface morphology.   

Surface roughness systematics in manganite thin films grown by pulsed laser deposition

Surface roughness is a critical quality factor in high quality heterointerface-based devices for oxide electronics. Given the significance of manganite films in the potential spin valve and magnetic tunnel junction type spintronics devices, we have examined the roughness systematics of the corresponding films grown by pulsed laser deposition (PLD) on key substrates such as (001) LaAlO$_{3}$ (LAO) and (001) SrTiO$_{3}$ (STO). Thus films of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ were grown by PLD at different substrate temperatures in the range 600-800 $^{\circ}$C at oxygen pressures in the range 20-200 mTorr. Films with thickness in the range between a few nanometers up to several hundred nanometers were examined for surface roughness (by using Atomic Force Microscopy) as well as various other properties such as crystallinity (x-ray diffraction and Rutherford backscattering channeling), four probe resistivity and magnetization (squid magnetometry). Films thinner than about 20 nm were found to be extremely flat on both the substrates. The surface roughness increase with thickness showed a two step transition-like structure in the case of LAO while a single step structure in the case of the STO substrate. The transport and magnetization properties exhibited a monotonic change with thickness in the case of STO, but a non-monotonic behavior in the case of LAO substrate.   

Session N41: Correlated Electrons: Magnetism in Cobaltates, Ruthenates and Skutterudites

Doping dependence of electron-electron scattering in Na$_x$CoO$_2$

The in-plane resistivity $\rho$ was measured down to 40 mK for a single crystal of Na$_x$CoO$_2$ ($x$ = 0.75), which has spin-density-wave order below $T_c$ = 22 K. We show its Fermi-liquid ground state by observing a $T^2$ dependence of $\rho$ at low temperature, $\Delta \rho = AT^2$. The measured value of coefficient $A$ = 2.60 $\mu \Omega$ cm K$^{-2}$ is about 100 and 3 times of that in Na$_{0.31}$CoO$_2$ and Na$_ {0.70}$CoO$_2$, respectively, indicating an increase of electron-electron scattering upon Na doping. The enormous $A$, and the moderate electron specific heat coefficient $\gamma$, gives an even larger Kadowaki-Woods ratio $A/\gamma^2$ than that previously reported in Na$_{0.70}$CoO$_2$ (Li {\it et al.}, Phys. Rev. Lett. {\bf 93}, 056401 (2004)).   

Magnetic-field-induced Fermi surface reconstruction in Na$_{0.5}$CoO$_2$

We have performed an electrical transport study in Na$_{0.5} $CoO$_{2}$ at high fields $B$ and low temperatures $T$. We find that the charge ordered state observed below $T_{CO}$ = 53 K can be suppressed by large in- plane magnetic fields, but not by fields applied along the inter-plane direction. For $B$ rotating within the conductive CoO$_{2}$ layers we observe angular magnetoresistance oscillations of essentially two-fold periodicity consistent with the reported orthorhombic symmetry of Na$_{0.5} $CoO$_{2}$. As $B$ increases however, a new 6-fold periodicity emerges indicating the stabilization of a hexagonal FS as reported by the ARPES measurements. This observation suggests on the one hand, that the Na superstructure defines the geometry of the FS at low temperatures, and on the other, that the charge order in the conducting plane is suppressed by high in-plane fields. At low temperatures Shubnikov de Haas oscillations (SdH) of very small frequencies are observed for $B$ // c-axis, indicating that almost the entire FS reported for $x$ = 0.6 and 0.7 disappears below $T_{CO}$ for $x$ = 0.5. Our results strongly indicate that the charge ordering involves the coupling with the Na order and involves the large hole pocket.   

Counter-Nagaoka-Thouless Problem On the Triangular Lattice

On the frustrated triangular lattice, the Nagaoka -Thouless theorem predicts ferromagnetism in the case of a single hole in the $U=\infty$ Hubbard model for a particular sign of hopping ( $t>0$), while for the counter case ($t<0$), nothing is known exactly or rigorously. We study the counter case for the triangular lattice motivated by the connection with the physics of sodium cobaltate $Na_x Co O_2$, which realizes this physical situation. We investigate the competition between charge ordered states, N\'{e}el ordered states and spin liquid states, by studying small clusters variationally as well as using exact diagonalization. While the ground state always seems to be a singlet for clusters with an even number of electrons, the full spectrum gives insights into the possibility of long ranged order. Comparing the spin configurations for specific locations of the hole with those of an {\em odd site} Heisenberg model followed by spin projection, enable us to examine as well as evaluate the notion of the charge carriers being solitons or holons in a well prepared spin background.   

Large anisotropy in the paramagnetic susceptibility of SrRuO$_{3}$ films

By using the extraordinary Hall effect in SrRuO$_{3}$ films we performed measurements of the paramagnetic susceptibility in this itinerant ferromagnet, from $T_{c}$ ($\sim 150$ K) to $300$ K. These measurements, combined with measurements of magnetoresistance, reveal that the susceptibility, which is almost isotropic at $300$ K, becomes highly anisotropic as the temperature is lowered, diverging along a single crystallographic direction in the vicinity of $T_{c}$. The results provide a manifestation of the effect of large magnetocrystalline anisotropy in the paramagnetic state of a \textit{4d} itinerant ferromagnet [e-print: cond-mat/0311341]. They also shed light on the controversy regarding the nature of the universality class of the phase transition in SrRuO$_{3}$ [D. Kim \textit{et al}, Phys. Rev. B \textbf{67}, 100406(R) (2003)].   

Comparing the Effects of Varying Carrier Density and Isoelectronic Doping on Magnetic Properties of SrRuO_3

We change the carrier density by introducing Ru vacancies to form SrRu$_{1-v}$O$_3$ and introduce lattice distortions with the substitution of Ca to form Sr$_{1-x}$Ca$_x$RuO$_3$. The hyperfine field falls with increasing $x$ in Sr$_{1-x}$Ca$_x$RuO$_3$ and is less than 1T at x=0.8. For all values of x$<0.8$ the isomer shift and the quadrupole splitting remain unchanged. This fall in the value of the hyperfine field is consistent with what others have found for T$_c$. $T_c$ rapidly drops with increasing number of Ru vacancies so that T$_c$=45K when $v$=0.12 suggesting that carrier density has a stronger effect on T$_c$ than lattice distortions. The hyperfine field remains unchanged which may be explained by assuming that the polarization of the s-electrons responsible for the hyperfine field is due to localized d-electrons which give rise to the local moment present above T$_c$.   

Magnetic and Transport Study of Single Crystal SrRu$_{1-x}$Cr$_{x}$O$_{3}$ with Increased Curie Temperature

We report results of a structural, magnetic and transport study of single crystal SrRu$_{1-x}$Cr$_{x}$O$_{3}$ (0$\le $ x$<$0.15), i.e., Cr doped SrRuO$_{3}$. Unlike other impurity doping (such as Ca, Mn, Fe), the Cr doping systematically enhances the Curie temperature from Tc=165 K for x=0 to Tc=190 K for x=0.13. In the meantime, the Cr doping also significantly increases the saturation moment from 1.1 for x=0 to as large as 1.5 Bohr magneton/Ru for x$>$0. The system stays itinerant for the entire doping range with resistivity showing the Fisher-Langer behavior at Tc. Magnetic anisotropy and negative magnetoresistance are also observed. All results will be presented and discussed along with comparisons with other related systems.   

Magnetic ordering transition in the spin-gap compound Sr$_2$Cu(BO$_3$)$_2$

Sr$_2$Cu(BO$_3$)$_2$ is a weakly coupled dimer compound with an accessible spin gap between the singlet ground state and the lowest triplet excited state. The critical field (H$_{c} \simeq 56T$) above which the compound magnetically orders is significantly smaller than the intradimer coupling (J$ \simeq 100K$), indicating triplet delocalisation due to the effect of interdimer exchange. Here, we present results of anisotropic high field magnetisation measurements that reveal the effects of Dzyaloshinskii-Moriya exchange interactions in determining the nature of the ordered state for H $>$ H$_c$. We also present results that demonstrate how chemical pressure (the substitution of Ba for Sr) can be used to tune the spin gap in this material.   

Oxygen Anneal Effect on Single Crystalline Sr$_2$RhO$4-x$

Two dimensional perovskite-type transition metal oxides provide rich issues which originate from strong electron-electron correlation such as a spin triplet superconductivity in Sr2RuO4, quantum criticality at around the metamagnetic transition in Sr3Ru2O7 and antiferromagnetic metal phase in Ca3Ru2O7. We regard Rh oxides as another intriguing materials because of the similarity. Especially, Sr2RhO4 has been studied using single crystals grown by a floating-zone method. The importance of the oxygen content in Sr2RhO4 will be discussed based upon the results of electrical resistivity, magnetic susceptibility and specific heat.   

Magnetic Susceptibility, Electrical Resistivity, and Specific Heat Measurements of the filled Skutterudite PrOs$_4$As$_{12}$

Single crystals of PrOs$_{4}$As$_{12}$ were grown by a flux method. Measurements on single crystals of PrOs$_{4}$As$_{12}$ reveal antiferromagnetic ordering at a N\'{e}el temperature near 2.2 K. Low-temperature electrical resistivity, $\rho $(T), measurements indicate that T$_{N}$ is completely suppressed by fields of 1.5 T, while ac magnetic susceptibility and magnetization measurements reveal a metamagnetic transition at temperatures near the N\'{e}el temperature for low fields. High temperature Curie-Weiss fits yield an effective moment, $\mu _{eff}$ = 3.8 $\mu _{B}$, higher than the Pr$^{3+}$ free ion value $\mu $ = 3.58 $\mu _{B}$ derived from by Hund's rules. Fits to specific heat data above T$_{N}$ reveal a moderately enhanced electronic specific heat coefficient, $\gamma \quad \sim $ 100 mJ/mol K$^{2}$. The temperature dependence of $\rho $ and the negative magnetoresistance are indicative of Kondo lattice behavior. Research at UCSD supported by the U.S. DOE and NSF.   

Temperature dependence of crystal field transitions in PrOs$_4$Sb$_{12}$

PrOs$_4$Sb$_{12}$ has attracted considerable attention as a heavy fermion superconductor, in which quadrupolar fluctuations play an important role in the pairing mechanism. Recently, we determined the crystal field potential using inelastic neutron scattering [Goremychkin \textit{et al} Phys. Rev. Lett. \textbf{93}, 157003 (2004)], and determined that the ground state is a a $\Gamma_1$ singlet. Although this does not favor quadrupolar Kondo models of the superconducting pairing, we have proposed that inelastic quadrupolar, or aspherical Coulomb, scattering of the conduction electrons by the 4f CF levels plays a significant role in enhancing T$_c$ compared to the isoelectronic lanthanum compound. Comparison with the crystal field level scheme in PrRu$_4$Sb$_{12}$ [Goremychkin \textit{et al} Physica B, in press], where T$_c$ has been reduced by praseodymium substitution, is consistent with this model. We report on recent high resolution neutron measurements of the temperature dependence of the transition to the $\Gamma_5$ triplet at 0.63 meV, whose linewidth provides a probe of \textit{s-f} interactions.   

Ordered Magnetic State in PrFe$_{4}$Sb$_{12}$ Single Crystals

Single crystals of the filled skutterudite compound PrFe$_{4}$Sb$_{12}$ were prepared and characterized via X-ray and neutron diffraction, specific heat, electrical resistivity, and magnetization measurements. Long range magnetic ordering occurs at $T_{\mathrm{c}} \approx 4.1$~K. The magnetic structure consists of ordered moments on both Pr and Fe sites and may be ferrimagnetic. The electrical resistivity exhibits a very weak dependence on both applied magnetic field and pressure. Specific heat measurements indicate an enhanced effective mass. This work was supported by the DOE, NSF, and NNSA through SSAA. NHMFL is supported by the DOE, NSF, and State of Florida.   

Impurity and magnetic field investigation of PrOs$_{4}$Sb$_{12}$

\\ We have performed a low level doping study of PrOs$_{4}$Sb$_{12}$. Samples with small amounts of La and Nd substituted for Pr were investigated by magnetic susceptibility and specific heat in magnetic fields to 14 T. The goal of this study was to search for correlations between superconductivity, effective electron mass, crystalline electric fields, and field-induced antiferro-quadropolar order. In agreement with our previous study $^{1}$, we find T$_{c}$ to be only weakly affected by the alloying while other characteristics, such as the specific heat discontinuity at T$_{c}$ and the width of the transition, show a dramatic dependence on the concentration of impurities. Results will be discussed in relation to considered models of superconductivity and heavy fermion behavior. Supported by Department of Energy, grant No. DE-FG02-99ER45748. \\ \\ $^{1}$ C. R. Rotundu, P. Kumar, B. Andraka, cond-mat/0402599   

Large Magnetoelastic Coupling in Pr$_{2}$CuO$_{4}$ Single Crystals

The in-plane thermal conductivity, $\kappa _{a}$, of insulating Pr$_{2}$CuO$_{4}$ was measured at low temperatures and in magnetic fields of up to 8 Tesla. At 5 K and 8 Tesla $\kappa _{a}$ increases by 50{\%} for H // \textbf{a}, and 300{\%} for H // \textbf{c}, relative to its value in zero magnetic field. This increase is most likely due to the ability of the magnetic field to mix the non-magnetic ground state of the Pr 4f crystal field level with the first excited crystal field level at 18 meV. The magnetic field produces a substantial magnetic moment at each Pr site as well as a significant change in the 4$f$ charge distribution. Either effect could significantly alter the heat conducted by acoustic phonons. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725.   

Thermodynamic Properties of Pr3RuO7

We have measured the thermal and magnetic properties of single crystals of Pr$_{3}$RuO$_{7}$. The magnetic measurements indicate an antiferromagnetic transition with spins aligned along the RuO chains (i.e. c-axis) at T$_{N}$ = 54 K at which there is a large step anomaly in the specific heat, $\Delta $c $\sim $ R, but the estimated entropy change is small, $\Delta $s $\sim $ R/10, suggesting only incomplete spin ordering in zero field. On the other hand, there is a metamagnetic transition at low temperature at B$_{c} \quad \sim $ 3T with a large saturated moment of $\sim $ 5 $\mu _{B}$. Surprisingly, there is a large linear contribution to the specific heat at low temperatures, $\gamma \quad \sim $ 0.16 J/mol{\textbullet}K$^{2}$ and large temperature independent contribution to the susceptibility, with a Wilson ratio $\chi _{0}$/$\gamma \quad >$ 1, suggesting that, despite its insulating behavior, there is a finite density of states of strongly correlated electrons at the Fermi surface.   

Magnetic properties of iron doped magneto-electric LiNiPO$_4$ single crystals

Neutron scattering and magnetic susceptibility studies of pure and iron-doped magneto-electric LiNiPO$_4$ single crystals are compared. Elastic neutron scattering of single-crystal LiNiPO$_4$ reveal a spontaneous first-order commensurate- incommensurate magnetic phase transition (at $T_{C-IC}$ = 20.8 K) accompanied by a second transition ($T_{IC}$ = 21.7 K) from long- to short-range IC structure. These transitions are also identified in the magnetic susceptibility measurements. The modulated structure has a predominant antiferromagnetic component giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic peak, and a ferromagnetic component giving rise to peaks at small momentum transfers around the origin $(0,\pm Q,0)$. The wavelength of the modulated magnetic structure varies continuously with temperature. Magnetic susceptibilities of the iron doped crystals show magentic-field dependent properties characteristic of spin-glass systems.   

Session N42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures VI

Magnetic Properties of Chemically Synthesized FePt Nanoparticles

Chemically synthesized FePt nanoparticles have attracted considerable attention in recent years because of their potential use in ultra-high density magnetic recording media. In the original procedure described by Sun et al., the as-synthesized nanoparticles have the fcc phase and must be thermally annealed to achieve the high-anisotropy L1$_{0}$ phase [1]. We have been addressing some of the materials problems associated with obtaining the L1$_{0}$ phase. These include lowering the ordering temperature, reducing sintering during annealing, orienting the easy axes, and understanding the size effect on chemical ordering. Additive Au and Ag significantly lower the ordering temperature, while additive Cr and Cu increase the ordering temperature; however, the onset of ordering is correlated with sintered grain growth. Sintering can be reduced by encapsulating the nanoparticles with a shell such as silicon oxide or copper. Easy-axis orientation has been achieved using L1$_{0}$ FePt nanoparticles that were directly synthesized using a high-temperature solvent [2]. The nanoparticles were dispersed in a PVC binder and oriented by drying the dispersion in a magnetic field. [1] S. Sun et al., Science 287, 1989 (2000). [2] S. Kang et al., Appl Phys. Lett. (in press).   

Magnetic Properties of Core-Shell FePt(CFx) Nanocluster Films

A core-shell FePt nanocluster system, in which the magnetic core is coated with a layer of a non-magnetic shell, is of great interest for study and tailoring magnetic properties such as magnetization, anisotropy and interparticle interactions. In this study core-shell FePt clusters with fluorocarbon (CF$_{x})$ shell are synthesized by a cluster-deposition system with gas-aggregation technique. Monodispersed core-shell structure FePt(CF$_{x}) \quad _{ }$clusters are produced with average diameter of 4 nm and with a uniform size distribution. High magnetic anisotropy L1$_{0}$ phase FePt(CF$_{x})$ cluster-assembled films were realized via post-deposition annealing. Crystal structure and nanostructure of the films were studied by XRD and TEM. Magnetic properties of the films were measured at temperatures between 10 K and 300 K. Results show that the FePt L1$_{0}$ ordering temperature is decreased by addition of CF$_{x}$.$_{ }$Interparticle interactions were studied by measuring the $\Delta $M curves. Thermal stability of the films was also studied by fitting the temperature dependence of coercivity with the Sharrock formula. Our results indicate that the magnetic properties of the core-shell FePt(CF$_{x})$ nanoclusters are tunable for various nanomagnetic applications.   

Magnetic and Transport Properties of Co Nanoparticles Fabricated With a Cluster Gun

Cluster guns have been found to be suitable for the fabrication of nanoparticles in a wide range of materials with the additional advantage of in-situ processing (annealing, surface passivation, etc.) of the nanoparticles inside the sputtering chamber [1, 2]. In this study, we look to optimize parameters for fabricating Co nanoparticles and embed them in a carbon matrix. Magnetic and transport properties are measured over a wide temperature range. The size and distribution of the nanoparticles can be controlled by varying the target-orifice distance, Argon pressure, sputtering time and Co magnetron power. At the lowest power used the Co nanoparticles are less than 5nm in size. At this size thermo-magnetic measurements indicate a blocking temperature of 115 K indicative of superparamagnetism in this sample. As the power is increased there is an increase of the blocking temperature commensurate with the increase in nanoparticle size as seen in bright field electron microscopy. The transport studies of these samples show a cross-over from metallic to semi-conducting behavior as the inter-particle spacing is varied. The origin of the cross-over is under investigation and the results will be reported. [1] Stoyanov S, et al., J. Appl. Phys., 93 (10): 7190 (2003). [2] Skumryev V, et al., Nature, 423 (6942): 850 (2003).   

High coercivity in FePt nanoparticle assemblies

Ultra-fine FePt nanoparticles have been synthesized via a novel chemical solution synthesis route. Without using a reducing agent, the stoichiometric FePt nanoparticles were produced by the decomposition of iron acetylacetonate and platinum acetylacetonate in octyl ether in the presence of oleic acid and oleyl amine. The particle size was found by transmission electronic microscopy observation to be around 2 nm. The particles were then deposited on a substrate to form thin-film-like assemblies and undertaken heat treatments. Upon annealing the as-synthesized nanoparticles were expected to transform from FCC structure to the high anisotropic FCT structure and therefore magnetic hardening was developed in the assemblies. Coercivity up to 2.7 T has been obtained in the samples with the Fe:Pt molar ratio of 1.2:1 after being annealed at 650\r{ }C for 1 hour in forming gas (Ar + 7{\%} H$_{2})$. The high coercivity indicates a highly completed phase transition from the FCC structure to the FCT structure.   

Magnetism of Pd7Ni3 nano alloy particles

The magnetic properties of Pd$_{7}$Ni$_{3}$ alloy nano-particles, $\sim $10 nm diameter, prepared from chemical precipitation followed by reduction reaction are reported. Magnetic palladium alloys is interesting for the enhanced magnetic moments due to Ni doping into Pd, ordinarily paramagnetic, with ferromagnetic transition temperature T$_{C}$, for bulk, of about 327 K. While prepared in the nano size, depending on preparation procedures and dispersity, samples exhibit super-paramagnetism, spin-glass-like or even Curie-Weiss-like behaviors. However, enhancement of the magnetic moment remains with an enhancement magnitude of about 0.3 $\mu _{B}$ smaller and is preparation method dependent. The complicated magnetic behavior observed may suggest a surface spin effect. Possible core-shell magnetic structure for particles of such small diameter along with the chemically observed core-shell nonuniformity by EXAFS further complicated the observed magnetic behavior.   

Magnetic Properties of CoFe2O4 Nanopillars

Ferrimagnetic CoFe$_{2}$O$_{4}$ spontaneously forms nanopillars embedded in a BaTiO$_{3}$ or BiFeO$_{3}$ matrix during thin film growth by pulsed laser deposition. Such thin film nanostructures show three dimensional heteroepitaxy. All the films have a large uniaxial magnetic anisotropy with an easy axis normal to the film plane. It is calculated that stress anisotropy is the main contribution to the anisotropy field. We studied the magnetic behavior of the CoFe$_{2}$O$_{4}$ nanopillars formed at different growth temperatures, with different film thickness and on various substrates.   

Magnetic Oxide nanoparticles

We have fabricated nanopillar arrays of epitaxial magnetic oxide thin films and heterostructures consisted of SrRuO$_{3}$, La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ and insulating barrier. The films were grown on TiO$_{2}$ surface terminated (001) SrTiO$_{3}$ substrates with atomic layer control by pulsed laser deposition with in situ high pressure RHEED, and were patterned into nanopillars using e-beam lithography and neutralized Ar ion milling with Ti and Au as milling mask materials. Scanning electron and atomic force microscopy measurements confirmed that we have produced well defined diameter 100 nm and 40 nm tall pillar arrays, which are, to our knowledge, the smallest pillars made from magnetic perovskite oxides. The LSMO pillars whose dimensions are smaller than the domain size ($\sim $150nm) and comparable to the exchange length ($\sim $50nm) are ferromagnetic at room temperature as shown by magnetic force microscopy. Using multilevel e-beam lithography we made single nano-ellipses from LSMO and SRO and wired them individually with Au leads. We performed electron transport measurements at 5K aiming to measure anisotropic magnetoresistance and coercive fields of single nano-ellipses ranging in size from 850 x 400 nm$^{2}$ to 400 x 150nm$^{2}$. Supported by NSF-ECS 0210449.   

Volume Dependence of Thermoinduced Magnetization in Antiferromagnetic Nanoparticles

Monte Carlo methods have been applied to classical Heisenberg models to study the thermoinduced magnetization in nanoparticles of antiferromagnetic materials. In the presence of uniaxial anisotropy, the average magnetization per spin of the individual particles is found to decrease as approximately the square-root of the particle volume, $M \sim V^{-1/2}$, which is significantly different from the $M \sim V^{-1}$ predicted by a recent theory [S. M{\o}rup and C. Frandsen, Phys.\ Rev.\ Lett.\ {\bf 92}, 217201 (2004)]. The exact value of the exponent depends on the strength of the uniaxial anisotropy, and approaches $-1/2$ from below as the strength of the anisotropy increases. In addition, the magnitude of the thermoinduced magnetization decreases as the strength of the anisotropy increases, and it vanishes in the infinite anisotropy, i.e. Ising, limit. This indicates that spin-waves are essential to thermoinduced magnetization.   

Anomalous Temperature Dependence of Magnetic Moment in Monodisperse Antiferromagnetic Nanoparticles

1 Condensed Matter Sciences Division, Oak Ridge National Laboratory*, TN 37831 2 Department of Physics and Astronomy, The University of Tennessee, TN 37996 3 Environmental Sciences Division, Oak Ridge National Laboratory*, TN 37831 Recent experiments [1] and theory [2] from AFM nanoparticles showed that they exhibit sizable net magnetization, which increases with increasing temperature. In order to further understand such peculiar temperature dependence, we have measured the magnetic properties of monodisperse hematite ($\alpha$-Fe2O3) nanoparticles, grown using a microemulsion precipitation technique, which minimizes the impact of the particle moment distribution on the measured properties of the samples. Our measured results indicate that the net magnetization of these nanoparticles, when small, indeed increases linearly with increasing temperature. This is in sharp contrast to the bulk-like behavior of $\alpha$-Fe2O3, which was observed in particles with size larger than 120 nm. [1] M. Seehra et al, Phys. Rev. B 61, 3513 (2000) [2] S. Mørup, C. Frandsen, Phys. Rev. Lett. 92, 217201 (2004) *Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725   

Tailoring magnetic properties of Co-ferrite soft magnetic nanoparticles

Monodisperse Co-ferrite soft magnetic nanoparticles with particle size from 3 nm to 20 nm and different Co concentration have been synthesized by chemical solution methods. The composition was controlled by varying the mole ratios of the precursors in the solution. It has been found that magnetic properties of the nanoparticles can be tailored by changing the composition, particle size, as well as by subsequent heat treatments. Magnetization value of 223 emu/g was obtained after reduction. Particles with size less than 12 nm showed ferromagnetism-superparamagnetism transitions at temperatures between 10 K and 300K. The magnetic interaction of the nanoparticles was studied by zero-field-cooling and field-cooling experiments. The blocking temperature of the nanoparticles was found to increase with increasing particle size. The 4 nm and lesser sized particles showed exchange bias at 10 K. A coercivity value of 1.8 T was obtained at 10 K for the 20 nm particles.   

Magnetocaloric effect in ferrite nanoparticles

Miniaturization of the electronic devices for space, military and consumer applications requires cooling devices to be fabricated on a chip for power efficient, noise-free operations. Refrigeration based on the adiabatic-demagnetization has been used for several decades for cooling down to sub-kelvin temperatures. Superparamagnetic particles also hold tremendous potential towards this application. We have studied magnetocaloric effect (MCE) properties in chemically synthesized ferrite nanoparticles over a broad range in temperature and magnetic fields. Nanoparticles investigated include Fe$_{3}$O$_{4}$ (average size = 8 nm, synthesized using co-precipitation method), MnZnFe$_{2}$O$_{4}$ (average size = 15 nm, synthesized using reverse-micelle technique) and CoFe$_{2}$O$_{4}$ (average size 8 nm, synthesized using pyrolectic technique). The magnetic entropy change was calculated by applying Maxwell's relations to magnetization vs magnetic field curves at various temperatures. Our results indicate that the single-domain particles in their superparamagnetic state show a considerable entropy change near the blocking temperature. The influence of interactions on MCE effect will also be discussed. Work supported by NSF through Grant No. CTS-0408933   

Magnetic characterization of nano-sized iron oxide particles embedded in alginate hydrogel

We have examined the magnetic properties of a nanoparticle system (gamma-Fe2O3 alginate nanocomposite) prepared using eight iterations of a standard matrix-mediated precipitation reaction. Earlier measurements on the magnetic properties of lower generations of this system showed sensitivity to the number of iterations of the reaction[1]. We used DC magnetization measurements to determine the temperature and field dependence of the blocking temperature, saturation magnetization, and the coercive field. We find a low-field blocking temperature T$_B$=72K, higher than the lower generation samples. We probed the blocking dynamics of the system by measuring the temperature and frequency dependent complex AC susceptibility. These measurements yield a zero-field activation energy of approximately 2770K, which yields important information about the magnetic anisotropy in these systems. [1] R. Naik et al, J. Appl. Phys. (accepted for publication)   

Reduction of Ordering Temperature in Substituted FePtM (M = Ni,Cu) Nanoparticles Formed by Chemical Synthesis

FePt and CoPt-type nanoparticles made by chemical synthesis have recently become promising candidates for ultra-high density magnetic recording media [1]. However, at high annealing temperatures, the particles sinter together and array formation is lost. Recent studies [2] have reported reduced transformation temperatures with the addition of Ag and Au in the FePt nanoparticles. In this study, we used M = Ni, Cu substitution to reduce the transformation temperature. The as-made FePtNi and FePtCu have the disordered fcc structure with zero coercivity at room temperature. After annealing at temperatures in the range of 300-600 $^{o}$C, the particles become fct with a coercivity of 4 kOe in FePtCu at 400 $^{o}$C and 6 kOe in FePtNi at 500 $^{o}$C. The high coercivity obtained at the lower annealing temperature suggests a lower transformation temperature in both alloys. Preliminary DSC studies showed a reduced transformation temperature in the Ni substituted samples. The structural transformations that occur after annealing and their effects on magnetic properties are currently being investigated by HREM and M\"{o}ssbausser spectroscopy. References [1] S. Sun, et al. Science, 287, 1989-1992 (2000) [2] S. Kang, et al., IEEE T MAGN, 39, 5, 2753 (2003)   

Session N43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides II

The Relationship between Local Structure Changes and Magnetization as a Function of Hole Concentration in Doped Perovskite Manganites

We present X-ray Absorption Fine Structure (XAFS) measurements on several perovskite manganite samples, La$_{1-x}$Ca$_{x}$MnO$_{3}$, in the CMR region (0.2 $\le \quad x \quad \ge $ 0.5) as a function of temperature and applied magnetic field. These results indicate that polaron-induced changes in the local structure depend only on magnetization for a given sample, irrespective of whether the sample magnetization, M, is achieved through a decrease in temperature or an applied magnetic field. Furthermore, the relationship between changes in local structure and magnetization is clearly a function of hole concentration, $x$, demonstrated by a change in slope at roughly $\textstyle{M \over {M_0 }}\approx 2x$ (M$_{0}$ is the saturation magnetization at low T). These results lead to a proposed model for the magnetization process, in which the magnetization initially develops via Mn pairs throughout the sample. This model is similar to the cluster and phase separated models proposed by others but the clusters are at the nanoscale. NSF DMR0301971.   

Complex interactions between structural, magnetic and electronic properties of epitaxial thin films of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

The bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ resides within the Ruddlesden-Popper (RP) family of materials and consists of interleaved blocks of two metallic/ferromagnetic (La,Sr)MnO$_{3}$ layers and one insulating (La,Sr)O layer. We have grown epitaxial thin films on (110)-oriented SrTiO$_{3}$ substrates by pulsed laser deposition with the $c$-axis aligned in the plane of the film. These in-plane aligned films are a model system for probing the anisotropic magnetic and electronic properties of this bilayer manganite. The films display similar properties to their bulk counterparts with the easy direction lying within the $a-b$ planes and coincident metal/insulator and ferromagnet/paramagnet transitions occurring at a suppressed $T_{c}\sim $90K (120K for bulk). While the magnetic properties are robust to the presence of defects, the electronic properties are highly sensitive to these defects, which include amorphous regions, antiphase boundaries and trace amounts of other RP phases.   

Anisotropic Local Distortion of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ Through the Ferromagnetic Transition Temperature

In previous temperature-dependent EXAFS studies of the quasi-cubic manganite, La$_{1-x}$Ca$_{x}$MnO$_{3}$, excess broadening of the Mn-O pair-distribution function has been observed near and above the Curie temperature, T$_{c}$=120, attributed to the appearance of Jahn-Teller polarons. A similar, yet anisotropic, broadening is expected in the bilayer system, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$. We report a temperature-dependent, polarized EXAFS study at the Mn-K edge for the x=0.4 sample. This analysis shows similar broadening for Mn-O pairs in both the ab-plane and the c-axis, but the magnitude of the distortion in the ab-plane is about 4 times greater than along the c-axis. The data also show a small excess of broadening below T$_{c }$ as well, indicating that polaron distortions are present even at 100K where the magnetization becomes constant. Analysis of further Mn neighbors is currently underway. Details of the analysis and a discussion of the implications for CMR in the bilayer manganites will be presented. Support: NSF DMR 0301971.   

Multi-scale Phase Competition and Coexistence in Strongly Correlated Materials

The intriguing interplay between charge/spin/lattice/orbital degrees of freedom gives rise to various competing phases as ground states in strongly correlated materials such as manganites. The delicate balance between these ground states leads to new metastable ground state which is characterized by the coexistence of phases with distinct electronic/magnetic ground state which is characterized by the coexistence of phases with electronic/magnetic properties such as the insulating charge ordered phase and the metallic ferromagnetic phase. The length scale of the phase separated system was found to vary from micrometer to nanometer in various manganites and under different situation. The CMR is closely related to the coexistence of the phase separated inhomogenieties. Lattice strain apparently plays a very significant role affecting the length scale of these phase separated systems. Nanoscale phase separation is often observed at temperatures above the characteristic temperature associated with formation of each particular phase. For example, we have observed the formation of nano-scale ferromagnetic droplets in a well defined temperature window preceding the long-range ferromagnetic ordering. The complexity of the electronic phase separation in different length scales will be discussed. This work is now doing in collaboration with C.H.Chen, S-W.Cheong, T.Asaka, Y.Horibe, Y.Matsui, T.Katsufuji, M.Uehara, Y.Moritomo, H.Kuwahara and their team members.   

Epitaxial strain and phase separation in La$_{0.7}$Ca$_{0.3}$MnO$_3$ manganite

Epitaxial strain has been explored as a method of tailoring structural distortions to examine their influence on physical properties of thin films of colossal magnetoresistance manganites. However depressed saturation magnetizations have been found in nanometer thick samples both for tensile and compressive in plane strains, and a clear picture of the effect of strain on the phase separation picture has not emerged yet. In this paper we present results of ultrathin La$_{0.67}$Ca$_{0.33}$MnO$_{3 }$films grown on SrLaAlO$_{4}$ (a=0.375 nm) under high in plane compressive strain (-3.1 {\%}). We show 2D dimensional epitaxial growth below the critical thickness what allows exploring the effect of lattice distortions on the PS in highly ordered films with thickness in the 2 - 6.5 nm range. Aberration corrected scanning transmission electron microscopy (STEM) with atomic resolution combined with energy loss spectroscopy are used to show evidence for strain induced nanoscale phase separation.   

Current images and anomalous transport properties in phase-separated manganites

We have observed inhomogeneous current distributions in the phase-separated manganite crystals by using magneto-optical imaging technique. Increase of the current causes a change of conduction paths from inhomogeneous to homogeneous concomitantly with sharp increase in resistivity. Using this anomalous current-voltage relation, we demonstrate the possibility of low- field colossal magnetoresistance effects. In addition, we found oscillations of the current at a constant voltage condition. These anomalous transport properties are reasonably explained by taking into account the effects of Joule heating.   

Charge, orbital and magnetic ordering in the bandwidth-control manganites $Pr_0.55(Ca_{1-y}Sr_{y})_0.45MnO_3$ ($y=0.15,0.20$)

$\rm Pr_{0.55}(Ca_{1-y}Sr_{y})_{0.45}MnO_3$ has been reported to display bicritical features near y=0.25. We have utilized neutron and synchrotron x-ray scattering techniques to study the interactions between FM, AFM and charge/orbital ordering (CO-OO) for samples with y=0.15 and y=0.20 at H=0 and in a magnetic field up to 7 Tesla. In the absence of magnetic field, both samples display insulating AFM ordered ground states. The CO-OO shows an incommensurate-commensurate transition as the temperature is lowered and the correlation lengths of the CO-OO remain much shorter than those for the AFM ordering at all temperatures. With the application of a magnetic field there is a sharp first-order like transition from the CO-OO insulating phase to a metallic ferromagnetic state. This work was supported by the US DOE under Contract No. DE-AC05- 00OR22725 with UT-Batelle, LLC and by U.S. NSF DMR-0139882.   

Strain-induced metal-insulator phase coexistence in perovskite manganites

The observed coexistence of distinct metallic and insulating electronic phases within the same sample of a perovskite manganite, such as La$_{1-x-y} $Pr$_y$Ca$_x$MnO$_3$, has been a puzzle to both theorists and experimentalists. In particular, colossal magnetoresistance in these materials is considered to be closely related to the texture owing to nanometer- and micrometer-scale heterogeneities. In this talk, we show that such texturing can be due to the intrinsic complexity of a system with strong coupling between the electronic and elastic degrees of freedom. More specifically, we demonstrate, using an atomic scale description of lattice distortions, that the presence of multiple local energy minimum states with different lattice distortions and different electronic properties, and the long-range interaction between strain fields provide a natural mechanism for such self-organized multiphase coexistence within the same material. This framework provides a basis for engineering nanoscale patterns of metallic and insulating phases, and for understanding other novel features observed in manganites, such as: precursor short range ordering and quasielastic scattering near the phase transition temperature; hysteretic and glassy dynamics; metastability; and photoinduced insulator-metal transition.   

Reentrant Charge Ordering In Manganites as Experimental Evidence for a ``Strain Glass"

A reentrant charge-ordering transition occurs within the micron scale phase separated manganite (La,Pr)$_{5/8}$Ca$_{3/8}$MnO$_{3}$. This low temperature state, in which charge-ordered and ferromagnetic-metallic phases coexist, accompanies spin glass-like magnetism. Furthermore, thermal conductivity measurements reveal an irreversibility characteristic of a freezing transition in the lattice degrees of freedom, strongly suggesting the presence of inhomogeneous long-range strain. Our results point to a unique phase transition from a ``strain liquid'' to a ``strain glass'' state where phase-separated regions strongly interact via martensitic accommodation strain resulting in a cooperative freezing of the combined charge/spin/strain degrees of freedom.   

Charge ordering structure and Mn valence in La0.33Ca0.67MnO3

Charge ordering (CO) and its structure is one of the long standing questions about manganites. Previously, X-ray, neutron and electron diffraction had been used to explore the crystallographic structure in charge ordered La1-xCaxMnO3 [2-3]. Two possible models, ``Wigner-crystal'' model and ``Bi-stripe'' model of the CO structure were proposed and supported by different evidences. Convergent beam electron diffraction (CBED) has unique capability on crystallographic symmetry determination and for probing crystal charge density [1]. The CBED patterns are sensitive to the charge states of atoms. The experimental CBED patterns show clearly the symmetry against the ``Bi-stripe'' model. The experimental CBED patterns are further compared with simulation using different charge valence models. The results show a surprisingly small difference between Mn(3+) and Mn(4+). The significance of this work will be discussed. Reference: [1] J. M. Zuo, Rep. Prog. Phys, 67, 2053 (2004) [2] P. G. Radaelli et al., Phys. Rev. B 55, 3015 (1997) [3] S. Mori, C. H. Chen, and S.-W. Cheong, Nature 392, 473 (1998)   

Energetics of reversible and non-reversible charge disordering effect in manganites induced by external beam irradiation

The reversible electron beam-induced melting and reentrant behavior of charge-ordered state in (Bi,Ca)MnO$_{3 }$phenomenon is in sharp contrast with the non-reversible effect in (Pr,Ca)MnO$_{3}$ observed under similar experimental conditions. In the Pr-manganites the charge disordering effect depends upon the total energy deposited on the sample by the incident beam, whereas in the Bi-manganites the reversible effect is a function of the power input from the incident beam. The energetics of the electron beam-induced effect in these two systems can be understood in the frame work of lifetime of the charge disordered clusters induced by the external irradiation. .   

Session N44: Session on Refereeing

Session on Refereeing

Representative editors from Physical Review Letters and the Physical Review will provide useful information and tips for referees. The information presented will be relevant to anyone who has recently been asked to referee for a Physical Review journal, or who would like to add to their knowledge and experience of the refereeing process. It will also be of interest to authors who want to know more about the referee reports they receive. Topics we will cover include: (1) how to write a good referee report, (2) the differences between reports for PRL and the PR journals, (3) the role of the referee in the review process, (4) how to submit a referee report, (5) how to use the referee web interface, etc. Following the short presentations from the PRL and PR editors, there will be a moderated discussion where you can ask questions relevant to refereeing.