bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunable vibrational band gaps in one-dimensional diatomic granular crystals with three-particle unit cells
Authors: Boechler N. , Yang J. , Theocharis G. , Kevrekidis P. G. , Daraio C.
Published: April 2011

We investigate the tunable vibration filtering properties of statically compressed one-dimensional diatomic granular crystals composed of arrays of stainless steel spheres and cylinders interacting via Hertzian contact. The arrays consist of periodically repeated three-particle unit cells (sphere-cylinder-sphere) in which the length of the cylinder is varied systematically. We investigate the response of these granular crystals, given small amplitude dynamic displacements relative to those due to the static compression, and characterize their linear frequency spectrum. We find good agreement between theoretical dispersion relation analysis (for infinite systems), state-space analysis (for finite systems), and experiments. We report the observation of three distinct pass bands separated by two finite band gaps, and show their tunability for variations in cylinder length and static compression.
High surface area electrodes in ionic polymer transducers: Numerical and experimental investigations of the electro-chemical behavior
Authors: Akle Barbar J. , Habchi Wassim , Wallmersperger Thomas , Akle Etienne J. , Leo Donald J.
Published: April 2011

Ionomeric polymer transducer (IPT) is an electroactive polymer that has received considerable attention due to its ability to generate large bending strain (>5%) and moderate stress at low applied voltages (±2 V). Ionic polymer transducers consist of an ionomer, usually Nafion, sandwiched between two electrically conductive electrodes. A novel fabrication technique denoted as the direct assembly process (DAP) enabled controlled electrode architecture in ionic polymer transducers. A DAP built transducer consists of two high surface area electrodes made of electrically conducting particles uniformly distributed in an ionomer matrix sandwiching an ionomer membrane. The purpose of this paper is to investigate and simulate the effect of these high surface area particles on the electro-chemical response of an IPT. Theoretical investigations as well as experimental verifications are performed. The model used consists of a convection-diffusion equation describing the chemical field as well as a Poisson equation describing the electrical field. The two-dimensional model incorporates highly conductive particles randomly distributed in the electrode area. Traditionally, these kinds of electrodes were simulated with boundary conditions representing flat electrodes with a large dielectric permittivity at the polymer boundary. This model enables the design of electrodes with complicated geometrical patterns. In the experimental section, several transducers are fabricated using the DAP process on Nafion 117 membranes. The architecture of the high surface area electrodes in these samples is varied. The concentration of the high surface area RuO2 particles is varied from 30 vol% up to 60 vol% at a fixed thickness of 30 μm, while the overall thickness of the electrode is varied from 10 μm up to 40 μm at a fixed concentration of 45%. The flux and charge accumulation in the materials are measured experimentally and compared to the results of the numerical simulations. Trends of the experimental and numerical investigations are in agreement, while the computational capacity is limiting the ability to add sufficient amount of metal particle to the electrode in order to match the magnitudes.
Monohydrates of cuprous chloride and argentous chloride: H2O⋅⋅⋅CuCl and H2O⋅⋅⋅AgCl characterized by rotational spectroscopy and ab initio calculations
Authors: Mikhailov Victor A. , Roberts Felicity J. , Stephens Susanna L. , Harris Stephanie J. , Tew David P. , Harvey Jeremy N. , Walker Nicholas R. , Legon Anthony C.
Published: April 2011

Pure rotational spectra of the ground vibrational states of ten isotopologues of each of H2O⋅⋅⋅CuCl and H2O⋅⋅⋅AgCl have been measured and analyzed to determine rotational constants and hyperfine coupling constants for each molecule. The molecular structure and spectroscopic parameters determined from the experimental data are presented alongside the results of calculations at the CCSD(T) level. Both experiment and theory are consistent with structures that are nonplanar at equilibrium. The heavy atoms are collinear while the local C2 axis of the water molecule intersects the axis defined by the heavy atoms at an angle, ϕ = 40.9(13)° for Cu and ϕ = 37.4(16)° for Ag. In the zero-point state, each molecule is effectively planar, undergoing rapid inversion between two equivalent structures where ϕ has equal magnitude but opposite sign. The equilibrium geometry has Cs symmetry, however. The ab initio calculations confirm that the timescale of this inversion is at least an order of magnitude faster than that of rotation of the molecule in the lowest rotational energy levels. The molecular geometries are rationalized using simple rules that invoke the electrostatic interactions within the complexes. Centrifugal distortion constants, ΔJ and ΔJK, nuclear quadrupole coupling constants, χaa(Cu), χaa(Cl), (χbb – χcc)(Cu), and (χbb – χcc)(Cl), and the nuclear spin-rotation constant of the copper atom, $C_{bb}^{{\rm Cu}} + C_{cc}^{{\rm Cu}}$Cbb Cu +Ccc Cu , are also presented.
About the determination of the Schottky barrier height with the C-V method
Authors: Bozhkov V. G. , Torkhov N. A. , Shmargunov A. V.
Published: April 2011

On the basis of general notions about Schottky barrier contacts (SBC) with the insulating layer and interface states (ISs) communicating with semiconductor and metal (when their influence results in the linear bias-dependence of the barrier height and the ideality factor n=const), it is shown that the barrier height determined with C-V method is defined with a simple expression: ϕbc=nϕb0-(n-1)(ϕs+V2), practically corresponding to the flatband barrier height expression determined from I-V-characteristic: ϕbf=nϕb0-(n-1)ϕs. The apparent difference is related to the difference in implementation of the flatband condition in both cases. Earlier, the close correspondence of values ϕbc and ϕbf was only known for the ideal Bardeen model, practically excluding values n>1. The received result is also proved by the detailed analysis of possible conditions of determining the SBC capacity (different frequencies of the test signal, presence or absence of ISs, and their communication with semiconductor and/or metal). It is essential that the measured barrier height ϕbc remains almost independent of the frequency of the test signal and is determined with the relation between the applied voltage and its part dropping in the barrier only.
Role of Coulomb blockade and spin-flip scattering in tunneling magnetoresistance of FeCo-Si-O nanogranular films
Authors: Kumar Hardeep , Ghosh Santanu , Bürger Danilo , Li Lin , Zhou Shengqiang , Kabiraj Debdulal , Avasthi Devesh Kumar , Grötzschel Rainer , Schmidt Heidemarie
Published: April 2011

In this work, we report the effect of FeCo atomic fraction (0.33 < x < 0.54) and temperature on the electrical, magnetic, and tunneling magnetoresistance (TMR) properties of FeCo-Si-O granular films prepared by atom beam sputtering technique. Glancing angle x-ray diffraction and TEM studies reveal that films are amorphous in nature. The dipole–dipole interactions (particle–matrix mixing) is evident from zero-field cooled and field-cooled magnetic susceptibility measurements and the presence of oxides (mainly Fe-related) is observed by x-ray photoelectron spectroscopy analysis. The presence of Fe-oxides is responsible for the observed reduction of saturation magnetization and rapid increase in coercivity below 50 K. TMR has been observed in a wide temperature range, and a maximum TMR of −4.25% at 300 K is observed for x = 0.39 at a maximum applied field of 60 kOe. The fast decay of maximum TMR at high temperatures and lower TMR values at 300 K when compared to PFeCo2/(1+PFeCo2), where PFeCo is the spin polarization of FeCo are in accordance with a theoretical model that includes spin-flip scattering processes. The temperature dependent study of TMR effect reveals a remarkably enhanced TMR at low temperatures. The TMR value varies from −2.1% at 300 K to −14.5% at 5 K for x = 0.54 and a large MR value of −18.5% at 5 K for x = 0.39 is explained on the basis of theoretical models involving Coulomb blockade effects. Qualitatively particle–matrix mixing and the presence of Fe-oxides seems to be the source of spin-flip scattering, responsible for fast decay of TMR at high temperatures. A combination of higher order tunneling (in Coulomb blockade regime) and spin-flip scattering (high temperature regime) explains the temperature dependent TMR of these films.
Validity of the thermal activation model for spin-transfer torque switching in magnetic tunnel junctions
Authors: Heindl R. , Rippard W. H. , Russek S. E. , Pufall M. R. , Kos A. B.
Published: April 2011

We have performed spin-transfer torque switching experiments with a large number of trials (up to 107 switching events) on nanoscale MgO magnetic tunnel junctions in order to test the validity and the limits of the thermal activation model for spin-torque-assisted switching. Three different methods derived from the model (“read disturb rate,” “switching voltage versus pulse duration,” and “switching voltage distribution” measurements) are used to determine the thermal stability factor and the intrinsic switching voltage. The results obtained from the first two methods agree well with each other as well as with values obtained from quasistatic measurements, if we use only the data for which the voltage is smaller than approximately 0.8 of the intrinsic switching voltage. This agreement also shows that, in our samples, in the low voltage region, the influence from other factors contributing to the switching (such as current-induced heating and field-like torque) is negligible. The third method (switching voltage distribution measurements) yields incorrect values for the time-scales (<1μs) at which the experiments are performed. Macrospin simulations confirm our findings that the model must be applied only in the low voltage limit, and that in certain devices this limit can extend up to about 0.9 of the intrinsic switching voltage.
Magnetic-flux-induced persistent currents in nonlinear mesoscopic rings
Authors: Zhang R. L. , Qi D. X. , Peng R. W. , Li J. , Fan R. H. , Huang R. S. , Wang Mu
Published: April 2011

We investigate magnetic-flux-induced persistent currents (PCs) in a one-dimensional nonlinear mesoscopic ring based on the Frenkel–Kontorova (FK) model. By applying a transfer-matrix technique, the energy spectra, the PCs, and the Thouless exponent are theoretically obtained. It is shown that the energy spectrum splits into sub-bands when the on-site energy is gradually increased, and in the flux-dependent energy spectra, the energy levels show different behaviors over the transition by breaking of analyticity. Meanwhile, the PC is determined by the magnetic flux, the on-site energy, and the Fermi level. The increment of the on-site energy leads to a dramatic suppression of the PC. When the Fermi level is in the vicinity of “band” gaps, the PC is limited considerably; otherwise, the PC increases by several orders of magnitude. The suppressed PC is related to the electronic localization of the FK ring, which is described by the Thouless exponents. Our investigations provide detailed information about the influence of nonlinear structure on the PC and contribute to its potential application in quantum devices.
119Sn Mössbauer spectroscopy study of UCo1−xRuxSn solid solutions
Authors: Krylov V. I.
Published: April 2011

The influence of the substitution of Co for Ru on the magnetic hyperfine fields Bhf on 119Sn nuclei in UCo1−xRuxSn solid solutions was investigated. The compounds were crystallized in a hexagonal crystal structure of the ZrNiAl type. This structure is characterized by two nonequivalent crystal positions of d-metal. It was shown that Bhf measured at 5 K decreases with an increase in Ru content. Such a dependence of Bhf is opposite to the concentration dependence of the uranium magnetic moments. Therefore, the different mechanisms and electron groups are responsible for the formation of the U magnetic moments and the hyperfine magnetic field on the 119Sn nuclei. It has been concluded from analysis of the obtained results that (i) the observed linear reduction of electron polarization on 119Sn nuclei can be explained by the decrease in density of the s-electrons due to the hybridization of the s-electrons of Sn with Ru/Co atoms, and (ii) the Ru/Co atoms in the two crystal positions make significantly different contributions not just to the 5f-d hybridized band but also to the formation of the magnetic hyperfine fields on 119Sn nuclei.
Visualization of vortex motion in FeAs-based BaFe1.9Ni0.1As2 single crystal by means of magneto-optical imaging
Authors: Lin Zhi Wei , Li Yong Jian , Zhu Jian Guo , Wang Xiao Lin , Dou Shi Xue , Guo You Guang , Lei Gang , Wang Yi , Philips Matthew , Cortie Michael , Li Yan Cheng , Choi Ki-Young , Shi Xun
Published: April 2011

Superconductivity has been found in newly discovered iron-based compounds. This paper studies the motion of magnetic vortices in BaFe1.9Ni0.1As2 single crystal by means of the magneto-optical imaging technique. A series of magneto-optical images reflecting magnetic flux distribution at the crystal surface were taken when the crystal was zero-field cooled to 10 K. The behavior of the vortices, including penetration into and expulsion from the single crystal with increasing and decreasing external fields, respectively, is discussed. The motion behavior is similar to that observed in high-Tc superconducting cuprates with strong vortex pinning; however, the flux-front is irregular due to randomly distributed defects in the crystal.
Is the magnetic anisotropy proportional to the orbital moment?
Authors: Skomski R. , Kashyap A. , Enders A.
Published: April 2011

The relation between orbital moment and magnetic anisotropy is investigated by model calculations, which show that only a part of the spin-orbit coupling contributes to the anisotropy. A large part of the anisotropy energy, about 50% for iron series elements and nearly 100% for rare-earths, is stored in the nonrelativistic part of the Hamiltonian. A feature important for x-ray magnetic circular dichroism is that the orbital moment of heavy atoms rotates with the spin moment, whereas in light atoms, the orbital moment is recreated in each different direction. In the discussion, we consider three examples of current interest in different areas of magnetism, namely, spin-orbit coupling in Gd3+ and Eu2+, surface anisotropy of Nd2Fe14B, and multiferroic magnetization switching using rare-earths.
Anisotropic SmCo5 flakes and nanocrystalline particles by high energy ball milling
Authors: Cui B. Z. , Zheng L. Y. , Waryoba D. , Marinescu M. , Hadjipanayis G. C.
Published: April 2011

This work is focused on the evolution of structure, texture and magnetic properties in SmCo5 powders during high energy ball milling (HEBM) in heptane with and without oleic acid (OA). Single-crystal micron SmCo5 flakes were formed in the early stage of HEBM (0–0.5 h). However, HEBM of 1–2 h without OA led to the formation of a mixture of single-crystal microflakes and loosely agglomerated polycrystalline microparticles with the highest coercivity of 15.7 kOe. HEBM of 3–5 h without OA led to the formation of isotropic, more or less equiaxed SmCo5 particles with a size of 2–30 μm and a polycrystalline nanostructure due to the coalescence of flakes and particles. In the case of HEBM in heptane with 15 wt. % OA, prolonged milling (from 1 to 5 h) led first to the formation of SmCo5 single-crystal submicron flakes and later to submicron- and nanosize-nanocrystalline flakes with a [001]-out-of-plane texture. OA plays an essential role in the formation of anisotropic SmCo5 flakes via the impediment of cold welding and agglomeration of flakes. The highest coercivity of 17.7 kOe was obtained in a flake sample milled for 5 h with a thickness of 8–80 nm, an average grain size of 8 nm and an aspect ratio of 102–103.
Fabrication of FePt type exchange coupled composite bit patterned media by block copolymer lithography
Authors: Wang Hao , Rahman M. Tofizur , Zhao Haibao , Isowaki Yosuke , Kamata Yoshiyuki , Kikitsu Akira , Wang Jian-Ping
Published: April 2011

We fabricated exchange coupled composite (ECC) L10 ordered, (001) oriented FePt (5nm)/Fe (5 nm) bit patterned media over a large area by diblock copolymer lithography. We formed the dot arrays of the copolymer directly on the magnetic film and used them as the etching mask. The average size of the ECC FePt/Fe pillars was about 32 nm, with a center to center distance of about 35 nm and a size distribution of about 8%. The perpendicular coercivity (Hc) of the ECC FePt/Fe patterned structures was about 4.3 kOe. Both the coercivity and the saturation field of the ECC FePt/Fe patterned structure were reduced by about 50% due to the exchange coupling between FePt and Fe in the FePt/Fe patterned structure compared to the FePt patterned structure with similar dot size and distribution. The thermal stability and gain factor of the FePt/Fe ECC structure were about 260 kBT and 1.35, respectively.
Magnetic properties in polycrystalline and single crystal Ca-doped LaCoO3
Authors: Zeng R. , Debnath J. C. , Chen D. P. , Shamba P. , Wang J. L. , Kennedy S. J. , Campbell S. J. , Silver T. , Dou S. X.
Published: April 2011

Polycrystalline (PC) and single crystalline (SC) Ca-doped LaCoO3 (LCCO) samples with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on dc magnetization, ac susceptibility, exchange-bias, and the magnetocaloric effect. These findings suggest that complex structural phases, ferromagnetic (FM), and spin-glass/cluster-spin-glass (CSG), and their transitions exist in PC samples, while there is a much simpler magnetic phase in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect, and that a large inverse irreversible magnetocaloric effect exists in this system.
Microstructure and ordering parameter studies in multilayer [FePt(x)/Os]n films
Authors: Chiang D. P. , Chen S. Y. , Yao Y. D. , Ouyang H. , Yu C. C. , Chen Y. Y. , Lin H. M.
Published: April 2011

The microstructure, ordering parameter, and magnetic properties of multilayer [FePt(x)/Os]n films on glass substrate by dc-magnetron sputtering (with x being thickness in nm; Os with a fixed thickness 5 nm; n being the number of layers) have been studied as a function of the annealing temperatures between 300 and 900 °C. The grain size of multilayer films can be controlled by annealing temperature and thickness of the FePt layer with Os space layer. The coercivity as a function of the annealing temperature for samples with n = 1 and pure FePt behaves roughly saturated after annealing above 700 °C. However, for samples with n > 4 the value of Hc seems still increasing with increasing annealing temperature between 600 and 900 °C, and the ordering parameter decreases with increasing the number of Os layers. Our experimental results are reasonably well to describe the effect of strain-assisted transformation.
Orbital moment determination in (MnxFe1−x)3O4 nanoparticles
Authors: Pool V. L. , Jolley C. , Douglas T. , Arenholz E. A. , Idzerda Y. U.
Published: April 2011

Nanoparticles of (MnxFe1−x)3O4 with a concentration ranging from x = 0 to 1 and a crystallite size of 14–15 nm were measured using X-ray absorption spectroscopy and X-ray magnetic circular dichroism to determine the ratio of the orbital moment to the spin moment for Mn and Fe. At low Mn concentrations, the Mn substitutes into the host Fe3O4 spinel structure as Mn2+ in the tetrahedral A-site. The net Fe moment, as identified by the X-ray dichrosim intensity, is found to increase at the lowest Mn concentrations then rapidly decrease until no dichroism is observed at 20% Mn. The average Fe orbit/spin moment ratio is determined to initially be negative and small for pure Fe3O4 nanoparticles and quickly go to 0 by 5%–10% Mn addition. The average Mn moment is anti-aligned to the Fe moment with an orbit/spin moment ratio of 0.12 which gradually decreases with Mn concentration.
Structural and magnetic properties of rapidly quenched (Sm,Zr)(Fe,Co)11BX (x = 0–1) ribbons
Authors: Horiuchi Yosuke , Sakurada Shinya
Published: April 2011

The structural and magnetic properties of rapidly quenched (Sm,Zr)(Fe,Co)11Bx (x = 0–1) ribbons have been studied. Boron addition increases the amorphous forming ability and fine-uniform TbCu7 grains were obtained by rapid quenching with relatively low roll surface velocity. The magnetic properties of Mr = 0.99 T and Hcj = 273 kA m− 1 were obtained for (Sm0.7Zr0.3)(Fe0.8Co0.2)11B0.7 ribbons fabricated by rapid quenching with a surface velocity of 20 ms− 1 and subsequent annealing at 998 K for 0.9 ks.
Influence of magnetic field annealing on saturation magnetostriction and μi–T curves for FeCo-based nanocrystalline alloy
Authors: Shi Rui-min , Wang Zhi , Jia Yun-yun , Fu Bin , Long Yi
Published: April 2011

The influence of magnetic field annealing on the saturation magnetostriction and μi–T curves for the nanocrystalline (Fe0.5Co0.5)73.5Cu1Nb3Si13.5B9 alloy was investigated. In contrast to the nonmagnetic field annealing, the magnetic field annealing reduces λS and remodels the μi–T curves of the samples dramatically. After nonmagnetic annealing at different temperatures, the λS of samples increases with the annealing temperature (Ta), and the value of μi decreases with T increasing at different rates. However, the λS of magnetic annealed samples decreases with increasing Ta because of the ordered rearrangement of the magnetic domain along with the applied field direction. The μi of magnetic annealing samples tends to increase with Ta, but it behaves in a different fashion with increasing T, which is correlated with the lower magnetostriction and the induced uniaxial anisotropy by magnetic annealing.
Electric field modulation of surface anisotropy and magneto-dynamics in multiferroic heterostructures
Authors: Lou J. , Liu M. , Reed D. , Ren Y. H. , Sun N. X.
Published: April 2011

The effects of an electric field on the magneto-dynamics of a FeGaB/PZN-PT (lead zinc niobate-lead titanate) multiferroic heterostructure were investigated. It was found that with the increase of electric field applied on the PZN-PT layer, the magnetoelectric coupling is increased, and the magnetization of the FeGaB film changes gradually from an unpinned to a pinned boundary condition at the FeGaB/PZN-PT interface. This change leads to the observation of the first-order standing spin wave mode and the varied resonance intensity ratio between the standing mode and the uniform mode from 1 to 5% in the FMR spectra. The E-field induced surface anisotropy change confirms the strain mediated magnetoelectric mechanism through the interface between multiferroic heterostructures.
Electric-poling-induced magnetic anisotropy and electric-field-induced magnetization reorientation in magnetoelectric Ni/(011) [Pb(Mg1/3Nb2/3)O3](1-x)-[PbTiO3]x heterostructure
Authors: Wu Tao , Bur Alexandre , Wong Kin , Leon Hockel Joshua , Hsu Chin-Jui , Kim Hyungsuk K. D. , Wang Kang L. , Carman Gregory P.
Published: April 2011

This study reports the influence of poling a PMN-PT single crystal laminated structure on the magnetic properties of a 35 nm polycrystalline Ni thin film. During the poling process, a large anisotropic remanent strain is developed in the PMN-PT that is transferred to the ferromagnetic film creating a large predefined magnetic anisotropy. Test results show that operating the PMN-PT substrate in the linear regime following poling produces sufficient anisotropic strain to reversibly reorient the magnetization toward an easy axis oriented 90° to the magnetic easy axis induced during poling. The influence of poling prestress on the magnetic anisotropy field, coercive field and magnetic remanence is discussed.
High-sensitivity detector for molecular sensing using magnetic particles
Authors: Roy D. Le , Yang W. , Yin X. , Lai R. Y. , Liou S.-H. , Sellmyer D. J.
Published: April 2011

A scheme for molecular sensing using magnetic tracer particles and a microcantilever torsional magnetometer is investigated. The present report deals with the example of explosive 2,4,6-trinitrotoluene (TNT) detection. The sensors consist of silicon microcantilevers functionalized with TNT aptamers that are weakly bonded to magnetic particles via TNT-analog molecules. When exposed to TNT, the magnetic signal, initially maximum, is expected to undergo a steplike decrease as the TNT molecules replace the magnetic particles on the TNT receptors. We demonstrate the feasibility of this detection technique in terms of chemical reactions and our magnetometer sensitivity that reaches the range of 10−11 emu at room temperature with commercial atomic force microcopy cantilevers.
Tuning of the surface plasmon resonance in TiO2/Au thin films grown by magnetron sputtering: The effect of thermal annealing
Authors: Torrell M. , Kabir R. , Cunha L. , Vasilevskiy M. I. , Vaz F. , Cavaleiro A. , Alves E. , Barradas N. P.
Published: April 2011

Nanocomposites consisting of a dielectric matrix, such as TiO2, with embedded noble metal nanoparticles (NPs) possess specific optical properties due to the surface plasmon resonance (SPR) effect, interesting for several applications. The aim of this work is to demonstrate that these properties are sensitive to the nanostructure of magnetron-sputtered TiO2/Au thin films, which can be tuned by annealing. We study the role of the shape and size distribution of the NPs, as well as the influence of the crystallinity and phase composition of the host matrix on the optical response of the films. All these characteristics can be modified by vacuum annealing treatments of the deposited films. A theoretical interpretation and modeling of the experimental results obtained is presented. The model involves a modified Maxwell-Garnett approach for the effective dielectric function of the composite (describing the SPR effect) and the transfer matrix formalism for multilayer optics. Input data are based on the experimental information obtained from the detailed structural characterization of the films. It is shown that the annealing treatments can be used for controlling the optical properties of the composite films, making them attractive for decorative coatings.
Improving the stretchability of as-deposited Ag coatings on poly-ethylene-terephthalate substrates through use of an acrylic primer
Authors: Sim Gi-Dong , Won Sejeong , Jin Chun-yan , Park Inkyu , Lee Soon-Bok , Vlassak Joost J.
Published: April 2011

In this paper, we report that silver films evaporated on poly-ethylene-terephthalate (PET) substrates coated with an acrylic primer can be stretched beyond 70% without fracture. As-deposited films show a larger failure strain than annealed coatings. These observations are rationalized in light of a ductile fracture mechanism where debonding from the substrate coevolves with strain localization. The results of this study indicate that PET substrates coated with an acrylic primer layer may be suitable for stretchable electronics.
Finite element modeling of atomic force microscopy cantilever dynamics during video rate imaging
Authors: Howard-Knight J. P. , Hobbs J. K.
Published: April 2011

A dynamic finite element model has been constructed to simulate the behavior of low spring constant atomic force microscope (AFM) cantilevers used for imaging at high speed without active feedback as in VideoAFM. The model is tested against experimental data collected at 20 frame/s and good agreement is found. The complex dynamics of the cantilever, consisting of traveling waves coming from the tip sample interaction, reflecting off the cantilever-substrate junction, and interfering with new waves created at the tip, are revealed. The construction of the image from this resulting nonequilibrium cantilever deflection is also examined. Transient tip-sample forces are found to reach values up to 260 nN on a calibration grid sample, and the maximum forces do not always correspond to the position of steepest features as a result of energy stored in the cantilever.
On the determination of partial differential cross sections for photodetachment and photoionization processes producing polyatomic molecules with electronic states coupled by conical intersections
Authors: Han Seungsuk , Yarkony David R.
Published: April 2011

A formalism is derived for the computation of partial differential cross sections for electron photodetachment and photoionization processes that leave the residual or target molecule in electronic states that are strongly coupled by conical intersections. Because the electronic states of the target are nonadiabatically coupled, the standard adiabatic states approach of solving the electronic Schrödinger equation for the detached electron at fixed nuclear geometries and then vibrationally averaging must be fundamentally modified. We use a Lippmann–Schwinger equation based approach, which leads naturally to a partitioning of the transition amplitude into a Dyson orbital like part plus a scattering correction. The requisite Green's function is that developed in our previous paper for the direct determination of total integral cross sections. The method takes proper account of electron exchange, possible nonorthogonality of the orbital describing the detached electron, and nonadiabatic effects in the product molecule. The Green's function is constructed in an L2 basis using complex scaling techniques. The accurate treatment of nonadiabatic effects in the residual molecule is accomplished using the multimode vibronic coupling model. For photodetachment, an approximate approach, which is less computationally demanding, is suggested.
Quantum process tomography of excitonic dimers from two-dimensional electronic spectroscopy. I. General theory and application to homodimers
Authors: Yuen-Zhou Joel , Aspuru-Guzik Alán
Published: April 2011

Is it possible to infer the time evolving quantum state of a multichromophoric system from a sequence of two-dimensional electronic spectra (2D-ES) as a function of waiting time? Here we provide a positive answer for a tractable model system: a coupled dimer. After exhaustively enumerating the Liouville pathways associated to each peak in the 2D-ES, we argue that by judiciously combining the information from a series of experiments varying the polarization and frequency components of the pulses, detailed information at the amplitude level about the input and output quantum states at the waiting time can be obtained. This possibility yields a quantum process tomography (QPT) of the single-exciton manifold, which completely characterizes the open quantum system dynamics through the reconstruction of the process matrix. In this manuscript, we present the general theory as well as specific and numerical results for a homodimer, for which we prove that signals stemming from coherence to population transfer and vice versa vanish upon isotropic averaging, therefore, only allowing for a partial QPT in such case. However, this fact simplifies the spectra, and it follows that only two polarization controlled experiments (and no pulse-shaping requirements) suffice to yield the elements of the process matrix, which survive under isotropic averaging. Redundancies in the 2D-ES amplitudes allow for the angle between the two site transition dipole moments to be self-consistently obtained, hence simultaneously yielding structural and dynamical information of the dimer. Model calculations are presented, as well as an error analysis in terms of the angle between the dipoles and peak amplitude extraction. In the second article accompanying this study, we numerically exemplify the theory for heterodimers and carry out a detailed error analysis for such case. This investigation reveals an exciting quantum information processing (QIP) approach to spectroscopic experiments of excitonic systems, and hence, bridges an important gap between theoretical studies on excitation energy transfer from the QIP standpoint and experimental methods to study such systems in the chemical physics community.
Lattice thermal conductivity of graphene nanoribbons: Anisotropy and edge roughness scattering
Authors: Aksamija Z. , Knezevic I.
Published: April 2011

We present a calculation of the thermal conductivity of graphene nanoribbons (GNRs), based on solving the Boltzmann transport equation with the full phonon dispersions, a momentum-dependent model for edge roughness scattering, as well as three-phonon and isotope scattering. The interplay between edge roughness scattering and the anisotropy of the phonon dispersions results in thermal conduction that depends on the chiral angle of the nanoribbon. Lowest thermal conductivity occurs in the armchair direction and highest in zig-zag nanoribbons. Both the thermal conductivity and the degree of armchair/zig-zag anisotropy depend strongly on the width of the nanoribbon and the rms height of the edge roughness, with the smallest and most anisotropic thermal conductivities occurring in narrow GNRs with rough edges.
Correlation of thermoelectric and microstructural properties of p-type CeFe4Sb12 melt-spun ribbons using a rapid screening method
Authors: Yan Y. G. , Wong-Ng W. , Kaduk J. A. , Tan G. J. , Xie W. J. , Tang X. F.
Published: April 2011

We have demonstrated the effect of gradients in phase composition and microstructure of p-type CeFe4Sb12 melt-spun ribbons on Seebeck coefficient by using a thermoelectric (TE) screening tool in conjunction with x-ray diffraction. The spatial Seebeck coefficient variation was found to correlate with the changes in the phase composition and microstructure. The observed gradient was the result of a postannealing effect caused by the thermal lag between the contact surface with the roller and the free surface. Our results illustrate the application of our screening tool to the three-dimensional variation in Seebeck coefficient in a model TE material.
Accelerated convergence of molecular free energy via superposition approximation-based reference states
Authors: Somani Sandeep , Gilson Michael K.
Published: April 2011

The free energy of a molecular system can, at least in principle, be computed by thermodynamic perturbation from a reference system whose free energy is known. The convergence of such a calculation depends critically on the conformational overlap between the reference and the physical systems. One approach to defining a suitable reference system is to construct it from the one-dimensional marginal probability distribution functions (PDFs) of internal coordinates observed in a molecular simulation. However, the conformational overlap of this reference system tends to decline steeply with increasing dimensionality, due to the neglect of correlations among the coordinates. Here, we test a reference system that can account for pairwise correlations among the internal coordinates, as captured by their two-dimensional marginal PDFs derived from a molecular simulation. Incorporating pairwise correlations in the reference system is found to dramatically improve the convergence of the free energy estimates relative to the first-order reference system, due to increased conformational overlap with the physical distribution.
A novel algorithm for quick and continuous tracing the change of radon concentration in environment
Authors: Tan Yanliang , Xiao Detao
Published: April 2011

Several measurements of the radon concentration are performed by RAD7 in the University of South China. We find that 30–40 min is needed for RAD7 for tracing the concentration of the standard radon chamber. There are two reasons. The first is that the sufficient time of air cycle is needed for the radon concentration in internal cell of RAD7 equal to that of the environment; and the second is that the sufficient decay time is needed for the 218Po concentration in internal cell of RAD7 equal to that of the radon. We used a zeroth order approximation to describe the evolution of the environment radon concentration, and obtained a novel algorithm for quick and continuous tracing the change of radon concentration. The corrected radon concentration obtained through this method is in good agreement with the reference value. This method can be applied to develop and improve the instruments for tracing the change of radon concentration quickly.
Nanograting transfer for light extraction in organic light-emitting devices
Authors: Wang Ziyao , Hauss Julian , Vannahme Christoph , Bog Uwe , Klinkhammer Sönke , Zhao Dong , Gerken Martina , Mappes Timo , Lemmer Uli
Published: April 2011

Damage-free transfer of nanogratings as waveguide mode extraction elements onto the top of top-emission organic light-emitting devices (OLEDs) was realized with the assistance of cyclic olefin copolymer molds. Photoluminescence and electroluminescence measurements together with transfer matrix calculations were used to identify the extracted waveguide modes. The presented one-step, wet-free, and etchless nanotransfer approach can avoid damage on the top-emission OLED structure and does not adversely affect the OLED performance. It offers the possibility for the independent optimization of OLED fabrication and nanofabrication, and has potential applications in organic optoelectronic devices, especially top-emission OLEDs.
Fabrication and characterization of topological insulator Bi2Se3 nanocrystals
Authors: Zhao S. Y. F. , Beekman C. , Sandilands L. J. , Bashucky J. E. J. , Kwok D. , Lee N. , LaForge A. D. , Cheong S. W. , Burch K. S.
Published: April 2011

In the recently discovered class of materials known as topological insulators, the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change in invariants at the interface results in metallic, time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi2Se3 via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi2Se3 nanocrystals.
Intermolecular potential energy surface and second virial coefficients for the water–CO2 dimer
Authors: Wheatley Richard J. , Harvey Allan H.
Published: April 2011

A five-dimensional potential energy surface is calculated for the interaction of water and CO2, using second-order Møller–Plesset perturbation theory and coupled-cluster theory with single, double, and perturbative triple excitations. The correlation energy component of the potential energy surface is corrected for basis set incompleteness. In agreement with previous studies, the most negative interaction energy is calculated for a structure with C2v symmetry, where the oxygen atom of water is close to the carbon atom of CO2. Second virial coefficients for the water–CO2 pair are calculated for a range of temperatures, and their uncertainties are estimated. The virial coefficients are shown to be in close agreement with the available experimental data.
Geometry of non-Hausdorff spaces and its significance for physics
Authors: Heller Michael , Pysiak Leszek , Sasin Wiesław
Published: April 2011

Hausdorff relation, topologically identifying points in a given space, belongs to elementary tools of modern mathematics. We show that if subtle enough mathematical methods are used to analyze this relation, the conclusions may be far-reaching and illuminating. Examples of situations in which the Hausdorff relation is of the total type, i.e., when it identifies all points of the considered space, are the space of Penrose tilings and space-times of some cosmological models with strong curvature singularities. With every Hausdorff relation a groupoid can be associated, and a convolutive algebra defined on it allows one to analyze the space that otherwise would remain intractable. The regular representation of this algebra in a bundle of Hilbert spaces leads to a von Neumann algebra of random operators. In this way, a probabilistic description (in a generalized sense) naturally takes over when the concept of point looses its meaning. In this situation counterparts of the position and momentum operators can be defined, and they satisfy a commutation relation which, in the suitable limiting case, reproduces the Heisenberg indeterminacy relation. It should be emphasized that this is neither an additional assumption nor an effect of a quantization process, but simply the consequence of a purely geometric analysis.
Toward canonical ensemble distribution from self-guided Langevin dynamics simulation
Authors: Wu Xiongwu , Brooks Bernard R.
Published: April 2011

This work derives a quantitative description of the conformational distribution in self-guided Langevin dynamics (SGLD) simulations. SGLD simulations employ guiding forces calculated from local average momentums to enhance low-frequency motion. This enhancement in low-frequency motion dramatically accelerates conformational search efficiency, but also induces certain perturbations in conformational distribution. Through the local averaging, we separate properties of molecular systems into low-frequency and high-frequency portions. The guiding force effect on the conformational distribution is quantitatively described using these low-frequency and high-frequency properties. This quantitative relation provides a way to convert between a canonical ensemble and a self-guided ensemble. Using example systems, we demonstrated how to utilize the relation to obtain canonical ensemble properties and conformational distributions from SGLD simulations. This development makes SGLD not only an efficient approach for conformational searching, but also an accurate means for conformational sampling.
Robust acceleration of self consistent field calculations for density functional theory
Authors: Baarman K. , Eirola T. , Havu V.
Published: April 2011

We show that the type 2 Broyden secant method is a robust general purpose mixer for self consistent field problems in density functional theory. The Broyden method gives reliable convergence for a large class of problems and parameter choices. We directly mix the approximation of the electronic density to provide a basis independent mixing scheme. In particular, we show that a single set of parameters can be chosen that give good results for a large range of problems. We also introduce a spin transformation to simplify treatment of spin polarized problems. The spin transformation allows us to treat these systems with the same formalism as regular fixed point iterations.
Electron and hole transport in poly(fluorene-benzothiadiazole)
Authors: Zhang Yuan , Blom Paul W. M.
Published: April 2011

We investigate the electron and hole transport in poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT). An Ohmic hole contact on F8BT is achieved by using the high work function anode MoO3 as hole injection contact, enabling the occurrence of space-charge limited currents. The electron transport in F8BT is trap-limited and the traps can be deactivated using n-type doping by decamethylcobaltocene (DMC). Due to the alignment of the energy levels of DMC and F8BT the electrons from the DMC donor not only fill the traps but also fill up the lowest unoccupied molecular orbital of F8BT such that the electron transport can be enhanced beyond the hole transport.
Mixed homopolymer brushes grafted onto a nanosphere
Authors: Wang Yueqiang , Yang Guang , Tang Ping , Qiu Feng , Yang Yuliang , Zhu Lei
Published: April 2011

Microphase separation of mixed A/B polymer brushes grafted onto a nanosphere with its radius comparable to the size of polymers is investigated by numerical implementation of the self-consistent field theory. The idea is to embed the sphere within a larger cubic computational cell and use a “masking” technique to treat the spherical boundary. The partial differential equations for the chain propagator on the sphere can thus be readily solved with an efficient and high-order accurate pseudospectral method involving fast Fourier transform on a cubic cell. This numerical technique can circumvent the “pole problem” due to the use of a spherical coordinate system in conventional finite difference or finite element grid. We systematically investigate the effect of the total grafting density, composition, chain length asymmetry between two grafted homopolymers as well as spherical radius, i.e., substrate curvature on the formation of island structure with specific arrangement in a regular lattice. A series of island structures with different island numbers representing specific structure symmetry ranging from 2 to 12 except for 11 are found, in contrast to conventional hexagonal arrangement for polymer brushes on a planar substrate. Among these parameters, the spherical radius plays a significant role in determining the type of island structures, i.e., the morphology formed on the sphere.
Thermal-vacancy-assisted phase transition in FePt thin films
Authors: Li X. H. , Wang F. Q. , Liu B. T. , Guo D. F. , Zhang X. Y.
Published: April 2011

Understanding the ordering transition from A1 to L10 structure in FePt thin films is of great significance for developing L10-FePt films as ultrahigh density magnetic recording media. Here, the L10-ordering transition of FePt films has been investigated based on activation volume measurements. A large activation volume ΔV∗=10–11 Å3=(0.75–0.8) Ω, where Ω is average atomic volume of FePt, is determined for atomic diffusions in the L10-ordering transition, indicating a thermal-vacancy-assisted phase transition. This transition is suggested to be predominantly dependent on the diffusion of Fe atoms. These findings have direct implications for yielding L10-FePt films at low temperatures and optimizing their microstructures.
Michaelis–Menten speeds up tau-leaping under a wide range of conditions
Authors: Wu Sheng , Fu Jin , Cao Yang , Petzold Linda
Published: April 2011

This paper examines the benefits of Michaelis–Menten model reduction techniques in stochastic tau-leaping simulations. Results show that although the conditions for the validity of the reductions for tau-leaping remain the same as those for the stochastic simulation algorithm (SSA), the reductions result in a substantial speed-up for tau-leaping under a different range of conditions than they do for SSA. The reason of this discrepancy is that the time steps for SSA and for tau-leaping are determined by different properties of system dynamics.
Time-resolved gas temperature evolution in pulsed Ar–N2 microwave discharge
Authors: Britun Nikolay , Godfroid Thomas , Konstantinidis Stephanos , Snyders Rony
Published: April 2011

Temporal evolution of the gas temperature (Tg) in a pulsed microwave surfaguide discharge is studied by measuring the N2 rotational temperature. We found that at high power applied per pulse, gas temperature grows linearly, and saturates after about 150 μs. This effect is absent at low power values, or at short pulse durations. Observed Tg behavior correlates with time-resolved measurements of the N2 vibrational temperature, as well as with N emission lines. Consequently, Tg time behavior was related to N atoms production in plasma. Using obtained Tg growth rates, the effective power used for plasma heating is determined.