bluuebunny/crossref_metadata_2025 huggingface dataset:
Room temperature nonlinear operation of a graphene-based three-branch nanojunction device with chemical doping
Authors: Fadzli Abd Rahman Shaharin , Kasai Seiya , Manaf Hashim Abdul
Published: May 2012

A chemically doped graphene-based three-branch nanojunction device is fabricated on a SiO2/p-Si substrate, and its nonlinear operation is characterized at room temperature (RT). By polyethyleneimine doping, the fabricated device shows improved field effect mobility of 14 800 and 16 100 cm2/Vs for electron and holes, respectively. The device clearly exhibits nonlinearity in voltage transfer curves at RT. The curvature of the transfer curve can be controlled by using the back gate voltage, and its polarity abruptly switches near the Dirac point because of the carrier type change. The observed behaviour can be quantitatively explained in terms of the difference in the amounts of gate-induced carriers in the two input branches.
Electromigration in Cu(Al) and Cu(Mn) damascene lines
Authors: Hu C.-K. , Ohm J. , Gignac L. M. , Breslin C. M. , Mittal S. , Bonilla G. , Edelstein D. , Rosenberg R. , Choi S. , An J. J. , Simon A. H. , Angyal M. S. , Clevenger L. , Maniscalco J. , Nogami T. , Penny C. , Kim B. Y.
Published: May 2012

The effects of impurities, Mn or Al, on interface and grain boundary electromigration (EM) in Cu damascene lines were investigated. The addition of Mn or Al solute caused a reduction in diffusivity at the Cu/dielectric cap interface and the EM activation energies for both Cu-alloys were found to increase by about 0.2 eV as compared to pure Cu. Mn mitigated and Al enhanced Cu grain boundary diffusion; however, no significant mitigation in Cu grain boundary diffusion was observed in low Mn concentration samples. The activation energies for Cu grain boundary diffusion were found to be 0.74 ± 0.05 eV and 0.77 ± 0.05 eV for 1.5 μm wide polycrystalline lines with pure Cu and Cu (0.5 at. % Mn) seeds, respectively. The effective charge number in Cu grain boundaries Z*GB was estimated from drift velocity and was found to be about −0.4. A significant enhancement in EM lifetimes for Cu(Al) or low Mn concentration bamboo-polycrystalline and near-bamboo grain structures was observed but not for polycrystalline-only alloy lines. These results indicated that the existence of bamboo grains in bamboo-polycrystalline lines played a critical role in slowing down the EM-induced void growth rate. The bamboo grains act as Cu diffusion blocking boundaries for grain boundary mass flow, thus generating a mechanical stress-induced back flow counterbalancing the EM force, which is the equality known as the “Blech short length effect.”
Experimental and computational investigation of the group 11–group 2 diatomic molecules: First determination of the AuSr and AuBa bond energies and thermodynamic stability of the copper- and silver-alkaline earth species
Authors: Ciccioli A. , Gigli G. , Lauricella M.
Published: May 2012

The dissociation energies of the intermetallic molecules AuSr and AuBa were for the first time determined by the Knudsen effusion mass spectrometry method. The two species were produced in the vapor phase equilibrated with apt mixtures of the constituent elements, and the dissociation equilibria were monitored mass-spectrometrically in the temperature range 1406–1971 K (AuSr) and 1505–1971 K (AuBa). The third-law analysis of the equilibrium data gives the following dissociation energies ($D_0^{\circ}$D0∘, in kJ/mol): 244.4 ± 4.8 (AuSr) and 273.3 ± 6.3 (AuBa), so completing the series of $D_0^{\circ}$D0∘s for the AuAE (AE = group 2 element) diatomics. The AuAE species were also studied computationally at the coupled cluster including single, double and perturbative triple excitation [CCSD(T)] level with basis sets of increasing zeta quality, and various complete basis set limit extrapolations were performed to calculate the dissociation energies. Furthermore, the entire series of the heteronuclear diatomic species formed from one group 11 (Cu, Ag) and one group 2 (Be, Mg, Ca, Sr, Ba) metal was studied by DFT with the hybrid meta-GGA TPSSh functional and the def2-QZVPP basis set, selected after screening a number of functional-basis set combinations using the AuAE species as benchmark. Dissociation energies, internuclear distances, vibrational frequencies, and anharmonic constants were determined for the CuAE and AgAE species and their thermal functions evaluated therefrom. On this basis, a thermodynamic evaluation of the formation of these species was carried out under various conditions.
Performance analysis and parametric optimum criteria of the nanothermoelectric engine with a single-level quantum dot at maximum power
Authors: Wang Hao , Wu Guoxing , Fu Yueming , Chen Daojiong
Published: May 2012

In this article, we have studied a simple model of the nanothermoelectric engine with a single level quantum dot. Based on the model, expressions for the power output and efficiency of the nanothermoelectric engine are derived. The effects of a spin-degenerate level and the temperature ratio of the two reservoirs on the performance of the nanothermoelectric engine are revealed. The optimal performance characteristics of the nanothermoelectric engine are analyzed by a numerical calculation and graphic method. Furthermore, some important operating regions, including the power output, efficiency, and temperatures of the cyclic working substance, are determined and evaluated optimally. At last, we discussed the relation to the thermoelectric figure of merit ZT.
Stability of nano-scale ferroelectric domains in a LiNbO3 single crystal: The role of surface energy and polar molecule adsorption
Authors: Sun X. , Su Y. J. , Li X. , Gao K. W. , Qiao L. J.
Published: May 2012

The stability of nano-scale ferroelectric domains in a LiNbO3 single crystal under varied atmospheric humidity levels was studied using piezoelectric force microscopy. Experimental results showed that the nano-scale domains fabricated by the tip field of the atomic force microscope changed as the environmental humidity changed; the c− domains expanded or shrank with increases or decreases in the environmental humidity (that is, with the amount of adsorbed H2O molecules on the domain surface), while the c+ domains transformed in the opposite sense. The surface energy of the domains is responsible for these transformations.
First-principles study of intrinsic point defects in hexagonal barium titanate
Authors: Dawson J. A. , Harding J. H. , Chen H. , Sinclair D. C.
Published: May 2012

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states, and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results.
Acousto-microfluidics: Transporting microbubble and microparticle arrays in acoustic traps using surface acoustic waves
Authors: O’Rorke R. D. , Wood C. D. , Wälti C. , Evans S. D. , Davies A. G. , Cunningham J. E.
Published: May 2012

We demonstrate that aqueous suspensions of microbubbles, formed into arrays using standing surface acoustic waves (SSAWs), can be transported by controlled modulation of the SSAW frequency. The array is repeatedly captured at a sequence of spatial positions along the acoustic beam path and long-range transportation is achieved by periodic cycling of the applied frequency across the transducer bandwidth. We also demonstrate that controllable alignment and transport can be achieved in a detachable microfluidic device, where the microfluidic channel, in which particle transport occurs, is separated from the piezoelectric substrate by an acoustic coupling gel. Proof-of-concept transport is first discussed using a test system of latex particles before the non-invasive manipulation technique is applied to arrays of microbubbles. We explore the role of acoustic radiation forces in the spatial control of particles by analysing the dynamics of particle manipulation by SSAWs. Our results highlight the exquisite control which we have over the position and transport of particles and we anticipate that this technique could find wide applications for the accurate and programmable, non-invasive ordering and transport of biological samples in microfluidic systems.
Cold rolled Fe-6.5 wt. % Si alloy foils with high magnetic induction
Authors: Fang X. S. , Liang Y. F. , Ye F. , Lin J. P.
Published: May 2012

Fe-6.5 wt. % Si alloy foils with 95 mm in width and 0.30 mm in thickness were successfully fabricated by cold rolling process. Excellent magnetic properties (Hc = 20.4 A/m, µm = 22 200, and Bs = 1.69 T) were obtained after annealing at the temperature of 1273 K for 1.5 h. This high magnetic induction is considered to be due to the formation of {hk0}〈001〉 textures. Cut cores from this material have a very low iron loss at frequencies from 400 Hz to 10 kHz.
Calibration-quality adiabatic potential energy surfaces for ${\rm H}_3^+$H3+ and its isotopologues
Authors: Pavanello Michele , Adamowicz Ludwik , Alijah Alexander , Zobov Nikolai F. , Mizus Irina I. , Polyansky Oleg L. , Tennyson Jonathan , Szidarovszky Tamás , Császár Attila G.
Published: May 2012

Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+. The underlying Born–Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born–Oppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41 655 ab initio points is presented which gives a standard deviation better than 0.1 cm−1 when restricted to the points up to 6000 cm−1 above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The ro-vibrational transition frequencies for \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+, H2D+, and \documentclass[12pt]{minimal}\begin{document}${\rm HD}_2^+$\end{document} HD 2+ are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute ro-vibrational energy levels, which makes explicit allowance for the inclusion of non-adiabatic effects, reproduces all the known ro-vibrational levels of the \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ isotopologues considered to better than 0.2 cm−1. This represents a significant (order-of-magnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of non-adiabatic effects in the \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16 000 cm−1, and (c) results suggest that we can predict accurately the lines of \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ towards dissociation and thus facilitate their experimental observation.
Distance dependent quenching effect in nanoparticle dimers
Authors: Polemi Alessia , Shuford Kevin L.
Published: May 2012

In this paper, we investigate the emission characteristics of a molecule placed in the gap of a nanoparticle dimer configuration. The emission process is described in terms of a local field enhancement factor and the overall quantum yield of the system. The molecule is represented as a dipolar source, with fixed length and fed by a constant current. We first describe the coupled dimer-molecule system and compare these results to a single sphere-molecule system. Next, the effect of dimer size is investigated by changing the radius of the nanoparticles. We find that when the radius increases, a saturation effect occurs that trends towards the case of a radiating dipole between two flat interfaces, which we refer to as a parallel plate waveguide geometry. An analytical solution for the parallel plate waveguide geometry is presented and compared to the results for the spherical dimer configuration. We use this approximation as a reference solution, and also, it provides useful guidelines to understand the physical mechanism behind the energy transfer between the molecule and the dimer. We find that the emission intensity undergoes a quenching effect only when the inter-nanoparticle gap distance of the dimer is very small, meaning that strong coupling prevails over energy engaged in the heating process unless the molecule is extremely close to the metal surface.
Sub-0.1 nm-resolution quantitative scanning transmission electron microscopy without adjustable parameters
Authors: Dwyer C. , Maunders C. , Zheng C. L. , Weyland M. , Tiemeijer P. C. , Etheridge J.
Published: May 2012

Atomic-resolution imaging in the scanning transmission electron microscope (STEM) constitutes a powerful tool for nanostructure characterization. Here, we demonstrate the quantitative interpretation of atomic-resolution high-angle annular dark-field (ADF) STEM images using an approach that does not rely on adjustable parameters. We measure independently the instrumental parameters that affect sub-0.1 nm-resolution ADF images, quantify their individual and collective contributions to the image intensity, and show that knowledge of these parameters enables a quantitative interpretation of the absolute intensity and contrast across all accessible spatial frequencies. The analysis also provides a method for the in-situ measurement of the STEM’s effective source distribution.
Lattice thermal conductivity diminution and high thermoelectric power factor retention in nanoporous macroassemblies of sulfur-doped bismuth telluride nanocrystals
Authors: Zhang Yanliang , Mehta Rutvik J. , Belley Matthew , Han Liang , Ramanath Ganpati , Borca-Tasciuc Theodorian
Published: May 2012

We report ultralow lattice thermal conductivity in the 0.3 ≤ κL ≤ 0.6 W m−1 K−1 range in nanoporous bulk bismuth telluride pellets obtained by sintering chemically synthesized nanostructures, together with single-crystal-like electron mobilities and Seebeck coefficients at comparable charge carrier concentrations. The observed κL is up to 35% lower than classical effective medium predictions, and can be quantitatively explained by increased phonon scattering at nanopores and nanograins. Our findings are germane to tailoring nanoporous thermoelectric materials for efficient solid-state refrigeration, thermal energy harvesting, and thermal management applications.
Low-κ organic layer as a top gate dielectric for graphene field effect transistors
Authors: Mordi G. , Jandhyala S. , Floresca C. , McDonnell S. , Kim M. J. , Wallace R. M. , Colombo L. , Kim J.
Published: May 2012

We demonstrate the characteristics of dual gated graphene field effect transistors using a thin layer (∼7 nm) of parylene-C as a top-gate dielectric. Our devices exhibit good dielectric properties with minimal doping, low leakage current (∼10−6 A/cm2 at ±2 V), and a dielectric constant of ∼2.1. Additionally, Raman spectroscopy did not reveal any process induced defects after dielectric deposition. Electrical characterization performed in air showed a carrier mobility of ∼5050 cm2/Vs with hysteresis less than 30 mV during top gate operation (−2.5 V to 2.5 V) which indicates that parylene and its interface with graphene does not have a significant amount of trapped charges.
Utility of the inverse partial fluorescence for electronic structure studies of battery materials
Authors: Wadati H. , Achkar A. J. , Hawthorn D. G. , Regier T. Z. , Singh M. P. , Truong K. D. , Fournier P. , Chen G. , Mizokawa T. , Sawatzky G. A.
Published: May 2012

X-ray absorption spectroscopy is one of the most widely used experimental techniques to study the electronic and spatial structure of materials. Fluorescence yield mode is bulk-sensitive, but has several serious problems coming from saturation effects. In this study, we show the usefulness of partial fluorescence yields in addressing these problems. We discuss the different behaviors of La2NiMnO6 and LiMnO2 at the Mn 2p absorption edges. The total fluorescence yield produces misleading spectra for LiMnO2 due to the absence of high-Z (Z: atomic number) elements. We conclude that the measurement of the inverse partial fluorescence yield is essential in studies of LiMnO2, which is a hotly debated Li-ion battery material.
Investigation of CuSb4Te2 alloy for high-speed phase change random access memory applications
Authors: Lu Yegang , Song Sannian , Song Zhitang , Rao Feng , Wu Liangcai , Zhu Min , Liu Bo , Yao Dongning
Published: May 2012

The thermal stability of amorphous Sb2Te film can be significantly improved by the addition of Cu. CuSb4Te2 alloy is considered to be a potential candidate for phase change random access memory (PCRAM), as evidenced by a higher crystallization temperature, a better data retention ability, and a faster switching speed in comparison with those of Ge2Sb2Te5. A reversible switching between set and reset states can be realized by an electric pulse as short as 7 ns for CuSb4Te2-based PCRAM cell. In addition, CuSb4Te2 shows endurance up to 1.5 × 105 cycles with a resistance ratio of about two orders of magnitude.
A temperature behavior of the frustrated translational mode of adsorbate and the nature of the “adsorbate–substrate” interaction
Authors: Ignatyuk V. V.
Published: May 2012

A temperature behavior of the frustrated translational mode (T-mode) of a light particle, coupled by different regimes of ohmicity to the surface, is studied within a formalism of the generalized diffusion coefficients. The memory effects of the adsorbate motion are considered to be the main reason of the T-mode origin. Numerical calculations yield a thermally induced shift and broadening of the T-mode, which is found to be linear in temperature for Ohmic and super-Ohmic systems and nonlinear for strongly sub-Ohmic ones. We obtain analytical expressions for the T-mode shift and width at weak coupling for the systems with integer “ohmicity” indexes n = 0÷2 in zero temperature and high temperature limits. We provide an explanation of the experimentally observed blue- or redshifts of the T-mode on the basis of a comparative analysis of two typical times of the system evolution: a time of decay of the “velocity–velocity” autocorrelation function, and a correlation time of the thermal bath random forces. A relation of the T-mode to the multiple jumps of the adsorbate is discussed, and generalization of conditions of the multiple hopping to the case of quantum surface diffusion is performed.
Electron impact total cross section for acetylene over an extensive range of impact energies (1 eV–5000 eV)
Authors: Vinodkumar Minaxi , Barot Avani , Antony Bobby
Published: May 2012

Comprehensive study on electron impact for acetylene molecule is performed in terms of eigenphase diagram, electronic excitation cross sections as well as total cross section calculations from 1 eV to 5000 eV in this article. Computation of cross section over such a wide range of energy is reported for the first time. We have employed two distinct formalisms to derive cross sections in these impact energies. From 1 eV to ionization threshold of the target we have used the ab initio R-matrix method and then spherical complex optical potential method beyond that. At the crossing point of energy, both theories matched quite well and hence prove that they are consistent with each other. The results presented here expectedly give excellent agreement with other experimental values and theories available. The techniques employed here are well established and can be used to predict cross sections for other targets where data are scarce or not available. Also, this methodology may be integrated to online database such as Virtual Atomic and Molecular Data Centre to provide cross section data required by any user.
Direct assembly of three-dimensional mesh plasmonic rolls
Authors: Min Huang Fu , Sinha Jatin K. , Gibbons Nicholas , Bartlett Philip N. , Baumberg Jeremy J.
Published: May 2012

A direct-assembly method to construct three-dimensional (3D) plasmonic nanostructures yields porous plasmonic rolls through the strain-induced self-rolling up of two-dimensional metallic nanopore films. This route is scalable to different hole sizes and film thicknesses, and applicable to a variety of materials, providing general routes towards a diverse family of 3D metamaterials with nano-engineerable optical properties. These plasmonic rolls can be dynamically driven by light irradiation, rolling or unrolling with increasing or decreasing light intensity. Such dynamically controllable 3D plasmonic nanostructures offer opportunities both for sensing and feedback in active nano-actuators.
Excitation energies from range-separated time-dependent density and density matrix functional theory
Authors: Pernal Katarzyna
Published: May 2012

Time-dependent density functional theory (TD-DFT) in the adiabatic formulation exhibits known failures when applied to predicting excitation energies. One of them is the lack of the doubly excited configurations. On the other hand, the time-dependent theory based on a one-electron reduced density matrix functional (time-dependent density matrix functional theory, TD-DMFT) has proven accurate in determining single and double excitations of H2 molecule if the exact functional is employed in the adiabatic approximation. We propose a new approach for computing excited state energies that relies on functionals of electron density and one-electron reduced density matrix, where the latter is applied in the long-range region of electron-electron interactions. A similar approach has been recently successfully employed in predicting ground state potential energy curves of diatomic molecules even in the dissociation limit, where static correlation effects are dominating. In the paper, a time-dependent functional theory based on the range-separation of electronic interaction operator is rigorously formulated. To turn the approach into a practical scheme the adiabatic approximation is proposed for the short- and long-range components of the coupling matrix present in the linear response equations. In the end, the problem of finding excitation energies is turned into an eigenproblem for a symmetric matrix. Assignment of obtained excitations is discussed and it is shown how to identify double excitations from the analysis of approximate transition density matrix elements. The proposed method used with the short-range local density approximation (srLDA) and the long-range Buijse-Baerends density matrix functional (lrBB) is applied to H2 molecule (at equilibrium geometry and in the dissociation limit) and to Be atom. The method accounts for double excitations in the investigated systems but, unfortunately, the accuracy of some of them is poor. The quality of the other excitations is in general much better than that offered by TD-DFT-LDA or TD-DMFT-BB approximations if the range-separation parameter is properly chosen. The latter remains an open problem.
Evaluation of the grand-canonical partition function using expanded Wang-Landau simulations. I. Thermodynamic properties in the bulk and at the liquid-vapor phase boundary
Authors: Desgranges Caroline , Delhommelle Jerome
Published: May 2012

The Wang-Landau sampling is a powerful method that allows for a direct determination of the density of states. However, applications to the calculation of the thermodynamic properties of realistic fluids have been limited so far. By combining the Wang-Landau method with expanded grand-canonical simulations, we obtain a high-accuracy estimate for the grand-canonical partition function for atomic and molecular fluids. Then, using the formalism of statistical thermodynamics, we are able to calculate the thermodynamic properties of these systems, for a wide range of conditions spanning the single-phase regions as well as the vapor-liquid phase boundary. Excellent agreement with prior simulation work and with the available experimental data is obtained for argon and CO2, thereby establishing the accuracy of the method for the calculation of thermodynamic properties such as free energies and entropies.
Evaluation of the grand-canonical partition function using expanded Wang-Landau simulations. II. Adsorption of atomic and molecular fluids in a porous material
Authors: Desgranges Caroline , Delhommelle Jerome
Published: May 2012

We propose to apply expanded Wang-Landau simulations to study the adsorption of atomic and molecular fluids in porous materials. This approach relies on a uniform sampling of the number of atoms and molecules adsorbed. The method consists in determining a high-accuracy estimate of the grand-canonical partition function for the adsorbed fluids. Then, using the formalism of statistical mechanics, we calculate absolute and excess thermodynamic properties relevant to adsorption processes. In this paper, we examine the adsorption of argon and carbon dioxide in the isoreticular metal-organic framework (IRMOF-1). We assess the reliability of the method by showing that the predicted adsorption isotherms and isosteric heats are in excellent agreement with simulation results obtained from grand-canonical Monte Carlo simulations. We also show that the proposed method is very efficient since a single expanded Wang-Landau simulation run at a given temperature provides the whole adsorption isotherm. Moreover, this approach provides a direct access to a wide range of thermodynamic properties, such as, e.g., the excess Gibbs free energy and the excess entropy of adsorption.
Efficiency degradation behaviors of current/thermal co-stressed GaN-based blue light emitting diodes with vertical-structure
Authors: Liu Lilin , Ling Minjie , Yang Jianfu , Xiong Wang , Jia Weiqing , Wang Gang
Published: May 2012

With this work, we demonstrate a three-stage degradation behavior of GaN based LED chips under current/thermal co-stressing. The three stages in sequence are the initial improvement stage, the platform stage, and the rapid degradation stage, indicating that current/thermal co-stressing activates positive effects and negative ones simultaneously, and the dominant degradation mechanisms evolve with aging time. Degradation mechanisms are discussed. Electric current stress has dual characters: damaging the active layers by generating defects and at the same time improving the p-type conductivity by activating the Mg-dopant. High temperature stresses will promote the effects from electric current stresses. The activation of the Mg-dopant will saturate, whereas the generation of defects is carried on in a progressive way. Other mechanisms, such as deterioration of ohmic contacts, also operate. These mechanisms compete/cooperate with each other and evolve with aging time, resulting in the observed three-stage degradation behavior. There exist risks to predict the lifetime of LEDs by a model with a constant accelerated factor.
Direct calculation of the attempt frequency of magnetic structures using the finite element method
Authors: Fiedler G. , Fidler J. , Lee J. , Schrefl T. , Stamps R. L. , Braun H. B. , Suess D.
Published: May 2012

A numerical implementation of the transition state theory is presented which can be used to calculate the attempt frequency f0 of arbitrary shaped magnetic nanostructures. The micromagnetic equations are discretized using the finite element method. The climbing image nudged elastic band method is used to calculate the saddle point configuration, which is required for the calculation of f0. Excellent agreement of the implemented numerical model and analytical solutions is obtained for single domain particles. The developed method is applied to compare f0 for single phase and graded media grains of advanced recording media. f0 is predicted to be comparable if the maximum anisotropy is the same in these two media types.
Fourth virial coefficients of asymmetric nonadditive hard-disk mixtures
Authors: Saija Franz , Santos Andrés , Yuste Santos B. , López de Haro Mariano
Published: May 2012

The fourth virial coefficient of asymmetric nonadditive binary mixtures of hard disks is computed with a standard Monte Carlo method. Wide ranges of size ratio (0.05 ⩽ q ⩽ 0.95) and nonadditivity (−0.5 ⩽ Δ ⩽ 0.5) are covered. A comparison is made between the numerical results and those that follow from some theoretical developments. The possible use of these data in the derivation of new equations of state for these mixtures is illustrated by considering a rescaled virial expansion truncated to fourth order. The numerical results obtained using this equation of state are compared with Monte Carlo simulation data in the case of a size ratio q = 0.7 and two nonadditivities Δ = ±0.2.
Voltage response of non-uniform arrays of bi-superconductive quantum interference devices
Authors: Longhini Patrick , Berggren Susan , Leese de Escobar Anna , Palacios Antonio , Rice Sarah , Taylor Benjamin , In Visarath , Mukhanov Oleg A. , Prokopenko Georgy , Nisenoff Martin , Wong Edmond , De Andrade Marcio C.
Published: May 2012

Multi-loop arrays of Josephson junctions (JJs) with non-uniform area distributions, which are known as superconducting quantum interference filters (SQIFs), are the most highly sensitive sensors of changes in applied magnetic field as well as the absolute magnitude of magnetic fields. The non-uniformity of the loop sizes allows the array to produce a unique collective voltage response that has a pronounced single peak with a large voltage swing around zero magnetic field. To obtain high linear dynamic range, which is critical for a wide variety of applications, the linearity of the slope of the anti-peak response must be improved. We propose a novel scheme for enhancing linearity—a new configuration combining the SQIF array concept with the recently introduced bi-superconductive quantum interference device (SQUID) configuration, in which each individual SQUID loop is made up of three JJs as opposed to using two JJs per loop in standard dc SQUIDs. We show, computationally, that the additional junction offers a viable linearization method for optimizing the voltage response and dynamic range of SQIF arrays. We have realized SQIF arrays based on bi-SQUID cells and present first experimental results.
Effects of media stray field on electromigration characteristics in current-perpendicular-to-plane giant magnetoresistance spin-valve read sensors
Authors: Gui Zeng Ding , Lee Kyoung-il , Chung Kyung-Won , Bae Seongtae
Published: May 2012

Effects of magnetic stray field retrieved from both longitudinal and perpendicular magnetic recording media (denoted by “media stray field”) on electromigration (EM) characteristics of current-perpendicular-to-plane (CPP) giant magnetoresistance spin-valve (GMR SV) read sensors have been numerically studied to explore the electrical and magnetic stability of the read sensor under real operation. The mean-time-to-failure (MTTF) of the CPP GMR SV read sensors was found to have a strong dependence on the physical parameters of the recording media and recorded information status, such as the pulse width of media stray field, the bit length, and the head moving velocity. According to the numerical calculation results, it was confirmed that in the longitudinal media, the shorter the stray field pulse width (i.e., the sharper the media transition) allows for the longer MTTF of the CPP GMR SV read sensors; while in the perpendicular media, the sharper the media transition gives rise to a shorter MTTF. Interestingly, it was also revealed that the MTTF could be improved by reducing the bit length as well as increasing the head velocity in both longitudinal and perpendicular media. Furthermore, the bit distribution patterns, especially the number of consecutive ‘0’ bits strongly affected the MTTF of GMR SV read sensors. The strong dependences of MTTF on the media stray field during CPP GMR SV sensor operation are thought to be mainly attributed to the thermal cycling (temperature rise and fall) caused by the resistance change due to GMR effects.
Transparent conductivity of fluorine-doped anatase TiO2 epitaxial thin films
Authors: Mohri Satoru , Hirose Yasushi , Nakao Shoichiro , Yamada Naoomi , Shimada Toshihiro , Hasegawa Tetsuya
Published: May 2012

Transparent conductive anatase TiO2−xFx(F:TiO2) epitaxial thin films were fabricated by reactive pulsed laser deposition with a solid fluorine source. F-doping as high as 1.6 × 1021 cm−3 (TiO1.95F0.05) was achieved under optimal growth conditions, and the obtained anatase TiO1.95F0.05 film had a low resistivity of 1.6 × 10−3 Ωcm and a high internal transmittance of >95%. Furthermore, the refractive index of a TiO1.95F0.05 film was >0.2 less than that of undoped TiO2. By comparing the refractive indices of F:TiO2 and Nb:TiO2, we concluded that the decreased refractive index in F:TiO2 can be attributed to a reduced electronic polarizability due to increased bond ionicity as well as doped electrons.
Retrofitting an atomic force microscope with photothermal excitation for a clean cantilever response in low Q environments
Authors: Labuda Aleksander , Kobayashi Kei , Miyahara Yoichi , Grütter Peter
Published: May 2012

It is well known that the low-Q regime in dynamic atomic force microscopy is afflicted by instrumental artifacts (known as “the forest of peaks”) caused by piezoacoustic excitation of the cantilever. In this article, we unveil additional issues associated with piezoacoustic excitation that become apparent and problematic at low Q values. We present the design of a photothermal excitation system that resolves these issues, and demonstrate its performance on force spectroscopy at the interface of gold and an ionic liquid with an overdamped cantilever (Q < 0.5). Finally, challenges in the interpretation of low-Q dynamic AFM measurements are discussed.
Semiclassical approach to model quantum fluids using the statistical associating fluid theory for systems with potentials of variable range
Authors: Trejos Víctor M. , Gil-Villegas Alejandro
Published: May 2012

Thermodynamic properties of quantum fluids are described using an extended version of the statistical associating fluid theory for potentials of variable range (SAFT-VR) that takes into account quantum corrections to the Helmholtz free energy A, based on the Wentzel-Kramers-Brillouin approximation. We present the theoretical background of this approach (SAFT-VRQ), considering two different cases depending on the continuous or discontinuous nature of the particles pair interaction. For the case of continuous potentials, we demonstrate that the standard Wigner-Kirkwood theory for quantum fluids can be derived from the de Broglie-Bohm formalism for quantum mechanics that can be incorporated within the Barker and Henderson perturbation theory for liquids in a straightforward way. When the particles interact via a discontinuous pair potential, the SAFT-VR method can be combined with the perturbation theory developed by Singh and Sinha [J. Chem. Phys. 67, 3645 (1977); Singh and Sinha J. Chem. Phys. 68, 562 (1978)]. We present an analytical expression for the first-order quantum perturbation term for a square-well potential, and the theory is applied to model thermodynamic properties of hydrogen, deuterium, neon, and helium-4. Vapor-liquid equilibrium, liquid and vapor densities, isochoric and isobaric heat capacities, Joule-Thomson coefficients and inversion curves are predicted accurately with respect to experimental data. We find that quantum corrections are important for the \documentclass[12pt]{minimal}\begin{document}$\it global$\end{document}global behavior of properties of these fluids and not only for the low-temperature regime. Predictions obtained for hydrogen compare very favorably with respect to cubic equations of state.
Physics of shell assembly: Line tension, hole implosion, and closure catastrophe
Authors: Luque Antoni , Reguera David , Morozov Alexander , Rudnick Joseph , Bruinsma Robijn
Published: May 2012

The self-assembly of perfectly ordered closed shells is a challenging process involved in many biological and nanoscale systems. However, most of the aspects that determine their formation are still unknown. Here we investigate the growth of shells by simulating the assembly of spherical structures made of N identical subunits. Remarkably, we show that the formation and energetics of partially assembled shells are dominated by an effective line-tension that can be described in simple thermodynamic terms. In addition, we unveil two mechanisms that can prevent the correct formation of defect-free structures: “hole implosion,” which leads to a premature closure of the shell; and “closure catastrophe,” which causes a dramatic production of structural disorder during the later stages of the growth of big shells.
Visual and quantitative measurement of the temperature distribution of heat conduction process in glass based on digital holographic interferometry
Authors: Wang Qian , Zhao Jianlin , Jiao Xiangyang , Di Jianglei , Jiang Hongzhen
Published: May 2012

The temperature distribution of heat conduction process in transparent solid medium is visually and quantitatively measured based on digital holographic interferometry. A series of phase maps reflecting the temperature distribution of a glass sample during the heat conduction process are numerically reconstructed from the digital holograms. Then, based on the derived relationship between temperature variation and phase change, we obtained the full field temperature distribution of the glass sample by the method of sample point calibration. By seriating maps of the temperature distribution, a movie is produced to show the heat conduction phenomenon more vividly. What is more, based on the thermodynamic model of heat conduction, we numerically figured out the temperature distribution of the glass sample using the finite element algorithm. It turns out that the experimental results are consistent with the numerical simulation results very well.
A computational study of high-frequency behavior of graphene field-effect transistors
Authors: Chauhan Jyotsna , Liu Leitao , Lu Yang , Guo Jing
Published: May 2012

High Frequency potential of graphene field-effect transistors (FETs) is explored by quasi-static self-consistent ballistic and dissipative quantum transport simulations. The unity power gain frequency fMAX and the cut-off frequency fT are modeled at the ballistic limit and in the presence of inelastic phonon scattering for a gate length down to 5 nm. Our major results are (1) with a thin high-κ gate insulator, the intrinsic ballistic fT is above 5 THz at a gate length of 10 nm. (2) Inelastic phonon scattering in graphene FETs lowers both fT and fMAX, mostly due to decrease of the transconductance. (3) fMAX and fT are severely degraded in presence of source and drain contact resistance. (4) To achieve optimum extrinsic fMAX performance, careful choice of DC bias point and gate width is needed.
Modeling of high-frequency capacitance-voltage characteristics to quantify trap distributions near SiO2/SiC interfaces
Authors: Basile A. F. , Mooney P. M.
Published: May 2012

A procedure to calculate capacitance-voltage (C-V) characteristics from numerical solutions of the Poisson equation for metal-oxide-semiconductor (MOS) capacitors with traps located both at the oxide/semiconductor interface and in the semiconductor is presented. This method is tested for the simple case of a uniform distribution of a single acceptor or donor level in the semiconductor, for which an approximate analytical solution was derived by analogy with the results obtained for a Schottky diode within the depletion approximation. This method is then applied to model the high-frequency C-V curves of n-type 4H- and 6H-SiC MOS capacitors, which show a kink at depletion bias voltages that broadens with decreasing temperature below 150 K. This feature of the high-frequency capacitance occurs at the same temperature and voltage range as a signal detected by constant capacitance deep-level-transient spectroscopy (CCDLTS) measurements and attributed to SiC traps. When only interface traps are considered, the trap energy distribution that is required to reproduce the kink in the C-V curves is not consistent with the trap energy distribution determined from CCDLTS measurements. Numerical simulations show that traps in the SiC epi-layer near the SiO2/SiC interface as well as interface traps with energies close to the SiC conduction band are necessary to explain both the CCDLTS measurement results and the temperature dependence of C–V curves.
Ferromagnetism in ZrFe12−xAlx and HfFe12−xAlx (x = 6.0, 6.5, 7.0)
Authors: McGuire Michael A. , Ghimire Nirmal , Singh David J.
Published: May 2012

Results of crystal structure, microstructure, and magnetic properties studies of arcmelted and annealed ZrFe6Al6, ZrFe5.5Al6.5, ZrFe5Al7, HfFe6Al6, HfFe5.5Al6.5, and HfFe5Al7 are reported. These compounds adopt the ThMn12 structure-type (space group 139, I4/mmm), like their more common rare-earth analogues. Analysis of powder x-ray diffraction data show that the 8f site is occupied by Fe, the 8i site by Al, and the 8j site by a mixture of Fe and Al. All of the compounds undergo a ferromagnetic transition at temperatures ranging from 170 to 270 K. Three effects on the magnetic properties are noted as the Fe content is increased: decrease in the Curie temperature, decrease in the magnetic moment per Fe at high fields, and the development of non-monotonic temperature dependence of the low-field magnetization below the Curie temperature. First principles calculations indicate a ferromagnetic ground state, but find antiferromagnetic interactions among Fe moments on 8j sites which is likely important in understanding the observed behavior.
First-principles study of Cs and Sr sorption on carbon structures
Authors: Londono-Hurtado Alejandro , Morgan Dane , Szlufarska Izabela
Published: May 2012

Metal sorption on carbon plays an important role in a range of applications, including batteries, supercapacitors, and nuclear reactors. In particular, understanding sorption of radioactive isotopes, such as Cs and Sr in different graphite materials, is important for establishing nuclear reactor safety. This work uses ab initio methods to predict the binding behavior of Cs and Sr adatoms to perfect graphite, amorphous carbon, and diamond structures. The aim of these simulations is to analyze the influence of sp2 / sp3 binding and graphite defects on adatom binding strength. Results show a tendency of defects in graphite and carbon structures with sp3 bonds to form stronger bonds with the adatoms than perfect sp2 graphite. Comparison with experimental sorption data suggests that defects and sp3 bonded regions play a critical role in the sorption behavior of Cs and Sr on graphite.
Diluted antiferromagnet effect on magnetic and microwave characteristics of exchange-biased multilayered thin films
Authors: Phuoc Nguyen N. , Ong C. K.
Published: May 2012

A comprehensive experimental study of the effect of antiferromagnetic (AF) dilution on the magnetic properties and microwave characteristics of Fe70Co30/(Mn75Ir25)1−x(Al2O3)x multilayered thin films with the variation of AF thickness was performed. Our result reveals a linear relationship between the critical AF thickness and the dilution of the AF layers providing an evidence of a volume effect in the AF layers consistent with the domain state model. The AF magnetic anisotropy deduced from the critical AF thickness was found to reduce monotonically with the AF dilution. The variations of exchange bias, rotational magnetic anisotropy, ferromagnetic resonance frequency, and effective damping factor with the AF dilution and thickness have also been presented and discussed.
Study of charge distribution and charge loss in dual-layer metal-nanocrystal-embedded high-κ/SiO2 gate stack
Authors: Lwin Z. Z. , Pey K. L. , Zhang Q. , Bosman M. , Liu Q. , Gan C. L. , Singh P. K. , Mahapatra S.
Published: May 2012

In this work, we present a comprehensive experimental study of charge loss mechanisms in a dual-layer metal nanocrystal (DL-MNC) embedded high-κ/SiO2 gate stack. Kelvin force microscopy characterization reveals that the internal-electric-field assisted tunneling could be a dominant charge loss mechanism in DL devices that mainly depends on the charge distribution in two MNC-layers and inter-layer dielectric (ILD) thickness between the two layers of nanocrystals. Our findings suggest that an optimized DL-MNCs embedded memory cell could be achieved by defining the ILD thickness larger than the average MNC-spacing for enhancement of retention ability in MNC embedded gate stacks. It implies the possibility of reducing MNC spacing in DL structure of scaled memory devices by controlling the thickness of ILD.
Dual conductance, negative differential resistance, and rectifying behavior in a molecular device modulated by side groups
Authors: Wan Haiqing , Xu Ying , Zhou Guanghui
Published: May 2012

We investigate the electronic transport properties for a molecular device model constructed by a phenylene ethynylene oligomer molecular with different side groups embedding in a carbon chain between two graphene electrodes. Using the first-principles method, the unusual dual conductance, negative differential resistance (NDR) behavior with large peak to valley ratio, and obvious rectifying performance are numerically observed in such proposed molecular device. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals (MOs) as well as transmission coefficients under various external voltage biases gives an inside view of the observed results, which suggests that the dual conductance behavior and rectifying performance are due to the asymmetry distribution of the frontier MOs as well as the corresponding coupling between the molecule and electrodes. But the NDR behavior comes from the conduction orbital being suppressed at certain bias. Interestingly, the conduction properties can be tuned by introducing side groups to the molecule and the rectification as well as the NDR behavior (peak to valley ratio) can be improved by adding different side groups in the device model.
Enhanced charge transfer by phenyl groups at a rubrene/C60 interface
Authors: Mou Weiwei , Ohmura Satoshi , Hattori Shinnosuke , Nomura Ken-ichi , Shimojo Fuyuki , Nakano Aiichiro
Published: May 2012

Exciton dynamics at an interface between an electron donor, rubrene, and a C60 acceptor is studied by nonadiabatic quantum molecular dynamics simulation. Simulation results reveal an essential role of the phenyl groups in rubrene in increasing the charge-transfer rate by an order-of-magnitude. The atomistic mechanism of the enhanced charge transfer is found to be the amplification of aromatic breathing modes by the phenyl groups, which causes large fluctuations of electronic excitation energies. These findings provide insight into molecular structure design for efficient solar cells, while explaining recent experimental observations.