bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of bibliometric indicators to determine citation bias
Authors: Simko Ivan
Published: June 2015

Citations of research papers and citation-related indicators are frequently used factors in determining research priorities, allocating funding, and deciding appointments, promotions and tenures. The main problem with using citations for a variety of evaluations is a substantial difference in the average number of citations received by papers in different research fields and subfields. A large number of species are subjects of biological research, but the distributions of their citations have not been studied in detail. The key objective of the present work was to determine whether the choice of experimental subjects influence bias in citations. A case study was performed on papers from 108 plant species and five research fields. Funnel plot analyses and computer simulations identified species-related citation bias within all research fields. Relationships between bibliometric indicators imply that species with a fast growing number of publications in recent years (for example, new model organisms) generally have a higher average number of citations per paper than is the overall mean for the research field. In contrast, the average number of citations received by the five most prominent papers of the species was strongly correlated with the total number of published papers from laboratories working with the species. The current study indicates that despite the high frequency of cross-species citations, citations of species show a pattern similar to separate subfields. Although these analyses were performed on plant research papers, the findings have relevance for other areas of research where experimental subjects tend to form separate subfields. To reliably compare citations across subfields, a new type of bibliometric indicator is needed. In the meantime committees evaluating quality of research should take into consideration that citations of papers within a research field might be biased due to the experimental subjects used in the studies.
Suppression of spin pumping between Ni80Fe20 and Cu by a graphene interlayer
Authors: Gannett Will , Keller Mark W. , Nembach Hans T. , Silva Thomas J. , Chiaramonti Ann N.
Published: June 2015

We compare ferromagnetic resonance measurements of Permalloy Ni80Fe20 (Py) films sputtered onto Cu(111) films with and without a graphene (Gr) interlayer grown by chemical vapor deposition before Py deposition. A two-angle sputtering method ensured that neither Gr nor Py was degraded by the sample preparation process. We find the expected damping enhancement from spin pumping for the Py/Cu case and no detectable enhancement for the Py/Gr/Cu case. Since damping is sensitive to effects other than spin pumping, we used magnetometry to verify that differences in Py magnetostatic properties are not responsible for the difference in damping. We attribute the suppression of spin pumping in Py/Gr/Cu to the large contact resistance of the Gr/Cu interface.
Thermal conductivity of nanostructured SixGe1−x in amorphous limit by molecular dynamics simulation
Authors: Norouzzadeh Payam , Nozariasbmarz Amin , Krasinski Jerzy S. , Vashaee Daryoosh
Published: June 2015

We report the thermal conductivity of amorphous SixGe1−x compound calculated versus composition and temperature. The result sets the minimum value of thermal conductivity which is achievable by nanostructuring. We employed molecular dynamics with Tersoff's potential for the calculations. It was found that, contrary to the crystalline SixGe1−x, the thermal conductivity of amorphous phase is a weak function of the material composition. For the most popular composition Si0.8Ge0.2, the thermal conductivity of the amorphous phase is less than 1 W m−1 K−1 with small reduction as the temperature increases from 300 K to 1400 K. The thermal conductivity of amorphous SixGe1−x for any value of x is approximately an order of magnitude smaller than the minimum thermal conductivity of crystalline SixGe1−x alloy, which occurs near x = 0.5. It is known that alloying with germanium is more efficient than nanostructuring to reduce the thermal conductivity of silicon; however, it was found that the amorphization process is even more effective than alloying for that purpose. It was also shown that the reduction of the thermal conductivity of silicon due to alloying with germanium is more efficient in crystalline phase than in amorphous phase.
Dynamics of cylindrical converging shock waves interacting with aerodynamic obstacle arrays
Authors: Vignati F. , Guardone A.
Published: June 2015

Cylindrical converging shock waves interacting with an array of aerodynamic obstacles are investigated numerically for diverse shock strengths and for different obstacle configurations in air in standard conditions. The considered number of obstacles N is 4, 6, 8, 16, and 24. Obstacles are lenticular airfoils with thickness-to-chord ratios tc of 0.07, 0.14, and 0.21. The distances of the airfoil leading edge from the shock focus point rLErLEref are 1, 2, and 2.5, where rLEref=7 is the dimensionless reference distance from the origin. Considered impinging shock Mach numbers Ms are 2.2, 2.7, and 3.2 at the reference distance from the origin. The reference experimental configuration (N=8,tc=0.14,rLE=7,Ms=2.7) was proposed by Kjellander et al. [“Thermal radiation from a converging shock implosion,” Phys. Fluids 22, 046102 (2010)]. Numerical results compare fairly well to available one-dimensional models for shock propagation and to available experimental results in the reference configuration. Local reflection types are in good agreement with the classical criteria for planar shock waves. The main shock reshaping patterns are identified and their dependence on the shock strength and obstacle configuration is exposed. In particular, different shock patterns are observed after the leading edge reflection, which results in polygonal shock wave with N, 2N, 3N, and 4N sides. The largest temperature peak at the origin is obtained for the 8- and the 16-obstacle configurations and for the smallest thickness to length ratio, 0.07, located at distance from the origin of 2rLEref. In terms of compression efficiency at the origin, the 16-obstacle configuration is found to perform slightly better than the reference 8-obstacle configuration—with an efficiency increase of about 2%-3%, which is well within the model accuracy—thus confirming the goodness of the obstacle arrangement proposed by Kjellander and collaborators.
An experimental investigation of vortex-induced vibration of a rotating circular cylinder in the crossflow direction
Authors: Seyed-Aghazadeh Banafsheh , Modarres-Sadeghi Yahya
Published: June 2015

Vortex-induced vibration of a flexibly-mounted circular cylinder free to oscillate in the crossflow direction with imposed rotation around its axis was studied experimentally. The rotation rate, α, defined as the ratio of the surface velocity and free stream velocity, was varied from 0 to 2.6 in small steps. The amplitudes and frequencies of oscillations as well as the flow forces were measured in a Reynolds number range of Re = 350 -1000. The maximum amplitude of oscillations was limited to values less than a diameter of the cylinder at high rotation rates. Also, the lock-in range became narrower at higher rotation rates and finally the oscillations ceased beyond α = 2.4. Vortex shedding pattern was found to be 2S (two single vortices shed per cycle of oscillations) for rotation rates up to α = 1.4 and transitioned toward an asymmetric P shedding (one pair of vortices shed in a cycle of oscillations) for rotation rates within the range of 1.4 ≤ α ≤ 1.8. Vortex shedding was found to persist up to higher rotation rates than those observed for a non-oscillating cylinder. The phase difference between the flow forces and displacement of the cylinder in the crossflow direction was influenced as the rotation rate was increased: At high reduced velocities, the phase difference decreased from 180° for a non-rotating cylinder to values close to 90° for a rotating cylinder at large rotation rates. Different shedding patterns resulted in flow forces with different frequencies. In the crossflow direction, the dominant frequency of flow forces was found to be close to the system’s natural frequency for all the rotation rates tested with either 2S or P vortex shedding pattern. In the inline direction, however, the change from 2S to P shedding at high rotation rates resulted in a shift of the ratio of the dominant frequency of the inline flow forces to the natural frequency of the system from 2:1 to 1:1.
Detailed investigation of optoelectronic and microstructural properties of plasma polymerized cyclohexane thin films: Dependence on the radiofrequency power
Authors: Manaa C. , Bouaziz L. , Lejeune M. , Kouki F. , Zellama K. , Benlahsen M. , Mejatty M. , Bouchriha H.
Published: June 2015

Optical properties of polymerized cyclohexane films deposited by radiofrequency plasma enhanced chemical vapor deposition technique at different radiofrequency powers onto glass and silicon substrates, are studied and correlated with the microstructure of the films, using a combination of atomic force microscopy, Raman and Fourier Transformer Infrared spectroscopy and optical measurements. The optical constants such as refractive index n, dielectric permittivity ε and extinction k and absorption α coefficients, are extracted from transmission and reflection spectra through the commercial software CODE. These constants lead, by using common theoretical models as Cauchy, Lorentz, Tauc and single effective oscillator, to the determination of the static refractive index ns and permittivity εs, the plasma frequency ωp, the carrier density to effective mass ratio N/me*, the optical conductivity σoc, the optical band gap Eg and the oscillation and dispersion energies E0 and Ed, respectively. We find that n, εs, ωp, N/me*, Ed, increase with radiofrequency power, while Eg and E0 decrease in the same range of power. These results are well correlated with those obtained from atomic force microscopy, Raman and infrared measurements. They also indicate that the increase of the radiofrequency power promotes the fragmentation of the precursor and increases the carbon C-sp2 hybridization proportion, which results in an improvement of the optoelectronic properties of the films.
Coupled dopant diffusion and segregation in inhomogeneous SiGe alloys: Experiments and modeling
Authors: Lin Yiheng , Yasuda Hiroshi , Schiekofer Manfred , Xia Guangrui (Maggie)
Published: June 2015

A coupled diffusion and segregation model was derived, where the contributions from diffusion and segregation to dopant flux are explicitly shown. The model is generic to coupled diffusion and segregation in inhomogeneous alloys, and provides a new approach in segregation coefficient extraction, which is especially helpful for heterostructures with lattice mismatch strains. Experiments of coupled P diffusion and segregation were performed with graded SiGe layers for Ge molar fractions up to 0.18, which are relevant to pnp SiGe heterojunction bipolar transistors. The model was shown to describe both diffusion and segregation behavior well. The diffusion-segregation model for P in SiGe alloys was calibrated and Eseg=0.5 eV is suggested for the temperature range from 800 °C to 950 °C.
Hydrodynamic segregation in a bidisperse colloidal suspension in microchannel flow: A theoretical study
Authors: Kanehl Philipp , Stark Holger
Published: June 2015

Colloids in suspension exhibit shear-induced migration towards regions of low viscous shear. In dense bidisperse colloidal suspensions under pressure driven flow large particles can segregate in the center of a microchannel and the suspension partially demixes. To develop a theoretical understanding of these effects, we formulate a phenomenological model for the particle currents based on the work of Phillips et al. [Phys. Fluids 4, 30 (1992)]. We also simulate hard spheres under pressure-driven flow in two and three dimensions using the mesoscale simulation technique of multi-particle collision dynamics. Using a single fit parameter for the intrinsic diffusivity, our theory accurately reproduces the simulated density profiles across the channel. We present a detailed parameter study on how a monodisperse suspension enriches the channel center and quantitatively confirm the experimental observation that a binary colloidal mixture partially segregates into its two species. In particular, we always find a strong accumulation of large particles in the center. Qualitative differences between two and three dimensions reveal that collective diffusion is more relevant in two dimensions.
Mesoscale elucidation of laser-assisted chemical deposition of Sn nanostructured electrodes
Authors: Liu Zhixiao , Deng Biwei , Cheng Gary J. , Deng Huiqiu , Mukherjee Partha P.
Published: June 2015

Nanostructured tin (Sn) is a promising high-capacity electrode for improved performance in lithium-ion batteries for electric vehicles. In this work, Sn nanoisland growth for nanostructured electrodes assisted by the pulse laser irradiation has been investigated based on a mesoscale modeling formalism. The influence of pertinent processing conditions, such as pulse duration, heating/cooling rates, and atom flux, on the Sn nanostructure formation is specifically considered. The interaction between the adsorbed atom and the substrate, represented by the adatom diffusion barrier, is carefully studied. It is found that the diffusion barrier predominantly affects the distribution of Sn atoms. For both α-Sn and β-Sn, the averaged coordination number is larger than 3 when the diffusion barrier equals to 0.15 eV. The averaged coordination number decreases as the diffusion barrier increases. The substrate temperature, which is determined by heating/cooling rates and pulse duration, can also affect the formation of Sn nanoislands. For α-Sn, when applied low heating/cooling rates, nanoislands cannot form if the diffusion barrier is larger than 0.35 eV.
Building proper invariants for eddy-viscosity subgrid-scale models
Authors: Trias F. X. , Folch D. , Gorobets A. , Oliva A.
Published: June 2015

Direct simulations of the incompressible Navier-Stokes equations are limited to relatively low-Reynolds numbers. Hence, dynamically less complex mathematical formulations are necessary for coarse-grain simulations. Eddy-viscosity models for large-eddy simulation is probably the most popular example thereof: they rely on differential operators that should properly detect different flow configurations (laminar and 2D flows, near-wall behavior, transitional regime, etc.). Most of them are based on the combination of invariants of a symmetric tensor that depends on the gradient of the resolved velocity field, G=∇u¯. In this work, models are presented within a framework consisting of a 5D phase space of invariants. In this way, new models can be constructed by imposing appropriate restrictions in this space. For instance, considering the three invariants PGGT, QGGT, and RGGT of the tensor G GT, and imposing the proper cubic near-wall behavior, i.e., νe=O(y3), we deduce that the eddy-viscosity is given by νe=(Cs3pqrΔ)2PGGTpQGGT−(p+1)RGGT(p+5/2)/3. Moreover, only RGGT-dependent models, i.e., p > − 5/2, switch off for 2D flows. Finally, the model constant may be related with the Vreman’s model constant via Cs3pqr=3CVr≈0.458; this guarantees both numerical stability and that the models have less or equal dissipation than Vreman’s model, i.e., 0≤νe≤νeVr. The performance of the proposed models is successfully tested for decaying isotropic turbulence and a turbulent channel flow. The former test-case has revealed that the model constant, Cs3pqr, should be higher than 0.458 to obtain the right amount of subgrid-scale dissipation, i.e., Cs3pq = 0.572 (p = − 5/2), Cs3pr = 0.709 (p = − 1), and Cs3qr = 0.762 (p = 0).
Isotope analysis of diamond-surface passivation effect of high-temperature H2O-grown atomic layer deposition-Al2O3 films
Authors: Hiraiwa Atsushi , Saito Tatsuya , Matsumura Daisuke , Kawarada Hiroshi
Published: June 2015

The Al2O3 film formed using an atomic layer deposition (ALD) method with trimethylaluminum as Al precursor and H2O as oxidant at a high temperature (450 °C) effectively passivates the p-type surface conduction (SC) layer specific to a hydrogen-terminated diamond surface, leading to a successful operation of diamond SC field-effect transistors at 400 °C. In order to investigate this excellent passivation effect, we carried out an isotope analysis using D2O instead of H2O in the ALD and found that the Al2O3 film formed at a conventional temperature (100 °C) incorporates 50 times more CH3 groups than the high-temperature film. This CH3 is supposed to dissociate from the film when heated afterwards at a higher temperature (550 °C) and causes peeling patterns on the H-terminated surface. The high-temperature film is free from this problem and has the largest mass density and dielectric constant among those investigated in this study. The isotope analysis also unveiled a relatively active H-exchange reaction between the diamond H-termination and H2O oxidant during the high-temperature ALD, the SC still being kept intact. This dynamic and yet steady H termination is realized by the suppressed oxidation due to the endothermic reaction with H2O. Additionally, we not only observed the kinetic isotope effect in the form of reduced growth rate of D2O-oxidant ALD but found that the mass density and dielectric constant of D2O-grown Al2O3 films are smaller than those of H2O-grown films. This is a new type of isotope effect, which is not caused by the presence of isotopes in the films unlike the traditional isotope effects that originate from the presence of isotopes itself. Hence, the high-temperature ALD is very effective in forming Al2O3 films as a passivation and/or gate-insulation layer of high-temperature-operation diamond SC devices, and the knowledge of the aforementioned new isotope effect will be a basis for further enhancing ALD technologies in general.
Control of optical response of a supported cluster on different dielectric substrates
Authors: Iida Kenji , Noda Masashi , Nobusada Katsuyuki
Published: June 2015

We develop a computational method for optical response of a supported cluster on a dielectric substrate. The substrate is approximated by a dielectric continuum with a frequency-dependent dielectric function. The computational approach is based on our recently developed first-principles simulation method for photoinduced electron dynamics in real-time and real-space. The approach allows us to treat optical response of an adsorbate explicitly taking account of interactions at an interface between an adsorbate and a substrate. We calculate optical absorption spectra of supported Agn (n = 2, 54) clusters, changing the dielectric function of a substrate. By analyzing electron dynamics in real-time and real-space, we clarify the mechanisms for variations in absorption spectra, such as peak shifts and intensity changes, relating to various experimental results for optical absorption of supported clusters. Attractive and repulsive interactions between an adsorbate and a substrate result in red and blue shifts, respectively, and the intensity decreases by energy dissipation into a substrate. We demonstrate that optical properties can be controlled by varying the dielectric function of a substrate.
Photoluminescence excitation measurements using pressure-tuned laser diodes
Authors: Bercha Artem , Ivonyak Yurii , Mędryk Radosław , Trzeciakowski Witold A. , Dybała Filip , Piechal Bernard
Published: June 2015

Pressure-tuned laser diodes in external cavity were used as tunable sources for photoluminescence excitation (PLE) spectroscopy. The method was demonstrated in the 720 nm-1070 nm spectral range using a few commercial laser diodes. The samples for PLE measurements were quantum-well structures grown on GaAs and on InP. The method is superior to standard PLE measurements using titanium sapphire laser because it can be extended to any spectral range where anti-reflection coated laser diodes are available.
Antiparallel interface coupling evidenced by negative rotatable anisotropy in IrMn/NiFe bilayers
Authors: Schafer D. , Grande P. L. , Pereira L. G. , Azevedo G. M. , Harres A. , de Sousa M. A. , Pelegrini F. , Geshev J.
Published: June 2015

Negative rotatable anisotropy is estimated via ferromagnetic resonance measurements in as-made, annealed, and ion-irradiated IrMn3/Ni81Fe19 bilayers. Opposite to previous observations, inverse correlation between rotatable anisotropy and coercivity is observed. The exchange-bias field, determined from hysteresis loop measurements, is higher than that obtained from ferromagnetic resonance for all samples. The results are discussed in terms of majority antiparallel coupling and magnetic-field-induced transitions from antiparallel to parallel states of uncompensated spins at ferromagnet/antiferromagnet interface. We affirm that an observation of negative rotatable anisotropy evidences antiparallel coupling even in systems presenting conventional exchange bias.
Hydrogen accumulation as the origin of delamination at the a-carbon/SiO2 interface
Authors: Segura-Ruiz J. , Gutfreund P. , Imbert G. , Ponard A. , Cubitt R.
Published: June 2015

This work reports the characterization of the interface amorphous carbon (a-C)/SiO2 by neutron and X-ray reflectometry. Neutrons have shown the existence of an intermediate layer (IL) between the a-C and the SiO2 layers that was not evidenced by XRR. This IL has been associated with the accumulation of H inside the SiO2 layer near the interface with the a-C. The characteristics of this layer, in particular, its H-concentration and thickness, seem to be correlated with the weakness of this interface. A plot of the molecular weight as a function of the mass density for the SiO2 and the IL layers graphically demonstrates the risk of delamination of each sample. The combination of NR and XRR is shown to be a powerful technique in the characterization of layers and interfaces used in the micro/nanoelectronics industry. The same approach can be extended to other interfaces of interest involving hydrogen.
Controlling the interface charge density in GaN-based metal-oxide-semiconductor heterostructures by plasma oxidation of metal layers
Authors: Hahn Herwig , Pécz Béla , Kovács András , Heuken Michael , Kalisch Holger , Vescan Andrei
Published: June 2015

In recent years, investigating and engineering the oxide-semiconductor interface in GaN-based devices has come into focus. This has been driven by a large effort to increase the gate robustness and to obtain enhancement mode transistors. Since it has been shown that deep interface states act as fixed interface charge in the typical transistor operating regime, it appears desirable to intentionally incorporate negative interface charge, and thus, to allow for a positive shift in threshold voltage of transistors to realise enhancement mode behaviour. A rather new approach to obtain such negative charge is the plasma-oxidation of thin metal layers. In this study, we present transmission electron microscopy and energy dispersive X-ray spectroscopy analysis as well as electrical data for Al-, Ti-, and Zr-based thin oxide films on a GaN-based heterostructure. It is shown that the plasma-oxidised layers have a polycrystalline morphology. An interfacial amorphous oxide layer is only detectable in the case of Zr. In addition, all films exhibit net negative charge with varying densities. The Zr layer is providing a negative interface charge density of more than 1 × 1013 cm–2 allowing to considerably shift the threshold voltage to more positive values.
Numerical simulation of vortex-induced vibration of a circular cylinder in a spanwise shear flow
Authors: Zhao Ming
Published: June 2015

Vortex-induced vibration of a circular cylinder with a length-to-diameter ratio of 19.2 in a spanwise shear flow is investigated numerically. The Reynolds numbers based on the velocity at the centre of the cylinder and the mass ratio are 500 and 2, respectively. The responses of the cylinder in shear flows with shear factors of 0.05 and 0.1 are compared with that in the uniform flow. Although the oscillation of the lift force for a stationary cylinder in a sheared flow is very weak, it is found that if the cylinder is allowed to vibrate, the lock-in regime and the maximum response amplitude are comparable with their counterparts for a cylinder in a uniform flow. The maximum response amplitude for a shear factor of 0.05 is found slightly greater than that for a uniform flow. In the lock-in regime, the vortex shedding and the oscillation of the sectional lift coefficient are found to synchronize (have a same frequency) along the cylinder span, leading to strong vibration of the cylinder. The sectional lift coefficient changes from being in phase to being out of phase with the response displacement at a location on the cylinder span, and the location where the lift coefficient changes its phase depends on the reduced velocity. The phase change of the lift coefficient corresponds to the change in the vortex shedding mode. The role of the sectional lift coefficient in the vibration varies along the cylinder span. For a small reduced velocity in the lock-in regime, the sectional lift forces near the high-velocity end of the cylinder excite the vibration, while those at the rest of the cylinder span damp the vibration. With increasing reduced velocity, the location where the sectional lift forces excite the vibration moves towards the low-velocity end of the cylinder.
Local structure of solid Rb at megabar pressures
Authors: De Panfilis S. , Gorelli F. , Santoro M. , Ulivi L. , Gregoryanz E. , Irifune T. , Shinmei T. , Kantor I. , Mathon O. , Pascarelli S.
Published: June 2015

We have investigated the local and electronic structure of solid rubidium by means of x-ray absorption spectroscopy up to 101.0 GPa, thus doubling the maximum investigated experimental pressure. This study confirms the predicted stability of phase VI and was completed by the combination of two pivotal instrumental solutions. On one side, we made use of nanocrystalline diamond anvils, which, contrary to the more commonly used single crystal diamond anvils, do not generate sharp Bragg peaks (glitches) at specific energies that spoil the weak fine structure oscillations in the x-ray absorption cross section. Second, we exploited the performance of a state-of-the-art x-ray focussing device yielding a beam spot size of 5 × 5 μm2, spatially stable over the entire energy scan. An advanced data analysis protocol was implemented to extract the pressure dependence of the structural parameters in phase VI of solid Rb from 51.2 GPa up to the highest pressure. A continuous reduction of the nearest neighbour distances was observed, reaching about 6% over the probed pressure range. We also discuss a phenomenological model based on the Einstein approximation to describe the pressure behaviour of the mean-square relative displacement. Within this simplified scheme, we estimate the Grüneisen parameter for this high pressure Rb phase to be in the 1.3–1.5 interval.
High resolution quartz flexure accelerometer based on laser self-mixing interferometry
Authors: Wang Cuo , Li Xingfei , Kou Ke , Wu Tengfei , Xiang Hongbiao
Published: June 2015

As common high-precision inertial sensors, quartz flexure accelerometers have a wide application prospect in low-cost inertial navigation systems. To ameliorate their resolution performance restricted by differential capacitance detection, we proposed a modified type of quartz flexure accelerometer based on an emerging optical technique named laser self-mixing interferometry, which is utilized to sense the displacement of a quartz pendulous reed, and then an equal and opposite force is accordingly produced to maintain the reed motionless relative to the inertial frame. The configuration and working principle of the improved accelerometer have been introduced and analyzed. The preliminary experiments indicate that its bias stability reaches 0.75-0.85 μg, which shows some progress when compared to the traditional type. Further improvements are mainly limited by the characteristics of the laser diode and the multiple reflections from the pendulous reed.
Influence of the density of states of graphene on the transport properties of graphene/MoS2/metal vertical field-effect transistors
Authors: Moriya Rai , Yamaguchi Takehiro , Inoue Yoshihisa , Sata Yohta , Morikawa Sei , Masubuchi Satoru , Machida Tomoki
Published: June 2015

We performed detailed studies of the current–voltage (I–V) characteristics in graphene/MoS2/metal vertical field-effect transistors. Owing to its low density of states, the Fermi level in graphene is very sensitive to its carrier density and thus the external electric field. Under the application of a bias voltage VB between graphene and the metal layer in the graphene/MoS2/metal heterostructure for driving current through the van der Waals interface, the electric field across the MoS2 dielectric induces a shift in the Fermi level of graphene. When the Fermi level of graphene coincides with the Dirac point, a significant nonlinearity appears in the measured I–V curve, thus enabling us to perform spectroscopy of the Dirac point. By detecting the Dirac point for different back-gate voltages, we revealed that the capacitance of the nanometer-thick MoS2 layer can be determined from a simple DC transport measurement.
Continuous third harmonic generation in a terahertz driven modulated nanowire
Authors: Hamilton Kathleen E. , Kovalev Alexey A. , De Amrit , Pryadko Leonid P.
Published: June 2015

We consider the possibility of observing continuous third-harmonic generation using a strongly driven, single-band one-dimensional metal. In the absence of scattering, the quantum efficiency of frequency tripling for such a system can be as high as 93%. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive a semiclassical master equation for a one-dimensional band of strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We use the Kronig-Penney model with varying effective mass to establish the growth parameters of an InAs/InP nanowire near optimal for third harmonic generation at terahertz frequency range.
Effect of incorporation of ethylene glycol into PEDOT:PSS on electron phonon coupling and conductivity
Authors: Lin Yow-Jon , Ni Wei-Shih , Lee Jhe-You
Published: June 2015

The effect of incorporation of ethylene glycol (EG) into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on electron phonon coupling and conductivity is investigated. It is shown that the carrier density (NC) increases significantly and the carrier mobility (μ) increases slightly at 300 K. The increased intensity of the Raman spectrum between 1400 and 1450 cm−1, following EG treatment (that is, the quinoid-dominated structures of the PEDOT chain), leads to an increase in the number of polarons (bipolarons), which leads to an increase in NC. In addition, μ in PEDOT:PSS samples with or without EG addition exhibits a strong temperature dependence, which demonstrates the dominance of tunneling (hopping) at low (high) temperatures. The high conductivity of PEDOT:PSS samples with the addition of EG is attributed to the combined effect of the modification of the electron-phonon coupling and the increase in NC (μ).
Carbon-rich hexagonal (BN)C alloys
Authors: Uddin M. R. , Li J. , Lin J. Y. , Jiang H. X.
Published: June 2015

Thin films of hexagonal boron nitride carbon, h-(BN)1−x(C2)x, alloys in the C-rich side have been synthesized by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. X-ray diffraction measurements confirmed single hexagonal phase of h-(BN)1−x(C2)x epilayers. Electrical transport and Raman spectroscopy measurements results revealed evidences that homogenous h-(BN)1−x(C2)x alloys with x ≥ 95% can be synthesized by MOCVD at a growth temperature of 1300 °C. The variable temperature Hall-effect measurements suggested that a bandgap opening of about 93 meV with respect to graphite has been obtained for h-(BN)1−x(C2)x with x = 0.95, which is consistent with the expected value deduced from the alloy dependence of the energy gap of homogenous h-(BN)1−x(C2)x alloys. Atomic composition results obtained from x-ray photoelectron spectroscopy measurements revealed that the carrier type in C-rich h-(BN)1−x(C2)x alloys is controlled by the stoichiometry ratio between the B and N via changing the V/III ratio during the growth. The demonstration of bandgap opening and conductivity control in C-rich h-(BN)1−x(C2)x alloys provide feasibilities for realizing technologically significant devices including infrared (IR) emitters and detectors active from near to far IR and multi-spectral IR emitters and detectors.
Electrokinetics of scalable, electric-field-assisted fabrication of vertically aligned carbon-nanotube/polymer composites
Authors: Castellano Richard J. , Akin Cevat , Giraldo Gabriel , Kim Sangil , Fornasiero Francesco , Shan Jerry W.
Published: June 2015

Composite thin films incorporating vertically aligned carbon nanotubes (VACNTs) offer promise for a variety of applications where the vertical alignment of the CNTs is critical to meet performance requirements, e.g., highly permeable membranes, thermal interfaces, dry adhesives, and films with anisotropic electrical conductivity. However, current VACNT fabrication techniques are complex and difficult to scale up. Here, we describe a solution-based, electric-field-assisted approach as a cost-effective and scalable method to produce large-area VACNT composites. Multiwall-carbon nanotubes are dispersed in a polymeric matrix, aligned with an alternating-current (AC) electric field, and electrophoretically concentrated to one side of the thin film with a direct-current (DC) component to the electric field. This approach enables the fabrication of highly concentrated, individually aligned nanotube composites from suspensions of very dilute (ϕ=4×10−4) volume fraction. We experimentally investigate the basic electrokinetics of nanotube alignment under AC electric fields, and show that simple models can adequately predict the rate and degree of nanotube alignment using classical expressions for the induced dipole moment, hydrodynamic drag, and the effects of Brownian motion. The composite AC + DC field also introduces complex fluid motion associated with AC electro-osmosis and the electrochemistry of the fluid/electrode interface. We experimentally probe the electric-field parameters behind these electrokinetic phenomena, and demonstrate, with suitable choices of processing parameters, the ability to scalably produce large-area composites containing VACNTs at number densities up to 1010 nanotubes/cm2. This VACNT number density exceeds that of previous electric-field-fabricated composites by an order of magnitude, and the surface-area coverage of the 40 nm VACNTs is comparable to that of chemical-vapor-deposition-grown arrays of smaller-diameter nanotubes.
Thermal properties measurement of binary carbonate salt mixtures for concentrating solar power plants
Authors: Jo Byeongnam , Banerjee Debjyoti
Published: June 2015

Specific heat capacity of an alkali molten salt mixture which is composed of lithium carbonate and potassium carbonate was measured using a differential scanning calorimeter (DSC). The specific heat capacity measurement was performed for 14 different composition ratios of the molten salt mixture to examine the effect of composition ratios on the specific heat capacity. The measured specific heat capacity values were compared with theoretically predicted values by the thermal equilibrium model. Additionally, changes in both melting point and latent heat of fusion were also investigated with change in the composition of two salts. In results, according to the heat flow curves in liquid phase obtained from DSC, the carbonate molten salt mixture could be categorized to three distinct groups: (1) gradually increased specific heat capacity, (2) dramatic decrease in the specific heat capacity, and (3) uniform specific heat capacity. Moreover, the specific heat capacity of the carbonate salt mixtures was strongly dependent upon the mole fraction of lithium carbonate. The specific heat capacity of the salt mixtures was drastically increased up to that of pure lithium carbonate in liquid phase, and decreased down to that of pure potassium carbonate in solid phase. In comparison with theoretical prediction, while the predictions were linearly increased with mole fraction of lithium carbonate, however, the measured data showed no linearity in the change of the specific heat capacity. The effect of the composition was also shown in the melting temperature and latent heat of fusion.
Nanosecond pump and probe observation of bimolecular exciton effects in rubrene single crystals
Authors: Ward Kebra A. , Richman Brittany R. , Biaggio Ivan
Published: June 2015

Transient grating pump and probe experiments are used to investigate excitonic processes on the nanosecond time scale in rubrene single crystals. We find that bimolecular interactions cause a photoinduced excited state density on the order of 0.5 × 1020 cm−3—corresponding to an average distance of ∼3 nm between individual states—to decrease by a factor of 2 after 2 ns, following a typical power-law decay. We assign the observed power-law decays to high-density interactions between excited states. Because of the high efficiency singlet exciton fission observed in rubrene, these bimolecular interactions are likely those between triplet excitons or between coherent quantum superpositions of a singlet and a pair of triplet-excitons.
Anisotropy engineering using exchange bias on antidot templates
Authors: Goncalves F. J. T. , Desautels R. D. , Su S. , Drysdale T. , van Lierop J. , Lin K.-W. , Schmool D. S. , Stamps R. L.
Published: June 2015

We explore an emerging device concept based on exchange bias used in conjunction with an antidot geometry to fine tune ferromagnetic resonances. Planar cavity ferromagnetic resonance is used to study the microwave response of NiO/NiFe bilayers with antidot structuring. A large frequency asymmetry with respect to an applied magnetic field is found across a broad field range whose underlying cause is linked to the distribution of magnetic poles at the antidot surfaces. This distribution is found to be particularly sensitive to the effects of exchange bias, and robust in regards to the quality of the antidot geometry. The template based antidot geometry we study offers advantages for practical device construction, and we show that it is suitable for broadband absorption and filtering applications, allowing tunable anisotropies via interface engineering.
Simulation study of 14-nm-gate III-V trigate field effect transistor devices with In1−xGaxAs channel capping layer
Authors: Huang Cheng-Hao , Li Yiming
Published: June 2015

In this work, we study characteristics of 14-nm-gate InGaAs-based trigate MOSFET (metal-oxide-semiconductor field effect transistor) devices with a channel capping layer. The impacts of thickness and gallium (Ga) concentration of the channel capping layer on the device characteristic are firstly simulated and optimized by using three-dimensional quantum-mechanically corrected device simulation. Devices with In1−xGaxAs/In0.53Ga0.47As channels have the large driving current owing to small energy band gap and low alloy scattering at the channel surface. By simultaneously considering various physical and switching properties, a 4-nm-thick In0.68Ga0.32As channel capping layer can be adopted for advanced applications. Under the optimized channel parameters, we further examine the effects of channel fin angle and the work-function fluctuation (WKF) resulting from nano-sized metal grains of NiSi gate on the characteristic degradation and variability. To maintain the device characteristics and achieve the minimal variation induced by WKF, the physical findings of this study indicate a critical channel fin angle of 85o is needed for the device with an averaged grain size of NiSi below 4x4 nm2.
In-situ NC-AFM measurements of high quality AlN(0001) layers grown at low growth rate on 4H-SiC(0001) and Si(111) substrates using ammonia molecular beam epitaxy
Authors: Chaumeton Florian , Gauthier Sébastien , Martrou David
Published: June 2015

Nitride wide-band-gap semiconductors are used to make high power electronic devices or efficient light sources. The performance of GaN-based devices is directly linked to the initial AlN buffer layer. During the last twenty years of research on nitride growth, only few information on the AlN surface quality have been obtained, mainly by ex-situ characterization techniques. Thanks to a Non Contact Atomic Force Microscope (NC-AFM) connected under ultra high vacuum (UHV) to a dedicated molecular beam epitaxy (MBE) chamber, the surface of AlN(0001) thin films grown on Si(111) and 4H-SiC(0001) substrates has been characterized. These experiments give access to a quantitative determination of the density of screw and edge dislocations at the surface. The layers were also characterized by ex-situ SEM to observe the largest defects such as relaxation dislocations and hillocks. The influence of the growth parameters (substrate temperature, growth speed, III/V ratio) and of the initial substrate preparation on the dislocation density was also investigated. On Si(111), the large in-plane lattice mismatch with AlN(0001) (19%) induces a high dislocation density ranging from 6 to 12×1010/cm2 depending on the growth conditions. On 4H-SiC(0001) (1% mismatch with AlN(0001)), the dislocation density decreases to less than 1010/cm2, but hillocks appear, depending on the initial SiC(0001) reconstruction. The use of a very low growth rate of 10 nm/h at the beginning of the growth process allows to decrease the dislocation density below 2 × 109/cm2.
Combination of Pyridinium and Isoquinolinium Ylides with Phenylisocyanate and Isothiocyanates: Synthesis, Characterisation, and X-Ray Crystal Structures of Mesoionic Monosubstituted 3-Oxo-Propanamides or Thioamides
Authors: Seifi Mohammad , Ebrahimipour S. Yousef , Simpson Jim , Dusek Michal , Eigner Vaclav , Sheibani Hassan
Published: June 2015

Pyridinium ylides derived from 2-bromoacetophenone or methyl bromoacetate have been reacted with phenylisocyanate, phenyl- or methylisothiocyanate to afford mesoionic monosubstituted 3-oxo-propanamides or thioamides, via self-protonation of the intermediate N-anion. A similar reaction under the same conditions of isoquinolinium ylides with phenylisocyanate or phenyisothiocyante also produced the corresponding mesoionic compounds. In order to establish the exact structure of the mesoionic compounds, single crystal X-ray structures were obtained for three of the pyridinium ylides.
Rab3-interacting molecules 2α and 2β promote the abundance of voltage-gated Ca
V
1.3 Ca
2+
channels at hair cell active zones
Authors: Jung Sangyong , Oshima-Takago Tomoko , Chakrabarti Rituparna , Wong Aaron B. , Jing Zhizi , Yamanbaeva Gulnara , Picher Maria Magdalena , Wojcik Sonja M. , Göttfert Fabian , Predoehl Friederike , Michel Katrin , Hell Stefan W. , Schoch Susanne , Strenzke Nicola , Wichmann Carolin , Moser Tobias
Published: June 2015

Significance

Sound encoding relies on Ca
2+
-regulated transmitter release from inner hair cells (IHCs). Here we demonstrate a role of Ras-related in brain 3 (Rab3)–interacting molecule 2 (RIM2) in Ca
2+
channel-clustering and vesicle-tethering at the active zones of IHCs. Active zones of RIM2α-deficient IHCs cluster fewer synaptic voltage-gated Ca
V
1.3 Ca
2+
channels, resulting in reduced synaptic Ca
2+
influx. Exocytosis was diminished in RIM2α-deficient IHCs, likely contributing to the mild hearing impairment of RIM2α knockout mice. Hair cell-specific disruption of all RIM2 isoforms caused a stronger decrease of Ca
2+
current and exocytosis in IHCs and impaired the encoding of sound onset in spiral ganglion neurons. We conclude that RIM2α and RIM2β promote synaptic clustering of Ca
2+
channels at IHC active zones and are required for normal hearing.
Imaging transient melting of a nanocrystal using an X-ray laser
Authors: Clark Jesse N. , Beitra Loren , Xiong Gang , Fritz David M. , Lemke Henrik T. , Zhu Diling , Chollet Matthieu , Williams Garth J. , Messerschmidt Marc M. , Abbey Brian , Harder Ross J. , Korsunsky Alexander M. , Wark Justin S. , Reis David A. , Robinson Ian K.
Published: June 2015

Significance

Despite phase transitions, such as melting, being ubiquitous in nature, understanding what occurs at the nanoscale (such as in nanocrystals) has so far remained challenging. With ensemble studies of nanocrystals it is often difficult to discriminate between intrinsic size-dependent properties and effects due to sample size and shape dispersity. Here, using an X-ray free electron laser we image the reversible melting of an individual nanocrystal induced by an ultrashort laser. It is revealed that the melting occurs transiently, repeatably, and inhomogeneously. This is consistent with a core-shell model where the exterior is melted and a solid core remains. These findings reveal, unambiguously, that core-shell melting occurs, which has important implications for understanding nanoscale phenomena.
New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods
Authors: Stork Nigel E. , McBroom James , Gely Claire , Hamilton Andrew J.
Published: June 2015

Significance

Many suggest we are approaching a sixth mass extinction event, and yet estimates of how many species exist, and thus how many might become extinct, vary by as much as an order of magnitude. There are few statistically robust methods to estimate global species richness, and here we introduce several new methods, including one that builds on the observation that larger species are often described before smaller species. We combine these, giving equal weight to each, to provide mean global species estimates for the most speciose order, class, and phylum on Earth, beetles, insects, and arthropods (terrestrial). We attempt to aid conservation planning by broadening the range of methods used and bringing greater stability to global estimates for these taxa.
Precision-guided antimicrobial peptide as a targeted modulator of human microbial ecology
Authors: Guo Lihong , McLean Jeffrey S. , Yang Youngik , Eckert Randal , Kaplan Christopher W. , Kyme Pierre , Sheikh Omid , Varnum Brian , Lux Renate , Shi Wenyuan , He Xuesong
Published: June 2015

Significance

The lack of effective tools to modulate individual species within a complex microbial community poses a major barrier to studying human microbiome and its associated diseases. We showed targeted removal of human cariogenic
Streptococcus mutans
within an in vitro oral multispecies community using a high-efficacy antimicrobial peptide—C16G2—as well as drastic reconstruction of the microbial structure following treatment. The findings suggest the potential of using targeted antimicrobials to modulate microbiome and study the ecological function of specific bacterial species.
Asymptomatic spread of huanglongbing and implications for disease control
Authors: Lee Jo Ann , Halbert Susan E. , Dawson William O. , Robertson Cecile J. , Keesling James E. , Singer Burton H.
Published: June 2015

Significance

Huanglongbing (HLB) is a vector-transmitted bacterial infection of citrus trees that poses a major threat to the citrus industry in Florida, Texas, and California. Current control strategies that focus on the vector, the Asian citrus psyllid
Diaphorina citri
, are usually initiated when the trees become symptomatic, anywhere from 10 mo to several years after initial infection. We show, experimentally, that newly infected young leaves can become infectious within 10–15 d after receiving an inoculum of bacteria from an adult psyllid. We then show by microsimulation of the asymptomatic spread of HLB through a grove under different invasion scenarios and control strategies that reduction of up to 75% of adult psyllids and nymphs can enhance citrus production.