bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic nanoparticles formed in glasses co-doped with iron and larger radius elements
Authors: Edelman I. , Ivanova O. , Ivantsov R. , Velikanov D. , Zabluda V. , Zubavichus Y. , Veligzhanin A. , Zaikovskiy V. , Stepanov S. , Artemenko A. , Curély J. , Kliava J.
Published: October 2012

A new type of nanoparticle-containing glasses based on borate glasses co-doped with low contents of iron and larger radius elements, Dy, Tb, Gd, Ho, Er, Y, and Bi, is studied. Heat treatment of these glasses results in formation of magnetic nanoparticles, radically changing their physical properties. Transmission electron microscopy and synchrotron radiation-based techniques: x-ray diffraction, extended x-ray absorption fine structure, x-ray absorption near-edge structure, and small-angle x-ray scattering, show a broad distribution of nanoparticle sizes with characteristics depending on the treatment regime; a crystalline structure of these nanoparticles is detected in heat treated samples. Magnetic circular dichroism (MCD) studies of samples subjected to heat treatment as well as of maghemite, magnetite, and iron garnet allow to unambiguously assign the nanoparticle structure to maghemite, independently of co-dopant nature and of heat treatment regime used. Different features observed in the MCD spectra are related to different electron transitions in Fe3+ ions gathered in the nanoparticles. The static magnetization in heat treated samples has non-linear dependence on the magnetizing field with hysteresis. Zero-field cooled magnetization curves show that at higher temperatures the nanoparticles occur in superparamagnetic state with blocking temperatures above 100 K. Below ca. 20 K, a considerable contribution to both zero field-cooled and field-cooled magnetizations occurs from diluted paramagnetic ions. Variable-temperature electron magnetic resonance (EMR) studies unambiguously show that in as-prepared glasses paramagnetic ions are in diluted state and confirm the formation of magnetic nanoparticles already at earlier stages of heat treatment. Computer simulations of the EMR spectra corroborate the broad distribution of nanoparticle sizes found by “direct” techniques as well as superparamagnetic nanoparticle behaviour demonstrated in the magnetization studies.
Investigation of the recombination mechanism of excess carriers in undoped BaSi2 films on silicon
Authors: Hara K. O. , Usami N. , Toh K. , Baba M. , Toko K. , Suemasu T.
Published: October 2012

Excess-carrier recombination mechanisms in undoped BaSi2 epitaxial films grown by molecular beam epitaxy on n-type silicon substrates have been studied by the microwave-detected photoconductivity decay measurement. The measured excess-carrier decay is multiexponential, and we divided it into three parts in terms of the decay rate. Measurement with various excitation laser intensities indicates that initial rapid decay is due to Auger recombination, while the second decay mode with approximately constant decay to Shockley-Read-Hall recombination. Slow decay of the third decay mode is attributed to the carrier trapping effect. To analyze Shockley-Read-Hall recombination, the formulae are developed to calculate the effective lifetime (time constant of decay) from average carrier concentration. The measurement on the films with the thickness of 50–600 nm shows that the decay due to Shockley-Read-Hall recombination is the slower in the thicker films, which is consistent with the formulae. By fitting the calculated effective lifetime to experimental ones, the recombination probability is extracted. The recombination probability is found to be positively correlated with the full width at half-maximum of the X-ray rocking curves, suggesting that dislocations are acting as recombination centers.
Analytic potential and charge model for III-V surrounding gate metal-oxide-semiconductor field-effect transistors
Authors: Marin E. G. , Ruiz F. G. , Tienda-Luna I. M. , Godoy A. , Sánchez-Moreno P. , Gámiz F.
Published: October 2012

In this work, an analytical model is proposed to calculate the potential and the inversion charge of III-V cylindrical Surrounding-Gate metal-oxide-semiconductor field-effect transistors (MOSFETs). The model provides expressions for the calculation of the subband energies and their corresponding wavefunctions, taking into account their penetration into the gate insulator and the effective mass discontinuity in the semiconductor-insulator interface for this kind of devices. The model considers Fermi-Dirac statistics and the two-dimensional quantum confinement of the carriers. We demonstrate that our analytical solution fits very well the numerical solution in all operating regimes and for different device sizes and materials.
Thermally driven unipolar and bipolar spin diode based on double quantum dots
Authors: Qi Fenghua , Ying Yibo , Jin Guojun
Published: October 2012

Using the master equation approach, we theoretically propose a unipolar spin diode consisting of two serially coupled quantum dots connected to two normal metal electrodes, in which the two electrodes have different temperatures, and the two dots have different Zeeman splittings. The current through this system can be from zero to nearly 100% spin-polarized by tuning the difference of the electrode temperatures or the gate voltages applied to the quantum dots. This particular transport property is attributed to the interplay of the strong intradot Coulomb interactions, the asymmetrical energy level distributions in the two dots induced by the different Zeeman splittings and different Fermi distributions in the two electrodes caused by different temperatures. Also, a bipolar spin diode can be designed using the two unipolar spin diodes. These configurations may provide the essential elements for the design of logic circuits and even magnetic heads.
Thermosize effects and irreversibility on the performance of a macro/nano scaled refrigeration cycle
Authors: Wang Hao , Wu Guoxing
Published: October 2012

Based on thermosize effects, we have established an irreversible macro/nano thermosize refrigerator model. Expressions for several important performance parameters, such as the coefficient of performance, cooling rate and power input are derived. The optimal performance parameters are obtained at the maximum cooling rate. By means of numerical calculation and illustration, the optimization criterion for the refrigerator is obtained. Finally, several special cases are discussed in detail. The results obtained here will provide theoretical guidance for the design some new macro/nano scaled devices.
Influence of excited state spatial distributions on plasma diagnostics: Atmospheric pressure laser-induced He-H2 plasma
Authors: Monfared Shabnam K. , Hüwel Lutz
Published: October 2012

Atmospheric pressure plasmas in helium-hydrogen mixtures with H2 molar concentrations ranging from 0.13% to 19.7% were investigated at times from 1 to 25 μs after formation by a Q-switched Nd:YAG laser. Spatially integrated electron density values are obtained using time resolved optical emission spectroscopic techniques. Depending on mixture concentration and delay time, electron densities vary from almost 1017 cm−3 to about 1014 cm−3. Helium based results agree reasonably well with each other, as do values extracted from the Hα and Hβ emission lines. However, in particular for delays up to about 7 μs and in mixtures with less than 1% hydrogen, large discrepancies are observed between results obtained from the two species. Differences decrease with increasing hydrogen partial pressure and/or increasing delay time. In mixtures with molecular hydrogen fraction of 7% or more, all methods yield electron densities that are in good agreement. These findings seemingly contradict the well-established idea that addition of small amounts of hydrogen for diagnostic purposes does not perturb the plasma. Using Abel inversion analysis of the experimental data and a semi-empirical numerical model, we demonstrate that the major part of the detected discrepancies can be traced to differences in the spatial distributions of excited helium and hydrogen neutrals. The model yields spatially resolved emission intensities and electron density profiles that are in qualitative agreement with experiment. For the test case of a 1% H2 mixture at 5 μs delay, our model suggests that high electron temperatures cause an elevated degree of ionization and thus a reduction of excited hydrogen concentration relative to that of helium near the plasma center. As a result, spatially integrated analysis of hydrogen emission lines leads to oversampling of the plasma perimeter and thus to lower electron density values compared to those obtained from helium lines.
Impact of atomic layer deposition temperature on HfO2/InGaAs metal-oxide-semiconductor interface properties
Authors: Suzuki Rena , Taoka Noriyuki , Yokoyama Masafumi , Kim Sang-Hyeon , Hoshii Takuya , Maeda Tatsuro , Yasuda Tetsuji , Ichikawa Osamu , Fukuhara Noboru , Hata Masahiko , Takenaka Mitsuru , Takagi Shinichi
Published: October 2012

We have studied the impact of atomic-layer-deposition (ALD) temperature on the HfO2/InGaAs metal-oxide-semiconductor (MOS) interface with a comparison to the Al2O3/InGaAs interface. It is found that the interface properties such as the C-V characteristics and the interface trap density (Dit) and the interface structure of HfO2/InGaAs have strong dependence on the ALD temperature, while the Al2O3/InGaAs interfaces hardly depend on it. As a result, we have achieved the HfO2/InGaAs interfaces with low Dit comparable to that in the Al2O3/InGaAs interface by lowering the ALD temperature down to 200 °C or less. Also, we have found that As2O3 and Ga2O3 formed at the interface during ALD increase with a decrease in the ALD temperature. Combined with the ALD temperature dependence of the electrical characteristics, the better C-V characteristics and the lower Dit obtained at the lower ALD temperature can be explained by the As2O3 and Ga2O3 passivation of the HfO2/InGaAs interfaces, which is consistent with a reported theoretical result on the effective passivation of III-V MOS interfaces by trivalent oxides.
The time-of-flight experiment and the average transit-time
Authors: Rocha Rodrigo P. , Freire José A.
Published: October 2012

The time-of-flight experiment is a transient photo-current measurement commonly used to measure the mobility of organic semiconductors. The experiment has a well established procedure to extract the average transit-time of the photo-generated carriers across the sample from the current versus time signal. In this work, we compare, for a dispersive signal, the transit-time as inferred from the experimental procedure with the statistically defined average transit-time. We show that the two quantities are not only numerically different, the experimental transit-time being always smaller than the exact one, but they also have markedly different electric field and sample length dependencies. Our results are relevant for any attempt to model an experimental time-of-flight mobility.
Magnetic and electrical properties of In doped cobalt ferrite nanoparticles
Authors: Nongjai Razia , Khan Shakeel , Asokan K. , Ahmed Hilal , Khan Imran
Published: October 2012

Nanoparticles of CoFe2O4 and CoIn0.15Fe1.85O4 ferrites were prepared by citrate gel route and characterized to understand their structural, electrical, and magnetic properties. X-ray diffraction and Raman spectroscopy were used to confirm the formation of single phase cubic spinel structure. The average grain sizes from the Scherrer formula were below 50 nm. Microstructural features were obtained by scanning electron microscope and compositional analysis by energy dispersive spectroscopy. The hysteresis curve shows enhancement in coercivity while reduction in saturation magnetization with the substitution of In3+ ions. Enhancement of coercivity is attributed to the transition from multidomain to single domain nature. Electrical properties, such as dc resistivity as a function of temperature and ac conductivity as a function of frequency and temperature were studied for both the samples. The activation energy derived from the Arrhenius equation was found to increase in the doped sample. The dielectric constant (ε′) and dielectric loss (tan δ) are also studied as a function of frequency and temperature. The variation of dielectric properties ε′, tan δ, and ac conductivity (σac) with frequency reveals that the dispersion is due to Maxwell–Wagner type of interfacial polarization in general and the hopping of charge between Fe2+ and Fe3+ as well as between Co2+ and Co3+ ions at B-sites. Magnetization and electrical property study showed its dominant dependence on the grain size.
Reversible solid-state hydrogen-pump driven by magnetostructural transformation in the prototype system La(Fe,Si)13Hy
Authors: Krautz Maria , Moore James D. , Skokov Konstantin P. , Liu Jian , Teixeira Cristiano S. , Schäfer Rudolf , Schultz Ludwig , Gutfleisch Oliver
Published: October 2012

In magnetocaloric La(Fe,Si)13 alloys, it is known that hydrogen addition shifts the magnetic transition temperature TC near room temperature. By partial hydrogen desorption, TC can be adjusted precisely towards the working temperature of a magnetic cooling device. In this work, we studied the stability of partially desorbed LaFe11.6Si1.4Hy and show that the large volume difference of ferro- and para-magnetic phases drives the system from a single phase to a stable two-phase configuration. The hydrogen concentration separates on a macroscopic scale. The phase-coexistence is visualized by Kerr Microscopy. Differential scanning calorimetry measurements show that hydrogen can be reversibly recombined and separated again. We explain the separation by the coexistence of a high-volume ferromagnetic and low-volume paramagnetic phase that can be induced either by temperature or other external stimuli. This phenomenon can be applied to material systems that show a coupling of physical and structural transitions.
Effect of interfacial lattice mismatch on bulk carrier concentration and band gap of InN
Authors: Kuyyalil Jithesh , Tangi Malleswararao , Shivaprasad S. M.
Published: October 2012

The issue of ambiguous values of the band gap (0.6 to 2 eV) of InN thin film in literature has been addressed by a careful experiment. We have grown wurtzite InN films by PA-MBE simultaneously on differently modified c-plane sapphire substrates and characterized by complementary structural and chemical probes. Our studies discount Mie resonances caused by metallic In segregation at grain boundaries as the reason for low band gap values (≈ 0.6 eV) and also the formation of Indium oxides and oxynitrides as the cause for high band gap value (≈ 2.0 eV). It is observed that polycrystallinity arising from azimuthal miss-orientation of c-oriented wurtzite InN crystals increases the carrier concentration and the band gap values. We have reviewed the band gap, carrier concentration, and effective mass of InN in literature and our own measurements, which show that the Moss-Burstein relation with a non-parabolic conduction band accounts for the observed variation of band gap with carrier concentration.
Electromigration-driven complex dynamics of void surfaces in stressed metallic thin films under a general biaxial mechanical loading
Authors: Dasgupta Dwaipayan , Sfyris Georgios I. , Maroudas Dimitrios
Published: October 2012

We report results of a systematic computational study of the electromigration-driven complex surface dynamics of voids in mechanically stressed thin films of face-centered cubic metals with 〈100〉-oriented film planes. The films are subjected to an external electric field simultaneously with biaxial mechanical loading, which can be either purely compressive, ranging from purely isotropic to strongly anisotropic including uniaxial, or a mixed type of loading with both tensile and compressive stress components in the applied stress tensor. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological evolution following a well validated, two-dimensional, and fully nonlinear model. We find that depending on the electromechanical conditions, void size, and surface diffusional anisotropy, two types of asymptotic states can be stabilized in the void surface dynamical response, namely, morphologically steady or time-periodic traveling voids, and film failure can be caused by void tip extension. The loading mode as well as the loading anisotropy are found to be the significant factors in determining the void morphological stability domains and can be tailored to stabilize steady or time-periodic states and to increase the film's resistance to failure. Under a mixed (tensile + compressive) loading mode, we find that it is impossible to stabilize steady states in the void morphological response and that the stress levels that the film can sustain prior to failure are much lower than those under purely tensile or purely compressive biaxial loading.
Coordination state probabilities and the solvation free energy of Zn2+ in aqueous methanol solutions
Authors: Tam Hok Hei , Asthagiri Dilip , Paulaitis Michael E.
Published: October 2012

Coordination state probabilities for the [Zn(H2O)n(CH3OH)m]2+ complex in aqueous methanol solutions are calculated as a function of the bulk solution concentration, and the number of methanol ligands, m = 0, 1, …, 6 with n+m = 6. Zinc ion solvation free energies, which serve to normalize these probabilities, also reproduce the methanol concentration dependence of the experimentally derived free energy of zinc ion transfer from water to aqueous methanol solutions. Coordination state probabilities, p(n, m), are derived by extending quasi-chemical theory of ion hydration to solvent mixtures and mixed ligands. Free energy contributions to p(n, m) include the free energy of forming the mixed-ligand complex in the ideal gas, obtained by quantum chemical calculations, and the solvation free energy of the complex, approximated by a dielectric continuum model. We find that replacing water ligands with methanol ligands preferentially stabilizes methanol-rich complexes in the ideal gas. Conversely, water-rich complexes are stabilized by the solvation free energy contribution, such that the [Zn(H2O)6]2+ complex is the dominant species in solution for all methanol concentrations considered. Stabilization of the methanol-rich complexes is a consequence of the local coordination chemistry, dominated by the delocalization of charge on the zinc ion, while the stabilization of water-rich complexes is a consequence of favorable ion-solvent electrostatic interactions and smaller dielectric cavities for the water-rich complexes at fixed total charge in the dielectric continuum model. Our analysis also highlights an entropic contribution associated with the reversible work required to remove n water and m methanol molecules from bulk solution to form the [Zn(H2O)n(CH3OH)m]2+ complex, which captures the methanol concentration dependence of the solvation free energy of the zinc ion.
Shallow flow past a cavity: Coupling with a standing gravity wave
Authors: Wolfinger M. , Ozen C. A. , Rockwell D.
Published: October 2012

Shallow, fully turbulent inflow past a cavity can give rise to highly organized oscillations, due to coupling between: (i) the inherent instability of the separated turbulent layer along the opening of the cavity, and (ii) a gravity standing wave within the cavity. Techniques of particle image velocimetry and pressure measurements are employed to characterize the occurrence of the fully coupled state of the oscillation, relative to its uncoupled state. At a threshold value of inflow velocity, the frequency of the inherent instability of the turbulent separated shear layer locks-on to the frequency of the gravity standing wave. Moreover, the amplitude of the spectral peak of the pressure fluctuation, both at the impingement corner of the cavity and within the cavity, attains maximum values when lock-on occurs. Occurrence of a fully coupled or locked-on state substantially alters the time-averaged flow structure. Enhanced magnitudes of patterns of Reynolds stress and transverse velocity fluctuation occur along the cavity shear layer. Simultaneously, increased flow from within the cavity towards the separated shear layer occurs at a location immediately downstream of separation at the leading-edge of the cavity and, correspondingly, increased flow into the cavity occurs in the impingement region. The influence of the locked-on state is therefore global: both the separated shear layer along the mouth of the cavity and the flow within the cavity are substantially altered. Phase-averaged representations of the flow structure show highly coherent, phase-locked patterns of vorticity along the cavity opening during the locked-on state. These vorticity patterns are associated with large, phase-locked excursions of the transverse component of velocity, within both the separated the shear layer and the cavity.
Investigations on the local entrainment velocity in a turbulent jet
Authors: Wolf M. , Lüthi B. , Holzner M. , Krug D. , Kinzelbach W. , Tsinober A.
Published: October 2012

We report an experimental analysis of the local entrainment velocity in the self-similar region of a turbulent jet. Particle tracking velocimetry is performed to determine the position of the convoluted, instantaneous turbulent/non-turbulent interface and to compute velocity and velocity derivatives in the proximity of the interface. We find that the local entrainment velocity is mostly governed by a viscous component and that its magnitude depends on the local shape of the interface. It is illustrated that local entrainment is faster for surface elements concave towards the turbulent region. A closer analysis of the plane spanned by mean and Gaussian curvature reveals that depending on the surface shape, different small-scale mechanisms are dominant for the local entrainment process, namely, viscous diffusion for concave shapes and vortex stretching for convex shapes. Key quantities influencing viscous diffusion and vortex stretching in the entrainment process are identified. It is illustrated that the viscous advancement of the interface into the non-turbulent region mostly depends on the shape of the enstrophy profile normal to the interface. The inviscid contribution is intimately related to the alignment of vorticity with the eigenvectors of the rate of strain tensor. Finally, the analysis substantiates that the convolution of the instantaneous interface is driven by the advection of the underlying fluid together with a contribution from the local entrainment velocity, with the advection velocity being the governing part.
Nanostructured and amorphous-like tungsten films grown by pulsed laser deposition
Authors: Dellasega D. , Merlo G. , Conti C. , Bottani C. E. , Passoni M.
Published: October 2012

An experimental investigation of nanostructured, micrometer-thick, tungsten films deposited by pulsed laser deposition is presented. The films are compact and pore-free, with crystal grain sizes ranging from 14 nm to less than 2 nm. It is shown how, by properly tailoring deposition rate and kinetic energy of ablated species, it is possible to achieve a detailed and separate control of both film morphology and structure. The role of the main process parameters, He background pressure, laser fluence, and energy, is elucidated. In contrast with W films produced with other PVD techniques, β-phase growth is avoided and the presence of impurities and contaminants, like oxygen, is not correlated with film structure. These features make these films interesting for the development of coatings with improved properties, like increased corrosion resistance and enhanced diffusion barriers.
Magnetic and magnetotransport properties of Ba2FeMoO6 pulsed laser deposited thin films
Authors: Kim Kyeong-Won , Ghosh Siddhartha , Buvaev Sanal , Mhin Sungwook , Jones Jacob L. , Hebard Arthur F. , Norton David P.
Published: October 2012

Phase pure Ba2FeMoO6 thin films were grown by pulsed laser deposition at substrate temperatures ranging from 600 to 900 °C, and the structural, magnetic, and magnetotransport properties were investigated. The grain size, the conductivity, the saturation magnetization, and the coercive field all increase with increasing growth temperature, while the Curie temperature remains relatively constant. All samples showed negative magnetoresistance which is consistent with the half metallic property of double perovskites. Interestingly, the samples grown on the higher temperature substrates showed a small low-field positive magnetoresistance which is not understood. We also have observed an anomalous Hall effect, due to spin-polarized itinerant electrons, in which the saturation field of the hole-like anomalous portion decreases with increasing temperature and at higher temperatures manifests a negative slope (ordinary Hall effect) indicative of negative charge carriers.
Bulk-heterojunction morphology control during spin coating: Modelling diffusion assisted phase separation
Authors: Ghosh S. S. , Lonkar G. S. , Mahajan M. S. , Jadkar S. R. , Waman V. S. , Kamble M. M. , Ganesan V. , Sali J. V.
Published: October 2012

We demonstrate the use of controlling the ambient during spin coating to finely control the solvent evaporation rate in order to obtain varying degree of phase separation in P3HT:PCBM blend films and study the effect on device performance. To understand the basic phenomenon driving the phase separation, modelling study has been performed, which show that the phase separation is due to PCBM diffusion towards the nucleation site once its solubility limit is crossed during spin coating process.
A close look into the excluded volume effects within a double layer
Authors: Frydel Derek , Levin Yan
Published: October 2012

We explore the effect of steric interaction on the ionic density distribution near a charged hard wall. For weakly charged walls, small particles, and monovalent ions, the mean-field Poisson-Boltzmann equation provides an excellent description of the density profiles. For large ions and large surface charges, however, deviations appear. To explore these, we use the density functional theory. We find that local density functionals are not able to account for steric interactions near a wall. Based on the weighted density approximation, we derive a simple analytical expression for the contact electrostatic potential, which allows us to analytically calculate the differential capacitance of the double layer.
Dielectric functions and interband critical points of anisotropic chain structured TlSe single crystals
Authors: Isik M. , Gasanly N. M.
Published: October 2012

Spectroscopic ellipsometry measurements were carried out on TlSe single crystals for orientations of electric field, parallel (E∥c), and perpendicular (E⊥c) to optic axis c. The experiments were performed in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the pseudodielectric function as well as pseudorefractive index and pseudoextinction coefficient were calculated from the analysis of ellipsometric data under the light of ambient-substrate optical model. The energies of interband transitions (critical points) have been found from the analysis on second derivative spectra of the pseudodielectric function. The analysis revealed four and five interband transition structures for E∥c and E⊥c configurations, respectively. The obtained critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure given in literature.
Transition in spin dependent transport from superparamagnetic-superparamagnetic to superparamagnetic-ferromagnetic in sputtered Cu100–xCox granular films
Authors: Kumar Dinesh , Chaudhary Sujeet , Pandya Dinesh K.
Published: October 2012

The transition in the spin-dependent transport from superparamagnetic-superparamagnetic (SPM-SPM) to superparamagnetic-ferromagnetic (SPM-FM) in room temperature co-sputtered granular Cu100−xCox (x = 15.1–30.9 at. %) thin films is tracked by varying the cobalt concentration. It is found that at lower cobalt concentrations of x ≤ 20.9, the spin dependent transport is governed by the scattering which electrons undergo while they move through SPM-SPM network. At higher cobalt concentration x > 20.9, the transport behavior changes due to predominant electronic scattering through SPM-FM networks. From the isothermal magnetoresistance behavior in 20–300 K range, transmission electron microscopy analysis, and magnetization behavior, three different composition regimes are identified. These are (i) x ≤ 15.1, consisting of nearly spherical monodispersed single uncoalesced non-interacting small SPM particles only; (ii) 15.1 < x ≤ 20.9, having bimodal distribution with small monodispersed SPM and weakly interacting bigger SPM particles, and (iii) x > 20.9, having monodispersed small SPM particles and FM clusters having broad distribution with stronger interactions. The work provides an insight to understand the transition of spin dependent transport from SPM-SPM to SPM-FM and the gradual increase in the strength of magnetic interaction among the particles vis-à-vis cobalt concentration.
Dependence of contact electrification on the magnitude of strain in polymeric materials
Authors: Sow Mamadou , Lacks Daniel J. , Mohan Sankaran R.
Published: October 2012

We present a new experimental approach to systematically study the effect of material strain on contact electrification. Thin sheets of latex rubber are variably strained by stretching over a hollow cylinder, analogous to a drum. The strained sheets are repeatedly contacted with another surface by a computer-controlled apparatus. The surface potential on the latex rubber is measured in real time by a non-contact electrostatic voltmeter. For unstrained latex rubber, we find that contact with polytetrafluoroethylene (PTFE) leads to a negative surface potential that progressively increases in magnitude to more negative values with increasing number of contacts. When the latex rubber is strained, contact with PTFE causes the surface potential of the latex rubber to shift to less negative values. At strains of more than ∼50%, the polarity of the surface potential is reversed, such that contact with PTFE causes the latex rubber to charge positively. These results indicate that contact electrification intimately depends on the degree of material strain, and may explain how spatial inhomogeneities of charge exist on contacted surfaces and why there is a lack of reproducibility in contact charging experiments.
High-resolution structural studies and covalent bond interactions in BiFeO3-PbTiO3 compounds: The role of ferroism
Authors: Cotica L. F. , Freitas V. F. , Catellani I. B. , Santos I. A. , Garcia D. , Eiras J. A.
Published: October 2012

The origin of enhanced ferroism in La-modified BiFeO3-PbTiO3 compounds is investigated by examining chemical bonding in high-resolution structural measurements. It is shown that structural modifications in the perovskite A-site, such as absent or inactive lone-pair ions, lead to reduced covalence in A-O bonds and increased covalence in B-O bonds for the tetragonal phase. This change in bond character results in a structural accommodation that favors the rhombohedral over the tetragonal phase, which also enhances the ferroic properties. These observations highlight the influence of modifiers on BiFeO3-PbTiO3 bonds and phase stabilization mechanisms.
Impact of forming gas annealing on the dielectric properties of SrBi2Ta2O9 thin films prepared by metalorganic decomposition
Authors: Wang Dong-Sheng
Published: October 2012

SrBi2Ta2O9 (SBT) thin films were prepared on Pt/TiO2/SiO2/Si substrates by metalorganic decomposition method. The dielectric properties of SBT films strongly depend on annealing conditions and annealing time. Compared with films not annealed in forming gas, the relative dielectric constant and the dissipation factor for SBT films annealed at 400 °C decrease by 23.4% and 30.6%, respectively. It implies that the dominant dielectric loss mechanism is related to the degradation of films. Forming gas ambient may have played an important role in the increase of oxygen vacancies in SBT thin films and the degradation of dielectric properties.
Thermal imaging of nickel-aluminum and aluminum-polytetrafluoroethylene impact initiated combustion
Authors: Densmore John M. , Biss Matthew M. , Homan Barrie E. , McNesby Kevin L.
Published: October 2012

Combustion temperatures from impact initiated nickel-aluminum (NiAl) and aluminum-polytetrafluoroethylene (Al-PTFE) materials have been measured using a high-speed two-camera imaging pyrometer. The materials were launched with a nominal velocity of 1700 m/s into a sealed chamber. Upon impact into a steel anvil chemical reactions were initiated and a flame propagated through the chamber. The measured temperature after impact was 3600 K (NiAl) and 3300 K (Al-PTFE).
Why have not the hairs on the feet of gecko been smaller?
Authors: Su Yewang , He Shijie , Hwang Keh-Chih , Ji Baohua
Published: October 2012

The nanometer size of the tiny hair is the key to the secret of strong stickiness of gecko's feet, by which the hair can achieve the maximum adhesion strength that is insensitive to the interfacial flaws with substrate surface. But the question why the hairs have not been smaller is not answered yet. In this study, we derived a geometric parameter of the surface structures considering lateral interaction among hairs, which gives a critical size below which these hairs will bunch together and cause failure of the adhesion, suggesting a lower limit of the dimension of hairs on gecko's feet.
Tunable phononic crystals based on cylindrical Hertzian contact
Authors: Li Feng , Ngo Duc , Yang Jinkyu , Daraio Chiara
Published: October 2012

We investigate the tunability of phononic crystals consisting of arrays of cylindrical elements using theoretical, numerical, and experimental approaches. We show that when these systems are excited by a continuous dynamic signal under large static precompression, they support a characteristic band structure whose cutoff frequency can be controlled by changing the alignment angles or the static precompression. Furthermore, we report the formation of an additional pass band when there exists particles' eccentricity, which is caused by the coupling mechanism between longitudinal and shear modes.
Electric and magnetic contributions to spatial diffusion in collisionless plasmas
Authors: Smets R. , Belmont G. , Aunai N.
Published: October 2012

We investigate the role played by the different self-consistent fluctuations for particle diffusion in a magnetized plasma. We focus especially on the contribution of the electric fluctuations and how it combines with the (already investigated) magnetic fluctuations and with the velocity fluctuations. For that issue, we compute with a hybrid code the value of the diffusion coefficient perpendicular to the mean magnetic field and its dependence on the particle velocity. This study is restricted to small to intermediate level of electromagnetic fluctuations and focuses on particle velocities on the order of few times the Alfvén speed. We briefly discuss the consequences for cosmic ray modulation and for the penetration of thermal solar wind particles in the Earth magnetosphere.
Stationary self-focusing of intense laser beam in cold quantum plasma using ramp density profile
Authors: Habibi M. , Ghamari F.
Published: October 2012

By using a transient density profile, we have demonstrated stationary self-focusing of an electromagnetic Gaussian beam in cold quantum plasma. The paper is devoted to the prospects of using upward increasing ramp density profile of an inhomogeneous nonlinear medium with quantum effects in self-focusing mechanism of high intense laser beam. We have found that the upward ramp density profile in addition to quantum effects causes much higher oscillation and better focusing of laser beam in cold quantum plasma in comparison to that in the classical relativistic case. Our computational results reveal the importance and influence of formation of electron density profiles in enhancing laser self-focusing.
Micro-imaging of transient guest profiles in nanoporous host systems of cylindrical symmetry
Authors: Binder Tomas , Hibbe Florian , Chmelik Christian , Kärger Jörg , Martinez-Joaristi Alberto , Gascon Jorge , Kapteijn Freek , Ruthven Douglas
Published: October 2012

Nanoporous host materials giving rise to transient guest profiles of cylindrical symmetry during molecular uptake and release are shown to provide particularly advantageous conditions for the study of guest diffusion by micro-imaging. Considering zeolites of structure type DDR (Deca-dodecasil 3R) as a host system and short-chain length hydrocarbons as guest molecules, the benefits thus attainable in micro-imaging studies using interference microscopy are shown to include the determination of transient concentration profiles with improved accuracy, the option to overcome the disturbing impact of surface imperfections, and easy access to concentration-dependent diffusivities.
Ultra-high hole mobility exceeding one million in a strained germanium quantum well
Authors: Dobbie A. , Myronov M. , Morris R. J. H. , Hassan A. H. A. , Prest M. J. , Shah V. A. , Parker E. H. C. , Whall T. E. , Leadley D. R.
Published: October 2012

In this paper, we report a Hall mobility of one million in a germanium two-dimensional hole gas. The extremely high hole mobility of 1.1 × 106 cm2 V−1 s−1 at a carrier sheet density of 3 × 1011 cm−2 was observed at 12 K. This mobility is nearly an order of magnitude higher than any previously reported. From the structural analysis of the material and mobility modeling based on the relaxation time approximation, we attribute this result to the combination of a high purity Ge channel and a very low background impurity level that is achieved from the reduced-pressure chemical vapor deposition growth method.
Effects of laser energy fluence on the onset and growth of the Rayleigh–Taylor instabilities and its influence on the topography of the Fe thin film grown in pulsed laser deposition facility
Authors: Mahmood S. , Rawat R. S. , Wang Y. , Lee S. , Zakaullah M. , Tan T. L. , Springham S. V. , Lee P.
Published: October 2012

The effect of laser energy fluence on the onset and growth of Rayleigh–Taylor (RT) instabilities in laser induced Fe plasma is investigated using time-resolved fast gated imaging. The snow plow and shock wave models are fitted to the experimental results and used to estimate the ablation parameters and the density of gas atoms that interact with the ablated species. It is observed that RT instability develops during the interface deceleration stage and grows for a considerable time for higher laser energy fluence. The effects of RT instabilities formation on the surface topography of the Fe thin films grown in pulsed laser deposition system are investigated (i) using different laser energy fluences for the same wavelength of laser radiation and (ii) using different laser wavelengths keeping the energy fluence fixed. It is concluded that the deposition achieved under turbulent condition leads to less smooth deposition surfaces with bigger sized particle agglomerates or network.
Simulation guided design of a microfluidic device for electrophoretic stretching of DNA
Authors: Hsieh Chih-Chen , Lin Tsung-Hsien , Huang Chiou-De
Published: October 2012

We have used Brownian dynamics-finite element method (BD-FEM) to guide the optimization of a microfluidic device designed to stretch DNA for gene mapping. The original design was proposed in our previous study [C. C. Hsieh and T. H. Lin, Biomicrofluidics 5(4), 044106 (2011)] for demonstrating a new pre-conditioning strategy to facilitate DNA stretching through a microcontraction using electrophoresis. In this study, we examine the efficiency of the original device for stretching DNA with different sizes ranging from 48.5 kbp (λ-DNA) to 166 kbp (T4-DNA). The efficiency of the device is found to deteriorate with increasing DNA molecular weight. The cause of the efficiency loss is determined by BD-FEM, and a modified design is proposed by drawing an analogy between an electric field and a potential flow. The modified device does not only regain the efficiency for stretching large DNA but also outperforms the original device for stretching small DNA.
Three-photon near-infrared quantum cutting in Tm3+-doped transparent oxyfluoride glass ceramics
Authors: Yu D. C. , Zhang J. P. , Chen Q. J. , Zhang W. J. , Yang Z. M. , Zhang Q. Y.
Published: October 2012

Efficient three-step sequential three-photon near-infrared (NIR) quantum cutting in Tm3+-doped transparent oxyfluoride glass ceramics has been demonstrated, where an absorbed blue photon could be cut into three NIR photons at 1190, 1460, and 1800 nm with quantum yield greater than unity. On the basis of static and dynamic photoemission, monitored excitation, and time-resolved fluorescence spectra, we investigate in detail the underlying optoelectronic mechanism. Further development of an efficient triply-cutting material might open up a path towards ultra-efficient photonic devices, which enables more photons emitted than absorbed in the excitation process.
Fe-doped InN layers grown by molecular beam epitaxy
Authors: Wang Xinqiang , Liu Shitao , Ma Dingyu , Zheng Xiantong , Chen Guang , Xu Fujun , Tang Ning , Shen Bo , Zhang Peng , Cao Xingzhong , Wang Baoyi , Huang Sen , Chen Kevin J. , Zhou Shengqiang , Yoshikawa Akihiko
Published: October 2012

Iron(Fe)-doped InN (InN:Fe) layers have been grown by molecular beam epitaxy. It is found that Fe-doping leads to drastic increase of residual electron concentration, which is different from the semi-insulating property of Fe-doped GaN. However, this heavy n-type doping cannot be fully explained by doped Fe-concentration ([Fe]). Further analysis shows that more unintentionally doped impurities such as hydrogen and oxygen are incorporated with increasing [Fe] and the surface is degraded with high density pits, which probably are the main reasons for electron generation and mobility reduction. Photoluminescence of InN is gradually quenched by Fe-doping. This work shows that Fe-doping is one of good choices to control electron density in InN.
Intense plasma emission induced by jet-to-jet coupling in atmospheric pressure plasma arrays
Authors: Kim Sung-O , Kim Jae Young , Kim Do Yeob , Ballato John
Published: October 2012

Intense plasma emissions were achieved via jet-to-jet coupling in a multi-tube array-based plasma device in ambient air. The plasma array device consisted of a central glass tube encircled by an array of hollow glass tubes. A single plasma jet was induced via jet-to-jet coupling and enabled significantly increased plasma emission despite a negligible change in power consumption. An increase in the number of outer tubes yielded a greater number of charged particles involved in the plasma process and resulting in the achievement of higher plasma emission in the coupled system.
Magneto-optical Kerr effect investigation on magnetoelectric coupling in ferromagnetic/antiferroelectric multilayer thin film structures
Authors: Mirshekarloo Meysam Sharifzadeh , Yakovlev Nikolai , Wong Meng Fei , Yao Kui , Sritharan Thirumany , Bhatia Charanjit Singh
Published: October 2012

Magnetoelectric (ME) membranes comprising soft ferromagnetic Ni and antiferroelectric (AFE) (Pb,La)(Zr,Sn,Ti)O3 (PLZST) layers were proposed and fabricated through a bulk micro-machining process on silicon wafers. An AC-mode magneto-optical Kerr effect technique was proposed to examine the magnetoelectric coupling in the multilayer membranes, in which the electric field-induced magnetization rotation was analyzed for understanding the underlying coupling mechanisms. The AFE to ferroelectric phase transformation of PLZST induced a rotation of magnetization of about 0.5° in Ni, persuaded by strain-induced anisotropy of about −0.5 kJ/m3.
Microcantilever Q control via capacitive coupling
Authors: Huefner Magdalena , Pivonka Adam , Kim Jeehoon , Ye Cun , Blood-Forsythe Martin A. , Zech Martin , Hoffman Jennifer E.
Published: October 2012

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ∼10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO2 thin film in high vacuum.
Variable-thickness multilayered polyvinylidene fluoride transducer with improved sensitivity and bandwidth for photoacoustic imaging
Authors: Xi Lei , Li Xiaoqi , Jiang Huabei
Published: October 2012

The performance of photoacoustic imaging (PAI) is critically dependent on the sensitivity and bandwidth of the transducer. Here, we report the design and fabrication of a polyvinylidene fluoride ultrasonic transducer with variable-thickness layers that demonstrates significantly improved sensitivity and bandwidth. We show that the multilayered transducer provides more than 2-fold increase in sensitivity and more than 65% improvement in bandwidth compared with the conventional single-layer transducer. Photoacoustic imaging of mouse brain with the intact skull and scalp is conducted to demonstrate the power of the multilayered transducer for resolving brain structure in an in vivo setting.
Growth mechanisms of in situ TiC in laser melted Ti–Si–C ternary system
Authors: Gu Dongdong , Dai Donghua , Zhang Guoquan , Wang Hongqiao
Published: October 2012

Laser rapid melting and recrystallization of in situ crystals is an interesting issue in research fields of applied physics and materials science. This letter investigated the growth mechanisms of in situ dendritic TiC from laser melted Ti–Si–C ternary system. The growth of TiC dendritic trunks was kinetically dependent on laser scan speed and was influenced negligibly by laser power. A higher scan speed produced the elongated dendritic trunks. The development of TiC dendritic arms was temperature-dependent. An increase in laser energy density by increasing laser power or decreasing scan speed enhanced the growth of primary and even secondary dendritic arms.
Anomalous transport of Sb in laser irradiated Ge
Authors: Bruno E. , Scapellato G. G. , La Magna A. , Cuscunà M. , Napolitani E. , Boninelli S. , Priolo F. , Fortunato G. , Privitera V.
Published: October 2012

Excimer laser annealing is shown to be very promising to promote Sb incorporation in Ge up to concentrations as high as 1 × 1021 at./cm3. However, we demonstrate that when Ge is melted by laser irradiation, a high excess of vacancies is generated in the molten region. These vacancies induce Sb electrical deactivation at the melt depth through the formation of Sbm-Vn complexes that act as a sink for further Sb atoms, even leading Sb to back-diffuse towards the surface, against the concentration gradient. These results are fundamental for the realization of new generation Ge-based micro and optoelectronic devices.
Space-charge-based electrostatic plasma confinement involving relaxed plasma species
Authors: Pacheco J. L. , Ordonez C. A. , Weathers D. L.
Published: October 2012

A numerical study is reported on the equilibrium properties of a surface-emitted or edge-confined non-drifting plasma. A self-consistent finite-differences evaluation of the electrostatic potential is carried out for a non-neutral plasma that follows a Boltzmann density distribution. The non-neutral plasma generates an electrostatic potential that has an extremum at the geometric center. Poisson's equation is solved for different ratios of the non-neutral plasma size to the edge Debye length. The profiles of the electrostatic potential and the plasma density are presented for different values of that ratio. A second plasma species is then introduced for two-plasma-species confinement studies, with one species confined by the space charge of the other, while each species follows a Boltzmann density distribution. An equilibrium in which a neutral region forms is found. An equilibrium is also found in which the two species have equal temperatures and charge states.