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Shape memory polymers (SMPs) are one of the intriguing functional materials and have been widely and intensively studied. In order to apply these new polymers to load bearing engineering structures and devices, developing physics-based thermomechanical constitutive models is mandatory. The aim of this Tutorial is to demonstrate how to establish a thermomechanical constitutive model for SMPs. It begins with classifications of SMPs, followed by a discussion on the underlying physics for different SMPs. After that, three classical SMP thermomechanical modeling frameworks are introduced, which include the visco-elasto-plastic based rheological framework, the storage strain-based phase transition framework, and the representative unit cell based multi-branch framework. Next, three commonly adopted new model establishment methods are presented within these frameworks with detailed examples. Finally, future perspectives on this research direction are discussed. We hope that this Tutorial will help readers understand the roadmap from physics to mathematical modeling of SMPs. [ABSTRACT FROM AUTHOR]

Disordered many-particle hyperuniform systems are exotic amorphous states of matter that lie between crystals and liquids. Hyperuniform systems have attracted recent attention because they are endowed with novel transport and optical properties. Recently, the hyperuniformity concept has been generalized to characterize two-phase media, scalar fields, and random vector fields. In this paper, we devise methods to explicitly construct hyperuniform scalar fields. Specifically, we analyze spatial patterns generated from Gaussian random fields, which have been used to model the microwave background radiation and heterogeneous materials, the Cahn-Hilliard equation for spinodal decomposition, and Swift-Hohenberg equations that have been used to model emergent pattern formation, including Rayleigh-Bénard convection. We show that the Gaussian random scalar fields can be constructed to be hyperuniform. We also numerically study the time evolution of spinodal decomposition patterns and demonstrate that they are hyperuniform in the scaling regime. Moreover, we find that labyrinth-like patterns generated by the Swift-Hohenberg equation are effectively hyperuniform. We show that thresholding (level-cutting) a hyperuniform Gaussian random field to produce a two-phase random medium tends to destroy the hyperuniformity of the progenitor scalar field. We then propose guidelines to achieve effectively hyperuniform two-phase media derived from thresholded non-Gaussian fields. Our investigation paves the way for new research directions to characterize the large-structure spatial patterns that arise in physics, chemistry, biology, and ecology. Moreover, our theoretical results are expected to guide experimentalists to synthesize new classes of hyperuniform materials with novel physical properties via coarsening processes and using state-of-the-art techniques, such as stereolithography and 3D printing. [ABSTRACT FROM AUTHOR]

In this paper, we propose a class of pentamode metamaterials for which the frequency range of the acoustics band gaps can be tuned and large figures of merit can be obtained. The band structures of the pentamode metamaterials are calculated systematically by using the finite element method. The numerical results show that the lower edge frequency of the first acoustics band gaps of pentamode metamaterials can be tuned between 3.72 kHz and 10.6 kHz by changing the diameters of the bottom and top touch cones slightly, and the relative bandwidth of the first acoustics band gaps can also be expanded. In addition, compared with the results seen in the previous research in this area, the volume filling fraction of pentamode metamaterials can be decreased by 15.7%-24.4% and the maximum figure of merit can be increased by 39.2%. [ABSTRACT FROM AUTHOR]

The phonon Boltzmann transport equation (BTE) is widely utilized to study non-diffusive thermal transport. We find a solution of the BTE in the thin film transient thermal grating (TTG) experimental geometry by using a recently developed variational approach with a trial solution supplied by the Fourier heat conduction equation. We obtain an analytical expression for the thermal decay rate that shows excellent agreement with Monte Carlo simulations. We also obtain a closed form expression for the effective thermal conductivity that demonstrates the full material property and heat transfer geometry dependence, and recovers the limits of the one-dimensional TTG expression for very thick films and the Fuchs-Sondheimer expression for very large grating spacings. The results demonstrate the utility of the variational technique for analyzing non-diffusive phononmediated heat transport for nanostructures in multi-dimensional transport geometries, and will assist the probing of the mean free path distribution of materials via transient grating experiments. [ABSTRACT FROM AUTHOR]

Narrow bandwidth and bulky configuration are the main obstacles for the realization and application of invisible cloaks. In this paper, we present an effective method to achieve broadband and thin acoustic cloak by using an acoustic metasurface (AMS). In order to realize this cloak, we use slitted unit cells to design the AMS due to the advantage of less energy loss, broad operation bandwidth, and subwavelength thickness. According to the hyperboloidal phase profile along the AMS, the incident plane waves can be focused at a designed focal spot by the flat lens. Furthermore, broadband acoustic cloak is obtained by combining two identical flat lenses. The incident plane waves are focused at the center point in between of the two lenses by passing through one lens, and then recovered by passing through the other one. However, they cannot reach the cloaked regions in between of the two lenses. The simulation results can verify the nondetectability effect of the acoustic cloak. Our study results provide an available and simple approach to experimentally achieve the acoustic cloak, which can be used in acoustic nondetectability for large objects. [ABSTRACT FROM AUTHOR]

The recognition of subsurface defects is an important aspect of inverse heat transfer problems. The solution of direct problems is the basis of the inverse problems. In this paper, based on the non- Fourier heat conduction equation and using the image method and the wave function expansion method, an analytical method to study the multiple scattering of thermal waves and temperatures from the subsurface defect in semi-infinite functionally graded materials (FGMs) is presented. A general solution of the scattered fields of thermal waves is obtained. Without loss of generality, the incidence of thermal waves excited by the periodically modulated laser and the subsurface defect are treated as the line heat source and a cylindrical cavity with thermal insulation in the modelling, respectively. The amplitude of the changes in temperature at the frontal surface in the cylindrical defect due to different physical and geometrical parameters is analysed, discussed, and presented graphically using the analytical method. The significant changes in the temperature were found to occur at the frontal surface because of the existence of conducting defects. In addition, the influence of the defect state on the temperature was found to also be related to the nonhomogeneous parameters of FGMs, the wavenumber, the frequency of the thermal waves, and the depth of the buried defects. The numerical results and method proposed in this paper are expected to be applied for the analysis of infrared thermal waves in non-destructive evaluations of functionally graded materials and inverse problems of mathematical physics. [ABSTRACT FROM AUTHOR]

An excellent high-temperature magnetic softness was observed in a Si-rich nanocrystalline Fe74.5Cu1Nb2Si17.5B5 alloy. The Curie temperatures of amorphous and crystal phases, TCA* and TCcry, for this alloy were detected to be 365 °C and 580 °C, respectively. For the 480 °C-annealed alloy, the initial permeability μi drops to nearly zero just above TCA*; however, for the 540 °C-annealed alloy, the μi of about 10 000 at f = 10 kHz has no perceivable decline in this temperature range and can hold up to more than 400 °C. Such a magnetic softness at elevated temperature is superior to that of Finemet-type Fe-based nanocrystalline alloys ever reported. The origin of the high temperature magnetic softness was interpreted by the enhancement effect of Curie temperature in residual amorphous matrix. [ABSTRACT FROM AUTHOR]

Nd-Co amorphous magnetic films with perpendicular magnetic anisotropy have been grown on nanostructured templates prepared with self-organized di-block poly(styrene)-block-poly(4-vinylpyridine) copolymer layers with a periodic structure of 60 nm spaced pores. These templates modify both the magnetic film topography and mechanical strain on a local scale. The effect of these structural changes is particularly noticeable in the low thickness range of the magnetic films where the transition from in-plane to out-of plane magnetization takes place. The Nd-Co films grown on the copolymer template present lower perpendicular magnetic anisotropy and, also, stronger stripe domain pinning effects in comparison with reference films grown on flat Si substrates. [ABSTRACT FROM AUTHOR]

Current-voltage characteristics of symmetric ferroelectric tunnel junction were studied in an ultrathin ferroelectric barrier with the consideration of the thickness and strain dependent converse-piezoelectric effect. With proper boundary conditions, large piezoelectric strain can be achieved in the ferroelectric barrier when its thickness is in the vicinity of the critical thickness. Because of the exponential relationship between the tunneling current and the effective film thickness, tunneling current can be greatly enhanced by the external electric field and substrate induced strain. [ABSTRACT FROM AUTHOR]

Dispersive characteristics of elastic waves propagating through laminated piezoelectric phononic crystal with imperfect interfaces are studied in this paper. First, the transfer matrix method and the Bloch theorem are used to derive the dispersion equation. Next, the imperfect interfaces with the jump of mechanical quantity across interface are considered. In the spring model of imperfect interface, the tractions are continuous, but displacements are discontinuous across the interface. In the mass model of imperfect interface, displacements are continuous, but tractions are discontinuous. In the spring-mass model, both traction and displacement are discontinuous. The effect of imperfect interface is introduced by inserting an additional interface transfer matrix in the calculation of total transfer matrix. Finally, the dispersion equation is solved numerically and the dispersive curves are shown in the Brillouin zone. Band gaps of phononic crystal with imperfect interface are compared with that of phononic crystal with perfect interface. The influences of the imperfect interface on the dispersive curves and the band gaps of periodic laminated piezoelectric composite are discussed. [ABSTRACT FROM AUTHOR]

Raman spectra of pure pyridine, pyridine aqueous solution, and pyridine in methanol under high pressure were measured separately. Behaviors of two Fermi doublets, v1 and v12, v1+v6 and v8, occurred simultaneously in one pyridine molecule are analyzed according to their spectra, which indicates that the v1 Raman activity decreased with increasing pressure and disappeared eventually, which induced weakness and even disappearance of the Fermi resonance between v1 and v12, while the v1 Raman intensity variation had no effect on the presence of the Fermi resonance between v1+v6 and v8 as well as its variation law with increasing pressure. Those phenomena were interpreted by group theory in this article. It also indicates experimentally that all Raman bands of pyridine appeared blueshift with increasing pressure except that OH group appeared redshift. Moreover, frequency v1 shifted more quickly with increasing pressure than their counterparts did in neat liquid, so did its Raman intensity variation. [ABSTRACT FROM AUTHOR]

A new fluid-structure interaction model that considers high gas compressibility is developed using the Rankine-Hugoniot relations. The impulse conservation between the gas and structure is utilized to determine the reflected pressure profile from the known incident pressure profile. The physical parameters of the gas such as the shock front velocity, gas density, local sound velocity, and gas particle velocity as well as the impulse transmitted onto the structure are also evaluated. A series of one-dimensional shock loading experiments on free standing monolithic aluminum plates were conducted using a shock tube to validate the proposed model. The momentum was evaluated using high speed digital imagery. The experimental peak reflected pressure, the reflected pressure profile, and the momentum transmitted onto the plate were compared with the predicted results. The comparisons show that the gas's compressibility significantly affects the fluid structure interaction behavior, and the new model can predict more accurate results than existing models. The effect of factors, such as the areal density of a plate and the peak incident pressure on momentum transfer are also discussed using the present model. Moreover, the maximum achievable momentum and the fluid structure interaction time are defined and calculated. [ABSTRACT FROM AUTHOR]

Theoretical and experimental AC loss data on a superconducting pancake coil wound using second generation (2 G) conductors are presented. An anisotropic critical state model is used to calculate critical current and the AC losses of a superconducting pancake coil. In the coil there are two regions, the critical state region and the subcritical region. The model assumes that in the subcritical region the flux lines are parallel to the tape wide face. AC losses of the superconducting pancake coil are calculated using this model. Both calorimetric and electrical techniques were used to measure AC losses in the coil. The calorimetric method is based on measuring the boil-off rate of liquid nitrogen. The electric method used a compensation circuit to eliminate the inductive component to measure the loss voltage of the coil. The experimental results are consistent with the theoretical calculations thus validating the anisotropic critical state model for loss estimations in the superconducting pancake coil. [ABSTRACT FROM AUTHOR]

We apply general thermodynamics-based wave analysis methods to a gas-gun-driven plate impact experiment designed to derive strength information from tantalum at pressures of 10-25 GPa. The analysis provides estimates of the complete deformation paths in terms of the coupled evolution of mean stress, deviatoric stress, plastic strain, and plastic strain rate, yielding detailed information for direct comparison to strength models. This inverse analysis (deriving estimates of strength behavior directly from the measurements, with no strength model assumed) is compared to forward analysis (hydrodynamic simulations with specific strength models, in general adjusting parameters to optimally match the experiment). This comparison fulfills three goals. (1) To determine the parameter sensitivity and overall stability of the inverse analysis by analyzing simulated data as if it were experimental data. We find that, in reasonably favorable cases, precision to ∼10% is possible for the flow curve during loading and ∼30% for the shape of the curve during unloading. (2) To distinguish the ability of different strength models to account for the measurements. In particular we find that a new multiscale strength model seems to capture the rate-dependent release behavior very well but that it is difficult to capture the effects of a particular material's microstructure and texture. (3) To bracket our understanding of the actual strength behavior in the experiment and enhance our confidence in both the forward and inverse calculations. The results show a peak deviatoric stress of ∼0.7-1.4 GPa occurring nearly at the point of peak plastic strain rate, followed by a complex evolution in which the material's internal relaxation and strain-hardening properties interact with the rest of the loading wave, the post-shock plateau, and the unloading wave. The results show the importance of extreme precision in measurement timing and equation-of-state calibrations, particularly at higher pressures. [ABSTRACT FROM AUTHOR]

A transmission line model for the calculation of optical interference phenomena in dielectric multilayered structures is adopted as an alternative option to the transfer matrix model (TMM). The method is based on the transmission line theory and is exact, easy to implement and uses closed iterative forms instead of the TMMs matrix formalism. The proposed model has been appropriately modified and then applied for performance evaluation of a typical organic photovoltaic device under inclined illumination. Optical field distribution, short-circuit photocurrent and reflectivity have been calculated under different angles of light incidence. The theoretical simulations have been discussed and compared with experimental photocurrent measurements, while the influence of the photoactive layer thickness on the device efficiency has been evaluated for different angles of light incidence, taking into account its extinction coefficient anisotropy. [ABSTRACT FROM AUTHOR]

The switching process in ferroelectrics with a maximum in time for the switching current as observed by Merz [Phys. Rev. 95, 690 (1954)] is simulated. To this end, the Weiss molecular mean-field model is combined with the fluctuation of dipoles in double-well potentials. A first-order rate equation for the polarization with time-dependent coefficients results. This equation is solved for the time-dependent switching process, as well as for the static polarization hysteresis loop. [ABSTRACT FROM AUTHOR]

Breakdown in a microwave field leads to the formation of plasmoids that expand in a direction parallel to the incident electric field and form microwave streamers. A two-dimensional model based on solutions of Maxwell's equations coupled with a quasineutral description of the plasma is used to study the formation of a single streamer formed and confined at the antinode between two incident linearly polarized waves with opposed wave vectors. We discuss the parameters controlling the plasma density in the streamer, the field distribution and elongation speed for incident fields of frequency F = 110 GHz at atmospheric pressure, p = 760 Torr, in dry air (other conditions with the same F/p can be deduced from simple scaling laws). The results show that the large electric field at the tips of the streamer (responsible for the streamer elongation parallel to the incident field) presents successive maxima and minima during the streamer extension. These maxima and minima are associated with resonant effects. When the applied field is only slightly above the critical field, the streamer growth may stop at the first field minimum. [ABSTRACT FROM AUTHOR]

A computational model is developed to investigate the effect of vertical composition gradient on the photovoltaic properties of organic photovoltaic cell based on poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). When the photovoltaic properties of model devices with systematic variation of vertical composition distribution are calculated by using this model, it is found that the efficiency increases almost linearly as the concentrations of P3HT and PCBM become richer near anode and cathode, respectively. The larger exciton generation near anode due to enrichment of P3HT near the anode contributes mainly to an increase of the device efficiency by compensating the lower hole mobility of P3HT with shorter pathway to the anode. [ABSTRACT FROM AUTHOR]

Magnetic and magneto-transport properties of granular ferromagnetic Cox-(Al2O3)1-x films with a large range of cobalt concentrations x are investigated. The films with x below the percolation threshold xp demonstrate the presence of oriented uniaxial anisotropy of the granules with the anisotropy field Ha ∼ 2 kOe and easy anisotropy axis perpendicular to the film plane. It is shown that for the values of x close but below xp, field dependencies of the magnetoresistance in the magnetic fields applied in the film plane, possess an anomalous field dependence with a positive magnetoresistance and a maximum in the range of field magnitudes 0 < H < Ha. The positive magnetoresistance effect is isotropic in the film plane and reduces as the field direction is rotated out of the film plane up to the complete quenching for the field perpendicular to the film plane. The effect is not related to an anisotropic magnetoresistance manifestation and is temperature-dependent. We propose a model, which explains the phenomenon and bases on an accounting of thermal fluctuations of uniaxial particle magnetic moments in the fields perpendicular to the particle easy anisotropy axis in the presence of the ferromagnetic interaction between the particles. [ABSTRACT FROM AUTHOR]

We have corrected the field enhancement factor for the “floating sphere at emitter-plane potential” model with the finite anode-cathode distance. If ρ is the radius of sphere, h is the distance from cathode to the center of sphere, and l is the distance from the center to the anode, then the field enhancement factor is given as the following expression βsph=(2+7η-η2)(λ2-2λ+2)/[2η(1-λ)(2-λ)], where η=ρ/h, λ=ρ/l. This expression demonstrates reasonable behavior for three limiting cases: if h→ρ, if l→∞, and if l→ρ. We have compared our factor βsph with the field enhancement factor βtube for the “hemisphere on a post” model and the factor βell for the “hemiellipsoid on plane” model. We have shown realization of the approximate evaluation βtube≈(βsph+βell)/2. [ABSTRACT FROM AUTHOR]

Based on a mode match transfer matrix method and the quantum coherent transport theory of Mireles and Kirczenow, we investigate the coherent electron tunneling in FM/S1/I/S2/FM (FM represents the ferromagnetic metal layer, S1 and S2 represents the different semiconductor layer, respectively, I represents the insulator layer.) system. The effects of the thickness of the semiconductor layers and the Rashba spin-orbit coupling on the spin-dependent tunneling transmission coefficient and the properties of the tunnel magnetoresistance (TMR) are studied. It is found that the variations of tunneling transmission coefficients and the tunnel magnetoresistance TMR with Rashba spin-orbit coupling and the thickness of semiconductor layer, show typical resonant properties and the TMR can be enhanced and its sign can switch from positive to negative by increasing the ratio of Rashba spin-orbit coupling strength between two semiconductor layers. [ABSTRACT FROM AUTHOR]

Electrical transport properties in ferromagnetic shape memory Ni–Mn–Ga single crystal have been investigated both in experiment and theory by analyzing electrical resistivity along different crystallographic directions during heating. The experimental results show a clear first-order martensitic transformation and a large anisotropic resistivity (AR) of 23.7% at the tetragonal martensitic phase. The theoretical conductivity (σ=1/ρ), estimated using first-principles calculations combined with classical Boltzman transport theory, proves essential crystallographic anisotropic resistivity (AR=31%) in the martensitic phase and agrees well with experimental results. The AR in the martensitic phase is reveled to mainly originate from the splitting of the minority-spin Ni 3d and Ga 4p states near the Fermi level and hence reconstruction of the minority-spin Fermi surface upon martensitic transformation. [ABSTRACT FROM AUTHOR]

Nonradiative transitions occurring in semiconductors result in thermal emissions carrying information on the material’s thermal and electronic properties. A simple one-dimensional theoretical model is devised which accounts for the photothermal signal variations due to nonradiative transitions occurring in semiconductor thinfilms. The theory was verified by determining the transport properties of p-type silicon wafer. We could get the thermal diffusivity, minority carrier lifetime, surface recombination velocity, and minority carrier mobility of CuInS2 thin films, thereby proving the efficiency and simplicity of photothermal beam deflection technique for real time characterization of semiconductor thin films. The film fabrication history, composition, and post deposition treatments play crucial role in determining the transport properties and the effect of these conditions on transport properties of the film as well as on the solar cell parameters is discussed. [ABSTRACT FROM AUTHOR]

The effect of partial substitution of Zr by Al in Zr35-xTi30Cu7.5Be27.5Alx (x≤10) alloys on the glass forming ability (GFA) is studied. The thermal and the elastic criteria in evaluating the GFA of the alloys are examined. The calorimetric quantities are measured to determine the width of the supercooled liquid region ΔTx and γ indexes. Parallel studies are performed to measure the elastic moduli with the emphasis of the ratio of the shear to the bulk modulus G/B. The addition of Al is found to have remarkable influence on the GFA of the alloys. While ΔTx and γ decrease with the deterioration of the GFA, G/B is observed to decrease as well, contrasting with the general tendency among various glass forming metallic systems. The thermal and elastic criteria of GFA are discussed. [ABSTRACT FROM AUTHOR]

Distinct, periodic arc-shaped ripples are observed on the surface of the weld bead in almost any welded components which have a significant effect on weld quality. This article presents the complex transport phenomena and their effect on the formation of ripples in three-dimensional moving gas metal arc welding. The transient distributions of the melt flow velocity and temperature in the weld pool, weld pool shape and dynamics, and solidified weld bead are calculated. It is found that the surface ripples are formed by the interplay between the up-and-down weld pool dynamics, caused mainly by the periodic droplet impingements, and the rate of weld pool solidification. The effects of various welding parameters, including the welding current, droplet size, droplet frequency, droplet impinging velocity, and travel speed on the pitch (distance between two ripples) and height of the ripple are investigated. This study provides fundamental understanding to the underlying physics that cause the formation of ripples with different pitches and heights. [ABSTRACT FROM AUTHOR]

As-grown and oxygen plasma treated n-type tin dioxide (SnO2) (101) thin films were investigated regarding the properties of Au contacts, surface structure, and band bending. The plasma treatment was performed in a conventional oxygen plasma cleaning system with maximum oxygen ion energies of 400 eV. Whereas the as-deposited SnO2 film formed non-Schottky contacts with Au, the oxygen plasma treated films formed Schottky contacts with Au. Capacitance-voltage and differential Hall measurements indicated the introduction of bulk electron traps up to several 100 nm below the surface due to the oxygen plasma treatment. Angle resolved x-ray photoelectron spectroscopy (AR-XPS) revealed a surface accumulation layer on the as-grown film that was absent after the plasma treatment. These measurements further revealed chemical differences between the as-deposited and the plasma treated SnO2 surfaces. All SnO2 films had Sn2+–O bonds. The AR-XPS Sn2+–O signal intensity increased after plasma treatment, indicating that the oxygen plasma damaged the SnO2 surface. Additionally, an O2 adlayer was formed due to the oxygen plasma treatment. The damaged layer due to oxygen ion bombardment, oxygen adsorption layer, and possibly bulk traps depleted the surface carriers which enabled the formation of a Schottky contact with Au. [ABSTRACT FROM AUTHOR]

For carbon tetrachloride, a temperature Hugoniot at 7–23 GPa and a distribution of the constant-volume specific heat on the Hugoniot CV(T) at 1057–3275 K are simultaneously estimated from the Walsh–Christian (WC) equation such that the Hugoniot fits well to the existing measured data. The estimated CV(T) distribution reveals the significance of the contribution of electrons and ions to the specific heat. That is, in contrast to the almost uniform distribution of the specific heat predicted from the Debye equation, the CV(T) distribution increases significantly with an increase in the Hugoniot temperature due to thermal excitation of electrons at 1057–1500 K (7–10.1 GPa), additional activation of the dimerization reaction at 1500–2350 K (10.1–16 GPa), and also additional activation of the polymerization reaction at 2350–3275 K (16–23 GPa). As an example, evidence is presented that carbon tetrachloride is a semiconductor at 1500 K. The CV(T) distribution in each temperature range is formulated and a temperature Hugoniot is reevaluated from the WC equation using the specific heat equations formulated. It is confirmed by a good fit of the reevaluated Hugoniot to the existing measured data that the specific heat equations express the CV(T) distribution appropriately. [ABSTRACT FROM AUTHOR]

We experimentally and numerically investigate the transmission of single cycle terahertz pulses through subwavelength slit apertures featuring zero cutoff frequency and very low attenuation. Employing a polaritonic approach we demonstrate that wave forms transmitted through slit samples with slit widths as small as λ/1000 can be visualized and analyze the applicability of this approach to terahertz near-field microscopy. Finite element simulations are used to quantitatively investigate resolution limitations due to imperfect experimental configurations. Our results show that resolutions on the scale of the slit width are possible; however, they demand an accurate control of the distance between the imaging aperture and the sample. This is because the presence of small gaps leads to around-the-bend waveguiding effects resulting in a significant reduction of the attainable resolution. [ABSTRACT FROM AUTHOR]

We report a microscopic investigation of the spectrum of voltage fluctuations in nanometric n-Si layers. Theory makes use of a Monte Carlo simulator self-consistently coupled with a two-dimensional Poisson solver. We consider layers of variable thickness W in the range of 2–100 nm and variable length L in the range of 10–1000 nm embedded in an external dielectric medium. Calculations are performed at T=300 K for different doping levels and in the presence of an applied voltage of increasing strength. The spectra are found to exhibit peaks centered on the terahertz region. For W≥100 nm and carrier densities of 5×1017 and 5×1018 cm-3, the frequency peaks agree with the value of the three dimensional plasma frequency. For W≤100 nm, the results exhibit a plasma frequency that depends on L, thus implying that the oscillation mode is dispersive. The corresponding frequency covers a wide range of values of 0.2–10 THz and is in agreement with the values of the two-dimensional plasma frequency predicted by existing analytical models. At sufficiently high voltages, the two-dimensional plasma peak is washed out and we observe the onset of a peak in the subterahertz region which is associated with transit time instabilities induced by current saturation conditions. [ABSTRACT FROM AUTHOR]

Complex acoustic impedance is determined in a prototype refrigerator that can mimic orifice-type, inertance-type, and double inlet-type pulse tube refrigerators from simultaneous measurements of pressure and velocity oscillations at the cold end. The impedance measurements revealed the means by which the oscillatory flow condition in the basic pulse tube refrigerator is improved by additional components such as a valve and a tank. The working mechanism of pulse tube refrigerators is explained based on an electrical circuit analogy. [ABSTRACT FROM AUTHOR]

Based on the principle of minimal energy, the angular dependence of exchange bias has been investigated in detail with noncollinear easy axes of unidirectional and uniaxial anisotropies in the ferromagnetic/antiferromagnetic bilayers. The competition between unidirectional and uniaxial anisotropies divides the initial magnetization state of the bilayers into monostable state and bistable state, which determine the angular dependence of exchange bias directly. When the external field is applied along the intrinsic easy axes and intrinsic hard axes, it is found that one of the coercive fields at the descending or ascending branch of the hysteresis loop makes an abrupt change, whereas the other coercive field keeps continuity by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show the jump phenomenon in the angular dependence of exchange bias. This jump phenomenon of the exchange bias is found to be an intrinsic property of the bilayers which is dependent on the relative magnitudes and orientations between unidirectional and uniaxial anisotropies. The numerical calculations indicate that both the exchange bias field and the coercivity are larger in the magnitude at the points of the jumps. At the jumping points of the intrinsic easy axes, the coercivity reaches the maximum; at the jumping points of the intrinsic hard axes, the exchange bias field reaches the maximum, at the meantime the coercivity can vanish itself suddenly. These features of the jump phenomenon are useful in the technological applications to achieve the largest exchange bias field. [ABSTRACT FROM AUTHOR]

Future monolithic point of load switched mode power supplies will be expected to meet the energy requirements of miniaturized, high functionality electronic devices. Recently, Ni45Fe55 has emerged as a potentially important material choice for use as a soft magnetic core material within high frequency integrated passive magnetic components. The operating frequency range of the integrated passives which form a key part of the point of load power supply must increase to allow for inductor/transformer miniaturization to become monolithic with power integrated circuits. In this work, an analysis of the high frequency permeability spectra of an electroplated Ni45Fe55 thin film has been carried out to quantitatively analyze the material’s high frequency performance. Complex permeability spectra of the film have been investigated at frequencies up to 9 GHz to identify both the film’s spectroscopic splitting factor (g) and its effective dimensionless damping parameter (α). The Kittel equation is utilized to identify g as 2.128, while α is determined to be approximately 0.045. The critically damped condition for the film is also examined to extract α in the critically damped case under a range of externally applied bias fields. It is concluded that for monolithic power inductors, improved device performance can be achieved when the ferromagnetic core is in an underdamped state up to a critical frequency. [ABSTRACT FROM AUTHOR]

The Vlasov and Maxwell’s equations are established and solved numerically to describe the effects of toxin/pollutant gas pressure and functionalized single wall carbon nanotube (SWCNT) sensor in a perturbed microwave resonant cavity. The dependence of the absorption coefficient on incident frequency, toxin/pollutant gas pressure, electron density, and collision frequency is presented. The numerical results illustrate that the resonant frequency shifts by a suitable amount for modest changes in toxin/pollutant gas pressure. It is also illustrated that high density and low collision of the blend of toxin/pollutant gas and SWCNT sensor in a microwave resonant cavity can be employed as broadband absorption of microwave and the detection of toxin/pollutant gas characteristics through adjustments of the amount of toxin/pollutant gas pressure and functionalized SCWNT sensor. The numerical results additionally illustrate that the microwave absorption spectra of the blend of toxin/pollutant gas and SWCNT sensor in a microwave resonant cavity are in good agreement with the available experimental data. The present method is, in principle, applicable to any kind of a single nanofiber, nanowire, silica gel, cotton fiber, and even various types of nanotubes. [ABSTRACT FROM AUTHOR]

We investigate the current noise autocorrelations and cross correlations through two interacting quantum dots coupled to four external leads. The results indicate sign reversal of the cross correlation in different tunneling regions, respect to competing or supporting processes. By adjusting the dot-lead coupling in one dot, shot noise in another system can be modulated from sub- to super-Poissonian in the Coulomb blockade regime. Furthermore, spin injection can induce super-Poissonian shot noises in both dots due to the corporation of spin blockade and Coulomb blockade. [ABSTRACT FROM AUTHOR]

Size dependent domain configuration and its evolution under an external electric field are investigated for ultrathin ferroelectric films using an unconventional phase field method. The simulation reveals a series of domain configurations at different thicknesses, including zigzag patterns with eight variants or four variants coexisting, a vortex pattern with four variants coexisting, and a stripe pattern with two variants coexisting. When the film thickness falls below a critical value of 2.8 nm, the polarization vanishes, indicating the suppression of ferroelectricity. The evolution of domain configuration under an alternating electric field is also investigated, and the reduction in remnant polarization and coercive field with respect to decreasing thickness is observed. [ABSTRACT FROM AUTHOR]

A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology. [ABSTRACT FROM AUTHOR]

A cover layer of CdS enhances the efficiency of CdTe and CuInSe2 based solar cells. The experimental results are summarized, giving the reasons why a CdS layer causes Voc and the fill factor to increase. Field quenching by Frenkel–Poole excitation in copper doped CdS leads to a negative differential conductivity and a high-field domain with a field of 50 kV/cm, limiting the maximum field in the CdS/CdTe junction. This prevents tunneling through the junction and reducing electron leakage. Other evidence indicates an adjustment of the electron affinity, hence in a shift of the band connection at different bias conditions. A band model of the CdS/CdTe cell is proposed. [ABSTRACT FROM AUTHOR]

Electrical conduction in chaotic media near the percolation threshold is investigated by means of Monte Carlo techniques. The main target is to demonstrate the possibility of modulating the conduction of a chaotic film by means of lateral gate electrodes fabricated in the plane of the film. Exponential variations in the conductivity due to the modification of random paths in the chaotic film can be exploited for the realization of devices with high on/off ratio. In this work it is demonstrated that despite the chaotic conduction driven by charge hopping between localized sites, an almost deterministic behavior can be obtained for suitable site concentrations. Transistors based on this concept could be realized on different layers, on the same substrate, with technologies similar to those actually used for conventional integrated circuits: this would make possible an effective three-dimensional integration of devices and circuits. [ABSTRACT FROM AUTHOR]

Fluid motion induced on the surface of 100 MHz focused surface acoustic wave (F-SAW) devices with concentric interdigital transducers (IDTs) based on Y-cut Z-propagating LiNbO3 substrate was investigated using three-dimensional bidirectionally coupled finite element fluid-structure interaction models. Acoustic streaming velocity fields and induced forces for the F-SAW device are compared with those for a SAW device with uniform IDTs (conventional SAW). Both, qualitative and quantitative differences in the simulation derived functional parameters, such as device displacements amplitudes, fluid velocity, and streaming forces, are observed between the F-SAW and conventional SAW device. While the conventional SAW shows maximum fluid recirculation near input IDTs, the region of maximum recirculation is concentrated near the focal point of the F-SAW device. Our simulation results also indicate acoustic energy focusing by the F-SAW device leading to maximized device surface displacements, fluid velocity, and streaming forces near the focal point located in the center of the delay path, in contrast to the conventional SAW exhibiting maximized values of these parameters near the input IDTs. Significant enhancement in acoustic streaming is observed in the F-SAW device when compared to the conventional ones; the increase in streaming velocities was computed to be 352% and 216% for tangential velocities in propagation and transverse directions, respectively, and 353% for the normal velocity. Consequently, the induced streaming force for F-SAW is 480% larger than that for conventional SAW. In biosensing applications, this allows for the removal of smaller submicron sized particles by F-SAW which are otherwise difficult to remove using the conventional SAW. The F-SAW presents an order of magnitude reduction in the smallest removable particle size compared to the conventional device. Our results indicate that the acoustic energy focusing and streaming enhancement brought about by the F-SAW device manifests itself as enhanced biofouling removal efficiency of F-SAW throughout the device delay path compared to the conventional device, thereby providing enhanced device sensitivity, selectivity, and reusability. Furthermore, contrary to the conventional SAW in which the smallest particle is removable near the input IDTs, the F-SAW device removes the smallest particle near the device focal point. The results of this work are shown to have significant implications in typical biosensing and microfluidic applications. In a broader context, the results of the present study demonstrate a technique of enhancing streaming induced flows, which is of great importance to contemporary problems involving microfluidic and sensing applications of piezoelectric devices. [ABSTRACT FROM AUTHOR]

We have studied the bias stressing voltage dependence of the negative bias temperature instability (NBTI) in p-channel field effect transistors with nominal SiON/HfSiON gate stacks. Measurements were made at the temperature of 448 K. Three different methods were used to extract device parameters such as the NBTI induced threshold voltage shifts ΔVth. One of the methods provided access to all of the NBTI induced trapped charge (both fast and slow relaxing) while the other two methods necessarily determined only the slow trap component. Assuming a model of the form ΔVth=Atα, where t is the bias stressing time, we determine that both A and α are bias voltage dependent for the data from all three extraction methods. Furthermore, slow and slow plus fast trap values were significantly different, suggesting that a unique set of A and α values cannot be used to describe all the traps. This result raises concerns about protocols used with NBTI data to determine the reliability lifetime. [ABSTRACT FROM AUTHOR]

Hard and refractory particles with various shapes have been added to ceramic materials to enhance creep resistance. Also, many of the particles in food emulsions, magnetic particle suspensions, and other industrial systems may have nonspherical shapes. Hence, it is important to understand the effects of particle shape on the viscosity of suspensions. While the shape of many particles can be approximated as prolate or oblate spheroids, the purpose of the present study is to model the effective viscosity of semidilute suspensions of rigid ellipsoids. Closed-form expressions are obtained; however, their formulations are formidable. For the special cases of rodlike or disklike particles, simple closed-form expressions can be obtained. The present solutions are compared with existing solutions. [ABSTRACT FROM AUTHOR]

The idea of localized resonance in phononic crystals is introduced to improve the low-frequency acoustic absorption of viscoelastic materials, which are widely used underwater to absorb the unwanted noise and meet a difficult challenge to enhance low-frequency absorption. Experimental measurements for acoustic absorptances of various viscoelastic polymer slabs embedded with locally resonant scatterers, i.e., heavy spheres coated by soft silicon rubber, are reported. Theoretical investigations using a layer-multiple-scattering method show good agreement with the experiment results, which supports unambiguously the observed low-frequency absorption phenomenon. The physical mechanism for the low-frequency absorption is also investigated by referring the elements of the Mie scattering matrix. The observations suggest that the scattered energy tends to remain in transverse waves, which enhance the multiple scattering, damp rapidly in viscoelastic polymer and dominate the acoustic absorption. [ABSTRACT FROM AUTHOR]

The effects of O vacancy (VO) on the electronic and magnetic properties of Co-doped ZnO are examined through first-principles total-energy calculations. Our results suggest that due to the presence of VO, O 2p states are coupled with the Co 3d states. Co 3d empty minority states are broadened and move up towards the Fermi level. VO at metastable sites can induce the ferromagnetic (FM) coupling between nearest-neighbor (NN) Co ions, and the atoms around the FM-coupled NN Co ions are spin-polarized which leads to the long-ranged FM coupling between Co ions. Our results based on first-principles total-energy calculations give a possible explanation for the controversial magnetic properties of Co-doped ZnO reported in the literature. [ABSTRACT FROM AUTHOR]

Tunnel field-effect transistors (TFETs) are potential successors of metal-oxide-semiconductor FETs because scaling the supply voltage below 1 V is possible due to the absence of a subthreshold-swing limit of 60 mV/decade. The modeling of the TFET performance, however, is still preliminary. We have developed models allowing a direct comparison between the single-gate, double-gate, and gate-all-around configuration at high drain voltage, when the drain-voltage dependence is negligible, and we provide improved insight in the TFET physics. The dependence of the tunnel current on device parameters is analyzed, in particular, the scaling with gate-dielectric thickness, channel thickness, and dielectric constants of gate dielectric and channel material. We show that scaling the gate-dielectric thickness improves the TFET performance more than scaling the channel thickness and that improvements are often overestimated. There is qualitative agreement between our model and our experimental data. [ABSTRACT FROM AUTHOR]

Current-induced spin-transfer torque in spin valve structures with magnetic layers made of different materials can exhibit a nonstandard, i.e., wavy like, angular variation. Following the diffusive spin transport model, the wavy torque was theoretically found in the Co(8 nm)/Cu(10 nm)/Py(8 nm) spin valve, and later was also experimentally confirmed. However, the single domain (macrospin) approximation fails to describe some features of magnetic dynamics observed in experiments, especially the frequency agility with current and its slope for the out-of-plane precessions. Therefore, we have revisited the abovementioned pillar structure and analyzed the conditions for the appearance of the out-of-plane oscillations. In particular, we have studied the possibility of triggering these modes by means of a moderate external magnetic field. We have identified modes supported within the macrospin model and verified the results by full micromagnetic simulations, revealing numerous qualitative discrepancies. For instance, the out-of-plane oscillations have been found in the macrospin model, whereas no such precessions have been observed in micromagnetic simulations even with an external magnetic field. [ABSTRACT FROM AUTHOR]

One of the general challenges for the evaluation and interpretation of optical emission spectroscopy measurements is the occurrence of radiation trapping, which is also named self-absorption or opacity. A convenient technique to treat radiation trapping is the introduction of a parameter, which is called escape factor and gives a measure for the amount of radiation trapping. In this paper evaluations on the concept of escape factors are presented for an inductively coupled plasma in argon. Especially, the strong argon line at 811.53 nm, which arises from the transition of 2p9-1s5, is under consideration. To estimate escape factors for this line, a particular method is proposed and presented here. First experimental results are obtained under the restrictive assumptions that transitions into the resonant levels 1s2 and 1s4 are sufficiently optically thin and ratios of population densities are constant. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) supported by photoluminescence (PL) has been applied to study (i) the surface band bending, (ii) the band gap bowing, and (iii) the Stokes shift for InGaN layers grown by molecular beam epitaxy with 0.14≤In≤0.36. The type of surface band bending has been investigated on the basis of the shape of CER resonance. It has been found that the surface band bending changes from n-type for layers with low indium content (In<27%) to flatband (or weak p-type band) for layers with In∼35%. The band gap bowing has been determined to be 1.4±0.2 and 2.1±0.3 eV for CER data with and without strain corrections, respectively. From this analysis it has been concluded that the reliable value of the bowing parameter for unstrained InGaN should be between 1.4 and 2.1 eV. Comparing CER with PL data it has been found that the Stokes shift rises from 20 to 120 meV when the indium concentration increased from 14% to 36%. In addition, it has been observed that the intensity of PL from InGaN layers decreased exponentially with the increase in the indium content. The last two findings are attributed to an easier formation of native point defects and stronger indium segregation in InGaN alloys with higher indium concentrations. [ABSTRACT FROM AUTHOR]

The light extraction efficiency of an organic light-emitting diode (OLED) was significantly enhanced upon incorporating a photonic-crystal (PC) pattern consisting of ZnS pillars residing on top of the transparent anode-glass substrate. The luminance efficiency of the PC-OLED is enhanced by a factor of 2.1 to 2.4 depending on the periodicity of the PC structure, when compared to that of a control OLED operating under the same conditions. [ABSTRACT FROM AUTHOR]

Optical emission spectroscopy is applied to the study of a radiofrequency (13.56 MHz) discharge in methane used to obtain hydrogenated carbon films and particles. The methane dissociation allows the creation of species in the plasma bulk as H2, H, and CH. The emission lines of these species are studied as a function of time and of incident rf power. The electron temperature is determined from the two line radiance ratio method and the corona balance model using the Balmer lines (Hα, Hβ, and Hγ). The incident rf power enhancement in the range 40–120 W leads to the increase in the emission line intensities as the electron temperature decreases. The temporal variations of CH and hydrogen emission lines, of the dc self-bias voltage, and of the electron temperature are correlated both with the particle behavior and growth in the plasma, and with the coating that grows onto the powered electrode. [ABSTRACT FROM AUTHOR]

The ac field (Eac) amplitude dependence of dielectric response has been investigated for poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor terpolymer films. The frequency dependence of the temperature of the permittivity maximum Tm can be well described by the Vogel–Fulcher relation for Eac in the range from 1.33 to 80 MV/m. Tm remains nearly constant at an Eac level lower than 13.3 MV/m, and starts decreasing with Eac increases further. The two parameters fitting to the Vogel–Fulcher relation, freezing temperature Tf and activation energy Ea, vary little for Eac below 26.7 MV/m and then Tf decreases, while Ea increases for higher Eac level. [ABSTRACT FROM AUTHOR]