Key Physical Parameters Research Articles

Quantifying Interdopant Exciton Processes in Organic Light Emitting Diodes

Interdopant exciton transfer processes play a critical role in engineering emission spectra from multiple emitters in white organic light emitting diodes (OLEDs). Developing experimental techniques for probing these energy transfer processes are thus vital to gain a better understanding of the device physics and to eventually engineer better white OLEDs. In this article, we present a simple method, based on electroluminescence spectra, to study and quantify the energy transfer mechanisms in a two-dopant (triplet–singlet) system. Through a combination of experimental spectra obtained from a collection of donor–acceptor separations and theoretical simulations of spectral variations, we demonstrate that the key physical parameters such as the Dexter van der Waals radii and the Förster radius can be quantified.

Cardinal kinematics: I. Rotation fields of the APOGEE Survey

Correlations between stellar chemistry and kinematics have long been used to gain insight into the evolution of the Milky Way Galaxy. Orbital angular momentum is a key physical parameter and it is often estimated from three-dimensional space motions. We here demonstrate the lower uncertainties that can be achieved in the estimation of one component of velocity through selection of stars in key directions and use of line-of-sight velocity alone (i.e. without incorporation of proper motion data). In this first paper we apply our technique to stars observed in the direction of Galactic rotation in the APOGEE survey. We first derive the distribution of azimuthal velocities, $v_\phi$, then from these and observed radial coordinates, estimate the stellar guiding centre radii, $R_g$, within $6.9\leq R \leq 10$ kpc with uncertainties smaller than (or of the order of) 1kpc. We show that there is no simple way to select a clean stellar sample based on low errors on proper motions and distances to obtain high-quality 3D velocities and hence one should pay particular attention when trying to identify kinematically peculiar stars based on velocities derived using the proper motions. Using our azimuthal velocity estimations, we investigate the joint distribution of elemental abundances and rotational kinematics free from the blurring effects of epicyclic motions, and we derive the $\partial v_\phi / \partial R_g$ trends for the thin and thick discs as a function of radius. Our analysis provides further evidence for radial migration within the thin disc and hints against radial migration playing a significant role in the evolution of the thick disc.

Direct Nanoscale Sensing of the Internal Electric Field in Operating Semiconductor Devices Using Single Electron Spins.

The electric field inside semiconductor devices is a key physical parameter that determines the properties of the devices. However, techniques based on scanning probe microscopy are limited to sensing at the surface only. Here, we demonstrate the direct sensing of the internal electric field in diamond power devices using single nitrogen-vacancy (NV) centers. The NV center embedded inside the device acts as a nanoscale electric field sensor. We fabricated vertical diamond p-i-n diodes containing the single NV centers. By performing optically detected magnetic resonance measurements under reverse-biased conditions with an applied voltage of up to 150 V, we found a large splitting in the magnetic resonance frequencies. This indicated that the NV center senses the transverse electric field in the space-charge region formed in the i-layer. The experimentally obtained electric field values are in good agreement with those calculated by a device simulator. Furthermore, we demonstrate the sensing of the electric field in different directions by utilizing NV centers with different N-V axes. This direct and quantitative sensing method using an electron spin in a wide-band-gap material provides a way to monitor the electric field in operating semiconductor devices.

Physical Analysis of the Initial Core and Running-In Phase for Pebble-Bed Reactor HTR-PM

The pebble-bed reactor HTR-PM is being built in China and is planned to be critical in one or two years. At present, one emphasis of engineering design is to determine the fuel management scheme of the initial core and running-in phase. There are many possible schemes, and many factors need to be considered in the process of scheme evaluation and analysis. Based on the experience from the constructed or designed pebble-bed reactors, the fuel enrichment and the ratio of fuel spheres to graphite spheres are important. In this paper, some relevant physical considerations of the initial core and running-in phase of HTR-PM are given. Then a typical scheme of the initial core and running-in phase is proposed and simulated with VSOP code, and some key physical parameters, such as the maximum power per fuel sphere, the maximum fuel temperature, the refueling rate, and the discharge burnup, are calculated. Results of the physical parameters all satisfy the relevant design requirements, which means the proposed scheme is safe and reliable and can provide support for the fuel management of HTR-PM in the future.

Structural health monitoring system of bridges

The main scope of the research is to develop an integrated monitoring system for durability assessment of bridges. The system must interface and integrate the actual practice mainly based on visual inspections and combine the response of a number of different reliable sensors, installed on the structure to monitor the progress of damage, with enhanced realistic deterioration models. The system and the sensors were developed to cover the parameters for the most important deterioration mechanisms: corrosion of reinforcement in bridges, carbonation of concrete, freeze-thaw cycles, alkali-silica reaction and mechanical damage, as well as the changes in the structures behavior and safety: static deformation, strains; crack widths and vibrations (frequencies, amplitudes, accelerations and vibration modes).The progress of the various types of damage mechanisms can be predicted by monitoring the key physical and chemical parameters of the materials (such as temperature, humidity and pH for the concrete, measured on the surface of the structure and as a profile through the concrete thickness, and/rate of corrosion of reinforcement), and key mechanical parameters (such as strains, deflections, vibrations).The use of permanent monitoring systems has several advantages once the system is installed: (i) to reduce the operating costs of inspections and maintenance by 25%, and the traffic-related costs by 30 % by reducing the number and extent of site inspections and (ii) to reduce the overall life costs of bridges by 10 % by applying the improved lifetime prediction models already from the design stage.

Parameter-free prediction of DNA dynamics in planar extensional flow of semidilute solutions

The dynamics of individual Deoxyribonucleic acid (DNA) molecules in semidilute solutions undergoing planar extensional flow is simulated using a multiparticle Brownian dynamics algorithm, which incorporates hydrodynamic and excluded volume interactions in the context of a coarse-grained bead-spring chain model for DNA. The successive fine-graining protocol [P. Sunthar and J. R. Prakash, Macromolecules 38, 617–640 (2005); R. Prabhakar et al., J. Rheol. 48, 1251–1278 (2004)], in which simulation data acquired for bead-spring chains with increasing values of the number of beads Nb, is extrapolated to the number of Kuhn steps NK in DNA (while keeping key physical parameters invariant), is used to obtain parameter-free predictions for a range of Weissenberg numbers and Hencky strain units. A systematic comparison of simulation predictions is carried out with the experimental observations of Hsiao et al. [J. Rheol. (in press)], who have recently used single molecule techniques to investigate the dynamics of dilute and semidilute solutions of λ-phage DNA in planar extensional flow. In particular, they examine the response of individual chains to step-strain deformation followed by cessation of flow, thereby capturing both chain stretch and relaxation in a single experiment. The successive fine-graining technique is shown to lead to quantitatively accurate predictions of the experimental observations in the stretching and relaxation phases. Additionally, the transient chain stretch following a step strain deformation is shown to be much smaller in semidilute solutions than in dilute solutions, in agreement with experimental observations.

Synthesis of zeolitic imidazolate framework-8 particles of controlled sizes, shapes, and gate adsorption characteristics using a central collision-type microreactor

Controlling the particle size and shape of soft metal-organic framework compounds is crucial for understanding and regulating the adsorption-induced structural transitions that lead to the unique stepwise adsorption behavior of these materials. The present study demonstrates that the intensive mixing facilitated by a microreactor enables the synthesis of monodisperse zeolitic imidazolate framework-8 (ZIF-8) particles with high reproducibility. In the flow microreactor process used in this study, the particle size is predominantly determined by the reaction conditions during the initial mixing in the microreactor. On the other hand, the particle shape depends upon the temperature during the subsequent aging process. The technique proposed in this study is additive-free and allows control of both particle size from 50nm to 2μm and particle shape from cube to chamfered cube to rhombic dodecahedron, simply by adjusting the concentration of feed solutions and reaction temperature. Furthermore, we demonstrate that both particle size and particle shape are key physical parameters determining the adsorption characteristics of ZIF-8 particles.

Dynamic Modeling and Observer-based Servomechanism Control of a Towing Rope System

This paper presents a control-oriented dynamical model of a towing rope system with variable-length. In this system, a winch driven by a motor's torque uses the towing rope to pull a cart. In general, it is a difficult and complicated process to obtain an accurate mathematical model for this system. In particular, if the rope length is varied by operating the winch, the varying rope dynamics needs to be considered, and the key physical parameters need to be re-identified... However, real time parameter identification requires long computation time for the control scheme, and hence undesirable control performance. Therefore, in this article, the rope is modeled as a straight massless segment, with the mass of rope being considered partly with that of the cart, and partly as halfway to the winch. In addition, the changing spring constant and damping constant of the towing rope are accounted for as part of the dynamics of the winch. Finally, a reduced-order observer-based servomechanism controller is designed for the system, and the performance is evaluated by computer simulation.

Reactivity Estimation Based on an Extended State Observer of Neutron Kinetics

Reactivity is a key physical parameter that directly reflects the balance between neutron generation and consumption in every nuclear fission reactor. In this paper, by regarding the reactivity as an extended state-variable of point kinetics, a novel extended state-observer (ESO) is proposed. Theoretic analysis shows that this newly-built ESO provides globally asymptotically bounded estimation for the reactor neutron flux, concentrations of delayed neutron precursors and reactivity. Numerical simulation results illustrate the influence of the observer parameter to its performance, and show that this ESO can provide better reactivity estimation than the classical inverse point kinetics (IPK) method. The ESO has a simple form, and has only one parameter to be tuned online, which can induce an easy engineering implementation.

Evaluation of the Key Physical Parameters of Compressive Strained Ge1-x Snx for Optoelectronic Devices

Both strain technology and alloying technology can change the band structures of Germanium semiconductor. This paper focus on evaluation of the key physical parameters, such as energy levels and effective mass, of germanium under strain and alloy conditions, on the basis of deformation potential theory and kp perturbation theory. The results show that: (1), The bandgap transition in Ge1-xSnx alloy cannot occur under strain. So the transformation efficiency of the strained Ge1-xSnx/(001)Ge based devices can not be improved; (2), The various hole effective masses of strained Ge1-xSnx/(001)Ge decrease with the increase of the stress, which benefits to the pMOS performance improvement. Our valid models can provide the valuable references to the design of modified Ge semiconductor and optoelectronic devices.

Methane storage in nanoporous material at supercritical temperature over a wide range of pressures.

The methane storage behavior in nanoporous material is significantly different from that of a bulk phase, and has a fundamental role in methane extraction from shale and its storage for vehicular applications. Here we show that the behavior and mechanisms of the methane storage are mainly dominated by the ratio of the interaction between methane molecules and nanopores walls to the methane intermolecular interaction, and a geometric constraint. By linking the macroscopic properties of the methane storage to the microscopic properties of a system of methane molecules-nanopores walls, we develop an equation of state for methane at supercritical temperature over a wide range of pressures. Molecular dynamic simulation data demonstrates that this equation is able to relate very well the methane storage behavior with each of the key physical parameters, including a pore size and shape and wall chemistry and roughness. Moreover, this equation only requires one fitted parameter, and is simple, reliable and powerful in application.

Phase-field model of cell motility: Traveling waves and sharp interface limit

In this paper, we report our recent results on the asymptotic analysis of a PDE model for the motility of an eukaryotic cell. We formally derive the sharp interface limit, which describes the motion of the cell membrane. In the 1D case, we rigorously justify the limit, and, using numerical simulations, observe some surprising features, such as discontinuity of interface velocities and hysteresis. We show that nontrivial traveling wave solutions appear when the key physical parameter exceeds a critical value.

Complex quantum transport in a modulation doped strained Ge quantum well heterostructure with a high mobility 2D hole gas

The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm2/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2 K, the hole effective mass is found to be 0.065 m0. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW.

Effective anisotropic compliance relationships for wing-cracked brittle materials under compression

The failure mechanism for brittle materials under compressive loading is generally assumed to be dominated by the development of wing-cracks. Due to the preferential direction of the wing-crack development, material damaged in this mode exhibits damage induced anisotropy and volume dilatancy. The effective compliance of such wing-crack damaged materials is important for developing micromechanics-based constitutive models. This work addresses the scenario of a brittle material with periodically distributed wing-cracks under uniaxial compressive load. Closed form expressions for the instantaneous anisotropic compliance tensor with respect to the key physical parameters of the wing-cracks (the number density of cracks, the pre-existing flaw orientations and flaw sizes, the instantaneous length of wing-cracks, and the friction coefficient on the flaw surfaces) are derived through a kinematic approach. Accordingly, finite element models with perturbing compressive loads and periodic boundary conditions are carried out based on the above ideological wing-crack model, in order to verify and parameterize the analytically-based model. Good agreement is found between the finite element results and the analytical expression.

Testing the completeness of the SDSS colour selection for ultramassive, slowly spinning black holes

We investigate the sensitivity of the colour-based quasar selection algorithm of the Sloan Digital Sky Survey to several key physical parameters of supermassive black holes (SMBHs), focusing on BH spin ($a_{\star}$) at the high BH-mass regime ($M_{BH} \geqslant10^9\, M_{\odot}$). We use a large grid of model spectral energy distribution, assuming geometrically-thin, optically-thick accretion discs, and spanning a wide range of five physical parameters: BH mass $M_{BH}$, BH spin $a_{\star}$, Eddington ratio $L / L_{Edd}$ , redshift $z$, and inclination angle $inc$. Based on the expected fluxes in the SDSS imaging ugriz bands, we find that $\sim 99.8\%$ of our models with $M_{BH} \leqslant 10^{9.5}\, M_{\odot}$ are selected as quasar candidates and thus would have been targeted for spectroscopic follow-up. However, in the extremely high-mass regime, $\geqslant 10^{10} M_{\odot}$, we identify a bias against slowly/retrograde spinning SMBHs. The fraction of SEDs that would have been selected as quasar candidates drops below $\sim50\%$ for $a_{\star} <0$ across $0.5<z<2$. For particularly massive BHs, with $M_{BH} \simeq 3\times10^{10}\, M_{\odot}$, this rate drops below $\sim20\%$, and can be yet lower for specific redshifts. We further find that the chances of identifying any hypothetical sources with $M_{BH} = 10^{11}\, M_{\odot}$ by colour selection would be extremely low at the level of $\sim 3\%$. Our findings, along with several recent theoretical arguments and empirical findings, demonstrate that the current understanding of the SMBH population at the high-$M_{BH}$, and particularly the low- or retrograde-spinning regime, is highly incomplete.

Diffusion in Ni-Based Single Crystal Superalloys with Density Functional Theory and Kinetic Monte Carlo Method

AbstractIn the paper, we focus on atom diffusion behavior in Ni-based superalloys, which have important applications in the aero-industry. Specifically, the expressions of the key physical parameter – transition rate (jump rate) in the diffusion can be given from the diffusion theory in solids and the kinetic Monte Carlo (KMC) method, respectively. The transition rate controls the diffusion process and is directly related to the energy of vacancy formation and the energy of migration of atom from density functional theory (DFT). Moreover, from the KMC calculations, the diffusion coefficients for Ni and Al atoms in the γ phase (Ni matrix) and the γʹ phase (intermetallic compound Ni3Al) of the superalloy have been obtained. We propose a strategy of time stepping to deal with the multi-time scale issues. In addition, the influence of temperature and Al concentration on diffusion in dilute alloys is also reported.

Anisotropic magnetization relaxation in ferromagnetic multilayers with variable interlayer exchange coupling

The FMR linewidth and its anisotropy in F$_1$/f/F$_2$/AF multilayers, where spacer f has a low Curie point compared to the strongly ferromagnetic F$_1$ and F$_2$, is investigated. The role of the interlayer exchange coupling in magnetization relaxation is determined experimentally by varying the thickness of the spacer. It is shown that stronger interlayer coupling via thinner spacers enhances the microwave energy exchange between the outer ferromagnetic layers, with the magnetization of F$_2$ exchange-dragged by the resonance precession in F$_1$. A weaker mirror effect is also observed: the magnetization of F$_1$ can be exchange-dragged by the precession in F$_2$, which leads to anti-damping and narrower FMR linewidths. A theory is developed to model the measured data, which allows separating various contributions to the magnetic relaxation in the system. Key physical parameters, such as the interlayer coupling constant, in-plane anisotropy of the FMR linewidth, dispersion of the magnetic anisotropy fields are quantified. These results should be useful for designing high-speed magnetic nanodevices based on thermally-assisted switching.

Circuit-Based Electrothermal Simulation of Power Devices by an Ultrafast Nonlinear MOR Approach

This paper presents an efficient circuit-based approach for the nonlinear dynamic electrothermal simulation of power devices and systems subject to radical self-heating. The strategy relies on the synthesis of a nonlinear compact thermal network extracted from a finite-element model by a novel model-order reduction method requiring a computational time orders of magnitude lower than conventional techniques. Unlike commonly employed approaches, the proposed network allows reconstructing the whole time evolution of the temperature field in all the points of the domain with high accuracy. Electrothermal simulations are enabled in a commercial SPICE-like simulator by coupling such a network with subcircuits that describe the electrical device behavior by accounting for the temperature dependence of the key physical parameters. As a case study, the dynamic electrothermal analysis of a packaged silicon carbide power MOSFET undergoing a short-circuit test is performed, showcasing the performance of the approach and highlighting the need of including the thermal nonlinearities to achieve reliable results.

Simultaneous Determination of Chlorofluorocarbons and Sulfur Hexafluoride in Seawater Based on a Purge and Trap Gas Chromatographic System

Abstract Chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) are types of anthropogenic halogenated compounds, and are very important for ocean processes, including air-sea and water mass exchanges. Meanwhile, they can also be used to calculate some key physical and biogeochemical parameters such as the apparent oxygen utilization rate and anthropogenic CO2. Compared to CFCs, SF6 is quite difficult to measure in seawater due to its low solubility. However, it is useful to study the ocean processes based on both CFCs and SF6. In this study, a purge and trap gas chromatography system was developed to simultaneously measure CFC-12 and SF6 in seawater. The optimal conditions were as follows: trap temperature of −70 °C, purge time of 8 min, purge gas pressure of 310 kPa, desorption time of 30 s, desorption temperature of 90 °C. The method was simple and sensitive. The detection limit were 0.02 pmol kg−1 for CFC-12 and 0.03 fmol kg−1 for SF6. The analytical precision of CFC-12 was ±1.2% and that of SF6 was ±0.5%. The correlation coefficient (R2) values of all calibration curves were greater than 0.9995. This method was successfully used in the analysis of sea water samples collected during the 6th Chinese Arctic Research Expedition.

Galactic cold cores

The association of filaments with protostellar objects has made these structures a priority target in star formation studies. The datasets of the Herschel Galactic Cold Cores Key Programme allow for a statistical study of filaments with a wide range of intrinsic and environmental characteristics. Characterisation of this sample can be used to identify key physical parameters and quantify the role of environment in the formation of supercritical filaments. Filaments were extracted from fields at D<500pc with the getfilaments algorithm and characterised according to their column density profiles and intrinsic properties. Each profile was fitted with a beam-convolved Plummer-like function and quantified based on the relative contributions from the filament 'core', represented by a Gaussian, and 'wing' component, dominated by the power-law of the Plummer-like function. These parameters were examined for populations associated with different background levels. We find that filaments increase their core (Mcore) and wing (Mwing) contributions while increasing their total linear mass density (Mtot). Both components appear to be linked to the local environment, with filaments in higher backgrounds having systematically more massive Mcore and Mwing. This dependence on the environment supports an accretion-based model for filament evolution in the local neighbourhood (D<500pc). Structures located in the highest backgrounds develop the highest central Av, Mcore, and Mwing as Mtot increases with time, favoured by the local availability of material and the enhanced gravitational potential. Our results indicate that filaments acquiring a significantly massive central region with Mcore>Mcrit/2 may become supercritical and form stars. This translates into a need for filaments to become at least moderately self-gravitating in order to undergo localised star formation or become star-forming filaments.