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The carrier collection efficiencies of InGaAs/GaAsP superlattice (SL) photovoltaic structures were optimized by choosing adequate manufacturing parameters, such as the composition and thickness of the quantum wells (QWs) and barrier layers. However, no insights have been observed from the viewpoint of the nonradiative transition of photoexcited carriers. In this study, piezoelectric photothermal (PPT) and photoluminescence (PL) measurements were performed as a function of temperature from 100 to 340 K. Using a piezoelectric transducer, the PPT signal detected the heat generated by nonradiative recombination (NR). The indium composition of the QW layer was fixed at 0.3, and the phosphorus composition x[P] in the barrier layer was changed from 0.4 to 0.6. The observed temperature dependences of the PPT and PL signal intensities were analyzed using a rate equation for the photoexcited carriers in e1 and hh1 quantized levels. Four carrier dissipating processes, namely, radiative recombination, NR, thermal escape from the QW thermal excitation (TE), and tunneling after thermal excitation (TATE), were considered for both electrons and holes. Thermal activation energies were included in the NR, TE, and TATE processes. Because nonradiative and radiative transition components cause PPT and PL signals, curve fitting of the temperature behavior enabled us to determine the activation energies. We then found that the activation energy of the NR process reached a maximum at x[P] = 0.45. No such maxima were observed for the TE and TATE process. This result was explained by a trade-off between the strain valance condition over the entire range of the SL structure and the local residual strain at the interfaces between the QW, interlayer, and barrier layer when x[P] increased. Because no software can theoretically calculate the activation energy of the NR process, we demonstrated the usefulness of the present PPT experimental methodology for investigating carrier transport properties. [ABSTRACT FROM AUTHOR]

Electromagnetically induced slow-light medium is a promising system for quantum memory devices, but controlling its noise level remains a major challenge to overcome. This work considers the simplest model for such medium, comprised of three-level Λ-systems interacting with bosonic bath, and provides a new fundamental trade-off relation in light–matter interaction between the group velocity of light and the Fano factor of photon current due to radiative transitions. Considering the steady state limits of a newly derived Lindblad-type equation, we find that the Fano factor of the photon current maximizes to 3 at the minimal group velocity of light, which holds true universally regardless of detailed values of parameters characterizing the medium. [ABSTRACT FROM AUTHOR]

Ram pressure stripping (RPS) is a process that removes the interstellar medium (ISM) quickly, playing a vital role in galaxy evolution. Previous RPS studies have treated the ISM as single-phase or lack the resolution and physical processes to properly capture the full multiphase ISM. To improve this simplification, we introduce an inflowing, hot intracluster medium (ICM) into a self-consistently modeled ISM in a local patch of star-forming galactic disks using the TIGRESS framework. Our simulations reveal that the workings of RPS are not only direct acceleration of the ISM by ICM ram pressure but also mixing-driven momentum transfer involving significant phase transition and radiative cooling. The hot ICM passes through the low-density channels of the porous, multiphase ISM; shreds the cool ISM; and creates mixing layers. The ICM momentum is transferred through the mixing layers while populating the intermediate-temperature gas and radiating thermal energy away. The mixed gas extends beyond galactic disks and forms stripped tails that cool back unless the ICM fluxes are large enough to prevent cooling until they escape the simulation domain. The mixing-driven momentum transfer predicts that the more ICM mixes in, the faster the ISM moves, resulting in the anticorrelation of outflow velocity and gas metallicity of the stripped ISM. The compression of the ISM disks due to the ICM ram pressure enhances star formation rates up to 50% compared to the model without ICM. With the ICM ram pressure higher than the disk anchoring pressure, star formation is quenched within ∼100 Myr. [ABSTRACT FROM AUTHOR]

Results of experimental and theoretical investigations into the electron-impact excitation of the resonance 6 s 6 p 1 P 1 ° â†' 6 s 21 S 0 (132.2 nm), as well as the cascade 6 s 7 s 1 S 0 â†' 6 s 6 p 1 P 1 ° (309.2 nm) and 6 p 21 D 2 â†' 6 s 6 p 1 P 1 ° (150.8 nm) spectral transitions from the ground 6 s 21 S 0 level in the thallium ion, are presented. Crossed beams of electrons and Tl+ ions in combination with a spectroscopic method were used in the experiment. Measured cross sections have a distinct resonance structure arising mainly from the electron decay of atomic 5 d 96 s 26 p 2, 5 d 106 s 7 s (1 S) np (n â©ľ 7), 5 d 106 s 6 d (1 D) np (n â©ľ 6), 5 d 106 s 6 d (1 D) nd (n â©ľ 6) and ionic 5 d 96 s 2 np, 5 d 96 s 2 nd, 5 d 96 snp (n â©ľ 6) autoionizing levels, as well as radiative transitions from the higher 6 s 7 s 1 S 0, 6 s 6 d 1 D 2, and 6 p 21 D 2, 1 S 0 ionic levels. Relativistic distorted wave (RDW) calculations are performed to obtain the effective cross sections for the above transitions. The absolute values of the cross sections are found to be 1.39 Ă— 10âˆ'16 cm2 at 300 eV for the λ 132.2 nm resonance line, and 0.11 Ă— 10âˆ'16 cm2 and 0.56 Ă— 10âˆ'16 cm2 at 100 eV for the λ 309.2 nm and λ 150.8 nm lines, respectively. The contribution of the cascade transitions being studied to the effective cross section of the λ 132.2 nm resonance line at the energy of 100 eV is about 30%. A calculation using the semi-empirical Van Regemorter formula is also performed to obtain the effective cross section of the resonance λ 132.2 nm line. The absolute value of the cross section at 300 eV is found to be nearly the same as that given by the RDW calculation. [ABSTRACT FROM AUTHOR]

Self-assembled GaSb/GaAs quantum-ring-with-dot structures (QRDSs) are the nanostructures exhibiting type-II band alignment. Each QRDS consists of both quantum ring (QR) and quantum dot (QD) parts which have their own energy levels. In this work, the carrier dynamics in the GaSb/GaAs QRDSs are explored and quantitatively described by a rate equation model which is developed from experimental photoluminescence (PL) spectra in literature. The model is comprised of the carrier transition rates and activation energies involved the transfer processes of thermal-excited carriers. The difference between the QR and QD PL intensities is also taken into account in the model. Electronic structures of a single QRDS are calculated in order to determine the overlap between electron and hole wave functions that can be used for estimating the radiative transition rates. Numerical values of the radiative and non-radiative transition rates, carrier capture and escape rates, and activation energies are presented. The PL spectra obtained from the model are consistent with the reported experimental data. [ABSTRACT FROM AUTHOR]

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr ii triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca ii triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr ii and Ca ii lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca ii line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25. [ABSTRACT FROM AUTHOR]

Thermal corrections to the one-photon spontaneous and induced transition probabilities for hydrogen and hydrogen-like ions are evaluated. The found thermal corrections are given by the vertex Feynman graph, where the vertex represents the thermal interaction between the bound electron and the nucleus. All derivations of thermal corrections to bound–bound transitions for an atom exposed to blackbody radiation are made in a fully relativistic approach within the framework of the adiabatic S-matrix formalism. It is found that the vertex-type radiative corrections to the transition rates can be at the level of a few percent to corresponding spontaneous rates for highly excited states in the hydrogen atom. A comprehensive analysis of the vertex-type thermal corrections for hydrogen-like atomic systems is presented. [ABSTRACT FROM AUTHOR]

With the recent advent of circular polarization capabilities at the Atacama Large Millimeter/submillimeter Array (ALMA), Zeeman effect measurements of spectral lines are now possible as a means to directly probe line-of-sight magnetic fields in protoplanetary disks (PPDs). We present a modeling study that aims to guide physical interpretation of these anticipated observations. Using a fiducial density structure based on a typical ringed disk, we simulate line emission for the hyperfine components of the CN J = 1−0 transition with the POLARIS radiative transfer code. Since the expected magnetic field and typical CN distribution in PPDs remain largely unconstrained, we produce models with several different configurations. Corresponding integrated Stokes I and V profiles and 0.4 km s−1 resolution, 1″ beam convolved channel maps are presented. We demonstrate that the emission signatures from toroidally dominated magnetic fields are distinguishable from vertically dominated magnetic field based on channel map morphology. Due to line-of-sight and beam cancellation effects, disks with toroidal -field configurations result in significantly diminished Stokes V emission. Complex magnetic fields therefore render the traditionally used method for inferring line-of-sight magnetic field strengths (i.e., fitting the derivative of the Stokes I to the Stokes V profile) ambiguous, since a given intrinsic field strength can yield a variety of Stokes V amplitudes depending on the magnetic field geometry. In addition, gas gaps can create structure in the integrated Stokes V profile that might mimic magnetic substructure. This method should therefore be applied with caution in PPD environments and can only confidently be used as a measure of magnetic field strength if the disk's magnetic field configuration is well understood. [ABSTRACT FROM AUTHOR]

We model the (v′ = 1, v″ = 0) band of the Birge–Hopfield I band system of molecular nitrogen, N2, i.e. , emitted by nonthermal gas discharges in low pressure air. In particular, we calculate the rate of electron impact excitation of N2, which is thermally equilibrated at room temperature, to rotational levels J′ < 31 of N2(b1Πu, v′ = 1, J′). Subsequently, we calculate the intensity of emission lines resulting from radiative transitions from the aforementioned rotational levels to. Predissociation is the only nonradiative channel considered for the decay of N2(b1Πu, v′ = 1, J′) levels. [ABSTRACT FROM AUTHOR]

Hexagonal boron nitride (hBN) has become a prominent material for nanophotonic and quantum technology studies. Its wide bandgap can accommodate room temperature radiative optical transitions originating from defect states in different atomic structures. Here, we report the engineering of visible defects in chemical vapor deposited monolayer hBN irradiated by femtosecond pulses (0.1–0.5 W) at room temperature. Photoluminescence and transient photoluminescence measurements reveal the presence of a sharp emission at 630 nm with a time constant of approximately 3 ns randomly distributed around the irradiated region. We imaged the distribution of the photogenerated electrons by time-resolved photoemission electron microscopy in the picosecond and nanosecond timescales with 100 nm spatial resolution. We determined the precise location of the defects within the region of interest corresponding to an optical transition between 1.95 eV and 3.90 eV above the valence band maximum of hBN, ascribed to NBVN color centers. [ABSTRACT FROM AUTHOR]

Si quantum dots (QDs) have a significant improvement in luminous efficiency compared with bulk Si, achieved by alleviating the forbiddance of no-phonon Γ–Γ radiative transition determined by the law of momentum conservation. Two divergent mechanisms have been proposed to account for the breakdown of momentum conservation in Si QDs, one is due to the space-confinement-induced spread of k -space wave functions associated with Heisenberg uncertainty principle Δr · Δk > 1/2, and the other is due to the interface-effect-induced intervalley mixing between indirect and direct bandgap states. Both mechanisms could cause a small overlap of the electron and hole wave functions in k -space and make vertical transitions allowed, which leads to the zero-phonon light emission. In this work, we unravel the hierarchical relationship between these two primary mechanisms in the process of zero-phonon light emission from indirect bandgap QDs, by performing semiempirical pseudopotential calculation including many-body interaction on the room-temperature luminescent properties of a series of Si, Ge, and Ge/Si core/shell QDs. We show that the space confinement mechanism is dominant in both Si and Ge indirect bandgap QDs, and the interface-induced intervalley coupling mechanism plays a minor role. While in Ge/Si core/shell QDs, the interface-induced intervalley coupling mechanism has a more pronounced contribution to enhanced light emission, implying one can further enhance light emission via engineering interface based on the intervalley coupling mechanism. Given this, we further engineer the Ge QD interface by bringing four motifs of Si/Ge multiple layers from previously inverse designed Si/Ge superlattices and core/shell nanowires for light emitters. We show that two out of four motifs always give rise to two orders of magnitude enhancement in light emission relative to the Ge and Si QDs. We demonstrate that the interface engineering can enhance light emission in indirect bandgap QDs substantially and regulate the intervalley coupling mechanism as the primary factor over the space confinement mechanism in breaking the momentum conservation law. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the origin of point defects revealed by electron spin resonance (ESR) and photoluminescence (PL) emission in correlation with the photocatalytic activity of ZnO nanocrystals subjected to thermal annealing at various temperatures. Two ESR signals at g ∼ 1.96 and ∼2.003 were consistently observed in all annealed ZnO samples. However, their relative intensities have changed, indicating that the point defect densities were influenced by the annealing temperature. Interestingly, when doping nanoZnO with Cr3+, the Q-band ESR measurements at T = 100 K did show that the g ∼ 1.96 signal was completely passivated, suggesting that the origin of the signal lies in the electrons located near the conduction band, i.e. at a shallow-donor level. The intensity of the g ∼ 2.003 signal decreased by rising the annealing temperature, and this is attributed to the depopulation of zinc interstitials through the thermally induced migration process and/or recombination with the zinc vacancies. The green PL emission line at ∼520 nm, which is dominant in the 700 °C annealed ZnO sample, shows a correlation with the ESR signal at g ∼ 1.96, whose origin is attributed to the radiative transition of the electron from the shallow donor level to the singly ionized zinc vacancy. Furthermore, the high density of the shallow donor electron states was found to be primarily responsible for the high photocatalytic activity in the degradation of methylene blue. [ABSTRACT FROM AUTHOR]

We model emissions from the Carroll-Yoshino (v′ = 0, v″ = 0) band of molecular nitrogen N2, i.e. due to nonthermal gas discharges in low pressure air and N2. In particular, we calculate the rate of excitation of thermally equilibriated N2 at room temperature (i.e. 300 K) to various rotational levels J′ of. Subsequently, we calculate the intensity of 47 emission lines resulting from radiative transitions from the aforementioned rotational levels. It is assumed that predissociation is the only nonradiative channel for the decay of excited levels. [ABSTRACT FROM AUTHOR]

State selective charge transfer cross sections are calculated in Mg12+ collisions with H and He atoms at incident energies of 1.0, 3.0, and 5.0 keV amu−1 using a time-dependent lattice method. Total capture cross sections using the time-dependent lattice method are found to be larger than those previously calculated using the Landau–Zener method. Lyman line ratios are calculated using the time-dependent lattice capture cross sections and standard radiative transitions for both the H and He atoms. Lyman line probabilities are presented for both H and He. The strongest probability for both H and He is the Lyman α emission, but relatively strong probabilities for He at an incident energy of 1.0 keV amu−1 are found for Lyman β, γ, δ, and ϵ emission. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) Y2Ti2O7 (YTO): Ho3+/Yb3+ nanofibers with diameters ranging from 300 to 400 nm were synthesized by an electrospinning technology and the structure of YTO nanofibers was identified as face-centered cubic pyrochlores. The detected green and red signals in the upconversion emission spectra under 980 nm laser excitation are due to the radiative transitions (5F4, 5S2) ? 5I8 and 5F5 = 5I8 of Ho3+ ions, respectively. YTO: Ho3+/Yb3+ nanofibers have the characteristics of porosity, threedimensional braidability, high specific surface area and controllable structure, which improves the performance of optical temperature sensing. Furthermore, the nanofiber membranes consisting of YTO: Ho3+/Yb3+ with temperature sensing performance can work steadily at medium temperature range in view of its fast responses and high stability, moreover, a maximum sensitivity about 0.18% K-1 at 423 K, which indicates that the nanofibers have good thermal sensitivity. 1D nanostructure rare earth iondoped cubic pyrochlore material was proven to obtain excellent upconversion luminescence properties, which provides a new route for the development of sensitive optical temperature-sensing materials. [ABSTRACT FROM AUTHOR]

Metronomes placed on the perimeter of a disc-shaped platform, which can freely rotate in a horizontal plane, are used for a simple classroom illustration of the Kuramoto-type phase transition. The rotating platform induces a global coupling between the metronomes, and the strength of this coupling can be varied by tilting the metronomes' swinging plane relative to the radial direction on the disc. As a function of the tilting angle, a transition from spontaneously synchronized to unsynchronized states is observable. By varying the number of metronomes on the disc, finite-size effects are also exemplified. A realistic theoretical model is introduced and used to reproduce the observed results. Computer simulations of this model allow a detailed investigation of the emerging collective behaviour in this system. [ABSTRACT FROM AUTHOR]

High resolution two-photon spectrum of the transitions 6S1/2 → 6P3/2 → 8S1/2, 9S1/2 and 6S1/2 → 6P1/2 → 7D3/2 in neutral 133Cs are presented in a room-temperature vapor cell using a femtosecond optical frequency comb. Spectra are obtained by scanning the repetition frequency of the femtosecond optical frequency comb over the two-photon hyperfine structure. The centroid frequency of the 6S1/2 → 8S1/2, 9S1/2, 7D3/2 transitions are 729009798.80(17) MHz, 806761363.96(11) MHz, and 780894762.595(23) MHz, respectively. The hyperfine coupling constants of the corresponding states are also obtained. The results are consistent with the previous measurements. [ABSTRACT FROM AUTHOR]

Recent ambitions have been aimed at modelling of a free-burning arc in argon by means of a self-consistent description of the arc plasma and the electrodes avoiding assumptions of equilibrium. As a next step, the present work deals with the impact of the reaction chemistry used in non-equilibrium modelling. A set of reactions accounts for elastic scattering, excitation and de-excitation in collisions with electrons and atoms, step-wise ionization, recombination and spontaneous emission. A simple chemistry model based on the approximation of prompt ionization, a model with an effective excited level, and a model implying higher excited levels of argon atoms are considered. The results obtained show that all three models are capable of predicting the arc characteristics. They deliver values of electric current density, electric conductivity, arc voltage, electron and heavy particle temperature which are close to each another. The simple model can serve as a fast-track evaluation of the arc characteristics, and for sensitive studies of the impact of model parameters. The extended model allows obtaining the populations of individual and effective levels giving rise to important radiative transitions. [ABSTRACT FROM AUTHOR]

Transition probabilities and oscillator strengths for electric dipole radiation in triply ionized praseodymium are reported for the first time in this paper. They were computed using a semi-empirical relativistic Hartree-Fock approach including core-polarization effects. Due to the lack of experimental data in the Pr IV spectrum, the accuracy of our results is estimated and discussed on the basis of comparisons between calculations performed with a similar physical model and laboratory measurements previously published for the isoelectronic ion Ce2+. In view of their great interest in optical materials and nanophotonics, radiative rates for forbidden lines within the 4f² ground-state configuration of Pr3+ were also calculated in our work. [ABSTRACT FROM AUTHOR]

Resonant enhancement of high harmonic generation can be obtained in plasmas containing ions with strong radiative transitions resonant with harmonic orders. The mechanism for this enhancement is still debated. We perform the first temporal characterization of the attosecond emission from a tin plasma under near-resonant conditions for two different resonance detunings. We show that the resonance considerably changes the relative phase of neighboring harmonics. For very small detunings, their phase locking may even be lost, evidencing strong phase distortions in the emission process and a modified attosecond structure. These features are well reproduced by our simulations, allowing their interpretation in terms of the phase of the recombination dipole moment. [ABSTRACT FROM AUTHOR]

The type-III ELMy H-mode might be the solution for an integrated ITER operation scenario fulfilling the fusion power amplification factor (output fusion power to input heating power) of Q = 10 with simultaneous acceptable steady-state and transient power loads to the plasma-facing components. This highly radiative type-III ELMy H-mode is achieved by nitrogen seeding. Experiments on the tokamak ASDEX Upgrade, in which all plasma-facing components are coated with tungsten, showed favourable confinement conditions (H98(y,2) = 1) at high plasma pressure (ßN = 2.4). The power load to the plasma-facing components is as low as ≈3 MWm-2 during the peak heat loads due to edge localized modes (ELMs) at high radiative power fractions of frad ≈ 0.75. In those high-density discharges the central impurity concentration is very low as a result of hollow nitrogen density profiles and minimal erosion of tungsten. The tungsten erosion is not only suppressed in between ELMs, when the divertor is detached, but also during the type-III ELM activity. Such low impurity concentrations in the plasma core might lead to even higher fusion amplification factors in ITER than 10. This is a demonstration of the compatibility of the radiating type-III ELMy H-mode with a tungsten divertor and main chamber wall, with sufficient confinement and favourable power exhaust characteristics. Together with previous results in all-carbon devices this demonstration strengthens the case for a potential application of a Q = 10 scenario on ITER. [ABSTRACT FROM AUTHOR]

By adopting a finite square-well potential, the analytic expressions of vorticity strengths, which are related to transverse currents, are derived for some selected E1 and E2 transitions. From these analytic expressions it can be seen clearly that, like the threshold strengths, the vorticity strengths are also sharply distributed just above the threshold in nuclei near the drip-line and mainly come from the orbits with small binding energy. [ABSTRACT FROM AUTHOR]

The radiative and Auger decay processes of K-shell ionized Np ions are studied theoretically using the flexible atomic code (FAC). Relativistic effects, Breit interaction, QED corrections and nuclear finite mass and volume effects are considered systematically. The resulting calculated K x-rays and Auger spectra of Np are compared with the measured spectra emitted after the electron-capture (EC)-decay of mass-separated 237Pu. The general consistency between theory and experiment is good; the relative intensities and relative positions of the peaks in the measured spectra are reproduced with good accuracy, in spite of the existence of different ways to produce the primary K-shell vacancy, enabling identification of the observed structure in the experimental spectra. We find that most of the radiative transition rates are greater than the Auger transition rates, and the latter decrease rapidly with the transfer of initial vacancies to the outer shell. [ABSTRACT FROM AUTHOR]

A detailed numerical calculation on the phonon-assisted intersubband transition rates of electrons in wurtzite GaN/InxGa1-xN quantum wells is presented. The quantum-confined Stark effect, induced by the built-in electric field, and the ternary mixed crystal effect are considered. The electron states are obtained by iteratively solving the coupled Schr¨odinger and Poisson equations. The dispersion properties of each type of phonon modes are considered in the derivation of Fermi's golden rule to evaluate the transition rates. It is indicated that the interface and halfspace phonon scattering play an important role in the process of 1-2 radiative transition. The transition rate is also greatly reduced by the built-in electric field. This work can be helpful for the structural design and simulation of new semiconductor lasers. [ABSTRACT FROM AUTHOR]

Weak interactions break time-reversal (T) symmetry in the two-state system of neutral K-mesons. We present and discuss a two-state mechanical system, i.e. a Foucault-type pendulum on a rotating table, for a full representation of K0&Kmacr;0 transitions by the pendulum motions including T violation. The pendulum moves with two different oscillation frequencies and two different magnetic dampings. Its equation of motion is identical to the differential equation for the real part of the CPT-symmetric K-meson wavefunction. The pendulum is able to represent microscopic CP and T violation with CPT symmetry owing to the macroscopic Coriolis force, which breaks the symmetry under reversal of motion. Video clips of the pendulum motions are given as supplementary material. [ABSTRACT FROM AUTHOR]

We investigate the radiative dileptonic decays B2 → ι+ι- γ(&igr;=e,&mgr; and T ) in the framework of the topcolorassisted technicolor (TC2) model. The contributions of the new particles predicted by this model to the branching ratios and the forward-backward asymmetry (AFB) of these decays are considered. It is found that the values of their branching ratios are greater than the standard model (SM) predictions by several times up to one order of magnitude and |AFB| can also be enhanced by several times in a wide range of the parameter space. [ABSTRACT FROM AUTHOR]

An important contribution to the magnetohydrodynamic equilibrium at the tokamak edge comes from the Pfirsch- Schlüter current. The parallel electric field that can be associated with these currents is necessarily poloidally asymmetric and makes a similarly nonuniform contribution to the radial electric field on a flux surface. Here the role of the poloidal variation of this radial electric field in the L-H transition power threshold is investigated. Dependence of the resulting electric fields on magnetic topology, geometric factors such as the upper/lower triangularity and elongation, and the relative position of the X-point(s) in the poloidal plane are examined in detail. Starting with the assumption that an initially more negative radial electric field at the edge helps lower the transition power threshold, we find that our results are in agreement with a variety of experimental observations. In particular, for a 'normal' configuration of the plasma current and toroidal field we show the following. (i) The net radial electric field contribution by the Pfirsch-Schlüter currents at the plasma edge is negative for a lower single null and positive for a corresponding upper single null geometry. (ii) It becomes more negative as the X-point height is reduced. (iii) It also becomes more negative as the X-point radius is increased. These observations are consistent with the observed changes in the L-H transition power threshold PLH under similar changes in the experimental conditions. In addition we find that (iv) in USN with an unfavourable ion ∇B drift direction, the net radial electric field contribution is positive but decreases as the X-point radius decreases. This is consistent with the C-Mod observation that an L-I mode transition can be triggered by increasing the upper triangularity in this configuration. (v) Locally the radial electric field is positive above the outer mid-plane and reverses sign with reversal of the toroidal field, consistent with DIII-D observations in low-power L-mode discharges. Thus, taken as a whole, the Pfirsch-Schlüter current-driven fields can explain a number of observations on the L-H or L-I transition and the required power threshold PLH levels not captured by simple scaling laws. They may indeed be an important 'hidden variable'. [ABSTRACT FROM AUTHOR]

Energy levels, radiative transition probabilities and autoionization rates for [Cd]4f145p65l'nl, [Cd]4f145p66l''nl, [Cd]4f145p55d2nl, [Cd]4f145p55d6l''nl, [Cd]4f135p65d2nl and [Cd]4f135p65d6l''nl (l' = d, f, g, l'' = s, p, d, l = s, p, d, f, g and n = 5–7) states of Yb-like tungsten (W4 +) are calculated using the relativistic many-body perturbation theory method (RMBPT code), the multiconfiguration relativistic Hebrew University–Lawrence Livermore Atomic Code (HULLAC code) and the Hartree–Fock relativistic method (COWAN code). Branching ratios relative to the [Cd]4f145p65d, [Cd]4f145p66s and [Cd]4f145p66p thresholds in Tm-like tungsten and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the singly excited, as well as non-autoionizing core-excited states in Yb-like tungsten. Contributions from the autoionizing doubly excited states [Cd]4f145p65fnl, [Cd]4f145p66l''nl and core-excited [Cd]4f145p55d2nl, [Cd]4f145p55d6l''nl, [Cd]4f135p65d2nl, [Cd]4f135p65d6l''nl states (with n up to 100), which are particulary important for calculating the total DR rates, are estimated. Synthetic dielectronic satellite spectra from Yb-like W are simulated in a broad spectral range from 200 to 1400 Å. These calculations provide recommended values critically evaluated for their accuracy for a number of W4 + properties useful for a variety of applications including for fusion applications. [ABSTRACT FROM AUTHOR]

We characterize the temporal profile of the high-order harmonic emission from ablation plasma plumes using cross-correlations with the infrared (IR) laser beam provided by two-photon harmonic+IR ionization of rare gas atoms.We study both non-resonant plasmas (lead, gold and chrome) and resonant plasmas (indium and tin), i.e. plasmas presenting in the singly charged ions a strong radiative transition coinciding with a harmonic order. The cross-correlation traces are found to be very similar for all harmonic orders and all plasma targets. The recovered harmonic pulse durations are very similar to the driving laser, with a tendency towards being shorter, demonstrating that the emission is a directly laser-driven process even in the case of resonant harmonics. This provides a valuable input for theories describing resonant-harmonic emission and opens the perspective of a very high flux tabletop XUV source for applications. [ABSTRACT FROM AUTHOR]

The CHIANTI spectral code consists of an atomic database and a suite of computer programs to calculate the optically thin spectrum of astrophysical objects and carry out spectroscopic plasma diagnostics. The database includes atomic energy levels, wavelengths, radiative transition probabilities, collision excitation rate coefficients, and ionization and recombination rate coefficients, as well as data to calculate free--free, free--bound, and two-photon continuum emission. Version 7 has been released, which includes several new ions, significant updates to existing ions, as well as Chianti-Py, the implementation of CHIANTI software in the Python programming language. All data and programs are freely available at http://www.chiantidatabase.org, while the Python interface to CHIANTI can be found at http://chiantipy.sourceforge.net. [ABSTRACT FROM AUTHOR]

We show that it is possible to generate Einstein-Podolsky-Rosen (EPR) entangled radiation using an atomic reservoir controlled by coherent population trapping. A beam of three-level atoms is initially prepared in near-coherent population trapping (CPT) state and acts as a long-lived coherence-controlled reservoir. Four-wave mixing leads to amplification of cavity modes resonant with Rabi sidebands of the atomic dipole transitions. The cavity modes evolve into an EPR state, whose degree of entanglement is controlled by the intensities and the frequencies of the driving fields. This scheme uses the long-lived CPT coherence and is robust against spontaneous emission of the atomic beam. At the same time, this scheme is implemented in a one-step procedure, not in a two-step procedure as was required in Phys. Rev. Lett. 98 (2007) 240401. [ABSTRACT FROM AUTHOR]

We have studied, both theoretically and experimentally, the mechanism of photoluminescence (PL) of Ge nanocrystals in SiO2 and GeO2 matrices. Ge quantum dots (QDs) have been created by pulsed laser deposition (PLD). Time-resolved PL spectra in the photon energy range of 1.4- 3.2 eV have been measured within the time range 50 ns-20 μs. We have calculated the exciton binding and radiative transition energies accounting for both finite potential barriers and heterointerface polarization in the system. PL spectra have been calculated and compared with experimental results accounting for quantum-mesoscopic fluctuations and possible oscillations in exciton radiative lifetime occurring as QD size decreases. Good agreement between calculated and experimental PL spectra supports an assumption on the excitonic character of PL in Ge QDs and enables the parameters of the PL model to be determined. [ABSTRACT FROM AUTHOR]

The optical property of a GaAsSb/InGaAs type-II quantum well (QW) was studied and the conduction band discontinuity (ΔEc) was determined by spatially indirect radiative transitions. ΔEc between GaAs0.73Sb0.27 and In0.19Ga0.81As was measured to be -210 meV, which showed that the heterojunction was highly staggered. For the type-II QW, radiative transition efficiency was decreased with the increase of interband transition wavelength. It was explained by the reduction of overlap between electron and hole functions. By optimizing transition wavelength and efficiency of the type-II QW, strong photoluminescence was observed around 1.3 µm at room temperature. [ABSTRACT FROM AUTHOR]

We systematically study the decay properties of the K-shell excited F-like ions with 10≤ Z ≤36 based on the multiconfiguration Dirac-Fock method. The Breit interaction, the QED corrections and the nuclear finite mass effects are also considered as perturbation. Auger transition rates, radiative, Auger and natural widths, as well as fluorescence and Auger yields for K-shell excited F-like ions are presented. It is shown by means of concrete figures that the decay properties change significantly with the increase of the atomic number Z; the Auger rate is overtaken at Z = 30 by the radiative decay rate. Several fitting formulae for the radiative and Auger widths and the fluorescence yields have been evaluated which is expected to be useful in plasma analysis and plasma modeling. [ABSTRACT FROM AUTHOR]

The paramagnetic Y3−0.02−xEr0.02YbxAl5O12 (x = 0.02, 0.06, 0.10, 0.12, 0.18, 0.20) nanocrystals (NCs) were synthesized by the microwave-induced solution combustion method. The XRD, TEM and SEM techniques were applied to determine the NCs’ structures and sizes. The XRD patterns confirmed that the NCs have for the most part a regular structure of the Y3Al5O12 (YAG) phase. The changes of the distance between donor Yb3+ (sensitizer) and acceptor Er3+ (activator) were realized by changing the donor’s concentration with a constant amount of acceptor. Under 980 nm excitation, at room temperature, the NCs exhibited strong red emission near 660 and 675 nm, and green upconversion emission at 550 nm, corresponding to the intra 4f transitions of Er3+ (4F9/2,2H11/2, 4S3/2) → Er3+ (4I15/2). The strongest emission was observed in a sample containing 18% Yb3+ ions. The red and green emission intensities are respectively about 5 and 12 times higher as compared to NCs doped with 2% of Yb3+. In order to prove that the main factor responsible for the increase of the upconversion luminescence efficiency is reduction of the distance between Yb3+ and Er3+, we examined, for the first time the influence of hydrostatic pressure on luminescence and luminescence decay time of the radiative transitions inside donor ion. The decrease of both luminescence intensity and luminescence decay times, with increasing hydrostatic pressure was observed. After applying hydrostatic pressure to samples with e.g. 2% and 6% Yb3+, the distance between the donor and acceptor decreases. However, for higher concentrations of the donor, this distance is smaller, and this leads to the effective energy transfer to Er3+ ions. With increasing pressure, the maximum intensity of near infrared emission is observed at 1029, 1038 and 1047 nm, what corresponds to 2F5/2 → 2F7/2 transition of Yb3+. [ABSTRACT FROM AUTHOR]

The x-ray energies and transition rates associated with single and double electron radiative transitions from the double K hole state 2s2p to the 1s2s and 1s2 configurations of 11 selected He-like ions (10 ≤ Z ≤ 47) are calculated using the fully relativistic multi-configuration Dirac–Fock method (MCDF). An appropriate electron correlation model is constructed with the aid of the active space method, which allows the electron correlation effects to be studied efficiently. The contributions of the electron correlation and the Breit interaction to the transition properties are analyzed in detail. It is found that the two-electron one-photon (TEOP) transition is correlation sensitive. The Breit interaction and electron correlation both contribute significantly to the radiative transition properties of the double K hole state of the He-like ions. Good agreement between the present calculation and previous work is achieved. The calculated data will be helpful to future investigations on double K hole decay processes of He-like ions. [ABSTRACT FROM AUTHOR]

Spectroscopy based on Stark broadening is a very efficient and successful method for diagnosing the physical conditions in laboratory or astrophysical plasmas. The studies are often focused on the lines of hydrogen or hydrogen-like ions due to the simple atomic structure. However, the Stark modeling may be difficult even for one-electron systems, in particular when radiative transitions with highly-degenerate Rydberg states are considered. Besides, the computations may become challenging for the modeling of line shapes in an optically thick plasma and accounting for non-local thermodynamical equilibrium (non-LTE) effects on the state populations. The statistical approach is a powerful technique which can be used to simplify complex coupled problems by replacing the numerous levels or lines by a single continuous distribution characterized by a reduced number of exact moments: area, center-of-gravity, variance, etc. In this paper, we study various statistical properties of a hydrogenic atom broadened by Stark effect. First, we derive the moments of level energies accounting for linear and quadratic Stark effects, electric-quadrupole and fine structure corrections. The resulting formulas allows one to estimate the relative importance of these corrections on the energies. We then derive the moments of the strength-weighted line energies for the Lyman and Balmer series in the framework of Linear-Stark effect. The moments are then used in a Gram–Charlier expansion series to represent the line shape, the resolution of the spectrum being controlled by the order at which the series is truncated. We show that the line shape of the transition can be expressed using universal functions of n and , independent of the atomic number and of the field intensity. This allows for fast Stark broadening calculations which can be used in more complex simulations (in opacity or emissivity codes, for radiative transfer simulations or non-LTE models). [ABSTRACT FROM AUTHOR]

We present lifetime measurements using beam-foil techniques for radiative transitions from the 3p4(1S)4s2S1/2, 3p4(3P)5 , and 3p4(3P)3d2F5/2 levels in Cl I and the corresponding results of the oscillator strengths for transitions at 1004.68, 1079.88, 1090.73, and 1094.77 Å, respectively. We compare our experimental results with available theoretical calculations and astronomical observations in an effort to resolve discrepancies among them. [ABSTRACT FROM AUTHOR]

By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth’s oceans, fingering convection in stellar atmospheres, and moist convection in Earth’s atmosphere are derived from the same general diabatic convective instability. We also show that “radiative convection” triggered by the CO/CH4 transition with radiative transfer in the atmospheres of brown dwarfs is analogous to moist and thermohaline convection. We derive a generalization of the mixing-length theory to include the effect of source terms in 1D codes. We show that CO/CH4 “radiative” convection could significantly reduce the temperature gradient in the atmospheres of brown dwarfs similarly to moist convection in Earth’s atmosphere, thus possibly explaining the reddening in brown dwarf spectra. By using idealized 2D hydrodynamic simulations in the Ledoux unstable regime, we show that compositional source terms can indeed provoke a reduction of the temperature gradient. The L/T transition could be explained by a bifurcation between the adiabatic and diabatic convective transports and seen as a giant cooling crisis: an analog of the boiling crisis in liquid/steam-water convective flows. This mechanism, with other chemical transitions, could be present in many giant and Earth-like exoplanets. The study of the impact of different parameters (effective temperature, compositional changes) on CO/CH4 radiative convection and the analogy with Earth moist and thermohaline convection is opening the possibility of using brown dwarfs to better understand some aspects of the physics at play in the climate of our own planet. [ABSTRACT FROM AUTHOR]

The Zombie Vortex Instability (ZVI) occurs in the dead zones of protoplanetary disks (PPDs), where perturbations excite baroclinic critical layers, generating “zombie” vortices and turbulence. In this work, we investigate ZVI with nonuniform vertical stratification; while ZVI is triggered in the stratified regions away from the midplane, the subsequent turbulence propagates into and fills the midplane. ZVI turbulence alters the background Keplerian shear flow, creating a steady-state zonal flow. Intermittency is observed, where the flow cycles through near-laminar phases of zonal flow punctuated by chaotic bursts of new vortices. ZVI persists in the presence of radiative damping, as long as the thermal relaxation timescale is more than a few orbital periods. We refute the premature claim by Lesur & Latter that radiative damping inhibits ZVI for disk radii . Their conclusions were based on unrealistically short cooling times using opacities with virtually no grain growth. We explore different grain growth and vertical settling scenarios, and find that the gas and dust in off-midplane regions are not necessarily in local thermodynamic equilibrium (LTE) with each other. In such cases, thermal relaxation timescales can be orders of magnitude longer than the LTE optically thin cooling times because of the finite time for energy to be exchanged between gas and dust grains via collisions. With minimal amounts of grain growth and dust settling, the off-midplane regions of disks are susceptible to ZVI, and much of the planet-forming regions can be filled with zombie vortices and turbulence. [ABSTRACT FROM AUTHOR]

We build a highly predictive 3-3-1 model, where the field content is extended by including several SU(3)L scalar singlets and six right handed Majorana neutrinos. In our model the gauge symmetry is supplemented by the discrete group, which allows to get a very good description of the low energy fermion flavor data. In the model under consideration, the discrete group is broken at very high energy scale down to the preserved Z2 discrete symmetry, thus generating the observed pattern of SM fermion masses and mixing angles and allowing the implementation of the loop level inverse seesaw mechanism for the generation of the light active neutrino masses, respectively. The obtained values for the physical observables in the quark sector agree with the experimental data, whereas those ones for the lepton sector also do, only for the case of inverted neutrino mass spectrum. The normal neutrino mass hierarchy scenario of the model is ruled out by the neutrino oscillation experimental data. We find an effective Majorana neutrino mass parameter of neutrinoless double beta decay of mee = 46.9 meV, a leptonic Dirac CP violating phase of −81.37° and a Jarlskog invariant of about 10−2 for the inverted neutrino mass hierarchy. The preserved Z2 symmetry allows for a stable scalar dark matter candidate. [ABSTRACT FROM AUTHOR]

This paper presents a detailed hydrostatic model of the upper atmosphere of HD 189733b, with the goal of constraining its temperature, particle densities, and radiation field over the pressure range 10–4 to 10 μbar, where the observed Hα transmission spectrum is produced. The atomic hydrogen level population is computed including both collisional and radiative transition rates. The Lyα resonant scattering is computed using a Monte Carlo simulation. The model transmission spectra are in broad agreement with the data. Excitation of the H(2ℓ) population is mainly by Lyα radiative excitation due to the large Lyα intensity. The density of H(2ℓ) is nearly flat over two decades in pressure and is optically thick to Hα. Additional models computed for a range of the stellar Lyman continuum (LyC) flux suggest that the variability in Hα transit depth may be due to the variability in the stellar LyC. Since metal lines provide the dominant cooling of this part of the atmosphere, the atmosphere structure is sensitive to the density of species such as Mg and Na, which may themselves be constrained by observations. Since the Hα and Na D lines have comparable absorption depths, we argue that the centers of the Na D lines are also formed in the atomic layer where the Hα line is formed. [ABSTRACT FROM AUTHOR]

Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as Hα 6563 and Ca ii 8542 . However, whether there is a response to the surge appearance and evolution in the Si iv lines or, in fact, in many other transition region lines has not been studied. In this paper, we analyze a simultaneous episode of an Hα surge and a Si iv burst that occurred on 2016 September 03 in active region AR 12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. For the first time, we report emission of Si iv within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si iv patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si iv, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the nonequilibrium ionization of silicon. Through spectral synthesis, we explain several features of the observations. We show that the presence of Si iv emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes. [ABSTRACT FROM AUTHOR]

We present precise values of the dipole polarizabilities (α) of the ground and metastable states of Yb+, that are important in reducing systematics in the clock frequency of the transition. The static values of α for the ground and states are estimated to be and , respectively, while the tensor contribution to the state as compared to the experimental value . This corresponds to the differential scalar polarizability value of the above transition as −7.8(5) in contrast to the available experimental value −6.9(1.4) J m2 V−2 . This results in the black-body radiation shift of the clock transition as Hz at the room temperature, which is large as compared to the previously estimated values. Using the dynamic α values, we report the tune-out and magic wavelengths that could be of interest to subdue systematics due to the Stark shifts and for constructing lattice optical clock using Yb+. [ABSTRACT FROM AUTHOR]

We investigate the rate of change of energy for a static two-level atom interacting with a massless quantum scalar field in global monopole spacetime and separately calculate the contributions of thermal fluctuations and radiation reaction. We discuss two different kinds of atom-field interactions separately. The behaviors of the atomic transition rates are analyzed in different circumstances such as near distance and big solid angle deficit. Moreover, we compare the results with those in Minkowski spacetime so as to reveal the effects of the global monopole. In general, as the atom-monopole distance increases, the transition rates oscillate around the results in Minkowski spacetime and the amplitude of oscillation gradually decreases. The oscillation is more severe for larger solid angle deficit. Our works suggest that the transition rates can profoundly change with different atom-field interactions and different types of scalar field. [ABSTRACT FROM AUTHOR]

It has been shown that the femtosecond laser polarization modulation is a very simple and well-established method to control the multi-photon absorption process by the light–matter interaction. Previous studies mainly focused on the multi-photon absorption control in the weak field. In this paper, we further explore the polarization control behavior of multi-photon absorption process in the intermediate femtosecond laser field. In the weak femtosecond laser field, the second-order perturbation theory can well describe the non-resonant two-photon absorption process. However, the higher order nonlinear effect (e.g., four-photon absorption) can occur in the intermediate femtosecond laser field, and thus it is necessary to establish new theoretical model to describe the multi-photon absorption process, which includes the two-photon and four-photon transitions. Here, we construct a fourth-order perturbation theory to study the polarization control behavior of this multi-photon absorption under the intermediate femtosecond laser field excitation, and our theoretical results show that the two-photon and four-photon excitation pathways can induce a coherent interference, while the coherent interference is constructive or destructive that depends on the femtosecond laser center frequency. Moreover, the two-photon and four-photon transitions have the different polarization control efficiency, and the four-photon absorption can obtain the higher polarization control efficiency. Thus, the polarization control efficiency of the whole excitation process can be increased or decreased by properly designing the femtosecond laser field intensity and laser center frequency. These studies can provide a clear physical picture for understanding and controlling the multi-photon absorption process in the intermediate femtosecond laser field, and also can provide a theoretical guidance for the future experimental realization. [ABSTRACT FROM AUTHOR]

The ELMy H-mode plasmas realized with the supersonic molecular beam injection (SMBI) are studied in relation to the energy confinement and the heating power for the L–H transition ( in the HL-2A tokamak. A database is assembled for this study based on the ELMy H-mode discharges during the experimental campaigns in the period 2009–2013 at the HL-2A tokamak. The statistical results show that the SMBI is favourable for reaching the H-mode by reducing the heating power at the L–H transition and for the H-mode performance by improving the energy confinement compared with the ordinary gas puffing (GP). The reduction of is about 20% when the density is low, and the energy confinement enhancement factor of 1.5 is achieved with the SMBI. Note that in the database the density dependence of is non-monotonic with the at which the is minimum. Most of data are on the low density branch where the increases with the decrease in density. The minimum of the in HL-2A is comparable to the ITPA multi-machine threshold power scaling . The physics behind the reduction of the with the SMBI is also investigated in relation to the change of the density gradient at the plasma edge, the gas fuelling efficiency, and the recycling. [ABSTRACT FROM AUTHOR]

In the era of large stellar spectroscopic surveys, there is an emphasis on deriving not only stellar abundances but also the ages for millions of stars. In the context of Galactic archeology, stellar ages provide a direct probe of the formation history of the Galaxy. We use the stellar evolution code MESA to compute models with atomic diffusion—with and without radiative acceleration—and extra mixing in the surface layers. The extra mixing consists of both density-dependent turbulent mixing and envelope overshoot mixing. Based on these models we argue that it is important to distinguish between initial, bulk abundances (parameters) and current, surface abundances (variables) in the analysis of individual stellar ages. In stars that maintain radiative regions on evolutionary timescales, atomic diffusion modifies the surface abundances. We show that when initial, bulk metallicity is equated with current, surface metallicity in isochrone age analysis, the resulting stellar ages can be systematically overestimated by up to 20%. The change of surface abundances with evolutionary phase also complicates chemical tagging, which is the concept that dispersed star clusters can be identified through unique, high-dimensional chemical signatures. Stars from the same cluster, but in different evolutionary phases, will show different surface abundances. We speculate that calibration of stellar models may allow us to estimate not only stellar ages but also initial abundances for individual stars. In the meantime, analyzing the chemical properties of stars in similar evolutionary phases is essential to minimize the effects of atomic diffusion in the context of chemical tagging. [ABSTRACT FROM AUTHOR]

Neglecting the self-force, self-energy and radiative effects, we follow the spirit of Wald’s gedanken experiment and further discuss whether an extremal Kerr–Newman-AdS (KNA) black hole can turn into a naked singularity when it captures charged and spinning massive particles. It is found that feeding a test particle into an extremal KNA black hole could lead to a violation of cosmic censorship for the black hole. [ABSTRACT FROM AUTHOR]