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A 0.65 THz Solid-state Source Interferometer–Polarimeter (SSIP) with vertical viewing has been installed for line-integrated density and Faraday rotation measurements on Experimental Advanced Superconducting Tokamak (EAST). The SSIP utilizes three independent solid-state diode sources based on frequency multiplier (X48) to provide the probing beam at a fixed frequency of 0.65 THz with ∼2.5 mW output power, which provide an Intermediate Frequency (IF) with wide range, and the highest IF is up to ∼10 MHz. The mixers optimized for high sensitivity, ∼750 mV/mW, are used in the SSIP system, which permits multichannel interferometer–polarimeter on EAST with a low phase noise. The resolution of the integrated density is 3.3 × 1017 m−2, and the Faraday rotation is about 1.5°. The density measurement has been implemented with IF ∼0.85 MHz, in a plasma with plasma current ∼300 kA on EAST. The line-averaged density, 3.5 × 1019 m−3, measured by the SSIP shows good agreement with the results measured by the POlarimeter–INTerferometer (POINT) system and the CO2 dispersion interferometer system in the same discharge. [ABSTRACT FROM AUTHOR]

This study addresses the challenge of intermediate frequency (IF) instability in the far-infrared polarimeter/interferometer (POINT) of the Experimental Advanced Superconducting Tokamak (EAST) to ensure the accuracy and stability of electron density measurements. Sudden and extensive IF shifts of the lasers can cause instability and even measurement errors of the diagnostic system. This paper introduces a comprehensive solution for stabilizing IF fluctuations. First, analog IF is converted into digital form using an analog-to-digital converter, and the digitalized signal is processed by a digital signal system based on a ZYNQ processor. The exact value of the IF is obtained by acquiring the point of maximum amplitude through the fast Fourier transform method, while the ZYNQ processor loaded with a fuzzy control algorithm will precisely adjust the laser cavity length via piezoelectric ceramics, achieving frequency stabilization within a target range. Comparative analyses confirm the method's efficacy in managing sudden frequency shifts, maintaining stability within 850 ± 100 kHz (a central frequency of 850 kHz, fluctuating within a range of ±100 kHz), with a control speed of 0.5 s per action and robust against variations up to 270 kHz. This efficient and rapid control mechanism fulfills the critical need for IF stabilization, ensuring the stability and precision of the POINT system in the EAST discharge experiments. [ABSTRACT FROM AUTHOR]

High-k turbulence (1 < kθρs < 5) spectrograms have been directly measured in density modulation experiments of the EAST tokamak using the collective Thomson scattering diagnostic system. Density-peaking modulation is achieved in Lower Hybrid Wave (LHW) modulation experiments, revealing that the broadband turbulence is sensitive to the power modulation of LHW and the concomitant variation of density-peaking. Additionally, the contributions of 2.45/4.6 GHz LHW to density peaking differ when Neutral Beam Injection (NBI) is on/off, displaying distinct responses of broadband turbulence in low/high frequency ranges. It is found by cross-coherence analysis that the phase angle is negative in cases without NBI, while in NBI cases it can be modulated from negative to positive, indicating a significant transition of radial turbulence propagation. These findings highlight the active control of density-peaking via auxiliary heating in tokamak plasmas and suggest the potential role of high-k turbulence in density-peaking modulation. [ABSTRACT FROM AUTHOR]

Here, we study a magnonic crystal made of low-damping yttrium iron garnet that utilizes pseudo-caustic spin wave beams generated from subwavelength square well features arranged in a two-dimensional array. The lattice symmetry and the angle between the caustic beam propagation direction and the applied magnetic field were tailored to optimize the interaction of spin waves with the engineered defects. A prominent, narrow 3 MHz feature with large rejection efficiency is observed in the spin wave transmission spectrum that could be useful as a narrowband notch filter, and time- and space-resolved Brillouin light scattering (BLS) measurements suggest that both caustic interference and edge effects may contribute to this notch feature. Furthermore, the BLS measurements show that caustics are generated efficiently at the laser ablated wells, and by tuning the frequency by 30 MHz, the caustic beam angles and, hence, the details of how the spin wave caustics hit the ablated wells change sufficiently to add and remove caustic beams, which can be used to create additional device functionality. The generation and conversion of caustic beams show promise for applications that require directional energy transport and for magnonic devices. [ABSTRACT FROM AUTHOR]

We report on an all-optical investigation of coupled spin excitation modes in a series of magnetic trilayer structures. Using time-resolved magneto-optic Kerr effect (tr-MOKE) magnetometry, we observe multi-mode coherent spin excitations in Ni 81 Fe 19 /Ru/ Co 49 Fe 49 V 2 multilayers even though the tr-MOKE optical detection is sensitive only to the Co 49 Fe 49 V 2 magnetization dynamics. Frequency shifts of the different modes indicate that the coupling between the Ni 81 Fe 19 and Co 49 Fe 49 V 2 layers varies from anti-ferromagnetic to ferromagnetic to uncoupled as the Ru spacer layer thickness is increased from 8 Å to 200 Å. The lifetime of the high frequency coherent oscillations in the Co 49 Fe 49 V 2 layer increases by over 200%–300% even in the case of uncoupled Ni 81 Fe 19 and Co 49 Fe 49 V 2 layers with a 200 Å thick Ru spacer. The results suggest an additional method to decrease the damping of high-moment alloys in layered magnetic nanostructures. [ABSTRACT FROM AUTHOR]

We present all-optical studies of spin dynamics in two classes of ferrimagnets. Both sets of experiments use table-top laser-based pump–probe techniques to examine the ultrafast and longer timescale spin excitations. We use the visible/near infra-red time-resolved magneto-optical Kerr effect (tr-MOKE) to follow the spin dynamics of a series of metallic (FeCo) 1 − x Gd x thin films with varying Gd content. Magnetic compensation in the films occurs at a Gd concentration of ≈ 26 % , and the spin dynamics of the films exhibit a non-monotonic variation in effective magnetization. We also examine spin dynamics in an insulating NiFe 2 O 4 spinel using ultrafast techniques up at extreme ultraviolet energies, which permit element-specific investigations. The element and time-resolved delay scans reveal a non-trivial interaction between spin excitations on the different magnetic sub-lattices of the magnetic insulator. [ABSTRACT FROM AUTHOR]

An E-band (60–90 GHz) multi-channel Doppler backscattering (DBS) system with X-mode polarization has been installed on the Experimental Advanced Superconducting Tokamak (EAST), which can measure the turbulence at five different radial locations simultaneously. This system can launch 31 fixed microwave frequencies in the range of 60–90 GHz with a 1 GHz interval into the plasma, and five probing signals are selected by employing a reference signal and multiple filters. During experiments, the frequency of the reference signal is tunable in the E-band, and the selected probing signals can be changed as needed without any other adjustments, which can be performed in one shot or between shots. Furthermore, the incident angle can be adjusted from −10° to 20°, and the wavenumber range is 4–25 cm−1 with a wavenumber resolution of Δk/k ≤ 0.35. Ray tracing simulations are employed to calculate the scattering locations and the perpendicular wavenumber. In this article, the hardware design, ray tracing, and initial results obtained from the EAST plasma will be presented. [ABSTRACT FROM AUTHOR]

The pressure crashes observed in shot No. 71326 in the Experimental Advanced Superconducting Tokamak are numerically investigated with the three-dimensional, toroidal, and full resistive-magnetohydrodynamics code (CLT). Based on the experimental observations, the pressure crash is caused by the nonlinear evolution of the m/n = 2/1 double tearing mode (DTM), where n and m are the toroidal and poloidal mode numbers, respectively. However, we find that the m/n = 2/1 DTM is stable based on the safety factor (q) profile from the equilibrium code EFIT, which indicates that the original q profile is somewhat inconsistent with the actual profile due to q measurement uncertainty. Since there is no motional Stark effect diagnostic for this shot, the local information of the magnetic field is missing, which leads to the largest contribution to the discrepancy. If other information is perfectly known and the q profile is the only uncertainty, then we could provide some information for the EFIT reconstruction by comparing our simulation results with electron cyclotron emission signals to constrain the uncertainty of the q profile to a much smaller region. The influence of plasma rotation and the two-fluids effect is also discussed. [ABSTRACT FROM AUTHOR]

A fully analytical model is established for the thermal fluctuations of the beatnote phase of an optically pumped dual-frequency vertical-external-cavity surface-emitting laser (VECSEL). This model starts with the resolution of the heat equation inside the semiconductor chip structure and follows with the evaluation of the induced thermo-optic phase shift. Both the fluctuations of the heat induced by the optical pumping and the thermodynamic fluctuations at room temperature are considered. On the one hand, the thermal response of the structure is investigated and a significant thermal lens effect caused by the pump is deduced. On the other hand, the power spectral density of the frequency noise is calculated in the presence of diffusion spatial anisotropy. The present model is in very good agreement with the phase noise measured for a dual-frequency VECSEL at 852 nm for application to metrology and the validity of the usual low-pass filter model is discussed. [ABSTRACT FROM AUTHOR]

The effects of compressibility and guiding field on the Kelvin–Helmholtz (KH) instability in a double resonant surface system are numerically investigated by using a compressible magnetohydrodynamics model. The linear growth rate and the nonlinear saturation amplitude are analyzed for different parameters. A transition from double tearing modes to the KH instability occurs depending on the compressibility and guiding field with a weak magnetic shear. These results may deepen our understanding of the properties of the KH instability with double resonant surfaces in both space and laboratory plasmas. [ABSTRACT FROM AUTHOR]

Microinstabilities such as ion temperature gradient (ITG) mode and trapped electron mode (TEM) in quasi-axisymmetric (QA) stellarator are investigated by using gyrokinetic Vlasov simulations, where the QA configuration of Chinese first quasi-axisymmetric stellarator is utilized. Extensive parameter scans with respect to the density and temperature gradients for the zero beta cases identify the microinstability maps that indicate the transition boundary among ITG and TEM. It is found that, in particular for the modes with the ion-gyroradius scale, a relatively more stabilized regime appears along the transition boundary of ITG–TEM. Such a valley-like stability structure is not observed in the other cases with the sub-ion-gyroradius-scale. A stabilization effect attributed to the hollow density profile is also demonstrated. [ABSTRACT FROM AUTHOR]

The effect of post-growth annealing (PGA) on dot-in-well (DWELL) structures grown on Si substrates has been studied. The photoluminescence (PL) measurements showed that, compared to the DWELL structures directly grown on GaAs, the PGA process induces a distinct difference in the tuning of the emission properties. Then, transmission electron microscopy imaging of the samples revealed that PGA improved the uniformity of quantum dots (QDs) while the size of the QDs increased, in agreement with a corresponding red shift and a decrease of the full width at half maximum in the PL emission spectrum. Finally, energy-dispersive x-ray linescan provided a quantitative analysis of the composition change of DWELL grown on Si in the as-grown, 700 °C annealed, and 800 °C annealed samples. The change in the InL/GaK concentration ratio became gradual between the QDs and surrounding materials after 800 °C annealing. The analysis of the optical properties, morphology evolution, and compositional change of the QDs as a function of the annealing temperature showed good agreement. [ABSTRACT FROM AUTHOR]

Since the origin of magnetism in ZnO-based diluted magnetic semiconductors (DMSs) is still controversial, in this work, we presented a detailed study on the magnetic, structural, and electronic properties of wurtzite ZnO-based DMS systems with point and complex intrinsic defects. Two outer electrons from neutral oxygen vacancy (VO) occupy the a1 orbital, making the inducted magnetic moment to be zero, while a cluster including three VOs leads to a magnetic moment of ∼1 μB. The magnetic moment of the system with a Zn vacancy (VZn) is 1.65 μB. When two neutral VZns in different relative distances were created in respective supercells, the systems showed different magnetic moments induced by the unequal level between the highest electron occupied orbital of the defect state introduced by different VZn sites and the valence band maximum. The system of a neutral O occupying an octahedral site gives rise to a magnetic moment of 2 μB, while zinc interstitial and antisite defects do not cause spin polarization. The system with a complex defect of VO and VZn is magnetic when those vacancies are adjacent but still do not cause the compensation effect. The oxygen interstitial defect is unstable, and VZn easily turns into the complex defect. We suggest that VO clusters and VZn complex defects could likely be the origin of ferromagnetism in undoped ZnO. [ABSTRACT FROM AUTHOR]

The CuPt ordering of the group III elements in GaxIn1-xP (x ≃ 0.5) has been observed to vary during growth by metalorganic vapor-phase epitaxy of InAs quantum dots capped with GaAs in a GaInP matrix. While ordering is not affected by the insertion of a GaAs layer, the growth of InAs quantum dots capped with GaAs results in ordered, partially ordered, or fully disordered GaInP. We show that the degree of ordering depends on the deposition time of the InAs quantum dots and on the thickness of the GaAs capping layer. Our results indicate that disordered GaInP occurs in the presence of excess indium at the growth surface, which results from the growth of strained InAs quantum dots. Ordering resumes when the excess indium is consumed. Cathodoluminescence, scanning transmission electron microscopy, and atomic force microscopy are used in this study to correlate the electronic properties and the microstructure of the thin films. [ABSTRACT FROM AUTHOR]

The effect of the divacancy defect on magnetic properties of Fe94V6 alloys was investigated using the first-principles calculations based on density functional theory. The model of Fe28V2 super-lattice for a divacancy in Fe94V6 alloy was established, in comparison with the perfect Fe30V2 super-lattice. For the first time, the effect of the on-site Coulomb repulsion term was considered to correct the underestimation for the bandgap. The magnetism of the Fe, V atoms, and the super-lattice were analyzed by the electron hybridization of the atoms, and the interactions between V and four different neighboring Fe atoms were also investigated. The results show that both Fe28V2 and Fe30V2 super-lattices are ferromagnetic, which is determined mainly by the strong ferromagnetism of Fe 3d electrons, and the hybridizations between Fe 3d electrons and Fe 2p and V 3d electrons account for the weak anti-ferromagnetism. When the Fe divacancy exists, the average magnetic moments of the Fe28V2 super-lattice and Fe atoms inside increase, while that of V atoms decreases due to the change of the Fe-V charges. The variation of the magnetic moments of different neighboring Fe atoms with respect to V atoms depends on the charge transfer and the Fe-V interaction. [ABSTRACT FROM AUTHOR]

Vertical position stability plays a crucial role in maintaining safe and reliable plasma operation for long-pulse fusion devices. In general, the vertical position is measured by using inductive magnetic coils installed inside the vacuum vessel; however, the integration drift effects are inherent for steady-state or long-pulse plasma operation. Developing a non-magnetic approach provides a fusion reactor-relevant steady-state solution that avoids the negative impact of integration drift. In this paper, we compare the non-inductively determined vertical position achieved by line-integrated interferometer and polarimeter measurements to that employing an inductive flux loop for a 1056 s discharge recently achieved on EAST (Experimental Advanced Superconducting Tokamak). Experimental results show that the non-inductive measurement is more robust than flux loops after 300 s if the integrator is not reset to suppress integrator drift. Real-time vertical position control using the non-inductive system is proposed for the next EAST experimental campaign. [ABSTRACT FROM AUTHOR]

In this work, we give a direct interpretation of micrographs of the 60° and 90° defect core at the GaAs/Si interface using aberration corrected scanning transmission electron microscopy. We investigate the post-growth annealing effects on dislocation rearrangement at the interface as well as the threading dislocations in buffer layers; finally, the density of threading dislocations has been calculated as a function of annealing temperature. [ABSTRACT FROM AUTHOR]

The voltage noise characteristic and sensitivity of magnetic tunnel junction sensors are crucial for ultralow field detection. In this work, we used a soft magnetic material electrode Co70.5Fe4.5Si15B10 as a sensing layer to improve the sensitivity. Then, a bias field along the easy axis of a free layer was applied to improve the linearity and manipulate the sensitivity of magnetic tunnel junction sensors. More importantly, random telegraph noise was suppressed by the bias field, resulting in hysteresis-free performance. The highest sensitivity of 3.9%/Oe and the best field detectivity of 4.5nT/√Hz at 10 Hz without hysteresis have been achieved. The sensors showed excellent performance with CoFeSiB electrodes, indicating that it is an effective way to improve the performance of sensors by introducing the bias field. [ABSTRACT FROM AUTHOR]

It appears to be a common sense to measure the crowdedness of a fluid system by the densities of the species constituting it. In the present work, we show that this ceases to be valid for confined fluids under some conditions. A quite thorough investigation is made for a hard sphere (HS) fluid adsorbed in a hard sphere matrix (a quench-annealed system) and its corresponding equilibrium binary mixture. When fluid particles are larger than matrix particles, the quench-annealed system can appear much more crowded than its corresponding equilibrium binary mixture, i.e., having a much higher fluid chemical potential, even when the density of each species is strictly the same in both systems, respectively. We believe that the insight gained from this study should be useful for the design of functionalized porous materials. [ABSTRACT FROM AUTHOR]

Microstructure evolution of three Al-Ag alloys with different Ag contents (1 wt. % Ag, 5 wt. % Ag, and 15wt. % Ag) was studied by positron annihilation spectroscopy during the aging process. In situ measurements of the positron lifetime and Doppler broadening of annihilation radiation indicate the fast formation of Ag-rich clusters during natural aging of the alloys. The formation of Ag-rich clusters was further confirmed by coincidence Doppler broadening measurements. The Ag signal reflected by the Coincidence Doppler broadening spectrum increases with increasing Ag content and is further enhanced after subsequent artificial aging at 140 °C. This might be due to the increase in the size of Ag clusters. The temperature dependence of the Doppler broadening spectra between 10K and 290K was measured for the Al-Ag alloys after natural and artificial aging. Detrapping of positrons from Ag clusters with increasing temperature was observed for all the three Al-Ag alloys after natural aging and for the Al-1wt. % Ag after artificial aging. This indicates that Ag clusters act as shallow positron trapping centers. The thermal detrapping of positrons becomes ambiguous with increasing Ag content in the alloy and is nearly invisible in the artificially aged Al-5wt. % Ag and Al-15wt. % Ag. The positron binding energy of the Ag cluster is roughly estimated to be about 18.8 meV and 50 meV in the Al-1 wt. % Ag sample after natural aging and artificial aging at 140 °C, respectively, which suggests that the confinement of positrons in the quantum-dot like state depends on the size or chemical composition of clusters. Theoretical calculations confirm positron trapping by Ag nanoclusters, and the confinement of positrons is enhanced with increasing Ag cluster size. [ABSTRACT FROM AUTHOR]

Linear destabilization and nonlinear dynamics of m/n = 2 tearing modes in tokamak plasmas with locally reversed shear have been investigated in a reduced magnetohydrodynamic model. It is found that a transformation from the single tearing mode structure to the global double tearing mode structure takes place depending on the local shear parameter. The resistivity scaling law of the mode growth rate also changes from ∼SHp−3/5 to ∼ SHp−1/3 with qmin decreasing, where SHp is the magnetic Reynolds number, and qmin is the local minimum q value at position outsider q = 2 surfaces. Nonlinear simulation shows that once qmin < 2, the fast growing islands on the outer rational surfaces interact each other and trigger a plasmoid instability, which can couple with the inner tearing mode and generate a turbulent structure finally. This might provide an implication for understanding of relative nonlinear physics in ITER or astrophysical plasmas. [ABSTRACT FROM AUTHOR]

Laser-aided interferometers are employed as the common diagnostics for electron density measurements. On the EAST (Experimental Advanced Superconducting Tokamak), we have four main electron density diagnostics—the far-infrared hydrogen cyanide interferometer, polarimeter/interferometer, carbon dioxide (CO2) dispersion interferometer, and solid-state source interferometer. For interferometers, the phase difference brought by the change in electron density can be acquired via mixers. A real-time data acquisition (DAQ) system is needed to calculate the highly precise density values. In this paper, a field-programmable gate array is employed to implement the system with fast Fourier transform-based demodulation technology. A highly precise analog/digital converter is employed for readout and digitization of the waveform including multiple frequency signals from high-performance Schottky mixers. The system has been simulated, and the results show that it can meet the requirements of the accuracy of interferometers on the EAST. Furthermore, the framework of the DAQ system also offers a reference of the data processing method to the future laser-aided interferometers. [ABSTRACT FROM AUTHOR]

Spin waves in micrometer-sized, patterned Y-shaped Permalloy structures were studied using micro-focus Brillouin light scattering (BLS) with a magnetic field applied in-plane. For in-plane magnetized thin films and microstrips, the dispersion relations depend on the angle of the magnetization with respect to the microstrip axis. BLS measurements show that spin waves generated in the two arms that form the top of the Y structure can be channeled into a longer magnetic microstrip that forms the base when the applied field is oriented perpendicular to the long axis of the base. In this configuration, the base supports surface spin waves. A comparison of the BLS data with micromagnetic simulations reveals that low- k spin waves generated by a microstrip antenna in the arms are converted to higher- k spin waves in the base, which may be useful for nanomagnonic applications. [ABSTRACT FROM AUTHOR]

Using first-principles method and Boltzmann theory, we provide an accurate prediction of the electronic band structure and thermoelectric transport properties of α-MgAgSb. Our calculations demonstrate that only when an appropriate exchange-correlation functional is chosen can we correctly reproduce the semiconducting nature of this compound. By fine tuning the carrier concentration, the thermoelectric performance of α-MgAgSb can be significantly optimized, which exhibits a strong temperature dependence and gives a maximum ZT value of 1.7 at 550 K. We also provide a simple map by which one can efficiently find the best doping atoms and optimal doping content. [ABSTRACT FROM AUTHOR]

The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. [ABSTRACT FROM AUTHOR]

The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. [ABSTRACT FROM AUTHOR]

A two-step treatment, first chemical etching then thermal treatment, is proposed to achieve an atomically flat and thermally stable TiO2-terminated SrTiO3 substrate. LaAlO3 films were then grown on those TiO2-terminated and as-received substrates. LaAlO3 films on the TiO2-terminated SrTiO3 substrate maintained the layer-by-layer growth mode with a sharp interface, while films on the as-received substrates easily underwent reconstruction adverse to the sharp interface. Both LaAlO3/SrTiO3 interfaces displayed metallic conductive behavior, while the difference in magnetotransport properties indicated the difference in origin for interface conductivity. Large positive magnetoresistance implied that the LaAlO3/as-received substrate interface was a 3D conductive interface dominated by oxygen vacancies. However, the annealed-LaAlO3/treated-substrate interface preserved intrinsic quasi-2D interface magnetism as evidenced by large negative magnetoresistance. [ABSTRACT FROM AUTHOR]

In saturated Ohmic confinement regime of the experimental advanced superconducting tokamak, significant changes in the relative electron temperature fluctuations T ̃ e / T ¯ e , measured by a correlation electron cyclotron emission system, have been observed. It was found that T ̃ e / T ¯ e is strongly dependent on the normalized electron temperature and density gradient, R / L T e = R ∇ T e / T e and R / L n e = R ∇ n e / n e . T ̃ e / T ¯ e stay at a high level when R / L T e is within the range of 17 − 25 and R / L n e is within the range of 4 − 6. It has been also observed that the energy confinement degradation is correlated with the increase in electron temperature fluctuations. The linear stability analysis results show that the electron temperature fluctuations are mainly attributed to the ion temperature gradient modes. [ABSTRACT FROM AUTHOR]

Synergistic effects between two frequencies of lower hybrid (LH) waves—operating at 2.45 and 4.6 GHz—were observed in experiment on EAST for the first time. At low density ( n e , lin ≈ 2.0 × 1 0 19 m − 3 ), simultaneous injection of a 65/35 mix of 2.45/4.6 GHz power achieved an lower hybrid current drive efficiency that was 25% higher than what should be expected from the linear combination of the two sources. The experiment was interpreted with time-dependent simulations, using the equilibrium and transport solver TRANSP, coupled with the ray-tracing code GENRAY and the Fokker–Planck solver CQL3D. For each discharge, profiles of current and hard x-rays from simulation and measurement agree within uncertainties. An examination of the electron distribution function indicates that the LH synergy is supported by the increased width of the LH resonance plateau in the simultaneous injection case compared to independent injection. [ABSTRACT FROM AUTHOR]

Hot-electron energy relaxation time is deduced for Ga-doped ZnO epitaxial layers from pulsed hotelectron noise measurements at room temperature. The relaxation time increases from ~0.17 ps to ~1.8 ps when the electron density increases from 1.4×1017 cm-3 to 1.3×1020 cm-3. A local minimum is resolved near an electron density of 1.4×1019cm-3. The longest energy relaxation time (1.8 ps), observed at the highest electron density, is in good agreement with the published values obtained by optical time-resolved luminescence and absorption experiments. Monte Carlo simulations provide a qualitative interpretation of our observations if hot-phonon accumulation is taken into account. The local minimum of the electron energy relaxation time is explained by the ultrafast plasmon-assisted decay of hot phonons in the vicinity of the plasmon-LO-phonon resonance. [ABSTRACT FROM AUTHOR]

Presents information on a study which obtained the temperature distribution and evolution of metal field-ion emitters under pulsed and continuous-wave laser irradiation using implicit alternating direction method. General aspect of the problem; Description of the method; Results and discussion; Conclusions.

Boson peaks (BP) have been observed from phonon specific heats in 10 studied amorphous alloys. Two Einstein-type vibration modes were proposed in this work and all data can be fitted well. By measuring and analyzing local atomic structures of studied amorphous alloys and 56 reported amorphous alloys, it is found that (a) the BP originates from local harmonic vibration modes associated with the lengths of short-range order (SRO) and medium-range order (MRO) in amorphous alloys, and (b) the atomic packing in amorphous alloys follows a universal scaling law, i.e., the ratios of SRO and MRO lengths to solvent atomic diameter are 3 and 7, respectively, which exact match with length ratios of BP vibration frequencies to Debye frequency for the studied amorphous alloys. This finding provides a new perspective for atomic packing in amorphous materials, and has significant implications for quantitative description of the local atomic orders and understanding the structure-property relationship. [ABSTRACT FROM AUTHOR]

The efficacy of strained layer threading dislocation filter structures in single crystal epitaxial layers is evaluated using numerical modeling for (001) face-centred cubic materials, such as GaAs or Si1-xGex, and (0001) hexagonal materials such as GaN. We find that threading dislocation densities decay exponentially as a function of the strain relieved, irrespective of the fraction of threading dislocations that are mobile. Reactions between threading dislocations tend to produce a population that is a balanced mixture of mobile and sessile in (001) cubic materials. In contrast, mobile threading dislocations tend to be lost very rapidly in (0001) GaN, often with little or no reduction in the immobile dislocation density. The capture radius for threading dislocation interactions is estimated to be approximately 40 nm using cross section transmission electron microscopy of dislocation filtering structures in GaAs monolithically grown on Si. We find that the minimum threading dislocation density that can be obtained in any given structure is likely to be limited by kinetic effects to approximately 104-105cm-2. [ABSTRACT FROM AUTHOR]

Previous experimental work has shown that a phase transition in C60 multifragmentation induced by nanosecond laser occurs at almost constant temperature covering a wide range of laser fluency. Here the relative yields of ionic fragments (IFs) Cn+ (n = 1-20) resulting from the multifragmentation are measured within the phase transition region. By excluding two small IFs and magic IFs due to their abnormal behavior, the data for residual IFs are used to estimate the size distributions of primary intermediate-mass IFs in the multifragmentation regime. The distributions are found to obey power laws n- T . Furthermore, the exponent T values have sensitive dependence on lower laser fluency and converge to a constant of about 2.4 ± 0.2 for larger fluencies. These observations are in good agreement with an explanation based on the Fisher droplet model, offering the tantalizing possibility of a liquid-to-gas phase transition in C60 systems. [ABSTRACT FROM AUTHOR]

The electronic structure of a quintuple layer (QL) of Bi2Te3 is calculated using the first-principles pseudopotential method. It is found that the band gap of an isolated QL is considerably larger than that of bulk Bi2Te3. The electronic transport of the QL is, then, evaluated using the semiclassical Boltzmann theory within the relaxation time approximation. By fitting the energy surface from first-principles calculations, a suitable Morse potential is constructed and used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. By optimizing the carrier concentration of the system, the ZT of Bi2Te3 QL can be enhanced to a relatively high value. Moreover, the ZT value exhibits strong temperature dependence and can reach as high as 2.0 at 800 K. This value can be further increased to 2.2 by the substitution of Bi atoms with Sb atoms, giving nominal formula of (Bi0.25Sb0.75)2Te3. The significantly enhanced ZT value makes QL a very appealing candidate for thermoelectric applications. [ABSTRACT FROM AUTHOR]

The electronic structure of a quintuple layer (QL) of Bi2Te3 is calculated using the first-principles pseudopotential method. It is found that the band gap of an isolated QL is considerably larger than that of bulk Bi2Te3. The electronic transport of the QL is, then, evaluated using the semiclassical Boltzmann theory within the relaxation time approximation. By fitting the energy surface from first-principles calculations, a suitable Morse potential is constructed and used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. By optimizing the carrier concentration of the system, the ZT of Bi2Te3 QL can be enhanced to a relatively high value. Moreover, the ZT value exhibits strong temperature dependence and can reach as high as 2.0 at 800 K. This value can be further increased to 2.2 by the substitution of Bi atoms with Sb atoms, giving nominal formula of (Bi0.25Sb0.75)2Te3. The significantly enhanced ZT value makes QL a very appealing candidate for thermoelectric applications. [ABSTRACT FROM AUTHOR]

The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Infrared and Raman-scattering spectroscopies were employed to explore the lattice and spin dynamics of Bi1-xDyxFeO3 nanoparticles. With increasing substitution of Bi by Dy in the range of 0.00⩽x⩽0.40, we observe (1) evidence for an increase of local lattice distortion of the FeO6 octahedra, and significant changes of phonon parameters in the x=0.15 sample, corresponding to the structural transformation from rhombohedral to orthorhombic, (2) the chemically substituted enhanced dielectric constant of 43.4 in the x=0.20 sample, (3) the development of two-magnon excitations in Dy substituted samples, which sensitively gauges the modification of magnetic structures from a cycloidal spin to a homogeneous magnetized state, and (4) Dy substitution disrupts the lattice-spin interactions at high temperatures. These findings extend our understanding of tailoring the structural and magnetic properties of chemically substituted multiferroic nanoparticles and advance the technologically important development of these materials. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

The structure, ionic valences, magnetism, and magneto-transport behaviors of mixed valence oxides La1–xCaxMn1–xRuxO3 are systematically investigated. The simultaneous substitutions of La3+ and Mn3+ ions by Ca2+ and Ru4+, respectively, are confirmed by the structural and ionic valence characterizations, excluding the presence of Mn4+ and Ru3+ ions. The enhanced ferromagnetism, induced metal-insulator transition, and remarkable magnetoresistance effect are demonstrated when the substitution level x is lower than ~0.6, in spite of the absence of the Mn3+-Ru4+ eg-orbital double-exchange. These anomalous magnetotransport effects are discussed based on the competing multifold interactions associated with the Mn3+-Ru4+ super-exchange and strong Ru4+-Ru4+ hopping, while the origins for the metal-insulator transition and magnetoresistance effect remain to be clarified. [ABSTRACT FROM AUTHOR]

Theoretical studies and numerical simulations on design, performance, and improvements of terahertz quantum-well photodetector (THz QWP) are presented. In the first part of this paper, we discuss the device band structure resulting from a self-consistent solution and simulation results. First, the temperature dependence of device characteristics is analyzed. Next, we deduce the condition of optimal doping concentration for maximizing dark current limited detectivity Ddet* when QWP is lightly doped. Accordingly, unlike in previously published reports, doping concentration is not fixed and is selected by the above condition. In the second part of this paper, we propose two schemes for improving operation temperature. The first is to incorporate an optical antenna which focuses incident THz wave. Numerical results show that the QWP with peak frequency higher than 5.5 THz is expected to achieve background-noise-limited performance at 77 K or above when employing a 106 times enhancement antenna. The second scheme is to use a laser as the signal source to achieve photon-noise-limited performance (PLIP) at high temperatures. Simulations show that when operating below critical temperature QWPs in the range of 1 ∼ 7 THz can reach PLIP under practical illumination intensities. [ABSTRACT FROM AUTHOR]

We report a resistive switching memory structure based on silicon wafers by employing both materials and processing fully compatible with complementary metal-oxide semiconductor technology. A SiOx nanolayer was fabricated by direct plasma-oxidation of silicon wafers at room-temperature. Resistive switching behaviors were investigated on both p- and n-Si wafers, whereas self-rectifying effect was obtained in the Cu/SiOx/n-Si structure at low-resistance state. The self-rectifying effect was explained by formation of the Schottky barrier between the as-formed Cu filament and the n-Si. These results suggest a convenient and cost-efficient technical-route to develop high-density resistive switching memory for nowadays Si-based semiconductor industry. [ABSTRACT FROM AUTHOR]

Negative differential resistance (NDR) behavior existing in dark current-voltage (IV) curves of terahertz quantum-well photodetectors (QWPs) is theoretically investigated. Due to electron-electron scattering, the localized two-dimensional (2D) electrons in terahertz QWPs are thermalized. In steady state, the effective temperature of the 2D electrons is found to be higher than that of lattice. A self-consistent model is used to simulate the dark IV curves of terahertz QWPs, taking into account the thermalization effect of the 2D electrons. The NDR behavior is qualitatively reproduced. The periodic structures of electric-field domain and 2D electron occupation are formed in the NDR regime. The improved self-consistent model is useful for further understanding of the electron transport properties and improving the performance of terahertz QWPs. [ABSTRACT FROM AUTHOR]

We designed and demonstrated a terahertz quantum cascade laser based on indirect pump injection to the upper lasing state and phonon scattering extraction from the lower lasing state. By employing a rate equation formalism and a genetic algorithm, an optimized active region design with four-well GaAs/Al0.25Ga0.75As cascade module was obtained and epitaxially grown. A figure of merit which is defined as the ratio of modal gain versus injection current was maximized at 150 K. A fabricated device with a Au metal-metal waveguide and a top n+ GaAs contact layer lased at 2.4 THz up to 128.5 K, while another one without the top n+ GaAs lased up to 152.5 K (1.3hω/kB). The experimental results have been analyzed with rate equation and nonequilibrium Green's function models. A high population inversion is achieved at high temperature using a small oscillator strength of 0.28, while its combination with the low injection coupling strength of 0.85 meV results in a low current. The carefully engineered wavefunctions enhance the quantum efficiency of the device and therefore improve the output optical power even with an unusually low injection coupling strength. [ABSTRACT FROM AUTHOR]

We experimentally investigated the effect of oscillator strength (radiative transition diagonality) on the performance of resonant phonon-based terahertz quantum cascade lasers that have been optimized using a simplified density matrix formalism. Our results show that the maximum lasing temperature (Tmax) is roughly independent of laser transition diagonality within the lasing frequency range of the devices under test (3.2-3.7 THz) when cavity loss is kept low. Furthermore, the threshold current can be lowered by employing more diagonal transition designs, which can effectively suppress parasitic leakage caused by intermediate resonance between the injection and the downstream extraction levels. Nevertheless, the current carrying capacity through the designed lasing channel in more diagonal designs may sacrifice even more, leading to electrical instability and, potentially, complete inhibition of the device's lasing operation. We propose a hypothesis based on electric-field domain formation and competition/switching of different current-carrying channels to explain observed electrical instability in devices with lower oscillator strengths. The study indicates that not only should designers maximize Tmax during device optimization but also they should always consider the risk of electrical instability in device operation. [ABSTRACT FROM AUTHOR]

Resistive switching mechanism of Ti/HfOx/Pt memory devices was studied using X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy images. Spatial distributions of valence of Hf demonstrated that the fraction of Hf4+ increased from Ti/HfOx interface to HfOx/Pt interface in high resistance state (HRS), but it maintained a constant level in low resistance state (LRS). Rupture of oxygen vacancies formed conducting paths occurred near the HfOx/Pt interface. The cross sectional images of active switching region also varied with HRS and LRS. A dynamic model of interface processes was proposed to interpret interfaces migration of oxygen vacancies near both the top and bottom electrodes. [ABSTRACT FROM AUTHOR]

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed. [ABSTRACT FROM AUTHOR]

With a newly installed 5-channel W-band Doppler reflectometer (DR) in EAST tokamak, many interesting coherence modes and turbulences have been observed. In this paper, the tearing mode (TM), a novel inter-edge localized mode (ELM) electromagnetic mode, and the eigenmode geodesic acoustic mode (GAM) accompanied by a broadband quasi-coherent mode (QCM) during H-mode stage have been reported. The TM could be directly observed in both density fluctuation and perpendicular velocity fluctuation at the DR channel localized in the magnetic island region; moreover, many high-order TM frequency harmonics, dual turbulence peaks inside and outside the island separatrix, and strong flow shear across the island separatrix could also be observed. A high-frequency (∼ 230 kHz) electromagnetic mode (toroidal mode number n = 1) could be identified. The mode is generated dozens of milliseconds before the ELM crash and may affect the ELM dynamic process. The eigenmode GAM excited during the H-mode stage is generated near the top of the density pedestal and then propagates radially inward with a radial wavenumber of about 1 cm − 1 . A broadband QCM with a frequency range of 20 – 100 kHz is always accompanied by eigenmode GAM and nonlinearly coupled with both GAM and the background turbulence from the bispectrum analysis. All these results convince the validity of the Doppler reflectometer in multi-scale measurement from the mesoscale to microscale. [ABSTRACT FROM AUTHOR]

Flow focusing of liquid in a gaseous medium is typically axisymmetric, and it is based on a round capillary positioned on top of a circular aperture, which restricts the gaseous flow from forming a pressure drop that accelerates the liquid into a fine jet. We report an experimental study on the two dimensional flow focusing enabled by a wedge over a slit that provides similar gas flow restriction. As the wedge-to-slit distance is gradually reduced, the liquid dripping transforms into a single continuous jet that then splits into two, three, and more approximately equally-spaced jets. Below a critical wedge-to-slit separation, the liquid undergoes random atomization. The complete set of phenomena is rationalized by the dispersion relation that suggests that the jet spacing is inversely proportional to the square root of the local pressure gradient of the gas flow field. Typical experiments in the incompressible gaseous flow regime can achieve the jet spacing as short as ∼100 µm at the pressure gradient maxima. [ABSTRACT FROM AUTHOR]