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It is crucial to better understand the magnetization reversal process between soft and hard magnets and to achieve a high maximum energy product in thick composite multilayers. In this study, we find that the exchange interactions dominate in soft–hard-magnetic composite bilayers, while dipole interactions are predominant in soft–hard-magnetic composite trilayers. Based on the first-order reversal curve, magnetization reversal models are developed for both the thick composite bilayer and trilayer. Dipole interactions play an important role in the long range, resulting in higher coercivity and remanence in the thick trilayer. A multilayer in a stacked trilayer structure is achieved, which is composed of thick films with a perpendicular magnetic anisotropy at a thickness of up to 16 μm. The enhanced dipole interactions lead to a remanent polarization of 1 T and a maximum energy product of 22.5 MGOe. This work contributes to the preparation of thick films with a high maximum energy product for applications in magnetic microelectromechanical systems. [ABSTRACT FROM AUTHOR]

Composited p-type electrodes with high reflectivity have been investigated in AlGaN-based ultraviolet light emitting diodes (UV-LEDs) to improve the light extraction efficiency, which are composed of a patterned ITO layer and an Al reflector. It is verified that the patterned ITO with a thickness of 30 nm can not only well form Ohmic contact with p-GaN capping layer, but also be nearly 90% transparent to ultraviolet light, and thus presenting a reflectivity of 73% at 280 nm when combined with an Al reflector. Further experimental efforts confirm that the performance of the UV-LEDs is dramatically improved with such p-type electrodes. The maximum light output power and wall plug efficiency in the current range of 0–100 mA are severally increased by 49.8% and 54.2% compared to the device with traditional Ni/Au electrodes. [ABSTRACT FROM AUTHOR]

In this study, we have studied the influence of nitrogen content on the mechanical properties of nanostructure medium entropy (TiVCrAl)Nx films prepared by magnetron sputtering. A transition from body-centered cubic nanocrystalline phases to an face-centered cubic columnar crystal structure has been observed in the medium entropy films as the nitrogen flow increases. With increasing nitrogen flow, the lattice constant of the film first increases and then decreases due to the change of composition from substoichiometric to overstoichiometric, resulting in the different bond length of Metal-N. The hardness of the films first increases and then decreases with the increase of nitrogen flow. When the nitrogen flow was 25 sccm, the hardness reached the maximum value of 23.1 GPa. Moreover, the film has shorter cracks length and a higher hardness/modulus ratio, exhibiting enhanced fracture toughness. The increased hardness primarily results from the formation of bonds between metals and nitrogen and the strengthening of a solid solution and a grain boundary. [ABSTRACT FROM AUTHOR]

Influence of the barrier layer on the electrical properties of V/Al-based Ohmic contact is investigated by comparing the surface morphology and alloying results of V/Al/Ni/Au (with barrier Ni) and V/Al/Cr/Au (with barrier Cr) contacts on n-AlGaN (Al molar fraction>60%) after 850 °C annealing. Due to the clustering of Ni during annealing, a thin Al layer (AlOx) and a number of Al-Au alloy clusters are formed at the contact interface of n-AlGaN, which increases the contact resistance of the V/Al/Ni/Au Ohmic contacts. In contrast, the annealed V/Al/Cr/Au electrodes cover the surface uniformly, which not only contributes to the increased contact area, but also suppresses the formation of high resistance products. Thanks to the Cr barrier layer, the specific contact resistivity of V/Al-based Ohmic contact is reduced by 26%. [ABSTRACT FROM AUTHOR]

This study proposes a novel machine learning (ML) methodology for the efficient and cost-effective prediction of high-fidelity three-dimensional velocity fields in the wake of utility-scale turbines. The model consists of an autoencoder convolutional neural network with U-Net skipped connections, fine-tuned using high-fidelity data from large-eddy simulations (LES). The trained model takes the low-fidelity velocity field cost-effectively generated from the analytical engineering wake model as input and produces the high-fidelity velocity fields. The accuracy of the proposed ML model is demonstrated in a utility-scale wind farm for which datasets of wake flow fields were previously generated using LES under various wind speeds, wind directions, and yaw angles. Comparing the ML model results with those of LES, the ML model was shown to reduce the error in the prediction from 20% obtained from the Gauss Curl hybrid (GCH) model to less than 5%. In addition, the ML model captured the non-symmetric wake deflection observed for opposing yaw angles for wake steering cases, demonstrating a greater accuracy than the GCH model. The computational cost of the ML model is on par with that of the analytical wake model while generating numerical outcomes nearly as accurate as those of the high-fidelity LES. [ABSTRACT FROM AUTHOR]

Pool boiling has been considered as an effective method for heat transfer, which is extensively used in semiconductor microprocessors and aerospace. The prevention of critical heat flux (CHF) and improvement of heat transfer coefficient (HTC) are the keys to strengthen the pool boiling heat transfer. It has been reported that porous pillars can delay CHF by reducing vapor–liquid counter flow and nano-porous structure can significantly improve the liquid replenishment capacity and HTC. Herein, we modulate micro-nano porous structures by etching nanostructures on the surface of microporous pillars in order to utilize the advantages of above two structures. The boiling phenomenon inside the micro-nano composite pores is clearly demonstrated based on our visualization experiment. It is observed that the main vapor motion is regular periodic growth and recession. Through the quantitative analysis of the vapor–liquid interface area and vapor movement frequency, we find that they are both positively correlated with the heat transfer performance of porous structures. Hence, it is proved that the vapor–liquid phase distribution is the decisive factor affecting the heat transfer performance of micro-nano porous structures. Following the principle of maintaining larger vapor–liquid interface area and accelerating vapor movement frequency, the micro-nano porous structures with better performance can be obtained. Our approach establishes the relationship between the internal boiling phenomenon and the performance of micro-nano porous structures. In addition, it also provides a feasible direction for improving the performance of engineered boiling micro-nano structures. [ABSTRACT FROM AUTHOR]

We present a design for a quantum photodetector operating in the terahertz range, at 3.45 THz (15 meV, 87 μm). Our device relies on biased GaAs/AlGaAs heterostructure, designed to exploit LO phonon scattering as an extraction mechanism. In our design, the external potential due to the applied bias forms an extraction miniband and allows accommodating an LO phonon transition (36 meV) and use it as an extraction mechanism, even though its energy exceeds the detector's absorbing transition at 15 meV. Spectral-resolved measurements performed on arrays of patch antenna microcavities reveal a peak photocurrent at the designed photon energy with a responsivity of 80 mA/W at 20 K. The maximum operating temperature of the photodetector is found to be 40 K. Detector characterizations were performed both with a black-body source as well as with a terahertz quantum cascade laser emitting at 3.5 THz. [ABSTRACT FROM AUTHOR]

Magnetic properties and coercivity mechanism of an anisotropic Ce–Nd–Fe–B film with a high Ce content (70 at. % Nd is replaced by Ce) have been investigated. After grain boundary diffusion with Tb layers, an enhancement of coercivity from 4150 to 9250 Oe is observed. Combining the initial magnetization curves, micromagnetic theory, and in-siut observation of magnetic domains in the demagnetization process, it is confirmed that the coercivity mechanism for the high-Ce-content magnets is the mixed type dominated by the pinning mechanism. Moreover, as the thickness of the Tb diffusion layer increases, the pinning center of domain walls changes from narrow planar inhomogeneities to wide planar inhomogeneities. A growing role of pinning plays in determining the coercivity of samples with increasing the thickness of Tb layer due to the increase in magnetocrystalline anisotropy after Tb substitution in the RE2Fe14B phase. In addition, the CeFe2 intergranular phase leads to the enhancement of coercivity due to decoupling the hard magnetic phase grains. Our results provide an insight into the coercivity mechanism of high-Ce-concentration Ce–Nd–Fe–B magnetic materials and promote the comprehension of the effect of Tb diffusion in the magnetization reversal process. [ABSTRACT FROM AUTHOR]

The performance of ultraviolet (UV) laser diodes (LDs) with a low Al mole fraction AlGaN cladding layer was investigated by varying the thicknesses of the waveguide layer. It is found that (1) the loss of carriers in the waveguide layer is much larger than that in blue or green LDs due to the shallower quantum well and consequently a weaker carrier confinement of UV lasers. (2) Carrier loss in the waveguide layer can be suppressed by using a thinner waveguide layer. Therefore, the threshold current of LDs can be reduced. (3) The ultraviolet GaN/AlGaN single quantum well laser diodes lasing at 366 nm are fabricated by using relatively thin AlGaN waveguide layers and low Al mole fraction AlGaN cladding layers. [ABSTRACT FROM AUTHOR]

In recent years, MnFePGe magnetocaloric materials have been widely studied as promising candidates for magnetic refrigeration materials. The Curie temperature of MnFePGe can be adjusted to around room temperature by changing the element ratio or doping with other elements. Due to its first-order magnetic and structural transition, it engenders a large entropy change but unfortunately also exhibits a large thermal hysteresis during the phase transition, which leads to energy loss and lower refrigeration capability. In this paper, we establish a correlation between the in-plane covalent bonding and Curie temperature (TC), thermal hysteresis (ΔThys), two-phase coexistence zone (ΔTcoex), and entropy change (ΔSDSC) using 54 Mn2−xFexPyGe1−yMz (where M is a metallic or nonmetallic doped element) samples with different components. Neutron diffraction and XRD diffraction data and refinements have been employed to allow a detailed electron density reconstruction of six typical samples with the maximum entropy method. We find that the length of the in-plane bonding is closely correlated with the TC and ΔThys, while the TC, ΔThys, ΔTcoex, and ΔSDSC have no significant correlation with the length of the interlayer covalent bond. Moreover, we find that the ΔThys correlates most strongly with the change in the bond length when undergoing the paramagnetic-to-ferromagnetic phase transition rather than the absolute value of the bond length. These results provide an understanding of how to control the properties, enabling effective ways to tune the composition of magnetic refrigeration materials to tailor magnetocaloric properties for optimal performance. [ABSTRACT FROM AUTHOR]

AlGaN-based deep-ultraviolet light emitting diodes (DUV-LEDs) with thin p-GaN capping layer have been one of the most promising configurations, thanks to their excellent light extraction potential, which are, however, generally accompanied by insufficient hole supply. In this work, multi-graded p-AlGaN layers are adopted as an integrated p-type region, in which the electron blocking layer (EBL) is taken off to promote the carrier transport. The experimental results show that both the operation voltage and light output power of the DUV-LEDs are improved compared to the traditional ones, leading to a remarkable increase (by 114%) in their wall-plug efficiency. Further analysis confirms that the integrated p-type region serving as the hole supply layer helps holes to avoid the barrier between the p-GaN and integrated p-type region during their transport, and the absence of the EBL further eliminates part of the obstacles for hole drifting, giving rise to an enhanced hole concentration in quantum wells. Meanwhile, thanks to the modulation of the negative polarization induced bulk charges for conduction band, which is introduced within the integrated p-type region, the electron leakage is effectively suppressed even without the EBL, thus improving the device performance dramatically. [ABSTRACT FROM AUTHOR]

Due to its particular geometry, stimulated Raman side-scattering (SRSS) drives scattered light emission in non-usually diagnosed directions, leading to scarce and complex experimental observations. Direct-irradiation campaigns at the SG-II Upgrade facility have measured the scattered light driven by SRSS over a wide range of angles. Typical interaction conditions were as follows: an overlapped laser intensity of 1.2 × 10 15 W cm−2 propagated into a plasma with a density scale length L n c / 4 ≈ 250 μm and an electron temperature T e ≈ 2.2 keV. It indicated an emission at large polar angles over a broad azimuthal range, sensitive to the plasma profile, resulting in a loss of about 5% of the total laser energy. Direct comparison with back-scattering measurement, both in the full-aperture back-scattered direction and sampled at smaller polar angles in the same azimuthal plane, has evidenced SRSS as the dominant Raman scattering process. The predominance of SRSS was confirmed by two-dimensional particle-in-cell simulations, and its angular spread has been corroborated by ray-tracing simulations. The main implication is that a complete characterization of the SRS instability and an accurate measurement of the energy losses require the collection of the scattered light in a broad range of directions. Otherwise, spatially limited measurement could lead to an underestimation of the energetic importance of stimulated Raman scattering. [ABSTRACT FROM AUTHOR]

The optically pumped rare gas laser (OPRGL) is an innovative type of gas laser, which has the potential to be a candidate for high-energy lasers (HELs), in which metastable rare gas atoms generated by gas discharge are used as active particles. To realize these kinds of lasers as HELs in field application, high efficiency and low cost are necessary. For this purpose, we demonstrated a closed-chamber operation with repetitively pulsed gas discharge for an OPRGL of argon metastables. The pulsed discharge was powered by a home-made LC generator, which provided a high efficiency for the deposition of electric energy. The metastable density produced was higher than 1012 cm−3, quite enough for efficient laser operation. The dependencies of laser radiation on the parameters of discharge voltage, gas pressure, and argon concentration were studied, which showed an optimized argon concentration of 1%. Long-time operations were also performed, which demonstrated the practicability of the operation of OPRGL in a closed chamber, with the avoiding of the running away of rare gases in the open cycle of flowing operation. [ABSTRACT FROM AUTHOR]

The state of burning plasma had been achieved in inertial confinement fusion (ICF), which was regarded as a great milestone for high-gain laser fusion energy. In the burning plasma, alpha particles incident on the cryogenic (warm dense) fuels cannot be simply regarded as single particles, and the new physics brought about by the density effects of alpha particles should be considered. In this work, the collective interaction between them has been considered, namely, the effect of the superposition of wake waves. The stopping power of alpha-particle clusters, i.e., the rate of energy loss per unit distance traveled has been calculated using both analytical and simulation approaches. In theory, we obtain the stopping power of alpha clusters in cryogenic (warm dense) fuel by the dielectric function method, which illustrates the importance of the effective interaction between particles. Simulation results using the LAPINS code show that the collective stopping power of the alpha cluster is indeed increased via coherent superposition of excitation fields (the excitation of high-amplitude wake waves). However, the comparison between simulation and theoretical results also illustrates a coherent–decoherent transition of the stopping power of the cluster. The initial conditions with various sizes and densities of the alpha clusters have been considered to verify the condition of decoherence transition. Our work provides a theoretical description of the transport properties of high-density alpha particles in warm dense cryogenic fuel and might give some theoretical guidance for the design of actual fusion processes. [ABSTRACT FROM AUTHOR]

Lithium iron phosphate (LiFePO4) with an olivine-type structure has garnered great interest due to its electrochemical activity. It is considered one of the most promising cathode materials for lithium-ion batteries because of its high discharge platform, large theoretical capacity, and excellent cycling performance and stability. However, the typical crystal structure of LiFePO4 restricts the formation of efficient electron conduction pathways and hampers the embedding and removal motion of lithium ions, resulting in low electron conductivity and a low ion diffusion rate. The experimental results show that element doping can reduce the activation energy of diffusion, increase the diffusion rate of lithium ions, improve the rate capability of lithium-ion batteries, and contribute to the improvement of its conductivity. In this work, the electronic structure and diffusion mechanism of a S/Co/Mn co-doped LiFePO4 system are systematically studied. The introduction of S, Co, and Mn atoms leads to the formation of impurity bands, resulting in a significant reduction in the bandgap from 3.593 eV to 0.190 eV, which is beneficial for improving the conductivity of LiFePO4. Moreover, the diffusion activation energy of lithium ions decreases from 0.73 eV to 0.34 eV, and the diffusion rate of lithium ions increases from 2.7 × 109 cm/s to 1.0 × 1016 cm/s. These results demonstrate that co-doping of S, Co, and Mn can effectively enhance the conductivity and diffusion rate of lithium ions in LiFePO4, providing a reliable reference for improving the electrochemical performance of lithium iron phosphate. [ABSTRACT FROM AUTHOR]

In composite magnets, an in-depth understanding of magnetization reversal behaviors promotes optimizing the structure design of a magnet and improving its performance. In this work, the perpendicular magnetic anisotropic Ta/La–Pr–Fe–B/Dy/La–Pr–Fe–B/Ta multilayers have been prepared by adjusting the thickness of the Dy layer. The domain reversal evolutionary procedure has been investigated in different aspects including the characterization of domain morphology, micromagnetic analysis, and irreversible reversal distribution. It is confirmed that the nucleation mechanism is dominant in determining the coercivity of the multilayers with Dy diffusion. Dy diffusion helps to enhance the coercivity of the multilayers. The formation of Dy-containing hard magnetic phases and rare-earth-rich grain boundary phases by adding the Dy element leads to a strong nucleation field and isolation of hard magnetic phase grains, respectively. Our results aid in the understanding of magnetization reversal behaviors and enhance the magnetic properties of highly abundant rare-earth permanent magnetic multilayer films with the doping element. [ABSTRACT FROM AUTHOR]

In the double-cone ignition scheme, two deuterium–tritium shells in a pair of head-on Au cones are compressed and accelerated spherically [Zhang et al., Philos. Trans. R. Soc. A. 378 (2184), 20200015 (2020)]. The high-speed plasma jets from the cone tips collide and form a stagnating plasma with a higher density during the stagnation stage, preheating the plasma by the Coulomb potential. The preheated plasma is then rapidly heated up further to the ignition temperature by fast electrons generated by a powerful laser pulse of 10 ps. The conditions of the stagnating plasma strongly affect the fast-heating efficiency and consequently the success of ignition. In order to understand dynamical process in the stagnation stage, a special experimental campaign was conducted, where the evolution of the stagnating plasma was diagnosed through the temporal resolved self-emission signals. The spatial-temporal distributions of temperature and density of the colliding plasma were analyzed by the Abel inversion algorithm and the Legendre polynomial fitting. The stagnation period was found to be about 300 ps, the temperature of the core area of the stagnated plasma was between 340 and 390 eV, while the aspect ratio of the colliding plasma was about 0.78. [ABSTRACT FROM AUTHOR]

A polycrystalline TmNiAl2 compound with transition temperature near the liquid helium boiling point was successfully synthesized. Magnetic measurements show that FM (ferromagnetic) to FM and FM to PM (paramagnetic) transitions take place at 2.4 K and 4.0 K, respectively. Magnetic entropy change (–ΔSM) is calculated, and its maximal value [(–ΔSM)max] reaches as high as 20.7 J/kg K with the field change of 0–5 T. The low transition temperature together with large (–ΔSM)max at the liquid helium temperature zone originates from the weak spin-spin exchange interaction between Tm atoms. For a clear comparison, the magnetocaloric effect (MCE) of polycrystalline RNiAl2 (R = Tb and Gd) samples was also prepared and investigated. For the TbNiAl2 compound, FIM (ferrimagnetic) to AFM (antiferromagnetic) and AFM to PM transitions occur at 11.0 K and 21.5 K, respectively. As for the GdNiAl2 compound, an FM to PM transition occurs at 30.5 K. The value of (–ΔSM)max under the field change of 0–5 T is calculated to be 11.8 J/kg K and 17.3 J/kg K for TbNiAl2 and GdNiAl2, respectively. Compared with other RNiAl2-series MCE materials, TmNiAl2 exhibits the lowest working temperature and relatively larger (–ΔSM)max. The large MCE at low temperature indicates that TmNiAl2 is competitive among the MCE materials working at a liquid helium temperature zone. [ABSTRACT FROM AUTHOR]

Based on the first-principles calculations, the ferromagnetic origin and magnetic mechanisms of the Li(Zn, Mn)N system were investigated. The systems with different concentrations of magnetic moments and carriers were established. Compared with the Li16+y(Zn15Mn)N16 systems, the Li16+y(Zn14Mn2)N16 systems are more stable. Different from the Li15(Zn14Mn2)N16 system, Li16(Zn14Mn2)N16 and Li18(Zn14Mn2)N16 systems are anti-ferromagnetic semiconductors; the ferromagnetic state of the Li17(Zn14Mn2)N16 system is the preferred state. The spin magnetic moments mostly originated from Mn-3d states strongly hybridized with the N-2p and Li-2s states around the Fermi level. The ferromagnetic mechanism can be explained by the Mn-N-Mn chain with the p-d hybridization. [ABSTRACT FROM AUTHOR]

The one degree of freedom (1-DOF) manipulator with nano-resolution is a significant component in the micro-/nano-manipulation. In order to simultaneously achieve a large stroke and high precision, a piezo-driven 1-DOF flexure-based manipulator consisting of an enhanced double Scott-Russell mechanism (EDSRM), a lever type mechanism, and a Z-shaped mechanism is proposed in this paper. Analytical models are developed to examine the kinetostatic and dynamic properties of the manipulator. A finite element analysis is further performed to evaluate the characteristics of the EDSRM and the complete manipulator. The prototype is fabricated on monolithic AL7075, and various experimental tests have been carried out to investigate the correctness of the modeling. The experimental results show that the proposed manipulator has a satisfactory amplification ratio, static stability, and dynamic performance. [ABSTRACT FROM AUTHOR]

To fulfill the increasing needs of diagnostic support for researchers in plasma technology, a portable diagnostic package (PDP) equipped for both laser Thomson scattering (TS) and optical emission spectroscopy has been designed and constructed at Oak Ridge National Laboratory (ORNL), aiming to measure the temperature and number density of electrons and temperatures of ions in plasma devices. The PDP has been initially implemented on a high density and low temperature electrothermal arc source (ET-arc) at ORNL to test its TS capability. TS from the plasmas in the ET-arc has been obtained using the PDP. The electron temperature and number density were determined from TS spectra. These results were then compared to measurements from previous studies on the ET-arc. The TS diagnostic measured 0.8 ± 0.1, 1.3 ± 0.2, and 0.7 ± 0.1 eV and (4.4 ± 0.5) × 1021, (5.9 ± 0.7) × 1021, and (4.3 ± 0.5) x 1021 m-3, respectively, from three lines of sight that transect the plasma column. [ABSTRACT FROM AUTHOR]

An Advanced Research Projects Agency-Energy funded diagnostic system has been deployed to the Princeton field-reversed configuration 2 (PFRC-2) device, located at Princeton Plasma Physics Laboratory. The Portable Diagnostic Package (PDP), designed at Oak Ridge National Laboratory, allows for the measurement of Thomson Scattering (TS) for electron density and temperature and Optical Emission Spectroscopy (OES) for ion temperature, impurity density, and ion velocity. A tunable spectrometer on the PDP with three gratings provides the flexibility to measure low (1 eV) and high (1000 eV) electron temperature ranges from TS. Additionally, using a second spectrometer, the OES diagnostic can survey light emission from various ion excitation levels for wide wavelength ranges. The electron density (<2 × 1019 m−3) of plasmas generated in PFRC-2 has been below the PDP TS discrimination threshold, which has made TS signal detection challenging against a high-background of laser stray light. The laser stray light was iteratively reduced by making modifications to the entrance and exit geometry on PFRC-2. Rayleigh scattering experiments on PFRC have yielded the TS discrimination sensitivity to be >1 × 1020 m−3 for the PDP. A recently implemented narrow-band notch spectral filter that masks the second harmonic 532 nm Nd:YAG laser wavelength has increased the system's TS light discrimination sensitivity 65 times compared to the instance when the notch filter was not implemented. The hardware implementation including design changes to the flight tubes and Brewster windows will be discussed, along with results from Rayleigh and rotational Raman scattering sensitivity analyses, which were used to establish a quantitative figure of merit on the system performance. The Raman scattering calibration with the notch filter has improved the PDP electron density threshold to 1 ± 0.5 × 1018 m−3. [ABSTRACT FROM AUTHOR]

The rheological and self-healing behavior of a class of catalytically synthesized amine-functionalized polyolefins is investigated. We demonstrate that these materials possess tunable rheological properties according to the molecular weight and display autonomous self-healing. The linear viscoelastic properties are modeled using a tube-based model developed by Hawke et al. [J. Rheol., 60, 297–310, (2016)] to calculate several model parameters that describe the individual chain dynamics. The self-healing response is described by findings from the reptation model as well as recent theory on associating polymer networks with reversible bonds. The cooperation between experiments, modeling, and theory provide insight into designing new materials with programmable rheological properties and superior self-healing ability. [ABSTRACT FROM AUTHOR]

Heterointerfaces between two insulators play a central role in the study of oxide electronics owing to a spectrum of emergent properties. Manipulating transport of the interface by light can result in significant modulation of the ground state and excite localized states. However, their dynamics and mechanisms of photogenerated carries remain unclear. Here, this study presents the dynamics of carrier density and mobility under and after light illumination by Hall effect over time. It is discovered that the density and mobility after light illumination obey a stretched exponential expression, further indicating that the variation of mobility caused by the electron-electron scattering plays an important role in the recovery process in addition to the reduction of carrier density. Meanwhile, a non-linear Hall resistance at the LaAlO3/SrTiO3 interface under the illumination of a 360 nm laser at low temperature is observed. Furthermore, the gating effect can tune the recovery process after light illumination and induce a disappearance of non-linear Hall resistance. The results provide the experimental support for detailed mechanisms of the nonequilibrium process and developing of all-oxide electronic devices based on heterointerfaces. [ABSTRACT FROM AUTHOR]

Defects in semiconductors lead to deleterious effects in electron devices, but identifying their physical sources can be difficult. An example of this in gallium nitride (GaN) high electron mobility transistors is the well-known trap state located at approximately EC-0.57 eV. This trap is strongly correlated with output power degradation and reliability issues, but despite two decades of study, its specific physical source is still unknown. To address this long-standing question, two complementary nm-resolution characterization techniques—scanning probe deep level transient spectroscopy (SP-DLTS) and electron channeling contrast imaging (ECCI)—were used to spatially map the lateral distribution of these traps and to image and characterize their relation to residual threading dislocations within NH3-MBE-grown n-type GaN. Direct comparison of the SP-DLTS and ECCI measurements on the same sample region reveals highly localized concentrations of EC-0.57 eV traps that are spatially correlated with pure edge type threading dislocations in the GaN, but not with mixed and/or screw type dislocations, indicating that the specific dislocation character is a defining factor for this particular defect level. This work demonstrates the efficacy of combining these two techniques to obtain energy-, location-, and structure-resolved characterization of defects in a functional device structure. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) MoS2 has been considered to be one of the most promising semiconducting materials with the potential to be used in novel nanoelectronic devices. High carrier mobility in the semiconductor is necessary to guarantee a low power dissipation and a high switch speed of the corresponding electronic device. Strain engineering in 2D materials acts as an important approach to tailor and design their electronic and carrier transport properties. In this work, strain is introduced to MoS2 through perpendicularly building van der Waals heterostructures MoSe2-MoS2. Our first-principles calculations demonstrate that acoustic-phonon-limited electron mobility can be significantly enhanced in the heterostructures compared with that in pure multilayer MoS2. It is found that the effective electron mass and the deformation potential constant are relatively smaller in the heterostructures, which is responsible for the enhancement in the electron mobility. Overall, the electron mobility in the heterostructures is about 1.5 times or more of that in pure multilayer MoS2 with the same number of layers for the studied structures. These results indicate that MoSe2 is an excellent material to be heterostructured with multilayer MoS2 to improve the charge transport property. [ABSTRACT FROM AUTHOR]

High power microwave (HPM) sources usually require a well-defined rectangular pulse waveform, which is especially true for the case of long pulse (>100 ns), stable, and high efficiency operation. Most long pulse HPM drivers are realized with pulse forming networks. This paper presents a long pulse driver composed of a conventional Marx generator and metal-oxide varistors (MOVs), utilizing the MOV's nonlinear V–I characteristic. This method is easy to implement, and it has an additional benefit that the voltage can be stabilized even if the load's impedance changes slightly. A low inductance zig-zag folding structure of the MOV is designed to decrease its size and self-inductance. An LC filter is used to reduce the energy loss in the MOV. In the experiment, a 400 kV, 800 ns long pulse is achieved at a foil-less electron diode, and longer than 300 ns HPM generation is obtained. [ABSTRACT FROM AUTHOR]

Shape morphing is one of the most attractive functionalities of materials that are desired in many applications, including robotic grippers, medical stents, wearable electronics, and so on. Shape morphing can be implemented by using mechanical metamaterials that combine building blocks with properly designed mechanical or material properties. The design approaches are, however, mostly ad hoc or require materials with special properties. This work proposes two automated design strategies for programmable shape morphing and validates them on structures 3D-printed from a widely available commercial Stereolithography Durable resin. We proposed a so-called rose-shaped metamaterial with reduced stress concentration due to the absence of sharp corners and with a large range of tailorable Poisson's ratios, from −0.5 to 0.9, governed by a single design parameter. We programmed the shape of the rose-shaped metamaterial sheets aiming at high shape comfortability or uniform effective stiffness. The shape-morphing performance is demonstrated in the linear (0.1% strain) and non-linear (20% strain) deformation regimes, and it agrees well with the tensile test results. Our findings show the potential to develop complex practical metamaterial structures at comparatively low costs. [ABSTRACT FROM AUTHOR]

A criterion is proposed to predict the transition from slip to twinning in the face-centered cubic (FCC) metals within the framework of the generalized stacking fault energy (GSFE) curves, which are calculated by using the full potential linearly augmented plane wave method incorporating local orbital based on density functional theory. The criterion is a function of the stacking fault energy, unstable stacking fault energy, and unstable twinning fault energy, which are obtained from the GSFE curves. Based on the competition between slip and twinning, a new parameter is proposed to rank the sequence of the twinnability for ten typical FCC metals. The ranking results are in good agreement with the experimental data available. Compared with the previous studies of heterogeneous nucleation of twin, it is found that the heterogeneous nucleation sites of twinning such as crack tip and grain boundary do not change the ranking sequence. It is suggested that the GSFE curve may be regarded as the dominant factor determining the twinnability of FCC metals. [ABSTRACT FROM AUTHOR]

In the practical aerospace industry, the supersonic rarefied effect presents multiscale characteristics from the near-continuum regime to the free molecular regime. In this paper, a simple hydrodynamic-particle method (SHPM) is proposed to efficiently capture the multiscale properties for the supersonic rarefied flow. To combine the conventional computational fluid dynamics solver with the particle-based method, the weights are theoretically derived from the integral solution of the Boltzmann Bhatnagar–Gross–Krook equation. The present numerical method is validated by test cases of supersonic shock wave structure, Sod shock-tube, and supersonic flow around the circular cylinder. Numerical results demonstrate that the SHPM could capture the multiscale properties from the continuum regime to the rarefied regime. [ABSTRACT FROM AUTHOR]

We developed an angular-resolved scattered-light diagnostic station (ARSDS) to extend the study of laser-plasma instabilities (LPIs) by simultaneously diagnosing their features at different angles in a single shot. The ARSDS angularly samples the scattered light using an array of fibers with flexible setups. The collected light is detected with an imaging spectrometer, a streaked spectrometer, or a fiber-optic spectrometer to provide time-integrated/time-resolved spectral information. The ARSDS was implemented at Shenguang-II Upgrade laser facility for the double-cone ignition campaigns. Preliminary results confirm the importance of an angular-resolved detection due to the angular dependence of LPI processes, such as stimulated Raman scattering. [ABSTRACT FROM AUTHOR]

As a new type of two-dimensional semiconductor material, violet phosphorus (VP) has recently attracted much attention for applications in the field of optoelectronics. Although mechanical exfoliation has been used to prepare VP nanoribbons or nanosheets, production of the high quantum yield VP quantum dots (VPQDs) remains a challenge. Herein, the VPQDs are prepared via liquid exfoliation in ethanol and subsequent solvothermal treatment under 180 °C in dimethylformamide. The prepared VPQDs have a mean size of about 2 nm and can emit a strong blue photoluminescence (PL) when exchanged in ethanol solution. The quantum yield can reach 8.6%. The strong PL arises from both the band-to-band recombination related to quantum confinement effects and P-O-C related surface bonding state. Furthermore, the VPQDs solution has a good photostability under ambient conditions, suggesting their possible potential in fluorescent biological imaging and photocatalytic reactions. [ABSTRACT FROM AUTHOR]

The self-propulsion of a deforming sphere through an unbounded inviscid fluid is investigated analytically. Its motion is only induced by the coupling of its radial alteration, centroid shift, and rotation of the internal masses without vortex shedding and external forces. The Lagrange equations are used to describe such self-motion since the fluid-body system is conservative. Then the expressions for translational and rotational velocities of the deforming body are obtained in algebraic forms. Several cases show that some typical moving patterns of the sphere would be obtained as long as its radius variation and internal mass shift are properly coupled. [ABSTRACT FROM AUTHOR]

Photoexcited-carrier transport properties at the surface and the interface of barium strontium titanate/strontium titanate heterostructures are reported. Under a 365 nm light irradiation, the surfaces of barium strontium titanate films exhibit a metal-to-insulator transition, while the interfaces favor the metallic conduction with increasing temperatures. By analyzing, we consider that these results might be attributed to the intrinsic features of strontium titanate and the polarization state of barium strontium titanate films under the irradiation. Our results would contribute to further understanding of the photocarrier effect at the interface and demonstrate great potential applications in optoelectronic devices of all-oxide heterostructures. [ABSTRACT FROM AUTHOR]

The magnetic properties and the possible interaction mechanisms of anisotropic soft- and hard-magnetic multilayers have been investigated by altering the thickness of different kinds of spacer layers. The metal Ta and the insulating oxides MgO, Cr2O3 have been chosen as spacer layers to investigate the characteristics of the interactions between soft- and hard-magnetic layers in the anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer system. The dipolar and exchange interaction between hard and soft phases are evaluated with the help of the first order reversal curve method. The onset of the nucleation field and the magnetization reversal by domain wall movement are also evident from the first-order-reversal-curve measurements. Reversible/irreversible distributions reveal the natures of the soft- and hard-magnetic components. Incoherent switching fields are observed and the calculations show the semiquantitative contributions of hard and soft components to the system. An antiferromagnetic spacer layer will weaken the interaction between ferromagnetic layers and the effective interaction length decreases. As a consequence, the dipolar magnetostatic interaction may play an important role in the long-range interaction in anisotropic multilayer magnets. [ABSTRACT FROM AUTHOR]

Thus far, despite many investigations have been carried out for photo-triggered drug delivery systems, most of them suffer from an intrinsic drawback of without real-time monitoring mechanism. Incident intensity of light is a feasible parameter to monitor the drug release profiles. However, it is difficult to measure the incident laser power irradiated onto the photo-triggered carriers in drug delivery systems during in vivo therapy. We design an online measurement method based on the fluorescence intensity ratio (FIR) technique through upconversion nanoparticles. FIR value varies with temperature of sample due to the thermal effect induced by the incident laser, which validates the laser power measurement. Effects of rare earth doping concentration, as well as experimental conditions including laser spots and wavelengths on the measurement behavior were also investigated. [ABSTRACT FROM AUTHOR]

The impact of proton irradiation on the threshold voltage (VT) of AlGaN/GaN heterostructures is systematically investigated to enhance the understanding of a primary component of the degradation of irradiated high electron mobility transistors. The value of VT was found to increase monotonically as a function of 1.8MeV proton fluence in a sub-linear manner reaching 0.63V at a fluence of 1×1014 cm-2. Silvaco Atlas simulations of VT shifts caused by GaN buffer traps using experimentally measured introduction rates, and energy levels closely match the experimental results. Different buffer designs lead to different VT dependences on proton irradiation, confirming that deep, acceptorlike defects in the GaN buffer are primarily responsible for the observed VT shifts. The proton irradiation induced VT shifts are found to depend on the barrier thickness in a linear fashion; thus, scaling the barrier thickness could be an effective way to reduce such degradation. [ABSTRACT FROM AUTHOR]

Gd-doped ZnO thin films were prepared using pulsed laser deposition at different oxygen pressures and varied Gd concentrations. The effects of oxygen deficiency-related defects on the Gd incorporation, optical and structural properties, were explored by studying the impact of oxygen pressure during deposition and post-growth thermal annealing in vacuum. Rutherford Backscattering Spectrometry revealed that the Gd concentration increases with increasing oxygen pressure for samples grown with the same Gd-doped ZnO target. Unexpectedly, the c-lattice parameter of the samples tends to decrease with increasing Gd concentration, suggesting that Gd-defect complexes play an important role in the structural properties. Using low-temperature photoluminescence (PL), Raman measurements and density functional theory calculations, we identified oxygen vacancies as the dominant intrinsic point defects. PL spectra show a defect band related to oxygen vacancies for samples grown at oxygen deficiency. [ABSTRACT FROM AUTHOR]

NiAs-type manganese telluride (MnTe) films with preferred crystallographic plane of (110) were prepared on Si/SiO2 substrates by pulsed laser deposition. X-ray diffraction(XRD) of the films was studied at different temperatures. The XRD peak of MnTe (110) films shifts to higher angle with decreasing temperature, showing the decrease of the lattice parameter. Resistivity of the films was studied in the temperature range of 2-350 K. The bump between 150 and 250 K was observed in the films, which may be related to the special s-d and p-d overlaps induced by the compressed lattice. The magnon drag effect near its Néel temperature TN and enlarged magnetic-elastic coupling below 100 K were observed and analyzed in details. [ABSTRACT FROM AUTHOR]

This paper describes a method to optimally combine wide band gap Al2O3 with high dielectric constant (high-κ) Ta2O5 for gate dielectric applications. (Ta2O5)x(Al2O3)1-x thin films deposited by thermal atomic layer deposition (ALD) on GaN-capped AlxGa1-xN/GaN high electron mobility transistor (HEMT) structures have been studied as a function of the Ta2O5 molar fraction. X-ray photoelectron spectroscopy shows that the bandgap of the oxide films linearly decreases from 6.5 eV for pure Al2O3 to 4.6 eV for pure Ta2O5. The dielectric constant calculated from capacitance-voltage measurements also increases linearly from 7.8 for Al2O3 up to 25.6 for Ta2O5. The effect of post-deposition annealing in N2 at 600 °C on the interfacial properties of undoped Al2O3 and Ta-doped (Ta2O5)0.12(Al2O3)0.88 films grown on GaN-HEMTs has been investigated. These conditions are analogous to the conditions used for source/drain contact formation in gate-first HEMT technology. A reduction of the Ga-O to Ga-N bond ratios at the oxide/HEMT interfaces is observed after annealing, which is attributed to a reduction of interstitial oxygen-related defects. As a result, the conduction band offsets (CBOs) of the Al2O3/GaN-HEMT and (Ta2O5)0.16(Al2O3)0.84/GaN-HEMT samples increased by ~1.1 eV to 2.8 eV and 2.6 eV, respectively, which is advantageous for n-type HEMTs. The results demonstrate that ALD of Ta-doped Al2O3 can be used to control the properties of the gate dielectric, allowing the κ-value to be increased, while still maintaining a sufficient CBO to the GaN-HEMT structure for low leakage currents. [ABSTRACT FROM AUTHOR]

The impact of annealing of proton irradiation-induced defects in n-type GaN devices has been systematically investigated using deep level transient and optical spectroscopies. Moderate temperature annealing (>200-250 °C) causes significant reduction in the concentration of nearly all irradiation-induced traps. While the decreased concentration of previously identified N and Ga vacancy related levels at EC-0.13 eV, 0.16 eV, and 2.50 eV generally followed a first-order reaction model with activation energies matching theoretical values for NI and VGa diffusion, irradiation-induced traps at EC-0.72 eV, 1.25 eV, and 3.28 eV all decrease in concentration in a gradual manner, suggesting a more complex reduction mechanism. Slight increases in concentration are observed for the N-vacancy related levels at EC-0.20 eV and 0.25 eV, which may be due to the reconfiguration of other N-vacancy related defects. Finally, the observed reduction in concentrations of the states at EC-1.25 and EC-3.28 eV as a function of annealing temperature closely tracks the detailed recovery behavior of the background carrier concentration as a function of annealing temperature. As a result, it is suggested that these two levels are likely to be responsible for the underlying carrier compensation effect that causes the observation of carrier removal in proton-irradiated n-GaN. [ABSTRACT FROM AUTHOR]

The irradiation of a target with high laser intensity can lead to self-generation of an intense magnetic field (B-field) on the target surface. It has therefore been suggested that the sheath-driven acceleration of high-energy protons would be significantly hampered by the magnetization effect of this self-generated B-field at high enough laser intensities. In this paper, particle-in-cell simulations are used to study this magnetization effect on sheath-driven proton acceleration. It is shown that the inhibitory effect of the B-field on ion acceleration is not as significant as previously thought. Moreover, it is shown that the magnetization effect plays a relatively limited role in high-energy proton acceleration, even at high laser intensities when the mutual coupling and competition between self-generated electric (E-) and B-fields are considered in a realistic sheath acceleration scenario. A theoretical model including the v × B force is presented and confirms that the rate of reduction in proton energy depends on the strength ratio between B- and E-fields rather than on the strength of the B-field alone, and that only a small percentage of the proton energy is affected by the self-generated B-field. Finally, it is shown that the degraded scaling of proton energy at high laser intensities can be explained by the decrease in acceleration time caused by the increased sheath fields at high laser intensities rather than by the magnetic inhibitory effect, because of the longer growth time scale of the latter. This understanding of the magnetization effect may pave the way to the generation of high-energy protons by sheath-driven acceleration at high laser intensities. [ABSTRACT FROM AUTHOR]

This study presents an effective method to calculate the ideal tensile strength of six face-centered-cubic (fcc) crystals (Cu, Au, Ni, Pt, Al, and Ir) along an arbitrary tensile direction by considering the coupling effect of normal stress and shear stress on a given crystallographic plane. Meanwhile, the fracture modes of the six crystals can also be derived from the competition between shear and cleavage fracture along different crystallographic planes. The results show that both the intrinsic factors (the ideal shear strength and cleavage strength of low-index planes) and the orientation may affect the tensile strength and fracture modes of ideal fcc crystals, which may give the reliable strength limit of fcc metals and well interpret the observed high strength in nano-scale mechanical experiments. [ABSTRACT FROM AUTHOR]

We investigated the effect of magnetic doping on magnetic and transport properties of Bi2Te3 thin films. CrxBi2-xTe3 thin films with x=0.03, 0.14, and 0.29 were grown epitaxially on mica substrate with low surface roughness (∼0.4 nm). It is found that Cr is an electron acceptor in Bi2Te3 and increases the magnetization of CrxBi2-xTe3. When x=0.14 and 0.29, ferromagnetism appears in CrxBi2-xTe3 thin films, where anomalous Hall effect and weak localization of magnetoconductance were observed. The Curie temperature, coercivity, and remnant Hall resistance of thin films increase with increasing Cr concentration. The Arrott-Noakes plot demonstrates that the critical mechanism of the ferromagnetism can be described better with 3D-Heisenberg model than with mean field model. Our work may benefit for the practical applications of magnetic topological insulators in spintronics and magnetoelectric devices. [ABSTRACT FROM AUTHOR]

As semiconductor devices reach ever smaller dimensions, the challenge of power dissipation and quantum effect place a serious limit on the future device scaling. Recently, a multiferroic tunnel junction (MFTJ) with a ferroelectric barrier sandwiched between two ferromagnetic electrodes has drawn enormous interest due to its potential applications not only in multi-level data storage but also in electric field controlled spintronics and nanoferronics. Here, we present our investigations on four-level resistance states, giant tunneling electroresistance (TER) due to interfacial magnetoelectric coupling, and ferroelectric control of spin polarized tunneling in MFTJs. Coexistence of large tunneling magnetoresistance and TER has been observed in manganite/(Ba, Sr)TiO3/manganite MFTJs at low temperatures and room temperature four-resistance state devices were also obtained. To enhance the TER for potential logic operation with a magnetic memory, La0.7Sr0.3MnO3/BaTiO3/La0.5Ca0.5MnO3 /La0.7Sr0.3MnO3 MFTJs were designed by utilizing a bilayer tunneling barrier in which BaTiO3 is ferroelectric and La0.5Ca0.5MnO3 is close to ferromagnetic metal to antiferromagnetic insulator phase transition. The phase transition occurs when the ferroelectric polarization is reversed, resulting in an increase of TER by two orders of magnitude. Tunneling magnetoresistance can also be controlled by the ferroelectric polarization reversal, indicating strong magnetoelectric coupling at the interface. [ABSTRACT FROM AUTHOR]

The microstructure and magnetic properties of Fe3Se4 nanowire (NW) arrays in anodic aluminum oxide (AAO) porous membrane are studied. Cross-sectional SEM and plane-view TEM images show that the mean wire diameter (dw) and the center-to-center spacing (D) of Fe3Se4 nanowires are about 220 nm and 330 nm, respectively. The field-cooled magnetization dependent on the temperature indicates a Curie temperature around 334K for the Fe3Se4 nanowires. The coercivities of Fe3Se4 nanowires at 10 K, obtained from the in-plane and out-of-plane hysteresis loops, are as high as 22.4 kOe and 23.3 kOe, which can be understood from the magnetocrystalline anisotropy and the magnetization reversal process. [ABSTRACT FROM AUTHOR]

We investigate the magnetoelectric effect in Sr2CoGe2O7, which has a two-dimensional magnetic interaction between Co ions in the ab plane. This compound shows a weak magnetism and field-induced magnetic transition below the temperature of 7K. It does not exhibit electric polarization under zero magnetic field. However, by applying an external magnetic field, the electric polarization is induced around its Néel temperature and increases with increasing magnetic field. The induced electric polarization in Sr2CoGe2O7 is almost two orders of magnitude smaller than that of Ba2CoGe2O7. This result suggests that the different ionic radius of the strontium and the barium anion plays a key role in determining the property of electric polarization. The origins of electric polarization and magnetoelectric effect are discussed. [ABSTRACT FROM AUTHOR]

The spin configuration and spontaneous magnetostriction λ111 of TbxDy0.92-xNd0.1(Fe0.8Co0.2)1.93 (0.20⩽x⩽0.60) alloys have been investigated. The easy magnetization direction (EMD) at room temperature was observed towards the (111) axis when 0.40⩽x⩽0.60, accompanied by a rhombohedral distortion with large spontaneous magnetostriction coefficients λ111, which increases from 1640 ppm for x=0.40 to 1900 ppm for x=0.60. The strong (111)-oriented pseudo 1-3 particulate composite was fabricated by embedding and aligning particles in a passive epoxy matrix under an applied magnetic field. An enhanced magnetostrictive effect, the large low-field magnetostriction, λa, as high as 480 ppm at 3 kOe, was obtained for the sample of x=0.40, in an excess of 75% of its polycrystalline alloy although it only contains 27 vol.% alloy particles. This enhanced effect can be attributed to its low magnetic anisotropy, anisotropic magnetostrictive nature (e.g., λ111 ≠λ100, (111)EMD), chain structure, and the (111)-textured orientation. In addition, we give a direct experimental support that the (111)EMD of particle and the strong (111)-textured orientation are the crucial factors to realize the enhanced magnetostrictive effect in 1-3 particulate composites. [ABSTRACT FROM AUTHOR]