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Properties of ferroelectric semiconductors have garnered significant research interest, particularly due to their non-volatile memory. Meanwhile, studies on the characteristics of two-dimensional (2D) ferroelectrics have appeared as a crucial topic in solar cells, i.e., bulk photovoltaic effects. In this work, we propose two heterostructures: Cs2PbI2Cl2/Sc2CO2-UP (CSUP) and Cs2PbI2Cl2/Sc2CO2-DOWN (CSDN) for solar cells, to examine their photoelectric properties by using first-principles. Our findings indicate that such two heterostructures may have both high exciton binding energies and strong optical absorption coefficients in the ultraviolet region, with the CSDN showing exceptional carrier mobility as well. Moreover, we explore their characteristics by means of modulations of electric fields and stresses. The results reveal that the transition of band alignment in the CSUP can be engineered from type-II to type-I under the control of the electric fields, which may significantly increase the power conversion efficiency in actual solar cells. Moreover, both may have good potential in the application of logic devices. All these outputs may imply that, by means of fine modulations on photoelectric properties, the Cs2PbI2Cl2/Sc2CO2 possess immense potential to become multifunctional devices in ultraviolet photodetectors, solar cells, and logic devices. [ABSTRACT FROM AUTHOR]

Multiferroic materials have important research significance in the fields of magnetic random-access memory, ferroelectric random-access memory, resistive random-access memory, and neuromorphic computing devices due to their excellent and diverse physical properties. In this work, a solution of Bi2FeMnO6 was prepared using a solution-based method, and an Au/Bi2FeMnO6/TiO2 heterostructure device was fabricated on a Si substrate. X-ray diffraction and transmission electron microscopy data indicate that the Bi2FeMnO6 films have hexagonal R3c symmetry structures. The Bi2FeMnO6 film exhibits ferroelectricity with a fine remanent polarization. In addition, the Bi2FeMnO6-based devices have excellent switching ratios of 6.37 × 105. A larger switching ratio can provide a multi-resistance state for the device, which is beneficial for the simulation of synapses. Hence, it effectively emulates excitatory postsynaptic currents, paired-pulse facilitation, and long-term plasticity of synapses and achieves recognition accuracy of 95% in neuromorphic computing. We report a promising material for the development of various nonvolatile memories and neuromorphic synaptic devices. [ABSTRACT FROM AUTHOR]

To completely record a light field, photodetectors should be able to obtain corresponding parameters, including the intensity, position, propagation direction, polarization, wavelength, and time. Recently, metasurface-mediated two-dimensional (2D) material photodetectors have provided solutions for compact and integrated devices to obtain the characteristics of a light field, and most current metasurface-mediated 2D material photodetectors have focused on certain criteria. However, few efforts have been devoted to integrating multidimensional photodetection because of conflicts between the different requirements for distinct parameters and difficulties in fabrication. Problems for multidimensional photodetection are discussed, and the solutions may provide insight into next-generation photodetectors. [ABSTRACT FROM AUTHOR]

At high temperatures, serrated flows are frequently observed on the stress–strain curves of single-phase high-entropy alloys, indicating their unique mechanical behavior. In the current study, the CoCrFeMnNi (Cantor alloy) alloy was selected as a model material to further verify the temperature dependence on the serrated flow behavior through continuous temperature variation during tensile tests, which shows a certain strengthening effect of the serrated flow. The lattice strain and dislocation density were calculated at different conditions using the Williamson–Hall (W-H) analysis method. Based on the strain dependence of mobile dislocation density and forest dislocation density, a flow stress model related to the Portevin–Le Chatelier (PLC) effect was proposed to quantify the variation in the PLC behavior with the temperature and its influence on flow stress. The model is beneficial to accurately account for the flow stress during plastic deformation at high temperatures. [ABSTRACT FROM AUTHOR]

The Portevin–Le Chatelier (PLC) effect induced by dynamic strain aging is commonly observed in various alloys. The stress-drop magnitude of serrations and the critical strain at which serrations occur are two pivotal features characterizing serrated flow. This study investigates the temperature (160–460 °C) and strain rate (6 × 10−4 s−1–2 × 10−2 s−1) dependence of the serrated flow behavior in the CoNiV medium-entropy alloy through uniaxial tensile tests. Under the guidance of the mean-field theory, a scale theory has been applied to derive a scale function and scale indices that can be used to predict the distribution of stress drops. A normal PLC behavior of critical strain is fitted using the dislocation pinning model, demonstrating the presence of specific solute atoms conforming to the dislocation pinning model within medium-entropy alloys. [ABSTRACT FROM AUTHOR]

Half-valley semimetal (HVSM) and single-valley states are the hallmark of valleytronics in two-dimensional honeycomb lattice materials, but their quasi-one-dimensional analog that takes advantage of quantum manipulation has not yet been realized. We propose a double-helical ladder model described by a coupled double Su–Schrieffer–Heeger chain, wherein the interchain coupling controlled by magnetic flux induces time-reversal and particle-hole symmetry breaking and preserves only the chiral symmetry, which is classified into the AIII symmetry class. It realizes valley polarization, single-valley topological insulator, and HVSM as the topological quantum criticality (TQC), signaling well valley filter or valve effects. Furthermore, the TQC produces the largest inverse topological caloric effect accompanied by a T-linear relation of isothermal entropy change at ultra-low temperatures. Our findings not only open alternative perspectives for multifunctional quantum devices in valleytronics but also shed light on the thermodynamic characterization of TQC and promote the rapid development of topological quantum refrigeration technology. [ABSTRACT FROM AUTHOR]

In this study, a facile and effective approach for the preparation of Cu2O/ZnO heterostructures on flexible carbon materials to enhance the piezoelectric performance is reported. The Cu2O/ZnO heterostructures are deposited directly on carbon fibers (CFs) by a two-step method using plasma–liquid interaction strategy. The Cu2O microparticles are first deposited on the surface of CFs, and then, the ZnO nanostructures are grown on the surface of Cu2O microparticles and CFs. The as-prepared CFs show an improved piezoelectric response of d33 = 7.95 pm/V compared to intrinsic CFs (d33 = 2.64 pm/V), which is based on the nano-heterostructures principle. This work demonstrates a simple and feasible approach to prepare flexible carbon materials with enhanced piezoelectric performance, which may provide an ecologically friendly option for the deposition of semiconductor heterostructures. [ABSTRACT FROM AUTHOR]

We report the mechanical properties of cubic boron nitride (c-BN) and diamond under the combined impact of dynamical pressure and temperature, calculated using ab initio molecular dynamics. Our study revealed a pronounced sensitivity of the mechanical properties of c-BN to applied pressure. Notably, c-BN undergoes a brittle-to-ductile transition at ∼220 GPa, consistent across various dynamical temperatures, while diamond exhibits no such transition. Furthermore, the Vickers hardness profile for c-BN closely mirrors that of diamond across a spectrum of temperature–pressure conditions, highlighting c-BN's significant mechanical robustness. These results underscore the superior resilience and adaptability of c-BN compared to diamond, suggesting its potential as an ideal candidate for applications in extreme environments. [ABSTRACT FROM AUTHOR]

Hybrid organic–inorganic metal halide perovskites have emerged as a new generation of photovoltaic devices due to their impressive performance. Many experiments show that a small amount of pseudohalide BF4 doping significantly enhances the performance of the most studied CH3NH3PbI3 compound. We demonstrate, using real-time time-dependent density functional theory and ab initio non-adiabatic molecular dynamics, that a 2% concentration of BF4 doping can reduce the anharmonicity of the soft inorganic lattice, weaken the electron–phonon interaction, suppress non-radiative electron–hole recombination, and finally result in an 80% enhancement of charge carrier lifetime in CH3NH3PbI3 perovskite. More than that, low concentration of BF4 doping can keep the suitable bandgap and strengthen the optical absorption, whereas high concentration doping would extremely increase the bandgap so that should be prohibited in perovskite solar cells. Our results rationalize why low concentration of BF4 doping can suppress nonradiative charge carrier recombination and greatly improve the photovoltaic performance in CH3NH3PbI3 perovskite. [ABSTRACT FROM AUTHOR]

Here, we propose to generate the nonreciprocal macroscopic entanglement and steering of magnon modes from three yttrium iron garnet spheres, which are placed in a spinning microwave resonator damped by a squeezed reservoir. Strikingly, the genuine entanglement and steering among three magnons can be achieved due to the correlation transfer from squeezed microwave to three magnons and the steady-state entanglement and steering show strong robustness against temperature. Furthermore, the nonreciprocal tripartite entanglement and steering are simultaneously existent based on Fizeau light-dragging effect by spinning the resonator at the steady state, which provides an alternative way to manipulate nonreciprocal effects in a cavity magnonical system and may have potential applications in manipulating the macroscopic quantum states of the multipartite system. [ABSTRACT FROM AUTHOR]

By combining phase-field simulations and first-principles calculations, we proposed to modulate the monoclinic phases of high-index-oriented PbTiO3 films through epitaxial anisotropic strain to obtain superior piezoelectric performance. It was found that the piezoelectric coefficients e22 were remarkably enhanced (up to 2400%) in both (111)- and (110)-oriented PbTiO3 films. The excellent piezoelectric property is mainly attributed to the polarization rotation due to the flatter energy landscapes in the monoclinic phases, while the variation of polarization magnitude contributes little to piezoelectricity. Our research suggests modulating material properties through anisotropic strain, which might bring some insight to other functional materials. [ABSTRACT FROM AUTHOR]

Vanadium chalcogenides have been extensively studied owing to the diverse crystallographic structures with various stoichiometric ratios. The metal-to-insulator transition (MIT) widely reported in vanadium chalcogenides is a rapid reversible phase transition that requires small energy, demonstrating potential applications in memory devices. In this work, two-dimensional (2D) vanadium telluride (VTe2) nanosheets are prepared by the chemical vapor deposition method. The synthesized VTe2 nanosheets exhibit volatile threshold switching (TS) behaviors due to the MIT phase transition, which can be further confirmed by the temperature dependent TS behaviors. The TS memristor demonstrates good stability and high reliability with up to 1000 continuous and repeatable writing/erasing operations. Furthermore, based on the TS behaviors, the fabricated memristor can be utilized to implement basic Boolean logic operations of "OR," "AND," and "NOT." This study not only demonstrates the TS behaviors in the 2D VTe2 nanosheets owing to the MIT phase transition but also shows the potential applications of the TS devices in Boolean logic operations. [ABSTRACT FROM AUTHOR]

A hybrid memristor based on the bilayer structure of indium gallium zinc oxide (IGZO)/polyvinyl alcohol (PVA) is developed, which demonstrates device state updates in an analog manner with high reliability. The IGZO/PVA heterojunction is crucial for the realization of the memristive characteristics, presumably associated with oxygen ion redistribution across the IGZO/PVA interface. The hybrid memristor may act as an electronic synapse, being capable of emulating synaptic potentiation with good linearity, synaptic depression, and paired-pulse facilitation. It highlights potential applications of the oxide-polymer heterojunction in the exploration of neuromorphic devices. [ABSTRACT FROM AUTHOR]

The rapid development of semiconductor materials and devices has brought tremendous development opportunities to optoelectronics, intelligent manufacturing, Internet of Things, power electronics, and even innovative energy technologies. Among them, the third and fourth generation semiconductors represented by ZnO, GaN, SiC, and Ga2O3 are two kinds of emerging strategic material systems. Due to their large energy bandgaps, they exhibit excellent performance in application scenarios of high voltage, high frequency, and high temperature resistance, making them great candidates in high-power, radio frequency, and optoelectronic devices. The third and fourth generation semiconductors usually possess non-centrosymmetric crystal structures, which makes the piezoelectric polarization effect a fundamental characteristic for the third and fourth generation semiconductors in contrast to the first and second generation semiconductors as represented by Si, Ge, and GaAs. Research studies on the coupling of piezoelectricity, semiconductor, and light excitation properties were coined as piezotronics and piezo-phototronics in 2007 and 2010, respectively, by Zhong Lin Wang. The piezotronic and piezo-phototronic effects open another avenue for further improvement of the performance of electronic and optoelectronic devices. This Perspective will first introduce the basic concepts and principles of piezotronics and piezo-phototronics and the basic characteristics of the third and fourth generation semiconductors. Then, progress, challenges, and opportunities of ideal materials, comprehensive physical models, and outstanding applications based on piezotronics and piezo-phototronics are presented with emphasis. Finally, conclusions and outlooks are drawn for the piezotronics and piezo-phototronics based on the third and fourth generation semiconductors. [ABSTRACT FROM AUTHOR]

Skin effect is one of the intriguing phenomena exhibited by non-Hermitian wave systems. It reflects the localization of the modes at the boundaries of the structure. We demonstrated the skin effect for elastic waves propagating in a non-Hermitian phononic plate containing piezoelectric components in their unit cells. The latter behave as sensors and actuators by using the direct and inverse piezoelectric effects. The demonstration is based on the calculation of the complex non-reciprocal dispersion curves and their analysis for any direction of the wavevector in the two-dimensional space. Therefore, localization phenomena at different boundaries and corners of a finite square structure are presented. Furthermore, by applying different levels of non-Hermiticity in different parts of a square structure, it is shown that the localized features can appear at different positions and with various shapes. These localized phenomena can be reconfigured by acting on the non-Hermiticity parameters. Our results provided a feedback control strategy to introduce the non-Hermitian skin effect in two-dimensional elastic systems for potential applications, such as vibration control, energy harvesting, and sensing. [ABSTRACT FROM AUTHOR]

A special wake-up effect from antiferroelectric-like to ferroelectric (AFE-FE) characteristics in HfxZr1−xO2 thin films has been discussed intensively in terms of endurance performance enhancement. However, its physical origin and general impact on endurance remain unclear. In this work, the influence of various process parameters on the AFE-FE wake-up effect as well as on endurance performance and the material changes during AFE-FE wake-up are systematically studied. It is found that various parameters induce the AFE-FE wake-up effect and enhance endurance performance in the same way with enhancing tetragonal phase formation in HfxZr1−xO2 films, and the cycles of wake-up are universally associated with those of total endurance. In addition, via synchrotron-based grazing incidence x-ray diffraction, a tetragonal-orthorhombic-monoclinic phase transition is observed during AFE-FE wake-up. On the basis of these results, a correlation among crystalline composition, the AFE-FE wake-up effect, and the endurance performance of HfxZr1−xO2 thin films is established. This provides a clear guideline to a viable solution for the high endurance of HfxZr1−xO2 FE memory devices via crystal phase engineering. [ABSTRACT FROM AUTHOR]

Multimode integrated waveguides have large fabrication tolerances and enable low propagation losses. Multimode waveguide bends have, therefore, been used for high-quality (Q) factor multimode resonators. Conventional multimode circular bends typically require large bend radii to avoid the excitation of the higher-order modes. In this paper, we make use of multimode adiabatic bends in a multimode silicon racetrack resonator with a compact footprint of 0.16 mm2. The adiabatic bends help suppress the higher-order modes. The Q factor of the racetrack resonator has an average value of 2.2 × 106 in the wavelength range of 1260–1480 nm. Benefiting from the broadband high-Q multimode racetrack resonator, we experimentally demonstrated a continuous-wave Raman laser with a widely tunable wavelength range of 157 nm. The Raman laser has a threshold power of 0.2 mW and one of the highest slope efficiencies of 27.5%. This work shows how a high-efficiency integrated Raman laser can be achieved with a wide tunable wavelength range, compact footprint, and low threshold power for a tunable source that can extend the output wavelength beyond the direct output range of the pump laser. [ABSTRACT FROM AUTHOR]

Metal halide perovskites with excellent optoelectronic properties show great promise in optical imaging. However, the complex fabrication process of high-density perovskite image sensors limits its development. Herein, we report a simple perovskite color imaging system by combining a single high-performance perovskite photodetector (PD) and the advanced Fourier single-pixel imaging algorithm. We introduce an atomic layer deposition-TiO2 layer and a Spiro-OMeTAD layer to optimize the band energy level arrangement of the perovskite PD, suppressing the dark current and improving the photocurrent of the device, respectively. The optimized perovskite PD with flat spectral response covering all visible wavelength exhibits a high responsivity of 238 mA/W at zero bias, a high special detectivity of 1.13 × 1013 Jones, and a linear dynamic range of 132 dB. In order to bypass the fabrication of the high-density perovskite PD array, we set up a single-pixel imaging system based on an optimized high-performance perovskite PD, demonstrating a high-quality color imaging result. This work provides a perspective for perovskite image sensors and injects vitality into the single-pixel imaging technology. [ABSTRACT FROM AUTHOR]

A magnetically shielded Hall thruster (MSHT) has a significantly longer service life than a conventional unshielded Hall thruster (USHT) as it shifts the magnetic field of maximum strength from the channel interior to the exterior. In this study, it was experimentally determined that the influence of the radial position of an externally mounted hollow cathode on the performance of an MSHT is quite different from that of a USHT. The measured plasma parameters demonstrate that in addition to the coupling voltage loss and plume divergence loss, the electron cross-field transport loss (not considered previously and introduced by the outward shifting of the magnetic field) is also important to explaining the performance variation in the MSHT. This study refines the coupling theory between the cathode and Hall thruster and provides a valuable reference for determining the radial position of an externally mounted hollow cathode in an MSHT. [ABSTRACT FROM AUTHOR]

Phonon hydrodynamics, a collective motion of phonons, has recently attracted renewed attention since its temperature window has been greatly extended in layered materials. The exploration of phonon hydrodynamics bears importance in understanding phonon collective behavior, and its window is crucial for determining the phonon transport regime and engineering the heat transport. Thus, strategies for continuous tuning of the hydrodynamic window are needed, but it remains a challenge. In this work, we demonstrate that the phonon hydrodynamic window in bilayer graphene can be strongly altered by the strain based on theoretical calculations. In particular, the phonon hydrodynamics can be observed at 60 K in unstrained bilayer graphene, while only 0.25% strain can reduce this temperature to 28 K. This strong strain dependence not only provides an efficient way of modulating the phonon collective behavior but also renders a possibility of strain-induced transition of phonon transport regime. [ABSTRACT FROM AUTHOR]

Off-axis interferometric modules built on an ordinary bright field microscope make it possible to achieve single-shot quantitative phase imaging (QPI) by adding sufficient spatial carrier into the interferograms. However, compared with its on-axis counterparts, imaging configurations for off-axis interferometric modules have several disadvantages regarding optical aberration, stability, and space-bandwidth utilization of the lenses system. Herein, by demultiplexing technology based on polarization gratings, we propose a single-shot spatial light interference microscopy named as polarization-multiplexing light interference microscopy (PLIM) with on-axis imaging configuration to realize single-shot QPI. Although the imaging system is on-axis, the PLIM system still can generate sufficient spatial carrier, so we can adjust the orientation and absolute value of the spatial carrier independently of the magnification ratio of the imaging system. The experimental results prove that the PLIM system has better temporal phase stability compared with conventional grating-based QPI technologies and is suitable for high resolution QPI. [ABSTRACT FROM AUTHOR]

We report a CrISe monolayer as a room temperature ferromagnetic (FM) semiconductor with the Curie temperature (TC), magnetic anisotropy energy (MAE), and bandgap being 322 K, 113 μeV, and 1.76 eV, respectively. The TC and MAE can be further enhanced up to 385 K and 313 μeV by a tensile strain. Interestingly, the magnetic easy axis can be switched between off-plane and in-plane by compressive strain. Particularly, due to the broken inversion symmetry and strong spin–orbital coupling of Se atoms, a large Dzyaloshinskii–Moriya interaction (DMI) of 2.40 meV is obtained. More importantly, by micromagnetic simulations, stable skyrmions with sub-10 nm radius are stabilized by the large DMI above room temperature in a wide range of strain from −2% to 6%. Our work demonstrates CrISe as a promising candidate for next-generation skyrmion-based information storage devices and provides guidance for the research of DMI and skyrmions in room temperature FM semiconductors. [ABSTRACT FROM AUTHOR]

We investigated the thermal conductivity (TC) of a two-dimensional composite consisting of matrices with low TC and nanowire fillers with high TC using the Monte Carlo method and the finite element method. Compared to the electrical percolation that requires direct contact between fillers, an additional thermal transport channel through non-contact fillers is found to be important for the thermal percolation in composites. This additional channel can lead to an effective expansion of thermal percolation network by including extra fillers into the network. Thus, it can be utilized to enhance the TC of the composite that is helpful to improve the performance of thermal interface materials. [ABSTRACT FROM AUTHOR]

The fact that the thermoelectric performance is far inferior to that of p-type PbTe has inspired many strategies to develop n-type PbTe thermoelectrics. Alloying PbS in n-type PbTe effectively changes the shape of a valley to trigger a heavier conduction band for improving the Seebeck coefficient, while the resulting small orbital overlap inevitably leads to phase separation hindering electron transport. The effect of vacancies on the solubility of sulfur in n-type PbTe is ambiguous; especially, the heterostructure due to phase separation in high-content PbS-alloyed PbTe also requires sufficient modification to optimize the electroacoustic transport. This motivates the current work on the introduction of vacancies by charge-balancing doping via Sb2Te3 and discovers striking new insight that the introduced vacancies can induce a new heterostructure of Pb2Sb2S5 and suppress the aggregation of Sb and PbS in high-solubility n-type PbTe–PbS. The modification of the band structure and optimization of the electron transport give rise to a prominent enhancement in electronic performance. Furthermore, the Debye–Callaway model validates the dramatic contribution of vacancy aggregation and heterostructures to lattice thermal conductivity. As a result, the synergistic modulation of electroacoustic characteristics achieves a significant improvement in both the maximum zT and the near-room-temperature zT. Understanding such unique findings is critical for applicability to other thermoelectric materials. [ABSTRACT FROM AUTHOR]

Epitaxial thin films of cobalt ferrite (CoFe2O4) are grown on two isostructural substrates, (001)-oriented MgGa2O4 and ZnGa2O4, using pulsed laser deposition. The substrates have a lattice mismatch of 1.26% and 0.70% with bulk CoFe2O4 (CFO) crystal. We have systematically investigated the structural and magnetic properties of the epitaxial CFO films on these substrates. X-ray diffraction and transmission electron microscopy result analysis reveal that the films deposited on spinel ZnGa2O4 are essentially free of defects and are under a small compressive strain, while films on MgGa2O4 show partial strain relaxation along with defect formation. Room temperature magnetization data indicate that CFO grown on ZnGa2O4 substrates have a bulk-like saturation magnetization of 420 emu/cc and a uniaxial substrate-induced anisotropy value of − 22.9 × 10 6 erg/cm3 with an anisotropy field as low as 60 kOe. [ABSTRACT FROM AUTHOR]

Enhanced thermoelectric performance is restricted greatly by the interaction of various transport parameters, and this bottleneck urgently requires a solution. In this paper, first-principles calculations and Boltzmann transport theory are used to study the thermoelectric performance of two-dimensional (PbX) 2 (X = S , Se , Te) monolayers, and it is found that the thermoelectric performance can be enhanced significantly by applying a biaxial tensile strain. The room-temperature ZT values of the p-type (PbS) 2 , (PbSe) 2 , and (PbTe) 2 in zigzag (armchair) directions are boosted as high as 1.97 (1.35), 2.26 (1.31), and 2.45 (1.59), respectively. The results show that it is mainly attributed to the significantly reduced phonon thermal conductivity. Moreover, the sharply reduced phonon thermal conductivity is mainly due to the enhancement of the phonon scattering rate caused by strong phonon anharmonicity. In addition, the excellent ZT value of the p-type (PbX) 2 (X = S , Se , Te) monolayer exhibits their potential application in the thermoelectric field, and the external strain has a good prospect in enhancing the thermoelectric performance. [ABSTRACT FROM AUTHOR]

Non-Hermitian systems have gained a great deal of interest in various wave problems due their ability of exhibiting unprecedented phenomena such as invisibility, cloaking, enhanced sensing, or the skin effect. The latter manifests itself by the localization of all bulk modes in a specific frequency range at a given boundary, with an unconventional bulk-boundary correspondence. In this work, we propose to realize the skin effect for flexural waves in a non-Hermitian piezoelectric phononic beam with feedback control between a sensor and an actuator in each unit cell. By implementing a non-Hermitian parameter, effective gain and loss can be achieved in the phononic beam characterized by complex eigen frequencies, and non-reciprocal pass bands are obtained. We highlight that the split point separating the gain and loss areas can occur not only at the edges of the Brillouin zones but also inside the same Brillouin zone. We further analyze the influence of the geometric and non-Hermitian parameters on the complex dispersions and the split point. The topology of the complex bands is characterized by the winding number, which supports the skin effect together with the non-reciprocity. The localization degree of the skin mode manifested by the enhanced beam's vibration energy at one boundary is related to the strength of the non-reciprocity, and the skin mode can be always excited regardless of the source position. Our results provide a potential platform to introduce non-Hermiticity into phononic or metamaterial systems with novel functions for elastic waves such as topological insulators, vibration attenuation or amplification, and energy harvesting. [ABSTRACT FROM AUTHOR]

Optical cooling techniques of solid-state refrigerators, especially those toward the cryogenic temperature range, have attracted considerable attention in the fields of space exploration, precise measurement, material sciences, and so forth. Here, we report the laser cooling of the 7.5% Yb3+-doped LuLiF4 crystal down to 121 K reaching NIST's designated range of cryogenic temperatures (<123 K). Further results based on the cooling window indicate a promising cooling limit of 59 K, provided with enhancement in pump absorbance and heat load management of the sample. Our work, therefore, can motivate an all-solid-state optical refrigeration application beyond the liquid nitrogen boiling point, thus bringing great opportunity to realize cryogenic coolers and radiation-balanced lasers in miniaturized systems. [ABSTRACT FROM AUTHOR]

We report on time-resolved broadband transient reflectivity (tr-bb-TR) measurements performed on a bulk single crystal of Td-MoTe2 as a function of the incident pump fluence (F). Tr-bb-TR data unveil photoinduced electronic changes progressing on the sub-picosecond timescale as well as the dynamics of the coherent low-frequency 1A1 interlayer shear phonon. Our results indicate a gradual evolution of both the TR and the 1A1 Fourier intensity spectra as a function of F, ruling out the threshold-like change that has been associated with the ultrafast photoinduced Td → 1T ′ phase transition. We also observe a large redshift of the 1A1 Fourier spectral features, which implies that large renormalization effects are taking place on the interband transitions that are dielectrically susceptible to the 1A1 interlayer shear phonon displacement. [ABSTRACT FROM AUTHOR]

Neuromorphic computing has the potential to accelerate high performance parallel and low power in-memory computation, artificial intelligence, and adaptive learning. Despite emulating the basic functions of biological synapses well, the existing artificial electronic synaptic devices have yet to match the softness, robustness, and ultralow power consumption of the brain. Here, we demonstrate an all-inorganic flexible artificial synapse enabled by a ferroelectric field effect transistor based on mica. The device not only exhibits excellent electrical pulse modulated conductance updating for synaptic functions but also shows remarkable mechanical flexibility and high temperature reliability, making robust neuromorphic computation possible under external disturbances such as stress and heating. Based on its linear, repeatable, and stable long-term plasticity, we simulate an artificial neural network for the Modified National Institute of Standards and Technology handwritten digit recognition with an accuracy of 94.4%. This work provides a promising way to enable flexible, low-power, robust, and highly efficient neuromorphic computation that mimics the brain. [ABSTRACT FROM AUTHOR]

Low direct-current (DC) conductivity is one of the most desired characteristics for crosslinked polyethylene (XLPE) as a high-voltage DC cable insulation material. In this Letter, a correlation between the DC conductivity and cross-linking characteristics of XLPE-polystyrene (PS) composites at 50 °C was studied. Experimental results show that by adjusting the cross-linking structure, different trap distribution characteristics for XLPE-PS composites were realized. With the increase in the cross-linking agent content, DC conductivities of XLPE-PS composites tend to decrease, and the introduced average trap depth increases correspondingly. An increase of 0.07 eV for average charge trap depth in XLPE-PS composites could be acquired in the test range. It is considered that the increase in the average trap depth reduces the carrier mobility, contributing to the reduction of DC conductivity for XLPE-PS composites. Thus, the DC conductivity and average trap depth of XLPE composites show a strong relevance. The results suggest that the interaction between PS particles and the matrix introduced by cross-linking plays a dominant role in determining the charge conduction for XLPE-PS composites. [ABSTRACT FROM AUTHOR]

We report the spectral dependence of the three-photon absorption coefficient γ and third-order nonlinear refractive index n2 in pristine, Sn-doped and Fe-doped β-Ga2O3 crystals using 190 fs pulses in the range of 540–750 nm, i.e., photon energy between half and one-third of the bandgap Eg. The nonlinear coefficient n2 is found to be independent of Sn or Fe doping, and the maximum value of n2 is about 4 × 10−19 m2/W when the photon energy is near Eg/2. However, we observe that Fe doping can significantly enhance the γ value in the range of 540–600 nm (near Eg/2) while doping has negligible impact on γ for wavelength larger than 620 nm. Femtosecond pump-probe measurements show that Fe doping induced a fast (∼50 ps) carrier decay process after photo excitation, which could be attributed to the Fe induced deep trap states. We obtain three-photon figures of merit that allow operational intensities up to 100 GW/cm2. Our results indicate that β-Ga2O3 has great potential for all-optical switching applications in red and near IR regimes. [ABSTRACT FROM AUTHOR]

High-speed optical communication systems are built for real-time, massive, and remote information exchange; however, it is strongly reliant on the applied power. Herein, we developed a self-driven optical communication system based on a high-performance graphene/n-Si (Gr/n-Si) hybrid photodetector. Under zero bias, the Gr/n-Si device presents good performance at a wavelength of 520 nm, including the photoresponsivity of 0.27 A W−1, specific detectivity of 9.39 × 1011 Jones, and on/off ratio of 104 with a rise and fall time of 128 and 131 ns, respectively. This hybrid device also exhibits 3 dB bandwidth of 2.18 MHz as well as a small noise equivalent power of 1.68 × 10−17 W Hz−1/2. Furthermore, an optical communication system was constructed based on this hybrid photodetector, through which the audio and text signals were steadily and accurately transmitted under zero bias. Our work lays a solid foundation to demonstrate a promising application of Gr/n-Si hybrid devices toward self-driven optical communications. [ABSTRACT FROM AUTHOR]

Metal–semiconductor junctions are at the heart of modern electronics. Various two-dimensional materials (2D) based electronic devices have been reported. However, Ohmic contacts between evaporated metal contacts and semiconducting 2D materials are still hard to achieve. Thus, design and fabrication of Schottky diodes based on layered WSe2 remain challenging. Here, we report a doping-free strategy to achieve Ohmic contacts in WSe2 diodes via van der Waals (vdW) contacts. We designed and fabricated vertical WSe2/Pd Schottky diodes, in which Ohmic and Schottky junctions can be realized simultaneously via vdW contacts and evaporated contacts, respectively. The specific contact resistance of the vdW contact is about 74.5 kΩ·μm2. The Schottky diodes exhibit strong rectification behavior with rectification ratio up to 105. [ABSTRACT FROM AUTHOR]

We propose a type of thermal interface materials incorporating insulating nanowires with partially metallic coating in insulating polymer matrix. Large thermal conductivity can be obtained due to thermal percolation while the electrical insulation is maintained by controlling C M φ < φ c e and φ > φ c th , where φ is the volume fraction of fillers, C M is the metallic coating fraction, and φ c e and φ c th are the electrical and thermal percolation thresholds, respectively. The electrical conductivity of such composite materials can further be regulated by coating configuration. In this regard, we propose the concept of "thermal-percolation electrical-insulation," providing a guide to design efficient hybrid thermal interface materials. [ABSTRACT FROM AUTHOR]

Ferroelectric thin films have been systematically investigated via scanning probe microscopy in recent years. Research indicates that the surface potential is the combined result of injected and polarization charges. The relationships between surface potentials and the two types of charges are usually investigated via scanning Kelvin probe microscopy (SKPM). Typically, SKPM investigations indicate that the surface charge distribution is dominated by injected charges trapped during poling using a conductive AFM tip rather than by polarization charges. The presence of injected charges leads to controversy concerning the origins of resistive switching behavior. In this study, relaxation of injected charges was observed during an optimized thermal treatment. This caused polarization charges to dominate over injected charges. Different electroresistance switching characterizations were observed via conductive atomic force microscopy (C-AFM) of injected and polarization charge-dominated films. Our research extends the methods of distinguishing whether electroresistance switching behavior is driven by charge trapping/detrapping or ferroelectric polarization. This provides an effective approach to classifying the origins of electroresistance switching in ferroelectric thin films by combining piezoelectric force microscopy, SKPM, and C-AFM. [ABSTRACT FROM AUTHOR]

Strain engineering of noble metals may optimize their adsorption properties and catalytic performance. On the other hand, while it has been shown that lattice strain may change surface reconstruction for some crystals, it remains to be seen whether strain-induced surface phase transition will modify the adsorption and catalysis properties. In this contribution, we address this issue by density functional methods. We show that 4d elements of Rh, Pd, and Ag change from unreconstruction to 1 × 2 missing row reconstruction under negative strains, while 5d ones including Ir, Pt, and Au change from reconstruction to unreconstruction under positive strains. Strain-induced surface phase transition results in abrupt changes in hydrogen adsorption strength. The present results imply that it is important to consider possible modifications in surface reconstruction to understand catalytic performance of strained noble metals. [ABSTRACT FROM AUTHOR]

It is widely believed that the undoped Mott insulator is crucial to understand the origin of high-TC superconductivity and the complex phase diagram in cuprates. Tetragonal CuO, the simplest structure among cuprates, is regarded as an archetypal model to explore the ground state physics of Mott insulator. Here, we report on the epitaxial growth and surface characterization of tetragonal CuO thin film by combining ozone-assisted molecular beam epitaxy and multi-analytical techniques. In situ scanning tunneling microscopy measurement reveals two sets of Cu2+ square lattices with a reversed contrast of their apparent heights between neighboring domains. A spectroscopy study demonstrates an upper bound of the charge transfer gap as 3.68 eV, and quantitatively agrees with the previous photoemission and x-ray photoelectron results. Our work will deepen the understanding of high-energy electronic structures in parent cuprates. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a method for reconstructing the topology of optical vortex lines by utilizing digital holography. Compared with the traditional methods that need complicated spatial scanning procedure, the proposed method only requires a single exposure of hologram. It enables this method to be particularly well suited to observe the dynamic change of vortex topology. With the method, we experimentally reconstructed the dynamically rotating three-dimensional topology. These results are expected to provide a powerful tool for the experimental observation of the dynamical evolution of light fields and can be used to improve efficiency in information demodulation of topology carrying beams. [ABSTRACT FROM AUTHOR]

Conductive polymers that combine the advantages of traditional polymers and organic conductors have recently received more and more attention in applications such as sensors, smart electronics, display, and energy conversion/storage. Interface properties including conductivity, mechanical properties, and mass transport/immobilization are key factors affecting the performance of these devices. Doping engineering provides an effective method to enhance interface properties of conducting polymers and bring new functions for practical applications. In this Perspective, we review some application examples of doping-tunable interfacial properties of conducting polymers with advanced features, i.e., enhancing conductivity by doping; tunable self-assembly of conductive polymers; structure-derived elasticity and its application in flexible pressure sensors; and enhanced interfacial transport and its applications in biosensors and lithium ion battery. Furthermore, this Perspective also discusses the challenges of this field and some possible solutions. [ABSTRACT FROM AUTHOR]

Threshold voltage (VTH) instability has been studied in the as-fabricated p-GaN gated enhancement-mode high electron mobility transistors (p-GaN E-HEMTs) under a positive gate stress. A negative VTH shift (ΔVTH) obtained by dynamic measurement has been observed more severely when compared to the static one. The VTH deviation is attributed to the complicated influence of carrier transport behaviors in the p-GaN gate. The impacts of hole accumulation, trapping, and consumption on the VTH instability and drain current variation can be effectively distinguished according to the transfer characteristics obtained from the pulse I–V measurement. Moreover, the density of hole traps in the p-GaN gate is estimated to be around ∼2 × 1011 cm−2 by the capacitance–voltage measurement, and the energy level is calculated to be around EV + 0.62 eV by fitting the recovery curve of gate current after positive gate bias. This study focusing on the in-depth influence of different carrier behaviors on the gate performance can help with the understanding and further improvement of the dynamic instability and reliability of the GaN-based HEMTs. [ABSTRACT FROM AUTHOR]

Third-order elastic constants (TOEC) play an important role in nonlinear material characterization, but measurements of TOEC are laborious with large error margins. This Letter presents the equations of wave velocity changes caused by homogeneous temperature variation and uniaxial stress in isotropic media and the expression of TOEC in terms of thermally induced velocity change and thermal strain. TOEC of an aluminum sample were experimentally determined by measuring ultrasonic wave velocity changes in the uniaxial loading test and the thermal modulation test. Experimental results showed good agreement between the two test methods. Owing to the simple test setup and high measurement sensitivity, the thermal modulation test is a potential experimental method to determine TOEC and absolute acoustic nonlinearity parameters. [ABSTRACT FROM AUTHOR]

This Letter studies the reverse leakage and breakdown mechanisms of vertical GaN-on-Si Schottky barrier diodes (SBDs) with and without argon-implanted termination (ArIT). The electrical leakage characteristics in the vertical GaN-on-Si SBD without edge termination sequentially go through the thermionic field emission, variable range hopping (VRH), and trap-assisted tunneling conduction mechanisms as the reverse bias increases gradually. Its leakage and breakdown mechanisms are limited by the edge electric field crowding effect. While for the vertical GaN-on-Si SBD with ArIT (ArIT-SBD), the electrons conduction at a low reverse bias, following the space-charge-limited conduction (SCLC) model, is limited by the damage-induced traps in the implanted GaN. As the reverse bias increases up to the occurrence of breakdown, the VRH and SCLC dominate the leakage mechanism of the ArIT-SBD, which stem from intrinsic traps in GaN grown on Si. A rapidly growing leakage under a low reverse bias and enhanced breakdown voltage performance in the ArIT-SBD is attributed to the charging of the damage-induced traps in implanted GaN. This Letter not only gives in-depth insights of vertical GaN-on-Si SBDs but also provides a useful design guidance of implanted termination for high-voltage power devices. [ABSTRACT FROM AUTHOR]

Cubic heteroepitaxial (001) SrMnO3-δ (SMO) films were grown on SrTiO3 substrates by atomic alternating layer molecular beam epitaxy. Precise control of cation stoichiometry was achieved by in situ reflection high-energy electron diffraction (RHEED) oscillation. During SMO film growth, a correlation between RHEED oscillation features and the cation stoichiometry/monolayer dose was established. In non-stoichiometric films, there were out-of-plane lattice expansions due to off-stoichiometry-dependent defects, including Ruddlesden–Popper SrO planar faults in Sr-rich films and Sr vacancy defect clusters in Mn-rich films. [ABSTRACT FROM AUTHOR]

Inspired by the contact-separation mode triboelectric nanogenerator (TENG), we propose a technique for local surface charge density measurement based on atomic force microscopy. It is named as scanning TENG, in which a conductive tip tapping above a charged dielectric surface produces an AC between the tip and the dielectric bottom electrode due to electrostatic induction. The Fourier analysis shows that the amplitude of the first harmonic of the AC is linearly related to surface charge density. The results demonstrate that the scanning TENG is a powerful tool for probing nanoscale charge transfer in contact-electrification. [ABSTRACT FROM AUTHOR]

In this work, the authors propose a graphene-based tunable perfect metamaterial absorber at microwaves that can achieve perfect absorption at all angles. This is realized by stacking an unpatterned graphene sandwich structure and a dielectric layer on top of a mushroom-type high impedance surface whose resonant frequency is stable for all angles of incidence. A perfect-absorption-angle tunable absorber working at TM polarization is designed and realized, achieving perfect absorption covering a wide range of angles from normal to grazing incidence at the same frequency when the impedance of graphene is modulated by different bias voltages. The design concept of integrating different tunable materials and meta-structures opens up promising possibilities for wave manipulation with applications in smart and multifunctional electromagnetic devices. [ABSTRACT FROM AUTHOR]

Nickel ferrite (NiFe2O4) thin films are grown epitaxially on two different isostructural substrates, (100)-oriented MgGa2O4 and ZnGa2O4, using pulsed laser deposition. These spinel substrates have a lattice mismatch of 0.62% and 0.04%, respectively, with NiFe2O4 crystal. While the films grown on MgGa2O4 substrates exhibit significant strain resulting in a tetragonal distortion of the crystal structure, the films on ZnGa2O4 substrate are essentially strain-free and retain their cubic structure because of the near-perfect lattice match. Magnetometry data suggest that film strain is the principal factor determining the anisotropy of these NiFe2O4 films. This is also confirmed by the effective magnetization values obtained from ferromagnetic resonance (FMR) measurements. While there is only a modest decrease in effective Gilbert damping constant with strain reduction, an enhancement in the spin voltage is observed in the spin Seebeck effect (SSE) measurements for NiFe2O4 films grown on ZnGa2O4 substrate with thickness ≤ 200 nm. [ABSTRACT FROM AUTHOR]

The Yb3+:YAG crystal has been one of the most widely used active media in the solid-state lasers of high power, mainly thanks to its excellent thermal, mechanical, and optical properties. Thermal effect due to heat deposition in the active medium, however, greatly deteriorates the beam quality of the laser output and sets a limit on its maximum power available. Although the cooling proposal of anti-Stokes fluorescence can help realize the heat-free high-power lasers with good beam quality, so-called radiation-balanced lasers, there is no substantial advancement in the optical cooling of Yb3+:YAG crystals since its latest experimental report with a temperature drop of about 9 K. Here we demonstrate experimentally a remarkable temperature drop of about 80 K in a 3% Yb3+-doped YAG single crystal pumped by a fiber laser at 1030 nm. Further analysis predicts that the cooling limit of the titled crystal can reach as low as 180 K from the room temperature. Our work therefore reveals a key pathway to facilitate the optical refrigeration of the Yb3+:YAG crystal down to the thermoelectric cooling limit, thus offering a unique entry point to practical radiation-balanced lasers. [ABSTRACT FROM AUTHOR]

As an emerging phenomenon in oxide heterostructures, the ferroelectric control of electronic transport is attracting considerable research attention. However, the effect of ferroelectric polarization on resistive switching (RS) remains controversial. In this study, to determine the effects of ferroelectric polarization on memory characteristics, ferroelectric and non-ferroelectric BaTiO3 (BTO) films were fabricated using different oxygen partial pressures (Po2) during film growth. A correlated electron oxide, SmNiO3 (SNO), was selected as the bottom electrode. Based on piezoelectric force microscopy, ferroelectricity was verified in the BTO films prepared at Po2 ≥ 3 Pa, whereas the films fabricated at lower Po2 did not exhibit ferroelectricity. Remarkable RS with an ON/OFF ratio of 104% was clearly observed in heterostructures containing ferroelectric BTO, while RS was negligible in structures with non-ferroelectric BTO. The strong ferroelectricity dependence of RS behavior on the BTO/SNO heterostructures was attributed to the ferroelectric control of device transport between bulk-limited current in the low-resistance state and interface-limited conductance in the high-resistance state, which results from the modulation of the potential barrier at the BTO/SNO interface. The findings provide strong evidence for the dominant effect of ferroelectric polarization of BTO on the transport properties of BTO/SNO heterostructures. [ABSTRACT FROM AUTHOR]

We present a laser machining method for fabricating an all-fiber pillar-in-bubble Fabry-Perot interferometer (FPI), which is used for strain sensors with high sensitivity. The micro-structure of the air-bubble is fully controllable, especially the cavity length and sidewall thickness. The measured sensitivity of this strain sensor is as high as 56.69 pm/με, which is several times higher than that of most FPI strain sensors reported to date. This sensor also has a low-temperature sensitivity of 0.682 pm/°C, reducing the cross-sensitivity between tensile strain and temperature to 0.012 με/°C. Furthermore, such a sensor has the benefits of flexible design, simple fabrication, and high reproducibility, making it attractive for practical applications. [ABSTRACT FROM AUTHOR]