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We present an optical inference platform based on the nonlinear interaction of a spatially modulated optical waveform in a second-harmonic-generation nonlinear crystal. Tuning the phase-matching condition allows for both efficient nonlinearity, required for general all-optical computing, and for optimizing the system performance, matching it to the spectral distribution of the input data to be processed. [ABSTRACT FROM AUTHOR]

We report on a cascade nanosecond CdSe optical parametric oscillator with an output energy of 0.54 mJ at 100 Hz, temperature tuning within 9245–9285 nm, and a narrow (∼6 cm−1) spectral width achieved by a volume Bragg grating acting on the first stage. The CdSe crystal is pumped intracavity by the signal wave from the PPKTP-based first stage, and both stages operate under noncritical phase-matching conditions. The simple, compact, and robust setup produced the highest overall conversion efficiency for an all-solid-state laser system based on CdSe with 1-μm pumping, and the corresponding quantum conversion efficiency exceeded 13%. [ABSTRACT FROM AUTHOR]

Recently, quasi-one-dimensional van der Waals crystal Ta2PdS6 has been reported as a promising thermoelectric material with an extraordinarily high power factor. However, element doping to tune the thermoelectric properties has not been studied yet. Here, we systematically investigated the effect of Se doping on the phase composition, charge transport properties and thermoelectric performance of Se-doped Ta2Pd(S1−xSex)6 (x = 0, 0.02, 0.05, and 0.07) polycrystalline bulk materials. Upon doping Se at the S sites to increase the carrier concentration and mobility, the electrical conductivity of Ta2Pd(S0.93Se0.07)6 is dramatically enhanced, while the S is slightly reduced, yielding a significantly improved power factor compared to that of pristine Ta2PdS6. Consequently, Ta2Pd(S0.93Se0.07)6 exhibits a peak ZT of 0.29 at 700 K when the doping content x = 0.07, which is more than twice that of pristine Ta2PdS6. [ABSTRACT FROM AUTHOR]

Recently, quasi-one-dimensional van der Waals crystal Ta2PdS6 has been reported as a promising thermoelectric material with an extraordinarily high power factor. However, element doping to tune the thermoelectric properties has not been studied yet. Here, we systematically investigated the effect of Se doping on the phase composition, charge transport properties and thermoelectric performance of Se-doped Ta2Pd(S1−xSex)6 (x = 0, 0.02, 0.05, and 0.07) polycrystalline bulk materials. Upon doping Se at the S sites to increase the carrier concentration and mobility, the electrical conductivity of Ta2Pd(S0.93Se0.07)6 is dramatically enhanced, while the S is slightly reduced, yielding a significantly improved power factor compared to that of pristine Ta2PdS6. Consequently, Ta2Pd(S0.93Se0.07)6 exhibits a peak ZT of 0.29 at 700 K when the doping content x = 0.07, which is more than twice that of pristine Ta2PdS6. [ABSTRACT FROM AUTHOR]

High-temperature piezoelectric vibration sensors play a crucial role in the accurate monitoring of the dynamic mechanical conditions in aerospace, automotive, and energy generation systems. However, the use of conventional piezoelectric materials in high-temperature environments is restricted owing to their limited Curie temperatures. In this study, we grew a piezoelectric crystal Bi12SiO20 (BSO) with the crystal cut optimized for high longitudinal piezoelectric coefficient and low piezoelectric crosstalk behaviors. Subsequently, a compression-type piezoelectric vibration sensor utilizing the BSO bulk crystal was developed and fabricated for structural health monitoring under high temperatures. The impact of pre-tightening torques on the sensor performance was investigated. Moreover, the sensor performance was analyzed under temperatures up to 650 °C. The BSO-based sensor exhibited an average sensitivity of ∼3.89 pC/g between 25 and 650 °C under 160 Hz frequency, with a variation of 5.5%. Additionally, the BSO-based sensor demonstrated ultra-stable sensitivity at 600 °C, highlighting its strong sensing capabilities and reliability under high temperatures. Thus, the BSO-based vibration sensor is a promising option for structural health monitoring applications under high temperatures. [ABSTRACT FROM AUTHOR]

In this Letter, we investigate coherent phonon dynamics in the incommensurate LaVS3 crystal by femtosecond pump-probe spectroscopy. Two coherent phonon modes are systematically observed in the transient reflectivity, centered at 1.8 and 2.85 THz, respectively, while a third mode centered at 4.5 THz is observed only at high pump fluence. The experimental results obtained at two different polarization configurations as well as a comparison with recent theoretical results allow to assign the two main modes to the interlayer shearing mode and to an intralayer mode, respectively. Two possible assignments are discussed for the third mode, by invoking a possible emergence of nonlinear phonon processes. [ABSTRACT FROM AUTHOR]

The external quantum efficiency (EQE) and internal quantum efficiency (IQE) of radiation are quantified by omnidirectional photoluminescence measurements using an integrating sphere for two types of GaN crystals with different carbon concentrations ([C] = 1 × 10 14 cm − 3 , 2 × 10 15 cm − 3 ). In the sample with lower [C], when the excitation density is 140 W cm − 2 , the EQE and IQE for near-band-edge (NBE) emission are 0.787% and 21.7%, respectively. The relationship between [C] and the IQE for NBE emission indicates that carbon impurities work as effective nonradiative recombination centers (NRCs) in n-type GaN, and major NRCs switch from carbon impurities to intrinsic NRCs, such as vacancies, when [C] falls below 3.5 × 10 14 cm − 3 . [ABSTRACT FROM AUTHOR]

Defect engineering via doping exhibits considerable potential for improving the performance of environment-friendly lead-free piezoelectric materials. Owing to the susceptibility to lattice vibrations and the micro-local chemical environment, the readily available Mn/Fe transition metal elements (TMEs) facilitate the construction of defect structures. However, the role of TMEs in shaping the domain structures and the corresponding promotional mechanism of piezoelectricity need to be further decoded. Herein, we propose the different influence mechanisms of Mn and Fe ions on the ferroelectric domain and piezoelectric properties. Different concentrations of (M n Nb/Ta ′ - V O • • ) • and (F e Nb/Ta ″ - V O • • ) × defect dipoles are obtained based on the synergy of Mn/Fe ions with oxygen vacancies. Diverse ferroelectric behaviors resulting from (M n Nb/Ta ′ - V O • • ) • and (F e Nb/Ta ″ - V O • • ) × defect dipoles are observed. Furthermore, the variation of the dielectric diffusiveness with the defect dipole concentration is investigated. Trace concentration of (M n Nb/Ta ′ - V O • • ) • generates strong diffusiveness. With the characterization of the ferroelectric domain, this strong diffusiveness is attributed to the lattice-like domain structure. Thereafter, the mechanisms of Mn/Fe defect dipoles on the formation of domain structures are clarified. Macroscopically, the dielectric and piezoelectric properties are measured with Mn/Fe ion components. Trace Mn doping and the resulting lattice-like domain significantly enhance the piezoelectric coefficient, resulting in an increase of nearly 50% for K(Ta,Nb)O3 single crystals. This work highlights the tremendous potential of TME-induced defect dipoles for modifying the ferroelectric domain and provides a reference paradigm for improving piezoelectricity through defect engineering. [ABSTRACT FROM AUTHOR]

To investigate the carrier recombination processes in GaN crystals grown by the low-pressure acidic ammonothermal (LPAAT) method, the photoluminescence (PL) spectra and PL lifetimes of LPAAT GaN crystals grown on acidic ammonothermal (AAT) GaN seed crystals were correlated with the growth polarity and species/concentration of point defects. The PL spectra of LPAAT GaN grown toward the (000 1 ¯ ) direction (−c region), which provided the highest growth rate, exhibited a predominant near-band edge (NBE) emission. Neither bandgap narrowing nor Burstein–Moss shifts due to high concentration residual impurities were observed in the NBE emissions, indicating higher purity than the previously reported AAT GaN crystals. In addition, strain-induced energy shift or energy broadening of excitonic emission peaks was not observed, indicating excellent crystal coherency. Because of the reduced concentration of midgap recombination centers, a record-long room-temperature PL lifetime for the NBE emission of ammonothermal GaN (40 ps) was obtained from the −c region. Meanwhile, the PL spectra also exhibited the yellow and blue luminescence bands originating from particular deep-state radiative recombination centers. The major vacancy-type defects acting as midgap recombination centers are identified as vacancy complexes comprising a Ga vacancy (VGa) and a few N vacancies (VN), namely, VGa(VN)n buried by H and/or O, where n is an integer. Further reduction of such defect complexes will allow less compensated stable carrier concentration in the LPAAT GaN crystals. [ABSTRACT FROM AUTHOR]

Recently, researchers have devoted their intense efforts to investigating Weyl physics in synthetic space. In this Letter, we study the intriguing topological rotated Weyl physics in a three-dimensional parameter space, which consists of two extra structural parameters and the wave vector of a simple one-dimensional sonic crystal. In our ultrasonic experiments, we observe that the topological interface states propagate along the interface formed by two sonic crystals of distinct chirality caused by the rotated Weyl points. We detect the rotated synthetic Weyl points and measure the singularities of the reflection phase, which results in the robustness of the interface states. At the same time, it also shows the advantages of synthetic dimensions in exploring high-dimensional physics problems in low-dimensional systems. [ABSTRACT FROM AUTHOR]

Here, we report the results of strain-controlled spin-wave propagation regimes in a double-period multiferroic structure. It consists of an alligator-type magnonic crystal with a period of 250 μm and a piezoelectric layer, featuring a periodic counter-pin-type electrode system with a period of 125 μm. Employing microwave measurements, we acquired the transmission and dispersion of spin waves under various external electric field configurations applied to the piezoelectric layer. The formation of bandgaps in the magnon spectrum and the variation of the spin-wave transmission when altering the configurations of the external electric field are demonstrated. A finite element method reveals that the combination of the non-uniformity in the initial internal magnetic field of the magnonic crystal, which is caused by the presence of periodic alligator-type regions, together with elastic deformations, heightens the amplitude of the modulation of the internal magnetic field. Micromagnetic modeling has demonstrated that this modulation enhancement results in the variation of the spin-wave transmission at the frequency of the magnonic bandgap center of the magnonic crystal. The proposed design of the reconfigurable magnonic crystal creates a condition for the nucleation of the spin-wave bandgap, with further enhancement of the spin-wave reflection from the periodic grating induced by strain. We demonstrate the potential use of the proposed device as a multi-band NAND/NXOR spin-wave based logic gate. [ABSTRACT FROM AUTHOR]

We demonstrate an experimental and theoretical comparison of non-oxide LiGaS2, HgGa2S4, and AgGaS2 crystals performance for wavelength conversion into the near and mid-IR range 1.5–8 μm in optical parametric amplifier pumped by Cr:Forsterite laser, delivering 100 fs pulses at 1.24 μm. It is shown that exceptionally high total energy conversion efficiency into the idler (4–5 μm) and signal (1.65–1.8 μm) waves up to 18% can be achieved using the HGS crystal, providing high nonlinearity, while the LGS crystal is more preferable for generating few-cycle mid-IR pulses due its unique dispersive properties. Our source features high peak power in gigawatt regime (0.4–2.4 GW) with pulse duration below 80 fs and optical synchronization with high harmonic generation (HHG) and THz beamlines, which is ideal for pump-probe experiments of nonlinear and strong-field physics. [ABSTRACT FROM AUTHOR]

We report on measurements of the anisotropy of velocities and attenuation of GHz-frequency acoustic phonons in a cubic MgO crystal at room temperature. They are used to quantify the strong anisotropy of the Grüneisen parameter and calculate the attenuation anisotropy for Rayleigh surface acoustic waves. These observations constitute important building blocks for better understanding of ultrafast laser-based magneto-acoustic and nonlinear acoustic experiments at ultrahigh frequencies. [ABSTRACT FROM AUTHOR]

Phonons, as quantized vibrational modes in crystalline materials, play a crucial role in determining a wide range of physical properties, such as thermal and electrical conductivity, making their study a cornerstone in materials science. In this study, we present a simple yet effective strategy for deep learning harmonic phonons in crystalline solids by leveraging existing phonon databases and state-of-the-art machine learning techniques. The key of our method lies in transforming existing phonon datasets, primarily represented in interatomic force constants, into a force–displacement representation suitable for training machine learning universal interatomic potentials. By applying our approach to one of the largest phonon databases publicly available, we demonstrate that the resultant machine learning universal harmonic interatomic potential not only accurately predicts full harmonic phonon spectra but also calculates key thermodynamic properties with remarkable precision. Furthermore, the restriction to a harmonic potential energy surface in our model provides a way of assessing uncertainty in machine learning predictions of vibrational properties, essential for guiding further improvements and applications in materials science. [ABSTRACT FROM AUTHOR]

Higher-order topological insulators exhibit intriguing capacity to confine energy on lower-dimensional boundaries owing to the unique bulk-boundary correspondence. Many potential practical applications of the higher-order states in classical wave systems have been proposed and achieved. In this work, we implement a second-order topological insulator in a waterborne acoustic crystal by drilling and grooving a copper plate based on a two-dimensional Su–Schrieffer–Heeger lattice with different intracell and intercell couplings. The far-field transmission spectrum and near-field pressure field distributions verify the existence of the one-dimensional edge states and zero-dimensional corner states in the bulk bandgap. Due to the highly localized edge and corner states, the polystyrene particles are trapped at the edges and corners by the acoustic gradient force. Our findings provide a good platform to manipulate underwater acoustic wave and may inspire topological acoustic applications. [ABSTRACT FROM AUTHOR]

Exploring efficient and stable long-persistent luminescence (LPL) materials is of great value for promoting their advanced applications. Here, the metal halide CsCdCl3:Mn2+ crystals with tunable afterglow properties and good stability were grown by a facile solution method. Upon ultraviolet excitation, CsCdCl3:Mn2+ crystals exhibited a bright orange emission at 590 nm from the d–d transition of Mn2+ ions. Mn2+ doping concentrations matter for the LPL performance. By optimizing the doping amount of Mn2+, an enhanced afterglow duration up to 12 000 s was achieved, compared with undoped sample, originating from a trap redistribution. The deep traps in CsCdCl3:0.1Mn2+ crystal that provide little contribution to the LPL at room temperature shifted to shallow levels, thus synergistically enhancing the afterglow intensity and duration. Based on the variable afterglow durations by tuning Mn2+ doping concentrations, a multi-dimensional information storage encryption model was designed. This work gives deep understanding in doping effect on the afterglow and provides examples for the development of multi-dimensional information encryption. [ABSTRACT FROM AUTHOR]

Thermoelectrics (TEs) have been considered a sustainable and eco-friendly energy technology. However, due to limitations in energy conversion efficiency, TE devices have not yet been widely adopted. Here, we proposed a class of TE materials, trigonal XPt2Se3 (X = K, Rb), with the same crystal structure as Bi2Te3. At room temperature, with quartic anharmonicity correction, the lattice thermal conductivity (κL) of KPt2Se3 and RbPt2Se3 in the x-direction is only 0.57 and 0.46 W m−1 K−1, respectively. The ultralow κL arises from their layered structure, strong lattice anharmonicity, weak bonding nature, rattling motion of guest alkali metal atoms, and large scattering space. Simultaneously, the large density of states contributes to large power factors. At 800 K, both under n-type and p-type doping, KPt2Se3 exhibits ZT values that can exceed 4 in specific directions, while RbPt2Se3's ZT values can surpass 3, which is significantly higher than traditional TE materials. Our research not only elucidates that the layered trigonal crystals XPt2Se3 (X = K, Rb) represent a category of potential TE materials with ultralow κL and high TE performance but also provides directions for exploring TE materials. [ABSTRACT FROM AUTHOR]

A layered van der Waals ferromagnet CrGeTe3 has attracted considerable interest due to the great potential for exploring low-power functional spintronic devices. However, research on the incorporation of chemical substitution in CrGeTe3 to tune the lattice vibrations and magnetic interactions is still lacking. In this work, an understanding of the phonon property and magnetic orders in Mn lightly doped Cr1−xMnxGeTe3 single crystals has been reported. It is found that the phonon symmetries are preserved over the studied Mn substitution content, as confirmed by angle-resolved polarized Raman spectroscopy and first-principles calculations. Furthermore, clear signatures of improved ferromagnetic Curie–Weiss temperature and magnetic moment are observed by detailed magnetic measurements. The higher magnetic moment of Mn ions, the variation of Mn–Te–Cr angle, Mn-induced hole doping, and the concomitant strain effect may all contribute to the enhanced ferromagnetism. Overall, our results demonstrate that chemical substitution is an effective pathway to engineering the lattice and magnetism of CrGeTe3. [ABSTRACT FROM AUTHOR]

Symmetry and non-Hermiticity play pivotal roles in photonic lattices. While symmetries, such as parity-time (P T) symmetry, have attracted ample attention, more intricate crystalline symmetries have been neglected in comparison. Here, we investigate the impact of the 17 wallpaper space groups of two-dimensional crystals on non-Hermitian band structures. We show that the non-trivial space group representations enforce degeneracies at high symmetry points and dictate their dispersion away from these points. In combination with either T or P T , the symmorphic p4 mm symmetry and the non-symmorphic p2mg, p2gg, and p4gm symmetries protect exceptional chains intersecting at the pertinent high symmetry points. [ABSTRACT FROM AUTHOR]

We measured and simulated the spectrally resolved angular distributions of 0.3–1-THz emission from the two-color filament with its plasma length (∼40 mm) exceeding the dephasing length (∼25 mm) between the fundamental (740 nm) and the second harmonic (370 nm) pulses in air. We show that only the forwardly propagating on-axis terahertz (THz) radiation is sensitive to the variation of the phase offset φ between fundamental and second harmonics, while the ring-like THz beam carrying ≳ 80 % of the overall THz yield is independent of φ. Utilization of the THz ring allows one to omit the tedious adjustment of the frequency-doubling crystal position in the experiment. [ABSTRACT FROM AUTHOR]

One of the most important features of organic cocrystals is the interaction between electrons and lattices, which brings interesting physical phenomena. In this work, we prepared two isomeric crystals in which the isomeric donors are used to fabricate charge transfer crystals. Isomeric crystals present different electron–lattice coupling strengths to further affect the charge localization, which leads to different dielectric constants and fluorescence lifetimes. Furthermore, external magnetic field and electric field exhibit different tunability on the photoluminescence and dielectric constants of the isomeric cocrystals due to the significant differences in the coupling between the charge and lattice vibration. Overall, through fabricating isomeric organic crystals, neighbor molecular vibration dependence of electron–lattice coupling is studied, which provides a platform to further understand the lattice vibration dependent spin and optics. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) spectra of bulk α-In2Se3 crystals before and after 10 MeV electrons irradiation with the 1015 and 1017 cm−2 fluences were studied in the temperature range from 7 to 340 K. Three main types of radiative recombinations corresponding to band-to-tail (BT), deep defects, and band-to-band (BB) recombination were manifested in the non-irradiated α-In2Se3 crystals. Also recombinations that can be associated with exciton recombinations at temperatures below 45 K are observed. After electron irradiation, noticeable changes in the PL spectra are observed. We detected a slight increase in activation energy of the BT recombination. An increase in the concentration of deep defects is also noted. The significant decrease in PL intensity of the BB recombinations indicates the formation of non-radiative recombination centers after electron irradiation with the 1017 cm−2 fluence. Our study may be useful for understanding the effects of high-energy electrons irradiation on the performance of electronic and photovoltaic devices based on α-In2Se3. [ABSTRACT FROM AUTHOR]

As a quantum state of frequency extrema in the momentum space of acoustic systems, sonic valley pseudospin provides a new degree of freedom for controlling acoustic waves. Higher-order topological insulators (HOTIs) have extended the traditional bulk-edge correspondence principle and are a crucial concept for classic wave regulation. However, HOTIs in valley sonic crystals (VSCs) only appear in a single bandgap, which limits the multi-frequency selectivity of the corner state and is not conducive to the design of multi-frequency acoustic communication devices. Here, we demonstrate "Y-shaped" acoustic crystals with C3 symmetry that form a double-band VSC, and the topological phase transitions in both low- and high-frequency band gaps coincide. We realize theoretically and experimentally higher-order states in dual-band valley sonic crystals. Our work enriches the application of HOTIs in acoustic multi-frequency regulatory systems and provides different avenues for designing of multi-band acoustic devices. [ABSTRACT FROM AUTHOR]

High-quality aluminum nitride (AlN) crystals are the key material for the development of high-performance solid-state solar-blind vacuum-ultraviolet (VUV) photodetectors. However, the commonly used epitaxial method to grow AlN crystals would limit this development due to the existence of indispensable substrates. This study addresses this issue using free-standing lamellar AlN single crystals that are grown using the physical vapor transport method. The large lateral dimension of the crystal enables the construction of an Au-AlN-graphene van der Waals heterojunction, which can function as a vertical VUV photodetector with the graphene serving as the light window. The asymmetric junctions formed on the two sides of the crystal and the limited penetration of the VUV endow the device with a bias polarity-dependent photoresponse feature arising from different photoelectric processes. Furthermore, the device demonstrates a high responsivity of 5.77 A W−1 and a high specific detectivity of 1.71 × 1013 cm Hz1/2 W−1 under the illumination of a 193 nm laser. The high crystallinity of the AlN guarantees a high spectral selectivity of responsivity with a 193 nm/280 nm rejection ratio of 3 × 102. This work would inspire the development of wide-bandgap-semiconductor-based VUV photodetectors in terms of methodology and mechanism. [ABSTRACT FROM AUTHOR]

Domain engineering is an active field of research that aims to enhance the functional properties of ferroelectric materials for various applications. This work presents a voltage-free method for fabricating topological superdomain arrays in 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-28PT) crystals. During the cooling process from high-temperature paraelectric phase to room temperature ferroelectric phase, the out-of-plane polarization of PMN-28PT crystals in contact with aluminum can be fully regulated downward, attributed to the difference in the work function of aluminum and PMN-28PT. In conjunction further with photolithography and ultraviolet light illumination, it is easy to obtain periodic strip domain structures with alternating up and down polarization. Moreover, this method can also control in-plane polarization, resulting in a large-scale central divergent/convergent topological superdomain array. These findings provide insights into ferroelectric domain engineering and have implications for the development of electromechanical, piezoelectronic, and photonic devices. [ABSTRACT FROM AUTHOR]

Ion implantation is a promising method for the development of β-Ga2O3-based technologies and devices. However, the physical principles of ion implantation for this particular semiconductor are still at the early stage of development. One of the primary tasks is the study of electrical properties of the ion-doped layers. In this work, we have investigated the electrical parameters of layers produced by ion implantation of a shallow donor impurity—silicon—into a semi-insulating β-Ga2O3 doped with iron and having a surface orientation of (−201). It is established that the activation efficiency of the implanted impurity significantly exceeds unity after post-implantation annealing at high temperatures. This indicates that not only silicon itself contributes to conductivity, but also defects formed with its (and, probably, iron) participation are involved. The temperature dependence of electron mobility is consistent with the theoretically calculated one under the assumption that, apart from shallow donors, there are also deep defect-associated donors and acceptors. It is assumed that the established properties are specific for the case of direct Si implantation into β-Ga2O3 doped with Fe. [ABSTRACT FROM AUTHOR]

A wafer-scale single crystal thick film of rare-earth iron garnet (RIG) has been successfully produced on a 3-in. gadolinium gallium garnet (GGG) substrate using the liquid phase epitaxy method. The RIG crystal's thickness measures ∼550 μm. By removing the GGG substrate through polishing, we improved the terahertz (THz) transmittance of the RIG crystal. In the frequency range of 0.1–1.0 THz, the RIG material exhibits a large refractive index of around 4.50, with a transmittance of ∼60% and an absorption rate of only 10–50 cm−1. Furthermore, we investigated the THz magneto-optical effect in the RIG material through THz time-domain spectroscopy. The observed results demonstrate the presence and significance of the magneto-optical effect in the RIG crystal. To provide further insights, we measured the THz Faraday rotation angle of the 550 μm-thick RIG crystal using the THz-TDS system under an external magnetic field of 0.17 T. The measured Faraday rotation angle reached 22°, and the calculated Verdet constant for the RIG sample was ∼120°/mm/T. Considering these findings, our study highlights the unique properties of this wafer-scale single crystal thick film of RIG, including its low loss, high transmission, and strong magneto-optical effect in the THz range. These characteristics make it a promising candidate for various applications, such as THz magnetic polarization conversion, non-reciprocal phase shifters, and isolators. [ABSTRACT FROM AUTHOR]

We reported the realization of a 174Yb+–113Cd+ bi-species Coulomb crystal comprising 174Yb+, a heavier ion, as coolant and verified the potential of applying a coolant ion with a greater mass than the clock ion to sympathetically cooled microwave frequency standard. The two species of massive ions stably trapped in a Paul trap make up this large two-component crystal. The 113Cd+ ions are trapped in the center, which reduces considerably RF heating and excess micro-motion to which the 113Cd+ ions are subjected. Under this scheme, the uncertainty due to the second-order Doppler effect is reduced to 5 × 10 − 16 , which represents an order of magnitude improvement over sympathetic cooled 40Ca+–113Cd+ crystal. The second-order Zeeman frequency shift, which contributes the largest term of the systematic frequency shifts to the microwave ion frequency standard, is estimated to be as low as 7.133 × 10 − 13 corresponding to the low value of the operating magnetic field. The relevant AC Stark shift uncertainty is estimated to be 4 × 10 − 19 . These results demonstrate that using a heavier coolant ion is far superior and confirm the feasibility of a sympathetic-cooled cadmium-ion microwave clock system employing a 174Yb+–113Cd+ bicrystal. [ABSTRACT FROM AUTHOR]

KTa1−xNbxO3 crystals exhibit excellent piezoelectric, ferroelectric, and electro-optical properties near the phase transition point, owing to the evolution of polar nanoregions (PNRs). This paper discusses the physical properties of the PNRs in three characteristic temperature ranges and analyzes the dielectric relaxation properties of the crystals. The effects of different heating rates and initial temperatures on the pyroelectric effect are discussed, and the influence of small electric fields on increasing the pyroelectric coefficient is investigated. The polarization characteristics of the crystal were obtained according to the change in the pyroelectric performance, and the influence of different conditions on the dynamic response of the PNRs was analyzed. It is proposed that the evolution of PNRs, induced by external coupling with an internal electric field, significantly affects macroscopic performance. [ABSTRACT FROM AUTHOR]

Nonlinear infrared (IR) crystals for radiation conversion are of paramount importance for realization of advanced laser spectrometers for medical diagnostics, environmental monitoring, and advanced sensing. However, performance of such crystals suffers from substantial surface reflectivity coming from rather high (over 2.5) refractive index of the key nonlinear materials used. Here, based on the example of promising BaGa4Se7 nonlinear crystal, we attested direct surface patterning with ultraviolet (257 nm) femtosecond laser pulses used to engrave anti-reflective microstructures (ARMs) directly on both output sides of the crystal. Imprinted surface nanotrenches arranged into a fish-net morphology with a periodicity down to 500 nm was found to increase transmittance of the crystals from 65% to 84% within a practically relevant shortwave IR spectral range. Formation of the ARMs with an optimized geometry is expected to weakly reduce the laser damage threshold of a pristine crystal material as it was also evidenced from supporting full-wave simulations and tests. [ABSTRACT FROM AUTHOR]

A carbon-regulated Si substrate engineering has been adopted to reduce the RF loss of GaN-based HEMT buffer stacks. By implanting the substrate with high-dose carbon, undersaturation of Si self-interstitials is formed, and the self-interstitial-assisted aluminum diffusion into the Si substrate during the growth can be significantly suppressed. Consequently, the formation of parasitic conductive channel is suppressed, and the RF loss of the buffer stacks can be reduced. By combining the substrate engineering with low-temperature growth, the RF loss of the buffer stacks is reduced to as low as 0.13 dB/mm at 10 GHz. In addition, the crystal quality of the buffer stacks grown on the regulated substrates does not degrade. This work shows a great potential for fabrication of high-quality and low-loss GaN-on-Si RF devices. [ABSTRACT FROM AUTHOR]

Reliable methods to create quantum emitters in hexagonal boron nitride (hBN) are highly sought after for scalable applications in quantum photonic devices. Specifically, recent efforts have focused on defects in hBN with a zero phonon line at 2.8 eV (436 nm). Here, we employ carbon-doped hBN crystals that were irradiated by an electron beam to generate these emitters and perform annealing treatments to investigate the stability of the emitters. We find that the blue emitters are stable up to ∼800 °C. However, upon annealing to 1000 °C, the emitters disappear, and a family of other emitters appears in the region of hBN that had been irradiated by an electron beam. Our findings contribute to the understanding of emitter species and emitter formation in hBN. [ABSTRACT FROM AUTHOR]

Thermoelectric materials, based on photo-thermoelectric effect (PTE), may be promising in photo-detection because of their self-power, extremely broad-band, and free of cryogenic attachments. Up to now, the performance of PTE is mainly optimized through enhancement of extrinsic absorption such as using optical metamaterials. Instead, we here improve the PTE through materials engineering, accordingly systematically investigated the PTE of both P- and N-type SnSe crystals with different carrier concentrations (1017–1019 cm−3). P-type SnSe has much better photo-thermoelectric performance than the N-type one. Among P-type SnSe, the SnSe crystal with the largest carrier concentration (∼1019 cm−3 at room temperature) demonstrates the highest photo-thermoelectric performance. Analysis by a modified two-temperature model suggests that the key parameter of enhanced PTE is the high electrical conductivity, which leads to large optical absorption and large temperature difference. Our work provides a guideline on how to engineer thermoelectric materials to enhance their photo-thermoelectric performance. [ABSTRACT FROM AUTHOR]

α-RuCl3 is considered to be the top candidate material for the experimental realization of the celebrated Kitaev model, where ground states are quantum spin liquids with interesting fractionalized excitations. It is, however, known that additional interactions beyond the Kitaev model trigger in α-RuCl3 a long-range zigzag antiferromagnetic ground state. In this work, we investigate a nanoflake of α-RuCl3 through guarded high impedance measurements aimed at reaching the regime where the system turns into a zigzag antiferromagnet. We investigated a variety of temperatures (1.45–175 K) and out-of-plane magnetic fields (up to 11 T), finding a clear signature of a structural phase transition at ≈ 160 K as reported for thin crystals of α-RuCl3, as well as a thermally activated behavior at temperatures above ≈ 30 K, with a characteristic activation energy significantly smaller than the energy gap that we observe for α-RuCl3 bulk crystals through our angle resolved photoemission spectroscopy (ARPES) experiments. Additionally, we found that below ≈ 30 K, transport is ruled by Efros–Shklovskii variable range hopping (VRH). Most importantly, our data show that below the magnetic ordering transition known for bulk α-RuCl3 in the frame of the Kitaev–Heisenberg model (≈ 7 K), there is a clear deviation from VRH or thermal activation transport mechanisms. Our work demonstrates the possibility of reaching, through specialized high impedance measurements, the thrilling ground states predicted for α-RuCl3 at low temperatures in the frame of the Kitaev–Heisenberg model and informs about the transport mechanisms in this material in a wide temperature range. [ABSTRACT FROM AUTHOR]

High-performance electro-optic (EO) crystals play a key role in optical communication and information processing. Potassium tantalate niobate (KTa1−xNbxO3, KTN) crystals advantageously have an ultrahigh quadratic electro-optic (QEO) coefficient and a wide transparency range, and their excellent QEO properties are closely related to polar nanoregion (PNR)-related polar units. Owing to the PNR-related polar units, KTN crystals have excellent strain properties, which makes them a potential multifunctional material. However, the relationship between strain behavior and QEO properties in nanodisordered KTN crystals has rarely been reported. The promotion of EO activity by regulating the PNRs-related polar units still needs to be studied. Here, we study the coupling effect of strain behavior and QEO properties in KTN crystals. The QEO, elastic-optic, electrostrictive, and elastic coefficients of KTN crystals are characterized, and the strain–QEO property coupling relation is established. The results show that the strain-induced QEO response significantly affects the QEO properties. The existence and response of PNRs near ferroelectric–paraelectric phase transition are demonstrated by variable-temperature x-ray diffraction and Raman spectroscopy. Combined with the heating and cooling QEO response time tests, the response of PNRs is shown to be an important reason for the enhancement of the QEO response of KTN crystals through the promotion of the strain-induced QEO response. This study provides guidance for clarifying the origin of the QEO properties of nanodisordered KTN crystals and is beneficial for further improving the EO properties of perovskite crystals by manipulating the strain behavior. [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]

A nonlinear phase shift of low-power spin waves (SWs) induced by a high-power pumping SW excited both inside and outside the magnonic band-gaps of a magnonic crystal has been studied. The magnonic crystal with spatially periodic thickness is fabricated from an yttrium iron garnet film by chemical etching. The results show that the phase shift of the low-power SWs can be effectively controlled by variation of power level of the pumping SW. This induced nonlinear phase shift is weakened if the pump frequency lies in the magnonic bandgap. The data obtained are well explained by contradirectional coupling of the high-power forward and reflected spin waves. A theoretical model for this effect is presented. Our findings are important for the further progress in SW computing. [ABSTRACT FROM AUTHOR]

Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer. [ABSTRACT FROM AUTHOR]

Poly(lactic acid)-based (PLA) films with biodegradability and ease of processing are promising dielectrics for application in electrostatic capacitors. Nevertheless, the relative low energy storage capability of the PLA film greatly hampers the development of its further application. Herein, a biodegradable polymer of stereocomplex crystals of poly(lactic acid) is prepared utilizing a solution blending method of the co-blended poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) in a 1:1 ratio. The stereocomplex crystals could prominently improve insulation and mechanical properties of PLLA/PDLA blend films, thus achieving an outstanding energy density (Ud ∼ 13.7 J/cm3) accompanied with high efficiency (η ∼ 92.8%) at a breakdown electric field of 750 MV/m. Additionally, the PLLA/PDLA blend film also exhibit good high-temperature performance and reliability. This work provides a reasonable tailoring strategy for biodegradable polymer dielectrics and demonstrates a great potential of biodegradable polymers for practical applications in environmentally friendly power electronics and electrical systems. [ABSTRACT FROM AUTHOR]

In this investigation, the dynamic behavior of the switching and the non-switching polarization loops has been studied for ⟨001⟩pc oriented PMN-PT and Sm:PMN-PT crystals. A five-pulse triangular wave sequence was employed to measure the switching and non-switching polarization loops separately as a function of amplitude and frequency of the poling field. Dynamic scaling of the switching loops was performed according to the power law < A S W > ∝ E α f β . The lower negative value of β for Sm:PMN-PT suggests a shorter response time of crystallographic domains for it as compared to that for PMN-PT. The value of exponent α for PMN-PT was found to be higher in the low-field regime where 180° switching dominates the domain reorientation process. For Sm:PMN-PT, on the other hand, the value of α was found to be higher at high fields, which implies a superior ferroelastic domain reorientation. A similar power law scaling was also performed on the non-switching loops, which suggests a faster dynamic response of intrinsic polarization in Sm:PMN-PT as compared to that for PMN-PT. However, the intrinsic polarization in Sm:PMN-PT was found to be less responsive to the increasing field as compared to that for PMN-PT. We believe that the findings of this investigation would add another dimension to the understanding of the mechanism responsible for the high piezoelectric activity in samarium doped PMN-0.28PT crystal and polycrystalline ceramics. [ABSTRACT FROM AUTHOR]

The dispersion relationships in antiferromagnetically coupled magnonic crystals (MCs) were investigated using micromagnetic simulations. In contrast to traditional MCs, antiferromagnetically coupled MCs have two oppositely polarized modes, enabling the realization of synthetic ferrimagnetic and synthetic antiferromagnetic MCs. The magnon flatband effect was discovered, and a large bandgap of the dispersion relation was also realized in this structure. We found that the center frequency and width of the dispersion bands with a specific polarization were influenced by the thickness and thickness ratio of the spin-up and spin-down magnetic sublattices. Based on these results, spin-wave filtering devices were proposed. Our study uncovered the magnon dispersion relations of a type of MC, which provides fresh insights into the development of ultra-efficient magnonic devices. [ABSTRACT FROM AUTHOR]

Lonsdaleite is a hexagonal allotrope of carbon found in nature in meteorites and at meteorite impact sites. It has been predicted to have an indentation hardness greater than cubic diamond by first principles calculations. However, this has not been demonstrated experimentally. Here, nanoindentation was used to measure the hardness of two different lonsdaleite samples. One contains nanocrystalline lonsdaleite synthesized by high pressure compression of glassy carbon. The other is from a ureilite meteorite that contains lonsdaleite crystals up to ∼ 1 μ m. The hardness of these two samples was determined using both the Oliver–Pharr and Meyer methods. Our results show that the hardness of the lonsdaleite samples is similar to that of diamond; therefore, there is no evidence that these forms of polycrystalline lonsdaleite are significantly harder than similar forms of diamond. [ABSTRACT FROM AUTHOR]

(Na0.5Bi0.5)TiO3 (NBT)-based materials have been widely studied for their large electric-field-induced strains. However, a high coercive field (Ec) has long hindered the applications of NBT-based materials. Here, we propose a simple method to significantly reduce the Ec of rhombohedral NBT-based crystals through domain engineering. By applying an alternating current (AC) electric field along the [001] direction, the Ec of Mn-doped (Na0.485K0.015Bi0.5)TiO3 single crystal can be reduced from 70 to 20 kV/cm after about 25 cycles without sacrificing the ferroelectric polarization. Meanwhile, the piezoelectric coefficient d33 and the optical transparency of the crystals are also enhanced compared with those after direct-current electric field poling. The domain structure characterization shows that the AC cycles can form a laminar domain configuration, in which the 109° domain walls are parallel to (001) planes. It is demonstrated that in the laminar domain configuration, almost only 71° polarization switching occurs when the external electric field is reversed. The required energy for polarization reversal is significantly lower than that of the "4R" domain configuration; thus, the Ec is reduced greatly. The low Ec is maintained after depolarization at 250 °C, evidencing good thermal stability of the laminar domain configuration. Furthermore, this method is also applicable to other rhombohedral single crystals and may be applied to [001]-textured polycrystalline ceramics in the future; thus, it may indeed benefit the practical applications of NBT-based piezoelectric devices. [ABSTRACT FROM AUTHOR]

We report on sharp spin-valve-like magnetoresistance (MR) below ferromagnetic order (TC), which has not been discussed in the pristine as well as doped crystals. In addition, typical manifestation of exchange bias (EB) effect is also observed by systematic shifts of the loop observed in MR driven by the cooling field. The EB effect appears below a characteristic temperature (TA), below which ferromagnetic and antiferromagnetic phases have been suggested to coexist. As a result of In doping, the sharp spin-valve-like evidence and EB effect over a wide temperature range, which are observed even at much higher temperatures than the liquid nitrogen temperature, suggest the system as a potential candidate for application in topological spintronics. [ABSTRACT FROM AUTHOR]

We investigate a band structure ω (k) of a photonic time crystal with periodic square (step) modulation in time of its permittivity ε (t) , oscillating between the value ε 1 (sustained for a fraction of time τ of the period) and the value ε 2 [fraction (1 − τ)]. The strength of modulation is m = (ε 1 − ε 2) / (ε 1 + ε 2). We find that ω (k) can be periodic in a wave number k (in addition to the frequency ω), provided that a certain function f (m , τ) of the parameters m and τ is an irreducible rational number. However, even for arbitrary values of m and τ, f (m , τ) can be approximated by a fractional number to any desired degree of periodicity. Hence, for square modulation, a photonic band structure is necessarily periodic or quasi-periodic in the wave number. Moreover, for appropriate sets of the parameters m and τ, the modes associated with k values within the band gaps can have identical values of the imaginary part of ω. For simultaneous excitation of these modes, all the fields would grow in time at the same rate, resulting in powerful amplification. [ABSTRACT FROM AUTHOR]

We study optical rectification in GaSe by performing THz generation with femtosecond laser pulses whose wavelength is tuned from below to above the GaSe bandgap. As expected from a theory, we observed a first THz emission peak at 1.77 eV, where phase matching is realized. A second THz emission peak was recorded, when the pump photon energy reaches the crystal bandgap (2.205 eV). This can be attributed to a resonance of the GaSe nonlinearity. In crystals thinner than the coherence length, the bandgap peak is stronger than the phase-matched one. [ABSTRACT FROM AUTHOR]

Employing polarization sensitive terahertz (THz) transmission spectroscopy, we explored how the waveform of initially single-cycle linearly polarized THz pulses changes upon propagation through a thick antiferromagnetic crystal of CoF2. The changes upon propagation through CoF2 are found to depend strongly on both the incoming polarization and temperature. In particular, the ellipticity and polarization rotation acquired by initially linearly polarized light are quantified and explained in terms of magnetic linear birefringence and dichroism. Although the magneto-optical effects are often considered to be relatively weak, our experiments reveal that the polarization of the THz pulse substantially changes along the pulse duration. The pulse shape is further complicated by features assigned to the formation of magnon-polaritons. The findings clearly show the importance of accounting for propagation effects in antiferromagnetic spintronics and magnonics. [ABSTRACT FROM AUTHOR]

The emergence of anisotropic two-dimensional (2D) materials provides a platform for the cutting-edge nano- and optoelectronic devices. Exploring low-dimensional materials and revealing their anisotropic behavior are crucial for designing angle-dependent nanodevices. The metallicity of molybdenum dioxide (MoO2) crystal differentiates it from the most commonly studied semi-conductive anisotropic 2D materials. However, the studies on its optical anisotropy are still lacking. Here, two most commonly obtained shapes of rhombic and hexagonal MoO2 were synthesized by one step method of chemical vapor deposition. The rhombic and hexagonal MoO2 display a slight frequency shift of 1–5 cm−1 depending on the variation modes, but the Raman modes at 366 cm−1 remain unaltered. Using a combination of differential reflectance spectroscopy and reflectance difference spectroscopy, we revealed the unusual narrow-band optical anisotropies of rhombic and hexagonal MoO2 crystals in the visible wavelength region due to its unique metallic properties. Furthermore, it is found that the center wavelengths of the narrow-band optical anisotropy of the MoO2 crystal can be effectively adjusted by coherent optical interference. Our results present an interesting anisotropic metallic 2D candidate and an effective cavity-based approach to regulate the center wavelengths of as-obtained narrow-band optical anisotropy, which is highly beneficial for the wavelength-selected devices. [ABSTRACT FROM AUTHOR]

Backscattering immunity of valley edge states for a monolayer sonic crystal provides a basis for designing devices with unconventional functions. Recently, the valley edge state of a bilayer sonic crystal, regarded as a new degree of freedom, gives a powerful platform for manipulating acoustic waves. In this Letter, we realize valley spin insulators in a bilayer sonic crystal and find that the layer-mixed, layer-polarized, and layer-locked edge states exist on both zigzag and armchair interfaces. There are significant differences between the armchair edge states and the zigzag edge states at the interfaces formed by two domains of distinct acoustic layer-valley Hall phases. Based on the projected dispersion relations for a bilayer sonic crystal, we achieve theoretically and experimentally bilayer multichannel transmission with different layers locking. This paper provides a design approach for applications of bilayer acoustic multichannel communication devices. [ABSTRACT FROM AUTHOR]

Engineering the heat flux between two surfaces kept at different temperatures relies on the ability to tailor the dispersion of modes sustained by the system. Metasurfaces made of ordered arrays of subwavelength spherical nanoparticles have an optical response that depends not only on materials but also on their geometrical parameters. Our system is modeled by using an effective medium approximation allowing the homogenization of individual metasurfaces and replacing them with anisotropic layers. Excitation or suppression of surface and hyperbolic modes can be controlled by means of different degrees of freedom offered by the metasurfaces. By leveraging this flexibility, we theoretically show that the near-field radiative heat transfer between two such metasurfaces can be effectively geometrically tuned. [ABSTRACT FROM AUTHOR]