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Magnonic Fabry–Pérot resonators as programmable phase shifters
We explore the use of magnonic Fabry–Pérot resonators as programmable phase shifters for spin-wave computing. The resonator, composed of an yttrium iron garnet film coupled with a CoFeB nanostripe, operates through dynamic dipolar coupling, leading to wavelength downconversion and the formation of a magnonic cavity. Using super-Nyquist sampling magneto-optical Kerr effect microscopy and micromagnetic simulations, we demonstrate that these resonators can induce a π phase shift in the transmitted spin wave. The phase shift is highly sensitive to the magnetization alignment within the resonator, allowing for on-demand control via magnetic switching. This feature, combined with low-loss transmission, positions the magnonic Fabry–Pérot resonator as a promising component for reconfigurable magnonic circuits and spin-wave computing devices.
Temperature-dependent random telegraph signal in Co/CoO/Co point contact devices
Nanoscale Co/CoO/Co point contact devices are promising for spintronics, magnetic sensors, single-electron transistors, and memory devices. We fabricated Co/CoO/Co point contact devices and studied their random telegraph signal (RTS) characteristics at various temperatures. In the time domain, the current fluctuates asymmetrically with respect to zero bias, with RTS behavior emerging above a certain voltage threshold. In the frequency domain, the RTS power spectral density exhibits Lorentzian lines. As temperature increases from 50 to 300 K, two RTS behaviors are observed: at ≤100 K, the high current state dominates; at ≥200 K, the low current state prevails, with RTS absent at 150 K. Single exponential fitting shows that RTS state lifetimes decrease with temperature. The charge capture and release model explains the RTS origins and temperature-dependent behavior from an energy band perspective. This study provides insights into charge dynamics, single-electron transport, and the noise mechanisms affecting the reliability of nanoscale devices.
Strong coupling of excitons in a two-dimensional atomic crystal with quasi-bound state in the continuum supported by a single nanoparticle
Bound states in the continuum (BICs) supported by high-index dielectric nanoparticles have garnered significant attention due to their ability to achieve subwavelength confinement with high-quality factors, offering great potential for both fundamental studies of light–matter interactions and the development of compact photonic devices. However, the strong coupling of BICs with quantum emitters in single-particle systems has not been reported. In this paper, we propose a dielectric nanodisk integrated with a monolayer of transition metal dichalcogenides. A supercavity mode known as Friedrich–Wintgen BIC is launched in the nanodisk to enable strong coupling of BIC with WS2 excitons. We demonstrate that the reduction in coupling strength as the quasi-BIC quality factor increases is due to a decrease in the number of excitons participating in the coupling. Additionally, the coupling strength can be manipulated by adjusting the shape and aspect ratio of the nanoresonators, enabling a transition from strong to weak coupling regimes. Near-field analysis shows that the manipulation of the BIC–exciton interaction arises from the overlap of the BIC mode's near-field with the excitons. Our findings offer a promising strategy for manipulating light–matter interactions and enhance the potential applications of BIC resonances in photonics.
Tailoring transverse magneto-optical Kerr effect enhancement in Mie-resonant nanowire-based metasurfaces
Tailoring of the transverse magneto-optical Kerr effect (TMOKE) in hybrid metasurfaces comprising rectangular silicon nanowires coupled with a nickel substrate is demonstrated. The excitation of Mie modes of different orders in nanowires causes TMOKE enhancement. The in-plane magnetic dipole mode leads to the largest TMOKE enhancement compared to other Mie modes. Changing the width of silicon nanowires entails a modification of that mode, thereby ensuring tailoring of the TMOKE within the range of 2.2%–3.8%. This tunability is associated with the modification of the near-field localized at the Si/Ni interface and the far-field response of the excited magnetic dipole mode. Adjusting these two quantities allows one to achieve the highest values of the TMOKE caused by individual Mie modes in silicon nanowires.
Effects of the RF bias power of ICP etching on the electrical properties of n-type Ohmic contact on high-Al-fraction AlGaN
The effects of the radio frequency (RF) bias power of inductively coupled plasma etching on the electrical properties of n-type Ohmic contact have been investigated. By reducing the RF bias power, a high-quality n-type Ohmic contact has been achieved on n-Al0.70Ga0.30N, with a specific contact resistivity of 1.2 × 10−4 Ω cm2. It is confirmed that low-power etching introduces fewer acceptor-state defects on the etched surface, which not only reduces the compensation for electrons but also reduces the degree of oxidation on the etched surface, providing favorable conditions for improving the electrical properties of metal–semiconductor contacts.
Modulation of nonlinearity and asymmetry in a spin–orbit torque driven artificial synapse
Unconventional computing schemes inspired by biological neural networks are being explored with ever growing interest to eventually replace traditional von Neumann architecture-based computation. Realization of such schemes necessitates the development of device analogs to biological neurons and synapses. Particularly, in spin-based artificial synapses, the spin–orbit torque (SOT) can be utilized for changing between multiple resistance states of the synapse. In this work, we demonstrate synaptic behavior, namely long-term potentiation and long-term depression in a ferrimagnet (GdFe) via SOT generated using a heavy metal (Pt). The dependence of the synapse-like output on the input parameters is extensively investigated. Synaptic arrays based on experimental results are simulated and used to perform the classification of a handwritten digit dataset. Correlating the classification accuracy with the experimentally observed synaptic behavior, the performance of the synapse is found to depend on the critical switching currents. Understanding the correlation between the input parameters and synaptic performance could accelerate the development of artificial spintronic synapses possessing high operation speed, nonvolatility and plasticity, thereby enabling efficient compute in-memory systems in the near future.
Large positive VFB shift achieved by a band alignment modulated with insertion of Ga2O3 dipole layer
In this Letter, a Ga2O3 dipole layer deposited at the SiO2/HfO2 interface by in situ atomic layer deposition (ALD) has been demonstrated to be a great positive dipole. Through the in situ ALD of 10–30 cycles Ga2O3 dipole layer, a 1.09–1.59 V flatband voltage positive modulation range is obtained with a small 0.05–0.36 nm equivalent oxide thickness penalty, respectively. The modification of the band alignment is verified by x-ray photoelectron spectroscopy measurement on the gate stacks before and after Ga2O3 dipole layer insertion. The valence band offset between SiO2/HfO2 has been suppressed by the insertion of Ga2O3, proving a p-type dipole nature of Ga2O3 dipole layer. The first interface of SiO2/Ga2O3 more dominantly decides the p-type nature, and the second Ga2O3/HfO2 suppresses it. This work indicates that ALD of Ga2O3 is a promising positive dipole candidate for multiple threshold voltage technology in advanced process nodes.
Valley polarization in two-dimensional zero-net-magnetization magnets
Valleytronics in two-dimensional (2D) zero-net-magnetization magnets exhibits ultradense and ultrafast potential due to their intrinsic advantages of zero stray field and terahertz dynamics. The zero-net-magnetization magnets mainly include PT-antiferromagnet [the joint symmetry (PT) of space inversion symmetry (P) and time-reversal symmetry (T)], altermagnet, and fully compensated ferrimagnet. In these magnets, achieving controllable valley polarization is extremely important to the application of valleytronics. In this perspective article, we provide some possible design strategies to achieve valley polarization and spin-splitting in 2D zero-net-magnetization magnets. Furthermore, the anomalous valley Hall effect can be achieved in these zero-net-magnetization magnets. These proposed design strategies can encourage more theoretical and experimental works to explore valley polarization in these eminent magnets.
Tunable sliding ferroelectricity in two-dimensional van der Waals RuX2 (X = Cl, Br, and I) multiferroic layers
Two-dimensional (2D) van der Waals (vdW) materials offer vast potential for designing ferroelectrics with desired properties through simple layer stacking. Here, based on first principles, we demonstrate that the vdW layered crystals RuX2 (X = Cl, Br, and I) are a class of 2D multiferroic sliding ferroelectrics. The stacking of two magnetic RuX2 monolayers with the same orientation breaks the spatial inversion symmetry, resulting in a stable vertical polarization. In addition, the direction of polarization can be reversed through slight interlayer sliding, in which it only needs to overcome the small energy barrier of 7.16 meV. Among these layered crystals, the bilayer RuI2 not only possesses a remarkable sliding ferroelectricity of 0.49 pC/m but also exhibits stable long-range magnetic order due to its large magnetic anisotropy energy. When the RuI2 stack is increased to trilayers, the polarization significantly increases to 1.03 pC/m, which is much larger than that of its bilayer structure. Furthermore, the application of compressive strain results in a substantial increase in vertical polarization. This work provides an efficient method for designing 2D multiferroic sliding ferroelectric materials by stack engineering.
Band engineering for large perpendicular magnetocrystalline anisotropy and low magnetic Gilbert damping constant by anion substitution at Fe/MgO interface
Large interfacial perpendicular magnetocrystalline anisotropy (iPMA) and low Gilbert magnetic damping constant (α) in magnetic tunnel junctions (MTJs) are desired to achieve higher storage density and lower standby power operations in magnetic random-access memory. This work theoretically investigates effects of nitrogen and fluoride anions (N-anion and F-anion) substitution on the MgO barrier interface of Fe/MgO/Fe MTJ for iPMA and α using first-principles calculations. We find that the N-anion substitution significantly enhances iPMA by four times and reduces α by 65% compared to the pristine Fe/MgO/Fe, indicating a guideline toward an MTJ with large iPMA and low α simultaneously. The mechanism is explained by a band realignment at the Fermi level (EF) where Fe d±1 (dxz,dyz) orbitals at the interface are pushed above and below EF but Fe d±2 (dxy, dx2−y2) orbitals remain at EF by the N-anion substitution.
Efficient quantum frequency conversion of ultra-violet single photons from a trapped ytterbium ion
Ion trap system is a leading candidate for quantum information science benefitting from its long coherence time, high-fidelity gate operations. In addition, the ion photon entanglement provides a versatile tool to realize quantum networks by generating an ideal pair of a stationary memory qubit and a flying communication qubit. Rapid developments in nonlinear quantum frequency conversion techniques have enhanced the potential for constructing a trapped ion quantum network via optical fiber connections. The generation of long-distance entanglement has been demonstrated with ions such as Ca+ and Ba+, which emit photons in visible or near-infrared range naturally. On the other hand, as the qubit-native photons reside in ultra-violet (UV) spectrum, the Yb+ ion has not been considered as a strong competitor for telecommunication qubits despite extensive research on it. Here, we demonstrate an efficient difference-frequency conversion of UV photons, emitted from a trapped Yb+ ion, into a visible range. We provide experimental evidence that confirms the converted photons are radiated from the Yb+ ion. Our results provide a crucial step toward realizing a long-distance trapped ion quantum network based on Yb+ ions through quantum frequency conversion.
Optical absorption stacking modulation in atypical multilayer graphene
With graphene layers increasing, the interlayer charge transfers would generate an out-of-plane electric dipole moment, which is directly related to the optical absorption according to the Fermi Golden rule. Thus, in this work, we investigated the relationship between the absorption of multilayer graphene with different stacking orders from the transition dipole moment (TDM) and joint density of states (JDOS). It is found that the interlayer linkage of different stacking sequences leads to the varied chiral pseudospin doublets, which results in changes in TDM, consequently influencing the absorption spectra in the infrared and visible regions. While the enhanced absorption in the thicker multilayers from the ultraviolet to ionizing radiation region could be attributed to the increasement of JDOS. The comprehensive understanding of the absorption mechanism behind the multilayer graphene could provide another way of thinking about the optical absorption stacking modulation in atypical multilayer graphene.
Growth of nitrogen-doped (010) <i>β</i>-Ga2O3 by plasma-assisted molecular beam epitaxy using an O2/N2 gas mixture
In this study, we report on the intentional nitrogen doping of plasma-assisted MBE (PAMBE)-grown (010) β-Ga2O3 films by generating the growth plasma with an O2 and N2 gas mixture. A nitrogen doping range of 1.3 × 1018 to 4.5 × 1019 cm−3 was achieved. The nitrogen doping profiles have top-hat shapes with sharp turn-on and turn-off. Nitrogen doping was found to have a negligible impact on surface morphology. Nitrogen incorporation was unaffected by gallium flux and growth temperatures over a large growth window. An incorporation dependence on plasma power and total gas flow rate was observed. This was attributed to the N2 triple bond strength compared to the O2 double bond strength, which makes N2 harder to crack in the plasma source. Increasing the plasma power from 140 to 230 W increased nitrogen incorporation by 1.6×, while increasing the total gas flow rate from 0.8 to 2.0 sccm decreased incorporation by 3.2×. The compensation effect of nitrogen was verified by characterizing effective carrier concentration in conducting tin and nitrogen co-doped films. A 2.1 V built-in voltage was extracted from a nitrogen-doped, n− junction device using CV measurements. The device was found to exhibit rectifying behavior with a 100 A/cm2 current density at 2.9 V. The junction demonstrated here can play a key role in field management and electrostatic engineering for β-Ga2O3-based power devices. Overall, the controllability of PAMBE nitrogen doping and the properties of nitrogen-doped films suggest PAMBE-grown, nitrogen-doped layers have promising power device applications.
Aberration measurement by electron ptychography and consistency among different algorithms
Control over and knowledge of the electron probe is important in all scanning transmission electron microscopy (STEM) techniques. This is emphasized especially in electron ptychography, where the accurate probe wave function is required to deconvolve illumination from the specimen. The majority of ptychographic algorithms, such as the extended ptychographic iterative engine, reconstruct the electron probe on a pixelated grid numerically self-consistently. Solutions are thus not necessarily bound to wave functions physically realizable by the optical system. A method is presented to characterize reconstructed probes by conventional lens aberrations. The fitted aberrations are then used to investigate the quality of the retrieved probes, and their consistency is examined in a systematic study using a 4D-STEM focal series recorded for a thin SnS2 2D flake. Additionally, the influences of partial coherence and limited electron dose on the retrieved probes are analyzed, and the usefulness of the retrieved probes for different ptychographic methods, such as single sideband, Wigner distribution deconvolution ptychography, and gradient descent-based schemes, is elucidated. Finally, applications for ptychography-driven alignments of aberration-correcting electron optics are outlined.
High-frequency/high-field electron paramagnetic resonance generalized spectroscopic ellipsometry characterization of Cr3<b>+</b> in <b> <i>β</i> </b>-Ga2O3
Electron paramagnetic resonance of Cr3+ ions in β-Ga2O3 is investigated using terahertz spectroscopic ellipsometry under magnetic field sweeping, a technique that enables the polarization resolving capabilities of ellipsometry for magnetic resonance measurements. We employed a single-crystal chromium-doped β-Ga2O3 sample, grown by the Czochralski method, and performed ellipsometry measurements at magnetic field strengths ranging from 2 to 8 T, at frequencies from 82 to 125 and 190 to 230 GHz, and at a temperature of 15 K. Analysis of the frequency-field diagrams derived from all Mueller matrix elements allowed us to differentiate between the effects of electron spin Zeeman splitting and zero-field splitting and to accurately determine the anisotropic Zeeman splitting g-tensor and the zero-field splitting parameters. Our results confirm that Cr3+ ions predominantly substitute into octahedral gallium sites. Line shape analysis of Mueller matrix element spectra using the Bloch–Brillouin model provides the spin volume concentration of Cr3+ sites, showing very good agreement with results from chemical analysis by inductively coupled plasma-optical emission spectroscopy and suggesting minimal occupation of sites with inactive electron paramagnetic resonance. This study enhances our understanding of the magnetic and electronic properties of chromium-doped β-Ga2O3 and demonstrates the effectiveness of high-frequency/high-field electron paramagnetic resonance generalized spectroscopic ellipsometry for characterizing defects in ultrawide-bandgap semiconductors.
Elegant high-order harmonic vortices generation
High-order harmonic generation is a cornerstone of attosecond science, with applications spanning from spectroscopy to the creation of ultrashort light pulses with temporal duration falling in the attosecond regime. In addition, light beams carrying orbital angular momentum (OAM) allow studies of light–matter interactions mediated by OAM couplings. In this work, we present an alternative approach to generating high-order harmonic vortices using elegant Laguerre–Gaussian (eLG) beams. We examine the spatiotemporal characteristics of these harmonic vortices in the far-field regime and demonstrate how the low divergence of eLG beams makes them suitable for producing extreme ultraviolet (XUV) twisted attosecond pulses. Additionally, by solving the far-field Fraunhofer integral, we analyze the influence of azimuthal and radial indices on the spatial profile of vortex beams, thereby exploring the impact of larger topological charges. This study extends the concept of harmonic vortices generated by Laguerre–Gaussian beams to applications beyond the paraxial approximation.
A site-selection multi-element co-doping strategy in three-layered bismuth-based layered perovskite-like structure ferroelectrics leads to large energy storage capability
Improving the energy storage density of dielectric capacitors, which are widely used in power electronic devices, is a continuous challenge. In this work, a site-selection multi-element co-doping strategy was used by doping Pr atoms at the A-site and then doping Mn atoms at the B-site in bismuth-based layered perovskite-like structure prototype ferroelectrics Bi4Ti3O12 due to its large polarization and high Curie temperature. On the one hand, the substitution of Bi3+ with Pr3+ at the A-site introduces significant cation disorder, which disrupts the long-range ferroelectric order, consequently leading to a reduction in remnant polarization. On the other hand, the substitution of Ti4+ by Mn4+ at the B-site results in delayed polarization saturation due to the different electronic configurations between d3 Mn4+ and d0 Ti4+. In addition, the leakage current of the thin film exhibits a continuous decrease with doping concentration, which can be attributed to microstructural modifications including reduced grain size and the formation of amorphous regions, consequently leading to an enhanced breakdown field. Finally, a recoverable energy storage density of 42.1 J cm−3 and an efficiency of 69.9% were obtained for the BPT film, and a recoverable energy storage density of 81.8 J cm−3 and an efficiency of 73% were achieved in the BPTM film. The enhanced energy storage density can be attributed to the synergistic effect of co-doping of A- and B-sites on polarization and breakdown. Besides, the doped films have excellent frequency, temperature, and cycling stability. This work provides a guide to substantially enhance dielectric energy storage by a site-selection multi-element co-doping strategy.
Ultrafast 0D/1D ZnO/CuO photodetector in nanosecond scale by engineering the type-II heterostructure
Zero-dimensional/one-dimensional (0D/1D) heterojunctions have excellent potential in the field of optoelectronic devices due to the synergy effect of different dimensions. Most reported 0D/1D heterojunction photodetectors only focus on optimizing the separation efficiency of photogenerated carriers at the interface. However, the carriers within the quantum dots (QDs) cannot be transferred to the electrodes, resulting in recombination of photogenerated carriers separated at the interface. Therefore, the response speed of most 0D/1D heterojunction photodetectors is still limited to the order of seconds (s) and milliseconds (ms). In our work, we demonstrate a nanosecond (ns) scale ZnO/CuO heterojunction photodetector with efficient photoelectric conversion by engineering the type-II 0D/1D heterojunction interface. Herein, the surface defect states of ZnO QDs are deliberately introduced as “electrons storage pool” to suppress carrier recombination and further promote separation, which has been confirmed by photoluminescence (PL) and time-resolved photoluminescence (TRPL). As a result, the photodetector exhibited excellent performance with ultrafast response speed of 20 ns, responsivity of 213 A/W, and detectivity of 2.95 × 1011 Jones, respectively. This defect related interface engineering provides a feasible strategy for the development of high-performance 0D/1D heterojunction photodetectors.
Multi-band nonreciprocal thermal radiation based on Weyl semimetals with epsilon-near-zero multilayers
Although various nonreciprocal thermal emitters have been suggested to break the balance between absorption and emission, few structures can achieve strong nonreciprocity in more than three bands. To break this constraint, we propose a nonreciprocal thermal emitter device based on GaN/AIN/SiC/Weyl semimetal (WSM), which is capable of three discrete pairs of near-perfect absorption and emission, leading to perfect hexa-band strong nonreciprocal radiation. The enhanced nonreciprocal thermal radiation is attributed to the field enhancement of epsilon-near-zero layer and nonreciprocal guided resonances excited in the WSM film. By studying the magnetic field distribution, the physical mechanism of multi-band nonreciprocal thermal radiation is revealed, which can be verified by the impedance matching theory. Furthermore, the dependence of the structure dimensions and the axial vector b of the Weyl semimetal on the performance of the nonreciprocal radiation is investigated in detail. We believe that this work can provide an approach to the development of energy conversion devices and frequency selective detectors.
Characterizing self-heating dynamics using cyclostationary measurements
Self-heating in surround gate (e.g., nanosheet, nanowire, and FinFET) transistors degrades their on-current performance and reduces their lifetime. If a transistor heats/cools with time constants much shorter than the inverse of the operating frequency, predictable, frequency-independent performance is expected; if not, the operating frequency must be optimized for the highest performance. Typically, time constants are measured by expensive, ultra-fast instruments with high temporal resolution. Instead, here, we demonstrate an alternate, inexpensive, cyclostationary measurement technique to characterize self-heating (and cooling) with sub-microsecond resolution. The results are independently confirmed by direct imaging of the transient heating/cooling of the channel temperature by the thermoreflectance method. Routine use of the proposed technique will help improve the design of the surrounding gate transistors and shorten their design cycle.