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Magnetic skyrmions in two-dimensional half-metallic MoX2 (X = S, Se)
Magnetic skyrmions hosted in two-dimensional (2D) magnets offer exciting prospects for next-generation spintronic information storage and processing. Nevertheless, progress on skyrmions in 2D systems has been limited, mainly owing to the inversion symmetry of most 2D magnets, which in turn leads to the lack of the key factor Dzyaloshinskii–Moriya interaction (DMI) for generating topological magnetic structures. In this study, 2D half-metallic MoX2 (X = S, Se) monolayers are investigated by first-principles calculations and atomic spin simulations, which reveal their high Curie temperatures, strong DMI, and intrinsic chiral spin textures. Crucially, both MoS2 and MoSe2 can host stable skyrmion states over a wide range of external magnetic fields. Moreover, a dimensionless criterion is used to clarify the relationship between magnetic parameters and spin textures. The findings highlight that MoX2 monolayers are promising candidates for developing spintronic devices based on topological spin textures.
Scattering-induced loss in ferroelectric photonic devices
Ferroelectric materials have colossal optical nonlinearities, but their integration into quantum photonic chips is made challenging by the additional loss mechanisms that they introduce. Here, we present a perturbative theory that expresses non-absorptive (elastic) photon scattering-induced loss as a functional of a general spectral density for spatial fluctuations of electric permittivity. We apply the theory to calculations of attenuation coefficients α in slab waveguides in order to compare two distinct loss mechanisms: Interface roughness and ferroelectric domain disorder. Our theory can account for realistic roughness without special symmetry considerations, and it demonstrates how to use electron microscopy images of ferroelectric domains to obtain explicit numerical predictions for α. Loss is maximum when the mean domain length is comparable to the wavelength of light (Mie regime), indicating that, for telecom wavelengths, sub-micron domains (Rayleigh regime) or single domain waveguides provide equivalent strategies for reducing loss.
Multiband quantum oscillations and Kohler's rule in the extremely large magnetoresistance material pyrite PtBi2
We present magnetotransport, Seebeck effect, and magnetic torque measurements conducted on the Dirac semimetal PtBi2 single crystals with a temperature range of 1.8–300 K and magnetic field strength up to 14 T. A substantial, nonsaturating magnetoresistance (MR) that reaches 5.15 × 105% at 1.8 K and 9 T obeys a single-parameter scaling, suggesting an effective unified scattering rate across a broad temperature range from 1.8 to 250 K. Importantly, distinct quantum oscillations in the Seebeck coefficient identify multiple extremal orbits with frequencies F = 62.5–1476 T, which are mutually verified by the torque de Haas–van Alphen (dHvA) oscillations and result in small cyclotron masses m* ≈ 0.072–0.16me via Lifshitz–Kosevich analysis. The angular evolution of the dominant frequencies supports three-dimensional multiband pockets/four groups of pockets, and an additional weak branch at F0 ≈ 62.5 T is robust against data-processing variations. Our results establish comprehensive quantum oscillations as a sensitive probe of multiband fermiology and scattering scaling in Dirac semimetals, with implications for understanding the mechanism of extreme MR behavior.
N-polar AlGaN HEMTs on silicon for reduced contact resistance
Nitrogen-polar (N-polar) AlGaN-channel high electron mobility transistors (HEMTs) offer a promising pathway to overcome the limitations of equivalent metal-polar (M-polar) devices, particularly the high contact resistance associated with increased aluminum content. In this work, we report the growth and fabrication of N-polar AlGaN-channel HEMTs on a silicon substrate by ammonia molecular beam epitaxy (NH3-MBE). N-polarity is achieved using an epitaxial NbN polarity-inversion layer, enabling the growth of a buffer stack with adequate structural and electrical quality. Two heterostructure designs are investigated. Capacitance–voltage and Hall measurements confirm the formation of a high-density two-dimensional electron gas with sheet carrier densities up to 2.4×1013 cm−2. Owing to the elimination of the Al-rich barrier between the surface and the channel (present in M-polar configuration) and improving surface morphology, encouraging low-resistance Ohmic contacts are achieved, with contact resistance reduced below 1 Ω mm on the Al0.2Ga0.8N channel. These results demonstrate the viability of N-polar AlGaN-channel HEMTs on silicon and highlight their potential for next-generation ultra-wide-bandgap power devices on Si.
A tunable coincidence counter based on superconducting nanowire cryotrons
Single-photon coincidence counters are essential components in integrated quantum photonics, enabling efficient logic discrimination and real-time error correction at the chip level. However, monolithic integration at cryogenic temperature remains challenging. Here, we demonstrate a coincidence counter based on superconducting nanowire cryotrons (nTron). The circuit comprises five nTron devices, including delay gates, buffer gates, and an AND gate, achieving a maximum bias margin of 22% at a bit error rate (BER) of 10−5. Operating at 1 MHz, the counter exhibits a static power consumption of 282 nW and a dynamic power consumption of approximately 2 nW at a maximum operation frequency of 17 MHz. The coincidence time window is tunable, with a minimum width below 1 ns, and its position can be adjusted via bias currents. This design offers compatibility with superconducting nanowire single-photon detectors in fabrication and operation, supporting monolithic integration for scalable quantum photonic systems.
Near-surface-weighted non-equilibrium viscoelastic transitions during mist-CVD growth of amorphous TiO <i>x</i> revealed by dual-frequency quartz crystal microbalance
We demonstrate that the viscoelastic evolution during mist chemical vapor deposition (mist-CVD) of amorphous TiOx (a-TiOx) is governed by near-surface-weighted non-equilibrium transitions rather than spatially uniform processes. Using dual-frequency quartz crystal microbalance (QCM) measurements at 9 and 27 MHz, we show that higher-frequency data resolve discrete viscoelastic regimes that remain hidden in conventional depth-averaged measurements. To enhance analytical reliability, the characteristic relaxation time τ was extracted via a unified exponential fitting protocol with residual-based uncertainty estimation. Arrhenius analysis with error bars reveals a clear deviation from linearity, indicating that the relaxation cannot be described by a single activation energy. This non-Arrhenius behavior is further supported by the lognormal distribution of τ and its correlation with the viscoelastic slope parameter S, demonstrating the coexistence of multiple relaxation pathways. These findings establish multi-frequency QCM as a statistically robust, depth-selective probe for investigating near-surface-weighted non-equilibrium dynamics during solution-derived oxide growth.
Investigation on AZO/4H-SiC n-i-p ultraviolet photodiodes with high response via Al-composition engineering
Transparent conductive materials can be employed in advanced ultraviolet (UV) photodetectors for high performance. To fabricate an Al-doped ZnO (AZO)/4H-SiC n-i-p UV photodiode, an ultrathin AZO film is studied and utilized. A range of Al in AZO content from 3.46% to 7.36% is investigated. As the Al content increases, the bandgap of the AZO film expands, resulting in higher UV transmittance. However, the film's resistivity increased correspondingly. By optimization, A 25 nm thick AZO film with a 6.00% Al content demonstrates a notable 92% transmittance at 280 nm wavelength and an electrical resistivity of 0.131 Ω cm. By using the AZO film as the n-type layer, the AZO/4H-SiC n-i-p ultraviolet photodiode achieved a low dark current of 0.245 pA at −10 V and a high spectral response peak of 0.181 A/W at 290 nm wavelength, corresponding to a quantum efficiency of 77.5%, which is better than that of the conventional pure 4H-SiC n-i-p UV photodiode.
Correlating carrier concentration and mobility in graphene oxide-doped PEDOT:PSS with electroluminescence efficiency of polymer light-emitting diodes
We report a quantitative decoupling of carrier concentration and mobility in graphene oxide (GO)-doped poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and identify the dominant transport factor associated with electroluminescence (EL) efficiency in polymer light-emitting diodes. Combined Hall-effect and space-charge-limited current analyses reveal no systematic correlation between carrier concentration and device performance, whereas hole mobility exhibits a pronounced maximum at an optimal GO loading and closely follows the EL efficiency trend. This non-monotonic behavior is attributed to the competition between improved percolation pathways and disorder-induced carrier scattering. Despite minor variations in optical transmittance and work function, device performance is significantly improved, supporting the dominant role of effective vertical transport in device operation. GO incorporation optimizes effective injection/transport pathways and facilitates field-assisted hole transport into the emissive layer, which may contribute to more favorable recombination conditions. The optimized device achieves a maximum luminance of 3910 cd/m2 and a current efficiency of 2.48 cd/A, corresponding to an order-of-magnitude enhancement. These results establish mobility-governed effective vertical transport as a key mechanism and provide a physically grounded design principle for optimizing PEDOT:PSS-based optoelectronic devices.
Spatial control of single-photon emitters in gallium nitride via dose-engineered focused ion beam processing
Single-photon emitters (SPEs) in solid-state materials are key components for emerging quantum technologies, but their random spatial distribution and lack of spatial isolation remain major obstacles to scalable device integration. Here, we demonstrate that dose-engineered Ga+ focused ion beam irradiation enables control over defect-related luminescence and spatial localization of SPEs in gallium nitride (GaN). High-dose irradiation effectively suppresses defect-related emission and deactivates optically active centers in the surrounding regions, while low-dose irradiation followed by thermal annealing promotes the formation of SPEs. By combining low- and high-dose regions within a single patterned structure, emitters are preferentially formed and emerge within predefined low-dose areas, while competing emission is eliminated in the surrounding regions. These results establish a practical and fabrication-compatible strategy for spatial control of quantum emitters in GaN, providing a pathway toward scalable integrated quantum photonic devices.
Experimental and numerical demonstration of threshold voltage asymmetry and synaptic plasticity in MoS2 transistors
Thanks to their advanced tunable electrical properties, two-dimensional materials have emerged as a promising platform for neuromorphic computing, offering unique flexibility and scalability. In this work, we report the fabrication and experimental characterization of MoS2-based transistors exhibiting counterclockwise hysteresis and synaptic plasticity. Our devices demonstrate multilevel conductivity modulation under pulsed excitation, and a pronounced dependence of the threshold voltages on the sweep rate of the applied gate signal. An in-house physics-based numerical simulator is exploited to rationalize the measured hysteresis and the frequency-dependent behavior, providing further insights into the underlying memristive mechanism, i.e., the delayed migration of oxygen ions. Moreover, simulations reveal that ion migration governs the observed plasticity, enabling control of current modulation through pulse duration. These findings establish the relevance of ionic dynamics in shaping device performance and highlight the potential of MoS2-based transistors for artificial neural network hardware.
The equation of state of tantalum to 433 GPa
The volume compression of body-centered cubic tantalum has been measured to 433 GPa in the standard bevel and toroidal diamond anvil cell (DAC) using synchrotron-based angle-dispersive powder x-ray diffraction. Six experiments were carried out achieving a maximum pressure of 433(6) GPa. Pressures were estimated using an equation of state (EoS) of Cu or Bi for all experiments. We additionally calculated the elastic constants of Ta and Bi and used our previously calculated values for Cu to inform the stress state of the sample and an EoS of Ta for comparison to our experimental data. Our experimental pressure–volume data for Ta was fit with a fourth-order Vinet EoS, resulting in parameters K0 = 192(5) GPa, η = 6.4(6), β = −35(5), and Ψ = 141(13). This Ta EoS is consistent within uncertainty with the ultrasonically derived ambient isothermal bulk modulus, with existing DAC data up to 100 GPa, and with Ta ramp EoS measurements that exceed the pressures of the current study.
Harnessing Pt/W spin Hall superlattices for efficient field-free spin–orbit torque switching
Spin–orbit torque (SOT) offers a promising route to achieve energy-efficient magnetization switching. However, materials with a perpendicular magnetic anisotropy (PMA) require an external magnetic field for deterministic magnetization switching. Achieving SOT-driven field-free switching of PMA materials is crucial for next-generation spintronic devices. Here, we demonstrate the field-free SOT switching in [Pt/W]n/Co heterostructures using Pt/W multilayers. The competing spins from the Pt and W bilayer generate an out-of-plane SOT field that enables magnetization switching without the need for an external in-plane magnetic field. These multilayers generate unconventional spin currents, as confirmed by anomalous Hall resistance loop-shift measurements. We observed a maximum field-free switching of 92% in domain wall devices. In addition, the sample with 7 bilayers exhibits a 2.7-fold enhanced damping-like efficiency compared to a single Pt layer. This work advances practical SOT device development and deepens understanding of deterministic perpendicular magnetization switching.
Ultra-stable multiple emission wavelengths produced by Tb-doped Al <i>x</i> Ga1− <i>x</i> N-based light-emitting diodes
Tb-doped semiconductors are candidates for novel full-color light sources in next-generation high-definition displays. Particularly, Tb-doped AlxGa1−xN (AlxGa1−xN:Tb) has attracted much attention because Tb3+ ions doped into AlxGa1−xN show ultra-stable multi-color emissions consisting of blue, green, yellow, and red. Herein, we demonstrate room-temperature operation of AlxGa1−xN:Tb light-emitting diodes (LEDs) under current injections and characterize the optical/electrical properties. Ultra-stable Tb3+ emissions with narrow linewidths are clearly observed, and it is revealed that the quantum efficiencies monotonically increase for Al-richer conditions. Furthermore, the LEDs fabricated on AlN templates show superior optical and electrical properties to those on GaN templates because Al-rich AlxGa1−xN:Tb layers are coherently grown on the AlN templates due to the decreased lattice mismatch, resulting in higher crystal quality and shortening the radiative lifetime (from 280 to 227 μs) related to 5D4–7FJ (J = 3, 4, 5, 6) transitions in Tb3+ ions. Through color filters, RGB emissions originating from the respective transitions are selectively obtained at the same light-emitting area. The multi-color AlxGa1−xN:Tb-based LEDs would be a novel light source for wide applications such as micro-LED displays.
Flexoelectric polarization synergy enables ultrafast charge transfer for high-efficiency photoconversion in mixed-dimensional MoS2/WSe2 heterostructures
Transition metal dichalcogenide nanotubes (NTs) and their heterostructures hold promise for advanced photonics, yet the combined role of flexoelectricity and heterointerface electronegativity mismatch in their performance is unknown. Here, using the atomic-bond-relaxation method, Marcus theory, and the detailed balance principle, we show that in one-dimensional/two-dimensional MoS2-NT/WSe2 heterostructures, the synergistic action of a strain gradient induced flexoelectric field and an interfacial built-in field drives an ultrahigh charge-transfer rate of 8×1013s−1. This mechanism, coupled with the enhanced photoconfinement effect of NT, yields a remarkable photoconversion efficiency exceeding 10%. Our work reveals strain and interface engineering as a powerful strategy for achieving high-performance optoelectronics in low-dimensional systems.
Photoluminescence-based ultrasensitive DNA detection of typhoid using MgO nanomaterials
An ultrasensitive optical DNA biosensor based on magnesium oxide (MgO) nanoparticles is reported for the detection of Salmonella typhimurium. The sensing mechanism relies on photoluminescence (PL) modulation governed by defect-mediated charge transfer interactions known as photoinduced electron transfer upon DNA hybridization. The MgO nanostructures exhibit strong emission centered at ∼780 nm, which is significantly enhanced upon probe DNA immobilization due to surface defect passivation. Subsequent hybridization with target DNA induces concentration-dependent PL quenching, enabling quantitative detection in the range of 30–150 aM. A linear calibration response with a high correlation coefficient (R2 = 0.96) is obtained, yielding a limit of detection of 4.2 aM. The biosensor demonstrates excellent specificity, with a ∼6.5-fold higher response for complementary DNA compared to non-complementary sequences. The observed PL modulation is attributed to radiative recombination suppression via charge transfer interactions following dsDNA formation. The developed platform offers a rapid, label-free, and highly sensitive approach for pathogen detection, highlighting the potential of MgO-based optical biosensors for advanced diagnostic applications.
Super-resolution phenomenon induced by surface spin-wave diffraction through a hole in in-plane magnetized YIG film
Diffraction of a surface spin wave by a through hole formed inside the in-plane magnetized yttrium iron garnet (YIG) film is investigated both experimentally and theoretically for the case where the hole diameter is significantly smaller than the wavelength. The study reveals a super-resolution and super-directional propagation phenomenon characterized by the formation of a distinct shadow trace from the hole in the spin-wave profile. This shadow is elongated at a long distance from the hole. This occurs along the directions of super-directional wave propagation, corresponding to the inflection points of the isofrequency curve. Theoretical predictions show excellent agreement with experimental observations of super-directional magnon beams engineered by anisotropic dispersion in YIG and are further validated by micromagnetic simulations. The observed formation and splitting of spin-wave beams, along with the extended non-diffracted beam propagation distance, highlight the potential application of these effects in spin-wave multiplexers for magnonic logic devices.
Verification of 100 nm resolution in a superconducting nanostrip single-photon detector for hard x-ray image reconstruction
We propose a method using a superconducting nanostrip single-photon detector (SNSPD) that can significantly reduce resolution compared with conventional imaging detectors. An SNSPD is advantageous for miniaturization because it consists of a single superconducting strip and allows the direct detection of hard x rays when a thick superconductor with a high atomic number is utilized. A two-dimensional image can be obtained by arranging superconducting strips in a one-dimensional array, rotating the detector or sample, and reconstructing the image using computed tomography technology. One-dimensional SNSPD arrays made of tantalum nitride with a pitch of 100 nm, corresponding to the resolution, were fabricated. A prototype detector was assembled using a counter located at room temperature, incorporating flexible printed circuits with 100 signal cables connected the SNSPD array to the counter via a relay connector to dissipate heat at room temperature. It was demonstrated that the heat flow to the SNSPD array and the signal loss during x-ray detection could be effectively suppressed. By simultaneously obtaining x-ray detection signals from two adjacent strips, it was observed that crosstalk occurred; however, increasing the film thickness helped reduce the occurrence of crosstalk. For strips with a thickness of 270 nm, the crosstalk occurrence rate was approximately 35%, which was deemed acceptable. We conclude that a resolution of 100 nm for hard x rays can be achieved using the SNSPD array.
Low-droop and high-efficiency (&gt;40%) UVA emitters enabled by precursor modulated quantum wells
We demonstrate a precursor-modulation growth scheme to overcome the efficiency limitations of ultraviolet-A (UVA) light emitting diodes (LEDs) caused by interface intermixing and poor carrier confinement. By incorporating intentional growth interruptions, we engineered more abrupt interfaces with Al-rich interfacial “spikes.” Scanning transmission electron microscopy energy-dispersive x-ray spectroscopy mapping and Schrödinger–Poisson simulations confirm these spikes increase conduction- and valence-band offsets by over 50%, improving electron–hole wavefunction overlap by approximately 20%. The resulting 380-nm micro-LEDs achieved a peak external quantum efficiency of 41%. Notably, the devices maintain high efficiency above 40% up to a current density of 200 A/cm2 with negligible droop. This approach provides a scalable and robust pathway for high-power, high-efficiency solid-state UVA emitters.
Sign reversal of current-induced torque in perpendicularly magnetized Pd/Co2MnGa and Pd/Co2MnSi
We studied the effective magnetic field Heff arising from the spin–orbit torque (SOT) and/or orbital torque (OT) by measuring shifts of out-of-plane hysteresis loops under various in-plane currents Is and in-plane magnetic fields Hxs along I in perpendicularly magnetized Pd/Co2MnGa (CMG) and Pd/Co2MnSi (CMS). The Heff for Pd/CMS is explained well by the SOT originating from the spin Hall effect (SHE) in Pd, whereas that for Pd/CMG points in the opposite direction under a given polarity of I and Hx. Neither the self-induced SOT in CMG nor the SOT originating from the interfaces is responsible for the Heff reversing in Pd/CMG. The magnitude of the effective spin/orbital Hall angle, derived from the Heff, increases with the thickness of both the CMG and Pd layers in Pd/CMG, which is consistent with the predicted behavior of the OT originating from the orbital Hall effect in Pd. Considering these results and the fact that CMG has stronger spin–orbit interaction than CMS, the OT, which is larger than and opposite to the SOT originating from the SHE in Pd, is a plausible candidate for reversing the Heff in Pd/CMG.
Reconfigurable WSe2 transistors enabled by complementary electrostatic shielding of source and drain electrodes
Reconfigurable field-effect transistors (RFETs) based on ambipolar two-dimensional (2D) semiconductors provide a versatile platform for multifunctional logic and neuromorphic computing. Traditional RFET architectures typically rely on horizontally arranged multi-gate structures to create tunable homojunctions, a design that imposes significant constraints on lateral scaling due to the required physical gaps between adjacent gates. In this work, we report a reconfigurable WSe2 transistor that achieves polarity control through a complementary electrostatic shielding mechanism. By employing an asymmetric contact configuration consisting of a top-contacted source and a bottom-contacted drain, we demonstrate that these electrodes can selectively shield the top-gate and back-gate fields, respectively. This architecture ensures that the back gate independently modulates carrier injection at the source junction while the top gate governs injection at the drain junction. Such a vertically decoupled dual-gating scheme enables a single device to exhibit robust reconfigurable n- and p-type characteristics with a scaled lateral channel length of ∼500 nm, which is a notable improvement over conventional split-gate RFETs based on 2D materials (∼2 μm). Furthermore, we demonstrate a complementary logic inverter based on this device that achieves a static power consumption below 10 pW. This study introduces a novel physical principle for RFET operation and offers a promising pathway for the continuous scaling of reconfigurable 2D electronics.