Field-effect transistors are widely used to control carrier density in a channel of a semiconductor by a gate voltage. Mott transistors employing strongly correlated electron systems as a channel are particularly intriguing due to their switching mechanism being cooperative phase transitions. In contrast to conventional approach of modulating carrier density or filling in a Mott insulator, we propose an alternative approach of tuning effective bandwidth. We employ SrVO3 double quantum well (DQW) channel, where metal–insulator transition is induced by gate-controlling the tunneling probability of the localized V 3d electrons between two QWs. In contrast to conventional unipolar accumulation/depletion responses in single QW channels, DQW exhibits conductance suppression in both of positive and negative gate voltages. The observed switching behavior can be ascribed to the misalignment of quantized energy levels, suppressing the resonant tunneling effect and enhancing electron correlations.
Functional materials are typically employed as either capacitive- or resistive-mode sensors. Here, we present a radio frequency electromagnetic temperature-sensing approach based on functional material-loaded lossy transmission lines. The functional material consists of a polydimethylsiloxane–carbon fiber (PDMS–CF) thermistor composite embedded within a coplanar waveguide, filling a gap in the signal line. The composite bridges the signal line, forming a lossy section whose transmission magnitude depends on both the permittivity and conductivity of the sensing material. The proposed configuration, therefore, establishes a direct relationship between the composite’s permittivity and conductivity and the transmission coefficient of the sensor. The PDMS–CF-loaded sensor exhibits a resistance-dominated response in the lower temperature range (below 125 °C) and a capacitance-dominated response in the higher temperature range (above 125 °C). Each temperature region is modeled using a piecewise linear fit, enabling temperature to be determined from the measured S-parameters across the full frequency span of a vector network analyzer (VNA). A simplified sensing platform is demonstrated using a portable, open-source data-acquisition system (nanoVNA), with performance validated against a traceable benchtop VNA under periodic thermal cycling. Finally, we demonstrate a fully circular sensor architecture based on liquid-metal (Galinstan) conductors embedded in 3D-printed channels, providing a viable route for recycling both the conductive elements and the sensing material.
We report the observation of two temperature-driven polarity reversals of the anomalous Hall effect (AHE) in an ultrathin thulium iron garnet Tm3Fe5O12, TmIG and platinum (Pt) heterostructure, grown by scalable on-axis sputtering. Hall-bar devices fabricated from a 6-nm-thick, epitaxially strained TmIG film grown on (111) gadolinium gallium garnet exhibit polarity reversals of the anomalous Hall resistance at ∼81 and 124 K. A pronounced divergence of the coercive field is observed only at the lower temperature, indicating a compensation-like transition. True magnetic compensation has not previously been reported in TmIG owing to the weak exchange coupling between the Tm and Fe sublattices. We attribute the two polarity reversals to distinct physical mechanisms: a crossover in the dominant contribution to the AHE within the Pt layer, and a strain-induced modification of the magnetic structure of the TmIG layer, detected via the magnetic proximity effect. These results demonstrate strain as an effective control parameter for engineering proximity-induced magnetotransport in heavy-metal/rare-earth iron garnet heterostructures.
Spintronic terahertz (THz) emitters generate ultra-broadband radiation via the inverse spin Hall effect within an exceptionally compact structure, eliminating the need for complex optical alignment and cryogenic systems. However, the optical absorption of spintronic THz emitters is only about 15%–20%, which limits the output power and energy utilization efficiency of such devices. Here, we demonstrate a hybrid THz source integrated on a single chip, combining a W/CoFeB/Pt spin trilayer with a photoconductive antenna fabricated from low-temperature-grown gallium arsenide. In this configuration, the voltage-controlled photoconductive antenna enhances the output by recovering the laser energy transmitted through the spintronic structure. Under low bias, the THz waves generated by the two emitters undergo coherent superposition in space, enabling spectral and power modulation of the THz signal by tuning the bias-controlled power ratio between the two structures. Under high bias, the hybrid device reaches an estimated peak field of ∼25.8 kV/cm (2.58 × 106 V/m) and a broadband output up to 6 THz under a laser pump pulse energy of only 1.25 nJ. This work provides a pathway toward electrically tunable, integrated spin-based THz sources through synergistic on-chip integration.
Fe-doped InSb is an n-type ferromagnetic semiconductor with high Curie temperatures above 300 K, offering a promising platform for semiconductor spintronics. To elucidate the microscopic origin of ferromagnetism in heavily Fe-doped InSb, we systematically investigate Fe defect configurations, magnetic interactions, and electronic structures using first-principles calculations based on cluster expansion and the Korringa–Kohn–Rostoker (KKR) Green’s function method. Our cluster expansion analysis indicates that Fe atoms occupy In substitutional sites below an Fe concentration of 20%, whereas interstitial Fe defects emerge above 20% and coexist with substitutional ones. This change in the dominant defect configurations is consistent with the experimentally observed behavior of the lattice constant: Vegard’s law is followed below 20%, whereas a slight upward deviation appears above 20%. Exchange-coupling analyses reveal that the coexistence of substitutional and interstitial Fe defects yields robust ferromagnetic networks and enables Curie temperatures to exceed room temperature. In particular, nearest-neighbor octahedral–tetrahedral interstitial Fe pairs exhibit exceptionally strong ferromagnetic coupling, which is attributed to direct exchange driven by orbital hybridization. Overall, these results provide first-principles insights into the defect-driven magnetic interactions underlying high-temperature ferromagnetism in Fe-doped InSb and offer practical guidelines for the design of semiconductor spintronic materials.
Inducing a superconducting gap in topological materials via the proximity effect has been a widely applied approach to exploring topological superconductivity. However, the inverse proximity effect, which can provide crucial insights for understanding topological insulator-superconductor (TI-SC) Josephson junctions, remains largely unexplored thus far. In this study, we report the successful epitaxy of a TI-SC heterostructure using NbTiN as the superconductor layer and investigate the inverse proximity effect in it. Electrical transport measurements reveal a systematic suppression of the superconducting transition temperature with increasing Bi2Te3 thickness, reaching ∼23% in the 10 nm NbTiN samples and tending toward saturation at larger Bi2Te3 thickness. This behavior resembles the inverse proximity effect in conventional superconductor-normal metal bilayers. The perpendicular upper critical field yields a coherence length with only weak thickness dependence. Angle-resolved photoemission spectroscopy resolves the Bi2Te3 Dirac surface state and shows no detectable thickness-dependent shift of the Dirac point. Our results establish a model system to investigate the inverse proximity effect in TI-SC heterostructures with sharp superconducting transitions at ultrathin thickness, offering important insights into a deep understanding of TI-SC Josephson junctions.
Epitaxial strain plays a key role in determining the structure and functionality of thin films, with the choice of substrate being traditionally used to control the magnitude of the applied strain. However, even in the large family of perovskite materials, this allows for only a limited, discrete set of strain states to be achieved. Here, we report on an approach to controlling epitaxial strain during the growth of perovskite materials by using a single SrTiO3 substrate (the most available perovskite in single crystal form) and a buffer layer consisting of the solid solution Sr(Sn,Ge)xTi1–xO3, whose lattice parameter can be tuned continuously from 3.880 Å to 4.007 Å while maintaining coherent epitaxial growth on SrTiO3 with high quality interfaces. Using a BaTiO3 overlayer as a model system, we show that changes to the buffer layer composition, i.e., an increase in the in-plane lattice parameter, change the strain state of BaTiO3 from fully relaxed, through highly compressively strained, to an exotic state showing ’inverted’ epitaxy in which the buffer layer is relaxed from the substrate but lattice matched to the overlayer.
Lithium niobate (LiNbO3) thin films, serving as next-generation optoelectronic integration platforms, enable nonvolatile memristors with exceptional convolutional computing capabilities through inherent ferroelectricity. Although volatile memristors offer distinct advantages for temporal information processing due to their transient dynamics, such devices remain underexplored in LiNbO3 systems. Critically, grain boundary-associated conductive channel formation provides an effective approach for regulating volatile resistive switching behavior. Herein, we report a polycrystalline LiNbO3-based volatile memristor fabricated via pulsed laser deposition. Oxygen-vacancy-related dynamics contribute to the volatile switching behavior, while the polycrystalline grain-boundary structure provides favorable environments for localized conductive-channel evolution, which is essential for emulating the transient nature of biological neural signals. This memristor is capable of accurately emulating biorealistic synaptic functions, including paired-pulse facilitation (PPF; τ1 = 1.2 ms and τ2 = 11.6 ms) and spike-dependent synaptic plasticity. As a seven-bit physical reservoir computing unit, it achieves accuracy of 94.11% in MNIST digit classification by leveraging its intrinsic nonlinear dynamics. The memristor’s simplified structure and fabrication process ensure excellent scalability, while its inherent optical/piezoelectric multimodality enables interaction with optical and piezoelectric signals. This work lays the foundation for advanced intelligent sensing and neuromorphic computing systems.
We studied the dip-coating method for developing active fibers with a volumetric distribution of nanocrystals doped with rare-earth ions in the optical fiber core. The influence of the two-step fiber drawing process on the structure and luminescence properties of nanocrystals was investigated. Neodymium-doped yttrium aluminum garnet (Nd:YAG, 1 mol. %) nanocrystals were incorporated on a set of Ge-doped silica rods, which were stacked together in a hexagonal lattice, forming a preform of the fiber core. After the first drawing of the preform into the fiber subpreform, we observed the extended emission lifetime of Nd3+ cations at 1064 nm (τ = 457 μs), broadened luminescence bands compared to the crystalline Nd:YAG, and the lack of Raman shifts in the range of 100–900 cm−1 characteristic for the crystals. These results indicate the dissolution of Nd:YAG crystals in the glass during thermal treatment. After the second thermal drawing process, the final active fiber was developed. It had attenuation below 1 dB/m for 750–2050 nm, a cutoff wavelength at 1670 nm, and a numerical aperture NA = 0.159. Moreover, its photoluminescence spectra and lifetime (τ = 468 μs) were examined. These results show that although the Nd:YAG nanocrystals dissolved in the glass, the dip-coating method can easily introduce active ions into the optical fiber, with a uniform volumetric distribution in the core and without clustering of active cations.
Cryogenic-temperature sputtering deposition at 100 K was used to fabricate crystalline CoFe/Fe stacked ultrathin films grown on a MgO tunneling barrier. The CoFe/Fe stacked layers exhibited a high perpendicular magnetic anisotropy (PMA) energy density of 0.78 mJ/m2 and a voltage-controlled magnetic anisotropy coefficient of 161 fJ/Vm even after annealing at 673 K. The high PMA can be attributed to the addition of the Fe overlayer, which enhances the interfacial PMA originating from orbital hybridization between Fe and O atoms. Although the CoFe/Fe stacked free layers exhibited a clear PMA, Fe films directly grown on MgO exhibited rounded magnetization curves, indicating that the CoFe layer helped grow ultrathin Fe films. Stacked crystalline ultrathin films are expected to be used for the development of free layers, and the CoFe/Fe stacked layers developed in this study can be applied to various applications based on perpendicular magnetic tunnel junctions, e.g., voltage-controlled magnetoresistive random-access memory, achieving extremely low power consumption.
The performance of electron accelerators and photon detectors heavily depends on the efficiency and stability of the photocathode. The emitted beam current and brightness can be limited by surface and bulk disorder in polycrystalline photocathode materials. Epitaxial growth offers a promising route to mitigate these limitations and enable high-current, high-brightness electron beams. In this work, we present the first demonstration of the epitaxial growth of a bi-alkali antimonide photocathode, K2CsSb, utilizing both pulsed laser deposition-assisted and thermal evaporation techniques. In our study, the growth of the epitaxial layer with a flat surface was confirmed by reflection high-energy electron diffraction for the K2CsSb thin film. In situ and real-time x-ray characterization were utilized to confirm the film stoichiometry, surface roughness, and crystallinity. Quantum efficiency (QE) of 3% at 530 nm and 9% at 450 nm was obtained from an epitaxial K2CsSb thin film with a film thickness as thin as 6 nm with an ultrasmooth surface of 0.4 nm roughness on a Gr/4H–SiC substrate. Increasing the thickness to 20 nm enhanced the QE to 4.5% and 14.5% at 530 and 450 nm, respectively, with the surface roughness remaining under 1 nm. In a follow-up experiment, QE of over 9% (530 nm) and 21% (450 nm) were achieved from an epitaxial K2CsSb thin film grown on a 4H–SiC single-crystal substrate.
Digital printing of porous conductive inks is essential for circuit applications that require the printed traces to have a high surface area. Here, we introduce a method for direct ink writing (DIW) of porosity-tunable conductive inks that is free of post-processing steps. This is accomplished using an in situ non-solvent-induced phase separation mechanism in which silver flakes, eutectic gallium-indium droplets, and carbon nanotubes are suspended within a ternary phase solution of polycaprolactone, toluene, and dimethylformamide. The resulting ink has an adjustable electrical conductivity spanning 103–105 S/m and can achieve a tunable porosity of 5%–21% with a pore size range of 1–12 μm. By integrating the in situ phase separation mechanism into DIW, we can combine micrometer-scale phase-separated pores with sub-millimeter-scale printed pores to achieve multi-scale porosity. This conductive ink formulation demonstrates a way to directly tune material properties with printing, which has the potential to extend to other compositions to enable practical applications.
Merging electrochemistry with magnetism offers a compelling pathway for next-generation memory and computing technologies by enabling voltage-driven control of magnetic properties through ionic motion and interfacial redox reactions. This approach allows continuous, analog, and history-dependent tuning of magnetism, in contrast to conventional electronic methods based on binary switching, therefore opening opportunities for adaptive functionalities. By dynamically reshaping the material landscape (i.e., modifying magnetic order, anisotropy or exchange interactions) electrochemical modulation supports neuromorphic and in-memory computing architectures where processing and storage are intrinsically coupled. This Editorial highlights recent progress in electrochemically controlled magnetism while addressing key challenges, including time-scale mismatches between ionic and electronic processes, long-term reliability, variability, integration with existing technologies, and the need for standardized performance metrics. More broadly, this emerging computing paradigm points toward a shift from static circuits to dynamic materials that evolve, adapt and compute, embedding intelligence directly within their chemical state and structure.
GeSn alloys have garnered significant interest for electronic and optoelectronic applications over the past decade, primarily because of their monolithic compatibility with complementary metal–oxide–semiconductor fabrication platforms, tunable band structure, extended optical absorption into the short-wave infrared range, and excellent charge-transport properties. Recently, ion-implanted GeSn has been explored for the fabrication of photoconductive terahertz (THz) devices as a competitive alternative to low-temperature-grown III–V compound semiconductors. Since their ultrafast photoconductivity response governs the THz performance of such devices, this work provides a comprehensive characterization of the transient optoelectronic properties of ion-implanted GeSn alloys under different implantation doses and energies. In particular, optical pump–THz probe measurements were performed to assess the photoconductivity dynamics more accurately, while the Drude–Smith model was employed to extract the transient optoelectronic parameters of the samples under ultrafast excitation. Complementary characterization techniques, such as x-ray diffraction (XRD), photoluminescence (PL), and scanning electron microscopy (SEM), were also used to gain deeper insight into the properties of the ion-implanted GeSn samples. The results show that GeSn treated with moderate ion-implantation dose and energy exhibits an optimized balance between short carrier lifetime, good crystallinity, and high mobility, highlighting the importance of defect engineering for tailoring the transient optoelectronic properties of GeSn for ultra-broadband THz sensors. These findings are expected to support the development of next-generation GeSn-based functional devices.
We report on experimental and theoretical studies of deep-level defects in BGaAs alloys, with a particular focus on the effects of rapid thermal annealing (RTA) on their electrical and optical properties. The n- and p-type BxGa1−xAs layers, with varying boron concentration ranging from 0% to 2.0% (dilute borides), were grown by molecular beam epitaxy on GaAs at a substrate temperature of 400 °C and subsequently ex-situ annealed at 700 °C. Deep level transient spectroscopy (DLTS) was employed to investigate the properties of deep-level defects and their evolution upon annealing. In addition, density functional theory (DFT) was used to calculate defect formation energies and charge transition levels, enabling the identification of native point defects and defect complexes observed in DLTS experiments. Based on the obtained DFT results, we discuss the origin of the observed trap levels, with particular emphasis on B-related defects. We also found that RTA significantly affected both the carrier concentration and the total trap concentration in n- and p-type BGaAs as the boron content increased. Furthermore, annealing impacts the emission properties and overall optical quality of these dilute borides.
X-ray detection underpins a wide range of applications in medicine, security, industrial inspection, scientific research for non-destructive imaging, and material analysis. The rapid development of Ga2O3-based x-ray detectors offers a promising pathway toward next-generation detectors with high sensitivity, low noise, and harsh environment applications, benefiting from its intrinsic material properties, such as high density, wide bandgap energy, and high thermal–chemical stability. However, the underlying device operating mechanisms, including both carrier excitation and transport processes, have not yet been adequately studied, largely due to the misuse of x-ray sources in previous studies. In addition, benchmarking of device characteristics has been problematic due to experimental or data analysis issues, as well as misunderstandings of the applied equations associated with parameter definitions. In this work, we have designed and performed instructive research based on epitaxial β–Ga2O3:Si and its planar Schottky detectors, measured with energy-tunable monochromatic x-ray beams on a synchrotron beamline, clarifying the device excitation and carrier transport mechanisms with properly benchmarked device performance. In the end, we propose a set of protocols for correctly measuring and analyzing the device performance. The proposed protocols are broadly applicable and can be readily extended to other semiconductor x-ray detectors.
This study investigates the enhancement of double magnetic tunnel junction (DMTJ) performance through the integration of an assistance layer with perpendicular magnetic anisotropy. The diameter of the device and the thickness of the storage layer are identified as the key parameters that define thermal stability and switching speed. Both macrospin-model simulations and experimental measurements confirm that an increase in effective anisotropy produces faster switching. Previous reports confirmed that dual-spin torque mechanisms can be achieved with a hard polarizer or by introducing a thinner assistance layer alternative, but this can reduce tunneling magnetoresistance. In this work, by tuning the capping magnesium oxide, higher tunneling-magneto resistance values are achieved. Quasi-static measurements reveal that the highest spin-transfer torque efficiency is obtained when the effective anisotropy is maximized. Write-error-rate measurements, performed as a function of pulse width down to 7 ns, are analyzed with a new unified model introduced in this work, which combines ballistic and thermally activated regimes. One main result of this work shows that reducing the thicknesses of storage and assistance layers decreases the characteristic switching time τsw and that τsw further exhibits a clear dependence on device diameter, consistent with domain-wall-mediated switching dynamics, thereby indicating enhanced switching speed in the ballistic regime. The unified model introduces a characteristic switching time associated with minimum reversal energy, enabling the definition of a new figure of merit based on this minimum energy. The analysis highlights a trade-off where enhancing the ratios of thermal stability to current leads to longer characteristic switching times and higher write energy. These findings provide a comprehensive understanding of the factors that influence DMTJ performance and offer strategies to optimize their design for low-energy magnetic memory applications.
