Physics

Non-destructive avalanche breakdown is crucial for enhancing the robustness of power devices against overvoltage stresses and surge energy, whereas state-of-the-art GaN HEMTs present no avalanche capability. In this work, we demonstrate a semiconducting p-GaN gate HEMT (SG-HEMT) that achieves avalanche-like non-destructive breakdown capability. A thin p-GaN layer covering the AlGaN layer serves as the intrinsic gate, with a control electrode formed outside the active region. This thin p-GaN layer (i.e., the SG) depletes the 2DEG in the channel and creates an energy barrier for electrons. During the blocking state, the depletion region gradually expands within the SG under high drain stress. Once the SG is fully depleted, the electron barrier in the channel is eliminated, allowing electron current to flow and consequent avalanche-like non-destructive breakdown. While this non-destructive breakdown exhibits similar electrical characteristics as the avalanche process, the SG-HEMT operates on a distinct physical mechanism. Instead of impact ionization and carrier multiplication, breakdown of the SG-HEMT is triggered by the full depletion of the SG, facilitating unipolar channel electron conduction. Thus, the well-designed SG-HEMT with an SG length of 9 μm exhibits a non-destructive breakdown voltage of ∼483 V, targeting 400 V applications. Moreover, the SG-HEMT shows no current degradation after ten cycles of repetitive breakdown tests. Meanwhile, owing to the suppression of trapping effects by the SG, an ultra-low dynamic RON/static RON ratio of 1.09 is achieved after 400 V stress. These results indicate that the SG-HEMT offers a promising pathway to construct highly robust GaN power systems.

Acoustic vortex beams carrying orbital angular momentum (OAM) have attracted a broad research interest in the past 20 years. Due to the orthogonality of OAM beams with different orders, a wealth of interesting applications have been developed, including communications, particle manipulation, and imaging. Although many techniques have been reported to generate acoustic OAM beams, it still remains a formidable challenge to generate reconfigurable and order-controllable OAM beams via a cost-effective manner. In this work, we propose a balanced-ternary-based approach to generate reconfigurable order-controllable acoustic OAM beams in water. We cascade three engineered spiral phase plates and manipulate them in a reconfigurable manner to generate 26 different OAM modes at ultrasonic frequencies. Experimental results demonstrate the good quality and high purity of all the generated OAM beams. According to this method and theory, we can flexibly extend our design to generate acoustic OAM beams with arbitrarily high orders. This work provides a new paradigm for the reconfigurable and order-controllable generation of acoustic OAM beams and may facilitate many OAM-based applications.

Bismuth-based pyrochlore phases, as linear dielectrics, offer a compelling combination of ultralow loss, moderate permittivity, and high breakdown strength, rendering them promising for dielectric energy storage applications. However, the advancement of related research is impeded by the inherent thermodynamic instability in the Bi2O3–TiO2 phase diagram, which makes the synthesis of phase-pure Bi2Ti2O7 particularly challenging. In this work, we demonstrate that the stability of Bi2Ti2O7-based phase is significantly enhanced through careful control of the thermal processing temperature. The resulting materials, therefore, show ultralow dielectric loss and reduced polarization switching hysteresis as well as improved breakdown field. Consequently, a high energy density of 65.5 J/cm3 and an excellent energy efficiency of 84.3% are achieved concurrently. The findings reported herein help to elucidate the relationship between pyrochlore structure stabilization and thermal treat process, thereby providing an effective way for improving the energy storage performance of Bi2Ti2O7-based thin films.

We report a pressure-threshold phenomenon in nanosecond air discharges: explosive cathode processes become the dominant source of a dense highly ionized plasma at air pressures above ∼100 Torr. Picosecond laser imaging reveals that these explosive processes, while competing with air ionization, result in the ejection of cathode material over a time scale shorter than 1 ns to create a plasma with electron densities as high as ≈6 × 1019 cm−3, significantly exceeding values achievable through air ionization alone. The explosive processes exhibit a sharp onset, disappearing when the air pressure drops by as little as tens of Torr relative to 100 Torr. Our findings demonstrate that, at high air pressures, cathode vaporization—rather than gas ionization—initiates the generation of such a dense plasma and enables electric spark formation. At intermediate pressures, below ∼100 Torr, yet above vacuum conditions, this explosive mechanism is suppressed, and the discharge transitions to a regime governed by volumetric gas ionization. This behavior contrasts with vacuum discharges, where cathode material is the dominant plasma source. The revealed threshold mechanism is crucial for controlling plasma in applications ranging from nanoparticle synthesis to pulsed power systems.

Graphene exhibits optical chirality only when its in-plane mirror symmetries are broken—by strain corrugation, buckling, interlayer twist, chemical corrugation, or planar patterning—yet the microscopic route by which such interfacial handedness is transferred to adjacent otherwise achiral soft matter needs to be understood in full theoretical depth. We present a symmetry-based, predictive model showing that non-collinear corrugations of a graphene sheet generate a geometric pseudoscalar that acts as a chiral field at the graphene–liquid crystal (LC) interface. This field deracemizes configurationally achiral smectic-A LC molecules aligned by π–π stacking on the graphene surface and, under an out-of-plane electric field, drives a surface electroclinic effect (ECE)—a measure of chirality in the LC. The framework yields clear predictions: a single-pole frequency response; a rise-then-saturate behavior of the electroclinic coefficient with increasing graphene corrugation amplitude; and a monotonic increase of the cutoff frequency with roughness. The predictions are consistent with previously reported experimental observations, including graphene circular-dichroism studies and our companion LC measurements. The model explains why ultraflat graphene is optically achiral, yet strained graphene induces a robust interfacial ECE, and it provides practical design knobs for engineering chiral responses in 2D/soft-matter hybrids, opening a broadly applicable route to interfacial chirality control.

To investigate the mechanism of the linear electro-optic effect in strained silicon and advance the practical application of silicon-based optical modulators, this paper addresses the limitations of existing theoretical models due to their lack of continuous strain tuning capability. By employing a self-designed uniaxial stress setup, continuous and tunable strain was applied to silicon, enabling systematic measurement of the relationship between the output electro-optic signal and strain. Through controlled variable analysis, the synergistic influence of strain and modulation electric fields was qualitatively examined. Signal separation was performed by exploiting the polarization dependence differences between the plasma dispersion effect and the Pockels effect. The results indicate that the electro-optic signal primarily originates from the plasma dispersion effect, and both this effect and the contribution from the Pockels effect enhance with increasing strain. When strain reaches the order of 10−4, the electro-optic tensor of silicon attains a magnitude of approximately 0.01 pm/V. An experimental formula describing the variation of the second-order nonlinear susceptibility tensor with strain is also provided. This study offers direct experimental evidence for elucidating the mechanism of the linear electro-optic effect in strained silicon and holds significant implications for accelerating the integration of silicon-based electro-optic modulators.

The structural properties of a β-Ga2O3 single crystal grown by the oxide crystal growth from cold crucible (OCCC) method were investigated using synchrotron radiation x-ray topography and x-ray reticulography. The region grown beneath the seed exhibits high crystalline quality with a rocking curve full width at half maximum of about 26 arc sec. During diameter enlargement, a twist-type lattice misorientation develops between the central and laterally expanded regions, originating near the shoulder and propagating along boundaries parallel to the 〈010〉 growth direction. Dislocation analysis reveals that 〈010〉-oriented screw dislocations dominate the defect structure with densities of ∼105 cm−2, while higher densities (∼106 cm−2) appear in the wing region (i.e., the laterally expanded region formed during diameter enlargement). These results clarify defect formation in OCCC-grown β-Ga2O3 and provide insights into optimizing growth conditions.

As a typical ionic crystal exhibiting liquid-like behavior, p-type semiconductor Cu2Se has been a research hotspot in thermoelectric materials due to its low thermal conductivity. In this work, by varying the temperature and pressure conditions during synthesis, different morphologies and quantities of microstructural defects were introduced in Li+ and S2− co-doped copper selenide samples. The influence of these defects on the thermoelectric performance of the samples was subsequently investigated. The synthesized samples were characterized for their phase composition, microstructure, electrical transport properties, thermal transport properties, and thermoelectric performance. The results indicate that cracks are the primary defects in samples sintered at high temperature and atmospheric pressure, while impurity phases and micron-sized pores are the main defects in samples sintered under high pressure and low temperature. These defects significantly affect the electrical and thermal transport properties of the doped Cu2Se samples. The presence of residual Cu3Se2 and elemental Cu in the sample sintered at 5 GPa/room temperature leads to anomalous electrical transport behavior. When the pressure and temperature exceed 5 GPa and 1200 °C, the shape of pore defects is markedly altered, their number is reduced, and the propagation of crack defects is simultaneously suppressed. By employing Li+ and S2− co-doping combined with high-temperature and high-pressure sintering, the ZT value of the doped Cu2Se material was enhanced from 2.1 to a maximum of approximately 2.4.

Developing lead-free piezoelectric materials with high thermal stability is essential for sensing and actuation applications in harsh environments. In this work, trace amounts of lithium niobate (LiNbO3) were introduced into the 0.75BiFeO3–0.25BaTiO3 system to tailor the phase structure, lattice distortion, and defect chemistry. The substitution of Li+/Nb5+ for A-/B-site elements promotes partial rhombohedral (R) to tetragonal (T) phase transition, enhances tetragonal distortion, reduces oxygen vacancy formation, and simultaneously strengthens phase separation. This slightly weakens long-range ferroelectric order and facilitates the formation of a suitable amount of nanodomains. The optimized composition (x = 0.001) exhibits highly stable piezoelectric constant (d33) around 107 pC/N (at room temperature) over a broad temperature range of 30–322 °C, with fluctuations below 10%, and retains a high piezoelectric response (d33 ∼ 103 pC/N) even after aging at 300 °C for 12 h. This enhanced stability stems from the balanced interplay among the thermal disturbance de-pinning effect, thermally driven dipole-moment attenuation effect, and thermally induced ferroelectric domain disorder effect. This study offers an effective defect-phase-domain design strategy for realizing lead-free piezoelectric ceramics with high thermal reliability.

Facet degradation in blue GaN-based laser diodes operating at output optical powers exceeding 17 W is investigated. Facet coating failure in large optical cavity devices is shown to originate from an inhomogeneous temperature distribution at the front facet caused by non-uniform optical intensity across the emitting region under high-power operation. Localized heat accumulation leads to partial melting of the epitaxial material near the facet, and the resulting thermal stress induces coating delamination and spalling, constituting catastrophic optical damage (COD). Notably, the coating structure and interfaces within the damaged regions remain well defined after COD, indicating that failure is initiated by localized thermal degradation of the epitaxial material, which subsequently triggers coating failure. These results demonstrate that suppressing optical intensity non-uniformity or reducing the overall junction temperature is critical for mitigating facet COD in high-power GaN-based laser diodes.

We present a model dielectric function composed of critical point functions in order to parameterize the temperature and wavelength dependencies of the dielectric function of InAs. This model is based on Adachi’s critical point model, with simple wavelength-dependent analytical functions whose parameters change linearly with temperature. The calculated dielectric function at room temperature is in excellent agreement with previously published data. We apply this model in the spectral range of 0.7–5 eV and in the temperature range of room temperature to 250°C with in situ spectroscopic ellipsometry measurements on an InAs substrate. Spectroscopic measurements were performed continuously while slowly ramping sample temperature in a stepwise manner in the controlled ambient environment of an atomic layer deposition system. We find that our model matches excellently with all experimental data with deviations less than 2% in pseudoepsilon. Our model permits smooth interpolation of the dielectric function of InAs for any intermediate temperature in the range studied and therefore can be used to monitor temperature, for example, during thin film deposition processes by in situ spectroscopic ellipsometry. We propose that this model can be applied to other semiconductors as well as wider temperature ranges.

Temperature-dependent charge transport in epitaxial GeSiSn films with varying thicknesses and Si and Sn content, grown on Ge/Si(001) substrates, was investigated using admittance spectroscopy. The conductance exhibits a crossover from Mott variable-range hopping at low temperatures to thermally activated conduction at higher temperatures. Below 150 K, carriers move via localized states in the band tails, with hopping parameters governed by the density and spatial distribution of disorder-induced states. Increasing nominal Sn concentration and thickness enhances structural disorder and Sn segregation, leading to a higher density of states and reduced hopping length. Scanning capacitance microscopy reveals variations in charge-carrier concentration in high-Sn films, indicating the presence of coexisting p-type and n-type regions at the microscale, consistent with compositional fluctuations, Sn segregation, and microstrain. These results demonstrate that transport in GeSiSn alloys is primarily dominated by disorder-assisted hopping at low temperatures, establishing a quantitative link between microscopic disorder and macroscopic electrical response in metastable group-IV semiconductors.

We present a systematic first-principles study of dopant-induced phase stabilization in HfO2 across ferroelectric (FE) and non-ferroelectric polymorphs for a large set of dopants. To overcome the strong configuration dependence of defect energetics, we develop a multi-stage screening workflow that identifies low-energy dopant–vacancy configurations. In an equilibrated-solution [“PVD” (physical vapor deposition)] model, bulk T = 0 K energetics indicate that ionically compensated doping alone is insufficient to stabilize the orthorhombic-ferroelectric phase over the monoclinic phase; certain dopants even increase its relative energy. Configurational-entropy corrections are small at device-relevant temperatures, and vibrational contributions—benchmarked against explicit phonon calculations—primarily stabilize the tetragonal phase, remaining inadequate to reverse bulk phase ordering at moderate anneal temperatures. We further observe that many rhombohedral-FE supercells lose phase identity upon relaxation, typically collapsing toward orthorhombic-FE motifs, underscoring sensitivity to local defect arrangements. To assess deposition effects, we introduce a planar (“atomic layer deposition”) model that mimics dopant layering; it amplifies dopant-identity sensitivity and can reshape phase competition compared to the equilibrated-solution limit. Overall, our results suggest that experimentally observed ferroelectric stabilization in thin films arises from a combination of interfacial/finite-size terms and deposition-induced dopant distributions, rather than bulk thermodynamics alone.

In this work, we present the design and optimization of a silicon-based selective emitter (SE) for thermophotovoltaic (TPV) systems with enhanced spectral efficiency. The proposed SE combines a broadband infrared absorber, a periodic triangular arrays of heavily doped silicon in this case, and a one-dimensional (1D) dielectric photonic crystal filter alternating silicon and silicon dioxide (Si/SiO2) layers. Both parts are integrated within a single monolithic structure that forms the selective emitter. The broadband absorber ensures high emissivity in the in-band region (i.e., for wavelengths shorter than the bandgap wavelength of the TPV cell), while the photonic crystal filter selectively suppresses long-wavelength emission in the out-of-band region (i.e., for wavelengths longer than the bandgap wavelength of the TPV cell), resulting in a spectrally selective emission profile over a wide infrared range up to 8 μm. To enhance the performance of the structure, a Particle Swarm Optimization algorithm was implemented to optimize the thicknesses of the Si/SiO2 multilayers. The optimized design demonstrates a spectral efficiency improvement exceeding 20% compared to conventional quarter-wave photonic crystal structures, and more than 45% relative to a blackbody emitter at an operating temperature of 1500 K. Beyond the specific structure investigated in this study, the proposed optimization framework provides a general and flexible methodology that can be readily extended to other broadband emitters and tailored to different TPV cell bandgaps.

Low threading dislocation density (TDD) GaN substrates are essential for fabricating vertical GaN power devices. In this study, we fabricated large-diameter, low-TDD GaN wafers using the Na-flux multipoint seed (MPS) method. However, the MPS method leads to regions with high-TDD values (>105 cm−2) at the crystal coalescence regions. We focused on the Ga composition of a Ga–Na melt and investigated TDD reduction by modifying the crystal growth mode. As the Ga composition was reduced from 27 to 19 mol. %, crystal shape uniformity improved, and the maximum TDD above the coalescence regions decreased from 4.41 × 105 to 3.01 × 105 cm−2. This reduction suggests that the improved shape uniformity promotes the efficient concentration of dislocations toward the coalescence boundaries. We also found that reducing the Ga composition effectively suppressed the formation of inclusions in the GaN crystals, contributing to the reduction of inclusion-induced dislocations. At 19 mol. % Ga composition, the mean TDD in regions excluding coalescence boundaries was in the low 104 cm−2 range. These findings are expected to contribute to the fabrication of high-quality GaN substrates using the Na-flux method.

Dipole–dipole interactions are central to energy-transfer processes. Understanding the interaction dynamics is essential across diverse fields, including solar energy harvesting, organic light-emitting diodes, long-range energy transport, and molecular biosensing. Such interactions are fully described by the photonic Green function, which defines the electromagnetic response of a point dipole source. However, direct experimental access to the individual components of Green’s function at the single-dipole level remains challenging. Here, we employ a double-probe terahertz (THz) near-field microscope to directly map the dipole–dipole interactions in free space and near a resonant dielectric interface formed by a pellet of α-lactose and air. In free space, we observe highly anisotropic energy-transfer dynamics arising from the non-radiative near-field contribution of Green’s function, including the pronounced suppression associated with the magic-angle condition. Near the air/α–lactose interface, we reveal strong modifications of Green’s function along both the in-plane and out-of-plane directions: a reduction in the energy-transfer rate near the vibrational resonance of the medium and pronounced oscillations in intensity and phase due to interference between the direct dipole–dipole interaction and the surface-mediated contribution. Our results provide direct access to the non-radiative components of Green’s function and establish a powerful framework for probing and engineering dipole–dipole interactions in complex and resonant photonic media.

Metal organic frameworks (MOFs) have emerged as a promising structure to improve the physicochemical performance of semiconductors by constructing MOF-based structures. In this study, ZnO photocatalysts were successfully synthesized from zeolitic imidazolate framework-8 (ZIF-8) utilizing an elementary notion aqueous approach, which requires mixing at room temperature. By adjusting the heating temperature and total amount of organic ligands in the MOF structure, the optical and structural properties of the resultant ZnO were successfully controlled. The resulting ZnO derived from ZIF-8 was applied as water pollutant removal by degrading methylene blue (MB) under visible light irradiation, and the ZnO-650 sample outperformed the other samples, removing approximately 91% of MB within 30 min. The enhanced activity was primarily due to the sample’s improved crystallinity and increased specific surface area preserved from the ZIF-8 precursor, as well as the decreased photoluminescence intensity, which reduced electron–hole recombination. Furthermore, density functional theory was used to simulate the ZnO-derived ZIF-8 structures and calculate the samples’ energy bandgap and work function, which explain the charge migration process sufficiently to better understand the photocatalytic pathway of ZnO derived from ZIF-8, allowing us to propose a potential MB degradation mechanism. Overall, this study will offer an effective procedure for creating and establishing efficient semiconductor photocatalysts that utilize the advantage of MOFs’ superb properties.

ABSTRACT The senescence of nucleus pulposus cells (NPCs) is a hallmark pathological feature of intervertebral disc degeneration. Senescent NPCs exhibit mitochondrial dysfunction and impaired mitophagy, thus compromising mitochondrial turnover and quality control. Current therapeutic strategies predominantly target external risk factors but do not restore mitophagy. In this study, a proline carbon dot‐composited poly(L‐methionine) (EG 45 M 25 /Pro‐CD) hydrogel is developed and employed to re‐establish the impaired mitophagy pathway. Expression levels of Parkin and phosphorylated Parkin increase, indicating successful activation of the mitophagy pathway. Restoration of mitochondrial function markedly reduces oxidative stress and the senescence‐associated secretory phenotype in NPCs. The levels of type II collagen and aggrecan recover to 84.63% and 77.31% of their normal ones, respectively. In an animal model, the administration of Pro‐CD in a thermo‐sensitive and injectable hydrogel effectively preserves the water content and height of intervertebral discs while maintaining the structural integrity and functional activity of NPCs. The mean Pfirrmann grade improves from 5.00 in the Control group to 3.33 after treatment, whereas the disc height index recovers to 82.06% of the physiological value. Collectively, our findings demonstrate that the EG 45 M 25 /Pro‐CD composite hydrogel has therapeutic potential to promote endogenous disc regeneration by enhancing mitophagy, re‐establishing redox homeostasis, and alleviating NPC senescence.

ABSTRACT Although chalcogenide‐based catalysts offer significant potential for enhancing lithium‐sulfur (Li‐S) battery performance, the absence of reliable descriptors linking the d ‐band center to sulfur conversion kinetics hinders the rational design of electrochemical systems. Herein, we address this limitation by engineering a catalytic interlayer through modification of 2H‐MoS 2 electronic structure, achieved via substitutional doping of n‐type Co/Fe transition metals (TM) at Mo sites. Comprehensive findings elucidate that such doping initiates a distinct S‐mediated d ‐ p hybridization involving Mo 4 d— S 3 p— TM 3 d orbitals, thereby modulating electronic density of states near the Fermi level. Specifically, in the CoFe‐MoS 2 @carbon paper (CP) interlayer, synergistic effect of co‐doping with two different TM drives optimized downshift of the Mo 4 d ‐band center to intermediate energy states, fostering moderate catalyst‐reactant interaction. Furthermore, the simultaneously lowered S 3 p ‐band center enhances the degree of d ‐ p orbital overlap. These electronic redistributions enhance both electrical and ionic conductivity, thereby facilitating accelerated redox kinetics with reduced activation energy, while mitigating the shuttle effect and promoting uniform Li 2 S deposition. Consequently, the assembled cell delivers outstanding stability with a low decay rate of 0.024% for 2000 cycles even at 10C. This work emphasizes that a balanced d ‐band center is key to achieving highly active chalcogenide‐based materials for advanced Li‐S batteries.