Physics

Temperature, a key environmental factor affecting photovoltaic devices, strongly influences the electrical performance of CIGS solar cells. This study examines cells with 30 and 60 nm CdS buffer layers using temperature-dependent current–voltage and capacitance–voltage (C–V) measurements. While both structures show similar efficiency temperature coefficients, the underlying recombination mechanisms differ: thin-buffer cells are dominated by interface-related processes, whereas standard-buffer cells exhibit more bulk-controlled behavior. C–V profiling indicates that thermally activated changes in carrier concentration shift with buffer thickness, occurring near the heterojunction in thin-buffer cells and deeper in the absorber for thicker layers. These spatial differences are consistent with thermally activated (VSe−VCu) defect transformations. Overall, temperature-dependent electrical characterization provides insight into dominant recombination pathways and reveals how buffer thickness influences the location and impact of thermally activated defect states in CIGS heterostructures.

We investigated the impact of growth temperature on hole concentration in polarization-doped AlGaN with high AlN mole fraction on AlN, without Mg doping. A significantly lower growth temperature (around 800 °C) results in a higher hole concentration of 5 × 1018 cm−3 at room temperature in polarization-doped AlGaN without Mg doping, compared with traditional growth temperatures above 1000 °C. Furthermore, a hole concentration of 1 × 1018 cm−3 was observed even at 80 K in such an AlGaN layer without Mg doping, indicating that the hole concentration of 1 × 1018 cm−3 was induced by polarization doping alone. Previously, the hole concentrations in such AlGaN layers relied on a combination of polarization doping and Mg doping. Our results demonstrate that low growth temperature enables high hole concentration even without Mg doping.

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.

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.

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.

Semi-insulating GaN layers for high-electron mobility transistors (HEMTs) used in radio frequency applications are typically achieved through Fe-doping, despite its known adverse effects on transport properties. In this paper, we investigate the possibility of using Mn as an alternative dopant to Fe for GaN buffer layers grown by metal-organic chemical vapor deposition and assess its incorporation onto the subsequent non-intentionally doped (nid) layers. For this purpose, two studies were conducted: first, epitaxial heterostructures for HEMTs were grown with varying the thickness of the nid GaN layer, while keeping the total GaN stack thickness and the Mn-doping level, followed by varying the Mn-doping level and keeping the GaN stack thickness. The Mn-doped buffer layers showed a much faster concentration decay into the subsequent nid GaN layers than Fe. The samples maintained good surface morphology and crystalline quality, and their transport properties did not degrade with different GaN stack thicknesses and doping levels, showcasing Mn as a promising dopant for semi-insulating GaN buffer layers.

Pseudo-magnetic fields, as artificially synthesized gauge fields, have been a subject of significant research attention in classical wave systems. In this study, the pseudo-magnetic field control is realized through bidirectional parameter modulation applied to a two-dimensional photonic crystal. By lowering the symmetry of the photonic crystal, the shift of Dirac cones is induced, thereby forming quantized photonic Landau levels. Experimental observation of quantum Hall-like edge states between Landau levels enables controllable unidirectional transport of electromagnetic waves along both x and y directions. By designing opposite pseudo-magnetic fields within the photonic systems, we can achieve the snake-like interface state. Our research has expanded the physical properties of the pseudo-magnetic field and provided a new approach to its potential applications in electromagnetic wave control.

Betavoltaic cells harness the decay of beta-radiation to directly generate electrical power—making them of interest for powering remote or isolated devices. Compound semiconductors are promising materials for use in advanced betavoltaic cells owing to their excellent electronic properties combined with radiation hardness. Modifying the device architecture is one way the electrical power generated from absorbed incoming beta-radiation can be enhanced. In this study, an epitaxial layer is added to a SiC Schottky diode to create a betavoltaic device with greater power output. This output is quantified using simulated beta radiation (electron beam-induced current) and beta radiation from either a 63Ni or a 90Sr/90Y emitter. The epitaxial layer-related enhancement increases as the energy of the incoming simulated radiation increases, which is confirmed in practice for the 90Sr/90Y exposed device having Voc = 0.37 V, Jsc = 0.054 μA/cm2, and Pmax = 0.014 W/cm2.

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.

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.

Ultrafast optical manipulation of magnetism provides a promising pathway for next-generation spintronic technologies. Here, we theoretically demonstrate that terahertz-driven nonlinear phononics can induce transient ferromagnetic polarization in antiferromagnetic MnF2. Using first-principles calculations and nonlinear lattice-dynamics modeling, we show that the simultaneous excitation of two degenerate Eu infrared-active phonons drives a rectified displacement of the intrinsic B2g Raman mode through trilinear phonon coupling. The resulting lattice distortion modifies magnetic exchange interactions and produces a finite magnetization in an otherwise collinear antiferromagnetic state. Furthermore, a tailored two-pulse terahertz excitation scheme with distinct pulse widths and controlled delay enhances the rectified Raman displacement and the induced magnetization. Magnetization-dynamics simulations reveal picosecond-scale oscillations and a sizable light-induced magnetic moment approaching 1 μB per unit cell under strong excitation. These results establish a phonon-mediated pathway for ultrafast optical manipulation of antiferromagnetic order and suggest a strategy for controlling magnetism in antiferromagnetic materials using engineered terahertz fields.

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.

Controlling phonon-mediated heat transport in two-dimensional (2D) materials through intrinsic and disorder-free mechanisms remains a fundamental challenge. Here, we demonstrate that fractional-layer engineering provides a general route to intrinsically suppress thermal conductivity in 2D materials. Using first-principles calculations combined with the Boltzmann transport equation, thermal conductivity of monolayer MoSe2 and fractional-layer MoSe was investigated. Specifically, fractional-layer reconstruction leads to an almost twofold reduction in thermal conductivity. Phonon analysis shows that thermal conductivity suppression is dominated by strongly enhanced four-phonon scattering of acoustic phonons, with splitting processes playing the leading role. Additionally, fractional-layer engineering drives a sign reversal and a pronounced enhancement of the Grüneisen parameter for the out-of-plane acoustic mode, indicating strengthened phonon anharmonicity. Our results identify fractional-layer engineering as a broadly applicable strategy for intrinsic phonon and thermal-transport regulation in 2D materials.

The intrinsic metallicity of diamond (100) surfaces, arising from unsaturated dangling bonds, remains a critical bottleneck for their integration into high-performance electronic devices. Here, we conduct a systematic first-principles investigation into the modulation of electronic and thermal properties via surface reconstruction and chemical passivation, complemented by homogeneous nonequilibrium molecular dynamics simulations. Our results demonstrate that surface reconstruction via dangling-bond saturation drives a transition from metallic to semiconducting characteristics, with the 2 × 1 reconstruction exhibiting the lowest surface energy (0.294 eV/Å2) and optimal stability. Furthermore, the introduction of functional groups (–F, –H, –O, –OH, and –NH2) enables precise modulation of the band structure and work function of the diamond (100) surface. Among the investigated functional groups, the H-terminated surface stands out as the optimal configuration, achieving a high acoustic-phonon-limited hole mobility (∼2.09 × 104 cm2 V−1 s−1) while maintaining a remarkable thermal conductivity of 726.2 W m−1 K−1. Our findings provide critical theoretical guidance for the development of high-performance diamond-based power electronics and thermal management systems.

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.

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.

Electrodeposition technology has shown unique application potential in perovskite solar cell fabrication due to its characteristics of requiring only water and alcohol solvents without the need for high-vacuum environments. However, current related research is still in the preliminary exploration stage (fewer than 50 publications), and most are concentrated on manual laboratory preparation, with prominent issues such as rough process control and poor reproducibility, with few systematic studies on automated control of electrodeposition processes and synergistic regulation of additives. Addressing these issues, this work designs and constructs a fully automated electrodeposition preparation system, which achieves high reproducibility in the preparation of MAPbI3 perovskite solar cells. The resulting devices exhibit an efficiency of 7.29%, a 16% improvement over manual preparation, with the efficiency standard deviation reduced from 1.47% to 1.22%. On this basis, by further introducing MACl and FABr additives, the champion efficiency was improved to 15.15%, with the efficiency standard deviation reduced to 0.93%. The automated preparation system and additive regulation strategy proposed in this study provide effective technical support and equipment reference for developing low-cost and environmentally friendly preparation of perovskite solar cells.

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.

Extending the spectral content of femtosecond lasers to span at least one octave is key for their stabilization through self-referencing. An effective way to achieve this goal is to leverage soliton dynamics and dispersive wave (DW) generation in third-order nonlinear waveguides or fibers. Tuning of the DW position at the second harmonic of the pump is still a challenge, and it results in an excessive power requirement on the input pulse. Here, we show a new tuning mechanism of DW position in GaN waveguides by controlling the thickness of the cladding deposition. We demonstrate efficient octave-spanning supercontinuum generation by controlling the position of the DW and detecting the carrier-envelope offset frequency with only 22 pJ of pulse energy with, respectively, 100 kHz resolution and 100 Hz video bandwidth.

Localized thermal excitation is ubiquitous in practical heat-flux sensing but deviates from the uniform-heating assumption underlying conventional transverse thermoelectric (TTE) models. Here, we demonstrate that TTE thin-film devices exhibit an intrinsic spatial response under point-like heating, where the output voltage depends strongly on the excitation position. Finite-element simulations reveal that the cross-plane temperature difference remains nearly invariant, while the in-plane temperature gradient varies significantly with heating location. This lateral gradient introduces an additional thermoelectric contribution that can enhance, suppress, or even reverse the output signal. The same spatial-dependence mechanism is further reproduced in finite-element simulations of Cu-Constantan multilayer devices, showing consistent behavior without polarity reversal. These results establish that the TTE response arises from the coupling of cross-plane and lateral thermal gradients, extending the conventional ΔTz-dominated interpretation to localized-heating conditions and providing a physical basis for position-dependent calibration and spatial-error assessment in localized heat-flux sensing.

This paper reports a compact, efficient, low-threshold continuous-wave 320 nm ultraviolet laser based on a linear cavity with Pr:YLF and LiB3O5 (LBO) crystals. Thermal effects were analyzed via a three-dimensional temperature field model and thermal lens calculations, guiding the selection of crystal parameters that balance pump absorption and heat load. Using a 0.5 at. % doped Pr:YLF crystal of 8 mm length and a 12 mm long LBO crystal, the laser delivers 3.03 W of continuous-wave 320 nm output with a slope efficiency of 35.2% and a threshold power of 60 mW—the highest slope efficiency reported for a CW Pr:YLF laser at this wavelength. The compact linear cavity minimizes length and loss, offering a viable design for future highly integrated ultraviolet lasers using crystal bonding technology.

We demonstrate exchange bias-driven large topological Hall effect in epitaxial NiFeMo/NiFeMoO heterostructures fabricated on Al2O3 (0001) substrates. Structural analyses confirm high-quality epitaxial growth with distinct strain states in single-layer NiFeMo and bilayer NiFeMo/NiFeMoO thin films. Magnetometry reveals that NiFeMo remains ferromagnetic over the entire temperature range, whereas NiFeMoO undergoes a suggested paramagnetic-to-antiferromagnetic transition near 175 K, inducing robust exchange bias in field cooled NiFeMo/NiFeMoO heterostructures. Magnetic force microscopy, supported by micromagnetic simulations incorporating Dzyaloshinskii–Moriya interactions, uncovers non-coplanar spin textures at room temperature under moderate magnetic fields. Magnetotransport studies identify a pronounced topological Hall-like signal, strongly enhanced by exchange bias at low temperatures. These results establish that epitaxial strain and interfacial exchange coupling act synergistically to stabilize non-coplanar spin textures and tailor the topological Hall response from 10 to 300 K, offering a promising pathway for oxide/metal interface-based topological spintronic applications.

We investigated the transport properties of 2DEG in ScAlN/GaN heterostructures prepared by plasma-assisted molecular beam epitaxy. Four samples with Sc compositions of 3%–17% and ScAlN barrier thickness of 4–6.6 nm were grown on GaN/SiC template substrates. In situ reflection high energy electron diffraction patterns and atomic force microscopy images confirmed the atomically smooth surface. The atomic-resolution scanning transmission electron microscopy image demonstrated an abrupt ScAlN/GaN interface. At room temperature, the sample exhibited a sheet electron density of 2.0–3.1 × 1013 cm−2 and an electron mobility of 179–468 cm2/Vs. The sheet electron density remained nearly constant across temperatures from 2 to 400 K, indicating that the 2DEG is induced solely by the polarization effect. As the temperature decreased, the mobility increased and eventually saturated. The scattering mechanisms limiting electron mobility were analyzed, accounting for the increased effective mass due to the non-parabolicity of the conduction band at a high sheet electron density. The calculated total mobility shows excellent agreement with the experimental data, suggesting that temperature-independent interface roughness scattering is the dominant mechanism. These findings provide critical insights for understanding and improving the transport properties of 2DEG in ScAlN/GaN for HEMT applications.

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.

According to the classical laws of magnetism, the shape of magnetically soft objects limits the effective susceptibility. For example, spherical soft magnets cannot display an effective susceptibility larger than 3. Although this is true for macroscopic multi-domain magnetic materials, we explain why magnetic nanoparticles in a single-domain state do not suffer from this limitation. For single-domain particles, the differences between demagnetization factors along the principal axes are relevant and can influence susceptibility but do not limit it to an upper value as in the case for multi-domain particles. We experimentally validated this result on spherical nanoparticles with varying diameters (9–150 nm) and varying volume fractions (0.1–47 vol. %). In agreement with our predictions, we measure single-domain particle susceptibilities largely above 3, in fact up to more than 250. Moreover, contrary to an existing model for assemblies of particles, we find that the susceptibility of materials composed of non-interacting single-domain particles in a non-magnetic matrix scales linearly with the volume fraction of particles. This implies that high susceptibilities (>100) are achievable for nanoparticle-based composites and is relevant for the design of magnetically soft materials that are operational at MHz–GHz frequencies with negligible power losses.