Physics

Ammonia is essential for fertiliser production and is also considered a promising carbon-free energy carrier; however, its large-scale synthesis through the Haber-Bosch process remains highly energy-intensive. Carbon-supported Ru catalysts promoted by Ba have shown high activity for ammonia synthesis under mild conditions, yet the atomic-level role of Ba in N 2 adsorption, activation, and initial N–N cleavage tendency remains incompletely understood. In this study, density-functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations were employed to investigate defective graphene-supported Ru/Ba systems containing either isolated Ru atoms or Ru 11 clusters. Structural and electronic analyses, including Bader charge, charge-density difference, density of states, and Crystal Orbital Hamilton Population (COHP) analyses, show that Ba acts as an auxiliary electronic promoter that pre-modulates the Ru electronic structure, enhances Ru-mediated back-donation to N 2 , and weakly stabilises the Ru-polarised distal N atom at the Ru–Ba–N 2 interface, thereby strengthening Ru–N interactions and weakening the N ≡ N bond.

In van der Waals heterostructures, the quantum-confined Stark effect enables electrical control of optical transitions, yet it carries an intrinsic penalty: every field increment that redshifts the emission also pulls the electron and hole wavefunctions apart, eroding the oscillator strength on which device efficiency depends. We report a dual-modulation strategy for MoS2/WSe2 bilayers that pairs biaxial tensile strain (0%–2.0%) with a reduced perpendicular electric field (0–200 kV/cm). The strain pre-conditions the permanent interlayer dipole moment, so a smaller field achieves a given spectral shift while wavefunction separation stays limited. We assembled a 5000-point density functional theory with Grimme's D3 dispersion correction and Becke–Johnson damping [DFT-D3(BJ)]/Perdew–Burke–Ernzerhof exchange-correlation functional (PBE) and technology computer-aided design dataset by Latin hypercube sampling and trained a deep neural network surrogate (R2 > 0.98); it outperformed Gaussian process, random forest, and polynomial response-surface baselines, most clearly in the super-additive nonlinear interaction regime. NSGA-II optimization mapped the full Pareto frontier. At ɛ = 1.5% and F = 85 kV/cm, the balanced operating point delivers a 340 nm redshift at a normalized oscillator strength of 0.48, a 218% gain over the field-only reference at the same shift. First-order perturbation analysis attributes this to a strain-induced reduction of the dipole moment from 0.42 to 0.31 eÅ, which lowers the required field by 43% and reduces wavefunction separation in proportion. HSE06 benchmarking of five Pareto configurations confirms that relative PBE trends hold to within 6%. Under ±5% fabrication tolerances, Monte Carlo simulation gives an efficiency variation of 3.3% for this configuration, against 41% for the field-only device, a 12.5-fold gain in process robustness.

Domain switching by femtosecond (fs) light pulses was detected recently in several ferroelectric materials, including lithium niobate (LN), and attracted a strong research interest. Qualitative explanations of this phenomenon by the generation of pyro- and thermoelectric fields, given in the experimental papers, are misleading. We propose a model of fs switching as applied to LN crystals. It includes new ingredients and provides a clear physical picture of the detected nontrivial physical phenomenon. In particular, it dismisses the pyroelectric and thermoelectric fields from the primary reasons of the reversal. The multiphoton band-to-band absorption within the transparency window causes not only the local heating of the crystal but also the generation of free charge carriers. The resulting photo-conductivity quickly compensates the pyroelectric fields and provides, after trapping of free carriers, a negative replica of the compensated electric fields. This replica possesses the sign necessary for the polarization reversal and, under proper parameters of the pulses, exceeds the coercive fields not far from the sample interface.

Acoustic energy harvesting has attracted considerable attention due to its promising applications in low-power sensors, self-powered monitoring systems, and Internet of Things (IoT) nodes. In this study, the thermoacoustic effect is leveraged to modulate the amplitude of acoustic oscillations within a standing-wave acoustic resonator, providing a novel strategy for enhancing the performance of acoustic energy harvesters beyond conventional approaches. First, an experimental prototype of the acoustic resonator driven by an external loudspeaker was designed and constructed, and its forced response was systematically characterized. Subsequently, the dynamic behavior of the loudspeaker-driven acoustic resonator integrated with an inhomogeneously heated thermoacoustic stack was investigated, with particular emphasis on the effects of driving frequency and voltage. The results show that, as the temperature difference across the thermoacoustic stack increases, acoustic oscillations inside the loudspeaker-driven acoustic resonator can be either amplified or suppressed. Distinct frequency bands are identified in which amplification and suppression of acoustic oscillations take place. Specifically, amplification occurs within the range of 70–130 Hz, whereas suppression is observed over 40–70 and 130–250 Hz. In addition, an optimal driving voltage of 5.5 V is observed, at which the modulation effect becomes most pronounced, yielding a maximum pressure ratio of 1.48 and a minimum pressure ratio of 0.81. The amplitude modulation strategy proposed in this study not only expands the application scenarios of thermoacoustic technology but also offers valuable guidance for the optimization and performance enhancement of acoustic energy harvesting systems.