Perovskite materials are considered to be promising electrolyte membrane candidates for electrochemical applications owing to their excellent proton- or oxide-ion-conducting properties. RbBiNb₂O₇ is a double-layered Dion–Jacobson perovskite oxide, with Pmc2₁ symmetry. In this study, the electronic structure and proton-diffusion properties of bulk RbBiNb₂O₇ were systematically investigated using first-principles calculations. The unique layered crystal structure of RbBiNb₂O₇ plays a crucial role in proton storage and proton conductivity. Different proton-diffusion steps in RbBiNb₂O₇ were considered, and the activation energies of the relevant diffusion steps were evaluated using the climbing image-nudged elastic band (CI-NEB) technique. The proton diffusion in RbBiNb₂O₇ presents a two-dimensional layered characteristic in the a-b plane, owing to its layered crystalline nature. According to the transition state calculations, our results show that the bulk RbBiNb₂O₇ exhibits good proton-transport behavior in the a-b plane, which is better than many perovskite oxides, such as CaTiO₃, CaZrO₃, and SrZrO₃. The proton diffusion in the Rb–O and Nb–O layers is isolated by a higher energy barrier of 0.86 eV. The strong octahedral tilting in RbBiNb₂O₇ would promote proton transport. Our study reveals the microscopic mechanisms of proton conductivity in Dion–Jacobson structured RbBiNb₂O₇, and provides theoretical evidence for its potential application as an electrolyte in solid oxide fuel cells (SOFCs).

Two-dimensional (2D) materials have potential applications in nanoscale sensors and spintronic devices. Herein, motivated by experimental synthesis of a CrI${}_3$ monolayer possessing intrinsic magnetism and a Janus MoSSe monolayer with piezoelectricity, we propose a 2D Janus Cr${}_2$I${}_3$F${}_3$ monolayer as a multifunctional material exhibiting both piezoelectricity and ferromagnetism. Using density functional theory calculations, we systematically investigated the structural stability and the electronic, magnetic, and piezoelectric properties of the Janus Cr${}_2$I${}_3$F${}_3$ monolayer. We predicted that a vertical polarization of up to −0.155 × 10${}^{-10}$ C/m is induced in the Cr${}_2$I${}_3$F${}_3$ monolayer due to the breaking of symmetry. The origination mechanism of polarization was demonstrated in terms of a local dipole moment calculated by maximally localized Wannier functions. Meanwhile, it was found that a remarkable piezoelectric response can be produced under a uniaxial strain in the basal plane. The calculated piezoelectric coefficients of the Cr${}_2$I${}_3$F${}_3$ monolayer compare favorably with those of the frequently used bulk piezoelectric materials such as $\alpha$–quartz and wurtzite AlN. Particularly, the e${}_{31}$ and d${}_{31}$ values of the Cr${}_2$I${}_3$F${}_3$ monolayer are nearly 10 times as large as that of Mo-based transition metal dichalcogenides. We also found that the magnitude of e${}_{31}$ mainly arises from the ionic contribution, while the electronic contribution can be nearly neglected. The considerable piezoelectric response combined with the intrinsic magnetism make the Janus Cr${}_2$I${}_3$F${}_3$ monolayer a potential candidate for novel multifunctional devices integrating both piezoelectric and spintronic applications.

Comprehensive analysis of compositional effects on wurtzite Ga_xIn_1−xN ternary alloys by first-principles study. Observed nuanced deviations from Vegard's law, especially in the band gap, elastic, piezoelectric, and dielectric constants.

Utilizing a continuum model, this study systematically investigates the Fermi arcs of topological surface states in three-dimensional multi-Weyl semimetals. We analyzed the energy spectra and wave functions for bulk quadratic and cubic-Weyl semimetals, each with a single Weyl point. The Fermi arcs of the topological surface states in these multi-Weyl semimetals were derived analytically. Those results reveal a direct proportionality between the number of arcs emanating from a Weyl point and its winding number. Furthermore, we elucidate the topological Lifshitz transition of Fermi arcs, which depends on boundary condition parameters, and pinpoint the critical points associated with the Lifshitz transition in these semimetals. These findings highlight the evolution of Fermi arcs of topological surface states in multi-Weyl semimetals, potentially opening new avenues for modulating transport properties of topological quantum materials. Our study provides a robust analytic framework for understanding the behavior of Fermi arcs in multi-Weyl semimetals, offering valuable insights for developing topological quantum devices.

The in-plane and out-of-plane A ₁ modes in the Janus Y ₃ –Cr ₂ –X ₃ monolayer are both infrared and Raman active and may manifest significant magnetic-phonon coupling and magneto-optical effects in their optical spectra.

A micro-perforated panel (MPP) absorber with extended necks is proposed to seek low-frequency efficient sound absorption in a deep sub-wavelength thickness. The effects of the extension direction and the design parameters of the extended necks on its sound absorption characteristics have shortcomings and are indeterminate, further affecting the application of combining multiple absorption cells. The theoretical model is formulated to account for the different extended necks: e.g. the outside extended necks, the inside extended necks, and the inside-outside extended necks. The sound absorption curves of the MPP absorbers with the different extended necks are theoretically and numerically analyzed. The length of the inside extended necks is more sensitive to the resonance frequency of the MPP absorber; also, its perforation diameter is more significant to the absorption peak. Additionally, the variation of the outer diameter of the inside extended necks and the thickness of the backed air cavity promotes the resonance frequency of the MPP absorbers to change, but its absorption peak and 0.5 absorption bandwidth remain unchanged. An experimental measurement is carried out to verify the results. This paper provides an exploration for a novel MPP absorber, alleviating the low-frequency sound absorption bandwidth and total thickness.