Simultaneous Dirac-like Cones at Two Energy States in Tunable Phononic Crystals: An Analytical and Numerical Study

Simultaneous occurrence of Dirac-like cones at the center of the Brillouin zone (Г) at two different energy states is termed Dual-Dirac-like cones (DDC) in this article. The occurrence of DDC is a rare phenomenon. Thus, the generation of multiple Dirac-like cones at the center of the Brillouin zone is usually non-manipulative and poses a challenge to achieve through traditional accidental degeneracy. However, if predictively created, DDC will have multiple engineering applications with acoustics and vibration. Thus, the possibilities of creating DDC have been identified herein using a simple square periodic array of tunable square phononic crystals (PnCs) in air media. It was found that antisymmetric deaf bands may play critical roles in tracking the DDC. Hence, pivoting on the deaf bands at two different energy states, an optimized tuning parameter was found to achieve Dirac-like cones at two distinct frequency states, simultaneously. Orthogonal wave transport identified as key Dirac phenomena was achieved at two frequencies, herein. It was identified that beyond the Dirac-like cone, the Dirac phenomena remain dominant when a doubly degenerated state created by a top band with positive curvature and a near-flat deaf band are lifted from a bottom band with negative curvature. Utilizing a mechanism of rotating the PnCs near a fixed deaf band, frequencies are tracked to form the DDC, and orthogonal wave transport is demonstrated. Exploiting the dispersion behavior, unique acoustic phenomena, such as ballistic wave transmission, pseudo diffusion and acoustic cloaking are also demonstrated at the Dirac frequencies using numerical simulation. The proposed tunable acoustic PnCs will have important applications in acoustic and ultrasonic imaging, waveguiding and even acoustic computing.

Two-step Brillouin zone sampling for efficient computation of electron dynamics in solids

We develop a numerical Brillouin-zone integration scheme for real-time propagation of electronic systems with time-dependent density functional theory. This scheme is based on the decomposition of a large simulation into a set of small independent simulations. The performance of the decomposition scheme is examined in both linear and nonlinear regimes by computing the linear optical properties of bulk silicon and high-order harmonic generation. The decomposition of a large simulation into a set of independent simulations can improve the efficiency of parallel computation by reducing communication and synchronization overhead and enhancing the portability of simulations across a relatively small cluster machine.

Non-Hermitian Bulk-Boundary Correspondence and Singular Behaviors of Generalized Brillouin Zone

The bulk boundary correspondence, which connects the topological invariant, the continuum band and energies under different boundary conditions, is the core concept in the non-Bloch band theory, in which the generalized Brillouin zone (GBZ), appearing as a closed loop generally, is a fundamental tool to rebuild it. In this work, it can be shown that the recovery of the open boundary energy spectrum by the continuum band remains unchanged even if the GBZ itself shrinks into a point. Contrastively, if the bizarreness of the GBZ occurs, the winding number will become illness. Namely, we find that the bulk boundary correspondence can still be established whereas the GBZ has singularities from the perspective of the energy, but not from the topological invariant. Meanwhile, regardless of the fact that the GBZ comes out with the closed loop, the bulk boundary correspondence can not be well characterized yet because of the ill-definition of the topological number. Here, the results obtained may be useful for improving the existing non-Bloch band theory.

Slow acoustic surface modes through the use of hidden geometry

The acoustic surface modes supported by a partly covered periodic meander groove structure formed in an assumed perfectly rigid plate are investigated. This allows one to create a slower acoustic surface wave than can be achieved with the same uncovered meander structure. By changing the size of the uncovered section the phase and group speeds can be tuned. When the uncovered section of the meander structure is centred along the grooves then the distance along the grooves between neighbouring holes is the same on both sides of the structure so no band gap is observed at the first Brillouin zone boundary due to glide symmetry. This then gives quite linear dispersion. As the uncovered section’s position is moved away from the centre of the meander structure a band gap opens at the Brillouin zone boundary.

Frequencies analysis in an infinite beams array

This model consists of a periodic structure formed by solid beams equidistant from each other submerged in a fluid. The beams are clamped at both ends. The distance between the beams, the elastic properties of the solid and the fluid; and the geometric parameters of the beams determine a relationship between the frequencies of the mechanical waves that can propagate through the structure and the wave vector. Analysis within the first Brillouin zone with the Bloch periodicity condition gives rise to frequency bands in which there is the propagation of mechanical waves and bands in which no waves are propagated. Some propagation bands and forbidden regions were found in the examined frequency ranges for various geometric configurations.

Transformation of guided modes into bound states in the continuum

We study a silicon-based structure composed of parallel rods arranged in a periodic array. The structure supports guided modes which due to the deformation of structure in general become leaky modes. At the same time the modes split at the Brillouin zone surface by a pair with certain symmetry. Here we report on guided modes transformation into symmetry-protected BIC under deforming the structure, because of inherent field distribution governed by the Bragg-related mode hybridization.

Critical Temperature in Zigzag Graphene Nanoribbon: a First-principles Study

The critical (Néel) temperature in the zigzag graphene nanoribbon was calculated using the mean-eld approximation within the generalized Bloch theorem. This calculation was carried out over the Brillouin zone of the magnon spectrum. We found a nearly at magnon dispersion at the high energy in one-third of the Brillouin zone. Our calculation showed the critical temperature below room temperature, in good agreement with the prediction in the previous works. Our last work (Prayitno 2021 Physica E 129 114641) revealed that the critical temperature may be enhanced by increasing the ribbon width. In this brief report, we justified that the critical temperature becomes almost constant up to a certain ribbon width. This result indicates that the critical temperature in the graphene nanoribbon will never reach room temperature for any ribbon widths, thus it is likely difficult to apply pristine graphene nanoribbon in any practical devices working near room temperature.

Variation of the silicon optical parameters after rapid heat treatment

The results of the effect of rapid heat treatment on the optical characteristics of a silicon wafer surface in the region of the G-point in the Brillouin zone are presented for different types of silicon wafers conductivity, their doping level, the covalent radii of dopants and the crystallographic orientation of the wafer surface. The absorption coefficient and refractive index of the initial 100 mm diameter samples KDB-12 <100>, KDB-10 <111>, KDB-0.005 <100> and KES-0.015 <100>, underwent standard chemical-mechanical polishing, was measured on a Uvisel 2 ellipsometer (Horiba Scientific, France) in the spectral range 0.6–6.0 eV (200–2100 nm) before and after rapid heat treatment. The incidence angle of the light beam was 70° relative to the sample plane. It is shown that the changes in the optical characteristics of the silicon surface in the spectral region of the location of the G-point in the Brillouin zone after rapid heat treatment is due to a decrease in the surface deformation potential due to solid-phase recrystallisation of the mechanically damaged layer. It has been established that carrying out the rapid heat treatment of silicon samples with a high boron concentration leads to a more significant decrease in the refractive index and absorption compared with silicon with a low boron concentration, due to an increase in the depletion of the silicon surface with boron as a result of diffusion processes at the silicon – silicon dioxide interface.