Semi-Dirac points in two-dimensional phononic crystals

Abstract

A two-dimensional phononic crystal (PC) composed of a triangular array of square iron cylinders embedded in water is designed, in which the accidental degeneracy of the Bloch eigenstates is utilized to realize a semi-Dirac point at the Brillouin zone center. In the vicinity of the semi-Dirac point, the dispersion relation is linear along the Y direction but quadratic along the X direction. Rotating the iron cylinders around their axis by 45 and slightly tuning the side length of the cylinders, a new semi-Dirac point can be realized at the Brillouin zone center, where the dispersion relation is quadratic along the Y direction but linear along the X direction. To gain a deeper understanding of the semi-Dirac point, a k p perturbation method is used to investigate this peculiar dispersion relation and study how the semi-Dirac point is formed. The linear slopes of dispersion relations along any direction around the semi-Dirac point can be accurately predicted by the perturbation method, and the results agree very well with the rigorous band structure calculations. Furthermore, the mode-coupling integration between the degenerate Bloch eigenstates is zero in one direction but non-zero in the perpendicular direction, and this is the ultimate reason for the forming of a semi-Dirac point. With the help of the perturbation method, an effective Hamiltonian can be constructed around the semi-Dirac point, so that the Berry phase can be calculated, which is found to be zero. Actually, the different values of Berry phase indicate an important distinction between the semi-Dirac points and Dirac points. In addition, the acoustic wave transmission through the corresponding PC structure has been studied, and a switch-like behavior of the transmittance is observed along different directions. Along some particular direction, there exist deaf bands around the semi-Dirac point, and these bands cannot be excited by the externally incident plane waves due to the mismatch in mode symmetry. But the situation is different along the other direction, where the bands are active ones and therefore can be excited by the incident plane waves. Actually, such properties of the bands can be easily changed as long as the iron cylinders are rotated around their axis. The work described in this paper is helpful to the understanding of semi-Dirac point in phononic crystals and suggests possible applications in diverse fields.