Molecular orbitals of an elastic artificial benzene

Abstract

Benzene, a hexagonal molecule with formula C$_6$H$_6$, is one of the most important aromatic hydrocarbons. Its structure arises from the $sp^{2}$ hybridization from which three in-plane $\sigma$-bonds are formed. A fourth $\pi$-orbital perpendicular to the molecular plane combines with those arising from other carbon atoms to form $\pi$-bonds, very important to describe the electronic properties of benzene. Here this $\pi$-system is emulated with elastic waves. The design and characterization of an artificial mechanical benzene molecule, composed of six resonators connected through finite phononic crystals, is reported. The latter structures trap the vibrations in the resonators and couple them through evanescent waves to neighboring resonators establishing a tight-binding regime for elastic waves. Our results show the appearance of a spectrum and wave amplitudes reminiscent of that of benzene. Finite element simulations and experimental data show excellent agreement with the H\"uckel model for benzene.