Phonon interference induced by defect pairs
The coherent dynamics of lattice waves is of significant importance for exploring the nonadiabatic phenomena of the electron-phonon coupling under nonequilibrium conditions. Here, we propose a theoretical model for phonon interference induced by a defect pair, in which lattice waves with the selective modes are launched independently by each of defects due to the strongly local electron-phonon coupling. We find that the intensities of phonon interference fringes could be directly evaluated by a measurable quantity in experiments--Huang-Rhys factor, and the geometry of interference patterns could be modified by the internal parameters of defect pair, including the charge states, spatial distance, energy-level depth and localization strength. In particular, several novel quantum states, such as the Dirac-like and nodal-line topological states are emerged for the appropriate internal parameters. Furthermore, using Huang-Rhys model, we simulate the ultrafast optical spectroscopy in the phonon interference duration when defect pairs are excited as the color centers, where interference fringes are perfectly matched by intensities of spectrum, similar to the $\textit{lock-key}$ pattern. These results show that defect pair, serving as the driven force for the coherent dynamical processes for multidegrees of freedom, offers an ideal platform to explore fundamental quantum phenomena in nonequilibrium and ultrafast physics.