A flat-mode perspective on the boson peak in amorphous solids

Abstract: The boson peak is a characteristic anomaly of amorphous solids broadly defined as a low-energy excess in the density of states and heat capacity compared to the textbook predictions of Debye theory. The origin of this anomaly has long been the subject of ongoing debate and remains a topic of active controversy. While remaining agnostic about the microscopic origin of the phenomenon, we propose that the boson peak (BP) may universally originate from a dispersionless, optic-like excitation, which we refer to as the 'flat mode'. We revisit both experimental and simulation data from the literature through this lens and conduct further simulations in 2D and 3D amorphous systems. These analyses collectively provide supporting evidence for this interpretation. Notably, if this is indeed the case, a striking analogy emerges with similar anomalies observed in crystalline materials, where the nonphononic flat mode is effectively replaced by anomalously low-energy optical phonons.