 In the context of phonon dispersion relations, these oscillations appear as a vertical tangent in the plot of formula_6, called the Kohn anomalies.Many different systems exhibit Kohn anomalies, including graphene, bulk metals, and many low-dimensional systems (the reason involves the condition formula_7, which depends on the topology of the Fermi surface).However, it is important to emphasize that only materials showing metallic behaviour can exhibit a Kohn anomaly, as we are dealing with approximations that need a homogeneous electron gas.