▎ 摘　　要

We have identified the cyclotron resonance response of the purest graphene ever investigated, which can be found in nature on the surface of bulk graphite, in the form of decoupled layers from the substrate material. Probing such flakes with Landau level spectroscopy in the THz range at very low magnetic fields, we demonstrate a superior electronic quality of these ultralow density layers (n(0) approximate to 3 X 10(9) cm(-2)) expressed by the carrier mobility in excess of 10(7) cm(2)/(V.s) or scattering time of tau approximate to 20 ps. These parameters set new and surprisingly high limits for intrinsic properties of graphene and represent an important challenge for further developments of current graphene technologies.