Origin of Two Larmor Frequencies in the Coherent Spin Dynamics of Colloidal CdSe Quantum Dots Revealed by Controlled Charging

The coherent spin dynamics are studied by pump-probe ellipticity and Faraday rotation in colloidal CdSe quantum dots, whose surface is modified chemically by adding electron and hole acceptors or capped by a ZnS shell. Surface states are found to play a key role in the appearance of the spin signals. Two Larmor precession frequencies are found in the spin signals and we exploit the photocharging approach to identify their origin. Both frequencies are related to the coherent spin precession of electrons about the external magnetic field. The lower one is due to the confined electron centered in the middle of a quantum dot and the higher one to the electron localized in the vicinity to the surface. By that we solve the long-standing problem on the origin of the high frequency signal in CdSe quantum dots often assigned in literature to the exciton Larmor precession.