Abstract
We study local moment formation for adatoms on bilayer graphene (BLG) within a mean-field theory of the Anderson impurity model. The wavefunctions of the BLG electrons induce strong particle–hole asymmetry and band dependence of the hybridization, which is shown to result in unusual features in the impurity model phase diagram. We also study the effect of varying the chemical potential, as well as varying an electric field perpendicular to the bilayer; the latter modifies the density of states of electrons in BLG and, more significantly, shifts the impurity energy. We show that this leads to regimes in the impurity phase diagram where local moments can be turned on or off by applying modest external electric fields. Finally, we show that the RKKY interaction between local moments can be varied by tuning the chemical potential (as has also been suggested in monolayer graphene) or, more interestingly, by tuning the electric field so that it induces changes in the band structure of BLG.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Bilayer graphene is of great interest as a tunable gap semiconductor—its gap can be varied by applying an electric field perpendicular to the bilayer. As a result, the physics of local moment formation and local moment interactions, effects which rely on the properties of the electronic host material in which the impurities reside, can be dramatically tuned, unlike in conventional metals. While the physics of impurity adatoms has been explored in monolayer graphene, very little is known about bilayer or multilayer graphene.
Main results. We study local moment formation for adatoms on bilayer graphene and obtain the mean-field phase diagram as a function of chemical potential, impurity energy and bias electric field. Our main results are twofold. First, we show that a perpendicular electric field results in two effects—a tuning of the gap and a tuning of the impurity energy level which sits closer to the top layer. The latter leads to regimes of the local moment phase diagram where moments can be turned off or on with modest electric fields. Second, we show that one can use the electric field, which controls the density of states, to control the magnitude as well as the sign of the RKKY interaction between magnetic adatoms.
Wider implications. Our paper illustrates the ability to manipulate and switch on/off adatom magnetic moments and their interactions—this is of great interest to the nanoscience and quantum computation communities.