Abstract
Kondo resonances in heterostructures formed by magnetic molecules on a metal require free host electrons to interact with the molecular spin and create delicate many-body states. Unlike graphene, semiconducting graphene nanoribbons do not have free electrons due to their large bandgaps, and thus they should electronically decouple molecules from the metal substrate. Here, we observe unusually well-defined Kondo resonances in magnetic molecules separated from a gold surface by graphene nanoribbons in vertically stacked heterostructures. Surprisingly, the strengths of Kondo resonances for the molecules on graphene nanoribbons appear nearly identical to those directly adsorbed on the top, bridge and threefold hollow sites of Au(111). This unexpectedly strong spin-coupling effect is further confirmed by density functional calculations that reveal no spin–electron interactions at this molecule-gold substrate separation if the graphene nanoribbons are absent. Our findings suggest graphene nanoribbons mediate effective spin coupling, opening a way for potential applications in spintronics.
Highlights
Kondo resonances in heterostructures formed by magnetic molecules on a metal require free host electrons to interact with the molecular spin and create delicate many-body states
We investigate electronic and spintronic properties of vertically stacked heterostructures formed by TBrPP-Co molecules adsorbed on armchair edge graphene nanoribbons (AGNRs) on a Au(111) surface using low-temperature scanning tunnelling microscopy and spectroscopy at the atomic limit supported by density functional theory (DFT) calculations
The long axes of the AGNRs mostly align along the [110] surface close-packed row directions of Au(111) they often orient to other surface directions
Summary
Kondo resonances in heterostructures formed by magnetic molecules on a metal require free host electrons to interact with the molecular spin and create delicate many-body states. The strengths of Kondo resonances for the molecules on graphene nanoribbons appear nearly identical to those directly adsorbed on the top, bridge and threefold hollow sites of Au(111) This unexpectedly strong spin-coupling effect is further confirmed by density functional calculations that reveal no spin–electron interactions at this molecule-gold substrate separation if the graphene nanoribbons are absent. The geometrically relaxed DFT calculations reveal a large vertical distance (7.5 Å) between the molecule and the underlying Au(111) surface when an AGNR is sandwiched in between This separation would effectively decouple the TBrPP-Co from the Au(111) surface electronically. We have detected strong Kondo resonances on TBrPP-Co adsorbed on AGNR with three different Kondo temperatures tracing atomic details of the adsorption sites on the underlying Au(111) surface This suggests that AGNRs are rather effective in mediating the interaction of the molecule magnetic moment with electronic spins in gold. Such mediation of spin interactions is reminiscent of the transparency of graphene seen on Cu(111) surfaces[24], which projects a long-range electronic density with copper character away from the surface, and is uniquely made evident here by the subtle Kondo probe
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