Kondo effect in transport through molecules adsorbed on metal surfaces: From Fano dips to Kondo peaks

The Kondo effect observed in recent scanning tunneling microscopy (STM) experiments on transport through CoPc and TBrPP-Co molecules adsorbed on Au(111) and Cu(111) surfaces, respectively, is discussed within the framework of a simple model [G. Chiappe and E. Louis, Phys. Rev. Lett. 97, 076806 (2006)]. Both molecules show a four lobed structure with Co placed in its center, the active Co d orbital lying at lower energy in the TBrPP-Co molecule. In the Kondo regime, and by varying the ratio r of STM tip/Co to STM tip/lobes hoppings, it is possible to produce a crossover from a conductance Kondo peak (CoPc, high r) to a conductance Fano dip (TBrPP-Co, r around 1). The essential role of the internal structure of the molecule in controlling even the entrance into the Kondo regime is highlighted. In particular, it is shown that the crossover from Fano dip to Kondo peak can also be produced, changing the sign of the lobe/lobe hopping while keeping that ratio constant. In the case of TBrPP-Co/Cu(111), we show that varying the density of states at the metal surface, or distorting the molecule, changes the shape of the Fano dip and the Kondo temperature, and shifts the dip minimum. These features are in line with experimental observations, indicating that our simple model contains the essential physics underlying the transport properties of such complex molecules.