Kondo effect in binuclear metal-organic complexes with weakly interacting spins.

We report a combined experimental and theoretical study of the Kondo effect in a series of binuclear metal-organic complexes of the form [(Me(hfacac)2)2(bpym)]0, with Me = nickel (II), manganese (II), zinc (II); hfacac = hexafluoroacetylacetonate, and bpym = bipyrimidine, adsorbed on Cu(100) surface. While Kondo features did not appear in the scanning tunneling spectroscopy spectra of nonmagnetic Zn2, a zero bias resonance was resolved in magnetic Mn2 and Ni2 complexes. The case of Ni2 is particularly interesting as the experiments indicate two adsorption geometries with very different properties. For Ni2 complexes we have employed density functional theory to further elucidate the situation. Our simulations show that one geometry with relatively large Kondo temperatures TK∼10 K can be attributed to distorted Ni2 complexes, which are chemically bound to the surface via the bipyrimidine unit. The second geometry we assign to molecular fragmentation: we suggest that the original binuclear molecule decomposes into two pieces, including Ni(hexafluoroacetylacetonate)2, when brought into contact with the Cu substrate. For both geometries our calculations support a picture of the (S=1)-type Kondo effect emerging due to open 3d shells of the individual Ni2+ ions. [ABSTRACT FROM AUTHOR]