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

TL;DR

This study investigates the Kondo effect in binuclear metal-organic complexes on copper surfaces, revealing different behaviors based on geometry and metal type, with theoretical insights into the underlying electronic interactions.

Contribution

It provides combined experimental and theoretical analysis of the Kondo effect in specific binuclear complexes, highlighting geometry-dependent phenomena and the role of open 3d shells in magnetic interactions.

Findings

Kondo resonance observed in Mn_2 and Ni_2 complexes but not in Zn_2.

Two distinct adsorption geometries for Ni_2 with different Kondo temperatures.

Theoretical models support a spin-1 Kondo effect due to open 3d shells.

Abstract

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 non-magnetic Zn_2, a zero bias resonance was resolved in magnetic Mn_2 and Ni_2 complexes. The case of Ni_2 is particularly interesting as the experiments indicate two adsorption geometries with very different properties. For Ni_2-complexes we have employed density functional theory to further elucidate the situation. Our simulations show that one geometry with relatively large Kondo temperatures T_K ~ 10K can be attributed to distorted Ni_2 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 Ni^{2+} ions.