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

Abstract

We report a combined experimental and theoretical study of the Kondo effect in a series of binuclear metal-organic complexes of the form $[{(\mathrm{Me}(\mathrm{hfacac})}_{2}{)}_{2}(\mathrm{bpym})]{}^{0}$, 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 ${\mathrm{Zn}}_{2}$, a zero bias resonance was resolved in magnetic ${\mathrm{Mn}}_{2}$ and ${\mathrm{Ni}}_{2}$ complexes. The case of ${\mathrm{Ni}}_{2}$ is particularly interesting as the experiments indicate two adsorption geometries with very different properties. For ${\mathrm{Ni}}_{2}$ complexes we have employed density functional theory to further elucidate the situation. Our simulations show that one geometry with relatively large Kondo temperatures ${T}_{\mathrm{K}}\ensuremath{\sim}10$ K can be attributed to distorted ${\mathrm{Ni}}_{2}$ 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 ${\mathrm{Ni}}^{2+}$ ions.