Mechanical control of spin states in single molecules

We study individual Co(tpy-SH)$_2$ complexes by connecting them within mechanically controllable break-junction devices that allow us to controllably stretch the molecule while measuring its electrical conductance. At low temperature, this molecule produces the Kondo effect, observed as a peak in the conductance at zero bias. We find that as a function of stretching the Kondo peak splits in two, in distinct contrast to behavior observed in spin-1/2 molecules. The temperature dependence of the Kondo signal for the unstretched molecule is in agreement with the scaling prediction for an underscreened $S$ = 1 Kondo effect. The splitting of the Kondo resonance by mechanical stretching can be explained by a spin-orbit-induced lifting of the degeneracy of the $S$ = 1 triplet upon distortion from octahedral symmetry of the Co ion. We observe evidence of the resultant spin anisotropy in the magnetic-field dependence of the Kondo peaks.