Kondo-Transport Spectroscopy of Single Molecule Magnets

We demonstrate that in a single molecule magnet strongly coupled to electrodes the Kondo effect involves all magnetic excitations. This Kondo effect is induced by the quantum tunneling of the magnetic moment. Importantly, the Kondo temperature $T_K$ can be much larger than the magnetic splittings. We find a strong modulation of the Kondo effect as a function of the transverse anisotropy parameter or a longitudinal magnetic field. Both for integer and half-integer spin this can be used for an accurate transport spectroscopy of the magnetic states in low magnetic fields on the order of the easy-axis anisotropy parameter. We set up a relationship between the Kondo effects for successive integer and half-integer spins.

Figure 1

Energy diagram for $H_{\mathrm{M}}$ with $B_2\ll D$ for various magnetic fields. The two subsets of eigenstates of different magnetic symmetry are indicated by full and dashed bars.

Figure 2

Dependence of $T_K$ on the transverse anisotropy $B_2/D$: (a) $T_K$ from the full NRG calculation, (b) spin-binding energy estimate $\Delta E$. Parameters: $S=7/2$, $D={10}^{-4}\mathrm{W}$.

Figure 3

Spectral function, qualitatively equivalent to $dI/d{V}_{\mathrm{bias}}$ as function of $H_z/D$ and frequency $\omega$ (a) for smallest nontrivial integer spin $S=1$, (b) half-integer spin $S=3/2$, and (c) large integer $S=3$, (d) half-integer spin $S=7/2$. Parameters: $J=0.15\mathrm{W}$, $D={10}^{-4}\mathrm{W}$, and $B_2/D=0.1$.