Spin Excitations of a Kondo-Screened Atom Coupled to a Second Magnetic Atom

Screening the electron spin of a magnetic atom via spin coupling to conduction electrons results in a strong resonant peak in the density of states at the Fermi energy, the Kondo resonance. We show that magnetic coupling of a Kondo atom to another unscreened magnetic atom can split the Kondo resonance into two peaks. Inelastic spin excitation spectroscopy with scanning tunneling microscopy is used to probe the Kondo effect of a Co atom, supported on a thin insulating layer on a Cu substrate, that is weakly coupled to a nearby Fe atom to form an inhomogeneous dimer. The Kondo peak is split by interaction with the non-Kondo atom, but can be reconstituted with a magnetic field of suitable magnitude and direction. Quantitative modeling shows that this magnetic field results in a spin-level degeneracy in the dimer, which enables the Kondo effect to occur.

Figure 1

Left inset: Topographic STM image ($5\times 5{\mathrm{nm}}^2$) of a dimer consisting of an Fe atom and a Co atom on an island of ${\mathrm{Cu}}_2\mathrm{N}$ on Cu(100). The adsorption sites are shown in the right inset. Main panel: Conductance spectra $dI/dV$ taken on the Fe (lower) and Co (upper) atoms in the structure. Light gray curves show respective spectra taken on isolated atoms on ${\mathrm{Cu}}_2\mathrm{N}$. Both upper curves are offset by $0.1\mathrm{nA}/\mathrm{mV}$ for clarity. Before taking each spectrum the tip height was adjusted to create a $10\mathrm{M}\Omega$ tunnel junction (10 mV, 1 nA).

Figure 2

Conductance spectra $dI/dV$ measured with the tip positioned over each of the atoms in a dimer, when a magnetic field is applied perpendicular to [(a)  for Co, (c)  for Fe] and along the dimer [(b)  for Co, (d)  for Fe]. Spectra are offset by intervals of $0.1\mathrm{nA}/\mathrm{mV}$ for clarity.

Figure 3

Small dots: lowest 12 eigenvalues of Eq. (1) with $J=0.13\mathrm{meV}$, $g_{\mathrm{Fe}}=2.11$, $g_{\mathrm{Co}}=2.16$, $D_{\mathrm{Fe}}=-1.53\mathrm{meV}$, $E_{\mathrm{Fe}}=0.31\mathrm{meV}$ and $D_{\mathrm{Co}}=2.70\mathrm{meV}$ for $B=0$ to 7 T in increments of 0.1 T along $x$. Color indicates the values of $I_{0\to n}^{(\mathrm{Co})}$ (blue) and $I_{0\to n}^{(\mathrm{Fe})}$ (red) as indicated by the color key on the left. The ground state is colored black. Large dots: experimental data points indicating positions of peaks (open circles) and steps (filled circles) in Figs. 2a (blue, Co) and 2c (red, Fe), all plotted relative to the calculated ground state. The zero of the energy axis is arbitrary, as only energy differences between states are observable. Arrows indicate the magnetization along the $x$ direction of the Co spin (short arrows) and the Fe spin (long arrows) for the four lowest-energy states. This labeling also applies at low fields, as if the avoided crossing near 1 T were a true crossing. At the ground-state level crossing at 2.1 T, only the Co spin changes direction.