Figure 2: Sign and strength of the exchange interaction between atoms.
From: Exploring the phase diagram of the two-impurity Kondo problem
(a,b) Energy versus transverse magnetic field of the four lowest energy states for two example dimers, corresponding to the instances presented in panels (c,d) with AFM (J>0) and FM (J<0) coupling respectively. In the AFM case, a state crossing is expected at a field Bc. (c,d) Example of zero-field differential conductance spectra taken on a Co atom in two different dimers. Red and green arrows show, respectively, the exchange and anisotropy energies. In the FM spectrum, an additional step can be resolved at 3/2J. (e) Colour map of the experimentally extracted coupling strength between Co atoms on Cu2N. Each circle corresponds to the position of the second Co atom atop a Cu atom of the lattice, with respect to a reference Co atom (blue sphere). Light grey represent Cu positions too close to allow a Co dimer to be built while the small grey circles are the N atoms. The axes identify the coordinate system for the reference atom, with the nitrogen direction defined as z-axis. (f) Coupling strength versus inter-dimer distance. For comparative purposes, an isotropic three-dimensional RKKY curve with Fermi wavelength of bulk Cu and horizontal offset of 0.15 nm, corresponding to a phase shift of 1.3π, is shown (dashed line). The point represented with an open circle is the {2, 0} dimer, whose coupling cannot be explained in terms of RKKY interaction only. Errors are estimated based on variance found during fitting.
