Kondo Effect in Single Atom Contacts: The Importance of the Atomic Geometry

Co single atom junctions on copper surfaces are studied by scanning tunneling microscopy and ab initio calculations. The Kondo temperature of single cobalt atoms on the Cu(111) surface has been measured at various tip-sample distances ranging from tunneling to the point contact regime. The experiments show a constant Kondo temperature for a whole range of tip-substrate distances consistently with the predicted energy position of the spin-polarized $d$ levels of Co. This is in striking difference to experiments on $\mathrm{Co}/\mathrm{Cu}(100)$ junctions, where a substantial increase of the Kondo temperature has been found. Our calculations reveal that the different behavior of the Co adatoms on the two Cu surfaces originates from the interplay between the structural relaxations and the electronic properties in the near-contact regime.

Figure 1

Conductance and $dI/dV$ spectra for isolated Co atoms on a Cu(111) surface achieved at different tip-substrate distances $\Delta Z$ from tunneling to point contact. Prior to the point contact measurements the tip was stabilized at a defined sample distance, setting the tunneling conditions at 8 meV and 5 nA. In the inset a representative current vs tip displacement is shown. $dI/dV$ spectra have been recorded at the positions indicated by circles in the inset. The curves are normalized to the tunneling current at the tip height location and a vertical offset has been added for a better visualization. The Kondo temperature $T_K$ given on the right side of the image has been obtained fitting the curves with a Fano line shape (light line).

Figure 2

Atomic relaxation at the single Co atom junction as a function of the tip displacement. The initial tip-adatom distance (H) is the same for Cu(111) and Cu(100) surfaces. The adatom-substrate (L) distances for Co adatom on the two surfaces is shown in panel (b). The dashed lines show the Co-substrate distance in point contact conditions. (A), (B), and (C) indicate the position where the LDOS shown in Fig. 3 have been calculated; the values in the brackets are the tip-substrate distances for these three positions [23].

Figure 3

Influence of the tip proximity on the $d$ levels of Co atoms on Cu(111). (a) Spin-polarized LDOS for the $d$ levels. The curves are calculated for tip-substrate displacements as denoted in Fig. 2. (b) Energy position of the occupied $d$-band center on Cu(111) and Cu(100) at different tip-substrate positions. The lines connecting the points are a guide to the eye.