Signature of a Chemical Bond in the Conductance between Two Metal Surfaces

Conductance in monatomic metal contacts is quantized; it increases in discrete steps of one conductance quantum $2e^2/h$. By contrast, in a vacuum barrier between two metal surfaces we find that conductance increases linearly and continuously with the interaction energy between individual atoms. This behavior shows unambiguously that current flow between single atoms is a measure for their chemical interaction. In the controlled environment of a scanning tunneling microscope it should allow us to study the formation of covalent bonds up to the point where these atoms finally jump into contact.

Figure 1

Interaction energy (solid squares), tunneling conductance (black curve) in units of $G_0=2e^2/h$, the conductance quantum, and predicted interaction energy (dashed curve) in an Au(111)-W(111) system. The predicted interaction energy over the whole distance range has been determined from a single point of the calculation for the coupled system (empty circle). Interaction energies between Au and W atoms in a dimer (solid circles) are given for comparison.

Figure 2

Interaction energy, tunneling conductance, and predicted interaction energy in a Cu(100)-Cu(100) system. The interaction energies between Cu atoms in a dimer are given for comparison.

Figure 3

Attractive forces between the surfaces. (a) Predictions for the Au(111) surface (dashed line) are compared with previous calculations (solid triangles) [13]. Up to the jump into contact, indicated by the sudden onset of large displacements (see arrows), the predicted force is equal to the force calculated from the coupled system. (b) An identical behavior is observed for the Cu(100) surface. The jump into contact (see arrows) occurs at a lower distance due to the different decay of $5d$ and $3d$ states.