Playing Pinball with Atoms
(Result of the month 01/2009)

(a) An STM topograph of atomic chains on Ge(001) at 4.7 K, gap voltage -1.5 V and tunneling current 0.5 nA. Bright protrusions are the atoms that make up the atomic width chains in the image. As a result of a Peierls instability the chains exhibit a 4-fold periodicity with dimers paired together, giving a periodic low-high-high-low appearance of the atoms in the chains.
(b) Top view of a regular dimer pair at 77 K, gap voltage -1.0 V and tunneling current 0.8 nA. Two terrace atoms can be seen to protrude from the Ge(001) surface indicated by the white arrows.
(c,d) Two subsequent images of a dimer pair that exhibits mobility. The reconfiguration of the dimer pairs is too fast to image and shows up as a discontinuity as the tip is scanned across the chain. (e) Schematic diagram of the dimerized atomic chain and the underlying substrate

The boundaries of the dimer pair at 87% maximum height measured from STM topographs. The narrow lines designate the measured boundary positions. The atoms of the dimer pairs are identified as pivot atoms (P) or revolving atoms (R). The motion of the dimers, indicated by the red arrows, resembles that of the flippers of an atomic pinball machine. The thick dashed lines designate a down-down (orange), down-up (blue) and up-up.

(a) The measured flip-flop frequency of the dimers as a function of tunnel current. The frequency depends linearly on the tunnel current and passes through the origin. Each plotted data point is the average of 100 values.
(b) Telegraphic signal resulting from different dimer flipping modes at 77 K, gap voltage -1.0 V, open feed-back loop, and an initial tunnel current of 1.0 nA. Slow variations in tunnel current resulting from drift of the STM tip have been corrected for in this graph. The dimer pair switches between six well-defined states, indicated by the red lines in the graph.