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Zoom into the Nanoworld: Surface State Waves as Movie Stars

The noble metals copper, silver and gold possess a two dimensional electron state localized at their closed packed (111) surface. The energy of this surface state is very close to the Fermi energy and covers the occupied as well as the unoccupied region depending on the wave vector parallel to the surface. The measuring of this wave vector dependent E(k) is a demanding task which generally requires a combination of different techniques (e.g. Photoemission and Inverse Photoemission) and an extremely high energy resolution.

Utilizing the imaging of standing waves which occur at step edges and point defects we have now performed a E(k) study on the Ag(111) surface with our low-temperature STM (click here for more details). Combining a point-by-point mapping of spectroscopic data with a lock-in detection of dI/dU for the determination of the local density of states, we have imaged the standing wave pattern for 14 different energies covering the range below as well as above the Fermi energy from E = -0.06 eV to E = +0.15 eV. Performing this measurement at T = 5 K not only gives the required energy resolution but also guarantees an extremely high stability of the tunneling process which was needed because the data acquisition took about 17 hours of continuous measurement.

The results arranged in a film sequence (click Figure) visualize the wave properties of the surface state and ist energy dependence and demonstrate the opportunities which are offered by the local probe of a scanning tunneling microscope especially if it is operated at low temperatures.

Wave properties of surface state

(Click to start animation. (1.1 Mb))

Maps of dI/dU with Usample = -0.06 ... +0.15 V of an Ag(111) surface. In the 50 x 50 nm2 frame standing wave patterns form at the step edge crossing from top left to bottom right and at several point defects. The periodicity of the wave pattern is energy dependent with a decreasing wavelength for increasing Usample. The 14 images are arranged in a continuous loop. (Data from PhD work Burkhard Grimm)