–like surface state appear as peaks in the tunneling spectra above Gd islands with a local coverage of 4–15 ML but not above the first ML. Small disturbances, induced by the tunneling tip by the application of a bias voltage which exceeds the tunneling barrier height, lead to a collapse of the surface state. This collapse is visible in the apparent height of constant–current STM–images. Scanning tunneling spectroscopy addresses the origin of the change to the suppression of the Gd(0001) surface state by hydrogen transferred from the tip to the surface.
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