Thickness and Morphology Dependent Electronic Properties

Gd island at any sample bias (cf. Fig. 3.10c-e) in the voltage range under study (—0.6 V U + 0.9 V). At / = +0.47 V the contrast between the island surface and the monolayer vanishes. Comparison with the spectra of Fig. 3.10b reveals that at this sample bias the dlIdU signal of the Gd monolayer is equal to Gd(OOOl) islands. The contrast inverts if the sample bias is further reduced. For instance Fig. 3.10e shows a map of the dlIdU signal at f/ = —0.1 V, i.e. close to the energetical position of the occupied surface state. 

It was shown until now that the electronic stmcture which is manifested in the dlIdU spectra of clean Gd(OOOl) islands, i.e. the energetical position of the surface state and its intensity, does not dependent on the local coverage for loc 4 ML. It is known, however, that the first monolayer of GdAV(llO) does not exhibit the surface state (cf. Fig. 3.8b). Consequently, the question arises What is the critical thickness for the surface state to appear in the tunneling spectra To unravel this 

3 Structural and Electronic Properties of Rare Earth Metal Systems 

The difference becomes obvious in the tunneling spectra measured at sites with different local coverages as plotted in Fig. 3.12. To prevent an unwanted overlap the spectra have been shifted relative to each other. As already described for room temperature measurements (cf. Fig. 3.8b) the Gd monolayer exhibits an asymmetric peak centered at a sample bias U — +0.3V which does not represent a surface state. In contrast, the spectrum measured above a double layer patch shows two maxima of the dUdU signal a distinct peak at U x +0.45 V and a weak shoulder at 17 —0.1 V. Similar to the results previously shown in Fig. 3.9 experiments were performed which show that both features vanish upon contamination (not shown here). 

Therefore it can be concluded that the surface state already exists on Gd patches with a thickness of two atomic layers and down to an area of at least 25 nm. If the local coverage is increased the occupied part of the surface state shifts to a higher binding energy. For loc = 3 ML it amounts to —140 meV until it converges for loc 4 ML to the thick film limit of —180 meV. The binding energy of the empty part of the surface state exhibits a much weaker dependence 

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