Stability, structural, and electronic properties of atomic chains onYb/Ge(111)3×2studied by STM and STS

Abstract

By means of scanning tunneling microscopy and spectroscopy ͑STM/STS͒, we have investigated the stability and the structure of atomic chains on Yb/ Ge͑111͒3 ϫ 2. STM allows the identification of different building blocks of this reconstruction, depending on the bias polarity and voltage, and validates the honeycomb chainchannel ͑HCC͒ structure with the Gev Ge double bond and metal coverage of 1/6 ML for Yb/ Ge͑111͒3 ϫ 2, in agreement with the recent photoemission study ͓Kuzmin et al., Phys. Rev. B 75, 165305 ͑2007͔͒. The Yb atoms are found to be adsorbed on similar sites in the well-defined ϫ2 rows. Locally, such rows are distorted, leading to the ϫ4 periodicity, where the Yb atoms are adsorbed on two different sites that are well consistent with T4 and H3 sites. It is also assumed that Yb atoms can fluctuate rapidly between the neighboring T4 and H3 sites, leading to continuous rows observed together with the ϫ2 rows in STM images. The stability of Ge honeycomb chain is controlled by the presence of Yb atom per two ͑3 ϫ 1͒ surface units in average, which results in the donation of one electron from Yb to the surface per ͑3 ϫ 1͒ unit. When this density is locally changed, the Ge honeycomb chain is found to be broken. The inner structure of the Ge honeycomb chain is visualized in STM and shows dimerized features without any apparent buckling. The STM observations also account for why the double periodicity is missing in the low-energy electron diffraction pattern from Yb/ Ge͑111͒3 ϫ 2. The local electronic structure of this reconstruction, namely the Yb rows and Ge honeycomb chains, is studied by STS. The results support the HCC structure with the Gev Ge double bond. It is believed that the present study elucidates the difference between the ͑3 ϫ 2͒ reconstructions of Yb and Eu on Ge͑111͒ and those of alkaline-earth and rare-earth metals on Si͑111͒.