Much effort has been made over the last years in order to understand the properties of the reconstructions of 1/3 ML Sn/Ge(111), which becomes (3x3) below 200 K. This system has been the subject of Fermi surface (FS), structural studies and theoretical calculations concerning its ground state. Recently, it has been shown that its fundamental state is a Mott insulator that stabilizes below 30 K [1].
In spite of the many studies on this interface, the Sn 4d core level (CL) line shape and the atomic structure of the (3x3) phase are still controversial. Theoretical calculations, STM experiments and surface x-ray diffraction measurements support a surface unit cell with one Sn atom in a higher position than the other two (1U2D model). However, the Sn 4d CL exhibits a line shape which apparently cannot be understood within this structural model. Due to this fact, some experiments have even been interpreted as supporting the opposite situation, with two higher Sn atoms per unit cell (2U1D model) [2]. Beyond the application to this particular system, this controversy is of fundamental relevance, as it affects our understanding of the basic mechanisms behind surface core level shifts.
In this work we present a new study of the Sn 4d CL line shape, including an analysis of the FS and STM images. We have performed ultrahigh resolution PES experiments as a function of coverage and temperature for the (3x3) phase. The improvement of the experimental conditions allows us to unambiguously observe a Sn 4d CL line shape in agreement with the 1U2D model and within the initial state interpretation. STM pictures show unambiguously just one Sn atom in a high position, supporting thus the 1U2D model, and also is in agreement with ab initio calculations. Finally, the analysis of the FS and other constant energy contours of the valence band, further supports the 1U2D structure and discard the existence of any 2U1D configuration [3]. These results solve a long-standing polemic and provide the clue to understand the behavior of low-temperature phases.
[1] R. Cortés, et al, Phys. Rev. Lett. 96 (2006) 126103
[2] T.-L. Lee, et al, Phys. Rev. Lett. 96 (2006) 46103; M.E. Dávila, et al, Phys. Rev. B 70 (2004) 241308
[3] O. Pulci, et al. Appl. Phys. A 85 (2006) 361
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