Metallic Nature of the alpha-Sn/Ge(111) Surface down to 2.5 K

Low temperature (down to 2.5 K) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are presented to assess the nature of the alpha-Sn/Ge(111) surface. Bias-dependent STM and STS measurements have been used to demonstrate that such a surface preserves a metallic 3×3 reconstruction at very low temperature. A tip-surface interaction mechanism becomes active below about 20 K at the alpha-Sn/Ge(111) surface, resulting in an apparent unbuckled (sqrt(3)×sqrt(3)) reconstruction when filled states STM images are acquired with tunneling currents higher than 0.2 nA.