Incommensurate surface superstructure observed on epitaxially grown Al(111) films by scanning tunneling microscopy at mK temperature

Superconducting aluminum samples are generally used as a standard reference for testing the energy resolution of cryogenic scanning tunneling microscopes operating at ultra-low temperatures by measuring its superconducting tunneling gap spectrum. However, only a few of the atomic structure of the aluminum surface have been reported in those studies because of the difficulties in preparing atomically clean surfaces due to its strong reactivity. In this presentation, we report the observation of an incommensurate superstructure on Al(111) films grown epitaxially on graphene on SiC substrates. The observed superstructure has 3-fold symmetry with a 3.3 periodicity. The superstructure closely resembles a charge density wave structure, for example, similar to that typically observed on NbSe₂. We therefore speculate that possibly Al(111) may support a surface CDW at ultra-low temperatures due to same electron-phonon coupling which is responsible its superconductivity. Other possible explanations for a surface or bulk CDW phase include such soft phonons resulting from strain effects [1], which will be discussed.

[1] Phonon Instabilities and the Ideal Strength of Aluminum, D.M. Clatterbuck, C. R. Krenn, Marvin L. Cohen, and J.W. Morris, Jr., Phys. Rev. Lett. 91, 135501 (2003).