Jean-Marie Dubois

Abstract:

This SISAM 2022 symposium is about the lifetime achievements of Prof. Uichiro Mizutani. Among many other breakthroughs, Mizutani and his collaborators were able to define in an unambiguous way the so-called e/a ratio [1], which is a measure of the number of electrons an atom shares with the Fermi sea in an intermetallic to achieve structural stability of a given crystal architecture. Although computation of this number requires periodicity of the lattice, the concept can be extended to aperiodic metallic systems such as quasicrystals [2]. It turns out that the e/a ratio found for truly aperiodic quasicrystals as well as weakly periodic crystals of giant unit cell (the so-called approximants) corresponds to a very specific value: e/a=2.2 ± 0.1 e-/at [3]. This way, the basic assumption of An Pang Tsai [4], who discovered most of the thermodynamically stable quasicrystals using e/a, was confirmed, yet with e/a defined with no questionable assumption and a clearly assessed mechanism for the contribution of the individual atoms to the valence band.
The formation of a pseudo-gap at the Fermi energy was discovered and documented using soft X-ray spectroscopy by my late colleague, Esther Belin-Ferré [5,6]. It was well illustrated in a family of Al-based intermetallics spanning a broad range of e/a values. The deepening of the pseudo-gap around the e/a=2.2 e-/at value is clearly observable for stable compounds. The talk will report how this data helped us to understand two properties of practical interest of quasicrystals and related compounds. The first is the reduced wetting observed against polar liquids (like water) deposited on the polished surface of a quasicrystal equipped with its layer of native oxide in air [7]. The second is friction or solid-solid adhesion measured in vacuum against metallic antagonists like hard-Cr steel [8]. Both properties emphasize the role of the reduced density of free electrons in the material and indeed correlate with the electronic conductivity of these specific materials.