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Jorge Lobo-Checa - Zaragoza Espagne

The step effect in Bismuth studied through curved crystals : Evolution of the pristine and BiAg2 alloy surface states


Bismuth is a layered material that has attracted a great deal of attention not only because it is the heaviest stable element, but also due to its significant spin-orbit interaction [1]. A prominent number of compounds showing relevant surface electronic properties include bismuth as key elements, for example topological insulators [2] or surface alloys [3,4]. Generally these materials exhibit spin degeneracy lifting via space inversion symmetry breaking, bearing the prospect of future spintronic applications.

Unraveling the fundamental scattering aspects of electronic states which exhibit no spin degeneracy is nowadays a key goal in the Solid State scientific community. An observation that has fostered this research activity is the lack of back-reflected waves near steps at the surface of topological insulators [5]. The reduction of scattering has been attributed to the topological protection of surface states in the material, which is directly related to the strong spin-orbit interaction. Instead of using vicinal surfaces, i.e., crystal surfaces close to a high symmetry orientation, we use crystals with curved shape, which allows us to selectively test different vicinal surface planes on the same sample. The beauty of these substrates is that the density of steps changes progressively as we move along the curvature of the crystal so that the terrace size is continuously tuned from 40 to 1nm [6-9].

Studying these curved samples by means of high resolution ARPES, I will show the effects that steps induce upon the electronic structure of two element related systems : Pristine Bi (111) [1] and the surface alloy of BiAg2[3]. In the first case we observe a clear 2D to 1D transition which is correlated to the local structure obtained from STM and LEED. We find that this surface state transition is different from the noble metal curved surfaces [6-8] since electron- and hole- pockets of the flat surface are strongly transformed into non-dispersive 1D state perpendicular to the steps which is located close to the Fermi energy [10]. In the second system, the scattering of the surface electron wavefunctions by the steps is rather weak, much smaller than the underlying substrate of Ag with transmission values close to topological insulators [10]. Both systems are in the proximity of being topologically protected.

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