Atomically precise step grids for the engineering of helical states

Abstract

Conventional spin-degenerated surface electrons have been effectively manipulated by using organic and inorganic self-assembled nano-arrays as resonators. Step superlattices naturally assembled in vicinal surfaces are a particularly interesting case since they represent simple one-dimensional (1D) models for fundamental studies, and can imprint strong anisotropies in surface electron transport in real devices. Here we present the first realization of periodic resonator arrays on the BiAg2 atom-thick surface alloy with unprecedented atomic precision, and demonstrate their potential ability for tuning helical Rashba states. We employ a Ag cylindrical crystal curved around the Ag(645) vicinal surface [c-Ag(645) sample] to be able of selecting local vicinal planes with kinked steps. After sublimation and post-annealing of a 0.3 ML Bi layer, we achieve a homogenous BiAg2 surface alloy, featuring sharp arrays of extraordinarily straight Bi-terminated, monoatomic steps, with variable (tunable) density across the curved surface. Scanning the ultraviolet light beam on such Bi/c-Ag(645) interface in angle-resolved photoemission (ARPES) experiments allowed us to observe the characteristic signatures of strong and coherent step-lattice scattering in the spin-textured bands of the BiAg2 alloy, that is, step-umklapps, and a large, step-density-dependent Rashba band shift. First-principle DFT calculations reveal a complex terrace/step spin/orbital composition, which in turn explains the large spectral broadening and strong photon-energy dependence observed in ARPES. DFT also predicts the split-off of 1D electron-like step and terrace bands dispersing parallel to surface steps.