Atomic Chains at Surfaces

Abstract

It has become possible to assemble one-dimensional atom chains at stepped surfaces with atomic precision. These form a new class of materials for exploring electrons in one dimension. Theory predicts a radically different behavior compared to higher dimensions. The single-electron picture has to be abandoned, because electrons cannot avoid each other when moving along a line. This article gives an overview of the phenomena that have been observed for electrons in onedimensional chain structures, many of them quite unexpected, such as a fractional electron number per chain atom, a doublet of nearly half-filled bands instead of a single filled band, and spin-polarized bands in non-magnetic materials. First, the basic methods for analyzing electrons in atomic wire structures are outlined. Metal surfaces with free-electron-like surface states serve as model cases for explaining the quantization phenomena induced by steps and terraces. These self-assemble into lateral superlattices at vicinal surfaces. The periodicity can be tuned by the miscut angle. One can distinguish two regimes, i.e., quantum-well states con.ned within each terrace and superlattice states extending over the whole step array. Then, we move on to semiconductor surfaces, where metal atom chains and broken bond chains can be combined into more complex structures. The chain atoms are locked rigidly to the substrate, but the electrons near the Fermi level completely decouple from the substrate, because they lie in the band gap of the semiconductor. The dimensionality can be controlled by adjusting the step spacing with intra- and inter-chain coupling ratios ranging from 10 : 1 to > 70 : 1.