Christian Blumenstein (PhD thesis)

Christian Blumenstein (PhD thesis): One-dimensional electron liquid at a surface: Gold wires on Ge(001)

Abstract

Self-organized nanowires at semiconductor surfaces offer the unique opportunity to study electrons in reduced dimensions. Notably the dimensionality of the system determines its electronic properties, beyond the quasiparticle description. In the quasi-one-dimensional (1D) regime with weak lateral coupling between the chains, a Peierls instability can be realized. A nesting condition in the Fermi surface leads to a back-folding of the 1D electron band and thus to an insulating state. It is accompanied with a charge density wave (CDW) in real space that corresponds to the nesting vector. This effect has been claimed to occur in many surface-defined nanowire systems, such as the In chains on Si(111) or the Au reconstruction on the terraced Si(553) and Si(557) surfaces. Therefore a weak coupling between the nanowires in these systems has to be concluded. However theory proposes another state in the perfect 1D limit, which is completely destroyed upon slight coupling to higher dimensions. In this so-called Tomonaga-Luttinger liquid (TLL) state, the quasiparticle description of the Fermi liquid breaks down. Since the interaction between the electrons is enhanced due to the strong confinement, only collective excitations are allowed. This leads to novel effects like spin charge separation, where spin and charge degrees of freedom are decoupled and allowed to travel independently along the 1D-chain. Such rare state has not been realized at a surface until today.
This thesis uses a novel approach to realize nanowires with improved confinement by studying the Au reconstructed Ge(001) surface. A new cleaning procedure using piranha solution is presented, in order to prepare a clean and long-range ordered substrate. To ensure optimal growth of the Au nanowires the phase diagram has been extensively studied by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The structural elements of the chains are revealed and described in high detail. Remarkably a structural phase transition of the delicate wire structure is found to occur above room temperature. Due to the lack of energy gaps a Peierls transition can be excluded as its origin. The transition is rather determined as 3D Ising type and therefore includes the substrate as well.
Two hallmark properties of a TLL are found in the Au/Ge(001) wires by spectroscopic studies: Power-law suppression of the density of states (DOS) and universal scaling. This impressively proves the existence of a TLL in these chains and opens up a gateway to an atomic playground. Local studies and manipulations of a TLL state become possible for the first time. These comprise (i) doping by alkaline atoms, (ii) studies on chain ends and (iii) tunable coupling between the chains by additional Au atoms. Most importantly these manipulations offer input and test for theoretical models and predictions, and are thereby ultimately advancing the field of correlated electrons.

You find the complete pdf version here.