Electronic Phase Separation and Atomic Structure of One-dimensional Electronic Systems

Abstract

One-dimensional (1D) electronic systems have been intensively investigated because of exotic and intriguing electronic properties and their possibilities of application on the new generation of logic and memory devices. Because of the enhanced many-body interactions between electrons and electrons or quasiparticles as the dimension decreases, the exotic physical phenomena could emerge. In this thesis, I studied two self-assembled atomic wires on semiconductor surface, the In atomic wire on Si(111) and the Gd silicide nanowire on Si(001), in detail at atomic scale by using scanning tunneling microscope ad spectroscopy (STM/S). Electronic phase separation (EPS), the coexistence of different electronic or magnetic phases at low temperature, may provide an extra control knob on the functionality based on those competing states. A microscopic investigation in real space is essential to understand the underlying physics and the functionality. We investigated the atomic scale domain boundaries formed on the electronically phase separated In/Si(111) surface at the various temperatures. Atom-resolved STM study reveals that the atomic scale phase flip defects constrain the CDW states near the defects and the overlapped constraint has key roles to form the EPS domain boundaries. Well arranged phase defects can assist the formation of the 8 × 2 phase locally, while the randomly located phase defects suppress the CDW phase transition globally. This result emphasizes the importance of the local approach to understanding the mechanism of the electronic phase separation and the phase transition related with defects. We investigate the various atomic-scale metal-insulator junctions emerging from a quasi-1D charge-density-wave (CDW) state of the EPS In atomic wire array on a Si(111) surface. Spatial variations of the CDW gap and amplitude are quantified for various interfaces of metallic and insulating CDW domains by STM and STS. The strong anisotropy in the metal-insulator junctions is revealed with an order of magnitude difference in the intrawire and interwire junction lengths of 0.4 and 7 nm, respectively. The intrawire junction length is reduced dramatically by an atomic scale impurity, indicating the tunability of the metalinsulator junction in atomic scale. Density functional theory calculations disclose the dynamical nature of the intrawire junction formation and tunability. The physics and insight delivered in the present work can widely be applied in various electron-phonon-coupled systems where the separation and the switching of electronic phases are important. Self-assembled rare-earth (RE) silicide nanowires (NWs) on semiconductor surfaces are considered as good candidates for creating and investigating onedimensional electron systems. While detailed atomic structures are essential to understand electronic properties of these NWs, there have been only few successful observations of atomic structures with microscopy and reliable structure models are lacking. We reinvestigate gadolinium silicide NWs with high resolution STM. STM images reveal distinct atomic structures of narrow (4a0 and 5a0 wide) NWs and wider ones clearly. The narrow NWs have double buckled dimer rows with a a0 periodicity along the wire and the wider wires has extra central row structures with a 2a0 periodicity. The central structure of a wider wire can be understood from that of a two dimensional silicide. Based on these STM observations, we propose new structure models of Gd silicide NWs. This work would provide an important basis for the study of intriguing electronic properties found in various self-assembled RE silicide NWs