STM Study on 2D Heterostructures with Strong Spin-Orbit Coupling and Proximity Effect

Abstract

Various crystal structures, strong spin-orbit coupling (SOC), and many-body effects in low-dimensional materials has provided a rich area of research for discovering unique electronic, magnetic properties, and various functionalities. By designing artificial two-dimensional (2D) heterostructure composed of low-dimensional materials, the interplay between crystal symmetry, SOC, and many-body interaction can be induced through interfacial interactions, which provide a route to topological superconductors and innovative quantum devices. In this thesis, I study on 2D heterostructures composed of elemental 2D material with strong SOC and 2D material with many-body quantum states such as superconductivity (SC) and charge-density-wave (CDW) by using scanning tunneling microscopy and spectroscopy. As a deposited layer, the elemental 2D materials composed of bismuth have various types of allotropes with the strongest SOC, which has been expected to realize the exotic electronic and spintronic properties such as topological properties, helical spin textures, and Dirac fermions. As substrates, I pick up Pb/Si(111) which is a well-known SC system and 2H-NbSe2 which is a layered transition metal chalcogenides exhibiting SC and CDW.

First, I conduct a prestudy on the CDW state in 2H-NbSe2. In 2D electron system, the interplay between SC and CDW is an important issue and there have been unsolved issues about microscopic CDW structures in 2H-NbSe2. I discover two different, energetically competing, CDW structures and formation of unconventional domain walls, revealing the origin of incommensuration CDW ground state. I investigate native atomic defects and observe early condensation of selective CDW structure depending on defect structure, understanding in-depth microscopic mechanism of defect-CDW interaction. Furthermore, I conduct a detailed analysis of the incomplete CDW phase inversion and find the hidden signature of CDW gap, providing insights in understanding partially-gapped mutliband CDW materials. This prestudy provides a foundation for further studies on 2D heterostructures including 2H-NbSe2, by giving an understanding of the electronic states.

Second, I study on the electronic properties of the various bismuth monolayers grown on two substrates. I observe the formation of a square-lattice bismuth monolayer on Pb. This distinct monolayer has been predicted to host Dirac fermions, while it has been rarely reported experimentally. In the Bi/Pb heterostructure, Rashba-type spin-split Dirac bands is predicted at the Fermi level and proximity-induced SC and moire superstructure on Bi monolayer are detected with analysis of detailed atomic and electronic structures. This makes Bi/Pb heterostructure as interesting platform with 2D Dirac bands, topological SC, and the moire superstructure. I observe the formation of a doped Bi(110) puckered monolayer on 2H-NbSe2. This distinct monolayer has been predicted to have ferroelectric phase from inversion symmetry breaking, while it has been expected to be challenging to experimental realization. In Bi/NbSe2 heterostructure, enhanced Berry curvature dipole and persistent spin texture from Rashba effect is predicted with characterization of detailed atomic and electronic structures. Additionally, ferroelectric domain walls and formation of various moire superstructures are observed on monolayers grown in various directions. This heterostructure becomes a promising platform not only to exploit nonlinear Hall effect and coherent spin transport properties, but also for further investigation of 2D ferroelectricity and moire superstructure.