Tunable many-body interactions in a helical Luttinger liquid

• Main Author: Jia, Junxiang
• Other Authors: Bent Weber
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/169878
• Institution: Nanyang Technological University

The effect of electronic many-body interactions in one-dimensional (1D) systems is drastically enhanced compared to higher dimensions due to the restricted phase-space, often leading to strong electronic correlations. It can be shown that due to a breakdown of the Fermi liquid theory, the ground state at low temperatures becomes a Tomonaga-Luttinger liquid (TLL). In a TLL, the strength of electronic correlations is characterized by the Luttinger parameter K, ranging between K=0 and K=1, where K=1 describes non-interacting electrons. Various systems, such as single-walled carbon nanotubes, 1D atomic metal chains, and MoS2 mirror twin boundaries, have been reported to host TLLs, in which spin-degeneracy is intact. Recently, TLLs have been reported also in the edge channels of two-dimensional (2D) topological insulators, quantum spin Hall (QSH) insulators. Unlike spinful TLLs, QSH insulator edges host helical TLLs in which the 1D states are spin-momentum locked with a linear (Dirac) dispersion. In this thesis, we investigate the presence and tunability of a helical TLL in the edges of the QSH insulator, monolayer 1T'-WTe2, grown on two substrates, epitaxial bilayer graphene on silicon carbide (BLG/SiC) and highly oriented pyrolytic graphite (HOPG). Measuring the local density of states (LDOS) by scanning tunneling spectroscopy (STS), we confirm a QSH gap of ~65 meV and a metallic edge state. By fitting the temperature-dependent edge STS near the Fermi level to the TLL theory, we further confirm the presence of helical TLL in the QSH edges on both substrates. On the edge, we observe a V-shaped suppression in the edge state LDOS, the zero bias anomaly (ZBA). Fitting ZBA to TLL theory, we extract K across both substrates and edge types. A detailed statistical analysis on tens of measurements across edge types and substrates shows that the Luttinger parameter can be tuned between K~0.2 and K~0.4. We explain our findings as a result of a) dielectric screening induced by substrate, and b) Fermi velocity of electrons in the edge state. The former and the latter alter Coulomb potential and kinetic energies of electrons, respectively. Both cases lead to the tunability of the Luttinger parameter. We believe that our results pave the way for future investigation of the interplay of strong many-body correlations and topology in 1D helical systems towards realizing non-Abelian parafermions.