Novel electronic transport properties in topological nodal-line fermions

Abstract

Among various types of topological semimetals, nodal-line semimetals, exhibit band-crossing points that form one-dimensional lines in momentum space, have attracted significant attention in condensed matter physics. The nodal line semimetal is known for their potential to exhibit exotic charge and spin transport phenomena, including local two-dimensional quantum interference, spin Hall effect, quantum oscillation phase shift, anomalous Hall effect, and chiral anomaly. Furthermore, when nodal-line semimetals are in proximity to the topological phase transition between nodal-line semimetals and direct band-gap semiconductors, the interplay of electron interaction with various energy scales can lead to various electronic phases. In the weak coupling regime, these materials can exhibit topological insulator or Dirac/Weyl semimetal states, while in the strong coupling regime, they can display complex many-body states such as spin density waves, charge density waves, excitonic insulators, and superconducting states.

However, these unique phenomena of nodal-line fermion have often been masked by the complexity in band crossings or the coexisting topologically trivial states. Therefore, it is crucial to identify suitable materials that host isolated nodal-line fermions for experimental investigations of nodal-line fermions. Recently, a series of alkaline triarsenides AEAs3 (AE = Sr and Ca) has been proposed as a model system hosting not only isolated nodal-line fermions, but also topological phase transitions between the direct-gap insulator and nodal-line semimetal phases. In this thesis, we will present experimental evidence demonstrating the topological transport propertiesexhibited by the nodal-line fermions in AEAs3.

In the first part, we focus on the topological band properties of SrAs3, a rare system with isolated nodal-line states. We demonstrate that the single-loop nodal-line states in slightly hole-doped SrAs3 are well-isolated from trivial states through band structure calculations and angle-resolved photoemission spectroscopy (ARPES). We also investigate the characteristic torus-shaped Fermi surface and the associated encircling Berry flux of nodal-line fermions, which are clearly observed through quantum oscillations.

In the second part, we present another topological transport of nodal line semimetal SrAs3, the local two-dimensional (2D) weak antilocalization (WAL) in the three-dimensional (3D) Fermi surface. The unique magnetic field and temperature dependencies of the magnetoconductivity in SrAs3 are found to be well described by 2D scaling. Furthermore, we present dimensionality effect on WAL, where the strength of WAL increase as the local 2D nature of torus-shaped FS become strong, resulting in the largest WAL ratio among topological semimetals.

In the last part, we introduce CaAs3, sister material of the topological semimetal SrAs3. Although CaAs3 is a topologically trivial semiconductor with small gap E0 ~130 meV, it is located near the quantum critical point to the nodal-line semimetal phases, where complex many-body states are expected to emerge. We present experimental observations of exotic temperature-dependent resistivity with three insulating phases, 2D quantum oscillations in the insulating state, and a metal-insulator transition with a small threshold field. These observations can be well explained by the presence of inverted bands, nodal-line fermions, in the surface states, with gap opening due to an excitonic phase.

Our studies, presented in this thesis, introduce well isolated nodal-line fermion in the both bulk (SrAs3) and surface (CaAs3) states. Our investigations contribute to a deeper understanding of exotic topological responses, quantum phase transitions, and topological many-body states with strong electron correlations. The strong electromagnetic response exhibited by electrons in these materials, combined with their band topology and strong correlations, may offer new possibilities for information carriers and control mechanisms in electronic and spintronic applications. Our findings would provide a promising material platform for future electronic or spintronic technologies such as dissipation-less electronics or topological spintronic applications.