Multiple Topological States in LaAgAs2, a Failed Square-Net Semimetal

TL;DR

This study comprehensively investigates LaAgAs2, revealing its complex electronic structure, multiple topological states, and the influence of structural distortion, thereby guiding the design of new topological materials.

Contribution

It provides the first detailed analysis of LaAgAs2's electronic and topological properties, highlighting the role of structural distortion in topological state formation.

Findings

LaAgAs2 hosts multiple topological states including a Z2 surface state and a bulk Dirac state.

Cis-trans distortion transforms Dirac bands into quasi-1D trivial bands.

The electronic structure is well explained by DFT considering structural distortion.

Abstract

The rational design of new materials emerges as an important direction to explore new topological materials, which is based on the understanding of the correlation between crystal and electronic structures. In this paper, we perform a comprehensive study on the crystal and electronic structures in LaAgAs2 through a combination of single-crystal x-ray diffraction (XRD), quantum oscillation, and angle-resolved photoemission spectroscopy (ARPES) experimental measurements, and density functional theory (DFT) calculations. Single-crystal XRD measurements reveal that LaAgAs2 crystallizes into a HfCuSi2-derived structure with the square net distorted into cis-trans chains. Quantum oscillation measurements reveal two frequencies with small effective masses and quasi-two-dimensional (2D) characters. ARPES measurements reveal an electronic structure strikingly different from the square-net-based semimetals, such as LaAgAs2. The Fermi surface is quasi-two-dimensional (2D), with Dirac-like hole pockets at the zone center and a quasi-1D elliptical electron pocket at the zone boundary. Based on the DFT calculations, the measured electronic structure can be well understood regarding the cis-trans distortion, which transforms the two-dimensional square net-derived Dirac bands into quasi-1D trivial bands. Intriguingly, multiple topological states can be identified around the zone center, including a nontrivial Z2 topological surface state and a bulk Dirac state. Our study clarifies the impact of cis-trans distortion and identifies LaAgAs2 as a topological material with multiple topological states near the Fermi level, providing a guideline for intentionally designing new topological materials.