[Paper Review] Observation of topological nodal-loop state in RAs3 (R = Ca, Sr)

M. Mofazzel Hosen, Baokai Wang+9 more|arXiv (Cornell University)|Dec 15, 2018

Topological Materials and Phenomena41 references4 citations

TL;DR

This study reports the first experimental observation of a topological nodal-loop state and drumhead surface states in SrAs₃ using high-resolution angle-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. The nodal loop is centered at the Y point in the Brillouin zone, with surface states forming a drumhead-like dispersion around the Γ point, while CaAs₃ remains topologically trivial under similar conditions, establishing RAs₃ as a minimal, symmetry-protected platform for studying topological quantum phases.

ABSTRACT

Topological nodal-line semimetals (NLSs) are unique materials, which harbor one-dimensional line nodes along with the so-called drumhead surface states arising from nearly dispersionless two dimensional surface bands. However, a direct observation of these drumhead surface states in the currently realized NLSs has remained elusive. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with parallel first principles calculations, we examine the topological characteristics of SrAs3 and CaAs3. SrAs3 is found to show the presence of a topological nodal-loop, while CaAs3 is found to lie near a topologically trivial phase. Our analysis reveals that the surface projections of the bulk nodal-points in SrAs3 are connected by drumhead surface states. Notably, the topological states in SrAs3 and CaAs3 are well separated from other irrelevant bands in the vicinity of the Fermi level. These compounds thus provide a hydrogen-like simple platform for developing an in-depth understanding of the quantum phase transitions of NLSs.

Motivation & Objective

• To identify and characterize topological nodal-loop states in RAs₃ (R = Ca, Sr) using experimental and theoretical methods.
• To determine whether CaAs₃ and SrAs₃ host topological surface states, particularly drumhead states, and how they differ in topological character.
• To investigate the role of crystal symmetry and spin-orbit coupling in stabilizing or closing the nodal-loop state.
• To establish RAs₃ as a minimal, clean platform for studying topological phase transitions between nodal-line semimetals and topological insulators.
• To provide a hydrogen-like prototype system with minimal symmetry protection and isolated topological states near the Fermi level.

Proposed method

• High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to map the electronic band structure of SrAs₃ and CaAs₃ surfaces.
• First-principles calculations within density functional theory (DFT) were performed to model the bulk band structure and predict topological features.
• Calculations were carried out both with and without spin-orbit coupling (SOC) to assess its impact on nodal-loop stability and surface state formation.
• Photon energy-dependent ARPES measurements were used to extract momentum-resolved dispersion maps and confirm surface state connectivity.
• The surface projections of bulk nodal points were analyzed to verify the presence of drumhead states connecting nodal points in momentum space.
• Transport measurements and resistivity data were used to corroborate the electronic structure findings, particularly the large resistivity in CaAs₃ indicating a gap.

Experimental results

Research questions

• RQ1 Does SrAs₃ host a topological nodal-loop state protected by time-reversal and inversion symmetry?
• RQ2 Are drumhead surface states observed in SrAs₃, and do they connect the projections of bulk nodal points in momentum space?
• RQ3 How does spin-orbit coupling affect the topological nature of CaAs₃, and does it drive a topological phase transition?
• RQ4 Why is the topological state in RAs₃ isolated from other metallic bands near the Fermi level, enabling clean observation?
• RQ5 Can the RAs₃ system serve as a minimal, symmetry-protected platform for studying topological quantum phase transitions?

Key findings

• A topological nodal-loop state was experimentally observed in SrAs₃ centered at the Y point of the Brillouin zone, with a projected loop size of approximately 0.30 Å⁻¹ along the k_y direction.
• Drumhead surface states were directly observed in SrAs₃, forming a closed, in-plane dispersion around the Γ point, marking the first such observation in this geometry.
• CaAs₃ exhibits a nearly flat surface state when spin-orbit coupling is included, but remains topologically trivial due to a clear gap near the Fermi level.
• The nodal-loop in SrAs₃ is protected by time-reversal symmetry and inversion symmetry, with minimal crystal symmetry requirements, making it a minimal topological system.
• ARPES data show that the topological states in both SrAs₃ and CaAs₃ are well separated from other bands near the Fermi level, enabling clean identification of topological features.
• Transport measurements confirm a large resistivity of 260 Ω·cm in CaAs₃ at 2 K, consistent with a gapped, topologically trivial state.