Experimental observation of drumhead surface states in SrAs3

Observation of Type-II Topological Nodal-Line Fermions in ZrSiSe

Recently, there has been significant interest in topological nodal-line semimetals due to their linear energy dispersion with one-dimensional nodal lines or loops. These materials exhibit fascinating physical properties, such as drumhead surface states and 3D anisotropic nodal-line structures. Similar to Weyl semimetals, type-II nodal-line semimetals have two crossing bands that are both electron-like or hole-like along a certain direction. However, the direct observation of type-II nodal-line Fermions has been challenging due to the lack of suitable material platforms and the low density of states. Here we present experimental evidence for the coexistence of both type-I and type-II nodal-line Fermions in ZrSiSe, which was obtained through magneto-optical and angle-resolved photoemission spectroscopy (ARPES) measurements. Our density functional theory calculations predict that the type-II nodal-line structure can be developed in the Z-R line of the first Brillouin zone based on the lattice constants of the grown single crystal. Indeed, ARPES measurements reveal the type-II nodal-line band structure. The extracted type-II Landau level transitions from magneto-optical measurements exhibit good agreement with the calculated type-II energy dispersion model based on the band structure. Our experimental results demonstrate that ZrSiSe possesses two types of nodal-line Fermions, distinguishing it from other ZrSiX (X = S, Te) materials and positioning it as an ideal platform for investigating type-II nodal-line semimetals.

Optical Transitions of a Single Nodal Ring in SrAs_{3}: Radially and Axially Resolved Characterization

SrAs_{3} is a unique nodal-line semimetal that contains only a single nodal ring in the Brillouin zone, uninterrupted by any trivial bands near the Fermi energy. We performed axis-resolved optical reflection measurements on SrAs_{3} and observed that the optical conductivity exhibits flat absorption up to 129 meV in both the radial and axial directions, confirming the robustness of the universal power-law behavior of the nodal ring. The axis-resolved optical conductivity, in combination with theoretical calculations, further reveals fundamental properties beyond the flat absorption, including the overlap energy of the topological bands, the spin-orbit coupling gap along the nodal ring, and the geometric properties of the nodal ring such as the average ring radius, ring ellipticity, and velocity anisotropy. In addition, our temperature-dependent measurements revealed a spectral weight transfer between intraband and interband transitions, indicating a possible violation of the optical sum rule within the measured energy range.

Evidences of Topological Surface States in the Nodal-Line Semimetal SnTaS2 Nanoflakes

Exploring the topological surface state of a topological semimetal by the transport technique has always been a big challenge because of the overwhelming contribution of the bulk state. In this work, we perform systematic angular-dependent magnetotransport measurements and electronic band calculations on SnTaS₂ crystals, a layered topological nodal-line semimetal. Distinct Shubnikov-de Haas quantum oscillations were observed only in SnTaS₂ nanoflakes when the thickness was below about 110 nm, and the oscillation amplitudes increased significantly with decreasing thickness. By analysis of the oscillation spectra, together with the theoretical calculation, a two-dimensional and topological nontrivial nature of the surface band is unambiguously identified, providing direct transport evidence of drumhead surface state for SnTaS₂. Our comprehensive understanding of the Fermi surface topology of the centrosymmetric superconductor SnTaS₂ is crucial for further research on the interplay of superconductivity and nontrivial topology.

Nontrivial topological states in new two-dimensional CdAs

By using first-principles calculations and symmetry analysis, we propose two topological nontrivial two-dimensional (2D) materials: CdAs-164 and CdAs-187. The results of binding energies, phonon dispersions, mechanical constants and thermodynamic stability demonstrate that the two materials are stable and may be synthesized in future experiments. When spin-orbit coupling (SOC) is not considered, the former is a typical Dirac semimetal with six equivalent Dirac points on the paths of Γ-M. These Dirac points are protected by vertical mirror symmetry. The latter is a nodal ring semimetal with the coexistence of two type-I nodal rings and one type-II nodal ring, and these nodal rings are protected by the horizontal mirror operationσ_h. After SOC is considered, both of the two materials turn into topological insulators withZ₂= 1. Our findings indicate that CdAs-164 and CdAs-187 are excellent candidates to explore the nontrivial topological states of 2D materials.

Prediction of topological nontrivial semimetals and pressure-induced Lifshitz transition in 1T'-MoS2 layered bulk polytypes

Recently, bulk MoS2 crystals stacked by 1T'-MoS2 monolayers have been synthesized successfully, but little is known about their stacking sequences and topological properties. Based on first-principles calculations and symmetry-based indicator theory, we discovered that three predicted bulk structures of MoS2 (named 2M-, 1T'- and β-MoS2) stacked by 1T' monolayers are topological insulators and nodal line semimetals with and without spin-orbit coupling. Their stacking stability, electronic structure and the topology origin were systematically investigated. Further research proves that in the absence of SOC the open- and closed-type nodal lines can coexist in the momentum space of 2M-MoS2, which also possesses drumhead-like surface state. Moreover, we predicted a pressure-induced Lifshitz transition at about 1.3 GPa in 2M-MoS2. Our findings greatly enrich the topological phases of MoS2 and probably bring MoS2 to the rapidly growing family of layered topological semimetals.