Endless Dirac nodal lines in the nonmagnetic kagome metal Ni3In2S2

The study investigates whether the nonmagnetic kagome compound Ni3In2S2 hosts Dirac nodal lines near the Fermi energy and how these topological features influence its electronic and transport properties. Establishing Dirac nodal lines in a nonmagnetic kagome lattice is important for realizing high-mobility carriers and large magnetoresistance and for providing a platform to engineer diverse topological electronic states.

The paper situates Ni3In2S2 within the broader context of kagome-lattice materials, which have exhibited Dirac fermions, flat bands, Weyl semimetal phases, and unconventional transport phenomena. Prior work on kagome metals (e.g., CoSn, FeSn, Co3Sn2S2 family) demonstrated topological bands and large anomalous transport signals, primarily in magnetic systems. The authors also reference symmetry-based indicators and topological quantum chemistry frameworks for diagnosing topological band structures, and earlier theory on nodal-line semimetals.

Crystals of Ni3In2S2 were grown and characterized (details in Supplementary Materials). Single-crystal X-ray diffraction confirmed the reported structure. ARPES: Measurements were performed on the (001) surface at low temperature (stated as ~10 K in the ARPES section; elsewhere the sample stage was maintained at T=15 K) using photon energy hv ≈ 125 eV (corresponding to kz ≈ 0). A Scienta-Omicron DA30 electron analyzer at the 21-ID-1 beamline of the National Synchrotron Light Source II was used. The total energy resolution was ~15 meV. Constant-energy maps were collected at EB = 0, 0.2, and 0.4 eV relative to EF. Band dispersions were measured along surface high-symmetry directions (MK and Γ–M). DFT: First-principles calculations (generalized gradient approximation, PBE) were carried out in the primitive cell (rhombohedral BZ). Calculated electronic structures and Fermi surfaces were folded into the surface Brillouin zone for comparison with ARPES. Standard DFT methods and tools (e.g., VASP; details and any Wannierization procedures referenced) are cited, with computational details provided in the Supplementary Materials. Transport and thermodynamic measurements: Magnetoresistance and heat capacity were measured in a Quantum Design Dynacool cryostat. Isothermal magnetization was measured using the vibrating sample magnetometer option, and temperature-dependent magnetic susceptibility was measured on a Quantum Design MPMS3 with the DC option. Electrical contacts for transport were made with Epotek H20E silver epoxy. Measurement geometries for longitudinal and transverse magnetoresistance followed the orientations depicted in the figures.

- ARPES constant-energy maps at EB = 0, 0.2, and 0.4 eV reveal Fermi surface features near the M point that evolve from parallel lines (EB=0) to a node near EB ≈ 0.2 eV and then to an oval centered at M (EB=0.4 eV), indicative of a Dirac point at M. - ARPES band dispersion along MK clearly resolves a Dirac-cone-like structure consistent with DFT. Along Γ–M, only part of the calculated bulk bands are observed, attributed to ARPES matrix-element effects and zone folding. - DFT, performed in the primitive (rhombohedral) BZ and folded into the surface BZ, reproduces the observed Fermi surface evolution and indicates Dirac nodal lines near EF. - Transport: Longitudinal magnetoresistance at 1.8 K is nonsaturating up to 9 T and reaches ~2000%, evidencing giant magnetoresistance consistent with Dirac nodal-line carriers. Transverse MR is over an order of magnitude larger than longitudinal MR, consistent with quasi-2D electronic structure. - Quantum oscillations: de Haas–van Alphen oscillations show multiple frequencies. Fourier analysis yields components corresponding to Fermi surface cross-sectional diameters kp ≈ 0.12, 0.15, 0.20, 0.22, and 0.247 (with a the lattice constant) on the kz≈0 plane, in agreement with DFT.

Combined ARPES and DFT establish Dirac nodal lines near EF in Ni3In2S2. The observed Dirac-cone-like dispersion at M and its constant-energy evolution match the folded DFT electronic structure, supporting a nodal-line topology. Matrix-element and zone-folding effects explain partial visibility along Γ–M in ARPES. The transport results—giant, nonsaturating magnetoresistance and clear quantum oscillations with multiple frequencies—corroborate high-mobility Dirac carriers associated with nodal lines. As a nonmagnetic kagome system exhibiting ‘endless’ Dirac nodal lines near EF, Ni3In2S2 provides a promising platform to explore and engineer topological electronic phenomena without complications from magnetic order.

The work identifies Ni3In2S2 as, to the authors’ knowledge, the first nonmagnetic kagome material hosting Dirac nodal lines near the Fermi level. ARPES directly visualizes Dirac-like dispersions consistent with DFT, and transport measurements reveal giant magnetoresistance and quantum oscillations consistent with high-mobility Dirac carriers. This establishes Ni3In2S2 as a platform for realizing and tuning diverse topological electronic states in kagome lattices. Potential future directions include tuning the Dirac nodal lines via strain, chemical substitution, or pressure; exploring thickness and surface termination effects; probing Berry-phase signatures in quantum oscillations; and investigating device-relevant magnetotransport at higher fields and varying orientations.

- ARPES along Γ–M shows only partial agreement with DFT, likely due to ARPES matrix-element effects combined with Brillouin-zone folding from the bulk to the surface BZ, leading to weak intensity of folded bands. - ARPES measurements reported were conducted at a single photon energy (hv ≈ 125 eV), corresponding to kz ≈ 0, limiting kz mapping within the presented data. - Detailed sample growth procedures and some computational parameters are deferred to Supplementary Materials, which may limit immediate reproducibility from the main text alone.