Endless Dirac nodal lines in the nonmagnetic kagome metal Ni3In2S2

Kagome materials, characterized by their unique lattice structure, have emerged as a fertile ground for exploring exotic topological electronic states. The specific arrangement of atoms in a kagome lattice can lead to flat bands and Dirac points, which are crucial for realizing novel quantum phenomena. Numerous recent studies have investigated kagome magnets, uncovering fascinating properties such as Weyl semimetals and topological phases. However, relatively few studies have explored nonmagnetic kagome materials, presenting a significant gap in our understanding. This study focuses on Ni3In2S2, a nonmagnetic kagome metal, to investigate its electronic structure and the presence of topological features. The primary research question is to determine if Ni3In2S2 exhibits Dirac nodal lines, which are one-dimensional topological features in the electronic band structure, potentially impacting its transport properties. The study's importance lies in expanding the knowledge base of topological materials beyond the realm of magnetic systems and opening new avenues for designing novel materials with tailored electronic properties. Understanding the electronic structure of Ni3In2S2 can lead to the development of new technological applications leveraging its unique topological properties. The nonmagnetic nature of Ni3In2S2 also offers an advantage in terms of reduced magnetic scattering, potentially leading to higher carrier mobility and enhanced transport properties.

Extensive research on kagome materials has unveiled a rich tapestry of topological phenomena. Studies on magnetic kagome materials, such as those involving CoSn, FeSn, and various other transition metal compounds, have reported the observation of Dirac fermions, flat bands, and Weyl semimetals. These findings have highlighted the importance of the kagome lattice structure in generating unique topological electronic states. However, the investigation of nonmagnetic kagome materials remains relatively unexplored. Previous theoretical work has predicted the existence of Dirac nodal lines in specific materials, but experimental confirmation remains crucial. This study builds upon the existing literature by focusing on a nonmagnetic kagome metal, aiming to verify theoretical predictions and advance our understanding of topological states in this class of materials.

The research employed a multi-pronged approach combining theoretical calculations and experimental measurements. First, density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) with the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) to determine the electronic band structure and Fermi surface of Ni3In2S2. The calculations employed a plane-wave basis set and considered the primitive cell of the material. The calculated electronic structure was then folded to match the experimental surface Brillouin zone obtained from ARPES measurements. Secondly, high-quality single crystals of Ni3In2S2 were grown, and their structural properties were verified using single-crystal X-ray diffraction. The crystals exhibited metallic behavior with high residual resistivity ratios, and Pauli paramagnetism was observed without any evidence of phase transitions. Angle-resolved photoemission spectroscopy (ARPES) measurements were performed on the (001) surface of Ni3In2S2 crystals at 10 K using a synchrotron light source and a Scienta-Omicron DA30 electron analyzer. This provided experimental data on the electronic structure and Fermi surface. The energy resolution of the ARPES measurement was approximately 15 meV. The ARPES data was analyzed to identify the presence of Dirac nodal lines. The total energy resolution of the ARPES measurement was ~15 meV. Transport property measurements, including transverse and longitudinal magnetoresistance, were conducted at various temperatures using a Quantum Design DynaCool cryostat. Magnetization measurements, including de Haas-van Alphen oscillations, were also performed to characterize the material's electronic transport properties. These experiments aimed to observe the effects of the Dirac nodal lines on the transport properties of Ni3In2S2, specifically looking for evidence of giant magnetoresistance and quantum oscillations. The data analysis involved Fourier transformation of the magnetization data to identify different oscillation frequencies related to the Fermi surface dimensions.

ARPES measurements revealed a Fermi surface consistent with the presence of Dirac nodal lines near the M point. As the binding energy increased, parallel lines in the Fermi surface mapping evolved into a node and eventually an oval shape, further suggesting the presence of a Dirac point. DFT calculations corroborated these findings, demonstrating qualitative agreement with the ARPES data. However, the experimental resolution of certain bands was limited due to ARPES matrix element effects and zone folding. Transport measurements exhibited a giant longitudinal magnetoresistance reaching 2000% at 1.8 K up to 9 T. This non-saturating behavior strongly indicates the influence of the Dirac nodal lines on the material's conductivity. The longitudinal magnetoresistance was significantly smaller than the transverse magnetoresistance, consistent with a quasi-two-dimensional electronic structure. Furthermore, clear quantum oscillations were observed in the magnetization data, and Fourier transformation of these oscillations yielded several frequencies corresponding to different Fermi surface diameters. These diameters agreed well with the DFT calculations. The results demonstrate that Ni3In2S2 is the first nonmagnetic kagome material identified to host endless Dirac nodal lines near the Fermi energy (EF).

The findings confirm the existence of endless Dirac nodal lines in the nonmagnetic kagome metal Ni3In2S2. The agreement between ARPES and DFT data provides strong evidence for the topological nature of the electronic structure. The giant magnetoresistance and quantum oscillations observed in transport measurements further underscore the impact of the Dirac nodal lines on the material's electronic transport properties. The nonmagnetic nature of Ni3In2S2 sets it apart from previously studied kagome materials, significantly expanding our understanding of topological states in nonmagnetic systems. These results suggest Ni3In2S2 is a promising platform for manipulating topological electronic states and potentially exploring novel quantum phenomena. The observed high carrier mobility associated with the Dirac bands contributes to the giant magnetoresistance, highlighting the potential of this material for technological applications.

This study establishes Ni3In2S2 as the first nonmagnetic kagome material exhibiting endless Dirac nodal lines near the Fermi level. The combined ARPES and DFT results, along with the observation of giant magnetoresistance and quantum oscillations, conclusively demonstrate the presence and influence of these topological features. The nonmagnetic nature of this material opens up new possibilities for manipulating and exploring topological phenomena. Future research could focus on investigating the influence of doping or external fields on the Dirac nodal lines and their impact on the material’s transport and optical properties.

The ARPES measurements were limited by matrix element effects and zone folding, which resulted in some bands being less clearly resolved than others. The resolution of the experimental data might also limit the precise determination of the Dirac nodal line parameters. Further investigations with improved experimental techniques could potentially resolve these issues. Future studies may also explore different surface terminations of the material to assess their impact on the surface states and Dirac nodal lines.