Observation of topological nodal fermion semimetal phase in ZrSiS

Unveiling new topological phases of matter is one of the current objectives in condensed matter physics. Recent experimental discoveries of Dirac and Weyl semimetals prompt the search for other exotic phases of matter. Here we present a systematic angle-resolved photoemission spectroscopy study of ZrSiS, a prime topological nodal semimetal candidate. Our wider Brillouin zone (BZ) mapping shows multiple Fermi surface pockets such as the diamond-shaped Fermi surface, elliptical-shaped Fermi surface, and a small electron pocket encircling at the zone center $\left(\mathrm{\Gamma }\right)$ point, the $M$ point, and the $X$ point of the BZ, respectively. We experimentally establish the spinless nodal fermion semimetal phase in ZrSiS, which is supported by our first-principles calculations. Our findings evidence that the ZrSiS-type of material family is a new platform on which to explore exotic states of quantum matter; these materials are expected to provide an avenue for engineering two-dimensional topological insulator systems.

Figure 1

Crystal structure and sample characterization of ZrSiS. (a) Tetragonal crystal structure of ZrSiS. The atoms form clear two-dimensional structures. Zr layers are separated by two neighboring S layers, where both are sandwiched between Si atoms forming a square net. (b) Temperature dependence of the electrical resistivity of single-crystalline ZrSiS measured with the electrical current flowing within the $ab$ plane, in zero magnetic field applied along the $c$ axis of the tetragonal unit cell. The inset shows a picture of the sample used in our measurement. (c) Core level spectroscopic measurement of ZrSiS, which shows various core energy levels of the constituting elements. Here the vertical dashed line at zero energy represents the Fermi level. (d) Calculated band structure along various high-symmetry directions. Small gaps are observed due to the spin-orbit coupling. (e), (f) Measured Fermi surface maps covering multiple Brillouin zone regions. Photon energies of the measurements are denoted in the plots. (g) Calculated bulk Fermi surface maps over the wider BZ regions. High-symmetry points are also marked on the plot. ARPES data were collected at the SIS-HRPES end station at the SLS, PSI.

Figure 2

Electronic structure measurements of ZrSiS. (a) Fermi surface map (with binding energy of 0 meV shown in the top left panel) and constant energy contours. The value of the binding energy is denoted at the constant energy contour plots. (b) Dispersion map of ZrSiS along the high-symmetry directions obtained by using incident photon energy of 50 eV at a temperature of 20 K. ARPES data were collected at the SIS-HRPES end station at the SLS, PSI. The high-symmetry directions are marked on the plots. (c) Slab calculations along high-symmetry directions. Blue and red curves represent the surface and bulk bands, respectively. A small bulk gap is observed at the $X$ point along the $M\text{−}X\text{−}M$ high-symmetry direction.

Figure 3

Dispersion maps for ZrSiS. (a) Photon energy dependent ARPES dispersion maps. The measured photon energies are given on the plots. The linearly dispersive states are observed, which do not show any dispersion with photon energy. (b) ARPES energy-momentum dispersion map measured with photon energy of 70 eV. (c) Calculations showing the dispersion map along the $M\text{−}\mathrm{\Gamma }\text{−}M$ high-symmetry direction. The inset shows a calculated Fermi surface formed by Dirac line nodes. Experimental data qualitatively agrees with the calculations.