Dirac nodal-line semimetal zinc polynitride at high pressure

Topological semimetals exhibit novel quantum states and electronic structures with interesting Fermi surfaces. In contrast to Weyl and Dirac nodal points, nodal lines can have multiple topological configurations in momentum space that lead to new properties, such as long-range Coulomb interaction and flat Landau levels. Herein, we report the discovery of a Dirac nodal-line semimetal polynitride ${\mathrm{ZnN}}_4$, synthesized from the elements under high pressure via laser-heated diamond anvil cell technique. The crystal structure of ${\mathrm{ZnN}}_4$ has an $Ibam$ space group, and features infinite one-dimensional nitrogen chains in the pressure range of 14–130 GPa. The Dirac cone is observed midway between $S$ and $X$ points in the Brillouin zone and the Dirac nodal-line is found near the $T$ point. The measured electrical resistance of the ${\mathrm{ZnN}}_4$ sample shows an anomalous increase with pressure, which is attributed to the localization of valence electrons. The case of ${\mathrm{ZnN}}_4$ provides a unique example of Dirac nodal-line semimetal in nitrides.

Figure 1

(a) Schematic illustration of a laser-heated DAC and sample. Bottom: Transmitted and reflected light optical image of a sample (inside a DAC) before and after laser heating. The reacted part is indicated with a red circle. (b), (c) Full-profile refinements of XRD patterns in a Zn-N sample at (b) 90 GPa and (c) 82 GPa. The incident wavelength is 0.6199 Å. Blue, green, and orange bars in the bottom panel represent the calculated peak positions of the $P\overline{1}\text{−}{\mathrm{ZnN}}_4$, Ibam-${\mathrm{ZnN}}_4$, and $P{6}_3/mmc\text{−}\mathrm{Zn}$ phase, respectively.

Figure 2

Projections of the $Ibam\text{−}{\mathrm{ZnN}}_4$ structure along the (a) $a$ axis and (b) $b$ axis in the unit cell. Blue and green spheres denote zinc and nitrogen atoms, respectively. (c) Bond distances and angles in ${\left[{{\mathrm{N}}_4}^{2-}\right]}_{\mathrm{n}}$ chains. Nitrogen atoms at different Wyckoff positions are differentiated by colors. (d) Calculated 2D ELF of 1D nitrogen chains under 120 GPa.

Figure 3

(a) 2D band structure of $Ibam\text{−}{\mathrm{ZnN}}_4$ and correspondingly PDOS at 60 GPa. Green lines (top left) emphasize a Dirac nodal-line (DNL) and Dirac cone near the Fermi level. (b) 3D band structure of $Ibam\text{−}{\mathrm{ZnN}}_4$ on the $k_a\text{−}{k}_c$ plane under 60 GPa. The DNL is emphasized in pink, and the green lines represent the anisotropic Dirac cone near the $T$ point in Fig. 3. (c) Projection of the band strructure on the $k_a\text{−}{k}_c$ plane near the $T$ point. The path of four yellow dot-dashed lines marks the positions of the DNL around the $T$ point.

Figure 4

(a) Full-profile refinement of the XRD pattern in a non-magnetic steel DAC at 105 GPa. The proportion of Zn and ${\mathrm{ZnN}}_4$ is about 4:1. (b) Temperature dependence of the electrical resistance of ${\mathrm{ZnN}}_4$ in the temperature range of 3–250 K. The cryogenic zone is enlarged for illustration. (c) Measured resistance along isotherms 3 and 200 K at different pressures. Inset: Photograph of the electrical cell. (d) Calculated band structure of ${\mathrm{ZnN}}_4$ from 110 to130 GPa. The blue square area highlights the band gap. (e) Enlarged view of panel (d). (f) Cross sections of the ELF in the $Ibam\text{−}{\mathrm{ZnN}}_4$ phase at 60 GPa (left) and 300 GPa (right).