Realization of Z${}_2$ topological metal in single-crystalline nickel deficient NiV${}_2$Se${}_4$

Temperature-dependent electronic and magnetic properties are reported for nickel-deficient NiV${}_2$Se${}_4$. Single-crystal X-ray diffraction shows it to crystallize in the monoclinic Cr${}_3$S${}_4$ structure type with space group I2 / m and vacancies on the Ni site, resulting in the composition Ni${}_{0.85}$V${}_2$Se${}_4$ in agreement with our electron-probe microanalysis. Structural distortions are not observed down to 1.5 K. Nevertheless, the electrical resistivity shows metallic behavior with a broad anomaly around 150-200 K that is also observed in the heat capacity data. This anomaly indicates a change of state of the material below 150 K. It is believed that this anomaly could be due to spin fluctuations or charge-density-wave fluctuations, where the lack of long-range order is caused by vacancies at the Ni site of Ni${}_{0.85}$V${}_2$Se${}_4$. The non-linear temperature dependence of the resistivity as well as an enhanced value of the Sommerfeld coefficient γ = 104.0 (1) mJ mol${}^{-1}$ K${}^{-2}$ suggest strong electron-electron correlations in this material. First-principles calculations performed for NiV${}_2$Se${}_4$, which are also applicable to Ni${}_{0.85}$V${}_2$Se${}_4$, classify this material as a topological metal with Z${}_2$ = (1; 110) and coexisting electron and hole pockets at the Fermi level. The phonon spectrum lacks any soft phonon mode, consistent with the absence of periodic lattice distortion in the present experiments. (© 2023 The Authors. Annalen der Physik published by Wiley‐VCH GmbH)