We report on a systematic study of the thermodynamic, electronic and charge transport properties of high-quality single crystals of BaNiS 2 , the metallic end-member of the quasi-twodimensional BaCo 1−x Ni x S 2 system characterized by a metal-insulator transition at x cr = 0.22. Our analysis of magnetoresistivity and specific heat data consistently suggests a picture of compensated semimetal with two hole-and one electron-bands, where electron-phonon scattering dominates charge transport and the minority holes exhibit, below ∼100 K, a very large mobility, µ h ∼ 15000 cm 2 V −1 s −1 , which is explained by a Dirac-like band. Evidence of unconventional metallic properties is given by an intriguing crossover of the resistivity from a Bloch-Grüneisen regime to a linear−T regime occurring at 2 K and by a strong linear term in the paramagnetic susceptibility above 100 K.We discuss the possibility that these anomalies reflect a departure from conventional Fermi-liquid properties in presence of short-range AF fluctuations and of a large Hund coupling.

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By means of Shubnikov-de-Haas and de-Haas-van-Alphen oscillations, and ab initio calculations, we have studied the Fermi surface of high-quality BaNiS 2 single crystals, with mean free path l ∼ 400Å. The angle and temperature dependence of quantum oscillations indicates a quasi-twodimensional Fermi surface, made of an electron-like tube centred at Γ, and of 4 hole-like cones, generated by Dirac bands, weakly dispersive in the out-of-plane direction. Ab initio electronic structure calculations, in the density functional theory framework, show that the inclusion of screened exchange is necessary to account for the experimental Fermi pockets. Therefore, the choice of the functional becomes crucial. A modified HSE hybrid functional with 7% of exact exchange outperforms both GGA and GGA+U density functionals, signalling the importance of non-local screened-exchange interactions in BaNiS 2 , and, more generally, in 3d compensated semimetals.

High pressure is a proven effective tool for modulating inter-layer interactions in semiconducting transition metal dichalcogenides, which leads to significant band structure changes. Here, we present an extended infrared study of the pressure-induced semiconductor-to-metal transition in 2H-$$\hbox {MoTe}_2$$ MoTe 2 , which reveals that the metallization process at 13–15 GPa is not associated with the indirect band-gap closure, occurring at 24 GPa. A coherent picture is drawn where n-type doping levels just below the conduction band minimum play a crucial role in the early metallization transition. Doping levels are also responsible for the asymmetric Fano line-shape of the $$\hbox {E}_{1u}$$ E 1 u infrared-active mode, which has been here detected and analyzed for the first time in a transition metal dichalcogenide compound. The pressure evolution of the phonon profile under pressure shows a symmetrization in the 13–15 GPa pressure range, which occurs simultaneously with the metallization and confirms the scenario proposed for the high pressure behaviour of 2H-$$\hbox {MoTe}_2$$ MoTe 2 .