Anomalous metallic state in quasi-two-dimensional ${\text{BaNiS}}_2$

We report on a systematic study of the thermodynamic, electronic, and charge transport properties of high-quality single crystals of ${\mathrm{BaNiS}}_2$, the metallic end member of the quasi-two-dimensional ${\mathrm{BaCo}}_{1-x}{\mathrm{Ni}}_x{\mathrm{S}}_2$ 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 bands and one electron band, where electron-phonon scattering dominates charge transport and the minority holes exhibit, below $\sim 100$ K, a very large mobility, ${\mu }_h\sim 15000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, 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 $\text{linear-}T$ 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.

Figure 1

Tetragonal structure of ${\mathrm{BaNiS}}_2$. The unit cell is drawn using solid lines.

Figure 2

Temperature dependence of the isobaric specific heat, $C_P$, of a bunch of ${\mathrm{BaNiS}}_2$ single crystals. In the inset, the broken line is a fit of the low-temperature data that yields $\gamma$ and ${\mathrm{\Theta }}_D$. The red solid line is the theoretical Debye curve described by Eq. (1).

Figure 3

Magnetic susceptibility of a ${\mathrm{BaNiS}}_2$ single crystal. ${\chi }_{ab}$ and ${\chi }_c$ indicate the measurements taken with field in the $ab$ plane and along the $c$ axis, respectively. The average is given by ${\chi }_{\mathrm{avg}}=\frac{2}{3}{\chi }_{ab}+\frac{1}{3}{\chi }_c$. The data have been corrected from the core diamagnetic susceptibility estimated to be ${\chi }_{\mathrm{core}}\approx -1.2\times {10}^{-4}$ emu ${\mathrm{mol}}^{-1}$.

Figure 4

(a) $ab\text{-plane}$ and (b) $c\text{-axis}$ resistivity of a representative ${\mathrm{BaNiS}}_2$ single crystal with $RRR=9.4$. The red lines are a data fit using Eq. (2). The inset shows the low-temperature resistivity curve obtained on a sample with $RRR=15.8$. Black and red circles indicate data obtained in a dilution cryostat and in a PPMS, respectively. The blue line is a linear fit $\rho ={\rho }_0^{\prime }+AT$ with $A=1.91\times {10}^{-3}\mathrm{m}\mathrm{\Omega }\mathrm{cm}{\mathrm{K}}^{-1}$ and ${\rho }_0^{\prime }=5.5\times {10}^{-3}\mathrm{m}\mathrm{\Omega }\mathrm{cm}$.

Figure 5

(a) Transverse and (b) longitudinal magnetoresistivity of a ${\mathrm{BaNiS}}_2$ single crystal with $RRR=15.8$ at temperatures ranging from 1.8–300 K. The magnetic field was applied perpendicular to the current.

Figure 6

Magnetoresistance $\mathrm{\Delta }\rho /\rho (B=0)$ plotted as a function of $B^2/{\rho }^2(B=0)$. Note the deviation from the Kohler's rule at high temperature.

Figure 7

Field dependence of the components of the magnetoconductivity tensor, (a) ${\sigma }_{xx}$ and (b) ${\sigma }_{xy}$. Black solid lines are fits of the curves using Eqs. (3), (4) and a three-band model with two hole bands and one electron band.

Figure 8

Carrier mobilities extracted from the analysis of the temperature and field dependence of the longitudinal and transverse conductivity for two ${\mathrm{BaNiS}}_2$ samples with $RRR=7.9$ (left) and $RRR=15.8$ (right). The dashed lines above 100 K indicate the extrapolated behavior of the mobility by assuming a conventional ${\mu }_i\propto 1/T$ dependence expected for a metal with $\rho \propto T$.

Figure 9

Calculated electronic bands and total density of states. The dotted line indicates the Fermi level, ${\epsilon }_F$.