High-pressure study of transport properties in Co${}_{0.33}$NbS${}_2$

This is the first study of the effect of pressure on transition metal dichalcogenides (TMDs) intercalated by atoms that order magnetically. Co${}_{0.33}$NbS${}_2$ is a layered system where the intercalated Co atoms order antiferromagnetically at $T_N$ = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity ($\rho$), thermoelectric power ($S$), and thermal conductivity ($\kappa$). At ambient pressure the magnetic transition corresponds to a well-pronounced peak in $dS/dT$, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to $\kappa$. Under pressure the resistivity increases in the high-temperature range, contrary to all previous measurements in other layered TMDs. In the low-temperature range the strong dependences of thermopower and resistivity on pressure are observed below $T_N$, which, in turn, also depends on pressure at rate of $d{T}_N/dp$ ∼ −1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.

Figure 1

The ambient pressure dc-resistivity ρ($T$) (open blue circles) and thermoelectric power $S$($T$) (open red diamonds) of Co${}_{0.33}$NbS${}_2$. Temperature $T_N$, at which antiferromagnetic ordering sets in, is marked by an arrow.

Figure 2

(a) Log-log plot of the measured thermal conductivity ${\kappa }_{\mathit{tot}}$ ($T$) (open red circles) and its decomposed lattice ${\kappa }_{\mathit{ph}}$($T$) (open blue triangles) and electronic contributions ${\kappa }_{\mathit{el}}$($T$) (dashed line), as described in the text. The low-temperature part of the ${\kappa }_{\mathit{ph}}$($T$) follows the power low ($T^{\nu }$), where the fit (indicated by the black line) yields ν $=$ 1.4 ± 0.02. The inset shows a linear plot of the same data. (b) Temperature dependence of $d{\kappa }_{\mathit{tot}}$($T$)/dT (open red circles) and $d{\kappa }_{\mathit{el}}$($T$)/dT (filled green triangles) in the narrow temperature range around $T_N$ reveals an involvement of structural degrees of freedom in the transition.

Figure 3

(a) Pressure dependence of the low-temperature dc-resistivity. (b) dc-resistivity in the whole measured temperature range for selected values of pressure (for each curve pressure is determined at 4 K). Note that the resistivity increases with increasing pressure in the high-temperature region.

Figure 4

Pressure dependence of the derivative of electric resistivity $d\rho \left(T,p\right)/dT$ in the low-temperature region. The temperature of magnetic ordering corresponds to the minimum of the derivative.

Figure 5

The temperature dependence of the thermoelectric power and its pressure behavior $S$($T$, $p$) in the low-temperature range. Inset: The pressure dependence of the transition temperature $T_N$, as determined from the minimum of the derivative $d\rho \left(T\right)/dT$.

Figure 6

Pressure dependence of the derivative of the thermoelectric power $dS\left(T\right)/dT$ in the low-temperature range. $T_N$ corresponds to the minimum of $dS\left(T\right)/dT$ [i.e., the temperature for which the temperature dependence of $S\left(T\right)$ is the steepest].