Low-temperature conducting state in two candidate topological Kondo insulators: ${\mathrm{SmB}}_6$ and ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$

We have investigated the low-temperature conducting state of two Kondo insulators, ${\mathrm{SmB}}_6$ and ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$, which have been theoretically predicted to host topological surface states. Through comparison of the specific heat of as-grown and powdered single crystals of ${\mathrm{SmB}}_6$, we show that the residual term that is linear in temperature is not dominated by any surface state contribution, but rather is a bulk property. In ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$, we find that the Hall coefficient is independent of sample thickness, which indicates that conduction at low temperatures is dominated by the bulk of the sample, and not by a surface state. The low-temperature resistivity of ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$ is found to monotonically decrease with low concentrations of disorder introduced through ion irradiation. This is in contrast to ${\mathrm{SmB}}_6$, which is again indicative of the contrasting origins of the low-temperature conduction. In ${\mathrm{SmB}}_6$, we also show that the effect of low concentrations of irradiation damage of the surface with ${\mathrm{Fe}}^{+}$ ions is qualitatively consistent with damage with nonmagnetic ions.

Figure 1

Specific heat divided by temperature $C/T$ as a function of $T$ for three crystals of ${\mathrm{SmB}}_6$ from the same batch. Two are pristine single crystals; one was ground to a powder and sieved to leave grain sizes in the stated range. The data for the ground powder include the contribution of the GE varnish which is negligible at low $T$.

Figure 2

Temperature dependence of the Hall coefficient of ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$ at two points along a wedge-shaped sample to demonstrate thickness dependence. Inset shows the ratio of the Hall resistivity $R_{xy}$ at the two points, and the same ratio for previous measurements on ${\mathrm{SmB}}_6$ [6].

Figure 3

Temperature dependence of the resistance of (a) ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$ and (b), (c) ${\mathrm{SmB}}_6$ after differing concentrations of ion-irradiation damage to the top and bottom face to the stated depth using the stated ion. Panel (b) is reproduced from a previous study [7].