Isosbestic points in doped SmB6 as features of universality and property tuning

Abstract

Recent theoretical models and experiments suggest that $\mathrm{Sm}{\mathrm{B}}_{6}$ is a topological Kondo insulator. Here we report on heat capacity $C(T)$ investigations of three single-crystalline $\mathrm{Sm}{\mathrm{B}}_{6}$ samples having different quality, of $\mathrm{Sm}{\mathrm{B}}_{6}$ single crystals doped with various concentration of trivalent lanthanum ions ($\mathrm{S}{\mathrm{m}}_{1--x}\mathrm{L}{\mathrm{a}}_{x}{\mathrm{B}}_{6}$ solid solutions) and divalent ytterbium ions ($\mathrm{S}{\mathrm{m}}_{1--x}\mathrm{Y}{\mathrm{b}}_{x}{\mathrm{B}}_{6}$), and of samarium-deficient sintered samples ($\mathrm{S}{\mathrm{m}}_{1--x}{\mathrm{B}}_{6}$). Isosbestic (crossing) points, which can be considered as a feature of universality, were observed on $C$($T, x$) dependencies for all investigated types of samples at ${T}_{\mathrm{iso}}(\mathrm{S}{\mathrm{m}}_{1--x}\mathrm{L}{\mathrm{a}}_{x}{\mathrm{B}}_{6})\ensuremath{\approx}17\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{iso}}(\mathrm{S}{\mathrm{m}}_{1--x}\mathrm{Y}{\mathrm{b}}_{x}{\mathrm{B}}_{6})\ensuremath{\approx}31\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{iso}}(\mathrm{S}{\mathrm{m}}_{1--x}{\mathrm{B}}_{6})\ensuremath{\approx}21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In connection with this as well as with the detailed analysis of the low-temperature $C(T)$ dependencies it can be assumed that by appropriate doping the properties of $\mathrm{Sm}{\mathrm{B}}_{6}$ may be changed/tuned. To obtain the evolution of the gap structure of $\mathrm{Sm}{\mathrm{B}}_{6}$ with changing doping, a simple phenomenological model based on rectangular densities of states was proposed. Additionally, for $\mathrm{S}{\mathrm{m}}_{1--x}\mathrm{L}{\mathrm{a}}_{x}{\mathrm{B}}_{6}$ solid solutions which were available with a high La concentration, it was shown that the insulator-metal transition appears around ${x}_{\mathrm{La}}\ensuremath{\approx}0.40$.