We report a study of the temperature-dependent electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and ${}^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) in Heusler-type ${\mathrm{Ru}}_2\mathrm{TaAl}$, to shed light on its semimetallic behavior. While the temperature dependence of the electrical resistivity exhibits semiconductorlike behavior, the analysis of low-temperature specific heat reveals a residual Fermi-level density of states (DOS). Both observations can be realized by means of a semimetallic scenario with the Fermi energy located in the pseudogap of the electronic DOS. The NMR Knight shift and spin-lattice relaxation rate show activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap in ${\mathrm{Ru}}_2\mathrm{TaAl}$. From the first-principles band structure calculations, we further provide a clear picture that an indirect overlap between electron and hole pockets is responsible for the formation of a pseudogap in the vicinity of the Fermi level of ${\mathrm{Ru}}_2\mathrm{TaAl}$. In addition, an effort for improving the thermoelectric performance of ${\mathrm{Ru}}_2\mathrm{TaAl}$ has been made by investigating the thermoelectric properties of ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$. We found significant enhancements in the electrical conductivity and Seebeck coefficient and marked reduction in the thermal conductivity via the off-stoichiometric approach. This leads to an increase in the figure-of-merit $ZT$ value from $6.1\times {10}^{-4}$ in ${\mathrm{Ru}}_2\mathrm{TaAl}$ to $3.4\times {10}^{-3}$ in ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$ at room temperature. In this respect, a further improvement of thermoelectric performance based on ${\mathrm{Ru}}_2\mathrm{TaAl}$ through other off-stoichiometric attempts is highly probable.

• Received 7 June 2017
• Revised 24 August 2017

DOI:https://doi.org/10.1103/PhysRevB.96.125106