Unusual transport and large diamagnetism in the intermetallic semiconductorRuAl2

Abstract

We report measurements of the resistivity, thermopower, thermal conductivity, Hall coefficient, and magnetic susceptibility of ${\mathrm{RuAl}}_2$ over a wide temperature range. Room-temperature sound velocity measurements are also reported. We find that at low temperature ${\mathrm{RuAl}}_2$ behaves as a low carrier-density semimetal. As the temperature is raised, excitations across a 0.6-eV pseudogap dominate the transport, and ${\mathrm{RuAl}}_2$ behaves effectively as a semiconductor. The diamagnetic susceptibility of ${\mathrm{RuAl}}_2$ is anomalously large, and has an unusual temperature dependence that can be attributed to a Pauli term that reflects underlying structure in the density of states. We also suggest that the large diamagnetic response is a universal feature of hybridization gap semiconductors. The potential of ${\mathrm{RuAl}}_2$ for thermoelectric applications is evaluated. We find that even if the lattice component of the thermal conductivity could be reduced to its minimum value, the electrical properties of ${\mathrm{RuAl}}_2$ are not good enough to make it an attractive thermoelectic material.