Unusual transport and large diamagnetism in the intermetallic semiconductor RuAl2

We report measurements of the resistivity, thermopower, thermal conductivity, Hall coefficient, and magnetic susceptibility of RuAl2 over a wide temperature range. Room-temperature sound velocity measurements are also reported. We find that at low temperature RuAl2 behaves as a low carrier-density semimetal. As the temperature is raised, excitations across a 0.6-eV pseudogap dominate the transport, and RuAl2 behaves effectively as a semiconductor. The diamagnetic susceptibility of RuAl2 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 RuAl2 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 RuAl2 are not good enough to make it an attractive thermoelectic material.