Unusual violation of the Wiedemann–Franz law at ultralow temperatures in topological compensated semimetals

Thermal conductivity and electrical resistivity at ultralow temperatures and high magnetic fields are studied in the topological compensated semimetals TaAs2, NbAs2, and NdSb. A striking phenomenon is observed where the thermal conductivity shows a T4 scaling at very low temperatures, while the resistivity shows a T-independent residual term. This indicates a strong violation of the Wiedemann–Franz (WF) law, since the field dependence of κ shows that the low-temperature thermal conductivity is dominated by electronic transport. The obtained Lorenz ratio is hundreds of times lower than Sommerfeld’s value even when approaching the zero-temperature limit. The strong downward deviation of the WF law at very low temperatures point to a non-Fermi liquid state in these materials. In addition, the giant thermal quantum oscillations accompanied by antiphase characteristics have been observed. Our findings not only point to a possible non-Fermi liquid ground state of these topological compensated semimetals, but also reveal an unusual T4 temperature dependence for the electronic thermal conductivity.