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

Abstract

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.