Anomalously high electronic thermal conductivity and Lorenz ratio in Bi₂Te₃ nanoribbons far from the bipolar condition

The Lorenz number (L) of a conductor is the ratio between its electronic thermal conductivity and electrical conductivity. It takes the Sommerfeld value of L0=π2/3kB/e2 in simple, metallically electronic systems where charge and heat are both carried by the same group of quasi-particles that experience elastic scattering. Higher values of L than L₀ are possible in semiconductors where both electrons and holes co-exist at high densities, that is, in bipolar conduction. As a narrow-bandgap semiconductor, Bi₂Te₃ exhibits L > L₀ which has been generally attributed to such bipolar conduction mechanisms. However, in this work, we report that L > L₀ is still observed in individual, single-crystal Bi₂Te₃ nanoribbons even at low temperatures and when degenerately doped, that is, far from the bipolar conduction condition. This discovery calls for different mechanisms to explain the unconventional electronic thermal transport behavior in Bi₂Te₃.