Thermal conductivity tensor of NbO2

Abstract

Among several transition metal oxides showing metal to insulator transition, NbO2 has attracted attention as the active material of memory devices due to its high thermal stability. Although clarification of thermal conductivity (κ) is essentially important for optimizing the NbO2 device performance, it has not been reported thus far probably due to the anisotropic crystal structure. Here we show the thermal conductivity tensor of NbO2 crystal. We measured the thermal conductivities of (1 1 0)-, (0 0 1)-, and (1 1 1)-oriented NbO2 films and found 〈1 1 1〉 -direction shows ∼1 W m−1 K−1 higher thermal conductivity compared with the other directions (∼3 W m−1 K−1). Using the measured κ-values, we successfully determined thermal conductivity tensor components κ11, κ13, and κ33. The diagonal components of the thermal conductivity tensor were ∼2.5 W m−1 K−1 while the nonzero off-diagonal component was ∼1 W m−1 K−1. We believe that the present results are of great value in optimizing the performance of NbO2 based devices.