Thermal conductance of Nb thin films at sub-kelvin temperatures

Abstract

We determine the thermal conductance of thin niobium (Nb) wires on a silica substrate in the temperature range of 0.1–0.6 K using electron thermometry based on normal metal-insulator-superconductor tunnel junctions. We find that at 0.6 K, the thermal conductance of Nb is two orders of magnitude lower than that of Al in the superconducting state, and two orders of magnitude below the Wiedemann-Franz conductance calculated with the normal state resistance of the wire. The measured thermal conductance exceeds the prediction of the Bardeen-Cooper-Schrieffer theory, and demonstrates a power law dependence on temperature as T4.5, instead of an exponential one. At the same time, we monitor the temperature profile of the substrate along the Nb wire to observe possible overheating of the phonon bath. We show that Nb can be successfully used for thermal insulation in a nanoscale circuit while simultaneously providing an electrical connection.

Joule heating effects in high-transparency Josephson junctions

“…For SGS junctions with Al/Ti leads on the Si/SiO 2 substrate the exponents γ ≈ 4 [23] and γ = 3.1 [51], which agrees with our findings, have been previously reported. Power law thermal transport with the exponent γ = 4.5 has also been observed in superconducting Nb at low temperatures k B T [52]. The origin of the exponent γ ≈ 3 observed in our suspended device is not fully clear.…”

Colloquium: Quantum heat transport in condensed matter systems

“…The measured thermal conductivity of Al closely follows Eq. ( 15), as shown by Peltonen et al (2010) and Feshchenko et al (2017). At the same time Nb films suffer from a nonvanishing subgap DOS, leading to power-law thermal conductance in T, i.e., poor thermal insulation in the low temperature regime.…”

Damping in a superconducting mechanical resonator

“…For the lowest value of 0.5 K in the temperature range seen in Fig. 2 the thermal conductivities are estimated to be κ Al ≃ 4 × 10 −2 W K −1 m −1 for aluminum [27] and κ SiN ≃ 10 −2 W K −1 m −1 for silicon nitride [28,29]. For these values the estimated temperature p-2…”