Microscopic origin of anomalous cooling and thermometer features

Identify the microscopic origin of the reduced cooling performance near 1.3–1.5 K and the accompanying jump in the zero-bias resistance of the TiW-Al normal-metal island thermometer.

Background

The device exhibits unexpectedly reduced cooling performance between 1.3 K and 1.5 K, together with a jump in the thermometer’s zero-bias resistance between 1.4 K and 1.3 K. The authors note that 1.4 K coincides with the critical temperature of a 20-nm-thick Al film, but they do not identify the microscopic mechanism responsible for either observation.

References

While we cannot assign a microscopic origin to these features, we note that the temperature of 1.4~K coincides with the critical temperature of a 20-nm-thick Al film.

— Electronic cooling of a TiW alloy normal-metal island using Nb-based superconducting tunnel junctions  (2608.24455 - Hätinen et al., 25 Aug 2026) in Main text, paragraph discussing the cooling curves and thermometer calibration near 1.3–1.5 K

While the coincidence suggests that the superconductivity in the Al layer may influence the local density of states and therefore the thermometer response, the present data do not establish a direct connection.

— Electronic cooling of a TiW alloy normal-metal island using Nb-based superconducting tunnel junctions  (2608.24455 - Hätinen et al., 25 Aug 2026) in Main text, paragraph discussing the discontinuity in the thermometer calibration