Low dissipation thermometry using superconducting tunnel junctions

Abstract

Thermometers are a cornerstone of experimental physics, starting in XVII th century with the premises of thermodynamics. Nowadays, the state of the art thermometers are employed as sensors in bolometers and calorimeters, probing the light originating from the Big Bang. The limitation of these devices is intrinsic. Performing at millikelvin temperatures, their temperatures start to fluctuate as predicted by the Fluctuation-Dissipation Theorem. In this thesis we investigate thermometers able to measure the temperature of an electron gas at sub-kelvin temperatures. We based our approach on tunnel junctions (I) between a superconductor (S) and the system under study, either a normal metal (N) or a weaker superconductor (S'). We tried to decoupled the system from its immediate surrounding by constructing SINIS or SIS'IS structure. This way, the central island is protected from noises and heat coming from the measurement contacts. We focused our work on characterizing a non-invasive thermometer, thus we reduced dissipation in the thermometer to its minimum value. In order to achieve that, we mainly investigate how the low-bias impedance of SINIS and SIS'IS devices reacts with temperature. For SINIS structures, we observed a saturation of the temperature response due to the presence of leakage through the junction. Ballistic Andreev reflection are one of the source of these leakage, and set a minimum working temperature for these device. In the special case where diffusive Andreev reflection are dominating the sub-gap conductance, the low bias impedance of a SINIS structure is responsive toward lower temperatures. The achievable temperature range was only limited by a spurious heat load. In SIS'IS structures, we monitor the transition of the weaker of the superconductor by measuring the supercurrent through the whole device. Such a thermometer response is really large, but the temperature range is also really narrow around the transition temperature of S'. In a sense, this kind of devices is similar to Transition-Edge Sensors (TES), but the presence of tunnel junctions increases the responsivity and reduces the heat leak through the measurement contacts.For all these devices, we developed a unified model allowing ones to reproduce quantitatively the zero bias impedance response to temperature. This model allows one to compare and optimize the sensitivity of the thermometers, given as a Noise Equivalent Temperature (NET). NET as low as a few *mu*K/*sqrt*<Hz> have been observed for a SIS'IS device, and a SINIS device demonstrated a NET within a factor of two of its theoretical limit set by the temperature fluctuations.