| | | An array of SIN tunnel junctions as a sensitive thermometer I J Agulo12 and L Kuzmin1 Quantum Device Physics Laboratory, Dept. of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg 412 96, Sweden 2(on study leave) Dept. of Physics, University of the Philippines - Baguio, Gov. Pack Road, Baguio City 2600, Philippines E-mail: leonid.kuzmin@mc2.chalmers.se Abstract We have fabricated and measured an array of superconductor-insulator-normal metal (SIN) tunnel junctions for the purpose of using it as a sensitive thermometer. An increase of the responsivity to temperature, dV/dT ~5 |j,V/mK is observed compared to that of a single junction responsivity, dV/dT~1 |j,V/mK. We then used such array thermometer to measure the temperature stability of the Heliox AC-V Cryogenfree System developed by Oxford Instruments. We have been able to measure a temperature stability of ±100 |j,K over a period of time. | | |
| | | Introduction Low temperature thermometry represents a very complicated task. In addition to the usual requirements of a thermometer, such as accuracy, sensitivity and the range of temperature measuring [1], the matching electronics set-up also adds to the difficulty of thermometer fabrication. Many different professional thermometers using microscale electron gas subsystem have already been developed [2] with high accuracy and sensitivity. The main problem is usually the matching with the external professional amplifier. The best temperature sensitivity to our knowledge has been achieved by Schmidt, et. al., where they embedded the SIN thermometer in an LC resonance circuit connected to a high frequency cryogenic amplifier [3]. However, this system is too complicated for standard measurements at low temperatures in cryogenic systems. The novel concept of a thermometer proposed by Kuzmin [4] gives high temperature resolution that could be matched with any standard room-temperature amplifier. According to the model, the signal of the thermometer may overcome the noise limitation of the amplifier by increasing the number of junctions, N. In this way, the sensitivity of the thermometer could be proportionally increased. The operation of the thermometer based on SIN array can be analyzed using the heat balance equation developed for single bolometer [4]: | | Here, ZA(7f - Tph) is the heat flow from electron to the phonon subsystems in the normal metal, E is a material constant, A - a volume of the absorber, Te and T h are, respectively, the electron and phonon temperatures of the normal metal; PSIN(V,Te,Tph) is cooling power of the SIN tunnel junctions; Cv = yTe is the specific heat capacity of the normal metal; P0 a background rf power, 2V2/RS is the heat load due to the subgap leakage resistance, RS. A thermometer is characterized by its responsivity, noise equivalent temperature and the time constant. In the current-biased mode, the responsivity, SV, is described by the voltage response to the change of temperature | | |