Highlights of recent research:
Coulomb Blockade Thermometer (CBT) is a primary thermometer based on electric conductance characteristics of tunnel junction arrays.
Based on our long-term experience in thermal properties of mesoscopic structures and thermometry, a metrological Coulomb blockade thermometer is being developed in collaboration with Mikes. The key issue here is to find a reliable a fabrication technique for obtaining high quality opaque tunnel junctions and sufficiently large cooling fins to thermalize the device at temperatures down to 10 mK. Our recent results would indicate that the objective of a 10 mK Coulomb blockade thermometer is a realistic objective.
Fig 1: Schematic picture of Coulomb blockade thermometer with multiple islands. Large cooling fins have been attached to help the thermometer to function at lower temperatures.
Fig 2: A way to thermalize electrons at temperatures as low as 10mK: even big (20x40µm) thin-film cooling fins have no sufficient volume to guarantee low enough electron temperatures. The solution is to go into the third dimension and to deposit very thick (~10µm) copper blocks.
We developed the concept and theory and demonstrated experimentally primary thermometry based on a four-probe measurement of a single tunnel junction embedded within four arrays of junctions (SJT). We showed that random sample specific and environment-related errors can be avoided in this configuration. This method relates temperature directly to Boltzmann constant, which will form the basis of the definition of temperature and realization of official temperature scales in the future.
Fig 3: A single-junction thermometer. The junction (framed) is embedded within arrays to protect it from environment noise. The graphs show temperature measurements at 42 K and 77 K.