Coulomb Blockade Thermometry and Other Aspects of Tunnel Junction Arrays
This report describes experimental results on a few different aspects of two-dimensional tunnel junction arrays: Coulomb blockade thermometry (CBT), Hall effect and Josephson array magnetometry (JAM).
CBT is a primary thermometry method which is only a few years old. It is suitable for cryogenic temperatures between 20 mK and 30 K. The primary nature, the small size, the speed of operation and the low sensitivity to magnetic field, neutron radiation and nonuniformities in the array makes it a good choice for general laboratory work. We have investigated some new possibilities which can improve this technique. We show that in addition to one-dimensional (1D) arrays which were originally used, we can also use 2D arrays and achieve better results above 1 K. The main advantages are better robustness to nonuniformities in the array or broken/shorted junctions, lower noise and faster measurement due to lower resistance.
We present a faster primary measurement method for CBT which is based on the third derivative of the current-voltage characteristics. This third derivative has a zero crossing at a well defined temperature dependent voltage. We demonstrate a feedbacked measurement circuit which is able to follow this zero crossing.
We have also measured Hall resistance in superconducting arrays. We report a complex structure in the magnetic field dependence of the Hall resistance. We see that the Hall resistance only appears in certain ranges of the junction parameters. We also see that in some cases a gate voltage can affect the Hall voltage strongly, even change the sign of the Hall resistance. We were also able to increase the periodic fluctuations of the ordinary resistance in an array by ten times by aplying a magnetic field of 30-50 mT.
The JAM experiments are very preliminary and we just briefly report on the first data we have taken. We hope to be able to use the S-I transition in a Josephson junction array as a megnetic field sensor.