New sensor for weak magnetic fields - SQUIPT

Researchers at the Low Temperature Laboratory of Aalto University (Helsinki, Finland) and at Scuola Normale Superiore (SNS, Pisa, Italy) have developed a new type of a sensor for the detection of tiny magnetic fields. The device bears a strong resemblance to traditional SQUID magnetometers (Superconducting Quantum Interference Device), found in numerous applications. The new structure benefits from very small power dissipation and a simple method of measurement at low frequencies.

The key element of the device is a loop made of a thin film of superconducting metal deposited on an insulating substrate. A small section of the loop, with a typical length between few tens and few hundreds of nanometers, is replaced by a normal, non-superconducting metal. Due to the so-called proximity effect, the normal-conducting metal acquires features reminiscent of a weak superconductor. This phenomenon takes place in the interface region whenever a normal metal is placed in good electrical contact with a superconductor. Most importantly for the new structure, entitled SQUIPT (Superconducting Quantum Interference Proximity Transistor), certain properties of the normal metal start to depend on the amount of magnetic flux threading the loop. Unlike in SQUID sensors, the SQUIPT converts a change in the flux into a change in electric current or voltage in a tunneling junction between the normal metal and an additional superconducting electrode. In such a structure, electrons tunnel quantum mechanically through a thin insulating oxide layer separating the normal and superconducting electrodes.

The SQUIPT utilizes the well known electrical transport properties of metallic tunnel junctions, as well as the flexibility, originating from the superconducting proximity effect, in choosing the materials and device dimensions. So far, measurements have been performed mostly at temperatures of approximately 0.1 degree above the absolute zero. With a suitable choice of the superconducting material, however, the device will function also at liquid helium temperatures (around 4 Kelvins, or -269°C) relevant for many practical applications.

"Our original idea was to study the influence of the proximity effect on the heat flow between the normal metal electrons and lattice vibrations, tells Prof. Jukka Pekola from the Low Temperature Laboratory, leader of the Finnish part of the joint research effort. "Soon we noticed that the structure performs well as a sensitive magnetometer. The prototype structures were not optimized in several respects, but the predicted sensitivities and noise properties seem promising", he clarifies.

The research was reported in the online edition of Nature Physics on Feb 28th, 2010.

F. Giazotto, J. T. Peltonen, M. Meschke, and J. P. Pekola, SQUIPT - Superconducting Quantum Interference Proximity Transistor http://dx.doi.org/10.1038/nphys1537


For additional information, please contact:

Professor Jukka Pekola Low Temperature Laboratory, Aalto University PICO-group, Micronova tel. +358 9 470 24913 jukka.pekola(at)tkk.fi

Senior researcher Matthias Meschke matthias.meschke(at)ltl.tkk.fi

Researcher Joonas Peltonen joonas.peltonen(at)ltl.tkk.fi