Superconducting Quantum Interference Proximity Transistor

Francesco Giazotto

NEST CNR-INFM and Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy

Proximity effect [1] is a phenomenon which can be described as the induction of superconducting correlations into a normal-type conductor. One striking consequence of this effect is the modification of the local density of states (DOS) in the normal metal [2], and the opening of a minigap whose amplitude can be controlled by changing the macroscopic phase of the superconducting order parameter [3]. Proximity effect was experimentally demonstrated in mesoscopic hybrid structures at milliKelvin temperatures both in transport measurements [4] as well as recently with scanning tunneling microscopy (STM) [5]. Here we report the realization of a novel interferometer, the superconducting quantum interference proximity transistor (SQUIPT), whose operation relies on the modulation with the magnetic field of the DOS of a proximized metal embedded in a superconducting loop. Flux sensitivities down to ~10^-5 F₀/Hz^-1/2 (F₀ » 2´10^-15 Wb is the flux quantum) can be achieved even for a non-optimized design, with an intrinsic dissipation which is several orders of magnitude smaller than in conventional superconducting interferometers. Optimizing the device parameters promises to largely increase the sensitivity for the detection of tiny magnetic fields. The SQUIPT has a number of features which make it attractive for a variety of applications: (1) only a simple DC read-out scheme is required, similarly to DC SQUIDs; (2) either current- or voltage-biased measurement can be conceived depending on the setup requirements; (3) a large flexibility in the fabrication parameters and materials, such as semiconductors, carbon nanotubes or grapheme instead of normal metals, is allowed to optimize the response and the operating temperature; (4) ultralow dissipation (~1-100 fW) which makes it ideal for nanoscale applications; (5) ease of implementation in a series or parallel array for enhanced output; (6) ease of integration with superconducting refrigerators [6] to actively tune the device working temperature. This approach opens the way to magnetic-field detection based on ``hybrid'' interferometers which take advantage of the flexibility intrinsic to proximity metals.

References

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[5] H. le Sueur, P. Joyez, H. Pothier, C. Urbina, and D. Esteve, Phys. Rev. Lett. 100, 197002 (2008).

[6] F. Giazotto, T. T. Heikkilä, A. Luukanen, A. M. Savin, and J. P. Pekola, Rev. Mod. Phys. 78, 217 (2006).