The Editors of the journal Physical Review Letters recently awarded the label "Editor's suggestion" to an article from the Quantronics group of SPEC (Laboratory of Condensed Matter Physics) : "Phase Controlled Superconducting Proximity Effect Probed by Tunneling Spectroscopy, Phys. Rev. Lett. 100 (2008) 197002". By this label publishers aim to put forward a small number of items they regard as particularly clear and likely to attract readers outside their specialty.
This paper provides for the first time a neat, clear and complete overview of "the superconducting proximity effect." Such an effect occurs at the interface between metal superconductor (S) and metals with "normal" resistivity (N), in which superconductivity can locally expand within the normal metal and make it non-resistive.
Research to understand the superconducting proximity effect (and more generally inhomogeneous superconductivity) started in the 60's, notably under the leadership of P.-G. De Gennes when he was in Orsay. The theoretical progress has been rapid since the proximity effect was theoretically understood in the early 70's. However, as the phenomenon occurs usually at the sub-micron scale, it was not until the development in nanofabrication techniques and in mesoscopic physics in the 90's that experimental tests became possible. Like the pieces of a puzzle, numerous (and scattered) experimental works, have worked the way up to confirming the theoretical vision of the phenomenon. Despite this renewed interest in the topic, however, some key pieces of the puzzle remained missing.
The experiment in the Quantronics group not only completes this puzzle, but alone provides a complete view of the proximity effect for the first time. In this experiment, a direct and highly accurate mapping of the electronic states in metals near N-S interfaces, which is a fundamental theoretical quantity, was obtained. Thus, without any sophisticated data interpretation, a comparison between theory and experimental results and shows an excellent agreement for the full range of parameters.
This fundamental experiment in the superconductivity field was made possible by developing a combined scanning tunneling and atomic force microscope operating at very low temperature (35 mK), unprecedented in the world today. This versatile device could be used to perform many other experiments in mesoscopic physics.
Lastly, the importance of this work was also recognized by the Academy of Sciences who has just awarded Hélène le Sueur by its thematic "Madeleine Lecoq" prize, for the development of the microscope and this experiment, which is the subject of her PhD thesis.
Phase Controlled Superconducting Proximity Effect Probed by Tunneling Spectroscopy
H. le Sueur, P. Joyez, H. Pothier, C. Urbina, and D. Esteve, Phys. Rev. Lett. 100 (2008) 197002.