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Electrical symbol for a Josephson junction

A research conducted by Joseph Lombardo, PhD student at the Laboratory of Experimental Physics of Nanostructured Materials under the direction of Prof. Alejandro Silhanek, in collaboration with researchers from K.U. Leuven, UAntwerp and the University Grenoble-Alpes, has made it possible to highlight a new method for the manufacture of nanoscopic Josephson junctions in a simpler, more practical and inexpensive way and to obtain control of their properties after their creation. These results were recently published in Nanoscale (1).

C

onventional superconducting materials are home to many curious physical phenomena. Their first basic property is to lose all electrical resistance below a so-called "critical" temperature around -273°C (close to absolute zero). In other words, below this temperature, a superconductor carries an electric current without any loss of energy. This property makes it extremely attractive from a technological point of view, although it requires complex cooling systems to operate.  The second basic property of superconductors is their ability to expel magnetic fields, which means that the magnetic field inside a superconductor is permanently zero. This property gives rise to a rather spectacular physical phenomenon: quantum levitation. This one sends us back to the familiar image that we have of the future and which is conveyed by science fiction, a little like Marty McFly's hoverboard in the movie Back to the Future.

The Josephson junctions are the result of the work of Prof. Brian D. Josephson who, in 1962, questioned the behaviour of two superconductors separated by an insulating barrier. This British physicist will not only discover that under these conditions, a "super" electric current can pass from one superconductor to another without losing energy, but that this current is also very sensitive to the external magnetic field. This discovery earned him the Nobel Prize in Physics in 1973 and was the basis of the modern definition of the "Volt", the unit of measurement of electrical potential, as well as the birth of the most sensitive magnetic field and temperature detectors of our time called "SQUID's" (from the English "Superconducting Quantum Interference Devices").

Since then, many scientists have become interested in making Josephson junctions. Often, the most advanced current methods permit to create such junctions within sizes in the order of several tens of nanometres, but these often prove complex and require a large investment in terms of time but also cost. Moreover, once manufactured, the properties of these joints are fixed and can therefore no longer be adapted.

The researchers of the Laboratory of Experimental Physics of Nanostructured Materials of ULiège have been interested in this problem and have just developed a new process to manufacture this type of superconducting junction. "We have developed a method that makes manufacturing simpler, universal, more practical and less expensive," explains Joseph Lombardo. This new technique is also quite innovative insofar as it also allows us to obtain a control of the properties of the joints after their fabrication». The mastering of these junctions is important because they are also at the heart of the emerging quantum processors where they play the role of "quantum bits", i.e. the fundamental unit of storage of this type of computer whose development is a major subject of contemporary physics.

"The basic principle is to make superconducting niobium nanowires using a very classical and easily reproducible method," explains Joseph Lombardo. These nanowires have the particularity of having a "bow tie" shape that concentrates the electric current in the center of it. "When the current reaches a sufficiently high value, it changes the structure of the material in the center of the nanowire, and thus also its superconducting properties. "In particular, the superconducting critical temperature in the central nanowire region is decreased," says the young researcher. This reduction in critical temperature is such that the center of the junction acts as a normal barrier between the two outer parts of the superconducting nanowire"

LOMBARDO Josephson Junctions

Modification of the center of a niobium nanowire and appearance of Josephson junction properties.

By subsequently examining the behaviour of the current through the modified nanowire as a function of the external magnetic field, the researchers were able to highlight the unique characteristics of a superconducting Josephson junction. Increasing the current flowing through the nanowire allows changing the current response to the magnetic field, demonstrating the adjustable post-nanofabrication nature of the junction.

This research adds a stone to the edifice of nanotechnology. Building on this success in the manufacture of Josephson junctions, Prof. Silhanek's team is now working on the manufacture of a SQUID based on this method, which will certainly contribute to making these devices more accessible.

Scientific reference

(1) J. Lombardo et al., In situ tailoring of superconducting junctions via electro-annealing, Nanoscale, 2018, 10, 1987-1996

Contact

Laboratory of Experimental Physics of Nanostructured Materials  I CESAM Research Unit

Joseph LOMBARDO - J.Lombardo@uliege.be

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