Sandwiched between superconductors, graphene adopts exotic electronic states | MIT News

In regular conductive supplies corresponding to silver and copper, electrical present flows with various levels of resistance, within the type of particular person electrons that ping-pong off defects, dissipating power as they go. Superconductors, against this, are so named for his or her outstanding potential to conduct electrical energy with out resistance, by the use of electrons that pair up and transfer by a cloth as one, producing no friction.

Now MIT physicists have discovered {that a} flake of graphene, when introduced in shut proximity with two superconducting supplies, can inherit a few of these supplies’ superconducting qualities. As graphene is sandwiched between superconductors, its digital state adjustments dramatically, even at its heart.

The researchers discovered that graphene’s electrons, previously behaving as particular person, scattering particles, as a substitute pair up in “Andreev states” — a elementary digital configuration that permits a standard, nonsuperconducting materials to hold a “supercurrent,” an electrical present that flows with out dissipating power.

Their findings, revealed this week in Nature Physics, are the primary investigation of Andreev states on account of superconductivity’s “proximity impact” in a two-dimensional materials corresponding to graphene.

Down the highway, the researchers’ graphene platform could also be used to discover unique particles, corresponding to Majorana fermions, that are thought to come up from Andreev states and could also be key particles for constructing highly effective, error-proof quantum computer systems.

“There’s a big effort within the condensed physics group to search for unique quantum digital states,” says lead creator Landry Bretheau, a postdoc in MIT’s Division of Physics. “Particularly, new particles known as Majorana fermions are predicted to emerge in graphene that’s linked to superconducting electrodes and uncovered to giant magnetic fields. Our experiment is promising, as we’re unifying a few of these substances.”

Landry’s MIT co-authors are postdoc Joel I-Jan Wang, visiting scholar Riccardo Pisoni, and affiliate professor of physics Pablo Jarillo-Herrero, together with Kenji Watanabe and Takashi Taniguchi of the Nationwide Institute for Supplies Science, in Japan.

The superconducting proximity impact

In 1962, the British physicist Brian David Josephson predicted that two superconductors sandwiching a nonsuperconducting layer between them may maintain a supercurrent of electron pairs, with none exterior voltage.

As an entire, the supercurrent related to the Josephson impact has been measured in quite a few experiments. However Andreev states — thought of the microscopic constructing blocks of a supercurrent — have been noticed solely in a handful of programs, corresponding to silver wires, and by no means in a two-dimensional materials.  

Bretheau, Wang, and Jarillo-Herrero tackled this subject by utilizing graphene — an ultrathin sheet of interlinked carbon atoms — because the nonsuperconducting materials. Graphene, as Bretheau explains, is a particularly “clear” system, exhibiting little or no scattering of electrons. Graphene’s prolonged, atomic configuration additionally permits scientists to measure graphene’s digital Andreev states as the fabric is available in contact with superconductors. Scientists can even management the density of electrons in graphene and examine the way it impacts the superconducting proximity impact.

The researchers exfoliated a really skinny flake of graphene, only a few hundred nanometers broad, from a bigger chunk of graphite, and positioned the flake on a small platform produced from a crystal of boron nitride overlaying a sheet of graphite. On both finish of the graphene flake, they positioned an electrode produced from aluminum, which behaves as a superconductor at low temperatures. They then positioned the whole construction in a dilution fridge and lowered the temperature to twenty millikelvin — properly inside aluminum’s superconducting vary.

“Pissed off” states

Of their experiments, the researchers various the magnitude of the supercurrent flowing between the superconductors by making use of a altering magnetic subject to the whole construction. In addition they utilized an exterior voltage on to graphene, to range the variety of electrons within the materials.

Below these altering circumstances, the workforce measured the graphene’s density of digital states whereas the flake was in touch with each aluminum superconductors. Utilizing tunneling spectroscopy, a typical approach that measures the density of digital states in a conductive pattern, the researchers have been in a position to probe the graphene’s central area to see whether or not the superconductors had any impact, even in areas the place they weren’t bodily touching the graphene.

The measurements indicated that graphene’s electrons, which usually act as particular person particles, have been pairing up, although in “pissed off” configurations, with energies depending on magnetic subject.

“Electrons in a superconductor dance harmoniously in pairs, like a ballet, however the choreography within the left and proper superconductors will be completely different,” Bretheau says. “Pairs within the central graphene are pissed off as they attempt to fulfill each methods of dancing. These pissed off pairs are what physicists know as Andreev states; they’re carrying the supercurrent.”

Bretheau and Wang discovered Andreev states range their power in response to a altering magnetic subject. Andreev states are extra pronounced when graphene has a better density of electrons and there’s a stronger supercurrent operating between electrodes.

“[The superconductors] are literally giving graphene some superconducting qualities,” Bretheau says. “We discovered these electrons will be dramatically affected by superconductors.”

Whereas the researchers carried out their experiments underneath low magnetic fields, they are saying their platform could also be a place to begin for exploring the extra unique Majorana fermions that ought to seem underneath excessive magnetic fields.

“There are proposals for the best way to use Majorana fermions to construct highly effective quantum computer systems,” Bretheau says. “These particles may very well be the elementary brick of topological quantum computer systems, with very robust safety towards errors. Our work is an preliminary step on this path.”

This work was supported, partially, by the U.S. Division of Vitality and the Gordon and Betty Moore Basis.

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