Forskere kortlægger små drejninger i magisk vinkelgrafen

Lavet af et enkelt lag af kulstofatomer forbundet i et sekskantet bikagemønster, graphens struktur er enkel og tilsyneladende delikat. Siden opdagelsen i 2004, forskere har fundet ud af, at grafen faktisk er usædvanligt stærkt. Og selvom grafen ikke er et metal, det leder elektricitet ved ultrahøje hastigheder, bedre end de fleste metaller.

I 2018, MIT-forskere ledet af Pablo Jarillo-Herrero og Yuan Cao opdagede, at når to ark grafen stables sammen i en let forskudt "magisk" vinkel, den nye "snoede" grafenstruktur kan enten blive en isolator, fuldstændig blokering af elektricitet fra at strømme gennem materialet, eller paradoksalt nok, en superleder, i stand til at lade elektroner flyve igennem uden modstand. Det var en monumental opdagelse, der hjalp med at lancere et nyt felt kendt som "twistronics, "studiet af elektronisk adfærd i snoet grafen og andre materialer.

Nu rapporterer MIT-teamet deres seneste fremskridt inden for grafen twistronics, i to artikler offentliggjort i denne uge i tidsskriftet Natur .

I den første undersøgelse, forskerne, sammen med samarbejdspartnere ved Weizmann Institute of Science, har afbildet og kortlagt en hel snoet grafenstruktur for første gang, med en opløsning, der er fin nok til, at de er i stand til at se meget små variationer i den lokale vridningsvinkel på tværs af hele strukturen.

Resultaterne afslørede områder i strukturen, hvor vinklen mellem grafenlagene svingede lidt væk fra den gennemsnitlige forskydning på 1,1 grader.

Holdet opdagede disse variationer ved en ultrahøj vinkelopløsning på 0,002 grader. Det svarer til at kunne se et æbles vinkel mod horisonten på en kilometers afstand.

De fandt ud af, at strukturer med et snævrere udvalg af vinkelvariationer havde mere udtalte eksotiske egenskaber, såsom isolering og superledning, kontra strukturer med et bredere udvalg af snoningsvinkler.

"Dette er første gang, en hel enhed er blevet kortlagt for at se, hvad der er snoningsvinklen i et givet område i enheden, " siger Jarillo-Herrero, Cecil og Ida Green professor i fysik ved MIT. "Og vi ser, at man kan have en lille smule variation og stadig vise superledning og anden eksotisk fysik, men det kan ikke være for meget. Vi har nu karakteriseret hvor meget twist variation du kan have, og hvad er nedbrydningseffekten af ​​at have for meget."

I den anden undersøgelse, teamrapporten skaber en ny snoet grafenstruktur med ikke to, men fire lag grafen. De observerede, at den nye firelags magiske vinkelstruktur er mere følsom over for visse elektriske og magnetiske felter sammenlignet med dens to-lags forgænger. Dette tyder på, at forskere måske lettere og mere kontrollerbart kan studere de eksotiske egenskaber af magisk vinkelgrafen i firelagssystemer.

"Disse to undersøgelser sigter mod bedre at forstå den forvirrende fysiske adfærd af enheder med magiske vinkler, " siger Cao, en kandidatstuderende ved MIT. "Når man først har forstået, fysikere mener, at disse enheder kan hjælpe med at designe og konstruere en ny generation af højtemperatur-superledere, topologiske enheder til kvanteinformationsbehandling, og lavenergiteknologier."

Som rynker i plastfolie

Siden Jarillo-Herrero og hans gruppe først opdagede magisk vinkel grafen, andre har kastet sig over chancen for at observere og måle dens egenskaber. Flere grupper har afbildet magiske vinkelstrukturer, ved hjælp af scanning tunneling mikroskopi, eller STM, en teknik, der scanner en overflade på atomniveau. Imidlertid, forskere har kun været i stand til at scanne små pletter af magisk vinkel grafen, spænder højst over et par hundrede kvadratnanometer, ved at bruge denne tilgang.

"At gå over en hel mikron-skala struktur for at se på millioner af atomer er noget, som STM ikke er bedst egnet til, " siger Jarillo-Herrero. "I princippet kunne det lade sig gøre, men det ville tage enormt lang tid."

Så gruppen rådførte sig med forskere ved Weizmann Institute for Science, who had developed a scanning technique they call "scanning nano-SQUID, " where SQUID stands for Superconducting Quantum Interference Device. Conventional SQUIDs resemble a small bisected ring, the two halves of which are made of superconducting material and joined together by two junctions. Fit around the tip of a device similar to an STM, a SQUID can measure a sample's magnetic field flowing through the ring at a microscopic scale. The Weizmann Institute researchers scaled down the SQUID design to sense magnetic fields at the nanoscale.

When magic-angle graphene is placed in a small magnetic field, it generates persistent currents across the structure, due to the formation of what are known as "Landau levels." These Landau levels, and hence the persistent currents, are very sensitive to the local twist angle, for eksempel, resulting in a magnetic field with a different magnitude, depending on the precise value of the local twist angle. In this way, the nano-SQUID technique can detect regions with tiny offsets from 1.1 degrees.

"It turned out to be an amazing technique that can pick up miniscule angle variations of 0.002 degrees away from 1.1 degrees, " Jarillo-Herrero says. "This was very good for mapping magic-angle graphene."

The group used the technique to map two magic-angle structures:one with a narrow range of twist variations, and another with a broader range.

"We placed one sheet of graphene on top of another, similar to placing plastic wrap on top of plastic wrap, " Jarillo-Herrero says. "You would expect there would be wrinkles, and regions where the two sheets would be a bit twisted, some less twisted, just as we see in graphene."

They found that the structure with a narrower range of twist variations had more pronounced properties of exotic physics, such as superconductivity, compared with the structure with more twist variations.

"Now that we can directly see these local twist variations, it might be interesting to study how to engineer variations in twist angles to achieve different quantum phases in a device, " Cao says.

Tunable physics

Over the past two years, researchers have experimented with different configurations of graphene and other materials to see whether twisting them at certain angles would bring out exotic physical behavior. Jarillo-Herrero's group wondered whether the fascinating physics of magic-angle graphene would hold up if they expanded the structure, to offset not two, but four graphene layers.

Since graphene's discovery nearly 15 years ago, a huge amount of information has been revealed about its properties, not just as a single sheet, but also stacked and aligned in multiple layers—a configuration that is similar to what you find in graphite, or pencil lead.

"Bilayer graphene—two layers at a 0-degree angle from each-other—is a system whose properties we understand well, " Jarillo-Herrero says. "Theoretical calculations have shown that in a bilayer-on-top-of-bilayer structure, the range of angles over which interesting physics would happen is larger. So this type of structure might be more forgiving in terms of making devices."

Partly inspired by this theoretical possibility, the researchers fabricated a new magic-angle structure, offsetting one graphene bilayer with another bilayer by 1.1 degrees. They then connected the new "double-layer" twisted structure to a battery, applied a voltage, and measured the current that flowed through the device as they placed the structure under various conditions, such as a magnetic field, and a perpendicular electric field.

Just like magic-angle structures made from two layers of graphene, the new four-layered structure showed an exotic insulating behavior. But uniquely, the researchers were able to tune this insulating property up and down with an electric field—something that's not possible with two-layered magic-angle graphene.

"This system is highly tunable, meaning we have a lot of control, which will allow us to study things we cannot understand with monolayer magic-angle graphene, " Cao says.

"It's still very early in the field, " Jarillo-Herrero says. "For the moment, the physics community is still fascinated just by the phenomena of it. People fantasize about what type of devices we could make but realize it's still too early and we have so much yet to learn about these systems."

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation og undervisning.