The carbon atoms hold strongly to one another, making imperfections in the pattern uncommon. Graphene is a repeating honeycomb sheet with a carbon atom residing at every corner. Perhaps surprisingly, graphene's wealth of quantum research opportunities is tied to its physical simplicity. The topic of stacked graphene is extensively represented in scientific journals, and the online arXiv preprint server has over 2,000 articles posted about "bilayer graphene"-nearly 1,000 since 2018. The 2021 American Physical Society March Meeting included 13 sessions addressing the topics of graphene or twisted bilayers, and Das Sarma hosted a day long virtual conference in June for researchers to discuss twisted graphene and the related research inspired by the topic. The richness and diversity of the electrical behaviors in graphene stacks has inspired a stampede of research.
"Sounds like magic or science fantasy, except it is happening every day in at least ten laboratories in the world." "Here is a system where almost every interesting quantum phase of matter that theorists ever could imagine shows up in a single system as the twist angle, carrier density, and temperature are tuned in a single sample in a single experiment," says Das Sarma, who is also the Director of the CMTC. One of the most promising veins for scientific treasure is the appearance of superconductivity (lossless electrical flow) in stacked graphene. And there is a lot to map the phenomena in graphene range from the familiar like magnetism to more exotic things like strange metallicity, different versions of the quantum Hall effect, and the Pomeranchuk effect-each of which involve electrons coordinating to produce unique behaviors. Researchers at JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland, including JQI Fellows Sankar Das Sarma and Jay Sau and others, are busy creating the theoretical physics foundation that will be a map of this new landscape. Research into these stacked sheets of graphene is like the Wild West, complete with the lure of striking gold and the uncertainty of uncharted territory. Researchers have discovered that stacking layers of graphene two or three at a time (called, respectively, bilayer graphene or trilayer graphene) and twisting the layers relative to each other opens fertile new territory for scientists to explore. In recent years, graphene has kept on giving. In 2010 early experiments demonstrating the quantum richness of graphene earned two researchers the Nobel Prize in physics. Research into individual atom-thick sheets of graphite-called graphene-took off after 2004 when scientists developed a reliable way to produce it (using everyday adhesive tape to repeatedly peel layers apart). This last form, graphite, is at first glance the most mundane, but thin sheets of it host a wealth of uncommon physics.
Even just carbon on its own is extraordinarily adaptable: It is the only ingredient in (among other things) diamonds, buckyballs and graphite (the stuff used to make pencil lead). And strong, lightweight carbon fibers are used in cars, planes and windmills. Carbon is the essential ingredient for turning iron into steel, which underlies technologies from medieval swords to skyscrapers and submarines. Carbon is the backbone of life on earth and the fossil fuels that have resulted from the demise of ancient life.