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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 1 4 SPIN TRIPLETS LEAD TO BETTER QUBITS Scientists at the National Institute of Standards and Technology (NIST), Gaithersburg, Md., have discovered a potentially useful material for building quantum computers. The supercon- ductor (SC) could address one of the primary challenges facing effective quantum logic circuits. Newly discovered properties in the compound uranium ditelluride, or UTe2, show that it could prove high- ly resistant to one of the nemeses of quantum computer development—the challenge of making such a computer’s qubits, or memory storage switches, function long enough to finish a compu- tation before losing the delicate phys- ical relationship that allows them to operate as a group. The compound’s unusual and strong resistance to magnetic fields makes it a rare bird among supercon- ducting materials, which offer distinct advantages for qubit design—chiefly, their resistance to the errors that can easily creep into quantum compu- tation. UTe2’s exceptional behaviors could make it attractive to the nascent quantum computer industry, ac- cording to the research team. “This is potentially the silicon of the quan- tum information age,” says NIST’s Nick Butch. “You could use uranium ditelluride to build the qubits of an efficient quantum computer.” In all SCs, elec- trons formCooper pairs, and the electromag- netic interactions that cause these pairings are responsible for UTe2’s superconductivity. What is specifically important to this material’s Cooper pairing is the ability of its electrons’ quantum spins to orient in one of three combina- tions, making them spin triplets. Most spin-triplet SCs are predicted to be to- pological as well, possessing a highly useful property in which the supercon- ductivity would occur on the surface of the material and would remain super- conducting even in the face of external disturbances. This resistance might also help sci- entists understand the nature of UTe2 and perhaps superconductivity itself. nist.gov. GRAPHENE’S MAGIC ANGLE LAUNCHES TWISTRONICS Researchers at California Institute of Technology (Caltech), Pasadena, are building on last year’s MIT discovery of the “magic angle” for stacked sheets of graphene. They are directly studying the material using a scanning tunneling microscope that can image electronic properties at atomic-length scales. Understanding the “magic an- gle”—a specific orientation between EMERGING TECHNOLOGY sheets of graphene that yields special electric properties—could pave the way for production of room-temperature superconductors, which could transmit enormous electric currents while pro- ducing zero heat. In early 2018, researchers at MIT discovered that, at a certain orienta- tion, the graphene’s bilayer material be- comes superconducting and moreover, the superconducting properties can be controlled with the electric fields. Their discovery launched a new field of research into magic angle-oriented graphene, known as “twistronics.” Research on the magic angle re- quires an extreme level of precision to get the two sheets of graphene aligned at just the right angle. The Caltech team expanded on MIT’s discovery by devel- oping a new method of creating sam- ples of magic angle-twisted graphene that can be used to align the two sheets of graphene very precisely while leaving it exposed for direct observation. Using this technique, the research- ers could learn more about the elec- tronic properties of the material at the magic angle as well as study how these properties change as the twist angle moves away from the magic value. Their work provides several key insights that will guide future theoretical mod- eling and experiments in twistronics. caltech.edu. This whimsical illustration shows the superiority of one superconductor’s newfound properties. Most known superconductors are spin singlets, on the left island. Uranium ditelluride, however, is a rare spin triplet, on the right island and also at the top of a mountain representing its high resistance to magnetic fields. Courtesy of NIST/Natasha Hanacek. Scanning tunneling microscopy topographic image of twisted bilayer graphene. Courtesy of Stevan Nadj-Perge.