Physicists have examined the flow of electron pairs in ‘magic-angle’ graphene, marking significant progress in comprehending how this peculiar material exhibits superconductivity. By understanding the ease with which electron pairs traverse the material, researchers are unraveling its exceptional properties.
Superconducting materials are akin to carpool lanes on a busy highway. Just as carpoolers can bypass heavy traffic, paired electrons can glide through these materials without any friction.
However, the flow of electron pairs is influenced by various factors, notably the density of these pairs within the material. This characteristic, known as “superfluid stiffness,” indicates how easily a current of electron pairs can flow and is crucial in assessing a material’s superconductivity.
A team of physicists from MIT and Harvard University has successfully measured the superfluid stiffness in magic-angle graphene for the first time. This unique material is composed of two or more atomically thin graphene sheets twisted at a specific angle, resulting in a range of remarkable properties, including uncommon superconductivity.
The superconductivity observed in magic-angle graphene holds potential for advanced quantum computing technologies, yet the precise mechanisms of its superconductivity remain unclear. Understanding the material’s superfluid stiffness is pivotal for scientists aiming to decode how superconductivity functions in magic-angle graphene.
The results indicate that the superconductivity in magic-angle graphene is largely determined by quantum geometry, which pertains to the conceptual “shape” of the quantum states existing within a material.
This research, published in the journal Nature, marks the initial instance of directly measuring superfluid stiffness in a two-dimensional material. The team employed a novel experimental method that could also apply to assess other two-dimensional superconductors.
“There’s a whole host of 2D superconductors waiting to be explored; we are just beginning,” remarks Joel Wang, co-lead author and research scientist at MIT’s Research Laboratory of Electronics (RLE).
The co-authors from MIT comprise co-lead author Miuko Tanaka, along with Thao Dinh, Daniel Rodan-Legrain, Sameia Zaman, Max Hays, Bharath Kannan, Aziza Almanakly, David Kim, Bethany Niedzielski, Kyle Serniak, Mollie Schwartz, Jeffrey Grover, Terry Orlando, Simon Gustavsson, Pablo Jarillo-Herrero, and William D. Oliver. They collaborated with Kenji Watanabe and Takashi Taniguchi from Japan’s National Institute for Materials Science.
Magic Resonance
Since its discovery in 2004, graphene has been hailed as an extraordinary material. It consists of a single layer of carbon atoms structured in a precise, chicken-wire lattice. This unique arrangement endows graphene with exceptional strength, durability, and impressive capabilities for conducting electricity and heat.
In 2018, Jarillo-Herrero found that stacking two graphene sheets at an exact, “magic” angle leads to magic-angle twisted bilayer graphene (MATBG), which displays entirely new properties, including superconductivity, where electrons pair instead of repelling each other as they typically do. These Cooper pairs can create a superfluid capable of moving through a material effortlessly and without friction.
“Despite Cooper pairs having no resistance, they still require some kind of push, like an electric field, to initiate movement,” Wang explains. “Superfluid stiffness indicates how readily these particles can be mobilized to induce superconductivity.”
To measure superfluid stiffness in superconducting materials, scientists traditionally use a technique involving a microwave resonator—a device that resonates at specific microwave frequencies, much like a vibrating violin string. Inserting a superconducting material into the resonator alters its resonance frequency and kinetic inductance, metrics that correlate directly to the superfluid stiffness.
Historically, this method has only suited larger, thicker samples. The MIT team recognized that measuring superfluid stiffness in materials as thin as MATBG would necessitate a novel technique.
“In contrast to MATBG, typical superconductors examined with resonators can be 10 to 100 times thicker and larger,” Wang says. “We were uncertain if such a minuscule sample would yield any measurable inductance.”
A Captured Signal
The primary challenge of studying superfluid stiffness in MATBG involved attaching the extremely delicate material seamlessly to the microwave resonator surface.
“For this process to function optimally, we need a superconducting connection between the two materials to avoid degrading the microwave signal or having it bounce back instead of entering our target material,” Wang points out.
Will Oliver’s group at MIT has been enhancing techniques to accurately link fragile two-dimensional materials for developing novel quantum bits for upcoming quantum computing devices. In this new study, Tanaka, Wang, and their team applied these methods to effectively connect a tiny sample of MATBG to an aluminum microwave resonator. They began by assembling MATBG using standard techniques, followed by encasing it between two insulating layers of hexagonal boron nitride to preserve MATBG’s atomic structure and properties.
“Aluminum, a material present in our superconducting quantum computing research, such as aluminum resonators for reading aluminum qubits, worked well for us,” Oliver explains. “So we decided to use aluminum for most of the resonator and connect a small piece of MATBG to its end. This turned out to be advantageous.”
“To achieve contact with the MATBG, we precisely etch it, akin to slicing layers of cake with a sharp knife,” Wang describes. “We expose a side of the freshly cut MATBG, onto which we deposit aluminum—the same material in the resonator—to ensure a proper contact and form an aluminum lead.”
Subsequently, the researchers connected the aluminum leads of the MATBG structure to the larger aluminum microwave resonator, sending a microwave signal through it and measuring the changes in resonance frequency, which allowed them to deduce MATBG’s kinetic inductance.
Upon translating the measured inductance to superfluid stiffness, the researchers discovered a value significantly surpassing conventional superconductivity theories. They suspected this unexpected finding was related to the quantum geometry of MATBG—the correlation of the quantum states of electrons.
“We observed a tenfold increase in superfluid stiffness far beyond conventional expectations, with a temperature dependency aligning with quantum geometry theory predictions,” Tanaka comments. “This provided compelling evidence pointing to quantum geometry’s role in determining superfluid stiffness in this two-dimensional material.”
“This research exemplifies how advanced quantum technology, typically used in quantum circuits, can be leveraged to explore condensed matter systems with strongly interacting particles,” adds Jarillo-Herrero.
This investigation received funding from the US Army Research Office, the National Science Foundation, the Air Force Office of Scientific Research, and the Under Secretary of Defense for Research and Engineering.
A related study focusing on magic-angle twisted trilayer graphene (MATTG), developed through collaboration between Philip Kim’s group at Harvard University and Jarillo-Herrero’s team at MIT, is also featured in this issue of Nature.
