Graphene Supermoiré Lattice Shows Strong Superconductivity

Researchers have found strong superconductivity and electron correlations in a special graphene setup known as a supermoiré lattice. This work comes from a team led by Mitali Banerjee at EPFL, working with groups from Freie Universität Berlin, Japan's National Institute for Materials Science, the US National High Magnetic Field Laboratory, Florida State University, and EPFL's Center for Quantum Science and Engineering. They built the structure using three graphene layers twisted at different angles and published their results in Nature Physics.

Background

Graphene is a single layer of carbon atoms arranged in a honeycomb pattern. It conducts electricity well because electrons move fast through it. When scientists stack two graphene layers and twist them by a small angle, they create a moiré lattice. This pattern looks like overlapping grids and slows down the electrons. At certain twist angles, called magic angles, the electrons interact strongly with each other. This leads to unusual states like superconductivity, where electricity flows without resistance, or insulation, where it does not flow at all.

These moiré effects have been studied a lot in recent years. They mimic behaviors seen in high-temperature superconductors, which could change how we use energy one day. But most work focused on two-layer graphene with mirror symmetry, where the top and bottom look the same. Now, teams are looking at more layers to see what happens.

In this case, the researchers used three layers. They twisted the top layer relative to the middle one at one angle and the bottom layer at a slightly different angle. This broke the mirror symmetry. The two moiré patterns from these twists overlapped and formed a larger pattern, the supermoiré lattice. This bigger structure has a longer wavelength, on the scale of tens of nanometers.

The idea came from wanting to see if superconductivity could still happen without mirror symmetry. Banerjee's team built devices with these layers sandwiched between insulators and metal gates. They cooled them to near absolute zero, about 0.3 Kelvin, and measured how electricity flowed.

Key Details

The team tuned two things: carrier density and displacement field. Carrier density controls how many electrons are available, using gate voltages. Displacement field is an electric field across the layers, set by top and bottom gates.

They measured electrical resistance as they changed these. Resistance dropped to near zero in some spots, a sign of superconductivity. To confirm, they checked how it behaved with temperature and current. As temperature rose, the zero-resistance state vanished. High currents or magnetic fields also killed it, matching superconductor traits.

Phase Diagram Mapping

The phase diagram showed a lot going on. There were many peaks in resistance, pointing to insulating states. Between them sat superconducting regions. The supermoiré lattice split the main superconductivity area into smaller domes. Insulating states sat at half-filling of the supermoiré bands, where electrons fill half the spots.

They saw Brown-Zak oscillations and Hofstadter butterfly patterns in magnetic fields. These repeating resistance patterns proved a larger periodic structure steered the electrons—the supermoiré lattice.

The supermoiré creates mini-flat bands and mini-Dirac bands. Flat bands mean electrons barely move on their own; interactions dominate. Dirac bands relate to graphene's linear electron paths. The supermoiré potential breaks symmetry in these bands, driven by electron repulsion.

This leads to a cascade: superconductor to insulator shifts as density changes. Superconductivity held up even without mirror symmetry, with clear critical temperatures around 1 Kelvin and magnetic field limits.

"Despite this symmetry breaking, we still observed strong superconducting regions with clear critical temperatures and critical magnetic fields," said Mitali Banerjee, lead researcher at EPFL.

Zekang Zhou, the first author, spotted key features during measurements. The team used out-of-plane magnetic fields up to 10 Tesla to map the bands.

What This Means

This work shows supermoiré lattices add a new way to control electrons in graphene. The larger pattern tunes quantum states, fragmenting superconductivity into parts. It proves strong superconductivity needs no mirror symmetry, challenging some ideas.

The findings map how moiré and supermoiré interplay with correlations. Mini-flat bands host these states, offering a tool to study electron pairing in superconductors.

For tech, this points to designing materials with tailored phases. Quantum devices might use these for lossless switches or sensors. The broken symmetry setup is easier to make than perfect mirror ones.

Banerjee's team plans more work. They want to find when supermoiré lattices form best and why superconductivity appears away from magic angles. They also eye moiré quasicrystals with supermoiré.

"The rich phase diagram inspired us to pursue this system," Banerjee said. "It ultimately revealed that the supermoiré degree of freedom provides a powerful new way to engineer and explore novel quantum phases in graphene-based systems."

Other groups watch closely. Similar twisted multilayers could yield more surprises. The precise control of density and field lets researchers probe deep into correlated states. This builds on bilayer graphene but adds layers for richer physics.

The study fills gaps in how higher-order moiré works. It shows supermoiré potentials reshape bands and boost interactions. Future devices might stack these for 2D circuits with zero resistance paths.

Teams elsewhere confirm moiré superconductivity, but this trilayer twist stands out for its cascade and symmetry break. It sets a benchmark for multilayer systems.