In 2018, a graduate student at MIT performed what seemed like an absurdly simple experiment. Pablo Jarillo-Herrero's team took two sheets of graphene—each just one atom thick—stacked them on top of each other, and twisted one layer by 1.1 degrees. What emerged from this elementary operation was something nobody had predicted: superconductivity at temperatures that made theorists scramble to understand what they were witnessing.

This discovery opened a door into a realm where the traditional boundaries of condensed matter physics begin to dissolve. Moiré materials—named for the interference patterns that emerge when lattices overlap at small angles—have become perhaps the most exciting playground for quantum many-body physics in a generation. Here, phenomena that typically require extreme conditions or exotic compounds can be dialed in by adjusting a twist angle, applying a gate voltage, or varying carrier density.

What makes this platform so remarkable is not just the exotic phases it hosts, but the unprecedented control it offers. We can now engineer the electronic structure of materials almost at will, creating flat bands where electrons slow to a crawl and begin to interact strongly with one another. The result is a laboratory for studying correlated quantum matter that combines the tunability of cold atomic systems with the richness of real electronic materials.

The key to moiré physics lies in a counterintuitive principle: slow electrons are interesting electrons. In ordinary metals, electrons move with high velocities through the crystal lattice, largely ignoring each other as they zip past. Their kinetic energy dominates, and quantum correlations remain weak. But when you can suppress that kinetic energy—when you can force electrons to nearly stand still—something profound happens.

Twisting two atomically thin layers at certain magic angles achieves exactly this. The moiré pattern creates a new, much larger periodic structure called a superlattice. Electrons navigating this superlattice experience interference effects that can flatten their energy bands almost completely. In twisted bilayer graphene at 1.1 degrees, the bandwidth drops to just a few millielectronvolts—roughly a hundred times smaller than the Coulomb interaction energy between electrons.

This flat band condition represents a kind of phase space engineering. By making kinetic energy negligible compared to interaction energy, we enter a regime where correlation effects dominate. Every electron feels every other electron intensely. The system becomes exquisitely sensitive to small perturbations, and exotic many-body ground states become energetically favorable.

The tunability is remarkable. Unlike cuprate superconductors or heavy fermion compounds where the electronic structure is fixed by chemistry, moiré systems can be adjusted continuously. Gate voltages change the carrier density. Pressure or strain modifies interlayer coupling. Even the twist angle itself can be varied in situ using novel device geometries. Each knob provides access to different regions of a vast phase diagram.

This engineering capability has transformed flat bands from a theoretical curiosity into an experimental tool. Researchers can now systematically explore how correlated electron physics evolves across parameter space, testing theories against a tunable quantum simulator that operates at temperatures far more accessible than traditional strongly correlated materials.

Takeaway

When you suppress motion, interactions dominate. Flat bands create a regime where electrons can no longer ignore each other, and the resulting correlations give rise to physics impossible in conventional materials.

The superconductivity discovered in twisted bilayer graphene arrived without warning. Graphene itself is decidedly not a superconductor—it is a semimetal with exceptional conductivity precisely because its electrons move freely. That two sheets of this material, twisted slightly, would suddenly conduct electricity with zero resistance below about 1.7 Kelvin struck the community as deeply mysterious.

The mystery deepens when you examine the phase diagram. Superconductivity appears adjacent to correlated insulating states, much like the topology seen in cuprate high-temperature superconductors. This proximity is unlikely to be coincidental. In cuprates, superconductivity emerges when you dope a Mott insulator—adding charge carriers to a system frozen by electron-electron repulsion. The same pattern appears in magic-angle graphene.

This resemblance has ignited fierce debate about mechanism. Is the superconducting pairing conventional, mediated by phonons in some unusual geometry? Or are we seeing something more exotic—spin fluctuations, or an entirely novel pairing glue arising from the correlated insulating state itself? The relatively low critical temperatures might suggest weak coupling, but the phase diagram topology points toward strong correlation physics.

Recent experiments have added complexity. Superconductivity has now been observed in trilayer graphene, in twisted transition metal dichalcogenides, and in various heterostructures combining different two-dimensional materials. Each system shows variations: different symmetries, different density dependencies, different magnetic field responses. The diversity suggests we may not be dealing with a single mechanism but rather a family of unconventional superconductors.

Perhaps most intriguingly, these systems offer an opportunity that cuprate research never provided: systematic tunability. We cannot easily change the copper-oxygen plane geometry in YBCO. But in moiré materials, we can vary twist angle, layer number, material composition, and doping level while watching superconductivity appear and disappear. This experimental control may finally crack problems that have resisted solution for four decades.

Takeaway

The phase diagram of moiré superconductors mirrors that of cuprates—superconductivity emerging from correlated insulators—but with tunability that enables systematic investigation of mechanisms that have remained mysterious for decades.

Superconductivity may have grabbed headlines, but the correlated insulator phases in moiré materials are equally profound. At certain filling fractions—when the flat bands are partially occupied—electrons spontaneously organize into insulating states. These are not band insulators, where gaps arise from the lattice structure. They are Mott insulators, where electrons stop moving because their mutual repulsion makes motion energetically prohibitive.

The flat band physics amplifies this correlation effect dramatically. With essentially zero kinetic energy, electrons have no way to screen their interactions. The system becomes extraordinarily sensitive to filling. At integer fillings per moiré unit cell, strong correlations can lock electrons in place, producing insulating behavior in what should be a metallic system. These states often break symmetries spontaneously, exhibiting magnetic order that emerges purely from electron interactions.

But the phase diagram extends far beyond simple Mott physics. Researchers have observed orbital magnets—states where magnetic order arises not from electron spins but from orbital currents circulating within each moiré unit cell. Even more exotic are the fractional quantum anomalous Hall states, where electrons fractionalize into quasiparticles carrying fractions of the electron charge, stabilized not by strong magnetic fields but by the intrinsic topology of the flat bands.

The variety is staggering. Depending on twist angle, carrier density, and applied fields, moiré systems can host metallic states, superconductors, integer and fractional Chern insulators, orbital ferromagnets, antiferromagnets, strange metals, and phases that resist classification entirely. Each represents a different solution to the quantum many-body problem, made accessible by the flatness of the bands.

This zoo of phases transforms moiré materials into something unprecedented: a quantum simulator for strongly correlated electrons that operates with real electrons in a solid-state environment. The ability to smoothly tune between phases offers direct insight into the competitions and instabilities that govern quantum matter, revealing how different ground states emerge from the same underlying interactions.

Takeaway

Moiré materials host a zoo of correlated phases—Mott insulators, orbital magnets, fractional quantum Hall states—each representing a different solution to the strongly interacting quantum many-body problem, all accessible through continuous tuning.

Moiré materials represent something genuinely new in condensed matter physics: a platform where the electronic structure itself becomes a design variable. The implications extend beyond any single phenomenon. We are developing an experimental methodology for studying strongly correlated quantum matter, one that combines the clean tunability of artificial systems with the complexity of real materials.

The questions being addressed here are fundamental. How do electrons organize when forced to interact strongly? What determines whether a system becomes superconducting, magnetic, or topological? These are among the deepest problems in physics, and moiré materials offer a path toward answers that seemed inaccessible just a decade ago.

Looking further ahead, one cannot help but wonder what other surprises hide in twist space. If a simple twist transforms graphene into a superconductor, what might emerge from carefully designed moiré heterostructures combining multiple materials? The frontier remains vast, and we have only begun to map its contours.