Two Dimensional Materials have gone crazy!

In case you haven't been paying attention, two-dimensional materials have started to show many phases of matter we have only dreamed of.

There are a ton of two-dimensional materials these days. You’ve probably heard of graphene, a single layer of carbon atoms arranged in a hexagonal grid.

In graphene, carbon atoms sit at the vertices of these hexagons. Photo by Andrew Draper on Unsplash

In 2018, everything changed when two layers of graphene were twisted to reveal superconductivity! The twist itself is interesting (I briefly discussed it in a previous post), but the key takeaway is that these materials now come with an extra knob for accessing new phases of matter. It’s remarkable. We can first think of these materials like Lego blocks:

Photo by Omar Flores on Unsplash

Each layer is a different material: mix and match, and you might discover an exotic new phase. This “Lego” idea had already been in the air before 2018, but the physics since then has shown that it’s not just about stacking—we can twist too, creating not just patterns, but new ways for electrons to move.

Two hexagonal layers twisted on top of each other, creating a moiré pattern.

We knew these patterns would occur, but we didn’t realize we could make it superconduct. Now we can stack and twist to great effect. Of course, twisted bilayer graphene isn’t about to revolutionize high-speed trains (it goes superconducting at only 4K¹), but the way it goes superconducting is eerily reminiscent of higher-temperature superconductors. That means it might help us understand those other materials better.

And once people started twisting, they didn’t stop. We now have twisted multilayers of graphene, transition-metal dichalcogenide (TMD) bilayers², and more. But it doesn’t end there; you can also apply magnetic fields, electric fields, and pattern the lattice in sophisticated ways. With all that in mind, here’s a short and incomplete survey of some of the exotic phases in these materials:

“Fractional… what now?”

All of these phases are exceptionally hard to understand and model. Some of the best minds in the field are actively working on them. One particularly exciting phase is the fractional Chern insulator, which could be useful for quantum computing.

But even setting aside applications, what’s astonishing is that all of these phenomena come from nothing more than electrons moving on a lattice and experiencing a few fields. Nature seems to treat electrons like Play-Doh, shaping them into wildly different quantum phases.

This is a deep and fundamental question: What can be accomplished using electrons alone?

1. That’s -452.47 degrees Fahrenheit.

2. To this day, I still can’t say the full name, so I just say “TMD.”