Yuan Cao — Twisting Graphene into New Physics

UC Berkeley · Department of Physics & EECS · Reconfigurable Quantum Materials Lab

A condensed summary of the research of Yuan Cao, the physicist who discovered that two sheets of carbon, rotated by roughly one degree, can switch between being an insulator and a superconductor.

The idea in one paragraph

Graphene is a single layer of carbon atoms in a honeycomb lattice. Stack two layers and rotate them slightly and the mismatch creates a moiré superlattice — a large-scale interference pattern. In 2011 Allan MacDonald and Rafi Bistritzer predicted that near a special twist of about 1.1° the electrons would sit in nearly flat energy bands: they slow almost to a halt, so their mutual repulsion dominates their kinetic energy. Strongly interacting electrons are where the most interesting phases of matter live — magnetism, correlated insulators, and unconventional superconductivity. Cao’s achievement was to actually build that system and measure it.

How he did it: the “tear-and-stack” trick

Hitting 1.1° reliably is a fabrication problem. Cao pioneered a method of tearing a single graphene flake with the tip used to pick it up, so the two halves keep an identical crystal orientation, then laying the second half back down at a deliberately chosen small angle. He also re-tuned the dilution refrigerator to reach temperatures (~1.7 K) low enough for the superconducting state to emerge cleanly. His advisor, Pablo Jarillo-Herrero, calls him “a tinkerer” — his MIT office was reportedly strewn with disassembled computers and pieces of homemade telescopes.

What the Berkeley lab does now

The Reconfigurable Quantum Materials Lab builds moiré devices whose quantum phase can be dialed in situ — with gate voltage, displacement field, pressure, or twist angle. Themes:

• Twistronics beyond graphene: twisted transition-metal dichalcogenides and multilayer stacks hosting correlated insulators, ferroelectricity, and fractional Chern insulators.
• Reconfigurable superconductivity: understanding the pairing “glue” in flat-band systems and switching it on and off electrically.
• New fabrication: cleaner interfaces and dynamically tunable twist (“nano-origami”) so one device can sweep a whole phase diagram.

The through-line of Cao’s career: treat a material not as fixed, but as a knob-covered instrument whose ground state you get to choose.

Sources

• UC Berkeley Physics — faculty profile & 2026 Sloan Fellow announcement
• UC Berkeley VC for Research — Reconfigurable Quantum Materials Lab
• Cao et al., “Unconventional superconductivity in magic-angle graphene superlattices,” Nature 556, 43–50 (2018)
• Cao et al., “Correlated insulator behaviour at half-filling in magic-angle graphene superlattices,” Nature 556, 80–84 (2018)
• Nature — “Nature’s 10: 2018” (the graphene wrangler)
• Quanta Magazine — “How Twisted Graphene Became the Big Thing in Physics” (2019)
• Google Scholar profile kJhEIh0AAAAJ (citation total)