• Theory of the strange metal Sr₃Ru2O₇ and related materials.
“Strange metals” are systems where the usual rules for how electrons move in metals simply fail. Understanding them has been one of the central puzzles of condensed matter physics for decades. The compound Sr₃Ru2O₇ provides one of the clearest examples of this strange behavior. We developed a theory that successfully explains much of its unusual physics. The theory makes several unexpected predictions—some already verified by experiments—and provides new directions for exploring related materials. This marks a major step toward a general understanding of strange metals.
• Hierarchy of energy scales near quantum criticality.
When a metal is tuned to the brink of a phase transition—say, on the verge of becoming magnetic—it enters the realm of “quantum criticality.” These systems are rich in exotic phenomena and often host superconductivity at unexpectedly high temperatures. Despite their apparent simplicity, they are extremely challenging to describe theoretically. Using large-scale computer simulations, we developed a method that allows us to probe these metals without uncontrolled approximations. This enabled us to map out the sequence of “crossovers” the system undergoes as it approaches the critical point. By combining numerical simulations with analytical insights, we established a clear physical picture of quantum critical metals—an advance that helps explain why these materials are fertile ground for superconductivity.
• Numerically exact simulations for moiré quantum critical metals.
One of the most exciting developments of recent years has been the discovery of exotic phases of matter, including superconductivity, in twisted bilayer graphene. At a special “magic” twist angle, the electronic bands become extremely flat, amplifying the effects of interactions between electrons. Theoretical tools to describe such systems are scarce. We developed a new simulation method for twisted bilayer graphene at charge neutrality that captures the essential physics with minimal approximations, while still being computationally feasible. This approach opens a new window into the unusual behavior of graphene moiré systems and provides a benchmark for future theoretical work.
• Cascade of phase transitions and the Pomeranchuk effect in twisted bilayer graphene.
In close collaboration with experimental groups, we studied the puzzling electronic phases of magic-angle twisted bilayer graphene (MATBG). Together with S. Ilani’s group at Weizmann, we explained striking measurements of electronic compressibility in terms of a “cascade” of phase transitions, where electrons fill different spin and valley states one by one as the density is tuned. Further work with the Ilani (WIS) and Young (UCSB) groups uncovered an electronic version of the “Pomeranchuk effect” (originally known in liquid helium-3), where a liquid-like electronic state transforms into a more resistive state due to the entropy carried by electron spins. Our theoretical predictions of unusually large entropy were borne out experimentally. These discoveries reshaped the field’s understanding of MATBG and were widely covered in the scientific community and the popular press.
• Superconductivity and correlated states in multilayer graphene.
Beyond twisted bilayers, researchers have turned to graphene stacks with three or more layers, where the stacking order itself creates new possibilities. Our theoretical work showed that rhombohedral trilayer and tetralayer graphene naturally host electronic states with an annular-shaped Fermi surface—composed of two concentric rings in momentum space. Remarkably, this geometry allows repulsive electron interactions to stabilize superconductivity, often with exotic chiral (handed) pairing. In collaboration with experiments, we identified how such states can be tuned using gate voltages and electric fields. Experiments revealed highly spin-polarized metallic phases—sometimes called half- and quarter-metals—providing clear evidence of strong electron correlations. Additional theoretical studies uncovered “intervalley coherence,” a new type of ordering where electrons in graphene’s two valleys lock together. These advances show that multilayer graphene is not just another material system, but a highly tunable platform where superconductivity, magnetism, and novel quantum orders naturally intertwine. Together, these findings position multilayer graphene as one of the most promising playgrounds for discovering and controlling new states of quantum matter.
