THE COLLAPSE OF A MANY-BODY GAP ENABLES ULTRASENSITIVE FAR-INFRARED DETECTION
One of the central ideas in modern physics is the phase transition — a sudden transformation of the state of a material. We encounter phase transitions throughout everyday life: water freezes into ice, wax melts in the warmth of a flame, and water vapor condenses into droplets on a cold window.
In these familiar examples, the atoms themselves rearrange into a new structure, giving the material entirely different properties. But some of the most fascinating phase transitions are invisible to the naked eye. Instead of atomic rearrangement, it is the electrons that collectively reorganize into a new quantum state. A subtle change in external parameters — temperature, magnetic field, or even exposure to light — can completely transform how such a material behaves.
Superconductivity is perhaps the most celebrated example of an electronic phase transition. For over a century, the superconducting state — carrying electrical current without any resistance — has captivated physicists. Yet the transition itself is just as remarkable as the state it leads to. Near the transition point, even a tiny external stimulus produces a dramatic change in the material’s properties. This extraordinary sensitivity has made superconductors the foundation of some of the world’s most sensitive detectors, capable of registering individual photons arriving from distant galaxies.
In our laboratory at Institute for Functional Intelligent Materials (IFIM), National University of Singapore, we investigate how emerging quantum materials interact with far-infrared (FIR) radiation — an important band of the electromagnetic spectrum notoriously difficult to detect.
In a recent study published in Nature Communications, we showed that superconductors are not the only materials whose phase transition can be harnessed for ultrasensitive light detection. We found that even low-intensity FIR radiation is enough to switch magic-angle twisted bilayer graphene from a correlated insulator – a state in which interactions bind the electrons into a fragile collective order that blocks the flow of current – into a metal. The mechanism is strikingly simple: the absorbed radiation heats the electrons — while the crystal lattice itself stays cold — and even a slight rise in their temperature is enough to “melt” the fragile insulating state, restoring metallic conduction.
For scientific community, the result is intriguing in its own right. Watching a correlated quantum state dissolve under a faint beam of FIR light opens a new window into some of the most delicate forms of collective electron behaviour — how such fragile phases arise, how they vanish, and how they can be controlled.
But the discovery also carries a practical promise. The FIR range remains one of the least mastered regions of the electromagnetic spectrum: lying between the domains of electronics and optics, it is notoriously difficult to generate and detect — even though it is precisely where many emerging technologies operate, from medical diagnostics to security screening and the observational astronomy. Progress in all of these fields ultimately depends on fast and sensitive detectors. Our results establish magic-angle graphene as a new platform for exactly that — a quantum material in which the insulating state itself can be switched on and off with a simple gate voltage, and where a whisper of FIR light is enough to flip an electronic switch.
Artistic illustration of the melting of the correlated insulating state in magic-angle twisted bilayer graphene under far-infrared irradiation.