Heating only the electrons in twisted graphene raises its resistance by kilohms

The devices were small bars of twisted graphene sealed inside hexagonal boron nitride, with graphite gates to set the number of charge carriers and metal antennas to funnel the radiation in ^[10]. Each photon of that 0.14-terahertz beam carried 0.6 millielectron volts, too little to push an electron from one relevant band to another ^[11]. So nothing was excited across a gap. The beam stirred the carriers already present, and they shared the energy among themselves within femtoseconds, settling into a hot electronic state before much of that heat could leak into the lattice ^[12].

In a conventional Fermi liquid, resistance from electron interactions often grows with the square of temperature, while a linear rise can point to phonon scattering and has also been associated with strange metals that the standard description does not cover ^[8]. Twisted bilayer graphene has displayed both trends, with the exponent changing as researchers vary the twist angle and the number of carriers ^[9]. "In twisted bilayer graphene, the same temperature dependence can have more than one plausible microscopic origin," said Artur Shilov, a doctoral student at I-FIM and the paper's first author ^[6]. "We needed another control knob," he said ^[5].

The twist angle changes the band structure. Stacking two honeycomb sheets slightly out of alignment produces a moire superlattice that reshapes the energy landscape carriers move through ^[17]. Near 1.1 degrees, some of those bands become unusually flat, so electrons move more slowly and feel one another more strongly, and the material hosts correlated insulating states and superconductivity ^[16].

The two heatings differ by more than a factor of 20 ^[1]. In absolute terms the electrons sat near 22 kelvin while the lattice stayed near 2, roughly eleven times as hot ^[2]. "Conventional transport measurements heat the electrons and the lattice together, so their fingerprints are superimposed," said Denis Bandurin, the I-FIM assistant professor who led the study ^[3]. "We wanted to separate those two temperatures and ask what the electrons themselves were doing," he said ^[4].

The measurement answers a narrow question well. It shows that warming the carriers alone pushes resistance up in a regime where phonons were the natural candidate ^[2]. It does not split an ordinary temperature sweep between the two channels, because in that sweep both temperatures still rise together ^[7]. The electron temperature is an estimate ^[13], and the demonstration sits at a lattice near 2 kelvin with electrons no hotter than about 22 ^[2].