Researchers of SPEC, in collaboration with teams of the C2N (Palaiseau, France) and NIMS (Tsukuba, Japan) as well as the startup CRYOHEMT (Orsay, France) recently published an article in Nature physics.
The authors :
- Raphaëlle DELAGRANGE
- Manjari GARG
- Gaëlle LE BRETON
- Aifei ZHANG
- Quan DONG
- Yong JIN
- Kenji WATANABE
- Takashi TANIGUCHI
- Preden ROULLEAU
- Olivier MAILLET
- Patrice ROCHE
- François PARMENTIER
Under high perpendicular magnetic field and at low temperatures, graphene develops an insulating state at the charge neutrality point. This state, dubbed nu=0, is due to the interplay between electronic interactions and the four-fold spin and valley degeneracies in the flat band formed by the n=0 Landau level. Determining the ground state of nu=0, including its spin and valley polarization, has been a theoretical and experimental undertaking for almost two decades. Here, we present experiments probing the bulk thermal transport properties of monolayer graphene at nu=0, which directly probe its ground state and collective excitations. We observe a vanishing bulk thermal transport, in contradiction with the expected ground state, predicted to have a finite thermal conductance even at very low temperature. Our result highlight the need for further investigations on the nature of nu=0.
© François Parmentier – CNRS
The four possible ground states of ν=0, shown as two spins (red and blue arrows) distributed on the honeycomb lattice of graphene.
Yellow: antiferromagnetic phase: the two opposite pins reside on a separate sublattice.
Purple: ferromagnetic phase: the two spins are aligned, each on its sublattice.
Orange: Kekule distortion phase: the two opposite spins live on a superposition of the two sublattices.
Cyan: sublattice polarized phase: the two opposite spins live on the same sublattice.
The central cartoon depicts the principle of the experiment, where heat is carried for a hot electrode (red) to a cold one (purple) across ν=0. Only the antiferromagnetic and Kekule distortion phase are thermal conductors at low temperature.