J. Hone

Even-denominator fractional quantum Hall states in bilayer graphene

Exotic states pop up in bilayer graphene Particles with exotic quantum statistics are expected to be able to support an especially appealing flavor of quantum computing (QC) called topological QC. A particular fractional quantum Hall state in the semiconductor GaAs has long been thought to possess excitations with these favorable properties, but proving so has turned out to be tricky. Working with bilayer graphene instead of GaAs, Li et al. found four states that appear to be consistent with the theoretical description of states with the required quantum statistics. The researchers were able to tune the properties of these states by applying an electric field, adding a valuable control knob. Science, this issue p. 648 Transport measurements in dual-gated bilayer graphene suggest that four fractional quantum Hall states may host exotic excitations. The distinct Landau level spectrum of bilayer graphene (BLG) is predicted to support a non-abelian even-denominator fractional quantum Hall (FQH) state similar to the 52 state first identified in GaAs. However, the nature of this state has remained difficult to characterize. Here, we report transport measurements of a robust sequence of even-denominator FQH in dual-gated BLG devices. Parallel field measurement confirms the spin-polarized nature of the ground state, which is consistent with the Pfaffian/anti-Pfaffian description. The sensitivity of the even-denominator states to both filling fraction and transverse displacement field provides new opportunities for tunability. Our results suggest that BLG is a platform in which topological ground states with possible non-abelian excitations can be manipulated and controlled.

Simultaneous determination of structure and optical transitions of individual single-walled carbon nanotubes

By combining electron diffraction with Rayleigh scattering spectroscopy, we determine the physical structure and optical transition energies of individual single-walled carbon nanotubes. This permits study of the energy-level structure as a function of nanotube chirality.