Research Team Discovers Insulator Property of Bilayer Graphene

A team of researchers have found that when the number of electrons on a bilayer graphene (BLG) sheet is close to zero, the material becomes insulating (resists flow of electrical current). This finding suggests that multi-layer graphene might be suitable as an electronic material in the semiconductor and electronics industries. Their research was published recently in Nature Nanotechnology. The research was supported by grants from the National Science Foundation, Office of Naval Research, FENA Focus Center, and other agencies.

Bilayer graphene schematic. The blue beads represent carbon atoms. Source: Lau lab, UC Riverside
Bilayer graphene schematic. The blue beads represent carbon atoms.
Source: Lau lab, UC Riverside

Graphene is the thinnest elastic material in nature. It is a one-atom thick sheet of carbon atoms arranged in a hexagonal lattice. As a result of graphene’s planar and chicken wire-like structure, sheets of it can be stacked. When two graphene sheets are stacked in a special manner, bilayer graphene is formed. BLG, like graphene, has high current-carrying capacity. This is known as high electron conductivity. The high current-carrying capacity results from the extremely high velocities that electrons can acquire in a graphene sheet.

An important finding of the research team is that the intrinsic energy gap in BLG grows with increasing magnetic field. In solid state physics, an energy gap (or band gap) refers to an energy range in a solid where no electron states can exist. The size of the energy gap of a material generally determines whether it is a metal (no gap), semiconductor (small gap) or insulator (large gap). The presence of an energy gap in silicon is important to the semiconductor industry because this can be used to turn the device on (conductive), and off (insulating) in digital applications. While bilayer graphene can be turned off, single layer graphene (SLG) will always remain metallic and conductive because SLG is gapless.

A scanning electron microscope image of a graphene sheet (red) suspended between two electrodes. The length of the graphene sheet shown is about 1/100 of the width of a human hair. Source: Lau lab, UC Riverside
A scanning electron microscope image of a graphene sheet (red) suspended between two electrodes. The length of the graphene sheet shown is about 1/100 of the width of a human hair. Source: Lau lab, UC Riverside

The research team consists of Chun Ning Lau (associate professor of physics and astronomy at University of California, Riverside and lead author of the research paper); Allan MacDonald (Sid W. Richardson Foundation Regents Chair in the Department of Physics at The University of Texas at Austin and co-author on the research paper); J. Velasco Jr. (first author of the research paper); L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, and C. Varma at UC Riverside; R. Stillwell and D. Smirnov at the National High Magnetic Field Laboratory, Tallahassee, Fla.; and Fan Zhang and J. Jung at The University of Texas at Austin.

Bilayer Graphene Research Paper Abstract

Bilayer graphene is an attractive platform for studying new two-dimensional electron physics because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron-electron interaction effects. Theory predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps are analogous to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects accompanied by spontaneous quantum Hall effects. Although recent experiments have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ~15 mV nm-1, but it increases monotonically with magnetic field, with an apparent particle-hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.

More info: University of California, Riverside