January 25, 2012 — A research team led by University of California, Riverside identified the "Higgs boson" property of bilayer graphene (BLG): when the number of electrons on the BLG sheet is close to 0, the material becomes insulating. "BLG becomes insulating because its electrons spontaneously organize themselves when their number is small," said Chun Ning (Jeanie) Lau, an associate professor of physics and astronomy who led the research. "This is called ‘spontaneous symmetry breaking’ in physics, and is a very important concept since it is the same principle that ‘endows’ mass for particles in high energy physics."
BLG is formed when two graphene sheets are stacked in a special manner. It shares a high current-carrying capacity with single-layer graphene, having electrons with extremely high velocities. A typical conductor has a huge number of electrons, which move around randomly. When BLG has only a few electrons, interactions between them cause the electrons to behave in an orderly manner.
 |
| Figure 1. A bilayer graphene schematic. The blue beads represent carbon atoms. SOURCE: Lau lab, UC Riverside. |
The team measured the mass of a new type of massive quantum particle found only inside BLG crystals, explained Allan MacDonald, the Sid W. Richardson Foundation Regents Chair in the Department of Physics at The University of Texas at Austin and a coauthor on the research. The physics at work here is "closely analogous" to the physics that makes a proton’s mass inside an atomic nucleus much larger than the mass of the quarks from which it is formed, said MacDonald, noting that in this research, the particle is made of electrons, not quarks.
In 2011, researchers at Madrid’s Institute for Material Science theorized that ripples in graphene arose from this spontaneous symmetry-breaking process. (Higgs boson and graphene: Shared negative curvature )
 |
| Figure 2. 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. |
MacDonald explained that the experiment the research team conducted was motivated by theoretical work which anticipated that new particles would emerge from the electron sea of a BLG crystal. Further experimentation will shed light on the theories about BLG properties, he said.
The research team found that the intrinsic bandgap in BLG grows with increasing magnetic field. Generally, the size of the energy gap of a material determines whether it is a metal (no gap), semiconductor (small gap) or insulator (large gap).
Also read: Graphene: semimetal, not semiconductor, insulator, or metal
Single layer graphene (SLG) is gapless, unlike silicon, and therefore cannot be implemented as a silicon replacement in semiconductor applications. SLGF always remains metallic and a conductor no matter its electron count. However, BLG can be switched on and off. "Our research is in the initial phase, and, presently, the band gap is still too small for practical applications," Lau said, but looked to trilayer graphene and tetralayer graphene as possible materials for digital and infrared technologies. The researchers have begun investigating these possibilities.
The research is reported online (Jan. 22) in Nature Nanotechnology. Lau and MacDonald were joined in the research by J. Velasco Jr. (the first author of the research paper), L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, and C. Varma at UCR; 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.
The research was supported by grants from the National Science Foundation, Office of Naval Research, FENA Focus Center, and other agencies.
Learn more about UC Riverside at www.ucr.edu.
More graphene research:
Graphene nanowiggles exhibit specific bandgap and magnetic properties
Graphene doping doesn’t need its own step when done on the edge