Listening to graphene’s quartet harmonies

September 13, 2010 – Researchers at the National Institute of Standards and Technology (NIST) say they have discovered how graphene splits into different sets of energy levels at low temperatures and high magnetic fields, raising new fundamental physics questions and possibly new capabilities for the material.

Graphene, the single-atom-thick sheet of carbon atoms, has long been studied for its desirous properties — its electrons act as if they have no mass, it has nearly 100× the electron mobility of silicon, and the electron speed is independent of their energy (i.e. other materials require an applied voltage, which creates extra heat).

NIST has built what it calls "the world’s most powerful" scanning-probe microscope to study graphene in unprecedented conditions — ultrahigh vacuum, high magnetic field, and ultralow temperature (down to 10 mK, or 0.0001°C above absolute zero — to get a better look at the atom-by-atom differences in graphene’s electron energies.

What they found, they report in Nature, was somewhat surprising. Graphene’s structural geometry and electromagnetic properties enable its electrons to populate four possible sublevels (a "quartet"); scientists have theorized that applying a magnetic field would split this quartet of levels into different energies. No tool so far had been sensitive enough to resolve those differences. Using the new SPM, they observed that increasing the magnetic field at these extreme low temperatures shows an apparent "man-body effect" in which electrons interact strongly with one another in ways that affect their energy levels. They speculate this could be due to formation of "condensate" which causes them to act as a single coordinated unit instead of moving independently. (Separate research into graphene, published in Nature Physics, describes how the energy levels of graphene’s electrons vary with position as they move along the material’s crystal structure, and suggests involvement of interactions between electrons in neighboring layers.)

This observation could have implication in using graphene for very low heat-producing, highly energy-efficient electronic devices.

Artist’s rendition of electron energy levels in graphene. An electron in any given energy level (wide purple band) comprises four quantum states (the four rings), called a "quartet," which split into different energies when immersed in a magnetic field. The two smaller bands on the outermost ring represent the further splitting of a graphene electronic state. (Credit: T. Schindler and K. Talbott/NIST)



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