The quantum Hall effect warms up

Observing the quantum Hall effect (QHE) in a two-dimensional electron system requires exquisite care and ultralow temperatures, typically below the boiling point of liquid helium. Graphene, a one-atom-thick sheet of carbon atoms tightly packed in a honeycomb lattice, is a remarkable exception. By fashioning graphene into an FET and using the gate voltage to adjust the charge-carrier density in a fixed magnetic field of 29 T, Andre Geim (University of Manchester, UK), Philip Kim (Columbia University), and their colleagues noticed QHE signatures emerge even at room temperature. Graphene’s band structure accounts for the observation: Its electrons behave like massless, relativistic fermions (see Physics Today, January 2006, page 21 ). In the presence of a magnetic field, the allowed electron energies split into discrete Landau levels. In graphene, unlike other, “nonrelativistic” QHE materials, the gap between the ground state and first excited level exceeds the thermal energy at room temperature by an order of magnitude for a magnetic field of 45 T. Researchers currently harvest their graphene by simply peeling off monolayer flakes from bulk graphite. With more careful preparation to reduce defects, the authors expect that QHE-based resistance standards using graphene in magnetic fields as low as 5 T may soon be developed. ( K. S. Novoselov et al., Science 315 , 1379, 2007. http://dx.doi.org/10.1126/science.1137201 )