  When electrons are confined in two-dimensional (2D) materials, quantum
mechanically enhanced transport phenomena, as exemplified by the quantum Hall
effects (QHE), can be observed. Graphene, an isolated single atomic layer of
graphite, is an ideal realization of such a 2D system. Here, we report an
experimental investigation of magneto transport in a high mobility single layer
of graphene. Adjusting the chemical potential using the electric field effect,
we observe an unusual half integer QHE for both electron and hole carriers in
graphene. Vanishing effective carrier masses is observed at Dirac point in the
temperature dependent Shubnikov de Haas oscillations, which probe the
'relativistic' Dirac particle-like dispersion. The relevance of Berry's phase
to these experiments is confirmed by the phase shift of magneto-oscillations,
related to the exceptional topology of the graphene band structure.
