Single valley Dirac fermions in zero-gap HgTe quantum wells

Dirac fermions have been studied intensively in condensed matter physics in recent years. Many theoretical predictions critically depend on the number of valleys where the Dirac fermions are realized. We now report the discovery of a two-dimensional system with a single, spin-degenerate Dirac valley. We study the transport properties of HgTe quantum wells grown at a critical thickness where the band gap vanishes. In a magnetic field, we observe quantum Hall plateaux that exhibit the anomalous sequence characteristic of Dirac systems. The filling factors at which the plateaux occur are direct evidence of the presence of a single Dirac valley in the system. Also the conductivity at the Dirac point and its temperature dependence can be understood from single valley Dirac fermion physics. Our observations pave the way to study quantum interference and ballistic transport phenomena of Dirac fermions, which so far have been hard to access.