${\text{MoS}}_2$, a member of transition metal dichalcogenides (TMDs), has recently emerged as an interesting two-dimensional material due to its unique mechanical, thermal, electronic and optical properties. Unlike graphene which possesses massless Dirac fermions with ultrahigh electron mobility, monolayer ${\text{MoS}}_2$ is a direct band gap semiconductor. An interesting question arises: Can monolayer ${\text{MoS}}_2$ also possess massless Dirac fermions with ultrahigh electron mobility? Here, using first-principles calculations, we show that a monolayer ${\text{MoS}}_2$ allotrope, which consists of repeated square-octagon rings (abbreviated as so-${\text{MoS}}_2$ to distinguish it from the normal hexagonal lattice, h-${\text{MoS}}_2$) possesses both massless Dirac fermions and heavy fermions. Distinct from the $p$-orbital Dirac fermions of graphene, the Dirac fermions of so-${\text{MoS}}_2$ are $d$ electrons and possess a Fermi velocity comparable to that of graphene. The Dirac cone structure in so-${\text{MoS}}_2$ demonstrated here greatly enriches our understanding on the physical properties of TMDs and opens up possibilities for developing high-performance electronic or spintronic devices.

• Received 22 January 2014
• Revised 17 April 2014

DOI:https://doi.org/10.1103/PhysRevB.89.205402