  In topological quantum materials the conduction and valence bands are
connected at points (Dirac/Weyl semimetals) or along lines (Line Node
semimetals) in the momentum space. Numbers of studies demonstrated that several
materials are indeed Dirac/Weyl semimetals. However, there is still no
experimental confirmation of materials with line nodes, in which the Dirac
nodes form closed loops in the momentum space. Here we report the discovery of
a novel topological structure - Dirac node arcs - in the ultrahigh
magnetoresistive material PtSn4 using laser-based angle-resolved photoemission
spectroscopy (ARPES) data and density functional theory (DFT) calculations.
Unlike the closed loops of line nodes, the Dirac node arc structure resembles
the Dirac dispersion in graphene that is extended along one dimension in
momentum space and confined by band gaps on either end. We propose that this
reported Dirac node arc structure is a novel topological state that provides a
novel platform for studying the exotic properties of Dirac Fermions.
