  Topological metals and semimetals are new states of matter which attract
great interest in current research. Here, based on first-principles
calculations and symmetry analysis, we propose that the family of
titanium-based compounds Ti3X (X=Al, Ga, Sn, Pb) are unexplored topological
semimetals. These materials feature the coexistence of a nodal loop and a nodal
surface in their low-energy band structure. Taking Ti3Al as an example, we show
that the material has an almost ideal nodal loop in the sense that the loop is
close to the Fermi level and it is nearly flat in energy with energy variation
<0.25 meV. The loop is protected by either one of the two independent
symmetries: the combined spacetime inversion symmetry and the mirror reflection
symmetry. The nodal surface at the k_z=\pi plane is guaranteed by the
nonsymmorphic screw rotational symmetry and the time reversal symmetry. We
discuss the effect of spin-orbit coupling and construct an effective model for
describing the nodal loop. Our findings indicate that the Ti3Al family
compounds can serve as an excellent material platform for studying new
topological phases and particularly the interplay between nodal-loop and
nodal-surface fermions.
