Nearly flat nodal surface states in pseudo-one-dimensional molybdenum monochalcogenides X(MoS) 3 (X = K, Rb, and Cs)

Topological semimetals have aroused great scientific interest due to their special properties in materials science. Among the different types of topological semimetals, nodal surface semimetals (NSSs) are still not fully understood and new materials are in great demand. Herein, we propose a family of pseudo-one-dimensional compounds X(MoS) 3 (X = K, Rb, and Cs) as excellent NSSs with a nearly flat nodal surface near the Fermi level. Their nodal surface states are protected by non-symmorphic twofold screw rotation symmetry and time reversal symmetry. Compared with the NSSs proposed previously, the current materials have several advantages: their nodal surface is almost flat with a relatively small energy variation, in particular for Cs(MoS) 3 ; the location of their nodal surface is very close to the Fermi energy level; and their linear dispersion range is very large. Moreover, their dynamic and mechanical properties were examined, and strong elastic anisotropy was observed. Through uniform strain, the total energy variation of the nodal surface can be effectively reduced with the linear dispersion range preserved. Under the spin orbit coupling effect, the nodal surface opens a gap, but it is so small it can be safely neglected. Overall, these materials have excellent electronic properties and are ideal candidates for further investigation of the interesting physics associated with the nodal surface state. Additionally, since all the X(MoS) 3 compounds are existing materials, corresponding experimental characterizations and measurements can immediately be facilitated and advanced.