Robust topological nodal lines in halide carbides.

A topological nodal line semimetal is a novel state of matter in which the gapless bulk state extends along the Brillouin zone forming a closed 1D Fermi surface. Here, we theoretically predict that the layered halide carbide Y2C2I2 is a novel metal containing interconnected nodal lines protected by the parallel mirror planes kz = 0 and kz = 0.5, characterized by independent mirror Chern numbers (MCNs) |μ1| = |μ| = 1. Because of the interlocking 2D nodal lines stacked along the c-axis direction, the topological property is robust both in the 3D bulk as a topological chain metal and in the 2D nanostructure as a topological nodal line. Consequently, Y2C2I2 and other related layered halide carbide materials are unique candidates to realize functional topological devices due their size-independent topological properties.