  High spin Chern-number insulator (HSCI) emerges as a novel 2D topological
phase of condensed matter that is beyond the classification scheme of
topological quantum chemistry. In this work, we report the observation of a
semimetallic Sb monolayer with the same band topology as HSCI. Our
first-principles calculations further indicate a moderate lattice strain can
make Sb monolayer an insulator while retaining the nontrivial topological
order. A quantized spin Hall conductivity is found in the HSCI phase of Sb
monolayer. The results suggest strained Sb monolayer as a promising platform
for exploring intriguing physical properties of the exotic HSCI topological
matter
