  We apply a topological material design concept for selecting a bulk topology
of 3D crystals by different van-der-Waals stacking of 2D topological insulator
layers, and find a bismuth halide Bi4Br2I2 to be an ideal weak topological
insulator (WTI) with the largest band gap (~230 meV) among all the WTI
candidates, by means of angle-resolved photoemission spectroscopy (ARPES),
density functional theory (DFT) calculations, and resistivity measurements. Our
results vastly expand future opportunities for fundamental research and device
applications with a robust WTI.
