Fragile topological insulators protected by rotation symmetry without spin-orbit coupling

We present a series of models of three-dimensional rotation-symmetric fragile topological insulators in class AI (time-reversal symmetric and spin-orbit-free systems), which have gapless surface states protected by time-reversal (T and nfold rotation ({C}_{n} symmetries (n=2,4,6. Our models are generalizations of Fu's model of a spinless topological crystalline insulator, in which orbital degrees of freedom play the role of...