  In this work, we theoretically study a modified Su-Schrieffer-Heeger (SSH)
model in which each unit cell consists of three sites. Unlike existing
extensions of the SSH model which are made by enlarging the periodicity of the
(nearest-neighbor) hopping amplitudes, our modification is obtained by
replacing the Pauli matrices in the system's Hamiltonian by their higher
dimensional counterparts. This in turn leads to the presence of next-nearest
neighbor hopping terms and the emergence of different symmetries than those of
other extended SSH models. Moreover, the system supports a number of edge
states that are protected by the time-reversal symmetry rather than the usual
chiral symmetry. Finally, our system could be potentially realized in various
experimental platforms including superconducting circuits as well as
acoustic/optical waveguide arrays.
