Su–Schrieffer–Heeger model
In condensed matter physics, the Su–Schrieffer–Heeger (SSH) model is a one-dimensional lattice model that presents topological features. It was devised by Wu-Pei Su, John Robert Schrieffer, and Alan J. Heeger in 1979, to describe the increase of electrical conductivity of polyacetylene polymer chain when doped, based on the existence of solitonic defects. It is a quantum mechanical tight binding approach, that describes the hopping of spinless electrons in a chain with two alternating types of bonds. Electrons in a given site can only hop to adjacent sites.
Note that each carbon atom is connected to two kinds of bonds. The SSH chain was originally used to model the dimerization of this molecule.
Depending on the ratio between the hopping energies of the two possible bonds, the system can be either in metallic phase (conductive) or in an insulating phase. The finite SSH chain can behave as a topological insulator, depending on the boundary conditions at the edges of the chain. For the finite chain, there exists an insulating phase, that is topologically non-trivial and allows for the existence of edge states that are localized at the boundaries.