B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine Kharkiv 61103, Ukraine V. N. Karazin Kharkiv National University, Kharkiv 61022, Ukraine
Received January 10, 2022, published online March 25, 2022
The ground state behavior of the biaxial spin-1/2 chain with free open edges is studied. Using the exact Bethe ansatz solution we show that there can exist boundary bound states for many finite values of the exchange coupling
constants. The non-trivial interaction between spins produces charging of the vacua of the model, and boundary bound states. Our theory also describes the behavior of the spinless fermion chain with pairing (the Kitaev chain) and an interaction between fermions at neighboring sites for free open boundaries. There for the simple case of noninteracting fermions simplest boundary states are Majorana edge modes.