Field Induced Chiral Soliton Phase in the Kitaev Spin Chain

The bond-dependent Ising interaction present in the Kitaev model has attracted considerable attention. The appearance of an unexpected intermediate phase under a magnetic field is particularly intriguing, and one may wonder if a similar phase occurs in the Kitaev spin chain with alternating $x$- and $y$-bond Ising interactions. Previous studies have focused on a transverse field, $h_z$, and reported a direct transition to the polarized state. Here, we investigate phases with arbitrary angle of two longitudinal fields, $h_x$ and $h_y$. For a magnetic field applied along the diagonal, $h_x$=$h_y$, the chain remains gapless up to a critical field $h^{c_1}_{xy}$. Surprisingly, above $h^{c1}_{xy}$ it enters an unusual intermediate phase before reaching the polarized state at $h^{c_2}_{xy}$. This phase is characterized by a staggered vector chirality and for periodic boundary conditions, a two-fold degeneracy with a finite gap. For open boundary systems the ground-state exhibits a single soliton, lowering the energy, and gapless excitations. However, the corresponding anti-soliton raises the energy sufficiently that a gap appears for soliton and anti-soliton pairs in periodic systems. An intuitive variational picture is developed describing the soliton phase.