Spin Berry phase in the Fermi-arc states

An unusual electronic property of a Weyl semimetallic nanowire is revealed. Its band dispersion exhibits multiple subbands of partially flat dispersion, originating from the Fermi-arc states. Remarkably, the lowest energy flat subbands bear a finite size energy gap, implying that electrons in the Fermi-arc surface states are susceptible to the spin Berry phase. This is shown to be a consequence of spin-to-surface locking in the surface electronic states. We verify this behavior and the existence of the spin Berry phase in the low-energy effective theory of Fermi-arc surface states on a cylindrical nanowire by deriving the latter from a bulk Weyl Hamiltonian. We point out that in any surface state exhibiting a spin Berry phase $\pi$, a zero-energy bound state is formed along a magnetic flux tube of strength ${\Phi }_0/2=hc/\left(2e\right)$. This effect is highlighted in a surfaceless bulk system pierced by a dislocation line, which shows a 1D chiral mode along the dislocation line.