Broken symmetry and competing orders in Weyl semimetal interfaces with Fermi arc states*

 

Weyl semimetal (WSM) slabs hosting Fermi arc surface states in proximity can give rise to broken symmetry phases, due to emergent excitonic order, in presence of Coulomb interactions.  In such a phase, correlations between low-energy interlayer fermions become non-zero even without any tunneling processes. The recently found WSM Co₃Sn₂S₂ and the proposed WSM 4-2 Spinel compound, which respectively hosts two and three Fermi arcs in the surface Brillouin zone, provide platforms to examine the effects of multiple Fermi arcs in such phases.  We find that such a system hosts different excitonic orders, such as zero momentum exciton modes as well as finite momentum exciton modes and charge-density-wave orders. These orders break a number of U(1) symmetries, whereas charge density wave orders also break translation symmetry. Broken U(1)-symmetries give rise to Goldstone modes with interesting transport signatures and potential applications in quantum technologies.