Surface states and arcless angles in twisted Weyl semimetals

Fermi arc states are features of Weyl semimetal (WSM) surfaces which are robust due to the topological character of the bulk band structure. We demonstrate that Fermi arcs may undergo profound restructurings when surfaces of different systems with a well-defined twist angle are tunnel coupled. The twisted WSM interface supports a moiré pattern which may be approximated as a periodic system with a large real-space unit cell. States bound to the interface emerge, with interesting consequences for the magneto-oscillations expected when a magnetic field is applied perpendicular to the system surfaces. As the twist angle passes through special "arcless angles,"for which open Fermi arc states are absent at the interface, Fermi loops of states with no connection to bulk states appear. Such states have interesting resonance signatures in the optical conductivity of the system in a magnetic field perpendicular to the interface. These signatures are independent of the bulk thickness or disorder.