Finite-size topological phases from semimetals

Topological semimetals are some of the topological phases of matter most intensely studied experimentally. The Weyl semimetal phase, in particular, has garnered tremendous, sustained interest given fascinating signatures such as the Fermi arc surface states and the chiral anomaly, as well as the minimal requirements to protect this three-dimensional (3D) topological phase. Here, we show that thin films of Weyl semimetals [which we call quasi-(3 - 1)-dimensional, or q(3 - 1)D] generically realize finite-size topological phases distinct from 3D and 2D topological phases of established classification schemes: response signatures of the 3D bulk topology coexist with topologically protected, quasi-(3 - 2)D Fermi arc states or chiral boundary modes due to a second, previously unidentified bulk-boundary correspondence. We show these finite-size topological semimetal phases are realized by Hamiltonians capturing the Fermiology of few-layer van der Waals material MoTe2 in experiment. Given the broad experimental interest in few-layer van der Waals materials and topological semimetals, our work paves the way for extensive future theoretical and experimental characterization of finite-size topological phases.