Transport properties of surface states in topological semimetals

Topological materials, characterized by non-trivial bulk topology and exotic surface states, have attracted tremendous research interest in recent years. Transport measurement is one of the most common and effective methods for studying topological electronic systems, which is closely related to the exploration of novel physical phenomena and the application of electronic devices. Transport studies based on topological insulators have been widely conducted, where the transport signal is contributed solely by the topological surface states. However, in topological semimetals, both bulk and surface states coexist, which brings complexity to transport studies but also suggests that more diverse physical phenomena can be discovered. Most of the research on the transport properties of topological semimetals has focused on their bulk states, and the contribution of their surface states is usually considered to be negligible. This review introduces several novel transport properties caused by surface states in typical topological semimetal systems, including (1) resonant spin-flip reflection and its transport signal induced by drumhead surface states in topological nodal-line semimetals and (2) peculiar transport properties of Fermi arc surface states in Weyl semimetals and their responses to external fields. Drumhead surface states in nodal-line semimetals are a set of bound states surrounded by the projection of the nodal loop. In nodal-line semimetals with spin-orbit coupling, the spin-polarized drumhead surface states correspond to the resonant spin-flip reflection process at the interface between normal metal and nodal-line semimetal. By solving the scattering process at the interface of the heterojunction within the nodal loop, the probability spectrum of spin-flip reflection shows a sharp resonance peak near zero energy. By injecting spin-polarized electrons from a ferromagnetic electrode into the heterojunction between normal metal and nodal-line semimetal, electrons can undergo spin-flip reflection at the interface, resulting in a nearly pure spin current near the resonance energy. Moreover, the charge flow between two antiparallel ferromagnets can also reflect the resonant spin-polarized reflection process of the electrons, which manifests as a resonance peak in the differential conductance. The subsequent mechanism explores a series of transport signatures of Fermi arc surface states in Weyl semimetals. The discontinuous shape of the Fermi arc in the surface Brillouin zone of Weyl semimetals, as well as the chiral Landau levels inside the Weyl semimetal under external magnetic fields, can be utilized to design devices that realize special Andreev reflection and Fabry-Perot interference mechanisms. By designing suitable surface transport devices, such mechanisms can be detected as specific electrical signals in the conductivity spectrum, providing an effective criterion for the detection of Fermi arc surface states. These devices can also provide measurement schemes for Fermi arc Lifshitz transitions, which are reflected in special response of conductance spectra to magnetic field. To summarize, research introduced in this review enhances our understanding of topological semimetal materials and opens new avenues for detecting and manipulating topological surface states. In addition, studying the transport measurements of surface states in topological semimetals will also contribute to the design of novel quantum devices.