Ultra-high surface conductivity in Weyl semimetal NbAs

In two-dimensional (2D) systems, high mobility is typically achieved in low-carrier-density semiconductors and semimetals. Increasing the carrier density in these systems to be comparable to typical metals usually degrades the mobility value, owing to an enhanced scattering probability from the large Fermi wave vector and possible charged impurities from the doping process. Here, we discover that the surface state of Weyl semimetal NbAs can overcome such a limit and maintain a high mobility even in the presence of a high carrier density. To study its surface transport properties, we first develop a new growth scheme to synthesize high-quality nanostructures of NbAs with tunable Fermi levels. Owing to the large surface-to-bulk ratio, the 2D surface state exhibits dominant quantum oscillations with a high carrier density. Combined with the high mobility value, a record-high surface sheet conductance among non-superconducting 2D systems is achieved up to 5-100 S. Corroborated by theory, we attribute the origin of the ultra-high surface conductance to the disorder-tolerant nature of Fermi arcs. Our results present the first transport evidence for the low-dissipation property of NbAs surface state and establish it as an excellent 2D metal with supreme conductivity.