The acoustic surface plasmon (ASP) with a low and linear dispersion relation is usually attributed to the strong Coulomb screening of surface states by bulk electrons, offering a promising avenue for studying fundamental quantum phenomena such as pairing glue for unconventional superconductivity and practical plasmonic applications. However, whether ASPs can persist in systems with reduced electronic screening remains unclear. Topological semimetals feature linear band touching, where the low density of states near the nodes leads to insufficient screening of the long-range Coulomb interactions and give rise to intriguing physical properties, providing a natural platform for exploring ASPs under weak-screening conditions. Here, using high-resolution electron energy loss spectroscopy with precise beam energy tuning, we unambiguously observe an ASP mode over a wide energy-momentum range on prototypical nodal-line semimetals, ZrSiS and ZrSiSe. We further find the ASP mode originates from the interplay between the floating surface states and substantially lower bulk charge carrier densities than conventional metals. Our results establish the physical foundation for the realization and application of ASPs on the surface of nodal-line semimetals.

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• Surface plasmons
• Surface states
• Nodal-line semimetals
• Topological materials
• Electron energy loss spectroscopy