Thermal Properties of Topological Semimetal NbP Thin Films and Nanoribbons

Topological semimetals (TSMs) have emerged as materials with unconventional resistivity scaling in ultrathin films, unlike the scaling observed in common metals like copper. For example, NbP displays decreasing resistivity with decreasing film thickness, dropping below the resistivity of copper in sub-3 nm thin films. While the scaling of NbP electrical resistivity was recently reported, here we present the first systematic thermal study of NbP thin films and nanoribbons. NbP was sputtered with a Nb seed layer on sapphire and thermally evaluated both as blanket films (few-nm to 100 nm thick) and as nanoribbons of varying widths (30 nm to 250 nm) and lengths (150 nm to 5 μm). Electrical self-heating measurements and thermal modeling enabled an estimate of in-plane thermal conductivity for the thinnest NbP nanoribbons. Time-domain thermoreflectance (TDTR) measurements of the blanket films provided cross-plane thermal conductivities. Our results show pronounced anisotropy: effective cross-plane thermal conductivity decreases with reduced thickness due to size effects, while in-plane thermal conductivity partly scales with the Wiedemann–Franz law. These findings offer new insights into the thermal transport of ultrathin TSMs, with important implications for their application as next-generation nanoscale interconnect materials.