Robust topological surface state in Kondo insulator SmB₆ thin films

Fabrication of smooth thin films of topological insulators with true insulating bulk are extremely important for utilizing their novel properties in quantum and spintronic devices. Here, we report the growth of crystalline thin films of SmB₆, a topological Kondo insulator with true insulating bulk, by co-sputtering both SmB₆ and B targets. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy indicate films that are polycrystalline with a (001) preferred orientation. When cooling down, resistivity ρ shows an increase around 50K and saturation below 10K, consistent with the opening of the hybridization gap and surface dominated transport, respectively. The ratio ρ_2K/ρ_300K is only about two, much smaller than that of bulk, which indicates a much larger surface-to-bulk ratio. Point contact spectroscopy using a superconductor tip on SmB₆ films shows both a Kondo Fano resonance and Andeev reflection, indicating an insulating Kondo lattice with metallic surface states.