Electron spin resonance study of proton-irradiated topological insulator Bi₂Se₃ nanoparticles

We have studied the proton-irradiation effects on the topological insulator Bi₂Se₃ nanoparticles by means of electron spin resonance (ESR) measurements. The ESR signals from the hydrogen-doped Bi₂Se₃ nanoparticles, ascribed to the metallic surface of the topological insulator, were enhanced at high enough proton irradiation doses, which was explained by the enhanced surface effect. Our density functional theory calculations indicated that the hydrogen defects act as donors whose transition levels are in the conduction band. Thus, weak potential fluctuations are taken to be responsible for the small activation energy of ~5 meV, obtained from the temperature-dependent ESR intensity.