Nanoscale observation of subgap excitations in β-Si3N4 with a high refractive index using low-voltage monochromated STEM: a new approach to analyze the physical properties of defects in dielectric materials (Nanoscale observation of subgap excitations in β-Si3N4 with a high refractive index using low-voltage monochromated STEM: a new approach to analyze the physical properties of defects in dielectric materials)