Site-selective 11B NMR studies on YbAlB4

β-YbAlB4 is a distinctive heavy fermion superconductor that exhibits unconventional quantum criticality without tuning in a strongly intermediate valence state. In this paper, we report the result of 11B NMR measurements on the single crystals of, β-YbAlB4 and α- YbAlB4, the locally isostructural polymorph of β-YbAlB4. All 11B NMR lines for both samples were successfully assigned to inequivalent crystallographic sites by comparing the experimental results and the ab-initio calculation of the electric field gradient. In both compounds, the anisotropy of the Knight shift exhibits a characteristic radial pattern, indicating approximate axial symmetry of the hybridization between the Yb-4f electrons and the conduction electrons.