  Topological insulators (TIs) containing 4f electrons have recently attracted
intensive interests due to the possible interplay of their non-trivial
topological properties and strong electronic correlations. YbB6 and SmB6 are
the prototypical systems with such unusual properties, which may be tuned by
external pressure to give rise to new emergent phenomena. Here, we report the
first observation, through in-situ high pressure resistance, Hall, X-ray
diffraction and X-ray absorption measurements, of two pressure-induced quantum
phase transitions (QPTs) in YbB6. Our data revealthat the two insulating phases
are separated by a metallic phase due to the pressure-driven valence change of
Yb f-orbitals. In combination with previous studies, our results suggest that
the two insulating states may be topologically different in nature and
originate from the d-p and d-f hybridization, respectively. The tunable
topological properties of YbB6 revealed in this study may shed light on the
intriguing correlation between the topology and the 4f electrons from the
perspective of pressure dependent studies.
