  As the thickness of a three-dimensional (3D) topological insulator (TI)
becomes comparable to the penetration depth of the surface states, quantum
tunneling between surfaces turns their gapless Dirac electronic structure into
a gapped surface state. Analytical formulation suggests that the hybridization
gap scales exponentially with decrease in number of layers while the system
oscillates between topologically trivial and non-trivial insulators. This work
explores the transport properties of a 3D TI in the inter-surface hybridization
regime. By experimentally probing the hybridization gap as a function of
BiSbTeSe2 thickness using three different methods, we map the crossover from
the 3D to 2D state. In the 2D topological state, we observe a finite
longitudinal conductance at ~2e2/h when the Fermi level is aligned within the
surface gap, indicating a quantum spin Hall (QSH) state. Additionally, we study
the response of trivial and non-trivial hybridization gapped states modulated
by external out-of-plane magnetic and electric fields. Our revelations of
surface gap-closing and/or reopening features are strongly indicative of
topological phase transitions (TPTs) in the hybridization gap regime, realizing
magnetic/electric field switching between band insulating and QSH states with
immense potential for practical applications.
