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As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped spectrum. Whether the surface hybridization gap can host topological edge states is still an open question. Herein, we provide transport evidence of 2D topological states in the quantum tunneling regime of a bulk insulating 3D TI BiSbTeSe₂. Different from its trivial insulating phase, this 2D topological state exhibits a finite longitudinal conductance at ~2e²/h when the Fermi level is aligned within the surface gap, indicating an emergent quantum spin Hall (QSH) state. The transition from the QSH to quantum Hall (QH) state in a transverse magnetic field further supports the existence of this distinguished 2D topological phase. In addition, we demonstrate a second route to realize the 2D topological state via surface gap-closing and topological phase transition mechanism mediated by a transverse electric field. The experimental realization of the 2D topological phase in a 3D TI enriches its phase diagram and marks an important step toward functionalized topological quantum devices.

Despite numerous studies on three-dimensional topological insulators (3D TIs), the controlled growth of high quality (bulk-insulating and high mobility) TIs remains a challenging subject. This study investigates the role of growth methods on the synthesis of single crystal stoichiometric BiSbTeSe₂(BSTS). Three types of BSTS samples are prepared using three different methods, namely melting growth (MG), Bridgman growth (BG) and two-step melting-Bridgman growth (MBG). Our results show that the crystal quality of the BSTS depend strongly on the growth method. Crystal structure and composition analyses suggest a better homogeneity and highly-ordered crystal structure in BSTS grown by MBG method. This correlates well to sample electrical transport properties, where a substantial improvement in surface mobility is observed in MBG BSTS devices. The enhancement in crystal quality and mobility allow the observation of well-developed quantum Hall effect at low magnetic field.

Spin waves, quantized as magnons, have low energy loss and magnetic damping, which are critical for devices based on spin‐wave propagation needed for information processing devices. The organic‐based magnet [V(TCNE)_x; TCNE = tetracyanoethylene;x≈ 2] has shown an extremely low magnetic damping comparable to, for example, yttrium iron garnet (YIG). The excitation, detection, and utilization of coherent and non‐coherent spin waves on various modes in V(TCNE)_xis demonstrated and show that the angular momentum carried by microwave‐excited coherent spin waves in a V(TCNE)_xfilm can be transferred into an adjacent Pt layer via spin pumping and detected using the inverse spin Hall effect. The spin pumping efficiency can be tuned by choosing different excited spin wave modes in the V(TCNE)_xfilm. In addition, it is shown that non‐coherent spin waves in a V(TCNE)_xfilm, excited thermally via the spin Seebeck effect, can also be used as spin pumping source that generates an electrical signal in Pt with a sign change in accordance with the magnetization switching of the V(TCNE)_x. Combining coherent and non‐coherent spin wave detection, the spin pumping efficiency can be thermally controlled, and new insight is gained for the spintronic applications of spin wave modes in organic‐based magnets.