Finite-Size and Composition-Driven Topological Phase Transition in (Bi_1–xIn_x)₂Se₃ Thin Films

Abstract

In a topological insulator (TI), if its spin-orbit coupling (SOC) strength is gradually reduced, the TI eventually transforms into a trivial insulator beyond a critical point of SOC, at which point the bulk gap closes: this is the standard description of the topological phase transition (TPT). However, this description of TPT, driven solely by the SOC (or something equivalent) and followed by closing and reopening of the bulk band gap, is valid only for infinite-size samples, and little is known how TPT occurs for finite-size samples. Here, using both systematic transport measurements on interface-engineered (Bi1-xInx)2Se3 thin films and theoretical simulations (with animations in the Supporting Information), we show that description of TPT in finite-size samples needs to be substantially modified from the conventional picture of TPT due to surface-state hybridization and bulk confinement effects. We also show that the finite-size TPT is composed of two separate transitions, topological-normal transition (TNT) and metal-insulator transition (MIT), by providing a detailed phase diagram in the two-dimensional phase space of sample size and SOC strength.

Solution to the Hole-Doping Problem and Tunable Quantum Hall Effect in Bi2Se3 Thin Films

“…As summarized in the phase diagram in Figure 2b, 7 these observations show that counter-doping becomes gradually more difficult, requiring a higher level of Ca doping, as Bi2Se3 films become thinner. This suggests that even in these interfaceengineered Bi2Se3 films, the majority of defects still originate from the interfaces rather than from the bulk of the films.Even though p-type Bi2Se3 films are achieved for all thicknesses from 50 QL down to 6 QL at 2% of Ca, the films again become n-type with further increase (above 4.4%) in Ca doping except for 6 QL, which becomes insulating probably due to a disorder-driven topological phase transition 31,33 . This is because Ca doping inevitably introduces disorder and thus degrades both electronic and structural properties of the Bi2Se3 films.…”

Layered Topological Insulators and Semimetals for Magnetoresistance Type Sensors

“…Angleresolved photoemission spectroscopy (ARPES) results revealed a small bulk gap (0.1 eV) in (Bi 1-x In x ) 2 Se 3 (x = 0.08) due to the topological phase transition (TPT) (Figure 7d). TPT can induce a small bulk gap [25,[86][87][88][89] and a probable coupling of the top and bottom surfaces. [90][91][92] The enhanced intersurface coupling induces a net spin polarization along the in-plane magnetic field (Figure 7e).…”

Local structural analysis of In‐doped Bi2Se3 topological insulator using X‐ray fluorescence holography

“…Such a broad tunability of the electronic states is a unique feature of this system and favorable to the applications as electronic devices. Thus, extensive works have been performed for understanding the variation in the physical properties as a function of x .…”

“…Thus, extensive works have been performed for understanding the variation in the physical properties as a function of x. [7][8][9][10] However, the effect of the doping on the electronic states has not been clearly understood so far. In the ARPES study of (In x Bi 1−x ) 2 Se 3 , 6 ARPES spectra were systematically measured at several concentrations of x = 0.02, 0.04, 0.08, 0.20, and 0.30, where the TI to non-TI transition was observed at about x = 0.04.…”