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

Bi₂Se₃, one of the most widely studied topological insulators (TIs), is naturally electron-doped due to n-type native defects. However, many years of efforts to achieve p-type Bi₂Se₃ thin films have failed so far. Here, we provide a solution to this long-standing problem, showing that the main culprit has been the high density of interfacial defects. By suppressing these defects through an interfacial engineering scheme, we have successfully implemented p-type Bi₂Se₃ thin films down to the thinnest topological regime. On this platform, we present the first tunable quantum Hall effect (QHE) study in Bi₂Se₃ thin films and reveal not only significantly asymmetric QHE signatures across the Dirac point but also the presence of competing anomalous states near the zeroth Landau level. The availability of doping tunable Bi₂Se₃ thin films will now make it possible to implement various topological quantum devices, previously inaccessible

Finite-Size and Composition-Driven Topological Phase Transition in (Bi_1–<i>x</i>In_<i>x</i>)₂Se₃ Thin Films

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 (Bi_1–<i>x</i>In_<i>x</i>)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

Magnetic Field-Induced Spin Nematic Phase Up to Room Temperature in Epitaxial Antiferromagnetic FeTe Thin Films Grown by Molecular Beam Epitaxy

Electronic nematicity, where strong correlations drive electrons to align in a way that lowers the crystal symmetry, is ubiquitous among unconventional superconductors. Understanding the interplay of such a nematic state with other electronic phases underpins the complex behavior of these materials and the potential for tuning their properties through external stimuli. Here, we report magnetic field-induced spin nematicity in a model system tetragonal FeTe, the parent compound of iron chalcogenide superconductors, which exhibits a bicollinear antiferromagnetic order. The studies were conducted on epitaxial FeTe thin films grown on SrTiO3(001) substrates by molecular beam epitaxy, where the bicollinear antiferromagnetic order was confirmed by in situ atomic resolution scanning tunneling microscopy imaging. A 2-fold anisotropy is observed in in-plane angle-dependent magnetoresistance measurements, indicative of magnetic field-induced nematicity. Such 2-fold anisotropy persists up to 300 K, well-above the bicollinear antiferromagnetic ordering temperature of 75 K, indicating a magnetic field-induced spin nematic phase up to room temperature in the antiferromagnet FeTe

Composition Control of Plasmon–Phonon Interaction Using Topological Quantum-Phase Transition in Photoexcited (Bi_1–<i>x</i>In_<i>x</i>)₂Se₃

Plasmonics is a technology aiming at light modulation via collective charge oscillations. Topological insulators, where Dirac-like metallic surfaces coexist with normal insulating bulk, have recently attracted great attention in plasmonics due to their topology-originated outstanding properties. Here, we introduce a new methodology for controlling the interaction of a plasmon with a phonon in topological insulators, which is a key for utilizing the unique spectral profiles for photonic applications. By using both static and ultrafast terahertz spectroscopy, we show that the interaction can be tuned by controlling the chemical composition of (Bi_1–<i>x</i>In_<i>x</i>)₂Se₃ microribbon arrays. The topological quantum-phase transition induced by varying the composition drives a dramatic change in the strength of the plasmon–phonon interaction. This was possible due to the availability of manipulating the spatial overlap between topological surface plasmonic states and underlying bulk phonons. Especially, we control the laser-induced ultrafast evolution of the transient spectral peaks arising from the plasmon–phonon interaction by varying the spatial overlap across the topological phase transition. This study may provide a new platform for realizing topological insulator-based ultrafast plasmonic devices

Control over Electron–Phonon Interaction by Dirac Plasmon Engineering in the Bi₂Se₃ Topological Insulator

Understanding the mutual interaction between electronic excitations and lattice vibrations is key for understanding electronic transport and optoelectronic phenomena. Dynamic manipulation of such interaction is elusive because it requires varying the material composition on the atomic level. In turn, recent studies on topological insulators (TIs) have revealed the coexistence of a strong phonon resonance and topologically protected Dirac plasmon, both in the terahertz (THz) frequency range. Here, using these intrinsic characteristics of TIs, we demonstrate a new methodology for controlling electron–phonon interaction by lithographically engineered Dirac surface plasmons in the Bi₂Se₃ TI. Through a series of time-domain and time-resolved ultrafast THz measurements, we show that, when the Dirac plasmon energy is less than the TI phonon energy, the electron–phonon coupling is trivial, exhibiting phonon broadening associated with Landau damping. In contrast, when the Dirac plasmon energy exceeds that of the phonon resonance, we observe suppressed electron–phonon interaction leading to unexpected phonon stiffening. Time-dependent analysis of the Dirac plasmon behavior, phonon broadening, and phonon stiffening reveals a transition between the distinct dynamics corresponding to the two regimes as the Dirac plasmon resonance moves across the TI phonon resonance, which demonstrates the capability of Dirac plasmon control. Our results suggest that the engineering of Dirac plasmons provides a new alternative for controlling the dynamic interaction between Dirac carriers and phonons