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Terahertz (THz) spectroscopy can characterize the collective oscillations of free particles in two-dimensional (2D) materials. The resonant response appearing in the transmitted THz spectra is relevant to the 2D plasmon mode. Here, we investigate the spectral extinction of the THz wave transmitted through the graphene-integrated Bi2Se3 microstructure, where the bias voltage applied to the gate electrode controls the device sheet conductance. Comparing the spectral response of the device with the EM wave simulation result, we observe a consistent spectral modulation as a function of the input particle density. The simulation result further characterizes the Bi2Se3 Dirac plasmon polariton (DPP) coupled to graphene. We find that the large graphene polarizability enables efficient control of the Bi2Se3 DPP mode up to 70% using a moderate gate voltage range of −1 to 1 V. Our result can be used to understand the interlayer long-range Coulomb interaction between Dirac materials. 

Thermoelectric materials can convert thermal energy into electricity, making them promising candidates for harvesting waste heat, an increasingly important challenge in the energy-intensive modern world. The search for improved thermoelectric materials is therefore an active area of research in materials physics. Despite their fundamental and practical significance, thermoelectric properties—such as the Seebeck coefficient and power factor—are rarely explored in student labs due to the complexity in measurement schemes and requirement for sophisticated equipment. In this work, we present a user-friendly, low-cost, and efficient thermoelectric measurement system built with Arduino and LabVIEW, which can simultaneously measure Seebeck coefficients and power factors as a function of temperature. This was made possible by improving the resolution of Arduino over ∼1000 times with amplifiers and noise reduction schemes. With a total cost of only ∼$100 and simple measurement protocols, this setup is well suited not only for student labs but also for efficient thermoelectric research. 

We report terahertz time-domain spectroscopy experiments demonstrating strong light–matter coupling in a terahertz LC metamaterial (MM) in which the phonon resonance of a topological insulator thin film is coupled to the photonic modes of an array of electronic split ring resonators. As we tune the MM resonance frequency through the frequency of the low-frequency α mode of (BixSb1–x)2Te3 (BST), we observe strong mixing and level repulsion between the phonon and MM resonance. This hybrid resonance is a phonon polariton. We observe a normalized coupling strength, η = ΩR/ωc ≈ 0.09, using the measured vacuum Rabi frequency and cavity resonance. Our results demonstrate that one can tune the mechanical properties of these materials by changing their electromagnetic environment and therefore modify their magnetic and topological degrees of freedom via coupling to the lattice in this fashion. 

Using molecular beam epitaxy (MBE) to grow multielemental oxides (MEOs) is generally challenging, partly due to difficulty in stoichiometry control. Occasionally, if one of the elements is volatile at the growth temperature, stoichiometry control can be greatly simplified using adsorption-controlled growth mode. Otherwise, stoichiometry control remains one of the main hurdles to achieving high-quality MEO film growths. Here, we report another kind of self-limited growth mode, dubbed diffusion-assisted epitaxy, in which excess species diffuses into the substrate and leads to the desired stoichiometry, in a manner similar to the conventional adsorption-controlled epitaxy. Specifically, we demonstrate that using diffusion-assisted epitaxy, high-quality epitaxial CuCrO2 films can be grown over a wide growth window without precise flux control using MBE. 

Since the notion of topological insulator (TI) was envisioned in late 2000s, topology has become a new paradigm in condensed matter physics. Realization of topology as a generic property of materials has led to numerous predictions of topological effects. Although most of the classical topological effects, directly resulting from the presence of the spin-momentum-locked topological surface states (TSS), were experimentally confirmed soon after the theoretical prediction of TIs, many topological quantum effects remained elusive for a long while. It turns out that native defects, particularly interfacial defects, have been the main culprit behind this impasse. Even after quantum regime is achieved for the bulk states, TSS still tends to remain in the classical regime due to high density of interfacial defects, which frequently donate mobile carriers due to the very nature of the topologically-protected surface states. However, with several defect engineering schemes that suppress these effects, a series of topological quantum effects have emerged including quantum anomalous Hall effect, quantum Hall effect, quantized Faraday/Kerr rotations, topological quantum phase transitions, axion insulating state, zeroth-Landau level state, etc. Here, we review how these defect engineering schemes have allowed topological surface states to pull out of the murky classical regime and reveal their elusive quantum signatures, over the past decade. 

Abstract This work focuses on the low frequency Drude response of bulk-insulating topological insulator (TI) Bi 2 Se 3 films. The frequency and field dependence of the mobility and carrier density are measured simultaneously via time-domain terahertz spectroscopy. These films are grown on buffer layers, capped by Se, and have been exposed in air for months. Under a magnetic field up to 7 Tesla, we observe prominent cyclotron resonances (CRs). We attribute the sharp CR to two different topological surface states from both surfaces of the films. The CR sharpens at high fields due to an electron-impurity scattering. By using magneto-terahertz spectroscopy, we confirm that these films are bulk-insulating, which paves the way to use intrinsic TIs without bulk carriers for applications including topological spintronics and quantum computing.