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  1. Abstract

    The interface between 2D topological Dirac states and ans‐wave superconductor is expected to support Majorana‐bound states (MBS) that can be used for quantum computing applications. Realizing these novel states of matter and their applications requires control over superconductivity and spin‐orbit coupling to achieve spin‐momentum‐locked topological interface states (TIS) which are simultaneously superconducting. While signatures of MBS have been observed in the magnetic vortex cores of bulk FeTe0.55Se0.45, inhomogeneity and disorder from doping make these signatures unclear and inconsistent between vortices. Here superconductivity is reported in monolayer (ML) FeTe1–ySey(Fe(Te,Se)) grown on Bi2Te3by molecular beam epitaxy (MBE). Spin and angle‐resolved photoemission spectroscopy (SARPES) directly resolve the interfacial spin and electronic structure of Fe(Te,Se)/Bi2Te3heterostructures. Fory = 0.25, the Fe(Te,Se) electronic structure is found to overlap with the Bi2Te3TIS and the desired spin‐momentum locking is not observed. In contrast, fory = 0.1, reduced inhomogeneity measured by scanning tunneling microscopy (STM) and a smaller Fe(Te,Se) Fermi surface with clear spin‐momentum locking in the topological states are found. Hence, it is demonstrated that the Fe(Te,Se)/Bi2Te3system is a highly tunable platform for realizing MBS where reduced doping can improve characteristics important for Majorana interrogation and potential applications.

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  2. Free, publicly-accessible full text available August 9, 2024
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  4. Free, publicly-accessible full text available June 28, 2024
  5. Abstract PdCoO 2 layered delafossite is the most conductive compound among metallic oxides, with a room-temperature resistivity of nearly $$2\,\mu \Omega \,{{{{{\rm{cm}}}}}}$$ 2 μ Ω cm , corresponding to a mean free path of about 600 Å. These values represent a record considering that the charge density of PdCoO 2 is three times lower than copper. Although its notable electronic transport properties, PdCoO 2 collective charge density modes (i.e. surface plasmons) have never been investigated, at least to our knowledge. In this paper, we study surface plasmons in high-quality PdCoO 2 thin films, patterned in the form of micro-ribbon arrays. By changing their width W and period 2 W , we select suitable values of the plasmon wavevector q , experimentally sampling the surface plasmon dispersion in the mid-infrared electromagnetic region. Near the ribbon edge, we observe a strong field enhancement due to the plasmon confinement, indicating PdCoO 2 as a promising infrared plasmonic material. 
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  6. 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.

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  7. 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. 
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  8. 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. 
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