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1. An all-epitaxial nitride heterostructure with concurrent quantum Hall effect and superconductivity

   https://doi.org/10.1126/sciadv.abf1388  

   Dang, Phillip; Khalsa, Guru; Chang, Celesta S.; Katzer, D. Scott; Nepal, Neeraj; Downey, Brian P.; Wheeler, Virginia D.; Suslov, Alexey; Xie, Andy; Beam, Edward; et al (February 2021, Science Advances)

   null (Ed.)

   Creating seamless heterostructures that exhibit the quantum Hall effect and superconductivity is highly desirable for future electronics based on topological quantum computing. However, the two topologically robust electronic phases are typically incompatible owing to conflicting magnetic field requirements. Combined advances in the epitaxial growth of a nitride superconductor with a high critical temperature and a subsequent nitride semiconductor heterostructure of metal polarity enable the observation of clean integer quantum Hall effect in the polarization-induced two-dimensional (2D) electron gas of the high-electron mobility transistor. Through individual magnetotransport measurements of the spatially separated GaN 2D electron gas and superconducting NbN layers, we find a small window of magnetic fields and temperatures in which the epitaxial layers retain their respective quantum Hall and superconducting properties. Its analysis indicates that in epitaxial nitride superconductor/semiconductor heterostructures, this window can be significantly expanded, creating an industrially viable platform for robust quantum devices that exploit topologically protected transport. 

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2. Weyl points and topological surface states in a three-dimensional sandwich-type elastic lattice

   https://doi.org/10.1088/1367-2630/ab9e31  

   Ganti, Sai Sanjit; Liu, Ting-Wei; Semperlotti, Fabio (August 2020, New Journal of Physics)

   Abstract Following the realization of Weyl semimetals in quantum electronic materials, classical wave analogues of Weyl materials have also been theorized and experimentally demonstrated in photonics and acoustics. Weyl points in elastic systems, however, have been a much more recent discovery. In this study, we report on the design of an elastic fully-continuum three-dimensional material that, while offering structural and load-bearing functionalities, is also capable of Weyl degeneracies and surface topologically-protected modes in a way completely analogous to its quantum mechanical counterpart. The topological characteristics of the lattice are obtained byab initionumerical calculations without employing any further simplifications. The results clearly characterize the topological structure of the Weyl points and are in full agreement with the expectations of surface topological modes. Finally, full field numerical simulations are used to confirm the existence of surface states and to illustrate their extreme robustness towards lattice disorder and defects. 

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3. From classical to quantum regime of topological surface states via defect engineering

   https://doi.org/10.21468/SciPostPhysLectNotes.58  

   Salehi, Maryam; Yao, Xiong; Oh, Seongshik (July 2022, SciPost Physics Lecture Notes)

   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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4. Fragility of surface states in topological superfluid 3He

   https://doi.org/10.1038/s41467-021-21831-y  

   Heikkinen, P. J.; Casey, A.; Levitin, L. V.; Rojas, X.; Vorontsov, A.; Sharma, P.; Zhelev, N.; Parpia, J. M.; Saunders, J. (December 2021, Nature Communications)

   null (Ed.)

   Abstract Superfluid 3 He, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing. In topological insulators and quantum Hall systems, the surface/edge states, arising from bulk-surface correspondence and the momentum space topology of the band structure, are robust. Here we demonstrate that in topological superfluids and superconductors the surface Andreev bound states, which depend on the momentum space topology of the emergent order parameter, are fragile with respect to the details of surface scattering. We confine superfluid 3 He within a cavity of height D comparable to the Cooper pair diameter ξ 0 . We precisely determine the superfluid transition temperature T c and the suppression of the superfluid energy gap, for different scattering conditions tuned in situ, and compare to the predictions of quasiclassical theory. We discover that surface magnetic scattering leads to unexpectedly large suppression of T c , corresponding to an increased density of low energy bound states. 

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5. Tunable Acoustic Topological Insulator

   Darabi, Amir; Leamy, Michael (June 2019, PHONONICS 2019: 5th International Conference on Phononic Crystals/Metamaterials, Phonon Transport and Topological Phononics)

   Inspired by the quantum spin Hall effect (QSHE), we propose and numerically verify at tuna-ble topological insulator by employing asymmetry and nonlinearity, to exhibit topologically protected edge states (TPES). These properties may open new research directions in realizing the quantized anomalous Hall effects and topological insulators. 

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