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1. Spin-Filtered Tunneling Device Using a Topological Insulator

   Berlioz, Jose R. (April 2021, Masters Thesis of Engineering)

   null (Ed.)

   There has been much interest in the study of topological insulators (TI) recently. Due to their unique electronic structure, these new materials have been an active area of research to discover new quantum phenomena and their application in new technologies. Unlike the electronic structure observed in traditional semiconductors, the strong spin-orbit coupling induces a band inversion in the electronic structure of TIs. One of the side effects of this band inversion is creating metallic-like surface states at the material's surface that are protected by time invariance and whose spin angular momentum is locked to the direction of the momentum of the electron. These surface states are essentially resistant to scattering events that otherwise affect other materials. Leveraging the characteristic scattering resistance, the spin-momentum locking of the surface states, and the Dirac cone structure, a spin-resonant tunneling diode using topological insulators has been investigated to implement a negative differential resistance device. Utilizing the spin texture of the surface states, an additional spin-filter can help to suppress the valley current in a negative differential resistance device. In the spin-resonant tunneling diode, the tunneling process would also benefit from having protection from conventional scattering processes due to defects and thickness or line edge roughness. This research is focused on the manufacturing of a spin-filtered tunnel diode. Using molecular beam epitaxy to grow a three-layer heterostructure, with two layers of bismuth selenide as the topological insulator separated by a thin layer of tungsten diselenide as a tunnel barrier. The alignment of the Fermi levels of the topological insulator layers and the thickness of the tunnel barrier were investigated using X-ray Photoelectron Spectroscopy. The fabrication and initial electrical measurements of the spin-filtered tunnel diode were also investigated. 

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2. On the Remarkable Superconductivity of FeSe and Its Close Cousins

   https://doi.org/10.3390/sym12091402  

   Kreisel, Andreas; Hirschfeld, Peter J.; Andersen, Brian M. (September 2020, Symmetry)

   null (Ed.)

   Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity. 

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3. Non-equilibrium states and interactions in the topological insulator and topological crystalline insulator phases of NaCd4As3

   https://doi.org/10.1063/4.0000273  

   Kafle, Tika R; Zhang, Yingchao; Wang, Yi-yan; Shi, Xun; Li, Na; Sapkota, Richa; Thurston, Jeremy; You, Wenjing; Gao, Shunye; Dong, Qingxin; et al (January 2025, Structural Dynamics)

   Topological materials are of great interest because they can support metallic edge or surface states that are robust against perturbations, with the potential for technological applications. Here, we experimentally explore the light-induced non-equilibrium properties of two distinct topological phases in NaCd4As3: a topological crystalline insulator (TCI) phase and a topological insulator (TI) phase. This material has surface states that are protected by mirror symmetry in the TCI phase at room temperature, while it undergoes a structural phase transition to a TI phase below 200 K. After exciting the TI phase by an ultrafast laser pulse, we observe a leading band edge shift of >150 meV that slowly builds up and reaches a maximum after ∼0.6 ps and that persists for ∼8 ps. The slow rise time of the excited electron population and electron temperature suggests that the electronic and structural orders are strongly coupled in this TI phase. It also suggests that the directly excited electronic states and the probed electronic states are weakly coupled. Both couplings are likely due to a partial relaxation of the lattice distortion, which is known to be associated with the TI phase. In contrast, no distinct excited state is observed in the TCI phase immediately or after photoexcitation, which we attribute to the low density of states and phase space available near the Fermi level. Our results show how ultrafast laser excitation can reveal the distinct excited states and interactions in phase-rich topological materials. 

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4. Ordinary lattice defects as probes of topology

   https://doi.org/10.1088/1361-6633/ae5bcb  

   Mains, Aiden J; Zhong, Jia-Xin; Jing, Yun; Roy, Bitan (April 2026, Reports on Progress in Physics)

   Abstract In addition to topological lattice defects such as dislocations and disclinations, crystals are also accompanied by unavoidable ordinary defects, devoid of any non-trivial geometry or topology, among which vacancies, Schottky defects, substitutions, interstitials, and Frenkel pairs are the most common. In this work, we demonstrate that these ubiquitous ordinary lattice defects, although geometrically trivial, can nonetheless serve as universal probes of the non-trivial topology of electronic Bloch bands, and any change in the local topological environment in an otherwise normal insulator in terms of mid-gap bound states in their vicinity. We theoretically establish these generic findings by implementing a minimal model Hamiltonian describing time-reversal symmetry breaking topological and normal insulators on a square lattice, fostering such point defects. The defect-bound mid-gap modes are also shown to be robust against sufficiently weak random point-like charge impurities. Furthermore, we showcase experimental observation of such bound states by embedding ordinary crystal defects in two-dimensional acoustic Chern lattices, where precision-controlled hopping amplitudes are implemented via active meta-atoms and Green’s-function-based spectroscopy is used to reconstruct spectra and eigenstates. Our combined theory-experiment study establishes ordinary lattice defects as probes of topology that should be germane in crystals of any symmetry and dimension, raising the possibility of arresting localized Majorana modes near such defects in the bulk of topological superconductors and to emulate ordinary-defect-engineered topological devices. 

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5. Tuning interactions between spins in a superconductor

   https://doi.org/10.1073/pnas.2024837118  

   Ding, Hao; Hu, Yuwen; Randeria, Mallika T.; Hoffman, Silas; Deb, Oindrila; Klinovaja, Jelena; Loss, Daniel; Yazdani, Ali (March 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Novel many-body and topological electronic phases can be created in assemblies of interacting spins coupled to a superconductor, such as one-dimensional topological superconductors with Majorana zero modes (MZMs) at their ends. Understanding and controlling interactions between spins and the emergent band structure of the in-gap Yu–Shiba–Rusinov (YSR) states they induce in a superconductor are fundamental for engineering such phases. Here, by precisely positioning magnetic adatoms with a scanning tunneling microscope (STM), we demonstrate both the tunability of exchange interaction between spins and precise control of the hybridization of YSR states they induce on the surface of a bismuth (Bi) thin film that is made superconducting with the proximity effect. In this platform, depending on the separation of spins, the interplay among Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, spin–orbit coupling, and surface magnetic anisotropy stabilizes different types of spin alignments. Using high-resolution STM spectroscopy at millikelvin temperatures, we probe these spin alignments through monitoring the spin-induced YSR states and their energy splitting. Such measurements also reveal a quantum phase transition between the ground states with different electron number parity for a pair of spins in a superconductor tuned by their separation. Experiments on larger assemblies show that spin–spin interactions can be mediated in a superconductor over long distances. Our results show that controlling hybridization of the YSR states in this platform provides the possibility of engineering the band structure of such states for creating topological phases. 

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