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1. Monolayer Superconductivity and Tunable Topological Electronic Structure at the Fe(Te,Se)/Bi ₂ Te ₃ Interface

   https://doi.org/10.1002/adma.202210940  

   Moore, Robert_G; Lu, Qiangsheng; Jeon, Hoyeon; Yao, Xiong; Smith, Tyler; Pai, Yun‐Yi; Chilcote, Michael; Miao, Hu; Okamoto, Satoshi; Li, An‐Ping; et al (April 2023, Advanced Materials)

   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 FeTe_0.55Se_0.45, inhomogeneity and disorder from doping make these signatures unclear and inconsistent between vortices. Here superconductivity is reported in monolayer (ML) FeTe_1–ySe_y(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. Elastocaloric determination of the phase diagram of Sr2RuO4

   https://doi.org/10.1038/s41586-022-04820-z  

   Li, You-Sheng; Garst, Markus; Schmalian, Jörg; Ghosh, Sayak; Kikugawa, Naoki; Sokolov, Dmitry A.; Hicks, Clifford W.; Jerzembeck, Fabian; Ikeda, Matthias S.; Hu, Zhenhai; et al (July 2022, Nature)

   Abstract One of the main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagrams that also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research 1 . In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method of tuning 2,3 , leading to experiments that have advanced our understanding of the fascinating unconventional superconductor Sr 2 RuO 4 (refs.  4–9 ). Here we map out its phase diagram using high-precision measurements of the elastocaloric effect in what we believe to be the first such study including both the normal and the superconducting states. We observe a strong entropy quench on entering the superconducting state, in excellent agreement with a model calculation for pairing at the Van Hove point, and obtain a quantitative estimate of the entropy change associated with entry to a magnetic state that is observed in proximity to the superconductivity. The phase diagram is intriguing both for its similarity to those seen in other families of unconventional superconductors and for extra features unique, so far, to Sr 2 RuO 4 . 

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3. Controllable suppression of the unconventional superconductivity in bulk and thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ via high-energy electron irradiation

   https://doi.org/10.1103/PhysRevResearch.6.033178  

   Ruf, Jacob P; Noad, Hilary_M L; Grasset, Romain; Miao, Ludi; Zhakina, Elina; McGuinness, Philippa H; Nair, Hari P; Schreiber, Nathaniel J; Kikugawa, Naoki; Sokolov, Dmitry; et al (August 2024, Physical Review Research)

   In bulk ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the relationship between $T_{\text{c}}$and the residual resistivity ${\rho }_0$, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we show that $T_{\text{c}}$is suppressed in both systems at nearly identical rates. This suggests that part of ${\rho }_0$in films comes from defects that do not contribute to superconducting pairbreaking and establishes a quantitative link between the superconductivity of bulk and thin-film samples. Published by the American Physical Society2024 

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4. Nickelate Superconductivity without Rare‐Earth Magnetism: (La,Sr)NiO ₂

   https://doi.org/10.1002/adma.202104083  

   Osada, Motoki; Wang, Bai_Yang; Goodge, Berit_H; Harvey, Shannon_P; Lee, Kyuho; Li, Danfeng; Kourkoutis, Lena_F; Hwang, Harold_Y (September 2021, Advanced Materials)

   Abstract The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite‐layer nickelate (Nd,Sr)NiO₂and (Pr,Sr)NiO₂thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare‐earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La_1−xSr_xNiO₂phase diagram can enter the regime of coherent low‐temperature transport (x = 0.14 ‐ 0.20), with subsequent superconducting transitions and a maximum onset of ≈9 K atx = 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO₂is observed, which likely reflects the self‐doped nature of the electronic structure. Combining the results of (La/Pr/Nd)_1−xSr_xNiO₂reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron‐hole populations across the lanthanides. 

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5. Edge and Point‐Defect Induced Electronic and Magnetic Properties in Monolayer PtSe ₂

   https://doi.org/10.1002/adfm.202110428  

   Li, Jingfeng; Joseph, Thomas; Ghorbani‐Asl, Mahdi; Kolekar, Sadhu; Krasheninnikov, Arkady_V; Batzill, Matthias (January 2022, Advanced Functional Materials)

   Abstract Edges and point defects in layered dichalcogenides are important for tuning their electronic and magnetic properties. By combining scanning tunneling microscopy (STM) with density functional theory (DFT), the electronic structure of edges and point defects in 2D‐PtSe₂are investigated where the 1.8 eV bandgap of monolayer PtSe₂facilitates the detailed characterization of defect‐induced gap states by STM. The stoichiometric zigzag edge terminations are found to be energetically favored. STM and DFT show that these edges exhibit metallic 1D states with spin polarized bands. Various native point defects in PtSe₂are also characterized by STM. A comparison of the experiment with simulated images enables identification of Se‐vacancies, Pt‐vacancies, and Se‐antisites as the dominant defects in PtSe₂. In contrast to Se‐ or Pt‐vacancies, the Se‐antisites are almost devoid of gap states. Pt‐vacancies exhibit defect induced states that are spin polarized, emphasizing their importance for inducing magnetism in PtSe₂. The atomic‐scale insights into defect‐induced electronic states in monolayer PtSe₂provide the fundamental underpinning for defect engineering of PtSe₂‐monolayers and the newly identified spin‐polarized edge states offer prospects for engineering magnetic properties in PtSe₂nanoribbons. 

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