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1. 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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2. In Situ Scattering Studies of Superconducting Vacancy‐Ordered Monoclinic TiO Thin Films

   https://doi.org/10.1002/apxr.202300086  

   Baksi, Merve ; Hong, Hawoong ; Kumah, Divine P. ( November 2023 , Advanced Physics Research)

   The structural and transport properties of vacancy‐ordered monoclinic superconducting titanium oxide (TiO) thin films grown by molecular beam epitaxy are investigated. The evolution of the crystal structure during growth is monitored by in situ synchrotron X‐ray diffraction. Long‐range ordering of Ti and O vacancies in the disordered cubic phase stabilizes the vacancy‐ordered monoclinic TiO phase. The reduced structural disorder arising from vacancy‐ordering is correlated with a superconductor‐metal transition (SMT) in contrast to the superconductor‐insulator transition (SIT) observed in cubic TiO, orthorhombicTi₂O₃, and the Magneli γ −Ti₃O₅and γ −Ti₄O₇phase. Magnetoresistance measurements for the SIT phases indicate superconducting fluctuations persisting in the normal phase. These results confirm the role of disorder related to Ti and O vacancies and structural inhomogeneity in determining the electronic properties of the normal state of titanium oxide‐based superconductors.

    

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3. 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)

   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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4. Strain‐Driven Mixed‐Phase Domain Architectures and Topological Transitions in Pb 1− x Sr x TiO 3 Thin Films

   https://doi.org/10.1002/adma.202203469  

   Kavle, Pravin ; Zorn, Jacob A. ; Dasgupta, Arvind ; Wang, Bo ; Ramesh, Maya ; Chen, Long‐Qing ; Martin, Lane W. ( August 2022 , Advanced Materials)

   The potential for creating hierarchical domain structures, or mixtures of energetically degenerate phases with distinct patterns that can be modified continually, in ferroelectric thin films offers a pathway to control their mesoscale structure beyond lattice‐mismatch strain with a substrate. Here, it is demonstrated that varying the strontium content provides deterministic strain‐driven control of hierarchical domain structures in Pb_1−xSr_xTiO₃ solid‐solution thin films wherein two types,c/aanda₁/a₂, of nanodomains can coexist. Combining phase‐field simulations, epitaxial thin‐film growth, detailed structural, domain, and physical‐property characterization, it is observed that the system undergoes a gradual transformation (with increasing strontium content) from droplet‐likea₁/a₂ domains in ac/adomain matrix, to a connected‐labyrinth geometry ofc/adomains, to a disconnected labyrinth structure of the same, and, finally, to droplet‐likec/adomains in ana₁/a₂ domain matrix. A relationship between the different mixed‐phase modulation patterns and its topological nature is established. Annealing the connected‐labyrinth structure leads to domain coarsening forming distinctive regions of parallelc/aanda₁/a₂ domain stripes, offering additional design flexibility. Finally, it is found that the connected‐labyrinth domain patterns exhibit the highest dielectric permittivity.

    

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5. Local structure of Sr 2 CuO 3.3 , a 95 K cuprate superconductor without CuO 2 planes

   https://doi.org/10.1073/pnas.1918890117  

   Conradson, Steven D. ; Geballe, Theodore H. ; Jin, Changqing ; Cao, Lipeng ; Baldinozzi, Gianguido ; Jiang, Jack M. ; Latimer, Matthew J. ; Mueller, Oliver ( February 2020 , Proceedings of the National Academy of Sciences)

   The local structure of the highly “overdoped” 95 K superconductor Sr₂CuO_3.3determined by Cu K X-ray absorption fine structure (XAFS) at 62 K in magnetically oriented samples shows that 1) the magnetization is perpendicular to thecaxis; 2) at these levels of precision the Cu sublattice is tetragonal in agreement with the crystal structure; the O sublattice has 3) continuous -Cu-O- chains that orient perpendicular to an applied magnetic field; 4) approximately half-filled -Cu-O- chains that orient parallel to this field; 5) a substantial number of apical O vacancies; 6) O ions at some apical positions with expanded Cu-O distances; and 7) interstitial positions that imply highly displaced Sr ions. These results contradict the universally accepted features of cuprates that require intact CuO₂planes, magnetization along thecaxis, and a termination of the superconductivity when the excess charge on the CuO₂Cu ions exceeds 0.27. These radical differences in charge and structure demonstrate that this compound constitutes a separate class of Cu-O–based superconductors in which the superconductivity originates in a different, more complicated structural unit than CuO₂planes while retaining exceptionally high transition temperatures.

    

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