Realization of a 2D Lieb Lattice in a Metal-Inorganic Framework with Partial Flat Bands and Topological Edge States.

Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice's morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology.

Uncovering an Interfacial Band Resulting from Orbital Hybridization in Nickelate Heterostructures.

The interaction of atomic orbitals at the interface of perovskite oxide heterostructures has been investigated for its profound impact on the band structures and electronic properties, giving rise to unique electronic states and a variety of tunable functionalities. In this study, we conducted an extensive investigation of the optical and electronic properties of epitaxial NdNiO3 synthesized on a series of single-crystal substrates. Unlike nanofilms synthesized on other substrates, NdNiO3 on SrTiO3 (NNO/STO) gives rise to a unique band structure featuring an additional unoccupied band situated above the Fermi level. Our comprehensive investigation, which incorporated a wide array of experimental techniques and density functional theory calculations, revealed that the emergence of the interfacial band structure is primarily driven by orbital hybridization between the Ti 3d orbitals of the STO substrate and the O 2p orbitals of the NNO thin film. Furthermore, exciton peaks have been detected in the optical spectra of the NNO/STO film, attributable to the pronounced electron-electron (e-e) and electron-hole (e-h) interactions propagating from the STO substrate into the NNO film. These findings underscore the substantial influence of interfacial orbital hybridization on the electronic structure of oxide thin films, thereby offering key insights into tuning their interfacial properties.

Orbital-Hybridization-Driven Charge Density Wave Transition in CsV3 Sb5 Kagome Superconductor.

Owing to its inherent non-trivial geometry, the unique structural motif of the recently discovered kagome topological superconductor AV3 Sb5 (A = K, Rb, Cs) is an ideal host of diverse topologically non-trivial phenomena, including giant anomalous Hall conductivity, topological charge order, charge density wave (CDW), and unconventional superconductivity. Despite possessing a normal-state CDW order in the form of topological chiral charge order and diverse superconducting gaps structures, it remains unclear how fundamental atomic-level properties and many-body effects including Fermi surface nesting, electron-phonon coupling, and orbital hybridization contribute to these symmetry-breaking phenomena. Here, the direct participation of the V3d-Sb5p orbital hybridization in mediating the CDW phase transition in CsV3 Sb5 is reported. The combination of temperature-dependent X-ray absorption and first-principles studies clearly indicates the inverse Star-of-David structure as the preferred reconstruction in the low-temperature CDW phase. The results highlight the critical role that Sb orbitals play and establish orbital hybridization as the direct mediator of the CDW states and structural transition dynamics in kagome unconventional superconductors. This is a significant step toward the fundamental understanding and control of the emerging correlated phases from the kagome lattice through the orbital interactions and provides promising approaches to novel regimes in unconventional orders and topology.

Thickness dependence of structural and superconducting properties of Co-doped BaFe2As2 coated conductors.

High-quality Co-doped BaFe2As2 thin films with thickness up to 2 µm were realized on flexible metal tapes with LaMnO3 as buffer layers fabricated by an ion beam-assisted deposition technique. Structural analysis indicates that increasing thickness does not compromise the film crystallinity, except for a small amount of impurities. Two types of thickness dependence of critical current density (J c) were found: one is almost thickness independent in the range of 0.6-1.5 µm and the other is highly thickness dependent. In addition, the maximum value for crucial current I c at 9 T and 4.2 K is about 55 A/12 mm-W for the 1.5-µm-thick film. Anisotropic Ginzburg-Landau scaling demonstrates that dominant pinning centers develop from correlated to uncorrelated with increasing film thickness. The further theoretical analysis shows that with film thickness increasing the pinning mechanism evolves progressively from a δl pinning to the δT c pinning mechanism.

REBCO mixtures with large difference in rare-earth ion size: superconducting properties of chemical solution deposition-grown Yb1-x Sm x Ba2Cu3O7- δ films.

The main objective of this work was to study the superconducting properties of REBCO films with a mixture of rare-earth (RE) ions with large difference in ion size, in particular Sm3+ and Yb3+. These Yb1-x Sm x Ba2Cu3O7- δ films have been successfully prepared for the first time by chemical solution deposition following the extremely low-fluorine route, which allows reducing the fluorine content by 93% with respect to standard full trifluoroacetate solutions. On the one hand, critical temperature T c remains stable at approximately 90 K with Sm content up to x = 0.5 where T c starts to increase towards the values of pure SmBCO films of approximately 95 K. On the other hand, the critical current densities J c of the pure SmBCO films are the largest at 77 K, where the influence of a higher T c is very relevant, while at lower temperatures and low fields, the mixed films reach larger values. This demonstrates that mixing rare-earth elements RE in REBa2Cu3O7- δ causes a change in the pinning properties of the films and reveals the importance of selecting adequate REBCO compounds according to the temperature and magnetic field region of a desired application.

Multi-Aperture Shower Design for the Improvement of the Transverse Uniformity of MOCVD-Derived GdYBCO Films.

A multi-aperture shower design is reported to improve the transverse uniformity of GdYBCO superconducting films on the template of sputtered-LaMnO3/epitaxial-MgO/IBAD-MgO/solution deposition planarization (SDP)-Y2O3-buffered Hastelloy tapes. The GdYBCO films were prepared by the metal organic chemical vapor deposition (MOCVD) process. The transverse uniformities of structure, morphology, thickness, and performance were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), step profiler, and the standard four-probe method using the criteria of 1 µV/cm, respectively. Through adopting the multi-aperture shower instead of the slit shower, measurement by step profiler revealed that the thickness difference between the middle and the edges based on the slit shower design was well eliminated. Characterization by SEM showed that a GdYBCO film with a smooth surface was successfully prepared. Moreover, the transport critical current density (Jc) of its middle and edge positions at 77 K and self-field were found to be over 5 MA/cm² through adopting the micro-bridge four-probe method.

The Interface Structure of FeSe Thin Film on CaF2 Substrate and its Influence on the Superconducting Performance.

The investigations into the interfaces in iron selenide (FeSe) thin films on various substrates have manifested the great potential of showing high-temperature-superconductivity in this unique system. In present work, we obtain FeSe thin films with a series of thicknesses on calcium fluoride (CaF2) (100) substrates and glean the detailed information from the FeSe/CaF2 interface by using scanning transmission electron microscopy (STEM). Intriguingly, we have found the universal existence of a calcium selenide (CaSe) interlayer with a thickness of approximate 3 nm between FeSe and CaF2 in all the samples, which is irrelevant to the thickness of FeSe layers. A slight Se deficiency occurs in the FeSe layer due to the formation of CaSe interlayer. This Se deficiency is generally negligible except for the case of the ultrathin FeSe film (8 nm in thickness), in which the stoichiometric deviation from FeSe is big enough to suppress the superconductivity. Meanwhile, in the overly thick FeSe layer (160 nm in thickness), vast precipitates are found and recognized as Fe-rich phases, which brings about degradation in superconductivity. Consequently, the thickness dependence of superconducting transition temperature (Tc) of FeSe thin films is investigated and one of our atmosphere-stable FeSe thin film (127 nm) possesses the highest Tconset/Tczero as 15.1 K/13.4 K on record to date in the class of FeSe thin film with practical thickness. Our results provide a new perspective for exploring the mechanism of superconductivity in FeSe thin film via high-resolution STEM. Moreover, approaches that might improve the quality of FeSe/CaF2 interfaces are also proposed for further enhancing the superconducting performance in this system.

Crystal structure and magnetic properties of LaCa0.143†(4)O0.857†(4)F0.143†(4)Bi0.857†(4)S2.

The synthesis, structure, and magnetic properties of lithium dibarium calcium oxide fluoride di-sulfide are reported. LaCa0.143 (4)O0.857 (4)F0.143 (4)Bi0.857 (4)S2 crystallizes in the tetra-gonal space group P4/nmm. The structure exhibits disorder of the Ca(2+) and Bi(3+) cations, and the O(2-) and F(-) anions. The structure is composed of a stacking of [(O,F)2La2] layers and double [(Bi,Ca)S2] layers. Magnetic property measurements indicate a very small magnetization at 300 K and the existence of weak ferromagnetism at 2 K.

Tuning Superconductivity in FeSe Thin Films via Magnesium Doping.

In contrast to its bulk crystal, the FeSe thin film or layer exhibits better superconductivity performance, which recently attracted much interest in its fundamental research as well as in potential applications around the world. In the present work, tuning superconductivity in FeSe thin films was achieved by magnesium-doping technique. Tc is significantly enhanced from 10.7 K in pure FeSe films to 13.4 K in optimized Mg-doped ones, which is approximately 1.5 times higher than that of bulk crystals. This is the first time achieving the enhancement of superconducting transition temperature in FeSe thin films with practical thickness (120 nm) via a simple Mg-doping process. Moreover, these Mg-doped FeSe films are quite stable in atmosphere with Hc2 up to 32.7 T and Tc(zero) up to 12 K, respectively, implying their outstanding potential for practical applications in high magnetic fields. It was found that Mg enters the matrix of FeSe lattice, and does not react with FeSe forming any other secondary phase. Actually, Mg first occupies Fe-vacancies, and then substitutes for some Fe in the FeSe crystal lattices when Fe-vacancies are fully filled. Simultaneously, external Mg-doping introduces sufficient electron doping and induces the variation of electron carrier concentration according to Hall coefficient measurements. This is responsible for the evolution of superconducting performance in FeSe thin films. Our results provide a new strategy to improve the superconductivity of 11 type Fe-based superconductors and will help us to understand the intrinsic mechanism of this unconventional superconducting system.

Epitaxial growth of BaHfO3 buffer layer and its structure degeneration analysed by Raman spectrum.

BaHfO3 (BHO) has been proposed as a new cap layer material for YBa2Cu3O7-δ (YBCO) coated conductors. Highly c-axis oriented BHO cap layer has been deposited on ion-beam assisted deposition-MgO buffered Hastelloy tapes by direct-current-magnetron sputtering method. The epi-growth of BHO films combined with its properties is investigated in details. The degenerated cubic crystal structure of BHO film is confirmed by Raman spectrum analysis. XRD θ-2θ scan, φ-scan and ω-scan reveal an excellent c-axis alignment with good in-plane and out-of-plane textures for BHO cap layers. SEM and AFM investigations show BHO cap layer a dense and crack-free morphology. Subsequently pure c-axis orientation YBCO film was epitaxial grown on such BHO cap layer successfully, shown BaHfO3 a potential cap layer material for coated conductors.