Emergent ferromagnetism with superconductivity in Fe(Te,Se) van der Waals Josephson junctions.

Ferromagnetism and superconductivity are two key ingredients for topological superconductors, which can serve as building blocks of fault-tolerant quantum computers. Adversely, ferromagnetism and superconductivity are typically also two hostile orderings competing to align spins in different configurations, and thus making the material design and experimental implementation extremely challenging. A single material platform with concurrent ferromagnetism and superconductivity is actively pursued. In this paper, we fabricate van der Waals Josephson junctions made with iron-based superconductor Fe(Te,Se), and report the global device-level transport signatures of interfacial ferromagnetism emerging with superconducting states for the first time. Magnetic hysteresis in the junction resistance is observed only below the superconducting critical temperature, suggesting an inherent correlation between ferromagnetic and superconducting order parameters. The 0-π phase mixing in the Fraunhofer patterns pinpoints the ferromagnetism on the junction interface. More importantly, a stochastic field-free superconducting diode effect was observed in Josephson junction devices, with a significant diode efficiency up to 10%, which unambiguously confirms the spontaneous time-reversal symmetry breaking. Our work demonstrates a new way to search for topological superconductivity in iron-based superconductors for future high Tc fault-tolerant qubit implementations from a device perspective.

Atomic Properties of Monoclinic Ag2Se Thin Film Grown on SrTiO3 Substrate by Molecular Beam Epitaxy.

Silver chalcogenides have attracted a great deal of interest due to their promise for exhibiting novel topological properties. Using scanning tunneling microscopy/spectroscopy (STM/S), we have characterized the atomic structure and electronic properties of a monoclinic Ag2Se thin film, similar to ß-Ag2Te, grown on a SrTiO3 (STO)(001) substrate by molecular beam epitaxy (MBE). Three different types of Ag2Se atomic terminations are observed on the surface: (i) homogeneous hexagonal-like, (ii) rough mixed, and (iii) flat zigzag-striped structures. Structural analysis indicates that the different atomic terminations stem from different growth directions, which can be attributed to the lattice mismatch between the substrate and the Ag2Se film. STS analysis of these atomic terminations uncovers different features near the Fermi level, indicating constituent- and direction-dependent electronic properties. This Letter presents a practical method to grow monoclinic thin film Ag2Se and provides insight into its physical properties.

Interfacial Superconductivity Achieved in Parent AEFe2As2 (AE = Ca, Sr, Ba) by a Simple and Realistic Annealing Route.

Materials with interfaces often exhibit extraordinary phenomena exemplified by rich physics, such as high-temperature superconductivity and enhanced electronic correlations. However, demonstrations of confined interfaces to date have involved intensive effort and fortuity, and no simple path is consistently available. Here, we report the achievement of interfacial superconductivity in the nonsuperconducting parent compounds AEFe2As2, where AE = Ca, Sr, or Ba, by simple subsequent annealing of the as-grown samples in an atmosphere of As, P, or Sb. Our results indicate that the superconductivity originates from electron transfer at the interface of the hybrid van der Waals heterostructures, consistent with the two-dimensional superconducting transition observed. The observations suggest a common origin of interfaces for the nonbulk superconductivity previously reported in the AEFe2As2 compound family and provide insight for the further exploration of interfacial superconductivity.

Atomic Structure and Electronic Properties of Intermetallic CaBi2 Thin Films.

Intermetallic bismuth-based compounds have attracted great interest as promising candidates for novel topological superconductivity. Among them, CaBi2 is a newly discovered member for which the atomic structure and electronic properties have never been systematically explored. Using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically characterized the atomic structure and electronic properties of CaBi2(010) thin films grown by molecular beam epitaxy (MBE) and found that their growth follows a Stranski-Krastanov mode. A nonreconstructed IBi layer and a (1 × 2) reconstructed IICa layer were found to be the most common surfaces. Nonreconstructed IIIBi and VCa layers were further exposed with reduced bismuth growth flux. All of these constituent layers exhibit unique features in the STS spectra, indicating that unique electronic properties exist in each specific constituent layer. Our findings provide for deeper understanding of the physical properties of this compound and suggest further studies of the two-dimensional (2D) layered materials family.