Atomic characterization on tetragonal FeAs layer and engineering FeAs superlattices is highly desirable to get deep insight into the multi-band superconductivity in iron-pnictides. We fabricate the tetragonal FeAs layer by topotactic reaction of FeTe films with arsenic and then obtain K_xFe₂As₂ upon potassium intercalation using molecular beam epitaxy. The in-situ low-temperature scanning tunneling microscopy/spectroscopy investigations demonstrate characteristic $\sqrt{2}\times \sqrt{2}$ reconstruction of the FeAs layer and stripe pattern of K_xFe₂As₂, accompanied by the development of a superconducting-like gap. The ex-situ transport measurement with FeTe capping layers shows a superconducting transition with an onset temperature of 10 K. This work provides a promising way to characterize the FeAs layer directly and explore rich emergent physics with epitaxial superlattice design.

Chiral switching is a fascinating topic and plays an important role in construction of homochirality. Nevertheless, due to the complexity and flexibility of noncovalent interactions, switching the chirality of entire supramolecular assemblies has hitherto remained a challenge. Here we report the electric field-controlled chirality switching of pentacene pinwheel arrays and two-dimensional (2D) network domains. Pentacene molecules on Cd(0001) surface form the porous network structure with building blocks of hexamer pinwheels. Driven by the electric field from a scanning tunneling microscopy (STM) tip, the supramolecular chirality of pentacene pinwheels and the organizational chirality of entire network domains can be simultaneously switched from one enantiomorph to another. Furthermore, such chiral switching is reversible and repeatable under successive voltage pulses. First-principles calculations demonstrate that electric field significantly modulates the interfacial charge transfer and induces the Coulomb expansion of pentacene layers, and the subsequent reaggregation leads to the chiral flipping of the supramolecular pinwheels and 2D domains. Our results provide a new strategy for dynamic control of the 2D chiral structures and help to steer the supramolecular assembly toward homochirality.

We have studied the magnetic and electrical transport properties of epitaxial NiAs-type CrTe thin films grown on SrTiO₃(111) substrates. Unlike rectangle hysteresis loops obtained from magnetic measurements, we have identified intriguing extra bump/dip features from anomalous Hall experiments on the films with thicknesses less than 12 nm. This observed Hall anomaly is phenomenologically consistent with the occurrence of a topological Hall effect(THE) in chiral magnets with a skyrmion phase. Furthermore, the THE contribution can be tuned by the film thickness, showing the key contribution of asymmetric interfaces in stabilizing Néel-type skyrmions. Our work demonstrates that a CrTe thin film on SrTiO₃(111) substrates is a good material candidate for studying real-space topological transport.

Pb nanobridges with a thickness of less than 10 nm and a width of several hundred nm have been fabricated from single-crystalline Pb films using low-temperature molecular beam epitaxy and focus ion beam microfabrication techniques. We observed novel magnetoresistance oscillations below the superconducting transition temperature (T_C) of the bridges. The oscillations—which were not seen in the crystalline Pb films—may originate from the inhomogeneity of superconductivity induced by the applied magnetic fields on approaching the normal state, or the degradation of film quality by thermal evolution.