Advanced synchrotron radiation focusing down to a size of 300 nm has been used to visualize nanoscale phase separation in the K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ superconducting system using scanning nanofocus single-crystal x-ray diffraction. The results show an intrinsic phase separation in K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ single crystals at $T$ < 520 K, revealing the coexistence of (i) a magnetic phase characterized by an expanded lattice with superstructures due to Fe vacancy ordering and (ii) a nonmagnetic phase with an in-plane compressed lattice. The spatial distribution of the two phases at 300 K shows a frustrated or arrested nature of the phase separation. The space-resolved imaging of the phase separation permitted us to provide direct evidence of nanophase domains smaller than 300 nm and different micrometer-sized regions with percolating magnetic or nonmagnetic domains forming a multiscale complex network of the two phases.

Twisted interfaces between stacked van der Waals (vdW) cuprate crystals present a platform for engineering superconducting order parameters by adjusting stacking angles. Using a cryogenic assembly technique, we construct twisted vdW Josephson junctions (JJs) at atomically sharp interfaces between Bi₂Sr₂CaCu₂O_8+xcrystals, with quality approaching the limit set by intrinsic JJs. Near 45° twist angle, we observe fractional Shapiro steps and Fraunhofer patterns, consistent with the existence of two degenerate Josephson ground states related by time-reversal symmetry (TRS). By programming the JJ current bias sequence, we controllably break TRS to place the JJ into either of the two ground states, realizing reversible Josephson diodes without external magnetic fields. Our results open a path to engineering topological devices at higher temperatures.

Control of magnetism in heterostructures: The interface between two different materials in a heterostructure can exhibit properties unique to either of the two materials alone. A well-known example is a conducting gas that forms when LaAlO ₃ is grown on SrTiO ₃ , but only if the LaAlO ₃ layer is at least four unit cells thick. Wang et al. report a similarly abrupt magnetic transition in a heterostructure formed by another oxide (LaMnO ₃ ) on the same SrTiO ₃ substrate. Even though bulk LaMnO ₃ is an antiferromagnet, when six or more unit-cell layers of it were deposited on SrTiO ₃ , it behaved like a ferromagnet. Science , this issue p. 716