Interface-enhanced superconductivity in multi-grain (FeSe)η(SrTiO3)1-η composites

Interface superconductivity, realized in multiple artificial crystalline heterostructures, is one of the most exciting directions to search for high-temperature superconductivity. In this work, we prepare bulk (FeSe)(eta)(SrTiO3)(1-eta) multi-grain composites by a simple facile liquid-phase compaction method using a spark-plasma-sintering technique. Combining transmission electron microscopy/scanning electron microscopy and x-ray diffraction investigations, we demonstrate that the composites consist of micron-scale SrTiO3 grains surrounded by [001]-compressed beta-FeSe grains. Transport measurements for the composites with FeSe mole fraction eta > 0.06 reveal that two superconducting channels, one T-c similar to 13 K phase from FeSe/SrTiO3 interfaces and another T-c similar to 7 K phase from FeSe grains, cooperatively induce macroscopic superconducting behavior with isotropic upper critical fields above 40 T. This work points out a straightforward method to enhance T-c in the multi-grain (FeSe)(eta)(SrTiO3)(1-eta) composites by reducing the crystalline grains to nanoscale and finely tuning the stoichiometries of FeSe and SrTiO3.