Controllable Phase Transition for Layered β-FeSe Superconductor Synthesized by Solution Chemistry

Low-temperature synthesis of beta-FeSe superconductor from soluble precursors is a great challenge in the chemical solution approaches. Here, we develop a new and facile solution-based synthetic route to first fabricate narrow-phased beta-FeSe superconductor with soluble iron and selenium sources as starting materials. The growth mechanism of beta-FeSe superconductors is discussed by kinetically controllable syntheses in various reaction conditions. Chemically engineering the stoichiometry of beta-FeSe products by selenium-diffusion process gives rise to a transition of antiferromagnetic-superconducting-antiferromagnetic (AFM-SC-AFM) order. Once the AFM order is suppressed, SC beta-FeSe nanosheets show a tunable initial superconducting transition temperature (T-C) from 3.2 to 10 K in the superconducting regime. Electrical measurements on superconducting beta-FeSe exhibit an upper critical magnetic field higher than 14 T, showing potential application of beta-FeSe nanosheet for superconducting device. This method provides guidance for future applications in such chemical solutions for diffusion controlled synthesis of narrow-phased functional materials, which are enriched of abundant fundamental physics and potentials for future applications.