Flux free growth of superconducting FeSe single crystals

We report flux free growth of superconducting FeSe single crystals by an easy and versatile high temperature melt and slow cooling method for first time. The room temperature x-ray diffraction (XRD) on the surface of the piece of such obtained crystals showed single [101] plane of beta-FeSe tetragonal phase. The bulk powder XRD, being obtained by crushing the part of crystal chunk showed majority (similar to 87%) beta-FeSe tetragonal (space group P4/nmm) and minority (similar to 13%) delta-FeSe hexagonal (space group P6(3)/mmc) crystalline phases. Detailed high resolution transmission electron microscope images along with selected area electron diffraction showed the abundance of both majority beta-FeSe and minority delta-FeSe phases. Both transport (rho-T) and magnetization exhibited superconductivity at below around 10 K. Interestingly, the magnetization signal of these crystals is dominated by the magnetism of minority delta-FeSe magnetic phase, and hence the isothermal magnetization at 4 K was seen to be ferromagnetic like. Transport (rho-T) measurements under magnetic field showed superconductivity onset at below 12 K, and rho = 0 (T-c) at 9 K. Superconducting transition temperature (T-c) decreases with applied field to around 6 Kat 7 T, with dT(c)/dH of similar to 0.4 KT-1, giving rise to an H-c2(0) value of around 50, 30 and 20 T for normal resistivity rho(n) = 90%, 50% and 10% respectively, which are calculated from conventional one band Werthamer-Helfand-Hohenberg equation. FeSe single crystal activation energy is calculated from thermally activated flux flow model which is found to decreases with field from 12.1 meV for 0.2 T to 3.77 meV for 7T.