High-pressure growth of SmFeAsO$_{1-x}$ single crystals without fluorine doping

We synthesized fluorine-free SmFeAsO$_{1-x}$ single crystals for a nominal value of $x$=0.15 at a pressure of 3.3 GPa and at a temperature of 1350-1450 $^{\circ}$C by using a self-flux method. Plate-shaped single crystals were obtained with the lateral size of a few-150 $\mu $m. Single crystals showed the superconducting transition at about 53.5 K with a narrow resistive transition width of 0.5 K. The synchrotron-irradiated X-ray diffractometry (XRD) peaks and the high-resolution scanning transmission electron microscopy (HR-STEM) images indicate the good crystallinity of our single crystals, which have a well-defined layered tetragonal structure. The chemical composition of the crystals was examined with the electron-probe X-ray microanalysis. A sharp transition, a low residual resistivity, and a large residual resistivity ratio indicate high quality of our single crystals. Fluorine-free single crystals of Fe-based superconductors are expected to provide an easier route to find the optimum conditions for the crystal formation in this family of materials.