Ambient-pressure synthesis of single-crystal MgB2 and their superconducting anisotropy

We synthesized single crystalline ${\mathrm{MgB}}_{2}$ under ambient pressure by using conventional materials and equipment. The single crystals of ${\mathrm{MgB}}_{2}$ were of good quality, where the crystal structure refinements were successfully converged with $R=0.020.$ The measurements of the magnetic properties yielded a sharp superconducting transition at 38 K with transition width $\ensuremath{\Delta}{T}_{c}=0.8\mathrm{K}.$ The upper critical field for applied field parallel to the ab plane ${(H}_{c2}^{\mathrm{ab}})$ reveals a positive curvature, while ${H}_{c2}$ parallel to the c axis ${(H}_{c2}^{c})$ increases linearly in temperature dependence, which yields a temperature dependence of the superconducting anisotropy ratio of $\ensuremath{\gamma}{=H}_{c2}^{\mathrm{ab}}{/H}_{c2}^{c}$ with $\ensuremath{\gamma}\ensuremath{\sim}1$ near ${T}_{c}$ and 4.0 at 25 K.