Single crystal growth and characterization of tetragonal FeSe1-x superconductors

The plate-like single crystals of tetragonal (P4/nmm) FeSe1-x superconductors were grown using the KCl-AlCl3 flux technique which produced single crystalline tetragonal samples of about 4 x 4 x 0.1 mm(3) dimensions. The energy dispersive X-ray spectroscopy established a ratio of Fe : Se = 1 : 0.96 +/- 0.02. The resistivity and magnetization measurements revealed a sharp superconducting transition at T-c = 9.4 K. Multiple Andreev reflections spectroscopy pointed to the existence of two-gap superconductivity with the gap values Delta(L) = 2.4 +/- 0.2 meV and Delta(S) = 0.75 +/- 0.1 meV at 4.2 K.