Evolution of superconductivity in isovalent Te-substituted KxFe2-ySe2   crystals
  We report the evolution of superconductivity and the phase diagram of the KxFe2-ySe2-zTez (z=0-0.6) crystals grown by a simple one-step synthesis. No structural transition is observed in any crystals, while lattice parameters exhibit a systematic expansion with Te content. The Tc exhibits a gradual decrease with increasing Te content from Tconset = 32.9 K at z = 0 to Tconset = 27.9 K at z = 0.5, followed by a sudden suppression of superconductivity at z = 0.6. Upon approaching a Te concentration of 0.6, the shielding volume fraction decreases and eventually drops to zero. Simultaneously, hump positions in r-T curve shift to lower temperatures. These results suggest that isovalent substitution of Te for Se in KxFe2-ySe2 crystals suppresses the superconductivity in this system. 
Toshinori Ozaki, Hiroyuki Takeya, Keita Deguchi, Satoshi Demura,   Hiroshi Hara, Tohru Watanabe, Denholme Saleem James, Hiro...
cond-mat.supr-con