Pressure Tuned Enhancement of Superconductivity and Change of Ground   State Properties in LaO0.5F0.5BiSe2 Single Crystals
  By using a hydrostatic pressure, we have successfully tuned the ground state and superconductivity in LaO0.5F0.5BiSe2 single crystals. It is found that, with the increase of pressure, the original superconducting phase with Tc about 3.5 K can be tuned to a state with lower Tc, and then a new superconducting phase with Tc about 6.5 K emerges. Accompanied by this crossover, the ground state is switched from a semiconducting state to a metallic one. Accordingly, the normal state resistivity also shows a nonmonotonic change with the external pressure. Furthermore, by applying a magnetic field, the new superconducting state under pressure with Tc about 6.5 K is suppressed, and the normal state reveals a weak semiconducting feature again. These results illustrate a non-trivial relationship between the normal state property and superconductivity in this newly discovered superconducting system. 
Jianzhong Liu, Sheng Li, Yufeng Li, Xiyu Zhu, Hai-Hu Wen
cond-mat.supr-con