Pressure effect on superconductivity of AxFe2Se2 (A = K and Cs)

We have performed high hydrostatic pressure resistivity measurements (up to 1.7 GPa) on the newly discovered superconductor A(x)Fe(2)Se(2) (A = K and Cs) single crystals. Two batches of single crystals KxFe2Se2 with different transition temperatures (T-c) were used to study the effect of pressure. The T-c of the first one gradually decreases with increasing pressure from 32.6K at ambient pressure. While a dome-like behavior was observed for the crystal with T-c = 31.1 K, T-c reaches its maximum value of 32.7K at the pressure of 0.48 GPa. This indicates that there exists an optimal doping with maximum T-c of 32.7K in the KxFe2Se2 system. The behavior of T-c versus pressure for CsxFe2Se2 also shows a dome-like behavior, and T-c reaches its maximum value of 31.1K at a pressure of 0.82 GPa. The hump observed in the temperature dependence of resistivity for all of the samples tends to shift to high temperature with increasing pressure. The resistivity hump could arise from the vacancy of Fe or Se.