Pressure-restored superconductivity in Cu-substituted FeSe

Copper doping of FeSe destroys its superconductivity at ambient pressure, even at low doping levels. Here we report the pressure-dependent transport and structural properties of Fe_1.01-xCu_xSe with 3% and 4% Cu doping and find that the superconductivity is restored. Metallic resistivity behavior, absent in Cu-doped FeSe, is also restored. At the low pressure of 1.5 GPa, superconductivity is seen at 6 K for 4% Cu doping, somewhat lower than the 8 K T_c of undoped FeSe. T_c reaches its maximum of 31.3 K at 7.8 GPa, lower than the maximum superconducting temperature in the undoped material under pressure (T_c max of 37 K) but still very high. X-ray diffraction shows that applied pressure decreases the lattice parameter in the basal plane, counteracting the structural effect of Cu doping, providing a possible explanation for the restoration of the superconductivity.