Intrinsic dependence of T_c on hydrostatic (He-gas) pressure for superconducting LaFePO, PrFePO, and NdFePO single crystals

摘要

Since their discovery in 2008, the Fe-based superconductors have attracted a great deal of interest. Regrettably, the mechanism(s) responsible for the superconductivity has yet to be unequivocally identified. High pressure is an important variable since its application moderates the pairing interaction. Thus far, the LnFePO (Ln = La, Pr, Nd, Sm, Gd) family of superconductors has received relatively little attention. Early high-pressure studies on LaFePO found that T_c initially increased with pressure before passing through a maximum at higher pressures. The present studies on both polycrystalline and single-crystalline LaFePO, PrFePO, and NdFePO utilize the most hydrostatic pressure medium available, i.e., dense He. Surprisingly, for all samples, T_c is found to initially decrease rapidly with pressure at the rate dT_c/dP ≌ -2 to-—3 K/GPa. Less hydrostatic pressure media thus appear to enhance the value of T_c in these materials. These results give yet further evidence that the superconducting state in Fe-based superconductors is extraordinarly sensitive to lattice strain.