Hydrostatic high-pressure studies to 25 GPa on the model superconducting pnictide LaRu_2P_2

摘要

Prior to the discovery of the Fe-pnictides in 2008, the ruthenium phosphide LaRu_2P_2 possessed the highest value of the superconducting transition temperature, T_c ≈ 4 K, in the entire pnictide family. Recently, there has been renewed interest in this compound in an effort to better understand why the Fe-pnic tides have much higher values of Tc. In related phosphides superconductivity appears to only be present if the separation between the phosphor ions d_(p-p) in neighboring Ru_2P_2 planes is greater than the critical value 2.8 ?, too great for a P-P covalent bond to be formed. For example, in superconducting LaRu_2P_2, the value of dp_p is 3.0 ?. To test these ideas directly, we have carried out hydrostatic high-pressure studies on single-crystalline LaRu_2P_2 in a diamond-anvil cell using He pressure medium to pressures as high as 25 GPa and temperatures as low as 1.5 K. We find that T_c initially increases under pressure, but suddenly disappears above 2.1 GPa. Since d_(p-p) decreases under pressure, the sudden disappearance of superconductivity is likely due to the formation of a covalent P-P bond between adjacent Ru_2P_2 planes and a possible structural phase transition.