Metallicity of Ca 2 Cu 6 P 5 with single and double copper-pnictide layers
Here, we report thermodynamic and transport properties, and also theoretical calculations, for Cu-based compound Ca 2 Cu 6 P 5 and compare with CaCu 2 -dP 2 . Both materials have layers of edge-sharing copper pnictide tetrahedral CuP 4 , similar to Fe–As and Fe–Se layers (with FeAs 4 , FeSe 4 ) in the iron-based superconductors. Despite the presence of this similar transition-metal pnictide layer, we find that both Ca 2 Cu 6 P 5 and CaCu 2-δ P 2 have temperature-independent magnetic susceptibility and show metallic behavior with no evidence of either magnetic ordering or superconductivity down to 1.8 K CaCu 2-δ P 2 is slightly off-stoichiometric, with δ = 0.14. Theoretical calculations suggest that unlike Fe 3d-based magnetic materials with a large density of states (DOS) at the Fermi surface, Cu have comparatively low DOS, with the majority of the 3d spectral weight located well below Fermi level. The room-temperature resistivity value of Ca 2 Cu 6 P 5 is only 9 μΩ-cm, due to a substantial plasma frequency and an inferred electron-phonon coupling λ of 0.073 (significantly smaller than that of metallic Cu). Also, microscopy result shows that Cu–Cu distance along the c-axis within the double layers can be very short (2.5 Å), even shorter than metallic elemental copper bond (2.56 Å). The value of dρ/dT for CaCu 2-δ P 2 at 300 K is approximately three times larger than in Ca 2 Cu 6 P 5 , which suggests the likelihood of stronger electron-phonon coupling. This study shows that the details of Cu-P layers and bonding are important for their transport characteristics. In addition, it emphasizes the remarkable character of the DOS of ‘122’ iron-based materials, despite much structural similarities.