Correlation between site preference of ternary Mn addition in LaAg and superconductivity

Abstract

Investigation of structure, surface morphology, composition, and the superconducting-normal phase diagram of LaAg_1-cMn_c superconducting alloys with nominal composition c=0.0, 0.025, 0.05, 0.1, 0.2, and 0.3, reveals the following. The alloys with c=0, 0.025, and 0.05 are essentially single phase alloys with the actual Mn concentration x same as the nominal one, i.e., c=x, whereas in the alloys with c=0.1, 0.2, and 0.3, the actual Mn concentration of the majority phase (crystalline grains) is x=0.050(1), 0.080(1), and 0.100(1), respectively. The ternary Mn addition does not alter the CsCl structure of the parent compound LaAg. Neither a structural phase transition nor a long-range antiferromagnetic order is observed at any temperature in the range 1.8 K≤T≤50 K in any of the Mn containing alloys. Mn has exclusive La (Ag) site preference in the alloy (alloys) with x=c=0.025 (x≥0.05 or c≥0.1) whereas in x=c=0.05, Mn has essentially no site preference in that all the Mn atoms either occupy the La sites or the Ag sites. When Mn substitutes for La at the La sublattice sites (as is the case in x=c=0.025), conventional phonon-mediated (BCS) superconductivity prevalent in the parent LaAg compound is destroyed but when Mn replaces Ag at the Ag sublattice sites (as in x≥0.05), non-BCS superconductivity at elevated temperatures is observed.