Low temperature specific heat of superconducting ternary intermetallics La₃Pd₄Ge₄, La₃Ni₄Si ₄, and La₃Ni₄Ge₄ with U ₃Ni₄Si₄-type structure

A systematic investigation on the thermodynamic properties of La-based ternary intermetallic superconductors crystallizing in a U₃Ni ₄Si₄-type structure is presented. The U₃Ni ₄Si₄-type structure consists of a characteristic intergrowth of periodic BaAl₄ (ThCr₂Si₂)- and AlB₂-type segments. Pristine low temperature specific heat data for recently discovered members La₃Ni₄Si₄ and La₃Ni₄Ge₄ with T_cs of 1.0 and 0.7 K, respectively, are presented as well as La₃Pd₄Ge ₄ with the highest T_c of 2.5 K in the U₃Ni ₄Si₄-type group. Owing to the higher T_cs of U₃Ni₄Si₄-type superconductors than the related ThCr₂Si₂-type compounds, comparisons are drawn in our investigations of the ternary intermetallics of LaPd₂Ge₂, LaNi₂Si₂, and LaNi₂Ge₂ having a ThCr₂Si₂-type structure. Our investigations of the thermodynamic properties show that La₃Ni₄Si₄ and La₃Ni₄Ge₄ have higher values of γ_n, N(E_F), and Θ_D than La ₃Pd₄Ge₄. The same trend was found in ThCr ₂Si₂-type compounds of LaPd₂Ge₂, LaNi₂Si₂, and LaNi₂Ge₂. It turns out that the difference in T_c between La₃Pd ₄Ge₄, La₃Ni₄Si₄, and La₃Ni₄Ge₄, as well as the relatively higher T_c of the U₃Ni₄Si₄-type superconductors than of the related ThCr₂Si₂-type compounds, are largely due to the strength of electron-phonon coupling.