Specific heat of novel ternary superconductors La₃Ni₄X₄ (X = Si and Ge)

A new superconductivity in ternary intermetallics of La₃Ni₄X₄, where X = Si and Ge, has been found. The superconducting critical temperatures, T_cs, of La₃Ni₄Si₄ and La₃Ni₄Ge₄ defined as the onset of diamagnetism are 1.0 and 0.70 K, respectively. These superconductors have U₃Ni₄Si₄-type crystal structure, with a combination of BaAl₄-(ThCr₂Si₂-) and AlB₂-type alternating layers. In this work, we present studies on low temperature specific heat in these nickel compounds to shed light on the superconducting natures. The Sommerfeld coefficient, γ_n, for both compounds showed relatively high values; γ_n = 37.4 mJ/mol K² and 36.9 mJ/mol K² are derived for La₃Ni₄Si₄ and La₃Ni₄Ge₄, respectively. The Debye temperature Θ_D in La₃Ni₄Si₄ is estimated as 321 K, while that in La₃Ni₄Ge₄ is given as 284 K. As well as the electron-phonon coupling constant, the electronic density of states at the Fermi level is estimated. It turned out that these materials have high density of states, which is in common to the analogous of La₃Pd₄Ge₄. A new superconductivity in the compounds with U₃Ni₄Si₄-type structure is discussed based on the thermodynamic properties.