Experimental and first-principles studies on superconductivity in noncentrosymmetric La3Se4

We report the synthesis and detailed characterization of superconducting ${\mathrm{La}}_{3}{\mathrm{Se}}_{4}$, with ${T}_{c}\ensuremath{\sim}8.5\ifmmode\pm\else\textpm\fi{}0.1$ K, using x-ray diffraction, electrical transport, magnetization, and heat capacity measurements. ${\mathrm{La}}_{3}{\mathrm{Se}}_{4}$ crystallizes in the noncentrosymmetric cubic ${\mathrm{Th}}_{3}{\mathrm{P}}_{4}$-type structure with space group $I\overline{4}3d$. Characteristic superconducting parameters such as the lower critical field, upper critical field, thermodynamic critical field, coherence length, penetration depth, and Ginzburg-Landau parameter have been determined. The specific heat jump at ${T}_{c}$, $\mathrm{\ensuremath{\Delta}}C/\ensuremath{\gamma}{T}_{c}=2.04\ifmmode\pm\else\textpm\fi{}0.05$, exceeds the value for a weakly coupled BCS superconductor, and the electron-phonon coupling constant is found to be ${\ensuremath{\lambda}}_{ep}=0.87\ifmmode\pm\else\textpm\fi{}0.02$, suggesting superconductivity in ${\mathrm{La}}_{3}{\mathrm{Se}}_{4}$ is in the strong-coupling regime. The estimated upper critical field is well below the calculated Pauli limit, and the Maki parameter value ($\ensuremath{\alpha}<$ 1) indicates that the superconducting upper critical field is dominated by orbital pair breaking. From density functional theory based first-principles simulations we observe the number of states at the Fermi energy is dominated mainly by $d$ and $f$ electrons of La. Furthermore, we observe band crossings along the high-symmetry $k$ lines in the vicinity of the Fermi energy. These bands are observed to split due to the removal of spin degeneracy associated with spin-orbit coupling, with the splitting energy ${E}_{\mathrm{ASOC}}\ensuremath{\approx}$ 65 meV.