Two distinct superconducting phases and pressure-induced crossover from type-II to type-I superconductivity in the spin-orbit-coupled superconductors BaB i3 and SrB i3

We report two distinct superconducting states with different crystal structures and a crossover from a type-II to a type-I superconductor (SC) in (Ba,Sr)Bi3. The superconducting parameters are revealed to classify two SCs: BaBi3 is in the weak-coupling limit on the basis of ΔC/γnTc∼0.67 and 2Δ/kBTc∼3.28 while SrBi3 is a strong-coupling SC with ΔC/γnTc∼2.41 and 2Δ/kBTc∼6.09. A large Kadowaki-Woods ratio (RKW∼3.53a0) suggests an enhanced electron-electron scattering in BaBi3. With increasing the pressure, the Tc of BaBi3 decreases linearly at first, and then shows an abrupt increase up to 6.2 K at 0.88 GPa. This behavior can be attributed to a pressured-induced structural transition and the resulting variations of spin-orbit coupling and Fermi structures. Tc of SrBi3 is suppressed monotonously by pressure. The Ginzburg-Landau parameter κGL of BaBi3 decreases from 10.35 at ambient pressure (AP) to 0.86 at 1.75 GPa, and then tends to saturation. κGL of SrBi3 decreases from 0.76 at AP to 1/122 at 1.20 GPa, which manifests pressured-induced crossover from a type-II to a type-I SC. Possible physical mechanisms are proposed.