Enhanced Electron–Phonon Coupling of Superconductivity in Indium-Doped Topological Crystalline Insulator SnTe

Indium-doped SnTe (Sn_1-xIn_xTe) provides a model platform for exploring the emergence of superconductivity within a topological crystalline insulator. Here, we present a systematic investigation of the structural, transport, and thermodynamic properties of high-quality single crystals with 0.0 ≤ x ≤ 0.5. All compositions up to x = 0.4 form a single-phase cubic structure, enabling a controlled study of the superconducting state. Electrical resistivity and specific heat measurements reveal a bulk, fully gapped s-wave superconducting phase whose transition temperature increases monotonically with In concentration, reaching <i>Tc</i> ≈ 4.7 K at x = 0.5. Analysis of the electronic specific heat and McMillan formalism shows that the electron-phonon coupling constant <i>λ_el_-_ph</i> systematically increases with doping, while the Debye temperature systematically decreases, resulting in the lattice softening. This behavior, together with the observed evolution of the normal-state resistivity exponent from Fermi-liquid (<i>n</i> ≈ 2.04) toward non-Fermi-liquid values (<i>n</i> ≈ 1.72), demonstrates a clear crossover from weak to strong interaction with increasing In content. These results establish Sn_1-xIn_xTe as a tunable superconducting system in which coupling strength can be continuously controlled, offering a promising platform for future studies on the interplay between phonon-mediated superconductivity and crystalline topological band structure.