Unveiling Quasiparticle Excitations in FeSe_0.45Te_0.55: A Thermal Transport Perspective on Superconducting Symmetry

We present a comprehensive investigation of the superconducting and transport properties of FeSe_0.45Te_0.55 single crystals using magnetic susceptibility, electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. Magnetization and resistivity data confirm bulk superconductivity with a sharp transition near 14.5 K, marked by strong Meissner screening and zero resistance. The Seebeck coefficient exhibits a sign change near 107 K, indicating a crossover in dominant carrier type from holes to electrons, consistent with multiband conduction. A distinct anomaly at 14.5 K aligns with the superconducting onset, while a dip near 30 K suggests enhanced scattering or band reconstruction. Thermal conductivity measurements reveal corresponding anomalies at ~12.7 K and ~107 K, highlighting the interplay between carrier dynamics and heat transport. Low-temperature κ/T data under varying magnetic fields were used to probe the superconducting gap symmetry. A finite residual linear term of 0.01346 WK^-2m^-1 in κ/T as (T to 0) at zero field provides direct evidence for low-energy quasiparticles, ruling out a fully gapped s-wave state. The monotonic field dependence and near-quadratic scaling with (H²) are consistent with vortex-induced quasiparticle transport and the Volovik effect, supporting a nodal or strongly anisotropic gap structure. These results, grounded in bulk-sensitive transport measurements, confirm the presence of unconventional superconductivity in FeSe_1-xTe_x and establish it as a robust platform for exploring correlated electronic states and pairing mechanisms.