Signatures of unconventional pairing and gapless superconductivity in the kagome superconductors Cs(V_1-xTi_x)₃Sb₅

Although the recently discovered vanadium-based kagome superconductors AV₃Sb₅ (A = K, Rb, and Cs) are promising candidates for hosting unconventional superconductivity, the true nature of their superconducting gaps remains elusive. Here, we report temperature- and magnetic-field-dependent thermal conductivity (κ) measurements on Ti-substituted Cs(V₁₋ₓTiₓ)₃Sb₅ (x = 0.0–0.06). For samples within the first superconducting dome (0.0 ≤ x ≤ 0.02), κ(T) revealed excitations consistent with a small full gap and a monotonic suppression of the gap minimum with increasing x. In the same range, κ(H) showed that quasiparticles were easily excited and delocalized by small fields along the c axis, resembling the behavior of nodal superconductors. Moreover, the electronic κ approached its normal-state value well below Hc2, uncovering an anomalous gapless superconducting state. These results strongly support unconventional chiral d + id pairing in the first dome, characterized by a fragile full gap susceptible to time–reversal–symmetry–breaking perturbations.