Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO₃ single crystals

We report on the Nernst effect, specific heat and transport measurements performed in high quality (K,Ba)BiO₃ single crystals close to optimal doping (T_c~31 K). We show that a nonzero Nernst effect remains visible well above the upper critical field unambiguously deduced from the onset of the specific heat anomaly. This finite Nernst signal is attributed to fluctuations of the amplitude of the order parameter in a region where the free energy is smaller than k_BT. Despite the absence of any vortex liquid phase (and hence of any significant phase fluctuations), the field and temperature dependence of the Nernst coefficient is very similar to the one obtained in electron-doped cuprates.