Anomalous Magnetic Field Dependence of the Thermodynamic Transition Line in the Isotropic Superconductor [Formula presented]

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality [Formula presented] single crystals. The temperature dependence of the magnetic field [Formula presented] corresponding to the onset of the specific heat anomaly presents a clear positive curvature. [Formula presented] is significantly smaller than the field [Formula presented] for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization presents a strong deviation from the Ginzburg-Landau theory emphasizing the peculiar nature of the superconducting transition in this material.