Temperature dependent change in the symmetry of the order parameter in an electron-doped high-temperature superconductor

We present specific heat measurements which show an unexpected phase transition from d-wave symmetry to s-wave symmetry as the temperature is reduced in electron-doped Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta}$ (PCCO), in both optimal and over-doped single crystals. The field dependence of electronic specific heat (C$_{el}$) is linear at T=2K, consistent with s-wave symmetry, and non-linear, consistent with d-wave symmetry, at T$\geq$3K. This behavior is most consistent with a phase transition in the symmetry of the order parameter as the temperature is changed. Such a phase transition could be an explanation for the previous controversial results of different experiments, performed at different temperatures in the electron-doped cuprates.