We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-y}$ and ${\mathrm{Pr}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-y}$ as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature $T_c.$ In contrast, below $T_c$ for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below $T_c.$ Possible interpretations of this observation are the existence of a normal-state pseudogap in the electron doped HTS or the existence of a spatially nonuniform superconducting state.

• Received 9 October 2000

DOI:https://doi.org/10.1103/PhysRevB.63.100507