  We have investigated the vortex-glass transition for the cuprate
superconductor
  Nd2-xCexCuOy (NCCO) in the region of low temperatures (T < 4.4 K) and
moderate up to high magnetic fields (3 T < B < 5 T) compared to the upper
critical field Bc2 (Bc2 approx. 6 T for NCCO). We analyzed our data using the
vortex-glass theory and found an excellent scaling behavior of the
current-voltage characteristics in this temperature region. The critical
exponents have been calculated and were compared to the few available
investigations for NCCO and to the results for other materials. Additionally we
determined the melting line for low temperatures and compared it to the
available literature data. In this way we could draw a complete picture of the
mixed-state phase diagram for this material. The knowledge about the phase
diagram is crucial for the understanding of the transport properties of the
high-Tc cuprates in the mixed state. It is found that for NCCO, thermal
fluctuations play an important role even at temperatures below 4.2 K. This is
possibly due to the relatively high anisotropy of this material. An
extrapolation procedure is proposed in order to overcome our experimental
limitation of the temperature range and to obtain the melting line for T > 4.4
K. A kink in the extrapolated portion of the melting line is found and
explained in terms of a loss of universality of the critical exponents for
temperatures T > 6 K.
