Raman Spectroscopy of the electron-doped cuprates in magnetic field

We investigate the influence of magnetic field on the electronic excitations across the superconducting (SC) gap of the electron-doped cuprates R$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ (R = Pr, Nd) for 0.13$<$x$<$0.18. We report the anomalous result that the 2$\Delta $ coherence peak energy decreases rapidly with increasing field. In sharp contrast, the magnetic field has only a weak effect on the coherence peak energy in the hole-doped cuprates and conventional superconductors. We determine effective upper critical field lines H$^{\ast }_{c2}$ at which the superfluid stiffness vanishes and H$^{2\Delta }_{c2}$ at which the SC amplitude is suppressed. We find that H$^{2\Delta }_{c2}$ is larger than H$^{\ast }_{c2}$ for all dopings, especially for x\underline {$<$}0.15. Both H$^{2\Delta }_{c2}$ and H$^{\ast }_{c2}$ decrease more rapidly with doping than T$_{c}$ or 2$\Delta $ for x$>$0.15 (over-doped). The rapid increase of SC coherence length (several hundred Angstrom) with increasing doping implies a weak pair potential with less momentum dependence. This work was supported by NSF grant No. DMR 01-02350.