Field Effect on the Superconducting Magnetic Gap in Nd$_{2-x}$Ce$_x$CuO$_4$

Superconductivity in the cuprates occurs in close proximity to antiferromagnetic phases, and determining the nature of these phases is essential in arriving at a satisfactory understanding of these complex oxides. A unique opportunity exists in the prototypical electron-doped compound Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO), for which the upper critical field is relatively small ($\sim 10$ T). Although a magnetic field enhances the elastic response at the antiferromagnetic position $(\pi,\pi)$, it has been shown [1] that this effect is the paramagnetic response of the epitaxial secondary phase (Nd,Ce)$_2$O$_3$, and not due to the weakening of superconductivity. Our recent inelastic neutron scattering results demonstrate a real field effect on the magnetic excitations at $(\pi,\pi)$. It is known that in NCCO, these excitations become gapped below the superconducting critical temperature $T_c$ [2]. Here, we report on the effect of a magnetic field on this gap. In addition to the field-effect results, we report on the doping-dependence of two-dimensional antiferromagnetic correlations in non-superconducting oxygen-reduced NCCO, and compare the results to previous studies on as-grown NCCO [3]. [1] P.K. Mang et al, Nature 426, 139 (2003); Phys. Rev. B 70, 094507 (2004). [2] K. Yamada et al., Phys. Rev. Lett. 90, 137004 (2003). [3] P.K. Mang et al, Phys. Rev. Lett. 93, 027002 (2004).