  The quantum spin fluctuations of the S = 1/2 Cu ions are important in
determining the physical properties of the high-transition temperature
(high-Tc) copper oxide superconductors, but their possible role in the electron
pairing for superconductivity remains an open question. The principal feature
of the spin fluctuations in optimally doped high-Tc superconductors is a well
defined magnetic resonance whose energy (Er) tracks Tc (as the composition is
varied) and whose intensity develops like an order parameter in the
superconducting state. We show that the suppression of superconductivity and
its associated condensation energy by a magnetic field in the electron-doped
high-Tc superconductor, Pr0.88LaCe0.12CuO4-d (Tc = 24 K), is accompanied by the
complete suppression of the resonance and the concomitant emergence of static
antiferromagnetic (AF) order. Our results demonstrate that the resonance is
intimately related to the superconducting condensation energy, and thus suggest
that it plays a role in the electron pairing and superconductivity.
