  In contrast to hole-doped cuprates, electron-doped cuprates consistently
exhibit strong antiferromagnetic correlations with a commensurate ({\pi},
{\pi}) ordering wave vector, leading to the prevalent belief that
antiferromagnetic spin fluctuations mediate Cooper pairing in these
unconventional superconductors. However, early investigations produced two
paradoxical findings: while antiferromagnetic spin fluctuations create the
largest pseudogap at "hot spots" in momentum space, Raman scattering and
angle-resolved photoemission spectroscopy measurements using the leading-edge
method seem to suggest the superconducting gap is also maximized at these
locations. This presented a dilemma for spin-fluctuation-mediated pairing:
Cooper pairing is strongest at momenta where normal state low energy spectral
weight is most suppressed. Here we investigate this dilemma in Nd2-xCexCuO4
using angle-resolved photoemission spectroscopy under significantly improved
experimental conditions. The unprecedented signal-to-noise ratio and resolution
allow us to directly observe the Bogoliubov quasiparticles, demonstrating the
existence and importance of two sectors of states: 1. The reconstructed main
band and the states gapped by the antiferromagnetic pseudogap around the hot
spots. 2. The gossamer Fermi surface states with distinct dispersion inside the
pseudogap, from which Bogoliubov quasiparticle coherence peaks emerge below Tc.
Supported by numerical results, we propose that the non-zero modulus of the
antiferromagnetic order parameter causes the former, while fluctuations in the
antiferromagnetic order parameter orientation are responsible for the latter.
Our revelations of the gossamer Fermi surface reconcile the paradoxical
observations, deepening our understanding of superconductivity in
electron-doped cuprates in particular, and unconventional superconductivity in
general.
