1. Department of Physics, Stanford University, Stanford, California 94305, USA
   
2. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
   
3. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
   
4. Stanford Synchrotron Radiation Laboratory, Stanford, California 94309, USA

Correspondence to: M. Greven^3,4 Correspondence and requests for materials should be addressed to M.G. (Email: greven@stanford.edu).

High-transition-temperature (high-T_c) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping^{1, 2} and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd_2-xCe_xCuO₄ (NCCO) shows bulk superconductivity above x  0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x  0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases⁷. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies^{8, 9, 10, 11}, arises from a build-up of spin correlations, in agreement with recent theoretical proposals^{12, 13}.

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