A hidden pseudogap under the 'dome' of superconductivity in electron-doped high-temperature superconductors

The ground state of superconductors is characterized by the long-range order of condensed Cooper pairs: this is the only order present in conventional superconductors. The high-transition-temperature (high-T-c) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap(1,2)-a suppression of the accessible electronic states at the Fermi level in the normal state of high-T-c superconductors has been interpreted as either a precursor to superconductivity(3,4) or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point(5,6). Here we report the existence of a second order parameter(7) hidden within the superconducting phase of the underdoped (electron-doped) high-T-c superconductor Pr2-xCexCuO4-y and the newly synthesized electron-doped material La2-xCexCuO4-y (ref. 8). The existence of a pseudogap when superconductivity is suppressed excludes precursor superconductivity as its origin. Our observation is consistent with the presence of a (quantum) phase transition at T = 0, which may be a key to understanding high-T-c superconductivity. This supports the picture that the physics of high-T-c superconductors is determined by the interplay between competing and coexisting ground states.