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-Tc) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap1,2—a suppression of the accessible electronic states at the Fermi level in the normal state of high-Tc superconductors—has been interpreted as either a precursor to superconductivity3,4 or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point5,6. Here we report the existence of a second order parameter7 hidden within the superconducting phase of the underdoped (electron-doped) high-Tc 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-Tc superconductivity. This supports the picture that the physics of high-Tc superconductors is determined by the interplay between competing and coexisting ground states.