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 superconductorshas 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 parameter⁷ hidden within the superconducting phase of the underdoped (electron-doped) high-T_c superconductor Pr_{2_x}Ce_xCuO_{4_γ} and the newly synthesized electron-doped material La_2-xCe_xCuO_4-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.