Hidden pseudogap and superconductivity in electron doped high-temperature superconductors

The ground state of superconductors is characterized by the long range order of condensed Cooper-pairs. While this is the only order in conventional superconductors, in high-temperature (high-T-c) superconductors the presence of other competing groundstates might be responsible for their complicated phase diagram. A suppression of the accessible electronic states at the Fermi-level in the normal state of high-T-c superconductors which is called pseudogap [1,2] is interpreted as precursor superconductivity [3,4] or as indication of another near-by groundstate which can be separated from the superconducting state by a quantum critical point governing the interplay between the two states [5,6]. Here we report the existence of a second hidden order parameter [7] 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 [8]. Our observation is consistent with the presence of a (quantum) phase transition at T = 0 which is discussed to be a key concept to understand high-temperature superconductivity. The existence of a pseudogap when superconductivity is suppressed excludes clearly precursor superconductivity as its origin [9]. This supports the picture that the physics of high-T-c superconductors is determined by the interplay between competing and coexisting groundstates.