title: A hidden pseudogap under the 'dome' of superconductivity in electron-doped high-temperature superconductors.
abstract: 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--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 or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point. Here we report the existence of a second order parameter hidden within the superconducting phase of the underdoped (electron-doped) high-T(c) superconductor Pr2-xCe(x)CuO4-y and the newly synthesized electron-doped material La2-xCe(x)CuO4-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.
authors.affiliation: Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany. lambert.alff@wmi.badw.de
authors.forename: L
authors.initials: L
authors.lastname: Alff
authors.forename: Y
authors.initials: Y
authors.lastname: Krockenberger
authors.forename: B
authors.initials: B
authors.lastname: Welter
authors.forename: M
authors.initials: M
authors.lastname: Schonecke
authors.forename: R
authors.initials: R
authors.lastname: Gross
authors.forename: D
authors.initials: D
authors.lastname: Manske
authors.forename: M
authors.initials: M
authors.lastname: Naito