Abstract
In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe(1-x)CoxAs. We reveal a large and robust superconducting gap on a band sinking below the Fermi level on Co substitution. The observed Fermi-surface-free superconducting order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the order parameter is not tied to the Fermi surface as a result of a surface instability.
Highlights
In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface instability, in which the low-energy electrons are paired by attractive interactions
The pairing is caused by local antiferromagnetic exchange couplings, well defined in the real space, which lead to a SC order parameter (OP) that is fixed in the momentum space and relatively insensitive to small changes of the electronic structure near the Fermi surface (FS)
Since the cut on the g band measures the largest gap on the electron FSs15,16, our results prove that the SC gap on the a band is the largest over the momentum space and rule out the possibility of a proximity effect causing by the pairing on other FS sheets
Summary
In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface instability, in which the low-energy electrons are paired by attractive interactions. Two main categories of theoretical descriptions arise when trying to describe the high-Tc superconductivity of the iron-based superconductors (IBSCs): the weak coupling approach, which involves only the low-energy electronic structure near the Fermi level (EF) (refs 1–5), and the strong coupling approach, which emphasizes the local magnetic moments and strong Coulomb interactions[6,7,8,9,10]. In the former, superconductivity emerges as a Fermi surface (FS) instability and is, in principle, sensitive to FS changes. The observed FS-free SC gap is the largest over the momentum space, which rules out a proximity effect origin and indicates that the SC OP is not tied to the FS as a result of a FS instability
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