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Quantum-Critical Spin-Density Waves in Iron-Selenide High-Tc Superconductors

Hidden spin-density waves (hSDW) with Neel ordering vector (pi,pi) have been proposed recently as parent groundstates to electron-doped iron-selenide superconductors. Doping such groundstates can result in visible electron-type Fermi surface pockets and faint hole-type Fermi surface pockets at the corner of the folded Brillouin zone. A Cooper pair instability that alternates in sign between the electron-type and the hole-type Fermi surfaces has recently been predicted. The previous is due to the interaction of electrons and holes with hidden spin fluctuations connected with hSDW order that is near a quantum-critical point. Quantum criticality is tuned in by increasing the strength of Hund's Rule from the hSDW state. We find that the exchange of hidden spin fluctuations by electrons/holes in the critical hSDW state results in asymptotic freedom. In particular, the strength of spin-flip interactions becomes weaker and weaker on length scales that are shorter and shorter compared to the range of hSDW order. We then argue that string states that connect well-separated particle/hole excitations in the hSDW are robust. This suggests a picture where the hole degrees of freedom mentioned previously are confined.