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

TL;DR

This paper investigates the quantum-critical behavior of hidden spin-density waves in iron-selenide superconductors, revealing how spin fluctuations influence electron interactions and potentially lead to confinement of hole states.

Contribution

It introduces a novel analysis of quantum-critical hSDW states, showing asymptotic freedom of spin-flip interactions and the robustness of string states connecting particle-hole pairs.

Findings

Spin-flip interactions weaken at short distances near quantum criticality.

Hidden spin fluctuations induce asymptotic freedom in electron-hole interactions.

String states connecting particle-hole excitations are robust, implying confinement of holes.

Abstract

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.