Entangled Orbital Triplet Pairs in Iron-Based Superconductors

TL;DR

This paper proposes that the s$^{{plus-minus}}$ pairing in iron-based superconductors involves entangled orbital and angular momentum states, explaining nodal gaps and phase transitions through a high-to-low spin transition of Cooper pairs.

Contribution

It introduces a novel orbital entanglement framework for understanding pairing symmetry and phase transitions in iron-based superconductors.

Findings

Orbital degrees of freedom lead to internal d-wave structure in s$^{{plus-minus}}$ pairs.

Nodal gap in KFe$_{2}$As$_{2}$ explained as high spin orbital configuration.

Pressure-induced transition interpreted as a high-to-low spin phase change.

Abstract

A key question in high temperature iron-based superconductivity is the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. While electronically paired superconductors generally avoid the Coulomb interaction through the formation of nodal, higher angular momentum pairs, iron based superconductors appear to form singlet s-wave (s$^{\pm}$) pairs. By taking the orbital degrees of freedom of the iron atoms into account, here we argue that the s$^{\pm}$ state in these materials possesses internal d-wave structure, in which a relative d-wave ($L=2$) motion of the pairs entangles with the ($I=2$) internal angular momenta of the d-orbitals to form a low spin $J=L+I=0$ singlet. We discuss how the recent observation of a nodal gap with octahedral structure in KFe$_{2}$As$_{2}$ can be understood as a high spin ($J=L+I=4$) configuration of the orbital and isospin angular momenta; the observed pressure-induced phase transition into a fully gapped state can then interpreted as a high-to-low spin phase transition of the Cooper pairs.