Magnetism in Fe-based superconductors

TL;DR

This review summarizes experimental findings on magnetism in Fe-based superconductors, highlighting their phase diagram, magnetic order, spin dynamics, and the resonance in the superconducting state, drawing parallels with cuprates.

Contribution

It provides a comprehensive synthesis of experimental studies on magnetism in Fe-based superconductors, emphasizing similarities and differences with cuprates and analyzing spin interactions and resonance phenomena.

Findings

Parent compounds exhibit magnetic order and structural transitions suppressed by doping.

The spin arrangement in Fe-based superconductors is stripe-like and similar across different compounds.

A resonance in spin excitation spectrum correlates with superconducting transition temperature.

Abstract

In this review, we present a summary of experimental studies of magnetism in Fe-based superconductors. The doping dependent phase diagram shows strong similarities to the generic phase diagram of the cuprates. Parent compounds exhibit magnetic order together with a structural phase transition both of which are progressively suppressed with doping allowing superconductivity to emerge. The stripe-like spin arrangement of Fe moments in the magnetically ordered state shows the identical in-plane structure for the RFeAsO (R=rare earth) and AFe2As2 (A=Sr, Ca, Ba, Eu and K) parent compounds, notably different than the spin configuration of the cuprates. Interestingly, Fe1+yTe orders with a different spin order despite very similar Fermi surface topology. Studies of the spin dynamics in the parent compounds shows that the interactions are best characterized as anisotropic three-dimensional (3D) interactions. Despite the room temperature tetragonal structure, analysis of the low temperature spin waves under the assumption of a Heisenberg Hamiltonian indicates strong in-plane anisotropy with a significant next-near neighbor interaction. In the superconducting state, a resonance, localized in both wavevector and energy is observed in the spin excitation spectrum as in the cuprates. This resonance is observed at a wavevector compatible with a Fermi surface nesting instability independent of the magnetic ordering of the relevant parent compound. The resonance energy (Er) scales with superconducting transition temperature (TC) as Er ~4.9 kBTC consistent with the canonical value of ~5 kBTC observed in the cuprates. Moreover, the relationship between the resonance energy and the superconducting gap, \Delta, is similar to that observed in many unconventional superconductors (Er/2\Delta ~ 0.64).