Elementary Excitations due to Orbital Degrees of Freedom in Iron Based Superconductors

TL;DR

This paper reviews the role of orbital degrees of freedom in elementary excitations of iron-based superconductors, proposing that analyzing these excitations could distinguish between orbital- and spin-based theories of the structural phase transition.

Contribution

It highlights the potential of elementary excitations to differentiate orbital-based mechanisms from spin-based ones in the structural transition of iron superconductors.

Findings

Elementary excitations differ fundamentally between orbital- and spin-based scenarios.

Bulk thermodynamic measurements cannot distinguish the two scenarios.

Theories suggest excitations as a key to resolving the debate.

Abstract

One central issue under intense debate in the study of the iron based superconductors is the origin of the structural phase transition that changes the crystal lattice symmetry from tetragonal to orthorhombic. This structural phase transition, occurring universally in almost every family of the iron-based superconductors, breaks the lattice $C_4$ rotational symmetry and results in an anisotropy in a number of physical properties. Due to the unique topology of the Fermi surface, both orbital- and spin-based scenarios have been proposed as the driving force. In this review, we focus on theories from the orbital-based scenario and discuss several related experiments. It is pointed out that although both scenarios lead to the same macroscopic phases and are not distinguishable in bulk measurements of the thermodynamic properties, the elementary excitations could be fundamentally different, and provide us with the possibility to resolve this long-standing debate between orbital- and spin-based theories.