Theory of Valley-Density Wave and Hidden Order in Iron-Pnictides

TL;DR

This paper develops a theoretical framework for understanding the complex density wave orders in iron-pnictides, revealing a hidden order that explains structural transitions and coexistence of different density waves.

Contribution

It introduces a unified symmetry-based theory of density waves in iron-pnictides, predicting hidden orders and their role in structural and magnetic transitions.

Findings

Prediction of coexisting SDW and PDW with perpendicular orientations

Explanation of structural transition as a consequence of hidden order

Identification of observable signatures like orbital ferromagnetism

Abstract

In the limit of perfect nesting, the physics of iron-pnictides is governed by the density wave formation at the zone-edge vector M. At high energies, various spin- (SDW), charge- (CDW), orbital/pocket- (PDW) density waves, and their linear combinations, all appear equally likely, unified within the unitary order parameter of U(4)XU(4) symmetry. Nesting imperfections and low-energy interactions reduce this symmetry to that of real materials. Nevertheless, the generic ground state preserves a distinct signature of its highly symmetric origins: a SDW along one axis of the iron lattice is predicted to coexist with a perpendicular PDW, accompanied by weak charge currents. This "hidden" order induces the structural transition in our theory, naturally insures T_s >= T_N, and leads to orbital ferromagnetism and other observable consequences.