Infrared study of the spin reorientation transition and its reversal in the superconducting state in underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $

TL;DR

This study uses infrared spectroscopy to explore the spin reorientation transition and its reversal in underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, revealing a new double-$ extbf{Q}$ antiferromagnetic phase that impacts superconductivity.

Contribution

It provides experimental evidence of the transition from single-$ extbf{Q}$ to double-$ extbf{Q}$ antiferromagnetic structure and its effects on the superconducting state in underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$.

Findings

A distinct SDW develops in the t-AF phase.

The SDW in the t-AF phase has more low-energy spectral weight.

Additional phonon modes suggest Brillouin-zone folding due to double-$ extbf{Q}$ magnetic structure.

Abstract

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $. In agreement with the predicted transition from a single-$\mathbf{Q}$ to a double-$\mathbf{Q}$ AF structure, we found that a distinct spin density wave (SDW) develops in the t-AF phase. The pair breaking peak of this SDW acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-$\mathbf{Q}$ magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment.