Optical study of the antiferromagnetic ordered state in electron-overdoped Ca0.77Nd0.23FeAs2

TL;DR

This study uses optical spectroscopy to analyze the antiferromagnetic state in overdoped Ca0.77Nd0.23FeAs2, revealing localization effects and contrasting behavior with other iron-based superconductors.

Contribution

It provides new insights into the magnetic order mechanism in overdoped iron pnictides, highlighting a localized origin rather than Fermi surface nesting.

Findings

Spectral weight decreases at low frequencies upon AFM transition.

No spin-density-wave gap observed in optical spectra.

Spectral weight shifts to higher frequencies, indicating localization.

Abstract

In Ca1-xRExFeAs2 (RE= rare earth), an antiferromagnetic (AFM) phase as well as a structural transition has been reported, even in the electron-overdoped regime. Here we investigated the temperature-dependent in-plane optical spectroscopy of overdoped Ca0.77Nd0.23FeAs2. Upon entering the AFM state, we found an abrupt reduction of low-frequency (500-2 000 cm-1) spectral weight in the optical conductivity. In sharp contrast to the parent compounds of 122 system, where spin-density-wave gaps have been clearly observed in the AFM state, a gap signature is absent in Ca0.77Nd0.23FeAs2. This may be a consequence of the poor nesting condition between hole and electron pockets. However, a spectral weight analysis shows that the reduced spectral weight at low frequency is transferred to the high frequency range (> 4 000 cm-1), pointing to a localization effect. These observations suggest that the AFM order in Ca0.77Nd0.23FeAs2 is most likely to originate from a localized nature rather than Fermi surface nesting.