Muon spin rotation and infrared spectroscopy study of magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $

TL;DR

This study investigates the interplay between magnetism and superconductivity in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ using muon spin rotation and infrared spectroscopy, revealing how magnetic moments and superconducting properties evolve across doping levels.

Contribution

It provides detailed insights into the coexistence and competition of magnetic and superconducting orders in hole-doped BaKFe2As2, highlighting similarities with electron-doped counterparts.

Findings

Magnetic moments are moderately suppressed in underdoped regions.

Superconducting and magnetic orders coexist on the nano-scale in underdoped samples.

Superconducting condensate density decreases in the underdoped regime.

Abstract

Using muon spin rotation and infrared spectroscopy we study the relation between magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the N\'{e}el-temperature, $ T^{\mathrm{N}} $. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair breaking peak in the infrared response. In the moderately underdoped region, superconducting and static magnetic orders co-exist on the nano-scale and compete for the same electronic states. The static magnetic moment disappears rather sharply near optimal doping, however, in the slightly overdoped region there is still an enhancement or slowing down of spin fluctuations in the superconducting state. Similar to the gap magnitude reported from specific heat measurements, the superconducting condensate density is nearly constant in the optimally- and slightly overdoped region, but exhibits a rather pronounced decrease on the underdoped side. Several of these observations are similar to the phenomenology in the electron doped counterpart Ba(Fe$ _{1-y} $Co$ _{y} $)$ _{2} $As$ _{2} $.