Competition of magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2

TL;DR

This study investigates the interplay between magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2, revealing coexistence of magnetic order and superconductivity without phase separation, and highlighting differences from single crystal behaviors.

Contribution

It provides the first comprehensive analysis of magnetic and superconducting phases in polycrystalline underdoped (Ba1-xKx)Fe2As2, demonstrating homogeneous coexistence near optimal doping.

Findings

Magnetic order persists in underdoped samples with x <= 0.2.

Superconductivity appears in all samples except at very low doping.

No mesoscopic phase separation observed near optimal doping.

Abstract

Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad SDW phase transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2, where C changes by 0.1 J/K at Tc = 37.3 K. 57Fe-Moessbauer-spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic ordering at 4.2 K in samples with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF magnetic ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about single crystal (Ba1-xKx)Fe2As2.