Distinct high-T transitions in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

TL;DR

This study reveals that in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, the structural and magnetic phase transitions occur at different temperatures, with superconductivity coexisting with antiferromagnetism below 20 K.

Contribution

The paper demonstrates that the structural and magnetic transitions are separate and sequential in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, contrasting previous assumptions of simultaneous transitions.

Findings

Structural transition at T_S ≈ 110 K

Magnetic transition at T_N ≈ 102 K

Superconductivity coexists with antiferromagnetism below T_c=20 K

Abstract

In contrast to the simultaneous structural and magnetic first order phase transition $T_{0}$ previously reported, our detailed investigation on an underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously revealed that the transitions are not concomitant. The tetragonal ($\tau$: I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at $T_{S}\simeq$ 110 K, followed by an adjacent antiferromagnetic (AFM) transition at $T_{N}\simeq$ 102 K. Hysteresis and coexistence of the $\tau$ and $\vartheta$ phases over a finite temperature range observed in our NMR experiments confirm the first order character of the structural transition and provide evidence that both $T_{S}$ and $T_{N}$ are strongly correlated. Our data also show that superconductivity (SC) develops in the $\vartheta$ phase below $T_{c}$ = 20 K and coexists with long range AFM. This new observation, $T_{S}\neq T_{N}$, firmly establishes another similarity between the hole-doped BaFe$_{2}$As$_{2}$ via K substitution and the electron-doped iron-arsenide superconductors.