A neutron scattering study of the under-doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, x=0.09 and 0.17 self-flux grown single crystals and the universality of the tricritical point

TL;DR

This neutron scattering study investigates the magnetic and superconducting properties of under-doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ crystals, revealing a possible universal tricritical point and competition between superconductivity and antiferromagnetism.

Contribution

The paper provides new neutron scattering data on under-doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, suggesting the universality of the tricritical point and detailed insights into the interplay of magnetic order and superconductivity.

Findings

Magnetic order parameter shows a gradual temperature dependence near the tricritical point.

Superconductivity suppresses T$_{c}$ under magnetic field, with magnetic Bragg peak intensity increasing.

Magnetic Bragg peaks remain stable under high magnetic field within measurement error.

Abstract

We present a combination of elastic neutron scattering measurements in zero and 14.5 T and magnetization measurements in zero and 14 T on under-doped superconducting Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ x=0.17, and the same measurements in zero field on a non-superconducting crystal with x=0.09. The data suggest that the under-doped materials may not be electronic phase separated but rather have slightly inhomogeneous potassium doping. The temperature dependence of the magnetic order parameter (OP) below the transition of the sample with x=0.09 is more gradual than that for the case of the un-doped BaFe$_{2}$As$_{2}$, suggesting that this doping may be in the vicinity of a tricritical point. We advance therefore the hypothesis that the tricritical point is a common feature of all superconducting 122s. For the x=0.17 sample, while T$_{c}$ is suppressed from $\approx$17 K to $\approx$8 K by a magnetic field of 14 T, the intensity of the magnetic Bragg peaks (1 0 3) at 1.2 K is enhanced by 10$%$ showing competition of superconductivity (SC) and antiferromagnetism (AFM). The intensity of the magnetic Bragg peaks (1 0 3) in the (T$_{c}$, T$_{N}$) temperature interval remain practically unchanged in 14.5 T within a 10$%$ statistical error. The present results are discussed in the context of the existing literature.