Suppression of the antiferromagnetic order when approaching the superconducting state in a phase-separated crystal of K$_x$Fe$_{2-y}$Se$_2$

TL;DR

This study investigates how antiferromagnetic order diminishes near the superconducting transition in K$_x$Fe$_{2-y}$Se$_2$, revealing phase separation and the interplay between magnetic and superconducting states through neutron scattering and other measurements.

Contribution

It provides evidence for phase separation and suppression of magnetic order near the superconducting transition in K$_x$Fe$_{2-y}$Se$_2$ using combined neutron scattering and magnetic measurements.

Findings

Magnetic order development is interrupted at ~42 K before superconductivity sets in.

Coexistence of spin-wave excitations and superconductivity resonance observed.

Phase separation between antiferromagnetic and superconducting regions confirmed.

Abstract

We have combined elastic and inelastic neutron scattering techniques, magnetic susceptibility and resistivity measurements to study single-crystal samples of K$_x$Fe$_{2-y}$Se$_2$, which contain the superconducting phase that has a transition temperature of $\sim$31 K. In the inelastic neutron scattering measurements, we observe both the spin-wave excitations resulting from the block antiferromagnetic ordered phase and the resonance that is associated with the superconductivity in the superconducting phase, demonstrating the coexistence of these two orders. From the temperature dependence of the intensity of the magnetic Bragg peaks, we find that well before entering the superconducting state, the development of the magnetic order is interrupted, at $\sim$42 K. We consider this result to be evidence for the physical separation of the antiferromagnetic and superconducting phases; the suppression is possibly due to the proximity effect of the superconducting fluctuations on the antiferromagnetic order.