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

We have combined elastic and inelastic neutron scattering techniques, magnetic susceptibility, and resistivity measurements to study single-crystal samples of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$, 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.

Figure 1

Integrated intensity of the scans along the [120] direction through the magnetic Bragg peak (0.2, 0.4, 0.5) [scans are shown in Fig. 3]. Lines through data are guides to the eye. The dashed line is the extrapolation to the high-temperature data. Throughout the paper, error bars represent one standard deviation. The data plotted here were collected on HB1A, and were confirmed on HB1.

Figure 2

(a) Left axis: magnetic susceptibility measured under zero-field-cooling conditions with a magnetic field of 10 Oe applied along $c$ axis. Right axis: resistivity measured in the $a-b$ plane in zero field. (b) Contour map of the time-of-flight data projected onto the ($H,K,0$) plane at 5 K with energies ranging from 12–16 mev. The positions of the spin-wave excitations and the resonance mode have been marked. (c) Energy scans at $\mathit{Q}=(0.25,0.75,0)$ at 5 and 45 K. These are the raw data without subtracting the backgrounds. We counted 15 min for each data point and normalized the counts to 5 min per point. Lines through data are fits using Lorentzian functions. The shade illustrates the intensity gain at 5 K. (d) Integrated intensities obtained from the fits to the $\mathit{Q}$ scans at 14.5 meV through (0.25, 0.75, 0) along the [110] direction. The solid line is a fit to the data using the BCS gap function. The results in (b) were obtained on ARCS, and those in (c) and (d) were obtained on HB1 and confirmed on 1T.

Figure 3

(a) and (b), linear scans through the magnetic Bragg peak (0.2, 0.4, 0.5) along the [120] and [001] directions, respectively. Lines through data are fits with Gaussian functions. Scan trajectories are shown in the inset. Horizontal bars illustrate the instrumental resolutions. (c) Scans through the nuclear Bragg peak (120) at 30 and 55 K. The data plotted here were collected on HB1A, and were confirmed on HB1.