Observing the Suppression of Superconductivity in ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ by Correlated Magnetic Fluctuations

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. These results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$, with important implications for future investigations of physical phenomena arising from the interplay between them.

Figure 1

Temperature dependence of the in-plane resistivity of ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ near the superconducting transition and the magnetic susceptibility of a zero-field-cooled ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ single crystal with a 10 Oe magnetic field applied along the $c$ axis. The inset shows one unit cell of ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$, where the magnetic structure of the Eu sublattice is indicated.

Figure 2

(a) Optical micrograph of a ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ single crystal after cleaving and deposition of a conductive coating. The approximate location of the images taken in Fig. 3 is marked by the white cross. (b)–(f) Three-dimensional SHM images of vortices in a ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ single crystal after field cooling to 30 K in effective perpendicular fields between $-2.1$ and 1.3 Oe [39]. The scan size is $12.6\times 12.6\mu \mathrm{m}$. The full magnetic field range of the images (vertical scale) span 0.4 ($-2.1$), 0.5 ($-1.25$), 0.8 ($-0.45$), 0.7 (0.4), 0.5 G (1.3 Oe).

Figure 3

(a)–(c) Vortex-resolved SHM images after field cooling in $H_z^{\mathrm{eff}}=-0.8\mathrm{Oe}$ from above $T_c$ to three different temperatures, illustrating the evolution of vortex profiles as $T_m$ is approached from above. The field of view in each of these images is $6.5\times 6.5\mu \mathrm{m}$ and vertical scales span 0.9 G. (d) Vortex profiles extracted from SHM images in the sequence shown in (a)–(c) with superimposed fits to a modified Clem model, Eq. (1).

Figure 4

Temperature dependence of the normalized superfluid density ${\rho }_s(T)/{\rho }_s(0)$ (solid symbols) and a fit to the model described in the text (dashed line). Inset: plot of the penetration depth as a function of temperature extracted from fits to a modified Clem model, Eq. (1), for one of the vortices shown in Figs. 3. The dashed light blue line shows the assumed bare penetration depth dependence ${\lambda }_0(T)=\lambda (0)/\sqrt{1-(T/T_c{)}^2}$.