Spontaneous C₄ symmetry breaking has been widely reported in iron-based superconductors. Such a novel phenomenon named "nematic superconductivity" draws a lot of attention. In particular, novel nematic states with variable direction of the principal axis has been reported in hole-doped 122-type (Ba_1-xRb_x)Fe₂As₂ [1]. On the other hand, in contrast to (Ba_1-xK_x)Fe₂As₂, not much is known on the normal state and superconducting properties of (Ba_1-xRb_x)Fe₂As₂.

In this study, we synthesized high-quality single crystals of (Ba_1-xRb_x)Fe₂As₂ close to the optimal composition of x ≈ 0.4, and have characterized normal state and superconducting properties for pristine/proton irradiated samples. The highest T_c was found to be ~38 K for pristine samples. The critical current density (J­_c­) in (Ba_0.56Rb_0.44)Fe₂As₂ at 2 K under self-field (sf) is ~1.7 MA/cm­², which is comparable to that in (Ba_0.6K_0.4)Fe₂As₂. After the H⁺ irradiation, J_c(2 K, sf) reaches 12 MA/cm­² at a dose of 3 × 10¹⁶ H⁺/cm², which is comparable to (Ba_1-xK_x)Fe₂As₂ (11 MA/cm­² at x = 0.4, 5.8 × 10¹⁶ H⁺/cm² [2]), but larger than those in Ba(Fe­_1-xCo_x)₂As₂ (2.5 MA/cm² at x = 0.07, 1.2 × 10¹⁶ H⁺/cm² [3]), and BaFe₂(As_1-xP_x)­₂ (4.5 MA/cm² at x = 0.33, 1.0 × 10¹⁶ H⁺/cm² [4]). J_c at low temperatures follows the power-law dependence on the magnetic field (J_c H^-α) with α changed from ~ 1/2 to ~ 1/3 after the H⁺ irradiation. Unusual peak effects with their peaks at around the self-field were observed in H⁺ irradiated samples. We tentatively interpret this phenomenon as being caused by the one-dimensional correlation among introduced point defects.

In addition, magneto-optical imaging based on the Faraday effect of an indicator garnet film will be employed to image superconducting state at temperatures below T_c. This technique may also allow us to image nematic domains, which will help us further explore its novel nematic state. In preparation for this observation at low temperatures, the presence of twin-boundaries is confirmed optically in underdoped samples below the structural transition.

[1] K. Ishida et al., Proc. Natl. Acad. Sci. USA, 117, 6424 (2020).
[2] T. Taen et al., Supercond. Sci. Technol. 28, 085003 (2015).
[3] T. Taen et al., Phys. Rev. B 86, 094527 (2012).
[4] A. Park et al., Phys. Rev. B 101, 224507 (2020).

Keywords: (Ba1-xRbx)Fe2As2, critical current density, H+ irradiation, nematicity