Interplay of stripe and double-Q magnetism with superconductivity in $\mathrm{Ba}_{1-x}\mathrm{K}_{x}\mathrm{Fe}_{2}\mathrm{As}_{2}$ under the influence of magnetic fields

TL;DR

This study investigates how magnetic fields influence the coexistence and competition of stripe and double-Q magnetism with superconductivity in Ba₁₋ₓKₓFe₂As₂, revealing field-dependent suppression effects and anisotropic behavior.

Contribution

It provides detailed experimental insights into the magnetic field effects on magneto-structural transitions and superconductivity in Ba₁₋ₓKₓFe₂As₂, supported by a phenomenological Ginzburg-Landau model.

Findings

Magnetic fields suppress the orthorhombic reentrant phase more than superconductivity.

Out-of-plane fields have a stronger suppression effect than in-plane fields.

The suppression correlates with the anisotropy of the superconducting state.

Abstract

At $x\approx0.25$ $\mathrm{Ba}_{1-x}\mathrm{K}_{x}\mathrm{Fe}_{2}\mathrm{As}_{2}$ undergoes a novel first-order transition from a four-fold symmetric double-Q magnetic phase to a two-fold symmetric single-Q phase, which was argued to occur simultaneously with the onset of superconductivity (B\"ohmer et al., Nat. Comm. 6, 7911 (2015)). Here, by applying magnetic fields up to 10T, we investigate in more detail the interplay of superconductivity with this magneto-structural transition using a combination of high-resolution thermal-expansion and heat-capacity measurements. We find that a magnetic field suppresses the reentrance of the single-Q orthorhombic phase more strongly than the superconducting transition, resulting in a splitting of the zero-field first-order transition. The suppression rate of the orthorhombic reentrance transition is stronger for out-of-plane than for in-plane fields and scales with the anisotropy of the superconducting state. These effects are captured within a phenomenological Ginzburg-Landau model, strongly suggesting that the suppression of the reentrant orthorhombic single-Q phase is primarily linked to the field-induced weakening of the superconducting order. Not captured by this model is however a strong reduction of the orthorhombic distortion for out-of-plane fields, which deserves further theoretical attention.