Spin waves in layered superconductors with helical magnetic structure: the case of RbEuFe₄As₄

We evaluated the spin-wave spectrum and dynamic susceptibility in a layered superconductor with helical interlayer magnetic structure [1]. We especially focus on the structure in which the moments rotate 90 degrees from layer to layer realized in the iron pnictide RbEuFe₄As₄. While in nonmagnetic superconductors low-frequency magnetic field decays on the distance of the order of the London penetration depth, spin waves mediate its propagation to much larger distances limited by external dissipation mechanisms. The spin-wave spectrum in superconductors is strongly renormalized due to the long-range electromagnetic interactions between the oscillating magnetic moments. This leads to strong enhancement of the frequency of the mode coupled with uniform field and this enhancement exists only within a narrow range of the c-axis wave vectors of the order of the inverse London penetration depth.  The key feature of materials like RbEuFe₄As₄ is that this uniform mode corresponds to the maximum frequency of the spin-wave spectrum with respect to the c-axis wave vector.  As a consequence, the high-frequency surface resistance acquires a very distinct asymmetric feature spreading between the bare and renormalized frequencies. For parameters of RbEuFe₄As₄, this feature is expected in the range 10-20 GHz.