Seminars

Influence of correlated magnetic fluctuations on parameters of magnetic superconductors

Alexei E. Koshelev

Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue,
Lemont, Illinois 60439, USA

Several superconducting materials host sublattices of rare-earth local magnetic moments weakly interacting with Cooper pairs. These moments may order inside superconducting state. In this case, emerging magnetic fluctuations suppress superconducting parameters. A notable example of such a material is recently discovered iron pnictide RbEuFe₄As₄ with the superconducting transition at 36.7 K and the magnetic transition at 15 K. As a model for this material, we consider a clean layered superconductor containing magnetic-moments sublattice in which a magnetic order establishes inside a superconducting state without destruction of superconductivity.  We investigate the corrections to the superconducting gap and London penetration depth caused by the weak exchange interactions of Cooper pairs with correlated magnetic fluctuations [1].  The influence of nonuniform exchange field on superconducting parameters is very sensitive to the relation between the magnetic correlation length, x_h, and superconducting coherence length x_s defining the 'scattering' (x_h < x_s) and 'smooth' (x_h > x_s) regimes.  We quantified this 'scattering-to-smooth' crossover for the case of quasi-two-dimensional magnetic fluctuations realized in RbEuFe₄As₄.  In the ‘scattering’ regime, the suppression of superconductivity is similar to the case of magnetic impurities [2] and the exchange corrections are proportional to the magnetic scattering rate, which grows ∝ x_h until x_h ≪ x_s. In the opposite limit, when x_h exceeds x_s, smoothening of spatial variations of the exchange field strongly diminishes its effect on superconducting parameters leading to much weaker dependence of the corrections on x_h. Moreover, the gap correction may even decrease with increasing of x_h in the immediate vicinity of the magnetic transition if it is located at a temperature much lower than the superconducting transition. The crossover between the regimes occurs to be unexpectedly broad: the scattering approximation becomes inaccurate already when x_h is substantially larger than x_s. We applied the developed theoretical framework to modelling the observed behaviour of the London penetration depth extracted from the vortex imaging in RbEuFe₄As₄ [3].

Acknowledgements: This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

References:

[1] A. E. Koshelev, “Suppression of superconducting parameters by correlated quasi-two-dimensional

magnetic fluctuations”, Phys. Rev. B 100, 014518 (2019).

[2] A. A. Abrikosov and L. P. Gor’kov, “Contribution to the theory of superconducting alloys with paramagnetic impurities”, Sov. Phys. JETP 12, 1243 (1961); S. Skalski, O. Betbeder-Matibet, and P. R. Weiss, “Properties of superconducting alloys containing paramagnetic impurities”, Phys. Rev. 136, A1500 (1964); V. G. Kogan, R. Prozorov, and V. Mishra, “London penetration depth and pair breaking”, Phys. Rev. B 88, 224508 (2013).

[3] D. Collomb et al., “Observing the suppression of superconductivity in RbEuFe₄As₄ by correlated magnetic fluctuations”, Phys. Rev. Lett. 126, 157001 (2021).