Martin Wood Complex, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU
Professor Simon Bending, University of Bath
Prof Amalia Coldea
Abstract
The possible coexistence of superconductivity and magnetism has fascinated scientists for many decades since conventional wisdom dictates that the strong ferromagnetic exchange interaction should destroy singlet Cooper pairing. In this talk I will present the results of two recent case studies of magnetic iron-based superconductors which illustrate full coexistence with two distinct forms of Eu2+ magnetic order. RbEuFe4As4 is a stoichiometric superconductor with Tc ≃ 37 K that exhibits a transition to a helical antiferromagnetic state at Tm ≃ 15 K. Previous optical conductivity and angle resolved photoemission spectroscopy measurements on this material suggested almost complete separation between the superconducting and magnetic sublattices. In contrast our scanning Hall microscopy images of vortices revealed a dramatic increase in the penetration depth and suppression of the superfluid density near the magnetic ordering temperature [1]. Our observations are in good agreement with a recently developed model of the suppression of superconductivity by correlated magnetic fluctuations [2]. Isovalent P-doped EuFe2(As1−xPx)2 exhibits superconductivity within the range 0.15 < x < 0.3, reaching a maximum transition temperature of Tc ≃ 25 K at x ≃ 0.21. Remarkably, superconductivity coexists fully with a uniaxial ferromagnetic state below TFM ≃ 19 K. Previous Magnetic force microscopy (MFM) measurements [3] have demonstrated the existence of two distinct stripe ferromagnetic domain structures below TFM: the Domain Meissner State where the domain period is renormalized by screening Meissner currents and the Domain Vortex State characterised by the spontaneous nucleation of vortices and antivortices at lower temperatures. To better understand the vortex dynamics in these regimes, we have performed bulk magnetometry and magnetic relaxation measurements in conjunction with high-field MFM. Our results indicate that the magnetic irreversibility is strongly dependent on the presence of both the magnetic and superconducting orders, and just below TFM we observe a pronounced peak in the creep activation energy while MFM measurements reveal the presence of very closely spaced (w ≪ λ) vortex clusters. We attribute these observations to the formation of vortex polarons arising from local modifications of the stripe domain structure by the vortex fields and develop a qualitative theory for these. Our results suggest new routes for the magnetic enhancement of vortex pinning with potential applications in high-current conductors.
[1] D. Collomb et al., Phys. Rev. Lett. 126, 157001 (2021).
[2] A.E. Koshelev, Phys. Rev. B 102, 054505 (2020).
[3] V. S. Stolyarov et al., Sci. Adv. 4, eaat1061 (2018).