EPSD Colloquium

Ferromagnetism and superconductivity are usually exclusive to each other, with very few materials showing both phenomena simultaneously. One of these is the iron pnictide superconductor EuRbFe4As4. This compound is a high-temperature superconductor with a superconducting transition temperature of about 35 K. When cooled down below 16 K, magnetic order sets in in the planes of Eu atoms. How does the presence of this magnetism in the Eu planes affect the superconductivity that resides in the neighboring FeAs planes? We study this experimentally using scanning tunneling microscopy, supported by ARPES and optical measurements. I will show that the interaction between magnetism and superconductivity leads to a modulated superconducting order in real space. This order can take one of two forms - one, where the phase of the superconductor is constant in space, while the amplitude is modulated, while the other features a pure phase modulation with a constant amplitude in space. I will show that one of the consequences of such modulated superconductivity is the presence of gapless quasiparticle excitations that arise from the Doppler shift of pairs in a phase-modulated superconductor.