Chirality-resolved spectroscopy of Caroli–de Gennes–Matricon states in multiband FeTe_1−xSe_x superconductors

G. Blumberg,^1,2 T. Rõõm,² U. Nagel,² A. Glezer Moshe,² R. Nagarajan,² H.-T. Yi,¹ and S. Oh¹

¹Department of Physics & Astronomy, Rutgers University, New Jersey, USA

²National Institute of Chemical Physics and Biophysics, Tallinn, Estonia

We employ terahertz Faraday magneto-optical spectroscopy to probe the relaxation dynamics of quantized helical Caroli–de Gennes–Matricon (CdGM) states in epitaxial FeTe_1−xSe_x thin films, nodeless multiband superconductors with short coherence lengths in the moderately clean limit. By exploiting polarization-selective optical transitions, we directly resolve the helicity and band origin of vortex-core quasiparticles. We observe long-lived CdGM resonances with opposite circular polarizations for electron- and hole-like bands. This enables independent, band-resolved determination of quasiparticle lifetimes, vortex masses, coherence lengths, and upper critical fields, and reveals their systematic evolution with isovalent substitution x. The results establish terahertz magneto-optics as a direct probe of helical vortex-core excitations and provide dynamical evidence for multiband CdGM states in iron-based superconductors.

Paper under consideration in Phys. Rev. Letters.

Supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 885413).