Prof. Amalia Coldea
University of Oxford

Abstract: Stabilization of the spin-density waves in high magnetic fields in iron-chalcogenides superconductors FeSe1-xSx

Spin-density-wave (SDW) order often competes or coexists with unconventional superconductivity. SDW phases are characterized by a reconstructed Fermi surface and provide the low-energy spin fluctuations that mediate superconducting pairing [1]. The iron chalcogenide FeSe1−x​Sx​ exhibits a nematic electronic phase, but SDW order is stabilized only under applied pressure, in contrast to other isoelectronic iron pnictides.

In this talk, I will present a series of experimental studies performed in high magnetic fields to suppress superconductivity and expose the competing electronic ground state. Magnetotransport measurements of FeSe0.96​S0.04​ under hydrostatic pressure reveal sharp resistivity upturns inside the nematic phase, consistent with the stabilization of different SDW phases both inside and outside the nematic phase boundary at high pressure [2]. In addition, we detect a field-induced incipient SDW through resistivity upturns, once superconductivity is quenched, within the nematic B phase of FeSe1−x​Sx​ (0.1<x<0.18) [3]. Direct comparison between chemical substitution and applied pressure tuning demonstrates the presence of Fermi-surface reconstruction, consistent with the stabilization of SDW order within the nematic phase of FeSe1−x​Sx​ through different routes. We also find that the SDW state is rather fragile with respect to substitution, like Cu doping [4], despite the fact that superconductivity remains robust in the high-pressure phase [5]. These findings highlight the important role of SDW order in understanding the superconducting pairing mechanism in iron chalcogenides.

[1] Rafael M. Fernandes et al., Nature, 601, 35 (2022).
[2] Z. Zajicek et al., Phys. Rev. B 113, 075135 (2026).
[3] I. Paulescu et al, submitted (2026).
[4] Z. Zajicek et al., Phys. Rev. Research 4, 043123 (2022).
[5] P. Reiss et al., npj Quantum Materials, 9, 73 (2024).