We report neutron, X-ray absorption, and resonant X-ray spectroscopy of magnetic excitations in the new heavy-fermion van-der-Waals superconductor CeSiI. We determined effective Hamiltonians and ground states of crystal electric fields and magnons. Isotropic Heisenberg interactions on a quasi two dimensional lattice, including ferromagnetic nearest-neighbor exchange as the dominant interaction, provide an excellent account to the low-energy measured dynamics and stabilize a co-rotating spin cycloid. Our study provides the basis to model CeSiI from first principles, thereby laying the ground for microscopic understanding of heavy-fermion physics, their unconventional superconductivity, and quantum criticality.
@article{arxiv.2605.28698,
title = {Magnetic order, magnons, and crystal fields in van der Waals CeSiI},
author = {Wolfgang Simeth and Connor A. Occhialini and Michael E. Ziebel and Nethmi W. Hewage and Sabrina J. Li and Daniel Pajerowski and Taehun Kim and Ben Zager and Jonathan Pelliciari and Kipton Barros and Daniel Rehn and Abhay N. Pasupathy and Valentina Bisogni and Xavier Roy and Allen Scheie},
journal= {arXiv preprint arXiv:2605.28698},
year = {2026}
}