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arXiv:2603.17635

Field-induced quasi-bound state within the two-magnon continuum of a square-lattice Heisenberg antiferromagnet

Frank Elson, M. Nayak, Alexander A. Eberharter, M. Skoulatos, Simon Ward, U. Stuhr, Niels Bech Christensen, D. Voneshen, C. Fiolka, K. W. Krämer, Christian Rüegg, Henrik Moodysson Rønnow, Bruce Normand, M. Mourigal, Frédéric Mila, Andreas Martin Läuchli, Martin Må̊nsson

Published Mar 18, 2026• 0 citations
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Abstract

Quantum magnets in two dimensions display strong quantum interaction effects even when magnetically ordered. Using the metal-organic framework material CuF$_2$(D$_2$O)$_2$(pyz), we investigate the field-dependent spin dynamics of the $S = 1/2$ square-lattice Heisenberg antiferromagnet by high-resolution inelastic neutron scattering to applied fields beyond one third of saturation. We discover an anomalously sharp, dispersive ``shadow mode'' residing within the two-magnon continuum, which shadows the dispersion of the transverse one-magnon branches across the Brillouin zone at an offset equal to the Larmor energy. We perform cylinder matrix-product-state (MPS) calculations that reproduce the field-induced spectrum quantitatively and apply a spectrally consistent $1/S$ spin-wave theory to deduce that the ``Larmor-shadow mode'' is a composite two-magnon resonance: a dispersing magnon at wavevector ${\bf Q}$ couples to the uniform Larmor precession at $Γ$, its small intrinsic linewidth indicating a non-perturbative effect of attractive magnon-magnon interactions. Another quantum-fluctuation phenomenon, the zero-field $(π,0)$ anomaly, is lost at increasing fields, which tighten the spectral weight into the one-magnon and Larmor-shadow modes. To our knowledge, these results constitute the first observation of a sharp quasi-bound state embedded in the continuum of a gapless two-dimensional antiferromagnet.

DOI
10.48550/arxiv.2603.17635

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