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arXiv · 2607.20036

Vanishing spin stiffness in weakly disordered two-dimensional Heisenberg ferromagnets

Jacopo Niedda·Aldo Coraggio·Giacomo Bracci-Testasecca·Antonello Scardicchio
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Abstract

We show that a small fraction of antiferromagnetic bonds qualitatively alters the long-wavelength dynamics of two-dimensional Heisenberg ferromagnets. Although the classical ground state remains magnetized, weak bond frustration generates logarithmically correlated spatial fluctuations of the local spin stiffness, despite the microscopic disorder being short ranged. A replica field theory calculation shows that the effective disorder strength grows under coarse graining, while the spin stiffness decreases, yielding anomalously soft magnons with a scale-dependent dynamical exponent $z > 2$. Numerical diagonalization of the semiclassical spin-wave Hamiltonian confirms the anomalous low-energy scaling. The flow is toward an infinite-disorder, zero stiffness regime.

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Jacopo Niedda, Aldo Coraggio, Giacomo Bracci-Testasecca, Antonello Scardicchio. 2026-07-22. Vanishing spin stiffness in weakly disordered two-dimensional Heisenberg ferromagnets. https://arxiv.org/abs/2607.20036

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