Spontaneous magnetization in the quantum Heisenberg ferromagnet
For every dimension d ≥ 3 and every spin $S\in\{\frac12,1,\frac32,\ldots\}$, we prove that the nearest-neighbor isotropic quantum Heisenberg ferromagnet has a translation-invariant, spontaneously magnetized equilibrium state at every sufficiently low positive temperature. The same state satisfies the KMS condition for the zero-field dynamics and has magnetization at least $S/4$. This resolves the low-temperature ordering problem in the spontaneous-magnetization formulation.
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