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Large effects of boundaries on spin amplification in spin chains

We investigate the effect of boundary conditions on spin amplification in spin chains. We show that the boundaries play a crucial role for the dynamics: A single additional coupling between the first and last spins can macroscopically modify the physical behavior compared to the open chain, even in the limit of infinitely long chains. We show that this effect can be understood in terms of a "bifurcation" in Hilbert space that can give access to different parts of Hilbert space with macroscopically different physical properties of the basis functions, depending on the boundary conditions. On the technical side, we introduce semiclassical methods whose precision increase with increasing chain length and allow us to analytically demonstrate the effects of the boundaries in the thermodynamic limit.