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MA: Fachverband Magnetismus

MA 52: Poster Magnetism III

MA 52.2: Poster

Donnerstag, 12. März 2026, 15:00–17:00, P4

Holstein-Primakoff Expansions and the Goldstone Theorem for Ferromagnetic Spin Chains — •Hendrik Wächter, Fred Hucht, and Jürgen König — University of Duisburg-Essen, Faculty of Physics, Duisburg, Germany

A common approach to investigate collective magnetic excitations is to represent spins as bosons. A popular representation that achieves this is the Holstein-Primakoff transformation, which comes at the cost of introducing square roots containing the occupation number operator to the spin ladder operators. For many applications, it is preferable to expand these square roots in power series. Typically, this is done by a Taylor series expansion in powers of 1/S, sacrificing essential virtues of the Holstein-Primakoff transformation by coupling to an unphysical Hilbert space. We consider Newton series expansions as an alternative [1]. Ferromagnetic spin chains are used to compare the efficacy of both expansions, focusing on ground-state and low-energy excitation properties, notably including Goldstone modes. Matrix product states and the density-matrix renormalization group method are used to obtain highly accurate numerical results. We find that the Newton series expansion up to a finite order respects the existence of gapless spin excitations within the relevant Hilbert space.

[1] J. König and A. Hucht, SciPost Phys. 10, 007 (2021)

Keywords: Magnons; Holstein-Primakoff; Goldstone Theorem; Newton Series Expansion