Quantized ΔS=2 excitation spectra by confinement in an S=1 Ising spin chain under magnetic fields

We calculate the dynamical spin-structure factor of the S = 1 Ising spin chain with negative single-ion anisotropy in transverse and longitudinal magnetic fields using the infinite time-evolving-block-decimation algorithm [1]. We show that when only a transverse magnetic field is applied, both the ΔS = 2 excitation continuum and one-magnon mode appear in the low-lying excitation. When a longitudinal magnetic field is further applied, the excitation continuum changes into quantized spectra. The quantized ΔS = 2 excitation spectra originate from the confinement of two domain walls, each of which carries ΔS = 1. The quantized excitation energies are explained by the zeros in the Airy function. [1] T. Suzuki and S. Suga, J. Phys. Soc. Jpn. 88, 053702 (2019).