Motivated by the intriguing mode splittings in a magnetic field recently observed with inelastic neutron scattering in the spin ladder compound (C5H12N)2CuBr4 (BPCB), we investigate the nature of the spin ladder excitations using DMRG and analytical arguments. Starting from the fully frustrated ladder, for which we derive the low-energy spectrum, we show that bound states are generically present close to k=0 in the dynamical structure factor of spin ladders above Hc1, and that they are characterized by a field-independent binding energy and an intensity that grows with H−Hc1. These predictions are shown to explain quantitatively the split modes observed in BPCB.
@article{arxiv.1912.01576,
title = {Magnetic field induced bound states in spin-$\frac{1}{2}$ ladders},
author = {Mithilesh Nayak and Dominic Blosser and Andrey Zheludev and Frédéric Mila},
journal= {arXiv preprint arXiv:1912.01576},
year = {2020}
}