Magnetic excitations in a magnetized double ladder Cu-CPA

Quantum spin ladders bridge the gap between one- and two-dimensional quantum magnets. They feature a complex excitation spectrum that depends on the relative strengths of the leg and rung exchange interactions. While realizations of strong-rung ladders are common, only one strong-leg spin ladder has been studied in detail to date. However, a recent breakthrough in material synthesis, allows us to study a second candidate for the strong-leg regime: (C5H9NH3)2CuBr4 (or Cu-CPA for short). Combining X-ray and neutron scattering, we have found that Cu-CPA, which was initially believed to be a simple spin ladder system, represents the more complex case of a double ladder system, which remains to be explored [1]. Our zero-field inelastic neutron-scattering experiment shows no dispersion perpendicular to the ladder propagation direction, confirming this is indeed a purely one-dimensional system. Additionally, we observed two distinct low-energy modes that are consistent with a double ladder [2]. Preliminary measurements under applied magnetic field hint toward only a partial mode-softening. Therefore, we propose to use neutron spectroscopy under externally applied magnetic fields to study the condensation of magnons and understand the magnetization processes in this double ladder system. [1] Philippe et. al., submitted to PRB, https://arxiv.org/abs/2404.08274 [2] Philippe et. al., in preparation