Abstract
We study excitations in weakly interacting pairs of quantum spin ladders coupled through geometrically frustrated bonds. The ground state is a disordered spin liquid, at high fields replaced
by an ordered chiral helimagnetic phase. The spectra observed by high-field inelastic neutron scattering experiments on the prototype compound Sul–Cu2Cl4 are qualitatively different from those in the previously studied frustration-free spin liquids. Beyond the critical field Hc = 3.7 T, the
soft mode that drives the quantum phase transition spawns two separate excitations: a gapless
Goldstone mode and a massive magnon. Additional massive quasiparticles are clearly visible below
Hc, but are destroyed in the ordered phase. In their place one observes a sharply bound excitation
continuum.
