  We study excitations in weakly interacting pairs of quantum spin ladders
coupled through geometrically frustrated bonds. The ground state is a
disordered spin liquid, that at high fields is 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.
