Momentum-resolved spin-conserving two-triplon bound state and continuum in a cuprate ladder

Studying multi-particle elementary excitations has provided unique access to understand collective many-body phenomena in correlated electronic materials, paving the way towards constructing microscopic models. In this work, we perform O K-edge resonant inelastic X-ray scattering (RIXS) on the quasi-one-dimensional cuprate $${{{{{{\rm{Sr}}}}}}}_{14}{{{{{{\rm{Cu}}}}}}}_{24}{{{{{{\rm{O}}}}}}}_{41}$$ with weakly-doped spin ladders. The RIXS signal is dominated by a dispersing sharp mode ~ 270 meV on top of a damped incoherent component ~ 400-500 meV. Comparing with model calculations using the perturbative continuous unitary transformations method, the two components resemble the spin-conserving ΔS = 0 two-triplon bound state and continuum excitations in the spin ladders. Such multi-spin response with long-lived ΔS = 0 excitons is central to several exotic magnetic properties featuring Majorana fermions, yet remains unexplored given the generally weak cross-section with other experimental techniques. By investigating a simple spin-ladder model system, our study provides valuable insight into low-dimensional quantum magnetism. Quasi-one-dimensional spin ladders are useful models to study collective many-body phenomena. Here, the authors investigate the excitations of weakly hole-doped cuprate ladders using oxygen K-edge resonant inelastic X-ray scattering revealing fingerprints of spin singlet multi-triplon bound state and continuum modes that are relevant to understanding various collective spin phenomena, such as Majorana fermions.