Crossover of high-energy spin fluctuations from collective triplons to localized magnetic excitations in Sr14-xCaxCu24O41 ladders

We studied the magnetic excitations in the quasi-one-dimensional (q-1D) ladder subsystem of Sr14-xCaxCu24O41 (SCCO) using Cu L-3-edge resonant inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra with high (x = 12.2) and without (x = 0) Ca content, we track the evolution of the magnetic excitations from collective two-triplon (2 T) excitations (x = 0) to weakly-dispersive gapped modes at an energy of 280 meV (x = 12.2). Density matrix renormalization group (DMRG) calculations of the RIXS response in the doped ladders suggest that the flat magnetic dispersion and damped excitation profile observed at x = 12.2 originates from enhanced hole localization. This interpretation is supported by polarization-dependent RIXS measurements, where we disentangle the spin-conserving Delta S = 0 scattering from the predominant Delta S = 1 spin-flip signal in the RIXS spectra. The results show that the low-energy weight in the Delta S = 0 channel is depleted when Sr is replaced by Ca, consistent with a reduced carrier mobility. Our results demonstrate that off-ladder impurities can affect both the low-energy magnetic excitations and superconducting correlations in the CuO4 plaquettes. Finally, our study characterizes the magnetic and charge fluctuations in the phase from which superconductivity emerges in SCCO at elevated pressures.

Automated summary

In this study, we investigated the magnetic excitations in the quasi-one-dimensional ladder subsystem of Sr14-xCaxCu24O41 using Cu L-3-edge resonant inelastic X-ray scattering. The evolution of magnetic excitations from collective two-triplon excitations to weakly-dispersive gapped modes was tracked by comparing RIXS spectra with and without Ca content. Density matrix renormalization group calculations showed that the observed flat magnetic dispersion and damped excitation profile at high Ca content originated from enhanced hole localization. Our results also demonstrated that off-ladder impurities can affect low-energy magnetic excitations and superconducting correlations in SCCO.