A light-induced charge order mode in a metastable cuprate ladder

Abstract

We report the observation of an emergent charge order mode in the optically-excited cuprate ladder Sr$_{14}$Cu$_{24}$O$_{41}$. Near-infrared light in the ladder plane drives a symmetry-protected electronic metastable state together with a partial melting of the equilibrium charge order. Our time-resolved resonant inelastic x-ray scattering measurements at the upper Hubbard band reveal a gapless collective excitation dispersing from the charge-order wavevector up to 0.8 eV with a slope on the order of the quasiparticle velocity. These findings reveal a regime where correlated carriers acquire itinerant character at finite momentum, and charge order becomes dynamically fluctuating, offering a platform to explore light-induced pairing instabilities.

Figures (3)

• Figure 1: (a) Crystal structure of Sr$_{14}$Cu$_{24}$O$_{41}$ featuring chain and ladder sublattices. Effectively independent at equilibrium, these units become transiently coupled by near-infrared (1.55 eV) excitation, giving rise to a metastable chain-to-ladder hole transfer. (b) Time-resolved x-ray absorption spectroscopy (trXAS) at the O $K$-edge probes the low energy electronic structure of chains and ladders, including upper Hubbard band (UHB) and Zhang-Rice singlet (ZRS) states. (c) Equilibrium (black) and transient (blue, 8 mJ/cm$^2$ fluence) O $K$-edge trXAS at $t = 0.4$ ps. Pump and probe pulses are polarized along the ladder legs ($E \parallel c$). (d-f) Differential trXAS intensity [$I_\mathrm{XAS}(t) - I_\mathrm{XAS}(t < 0)$] at $t = 0.4$ ps, 3 ps, and 101 ps, respectively. The pump-induced spectral reshaping is metastable. The solid lines are fits to the data (see SM Section 1).
• Figure 2: (a) Sketch of the time-resolved resonant inelastic X-ray scattering (trRIXS) experiment. Following optical excitation, O $K$-edge X-ray pulses probe charge dynamics in the ladder via scattering into a grating spectrometer. Black arrows represent spins, pink halos represent charge-ordered holes in the ground state, and the yellow halo a hole transferred from the chain to the ladder. $\lambda_\mathrm{CO}$ denotes the charge-order wavelength. (b) Equilibrium (grey), transient (black), and difference (red) trRIXS spectra at $t$ = 0.4 ps, at momentum transfers $q_\mathrm{leg}$ = 0.16 and 0.28 reciprocal lattice units (r.l.u.) and at resonance with the UHB peak of the XAS spectrum. Here, $q_\mathrm{leg} \parallel c$ and $q_\perp \parallel b$. (c-e) Equilibrium, transient, and difference trRIXS momentum-energy intensity maps, as a function of $q_\mathrm{leg}$ and $q_{\perp}$, respectively. $q_\mathrm{CO}$ marks the charge order wavevector abbamonte2004crystallization. Arrows in (e) indicate the momentum transfer of the spectra in panel (b). Dashed lines in (e) are guides to the eye.
• Figure 3: (a,b) Momentum-dependent RIXS spectra in and out of equilibrium, vertically offset for clarity. Momentum is given in r.l.u. along the ladder direction. Equilibrium spectra in (a) are fit with a Gaussian for the quasielastic peak and an asymmetric Lorentzian for the $\Delta S = 0$ two-triplon (thin black lines). Transient spectra in (b) include an additional Gaussian component (shaded red). (c) Dispersion of the emergent collective mode (red markers) extracted from fits in (b). The shaded gray area denotes the theoretical $\Delta S = 0$ two-triplon continuum from Ref. tseng2023momentum, while blue markers indicate the lower boundary of the $\Delta S = 1$ two-triplon continuum from Ref. padma2025beyond. Error bars represent instrumental uncertainties in momentum transfer and energy-loss reference point.