Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy

TL;DR

This study uses time-resolved X-ray photoelectron spectroscopy to observe how ultrashort infrared pulses induce charge redistribution in a high-temperature superconductor, revealing distinct behaviors of oxygen ions in different planes.

Contribution

It demonstrates element-specific, time-resolved insights into charge dynamics in copper-oxide superconductors, highlighting the differential response of oxygen ions in Cu-O planes versus Sr-O planes.

Findings

Oxygen in Cu-O planes is significantly perturbed by infrared pulses.

Apical oxygen remains unaffected by the infrared excitation.

The technique enables studying out-of-equilibrium electronic structures in high-temperature superconductors.

Abstract

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8}+ \delta}$ crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O\,$1s$ component arising from the Cu-O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu-O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O\,$1s$ core-level emission originating from the oxygen ions in the Cu-O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu-O plane in the normal and superconducting phases.