Crossover from Collective to Incoherent Spin Excitations in Superconducting Cuprates Probed by Detuned Resonant Inelastic X-ray Scattering

TL;DR

This study uses resonant inelastic x-ray scattering to explore how spin excitations in cuprate superconductors evolve from collective to incoherent behavior as doping increases, revealing a crossover near optimal doping.

Contribution

It provides experimental evidence of a crossover from collective to incoherent spin excitations in cuprates, supported by theoretical modeling with the Hubbard model.

Findings

Optimal doping spectra show paramagnon features consistent with prior underdoped results.

Beyond optimal doping, spectra shift to incoherent particle/hole excitations with fluorescence-like behavior.

Results suggest a quantum phase transition near optimal doping.

Abstract

Spin excitations in the overdoped high temperature superconductors Tl$_2$Ba$_2$CuO$_{6+\delta}$ and (Bi,Pb)$_2$(Sr,La)$_{2}$CuO$_{6+\delta}$ were investigated by resonant inelastic x-ray scattering (RIXS) as functions of doping and detuning of the incoming photon energy above the Cu-$L_3$ absorption peak. The RIXS spectra at optimal doping are dominated by a paramagnon feature with peak energy independent of photon energy, similar to prior results on underdoped cuprates. Beyond optimal doping, the RIXS data indicate a sharp crossover to a regime with a strong contribution from incoherent particle/hole excitations whose maximum shows a fluorescence-like shift upon detuning. The spectra of both compound families are closely similar, and their salient features are reproduced by exact-diagonalization calculations of the single-band Hubbard model on a finite cluster. The results are discussed in the light of recent transport experiments indicating a quantum phase transition near optimal doping.