Cooper-Pair Localization in the Magnetic Dynamics of a Cuprate Ladder

TL;DR

This study reveals that holes in a cuprate ladder form strongly bound localized Cooper pairs, affecting magnetic dynamics, with extended models needed to explain experimental observations, which may also inform understanding of 2D cuprates.

Contribution

It demonstrates the importance of additional interactions beyond simple models to explain magnetic and pairing phenomena in cuprate ladders.

Findings

Holes form strongly bound localized Cooper pairs.

Standard Hubbard model does not match spectral features.

Extended attractive interaction model explains the results.

Abstract

We investigate the spin dynamics of the cuprate ladder Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ to elucidate the behavior of its intrinsically doped holes. Combining high-resolution neutron spectroscopy and density matrix renormalization group calculations enables a comprehensive analysis of the collective magnetic dynamics. We find a general absence of magnetic signatures from unpaired charges, indicating holes within the system form strongly bound localized Cooper pairs. A one-band Hubbard model fails to match the spectral features but a straightforward extension to a large attractive nearest-neighbor interaction quantitatively explains our results. Our finding shows the significance of additional interactions beyond the long-predicted quantum spin pairing in the ($d$-wave) charge pairing process. Considering the parallels between ladders and two-dimensional cuprates, these results are potentially relevant for square lattices as well.