Cuprate superconductors as viewed through a striped lens

TL;DR

This paper reviews the complex interplay of charge, spin, and pairing phenomena in cuprate superconductors, emphasizing stripe order's role in their unconventional superconductivity and the balance between pairing strength and phase coherence.

Contribution

It provides a comprehensive, updated understanding of cuprate superconductivity through the lens of stripe order and emergent electronic correlations.

Findings

Stripe order frustrates bulk superconductivity but enables pairing.

Superconductivity involves a compromise between pairing scale and phase coherence.

In-phase and antiphase superconductivity depend on spin stripe energy gaps.

Abstract

Understanding the electron pairing in hole-doped cuprate superconductors has been a challenge, in particular because the "normal" state from which it evolves is unprecedented. Now, after three and a half decades of research, involving a wide range of experimental characterizations, it is possible to delineate a clear and consistent cuprate story. It starts with doping holes into a charge-transfer insulator, resulting in in-gap states. These states exhibit a pseudogap resulting from the competition between antiferromagnetic superexchange $J$ between nearest-neighbor Cu atoms (a real-space interaction) and the kinetic energy of the doped holes, which, in the absence of interactions, would lead to extended Bloch-wave states whose occupancy is characterized in reciprocal space. To develop some degree of coherence on cooling, the spin and charge correlations must self-organize in a cooperative fashion. A specific example of resulting emergent order is that of spin and charge stripes, as observed in La$_{2-x}$Ba$_x$CuO$_4$. While stripe order frustrates bulk superconductivity, it nevertheless develops pairing and superconducting order of an unusual character. The antiphase order of the spin stripes decouples them from the charge stripes, which can be viewed as hole-doped, two-leg, spin-$\frac12$ ladders. To achieve superconducting order, the pair correlations in neighboring ladders must develop phase order. In the presence of spin stripe order, antiphase Josephson coupling can lead to pair-density-wave superconductivity. Alternatively, in-phase superconductivity requires that the spin stripes have an energy gap, which empirically limits the coherent superconducting gap. Hence, superconducting order in the cuprates involves a compromise between the pairing scale, which is maximized at $x\sim\frac18$, and phase coherence, which is optimized at $x\sim0.2$.