Can dd excitations mediate pairing ?

TL;DR

This paper investigates whether $dd$ excitations in high-$T_c$ cuprates can mediate pairing interactions, exploring a potential orbital mechanism beyond traditional electron-phonon coupling theories.

Contribution

It introduces the idea that $dd$ excitations could play a role in mediating pairing in high-$T_c$ cuprates, offering a new perspective on the pairing mechanism.

Findings

Evidence of strong coupling between $dd$ excitations and doped holes.

Proposes a novel orbital-mediated pairing mechanism.

Revisits the role of orbital degrees of freedom in high-$T_c$ superconductivity.

Abstract

The Cu-$3d$ states in the high-$T_c$ cuprates are often described as a single band of $3d_{x^2-y^2}$ states, with the other four $3d$ states having about 2 to 3 eV higher energy due to the lower-than-octahedral crystal field at the copper sites. However, excitations to these higher energy states observed with RIXS show indications of strong coupling to doped holes in the $3d_{x^2-y^2}$ band. This relaunches a decades-old question of the possible role of the orbital degrees of freedom that once motivated Bednorz and M\"uller to search for superconductivity in these systems. Here we explore a direction different from the Jahn-Teller electron-phonon coupling considered by Bednorz and M\"uller, namely the interaction between holes mediated by $dd$ excitations.