Collective mode in the SU(2) theory of cuprates

TL;DR

This paper investigates the SU(2) theory of cuprates by analyzing a collective charge mode's influence on charge susceptibility, connecting theoretical predictions with recent experimental techniques like MEELS and RIXS.

Contribution

It derives the impact of the SU(2) collective mode on charge susceptibility and relates it to experimental observations in cuprates.

Findings

Two peaks in charge susceptibility at finite energy.

Peaks evolve with the phase diagram.

Signature of SU(2) fluctuations in experimental data.

Abstract

Recent advances in momentum-resolved electron energy-loss spectroscopy (MEELS) and resonant inelastic X-ray scattering (RIXS) now allow one to access the charge response function with unprecedented versatility and accuracy. This allows for the study of excitations which were inaccessible recently, such as low-energy and finite momentum collective modes. The SU(2) theory of the cuprates is based on a composite order parameter with SU(2) symmetry fluctuating between superconductivity and charge order. The phase where it fluctuates is a candidate for the pseudogap phase of the cuprates. This theory has a signature, enabling its strict experimental test, which is the fluctuation between these two orders, corresponding to a charge 2 spin 0 mode at the charge ordering wave-vector. Here we derive the influence of this SU(2) collective mode on the charge susceptibility in both strong and weak coupling limits, and discuss its relation to MEELS, RIXS and Raman experiments. We find two peaks in the charge susceptibility at finite energy, whose middle is the charge ordering wave-vector, and discuss their evolution in the phase diagram.