Dynamic Susceptibility and Phonon Anomalies in the Bilayer $t$-$J$ Model

TL;DR

This paper investigates the bilayer $t$-$J$ model to understand its magnetic and phononic properties, revealing how interlayer interactions influence observable susceptibilities and phonon anomalies in high-temperature superconductors.

Contribution

It demonstrates that in bilayer systems, in-plane $d$-wave pairing prevents interlayer singlet order and computes susceptibility and phonon anomalies relevant to YBCO and BSCCO.

Findings

In-plane $d$-wave pairing excludes interlayer singlet order.

Calculated susceptibilities match experimental Fermi surfaces.

Identified phonon anomalies consistent with observations.

Abstract

We consider a bilayer version of the extended $t$-$J$ model, with a view to computing the form of certain experimentally observable properties. Using the slave-boson decomposition, we show at the mean-field level that in the bilayer system the existence of in-plane $d$-wave singlet pairing excludes any interplane singlet order for reasonable values of the interplane superexchange parameter. Restricting the analysis to the regime of no interplane singlet pairing, we deduce parameter sets reproducing the Fermi surfaces of YBCO- and BSCCO-like bilayer systems. From these we calculate the form of the dynamic susceptibility $\chi( {\bf q}, \omega )$ in both systems, and of the anomalies in frequency and linewidth of selected phonon modes in YBCO. We compare the results with experiment, and discuss the features which differ from the single-layer case.