Prediction of pseudogap formation due to $d$-wave bond-order in organic superconductor $\kappa$-(BEDT-TTF)$_2$X

TL;DR

This paper predicts a $d$-wave bond-order in the organic superconductor $$-(BEDT-TTF)$_2$X, explaining pseudogap phenomena through advanced theoretical methods, and aligns with experimental observations.

Contribution

It introduces the first prediction of $d$-wave bond-order in this material using fRG and density-wave theories, linking hidden order to pseudogap behavior.

Findings

Prediction of $d$-wave bond-order at specific wavevector

Correlation between bond-order and pseudogap features

Consistency with experimental NMR and density-of-states data

Abstract

Rich hidden unconventional orders with pseudogap formation, such as the inter-site bond-order (BO), attract increasing attention in condensed matter physics. Here, we investigate the hidden order formation in organic unconventional superconductor $\kappa$-(BEDT-TTF)$_2$X. We predict the formation of $d$-wave BO at wavelength $q=Q_B=(\delta,\delta)$ ($\delta=0.38\pi$) for the first time, based on both the functional renormalization group (fRG) and the density-wave equation theories. The origin of the BO is the quantum interference among antiferromagnetic spin fluctuations. This prediction leads to distinct pseudogap-like reduction in the NMR $1/T_1$ relaxation rate and in the density-of-states, consistently with essential experimental reports. The present theory would be applicable for other strongly correlated metals with pseudogap formation.