Magnetic impurities in a charge-ordered background

TL;DR

This study explores how magnetic impurities influence charge-density wave order in a disordered Hubbard-Holstein model, revealing suppression of CDW, emergence of bad insulator states, and enhanced pairing correlations at high doping.

Contribution

It demonstrates the impact of magnetic impurities on CDW order, critical temperature reduction, and the transition to bad insulator states using advanced quantum Monte Carlo simulations.

Findings

AFM correlations hinder CDW around impurities

Critical temperature drops with impurity concentration, vanishing near 40%

Disorder induces bad insulating states with suppressed gaps

Abstract

We investigate how magnetic impurities may affect a system exhibiting charge-density wave (CDW) in its ground state. We consider a disordered Hubbard-Holstein model with a homogeneous electron-phonon interaction, but with a (randomly chosen) fraction of sites displaying a non-zero Coulomb repulsion, $U$, and perform state-of-the-art finite-temperature quantum Monte Carlo simulations. For a single magnetic impurity, charge-charge correlations hamper the spin-spin ones around the repulsive site, thus requiring a strong enough value of $U$ to create non-negligible antiferromagnetic (AFM) correlations. As the number of magnetic impurities increases, these AFM correlations become deleterious to CDW order and its features. First, the critical temperature is drastically reduced, and seems to vanish around 40$\%$ of impurities (for fixed $U/\lambda=2$), which we correlate with the classical percolation threshold. We also notice that just a small amount of disorder suffices to create a \textit{bad insulating} state, with the suppression of both Peierls and spin gaps, even within the charge-ordered phase. Finally, we have also found that pairing correlations are enhanced at large doping, driven by the competition between CDW and AFM tendencies.