Collective excitations in unconventional charge-density wave systems

TL;DR

This paper investigates the collective excitations in the d-density-wave state of the t-J model, revealing characteristic features detectable via neutron scattering, with implications for understanding unconventional charge-density waves.

Contribution

It provides a theoretical framework for the density response in DDW states, including expressions applicable to various scattering experiments and numerical results highlighting collective excitation signatures.

Findings

Density response is mainly collective with broad, dispersive features.

Well-defined peaks appear at small momentum transfers below twice the DDW gap.

Inelastic neutron scattering can effectively detect these excitations.

Abstract

The excitation spectrum of the t-J model is studied on a square lattice in the large $N$ limit in a doping range where a $d$-$density$-$wave$ (DDW) forms below a transition temperature $T^\star$. Characteristic features of the DDW ground state are circulating currents which fluctuate above and condense into a staggered flux state below $T^\star$ and density fluctuations where the electron and the hole are localized at different sites. General expressions for the density response are given both above and below $T^\star$ and applied to Raman, X-ray, and neutron scattering. Numerical results show that the density response is mainly collective in nature consisting of broad, dispersive structures which transform into well-defined peaks mainly at small momentum transfers. One way to detect these excitations is by inelastic neutron scattering at small momentum transfers where the cross section (typically a few per cents of that for spin scattering) is substantially enhanced, exhibits a strong dependence on the direction of the transferred momentum and a well-pronounced peak somewhat below twice the DDW gap. Scattering from the DDW-induced Bragg peak is found to be weaker by two orders of magnitude compared with the momentum-integrated inelastic part.