Anomalous Excitation Spectra of Frustrated Quantum Antiferromagnets

TL;DR

This paper investigates the excitation spectra of frustrated quantum antiferromagnets using series expansions, revealing anomalous features that deviate from traditional theories and are explained by a spinon perspective.

Contribution

It provides new insights into the excitation spectra of frustrated antiferromagnets, highlighting anomalies and their interpretation via spinon models, supported by series expansion calculations.

Findings

High-energy spectra show anomalous features differing from linear spin-wave theory.

Deviations from LSWT increase with frustration, leading to roton-like minima.

Results align with experimental observations on Cs_2CuCl_4.

Abstract

We use series expansions to study the excitation spectra of spin-1/2 antiferromagnets on anisotropic triangular lattices. For the isotropic triangular lattice model (TLM) the high-energy spectra show several anomalous features that differ strongly from linear spin-wave theory (LSWT). Even in the Neel phase, the deviations from LSWT increase sharply with frustration, leading to roton-like minima at special wavevectors. We argue that these results can be interpreted naturally in a spinon language, and provide an explanation for the previously observed anomalous finite-temperature properties of the TLM. In the coupled-chains limit, quantum renormalizations strongly enhance the one-dimensionality of the spectra, in agreement with experiments on Cs_2CuCl_4.