Fractional and composite excitations of antiferromagnetic quantum spin trimer chains

Jun-Qing Cheng,Jun Li,Dao-Xin Yao,Zijian Xiong,Han-Qing Wu,Anders W Sandvik

doi:10.1038/s41535-021-00416-4

Abstract

Using quantum Monte Carlo, exact diagonalization, and perturbation theory, we study the spectrum of the S = 1/2 antiferromagnetic Heisenberg trimer chain by varying the ratio g = J2/J1 of the intertrimer and intratrimer coupling strengths. The doublet ground states of trimers form effective interacting S = 1/2 degrees of freedom described by a Heisenberg chain. Therefore, the conventional two-spinon continuum of width ∝ J1 when g = 1 evolves into to a similar continuum of width ∝ J2 when g → 0. The intermediate-energy and high-energy modes are termed doublons and quartons which fractionalize with increasing g to form the conventional spinon continuum. In particular, at g ≈ 0.716, the gap between the low-energy spinon branch and the high-energy band with mixed doublons, quartons, and spinons closes. These features should be observable in inelastic neutron scattering experiments if a quasi-one-dimensional quantum magnet with the linear trimer structure and J2 < J1 can be identified. Our results may open a window for exploring the high-energy fractional excitations.

Highlights

IntroductionMany quasi one-dimensional (1D) magnetic materials with spin
Many quasi one-dimensional (1D) magnetic materials with spinS = 1/2 moments harbor exotic phenomena originating from the physics of the Heisenberg antiferromagnetic chain (HAC) and its extensions[1]
Model The Hamiltonian of the spin-1/2 antiferromagnetic trimer chain with periodic boundary conditions reads and related methods formulated with matrix-product states (MPS)[40] are very powerful for 1D systems and are applicable for calculations of dynamical structure factors[41,42], primarily for systems with open boundaries, due to inefficiency in the case of periodic chains

Summary

Introduction

Many quasi one-dimensional (1D) magnetic materials with spin. The dynamic spin structure factor of KCuF32,3, measured using inelastic neutron scattering, exhibits the characteristic gapless two-spinon continuum[4] of the uniform HAC. A phase transition to a gapped dimerized state, driven by additional frustration and spin-phonon couplings has been realized in CuGeO35. The resonant inelastic X-ray scattering (RIXS) technique has enabled specific detection of multi-spinon excitations[12,13] in the HAC material Sr2CuO3, and string excitations have been identified by terahertz spectroscopy in the Heisenberg−Ising compounds SrCo2V2O814 and BaCo2V2O815. The case of strong intertrimer coupling has been investigated theoretically[18], and other quantum magnets with trimerized structure have attracted attention[27,28,29]

Objectives

Results

Conclusion