Low-energy structure of the intertwining double-chain ferrimagnets A_3_Cu_3_(PO_4_)_4_ (A=Ca,Sr,Pb)

TL;DR

This study investigates the low-energy excitations of a class of double-chain ferrimagnets, revealing that their excitation spectrum exhibits weak dispersion contrary to spin-wave theory predictions, with implications for understanding their magnetic properties.

Contribution

The paper provides a detailed analysis combining perturbation, exact diagonalization, and quantum Monte Carlo methods to clarify the excitation spectrum of A_3_Cu_3_(PO_4_)_4_ ferrimagnets, challenging previous spin-wave theory assumptions.

Findings

Weak but nonvanishing momentum dispersion in excitation spectrum

Spin-wave theory predicts flat bands regardless of bond alternation

Low-lying excitation mechanisms resemble those in alternating-spin linear-chain ferrimagnets

Abstract

Motivated by the homometallic intertwining double-chain ferrimagnets A_3_Cu_3_(PO_4_)_4_ (A=Ca,Sr,Pb), we investigate the low-energy structure of their model Hamiltonian H=\sum_n_[J_1_(S_{n :1}_+S_{n :3}_) +J_2_(S_{n+1:1}+S_{n-1:3}_)]\cdotS_{n:2}_, where S_{n:l}_ stands for the Cu^{2+}^ ion spin labeled l in the nth trimer unit, with particular emphasis on the range of bond alternation 0<J_2/J_1<1. Although the spin-wave theory, whether up to O(S^1^) or up to O(S^0^), claims that there exists a flat band in the excitation spectrum regardless of bond alternation, a perturbational treatment as well as the exact diagonalization of the Hamiltonian reveals its weak but nonvanishing momentum dispersion unless J_2_=J_1_ or J_2_=0. Quantum Monte Carlo calculations of the static structure factor further convince us of the low-lying excitation mechanism, elucidating similarities and differences between the present system and alternating-spin linear-chain ferrimagnets.