Electronic structure and magnetic properties of the linear chain cuprates Sr_2CuO_3 and Ca_2CuO_3

TL;DR

This study analyzes the electronic structure and magnetic interactions of Sr_2CuO_3 and Ca_2CuO_3 using band-structure calculations and experimental data, revealing differences in their magnetic coupling and providing a detailed model for their properties.

Contribution

It presents a comprehensive parameterization of extended Hubbard and Heisenberg models for these compounds based on band-structure and experimental data, highlighting differences in inter-chain interactions.

Findings

Ca_2CuO_3 has a lower bonding band energy than Sr_2CuO_3.

Inter-chain exchange constants are approximately 0.8 meV for Sr_2CuO_3 and 3.6 meV for Ca_2CuO_3.

The anisotropic Heisenberg model accurately reproduces magnetic properties like T_N and magnetization.

Abstract

Sr_2CuO_3 and Ca_2CuO_3 are considered to be model systems of strongly anisotropic, spin-1/2 Heisenberg antiferromagnets. We report on the basis of a band-structure analysis within the local density approximation and on the basis of available experimental data a careful analysis of model parameters for extended Hubbard and Heisenberg models. Both insulating compounds show half-filled nearly one-dimensional antibonding bands within the LDA. That indicates the importance of strong on-site correlation effects. The bonding bands of Ca_2CuO_3 are shifted downwards by 0.7 eV compared with Sr_2CuO_3, pointing to different Madelung fields and different on-site energies within the standard pd-model. Both compounds differ also significantly in the magnitude of the inter-chain dispersion along the crystallographical a-direction: \approx 100 meV and 250 meV, respectively. Using the band-structure and experimental data we parameterize a one-band extended Hubbard model for both materials which can be further mapped onto an anisotropic Heisenberg model. From the inter-chain dispersion we estimate a corresponding inter-chain exchange constant J_{\perp} \approx 0.8 and 3.6 meV for Sr_2CuO_3 and Ca_2CuO_3, respectively. Comparing several approaches to anisotropic Heisenberg problems, namely the random phase spin wave approximation and modern versions of coupled quantum spin chains approaches, we observe the advantage of the latter in the reproduction of reasonable values for the N\'eel temperature T_N and the magnetization m_0 at zero temperature. Our estimate of $J_{\perp}$ gives the right order of magnitude and the correct tendency going from Sr_2CuO_3 to Ca_2CuO_3. In a comparative study we also include CuGeO_3.