Mechanism of Basal-Plane Antiferromagnetism in the Spin-Orbit Driven IridateBa2IrO4

Vamshi M Katukuri,Jeroen Van Den Brink,Liudmila Siurakshina,Ioannis Rousochatzakis,Liviu Hozoi,Viktor Yushankhai

doi:10.1103/physrevx.4.021051

Abstract

By ab initio many-body quantum chemistry calculations, we determine the strength of the symmetric anisotropy in the 5$d^5$ j $\approx$ 1/2 layered material Ba$_2$IrO$_4$. While the calculated anisotropic couplings come out in the range of a few meV, orders of magnitude stronger than in analogous 3d transition-metal compounds, the Heisenberg superexchange still defines the largest energy scale. The ab initio results reveal that individual layers of Ba$_2$IrO$_4$ provide a close realization of the quantum spin-1/2 Heisenberg-compass model on the square lattice. We show that the experimentally observed basal-plane antiferromagnetism can be accounted for by including additional interlayer interactions and the associated order-by-disorder quantum-mechanical effects, in analogy to undoped layered cuprates.

Highlights

The few varieties of square-lattice effective spin models are emblematic in modern quantum magnetism and extensively investigated in relation to layered superconducting materials such as the copper oxides [1] and iron pnictides or chalcogenides [2]
The ab initio results reveal that individual layers of Ba2IrO4 provide a close realization of the quantum spin-1=2 Heisenberg-compass model on the square lattice
In 2D iridates, on the other hand, much less information is presently available on the magnitude of various electronic-structure parameters that enter the superexchange models. While estimates for these effective electronic-structure parameters are normally based on either density-functional band-structure calculations [15,16,18,24,25] or experiments [10,11,13,17,20], here we rely on many-body quantum-chemistry methods to directly obtain an ab initio assessment of both the NN Heisenberg exchange and the anisotropic couplings on the square lattice of Ba2IrO4

Summary

INTRODUCTION

The few varieties of square-lattice effective spin models are emblematic in modern quantum magnetism and extensively investigated in relation to layered superconducting materials such as the copper oxides [1] and iron pnictides or chalcogenides [2]. In 2D iridates, on the other hand, much less information is presently available on the magnitude of various electronic-structure parameters that enter the superexchange models While estimates for these effective electronic-structure parameters are normally based on either density-functional band-structure calculations [15,16,18,24,25] or experiments [10,11,13,17,20], here we rely on many-body quantum-chemistry methods to directly obtain an ab initio assessment of both the NN Heisenberg exchange and the anisotropic couplings on the square lattice of Ba2IrO4. As we discuss in the following, this symmetry analysis is useful in determining the nature of each of the low-lying many-body states in the quantum-chemistry calculations

GENERAL CONSIDERATIONS

Ab initio results

COMPARISON TO EFFECTIVE SUPEREXCHANGE MODELS

U t1sin2θ þ t2 2

GROUND-STATE PHASE DIAGRAM

CONCLUSIONS