Structural Distortion Stabilizing the Antiferromagnetic and Insulating Ground State of NiO

Ekkehard Krüger

doi:10.3390/sym12010056

Abstract

We report evidence that the experimentally observed small deformation of antiferromagnetic NiO modifies the symmetry of the crystal in such a way that the antiferromagnetic state becomes an eigenstate of the electronic Hamiltonian. This deformation closely resembles a rhombohedral contraction, but does not possess the perfect symmetry of a trigonal (rhombohedral) space group. We determine the monoclinic base centered magnetic space group of the antiferromagnetic structure within the deformed crystal which is strongly influenced by the time-inversion symmetry of the Hamiltonian. The antiferromagnetic state is evidently stabilized by a nonadiabatic atomic-like motion of the electrons near the Fermi level. This atomic-like motion is characterized by the symmetry of the Bloch functions near the Fermi level and provides in NiO a perfect basis for a Mott insulator in the antiferromagnetic phase.

Highlights

Nickel monoxide is antiferromagnetic with the relatively high Néel temperature TN = 523 K. fAbove TN, NiO possesses the fcc structure Fm3m = Γc Oh5 bearing the international number 225 [1].Cracknell and Joshua [2] found that, below TN, the magnetic structure is invariant under the magnetic group Cc 2/c (Number 90 in Table 7.4 of [3]), which will be given explicitly in Equation (1).The antiferromagnetic state is accompanied by a small contraction of the crystal along one of the triad axes often referred to as a rhombohedral deformation
In summary: In the forgoing Section 3, we report evidence that the antiferromagnetic ground state of NiO is stable only if M9 in Equation (12) is the magnetic group of the antiferromagnetic structure
We found narrow, partly filled bands with optimally localized symmetry adapted Wannier functions in the band structures of magnetic materials and of superconductors by allowing the Wannier functions: (i) to be adapted only to the magnetic group M of the magnetic structure or (ii) to be spin dependent, respectively

Summary

Introduction

The magnetic group Cc 2/c does not possess any trigonal (rhombohedral) subgroup This interpretation, if taken literally, seems to imply that the ground state of NiO does not possess any symmetry because, clearly, it cannot have two space groups. Having determined explicitly the magnetic group of the ground state of NiO, the group-theoretical nonadiabatic Heisenberg model (NHM) [4] becomes applicable. On the basis of the symmetry of the Bloch functions in the band structure of

Results

Discussion

Conclusion