Structural Distortion Stabilizing the Antiferromagnetic and Insulating Ground State of NiO

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.

Nonadiabatic Atomic-Like State Stabilizing Antiferromagnetism and Mott Insulation in MnO

“…In two forgoing papers [5,14], I can provide evidence that, first, the nonadiabatic atomic-like motion in a special partly-filled energy band of NiO is responsible for both the antiferromagnetic and the insulating state and that, secondly, the (small) rhombohedral-like distortion of the crystal is necessary to stabilize an antiferromagnetic eigenstate in a system invariant under time inversion.…”

“…The physical origin of the rhombohedral-like contraction in MnO is the same as in NiO because the concerned magnetic groups are identical. Therefore, in Sections 2 and 3, only a brief summary of Sections 3 and 4 of [5] is given to make the paper self-contained. These sections provide the magnetic group of the antiferromagnetic state and repeat the definition of the term "rhombohedral-like contraction" used in the following sections.…”

“…This section is a brief summary of Section 3 of [5] providing the terms needed in Sections 4 and 6. The symmetry operations of the type IV Shubnikov magnetic group [4]:…”

“…The unitary subgroup C2/c (15) of C c 2/c is based on the monoclinic base-centered Bravais lattice Γ b m and contains (besides the pure translations) the four elements given in Equation (2) of [5]. As in [5], I refer to the magnetic group C c 2/c as:…”

“…Above T N , both monoxides possess the fcc structure Fm3m = Γ f c O 5 h (225) (in parentheses, the international number), and below T N , the antiferromagnetic structure in both materials is invariant under the monoclinic base-centered magnetic group C c 2/c given in Equation (1) [1][2][3][4]. In both MnO and NiO, the antiferromagnetic state is accompanied by a small rhombohedral-like [5] contraction of the crystal. In addition, both compounds are Mott insulators in the antiferromagnetic, as well as in the paramagnetic phase [6].…”