XY magnetism, Kitaev exchange, and long-range frustration in theJeff=12honeycomb cobaltates,Physical Review B

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XY magnetism, Kitaev exchange, and long-range frustration in theJeff=12honeycomb cobaltates
Physical Review B ( IF 3.2 ) Pub Date : 2021-10-22 , DOI: 10.1103/physrevb.104.134425
Shreya Das ₁ , Sreekar Voleti ₂ , Tanusri Saha-Dasgupta ₁ , Arun Paramekanti ₂

Affiliation

The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with

3 d

transition-metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates

{CoTiO}_{3},

{BaCo}_{2} {({PO}_{4})}_{2}

, and

{BaCo}_{2} {({AsO}_{4})}_{2}

, using ab initio density-functional theory, which takes into account realistic crystal-field distortions and chemical information, in conjunction with exact diagonalization numerics. We show that these

{Co}^{2 +}

magnets host

J_{eff} = 1 / 2

local moments with highly anisotropic

g

factors, and we extract their full spin Hamiltonians including longer-range and anisotropic exchange couplings. For

{CoTiO}_{3}

, we find a nearest-neighbor easy-plane ferromagnetic

X X Z

model with additional bond-dependent anisotropies and interlayer exchange, which supports three-dimensional Dirac nodal line magnons. In contrast, for

{BaCo}_{2} {({PO}_{4})}_{2}

and

{BaCo}_{2} {({AsO}_{4})}_{2}

, we find a strongly suppressed interlayer coupling, and significant frustration from additional third-neighbor antiferromagnetic exchange mediated by P or As. Such bond-anisotropic

J_{1} − J_{3}

spin models can support collinear zigzag or coplanar spiral ground states. We discuss their dynamical spin correlations which reveal a gapped Goldstone mode and argue that the effective parameters of the pseudospin-

1 / 2

models in these two materials may be strongly renormalized by coupling to a low-energy spin exciton. Our results call for reexamining proposals for realizing Kitaev spin liquids in the honeycomb cobaltates.

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