During the past few years, the Mo3O8 quantum magnets with kagome layers formed by Mo atoms have attracted a great deal of attention as a new candidate to realise quantum spin liquid states. The magnetic susceptibility of LiZn2Mo3O8 has been experimentally shown to follow a Curie-Weiss law with the low- and high-temperature regimes whose Curie constants are related as CL~1/3CH. Originally, the disappearance of 2/3 of the paramagnetic spins at low temperatures in LiZn2Mo3O8 was attributed to valence bond condensation on the trimerized lattice of the Mo3O13 clusters [1], and several possible scenarios have been proposed to describe this unusual behaviour [2,3]. However, their adequacy was questioned by recently synthesized Li2InMo3O8, that shows a conventional Neel 120° magnetic order, and its counterpart Li2ScMo3O8, where no magnetic order has been observed down to 0.5 K [4]. Motivated by these studies, we combine rigorous first-principles calculations and effective low-energy models to study the microscopic origin of magnetism in LiZn2Mo3O8, Li2InMo3O8, and Li2ScMo3O8. Considering the extended Hubbard model constructed in the basis of Wannier functions for the Mo d states on the anisotropic kagome lattice, we argue that the magnetic properties of these systems can be described by two different regimes of electron localization defined by the competition of kinetic energy and intersite Coulomb interactions. Being in the strong interaction limit, LiZn2Mo3O8 will be proposed to reveal a plaquette charge order due to the correlated collective motion of electrons, where the partial spin pairing decoupling 2/3 of the spin originates from the asymmetry of tunneling processes. On the other hand, Li2InMo3O8 and Li2ScMo3O8 will be shown to fall into a cluster Mott insulator regime with the electrons localized at the Mo3O13 clusters, where the former exhibits long-range antiferromagnetic order and the latter is more likely to reveal short-range order with quantum spin liquid-like excitations.References: [1] J. P. Sheckelton, J. R. Neilson, D. G. Soltan, T. M. McQueen, Nat. Mater., Vol. 11, p. 493 (2012). [2] Rebecca Flint and Patric A. Lee, Phys. Rev. Lett., Vol. 111, p. 217201 (2013). [3] Gang Chen, Hae-Young Kee, and Yong Baek Kim, Phys. Rev. B, Vol. 93, p. 245134 (2016). [4] Yuya Haraguchi, Chishiro Michioka, Masaki Imai et. al, Phys. Rev. B, Vol. 92, p. 014409 (2015). 

First-principles studies of quantum magnetism in Mo3O8 cluster systems

Being studied mostly in nonpolar chiral structures, skyrmions in polar crystals are of great interest owing to their interplay with electric polarization, giving rise to fascinating multiferroic properties. Recently, skyrmion lattices have been reported in lacunar spinel GaV4S8 [1], where a complex phase diagram comprising paramagnetic, ferromagnetic, skyrmion, and cycloidal states has been demonstrated, and the spin-driven excess polarization was assigned in each magnetic phase with a total value of ~100 μC/m2 [2]. The existence of multiple ferroelectric phases in GaV4S8 indicates an intricate interplay between charge, spin, and lattice degrees of freedom, making their theoretical description extremely important. In our studies, we present the microscopic theory of spin-driven electric polarization P in GaV4S8 and GaMo4S8 [3,4]. To this end, we extend the theory of superexchange for magnetic interactions to deal with the change of electric polarization depending on the relative direction of spins in each bond. Based on realistic Hubbard-type models derived for the magnetically active molecular orbitals formed in the V4 and Mo4 tetrahedra, we show that (i) although a skyrmion layer in these materials is mainly formed by the in-plane magnetic interactions, the spin-excess polarization perpendicular to the rhombohedral z axis is given by the interlayer electron transfer and originates from strongly competing isotropic and antisymmetric contributions; (ii) P arises from the stacking misalignment of such layers in the perpendicular direction, which is inherent to the lacunar spinel structure; (iii) while the interorbital Hund’s coupling is small, it is one of the key parameter responsible for P in lacunar spinels; (iv) an importnat aspect of multiferroicity in GaMo4S8, which can also potentially host skyrmionic states, is strong exchange anisotropy; and (v) the direction of rhombohedral distortions leads to opposite signs of P in GaV4S8 and GaMo4S8.References: [1] I. Kézsmárki, S. Bordács, P. Milde et. al, Nat. Mater., Vol. 14, 1116 (2015). [2] E. Ruff, S. Widmann, P. Lunkenheimer et. al, Sci. Adv., Vol. 10, e1500916 (2015). [3] S. A. Nikolaev and I. V. Solovyev, Phys. Rev. B, Vol. 99, 100401(R) (2019). [4] S. A. Nikolaev and I. V. Solovyev, Phys. Rev. B 102 014414 (2020).

Microscopic theory of spin driven electric polarization in lacunar spinels GaV4S8 and GaMo4S8

B4: Complex Oxide, Heusler Alloy, and 2D Systems

poster

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Sergey Nikolaev

2

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First-principles studies of quantum magnetism in Mo3O8 cluster systems

Microscopic theory of spin driven electric polarization in lacunar spinels GaV4S8 and GaMo4S8

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