Crystallography-driven molecularization of a two-dimensional spin-$3/2$ magnet

TL;DR

This paper reveals that the unique crystallography of Na$_2$Mn$_3$O$_7$ causes magnetic degrees of freedom to become molecularized, leading to a disordered quantum ground state despite strong antiferromagnetic interactions.

Contribution

It demonstrates how crystallographic inequivalence can induce molecularization of magnetic moments, stabilizing quantum-disordered states in large-spin 2D magnets.

Findings

Magnetic correlations develop in two stages within hexagons and across the lattice.

The material exhibits no magnetic order despite sizable antiferromagnetic interactions.

Crystallography-driven hierarchy isolates hexagons, preventing long-range order.

Abstract

Large-spin two-dimensional magnets are generally expected to develop conventional long-range order once the dominant exchange scale becomes appreciable. The layered spin-$3/2$ maple-leaf compound Na$_2$Mn$_3$O$_7$ defies this expectation: despite sizable antiferromagnetic interactions and no evident disorder, it exhibits no magnetic ordering and displays two well-separated thermodynamic crossover scales. We show that this behavior originates from a crystallography-driven molecularization of the magnetic degrees of freedom. The low-symmetry structure partitions the Mn sublattice into inequivalent exchange pathways, generating a pronounced hierarchy that nearly isolates antiferromagnetic hexagons. Magnetic correlations therefore develop in two stages: first within individual hexagons at a scale set by the dominant exchange, and only at much lower temperatures do frustrated inter-hexagon couplings attempt to establish coherence across the lattice. While isolated hexagons reproduce the two-step thermodynamic structure, the experimentally relevant temperature scales emerge only once the hexagons are embedded in the frustrated two-dimensional network. The resulting quantum ground state is magnetically disordered, characterized by strong intra-hexagon correlations and rapidly decaying inter-hexagon correlations. These results identify crystallographic inequivalence as a materials-level mechanism for stabilizing molecularized and quantum-disordered states even in large-spin two-dimensional magnets.