Magnetism and spin dynamics of a weakly interacting rare-earth stretched diamond lattice

Résumé :

In condensed matter physics, a quantum-disordered ground state is characterized by the absence of long-range order down to absolute zero temperature, where the disorder is driven by quantum fluctuations that persist even at T=0. On the other hand, the magnetism of rare-earth materials is particularly intriguing as it originates from their localized 4f electrons, which exhibit strong spin–orbit coupling, pronounced magnetic anisotropy, and relatively weak exchange interactions between magnetic ions. These unique characteristics, when combined with suitable lattice geometry, can give rise to a wide variety of unconventional magnetic ground states.

A diamond magnetic lattice is a bipartite network and in its ideal form, does not exhibit geometric frustration. However, “stretched” diamond lattices have recently gained attention where frustration can emerge due to competition between nearest-neighbor (J1) and next-nearest-neighbor (J2) exchange interactions, despite preserving the bipartite nature. In this context, we investigate the magnetic ground state of the rare-earth molybdate compound Na₅Yb(MoO₄)₄, which crystallizes in a stretched diamond magnetic lattice. This compound can be viewed as a complex derivative of the conventional scheelite- type ABO₄ structure, crystallizing in the tetragonal space group I4₁/a. The magnetic lattice in Na₅Yb(MoO₄)₄ is highly unusual, featuring a remarkably large nearest-neighbor Yb–Yb separation of approximately 6.3 Å, in contrast to previously studied stretched-diamond systems where the J1 distance typically lies in the range of 3–5 Å and is predominantly governed by superexchange interactions. Furthermore, the next-nearest-neighbor (J2) Yb–Yb distance exceeds 9 Å, significantly weakening the J2 exchange interactions. As a result, magnetic frustration arising from competing J1–J2 exchange is significantly suppressed in Na₅Yb(MoO₄)₄, distinguishing it from other frustrated diamond-lattice systems.

We employ neutron and synchrotron X-ray diffraction to elucidate the structural details of Na₅Yb(MoO₄)₄. The magnetic properties and ground state are investigated using bulk magnetic susceptibility measurements, specific heat studies, and muon spin relaxation (μSR) experiments. In addition, density functional theory calculations within the DFT+U framework are used to provide theoretical support for the experimental findings. Our results establish Na₅Yb(MoO₄)₄ as a rare example of a dipolar quantum paramagnet in which single-ion physics and long-range dipolar interactions dominate, while exchange interactions are suppressed to the millikelvin energy scale.

Reference

1. N. D. Kelly et al., Physical Review Materials 6, 044410 (2022).
2. A. Chauhan et al., Physical Review B 108, 134424 (2023).
3. J. Kumar et al., Physical Review B 111, 014411 (2025).
4. T. Arh et al., Nat. Mater. 21, 416 (2022).

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