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arXiv · 2510.12006

Evidence for easy-plane XY ferromagnetism in heavy-fermion quantum-critical CeRh6Ge4

Riku Yamamoto·Sejun Park·Zachary W. Riedel·Phurba Sherpa·Joe D. Thompson·Filip Ronning·Eric D. Bauer·Adam P. Dioguardi·Michihiro Hirata
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Abstract

We report $^{73}$Ge nuclear quadrupole resonance (NQR) and magnetic resonance (NMR) spectroscopy in the heavy-fermion quantum-critical ferromagnet CeRh$_6$Ge$_4$. NQR and NMR spectral measurements at the two non-equivalent Ge sites reveal electric field gradient tensors and the directions of their principal axes relative to the hexagonal basal plane. The spin-lattice relaxation rate $1/T_1$ experiments reveal a clear critical slowing down approaching the ferromagnetic transition. $1/T_1$ in the paramagnetic state is found to be predominantly caused by fluctuating 4$f$-local moments. The Knight shift shows Curie-Weiss behavior at high temperature and a deviation from this below $T^\ast$ $\approx$ 25 K possibly due to a mixture of crystalline electric field effects and Kondo screening. Order-parameter-like behavior of hyperfine fields at the Ge sites and ferromagnetic signal enhancement are observed in Zeeman-perturbed NQR, with uniform ferromagnetic order and a small ordered moment of $\approx$ 0.26 $\mu_B$/Ce confined within the $ab$-plane. The ordered moment shows a notable in-plane magnetic stiffness against out-of-plane radiofrequency fields and has an (XY-type) in-plane isotropic nature. Our results reveal a strong easy-plane anisotropy of 4$f$-electron moment and suggest an involved interplay of hybridization and local moment physics in this quantum critical heavy-fermion ferromagnet.

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Riku Yamamoto, Sejun Park, Zachary W. Riedel, Phurba Sherpa, Joe D. Thompson, Filip Ronning, Eric D. Bauer, Adam P. Dioguardi, Michihiro Hirata. 2025-10-13. Evidence for easy-plane XY ferromagnetism in heavy-fermion quantum-critical CeRh6Ge4. https://doi.org/10.1103/z17k-jz1s

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