Field-Induced Selective Spin Gap Closure and Quantum Criticality in BaNd$_2$ZnS$_5$

Sangyun Lee; A. J. Woods; B. Billingsley; Shengzhi Zhang; R. Movshovich; S. M. Thomas; C. A. Mizzi; B. Maiorov; Shuyi Li; Chunjing Jia; Tai Kong; Eun Sang Choi; Vivien S. Zapf; Minseong Lee

arXiv · 2604.20173

Field-Induced Selective Spin Gap Closure and Quantum Criticality in BaNd$_2$ZnS$_5$

Sangyun Lee·A. J. Woods·B. Billingsley·Shengzhi Zhang·R. Movshovich·S. M. Thomas·C. A. Mizzi·B. Maiorov·Shuyi Li·Chunjing Jia·Tai Kong·Eun Sang Choi·Vivien S. Zapf·Minseong Lee

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Abstract

We report thermodynamic evidence for field-induced mode-selective quantum criticality in the layered rare-earth magnet BaNd2ZnS5 (BNZS). Below the Neel temperature TN = 2.9 K, spin-orbit-entangled Nd3+ moments form two symmetry-inequivalent low-energy spin-excitation modes arising from Kramers doublet physics under a magnetic field, with distinct gaps Delta_L and Delta_H. For magnetic fields applied along the [110] direction, the lower-energy gap Delta_L softens continuously and collapses at a critical field Hc ~ 2 T, while the higher-energy gap Delta_H remains gapped, leaving the system in an intermediate partially critical phase. Despite the partial nature of the criticality, thermodynamic measurements reveal a continuous quantum phase transition. The ac susceptibility shows universal scaling behavior, with chi_ac(T, H) collapsing onto a single scaling function and following chi_ac ~ T^-0.2 at criticality. A finite residual Sommerfeld coefficient gamma_0 further indicates the emergence of gapless excitations confined to a single symmetry sector near the quantum critical point. In contrast to conventional quantum criticality based on global softening of low-energy excitations, BNZS exhibits a selective breakdown of Kramers-doublet excitations due to its strong anisotropic interactions. Our results establish BNZS as a spin-orbit-coupled rare-earth magnet where quantum criticality is not global but mode-selective, with anisotropic interactions enabling independent criticality in distinct excitation sectors.

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Sangyun Lee, A. J. Woods, B. Billingsley, Shengzhi Zhang, R. Movshovich, S. M. Thomas, C. A. Mizzi, B. Maiorov, Shuyi Li, Chunjing Jia, Tai Kong, Eun Sang Choi, Vivien S. Zapf, Minseong Lee. 2026-04-22. Field-Induced Selective Spin Gap Closure and Quantum Criticality in BaNd$_2$ZnS$_5$. https://arxiv.org/abs/2604.20173

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