Possible spin gap in a clean sample of the quantum spin liquid candidate: herbertsmithite

The mineral herbertsmithite (HS), ZnCu3(OD)6Cl2, is a prominent candidate to display a quantum spin liquid (QSL) phase. The Cu(2+) spins (s=1/2) in HS are arranged in a 2D kagome lattice and couple antiferromagnetically. The material has been found to not order down to 50 mK, even though the Curie-Weiss temperature is around -300 K. However, HS is prone to structural disorder between Zn and Cu, disrupting the kagome symmetry and introducing coupling between layers. For this reason, experimental results have been interpreted with caution. We have utilized solid state NMR to quantify the structural disorder that is almost invisible by diffraction methods. The samples with the lowest disorder signal display a clear peak in AC-susceptibility at 200 mK, suggesting either magnetic order or the opening of a spin gap of the order 20 micro-eV. Therefore, we believe it is timely to revisit the question of magnetic order and fluctuations in HS. We aim to study our powder sample using OSIRIS, high-resolution spectroscopy data.

矿物赫伯史密斯矿（herbertsmithite, HS），化学式为ZnCu₃(OD)₆Cl₂，是展现量子自旋液体（quantum spin liquid, QSL）物相的典型候选材料。HS中的二价铜离子（Cu²⁺）自旋（自旋量子数s=1/2）排列为二维 kagome 晶格，并发生反铁磁性耦合。尽管该材料的居里-外斯温度约为-300 K，但实验观测到其在低至50 mK的温度范围内均未出现磁有序。然而，HS易发生锌与铜之间的结构无序，破坏了kagome晶格的对称性并引入层间耦合，因此相关实验结果需谨慎解读。我们采用固态核磁共振（solid state NMR）技术，量化了衍射方法几乎无法探测到的结构无序程度。无序度最低的样品在200 mK处呈现出清晰的交流磁化率（AC-susceptibility）峰，这暗示体系要么发生了磁有序，要么打开了约20微电子伏特的自旋隙（spin gap）。因此，我们认为此时重新探讨HS中的磁有序与涨落问题恰逢其时。本研究计划借助高分辨率光谱仪OSIRIS对粉末样品开展测试分析。