kagome反铁磁体YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_x$]中自旋子局域化的核磁共振证据
NMR evidence of spinon localization in kagome antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_x$]
AI总结：
该研究通过核磁共振实验发现kagome反铁磁体YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_x$]中自旋子因杂质引起的符号涨落而局域化，形成近零能量平带，解释了低温下的异常激发行为。
摘要：
我们对kagome反铁磁体YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$]进行了核磁共振研究。从1 K降至0.05 K，Br中心峰未发现显著的核磁共振谱线展宽，表明不存在静态反铁磁有序。与30 K以上温度时占主导的二维kagome反铁磁涨落特征相反，当样品从30 K冷却至8 K时，Knight位移$K_{\ m{n}}$和自旋-晶格弛豫率$1/T_{1}$均增大，这可归因于强非磁性杂质对自旋激发的散射。异常的是，在接近2 K（远低于交换能）处，$K_{\ m{n}}$和$1/T_{2}$出现一个驼峰，表明存在接近零能量且具有大态密度的激发。这些现象通过具有键依赖交换相互作用的Heisenberg模型的平均场模拟得到了重现，其中由杂质引起的自旋子动能项中的符号涨落导致自旋子局域化，并在费米能量附近形成几乎平坦的能带。
英文摘要：
We performed nuclear magnetic resonance studies on a kagome antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$]. No significant NMR spectral broadening is found in the Br center peak from 1 K down to 0.05 K, indicating absence of static antiferromagnetic ordering. In contrast to signatures of dominant 2D kagome antiferromagnetic fluctuations at temperature above 30 K, both the Knight shift $K_{\rm{n}}$ and the spin-lattice relaxation rate $1/T_{1}$ increase when the sample is cooled from 30 K to 8 K, which can be attributed to the scattering of spin excitations by strong non-magnetic impurities. Unusually, a hump is observed in $K_{\rm{n}}$ and $1/T_{2}$ close to 2 K (far below the exchange energy), which indicates the existence of excitations with a large density of states close to zero energy. These phenomena are reproduced by a mean-field simulation of Heisenberg model with bond-dependent exchange interactions, where the sign fluctuations in the spinon kinetic terms caused by impurities result in localization of spinons and an almost flat band close to the Fermi energy.
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