真空退火LaO$_{0.5}$F$_{0.5}$BiS$_2$中的$^{19}$F NMR与缺陷自旋
$^{19}$F NMR and defect spins in vacuum-annealed LaO$_{0.5}$F$_{0.5}$BiS$_2$
AI总结：
本研究通过磁化与$^{19}$F NMR测量，发现生长态LaO$_{0.5}$F$_{0.5}$BiS$_2$中存在高浓度缺陷局域磁矩，其弛豫由导带涨落主导，且低温下无磁有序。
摘要：
我们报道了生长态LaO$_{0.5}$F$_{0.5}$BiS$_2$正常态下的磁化强度与$^{19}$F NMR（核磁共振）测量结果。磁化强度主要由与温度无关的抗磁分量，以及来自浓度约1000 ppm的局域磁矩的、与磁场和温度相关的顺磁贡献$M_μ(H,T)$主导，该浓度比测得的稀土杂质浓度所能解释的水平高出一个数量级。$M_μ(H,T)$可通过布里渊函数$B_J(x)$拟合，或者更符合实际的是，用缺陷晶体场中磁性Bi 6p离子的两能级$\tanh(x)$模型拟合。两种拟合都需要唯象的居里-外斯变量$x = μ_\text{eff}H/(T + T_W)$，其中$T_W \thickapprox 1.7$ K。在低至2 K的温度下没有磁有序的证据，且$T_W$的起源尚不明确。$^{19}$F的频移、线宽和自旋-晶格弛豫率与纯偶极$^{19}$F/缺陷自旋相互作用一致。从$^{19}$F自旋-晶格弛豫率得到的缺陷自旋关联时间$τ_c(T)$遵循科林加关系$τ_cT = \text{常数}$，表明弛豫由导带涨落主导。
英文摘要：
We report results of magnetization and $^{19}$F NMR measurements in the normal state of as-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution $M_μ(H,T)$ from a $\sim$1000~ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. $M_μ(H,T)$ can be fit by the Brillouin function $B_J(x)$ or, perhaps more realistically, a two-level $\tanh(x)$ model for magnetic Bi $6p$ ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument $x = μ_\mathrm{eff}H/(T + T_W)$, $T_W \approx 1.7$ K. There is no evidence for magnetic order down to 2 K, and the origin of $T_W$ is not clear. $^{19}$F frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar $^{19}$F/defect-spin interactions. The defect-spin correlation time $τ_c(T)$ obtained from $^{19}$F spin-lattice relaxation rates obeys the Korringa relation $τ_cT = \text{const.}$, indicating the relaxation is dominated by conduction-band fluctuations.
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