Temperature- and field-dependent ¹H-, ¹⁹F-, and ^79,81Br-NMR measurements together with zero - field ^79,81Br-NQR measurements on polycrystalline samples of barlowite, Cu₄(OH)₆FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the ^79,81Br-NMR spin-lattice relaxation rates 1/T₁ indicate a phase transition at T_N [Formula: see text] 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below T_N. Moreover, 1/T₁T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR- lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of v_Q ≃ 28.5 MHz and 24.7 MHz for ⁷⁹Br- and ⁸¹Br-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to η ≃ 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above T_N. This broadening and the relative large η value suggests a symmetry reduction at the Br- site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large η. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.

中文翻译：

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磁共振作为Barlowite Cu4（OH）6FBr中Kagomé磁性的局部探针。

对重晶石，Cu ₄（OH）₆ FBr的多晶样品进行了与温度和场相关的¹ H-，¹⁹ F-和^79,81 Br-NMR测量，以及零场^79,81 Br-NQR测量。在微观水平上研究磁性和可能的​​结构变形。^79,81 Br-NMR自旋晶格弛豫速率1 / T ₁的温度依赖性表明在T _N发生相变。 [公式：参见文本] 15 K是磁性起源，但具有异常弱的波动减缓作用低于T _ñ。而且1 / T ₁T与总体磁化率成线性比例，表明磁旋波动持续低至2K。四重共振（NQR）研究揭示了一对与Br的两个同位素相关的零场NQR谱线，其核自旋为I = 3/2 。四极耦合的v的常量_Q  ≃28.5 MHz和24.7兆赫为⁷⁹ BR-和⁸¹ BR-细胞核选自Br-NMR测定和电场梯度的非对称性参数估计η≃0.2。Br-NQR谱线与我们从Br-NMR得出的结果一致，它们相对较宽，甚至高于T _N。这种变宽和相对较大的η值表明在Br-位对称性降低，反映出晶格中存在局部变形。我们的密度泛函计算表明，位于kagome平面之间的Cu2原子的位移不能解释此相对较大的η。另一方面，完全的结构松弛，包括kagome平面的变形，导致与实验更好的吻合。