NMR evidence of spinon localization in the kagome antiferromagnet YCu₃(OH)₆Br₂[Br_1-x(OH)_x ]

We performed nuclear magnetic resonance studies on a kagome antiferromagnet YCu3(OH)6Br2[Br1-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 Kn and the spin-lattice relaxation rate 1/T1 increase when the sample is cooled from 30 K to 8 K, which can be attributed to the scattering of spin excitations by strong nonmagnetic impurities. Unusually, a hump is observed in Kn and 1/T2 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 the 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.