Spin dynamics and disorder effects in the S = 1 2 kagome Heisenberg spin-liquid phase of kapellasite

We report 35 Cl NMR, ESR, µSR and specific heat measurements on the S = 1/2 frustrated kagomé magnet kapellasite, α−Cu3Zn(OH)6Cl2, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction J1 and give an energy scale for the competing interactions |J| ∼ 10 K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term D, with |D/J| ∼ 3 %. These findings validate a posteriori the use of the J1 − J2 − J d Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagomé lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by 35 Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and µSR in the low-T , correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low energy modes.