Symmetry Reduction in the Quantum Kagome Antiferromagnet Herbertsmithite
  Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the three-fold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state. 
A. Zorko, M. Herak, M. Gomil\v{s}ek, J. van Tol, M. Vel\'azquez, P.   Khuntia, F. Bert, P. Mendels
cond-mat.str-el