Impurities and defects in strongly correlated quantum systems often produce significant effects over an extended spatial region, which can be studied by local probes such as nuclear or electron magnetic resonance (NMR or ESR). The best example is the edge states in Heisenberg spin chains. The spin 1/2 edge state in spin 1 Haldane chains is a direct consequence of the valence-bond-solid ground state of the pure system. The ESR experiments have played vital roles in identifying the edge spins and their interactions. The edge states are not localized at a single site but associated with local staggered magnetization due to the antiferromagnetic interaction of the bulk, and the spatial extent of such a polaronic structure is given by the correlation length of the bulk. The real space profile of spin polarons have been actually observed by NMR experiments in both spin 1 and spin 1/2 Heisenberg chains, from which the temperature dependence of the correlation length was deduced. Thus, by studying impurity-induced phenomena, one can get intrinsic information of the bulk.
In this letter, we report the observation of such a spin polaron in real space by 11B NMR experiments on Zn doped SrCu2(BO3)2 performed in a sufficiently high magnetic field to saturate unpaired spins. With the help of exact diagonalization results, a nearly complete assignment of the 15 additional boron sites has been achieved, leading to the determination of microscopic structure of a localized spin polaron with unprecedented accuracy.
Reference : M. Yoshida, H. Kobayashi, I. Yamauchi, M. Takigawa, S. Capponi, D. Poilblanc, F. Mila, K. Kudo, Y. Koike, and N. Kobayashi, Real Space Imaging of Spin Polarons in Zn-Doped SrCu2(BO3)2, Phys. Rev. Lett. 114, 056402 (2015)